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 pre-plateau Ranquil Huao lavas (Ba/La < 18; La/Ta < 14; Th/Ta < 2; Ta/U > 2·2; Figs 7 and 8) show no evidence of a subduction-like component. Entry of this component is signaled by the transitional arc-like low Ta/U (down to 1·4; Fig. 8) and high Ba/La (14–38) and Sr/La (25–70) (Fig. 7) in the plateau lavas. The presence of transitional arc-like ratios in some of the eastern and central plateau lavas shows that the effect of this component was not limited to the western Somuncura province. Particularly notable are ratios of La/Ta up to 19, Ba/La up to 38 and Sr/La up to 70 in early and main plateau lavas from the northeastern part of the Meseta de Somuncura and in the Coña Niyeu sequence (Fig. 7). A comparison of lavas within sequences and across the region shows that the arc-like affinities in terms of Ba/Ta, Ba/La, Sr/Ta, Sr/La, Th/Ta and Th/La increase from the pre-27 Ma pre-plateau Ranquil Huao lavas to the 27 Ma plateau lavas near the top of the Ranquil Huao sequence to the <26 Ma northeastern plateau and Coña Niyeu lavas (Figs 7 and 8). This pattern changes in the uppermost plateau and post-plateau lavas, which continue to have high Ba/Ta ratios but show a reversal to lower Sr/Ta, Sr/La, Th/La and Ta/U ratios (Figs 7 and 8).
The easiest way to explain entry of a subducted component into the magma source at the time of plateau magmatism is for a subducting slab and/or slab-modified mantle to dehydrate and trigger the melting event that produced the plateau magmas. Notably, the enrichments in fluid-mobile elements (Sr, Ba, and U) were accompanied by little to no arc-like depletion of the HFSE as evidenced by the lack of negative Ta anomalies in extended trace element plots in Fig. 5 and the restricted range of MORB- or OIB-like La/Ta ratios (most 10–15) in the majority of the plateau and post-plateau lavas (Fig. 7). One explanation is that the thermal processes that produced the plateau magmas destabilized oxide phases that formed either in or above the subducting slab (e.g. Reagan & Gill, 1989) releasing incorporated Ti and HFSE. Subduction zone models that attribute HFSE depletion to differential element mobility in fluids (e.g. see Elliot et al., 1997) provide no easy explanation as to why Ta was not depleted as Ba, Sr and U were enriched in the magma source.
Increases in Th/Ta and Th/La ratios, which accompany those in Ba/La and Sr/La in the plateau lavas, require addition of Th as well as Ba and Sr to the magma source (Figs 7 and 8). As Th is generally immobile in fluids, Th enrichment in arc magmas is commonly linked to melting of subducted sediment (see review by Elliot et al., 1997) or incorporation of crust into the mantle wedge by forearc subduction erosion (e.g. Stern, 1991; Kay et al., 2005). Both processes could play a role in the petrogenesis of the plateau lavas. Removal of the forearc margin by subduction erosion during the Oligocene oblique convergence regime, or as the plate margin adjusted to changes associated with the breakup of the Farallón plate, could explain why: (1) Th/Ta ratios are higher in some plateau than in post-plateau lavas; (2) 87Sr/86Sr ratios are higher and Nd values lower in plateau lavas at Comallo (0·7050–0·7051; Nd –2) than in late Oligocene lavas to the west (0·7035–0·704; Nd +3·3 to +5) (Fig. 9; Table 7).
Another issue is why Ba/La ratios remain high in the post-plateau lavas, whereas Sr/La (<30), Th/La, La/Ta (<16), and Ta/U ratios are close to pre-plateau levels (Figs 7 and 8). Lower Sr/La ratios are not attributable to plagioclase fractionation, as post-plateau lavas generally have higher Sr contents and can have positive Eu anomalies (Eu/Eu* up to 1·2). Relative decreases in ratios involving Sr, Ba, and Th are better explained by a depletion of subducted components in the magma source. The relatively high Ba/Ta and Ba/La ratios could be due to phlogopite formation in the lithospheric mantle during the plateau stage, consistent with the eruption of the high-K orendite suite near 20 Ma (Corbella, 1982c).
