Evolution of the late Paleozoic accretionary complex and overlying forearc-magmatic arc, south...

TECTONICS, VOL. 18, NO. 4, PAGES 582-605, AUGUST 1999

Evolution of the late Paleozoic accretionary complex and overlying forearc-magmatic arc, south central Chile (38ø-41øS): Constraints for the tectonic setting along the southwestern margin of Gondwana

Mark W. Martin • Servicio Nacional de Geologfa y MinerVa-Chile, Santiago

Terence T. Kato

Department of Geosciences, California State University, Chico

Carolina Rodriguez, Estanislao Godoy, Paul Duhart, Michael McDonough, and Alberto Campos 2 Servicio Nacional de Geologfa y Miner/a-Chile, Santiago

Abstract. Stratigraphic, structural, metamorphic, and geochronologic studies of basement rocks in the Andean foothills and Coast Ranges of south central Chile (39ø-41øS) suggest a protracted late Paleozoic to middle Mesozoic deformational and metamorphic history that imposes important constraints on the tectonic development of the southwestern Gondwana margin. In the study area the late Paleozoic paired metamorphic belt, coeval magmatic arc, and overlying Triassic sedimentary units preserve a record of Late Carboniferous to Early Permian subduction and arc magmatism, subsequent deep exhumation of the Western Series subduction complex, and di- minished uplift and erosion of the Eastern Series arc-forearc region by the Late Triassic. Late Paleozoic structural elements and metamorphic assemblages formed during early subduction and arc magmatism, collectively referred to as D1, are largely erased in the Western Series by the dominant D2 schistosity and lower greenschist grade metamorphism. D1 structural features, as well as original sedimentary textures, are relatively well preserved in the less penetratively deformed Eastern Se- ries. The regional distribution of late Paleozoic arc magmatism suggests that the late Paleozoic convergent margin devi- ated from a N-S trend north of this area to a NW-SE trend near

this latitude and faced an open marine environment to the southwest. A transition from F2 isoclinal folding to more open, larger-scale F3 folds, interpreted as change in ductility during differential uplift of the Western Series, is not apparent in the Eastern Series. Despite a lesser degree of uplift during the main exhumational D2 event, delineation of unconformi-

ties and U-Pb dating of detrital zircons and intrusions into the Eastern Series allow tighter constraints to be placed on timing

•Now at Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge.

2Deceased July 1996.

Copyright 1999 by the American Geophysical Union.

Paper number 1999TC900021. 0278-7407/99/1999TC900021512.00

of uplift and denudation of the Eastern Series than on that in the Western Series. A regional unconformity exposed in the Lake District that separates more highly deformed Eastern Se- ries lithologies from Late Triassic shallow marine to continental deposits suggests that substantial uplift also affected the inner forearc and magmatic arc region during the D2 event. We propose that dextral-oblique convergence, initiated during the middle Permian along this segment of the Gondwana margin, resulted in the transpressional uplift and juxtaposition of high pressure/temperature (P/T) Western Series against low P/T Eastern Series lithologies and culminated with deposition of Late Triassic, continental to shallow marine, coarse clastic

sedimentary rocks in fault-bounded strike-slip basins adjacent to the exhumed 'Western Series. Large-scale dextral transpression and northward displacement of the accretionary complex during Late Permian to Late Triassic time along the Chilean margin of Gondwana are synchronous and kinematically compatible with widespread regional transpression, extension, and silicic magmatism inboard of the southern Gondwana margin at this time.

1. Introduction

Although the general framework regarding the timing, kinematics, and architecture associated with the onset of con-

vergent margin evolution along the western margin of South America during the late Paleozoic is reasonably well constrained [Herv•, 1988; Mpodozis and Ramos, 1989], detailed constraints are lacking. Remarkably little is known about the detailed tectonic history of the southwestern margin of Gond- wana during late Paleozoic to early Mesozoic time. Much of what is known has been inferred from geochemical studies of arc magmatic and felsic volcanic rocks [Mpodozis and Kay, 1992; Martin et al., 1999] or from extensional basin development and widespread silicic magmatism inboard of the Gondwana margin [Uliana et al., 1989; Kay et al., 1989; Ra- mos and Kay, 1991]. In contrast, very little is known of the rocks in the forearc region along the southwestern margin of

582

MARTIN ET AL.: EVOLUTION OFTHE PAIRED METAMORPHIC BELT, CHILE 583

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Gondwana. In general, this is because rocks of this age are either no longer preserved owing to subsequent subduction erosion [Stern, 1991] or extremely limited in extent and volume of exposure. However, where exposed in the rock record, it is often the forearc basin and accretionary prism environment that faithfully record the details of much of the kinematic history, including tectonic uplift and exhumation, along convergent margins, such as California [Page et al., 1998], Alaska [Kusky et al., 1997], Japan [Wallis, 1998], and other parts of the Circum-Pacific region [Mann and Gordon, 1996].

In this paper, we present field and geochronologic data from the late Paleozoic paired metamorphic belt and associated magmatic arc, exposed in the Lake District of the Andean foothills and Coastal Ranges of southcentral Chile (38ø-41øS; Fig- ure 1), which link deformation and metamorphism in the subduction complex with that in the coeval magmatic arc-inner forearc region. These data place tighter age constraints on the initiation of pre-Andean subduction processes along this segment of the southwestern Gondwana margin. Subsequent regional exhumation and juxtaposition of high pressure/temperature (P/T) and low P/T assemblages occurred during a period of quiescence in arc magmatism, possibly during or prior to dextral oblique convergence along the margin between the Middle Permian and Late Jurassic.

2. Paired Metamorphic Belt in South Central Chile

Many of the Paleozoic metamorphic rocks that crop out along the Chilean coastline from Chafiaral (26øS) to Cape Horn (56øS) are interpreted to belong to a late Paleozoic forearc accretionary complex [Forsythe, 1982; Dalziel, 1984; Herv& 1988]. These rocks offer a unique opp6rtunity to study pre-Andean convergent margin evolution along the southwest facing Gondwana margin at this time. Lithologies indicative of accretionary complexes, including metaclastic rocks, metacherts, metabasalts, ultramafic rocks, serpentinites, retrograde high P/T metamorphic assemblages, low P/T metamorphic assemblages, and associated magmatic arc intrusions, are widely distributed latitudinally in the southern half of Chile (see Hervd et al. [1981 ] for discussion). Existing data regarding the age of these lithologies, timing of onset of deformation, metamorphism, and accretion of the complex are limited to poorly bracketed stratigraphic and relative crosscutting relations, Carboniferous to Permian Rb-Sr and K-Ar ages from metamorphic rocks within the retrograded high P/T belt, and Rb-Sr, K-Ar, and 4øAr-39Ar ages from the associated late Paleo- zoic batholith south of 34øS [Hervd et al., 1981, 1984; Muni- zaga et al., 1988; Beck et al., 1991 ]. At 37øS, from west to east, three regional lithotectonic units have been distin- guished (Figure 1): a high P/T Western Series (WS), a low P/T Eastern Series (ES), and a late Paleozoic granitic batholith intrusive into the Eastern Series [Aguirre et al., 1972; Hervd et al., 1974, 1981; Karo, 1985].

Figure 1, Regional map showing geology (35ø-42øS) from Chilean coastline to Argentine border. Areas of Figures 2 and 6 are inset by rectangles.

584 MARTIN ET AL.: EVOI_,UTION OF THE PAIRED METAMORPHIC BELT, CHILE

Eastern Series metasedimentary rocks, in general, except near the contact zone of the late Paleozoic batholith, consist

of weakly to moderately transposed metasandstones and metapelites, which in areas of low strain preserve sedimentary structures [Gonzdlez-Bonorino, 1970; Herv•, 1977]. Unlike Western Series lithologies, marie metavolcanic rocks, serpentinites, and metacherts have not been recognized in the Eastern Series assemblage. Eastern Series rocks are characterized by low P/T metamorphic assemblages that are spatially associated with the late Paleozoic Nahuelbuta granitic batholith [Gon- zdlez-Bonorino, 1970; Aguirre et al., 1972; Hervd et al., 1976].

The Western Series complex is composed of penetratively deformed, highly recrystallized metamorphosed chert, elastic, mafic volcanic, and ultramafic rocks containing pervasive lower greenschist grade (chlorite zone) metamorphic assemblages [Aguirre et al., 1972; Hervd et al., 1974, 1981; Kato, 1985]. Although rarely preserved, pillow structure in metabasalt, graded bedding, and fine lamination in metachert are sufficiently widespread to suggest a deep marine origin for West- ern Series lithologies. The tholeiitic affinity of the metabasalt has been established by geochemical data [Godoy, 1979, 1980]. The sporadic but widespread occurrence of the blueschist minerals, sodic amphibole [Aguirre et al., 1972], and lawsonite, reported in meta-argillite on Chilo6 Island [Sal- iot, 1969] as relict minerals in the lower greenschist assemblage, have assigned the Western Series to high P/T type metamorphism [Aguirre et al., 1972; Herv• et al., 1974, 1981; Kato, 1985]. The ubiquitous lower greenschist, retrograde metamorphic event is synchronous with intense planar transposition and schistosity (S2) development, which generally obliterates earlier metamorphic and structural elements within the Western Series schists [Kato, 1985].

The distribution of Western Series and Eastern Series

lithologies in the Central Valley is not well constrained (Fig- ure 1). West of the Central Valley, Western Series rocks, though poorly exposed owing to dense vegetation and weath- ering, constitute a continuous map unit in the Coast Range as far south as Chilo6 Island (42øS). East of the Central Valley, metasedimentary rocks are exposed in the Lake District as isolated, discontinuous outcrops between the north shore of Lago Calafqu•n and the southern shore of Lago Ranco; however, nei- ther the age nor the tectonic affinity of these rocks has been well documented. South of Temuco (38ø40'S), the Western Se- ries-Eastern Series contact is covered by coarse elastic Tertiary sediments of the Central Valley. Metasedimentary rocks in the

Lake District are bound on the east by intrusions of the North- ern Patagonia Batholith, which range in age from Late Paleo- zoic to Tertiary [Moreno and Parada, 1976; Munizaga et al., 1988; Beck et al., 1991].

