羌塘地体南缘侏罗纪-白垩纪花岗岩体低温热年代学及隆升和剥蚀过程研究

The pre-Cenozoic crustal thickness and elevation of the blocks within Tibet are essential to understand the uplift and expanding history of the Tibetan Plateau. The thick crust and high elevation of the Qiangtang block were achieved by crustal shortening during the Early Cretaceous, and stable isotope-based paleoaltimetry suggests that much of Qiangtang attained its modern elevation by the late Eocene. However, it is unclear that whether or not this high elevation can date back to the Early Cretaceous, a period with substantial crustal thickening. Since quantitative paleoaltimeters are incapable of reconstructing the pre-Cenozoic elevations, it is logical to estimate the paleoelevation by quantitative calculation of the pre-Cenozoic exhumation of the Qiangtang block. We focus on four Jurassic-Cretaceous granite plutons within the south Qiangtang magmatic arc that is sensitive to Andean-type subduction and continental collision. Low-temperature thermochronometers (AFT, AHe, ZFT and ZHe) and thermal modeling, in combination with Al-in-hornblende barometer are employed to reveal detailed thermal histories, and to constrain the timing and rates of exhumation. The thermal histories, the exhumation processes of the plutons and their underlying link to the Bangong-Nujiang Tethyan subduction and the Lhasa-Qiangtang/Indo-Asian collisions will shed new light on the process and mechanism of the Tibetan Plateau uplift since the Late Mesozoic.

青藏高原前新生代各地体地壳厚度和海拔高度是研究高原隆升和扩展历史的重要边界条件。羌塘地体早白垩世期间发生了显著的地壳缩短增厚并伴随着地形隆起，稳定同位素古高度计揭示了始新世羌塘地体已隆升至现今的海拔高度，但该高海拔特征能否回溯至白垩纪地壳增厚的时代？目前定量古高度计还不能有效重建新生代之前的古高度，因此定量研究晚中生代以来至始新世期间羌塘地体的剥蚀过程，是反演前新生代古高度演化过程的关键环节。本项目拟选取对洋-陆俯冲及陆-陆碰撞响应敏感的羌塘南缘陆缘弧内花岗岩体为研究对象，通过对侏罗纪-白垩纪4个岩体花岗岩锆石和磷灰石矿物对裂变径迹和(U-Th)/He年代学研究，利用花岗岩Al-in-hornblende压力计估算岩体定位压力和深度，结合热史模拟刻画T-t轨迹，计算剥蚀过程中的时间和速率，探讨其与班-怒洋俯冲消减、拉萨-羌塘和印度-亚洲碰撞的关系，深入认识青藏高原隆升过程与机制。

为探讨青藏高原羌塘地体南缘冷却剥露历史，揭示控制其T-t演化过程及地球动力学背景，本项目对羌塘地体南缘及相邻的班公湖-怒江缝合带（班怒带）出露的中、新生代花岗岩进行低温热年代学研究，通过磷灰石裂变径迹（AFT）和U-Th-He（AHe）和锆石U-Th-He（ZHe）等低温热年代学方法，结合黑云母Ar-Ar热年代学数据，获得了四个花岗岩体共计9个的低温热年代学年龄数据。并花岗岩中角闪石Al-in-hornblende温压计，对岩体就位深度进行估算，获得了岩体就位以来的剥蚀量及平均剥蚀速率。HeFTy热史模拟显示改则地区热拉措岩体早白垩世晚期（~100 Ma）快速冷却事件及随后至今（95 Ma至今）的缓慢冷却。东巧康日岩体则记录了晚白垩世（85-70 Ma）快速冷却事件及随后的缓慢冷却过程（65-0 Ma）。在班公湖地区，日土岩体记录了古新世-始新世早期（60-45 Ma）快速冷却事件及随后的缓慢冷却过程。在滇西贡山地区，位于怒江东岸崇山剪切带以北的路卡岩体则经历了32-30 Ma和6-4 Ma两期快速冷却事件。结合本项目岩浆岩成因及地球动力学背景研究，推测早白垩世晚期快速冷却事件受控于该时期的拉萨-羌塘碰撞事件；晚白垩世快速冷却受控于拉萨-羌塘碰撞后岩石圈及下地壳的拆沉所引起的重力反弹，或者拉萨地体向羌塘地体之下的俯冲导致的地壳增厚过程；班公湖地区日土岩体所记录的古新世-早始新世快速冷却事件受控于新生代印度-亚洲大陆碰撞过程；滇西贡山地区路卡岩体32-30 Ma快速冷却可能与崇山剪切带的活动相关，也可能是岩体就位以来的自然冷却过程所控制，而6-4 Ma快速冷却过程可能受控于怒江及其支流的快速下切作用。青藏高原不同地区冷却和剥露历史的显著差异需要更多的研究以揭示其控制因素。