Glimpses into the petrogenesis of some classical lithotypes from the Fen carbonatite complexes, Norway

Abstract

Carbonatites, rare igneous rocks rich in carbonate minerals, hold significant economic value due to their high concentrations of niobium, tantalum, and rare earth elements (REEs). Despite their importance, the origins and evolution of carbonatites remain subjects of ongoing scientific debate. In this study, I investigate the genesis of the Fen Carbonatite Complex in Norway through a comprehensive analysis of its mineralogy and geochemistry. Employing petrography, X-ray diffraction (XRD), X-ray fluorescence (XRF), and inductively coupled plasma mass spectrometry (ICP-MS), I've explored two primary hypotheses for the origin of carbonatites: magmatic crystallization and hydrothermal metasomatism. My findings reveal a multifaceted petrogenesis for the Fen Carbonatite Complex, with evidence supporting both magmatic and hydrothermal processes. The magmatic origin is indicated by significant REE enrichments, particularly heavy rare earth elements (HREEs), and the presence of primary magmatic minerals such as calcite, dolomite, and apatite. However, the exceptional enrichments in middle and light rare earth elements (MREEs and LREEs) point to substantial metasomatic modification, likely driven by external fluid interactions. Textural features observed through petrographic analysis, such as ghost crystals and pseudo cubic fluorite, alongside geochemical trends from major oxide analysis, further corroborate the role of metasomatic processes. The interplay of magmatic and metasomatic influences underscores the complex evolution of the Fen Carbonatite Complex. I conclude that while the Fen Carbonatite Complex exhibits strong magmatic characteristics, its development has been significantly shaped by hydrothermal and metasomatic processes. The integration of multiple analytical techniques has been essential in unraveling the intricate formation history of these rocks. This research highlights the necessity of considering both magmatic and metasomatic theories to fully understand the petrogenesis of carbonatites, contributing valuable insights to the broader geological understanding of these economically important rocks.