Pb Isotopic Geochemistry Of Ultra High Pressure Metamorphic Rocks And Mafic Igneous Rocks From The Dabie-Sulu Orogen

Abundant ultra-high pressure metamorphic (UHPM) rocks, such as coesite-bearing eclogites and gneisses, occur in the Dabie-Sulu orogen. Based on Pb isotope geochemical study of the UHPM rocks exposed on the surface, the South and North Dabie zones are considered to be exhumed slices of the subducted upper and lower crust, respectively. This thesis will focus on the Pb isotope geochemistry of three kinds of rocks. First, the Chinese Continental Scientific Drilling project on the Su-Lu UHPM belt provides an opportunity to investigate the Pb isotopic composition of the UHPM belt along vertical directions, providing constraints on exhumation mechanism of the UHPM rocks. Second, the Chengchong and Shanqi gabbros in the North Huaiyang area are not involved in continental subduction and related UHP metamorphism, thus provide an opportunity to investigate the geochemical characters of protoliths of the UHPM rocks. Third, Mesozoic mafic intrusive rocks occur in Dabie-Sulu orogen widely, carrying geochemical information from their mantle source. Therefore, they will help us understand the geochemical characteristics of the lithospheric mantle underneath the Dabie orogen and test whether deeply subducted continental crust can be recycled into the upper mantle and the influence of the recycling on the composition of upper mantle.1 Initial Pb composition of the Neoproterozoic mafic intrusive rocks in the Dabie orogenTo understand the variation of Pb isotopic composition of UHPM eclgite, it is important to know Pb isotopic composition of the Neoproterozoic mafic igneous rocks in the Dabie orogen. Two Neoproterozoic mafic instrusions in the North Huaiyang, the Chengchong and Shanqi gabbro, were not involved in deep subduction. Initial Pb composition of the Chengchong and Shanqi gabbros (²⁰⁶Pb/²⁰⁴Pb_t=750Ma= 16.68～17.09, ²⁰⁷Pb/²⁰⁴Pb_t=750Ma= 15.40～15.45, ²⁰⁸Pb/²⁰⁴Pb_t=750Ma = 37.72～37.85) are close or a little above the Mantle line in Zartman and Doe' Pb isotope evolution curve(Zartman and Doe, 1981). This suggests that their source was mainly from mantle and contaminated by crustal material. The high ²⁰⁸Pb/²⁰⁴Pb character of the...