3. Lithotectonic Relationships Within the Basement of the Lake District (Inner Forearc and Magmatic Arc)

Previous work on the basement rocks in the Lake District region separated it into several units: the Tralcfin Formation, the Panguipulli Formation, the Metamorphic Basement, and granitic rocks. The unmetamorphosed Tralc• Formation is exposed on Cerro Tralcfin on the west shore of Lago Rifiihue (Figure 2) and consists of medium- to thick-bedded continental red beds and conglomerates that dip shallowly to the south [Aguirre and Levi, 1964; Parada, 1975]. The age of the Tralcfin Formation is considered Late Triassic on the basis of preserved plant fossils [Tarera, 1971]. Consensus is lacking, however, as to the age of the rocks in the region which comprise the Panguipulli Formation and the Metamorphic Basement (Aguirre and Levi [1964]; and many others). These studies suggest that the Panguipulli Formation ranges in age from pre- Carboniferc, t•s to Late Triassic. Basement in the region is intruded by granitic intrusions of the Northern Patagonia Ba- tholith that have yielded Rb-Sr, K-Ar, and 4øAr-39Ar cooling ages that range from Tertiary to Carboniferous [Parada and Mu- nizaga, 1978; Munizaga et al., 1988; Beck et al., 1991]. U-Pb crystallization ages have not been published from these intrusions.

3.1 Field Criteria for Differentiation of Metasedimentary Rock Units

3.1.1. Traffin Sequence. Mapping of the basement rocks in the region indicates that the "Panguipulli Formation" can be subdivided into two units. The older unit, which we term the Traffin Sequence for exposures described by Illies[ 1960] in the Rfo Traffin area (Figure 2), is exposed west of the Northern Patagonia Batholith and is regionally deformed and metamorphosed. Where primary sedimentary features have not been overprinted, the Traffin Sequence consists of thin- to medium- bedded (thick to massive bedding occurs locally), rhythmically interbedded brown to gray, fine- to medium-grained cross- stratified sandstones, siltstones, mudstones, and locally, in-

Figure 2 (opposite). Map distribution of Traffn Sequence (Eastern Series), Pennslyvanian-Permian Batholith, Panguipulli Formation, and Tralcgn Formation in the Lake District. Solid circles on map are geochronological sample locations; unless denoted by an asterisk (K-Ar), a number sign (4øhr-39Ar; F. Munizaga, personal communication, 1998), or an "at" sign (4øAr-39Ar: C. Rod- riquez, personal communication, 1999), ages are U-Pb zircon determinations. Open circles on map are known middle to late Triassic plant fossil localities. Equal area stereoplots contain poles to both bedding (SO, solid circles), axial planar cleavage (S 1, pluses; S2, solid squares), D 1 fold axes (F1, open squares), D2 fold axes (F2, stars), and best fit D 1 and D2 fold axes (shaded circles and open circles, respectively). (Stereoplot program is by R. Allmendinger). Figure 4 is the area inset by rectangle.

MARTIN ET AL.: EVOLUTION OF THE PAIRED METAMORPHIC BELT, CHILE 585

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586 MARTIN ET AL.: EVOLUTION OF 22dE PAIRED METAMORPHIC BEUF, CHILE

tercalated angular-clast conglomerates (i.e., Isla Huapi on Lago Ranco, Figure 2). Massive sandstone horizons often contain mudstone rip-up clasts from underlying strata. On Isla Huapi, pelitic, rhythmically bedded, fine-grained sediments (Figure 3a) and massive sands are interbedded with and grade laterally into massive, coarse angular-clast conglomerates containing an assortment of igneous and metamorphic clasts up to several meters in diameter. Clasts are dominated by metamorphic (gneiss, schist, and slate), plutonic (felsic, intermediate, and mafic/ultramafic), and volcanic (felsic to intermediate) rocks (Thiele et al. [1976]; and this study). Bed- ding thickness, grain size, sedimentary structures, and repeti- tious nature of the sequence suggest a marine, mass-flow origin, probably a middle to lower fan slope environment.

3.1.2. Panguipulli Formation. The Panguipulli Formation is also exposed west of the Northern Patagonia Ba- tholith but is restricted to the northern part of the study area (Figure 2). Relative to the Traffin Sequence, the Panguipulli Formation is less deformed, very feebly recrystallized, and generally records subgreenschist grade metamorphism. Unlike the Traftin Sequence, it consists of repetitive packages of medium- to thick-bedded, fining upward conglomerates to coarse sands to fine sands and locally to shales (Figure 3b), some of which contain plant fossils [Arrondo et al., 1988]. Each pack- age is 1-4 m thick. Low-amplitude cross bedding indicates that the sequence is generally upright. Locally, basal conglomerate horizons contain shale rip-up clasts derived from the immedi- ate underlying shale bed. Conglomerate clasts are well rounded to subrounded and range in size from pebble- to cobble-sized clasts up to 15 cm. Conglomerate horizons consist domi- nantly of imbricated, milky, polycrystalline quartz and schist and locally of granitic clasts. The macroscopic and microscopic detrital components imply the exposure of granitic masses in the region during deposition of this unit (Hervg et al. [1976]; and this study). Repetitive, thin, graded bedding indicative of turbidite origin, similar to those ubiquitous in the Traffin Sequence, are not observed. A much shallower depositional environment (upper fan-slope channel) than that for the Traffin Sequence is inferred from fossil plant remains, bedding characteristics, coarse grain size, and imbricated clasts.

Confusion surrounding the age of the Panguipulli Forma- tion arises from two issues. One is the early tentative identifi- cation of upper Carboniferous-Permian plant fossils collected from shale horizons [Tavera, 1971; Minato, 1977] within conglomerate-to-shale packages in the Panguipulli Formation at Punta Peters along the western shoreline of Lago Pan- guipulli (Figure 2). Subsequent study of the plant flora from this locality and from the north shore of Lago Calafqu(•n in identical lithologies identified these fossils as belonging to the Middle to Late Triassic interval [Arrondo et al., 1988]. The second issue is that along the western shoreline of Lago Pan- guipulli, exposures of channel-fill conglomerates and coarse sandstones of the Panguipulli Formation are in close proxim- ity to turbidites that contain SE-vergent mesoscopic folds and cleavage; however, the contact between these two lithologies is never exposed. The turbidites and kinematics of the told structures are identical to those of the Traffm Sequence exposed to the south at Rfo Traffin (Figure 2). On the basis of these similarities and the contact relationship described in section 3.1.3, we group the turbidites of Lago Panguipulli with the

Trafiin Sequence. However, an alternative interpretation is that the turbidites exposed on Lago Panguipulli are lateral facies equivalent to the channel-fill conglomerate of the Panguipulli Formation and that folding and cleavage formation, observed only in the turbidites, is a function of the competency contrast of the two different lithologies.

3.1.3. Contact relationship between Traffin Sequence and Panguipulli Formation. The only known exposure of this contact relationship is exposed along the northern shoreline of Lago Calafqu6n (Figure 4). Here folds in metamorphosed (biotite grade)fine-grained, thin- to medium-bedded pelitic sedimentary rocks of the Traffin Sequence contain a west-vergent axial planar cleavage. These rocks and fabrics are intruded by a coarse-grained biotite, hornblende granodiorite that has yielded an 4øAr-39Ar hornblende age of 305 Ma (F. Munizaga, personal communication, 1998). At the contact, folded, cleaved, and metamorphosed, fine-grained metasedimentary rocks of the Traffin Sequence are unconformably overlain by moderately to shallowly east dipping unmetamorphosed fining upwards packages of conglomerate, coarse- grained red sandstone, sandstone, and shale of the Panguipulli Formation. The dominant conglomerate clast type is white polycrystalline quartz and fine-grained schist similar to Traffin Sequence lithologies; imbrication of conglomerate clasts is common. Near the base of the Panguipulli Formation, altered granite clasts are present, suggesting that uplift and erosion of the late Paleozoic intrusions had occurred by the time of Pan- guipulli deposition. It is from shales in the Panguipulli For- mation, between the contact with Traffin Sequence and Lic/in Ray, that Late Triassic plant fossils have been identified [Ar- rondo et al., 1988].

3.1.4. Intrusive rocks (late Carboniferous- Early Permian batholith). ParadaandMunizaga [1978] inferred that much of the batholith in the region outlined in Figure 2 was Middle Jurassic in age on the basis of a Rb-Sr isochron yielding an age of 160 + 20 Ma for samples collected over an area of several 100 km 2. Subsequent workers, using the Rb-Sr, K-Ar, and 4øAr-39Ar methods, found Carboniferous, Ju- rassic, Cretaceous, and Tertiary ages within the Panguipulli Batholith [Munizaga et al., 1988; Beck et al., 1991]. Textur- ally and compositionally, these intrusive phases range from very coarse grained to fine-grained and from granites to to- nalites and diorites [Parada and Munizaga, 1978]. As discussed in section 3, in several localities along the west side of the Northern Patagonia Batholith, granitic rocks intrude deformed and metamorphosed rocks of the Traffin Sequence.

3.2 Structural and Metamorphic Relationships

The Traffin Sequence records two kinematically distinct folding events (Figure 2), the older (D1) being SE-vergent and the younger being NE-vergent (D2). In the northern half of the area where D1 is little affected by D2, D1 is associated with NE-trending, tight to isoclinal folds (F1). F1 folds are typically characterized by northwest dipping, steeply to moderately inclined axial planar cleavage (S1). In the southeastern part of the area, D1 is significantly more intense, and locally, there is complete transposition of bedding and S1 into parallelism. On Isla Huapi in Lago Ranco (Figure 2), map-scale


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588 MARTIN ET AL.: EVOLUTION OF THE PAIRED METAMORPHIC BELT, CHILE

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Figure 4. Map relationship of Trafdn Sequence and Pan- guipulli Formation contact along north shore of Lago Ca- lafqu6n.

folds trend NE and verge to the SE [Thiele et al., 1976]. Our work along the shoreline of Isla Huapi indicates that many of the lithologies display SE-vergent cleavage, which is often preserved in conglomerate horizons as a ductile transposition of the fine-grained matrix or, locally, by transposition of clasts parallel to cleavage, at a high angle to bedding. South- east-vergent map-scale folds on Isla Huapi and S1 are interpreted to be D 1 structures.

Quartz veins are ubiquitous in the Traffin Sequence; however, they are most prevalent in the southern part of the region. Veins range from less than a centimeter in width and 2-3 cm in length up to a meter in width and several meters in length and, without exception, have tensile fracture morphologies. They are deformed by F1 folding and thus formed early in the deformational history of the Traffin Sequence. Where weakly or nondeformed, it is clear that quartz-filled frac- tures formed as a result of ductility contrast between fine- and coarse-grained beds. They were deformed during subsequent transposition of bedding and S 1 during F2 folding (Figure 3c). In exposures where bedding and cleavage relations are completely transposed, quartz veins provide the only strain marker for determining the degree of strain recorded in the Traffin Se- quence.

