Emplacement and implications of peridotite‐hosted leucocratic dykes, Vammala Mine, Finland

Abstract

Leucocratic dykes cut the lowermost layer of the Stormi ultramafic complex that hosts the Vammala Ni‐Cu‐Fe sulphide mine. The dykes and associated wallrock alteration zones formed when granitic melt, derived from migmatites during high‐grade amphibolite facies metamorphism, invaded 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 initiation and propagation were facilitated by contractional cooling. Lack of a secondary fabric overprinting the peridotite and wallrock alteration zones, the high‐grade metamorphic mineralogy in the zones, and the dynamic recrystallization textures of plagioclase in the dykes, are all consistent with peridotite emplacement and leucocratic dyke‐development after formation of the regional gneissosity (in D₂) and before or during the ensuing regional event (D₃). The same age relationships probably apply to other nickel‐bearing ultramafic bodies and their associated leucocratic dykes in this region of Finland.

Marshall, B., Smith, J.V. & Mancini, F., 1995: Emplacement and implications of peridotite‐hosted leucocratic dykes, Vammala Mine, Finland. GFF, Vol. 117 (Pt. 4, December), pp. 199–205. Stockholm. ISSN 1103–5897.

Keywords:

• Dyke‐emplacement
• host‐peridotite
• Ni‐Cu‐Fe sulphides
• wallrock alteration
• high‐temperature metasomatism