Plio–Pleistocene Basalts from the Meseta Del Lago Buenos Aires, Argentina: Evidence for Asthenosphere–Lithosphere Interactions During Slab Window Magmatism

Chemical Geology 193 (2003) 215–235 www.elsevier.com/locate/chemgeo

Plio–Pleistocene basalts from the Meseta del Lago Buenos Aires, Argentina: evidence for asthenosphere–lithosphere interactions during slab window magmatism

Matthew Gorringa,*, Brad Singerb, Jason Gowersa, Suzanne M. Kayc

a Department of Earth and Environmental Studies, Montclair State University, Upper Montclair, NJ 07043, USA b Department of Geology and Geophysics, University of Wisconsin-Madison, Madison, WI 53706, USA c Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY 14853, USA Received 9 July 2001; accepted 9 September 2002

Abstract

Plio–Pleistocene (3.4–0.125 Ma) post-plateau magmatism in the Meseta del Lago Buenos Aires (MLBA; 46.7jS) in southern Patagonia is linked with the formation of asthenospheric slab windows due to ridge collision along the Andean margin f 6 Ma ago. MLBA post-plateau lavas are highly alkaline (43–49% SiO2;5–8%Na2O+K2O), relatively primitive (6–10% MgO) mafic volcanics that have strong OIB-like geochemical signatures. Their relatively enriched Sr–Nd isotope ratios (87Sr/86Sr = 0.7041–0.7049; 143Nd/144Nd = 0.51264–0.51279), low 206Pb/204Pb (18.13–18.45), steep REE patterns (La/ Yb = 11–54), and low LILE/LREE and LILE/HFSE ratios (Ba/La < 15, La/Ta < 15, Ba/Ta < 180; Sr/La = 15–22; Th/La < 0.13; Ce/Pb>15) are distinctive from most other Neogene Patagonian slab window lavas. These data are interpreted to indicate contamination of OIB-like asthenosphere-derived slab window magmas with an EM1-type component derived from the Patagonian continental lithospheric mantle (CLM). The EM1-type signature in Patagonian slab window lavas are geographically associated with the Deseado Massif and indicate important regional differences in lithospheric mantle chemistry beneath southern Patagonia. We propose that hot, upwelling subslab asthenosphere in slab window tectonic settings can cause significant thermo-mechanical erosion and thinning of the continental lithospheric mantle and, thus, may be an important process in slab window magma petrogenesis. D 2003 Elsevier Science B.V. All rights reserved.

Keywords: Slab windows; Basalts; Patagonia; Asthenosphere; Lithosphere

1. Introduction continental subduction zones and offers a rare opportunity to investigate the chemistry of the astheno- Mafic slab window magmatism is produced by the spheric and lithospheric mantle beneath continents. collision and interaction of mid-ocean ridges with Slab window mafic magmas are thought to be the product of decompression melting as asthenospheric mantle upwells through the gap that opens between * Corresponding author. Tel.: +1-973-655-5409; fax: +1-973- the subducting oceanic plates (e.g. Thorkelson, 1996). 655-4072. Thus, magma sources are thought to dominantly E-mail address: [email protected] (M. Gorring). reflect the chemistry of the asthenospheric mantle

0009-2541/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. PII: S0009-2541(02)00249-8 216 M. Gorring et al. / Chemical Geology 193 (2003) 215–235 beneath the subducting plate (e.g., Thorkelson, 1996). Gorring et al., 1997; Johnston and Thorkelson, 1997; The strong OIB- and MORB-type chemistry of sev- D’Orazio et al., 2000). However, as with all conti- eral examples of Neogene mafic slab window lavas, nental basalts, the continental lithospheric mantle from British Columbia to the Antarctic Peninsula, (CLM) is a possible reservoir for enriched, OIB-type generally support the idea that mantle sources are mantle components (e.g., EM1 and EM2; Hofmann, dominated by the upwelling subslab asthenosphere 1997), and could potentially play an important role as (Johnson and O’Neil, 1984; Thorkelson and Taylor, a contaminant for asthenosphere-derived slab window 1989; Storey et al., 1989; Ramos and Kay, 1992; Hole magmas. Thus, the interpretation of mantle sources et al., 1995; Cole and Basu, 1995; Luhr et al., 1995; for mafic slab window lavas with OIB signatures and

