Geological Society of America Bulletin

doi: 10.1130/B25482.1 2004;116;1299-1317Geological Society of America Bulletin

 A. HamiltonJames M. McLelland, M.E. Bickford, Barbara M. Hill, Cory C. Clechenko, John W. Valley and Michael igneous zircon: Implications for AMCG complexesDirect dating of Adirondack massif anorthosite by U-Pb SHRIMP analysis of  

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ABSTRACT

The low abundance of igneous zircon in Proterozoic massif anorthosites has pre-sented a major obstacle to the acquisition of direct absolute ages of crystallization for these important rocks. Indirect dating that relies on zircon ages from associated mangerite-charnockite-granite granitoids assumes that they have a coeval relationship with anorthosite that requires documen-tation. SHRIMP (sensitive, high-resolu-tion ion-microprobe) U-Pb zircon-dating techniques provide a powerful means for directly dating the small populations of zircons in anorthositic rocks and for resolv-ing problems with inheritance. Within the Adirondack Mountains, 10 samples of mas-sif anorthosite have yielded more-than-suf-fi cient quantities of igneous zircon to estab-lish directly the ages of the region’s classic anorthosite occurrences (e.g., the Marcy and Oregon Dome massifs). In addition, a ferrogabbro, a ferrodiorite, and a coronitic olivine metagabbro, all crosscutting massif anorthosite, were dated. The average age of this suite of 13 anorthositic samples is 1154 ± 6 Ma (MSWD [mean square of weighted deviates] = 0.26, probability = 0.99). In addi-tion, eight associated granitoids have been

dated by SHRIMP techniques and comple-ment another fi ve previously dated by multi-grain thermal-ionization mass spectrometry (TIMS) methods. The 13 granitoids yield an average age of 1158 ± 5 (MSWD = 0.89, probability = 0.60) and are broadly coeval with the massif anorthosite. The overlap-ping ages provide evidence that these rocks constitute a single, composite anorthosite-mangerite-charnockite-granite (AMCG) suite intruded at ca. 1155 Ma, an age corre-sponding to the ages of major AMCG suites in the Grenville province in Canada (e.g., Morin and Lac St-Jean).

Although rocks of the Adirondack AMCG suite are now documented as broadly coeval, it does not follow that the constituent AMCG lithologies were comagmatic. Field relation-ships and mineral disequilibria in transi-tional zones are inconsistent with derivation from a single parental magma. Moreover, the presence of older (ca. 1.2–1.3 Ga) inherited cores in some zircons from AMCG granitoids confl icts with derivation of these rocks from magmas that formed anorthosite, gabbro, or ferrodiorite, or jotunite, in which zircons are highly soluble. The slightly older ca. 1158 Ma average age of the mangeritic and charnock-itic members of the AMCG suite is consistent with an origin as early lower-crustal ana-tectites that left behind pyroxene-plagio-clase restites. This refractory material then reacted (by assimilation–fractional crystal-lization [AFC]) with ponded, mantle-derived gabbroic magmas to produce plagioclase-rich crystal mushes with crustal isotopic

signatures, as proposed much earlier by R.F. Emslie. These magmas are considered to be parental to the Adirondack anorthosite, and upon ascent they were emplaced in proximity to still hot, earlier mangeritic and charnock-itic bodies where they underwent further fractionation. The composite nature of the Marcy massif documents that this process was repeated in several sequential pulses.

Keywords: anorthosite, AMCG, geochronol-ogy, zircon, SHRIMP, Adirondacks.

INTRODUCTION

Emslie (1978, 1985) and Emslie et al. (1994), among others, have reviewed the fi eld and chemical characteristics of Middle Proterozoic complexes consisting of anorthosite-mangerite-charnockite-granite intrusions and commonly referred to as anorthosite-mangerite-char-nockite-granite (AMCG) suites. Typically, the granitoids comprise orthopyroxene-bearing monzonitic, syenitic, and granitic plutons, all exhibiting high K

2O concentrations and

elevated Fe/(Fe + Mg). The anorthositic compo-nent commonly ranges from leucogabbronorite (plagioclase = ~70%) to true anorthosite (plagio-clase > 90%). Plagioclase compositions in these bodies are generally in the interval An

45–An

55.

A small volume of ferrodiorite accompanies the anorthositic suite and has been interpreted as residual magma that, together with associated cumulates, resulted from extensive crystalliza-tion of plagioclase-rich anorthosite (e.g., Emslie et al., 1994; McLelland et al., 1994).

Direct dating of Adirondack massif anorthosite by U-Pb SHRIMP analysis of igneous zircon: Implications for AMCG complexes

James M. McLelland†

Department of Geology, Colgate University, Hamilton, New York 13346, USA

M.E. BickfordBarbara M. HillDepartment of Earth Sciences, Heroy Geology Laboratory, Syracuse University, Syracuse, New York 13244-1070, USA

Cory C. ClechenkoJohn W. ValleyDepartment of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706, USA

Michael A. Hamilton‡

Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada

GSA Bulletin; November/December 2004; v. 116; no. 11/12; p. 1299–1317; doi: 10.1130/B25482.1; 12 fi gures; Data Repository item 2004168.

†Present address: Department of Geosciences, Skidmore College, Saratoga Springs, New York 12032, USA; e-mail: jmclelland@citlink.net.

‡Present address: Jack Satterly Geochronology Laboratory, Department of Geology, University of Toronto, Toronto, Ontario M5S 3B1, Canada.

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The origin of AMCG complexes has remained controversial for decades, and three compet-ing models have emerged. The fi rst of these proposes that most, if not all, members of the suite are comagmatic and formed by fractional crystallization of a parental magma that is com-monly considered to be of intermediate compo-sition and possibly of lower-crustal derivation (Duchesne et al., 1999; Vander Auwera et al., 1998, 2003; Longhi et al., 1999). The second model holds that granitoid and anorthositic members of the suite are broadly coeval but not comagmatic (Emslie, 1978, 1985; Emslie et al., 1994; Hamilton et al., 1994; Longhi and Ashwal, 1985; McLelland et al., 1996; among others), the anorthosites being mantle derived and con-taminated by crust, whereas the granitoids result from lower-crustal melting caused by intrusion of hot gabbroic magma. A third, less widely held model asserts that the granitoids and anorthosites are neither comagmatic nor coeval but are inde-pendent entities that may be separated by large intervals of time (Alcock et al., 2001; Isachsen et al., 2001). The resolution of this controversy has remained elusive, and unequivocal criteria have been diffi cult to establish.

The absolute ages of the granitoid and anor-thositic members of the suite are critical to any AMCG model. This information can establish a strictly contemporaneous relationship or demonstrate that the members of the suite are separated by long time intervals. In the same manner, absolute ages permit correlation of suite members with specifi c regional tectonic settings. The advent of U-Pb zircon dating made it possible to obtain ages for granitoid members of the suite even where high-grade metamor-phism had overprinted the rocks (e.g., Silver, 1969). However, early multigrain thermal-ionization mass spectrometry (TIMS) dating required relatively large quantities of zircons, and anorthositic rocks are notoriously poor in igneous zircon. Accordingly, anorthosites were dated indirectly by obtaining ages for associated granitoids thought to be coeval members of the AMCG suite (Emslie and Hunt, 1990). Within the Adirondacks, McLelland and Chiarenzelli (1990a) extended this method by dating AMCG granitoids that exhibited mutually crosscutting relationships with the anorthosite. Even this approach failed to provide an unequivocal set of relationships because it could be argued that the granitoids were already in place when the hot anorthosite intruded and caused melting and back-intrusion by the granitoids. Furthermore, the presence of inherited cores and metamor-phic overgrowths associated with zircons from the granitoids resulted in mixed ages that could not be entirely rectifi ed either by diligent hand-picking or by existing air-abrasion techniques.

More recently, a number of workers (Scoates and Chamberlain, 1995; Scoates et al., 1996; Mitchell et al., 1996; Hamilton et al., 1998; and references therein; Hamilton et al., 1994) have obtained precise ages for several anor-thosite massifs by utilizing small-fraction and single-grain TIMS techniques. These methods, however, are more diffi cult to apply when there exists a high-grade metamorphic overprint, as in the Adirondacks.

Resolution of Adirondack geochronological problems, as described above, has been made possible by the advent of high-transmission, high-mass-resolution ion microprobes such as the SHRIMP (sensitive high-resolution ion microprobe; Compston et al., 1984) that are capable of dating multiple, ~5–30 spots on a single zircon grain. As a result, the sparse, but almost ubiquitous, population of igneous zir-cons in anorthosites now provides more than enough grains to date these rocks directly and with confi dence that inherited components have been avoided. This technique also avoids diffi culties inherent in the dating of grains with metamorphic overgrowths. In this study, we have applied the SHRIMP technique to date zir-cons from Adirondack massif anorthosite and to obtain new ages from critical granitoid samples previously dated by multigrain TIMS methods. Igneous zircons from 13 samples of anorthositic rocks and related gabbros have been dated, and all yield ages (ca. 1155 Ma) similar to the ages of 13 associated granitoids. This result confi rms that anorthosite and granitoid members of the Adirondack AMCG suite are broadly coeval and were emplaced shortly after contractional orogeny at ca. 1200–1160 Ma (Wasteneys et al., 1999). Subsequent assertions by Alcock et al. (2001) and Isachsen et al. (2001) that the Marcy anorthosite was emplaced at ca. 1050 Ma, i.e., 100 m.y. after the AMCG units dated here, are shown to be incorrect. In the following, we report the results of our investigation and use them to address petrologic and tectonic aspects of Adirondack, and other, AMCG suites.

