巴基斯坦Naran榴辉岩变质作用及其对喜马拉雅西构造结抬升历史的制约

The uplift history of High pressure-Ultra- High pressure terran is an important question of subduction geodynamics, and the P-T-t path of eclogites can reveal the process of continental subduction and exhumation accurately. The newly found Pakistan Naran eclogites are near the Main Mantlt Thrust, and they have two stages of symplectite, clinopyroxene + Plagioclase and Amphiblole + Plagioclase, which indicate granulite facies and amphibolite facies added on eclogite facites after peak metamorphism. This may be different from previous model of quick exhumation. A comprehensive study of petrology, metamorphism and geochology from the Naran eclogites will be carried out in this project. Based on petrology and mineral chemistry we will calculate the P-T condition of metamorphic stages using phase equilibrium and geother-mobarometer. Zircon, Rutite, Mica and other monomineral dating by precise methods are processed in order to obtain metamorphic age of multi-exhumation stage as well as protolith age, then an accurate P-T-t path is established. The protolith property of Naran eclogites will be constrained by bulk main and minor element and Sr-Nd isotope. According to compare the evolution history of Naran eclogite with other eclogite from Western Himalaya Syntaxis, this project will reveal the different exhumation and uplift history of Western Himalaya Syntaxis.

高压-超高压地体的抬升历史是俯冲带动力学的重要科学问题，榴辉岩P-T-t轨迹研究能准确反映陆岩石圈俯冲和折返。巴基斯坦新发现的Naran榴辉岩位于喜马拉雅西构造结，靠近主地幔断裂，岩相学观察表明具有单斜辉石+斜长石和角闪石+斜长石两阶段后成合晶，这表明榴辉岩峰期变质之后可能有麻粒岩相和角闪岩相叠加，这与前人在西构造结榴辉岩获得的快速折返模式可能不同。本项目拟综合变质作用、年代学和地球化学三方面研究Naran榴辉岩。在岩相学和矿物原位微区化学的基础上运用相平衡和地质温压计计算各个变质阶段的P-T条件。通过对锆石、金红石和云母等单矿物精确定年，厘定原岩时代和多阶段折返的变质-冷却年龄，建立准确的P-T-t。通过全岩主微量、Sr-Nd同位素限定Naran榴辉岩的原岩属性。通过对比Naran榴辉岩和西构造结其他榴辉岩演化历史，揭示喜马拉雅西构造结差异折返及抬升历史。

印度-亚洲大陆碰撞发生在大约50Ma以前的古近纪。进行了数十年的研究西喜马拉雅地区发生的隆升历史和折返机制仍存在争议。我们在著名的Upper Kaghan Valley西南面约30公里处的Naran发现了一种新型的榴辉岩。Naran榴辉岩的相平衡计算记录了710-770°C和2.2-2.8 GPa的峰值变质条件。峰期变质之后发生了两阶段的折返，第一阶段在高压麻粒岩相条件下，第二阶段发生在角闪岩相条件下。锆石SIMS U-Pb年代学研究得出的变质年龄大约为46Ma，这个年龄限制沿印度板块西北缘俯冲碰撞的峰值变质时代。金红石SIMS U-Pb定年结果表明冷却年龄大约为26Ma，这代表角闪石相阶段变质年龄（PT条件为550-630°C和0.5-0.8 GPa）。印度岩石圈板块俯冲过程中发生在Naran构造岩片的折返速率(约3.4mm/yr)远低于Upper Kaghan Valley构造岩片的(约30-143mm/yr)。喜马拉雅西构造结在古近纪时的构造演化代显示出明显的时空变化，表明大陆构造岩片的差异性折返，这种多构造岩片的折返过程可能与印度岩石圈板片的撕裂作用有关。