西藏拉萨地块北部晚白垩世晚期基性岩墙的成因:来自锆石U-Pb年代学及地球化学的制约

拉萨地块北部阿索地区新发现的晚白垩世晚期基性岩墙对于讨论班公湖-怒江洋盆闭合后的碰撞过程具有重要研究意义。本文报道了阿索地区目思基性岩墙的全岩地球化学组成和LA-ICP-MS锆石U-Pb年龄。研究表明,目思基性岩墙中一组最年轻锆石的U-Pb加权平均年龄为74Ma。岩墙内的辉绿玢岩SiO2含量介于51.90%~53.55%之间,MgO含量介于3.98%~4.97%之间,Mg^#为50.0~57.5,低Cr(51.30×10^-6~79.48×10^-6)和Ni(55.94×10^-6~74.17×10^-6)含量。岩墙具有轻稀土元素富集的特征,明显的负Eu异常。在微量元素方面,富集Ba、Th、U、K等大离子亲石元素富集,亏损Nb、Ta、Ti等高场强元素,并具有Pb的正异常。La/Sm-Sm/Yb投图结果显示基性岩墙来自于尖晶石+石榴石二辉橄榄岩地幔源区1%~5%的部分熔融,并在演化过程中发生了橄榄石、单斜辉石及斜长石的结晶分异,同时在岩浆上升过程中受到地壳混染。结合区域地质背景,拉萨地块与羌塘地块碰撞后下地壳增厚并发生榴辉岩化,导致拆沉作用。目思基性岩墙的锆石U-Pb年龄表明拆沉作用持续到晚白垩世晚期。

Zircon U-Pb geochronology and geochemisty: Constraints on petrogenesis of the lately Late Cretaceous mafic dykes in northern Lhasa terrane, Tibet

The lately Late Cretaceous mafic dykes newly discovered in the Asa area in the northern Lhasa terrane have important research significance for discussing the collision process after the closure of the Bangong-Nujiang ocean basin. This paper reports whole-rock major and trace element compositions and LA-ICP-MS zircon U-Pb ages from Musi mafic dykes in Asa area. Our aim was to identify the petrogenesis of Musi mafic dykes and to constrain the time limit of the delamination. LA-ICP-MS zircon U-Pb isotopic dating indicates that the Musi mafic dykes emplaced at 74Ma. The diabase porphyrites from Musi mafic dykes were characterized by their high content of SiO₂ (51.90%~53.55%), MgO (3.98%~4.97%), Mg^# (50.0~57.5), Cr (51.30×10^-6~79.48×10^-6) and Ni (55.94×10^-6~74.17×10^-6). They are characterized by light rare earth elements (LREE) enriched, and show a strong negative Eu anomaly. The large ion lithophile elements (LILEs) such as Rb, Th, U, K are enriched, and the high field strength elements (HFSEs) such as Nb, Ta, Ti are depleted, with a positive Pb anomaly. The discrimination diagram of La/Sm-Sm/Yb shows that Musi mafic dykes were formed by the magmas derived from 1%~5% partial melting of the spinel+garnet lherzolite mantle. These magmas fractionated mafic minerals such as olivine, clinopyroxene and plagioclase during the evolution process, and assimilated of crustal material during the uplifting. After the collision between the Lhasa terrane and the Qiangtang terrane, the lower crust was thickened and eclogites were formed, resulting in the delamination. Our data, in combination with published geochronological and geochemical data, show that the model of delamination is the small-scale gradual delamination, rather than the whole delamination of the lower crust with mantle. The zircon U-Pb ages of Musi mafic dykes suggest that the delamination lasted until the lately Late Cretaceous.