U-Pb geochronology and Sr-Nd isotopic systematics of minerals from the ultrabasic-alkaline massifs of the Kola province

Abstract

This paper reports the results of U-Pb geochronological and Sr-Nd isotopic geochemical investigations (LA-ICP-MS) for perovskite, apatite, titanite, and calcite from the ultrabasic-alkaline rocks of the Paleozoic Kola alkaline province of the Fennoscandian Shield. Based on the obtained data, two main stages were distinguished in the history of Paleozoic intrusions in this province: (1) formation of ultrabasic-alkaline series of the Kovdor, Afrikanda, Turiy Mys, Ozernaya Varaka, Lesnaya Varaka, and other massifs, as well as the ultrabasic-alkaline series of the Khibiny and Lovozero massifs (385–375 Ma) and (2) formation of agpaitic syenites in the Khibiny and Lovozero calderas (375–360 Ma) and related apatite-nepheline deposits (370 Ma). The Sr-Nd isotopic geochemical investigations of perovskite, apatite, and titanite, which are the main hosts for the rare earth elements and Sr in ultrabasic-alkaline rocks, showed that variations in the Sr and Nd isotopic characteristics of these rocks are related to a large extent to crustal contamination during the ascent of their parental melts toward the surface and crystallization in magma chambers. As a result, the Sr and Nd isotopic characteristics of late minerals (apatite and titanite) do not reflect the initial Sr and Nd isotopic ratios of the primary magma. Initial ratios in the primary mantle melts are most closely approximated by the isotopic characteristics of phases crystallizing during early stages (e.g., perovskite).