北拉萨地块富集Hf同位素的中生代花岗质岩石对地壳基底的启示

It is still controversial about the tectonic nature of the Northern Lhasa subterrane. Previous studies suggested the Northern Lhasa subterrane was possibly a juvenile block attributed to the southward subduction of Bangong-Nujiang ocean without ancient crustal basement. However, the others proposed that there was ancient crustal basement underlying the Northern Lhasa subterrane or itself include several microcontinent with ancient crustal basement. It is convenient for us to explore the deep crustal nature based on the Mesozoic granitic rocks with enriched Hf isotope in the Northern Lhasa subterrane. This project is planning to conduct systematical geochronological, elemental, Sr-Nd isotopic, and in situ zircon Hf isotopic geochemical analysis on the Mesozoic granitic rocks with enriched Hf isotopic composition from Nagqu, Baingoin and Nyima areas. This project will reveal (1) spatial and temporal distribution, petrologic and geochemical characteristics of the enriched Hf isotopic granitic rocks in the Northern Lhasa subterrane. (2) Is there ancient crustal basement existed in the Northern Lhasa subterrane? (3) The relationship between the Northern and Central Lhasa subterrane. The project is expected to provide the valid geochemical evidence for the study of the Mesoizoic tectonic evolution of the Northern Lhasa Terrane and adjacent Bangong-Nujiang and Yongzhu-Namco suture.

北拉萨地块的构造属性一直存在争议，已有研究认为北拉萨地块可能为班公湖-怒江洋南向俯冲消减过程中的增生块体，无古老地壳基底；另外一种意见认为其本身就存在古老地壳基底或者地壳中包含多个古老微陆块。北拉萨地块上出露的具有明显富集Hf同位素的中生代花岗质岩石为我们研究其地壳的深部物质组成提供了天然样品。本项目拟对富集Hf同位素的那曲、班戈及尼玛地区的中生代花岗质岩石开展系统的年代学、元素和Sr-Nd同位素分析，并结合锆石Hf同位素测试结果，试图揭示北拉萨地块：（1）Hf同位素富集的花岗质岩石的时空分布，岩石学和地球化学特征；（2）是否存在古老地壳基底物质? (3)北拉萨与中拉萨地块之间的关系。本研究所取得的认识将为揭示北拉萨地块和两侧班公湖-怒江、永珠-纳木错缝合带的中生代构造演化提供可靠的地球化学制约。

拉萨地块的构造演化历史吸引了国内外地质学者的广泛关注，但是对北拉萨地块的基底性质认识还存在一些不足。本项目对北拉萨地块上广泛出露的花岗质岩石的野外分布、年代学、元素和Sr-Nd-Hf同位素组成的系统研究，限定了这些中生代花岗质岩石的时空分布，厘定出北拉萨地块上存在大量Hf同位素富集的花岗质岩石，结合它们富集的Sr-Nd同位素和类似大陆地壳的微量元素组成特征，我们认为它们很可能代表了北拉萨地块存在的元古代地壳基底物质。这些基底物质特征不仅出现在花岗质岩石中，且表现在这些花岗质岩石伴生的暗色包体里。通过与中拉萨地块已有的地壳基底物质对比，我们的研究指示出北拉萨地块局部地区很可能与中拉萨地块具有亲缘性。