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

The Patagonian Batholith was formed by numerous plutonic events that took p lace between the Jurassic and the Miocene. North of 47 degrees S, the young est 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 Mioce ne. The Queulat Complex, located at 44 degrees 30 ' S, includes two Miocene plutonic units: the Early Miocene Queulat diorite (QD) and the Late Miocen e Puerto Cisnes granite (PCG). The QD includes hornblende + clinopyroxene d iorites and tonalites, whereas the PCG includes slightly peraluminous garne t +/- sillimanite granites and granodiorites. Eleven mineral Ar-Ar ages and three apatite fission track ages were obtaine d from the Queulat Complex and surrounding host rocks. Hornblende and bioti te Ar-Ar ages of c. 16-18 Ma and 9-10 Ma, respectively, were obtained for t he 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 o f 6.6 +/- 0.3 Ma and 5.6 +/- 0.1 Ma, respectively, were obtained for the PC G. Biotites and muscovites from mylonites and pelitic hornfelses adjacent t o 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 19-24km range, which is substantially higher th an the 10km depth estimated previously for the PCG emplacement. Exhumation rates (upsilon) up to 2.0 mm/yr were calculated for the time elapsed betwee n the QD and PCG emplacements. A upsilon value of 1.0 mm/yr was calculated for the PCG subsequent to its emplacement. Using the silica-Ca-tschermak-an orthite 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 m etapelitic lower crust was possible at > 30km depth during a period when up silon was between 1.0 mm/yr and 2.0 mm/yr.