Crystallization History of Sector-Zoned Microcline Megacrysts from the Godani Valley Pluton, Nigeria

David Vaniman1
Board of Earth Sciences, University of California, Santa Cruz, California 95064
1Present address: Department of Earth and Space Sciences, State University of New York, Stony Brook, New York 11794.

Summary: Sector-zoning is brought out by mineral inclusions (plagioclase, biotite, hornblende, sphene, and apatite) in 2 to 5 cm microcline megacrysts of the Godani Valley pluton, Nigeria. The inclusions are concentrated in the megacryst rims beneath the faces {110} and {201¯}; the megacryst cores are not sector-zoned. Comparison with other occurrences (Portugal and Czechoslovakia) shows that this type of sector-zoning is not controlled by growth rate. Chemical sector-zoning does not occur in the Godani megacrysts. Growth from magma is proposed, with preference for inclusions on faces at a high angle to the feldspar a-chain. A superficially similar type of sector-zoning occurs in microcline megacrysts of both pluton and wall rock at Papoose Flat, California, suggesting that this feature can be generated either by growth from magma or by replacement.

Mineralogical Magazine; December 1978 v. 42; no. 324; p. 443-451; DOI: 10.1180/minmag.1978.042.324.06
© 1978, The Mineralogical Society
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