Magma mixing for the genesis of Neoproterozoic Mopanshan granitoids in the western Yangtze Block, South China

Abstract

Crust- and/or mantle-derived igneous rocks in the Neoproterozoic continental magmatic arc along the western Yangtze Block (South China) have been extensively studied, but the crust-mantle interaction that might have been involved in the formation of these rocks was poorly understood. In this paper, we present an integrated study on petrology, whole-rock compositions, in-situ plagioclase Sr and zircon U-Pb-Hf isotopes for granodiorites and monzogranites from the Neoproterozoic Mopanshan pluton in the western Yangtze Block. Our objective is to provide vital constraint on the magma mixing process in the formation of these rocks. The new LA-ICP-MS zircon U-Pb dating results reveal that both granodiorites and monzogranites of the Mopanshan pluton were coeval and formed at ca. 820 Ma. They are calc-alkaline to high-K calc-alkaline (SiO2 = 66.06–70.30 wt%, K2O = 2.24–3.34 wt%) and metaluminous to slightly peraluminous (A/CNK = 0.99–1.10) rocks, and characterized by middle-upper crustal trace element patterns (e.g., enrichment in Rb, K, Th, U, and Pb, and depletion in Nb, Ta, Sr, and Ti). The magma mixing process is supported by following ample evidence, including (1) the disequilibrium mineral textures and an abruptly increased An values (up to 60) as well as significant decrease of (87Sr/86Sr)i ratios from core to rim in plagioclase crystals, (2) higher Mg# values (45–50 > 40) than those of experimental melts from basalts at the same silica contents, (3) positive and negative whole-rock εNd(t) (–0.72 to + 1.01) values as well as variable whole-rock (87Sr/86Sr)i (0.702778–0.705404) and zircon εHf(t) (+1.76 to + 7.72) values. The special geochemical characteristics and element modeling support that the Mopanshan pluton was generated by magma mixing of ancient crust-derived melts and relatively mafic melts from metasomatized mantle source. The adakitic signatures observed in the Mopanshan pluton were attributed to subsequent hornblende-dominant intra-crustal fractional crystallization, rather than derived from a thickened lower crust source. In combination with regional geology and our compilation for Nd-Hf isotopes of Neoproterozoic igneous rocks, the Mopanshan pluton is magmatic response of intensive crust-mantle interaction induced by underplating of voluminous mantle-derived magma in a back-arc extension setting. This research highlights that the crust-mantle interaction is significant for the petrological and geochemical diversity of Neoproterozoic granitoids from the western Yangtze Block.