赣东北珍珠山花岗岩脉地球化学、锆石U-Pb定年及Hf同位素组成研究

Geochemistry, Zircon U-Pb Dating and Hf Isotope Composition of Zhenzhushan Granite in Northeast Jiangxi Province

赣东北塔前–朱溪–赋春成矿带是一条重要的铜、钨、钼多金属成矿带,该成矿带及外围成矿潜力巨大,具有良好的找矿前景。本次选取该带南缘珍珠山出露的两条花岗岩脉为研究对象,开展LA-ICP-MS锆石U-Pb测年、Hf同位素及地球化学研究,对其成因、成岩时代和成矿作用进行约束。结果表明,花岗岩脉锆石的206Pb/238U加权平均年龄为129.3±0.5 Ma(MSWD=0.99)和133.3±0.8 Ma(MSWD=0.29),为早白垩世;花岗岩具有高SiO_2(74.10%~75.78%)、富Na_2O+K_2O(6.99%~7.89%)、富Al_2O_3(13.94%~15.15%)、高FeO^T/MgO值(7.50~12.36)、低CaO(0.31%~0.42%)及贫MgO(0.07%~0.13%)的特征。其中Na_2O/K_2O=3.14~9.09,A/CNK=1.08~1.30,属过铝质碱性岩系列。稀土总量较低,轻、重稀土分异较轻(LREE/HREE平均2.48),具强负Eu异常(δEu平均0.07),呈弱右倾海鸥形态;微量元素Ga/Al值高(3.41×10^(–4)~4.27×10^(–4)),相对亏损K、Ba、Nd、Sr等大离子亲石元素,富集U、Th、Nb、Ta、P等高场强元素。其中锆石εHf(t)值为–5.09~–0.05,显示成岩物质以壳源为主,并有地幔成分参与,Hf单阶段模式年龄范围为761~948 Ma,两阶段模式年龄范围为1153~1463 Ma。综合分析认为珍珠山花岗岩属于A1型花岗岩,是在早白垩世构造体制转换结束后非造山伸展拉张环境下侵位,岩脉中Sn、Nb含量高,暗示该区具有较大的钨锡铌钽矿找矿潜力。

The discovery of the ultra-large skarn-type copper-tungsten deposit in the Taqian-Zhuxi-Fuchun metallogenic belt in Zhuxi, Northeast Jiangxi province, exhibits a huge metallogenie potential and prospect in the belt and its peripheral district. The Zhenzhushan area is located in the south margin of the Metallogenic belt where a number of granite veins exposed. Based on the detailed field geological survey, petrological and geochronological studies, LA-ICP-MS zircon U-Pb dating of two big veins of porphyritic muscovite albite granite yielded weighted average ages of 129.3±0.5 Ma （MSWD=0.99） and 133.3±0.8 Ma （MSWD=0.29）, respectively. The granites are rich in SiO2 （74.10% -75.78%）, NaEO＋K2O （6.99%-7.89%）, Al2O3 （13.94%- 15.15%）, poor in CaO （0.31%-0.42%） and MgO （0.07%-0.13%）,with FeOT/MgO ratios of 7.50-12.36, Na2O/K2O ratios vary between 3.14 and 9.09, A/CNK=1.08-1.30, belonging to peraluminous alkaline rock series. The granites are characterized by low EREE value and weak REE fractionation （LREE/HREE average value is 2.48）, and showing weakly right dipping seagull shapes with obvious negative Eu anomalies （6Eu average value is 0.07）. Moreover, the rocks are characterized by high Ga/A1 values （3.41× 10^-4-4.27× 10^-4）, and relatively depleted in K, Ba, Nd, Sr and other LILEs, yet enriched in U, Th, Nb, Ta, P and other HFSEs. The εHf（t） values of zircons range from -5.09 to-0.05, indicative of juvenile mantle components contribution. The granites were likely derived from mixed source of crust and mantle. The A1-type features of the granites in the Zhenzhushan area suggest that the area was in an anorogenic extensional setting. In addition, the high contents of Sn and Nb in the dykes implies a great potential for W-Sn-Nb-Ta prospecting in the area.