Journal of Petrology Volume 42 Number 6 Pages 1067-1094 2001
© Oxford University Press 2001
Mantle Processes and Sources of Neogene Slab Window Magmas from Southern Patagonia, Argentina
DEPARTMENT OF GEOLOGICAL SCIENCES, CORNELL UNIVERSITY, ITHACA, NY 14853, USA
Received June 3, 1999; Revised typescript accepted October 2, 2000
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONREGIONAL TECTONIC AND GEOLOGIC...PETROGRAPHYGEOCHEMISTRYTEMPORAL AND SPATIAL PATTERNS...P-T CONDITIONS FOR PARTIAL...MANTLE SOURCE REGIONS AND...PETROGENETIC MODEL FOR...DISCUSSIONCONCLUSIONSAPPENDIX A: ANALYTICAL METHODSAPPENDIX B: CRYSTAL...APPENDIX C: DISTRIBUTION...REFERENCES |
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Neogene plateau lavas in Patagonia, southern Argentina, east of the volcanic gap between the Southern and Austral Volcanic Zones at 46·5° and 49·5°S are linked with asthenospheric slab window processes associated with the collision of a Chile Ridge segment with the Chile Trench at 12 Ma. The strong ocean-island basalt (OIB)-like geochemical signatures (La/Ta <20; Ba/La <20; 87Sr/86Sr = 0·7035–0·7046; 143Nd/144Nd = 0·51290–0·51261; 206Pb/204Pb = 18·3–18·8; 207Pb/204Pb = 15·57–15·65; 208Pb/204Pb = 38·4–38·7) of these Patagonian slab window lavas contrast with the mid-ocean ridge basalt (MORB)-like, depleted mantle signatures of slab window lavas elsewhere in the Cordillera (e.g. Antarctic Peninsula; Baja California). The Patagonian lavas can be divided into a voluminous
12–5 Ma, tholeiitic main-plateau sequence (48–55% SiO2; 4–5% Na2O + K2O) and a less voluminous 7–2 Ma alkaline post-plateau sequence (43–49% SiO2; 5–8% Na2O + K2O). Moderately high FeOT (9–11%), and low heavy rare earth element (HREE), Y, and Sc concentrations in all lavas are consistent with melt generation just below the garnet–spinel transition at a depth of 70 km. The main-plateau lavas from the western back-arc can be modeled by 10–15% partial melting of an OIB-like asthenospheric mantle source with additions from slab fluid–melt components coupled with crustal contamination (AFC). A three-stage petrogenetic model is envisaged: (1) decompression melting and source region contamination of an OIB-like subslab asthenospheric source by slab melts of the trailing edge of the subducted Nazca Plate; (2) minor contamination of slab window melts with arc components ‘stored’ in the supraslab mantle wedge and/or basal continental lithosphere; (3) further modification by addition of crustal components during magma ascent. The main-plateau lavas from the eastern back-arc can be modeled by 7% partial melting of the same asthenospheric source as the influence of arc components diminishes and the intensity of mantle upwelling into the slab window decreases. All post-plateau lavas can be modeled as 1–4% partial melts of the pristine OIB-like asthenospheric source in the widening slab window. KEY WORDS: slab window magmatism; southern Patagonia; plateau basalt petrogenesis; mantle chemistry; back-arc processes
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONREGIONAL TECTONIC AND GEOLOGIC...PETROGRAPHYGEOCHEMISTRYTEMPORAL AND SPATIAL PATTERNS...P-T CONDITIONS FOR PARTIAL...MANTLE SOURCE REGIONS AND...PETROGENETIC MODEL FOR...DISCUSSIONCONCLUSIONSAPPENDIX A: ANALYTICAL METHODSAPPENDIX B: CRYSTAL...APPENDIX C: DISTRIBUTION...REFERENCES |
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Mafic slab window volcanism is produced by the interaction or collision of mid-ocean ridges with continental subduction zones and offers a rare opportunity to investigate (1) the shallow asthenospheric mantle beneath continents and (2) subduction zone mantle processes and sources. Slab window mafic magmas are the product of decompression melting as asthenospheric mantle upwells through the gap that opens between the subducting oceanic plates as a ridge system is subducted or overidden by the hanging-wall plate (e.g. Thorkelson & Taylor, 1989
; Hole et al., 1995; Thorkelson, 1996; Gorring et al., 1997). Thus, magma sources should dominantly reflect the chemistry of the asthenospheric mantle beneath the subducting plate (‘subslab’), as opposed to normal arc and back-arc magma sources which reflect the mantle above the subducting plate (mantle wedge or ‘supraslab’; e.g. Thorkelson, 1996). However, mafic slab window lavas erupted along continental subduction zones are derived by partial melting of complex sources involving asthenospheric, lithospheric mantle and subducted slab components, and are often modified by crustal contamination during their ascent to the surface. Furthermore, melt generation processes are likely to be controlled by tectonic variables such as relative extensional vs compressional stress regimes, the age of the subducting oceanic crust, lateral mantle flow and the presence of hotspots, and the history of the continental lithosphere.
The chemistry of the shallow, asthenospheric mantle beneath ocean basins is relatively well constrained from numerous geochemical and isotopic studies of mid-ocean ridge and oceanic island basalts [MORB and OIB; see Zindler & Hart (1986) and Hofmann (1997) for reviews]. The widely accepted view is that the upper mantle is dominated by depleted MORB-type mantle (e.g. DMM of Zindler & Hart, 1986) that is sampled at mid-ocean ridges (Ito et al., 1987), most oceanic island arc systems (Kay, 1980; Hawkesworth et al., 1993), and a few oceanic island hotspots (e.g. Galápagos; White et al., 1993). However, it is also clear that the asthenospheric mantle is heterogeneous at a variety of scales as a result of the recycling of various components (e.g. altered oceanic crust, sediment, delaminated continental lithosphere) back into the mantle and from contamination from upwelling deep mantle plumes (Zindler & Hart, 1986; Staudigel et al., 1991; Hofmann, 1997). In contrast to the oceanic environment, the chemistry of the asthenospheric mantle beneath continents is more difficult to constrain, primarily because subduction zone processes and interactions with continental lithospheric mantle and crust often obscure the asthenospheric signal in continental basalts (e.g. Gill, 1981; Hildreth & Moorbath, 1988; Carlson, 1995). Despite these complexities, recent studies of slab window lavas have successfully documented the nature of the asthenospheric mantle beneath active continental margins. The chemistry of Neogene slab window lavas from Baja California to British Columbia indicates that the shallow asthenospheric mantle along the western North American margin is similar to Northern Pacific E-MORB (Johnson & O’Neil, 1984; Storey et al., 1989; Thorkelson & Taylor, 1989; Cole & Basu, 1995; Luhr et al., 1995). In contrast, Neogene slab window lavas from the Antarctic Peninsula have HIMU-like signatures, suggesting that OIB-like heterogeneities might exist in the shallow asthenosphere beneath this region (Hole, 1988, 1990).
In this paper, the major and trace element geochemistry and Sr–Nd–Pb isotope composition of Neogene slab window lavas from the southern Patagonian back-arc of Argentina are used to investigate the chemistry of the asthenospheric mantle and to constrain the contribution of various components to the lavas. These lavas are interpreted to have been generated in response to the opening of slab windows associated with collisions of segments of the Chile Ridge with the Chile Trench (Fig. 1; see also Ramos & Kay, 1992; Gorring et al., 1997). Ramos & Kay (1992) and Gorring et al. (1997) also used the narrow range of incompatible trace element and Sr–Nd–Pb isotopic ratios of Patagonian slab window lavas to argue that they were derived from a relatively homogeneous OIB-like mantle source. In contrast to the more ‘depleted’ MORB- and HIMU-like signatures (lower 87Sr/86Sr and higher 143Nd/144Nd) of western North American and Antarctic Peninsula slab window lavas, Patagonian slab window lavas suggest an asthenospheric mantle with a more ‘enriched’ OIB-like signature (higher 87Sr/86Sr and lower 143Nd/144Nd). Unlike western North American and Antarctic Peninsula slab window lavas, most Patagonian slab window lavas show evidence for components derived from the subducted slab and/or from the continental lithosphere and crust. Finally, the geochemical data presented here are integrated with the tectonic models of Ramos & Kay (1992) and Gorring et al. (1997) to create an internally consistent petrogenetic model for slab window magmatism in southern Patagonia.
