SHRIMP chronology of the Magallanes Basin basement, Tierra del Fuego: Cambrian plutonism and Permian high-grade metamorphism.

Francisco Herve; Mauricio Calderon; Mark Fanning; Stefan Kraus; Robert Pankhurst

Andean Geology 37 (2): 253-275. July, 2010
formerly Revista Geológica de Chile
www.scielo.cl/andeol.htm

SHRIMP chronology of the Magallanes Basin basement, Tierra del Fuego: Cambrian plutonism and Permian high-grade metamorphism

Geocronología SHRIMP del basamento de la Cuenca de Magallanes, Tierra del Fuego: plutonismo Cámbrico y metamorfismo Pérmico de alto grado.

Francisco Hervé¹, Mauricio Calderón¹, C. Mark Fanning², Stefan Kraus³, Robert J. Pankhurst⁴

¹ Departamento de Geología, Universidad de Chile, Casilla 13518, Correo 21, Santiago, Chile. fherve@cec.uchile.cl; caldera@esfera.cl
² Research School of Earth Sciences, Australian National University, Milis Road, Canberra, ACT 0200, Australia. Mark.Fanning@anu.edu.au ³ Instituto Antartico Chileno, Plaza Muñoz Gamero 1055, Punta Arenas, Chile. skratis@inach.cl
⁴ NERC Isotope Geosciences Laboratory, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, United Kingdom. rjpt@nigl nerc ac uk

ABSTRACT. Five new SHRTMP U-Pb zircon ages are reported for gneisses and foliated plutonic rocks belonging to the Tierra del Fuego igneous and metamorphic basement complex (TFIMC), obtained from the bottom of borehole cores through the Magallanes Basin. Three of the samples yielded weighted mean ²⁰⁶Pb/²³⁸U ages (523±7 Ma, 522±6 Ma and 538±6 Ma), interpreted as indicating Early Cambrian igneous crystallization of the host rocks. A migmatitic gneiss shows peaks at ca. 950-1,100 Ma and 560-650 Ma from inherited zircon grains in addition to two grains with ages of ca. 525 Ma, suggesting involvement of Grenvillian and Brasiliano material in the protolith of a Cambrian migmatite. A cordierite-sillimanite-garnet gneiss contains igneous zircons of Cambrian age and a population of U-rich metamorphic Permian zircons, indicating that a Permian high-grade metamorphic and anatectic (P=2-3 kbar, T=730-770°C) event affected the Cambrian igneous rocks or sedimentary rocks derived from them. Cambrian/Ediacaran plutonic rocks are known from the basement of NW Argentina, the Sierra de la Ventana, the Cape Fold Belt in South Africa, and the Ross Orogen in Antarctica. The Permian metamorphic event is coeval with the deformation and low-grade metamorphism of the sedimentary successions that overlie the basement in many of these areas. In Tierra del Fuego at least 8 to 12 km of cover rocks were removed following the high-grade Permian metamorphic episode and the unconformable deposition of the Tobífera Formation volcanic rocks in the Middle to Late Jurassic. This eroded cover could nave been an important source of detritus for the conglomeratic Permian and Triassic? Successions of neighboring regions in South America, Africa and Antarctica.

Keywords: Cambrian plutonism, Permian metamorphism, U-Pb SHRIMP ages, Basement complex, Magallanes Basin.

RESUMEN Cinco nuevas edades radiométricas logradas mediante análisis U-Pb en circón utilizando el SHRIMP, fueron determinadas en gneises y rocas plutónicas foliadas obtenidas desde el fondo de pozos de sondajes en la Cuenca de Magallanes y pertenecientes al denominado Complejo ígneo y Metamór-fico de Tierra del Fuego. En tres de las muestras fueron calculadas edades del Cámbrico Temprano (523±7 Ma, 522±6 Ma y 538±6 Ma), interpretadas como edades de cristalización ígnea de las rocas estudiadas. Un gneis migmatítico presenta 'peaks' de circones heredados de ca. 950-1.100 Ma y de 560-650 Ma además de dos granos de edades de 525 Ma, indicando la participación de material Grenviliano y Brasiliano en el protolito de la migmatita cámbrica. Un gneiss de cordierita-sillimanita-granate presenta una población de circones ígneos de edad cámbrica y otro grupo de circones metamórficos, ricos en U, de edad pérmica, indicando que en el Pérmico un evento metamórfico del alto grado acompañado de anatexis (P=2-3 kbar, T=730-770°C) afectó a rocas ígneas cámbricas y/o a rocas sedimentarias derivadas de ellas. Rocas plutónicas cámbricas/ediacaranas han sido descritas en el NW de Argentina, en los basamentos de la Sierra de la Ventana y del Cinturón Plegado del Cabo (en el sur de África), y en el Orógeno de Ross en Antartica. El evento metamórfico Pérmico es contemporáneo con la deformación y metamorfismo de bajo grado, registrado en las sucesiones sedimentarias que sobreyacen el basamento en muchas de estas areas. Siguiendo al episodio Pérmico de metamorfismo de alto grado, en Tierra del Fuego fueron removidos al menos 8 a 12 km de rocas de cobertura antes de la depositación de las rocas volcánicas de la Formación Tobífera en el Jurásico Medio y Superior. Las rocas erosionadas representan una fuente importante de material para las sucesiones conglomerádicas pérmicas y triásicas? ubicadas en las regiones vecinas de América del Sur, África y Antartica.

