RECOGNITION OF PROTEROZOIC RAPAKIVI GRANITES FROM THE PALEOZOIC NORTH QAIDAM OROGEN, CENTRAL CHINA AND ITS IMPLICATIONS

Rapakivi granites of the middle Proterozoic occur on all continents，especially on the northern hemisphere. They are the most voluminous silicic magmatism on the Earth and are considered to be emplaced in an extensional tectonic setting not directly related to orogenic process. The Yingfeng Proterozoic rapakivi granites have been recognized in the Paleozoic North Qaidam orogen, western segment of the China Central Orogenic System (CCOS). SHRIMP zircon U–Pb dating for these granites yielded ages of 1778 ± 17 and 1778 ± 12 Ma. They exhibit typical rapakivi textures and are of typical A-type granite, characterized by high Ga/Al ratios, Na₂O + K₂O and rare earth elements (except Eu) contents, but low MgO, CaO and Sr contents. Whole-rock ε_Nd(t) values of the granites range from −6.09 to −5.74 with Nd model ages of 2762 to 2733 Ma, and their zircon ε_Hf(t) values are from−8.3 to −5.2 with two-stage Hf model ages of 2944 to 2800 Ma, suggesting that these rocks were derived from old continental crust. These rapakivi granites are very close to typical Proterozoic (~1700 Ma) rapakivi granites on the North China Craton (NCC) in ages, petrography and geochemistry. It indicates that the Yingfeng Proterozoic rapakivi granites, together with a part of the NCC were involved into the Paleozoic North Qaidam orogen. Therefore, the Yingfeng Proterozoic rapakivi pluton, similar to the other Proterozoic rapakivi plutons, was emplaced in an anorogenic extensional setting, and part of the basement of the North Qaidam orogen in the western CCOS is similar to that of the NCC or was probably derived from it.