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in diagonal rule) are from Stern et al. (1990) |
REGIONAL TECTONIC AND GEOLOGIC SETTING |
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TOPABSTRACTINTRODUCTIONREGIONAL TECTONIC AND GEOLOGIC...PETROGRAPHYGEOCHEMISTRYTEMPORAL AND SPATIAL PATTERNS...P-T CONDITIONS FOR PARTIAL...MANTLE SOURCE REGIONS AND...PETROGENETIC MODEL FOR...DISCUSSIONCONCLUSIONSAPPENDIX A: ANALYTICAL METHODSAPPENDIX B: CRYSTAL...APPENDIX C: DISTRIBUTION...REFERENCES |
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Neogene lavas, inferred to be related to slab window formation (e.g. Ramos & Kay, 1992
; Gorring et al., 1997), erupted over large areas of the southern Andean back-arc in southern Patagonia, Argentina, southeast of the modern Chile Triple Junction (Fig. 1) following a series of ridge collisions along the Chile Trench during mid- to late Miocene time (Forsythe & Nelson, 1985; Cande & Leslie, 1986; Forsythe et al., 1986). Slab window lavas are most abundant between 46·5° and 49·5°S, northeast of the ridge segment that collided at 12 Ma and 100–400 km east of the gap between the Quaternary Southern (SVZ) and Austral Volcanic Zones (AVZ) (Fig. 1). K/Ar and 40Ar/39Ar ages (Ramos & Kay, 1992; Gorring et al., 1997) suggest two periods of magmatism: (1) a voluminous, late Miocene to early Pliocene main-plateau sequence; (2) a younger, less voluminous, latest Miocene to Pleistocene post-plateau sequence (Fig. 2). In this segment of the back-arc, both main- and post-plateau lavas post-date ridge collision and systematically young to the northeast (Gorring et al., 1997). The main-plateau lavas form large, elevated plateaux of the Mesetas de la Muerte, Belgrano, del Lago Buenos Aires, Central, and the smaller mesetas in the northeast region (Fig. 2). The main-plateau lavas have a maximum total thickness of 100 m and their total eruptive volume is estimated at 1000–2000 km3 (Ramos & Kay, 1992; Gorring et al., 1997). The post-plateau lava sequence includes small scoria cones, lava flows, and pyroclastic deposits capping the main-plateau sequence with a total eruptive volume estimated at 100 km3 (Baker et al., 1981; Stern et al., 1990; Ramos & Kay, 1992; Gorring et al., 1997). The Neogene slab window lavas are interbedded with continental sediments, which unconformably overlie Mesozoic and older Cenozoic sedimentary and volcanic rocks. The entire sequence forms a dissected plateau that dips gently eastward from the Patagonian Cordillera towards the Atlantic Ocean. The basement beneath the western back-arc consists of mid- to late-Paleozoic accretionary wedge complexes (Ramos, 1989). Beneath the central and northeastern sectors, the basement is considerably older, consisting of latest Proterozoic (542 ± 60 Ma; Pezzuchi, 1978) to Grenville age (1100 Ma; Pankhurst et al., 1994) metamorphic rocks of the Deseado Massif (Fig. 1). Additional details on the regional geology have been summarized by Ramos (1989).
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A slab window tectonic model has been developed by Ramos & Kay (1992)
, Kay et al. (1993), and Gorring et al. (1997) to explain the sequence of magmatic events occurring along a SW–NE transect opposite the Chile Ridge segment that collided at 12 Ma as shown in Fig. 3. Normal arc volcanism, similar to that in the modern Andean SVZ, is assumed to have occurred in the modern arc gap before ridge collision in the early Miocene. By 12 Ma, the ridge had collided, causing volcanism in the former arc region to cease and shift into the back-arc. Melting of the young, hot, Nazca Plate produced the Cerro Pampa adakites (Kay et al., 1993). Contemporaneous eruptions of minor volumes of back-arc mafic alkaline lavas are best related to melting in the supraslab mantle wedge. These lavas have similar OIB-like characteristics to the younger slab window lavas, suggesting that the supraslab mantle wedge may have already had OIB-like components in it before ridge collision. From 10 to 2 Ma, mafic lavas with strong OIB-like chemical signatures erupted across the back-arc above a developing slab window. The OIB-like signatures are interpreted as derived primarily from upwelling subslab asthenosphere although contributions from arc-modified supraslab wedge and continental lithospheric mantle are also evident in these lavas. The back-arc magmatic activity ends with inception of arc volcanism in the AVZ above the leading edge of the Antarctic Plate (Stern & Kilian, 1996). Farther north, opposite the ridge segment that collided at 6 Ma, abundant Plio-Pleistocene (5–0·2 Ma) plateau lavas occur in the Meseta del Lago Buenos Aires region (Baker et al., 1981). These lavas have also been interpreted in the context of a slab window model (Ramos & Kay, 1992; Gorring et al., 1997) and will not be considered in detail here.
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PETROGRAPHY |
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TOPABSTRACTINTRODUCTIONREGIONAL TECTONIC AND GEOLOGIC...PETROGRAPHYGEOCHEMISTRYTEMPORAL AND SPATIAL PATTERNS...P-T CONDITIONS FOR PARTIAL...MANTLE SOURCE REGIONS AND...PETROGENETIC MODEL FOR...DISCUSSIONCONCLUSIONSAPPENDIX A: ANALYTICAL METHODSAPPENDIX B: CRYSTAL...APPENDIX C: DISTRIBUTION...REFERENCES |
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The southern Patagonian lavas analyzed as part of this study represent all major plateaux and volcanic centers between 46·5° and 49·5°S that occur along a NE–SW transect opposite the 12 Ma ridge collision event (Fig. 2). The lavas are extremely fresh with only a few samples showing minor alteration of olivine phenocrysts. The main-plateau lavas have a medium-grained, subophitic to intergranular assemblage of clinopyroxene, plagioclase (An45–65), olivine (Fo67–78), and Fe–Ti oxides. Post-plateau lavas are sparsely phyric, with 5–20% phenocrysts in a glassy or fine-grained groundmass. Mineral assemblages are dominated by euhedral olivine (Fo58–84), with minor clinopyroxene. Groundmass phases are microcrystalline plagioclase, clinopyroxene, olivine, Fe–Ti oxides, and minor glass. A few Meseta de la Muerte samples contain abundant megacrysts of resorbed, complexly zoned anorthoclase (An14Ab57Kf29), plagioclase (An44–68), and clinopyroxene. Anorthoclase megacrysts typically have sieve textures that have been interpreted as evidence for crustal contamination and/or magma mixing (Ramos & Kay, 1992
). Mantle xenoliths are generally lacking in the main-plateau lavas, but small (1–2 cm) xenoliths are common in the post-plateau lavas. An important xenolith locality occurs at the Estancia Lote 17 in the Meseta Central (Fig. 2), where a variety of large (10–15 cm) spinel peridotite, granulite, and granitoid xenoliths are found in a post-plateau pyroclastic surge deposit. The petrology of the Lote 17 peridotite xenolith suite will be discussed elsewhere (Gorring & Kay, 2000). |
GEOCHEMISTRY |
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TOPABSTRACTINTRODUCTIONREGIONAL TECTONIC AND GEOLOGIC...PETROGRAPHYGEOCHEMISTRYTEMPORAL AND SPATIAL PATTERNS...P-T CONDITIONS FOR PARTIAL...MANTLE SOURCE REGIONS AND...PETROGENETIC MODEL FOR...DISCUSSIONCONCLUSIONSAPPENDIX A: ANALYTICAL METHODSAPPENDIX B: CRYSTAL...APPENDIX C: DISTRIBUTION...REFERENCES |
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We present 162 new major and trace element and 45 new Sr, Nd, and Pb isotopic analyses of lavas from the Mesetas de la Muerte, Belgrano, Central, and the northeast region (see Appendix A for analytical methods). Representative major and trace element analyses are given in Tables 1 and 2; Sr–Nd–Pb isotope data are given in Tables 3 and 4. The entire dataset is available from the Journal of Petrology Web site, which can be found at http://www.petrology.oupjournals.org. Previously published data for the Meseta de la Muerte from Stern et al. (1990)
and Ramos & Kay (1992) are also included in subsequent figures. Additional data for southern Patagonian Neogene basalts are available for the Meseta del Lago Buenos Aires (Hawkesworth et al., 1979; Baker et al., 1981) and for the Pali–Aike volcanic field (Skewes & Stern, 1979; Stern et al., 1990) (Fig. 1). Pali–Aike data are plotted for comparison, to highlight the striking chemical differences. For clarity, data for the Meseta del Lago Buenos Aires are not plotted, and these will be discussed in detail in a future paper.
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Major elements, fractional crystallization, and liquid lines of descent
A striking feature of the major element chemistry of Patagonian slab window lavas is the consistent difference in alkalinity between the main- and post-plateau lavas (Fig. 4). Most main-plateau lavas are silica-saturated, tholeiitic to alkalic basalts and basaltic andesites with 48–55% SiO2 and 4–5% Na2O + K2O. In contrast, most post-plateau lavas are silica-undersaturated, alkali basalts, hawaiites, and basanites with 43–50% SiO2 and 5–8% Na2O + K2O. Main-plateau lavas have higher Na2O/K2O ratios than post-plateau lavas (Tables 1 and 2). Systematic temporal changes from strongly tholeiitic main-plateau to alkaline post-plateau magmatism are best developed in the Mesetas de la Muerte, Belgrano, and Central. In the northeast region, main- and post-plateau lavas overlap in major element composition and are mostly alkaline basalts.