Palabras clave: Plutonismo Cámbrico, Metamorfismo Pérmico, Edades U-Pb SHRIMP, Complejo de basamento, Cuenca de Magallanes.

1. Introduction

The Magallanes Basin developed in Mesozoic and Cenozoic times, extending over most of Tierra del Fuego and the adjoining mainland areas of southernmost South America. The evolution of the basin began with a Middle to Late Jurassic extensional phase, which generated half grabens that were filled first with sediments and then with silicio volcanic rocks of the Tobífera Formation. Subsequent thrusting over the mainland of the quasi-oceanic Rocas Verdes basin, which developed during this extensional phase in the Pacific continental margin, generated during mid-Cretaceous to late Cenozoic times a foreland basin filled by a km-thick sedimentary succession.

The basement of this basin, except for a thin strip of low grade metasedimentary rocks at the NW coast of Península Brunswick, does not crop out and is only attainable through drilling. The Tobífera Formation unconformably covers a complex of granitoids and orthogneisses, here named the Tierra del Fuego Igneous and Metamorphic Complex (TFIMC). The age of this pre-Middle Jurassic unit has been difficult to ascertain with Rb-Sr and K-Ar techniques. From the U-Pb data of Söllner et al. (2000) and Pankhurst et al. (2003) it is known that crystalline rocks of Cambrian age are buried 3,000-4,000 m below the Mesozoic to Cenozoic sedimentary infill of the Magallanes Basin. However, both these studies referred to samples from the same borehole (Gaviota Norte 6, Fig. 1), therefore the areal extent of the Early Cambrian rocks remained uncertain.

FIG. 1. Map of Tierra del Fuego and surroundings (a), showing the locations of the oil wells from which the samples analysed in this paper were taken (b). Thick solid lines in the inset (a) indicate the sections shown in figure 8. Dashed Unes are depth contours (in meters) that delinéate the top of the Jurassic volcanic rocks (Tobífera Formation; from Biddle et al., 1986).

In this paper, we present 5 new SHRIMP U-Th-Pb age determinations for zircons separated from gneisses and foliated intrusive rocks from the bottom of oil drill-holes in the Magallanes basin. These data, together with the previously published (Söllner et al., 2000; Pankhurst et al., 2003) and the two recently published (Hervé et al., 2010) U-Pb ages of TFIMC magmatic zircons, and a petrographic study of a larger number of samples of similar source, are discussed and interpreted to provide a better understanding of the Paleozoic geological development of this portion of the Gondwana supercontinent.

2. Geologicalosetting

Known occurrences of crystalline basement rocks in southern South America include the Sierra de la Ventana Fold Belt (Rapela et al., 2003), the North Patagonian Massif, the Deseado Massif further to the southeast (Pankhurst et al., 2003), and the rocks underlying the sedimentary successions of the Magallanes Basin in Tierra del Fuego (Söllner et al., 2000; Pankhurst et al., 2003).

According to recent paleotectonic reconstructions (Pankhurst et al., 2006), the continental crust of part of this area is considered to have drifted away from Gondwana during the Early Cambrian in order to form an independent parautochthonous terrane (southern Patagonia). This crustal block later collided during the mid-Carboniferous with the North Patagonian Massif, which was already forming part of Gondwana. Before the later break-up of Gondwana, southernmost Patagonia was probably continuous with part of East Antarctica and South Africa. The Weddellosea and South Atlantic started to open in the Late Jurassic as a consequence of Gondwana break-up and a number of different models have been developed for the early opening of the Weddellosea and the tectonic evolution of its neighbouring continents (e.g., Suárez, 1976; LaBrecque and Barker,1981; Lawver et al., 1992; LaBrecque and Ghidella, 1997; Ghidella et al., 2002; Konig and Jokat, 2006). Southern Patagonian basement rocks formed during the Latest Proterozoic-Earliest Paleozoic are the result of the initial stages of formation of the Terra Australis orogen (Cawood, 2005) which evolved until the Late Paleozoic along the Pacific margin of Gondwana. Rocks of similar age are also found in the metamorphic basement of northwestern Argentina (e.g., Ramos, 1988) and in the Deseado Massif in northern Patagonia (Pankhurst et al., 2003). They are the result of continental margin orogenic events known as Pampean in Argentina, which can be broadly correlated with the Ross-Delamerian orogenies in East Antarctica and Australia (Transantarctic Mountains, Adelaide Fold Belt) (Söllner et al., 2000).

3. Sample description

Samples for this study were obtained from the lowest parts of drill cores generated during Empresa Nacional del Petróleo (ENAP, Chile) hydrocarbon exploration activities in the Magallanes basin. Five samples were selected for U-Pb dating following microscopic examination of approximately 30 thin sections of the cores. The samples are geographically distributed in a roughly N-S stretching band, 50 km wide and 200 km long, west of the international border between Chile and Argentina (Fig.1b).

