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Revista geológica de Chile

versión impresa ISSN 0716-0208

Rev. geol. Chile v.34 n.2 Santiago jul. 2007

http://dx.doi.org/10.4067/S0716-02082007000200005 

 

Revista Geológica de Chile. Vol. 34. No. 2. p. 249-275. July 2007

Crust-mantle interactions and generation of silicic melts: insights from the Sarmiento Complex, southern Patagonian Andes

Interacciones corteza-manto y generación de fundidos silíceos: perspectivas desde el Complejo Sarmiento, Andes patagónicos australes.

 

Mauricio Calderón , Francisco Hervé.
Departamento de Geología, Universidad de Chile, Casilla 13518, Correo 21 .Santiago, Chile, caldera@esfera.cl, fherve@cec.uchile.cl

Umberto Cordaní
Centro de Pesquisas Geocronológicas, Universidade de Sao Paulo,
Rúa do Lago 562, Cidade Universitaria, Sao Paulo, CEP 05508-900, Brazil, ucordani@usp.br

Hans-Joachim Massonne
Institut für Mineralogie und Kristallchemie, Universitát Stuttgart,
Azenbergstrasse 18, D-70174, Stuttgart, Germany h-j.massonne@imi.uni-stuttgart.de


ABSTRACT

A Late Jurassic seafloor remnant of the Rocas Verdes basin in southern Chile, the Sarmiento Complex {ca. 52°S), bears lithological layers with bimodal meta-igneous rocks appropriate for a comprehensive investigation of magma genesis in part of a lateral lithological transition from continental rifting to initial seafloor spreading. Metamorphosed mafic rocks collected from different layers in the ophiolite pseudostratigraphy and a plagiogranite have positive sNd150 values (+ 1 and +2). Granophyres, which are crosscut by ophiolitic mafic dikes, have negative sNd150 values (-5). Dacitic dikes within thick successions of pillow basalt have the least negative sNd150 values (-3 and -4). Although mafic and felsic igneous rocks show contrasting isotopic signatures, thermochemical modeling (EC-AFC) suggests they can share a common origin. Models consider an arbitrary composition of the crustal assimilant (mostly metapelite with an average eNd150 value of-7) and evaluate the feasibility for generation of silicic melts through the interaction of mafic magmas and metasedimentary rocks. A quantitative evaluation of basaltic magma contaminated by crustal wall rocks requires a Ma7 Mc (mass of anatectic melt/ mass of cumulates) ratio of 0.04. Analyses using dikes of dacite (with MaVMc ratios ranging between 0.28-0.35) and granophyres (with MaVMc ratios of 0.63-0.89) require the silicic magmas to contain higher proportions of anatectic melts derived from metamorphic rocks. Isotopic differences among granophyres and dacites could be controlled by eruption dynamics, regional stress field and/or differences in thermal regimes in magma chambers. Bimodal magmatism in the earliest tectonic evolution of the Rocas Verdes basin could reflect regimes of slow extension of the continental crust along the Jurassic southwestern margin of Gondwana.

Key words: Rocas Verdes basin, Sarmiento Complex, Bimodal magmatism, Petrogenesis, Patagonia, Chile.


RESUMEN

Un remanente Jurásico del fondo oceánico de la cuenca de Rocas Verdes, el Complejo Sarmiento (ca. 52°S), contiene niveles con asociaciones de rocas bimodales apropiadas para la investigación de la génesis de magmas en parte de la transición litológica entre 'rifting' continental y etapas tempranas de la extensión oceánica. Rocas máficas metamorfizadas y plagiogranitos en distintos niveles de la pseudoestratigrafía ofiolítica, tienen valores positivos de sNd150 (+1 and +2). Granófiros, que se encuentran atravesados por diques máficos con afinidades ofiolíticas, tienen valores negativos de ENd150(-5). Diques de dacita que intruyen una sucesión de basaltos almohadillados también presentan valores de sNd150 negativos (-3 y -4). Aunque las rocas estudiadas muestran razones isotópicas contrastantes, un modelo termoquímico, energéticamente balanceado (EC-AFC) sugiere que ellas pueden compartir un origen en común. Los modelos consideran una composición arbitraria de asimilante cortical (principalmente metapelita con un valor promedio de sNd150 cercano a -7) y evalúa la posibilidad de generación de magmas silíceos a través de la interacción de magmas máficos y rocas metasedimentarias. Una evaluación cuantitativa de la contaminación de magma basáltico por rocas encajantes, indica razones de Ma*/Mc (masa de líquido anatéctico/ masa de cumulados) de 0,04. En cambio, análisis cuantitativos en diques de dacita (con MaVMc entre 0,28-0,35) y granófiros (con MaVMc entre 0,63-0,89), indican que magmas silíceos contienen mayor proporción de líquidos antécticos derivados de rocas metamórficas. Las diferencias isotópicas entre granófiros y dacitas podrían haber sido controladas por la dinámica de la erupción, régimen de esfuerzos tectónicos y/o por diferencias de regímenes termales en la cámara magmática. El magmatismo bimodal en las etapas tempranas de la evolución de la cuenca de Rocas Verdes, probablemente refleja regímenes con bajas tasas de extensión en la corteza continental del margen sudoccidental de Gondwana durante el Jurásico.

Palabras Claves: Cuenca Rocas Verdes, Complejo Sarmiento, Magmatismo bimodal, Petrogénesis, Patagonia, Chile.


