Magma differentiation and recharge in the petrogenesis of early paleozoic mafic intrusives in the Qilian orogen, northwestern China

Abstract

An integrated study of zircon UPb ages and LuHf isotopes, whole-rock major-trace elements and Sr-Nd-Hf isotopes as well as amphibole and plagioclase major-trace elements is presented for early Paleozoic mafic intrusives from the Heishishan area in the Qilian orogen, northwestern China. The results provide not only constraints on the origin of their mantle sources but also insights into the effects of magma differeniation and recharge on their compositional variation. Zircon UPb dating yields consistent ages of 449 ± 2 Ma and 449 ± 3 Ma, respectively, for hornblende gabbro and diorite. These mafic intrusives show arc-like trace element distribution patterns and weakly enriched Sr-Nd-Hf isotope compositions with whole-rock (87Sr/86Sr)i ratios of 0.7040 to 0.7063, εNd(t) values of −2.3 to −0.3 and εHf(t) values of 1.6 to 4.9, and zircon εHf(t) values of 0.2 to 5.9. These geochemical features indicate their origination from mantle sources that were generated through metasomatism by subducting Proto-Tethyan oceanic crust-derived fluids. Some samples exhibit high Th/Nb and (La/Yb)N ratios, suggesting that the metasomatic agents would be composed of seafloor sediment-derived hydrous melts and oceanic igneous crust-derived aqueous solutions. The consequent metasomatites in the mantle wedge would be lithochemically fertile and geochemically enriched and therefore are susceptible to partial melting for mafic magmatism. Amphibole and plagioclase in the studied igneous rocks show complex textural and compositional zonings, recording four stages of mineral crystallization during magma evolution. Combining the whole-rock and mineral geochemical characteristics together, we suggest that different batches of mafic melts would be produced by partial melting of the metasomatic mantle sources, experiencing variable degrees of magma differentiation and recharge prior to emplacement at the subsurface. Therefore, in addition to the nature of crust-mantle interaction at mantle depths, the processes of magma evolution and recharge at crustal depths would have also played an important role in the petrogenesis of arc-type mafic igneous rocks above the oceanic subduction zone.