GEOLOGIC SETTING AND CHARACTERISTICS OF ADIRONDACK ANORTHOSITE AND RELATED MANGERITE-CHARNOCKITE-GRANITE (AMCG SUITE)

Regional Relationships

The Adirondack Mountains represent a south-western extension of the Grenville province via the Frontenac arch through the Thousand Islands sector of the St. Lawrence River (Fig. 1). The region is divided topographically into the Adirondack Highlands, underlain largely by

granulite-facies orthogneisses, and Adirondack Lowlands, underlain by upper amphibolite–grade metasedimentary rocks, notably marbles. The Carthage-Colton mylonite zone (Fig. 2, CCZ) separates these two regions, both of which have undergone multiple phases of deformation resulting in major refolded isoclines. The oldest rocks recognized in the Adirondacks consist of calc-alkaline tonalites and granodiorites dated at ca. 1350–1250 Ma (Fig. 2, RMTG). Rocks of similar composition and age have been rec-ognized in the Green Mountains of Vermont (Ratcliffe et al., 1991) and in western Connecti-cut (Walsh et al., 2004). In addition, similar calc-alkaline plutonic rocks ranging in age from 1270 to 1220 Ma are common (Easton, 1992) within the western Central Metasedimentary belt of the Canadian Grenville province (Fig. 1). All of these calc-alkaline orthogneisses have been inter-preted as arc-related additions of juvenile crust emplaced during the interval ca. 1400–1220 Ma. Moore and Thompson (1980) proposed that this interval was characterized by repeated arc accre-tion to the southeast margin of Laurentia; they referred to the protracted sequence of events as the Elzevirian orogeny. Because Elzevirian plutonism and metamorphism were followed by sedimentation (Flinton Group) overprinted by ca. 1100–1000 Ma high-grade metamorphism, Moore and Thompson (1980) introduced the concept of a Grenville orogenic cycle consisting of an early Elzevirian orogeny and a younger (ca. 1100–1000 Ma) Ottawan orogeny. This con-cept replaced that of the Grenville orogeny and emphasized the existence of two major pulses of orogeny separated by an extensional interval of sedimentation in the Central Metasedimentary belt. In the Adirondacks, Elzevirian accretion appears to have extended to ca. 1200–1160 Ma when the Adirondack Highland–Green Moun-tain terrane collided with the leading edge of the Frontenac terrane of the Central Metasedi-mentary belt, which was at that time represented by the Adirondack Lowlands (Wasteneys et al., 1999). This culminating accretionary event resulted in widespread magmatic and tectono-thermal events that are referred to in adjacent Canada as the ca. 1190–1140 Ma Shawinigan orogeny (Rivers, 1997; Corrigan and van Bree-men, 1997), and this terminology is adopted here. Synorogenic Shawinigan magmatism is represented in the Adirondack Lowlands by the 1180–1170 Ma Hyde School and Rockport granites (Fig. 2, HSRG) and the 1200–1180 Ma Rossie diorite–Antwerp granitoid suite (Fig. 2, RDAG). Corresponding magmatic activity has not yet been identifi ed in the Highlands, but evidence for Shawinigan metamorphism and deformation is widespread (McLelland et al., 1988a; Heumann et al., 2004).

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Subsequent to the Shawinigan orogeny, little, if any, regional deformation is apparent in the Adirondacks until ca. 1090 Ma when, following a brief interval (ca. 1103–1090 Ma) of granitic magmatism (Hawkeye granite, Fig. 2, HWK), the entire Grenville province was affected by the Ottawan orogeny (McLelland et al., 1996; Rivers, 1997). This event is thought to have been the result of collision with a continental-scale craton (Amazonia?), the suture zone lying somewhere to the southeast of the present-day Appalachians. Geothermometry, geobarometry, and seismic investigations document that during the Ottawan orogeny, the crustal thickness of the Adirondack Highlands was doubled, and rocks currently exposed in the region under-

went granulite-facies metamorphism and nappe emplacement (Bohlen et al., 1985; Valley et al., 1990; McLelland et al., 1996). The unde-formed nature of the ca. 1050 Ma Wanakena and Ausable Forks granite members of the Lyon Mountain Granite (Fig. 2, LMG) documents that major contractional pulses of the Ottawan orogeny had terminated by that time (McLel-land et al., 2001a). Locally, younger events of lesser magnitude continued to affect the region until ca. 990 Ma, as recorded by metamorphic zircon and monazite.

Long-term controversy has existed over how and when the Adirondack anorthosite massifs (Fig. 2, ANT) and related granitoids (Fig. 2, MCG) fi t into the Grenville orogenic cycle.

Buddington (1939, 1969), Postel (1952), and Davis (1969) regarded the anorthosites as early, preorogenic intrusions that were slightly older than the AMCG granitoids that crosscut them. In contrast, deWaard and Romey (1969) and Lettney (1969) adopted a synorogenic, comagmatic model for the Adirondacks. Silver (1969) emphasized that ca. 1050 Ma metamor-phic zircons in the Marcy massif indicated that the anorthosite was older than the ca. 1050 Ma regional metamorphism and predated the ca. 1120 Ma (Silver’s age) AMCG granitoids that crosscut it. The U-Pb zircon results of McLel-land et al. (1988b) and McLelland and Chiaren-zelli (1990a, 1990b) confi rmed Silver’s (1969) interpretation by establishing the presence of the ca. 1090–1050 Ma Ottawan orogeny in the Adirondacks (see also Mezger et al., 1991) and documenting that the AMCG granitoids were emplaced at ca. 1145 ± 15 Ma, thus fi xing a minimum age for the anorthosites that they crosscut. This date placed AMCG magmatism after the Shawinigan orogeny and ~50 m.y. prior to the Ottawan orogeny, consistent with the ages of other large anorthosite massifs in the Canadian Grenville province (e.g., Emslie and Hunt, 1990). The results also suggest that, consistent with fi eld relationships, anorthosites and granitoids of the suite are coeval. However, Alcock et al. (2001) have subsequently argued that the Marcy massif is a posttectonic intrusion. If correct, its ca. 1150 Ma age requires that there was no Ottawan orogeny in the Adirondacks. In a later publication (Isachsen et al., 2001) these same authors reported a TIMS age of 1040 ± 2 Ma for a single grain of zircon collected from an anatectic leucosome in charnockitic gneiss bordering the Marcy anorthosite. They interpreted the anatexis to be the result of heat from the Marcy massif that, by implication, must have been emplaced at ca. 1050–1040 Ma (i.e., very late in the Ottawan orogeny). These authors have expressed skepticism about prior zircon investigations, noting that most of them employed multigrain techniques and that none of them dated the anorthosite directly. In con-trast to these earlier studies, the present inves-tigation directly dates Adirondack anorthosite. A further discussion of the tectonic setting of AMCG genesis will be presented in the Sum-mary and Discussion section following the establishment of age relationships within the AMCG suite.

Relationships Specifi c to the Adirondack AMCG Suite

The central and eastern Grenville province contains a number of Middle Proterozoic AMCG complexes cored by large anorthosite massifs

Figure 1. Generalized location map of the Adirondacks as a southwestern extension of the Canadian Grenville province whose three major tectonic belts (Rivers, 1997) are shown. ABT—Allochthonous Boundary Thrust; GFTZ—Grenville Front tectonic zone; TLB (dark gray)—Trans-Labrador batholith, FA—Frontenac axis, GM—Green Mountains, H—Hou-satonic Mountains, HH-RP—Hudson Highlands and Reading Prong. The major anorthosite massifs (with ages) of the region: 1—Oregon Dome (ca. 1155 Ma); 2—Marcy (ca. 1155 Ma); 3—Morin (ca. 1160 Ma); 4—Lac St-Jean (ca. 1150 Ma); 5—Riviere Pentecote (ca. 1360); 6—Havre–Saint-Pierre–Atikonak (ca. 1130 Ma); 7—Mealy Mountains (ca. 1650 Ma); 8—Harp Lake (ca. 1450 Ma); 9—Nain Plutonic Suite (ca. 1330–1300 Ma, southernmost fl ank shown only). Age references: 1 and 2—Hamilton et al. (2004); 3 to 7—Emslie and Hunt (1990) and Doig (1991); 4—Hervet et al. (1997) and Higgins and van Breemen (1996); 5—Machado and Martignole (1988) and Martignole et al. (1993); 6—van Breemen and Higgins (1993); 8—Krogh and Davis (1973); 9—Hamilton et al. (1994, 1998, and references therein).

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(Fig. 1) that account for ~20% of the region’s area (McLelland, 1989). The Marcy anorthosite massif (Figs. 1, 2), within the Adirondack Moun-tains, has long been regarded as typical of Middle Proterozoic massifs worldwide (Ashwal, 1993). The smaller Oregon Dome and Snowy Mountain bodies (Fig. 2, OD and SM) are composition-ally and mineralogically similar to the Marcy massif. Several other small bodies occur on a

scale too small to be shown in Figure 2. Adiron-dack anorthosites are of the andesine-labradorite (An

45–An

55) variety and contain magnetite and

ilmenite refl ecting a relatively low fO

2 (Anderson

and Morrison, 1992) and resembling other large massifs in the Grenville province. In addition to these low-f

O2 massifs, several smaller and distinc-

tive alkalic and hemoilmenite-bearing andesine (An

30Or

11–An

40Or

7) anorthosites occur in Quebec

(Owens and Dymek, 2001) and in the Proterozoic core of the southern Appalachians (Owens and Tucker, 2003). These late- to posttectonic bodies are clearly more oxidized and younger (ca. 1050–1010 Ma) than the larger, more reduced Marcy massif that forms the subject of this paper.

