Miocene to Late Quaternary Patagonian Basalts (46–478S): Geochronometric and Geochemical Evidence for Slab Tearing Due to Active Spreading Ridge Subduction

Home , Adakite, Basanite, Chile Triple Junction, Taitao Peninsula

Journal of Volcanology and Geothermal Research 149 (2006) 346–370 www.elsevier.com/locate/jvolgeores

Miocene to Late Quaternary Patagonian basalts (46–478S): Geochronometric and geochemical evidence for slab tearing due to active spreading ridge subduction

Christe`le Guivel a,*, Diego Morata b, Ewan Pelleter c,d, Felipe Espinoza b, Rene´ C. Maury c, Yves Lagabrielle e, Mireille Polve´ f,g, Herve´ Bellon c, Joseph Cotten c, Mathieu Benoit c, Manuel Sua´rez h, Rita de la Cruz h

a UMR 6112 bPlane´tologie et GeódynamiqueQ, Universite´ de Nantes, 2 rue de la Houssinie`re, 44322 Nantes, France b Departamento de Geologıá. Fac. Cs. Fı´sicas y Matema´ticas, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile c UMR 6538 bDomaines oceániquesQ, UBO-IUEM, place Nicolas-Copernic, 29280 Plouzane´, France d CRPG-CNRS UPR A2300, BP 20, 54501 Vandoeuvre-les-Nancy, France e UMR 5573, Dynamique de la Lithosphe`re, Place E. Bataillon, case 60, 34095, Montpellier Cedex 5, France f LMTG-OMP, 14 Avenue E. Belin, 31400 Toulouse, France g IRD-Departamento de Geologia de la Universidad de Chile, Chile h Servicio Nacional de Geologıá y Minerıá, Avda. Santa Marıá 0104, Santiago, Chile Received 18 May 2005; received in revised form 29 August 2005; accepted 14 September 2005

Abstract

Miocene to Quaternary large basaltic plateaus occur in the back-arc domain of the Andean chain in Patagonia. They are thought to result from the ascent of subslab asthenospheric magmas through slab windows generated from subducted segments of the South Chile Ridge (SCR). We have investigated three volcanic centres from the Lago General Carrera–Buenos Aires area (46–478S) located above the inferred position of the slab window corresponding to a segment subducted 6 Ma ago. (1) The Quaternary Rı´o Murta transitional basalts display major, trace elements, and Sr and Nd isotopic features similar to those of oceanic basalts from the 87 86 SCR and from the Chile Triple Junction near Taitao Peninsula (e.g., ( Sr/ Sr)o =0.70396–0.70346 and qNd=+5.5À+3.0). We consider them as derived from the melting of a Chile Ridge asthenospheric mantle source containing a weak subduction component. (2) The Plio-Quaternary (b3.3 Ma) post-plateau basanites from Meseta del Lago Buenos Aires (MLBA), Argentina, likely derive from small degrees of melting of OIB-type mantle sources involving the subslab asthenosphere and the enriched subcontinental lithospheric mantle. (3) The main plateau basaltic volcanism in this region is represented by the 12.4–3.3-Ma-old MLBA basalts and the 8.2–4.4-Ma-old basalts from Meseta Chile Chico (MCC), Chile. Two groups can be distinguished among these main plateau basalts. The first group includes alkali basalts and trachybasalts displaying typical OIB signatures and thought to derive from predominantly asthenospheric mantle sources similar to those of the post-plateau MLBA basalts, but through slightly larger degrees of melting. The second one, although still dominantly alkalic, displays incompatible element signatures intermediate between those of OIB and arc magmas (e.g., La/NbN1 and TiO2 b2 wt.%). These intermediate basalts differ from their strictly alkalic equivalents by having lower High Field Strength Element (HFSE) and higher qNd (up to +5.4). These features are consistent with their derivation from an enriched mantle source contaminated by ca. 10% rutile-bearing restite of altered oceanic crust. The petrogenesis of the studied Mio-Pliocene basalts from MLBA and MCC is consistent with contributions of the subslab

* Corresponding author. E-mail address: [email protected] (C. Guivel).

0377-0273/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2005.09.002 C. Guivel et al. / Journal of Volcanology and Geothermal Research 149 (2006) 346–370 347 asthenosphere, the South American subcontinental lithospheric mantle and the subducted Pacific oceanic crust to their sources. However, their chronology of emplacement is not consistent with an ascent through an asthenospheric window opened as a consequence of the subduction of segment SCR-1, which entered the trench at 6 Ma. Indeed, magmatic activity was already important between 12 and 8 Ma in MLBA and MCC as well as in southernmost plateaus, i.e., 6 Ma before the subduction of the SCR-1 segment. We propose a geodynamic model in which OIB and intermediate magmas derived from deep subslab asthenospheric mantle did uprise through a tear-in-the-slab, which formed when the southernmost segments of the SCR collided with the Chile Trench around 15 Ma. During their ascent, they interacted with the Patagonian supraslab mantle and, locally, with slivers of subducted Pacific oceanic crust that contributed to the geochemical signature of the intermediate basalts. D 2005 Elsevier B.V. All rights reserved.

Keywords: slab window; slab tear; plateau basalts; alkali basalts; ridge subduction; Patagonia

1. Introduction asthenospheric windows which opened successively when segments of the Chile ridge bounded by large Neogene and Quaternary magmatic activity in the fracture zones (FZ) were subducted. Fig. 1B shows Patagonian Andes displays numerous specific features that the subduction of these various segments, accord- which can be related to the subduction of the seg- ing to their magnetic anomaly patterns, started at ca. mented South Chile Ridge (SCR) beneath the South 15–14 Ma (SCR-4, south of Desolacio´n FZ), 14–13 American plate. During the last 15 Ma, the location of Ma (SCR-3, south of Madre de Dios FZ), 12 Ma this ridge subduction (the Chile Triple Junction, CTJ) (SCR-2, south of Esmeralda FZ), 6 Ma (SCR-1, be- migrated northwards as a result of the oblique colli- tween Esmeralda and Tres Montes FZ), 3 Ma (SCR0, sion between the Chile ridge and the South American between Tres Montes and Taitao FZ) and finally 0.3 margin (Herron et al., 1981; Cande and Leslie, 1986; Ma (SCR1, north of Taitao FZ), respectively (Cande Cande et al., 1987; Nelson et al., 1994; Bangs and and Leslie, 1986; Forsythe et al., 1986). Cande, 1997; Tebbens and Cande, 1997; Tebbens et In this paper, we test this model using new geochro- al., 1997). The present location of the CTJ, ca. 50 km nometric (K–Ar) and geochemical (major, trace element north of the Taitao Peninsula (Fig. 1A), is marked by and Sr and Nd isotopic data) on basalts from the Lago near-trench magmatic activity (Forsythe and Nelson, General Carrera–Buenos Aires area (46–478S) in south- 1985; Forsythe et al., 1986, 1995; Lagabrielle et al., ern Patagonia. This area is located at the latitude of the 1994, 2000; Bourgois et al., 1996; Guivel et al., 1999, present Chile Triple Junction position (Fig. 1B), along 2003) and a corresponding gap in the Andean calc- the Chile–Argentina border, south of Mt. Hudson, the alkaline volcanic belt between the southern part of the southernmost active volcano of the SSVZ. As shown in Southern Volcanic Zone (SSVZ, 41815V–468S) and the Fig. 1B, it overlies the SCR-1 slab window present Austral Volcanic Zone (AVZ, 49–548S) (Stern et al., position inferred from magnetic anomalies (Cande 1990; Ramos and Kay, 1992). East of the Andean and Leslie, 1986; Tebbens et al., 1997; Lagabrielle et chain, the Patagonian back-arc domain is characterised al., 2000). Three Miocene to Quaternary basaltic com- by numerous Neogene basaltic plateaus (Mesetas), the plexes are exposed in this area on both sides of the emplacement of which does not seem to be connected Argentina/Chile border (Fig. 1C): Meseta Chile Chico either with back-arc extension or with a topographic (Chile) which is capped by a basaltic pile dated back to swell or hotspot track (Ramos and Kay, 1992). Nu- 8.2–4.4 Ma (Espinoza et al., 2005), Meseta del Lago merous authors (Ramos and Kay, 1992; Kay et al., Buenos Aires (Argentina) for which available K–Ar 1993; Gorring et al., 1997, 2003; D’Orazio et al., and Ar–Ar ages range from 10.0 to 0.76 Ma (Ton- 2000, 2001, 2003; Gorring and Kay, 2001) have pro- That et al., 1999) and 10.1 Ma to b110 ka (Brown et posed that these basaltic magmas were produced by al., 2004), and finally Rı´o Murta (Chile) subglacial melting of subslab asthenospheric mantle upwelling basalts, previously considered Holocene (Demant et through slab windows generated from subducted al., 1994, 1998; Corgne et al., 2001). We will show ridge segments (Dickinson and Snyder, 1979; Thor- that the timing and geochemistry of most of these kelson, 1996; Murdie and Russo, 1999). Especially, basaltic eruptive events do not fit with the hypothesis Gorring et al. (1997) and Gorring and Kay (2001) of their derivation from the subslab asthenospheric pointed out that the spatial distribution, ages and mantle from the SCR-1 fragment, and that alternative chemistries of the Neogene basaltic plateaus of South- models of opening of asthenospheric windows or tears- ern Argentina fit apparently with the locations of in-the-slab need to be envisioned. 348 C. Guivel et al. / Journal of Volcanology and Geothermal Research 149 (2006) 346–370

2. Regional geology Ridge beneath the Andean continental margin (Fig. 1A). Plate reconstructions by Cande and Leslie Most authors consider that the Miocene–Recent (1986) indicate that initial ridge collision started at evolution of the Patagonian Andes has been controlled 15–14 Ma at ca. 558S, forming a triple junction (the by the oblique northward subduction of the Chile Chile Triple Junction, CTJ) between South America, C. Guivel et al. / Journal of Volcanology and Geothermal Research 149 (2006) 346–370 349

Fig. 2. Plot of the ages of the southern Patagonian basalts against latitude. Where reported, errors are in 1r. The latitudes and ages of arrival to the trench of South Chile Ridge segments SCR-3, SCR-2, SCR-1, SCRO and SCR1 are also shown. Sources of previously published ages: Pali Aike (D’Orazio et al., 2000), Estancia Glencross (D’Orazio et al., 2001), Condor Cliff (Gorring et al., 1997), Meseta de la Muerte (Gorring et al., 1997), Meseta Central (Gorring et al., 1997), Meseta Belgrano (Gorring et al., 1997), Northeast region (Gorring et al., 1997), Cerro Pampa (in Kay et al., 1993), Meseta del Lago Buenos Aires (Ton-That et al., 1999, Brown et al., 2004, and this work), Meseta de Chile Chico (Espinoza et al., 2005), Murta basalts (this work).

