Andean Geology 37 (2): 300-328. July, 2010
formerly Revista Geológica de Chile
www.scielo.cl/andeol.htm

 

Middle Miocene calc-alkaline volcanism in Central Patagonia (47°S): petrogenesis and implications for slab dynamics

Volcanismo calcoalcalino durante el Mioceno Medio en Patagonia Central (47°S): petrogénesis e implicaciones en la dinámica de placas

 

Felipe Espinoza^1,3*, Diego Morata¹, Mireille Polvé², Yves Lagabrielle³, René C. Maury⁴, Aude de la Rupelle⁴, Christèle Guivel⁵, Joseph Cotten⁴, Hervé Bellon⁴, Manuel Suárez⁶

¹   Departamento de Geología, Universidad de Chile, Casilla 13518, Correo 21, Santiago, Chile. dmorata@cec.uchile.cl
²   UMR-CNRS 5563 LMTG, Observatoire Midi-Pyrénées, Université Paul-Sabatier, 14 rué Edouard Belin, 31400 Toulouse, France. polve@lmtg.obs-mip.fr
³   UMR-CNRS 5234 Géosciences Montpellier, Université de Montpéllier 2, CC 60, Place Eugéne Bataillon, 34095 Montpellier Cedex 5, France. yves.lagabrielle@gm.univ-montp2.fr
⁴   UMR-CNRS 6538 Domaines Océaniques, Université de Bretagne Occidentale, 6 avenue le Gorgeu, C.S. 93837, 29238 Brest Cedex 3, France. rene.maury@univ-brest.fr; grolotte.aude@hotmail.fr; Jo.Cotten@univ-brest.fr; Herve.Bellon@univ-brest.fr
⁵   UMR-CNRS 6112 Planétologie et Géodynamique, Université de Nantes, 2 rué de la Houssiniére, B. P 92208, 44322 Nantes Cedex 03, France. christelle.guivel@univ-nantes.fr
⁶  Servicio Nacional de Geología y Minería, Avenida Santa María 0104, Providencia, Santiago, Chile. msuarez@sernageomin.cl
*Present address: Servicio Nacional de Geología y Minería, Avenida Santa María 0104, Providencia, Santiago, Chile. fespinoza@sernageomin.cl

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ABSTRACT. We present a chronological (K-Ar), petrologic and geochemical study (major and trace elements, Sr-Nd isotopes) of Middle Miocene (ca. 16-14 Ma) calc-alkaline rocks (basalts to andesites) extruded in the present-day back-arc region of Central Patagonia (Zeballos Volcanic Sequence (ZVS), 47°S). This magmatism started shortly after mafic plutonism ceased in the are region (ca. 16 Ma, 200 km west), and ended ca. 2 My before the onset of volumi-nous slab tear-related back-arc alkaline basaltic magmatism (ca. 12 to Pliocene). The studied calc-alkaline rocks have a typical subduction-related signature (high LILE/HFSE ratios, depletion in Nb, Ta and Ti; Ba/La >20; Ta/Hf <1.5; (⁸⁷Sr/⁸⁶Sr)_o=0.70366-0.70402, ε_Nd=+0,1±3,8). Major and trace elements contents are consistent with their evolution by closed system fractional crystallization of a presumed parental liquid similar in composition to the most basic rock of the suite. Moreover, a strong subducted sediment imprint is recognized (increasing Th/HFSE and decreasing Ce/Pb during differentiation). However, these rocks show striking similarities with volcanic complexes emplaced above areas where a gently dipping slab oceurs (high K contents; similar LREE/HREE, Nb/Zr, Ba/Nb; Th/Hf; Th/Ta, Ta/Hf<0.3), particularly the present-day Andean flat-slab region and the Neuquén Basin during the Late Miocene. A comprehensive tectono-magmatic model is here presented to explain the generation and extrusion of these calc-alkaline magmas during the Middle Miocene. The development of a transient low-angle subduction and the resulting eastward migration of the volcanic front are then proposed. Mixing between stored remnants of calc-alkaline magmatism and later primitive alkaline melts is envisioned as the most likely process accounting for the transitional signature (Le., intermediate between calc-alkaline and alkaline, La/Nb >1; TiO₂ <2 wt%) of some basalts extruded synchronously with genuine alkaline lavas in the Neogene Patagonian Plateau Lavas province.

Keywords: Central Patagonia back-arc, Calc-alkaline magmatism, Low-angle subduction, Miocene.

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RESUMEN. En este trabajo se presenta un estudio cronológico (K-Ar), petrológico y geoquímico (elementos mayores y trazas, isótopos de Sr-Nd) de rocas calcoalcalinas (basaltos a andesitas) del Mioceno Medio (16-14 Ma) que ocurren en una posición de trasarco en Patagonia Central (47°S; Secuencia Volcánica Zeballos, SVZ). Este magmatismo comenzó poco después de que el plutonismo máfico cesara en la region bajo el arco (ca. 16 Ma, 200 km al oeste), y terminó ca. 2 My antes del comienzo de un voluminoso evento magmático basáltico alcalino desarrollado en el trasarco, relacionado con ventanas astenosféricas, que continuó hasta el Pleistoceno. Las rocas calcoalcalinas estudiadas tienen una química típicamente relacionada con procesos de subducción (altas razones LILE/HFSE, empobrecimientos enNb, Ta y Ti; Ba/La>20; Ta/Hf<1,5; (⁸⁷Sr/⁸⁶Sr)_o=0,70366-0,70402, ε_Nd=+0,1±3,8); y sus contenidos de elementos mayores y trazas son consistentes con una evolución por cristalización fraccionada, en un sistema cerrado, desde un líquido parental de composición similar a la de la roca más básica del conjunto analizado. Se reconoce, además, la influencia de sedimentos subductados en la composición de estos magmas (aumento de la razón Th/HFSE y disminución de la razón Ce/Pb durante la diferenciación). Sin embargo, estas rocas muestran importantes similitudes composicionales con las de complejos volcánicos emplazados sobre areas donde ocurre subducción de bajo ángulo (altos contenidos de K, similares razones LREE/HREE, Nb/Zr, Ba/Nb; Th/Hf; Th/Ta, Ta/Hf<0,3), particularmente rocas de la actual region de subducción plana en Chile central-Argentina y de la cuenca de Neuquén durante el Mioceno Superior. Se propone aquí un modelo tectono-magmático que explica la generación y extrusion de magmas calcoalcalinos en una posición de trasarco durante el Mioceno Medio en Patagonia mediante el desarrollo de un fenómeno transiente de subducción de bajo ángulo y el consiguiente desplazamiento hacia el este del frente volcánico. Además, la mezcla entre remanentes de este magmatismo calcoalcalino almacenados en la corteza y fundidos alcalinos posteriores, provenientes de la astenósfera, se presenta como el proceso responsable de la generación de la signatura transicional (z'.e., intermedia entre calcoalcalina y alcalina, La/Nb>1; TiO₂<2 wt%) reconocida en algunos basaltos que ocurren contemporáneamente con otros puramente alcalinos en la provincia de las Lavas Neógenas de Plateau de la Patagonia.

Palabras clave: Trasarco de Patagonia Central, Magmatismo calcoalcalino, Subducción de bajo ángulo, Mioceno.

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1. Introduction

The displacement of a volcanic are front to regions behind a former are and far away from the trench has been previously recognized in several regions. Moreover, diverse tectonomagmatic models have been proposed to explain this are migration (Kay and Gordillo, 1994; Gutscher et al., 2000; Beate et al., 2001; Kay and Mpodozis, 2002; Bourdon et al., 2003; Ramos and Folguera, 2009). This phenomenon is here proposed to have oceurred during Early to Middle Miocene in Central Patagonia (47°S, Fig. 1) where calc-alkaline volcanic rocks were deposited above foreland sediments shortly after plutonism ceased in the early are region.

FIG. 1. a. Scheme of southern South America showing the location and ages of the Miocene calc-alkaline volcanic rocks occurring in the present-day back-arc region of Patagonia. Black squares: tuffs interbedded with the synorogenic deposits of Santa Cruz Formation and Zeballos Group; black triangles: intrusives (andesite porphyries, diorites, granites; Ramos, 2002; Ramos et al., 2004); grey triangles: Patagonian adakites (Kay et al., 1993; Ramos et al., 2004); white triangles: actual volcanic arc edifices. Diagonal pattern: Lower Miocene North Patagonian Batholith (NPB) granitoids (Pankhurst et al., 1999). Also shown the geometry of the Chile Ridge subduction along the continental border, black lines represent the present day position of the Chile Ridge segments under subduction at the Taitao Peninsula, grey lines represents past location of the Chile Ridge segments at 6 Ma, 10 Ma, 14 Ma and 18 Ma. Arrows indicate the convergence vector for Nazca Plate since 48 Ma (after Cande and Leslie, 1986; Pardo-Casas and Molnar, 1987; Somoza, 1998; Breitsprecher and Thorkelson, 2009). Location of Fig. 1b is shown by a frame; b. Regional geological map of the Aysén Region (Chile) and Santa Cruz Province (Argentina). The precise location of the study area is indicated. Location of the detailed stratigraphic section of the Santa Cruz Formation by Blisniuk et al. (2005) is indicated by a fuchsia arrow. LOFZ: Liquiñe-Ofqui Fault Zone; PFTB: Patagonian Fold and Thrust Belt; SVZ: Southern Volcanic Zones; AVZ: Austral Volcanic Zone; SCR: South Chile Ridge; CTJ: Chile Triple Junction; LGCBA: Lago General Carrera-Buenos Aires.

