Origin of Ti-rich garnets in the groundmass of Wajrakarur field kimberlites, southern India: insights from EPMA and Raman spectroscopy

摘要: Although Ti-rich garnets are commonly encountered in the groundmass of many alkaline igneous rocks, they comparatively rare kimberlites. Here we report on occurrence P-15 and KL-3 kimberlites from diamondiferous Wajrakarur field Eastern Dharwar craton southern India. These contain considerable Ti (11.7–23.9 wt.% TiO2), Ca (31.3–35.8 CaO), Fe (6.8–15.5 FeOT) Cr (0.04–9.7 Cr2O3), but have low Al (0.2–5.7 Al2O3). In case kimberlite display a range compositions andradite to schorlomite, with proportion grossular (andradite(17.7–49.9)schorlomite(34.6–49.5)-grossular(3.7–22.8)-pyrope(1.9–10.4)). A few grains also significant chromium represent solid solution between schorlomite uvarovite. The kimberlite, contrast, mostly schorlomitic (54.9─90.9 mol %) composition. these two intimately associated chromian spinels, perhaps suggesting that garnet formed through replacement spinel. From textural evidence, it appears unlikely could originated secondary alteration, rather seem process which early magmatic spinels reacted late circulating, residual fluids final stages crystallization magma. Raman spectroscopy provides evidence for crystallinity is likely be result their partial transformation into during reaction late-stage (kimberlitic) fluid. close chemical association TiO2-FeO-CaO space those reported ultramafic lamprophyres (UML) consistent results predicted by experimental studies, possibly implies genetic link UML magmas. similar composition Swartruggens orangeite Kaapvaal South Africa, as well other an orangeitic affinity (e.g. India), inferred reflection high Ca- Ti-, Al-nature, parent magma (i.e. Group II kimberlites).