Nazca and Antarctic plates. Then, the CTJ migrated as strombolian cones, maars and flows filling channels northwards up to its present position at ca. 468S(Fig. or paleolandscapes. 1B). On the continent, the last major compressive The ages of these basaltic plateaus, based on available phase which affected the Patagonian fold and thrust K–Ar and Ar–Ar dates, have been plotted against lati- belt started at ca. 15 Ma (Lagabrielle et al., 2004) and tude in Fig. 2. The general pattern suggests that mag- is generally considered as a consequence of ridge– matic activity started between 12 and 8 Ma all along the trench collision. Then, in the back-arc domain, an back-arc domain of the Patagonian thrust and fold belt, important Neogene magmatic event led to the emplace- from 528S to the present position of the CTJ at 468S. ment of large basaltic plateaus (Mesetas). It started at Unlike Ramos and Kay (1992), Kay et al. (1993), Gor- ca. 12 Ma, more or less simultaneously with the em- ring et al. (1997, 2003) and Gorring and Kay (2001),we placement of Cerro Pampa adakites which have been find no evidence for a trend towards younger ages north- interpreted as partial melts of the young subducted wards which might be correlated with the chronology of Pacific oceanic slab (Kay et al., 1993). Gorring et al. the subduction of the successive Chile Ridge segments. (1997, 2003) and Gorring and Kay (2001) have dis- Young (Pliocene–Late Quaternary) or relatively young tinguished two stages of building of the basaltic mese- volcanic activity is evidenced both at ca. 528S (Pali Aike tas. Thick lava flows, generally tholeiitic, were erupted volcanic field, D’Orazio et al., 2000), 6508S (Camusu´ during the main plateau stage. Then, after a quiescence Aike, D’Orazio et al., 2005), 49856.6V8S (Condor Cliff, period sometimes several million years long, volcanic Gorring et al., 1997) and near 47–468S in Meseta del activity resumed, emplacing smaller amounts of post- Lago Buenos Aires (Gorring et al., 2003; Brown et al., plateau basaltic lavas (usually alkali basalts or basa- 2004) and Rı´o Murta (Demant et al., 1998). nites) richer in incompatible elements than the main The studied area (Lago General Carrera–Buenos plateau basalts. Post-plateau basalts generally crop out Aires, 46–478S and 70–738W) is located within the

Fig. 1. Geological setting of the studied volcanic rocks. (A) Simplified tectonic sketch map of the present-day Chile Triple Junction (CTJ) showing the location of the studied area and the sense of motion (black arrows) of the Nazca and Antarctic plates with respect to the South American Plate; numbers are relative velocities in cm/yr (DeMets et al., 1990); (B) simplified map of the CTJ area and location of the fracture zones (FZ) and active segments of the South Chile Ridge (SCR) indicating the ridge collision ages (grey numbers) and the present-day inferred locations of subducted active ridges (SCR0, SCR-1, SCR-2) (simplified from Guivel et al., 1999; Lagabrielle et al., 2004). Grey triangles: southernmost volcanoes of the South Volcanic Zone (SVZ) and northernmost volcanoes of the Austral Volcanic Zone (AVZ); empty triangle: Cerro Pampa adakite; (C) simplified geological map of the Lago General Carrera–Buenos Aires area in Patagonia (46–478S) showing the location of the studied areas in which Neogene and Quaternary volcanic rocks are located (modified from Lagabrielle et al., 2004). Local geological sketch maps of these areas are shown in Fig. 3. 350 C. Guivel et al. / Journal of Volcanology and Geothermal Research 149 (2006) 346–370 modern volcanic arc gap, south of the SSVZ and north of intruded into Palaeozoic to Plio-Quaternary units (Fig. the AVZ. In this zone, the Miocene to Late Quaternary 3). Strongly deformed Palaeozoic low- to medium-grade magmatic rocks investigated were emplaced over and/or metasediments of the Eastern Andean Metamorphic

Fig. 3. Local geological sketch maps of the studied volcanic areas. (A) Rı´o Murta basalts (modified from SERNAGEOMIN 1:1,000,000 unpublished map); (B) Meseta Chile Chico (MCC) basaltic plateau (simplified from Espinoza et al., 2005); (C) Meseta del Lago Buenos Aires (MLBA) basaltic plateau (simplified from SEGEMAR 1:750,000 map). The whole rock K–Ar ages and the location of dated samples are indicated. C. Guivel et al. / Journal of Volcanology and Geothermal Research 149 (2006) 346–370 351

Complex crop out west and south of the Lago General dillera front, in the Cosmeli basin and below the basal- Carrera–Buenos Aires (Bell and Sua´rez, 2000). Middle tic sequences of the Meseta del Lago Buenos Aires. to Late Jurassic rhyolitic ignimbrites and lava flows They are mostly fluviatile with local marine intercala- (with minor andesitic to basaltic intercalated flows) be- tions and correspond to the Guadal, Galera (in Chile) longing to the large silicic Chon Aike Province (Pan- and Santa Cruz (in Argentina) Formations (Lagabrielle khurst et al., 1998, 2000; Fe´raud et al., 1999) et al., 2004). Plio-Quaternary moraines and glacial unconformably overlie the metamorphic rocks. These deposits are mostly located in the eastern side of the rocks are referred to as the Iba´n˜ez Group in Chile and Lago General Carrera–Buenos Aires. Plutonic rocks are the El Quemado Complex in Argentina. Late Jurassic to represented by Meso-Cenozoic subduction-related gran- Lower Cretaceous marine sedimentary rocks (Coyaique itoids of the North Patagonian Batholith (Pankhurst et Group) and subaerial volcanics associated with conti- al., 1999), and small Late Miocene and Pliocene satellite nental sedimentary rocks (Divisadero Group), which plutons (Pankhurst et al., 1999; Sua´rez and De La Cruz, overlie diachronically the Iba´n˜ez Group (Sua´rez et al., 2001; Thomson et al., 2001; Morata et al., 2002). 1996), are mostly cropping out in the north of the Quaternary volcanic structures belonging to the studied area and below the basaltic sequences of the SSVZ occur north of the studied area. The Cay, Meseta Chile Chico (Fig. 3B). Cenozoic sedimentary Maca, Isla Colorada and Hudson volcanoes (Fig. formations are mainly exposed to the East of the Cor- 1B) are the southernmost volcanic centres of the

Table 1 K/Ar age data. 1: Rio Murta; 2: Meseta Chile Chico, 3: Meseta Lago Buenos Aires 40 40 36 Lab. Sample Rock type Loc K2O Ar* Ar* Ar Weight Age (Ma) number (%) (Â10À 7 cm3/g) (%) (Â10À 9 cm3/g) (g) 6012-8 PG 01a Basaltic lava flow 1 1.00 0.291 10.5 0.844 1.0069 0.90F0.08 6023-9 PG 06a Basalt, pillow lava 1 0.65 0.178 8.0 0.694 1.0045 0.85F0.10 6298-9 PG 102 Basaltic lava flow 1 0.33 0.029 0.7 1.434 1.0083 0.27F0.39 6315-2 PG 102 Basaltic lava flow 1 0.33 0.034 1.1 1.921 1.7992 0.32F0.30 6316-3 PG 102 Basaltic lava flow 1 0.33 1.8001 0.21F0.27 6046-7 PG 37a Basaltic plug 2 1.42 2.123 42.7 0.971 1.0064 4.63F0.13 6340-5 PG-138a Basaltic lava flow 2 0.96 1.383 19.9 1.891 1.0033 4.46F0.22 FE01-36a Basaltic lava flow 2 0.99 0.168 89 4.4F0.8 CC317-2a Basaltic lava flow 2 1.57 0.468 58 7.6F0.4 FE01-23a Basaltic plug 2 1.13 0.348 59 7.9F0.4 CC-313a Basaltic lava flow 2 1.33 0.426 28 8.2F0.5 6063-7 PG 44 Basaltic lava flow 3 2.78 0.966 26.8 0.899 1.0074 1.08F0.04 6047-8 PG 52 Basaltic lava flow 3 1.20 3.968 56.6 1.033 1.0012 10.23F0.26 5962-7 PG 51 Basaltic lava flow 3 1.77 1.962 41.8 0.945 1.0235 3.44F0.10 6022-8 PG 65 Basaltic lava flow 3 1.23 1.979 37.9 1.100 1.0009 4.98F0.15 6032-9 PG 67 Basaltic lava flow 3 0.58 1.297 37.1 0.742 1.0038 6.95F0.24 6373-4 PG 69 Basaltic lava flow 3 1.14 1.589 17.7 2.505 1.0025 4.32F0.23 6037-4 PG 72 Basaltic lava flow 3 1.74 2.143 39.6 1.116 1.0077 3.91F0.11 6056-8 PG 75 Basaltic lava flow 3 0.85 1.319 14.1 2.754 1.0112 4.81F0.32 6301-3 PG 105 Basaltic dyke 3 1.26 2.659 26.1 2.557 1.0039 6.53F0.25 6297-8 PG 108 Basaltic lava flow 3 1.28 2.414 33.2 1.655 1.0077 5.80F0.19 6312-7 PG 109 Basaltic lava flow 3 1.30 2.366 32.0 1.702 1.0011 5.64F0.19 6287-7 PG 113 Basaltic lava flow 3 0.85 1.603 28.7 1.346 1.0005 5.84F0.21 6288-8 PG 114 Basaltic neck 3 1.10 3.854 50.7 1.273 1.0052 10.84F0.28 6313-8 PG 116 Basaltic lava flow 3 1.13 3.643 57.4 0.918 1.0016 9.97F0.25 6302-4 PG 119 Basaltic lava flow 3 0.97 3.360 44.1 1.442 1.0028 10.71F0.29 6278-6 PG 120 Basaltic lava flow 3 0.86 3.389 42.3 1.569 1.0016 12.18F0.34 6314-1 PG 121 Basaltic lava flow 3 2.25 0.864 14.5 1.722 1.0013 1.19F0.08 6279-7 PG 130 Basaltic lava flow 3 1.96 2.177 35.5 1.356 1.0133 3.44F0.11 6280-8 PG 132 Basaltic lava flow 3 2.38 2.549 39.2 1.389 1.0385 3.32F0.10 6286-6 PG 133 Basaltic lava flow 3 2.25 2.646 35.8 1.624 1.0108 3.64F0.11 6296-7 PG 134 Basaltic lava flow 3 1.76 2.210 27.9 1.936 1.0009 3.89F0.14 6317-4 PG 143 Teschenite 3 1.60 6.396 48.4 2.316 1.0045 12.36F0.33 Analytical method is detailed in the text. a Data from Espinoza et al. (2005). 352 C. Guivel et al. / Journal of Volcanology and Geothermal Research 149 (2006) 346–370

SSVZ, and their location is controlled by the dextral less enriched in large ion lithophile elements (D’Orazio transcurrent Liquin˜e–Ofqui fault zone (Lo´pez-Escobar et al., 2003). et al., 1995), which originated in response to the oblique subduction of the Nazca plate beneath South 3. Analytical methods America (Cembrano et al., 1996). Previous geochemical data on these volcanoes have been published by One hundred samples (12 from Rı´o Murta, 22 Lo´pez-Escobar et al. (1993), Demant et al. (1994) and from Meseta Chile Chico, 62 from Meseta del Lago D’Orazio et al. (2003). The geochemical signatures of Buenos Aires) were selected on the basis of their the Cay, Maca and Isla Colorada lavas are typically petrographic freshness (macroscopic and microscop- calc-alkaline, while the Hudson lavas are comparatively ic), low Loss on Ignition (LOI) values and geological

Table 2 Major and trace element data for Murta basalts Sample PG01 PG04a PG04b PG05a PG05b PG06a PG07v PG101 PG102 PG104a PG104b PG104c Lat. 8S46808V21 46812V58 46812V58 46812V23 46812V23 46812V23 46811V26 46808V41 46808V50 46812V25 46812V25 46812V25 Long. 8W72836V52 72848V23 72848V23 72848V21 72848V21 72848V21 72848V10 72837V01 72837V27 72848V17 72848V17 72848V17 wt.%

SiO2 49.60 47.90 48.00 48.00 48.80 48.00 48.00 48.80 48.80 48.00 48.20 47.60 TiO2 2.09 1.22 1.65 1.70 1.18 1.57 1.36 1.52 1.50 1.71 1.36 1.25 Al2O3 17.70 20.00 18.55 18.30 21.30 18.25 18.00 17.80 17.60 18.00 19.10 19.00 Fe2O3* 11.40 8.63 10.28 10.50 7.60 10.22 9.45 9.70 9.90 10.52 9.00 9.08 FeO 8.72 6.60 7.86 8.03 5.81 7.82 7.23 MnO 0.19 0.13 0.16 0.17 0.12 0.16 0.15 0.16 0.16 0.17 0.14 0.14 MgO 5.20 7.35 7.22 7.00 5.80 7.50 7.35 7.25 7.52 6.95 7.30 8.10 CaO 8.00 9.50 9.80 9.62 10.40 9.85 9.75 10.00 10.40 9.60 10.10 10.00

