Is Himalayan leucogranite a product by in situ partial melting of the Greater Himalayan Crystalline? A comparative study of leucosome and leucogranite from Nyalam, southern Tibet
Is Himalayan leucogranite a product by in situ partial melting of the Greater Himalayan Crystalline? A comparative study of leucosome and leucogranite from Nyalam, southern Tibet
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喜马拉雅淡色花岗岩是大喜马拉雅晶体原位部分熔融的产物吗？
DOI：
10.1016/j.lithos.2019.06.007
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发表时间：
2019-10
期刊：
影响因子：
3.5
通讯作者：
Wu Fu Yuan
中科院分区：
文献类型：
--
作者：
Yang Lei;Liu Xiao Chi;Wang Jia Min;Wu Fu Yuan
喜马拉雅造山带中广泛分布的浅色花岗岩被认为是由新生代造山作用期间大喜马拉雅结晶体（GHC）经历高级变质作用时就地部分熔融形成的。因此，浅色花岗岩及其伴生的混合岩可以作为GHC折返历史的制约因素。然而，GHC，浅色花岗岩和混合岩之间的岩石成因关系并没有很好的约束。因此，我们对西藏南部聂拉木地区的浅色体和浅色花岗岩进行了详细的岩石学、矿物学、年代学和地球化学研究。独居石U-（Th）-Pb定年表明，GHC的深熔作用发生在始新世晚期和中新世（40 - 14 Ma），而浅色花岗岩的侵位发生在27 - 14 Ma。混合岩和浅色花岗岩中的浅色体在野外地质、矿物学、地球化学等方面存在明显差异，表明两者的成因不同。在基质变晶岩中，白小体主要以袋状或与黑素体互层的形式出现。浅色体含奥长石和富铁黑云母，全岩K2O含量高（4.8-7.4wt%），K2O/Na2O比值高（1.35-2.97），Eu正异常（Eu/Eu *= 0.93-2.61），稀有金属（Li、Be、Cs、Sn、Ta）含量低。这些特征与GHC的白云母脱水熔融起源一致。然而，浅色花岗岩侵入GHC并沿着藏南拆离系以小岩体形式产出。与浅色体相反，浅色花岗岩中的斜长石和黑云母分别为钠长石和铁叶石。浅色花岗岩K_2O含量（4.3-4.7wt%）和K_2O/Na_2O比值（1.04-1.24）较低，稀土元素含量较高，Eu负异常（Eu/Eu~*= 0.47-0.70）明显，表明其成因为广泛的分离结晶作用。我们认为，浅色花岗岩的岩浆产生在较深的GHC在高峰变质，和随后的广泛的分离结晶发生在长距离，沿沿着藏南拆离系（STDS）的GHC折返过程中向上迁移的岩浆。在这些过程中，GHC的高级变质岩部分熔融，导致混合岩中的浅色体形成。因此，浅色体和浅色花岗岩具有不同的成因，在喜马拉雅造山作用研究中应分开考虑。
Widespread leucogranites in the Himalayan orogenic belt are thought to have originated by in situ partial melting of the Greater Himalayan Crystalline (GHC) when it underwent high-grade metamorphism during Cenozoic orogenesis. Therefore, the leucogranites and associated migmatites can be used to constrain the exhumation history of the GHC. However, the petrogenetic relationship between the GHC, leucogranites, and migmatites is not well-constrained. As such, we carried out a detailed petrographic, mineralogical, geochronological, and geochemical study of leucosomes and leucogranites from the Nyalam region in southern Tibet. Monazite U–(Th)–Pb dating indicates that anatexis of the GHC occurred during the late Eocene and Miocene (40–14 Ma), whereas leucogranite emplacement occurred from 27 to 14 Ma. There are marked differences between the leucosomes in migmatites and leucogranites in terms of field geology, mineralogy, and geochemistry, suggesting different origins. The leucosomes occur mainly as pockets or are interlayered with melanosomes in stromatic metatexites. The leucosomes contain oligoclase and Fe-rich biotite, and have whole-rock compositions with high K2O contents (4.8–7.4 wt%) and K2O/Na2O ratios (1.35–2.97), positive Eu anomalies (Eu/Eu* = 0.93–2.61), and low rare-metal (Li, Be, Cs, Sn, and Ta) contents. These features are consistent with an origin by muscovite dehydration melting of the GHC. However, the leucogranites intrude the GHC and occur as small plutons along the South Tibetan Detachment System. In contrast to the leucosomes, the plagioclase and biotite in the leucogranites are albite and siderophyllite, respectively. The leucogranites have relatively low K2O (4.3–4.7 wt%) contents and K2O/Na2O ratios (1.04–1.24), high rare-metal contents, and marked negative Eu anomalies (Eu/Eu* = 0.47–0.70), indicating an origin by extensive fractional crystallization. We propose that the leucogranites were magmas produced in the deeper GHC during peak metamorphism, and subsequent extensive fractional crystallization occurred during long-distance, upward migration of magma along the South Tibetan Detachment System (STDS) during exhumation of the GHC. During these processes, the high-grade metamorphic rocks of the GHC were partially melted, resulting in the formation of leucosomes within migmatites. Therefore, the leucosomes and leucogranites have different origins and should be considered separately in studies of Himalayan orogenesis.
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