拉萨地体南缘早始新世荣玛辉长岩年代学、岩石地球化学特征及其地质意义

印度-欧亚大陆初始碰撞后的新特提斯洋板片断离过程至今尚未得到较好的约束.对拉萨地体南缘荣玛地区早始新世辉长岩开展了锆石U-Pb定年、全岩主微量及Sr-Nd同位素分析,探讨了岩石成因及动力学意义,以进一步约束新特提斯洋板片断离过程.研究结果表明,荣玛辉长岩的锆石U-Pb年龄为51±1 Ma,形成于早始新世;地球化学特征显...

Geochronology and Geochemistry Implications for Early Eocene Rongma Gabbros in Southern Margin of Lhasa Terrane,Tibet

The slab breakoff of the Neo-Tethys Ocean after the initial India-Asia collisional has not been well constrained up to now. In this paper, zircon U-Pb dating, whole rock major and trace element and Sr-Nd isotopes analysis of the Early Eocene gabbros in the Rongma area on the southern margin of the Lhasa terrane have been carried out, the petrogenesis and geodynamical implication are discussed to further constrain the slab breakoff of the Neo-Tethyan oceanic slab.The results show that zircon U-Pb ages of 51±1 Ma have been obtained for Rongma gabbros, indicating they were formed in the Early Eocene.The Rongma gabbros are enriched in LILEs (such as Rb, Sr, Ba) and depleted in HFSEs (such as Nb, Ta, Ti) with initial 87Sr/86Sr=0.705 9-0.706 6 and ε_Nd(t)=+3.1-+3.3. Compared with typical arc magmas, the Rongma gabbros have high contents of Zr (134.28×10-6-230.07×10-6), TiO₂(1.04%-1.51%) and Nb (9.01×10-6-14.67×10-6), suggesting a geochemical intraplate-affinity. In addition, Rongma gabbros were derived from depleted lithospheric mantle which metasomatized by slab-derived fluids during Neo-Tethyan subduction, accompanied with significant contributions from deep asthenosphere mantle.Combined with the geochemical and geochronological data of the Cenozoic magmatic rocks from the southern Lhasa terrane, it further proposes that slab breakoff of the Neo-Tethys Ocean has occurred before 51 Ma.