Pillow talk

Journal of Volcanology and Geothermal Research, 4 (1978) 55--80 55 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

P I L L O W T A L K 1

MAARTEN J. DEWIT= and CHARLES STERN

Lamont-Doherty Geological Observatory of Columbia University, Palisades, N. Y. 10964 (U.S.A.)

(Received September 2, 1975; revised and accepted February 21, 1978)

ABSTRACT

deWit, M.J. and Stern, C., 1978. Pillow talk. J. Volcanol. Geotherm. Res., 4: 55--80.

Three distinct types of pillows and pillow lava sequences with different modes of origin have been recognized in the extrusive sequences comprising the upper parts of ophiolite complexes that represent the mafic portion of the floor of an Early Cretaceous back-arc basin in southern Chile. One type of pillow formed by non-explosive submarine effusion. A second type formed by magmatic intrusion into pre-existing aquagene tuff formed by explosive eruption. The third type of pillow occurs within dikes, forming pillowed dikes, possibly as a result of vapor streaming within a cooling dike. Where studied in southern Chile, aquagene tufts and intrusive pillows decrease and water-lain pillows increase in relative abundance from north to south. This variation corresponds with a north-to-south decrease in both the relative volume of extrusives to extensional dikes and the range and volume of differentiated rocks, suggesting a southward increase in rate of extension relative to rate of magma supply within the spreading ridges at which the ophiolites formed. In the northern part of the original basin where the rate of extension was small relative to the rate of magma supply, magma remained in magma chambers longer, resulting in a greater range and volume of differentiated rocks. The larger volume of more differentiated, cooler and more viscous magmas, in conjunction with the likelihood of more violent eruption of volatile-rich differentiates, may have been responsible for the large volume of aquagene tuff in the northern part of the original basin. These observations in southern Chile suggest that ophiolites which contain a great abundance of aquagene tufts and intrusive pillow lavas formed in tectonic environments in which the rate of extension was small relative to the rate of magma supply (island arcs, embryonic marginal basins). Ophiolites with predominantly water-lain pillowed and massive lavas formed in tectonic environments in which the rate of extension was large relative to the rate of magma supply (mid-ocean ridges, mature back-arc basins). Thus geologic field data may supplement geochemical data as a tool in distinguishing the original igneous-tectonic environments in which ophiolites originate.

INTRODUCTION

Investigations of ophiolites, in many cases representative of sections of oceanic crust (Gass, 1968; Vine and Moores, 1972; Coleman, 1971), have

Lamont-Doherty Geological Observatory Contribution No. 2682. = Present address: UNDP, P.O. Box 5580, Addis Ababa, Ethiopia.

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yielded invaluable data that cannot be obtained by direct observation of ocean floors (Upadhyay et al., 1971; Reinhardt, 1969; Gass and Smewing, 1973; Peterson et al., 1974; deWit and Stern, 1976; Stern and Elthon, 1978). In this paper we present some results of a field and geochemical study of the extrusive and intrusive components of ophiolites located in southern Chile (Fig.l). These lithologic units within ophiolites have been matched with the several seismic layers reasonably well defined within the ocean crust (Fox et al., 1973; Christensen and Salisbury, 1975). Geologic field data concerning these layers and their boundaries are obviously important for an understanding of the mechanisms involved in the creation and evolution of ocean crust. Moreover, since there are limitations on the use of chemical data to distinguish among all the igneous-tectonic environments (mid- ocean ridge, marginal basin, yomg island arc) in which ophiolites may form (Hynes, 1975;Kay and Senechal, 1976; Vallance, 1974; Church and Ricco, 1976), field relations may provide a useful additional tool if not the only criteria for making such distinctions. Kidd and Cann (1975) convincingly used field data to demonstrate that the Troodos ophiolite complex was probably formed at a spreading ridge.

The extrusive units of the ophiolite complexes which we have studied consist of 1--2 km of massive and pillowed lavas and associated breccias and tuffs. Detailed descriptions of pillow lavas and their associated hyaloclastic rocks are numerous (see Snyder and Fraser, 1963a, b for review of early literature; Carlisle, 1963; Moore, 1965, 1966; Sigvaldason, 1968; Jones, 1969; Fumes, 1972; Amdt, 1973). Arndt (1973) noted that in many descriptions, pillow lava sequences of different types have frequently been treated together and ascribed to a common origin. In our study of the mafic complexes of southern Chile we have distinguished three types of pillows whose characteristics indicate different modes of origin. One type of pillow lava sequence formed by non-explosive submarine eruption and is common- ly associated with pillow breccias formed by post-deposition breakup and sedimentary redistribution of similar sequences. A second pillow lava sequence is associated with aquagene tuffs formed by explosive submarine eruption. Pillows and breccias of this type formed by magmatic intrusion into the unconsolidated tuff. The third type of pillow occurs within dikes, forming pillowed dikes rather than lavas. Pillowed dikes may be formed when the dike intrudes a level with free water generating vapor which streams through the rapidly cooling dike.