 

INTRODUCTION

Crust-mantle interactions involve processes of mass, chemical and thermal exchange between mantle-derived basaltic magmas and crustal metamorphic rocks. Basaltic injection into the crust has been widely considered as an important mechanism to generate silicic melts in the continental and oceanic crust(e.gr.,Huppertand Sparks, 1988; Pankhurst and Rápela, 1995; Petford and Gallagher, 2001). Basalt can act either as a parental source or as the thermal trigger for crustal melting (Grunder, 1995). The latter process is controlled by the fertility of the crustal material, temperature and the water content of basaltic magma (Annen and Sparks, 2002). Evidences from field and geo-chemical studies and experimental petrology point to amphibolite, metapelite and metagreywacke as suitable crustal lithologies for partial melting and generation of silicic melts (e.g., Wolf and Wyllie, 1994; Rapp and Watson, 1995; Patino Douce and Beard, 1995; Milord etal., 2001). Even partial melting experiments performed with mixtures of dry basalt and metapelite yielded water-undersatured granitic melts (Patino Douce, 1995) at temperatures of 1,000°C and 5-15 kbar pressure.

Several ophiolite complexes and overlying hemipelagic sedimentary successions exposed along the southern Pacific margin of South America (Fig. 1) have been considered as seafloor remnants of the Late Jurassic-Early Cretaceous Rocas Verdes basin (Katz, 1964; Dalziel etal., 1974; Suárez and Pettigrew, 1976; FuenzalidaandCovacevich, 1988; Stern and de Wit, 2003). The Sarmiento Complex preserves a lithological pseudostratigraphy with mixed mafic-felsic layers and without the ultramafic components of 'classical' ophiolites. De Wit and Stern (1981) argued for its development over a diffuse zone, dominated by the interaction between mafic magmas and continental rocks, when two continental blocks were separated. SHRIMP U-Pb zircon geochronology constrain bimodal volcanism to the time from 152 to 147 Ma (Calderón, 2006; Calderón etal., in press) enhancing the Sarmiento Complex as a geological setting where petrological and tectonic aspects of bimodal magmatism and continental lithosphere rupturing can be studied. For this purpose, whole rock Sr and Nd isotopic ratios, mineral chemistry and 40Ar/39Ar ages have been obtained in an appropriate suite of rocks collected over several field seasons. The use of Nd isotopic ratios and the thermodynamic framework of the energy-constrained assimilation fractional-crystallization (EC-AFC) model for complex open magmatic systems (Spera and Bohrson, 2001) provide petrological and thermochemical insights into the generation of silicic magmas through the interaction of basalt and fertile metamorphic rocks.


GEOLOGICAL BACKGROUND

The Sarmiento Complex formed as part of a submarine and partitioned basin that was continuously filled with the hemipelagic sedimentary successions known as the 'black shales' of the Zapata Formation (Allen, 1982; Fildani and Hessler, 2005). Basin development involved widespread eruptions of subaerial and subaquatic felsic ignimbrites that were deposited in the fault-bounded grabens of the basin associated with the El Quemado Complex (ca. 155 Ma; Pankhurst et al. 2000) and the Tobifera Formation {ca. 148-142 Ma; Calderón etal., in press). The Rocas Verdes basin evolved to a backarc basin in Early Cretaceous times (Wilson, 1991; Calderón, 2006) when rhythmic successions of fine grained distal turbidites in the upper member of the Zapata Formation (cf. Fuenzalidaand Covacevich, 1988) were deposited. Basin closure and obduction onto the cratonic margin occurred in the mid-Cretaceous (Dalziel, 1981) before the main orogenic deformation and rapid uplift of the Andean mountain chain leading to the initiation of the Magallanes foreland basin at -92 Ma (Fildani etal., 2003).

The formation of the Sarmiento Complex was contemporaneous in part with the emplacement of the Late Jurassic and eastern plutonic belt of the South Patagonian batholith (Fig. 1), which is mainly composed of leucogranitewith some gabbro (Hervé etal.,2007). Narrow and discontinuous belts of andalusite and sillimanite gneisses as well as migmatites occur in close spatial relation to the granitic belt (Watters, 1964; Calderón, 2005). A Late Jurassic age for crustal anatexis has been established through dating of overgrowth zones of zircons from the sillimanite gneisses (Hervé et al., 2003). At the Puerto Edén Igneous and Metamorphic Complex (Fig. 1), granites are considered as mixtures of variable amounts of radiogenic crust-derived anatectic melts and less radiogenic mafic magmas (Calderón etal., 2007).

Late Paleozoic polydeformed metasedimentary rocks, grouped in the Staines Complex and the Eastern Andean Metamorphic Complex (Forsythe and Allen, 1980; Hervé et al., 1998; Hervé et al., 2003; Augustsson, 2003; Augustsson etal., 2006), crop outtotheeastofthe South Patagonian batholith (Stewart etal., 1971 )1. To the west of the batholith (at Archipiélago Madre de Dios; Fig. 1)thicksuccessions of limestone, basaltandchertarethoughtto represent fragments of seamounts accreted to the paleo-Pacific margin of Gondwana (Forsythe and Mpodozis, 1983) between Early Permian and Early Jurassic times (Thomson and Hervé, 2002). The turbidites of the Duque de York Complex were deposited on top of the limestone- and basalt- successions between post-Early Permian to pre-Early Jurassic times in an active margin setting (Faundez etal., 2002; Lacassie etal, 2006). The Diego de Almagro Metamorphic Complex, that crops out to the west and is separated from the Duque de York complex by the northwest-southeast trending Seno Arcabuz shear zone (Olivares et a/., 2003), consists of foliated meta-granite, paraschists and blueschists juxtaposed in a subduction zone setting in the Early Cretaceous (Willner et al., 2004).

OCURRENCE AND M NERALOGY OF STUD ED SAMPLES

Samples for this study were collected from the pre-Jurassic metasedimentary rocks of the Staines Complex, the bimodal meta-igneous rocks of the Sarmiento Complex, felsic metamorphosed tuffs of the Tobifera Formation and late small volume igneous rocks in the Sarmiento Complex.

STAINES COMPLEX

Greenschistfaciespsammopeliticschistsofthe Staines Complex (Stewart et al., 19711; Forsythe and Allen, 1980; Allen, 1982; Hervé et al., 2003) crop out to the west of the Sarmiento Complex (Fig. 2). They consist of fine-grained and polydeformed white-mica chlorite schists with folded and crenulatedmillimeter-to centimeter-wide bands of granoblastic quartz and restricted plagioclase. Minor amounts of chloritized blasts of biotite also occur. Tourmaline, zircon, apatite and opaque such as graphite and Fe-Ti oxides are accessory phases. The turbiditic protolith of meta-morphic rocks was deposited in the late Paleozoic (Hervé etal., 2003; Augustsson etal., 2006).

