Geochronology and geochemistry of volcanic rocks from the Shaojiwa Formation and Xingzi Group, Lushan area, SE China: Implications for Neoproterozoic back-arc basin in the Yangtze Block

Abstract

We present a systematic SHRIMP zircon U–Pb geochronological, geochemical and Nd isotopic study on the Shaojiwa basaltic–rhyolitic suite and Xingzi amphibolites in Lushan area, Yangtze Block. The protoliths of the amphibolites are low potassium tholeiitic basalts. Rhyolite and amphibolite yield SHRIMP zircon U–Pb ages of 828±6Ma and 811±12Ma respectively. These ages suggest that the Xingzi Group formed in the Neoproterozoic rather than Paleoproterozoic as previously suggested. The basalts from the Shaojiwa Formation have slightly positive ɛNd(t) values (0 to +0.7) and exhibit significant enrichment in LILE and LREE and strong depletion in HFSE, similar to arc-like volcanics. These geochemical and isotopic signatures suggest that the basalts might have been derived from a slightly depleted mantle source metasomatized by fluid/melt released from a newly subducted slab with high degree of mantle melting. The rhyolites exhibit similar Nd isotopic ratios as the associated basalts and show subparallel REE patterns which can be explained by fractional crystallization of the basalt magma. In contrast, the amphibolites from the Xingzi Group have relatively high ɛNd(t) values (+3.4 to +4.0) and are characterized by nearly flat REE distributions with slight LREE depletion and generally similar contents of moderately incompatible elements to those of the average N-MORB, but with relative enrichment in LILE, similar to the Mariana back-arc basin basalt (BABB). The precursor magma of the amphibolites might have been originated from an N-MORB source modified by slab-derived fluids with relatively low degree of mantle melting. The geochemical affinity to arc (or intra-continental BABB) of the Shaojiwa basalts and intra-oceanic BABB of the Xingzi amphibolites appears to support the development of a Neoproterozoic incipient to mature back-arc basin in the southeastern Yangtze Block during the 828±6 to 811±12Ma period in response to the rollback of subducted slab. Widespread Neoproterozoic igneous rocks in the Jiangnan Orogen with ages older than 800Ma most likely occurred in an arc-back-arc environment