Abstract

BELMAR, Mauricio  and  MORATA, Diego. Nature and P-T-t constraints of very low-grade metamorphism in the Triassic-Jurassic basins, Coastal Range, central Chile. Rev. geol. Chile [online]. 2005, vol.32, n.2, pp.189-205. ISSN 0716-0208.  http://dx.doi.org/10.4067/S0716-02082005000200002.

Triassic-Jurassic volcano-sedimentary sequences in the Coastal Range of central Chile (34° 45/35°20'S) from the Vichuquén-Tilicura and Hualañé-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 »1.3, T»190) at 181-184 Ma, under geothermal gradient of »35°Ckm^-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 hydrogarnet (+albite and quartz) are indicative of re-equilibration under prehnite-pumpellyite facies conditions (P »3 kbar, T »300°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 metamorphic conditions between Triassic-Lower Jurassic metapelites and Middle-Late Jurassic metabasites

Keywords : Very low-grade metamorphism; Metabasites; Triassic-Jurassic basins; Coastal Range; Central Chile.