3.1. Aguila 2 borehole, sampleA2 518 M4-Augen gneiss

This rock is predominately composed by coarse-grained aggregates of quartz, plagioclase and K-feldspar, separated by finer bands of deformed and partly chloritized green biotite. The K-feldspar crystals also show microfaults. Interstitial carbonate partly replaces plagioclase. Zircon crystals are abundant, apatite and opaque minerals are accessory. In the same core, 30 cm long sections of peraluminous granitic gneisses with quartz, plagioclase, biotite and cordierite are present, as well as faintly foliated orthogneiss.

3.2. Gaviota Norte 6 borehole, sample GN6 2273 Ml-Banded biotite orthogneiss

The sample consists of aggregates of quartz, plagioclase and biotite, the latter oriented and tending to form separate bands. Plagioclase is partially altered to calcite and chlorite, the biotite is mainly chloritized and altered peripherally to white mica. Opaque minerals are abundant. Euhedral zircon and apatite are common in the biotite bands. Microca-taclastic bands cross-cut the rock.

3.3. María Emilia 2 borehole, sample ME21619 M2-Breccia of banded gneiss

The rock has a fragmental texture. All the angular fragments, up to 4 cm long, consist of quartzo-feldspathic gneiss, composedof quartz-plagioclase-K feldspar bands alternating with finer biotite-rich bands. Quartz crystals show only slight undulose extinction. The fine grained matrix berween the clasts is not foliated; it contains small, undefor-med biotite crystals, some of which bear fusiform prehnite inclusions. Titanite is abundant. The rock resembles a monomict sedimentary breccia rather than a tectonic breccia.

3.4. Monte Aymond 20 borehole, sample MA20 1011 M2-Migmatitic granitoid

This is a mixed rock, which has granitic and schistose portions. The peraluminous unfoliated granitic component consists of quartz, plagioclase, K-feldspar, and biotite in aggregates and bands which bear residual garnet with curved crystal limits and pinnitized cordierite megacrysts. The biotite bands are rich in zircon embedded in the biotite crystals. A similar rock type, without cordierite, consisting of quartz, plagioclase and orthoclase bands alternating with biotite-rich, sillimanite- and garnet-bearing ones, occurs in the core drilled at Laguna Erna 1, which has not been dated in this study.

3.5. Punta Baja 1 borehole, sample PB1137 M2-Cordierite-siüimanite-garnet gneiss

In hand specimens, the rock shows 1-2 cm thick biotite rich (melanosome) and quartzo-feldespathic (leucosome) bands in similar proportions. A coarse-grained aggregate of quartz and partially chloritized biotite with oriented opaque mineral inclusions is predominant in the srudied thin section. Cordierite is abundant and has inclusions of garnet with irregular curved shapes and fibrolite aggregates. Cordierite is partially transformed to pinnite. Zircon crystals included in the cordierite have opaque linings developing in their peripheries and cleavage cracks. Rutile is present in rather large crystal aggregates.

Additionally, comparison is made with SHRIMP U-Pb zircon ages of two samples from Lago Mercedes (samples LMl and LM2; Hervé et al., inpress; Fig.1b): a foliated amphibolite and a banded orthogneiss.

The rocks of the TFIMC are mainly peraluminous cordierite granites, with variable tectonic transforma-tion into orthogneisses, some with garnet, cordierite and sillimanite. The latter association is, in general, characteristic of rather low pressure metamorphism in the higher-temperature amphibolite facies, probably beyond the stability of white mica, which is conspicuously absent. Metaluminous hornblende-bearing granitoids are scarce and in our petrographic datábase restricted to the Lago Mercedes area (Fig.1b).

4. Methodology

The SHRIMP U-Pb ages were obtained on zircon concentrates prepared in the Departamento de Geología, Universidad de Chile, using SHRIMP II or SHRIMP RG at the Research School of Earth Sciences, The Australian National University, Canberra. The measurement techniques are similar to those described by Williams (1998, and references therein). Cathodoluminescence (CL) images were first obtained on all the sectioned zircon grains and used to target specific areas for analysis within the grains. The Temora reference zircon (Black et al., 2003) was used to calibrate the U-Pb ratios, and the data were processed using the SQUID Excel macro of Ludwig (2001), plots and age calculations made using ISOPLOT (Ludwig, 2003). Tera-Wasserburg and relative probability diagrams for new crystallization ages of igneous rocks are shown in figure 2, and those for more complex samples in figure 3. CL images of five samples are shown in figures 4 and 5. The geological time-scale used is that of IUGS-ICS (http://www.stratigraphy.org).

FIG. 2. Tera-Wasserburg and Relative Probability diagrams for the samples that yield Cambrian igneous crystallization age.

FIG. 3. Tera-Wasserburg and Relative Probability diagrams for the patterns of two of the gneisses which have an extended ztrcon age pattern.

FIG. 4. Cathodolummescence (CL) images of zircons considered to indicate igneous crystallization of the host rocks a. augen gneiss A2 518 M4; b. banded biotite orthogneiss GN6 2273 MI and c. banded gneiss breccia ME2 1619 M2.

FIG. 5. Cathodolummescence (CL) images of zircons from samples which have more complex zircon populations a. migmatitic granitoid MA20 1011 M2 and b. biotite-cordierite-sillimanite-garnet gneiss PB1 137 M2.

