Igneous Rock Associations 25.Pre-Pliocene Andean Magmatism in Chile

摘要

Andean-type magmatism and the term ‘andesite’ are oftenused as the norm for the results of subduction of oceanic lith-osphere under a continent, and the typical rock formed.Although the Andes chain occupies the whole western marginof South America, the most comprehensively studied rocksoccur in the present-day Chilean territory and are the focus ofthis paper. Andean magmatism in this region developed fromthe Rhaetian-Hettangian boundary (ca. 200 Ma) to the presentand represents the activity of a long-lived continental mag-matic arc. This paper discusses Pre-Pleistocene volcanic, plu-tonic, and volcano-sedimentary rocks related to the arc thatcover most of the continental mass of Chile (between thePacific coast and the High Andes) between the latitudes of 18°and 50°S. They comprise most of the range of sub-alkalineigneous rocks, from gabbro to monzogranite and from basaltto rhyolite, but are dominated by the tonalite-granodiorite andandesite example members. Variations in the petrographiccharacteristics, major and trace element composition and iso-topic signature of the igneous rocks can be correlated tochanges in the physical parameters of the subduction zone,such as dip angle of the subducting slab, convergence rate andangle of convergence. Early Andean magmatic products(Jurassic to Early Cretaceous) are found along the CoastalCordillera in the westernmost part of the Andes. The rockrecord of the subsequent stages (Late Cretaceous, Pale-ocene–Early Eocene, Middle Eocene–Oligocene, Miocene) isprogressively shifted to the east, reflecting migration of themagmatic front towards the continent. Tectonic segmentationof the convergent margin, as attested by the magmatic record,may have occurred throughout the Andean life span but it isparticularly evident from the Eocene onwards, where the evo-lution of the northern part of the Chilean Andes (north of27°S latitude) is very different to that of the southern segment(south of 27°S latitude).