关键词: 龟山岩组, 商丹断裂带, 高Ti型玄武岩, 低Ti型玄武岩, OIB型玄武岩, E-MORB型玄武岩

Abstract: In this report, we present the geochemical characteristics and Sr-Nd isotopic compositions of Neoproterozoic metamorphic basalts from the Guishan complex in the Xinyang area, south of the Shang-Dan Fault. All basalt samples analyzed belonged to the sub-alkaline tholeiitic basaltic series and could be classified into high-Ti and low-Ti types. The low-Ti type basalts had higher Mg content, low degrees of LREE/HREE fractionation and enrichment of incompatible elements, and relatively enriched Sr-Nd isotopes, showing E-MORB characteristics. The high-Ti type basalts had higher Fe content, high degrees of LREE/HREE fractionation and enrichment of incompatible elements, and relatively depleted Sr-Nd isotopes—compatible with OIB features. The low-Ti type basalts might result from the partial melting of lithospheric mantle caused by a mantle plume, with some crustal contamination. The high-Ti type basalts, on the other hand, might be generated from the partial melting of a mantle plume with minor crustal influence. The geochemical discriminant diagrams of tectonic setting show that the metamorphic basalts erupted in the extensional mantle-plume environment, possibly as part of the Meso-Neoproterozoic magmatic activities in the extensional environment of the Shang-Dan Fault.

Key words: Guishan complex, Shang-Dan Fault, high-Ti basalt, low-Ti basalt, OIB-type basalt, E-MORB-type basalt