湘东北金鸡金矿床岩体成岩机制

江南造山带湖南段中早古生代花岗质岩石对于研究早古生代构造演化以及金成矿作用具有重要的意义。位于该区中段的金鸡金矿床钻孔中新发现有两类花岗质岩石,分别为花岗岩和花岗闪长岩。对两类岩体样品进行了锆石LA-ICP-MS U-Pb测年,获得的年龄分别为(425.2±1.5)Ma和(430.6±1.5)Ma。岩石地球化学数据表明,花岗岩属I型花岗岩,其来源于地壳中变泥质岩石的部分融熔;花岗闪长岩属埃达克岩,其起源于地壳中变砂质岩石的部分融熔。Sr-Nd同位素分析显示,金鸡花岗闪长岩具有较高的(87Sr/86Sr)i(0.722369~0.722488)、较低的(143Nd/144Nd)i(0.511941~0.511990)以及εNd(t)值较低(–8.2~–7.2),并且金鸡花岗闪长岩的二阶段Nd模式年龄值为1.75~1.84 Ga,与江南造山带变质基底的二阶段模式年龄(1.65~2.14 Ga)一致。金鸡金矿床花岗岩和花岗闪长岩的岩石地球化学、年代学以及Sr-Nd同位素特征表明二者是华南早古生代陆内造山事件的产物,岩体成因及地球动力学背景的研究将有助于揭示湘东北地区金矿形成的地球动力学机制。

Petrogenesis of the Granitoids in the Jinji Au Deposit, Northeastern Hunan Province

The Middle–Early Paleozoic granitoids in Hunan section of the Jiangnan orogenic belt are of great significance for the study of the Early Paleozoic tectonic evolution and gold mineralization.Two types of granitoids,i.e.,granite and granodiorite,were recently found in the Jinji gold deposit,which is located in the central section of the orogenic belt.Two types of rock samples were dated by zircon La-ICPMS U-Pb technique,and the results show that the granite and granodiorite yielded U-Pb zircon ages of(425.2±1.5)Ma and(430.6±1.5)Ma,respectively.The geochemical data showes that the granite belongs to I-type granite,which was derived from partial melting of metapelitic rocks in the crust;the granodiorite belongs to adakite,which was derived from partial melting of metasandy rocks in the crust.The Sr-Nd isotopic analysis shows that the granodiorite has higher(87Sr/86Sr)i values of 0.722369 and 0.722488,lower(143Nd/144Nd)i values of 0.511941 and 0.511990 and lowerεNd(t)values of 8.2 and 7.2,and the two-stage Nd model ages of the granodiorite are 1.75 Ga and 1.84 Ga,which are consistent with the ages of metamorphic basement in the Jiangnan orogenic belt(1.65~2.14 Ga).The petrogeochemical,geochronologic and Sr-Nd isotopic characteristics of granites and granodiorites in the Jinji gold deposit indicate that they were produced by Early Paleozoic intracontinental orogenic events in South China,and the results help to investigate the geodynamic mechanism for Au mineralization in northeastern Hunan Province.