The western border of the continent has been the site of oceanic subduction beneath the South American continental Plate since the Jurassic. Miocene geological evolution of Patagonia was characterized by the oceurrence of particular geodynamic and tectonic events, among them: i) the South Chile spreading ridge approached the Chile Trench and started to be subducted at ca. 15 Ma beneath the South American Plate at 55°S (Cande and Leslie, 1986), producing a northward progressing tearing of the subducted Nazca slab (Guivel et al., 2006); ii) convergence parameters (angle and rate) between the Nazca and South American plates change (increasing velocity and oblique subduction (Pardo-Casas and Molnar, 1987; Somoza, 1998); iii) contractional deformation, although not well constrained in space and time, oceurred in the Patagonian Fold and Thrust Belt (Ramos, 1989; Lagabrielle et al., 2006) associated with iv) a topographic uplift of the chain (either by tectonic stacking or simple vertical uplift), and v) generation of molasse sediments, syntectonic plutonism and granite emplacement (Blisniuk et al., 2005; Sánchez et al., 2008). Until the Miocene, typical subduction-related are magmatism genera-ted the wide, trench-parallel, Patagonian Batholith. Early Miocene (22-16 Ma) calc-alkaline granitoids are present in its northern part (44°-47°S), but no volcanic products coeval with this plutonio activity have been described in the region. However, in the present-day back-arc region of the Central Patagonian Andes (~47-49°S, Fig. 1), ~200 km east from Miocene are granitoids and more than 300 km from the Chile Trench, tuffaceous horizons (22 to 14 Ma, Blisniuk et al., 2005) oceur within Middle Miocene foreland continental sediments, and subvolcanic rocks of this age (including adakites) are also found. In the study area of El Zeballos valley (western border of the Meseta del Lago Buenos Aires basaltic plateau) outcrops of Early to Middle Miocene volcanic rocks (basalts, basaltic andesites and andesites) overlie the synorogenic detrital sediments. The purpose of this paper is to discuss the nature of this magmatism, its relationships with the Early Miocene plutonism of the North Patagonian Batholith (NPB) and to explore a genetic link with the subduction of young oceanic plate beneath the continent at that time. In addition, we propose that this magmatism contributed as a geochemical end-member to the origin of the transitional signature observed in younger OIB-type basalts, particularly in the Meseta del Lago Buenos Aires basaltic plateau.

2.      Geological Setting

The Aysén region in southern Chile (45º-48ºS) is characterized by the occurrence of the NPB adjacent to the Chile Trench (Figs. 1a, b), which forms the western and central areas of the Patagonian Cordillera. It corresponds to the northern part (north of the Golfo de Penas) of the Patagonian Batholith, a ca. 1,000 km long and 50-120 km wide belt of subduction-related plutonic rocks. The NPB, mostly consisting of metaluminous hornblende-biotite granodiorites (e.g., Pankhurst et al., 1999; Suárez and De la Cruz, 2001) has a well established zonal age pattern with Early Cretaceous margins and Cenozoic central portions. Early Miocene gabbros to granodiorites of the NPB crop out between 44º and 46ºS in the central part of the Batholith, ranging in age between 22-16 Ma (Pankhurst et al., 1999; Parada et al., 2000; Suárez and De la Cruz, 2001; Fig. 1a). According to Pankhurst et al. (1999), the wide compositional variation recognized in the batholith resulted from the contribution of a primitive subduction-related component stored underneath the continental crust (either as mafic magmas or underplated basalts) and that of an isotopically evolved end-member, here represented by the lower Patagonian crust. Simultaneous melting and magma-mixing relationships between these end-members, in proportions controlled by the subduction-related thermal regime, account for the complete range of rocks in the NPB. Intrusion and exhumation of these rocks were tectonically controlled during the Cenozoic by the continental-scale trench-parallel dextral strike-slip Liquiñe-Ofqui Fault Zone (Pankhurst et al., 1999; Cembrano et al., 2002; Thomson, 2002). The plutons intrude Paleo-zoic basement rocks (Eastern Andean Metamorphic Complex; Hervé, 1993; Bell and Suárez, 2000) and Jurassic to Late Cretaceous calc-alkaline volcanics and continental rocks (Ibáñez and Divisadero Group). In the studied region, these Mesozoic rocks form the eastern ranges of the Cordillera as thrust and folded blocks oriented N160-170 (Lagabrielle et al., 2004), defining the morphotectonic front of the chain.

The last orogenic phase of the Patagonian Andes occurred during the Late Miocene according to many authors (Ramos, 1989; Suárez et al., 2000; Thomson et al., 2001; Lagabrielle et al., 2004), and synorogenic foreland basin detrital products deposited in the present-day back-arc correspond to fine-to-coarse grained continental sandstones and conglomerates (Santa Cruz Formation and Zeballos Group; Ugarte, 1956; Marshall et al., 1986; Escosteguy et al., 2002). They crop out to the east in Argentina and in isolated intracordilleran basins (e.g., Cosmelli Basin; Flint et al., 1994), overlying the Mesozoic volcanics (Figs. 1b, 2a). The period of deposition of these sediments is well constrained between 22 and 14 Ma by dated tuffaceous horizons interbedded with the sediments (Feagle etal, 1995; Marshall et al., 1986; Blisniuk et al., 2005). This depositional phase was sealed by a major final contractional tectonic event, leading to the development of the main Cordillera frontal thrust between 14.5 and -12 Ma (Lagabrielle et al., 2004, 2006). Other records of Early to Middle Miocene volcanic activity in Central Patagonia correspond to scarce calc-alkaline subvolcanic bodies recognized close to the studied area (Cerro Negro del Ghío and Cerro Indio diorites; Ramos, 2002) and as far as 390 km south (Cerro Moyano and Cerro Elefante andesitic porphyries; Linares and González, 1990), together with three Middle Miocene adakitic dacites (Cerro Pampa, Chaltén and Puesto Nuevo; Kay et al., 1993; Ramos et al., 2004), occurring also in a back-arc position (Fig. 1a; Table 1). Other calc-alkaline rocks occur further south but close to the are position (Cerro Blamaceda: 15.43 0.23 Ma U-Pb, Sánchez et al., 2006 and Sánchez, 2010, personal communication; Punto Bajo hornblende andesite flow: 18.3 0.6 Ma, and Cerro Caleta granodiorite sill: 19.7 0.6 Ma; Morello et al., 2001).

The foreland sediments are locally unconformably overlain by Late Miocene-Early Pliocene alkaline basalts derived from the subslab asthenosphere (the Neogene Patagonian Plateau Lavas (NPPL) basaltic province e.g., Gorring et al., 1997; Fig. 1b). Geo-dynamic models suggest that these primitive partial melts ascended through a tear in the slab (Guivel et al., 2006) opened in the Nazca Plate in response to the Chile Ridge subduction at 55°S, starting at ca. 15 Ma (Cande and Leslie, 1986). Notably, and particularly in the Meseta del Lago Buenos Aires (MLBA) and Meseta Chile Chico (MCC) plateaux, this basaltic sequence ('main-plateau' stage: 12.18-3.32 Ma, Guivel et al., 2006) includes both pure OIB-type alkaline basalts and transitional basalts. The geochemical signature of the latter is intermedíate between alkaline and calc-alkaline magmas (La/Nb> 1; TiO₂<2 wt%; Stern et al., 1990; Gorring et al., 1997,2003; Gorring and Kay, 2001; Espinoza et al., 2005; Guivel et al., 2006). Younger Plio-Pleistocene 'post-plateau' bimodal magmatism shows typical alkaline signatures. It includes primitive basanitic melts which rose through a completely opened slab window (Gorring et al., 2003) and were later stored, contaminated and differentiated in shallow crustal reservoirs (Espinoza et al., 2008).

2.1.      Field observations and sampling in Cerro Zeballos

The analyzed samples were collected ftom the top of Cerro Zeballos (1,900 m a.s.1.), a remnant relief located in the middle of the glacial valley of El Zeballos/Alto Ghío rivers. This valley coincides with theN160-170 Zeballos Fault Zone which defines here the morphotectonic front of the Cordillera (Lagabrielle et al., 2004,2006), and shows evidence of Pliocene extensional tectonics (Lagabrielle et al., 2007) which in tun may explain the young morphology of Cerro Zeballos (Figs. 2a, 3a). The base of this hill consists of synorogenic sandstones and conglomerates of the Zeballos Group, characteristically cross-cut by a network of east-west-striking basaltic dikes which likely represent the feeders of the overlying MLBA lava flows. Towards the summit, the conglomerates crop-out as thick layers forming steep cliffs (Fig. 3a). The studied volcanic sequence (Zeballos Volcanic Sequence, ZVS; Figs. 2 and 3), ca. 25 m thick, rests unconformably over the sediments, forming the top of the hill. The ZVS includes a lower 10-12 m thick whitish andesitic flow bearing centrimetric amphibole phenocrysts, and the overlying upper dark-grey olivine basaltic flow, also 10-12 m thick, which shows a well-preserved scoriaceous surface (Fig. 3b; samples PAT-37 and PAT-36, respectively). Several blocks of local origin (~4 m in diameter) of coarse chaotic to well-bedded breccia (debris flow-type) occur on the gentle south slope of the hill (Figs. 3a, c, d). Rock fragments forming these blocks are metric and angular in the coarse layers and centimetric and well rounded in the stratified horizons, and are set in coarse and fine volcaniclastic granular matrixes, respectively. They are compositionally homogeneous and similar to the andesitic deposit (no basaltic fragments were recognized) (Figs. 3c, d). Two of these fragments were separated for dating and chemical analysis (PAT-32 & -33). Based on these observations, good stratigraphic correlations can be made between the ZVS rocks and a detailed column of the Sta. Cruz Formation measured ~65 km south of the srudy area by Blisniuk et al. (2005) (Fig. 2b). A rounded boulder of fresh porphyritic andesite collected in the surroundings of Cerro Zeballos was also analyzed (PAT-39). Commonly, these andesite boulders contain centimetric to decimetric rounded amphibole- and plagioclase-rich gabbroic nodules (Fig. 5a), one of which was separated for analysis (PAT-38X). Finally, an isolated block (probably ice-transported) of fresh porphyritic andesite (PAT-26) was collected at 2,000 m altitude, east from Cerro Zeballos, on the western scarps of the MLBA.

FIG. 2. a. Local geological map colored over the Digital Elevation Model (DEM, SRTM2 90 m) of the study area; location of Cerro Zeballos (also known as Cerro Plomo) is shown by a circle. Note its isolated occurrence in the Portezuelo area, which marks the divide in river's flow direction (indicated by empty arrows); b. Stratigraphic columns showing correlations between a 500 m detailed section of Santa Cruz Formation from Blisniuk et al. (2005) near Posadas Lake, and a generalized stratigraphic column of the Zeballos Volcanic Sequence at Cerro Zeballos. Ages of dated rocks in both localities are shown. See exact location of Blisniuk et al.'s work in figure 1b.

