Abstract
Shales of the Lakhanda Group of Late Mezoproterozoic age (1050–1000 Ma) from the southeastern Siberian craton in Russia have been analyzed for major elements and a number of trace elements, including the REE's. Shales along the Maya River formed as platform sediments in a deeper shelf facies, whereas, shales along the Belaya River formed in more active and open environments of an upper shelf carbonate ramp. The log of most elemental compositions to Al 2O 3 ratios are the same in the Maya and Belaya River samples, suggesting a similar source rock composition for rocks in the two areas. The log of SiO 2, MgO, Na 2O, K 2O, Rb, Ba, and Ni to Al 2O 3 ratios are significantly higher and the log of TiO 2 to Al 2O 3 ratios are significantly lower in shales from the Belaya River than the Maya River sections. The CIA (chemical index of alteration) is thus significantly lower in the Belaya shales than the Maya shales, suggesting less weathering of the in the Belaya shales than the Maya shales. The ICVs (Index of Compositional Variability=Fe 2O 3+K 2O+Na 2O+CaO+MgO+TiO 2/Al 2O 3) of the Lakhanda shales are less than 1, suggesting that they are compositionally mature and were likely dominated by recycling. Several samples have ICV>1, suggesting some first cycle input. The low K 2O/Al 2O 3 ratios of these shales suggest that minimal first cycle alkali feldspar was present in the initial source. Most shales of the Lakhanda plot parallel and along the A–K line in A–CN–K plots suggestive of intense chemical weathering (high CIA) and do not indicate any clear-cut evidence of K-metasomatism or direct weathering back to the original source. If K-metasomatism produced these rocks, then they could have formed from tonalites to basalts. If weathering produced these rocks then they could have been produced from varied amounts of mostly granodiorite to granite. Elemental ratios critical of provenance (La/Sc, La/Cr, La/Co, Th/Sc, Th/Cr, Th/Co, and Eu/Eu*) are not significantly different between the Maya River and Belaya River shales, and the ratios are similar to fine-fractions derived from the weathering of mostly granitoids and not basic rocks. The Eu/Eu*, Th/Sc and low K 2O/Al 2O 3 ratios of most shales suggest weathering from mostly a granodiorite source rather than a granite source, consistent with a source from old upper continental crust. Some samples at the bottom of the Belaya River section contain very low Eu/Eu* (0.35), suggesting significant input of first cycle detritus from highly differentiated granitoids similar to those from the Aldan Shield.
Published Version
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