Journal of Petrology Advance Access originally published online on September 21, 2006
Journal of Petrology 2007 48(1):43-77; doi:10.1093/petrology/egl053
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The Somuncura Large Igneous Province in Patagonia: Interaction of a Transient Mantle Thermal Anomaly with a Subducting Slab
1Earth and Atmospheric Sciences, Snee Hall, Cornell University, Ithaca, NY 14853, USA
2Dirección Nacional de Minería and Geología, Buenos Aires, Argentina
3Earth and Environmental Studies, Montclair State University, Upper Montclair, NJ 07043, USA
4Ciencias Exactas and Naturales, Ciudad Universitaria Pabellón II, 1428 Buenos Aires, Argentina
RECEIVED JUNE 13, 2005; ACCEPTED AUGUST 23, 2006
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONREGIONAL SETTING AND GENERAL...PHYSICAL CHARACTERISTICS,...DISCUSSION: ORIGIN OF THE...EVOLUTION OF THE SOMUNCURA...CONCLUSIONSSUPPLEMENTARY DATAAPPENDIXREFERENCES |
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The Oligo-Miocene Somuncura province is the largest (
55 000 km2) back-arc mafic volcanic field in Patagonia, and one of Earth's largest with no clear link to a hotspot or major extension. Major and trace element and Sr–Nd–Pb isotopic data suggest involvement of a plume-like component in the mantle magma source mixed with hydrous, but not high field strength element (HFSE)-depleted components, from a disintegrating subducting plate. Magmatism is attributed to mantle upwelling related to disturbances during plate reorganization, possibly at a time when the South America plate was nearly stationary over the underlying mantle. Melting was enhanced by hydration of the mantle during Paleogene subduction. Crustal contamination was minimal in a refractory crust that had been extensively melted in the Jurassic. Eruption began with low-volume intraplate alkaline mafic flows with depleted Nd–Sr isotopic signatures. These were followed by voluminous 29–25 Ma tholeiitic mafic flows with flat light and steep heavy rare earth element (REE) patterns, intraplate-like La/Ta ratios, arc-like Ba/La ratios and enriched Sr–Nd isotopic signatures. Their source can be explained by mixing EM1–Tristan da Cunha-like and depleted mantle components with subduction-related components. Post-plateau 24–17 Ma alkaline flows with steep REE patterns, high incompatible element abundances, and depleted Sr–Nd isotopic signatures mark the ebbing of the mantle upwelling. KEY WORDS: Somuncura plateau; slab interaction; Patagonia; large igneous province (LIP); plume-like upwelling
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONREGIONAL SETTING AND GENERAL...PHYSICAL CHARACTERISTICS,...DISCUSSION: ORIGIN OF THE...EVOLUTION OF THE SOMUNCURA...CONCLUSIONSSUPPLEMENTARY DATAAPPENDIXREFERENCES |
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Large basaltic provinces are commonly attributed to the activity of mantle plumes, recognized by associated regional thermal uplift and the time-transgressive tracks of the volcanic centers (e.g. Coffin & Eldholm, 1994
). Numerous papers have focused on large igneous provinces (LIPs) such as Hawaii, the Paraná continental flood basalt province in South America, and continental rift volcanic fields such as those in Ethiopia. Less discussed have been intermediate-size volcanic provinces whose origins are not easily related to intracontinental rifting, mantle hotspots or extensive back-arc rifting. The purpose of this paper is to examine the geochemistry of one of the more enigmatic of these provinces, the Somuncura province of northern Patagonia (40·5° to 43°S; Figs 1 and 2). This province consists of a series of Oligocene to early Miocene volcanic fields that cover more than 55 000 km2 in the Meseta de Somuncura and surrounding region.
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). Early Miocene high-K volcanic rocks and lower crustal xenoliths occur in the Plan-Luan region, high-Na volcanic rocks occur in the Queupuniyeu region, and mantle xenoliths occur in late Oligocene flows in the Praguaniyeu region. The Nirihuau basin is composed of Miocene volcaniclastic and sedimentary rocks. The figure is based on maps by Rapela & Kay (1988Regionally, the Somuncura province is the largest of the post-Eocene mafic volcanic fields east of the Andean arc in Patagonia (Fig. 1) and the most difficult to explain in a geodynamic context. Further to the south, the extensive Miocene to Holocene plateau volcanic fields east of the Chile Triple Junction near 46·5° to 49·5°S have been explained in terms of mantle melting related to the opening of an asthenospheric slab window in the subducting plate after collision of the Chile ridge with the Chile trench (Ramos & Kay, 1992
; Gorring et al., 1997; Gorring & Kay, 2001). To the north, the extensive Pliocene to Holocene ‘Payenia’ volcanic province east of the active Southern Volcanic Zone arc at 35° to 38°S has been related to melting of a hydrated mantle wedge above a steepening subduction zone (Kay, 2001; Kay et al., 2004, 2006). The Somuncura province resembles these provinces in that there is little evidence for extension in the back-arc at the time of the eruptions (e.g. Kay et al., 1993; Ardolino et al., 1999). The only obvious contemporaneous tectonic association is that the Somuncura province eruptions preceded and coincided with the breakup of the Farallón plate, which caused a major change from oblique to near-normal convergence along the Andean margin (e.g. Cande & Leslie, 1986; Somoza, 1998). Kay et al. (1992, 1993) suggested that the Somuncura province was associated with a transient mantle thermal anomaly at the time of plate reorganization; de Ignacio et al. (2001) speculated that the thermal anomaly was due to a shallow asthenospheric upwelling caused by a concave-up slab geometry; and Muñoz et al. (2000) argued for an association with an asthenospheric slab window.
