Petrology of the Malignant Cove dioritic complex

Abstract

The Malignant Cove Dioritic intrusion occupies an area of 5 sq km. and is situated on the N. W. shore of Nova Scotia, some 20 km north of Antigonish.

Detailed mapping of a section of this intrusion together with subsequent petrographic work, has shown it to comprise various lithologies ranging from minor lherzolites, clinopyroxenites and peridotites, through intermediate stages of diorite (constituting the bulk of the intrusion) to residual porphyritic phases, quartz diorites, anorthosites and pegmatites. The semi-circular stock intrudes actinolitic hornfels and andesitic lithologies thought to be part of the Cambro-Ordivician Brierly Brook Formation of the Brown's Mountain Group (Benson,1970) .

Within the intrusion, the widespread occurrences of rhythmic layering mineralogical banding or cumulate textures implies magmatic fractionation and crystal accummulation within the parent magma. Horneblende, frequently poikilitic in texture is intercumulus to olivine and pyroxene in the ultramafics, and plagioclase in the anorthositic phases and porphyritic quartz diorites.

The various igneous lithogies are thus considered to have formed through fractionation of a single parental melt, for which an intermediate composition is suggested by relative lithological abundances. The mode of emplacement of the ultramafics varies from in-situ segregation to forceful emplacement following differentiation and settling to form a cumulate sequence. Where forceful emplacement occurred, chilled margins are absent indicating a closed crystal mush system at a temperature similar to that of the melt.

Detailed petrographic descriptions of 20 thin sections are included in the appendix.