Conditions and Timing of Pumpellyite–Actinolite-facies Metamorphism in the Early Mesozoic Frontal Accretionary Prism of the Madre de Dios Archipelago (Latitude 50°20′S; Southern Chile)

摘要

The Madre de Dios Metamorphic Complex (MDMC) in southern Chile is a fossil frontal accretionary prism, which is mainly composed of metapsammopelitic rocks, intercalations of oceanic rocks (greenstone and metachert) and platform carbonate. We concentrated on the metabasite to decipher the metamorphic evolution. This rock type contains assemblages of the pumpellyite–actinolite facies: pumpellyite ± actinolite–chlorite ± grandite ± phengite ± epidote–albite–quartz–titanite ± K-feldspar ± calcite. The metamorphic phases mainly grew by prograde hydration reactions during various episodes of restricted fluid influx. Fundamental phase relations of the pumpellyite–actinolite facies and adjacent facies were reproduced by pseudosections calculated for the system K₂O–Na₂O–CaO–FeO–O₂–MgO–Al₂O₃–TiO₂–SiO₂–H₂O–CO₂ at 200–400°C and 1–9 kbar. The calculated stability fields of the metamorphic assemblages as realized in the MDMC metabasite indicate highest metamorphic conditions restricted to 290–310°C, 4–6 kbar for the MDMC, presumably as a result of the main fluid influx at these conditions. Nevertheless, earlier local equilibria are still preserved as a result of strongly kinetically controlled mineral reactions and a lack of recrystallization and compositional homogenization at thin-section scale. Hence, thermodynamic calculations of local multivariant mineral equilibria using the entire compositional variation of minerals in the MDMC show that the prograde PT path evolved from 4 ± 1 kbar, 200–220°C to 5 ± 1 kbar, 290–330°C. The prograde PT path reflects nearly horizontal particle paths after reaching the maximum depth typical for frontal accretionary prisms. Long residence at maximum depth resulted in thermal re-equilibration. 40Ar/39Ar spot ages were measured by in situ UV laser ablation of local phengite concentrations in a deformed metapelite at 233·2 ± 1·8 Ma and in an undeformed metabasite at 200·8 ± 2·4 Ma. Whereas the first age represents an age of accretion, the latter age can be attributed to mineral growth either during a younger stage of accretion or during a retrograde stage. 40Ar/39Ar isotopic analyses of two further metabasite samples reflect a prominent resetting of ages at 152·0 ± 2·2 Ma and white mica growth during external fluid access triggered by either a local intrusion or a late Jurassic extensional episode.