The Late Miocene Chivinar volcanic complex (24º14’S – 67º27’W, Central Andes): calcalkaline and intraplate magma sources at the intersection of the Calama-Olacapato-El Toro volcanic belt and the arc

The Miocene Chivinar volcanic complex is located in NW Argentina, at the intersection of the Calama-Olacapato-El Toro back-arc volcanic belt and the active arc (Fig. 1). Chivinar includes very different rocks: in a first phase of volcanic activity topaz-bearing rhyolitic lava domes emplaced, while, in a later phase, dacitic and andesitic lavas were erupted (Koukharsky et al., 1991; Orlandi et al., 2011; Gioncada et al., 2014). The topaz-bearing rhyolitic lavas have sanidine and oligoclase phenocrysts in a groundmass of quartz, sanidine, oligoclase, topaz and micaceous minerals and minor accessory phases. The composition is peraluminous to metaluminous, with very high Nb, Ta (Fig. 2), low LREE values and flat REE patterns. The andesites and dacites range in silica from 59 to 63 wt% and belong to the high-K calcalkaline series (Fig. 2). Their incompatible trace element spiderdiagrams show the moderate Nb and Ta and Ti troughs of subduction-related andesitic magmas (Fig. 2), suggesting a calcalkaline mantle source. The topaz rhyolites are unrelated to the andesitic magmas in terms of fractional crystallization and crustal assimilation. On the other hand, they do not correspond to the S-type silicic peraluminous magmas of crustal derivation, requiring an igneous protolith rather than of a pelitic one (Taylor and Fallick, 1997). Other rhyolitic lavas with magmatic topaz of the western United States and Mexico show the same major and trace elements characteristics (Christiansen et al., 2007). Petrologic studies explained the North America rhyolites with melting of continental crust intruded by mafic magmas with an intraplate signature (Christiansen et al., 2007; Rodríguez-Ríos et al., 2007), in agreement with their intraplate setting. Very similar tectono-magmatic conditions are found also for the topaz rhyolites of Chivinar, in the Central Andes. In fact, the main structures controlling the rhyolitic volcanism of Chivinar may be mostly related to the back-arc transtensive conditions of COT (Acocella et al., 2011), rather than to the arc structures. Even though the overall tectonic setting of the Central Andes is contractional, transtensive or extensional conditions are found along the NW-SE trending fault zones in the back-arc, as the transtensive COT (e.g. Riller et al., 2001). In this context, the compositional features of the Chivinar topaz rhyolites (Fig. 2) could indicate partial melting of a continental crust previously intruded by mafic igneous bodies in an extensional regime, similarly to the North America topaz rhyolites. These similarities suggest a connection between the transtensive/extensional features of the COT and intraplate extensional settings elsewhere, indicating that the eruption of rhyolitic magmas suitable to the crystallization of magmatic topaz is favoured by this tectonic regime.