Along the northern shoreline of Lago Ranco (Futrono Pen- insula) and directly to the north in the Trafdn River drainage (Figure 2), the overprinting of D1 structures by D2 folding is

well preserved. At the outcrop scale, S 1 has been folded about NW trending open to tight folds (F2). F2 folds are associated with a steeply SW dipping axial-planar crenulation cleavage •,S2) and metamorphism that does not exceed lower greenschist grade (Figure 3d). Locally, in the northern half of the area, D2 is expressed typically as a poorly defined, incipient-spaced cleavage in the fine-grained rocks of the Traffin Sequence (Fig- ure 2).

Petrographic studies indicate that D1 is roughly synchronous with chlorite to sillimanite grade regional metamorphism related to the emplacement of the late Carboniferous to earliest Permian batholith. The progressive change of mineral assemblages in pelitic metasediments toward the intrusive contact include the presence in the assemblage of (1) chlorite, (2) chlorite + biotite, (3) biotite, (4) andalusite + cordierire, and (5) cordierire + sillimanite +/- andalusite. Near contacts with these granites, metamorphic fabric is parallel to subparallel with magmatic foliation (defined by the planar alignment of biotite, hornblende, and feldspar)often observed along the margins of these intrusions; migmatitic textures are locally preserved.

Large porphyroblasts of the higher-grade metamorphic minerals (andalusite, sillimanite, and cordierire) in all cases appear to predate or form early in the formation of the S 1 mineral foliation. Pressure shadows, elongation of porphyroblasts parallel to foliation, and flattening of foliation around porphyroblasts all suggest growth prior to or during an early phase of the main deformation. Andalusite porphyroblasts in some samples have central, nonpoikiloblastic cores, often exhibiting a well-defined chiastolite "cross" surrounded by poikiloblastic andalusite elongated in 'the foliation plane, suggesting its initial static and then dynamothermal growth. Biotite and chlorite are aligned within the foliation and appear to form later in the D1 event. In some cases, they overprint the foliation as postkinematic porphyroblasts. The presence of cordierite and andalusite in medium-grade pelitic rocks and the absence of staurolite and garnet in the assemblages establish the low P/T type metamorphism in the region during the D1 event. The apparent close approach to textural equilibrium between andalusite, sillimanite, and cordierite in a few samples suggests maximum pressures of 2.8 kbar at 550øC [Baldwin et al., 1997] during D1.

In a few samples, retrograde metamorphism of higher-grade assemblages (andalusite replaced by chlorite + pyrophyllite) followed the D1 deformational phase. Other samples, however, show little evidence of retrogression except for incipient replacement of biotite by chlorite. The extent of retrograde reaction appears to be limited by access of fluids to the rock, possibly during the D2 event.

Unlike the Traffin Sequence, outcrop-scale folds have not been observed in the Panguipulli Formation; as a result, structural data of the quality found in the Traffin Sequence are lacking (Figure 2). Outcrops of Panguipulli Formation are typically homoclinal, although bedding may vary between exposures from near horizontal to vertical to slightly overturned. In addition, penetrative deformation (i.e., cleavage development) is rarely observed and mostly occurs along sharp lithologic boundaries where the ductility contrast is expected to be high (e.g., between shale and coarse sandstone). Though few data exist, orientations from bedding over a large area, in-

MARTIN ET AL.' EVOLUTION OF THE PAIRED METAMORPHIC BELT, CHILE 589

a [• XA-76 Granitic 0.0794

0.0694

b.[• XQ-119 • ,.,4,4 • Metasand

,., o, 41 6• I 400 / Depositional age <403 Ma

'.0614[ 207Pb / 235U 0.3675 0.4675 0.5675 0.6675 0.4936 0.8936 1.2936

Ca

0.0665

0.0565

0.0465

O.O365

XA-34 Sandstone • single-grain detrital zircon •

Depositional age <230 Ma

207P, b / 235U 0.• 0.3•7• 0.4'•77 0.56•

d XA-284 G

2./•• •::• • • • Age = 304.7 +_ 2.1 Ma

207Pb / 235U ,

0.3250 0.3350 0.3450

ea 0.0479

0.0469

0.0459

0.0449

•XC-90 Granite

•,--'• Intercepts at 824.8 + 135.9 Ma .,/-"• and 281.6 _+ 3.5 Ma

.•• (MSWD = 0.58) .... 207Pb/2, 35U

0.3225 0.3325 0.3425

• XA-224 Granite

0.1886

Age = 176.9 +_ 3.3 Ma

207Pb / 235U 0.1'896 ' 0.1'906 '

gl 0.0486

0.0466

0.0446

Granodi

/

• XA-41

29

Age = 300.5 __ 2.0 Ma

o.3•,• ' o.3;,• ' o.3;•

h. • XA-258 Granite

0.0466 • 300 / /• Inter ec Ptan•.t 8 _+0433•).1_+r•6 Ms

0.3161 0.3261 0.3361 0.3461

Figure 5. (a-m) U-Pb concordia diagrams. MSWD, mean square weighted deviate.

590 MARTIN ET AL.' EVOLUTION OF THE PAIRED METAMORPHIC BELT, CHILE

i.l• XC-67 Granodiorite • J.r• XA-124 Granite • Intercepts at ø'ø"•l• • ..• l• 2560.3_+217.0Ma / •:• 30•r• J I 00510l• and 297.7 •0.9 Ma 32•

i o.o.ot _ 0.046 0.0470J 30

• 207pb / 235U 0.3326 0.3366 0.3406 0.3446 0.3486 0.3406 0.3606 0.3806

k.

0.047!

0.046•

0.0451

,• XA-45 Granodiorite

2,,•'•ercePtS at 672.3 _+ 298.5 Ma x, / and 288.8 _+ 12.9 Ma

• (MSWD = 1.06) ..... ,207Pb / 235U

0.3225 0.3325 0.3425 0.3525

• XC-27 Granite 3•• •' • 307•/'

3

3,,• • Age - 305.5 + 2.5 Ma

301• "• 207pb / 235U 0.3457 0.3497 0.3537

rn,,

0.0135

0.0133

0.0131

a XO-20 Granite •• , (,•'•

?- 8

• 8

85

84 Age = 91.3 + 4.9 Ma

• 4.5) 207Pb / 235U 0.0862 0.0882 0.0902

Figure 5. (continued)

cipient-spaced cleavage, and penetrative foliation, developed along lithologic boundaries within the Panguipulli Formation, are consistent with D2 kinematics obtained from the Traffm

Sequence (Figure 2). Quartz veins that are ubiquitous in Traftin Sequence are not observed in the Panguipulli Formation. Fur- thermore, metamorphism of the Panguipulli Formation does not exceed lower greenschist grade. The structural and metamorphic history recorded in the Panguipulli Formation is therefore in sharp contrast to that of the Traftin Sequence.

3.3. Timing Constraints

The age of the Traffn Sequence is poorly constrained. Field and petrographic studies clearly indicate that the low P/T metamorphic grade recorded in the Traffin Sequence increases from west to east toward the Northern Patagonia Batholith. These rocks are intruded by granites that have yielded K-Ar and 4øAr-39Ar cooling ages that range from 282 to 309 Ma [Muni- zagaet al., 1988; Beck et al., 1991], placing a minimum age

592 MARTIN ET AL.: EVOLUTION OF THE PAIRED METAMORPHIC BELT• CHILE

6666 •6666 •6666

c• II

MARTIN ET AL.: EVOLU'llON OF THE PAIRED METAMORPHIC BELT, CttlLE 593

Table 2. K-Ar Analyses for Samples Collected in the Lake District, Chile

Sample Latitude-Longitude, deg. UTM Coordinates Lithology Material K% %At' %Aft Age (MaY XA-34 39ø39'10"S-72ø16'34"W 733700-5607250 sandstone clinochlor-muscovite 5.777 70.988 7 291.0 + 7.0

XA-41 39ø51'41"S-72ø17'07"W 732212-5584084 granodiorite 4 302.0 + 7 0 XA-50 40ø04'53"S-72ø17'56"W 730300-5559700 granodiori•e 6 316.0 + 7.0 XA-69 40ø18'29"S-72ø21'34"W 724400-5534700 granodiorite 10 286.0 :t: 9.0 XA-91 40ø07'07"S-72ø13'00"W 737200-5555350 granodiorite 8 267.0 :t: 8.0 XA-21• 39ø52'12"S-72ø16'37"W 732900-5583100 granite 3 296.0 :t: 7.0 XA-22,-: 39ø43'40"S-72ø10'50"W 741636-5598650 granite 5 180.0 :t: 4.0 XA-225 39ø44'13"S-72ø10'25"W 742210-5597590 tonalire 23 142.0 + 5.0

XA-239 39ø50'02"S-72ø04'39"W 750100-5586575 diorite 5 142.0 :t: 3.0

XA-25-: 39ø54'54"S-72ø10'33"W 741381-5577845 tonalire 20 167.0 :t: 4.0

XA-258 39ø52'35"S-72ø12'15"W 739111-5582220 granodiorite 4 182.0 :t: 4.0 XA-279 39ø45'11"S-72ø1 !'43"W 740293-5595871 tonalire 29 159.0 :t: 5.0

XE-57 39ø38'48"S-72ø05'22"W 749742-5607400 tonalite 14 257.0 :t: 8.0

XE-68 39ø45'33"S-72ø00'02"W 756942-5594646 granodiorite 26 178.0 :t: 6.0 XE-83 39ø58'59"S-72ø20'17"W 727300-55'20700 tonalire 5 301.0 + 7.0

biotite 7.084 90.064

biotite 6.975 93.656

amphibole 0.462 5.558 amphibole 0.696 7.798

biotite 7.544 94.447

biotite 6.963 51.165

amphibole 0.449 2.574 biotite-chlorite 6.443 37.064

biotite-chlorite 5.586 38.056

biotite 6.861 51.194

amphibole 0.411 2.663 amphibole-chlorite 0.6 6.444 amphibole-chlodte 0.336 2.443

biotite 6.444 82.129

percent radiogenic Ar as nL/g percent atmospheric Ar error reported as 2-sigma

on the unit. In an effort to place a better maximum age bracket on the Traffin Sequence, a medium-grained, altered granite clast, sample XA-76, was collected from an angular-clast conglomerate, which is interbedded with metaturbidites on Isla Huapi in Lago Ranco. Zircons from this sample were analyzed, and though the data are inconclusive, the age of this clast is interpreted to be younger than 383 Ma, the analysis with the

207 206

youngest Pb/ Pb date (Figure 5a, Table 1, and appendix). In addition, a sample of metasandstone (XQ-119) was collected from the Traffin Sequence in order to date detrital zircons. The

2ø7pb/2ø6pb youngest detrital zircon from this sample has a date of 403 Ma (Figure 5b, Table 1, and appendix), indicating this sample is Early Devonian or younger in age.

