广西右江褶皱带东南缘西大明山矿集区燕山期酸性岩浆锆石U-Pb年龄、Hf同位素和Ce(Ⅳ)/Ce(Ⅲ)特征

LA-ICP-MS Zircon U-Pb Dating,Hf Isotopic Composition and Ce^(4+)/Ce^(3+) Characteristics of the Yanshanian Acid Magma in the Xidamingshan Cluster,Southeastern Margin of the Youjiang Fold Belt,Guangxi

广西西大明山多金属矿集区位于右江褶皱带东南缘。LA-ICP-MS锆石U-Pb定年结果显示,区内那宁石英斑岩脉、罗维隐伏二长花岗岩和黑云母花岗岩岩体的侵位年龄分别为(93.11±0.64)Ma、(92.92±0.69)Ma和(92.5±1.1)Ma,说明三者都形成于晚白垩世。Hf同位素组成结果显示,石英斑岩中锆石εHf(t)值变化于-18.4^-1.2之间,两阶段模式年龄变化于1 236~2 326 Ma之间。二长花岗岩和黑云母花岗岩锆石εHf(t)值分别为-19.4^+0.3和-12.2^-3.6,两阶段模式年龄分别变化于1 139~2 385 Ma和1 386~1 933 Ma,表明它们主要来源于中元古代末期增生的地壳组分与少量古元古代地壳地质的共同熔融。通过与右江褶皱带周缘岩浆岩带锆石Hf同位素特征对比,认为右江褶皱带周缘岩浆岩可能具有一致的物源。二长花岗岩和黑云母花岗岩中锆石的Ce(Ⅳ)/Ce(Ⅲ)比值平均值分别为23.65和33.76,说明罗维隐伏岩体具有较低的氧逸度。通过对比右江褶皱带周缘前人的成矿年代学及同位素研究,显示该区成岩成矿时间较短(92~95 Ma),形成于晚白垩世区域伸展构造体制下,右江褶皱带岩石圈发生松弛和伸展垮塌,导致中元古代和少量古元古代地壳发生部分熔融,形成大规模岩浆作用。结合右江褶皱带周缘岩浆带大地构造位置及成岩时代,认为右江褶皱带燕山期岩浆岩的形成可能受到太平洋构造域和特提斯构造域双重作用,可能以后者为主。

The Xidamingshan cluster is located along the southeastern margin of the Youjiang fold belt,SouthChina. LA-ICP-MS zircon U-Pb dating constrains the crystallization of the quartz porphyry,monzogranite and biotite granite at（93. 11 ± 0. 64） Ma,（92. 92 ± 0. 69） Ma and（92. 5 ± 1. 1） Ma,respectively. They have the same ages with each other within errors,suggesting they are magmatism coevally emplaced during the Later Cretaceous. The εHf（t） values of the quartz porphyry vary from -18. 4 to -1. 2 with an average value of -7. 1,with two-stage model Hf ages（TDM2） of 1 236 to 2 326 Ma with an average value of 1 609 Ma. And the values of the monzogranites and biotite granite are from -19. 4 to ＋ 0. 3 with an average value of -5. 6 and -12. 2 to -3. 6 with an average value of -6. 7,correspond to two-stage model Hf ages（TDM2） of 1 139 to 2 385 Ma and 1 386 to 1 933 Ma,respectively. It indicates that the source material was derived from the partial melting of the Paleo-and Mesoproterozoic crust. Comparing with Hf isotopic characteristics of the Yanshanian granites in the boundary of the Youjiang fold belt,we suggest that all of the granites may have the same magma sources. The average Ce（Ⅳ）/Ce（Ⅲ） ratios of monzogranite and biotite granite are 23. 65 and 33. 76,respectively. It suggests that the Luowei concealed intrusion is with low oxygen fugacity. Combined the results from this study coupled with previous research,we proposed that the magmas formed in a regional extensional setting along the southeastern margin of the Youjiang fold belt in the South China during the Later Cretaceous.This caused the partial melting of the Paleo-and Mesoproterozoic crust due to the lithosphere relaxation and collapse,which meanwhile resulted in the large-scale magmatism in the Xidamingshan cluster. In the light of tectonic settings and geochronology,we deduct that the Yanshanian granites along the margin of the Youjiang fold belt are related to both Pacific and Tethyan tectonic domain.