The Nature and Origin of Orbicular Rocks from Near Deshai, Swat Kohistan, Pakistan

R. F. Symes, J. C. Bevan and M. Qasim Jan
Department of Mineralogy, British Museum (Natural History), Cromwell Road, London SW7 5BD
Institute of Geology and Mineralogy, University of Peshawar, Pakistan

Abstract: Orbicular dioritic-noritic rocks from an area of mixed metemorphic and igneous rocks in Swat Kohistan, northern Pakistan, have been examined petrographically and chemically in order to determine the nature and origin of the orbicular texture. Using textural and compositional sequences it has been possible to relate the apparently different orbs to one another, and obtain a sequence of orb formation. The majority of the orbs comprise a series of distinct layers (shells) surrounding a central zone (core). Plagioclase, clinopyroxene, orthopyroxene and hornblende form the bulk of the shells. The cores have been extensively recrystallized. The development of a ‘comb-layered’ texture in some orbs and in associated layered rocks is comparable to that commonly described from other occurrences. A dual igneous/metasomatic crystallization history is invoked to explain the features of the orbs in this locality, the oscillatory zoning of the orbicular structure being caused by the alteration of primary minerals, such as pyroxene to amphibole, due to fluctuations in the pH2O of the magma.

Keywords: orbicular rocks • diorite • norite • Swat Kohistan • Pakistan

Mineralogical Magazine; December 1987 v. 51; no. 363; p. 635-647; DOI: 10.1180/minmag.1987.051.363.02
© 1987, The Mineralogical Society
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