Are the La Pampa Gneisses Remnants of the Chilenia Basement?

Are the La Pampa Gneisses remnants of the Chilenia basement?

Javier Álvarez* 1, Constantino Mpodozis 2, Antonio García-Casco 3, 4 , Idael F. Blanco-Quintero 3, 5 and Diego Morata 1, 6 1 Departamento de Geología, Fac. de Cs. Físicas y Matemáticas, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile. 2 Antofagasta Minerals, Apoquindo 4001, Santiago, Chile. 3 Departamento de Petrología y Mineralogía, Universidad de Granada, España. 4 Instituto Andaluz de Ciencias de la Tierra (IACT, CSIC-UGR), España. 5 Departamento de Geociencias, Universidad de Los Andes, Bogotá, Colombia. 6 Centro de Excelencia en Geotermia de Los Andes (CEGA), Universidad de Chile, Chile.

* E-mail: [email protected]

Abstract. The La Pampa Gneisses (LPG) form a small López de Azarevich et al. (2009) locate them as part of outcrop of leucocratic gneisses embedded within the late Cuyania basement. Paleozoic-Triassic Chollay Batholith to the east of Vallenar (Chile). The LPG was previously considered one of the few preserved remnants of the Chilenia basement (accreted to the Gondwana margin in the Devonian). Zircons collected from these rocks show inherited cores with highly variable U/Pb ages and two principal peak of ca. 301 Ma, interpreted as the age of zircons in the protolith; and a younger peak at ca. 263 Ma, probably record the age of a metamorphic episode, related to a regional Permian intrusion event. Calculated P-T conditions show that peak conditions took place at ca. 780 ºC and 5.3 kbar. These new data indicate that the LPG were emplaced and metamorphosed during the late Paleozoic, attesting that they are not relicts of the Chilenia basement.

Keywords: Chilenia, U/Pb, Zircon, Geothermobarometry.

1 Introduction

During most part of the Paleozoic a variety of allocthonous terranes (Pampia, Famatina, Arequipa-Antofalla and Cuyania) were amalgamated to the stable Río de la Plata and Amazonia cratons (Fig. 1). The latest terrane to become part of Gondwana was the hypotetical Chilenia terrane (Ramos et al., 1986; Fig. 1), which accreted to the western margin during the Devonian (ca. 395 Ma; Davis et al., 2000; Willner et al., 2011). However, unlike other terranes such as Pampia or Cuyania (Ramos, 2009; 2010), the outcrops attributed to Chilenia are scarce and there is no direct evidence for its original affiliation. The Guarguaraz Complex, that occur close to Mendoza, Argentina (Fig.1) has been interpreted as an acretionary wedge developed between Chilenia and Cuyania during the Devonian (Willner et al., 2011 and references therein). Chilenia seems to have bounded to the west by late Figure 1. Map of tectonostratigraphic terranes of Central Chile and Argentina (Ramos, 2009). In black, outcrops of the main Paleozoic subduction originated after its collision with metamorphic complexes. TMC: El Tránsito Metamorphic Gondwana (Hervé, 1988; Fig. 1), which probably include Complex, LPG: La Pampa Gneisses, GC: guarguaraz Complex, the El Tránsito Metamorphic Complex located in the LPAC: Late Paleozoic Accretionary Complexes. Frontal Cordillera, east of Vallenar (Ribba et al, 1988; Álvarez et al., 2011). Another unit attributed to Chilenia is the Filo Gris Complex (Astini and Cawood, 2009; Fig. 1), in the Frontal One of the scarce outcrop attributed to Chilenia is Las Cordillera of La Rioja, Argentina. Finally, the only one Yaretas Gneisses (Keppie and Ramos, 1999). However, outcrop attributed to Chilenia basement in Chile

