Petrogenesis of Mesozoic Li-, Cs-, and Ta-rich (LCT) pegmatites from the Neoproterozoic Jiangnan Orogenic Belt, South China: An alternative origin model for the LCT type pegmatite

Abstract

According to the classical petrogenesis model, Li-, Cs-, and Ta-rich (LCT) pegmatites juxtaposed in the same regional zonation are generally formed by unidirectional successive crystallization differentiation of the common parental granitic melt. However, this model cannot explain the origin of the Mesozoic LCT pegmatites from the East Jiangnan Orogenic Belt (JOB), South China. Herein, we studied the geology, zircon U–Pb ages, Hf isotope compositions, monazite U–Th–Pb ages, and Nd isotope compositions of three types of pegmatites in Renli, East JOB, South China to reveal their petrogenesis. The Renli pegmatites distributed in or around the Mufushan batholith, show mineralization zoning with distance from the batholith. The studied pegmatites were formed at the Early Cretaceous, with a decreasing trend from ca. 141 Ma for two-mica microcline pegmatite (type I) to 134 Ma for muscovite–albite–spodumene pegmatite (type III). The Renli pegmatites have coupled Hf–Nd isotope compositions and consistent Hf isotope compositions with the Mufushan granites. From the spatial–temporal, source and differentiation relationships between the Renli pegmatites and the Mufushan granites, we propose that type I pegmatites originates from biotite monzogranite while type II (muscovite–microcline–albite pegmatite) and type III pegmatites originate from the two-mica monzogranite and muscovite monzogranite, respectively, indicating a new model of parallel differentiations of triple granitic melts for the origin of LCT pegmatites in South China. The Renli pegmatites have a similar Hf isotope composition to the Neoproterozoic granitoids and the backarc sedimentary rocks of the Lengjiaxi Group formed during the Jiangnan orogeny. Combining with previous studies, we suggest that the Mesozoic Renli LCT pegmatites originated from reworking materials of the Neoproterozoic JOB caused by slab roll back of the paleo-Pacific Ocean plate during subduction.