青藏高原北羌塘三叠纪花岗岩中发现新元古代的基底信息:来自锆石SIMS U-Pb年龄和Hf-O同位素的约束

羌塘地块基底研究工作是青藏高原地学领域的难点。羌塘地区是否具有前寒武纪基底一直以来存在着很大的争议,现有的年代学资料虽有报道,但至今仍缺乏统一的认识。本文对北羌塘中部双湖地区花岗岩进行了SIMS锆石U-Pb年代学、全岩主微量元素、Sr-Nd同位素和锆石Hf-O同位素地球化学研究。双湖花岗岩形成于晚三叠世(~217 Ma),并捕获~828 Ma的锆石。双湖花岗岩具有高的SiO₂、K₂O和低的MgO含量(SiO2=64.00%~69.54%,K2O=4.09%~5.17%,MgO=1.44%~3.34%),具有轻稀土元素富集的配分模式,以及富集的全岩εNd(t)值(-9.6)和锆石原位εHf(t)值(-10.8~-8.1),岩浆锆石的δ¹⁸O值为6.98‰~8.30‰。岩相学和地球化学特征表明双湖花岗岩主要来源于中下地壳的部分熔融。综合区域内时空演化格架以及大量相关地质事实,认为双湖晚三叠世花岗岩形成于后碰撞伸展构造背景。捕获锆石的阴极发光显示较明显的振荡环带,表明来自于酸性岩浆岩;其具有高的不均一的O同位素(δ¹⁸O=8.20‰~10.23‰),Hf模式年龄为1.7~1.9 Ga,表明其源岩是来自古元古代地壳物质重熔形成的S型花岗岩,代表了北羌塘地块的基底。综合区域地质、岩石地球化学特征及其捕获锆石信息,本文认为双湖花岗岩为北羌塘地块可能具有扬子型晋宁期基底提供了重要信息。

Neoproterozoic Basement Information Revealed in the Triassic Granites in the Northern Qiangtang,Tibetan Plateau: Constraints from Zircon SIMS U-Pb Age and Hf-O Isotopes

The basement of the Qiangtang block,Tibetan Plateau,has not yet been well-documented.The existence of a pre-Cambrian basement in the Qiangtang area is actively debated,even though previous chronological data suggest the pre-Cambrian basement may exist.In this study,we present SIMS zircon U-Pb dating results,whole rock major and trace elements,Sr-Nd and zircon Hf-O isotopic data for the Shuanghu granite in the central part of Northern Qiangtang.The Shuanghu granite was formed at Late Triassic(ca.217 Ma)and captured ca.828 Ma zircon xenocrysts.The Shuanghu granite rocks have high SiO₂,K₂O and low MgO contents(SiO₂=64.0%–69.5%,K₂O=4.1%–5.2%,MgO=1.4%–3.3%).They exhibit enrichment in light REEs,enriched whole rock Nd and zircon Hf isotopic compositions(εNd(t)=-9.6,εHf(t)=-10.8–-8.1)with zirconδ18O range from 6.98‰to 8.30‰.These petrographic and geochemical characteristics suggest that the Shuanghu granite was originated from partial melting of the middle-lower crust.Combined with regional tectonic evolution framework,the Shuanghu granite was formed in a post-collision extensional setting.The 828 Ma zircon xenocrysts exhibit obvious oscillatory zones in cathodoluminescene images,implying they were derived from acidic magmatic rocks.They have high and heterogeneousδ¹⁸O values(8.20‰–10.23‰)and ancient Hf model ages(1.7–1.9 Ga),indicating that they were sourced from S-type granites by remelting of the Paleoproterozoic crustal materials,which may represent the northern Qiangtang basement.Combining regional geological and rock geochemical characteristics and zircon xenocrysts,we suggest that the Shuanghu granite provide favorable evidence for the Yangtze-type Jinningian basement in the Northern Qiangtang block.