青藏高原南拉萨地块新生地壳增生时间与机制研究

The growth and evolution of continental crust has always been one of the important issues of Earth sciences. The early Mesozoic granites from the southern Lhasa terrane, Tibet show very high zircon Hf isotopes similar to that of depleted mantle, which suggests that the southern Lhasa terrane develops wholly juvenile crust. However, the newly identified late Paleozoic granites have low zircon Hf isotopes and Early-Middle Proterozoic model ages that imply the southern Lhasa terrane should have been underlain by an ancient basement. The change of crustal nature reflects large-scale growth of juvenile crust in the southern Lhasa terrane. However, the detailed time and mechanism for this is mysterious. This proposal is designed to reveal this process by means of systematic study about the late Paleozoic to early Mesozoic magmatic rocks in the southern Lhasa terrane. The youngest age of granites derived from partial melting of ancient crust can limit the earliest occurrence of juvenile crust. And the earliest age of Mesozoic magmatic rocks with juvenile nature can constrain the time of crustal growth, while its petrogenesis may reveal the mechanism of crustal growth.

大陆地壳的形成与演化一直是地球科学研究的重要问题之一。青藏高原南拉萨地块中生代早期花岗岩普遍具有高的锆石Hf同位素组成，与亏损地幔演化线接近，反映南拉萨地块整体具有新生地壳特征。然而，新发现的晚古生代花岗岩，具有富集的锆石Hf同位素组成和早-中元古代地壳模式年龄，表现出古老陆壳物质熔融特点，暗示南拉萨地块曾发育古老的陆壳基地。南拉萨地块从晚古生代到早中生代地壳性质的转变，反映了一次大规模的新生地壳生长事件。然而，该新生地壳增生的具体时间和机制还很不清楚。本项目拟通过对南拉萨地块古生代晚期-中生代早期岩浆岩的系统研究来解决这一问题。以晚古生代古老陆壳熔融成因花岗岩结束发育的时间，来限制地壳增生时间的下限；以早中生代新生陆壳熔融成因岩浆岩首次出现的时间来制约新生地壳增生开始的时间，并根据其岩石成因揭示新生地壳生长机制。

大陆地壳的形成与演化是地球科学研究的重要问题之一，新生地壳的形成机制研究可以为该问题提供重要约束。青藏高原南拉萨地块经历了从晚古生代古老陆壳至中生代新生地壳性质的转变，然而其新生地壳的增生时间与机制尚不清楚。本项目对南拉萨地块晚古生代和中生代早期岩浆记录进行了详细研究。研究发现南拉萨地块中、东部地区晚古生代古老陆壳熔融可以持续到约300 Ma，而中生代早期新生地壳来源长英质岩石从三叠纪中期（233-226 Ma）已经开始发育。根据中生代早期中、基性岩浆岩地球化学组成及演化特征，南拉萨地块中、东部地区在三叠纪早期（245-237 Ma）可能已经以新生地壳物质为主。南拉萨地块中、东部地区地壳增生过程与岩石圈伸展和软流圈来源物质的加入相关。南拉萨地块西部地区中生代时期仍以古老陆壳为主，但是其新生地壳分布具有从早到晚和从陆内到海沟逐渐增加的趋势，这与从晚三叠世开始的大洋俯冲导致的新生地壳增生过程相关。