MAGMATISMO BÁSICO JURÁSICO-CRETÁCICO EN LOS ANDES FUEGUINOS Y SU RELACIÓN CON LA CUENCA MARGINAL ROCAS VERDES

IVANA URRAZA; ROGELIO D ACEVEDO; MAURICIO GONZALEZ GUILLOT; MONICA P ESCAYOLA

Revista Asociación Geológica Argentina

AGA

Año: 2016 vol. 73 p. 1 - 22

Several basic sills, dykes and lavas crop-out flanking the ophiolitic remnants of the Rocas Verdes marginal basin, affected bylow-grade regional metamorphism. Three varieties are distinguished accordingly to their primary paragenesis: plagioclasepyroxenelavas and sills, plagioclase lavas and sills, and hornblende-bearing sills. The former two are intercalated in theLate Jurassic Lemaire Formation, but do not extend into overlying units (Yahgán and Beauvoir Formations). Several sillsof these two varieties show evidences of emplacement into unconsolidated sediments, which indicate they are coeval withLemaire Formation and basaltic lavas. The plagioclase-pyroxene lavas and sills are chemically similar, suggesting they are cogenetic.The plagioclase varieties appear towards the margins of some pyroxene-plagioclase varieties or as independent sills; itsgeochemistry suggests they derive from the pyroxene-plagioclase varieties by crustal assimilation. Based on field relations andchemistry, the origin of the plagioclase-pyroxene and plagioclase varieties is related to the initial stages of rifting, before thedevelopment of the oceanic spreading ridges. This interpretation, along with the field observation, suggests that in areas awayfrom the axis of the basin all basic rocks, except the hornblende diabases, were emplaced prior to the deposition of the YahgánFormation, which represents the most evolved stage of the marginal basin. Contrarily, the hornblende diabases cut the YahgánFormation and are petrographically and chemically distinct from the other varieties. Thus they constitute a different, youngerunit. The formal name Puente Quemado Gabbro is adopted for this unit.