Mineralogy and Petrology of the Highland Border Suite Serpentinites

N. P. Ikin* and R. S. Harmon
Department of Geology, University College, Cardiff
Department of Geological Sciences, Southern Methodist University, Dallas, Texas 75275, USA
*Address for correspondence: 68 Shirley Drive, Heolgerrig, Merthyr Tydfil, South Wales CF48 1SF.

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 of these peridotites occurred at low temperatures (75±50°C) and produced pseudomorphic-textured lizardite serpentinites in which the minor-element compositions of the primary phases were preserved in the lizardite grains. Olivine was almost completely serpentinized before orthopyroxene was hydrated. Concomitant peripheral alteration of chromespinels, during serpentinization of orthopyroxene, caused release of Cr and Al cations which were incorporated into the lizardite. Under greenschist-facies conditions (c. 350°C at 3–4 kbar), the lizardite serpentinites were subsequently metamorphosed and recrystallized to nonpseudomorphic-textured antigorite serpentinites. Postmetamorphic processes included high-level brittle deformation that resulted in fracturing and cross-fibre chrysotile vein-formation, and ductile shearing (T ∼ 200 °C, P > 2 kbar) which produced talc-chrysotile shear zones.

Mineralogical Magazine; September 1983 v. 47; no. 344; p. 301-310; DOI: 10.1180/minmag.1983.047.344.04
© 1983, The Mineralogical Society
Mineralogical Society (www.minersoc.org)