Within this geotectonic framework, the pre-plateau, plateau and post-plateau volcanic sequences of the Somuncura province are described. Major and trace element chemistry for 120 samples, 143Nd/144Nd and 87Sr/86Sr ratios for 34 samples, Pb isotopic analyses for 14 samples, and total-fusion 40Ar/39Ar ages for four samples are presented. These data are used to constrain the mantle and crustal sources and the tectonic setting of the Somuncura province magmas. The argument is made that the mantle signatures are little overprinted by crustal ones in accord with eruption through a crust whose low-temperature melting fraction was largely depleted in a Jurassic melting event (Kay et al., 1989; Pankhurst & Rapela, 1995). A change from intraplate-like chemical signatures in the low-volume pre-plateau lavas to arc-like fluid-mobile element signatures in the voluminous plateau lavas is argued to reflect incorporation of subducted components into the mantle source as the plateau magmas formed. Tectonically, the Somuncura province is linked to a regionally hot mantle, preconditioned to melt by hydration related to Tertiary subduction. Melting is argued to be provoked by mantle instabilities related to plate reorganization, and possibly to a near-stationary position of the South American plate over the underlying mantle.
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REGIONAL SETTING AND GENERAL DISTRIBUTION OF THE SOMUNCURA PROVINCE VOLCANIC ROCKS |
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TOPABSTRACTINTRODUCTIONREGIONAL SETTING AND GENERAL...PHYSICAL CHARACTERISTICS,...DISCUSSION: ORIGIN OF THE...EVOLUTION OF THE SOMUNCURA...CONCLUSIONSSUPPLEMENTARY DATAAPPENDIXREFERENCES |
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The late Oligocene to Miocene volcanic rocks of the Somuncura province (Figs 2 and 3) overlie a late Precambrian to Paleozoic magmatic and metamorphic basement that is covered by the extensive Jurassic silicic volcanic rocks of the Chon Aike province (e.g. Kay et al., 1989
; Pankhurst & Rapela, 1995) as well as Cretaceous to Tertiary volcanic and sedimentary rocks (e.g. Rapela & Kay, 1988; Rapela et al., 1988; Ardolino et al., 1999). The location of the Somuncura province lavas relative to other Eocene to Miocene volcanic fields in the region is shown in Fig. 2b and the regional stratigraphy is shown in Table 1. Units shown in Fig. 2b include: (1) the Eocene to early Oligocene back-arc alkaline rocks in Argentina generally included in the El Buitre Formation (see Coira et al., 1985; Lema & Cortés, 1987; Ardolino & Franchi, 1993; Ardolino et al., 1999); (2) the arc and near back-arc volcanic rocks of the Eocene Huitrera and late Oligocene to early Miocene Ventana Formations to the west (e.g. Rapela et al., 1988); and (3) the late Oligocene to middle Miocene arc and fore-arc rocks in Chile (Muñoz et al., 2000).
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, population centers). Map is based on Coira et al. (1985), Ardolino & Franchi (1993) and Servicio Geológico Minero Argentino (SERNAMIN) geological maps of the provinces of Rio Negro and Chubut.
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The Somuncura province volcanic rocks consist of mafic lava flows and smaller volumes of silicic volcanic rocks, associated with large shield volcanoes. The most important centers seen on Thematic Mapper satellite images (Fig. 2a) are associated with silicic volcanic rocks in the Alta Sierra de Somuncura, and the Sierras of Telsen, Chacays, Apas and Talagapa (Fig. 3; see also Corbella, 1984
; Remesal, 1988). Other possible centers are near Maquinchao and in the Meseta de Carri Laufquen region. Coalesced mafic flows from centers in the central and eastern parts of the region largely cover and surround the Meseta de Somuncura. This largely undissected plateau stands 200–700 m above the regional base level and has peaks rising to over 2000 m in the Alta Sierra de Somuncura. Some of the mafic flows from the Alta Sierra de Somuncura area reach the La Angostura, Ramos Mexía, and Aguada Cecilio regions, over 120 km to the north and east (Fig. 3). Observed flow sequence thicknesses range from several meters on the outer edge of the plateau to over 110 m in the Ranquil Huao valley (Fig. 3). Total thicknesses beneath the central Somuncura plateau are unknown. An indication of the initial area covered by the flows comes from remnants that cover more than 50 000 km2 in the Meseta de Somuncura and 6600 km2 in the Mesetas de Carri Laufquen, Coli Toro, and surrounding region (Fig. 3). For comparison, the flow remnants cover about a third of the estimated pre-erosional area (164 000 ± 5000 km2) of the Columbia River province in North America, the total volume of which is estimated at 174 000 ± 31 000 km3 (Tolan et al., 1989).