389 corresponds to the La Pampa Gneisses (LPG; Ribba et al., shows a field formed by sillimanite, biotite, plagioclase, 1988; Moscoso et al., 2010). According to Ribba et al. cordierite, garnet, quartz and melt, similar to the peak (1988), these rocks form a 2 km in diameter, subcircular metamorphic assemblage of the sample. This modelling, enclave within late Paleozoic granitoids of the Chollay and the fact that sillimanite is strongly deformed, whereas Batholith in the El Tránsito valley, east to Vallenar (Fig. 1). quartz and plagioclase are not, suggest that partial melting The LPG are surrounded by a migmatitic aureole including occurred during prograde metamorphism. The isopleths of metamorphic fragments in a tonalite to granodiorite observed mineral composition intersect in the stability neosome (Ribba et al., 1988) similar to the late Paleozoic field of the observed assemblage. Using these data the granitoids surrounding the metamorphic enclave peak metamorphic conditions are estimated at ca. 780 ºC (Mpodozis and Kay, 1992). The LPG are light colored and ca. 5.3 kbar. Considering that the closure temperature rocks with alternating biotite-rich and quartz-feldespatic in zircon U/Pb system is over 900 °C (Lee et al., 1997), we bands. Their mineralogy includes quartz, plagioclase and conclude that Carboniferous ( ca. 301 Ma) ages of LPG biotite plus muscovite and also sillimanite and cordierite. correspond to the protholith age. Ribba et al. (1988) suggested that they were “probably derived form a granodioritic protolith”. One of the reasons to suggest that the LPG could be part of the basement of Chilenia is a Silurian (415±4 Ma) three point whole rock Rb/Sr isochron age reported by Ribba et al. (1988). Two K/Ar ages (muscovite: 239±10 Ma, biotite: 236±6 Ma) together with a 246±18 Ma Rb/Sr whole rock-muscovite- biotite isochron were considered, by the same authors, as the product of a younger Triassic thermal event.

To check more carefully, the possible affinities of the LPG with the Chilenia basement it is necessary to employ more penetrative geochronological methods including zircon U/Pb dating. In this contribution, we present new zircon U/Pb age data for the LPG and some of the sourrounding plutons together with petrological (geothermobarometric and P-T path calculations) data for the LPG in order to evaluate its potential adscription to Chilenia.

2 New data from the La Pampa Gneisses

Geochronology : Figure 2 show the probability density plot of sample JA05, onsidered to be representative from the LPG. The analyzed sample is a banded gneiss formed by plagioclase, quartz, biotite, muscovite, cordierite and sillimanite. Ages of individual zircon grains extend from the Mesoproterozoic to the Permian. The oldest zircons (Mesoprotrozoic, Cambrian, Ordovician and Silurian) are doubtless inherited. The largest populations are late Paleozoic, and include two peaks at 301 (Carboniferous) and 263 Ma (Permian). Carboniferous ages come from zircons with euhedral and prismatic habit that suggest an igneous origin either "in situ" or, if they were detrital zircons, a source very close. Permian ages come instead from the rims of grains with evidence of crystallization, thus showing that would, have been, most li kely related to a Permian metamorphic event.

Geothermobarometry : The P-T evolution of LPG sample JA05 was evaluated by means of an isochemical PT projection (pseudosection) using the bulk chemical analysis of the sample, obtained by ICP-OES. Calculations Figure 2. LA-ICP-MS zircon probablity density ages plots were performed for the KNCFMASHT system using for analyzed grains. Perple_X software (Connolly, 2005). The pseudosection