Na2O 4.34 3.25 3.70 3.60 3.57 3.50 3.40 3.40 3.54 3.40 3.30 3.15 K2O 1.00 0.58 0.66 0.65 0.53 0.61 0.50 0.44 0.42 0.68 0.57 0.51 P2O5 0.51 0.20 0.28 0.29 0.20 0.28 0.23 0.25 0.25 0.28 0.24 0.23 LOI À0.32 1.26 À0.11 0.28 0.49 0.20 1.64 0.73 À0.20 0.50 0.27 0.30 Total 99.71 100.02 100.19 100.11 99.99 100.14 99.83 100.05 99.89 99.81 99.58 99.36 Mg# 51.53 66.50 62.07 60.84 64.01 63.10 64.45 63.53 63.90 60.62 65.40 67.52 ne,- hy 0.65 À0.70 2.95 1.73 0.98 1.75 0.15 À2.28 1.10 0.37 0.46 0.51 ppm Rb 15 9.7 12 12 9 11.5 12 12.8 7 11.4 9.7 9 Sr 480 540 495 485 600 485 435 475 480 480 522 530 Ba 285 88 110 118 90 116 100 98 165 115 101 100 Th 2.2 0.8 1.3 1.3 0.8 0.9 0.7 0.7 0.9 1 0.95 0.7 Sc 25 20.8 26.2 28.5 18.7 27.5 27 32 34 30 25 22 V 221 142 190 210 138 200 175 210 215 230 180 165 Cr 56 53 60 64 48 65 85 145 165 65 68 57 Co 29 36 38 38 30 40 41 37 36 38 38 42 Ni 33 70 59 49 53 62 72 74 74 51 69 84 Y 38 19 26 29.5 19 26 24.5 26.8 27 28.5 23 21 Zr 185 106 146 154 101 148 140 145 150 162 130 120 Nb 11 6.5 8.8 9 5.7 8.5 5.8 5 5.1 9 7.4 6.7 La 22.5 9 11.7 12 8.7 11.2 9.7 10.2 10.2 12.7 10.3 9.3 Ce 52.5 20 29 29.5 19.5 26 25 26 25.5 30 24 21.5 Nd 30 12 18 19 11.5 17.5 15.5 15.5 16.8 19.5 15 13.5 Sm 6.9 3.1 4.4 4.7 3 4.1 3.9 4.1 4.2 4.7 3.8 3.5 Eu 2.25 1.16 1.5 1.57 1.14 1.48 1.35 1.54 1.54 1.65 1.4 1.26 Gd 6.8 4.2 4.9 5.2 4 4.8 4.5 4.8 4.7 5.5 4.4 3.9 Dy 6.35 3.4 4.65 5 3.3 4.5 4.2 4.7 4.7 5.15 4.1 3.65 Er 3.4 2.1 2.7 3 2 2.7 2.5 2.7 2.7 2.8 2.3 2 Yb 3.2 1.84 2.4 2.6 1.85 2.4 2.3 2.5 2.6 2.7 2.15 1.92 Analytical method is detailed in the text. 2+ *Total Fe as Fe2O3; LOI: loss on ignition; Mg#=Mg/(Mg+Fe ) assuming Fe2O3/FeO=0.15; ne, -hy=wt.% normative nepheline or wt.% normative hypersthene (minus sign). C. Guivel et al. / Journal of Volcanology and Geothermal Research 149 (2006) 346–370 353 position from a set of ca. 150 samples collected Sr and Nd isotopic data were measured on a single during fieldwork investigations in 1998, 2001 and batch of HCl 2N leached whole rock powder; 70 mg of 2002. powder were dissolved in a HNO3–HF mixture from Mineral analyses (available on request to the authors) which Sr and Nd were eluted. Nd was run on a double were obtained using a five spectrometer Cameca SX-50 Re filament and Sr on a W filament with Ta activator. electron microprobe (Microsonde Ouest, Brest, France). Measurements were performed on a Finnigan MAT 261 Analytical conditions were 10–12 nA, 15 kV, counting and Thermo Triton T1 mass spectrometers (Universite´de time 6 s. A detailed account of the procedure is given in Bretagne Occidentale, Brest). Sr and Nd isotopic ratios Defant et al. (1991). Whole rock 40K–40Ar datings of were calculated at t =0 using the 40K–40Ar determined Meseta del Lago Buenos Aires and Rıó Murta basalts ages. Nd initial ratios are expressed as qNd. The errors on (Table 1) were performed on the 0.5- to 0.15-mm-size 87Sr/86Sr and 143Nd/144Nd ratios are reported in Table 4. fraction after crushing, sieving and cleaning with dis- tilled water of whole-rock samples. Analyses were car- 4. Field data and K–Ar geochronometry ried out at the Laboratoire de Geóchronologie, Universite´ de Bretagne Occidentale (Brest, France). 4.1. Rıó Murta basalts (Chile) One aliquot of sample was powdered for K analysis by atomic absorption after HF chemical attack and These basalts, previously investigated by Demant et 0.5–0.15-mm grains were used for argon isotopic anal- al. (1994, 1998) and Corgne et al. (2001), crop out in yses. Argon extraction was performed by the direct the bottom of the glacial valley of Rıó Murta, dug into technique under high vacuum (10À 5–10À 7 hPa) using the North Patagonian Batholith granitoids some 30 km induction heating of a molybdenum crucible. The argon SSE of Hudson volcano (Fig. 3A). Their total preserved content was measured by isotope dilution and argon volume is small (b1km3, Demant et al., 1998). They isotopes were analysed in a 1808 stainless steel mass occur either as columnar jointed basaltic flows in the spectrometer, according the original procedure de- Rıó Murta river bed, eroded down to a few metres by scribed by Bellon et al. (1981). Age calculations, fol- the stream, or as subglacial and sublacustrine volcanics. lowing the equation of Mahood and Drake (1982) and These include pillow lavas and lava tubes up to 3 m in using the Steiger and Ja¨ger’s (1977) recommended con- diameter with glassy chilled margins, as well as hyalo- stants, are given, with 1r error, in Table 1. The K–Ar clastic breccias associated with varved clays, moraines ages on Meseta de Chile Chico basalts discussed in this and tills. On the basis of field evidence for local sub- paper are taken from Espinoza et al. (2005). glacial emplacement followed by moderate erosion, Major and trace-element analyses (Tables 2 and 3) former authors have considered them as Holocene. were conducted on agate-ground powders by inductive- However, two out of three K–Ar dates given in Table ly coupled plasma–atomic emission spectroscopy (ICP- 1 point out to an emplacement at ca. 0.85–0.9 Ma, AES) except Rb which was determined with flame while the third one (b0.5 Ma) obtained on the upper- atomic emission spectroscopy, at the Universite´de most part of a flow occupying the main Rıó Murta Bretagne Occidentale (Brest, France) and checked stream bed might be consistent with an Holocene age. against IWG-GIT standards BE-N, AC-E, PM-S and WS-E. Relative standard deviation is ca. 1% for SiO2 4.2. Meseta Chile Chico (MCC, Chile) and 2% for the other major elements except for low values (b0.50% oxide) for which the absolute standard The MCC flood basalt cover (Fig. 3B), up to 900– deviation is 0.01%. For trace elements, relative standard 1000 m thick, is composed of two sequences (Espinoza deviation is ca. 5% except for concentrations below six et al., 2005). The basal Lower Basaltic Sequence times the detection limit, for which the absolute stan- (LBS), which unconformably overlies the Mesozoic dard deviation is about one third of the detection limit. Ibań˜ez Group and the Neocomian Cerro Colorado For- Detection limits are 2 ppm for Ba, V, Cr, Co, Ni, Zr and mation, is formed by a 500–550-m-thick pile of Eocene Ce; 1 ppm for Nd, Gd and Er; 0.5 ppm for Rb, Sr, Nb, basaltic lava flows (57–40 Ma, Charrier et al., 1979; La and Sm; 0.3 ppm for Y, Dy and Th; 0.15 ppm for Sc, Baker et al., 1981; Petford et al., 1996; Espinoza et al., Eu and Yb. Specific details for the analytical methods 2005) crosscut by several basanitic necks and diatrems. and sample preparation can be found in Cotten et al. These Eocene basalts can be correlated with the 57–45 (1995). The major and trace-element data on Meseta Ma Posadas Basalt (Baker et al., 1981; Kay et al., 2002) Chile Chico basalts discussed in this paper are taken and with the 42F6 Ma Balmaceda Basalts (Baker et from Espinoza et al. (2005). al., 1981; Demant et al., 1996), which crop out east and 354 Table 3 Selected major and trace element data for Meseta del Lago Buenos Aires volcanic rocks Sample PG143 PG65 PG67 PG75 PG105 PG113 PG114 PG132 PG133 PG51 PG52 PG69 PG72 PG108 Age 12.4 4.98 6.95 4.81 6.53 5.84 10.84 3.32 3.64 3.45 10.23 4.32 3.91 5.8 Lat. 8S46838V41 47804V 47804V 47803.602 46843V33 46846V47 46846V33.6 47809V28 47809V30 47810V14 47820V15 47804V 47804.08 294948 UTM Long. 8W71828V08 71848V 71848V 71849.141 71842V06 71842V07 71842V47,9VV 71833’25,6VV 71833V19 71832V22 70848V18 71848V 71847.995 4820953 UTM Type MP-inter MP-inter MP-inter MP-inter MP-inter MP-inter MP-inter MP-inter MP-inter MP-alk MP-alk MP-alk MP-alk MP-alk wt.% .Gie ta./Junlo ocnlg n etemlRsac 4 20)346–370 (2006) 149 Research Geothermal and Volcanology of Journal / al. et Guivel C. SiO2 48.50 47.00 47.35 50.00 47.90 48.30 47.15 55.00 51.80 48.80 48.70 47.75 47.80 47.50 TiO2 1.76 1.48 1.43 1.54 1.64 1.61 1.25 1.40 1.68 2.22 2.44 2.16 2.63 2.06 Al2O3 16.55 15.85 15.70 17.20 16.25 16.70 15.60 18.40 16.50 16.00 15.82 17.60 17.75 16.10 Fe2O3* 10.30 11.55 11.42 10.66 10.34 11.60 10.36 8.65 13.10 10.80 12.91 10.77 11.92 11.90 MnO 0.15 0.17 0.18 0.17 0.16 0.17 0.18 0.19 0.23 0.16 0.18 0.16 0.17 0.17 MgO 6.42 8.37 9.40 5.66 8.54 7.15 10.40 2.07 2.63 7.03 4.92 6.28 4.78 7.12 CaO 8.45 9.65 9.60 9.10 8.65 9.60 10.00 5.30 5.75 8.20 8.60 9.05 8.00 9.85 Na2O 3.97 2.80 3.28 3.42 3.70 3.38 2.47 5.85 4.58 4.14 3.56 3.68 3.76 3.54 K2O 1.60 1.21 0.61 0.89 1.48 1.03 1.28 2.60 2.33 1.80 1.16 1.41 1.66 1.42 P2O5 0.43 0.40 0.38 0.29 0.66 0.44 0.41 0.91 1.28 0.68 0.54 0.46 0.53 0.54 LOI 1.41 0.75 À0.11 0.75 0.28 À0.27 0.57 À0.26 À0.52 À0.50 0.39 0.43 0.64 À0.23 Total 99.54 99.23 99.24 99.68 99.60 99.72 99.67 100.11 99.36 99.33 99.22 99.75 99.64 99.97 Mg# 59.23 62.81 65.73 55.30 65.81 58.96 70.06 35.80 31.87 60.27 47.04 57.61 48.31 58.24 ne,-hy 3.68 0.74 1.66 À10.49 3.78 1.09 0.77 1.13 À10.39 4.40 À6.71 3.17 1.40 4.61 ppm Rb 40.5 27.5 22 17.5 34.5 21.5 39.5 46.5 41 31.5 20.5 23.5 27 23.5 Sr 685 725 708 444 870 555 700 718 534 742 595 688 682 682 Ba 345 310 287 190 455 260 340 965 600 470 440 315 415 320 Th 4.95 3.5 3.6 2.5 5.2 2.5 4 5.8 6.7 3.25 2.8 3.05 3.35 3 Sc 23 29 29 26 22 28 30 10 12 19 23.5 22 21 26 V 200 215 213 198 190 225 240 46 54 174 240 210 240 235 Cr 167 286 320 150 220 181 485 2 2 200 28 42 12 185 Co 35 43 45 35 39 41 45 11 20 38 42 38 36 43 Ni 68 162 182 59 180 95 212 1 2 115 32 50 26 107 Y 23 26 23.7 25 25 25 23 33 44 30 30 24.5 29.5 245 Zr 164 157 149 141 210 149 124 294 500 240 205 205 232 180 Nb 20 11.6 11.4 12 24.5 15.3 11 45 53 37 29 31 36.5 30 La 28 24.5 24 14.5 38.5 21 22 55 61 37 26 28.5 31 28 Ce 56 55 53 34 75 45 47.5 109 124 74 53 60 68 57 Nd 29 34 30 18 36.5 24 26 47 52 36 32 28 33 30.5 Sm 6.3 6.85 6.3 4.4 7 5.6 5.5 9.2 11.7 7.5 7 6.1 7 6.7 Eu 1.86 1.95 1.81 1.42 2.07 1.76 1.64 3.2 3.28 2.26 2.22 1.92 2.15 2.07 Gd 6.2 5.7 5.45 4.3 5.8 5 4.7 7.7 9.9 6.9 7.2 5.9 6.8 6 Dy 4.4 4.6 4.35 4.35 4.7 4.5 4 6.2 8 5.5 5.4 4.6 5.45 4.8 Er 2.2 2.3 2.2 2.25 2.4 2.3 2 3 4 2.7 2.8 2.1 2.6 2.2 Yb 1.9 2.18 2.12 2.12 2.1 2.16 2 2.76 3.8 2.6 2.3 1.95 2.32 1.94 Analytical methods detailed in the text. MP: main plateau; PP: post-plateau; Alk.: alkali; Int.: intermediate. K–Ar ages in Ma. 2+ *Total Fe as Fe2O3; LOI: Loss on ignition; Mg#=Mg/(Mg+Fe ) assuming Fe2O3/FeO=0,15; ne,- hy=wt.% normative nepheline or wt.% normative hypersthene (minus sign). Sample PG109 PG116 PG119 PG120 PG130 PG134 PG44 PG121 PG41 PG46 PG50 PG123 PG126 PG127 Age 5.64 9.97 10.71 12.18 3.44 3.89 1.08 1.19 Lat. 8S 294962 UTM 47804V29,2VV 47806V08 47806V13,6VV 47809V32 47810V27 46852V24 47806V28 46841V24 47803V35 47807V42 47803V05 47803V13 47803V35 Long. 8W 4820912 UTM 71801V21,1VV 70859V59,6VV 70859V34,5VV 71833V15,3VV 71832V29,4VV 70844V08 70859V16,5VV 70849V48 70846V54 70851V58 71802V55,4VV 71802V50,6 71801V44,4VV Type MP-alk MP-alk MP-alk MP-alk MP-alk MP-alk PP-alk PP-alk PP-alk PP-alk PP-alk PP-alk PP-alk PP-alk .Gie ta./Junlo ocnlg n etemlRsac 4 20)346–370 (2006) 149 Research Geothermal and Volcanology of Journal / al. et Guivel C. SiO2 47.00 47.10 47.20 47.20 49.00 48.50 45.90 46.85 43.50 44.50 46.00 47.80 47.60 48.10 TiO2 2.05 2.36 2.38 2.34 2.71 2.31 2.29 2.56 2.64 2.34 2.45 2.03 2.43 2.42 Al2O3 16.00 16.20 16.25 16.10 17.00 16.75 14.50 17.25 13.15 14.35 14.40 16.65 17.45 16.85 Fe2O3* 11.95 11.95 12.04 12.00 13.20 10.75 10.80 10.75 12.08 12.15 11.50 10.40 11.76 11.35 MnO 0.17 0.16 0.16 0.17 0.20 0.16 0.17 0.16 0.18 0.18 0.17 0.15 0.17 0.15 MgO 7.45 7.03 7.40 7.40 3.85 7.01 8.10 5.37 10.30 10.14 8.80 7.05 4.54 5.87 CaO 9.60 9.55 9.32 9.90 6.85 7.72 8.15 9.70 10.60 9.40 9.42 7.42 9.55 6.70 Na2O 3.48 3.54 3.80 3.63 4.35 4.16 5.00 4.04 3.50 3.55 3.65 4.65 3.67 5.19 K2O 1.35 1.14 1.17 1.10 2.04 1.85 2.90 2.22 2.31 1.90 2.28 2.13 1.27 2.33 P2O5 0.50 0.44 0.43 0.42 0.86 0.62 1.50 0.74 1.05 0.69 0.86 0.79 0.41 0.83 LOI 0.13 0.54 À0.59 À0.26 À0.67 0.06 0.29 À0.19 À0.05 0.08 À0.26 0.51 1.14 À0.27 Total 99.68 100.01 99.56 100.00 99.39 99.89 99.60 99.45 99.26 99.28 99.27 99.58 99.99 99.52 Mg# 59.23 57.82 58.89 58.97 40.47 60.31 63.61 53.79 66.52 66.04 64.07 61.24 47.36 54.65 ne,-hy 4.43 3.47 5.02 4.79 1.59 4.50 16.26 9.67 14.71 11.22 9.67 9.02 2.79 10.55