Subdivisions of Somuncura province volcanic rocks on existing regional geological maps are inconsistent as they have traditionally been mapped using rock types, stratigraphic criteria and K/Ar ages rather than in relation to eruptive centers (see Ardolino & Franchi, 1993; Ardolino et al., 1999; Remesal et al. 2002). The volcanic classification scheme used below and on the maps in Fig. 3 is shown in Table 1 relative to the regional formation names used by Ardolino et al. (1999). Roman numerals have been added to formation names when necessary to clarify age differences. The major volcanic divisions shown on the map in Fig. 3 are: (1) small-volume Eocene back-arc mafic lavas (El Buitre Formation); (2) small-volume mafic pre-plateau and voluminous plateau flows (respectively Somún Curá Formations I and II) that regionally overlie Sarmiento Formation I silicic tuffs that are not shown in Fig. 3; (3) mafic post-plateau flows (Quiñelaf Formation); (4) intermediate to silicic volcanic sequences (Sarmiento II, Quiñelaf Formations I and II). In the discussion below, the plateau units are locally subdivided into early, main and late divisions. Neither these divisions nor the pre-plateau flows are shown in Fig. 3 as they are not well differentiated across the entire region. Other Miocene volcanic rocks whose general localities are shown on Fig. 2b are the nephelinite, basinite, tephrite and phonolite flows described by Corbella (1984, 1989a, 1989b).
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PHYSICAL CHARACTERISTICS, CHEMISTRY AND AGES OF THE SOMUNCURA PROVINCE VOLCANIC ROCKS |
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TOPABSTRACTINTRODUCTIONREGIONAL SETTING AND GENERAL...PHYSICAL CHARACTERISTICS,...DISCUSSION: ORIGIN OF THE...EVOLUTION OF THE SOMUNCURA...CONCLUSIONSSUPPLEMENTARY DATAAPPENDIXREFERENCES |
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The descriptions of the physical characteristics, ages, petrology and geochemistry of the Somuncura pre-plateau, plateau and post-plateau volcanic groups below are based on published information, new field data, new 40Ar/39Ar ages in Table 2, the summary of K/Ar ages given by Ardolino & Franchi (1993
), new major and trace element data in Tables 3–6 and Electronic Appendix Tables 1–5
that can be downloaded from http://petrology.oxfordjournals.org/, and new Sr, Nd and Pb isotopic data in Table 7. The data are plotted in Figs 4–12. The new analyses are for samples from across the region with an emphasis on sequences from the Ranquil Huao–Telsen region, the Rincón Grande and Coña Niyeu areas, the Chipauquil area south of Valcheta, north and east of Gan Gan, south and east of Maquinchao, the south–central Meseta Carri Laufquen, and south of the village of Comallo (Fig. 3). Sample locations are indicated in Fig. 3 and described in Table A1.
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, pre-plateau lavas; {
}, plateau lavas; {
}, post-plateau lavas; gray filled circles, silicic volcanic rocks.
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Analytical methods
Major element analyses were performed on a JEOL-733 Superprobe electron microprobe on glasses made from whole-rock powders in the facilities of the Materials Science Center at Cornell University. Samples were ground in a shatter-box in alumina containers. Silicic glasses contain a Li2B4O7 flux. Analyses are averages of 4–6 spots in wavelength-dispersive mode with a 15 kV accelerating voltage, 15 nA beam current, 40 s count time, and a 30 µm beam diameter. Typical 2
precision is ±1–5% relative at >1 wt %.
Trace elements were analyzed by instrumental neutron activation analysis (INAA) using the techniques and standards discussed by Kay et al. (1987). Powders were packed in ultrapure Suprasil® quartz tubes, irradiated in a TRIGA reactor in Ward Laboratory at Cornell University at a power level of 400 kW for 3–4 h, and counted for 4–10 h after 6 and 40 days, respectively, on an Ortec intrinsic Ge detector. Precision (2) based on multiple analyses of Cornell internal standard PAL is ±2% for all elements except U, Sr, Nd, and Ni, which have precisions of ±8–12%. Y, Rb, Sr and Zr concentrations in samples GG2, GG5 and M12 were determined by X-ray fluorescence (XRF) analysis at the Universidad Nacional de Jujuy in Argentina using US Geological Survey (USGS) standards.
Isotopic ratios were measured on a multi-collector VG Sector 54 thermal ionization mass spectrometer in the Keck Geochemistry Laboratory at Cornell University using the chemical and analytical techniques described by White & Duncan (1996). Average standard values for over 50 measurements of NBS987 give 87Sr/86Sr = 0·710235 ± 34 (2); La Jolla 143Nd/144Nd = 0·511864 ± 14 (2); and NBS981 206Pb/204Pb = 16·895 ± 20, 207Pb/204Pb =15·438 ± 20 and 208Pb/204Pb = 36·541 ± 60. Mass fractionation corrections assume 86Sr/88Sr = 0·1194, 146Nd/144Nd = 0·7219, 206Pb/204Pb = 16·937, 207Pb/204Pb = 15·493 and 208Pb/204Pb = 36·705. Total procedural blanks for Sr, Nd, and Pb are <150 pg. 87Sr/88Sr and 143Nd/144Nd ratios are age-corrected based on trace element data in Tables 3–6 with Rb concentrations being estimated from similar analyses by Remesal (1988), Corbella (1989a), Corbella & Barbieri (1989) and Remesal et al. (2002) where unavailable. Errors from Rb estimation are small as Sr/Rb ratios are all 25 or greater.
40Ar/39Ar analyses were carried out by total fusion of bulk samples (10–350 mg, 250–350 µm size fraction) at Lehigh University on a VG3600 mass spectrometer operating in static mode at an accelerating voltage of 4·5 kV. Age uncertainties include an error in the J factor of <2%. All isotopic ratios are corrected for line blank contributions, mass discrimination, orifice corrections, and interfering reactions on K and Ca. Details of the analytical procedure have been given by Gorring et al. (1997).
