白垩纪“旦师庭组”火山岩的归属：对拉萨地块南缘中生代构造演化的指示

Mesozoic tectonic setting of the southern Lhasa subterrane and its genetic relationship with the subduction and closing of the Neo-Tethys are still unclear, due to the lacking of understanding of the petrogenesis and associated geological significance of related magmatic rocks, especially the Late Cretaceous volcanic rocks. The Cretaceous "Danshiting Formation" volcanic rocks, distributed in the Sangri County, is an ideal target to answer the above scientific questions. To achieve this aim, this project will mainly focus on the petrogenesis of "Danshiting Formation" volcanic rocks. Systematic field investigation, sampling, petrological, mineralogical, geochemical, zircon U-Pb dating and Hf isotope methods will carried out. The results will be applied to define temporal and spatial framework of the volcanic rocks, to understand the nature of magma sources and associated formation mechanism, to unravel the linkage between magmatic events and the subduction of the Neo-Tethys. Together with the result of previous from the intrusions, this project will provide constraint on the tectonic setting of the southern Lhasa subterrane, and further establish the deep-seated geodynamic processes and related controls through time. These results will provide vital magmatic evidences to test the long-term discussed tectonic setting of the southern Lhasa subterrane and its relationship with the subduction of Neo-Tethys during the Cretaceous.

拉萨地块南缘中生代时期的构造背景和新特提斯洋的演化历史及其之间的耦合对应关系仍然是不清楚的，特别是对拉萨地块南缘晚白垩世火山岩及其地质意义的研究目前尚属空白。桑日县周边地区展布的白垩纪"旦师庭组"火山岩为解决这些问题提供了非常理想的研究对象。本项目正是以此为突破点，通过详细的野外调研和样品采集，拟对其进行系统的岩石学、矿物学、元素和同位素地球化学及精确的锆石U-Pb年代学和锆石Hf同位素研究，来约束研究区岩浆活动的时空格架，查明岩浆作用的源区性质和成因机制，探讨其与新特提斯洋俯冲消减之间的耦合关系。然后结合前人侵入岩的研究成果，进一步限定其形成构造环境，为存在争议的白垩纪时期拉萨地块南缘构造背景及新特提斯洋的俯冲消减过程提供重要的火山岩证据。

新特提斯洋的俯冲消减历史对于理解青藏高原的早期构建过程具有重要启示，俯冲消减作用形成的弧火山岩则是示踪新特提斯洋演化历史的最佳产物。本研究首次对分布在拉萨地块南缘的旦师庭组火山岩进行了系统的年代学、主微量及Sr-Nd-Hf同位素等地球化学研究，表明旦师庭组火山岩为一套由玄武岩和安山岩组成的中基性岩，其喷发年龄为90.1±0.7Ma~97.1±0.8Ma，亏损Nb、Ta、Ti等高场强元素，具有相对低的(87Sr/86Sr)i值(0.704228~0.704542)和相对高的Nd(t)值（2.08~2.74），以及较高的Hf(t)(平均值为11.58)，显示典型弧火山岩的地球化学特征，结合岩石成因与构造背景的讨论后，认为旦师庭组火山岩形成于新特提斯洋北向俯冲的安第斯型俯冲时期的陆缘弧环境下，受到俯冲板片流体交代的地幔楔的部分熔融，在岩浆上升过程中受到了地壳物质的混染和镁铁质矿物的分离结晶。根据我们的研究并综合前人的成果，将新特提斯洋北向俯冲历史分为4个期次：侏罗纪时期（195-150Ma）岩浆活动受控于新特提斯洋板片北向陡俯冲，形成了叶巴组和比马组火山岩；晚侏罗世至早白垩世期间（150-110Ma）新特提斯洋以低角度的平板俯冲为主，岩浆活动较弱并向北迁移；晚白垩世期间（110-80Ma）新特提斯洋以安第斯型俯冲为主，主要形成旦师庭组火山岩；古近纪时期（70-40Ma）新特提斯洋经历了洋陆俯冲到陆陆碰撞的转变过程，此期间的火山岩响应为林子宗群火山岩。