Mineral phases of sample PB1 137 MI were analysed on a CAMEBAX microprobe using a wavelength-dispersive spectrometer at the University of Montpellier, France. The standard operating conditions included an accelerating voltage of 15 kV, a beam current of 10 nA, electrón beam of 10-5 um diameter and counting times ranging from 20 to 30 seconds depending on the element being analysed.

5. Previous age determinations

There have been a number of attempts to cons-train the ages of basement rocks in the Magallanes basin. Halpern (1973) published a whole-rock Rb-Sr errorchron of 306±150 Ma (7 samples), and biotite mineral ages of 254±10 Ma (Rb-Sr) and 251±12 Ma (K-Ar) from gneissic granodiorite drill core samples. Unpublished radiometric age data is also available for basement samples obtained from the bottom of drill holes in the Magallanes Basin (Table 1). Internal ENAP reports (F. Escobar, 2006, written communication) indicate that eight K-Ar biotite ages range between 231 and 266 Ma. In addition, a Rb-Sr isochron of 267±3 Ma based on two whole rocks, plagioclase -biotite, and whole-rock isochrons of 324±64 Ma and 649±62 Ma are reported. A Jurassic Rb-Sr whole rock-biotite isochron age of 155±20 Ma in granodiorite and a K-Ar biotite age 185±5 Ma have been obtained and also given in this unpublished report. The interpretation of these ages will be made clear below, as no interpretation was provided for them in the written communication consulted.

* Söllner et al. (2000); **Puig, A. (1984)¹; ***Péez de Arce, C. and Hervé, M. (1992)²; ****Moraga, J. (1994)³.

The first U-Pb zircon analyses are given in Söllner et al. (2000). A granodioríte gneiss from the Gaviota Norte 6 borehole (Fig. 1b) has zoned igneous zircons that yield a range of discordant ²⁰⁶Pb²³⁸U ages. A concordia upper intercept age of 529±7.5 Ma was calculated for a series of unabraided grains, the abraded central portions of other grains giving a yet older upper intercept date of 549±6 Ma. The whole-grain upper intercept of 529±7.5 Ma was interpreted as the time of intrusion. Additionally, Sollner et al. (2000) report a K-Ar age of 261±6 Ma on biotite from the gneiss. A Rb-Sr two-point isochron on apatite and K-feldspar from the same rock gave 161.6±4.5 Ma, considered to relate to the Jurassic tectonomagmatic event in the area. They also obtained K-Ar ages of 171±9 Ma for hornblende from a dacite and 158±5 Ma for a whole rock basalt from the same drill core, both rocks belonging to the Tobífera Formation. Pankhurst et al. (2003) obtained a SHRIMP U-Pb zircon age of 523±5 Ma for a separate sampling of the same Gaviota 6 borehole orthogneiss. This age is for the euhedral tips and outer zones of sectioned zircon grains.

6. Results

A summary of the geochronological results obtained in this work is shown in table 2, with the full analytical data set in tables 3 to 7. The Aguila 2 borehole augen-gneiss has elongate, euhedral zircon grains, many with bipyramidal terminations, and simple oscillatory zoned interiors under CL imaging. Some grains are subround, interpreted as a consequence of the gneiss forming event. The 2²⁰⁶Pb²³⁸U ages form a dominant, slightly bimodal peak at about 525 Ma with tails on both younger and older sides (Fig. 2a). There is one significantly older grain that likely reflects an inherited component (grain 7, -596 Ma), whilst the younger analyses are interpreted to result from radiogenic Pb loss during the metamorphic overprint. A weighted mean of 12 analyses gives 523±7 Ma (MSWD=1.4) and this is considered to date the time of crystallisation of the dominant, zoned igneous zircon.

The Gaviota Norte 6 banded biotite orthogneiss also has elongate, euhedral zircon grains with bi-pyramidal terminations, although subround grains are more evident than in the Aguila 2 sample. The CL images show a simple oscillatory zoned internal structure in elongated crystals with bipyramidal terminations. The data show a dominant age peak on the relative probability plot (Fig. 2b) although once again there is a single older analysis (grain 5, -570 Ma) and a younger tail interpreted to arise from radiogenic Pb loss during the metamorphic overprint. The weighted mean age of 522.5±5.5 Ma (MSWD=0.85) is the time of zoned igneous zircon crystallisation.

The María Emilia 2 borehole breccia of biotite gneiss fragments yielded a notably homogeneous population of elongate euhedral zircon grains with bipyramidal terminations. The CL images show mostly simple oscillatory zoning, though some grains have central discontinuities which likely indicate the presence of older zircon components. The data for this sample form an asymmetric major age peak with two clearly older analyses (Fig. 2c). The data in the main peak is somewhat dispersed, but if the older (inherited components) and two younger (Pb loss) analyses are excluded the weighted mean for the remaining 13 gives 536±6 Ma (MSWD=1.6). Once again, this constrains the time of zoned igneous zircon crystallization, but there is evidence for inherited zircon and radiogenic Pb loss during the metamorphic overprint, though no new zircon formed.

