Cryptic Variation in the Kapalagulu Layered Intrusion, Western Tanzania

W. J. Wadsworth, A. C. Dunham and A. A. Almohandis
Department of Geology, University of Manchester, Manchester M13 9PL, England
Department of Geology, University of Hull, Cottingham Road, Hull HU6 7RX, England
Department of Geology, Riyadh University, Riyadh, Saudi Arabia

Abstract: The Kapalagulu intrusion displays the following sequence of cumulus phase layering in a stratigraphic sequence of 1400 m: Basal Zone (BZ) olivine ± chromite → Intermediate Zone (IZ) olivine + plag + opx →, olivine + plag + opx + cpx → Main Zone (MZ) plag + opx + cpx → plag + cpx + Fe/Ti oxide + apatite. The corresponding cryptic variation is olivine Fo83 − 77 (limited to BZ and IZ), orthopyroxene En82 − 56, clinopyroxene Ca46Mg45Fe9 to Ca43Mg37Fe21 and plagioclase An88 − 80. Reversals of the cryptic variation occur at the base of MZ (minor reversal) and in the middle of MZ (major reversal), and are attributed to the influx of relatively primitive magma. The major reversal indicates that progressive mixing of fresh and residual magmas occurred. Because of the major reversal, inverted pigeonite appears twice in the layered sequence, but at different compositions (En65 and En56). Unlike the cumulus olivine and pyroxene, cumulus plagioclase exhibits a wide range of composition (5−10% An) in individual rocks and even in single crystals.

Mineralogical Magazine; 1982 v. 45; no. 337; p. 227-236; DOI: 10.1180/minmag.1982.045.337.26
© 1982, The Mineralogical Society
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