Sr and Nd isotopic and trace element compositions of Quaternary volcanic centers of the Southern Andes

Earth and Planetary Science Letters, 88 (1988) 253-262 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

253

[6]

Sr and Nd isotopic and trace element compositions of Quaternary volcanic centers of the southern Andes K i y o t o F u t a 1 a n d Charles R. Stern 2 l U.S. Geological Survey, M S 963, Denver Federal Center, Denver, CO 80225 (U.S.A.) 2 Department of Geological Sciences, University of Colorado, Boulder, CO 80309 (U.S.A.) Received May 5, 1987; revised version received February 16, 1988 Isotopic compositions of samples from six Quaternary volcanoes located in the northern and southern extremities of the Southern Volcanic Zone (SVZ, 33-46 o S) of the Andes and from four centers in the Austral Volcanic Zone (AVZ, 49-54 ° S) range for 87Sr//86Sr from 0.70280 to 0.70591 and for 143Nd/A44 Nd from 0.51314 to 0.51255. The ranges are significantly greater than previously reported from the southern Andes but are different from the isotopic compositions of volcanoes in the central and northern Andes. Basalts and basaltic andesites from three centers just north of the Chile Rise-Trench triple junction have 875r/86 Sr, 143N d / l ~ Nd, La/Yb, Ba/La, and H f / L u that lie within the relatively restricted ranges of the basic magmas erupted from the volcanic centers as far north as 35 °S in the SVZ of the Andes. The trace element and Sr and Nd isotopic characteristics of these magmas may be explained by source region contamination of subarc asthenosphere, with contaminants derived from subducted pelagic sediments and seawater-altered basalts by dehydration of subducted oceanic lithosphere. In the northern extremity of the SVZ between 33 ° and 34 o S, basaltic andesites and andesites have higher 87Sr/86Sr, Rb/Cs, and Hf/Lu, and lower 143Nd/144 Nd than basalts and basaltic andesites erupted farther south in the SVZ, which suggests involvement of components derived from the continental crust. In the AVZ, the most primitive sample, high-Mg andesite from the southernmost volcanic center in the Andes (54 o S) has Sr and Nd isotopic compositions and K / R b and Ba/La similar to MORB. The high La/Yb of this sample suggests formation by small degrees of partial melting of subducted MORB with garnet as a residue. Samples from centers farther north in the AVZ show a regionally regular northward increase in SiOz, K20, Rb, Ba, Ba/La, and 87Sr/86Sr and decrease in MgO, Sr, K / R b , Rb/Cs, and 143Nd/144 Nd, suggesting increasingly greater degrees of fractional crystallization and associated intra-crustal contamination.

1. Introduction

Orogenic volcanism in the Andes results from subduction of oceanic lithosphere along and below the western margin of South America. In the Andes, active volcanism occurs in four separate zones below which the subducted plate dips to the east at angles greater than about 20 ° [1]. These four zones, each having distinct petrochemical characteristics [2-6], have been termed the Northern Volcanic Zone (NVZ, 5 ° N - 2 ° S), the Central Volcanic Zone (CVZ, 16-28°S), the Southern Volcanic Zone (SVZ, 33-46°S), and the Austral Volcanic Zone (AVZ, 49-54 ° S). Previous Sr and Nd isotopic studies of orogenic volcanoes in the southern Andes have been concentrated in a limited portion (36-42°S) of the SVZ [4,7-9]. We report Sr and Nd isotopic and 0012-821X/88/$03.50

© 1988 Elsevier Science Publishers B.V.

trace element compositions for representative samples of Quaternary volcanoes from the northern (33-34°S) and southern (45-46°S) extremities of the SVZ and from the AVZ of the Andes (Fig. 1). The general geology, petrology, and the major element chemistry of volcanoes in these regions of the southern Andes have been presented elsewhere [2,10,11]. Active volcanism in the AVZ results from convergence of the Antarctic plate against southernmost South America, whereas volcanism in the SVZ results from subduction of the Nazca plate (Fig. 1). The southern boundary of the SVZ is at 46 °S where a triple junction is formed by the Chile Rise, an active spreading ridge, and the Chile Trench [2,12,13]. Another petrogenetically significant division of the southern margin of South America occurs in the vicinity of 35-38°S. South of these latitudes, in

