EMPLACEMENT AND IMPLICATIONS OF PERIDOTITE-HOSTED LEUCOCRATIC DYKES, VAMMALA MINE, FINLAND

Leucocratic dykes cut the lowermost layer of the Stormi ultramafic com plex that hosts the Vammala Ni-Cu-Fe sulphide mine. The dykes and asso ciated wallrock alteration zones formed when granitic melt, derived fr om migmatites during high-grade amphibolite facies metamorphism, invad ed a fracture system within the peridotitic host. The geometry of the fracture system reflects the gross shape of the peridotite body. It is consistent with a mode of formation in which granite-magma-driven ini tiation and propagation were facilitated by contractional cooling. Lac k of a secondary fabric overprinting the peridotite and wallrock alter ation zones, the high-grade metamorphic mineralogy in the zones, and t he dynamic recrystallization textures of plagioclase in the dykes, are all consistent with peridotite emplacement and leucocratic dyke-devel opment after formation of the regional gneissosity (in D-2) and before or during the ensuing regional event (D-3). The same age relationship s probably apply to other nickel-bearing ultramafic bodies and their a ssociated leucocratic dykes in this region of Finland.