Fig. 1. Tectonic setting of southern South America showing the distribution of Neogene Patagonian plateau lavas (black) from Panza and Nullo (1994) relative to fracture zones and Chile Ridge segments (Cande and Leslie, 1986; Lothian, 1995). Numbers associated with plateau lava locations are the published range of 87Sr/86Sr and 206Pb/204Pb ratios, respectively (Hawkesworth et al., 1979; Baker et al., 1981; Stern et al., 1990; Gorring, 1997; D’Orazio, 2000; Gorring and Singer, 2000; Gorring and Kay, 2001). Ridge collision times are shown in bold numbers (Cande and Leslie, 1986). Austral Volcanic Zone (AVZ) and southern Southern Volcanic Zone (SSVZ) volcanic centers (black triangles) are from Stern et al. (1990). Upper Proterozoic–lower Cambrian basement of the Deseado Massif is also shown (line stipple; Pankhurst et al., 1994). Outline of the Magallanes Basin (heavy line with small ticks) is from Ramos (1989). MFZ = Magallanes Fracture Zone (Klepeis, 1994). M. Gorring et al. / Chemical Geology 193 (2003) 215–235 217 the extent of asthenosphere–lithosphere interactions 2. Regional tectonic and geologic setting is still controversial (e.g., Johnston and Thorkelson, 1997; Gorring and Kay, 2001). The Late Cenozoic tectonic history of southern In this paper, we present new geochemical and Sr– South America is dominated by near orthogonal Nd–Pb isotope data on a suite of Plio–Pleistocene subduction of the Nazca Plate beneath the South slab window-related lavas from the Meseta del Lago American Plate. In southern Patagonia, convergence Buenos Aires (MLBA) in southern Patagonia that has been punctuated by collision of segments of the provide an opportunity to further examine this issue. Chile Ridge with the Chile Trench, preceding sub- The MLBA is one of the largest (f 6000 km2) and duction of the Antarctic Plate (Fig. 1). Plate recon- northernmost exposures of slab window-related, Late structions indicate initial ridge collision near the Miocene to Pleistocene basaltic plateaus that occur southern tip of South America at f 14–15 Ma between 46.5jS and 52jS in southern Patagonian (Cande and Leslie, 1986), forming a triple junction back-arc (Fig. 1). The MLBA is also part of the larger between the South American, Nazca, and Antarctic province of Plio–Pleistocene basaltic magmatism that Plates. The Chile Triple Junction (CTJ) has since occurs in the Andean back-arc as far north as 34jS migrated northwards to its present position (f 46j) (e.g., Stern et al., 1990). The MLBA is located in the by a series of Neogene ridge collisions (Cande and northwestern corner of the Santa Cruz province, Leslie, 1986; Forsythe et al., 1986) (Fig. 1). These Argentina, about 300 km southeast of the Chile Triple ridge–trench collisions are thought to be responsible Junction (CTJ) and about 150 km east of the modern for several unique geodynamic, structural, and mag- volcanic arc gap between the Southern (SVZ) and matic features of southern Patagonia, such as the Austral Volcanic Zones (AVZ) (Fig. 1). Based on modern volcanic arc gap between the SVZ and the available geochemical and radiometric age informa- AVZ (Stern et al., 1984), the eruption of adakitic tion, Ramos and Kay (1992) and Gorring et al. (1997) magmas in the back-arc (Kay et al., 1993) and in interpreted MLBA magmas to have been generated in the AVZ (Stern and Kilian, 1996), the Neogene uplift response to the opening of slab windows associated of this sector of the Andes, and development of the with collision of a segment of the Chile Ridge with Patagonian fold-thrust belt (Ramos, 1989); and the the Chile Trench at f 6Ma(Fig. 1). Previous geo- rapid (15–20 mm) of recent (< 500 years) isostatic chemical work on MLBA post-plateau lavas have rebound of the Campo de Hielo due to anomalously shown that these rocks are highly alkaline, nephe- low viscosity mantle (< 1 1020 Pa s) and thin litho- line-normative basanites and alkali basalts, and have spheric thickness (< 100 km; Ivins and James, 1999). some of the highest 87Sr/86Sr and lowest 143Nd/144Nd Another major geologic feature of southern Pata- ratios of all Neogene mafic rocks south of the CTJ gonia is the extensive Late Miocene to Pleistocene (Hawkesworth et al., 1979; Baker et al., 1981). These plateau lavas that were erupted over large areas of the isotope signatures are more enriched than other OIB- back-arc (Fig. 1). Based on available K–Ar and like Neogene slab window lavas from southern Pata- 40Ar/39Ar radiometric age dating, recent investiga- gonia (e.g., D’Orazio et al., 2000; Gorring and Kay, tions have shown a temporal and spatial link between 2001) and from the Antarctic Peninsula (e.g., Hole et ridge–trench collision and slab window formation al., 1995) that are interpreted to be primarily astheno- (e.g., Ramos and Kay, 1992; Gorring et al., 1997; sphere-derived (Fig. 1). Therefore, given the slab D’Orazio et al., 2000) and the eruption of these window tectonic setting and the preliminary evidence plateau lavas, of which the MLBA is one of the major for unique trace element and isotopic compositions, a examples. These slab window lavas are interbedded detailed geochemical investigation of the MLBA post- with Late Miocene to Pleistocene fluvial gravels and plateau lavas has primary importance for constraining glacial sediments and were erupted over a sequence of petrogenetic models for Late Cenozoic magmatism in Jurassic silicic volcanics and Cretaceous to Miocene southern Patagonia and for understanding the extent mixed marine and continental sediments of variable of asthenosphere–lithosphere interactions in the pro- thickness (1–4 km). Basement beneath the Magal- duction of basaltic magmas along active continental lanes Basin (Fig. 1) in the western and southern margins. Patagonian back-arc consists of a highly deformed, 218 M. Gorring et al. / Chemical Geology 193 (2003) 215–235 low-grade metasedimentary accretionary complex include cinder, spatter, and scoria cones, maars, and which has been tentatively assigned a Devonian– associated flows and pyroclastic debris. Several lava Carboniferous age by Riccardi and Rolleri (1980) flows from the larger centers have spilled over the based on fossil evidence and has a probable meta- eastern edge of the MLBA and flowed up to 30 km morphic age of 224 F 38 based on a Rb–Sr whole down the Rio Deseado and Rio Pinturas drainage rock date obtained by Herve´ et al. (1981). Further systems. Most of the monogenetic cones rise f 100 east, basement consists of poorly exposed, upper –150 m above the surface of the plateau; the largest Proterozoic–lower Cambrian low-grade metamorphic centers are up to 400 m above the surface. They have rocks of the Deseado Massif (Fig. 1) with a K–Ar age well-preserved morphology (e.g. scoriaceous crater of 549 F 20 Ma for amphibolite obtained by Pezzuchi rims, flows with pressure ridges, etc.) and, thus, appear (1978). Additional details on the regional geology can geologically very young. The average thickness of the be found in Ramos (1989) and Ramos and Kay post-plateau volcanic pile is not well constrained due to (1992). limited continuous vertical exposure. Baker et al. (1981) estimated maximum thicknesses of cones and surface flows to be as much 300 m locally. Based on 3. Late Miocene to Pleistocene magmatism in the field observations, we estimate that the average thick- MLBA ness is probably closer to 100 m, thus totally erupted volumes are on the order of 600 km3. Existing age Available K–Ar and 40Ar/39Ar ages (Sinito, 1980; constraints on MLBA post-plateau lavas consist of a Baker et al., 1981; Mercer and Sutter, 1982; Thon- total of 12 K–Ar (whole-rock) and 40Ar/39Ar (step- That et al., 1999) suggest two periods of magmatism: heating on basalt matrix and plagioclase separates) (1) a voluminous, late Miocene to early Pliocene radiometric ages that range from 3.4 to 0.125 Ma, but tholeiitic main-plateau sequence, and (2) a younger, most are V 1.8 Ma (Baker et al., 1981; Thon-That et less voluminous, highly alkaline, Plio–Pleistocene al., 1999). These ages indicate eruption f 3–5.9 Ma post-plateau sequence. MLBA magmatism begins after ridge collision, and therefore, according to geo- with the eruption of the older main-plateau unit. These dynamic models of Gorring et al. (1997), a fully are voluminous, tabular lava flows with exposed developed slab window would have existed beneath thicknesses typically of 10–30 m consisting of several the MLBA by this time. flow units 2–5 m thick each. On the extreme western side of the MLBA, exposed sections of the main- plateau unit are up to 300 m thick. This sequence of 4. Analytical methods lavas flows creates the overall planar, plateau-like geomorphology of the MLBA. Existing age con- All samples were sawed into slabs, reduced to straints on MLBA main-plateau lavas consist of a 0.25–0.5 cm in a hardened steel mortar, and pulver- total of four K–Ar and 40Ar/39Ar radiometric ages ized in an alumina ceramic shatterbox. Major elements that range from 10 to 4.5 Ma (Sinito, 1980; Baker et were determined by electron microprobe analysis on al., 1981; Mercer and Sutter, 1982; Thon-That et al., fused glasses using a JEOL JXA-8600 electron 1999) with the oldest lavas exposed on the southeast microprobe at both Cornell University and Rutgers edge of the plateau. Preliminary data indicate signifi- University. Techniques and standards used for microp- cant geochemical differences between this unit and the robe analyses of major elements are given in Kay et al. younger, post-plateau lavas (Gorring and Singer, (1987). Precision and accuracy (2r) are F 1–5% for 2000); however, this will be more fully explored in elements at >1 wt.% and F 10–20% at < 1 wt.% a future manuscript. This paper focuses only on the abundance levels based on replicate analysis of basal- geochemistry and geodynamic implications of the tic glass standards. Trace elements were determined by younger, MLBA post-plateau lava sequence. INAA at Cornell University. Techniques and standards The MLBA post-plateau unit erupted from more are given in Kay et al. (1987). INAA precision and than 150 individual monogenetic volcanic centers that accuracy based on replicate analysis of an internal formed above the basal main-plateau unit. These basalt standard are 2–5% (2r) for most elements and M. Gorring et al. / Chemical Geology 193 (2003) 215–235 219

