特提斯喜马拉雅带东段列麦白云母花岗岩年代学及成因

Geochronology and Petrogenesis Stduy of Liemai Muscovite Granite in the Eastern Tethyan Himalaya

列麦白云母花岗岩位于特提斯喜马拉雅东段,侵位于雅拉香波穹窿边部早古生代浅变质岩中。为揭示其形成时代及成因,本文对其开展LA-ICP-MS锆石U-Pb年代学、Hf同位素及岩石地球化学研究。结果表明列麦白云母花岗岩具有高SiO_2(71.08%～71.49%)、Al_2O_3(15.55%～15.72%)和K_2O(4.32%～4.57%),高的A/CNK比值(1.17～1.21),低的CaO/Na_2O比值(0.22～0.28),属于高钾钙碱性过铝质花岗岩;富集Rb、Th和Hf,亏损Ba、Nb、Sr和Ti,稀土元素呈轻稀土(LREE)富集,重稀土(HREE)相对亏损的向右倾斜的配分模式(LREE/HREE=16.57～17.91),具有弱负Eu异常(δEu=0.78～0.79)。与二云母花岗岩比较,列麦白云母花岗岩具有较高的Rb含量(204.1×10^(-6)～293.8×10^(-6))、较低的Sr(<134.6×10^(-6))及显著的重稀土分馏效应。锆石新生边年龄为48.5±1.1 Ma(MSWD=2.1),代表其结晶年龄,该年龄为目前报道的最早年龄,其初始Hf同位素组成ε_(Hf)(t)=-6.4～-2.3,显示物源为壳源性质,二阶段模式年龄介于731～839 Ma之间,表明成岩物质形成于新元古代;核部继承锆石年龄变化在135.7～3339.2 Ma之间,表明其源岩为早白垩世沉积岩,是在印度与欧亚大陆主碰撞阶段,陆-陆碰撞导致地壳缩短加压升温,引起早白垩世沉积岩部分熔融而形成的。

Liemai muscovite-granite, located in the eastern segment of the Tethyan Himalaya belt, intruded into the low grade metamorphic rocks of the marginal of Yardoi dome in early Paleozoic era. Zircon U-Pb dating, Hf isotopic composition and whole-rock geochemistry of the Liemai muscovite-granite rocks have been carried out to constrain its formation age and petrogenesis. The results show that the Liemai muscovite-granite rocks have high SiO2 （71.08%-71.49%）, Al2O3 （15.55% -15.72%）, K2O（4.32%- 4.57%）, A/CNK ratios （1.17-1.21） and low CaO/Na2O ratios （0.22-0.28）, suggesting that the rocks belong to high-K calc-alkaline peraluminous leucogranites. The REE diagram shows enrichment of Rb, Th, K, La, Nd, Hf and Y, and depletion of Ba, Nb, Sr and Ti. These rocks are highly enriched in LREE （LaN/YbN=59. 96-79. 85） and depleted in HREE （LaN/GdN=6.28-6.51）, with LREE/HREE=16.57 -17.91 and slightly negative Eu anomalies （δEu=0.78-0.79）. In contrast to two-mica granite from the margin of Yardoi Dome, the Liemai muscovite granites have relatively high Rb contents （204. 1 × 10^-6 -293.8×10^-6） and Rb/Sr ratios （1.52-2.20）, relatively low Sr contents （〈134.6×10^-6） and Sr/Y ratios（23.94 - 27.73）, and more obvious HREE fractionation. Zircon LA-ICP-MS dating yields the youngest rim age of 48.5±1.1Ma （MSWD= 2.1）, representing the crystallization age of the Liemai muscovite-granite, which should be earliest age reported to date. The εHf （t） values of the zircons vary from -6.4 to -2.3, implying a crustal origin. The two-stage Hf model age varies in the range of 731- 839Ma, indicating that source rocks are Neoproterozoic in age. The ages of inherited core zircons vary between 135.7Ma and 3339.2Ma, indicating that the source rocks of the Liemai Leucogranites may be Early-Cretaceous sedimentary rock. It can be proposed that, during the main collision stage between the Indian and the Eurasian plates, crustal thickening in company with increasing pressure and temperature resulted in partial melting of the Early Cretaceous sedimentary rocks, giving a rise to the formation of the Liemai muscovite-granite.