Samples from the two other localities yielded even more complex results and this is commensurate with the zircon populations. The migmatitic granitoid from the Monte Aymond 20 borehole (Fig. 3a) has a wide range of zircon morphologies, from elongate euhedral grains with pyramidal terminations to round and subround, equant to elongate grains. The CL images highlight the complex nature of these zircon grains; many have darker more homogeneous areas that rim either zoned or other homogeneous metamorphic zircon cores. The metamorphic rims are of variable thickness, in some grains the zoned igneous core predominates. The U-Pb data is equally complex, for example the well developed metamorphic rims to grains 15 and 16 were analysed in the expectation that they would record a single event. However, grain 15 is latest Neoproterozoic -560 Ma whereas grain 16 is 'Grenville' in age -1,090 Ma. Overall, there is no uniform pattern of CL structure and age. The main age peaks span 550-650 Ma and 950-1,100 Ma, with a single grain giving -1,900 Ma and so this sample appears to be a migmatite granite derived in part from the anatexis of metasediments with 'Grenville' and 'Brasiliano' provenance (and older), with a maximum age of 515-525 Ma as determined by the youngest two zircons dated.

The biotite-bearing cordierite-sillimanite-garnet gneiss from the Punta Baja 1 borehole (Fig. 3b) has a unique zircon population amongst this series of samples. The grains are notably smaller, most less than 100 um in length, many less than 50 um, and the grains are generally subhedral to subround. The CL images are consistently homogeneous grey in color, usually interpreted as indicating a metamorphic origin for the zircon. There are some grains that have a faint zoning, which may be relict from an original igneous precursor. Due to the small size of the grains, it is not possible to analyze core or rim structures, and in most cases the grains are homogeneous. For the twenty grains analysed there is a predominant peak at about 260-270 Ma, the remaining eight ²⁰⁶Pb²³⁸U ages are between 495 and 545 Ma, with an isolated grain at 370 Ma. The metamorphic nature of the Permian-age zircon grains is highlighted by very low Th/U ratios (<0.02), the Cambrian grains having ratios between 0.3 and 0.62 as is normal for igneous zircon (Rubatto, 2002) (Fig. 6). Aweighted mean for all 12 Permian grains has excess scatter (MSWD=3.3), but if the two oldest analyses are excluded the weighted mean is 267±3 Ma (MSWD=2.0). This metamorphic zircon probably formed during the episode that produced the garnet-sillimanite-cordierite assemblages, charactenstic of upper amphibolite to granulite metamorphic facies (Vavra et al., 1996).

FIG. 6. Th/U versus age diagram for the zircon crystals from sample PB1 137 M2, in which the Permian metamorphic zircons differ completely from the older igneous ones.

According to P-T pseudo-sections calculated for this rock, Fe/(Fe+Mg) and Fe/(Fe+Mg+Ca+Mn) isopleths for biotite and garnet, respectively, in the corresponding mineral assemblage, metamorphic conditions can be restricted to the range 730-770°C and 2-3 kbar (Fig. 7). The calculated mineral assemblage at these conditions is the same as observed in the rock. The predictedproportions of biotite (ca. 4 vol.%) and cordierite (4 vol.%) appear to be sma-Uer than in the studied rock. The calculated modal mineralogy consists of approximately 30 vol.% of quartz, 12 vol.% of feldspars, 10 vol.% of sillimanite, 8 vol.% of garnet and 30 vol.% of haplogranitic melt. Anatexis and subsequent processes of melt (leucosome) migration through restitic material (melanosome) may explain the differences in modal mineralogy. Cordierite growth at expenses of biotite and garnet suggests that maximum metamorphic temperature was attained after the partial or complete disappearance of biotite.

FIG. 7. Pressure-temperature pseudosection calculated for the biotite-cordierite-sillimanite-garnet gneiss (sample PBl 137 MI) at Punta Baja. Dashed white lines show the isopleths of the Fe/(Fe+Mg) ratio in biotite, ranging between 0.5 and 0.6 (see Table 8); dashed and black lines are the contour lines of the Fe/(Fe+Mg+Ca+Mn) ratio in garnet, ranging between 0.7 and 0.77. The dashed and thin black contour lines indicate the predicted volume % of melt in the PT compositional space. The mineral assemblage present in the rock, Fe/(Fe+Mg) and Fe/(Fe+Mg+Ca+Mn) isopleths of biotite and garnet, respectively, restrict the P-T metamorphic conditions to the range 730-770°C and 2-3 kbar (indicated by the ellipse). The calculated phases and their relative modal proportions at approximately 750^GC and 2.5 kbar pressure conditions are indicated. The procedures for the P-T pseudosection calculation were as follows. The minimum free energy for an average bulk rock composition was calculated for a net of P-T conditions with the computer program package PERPLE X (Connolly, 1990). Calculations were undertaken in the system MnO-Na₂-CaO-K₂-FeO-MgO- Al₂O₃-SiO₂-H₂O-O₂ for the P-T range 1-10 kbar and 600-1,000°C. The thermodynamic calculation excluded the TiO₂ component, which in the rock is mainly accommodated in biotite and rutile. We used the thermodynamic data set of Holland and Powell (1988, updated 2002) for mineral and aqueous fluid. The later solid-solution models (see Powell and Holland, 1999), being compatible with this data set, were selected from the downloaded versión of the PERPLE_ X solution-model file (newest format solut. dat): Bio(HP) for biotite, Pl(h) plagioclase, Gt(HP) for garnet, Pheng(HP) for potassic white mica, and melt(HP) for melt. K-feldspar was considered as a puré phase. Mineral abbreviations are: And, andalusite; Bt, biotite; Crd, cordierite; Gt, garnet; Kfd, K-feldspar; Ky, kyanite; Mt, magnetite; Pl, plagioclase; Qz, quartz; Sil, sillimanite; WM, white mica. The graphical results were taken as raw data. The final pseudosection was redrawn by smoothing curves as demonstrated by Connolly (2005). The major elemente bulk chemical composition of rock sample PBl 137 MI was determined with a PHILIPS PW 2400 X-ray fluorescence (XRF) spectrometer at Stuttgart University. The bulk rock composition (wt%) is SiO₂: 65.07, TiO₂: 0.81, Al₂O₃:17.61, Fe₂O₃: 6.54, MgO: 1.59, MnO: 0.06, CaO: 0.50, NaO₂: 0.96, K₂O: 2.80 and P₂O₅: 0.12. The original rock composition was simplified to fit the used system (without TiO₂). An oxygen content of 0.065 wt% was selected corresponding to 10% Fe³⁺ of the total iron (see Massonne et al., 2007) and a 1 wt% of water content was considered in the calculation.

