早白垩世末“捷嘎组”火山岩研究：对拉萨地块中北部中生代构造演化的指示

Mesozoic tectonic setting of the central-nouthern 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 Early-Cretaceous to early Late-Cretaceous volcanic rocks. The Cretaceous “Jiega Formation” volcanic rocks, distributed in the Shiquanhe-Geji-Cuoqin County, is ideal target to answer the above scientific questions. To achieve this aim, this project will mainly focus on the petrogenesis of “Jiega 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, this project will provide constraint on the tectonic setting of the central-nouthern 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 central-nouthern Lhasa subterrane and its relationship with the subduction of Neo-Tethys during the Cretaceous.

拉萨地块中北部中生代时期的构造背景及其与新特提斯洋演化历史之间的耦合对应关系仍然是不清楚的，特别是对中拉萨地块早、晚白垩世之交火山岩及其地质意义的研究目前尚属空白。狮泉河-革吉县-措勤县周边地区展布的白垩纪“捷嘎组”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中北部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。

拉萨地块广泛分布有中生代岩浆岩，了解它们的成因机制和形成构造背景对认识新特提斯洋的演化历史意义重大。本文首次对出露于中拉萨地块西部赛利普地区的捷嘎组火山岩进行了系统的年代学、全岩地球化学及Sr-Nd-Hf同位素研究。精确的LA-ICP-MS锆石U-Pb定年获得了六组年龄： 107.1±1.6Ma、108.2±2.5Ma、108.9±1.6Ma、109.2±1.6Ma、113.1±1.5Ma、114.5±1.9Ma，表明研究区捷嘎组火山岩形成于早白垩世晚期。捷嘎组火山岩主要由玄武岩、安山岩和英安岩组成，可分为中基性和中酸性两组。中基性火山岩以中钾钙碱性为主，富集Th、U和Sr等大离子亲石元素，相对亏损Rb、Nb、Ta等高场强元素，样品的(87Sr/86Sr)i和Nd(t)分别为0.70688~0.70910和-6.23~-0.19， 176Hf/177Hf值介于0.282424~0.282649之间，对应的εHf(t)范围在-9.80~-2.26之间；而中酸性火山岩则主要为高钾钙碱性系列，富集Th、U、Hf和Zr等元素，亏损Ba、Nb、Ta、Ti等元素，样品的(87Sr/86Sr)i和Nd(t)分别为0.70612~0.70923和-7.05~-1.28，176Hf/177Hf值介于0.282486~0.282746之间，对应的εHf(t)范围在-5.99~0.93之间。在稀土元素配分图和蛛网图中两者具有相似的分布特征，具有相似的Sr-Nd-Hf同位素特征，具有典型的活动大陆边缘弧火山岩特征，可能来源于受到早期俯冲物质富集的地幔楔的部分熔融，并在岩浆上升过程中经历了不同程度的分离结晶作用和浅部地壳物质的同化混染作用。综合前人研究成果，本文认为捷嘎组火山岩可能为狮泉河-永珠-纳木错-嘉黎蛇绿岩带所代表的古洋盆(Slainajap洋)在早白垩世向南俯冲消减的产物，表明在早白垩世同时存在班公湖-怒江洋盆和Slainajap洋盆的南向俯冲作用。