Rb 22 14.3 13.8 12.9 36 33 46.5 40 37 33 43 31.5 15.2 35 Sr 685 600 590 582 578 761 1290 855 1020 790 870 810 615 885 Ba 285 235 215 220 480 442 715 560 755 525 670 390 250 460 Th 3 1.75 1.8 1.6 4.6 3.7 9.4 6.1 7.4 5 6.15 3.3 2.15 4.8 Sc 25 23 24 25 20 19 14.5 25 28 22.5 24 17 25 13.3 V 225 232 235 255 175 165 153 240 265 225 240 150 200 130 Cr 190 205 220 240 2 175 225 83 345 270 240 140 100 98 Co 42 43 44 48 27 37 42 32 52 51 49 37 42 35 Ni 124 75 86 91 6 114 190 42 215 215 170 134 44 87 Y 24 23.5 22.5 22.5 37.5 29 28 27.5 27 22.5 25 25 25.5 25 Zr 185 159 152 158 395 252 420 267 300 225 275 252 167 268 Nb 29 24 24 23.5 48 38 98 61 76 62 67 38 21 60 La 27.5 19 18.4 17.5 47 36 90 47 67 45 54 32 18.3 42 Ce 54 39 40 38 94 73 153 89 117 85 97 65 40 81 Nd 29.5 22.5 22.5 22 45.5 34 63 40.5 56 39.5 46 32 22.5 38.5 Sm 6.55 5.5 5.25 5.2 10 7.3 11 8.15 9.4 7.5 8.5 6.8 5.15 7.9 Eu 2.05 1.82 1.8 1.78 2.92 2.25 3.2 2.41 2.91 2.23 2.45 2.14 1.83 2.53 Gd 5.6 5.1 5.2 5.1 9 6.7 8.25 7 7.4 6.4 6.8 5.9 5.8 6.9 Dy 4.7 4.3 4.35 4.2 7.15 5.6 5.6 5.25 5.35 4.6 5.05 4.75 4.8 5.15 Er 2.1 2.1 2 2 3.5 2.7 2.6 2.4 2.5 2.2 2.25 2.2 2.2 2 Yb 1.88 1.75 1.74 1.69 3.25 2.46 1.8 2.04 1.87 1.6 1.8 1.97 2.09 1.81 355 356 C. Guivel et al. / Journal of Volcanology and Geothermal Research 149 (2006) 346–370

Table 4 Sr and Nd isotopic data 87 86 143 144 Sample Loc. SiO2 Age Mg# TiO2 La/Nb ( Sr/ Sr)0 2s ( Nd/ Nd) 2r qNd PG102 1 48.80 0.27 63.90 1.50 2.00 0.703532 4 0.512920 10 5.50 PG01a 1 49.60 0.90 51.53 2.09 2.05 0.703958 3 0.512792 8 3.00 PG06a 1 48.00 0.85 63.10 1.57 1.32 0.703460 5 0.512916 7 5.42 IBA47a 1 46.75 65.00 1.51 0.703590 0.512900 5.11 FE01-36b 2 46.01 4.40 1.42 2.03 0.704140 0.512873 4.58 PG65 3 47.00 4.98 62.81 1.48 2.11 0.704109 11 0.512910 10 5.31 PG75 3 50.00 4.81 55.30 1.54 1.21 0.704260 10 0.512751 11 2.20 PG105 3 47.90 6.53 65.81 1.64 1.57 0.704364 3 0.512795 10 3.06 PG113 3 48.30 5.84 58.96 1.61 1.37 0.704362 4 0.512730 9 1.79 PG114 3 47.15 10.84 70.06 1.25 2.00 0.704215 6 0.512821 7 3.57 PG108 3 47.50 5.80 58.24 2.06 0.93 0.704367 5 0.512736 8 1.91 PG109 3 47.00 5.64 59.23 2.05 0.95 0.704394 5 0.512724 7 1.68 PG116 3 47.10 9.97 57.82 2.36 0.79 0.703996 7 0.512799 9 3.14 Analytical methods are detailed in the text. International standards (NBS 987, La Jolla) are run regularly on both mass spectrometers. Typical values are (1) Triton T1 87Sr/86Sr=0.710250F12, 143Nd/144Nd=0.511850F6 and (2) 87Sr/86Sr=0.710251F16 143Nd/144Nd=0.512104F6 (JNdi). Average blanks for this study are 0.2 for Nd and 0.4 ng for Sr. Loc.: location in Fig. 1 (1=Rı´o Murta; 2=Meseta Chile Chico; 3=Meseta del Lago Buenos Aires). Ages from Table 1; geochemical values from Tables 2 and 3. a Data from Demant et al. (1998). b Data from Espinoza et al. (2005). north of the MCC area, respectively. The rather flat these flows are interbedded with glacial tills (Ton-That upper surface of the MCC, which covers ca. 300 km2, et al., 1999). The MLBA post-plateau lavas have been corresponds to the 400-m-thick Upper Basaltic Se- studied in detail by Gorring et al. (2003) and dated by quence (UBS) of Miocene–Pliocene tabular basaltic Brown et al. (2004) and Singer et al. (2004). They lava flows and necks which provided K–Ar ages of erupted from more than 150 small monogenetic volca- 8.2, 7.9, 7.6, 4.6, 4.5 and 4.4 Ma (Espinoza et al., nic centres (strombolian and spatter cones and very 2005). They are underlain by two rhyolitic flows well-preserved maars). The flows form a volcanic pile dated at 13.1 and 9.8 Ma, respectively. On the basis usually ca. 100 m thick topping the MLBA, and many of their age range and field features, they can be of them poured down its eastern slopes. Brown et al. considered equivalent to the main plateau basaltic se- (2004) published 31 isochron 40Ar/39Ar ages ranging quence (Gorring et al., 2003) of the Meseta del Lago from 3.3 to less than 0.1 Ma for these post-plateau Buenos Aires (MLBA) documented below. Very young MLBA basalts. Different volcanic pulses have been cones and associated flows do not occur on the MCC, recognised by these authors at 3.2–3.0 Ma, 2.4 Ma, 1.7 which therefore has apparently not undergone any mag- Ma, 1.35 Ma, 1.0 Ma, 750 ka, 430–330 ka, and finally, matic event equivalent to the post-plateau volcanic b110 ka. Other 40Ar/39Ar and K–Ar ages obtained by phases of MLBA and other Patagonian plateaus. Singer et al. (2004) from lavas interbedded with moraine deposits range from 1016F10 ka to 109F3 ka. 4.3. Meseta del Lago Buenos Aires (MLBA, Argentina) We have measured 22 new K–Ar ages (Table 1 and Fig. 3C) on MLBA basalts: 2 on the post-plateau lavas This Meseta (Fig. 3C) is one of the largest (ca. 6000 (1.19F0.08 Ma and 1.08F0.04 Ma) and 20 on the km2) basaltic plateaus in the Patagonian back-arc domain plateau sequence. These latter ages allow to ex- main. Its main plateau sequence (Gorring et al., 1997, tend the known range of the main plateau activity from 2003) is composed of an up to 300-m-thick pile of 12.18F0.34 Ma (PG 120) to 3.32F0.10 Ma (PG 132) tabular basaltic lava flows overlying the Miocene and to identify a quiescence period between ca. 10 and molasse sediments of the Rı´o Zeballos Group (Santa 7 Ma. Cruz Formation). Previous K–Ar and 40Ar/39Ar ages of The oldest volcanic pile crops out, as noticed by these main plateau basalts (Sinito, 1980; Baker et al., previous authors, along the southeastern border of the 1981; Mercer and Sutter, 1982; Ton-That et al., 1999) plateau, and especially along the trail from Estancia La range from 10 to 4.5 Ma and more recently Brown et al. Vizcaina to Laguna del Sello. Three samples from (2004) obtained three 40Ar/39Ar isochron ages of 10.12, tabular lava flows from this cross section gave ages 7.86 and 7.71 Ma on lavas from this sequence. Some of of 12.18F0.34 Ma (PG-120, 1000 m), 10.71F0.29 C. Guivel et al. / Journal of Volcanology and Geothermal Research 149 (2006) 346–370 357