The Lago Mercedes amphibolite and orthogneiss have also yielded Cambrian crystallization ages, similar to those obtained during this work (Hervé et al., in press).

7. Interpretation

7. 1. General

The Mesozoic-Cenozoic successions of the Magallanes Basin in Tierra del Fuego and part of the adjacent continental areas to the north are underlain by a crystalline basement complex here called the Tierra del Fuego Igneous and Metamorphic Complex (TFIMC). This complex is dominated by foliated peraluminous biotite granitoids, some containing cordierite and garnet, variably transfor-med to orthogneisses, some of them with ocellar structure. The attitude of the foliation is difficult to determine, but is usually at high angles to the core length, suggesting that a low-dipping foliation is predominant (Fig. 8).

FIG. 8. Cross-sections of the Magallanes Basin, showing the east- and northward prograding thrust pile over the basin (Harambour, 1998); main structures and depositional contacts between Cretaceous and Cenozoic sequences are from seismic lines. The contact between pre-Jurassic and Jurassic (Tobífera) units is a rough approximation. Oil wells are located in the foreland basin area.

Some of the gneisses contain the mineral assemblage cordierite-sillimanite-garnet-biotite-plagioclase-orthoclase-quartz. Usually, complete alteration of the cordierite to pinnite is observed, as well as partial chloritization of biotite, and partial replacement of plagioclase by carbonates. Laterbrittle deformation of the rocks is expressed by undulose extinction in quartz, microfractures and microbreccias.

Without exception, the zircon age spectra of all samples have a Cambrian component. The samples from Aguila 2, Gaviota Norte 6 and María Emilia 2 contain only igneous zircons of this age (Table 2). These three populations are considered to reflect crystallization ages of the igneous protolith of the gneisses, with the zircons being dominantly simple, oscillatory zoned igneous grains as documented by CL images. Sample María Emilia 2 is a breccia composed of angular gneiss fragments, the scarce intervening matrix has a sedimentary character rather than a cataclastic one. The breccia probably originated from sedimentary strata covering the gneissic basement and consists entirely of reworked basement fragments.

The migmatitic granite MA20 1011 M2 contains complex zircon grains and appears to be derived from ametasediment with 'Grenville' and 'Brasiliano'pro-venance, with a maximum protolith age of 515-525 Ma as determined by the youngest two zircons dated.

The cordierite-sillimanite-garnet gneiss PB1 137 M2 features a predominant Permian (267±3 Ma) peak of metamorphic zircons, with some older grains in the range -495-545 Ma. This is interpreted as resulting from a high temperature Permian metamorphic event that affected either a Cambrian igneous protolith, probably similar to Aguila 2 and Gaviota Norte 6 rocks dated here, or a sedimentary rock derived from Cambrian igneous rocks.

A Permian event is also indicated by the Rb-Sr (see text) and K-Ar biotite ages (Table 1) previously obtained from the basement rocks, but which were considered to be reset ages representing opening of the isotopic systems or cooling ages. They can now be interpreted as dating the closure of Rb-Sr and K-Ar systems during or after the Permian metamorphic event, which very probably formed the biotite in the dated gneiss, and probably also in the regional foliated granitoids. It is possible that all the gneisses studied here developed their foliation during this Permian metamorphism, but that only the rocks at the highest grades developed Permian zircon. The K-Ar dating of biotite crystals (-265 to 230 Ma) in the gneisses of the TFIMC, either newly formed or reset, or both, indicate cooling of the rocks after the Permian tectonometamorphic event.

The TFIMC rocks experienced yet younger events that modified their isotopic composition. Söllner et al. (2000) obtained a Jurassic mineral isochron in the rock dated as Cambrian by U-Pb zircon, and another previous Rb-Sr age also recorded this Jurassic younger event in the granodiorite. These have been related to the magmatic event that occurred in the area during the Middle to Late Jurassic associated with extensión during the initial stages of Gondwana break-up, which also generated the Tobífera Formation (Pankhurst et al., 2000). Obviously this event did not result in the resetting of the U-Pb zircon ages, but may have locally modified less resistant isotopic systems.

