Multi-pulse cotectic evolution and in-situ fractionation of calc-alkaline tonalite-granodiorite rocks, Velasco Cambro-Ordovician batholith, Famatinian belt, Argentina

Abstract

The study of a magmatic series composed of Qtz-diorites to granites of the south part of the Sierra de Velasco batholith (Famatinian arc) in Argentina reveals that most rocks follow a coherent cotectic trend, which is comparable to a (CLL) traced from experimental liquid compositions of calc-alkaline (andesitic) systems. The identification of cotectic relationships is crucial to interpret the meaning of geochemical variation trends in terms of phase equilibria. We show the contrast between typical CLL trends defined by rocks of the Palanche pluton and rocks departing from CLL array, which are interpreted as resulting from local processes of bulk assimilation of a pelitic host. Zircon U-Pb age determinations with SHRIMP support a coeval zircon crystallization record from of about 480 to 450Ma of samples plotting on the CLL. The absence of intrusive contacts between these coeval cotectic granites implies that they were fractionated in situ at the level of emplacement. Other younger pulses (442 ±5 Ma), also fall on the CLL, denoting that uniform processes of melting and fractionation were operative for at least 40Ma at the same locus at the active continental margin of Gondwana. During this protracted magmatic activity, sporadic pulses of mafic magma were injected (e.g. at 456 ±7 Ma), revealing a complex process of pulse amalgamation in the building up of the batholith. These mafic magma pulses may have contributed to basification of the host granitoid by enclave dissolution at the low pressure of emplacement. The reported geochemical trends, as true CLL, together with observed textures and crystallization sequences, point to a parental intermediate system of andesite composition (SiO2>55 wt%) with a water content of about 2.5 wt% H2O, similar to those formed by sediment-basalt subducted mélanges. A sublithospheric origin for the mafic precursor is proposed in agreement with prediction by recently published thermo-mechanical modeling and experimental phase equilibria.