云开地区燕山晚期花岗岩的岩石成因及构造意义: 锆石U-Pb年龄及Hf同位素证据

相比较丰富的前寒武纪和早古生代地质记录, 云开地区燕山晚期花岗岩类分布很少, 且未见详细的研究报道.对云开地区广西陆川米场、三叉冲钨矿床以及松旺钨锡钼矿床等3个典型地区的燕山晚期花岗岩进行了系统的LA-MC-ICP-MS锆石U-Pb和Lu-Hf同位素研究.定年结果表明, 米场黑云母花岗岩、三叉冲黑云母花岗岩和松旺花岗岩的侵位年龄分别为113±1 Ma, 103±1 Ma和88±1 Ma, 为燕山晚期岩浆作用的产物.三叉冲黑云母花岗岩和松旺花岗岩具有较一致锆石ε_Hf(t)值(分别为-5.2~-2.7和-5.2~-3.6)和二阶段模式年龄T_DM2(分别为1.3~1.5 Ga和1.4~1.5 Ga), 指示为中元古代地壳物质再造的产物.米场黑云母花岗岩具有相对较高的ε_Hf(t)值(-2.3~1.4)和T_DM2(1.1~1.3 Ga), 并且其含有大量的镁铁质微粒包体(MME), 表明米场黑云母花岗岩很可能是壳源-幔源岩浆混合的产物.结合中国东南部构造-岩浆演化来看, 云开地区燕山晚期花岗岩的形成可能与白垩纪时太平洋板块向华南板块俯冲后板片的折返-断离有关. 

Petrogenesis and Tectonic Significance of Late Yanshanian Granites in Yunkai Area,Southeast China:Evidence from Zircon U-Pb Ages and Hf Isotopes

Compared with abundant Precambrian and Early Paleozoic geological records in the Yunkai area, southeast China, Late Yanshanian granitoids are sparsely distributed in the area and are thus reported less. In this study, we present LA-MC-ICP-MS U-Pb age and Hf isotope data of zircons from three representative granites in the Yunkai area, southeastern Guangxi, each from the Michang area, Sanchachong W and Songwang W-Sn-Mo deposits respectively. Zircon U-Pb results indicate that all the granites were formed in Late Yanshanian period, with emplacement ages of 113±1 Ma (Michang biotite granite), 103±1 Ma (Sanchachong biotite granite) and 88±1 Ma (Songwang granite) respectively. The Sanchachong biotite granite and Songwang granite have consistent zircon ε_Hf(t) values (-5.2 to -2.7 and -5.2 to -3.6 respectively) and two-stage Hf modal ages (T_DM2) (1.3-1.5 Ga and 1.4-1.5 Ga respectively), indicating that they were formed by reworking of Mesoproterozoic juvenile crust; whereas the Michang biotite granite has relatively higher zircon ε_Hf(t) values of -2.3 to 1.4 and corresponding T_DM2 of 1.1-1.3 Ga, which, combined with widespread microgranular mafic enclaves (MMEs) in the granite, suggests that it was formed by mixing of crust- and mantle-derived magmas. In combination of Mesozoic tectono-magmatic evolution in southeast China, the Late Yanshanian magmatism in the Yunkai area was likely related to the rollback and break-off of the subducted Pacific plate during the Cretaceous.