Mantle source components, melting percentages and depths of melting
The dominant component by volume in the Somuncura province magmas comes from the mantle. Unraveling the origin of the mantle-derived magma requires constraining the mantle source composition and the depth and temperature of melting. Much can be learned by analogies with oceanic intra-plate lavas, particularly the well-studied ones in Hawaii. A number of similarities are notable. (1) The LREE to HFSE ratio as measured by La/Ta ratios ranges from 10 to 16 in both oceanic intra-plate (e.g. Hickey et al., 1986; Huang & Frey, 2003) and most Somuncura lavas (Fig. 7). (2) Somuncura province lavas show the Hawaiian Haleakala-like pattern of low and high LILE concentrations combined respectively with more and less enriched Sr–Nd isotopic signatures (Figs 5 and 9) that was made famous by Chen & Frey (1985). Specifically, the plateau lavas are like Hawaiian shield and post-shield lavas in being characterized by low Th, Ba, Sr, and LREE concentrations and 143Nd/144Nd ratios along with high Ba/La, Th/La, and 87Sr/86Sr ratios, whereas the pre- and post-plateau lavas are like Hawaiian pre-shield and post erosional lavas in showing the opposite pattern of high and low values. (3) The shapes of Somuncura pre-plateau, plateau and post-plateau REE patterns are reminiscent of those of Hawaiian pre-shield, late main shield, and post-erosional lavas. The nearly flat LREEand steep HREE patterns in some of the plateau lavas (Fig. 5) are rare in continental and most oceanic island lavas (Fig. 6; review by Lassiter & DePaolo, 1997), but common in Hawaiian shield-forming flows (e.g. Feigenson et al., 1996; Yang et al., 1996). Understanding the origin of these patterns has been a longstanding problem in Hawaiian magmatism (e.g. Feigenson et al., 1996).
Trace element models provide constraints on mantle melting percentages and source compositions for the Somuncura magmas. Relative melting percentages can be estimated from Fig. 12, which shows Th concentrations (ppm) vs chondrite-normalized La/Sm ratios for Somuncura flows with >250 ppm Cr compared with calculated melting curves from Gorring & Kay (2001). Melting percentages inferred by comparisons with these model curves are minima as differentiation and AFC processes increase both Th contents and La/Sm ratios. The mantle sources in the Gorring & Kay (2001) models (described in the caption of Fig. 12) were chosen for Patagonian back-arc lavas in the Triple Junction province (Fig. 1). These models can be applied to the Somuncura lavas. An ‘enriched source’ (0·885 ppm La; 0·414 ppm Sm; 0·113 ppm Th) is needed to approximate the characteristics of the late plateau, pre-plateau and post-plateau lavas. A less enriched source (0·481 ppm La; 0·380 ppm Sm; 0·600 ppm Th), which is a mixture of 50% of a MORB-type source with a mantle source whose trace element concentrations are two times the Leedy chondrite (Masuda et al., 1973), is needed to explain the lower Th contents at a given La/Sm ratio in the early and main plateau lavas, and to keep their melting percentages at realistic values.
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The calculated melting models indicate higher relative partial melting percentages for the voluminous plateau lavas than for the less voluminous pre- and post-plateau lavas. The most depleted sources and nearly the highest melting percentages (
10%) are inferred for the Comallo lavas (Fig. 12) that occur just east of the Paleogene arc. Their relatively high Ba/La and La/Ta ratios (Fig. 7) are consistent with a hydrous slab-derived component enhancing melting of a relatively depleted source. Most early and main plateau lavas, like most Hawaiian lavas, plot between the curves for the more and less enriched sources in Fig. 12. Melting percentages are variable, with the highest percentage melts (>10%) corresponding to the long flows in the northeastern region and to Coña Niyeu region flows (Fig. 3). The late plateau flows plot along or above the curve for the enriched source and have inferred melting percentages from 4 to 7%. The modeling implies 2·5–3% partial melting of an enriched source for the post-plateau flows and <2·5% partial melting for the pre-plateau Ranquil Huao flows.