SARMIENTO COMPLEX

In the southern segment of the Sarmiento Complex the incomplete ophiolite pseudostratigraphy dips gently to the south. Metagabbros (sills and dikes) and plagiogranites occur in the northern parts of the complex (Seno Lolos), whereas extrusive rocks dominate the southern exposures to Canal Unión (Fig. 2). Field work in the Sarmiento Complex has shown that it is possible to distinguish three main lithological layers in the region, as discussed below:

1.A mafic extrusive layer: exposed along the Cordillera Sarmiento is composed of a kilometer-thick, flat-lying succession of pillow and massive basalt, pillow breccias, and restricted intercalations of radiolaria-bearing cherts and siltstones. Olivine basalts show porphyritic and branching textures with metamorphic assemblages consisting of pumpellyite, chlorite, quartz, plagioclase, epidote, titanite, carbonate and rare actinolite. Metamorphic phases occur in amygdules, phenocrysts, veins and the groundmass. Olivine is totally replaced by calcite and/or chlorite. Clinopyroxene is the only relict mafic igneous phase. Meter-wide dikes of fine grained dolerite composed mainly of subhedral plagioclase (35-45%), ophitic clinopyroxene (25-35%), interstitial chlorite (13-23%) and accessory quartz, titanite and epidote occur in this layer.

2.Amafic-felsicextrusive layer: exposed along Península Taraba and at Isla Young consists mainly of successions of pillow basalts with intercalations of silicic tuffs and hyaloclastites. These successions are cut by late meter-wide dikes of dacite and rhyolite. Porphyritic and amygdaloidal basalts show variable amounts (5-15%) of olivine phenocrysts that are totally retrogressed to chlorite. In some cases, they contain calcite. A branching texture in the groundmass is defined by an aggregate of plagioclase, clinopyroxene, interstitial chlorite, accessory Fe-Ti oxides and pyrite. Felsic dikes are porphyritic rocks with phenocrysts of embayed quartz and subhedral plagioclase, and accessory allanite and chloritized pseudomorphs. Some of these rocks are partially brecciated and have myarolitic cavities suggesting hypabyssal conditions during crystallization. The quarzofeldspathic and spherulitic groun-dmass is aphanitic. Apatite and zircon are accessory phases. The secondary assemblage consists of chlorite, epidote, quartz and feldspars.


The mafic-felsic extrusive layer includes a narrow belt of sheared felsic and mafic meta-igneous rocks with a north-northwest trending subvertical foliation. The metamorphic assemblage in strain fringes and other microstructures in mafic rocks consists of actinolite, quartz, plagioclase, chlorite and titanite. In thefoliated felsic rocks, the predominant metamorphic phases are chlorite, epidote, titanite, quartz and feldspar with minor amounts of white mica. At the southern tip of Península Taraba (Bahía Intricate; Fig. 2), a small pluton of fine-grained biotite tonalite is in contact with massive, hornfelsicbasaltsandthefolded sedimentary successions of the Zapata Formation. The biotite tonalite contains microscopic inclusions of metamorphic rocks composed of quartz, plagioclase and decussate biotite.

3. A mafic-felsic intrusive layer: exposed between the east-west trending Seno Lolos, Seno Encuentros and along the northern shore of Seno Benavente (Fig. 2), consists of fine to medium-grained plagioclase- and quartz-rich porphyritic rocks with granophyric and graphic textures, hereinafter called granophyres. They are characterized by the presence of accessory allanite with narrow and irregular rims of colorless epidote. The granophyres are intruded by a swarm of north-south trending subvertical dike complexes of coarse- and fine-grained metagabbro. The dikes have inter-granular textures with no preserved pyroxene. Although most mafic dikes are straight and show centimeter-wide chilled margins, some sinusoidal dikes occur. Sheeted dike complexes occur in higher topographic levels (deVVit and Stern, 1981). Rare fine-grained plagiogranite dikes are in subhorizontal contactwith subvertical maficdikes. The plagiogranite dikes are composed of plagioclase, quartz and metamorphic epidote and chlorite. At the base of the mafic-felsic intrusive layer, homogeneous medium-grained and 'amphibolitized' gabbroic sills or dikes are found that preserve intergranular and cumulate textures that are defined by interlocking tabular plagioclaseandxenomorphichornblende. Interstitial opaque minerals are Fe-Ti oxides and pyrite. Sills of hornblende tonalite with inclusions of amphibolite and mafic aphanitic rocks appear at the base of this layer. The amphibolites have granoblastic textures with amphibole and plagioclase (melanosome) and leucocratic veins of tonalite (leucosome). Hornblende grains in the melanosomes have scarce relict inclusions of clinopyroxene. Hornblende in the amphibolites is retrogressed to actinolite, chlorite, titanite, epidote and traces of biotite. Plagioclase, in some cases, is replaced by microcrystals of white mica.

LATE SMALL VOLUME HORNBLENDE-BEARING INTERMEDIATE ROCKS

Plutons of medium-grained and panhypidio-morphic hornblende tonalite with marginal gra-nodiorite components crop out at Seno Encuentro (Fig. 2). Porphyriticand medium-grained hornblende dacitic dikes, with subhedral phenocrysts of hornblende, plagioclase and quartz within an afanitic quartzofeldspathic groundmass occur at Seno Benavente. The hornblende in them is partially replaced by chlorite, epidote and carbonate. Plagioclase shows variable degree of sericitization.

