摘要
澜沧江西侧的临沧花岗岩带和怒江以西的腾冲花岗岩带是滇西最主要的两个花岗岩带,它们形成于不同构造环境。Pb、Sr 同位素研究表明,腾冲花岗岩主要来源于1200—2000Ma 的上地壳(或未分异的地壳)物质,同位素组成变化范围较小,说明源区物质组分较均一。临沧带 Pb 同位素组成变化较大,其物源是上地壳、造山带和上地幔不均匀的混合物,基底时代为800—1600Ma。因此,临沧地区和腾冲地区的基底时代和性质不同,应视为两个不同的基底地体。
It is known that no matter where, when and from what material the granite was formed, the lead isotopic composition in its feldspar can indicate the geochemi-cal characteristics and age of source material. Initial lead isotopic composition (including Pb in feldspar and accessory galena) and strontium isotopic characteristics in the granites in Tengchong and Linchang belts, West Yunnan, are evidently different. It indicates that the granites were formed in different tectonic environments and from different source materials, and also reflects different age and nature of their basementsThe Tengchong granite was formed in a back-arc environment, as its initial Pb isotopic composition varies in smaller range(1-2.3%)i.e. 206Pb/204Pb 18.510-18.943, 207Pb/204Pb 15.649-15.807, and 208Pb/204Pb 39.115-39.505. In according to the plumbo-tectonic model by Doe and Zartman (1981), the lead came from the upper crust. Lower initial ratio 87Sr/86Sr, 0.708-0.714, indicates that the granite was produced by the remelting of the crust, and materials in source region were relatively homo-geneous. But the lower Sr (i) value suggests the existence of more basic volcanic material in the upper crust, or it is of the nature of the undifferentiated upper crust Initial Pb isotopic composition in the Linchang granite varies in a large range (2.3-2.9%), 206Pb/204Pb 18.344-l8.885,287Pb/204Pb 15.4544-15.843,208Pb/204Pb 38.255-39.130. It shows the material may come from the upper mantle, orogenic belt and upper crust, and the components in source region for the granite is inhomogeneous, and consistent with a magmatic-tectonic environment of ancient island arc. But higher Sr (i) ratio in the Linchang granite, 0.718-0.727, is not consistent with the larger variation in Pb isotopes. It may be because the Linchang granite is a parauto-chtonous granite formed by metamorphism and metasomatism. Its source rocks are a mixture of multiple components, and the homogenization of Pb isotopes is lower. Thus, isotopes reflect a regional inhomogeneity, and Sr isotopes reflect local uncom-plete homogenization. Higher Sr (i) value only shows its characteristics of the components of shallow continental crust.The Linchang and Tengchong granites were formed in three stages, 250Ma±.150 Ma±, and 50-80Ma±. Based on initial lead isotopic composition in accordence with the plumbo tectonic model for source material by Hampton and Toylor (1983), it is inferred that in Tenchong belt the Earlier and Middle Proterozoic strata of 2000Ma may belong to the undifferentiated crust, and in the Linchang region, the oldest strata on the basement may be less than 1600Ma, which consist of the shallow continental crust.
出处
《地质科学》
CAS
CSCD
北大核心
1991年第2期174-183,共10页
Chinese Journal of Geology(Scientia Geologica Sinica)
