Heterogeneous crustal sources and recycling for silicic magmas in a Neoproterozoic continental arc in the western Yangtze block, South China

Identifying the source characteristics of silicic magmas (SiO2 >65 wt%) is significant for revealing the nature of the crust and recycling processes under a continental arc zone. Herein, we present a comprehensive dataset of zircon U-Pb-Hf-O isotopes and whole-rock major and trace elements as well as Nd isotopes for two groups of Neoproterozoic silicic volcanic magmas from the Mianning area in the western Yangtze block (South China), in order to place systematic constraints on the heterogeneous crustal sources as well as crustal reworking and recycling processes of a Neoproterozoic continental magmatic arc during the evolution of the supercontinent Rodinia. The studied dacites and rhyolites, dated at ca. 815−811 Ma, are generally characterized by high SiO2 (66.87−80.02 wt%) but low MgO (0.11−0.81 wt%), Mg# (8.94−30.0), Cr (0.50−4.54 ppm), and Ni (1.10−4.98 ppm) contents, enriched light rare earth elements (LREEs) and large ion lithophile elements (LILEs; e.g., Rb, K), and depleted high field strength elements (HFSEs; e.g., Nb, Ta, Ti, and P), implying a source with continental crust affinity. Considering the contrasting whole-rock Nd and zircon Hf-O isotopic characteristics, these silicic volcanic rocks can be divided into two groups that originated from heterogeneous crustal sources. Group 1 dacites display relatively depleted whole-rock Nd (εNd[t] = +0.51 to +1.65) and zircon Hf (εHf[t] = +1.80 to +8.86) isotopes as well as submantle zircon O isotopes (δ18O = 3.95‰−4.63‰), and therefore, they were likely formed by recent melting of juvenile mafic arc lower crust, which was supplemented by mantle wedge melts metasomatized by predominantly subducted slab melts. Group 2 dacites and rhyolites have negative whole-rock εNd(t) (−4.05 to −2.12), low and variable zircon εHf(t) (−2.84 to +3.44), and highly variable zircon δ18O values (4.50‰−9.90‰), which could be derived from a relatively ancient and enriched crustal source heated by melts from juvenile lower crust or metasomatized mantle. We propose that the silicic volcanics in this study represent the simultaneous crustal melting of different magmatic reservoirs under a continental arc zone, which are responsible for the petrological and geochemical diversities of these Neoproterozoic silicic magmas. In combination with a compilation of previous data from nearby Neoproterozoic silicic igneous rocks, the diverse silicic magmas showing distinct Nd-Hf-O isotopes can be regarded as the products of primarily crustal reworking and recycling from heterogeneous crust-derived components (i.e., juvenile and ancient/evolved crust sources), and therefore, they preserve comprehensive information about crustal differentiation and enrichment in a continental arc setting.