华南前寒武系基底变质杂岩和中-新生代地幔岩的稀有气体同位素组成及其大地构造意义

Isotope compositions of noble gases in the Precambrian basement metamorphic complex and Meso-Cenozoic mantle-derived rocks in South China and their geotectonic significance

对华南前寒武系变质岩浆杂岩稀有气体He、Ne、Ar和Xe的系统研究表明:扬子克拉通基底为含高3He的下地壳"原始岩石层",(3He/4He)×10-6比值为2.8～4.6;而华夏板块基底变质岩浆杂岩则是在缺乏3He、低(3He/4He)×10-8比值(3.15～17.7)的构造环境下形成的大陆中-上地壳变质岩浆杂岩层,反映出两者基底性质迥然不同。华南中-新生代爆破岩筒He同位素组成相反,相对稳定的扬子克拉通(3He/4He)×10-8比值仅0.18～4.22,而郯庐—四会—吴川断裂以东,中-新生代活动地块(太平洋构造域)(3He/4He)×10-8比值高达3.7～20.5。He同位素表明郯庐—四会—吴川断裂带为切割深至地幔的边界深大断裂,是扬子克拉通与华夏板块间的边界且控制了燕山期火山—侵入岩浆向西扩展。Ar同位素组成表明华南大陆中-新生代地幔形成接近"均一"的地幔组份。136Xe/130Xe—129Xe/130Xe相关组份表明它们具有地幔柱岩石同位素组成特征。

The systematic study on noble gases He,Ne,Ar and Xe in the Precambrian basement metamorphic complex in South China （including the Yangtze Craton and the Cathaysian plate ） showed an obvious difference in isotope compositions. The basement of the Yangtze Craton was characterized by higher ^3He contents and higher ^3He/^4He （×10^-6 ） ratios ranging from 2.8 to 4.6 which suggested that the basement was evolved from the＂ original rock formation＂ of the lower crust. However,the basement metamorphic complex of the Cathaysian plate was a formation of the middle-upper crust with the tectonic environment characterized by lower ^3He contents and lower （^3He/^4He） ×10^-8 ratios changing from 3.15 to 17.7. The He isotope compositions of the Meso-Cenozoic explosive pipes from South China presented a striking contrast with those of Precambrian basement metamorphic complex. In the relatively stable region of the Yangtze Craton, the ^3He/^4He （10^-8） ratios of the explosive pipes were 0.18-4.22 only, while in the active tectonic region of the Pacific tectonic province to the east of the Sihui-Wuchuan fault zone, the ^3He/^4He（×10^-8） ratios increased to 3.7 -20. 5. The data of Ne isotope compositions of Meso-Cenozoic explosive pipes from South China demonstrated that the ^20Ne （×10^-8cm^3STP/g）, ^20Ne/^22Ne and ^21Ne/^22Ne（×10^-2） values for the K-Mg lamprophyre from the Yangtze Craton were 0.26 -0.44,10.3 and 0.027 respectively, while those from the Cathaysian plate were 3.15,9.33 and 0.031 respectively. It was inferred that the former might be derived from the mantle containing abundant original ^20Ne isotope while the latter from perfectly ＂homogeneous＂ MORB metamorphosed or contaminated by continental rocks during the diagenetic period. The ^40Ar/^36Ar ratios in the Neoproterozoic spilites from the Yangtze Craton and the Cathaysian plate were 272 and 750 respectively. Furthermore,the ^40Ar/^36Ar ratios of the Neoproterozoic gabbros from the Yangtze Craton increased to 2370, indicating that the gabbros were products of the mantle plume activities in the Yangtze Craton. The ^136Xe/^130Xe and ^129Xe/^130Xe ratios also showed the characteristics of isotope compositions of the mantle plume rocks. Based on what mentioned above,it was thought that the nature of Precambrian basement complex and evolution of the materials in depths of the two tectonic units in South China should be widely different, and the both became a unified subcontinent after the mantle in South China had a nearly ＂homogeneous＂ composition during Mesozoic. The Sihui-Wuchuan fault should be a deep fracture cutting to the mantle, and an important tectonic boundary separating the Cathaysian plate from the Yantze Craton during Precambrian. In Mesozoic, the Sihui-Wuchuan connected togather with the Tan-Lu fault zone and restricted the westward extension of the Yanshanian volcanic-intrusive magmatism.