Melting depths for the Somuncura magmas can be estimated by looking at their major element compositions relative to those of experimental melts (e.g. Langmuir et al., 1992; Scarrow & Cox, 1995; Turner & Hawkesworth, 1995). Some relevant experiments are on Hawaiian lherzolite HK66 (Hirose & Kushiro, 1993). The premise for determining depth of melting is that FeO* (total Fe as FeO) contents in magmas derived from similar sources are strongly dependent on pressure and nearly independent of melting percentage. Calculated depths for the Somuncura flows in Fig. 13 are for FeO* contents that have been corrected to 8% MgO by numerically adjusting phenocryst percentages or graphical projection. Measured MgO contents range from 9 to 4% (Tables 3–5). Uncertainties in the extrapolations and the assumption of a common source make relative depths more meaningful than calculated depths. Lower MgO contents in pre- and post-plateau lavas than in the plateau lavas at a given FeO* result in shallower calculated melting depths for the plateau lavas. Following Langmuir et al. (1992), the calculated depths are average segregation depths of pooled magmas and are proportional to the range of depths over which melting occurred or the length of the melting column. Given a constant mantle potential temperature or Tp (temperature after adiabatic rise to surface assuming no melting or phase change), shallower depths imply incremental melting over a longer melting column, whereas greater depths imply shorter melting columns and conditions approaching batch melting. Greater melting depths generally translate into a thicker lithosphere.
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Other constraints on depth of melting come from garnet-free or garnet-bearing residual mineral assemblages inferred from trace element data. Garnet is important in constraining mantle potential temperature as it is unstable above
60 km in an anhydrous mantle with a Tp of 1300°C and above 80 km in a plume-like mantle with a Tp of 1500°C (e.g. McKenzie & O'Nions, 1991). A longstanding problem in Hawaiian magma genesis has been that experimental studies on Hawaiian peridotites (e.g. KLB-1; Herzberg & Zhang, 1996) indicate that garnet should be present in the mantle source for lavas with steep HREE patterns, whereas models for the LREE indicate that garnet should be melted out within a high-temperature plume-like mantle. Based on models for Hawaiian magmas, the flat LREE and steep HREE chondrite-normalized patterns (Figs 5 and 6) of the Somuncura plateau lavas can be explained either by batch melting of a garnet-bearing mantle source with a flat REE pattern, or by incremental or batch melting of a garnet-free source with a flat LREE and steep HREE pattern (see Feigenson et al., 1996).
An interesting pattern emerges when FeO*-based depths (Fig. 13) are compared with inferred mantle melting percentages (Fig. 12) and Nd–Sr isotopic ratios (Fig. 9). Fields representing five stages are shown in Fig. 13 along with comparative fields for some Hawaiian shield, post-shield and post-erosional magmas. Like Hawaiian magmas, the Somuncura magmas show a temporal trend from relatively ‘depleted’ (pre-plateau) to ‘enriched’ (plateau) to ‘depleted’ (post-plateau) isotopic signatures as melting percentages increase and depths first decrease and then reverse (Figs 9 and 13). Lassiter et al. (1996) interpreted the Hawaiian trend as reflecting variable melting conditions as the magmas developed over a migrating plume source. In the Hawaiian model, pre-shield melts develop over the approaching plume, shield and post-shield magmas form in the main part of the plume, and post-erosional magmas reflect incorporation of lithospheric sources into the departing plume. With some caveats, Somuncura stages 2–5 have parallels with Hawaiian pre-shield to post-erosional chemical and isotopic trends.
Another aspect of the Somuncura province lavas is the comparability of their isotopic ratios to those of mantle plume-related basalts. As seen in Figs 9 and 10, Sr–Nd–Pb isotopic ratios in Somuncura province lavas generally fall between those of Hawaiian-type and southern Atlantic Tristan da Cunha- and Walvis Ridge-type plume sources. Somuncura lava 87Sr/86Sr, 143Nd/144Nd, and 206Pb/204Pb are closer to the Atlantic-type sources and are similar to those of Etendeka Tafelkop lavas described by Ewart et al. (2004). Based on the mixing lines that Ewart et al. (2004) used to model the Etendeka lavas, the 87Sr/86Sr, 143Nd/144Nd, 208Pb/204Pb, and 206Pb/204Pb of the Somuncura lavas can be described as mixtures of 60% of a Tristan da Cunha-type mantle source component and 40% depleted mantle (DM) along with a minor amount of subducted sediment. The Somuncura data support a greater proportion of a Tristan da Cunha-like mantle source component in the plateau lavas than in the pre- or post-plateau lavas. Such a model is in accord with the higher 87Sr/86Sr ratios of the plateau lavas, reflecting asthenospheric mantle as well as crustal sources, and reduces the amount of crustal contaminant needed to explain their isotopic ratios. Elevated 207Pb/204Pb ratios in the Somuncura magmas require some contribution from old continental crust that is either from subducted sediment or within the crust. Overall, the Somuncura data support mixing of a Tristan da Cunha-type plume source and ambient DM with minor amounts of subducted and in situ continental crust (Figs 10 and 14).