390 3 U/Pb ages from late Paleozoic and from late Paleozoic accretionary complexes in north-central Chile (28°-32° S): Possible fingerprints of the Chilenia terrane. Journal Triassic intrusives of South American Earth Sciences, 32, 460-436. Astini, R., Cawood, P., 2009. A Proterozoic basement under the Along with the analysis in the LPG, we analyzed two northern Cordillera Frontal: A hint to Chilenia and the samples from the Chollay Batholith from localities north continuation of the accretionary prism east of Precordillera?. XII and east of the LPG outcrops. The first corresponds to a Congreso Geológico Chileno, Abstract S9_01 2, Santiago. Connolly, J., 2005. Computation of phase equilibria by linear hornblende-biotite tonalite sample from an large pluton programming: A tool for geodynamic modeling and its (termed here as La Totora pluton) that intruded the LPG application to subduction zone decarbonation. Earth and north of the El Tránsito river. This sample (JA01) yield a Planetary Science Letters 236, 524-541. 266,1±3,5 Ma (Fig. 2) crystallization age, concordant with Davis, J., Roeske, S., McClelland, W., Kay, S., 2000. Mafic and the age obtained in recrystallized borders within the LPG ultramafic crustal fragments of the southwestern Precordillera zircons. The second sample (JA02) corresponds to a coarse terrane and their bearing on tectonic models of the early Paleozoic in western Argentina. Geology 28, 171-174. grained leucocratic “pink” granites that from large Hervé, F., 1988. Late Paleozoic subduction and accretion in Southen outcrops to the east and were attributed by Mpodozis and Chile. Episodes, Vol. 11, p. 183-188. Kay (1992) to the Chollay intrusive unit of the Chollay Keppie, J., Ramos, V., 1999. Odyssey of Terranes in the Iapetus an Batholith. This sample has a crystallization age of Rheic Oceans during the Paleozoic. Geological Society of 247,7±3,4 Ma and is slightly older than another U/Pb age America, Special Paper 336, 267-276. (242,5±1.5 Ma) reported, for the same pluton by Martin et Lee, J., Williams, I., Ellis, D., 1997. Pb, U, and Th diffusion in natural zircon. Nature 390, 159 – 162. al. (1999), south to the study area. Both ages attest the López de Azarevich, V., Escayola, M., Azarevich, M., Pimentel, M., occurrence of a regional Triassic intrusive. These are older Tassinari, C., 2009. The Guarguaraz Complex and the than the K/Ar reported by Ribba et al (1988) for the LPG Neoproterozoic-Cambrian evolution of southwestern Gondwana: that probably correspond to regional colling ages. Geochemical signatures and geochronological constraints. Journal of South American Earth Sciences 28, 333-344. Martin, M., Clavero, J., Mpodozis, C. 1999. Late Paleozoic to early Jurassic tectonic development of the high Andean Principal 4 Discussion and Conclusions Cordillera, El Indio Region, Chile (29-30° S). Journal of South American Earth Sciences 12, 33-49. The outcrops of La Pampa Gneisses have been considered Moscoso, R., Mpodozis, C., Nasi, C., Ribba, L., Arévalo, C., 2010. Geología de la Hoja El Tránsito, Región de Atacama. Servicio as a potential candidate to be the only preserved relict of Nacional de Geología y Minería, Carta Geológica de Chile, Serie the Chilenia basement in Chile (Ribba et al., 1988). Preliminar 7, escala 1:250.000, Santiago. Nevertheless, the maximum age (ca. 301 Ma) of igneous Mpodozis, C., Kay, S., 1992. Late Paleozoic to Triassic evolution of emplacement indicated by LPG zircon cores, is younger the Gondwana margin: Evidence from Chilean Frontal than the time considered as the age of accretion of Chilenia Cordilleran batholiths (28°S to 31°S). Geological Society of to Gondwana (ca. 395 Ma; Davis et al., 2000). The data America Bulletin 104, 999-1014. Ramos, V., 2009. Anatomy and global context of the Andes: main presented here rules out the possibility that LPG are geologic features and the Andean orogenic cycle. In: Kay, S.M., remnants of the Chilenia basement. The La Pampa Ramos, V.A., Dickinson, W. (Eds.), Backbone of the Americas, Gneisses were formed after the time of Chilenia collision Shallow Subduction, Plateau Uplift and Terrane Collision. to Gondwana. Geological Society of America, pp. 31-66. Memoir 204. Ramos, V., 2010. The Grenville-age basement of the Andes. Journal of South American Earth Sciences 29, 77-91. The metamorphic episode can be linked, as shown by the Ramos, V., Jordan, T., Allmendinger, R., Mpodozis, C., Kay, S., age of zircon rims, to the intrusion of tonalite-granodioritic Cortés, J., Palma, M., 1986. Paleozoic terranes of the central phases of the Chollay Batholith during the Permian. By Argentine-Chilean Andes. Tectonics 5, 855-880. contrast, the younger Triassic intrusive event extensively Ribba, L., Mpodozis, C., Hervé, F., Nasi, C., Moscoso, R., 1988. El developed to the east, was unable to alter the balance of basamento del Valle del Tránsito, Cordillera de Vallenar: eventos the mineral phases. magmáticos y metamórphicos y su relación con la evolución de los andes chileno-argentinos. Revista Geológica de Chile 15, 126-149. Willner, A., Gerdes, A., Massone, H.J., Schmidt, A., Sudo, M., Aknowledgments Thomson, S.,Vujovich, G., 2011. The geodynamics of collision of a microplate (Chilenia) in Devonian times deduced by the pressure temperature time evolution withi part of a collisional This work was supported by Conicyt doctoral thesis and belt (Guarguaraz Complex, W-Argentina). Contributions to Departamento de Postgrado y Postítulo, Universidad de Mineralogy and Petrology, DOI 10.1007/s00410-010-0598-8. Chile grants, Fondecyt Project 1080468, Chile and MICINN Project CGL2009-12446, Spain.

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