El Gabro Rancho Lata: Magmatismo mesozoico off-axis de la cuenca marginal Rocas Verdes en los Andes Fueguinos de Argentina

Abstract

The Rocas Verdes marginal basin originated during an extensional tectonic event in the Late Jurassic–Early Cretaceous, which finally led to the break-up of the Gondwana supercontinent. It is composed of mafic tholeiitic rocks with mid-ocean ridge basalt (MORB) affinity, exposed at the southernmost tip of South America. Magmatism occurred at spreading centers and was coeval with a calc-alkaline volcanic arc built on continental crust to the pacific side of the basin. Mafic dykes that also intrude continental crust on both sides of the marginal basin are related to the initial stages of basin development. In this contribution we present new field, petrographic, geochemical and microchemical data from gabbroic stocks and sills (the Rancho Lata Gabbro, GRL) emplaced in the northern flank of the southern tip of the marginal basin. The rocks exhibit phaneritic (gabbros) and subvolcanic (dolerites) textures. The GRL experienced greenschist facies metamorphism and mylonitic deformation. Chemical and mineralogical composition of GRL suggest that dolerites and gabbros could derive from a common parental magma, by crystal fractionation, where dolerites represent more differentiated liquids and gabbros the cumulus fraction. The GRL has tholeiitic affinity, moderate enrichment in LILE and LREE compared with normal (N)-MORB and chondrite, respectively, and high LREE/HREE ratios similar to those of enriched (E)-MORBs. On the basis of mineralogical and chemical composition, metamorphism and ductile deformation, the GRL is correlationed to other mafic dykes flanking the ophiolites reported in literature, and to the ophiolites itself from the least evolved part of the marginal basin. Therefore, the GRL most probably generated during the initial stages of basin development in an enriched upper mantle. The Nb-Ta-Ti troughs seen on normalized trace element diagrams indicate contribution of subduction components to the source.