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The majority of both main- and post-plateau lavas have experienced moderate amounts of crystal fractionation from primitive mantle melts. This is shown by the whole-rock compositions ranging from 6 to 9% MgO, mg-number = 0·54–0·60, 100–300 ppm Cr, and 100–200 ppm Ni (Fig. 5; Tables 1 and 2). Volumetrically minor hawaiites, mugearites and trachybasalts with 50–54% SiO2, <5% MgO, <100 ppm Cr and Ni, and mg-number <0·5 are attributed to more extensive crystal fractionation (Figs 4 and 5; Tables 1 and 2). Differences in alkalinity, along with variable TiO2 and K2O at the same MgO concentration, indicate that the main- and post-plateau lavas (as well as many lavas from within the two sequences) are not cogenetic. Nevertheless, whole-rock major and compatible trace element variations in both sequences define reasonably coherent trends with MgO consistent with a major role for crystal fractionation. As illustrated in Fig. 5, main- and post-plateau lavas with >5% MgO show trends of rapidly decreasing Ni and Cr, slightly decreasing FeO and CaO, and increasing SiO2 and Al2O3 with decreasing MgO. These trends are best explained by variable amounts of olivine, clinopyroxene, and plagioclase fractionation with minor Cr-spinel and Fe–Ti oxides. Sharp decreases in CaO and relatively constant Al2O3 with decreasing MgO at 4% MgO (Fig. 5b and c) indicate a change from olivine-dominated fractionating assemblages to plagioclase + clinopyroxene-dominated assemblages.
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Major element modeling indicates that most of the lavas require 10–40% crystallization of an assemblage consisting of olivine + clinopyroxene ± plagioclase + Fe-oxides + Cr-spinel from primary, mantle-derived melts. Representative liquid lines of descent (LLD) for main- and post-plateau lavas calculated using an incremental addition technique (Appendix B) are shown in Fig. 5. The slopes of calculated LLDs match the slopes of most major element trends for both main- and post-plateau lavas fairly well. However, the very steep negative trends for SiO2 vs MgO for the main-plateau sequence would require large proportions of titanomagnetite fractionation (8–10% of the total assemblage) and therefore are unlikely to represent a true LLD. Assimilation of SiO2-rich upper-crustal country rocks can explain these major element trends. The calculated effects of crustal assimilation and fractional crystallization (e.g. AFC; DePaolo, 1981) on the LLD of a representative main-plateau lavas are shown in Fig. 5 and discussed more fully below.
Incompatible trace element and isotopic ratios: clues to source regions
Incompatible trace elements
The trace element characteristics of the Patagonian slab window lavas are illustrated in Figs 6–9. Representative analyses are given in Tables 1 and 2. The OIB-like chemistry of the suite is evident by comparison with southern Southern Volcanic Zone (SSVZ) mafic arc lavas (Hickey et al., 1986; López-Escobar et al., 1993) and average N-MORB and OIB compositions (Sun & McDonough, 1989) in Figs 6 and 7, and Tables 1 and 2. The OIB-like character of the Patagonian slab window lavas is defined by enrichments in LILE (large ion lithophile elements) and LREE (light rare earth elements) compared with N-MORB, and relatively small or no HFSE (high field strength elements) depletions relative to LILE and LREE compared with mafic southern SVZ arc lavas. The overall uniformity of the OIB-like signature in the Patagonian slab window suite is shown in Fig. 7. This plot demonstrates the similarity of Ba/La, La/Ta, and Ba/Ta ratios in the Patagonian lavas to the OIB field as defined by Hickey et al. (1986), and their low values relative to SSVZ arc lavas (Ba/Ta >700, La/Ta >35; off-scale in Fig. 7). Anomalously high Ba/La ratios (>20) in some Meseta de la Muerte lavas are probably due to the presence of megacrystic feldspar and are not discussed further. Only the Antarctic Peninsula slab window lavas (Hole, 1988, 1990) with the highest La/Ta and Ba/Ta ratios overlap with those of southern Patagonia (Fig. 7). Most other North American slab window lavas (e.g. Baja California; Luhr et al., 1995) and Pali–Aike lavas (Stern et al., 1990), have lower Ba/Ta and La/Ta ratios.
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Despite the overall OIB-like trace element signatures, the main-plateau lavas from the western region of the back-arc (Muerte, Central, and Belgrano; see Fig. 2) have distinctively higher Ba/Ta and La/Ta ratios than main-plateau lavas from the northeast region and most post-plateau lavas (Fig. 7). The main-plateau lavas also have lower Nb/U (15–30), Ce/Pb (8–17), and Rb/Cs (15–65) ratios, and slightly higher Th/La (0·12–0·20) ratios than OIB and MORB (Fig. 8) (Hofmann et al., 1986; Sun & McDonough, 1989; Weaver, 1991). Western main-plateau lavas also have small negative Nb and Ta and large positive Pb anomalies on multi-element diagrams (Fig. 6) and have high Sr/La ratios that are similar to southern SVZ mafic arc rocks (Fig. 9). This reflects the influence of minor arc and/or continental crustal components on the chemistry of these lavas. In contrast, main-plateau lavas from the northeast region and most post-plateau lavas fall within or near the established ranges for OIB as shown in Figs 7 and 8.
Sr, Nd, and Pb isotopes
The Sr, Nd, and Pb isotopic composition of Patagonian slab window lavas are fundamentally different from western North American and Antarctic Peninsula slab window lavas, Pali–Aike lavas, and MORB. They have 87Sr/86Sr ratios ranging from 0·7035 to 0·7047, 143Nd/144Nd ratios ranging from 0·512896 to 0·512614, and 206Pb/204Pb ratios ranging from 18·3 to 18·9, 207Pb/204Pb ratios from 15·58 to 15·65, and 208Pb/204Pb from 38·2 to 38·8 (Tables 3 and 4; excluding sample PL-9). Sr–Nd ratios plot within the global OIB array between MORB and Bulk Silicate Earth (BSE) components of Zindler & Hart (1986) (Fig. 10). Pb ratios are within the range of southern hemisphere OIB with positive Dupal signatures (e.g. 
7/4 and 8/4 > 0; Dupré & Allègre, 1983; Hart, 1984) and form a diffuse array above the Northern Hemisphere Reference Line (NHRL; Hart, 1984) (Fig. 11). Their 87Sr/86Sr ratios are distinctly higher and 143Nd/144Nd and 206Pb/204Pb ratios lower than slab window lavas from the Antarctic Peninsula (Hole, 1988, 1990) and Baja California (Storey et al., 1989; Luhr et al., 1995), and also for Pali–Aike lavas further to south in Patagonia (Stern et al., 1990).
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Patagonian slab window lavas show systematic spatial trends in Sr, Nd, and Pb isotopic composition that are related to additional components derived from subduction-related processes and the continental lithosphere and crust. The main-plateau lavas show a west to east increase in 87Sr/86Sr ratios, with the lowest ratios in Meseta de la Muerte and Belgrano lavas, and the highest ratios in Meseta Central and northeast region lavas (Figs 10 and 12). With the exception of a few lavas from the Meseta de la Muerte, post-plateau lavas have relatively uniform 87Sr/86Sr (
0·7038) and 143Nd/144Nd (0·51275) ratios. With respect to Pb isotopes, both main- and post-plateau lavas from the Mesetas de la Muerte and Belgrano have higher 206Pb/204Pb ratios and lower 8/4 than Meseta Central and northeast region lavas (Figs 11 and 12; Tables 3 and 4).