F 10% for U, Sr, Nd, and Ni. Pb concentrations were 6. Geochemical results determined by ICP-MS at Cornell University using a single collector, Finnigan MAT Element2 instrument 6.1. Major and transition metals with an external and accuracy of f 5% based on replicate analyses of USGS basalts standards BIR-1 The major element and transition metal concen- and BHVO-2. trations of MLBA lavas analyzed for this study are Sr, Nd, and Pb isotopes were analyzed at Cornell given in Table 1. MLBA post-plateau lavas are a University on a VG Sector 54 thermal ionization strongly alkaline series of volcanics (most have 0– mass spectrometer. Chemistry and analytical techni- 20% normative nepheline; 44–49 wt.% SiO2;5–8 ques are summarized in White and Duncan (1996). wt.% Na2O+K2O), with almost all samples plotting Pb isotope ratios were corrected for mass fractiona- in the alkali basalt, trachybasalt, and basanite fields tion assuming NBS981 Pb values of 206Pb/204Pb = on a total alkali–silica classification diagram (Fig. 2; 16.937, 207Pb/ 204Pb = 15.493, and 208Pb/204Pb = Table 1). They are considerably more alkaline (higher 36.705. Average measured values for NBS981 were in both Na2O and K2O) than other equivalent Neo- 206Pb/204Pb = 16.908 F 3(2r), 207Pb/204Pb = gene southern Patagonian post-plateau slab window 15.455 F 3, and 208Pb/ 204Pb = 36.595 F 10. Sr and lavas from the central Santa Cruz province (47–50jS; Nd isotope ratios were corrected for mass fractiona- Gorring and Kay, 2001) and from the Pali Aike tion assuming 86Sr/88Sr = 0.1194 and 146Nd/144Nd = Volcanic Field (PAVF, 52jS, D’Orazio et al., 2000) 0.7219. Average measured value for NBS987 Sr (Fig. 2). MLBA post-plateau lavas have relatively standard was 87Sr/86Sr = 0.710235 F 34 (2r). Aver- high Mg#s (54–69), MgO (6–10 wt.%), and Cr age measured value for the La Jolla Nd standard was (100–400 ppm) and Ni (75–225 ppm) (Table 1) that 143Nd/144Nd = 0.511864 F 14 (2r). support the idea that these lavas have suffered only moderate amounts of crystal fractionation from primitive, mantle-derived basalts. Rapidly decreasing Ni 5. Petrography and Cr with decreasing MgO suggests that olivine and included Cr-spinel played the dominant role during A total of 35 samples of MLBA post-plateau crystal fractionation. Two samples (004 and 010) volcanics were sampled during the austral summers have anomalously high silica (f 50.5%), low MgO of 1989–1990 (Kay), 1996–1997 (Singer), and (f 4 wt.%), and low Cr (f 40 ppm) and Ni (15–35 1998–1999 (Gorring and Singer). The samples are ppm) and, thus, have suffered more extensive differ- extremely fresh with only a few showing incipient entiation than the majority of MLBA post-plateau alteration (iddingsite, hematite) of olivine phenocrysts lavas. along rims and interior cracks upon microscopic inspection. MLBA post-plateau lavas are porphyritic, 6.2. Incompatible trace elements with 5–20% phenocrysts overwhelming dominated by euhedral olivine, but clinopyroxene, plagioclase, Key trace element characteristics of MLBA post- and Fe-oxides are also common. One sample (010, plateau lavas are shown in Figs. 3–6, and the full Table 1), classified as mugearite, contained abundant analytical results are listed in Table 1. All samples are (30–50%), large (3–5 cm) anorthoclase and horn- have LREE-enriched patterns (LaN = 75–250) on blende megacrysts. Groundmass textures are micro- chondrite-normalized REE plots (Fig. 3). HREE con- crystalline and vary between vitrophyric, intersertal, centrations are roughly the same (YbN f 7–8) for all and intergranular with plagioclase, clinopyroxene, samples, and thus, variation in LREE concentrations olivine, Fe–Ti oxides, and glass as the dominant produces a large range of La/Yb ratio (11–54). The phases. Glass content varies from < 10% to as much buffering of HREEs and the increasing La/Yb fractio- as 80%. Flow alignment of groundmass plagioclase nation with increasing incompatible trace concentra- imparts a weak trachytic texture in some samples. tions strongly suggest a systematic variation in the Small (1–2 cm) spinel lherzolite and harzburgite degree of partial melting of a garnet-bearing source xenoliths are also common. during the petrogenesis of MLBA post-plateau mag- 220 M. Gorring et al. / Chemical Geology 193 (2003) 215–235

Table 1 Major and trace element analyses and Sr–Nd–Pb isotope data of post-plateau lavas from the Meseta del Lago Buenos Airesa Sample AP-01ttb AP-01g AP-02s CV-01ttb CV-01s CV-02s CV-02g AT-01ttb AT-01s Latitude (S) 46j43.9V 46j43.9V 46j40.0V 46j40.0V 46j35.4V 46j35.4V 46j52.0V 46j52.0V Longitude (W) 70j50.0V 70j50.0V 70j47.0V 70j47.0V 70j40.5V 70j40.5V 70j44.0V 70j44.0V Classification alk bas alk bas alk bas basan basan basan basan basan basan

SiO2 (wt.%) 46.50 47.05 46.73 44.70 45.10 44.03 44.19 46.00 46.29 TiO2 1.97 2.02 2.15 2.72 2.72 2.77 2.85 2.40 2.42 Al2O3 15.50 15.44 15.85 15.60 16.18 14.39 14.51 15.30 15.46 FeO 10.15 10.44 9.95 10.70 10.09 10.16 10.45 9.34 9.19 MnO 0.17 0.24 0.21 0.17 0.14 0.21 0.23 0.17 0.19 MgO 10.25 9.61 9.02 8.51 7.65 9.58 9.57 8.58 8.53 CaO 9.18 9.65 9.71 9.48 9.80 10.92 10.79 7.69 8.09

Na2O 3.02 3.26 3.46 3.90 3.93 3.74 3.91 4.81 4.76 K2O 1.48 1.54 1.60 2.34 2.42 2.32 2.33 2.88 2.74 P2O5 0.49 0.53 0.54 0.98 1.01 1.11 1.12 1.33 1.25 Total 98.7 99.8 99.2 99.1 99.0 99.2 99.9 98.5 98.9 Mg# 67.9 65.9 65.5 62.5 61.4 66.4 65.7 65.8 66.0 ne, hy 4.0 5.4 6.8 13.0 12.8 15.4 16.1 14.9 14.5 La (ppm) 24 32 30 50 57 66 71 66 79 Ce 64 57 105 127 132 137 Nd 28 30 46 56 57 57 Sm 6.1 5.8 8.6 10.1 10.6 10.2 Eu 1.80 1.70 2.40 2.96 3.05 2.79 Tb 0.80 0.80 0.92 1.14 1.18 1.07 Yb 1.84 1.86 1.77 1.99 2.02 1.80 Lu 0.25 0.26 0.24 0.27 0.28 0.24 Sr 698 694 579 1094 988 1097 1121 1301 1075 Ba 427 399 419 751 729 739 715 731 662 Cs 0.58 0.81 0.48 0.44 0.58 0.57 Rb 32 40 52 U 0.80 0.85 1.48 1.90 1.91 1.62 Th 3.9 3.6 6.6 7.8 7.9 9.0 Pb 7 3.9 4.9 5.4 7.5 Y25 26 30 Zr 185 283 406 Hf 4.2 4.2 5.1 6.7 6.6 8.4 Nb 42 77 98 Ta 2.51 2.35 4.36 4.76 4.88 5.58 Sc 25 25 17 29 27 15 Cr 320 325 281 181 136 322 313 230 218 Ni 211 165 139 153 134 174 160 198 171 Co 79 49 46 82 42 47 47 58 40 87Sr/86Sr 0.704597 0.704393 0.704448 143Nd/144Nd 0.512708 0.512649 0.512667 206Pb/204Pb 18.233 18.127 207Pb/204Pb 15.629 15.592 208Pb/204Pb 38.582 38.501 a 2+ alk bas = alkali basalt; basan = basanite; hawa = hawaiite; mugea = mugearite. Mg# = Mg/(Mg + Fe ) 100 assuming Fe2O3/FeO (wt.%) = 0.15. ne, hy = wt.% normative nepheline or wt.% normative hypersthene (minus sign). b Analyses published in Thon-That et al. (1999). c Rb, Pb, Y, Zr, Nb by ICP-MS at Cornell University (Gorring and Kay, 2001). M. Gorring et al. / Chemical Geology 193 (2003) 215–235 221

AT-02s AT-03s LC-27 LC-28 001 002 003d 004 005 46j56.0V 46j52.0V 46j49.8V 46j49.4V 46j49.0V 46j48.3V 46j47.8V 70j43.0V 70j44.0V 71j03.1V 71j09.7V 71j12.0V 71j08.9V 71j06.3V basan hawa basan basan hawa hawa hawa hawa alk bas 45.72 45.68 45.57 46.31 51.42 48.49 48.49 50.56 49.28 2.28 2.37 2.33 2.38 1.42 2.33 2.35 1.91 2.00 14.73 14.31 15.06 15.19 16.38 17.23 16.57 18.40 17.32 9.38 9.37 9.87 9.71 8.33 9.42 9.31 8.69 9.57 0.16 0.17 0.22 0.17 0.13 0.14 0.15 0.14 0.14 8.39 10.18 8.37 8.49 8.23 7.28 5.94 3.97 5.99 8.31 8.99 8.12 7.83 8.19 7.79 9.43 8.44 9.54 4.60 4.09 5.40 4.84 3.64 4.12 4.02 4.18 3.46 2.94 2.32 2.68 2.71 1.99 2.47 1.96 2.37 1.47 1.54 1.24 1.58 1.36 0.36 0.76 0.52 0.60 0.41 98.1 98.7 99.2 99.0 100.1 100.0 98.7 99.3 99.2 65.2 69.5 64.0 64.7 67.4 61.8 57.2 49.0 56.7 14.6 12.1 18.3 14.2 0.6 6.8 6.5 3.4 0.9 94 72 99 86 32 40 32 39 27 165 132 178 159 65 80 67 78 57 70 58 75 64 29 39 35 38 29 11.7 9.8 12.1 11.2 6.2 7.4 7.4 7.3 6.1 3.22 2.76 3.41 3.34 1.71 2.12 2.08 2.07 1.82 1.17 1.04 1.32 1.20 0.84 0.94 0.93 0.93 0.88 1.83 1.71 1.85 1.89 2.20 2.17 1.98 2.14 2.18 0.24 0.23 0.27 0.29 0.32 0.31 0.28 0.32 0.32 1213 1030 1479 1343 715 817 780 778 595 711 626 738 653 338 549 407 584 302 0.85 0.47 0.76 0.52 1.81 0.58 1.42 1.03 0.66