TOBÍFERA FORMATION

Along the eastern flankofthe Cordillera Sarmiento and the main part of the Cordillera Riesco, a kilometer-wide, north-south trending belt of sheared silicic pyroclastic rocks intercalated with meter-sized horizons of sheared quartz-bearing siltstones has been mapped in the Tobifera Formation (Stewart et al., 19711; Bruhn etal., 1978; Allen, 1982; Galaz etal., 2005). These meta-rhyolites have crystallization ages of ca. 148 Ma (SHRIMP U-Pb zircon ages; Calderón, 2006). Geothermobarometry of syntectonic metamorphic assemblages consisting of phengite, chlorite, stilpnomelane, and quartz indicate medium pressure greenschist facies conditions during dynamic metamorphism (Hervé etal., 2004; Calderón et al., 2005). The sheared belt is intruded by meter-wide, north-south trending sills of medium-grained chloritized dolerite, that (in studied samples) show no evidence of dynamic recrystallization. Consequently, the injection of the mafic magmas in this belt occurred after shearing and, is not related to the ca. 150 Ma old mafic magmatic events.

Tobifera rocks crop out to the west and in thrust contact over the Sarmiento Complex. They share an unconformable depositional contact with Paleozoic metasedimentary rocks that are basal breccias and conglomerates intercalated with greywackes, sandstones, siltstones. Rhyolitic crystal lapilli tuffs are dominant in the upper part of the succession (Allen, 1982). Minor lithicfragments of metasedimentary rocks and porphyritic rocks that are common constituents in these pyroclastic rocks show discrete stylolitic bands and anastomosing cleavages indicating dynamic recrystalliza-tion that is less intense than in the eastern sheared belt. White mica, quartz and plagioclase constitute the metamorphic assemblage. Pyrite is a frequent opaque phase.

MINERAL PHASE CHEMISTRY OF METAMORPHOSED MAFIC ROCKS

Mineral compositions were measured at the Institut für Mineralogie und Kristallchemie, Uni-versitát Stuttgart, using a CAMECA SX-100 micro-probe with 5 wavelength-dispersive systems. Operating conditions were: 15kV and 15 nA, a beam size of 7-10 |imorafocussed beam (for very small crystals), 20 seconds counting time on the peak and on the background for each element. The standards used were natural wollastonite (Si, Ca), natural orthoclase (K), natural albite (Na), natural rhodonite (Mn), synthetic Cr203 (Cr), synthetic Ti02 (Ti), natural hematite (Fe), natural baryte (Ba), synthetic MgO (Mg), synthetic Al203 (AI) and synthetic NiO (Ni). The PaP correction procedure provided by Cameca was applied.

The analyses were chosen to illustrate the compositional ranges of pyroxene, plagioclase and amphibole found in the metamorphosed mafic rocks. Studied samples were two medium-grained amphibolitized metagabbros (samples ST0228 and ST0229A) collected from the mafic-felsic intrusive layer. An amphibolite, consisting of an amphibole-rich melanosome and a quartz-plagio-clase leucosome was also analyzed (sample ST0226A). A fine-grained and weakly foliated dike of metabasaltic-andesite (ST0204) and an olivine metabasalt (OVV9962) with lath-shaped crystals of plagioclase and needles of clinopyroxene in the groundmass were collected from the mafic-felsic extrusive layer. Mineral compositions are presented in tables 1-3. The location of studied samples is illustrated in figure 2.

PYROXENE

Partially affected by metamorphic processes, clinopyroxene, which is predominantly augite in composition (Fig. 3), shows variable Mg-number [(100*Mg/(Mg+Fe2+)] and minor element concentrations. No compositional variation was detected where relict crystals allowed core and rim domains to be analyzed (see analyses of samples ST0204 and ST0228 in table 1). Clinopyroxene is the main stable igneous phase in the mafic rocks and its chemistry reflects primary magmatic compositions. The most primitive pyroxene grains, found in metagabbro ST0228, have Mg-numbers ranging from 80 to 90 in most cases and Cr203 contents as high as 0.36 wt% (Fig. 4a). Lower Mg-numbers in pyroxenes in the other metagabbro ST0229A suggest crystallization from a more fractionated magma. The clinopyroxene in both the dikes and lavas have Mg-number that range from approximately 67 to 75. Metabasalt OVV9962 has clinopyroxene with conspicuously higher contents of Ti02 and Al203, and elevated Na20 contents relative to other lithologies (Figs. 4b-d). These clinopyroxene crystals also have higher Cr203 contents than those in the metabasaltic-andesite and the evolved metagabbro. Although a negligible acmite component is found in the clinopyroxene, sodium contents show a good correlation with Mg-number (Fig. 4d).


PLAGIOCLASE

All of the plagioclase in the metabasalts and metabasaltic-andesites is albite in composition (Fig. 5) indicating a metamorphic origin. The albite occurs along with chlorite, epidote, titanite and scarce K-feldspar. The plagioclase in the metagabbros is predominantly bytownite (An7387; ST0228) and labradorite (An5065; ST0229A). Its composition and fresh aspect indicate that this feldspar is a primary igneous phase. The bytownite composition is consistent with the primitive character of sample ST0228, which is indicated by the high Mg-number and Cr203 content of the clinopyroxene.

Most plagioclase in the melanosome of the amphibolite is andesine (An37 51). Some is oligoclase (An1224). The few crystals in the leucosomes that were analyzed are mostly andesine in composition (An3235).






AMPHIBOLE

Metamorphic amphibole in the studied samples is actinolite and magnesiohornblende (according to Leake et al., 1997; Table 3). Actinolite in the mafic-extrusive layer is only present in the sheared domain, where sample ST0204 was collected. Actinolite is an ubiquitous phase in the mafic-felsic intrusive layer. Mg/(Mg+Fe) ratios are significantly higher in the actinolite in the metagabbros than in the metabasaltic-andesite (Fig. 6a). This feature is consistent with the progressive variation in Mg-number of relict clinopyroxene, indicating a bulk compositional control in the chemistry of actinolite. Some crystals of magnesiohornblende, which are usually replaced by aggregates of actinolite along crystal edges, occur in metagabbros.