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Clues to the origin of the components giving rise to these isotopic signatures come from comparisons with MORB, OIB, and with SSVZ and AVZ arc lavas. In particular, 87Sr/86Sr ratios in Patagonian slab window lavas are lower at the same 143Nd/144Nd ratio than arc lavas from the SSVZ (Hickey et al., 1986; López-Escobar et al., 1993) and the AVZ (Stern & Kilian, 1996) (Fig. 10). This is consistent with Patagonian slab window lavas having an overall smaller contribution from slab-derived fluids as compared with the southern SVZ and AVZ arc lavas. However, the relatively high La/Ta, Ba/Ta, and Sr/La ratios in western main-plateau lavas compared with eastern main-plateau lavas are consistent with the greater influence of a subduction-related component in the western main-plateau lavas. Other factors such as east–west chemical differences in the Patagonian lithospheric mantle and crust, and the relative contribution from these sources, undoubtedly played a role in modifying the chemistry of Patagonian slab window lavas as well (e.g. Hickey et al., 1986; Stern et al., 1990). Another observation is that Patagonian slab window lavas tend to form arrays on isotope diagrams (Figs 10 and 12) between an EM I- or EM II-type OIB component and a more depleted component, similar to that found in some Chile Ridge MORB (Segment 4A, Klein & Karsten, 1995) (Figs 10–12). The segment 4A Chile Ridge lavas have similar isotopic characteristics to Indian Ocean MORB and southern hemisphere Dupal OIB (Klein & Karsten, 1995; Sturm et al., 1999). This lends support for an enriched OIB-like component in the subslab asthenospheric mantle beneath southern Patagonia. The enriched EMI- or EMII-type components in Patagonian slabs window lavas are probably derived from the continental lithospheric mantle and/or crust as the lavas travel through the continental lithosphere en route to the surface.
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TEMPORAL AND SPATIAL PATTERNS IN THE EXTENT OF PARTIAL MELTING |
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TOPABSTRACTINTRODUCTIONREGIONAL TECTONIC AND GEOLOGIC...PETROGRAPHYGEOCHEMISTRYTEMPORAL AND SPATIAL PATTERNS...P-T CONDITIONS FOR PARTIAL...MANTLE SOURCE REGIONS AND...PETROGENETIC MODEL FOR...DISCUSSIONCONCLUSIONSAPPENDIX A: ANALYTICAL METHODSAPPENDIX B: CRYSTAL...APPENDIX C: DISTRIBUTION...REFERENCES |
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Patagonian slab window lavas are best interpreted as a series of magmas fractionated from distinct parental magmas that were produced by variable degrees of mantle melting. Evidence for variable mantle melting percentages comes from temporal and spatial variations in absolute abundances of incompatible trace elements at the same MgO concentration and differences in REE patterns (Tables 1 and 2; Fig. 13; see also Ramos & Kay, 1992
; Gorring et al., 1997). Particularly distinctive are lavas from the Meseta de la Muerte in the western region (Fig. 13a) and the Meseta Central in the central region of the back-arc. In these regions, main-plateau lavas are distinctly tholeiitic and have low abundances of incompatible trace elements (e.g. Th = 1–4 ppm) and relatively shallow sloping REE patterns (La/Yb = 4–15), whereas post-plateau lavas are strongly alkaline and have high abundances of incompatible trace elements (e.g. Th = 3–11 ppm) and steep REE patterns (La/Yb = 12–45). In contrast, northeast region main- and post-plateau lavas lack systematic chemical differences and generally are alkaline with relatively high incompatible element concentrations and steep REE patterns (Fig. 13b). These first-order differences in trace element chemistry of Patagonian slab window lavas can be explained by variations in the degree of partial melting of a relatively homogeneous OIB-like mantle source.
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0·10 ol + liq (e.g. Gurenko & Chaussidon, 1995
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As such, REE patterns can be used to determine relative variations in extent of partial melting in the mantle and changes in residual mantle mineralogy. The extensive dataset reported here allows spatial and temporal variations in mantle melting to be examined across the entire Patagonian back-arc. Quantitative estimates of melting percentages are based on the nonmodal, incremental batch melting model (Plank & Langmuir, 1992
). This model assumes batch melting in small increments (0·5%) within a single, vertical melt column and that all incremental melts are pooled at the top of the column. Trace elements are corrected for crystal fractionation using the fractional crystallization equation of Shaw (1970) and distribution coefficients given in Appendix C. The total fractionated crystal assemblage is obtained from the LLD (liquid line of descent) modeling (see Appendix B). Calculations are restricted to lavas with >6% MgO to minimize uncertainties in fractionation corrected trace element abundances and to exclude samples most affected by crustal processes. Calculating mantle melting percentages requires knowledge of the chemistry of the mantle source region. The strong OIB-like chemical signatures of Patagonian slab window lavas argue for a trace element-enriched mantle source as a starting point. Forward modeling of REE data indicates that source compositions that fall within the range shown in Fig. 13 are most suitable for modeling Patagonian slab window lavas at melting percentages between 1 and 15%. The ‘enriched source’ (upper limit on source range in Fig. 13a; Appendix C) is most appropriate for all post-plateau lavas and main-plateau lavas from the northeast region. A mixture of 50% MORB source mantle and 50% of an enriched mantle with trace element abundances 2 x chondritic is most appropriate for some of the main-plateau lavas from the Mesetas de la Muerte, Central, and Belgrano in the western back-arc (lower limit on source range in Fig. 13a; Appendix C). This mixed MORB-enriched mantle source is necessary to model the relatively low (La/Sm)n ratios (1–2·5; Fig. 14) of these lavas at realistic melting percentages (<20%). Depleted heavy REE (HREE) concentrations in both main- and post-plateau requires melt generation with residual garnet up to 10–15% partial melting.
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Extended trace element patterns generated from the models are plotted in comparison with representative main- and post-plateau lavas in Fig. 13. Muerte region post-plateau lavas are best modeled by 1–3% melting; whereas Muerte main-plateau lavas are best modeled by 7–10% melting (Fig. 13a). These percentages are similar to those quoted by Ramos & Kay (1992) for Muerte samples based on simple batch melting models of Nakamura et al. (1989). Both main- and post-plateau lavas from the northeast region can be modeled by 2–5% melting (Fig. 13b). Calculated melting percentages for representative Patagonian slab window lavas corrected for crystal fractionation are summarized in the plot of (La/Sm)n vs Th concentration in Fig. 14. Fractionation-corrected lavas match the predicted trend of the nonmodal melting model. Main-plateau lavas from Muerte, Central, and Belgrano with low (La/Sm)n ratios and Th concentrations represent the highest melting percentages (6–15%), whereas post-plateau lavas with high (La/Sm)n and Th represent lower melting percentages (2–3%). All northeast region lavas can be modeled by relatively low melt percentages (2–7%) and show no systematic differences between main- and post-plateau lavas. Thus, main-plateau lavas document a systematic decrease in the extent of partial melting northeastward across the back-arc, whereas post-plateau lavas indicate relatively constant and small extents of partial melting.
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P–T CONDITIONS FOR PARTIAL MELTING IN THE PATAGONIAN BACK-ARC |
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TOPABSTRACTINTRODUCTIONREGIONAL TECTONIC AND GEOLOGIC...PETROGRAPHYGEOCHEMISTRYTEMPORAL AND SPATIAL PATTERNS...P-T CONDITIONS FOR PARTIAL...MANTLE SOURCE REGIONS AND...PETROGENETIC MODEL FOR...DISCUSSIONCONCLUSIONSAPPENDIX A: ANALYTICAL METHODSAPPENDIX B: CRYSTAL...APPENDIX C: DISTRIBUTION...REFERENCES |
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Gorring et al. (1997)
estimated melt generation depths between 60 and 80 km at temperatures between 1350° and 1400°C for Patagonian slab window lavas based on consistently low HREE and Y abundances that require residual garnet in the mantle source. To place additional constraints on melting depths, we compare FeO concentrations of Patagonian lavas with compositions of experimental melts generated at various pressures (e.g. Scarrow & Cox, 1995). Melting depths in this case are considered to be average depths of melt segregation as partial melting is likely to be a polybaric process (e.g. Langmuir et al., 1992). This method is relatively robust because systematic FeO variations in natural and experimental melts are primarily functions of pressure and source region composition, not the extent of melting (e.g. Langmuir et al., 1992; Hirose & Kushiro, 1993).
To calculate average melt segregation depths for Patagonian lavas, we used experimental results on fertile lherzolite HK-66 by Hirose & Kushiro (1993). These results are more appropriate for interpreting Fe- and Ti-rich continental and oceanic intraplate lavas than those for fertile lherzolite KLB-1 (e.g. Francis & Ludden, 1995). We use FeO instead of SiO2 concentrations (e.g. Scarrow & Cox, 1995) as a monitor because of the uncertainties in correcting SiO2 values for crystal fractionation and crustal AFC in Patagonian main-plateau lavas. Patagonian main- and post-plateau analyses with >6% MgO were projected back to 15% MgO (e.g. Scarrow & Cox, 1995) using the slopes for the post-plateau and AFC-corrected main-plateau LLD in Fig. 5. The fractionation-corrected FeO values for Patagonian lavas were then input into a regression equation constructed from experimental melts of HK-66 at various pressures (see Fig. 15 for details). As Fig. 15 shows, most main- and post-plateau lavas yield average melt segregation pressures between 2·0 and 2·3 GPa corresponding to depths of 65–75 km. This depth is consistent with melting within the garnet stability field in accord with the incompatible trace element data.
