New constraints on the source of the Late Jurassic granodiorites from Gutian porphyry Cu–Mo deposit in the southeast coastal area, South China

Abstract

The Gutian porphyry Cu–Mo deposit, which is localized at the Southeast Coastal Metallogenic Belt (SCMB) in South China, is a newly identified porphyry Cu deposit occurring as part of a granodioritic igneous complex. We report new zircon U–Pb, in–situ apatite Sr–Nd, zircon Hf–O isotopic, and rock geochemical data for the early stage granodiorite, granodioritic porphyry (P1) and mineralization-related granodioritic porphyry (P2) of the Gutian complex, so as to constrain the magmatic sources of the Late Jurassic porphyry Cu deposit and crust–mantle interaction during the Late Jurassic. Zircon U-Pb dating yielded weighted-mean ages of 161.0 ± 1.7 Ma (n = 22; MSWD = 0.42) for the granodiorite and 159.9 ± 1.9 Ma (n = 23; MSWD = 0.37) for the granodioritic porphyry (P2). The granodioritic complex is featured by the increased Sr level, reduced Y level, great (La/Yb)N and Sr/Y ratios, indicating adakitic affinity. Both intrusions exhibit homogeneous enriched Sr–Nd–Hf isotopic compositions ((87Sr/86Sr)i = 0.7084–0.7099; εNd(t) = –10.3 to –7.8; εHf(t) = –13.1 to –8.8, and high δ18O values of 6.96‰ to 9.29‰ with an average at 7.92‰. Hf model ages (T2DM) vary from 1.76 to 2.03 Ga with a peak at ca. 1.85 Ga. The similar emplacement ages and isotopic compositions suggest that both intrusions formed from a common magmatic source: Paleoproterozoic thickened lower crust with ~ 20% garnet-bearing amphibolite. As suggested by Nb/Ta and Nb/U ratios, Mg# values, and the isotopic data of binary mixing models Hf–O and Sr–Nd, ca. 30–40% of mantle-derived magma may have been added to the source region (HfM/HfC = 0.2). This result was verified by the increased εHf(t) and decreased δ18O values than those of Paleoproterozoic juvenile material (εHf(106 Ma) = –24.7; δ18O values of ca. 8‰ to 10‰, with a peak value of 8.55‰). The geochemical characteristics are similar to arc magmas and this, and the coeval syenites and basalts, indicate that the mantle would have been metasomatized by a subduction-related component during the Middle to Late Jurassic. The Gutian granitoids are part of a Jurassic magmatic arc and are the product of the crust-mantle interaction resulting from paleo-Pacific subduction, which is the most important dynamic process for porphyry deposits in Late Mesozoic Southeastern Coastal area.