SOURCES, GEODYNAMIC SETTING OF FORMATION, AND DIAMOND-BEARING POTENTIAL OF KIMBERLITES FROM THE NORTHERN MARGIN OF THE RUSSIAN PLATE: A SR-ND ISOTOPIC AND ICP-MS GEOCHEMICAL STUDY

Abstract

Potassic magmas (their derivatives, kimberlites and others) are among the products of incipient mantle melting, which occurs at an increase in heat flow (plume ascent). They provide reliable information on the composition and structure of the Earth's interiors. In contrast to other world's cratons, in the East European platform, the first kimberlite pipe was found as late as in 1980. It is located near Arkhangelsk, on the north of the Russian plate, where the basement of the craton and numerous igneous rocks including kimberlite crop out or covered by only a few tens of meters of sediments. Up to now, a nearly E-W trending array of kimberlite magmatism areas (in places, diamond-bearing) was distinguished. It is about 1000 km long and includes the regions of Kandalaksha, Terskii Bereg, Arkhangelsk, and Central Timan. These occurrences of potassic magmatism show similar ages (360-390 Ma) and geologic and tectonic settings of their formation (Paleozoic faults or grabens confined to Riphean aulacogens), which ensures the correctness of their comparison. A complex geochemical study of 47 elements was carried out by means of isotopic (Rb-Sr and Sm-Nd) and ICP-MS methods. Based on the obtained results, the first comparative analysis was performed for kimberlites from various areas of the northern Russian plate and typical kimberlite groups from other regions (southern and western Africa and northern Australia). It was shown that despite some compositional variations, a number of characteristics suggest that the kimberlites of the areas under consideration belong to a single kimberlite province. Furthermore, at least two geochemical varieties can be distinguished within this rock population. Except for the kimberlites of the Zolotitskoe and Verkhotinskoe fields (Arkhangelsk area), the rocks are mainly nonmicaceous kimberlites similar in contents and ratios of some trace elements to the group I kimberlites of southern Africa, and in some parameters, to the kimberlites of western Africa (Koidu). In particular, similar to the Koidu kimberlites, their Sr and Nd isotopic signatures are close to the Bulk Silicate Earth. Consequently, in contrast to the group I kimberlites of southern Africa, which were presumably derived from a purely asthenospheric source, their compositions bear evidence for the presence of lithospheric material in the source. The micaceous kimberlites of the Zolotitskoe and Verkhotinskoe fields are distinguished into a new geochemical type of kimberlites, which is characterized by very low enrichment in trace elements (e.g., 20-40 ppm of La and 20-70 ppm Nb). In addition, they show negative εNd values (up to -7.6) at εSr varying from negative (-9) to low positive (up to 30) values indicating their unusual source, slightly enriched lithospheric mantle of the EMI type. The character of trace element distribution (in particular, negative anomalies of Th, U, and Nb in the kimberlites of the Zolotitskoe field) suggests that the lithospheric source was metasomatized under the influence of fluids derived in a paleosubduction zone, probably of Early Proterozoic age. On the εNd-εSr diagram, the kimberlites of the Zolotitskoe field form the beginning of a distinct trend, whose continuation comprises lamproites from North America and Central Aldan, i.e., objects occurring within northern continents (Laurasian group). In contrast, the micaceous kimberlites of southern Africa (group II kimberlites) and Australian lamproites (objects of southern continents, the Gondwana group) form another trend related to a source in the enriched mantle of the second type (EMII). The differences between the sources (EMI and EMII) of kimberlites and lamproites from northern and southern continents probably reflect global geochemical mantle heterogeneity. However, it did not affect significantly the diamond content.