Complexly zoned Ti-rich melanite-schorlomite garnets from Ambadungar carbonatite-alkalic complex, Deccan Igneous Province, Gujarat State, Western India

Ti-rich garnet phenocrysts from a tephrite ('nephelinite') plug in the Amba dungar complex situated in the Chhota Udaipur alkalic subprovince show conc entric zoning. Based on paired orthogonal traverses across three selected c rystals (total 81 step-scan point analyses), andradite content ranges from 55 to 86 mol% (the remainder being almost entirely schorlomite), correspond ing to the following wt% oxide variations: TiO2 5.5-15.8, CaO 29.6-32.5, Mg O 0.3-1.6, Fe2O3 20-26, Al2O3 0.7-3.5%; MnO, V2O3, Na2O and ZrO2 each rarel y exceeds 0.5%, Zoning patterns in individual grains from this one rock dif fer considerably in several ways: (1) there may be 2-5 alternating pale and dark zones, the pale generally being more andradite-rich enriched in Ti, M g and usually Zr but impoverished in Al; (2) the two orthogonal traverse ma y or may not be mirror images; (3) monotonic trends (decreasing Ca, Al, inc reasing Mg, Zr from core to rim) may or may not be present; (4) oscillatory zoning varies in amplitude; and (5) apparent substitutions differ, althoug h all crystals clearly show the Si-Ti substitution inferred for Ti-rich gar nets elsewhere. An idealized case is developed from an observed complexly z oned phenocryst population to piece together a history of the alkaline host magma that experienced several events such as polybaric differentiation, m agma-mixing, and kinetic effects. (C) 2000 Elsevier Science Ltd. All rights reserved.