Fig. 3. Field photographs of a. view of Cerro Zeballos from the southwest, note the cliff in the upper part formed by the syn orogenic sandstones of the Zeballos group; b. part of the studied volcanic sequence outcropping on top of the hill; c. large (~4 m diameter) breccia blocks deposited on the south slope of the Zeballos hill; d. close up view of the fragments forming the breccia in c (centimetric scale bar in the photograph).

3.      Analytical methods

Whole rock ⁴⁰K-⁴⁰Ar ages were obtained at the Université de Bretagne Occidentale (Brest, France) geochronology laboratory. Analyses were performed on the 0.5- to 0.15 mm-size fraction after crushing, sieving and cleaning with distilled water of whole-rock samples. One aliquot of sample was powde-red for K analysis by atomic absorption after HF chemical dissolution and 0.5-0.15-mm grains were used for argon isotopic analyses. Argon extraction was performed by the direct technique under high vacuum (10-5 10-7 hPa) using induction heating of a molybdenum crucible. The argon content was measured by isotope dilution and argon isotopes were analyzed in an 1808-geometry stainless steel mass spectrometer, according to the original procedure described by Bellon et al. (1981). Age calculations, following the equation of Mahood and Drake (1982) and using the constants recommended by Steiger and Jáger (1977), are given, with 2σ error, in table 1.

Mineral chemistry (Table 2) was carried out at Microsonde Sud (Montpellier, France), using a five spectrometer Probe CAMEBAX SX-100 and at LMTG (Toulouse, France) using a three spectrometer Probe CAMEBAX SX-50. Analytical conditions were 10nA, 15-20 kV, using natural standards as reference and ZAF corrections in both probes. A detailed account of the procedure is given in Defant et al. (1991).

All major and selected trace element analyses (Se, V, Cr, Co, Ni, Zr) (Table 3) were performed on agate-ground powders by ICP-AES at the chemical laboratory of the Université de Bretagne Occidentale (Brest, France). All data were measured using IWG-GIT, BE-N, AC-E, PM-S and WS-E as standards. Specific details for the analytical methods and sample preparation can be found in Cotten et al. (1995).

Trace element analyses of the other five samples (Table 3) were performed by ICP-MS using an Elan 6000 Perkin Elmer quadripolar ICP-MS at the LMTG (Toulouse, France). Calibrations, internal standard and interferences corrections were done following the procedure described in Aries et al. (2000). Data quality was controlled by running BCR-2 standard. Relative standard deviations are generally <5%.

The Sr and Nd isotopic compositions of five selected samples (Table 4) were analyzed on a Finnigan-Mat261 multicollectormass spectrometer at the LMTG (Toulouse, France). Sr and Nd were first separated from the matrix using the Sr SPEC, LN-SPEC and TRU-SPEC resins, following the techniques et up by Pin et al. (1995). Srwasloaded on a W single filament with a TaCl 'activator' and Nd was loaded on one of two Re filaments. Isotopic ratios were corrected for any laboratory bias, as described in Benoit et al. (1996), using the NBS standard solution for Sr and the La Jolla standard for Nd. The present-day Chondritic Uniform Reservoir (CHUR) value used is 0.512638 for ¹⁴³Nd/¹⁴⁴Nd.

4.      Geochronology

The ages of four samples are presented in table 1. They range from uppermost Lower Miocene (16.52 Ma) to Middle Miocene (15.08, 14.82, 14.48 Ma). According to them, the final event of eflusive calc-alkaline andesitic volcanism occurred in the Zeballos area at ca. 14 Ma, two My after the cessation of plutonism in the NPB and two My before the start of basaltic flood volcanism in the back-arc. These ages agree with those obtained by Boutonnet et al. (2010) in similar rocks, and by Marshall et al. (1986), Feagle et al. (1995) and Blisniuk et al. (2005) on tuffs interbedded with sediments of the Santa Cruz Formation (22-14 Ma; Fig. 2b), and with the age of several subvolcanic rocks occurring east of the Patagonian Fold and Thrust Belt in the extra Andean zone (Cerro Indio and Cerro Negro del Ghío: 18-13 Ma, Ramos, 2002; Cerro Moyano: 16±1 Ma, Linares and González, 1990; Table 1). These results indicate that deposition of synorogenic sediments (i.e., the construction of the Patagonian Cordillera) was coeval with a volcanic episode, with a later generation of gravitational debris flows probably resulting from surface uplift. In addition, the existence far south of some coeval intrusives indicates that this Early-Middle Miocene magmatic episode developed all along tiie extra Andean zone (Michael, 1983; Sánchez et al., 2006).

The age of the youngest dated calc-alkaline sample (14.48 0.36 Ma) suggests a ca. 2 Ma quiescence of volcanic activity in the area until the beginning of the basaltic OIB-type volcanism in the Neogene Patagonian Plateau Lavas (12.4 0.4 Ma, Gorring et al., 1997), and especially in the adjacent MLBA (12.18 0.34 Ma, Guivel et al., 2006).

5.      Mineral chemistry

5.1.      Lavas

The studied rocks present typical porphyritic to trachytic textores with olivine phenocrysts (in the case of the basalt), pyroxene, amphibole and feldspar, plus Fe-Ti oxides set in a fine grained crystalline groundmass made up of plagioclase laths plus granular to prismatic crystals of clinopyroxene and opaque oxides. Representative electron micropobe analyses of minerals are summarized in table 2.

The basalt (PAT-36) is highly porphyritic; it consists of almost completely altered (iddingsite) subhedral oüvine (Fo_45-68) with Fe-rich rims (Fig. 4c), big (>1 mm) subhedral to euhedral oscillatory-zoned diopside (Wo_50-46En_43-34Fs_8-20) and zoned calcic plagio-clase phenocrysts (An_85-37Ab_14-55Or_4-8). The plagioclase show sieve textores, and are characterized by mineral-and fluid- inclusion rich zones alternating with clean ones. The intergranular groundmass of this sample is made up of tiny Ca-poor plagioclase, diopside prisms and abundant Fe-Ti oxides, with compositions similar to those of the phenocrysts (Fig. 4a, b).

FIG. 4. a. Ab-An-Or temary compositional diagram for analyzed feldspar, b. classification diagram for pyroxenes (Morimoto et al., 1988) in the studied rocks; c. Forsterite percentage content of olivine in the analyzed basalt. Compositional pattems of oscillatory zonation in plagioclase are indicated by arrows.

Basaltic andesites (PAT-26, -33, -37, -38A, -39) and a single andesite sample (PAT-32) are porphyri-tic and rather homogeneous. They contain variable amounts of subhedral zoned Ca-amphibole phenocrysts with compositions varying between edenite, pargasite, hastingsite and hornblende. Calcic to slightly sodic plagioclase (An_94-33Ab_6-64Or_0-3) with complex oscillatory zoning pattems is an abundant primary phase (Fig. 4a). Weakly normally zoned augite and diopside (Wo_49-46En_42-34Fs_9-20), unzoned clinoenstatite (En₆₇Wo₄Fs₂₉) plus Fe-Ti oxides complete the phenocryst assemblage. Groundmasses are fine-grained and made up of plagioclase laths, acicular clinopyroxene plus tiny granular opaque Fe-Ti oxides with compositions similar to those of the phenocrysts (Fig. 4 a, b). In some samples, altered olivine microphenocrysts were also found (Fig. 5d).

5.2.      Mafic nodules

Mafic nodules show sharp contacts with the host lavas (Fig. 5a). They are holocrystalline coarse-grained amphibole gabbros (Fig. 5b, c), composedby ferroan pargasite- ferroan pargasitic hornblende (45 modal %), anorthite-bytownite plagioclase (An₉₃ ._MAb₇₁₆,35 modal %; Fig. 4a), diopside (Wo_47-49En_38-83Fs_15-18, 10 modal %; Fig. 4b) partly replaced by amphibole (Fig. 5c), Fe-Ti oxides (5 modal %), and up to 5 modal % accessory apatite and zircon. Mineral intergrowths are common among these crystals. Amphibole and clinopyroxene have compositions similar to those of the host andesite, while plagioclases are Caricher in the nodule (Fig. 4).

FIG. 5. Microphotographs of rocks from the Zeballos Volcanic Sequence. a. Sharp contact between a gabbroic nodule and the host porphyntic lava (dashed line); b. mineral assemblage of the nodules (pl+amph+cpx-Hmt+ap+zr); c. Subsolidus disequilibrium texture of clinopyroxenes partially transformed into amphibole in a nodule; d. altered olivine microxenocrysts in an Hb-beariug andesite evidencing mixing with basaltic melts (see text for explanations). Scale bar in photographs.

6.      Geochemistry

6.1.      Major and trace elements

Samples are subalkaline (according to Irvine and Baragar, 1971) and define a high-K calc-alkaline trend evolving from basalt to basaltic andesites and andesites, in the SiO₂ versus K₂O diagram (Le Bas et al., 1986; Fig. 6; Table 3). High Al₂O₃ and low TiO₂ also indicate an arc-like signature for the Zeballos Volcanic Sequence. CIPW norm indicates silica oversaturation (qz-normative) for the intermedíate samples, whereas the basalt and the gabbroic nodule are ne-normative. Silica contents vary between 46.7 to 57.7 wt%, and the nodule displays apicritic basalt chemical composition (42 wt% SiO₂); the magnesium numbers (mg#) range from 0.52 to 0.40.

FIG. 6. SiO2 versus K2O wt% diagram (after Peccerillo and Taylor, 1976) for the studied rocks of the Zeballos Volcanic Sequence (ZVS). Miocene andesitic intrusives (black crosses; Ramos et al., 2004) and Lower Miocene North Patagonian Batholith (NPB) granitoids (black circles, Pankhurst et al., 1999; Parada et al., 2000) are also plotted. Fields for Meseta del Lago Buenos Aires (MLBA), Meseta Chile Chico (MCC), main-plateau transitional basalts from the Neogene Patagonian Plateau Lavas (NPPL, 12-4 Ma; Gorring et al., 1997; Gorring et al., 2003; Espinoza et al., 2005; Guivel et al., 2006), Patagonian Adakites (Kay et al., 1993; Ramos et al., 2004), Southern South Volcanic Zone (SSVZ; D'Orazio et al., 2003; López-Escobar et al., 1993; Futa and Stern, 1988) and some South America Fíat Slab region volcanic rocks (Pocho volcanic rocks, Kay and Gordillo, 1994; Chachahuén volcanic complex, Kay et al., 2006) were drawn for comparison.