The amphiboles in the amphibolite, both in the melanosome or leucosome, are magnesiohornblende. Those in the melanosome show lower silica contents (Si ca. 6.8 and 7.0 in tetrahedral site based on 23 oxygens; Fig 6a) than in the leucosome (tetrahedral Si 7.1 and 7.8). The composition of amphibole and plagioclase in granoblastic assemblages permits the use of the hornblende-plagio-clase geothermometer of Blundy and Holland (1994). Equilibration temperatures of 742±50°C are based on twenty-one pairs of magnesiohornblende and andesine plagioclase compositions (Fig. 6b) at pressures between 0 and 2 kbar. Lower temperatures (645±50°C) were calculated from five pairs with oligoclase plagioclase compositions.


40Ar/39Ar GEOCHRONOLOGY

Laser heating 40Ar/39Ar analyses of individual grains of metamorphic amphibole in rocks of the the Sarmiento Complex, were carried out in the Centro de Pesquisas Geocronológicas at the Universidade de Sao Paulo, Brazil. Detailed analytical procedures, including mineral separation, irradiation and mass spectrometric measurement procedures are presented by Vasconcelos etal. (2002). Although none of the analyses allow plateau ages to be calculated, some more or less concordant steps were used to calculate apparent 'forced plateau' ages. Results of 40Ar/39Ar analyses are listed in table 4.

The 40Ar/39Ar spectra of an individual amphibole grain separated from an amphibolite (sample ST0226B), displays little discordance except at one step in which a fall in the Ca/K ratio is related to an increase of the apparent age. The low Ca/K ratio of the analyzed grain (< 30) is akin to the ratios calculated for a low-silica magnesio-hornblende in an amphibolite (ST0226A) from the same outcrop. A forced plateau age of 172±5 Ma was calculated using four steps of the spectrum (Fig. 7a).

Two amphibole grains separated from a meta-gabbro (sample ST0229A), were analyzed. Both grains have Ca/K ratios between 50 and 100, which are similar to those for some actinolite grains in the same sample (magnesiohornblende in this sample have Ca/K ratios < 40). The spectrum of the first grain displays an old age in the lowest incremental heating step (Fig. 7b). Aforced plateau age for this sample that was calculated from three steps yields an age of 151±12 Ma. The spectrum of the other grain shows less discordance and has a higher Ca/K ratio. A forced plateau age of 143±4 Ma for this sample was calculated using the first two steps (Fig. 7c).

The apparent age calculated for an amphibole grain of the amphibolite is about 20 Ma older than a zircon SHRIMP U-Pb crystallization age near 149 Ma for a plagiogranite. This age probably reflects excess 40Ar in the amphibole grain that was incorporated either during amphibolite fades metamorphism and/or during the injection and crystallization of hornblende tonalitic melts. The ages of the actinolite in the metagabbro are concordant in part with the U-Pb zircon crystallization ages of the plagiogranite. Taking into consideration that perturbation of the K-Ar system in this rock is low (due to the lack of tonalitic segregations in the outcrop) and that the age calculated from the grain with the higher potassium content is more precise, an Early Cretaceous (ca. 140 Ma) metamorphic cooling age is suggested.



ISOTOPIC COMPOSITIONS

Sr and Nd isotopic analyses were performed in the Centro de Pesquisas Geocronológicas (Universidade de Sao Paulo). Rock samples for isotopic analysis consisted of 1-3 kg of material, which were crushed and split to 200g, and then powdered in a tungsten carbide mill. Rb and Sr contents were determined by X-ray fluorescence spectrometry. 87Sr/86Sr ratios were measured by thermal ionization mass-spectrometry, corrected for mass fractionation to 86Sr/88Sr=0.1194 normalization. Sm and Nd (and other lanthanides) were chemically separated in HDEHP columns supported on teflon powder. The Sm and Nd concentrations were obtained by isotopic dilution using a mixed 149Sm and 150Nd tracer. The isotopic ratios were calculated relative to 146Nd/144Nd=0.7219. All analyses of radiogenic isotopes were performed using a VG 354 Micromass spectrometer. At the time of the analyses the following standard values were obtained: 87Sr/86Sr=0.71026± 0.00002 (2s)forNBS-987 and 143Nd/144Nd=0.511847±0.00002 (2s) and 0.512662±0.000027 (2s) values for La Jolla and BCR-1 respectively.

SHRIMP U-Pb geochronological determinations on zircons from felsic dikes of the Sarmiento Complex and metamorphosed tuffs of the Tobifera Formation show that mafic and felsic magmatism occurred between 152 and 147 Ma (Calderón etal., in press). Age dependant isotopic ratios were calculated at 150 Ma for most samples with the exception of those collected from late (?) plutonic rocks and dikes in the Sarmiento Complex. For these rocks, we assumed an age of 140 Ma based on published radiometric ages (e.g., Stern et al. 1992; 40Ar/39Ar analyses in this study) and field relations. The sample with the lowest 147Sm/144Nd ratio (0.064; dolerite) shows the largest variation, one epsilon-neodynium unit (eNd) when calculated either at 100 or at 150 Ma. Other samples show variations of around 0.2 epsilon units. Whole rock Sr and Nd isotopic data are listed in table 5 and isotopic relations are plotted in figure 8.

Metasedimentary rocks yielded 87Sr/86Sr initial ratios (Sr150; calculated at 150 Ma) of 0.71757 (psammo-pelitic schist) and 0.72703 (pelitic schist) and correspondingly low initial eNd150 values of -4.4 and-9.1, respectively. Depleted mantle model ages (TDM; after DePaolo et al., 1991) are 1,322 and 1,660 Ma, respectively.

Mafic meta-igneous rocks in the Sarmiento Complex have relatively depleted isotopic compositions with eNd150 values ranging between +0.8 and +2.3 and TDM between 624 and 706 Ma. The higher values of eNd150 (+2 and +2.3) correspond to the finegrained dike of gabbro and the amphibolite. The pillow basalt yielded Sr150 of 0.70427 and eNd150 of + 1.1. The plagiogranite showed a eNd150 value of+2.2.