2.63 2.26 2.73 1.91 2.27 1.18 1.56 1.51 0.98 11.4 9.0 11.9 10.2 8.7 4.1 5.4 5.8 3.4 8.4 7.0 6.1 4.5

9.2 7.3 9.3 8.8 4.1 5.5 5.0 4.9 4.3

6.32 5.06 6.75 5.99 1.55 3.06 2.21 2.34 1.91 14 18 14 15 23 18 25 17 27 220 386 243 240 409 175 119 38 176 161 226 175 184 176 116 48 15 76 39 44 41 41 40 38 36 27 37 0.704335 0.704875 0.512733 0.512636 18.471 18.453 15.557 15.590 38.429 38.515 (continued on next page) 222 M. Gorring et al. / Chemical Geology 193 (2003) 215–235

Table 1 (continued) Sample 006 007 008 009 010 011 012 013 Latitude (S) 46j49.5V 46j49.7V 46j56.0V 46j58.2V 46j58.7V 46j55.6V 46j58.6V 47j00.4V Longitude (W) 71j06.4V 71j05.0V 71j20.1V 71j18.1V 71j17.2V 71j11.3V 71j05.9V 71j05.6V Classification alk bas alk bas basan hawa mugea basan hawa hawa

SiO2 (wt.%) 47.34 49.66 43.79 48.16 50.68 45.65 49.23 47.82 TiO2 2.52 1.98 2.76 2.25 2.21 2.34 2.27 2.18 Al2O3 15.74 16.69 14.87 16.37 18.07 16.63 17.59 16.67 FeO 9.95 9.91 10.79 9.69 9.93 9.86 8.80 9.50 MnO 0.20 0.15 0.15 0.08 0.12 0.13 0.12 0.16 MgO 8.15 6.96 7.86 7.58 3.94 7.38 5.13 7.66 CaO 10.06 8.96 8.79 6.94 5.16 8.43 8.00 7.56

Na2O 3.89 3.59 4.98 4.58 5.69 4.68 4.55 4.19 K2O 1.66 1.32 2.57 2.28 2.81 2.35 2.16 1.98 P2O5 0.56 0.51 1.52 0.89 0.84 1.01 0.81 0.70 Total 100.1 99.7 98.1 98.8 99.4 98.5 98.6 98.4 Mg# 63.2 59.5 60.4 62.1 45.4 61.1 55.0 62.8 ne, hy 8.6 0.0 18.6 7.6 8.0 14.0 5.8 6.1 La (ppm) 32 19 88 44 37 63 42 38 Ce 64 41 173 87 78 126 83 79 Nd 30 24 62 38 39 48 38 32 Sm 6.8 5.7 12.2 7.6 8.3 9.4 7.7 7.3 Eu 2.04 1.57 3.53 2.27 2.39 2.71 2.18 2.14 Tb 0.93 0.82 1.34 0.98 0.99 1.15 1.05 1.00 Yb 1.88 1.85 2.10 1.93 1.55 2.04 2.32 2.05 Lu 0.27 0.26 0.28 0.28 0.21 0.29 0.33 0.29 Sr 759 589 1335 878 794 1084 803 794 Ba 390 259 697 476 489 543 476 383 Cs 0.40 0.45 0.58 0.57 0.36 0.58 0.81 0.68 Rb U 0.86 0.59 2.24 1.36 1.30 1.51 1.16 1.15 Th 3.5 2.2 10.1 4.8 4.2 7.0 5.7 4.5 Pb 8.0 3.3 Y Zr Hf 4.8 3.1 8.6 6.2 6.6 7.6 5.7 5.4 Nb Ta 2.90 0.99 6.08 3.77 3.73 4.46 3.18 3.21 Sc 26 20 19 17 8 21 20 20 Cr 369 192 156 225 40 212 123 176 Ni 130 87 102 162 35 119 65 133 Co 44 40 42 41 30 41 32 41 87Sr/86Sr 0.704433 0.704094 143Nd/144Nd 0.512676 0.512795 206Pb/204Pb 18.208 18.303 207Pb/204Pb 15.607 15.574 208Pb/204Pb 38.522 38.408 mas. The classic, nonmodal batch melting equation of strongly suggests that MLBA lavas were generated Shaw (1970) was used to model REE patterns of well within the garnet stability field at depths in MLBA post-plateau lavas. Good agreement between excess of 65–70 km. model melts and actual MLBA REE patterns are The use of an enriched mantle source for the REE obtained by 1–5% partial melting of LREE-enriched, modeling is justified by the strong OIB-like trace garnet-bearing lherzolite mantle source (Fig. 3). This element signatures of MLBA post-plateau lavas that M. Gorring et al. / Chemical Geology 193 (2003) 215–235 223

014 015 016 017 018 LC-25c LC-26 025 026 47j02.9V 47j02.9V 47j04.7V 47j06.0V 47j06.9V 47j07.7V 47j04.0V 46j49.1V 46j49.1V 71j03.3V 71j03.3V 71j01.7V 71j00.5V 70j58.6V 70j52.0V 70j48.0V 70j54.9V 70j54.9V hawa hawa alk bas hawa hawa hawa hawa hawa alk bas 48.08 49.12 47.84 45.45 47.83 47.20 45.65 48.05 48.87 2.22 2.15 2.48 2.63 2.52 2.31 2.67 2.60 2.06 16.68 16.95 16.24 16.31 17.09 16.10 16.01 16.87 16.48 9.59 9.35 10.38 10.33 9.56 10.05 10.13 9.83 10.19 0.15 0.13 0.17 0.17 0.21 0.16 0.19 0.13 0.11 7.29 7.10 7.14 7.18 5.86 8.01 7.19 5.53 7.36 8.83 7.49 10.04 9.80 9.76 9.42 10.86 9.02 7.98 4.52 4.71 3.64 3.90 3.84 3.87 4.63 4.16 3.27 2.05 2.11 1.19 2.08 2.29 1.97 1.24 1.82 1.60 0.74 0.72 0.44 0.79 0.76 0.66 0.89 0.80 0.57 100.2 99.9 99.5 98.6 99.7 99.8 99.5 98.8 98.5 61.4 61.4 59.0 59.3 56.2 62.6 59.8 54.1 60.2 10.0 7.7 4.2 11.2 7.5 8.6 13.5 5.5 4.9 44 32 19 47 45 43 50 59 30 86 66 40 92 88 82 92 119 57 38 33 20 41 36 37 45 53 33 7.8 7.2 5.2 8.3 7.7 7.4 8.2 9.4 7.4 2.22 2.10 1.67 2.35 2.25 2.12 2.38 2.60 2.14 0.99 0.94 0.81 1.04 1.07 0.98 0.96 1.10 1.00 2.18 2.10 1.77 1.98 2.00 2.06 1.97 2.05 2.17 0.31 0.29 0.24 0.29 0.30 0.32 0.27 0.30 0.31 818 781 570 868 795 828 966 987 798 487 406 208 500 525 558 581 456 412 0.68 0.51 0.18 0.39 0.65 0.61 0.56 0.70 0.38 33 1.36 0.94 0.46 1.29 1.22 1.30 1.42 1.39 1.04 5.1 3.2 1.6 5.6 5.6 5.4 5.7 5.2 3.6 4.6 4.4 5.0 24 209 5.9 5.2 3.5 5.8 5.5 5.4 5.3 7.4 4.3 50 3.38 2.43 1.51 4.07 3.75 3.65 4.17 3.56 1.62 23 17 24 27 25 24 27 22 22 182 151 222 183 149 236 203 122 241 99 122 73 91 67 134 87 54 169 38 38 44 42 37 44 42 36 44 0.704681 0.704743 0.704106 0.512663 0.512676 0.512623 18.221 18.267 15.604 15.538 38.638 38.384 are well displayed on primordial mantle-normalized lavas is also indicated by their low LILE/LREE and patterns (Fig. 4). All MLBA lavas have all the classic LREE/HFSE ratios shown in Fig. 5. This plot dem- enrichments of LILE, LREE, and HFSE that charac- onstrates the similarity of Ba/La, La/Ta, and Ba/Ta terize basalts from intraplate continental and oceanic ratios of MLBA lavas to a global average of oceanic settings (e.g. Sun and McDonough, 1989). The strong alkali basalt (‘‘OIB’’, Sun and McDonough, 1989) intraplate, OIB-like chemical signature of MLBA and their low values relative to SSVZ arc lavas (Ba/ 224 M. Gorring et al. / Chemical Geology 193 (2003) 215–235