The Marcy anorthosite massif is roughly elliptical; its long dimension (northwest-south-east) extends ~100 km and its short (northeast-

Figure 2. Generalized geologic and geochronological map of the Adirondacks showing locations of dated AMCG samples. Units designated by patterns and initials consist of igneous rocks dated by U-Pb zircon geochronology with ages indicated. Units present only in the Highlands (HL): RMTG—Royal Mountain tonalite and granodiorite (southern Highlands only); HWK—Hawkeye granite, LMG—Lyon Mountain Granite, and ANT—anorthosite. Units present only in the Lowlands (LL): HSRG—Hyde School and Rockport granites (Hyde School also contains tonalite); RDAG—Rossie diorite and Antwerp granodiorite. Granitoid members of the AMCG suite (i.e., the MCG—mangerite-char-nockite-granite) are present in both the Highlands and Lowlands. Unpatterned areas consist of metasedimentary rocks, glacial cover, or undi-vided units. A—Antwerp; AF—Ausable Forks; CA—Canton; CCZ—Carthage-Colton mylonite zone; GO—Gouveneur; GM—Gore Moun-tain; IL—Indian Lake; LM—Lyon Mountain; OD—Oregon Dome; SM—Snowy Mountain; SL—Saranac Lake; R—Rossie; RB—Roaring Brook on Giant Mountain; W—Wanakena. Locations for samples (sample numbers given for ages reported in this paper) discussed in text: a—Rooster Hill megacrystic charnockite; b—Piseco leucogranitic ribbon gneiss; c—Oregon Dome ferrodiorite and anorthosite, BMH-01-09; d—Gore Mountain mangerite; e—Snowy Mountain mangerite; f—Schroon Lake granitic gneiss, 9-23-83-7; g—Minerva mangerite; h—North Hudson metagabbro, CGAB; i—Woolen Mill gabbro and anorthosites, BMH-01E1, 2, 3, 4; j—anorthositic pegmatite in Ausable River at Jay, BMH-01-04 (k and l are omitted); m—Bloomingdale mangerite, AC-85-10; n—mangeritic dike crosscutting anorthosite northeast of Tupper Lake Village, BMH-01-15; o—mangerite southeast of Tupper Lake Village, AC-85-6; p—rapakivi granite in Stark anticline; q—Oswegatchie leucogranite; r—Diana pyroxene syenite; s—Croghan granitic gneiss; t—Carthage anorthosite; T—anorthosite at Tahawus, BMH-01-11; u—southern Marcy massif coarse-grained anorthosite, BMH-01-02; v—Middle Saranac Lake coarse-grained anorthosite, BMH-01-05; w—Upper Saranac Lake coarse-grained anorthosite, AC-85-8; x—Northway coarse-grained anorthosite, BMH-01-19. (After Hamilton et al., 2004.)

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southwest) dimension measures ~60 km (Fig. 2). Gravity models (Simmons, 1964) suggest that it is a slab-like body ~3–5 km thick with two fun-nel-shaped feeder pipes extending to ~10 km depth. Like other igneous bodies of this size, the Marcy massif is a composite intrusion consist-ing of a variety of anorthositic and leucogab-broic members that were repeatedly emplaced in an unspecifi ed number of pulses. Following Buddington (1939, 1969), the principal rock types are anorthosite (>90% plagioclase), mafi c anorthosite (90%–75% plagioclase), leucogab-bro (75%–65% plagioclase), and gabbro (<65% plagioclase). Together, anorthosite and mafi c anorthosite constitute ~90% of the massif. Leu-cogabbro is concentrated along the borders of the massif where it occurs together with mafi c anorthosite to form a deformed, fi ner-grained, and more mafi c border facies surrounding a largely anorthositic core of coarse-grained, little-deformed, blue-gray plagioclase. The leucogabbro is commonly referred to as the Whiteface facies (Kemp, 1921), whereas the anorthositic varieties are known as the Marcy facies (Miller, 1919). Modern workers continue to use these designations but with the recogni-tion that two facies are insuffi cient to describe the complicated nature of the massif and its multiple intrusive pulses. Throughout the mas-sif, crosscutting relationships exist between the various anorthositic lithologies, and xenoliths of one variety are commonly enclosed within another, forming a complicated collage of mul-tiple events that attests to a composite origin. In addition, the margins of the massif commonly contain a border facies consisting of rocks that represent commingling of anorthositic and granitoid magmas. Miller (1919) named these hybrid rocks Keene Gneiss for their exposures near the village of Keene. The mixtures can be very complex because they represent a variety of AMCG magmas, including ferrodiorite. Within Keene Gneiss, plagioclase xenocrysts incorporated into granitoids or alkali feldspar incorporated into anorthosite exhibit reaction rims of perthite or plagioclase, respectively (Hamilton et al., 2004; McLelland et al., 2002). These rimmed feldspars exhibit replacement textures similar to those produced by Wark and Stimac (1992) and Stimac and Wark (1992) in experiments designed to duplicate incorporation of feldspars into melts with which they are not in equilibrium. Further complicating the border zones are inclusions of metasedimentary rocks metamorphosed to a variety of skarns (Valley et al., 1990). These zones of Keene Gneiss separat-ing AMCG anorthosite and granitoids represent the only transitional compositions between them and consist of disequilibria-dominated, physical mixtures between the two magma types. Davis

(1969) and Buddington (1969) provided an additional example of disequilibrium recorded by the absence of correlation between pyroxene (Mg# [= MgO/(MgO + FeO)]) and plagioclase (An percentage) compositions within Keene Gneiss, whereas these parameters consistently correlate within the anorthosites and granitoids themselves. These observations have been inter-preted as indicative of a bimodal, rather than a comagmatic, origin for the suite (Buddington, 1969; Davis, 1969; McLelland et al., 1994).

Buddington (1939), McLelland and Whitney (1990), Ashwal (1993), and McLelland et al. (1994) have emphasized that compositional trends for Adirondack anorthositic magmas move toward silica depletion, whereas granitoid trends display silica enrichment. Consistent with this fact is the observation that ferrodiorites are common in anorthositic rocks but are rare in associated granitoids. Moreover, thin veinlets of ferrodioritic material that occur throughout the anorthosite massifs are clearly of local origin and can be seen to occupy small fractures and shear zones as would be expected from fi lter pressing and remobilization of evolved residual liquid. In short, Adirondack ferrodiorite is a late derivative from anorthositic magmas (McLel-land et al., 1994) and, in contrast to interpre-tations made for the Rogaland anorthositic complex (Vander Auwera et al., 1998, 2003, 2003; Longhi et al., 1999), did not serve as a parental magma for either the anorthosite or the AMCG granitoids. Indeed, as admitted by these cited authors, the enormous volume of AMCG granitoids in the Adirondacks weighs against derivation by fractionation but is consistent with their production as deep-crustal melts resulting from mantle-derived heat transported by mantle-derived gabbroic magma. This issue will be treated in greater detail in the Discussion section of this paper.

Vander Auwera et al. (1998) cited deWaard and Romey (1969) to imply that “parts” of the Adirondack AMCG suite are examples of continuous fractionation from a jotunitic parental magma to produce all members of the AMCG suite as comagmatic derivatives. The seminal article setting forth this proposi-tion (deWaard, 1970) is based on a sequence of rocks representative of most of the Adirondack AMCG suite and exceptionally well exposed in Roaring Brook on Giant Mountain (Fig. 2, RB) in the eastern Marcy massif. McLelland (in Bohlen et al., 1992) reinvestigated this locality in detail and has shown that the relationships there are best accounted for by magma mix-ing, commingling, and hybridization. Indeed, enclaves originally misinterpreted as xenoliths of Grenville metasedimentary rocks (Kemp, 1898; Jaffe et al., 1983) in the granitoids can

be shown to be the result of commingling between gabbroic and mangeritic magmas. At Snowy Mountain (Fig. 2), where deWaard and Romey (1969) interpreted fi eld relationships to be supportive of a comagmatic model, dating reported by Hamilton et al. (2004) indicates that charnockitic rocks dated at 1174 Ma sur-round a ca. 1155 Ma anorthositic core, effec-tively precluding a comagmatic interpretation. Within the Adirondacks, a supportable case for a comagmatic AMCG suite has yet to be made and is not supported by geochronological results presented in this paper.

A striking feature of the Marcy facies is the presence within it of rafts of exception-ally coarse-grained anorthosite containing plagioclase crystals with long dimensions of 15–30 cm and, rarely, up to 45 cm. These occur in subophitic intergrowths with both ortho- and clinopyroxene, some crystals of which qualify as “giant pyroxenes.” Emslie (1975) discussed similar giant pyroxenes from the Lac St-Jean and Morin massifs and reported high concentra-tions of Al

2O

3 (4.5–9.5 wt%) indicative of high-

pressure and a polybaric differentiation history for these rocks. In almost all occurrences, these ultra-coarse-grained assemblages are in the form of rafts surrounded by fi ner-grained variet-ies of leucogabbro or mafi c anorthosite. In addi-tion to the exceptionally coarse-grained rafts of anorthosite, there exist large volumes of coarse-grained (4–10 cm) anorthosite consisting of blue-gray plagioclase and 10% mafi c minerals. Much of this material exhibits fl ow alignment of plagioclase laths and likely represents mobi-lized crystal-rich mush. The alignment is inter-preted as magmatic rather than tectonic because neither plagioclase grains nor interstitial subo-phitic pyroxene is deformed. The high (>90%) concentration of plagioclase laths in these rocks implies that they represent cumulates formed within the postemplacement massif. The paren-tal magma of these cumulates is thought to be similar in composition to the mafi c anorthosite and leucogabbro that enclose the coarse-grained rafts and also constitute the Whiteface facies (Buddington, 1939, 1969; McLelland et al., 1994). There exists no evidence to support jotunitic parent magmas for any major member of the Adirondack AMCG suite.

