Silicate Mineralogy of the Behelvie Cumulates, N E Scotland

W. J. Wadsworth
Department of Geology, The University, Manchester M13 9PL

Abstract: Ultramafic, troctolitic and gabbroic rocks at the northern end of the Belhelvie layered intrusion display progressive cryptic variation over a stratigraphic thickness of approximately 800 m in steeply-dipping cumulates, which young from W to E. This variation is shown by olivine (Fo87−77), orthopyroxene (En87−79), clinopyroxene (Ca45Mg48Fe7 to Ca44.5Mg45.5Fe10) and plagioclase (An81−75). The Belhelvie succession is believed to be equivalent to the poorly-exposed and structurally complex Insch Lower Zone. A laterally impersistent hypersthene-gabbro unit within the main sequence is re-interpreted as a downfaulted block of slightly more evolved cumulates. A repeat sequence of peridotites, troctolites and gabbros on the eastern side of the intrusion, and separated from the underlying main succession by a thin septum of country rock, is believed to represent a fresh influx of magma.

Keywords: silicates • cryptic variation • cumulates • Belhelvie • Scotland

Mineralogical Magazine; March 1991 v. 55; no. 378; p. 113-119; DOI: 10.1180/minmag.1991.055.378.09
© 1991, The Mineralogical Society
Mineralogical Society (