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Melting depths inferred for Patagonian slab window magmas place important constraints on mantle potential temperature (Tp). Average melt segregation pressures of 2·2 GPa (70 km) require a mantle Tp of 1400°C for melting on an anhydrous peridotite solidus (e.g. McKenzie & O’Nions, 1991). Therefore, strong residual garnet signatures in western main-plateau lavas are difficult to reconcile with a hot, anhydrous mantle as garnet would be unstable at depths shallower than 70 km and probably would not remain as a residual phase with large extents of melting. As proposed by Gorring et al. (1997), a more consistent scenario would be for melting to occur at a lower Tp of 1350°C on an H2O–CO2-undersaturated solidus. The presence of volatiles lowers the Tp at which peridotite melts (e.g. Kushiro, 1972; Green, 1973) and significantly increases extents of partial melting (e.g. Hirose & Kawamoto, 1995). Furthermore, experimental data from Gaetani & Grove (1994) indicate that garnet can be stable to lower pressures (e.g. 1·6 GPa at 1275°C) in the presence of H2O. Melting experiments at 1·0 GPa and H2O-saturated conditions have been shown to produce melts with higher SiO2 (Hirose & Kawamoto, 1995). The combination of high SiO2, moderate FeO, residual garnet signatures, and high melting percentages in western main-plateau lavas are best explained by volatile present melting at a depth of 70 km depth.
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MANTLE SOURCE REGIONS AND THE ROLE OF ADDITIONAL COMPONENTS |
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TOPABSTRACTINTRODUCTIONREGIONAL TECTONIC AND GEOLOGIC...PETROGRAPHYGEOCHEMISTRYTEMPORAL AND SPATIAL PATTERNS...P-T CONDITIONS FOR PARTIAL...MANTLE SOURCE REGIONS AND...PETROGENETIC MODEL FOR...DISCUSSIONCONCLUSIONSAPPENDIX A: ANALYTICAL METHODSAPPENDIX B: CRYSTAL...APPENDIX C: DISTRIBUTION...REFERENCES |
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The isotopic and trace element characteristics of Patagonian slab window lavas are consistent with their being predominantly derived from a mantle source with OIB-like trace element characteristics (Figs 6–12). This source has been interpreted to reflect the composition of both the supraslab mantle wedge and the subslab asthenospheric mantle that upwells through the opening slab window (see Fig. 3). However, distinctive chemical evidence (high SiO2 and Sr/La, low Ce/Pb and Nb/U, regional variations in Sr–Nd–Pb isotopes), particularly for western main-plateau lavas, strongly suggests that arc processes and/or continental lithospheric contamination have played an important role in Patagonian slab window lava petrogenesis. Additional components that need to be considered include subduction-related components (subducted sediments, slab melts, slab-derived fluids), and enriched continental lithospheric mantle and crustal components.
Continental crustal components
Enriched Sr–Nd–Pb isotopic signatures, relatively high SiO2 (52–55%) and Th/La ratios (0·15–0·2), and low Nb/U (<25), Ce/Pb (<10), and Rb/Cs (<50) ratios in main-plateau lavas from the western back-arc raise the issue of in situ crustal contamination by AFC processes or by asthenospheric source mantle contamination by subducted crustal materials. As illustrated in Fig. 16, main-plateau lavas form an array on a Ce/Pb vs 87Sr/86Sr plot that deviates from the OIB + MORB field toward crustal compositions. This array can be successively modeled either by in situ AFC (
20% crust) or by mantle source region contamination (2% subducted sediments). Main-plateau lavas also form similar arrays on plots of Nb/U and Th/La vs 87Sr/86Sr (not shown). Details of the contamination models place additional constraints on the composition of mantle source regions for Patagonian slab window lavas. The primitive main-plateau magma used in the AFC models is a 10% partial melt of the ‘50% 2 x chondrite–50% MORB-source’ in Appendix C. The use of this source and the extent of partial melting are justified by the results of the trace element modeling discussed above. This source has a 87Sr/86Sr ratio of 0·7032 and is based on mixing of a MORB source with 87Sr/86Sr of 0·7028 and an OIB-like asthenospheric source with 87Sr/86Sr of 0·7038 that would be appropriate for generating the post-plateau lavas (‘enriched source’ in Appendix C). The major implication of the modeling in Fig. 16 is that western main-plateau lavas require a more isotopically ‘depleted’ source before contamination than post-plateau lavas, to explain their low Ce/Pb at comparable (and relatively low) 87Sr/86Sr ratios.
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Additional support for minor crustal contamination comes from regional Pb isotopic variations in Patagonian lavas. Lavas from the Meseta Central and northeast region lavas have lower 206Pb/204Pb ratios than lavas from the Mesetas de la Muerte and Belgrano further west (Figs 11 and 12). Direct evidence for regional variations in crustal Pb isotopic composition is at present poorly defined. A single Pb isotope analysis (Gorring, 1997) of Middle Jurassic Chon–Aike rhyolite from the Cueva de los Manos region (see Fig. 2) has relatively high 206Pb/204Pb like slab window lavas from the western back-arc (Fig. 11). Two-pyroxene mafic granulite xenoliths from the Estancia Lote 17, from further east within the Desado Massif, have relatively low 206Pb/204Pb (Gorring, 1997), more similar to (but not as low as) lavas from the central and northeastern regions of the back-arc (Fig. 11). Alternatively, the similarity of Pb isotopic compositions of western slab window lavas to some Nazca Plate sediments (Unruh & Tatsumoto, 1976) and SVZ arc lavas (Hickey et al., 1986) could also reflect source region contamination by subducted sediments (Fig. 11). In this case, the subducted sediment signature is confined to the western back-arc and regional differences in Pb isotopic signatures in the basement are still reflected in the plateau lavas from further east.
Arc components
Chemical evidence for substantial arc-related components is lacking in most Patagonian slab window lavas. However, evidence for a weak arc signature is clearly shown by main-plateau lavas from the western back-arc (e.g. Mesetas de la Muerte and Belgrano). These lavas have slightly higher Ba/Ta, Ba/La, and La/Ta ratios, significantly higher Sr/La (>35) ratios, and lower Ce/Pb, Nb/U, Rb/Cs, and 87Sr/86Sr ratios (Figs 6–10) than most other Patagonian slab window lavas. The spatial restriction of this signature to the western back-arc is comparable with the west-to-east decrease in arc signatures for SVZ lavas (Hickey et al., 1986) and Plio-Pleistocene back-arc lavas from 38° to 52°S (Munoz & Stern, 1989; Stern et al., 1990).
Low Ce/Pb ratios (as well as low Nb/U and Rb/Cs) in western main-plateau lavas could be modeled as source region contamination by subducted sediments or tectonically eroded forearc crust as shown in Fig. 16. However, subducted crustal components cannot explain the high Sr/La ratios in most western main-plateau lavas (see Fig. 9), as both sediments in the Chile Trench and Patagonian continental crust have low Sr/La ratios (Behrmann & Kilian, 1995; Stern & Kilian, 1996). Therefore, high Sr/La is attributed to subduction-related components (e.g. slab melts, fluids, or arc magmas) that either were stored in the basal continental lithospheric mantle or had contaminated the supraslab asthenosphere, or both.
Enriched continental lithospheric components
Another source of contamination for asthenosphere-derived slab window lavas is the continental lithospheric mantle, which can become enriched in incompatible trace elements by infiltration of small-percentage partial melts of the asthenosphere (e.g. McKenzie & O’Nions, 1995), as well as by subduction-related processes (e.g. Fitton et al., 1991; Kepezhinskas et al., 1996). Over long time-scales (>1 Gy), enriched isotopic signatures can develop (e.g. Zindler & Hart, 1986). Patagonian continental lithosphere is thought to be relatively young (0·5 to 1·2 Ga; Ramos, 1988; Pankhurst et al., 1994) and derived from melt-depleted, MORB-source asthenosphere, and therefore is not likely to have evolved extreme isotopic signatures (e.g. Stern et al., 1989, 1990; Zartman et al., 1991). Constraints on the isotopic composition of the Patagonian lithosphere come from peridotite xenoliths from the Pali–Aike and Estancia Lote 17 localities. Garnet peridotite xenoliths from Pali–Aike (Stern et al., 1989, 1999) and spinel peridotites from Estancia Lote 17 (Gorring & Kay, 2000) have more depleted Sr–Nd isotopic compositions (87Sr/86Sr = 0·7027–0·7043; 143Nd/144Nd = 0·51312–0·51279) and have Pb isotope ratios similar to Pacific MORB that are unlike most Patagonian slab window lavas. This suggests that at least part of the continental lithospheric mantle may not have the appropriate composition to be an isotopically enriched (e.g. 87Sr/86Sr > 0·705; 143Nd/144Nd < 0·51264), end-member component for most Patagonian slab window lavas.