Fig. 4. Primitive mantle-normalized trace element diagrams for representative MLBA post-plateau lavas (fine solid lines) compared to typical OIB (dashed line), N-MORB (heavy solid line), southern Fig. 2. Alkali–silica classification diagram (Le Maitre, 1989) for Southern Volcanic Zone (SSVZ) mafic arc lavas (gray field; Hickey Plio–Pleistocene MLBA post-plateau lavas (filled diamonds: this et al., 1986; Lo´pez-Escobar et al., 1993). OIB, N-MORB, and study; crosses: Baker et al., 1981). Heavy dark line is boundary normalization factors from Sun and McDonough (1989). Nb* is between alkaline and subalkaline rocks of Irvine and Baragar (1971). equal to 17 times the Ta concentration (e.g. Sun and McDonough, Also plotted are data from slab window lavas from Pali Aike 1989) and is used for plotting purposes only. (diagonal rule; Stern et al., 1990; D’Orazio et al., 2000) and Neogene southern Patagonian slab window lavas (Gorring and Kay, 2001). and Th/La (0.08–0.13) that fall near or well within the limits of OIB (Fig. 6). Ta>700, La/Ta>35; off-scale in Fig. 5). The strong Despite the overall strong intraplate, OIB-like trace OIB-like signature of most MLBA post-plateau lavas element signatures of most MLBA lavas, some sam- is also supported by Ce/Pb (18–24), Nb*/U (35–60),

Fig. 3. REE plots for representative MLBA post-plateau lavas (fine Fig. 5. Plot of Ba/Ta versus La/Ta for MLBA post-plateau lavas solid lines) compared to calculated partial melts (gray field) using a (filled diamonds) showing strong OIB signature and low LILE/ simple nonmodal batch melting model. Source composition (shown HFSE and LREE/HFSE ratios compared to SSVZ mafic arc lavas for reference) and partition coefficients used to calculate partial (not shown, plot off-scale toward upper right corner). Labeled melts are given in Gorring and Kay (2001). A constant bulk source samples have anomalously high Ba/Ta and La/Ta ratios (see text) mode of 0.55 olivine, 0.22 orthopyroxene, 0.15 clinopyroxene, 0.06 and are interpreted to reflect minor subduction-related components garnet, and 0.02 Cr-spinel was used. Melt reaction and reaction in these samples. Also plotted are data from slab window lavas from coefficients used were as follows: 0.25 opx + 0.61 cpx + 0.20 Pali Aike (diagonal rule; Stern et al., 1990; D’Orazio et al., 2000) gar + 0.04 sp ! 0.10 ol + liq. Normalization factors are based on and the Antarctic Peninsula (fine stipple; Hole, 1988, 1990). Leedy chondrite (Masuda et al., 1973) and are La (0.378), Ce Approximate fields for EM1-, EM2-, and HIMU-type OIB are from (0.978), Nd (0.716), Sm (0.23), Eu (0.0866), Tb (0.0589), Yb Weaver (1991). Unfilled star is the average OIB composition of Sun (0.249), Lu (0.0387). and McDonough (1989). M. Gorring et al. / Chemical Geology 193 (2003) 215–235 225

can be clearly seen as positive Pb anomalies on primitive mantle-normalized multi-element diagrams (Fig. 4). These geochemical signatures reflect the influence of minor subduction-related (slab-derived fluids, sediments) and/or continental crustal components in some MLBA lavas.

6.3. Sr, Nd, and Pb isotopic ratios

The Sr, Nd, and Pb isotopic composition of 10 MLBA post-plateau volcanics generally overlap the isotope compositions of Neogene Patagonian slab window lavas to south (Gorring and Kay, 2001), and

Fig. 6. Plots of (a) Ce/Pb and (b) Th/La versus Nb*/U for MLBA post-plateau lavas (filled diamonds) showing that most samples plot near or within fields for oceanic basalts (OIB + MORB). Labeled samples are referred to in text and have ratios indicating the influence Fig. 7. Plot of 143Nd/144Nd versus 87Sr/86Sr for Patagonian slab of arc and/or crustal AFC components. Fields for OIB + MORB, window lavas (large filled diamonds: this study; small open altered oceanic crust (diagonal rule), arcs (white box), average diamonds: Hawkesworth et al., 1979) showing their relatively continental crust (CC), and marine sediment (horizontal rule) are enriched OIB-like signatures that contrast with the more depleted, after Klein and Karsten (1995). Curve 1 represents bulk mixing HIMU-like signature of slab window lavas from Pali Aike and the between a primary MLBA post-plateau lava (1% partial melt of 50% Antarctic Peninsula (see Fig. 5 for data sources). Also plotted are MORB–50% OIB source) and average S-type Patagonian crust. fields for other Neogene southern Patagonian slab window lavas Curve 2 is bulk addition (source region contamination) of average (Gorring and Kay, 2001), Chile Ridge lavas (Klein and Karsten, Chile Trench sediments to the estimated asthenospheric mantle 1995; Sturm et al., 1999), mafic southern Southern Volcanic Zone source composition (e.g. 50% MORB–50% OIB source). Tick (SSVZ; Hickey et al., 1986; Hickey-Vargas et al., 1989; Lo´pez- labels indicate percentage of crustal component for each curve. See Escobar et al., 1993), and Austral Volcanic Zone (AVZ; Stern and Table 3 for chemistry of the endmember compositions used. Kilian, 1996) arc lavas. MORB field is compiled from literature (e.g., Hofmann, 1997 and references therein). Mantle components ples (001, 003d, 004, 005, 007, 025, 026) have higher (EM1, EM2, HIMU, DMM) are from Zindler and Hart (1986). La/Ta (15–20), Ba/Ta (200–250), Th/La (0.15–0.27) Curves 1 and 2 are mixing models involving crustal components as ratios, and lower Ce/Pb (10–15) and Nb*/U (12–30) in Fig. 6. Curves 3 and 4 are binary mixing models showing ratios (Figs. 5 and 6) that are outside the ranges for percentage of EM1-type lithospheric components added to a 5% partial melt of the estimated asthenospheric mantle source typical OIB and MORB (Hofmann et al., 1986; Sun composition. Tick labels indicate percentage of crustal and EM1- and McDonough, 1989; Weaver, 1991). The low Ce/ type components for each curve. See Tables 2 and 3 for chemistry of Pb ratios in these samples are due to excess Pb which the endmember compositions used. 226 M. Gorring et al. / Chemical Geology 193 (2003) 215–235 fall within fields defined by many continental intraplate basalts and OIB (Figs. 7–9; Table 1). Sr–Nd isotope ratios range from 87Sr/86Sr = 0.7041–0.7049 and 143Nd/144Nd = 0.51262–0.51279; however, most samples cluster near 87Sr/86Sr f 0.7045 and 143Nd/ 144Nd f 0.51267 (Fig. 7). Pb isotope ratios range 206Pb/204Pb = 18.13–18.47, 207Pb/204Pb = 15.54– 15.63, 208Pb/204Pb = 38.38–38.64 and form a diffuse array above the Northern Hemisphere Reference Line (NHRL; Hart, 1984), and thus, are similar to southern hemisphere EM1- and EM2-type OIB that have positive DUPAL anomalies (e.g., Dupre´ and Alle`gre, 1983; Hart, 1984).

