Cognate Xenoliths in the Tertiary Ultrabasic Dykes of South-West Skye

Fergus G. F. Gibb1
Department of Geology, University of Toronto, Toronto 5, Canada
1Present address: Department of Geology, The University, Manchester 13.

Summary: A group of ultrabasic dykes occurring in south-west Skye is typified by often abundant and diverse ultrabasic xenoliths composed of essentially the same minerals as the dykes. The xenoliths, which vary considerably in shape and size, are in sharp contact with the host rocks and exhibit no evidence of remelting by, or reaction with, the dykes. Although extensive axial concentration of olivine occurred in the dykes during intrusion, the xenoliths appear to be randomly distributed throughout all but the extreme margins of the dykes and often exhibit a preferred orientation. The orientation and distribution of the xenoliths are attributed to the suppression of rotation and axial migration of the xenoliths during the emplacement of the dykes because of the relatively high viscosity of the suspending medium.

The ultrabasic xenoliths, unlike many of those occurring in basalts, kimberlites, etc., are considered unlikely to represent primary upper mantle material and it is suggested that they were probably derived by the disintegration of layered ultrabasic rocks genetically related to the dykes and hence are of cognate origin.

Mineralogical Magazine; December 1969 v. 37; no. 288; p. 504-514; DOI: 10.1180/minmag.1969.037.288.11
© 1969, The Mineralogical Society
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