Pre-plateau flows
On a regional scale, the pre-plateau flows erupted back-arc to a broad region of arc volcanic centers that were most active between 32 and 29 Ma (Cazau et al., 1987; Rapela et al., 1988; Muñoz et al., 2000). Volcanic rocks from these centers comprise the Ventana Formation in Argentina (see Rapela et al., 1988) and also occur near the Chilean coast some 300 km to the west (Muñoz et al., 2000). These centers erupted east of the subducting Farallón plate, which was converging at an angle of 55–60° NE with the South America plate at a velocity of <5 cm/year (Somoza, 1998).
The pre-plateau flows are volumetrically minor, back-arc alkali basalts and hawaiites that erupted from small volcanic centers that are now largely obscured by erosion or a cover of younger flows. Their overall distribution is not well known. The two localities examined here are the basal flows in the Quebrada Ranquil Huao and the Sierra de Chacays (Fig. 3). The setting of the Ranquil Huao flows has been described by Remesal et al. (2002). Their absolute age is uncertain. The age of the basal Sierra de Chacays flow is constrained by a new 40Ar/39Ar total-fusion age of 29·2 ± 1·5 Ma in Table 2.
The pre-plateau flows are chemically distinct from the younger plateau and post-plateau flows. Those in the Ranquil Huao sequence are olivine–clinopyroxene-bearing basanites and olivine–clinopyroxene–plagioclase-bearing hawaiites (Remesal et al. 2002), whereas those in the Sierra de Chacays are olivine–pyroxene–plagioclase-bearing alkali olivine basalts (Table 3 and Fig. 4). The Ranquil Huao flows are notable among Somuncura province flows for their high light rare earth element (LREE) contents (Ce = 85–150) and steep REE patterns (La/Yb = 24–34, La/Sm = 6·6–8·2; Figs 5a and 6); low ratios of alkali, alkaline earth, and Th to REE and high field strength elements (HFSE) (e.g. Ba/La < 13; Ba/Ta < 150; La/Ta = 10·6–12·1,Th/Ta = 1–1·5, Th/La < 0·15; Figs 7 and 8); low 87Sr/86Sr ratios (0·7034); high 
Nd values (+4·6); and low 207Pb/204Pb and 208Pb/204Pb ratios at a given 206Pb/204Pb (18·7) ratio (Table 7; Figs 9 and 10). Their chemical characteristics generally resemble those of nearby Eocene back-arc flows (see Figs 7 and 9; Kay et al., 1993, 2004) whose locations are shown in Figs 2b and 3. The Sierra de Chacays flows (CH1 in Fig. 5a) are similar to the Ranquil Huao flows in having high REE contents (Ce 57 ppm), steep REE patterns (La/Yb = 15–18; La/Sm = 4·4–4·8), low 87Sr/86Sr ratios (0·7040), and high Nd (+2·1 to 2·6) values (Figs 5 and 9) compared with the overlying plateau flows.
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Plateau flows
The plateau flows of the Meseta de Somuncura and surrounding mesetas, along with those in the Meseta de Canquel region, constitute the main volume of the Somuncura province and the main back-arc volcanism in Patagonia at this time (Figs 2 and 3). A relative contemporaneous volcanic lull from
28 to 25 Ma to the west of the Somuncura province is consistent with the K/Ar ages for volcanic rocks in that region given by Cazau et al. (1987), Rapela et al. (1988) and Muñoz et al. (2000). As elsewhere in the Patagonia back-arc region, there is no structural evidence for significant extension in the Somuncura region at this time. In contrast, Muñoz et al. (2000) have argued for extension to the west in Chile during this period.
The Somuncura plateau group includes the voluminous basaltic to mafic andesitic flows in the Alta de Somuncura, Apas–Chacays–Telsen, and surrounding regions (Fig. 3). Ardolino & Franchi (1993) assigned late Oligocene to early Miocene ages to these flows based on a compilation of K/Ar ages that range from 33 ± 3 Ma to 22 ± 3 Ma for the mafic flows, and from 32 ± 2 Ma to 22 ± 2 Ma for associated trachytic and rhyolitic flows and sub-volcanic bodies. Remesal (1988) used many of the same K/Ar data to argue that Somuncura province volcanism climaxed in a 3 Myr period near 27 Ma. This proposal is consistent with paleomagnetic data from flows in the northeastern Meseta de Somuncura that fit the magmatic reversal record from 27·5 to 25 Ma (Orgeira & Remesal, 1993). Evidence for contemporaneous volcanic activity in the Meseta de Canquel (Figs 1 and 2) comes from Deseadense (37–27 Ma) and Coluehuapense (25–23 Ma) mammal-bearing tuffs that bound a mafic volcanic sequence in which a flow yielded a K/Ar age of 27·9 ± 0·8 Ma (Marshall et al., 1983, 1986).