Fig. 9. Plot of 87Sr/86Sr versus 206Pb/204Pb for MLBA post-plateau showing the weak EM1-type signatures that contrast with HIMU- like signatures of Pali Aike and Antarctic Peninsula slab window lavas. Data sources, symbols, and components as in Figs. 5, 7, and 8. Mixing curves are as in Fig. 7.

Our data confirm earlier results reported by Haw- kesworth et al. (1979) on the Sr–Nd isotope composition of MLBA post-plateau lavas, and extend the range to the most enriched Sr–Nd isotope ratios and the most unradiogenic 206Pb/204Pb reported for any Neogene Patagonian slab window lavas south of the CTJ. It should also be noted that MLBA post-plateau magmas, along with other Patagonian slab window lavas (Gorring and Kay, 2001), are isotopically dis- tinct from Pali Aike (Stern et al., 1990; D’Orazio et al., 2000) and Antarctica Peninsula (Hole, 1988, 1990) slab window lavas that have more depleted, HIMU-type OIB isotopic signatures. The range of Sr–Nd–Pb isotope compositions of MLBA lavas suggests mixing between an enriched EM1- and/or EM2-type components and a more depleted, OIB-like Fig. 8. Plots of (a) 207Pb/204Pb and (b) 208Pb/204Pb versus 206Pb/204Pb showing positive Dupal and EM1-type Pb signatures asthenospheric mantle component. of MLBA post-plateau lavas. Symbols and data sources as in Figs. 5 and 7. Nazca Plate sediments are from Unruh and Tatsumoto (1976) and Dasch (1981). Crustal granulite xenoliths from Estancia Lote 17 7. Discussion locality and Jurassic Chon Aike high-Si rhyolite (open diamond with cross) are unpublished data from Gorring (1997). Average S- type Patagonian crust and average Chile Trench sediments are from The trace element and isotopic characteristics of Kilian and Behrmann (1997). Northern Hemisphere Reference Line MLBA post-plateau lavas are consistent with their (NHRL) is from Hart (1984). being derived from mantle sources with OIB-like trace M. Gorring et al. / Chemical Geology 193 (2003) 215–235 227 element and isotopic characteristics (Figs. 4–8). This tios could also be explained as continental crust nor- includes both the asthenospheric mantle that upwells mally has extremely low Ce/Pb ( < 4) and very high through an opening slab window and enriched mantle concentrations of Pb relative to basaltic magmas components derived from the Patagonian continental (Hofmann et al., 1986). Simple mass balance calcu- lithospheric mantle. However, some samples show lations indicate that samples with lowest Ce/Pb ratios some chemical evidence (e.g., low Ce/Pb and Nb*/ (AP-02s, 003d, and 005) could be explained by 10– U, high Th/La and Ba/Ta), which suggests that sub- 20% bulk assimilation of Patagonian crust (see curves duction-related processes and/or continental crustal in Figs. 6 and 10). contamination have played a role, albeit minor, in The HFSE depletion, low Ce/Pb, and Pb isotope MLBA magma petrogenesis. In the following section, data for MLBA lavas could equally be interpreted as we discuss this role of these components and present a reflecting mantle source region contamination by petrogenetic model for MLBA post-plateau lavas. subducted sediment or a slab-derived fluid component. Simple mass balance calculations indicate that 7.1. Role of subduction-related and continental samples with lowest Ce/Pb ratios (AP-02s, 003d, and crustal components 005) could be explained by V 1% of bulk addition of Chile Trench sediment (Kilian and Behrmann, 1997) Several MLBA post-plateau lavas analyzed in this to the mantle source (see curves in Figs. 6 and 10). study (001, 003d, 004, 005, 007, 025, 026) have Slab-derived fluids are also thought to be responsible anomalously low Ce/Pb and Nb*/U, and high Th/ for HFSE depletion and enrichment in fluid mobile La, Ba/Ta, La/Ta ratios compared to most of the other elements like Ba, Sr, Cs, and Pb relative to LREE in MLBA samples (Figs. 5 and 6). These samples also arc magmas (e.g., Plank and Langmuir, 1993; Wood- have some of the lowest incompatible trace element head et al., 1997 and references therein). Thus, some concentrations of all MLBA lavas (Table 1) and, thus, of the geochemical characteristics of these anomalous are most susceptible to contamination by a variety of MLBA lavas could be attributed to mantle source components derived from either subduction-related contamination by subduction-related components processes (slab-derived fluids/melts, subducted sedi- (subducted sediments and/or fluids) that were either ments) or in situ assimilation-fractional crystallization (AFC) within the Patagonian continental crust, or both. Resolving these two processes is difficult, if not impossible, with the present MLBA post-plateau dataset. Circumstantial evidence for crustal AFC processes comes from the fact that these lavas are not primary mantle-derived magmas. They must have suffered some crystal fractionation, most likely while ponded in crustal magma chambers, and thus, could have assimilated crustal material in the process. Pb isotope data lends support for crustal AFC in that these samples have distinctly higher 206Pb/204Pb ratios (f 18.45) compared to most other MLBA post-plateau lavas (Fig. 8). Pb isotope analyses of Middle Jurassic Chon Aike rhyolite, crustal xenoliths from the region (Gorring and Kay, 2001), and estimates of the Fig. 10. 87Sr/86Sr versus Ce/Pb for MLBA post-plateau lavas average Patagonian crustal composition (Kilian and showing influence of EM1-type lithospheric mantle component in Behrmann, 1997) all have relatively high 206Pb/204Pb most samples. Labeled samples are those referred to in text as (f 18.45–18.70; Fig. 8), and thus, crustal material having evidence for EM2-type crustal AFC components. Data fields, sources, and symbols as in Figs. 5 and 7. Field for ‘‘pristine’’ could be a viable endmember contaminant during southern Patagonian slab window lavas are from Gorring and Kay, AFC processes assuming parental MLBA post-plateau (2001) and represent samples uncontaminated by arc-related and/or 206 204 magmas had Pb/ Pb < 18.45. The low Ce/Pb ra- crustal AFC processes. Mixing curves are as in Fig. 6. 228 M. Gorring et al. / Chemical Geology 193 (2003) 215–235 stored in the basal continental lithospheric mantle or post-plateau lavas generally have more enriched Sr that had contaminated the supraslab asthenosphere, or and Nd isotopic ratios and lower 206Pb/204Pb ratios both. than slab window lavas from further south (Figs. 1, 7– Most MLBA post-plateau lavas have trace element 10). Therefore, unless the asthenospheric mantle and isotope ratios well within the accepted ranges for beneath southern Patagonia is extremely heterogene- OIB (Figs. 4–8) and, thus, show little evidence for ous, a CLM component is considered to also have significant amounts of subduction-related and/or con- played an important role in the petrogenesis of MLBA tinental crustal components. Their incompatible trace post-plateau lavas. element concentrations are generally equal to or much The main evidence for an enriched CLM compo- higher than concentrations in average upper and lower nent in MLBA post-plateau lavas comes from their crust (e.g., Rudnick and Fountain, 1995), therefore, moderately high 87Sr/86Sr and low 143Nd/144Nd and trace element and isotopic signatures in MLBA post- 206Pb/204Pb ratios, and strong OIB-like trace element plateau lavas are largely buffered against the affects of signatures (e.g. high Ce/Pb, low LILE/LREE and small to moderate amounts of crustal contamination LILE/HFSE). These chemical signatures have also via assimilation-fractional crystallization (AFC) pro- been recognized in other Neogene slab window lavas cesses. Furthermore, the common occurrence of peri- from the northeastern sector of the Patagonian back- dotite xenoliths and xenocrysts in these lavas indicates arc (Gorring and Kay, 2001) and have been similarly rapid ascent with little time to interact with the interpreted as having an important enriched CLM continental crust. In summary, the geochemical evi- component. Whether the enriched component is dence indicates that subduction-related and/or crustal EM1- or EM2-type is difficult to determine exactly. AFC processes are relatively minor and unlikely to be Arrays on Sr–Nd and Sr–Pb isotope plots are responsible for the overall strong OIB-like, trace generally elongated toward an ill-defined area bet- element and isotope characteristics of MLBA basaltic ween EM1 and EM2 with the samples that have magmas. Instead, the geochemical variability that is anomalously low Ce/Pb ratios controlling the appa- observed mostly reflects the characteristics of the rent elongation toward EM2 (Figs. 7 and 9). How- mantle source(s) and variations in degrees of partial ever, the majority of MLBA samples have OIB-like melting. Ce/Pb (>18), moderate 87Sr/86Sr (0.7040–0.7046) and relatively low 206Pb/204Pb ratios that are defi- 7.2. Mantle source signatures of MLBA post-plateau nitely elongated in the direction of EM1 on Pb–Pb lavas and Sr–Pb isotope plots (Figs. 8 and 9). As men- tioned before, this weak EM1-type signature is not The OIB-type chemical and isotopic characteristics due to crustal AFC or source region contamination of MLBA post-plateau lavas, without significant crus- as Patagonian crust and Chile Trench sediments have tal and/or arc-related contamination, indicate that they higher 206Pb/204Pb ratios compared to all of the reflect mantle source signatures. As with all continen- MLBA lavas (see Fig. 8), and mass balance calcu- tal basalts, two mantle source components are possible, lations show that significant crustal additions would namely the asthenosphere and the continental litho- dramatically change certain key trace element ratios, spheric mantle (CLM). Given the slab window tectonic which is only observed for a few samples (see Figs. setting of MLBA post-plateau lavas, the asthenosphere 6 and 10). Furthermore, neither source mixing with is thought to be the dominant mantle source compo- Chile Trench sediments nor bulk assimilation with nent (Thorkelson, 1996; Gorring et al., 1997). Recent average Patagonian continental crust can directly petrologic studies of slab window lavas further south in reproduce the Sr–Nd–Pb isotope systematics of M- the central Santa Cruz Province (Gorring and Kay, LBA lavas (see curves 1 and 2 in Figs. 7 and 9), 2001) and in the PAVF (Stern et al., 1990; D’Orazio et unless (1) the endmember mantle source/parental al., 2000) indicate that the southern Patagonian asthe- magmas had higher 87Sr/86Sr, lower 143Nd/144Nd, nospheric mantle has relatively depleted, OIB-type iso- and lower 206Pb/204Pb ratios, or (2) the petrogenesis tope characteristics (e.g., 87Sr/86Sr = 0.7032–0.7038; involves a two-step process of lithospheric EM1-type 143Nd/144Nd = 0.5128–0.5129). In contrast, MLBA contamination followed by crustal AFC processes. M. Gorring et al. / Chemical Geology 193 (2003) 215–235 229