GEOCHRONOLOGICAL RESULTS

The in situ U-Pb ages presented here were obtained through the use of the SHRIMP II ion microprobe at the Geological Survey of Canada (GSC) in Ottawa. SHRIMP analytical and data-reduction procedures followed those described by Stern (1997). Operating condi-tions were similar to those described in detail

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Figure 3. Cathodoluminescence images of representative zircons from anorthosite samples (sample numbers are given in lower left-hand side of each frame). Analytical spots (~30 µm) are shown together with age obtained. See text for discussion of specifi c samples (continued on following page).

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by Hamilton et al. (2004) and are only briefl y summarized here. Zircons from the samples described below were cast along with frag-ments of the GSC laboratory standard zircon (BR266 zircon: 206Pb/238U isotope dilution age = 559 Ma) in one of three 1-inch (2.54 cm) epoxy pucks used in this study (GSC mounts: IP253, IP263, and IP274). All mounts were pol-ished with diamond compound to reveal zircon midsections, and the grains were imaged with a Cambridge Instruments S360 scanning electron microscope operated in backscattered-electron (BSE) and cathodoluminescence (CL) mode to identify compositional zoning, cores and/or overgrowths, and fractures. Zircons on all mounts were analyzed by using an O— primary beam focused into a roughly 25 × 35 µm ellipti-cal spot. Each mount was analyzed in a separate analytical session and individually calibrated for instrumental bias of Pb/U ratios. Analyses were carried out with a primary beam strength that ranged, depending on specifi c analytical ses-sion, from ~9 to 20 nA, but was essentially uni-form during any given data-acquisition period. Corresponding empirical standard calibrations yielded bias corrections for the Pb/U ratios

with external errors of ±1.0%–1.6% (1σ); these uncertainties were then propagated, along with the counting errors, through to the Pb/U isotope compositions of the unknowns. Corrections for common Pb were made through direct monitor-ing of the total measured counts of 204Pb. Final corrected ratios and ages are presented with 1σ analytical errors (68% confi dence limits; Table DR11), as are the error ellipses shown in the concordia diagrams presented below. Asso-ciated plots of weighted-mean 207Pb/206Pb ages and calculated weighted-mean ages presented in the text are shown at 95% confi dence limits. Statistical calculations and graphical presenta-tion of U-Pb results were facilitated by using the Isoplot/Ex program of Ludwig (2001).

An interesting feature of the analytical results is that an array of zircon U-Pb ages, clustering at ca. 1155 Ma but ranging along the concordia

curve down to ca. 1000 Ma, was observed in most samples. Because all grains appear to be of primary igneous origin and older cores were rarely observed, we interpret the cluster of old-est ages to represent the igneous crystallization age of the rock from which the zircons were separated. Further, because very few of the zir-cons imaged and analyzed display overgrowths, the array of essentially concordant ages down to ca. 1000 Ma is interpreted to result principally from variable amounts of domainal Pb loss that occurred during the ca. 1080–1050 Ma Ottawan high-grade metamorphism such that the analytical points lie on a chord between ca. 1155 Ma and 1000 Ma. The ca. 1000 Ma ages may refl ect a distinct metamorphic pulse at that time. Metamorphic ages and processes will not be discussed in this paper except where neces-sary for clarifi cation.

Figures 3–10 present specifi c zircon images and data of the analyzed samples. Cathodolu-minescence images of representative zircons are shown in Figures 3 and 8. Concordia plots of analytical data for 14 samples are presented in Figures 4, 5, 6, 7, 9, and 10, and cumula-tive-frequency diagrams (upper left) as well

Figure 3. (continued)

1GSA Data Repository item 2004168, Appendix DR1, description and location of sample sites, and Table DR1, SHRIMP II U-Th-Pb results for Adiron-dack AMCG samples, is available on the Web at http://www.geosociety.org/pubs/ft2004.htm. Requests may also be sent to editing@geosociety.org.

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as weighted-mean 207Pb/206Pb age (lower right) plots are shown as insets in these fi gures. In concordia plots, igneous ages are shown as light gray error ellipses, metamorphic ages as dark gray error ellipses, and Pb-loss ages as white error ellipses.

Anorthositic Samples

Coarse-Grained Anorthosite, Southern Marcy Massif (BMH-01-02)

This sample yielded an abundant population of large (300–600 µm), elongate, and prismatic zircons with aspect ratios of ~4:1 and euhedral terminations (Fig. 3A). The majority of grains represent fragments, and original zircon lengths certainly exceeded their recovered dimen-sions. None of these grains contain visible cores. Within elongate grains, zoning occurs in straight, regular fashion parallel to the c-axis. More equant grains show extremely well devel-oped sector zoning. On the basis of zoning and external morphology, the population of large grains is interpreted to be magmatic in origin, and it was isolated as a discrete igneous frac-tion. Some large grains are rounded by narrow overgrowths of darker (CL) zircon interpreted to be metamorphic in origin. Also present in the rock is a population of small (50–100 µm) sub-equant grains (Fig. 3A) that display weak to no zoning and locally contain bright (CL) rims. On the basis of morphological characteristics, these small zircons were interpreted to be metamor-phic in origin, and a representative population was handpicked for ion-probe analysis.

SHRIMP analyses for 14 large grains (n = 14 spots, Table DR1 [see footnote 1]) show a dominant grouping around 1150 Ma (Fig. 4A), but there are other ages represented at ca. 1050 Ma and ca. 1000 Ma as well. A cumula-tive-probability and histogram plot of these data (Fig. 4A, inset) shows a principal mode at ca. 1153 Ma. The weighted mean of the 10 analyses yielding the oldest 207Pb/206Pb ages is 1160 ± 15 (MSWD [mean square of weighted deviates] = 0.52, prob. = 0.86). Analyses yielding younger 207Pb/206Pb ages cluster near 1000 Ma, and a single analysis yields a meta-morphic age of 1055 Ma.

SHRIMP data from the handpicked population of small (50–100 µm), apparently metamorphic, grains (Table DR1 [see footnote 1]) are plotted on a concordia diagram (Fig. 4B). Most of the data plot between ages of ca. 1100 and 950 Ma and fall into the same metamorphic age range as in Figure 4A. However, two analyses from small, core-free grains yield essentially concordant data giving ages of ca. 1150 Ma. Because this age coincides with the igneous emplacement age given by the large zircons, we interpret that these

Figure 4. Concordia plots of SHRIMP analytical data for anorthositic samples from the Marcy massif: (A) BMH-01-02, grains with igneous textures. (B) BMH-01-02, grains with metamorphic textures. (C) BMH-01-04, pegmatitic gabbroic anorthosite. Light gray ellipses represent igneous ages; white ellipses represent mixed ages due to incomplete Pb loss; dark gray ellipses represent metamorphic ages. Insets show graphs of cumulative frequency and weighted-means of 207Pb/206Pb ages.

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two grains are igneous in origin but that they were inadvertently handpicked into the metamorphic fraction because of their small size. If these two data points are combined with the 10 analyses from the distinct igneous grains discussed above, the resulting weighted-mean 207Pb/206Pb age is 1157 ± 10 Ma, which we take as the best value for the emplacement age of this pluton.

Pegmatitic Mafi c Anorthosite at Jay, Northeastern Marcy Massif (BMH-01-04)

This sample of pegmatitic mafi c anorthosite yielded numerous very large (100–800 µm) zir-cons (Fig. 3B), most of which had been reduced to fragments (100–400 µm) by grinding. The original morphology of most grains appears to have been elongate with an aspect ratio of ~2:1. Preserved terminations are euhedral, and broad (20–50 µm) oscillatory zoning and sector zon-ing are common. One small (~100 µm) grain with good sector zoning (Fig. 3B) exhibits a rectangular, multifaceted form, and it is possible that many of the fragments in the sample were derived from large grains of similar morphol-ogy. In all cases, both morphology and zoning are interpreted as igneous in origin. A number of small (~100 µm) grains exhibit bright (CL) cores surrounded by irregular, dark CL rims, and the latter dark (CL) rims that are interpreted as metamorphic in origin.

SHRIMP data for 16 analyses (Table DR1 [see footnote 1]) are plotted on a concordia dia-gram (Fig. 4C). The analytical points are distrib-uted along concordia between ca. 1160 Ma and ca. 1050 Ma. A probability plot (Fig. 4C, inset), however, shows a strong peak at ca. 1158 Ma. A weighted-mean of the nine analyses with the oldest 207Pb/206Pb ages is 1160 ± 13 Ma (MSWD = 0.64; prob. = 0.74), which we interpret as the magmatic crystallization age of this sample (Fig. 4C, inset).

Coarse-Grained, Blue-Gray Anorthosite, Middle Saranac Lake, Northern Marcy Massif (BMH-01-05)

Zircons recovered from this sample were abundant, large (200–500 µm), and elongate with aspect ratios of ~4:1 (Fig. 3F). All of the grains represent fragments, and the original zircons may have approached 1 mm in length. These elongate grains exhibit broad, straight zoning parallel to the c-axis. A few intact tips show euhedral ter-minations. A second population of 200–400 µm fragments appears to have been more equant in form and shows strong sector zoning. The mor-phology and zoning of both sets of large grains is interpreted to be magmatic in origin.

SHRIMP data for 12 analyses (Table DR1 [see footnote 1]) are plotted on a concordia diagram (Fig. 5A). The data points cluster strongly at ca.

