广西栗木矿区牛栏岭岩体印支期年龄的厘定及其意义

Confirmation of Indosinian Age of the Niulanling Granite from Limu Orefield,Guangxi Province and Its Significance

本文对牛栏岭细粒铁锂云母花岗岩和似斑状铁锂云母花岗岩进行了锆石u—Ph年代学和岩石地球化学研究，讨论了岩体的形成时代、成因类型、源区性质及其与成矿关系。两种岩性锆石的LA—ICP—MSU—Pb定年表明，其加权平均年龄分别为（223．6±1．7）Ma和（223．7±1．7）Ma，属印支晚期构造岩浆活动的产物。岩石相对富集SiO2，K2O，Na2O，P2O；和贫CaO，MgO，TiO2；A／CNK值为1．08～1．31，落入强过铝质花岗岩区域；CIPW计算结果给出的岩体标准矿物刚玉的含量为1．6％～4．5％；岩体的球粒陨石标准化稀土配分曲线呈轻微右倾形，具有强烈的Eu负异常，8Eu为0．01～0．05。岩体富集cs，Rb，K等大离子亲石元素（LILE）和U，Pb，Hf，Ta等高场强元素（HFSE），明显亏损Ba，Sr，n等元素；锆石饱和温度介于631℃～681℃之间，低于S型花岗岩的平均锆石饱和温度。岩体中发现有（248±4）Ma和（1947±23）Ma的继承锆石，暗示其源区可能为晚古元古代的壳源物质，印支早期岩浆物质也可能参与了岩体的形成。岩体的地质地球化学特征研究表明，岩体可能为牛栏岭地区的W—Sn成矿作用提供W—Sn来源，暗示牛栏岭地区可能存在有224Ma左右的成矿作用，印支期成矿作用可能成为栗木矿区乃至南岭西段今后找矿工作的一个重点方向。

The samples from the Niulanling fine-grained and porphyritic granites have been analyzed for zircon U-Pb ages and whole rock geochemical compositions to discuss the emplacement age, genetic type, source and relationship with the mineralization. LA-ICP-MS zircon U-Pb dating results of the two granite samples indicate that the granites formed in the Late Indosinian period with weighted average age of （223.6 ± 1.7） Ma and （223.7 ± 1.7） Ma, respectively. The granites are rich in SiO2, K20, Na20 and P205 and poor in CaO, MgO and TiO2. The granites are strongly peraluminous with the A/CNK values changing from 1.08 to 1.31. CIPW-normative calculation gives 1.6%-4.5% corundum. Chondrite-normalized REE diagrams of the granites have slightly right-leaning shape with strong negative Eu anomalies and 8Eu values vary from 0.01 to 0.05; The granites are enriched in LILE （Cs, Rb, K） and HFSE （ U, Pb, Hf, Ta ）, but are relatively low in Ba, Sr and Ti. The zircon saturation temperatures （tzr） are from 631 to 681 ℃, which are lower than the averageing value of S-type granites. The （1947 _± 23） Ma inherited zircon indicates that the source region of the granite is mainly Late Palaeoproterozoic crustal materials. The inherited zircon of （248 ± 4） Ma suggests an addition of the Early Indosinian magma in the formation of the granites. The geological and geochemical characteristics of the granites show that the granites could provide W and Sn for the W-Sn mineralization in the Niulanling district and there might be mineralization of about 224 Ma in this area. The Indosinian mineralization should be the key target in the Limu district and very important even for western Nanling Range as a whole in the further ore-prospecting work.