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COMPLEX STUDY OF DIAMONDIFEROUS SEDIMENTARY-VOLCANOGENIC ROCKS OF THE CARNIAN AGE OF THE BULKUR ANTICLINE (NORTH-EAST SIBERIAN PLATFORM)
Abstract
Complex of researches (chemical, mineralogical, X-ray phase, rare-earth, isotopic analyses) of volcanogenic-sedimentary rocks, composing basal horizon of the Osipayskaya formation of the Carnian stage in the middle course of Lena river ((Bulkur anticline) (Grakhanov, Smelov 2010, Proskurin et al, 2012), where diamonds of V and Vii variety according to L.Y. Orlov classification, was performed. Granite, ilmenite, spinellide, rutile are identified in rocks among minerals of heavy fraction. According to X-ray phase analysis, rocks contain the following minerals: chamosite, illite, quartz, goethite, apatite, calcite, albite, anatase, berthierine, jarosite, gypsum, hematite, rutile, forsterite and pyroxene. Sediment maturity was estimated according to ICV index (Cox et al. 1995), calculated values characterize rocks, mainly, as immature sediments of the first cycle and typical for active tectonic settings. Chemical index of alteration (CIA) (Nesbitt H.W. & Young G.M., 1982) quite often depends on metamorphic and diagenetic transformations, in order to exclude the latter, plagioclase (PIA (Fedo et al., 1995)) and chemical (CIW (Harnois, 1988)) indices of weathering were calculated. Obtained values indicate significant degree of rock weathering. Content of rare-earth elements (REE) is 96-205 g/t and is characterized by moderate fractionation of REE that is why spider diagram shows gentle character of distribution. Sm-Nd datings, calculated according to different models, show different values of their protolith age. The youngest values indicate 222+13 Ma, which is consistent with geologic age of rock origin. As a result of performed studies, it identified that volcanogenic-sedimentary deposits of the Carnian age of the Bulkur region are characterized as high-ferruginous immature sediments, formed at the place of their deposition, adequately weathered , containing pyroclastic material. In respect of primary source composition, they are closest to the rocks of mafic series, probably with alkali bias.
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