Early Cretaceous plutonic rocks exposed south of Copiapo ́ 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 Copiapo ́ 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 Copiapo ́ 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 d18O values for these rocks range from þ6.9 to þ8.6‰. 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 Copiapo ́ 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 Copiapo ́ complex were emplaced at a relatively shallow crustal level of 2–3 km. q 2003 Elsevier Ltd. All rights reserved.