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EVOLUTION OF THE SOMUNCURA PROVINCE: A MAGMATIC–TECTONIC MODEL FOR INTERACTION OF A PLUME-LIKE UPWELLING AND A SUBDUCTING PLATE |
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TOPABSTRACTINTRODUCTIONREGIONAL SETTING AND GENERAL...PHYSICAL CHARACTERISTICS,...DISCUSSION: ORIGIN OF THE...EVOLUTION OF THE SOMUNCURA...CONCLUSIONSSUPPLEMENTARY DATAAPPENDIXREFERENCES |
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Parallels between Somuncura and Hawaiian magma evolution, along with the lack of any other explanation, led Kay et al. (1992
, 1993) to suggest that a transient mantle thermal anomaly related to changes in plate convergence vectors caused the Somuncura province to form. Subsequently, Muñoz et al. (2000) argued that Oligocene to early Miocene magmas from the Meseta de Somuncura to the Chilean coast formed in an asthenospheric upwelling through a slab window, created by slab rollback as the shallowly dipping Farallón plate evolved into a steeply dipping Nazca plate. In contrast, de Ignacio et al. (2001) speculated that the thermal anomaly was caused by a shallow asthenospheric upwelling induced and focused into the Somuncura region by a concave-up slab geometry. Both models are predicated on assumed slab geometries. The new three-stage model shown in Fig. 14 and discussed below takes into account all of the available geochemical and geological data. The model calls for a mantle upwelling to intersect a relatively steeply subducting Farallón plate. As before, the upwelling is linked to a mantle disturbance as plate convergence vectors changed to the west. Another factor could be a near-stable position of the South American plate relative to the hotspot reference frame. Melting is argued to be localized by a mantle wetspot that formed in relation to a Paleogene volcanic arc.
Stage 1: pre-plateau volcanism in an oblique convergence regime (Fig. 14a)
The initiation of the Somuncura province is marked by the eruption of intraplate-like late Oligocene pre-plateau mafic and silicic magmas. In accord with their chemistry, these magmas are shown in Fig. 14a as erupting over an incipient plume-like mantle upwelling, east of any influence of a steeply subducting slab in an oblique convergence regime. Support for the pre-plateau flows being small-percentage melts of a relatively isotopically ‘depleted’ garnet-bearing mantle source comes from their trace element and isotopic signatures (Figs 7–10). Like the Eocene flows before them, their intraplate-like trace element characteristics and ‘depleted’ Nd–Sr isotopic ratios show little effect of crustal or slab-related components. Their FeO*-based depths of melting are consistent with batch melting below an 80–90 km thick continental lithosphere (field 1 in Fig. 13). Flows to the west with variable arc-like trace element signatures (La/Ta = 16–38) and relatively low 87Sr/86Sr ratios and high Nd (> +1·8) values (Figs 8 and 9; Kay & Rapela, 1987; Kay et al. 1993; Muñoz et al., 2000) are shown in Fig. 14a to be located over a region of slab-modified mantle affected by the oblique convergence regime between the Farallón and South American plates (Cande & Leslie, 1986; Somoza, 1998).
Stage 2: plateau volcanism in a changing convergence region (Fig. 14b)
The main development of the Somuncura province is marked by the eruption of the plateau lavas. The model in Fig. 14b explains their principal features by invoking a disintegrating subducting slab, a plume-like mantle upwelling, a thinning mantle lithosphere, ponding of magmas at the base of the crust before eruption, and fractionation and mixing of new melt batches with partially melted underplated magmas to produce the silicic magmas.