Coarse and fine-grained granophyres in the mafic-felsic intrusive layer yielded more crustal-like £Ndi5o va'ues ranging from -5.1 to -5.5 and TDM model ages around 1,300 Ma. The massive dike of allanite-bearing dacite and of brecciated dacite in the mafic-felsic extrusive layer, have lower eNd150values of-3.9 and -2.9, than the granophyres. Afoliated metatuff in this layer yielded a eNd150 value of -5.1 that is indistinguishable from the values in the granophyres.

The hornblende dacite yielded a Sr140 of 0.704420 and a eNd140 of +0.9, which is similar to that in the tonalitic leucosome in the amphibolite (+0.8). The hornblende tonalite yielded a Sr140 of 0.70425, a TDM model age of 577 Ma and an eNd140 value of+2.8. Both rocks, which were collected from the mafic-felsic intrusive layer, have similar ratios to the metamorphosed mafic rocks. The biotite tonalite, in the southern tip of the mafic-felsic extrusive layer, has a higher Sr140 of 0.70451, lowereNd140+1.0and slightly older TDM model age (724 Ma). The significantly more primitive igneous rock in the Sarmiento Complex is the dolerite collected from a dike in the mafic extrusive layer, which has a eNd140of+5.4,aTDMof369 Ma and an initial 87Sr/86Sr ratio of 0.70583.


Metatuffsofthe Tobifera Formation have crustal-like isotopiccompositions with eNd150varying between -5.1 and -5.4, TDM model ages of 1,139 and 1,169 Ma and Sr150 of ca. 0.707 (average value). The sample with the highest Sr150 (0.7121) is the metatuff, which contains Hthicfragments of metasedimentary rocks among its constituents.


DISCUSSION

The lithological, metamorphic and geochemical variations in the Sarmiento Complex have been considered to be like those recognized in diverse ophiolite complexes (Stern etal., 1976; Stern, 1979; Saunders etal., 1979; de Wit and Stern, 1981). The development of secondary phases without schistosity and the downward increase in metamorphic grade from zeolite to greenschist to amphibolite facies within a variable and short vertical sequence (1-3 km) have been associated with processes of seafloor hydrothermal metamorphism (Stern et al., 1976). Greenschist facies dynamic metamorphism and the development of sheared belts in the Sarmiento Complex and Tobifera Formation, are probably associated with processes of ophiolite emplacement that are beyond the scope of this paper. Nevertheless, it is possible that some of the shearing was oceanic in setting.

MANTLE SOURCES AND MAFIC MAGMATISM

Geochemical data from metamorphosed mafic rocks of the Sarmiento Complex (Stern, 1979; Saunders et al., 1979; Fildani and Hessler, 2005) put constraints on compositional features of the mantle source that produced the mafic magmas. The rare-earth-element (REE) patterns (mostly mafic lavas and dikes) show LaN/LuN ratios ranging between 1.6 and 3.3 after normalization to chondrites (according to Sun and Mcdonough, 1989; Fig. 9a). The lack of heavy REE depletion in the chondrite normalized patterns suggests partial melting processes in the upper mantle in the spinel- or plagioclase-lherzolite stability field (P < 15 GPa). Multi-element variation diagrams for metamorphosed mafic rocks after normalization to normal-mid-ocean-ridge-basalt (Sun and Mcdonough, 1989; Fig. 9b) display erratic and enriched values in large-ion- lithophile-elements (LILE), such as Rb, Baand K compared to high-field-strength-element such as Nb and Ta. They also show more or less pronounced Nb-Ta and weak Ti negative anomalies. These compositional features are similar to those in continental and island arc basalts (Pearce, 1982) and suggest that the source of the mafic magmas was affected by processes related with asupra-subduction setting. Alowdegree of partial melting of the mantle sources seems to be needed. A sign of fluid-or melt-driven metasomatism in the mantle source is the low initial 143Nd/144Nd ratios in the metamorphosed mafic rocks that indicate a time-integrated enrichment of Nd relative to Sm in the mantle source.


A primitive mantle source is required by the isotopic composition of the dolerite dike. Min-eralogical similarities with the late or post-tectonic dolerite dikes in the metarhyolites of the Tobifera Formation (at Cordillera Riesco) suggest that these rocks belong to a younger episode of mafic mag-matism in which mantle sources and tectoniccondi-tions were different. These dolerite dikes could be related to the Cretaceous mafic magmatism of the Barros Arana Formation, which was derived from depleted subcontinental mantle sources (cf. Stern, 1991).

Tholeiitic fractional crystallization trends during mafic magmatism have been indicated by Stern (1979), supported in part by progressive variations in Mg-number of clinopyroxene (Fig. 4a). The titanium-rich clinopyroxene compositions in the metabasalt resemble those in alkaline rocks on clinopyroxene discrimination diagrams [(Ca+Na) versus Ti (not shown); Leterrier et al., 1982]. Nevertheless, the high titanium content in this phase could be controlled by magma chemistry and fast cooling rate at the time of crystallization of basalts (cf. Tracy and Robinson, 1977). High titanium content in the clinopyroxene is consistent with an evolved composition of the basaltic magma (supported by the low Mg-number) and quenching processes are suggested by pillow structures and branching texture.

FELSIC MAGMATISM

The petrological meaning of the granophyre and dacite in the bimodal layers of the Sarmiento Complex is a key subject point. Whole rock chemical analyses and inherited components in zircon crystals from granophyres (dated at 147±10 Ma) have been taken as evidence of their derivation from melting of continental material bordering the mafic complexes into which the mafic magmas intruded (cf. Saunders et al., 1979; de Wit and Stern, 1981; Stern etal., 1992). Furthermore, Nd isotopic compositions of metamorphosed felsic rocks support the involvement of crustal components during magma genesis.