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255 the region referred to elsewhere as Province II of the SVZ [14], and here as the southern SVZ (SSVZ), the continental crust is less than 40 km thick [15]; the subducted oceanic plate, which is less than 25 Ma south of the Mocha Fracture Zone (Fig. 1) [16], dips to the east at approximately 25 ° [1]; the Chile Trench is filled with sediment [17]; and the volcanoes forming the volcanic front occur along the western edge of the Main Andean Cordillera and have basal elevations of less than 1000 m. North of these latitudes, in the region referred to elsewhere as Province I of the SVZ [14] and here as the northern SVZ (NSVZ), the age of the ocean floor being subducted jumps to over 35 Ma [16]; the sedimentary infill in the Chile Trench decreases [17]; and the continental crust begins to thicken northward gradually [15]. The elevation of the Main Andean Cordillera increases north of 37°S, perhaps in response to a decrease in the dip of the subducted slab which begins to flatten northward until north of 33°S it descends below a volcanically quiescent region at an angle of less than 20 ° [1]. The volcanic front moves eastward into the center of the Main Andean Cordillera, which together with the increased elevation of the cordillera results in an increase of the basal elevation of the volcanoes along the volcanic front to well over 1000 m and greater than 3000 m between 33 and 34°S [15]. The Central Valley, a fore-arc depression separating the Coastal from the Main Andean Cordillera, also narrows northward and disappears north of 33°S, suggesting a northward change in stress patterns within the continental lithosphere.

2. Sample geochemistry 2.1. The southern part of the Southern Volcanic Zone (SSVZ) The analyzed samples are from the Maca, Cay, and Hudson volcanoes, located between 45 and 46 ° S, just north of the Chile Rise-Trench triple junction at the southern extremity of the SVZ (Fig. 1). The Maca and Cay volcanoes consist dominantly of olivine-bearing, high-A1 basalts and basaltic andesites, and the samples MC-13 (Maca) and C-7 (Cay) have TiO 2 4 1.5%, K20 ~<1.0%, L a / Y b = 5-8, B a / L a = 17-18, and H f / L u = 6 - 7 (Fig. 2), which are typical of other basalts and

basaltic andesites erupted from volcanoes located farther north in the SSVZ [8,9]. The basaltic andesite from the Cay volcano, located 35 km east of the Maca volcano (Fig. 1) has higher K20, Rb, Ba, La and L a / Y b than the Maca basaltic andesite which is similar to transverse petrochemical variations observed farther north in the SSVZ [8,9]. The sample from Maca has R b / C s = 18, which is within the range for basalts erupted along the volcanic front in the SSVZ (Fig. 2); whereas, the sample from Cay has R b / C s = 32, which is higher than previously published values for basalts from the SSVZ. The basaltic andesites from Maca and Cay volcanoes have Sr and Nd isotopic compositions within the range of most basalts and basaltic andesites from the SSVZ (Figs. 2 and 3), and their values plot within the field of oceanic island basalts (OIB) as do high-A1 basalts from many convergent plate boundaries [18]. A dacite from Cay (C-l) has higher K 2 0 , Rb, Ba, REE, and Cs contents than, but nearly similar L a / Y b , B a / L a , H f / L u and R b / C s ratios and Sr and Nd isotopic compositions as, the associated basaltic andesites from Cay (Figs. 2 and 3). Other volcanic centers from the SSVZ are also characterized by near constancy of ratios of highly incompatible trace element and isotopic compositions throughout the series basalt-andesite-dacite [8,9]. Samples from the Hudson volcano, just north of the gap in recent volcanic activity between 46 and 49°S (Fig. 1), have high TiO2, FeO, F e O / M g O , and N a 2 0 compared to other basalts and basaltic andesites in the SSVZ. The basaltic andesite, H-l, has higher K 2 0 , Ba, Rb, and REE contents than the basaltic andesites from Maca and Cay, but L a / Y b , H f / L u , 87Sr/86Sr, and 143Nd/a44Nd are similar to other samples from the SSVZ of the Andes (Figs. 2 and 3). 2.2. The northern part of the Southern Volcanic Zone (NSVZ) The samples analyzed in this study are from three of the northernmost volcanic centers in the SVZ; T u p u n g a t o , M a r m o l e j o , and M a i p o volcanoes (Fig. 1). These volcanic centers are characterized by the predominance of basaltic andesite, andesites, and dacites of variable mineralogy, including biotite + hornblende as well as olivine andesites [11,19]. Basalts are either ab-

