Unmetamorphosed Volcanic and Sedimentary Xenoliths in the Kimberlites of Sierra Leone

Abstract

THE dominant xenolith types in the kimberlites of the Sefadu area of eastern Sierra Leone are granite, amphibolite and amphibole-schist derived from the wall rocks. Eclogite and dunite xenoliths are only rarely encountered. Small inclusions of a suite of virtually unaltered volcanic rocks and derived sediments for which no counterpart is found in the mapped country rocks of the area are a feature of the kimberlite now exposed by mining at Koidu. I recently collected a series which included undeformed amygdaloidal basalt, clastic basaltic sandstone, fine-grained basic tuff, fragmental andesitic tuff and calcareous tuff. The kimberlites occur in granite emplaced in the Pre-Cambrian Kambui Schists, a strongly metamorphosed schist series of largely volcano-sedimentary parentage^1,2. The unaltered xenolithic material in the kimberlites is most readily explained as being derived from an overlying sequence of volcanic and associated sedimentary rocks, now removed by erosion.