The plagiogranites, with Si02 contents between 66-75 wt%, relatively enriched and subparallel REE patterns as compared with the metabasalts, and showing Eu depletion (Fig. 9a), have been suggested to result from extreme fractional crystallization of mafic magmas (Stern, 1979). Nevertheless, diverse ophiolite studies have revealed that the roofs of axial magma chambers can be marked by polyphase magmatism, partial melting of amphibolites and intense interaction between hydrothermal fluids and rocks (e.g., Gillis and Coogan, 2002 and references therein). Hornblende-plagioclase geothermometry (ca. 740±50°C) in an amphibolitic inclusion hosted in the hornblende tonalite indicates that disequilibrium partial melting (e.g., T > 900°C at P = 0.2 GPa; cf. Koepke etal., 2004) and melt segregation processes occurred in deeper portions of the ophiolite pseudostratigraphy. Therefore, the origin of plagiogranites, and probably of the small volume hornblende-bearing lithologies in the Sarmiento Complex could involve water-satured melts derived through anatexis of amphibolitized gabbros. A similar origin has been proposed for the generation of silicicdifferentiates in sea-floorspreading centers, such as plagiogranites in ophiolites (Gillis and Coogan, 2002) and silicic volcanic rocks in Iceland (Gunnarsson etal., 1998).

THERMOCHEMICAL MODEL

The integration of energy, species and mass conservation into simulations of assimilation, and fractional crystallization processes in complex magmatic systems is possible with the energy-constrained assimilation fractional-crystallization (EC-AFC) model (Spera and Bohrson, 2001; Bohrson and Spera, 2001). This model follow the thermochemical evolution of an open-system magma body through solution of a system of coupled, non-linear ordinary differential equations that express conservation of energy, mass and species (trace elements and isotopes). The thermochemical approach used below considers two subsystems (for details see Spera and Bohrson, 2001; Bohrson and Spera, 2001): a homogeneous magma body of initial mass M0 and a finite mass of surrounding country rocks. Solution of the EC-AFC provides values for the assimilant temperature (Ta), mass of melt within the magma body(MJ, mass of cumulates (Mc), mass of anatectic melt assimilated (Ma*) and concentration of trace element and isotopic ratios in melt as a function of magma temperature (TJ. Input parameters include the equilibration temperature (T ), the initial temperature and isotopic composition of pristine magma (Tm° and em) and wall rock (Ta° and ea), temperature-dependent trace element distribution coefficients (Dm and Da), heats of transition for pristine melt and wall rock (Dm and sDha) and the isobaric specific heat capacity of pristine melt and assimilant(Cpmand C ). Melt-productivity functions [/(Tm) and /(Ta)] exhibit non-linear sigmoidal forms in meltfraction versustemperature atfixed pressure and volátiles fugacity. Phase transition enthalpies and specific heat capacities are taken from the literature (Spera, 2000; Bohrson and Spera, 2001; Fowler etal., 2004). The temperature dependence of the trace-element partition coefficient is estimated from literature. Because the Sm-Nd isotopic system seems to be unaffected by greenschist and amphibolite fades metamorphism (not the case for the Rb-Sr system; e.g., DePaolo, 1988), solely the Nd isotopic composition of the studied rocks are considered in the following discussion.

The thermochemical model forthepetrogenesis of the allanite-bearing granophyres and dacitic dikes in the bimodal layers (models 1 and 2) requires the choice of an appropriate Nd composition representative of the crustal assimilant at the time of the emplacement of mafic magmas. At the latitudes of the Sarmiento Complex, rifting in the Rocas Verdes basin probably began in the pre-Jurassic continental basement rocks of southwestern Gondwana. These rocks are represented in the studied area by the metasedimentary rocks of the Staines Complex. These psammopelitic and pelitic schists have eNd150 of -4.4 and -9.1, respectively. These are the most extreme isotopic values (Fig. 8) measured on Paleozoic metasedimentary rocks east of the South Patagonian batholith. Because the metasedimentary rocks of the Staines Complex share common petrographic features, structures and zircon detrital patterns, with metasedimentary rocks of the Eastern Andean Metamorphic Complex (cf. Hervé et al., 2003), the correlation of both complexes is suggested. Consequently the average Nd isotopic composition of existing data (Augustsson, 2003; Calderón et al., 2007) is considered representative of the composition of the crustal assimilant. The assumed average isotopic composition of the crustal assimilant used in the calculation is eNd of-7.3. The composition of the fine-grained gabbro in the mafic-felsic intrusive layer is considered representative of the pristine mafic magmas. Selected results of the EC-AFC simulation are illustrative of a basaltic magma intruded at a liquidus temperature of 1200°C (Tm°) into an upper crust with ambient temperatures of 300°C (Ta° in model 1) and 400°C (Ta° in model 2). These temperatures are within the range expected in a rift setting. The local liquidus and solidus temperatures of the pelitic crustal assimilant are T=715°C and T =900°C. The equilibration temperature Teq is 870°C for both cases. Model 3 judges the thermal interaction between the mafic magmas and the host amphibolitic mafic crust in producing the plagiogranite. In model 3, the isotopic composition of the amphibolite is considered to be that of the mafic assimilant. All input parameters and comprehensive information are listed in table 6 and the models are shown by the lines labelled M1, M2, and M3 in figure 10.



The good fit between the actual data and the models in figure 10 justifies using the calculated curves that are labeled M1 and M2 as tentative solutions to explain the isotopic variations seen in the mafic-felsic igneous layers of the Sarmiento Complex. As shown by Bohrson and Spera (2001), geochemical trends using the EC-AFC model display non-monotonic behaviours linked to energy conservation and partial melting of the country rock. Afterthe mafic magma temperature decreases by 180°C the subhorizontal fractional crystallization trend is interrupted and gives rise to a steep mixing curve between the mafic magma and the radiogenic crustal rock (Fig. 10). A quantitative evaluation of the contamination of the basaltic magma by the crustal wall rocks indicates a MaVMc (mass of anatectic melt assimilated/ mass of cumulates) ratio of 0.04 (in model 2; Ta=745°C). Although small compositional differences between the basaltic lava and dike could be due to source heterogeneities, the model is in agreement with fractional crystallization being the main process involved in the evolution of mafic magmas.