However, some Patagonian slab window lavas do show evidence for interaction with enriched lithospheric components. A few northeast region main-plateau lavas have relatively high 87Sr/86Sr (0·7046) and low 143Nd/144Nd (0·51271) ratios, and strong OIB-like trace element signatures that are best explained by an enriched lithospheric component (Fig. 16) that has yet to be documented by studies of Patagonian peridotite xenoliths. These lavas also have the lowest 206Pb/204Pb ratios, the most positive 7/4 and 8/4 values, and some of the most oceanic-like Ce/Pb and Nb/U ratios of all Patagonian slab window lavas (Fig. 8). This suggests that east–west regional variations in 206Pb/204Pb ratios could reflect EM1-type OIB components in the lithospheric mantle beneath the Deseado Massif.
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PETROGENETIC MODEL FOR PATAGONIAN SLAB WINDOW LAVAS |
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TOPABSTRACTINTRODUCTIONREGIONAL TECTONIC AND GEOLOGIC...PETROGRAPHYGEOCHEMISTRYTEMPORAL AND SPATIAL PATTERNS...P-T CONDITIONS FOR PARTIAL...MANTLE SOURCE REGIONS AND...PETROGENETIC MODEL FOR...DISCUSSIONCONCLUSIONSAPPENDIX A: ANALYTICAL METHODSAPPENDIX B: CRYSTAL...APPENDIX C: DISTRIBUTION...REFERENCES |
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A petrogenetic model for Patagonian slab window lavas that is consistent with their chemical characteristics, melting percentages and depths, and the tectonic model of Gorring et al. (1997)
is shown in Fig. 3. The model requires four components to account for the chemical variability of Patagonian lavas (see Fig. 16). These components are: (1) partial melts of a relatively homogeneous, OIB-like subslab asthenospheric mantle; (2) partial melts of the subducted Nazca Plate and/or ‘stored’ slab-derived fluid components in basal continental lithosphere; (3) a contribution from upper crust; (4) a contribution from an enriched lithospheric mantle component. Only three of the four components are necessary to describe any single lava; most can be described by one or two.
Western main-plateau lavas
Muerte, Belgrano, and most Central main-plateau lavas from the western back-arc fit a three-stage process illustrated in Fig. 17. In the first stage, small-percentage partial melts (8% batch melt) of the trailing Nazca Plate edge contaminate the upwelling OIB-like subslab asthenosphere (see Fig. 3). Although some segments of the Chile Rise near the Chile Trench are erupting lavas with anomalous trace element and isotopic compositions (Klein & Karsten, 1995; Sturm et al., 1999), there are no direct constraints on the composition of the subducted oceanic crust that would have melted beneath this region of the Patagonian back-arc at 10 Ma. Cerro Pampa adakites provide indirect evidence for partial melting of more ‘normal’ MORB beneath this part of the back-arc at 12 Ma (see Figs 10–12). Therefore, N-MORB is used as a first-order approximation to the trace element composition of the subducted oceanic crust (see Appendix C). Mixing of 1% of this slab melt with 99% of the pristine subslab asthenosphere produces the ‘modified slab window mantle’ shown in Fig. 17a. This mixture was found by forward modeling of trace element ratios and the Sr isotopic composition of a representative Muerte main-plateau lava (LC-21, Table 1) and represents the optimum slab melt-enriched mantle mixture that yields the required Sr/La ratios but still preserves the necessary LREE source abundances (e.g. 1 x primitive mantle). Melting of this ‘modified slab window mantle’ can explain the relatively low 87Sr/86Sr and LREE abundances, and high Sr/La ratios of most western main-plateau lavas. Source region contamination by slab melting is consistent with the predicted thermal condition of the Nazca Plate edge at this time (Peacock et al., 1994; Gorring et al., 1997) and the eruption of Cerro Pampa adakites (Kay et al., 1993) just before the eruption of the Muerte main-plateau lavas (Fig. 3).
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) is 8% batch melt of an eclogitic N-MORB composition (continuous line). Range of Cerro Pampa adakitic magmas in gray from Kay et al. (1993)
) of the ‘modified slab window mantle’ mixes with stored arc components at the base of the continental lithosphere to produce a model primary melt (
) is shown in gray.
In the second stage, the slab melt-enriched asthenosphere partially melts (12%) as it upwells through the slab window and impinges on the base of the continental lithosphere at 70 km depth (Figs 3 and 17b). An additional ‘stored’ Ba- and Sr-rich and Nb-poor slab-derived fluid or arc magma component in the lowermost lithosphere is added to the primary model melt. This component is necessary to model the very high Sr/La (>40) and low Nb/U (<20) ratios in some western main-plateau lavas. The gray areas beneath the model primary melt curve for LC-21 in Fig. 17b represent the contribution of this component to the overall arc signature. A hydrous ‘stored’ arc fluid–magma component is also consistent with the high SiO2 and Na2O concentrations in western main-plateau lavas. This component is attributed to subduction processes that occurred during earlier Mesozoic and Cenozoic subduction along the South American margin.
Finally, AFC processes occurred in crustal magma chambers as minor amounts (6%) of upper-crustal components are assimilated, which accounts for the low Ce/Pb ratios in LC-21 (Fig. 17c). The general two-stage model can explain the combination of high Sr/La, relatively low 87Sr/86Sr, and low Ce/Pb and Nb/U in other western main-plateau lavas. Small modifications to the proportions of slab melt–fluid, stored arc, and crustal components along with variable extents of melting of the modified slab window mantle source lead to successful modeling of other western main-plateau lavas. Crustal AFC also helps to explain the high SiO2 contents (Fig. 5) and possibly the distinctive regional Pb isotope variations (Figs 11 and 12).
Northeast region main-plateau lavas
Northeast region main-plateau lavas with high 87Sr/86Sr and strong OIB-like signatures fit a simpler two-stage model in which unmodified subslab asthenosphere melts and interacts with enriched components in the lowermost continental lithosphere (Fig. 3). Smaller percentages of partial melting (3–5%) and the lack of arc signatures in these lavas reflect the decreasing influence of slab melt source contamination and stored arc components in the lithospheric mantle eastward across the back-arc. This model requires that enriched lithospheric mantle components have high 87Sr/86Sr (
0·7046) and low 143Nd/144Nd ratios (0·51264).
Post-plateau lavas
The simplest magmas to explain are the post-plateau lavas, which can be modeled by a simple, one-stage process involving small-percentage partial melting (1–4%) of the pristine, OIB-like subslab asthenospheric end-member (Figs 13 and 16). However, some Meseta de la Muerte post-plateau lavas with relatively high 87Sr/86Sr and low Ce/Pb ratios (e.g. CL-7, S70; Tables 1 and 3; Fig. 16) require multi-component models involving OIB-like asthenospheric, enriched lithospheric, and/or crustal components to explain their chemistry.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONREGIONAL TECTONIC AND GEOLOGIC...PETROGRAPHYGEOCHEMISTRYTEMPORAL AND SPATIAL PATTERNS...P-T CONDITIONS FOR PARTIAL...MANTLE SOURCE REGIONS AND...PETROGENETIC MODEL FOR...DISCUSSIONCONCLUSIONSAPPENDIX A: ANALYTICAL METHODSAPPENDIX B: CRYSTAL...APPENDIX C: DISTRIBUTION...REFERENCES |
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Origin of the OIB-like subslab asthenosphere beneath Patagonia
Several workers have shown that the chemistry of slab window magmas dominantly reflects that of the subslab asthenosphere beneath active continental margins (e.g. Storey et al., 1989
; Cole & Basu, 1995; Hole et al., 1995; Thorkelson, 1996; Gorring et al., 1997). All of the geochemical evidence from Patagonian slab window lavas points to an OIB-like subslab asthenosphere. However, OIB-like subslab asthenosphere generally conflicts with the most widely accepted models that depict the upper mantle as mostly depleted MORB source mantle (e.g. Carlson, 1995; Hofmann, 1997). Possible models to explain OIB-like subslab asthenosphere beneath Patagonia include the following: (1) the ascent of deep mantle plumes (e.g. Cheadle & Petford, 1993); (2) lateral flow of enriched asthenosphere from southeastern Pacific or South Atlantic plume activity; (3) enriched, OIB-like heterogeneities (e.g. Hickey et al., 1989; Stern et al., 1990); (4) the entrainment of a ‘weak’ plume in the upper mantle (e.g. Gorring et al., 1997).
The direct enrichment from high-temperature, deep mantle plumes is unlikely in view of the lack of geophysical evidence (e.g. hotspot track, topographic swell; Ramos & Kay, 1992; Gorring et al., 1997). Furthermore, global seismic tomography generally shows that the deep mantle (>800 km) beneath Patagonia is not anomalously hot (Zhang & Tanimoto, 1993; Grand et al., 1997). This model would also require invoking the seemingly fortuitous coincidence of simultaneous upwelling of deep mantle plumes and the opening of slab windows.
