Response to M.D. Palmer's Comments re Article in J. Great Lakes Res. 21:138–154 by Tsanis et al.
1995; Elsevier BV; Volume: 21; Issue: 3 Linguagem: Inglês
10.1016/s0380-1330(95)71524-6
ISSN2773-0719
Autores Tópico(s)American Environmental and Regional History
ResumoTi-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: TiO2 5.5–15.8, CaO 29.6–32.5, MgO 0.3–1.6, Fe2O3 20–26, Al2O3 0.7–3.5%; MnO, V2O3, Na2O and ZrO2 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.
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