Early Neoproterozoic (∼840 Ma) arc magmatism: Geochronological and geochemical constraints on the metabasites in the Central Jiangnan Orogen

Abstract

The Jiangnan Orogen is traditionally considered the suture zone, along which the Yangtze and Cathaysia blocks collided to form the united South China Block. There are still major controversies surrounding the pre-collisional tectonic pattern and amalgamation process along the Orogen and the relationship of the united South China Block (SCB) with the Rodinia. Our recent field investigations identified the presence of the volcano-sedimentary unit of the Lengjiaxi Group at Jianxichong, with the thickness of over 1700m. A set of new zircon U–Pb geochronological, whole-rock elemental and Sr–Nd isotopic data for the metabasites have been obtained and presented in this paper. The layered diabase and tuffaceous samples from the Jianxichong volcano-sedimentary unit yielded the weighted mean ages of 837±5Ma and 835±6Ma, respectively. The U–Pb ages of detrital zircons from the phyllite interlayer form a main age-cluster at the peak age of 870Ma with the youngest apparent age of 845±23Ma. This indicates that the metabasites in the Jianxichong volcano-sedimentary unit formed at ∼845–835Ma rather than in the previously-thought Neoarchean in the Central Jiangnan Orogen. The metabasitic samples have SiO2 ranging from 48.78wt.% to 53.72wt.% and MgO from 7.71wt.% to 12.74wt.% with mg-number of 63–71, and hence can be classified as tholeiitic basalt and basaltic andesite. They demonstrate a sub-parallel and right-sloping chondrite-normalized REE pattern with slightly negative Eu anomalies and sub-parallel spiky PM-normalized patterns with enrichment in LILEs, depletion in HFSEs and P–Ti negative anomalies, similar to those of arc volcanic rocks. Their ɛNd(t) values range from −1.64 to −0.37 and initial 87Sr/86Sr values from 0.7046 to 0.7081, respectively. Such crust-like geochemical characteristics might have been inherited from a sub-arc source modified by fluid/melt released from the subduction sedimentary component. In conjunction with available data and geological observations, we herein propose the development of the early Neoproterozoic (∼840Ma) continental arc-basin system along the Central Jiangnan Orogen in response to an arc-continent accretionary assembly along the Rodinia exterior.