In this paper we attempt to relate changes in the relative abundance of these different types of pillows to changes in some of the tectonic factors that influence the formation of ophiolites, specifically rate of extension relative to rate of magma supply. Southern Chilean ophiolites represent the mafic portions of the floor of an Early Cretaceous back-arc basin, closed and uplifted in the mid-Cretaceous (Dalziel et al., 1974; Bruhn and Dalziel, 1977). Field mapping suggests that the original basin widened markedly south-southeastwards from 49°S (deWit, 1977). The results of a comparative

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s tudy of the Sarmiento and Tortuga Complexes (Fig.2), located in the northern and southern extremities, respectively, of the outcrop belt of ophiolites (Fig.l) , indicate that towards the south, in what was probably the wider part of the original basin, an increased rate of tectonic extension relative to rate of magma supply resulted in a greater volume of extensional dikes relative to extrusive lavas, and also decreased the residence time of magma within magma chambers resulting in fewer differentiated rocks. In conjunction with these north-to-south changes, the abundance of aquagene tufts and intrusive pillowed dikes and lavas decreases, while pillowed and massive lavas formed by non-explosive submarine eruption become more abundant. Increased tectonic extension relative to magma supply is one characteristic difference between mid-ocean ridges and young island arcs. Young island arcs are characterized by a greater volume of both more differentiated (Jakes and White, 1971; Jakes and Gill, 1970; Miyashiro, 1975) and more pyroclastic (Rit tman and Rittman, 1976) rocks than mid-ocean ridges. Based on our observations in southern Chile, we suggest that ophiolites formed in the base of island arcs and in embryonic back-arc basins will be characterized by a greater abundance of aquagene tufts and intrusive type pillow lavas, while ophiolites formed in more mature back-arc basins and mid-ocean ridges will contain greater volumes of pillowed and massive lavas formed by quiescent submarine eruptions. Thus, the type and relative abundance of pillow lava sequences and their associated hyaioclastic rocks may reveal a great deal about the igneous-tectonic environment in which ophiolites formed.

CHILEAN OPHIOLITES

The ophiolite complexes that crop ou t from 49°S to Cape Horn (Fig.l) in southern Chile are flanked on the western (Pacific) side by the Patagonian batholith which has been interpreted as the roots of an andesitic chain initi- ated in the Middle Jurassic (Fig.3; Stern and Stroup, 1978). This volcanic chain developed on pre-Jurassic continental crust which in many places crops out to the west of the batholith. The batholith was intruded into, and contains large roof-pendents of pre-Jurassic metasediments and Jurassic silicic volcanics similar to those found comprising South American continental crust on the eastern (Atlantic) side of the ophiolites. Intrusion of mafic magmas behind the andesitic volcanic chain in the latest Jurassic first produced swarms of dikes which cut the pre-Jurassic basement and Jurassic volcanics. These dike swarms now flank the ophiolite complexes. Continued intrusion of mafic magma along spreading centers formed the mafic port ion of the floor of a back-arc basin which separated the active calc-alkaline volcanic

Fig.2. Geological map of the Sarmiento and Tortuga Complexes. Note wider belt of Lower Cretaceous basin sediments near Tortuga Complex, suggesting a wider original back-arc basin.

60

chain from continental South America, and is now preserved as an en echelon chain of ophiolite complexes stretching over 1000 km (Fig. 1).