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Fig. 2. Variations of latitude of H f / L u , R b / C s , and 8VSr/86Sr in samples from recent volcanoes of the southern Andes. Larger symbols (circles = basalts and basaltic andesites, triangles = andesites and dacites, squares = rhyolites) are taken from Table 1. Smaller symbols include both previously published data [8,9] as well as unpublished data. Solid symbols are for samples from the volcanic front, open symbols for samples from east of the volcanic front. The vertical lines at the right of the figure are T = terrigenous materials taken as the range between " u p p e r crust" and "total crust" [26], P = pelagic sediments [18,24,27], and O = oceanic mantle [29].

sent or very scarce in the NSVZ, but voluminous rhyolitic pyroclastic flows were erupted from the Maipo volcanic center in the Pleistocene [20]. A basaltic andesite from the pre-caldera phase of the Maipo volcano (MP-8) and basaltic ande-

site samples from a Holocene(?) post-caldera parasitic cone (Casimiro) associated with Maipo [8,9] have higher K 2 0 , Ba, Rb, La and Sr contents and higher L a / Y b than the basaltic andesites from the Maca and Cay volcanoes. Andesites from

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’ Major elements (except Na,O) were determined by microprobe analysis of fused glasses except for CK3-198 which was determined by wet chemical analysis (Skyline Lab., Denver, Colorada). Fe0 = total iron. Na,O, SC, Hf, Th, U, Cs, La, Ce, Eu, Yb, and Lu were determined by instrumental neutron activation and are considered accurate to + 10%. Zr and Ba were determined by XRF, + 10%. Rb, Sr, Nd, and Sm determined by isotope dilution mass-spectrometry, +0.7%. s7Sr/86Sr, +O.Ol% (20) are normalized to 86Sr/88Sr = 0.1194, and t43Nd/ ‘“Nd , f0.0061 (2a), are normalized to ‘“Nd/ ‘&Nd = 0.7219. No age correction was applied to Sr and Nd isotopic ratios.

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Sample: Volcano: Latitude:

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Major element ( in oxide wt.%), trace element (in ppm), and Sr and Nd isotopic composition of samples southern (SSVZ) portions of the Southern Volcanic Zone and in the Austral Volcanic Zone (AVZ) a

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8~Sr/a6Sr Fig. 3. STSr/S6Sr versus a43Nd/a44Nd for samples from the southern Andes. Solid error ovals are basalts and basaltic andesites, solid error diamonds are andesites and the solid error box is a rhyolite from the Southern Volcanic Zone (SVZ). Open error diamonds are andesites and dacites from the Austral Volcanic Zone (A VZ). The fields indicated for the southern part of SVZ (SSVZ) and the northern part of SVZ (NSVZ) include both our new and previously published data [9]. Fields for the Northern Volcanic Zone (NVZ) and Central Volcanic Zone (CVZ) are from [6].

the Marmolejo (MA-1) and Tupungato (T-l) volcanoes have higher K20, Rb, Ba, Sr, and La contents than the dacite from Cay. In general, incompatible trace element abundances and L a / Y b , R b / C s , and H f / L u (Fig. 2) are higher in the NSVZ samples than for rocks of similar silica content in the SSVZ [7,9,11,19,20]• The samples from the three northernmost volcanic centers in the NSVZ also have higher 87Sr/86 Sr and lower 143Nd/144Nd than samples of similar silica content from the SSVZ (Figs• 2 and 3), although the isotopic compositions of these volcanoes are still distinct from the "Andean" andesites of the CVZ. The Sr isotopic compositions of these three samples from the NSVZ do not correlate with silica content, but 143Nd/144 Nd is higher in the basaltic andesites than the two rocks of intermediate silica content• The rhyolite from the Maipo volcanic center (MP-11) has high K 2 0 , Rb, and Ba, but low St,

Zr, and a significant negative Eu anomaly compared to the associated basaltic andesites and andesites. The rhyolite has R b / C s = 33 and H f / L u = 12, both values similar to continental crust (Fig. 2), and Sr and Nd isotopic compositions higher and lower respectively than any previously reported sample from the southern Andes. With respect to the Sr and Nd isotopic compositions, this rhyolite is similar to volcanic rocks from the CVZ of the Andes (Fig. 3). 2.3. The Austral Volcanic Zone (A VZ) The Austral Volcanic Zone consists of five volcanic centers in the very southernmost Andes, and this study includes samples from Lautaro, Aguilera, Mt. Burney, and Cook Island volcanoes (Fig. 1). All the volcanic centers in the AVZ consist of hornblende + pyroxene andesite and dacites, and neither basalts nor rhyolites have been observed in this region of the Andes [2,10].

