| Qinling orogenic belt had experienced the subduction of the Shangdan oceanic crust in early Paleozoic times, and had a full collision between NQB and SCB in Indosinian period, a ginat Orogenic Belt was evolved on the northern side of NQB-The Shangdan Suture, and a minor part of rapakivi graintes has generated right on the Shangdan suture. Zircons in sediments can not only reflect the tectonic settings of the basin, but can trace back to the provenance by plotting the distribution of the difference between the measured crystallization ages of individual zircon grains present in the sediment and the depositional age of the sediment. The sediments are widespread in SQB which provides a nature ground to analyze its detrital zircons based on different spatial and temporal distribution and furthermore to discuss the timing of the closure of Shangdan ocean. Meanwhile, by using Pb isotpoic analysis, not only can we trace back its end-member magma compositions, but can reveal its function on indicating the tectonic setting of the Qinling Orogenic Belt. We analyzed the north, center and south part of SQB by using LA-ICPMS, the results indicated that all the analyzed data has a peak age of 420-490Ma, we are able to link the data to the magma activities in NQB, and conclued that the Shangdan ocean had closed before Mid-Devonian.The analysis on Shahewan rapakivi shows that the host rocks and the xenoliths are in the same range of the Pb isotope ratio. Feldspar in host rocks show the range of 207Pb/204Pb 15.428?15.536, mean value 15.503,206Pb/204Pb 17.450?17.604, mean value 17.533. Xenoliths 206pb/204pb range between 17.448?17.633, mean value 17.525,207Pb/204Pbl5.382?15.538, mean value 15.461. The results indicated that the host rock and xenoliths might originate from a same source and furthermore the intruction of Shahewan rapakivi might not be connnetced with lithosphere dalamination and might not gone into the stetch tectionc back ground. |
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