In-sequence buoyancy extrusion of the Himalayan Metamorphic Core, central Nepal: Constraints from monazite petrochronology and thermobarometry
In-sequence buoyancy extrusion of the Himalayan Metamorphic Core, central Nepal: Constraints from monazite petrochronology and thermobarometry
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尼泊尔中部喜马拉雅变质核的连续浮力挤压：来自独居石岩石年代学和温压测量的限制
DOI：
10.1016/j.jaesx.2020.100025
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发表时间：
2020-09
影响因子：
--
通讯作者：
Lei Yang
中科院分区：
文献类型：
--
作者：
Gautam Khanal;Jia-Min Wang;Fu-Yuan Wu;Jiang-Gang Wang;Lei Yang
最近在喜马拉雅变质核内发现的构造-变质不连续面挑战了以前对大陆碰撞期间造山过程/模式的理解。然而，它们的确切位置、空间范围和时间演变仍然存在争议。应用独居石岩石年代学和温压测量方法，对尼泊尔中部吉隆-夏布鲁文斯-朗塘断面的变质泥岩进行了研究。用常规温压法和锆铟金红石测温法测定了变质峰温和峰压。P-T条件在结构上向上增加，从540-580°C，LHS的8-10Kbar增加到640-710°C，8-12Kbar在下GHC，680-720℃，6-9Kbar在上GHC。岩石年代学研究表明，LHS发生了进行性变质作用，下部GHC发生了20~18 Ma的部分熔融，上部GHC发生了29~23 Ma的持续部分熔融，显示了这些单元之间的历时性。Rasuwagadhi剪切带(Rsz)以T/深度的跳跃和从下部GHc(30°C/Km，∼29 Ma)开始的峰期变质时间为标志，并与最近在其他喜马拉雅断面处发现的区域高喜马拉雅不连续相对比。该剪切带在24~18 Ma左右与藏南滑脱几乎同步作用，18 Ma后沿中央主冲断带运动传递，呈南向顺层逆冲传播方式。因此，结合底侵作用和浮力机制，提出了一种混合模式--顺序浮力挤压模型来解释HMC的整体折返作用，并可能适用于类似大型热碰撞造山带中的变质核。
Recent identification of tectono-metamorphic discontinuities within the Himalayan metamorphic core has challenged previous understanding of mountain-building processes/models during continental collision. However, their exact position, spatial extent, and temporal evolution still remain in debate. Monazite petrochronology and thermobarometry have been applied to metapelites of the Gyirong-Syabrubensi-Langtang transect in central Nepal. Peak metamorphic temperatures and pressures were determined by conventional thermobarometry and Zr-in Rutile thermometry. P-T conditions increase structurally upward from 540–580°C, 8–10 kbar at LHS to 640–710°C, 8–12 kbar at lower GHC, and 680–720°C, 6–9 kbar at upper GHC. Petrochronology studies indicate prograde metamorphism in the LHS, partial melting of 20–18 Ma in the lower GHC and prolonged partial melting at 29–23 Ma in the upper GHC, demonstrating a diachronism across these units. The Rasuwagadhi Shear Zone (RSZ), is marked by a jump in T/depth and peak metamorphic timing from lower GHC (30°C/km, ∼29 Ma), and can be correlated with the regional High Himalayan Discontinuity recently identified within the GHC in other Himalayan transects. This shear zone acted almost synchronously with the South Tibetan Detachment around 24–18 Ma and was relayed by movement along the Main Central Thrust after 18 Ma, presenting south-ward in-sequence thrust propagation style. Therefore, a hybrid model, In-sequence Buoyancy Extrusion that involves a combination of underplating and buoyancy mechanisms is proposed to explain the overall exhumation of HMC, and perhaps be applicable to metamorphic core in similar large and hot collisional orogen.
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