259 Each individual volcanic center in the AVZ is apparently uniform in its petrochemical characteristics, but there are marked regional (south to north) changes in the chemistry of the different centers [2]. The andesite from Cook Island, the southernmost volcanic center in the Andes, has high MgO compared to other andesites in the AVZ as well as the rest of the Andean chain, but it has the low TiO 2, HREE, and Zr and high A1203 characteristics of orogenic magmas. Sample CK3-198, which is representative of this volcano, has very low K20, Ba, Rb, and Cs contents and high K / R b , which lies within the range of mid-ocean ridge basalts (MORB). This andesite also has very high Sr and La contents and high L a / Y b compared to both basalts and andesites from the SVZ. K / L a , B a / L a and R b / L a are very low and distinct from typical arc volcanic rocks and are similar to MORB. With respect to these trace element contents and ratios, the andesites from Cook Island are similar to the high-Mg andesites described by Kay [21] from the Aleutians. They are distinct from boninites, which also have high MgO and low TiO2, Zr and H R E E and are characterized by relatively high K 2 0 , Rb, and Ba contents and low La and L a / Y b [22]. Isotopically, the andesite from Cook Island has low 87Sr/86Sr (0.70280) and high 143Nd/144 Nd (0.51314) (Figs. 2 and 3). These values are lower and higher respectively than any values which have been previously reported for orogenic andesites or basalts from the Andes and are within the range of isotopic compositions of MORB. The sample analyzed for this study also has a Pb isotopic composition similar to MORB [231. The sample analyzed from Mt. Burney (MB-9 and MB-21P), Aguilera (A-2), and Lautaro (L-l) have MgO contents that are more similar to typical orogenic andesites and dacites. K20, Rb, Ba and Cs contents are higher in the Mt. Burney andesites than Cook Island andesites and lower than dacites from Aguilera and Lautaro. Samples from Lautaro and Aguilera have higher La and L a / Y b than Mt. Burney andesites. R b / C s for the samples from Mt. Burney are higher than most samples from the SSVZ (Fig. 2), whereas the samples from Aguilera and Lautaro have lower R b / C s . The Sr and Nd isotopic compositions of the samples from Mt. Burney are higher and lower respec-

tively than the sample from Cook Island, and the samples from Aguilera and Lautaro have even higher Sr and lower Nd isotopic compositions than the samples from Mt. Burney (Figs. 2 and 3). 3. Discussion

All the samples analyzed in this study have L a / Y b ratios greater than five, and no equivalents of island arc tholeiites are known from the Andes [7]. Except for the sample from Cook Island, the volcanic rocks from the southern Andes, as well as other convergent plate boundary volcanic arcs, are characterized by high ratios of K, Rb, Ba, and Cs to La, and high ratios of Cs to K and Rb compared to MORB and OIB. All the analyzed samples, with the exception of the rhyolite from the Maipo volcano, have Sr and Nd isotopic compositions that lie within the "mantle array" (Fig. 3). Samples from the northern and central Andes have been shown to have Sr and Nd isotopic compositions that are outside or at the extreme margins of the range of values of the "mantle array" [6]. Except for the sample from Cook Island, most of the volcanic centers in the southern Andes have Sr and Nd isotopic compositions similar to OIB, although the samples from the NSVZ and from the northernmost volcanoes in the AVZ have higher 87Sr/86Sr and lower 143Nd/144 Nd than most such basalts. The samples from the Maca, Cay and Hudson volcanoes are similar to published values for the volcanic centers from as far north as 36 °S with respect to their Sr and Nd isotopic compositions (Figs. 2 and 3), indicating that between 36 and 46 o S, the SSVZ forms a coherent zone in which petrogenetic processes are fairly uniform. Basalts and basaltic andesites generated in this zone of the Andes have a restricted range of 87Sr/87Sr (0.7037-0.7044) and 143Nd/144Nd (0.512900.51275), as well as O isotopic compositions which range from 8180 = +5.2 to +7.2 [6,11]. Trace element ratios for basalts and basaltic andesites from this region of the Andes are also relatively restricted in their range, with L a / Y b = 3-10, B a / L a = 15-28, and H f / L u = 5-10 (Fig. 2). Although the Sr and Nd isotopic compositions of the volcanic centers in the SSVZ are within the "mantle array" and similar to OIB, their Pb isotopic compositions [9] and their trace element