Ma (PG-119, 1050 m) and finally 9.97F0.25 Ma (PG- along the Paso Roballos road, the lowest lava flows of 116, 1170 m) from bottom to top, respectively. This the main plateau sequence gave ages of 5.84F0.21 sequence is unconformably overlain by younger lava Ma (PG 113), 5.80F0.19 Ma (PG 108) and flows (sample PG-121, 940 m, 1.19F0.08 Ma old) of 5.64F0.19 Ma (PG 109), respectively. These flows the post-plateau sequence (Fig. 3C) which poured out overlie the Rı´o Zeballos Group Miocene molasse down its slopes towards the plain near Estancias La which is crosscut by an older basaltic neck dated at Vizcaina and Casa de Piedra. 10.84F0.28 Ma (PG 114). A good cross section of the top of the main plateau Finally, a few mafic rocks cropping out away from sequence is exposed in the southern border of the MLBA also provided K–Ar ages consistent with those Meseta along the Hacienda El Ghio horse-trail near of the main plateau building stage. One of them, a Rı´o Torrentoso. Four samples of tabular flows from columnar jointed hypovolcanic intrusion locally re- this cross section provided K–Ar ages ranging from ferred to as a btescheniteQ body, located north of the 3.89F0.14 Ma (sample PG-134, 980 m) to 3.32F0.10 MLBA near Estancia Las Chicas, has been dated back Ma (sample PG-132, 1455 m). A 40-m-thick glacial till to 12.40F0.33 Ma (sample PG 143) and a basaltic lava unit is interbedded between this latter sample and two flow 5 km east of Bajo Caracoles, near the SE edge of slightly older flows dated at 3.44F0.11 Ma and the MLBA, at 10.23F0.26 Ma (sample PG 52). 3.64F0.11 Ma, respectively. These data suggest that the main plateau stage of the MLGA volcanism ended 5. Petrologic and geochemical data at 3.3 Ma, and thus, that there was no significant time gap separating it from the post-plateau stage which 5.1. Sample classification started at 3.3 Ma according to Brown et al. (2004). In addition, they provide a rather precise dating for one A large majority of the studied rocks are petrograph- of the Pliocene glacial events already documented in ically fresh and their Loss On Ignition (LOI) values the area by Ton-That et al. (1999). range from slightly negative to ca. 1 wt.%. According On the western border of the Meseta, near Estancia to the TAS diagram shown in Fig. 4, they are mostly Los Corrales located 38 km south of Los Antiguos basaltic (basalts, basanites and trachybasalts) although a

Fig. 4. Total alkali–silica classification diagram (Le Maitre et al., 1989) for the Miocene to Late Quaternary igneous rocks recalculated to 100 wt. %, anhydrous basis. The heavy line represents the boundary between alkaline and subalkaline series (Irvine and Baragar, 1971). Open diamonds: Rı´o Murta basalts. Open triangles: intermediate main plateau lavas from Meseta Chile Chico (MCC). Black triangles: alkaline main plateau lavas from MCC; Open squares: intermediate main plateau lavas from Meseta del Lago Buenos Aires (MLBA). Black squares: alkaline main plateau lavas from MLBA. Black diamonds: post-plateau lavas from MLBA. Data on Meseta Chile Chico (MCC) intermediate and primitive lavas are from Espinoza et al. (2005). 358 C. Guivel et al. / Journal of Volcanology and Geothermal Research 149 (2006) 346–370 few samples plot in the fields of basaltic trachyandesite (LILE) and Light Rare-Earth Elements (LREE), and and trachyandesite. We have classified MLBA samples sometimes as well by specific isotopic signatures. from our data set according to their main plateau or These features are clearly observed in our MLBA and post-plateau position, based on their field relationships MCC data set, in which about 30% of the samples and K–Ar ages. For the latter, we have postulated that depart from the usual compositional range of OIB by the transition between the main plateau and post-pla- displaying La/Nb ratios greater than unity (up to 3.7) teau stages occurred abruptly at 3.3 Ma (Brown et al., and TiO2 contents usually lower than 2 wt.% (Fig. 5A 2004 and our data as discussed above). As discussed and B). Their relative depletion in Nb is unlikely to above, no post-plateau activity can be identified in result from fractionation of Ti-magnetite during differ- MCC, where all the Mio-Pliocene lavas crop out either entiation because their Nb contents increase with de- as a tabular flow pile or as necks and dykes crosscutting creasing Mg numbers (Mg#, Fig. 5C). Consequently, them (Espinoza et al., 2005). we have identified by specific symbols in all the geo- A further discrimination has been operated within chemical diagrams the MLBA and MCC samples char- our data set. Indeed, basaltic lavas from both the acterised by La/NbN1. Stern et al. (1990) termed these MLBA and the MCC display chemical features very btransitionalQ basalts, but we will rather use this word to similar to those considered typical of Ocean Island describe the Rı´o Murta basalts which plot in between Basalts (OIB), as already shown by previous authors the fields of alkalic and subalkalic basalts in most (Hawkesworth et al., 1979; Baker et al., 1981; Ramos geochemical diagrams. Therefore, the btransitionalQ and Kay, 1992). However, Stern et al. (1990), Gorring basalts of Stern et al. (1990) will be referred to here et al. (1997, 2003), Gorring and Kay (2001) and Espi- as bintermediateQ samples (i.e., intermediate between noza et al. (2005) have shown that some Patagonian OIB and subduction-related lavas), as opposed to the main plateau and post-plateau basalts display weak to other samples, termed bcratonicQ basalts by Stern et al. moderate bsubduction-relatedQ geochemical imprints (1990) and which display typical OIB characteristics traduced by relative depletions in High Field Strength (including La/Nbb1). The petrogenesis of the Elements (HFSE) vs. Large Ion Lithophile Elements bintermediateQ lavas will be discussed separately below.

Fig. 5. Selected plots of major (recalculated to 100 wt.%, anhydrous basis) elements, major element parameters, and trace elements (in ppm). (A)

TiO2–Mg#; (B) TiO2–La/Nb; (C) Nb–Mg#; (D) normative nepheline–Mg#. Symbols as in Fig. 4. C. Guivel et al. / Journal of Volcanology and Geothermal Research 149 (2006) 346–370 359

5.2. Major elements and petrographic types plateau MLBA stage and from MCC. The intermediate group is only represented in MLBA (main plateau) and Most of the studied basalts are silica-undersaturated MCC by alkali basalts, trachybasalts, basaltic trachyan- and contain up to 22 wt.% normative nepheline (Fig. desites and trachyandesites. It is noteworthy that main 5D), while a minority (including 8 over 12 Rı´o Murta plateau lavas from both Meseta Chile Chico and Lago samples) is hypersthene-normative. In the TAS dia- Buenos Aires are silica-undersaturated (alkali basalts gram, the MLBA and MCC samples plot consistently and basanites), while most of other Neogene Patago- above Irvine and Baragar’s (1971) limit between alka- nian main plateaus are principally made up of tholeiitic lic and subalkalic compositions while Murta samples basalts (Gorring and Kay, 2001). spread around this limit. The Murta basalts also fit Middlemost’s (1975) requirements for the definition of 5.3. Mineral chemistry transitional basalts, i.e., they plot in the subalkalic field in the K2O–SiO2 diagram and in the alkalic The Murta basalts are porphyritic, with plagioclase field in the Na2O–SiO2 diagram. They will thus be (An73–48Ab26–50Or1–2), olivine (Fo87–72) and minor au- termed transitional. All MCC Mio-Pliocene mafic gite (Wo46En42Fs12) phenocrysts set in a microcrystal- lavas are alkali basalts, trachybasalts or basaltic tra- line groundmass of plagioclase (An68Ab31Or1), olivine chyandesites (Espinoza et al., 2005; Figs. 4 and 5) (Fo82–73), Ti-magnetite and acicular quenched Ti-rich whether or not they display bintermediateQ La/Nb augite (Wo50En25Fs25). Megacrysts of plagioclase (up ratios (N1). MLBA post-plateau basalts contain usual- to 2 cm in size) and clinopyroxene are frequent in some ly 8–22 wt.% normative nepheline (Fig. 5D) and plot samples. in the fields of basanites and tephrites in the TAS The Mio-Pliocene MCC basaltic lava flows, plugs diagram (Fig. 4). MLBA main plateau basalts, inter- and dykes (Espinoza et al., 2005) bear low amounts mediate or not, are merely alkali basalts, the CIPW of olivine (Fo83–65) and minor clinopyroxene (Wo45 norms of which contain either small amounts of neph- En40–46Fs8–15) phenocrysts set into a microlitic to eline (less than 5%) or of hypersthene (Fig. 5D and subdoleritic groundmass containing olivine (Fo75–54), Table 3). The TAS diagram of Fig. 4 also shows that augite (Wo49–43En44–37Fs13–14), plagioclase (An68–66) the MLBA-MCC alkali basaltic/basanitic samples dis- and Ti-magnetite. playing La/Nbb1 and TiO2 N2 wt.% plot consistently The MLBA main plateau basalts contain plagioclase in the basalt/basanite/trachybasalt fields while the in- (An55–70), clinopyroxene (Wo49En42Fs9) and olivine termediate samples are basalts, trachybasalts or basal- (Fo86–62) phenocrysts set into a microlitic groundmass tic trachyandesites. The basalts range from primitive containing plagioclase (An66), olivine (Fo81–60), rare (Mg#N65, MgON8 wt.%) to evolved (Fig. 5A, C, D) clinopyroxene, Ti-magnetite and glass. The MLBA and the patterns of major element variations vs. Mg# post-plateau basalts are very fresh vesicular aphyric to (not shown) suggest the occurrence of fractionation moderately porphyritic lava flows, with occasional pla- effects involving olivine, clinopyroxene, plagioclase gioclase (An66–72), clinopyroxene (Wo52En38Fs10) and and titanomagnetite. Normative nepheline contents olivine (Fo83–73) phenocrysts. Their glassy to microlitic tend to decrease with Mg# (Fig. 5D), a pattern often groundmass contains plagioclase, olivine (Fo69), clin- observed in alkali basalt series. opyroxene (Wo51En40Fs9), Ti-magnetite and glass. In short, the distribution of petrographic types within Corroded quartz xenocrysts, always rimmed by small the studied sample set is relatively simple: Rı´o Murta clinopyroxene aggregates, are often present in basaltic lavas are transitional basalts while all the other ones are lava flows from MCC and MLBA, indicating that they of dominant alkali affinity. Among the latter, two experienced some extent of crustal contamination. groups can be distinguished: a genuine alkali one The compositions of calcic clinopyroxene pheno- with typical OIB geochemical signatures (La/Nbb1 crysts from MCC and MLBA are typical of those and TiO2 N2 wt.%), which will be referred to as the from alkaline intraplate basalts. They are Ti- and Al- balkali groupQ and a second one (the bintermediate rich and plot consistently within the alkalic fields in groupQ) displaying incompatible element signatures in- Leterrier et al.’s (1982) diagrams (not shown). termediate between those of OIB and arc lavas (La/ NbN1 and TiO2 b2 wt.%). The alkali group includes 5.4. Trace-element features the post-plateau MLBA lavas, all of which are strongly silica-undersaturated (basanites and tephrites), together Compatible element contents of the Murta basalts with alkali basalts and trachybasalts from the main are consistently low (Table 2). Those of MLBA lavas 360 C. Guivel et al. / Journal of Volcanology and Geothermal Research 149 (2006) 346–370

