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Journal of Petrology Advance Access published online on February 7, 2006
Journal of Petrology, doi:10.1093/petrology/egi100
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© The Author 2006. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org
Received April 1, 2004
Accepted December 19, 2005

Article

Mg-Al Sapphirine- and Ca-Al Hibonite-bearing Granulite Xenoliths from the Chyulu Hills Volcanic Field, Kenya

A. ULIANOV 1 * and A. KALT 1

1 UNIVERSITY OF NEUCHÂTEL, INSTITUTE OF GEOLOGY, RUE EMILE-ARGAND 11, CH-2007 NEUCHÂTEL, SWITZERLAND

* To whom correspondence should be addressed.
A. ULIANOV, E-mail: Alexey.Ulianov{at}unil.ch


  Abstract

Basanites of the Chyulu Hills (Kenya Rift) contain mafic Mg-Al and Ca-Al granulite xenoliths. Their protoliths are interpreted as troctolitic cumulates; however, the original mineral assemblages were almost completely transformed by subsolidus reactions. Mg-Al granulites contain the minerals spinel, sapphirine, sillimanite, plagioclase, corundum, clinopyroxene, orthopyroxene and garnet, whereas Ca-Al granulites are characterized by hibonite, spinel, sapphirine, mullite, sillimanite, plagioclase, quartz, clinopyroxene, corundum, and garnet. In the Mg-Al granulites, the first generation of orthopyroxene and some spinel may be of igneous origin. In the Ca-Al granulites, hibonite (and possibly some spinel) are the earliest, possibly igneous, minerals in the crystallization sequence. Most pyroxene, spinel and corundum in Mg-Al and Ca-Al granulites formed by subsolidus reactions. The qualitative P-T path derived from metamorphic reactions corresponds to subsolidus cooling, probably accompanied, or followed by, compression. Final equilibration was achieved at T {approx} 600-740°C and P <8 kbar, in the stability field of sillimanite. The early coexistence of corundum and pyroxenes (± spinel), as well as the association of sillimanite and sapphirine with clinopyroxene and the presence of hibonite, makes both types of granulite rare. The Ca-Al hibonite-bearing granulites are unique. Both types enlarge the spectrum of known Ca-Al-Mg-rich granulites worldwide.

Keywords: granulite xenoliths; corundum; sapphirine; hibonite; Kenya Rift.
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[Abstract] [Full Text] [PDF]


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