The structure of the Paleozoic alkaline-ultrabasic intrusions of the Kola Peninsula according to gravimetric data

The deep structure of the Paleozoic carbonatite complexes in the Kola Province has been studied by an original 3D density modeling method based on gravity prospecting data. Analysis of intrusions to depths of 12-22 km shows that the studied objects are subvertical cylindrical bodies, massifs with a shallow magma chamber and a side feeder channel, and bodies which are the apical parts of large alkaline-ultrabasic intrusions. It is shown that the bodies making up the intrusion bottom are composed mainly of ultrabasic rocks, while the apical parts of the massifs are dominated by carbonatites. The reconstruction of the magma chamber structure suggests that most carbonatite intrusions of the Kola Province initially looked like a spindle-shaped stock 15-20 km long with the height-to-diameter ratio of the chamber 2:1. The denudation level in the Kola part of Fennoscandia was estimated by comparing data on the intrusion shape, location of the magma chamber bottom, and variations in matter composition in the vertical section of the magma chambers. The results show that for the post-Devonian period which had passed since the intrusion formation, the territory studied was eroded by 4-11 km, with its southeastern part being the most heavily eroded.