This work presents experimental and geochemical constraints on the composition and conditions of generation of Late Cenozoic intraplate basalts of the Baikal Rift. Comparison of major elements, their ratios (SiO 2 , TiO 2 , MgO, FeO, CaO/Al 2 O 3 , TiO 2 /Al 2 O 3 ) and the FCKANTMS parameter in natural basaltic melts with experimental melts from various mantle lithologies showed that a peridotitic source – both dry and with added volatiles – cannot reproduce the observed natural compositions. In contrast, melts derived from pyroxenites, especially carbonated-bearing ones, completely overlap the compositional variations of the Baikal Rift lavas. The melting source is localized within the interval of garnet coexistence in pyroxenites (~50–70 km), where the pyroxenite solidus is reached under the lithospheric geothermal gradient, whereas the host peridotite remains subsolidus. The origin of pyroxenites is attributed to delamination of overthickened mafic crust, formed as a result of the Cenozoic India–Eurasia collision, by a gravitational "drip" mechanism. Rift-related extension is not the driving force of melting but only provides tectonic conditions for the activation of inherited and newly formed structures through which basaltic magma rises. The proposed model for the generation of basaltic melts in the Baikal Rift does not require a mantle plume and explains the sporadic nature of intraplate magmatism by local melting of pyroxenite rocks at lithospheric depths.
Based on isotope-geochronological studies of intrusive complexes of the Fennoscandian Shield hosting deposits with economic PGE‑Cu‑Ni‑Cr mineralization (intrusions of the Monchegorsk District, Pechenga, Fedorovo‑Pansky, Ahmavaara, and Penikat) and Fe‑Ti‑V‑PGE mineralization (Kolvitsa clinopyroxenite–wehrlite massif), Sm-Nd data on sulfide and rock‑forming minerals have been summarized and systematically analyzed; together they provided a basis for substantiating the main stages of ore formation in Paleoproterozoic. Three main stages of ore genesis are distinguished, correlating with changes in geodynamic regimes: early magmatic PGE‑Cu‑Ni‑Cr mineralization (2.5–2.4 Ga), associated with a mantle plume activity at an early stage of rifting; Cu‑Ni mineralization (1.97–1.94 Ga), corresponding to the late stage of continental rifting; and post‑collisional Fe‑Ti‑V‑PGE mineralization (1.85–1.82 Ga) at the final stages of the Lapland–Kola orogen evolution. The εNd(T) isotopic characteristics allowed three source types to be distinguished: plume (2.5–2.4 Ga, εNd(T) from –2.5 to +1.8), depleted asthenospheric (1.97–1.94 Ga, εNd(T) from +2.9 to +5.1), and enriched (1.85–1.82 Ga, εNd(T) from –2.0 to –2.5). The established evolution of sources reflects changes in geodynamic regimes and largely controls the localization of large deposits of strategic raw materials in the northeastern part of the Fennoscandian Shield.
The geochemical and Nd isotopic characteristics of Proterozoic dolerites and basalts from the Biryusa region, East Sayan (southwestern Siberian craton) are considered: Paleoproterozoic dolerites of the Biryusa and Nersa rivers and Neoproterozoic basalts and andesibasalts from the upper reaches of the Kremenshet River. The Paleoproterozoic dolerites have the geochemical signature of continental tholeiites, display fractionated REE patterns (Gd/Yb (N) =1.4–2.2), weak negative Nb anomalies, and have moderately radiogenic ε Nd (1620) values ranging from –1.8 to –7.9. The Neoproterozoic basalts show geochemical characteristics of suprasubduction low Ti tholeiites, with strong negative Nb anomalies, positive Sr anomalies, unfractionated heavy REE patterns (Gd/Yb (N) =0.9–1.1), and have a strikingly different Nd isotopic composition from the Paleoproterozoic dolerites: unradiogenic, with ε Nd (700) values from –10.4 to –11.1.Two models are proposed for the formation of the lithospheric source of the Neoproterozoic rocks with unradiogenic Nd isotopic composition. Model 1 assumes a lithospheric mantle source and its Paleoproterozoic age. Enrichment of this source occurred in two episodes: suprasubduction at about 1.9 Ga and plume related (?) at about 1.6 Ga; during the latter episode, remobilization of the 1.9 Ga subduction component produced a network of pyroxenite veins in spinel peridotites. Melting of these veins at about 720 Ma provided the enriched component to the Neoproterozoic basalts. Model 2 assumes a crustal nature of the source and its Neoarchean age. In this model, lower-crustal granulite complexes of Neoarchean age, whose magmatic protoliths have a suprasubduction origin, are considered as the melt source. Lithospheric erosion of the southwestern Siberian craton induced by a plume at 720 Ma led to delamination of lower crustal fragments, their foundering into the mantle plume and melting due to high plume temperatures, thus producing basaltic magmatism with "ancient crustal" Nd isotopic signatures and lower crustal geochemical characteristics.
Lamprophyric dykes of the Turiy Cape swarm in the Kola alkaline carbonatite province preserve a record of the complex multistage evolution of the melt, reflected in composition and petrographic position of micas. The aim of this study is to evaluate the ability of phlogopite to be an indicator of magmatic, late-magmatic, and post-magmatic processes. Analysis has been made on 19 samples of lamprophyric dykes to study petrographic features of phlogopite, as a result of which four mica groups were distinguished, differing in Mg#, in Ti, Al, Na, Ba and Mn contents, and in position within the rock. Earlier Mg-rich micas crystallized from a CO2-bearing alkaline melt. The later groups exhibit changes in fractional crystallization conditions, involvement of carbonate and fluid components, resorption of earlier grains, and rim enrichment in Fe and Ti during the post-magmatic stage.The results show that phlogopite composition commonly reflects the transition from early to late and post-magmatic stage and can therefore serve as a sensitive indicator of the multistage evolution of lamprophyric system.
Five ore zones have been identified within the Burpala intrusion, the most of them have metasomatic origin. The study established that rare-metal and rare-earth (Zr, REE, Nb, Th) accessory mineralization is not associated solely to metasomatic rocks. This assemblage has also been identified in syenites weakly affected by metasomatic processes, where a rare-metal minerals occur as interstitial aggregates.Ore zone № 5 is composed of fine- to medium-grained alkaline and nepheline syenites with an agpaitic index of 0.80– 1.42, in which, in comparison to the previously studied metasomatic ore zones № 1 and 2, the REE-Nb-Zr mineralization is late-magmatic. The main Nb, REE, and Zr minerals are loparite-(Ce), eudialyte, zircon, and minerals of the wöhlerite group (låvenite, rosenbuschite); chevkinite-(Ce), apatite, and catapleiite are present in subordinate amounts. The morphological characteristics of the minerals and the absence of clear signs of metasomatic processes point to their late-magmatic origin during the differentiation of alkaline magma: poikilitic loparite and eudialyte crystals formed at conditions of high melt alkalinity and supersaturation, whereas a subsequent decrease in agpaiticity led to the destabilization of zirconosilicates (eudialyte, låvenite) and the extensive crystallization of interstitial zircon, as well as the replacement of loparite by a titanite–pyrochlore assemblage. The transitional position of the ore-bearing Burpala syenites between typically miaskitic and agpaitic varieties led to the formation of a mixed type of ore mineralization.
