Asymmetric textural and structural patterns of a granitic body emplaced at shallow levels: the La Chinchilla pluton, northwestern Argentina

Abstract

New mapping and a detailed structural study of the La Chinchilla stock, Sierra de Velasco, NWArgentina,suggest an asymmetrical shape of the pluton and provide strong evidence for its shallow emplacementdepth. The pluton is a Lower Carboniferous monzogranite composed of K-feldspar, quartz, plagioclaseand biotite. It exhibits an internal asymmetric textural zoning, defined by porphyritic granite in thesoutheastern region to equigranular granite in the northwestern region. The presence of subhorizontaldikes in the northwestern area, where the contacts dip shallowly, and subvertical dikes intruding thehost rock nearby steep-dipping intrusive contacts in the southeastern region are compatible with anoverall asymmetrical shape and internal structure of this pluton. Considering published crystallizationages, a dominant strain field occurring at around 12 Ma is inferred based on magmatic fabrics in thepluton and its host rock (the Huaco pluton), with a principal shortening direction oriented SWeNE,consistent with the general NWeSE strike of the body. Field evidence supports brittle fracturing andblock displacement as the dominant emplacement mechanism, suggesting that magmatic stopingdominated during the late stage of the evolution of the magma chamber.