Early Cretaceous adakite from the Atlas porphyry Cu-Au deposit in Cebu Island, Central Philippines: Partial melting of subducted oceanic crust

Abstract

In the Philippine island arc system, there are widely developed Cenozoic arc volcanic rocks, adakites and large Cu-Au deposits. Besides, only two early Cretaceous adakitic intrusions have been found in the entire Philippines so far, i.e., the Lutopan Diorite and the adjacent Kansi diorites in Cebu Island, Central Philippines. The Lutopan Diorite (diorite to quartz diorite in lithology) is the ore-forming intrusion of the giant Atlas porphyry Cu-Au deposit, which is the only Cretaceous porphyry deposit in the Philippines. In this study, geochronology and geochemistry of the Lutopan Diorite in the Carmen pit of Atlas porphyry Cu-Au deposit are investigated in detail, by comparing them with the previously studied Early Cretaceous Kansi diorites and arc volcanic rocks in Cebu Island, to reveal the petrogenesis and magma sources. The Carmen diorites are characterized by moderate SiO2 and MgO contents, coupled with high Na2O and low K2O contents, LREE-enriched and listric REE distribution patterns, low Yb and Y contents, high Sr contents and thereby high Sr/Y ratios, and low (87Sr/86Sr)i (0.7037–0.7038), typical of modern island arc adakites. Zircon U-Pb ages of the Carmen diorites are 109 Ma − 107 Ma in the Early Cretaceous, which is identical with that of the Kansi diorite in the north within error. Both the Carmen and Kansi diorites were formed in intra-oceanic island arc settings. The Carmen diorites show listric REE patterns and low Nb/Ta, high Zr/Sm ratios, suggesting they are formed by partial melting of subducted oceanic crust in amphibolite facies, which is different from the garnet amphibolite- to amphibole eclogite-melting genesis of the Kansi diorites. There is a small amount of subducted sediments in the magma source of Carmen diorite, as further revealed by εNd(t) (+7.1 to +10.6), zircon εHf(t) (+3.3 to +3.9) values, and low Th/U ratios. The Carmen adakitic magma did not undergo significant interaction with mantle peridotite during magma ascent as indicated by the moderate MgO content and Mg# value, and quite low Cr and Ni contents, distinguished from the Kansi diorite (high-Mg adakite) that have high MgO, Mg#, and Cr and Ni contents. We suggest that the nearly contemporary Lutopan Diorite and Kansi diorite are two independent Early Cretaceous adakitic intrusions in Cebu Island, although they are both closely associated with Cu-Au mineralization. The slab-melting genesis and high oxygen fugacities of the Lutopan Diorite are the most important factors to cause Cu enrichment in the magma, which is necessary to form the giant Atlas porphyry Cu-Au deposit.