The extrusive components of the ophiolite complexes are overlain by Lower Cretaceous sedimentary rocks. The lack of sediments within the extrusive units, except for minor chert and jasper high in the sequences is consistent with the formation of the mafic complexes along localized spreading ridges followed by their transportation away from the zone of magmatic intrusion by continued spreading. The Early Cretaceous back-arc basin sedimentary rocks consist of shales and mudstones on the east (continental) side, and volcaniclastic greywackes with minor interbedded andesitic lavas on the west (volcanic arc) side (Fig.3; Dalziel et al., 1974). Such asymmetric sediment distribution is typical of back-arc basins (Karig and Moore, 1975). The lack of ophiolites north of 49 ° S, the southward widening of the outcrop belt of back-arc basin sedimentary rocks, and the increasing divergence in the strike of the Patagonian batholith and silicic volcanics that flank the ophiolites suggest that the original basin widened markedly from the north to the south (deWit, 1977). deWit (1977) has suggested that southeast of the present outcrop belt of ophiolites, the basin floor may have widened to over 200 km and may be preserved as part of the central Scotia Sea. The width of the original basin floor in the regions in which the Sarmiento and Tortuga Complexes are now exposed (Fig.2) has been estimated as 25 km and 100 km, respectively (deWit, 1977), although the extent of the mafic portion o f the basin floor is indefinite.

The back-arc basin was closed and uplifted in the mid-Cretaceous (Bruhn and Dalziel, 1977). The penetrative deformation caused by this event is heterogeneously distributed, concentrated predominantly in rock assem- blages flanking and overlying the ophiolites (Fig.3). Associated regional metamorphism in the prehnite-pumpeUyite facies is evident in the basin sediments (Watters, 1965), but not within the undeformed ophiolite complexes. The Sarmiento and Tortuga Complexes have escaped both the regional penetrative deformation and metamorphism. In these locations lack of penetrative deformation plus continuous and three-dimensional exposure have enabled us to study in detail the pillow lavas and breccias of the extrusive sequences and the gabbros and dikes of the intrusive sequences and trace the boundary between these units.

The ophiolites have been altered prior to the closure of the basin by an ocean floor metamorphism characterized by the development of secondary minerals without schistosity and a steep vertical gradient passing from zeolite to amphibolite facies within 2 km followed by a transition to fresh gabbros (deWit and Stern, 1976; Stern and Elthon, 1978). deWit and Stern (1976) suggested that this alteration was due to circulation of seawater and igneous and tectonic processes at the spreading ridge at which the ophiolites formed. Pillow lavas exhibit chemical effects of spilitization, particularly increase of SiO2 and decrease of CaO, but in the deeper levels of the ophiolite complexes higher-grade metamorphism has been more nearly isochemical (Stem and

61

Middle and early Late Jurassic [ ] Silicic volconlcs subordinate mofic Icvas

[ ] Basement [ ] Anatectic melts

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Jatest Jurassic- Early Cretaceous ~ Gobbro ~ ] Basic sills

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Fig.3. Schematic cross sections of the evolution of the back-arc basin in southern Chile (Bruhn et el., 1978; Bruhn and Dalziel, 1977). Note the change from subaerial to submarine eruption between stage A and B; the different lithologies flanking the basin in stage B resulting in the asymmetric sediment distribution within the basin; and the restriction of the deformation in stage C to the rock units overlying and flanking the ophiolites.

Elthon, 1978). Dikes in both the Sarmiento and Tortuga Complexes exhibit a tholeiitic trend consistent with differentiation within shallow level magma chambers involving crystallization and separation of olivine, pyroxenes, calcic plagioclase, and titanomagnetite; phases found in the cumulate gabbros of both ophiolites (Stern, 1976). The extent of differentiation and the volume of differentiates is much greater within the Sarmiento Complex, where highly evolved ferro-basalts and silicic dikes and laves similar in chemistry to oceanic plagiogranites (Coleman and Peterman, 1975) are common (Table 1).

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F o r dikes from among the dike swarms flanking both ophiolites, which represent the earliest manifestation of mafic magmatism, and for dikes and lavas from the Sarmiento Complex in the narrowest part of the original basin, the least differentiated basalts (FeO/MgO = 0.95) have normalized La/Yb ratios (La/Yb)N = 1.9 with La = 19 times chondritic abundance (Table 1). For the most primitive basalts from the dikes and lavas of the Tortuga Complex in the widest part of the original basin, La = 8--10 times chondritic abundance with (La /Yb)s = 0.7 (Table 1). This difference between the least differentiated basalts indicates that with the localization of the region of intrusion of magmas along spreading centers and the widening of the back-arc basin, mafic magmas were either progressively derived from chemically distinct magma source regions or a single mantle source became increasingly depleted in large- ion lithophile elements due to extraction of successive melt fractions (Stern, 1976).

