Mafic-ultramafic rocks from the Fanjingshan region, southwestern margin of the Jiangnan orogenic belt: Ages, geochemical characteristics and tectonic setting

The Neoproterozoic mafic-ultramafic rocks are widely developed in the Fanjingshan region, southwestern margin of the Jiangnan orogen, the rocks are all belong to tholeiitic series and including pillow lava, mafic-ultramafic sills and hypabyssal intrusive, gabbro. The pillow lavas are characterized by rich in strong incompatible elements such as Rb, Ba, Th and U as well as light rare earth elements (LREE), deplete in high-field-strong-elements (HFSE) such as Nb and Ta, and lower epsilon(Nd)(t) values, different to the basalt formed in ocean ridge setting obviously, guess them formed by partial melting of enriched mantle in the tectonic setting of small ocean basin back arc-island arc related with subduction. The mafic-ultramafic sills are mainly composed of diabase and calc-pyroxene peridotite, there are abundant original calcite crystals in the ultramafic sills, indicating a extrusion (rift) tectonic setting. The gabbro may be a latest magmatic rock in the region, their SHRIMP zircon U-Pb age is 821 +/- 4Ma. The degrees of deplete in high-field-strong-elements and rich in light rare earth elements are depressing from pillow lava, mafic-ultramafic sills to gabbro, and the epsilon(Nd)(t) values from negative change to positive, show that the contribution of depleted mantle increase with time. Authors deduce that the gradation of Neoproterozoic magmatism in the Fanjingshan region is approximately as follow: pillow lava (similar to 840Ma)-> S-type granite (similar to 838Ma) -> calc-ultramafic sills -> mafic sills -> gabbro, the tectonic setting changed from subduction-collision to extended-rifting.