The geodynamics of collision of a microplate (Chilenia) in Devonian times deduced by the pressure-temperature-time evolution within part of a collisional belt (Guarguaraz Complex, W-Argentina)

Abstract

The Guarguaraz Complex in West Argentina formed during collision between the microplate Chilenia and South America. It is composed of neritic clastic metasediments with intercalations of metabasic and ultrabasic rocks of oceanic origin. Prograde garnet growth in metapelite and metabasite occurred between 1.2GPa, 470°C and 1.4GPa, 530°C, when the penetrative s2-foliation was formed. The average age of garnet crystallization of 390±2Ma (2σ) was determined from three four-point Lu-Hf mineral isochrones from metapelite and metabasite samples and represents the time of collision. Peak pressure conditions are followed by a decompression path with slight heating at 0.5GPa, 560°C. Fluid release during decompression caused equilibration of mineral compositions at the rims and also aided Ar diffusion. An 40Ar/39Ar plateau age of white mica at 353±1Ma (1σ) indicates the time of cooling below 350-400°C. These temperatures were attained at pressures of 0.2-0.3GPa, indicative of an average exhumation rate of ≥1mm/a for the period 390-353Ma. Late hydrous influx at 0.1-0.3GPa caused pervasive growth of sericite and chlorite and reset the Ar/Ar ages of earlier coarse-grained white mica. At 284-295Ma, the entire basement cooled below 280°C (fission track ages of zircon) after abundant post-collisional granitoid intrusion. The deeply buried epicontinental sedimentary rocks, the high peak pressure referring to a low metamorphic geotherm of 10-12°C/km, and the decompression/heating path are characteristics of material buried and exhumed within a (micro) continent-continent collisional setting.