The igneous pseudostratigraphy of both the Sarmiento and Tortuga Com- plexes, schematically illustrated in Fig.4, consists in part of a complex of 100% sheeted dikes indicating 100% extension which is consistent with their formation at a spreading ridge (Gass, 1968; Moores and Vine, 1971). Beneath

SARMIENTO TORTUGA

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Fig.4. Schematic pseudostratigraphy of the Sarmiento and Tortuga Complexes. The descrip- tion of different types of pillow lava sequence is in the text. Note the igneous contact between the sheeted dikes and plagiogranites in the Sarmiento Complex and the continuous transition between extensional dikes and diabases in the Tortuga Complex where plagiogranites are absent.

64

the sheeted dike complexes are chemically layered plutonic rocks which grade downward into coarsely crystalline gabbros. In the Sarmiento Com- plex the plutonic rocks exhibit the same large chemical diversity found in the lavas, with ferro-gabbros and silicic plagiogranites common just below the sheeted dike complex. The silicic plagiogranites intruded the sheeted dike complex by magrnatic stoping, forming a chilled subhorizontal contact with the dikes (Fig.4). In the Tortuga Complex plagiogranites are absent. Here the sheeted dike unit grades downward into a mixture of dikes and diabases in a continuous transition. Diabases first appear as large dikes 3--5 m across which grade downwards into diabase masses 10--30 m across as the number of thinner fine-grained dikes decreases. The diabase masses have fine- grained margins and are separated by 5--20 m of recognizable thin (,~ 1 m) fine-grained dikes. They seem to have intruded as very large extensional dikes into the bo t tom of the sheeted dike complex (Fig.4). Below this sheeted dike-diabase unit, coarse-grained mineralogically layered cumulate gabbros become common.

The tholeiite differentiation trends exhibited by the ophiolite dikes and lavas could only have been produced by the crystallization and separation of olivines significantly more magnesian than are found in the deepest exposed gabbros (Stern, 1978}; thus it is unlikely that the deepest level of mafic cumulate gabbros has been exposed in either the Sarmiento or Tortuga Com- plex. The mafic phases that are found in the deepest exposed gabbros of each complex (Fig.4) would have crystallized from evolved magmas, consistent with their occurrence in the upper part of a large, still incompletely exposed gabbro unit. Also, the peridotite member of these ophiolites is no- where exposed. The lack of exposed ultramafic rocks is considered to be due to the present level of exposure and the fact that the Chilean ophiolites were uplifted but not obducted (Fig.3; Dalziel et al., 1974; Bruhn and Dalziel, 1977; Saunders et al., 1978).

PILLOW LAVAS

Towards the tops of the sheeted dike complexes screens of pillows occur between dikes; these are followed by regular horizontal sequences of massive and pillowed lavas, breccias and tufts as the density of dikes decreases. The dikes, characteristically vertical within the gabbros and sheeted dike unit, become noticeably sinusoidal in form and lose their parallelism as the pillo.w lavas increase in abundance.

In the Tortuga Complex the extrusive unit is between 1 and 1.5 km thick and consists of both massive lavas and pillowed lavas and minor pillow breccias (Fig.4). Pillow lavas consist of close packed pillows which range from 10 cm to 1 m in diameter and vary in shape from spherical to cylindri- cal (Fig. 5). Chilled margins of these pillows vary in thickness but are usually up to several centimeters wide and their internal boundaries are not always distinct. Vesicles are common. Radial columnar jointing is almost ubiquitous

65

Fig. 5. Close-packed spherical to ellipsoidal-shaped pillows with thick (> 5 cm) chilled margins, interpreted as water-lain pillows. (Tortuga Complex.)

and is expressed on the outer surface of the pillows by a hexagonal to more irregular fracture pattern, which together with concentric jointing often gives the pillows a block-like appearance.

Pillow lavas withthese characteristics are generally regarded as having formed by non-explosive extrusion of magma into water (Sigvaldason, 1968; Moore, 1965, 1966; Jones, 1969; Fumes, 1972). Present-day examples are clearly recorded in Hawaii (Moore, 1965; Tepley and Moore, 1973) and the Mid-Atlantic Ridge (Bellaiche et al., 1974). Such pillow lavas have been described from many localities (Iceland, Sigvaldason, 1968; Cyprus, Gass and Masson-Smith, 1963; Caledonides-Appalachians, Fumes, 1972; Jones, 1969; California coast, Bailey et al., 1964). The different shapes of pillows as well as the occurrence of both pillowed and massive lavas can be explained in terms of topography (original slope of deposition), viscosity of the lavas, and the rate of emission (Sigvaldason, 1968, BeUaiche et al., 1974).