(Table 3), as well as those from MCC basalts, range sition of Primitive Mantle (Sun and McDonough, 1989) from concentrations close to those of near-primitive are shown in Fig. 6. basaltic magmas (Co=45–55 ppm, Ni=200–220 ppm, Patterns of selected Murta transitional basalts have Cr=300–450 ppm) to very low ones. They decrease been plotted in Fig. 6a together with those of represen- rather abruptly with Mg# (diagrams not shown), a tative basaltic samples from the Chile Ridge (Klein and feature consistent with the occurrence of olivine and Karsten, 1995) and from Hudson volcano (Lo´pez-Esco- clinopyroxene fractionation effects. Incompatible trace- bar et al., 1993). Murta basalts display slightly LILE- element abundance patterns normalised to the compo- and LREE-enriched patterns (average primitive mantle-

Fig. 6. Primitive mantle normalised (Sun and McDonough, 1989) incompatible multi-element patterns. (A) Rı´o Murta transitional basalts, Chile Ridge (bold patterns, samples D34-1 and D42-4; Klein and Karsten, 1995) and Hudson volcano basalts (dashed patterns, samples Hud-1 and Hud-3; Lo´pez-Escobar et al., 1993) are shown for comparison; (B) intermediate MCC lavas (analyses in Espinoza et al., 2005); (C) intermediate MLBA main plateau lavas; (D) alkaline MCC lavas (analyses in Espinoza et al., 2005); (E) alkaline main plateau MLBA lavas; (F) alkaline post-plateau MLBA lavas. C. Guivel et al. / Journal of Volcanology and Geothermal Research 149 (2006) 346–370 361

normalised (La/Sm)N =1.75) and are slightly but signif- related to differentiation processes (e.g., fractionation of icantly depleted in Nb relative to K and La. These Ti-magnetite) but is pristine and linked to the nature of features are very similar to those of Chile Ridge seg- their source. It is worth to note that the range of La/Nb ment 3 sample D42-4 (Klein and Karsten, 1995) dis- ratios found in intermediate main plateau basalts (MCC playing subduction-related geochemical affinities. and MLBA) is far greater (Fig. 5B) than the one (La/ Sample PG01a is slightly richer in all trace elements Nb*b1.3; Nb*=17ÂTa) found by Gorring et al. but Sr than other less differentiated Murta samples. (2003) in some post-plateau alkali basalts. However, its incompatible trace-element pattern is par- Main plateau alkali basalts from both MCC and allel to the others, a feature consistent with olivine and MLBA show trace-element patterns typical of OIB plagioclase fractionation. (Fig. 6D and E). They display levels of LILE and Incompatible trace-element patterns of intermediate LREE enrichment similar to those of intermediate main plateau basalts from MCC and MLBA are shown basalts (average (La/Sm)N ratios of 3.09 and 2.90, is Fig. 6B and C, respectively. Both groups of interme- respectively) but lack the negative Nb anomalies typical diate basalts share the same trace-element characteris- of the latter. tics. They display LILE- and LREE-enriched patterns Post-plateau alkali basalts from MLBA display (average primitive-mantle normalised (La/Sm)N =2.72 very homogeneous trace element compositions. Like for MCC basalts and (La/Sm)N=3.00 for MLBA the MLBA main plateau alkali basalts, they show basalts) and are noticeably but variably depleted in OIB-like trace element patterns (Fig. 6F) with an Nb. This depletion in Nb seems to be attenuated with average (La/Sm)N =3.93, slightly higher than that of increasing differentiation, as Nb contents in intermedi- the main plateau lavas. They display a wide range of ate basalts increase when Mg# decreases (Fig. 5C). This LREE concentrations ((La/Yb)N from 6.3 to 37) with feature implies that the more or less pronounced Nb almost constant HREE contents (YbN ranging from depletion of the intermediate basalts is unlikely to be 3.25 to 4.26).

87 86 Fig. 7. (A) Plot of ( Sr/ Sr)o against qNd for the studied lavas. White diamonds: basalts (b1 Ma) from Rı´o Murta (grey diamond, data from 87 86 Demant et al., 1998); white squares: intermediate basalts from MLBA; black squares: alkali basalts from MLBA. (B) Plot of ( Sr/ Sr)o against qNd for the studied lavas and isotopic ratios from other Patagonian magmatic rocks: fields of the alkaline post-plateau (b1 Ma) basalts from Meseta Buenos Aires (Gorring et al., 2003) and the main plateau sequences from Patagonian Basaltic Field as defined by Gorring and Kay (2001), the Cerro Pampa adakites from Kay et al. (1993); Pacific MORB from Peate et al. (1997); black circles: basaltic andesites from the Chile Trench Taitao Ridge (CTR) (Guivel et al., 2003); and white circles: heterogeneous South Chile Ridge (SCR) basalts (Klein and Karsten, 1995). For geochemical modelling, two mixing curves have been calculated (ticks every 10%). [A]: mixing model between mantle source similar to that of the alkali basalts and adakitic melt derived from slightly altered oceanic crust (black star); [B]: mixing model between similar mantle source than in model [A] and slightly altered mid-ocean ridge basalt (black star); (see Table 5 for model parameters and see text for details). 362 C. Guivel et al. / Journal of Volcanology and Geothermal Research 149 (2006) 346–370

5.5. Sr and Nd isotopic data SCR basalts. As the SCR-1 ridge segment which entered the trench 6 Ma ago is thought to be pres- The isotopic compositions of the studied Mio-Plio- ently located beneath the studied area (Fig. 1B), the cene magmatic rocks are listed in Table 4 and plotted Murta basalts may represent melts from the subslab in Fig. 7A and B. Published data from Murta basalts SCR-1 asthenospheric mantle passing through the as well as additional isotopic ratios from other Pata- slab window, as envisioned by previous authors gonian magmatic rocks are also plotted on this figure. (Demant et al., 1998; Corgne et al., 2001). Their 87 86 Initial strontium isotopic ratios ( Sr/ Sr)o are scat- chemical similarities with Hudson magmas might tered between 0.70346 and 0.70439 and corresponding suggest that this window (or its zone of influence) 143 144 ( Nd/ Nd)o between 0.51291 and 0.51272. The extended northwards to the edge of the SSVZ, as isotopic compositions closest to Mid-Oceanic Ridge already pointed out by D’Orazio et al. (2003). The Basalts (MORB) are those from Murta transitional ba- specific geochemical features of sample PG-01a, i.e., 87 86 143 144 salts (( Sr/ Sr)o =0.70346–0.70353; ( Nd/ Nd)o = lower Mg#, strong negative Nb anomaly and respec- 0.512916À0.512920; qNd=+5.4À+5.5), with the ex- tively more (Sr) and less (Nd) radiogenic isotopic 87 86 ception of sample PG-01a (( Sr/ Sr)o =0.70396; signature compared to the others, might reflect crust- 143 144 ( Nd/ Nd)o =0.512792; qNd=+3.0) which is more al contamination effects of subslab magmas, likely differentiated as shown by its Mg#=51.5 (Table 2) and by the Patagonian Batholith through which they is also characterised by a strong negative Nb anomaly ascended. (La/Nb=2, Table 2). The Murta samples plot within the mantle array. Their signature is clearly different 6.2. Origin of the MLBA and MCC main and post- from those of Cerro Pampa adakites, but rather plateau alkali basalts and basanites similar to that of basaltic andesites from the Chile Trench Taitao Ridge (Guivel et al., 2003). Main The petrogenesis of Plio-Pleistocene basalts from plateau MLBA alkali basalts plot in the same MLBA has been previously studied by Hawkes- field of Fig. 7B that MLBA post-plateau basalts worth et al. (1979), Baker et al. (1981) and Gor- (Gorring et al., 2003). In this diagram, MLBA ring et al. (2003). These works have demonstrated intermediate basalts define a sub-vertical trend root- the highly alkaline affinity of these basalts (nephe- ed in the former field and evolving outside this line-normative basanites and alkali basalts) with a field towards higher 143Nd/144Nd at rather constant strong OIB-like geochemical signature and relatively 87Sr/86Sr ratios. enriched Sr/Nd isotopic ratios (87Sr/86Sr= 0.7041– 0.7049; 143Nd/144Nd=0.51264–0.51279). These fea- 6. Discussion tures have been interpreted by Gorring et al. (2003) as consistent with their derivation from an OIB-like 6.1. A subslab asthenospheric origin for the Rıó Murta source involving the deep subslab asthenospheric basalts mantle, together with a contribution of the enriched subcontinental lithospheric mantle (predominantly EM Rıó Murta transitional basalts have trace elements I type). The study of peridotitic xenoliths within Pata- patterns very similar to some Hudson volcano lavas gonian basalts (Rivalenti et al., 2004) has documented (Lo´pez-Escobar et al., 1993) and also to those of a lithospheric mantle metasomatised by asthenosphere- Enriched MORB (E-MORB) from the anomalous derived alkali basaltic melts. Our new isotopic data on segment 3 of the SCR (Klein and Karsten, 1995) main plateau MLBA alkali basalts plot within the (Fig. 6A). These E-MORB from segment 3 of the previously determined field of Patagonian basalts SCR show trace-element patterns very similar to (Fig. 7B). Although we basically agree with the inter- those of convergent margin magmas. This feature pretations of Gorring et al. (2003), we did not find any has been interpreted as reflecting contamination of unquestionable evidence for a specific geochemical the SCR mantle source by various amounts of either imprint of the subslab asthenosphere opposed to that oceanic sediments and altered oceanic crust or melts/ the supraslab (i.e., mantle wedge) enriched astheno- fluids derived from (Klein and Karsten, 1995). The sphere proposed by Stern et al. (1990). Moreover, our Sr/Nd isotopic ratios of Rıó Murta transitional basalts data do not allow us to ascribe the origin of the EM I overlap those of the very heterogeneous SCR basalts signature to either the Patagonian lithospheric mantle (Fig. 7B). These features allow us to infer that their or to an heterogenous subslab or supraslab astheno- source may be identical to the mantle source of the spheric mantle. C. Guivel et al. / Journal of Volcanology and Geothermal Research 149 (2006) 346–370 363