Major oxides versus silica plots are presented in figure 7a. CaO, TiO₂, Fe_total and MgO decrease with increasing silica contents, while K₂O, Na₂O and Al₂O₃ contents tend to increase. The gabbro nodule plots always consistently at the low silica end of each trend defined by the other samples.

FIG. 7a. Selected major oxides (wt%, values on anhydrous basis) versus silica (wt%) diagrams for rocks of the Zeballos Volcanic Sequence. Lower Miocene North Patagonian Batholith (NPB) granitoids (Pankhurst et al., 1999; Parada et al., 2000) and Miocene intrusives (Ramos et al., 2004) are also plotted. Compositional fields for Meseta del Lago Buenos Aires (MLBA), Meseta Chile Chico (MCC) and the Neogene Patagonian Plateau Lavas (NPPL) transitional and alkaline basalts (ca. 12-4 Ma; Gorring et al., 1997; Espinoza et al., 2005; Guivel et al., 2006) and that for the Patagoman Adakites (Kay et al., 1993; Ramos et al., 2004) were drawn for comparison.

Variation diagrams for trace elements (ppm) versus silica (wt%) are shown in figure 7b. Highly incompatible elements such as Nb (Fig. 7b) and Zr, Th, Ta, Hf, Th (not shown) display positive correlations with SiO₂; while Ni, Cr, Se (Fig. 7b) and V, Co (not shown) decrease drastically as silica increases. In particular, the Cr contents of the two least differentiated samples (PAT-36 and -33; 89 and 85 ppm Cr, respectively) are significantly higher than those of the other analyzed rocks (3-29 ppm Cr), being similar to those of the Patagonian adakites (81-102 ppm) but lower than the contents of some mafic Miocene granitoids of the NPB (213 -340 ppm). The same behavior is observed to a lesser extent for Ni and Co. Ba and Sr increase together with silica in the whole series, while Rb content decreases. The most evolved andesitic sample (PAT-32) displays a very high Rb content (84 ppm) compared to the other samples (28-57 ppm Rb; Fig. 7b). Particularly, the Sr content is higher in íhe studied lavas than in the other calc-alkaline Miocene rocks ai similar Si contents, but lower than in the Patagonian adakites (Fig. 7b). The gabbro nodule has higher Sr and Y and lower Se, Ni, Cr (Fig. 7b) and V, Co (not shown) than the basaltic sample.

FIG. 7b. Selected trace elements (ppm) versus silica (wt%) diagrams for rocks of the Zeballos Volcanic Sequenee. Simbology as in figure 7a.

Rare Earth Elements (REE) parteras of analyzed samples are presented in figure 8a. Basaltic andesites and the andesite PAT-32 have similar sub-parallel fractionated patterns, enriched in light REE ((La/ Yb)_N=6.8-10.8) with subhorizontal heavy REE ((Ho/Yb)_N=1.2-1.3) and slightly concave down-ward middle REE patterns ((Dy/Yb)_N=1.2-1.3), respectively. No marked Eu anomaly is observed in these rocks. The basalt displays lower REE concentrations than the intermedíate samples, with a less fractionated pattern ((La/Yb)_N=4.5), no Eu anomaly, no middle REE concavity and slightly more fractionated heavy REE ((Ho/Yb)_N= 1.4). The gabbroic nodule pattern has a shape similar to that of the basalt ((Ho/Yb)_N=1 .5), but displays slightly higher middle to light REE concentrations. As a whole, the REE patterns of the ZVS plot within the field defined by the transitional basalts of the MCC and MLBAplateaus (Fig. 8a).

FIG. 8. a. Chondrite-normalized (Nakamura et al., 1973) rare earth elements diagrams and b. Primitive mantle-normalized (Sun and McDonough, 1989) trace elements for rock samples of the Zeballos Volcanic Sequence. Compositional fields for Meseta del Lago Buenos Aires (MLBA), Meseta Chile Chico (MCC) and the Neogene Patagonian Plateau Lavas (NPPL) transitional (light grey field) and alkaline basalts (segmented line) (ca. 12-4 Ma; Gorring et al., 1997; Espinoza et al., 2005; Guivel et al., 2006) were drawn for comparison. OIB patterns from Sun and McDonough (1989). Simbology as in figure 7a.

Primitive mantle-normalized diagrams show that the studied rocks are variably enriched in Large Ion Lithophile Elements (LILE) and homogeneously depleted in High Field Strength Elements (HFSE) (Fig. 8b). Marked negative Nb-Ta, Hf, Ti and positive K, Pb and Sr anomalies are observed for all samples. The analyzed basalt displays lower contents in all incompatible trace elements than the intermedíate lavas. The gabbroic nodule is extremely depleted in the most incompatible elements, and displays marked negative Rb, Nb-Ta, Hf-Zr and positive P anomalies. Once again, the studied rocks plot con-sistently within the field defined by the transitional basalts of MCC and MLBA, which are characterized by similar anomalies (Espinoza et al., 2005; Guivel et al., 2006).

6.2.      Sr-Nd isotopes

Four lava samples (a basalt, two basaltic ande-sites and an andesite) and the gabbroic nodule were analyzed for Sr andNd isotopic compositions (Table 4). The corresponding data, after age corrections, form a coherent trend in the second quadrant of the Sr-Nd diagram (Fig. 9). The initial ⁸⁷Sr/⁸⁶Sr of the five samples is restricted to values ranging between 0.70366-0.70402, and all but the andesite (the most evolved analyzed lava) display a narrow range of ε_Ed values between +2.1 and +3.8, while the andesite has a lower ε_Ed (+0.1). The analyzed rocks plot close to the Early Miocene NPB granitoids (Pankhurst et al., 1999), but differ significantly from Mesozoic intrusives and Late Miocene-Pliocene satellite bo-dies of the Batholith, which have a more radiogenic signature. In addition, the ZVS rocks plot within the field defined by the transitional basalts (~12-4 Ma) of the Neogene Patagonian plateau lavas (Gorring et al., 1997; Gorring and Kay, 2001; Espinoza et al., 2005; Guivel et al., 2006).

FIG. 9. (87Sr/6Sr)o versus εNd diagram for rocks of the Zeballos Volcanic Sequence. Lower Miocene granitoids of the North Patagonian Batholith (NPB, Pankhurst et al., 1999) are also plotted. Fields for Upper Miocene-Pliocene granitoids of the NPB (Pankhurst et al., 1999), Meseta del Lago Buenos Aires (MLBA), Meseta Chile Chico (MCC) and the Neogene Patagonian Plateau Lavas (NPPL) main-plateau basalts (transitional and alkaline) (Gorring et al., 1997; Espinoza et al., 2005; Guivel et al., 2006), Southern South Volcanic Zone (SSVZ; D'Orazio et al., 2003; López-Escobar et al., 1993; Futa and Stern, 1988), Patagonian adakites (Kay et al., 1993; Ramos et al., 2004), Pocho (Kay and Gordillo, 1994) and Chachahuén volcanic rocks (Kay et al., 2006) were drawn for comparison. Isotopic values recalculated at 15 Ma. See text for discussion.

7.      Discussion

7.1.      Evidence of evolución by fractional crysta-üization for the Zeballos Volcanic Sequence

Major and trace element variation diagrams suggest derivation by fractional crystallization of the Zeballos Volcanic Sequence from a parental basaltic liquid, similar in composition with basalt PAT-36. The compatible behavior of some major and trace elements such as CaO, TiO₂, F_total, MgO and Se, Ni, Cr and V, Co suggests early olivine and later clinopyroxene plus Fe-Ti oxide fractionation from this kind of magma. In addition, amphibole fractionation is suggestedby the decrease of Rb/Ba, K/Ba and Dy/Yb ratios (concave-downwards middle REE patterns of the intermediate lavas, with (Dy/Yb) _N=1.2-1.3), although apatite fractionation may also contribute to this feature. The rather regular increase of Na₂O and Al₂O₃ with silica contents of the lava samples, along with its fairly high Sr concentrations and the lack of significant Eu anomalies, suggest that plagioclase fractionation did not control the differentiation process at any stage. Similarly, alkali feldspar fractionation is not recorded in ZVS rocks chemistry, as indicated by the increase of Ba with silica contents. Highly incompatible elements as Th, Nb, Ta, Hf, Zr, Y and REEs are enriched in the most differentiated samples, suggesting that these elements were stored in the residual liquid as fractionation of major and accessory phases oceurred. Similar characteristics are partially recognized in other Miocene calc-alkaline Patagonian rocks (NPB granitoids and back-arc intrusives; Fig. 7a, b).

Depletions in Rb, Nb-Ta, Hf-Zr and enrichment in P in the gabbro nodule are consistent with crystallization of hornblende, zircon and apatite, which are present in high modal abundances in this rock. Panjasawatwong et al. (1995) have shown a strong control of melt CaO and Al₂O₃ on the plagioclase An content , concluding that An~₉₀ plagioclase can-not crystallize from intermediate magma under any P-T-H₂O conditions. Moreover, this composition can only be produced by crystallization from water-rich basaltic melts. Therefore, the nodules from ZVS lavas record the addition of considerable amounts of water either to the melting zone where calc-alkaline melts were produced or to primitive basaltic liquids (from which mineral phases in the nodules would have crystallized), which would have later interacted with intermediate melts (see below). Moreover, absence of Eu anomalies in the REE patterns of analysed rocks (Fig. 8a) suggest that magmas were stabilized under high pressure and/or oxidizing conditions, further suggesting water involvement (e.g., Aigner-Torres et al., 2007). The application of the Holland and Blundy (1994) geothermometer to several amphibole-plagioclase pairs evidencing equilibrium (as inclusions or in direct contact) yields crystallization temperatures between 980° and 880°C for pressures of 1 and 0.5 GPa, respectively.

