北羌塘北缘多彩三叠纪高硅花岗岩的成因及其构造意义

Petrogenesis and tectonic implications of the Duocai Triassic high-silica granites from the northern part of North Qiangtang terrane

高硅花岗质岩浆作用不仅是衡量大陆地壳成熟度的重要指标之一,而且能为示踪陆壳生长过程和稀有金属成矿提供非常关键的信息。然而,目前国内外学者对于这一特殊花岗质岩浆的起源、演化及其相关地球动力学背景还存在较大的分歧。本文选择青藏高原中部多彩高硅花岗岩为研究对象,开展岩石学、锆石U-Pb年代学、岩石地球化学和Sr-Nd同位素等综合研究,为深入理解高硅花岗岩的形成机制和青藏高原中部晚三叠世的构造演化提供新线索。研究结果显示,多彩高硅花岗岩的锆石U-Pb年龄为219±2 Ma,表明其为晚三叠世岩浆活动的产物。岩石样品的^(87)Sr/^(86)Sr初始比值(I_(Sr))为0.708 4,εNd(t)值为-8.12~-7.36,两阶段模式年龄(t_(2DM))为1.65~1.59 Ga,总体上与可可西里-巴颜喀拉-松潘-甘孜(HBSG)地体三叠纪浊积岩成分特征类似。结合岩石地球化学的研究结果可进一步确定,多彩高硅花岗岩的形成应该与变杂砂岩的部分熔融有关,并且经历了较高程度的分离结晶。综合区域上多学科研究成果,可以判断北羌塘北缘晚三叠世中酸性岩浆岩(包括高硅花岗岩)的产生很可能与甘孜-理塘古特提斯洋的板片回撤作用密切相关。

High-silica granitic magmatism is not only considered as one of the key indexes to measure the maturity of continental crust, but also can provide essential information for understanding the growth of continental crust and the rare-mental mineralization. However, the origin, evolution, and geodynamic environment of the high-silica granitic magmatism remains highly controversial. This paper presents a comprehensive study of zircon U-Pb ages, bulk-rock geochemistry, and Sr-Nd isotopes for the Duocai high-silica granites in the central Tibetan Plateau. Zircon U-Pb dating of the studied rocks yields ages of 219±2 Ma, indicating those rocks represent the products of the Late Triassic magmatism. The Duocai granites have I_(Sr)ratios of 0.708 4, εNd(t) values from-8.12~-7.36, and t_(2DM)ages of 1.65 ~ 1.59 Ga, similar to the Triassic turbidites in the Hohxil-Bayan Har-Songpan-Garzê( HBSG)terrane. Geochemical and isotopic signatures indicate that the Duocai high-silica granites might be derived from partial melting of meta-greywackes, and they had experienced a relatively high degree of fractional crystallization. Combining with the other regional studies, the Late Triassic magmatism in the north margin of the North Qiangtang( NQ) terrane was induced by slab rollback of the Garzê-Litang Palaeo-Tethyan ocean.