To better constrain the absolute age of the Panguipulli Formation, a coarse sandstone horizon was sampled (XA-34) from Punta Peters, and detrital zircon and muscovite were ana-

lyzed by the U-Pb and K-Ar methods, respectively (Figure 5c, Tables 1 and 2, and appendix). The U-Pb detrital zircon data indicate that the sandstone is younger than Middle Triassic (230 Ma), consistent with the Middle to Late Triassic plant flora preserved in this unit. The detrital clinochlorite- muscovite yielded an age of 291 Ma (Table 2). These data imply that uplift and erosion of late Paleozoic intrusions and metamorphic rocks of both the Traffm Sequence and Western Series had occurred by Middle Triassic time, thereby serving as a source for igneous and metamorphic detritus of this age in the Panguipulli Formation.

To constrain the crystallization ages of granitic phases within the Northern Patagonia Batholith of the Lake District and to place minimum and maximum age constraints on both the Traffin Sequence and the Panguipulli Formation, respectively, we collected nine samples of granite-granodiorite from widespread localities (Figure 2). New U-Pb zircon and K-Ar analyses (Figures 5c-5k, Tables 1 and 2, and appendix), with one exception, imply that a significant area is underlain by granitic intrusions that range in age from 280 to 316 Ma (late Carboniferous-Early Permian) and not by Jurassic age intrusions as suggested by Parada and Munizaga [ 1978].

3.4. Discussion of Basement Relationships in the Lake District

On the basis of the preceding evidence, the Traffin Sequence consists of late Carboniferous marine turbidires (middle-lower

fan slope), locally intercalated with coarse conglomerates that are interpreted to represent deep water channel deposits. The clast composition of these conglomerates indicates derivation from a plutonic-metamorphic terrain [Thiele et al., 1976] that could be derived from basement exposed in Argentina [Fran- szese, 1995], the Liquifie gneisses in Chile and Argentina, or the Western Series rocks [Thiele et al., 1976]. However, the presence of clasts of metamorphosed but discernable fine- grained volcanic rocks and only slightly foliated coarser mafic igneous rock and gneisses and the absence of strongly foliated, coarse crystalline, greenschist grade clasts and polycrystalline quartz clasts rules out the post-D2 Western Series schists (as discussed in section 4) as a likely source for the clasts of Traffin Sequence, consistent with its age, and implies deposition of this unit prior to final amalgamation of the Western Series complex.

Granitic intrusions yield U-Pb zircon and K-Ar ages that range from 280 to 316 Ma; one sample yielded a Middle Juras- sic age. Southeast-vergent D1 dynamothermal fabrics, along with early-formed high-grade porphyroblasts, in pelitic rocks of the Traffin Sequence are prekinematic to early synkinematic with emplacement of these intrusions. Where intrusive relationships are not observed, metamorphic grade in the Traffin Sequence increases toward exposures of late Carboniferous- Early Permian granites (e.g., islands of Lago Ranco). North- east-vergent, lower greenschist grade D2 fabric overprints the D 1 fabric.

The Panguipulli Formation rests unconformably on the Traffm Sequence, compared to which it records both lower greenschist grade metamorphism and significantly less strain. Detrital zircon U-Pb geochronology and plant fossils support a Middle to Late Triassic age for the Panguipulli Formation. Sandstones and conglomerates of the Tralcfin Formation contain Triassic plant flora and are assumed (contact not exposed)


to rest depositionally on Traffn Sequence rocks :[Aguirre and Levi, 1964]. The data presented here are consistent with the Tralc•in Formation being the lateral and subaerial•equivalent of Panguipulli Formation. Alternatively, since the Tralc• For- mation is undeformed and is nearly flat lying, unlike the Pan- guipulli Formation, the Tralc•in Formation may be somewhat younger than the Panguipu!li Formation.

The data presented here suggest a major change in thermal conditions and orientation of stress regime in the time span from Early Permian to Late Triassic. Though few data exist, orientations from bedding over a large area, incipient-spaced cleavage, and penetrative foliation developed along lithologic boundaries within the Panguipulli Formation are consistent with D2 kinematics obtained from the Traffn Sequence (Figure 2). At present, we believe that this deformation is :the D2 event recorded in the Traffn Sequence. Northwest of the region, between Talca and Temuco (35ø-39øS, Figure 1), Late Triassic sedimentary and volcanic rocks are folded [Hervd et al., 1976], however, folding in these rocks appears to preserve NW-SE di- rected shortening. South of the area, in the Chonos archipel- ago (45 ø S), Middle to Late Triassic sedimentary rocks are also folded and cleaved [Hervd, 1998]. Alternatively, D2 may be part of a protracted event which developed later in the evolution of D1 associated with the buildup of the Carboniferous- Permian batholith (316-280 Ma); the change in shortening direction from D1 to D2 time is a function of plate convergence vectors during the lifespan of the arc. In this scenario, D1 in the Panguipulli Formation may postdate D2 of the Trafiin Se- quence, and the two deformational events are not necessarily related.

On the basis of the stratigraphic, structural, metamorphic, and timing constraints presented here, we conclude that much of the area covered by lithologies previously mapped as Metamorphic Basement and Panguipulli Formation belong to one map unit that we include in the Traffn Sequence. Further- more, timing constraints and lithologic, structural, and metamorphic characteristics of the Traffn Sequence require correla- tion with Eastern Series rocks exposed NW of the region. These similarities include (1) fine-grained pelitic and psammitic lithologies, (2) low P/T metamorphic assemblages, (3) timing and style of deformation, and (4) tectonic position between high P/T Western Series rocks to the west and similar age and composition granitic batholith to the east. We conclude, therefore, that Eastern Series lithologies continue to the SE, from Pur6n, beneath the Central Valley, and into the Lake District (Figure 1).

4. Lithotectonic Relationships Within the Western Series (Subduction Complex and Outer Forearc)

In the Coastal Range west of the Lake District (Figure 1), the Western Series complex preserves t•w textural indicators of the original depositional environment and therefore generally lacks internal stratigraphic constraints. The structural grain of the schistose, highly recrystallized Western Series is, however, remarkably consistent. Between latitudes 39ø30'S and 40ø30'S a regional structural fabric striking N50øW to N70øW is evident in the strike of S2 schistosity (Figure 6). This trend is expressed as moderately south and north dipping

S2 schistosity that is folded about open to tight fold axes (F3). The orientation of S2 foliation in the northern half of the re-

gion is less consistent than that in the southern half, in part owing to its shallower dips. Relict S1 mineral foliation is poorly preserved only as microscopic tight to isoclinally folded graphitic inclusions within porphyroblastic albite and, where less recrystallized, as fine residual micaceous layers, isoclinally folded between the dominant S2 schistosity planes. Petrographically, primary sedimentary and volcanic textures have been obliterated by transposition and recrystal- lization related to the D2 event. However, rare primary features, such as graded bedding in quartz-mica schist, or pillow structure in mafic schist, can be observed at the outcrop scale. In several localities, near the town of Mehuin and at Pucatrihue

(Figure 1), overturned relict-graded bedding in recumbent isoclinal F2 folds suggests that low-angle nappe structures were formed during the D2 ductile deformation. Although tectonic discontinuities between nappes similar to those reported from the Sanbagawa terrane of Japan [Wallis, 1998] are not yet identified, their presence may be inferred from apparent ire- gional differences in pre- to late D2 metamorphic assemblages [Kato, 1985].

Along the southern coastline, near Bahia Mansa (Figure 1), dips on S2 axial planes are much steeper, often exceeding 40 ø, owing to less ductile F3 folding of S2 schistosity planes; however, large-scale isoclinal folding and overturning of F3 fold limbs are not observed (see stereoplot, Figure 6). Al- though small-scale structural features such as mylonitic fabric. and cataclastic zones crosscutting S2 are present locally, de- monstrative field evidence for large-scale thrusting associated with D3 is difficult to establish, primarily owing to lack of exposure. Although crenulation cleavage and brittle-ductile shearing is evident in outcrop, little evidence of metamorphism is evident in the D3 structures, which clearly postdate the main phase of lower greenschist metamorphism.

Aeromagnetic anomalies associated with the Western Se- ries basement complex west of the Lake District [Parra, 1980; and unpublished map corrected for regional gradient (G. Yafiez, personal communications, 1996)], are broadly divisible into northern and southern magnetic zones (Figure 6). The northern magnetic zone consists of basement-related anomalies exceeding 100 nT. South of this zone, linear anomalies have lower amplitudes (<50 nT) but consistently trend between N60øW and N65øW. Because of poor exposure and Cenozoic cover, the zone separating these magnetic domains is not exposed. Ex- posures in the poorly exposed northern magnetic zone include disrupted serpentinized ultramafic and metagabbroic "ophiolitic" lithologies, best exposed near Morro Bonifacio along the coast between Corral and Mehuin (Figure 6). These magnetite rich, high magnetic susceptibility rock types are structurally interleaved with lower susceptibility quartz-mica schists and mafic schists. Continuous linear magnetic anomalies, which characterize the southern part of the terrain, are due primarily to NW-SE striking magnetite rich mafic schists exhibiting high magnetic susceptibilities, and isolated discontinuous anomalies are related to discrete serpentinite bodies [Kato and Godoy, 1987; Godoy and Kato, 1990]. Soffia and Pincheira [1997] interpret structural data from an area immediately NE of the boundary separating northern and southern magnetic domains to support the presence of large-scale SW-vergent thrust

MARTIN ET AL.: EVOLUTION OF •12-1E PAlRED METAMORPHIC BELT, CHILE

Pacific

Ocean Mehuin

I

73øW T

ws

5 10 15 20 Scale (km)

N

Chaihuin

Cz

Cz

ws

ws

4oos ws

Cz

N

CI - 2.0 sig

Cz

Figure 6. Map of Western Series (WS) west of Lake District showing domains of different aeromagnetic anomaly domain boundaries and strike and dip of S2 foliation. Stereoplot is of poles to S2 (n=167 and contour interval equals 3 standard deviations). Stereoplot program is by R. Allmendinger. Cz, Cenozoic; gr, granite; CI, contour interval.

595

faults in the region. Given these data, a reasonable interpretation of the magnetic anomaly separating the northern and southern magnetic domains is that it represents a regional thrust duplex that emplaced serpentinized ophiolitic assemblages into a largely metasedimentary terrain during D3 deformation.

The area north of the northern magnetic zone and south of the Western Series-Eastern Series contact near Pur6n (Figures 1 and 6) is nearly devoid of magnetic anomalies (Magnetic Zone 1, Godoy and Kato, [1990]) and is characterized by subhori- zontal S2 foliation [Kato, 1985], essentially lacking evidence

of D3 deformation. The amount of NE-SW contractional strain accommodated within the Western Series during the D3 event thus appears to have been more pronounced regionally in the south.