7.2.      Possible origin of the transitional signature of some Neogene Patagonian plateau basalts

The transitional signature of Neogene Patagonian Plateau Basalts is deñned as an intermedíate between alkaline and calc-alkaline geochemical afíinities, characterized by arc-like magmatism features (Nb and Ti depletions, La/Nb>1, low Ce/Pb and Nb/U), combined with a clear OIB-type intraplate imprint (Ba/La= 10-20, high contents in both LILE and HFSE). The origin of this signature has been attributed to the generation of these plateau basalts from an enriched asthenospheric source combined with the contribution of a lithospheric component involving: i) subductedbasaltic slab-derivedmelts/ fluids, ii) subducted crustal sediment-derived fluids/ melts, and/or iii) older calc-alkaline magmas (not previously recognized) stored at crustal levéis (Gorring and Kay, 2001; Espinoza et al., 2005; Guivel et al., 2006). As shown by Guivel et al. (2006), the intensity of Nb and Ti depletion is attenuated during differentiation, suggesting that it is related to the 'local' source of these basalts rather than representing an overall regional mantle feature. Therefore, these anomalies are likely related to the occurrence of a restitic rutile-bearing residue of slab melts (produced at the edges of the slab tear, see Thorkelson and Breitsprecher, 2005; Guivel et al., 2006) in the shallow asthenospheric or deep lithospheric Patagonian mantle. Indeed, rutile retains only Nb, Ta and Ti and is commonly observed as a residual phase during partial melting of oceanic basalts under P-T conditions consistent with those of hot subduction zones (Ringwood, 1990; Foley et al., 2000; Schmidt et al., 2004). In addition, ca. 9 Ma old calc-alkaline rhyolitic tuffs interbedded with basalts of MCC were previously thought to be responsible (by mixing processes) for this signature (Espinoza et al., 2005). However, other major and arc-like trace element characteristics observed in the transitional basalts (e.g., slightly higher SiO₂, Th/Yb, Zr/Nb and lower Ce/Pb than those of alkali basalts; Fig. 10), are not explained by the process described above. Then, the influence of a subduction component in the chemistry of transitional basalt represented by the ZVS magmas, is supported by arc-like signature recognized in their trace element contents (La/Nb>1, high Th/Nb and Ba/Nb, low Nb/Zr and Ta/Hf<0.4; Fig. 10). Furthermore, trace elements contents and ratios (LILE/HFSE; HFSE/HFSE) of transitional basalts plot consistently between those of genuine alkali basalts and ZVS rocks, suggesting a mixing origin for them (e.g., Ce/Pb, Nb/U, La/Yb, LILE/ Nb; Fig. 10). In addition, disequilibrium features evidencing mixing are recognized in plagioclase from the intermediate lavas and the basalt as oscilla-tory zoning patterns (Fig. 4a), and in some basaltic andesites where altered olivine microphenocrystals are found (Fig. 5d). Further evidence supporting the mixing hypothesis is the fact that transitional basalts were emplaced randomly during the main-plateau volcanic stage (Guivel et al., 2006), and their fiows are interbedded with those of pure alkali basaltic composition.

FIG. 10. Trace elements ratio diagrams for rocks of the Zeballos Volcanic Sequence. Lower Miocene North Patagoman Batholith (NPB, Parikhurst et al., 1999; Parada et al., 2000) and Miocene intrusives (Ramos et al., 2004) are also plotted. Other compositional fields as in figure 6. a. Th/Yb versus Nb/Yb; b. Ba (or other LILE or fluid-mobile element) versus Nb (or Ta); Nb*=17xTa (after Sun and McDonough, 1989); c. Th/Hf versus Ta/Hf; d. Th/Nb versus Ba/Th; e. La/Yb versus SiO2 wt%; f. Nb/Zr versus SiO2 wt%, and g. Ce/Pb versus Nb/U. Arrow in d) indicates the fractionation trend of the ZVS rocks. Composición of Chile Trench sediments (Kilian and Behrmann, 2003) is indicated in some diagrams. Symbology in the figure. See text for discussion.

7.3.           Miocene calc-alkaline magma sources

The calc-alkaline nature of ZVS rocks suggests that these magmas would have been generated by some type of subduction-related magmatic processes, as reflected by its chemical signatura. Then, magma source of ZVS could have been variably influenced by subducting slab and/or subducted sediments fluids/ melts and/or continental crust material, as usually envisioned for subduction-related magmatism (e.g., Ellam and Hawkesworth, 1988). However, a clear crustal signal is not recognized in ZVS magmas neither in their trace elements features for in their isotopic ratios (see Figs. 7, 9). Rather low contents of source enrichment tracers (e.g., Ta, Hf, Nb) and unradiogenic isotopic ratios are consistent with de-rivation of the ZVS magmas from an incompatible element-depleted and unradiogenic mantle source. Besides, fíat HREE patterns ((La/Yb)_N=10-15; (Sm/Yb)_N=2.5-3.5; Fig. 10e) preclude their deriva-tion from a deep garnet-bearing source similar to that of NPPL basalts (Gorring et al., 1997; Gorring and Kay, 2001; Espinoza et al., 2005). On the other hand, subducted sediments fluids/melts involvement could be depicted by Th contents (e.g., Kilian and Behrman, 2003). Th is often metasomatically added to are mantle-source regions, a process which leads to accentuate the negative Nb anomalies characteristic of are magmas in multielemental spider diagrams (e.g., Fig. 8b). In the Nb/Yb versus Th/Yb diagram of Pearce and Peate (1995) (Fig. 10a), ZVS rocks plot towards higher Th at a given Nb than do NPPL alkali basalts, which lie well within the mantle array (consistent for rocks with very little or lacking subduction components), reflecting an arc-type sediment imprint in the source of studied rocks. Furthermore, fluid-mobile element signature varíes with differentiation (increasing LILE/HFSE: Th/Nb, Th/Hf, Ba/Nb, decreasing Ce/Pb; Fig. 10c, d, g), implying that the influence of subducted sediment components varied throughout magmatic evolution. In addition, the higher than-MORB Th/U ratio of ZVS rocks confírms the need for sediment addition to its mantle source.

Regarding Miocene magma sources, the lack of any important crustal signature both in the former westernmost are (NPB) and the studied volcanic rocks is noteworthy. Miocene granitoids are mostly mafic rocks with calc-alkaline affinity, directly related to subduction, having primitive isotopic signatures (Fig. 9). Crustal influence (high ⁸⁷Sr/⁸⁶Sr) is only recognized in older (Mesozoic) and younger (Late Miocene-Pliocene) granitoids from the NPB (Pankhurst et al., 1999). Thus, the fairly similar unradiogenic isotopic signatures of Miocene rocks suggest a rather common mantle source (Fig. 9). Coeval and later to ZVS, the Pa-tagonian adakitic rocks have high Cr, Ni and Sr contents, HREE depletion and strongly unradiogenic isotopic signatures. All these features are consistent with melts generated from the subducting slab and then ascending through an enriched mantle wedge, where compatible elements such as Cr and Ni would be included into the uprising liquids (Kay et al., 1993; Ramos et al., 2004). In the ZVS, the two less differentiated rocks have higher Cr contents, and to a minor extent Ni contents, trian those of the intermedíate lavas, and similar to those of Patagonian adakites and to some NPB granitoids (Fig. 7b). This evidences the interaction of parental ZVS liquids, and similarly, of NPB magmas, with an enriched component, probably represented by the mantle above their melting regions.

As shown previously, ZVS and Miocene intru-sives share geochemical features with the Miocene NPB granitoids, but also they differ significantly First, they belong to a high-K series, while coeval NPB granitoids display a low-K calc-alkaline trend (Fig. 6). Second, although similar behaviors are observed for some major and trace elements, discrepancies in MgO, Al₂O₃ and in Sr, Ba, Y, Nb and Th contents between them and NPB granitoids suggests that their magmatic evolution followed different fractionation trends (Fig. 7). La/Yb and Nb/Zr ratios of ZVS lavas (and of the intrusives) are intermediate between the NPPL alkali basalts, the NPB are granitoids and the active volcanoes from the southern Andes. Since the Nb/Zr ratio of the studied samples does not vary significantly with differentiation for, for example, with ¹⁴³Nd/¹⁴⁴Nd (not shown), it is unlikely to be related with fractionation or contaminationprocesses (e.g., sediment involvement in the source as suggested for some island ares, e.g., Vroon et al., 1995), then it should trace a primary feature of the ZVS magma source. Therefore, these ratios allow to discriminate the source region enrichments between the earlier are magmatism (NPB) and a later eastern calc-alkaline volcanism (ZVS), and also to introduce the hypotesis that the ZVS magmatism have similar source characteristics with flat-slab related calc-alkaline rocks of the Central Andes (Pocho and Chachahuén complexes; Kay and Gordillo, 1994; Kay et al., 2006; Figs. 10e, f).

7.4.      Proposed Model: Transient Middle Miocene shallow subduction related with the Chile Ridge colusion

To explain the oceurrence of subduction-like magmatism far from former are regions some geotec-tonic models have been developed. In the case of Central Chile-Argentina (in the so-called Andean flat-slab region), models involving the shallowing of the subducting Neogene Plate since ca. 8 Ma and the generation of a secondary dehydration front far away from the are position have been proposed (Kay and Gordillo, 1994; Kay and Mpodozis, 2002). This phenomena would induce the generation of sub-alkaline magmas by slab dehydration and mantle-wedge processes (e.g., Tatsumi and Eggins, 1995; Grove et al, 2003), resulting in the extrusion of high-K amphibole-bearing basaltic to dacitic magmas with a strong arc-like component near ca. 500-700 km east of the present trench and also a tectonic efect. In this case, the phenomenon triggering this process would have been the subduction of the Juan Fernández Ridge hotspot track (Foley et al., 2000; Yáñez et al., 2001). In Ecuador, the abnormal width of the active volcanic are (~120 km) and the abundance of adakites and magnesian andesites with adakitic affinity have been accounted for by the fíat or low-angle subduction of the Pacific Plate, linked to the arrival to the trench of the Carnegie Ridge (Gutscher et al., 2000; Beate et al., 2001; Bourdon et al., 2003). In Patagonia, shallow syncollisional subduction was suggested by Gorring et al (1997) in relation with the subduction at ca. 15 Ma of the Chile Ridge (Cande and Leslie, 1986) and the opening of a slab window under the continent, and by Suárez et al. (2000) regarding timing of defor-mation in the Patagonian fold and thrust belt, but no further analysis of this process has been carried out. In the next sections we present a model to show the development, during the Middle Miocene, of a transient low-angle subduction beneath Patagonia that provoked the eastward migration of the are front and the consequent generation and extrusion of calc-alkaline magmas.

