青藏高原南部拉萨地块中新世超钾质岩石中的锆石记录

Zircon records of Miocene ultrapotassic rocks from southern Lhasa subterrane,Tibetan Plateau

幔源岩浆上升的过程中捕获的锆石为揭示深部地壳"隐藏"的岩浆作用事件提供了宝贵机会。本文对采自南部拉萨地块学那地区的超钾质脉岩中的锆石进行了U-Pb年代学、微量元素和Hf同位素研究。研究结果表明,学那超钾质岩石中的锆石主要展示出4个主要的年龄峰值,分别是:<100Ma、300~400Ma、450~500Ma以及700~850 Ma。这些锆石高U/Yb比值、低Y含量的特征暗示起源于大陆地壳。而新生代-中生代(<100 Ma)和晚古生代(300~400Ma)的岩浆活动在南部拉萨地块上广泛发育,这表明南部拉萨地块新生地壳物质对学那超钾质岩浆活动的贡献。但是超钾质脉岩中早古生代和元古代(450~500Ma和700~850Ma)锆石捕掳晶的存在则证实印度大陆地壳物质的加入。此外,从大约55Ma左右开始,锆石颗粒的(Dy/Yb)N比值开始逐渐增高,εHf(t)值则从+10^+5迅速下降至-10^-25。考虑到南部拉萨地块新生地壳的同位素组成特征,超钾质脉岩中的这些锆石颗粒可能记录了印度-亚洲陆陆汇聚过程中地壳的显著加厚以及俯冲的印度大陆地壳物质对南部拉萨地块后碰撞岩浆作用的贡献。

Zircons entrained in mantle-derived magmas offer a prime opportunity to reveal cryptic magmatic episodes in the deep crust. We have investigated zircons from mantle-derived uhrapotassic veins in the Xuena area, southern Lhasa subterrane. Zircons in the Xuena uhrapotassic rocks reveal four major magmatic pulses around 〈 100Ma, 300 - 400Ma, 450 - 500Ma, and 700 - 850Ma. The high U/Yb ratios and low Y contents of these zircons demonstrate their continental origin. Cenozoic-Mesozoic and Late Paleozoic magmatism have been widely identified from the southern Lhasa subterrane, suggesting the contribution from overlying juvenile crust. But similar Proterozoic-Early Paleozoic age distributions （450 -500Ma and 700 -850Ma） between these zircon xenocrysts and those dating records in the Himalayan orogenic belt corroborate the input from underthrusted Indian continental crust. Furthermore, the increasing （Dy/Yb） N ratio since - 60Ma and rapid decreasing zircon εHf （t） values, from ＋ 10 - ＋ 5 to - 10 - - 25, are interpreted to reflect significant and progressive crustal thickening in response to India-Asia convergence and the contribution from subducted Indian continental crust to postcollisional magmatism in the southern Lhasa sub-terrane.