Figure 5. Concordia plots of SHRIMP analytical data for anorthositic samples: (A) BMH-01-05, coarse-grained anorthosite, Marcy massif. (B) AC85-8, coarse-grained anorthosite, Marcy massif. (C) BMH-01-09, coarse-grained anorthosite, Oregon Dome massif. Ellipse shading and insets as in Figure 4.

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1150 Ma, and a probability plot (Fig. 5A, inset) shows a strong peak at 1155 Ma. The weighted-mean 207Pb/206Pb age of the nine oldest analyses is 1154 ± 20 Ma (MSWD = 0.91, prob. = 0.51), which we interpret as the crystallization age of this sample (Fig. 5A, inset).

Coarse-Grained, Blue-Gray Anorthosite, Upper Saranac Lake, Northwestern Marcy Massif (AC85-8)

This sample was originally analyzed by mul-tigrain TIMS methods and reported by McLel-land and Chiarenzelli (1990a). Two discordant fractions consisting largely of small, equant zircons yielded an upper-intercept age of 1054 ± 20 Ma that was interpreted as the age of a late Ottawan metamorphic event. In addition, a small fraction of elongate, prismatic grains was handpicked and air-abraded. These zircons yielded a slightly discordant data set in which a 207Pb/206Pb age of 1113 Ma represented a mini-mum crystallization age. This early result pro-vided the fi rst direct indication that the Marcy anorthosite massif is essentially coeval with granitoids associated with it.

Inspection of the original zircon separate revealed the presence of enough remaining elongate, prismatic grains (Fig. 3D) to warrant further handpicking for SHRIMP analysis. Fourteen spots were analyzed on 10 grains with the SHRIMP. The resulting data (Fig. 5B) show a distinct cluster of relatively imprecise points (because of low levels of radiogenic Pb [Pb*]; Table DR1 [see footnote 1]) near 1150 Ma; other analyses were distributed near con-cordia between 1188 Ma and ca. 1000 Ma. A weighted-mean of the six most tightly clustered points having 207Pb/206Pb ages 1123–1188 Ma (Table DR1) is 1149 ± 35 Ma (MSWD = 0.34; prob. = 0.89), which we interpret as the crystal-lization age of the sample (Fig. 5B, inset).

Coarse-Grained Anorthosite of the Oregon Dome Massif (BMH-01-09)

Samples of the Oregon Dome anorthosite yielded abundant and very large (200–600 µm) zircons (Fig. 3E) that are clearly fragments from larger grains. The fragments, and still-intact smaller zircons, are elongate, and an aspect ratio of ~3:1 is apparent for intact grains. Broad, straight igneous zoning parallel to the c-axis is common, and preserved tips show euhedral ter-minations. The size, morphology, and zoning of these zircons indicate a magmatic origin.

SHRIMP analytical data for 23 analyses on 15 grains (Table DR1 [see footnote 1]) are plotted on a concordia diagram (Fig. 5C). The data are distributed between ca. 1200 Ma and 1000 Ma. A probability plot (Fig. 5C, inset), however, shows a concentration of ages centered

Figure 6. Concordia plots of SHRIMP analytical data for anorthosite samples, Marcy mas-sif: (A) BMH-01-11, ore-bearing anorthosite, Tahawus. (B) BMH-01-19, coarse-grained anorthosite, Northway. (C) CGAB, coronitic olivine gabbro, North Hudson. Ellipse shading and insets as in Figure 4. White ellipses nested with gray ellipses represent reversely discor-dant younger ages.

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at ca. 1154 Ma; the other analyses form a broad spectrum between ca. 1120 and 1020 Ma, sug-gesting that the ca. 1154 Ma peak represents the dominant age in the population. The weighted mean (Fig. 5C, inset) of the nine oldest 207Pb/206Pb ages that are >95% concordant is 1159 ± 12 Ma (MSWD = 0.75; prob. = 0.65), which we consider to be the best estimate of the emplace-ment age of the Oregon Dome anorthosite. The presence of very thin rims and some rounding of larger igneous grains (Fig. 3E) attests to minor, late metamorphic recrystallization, which we interpret to be responsible for the very incom-plete Pb loss producing the broad spread of younger 207Pb/206Pb ages.

Coarse-Grained, Ore-Bearing Anorthosite, Tahawus, South-Central Marcy Massif (BMH-01-11)

Magnetite-ilmenite–rich anorthosite at this locality yielded a small population of 50–150 µm zircons and zircon fragments of irregular shape. Only a few grains showed any

zoning (Fig. 3H), and only two to three grains exhibited any suggestion of igneous origin.

SHRIMP data for 15 analyses on 13 grains (Table DR1 [see footnote 1]) yielded a cluster of points on concordia (Fig. 6A) with 207Pb/206Pb ages between 1091 and 985 Ma. A weighted mean of the seven most-concordant (>95%) anal-yses within this younger age grouping is 1021 ± 22 Ma (MSWD = 1.03; prob. = 0.40). A single analysis of a relatively featureless grain center yielded a concordant 207Pb/206Pb age of 1165 ± 13 Ma (Fig. 6A). Because of the high solubility of zircon in magmas of this composition (Watson and Harrison, 1983), we think it highly unlikely that this grain was inherited, and accordingly we interpret 1165 ± 13 Ma as the time of emplace-ment of the Tahawus anorthosite and the accom-panying magnetite-ilmenite ores. The most likely interpretation of the zircon grains yielding younger 207Pb/206Pb ages is that they crystallized during the Ottawan orogeny as Zr was released from the magnetite-ilmenite ores during garnet-forming reactions (McLelland et al., 2001a).

Coarse-Grained Anorthosite, Interstate 87 at Exit 30S, Eastern Marcy Massif (BMH-01-19)

This sample yielded only fragments of zir-con grains ranging in size from 50 to >400 µm (Fig. 3G). Although the original size of the largest grains is unknown, they may have approached 800 µm. The shape of intact smaller grains suggests that the fragments may have been derived from approximately equant grains. This interpretation is consistent with patterns of oscillatory zoning that are com-monly developed in the population. On the basis of the widespread occurrence and delicate nature of the oscillatory zoning, the zircons are interpreted to be of magmatic origin.

SHRIMP data for 24 analyses from 18 grains (Table DR1 [see footnote 1]), plotted on a concordia diagram (Fig. 6B), show an almost continuous spectrum of concordant and near-concordant ages between ca. 1180 and 1000 Ma. However, a probability plot (Fig. 6B, inset) suggests a dominant age mode at ca. 1155 Ma. A weighted-mean 207Pb/206Pb age of

Figure 7. Concordia plots of SHRIMP analytical data for samples from Woolen Mill, northeastern Marcy massif: (A) BMH-01E1, pegma-titic anorthosite 1, (B) BMH-01E2, pegmatitic anorthosite 2. (C) BMH-01E3, metamorphosed ferrogabbro. (D) BMH-01E4, coarse-grained noritic anorthosite near dam. Ellipse shading and insets as in Figure 4 except for black ellipses, which designate anomalously old ages dis-cussed in text. White ellipses nested with gray ellipses represent reversely discordant younger ages.

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the 10 least discordant older analyses is 1155 ± 11 Ma (MSWD = 0.64; prob. = 0.76), which we interpret as the crystallization age of Marcy anorthosite at this locality (Fig. 6B, inset).

Coronitic Olivine Metagabbro, North Hudson (CGAB)

Adirondack olivine metagabbros typically contain little, if any, igneous zircon. However, this sample yielded an abundant population of elongate (100–200 µm), prismatic zircons with aspect ratios of ~3:1 (Fig. 3H). Zircon fragments

of the same size range are also present. Intact grains exhibit almost euhedral terminations that are slightly rounded by dark (CL), irregular overgrowths. Both concentric oscillatory zoning and that parallel to prism faces are common in the zircon population. Morphology and zoning both indicate that these grains are igneous in origin. In addition to the prismatic grains there is an unusual population of grains that exhibit a spotted, polygonally segmented appearance that is aptly described as “tortoise-shell tex-ture” (e.g., Fig. 3H, upper-right grain). This

texture is identical to that described in bodies of metagabbro from the Central Gneiss belt in Ontario by Davidson and van Breemen (1988), where coronas of polycrystalline metamorphic zircon are found replacing older, primary igne-ous baddeleyite. Although SEM investigation of CGAB grains on our SHRIMP mount did not reveal the presence of baddeleyite, McLelland and Chiarenzelli (1990a) reported and dated (>1109 Ma) this mineral from a different frac-tion of the same heavy-mineral concentrate. They also reported small, equidimensional

Figure 8. Cathodoluminescence images of representative zircons from granitoid samples associated with the Marcy anorthosite massif (sample numbers are given in lower left-hand side of frame). Note the older, inherited core (bright) in lower right corner of Fig 8A and the older (1237 ± 28 Ma) grain fragment at the upper left corner of Fig 8B. Analytical spots (~30 µm) are shown together with age obtained. See text for discussion of specifi c samples.

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zircons growing on the baddeleyite. On the basis of these observations, we interpret the tortoise-shell zircons as the outer zircon shell of metamorphic coronas on baddeleyite.

SHRIMP data from 20 analyses on 19 grains (Table DR1 [see footnote 1]), plotted on a con-cordia diagram (Fig. 6C), show a strong cluster of ages near 1150 Ma and a subordinate group at ca. 1050 Ma. The probability plot (Fig. 6C, inset) shows a strong maximum at ca. 1146 Ma and a shoulder in the 1100–1000 Ma range. The weighted-mean 207Pb/206Pb age of the oldest six grains that are >95% concordant is 1150 ± 14 Ma (MSWD = 0.17; prob. = 0.97), which we interpret as providing the best estimate for the emplacement age of the metagabbro. The “tortoise-shell” zircons, which yield 207Pb/206Pb ages in the 1060–1030 Ma range, are interpreted as products of Ottawan metamorphic reactions.

