西秦岭中段日多玛岩体锆石U-Pb年龄、地球化学特征及其构造意义

日多玛花岗闪长岩体位于西秦岭中段美武岩体附近,对该岩体进行了岩相学、锆石U-Pb年代学和岩石地球化学方面的研究。日多玛花岗闪长岩体含有较丰富的暗色微粒包体,花岗闪长岩和暗色微粒包体LA-ICP-MS锆石U-Pb年龄分别为(236.8±3.6) Ma(MSWD=4.1)和(242.7±1.6) Ma(MSWD=1.4),属于印支早期。日多玛岩体具有富钾(2.58%~2.75%)、富碱(Na₂O+K₂O=5.57%~5.76%)、弱过铝质(A/CNK=1.48~1.51)特征,属于高钾钙碱性I型花岗岩类。日多玛岩体稀土元素表现为轻重稀土元素分馏较明显(LREE/HREE=10.4~11.9)、呈右倾特征,具有中等Eu负异常(δEu=0.68~0.81),岩石富集K、Rb、Ba、Th和U等大离子亲石元素,明显亏损Nb、Ta、Ti和P等高场强元素。岩石地球化学特征表明,日多玛花岗闪长岩来源于下地壳高钾玄武质岩石的部分熔融。此外,花岗闪长岩体具有较高的Mg^#(57~61)、Cr(149×10^-6~185×10^-6)和Ni(36×10^-6~47×10^-6),显示有少量幔源物质加入,暗色微粒包体可能代表了这种幔源岩浆。结合区域地质背景分析,认为日多玛花岗闪长岩体形成于后碰撞构造环境,可能与俯冲洋壳的断离作用有关。

Zircon U-Pb Ages,Geochemical Characteristics and Tectonic Implications of the Riduoma Granodiorite in the Middle Section of Western Qinling Orogen

Riduoma granodiorite is located near the Meiwu pluton in the middle section of the Western Qinling Orogen. A integrated study of petrography, U-Pb zircon dating and whole-rock geochemistry was carried out for the Riduoma granodiorite. The granodiorite contains numerous mafic microgranular enclaves (MMEs). LA-ICP-MS zircon U-Pb dating yielded a weighted mean age of (236.8 ± 3.6) Ma (MSWD=4.1) for the granodiorite and (242.7±1.6) Ma (MSWD=1.4) for the MMEs. The Riduoma pluton is weakly peraluminous (A/CNK=1.48-1.51) and high-K calc-alkaline, with high contents of K₂O (2.58%-2.75%), K₂O+Na₂O (5.57%-5.76%), and belongs to I-type. The granodiorite is characterized by enrichment in LILEs (such as K, Rb, Ba, Th, and U) and depletion in HFSEs (such as Nb, Ta, Ti and P) with high LREE/HREE ratios (10.4-11.9) and moderately-negative Eu anomalies (δEu=0.68-0.81). Geochemical characteristics of the Riduoma granodiorite suggest that it was derived from partial melting of the lower crust high-K basaltic rocks. In addition, the granodiorite shows relatively high Mg ^# (57-61), Cr (149×10^-6-185×10^-6), and Ni (36×10^-6-47×10^-6), suggesting minor mantle-derived contribution. The MMEs may represent this source magma. Petrography, zircon U-Pb ages, and geochemical characteristics, combined with the regional setting, suggest that the Riduoma pluton emplacement was resulted from the subducted slab break-off in a post-collisional setting.