A key aspect of the model is the disintegration of the subducting slab at the time of the plume-like mantle upwelling. The release of subducted components helps to explain the higher Sr, Ba, U, and Th to LREE ratios, higher 87Sr/86Sr ratios, and lower Nd values in the plateau compared with the pre-plateau lavas. The model differs from that of Muñoz et al. (2000), who attributed the arc-like ratios in the Somuncura province magmas to components introduced into the lithosphere over a relatively shallow Paleogene subduction zone. The problem with their model is how to introduce subducted components into the lithosphere up to 800 km east of the modern trench between the eruption of the pre-plateau and the plateau lavas. A flat-slab geometry seems improbable given the short time, the obliquity of Paleogene convergence, and the lack of deformational or magmatic evidence for a Paleogene or late Mesozoic flat-slab geometry (e.g. Kay & Abbruzzi, 1996; James & Sacks, 1999). A slab disintegration model like that called upon by Stern et al. (1990) to explain the presence of arc-like components in Plio-Pleistocene back-arc lavas is better. Such a model works well for the Somuncura province magmas as a plume-like mantle upwelling can accentuate slab disruption and could facilitate the dispersion of subducted components far to the east. An association of arc-like fluid-mobile element signatures (high Ba/La and Sr/La) with weak to absent arc-like HFSE depletions (low La/Ta ratios) in the plateau lavas (Fig. 7) fits with HFSE-bearing oxide phases being destabilized as a result of the breakdown of the slab and the oxidizing environment that produced them (e.g. Reagan & Gill, 1989). Enrichments in Th can be attributed to melting of subducted sediment or crust removed by fore-arc subduction erosion as the plate margin adjusted to changing convergence vectors.
The other key feature of the model is the plume-like mantle upwelling, which explains the voluminous and widespread plateau lavas as decompression-induced melts of mixtures of DM and EM1-like Tristan de Cunha-type mantle (Figs 9 and 10). The changing chemical characteristics and inferred FeO*-based melting depths in the early, main and late-stage plateau lavas (fields 2–4 in Fig. 13) can be reconciled with a lengthening melting column below a thinning mantle lithosphere.
The early plateau stage is exemplified by the <29 Ma to 27 Ma Ranquil Huao flows erupted from the Apas–Chacays region center. Their chemistry can be interpreted as recording an increasing temperature as the plume-like upwelling intensified mantle melting. Evidence comes from La/Sm ratios and Th contents indicating higher partial melting percentages (6–8%) of a more enriched mantle (Fig. 12) at FeO*-calculated depths ranging from 100 km at the base to 80 km at the top of the sequence (field 2; Fig. 13). Higher 87Sr/86Sr ratios and lower Nd values (Fig. 9) than in the pre-plateau lavas fit with an increased contribution from a Tristan da Cunha-like plume source. High Ba/La, Sr/La and Th/La ratios (Figs 7 and 8) signal the entry of subducted components into the mantle source. Higher 207Pb/204Pb isotopic ratios (Fig. 10) require an additional old crustal contaminant.
The main plateau stage at the peak of melt production is exemplified by the large-volume 27–25 Ma flows from the Alta de Somuncura region center(s) in the east and the Comallo region flows in the west. A peak of melting coincident with the eastern flows is consistent with an increased melting percentage inferred from Th contents and La/Sm ratios (9–12% for Coña Niyeu and NE flows; Fig. 12). These flows have the highest 87Sr/86Sr and lowest Nd in the eastern region (Fig. 9), in accord with a hotter and deeper mantle source influenced by a Tristan da Cunha-like (EM1) mantle plume. Their FeO*-based melting depths of 80–60 km are consistent with a lengthening melt column and a peak in lithospheric thinning. The formation of the Comallo lavas to the west could also coincide with the peak of the thermal event. Their chemistry fits with a high partial melting percentage (>10%) of an LREE-depleted source (Fig. 12) at a shallow Fe*-based depth (<65 km, Fig. 13). Their LREE-depleted characteristics, transitional arc-like Ba/La (24–31), Sr/La and La/Ta (16–23) ratios, high 87Sr/86Sr ratios, and low Nd values are consistent with a depleted lithosphere source component enriched by hydrous subducted components. A plume-like asthenospheric component provides an explanation for a more ‘enriched’ isotopic signature than in the Ventana Formation arc lavas to the west (Table 6; Figs 2b and 9).