7.4.1.          Geodynamic constraints: convergence kine-matics and ridge subduction

Changes in the convergence parameters (angle and rate) between the Nazca and South American plates are reported during the Cenozoic. The ca. 15 Ma ridge-trench colusion at 55°S (Cande and Leslie, 1986) oceurred synchronously with one of the two events when subduction parameters changed, during which high convergence rates (from ca. 30 mm/yr at 30 Ma up to 110 mm/yr at 15 Ma) and slightly oblique subduction (ca. 79°) were registered (Pardo-Casas andMolnar, 1987; Somoza, 1998)(see Fig. 11). As established by Pankhurst et al. (1999), these periods coincide with intense plutonic activity in the North Patagonian Batholith, and generated magmas are mostly mafic with unradiogenic isotopic signatures. The subduction of young, hot and buoyant oceanic lithosphere coupled with an increase in the convergence rate would cause a decrease in the slab dip (see Jordán et al., 2001 and references therein). One problem arising from this hypothesis is that it would take many million years to equilíbrate the slab (i.e., to reach a low subduction angle) within a resistant-to-flow asthenosphere. For the Neogene Patagonian mantle it has been shown that it has a low density and stayed rather hot since, at least, Eocene times (e.g., Murdie et al., 2000; Heintz et al., 2005; Espinoza et al., 2005). This supports the possibility that the slab could reequilibrate with the asthenosphere not long after kinematics changed and young oceanic lithosphere subducted under Patagonia (as the Chile Ridge approached the continent), supporting the development of transient shallow subduction during the Middle Miocene.

FIG. 11. Schematic cross sections (not to scale) of Patagonian lithosphere during the Early Miocene near the latitude of Meseta del Lago Buenos Aires (MLBA, ~47°S) where petrogenetic and tectonic models supporting the generation of calc-alkaline magmatism in the present-day back-arc domain are presented. Sections include magmatic and tectonic events that occurred at different latitudes between 44° and 55°S. Pre- ~16 Ma: near orthogonal and relatively slow normal-angle subduction; shallowing of the slab begins as the Chile Ridge approaches the trench; mafic calc-alkaline primitive plutonism (in terms of Sr-Nd isotopes) is generated below the are associated with the Liquiñe-Ofqui Fault Zone (LOFZ) (Lower Miocene granitoids of the North Patagonian Batholith, NPB); deformation and uplift along the Patagonian Fold and Thrust Belt (PFTB); deposition of synorogenic sediments in the foreland together with sporadic explosive volcanism. ~16-14 Ma: changes in the convergence parameters occurred (convergence rate increases and oblique subduction); collision of the Chile Ridge with the trench at ca., 15 Ma far south (~55°S); transient low-angle subduction under Central Patagonia; end of plutonism below the are; high-K calc-alkaline magmatism in the back-arc domain (ZVS and intrusives); last compressive event in the PFTB. Post ~14 Ma: oblique subduction continúes and convergence rate decrease; calc-alkaline magmatism of the ZVS ceased when the tear in the Nazca slab (generated after Chile Ridge collided at 55°S and which propagated northward) arrived below the MLBA region; later, deep sub slab asthenosphere-derived alkaline basaltic melts aróse through the tear in the slab and extruded extensively in the back-arc of Patagonia (main-plateau stage); interaction of these melts with stored calc-alkaline magmas (related to the ZVS) produced transitional basalts which were extruded concomitantly with pure alkaline lavas; melts of slab edges extruded as adakitic magmas also in a back-arc position. For each descnbed period, the E-W component of the convergence rate (mm/yr) between the Nazca and South American plates (averaged for periods between major magnetic anomalies) and the convergence angle with respect to the present day North, are indicated by an oriented arrow (from the data of Pardo-Casas and Molnar, 1987 and Pankhurst et al., 1999). LOFZ tectonics inferred from Pliocene transpressional pop-up structure (Cembrano et al., 2002); NE: North East volcanic region in Santa Cruz, Argentina.

7.4.2.         Chemical similitudes with Andean Flat-slab volcanics

Further support for emplacement of the ZVS and the Miocene diorites over a gently dipping subducting slab comes from their chemical similarities with volcanic rocks of the Pocho volcanic complex (Kay and Gordillo, 1994) in the Chilean flat-slab region (28°-33°S, see Kay and Mpodozis, 2002) and of the Chachahuén volcanic complex (37°S; Kay et al., 2006) in the Neuquén Basin (see Kay and Ramos, 2006). Chemical similarities between the ZVS lavas, the Miocene diorites and these flat-slab magmas include: i) high K₂O contents (Fig. 6); ii) similar La/Yb, La/Sm and Sm/Yb ratios (Table 3); and iii) similar Nb/Zr, La/Nb, Ba/Nb, Th/Hf, Th/Ta, Ta/Hf ratios (Fig. 10). On the other hand, whereas long-term Andean flat-slab volcanism evolved towards a more arc-like signature (e.g., Kay and Gordillo, 1994; Kay et al., 2006), Patagonian magmas evolved temporally towards a more primitive end member (Pliocene 'post plateau' mafic volcanism), perhaps as a result of the transient nature of the proposed Middle Miocene shallow subduction event.

Our chemical data for the ZVS rocks are consis-tent with the development during Middle Miocene of a transient low-angle subduction under Patagonia (Fig. 11). The particular geodynamic framework in the southeastern Pacific at this time gives a consis-tent support for this hypothesis. Thermal and age characteristics of the subducting Nazca slab were globally favorable to a decrease in the subduction angle. However, the approach to the trench of the active Chile Ridge seems to be the main factor for triggering the decrease in the subduction angle. As the Chile Ridge approached to the continent and the Farallón (Nazca) Plate becames younger close to the trench, the shallowing of the slab begun and the first calc-alkaline magmas (ZVS) erupted in the MLBAregion (ca. 16 Ma). The shallowing slab would reach melting temperatures ca. 300 km east from the trench and released enough water (mainly from subducted sediments mineral dehydration) to stabilize amphibole (then generating oxidizing conditions). Under these thermal and geodynamic conditions, a new regime for magma genesis would be set. Calc-akaline liquids from this source (incompatible elements-depleted and isotopically unradiogenic) would interact with a primitive component (high Cr and Ni) in the mantle wedge above, modifying the chemistry of parental ZVS magmas. Arelatively high degree of melting of this source, either mantle wedge- or lithospheric-like, and later mineral frac-tionation with negligible involvement of the crust would produce the chemistry of the studied rocks.

The shallowing of subduction angle continued up to ca. 14-12 Ma when the tear in the Nazca slab arrived below the MLBA region. Afterwards, alkaline and transitional flood basalts extruded forming the plateau. South of the Lago General Carrera-Buenos Aires area, melts from the subducted Nazca Plate edges were emplaced as adakitic dacites, also in the present-day back-arc domain (Thorkelson and Breitsprecher, 2005; Breitsprecher and Thorkelson, 2009). Finally, Miocene uplift ended at ca. 14 Ma after a shortening and compressive phase (Blisniuk et al., 2005; Lagabrielle et al., 2006, 2007), contemporaneously with the end of calc-alkaline magmatism in the MLBA area.

8.      Conclusions

The Zeballos Volcanic Sequence (ZVS) rocks represent a Middle Miocene high-K calc-alkaline volcanic event in the Patagonian back-arc region.

The evolution of ZVS magmas by fractional crystallization from melts similar to the basalt present in the sequence is supported by major and trace elements; no crustal involvement in this process is recognized.

Amphibole-rich gabbroic nodules included in intermediate lavas are cogenetic implying the addition of considerable amounts of water to the magma source.

ZVS rocks are enriched in LILE and REE and have Nb/Zr, La/Yb, Ce/Pb, Nb/U ratios distinct from those of typical arc-related (Miocene and present) volcanic rocks. These features, as well as high Th, low Th/U and e_Nd variations, document a strong signature of H₂O-rich subducted sediments.

The are component recognized in the transitional signature of later Mio-Pliocene plateau basalts of the Neogene Patagonian Plateau Lavas (Gorring et al., 1997; Guivel et al., 2006) is thought to have arisen from mixing of alkaline melts with stored calc-alkaline magmas equivalent to those of the ZVS.

Similarities between the ZVS magmas and those from volcanic complexes emplaced above a gently dipping slab (present Andean flat-slab region in Central Chile-Argentina; Late Miocene Neuquén Basin), together with a suitable geodynamic confi-guration, suggest the oceurrence of transient Middle Miocene low-angle subduction in Patagonia, possibly associated with the arrival of the Chile Ridge and the later ridge-trench colusion at ca. 15 Ma.

Acknowledgements

This work form part of the first author's PhD thesis (2004-2006 CONICYT-BIRF Chile grant) and part of the ECOS-CONICYT C05U01 (D.M. and Y.L.) and DyETI-INSU-CNRS (YL.) projects. The authors would like to thank Dr. P. de Parseval (LMTG, Toulouse, France) for the assistance with the microprobe analyses, Mr. P. Brunet (LMTG, Toulouse, France) for the TIMS measurements and Estancia Sol de Mayo crew (Santa Cruz, Argentina) for the field facilities and hospitality. F.E. thanks to A. Sánchez and F. Gutiérrez for interesting patagonian discussion. Editorial supervisión by J. Cembrano and thoughtful and constructive reviews by C.R. Stern, R.J. Pankhurst and S. Jego are greatly acknowledged.

