Hoge-resolutie onderzoek naar de kristallisatie omstandigheden in pegmatieten en gerelateerde gesteenten: Inzichten vanuit de morfologie van toermalijn, de geochemie en insluitsels

Pegmatites are an important source of strategic and critical metals needed for the green transitions, as well as industrial minerals and gemstones. They represent some of the most evolved rocks in the crust. Examining the processes that occur in pegmatites are key to understanding the extensive magma evolution and enrichment in incompatible elements. Pegmatites are characterized by their very coarse grain-size as well as anisotropic textures, such as mineral intergrowths, in especially the outer zones of the pegmatite dike. They are often found in other rocks, such as a parental granite, quartz veins and metasomatized host rocks. These rocks together make up the pegmatite system. This study examines the pegmatite at two different scales using two case studies. The outer zones of the Emmons pegmatite (Maine, USA) are studied to examine the small-scale variations (centimeters to millimeters) in crystallization conditions. Pegmatites, quartz veins and metasomatized host rocks from Musha-Ntunga and Gatumba-Gitarama (Rwanda) are studied to examine the larger, regional variations. In both cases tourmaline is used as a proxy, due to its large chemical variability, large stability range and low diffusion rate of elements through the crystal structure. Geochemical and morphological studies of the tourmaline from the Emmons pegmatite revealed geochemical variations on three different scales: between different textures, between the core and rims of tourmaline crystals and between different morphologies in the intergrowths between tourmaline and quartz. Based on these studies as well as fluid and melt inclusion studies, a boundary layer model is suggested for the formation of the quartz-tourmaline textures, found in the outer zones. In this model excluded elements form a layer of melt with a different composition around the growing tourmaline. The enrichment of water and fluxes in the boundary layer leads to alterations of the melts properties causing nucleation delay and increasing undercooling, leading to the intergrowths of tourmaline and quartz. Furthermore, the studies indicate that the Emmons pegmatite formed under high degrees of undercooling. The studies of the Rwandan rocks show that crystallization conditions vary between the different rocks of the pegmatite system. The geochemistry indicates that all the tourmaline found is related to a magmatic fluid. The study of the host rocks indicates that fluid-rock interactions also influence the composition of this mineral.