Therefore, we interpret the isotope and trace element CLM since stabilization beneath the continent include characteristics to reflect the existence of a hetero- the following: (1) the middle Jurassic Chon Aike geneous CLM beneath the MLBA that contains rhyolite event associated with Gondwana breakup volumetrically minor amounts of an EM2-type com- (Kay et al., 1989; Pankhurst et al., 1998), (2) exten- ponent that is variably superimposed on a more sive Cenozoic plateau basalt magmatism (Ramos and prevalent, but weak EM1-type component. Kay, 1992), and (3) nearly continuous subduction since the early Cretaceous (Bruce et al., 1991) and 7.3. Origin of the EM-type mantle sources in episodic into the late Paleozoic (Forsythe, 1982; Patagonian CLM Herve´, 1988). Perhaps continuous metasomatism by small-percentage melts of MORB-like asthenophere The range of Sr–Nd–Pb isotope compositions over 500 Ma, coupled with periodic pulses of OIB- suggests that most MLBA post-plateau lavas resulted type melts during Gondwana breakup and Cenozoic from mixing and/or contamination processes requiring plateau magmatism, could have produced the weak the involvement of EM-type CLM components. The EM1-type signatures in the Patagonian CLM. Phaner- relatively low 206Pb/204Pb and 143Nd/144Nd ratios of ozoic subduction-related processes could potentially EM1-type components require time-integrated low U/ superimposed additional EM2-type heterogeneities Pb and Sm/Nd ratios in the source. Thus, the origin of (subducted sediments, H2O-rich fluids) on the CLM, EM1-type signatures in continental intraplate basalts but this component is difficult to distinguish from the (and OIB) is generally attributed to interaction with an affects of in situ, crustal AFC. ancient, silicate melt and/or CO2-rich fluid metasomatized CLM or a lower crustal granulite assemblage (e.g., Zindler and Hart, 1986; Carlson, 1995 and 8. Geodynamic implications references therein). Direct involvement of significant amounts lower crustal contamination seems unlikely Gorring and Kay (2001) proposed a four-component based on arguments given above, thus the EM1 dynamic slab window model to explain the incompat- signature is most likely derived from metasomatized ible element and isotopic variations of Neogene plateau CLM. The high 87Sr/86Sr and low 143Nd/144Nd ratios lavas from central Santa Cruz. The four components of EM2-type components in continental intraplate included the following: (1) a relatively depleted, OIB- basalts is generally attributed to contamination with type subslab asthenosphere component, (2) a subduc- upper continental crustal components either through tion-related component (e.g., slab-derived fluids and/or AFC processes in the crust or to the interaction with melts) stored in either the basal continental lithosphere subduction-modified CLM (e.g., Carlson, 1995). or in the supraslab asthenosphere, (3) an enriched, The Patagonian continental lithosphere is relatively EM1-type CLM component, and (4) an upper crustal young (V 0.5 Ma, see Section 2), and thus, EM-type component as an AFC-derived contaminant. The signatures (both EM1 and EM2) are not likely to be model presented here is similar to, and lends support extreme due to the lack of time-integrated growth of for, Gorring and Kay (2001)’s model; however, the daughter isotopes (Stern et al., 1989, 1990; Zartman et main difference is that MLBA post-plateau lavas have a al., 1991). This is generally supported by the relatively much larger contribution from the EM1-type CLM depleted, but heterogeneous, Sr–Nd isotopic com- component compared to Neoegene slab window lavas positions (87Sr/86Sr = 0.7027–0.7043; 143Nd/144Nd = from central Santa Cruz as well as those from the Plio– 0.51312–0.51279) of peridotite xenoliths from Pali Pleistocene Pali Aike field that have stronger astheno- Aike (Stern et al., 1989, 1999) and from Estancia Lote spheric signatures (Fig. 10). 17 (Gorring and Kay, 2000). These data suggest that the Our preferred model for the petrogenesis of MLBA Patagonian CLM was stabilized from melt-depleted, post-plateau lavas and their relationship with regional MORB-source asthenosphere at some point in the early tectonic evolution is shown in Fig. 11. The geochem- Phanerozoic (Stern et al., 1989; Zartman et al., 1991). ical data presented here and available K–Ar and Major geotectonic events that have likely affected 40Ar/39Ar dates strongly support a slab window origin the chemical evolution of the southern Patagonian for the MLBA post-plateau lavas (e.g., Ramos and Kay, 230 M. Gorring et al. / Chemical Geology 193 (2003) 215–235

Fig. 11. Schematic southern Patagonian lithospheric cross-section during the Plio–Pleistocene (1.8–0.1 Ma) opposite where ridge collision occurred at 6 Ma (see Fig. 1) that shows the geodynamic model for MLBA post-plateau lavas (modified from Gorring et al., 1997; Gorring and Kay, 2001). Upwelling of OIB-type subslab asthenosphere may have resulted in thermo-mechanical erosion of the basal Patagonian continental lithosphere. This interaction between asthenosphere and continental lithosphere could explain the isotopic variability and distinctive EM1-type isotopic signatures of MLBA post-plateau lavas. CC = continental crust; CLM = continental lithospheric mantle.

