秦岭造山带的差异隆升特征——花岗岩^(40)Ar/^(39)Ar年代学研究的证据

A differential uplifting of Qinling orogeny belt evidences from^(40)Ar/^(39)Ar thermochronology of granites

分别出露于东、西秦岭的曹坪和沙河湾岩体、老君山和秦岭梁岩体 ,是秦岭全面碰撞后于三叠纪末 (T3 )侵入的花岗岩体 ,其冷却历史记录了秦岭陆内造山阶段的初期隆升过程。通过对角闪石、黑云母和钾长石的40 Ar/ 3 9Ar年龄测定 ,以及对钾长石40 Ar/ 3 9Ar年龄谱进行的多重扩散域模拟计算 ,发现东、西秦岭经历了完全不同的冷却历史 :从晚三叠世末至早侏罗世 ,东、西秦岭同时由 5 0 0℃开始快速冷却 ,之后东秦岭经过一个短暂的稳定期 (约 2 0Ma)后又持续快速冷却 ,至中侏罗世末即已通过 15 0℃等温线 (约地表下 3～ 5km) ;而西秦岭在早侏罗世至晚白垩世初的近 10 0Ma中一直处于稳定平缓的状态 ,至晚白垩世中期才快速冷却至 15 0℃。这种不同的冷却历史可能反映了东、西秦岭的差异隆升过程。

Caoping and Shahewan granite plutons, and Laojunshan and Qinlingliang granite plutons occur on Shangdan suture zone at east and west segments of Qinling orogenic belt, respectively. They emplaced simultaneously at the end of the Triassic when the Qinling orogenic belt had finished its collision and the intraplate deformation commenced. Therefore, the granite plutons had experienced the deformation and recorded the uplifting process in their cooling histories. The^(40)Ar/^(39)Ar dating on hornblendes, biotites and alkali feldspars, combining the multi-domain diffusion modeling on alkali feldspars show that the east and west segments of Qinling orogeny belt experienced much different cooling histories: from the end of Late Triassic to the Early Jurassic, both segments simultaneously cooled down rapidly from 500 ℃; then the east Qinling belt experienced a short stationary period (nearly 20 Ma), and then steadily cooled down again so rapidly that the temperature already dropped down across the isotherm of 150 ℃ (nearly 3～5 km below the surface) at the end of Mid-Jurassic; whereas the west Qinling belt was in a stable status around 300 ℃ for about 100 million years from the Early Jurassic to the Late Cretaceous, and cooled down rapidly to 150 ℃ at the middle of the Late Cretaceous. These different cooling histories may reflect the differential uplifting of Qinling orogeny belt.