Middle Tonian calc-alkaline picrites, basalts, and basaltic andesites from the Jiangnan Orogen: Evidence for rear-arc magmatism

Abstract

The chemical composition of a basalt is a critical indicator of mantle thermal condition and tectonic setting. The Yangtze Block, which is a part of the Rodinia supercontinent, formed during the Neoproterozoic era presents an enigmatic thermal state of the mantle and tectonic evolution. We compiled a large database of petrogeochemical data on the Middle Tonian volcanic rocks covering the entire Jiangnan Orogen with a view to evaluate the evolution of the magma compute the composition of the primary magma, estimate the mantle potential temperature (Tp), and distinguish the tectonic setting. Calc-alkaline picrites, basalts, and basaltic andesites (type A) from ca. 845–825 Ma and tholeiitic basalts (type B) from ca. 860–830 Ma were both identified as interbedded layers accompanied with the Middle Tonian sedimentary sequences (the Fanjingshan Group and its equivalents) and geographically intersecting with each other along the Jiangnan Orogen. Geochemical and isotopic features of type A volcanic rocks indicate that they originated from an enriched mantle source. Both slab-derived fluids and fractionation of olivine and clinopyroxene played important roles during magma generation and evolution. The calculated composition of the primary magma is picritic and the estimated average Tp is 1404 °C. This temperature is consistent with the melting of a mantle wedge due to a rapid slab roll-back process, but it is lower by 166 °C than that produced by a hot mantle plume. Considering the occurrence of the continental arc basalts (CAB)-like type A volcanic rocks and the back-arc basin basalts (BABB)-like type B basalts along with contemporaneous sedimentary sequences with the bidirectional detritus sources in the Jiangnan Orogen, we hereby speculate a rear-arc setting for type A volcanic rocks and a back-arc setting for Type B basalts.