Upper Cretaceous intrusives in the Coastal Cordillera near Valdivia: forearc magmatism related to the passage of a triple junction?

摘要

Upper Cretaceous intrusives of limited extent crop out in the Coastal Cordillera near of Valdivia (39°48' S), 100 km west of the main topographic divide of the Andean Cordillera. Given that plutonic rocks of the same age crop out at the same latitudes in the high Andes the coastal intrusives emplaced in a forearc position in the upper plate of a subduction setting. They correspond to hypabyssal intrusives displaying mainly porphyritic texture and lithological variations with microtonalites (minor), porphyritic microgranodiorites (main) and microgranites. They intrude the Upper Paleozoic-Triassic accretionary complex of the Bahia Mansa Metamorphic Complex. These intrusives, that comprise the Chaihuín Pluton and minor stocks of porphyritic felsic rocks, have calc-alkaline affinities with metaluminous and peraluminous character. They are geochemically similar to the contemporaneous main arc-related plutonic rocks of the Gualletué Plutonic Group. The microgranitoids and dacitic rocks from Los Boldos, the low and Loncoche are peculiar because they show an apparently adakitic affinity in Sr/Y and LaN/YbN discriminant diagrams; nevertheless Sr contents of these rocks (400 ppm) preclude a true adakitic character. The petrogenesis of the Chaihuín Pluton, is consistent with an evolution from tonalite to granite by successive fractional crystallization of plagioclase, amphibole, biotite, Fe-Ti oxides, apatite and zircon. The initial 87Sr/86Sr ratio (0.70411-0.70745), εNd (+4.24 to ?3.09) and present-day Pb isotopic ratios (206Pb/204Pb: 18.616 to 18.708; 207Pb/204Pb: 15.620 to 15.635; 208Pb/204Pb: 38.573 to 38.662) of these rocks indicate that depleted mantle derived-magmas were contaminated by assimilation of crustal material at the base of the paleo accretionary prism or by subduction erosion. The heat required to explain mantle melting beneath the forearc crust could be supplied by the subduction of a young and hot oceanic slab and/or an active spreading ridge, as attested in similar settings in the world. According to plate reconstruction models the studied forearc intrusives would be generated during the migration of a triple junction that passed near Valdivia between 100 and 70 Ma.