Numerous workers have suggested that anomalous OIB-like chemical signals in some MORB reflect long-distance dispersal and lateral transport of plume components by shallow mantle convection (e.g. Mahoney et al., 1994; Michael, 1994; Storey, 1995; Johnston & Thorkelson, 1997; Niu et al., 1999). For example, Mahoney et al. (1994) and Niu et al. (1999) interpreted geochemical anomalies along the southern and northern East Pacific Rise as due to the eastward transport of French Polynesian and Hawaiian plume components, respectively. Contamination by lateral flow of enriched asthenosphere associated with plume-related breakup of Gondwana has also been invoked to explain anomalous Indian Ocean-type asthenosphere beneath southeast Asia (Hoang et al., 1996) and western Pacific marginal basins (Hickey-Vargas et al., 1995). Johnston & Thorkelson (1997) suggested that northeastward flow of enriched asthenosphere from the Galapagos plume through a slab window in Central American arc could explain the OIB-like characteristics of Neogene arc lavas in southern Costa Rica and Panama.
Although certainly plausible as an indirect mechanism for enriched upper-mantle heterogeneity, models that invoke lateral transport of plume-derived materials are unlikely for the direct enrichment of the subslab Patagonian mantle. The closest Pacific hotspot is more than 1000 km to the north, in the Juan Fernandez Islands. The Easter Microplate is a distant second at more than 5000 km to the west. Basalts from both the Easter Microplate and Juan Fernandez are distinctly different in their Pb isotope signatures (e.g. 206Pb/204Pb >18·9, Gerlach et al., 1986; Poreda et al., 1993) compared with most Patagonian slab window lavas. Furthermore, Easter Microplate and Juan Fernandez lavas also have much high 3He/4He signatures (e.g. R/RA = 8–18; Farley et al., 1993; Poreda et al., 1993; where R is the sample 3He/4He and RA is 3He/4He of air) compared with anomalous Chile Ridge MORB near the Chile Triple Junction (R/RA < 8; Sturm et al., 1999). Thus, it appears that enriched components derived from known eastern Pacific hotspots are not present in the Patagonian subslab mantle. Westward flow of enriched asthenosphere from South Atlantic plumes could have affected the supraslab asthenosphere in Patagonia, but subslab asthenosphere would have been shielded from contamination by subducting oceanic lithosphere since the breakup of Gondwana.
Partial melting of a heterogeneous, ‘plum-pudding’ subslab mantle (e.g. Zindler et al., 1984; Allègre & Turcotte, 1986) would be expected to lead to correlations between chemical parameters sensitive to the extent of partial melting and isotopic composition. To a first order, Patagonian slab window lavas lack the systematic isotopic variations between main- and post-plateau lavas needed to argue for melting of a ‘plum-pudding’ asthenosphere. Stern et al. (1990) made similar arguments based on a limited number of samples from a wider region of the Patagonian back-arc (38–52°S). However, chemical correlations could easily be obscured by complex interactions between upwelling subslab asthenosphere, subducted oceanic lithosphere, and continental lithosphere in the slab window setting. The model presented in Fig. 17 has flexibility to allow for main-plateau lavas to be derived from melting of a greater proportion of depleted MORB-type ‘pudding’; whereas the post-plateau lavas would mostly represent melting of enriched OIB-like ‘plum’ material.
An important aspect of Patagonian slab window magmatism not easily explained by any of the models is the absence of mid-Miocene slab window magmatism south of 49°S associated with the ridge collision that occurred at 13–14 Ma (see Fig. 1). Gorring et al. (1997) proposed that a pre-existing, small-scale convection cell or ‘weak’ plume (e.g. Turcotte, 1995; Malamud & Turcotte, 1999) was entrained in the opening slab window opposite the ridge segment that collided at 12 Ma. Weak plumes originating in the upper mantle (perhaps 670 km), would be only slightly hotter than normal mantle (e.g. Tp 1350–1400°C), and would not impart the geophysical signature of stronger plumes originating from the deep mantle (Turcotte, 1995). In British Columbia, recent teleseismic tomography has imaged a low-velocity anomaly where a slab window lies beneath the northern Cordillera (Frederiksen et al., 1998). This low-velocity anomaly is interpreted to represent a thermal anomaly of 100–200°C caused by upwelling asthenosphere through the slab window (Frederiksen et al., 1998). Thus, weak plume-like convection in the shallow mantle as a result of opening of slab windows may allow for slightly hotter than normal mantle to advectively upwell. The weak plume model is attractive because it could explain both the OIB-like subslab asthenospheric chemistry and the lack of mid-Miocene slab window lavas south of 49·5°S. Therefore, we suggest that regional differences in subslab asthenospheric temperature and/or volatile content associated with the presence (or absence) of weak plumes may control the timing, distribution, and chemistry of slab window-related magmatism in southern Patagonia.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONREGIONAL TECTONIC AND GEOLOGIC...PETROGRAPHYGEOCHEMISTRYTEMPORAL AND SPATIAL PATTERNS...P-T CONDITIONS FOR PARTIAL...MANTLE SOURCE REGIONS AND...PETROGENETIC MODEL FOR...DISCUSSIONCONCLUSIONSAPPENDIX A: ANALYTICAL METHODSAPPENDIX B: CRYSTAL...APPENDIX C: DISTRIBUTION...REFERENCES |
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Patagonian slab window lavas studied here erupted in response to the collision of a segment of the Chile Ridge at
12 Ma and the formation of asthenospheric slab windows between the subducting Nazca and Antarctic Plates. The lavas are divided into an older (12–5 Ma), voluminous, tholeiitic, main-plateau sequence and a younger (7–2 Ma), less voluminous, alkaline, post-plateau sequence (Gorring et al., 1997). First-order differences in major and trace element abundances between the two sequences are be modeled by variations in extents of partial melting of an OIB-like subslab asthenospheric source. Trace element modeling indicates that both main- and post-plateau lavas were generated within the garnet stability field and represent 7–15% and 1–4% partial melts, respectively. Major and trace element modeling indicates partial melting occurred at a Tp of 1350°C along an H2O–CO2-undersaturated solidus at depths between 65 and 75 km consistent with melting at or below the garnet stability field near the base of the Patagonian continental lithosphere.
Post-plateau lavas can be modeled by a simple one-stage melting model of a dominant OIB-like subslab asthenospheric source characterized by 87Sr/86Sr 0·7038 ± 0·0003 and 143Nd/144Nd 0·51275 ± 0·00002. Homogeneous post-plateau lava chemical signatures reflect replacement of the supraslab asthenospheric wedge with pristine OIB-like subslab asthenosphere coupled with relatively weak mantle flow through a widening slab window. In contrast, western main-plateau lavas show evidence for arc and crustal components indicated by consistently low Nb/U, Ce/Pb, Rb/Cs, and high Sr/La ratios, high SiO2 and Na2O concentrations, and relatively low 87Sr/86Sr and high 143Nd/144Nd ratios. These signatures reflect a three-stage model: (1) source region contamination (1%) of an OIB-like subslab asthenosphere by small-degree (8%) partial melts of the trailing Nazca Plate edge; (2) partial melting of the slab-melt modified, subslab asthenospheric source and mixing with ‘stored’ arc components at the base of the continental lithosphere; (3) minor crustal contamination (<20%) by in situ AFC processes in the crust. West to east decreases in extent of partial melting and arc signatures in main-plateau lavas result from decreasing amounts of ‘stored’ arc components and the decreasing intensity of mantle upwelling as the Nazca Plate edge subducts to greater depths. Main-plateau lavas from the northeast region with strong OIB-like trace element signatures and high 87Sr/86Sr (0·7046) and low 143Nd/144Nd (0·51271) ratios indicate an enriched continental lithospheric component in these lavas. Systematic east–west differences in 206Pb/204Pb ratios in both sequences reflect the influence of regionally distinctive Patagonian crustal and/or lithospheric mantle components.
The relatively enriched OIB-like signatures of Patagonian lavas contrast with other slab window-related lavas from western North America (Johnson & O’Neil, 1984; Storey et al., 1989; Thorkelson & Taylor, 1989; Cole & Basu, 1995; Luhr et al., 1995) and the Antarctic Peninsula (Hole, 1988, 1990) that have more ‘depleted’ MORB- or HIMU-like chemistry. The anomalous chemistry of Patagonian slab window lavas cannot be entirely explained by continental lithospheric contamination or by upwelling of deep mantle plumes, and therefore, they are attributed to OIB-like asthenospheric sources in the upper mantle (e.g. Ramos & Kay, 1992; Gorring et al., 1997). Possible explanations are (1) a small-scale thermal anomaly or ‘weak plume’ (e.g. Turcotte, 1995) that existed in the upper mantle before the opening of slab windows beneath Patagonia (Gorring et al., 1997) and/or (2) melting of small-scale, enriched OIB-like heterogeneities in a more ‘depleted’, MORB-like subslab asthenosphere.
