祁连山西段玉石沟地区上二叠统砂岩碎屑锆石年代学及其地质意义

玉石沟地区位于青藏高原东北缘,大地构造属于北祁连造山带南缘,其石炭纪—三叠纪是上叠盆地发育时期,表现为浅海相、海陆交互相至陆相稳定型沉积建造。对玉石沟北部紫红色粗砂岩样品进行LA-ICP-MS碎屑锆石U-Pb测年,其年龄结果主要分布在4个区间:260～350 Ma(峰值314 Ma)、400～500 Ma(峰值445 Ma)、1 700～2 000 Ma、2 200～2 600 Ma,800～1 000 Ma锆石数仅有2颗,另有1颗锆石为2 056 Ma。锆石CL图像显示:260～350 Ma锆石中既有变质锆石也有岩浆锆石,可能响应南祁连地区石炭纪—早二叠世的热事件。400～500 Ma锆石以岩浆锆石为主,表明北祁连造山带和中祁连地块广泛发育的弧岩浆岩和同碰撞花岗岩提供了物源。1 700～2 000 Ma和2 200～2 600 Ma锆石主要为变质锆石,反映了祁连地块基底变质岩的年龄信息,推测来源于基底变质岩的剥露。800～1 000 Ma年龄区间的锆石数量稀少,可能反映新元古代侵入体在该组沉积时期尚未大规模剥露。砂岩中最年轻的锆石年龄为(289±2)Ma,限定了其沉积时代的下限为早二叠世,结合实测地层剖面上的岩石组合和层序变化,将之归属于上二叠统红泉组。碎屑锆石年龄结构表明玉石沟地区红泉组兼具北祁连造山带和中—南祁连地块的年龄信息,红泉组沉积物可能具有南、北两个物源区。

Detrital Zircon Geochronology and Its Geological Significance of Upper Permian Sandstones in Yushigou Area of Western Qilian Mountains,China

Yushigou area is located in the northeastern margin of the Qinghai-Tibetan plateau, and lies tectonically on the southern margin of the North Qilian orogenic belt. We conducted LA-ICP-MS detrital zircon U-Pb dating on the red sandstones from Yushigou. The results yielded four age groups at 260-350 Ma (peak at 314 Ma), 400-500 Ma (peak at 445 Ma), 1,700-2,000 Ma, and 2,200-2,600 Ma. Besides, two zircon grains were dated to be 800-1,000 Ma and another zircon to be 2,056 Ma.Zircons of 260-350 Ma show mainly magmatic (and minor metamorphic) features in CL imaging, reflecting the occurrence of Carboniferous to Early Permian magmatic-metamorphic events. Zircons of 400-500 Ma have mainly a magmatic origin, suggesting that the detrital provenance of the Yushigou sandstones were from arc magmatic rocks and syn-collisional felsic rocks. Zircons of 1,700-2,000 Ma and 2,200-2,600 Ma are dominantly metamorphic with old age information, indicating that the highly metamorphosed basement rocks were the detrital provenance of the sandstones. The few 800-1,000 Ma zircons indicate that the Neoproterozoic batholith had not been significantly exhumed. The youngest detrital zircon age from these sandstones (289±2 Ma) constrains the maximum deposition age of the Yaogou Group to the Early Permian. Comparing the rock features along the geological section, the strata likely belong to the Upper Permian Hongquan Formation. The broad age spectrum reflects the tectono-magmatic events in both the North Qilian orogenic belt and the South-Central Qilian block, and indicates that the Hongquan Formation has two detrital provenances.