Geochemical Distinction Between Altered Oceanic Basalt- and Seafloor Sediment-Derived Fluids in the Mantle Source of Mafic Igneous Rocks in Southwestern Tianshan, Western China

Abstract

Abstract The role of subducting oceanic crust-derived fluids in generating mafic arc magmatism has been widely documented. However, the subducting oceanic crust is generally composed of basaltic igneous crust and seafloor sediment, which may give rise to different compositions of liquid phases causing metasomatism of the mantle wedge. Because of the similarity in enrichment of fluid-mobile incompatible elements in the two sources of subduction zone fluids, it has been a challenge to distinguish between them when studying the products of mafic arc magmatism. This difficulty is overcome by a combined study of whole-rock Li isotopes and zircon O isotopes in addition to whole-rock major-trace elements and Sr–Nd–Hf isotopes in Late Paleozoic mafic igneous rocks from southwestern Tianshan in western China. Zircon U–Pb dating yields consistent ages of 313 ± 3 Ma to 305 ± 1 Ma for magma crystallization. The mafic igneous rocks exhibit arc-like trace element distribution patterns and depleted whole-rock Nd–Hf isotopes but slightly high (87Sr/86Sr)i ratios of 0·7039– 0·7057. They also show positive zircon εHf(t) values and slightly higher zircon δ18O values of 5·2–7·6‰. There are covariations of whole-rock Sr isotopes with Th/La and Rb/Nb ratios, indicating a contribution from terrigenous sediment-derived fluids to their mantle source in addition to basaltic igneous crust-derived fluids. Based on the slightly higher zircon δ18O values but variably lower whole-rock δ7Li values of -0·8 to 3·5‰ for the target rocks than those of mantle respectively, both altered oceanic basalt- and terrigenous sediment-derived fluids are identified in the mantle source of these mafic igneous rocks. Model calculations for trace elements and Sr–Nd–Li isotopes further confirm that the geochemical compositions of these mafic igneous rocks can be explained by chemical reaction of depleted MORB mantle peridotite with the mixed fluids to generate ultramafic metasomatites at subarc depths. Therefore, chemical metasomatism of the mantle wedge is a key mechanism for the incorporation of crustal components into the source of arc-like mafic igneous rocks above oceanic subduction zones.