Most of the Somuncura plateau flows appear to have erupted from large centers in the Apas–Chacays, the Alta de Somuncura, and the Carri Laufquen regions (Fig. 3). Constraints on the ages of the Apas–Chacays region centers come from flows in the Ranquil Huao region, where K/Ar ages range from 29 to 27 Ma (Ardolino & Franchi, 1993), and a flow near the top of the Ranquil Huao sequence that yielded a 40Ar/39Ar total-fusion age of 26·9 ± 0·8 Ma (Table 2). K/Ar ages for flows in the Alta de Somuncura region mostly range from 26 to 24 Ma (Ardolino & Franchi, 1993). Those in the Meseta de Carri Laufquen region are 31 ± 2, 28 ± 2, and 24 ± 5 Ma (Coira et al., 1985).
The plateau sequences are dominated by olivine–clinopyroxene–plagioclase-bearing basaltic and orthopyroxene-bearing basaltic andesitic flows that are distinct in their major and minor element chemistry from the pre-plateau flows (Table 4). Overall, the plateau flows have transitional alkaline to subalkaline compositions with 49·2–54·5% SiO2 (Fig. 4). Among their distinctive features are relatively high FeO*/MgO ratios (1·5–2·4), low MgO (4·3–8·5%), Cr (most 130–380 ppm), and Ni (90–210 ppm) concentrations, high Sr (most 400–500 ppm) and Na2O (3·4–4·3%) concentrations, and small to absent Eu anomalies. Other differences from the pre-plateau flow include lower LREE abundances (Ce = 14–60 ppm) and flatter overall and LREE slopes (La/Yb = 3–15; La/Sm = 1·6–4·7) (Figs 5b–d and 6). Further contrasts are higher ratios of alkali/alkaline earth elements relative to REE and HFSE, with some flows having notably higher Sr/La (25–70, most >26), Ba/La (14–38, most >18), Ba/Ta (190–710), La/Ta (10–24), and Th/Ta (1·2–2·5) (Fig. 7). Isotopic differences include higher 87Sr/86Sr ratios (0·7043–0·7051), lower Nd values (+1·2 to +2·2), and slightly higher 207Pb/204Pb and 208Pb/204Pb ratios at a given 206Pb/204Pb ratio (Table 7; Figs 9 and 10).
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Several features of the plateau group are noteworthy. First, where stratigraphic relations are clearest, the younger (late plateau) flows tend to have steeper LREE patterns, and slightly lower 87Sr/86Sr ratios than the older (early and main plateau) flows (Figs 5 and 9). Second, long early and main plateau flows that reach the northern margin of the plateau (e.g. at Aguada Cecilio; Fig. 3) tend to have the flattest LREE and steepest heavy REE (HREE) slopes (La/Sm = 1·6–2·2; Sm/Yb ratios = 1·9–2·6; Figs 5b, c and 6), highest 87Sr/86Sr ratios, and lowest
Nd values (Fig. 9) in the Somuncura region. REE patterns like these are uncommon in continental alkali lavas, but are found in oceanic hotspot lavas such as those from Hawaii (Figs 5b and 6). Third, Comallo region flows, which are the furthest west and closest to the modern Southern Volcanic Zone (SVZ) arc (Fig. 3), are notable for their flat LREE patterns (La/Yb = 3–4, La/Sm = 1·6–1·9: Figs 5b and 6), high La/Ta ratios (18–24: Fig. 7), high 87Sr/86Sr ratios (0·7050–0·7051) and low Nd (–1·5 to –2·2) (Fig. 9).
Post-plateau flows
The post-plateau flows erupted in the nearly orthogonal convergence regime that emerged after the breakup of the Farallón plate (e.g. Pardo-Casas & Molnar, 1987). According to Somoza (1998), the Nazca plate had a convergence vector of 80° NE with the South American plate and the convergence velocity was 16 cm/yr at this time. Published K/Ar age data are consistent with contemporaneous arc volcanism being present to the west after 25 Ma with eruptions in the Ventana belt in Argentina (25–24 Ma; Cazau et al., 1987; Rapela et al., 1988) and in Chile (25–20 Ma; Muñoz et al., 2000). Widespread back-arc volcanism also took place to the north and south (Figs 1 and 2). To the north between 36° and 38°S, 24–20 Ma alkali olivine basalt flows were followed by 19–16 Ma basaltic andesitic to dacitic volcanic centers (Kay & Copeland, 2006). To the south, mafic flows with K/Ar ages of 22·1–19·6 Ma are found in the Meseta de Canquel (Marshall et al., 1986). Early Miocene extension to the west has been suggested based on studies of sedimentary and volcaniclastic sequences in the Cura Mallin basin near 36° to 39°S (Jordan et al., 2001), sedimentary sequences in the Nirihuau basin (Fig. 2b), where subsidence has been shown to peak during a marine ingression (Cazau et al., 1987), and volcanic and sedimentary sequences in Chile (Muñoz et al., 2000). The amount of extension is still the subject of debate.
The post-plateau flow sequences are more restricted in volume and distribution than the plateau flows. They are concentrated in a NW–SE-trending band that runs from the Meseta Carri Laufquen to the Telsen region along the western and southern margins of the Meseta del Somuncura (Fig. 3). Ardolino & Franchi (1993) assigned most of these flows to the early Miocene based on a compilation of K/Ar ages that range from 23·3 ± 2 to 15 ± 1 Ma. Coira et al. (1985) reported a K/Ar age of 20 ± 1 Ma on a flow in the Meseta de Carri Laufquen. These age assignments agree with the two new total-fusion 40Ar/39Ar ages in Table 2. The 20·6 ± 0·6 Ma age is from a flow SE of Maquinchao. The 16·6 ± 0·4 Ma age is from a flow in the Telsen region near where K/Ar ages of 17 ± 1 Ma and 15 ± 1 Ma have been reported previously (see Ardolino & Franchi, 1993). The post-plateau stage ended with the eruption of the Telsen region flows.