The alkali basalts from MLBA (main plateau) and 6.3. Origin of the MLBA and MCC intermediate lavas the basanites from MLBA (post-plateau; Gorring et al., 2003) display rather similar Sr and Nd isotopic ratios Alkali basalts and intermediate lavas from both (Fig. 7A and B and Table 4), suggesting their derivation MCC and MLBA (main plateau) display roughly sim- from a single (or similar) type(s) of enriched mantle ilar chemical characteristics and tend to plot along the source(s). However, the variable slopes of their incom- same trends in some diagrams. Because of the present patible trace element patterns (Fig. 6D–F) are consistent lack of isotopic data on MCC basalts, the following with variable degrees of partial melting of such a source discussion will be focused on MLBA (main plateau) (Gorring et al., 2003), the lowest ones corresponding to intermediate lavas. the post-plateau MLBA basanites which show the high- The MLBA intermediate lavas erupted synchro- est La/Yb ratios. Luhr et al. (1995) have plotted near- nously with the MLBA alkali basalts, and both types primitive basalts in a La/Yb vs. Yb diagram to docu- have roughly similar compositions in major elements ment partial melting degrees of an enriched (lherzolitic) (with the exception of TiO2) and trace elements (with mantle source with variable contributions of spinel and the exception of Nb). The intermediate lavas display- garnet. In Fig. 8, where we have used the source ing the highest Nd isotopic ratios are also charac- composition proposed by these authors (La=1.79 terised by high Mg# and strong depletions in Nb ppm and Yb=0.31 ppm), the position of post-plateau and Ti (Fig. 9). MLBA basanites is consistent with 1.5–5% melting of a These features, together with the trend they define source in which garnet is slightly more abundant than with MLBA alkali basalts in Fig. 7, could suggest that spinel. Main plateau MCC and MLBA alkali lavas their genesis was controlled by a mixing process be- would derive from somewhat larger (5–10%) melting tween a component related to the alkali basalts (or their degrees of a less garnet-rich source. However, these mantle source) and a bcontaminantQ characterised by a calculations are rather dependent from the assumed relatively unradiogenic Sr isotopic signature similar to composition of the source. For instance, using the that of the alkali basalts but with higher Nd isotopic source composition proposed by Gorring and Kay ratios (above the mantle array) and a selective depletion (2001), i.e., La=0.885 ppm and Yb=0.423 ppm, in Ti and Nb. leads to obtain very low melting degrees (0.1–2%) for Mature continental crust and oceanic sediments, al- post-plateau MLBA basanites, and lower ones (2–5%) though depleted in Ti and Nb, have Sr isotopic signa- for the main plateau lavas (diagram not shown). tures much more radiogenic than required for the

Fig. 8. Plot of La/Yb against Yb for primitive samples (Mg# N63) of our data set. Symbols as in Fig. 4. Results of non-modal batch melting (Shaw, 1970) of garnet and spinel lherzolite sources (modified from Luhr et al., 1995) are shown. Composition of the source: La=1.79 ppm and Yb=0.31 ppm. The mode of the garnet lherzolite is taken as ol/opx/cpx/gt=60:25:9:6 and that of spinel lherzolite as ol/opx/cpx/sp=58:30:10:2. Phase proportions entering the melt are taken as ol/opx/cpx/gt or sp=10:20:65:5. Partition coefficients for La and Yb were selected from literature values as 0.0002:0.002:0.069:0.01:0.002 and 0.0015:0.049:0.28:4.1:0.007 for the minerals ol/opx/cpx/gt/sp, respectively. 364 C. Guivel et al. / Journal of Volcanology and Geothermal Research 149 (2006) 346–370

Fig. 9. Plots of Mg#, TiO2, La/Nb and Nb against qNd. Symbols as in Fig. 4. bcontaminantQ (Andean continental crust and Leg ODP This mixing model (parameters and results given in site 141 Chile Trench sediments, 87Sr/86Sr=0.715 and Table 5), similar to the former one but for the compo- =0.708, respectively; Stern and Kilian, 1996) and can sition of the contaminant, accounts for the isotopic thus be discarded as potential candidates. Contamina- signature of intermediate MLBA lavas (curve B in tion by adakitic magmas may also be envisioned, as Fig. 7). In addition, the trace-element features of the their composition matches the required trace element basaltic magma derived from such a mix fits with the and isotopic features, providing the adakitic component trace elements patterns of the intermediate lavas except derived from the melting of slightly altered oceanic for the lack of negative Nb anomalies (Fig. 10B and crust with a Sr isotopic signature slightly higher than Model B in Table 5). Thus, other processes must be that of MORB. The addition of up to 10% adakitic melt envisioned to explain these anomalies. (isotopic and trace-element compositions given in Table The negative correlation between Nb and Mg# 5) to the mantle source of MLBA alkali basalts does not observed for intermediate lavas (Fig. 5C) suggests fit the Sr, Nd isotopic trend of the intermediate basalts that the negative Nb anomaly is attenuated when (curve A in Fig. 7B). Moreover, the trace-element differentiation progresses. This feature implies that patterns of basaltic melts derived from such a blend Nb depletion with respect to adjacent incompatible are inconsistent with the observed ones (Fig. 10A and elements is pristine and linked to the mantle source Model A in Table 5), as they display strong positive Sr of the intermediate basalts. It could correspond to the anomalies and Yb depletion typical of adakites, which bweak subduction componentQ detected in ultramafic do not exist in the intermediate basalts. xenoliths of southern Patagonian basalts by Rivalenti Thus, we have tested another model (model B) et al. (2004). However, this Nb depletion is unlikely to involving the mixing between up to 10% of a represent an overall feature of the asthenospheric slightly altered oceanic crust (87Sr/86Sr=0.70375; mantle, as it does not occur in the MLBA post-plateau 143Nd/144Nd=0.5131) and an alkali basalt mantle basanites and main plateau alkali basalts. The origin of source similar to that considered in the former model. this signature could thus be either lithospheric or C. Guivel et al. / Journal of Volcanology and Geothermal Research 149 (2006) 346–370 365

Table 5 Results of mixing calculations and compositions of the corresponding end-members 12345 Mantle source Adakitic melt MORB Model A Model B Rb 2.20 – 4.44 – 23.95 Ba 28.50 306.00 27.80 560.95 283.51 Th 0.30 4.90 0.49 7.59 3.18 Nb 2.90 11.73* 2.30 37.12 27.87 La 2.75 26.60 3.55 48.97 26.99 Ce 5.40 60.90 10.45 101.51 54.74 Sr 68.50 1,886.00 131.00 2,232.71 666.91 Nd 2.95 30.30 9.26 47.58 29.97 Sm 0.66 4.41 3.06 7.74 6.73 Eu 0.21 1.16 1.10 2.12 2.08 Gd 0.56 – 4.27 – 6.21 Dy 0.47 – 4.95 – 5.33 Y 2.40 – 29.30 – 29.49 Er 0.21 – 2.97 – 2.48 Yb 0.19 0.72 2.90 1.06 2.01 87Sr/86Sr 0.704394 0.703636 0.703636 0.703823 0.704261 143Nd/144Nd 0.512724 0.513069 0.513069 0.5129079 0.5128132 qNd 1.68 8.41 8.41 5.26 3.42 (1) Mantle source composition (PG109/10); (2) adakitic melt (RB5 from Cerro Pampa, Nb*=17ÂTa (Kay et al., 1993); (3) MORB composition (sample D20-1) from SCR1 (Klein and Karsten, 1995); (4) model A: 10% batch partial melt of 90% of (1) mixed with 10% of (2); (5) model B: 10% batch partial melt of 90% of (1) mixed with 10% of (3). A constant bulk source mode of 0.58 olivine, 0.275 opx, 0.095 cpx, 0.015 gt and 0.035 spinel was used. Partition coefficients used in calculations are from Gorring and Kay (2001). Isotopic compositions for adakitic melt and MORB are those of slightly altered oceanic crust (DSDP/ODP sites 417/418, flow 300; Staudigel et al., 1995). shallow asthenospheric, and related to a blocalQ—i.e., porated either in the shallow asthenospheric or the not widespread—component derived from slightly al- deep lithospheric Patagonian mantle. tered oceanic crust. Thorkelson (1996) and Thorkelson and Breitsprecher (2005) have shown that the slab 6.4. Tectonic setting of MLBA and MCC: successive edges of an asthenospheric window are able to melt opening of ridge-derived asthenospheric windows? at depth, generating adakitic magmas and leaving restite fragments which may become long-term resi- As discussed above, the petrogenetic features of the dents of the continental lithospheric mantle. However, studied Mio-Pliocene basalts from MLBA (main pla- if the restite becomes entrained in the asthenosphere, it teau) and MCC are consistent with possible contribu- may then undergo partial melting. Furthermore, as Nb tions of the deep subslab asthenosphere, the South concentrations in intermediate basalts are in average American subcontinental lithospheric and astheno- lower than in the alkali basalts (26 vs. 38 ppm), Nb spheric mantle and the subducted oceanic crust to should be retained in some residual mineral during their magma sources. In the slab window opening these processes. Rutile is the best candidate as it model previously developed by Ramos and Kay concentrates only Ti, Nb and Ta and is commonly (1992), Kay et al. (1993), Gorring et al. (1997, 2003) observed as a residual phase during partial melting of and Gorring and Kay (2001), melting may occur at the oceanic basalts under P–T conditions consistent with boundary between the ascending subslab asthenosphere those of hot subduction zones (Ringwood, 1990; and the overlying subcontinental lithosphere, with oc- Foley et al., 2000; Schmidt et al., 2004). Amphibole casional contributions of the downgoing basaltic crust and/or phlogopite could also be considered, but their or of its melting products (Cerro Pampa adakites). occurrence in the restite should be detectable from the Alternatively, subslab-derived melts may interact with behaviour of incompatible elements other than Nb and the subcontinental mantle or with the oceanic crust at Ti. Thus, we propose that the origin of the Ti- and the slab window edges during their ascent towards the Nb-depleted intermediate MLBA basalts could be surface. linked to the contribution to their source of rutile- However, a critical aspect of the slab window model bearing restites of partially melted oceanic crust of Gorring, Kay and co-authors is that they consider from the edges of the asthenospheric window, incor- that several windows developed successively beneath 366 C. Guivel et al. / Journal of Volcanology and Geothermal Research 149 (2006) 346–370

post-plateau lavas seems hardly consistent with their ascent through asthenospheric windows derived from the successive segments of the SCR. Especially, the early magmas of MLBA (main plateau stage) and MCC cannot have ascended through the asthenospheric window generated by the segment SCR-1 which entered the trench at 6 Ma only and is now thought to be located beneath the studied area. Indeed, the post-plateau lavas of MLBA (b3.3 Ma) and the Quaternary basalts of Rı´o Murta may have ascended through this window (Gorring et al., 1997, 2003; Demant et al., 1998; Corgne et al., 2001). In short, discrepancies between the ages of emplacement of Patagonian basaltic plateaus and the age of the subduction of ridge segments lead us to reconsider the modalities and timing of slab window opening beneath Patagonia.