Pegmatitic Anorthosite 1, Woolen Mill (BMH-01E1)

At Woolen Mill, medium- to coarse-grained gabbroic anorthosite (3–5 cm) surrounds a pegmatitic phase of anorthosite characterized by coarse-grained plagioclase (5–10 cm) and subordinate pyroxenes of similar dimensions. A sample of the pegmatitic anorthosite facies yielded a large population of euhedral, pris-matic, well-faceted, equidimensional zircons, mostly between 100 and 300 µm, exhibiting excellent concentric oscillatory zoning.

Results for a total of 18 SHRIMP spot analyses on 18 separate grains are presented in Table DR1 (see footnote 1) and on a concordia diagram in Figure 7A. The ten oldest and least discordant (<4%) analyses yield a weighted-mean 207Pb/206Pb age of 1153 ± 9 (MSWD = 0.78, prob. = 0.64), which we interpret as the crystallization age of the pegmatitic anortho-site. A single analysis yielding a concordant 207Pb/206Pb age of 1184 ± 6 Ma was obtained on a U-enriched (1000 ppm) external growth zone of a magmatic grain and is represented by a black ellipse in Figure 7A. This analysis was excluded from the calculated weighted-mean age (Fig. 7A, inset) because its signifi cance is not completely understood and because it falls outside of the error of the average age (1153 ± 9 Ma) for the 10 oldest grains.

Pegmatitic Anorthosite 2, Woolen Mill (BMH-01E2)

This sample of pegmatitic anorthosite is similar to BMH-01E1 and was collected a few meters away from it. Large populations of euhedral, prismatic, and both elongate and equant grains were recovered from the sample (Fig. 7B). Well-developed concentric oscillatory zoning, sector zoning, and edge-parallel zoning

Figure 9. Concordia plots of SHRIMP analytical data for granitoid samples associated with the Marcy anorthosite massif: (A) AC85-6, mangerite, south of Tupper Lake. (B) AC85-10, mangerite in transition zone near the margin of the Marcy anorthosite, Bloomingdale. (C) 9-23-85-7, charnockite, south end of Schroon Lake. Ellipses and insets as in Figure 4 except for black ellipses, which represent older inherited cores.

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are common and attest to the igneous origin of these zircons (Fig. 3J). Grain size ranges from 100 to 200 µm, but many of the longer grains occur as fragments and suggest much larger par-ent grains on the order of 400–500 µm.

A total of 17 spots on 15 separate grains were analyzed and plotted on concordia (Table DR1 [see footnote 1], Fig. 7B). The oldest 13 of these, with concordance of >98%, yield a weighted-mean 207Pb/206Pb age of 1153 ± 6 Ma (MSWD = 0.43, prob. = 0.94) that is interpreted as the age of crystallization of the gabbroic anorthosite (Fig. 7B, inset).

Woolen Mill Ferrogabbro (BMH-01E3)The Woolen Mill ferrogabbro contains ~18%

Fe2O

3 and represents a moderately evolved fer-

rogabbro to ferrodiorite associated with the Marcy massif. The ferrogabbro is interpreted as dense, fi lter-pressed interstitial magma expelled as dikes and sheets during crystalliza-tion of the anorthosite (McLelland et al., 1994). As such, it is not surprising that the sample yielded a large number of excellent zircons (Fig. 3K) consisting of both zoned igneous cores embayed by metamorphic overgrowths (bright in CL, Fig. 3K) and elongate, prismatic magmatic grains with straight igneous zoning. Grains with embayed cores average ~100 µm in length, whereas the elongate fragments are 300–400 µm in length and original grains must have been much longer. Aspect ratios of 3:1 are common in the elongate population.

A total of 29 SHRIMP spots (concordance of >94%) were analyzed on 25 separate grains (Table DR1 [see footnote 1]) and are plotted in a concordia diagram (Fig. 7C). The 11 old-est spot ages from igneous-textured cores and magmatic domains yield a relatively well clus-tered weighted-mean 207Pb/206Pb age of 1157 ± 9 (MSWD = 0.67, prob. = 0.75) that is interpreted as the crystallization age of the gabbro (Fig. 7C, inset). A single older spot age of 1198 ± 14 Ma (SPOT 28.2, Table DR1), measured on a textur-ally representative igneous core, is shown in black in Figure 7C. This analysis was excluded from the mean age calculation because it is comparatively discordant (5.9%) and because it falls outside the error of the average 207Pb/206Pb age of 1157 ± 9 Ma defi ned by the main group of older spot ages. The signifi cance of this single spot age determination is considered suspect.

Without exception, all of the well-developed irregular rims and overgrowths (bright in CL, Fig. 7K) present on these grains are low in U (<80 ppm U) and yield ages of ≤1061 Ma (Table DR1 [see footnote 1]). We interpret this younger age cluster, and the associated disper-sion of nearly concordant ages, to refl ect the broad timing of recrystallization and partial Pb

loss, respectively, during high-grade Ottawan metamorphism.

Coarse-Grained Anorthosite near the Former Woolen Mill Dam (BMH-01E4)

This sample produced a large population of euhedral, doubly terminated zircons that exhibit extremely well-developed concentric oscillatory and sector zoning and are clearly of igneous origin (Fig. 3L). Grains with aspect ratios of ~2:1 and long dimensions up to and in excess of 200 µm are common.

Data from SHRIMP analyses on nine separate grains (Table DR1 [see footnote 1]) are plotted in Figure 7D. The average zircon domain in this sample is characterized by relatively low U abundance (≤60 ppm) and resultantly low concentrations of radiogenic Pb; the zircons

accordingly yield spot ages with low precision. Nonetheless, the oldest eight of these yield a weighted-mean 207Pb/206Pb age of 1140 ± 18 Ma (MSWD = 0.18, prob. = 0.99) that is interpreted as the age of crystallization (Fig. 7D, inset).

Mangeritic and Charnockitic Samples

Mangerite at Moodys, South of Tupper Lake Village (AC85-6)

This sample was previously dated by multi-grain TIMS methods, including air abrasion, and yielded an upper-intercept age of 1134 ± 4 Ma (McLelland and Chiarenzelli, 1990a). Until the current geochronological investigation, this age was considered to be the best-constrained date for the Adirondack AMCG suite. The original zircon separate containing abundant large (100–

Figure 10. Concordia plot and fi eld relationships of granitic dike swarm crosscutting the Marcy anorthosite massif near its western margin east of Tupper Lake Village. (A) Photo-graph of a representative dike (white) crosscutting anorthosite (gray where moss has been pulled back). (B) Concordia plot of SHRIMP analytical data for zircons from granitic dike (BMH-01-15). Ellipses and insets as in Figure 4.

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>600 µm) and prismatic grains with aspect ratios of 4:1–6:1 (Fig. 8A) was handpicked for the current SHRIMP analysis. Most grains show euhedral terminations exhibiting pyramidal tips modifi ed by basal pinacoids, and many of these have slightly rounded, narrow overgrowths of metamorphic zircon. Oscillatory zoning is com-mon and interpreted to be igneous in origin.

The majority of SHRIMP U-Pb data for 14 analyses on 13 grains (Table DR1 [see footnote 1]), plotted on a concordia diagram in Figure 9A, show a tight, concordant and near-concordant cluster near 1170 Ma. A weighted-mean 207Pb/206Pb age of the seven most concordant (>96%), coherently grouped older spot analyses (gray ellipses) is 1170 ± 8 Ma (Fig. 9A, inset; MSWD = 0.67; prob. = 0.67), which we interpret as the crystallization age of the mangerite. Four slightly more discordant (>4%) analyses (black ellipses, Fig. 9A) are excluded from this average and yield ages near 1200 Ma and older; these date distinctly embayed cores (e.g., Fig. 8A, lower right) interpreted as grains inherited from the source rocks of this mangerite.

Mafi c Mangerite along River Road between Bloomingdale and Franklin Falls (AC85-10)

Previously, this sample was dated by multi-grain TIMS methods and yielded a discordant linear array producing an upper-intercept age of 1135 ± 43 Ma (McLelland and Chi-arenzelli, 1990a). Inspection of the original zircon separates revealed many (200–600 µm) grains of elongate, euhedral, prismatic zircons together with fragments of perhaps even larger grains (Fig. 8B). Many grains show pyramidal terminations modifi ed by basal pinacoids. Oscillatory zoning parallel to prism sides is common, yielding many cross sections with little apparent zonation. We interpret the size, morphology, and zoning of these zircons as indications that they are of magmatic origin. In rare instances, thin, dark (CL) overgrowths or recrystallized rims can be seen as embayments on grain cores. We conclude that these over-growths or rims represent zircon growth during postemplacement metamorphism.

SHRIMP data for 13 spot analyses on 11 grains (Table DR1 [see footnote 1]; Fig. 9B) form an array between ca. 1100 and 1180 Ma; a single analysis plots at ca. 1025 Ma. A prob-ability plot (Fig. 9B) shows a broad peak near 1153 Ma, whereas a weighted-mean 207Pb/206Pb age of the nine oldest, most concordant (>95%) analyses is 1164 ± 15 Ma (MSWD = 0.94; prob. = 0.48), which we interpret as the best estimate of the crystallization age of this sample (Fig. 9B, inset). Analysis of a fragment of inher-ited zircon, embayed by a younger overgrowth, yielded a slightly discordant age of 1237 ±

28 Ma (Fig. 8B). Along with a second inherited grain dated at 1204 ± 46 Ma, these are shown as black ellipses in Figure 9B.