Pillow breccias associated with the extrusive pillow lavas in the Tortuga Complex consist of very angular fragments in most cases clearly derived from broken up pillows. The jointing system in the pillows determines the size and shapes of the breccia fragments. There is every gradation from complete pillows with well-developed jointing to this type of pillow breccia. The breccias usually contain little tuff matrix and are often clearly bedded, indicating that erosion, gravity induced breakup and deposition of the pillow

66

fragments (possibly along fault scraps, cf. Bellaiche et al., 1974) was a more important mechanism than explosive eruption in the formation of these breccias.

The extrusive unit of the Sarmiento Complex is over 2 km thick, and consists of a significant volume of aquagene tuff associated with pillow lavas and pillow breccias (Fig.4}. Massive lavas have not been encountered. The tuffs consist of altered basaltic glass. They are never laminated and they show no sedimentary structures. The origin of similar aquagene ruffs described in Iceland (Sigvaldason, 1968), British Columbia {Carlisle, 1963} and from the ocean floor (Bonatti, 1967) is at tr ibuted to explosive submarine eruption.

Complete vertical and horizontal gradations occur from massive aquagene tuff to both isolated and broken pillow breccias set in a tuffaceous matrix (Figs. 6 and 7), to close-packed pillow lavas (Fig.8). Outcrops studied in the Sarmiento Complex indicate that often the aquagene tuffs exist prior to the formation of the pillows and breccias, and that both pillows and breccia fragments formed by intrusion of basaltic material into the tuff. Fig.9 shows the margin of a vertical dike which intrudes an unknown thickness of aquagene tuff. The lobate highly irregular edge of the dike displays frozen in all transitions to the pillows and pillow breccia observed away from the dike (Fig.10). The dike has a glassy margin identical to those of the breccia fragments. Evidently, as a lobate port ion of the dike grows in a plastic manner, possibly due to irregularities in magma advance, it becomes entirely sur- rounded by such a glassy margin. Away from the dike, large fragments and pillows may undergo further breakdown to smaller fragments. Elsewhere (Fig. 11), a dike with numerous subparallel chilled margins is seen cutting through breccia with a matrix of aquagene tuff. The subparallel chilled margins of the dike probably result from repeated pulses of magma moving through the center of the dike, each new phase forming its own chilled margins and pushing the older margins outwards. A number of the fragments in the breccia contain similar subparallel alternating zones of rapid and more slow cooling. These fragments have formed by breaking off the edge of the expanding dike in a brittle manner.

In some outcrops thin dikes may be seen to be inflated at their upper- most extremity, outlining or feeding a pillow (Fig.12). Such cases have also been observed in the Afar Rift (E. Bonatti, personal communication, 1975) and in the Troodos massif, Cyprus (I. Gass, personal communication, 1975}. Where the vertical exposure is extensive enough a vertical sequence has been observed in which a pillow forming at the end of a dike and isolated pillows within the surrounding aquagene tuff occur at the highest level. Lower in the sequence the density of isolated pillows increases. These pillows often are thinned and their vesicles streamlined in the vertical direction (Fig.6). The base of the sequence is formed by a pile of nearly close-packed pillows containing only minor aquagene tuff. These relations suggest that the pillow pile is formed by settling under the influence of gravity of pillows

Fig.6. Isolated pil low in aquagene tuff. The pillow is elongated, and vesicles are streamlined, in the vertical direction suggesting deformat ion during gravity settling through the tuff. The sinusoidal dike at the left of the pil low is terminated in an inflated pil low form just above the area of the photo. Just below the area of the photo is a pile of highly involuted, nearly close-packed pillows (Fig.8). The dike, the isolated pillow, a pillow from within the pil low pile, and the tuff all have the same chemistry, suggesting a co-magmatic origin. See text for discussion. (Sarmiento Complex.)

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Fig.7. Broken and isolated pillow breccia within aquagene tuff. (Sarmiento Complex.)

Fig.8. Massive pile of involuted pillows with thin glassy margins. Interstitial material is aquagene tuff. (Sarmiento Complex.)

