Geochemical Characteristics of Magmatism in the Southern Andes (45–46° S)
Along the continental margin of southern Chile (45–46°S), Mesozoic to Recent magmas of the Patagonian Batholith were emplaced through a metasedimentary basement which had developed as an accretionary sedimentary wedge during the late Palaeozoic-early Mesozoic. Despite the rather radiogenic character of this basement, initial 87Sr/86Sr ratios of the plutons are generally low (0.7038–0.7047), suggesting only limited involvement of basement in their petrogenesis. A zonation of magma types is observed, but there is no unidirectional migration with time in the locus of magmatism, a consequence of the Andean margin being subject to alternate periods of compression and extension. The main Mesozoic zones of plutonism are typically calc-alkaline. Alkaline or tholeiitic magmas were emplaced in the back-arc regions, including a bimodal volcanic suite erupted in a back-arc basin which formed in the late Jurassic-early Cretaceous. On the oceanward side of the calc-alkaline arc, an intra-arc basin developed during the mid-Tertiary into which basalts with transitional calc-alkaline-tholeiitic characteristics were erupted. Tertiary tonalite-trondhjemite plutons emplaced within this basin have tholeiitic trace element characteristics and flat rare-earth patterns. Closure of the basin resulted in major uplift and exposure of deep levels of the main calc-alkaline arc, made up of minor gabbros, diorites and tonalites (often gneissified), some of the latter having REE patterns with HREE depletion indicative of hornblende fractionation.
KeywordsPlutonic Complex Convergent Plate Margin Potassic Granite Unidirectional Migration Andean Margin
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