4.1. Metamorphism of the Western Series

In the area of study, the ubiquitous lower greenschist metamorphic assemblage in pelitic and psammitic schists in the Western Series is quartz + albite + muscovite + chlorite +/- (low Fe)epidote + graphite + Fe-(Ti) oxide. Almandine rich

596 MARTIN ET AL.: EVOLUTION OF TItE PAIRED METAMORPHIC BELT, CHILE

ao

•2(•,..', ' .... •-"•"• • •; •'•:•, .sprl. .•,' .•.- • ,...• :, • , •'..'.:•

' x-••••'••••••.••...;•.•..>-•,,.>,."•-•.•, ,•½,•->,...,..•.. ß .':, .

............... , ............... • • • '• • p ..... •., ........ ;.:,• ........ -.:. •. .... •.•.....-• .,,. .... ,; .:....... ,; .•.. •.:. . , .... ,...r•.:,-.. ,•---•n.)•-¾..•. , ' ' •- '-':' <' ';-:"'*) .... •' • • ......

'-':- • "•' 'a". -•.•' ..... ::-::.• , ' .... ..-, •"' Ma,,. ' C'•'.? :-

... • .:->.--?:: ..... ,•..•.Z;i.•. - xN.:.>,:,;:: ,•:.•.--'-.. ..? >, ... . .:-i•,, :.. '. :½' ".•:--." . '•' ..•'-.: -, •' ' '- -:----:. •" -- ' ' '- .::.;.'i:. "2

Width of photomicro•aph is 0.5•

Near Los Pabilos (Figure 1), crossite (sodic amphibole) occurs as relict cores rimmed by actinolite in a mafic schist. The actinolite, along with albite, chlorite, epidote, and magnetite, forms the S2 fabric of the rock. Crystals of stilpnomelane, nucleating on magnetite crystals, both parallel and crosscut the S2 fabric suggesting late syntectonic (D2) to posttectonic crystallization (Figure 7a). This suggests, in one sample, a crude, clockwise P/T path of transitional blueschist- greenschist to greenschist to lowermost greenschist conditions. In addition, both syn-S2 stilpnomelane and zussmanite have been reported just north of the previous area [Massonne et al., 1998].

The presence of rare relict crossite and epidote assemblages in the Western Series implies uplift of portions of the complex from depths in excess of 25 km (7 kbar, Figure 8). Many of the widespread lower greenschist grade assemblages, however, lack reliable relict pressure indicators and may never have been buried to lower crustal depths. Because of the widespread distribution of the rare, relict high-pressure phases, deep exhumation of a significant volume of the complex is inferred during retrograde metamorphism associated with the regional D2 event in the Western Series.

constituents.

Rare relict, in situ sodic amphibole is reported in schists at Mehuin [Massonne et al., 1998], and at the Mau16n Hills northeast of Valdivia [Huffmann et al., 1997], and just south of the area near Los Pabilos (Kato et al. [1997]; and this report) suggesting at least local high P/T conditions. However, the far more common occurrence of relict garnet and biotite suggests that temperatures were also elevated in most parts of the complex prior to D2 lower greenschist retrograde metamorphism. The rare but widespread occurrence of large stilpnomelane crystals crosscutting the S2 planar fabric (Figure 7a) and postkinematic enlargement of albite porphyroblasts in some schists suggest that the retrograde metamorphism, at least locally, outlasted the D2 deformation.

200 300 400 500 600

Temperature (øC)

Figure 8. Pressure-temperature diagram showing inferred minimum pressure conditions (curve I) for crossite-bearing mafic schists of the Western Series, and maximum pressure conditions (curve II) for cordierite-bearing metapelites of the Eastern Series. (Cz, clinozoisite; G1, glaucophane; Qz, quartz; Tr, tremolite; Chl, chlorite; Ab, albite; Act, actinolite; Ep, epidote; FeOx, iron oxide; Ky, kyanite; Sill, sillimanite; And, andalusite; Gt, garnet; Cd, cordierite; Sp, spinel; F1, fluid) Equilibria curves are after the following 1, Maruyama et al. [1986]; 2, Brown [1978]; 3, Maruyama et al. [1983]; 4, Kerrick [1968]; 5, Holdaway [1971]; and 6, Baldwin et al. [1997].

b. Width of photomicrograph is 1.2mm -" '.2' :-' "'" •' '" :?":,' :.."•:--':• •.'½,•:2"• "•:"-•:". ..... -•. '--' .... ' '

........... • . '. .... •..,..• .... .,

•' Biotite . "-'::..: • ....: .... ,..-.,, • • •. • • . .

. .., '/:...';" ':'•N •. ß •:-'• '•-•:':.• '"":.%.•;: ::.."

""- ' "-': • ' - . '",; '; -•' • .... "-.-." ' :--" :.•-<'--' '". .... • '- •'.'• : :"

• '" '" ......... •" ' ' ............. •"• .... ' '• •': ...... '"

stilpnomelane (stilp) crosscutting S2 pl•ar foliation (relict sødic amphibøle (Na-amph) rimmed bY actinølite (act))' (b) • Photomicrograph of Eastern Series schist: Biotite porphyro- blast overprinting S1 foliation in metasandstone. (Photomi- crographs were t•.en at 100x.); sph, sphene; mag, magnetite.

garnet •d biotite are widespread as relict phases texturally re-

by whi, schists (metabasites) contain the assemblage albite + chlorite + actinolite + epidote + sphene +/- quiz +/- magnetite. These lithologies comprise the main volume of the Western Series

complex, but serpentinite •d metachert •e also significant

MARTIN ET AL.: EVOLI_rlION OF THE PAIRED METAMORPHIC BELT, CltlLE 597

4.2. Timing of Deformation and Metamorphism in the Western Series

A maximum age of deformation in the region is constrained by U-Pb detrital zircon studies between 42 ø and 40øS which indicate that some metasandstones within the Western Series are

younger than Middle Devonian and, locally, younger than Early Permian [Duhart et al., 1997]. If 275-300 Ma detrital zircon recovered from these rocks [Duhart et al., 1997] is derived from the uplifted and eroded batholith of the same age (as discussed in section 3.3), then these sediments could be as

young as Triassic. A 20-step 4øAr-39Ar heating experiment defined a plateau age for white mica (coexisting with sodic amphibole) of 324 +/- 3 Ma from a surficial boulder that is interpreted to date blueschist metamorphism in the Los Pabilos region (Figure 1) [Kato et al., 1997]. This date accords reasonably well with a K-Ar date of 304 +/- 9 Ma for white mica from another blueschist boulder at the same locality [Kato and Godoy, 1995] and a 328 Ma K-Ar date on glaucophane from the Western Series blueschist locality north of the study area near the coastal town of Pichilemu (Figure 1) [Hervg et al., 1974].

Whole rock Rb-Sr isochron ages that range from 342 to 280 Ma from Western Series schists have been interpreted to constrain the timing of regional peak metamorphism [Hervg, 1988; Munizaga et al., 1988]. More recently, Permian to Early Triassic K-Ar dates from Western Series schists on white mica and hornblende range from 220 to 248 Ma [Duhart et al., 1997]. Large white micas amenable to K-Ar analysis in the Western Series schists are generally formed during the D2 event and, if unzoned, probably date the uplift and cooling through the Ar blocking temperature (,approximately 350øC) during the regional retrograde event.

A local minimum age constraint on D3 deformation and regional exhumation of the Western Series is provided by the intrusive age and contact aureole relationships associated with a small granodioritic pluton that intrudes the Western Series near Chaihuin, southwest of Corral (Figure 6). A large dike along the periphery of the pluton that crosscuts the Western Series schist and D2 and D3 structures was sampled and dated at 91.3 +/- 4.9 Ma by the U-Pb zircon method (Figure 51, Table 1, and appendix). A K-Ar date of 103 +/- 2 Ma [Munizaga et al., 1988] is also reported from the pluton. Within the contact aureole of the Chaihuin pluton are the only occurrences of biotite replacing chlorite and hastingsitic hornblende rimming actinolite known to the authors in the regional area of the Western Series. These features indicate an increase of T in a

clear thermal overprint of the lower greenschist assemblage by a very narrow, low-pressure, hornblende hornfels grade aureole at the intrusive contact.

Collectively, sharp intrusive contacts, a narrow contact metamorphic zone, and mineral zoning relationships are consistent with an epizonal intrusive depth of only a few kilometers. Exhumation from deeper crustal levels was therefore completed by the time of intrusion. Since the intrusion crosscuts D3 folds defined by field attitudes and aeromagnetic linea- tions (Figure 6), it postdates these structures. No evidence for subsequent contraction is observed in the area peripheral to the stock. The subsequent amount of uplift required to bring these rocks to the present erosion surface probably was accom- plished by Cenozoic high-angle faults which are observed on

seismic reflection profiles along the periphery of the coastal range [McDonough et al., 1997a, b].

These data saggest that peak (high P/T) metamorphism within the Western Series occun•ed between 328 and 304 Ma.

Permissive petrographic evidence suggests that this metamorphism was synchronous with D1. D2 was accompanied by nearly complete transposition and retrogression of D1 fabrics associated with tectonic exhumation and juxtaposition of high P/T assemblages with shallower sedimentary rocks within the forearc zone. The timing of D2 is constrained by the Early Permian age of the youngest detrital zircon dates (275 Ma) from Western Series [Duhart et al., 1997]. K-Ar dates [Duhart et al., 1997] indicate that temperatures were in the vicinity of 350øC between 250 and 220 Ma. This thermochronology is consistent with structural evidence (discussed in section 4.1

and this section) and experimental data that indicate a transition from ductile (quasi-plastic) to brittle (elasticofrictional) behavior (in this case, during the D2-D3 transition) in the 250-350øC temperature range [Dunlap et al., 1997]. We suggest that uplift of the Western Series during the D2 to D3 transition inversely transgressed both these boandaries, resulting in both Ar closure and a change from ductile to more brittle- ductile deformation. D3 deformation and regional uplift within the Western Series occurred prior to emplacement of shallow level intrusions in the Late Cretaceous.

5. Metamorphic and Structural Relationships Along the Eastern-Western Series Contact in the Pur6n Area

The contact separating the Eastern Series from the Western Series at the latitude of the Lake District is not exposed. How- ever, the contact is reasonably well exposed north of the region and has been interpreted as a transitional contact in the Concepci6n to Pichilemu area (Figure 1) by Gonzc•lez- Bonorino [1971], who at that time referred to the Western Se- ries as Curepto Zone III and the adjacent Eastern Series as Curepto Zone II. Hervg [1977] considered the N-S contact between the Western Series and Eastern Series along the western margin of the Nahuelbuta Range between Concepci6n (36ø40'S) and Carlere (37ø40'S) to be a regional fault with large vertical displacement which sustained post-Miocene and possibly older movement. The NW-SE trending segment near Pur6n (Figure 1) has been interpreted as transitional between highly transposed, schistose Western Series and less penetratively deformed Eastern Series metasediments [Hervg, 1977; Kato, 1985].