The late-plateau stage flows erupted across the region at 26–24 Ma. As seen in the upper flows at Coña Niyeu, these lavas have higher Th contents and lower La/Sm ratios that result in lower calculated partial melting percentages (6–3%; Fig. 12), providing evidence for the passing of the main thermal peak. Lower 87Sr/86Sr ratios and higher Nd values in the late plateau upper Coña Niyeu and Valcheta region flows (Fig. 9) can be interpreted as recording a return to an increased role for depleted lithospheric source components. FeO*-based depths of 80–60 km are consistent with the residual effects of a long melting column and a continued thin lithosphere (field 4, Fig. 13).
The final feature of the model is the ponding of the plateau magmas near or at the base of the crust prior to eruption, in accord with evidence for olivine and pyroxene fractionation (low Mg, Ni, and Cr contents) at the expense of plagioclase (small Eu anomalies) and non-primitive compositions. In this setting, silicic trachytic and rhyolitic magmas can form by partial melting of previously underplated magmas. Fractionation of mafic magmas and mixing with silicic melts can produce benmoreite and trachytic magmas. The Sr–Nd–Pb isotopic similarities between the mafic and silicic magmas (Figs 9 and 10) indicate that little pre-existing crust was incorporated, in accord with a refractory crust that had lost most of its low-temperature melting component in the formation of the Jurassic Chon Aike rhyolites.
Stage 3: post-plateau magmatism in a near-normal convergence regime (Fig. 14c)
The last stage of the Somuncura province volcanism is marked by eruption of the 24–17 Ma post-plateau lavas in a band from the Carri Laufquen to the Telsen region (Fig. 3). The origin of these lavas must be considered in the context of the Somuncura province, as well as the widespread Patagonian volcanism and evidence for extension to the west at this time (e.g. Cazau et al., 1987; Muñoz et al., 2000; Jordan et al., 2001; Kay and Copeland, 2006; Fig. 1).
The model in Fig. 14c explains the chemistry of the post-plateau lavas relative to the ebbing of a plume-like upwelling. Decreased mantle melting is signaled by steeper HREE patterns (Figs 5e and 6) and trace element evidence for lower partial melting percentages (2·5–3%; Fig. 12) of a garnet-bearing mantle. A general return to FeO*-based depths of 70–85 km (field 5a, Fig. 14) is consistent with shortening of the melting column under a cooling lithosphere. The persistence of high Ba/La ratios can be explained by breakdown of sequestered phlogopite in the lithospheric mantle. An 5 Ma lapse between plateau and post-plateau magmatism in some places resembles that east of the Chile Triple Junction (Fig. 1), which Gorring et al. (1997) attributed to decompression melting of a more CO2-enriched asthenosphere at the ebb of the thermal event. The relative increase in CO2 in the mantle would be expected in association with a return to more alkaline magmas (e.g. Hirose, 1997) with OIB- and MORB-like Sr/La, Sr/Ta and Ta/U ratios (Figs 7 and 8). In this context, the 20 Ma lavas near Maquinchao are distinctive for their lower 87Sr/86Sr ratios and higher Nd values (Fig. 9), and Fe*-based depths of 90–100 km (field 5b in Fig. 13). Their relatively high Th contents and La/Sm ratios suggest low partial melting percentages (<3%). At the same time, the 17 Ma flows near Telsen have Nd–Sr isotopic compossitions (Fig. 9) and trace element-based melting percentages similar to those of the late plateau flows (Fig. 12). One explanation for this contrast is that the mantle under the Maquinchao region was cooled by the nearly orthogonally subducting Nazca plate, whereas higher temperatures persisted east of the influence of the subducting plate in the mantle under the Telsen region.
The post-plateau flows are also temporally associated with Na-rich volcanic rocks on and north of the Meseta de Somuncura and high-K volcanic rocks to the south. The restriction of Na- and K-rich volcanic rocks to this part of Patagonia at this time points to a continued distinctive mantle history under the Somuncura region. The best explanation for these volcanic rocks is that they are largely low-percentage partial melts of a phlogopite- and amphibole-rich lithosphere. The presence of these hydrous phases in the lithosphere can be linked to the residual effects of dehydration of the disintegrating slab and mantle melting related to the plume-like mantle upwelling.
