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Orbicular gabbro from near Baskil, southeastern Turkey

Published online by Cambridge University Press:  05 July 2018

Evren Yazgan
Affiliation:
Department of Geological Sciences, University College, Gower Street, London WC1E 6BT
Roger Mason
Affiliation:
Department of Geological Sciences, University College, Gower Street, London WC1E 6BT

Abstract

The first occurrence of orbicular rocks in Turkey is reported. They are gabbros in a dyke in the Baskil island-arc magmatic suite, of the southern branch of the Alpine-Himalayan chain. The orbicules have lithologically varied cores, regular shells of troctolitic composition, with a radial arrangement of olivine and plagioclase crystals, and a matrix of vari-textured gabbro. Rock and mineral analyses indicate that magmatic crystallization began near the inner margin of the troctolitic shells. Metamorphic hydration of minerals in cores, shells and matrix followed directly after the later stages of magmatic crystallization, as part of a continuous process, and thus a magmatic-metamorphic origin for the orbicules is proposed. Alternative hypotheses for the development of the orbicules are discussed: one involving the development of a protocrystalline magma shell, the other rapid crystallization during upward migration of xenolith-bearing magma.

Type
Petrology
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1988

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