| 陕西洛南县石家湾钼矿相关花岗斑岩的年代学及岩石成因：锆石U-Pb年龄及Hf同位素制 约 |
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| 引用本文：梁慧山.2010.Chronology and petrogenesis of Shijiawan granite porphyry in Shannxi Province: Constrains from zircon U-Pb geochronology and Hf isotopic compositions[J].Mineral Deposits,29(1):143~157 |
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| 基金项目:中央级公益性科研院所基本科研业务费专项资金（K0816）和全国危机矿山接替资 源找矿项目（20089949、200841019） |
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| 中文摘要:石家湾钼矿床位于东秦岭钼成矿带的西部黄龙铺地区，矿体主要分布于石家湾花岗斑岩外接触带的围岩中，呈细脉-网脉浸染状产出，属斑岩型矿床。花岗斑岩岩石呈斑状结构，斑晶与基质成分相同，主要矿物包括钾长石、石英、斜长石和黑云母。锆石的LA-ICP-MS U-Pb年代学研究表明，花岗斑岩的年龄为139.9～145.6 Ma，加权平均年龄为（141.4±0.6）Ma（MSDW=1.00），侵位于早白垩世，与辉钼矿的Re-Os同位素年龄138 Ma相近。岩石地球化学分析表明，石家湾花岗斑岩具有高硅〔w(SiO2) 70.52%～73.04%〕、富碱〔w(Na2O)+w(K2O)为8.13%～9.12%〕、准铝质特征（A/CNK为0.99～1.13）；稀土元素总量中等偏低（∑REE为145.27×10-6～156.97×10-6），富集轻稀土元素（∑LREE为135.38×10-6～145.97×10-6），中等Eu亏损（δEu 0.65～0.71），标准化曲线呈右倾；微量元素以富集Rb、Ba、U而亏损P、Ti、Ta、Nb 元素为特征。岩石具有较高的Sr初始比值（0.7073～0.7077）及较低的εNd（t）值（-14.8～-16.0），与地壳的同位素组成特征相近。锆石Hf的原位分析表明，176Hf/177Hf值变化于0.282056～0.282312之间，εHf（141 Ma）值介于-25.01～-15.97，平均值为-20.64，极低的负值指示了岩浆源区以下地壳为主。此外，矿石硫同位素和辉钼矿中Re 的含量反映了源区地幔物质的混入。与区内金堆城花岗斑岩及老牛山花岗岩的地球化学特征对比表明，石家湾花岗斑岩与它们两者可能来源于同源母岩浆，均形成于地壳物质的部分熔融，SiO2与Al2O3、TiO2、P2O5、TFeO呈明显的负相关性，说明岩浆演化过程中经历了一定程度的结晶分异作用。结合区域资料分析认为，区内三叠世以来共发生过3次岩浆侵入活动，每次都伴有不同程度的钼矿化作用，并分别对应于东秦岭-大别地区3个中酸性岩浆活动和钼矿化的峰期。 |
| 中文关键词:地球化学 锆石U_Pb 定年 Hf同位素 花岗斑岩 石家湾 |
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| Chronology and petrogenesis of Shijiawan granite porphyry in Shannxi Province: Constrains from zircon U-Pb geochronology and Hf isotopic compositions |
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| Abstract:The Shijiawan Mo deposit is located in Huanglongpu area of the East Qinling molybdenum belt, and its ore bodies mostly occur within the wall rocks at the exocontact zone of Shijiawan granite porphyry. Rocks have porphyritic textures. Phen ocrysts and matrixes have similar minerals, mainly composed of potassic feldspar, quartz, plagioclase and biotite. The granite porphyry is characterized by the enrichment of silica w(SiO2)=70.52%～73.04%) and alkali 〔w(Na2O）+ w(K2O)=8.13%～9.12%〕,and para-aluminous nature (A/CNK=0.99～1.13). The chondrite-normalized REE patterns have REE concentrations ranging from 145.27-6 to 156.97-6, LREE enrichment and weak negative Eu anomalies. They are depleted in P, Ti, Ta, Nb a nd rich in Rb, Ba, U in the primitive mantle-normalized spidergrams. They have high initial 87Sr/86Sr ratios （0.7073～0.7077）and low εNd（t）values（-14.8～-16.0）. Zircons from the granite porphyry hav e U-Pb ages of 139.9 to 145.6 Ma, with a weighted mean age of (141.4±0.6) Ma, suggesting that the intrusion was emplaced in Early Cretaceous, corresponding to 138 Ma Mo mineralization age obtained by the Re-Os method. Zircon insitu analysis gives 176Hf/177Hf ratios ranging from 0.282056 to 0.282312, and the average εHf（141 Ma）of -20.64, indicating that their parental magma was mainly derived from the lower crust. This conclusion is consistent with the Sr, Nd isotopic results. In addition, sulfur isotopic analysis and Re content of molybdenites imply that there were some inputs from the mantle. In comparison of geochemical features with Laoniushan granite and Jinduicheng granite porphyry of the same area, Shijiawan may share a congenetic magmatic source with the others; they were all formed from the melts of the upper crust and unde rwent a strong crystal differentiation during the evolutionary process. Based on regional geological data, it can be concluded that this area have experienced three times of magmatic intrusion activities, which corresponded respectively to three times of Mo mineralization since Late Triassic. |
| keywords:geochemistry,U-Pb zircon dating, Hf isotope, granite porphyry, Shijiawan |
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