Crustal growth during back-arc closure: Cretaceous exhumation history of cordillera darwin, southern patagonia

The Cordillera Darwin metamorphic complex is unique in the Andes in exposing kyanite-staurolite schist north of the Beagle Channel in southern Patagonia. Garnet in amphibolite facies pelitic schists from Bahía Pia has patchy textures whereby some grains consist of clear, grossular-rich garnet with fine-grained S1 inclusion trails truncated by regions of turbid spessartine-pyrope-rich garnet with biotite, muscovite, plagioclase and quartz inclusions. Micron-scale aqueous inclusions in turbid garnet are consistent with recrystallization facilitated by fluid ingress; S2 inclusion trails indicate this was broadly contemporary with the growth of kyanite and staurolite in the matrix. Pseudosection modelling in Na₂O-CaO-K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O-TiO₂-Fe₂O₃ (NCKFMASHTO) is used to infer a P-T path dominated by decompression from 12 to 9kbar at T≈620°C, coupled with garnet mode decreasing from ∼5% to <1%. U-Th-Pb in situ dating of S2 monazite indicates that staurolite and kyanite growth and thus exhumation was underway before 72.6±1.1Ma. Contact aureoles developed adjacent to late granite intrusions include sillimanite-bearing migmatites formed at P≈6kbar after 72Ma. Metamorphism of southern Cordillera Darwin induced by continental underthrusting beneath the arc, related to closure of the Rocas Verdes back-arc basin, was terminated by thrusting-controlled exhumation, with the rocks at P≈9kbar by c. 73Ma and 6kbar by c. 70Ma.