Mineralogy and petrology of the Highland Border Suite serpentinites

ABSTRACT. Prior to serpentinization, the ultramafic rocks of the ophiolitic Highland Border Suite were depleted harzburgites, dunites, and orthopyroxenites. They may have been the residual mantle after melting of primary lherzolite that produced the basaltic rocks of the Suite. Serpentinization f these peridotites occurred at low temperatures (75+50 ~ and produced pseudo-morphic-textured lizardite serpentinites in which the minor-element compositions ofthe primary phases were preserved in the lizardite grains. Olivine was almost completely serpentinized before orthopyroxene was hydrated. Concomitant peripheral teration of chrome-spinels, during serpentinization of orthopyroxene, caused release of Cr and A1 cations which were incorporated into the lizardite. Under greenschist-facies conditions (c. 350 ~ at 3-4 kbar), the lizardite serpentinites were subsequently metamorphosed and recrystallized to non-pseudomorphic-textured antigorite serpentinites. Post-metamorphic processes included high-level brittle deformation that resulted in fracturing and cross-fibre chrysotile vein-formation, and ductile shearing (T 200 ~ P> 2 kbar) which produced talc-chrysotile shear zones. THE mafic and ultramafic rocks that outcrop along the Highland Border Fracture Zone (H.B.F.Z.)--The Highland Border Suite (Ikin, 1979)--have been interpreted asophiolitic remnants ofa Caledonian marginal basin of Cambrian or early Ordovician age (Henderson and Robertson, 1982; Curry et al., 1982; Ikin, 1983). Rocks of the Suite occur as tectonically emplaced fault-bounded slivers and pods varying in width from 30-500 m, and separated one from another by as much as 35 km. The H.B.F.Z. is a complex zone of faulting, monoclinal downwarping and unconformity which marks the boundary between the late Precambrian-Cambrian Dalradian metasediments of the exposed ortho