Geochemical and isotopic (Nd and Sr) characteristics of the Ordovician magmatism on the Southeastern Puna, NW - Argentina.

VIRAMONTE, J.M.; PIMENTEL, M.; BECCHIO, R.; VIRAMONTE, J. G.

Kiel Alemania

Simposio; 20th Colloquium on Latin American Earth Sciences; 2007

Universidad de Kiel y GEOMAR

The geodynamic evolution of the Proto-Andean margin of Gondwana during Lower Paleozoic is still a subject of controversy. On one hand, it was characterized as a repeated onset of subduction processes associated with the docking of several terranes. On the other hand, Damm et al., 1990; Becchio et al., 1999; Lucassen et al., 2000; Zimmermann and Bahlburg 2003 and Franz et al., 2006 among other authors, suggest a geodynamic evolution dominated by intracrustal recycling processes with minor contribution of juvenile magmatism. The Puna region of the NW-Argentina, records sedimentation, deformation, metamorphism and magmatism events since ca. 510 to 440 Ma where it is difficult to discriminate different tectono-thermal cycles. Structural, petrological  and sedimentological studies suggest that during the Lower Ordovician period the central-eastern Puna region evolved in an extensional intracontinental setting. In this way, provenance studies indicate that sedimentary detritus is generally composed of reworked crustal material. This idea supports previous works on the basis of field observations, petrological, geochemical and isotopic data from metamorphic basement (ca. 510-500 Ma) to Andean rocks (Miocene) The evolution of magmatism started during the Early Tremadocian and it comprises mainly intermediate and acidic plutonic rocks as well as volcano-sedimentary sequences. In general, this magmatism forms two N-S trend belts. Following previous studies, we present new geochemical and isotopic data of Ordovician magmatic rocks of the eastern border of southern Puna (24º00´-25º30´). In the eastern area of salar Centenario, a voluminous and widespread plutonic unit crops out. It is composed of three silica-rich facies with U/Pb zircon and monazite ages of 475 ± 5 – 463 ± 2 Ma. It is also exposed a 100`s m thick volcano-sedimentary unit of bimodal metavolcanic rocks, metabasites and felsic metarhyolites and metadacites, intercalated with phyllites and metagreywackes. A metarhyolite exposed at the base of  the sequence yields an age of 485 ± 5 Ma. The bimodal nature of the magmatism in the volcano-sedimentary unit is evident as the metavolcanic rocks have basalt and rhyolite compositions. Bimodality is also shown by trace elements and Sr and Nd isotope data. Amphibolites yield positive εNd(T) values between +0.3 and +2.5 and initial 87Sr/86Sr ratios of 0.7067. These values and enrichment in LILE with Nb anomaly suggest they were derived from a mantle source in a subduction setting. Also, the flat REE pattern (LaN/SmN=1.23-1.43 and LaN/YbN=1.39-1.48) on a chondrite-normalized diagram resemble transitional mid-ocean ridge basalts (T-MORB). On the other hand, plutonic and felsic metavolcanics rocks are mainly subalkaline and peraluminous (ASI ³ 1.1). They are enriched in light rare earth relative to heavy rare earth elements (LaN/SmN=2.17-3.26 GdN/YbN=0.94-1.47) with negative Eu anomalies and show negative Nb and Ta anomalies on mantle-normalized multielement diagram. These rocks yield negative εNd(T) values, between –3.1 to –7.5, Sr initial ratios of 0.7089 to 0.7372 and Mesoproterozoic model ages (TDM values in the interval between 1.5 and 1.7 Ga). These felsic magmas represent the product of melting of older continental crust. Isotopic and trace elements data indicate that the orthogneisses and/or metapelites-metagreywackes of the basement are its potential protolith. Our data combined with the literature about sedimentation, deformation and metamorphism suggest that a retroarc basin represents the most likely tectonic setting for the origin of the eastern magmatic rocks in southeastern Puna. According to this, the felsic magmatism could be related to extensive crustal melting associated to astenosphere upwelling after tectonic switching occurring in the suprasubduction zone in the frame of a long-lived “Hot Orogen”.