Granitic orthogneiss contributions to the generation of Himalayan leucogranites: insights from the eastern Himalayas

Multi-component crustal sources are universally acknowledged as the overriding factor in causing geochemical heterogeneities of the Cenozoic Himalayan leucogranites. In previous studies, metasedimentary rocks from the Greater Himalayan Crystalline Complex were always underlined to be the dominant origin for leucogranites after the Eocene and Oligocene transition (ca. <36 Ma). However, given that the petrological diversity of the Greater Himalayas, especially the widespread and high-grade metamorphosed granitic gneisses; this traditional standpoint has been increasingly questioned nowadays. To further demonstrate the role of granitic gneiss in leucogranite generation, a rounded compilation of geochronological and geochemical data for leucogranites, granitic gneisses, and other related rocks from the specified N – S striking Yardoi – Cuonadong – Tsona transect has been conducted. After making comprehensive comparison and discussion between leucogranites and granitic gneisses, we argue that Cenozoic Himalayan leucogranites may not be pure metasediments derived S-type granites as orthogneisses could be another important endmember for the provenance of them based on the following evidence: (1) Abundant relict zircons within the Himalayan leucogranites display two evident U – Pb age clusters at ca. 850–800 Ma and ca. 520–470 Ma, which are contemporaneous with the Neoproterozoic and early Paleozoic granitic magmatism, respectively. (2) Zircon Hf isotopes of Himalayan-aged rims (−11.21 to −4.82) could be perfectly constrained by two evolution lines derived from Neoproterozoic (−6.40 to 0.16) and early Paleozoic (−2.37 to 6.15) zircon groups. (3) In terms of whole-rock Sr – Nd isotopes (all corrected to 20 Ma), there is a notable overlap between leucogranites (0.7142 to 0.8429 for Sr; and −17.34 to −9.86 for Nd) and granitic gneisses (0.7703 to 0.8716 for Sr, and −16.27 to −9.80 for Nd). (4) Although the fertility of granitic gneisses should be poorest in the absence of a separate aqueous phase; however, the evolutionary P – T – XH₂O conditions triggered by compressional thrust activity of the Main Central Thrust and arc-parallel extension have remarkably modified original source structures (infiltration by LHS-derived fluids) and melting behaviours (more recognized fluid-present partial melting cases). Consequently, the role the granitic gneisses would be strengthened because of the greatly improved fertility via fluid-present melting; and the Sr – Nd isotopic signatures of Himalayan leucogranites would be spatially-temporally evolved.

多组分地壳源被公认为造成新生代喜马拉雅淡色花岗岩地球化学不均一性的首要控制因素。既往研究中，大喜马拉雅结晶杂岩（Greater Himalayan Crystalline Complex）中的变沉积岩一直被认为是始新世-渐新世过渡期（约<36 Ma）后淡色花岗岩的主要物质来源。然而，鉴于喜马拉雅造山带的岩石学多样性，尤其是广泛分布的高级变质花岗片麻岩，这一传统观点如今受到了越来越多的质疑。为进一步阐明花岗片麻岩在淡色花岗岩形成过程中的作用，本研究对指定的近南北向亚东-库拉岗日-错那剖面（Yardoi – Cuonadong – Tsona transect）内的淡色花岗岩、花岗片麻岩及相关岩石的年代学与地球化学数据进行了全面汇编。通过对淡色花岗岩与花岗片麻岩开展综合对比与讨论，本文提出：新生代喜马拉雅淡色花岗岩并非仅由变沉积岩成因的S型花岗岩，正片麻岩可作为其物源的另一重要端元，依据如下：(1) 喜马拉雅淡色花岗岩中发育大量残留锆石，呈现出约850–800 Ma与约520–470 Ma两个显著的U-Pb年龄簇，分别与新元古代和早古生代花岗质岩浆活动时代一致。(2) 喜马拉雅期锆石边部的Hf同位素组成（-11.21至-4.82）可被新元古代（-6.40至0.16）和早古生代（-2.37至6.15）两组锆石的两条演化线完美约束。(3) 就全岩Sr-Nd同位素（均校正至20 Ma）而言，淡色花岗岩（Sr比值为0.7142至0.8429，ε_(Nd)(t)为-17.34至-9.86）与花岗片麻岩（Sr比值为0.7703至0.8716，ε_(Nd)(t)为-16.27至-9.80）之间存在显著的同位素重叠。(4) 尽管在缺乏游离水相的情况下，花岗片麻岩的熔体生成能力本应最差；但主中央逆冲带（Main Central Thrust）的挤压逆冲活动及平行弧伸展作用所触发的压力-温度-水活度（P-T-XH₂O）演化条件，显著改造了原始源区结构（如下喜马拉雅序列（Lower Himalayan Sequence, LHS）衍生流体的渗透作用）与熔融行为（如更多被证实的含水部分熔融案例）。因此，通过含水熔融大幅提升的熔体生成能力将强化花岗片麻岩的物源贡献，同时喜马拉雅淡色花岗岩的Sr-Nd同位素组成也会随空间和时间发生演化。

搜集汇总

背景与挑战

背景概述

该数据集聚焦于喜马拉雅淡色花岗岩的成因研究，通过整合东喜马拉雅地区特定剖面的地球化学和年代学数据，论证了花岗质正片麻岩作为重要源岩对淡色花岗岩形成的贡献，挑战了传统认为淡色花岗岩主要源自变质沉积岩的观点。数据集包含锆石年龄、同位素证据等关键信息，支持了多组分地壳源岩的模型，并探讨了构造活动对源岩熔融行为的影响。