Northeast of the town of Pur6n (Figure 1), Eastern Series metasedimentary rocks, between the Western-Eastern Series contact and intrusive contact of the Nahuelbuta Batholith, ex-

hibit a progressive increase of metamorphic grade northeast- ward, in the direction of the Batholith, and a progressive but heterogeneous increase in postintrusive, penetrative strain southwestward, toward the Western-Eastern Series contact. The

higher-grade pelitic schists (andalusite and/or sillimanite bearing) exhibit a steep metamorphic gradient toward the intrusive contact, as described in this area and to the north, along the western contact zone of the Nahuelbuta Batholith [Hervg, 1977]. These higher-grade metamorphic rocks contain a crude


mineral foliation S1 defined by biotite and muscovite alignment, which postdates static porphyroblasts of cordierite and andalusite. At lower grades (biotite zone, upper greenschist facies), biotite porphyroblasts are consistently late D1, parallel to S1 in pelitic layers (Figure•7b), and overgrowing strained sandstone clasts. Metamorphic grade decreases irregularly toward the Western-Eastern Series contact from biotite to biotite

+ chlorite assemblages in the pelitic schists. Within 5 km of the Western-Eastern Series contact, biotite and chlorite exist

in apparent textural equilibrium, and a clearly defined crenulation cleavage crosscuts the S1 mineral foliation. Within a kilometer of the contact, transposition along cleavage planes becomes so pronounced that S1 minerals are completely transposed into parallelism with S2. The Western-Eastern Series contact is defined where transposition is so complete that only the S2 schistosity is apparent in outcrop [Hervd, 1977].

In contrast, within the Western Series, remnants of S1 are

only visible petrographically as minute graphitic layers within late syn- to post D2 albite porphyroblasts. The widespread textural replacement of biotite by chlorite in the West- em Series schist is due to a high variance, retrograde reaction of the following type: phengite + chlorite <=> biotite + muscovite + quartz + (H20) fluid. In addition, balanced retrograde reactions can be written for the disappearance of garnet and growth of stilpnomelane in high Fe/Mg Western Series mafic- intermediate schist [Kato, 1985]. These reactions support the influx of fluid during the regional retrograde (and decompres- sional) D2 and possibly D3 events that exhumed the deeply buried Western Series complex. The source of voluminous aqueous fluid input during these events, especially during a short time interval may have been due to rapid duplexing and tectonic mixing of upper crustal sediments with previously accreted and metamorphosed portions of the subduction complex. The tectonic mixing of older oceanic crustal constituents with younger sedimentary units is suggested to have occurred regionally in the Western Series [McDonough et al., 1997a, b].

Within the narrow Western-Eastern Series contact zone, layer parallel extension is visible as elongate boudins of deformed Eastern Series metasandstone enveloped in more highly strained and recrystallized schist and phyllite similar to Western Series schists. The incipient development of quartz segregations along tension gashes in Eastern Series metasediments near the contact grades across the boundary to large, meter-scale quartz boudins parallel to S2 in the Western Series schist. These quartz veins probably served as a source of the distinctive and ubiquitous milky white polycrystalline quartz clasts present in shallow marine to terrestrial Middle to Late Triassic conglomerates exposed between 35 ø and 40øS [Hervd et al., 1976] and for the Panguipulli Formation in the Lake District previously described.

The approximate minimum prograde thermal gradient for Eastern Series metamorphism is estimated at 196øC/kbar (65øC/km; Figure 8), based on the andalusite-sillimanite- cordierite equilibria discussed in section 3.2. The maximum thermal gradient for rare crossite + epidote assemblages in the Western Series is estimated as 60øC/kbar (17øC/kin; Figure 8). Since both the Western Series and adjacent portions of the Eastern Series (at Pur6n) were subjected to lower greenschist metamorphism during late stages of D2, a crude minimum es-

timate of 4 kbars (14 km)of differential uplift of the Western Series relative to the adjacent Eastern Series Js inferred prior to culmination of the D2 event when temperatures were approximately 300ø-350øC. If the D 1-related assemblage of andalusite + cordierite in the Eastern Series did not sustain later burial,

the total cumulative uplift of that level of the batholith aureole to the present erosion surface did not exceed 10 km. The region-wide extent of late D2 lower greenschist grade metamorphic assemblages in the Western Series and adjacent Eastern Series suggests that by the end of the D2 deformation both units were at similar crustal levels and at similar thermal condi-

tions.

6. Discussion and Tectonic Synthesis

6.1. Early Subduction and Accretionary Phase (Late Carboniferous to Early Permian)

The Late Paleozoic to Jurassic tectonic development at the latitude of the Lake District is summarized in Figure 9. Turbid- ites of the Traffin Sequence and the Eastern Series suggest a marginal marine environment whereas fine-grained clastic rocks, cherts, pillow lavas, and serpentinites of the Western Series are indicative of a more open marine and/or primitive arc-backarc environment on oceanic crust [Vivallo et al., 1988]. These two lateral time-equivalent environments were set outboard of the South American cratonal landmass. The

available data prevent placing accurate age constraints on the timing of deposition for each of these sequences; however, the data do indicate that the older components' are at least Early Devonian and younger and that Permian [Duhart et al., 1997] to possibly Triassic age sediments have been structurally interleaved. Successively younger sedimentary rocks deposited in the forearc would have been structurally incorporated with older sequences as deformation at the margin progressed (Fig- ure 9c).

During the late Paleozoic a convergent margin developed along nearly the entire length of the southern Gondwana margin (Figure 9) [de Wit and Ransome, 1992]. Subduction-related processes initiated in the Lake District by latest Carboniferous time as supported by the presence of high P/T metamorphic assemblages dated between 328 and 304 Ma [Hervd et al., 1974; Kato and Godoy, 1995; Kato et al., 1997]. Deformation within the arc and forearc region at this time supports NW-SE contraction (D1, SE-vergent in Eastern Series) consistent with late Carboniferous sinistral shear recorded in metapelitic rocks in the accretionary complex near 42ø30'S [Sanhueza, 1996], and suggests a component of oblique sinistral convergence along the margin at this time (Figure 9c.1).

Arc magmatism above the subduction zone was active con- currently and continued into the Early Permian. It was during this time frame that the assembly of the paired metamorphic belt began with amalgamation of the Western and Eastern Se- ries into an accretionary complex along this margin (Figure 9c.1). This setting is consistent with the timing of early arc magmatism recorded in north central Chile (30øS) in the Elqui- Limari batholith [Mpodozis and Kay, 1992]. However, arc magmatic rocks of this age or older have not been documented from farther south in the North patagonia Batholith [Hervd, 1988; Mpodozis and Ramos, 1989; Rapela and Kay, 1988], al-

MARTIN ET AL.: EVOLUTION OF THE PAIRED METAMORPHIC BELT, CHlLE 599

C.

es = Inner forearc complex + late Carboniferous batholith ws = Subduction complex + outer forearc complex

Pacific Ocean

(Dev? - e. Perm) Early Subduction

(I. Perm - m. Jur) Transpression/uplift

(3)

/

/

",, /

t ws [es \

WS '"

(I. Jur. - present) Andean Subductio•

b.

--20oS

--28oS

Figure 9c

--36oS

--44øS •

--52oS

Triassic rifts Cuyo basin

..... Choiyoi volcanics

Figure 9. Tectonic summary diagram. (a)Late Paleozoic-early Mesozoic Gondwana reconstruction modified from Kay et al. [1989] and de Wit and Ransome [1992] showing the distribution of Proterozoic and older crust (stippled) and younger Paleozoic accreted terranes (shaded). Subduction and accretion processes along the southern margin of Gondwana-are depicted by dashed barbed line, K-C, Karoo basin and Cape fold belt. (b) Gen- eralized Permian-Triassic paleogeography along southwestern South American margin (present-day coordinates) modified from Ramos and Kay [ 1991]. Heavy dashed line approximates boundary between Precambrian (:rust and early Paleozoic accreted terranes. In the regions of the Paran•i Basin (PB), Sierra Septentrionales (SS), Sierra Australeõ (SA), and Sierra Grande (SG), deformation of Permian-Triassic age supports NE-SW maximum horizontal contraction and NW-SE maximum horizontal extension (present--day coordinates) [Cobbold and Ga- pais, 1991; Cobbold et al., 1992].. (c) Infe_rred tectQnic setting along the Chilean Gondwana margin (present day coordinates) from (1) Devonian(?) to Early Permian to (2) Late Permian to Middle Jurassic to (3) Late Juras- sic to present day. Heavy double-headed arrows represent hypothesized relative plate convergence vector, and fold symbol represents E'fi0wn c6-ntractile deformation.

. ,: • . -=•__•.; .... :• .•__: ._.


though Carboniferous age, arc magmatism is known in Argen- tina, east and southeast of the Lake District [Pankhurst et al., 1992, 1993a, b]. Metasedimentary rocks exposed to the south of the Lake District in Chile and Argentina have been interpreted to belong to the late Paleozoic accretionary complex [Forsythe, 1982; Hervg, 1988] with some evidence that the complex contains progressively younger sedimentary and metamorphic components toward the south [Hervd, 1988, 1998].

The exact nature and geometry of the tectonic setting south of the Lake District, during the late Paleozoic, remains unclear (see Rapela and Kay [1988] for discussion). However, the data presented here suggest that the Carboniferous-Early Permian magmatic arc extended toward the south along the southwestern Gondwana margin into the Lake District and made an east-southeastward bend into present-day Argentina, and that an open marine environment existed to the S-SW. At this time the magmatic arc would have trended southeastward into the Argentine Patagonia (Figure 9c.1).

6.2. Exhumation and Transpressional Deformation and Translation of the Subduction-Arc Complex (Middle Permian to Middle Jurassic)

Both the subduction complex and magmatic arc were being exhumed and eroded during this time span, although deformation and metamorphism during this period were more intense in the subduction complex (Western Series). Components of the more deeply buried subduction complex were juxtaposed with overlying accretionary prism assemblages which contain U-Pb detrital zircon dates as young as Early Permian [Duhart et al., 1997]. We suggest that this uplift coincided approximately with K-Ar age distribution (220-250 Ma) [Duhart et al., 1997] and the change from ductile, D2, to more brittle, D3, deformational characteristics within the Western Series. The

main D2 deformational event coincided with pervasive lower greenschist grade metamorphism which involved decompres- sion (from blueschist facies) and retrograde metamorphism (from upper greenschist-epidote amphibolite grade) in various parts of the Western Series [Kato, 1985]. Eastern Series lithologies were also incipiently retrograded to lower greenschist grade during this time frame. The more weakly expressed D2 deformation in the Eastern Series is consistent with moder-

ate burial prior to differential uplift of the more deeply buffed Western Series.

