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Geochemical and Sr–Nd–Pb–O isotope composition of granitoids of the Early Cretaceous Copiapó plutonic complex (27°30′S), Chile

2003; Elsevier BV; Volume: 16; Issue: 5 Linguagem: Inglês

10.1016/s0895-9811(03)00104-4

1873-0647

Robert Marschik, Dénis Fontignie, Massimo Chiaradia, P. Voldet,

earthquake and tectonic studies

Early Cretaceous plutonic rocks exposed south of Copiapó form part of the Coastal Batholith of northern Chile. These rocks intrude arc-derived volcanic and volcaniclastic rocks and marine limestones that were deposited in the Early Cretaceous Atacama backarc basin. The Copiapó plutonic complex consists mainly of calc-alkaline, medium- to coarse-grained diorite, granodiorite, tonalite, monzodiorite, and quartz monzonite. The plutonic rocks are subalkaline to alkaline, metaluminous, magnetite-series, volcanic arc, I-type granitoids. Batholithic magmas are a heat, potential fluid, metal, and sulphur source for the hydrothermal iron oxide-rich Cu–Au mineralization in the Candelaria-Punta del Cobre district. Ore-related hydrothermal alteration affected large portions of the Copiapó complex. The least altered batholithic rocks have initial 87Sr/86Sr of 0.703070–0.703231; initial 143Nd/144Nd of 0.512733–0.512781; and 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb of 18.428–18.772, 15.550–15.603, and 38.127–38.401, respectively. The δ18O values for these rocks range from +6.9 to +8.6‰. Isotope signatures and trace element distributions suggest that the magmas are mantle derived. A subduction fluid-modified mantle source may explain the geochemical characteristics of the Copiapó complex. The ascent of magmas occurred along deep-rooted structures without significant crustal contamination, though minor contamination by relatively young (e.g. Jurassic) igneous rocks during ascent is possible. Intrusive rocks with high-K to shoshonitic characteristics probably represent residual liquids of less evolved magmas. The regional geologic context suggests that the plutons of the Copiapó complex were emplaced at a relatively shallow crustal level of 2–3 km.