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APPENDIX A: ANALYTICAL METHODS |
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TOPABSTRACTINTRODUCTIONREGIONAL TECTONIC AND GEOLOGIC...PETROGRAPHYGEOCHEMISTRYTEMPORAL AND SPATIAL PATTERNS...P-T CONDITIONS FOR PARTIAL...MANTLE SOURCE REGIONS AND...PETROGENETIC MODEL FOR...DISCUSSIONCONCLUSIONSAPPENDIX A: ANALYTICAL METHODSAPPENDIX B: CRYSTAL...APPENDIX C: DISTRIBUTION...REFERENCES |
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All samples were sawn into slabs, reduced to 0·25–0·5 cm size in a steel mortar, and pulverized in an aluminum oxide ceramic shatterbox. Major elements were determined by electron microprobe analysis on fused glasses using a JEOL-733 Superprobe at Cornell University. Techniques and standards used for microprobe analyses of major elements have been given by Kay et al. (1987)
. Precision and accuracy (2
) are ±1–5% for elements at >1 wt % and ±10–20% at <1 wt % abundance levels based on replicate analysis of basaltic glass standards.
Trace elements were determined by INAA and by ICP-MS at Cornell University. Techniques and standards have been described by Kay et al. (1987), Cheatham et al. (1993), and White & Duncan (1996). INAA precision and accuracy based on replicate analysis of an internal basalt standard are 2–5% (2) for most elements and ±10% for U, Sr, Nd, and Ni. ICP-MS analyses were carried out on a VG PlasmaQuad PQ2+ in peak-jumping mode for U, Th, and Pb, and in scan mode for all other elements. ICP-MS analytical precision and accuracy are ±5–10% for all elements.
Sr, Nd and Pb isotopes were analyzed at Cornell University on a multi-collector VG Sector 54 thermal ionization mass spectrometer. Chemistry and analytical techniques were summarized by White & Duncan (1996). Pb isotope ratios were corrected for mass fractionation assuming NBS981 Pb values of 206Pb/204Pb = 16·937, 207Pb/204Pb = 15·493, 208Pb/204Pb = 36·705. Average measured values for NBS981 were 206Pb/204Pb = 16·895 ± 20, 207Pb/204Pb = 15·438 ± 20, 208Pb/204Pb = 36·541 ± 60 based on 40 analyses. Sr and Nd isotope ratios were corrected for mass fractionation assuming 86Sr/88Sr = 0·1194 and 146Nd/144Nd = 0·7219. Average measured value for NBS987 Sr standard was 87Sr/86Sr = 0·710235 ± 34 (2) based on 67 analyses. The La Jolla Nd standard was 143Nd/144Nd = 0·511864 ± 14 (2) based on 10 analyses from July 1993 to August 1997 and 0·511817 ± 12 (2) from August to November 1990 based on 15 analyses. Total procedural blanks for Sr, Nd, and Pb were consistently <150 pg, and thus negligible. No blank or age corrections were made.
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APPENDIX B: CRYSTAL FRACTIONATION MODELING |
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TOPABSTRACTINTRODUCTIONREGIONAL TECTONIC AND GEOLOGIC...PETROGRAPHYGEOCHEMISTRYTEMPORAL AND SPATIAL PATTERNS...P-T CONDITIONS FOR PARTIAL...MANTLE SOURCE REGIONS AND...PETROGENETIC MODEL FOR...DISCUSSIONCONCLUSIONSAPPENDIX A: ANALYTICAL METHODSAPPENDIX B: CRYSTAL...APPENDIX C: DISTRIBUTION...REFERENCES |
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For the purposes of constructing major element liquid lines of descent (LLD) and correcting trace element compositions for crystal fractionation an incremental addition technique (e.g. Francis & Ludden, 1995
; Kinzler, 1997) was used to calculate approximate primary magma compositions of all Patagonian slab window lavas with >5% MgO. This technique involves iteratively adding equilibrium solid phases (olivine, clinopyroxene, plagioclase, titanomagnetite, Cr-spinel) to calculated melts in 1% increments until the calculated liquid is in equilibrium with mantle olivine with composition of Fo90. Olivine, clinopyroxene, and plagioclase compositions were computed at each increment based on mineral–melt equilibria, stoichiometry, and experimental data following the procedures of Grove et al. (1992). Spinel compositions were computed by linear parameterization of Al2O3, FeOT, MgO, and Cr2O3 variations of spinel inclusions with Fo-number of host olivine phenocrysts in Hawaiian picrites (Wilkinson & Hensel, 1988). Titanomagnetite compositions are simple mixtures of pure stoichiometric magnetite and ilmenite in 3:1 (main-plateau) and 2:1 (post-plateau) proportions. Least-squares models of the few viable parent–daughter pairs were used to constrain average fractionating mineral proportions for all lavas with 5–10% MgO. Within this range, the main-plateau lavas are modeled by incremental addition of an assemblage consisting of 40% olivine, 30% plagioclase, 24% clinopyroxene, 4% titanomagnetite, and 2% Cr-spinel. Post-plateau lavas are modeled by incremental addition of an assemblage of 60% clinopyroxene, 35% olivine, 3% titanomagnetite, and 2% Cr-spinel. For calculated liquids with >10% MgO, the fractionating assemblage was assumed to be 97% olivine and 3% Cr-spinel (e.g. Francis & Ludden, 1995). |
APPENDIX C: DISTRIBUTION COEFFICIENTS AND MANTLE SOURCES |
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TOPABSTRACTINTRODUCTIONREGIONAL TECTONIC AND GEOLOGIC...PETROGRAPHYGEOCHEMISTRYTEMPORAL AND SPATIAL PATTERNS...P-T CONDITIONS FOR PARTIAL...MANTLE SOURCE REGIONS AND...PETROGENETIC MODEL FOR...DISCUSSIONCONCLUSIONSAPPENDIX A: ANALYTICAL METHODSAPPENDIX B: CRYSTAL...APPENDIX C: DISTRIBUTION...REFERENCES |
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Distribution coefficients and mantle source compositions used for modeling partial melting and crystallization are given in Table C1 and C2, respectively. Values for distribution coefficients were compiled from the literature from numerous sources, many of which were referenced by Gurenko & Chaussidon (1995)
and McKenzie & O’Nions (1995). Some values have been extrapolated (e.g. Tb, Ho, Tm, and Lu) or arbitrarily set (e.g. P, Ta = Nb, and Pb = Ce) because of the lack of experimental data.
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ACKNOWLEDGEMENTS |
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The authors would like to give special thanks to Victor Ramos for his expert knowledge of the regional geologic framework and for collecting many of the original samples for the project. Invaluable effort and assistance in the field by Daniel Rubiolo and Marisa Fernandez are also gratefully acknowledged. We would also like to thank José Panza and Graciala Marín for graciously supplying detailed maps and literature of the area. Assistance in the laboratory, particularly the efforts of Mike Cheatham, Linda Godfrey, and Andy Kurtz with TIMS and ICP-MS, John Hunt on the electron microprobe, and the entire staff at Ward Laboratory with INAA are greatly appreciated. This research also benefited from discussions with R. W. Kay, D. Turcotte, W. M. White, M. J. Hole, W. McDonough, and C. Langmuir. The manuscript was greatly improved by constructive comments and reviews by M. Wilson, D. Thorkelson, C. Stern, and an anonymous reviewer. This research was supported by NSF grant EAR-9219328, GSA grant 5156-93, the Cornell Chapter of Sigma Xi, and the Servicio Geológico Nacional de Argentina. This is a contribution to IGCP project 345, ‘Lithospheric Evolution of the Andean Continental Margin’.
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FOOTNOTES |
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*Corresponding author. Present address: Department of Earth and Environmental Studies, Montclair State University, Upper Montclair, NJ 07043, USA. E-mail: gorringm{at}mail.montclair.edu
Extended dataset can be found at http://www.petrology.oupjournals.org 
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REFERENCES |
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TOPABSTRACTINTRODUCTIONREGIONAL TECTONIC AND GEOLOGIC...PETROGRAPHYGEOCHEMISTRYTEMPORAL AND SPATIAL PATTERNS...P-T CONDITIONS FOR PARTIAL...MANTLE SOURCE REGIONS AND...PETROGENETIC MODEL FOR...DISCUSSIONCONCLUSIONSAPPENDIX A: ANALYTICAL METHODSAPPENDIX B: CRYSTAL...APPENDIX C: DISTRIBUTION...REFERENCES |
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