The Kola Devonian alkaline province in the northeastern Fennoscandian Shield is composed of giant agpaitic syenite plutons, 20 carbonatite intrusions, alkaline volcanics and enormous alkaline dykes. Since 1960s numerous geochronological studies were performed in order to determine the age of Khibina and Lovozero massifs, ultrabasic alkaline intrusions (Kovdor, Afrikanda, Turiy Mys) and to compare the time span of agpaitic and carbonatitic magmatism.
The comprehensive isotope studies performed by U. Kramm, L. Kogarko, V. Kononova (Kramm et al., 1993, 1994) provided the short time span of the Palaeozoic alkaline magmatism in the NE Fennoscandia. During a period of 380 - 360 Ma agpaitic complexes of Khibina and Lovozero were formed simultaneously with numerous carbonatite intrusions of Kovdor, Turiy Mys, Afrikanda, Sokli, etc. The U-Pb age estimates for zircons in ultrabasic rocks and alkaline syenite of the Kurga intrusion which is located in immediate vicinity of the Lovozero complex gave the age of 387+7 Ma (Arzamastsev et al., 1999).
Zircon, the best precise U-Pb geochronometer, is absent in the most of agpaitic and ultrabasic alkaline rocks. This fact inspired geologists to test new geochronological methods in order to decipher the history of the Kola in Devonian. More than 140 age determinations of the Kola alkaline rocks include Rb-Sr and Sm-Nd mineral isochron data, U-Pb baddeleyite and apatite, Lu-Hf data of apatite, in-situ LA ICP MS data of perovskite, eudialyte, 40Ar/39Ar determinations of biotite, amphibole and feldspar.
Our recent studies of the Kola alkaline rocks include determination of time span of magmatism in Kola part of the Fennoscandian Shield in Paleozoic time.
Studies of tholeiitic magmatism in Kola. Among numerous Proterozoic dykes of the Pechenga, Barents Sea, and Eastern Kola swarms we found dolerites which were formed 380-390 Ma ago, i.e., directly before the main stage of the Paleozoic alkaline magmatism of the Kola province. Based on 40Ar/39Ar dating, the biotite and plagioclase fractions from several dykes of the three Barents Coast swarms yielded age of 385 ± 15 Ma. The Rb-Sr and Sm-Nd isotope systematics of the dolerites suggest that their origin from the mantle spinel lherzolite facies. The depleted mantle material from which tholeiite magma was derived shows no evidence for metasomatic event. The relatively short stage of mantle metasomatism directly after the emplacement of tholeiitic magmas was accompanied by significant enrichment of the mantle in HFSE and REE. In contrast to the other LIPs, the Kola province is characterized by systematic plume-lithosphere evolution process expressed in deepening of the level of magma generation, development of mantle metasomatism and accompanying fertilization of mantle materials, and systematic changes in the composition of melts reaching the surface (Arzamastsev et al., 2017).
Determination of time span of alkaline magmatism in Kola. Among the 19 ultrabasic-alkaline massifs of the Kola province, rocks from the Khabozero group were selected for U–Pb age determination and Sr–Nd isotopic analyses using in situ ion probe and laser ablation techniques. The main reason to choose these massifs is that each of them is represented by a single rock type: olivinite (Lesnaya Varaka Massif), pyroxenite (Afrikanda Massif), and foidolite (Ozernaya Varaka Massif). Thus, derivatives of the Khabozero group sequentially intruded through different conduits, and we believe that the absence of later derivatives minimizes resetting of isotopic systems of early differentiates and allows to obtain correct age estimates. Mineral separates of perovskite from the olivinite and pyroxenite exhibit the age of 385–377 Ma. Age estimates for perovskite from the more evolved rocks of the series range from 376 to 367 Ma. The analysis of rare perovskite grains from the rocks of the Khibiny Massif revealed significant differences between the ages of the pyroxenite and xenoliths of ultrabasic alkaline rocks (383 ± 7 Ma) and apatite–nepheline ores (370 ± 3 Ma), which is consistent with geological observations of rock relationships. Together with geological observations, these data allow us to distinguish the following stages of Paleozoic magmatism in the northeastern Fennoscandian Shield.
393–381 Ma. Emplacement of subalkaline dolerite dikes in the distal zone adjacent to the Barents Sea region of the Kola–Kanin monocline.
387±7 Ma. Origination of a series of faults in Late Archean tonalites, trondhjemites, and granodiorites; formation of the Kurga Massif and ultrabasic and subalkaline volcanics in the northeastern part of the future Lovozero ring structure.
388±6 Ma. Formation of a series of ring faults and the origin of the Khibiny caldera at the contact of the Late Archean gneiss complex and the Paleoproterozoic Pechenga–Imandra–Varzuga paleorift rock series; injection of the first portions of melanephelinite magma and formation of framing ring dikes.
380±5 Ma. Emplacement of ultrabasic alkaline melts in the peripheric parts of the Khibiny and Lovozero calderas. Formation of ultrabasic alkaline complexes with carbonatites in the Kovdor Massif, as well as the Turiy Mys, Afrikanda, Lesnaya Varaka, Ozernaya Varaka, Ivanovka, Pesochnyi and other massifs.
373–367 Ma. Formation of the plutonic complexes of agpaitic syenites in the Khibiny and Lovozero calderas. Intrusion of carbonatite and pulaskite stocks in the eastern part of the Khibiny caldera.
377–362 Ma. Several pulses of dyke emplacement and formation of diatremes of alkali picrite, kimberlite, olivine melanephelinite, nephelinite, and phonolite mainly in the framing of the Kandalaksha paleorift.
359±5 Ma. Formation of late microcline–albite veins with ilmenite and zircon in the framing of the Lovozero Massif.
347±8 Ma. Postmagmatic processes in the alkali syenites of the central part of the Lovozero Massif marking the termination of thermal activity in the Khibina and Lovozero calderas.
What we can do more is to test the most abundant minerals titanite and Ca-Ti garnet as geochronometers. The first experience is as follows.
Titanite by SHRIMP. The U-Pb age estimates for titanite fractions in agpaitic syenites and apatite ores of the Khibina and Lovozero massifs was performed using SHRIMP-II (Rodionov et al., 2018 in press). The obtained age of 374.1±3.7 Ma for Khibina agpaitic syenites coincide with the age of apatite mineralization (371.1±4.2 Ma). Age estimation of the Lovozero lujavrite-foyaite-urtite complex is significantly older 380.9±4.5 Ma), whereas the time of titanite mineralization in the Suluai titanite deposit (361.4±3.2 Ma) supports the long-term postmagmatic history of these two giant plutons.
U-Pb age of Ca-garnet. The first successful geochronological studies of Ca-Ti garnets in carbonatites of the Afrikanda massif (Salnikova et al., 2017) inspired for further U-Pb age determinations of garnets from foidolites in the other massifs of the Kola Province. Obtained U-Pb ages for Salmagorsky massif is 377±2 Ma, Sallanlatva 378±3 Ma, Vuorijarvi 373±5 Ma. These data fall the time span of the ultrabasic alkaline intrusions of the Kola Province which were obtained by (Kramm et al., 1993, 1994; Arzamastsev, Wu, 2014 and refs ibid).
Further geochronological investigations of the Kola alkaline magmatism will be focused on age determinations using titanite, perovskite and Ca-garnet.