Geochemical comparison of kimberlites from the Siberian and East European platforms: Problems of genesis and spatial zoning

Abstract

239. 1156 Kimberlites from two Russian provinces, the East European (EEP) and Siberian (SP) platforms, were compared in terms of their traceelement composition and the ages of emplacement and mantle enrichment (Ndmodel ages). Previous comparisons were focused on geological, geomorphological, and mineralogical data (1), whereas our works are mainly underlain by isotopegeochemical studies, which provide insight into key problems of kimberlite petrology. In order to exclude methodical errors, kimberlites were analyzed in several known laboratories (Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry; Institute of Precambrian Geology and Geochronology and the Geological Institute of the Russian Academy of Sciences; and the Institute of Mineralogy, Geochemistry, and Crystal Chemistry of Rare Elements) using a common scheme described in (2). Numerous varieties of kimberlites distinguished by statistical analyses of a great body of available data significantly complicated their comparison. There� fore, in this work, we limited ourselves to comparison on the basis of the TiO 2 content in kimberlites. It was taken into account that TiO 2 is the typomorphic ele� ment of all potassic ultrabasic rocks, being an indica� tor of the depth of melt formation. Three petrological types of kimberlites of EEP and SP. It was recently found that rocks previously ascribed to a single kimberlite clan are characterized by broad compositional variations. In 1985, two groups differing in Sr and Nd isotope composition were distinguished among the South Africa kimber� lites (3); two new kimberlite types enriched in trace elements were later recognized at Koidu (Sierra Leone, Africa) and Aries in Australia (4). In 1995, Mitchell (5) proposed to restrict the application of the term kimberlites to the rocks that were previously clas� sified as Group I kimberlites of South Africa. Since the composition of rocks and minerals of Group II kim� berlites of South Africa are transitional to lamproites (the presence of REErich phosphates, perovskite, as well potassium richterite in differentiated varieties), these kimberlites were referred to as orangeites. In the pastt decade, the number of kimberlite types increased: special attention was drawn to a new type of diamondiferous kimberlites which was referred to by us as the Zolotitsa type (2) after the Zolititsa occur� rence in the EEP, whose rocks are depleted in Ti and trace elements (Fig. 1). At the same time, Siberian geologists described geochemically similar (including low TiO 2 content) diamondiferous kimberlites in the Nakyn field (Yakutia) and Slave craton (Canada) and proposed to consider them as anomalous kimberlites. Since the TiO2 content is an indication of pressure, the low Ti content in the "Zolotitsa" and "anomalous" kimberlites indicates their highpressure origin. Kimberlites are subdivided into three petrologi