SOURCES OF HETEROGENEOUS MAGMATISM OF THE ARKHANGELSK DIAMONDIFEROUS PROVINCE

The Arkhangelsk diamondiferous province is situated within the Kola-Ku loi craton and Late Proterozoic folded areas of its framing. The provi nce comprises eight fields of Late Devonian-Early Carboniferous ultrab asic and basic magmatism D-3-C-1 in age (dozens of diatremes and sills of kimberlite, olivine melilitite, and tholeiitic basalt). The kimber lites of the province are comparable to olivine lamproites in some spe cific features of their mineral composition. These features include re latively low A1(2)O(3) at high TiO2 and BaO concentrations in phlogopi tes, as well as high FeOtot and Crl(2)O(3) contents and Cr/(Cr + Al) r atio in spinels. Such an affinity to diamondiferous lamproites could b e caused by similar conditions of magma evolution. Two groups of rocks that differ in major and trace element chemistry are distinguished am ong the kimberlites of the province: (a) diamond-rich rocks of the Zol otitsa field and (b) diamond-poor rocks of the other fields. The forme r are close in some characteristic chemical parameters to group II kim berlites of South Africa (Smith et al., 1985), whereas the latter rese mble group I kimberlites of South Africa, Yakutia, and other regions. Systematic analysis of the rock geochemistry and Nd-Sr isotope composi tion shows that the diversified magmatism of the province is contribut ed to from three genetically different mantle sources: (a) primitive u pper mantle (PUM) or bulk silicate earth (BSE) for diamond-poor group I kimberlites, (b) prevalent mantle (PREMA) for tholeiitic basalts, an d (c) type I enriched mantle (EMI) for group II kimberlites of the Zol otitsa held and olivine melilitites of the Izhmozero field. In the cou rse of formation, the parent magmas of kimberlites of the Zolotitsa fi eld mixed with up to 15% magmas of group I kimberlites at the base of the lithosphere. Prior to mixing with group I kimberlites, parent magn a of group II kimberlites of the Zolotitsa field had similar isotope c omposition, but higher Rb, Pa, LREE, and lower CaO, Al2O3, and Na2O co mpared to olivine melilitites. These differences could be caused by th e significant proportion of phlogopite in the kimberlite source, and p otassium richterite in the melilitite source. The formation of the lit hosphere source of the EMI-type resulted from the metasomatism of the lower part of the old depleted lithosphere involving H2O-, LILE- and L REE-rich fluids-melts, which were derived from a subduction zone in th e Late Archean-Early Proterozoic epoch of the cratonic stage of the re gional evolution.