Fig.10. Pillow breccia with an isolated pillow in an aquagene tuff matrix. The photo was taken approximately 3 m away from the edge of the dike shown in Fig.9.

which formed by intrusion into the pre-existing aquagene tuff. In two separate outcrops of such vertical sequences, the chemistry of the feeder dike, an isolated pillow, and a pillow within the close-packed pillow pile were found to be the same within analytical precision, consistent with their formation in a single eruptive event. In one case the tuff also had a similar chemistry and may have been co-magmatic; in the other case it was clearly distinct.

In the sequences described above the tuff has formed before the pillows, but the stratigraphic order is the opposite. Pillows of this type of pillow lava vary from spheres to irregular masses, of ten being greatly elongated and involuted (Figs.6, 8}. Chilled margins are thin and well defined. Jointing is never conspicuous and where present is of ten epigenetic (Carlisle, 1963, p. 55). Vesicles may or may not be presenk Snyder and Fraser (1963b) review the literature on other occurrences of intrusive pillows.

At the top of the sheeted dike complex in the Sarmiento Complex, within the zone where screens of pillows first occur and dikes begin to be sinusoidal in form, vertical sheets of pillows of the dimensions of dikes are observed which can be traced downwards into homogeneous dikes {Fig.4). These sheets of pillows do not therefore appear to simply represent remnant screens of earlier horizontal pillow lavas subsequently invaded by vertical dikes, which are also found. Similar pillowed dikes have been observed in Cyprus (I. Gass, personal communication, 1975).

Fig.11. Dike intruding aquagene tuff with breccia. Note the subparallel chilled margin formed by mult iple intrusion within the dike, and similar structures in some breccia fragments which have formed by brit t le fracture off the edge of the expanding dike. (Sarmiento Complex. )

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Penetration

of Seawater

1

Release of

Magmatic watel

~ ' ~ x ~ ,/ I

I I } \ \

I I ,,

Zone of pillow forrnotion:

thick bonds

Zone of degassing: thin bonds

Zone of rapid cooling

quench textures

Zone of slow cooling sub-ophitici textures

Fig.13. Model of the development of pillowed dikes as outlined in text. Note that the bands outlining pillows begin to appear in the same region as radial crystal intergrowths of plagioclase.

approximately similar sizes and shapes. The pillowed dikes have both sides marked by dark green margins identical to those of the bands (Fig.13).

Petrographic examination of thin sections cut from different levels in a single dike covering its transformation from a homogeneous to a pillowed dike shows that just below the first pillow shapes the sub-ophitic texture of the lower part of the dike changes with the appearance of many radial crystal clusters of plagioclase. These radial crystal clusters also appear higher in the dike within the pillow shapes (Fig.13). Such clusters are quench textures and suggest that the level of pillow formation was also a level of rapid crystallization.

We suggest that the development of the bands which outline the pillow shapes is produced by development of a vapor phase and subsequent out- gassing of a dike along channels that shape the crystalline material into pillow- like forms. Development of quench textures at the same level indicates simul- taneous rapid crystallization. Release of hydrostatic pressure due to rise of magma could result in exsolution of volatiles and crystallization of drier magma. Alternatively, rapid crystallization of magma could concentrate volatiles in residual liquid until their concentration exceeds their solubility in this liquid and a vapor phase form. The low initial water content of basaltic magma (Moore, 1972) argues agains~ these processes acting to such an extent as to produce the well-defined pillow forms. Introduction of water into a magma at a level where seawater penetrates along zones of structural weakness could cause both crystallization and development of a vapor phase.

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We prefer this mechanism for producing pillowed dikes becmlse of the strong evidence for hydrothermal circulation of seawater in close association with igneous and tectonic activity at spreading ridges (deWit and Stem, 1976; Bonatti, 1975; Stern and Elthon, 1978). This model is consistent with the restriction of these pillowed dikes to the boundary between the intrusive and extrusive units of the Sarmiento Complex, a likely zone of structural weakness.

SUBMARINE ERUPTION

Many factors may effect the final products of submarine eruptions, such as the depth and slope of deposition, the rate at which lava comes in contact with water, and the viscosity of the magma which depends on the magma composition, gas content, and temperature (see Bonatti, 1967, for review; Sigvaldason, 1968; Ballaiche et al., 1974). Explosive eruptions forming aquagene tuffs may result from violent degassing of magma or fragmentation due to anisotropic stresses developed between the surface and center of a rapidly quenched magma (Carlisle, 1963). Explosive degassing of a magma will not usually occur at depth greater than 500 m below sea level because of hydrostatic pressure (McBirney, 1963).

