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Mesozoic batholiths from the central and southern Andes, which are typical cordilleran batholiths characterized by I-type granites, include a small but important proportion of granites with some S-type characteristics (“S-like” granites). The latter were emplaced during the Triassic and Jurassic, although in the central Andes, Permian precursors exist.

The Andean S-like granites are somewhat intermediate in their nature between the “end members” of the S- and I-types. They could have formed from I-type intrusions followed by combined assimilation and fractional crystallization (AFC) during magma ascent through high 87Sr/86Sr initial-ratio wall rock. Alternatively, magmas generated by partial melting of the mantle could have been ponded at the base of the crust, melting it and developing large silicic magma chambers, which developed as S-like granites.

The heat source for the hybridization process could have been Paleozoic plutons, to account for the relatively long period of time probably required by the AFC processes to yield early Mesozoic granites with S-type characteristics. If this is correct it appears unlikely that there was a distinct boundary between the so-called Andean (post–lower-Middle Triassic) and pre-Andean (pre–lower-middle Triassic) magmatic history of the region.

It is now agreed in the literature that the tectonic setting of the Mesozoic Andean S-like granites, emplaced during subduction of the proto-Pacific lithosphere under South America, was that of an extensional regime related to the fragmentation of Gondwanaland.

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