Zircon SHRIMP U-Pb dating of granites from Dulan and the chronological framework of the North Qaidam UHP belt, NW China

Abstract

Zircon SHRIMP dating of granites from Dulan, east segment of North Qaidam UHP belt shows that they are 406.6±3.5 Ma for Yematan-E, 407.3±4.3 and 397±6 Ma for Balijiehatan-W, 404.5±4.0 and 397.0±3.7 Ma for Shuiwenzhan-N, 380.5±5.0 Ma for Shuiwenzhan-S, 382.5±3.6 and 372.5±2.8 Ma for Chachagongma. These granites from Dulan represent the products of the third and fourth periods of Paleozoic magmatism in North Qaidam. Geochemically, the granitoids with metalumious to weak peratuminous are quartz diorite, granodiorite, and granite in composition and mainly belong to calc-alkaline series, a few samples to calc or alkali-calc series. The third period of granites is a rock association of granodiorite + granite, with initial 87Sr/86Sr ratios from 0.7082 to 0.7110 and T2DM model ages from 1.41–1.90 Ga; and the fourth period of granites is a rock association of quartz diorite+granodiorite+granite, with initial 87Sr/86Sr ratios from 0.7072 to 0.7091 and T2DM model ages from 1.07–1.38 Ga. Therefore, the third period of granites has higher initial 87Sr/86Sr ratios and T2DM model ages. On the contrary, the fourth period of granites has Nd(t) values from 0.6 to 3.0, higher than that of the third granite with Nd(t) values 3.2 to 9.3. Thus, the data comparison indicates that the third granites may derive from Paleo-proterzoic continental crust with mantle material whereas the fourth granites may derive from the Meso-proterzoic basalt crust with continental material. Combined with regional geology, we thought that the third granites were formed relative to plate exhumation and the fourth granites to delamination of the lithospheric mantle.