The quantitative analyses of magma contamination in the granophyres indicate MaVMc ratios between 0.63-0.89 at T between 895-920°C and with T of 795-820°C. In the case of the dacites from the mafic-felsic extrusive layer, MaVMc ratios are lower (0.28-0.35), Tm are higher (975-945°C) and Ta are lower (770-780°C). The meta-tuffaceous rocks in the mafic-felsic extrusive layer show similar results to those for the granophyres. The differences in the degree of crustal assimilation among the metamorphosed felsic rocks could be related to different residence times for the silicic magmas in the magmatic reservoirs, controlled in part by the ease of eruption in response to the regional stress field.

The results for model 3 for the plagiogranite petrogenesis are consistent with the fractionation model of Stern (1979). They predict the 13% assimilation of the amphibolitic mafic crust (MaVMc ratio of 0.13).

TECTONO-MAGMATIC MODEL

The development of the Rocas Verdes basin probably involved periods of variable spreading rates that favored either the extrusion of large volumes of basaltic rocks or the emplacement and evolution of axial mafic magma chambers. The conceptual model for bimodal magmatism in the early tectonic evolution the Sarmiento Complex requires an early upper crustal magma chamber that formed by continuous basaltic injection from mantle reservoirs (Fig. 11). The accumulation of mafic magmas, which probably occurred after a period of rapid extension and voluminous submarine mafic volcanism, triggered crustal anatexis, segregation and accumulation of silicic melts with crustal isotopic compositions. The granophyres crystallized in the relatively 'quiet' tectoniczones, whereas the allanite-bearingdacitic dikes crystallized from magmas that travelled through remnant brittle deformed pathways along the rift axis.

A renewed pulse of extension and extrusion of the mafic magmas is represented by the dike complexes of fine-grained gabbro and basaltic flows in the mafic extrusive layer (Fig. 11). Subsequently, remnant gabbroic portions of the ophiolitic crust underwent hydrothermal meta-morphism and anatexis during this episode of enhanced magmatic heat flux from the mantle. Even though there is lack of age control in the hornblende-bearing lithologies in the Sarmiento Complex, the above processes must be considered in the generation of the water-saturated magmas (hornblende-bearing intermediate igneous rocks) along axial magma chambers (Fig. 11).

A magmatic origin similar to that of the alianite-bearing granophyres and dacitic dikes of the Sarmiento Complex is suggested for the metatuffs of the Tobifera Formation, which also formed part of the seafloor of the Rocas Verdes basin. These tuffs share similar eNd150 values (of ca. -5) with the granophyres. Nevertheless, differences in the Nd and Si02 contents between the metatuffs (Nd between 6-16 ppm and Si02 of 78-81 wt%) and the granophyres (Nd of ca. 30 ppm and Si02 of 70-73 wt%) suggest that variable thermal regimes of magmatic reservoirs, caused by different rates of mafic magma recharge during magmatic chamber evolution, crustal heterogeneities, and other factors, exerted control over processes of assimilation and fractional crystallization. The periodic influx of mafic magmas and their interaction with the metapelites in the volcanic rifted continental crust may represent a first-order heat contribution for crustal anatexis and silicic magmatism during the early Rocas Verdes basin formation.


CONCLUSIONS

According to the discussions and available geochemical and isotopic data, the following points are worthy of note.

1. Metamorphosed mafic rocks and a plagio-granite from different layers in the ophiolite pseudo-stratigraphy of the Sarmiento Complex are characterized by eNd150 values ranging between +1 and +2. Isotopic and trace element compositions suggest that mafic magmas were derived from an enriched mantle source, previously affected by metasomatism in a supra-subduction setting.

2. The progressive variation in Mg-number of the relict augitic pyroxene records the processes of fractional crystallization operating in the mafic magma chambers and along the magmatic/ structural pathways.

3. The felsic rocks in the bimodal layers of the Sarmiento Complex have crustal-like isotopic compositions. The eNd150 values of these granophyres range between -5 and -5.5, whereas those of the dacites are slightly higher (-3 to -4). These values exclude their derivation by fractional crystallization processes alone in the mafic magma chambers.

4. Quantitative thermochemical analyses indicate that the felsic rocks formed from magmas with mass of anatectic melt assimilated/mass of cumulates ratios (MaVMc) of 0.28-0.35 in the dacites and of 0.63-0.89 in the granophyres.

5. Model results for plagiogranite petrogenesis indicate that fractional crystallization is the dominant process, but also predict the assimilation of amphibolitic mafic crust by 13% (MaVMc of 0.13).

Notas

1 Stewart, J.; Cruzat, A.; Page, B.; Suárez, M.; Stambuk, V. 1971. Estudio geológico económico de la Cordillera Patagónica entre los paralelos 51 °00"y 53°30"Lat.S: provincia de Magallanes. Instituto de Investigaciones Geológicas (Inédito): 174. Santiago, Chile.

ACKNOWLEDGEMENTS

This study was financially supported by the Conicyt grant and the BAPRTD (M.C), Fondecyt grants 1010412 and 1050431 (F.H.) and Conicyt-BMBF grant to F.H. and H.-J. M. The Álvarez-McCloud family is acknowledged for their logistical and friendly support during the fieldwork in the remote sampling areas. We would like to thank A. Onoe, L. Petronilho, S. de Souza, H. Sonoki, I.Sonoki and V. Loios of the Centro de Pesquisas Geocronológicas (Universidade de Sao Paulo, Brazil) who helped with the acquiring and processing of the isotopic and geochronological data. T. Theye is acknowledged for his guidance during the work with the electron microprobe. We thankS. Kay, C. Stern, E. Mooresand P. Thy for their constructive reviews.

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Received: September 14, 2006; accepted: April 2, 2007.