The olivine–plagioclase-bearing mafic post-plateau flows largely have alkali basaltic, trachybasaltic, mugearitic and hawaiitic compositions (Fig. 4). Compared with the plateau flows, they are generally characterized by lower SiO2 (most 48–52%), MgO (3·7–5·2%), Cr (most 3–250 ppm) and Ni (most 25–45 ppm), and higher TiO2 (most 2–3·3%) and K2O (most 1·9–2·7%) contents (Table 5). They also have higher incompatible element abundances (Ce 55–110 ppm); steeper REE patterns (La/Yb 15–26; La/Sm 4–7) (Figs 5e and 6); and generally similar Ba/La (19–31), Ba/Ta (240–500), and La/Ta (10–16) ratios along with lower Sr/La (15–30) ratios (Fig. 7). In addition, their 87Sr/86Sr ratios range to lower values (most are 0·7040–0·7045), their Nd (–0·2 to +2·3) to higher values, and their Pb isotopic ratios overlap (Table 7; Figs 9 and 10). They differ from the pre-plateau flows in being more Na2O- and less K2O-rich (Table 3). They also have lower La/Sm ratios than the pre-plateau Ranquil Huao flows (Fig. 6), higher Sr/La, Ba/Ta, and Ba/La ratios and a greater range of La/Ta ratios (Fig. 7), and lower 87Sr/86Sr ratios and higher Nd values (Fig. 9).
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Intermediate and silicic volcanic rocks
More silicic volcanic rocks that are temporally and spatially related to the mafic lavas include trachytic to comenditic rhyolitic tuffs, benmoreite to mugearite domes, and subvolcanic intrusive rocks (Fig. 4; Corbella, 1982a
, 1982b, 1984; Ardolino, 1987; Remesal, 1988; Corbella, 1989a). They are most abundant in the Sierras de los Chacays, Telsen, Apas, Talagapa, and Alta Sierra de Somuncura regions (Fig. 3), where they seem to be associated with large eruptive centers (Corbella, 1984). Compiled K/Ar ages for silicic tuffs and domes in the Sierras de Apas and Chacays regions range from 31 ± 2 to 25 ± 5 Ma and coincide with those of the plateau flows. K/Ar ages for trachytic tuffs and domes near Telsen range from 22 ± 2 to 18·2 ± 2 Ma and coincide with those of post-plateau flows in that region (Corbella, 1982a, 1982b; Ardolino & Franchi, 1993).
New chemical and Sr–Nd–Pb isotopic data (Tables 6 and 7) for samples with 59–71% SiO2 from south of the Sierra de Apas and the Talagapa region (Fig. 3) supplement major and trace element analyses by Corbella (1982b, 1984, 1989a). Mineralogical characteristics described by Corbella (1982b, 1984, 1989a) and total alkali vs SiO2 and trace element characteristics shown in Figs 4 and 5f are like those in other alkaline continental and oceanic intraplate volcanic rocks. Particularly distinctive among their trace element characteristics are high REE, Th, U and Ta contents, low Sr and Ba contents, and negative Eu anomalies that reach an extreme in the trachytic tuffs (Fig. 5f and Table 6). Initial Pb, Sr, and Nd isotopic ratios of a benmoreite with 59% SiO2 (GG5 in Table 6) overlap those of the mafic plateau flows (Figs 9 and 10), as do the Pb isotopic ratios of a trachytic tuff with 71% SiO2 (GG2; Table 6). Initial Nd values of four samples (Table 7) decrease slightly (+0·1 to –1·2) with increasing SiO2 (59–72%). It is not possible to calculate initial 87Sr/86Sr in samples with >71% SiO2 because of high Rb/Sr ratios (>100) and imprecise ages.
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High-potassium and sodium-rich flows
Other Somuncura province region volcanic rocks include K-rich lavas with orenditic (47% SiO2, 5·5–7% K2O, 7–8% MgO) to latitic (60–67% SiO2) compositions that erupted at the southern margin of the Sierra de Chacays at Plan-Luan (Corbella, 1983
, 1989a), nephelinitic to phonolitic lavas in the Sierra de Queupuniyeu and Praguaniyeu regions (Corbella, 1982c, 1983, 1989b), and nephelinitic lavas in the Alta Sierra de Somuncura (Corbella, 1985; Fig. 3). Few trace element and isotopic data are available for these rocks. An early Miocene age for the high-K volcanic rocks is based on a 19·3 ± 3 Ma whole-rock Rb–Sr date that puts their initial 87Sr/86Sr ratio between 0·7058 and 0·7065 (Corbella & Barbieri, 1989). An early Miocene age for the sodic nephelinite, basanite, phonolitic tephrite, and aegerine–augite and analcime-bearing phonolite flows in the Sierra de Queupuniyeu area is based on a K/Ar age of 19·3 ± 1 Ma on a basanite flow (Corbella, 1982c). An undated Praguaniyeu nephelinite (39% SiO2) analyzed by Stern et al. (1990) has high La/Yb (51, Fig. 5) and low Ba/La (6·5) and La/Ta (12), and Sr–Nd–Pb isotopic ratios similar to those of the post-plateau mafic lavas (Figs 9 and 10). A garnet lherzolite xenolith from a Praguaniyeu flow yielded a Sm–Nd mineral isochron age of 29·4 ± 5·7 Ma (Bjerg et al., 2005). |
DISCUSSION: ORIGIN OF THE SOMUNCURA PROVINCE MAGMATIC ROCKS |
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TOPABSTRACTINTRODUCTIONREGIONAL SETTING AND GENERAL...PHYSICAL CHARACTERISTICS,...DISCUSSION: ORIGIN OF THE...EVOLUTION OF THE SOMUNCURA...CONCLUSIONSSUPPLEMENTARY DATAAPPENDIXREFERENCES |
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Understanding the origin of the Somuncura province magmas requires deconvolving their mantle and crustal components, source melting conditions, and ascent paths. The approach below is to evaluate first the role of crustal contaminants, then that of pre-existing lithosphere and subducted components, and finally that of asthenospheric components. Temporal and spatial changes are used as tools in differentiating crustal and mantle components, and major and trace element data in evaluating mantle melting percentages and depths. Comparisons with Hawaiian intraplate magmas are used to evaluate the role of an oceanic intraplate-like asthenospheric upwelling and the differences that can be explained by subducted components. Finally, the tectonic setting of the Somuncura province is reappraised in terms of local and regional constraints.