 

References

Aigner-Torres, M.; Blundy, J.; Ulmer, P; Pettke, T 2007. Laser Ablation ICPMS study of trace element partitioning between plagioclase and basaltic melts: an experimental approach. Contributions to Mineralogy and Petrology 153 (6): 647-667.        [ Links ]

Aries, S.; Valladon, M.; Polvé, M.; Dupré, B. 2000. A routine method for oxide and hydroxide interfer-ence corrections in ICP-MS chemical analyses of environmental and geological samples. Geostandards Newsletter 24: 19-31.        [ Links ]

Beate, B.; Monzier, M.; Spikings, R.; Corten, J.; Silva, J.; Bourdon, E.; Eissen, J.-P. 2001. Mio-Pliocene adakite generation related to flat subduction in Southern Ecuador: The Quimsacocha volcanic center. Earth and Planetary Science Letters 192: 561-570.        [ Links ]

Bell, C; Suárez, M. 2000. The Río Lácteo Formation of southern Chile. Late Paleozoic orogeny in the Andes of southernmost South America. Journal of South America Earth Sciences 13: 133-145.        [ Links ]

Bellon, H.; Quoc Buu, N.; Chaumont, J.; Philippet, J.C. 1981. Implantation ionique d'argon dans une célibe support: application au tracage isotopique d l'argon contenu dans les minéraux et les roches. Comptes Rendus de l'Académie des sciences 292:977- 980. París.        [ Links ]

Benoit, M.; Polvé, M.; Ceuleneer, G. 1996. Trace element and isotopic characterization of mafic cumulates in a fossil mantle diapir (Ornan ophiolite). Chemical Geology 134: 199-214.        [ Links ]

Blisniuk, P.M.; Stern, L.A.; Chamberlain, C.R; Idleman, B.; Zeitler, P.K. 2005. Climatic and ecologic changes during Miocene surface up lift in the southern Patagonian Andes: Earth and Planetary Science Letters 230: 125-142.        [ Links ]

Bourdon, E.; Eissen, J.-R; Gutscher M.-A.; Monzier, M; Hall, M.L.; Corten, J. 2003. Magmatic response to early aseismic ridge subduction: the Ecuadorian margin case (South America). Earth and Planetary Science Letters 205:123-138.        [ Links ]

Boutonnet, E.; Arnaud, N.; Guivel, C; Lagabrielle, Y.; Scalabrino, B.; Espinoza, F. 2010. Subduction of the South Chile active spreading ridge: A17 Ma to 3 Ma magmatic record in central Patagonia (western edge of Meseta del Lago Buenos Aires, Argentina). Journal of Volcanology and Geothermal Research 189:319-339.        [ Links ]

Breitsprecher, K.; Thorkelson, D.J. 2009. Neogene kinematic history of Nazca-Antarctic-Phoenixslab Windows beneath Patagonia and the Antarctic Peninsula. Tectonophysics 464 (1-4): 10-20.        [ Links ]

Cande, S.C.; Leslie, R.B. 1986. Late Cenozoic tectonics of the Southern Chile trench. Journal of Geophysical Research 91: 471-496.        [ Links ]

Cembrano, J.; Lavenu, A.; Reynolds, P; Arancibia, G.; López, G.; Sanhueza, A. 2002. Late Cenozoic trans-pressional ductile deformation north of the Nazca-South America-Antarctica triple junction. Tectonophysics 354: 289-314.        [ Links ]

Corten, J.; Le Dez, A.; Bau, M.; Caroff, M.; Maury, R.C.; Dulski, R; Fourcade, S.; Bohn, M.; Brousse, R. 1995. Origin of anomalous rare-earth elements and ytrium enrichments in subaereally exposed basalts: evidence from FrenchPolynesia. Chemical Geology 119:115-138.        [ Links ]

Defant, M.J.; Richerson, M.; de Boer, J.Z.; Stewart, R.H.; Maury, R.C.; Bellon, H.; Drummond, M.S.; Feigenson, M.D.; Jackson, T.E. 1991. Dacite genesis via both slab melting and differentiation: Petrogenesis of La Yeguada Volcanic Complex, Panamá. Journal of Petrology 32: 1101-1142.        [ Links ]

D'Orazio, M.; Innocenti, F.; Manetti, R; Tamponi, M.; Tonarini, S.; González-Ferrán, O.; Lahsen, A.; Omarini, 2003. The Quaternary calc-alkaline volcanism of the Patagonian Andes close to the Chile triple junction: geochemistry and petrogenesis of volcanic rocks from the Cay and Maca volcanoes (45°S, Chile). Journal of South American Earth Sciences 16 (4): 219-242.        [ Links ]

Ellam, R.M.; Hawkesworth, C.J. 1988. Elemental and isotope variations in subduction related basalts: evidence for a three component model. Contributions to Mineralogy and Petrology 98: 72-80.        [ Links ]

Escosteguy, L.; Dal Molin, C; Franchi, M.; Geima, S.; Lapido, 0.2002. Estratigrafía de la cuenca de los ríos El Zeballos y Jeinemeni, Noroeste de la provincia de Santa Cruz. In Congreso Geológico Argentino, No. 15, Actas 1: 653-658. El Calafate.        [ Links ]

Espinoza, R; Morata, D.; Pelleter, E.; Maury, R.C.; Suárez, M.; Lagabrielle, Y; Polvée, M.; Bellon, H.; Cotten, J.; De la Cruz, R.; Guivel, C. 2005. Petrogenesis of the Eocene and Mio-Phocene alkaline basaltic magmatism in Meseta Chile Chico, Southern Patagonia, Chile: evidence for the participation of two slab Windows. Lithos 82: 315-343.        [ Links ]

Espinoza, F.; Morata, D.; Polvé, M.; Lagabrielle, Y; Maury, R.C.; Guivel, C; Cotten, J.; Bellon, H.; Suárez, M. 2008. Bimodal back-arc alkaline magmatism after ridge subduction: Pliocene felsic rocks from Central Patagonia (47°S). Lithos 101 (1-2): 191-217.        [ Links ]

Feagle, J.G.; Bown, T.M.; Swisher, C; Buckley, G.A. 1995. Age of the Pinturas and Santa Cruz Formations. In Congreso Argentino de Paleontología y Bioestratigrafía, No. 6, Actas 1: 129-135. Trelew, Argentina.        [ Links ]

Flint, S.S.; Prior, D.J.; Agar, S.M.; Turner, P. 1994. Stratigraphic and structural evolution of the Tertiary Cosmelli Basin and its relationship to the Chile triple junction. Journal of the Geological Society of London 151:251-268.        [ Links ]

Foley, S.F.; Barth, M.G.; Jenner, G.A. 2000. Rutile/melt partition coefficients for trace elements and an assessment of the influence of rutile on the trace element characteristics of subduction zone magmas. Geochimica et Cosmochimica Acta 64 (5): 933-938.        [ Links ]

Futa, K.; Stern, C.R. 1988. Sr and Nd isotopic and trace element composition of Quaternary volcanic centers of southern Andes. Earth and Planetary Science Letters 88: 253-263.        [ Links ]

Gorring, M.; Kay, S. 2001. Mantle processes and sources of Neogene slab window magmas from southern Patagonia, Argentina. Journal of Petrology 42 (6): 1067-1094.        [ Links ]

Gorring, M.L.; Kay, S.M.; Zeitler, P.K.; Ramos, V.A.; Rubiolo, D.; Fernández, M.I.; Panza, J.L. 1997. Neo-gene Patagonian plateau lavas: Continental magmas associated with ridge colusion at the Chile Triple Junction. Tectonics 16: 1-17.        [ Links ]

Gorring, M.; Singer, B.; Gowers, J.; Kay, S. 2003. Plio-Pleistocene basalts from the Meseta del Lago Buenos Aires, Argentina: evidence for asthenosphere-lithosphere interactions during slab window magmatism. Chemical Geology 193: 215-235.        [ Links ]

Grove, T.L.; Elkind-Tanton, L.T.; Parman, S.W.; Chatterjee, N; Müntener, O.; Gaetani, G.A. 2003. Fractional crystallization and mantle-melting controls on calc-alkaline differentiation trends. Contributions to Mineralogy and Petrology 145: 515-533.        [ Links ]

Guivel, C; Morata, D.; Pelleter, E.; Espinoza, R; Maury, R.C.; Lagabrielle, Y; Polvé, M; Bellon, H.; Cotten, J.; Benoit, M.; Suárez, M.; De la Cruz, R. 2006. Miocene to Late Quaternary Patagonian basalts (46-47°S): Geochronometric and geochemical evidence for slab tearing due to active spreading ridge subduction: Journal of Volcanology and Geothermal Research 149:346-370.        [ Links ]

Gutscher, M.A.; Maury, R.C.; Eissen, J.-R; Bourdon, E. 2000. Can slab melting be caused by flat subduction? Geology 28: 535-538.        [ Links ]

Heintz, M.; Debayle, E.; Vauchez, A. 2005. Upper mantle structure of the South American continent and neighboring oceans from surface wave tomography. Tectonophysics 406: 115-139.        [ Links ]

Hervé, R 1993. Paleozoic metamorphic complexes in the Andes of Aysén, southern Chile (west of Occidentalia). In Proceedings of the First Circum-Pacific and Circum-Atlantic Terrane Conference: 64-65. Guanajuato, México.        [ Links ]

Holland, T.J.B.; Blundy J. 1994. Non-ideal interactions in calcic amphiboles and their bearing on amphibole-plagioclase thermometry. Contribution to Mineralogy and Petrology 116: 433-447.        [ Links ]

Irvine, T.N.; Baragar, W.R. A. 1971. A guide to the chemical classification of the common volcanic rocks. Canadian Journal of Earth Science 8: 523-548.        [ Links ]

Jordán, T.; Burns, W.; Veiga, R.; Pángaro, R; Copeland, P; Kelley, S.; Mpodozis, C. 2001. Extension andbasin formation in the Southern Andes caused by increased convergence rate: A Mid-Cenozoic trigger for the Andes. Tectonics 20: 308-324.        [ Links ]

Kay, S.M.; Gordillo, CE. 1994. Pocho volcanic rocks and the melting of depleted continental lithosphere above a shallowly dipping subduction zone in the Central Andes. Contributions to Mineralogy and Petrology 117:25-44.        [ Links ]

Kay, S.M.; Mpodozis, C. 2002. Magmatism as a probe to the Neogene shallowing of the Nazca Plate beneath the modern Chilean flat-slab. Journal of South American Earth Sciences 15 (1): 39-57.        [ Links ]

Kay, S.M.; Ramos, V. (editors) 2006. Evolution of an Andean margin: A tectonic and magmatic view from the Andes to the Neuquén Basin (35°-39°S lat). Geological Society of America Special Paper 407.        [ Links ]

Kay, S.M.; Ramos, V.A.; Márquez, M. 1993. Evidence in Cerro Pampa volcanic rocks for slab-melting prior to ridge-collision in southern South America. Journal of Geology 101: 703-714.        [ Links ]

Kay, S.M.; Mancilla, O.; Copeland, P. 2006. Evolution of the late Miocene Chachahuén volcanic complex at 37°S over a transient shallow subduction zone under the Neuquén Andes. In Evolution of an Andean margin: A tectonic and magmatic view from the Andes to the Neuquén Basin (35°-39°S lat) (Kay, S.M.; Ramos, V.; editors). Geological Society of America Special Paper 407: 215-246.        [ Links ]

