Abstract
Upper Cretaceous intrusives of limited extent crop out in the Coastal Cordillera near of Valdivia (39º48’ 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 (
Highlights
In the Coastal Range of South Central Chile, between 39° S and 40° S, outcrops of isolated intrusive bodies of limited extension (< to 24 km long) and of Upper Cretaceous age have been identified
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
The MORB-normalized multi-element diagram for the studied rocks (Fig. 12) shows typical subduction related patterns, which suggests the participation of two components in the genesis of the forearc and arc magmas: the asthenospheric mantle wedge and LILE-rich fluids derived from the subducted plate and/or eroded forearc, which metasomatized the mantle wedge and induced its partial melting (e.g., Iwamori, 1998)
Summary
In the Coastal Range of South Central Chile, between 39° S and 40° S, outcrops of isolated intrusive bodies of limited extension (< to 24 km long) and of Upper Cretaceous age (mainly 92-83 Ma; Table 1) have been identified. At that time and at same latitudes, the corresponding subduction-related magmatic arc was approximately 100 km to the east, in the present Main Andean Cordillera position (Fig. 1, Table). At that time and at same latitudes, the corresponding subduction-related magmatic arc was approximately 100 km to the east, in the present Main Andean Cordillera position (Fig. 1, Table1) The existence of such magmatic rocks in the Coastal Range represent magmatic suites emplaced in the forearc crust which is typically a nonmagmatic zone with low heat flow (Gill, 1981). Based on zircon and apatite fission track studies, Glodny et al (2007) suggested that Toltén and Chiloé forearc zone would have experienced a transient heating in the Upper Cretaceous. 39°53 ́58 ́ ́ 73°29 ́51 ́ ́ granodiorite 39°54 ́10 ́ ́ 73°30 ́00 ́ ́ granodiorite
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