From Middle Permian to Middle Triassic there is a con-

spicuous lack of arc magmatism in southern Chile and Argen- tina. This suggests that either the subduction zone shallowed to the point where magmatism was shut off, or subduction ceased entirely, possibly associated with the accretion of an exotic terrane suggested by recently reprocessed seismic data from the Lake District region (M. McDonough, unpublished data, 1998) and/or convergence became sufficiently oblique so that magmatism ceased. The evidence that both Western Series and Eastern Series rocks experienced uplift during this time frame and the initial development of prominent NW strike of S2 in the Western Series complex and D2 of Eastern Series suggest to us that there was a major component of dextral- oblique convergence along the Western-Eastern Series contact at this time (Figure 9c.2).

By Middle to Late Triassic time, uplift and erosion of the late Carboniferous-Early Permian paired metamorphic belt and granitic batholith are recorded in the coarse clastic detritus in the subaerial to submarine sediments belonging to the Pan- guipulli-Tralcfin Formations and equivalent stratigraphic sequences exposed between 35ø-39øS (Figure l) [Herv• et al., 1976]. These rocks, which locally contain clasts of exhumed Western Series and Paleozoic batholith, rest depositionally on the Eastern Series and batholith. Locally, these deposits are intercalated with rhyolitic volcanic rocks [Hervg et al., 1976], implying that volcanism had again commenced in the region. The coarseness of these deposits and the abrupt lateral and vertical facies transitions were interpreted by Hervg et al. [1976] as indicating that rapid basinal subsidence was an important aspect in controlling sedimentation at this time.

The contact separating the Eastern Series (Traffin Sequence) from the Western Series is not exposed west of the Lake Dis- trict; however, given the known distribution of the Western Series and Eastern Series lithologies between 40 ø and 39øS, the contact must trend N-S to slightly NE-SW (Figure 1). It seems likely, given the regional metamorphic and structural timing constraints, that the NW trending D2-D3 folds that formed during late stage uplift of the Western Series complex are coeval and kinematically related to D2 folds in the Lake District. The obliqueness of the N-S contact separating the Western Series from the Eastern Series and NW trending folds in both units suggest that this contact is a high-angle north-south trending fault (Figure 9c.2).

We speculate that deposition of the Panguipulli-Tralcfin Formations and equivalent sequences was controlled by strike- slip faulting associated with dextral-oblique convergence along the margin. Continued deformation along N-S trending strike-slip faults linked by either NW trending reverse faults or NE trending normal faults in this setting lead to subsequent deformation of these sediments. We hypothesize that the N-S trending contact separating the Eastern Series and Western Se- ries in the Lake District was a strike-slip fault connected to a subparallel fault of greater magnitude in the present offshore region west of the present Coast Range by NW-SE trending reverse faults within a strike-slip duplex-type setting (Figure 9c.2).

The timing of the events recorded from the Middle Permian to Middle Jurassic forearc region of the Lake District, in southern Chile, agrees well with the studies from Argentina and northern Chile that suggest transpressional deformation, and cessation of late Paleozoic subduction-related magmatism, and the transition to extension along the Gondwana margin during the middle Permian to Jurassic [Dalziel et al., 1987; Ulliana et al., 1989' Kay et al., 1989' Ramos and Kay, 1991' Cobbold and Gapais, 1991' Mpodozis and Kay, 1992]. Field studies by Cobbold and Gapais, [ 1991 ] and Cobbold et al. [ 1992] in eastern Argentina in the Sierras Septentrionales a•qd Australes and in the Sierra Grande (Figure 9b), indicate that these areas experienced dextral-oblique transpressional deformation associated with NE-SW contraction along the southwestern margin of Gondwana during Permian-Triassic time. These authors further point out that deformation of similar age and stress regime is known from the Paranti Basin in Brazil and the Karoo Basin

and the Cape fold belt in South Africa (Figure 9). Mpodozis and Kay [1992] have hypothesized the oblique


collision of an unexposed exotic terrane (Equis Terrane) to explain the apparent gradual termination of subduction-related magmatism during the m/d-Permian in central Chile (28 ø- 31øS). In addition, there is ample structural, sedimentological, and volcanological evidence for the development of extensional basins in Argentina and Chile during the ri'riassic (Fig- ure 9b)[Uliana et al., 1989] and Jurassic [Dalziel et al., 1987]. These basins are subparallel and crudely en echelon along N-S to NNW-SSE trends and are inferred to be controlled by reactivation of older Paleozoic NW and NE striking structures [Uliana et al., 1989]. Depending on the orientation of the southern Gondwana •nargin, the en echelon aspect of these basins supports a component of oblique convergence along the margin at this time, although the primary control on the geometry of these basins appears to have been the grain of pre- existing structures.

A north-south trending, dextral strike-slip tectonic environment for post-Middle Triassic deformation is also compatible with paleomagnetic evidence indicating large-scale (15 ø of latitude) northward translation of portions of the Coast Range now situated near 30øS during the Late Triassic to Late Jurassic time [Forsythe et al., 1987]. This timing of events also accords well with the model for postcollisional reinoval of the hypothesized Equis Terrane by strike-slip faulting, postulated by Mpodozis and Kay [1992] in central Chile (28ø-31øS), and possibly with the formation of extensional basins at this time [Charrier, 1979; Cornejo et al., 1993; Martin et al., 1999]. Kay et al. [1989] have suggested that late Paleozoic to Jurassic silicic magmatism that erupted inboard of the Gondwana margin in southern South America formed in an extensional setting without subduction during a period when Gondwana was relatively stationary.

Although the temporal distribution of this magmatism is only broadly constrained to middle Permian to Jurassic time, the work presented here, from the southern Chilean segment of the Gondwana margin, suggests that transpressional uplift and northward translation of subduction complex-outer forearc material were synchronous with widespread silicic magmatism, extensional basin formation, and transpression inboard of the margin. We believe that the dextral-oblique convergence at this time along the South American portion of Gondwana is kinematically compatible with each of these tectonic regimes. Widespread silicic magmatism in the absence of subduction is aided by little or no plate motion [Kay et al., 1989]. Kay et al. [1989] cite paleomagnetic data that support little latitudinal motion of Gondwana during this time frame, and they assumed minor east-west motion, which cannot be determined as pre- cisely from paleomagnetic data. Together, these regional data sets support differential lateral motion between southwestern Gondwana and the proto-Pacific plate.

Since Late Jurassic time, the north-south locus of arc magmatism is well established along the entire present-day Chil- ean Cordillera cutting across. the SE trending late Carbonifer- ous-Early Permian continental margin that likely existed south of the Lake District (Figure 9c.3). This suggests that the in- tervening time was a period of accretion in the Chilean Cordil- lera south of the Lake District, which was accompanied by oblique convergence without significant subduction-related magmatism at the latitude of the Lake District. This is most easily explained if from Middle Permian to Middle Jurassic

time the Gondwana margin south of the Lake District was more orthogonal to the plate convergence vector but the margin to the north was more oblique to this vector. The ultimate effect of northward translation of crustal fragments during the Permo- Triassic and subsequent southward accretion along this part of the Gondwana margin would have been to create a more linear north-south trending continental margin along which the later Andean arc could develop (Figure 9).

Appendix: U-Pb and K-Ar Geochronology

A1. Sample XA-76

Sample XA-76 is a leucocratic granite clast collected from an angular clast conglomerate unit in the Trafiin Sequence on Isla Huapi of Lago Ranco (Figure 2). The zircon syste•natics of this sample are not well behaved. Each fraction is strongly discordant (Figure 5a and Table 1) Individual fractions have 2ø7pb/2ø6pb dates that range from •83 to 738 Ma (Table 1). Given the leucocratic composition and the multiple zircon morphologies and evidence for zircon overgrowth present (not analyzed) in this sample, it is likely that certain of these zircon populations are xenocrystic. On the basis of these characteristics, we interpret the crude linear array to be associated with inheritance and not with Pb loss. A maximum age for the granite is interpreted to be the youngest Pb-Pb date, 383 Ma. The conglomerate is therefore younger than this but older than the late Carboniferous-Early Permian granites that intrude the Traftin Sequence.

A2. Sample XQ-119

Sample XQ-119 is a metasandstone from the Traffin Se- quence collected just north-northwest of the study area outlined by Figure 2. The sample yielded numerous zircons of various morphologies and color. The majority of zircons show varying degrees of rounding and frosting associated with mechanical transport; however, euhedral, doubly terminated zircon crystals with little evidence for rounding or frosting associated with mechanical transport are also present, though less common. There is no evidence for metamorphic zircon overgrowth. From the euhedral zircon population, five individual zircon grains were abraded and analyzed (Table 1 and Figure 5b). All five analyses are discordant and have 2ø7pb/2ø6pb dates that range from 1297 to 403 Ma. On the basis of these data, we interpret the depositional age of this metasandstone to be younger than 403 Ma (Early Devonian).

A3. Sample XA-34

Sample XA-34 is a sandstone collected from the type locality of the Panguipulli Formation at Punta Peters on the western shoreline of Lago Panguipulli (Figure 2). The sample yielded numerous zircons of various morphologies and color. The majority of zircons show varying degrees of rounding and frosting associated with mechanical transport; however, euhedral, doubly terminated zircon crystals without evidence of rounding or frosting associated with mechanical transport are also present, though less common. There is no evidence for metamor-

602 MARTIN ET AL.: EVOLUTION OF TIlE PAIRED METAMORPHIC BELT, CHILE

phic zircon overgrowth. Four single detrital zircon grains were abraded and analyzed from this sample (Table ! ..•d Figure 5c). The oldest zircon analyzed was we!! rounded, frosted and 207 • '206 ......

pale yellow in color and yielded a Phi Pb date (17% discordant) of 2629 Ma (Late Archcan). A slightly frosted, colorless, acicular euhedral zircon is within error of concordia at 461 Ma. A euhedral, colorless (not frosted), inclusion-free zircon is concordant at 340 Ma. A euhedral, colorless (not frosted) zircon (containing several small inclusions) yielded concordant dates (within error) at 230 Ma. These data indicate that this sandstone has detrital components that are Late Archcan, Ordovician, Carboniferous, and Middle Triassic in

age. We interpret the alepositional age of this sandstone to be younger than 230 Ma (Middle Triassic). In addition, a K-Ar age on detrital, coarse-grained clinochlorite-muscovite yielded an age of 291 Ma (Table 2).

