Isotopic Ages of Minerals from the Pre-Cambrian Complex of the Malverns

Abstract

THE Malvern Hills, Worcestershire, form one of many Pre-Cambrian inliers of the Welsh Borderland¹; rocks of Pre-Cambrian age are divided into Malvernian ‘schists and gneisses’ and the overlying Warren House volcanic group. The analyses reported in the following communication are from minerals occurring within the supposed metamorphic rocks and do not include any from the volcanic group, no suitable material having been obtained from the latter. The ‘schists and gneisses’ were identified as Malvernian more than a century ago², but the origin of their structures remained a puzzle, and it was a matter of some controversy as to whether the rocks were igneous or metamorphic^3,4. However, most of the rocks seen by us are igneous, usually hydrothermally altered and sheared, but still retaining igneous textures and occasionally igneous mineralogy. Such schistose rocks as have been observed are either in shear zones or in fault blocks, and true gneisses, in the usual sense of high-grade, banded, metamorphic rocks, are absent. The commonest rocks are of intermediate igneous type, classifiable as mafic diorite or tonalite. Amphibolites of various kinds, usually with igneous textures, are another common rock type. Ultramafic rocks are usually rich in tremolite or actinolite, with biotite or chlorite. Granitic rocks are rare, but syenitic veins and pegmatites frequently occur. The usual mineral assemblages are those of the greenschist or epidote–amphibolite metamorphic facies. It is generally agreed that the alteration of the igneous rocks of the Malvernian occurred in the Pre-Cambrian, as no equivalent hydrothermal or metamorphic activity has affected the Cambrian or later strata.