Charnockitic Gneiss along Schroon Lake North of Pottersville (9-23-85-7)

This sample was previously dated by multi-grain TIMS methods and, on the basis of three variably discordant analyses, yielded an upper-intercept age of 1125 ± 10 Ma (Chiarenzelli and McLelland, 1991). Many zircons remained from the original separates and provided an abundant choice of grains for SHRIMP analy-sis. Grains and grain fragments selected for analysis (Fig. 8C) were large (200–600 µm), elongate, and prismatic and had aspect ratios of 3:1–4:1. Most grains exhibited euhedral terminations formed by pyramidal and basal pinacoids. Fine-scale, concentric oscillatory zoning is common and well developed in the majority of grains (Fig. 8C). Inherited cores were not observed. On the basis of morphology and zoning, all of these zircons are interpreted as magmatic in origin.

SHRIMP data for 26 analyses from 16 grains (Table DR1 [see footnote 1]; Fig. 9C) show a strong cluster near 1169 Ma, as indicated by a cumulative-probability plot (Fig. 9C, inset). A weighted-mean 207Pb/206Pb age calculated for the 13 oldest analyses that are >95% concordant is 1167 ± 9 Ma (MSWD = 0.77; prob. = 0.68), and this is interpreted to be the crystallization age (Fig. 9C, inset) of the sample.

Mangeritic Dikes Crosscutting the Marcy Anorthosite Massif at Wabeek (BMH-01-15)

A sample from a set of dikes that intrude Marcy anorthosite (Fig. 10) yielded numerous, very large (600–800 µm) elongate, prismatic

zircon grains together with fragments in the 100–600 µm range (Fig. 8D). Many termina-tions are euhedral and range from pyramidal to basal pinacoids or a combination of these. Oscillatory zoning is common but is not present in all grains. Where present, the zones tend to comprise broad bands. No inherited cores were observed in any grains. On the basis of morphol-ogy and zoning, these grains are interpreted as igneous in origin.

SHRIMP data for 16 analyses on 16 grains (Table DR1 [see footnote 1]; Fig. 10A) show a cluster of points between ca. 1100 and 1200 Ma. A probability plot (Fig. 10A, inset) shows a peak at ca. 1162 Ma, and the weighted-mean 207Pb/206Pb age of the oldest 10 analyses that are >95% concordant is 1160 ± 10 Ma (MSWD = 0.78; prob. = 0.63), which we interpret as the crystallization age of the dike (Fig. 10A, inset).

DISCUSSION

Coeval Nature of the Adirondack AMCG Suite

Adirondack AMCG ages determined in the present investigation, together with those of Chiarenzelli and McLelland (1991) and Hamil-ton et al. (2004) are summarized graphically in Figures 11A and 11B. These plots emphasize the narrow range of zircon ages for both the anor-thositic rocks and associated granitoid rocks. This close correspondence of ages is further documented by the overlapping weighted-mean ages for the two groups (i.e., anorthosites, 1154 ± 6 Ma, and granitoids, 1158 ± 5 Ma; Figs. 12A and 12B). Closer inspection of the granitoid data reveals that the three granitic members (Fig. 11A) yield a weighted-mean age of 1149

Figure 11. Plot of U-Pb zircon ages of (A) AMCG granitoid and (B) anorthosite samples from the Adirondack Highlands. The vertical dashed line represents the mean age of each group. Single letters refer to localities in Figure 2, and names give the geographic locations of samples. Granitic samples indicated by (G).

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± 9 Ma (MSWD = 0.4, prob. = 0.93), whereas ten samples of mangerite and charnockite yield a weighted-mean age of 1163 ± 7 Ma (MSWD = 0.2, prob. = 0.93). The latter age may be especially relevant to models of AMCG genesis, because the less-evolved charnockites and man-gerites probably represent the earliest members of the group (Emslie et al., 1994).

Direct dating of the Adirondack AMCG suite demonstrates that it is internally coeval within the interval 1155 ± 10 Ma; this result serves as a powerful constraint on genetic models within the region. Documentation of the contempora-neity of members within this suite confi rms the long-held assumption that the age of undated Adirondack anorthosites may be inferred from the ages of associated AMCG granitoids. Although care must be taken, the ages of other AMCG granitoids that exhibit appropriate crosscutting relationships with anorthosite may now be assigned with reasonable confi dence to their associated, but not yet directly dated, anorthosite massifs (e.g., Emslie and Hunt, 1990). Such documentation of absolute age relationships greatly facilitates the goal of con-straining and establishing the origin, evolution, and tectonic setting of AMCG complexes. In the following text, we utilize the coeval nature of the suite to help construct a self-consistent tectonic-petrologic model for AMCG genesis. Many of the specifi cs of this discussion are not new, but their synthesis hinges upon the coeval nature of the rocks involved.

Tectonic Setting of the Adirondack AMCG Suite

Having established a coeval origin for the Adirondack AMCG suite, we now address the long-debated issue of its tectonic setting and implications for the tectonic setting of AMCG suites in general. Emslie (1978), McLelland and Whitney (1990), and Anderson and Morrison (1992), among others, proposed an “anoro-genic” environment for these rocks and argued that anorthosites represent derivatives from gabbroic parental material. A reasonable way to produce these derivatives is to pond gabbroic magma at the crust-mantle interface where it cannot rise through the less dense crustal roof. Here the gabbroic magma can evolve under quiescent conditions and accumulate plagio-clase-rich crystal mushes, fractionate olivine and pyroxene, and decrease magma density. This process results in elevated isotherms, due in large part to heat of crystallization, and causes anatexis of the lower crust to produce granitoid melts (Emslie, 1975, 1978; Longhi and Ashwal, 1985). In contrast, in an environment of rapid rifting the ponded gabbroic magma would

not remain ponded, but would quickly ascend and form basalt fl ows, new oceanic crust, etc., without evolving toward plagioclase-rich crystal mushes. If, instead, the environment were one of contraction, magmas from diverse sources would become hybridized and/or injected upward prior to fractionation. Accordingly, active tectonic regimes were thought by many to preclude the quiescent environments necessary for fractionation and the accumulation of a pla-gioclase-rich crystal mush. In contrast, so-called “anorogenic” environments were perceived as capable of providing conditions for quies-cent fractionation and genesis of anorthositic magmas and AMCG suites. The “anorogenic” model was also consistent with the observation that, whereas many AMCG suites occur within tectonic belts, they do not appear to be syntec-tonic (e.g., Nain Plutonic Suite).

Although a number of investigators expressed skepticism about “anorogenic” environments (cf. Ashwal, 1993), the most demonstrable fl aw in the concept arose from the expansion of U-Pb zircon geochronology in the Grenville province, including the Adirondacks. By the early 1990s it became evident that emplacement ages of AMCG granitoids overlapped in time with contractional events taking place within the same general region (McLelland et al., 1996; Corrigan and Hanmer, 1997). Accordingly, the concept of “anorogenic” emplacement was no longer tenable for these AMCG suites. At the same time, the role of delamination in over-thickened, contractional orogens was becoming better understood, especially as it applies to late and postorogenic magmatism (Turner et al., 1992). Both McLelland et al. (1996) and Cor-rigan and Hanmer (1997) applied the delamina-tion mechanism to the genesis of AMCG suites, especially within the late stages of contractional orogenesis. They argued that once a dense litho-spheric keel is delaminated from a thickened orogen, hot replacement asthenosphere rises to the crust-mantle interface and produces gabbroic

magma through decompression partial melting. Simultaneously, the orogenic crust, deprived of its high-density root, undergoes strong buoyancy forces that result in uplift and the neutralization of horizontal contractional forces that may con-tinue to exist. Orogen collapse along low-angle normal faults may take place, but the buoyant, tectonically “neutral,” orogen can stay relatively “quiescent” for several million years. This set of circumstances results in a tectonic environ-ment that provides all of the stability inherent in supposed “anorogenic” settings. The concept involving delamination and uplift, however, is superior to the anorogenic concept, because a delaminated, buoyant orogen is more consistent with both theory and observation.

An important corollary of the delamination-based model is that it places the genesis of AMCG suites within time intervals that coincide with late- to posttectonic magmatism, and this timing is consistent with observations in the Adirondacks and adjacent parts of the Canadian Grenville province. Specifi cally, the voluminous AMCG magmatism dated at ca. 1150 Ma (e.g., Marcy, Morin, Lac St-Jean massifs, Fig. 1) immediately followed the Shawinigan orogeny that affected this part of the Grenville province during the interval ca. 1200–1150 Ma (McLel-land et al., 1996; Corrigan and Hanmer, 1997; Wasteneys et al., 1999). To the extent that these complexes result from delamination of over-thickened orogens, they refl ect the widespread effects of the Shawinigan orogeny. It is possible that delamination-related associations may apply to the ca. 1300–1450 Ma Harp Lake, Mistastin, Michikamau, and Nain AMCG suites, for which a model associating them with funneled, fl at-slab subduction in the interval 1460–1230 Ma has been advanced (Gower and Krogh, 2002). In this scenario, an underriding slab may have involved a spreading center that provided plumb-ing for ascending mantle melts. Alternatively, instabilities and rollback in the slab may have led to episodes of delamination that allowed

Figure 12. Composite weighted-mean 207Pb/206Pb age plots for (A) AMCG granitoids and (B) anorthosite samples of Adirondack AMCG suite.

