Nature and P-T-t constraints of very low-grade metamorphism in the Triassic-Jurassic basins, Coastal Range, central Chile

Abstract

Triassic-Jurassic volcano-sedimentary sequences in the Coastal Range of central Chile (34 degrees 45/35 degrees 20'S) from the Vichuquen-Tilicura and Hualane-Gualleco basins display metamorphic assemblages indicative of very low-grade conditions. Triassic and Lower Jurassic metapelites achieved anchizonal conditions (IC, b(o), coal-rank and fluid inclusions data: P approximate to 1.3, T approximate to 190) at 181-184 Ma, under geothermal gradient of approximate to 35 degrees C km(-1) during extensional geodynamic conditions. Nevertheless, in stratigraphically higher mafic volcanic sequences, of Middle to Late Jurassic age, the presence of chlorite (Xc =0.98-0.99), three types of epidote (XFe3+ = 41.14 +/- 0.90, 30.08 +/- 1.53 and 18.05 +/- 1.75), titanite, Fe-rich and Fe-poor pumpellyite and hydrogamet (+albite and quartz) are indicative of re-equilibration under prehnite-pumpellyite facies conditions (P approximate to 3 kbar, T approximate to 300 degrees C) and variable oxygen fugacity. A change from an extensional setting (with maximum basin subsidence and development of burial very low-grade metamorphism in the Triassic-Lower Jurassic metapelitic sequences) to a compressional setting (with local overthrusting of Triassic and Lower Jurassic sequences over Middle to Late Jurassic volcano-sedimentary sequences) is proposed to explain the contrasting metannorphic conditions between Triassic-Lower Jurassic metapelites and Middle-Late Jurassic metabasites.