Why is there a hotspot-like mantle upwelling in the Somuncura region?
The late Oligocene to early Miocene Somuncura province is one of a series of Late Cretaceous to Recent magmatic provinces in Patagonia that show no pattern in space and time and cannot be related to a known hotspot (Fig. 1). On a global scale, the topographic expression of the Meseta de Somuncura is comparable with that of African plateaux such as Tibesti and Hoggar, which Burke & Wilson (1972) attributed to hotspots without trails as they occur on a stationary plate in a global hotspot reference frame. Such a model does not easily explain the Patagonian provinces on the more mobile South American plate. Nevertheless, there are similarities between the Somuncura and Cenozoic African centers. Like the Somuncura province, many of the African centers lie outside the boundaries of Cretaceous rift zones, are associated with broad basement uplifts, and have no obvious link to extension. Based on these characteristics and others, Wilson & Guiraud (1992) suggested that the magmas of the African volcanic centers largely formed from partial melts of basal sub-continental lithosphere that had been metasomatized by Mesozoic–Cenozoic plume activity. A similar model for the Somuncura magmas is difficult to reconcile with temporal changes in the erupted lavas. However, a common feature could be a nearly stable position of South America relative to the hotspot reference frame at the time. Additional support comes from Silver et al. (1998), who argued that a late Oligocene to early Miocene acceleration of the South America plate relative to the hotspot reference frame led to the uplift of the modern Andes. Kay & Copeland (2006) argued that this acceleration actually began after 20 Ma when post-plateau magmatism was waning in the Somuncura province.
An important question is what triggered the thermal anomaly beneath the Somuncura province. One answer is to relate the anomaly to a mantle instability associated with late Oligocene plate reorganization, as first suggested by Kay et al. (1992, 1993). The magmatism could be related to a plume-like upwelling that is hotter than normal mantle and whose existence is justified by global plume heat flux estimates that show that identified plumes account for only 25% of residual mantle heat flux not explained by subducted slabs (e.g. Davies, 1988a, 1988b; Sleep, 1990; Turcotte, 1995). Gorring et al. (1997) argued for entrainment of such an upwelling in the slab window east of the Chile Triple Junction to explain the large amounts of back-arc volcanism in that region. The Somuncura event has been argued to be one of a series of tectonically induced melting events of a regionally hot Patagonian mantle (Kay et al., 2004).
Another factor in explaining and localizing the Somuncura magmatic province could be hydration of the mantle by the Paleogene subduction zone that produced the arc lavas on the western margin of the Somuncura province (Fig. 1; Rapela et al., 1988; Ardolino et al., 1999). Importantly, Ramos & Kay (1992) pointed out that the Paleogene arc ends near the southern margin of the Somuncura province near 43°S. They correlated the end of the arc with the northern margin of a slab window(s) created by the collision of the Aluk–Farallón spreading ridge with the Chile trench. A relation with the position of the Paleogene arc could explain the location of the Somuncura province.
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(1) The Somuncura magmatic province constitutes the largest volcanic field in extra-Andean Patagonia. This voluminous magmatism cannot be associated with major extension and has no obvious link to any tectonic event other than late Oligocene plate reorganization to the west. The features of the Somuncura province magmas are best explained by a transient plume-like mantle upwelling that intersected a subducting plate at a mantle ‘wetspot’.
(2) Somuncura province volcanic rocks can be generally divided into pre-plateau, plateau and post-plateau groups. Low-volume late Oligocene pre-plateau flows are typically intraplate alkaline basalts and hawaiites with ‘depleted’ isotopic compositions (87Sr/86Sr < 0·704; Nd > +2). Voluminous 27 ± 2 Ma plateau flows are dominantly hypersthene-normative basalts and basaltic andesites (50–54% SiO2) with flat REE patterns (La/Yb = 4–12), relatively low LILE abundances, transitional to arc-like Ba/La, Sr/La, Th/Ta and U/Ta ratios, intraplate La/Ta ratios and ‘enriched’ Nd–Sr isotopic compositions (87Sr/86Sr > 0·7043; Nd < +1·3). Intermediate to low-volume 23–17 Ma post-plateau flows are dominantly alkali olivine basalts and hawaiites (48–51% SiO2) with steep REE patterns (La/Yb >15), high LILE abundances, high Ba/La ratios, intraplate Sr/La and U/Ta ratios, and ‘depleted’ isotopic compositions (87Sr/86Sr = 0·7034–0·7046; Nd = +0·9 to +4·5).
