北羌塘三叠纪辉石闪长玢岩的成因及其地球动力学背景

尽管大量的研究发现青藏高原中部地区发育有三叠纪大规模岩浆活动,然而对于这些岩浆活动的成因机制以及深部动力学背景等问题的认识还存在很大的分歧.通过对青藏高原中部北羌塘地区三叠纪典型辉石闪长玢岩开展了系统的锆石U-Pb年代学、矿物化学、岩石地球化学与Sr-Nd-Hf同位素研究,深入剖析其岩石成因及动力学背景,进而为深入探究青藏高原中部三叠纪岩浆作用形成机理与有关板块构造演化过程提供重要的证据.通过LA-ICP-MS锆石U-Pb测年方法获得该岩浆岩的结晶年龄为227±2 Ma(MSWD=0.86).岩石样品在地球化学组成上相对富硅和铝,属于钙碱性系列.样品轻重稀土分异较强,轻稀土相对于重稀土显著富集,并且具有明显的Eu负异常.在微量元素组成上,样品相对于原始地幔要富集轻稀土以及Th、U,显著亏损Nb、Ta和Ti,整体表现出火山弧岩浆岩的特征.样品均具有相对较低的εNd(t)值(-4.53^-4.99)、比较集中ISr值(0.70705~0.70714),以及正的εHf(t)值(+0.81^+2.48),表明其岩浆源区很可能以富集的地幔组分为主.综合岩石学、地球化学与Sr-Nd-Hf同位素等研究结果,确定该岩体的形成应该与受俯冲组分(例如沉积物)改造的的富集地幔的部分熔融有关.此外,结合区域上已有的多学科研究资料还可以证实北羌塘地区有关古特提斯洋的俯冲时间至少开始于227 Ma.

Petrogenesis and Geodynamic Setting of Triassic Pyroxene Diorite-Porphyrite from the North Qiangtang Terrane: Geochronology,Mineral Petrogeochemistry and Sr-Nd-Hf Isotope Constraints

Although large volume of Triassic magmatism has been identified in the central Tibetan Plateau,the petrogenesis and geodynamic setting of these igneous rocks are still poorly understood. In this paper,we carried out a detailed study of zircon U-Pb geochronology,petrology,and geochemisrty for the diorite porphyrite in North Qiangtang terrane,aiming to reveal its petrogenesis and constaint the exploration of the Triassic tectonic-magmatic evolution. The LA-ICP-MS zircon U-Pb dating for this magmatic rock yields a crystallization age of 227±2 Ma (MSWD=0.86). All the samples have high contents of silicon and aluminum and belong to calc-alkaline series in composition. Compared with the primitive mantle,they exhibit enrichment of LREE,Th,and U,with depletion of Nb,Ta and Ti. All the samples have relatively low ε_Nd(t) values (-4.53 to -4.99),relatively concentrated I_Sr values (0.707 05 to 0.707 14),and positive ε_Hf(t) values (+0.81 to +2.48),suggesting an enriched mantle. Geochemical and isotopic variations reveal that the diorite-porphyrite was derived from partial melting of an enriched mantle that was previously modified by subduction-related components(e.g.,sediments). In addition,combined with the existing multi-disciplinary research data in the region,it could be concluded that the subduction of the Paleo-Tethyan ocean in the North Qiangtang terrane lasted at least to 227 Ma.