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

Abstract

Ti-rich garnet phenocrysts from a tephrite (‘nephelinite’) plug in the Ambadungar complex situated in the Chhota Udaipur alkalic subprovince show concentric zoning. Based on paired orthogonal traverses across three selected crystals (total 81 step-scan point analyses), andradite content ranges from 55 to 86 mol% (the remainder being almost entirely schorlomite), corresponding to the following wt% oxide variations: TiO 2 5.5–15.8, CaO 29.6–32.5, MgO 0.3–1.6, Fe 2O 3 20–26, Al 2O 3 0.7–3.5%; MnO, V 2O 3, Na 2O and ZrO 2 each rarely exceeds 0.5%. Zoning patterns in individual grains from this one rock differ 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, Mg and usually Zr but impoverished in Al; (2) the two orthogonal traverse may or may not be mirror images; (3) monotonic trends (decreasing Ca, Al, increasing Mg, Zr from core to rim) may or may not be present; (4) oscillatory zoning varies in amplitude; and (5) apparent substitutions differ, although all crystals clearly show the Si–Ti substitution inferred for Ti-rich garnets elsewhere. An idealized case is developed from an observed complexly zoned phenocryst population to piece together a history of the alkaline host magma that experienced several events such as polybaric differentiation, magma-mixing, and kinetic effects.