桂北平英花岗岩锆石U-Pb年代学、Hf同位素、地球化学特征及其地质意义

Geochronology, Hf Isotopic Compositions and Geochemical Characteristics of the Pingying Granite Pluton in Northern Guangxi,South China, and Its Geological Significance

通过对桂北平英花岗质岩体详细的锆石U-Pb年代学、Hf同位素组成及岩石地球化学特征的研究,论证了岩体的形成时代、成因类型、源区性质及其与宝坛锡矿的成矿关系。该岩体中心相-粗粒黑云母花岗岩的锆石LA-MC-ICP-MS U-Pb定年表明,其^(206)Pb/^(238)U加权平均年龄为834.2±5.1 Ma,属新元古代构造岩浆活动的产物。平英花岗岩具有高硅、富碱、强过铝质的特征,岩石富集Cs、Rb、U、Ta而亏损Ba、Sr、Ti等元素。球粒陨石标准化稀土配分曲线呈右倾形和强烈的Eu负异常(Eu/Eu*=0.05~0.31)。花岗岩中锆石的εHf(t)值介于-12.6^-1.6之间,峰值在-4.8^-3.0之间;二阶段模式年龄T_(DM)~C(Hf)在1.83~2.51 Ga之间,峰值在1.9~2.0 Ga之间。这些特征表明平英岩体形成于该区古元古代富硼基底的部分熔融作用,并经历了高度的分异演化过程。桂北九万大山—元宝山地区的新元古代黑云母花岗岩具有良好的锡成矿潜力,是华南多时代花岗岩演化及锡多金属成矿系列的重要组成部分。

Granite samples from the Pingying plutons have been analyzed for zircon U-Pb ages, Hf isotope and whole-rock geochemical compositions to discuss the emplacement age, genetic type, source and relationship with the Baotan tin deposit. Zircon LA-MC-ICP-MS U-Pb dating gives a weighted mean 206Pb/238U age of 834.2±5.1 Ma for the coarse-grained biotite granite in the central phase, indicating that it was formed in the Neoproterozoic. The granite samples are strongly peraluminous with high silica and alkali contents and enriched in Cs, Rb, U and Ta, but relatively low in Ba, Sr and Ti. Chondrite-normalized REE diagrams of the granite are right inclined and show strongly negative Eu anomalies with Eu/Eu*values ranging from 0.05 to 0.31. TheεHf(t) values of zircon grains vary from-12.6 to-1.6 with a peak of-4.8^-3.0, and the two-stage Hf model ages range from 1.83 to 2.51 Ga with a peak of 1.9~2.0 Ga. These characteristics indicate that the Pingying granite was formed by partial melting of the Paleoproterozoic basement rich in boron, and experienced strong differentiation. This study shows that the Neoproterozoic biotite granite in the Jiuwandashan-Yuanbaoshan area has the potential for tin mineralization and it is an important part of the evolution of multi-period granite and tin polymetallic metallogenic series in South China as well.