A4. Sample XA-284

Sample XA-284 is a medium-grained granodiorite that was collected from peninsula Huichoco along the south shore of Lago Calafqu6n (Figure 2). Six zircon fractions were analyzed, all of which are slightly discordant (Table 1 and Figure 5d). Five of the six fractions fall along a linear array with an upper intercept of 345 + 111 Ma; one fraction falls off this array. However, we feel that this upper intercept age is unrealistic and is a function of both Pb loss and small amounts of inheritance

in these zircons. We believe that a better estimate of the age of this sample is the weighted mean of the 2ø7pb/2ø6pb dates for the five fractions that define the linear array, 304.7 + 2.1 Ma (mean square weighted deviate (MSWD) equals 1.57).

A5. Sample XC-90

Sample XC-90 was collected approximately 7 kms east of the northeast shore of Lago Panguipulli (Figure 2). The sample is a medium-to coarse-grained biotite, hornblende granodiorite. Five zircon fractions were analyzed from this sample (Table 1 and Figure 5e). The 2ø7pb/2ø6pb dates of these fractions and the discordia defined by the five fractions suggest that these zircon populations have a component of inherited lead. This interpretation is supported by the air abrasion of fraction (nm(0) +74, where nm stands for nonmagnetic and (0) is the angle of tilt) for 12 hours, which moved this fraction up the discordia array toward an older 2ø7pb/2ø6pb date. The age of this sample is interpreted to be defined by the lower intercept age of the discordia, 281.6 + 3.5 Ma.

A6. Sample XA-224

Sample XA-22-'. is a granite that was collected along the eastern north shoreline of Lago Panguipulli (Figure 2). Two zircon fractions were analyzed from this sample, one of which was a single crystal. Both fractions are concordant and yield identical dates within error (Figure 50. The age of this granite

207 206

is interpreted to be the weighted mean of the Pb/ Pb dates of both fractions, 176.9 __ 3.3 Ma. Biotite from this sample was analyzed by K-Ar and yielded a date of 180.0 + 4.0 Ma (Ta- ble 2).

A7. Sample XA-41

Sample XA-41 was collected at Punta Torombo along the southern shoreline of the Lago Rifiihue (Figure 2) and is a coarse-grained, well-foliated (magmatic foliation) biotite granodiorite (63% SiO2). Except for the magmatic foliafion it is representative of the igneous rocks exposed along the southern and northern shoreline of Lago Rifiihue. The Lago Rifii- hue granodiorite intrudes fine-grained metaturbidites of Traffin Sequence.

Six zircon fractions from this sample were analyzed (Table 1 and Figure 5g). The systematics of the zircon fractions in this sample suggest that these zircons suffer both from Pb inheritance and from Pb loss. Four of the six fractions cluster

near 300 Ma. Relative to this cluster, the older 2ø7pb/2ø6pb date of fraction (nm(-3) <<74) suggests a minor amount of inherited lead, whereas fraction (nm(-1) +74) appears to have experienced Pb loss. In order to better access the significance of Pb loss in fraction (nm(-1) +74), nine gem quality zircons were selected from this fraction and analyzed. This fraction plots nearer to concordia at 300 Ma. In addition, a fraction of 12 gem quality zircons was selected from the finer size fraction of (nm(ol) +74) and (nm(-1) o74hp, where hp stands for hand- picked), with the result that this fraction plotted within error of concordia at 300.4 Ma. On the basis of these U-Pb systematics, the age of this sample is interpreted to be 300.4 + 5.0 Ma. Biotite from this sample was analyzed by K-Ar and yielded a date of 302.0 + 7.0 Ma (Table 2).

A8. Sample XA-258

Sample XA-258 is a granodiorite that was collected along the eastern north shoreline of Lago Rifiihue (Figure 2). Three single zircon crystals were analyzed, and the interpretation of their systematics is straightforward. These systematics are indicative of simple present-day Pb loss, and the 2ø7pb/2ø6Pb dates for the three zircons are identical within error (Figure 5h). A linear regression through these data yields an upper intercept of 304.9 + 2.6 Ma (MSWD equals 0.02), which we interpret as the age of crystallization of this intrusion. Biotite from this sample was analyzed by K-At and yielded a date of 182.0 + 4.0 Ma (Table 2), suggesting that the Ar system has been thermally reset by younger intrusions in the area.

A9. Sample XC-67

Sample XC-67 was collected approximately 10 kms south of the eastern part of Lago Rifiihue (Figure 2). The sample is a coarse-grained biotite granodiorite (61% SiO2). Five zircon fractions from this sample were analyzed (Table 1 and Figure 5i). The initial four fractions analyzed from this sample suggested an age range between 299 and 292 Ma. The coarsest fraction (nm(-1) ++74) is just in error of concordant at 299 Ma, and three fractions fall on a discordia with a lower intercept of 292 Ma. In general, this type of behavior can be attributed to small amounts of lead loss and inheritance in the zircon populations. Ten gem quality zircons were selected from the most concordant fraction (nm(-1) ++74) and air abraded for 12 hours, yielding a discordant point [nm(-1) ++74aa] that falls on the discordia defined by the other three fractions. This type of be-


havior is difficult to explain unless fraction (nm(-1) ++74aa, where aa stands for air abrasion for 12 or more hours) con- tained a xenocrystic zircon component from a slightly older rock. On the basis of the systematics of these zircon fractions, the age of this sample is interpreted to be represented b y the lower intercept defined by the discordia through four fractions, 291.0 +_ 29.7 Ma.

A10. Sample XA-124

Sample XA-124 was collected from'the upper reaches of Rfo Traffin north of Lago Ranco (Figure 2). The sample is a coarse-grained biotite granite (71% SiO2). This sample intrudes metaturbidites of Traffn Sequence. Four zircon fractions were analyzed from this sample (Table 1, Figure 5j). The behavior of these fractions suggests that inheritance controls

the lar•er zircons of similar magnetic their systematics as 2ø7Pb/2ø•Pb properties have older dates. Omitting the most discordant fraction yields a linear regression through the three most concordant fractions with a lower intercept age of 296.9 _+ 18.6 Ma. Although the interpretation that the lower intercept represents the crystallization .age of this sample was not tested by air abrasion techniques, on the basis of available data the lower intercept age of the discordia through all the data is interpreted as the age of this sample, 297.7 +_ 0.9 Ma.

All. Sample XA-45

Sample XA-45 was collected from just above the northern shoreline of Lago Ranco on Cerro Traffi (Figure 2). The sample is a medium- to coarse-grained biotite, hornblende granodiorite (61% SiO 0 that contains a weak to moderately developed magmatic foliation defined by the alignment of mafic mineral phases. This intrusion is similar in texture and composition to granodiorites exposed north of Lago Ranco and to exposures on various islands in and along the southern shoreline of Lago Ranco. Four zircon fractions were analyzed from this sample (Table 1 and Figure 5k). The systematics of the first three fractions analyzed suggested a simple inheritance array, with the lower intercept age being the probable age of the sample. In order to test this hypothesis, eight gem quality grains were selected from fraction (nm(-2) ++74) and air abraded for 12 hours with the resulting analysis moving up along the discordia array. On the basis of this behavior, the age of this sample is interpreted to be 288.8 + 12.9 Ma.

A12. Sample XC-27

Sample XC-27 was collected from the 'Chenos island aot•th- east of Isla Huapi of Lago Ranco (Figure 2). The sample is a

medium-to coarse-grained biotite, hornblende granodiorite (64% SiO2). Four zircon fractions were analyzed (Table 1 and Figure 51), three of which are in error of concordia between 304 and 306 Ma. The fourth fraction is slightly discordant and has a 2ø?pb/2ø6pb date of 309 Ma. Because each of these fractions represents between two and five individual zircons, all of which were abraded for 30 hours, it is difficult to establish the

significance and degree of Pb loss versus inheritance in these zircons; although given these data it appears that both may play a small role. On the basis of these data, we interpret the age of this sample to be best represented by the weighted mean

207 206

of the Pb/ Pb dates of the three most concordant fractions, 305.9 + 1.9 Ma.

A13. Sample XC-27

Sample XO-20 was collected from near the Pacific coast south of Bahia de Corral (Figure 6). The sample is a medium- grained granodiorite dike (68% SiO2) that intrudes and trun- cates a schistose fabric and F3 folds in the surrounding metabasites and that is itself intruded by ix younger porphyry intrusion. Four fractions from this sample were analyzed (Table 1 and Figure 5m). In general, the systematics of these fractions suggest that these zircons suffer from a small amount of Pb loss and inheritance. The three largest (by.weight) and most magnetic fractions (nm(0) -74, nm(- 1) +74, nm(- 1) ++74) rest just off concordia between 85 and 86 Ma. The least magnetic and smallest (by weight) fraction analyzed is within error of concordia at 85.9 Ma. The weighted mean of the 2ø?pb/2ø6pb dates gives an age of 91.3 _+ 4.9 Ma. (MSWD equals 4.5), which we interpret as the age of this sample.

Various other intrusive samples were collected in the study area oulined by Figure 2 and analyzed by the K-Ar method (Ta- ble 2).

Acknowledgments. We thank C. Mpodozis, M. Gardeweg, and J. Mufioz of the Servicio de Geologia y Miner/a de Chile (SERNAGEOMIN) for their support of this work. Fruitful discussions with N. Blanco, F. Herv6, H. Moreno, C. Mpodozis, and F. Munizaga have aided in our understanding of the geology of the region. The hard work by the staff • of SERNAGEOMIN•,s Puerto Varas office is gra- ciously appreciated. We thank J.D. Walker and W.R.Van Schmus at the University of Kansas for allowing MWM use of their U-Pb and mass spectrometer facilities and J. Vargas and the staff of SERNAGEOMIN's geochemistry laboratory for their assistance in this project. F. Munizaga allowed us to cite an unpublished 4øAr-39Ar date. We thank G. Yafiez for access to aeromagnetic data. T. Kato wishes to thank W. G. Ernst. Comments by I. Dalziel, S. Kay, and V. Ramos helped clarify ideas presented in this paper and are greatly appreciated. This work is dedicated to our friend and colleague Alberto Campos C., who died in a climbing accident on Calbuco Volcano, 1996.

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M.W. Martin, Department of Earth, Atmospheric and Planetary Sciences, MIF, Cambridge, MA 02139. ([email protected])

(Received October 20, 1998; revised April 7, 1999; accepted April 29, 1999.)

Evolution of the late Paleozoic accretionary complex and overlying forearc-magmatic arc, south...

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