6.5. Proposition of a new tectonic model involving slab tearing linked to spreading ridge collision

As discussed above, emplacement of thick alkali basalt sequences started at ca. 12 Ma, i.e., after the Fig. 10. Primitive mantle normalised (Sun and McDonough, 1989) tectonic phase that built up the Cordillera of the south- incompatible multi-element patterns of compositions obtained from ern Patagonian Andes. Indeed, the Late Miocene basal- batch partial melting (Shaw, 1970) of mantle sources derived from tic flows are roughly horizontal and always appear to mixing calculations (Table 5); (A) model A and (B) model B. A post-date the main folding and thrusting event. This constant bulk source mode of 0.58 olivine, 0.275 opx, 0.095 cpx, contractional phase ended with a major tectonic event 0.0.15 gt and 0.035 spinel was used. Patterns of main plateau MLBA intermediate samples with Mg# N59 are shown for comparison. See recognized at the scale of the entire southern Patagonia, text for details. characterised by thrusting of the pre-Cenozoic rocks of the Cordillera over the frontal Oligocene–Miocene ma- Patagonia during the last 15 Ma, each segment of the rine to continental molasse (Lagabrielle et al., 2004). SCR stopping its activity and thus developing its own This phase occurred necessarily after 16.3 Ma (youn- slab window after colliding with the Chile trench. Such gest known age of the mammal fauna of the continental a process should result into a younging northward molasse) and before 12–10 Ma (the age of the basal pattern of plateau building (Fig. 1) that Gorring et al. flows of the alkali plateau basalts). It is also recorded (1997) claim to have identified from their age data. by fission track analysis east of the present-day topo- Presently available literature ages, combined with our graphic divide where rapid cooling and denudation own K–Ar results, are plotted against latitude in Fig. 2. ceased between 12 and 8 Ma (Thomson et al., 2001). No clear age decrease is observed from South to North, It must be noted that the initiation of the subduction of especially regarding the onset of magmatic activity, and the Chile Ridge at 15–14 Ma in southern Patagonia no connection between the ages and the timing of coincides with this last main contractional phase that arrival of the various SCR segments to the Chile trench affected the entire Cordillera. A period of very rapid is easily identifiable. Magmatic activity seems to begin erosion and peneplanation followed the tectonic uplift, between ca. 12 and 8 Ma for nearly all the dated producing a relatively flat surface on which the alkali volcanic centres including Estancia Glencross (528S), basalts were emplaced. Mesetas Belgrano, Central and de la Muerte, the North- Considering the ages at which segments SCR1, east volcanic region and finally MLBA and MCC (46– SCR0, SCR-1 and SCR-2 entered the trench (0.3, 3, 6 478S). Moreover, a phase of relative paucity of volcanic and 12 Ma Fig. 1B), and assuming that the Patagonian activity seems to have occurred around 7 Ma (except plateau and post-plateau basaltic magmas originated possibly in MLBA), followed by a new pulse starting at from a mantle that ascended through a slab window, ca. 5–4 Ma in many volcanic centres. Thus, the chro- it becomes clear that this (or these) window(s) opened nology of emplacement of the Patagonian plateau and well before the subduction of the corresponding ridge C. Guivel et al. / Journal of Volcanology and Geothermal Research 149 (2006) 346–370 367 segments. This has to be taken into account when volcanic activity started earlier in MLBA than in Estan- attempting to link plateau basalt emplacement to the cia Glencross (528S). opening of such a window. For this reason, we favor a A tentative sketch of our proposed tectono-magmatic model based on the process of slab tearing at depth model of Patagonian plateau basalt emplacement is when collision starts at the trench (van den Beukel, shown in Fig. 11. As compression occurs in the Cordil- 1990) and leading ultimately to slab breakoff (e.g., lera leading to active orogenesis, tension forces are Davies and von Blanckenburg, 1995; von Blancken- applied to the descending slab that will break off be- burg and Davies, 1995; Mason et al., 1998). In such a neath the continental plate. A slab tearing all along the model, slab tear would start when strong tectonic cou- active Patagonian margin results ultimately into the pling in the forearc occurs before the subduction of the detachment and sinking of the deep part of the sub- ridge axis itself. This situation may occur when a series ducted plate. OIB-type magmas would be generated of large spreading ridge segments approaches the by the partial melting of the subslab asthenospheric trench, that is for the segments SCR-4 to SCR-2, all mantle uprising through the tear-in-the-slab, possibly of which arrived in less than 3 Ma within the subduc- near its boundary zone with the overlying continental tion zone (15–14 Ma for SCR-4, 14–13 Ma for SCR-3, lithospheric mantle (Coulon et al., 2002). Alternatively, and 12 Ma for SCR-2). Slab tear may then have prop- heat transfer through the tear might have induced partial agated towards the north into slightly older lithosphere melting of the supraslab mantle (Davies and von emplaced at segment SCR-1. The lack of correlation Blanckenburg, 1995). Then, the ascending magmas between latitude and onset of volcanic activity (Fig. 2) would interact with the Patagonian lithospheric mantle suggests that this propagation was very rapid: indeed, and locally with altered Pacific crust from the edges of the slab tear (intermediate magmas) and ultimately be emplaced in the back-arc domain between ca. 528 and 468S. Finally, for all segments, after slab tear, the spreading axis will enter the trench. The final stages of this evolution will correspond to the opening of btrueQ (ridge-related) slab windows. The latter are responsible for the genesis and ascent of the most recent basaltic magmas, i.e., in the northern and eastern Mesetas (including MLBA post-plateau phase) and Rı´o Murta during a second magmatic pulse starting at ca. 5–4 Ma.

7. Conclusions

1. The Quaternary Rı´o Murta transitional basalts display obvious geochemical similarities to the SCR and CTJ oceanic basalts. We consider them as derived from the melting of a Chile Ridge asthenospheric mantle source containing a weak subduction component. Their position above the inferred location of the slab window corresponding to the SCR-1 segment subducted 6 Ma ago is consistent with a slab window opening model previously developed by Ramos and Kay (1992), Kay et al. (1993), Gor- ring et al. (1997, 2003) and Gorring and Kay (2001). 2. Two groups may be identified among the main plateau basalts of MLBA and MCC. The first one includes alkali basalts and trachybasalts displaying typical OIB signatures and thought to derive from the melting of OIB-type mantle sources involving the deep subslab asthenosphere and the subcontinen- Fig. 11. Cartoon showing the main stages of the proposed tectonic tal mantle, as previously shown by Gorring et al. model of slab tearing during ridge collision at the trench. (1997, 2003). 368 C. Guivel et al. / Journal of Volcanology and Geothermal Research 149 (2006) 346–370

3. The second group of samples, although dominantly Bell, M., Sua´rez, M., 2000. The Rıó Laćteo formation of southern alkalic, displays incompatible element signatures Chile. Late Paleozoic orogeny in the Andes of southernmost South America. J. South Am. Earth Sci. 13, 133–145. intermediate between those of OIB and arc magmas Bellon, H., Quoc Buu¨, N., Chaumont, J., Philippet, J.C., 1981. N b (e.g., La/Nb 1 and TiO2 2 wt.%). These interme- Implantation ionique d’argon dans une cible support: application diate basalts differ from their alkalic equivalents by au traçage isotopique de l’argon contenu dans les mine´raux et les their HFSE-depleted character and their higher qNd roches. C. R. Acad. Sci., Paris 292, 977–980. (up to +5.4). We ascribe these specific features to Bourgois, J., Martin, H., Lagabrielle, Y., Le Moigne, J., Frutos Jara, J., 1996. Subduction erosion related to spreading-ridge subduc- their derivation from an enriched mantle source tion: Taitao Peninsula (Chile margin triple junction area). Geology contaminated by ca. 10% rutile-bearing restite of 24, 723–726. altered oceanic crust, likely derived from the edges Brown, L.L., Singer, B.S., Gorring, M.L., 2004. Paleomagnetism and of a slab window or slab tear. 40Ar/39Ar chronology of lavas from Meseta del Lago Buenos 4. The chronology of emplacement of main plateau Aires, Patagonia. Geochem. Geophys. Geosyst. 5 (1), Q01H04. doi:10.1029/2003GC000526. basalts from MLBA (12.4–3.3 Ma) and MCC Cande, S.C., Leslie, R.B., 1986. Late Cenozoic tectonics of the (8.2–4.4 Ma) is inconsistent with their origin from Southern Chile Trench. J. Geophys. Res. 91, 471–496. an asthenospheric window opened as a consequence Cande, S.C., Leslie, R.B., Parra, J.C., Hobart, M., 1987. Interaction of the subduction of the Chile Ridge segment SCR-1 between the Chile ridge and the Chile trench: geophysical and which entered the trench at 6 Ma. This fact allows us geothermal evidence. J. Geophys. Res. 92, 495–520. Cembrano, J., Herve´, F., Lavenu, A., 1996. The Liquin˜e–Ofqui fault to question the model developed by Gorring et al. zone: a long-lived intra-arc fault system in southern Chile. Tecto- (1997) and Gorring and Kay (2001), in which the nophysics 259, 55–66. Neogene basaltic magmas of Southern Argentina Charrier, R., Linares, E., Niemeyer, H., Skarmeta, J., 1979. K–Ar ages plateaus ascended through asthenospheric windows of basalt flows of the Meseta Buenos Aires in southern Chile and which opened successively when segments SCR-4, their relation to the southeast Pacific triple junction. Geology 7, 436–439. SCR-3, SCR-2 and finally SCR-1 of the Chile Corgne, A., Maury, R., Lagabrielle, Y., Bourgois, J., Suarez, M., ridge were subducted. In our preferred geodynamic Cotten, J., Bellon, H., 2001. La diversite´ des basaltes de Patagonie model, OIB and intermediate magmas of MLBA a` la latitude du point triple du Chili (468–478 lat. S): donneés and MCC, as well as those of other Patagonian comple´mentaires et implications sur les conditions de la subduc- plateaus (Mesetas Belgrano, Central, de la Muerte tion. C. R. Acad. Sci., Paris 333, 363–371. Cotten, J., Le Dez, A., Bau, M., Caroff, M., Maury, R.C., Dulski, P., and the Northeast volcanic region) originated from Fourcade, S., Bohn, M., Brousse, R., 1995. Origin of anomalous deep asthenospheric mantle uprising through a tear- rare-earth element and yttrium enrichments in subaerially ex- in-the-slab subparallel to the trench, which formed posed basalts: evidence from French Polynesia. Chem. Geol. 119, when the southernmost segments of the SCR col- 115–138. lided with the Chile Trench around 15 Ma. Coulon, C., Megartsi, M., Fourcade, S., Maury, R.C., Bellon, H., Louni-Hacini, A., Cotten, J., Coutelle, A., Hermitte, D., 2002. Post-collisional transition from calc-alkaline to alkaline volcanism Acknowledgements during the Neogene in Oranie (Algeria): magmatic expression of a slab breakoff. Lithos 62, 87–110. Davies, J.H., von Blanckenburg, F., 1995. Slab breakoff: a model This research was supported by the cooperation of lithosphere detachment and its test in the magmatism and program ECOS-Sud ACU01 and was part of the Chi- deformation of collisional orogens. Earth Planet. Sci. Lett. 129, lean FONDECYT Project 1000125 and French DyETI 85–102. project 2004–2005. We thank Drs. M. D’Orazio and C. Defant, M.J., Richerson, M., de Boer, J.Z., Stewart, R.H., Maury, Stern for their pertinent and helpful reviews of the R.C., Bellon, H., Drummond, M.S., Feigenson, M.D., Jackson, T.E., 1991. Dacite genesis via both slab melting and differentia- manuscript. 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