Role of crustal contaminants
A question that arises for mantle-derived magmas that are erupted through continental crust is the extent to which these magmas are affected by crustal contaminants. Intraplate-like Th/La and Ta/U ratios (Fig. 8) and relatively low 87Sr/86Sr ratios and high Nd values (Fig. 9) like those in the Somuncura province pre- and post-plateau mafic lavas are generally taken as evidence against significant upper crustal contamination. In contrast, a number of the chemical characteristics of the plateau lavas could imply a larger role for crustal contamination. Relative to the pre- and post-plateau lavas, the plateau lavas have higher 87Sr/86Sr ratios and lower Nd values (Fig. 9), higher SiO2 contents (up to 53%; Fig. 4), higher large ion lithophile element (LILE)/REE ratios (e.g. Sr/La, Ba/La and Th/La ratios, Fig. 7) and an upper crustal contamination trend on a Ta/U vs Th/La plot (Fig. 8). At the same time, restrictions on the amounts of crustal contaminants in the plateau lavas come from their high MgO (6·8–7·8%), Cr (250–330 ppm), and Ni (180–210 ppm) contents, low LREE contents, flat light and steep HREE patterns, limited HFSE depletion (e.g. low La/Ta ratios), and relatively high Sr contents.
Possible upper crustal contaminants in the plateau magmas are the voluminous Jurassic Chon Aike rhyolitic and related magmatic rocks that were emplaced during a massive regional crustal melting event preceding and accompanying the opening of the South Atlantic Ocean (e.g. Kay et al., 1989; Pankhurst & Rapela, 1995). Support for contamination from these rhyolites comes from the presence of aggregates of corroded quartz and partially digested feldspar in the plateau lavas (Remesal & Parica, 1989). The studies of Rapela & Pankhurst (1993) and Pankhurst & Rapela (1995) show that two varieties of Chon Aike volcanic rocks (Table 6) are common in the Somuncura region. The first is a high-SiO2, LILE-depleted rhyolite with a very large negative Eu anomaly and an age-corrected 87Sr/86Sr ratio >0·760 at 27 Ma, and the second is a low-SiO2, more LILE-rich rhyolite with a 87Sr/86Sr ratio near 0·710 at 27 Ma. The high-SiO2 rhyolite could have played a role in elevating the SiO2 contents of the plateau lavas, but could not have had much effect on their LILE contents or 87Sr/86Sr, Ba/La, and Ba/Ta ratios based on mass-balance considerations.
Isotopic and trace element considerations indicate that neither the low-SiO2 Chon Aike rhyolite nor any average upper crustal assimilant is likely to have played a major role in shaping the distinctive LILE/REE ratios and isotopic characteristics of the plateau lavas. The first-order problems with upper crustal contaminants emerge in both simple mixing models and energy-constrained assimilation and fractional crystallization (EC-AFC) models that use the formulation of Spera & Bohrson (2001). The mantle-derived basalts used in the models are assumed to have the isotopic ratios of the pre-plateau lavas; however, as the pre-plateau lavas have high LILE concentrations and steep REE patterns, their trace element characteristics are inappropriate. As no suitable basalt is known in the Somuncura region, the trace element concentrations of a primitive Hawaiian tholeiite (R117 of Yang et al., 1996) whose flat LREE and steep HREE patterns approach those of the plateau lavas like B26 are used (Fig. 5c).
Illustrative simple mixing models (not shown) that combine 80–85% of this basaltic end-member with 15–20% of the low-SiO2 Chon Aike rhyolite approach the 87Sr/86Sr ratios (0·7047), Nd, and Th, K, and REE contents of plateau lavas such as VAL3-4, but fail to match their relatively high Sr, Ba and Ta contents. These models can be improved by using more Sr, Ba and Ta-rich basalts, but none of them simultaneously match the REE, Ta, and Th contents or the trends of Ba/Ta, Ba/La and Th/Ta ratios in the early and main plateau lavas (Fig. 7).
