福建龙岩大洋-莒舟花岗岩地球化学、年代学、铪同位素特征及其地质意义

Zircon U-Pb geochronology, geochemistry and Hf isotope compositions of the Dayang and Juzhou granites in Longyan, Fujian and their geological implications

为了揭示闽西南龙岩地区大洋、莒舟花岗岩体的成因、侵位时代及其与成矿的关系，在岩石学、地球化学研究基础上，利用LA-MC-ICPMS锆石U-Pb、Lu-Hf同位素测试方法对上述岩体进行年龄测定及铪同位素示踪。岩石学特征表明大洋、莒舟岩体具有多期次侵入特征，岩性主要为花岗岩、似斑状花岗岩以及花岗闪长岩。大洋岩体花岗闪长岩的年龄为（136．2±0．9）Ma，细粒花岗岩的年龄为（125．0±1．7）Ma，似斑状花岗岩的年龄为（132．0±0．2）Ma，菖舟似斑状花岗岩的年龄为（132．4±0．8）Ma。岩石地球化学成分上具有富硅、富铝、低钛、高钾钙碱性特征，富集大离子亲石元素（K、Rb、Cs、Pb等），亏损高场强元素（Nb、Ta、Zr、Ce、Ti等），微量元素表现出后造山伸展背景的特征。稀土含量较低，总稀土含量分别为92．76×10^-6-176．26×10^-6和116．63×10^-6-213．95×10^-6，具有显著的负铕异常（δEu=0．01-0．54）和稀土分布模式差异性特征。总体上具有较高的Na2O＋K2O值、Ga／Al值及Rb含量，低Zr含量，具有高分异Ⅰ型花岗岩成因特征。两个代表性样品锆石铀εHf（t）值分别为-16．47--5．95和-11．05--3．35，对应的二阶段模式年龄分别为1567～2221Ma和1400～1889Ma，来源于古元古一中元古代地壳物质重熔。在以上研究基础上，讨论了两个岩体的成因及其所代表的构造环境，进而探讨了大洋、莒舟花岗岩与成矿的关系。

In order to reveal the petrogenesis, emplacement age of the Dayang and Juzhou granites from Longyan area in Fujian Province and their relationships with the ore-forming events, the LA-MC-ICPMS was employed for zircons U-Pb and Lu-Hf dating to assess the ages and sources of the granites. The Dayang and Juzhou granites consists of granite, porphyritic granite, and granodiorite, and show the characteristics of polyphase intrusion. Results of high-precision LA-MC-ICPMS U-Pb dating of zircons in the Dayang and Juzhou granites yields weighted mean ^206Pb/^238U ages of （136.2±0.9） Ma, （125.0±1.7） Ma and （132.0±0.2） Ma for Dayang granite, and （132.4±0.8） Ma for Juzhou granite. Basedon these geochemical results, these granites can be attributed to the high-K calc-alkaline series, with high contents of silicon and calcium and aluminum. All these samples are enriched in LREEs and LILEs （K, Rb, Cs, Pb, et al.）, but depleted in HFSEs （Nb, Ta, Zr, Ce, Ti, et al.）. The Dayang and Juzhou granites have relatively low REEs contentc （∑REE 92.76×10^-6-176.26×10^-6, 116.63×10^-6- 213.95×10^-6）, with significant negative Eu anomalies （δEu = 0.01-0.54）, and show different REEs distribution patterns. These granites show highly fraetionated I type geochemical signatures with high Na2O＋K2O, and Ga/Al ratios and Rb contents, but low Zr contents. They display εHf（t） values in the range of-16.47 to -5.95 and -11.05 to -3.35, and Hf isotope two-stage model ages of 1.57 to 2.22 Ga and 1.40 to 1.89 Ga respectively. These characteristics suggest that the Early Cretaceous granite may have been derived from partial melting of the paleoproterozoic- Mesoproterozoic crust materials. Based on the above study, the authors discussed the petrogenesis of and the tectonic environment for the Dayang and Juzhou granites, and the relationship between the granites and mineralization.