Studies on Geochemistry, Zircon U-Pb Dating and Lu-Hf Isotope Composition of the Tongchang Diorites, Shaanxi Province

The paper presents geochemistry, Zircon U-Pb geochronolgoy and Lu-Hf isotope of Tongchang diorites, which has a close relationship with the Tongchang Copper Deposit in Shaanxi province. Geochemical studies have shown that the diorites belong to low K calc-alkali rock series, which are characterized by higher Al₂O₃, Na₂O, MgO content and lower K₂O content. The rock is enriched in large ion lithophile elements (LILE) such as Rb, Ba, Th, U, K, but comparatively depleted in high field strength elements(HFSE) such as Ti, Nb, Ta and HREE. Meanwhile, the content of Sr is low, signifying the characteristics of crustal genesis magmatic rock. According to Zircon features and U-Pb isotopic dating, the rock is formed in a period of 843.7±5.5 Ma, which indicates that it is the product of Neoproterozoic magmatism. Lu-Hf isotope study demonstrates that its εHf(t) is between -5.96 and -30.17, and the value of tDM changes greatly from 1816 Ma to 3344 Ma. Moreover, the existence of five groups of age peak (i.e. 1.81-1.95, 2.13-2.65, 2.72-2.78, 2.97-3.09 and ~3.34 Ga ) indicates that the formation of Tongchang diorites is the result of partial melting of ancient crustal rocks in the breaking of Rodinia supercontinent, and these important crustal accretion events of the Yangtze Block occured from Neoarchean to Paleoproterozoic. The emergence of 2.72-2.78 Ga model age shows that partial basic materials from Yudongzi Group may have joined the formation of Tongchang diorites magmatism.