We suggest that a major factor in producing the greater volume of aquagene tuff in the Sarmiento Complex was the greater volume of more differentiated magmas (Table 1). A greater degree of differentiation signifies a longer cooling history for a magma, indicating a lower temperature and con- sequently a higher viscosity at eruption. Bonatti (1967) has suggested that hot ter more fluid magmas react plastically to the sudden chilling effect of cold water, whereas more viscous magmas react with violent fragmentation. This conclusion is based both on the examination of the phenocryst content of massive and fragmented ocean floor lavas, and on the different behavior of blocky aa and more fluid paehoehoe flows as they enter water (Stearns and MacDonald, 1946). Thus the greater quanti ty of differentiated cooler magmas erupted in the Sarmiento Complex could result in the greater proport ion of aquagene tuff. Also effusion may have frequently been more rapid and violent in the Sarmiento Complex due to the higher volatile content of more differentiated magmas.

In conjunction with these effects, the original depth of the floor of the back-arc basin may have been less in the Sarmiento than in the Tortuga Com- plex. The development of the basin in the latest Jurassic/earliest Cretaceous is associated with a change from subaerial to submarine eruption (Fig.3; Bruhn et al., 1978). In its earliest stages the basin must have gone through a period of shallow depth. The narrowest part of the basin may never have been as deep as the widest parts. Since volatiles would be concentrated in more differentiated rocks, the greater volume of differentiated rocks in the Sarmiento Complex may have acted in conjunct ion with more shallow depth of eruption to produce explosive submarine deposits.

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The frequent intimate association of aquagene tuff formed by explosive eruption and pillow lavas produced by non-explosive eruption is well docu- mented (Carlisle, 1963; Sigvaldason, 1968; Nayudu, 1962). Different explana- tions have been offered to explain the formation of different eruptive products in single-eruption events. Carlisle (1963) noted that both passive and violent subaerial eruptions are observed to occur from different vents of the same volcanic center. Sigvaldason (1968) suggested that vertical sequences of pillow lavas overlain by tuff may result from buildup of a volcanic pile resulting in decreasing depth and hydrostatic confining pressure allowing more rapid devolatilization and increasing shattering of eruptive products. Nayudu (1962) suggested that similar vertical sequences found in seamounts result by the initial formation of a thick pile of tuff which blankets the ensuing effusives from direct contact with large volumes of seawater allowing them to cool into massive basalts.

The eruptive sequence we observe in Chile in which pillow lavas form by intrusion of lava into tuff is similar to that suggested by Nayudu (1962). As suggested by Nayudu (1962), the tuff may have acted as a blanket limiting the volume of water making contact with new magmas, allowing pillows to form. This mechanism could function whether the tuff and pillows were co- magmatic or not. In the case where the tuff and pillows are co-magmatic, the change from tuff to pillows may also result from the gradual decrease in the speed of effusion during a single eruptive event. The outcrops of dikes feeding pillows "frozen in" in southern Chile most likely did not form during rapid effusion of magma, but probably represent the final stage of eruption when effusion is slow.

FORMATION OF CHILEAN AND OTHER OPHIOLITES

The formation of the pseudostratigraphy of Chilean ophiolites resulted from a complex sequence of the processes of intrusion and extrusion. Magma in shallow-level magma chambers either rose through feeder dikes to form extrusives, or if unable to reach the surface, cooled to form cumulate gabbros and more differentiated magmas. Later dikes cut the gabbros, pre-existing dikes and extrusives. Sufficient time must have elapsed between these events to allow cooling since dikes have chilled margins against each other and against coarse-grained gabbros.

Repeated diking is the expression of extension. Both more prolonged and more rapid spreading may have played a part in creating the observed chemical and pseudostratigraphic differences between the Sarmiento and Tortuga Complexes, and causing the apparent southward widening of the original back-arc basin in southern Chile. As noted above, the differences in the rare earth element chemistry of the most primitive basalts from the Tortuga Com- plex, and from among both the dike swarms flanking the ophiolites and the Sarmiento Complex, indicate either progressive tapping of distinct magma

