Abstract
An Early Cambrian pluton, known as the Guasayán pluton, has been identified in the central area of Sierra de Guasayán, northwestern Argentina. A U-Pb zircon Concordia age of 533±4 Ma was obtained by LA-MC-ICP-MS and represents the first report of robustly dated Early Cambrian magmatism for the northwestern Sierras Pampeanas. The pluton was emplaced in low-grade metasedimentary rocks and its magmatic assemblage consists of K-feldspar (phenocrysts)+plagioclase+quartz+biotite, with zircon, apatite, ilmenite, magnetite and monazite as accessory minerals. Geochemically, the granitic rock is a metaluminous subalkaline felsic granodiorite with SiO2=69.24%, Na2O+K2O=7.08%, CaO=2.45%, Na2O/ K2O=0.71 and FeO/MgO=3.58%. Rare earth element patterns show moderate slope (LaN/YbN=8.05) with a slightly negative Eu anomalies (Eu/Eu*=0.76). We report the first in situ Hf isotopes data (εHft=-0.12 to -4.76) from crystallized zircons in the Early Cambrian granites of the Sierras Pampeanas, helping to constrain the magma source and enabling comparison with other Pampean granites. The Guasayán pluton might provide a link between Early Cambrian magmatism of the central Sierras Pampeanas and that of the Eastern Cordillera, contributing to define the western boundary of the Pampean paleo-arc.
Highlights
The Sierras Pampeanas were sub-divided into Western and Eastern sectors according to their dominant lithologies (Caminos, 1972) (Fig. 1), whereas modern geochronological studies demonstrate that they correspond to different geological histories, as summarized below.The Western Sierras Pampeanas are dominated by 1030-1330 Ma (“Grenville orogen”) igneous and metamorphic rocks, intruded by relatively scarce Ordovician granites of the Famatinian cycle (e.g., Casquet et al, 2001; Sato et al, 2003; Rapela et al, 2010; Varela et al, 2011 and references therein)
Cambrian magmatism recognized in the Eastern Cordillera and Eastern Sierras Pampeanas is known as Pampean magmatic arc (e.g., Rapela et al, 1998; Omarini et al, 2008; Hauser et al, 2011; Iannizzotto et al, 2013; von Gosen et al, 2014 and references therein)
A representative sample (GUA-1) from Sierra de Guasayán was analysed for whole-rock major and trace elements using a ThermoARL sequential X-ray fluorescence spectrometer, following the procedure described by Johnson et al (1999)
Summary
The Sierras Pampeanas were sub-divided into Western and Eastern sectors according to their dominant lithologies (Caminos, 1972) (Fig. 1), whereas modern geochronological studies demonstrate that they correspond to different geological histories, as summarized below. Understanding of the petrogenesis and timing of the Early Cambrian magmatism developed during the Pampean orogeny (e.g., Lira et al, 1997; Rapela et al, 1998; Omarini et al, 2008; Schwartz et al, 2008; Hongn et al, 2010; Hauser et al, 2011; Iannizzotto et al, 2013; Lira et al, 2014; von Gosen et al, 2014 and references therein) These studies were based on the large Sierra Norte-Ambargasta batholith (Fig. 1) and the existence of other outcrops in the Sierras Pampeanas being unproven. The geochronological, isotopical, petrological, and geochemical data of the GUA-1 sample from the Guasayán pluton is important in that: i) this is the first report of robustly dated Early Cambrian magmatism in Sierra de Guasayán, ii) it is the north westernmost example of Early Cambrian magmatism in Sierras Pampeanas and might represents the link between the Pampean magmatism of Sierras Pampeanas and Eastern Cordillera in the northwest of Argentina
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