Geochemistry of the Mesoproterozoic Lakhanda shales in southeastern Yakutia, Russia: implications for mineralogical and provenance control, and recycling

Shales of the Lakhanda Group of Late Mezoproterozoic age (1050-1000 Ma) fro m the southeastern Siberian craton in Russia have been analyzed for major e lements and a number of trace elements, including the REE's. Shales along t he Maya River formed as platform sediments in a deeper shelf facies, wherea s, shales along the Belaya River formed in more active and open environment s of an upper shelf carbonate ramp. The log of most elemental compositions to Al2O3 ratios are the same in the Maya and Belaya River samples, suggesti ng a similar source rock composition for rocks in the two areas. The log of SiO2, MgO, Na2O, K2O, Rb, Ba, and Ni to Al2O3 ratios are significantly hig her and the log of TiO2 to Al2O3 ratios are significantly lower in shales f rom the Belaya River than the Maya River sections. The CIA (chemical index of alteration) is thus significantly lower in the Belaya shales than the Ma ya shales, suggesting less weathering of the in the Belaya shales than the Maya shales. The ICVs (Index of Compositional Variability = Fe2O3 + K2O + N a2O + CaO + MgO + TiO2/Al2O3) of the Lakhanda shales are less than 1, sugge sting that they are compositionally mature and were likely dominated by rec ycling. Several samples have ICV > 1, suggesting some first cycle input. Th e low K2O/Al2O3 ratios of these shales suggest that minimal first cycle alk ali 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-me tasomatism produced these rocks, then they could have formed from tonalites to basalts. If weathering produced these rocks then they could have been p roduced from varied amounts of mostly granodiorite to granite. Elemental ra tios 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 R iver 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 K2O/Al2O3 ratios of most shales suggest weathering from mostly a granod iorite source rather than a granite source, consistent with a source from o ld upper continental crust. Some samples at the bottom of the Belaya River section contain very low Eu/Eu* (0.35), suggesting significant input of fir st cycle detritus From highly differentiated granitoids similar to those fr om the Aldan Shield. (C) 2000 Elsevier Science B.V. All rights reserved.