7.2. Comparison with neighbouring areas in Patagonia and Malvinas/Falkland islands

The Cape Meredith complex in the Malvinas/ Falkland islands is composed of Mesoproterozoic basement gneisses and granitoids which have been compared in broad terms to the Namaqua-Natal-Dronning Maud Land metamorphic belt in southern Africa and Antarctica (Groenewald et al., 1991). SHRIMP U-Pb zircon ages date the oldest felsic gneisses at 1,118±8 Ma and cross-cutting igneous phases at ca., 1,090, 1,067±9 and 1,003±16 Ma (Jacobs et al., 1999), similar to the inherited zircon ages obtained here from the sample from Monte Aymond 20 borehole.

No Cambrian plutonio rocks or (meta-) sedimentary rocks containing predominantly Cambrian zircons are known from the Deseado Massif (Pankhurst et al., 2003) in Argentine Patagonia (Fig. 9), although Pezzuchi (1978) reported a 540±20 Ma K-Ar amphi-bole date for an amphibolite. However, the basement rocks of the Sierra de la Ventana succession, north of the Río Colorado, include Early Cambrian A- and I-type granites (Rapela et al., 2003), of the same age as those studied here, and low-grade metasedimentary rocks in the eastern part of the North Patagonian Massif contain predominant Cambrian detrital zircons as their youngest component (Pankhurst et al., 2006). The metasedimentary rocks of the Eastern Andes Metamorphic Complex exhibit varied detrital zircon age patterns (Hervé et al., 2003; Augustsson and Bahlburg, 2007), most of them with prominent Cambrian components, but extending into the Silu-rian, Devonian, Carboniferous and Permian. The Cordillera Darwin Metamorphic Complex (Hervé et al., in press) presents a significant number of detrital zircon age spectra and some of the samples include groups with Cambrian ages, as well as Silurian to Devonian components. The Duque de York Complex in the Madre de Dios archipelago, on the contrary, has prominent Permian but no Cambrian components. However, these zircons are igneous in origin (Sepúlveda et al., this volume), and not metamorphic as we have observed and documented for the Punta Baja gneiss.

FIG. 9. Paleogeographic reconstruction modified from Konig and Jokat (2006) showing the main units mentioned in the text. Dark grey areas indicate cratons in the SW Gondwana assemblage. Paleolatitudes correspond to ca. 167 Ma. Limits of the Terra Australis Orogen taken from Cawood (2005). Abbreviations used in this figure are AP: Antarctic Peninsula; DM: Deseado Massif; DML: Dronning Maud Land; EWM: Ellsworth Whitmore Mountains; GFS: Gastre Fault System; HN: Haag Nu-nataks; Mil: Malvinas/Falkland Islands; MBL: Marie Byrd Land; NN: Namaqua-Natal Province; NPM: North Patagonian Massif; THÜ: Thurston Island; SV: Sierra de la Ventana; VP: View Point.

7.3. Comparison with the Antarctic Peninsula

In this context it is very important to consider also the evolution of the basement of the Ellsworth Mountains and of the Antarctic Peninsula, which, like Patagonia, represent fragments of the former southwestern Gondwana margin. Cambrian zircons are common in the Ellsworth Mountains metasedimentary succession (Flowerdew et al., 2007), deposition of which spans the whole of the Paleozoic era. However, only minor outcrops of Late Cambrian igneous rocks are known (Vennum et al., 1992). The early stages of this succession are thought to have been deposited in an active rift that later evolved to a passive margin. The basement to this succession is considered to be represented by the granitic gneisses and leucogranites that crop out on the isolated Haag Nunataks; these have yielded Rb-Sr whole-rock isochron ages of 1,176±76 Ma, 1,058±53 Ma and 1,008±18 Ma (Millar and Pankhurst, 1987), which may be considered as reasonably consistent with the dated events in the Cape Meredith complex. The sedimentary provenance may initially have been mainly from southernmost Africa, and later on from a more expansive source region within West Gondwana, probably including the Kaapvaal and Congo cratons of southern and western Africa.

In the Antarctic Peninsula, crystalline basement rocks comprising mainly granitic orthogneisses, migmatites and layered paragneisses, are exposed in small, isolated outcrops (Adie, 1962). The oldest of these rocks are paragneisses cropping out at Adie Inlet on the eastern coast of Graham Land (Fig. 9). They contain predominantly Cambrian zircons (18 grains at 520±8 Ma), but also some younger Paleozoic detrital grains, and experienced high-grade metamorphism and migmatization at 258±3 Ma (Millar et al., 2002). The Permian zircons feature low Th/U ratios (<0.08) and differ significantly from the zoned Cambrian zircons which display Th/U ratios of 0.41-0.88. These observations are strikingly similar to those presented here for the Punta Baja 1 sample, and indicate similar and contemporaneous processes affecting both areas. Permian (and Triassic) granitoids crop out spora-dically along the Antarctic Peninsula, and many of them are 'S-type' (Millar et al., 2002).