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source regions or increasing depletion in light rare earth elements of a single source; suggesting in either case a more prolonged evolution of the magma source region below the southern wider part of the original basin. A number of factors also indicate that the rate of extension was greater relative to magma supply in the Tortuga Complex. Firstly, the volume of the extensional dikes and diabases is greater relative to the volume of extrusive rocks than in the Sarmiento Complex (Fig.4). When extension is slow relative to magma supply, large volumes of extrusives are fed by fewer dikes (Moores and Vine, 1971). Magma which cannot reach the surface due to lack of diking remains in magma chambers. The greater range and volume of differentiated rocks in the Sarmiento Complex indicates that here magma remained longer within magma chambers, consistent with the intrusion by magmatic stoping of the most differentiated magmas into the overlying sheeted dike complex (Fig.4). Assimilation of the lower port ion of the sheeted dike complex during this process also acted to decrease the volume of the sheeted dikes relative to the extrusive unit. The greater amount of differentiated magmas resulting from low rate of extension relative to magma supply in the Sarmiento Com- plex, possibly in conjunct ion with more shallow depth of the basin in this region due to less prolonged development, would act to increase the volume of aquagene tuf f formed by explosive submarine eruption because of the greater viscosity of the magmas and the greater violence of their eruption as discussed in the previous section. More prolonged development and more rapid extension relative to magma supply in the wider part of the basin resulted in dominantly non-explosive submarine eruptions due to the smaller proport ion of cooler more viscous differentiated lavas as well as the greater depth of eruption.

Ophiolites may form at mid-oceanic ridges, within marginal basins, or possibly within the base of island arcs. Mid-ocean ridges are clearly regions of greater extension relative to magma supply than island arcs. Some of the characteristic differences between mid-ocean ridges and island arcs parallel the differences between the Tortuga and Sarmiento Complexes and may re- flect the difference in the rate of extension versus magma supply in these different igneous and tectonic environments. Firstly, the depth of eruption is greater within mid-ocean ridges than island arcs, in part because the arc represents a buildup of a large volume of extrusives. This buildup of the volcanic pile in island arcs is likely to cause the relative volume of extrusives to extensional dikes to be greater than within mid-ocean ridges. Also, in older disected island arcs, plutons are observed to have intruded into extrusives, completely obliterating the extensional dike unit (Hamilton and Myers, 1967). Secondly, it has been suggested that the volume of differentiated rocks in the island arc tholeiite series which forms the base of island arcs is greater than the volume of differentiated abyssal tholeiites within ocean ridges (Jake§ and Gill, 1970; Jake~ and White, 1971; Miyashiro, 1975).

Marginal basins are likely to be intermediate in their character, depending,

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on their degree of development (Karig, 1971). During initial rifting of thick continental or island arc crust, extension may be small relative to magma supply. Subsequent development of a spreading ridge within thin oceanic type crust would allow more rapid extension relative to magma supply. The depth of eruption will progressively change from shallow to deep during the development of a marginal basin. The differences observed between the Sar- miento and Tortuga Complexes are consistent with the formation of these ophiolites at different stages of development of a marginal basin.

The geochemical similarity of island arc and mid-ocean ridge tholeiites, particularly with respect to those elements which are less mobile during hydrothermal alteration, limits the certainty with which the original igneous- tectonic environment of an ophiolite sequence may be determined by geochemical methods (Hynes, 1975; Kay and Senechal, 1976). Detailed study of the nature of the extrusive sequences of ophiolites, and the relative pro- portions of plutonics, dikes and extrusives, may serve as guides to identify- ing the original tectonic environment in which ophiolites formed. Rittman and Rittman (1976) have observed that island arc extrusive sequences are characterized by a much greater volume of pyroclastic rocks than mid-ocean ridge extrusives. Based on our study of Chilean ophiolites we suggest that the extrusive units of ophiolites formed in young island arcs and embryonic marginal basins will contain greater volumes of aquagene tuff, possibly associated with intrusive pillow lavas, while the extrusive units of ophiolites formed in more mature marginal basins or at mid-ocean ridges will contain greater volumes of massive lavas and pillowed lavas formed by non-explosive submarine eruption.

ACKNOWLEDGEMENTS

We wish to thank B. Bergmann, R. Cortes, E. Gonzales and S. Harambour of ENAP of Chile, the captain and crew of the U.S.R.V. "Hero", and V. Alvarez for logistic support; and E. Bonatti, I.W.D. Dalziel, D. Elthon, R. Forsythe, I.G. Gass, It. Kay, R. Kidd, M. Perfit, R. Schweickert, M.A. Skewes and G. Wagner for helpful discussions and suggestions. This research was supported financially by the National Science Foundation through grants GV 19543 and OPP 74-21415 from the Office of Polar Programs, and DES 75-04076 and EAR 76-82456.

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