The Sr-Nd-Pb Isotopic Geochemistry Of Kongling High-grade Terrain Of The Yangtze Craton (South China) And Its Tectonic Implications

Because of deeper burial of most Precambrian basements of Yangtze craton, the size of Archean crust and the character of early crust and mantle are largely unknown. The Precambrian basements exposed in the Yangtze craton are only limited to the Kangdian area of the Western Sichuan, the Hanzhong area of the Southern Shaanxi, and the Haungling area of the Western Hubei. But Huangling area of Yichang city in Hubei province is only proved fully for existing Archean basement.Pb isotopic mapping of geological blocks is one of the important geochemical research subjects. Testable objects are not only limited to Pb isotopes of granites (including feldspar), but also include Pb isotopes of basement rocks of blocks and granites as probes of Pb isotopic compositions of crust.Sr, Nd and Pb isotopes were analyzed to understand the crust formation and evolution of Yangtze craton and provide constraints on crustal evolution of the Yangtze craton in this study.As we know, acquiring higher accurate and precise data is very important for geochemical research. Five U. S. Geological Survey (USGS) reference materials (AGV-1, AGV-2, BHVO-2, BCR-2 and G-2)were analyzed using multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). The results show that both the first and second generations of USGS reference materials suffered from contamination to varying degrees in sample preparation. The contamination caused by sample preparation was more severe than crushing environment for the first generation of USGS reference materials. However, the second generation of USGS reference materials show reversed effect. Each of the five USGS reference materials yield homogeneous isotopic compositions after leaching. Thus, leaching prior to dissolving was efficient for obtaining the true lead isotopic compositions of geological samples. It means that leached and unleached sample powders are different in Pb isotopes and it should be careful during sample preparation. Moreover, a simplified leaching procedure of lead isotope analysis of geological samples was proposed to remove the secondary phases or potential contaminations in this study. Comparison of our method with previous methods proposed by McDonough and Chauvel ( Earth Plan. Sci. Lett. , 1991, 105:397-404 ) and Weis and Frey (J.Geophy.Res.,1996, 101:13831-13849),the results show that our method decreases the overall background of whole procedure and improves the efficiency of sample preparation.Based on the method above, this paper studies Sr-Nd-Pb isotopic compositions of rocks from the Kongling complex in Huangling area of the Yangtze craton. Combined with age data, major elements and trace elements geochemistry, and Sm-Nd isotopic ratios from previous studies, following conclusions are summarised:1) Pb and Sr isotopic ratios of twenty-two metasediments, amphibolites, DTTG gneisses(dioritic, tonalitic, and trondhjemitic gneisses),and granitic gneisses from the Kongling terrain are analyzed sysmatically for the first time. Combined with Nd isotopic composition, the results show extensive and complicated Pb isotopes compositions for Kongling Craton, ²⁰⁶Pb/²⁰⁴Pb=15.139～23.168, ²⁰⁷Pb/²⁰⁴Pb=l4.968-16.211, ²08Pb/²⁰⁴Pb=35.316～41.639.2) The metasediments show higher Pb isotopic ratios, whereas the metaigneous rocks display lower Pb isotopic ratios. When back-calculated at 120Ma, compared with Pb isotopes ratios of feldspar of the Qinling, Dabie and Qilian orogens, the result indicates the sources of Qinling, Dabie and Qilian orogens are similar to the metasediments from the Kongling terrain. Moreover, the Pb isotopic compositions of the metaigneous rocks are consistent with those of feldspar of granites of the North China craton.3) In this study, some samples were leached to remove the potential contamination. The results show different Pb isotopic compositions between leached and unleached sample powders. Generally, the Pb isotopes of leached samples are lower than those of unleached samples. The Pb isotopes of DTTG gneisses and amphibolite show insignificant difference between leached and unleached sample powders. However, the metasediments and granitic gneisses reveal more different Pb isotopic composition. It was explained that leaching processes has an apparent impact on different lithological Pb isotopes, and it is inappropriate to leach metasediment by 6N HC1. The results also show that DTTG gneisses and amphibolite exhibit a close isotopic system.4) The Pb-Pb isochron age of 2706±780Ma of DTTG , consistent with the Sm-Nd isochron age, zircon U-Pb age,suggests that DTTG gneisses and amphibolite are in a close isotopic system.5) Nevertheless, Sr and prior Nd isotopic data of the metasediments and granitic gneisses exhibit higher ⁸⁷Sr/⁸⁶Sr ratios. This suggests an open Rb-Sr system during the metamorphicprocess. ⁸⁷ Sr/⁸⁶ Sr ratios of DTTG gneisses and amphibolite fall in the line of average continental crust and also imply a close isotopic system.