The Miocene plutonic event of the Patagonian Batholith at 44° 30′ S: thermochronological and geobarometric evidence for melting of a rapidly exhumed lower crust

Abstract

The Patagonian Batholith was formed by numerous plutonic events that took place between the Jurassic and the Miocene. North of 47° S, the youngest plutons occupy the axial zone adjacent to the Liquine-Ofqui Fault Zone, which is a major intra-arc strike-slip fault system active since the Miocene. The Queulat Complex, located at 44° 30' S, includes two Miocene plutonic units: the Early Miocene Queulat diorite (QD) and the Late Miocene Puerto Cisnes granite (PCG). The QD includes hornblende + clinopyroxene diorites and tonalites, whereas the PCG includes slightly peraluminous garnet ± sillimanite granites and granodiorites. Eleven mineral Ar-Ar ages and three apatite fission track ages were obtained from the Queulat Complex and surrounding host rocks. Hornblende and biotite Ar-Ar ages of c. 16-18 Ma and 9-10 Ma, respectively, were obtained for the QD. The youngest ages of the QD are similar to the age of emplacement of the PCG as previously determined. Ar-Ar ages for muscovites and biotites of 66 ± 0-3 Ma and 5-6 ± 0·1 Ma, respectively, were obtained for the PCG. Biotites and muscovites from mylonites and pelitic hornfelses adjacent to the PCG yielded Ar-Ar ages between 5·1 Ma and 5-5 Ma. The apatite fission track ages of the QD and PCG overlap within the error margin (2·2 ± 1·1-3·3 ± 1·4 Ma). The Al-in-hornblende geobarometer yielded pressures for the QD emplacement equivalent to depths in the 19-24 km range, which is substantially higher than the 10km depth estimated pieviously for the PCG emplacement. Exhumation rates (v) up to 2·0 mm/yr were calculated for the time elapsed between the QD and PCG emplacements. A v value of 1·0 mm/yr was calculated for the PCG subsequent to its emplacement. Using the silica-Ca-tschermak-anorthite geobarometer, we estimate the QD magma generation to be at c. 33 km, which is similar to the current crustal thickness. Melting of mafic and metapelitic lower crust was possible at >30km depth during a period when v was between 1·0 mm/yr and 2·0 mm/yr.