Petrology and SHRIMP U-Pb zircon geochronology of Cordilleran granitoids of the Bariloche area, Argentina

摘要

A petrological and geochronological study of Cordilleran granitoid intrusions in the Bariloche area (Argentina) point to a complex time-compositional evolution of magmatic processes in relation with oblique subduction of the Phoenix plate below the South America active margin during Jurassic times. The observed geochemical variations in both major and trace elements, together with the textural and mineralogical relations, point to a roughly defined, overall process of magmatic "filtering" linking all the intrusive batholithic rocks of the Bariloche area. These data suggest that the composition of the parental magma that underwent fractionation may be an intermediate magma with SiO_2 = 58-60 wt%, MgO = 2.5 wt%, FeO = 6.5 wt%, CaO = 6.1. These are coincident with the typical compositions of evolved andesites. Magnetite, amphibole and plagiodase are the main phases involved in the fractionation process. According to Hbl thermobarometry, fractionation may have taken place, at least in part, at shallow pressures (P = 0.5-1.5 kbar), possibly at the level of emplacement. The coupled observations of the two pressure dependent ratios, namely Sr/Y and La/Yb are pointing to a low-pressure, low-temperature final fractionation dominated by only PI. The geochronologic study by U-Pb SHRIMP zircon determinations of 14 samples from granites, tonalites and diorites yield a broad range of about 20 Ma, between 150 and 170 Ma at the Medium Jurassic. The batholith was accomplished by a protracted magmatic activity that lasted for about 20 Ma. This time is much longer than the time elapsed from intrusion to complete crystallization of shallow magma chambers. It is concluded that amalgamation of discrete magma pulses is the dominant process that built-up the batholith. The observed structures suggest that the fractures conditioning the emplacement of the magma batches were arranged en echelon and show a right-stepping. The resulting geometry is compatible with the activity of a large-scale, sinistral, N-S trending, strike-slip fracture zone permitting the emplacement of each magma pulse. This major, strike-slip fault system should be deeply entrenched in the crust to allow intruding magmas generated and fractionated at depth. Because batholith generation is a direct consequence of subduction, structural relations and ages can be used to constraint the plate motion relations during Jurassic in this region of the South America active margin.