Mafic microgranular enclaves from the Tieshan batholith, eastern China: Implications for petrogenesis of magma associated with Fe-Cu skarn mineralization

Abstract

Mineralogy, geochemistry, and geochronology of the mafic microgranular enclaves (MMEs) and their host diorite from the Tieshan batholith, Edong district, eastern China, were studied to constrain their petrogenesis and relationship with Fe-Cu skarn mineralization. The MMEs are diorite in composition and characterized by mixed apatite morphologies (elongated thin needles and stubby prismatic form) and bladed biotite, as well as clots of host diorite and megacrystic alkali-feldspar xenocrysts. The MMEs have similar zircon U-Pb age as their host diorite (~139 Ma). Most of the major and trace elements of the MMEs show a linear trend on Harker diagrams and plot along the low-silica trend of the host diorite. They are also characterized by enrichment of LREE and large ion lithophile elements (LILE), but are depleted in high field strength elements (HFSE). The MMEs display similar initial Sr isotopes and εNd(t), and slightly lower εHf(t) compared to that of host diorite. These results indicate that the Tieshan batholith formed via partial melting of an enriched lithospheric mantle, which is believed to be in equilibrium with a garnet-bearing residue (>50 km), followed by contamination of low crustal materials and fractional crystallization, whereas the MMEs appear to be the products of magma mixing. The presence of Cu- and Ni-bearing minerals in the MMEs probably indicates upper mantle or lower crust accumulation of sulfide which may have played a key role in controlling the formation of the parental magma associated with the skarn-porphyry Cu deposit (148 to 135 Ma) from the Middle-Lower Yangtze River metallogenic belt (MLYRMB). In contrast, the thinner, normal crust (~30 km) at ~ 130 Ma, believed to be in equilibrium with a plagioclase-rich residue, was either unsuitable for producing Cu-bearing accumulations or failed to transfer the deep accumulations to shallower levels to form Cu-bearing skarn systems, resulting in Fe-only mineralization. This may imply that skarn-porphyry Cu deposits prefer to form under > 50 km crustal settings, whereas Fe skarns may preferentially occur under normal (~30 km) crustal thickness.