260 ratios are distinct from OIB, e.g. K / L a , B a / L a , and R b / L a are higher and K / C s and R b / C s are lower. Hickey et al. [9] have explained these features in SSVZ basalts by a source region contamination process, in which K, Rb, Ba, Pb, and Cs are released from the subducted oceanic lithosphere into the overlying zone of magma generation. Similar models have been proposed to explain high K / L a , B a / L a , and R b / L a and low K / C s and R b / C s ratios in island arc magmas compared to oceanic island basalts [24,25]. Below intra-oceanic island arcs these components may be subducted in the form of oceanic sediments and seawater-altered oceanic crust, whereas below the western margin of South America terrigenous sediments may be significant as well [11]. However, the low R b / C s and 87Sr/86 Sr of the basalts and basaltic andesites in the SSVZ are better explained by the modification of their mantle source by addition of components derived from pelagic rather than terrigenous sediments. This is because the available data suggest that pelagic sediments have both lower R b / C s and 8VSr/86Sr than terrigenous materials, as would be expected from their high clay content and prolonged interaction with seawater [18,26]. Magmas from the SSVZ of the Andes also have Pb isotopic compositions suggesting mixing of source mantle with pelagic sediments [4,9] and have the low H f / L u characteristics of pelagic as opposed to terrigenous sediments [27]. The basaltic andesites from Cay have higher incompatible-element abundances, L a / Y b , and R b / C s and lower B a / L a than those from Maca, which is similar to transverse chemical variations documented for other convergent plate boundary magmatic arcs [18]. Progressive down-dip dehydration of the subducted slab, beginning below the volcanic front, would leave a residue increasingly more depleted in Ca relative to Rb and Ba relative to La, which could be an additional factor in producing the higher R b / C s and lower B a / L a observed in magmas erupted farther from the trench in the Andean and other arc systems. Compared to the SSVZ, fewer mafic magmas reach the surface in the NSVZ, which is also characterized by greater volumes of andesites and rhyolites [11,20]. These characteristics, with respect to which the NSVZ is more similar to the CVZ than the SSVZ, suggest that mantle derived

magmas undergo more extensive evolution as they move towards the surface in the NSVZ relative to the SSVZ, probably due to a combination of factors including a change in the crustal stress regime as suggested by the northward narrowing of the Central Valley, northward thickening of the crust, and increase in basal elevation of the volcanic centers which increases the energy required to erupt dense mafic magmas. The relatively high 87Sr/86Sr, R b / C s , and H f / L u , and low 143Nd/144Nd of the basaltic andesites, andesites, and rhyolites from the three northernmost volcanoes in the NSVZ compared to the volcanoes in the SSVZ (Figs. 2 and 3) suggest an increased role of continental materials in petrogenesis in the NSVZ. These components may be incorporated in NSVZ magmas by source region contamination or by intra-crustal assimilation, but distinguishing between these two possibilities is difficult because of the lack of basalts in NSVZ volcanoes. In the case of the rhyolites from the Maipo volcanic center, their high 878r//86Sr and low 143Nd/144 Nd compared to the basaltic andesites and andesites from this same volcanic center suggest that intra-crustal assimilation was a significant process in their petrogenesis. In contrast, the available data for basaltic andesites, andesites, and dacites from the volcanic centers between 33 and 34°S show no systematic change in Sr or O [11] isotopic composition with increasing SiO 2. These data suggest that in the NSVZ as in the SSVZ, the sequence basaltic andesite to dacite is produced by fractional crystallization of mafic parental magmas without associated crustal contamination. This is also supported by the observation that Sr contents are higher in intermediate relative to basic rocks in the NSVZ, which is inconsistent with the trend expected from crustal contamination. The lack of evidence for intra-crustal assimilation in the formation of intermediate compositions in the NSVZ, and the fact that the O isotopic composition of both basic and intermediate rocks from the NSVZ are similar to those from the SSVZ [11], suggests that continental components were incorporated in the magmas of the NSVZ by source region contamination. Increased subduction of continental material north of 36 o S, related either to increased rates of tectonic erosion or increased proportions of terrigenous relative to pelagic sediment being