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hot, new asthenosphere access to the base of the crust so as to produce AMCG magmatism. On a much smaller scale, the Labrieville, St. Urbain, Roseland, and Montpelier anorthosites fall into the range ca. 1050–1000 Ma, coeval with late- to posttectonic events associated with the Ottawan orogeny (Owens and Dymek, 2001; Owens et al., 1994) and its proposed delamination (McLelland et al., 1996).

The delamination-related model presented above does not preclude derivation of AMCG magmatism from hotspot-related mechanisms such as those found in Paleozoic ring complexes of Niger, West Africa (Husch and Moreau, 1982). The essential factor in the produc-tion of such suites is the ponding of gabbroic magma and elevation of isotherms at the base of the crust followed by a period of quiescent fractionation. Both hotspots and delamination can accomplish these requirements as long as lithosphere is broken through so that isotherms become elevated owing to infl uxes of hot new asthenosphere.

Implications for AMCG Petrogenesis

Currently two major petrogenetic models have been put forward to account for the gen-esis of AMCG suites. The fi rst is based upon bimodal magmatism and considers the suite to be coeval but not comagmatic. It has been described in detail by Emslie (1985), Longhi and Ashwal (1985), Fram and Longhi (1992), Emslie et al. (1994), and Weibe (1994) and rep-resents the most commonly held view of AMCG magmatism. Its principal characteristics are that (1) anorthosites are mantle derived and (2) their associated granitoids (i.e., the mangerite-char-nockite-granite rocks of the AMCG suite) are the result of melting of the lower crust due to heat related to elevated isotherms and the mantle melts cited in (1). The second model utilizes experimental and isotopic evidence to argue for a comagmatic origin for most AMCG suites and is characterized by a mafi c (two pyroxenes and plagioclase) lower-crustal (P = ~10–13 kbar) source region that melts to produce jotunitic or ferrodioritic compositions similar to high-alu-mina gabbro but containing higher concentra-tions of Fe, Ti, P, and K (Vander Auwera et al., 1998, 2003, 2003; Longhi et al., 1999). Frac-tionation of the jotunitic magmas is thought to yield the entire range of AMCG compositions.

In studies of the Laramie, Wyoming, AMCG complex, Mitchell et al. (1996) and Scoates et al. (1996) presented models in which mantle-derived, high-alumina gabbroic melts fraction-ated to anorthositic cumulates and residual ferrodioritic liquids that then differentiated to monzodioritic compositions exhibiting little

silica enrichment. Open-system interaction of these magmas with crustal rocks yielded quartz-bearing differentiates, and associated large bod-ies of granite (e.g., the Sherman batholith) are considered to be primarily of crustal derivation.

Emslie et al. (1994) pointed out that heating of the lower crust by ponded gabbroic magma would lead to early production of granitoid melts, leaving behind hot, residual plagioclase-pyroxene restites that could be assimilated by the already-fractionating gabbroic magmas. Assimilation-fractional crystallization (AFC) processes would produce anorthositic magmas characterized by isotopic and geochemical signatures consistent with lower-crustal con-tamination. Lower-crustal signatures have been widely noted in anorthosites (e.g., Taylor et al., 1984; Clifford et al., 1995; Vander Auwera et al., 1998, 2003, 2003; Longhi et al., 1999; Schiellerup et al., 2000; Hannah and Stein, 2002; Wiszniewska et al., 2002) and are central to both comagmatic and bimodal models. How-ever, as shown by Emslie et al. (1994), these critical signatures do not preclude derivation of anorthositic parent magmas from mantle-derived gabbroic melts that become contami-nated by interaction with lower-crustal restites stripped of their granitoid fractions. Emslie et al. (1994) suggested that this sequence of events is consistent with the observation that, in the AMCG suites that they investigated, large granitoid batholiths commonly represent some of the earliest intrusions. In the Adirondacks, the slightly older ages (Fig. 11) reported for all 13 granitoids (1158 ± 5 Ma)—and for the 10 granitoids that are charnockites and mangerites (1163 ± 7 Ma)—are consistent with the model of Emslie et al. (1994). In contrast, the comag-matic model would predict a slightly younger age for the granitoid suite. In the bimodal model, late, crosscutting granitoids (Fig. 10A) are easily accounted for by multiple, crosscut-ting AMCG pulses. Indeed, small to moder-ate volumes of late granitic magmas, locally crosscutting older mafi c (i.e., anorthositic) rocks are not uncommon in the Nain Plutonic Suite. That the ages of some of these plutons postdate the youngest anorthositic units by as much as 10 m.y. attests to a long-lived supply of heat to the lower crust in that region (Hamilton, 1997). In the large body of Adirondack age data shown in Figure 11, six of the seven oldest ages are from granitoids, and this result supports a coeval, but not comagmatic, bimodal origin for the Adirondack AMCG suite consistent with the conclusions of Emslie et al. (1994) elsewhere in northeastern Canada.

Vander Auwera et al. (1998) and Longhi et al. (1999) presented experimental results within the system olivine-wollastonite-plagioclase-

quartz with starting materials that included several synthetic glasses and high-alumina gabbroic or jotunitic compositions considered to be reasonably good candidates for magmas parental to anorthosite. They interpreted their results as indicating that jotunitic or ferrodio-ritic parental magmas assumed for the Rogaland anorthositic complex can fractionate into both anorthositic cumulates and mangeritic to char-nockitic residues. However, they concluded that for many massif anorthosites (e.g., Marcy, Harp Lake, Nain, and Laramie complexes), the low normative-quartz compositions of likely parent magmas result in differentiation trends that do not move toward silica saturation. Conse-quently, granitoids associated with these intru-sions are best explained as crustal melts within bimodal AMCG complexes. This conclusion is consistent with the enormous volume of such granitoids as well as with compositional gaps between them and the more mafi c rocks such as ferrodiorites (Emslie et al., 1994). Moreover, a bimodal origin is almost certainly required by the observation, made earlier in this paper, that zircons within Adirondack AMCG granit-oids commonly contain older (ca. 1.2–1.3 Ga) embayed cores (Figs. 8A and 8D) that are read-ily explained as having been entrained from deep-crustal, arc-related rocks of Elzevirian age. In contrast, the presence of the older zircon cores in granitoids precludes their derivation by fractionation of jotunitic or ferrodioritic parental magmas. This is so because such liq-uids would dissolve entrained zircon grains, as documented by application of the Watson and Harrison (1983) zircon solubility relationship to jotunitic compositions such as those given by Vander Auwera et al. (1998) or found in the Adirondacks (McLelland et al., 1994). In such liquids, and at temperatures of ~1000 °C, zircon saturation is not encountered until con-centrations of 2000–5000 ppm are reached (i.e., 10 times the actual concentrations observed). Accordingly, the observed presence of inherited zircon cores in Adirondack AMCG granitoids demonstrates that these rocks are not the result of fractionation of mafi c magmas parental to associated anorthosites; i.e., the Adirondack AMCG suite is bimodal in origin and so is any other AMCG suite in which granitoid zircons contain inherited cores. Emslie and Hunt (1990) reported minor inheritance in zircon popula-tions of granitoids of the Rivière Pentecôte and Atikonak River AMCG suites in Quebec, indicating that these complexes are bimodal in origin. An examination of the zircons of the granitoids of other AMCG suites—including those of the Rogaland anorthositic complex in southern Norway—should be undertaken to seek the presence of inherited zircon cores.

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Investigations applying the Re-Os isotope system have led some workers to propose lower-crustal sources similar in composition to layered mafi c intrusions for magmas parental to massif-type anorthosites (Schiellerup et al., 2000; Wiszniewska et al., 2002). However, other workers (Hannah and Stein, 2002) have presented evidence that the Os isotope charac-teristics of anorthositic rocks can be explained by assimilation of appropriate crustal materials. Emslie et al. (1994) exemplifi ed this approach in demonstrating that crust-derived isotopic signatures of anorthosites (cf. Taylor et al., 1984; Clifford et al., 1995) are readily accounted for by magmatic assimilation (AFC) of lower-crustal pyroxene-plagioclase restites by dry mantle-derived gabbroic magmas. As pointed out by Emslie et al. (1994), the assimilating magma need not even have a high-alumina gabbro com-position, because AFC processes will replenish plagioclase as a liquidus phase in the gabbroic magma. We suggest that coeval olivine gabbro intimately associated with the Marcy anorthosite (e.g., sample CGAB, 1153 ± 13 Ma) represents an only slightly evolved gabbroic melt of the sort ponded at the crust-mantle interface. Assimila-tion of mafi c crustal residues by such gabbroic melts caused fractionation, and volumes of pla-gioclase-rich magmas and mushes accumulated by fl oating. Heat of crystallization drove further crustal melting, and recharges of mantle-derived gabbroic melt produced additional plagioclase cumulate along with mafi c interstitial liquid characterized by isotopic signatures that refl ect a blend of mantle and crustal components. In this way the plagioclase-rich crystal mushes parental to anorthosites were built up in a straightforward and self-consistent fashion. This model is wholly in accord with fi eld and chemical data from the Adirondack AMCG suite, accounts for zircon inheritance in its granitoids, and requires the observed slightly older ages of the charnockitic and mangeritic members. We conclude that a bimodal origin best accounts for all aspects of Adirondack AMCG rocks and probably for most AMCG suites as well.

ACKNOWLEDGMENTS

This research has been supported by National Sci-ence Foundation grants EAR-0125312 (to Bickford and McLelland) and by funds from the Colgate Uni-versity Research Council, all of which are gratefully acknowledged. Matt Heumann provided tireless assis-tance in sample preparation. We thank James Scoates, David Corrigan, and James Connelly for very helpful comments and reviews.

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