Inception of a Devonian subduction zone along the southwestern Gondwana margin: 40Ar–39Ar dating of eclogite–amphibolite assemblages in blueschist boulders from the Coastal Range of Chile (41°S)

Abstract

Coarsely crystalline residual blueschist boulders from the Coastal Range of south-central Chile (41°S) contain relict eclogite–amphibolite assemblages that provide evidence of pre-Carboniferous high pressure relative to temperature (high P/T) metamorphism along the southwestern continental margin of Gondwana. Early assemblages in the exotic boulders include omphacite, garnet, and hornblende that indicate eclogite-facies conditions of T = 553 ± 30 °C and P > 1.32 ± 0.04 GPa during metamorphism, corresponding to a low geothermal gradient of <12.5 °C/km. These phases are replaced to varying degrees by sodic amphibole + epidote assemblages. Relict hornblende from an early garnet amphibolite assemblage within blueschist yields an 40Ar–39Ar plateau age of 361 ± 1.7 Ma, providing a minimum age for early high P/T metamorphism. Coarse white micas that partially replace the hornblende and are in textural equilibrium with glaucophane yield plateau and near-plateau ages of 325 ± 1.1 Ma and ∼320 Ma, respectively. We interpret these data to indicate that late Paleozoic high P/T metamorphism related to subduction of oceanic lithosphere along the southwestern paleo-Pacific margin of Gondwana began prior to 361 Ma (Late Devonian). Subsequent retrograde metamorphism involving fluid infiltration and decreasing thermal gradients resulted in conversion of coarse eclogite–amphibolite to blueschist by ∼325 Ma in the dated samples.