The Arzybei Block as a Fragment of Granite-Greenstone Terrane: Geochemistry and Problems of Origin of Amphibolite-Gneissic Units and Granitoids

Precambrian metamorphic complexes and granitoids of the Arzybei Block are proven to be related to a granite-greenstone terrane of the East Sayan. Stratified deposits are subdivided into four units: biotite plagiogneisses, amphibolites, biotite gneisses and biotite-amphibole gneisses. The unit of biotite plagiogneisses contains a granite-gneissic dome. The lower part of the section composed of plagiogneisses and granitoids of tonalitic-trondhjemitic composition is identified as an infracrustal (“gray gneiss”) complex. Extreme depletion in light rare earths and large-ion lithophile elements is typical of this complex. Such tonalites and trondhjemites seemed to be the result of melting of the metabasalt protocrust with abundances of rare earth elements not exceeding 5 chondrite levels. The upper supracrustal metasedimentary-volcanic complex corresponds to primary dacite-andesite-basalt and graywacke assemblages; its section is comparable with greens tone belts of calc-alkaline type. Supracrustal acid metavolcanic and metasedimentary rocks are depleted in Rb, Th, U,and LREE compared with similar formations of the Kansk greenstone belt. It is shown that the ancient sialic basement of the Siberian Platform margin is geochemically heterogeneous.