Geochronology of Miocene plutons and exhumation history of the El Teniente region, central Chile (34-35°S)

⁴⁰Ar/³⁹Ar mineral dating and whole rock chemical analyses of Miocene to Pliocene Andean granitoids near the El Teniente copper deposit (34°S) provide new evidence for rapid Neogene exhumation. This exhumation is attributed to crustal thickening that culminated in the Late Miocene-Early Pliocene coincident with the emplacement of the ore deposit. Three groups of Neogene plutons in the forearc of the active Southern Volcanic Zone (SVZ) are considered. The oldest and westernmost is represented by the La Obra pluton (19.9 ± 0.5 Ma, biotite) which has chemical affinities with the host Late Oligocene-Early Miocene Coya-Machali Formation volcanic rocks. Slow cooling of this pluton is required by a 3.4 my difference between biotite and K-feldspar ⁴⁰Ar/³⁹Ar ages. Modeling shows that this cooling is consistent with an exhumation rate of ~0.55 mm/yr between 19.6 Ma and 16.2 Ma. The second group, termed the El Teniente Plutonic Complex, consists of plutons with biotite ages clustering at ~11 to 12 Ma and ~8 to 9 Ma. These plutons have chemical affinities with the Middle to Late Miocene Teniente Complex (Farellones Formation) volcanic rocks whose chemical characteristics suggest that they erupted through a thicker crust than the Coya-Machali units. A 0.7 my difference between biotite (8.4±0.3 Ma) and K-feldspar ages in the Nacimiento Rio Cortaderal pluton requires an exhumation rate of ~3 mm/yr betwween 8.4 and 7.7 Ma. Although modeled exhumation rates depend on estimates of mineral closure temperatures, paleo-geothermal gradients, and errors in age determinations, a higher exhumation rate for the Nacimiento Rio Cortaderal pluton than the La Obra pluton is a robust result of modeling. A third group of plutons (Young Plutonic Complex) farther west, is characterized by biotite ages of 6.6 to 5.6 Ma. Their steeper rare earth element patterns and more enriched isotopic signatures are consistent with emplacement in a crust even more thickened by Late Miocene compressional deformation. An elevated paleo-geothermal gradient, consistent with the Miocene magmatic-arc environment, best explains the cooling histories of these plutons. Their mineral ages are interpreted as being controlled by exhumation associated with crustal thickening due to compressional deformation related to crustal shortening. The data are consistent with a moderate regional deformation associated with eastward shift of the magmatic front between 20 to 16 Ma, and a stronger regional deformation associated with frontal arc migration between 8 and 5 Ma.