Kilian, R.; Behrmann, J. 2003. Geochemical constraints on the sources of Southern Chile Trench sediments and their recycling in are magmas of the Southern Andes. Journal of the Geological Society of London 160: 57-70.        [ Links ]

Lagabrielle, Y.; Suárez, M.; Rossello, E.A.; Hérail, G.; Martinod, J.; Régnier, M.; De la Cruz, R. 2004. Neogene to Quaternary tectonic evolution of the Patagonian Andes at the latitude of the Chile triple junction: Tectonophysics 385 (1-4): 211-241.        [ Links ]

Lagabrielle, Y; Bellón, H.; Espinoza, R; Guivel, C; Malavieille, J.; Maury, R.; Morata, D.; Polvé, M.; Rossello, E.; Suárez, M. 2006. Post-Phocene Deformation and Uplift of the Patagonian Andes in response to the subduction of the active Chile Spreading Ridge (CSR). In Backbone of the Americas 4-2: 47. Mendoza, Argentina.        [ Links ]

Lagabrielle, Y; Suárez, M.; Malavieille, J.; Morata, D.; Espinoza, R; Maury, R.C.; Scalabrino, B.; Barbero, L.; De la Cruz, R.; Rossello, E.; Bellon, H. 2007. Pliocene extensional tectonics in the Eastern Central Patagonian Cordillera: geochronological constraints andnew field evidence. Térra Nova 19 (6): 413-424.        [ Links ]

Le Bas, M.J.; Le Maitre, R.W.; Streckeisen, A.; Zanettin, B. 1986. A chemical classification of volcanic rocks based on the total alkali-silica diagram. Journal of Petrology 27: 745-750.        [ Links ]

Linares, E.; González, R.R. 1990. Catálogo de edades radiométricas de la República Argentina 1957-1987. Asociación Geológica Argentina, Publicaciones Especiales Serie B, Didáctica y Complementaria 19: 1-628. Buenos Aires.        [ Links ]

López-Escobar, L.; Kilian, R.; Kempton, P; Tagiri, M. 1993. Petrography and geochemistry of Quaternary rocks from the Southern Volcanic Zone of the Andes between 41°30' and 46°00'S, Chile. Revista Geológica de Chile 20 (1): 33-55.        [ Links ]

Mahood, G.; Drake, R.E. 1982. K-Ar dating young rhyolitic rocks: a case study for the Sierra La Primavera, Jalisco, México. Geological Society of America Bulletin 93: 1232-1241.        [ Links ]

Marshall, L.G.; Drake, R.E.; Curtis, G.H.; Butler, R.F.; Flanagan, KM.; Naeser, C.W. 1986. Geochronology of type Santacrucian (middle Tertiary) land mammal age, Patagonia, Argentina. Journal of Geology 94:449-457.        [ Links ]

Michael, PJ. 1983. Emplacement and differentiation of Miocene plutons in the foothills of the southernmost Andes. Ph.D. Thesis (Unpublished), Columbia University: 367 p.        [ Links ]

Morello, R; San Ramón, M.; Prieto, A.; Stern, C.R. 2001. Nuevos antecedentes para una discusión arqueológica en torno a la obsidiana verde en Patagonia meridional. Anales del Instituto de la Patagonia 29: 129-148.        [ Links ]

Morimoto, N.; Fabries, J.; Ferguson, A.K.; Ginzburg, I.V.; Ross, M.; Seifert, F.A.; Zussmann, J.; Aoki, K.; Gottardi, G. 1988. Nomenclature of piroxenes. American Mineralogist (173): 1123-1133.        [ Links ]

Murdie, R.; Styles, R; Prior, D.J.; Daniel, A.J. 2000. A new gravity map of southern Chile and its preliminary interpretation. Revista Geológica de Chile 27 (1): 49-63.        [ Links ]

Nakamura, N. 1974. Determination of REE, Ba, Fe, Mg, Na and K in carbonaceous and ordinary chondrites. Geochimica Cosmochimica Acta (38): 757-775.        [ Links ]

Panjasawatwong, Y.; Danyushevsky, L.V.; Crawford, A.J.; Harris, K.L. 1995. An experimental study of the effects of melt composition on plagioclasemelt equilibria at 5 and 10 kbar: implications for the origin of magmatic high-An plagioclase. Contributions to Mineralogy and Petrology 118 (4): 420-432.        [ Links ]

Pankhurst, R.J.; Weaver, S.D.; Hervé, R; Larrondo, P. 1999. Mesozoic-Cenozoic evolution of the North Patagonian Batholith in Aysén, southern Chile: Journal of the Geological Society 156: 673-694. London.        [ Links ]

Parada, M.A.; Lahsen, A.; Palacios, C. 2000. The Miocene plutonio event of the Patagonian Batholith at 44°30'S: thermocronological and geobarometric evidence for melting of a rapidly exhumed lower crust. Transactions of the Royal Society of Edinburgh: Earth Sciences 91: 169-179.        [ Links ]

Pardo-Casas, R; Molnar, P. 1987. Relative motion of the Nazca (Farallón) and South American Plates since Late Cretaceous time. Tectonics 6: 233-248.        [ Links ]

Pearce, J.A.; Peate, D.W. 1995. Tectonic implications of the composition of volcanic are lavas. Annual Review of Earth and Planetary Sciences 23: 251-285.        [ Links ]

Peccerillo, A.; Taylor, S.R. 1976. Geochemistry of Eocene Calc-Alkaline Volcanic Rocks from the Kastamonu Area, Northern Turkey. Contribution to Mineralogy and Petrology 58: 63-81.        [ Links ]

Pin, C; Telouk, R; Imbert, J-L. 1995. Direct determination of the samarium: neodymium ratio in geological materials by inductively coupled plasma quadrupole mass spectrometry with cryogenic desolvation. Comparison with isotope dilution thermal ionization mass spectrometry. Journal of Analytical Atomic Spectrometry 10: 93-98.        [ Links ]

Ramos, V. A. 1989. Andean foothills structures in northern Magallanes Basin, Argentina. American Association of Petroleum Geologist Bulletin 73: 887-903.        [ Links ]

Ramos, V.A. 2002. El magmatismo neógeno de la Cordillera Patagónica. In Geología y Recursos Naturales de Santa Cruz (Haller, M.J; editor). Congreso Geológico Argentino, No. 15, Velatorio I (13): 187-199. El Calafate.        [ Links ]

Ramos, V.A.; Folguera, A. 2009. Andean flat-slab sub-duction through time. Geological Society of London, Special Publications 327: 31-54.        [ Links ]

Ramos, V.; Kay, S.M.; Singer, B.S. 2004. Las adakitas de la cordillera Patagónica: Nuevas evidencias geoquímicas y geocronológicas. Revista de la Asociación Geológica Argentina 59 (4): 693-706.        [ Links ]

Ringwood, A.E. 1990. Slab-mantle interactions. 3. Petrogenesis of intraplate magmas and structure of the upper mantle. Chemical Geology 82 (3-4): 187-207.        [ Links ]

Sánchez, A.; Hervé, R; De Saint-Blanquat, M. 2008. Relations between plutonism in the back-arc region in southern Patagonia and Chile Rise subduction: A geochronological review. In International Symposium on Andean Geodynamics: 485-488. Nice.        [ Links ]

Schmidt, M.W.; Dardon, A.; Chazot, G; Vanucci, R. 2004. The dependance of Nb and Ta rutilemelt partitioning on melt composition and Nb/Ta fractionation during subduction processes. Earth and Planetary Science Letters 226: 415-432.        [ Links ]

Somoza, R. 1998. Updated Nazca (Farallon)-SouthAmerica relative motions during the last 40 my: implications for mountain building in the central Andean region. Journal of South American Earth Sciences 11:211-215.        [ Links ]

Steiger, R.H.; Jager, E. 1977. Subcommission on geo-chronology: convention on the use of decay constants in geo- and cosmochronology. Earth and Planetary Science Letters 36: 359-362.        [ Links ]

Stern, C.R.; Frey. F.A.; Futa, K.; Zartman, R.E.; Peng, Z.; Kyser, T.K. 1990. Trace-element and Sr, Nd, Pb and O isotopic composition of Pliocene and Quaternary basalts of the Patagonian Plateau lavas of southernmost South America. Contribution to Mineralogy and Petrology 104: 294-308.        [ Links ]

Suárez, M.; De la Cruz, R. 2001. Jurassic to Miocene K-Ar dates from eastern central Patagonian Cordillera plutons, Chile (45°-48°S). Geological Magazine 138 (1): 53-66.        [ Links ]

Suárez, M.; De la Cruz, R.; Bell, C.M. 2000. Timing and origin of deformation along the Patagonian fold and thrustbelt. Geological Magazine 137: 345-353.        [ Links ]

Sun, S.; McDonough, W.F. 1989. Chemical and isotopic systematics of oceanic basalto; implications for mantle composition and processes. In Magmatism in the Ocean Basins (Saunders, A.D.; Norry, J.M.; editors). Geological Society of London, Special Publication 42: 313-345.        [ Links ]

Tatsumi,Y.;Eggins, S. 1995. SubductionZone magmatism. Blackwell Publishing: 224 p.        [ Links ]

Thomson, S.N.; Hervé, F.; Stockhert, B. 2001. Mesozoic-Cenozoic denudation history of the Patagonian Andes (southern Chile) and its correlation to different subduction processes. Tectonics 20: 693-711.        [ Links ]

Thorkelson, D.J.; Breitsprecher, K. 2005. Partial melting of slab window margins: genesis of adakitic and non-adakitic magmas. Lithos 79: 25-41.        [ Links ]

Ugarte, F. 1956. El Grupo Río Zeballos. Revista de la Asociación Geológica Argentina 11 (3): 202-216.        [ Links ]

Vroon, P.Z.; van Bergen, M. J.; Klaver, G J.; White, W.M. 1995. Strontium, neodymium, and lead isotopic and trace-element signatures of the East indonesian sediments: provenance and implications for banda are magma genesis. Geochimica et Cosmochimica Acta 59 (12): 2573-2598.        [ Links ]

Yáñez, G.A.; Ramiro, C; von Huene, R.; Díaz, J. 2001. Magnetic anomaly interpretation across the southern central Andes (32°-34°S): The role of the Juan Fernández Ridge in the late Tertiary evolution of the margin. Journal of Geophysical Research 106: 6325-6345.        [ Links ]

 

Manuscript received: October 10,2009; revised/accepted: May 11,2010.