Geology and geochemistry of the Patagonian batholith (45°-46°S), Chile
The sector of the Chilean continental margin between 45 and 46 S has been the site of arc volcanism and plutonism since the mid-Jurassic, when magmatism shifted several hundred kilometres westwards from its earlier position in Argentina and became focussed in the fore-arc accretionary wedge of mainly low-grade, continent-derived metasediments. Despite the fact that the metasedimentary basement forms readily fusible crust, Rb-Sr isotopic analysis indicates that it can only have contributed a very minor component to the Jurassic - Tertiary Patagonian batholith. The arc has been subjected to several phases of extension and compression. A restricted back-arc, largely sedimentary, marine basin formed in the Late Jurassic and was destroyed in the mid-Cretaceous without significant deformation of the basin infill, when uplift and voluminous silicic plutonism and volcanism occurred in the back-arc regions. Extension in the western part of the arc gave rise to an intra-arc basin that was the site of submarine mafic volcanism. A major compressive phase in the Miocene caused underthrusting of the crust of the intra-arc basin beneath the main arc to the east, where deep parts of the Patagonian batholith are now exposed in a foliated plutonic complex consisting mainly of gabbros, diorites, quartz diorites and tonalites. At the junction between the intra-arc basin and the plutonic complex are metamorphic complexes formed from the volcanics and sediments of the basin. The alternating phases of extension and compression influenced the compositions of magmas generated and emplaced within the arc. Extensional phases enabled magmas to traverse the crust rapidly in small, compositionally diverse batches (e.g. calc-alkaline, MORB-like and enriched, within-plate magma types). On the other hand, compressive phases hindered the passage of mantle-derived magmas through the crust, favouring their accumulation at depth. There they mixed to produce calc-alkaline mafic magmas of more uniform character and caused anatexis of amphibolitic lower crust and the generation of large volumes of silicic magma which mostly crystallised within the crust.