Sources, geodynamic setting of formation, and diamond-bearing potential ofkimberlites from the northern margin of the Russian plate: A Sr-Nd isotopic and ICP-MS geochemical study

Potassic magmas (their derivatives, kimberlites and others) are among the p roducts of incipient mantle melting, which occurs at an increase in heat fl ow (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 pla te, where the basement of the craton and numerous igneous rocks including k imberlite 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 plac es, diamond-bearing) was distinguished. It is about 1000 km long and includ es the regions of Kandalaksha, Terskii Bereg, Arkhangelsk, and Central Tima n. These occurrences of potassic magmatism show similar ages (360-390 Ma) a nd geologic and tectonic settings of their formation (Paleozoic faults or g rabens confined to Riphean aulacogens), which ensures the correctness of th eir 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 obt ained results, the first comparative analysis was performed for kimberlites from various areas of the northern Russian plate and typical kimberlite gr oups from other regions (southern and western Africa and northern Australia ). It was shown that despite some compositional variations, a number of cha racteristics suggest that the kimberlites of the areas under consideration belong to a single kimberlite province. Furthermore, at least two geochemic al varieties can be distinguished within this rock population. Except for t he kimberlites of the Zolotitskoe and Verkhotinskoe fields (Arkhangelsk are a), the rocks are mainly nonmicaceous kimberlites similar in contents and r atios of some trace elements to the group I kimberlites of southern Africa, and in some parameters, to the kimberlites of western Africa (Koidu). In p articular, similar to the Koidu kimberlites, their Sr and Nd isotopic signa tures are close to the Bulk Silicate Earth. Consequently, in contrast to th e group I kimberlites of southern Africa, which were presumably derived fro m 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,ge ochemical type of kimberlites, which is characterized by very low enrichmen t in trace elements (e.g., 20-40 ppm of La and 20-70 ppm Nb). In addition, they show negative epsilon (Nd) values (up to -7.6) at epsilon (Sr) varying from negative (-9) to low positive (up to 30) values indicating their unus ual source, slightly enriched lithospheric mantle of the EMI type. The char acter of trace element distribution (in particular, negative anomalies of T h, U, and Nb in the kimberlites of the Zolotitskoe field) suggests that the lithospheric source was metasomatized under the influence of fluids derive d in a paleosubduction zone, probably of Early Proterozoic age. On the epsi lon (Nd)-epsilon (Sr) diagram, the kimberlites of the Zolotitskoe field for m the beginning of a distinct trend, whose continuation comprises lamproite s from North America and Central Aldan, i.e., objects occurring within nort hern continents (Laurasian group). In contrast, the micaceous kimberlites o f 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 lampr oites from northern and southern continents probably reflect global geochem ical mantle heterogeneity. However, it did not affect significantly the dia mond content.