1992; Gorring et al., 1997). This model involves two derived from subducted sediments and/or slab-derived mantle source components that can be explained as an fluids stored within the CLM and/or during AFC OIB-like subslab asthenosphere and an enriched CLM processes in crust. component with ‘young’ EM1-type signatures. Magma An important regional implication of this study for generation most likely occurred near the astheno- Patagonian back-arc magmatism is the observation that sphere–lithosphere boundary as asthenosphere up- enriched EM1-type components appear geographically wells through the slab window and impinged on the restricted to basalts from the north and northeastern base of the CLM. Based on the HREE depleted patterns sectors of the back-arc. Indeed, when one examines the of MLBA lavas, the minimum depth to the astheno- isotope data for all Neoegene slab window lavas from sphere–lithosphere boundary was at least within the 46.8jSto52jS, a systematic regional pattern emerges garnet stability field in excess of 65–70 km. In this with more depleted Sr and more radiogenic Pb isotope model, upwelling subslab asthenosphere supplies both signatures (87Sr/86Sr < 0.7038; 206Pb/204Pb>18.5) in heat and magma, and thus, mixing at the base (or the south and southwestern part of the back-arc within within) the CLM is an in situ, binary process. Binary the Magallanes Basin, to gradually more enriched mixing of asthenospheric melts with partial melts of EM1-type signatures (87Sr/86Sr>0.7038; 206Pb/204Pb < fertile portions of the CLM with EM1-type signatures 18.5) in the north and northeast within or near the produces the range of compositions observed in the Deseado Massif (see Fig. 1). We interpret this to reflect MLBA post-plateau lavas. Mixing curves between important regional differences in the chemistry of the asthenospheric slab window magmas and EM1-type Patagonian CLM that may not have been previously lithospheric components in Figs. 7 and 9 and mass recognized (Table 3). balance calculations in Table 2 indicate roughly 10– The results of this study also suggest that astheno- 40% CLM component in MLBA post-plateau lavas. sphere–lithosphere interactions are an important pet- This mixing could either have taken place fully within rogenetic process for slab window lavas on a global the lowermost CLM as asthenospheric melts passed basis. In our model for MLBA, we envision that though it, or by preferential melting and mixing of pristine, subslab asthenosphere upwelling through thermo-mechanically eroded EM1-type ‘plums’ that the slab window will not only partially melt, but also were intermingled in a ‘pudding’ of upwelling astheno- causes thermo-mechanical erosion and thinning of the sphere near the base of the CLM. In addition, several CLM. This is physically analogous to extensional- MLBA magmas have acquired weak arc signatures induced and plume-related interactions between these from further interactions with EM2-type components two mantle reservoirs in the petrogenesis of continen- .Grige l hmclGooy13(03 215–235 (2003) 193 Geology Chemical / al. et Gorring M.

Table 2 Mixing models showing quantities of EM1-type lithospheric components required to produce trace element and isotopic compositon of MLBA post-plateau lavas 12345 6 789ABCDE La 26 8.3 151.7 240 66 79 47 43 59 32 42 27 24 15 Nd 25.4 13.8 74.7 207 56 57 41 37 53 62 64 45 38 15 Sr 678 300 2119 1530 1097 1075 868 828 987 29 28 31 29 36 Pb 2.2 0.85 9.73 16 5.4 7.5 4.6 4.4 5 42 70 42 40 20 87Sr/86Sr 0.7036 0.7036 0.705 0.705 0.704393 0.704448 0.704681 0.704743 0.704106 29 33 32 39 20 143 Nd/144 Nd 0.51285 0.51285 0.51255 0.51255 0.512649 0.512667 0.512663 0.512676 0.512623 41 35 23 20 28 206 Pb/204 Pb 18.8 18.8 17.8 17.8 18.23 18.127 18.221 18.267 23 32 11 14 Average 37 43 30 32 21 1 and 2 = 1% and 5% partial melt of 50% MORB–50% OIB asthenospheric source mantle, respectively (Gorring and Kay, 2001); 3 = 0.3% partial melt of an enriched lithospheric mantle component; 4 = average lamproite (Bergman, 1987) with a ‘young’ EM1 isotope signature; 5 to 9 = MLBA post-plateau lavas (CV-02s, AT-01s, 017, LC-25, and 025, respectively); A= percentage of (1) mixed with (3) to make (5); B = percentage of (1) mixed with (3) to make (6); C = percentage of (2) mixed with (3) to make (7); D = percentage of (2) mixed with (3) to make (8); E = percentage of (1) mixed with (4) to make (9). All concentrations in 1–9 are in ppm. 231 232 M. Gorring et al. / Chemical Geology 193 (2003) 215–235

Table 3 MLBA post-plateau lavas have variably enriched Chemical compositions of MLBA magmas and crustal components Sr–Nd isotopes (87Sr/86Sr = 0.7041–0.7049; 143Nd/ used to model crustal contamination 144Nd = 0.51264–0.51279) and relatively low 206Pb/ 1234 204Pb ratios (18.13–18.45). Two mantle components La 0.481 26 21.5 24.6 are recognized as major potential sources: a relatively Ce 1.337 55.9 47.3 52.9 depleted OIB-like subslab asthenosphere, and an Nd 1.12 25.4 22.6 28.6 Sr 20.6 678 305 173 enriched CLM component with EM1-type signatures. Pb 0.05 2.2 13.3 11.1 The isotopic variability of most MLBA lavas can be Th 0.05 5.3 6.9 8.4 considered in terms of mixing of these two compo- U 0.016 1.6 1.9 1.5 nents in variable proportions. The origin of EM1-type Nb 0.65 51.5 9.1 14.3 components in the Patagonian lithosphere is tenta- 87Sr/86Sr 0.7036 0.7036 0.7077 0.72184 143Nd/144Nd 0.51285 0.51285 0.51253 0.51232 tively attributed to continuous basaltic melt metaso- 206Pb/204Pb 18.8 18.8 18.65 18.67 matism over the last 500 Ma, with enhanced meta- 1 and 2 = 50% MORB–50% OIB asthenospheric source mantle somatism during the Mesozoic Gondwana breakup and 1% partial melt of this source mantle, respectively (Gorring and Cenozoic basaltic plateau magmatism. The EM1- and Kay, 2001); 3 and 4 = average of Chile Trench sediments from type signatures of MLBA post-plateau lavas are dis- ODP Leg 141 and estimate of S-type Patagonian crust, respectively tinctive from most other Late Cenozoic Patagonian (Kilian and Behrmann, 1997). All concentrations in ppm. slab window lavas from further south that have more depleted, OIB-type isotope characteristics. This is tal intraplate basalts (e.g. McKenzie and Bickle, 1988; interpreted here to reflect an important regional Ellam and Cox, 1991; Arndt and Christensen, 1992; heterogeneity in the CLM that influences the chem- Gallagher and Hawkesworth, 1992). Although slab istry of southern Patagonian slab window basaltic window tectonic settings are arguably the best places magmas. The results of this study indicate that the on Earth to investigate the chemistry of the astheno- interaction of upwelling, hot subslab asthenosphere spheric mantle beneath active continental margins, with the basal CLM may be an important (and results of this study emphasizes the need to exercise commonly overlooked) geodynamic process in slab caution when interpreting the OIB-like signatures in window tectonic settings that can significantly mod- slab window lavas as entirely asthenosphere-derived. ify the asthenospheric chemical signatures of slab window magmas.

9. Conclusions Acknowledgements Abundant (f 600 km3) Plio–Pleistocene post-plateau mafic magmatism in the MLBA region is linked The authors would like to give special thanks to to ridge collision along the Andean margin f 6Ma Coco and Petty Nauta at the Estancia Telken for their ago. Most of the lavas are relatively primitive (6–10 gracious hospitality and their invaluable logistical wt.% MgO; Ni>80 ppm and Cr>150 ppm), highly support in the field. Special recognition also goes out alkaline (5–8 wt.% total alkalis; < 48% SiO2) basan- to Victor Ramos for his expert knowledge of the ites, hawaiites, and alkali basalts. Their incompatible regional geologic framework. Assistance in the trace element characteristics reveal strong intraplate, laboratory, particularly the efforts of Linda Godfrey OIB-like signatures with little evidence for the influ- on the TIMS and ICP-MS, Jeremy Delaney (Rutgers) ence of arc and/or crustal components. REE patterns and John Hunt (Cornell) on the electron microprobes, are all LREE-enriched with high LREE/HREE ratios and Bob Kay with INAA, are greatly appreciated. The (La/Yb = 11–54) and buffered HREE concentrations manuscript was greatly improved by constructive at f 6–8 chondrite. These data indicate variable comments and reviews by C. Hawkesworth and two extents of partial melting (f 1–5%) of a OIB-like, anonymous reviewers. This research was generously LREE-enriched mantle source at depths within the supported by grants from Montclair State University garnet stability field (>65–70 km). to MLG, NEGSA undergraduate research grant to JG, M. Gorring et al. / Chemical Geology 193 (2003) 215–235 233

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