DBPapers
DOI: 10.5593/SGEM2014/B11/S1.008

ECLOGITIC GARNETS FROM UPPER TRIASSIC DIAMONDFIFEROUS SEDIMENTARY-VOLCANOGENIC DEPOSITS, NORTHTEASTERN SIBERIAN PLATFORM

A. Biller, A.Smelov
Wednesday 1 October 2014 by Libadmin2014

References: 14th International Multidisciplinary Scientific GeoConference SGEM 2014, www.sgem.org, SGEM2014 Conference Proceedings, ISBN 978-619-7105-07-0 / ISSN 1314-2704, June 19-25, 2014, Book 1, Vol. 1, 63-70 pp

ABSTRACT
A unique association of placer diamonds has been reported from the northeastern
Siberian Platform, which includes a peculiar variety diamonds (with light carbon
isotopic ratio) not found in any of the Yakutian kimberlite pipes. The nature of the
bedrock source for this placer deposits and the conditions of its formation are still
unknown. According to recent data, the diamondiferous horizon is made of tuffites
dated at 226-228 Ma (U-Pb zircon dating). The tuffites contain large amounts of the
diamond associate minerals such as garnets and chromium spinel. The paper presents the results of studying eclogitic garnets from the carnian tuffites. Garnets were separate to “crustal” and “mantle-derived” groups, according to major element classification scheme of Schulze (2003). Numerous mineral inclusions were obtained in both groups garnets. The inclusions are made of rutile, ilmenite, apatite, shaped-form quarts, srilankite, zircon and pyroxene. Needle-like rutile and ilmenite are oriented in the structure of garnet, sometimes they occur as ingrowths. In some cases rutile and ilmenite preserve significant concentration of ZrO2 (up to first wt. %). Srilankite forms irregular aggregates in rutile. Composite polymineral inclusions occur and consist of rutile, ilmenite, apatite, and unusal phase (majoritic garnet?, pfu Si 3.32- 3.62). The presence in the garnets of oriented Ti-bearing needle inclusions has resulted, according to some researchers, from decompositions of the original super-titanic garnet and indicative of high-pressure conditions of garnet formation. This is also supported by the finds of majoritic garnet and coesite in the inclusions from placer diamonds in the region. These data are indicative of a deep sublitosheric source for the placer diamonds and their associated minerals. High contents of rutile, ilmenite and Zr-bearing phases inclusions in garnets strongly suggest an episode of metasomatism for their mantle source.

Keywords:eclogitic garnets, oriented inclusion in garnet, diamond placers, Siberian platform

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