Petrogenesis of Eocene mafic and felsic magmas in the New Caledonia ophiolite: geochemistry and geochronology constraints

Abstract

ABSTRACT Both felsic and mafic intrusions occur in mantle peridotites of the New Caledonia ophiolite (Peridotite Nappe) and record important information regarding the magmatic evolution during subduction initiation. A systematic petrological, geochronological and geochemical study on gabbro/diorite samples from the Peridotite Nappe is performed to explore their petrogenesis. The gabbro is mainly composed of plagioclase, clinopyroxene and amphibole. Most clinopyroxene have been altered to hornblende or actinolite, whereas some amphibole with high Al2O3 (>8 wt%) and low SiO2 (<46 wt%) contents are of a magmatic origin. This suggests that the gabbro crystallized from hydrous melts. The gabbro shows depleted Sr-Nd-Hf isotope compositions, suggesting that the parental magma of the gabbro could have been derived from the depleted MORB-type mantle (DMM) asthenosphere. The gabbro zircon have δ18O values of +4.74-+6.63‰, which are slightly higher than the mantle-like δ18O values of +5.3 ± 0.6‰, indicating the involvement of subduction-related fluids in their genesis. The diorite mainly consists of albite (An1-4) and tremolite. The felsic dikes within the New Caledonia ophiolite, including granitoids, high-Mg felsic dikes and diorites, are oceanic plagiogranites. Extremely low TiO2 contents, roughly negative correlation of Yb and SiO2 contents, REE patterns of plagiogranites and mafic rocks, and compositions of zircon suggest that the plagiogranites were not products of fractional crystallization of MORB but were generated by hydrous melting of mafic protoliths. The diorite zircon show core-rim textures. Both rims and cores yield identical U-Pb ages of 54.6 ± 0.9 Ma vs 54.6 ± 0.6 Ma, and no old inherited cores were identified based on significantly older dates. Our studies show zircon U-Pb ages of ~55 Ma for the gabbro/diorite and titanite U-Pb age of ~53 Ma for the gabbro, which are older than the New Caledonia CE-boninites (51–47 Ma). Therefore, the magmatic evolution of the New Caledonia ophiolite during subduction initiation has been reconstructed.