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

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

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。

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.