湘东南锡田钨锡多金属矿床锡石^40Ar/^39Ar直接定年  

Direct Dating of Cassiterite in Xitian Tungsten-Tin Polymetallic Deposit, South-East Hunan, by ^(40)Ar/^(39)Ar Progressive Crushing

作　　者：王敏[1] 白秀娟[2] 胡荣国[1] 程顺波[3] 蒲志平[1] 邱华宁[1] 

机构地区：[1]中国科学院广州地球化学研究所,同位素地球化学国家重点实验室,广东广州510640 [2]中国地质大学(武汉)资源学院,构造与油气资源教育部重点实验,湖北武汉430074 [3]中国地质调查局武汉地质调查中心,湖北武汉430205

出　　处：《大地构造与成矿学》2015年第6期1049-1060,共12页Geotectonica et Metallogenia

基　　金：973项目(2012CB416706);国家自然科学基金创新群体项目(41121002);中国科学院“135规划”(GIGCAS-135-Y234151001)联合资助

摘　　要：锡田矿床是近年来新发现的一个大型钨锡多金属矿床。为了探讨锡石直接定年的可能性,拓宽流体包裹体^(40)Ar/^(39)Ar定年对象,本文对锡田钨锡矿床一块矿石的共生白云母和锡石进行了^(40)Ar/^(39)Ar定年对比研究。白云母采用激光阶段加热分析,锡石采用真空击碎法提取流体包裹体进行定年,两者获得了非常一致的^(40)Ar/^(39)Ar年龄。白云母形成平坦的年龄谱,坪年龄为155.6±1.7 Ma(2σ)。锡石真空击碎分析形成了下降型阶梯状年龄谱,最初7个阶段表观年龄明显偏老且迅速下降,表明锡石中次生包裹体含有过剩氩;第8~18阶段形成了年龄坪,对应的^(39)Ar释放量占83.8%,坪年龄为154.3±3.0 Ma。锡石年龄坪数据点构成高度线性相关的等时线,等时年龄为155.1±7.0 Ma,代表了原生包裹体的年龄。锡石原生包裹体年龄与共生白云母年龄一致,代表了锡田矿床的成矿年龄,成矿作用发生在~155 Ma,属华南晚侏罗世大规模钨锡多金属成矿高峰期。在K-Cl-40Ar相关图解上,次生、原生包裹体数据点截然分开,表明次生、原生包裹体的流体性质完全不同,且根据这些图解可以获得锡石次生包裹体年龄约为104 Ma。The Xitian tungsten-tin polymetallic deposit is a large deposit in SE Hunan province discovered recently. In order to date the cassiterite directly by ^40Ar/^39 Ar progressive crushing, we selected paragenous muscovite and cassiterite from a piece of tin ore specimen from the Xitian tungsten-tin deposit. The muscovite by ^40Ar/^39 Ar laser stepwise heating forms a flat age spectrum with a plateau age of 155.6±1.7 Ma（2σ）. The ^40Ar/^39 Ar data of the cassiterite by crushing yield an age spectrum marked with abnormal old apparent ages declining step-by-step for the first seven steps, followed by a plateau with an age of 154.3±3.0 Ma for the last eleven steps（steps 8–18, about 83.8% of the total ^39 Ar）. The data points of cassiterite clearly distribute into two groups corresponding to secondary and primary fluid inclusions, respectively. Data points of the first several steps scatter on the inverse isochron diagram, and the apparent ages are abnormally older than the ages of the contemporaneous muscovite, indicating the presence of excess 40 Ar. The excess ^40Ar is most likely hosted in the secondary fluid inclusions distributing along the microcracks and is easily extracted by crushing. While data points of steps 8–18 yield an excellent isochron line corresponding to an age of 155.1±7.0 Ma with an initial ^40Ar/^6） Ar value of 294.8, which are interpreted as the contributions of the primary fluid inclusions, without excess ^40 Ar inside. The isochron age of the primary fluid inclusions in cassiterite is well concordant with that of the paragenous muscovite, indicating that the tin mineralization occurred at 155 Ma, coeval to the Late Jurassic large scale tungsten-tin mineralization in South China. The primary and secondary fluid inclusions could be easily distinguished by the polts of K-Cl-^40Ar, and thus, the ages of secondary and primary fluid inclusions can be obtained from these plots. This study reveals that cassiterite is a suitable mineral for ^40Ar/^39 Ar stepwise crushing.

关 键 词：锡石 流体包裹体 ^40Ar/^39Ar法 湖南锡田 

分 类 号：P597[天文地球—地球化学]