(3) Support for a relation between the Somuncura province magmas and a plume-like mantle upwelling comes from large-volume plateau flows (>100 km long) whose isotopic compositions can be approximated by mixtures of depleted mantle (DM) and a Tristan de Cunha plume-like mantle source component and whose chemistry has broad parallels with hotspot magmas such as those in Hawaii. These parallels include: (a) Hawaiian tholeiitic-like REE patterns with flat LREE and steep HREE slopes, (b) a similar pattern of FeO*-inferred melting depths, (c) similar partial melting percentages based on Th (ppm) and La/Sm ratios, and (d) a similar pattern of isotopically enriched, LREE- and LILE-poor tholeiitic lavas that erupted before and after isotopically depleted LREE- and LILE-enriched alkaline lavas.
(4) Support for incorporation of subducted components into the Somuncura mantle source associated with a disintegrating slab at the time of the mantle upwelling comes from high Ba, Sr, Th and U to LREE ratios in the plateau lavas. A lack of HFSE depletion in the plateau lavas supports resorption of HFSE-bearing phases that formed during the subduction process at this time. The subsequent depletion of fluid-mobile elements in the mantle source and sequestering of Ba and K in amphibole and phlogopite in the lithosphere is signaled by high Ba/La and low Sr, Th and U to LREE ratios in the post-plateau lavas and the eruption of high-K orenditic magmas. A return to CO2 as the more important mantle fluid phase is consistent with the intra-plate-like chemistry in the post-plateau flows.
(5) Evidence for minimal crustal contamination in the Somuncura plateau magmas comes from low LILE (Sr) and REE (Nd) abundances coupled with radiogenic isotopic compositions whose changing character can be largely explained by processes in the mantle. Limited crustal contamination is consistent with a crust whose low melting fraction was extensively depleted by massive crustal melting in the Mesozoic.
(6) Silicic Somuncura province magmas including trachytes, phonolites and rhyolites can be attributed to crystal fractionation, partial melting of underplated Somuncura type mafic magmas and mixing of these melts with mafic magmas.
(7) Generation of a late Oligocene plume-like mantle anomaly in the Somuncura region is best attributed to a local mantle thermal instability at the time of plate reorganization, possibly in combination with a nearly stable position of South America relative to the hotspot reference frame. Extensive mantle melting was aided by hydration of the mantle during Paleogene subduction in the Somuncura region.
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SUPPLEMENTARY DATA |
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Supplementary data are available at Journal of Petrology online.
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APPENDIX |
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TOPABSTRACTINTRODUCTIONREGIONAL SETTING AND GENERAL...PHYSICAL CHARACTERISTICS,...DISCUSSION: ORIGIN OF THE...EVOLUTION OF THE SOMUNCURA...CONCLUSIONSSUPPLEMENTARY DATAAPPENDIXREFERENCES |
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ACKNOWLEDGEMENTS |
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We thank C. W. Rapela, M. Franchi and R. W. Kay for discussion and participation in various aspects of the field work; D. L. Turcotte, W. M. White, B. Coira, and H. Corbella for discussion; J. A. Cortés for Eocene samples from the Meseta del Canquel region; and P. Ziegler for facilities for Ar/Ar dating at Lehigh University. The manuscript benefited from reviews by Marjorie Wilson, Julian Pearce, Charles Stern, and an anonymous reviewer. This research was funded in part by the US NSF (EAR-8803732, EAR-9219328 and EAR-0087515), the Argentine Geologic Survey (SEGEMAR), and an American Republics Fulbright fellowship to Kay.
*Corresponding author. Telephone: 1-607-255-4701. E-mail: smk16{at}cornell.edu
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