Abstract

Feldspar separates of 30 granitoid intrusions ranging in age from Neoproterozoic to Mesozoic were analyzed for Pb isotopic compositions, together with associated Precambrian basement whole rocks from the North and South Qinling orogenic belts. The two belts constitute the southern active margin of the North China craton and the northern passive margin of the Yangtze craton, respectively. Proterozoic basement rocks from the North and South Qinling belts are clearly distinguished in terms of both present-day Pb isotopes and calculated initials at the time of granite magmatism, with the former having considerably more radiogenic Pb. Granitoids of the South Qinling belt, irrespective of age and specific granitoid rock type, show a fairly homogeneous Pb isotopic composition that is similar to that of the South Qinling basement. In contrast, in the North Qinling belt the Pb isotopic composition of granitoids is correlated with their age: Neoproterozoic and early Paleozoic granitoids have radiogenic Pb isotopic compositions like those of the North Qinling Proterozoic basement rocks, but late Paleozoic and Mesozoic granitoids show significantly less radiogenic Pb isotopic compositions, similar to those of the South Qinling basement rocks. The unusual change is interpreted to reflect underthrusting of the South Qinling belt beneath the North Qinling belt at ca. 360 Ma. The process resulted in a change of the source for younger North Qinling granitoids from the radiogenic North Qinling to the less radiogenic South Qinling basement rocks. Thus, the Yangtze craton accreted to the North China craton no later than 360 Ma.

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