More complex isotopic and trace element EC-AFC models that allow for selective enrichment of incompatible elements are not any more successful (Fig. 11). The problem is that these models do not explain why Ba, Sr and Ta are enriched in the plateau lavas, whereas the REE are not. Another problem is that plateau lavas like B-26, which have high 87Sr/86Sr and low Sr contents, show the least evidence for LILE contamination. The failure of the EC-AFC models can be seen in the various plots in Fig. 11. Models that produce Sr and Nd isotopic trends like those in the Somuncura lavas (Fig. 11a) fail by overestimating Nd (Fig. 11b) and Th concentrations, and severely underestimating Sr (Fig. 11c) concentrations in plateau lavas such as Val3-4 and B26. Parental basalts similar to the average enriched mid-ocean ridge basalt (E-MORB) and ocean island basalt (OIB) of Sun & McDonough (1989), with lower Sr contents, produce even worse matches. As such, models that explain higher 87Sr/86Sr ratios and lower 143Nd/144Nd in the plateau lavas than in the pre- and post-plateau lavas by adding Chon Aike or upper crustal contaminants are inconsistent with LILE distributions.
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Three other observations also point to the differences among the Somuncura province lavas as being primarily due to other factors. The first is that Eocene and pre-plateau lavas that erupted though the same crust as the plateau lavas lack their high, arc-like Ba/La and Sr/La ratios. These high Ba/La and Sr/La ratios are also absent in plateau lavas that erupted through a similar type of crust east of the Chile Triple Junction to the south (Fig. 1; Ramos & Kay, 1992
; Gorring et al., 1997). The second is that a virtual overlap in 87Sr/86Sr and 143Nd/144Nd ratios in mafic and silicic Somuncura province volcanic rocks (Fig. 9) is inconsistent with a Chon Aike rhyolite contaminant shaping the properties of the silicic magmas. A similar case can be made for overlapping 87Sr/86Sr ratios in Plan-Luan region orenditic (0·7058–0·7066, 47% SiO2, >5% K2O) and K-latitic (0·7062–0·7067; 60–67% SiO2) volcanic rocks (Corbella & Barbieri, 1989). The third is that the high K/Na ratios in the high-K mafic to silicic Plan-Luan region volcanic rocks contrast with the high Na/K ratios in nephelinite to phonolitic lavas elsewhere in the region (Corbella, 1984). This contrast implies that mantle, rather than crustal components shape the chemical differences between these sequences.
Another issue is the role of lower crustal contaminants in the petrogenesis of the plateau magmas. The relatively high FeO*/MgO ratios (1·5–2·4) and relatively low MgO (4·3–8·5%), Cr (most 130–380 ppm), and Ni (90–210) contents of the plateau lavas are pertinent in that they indicate that the magmas have fractionated olivine and clinopyroxene. Likewise, high Sr (most 400–500 ppm) and Na2O (3·4–4·3%) contents and small negative Eu anomalies fit with minimal plagioclase removal. Along with the petrographic features summarized by Remesal (1988), these characteristics indicate crystallization of olivine and pyroxene before plagioclase, as expected in mafic magmas that fractionate at deep crustal levels where they could assimilate lower crust. Importantly, an approximation of the lower crustal composition in the region comes from granulite xenoliths in the Plan-Luan potassic volcanic rocks that Pankhurst & Rapela (1995) argued have the depleted chemistry (Nd <5·5 ppm; Rb <2 ppm; Sr 550–750 ppm; 87Sr/86Sr ratios = 0·7065–0·7078; Nd = –3 to –6·10) expected of lower crust that is residual to the Chon Aike rhyolites. EC-AFC models that use the same basaltic end-member as in the upper crustal models above and the parameters in Table A2 (see the Appendix) show that assimilants like the Plan-Luan xenoliths and generic lower crust also fail to produce the distinctive isotopic and trace element characteristics of the plateau lavas. As shown in Fig. 11, best-fit models that use Sr distribution coefficients near 0·05, which are in accord with plagioclase instability, produce Sr concentrations that approach those in the plateau lavas, but fail to match their Nd and Sr isotopic ratios (e.g. model with Plan-Luan xenolith in Fig. 11). Models that use higher Sr distribution coefficients (1·5) produce a better match for the Sr and Nd isotopic characteristics of the plateau lavas, but fail to produce their high Sr concentrations (see model with lower crust in Fig. 11a and c).
Role of subducted contaminants
The presence of subducted components in the mantle source can explain why Ba/Ta, Sr/Ta, Th/Ta, Ba/La, Sr/La, and Th/La ratios in the plateau lavas are higher than those in MORB and intraplate (OIB) lavas (Fig. 7) and why Ta/U ratios are intermediate between those in MORB and Andean SVZ arc magmas (Fig. 8). Support for a subducted component comes from the observation that the most arc-like ratios occur in the Comallo region plateau lavas (La/Ta = 18–24, Ba/La = 22–32; Th/Ta = 0·37; Ta/U < 2; Figs 7 and 8), which are closest to the SVZ arc front (Fig. 3). The values of these ratios indicate that the amount of subducted component in the Somuncura province lavas is less than in modern SVZ arc (Ba/La 21–28; Sr/La 53–68; Th/La 0·15–0·30; La/Ta 40–75; Th/Ta 8–21) and back-arc (Ba/La 15; Sr/La 17; Th/La 0·18; La/Ta 18; Th/Ta 5) lavas.
The OIB and MORB-like trace element ratios in Eocene and pr