261 subducted, may be due to a change in the dynamics of the interaction of the over-riding continental plate and the subducting oceanic plate as the angle of dip of the latter begins to flatten northwards [11]. Although the southern Andes has often been referred to as a single coherent petrogenetic province, the isotopic and trace element data confirm that the Austral Volcanic Zone is fundamentally distinct from the Southern Volcanic Zone [2]. Unlike the SSVZ, in which basalts are the dominant magma type, basalts have not been encountered in any of the volcanic centers of the AVZ. The most primitive rock observed in this region of the Andes is the high-Mg andesite from Cook Island, the southernmost volcanic center in the Andes. This andesite has features in common with other orogenic magmas such as high A120 3, and low TiO 2 and HREE, but is very distinct compared to orogenic lavas elsewhere in the Andes or other arcs with respect to certain trace element ratios, such as K / R b , K / L a , and B a / L a , and Sr, Nd, and Pb isotopic compositions; with respect to these isotopes the Cook Island andesite is similar to MORB. The high Sr content combined with the low F e O / M g O , K20, Ba, and Rb content suggests that this rock has not undergone significant nearsurface crystal-liquid fractionation. The MORBlike isotopic ratios are consistent with the formation of this rock by partial melting of a MORBtype source mantle or subducted MORB [2,28]. Kay [21] has proposed a similar origin for high-Mg andesites from the Aleutians. The high L a / Y b of the sample implies a low degree of partial melting in the source and suggests garnet as a residual phase. The low Ni and Cr contents of the sample from Cook Island suggest derivation from a midocean ridge basalt rather than MORB-type source mantle. The samples from Aguilera and Lautaro volcanoes have higher SiO2, K 2 0 , Ba, Rb, and Cs, and lower MgO, CaO, Sr, and Ni than the Cook Island andesite and have petrologic features such as extensively zoned phenocrysts, that are consistent with their derivation, at least in part, by fractional crystallization from a more primitive parental magma such as the Cook Island andesite, or possibly a basalt similar to those from the SSVZ. The samples from these two northernmost volcanic centers in the AVZ also have higher

875r//86 Sr and lower 143Nd/144Nd than both the Cook Island andesite or basalts from the SSVZ (Fig. 3), which suggest the participation of continental materials in their origin. The samples from Mt. Burney are intermediate between those of Cook Island and the Aguilera and Lautaro volcanoes with respect to K20, Ba, Rb, Sr, and Cs content, and Sr and N d isotopic compositions. Although the samples of the AVZ, considered as a coherent group, are spatially separated, the positive correlations between the small but regionally regular south to north increase in SiO 2 and the decrease in MgO, CaO and M g O / ( M g O + FeO), associated with more marked increase in K20, Ba, Rb, Cs, and 87Sr/86Sr and decrease in Sr, Ni, R b / C s and 143Nd//144 Nd (Figs. 2 and 3), are qualitatively consistent with a model in which the more evolved volcanic centers to the north form by fractional crystallization, combined with a small but significant amount of assimilation of continental crust, of a parental magma similar to the high-Mg andesite from Cook Island. The Cook Island andesite is considered to be the most appropriate parental magma for the other centers in the AVZ because of the lack of basalts in this volcanic zone of the Andes. The increasing degree of crystal fractionation and crustal contamination from south to north in this region of the southernmost Andes may result from regional variations in the nature of the stress in the continental lithosphere related to plate converge. In the northern part of the AVZ, close to the region where the Chile Rise is being subducted, magma residence time in the crust is apparently relatively long; whereas more to the south, where plate convergence changes gradually to strike-slip motion (Fig. 1), the lack of any continental component in the Cook Island andesite suggests rapid movement through the continental crust [2].

Acknowledgements Alexandra Skewes, Manuel Duran, Eric Leonard, Jorge Munoz, Michael Dobbs, Estanisloa Godoy, and Reynaldo Charrier collaborated in the collection of some of the samples; Manuel Suarez, Leo Dickenson, Ricardo Thiele, Godoy, and Charrier supplied others. The isotopic data were obtained in the Isotopic Branch of the Denver Center of the U.S.G.S. with the permission and

262 assistance of Carl Hedge, Zell P e t e r m a n , Bruce Doe, and R o b e r t Z a r t m a n . T h e final version of t h e m a n u s c r i p t b e n e f i t e d b y c r i t i c a l r e v i e w s o f F. F r e y , W . H i l d r e t h , B. B a r r e i r o , D . G e r l a c h , S. Goldich, and ported

by

B. M a r s h a l l . T h i s w o r k w a s s u p -

NSF

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EAR76-03816,

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20791, a n d E A R 8 3 - 1 3 8 8 4 .

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