The relative position of the Antarctic Peninsula is assumed to have been adjacent to and contiguous with Patagonia during the Early Mesozoic (Konig and Jokat, 2006; Hervé et al., 2006) before Gondwana break-up. Such a paleogeographic configuration would lócate Adie Inlet cióse to Tierra del Fuego.

7.4 Regional tectonics

Zircon ages of approximately 1,000 Ma ('Grenvillian') are common along the paleo-Pacific margin of Gondwana, whereas Cambrian igneous and me-tamorphic rocks are common in NW Argentina (the Pampean orogen) and in parts of Eastern Antarctica (Ross orogen). This suggests that the rocks studied here could have formed part of a previously more extensive (continuous?) magmatic province along the Gondwana margin, as noted by Söllner et al. (2000). However, Rapela et al. (2001) considered the Pampean orogen, which includes peraluminous granites and high-T metamorphic rocks, as a spatially restricted tectono-magmatic belt related to the colusion of a Pampean micro-continent with the margin of a larger continent to the east, and in this context no continuity can be established with the Tierra del Fuego basement rocks.

A similar microcontinent colusion has been advocated to explain the development of the Ross-Delamerian orogen rocks, ineluding the colusion of small terranes during subduction processes (Glen, 2005). If the peraluminous granitic rocks of the TFI-MC were also produced by a continental colusion, then involvement of another micro-continent would be necessary to explain this oceurrence. Alternatively, Dalziel (1997) proposed a model in which the subduction-generated Pampean magmatic are was related to the approach of a large craton, Laurentia, which would have collided with western South America in early Ordovician times.

Permian zireons of roughly the same age as those seen in the Punta Baja 1 sample (269 Ma) are common as detrital zireons in the Eastern Andes Metamorphic Complex and predominant in the Duque de York complex (Hervé et al., 2006). However, most of those detrital zireons are typical igneous ones, whereas those studied here have metamorphic characteristics. This event is contemporaneous with widespread Permian intrusive activity in the North Patagonian Massif, which may have been initiated when the subducted slab broke off, following the colusion between southern Patagonia and Gondwana (Pankhurst et al., 2006). It is possible that a southward prolongation of the Permian magmatic belt, which at present crops out only from the North Patagonian Massif northwards, lies hidden beneath younger sedimentary and/or volcanic units located between the North Patagonian Massif and the Magallanes basin. Recently, Ramos (2008) suggested that a Carboniferous magmatic are could be buried below the sedimentary rocks of the Magallanes basin forming the ridge known as the Dungeness High. The rocks studied here lie west of this are, and have not yielded zireons of Carboniferous age. It might also be suggested that a hypothetical micro-continental mass (the Antarctic Peninsula?) collided with the continental margin of Gondwana during Late Paleozoic times, as part of the amalgamation of Pangea.

A special geological feature of the TFIMC is the absence of the Paleozoic sedimentary cover typical for Cambrian basement elsewhere in the region (Sierra de la Ventana, the Cape Fold Belt in South Africa, the Ellsworth-Whitmore mountain block and northern Victoria Land in Antarctica). The TFIMC level presently covered by the Jurassic Tobífera Formation was 8 to 12 km below the surface during the Permian tectonometamorphic event, as suggested by the 2-3 kbar pressure determined here, and was exhumed before the Middle Jurassic; it would thus have been an important source of Cambrian to Permian aged detritus to the surrounding areas. The Permian conglomerates of the La Golondrina Formation in extra-Andean Patagonia (Pankhurst et al., 2003), in South Africa (Fildani et al., 2009), in the Antarctic Peninsula (View Point, Millar et al., 2002; Paradise Bay, Kraus 2007) and in the Duque de York Formation (Faúndez et al., 2002) may be the result of this exhumation process which might have involved températe glacial erosión processes in an orographic high.

Acknowledgements

The SHRIMP age determinations carried out at the Australian National University were financed by FONDECYT project 1050431 (FH) and MF research funds. This is also a contribution to FONDECYT projects 11075000 (MC) and 1095099 (FH), a CONICYT-BMBF project to Dr. H. Massonne (University of Stuttgart) and FH, and to Anillo Antartico Project ARTG-04 (CONICYT-World Bank). We thank Empresa Nacional del Petróleo (ENAP) geologists G. Romero and A. Chávez for providing the core samples from the basement of the Magallanes basin, and F. Escobar and J. Moraga for providing previously unpublished K-Ar and Rb-Sr age determinations. J. Vargas (Universidad de Chile) skilfully separated the zireons. Revisions by Drs. A. Willner, H. Miller and an anonymous referee greatly improved this contribution. We express our gratitude to Dr. A. Demant (Université d'Aix Marseille III) who helped with the microprobe analysis and to Dr. H. Massonne who helped with the necessary subtilities for the Pseudosection calculation.

Notas

¹ Puig, A. 1984. Empresa Nacional del Petróleo, intemal report (unpublished).

² Pérez de Arce, C; Hervé, M. 1992. Empresa Nacional del Petróleo, intemal report (unpublished).

³ Moraga, J. 1994. Empresa Nacional del Petróleo, intemal report (unpublished).

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Manuscript received: August 5,2009; revised/accepted: January 20,2010.