OLIVINE MELILITITES, MANTLE XENOLITHS, AND XENOCRYSTS OF THE TAKARINDIONA DISTRICT: PETROGENESIS, MAGMATIC EVOLUTION, AND THE SUB-CONTINENTAL LITHOSPHERIC MANTLE OF EAST-CENTRAL MADAGASCAR

The olivine melilitites from the southern part of the 6.8 Ma-old Takarindiona volcanic field (Eastern Madagascar) are olivine ± chromite -phyric lava flows, having zoned titanaugite, perovskite, melilite, nepheline, monticellite, Ba-mica and Fe-Ti oxides as microphenocrysts or groundmass phases. The volcanic rocks are very primitive ultrabasic rocks, rich in incompatible trace elements (e.g., Ba=1049 ± 153 ppm, Sr=1050 ± 167 ppm, Nb=98±13 ppm; La/Yb_n= 41±5; La/Nb=0.88±0.05), with a restricted range of ⁸⁷Sr/⁸⁶Sr (0.70391-0.70410) and ¹⁴³Nd/¹⁴⁴Nd (0.51272-0.51282). The rocks follow a differentiation trend controlled by approximately 20% removal/addition of phenocryst olivine ± chromite. The ol-melilititic magmas are the product of small degrees of partial melting (1-3%) of a peridotitic source, enriched in highly incompatible trace elements by CO₂^-, F^-, and H₂O-rich melts, located within the garnet stability field (3-3.5 GPa and ~100km depth) of sub-continental lithospheric mantle, where carbonates (dolomite) were stable phases. Mantle xenoliths within the volcanic rocks are mostly spinel harzburgites having mineral modes and chemical compositions suggesting variable degrees of “basalt” melt extraction. Based on textural and chemical evidence, and quantitative thermobarometric estimates, the xenoliths were incorporated at a pressure of ~1.1 GPa (~ 35-40 km depth), far shallower than the source of the melilititic magmas, and along a predictably cool geotherm beneath Archean continental lithosphere. Berman's TWQ program (1991, Can. Mineral. 29, 833-855 and its version for Windows) has been applied to calculate log (a Quartz) vs. T-X in the SiO₂-MgO-CaO-Na₂O system for 3kb pressure. At 900^oC the mel-ol assemblage is stable for log (a Quartz) between -1 and -1.5 (a Quartz - 0.1 to 0.03), and at 1000^oC, the balance between diopside and akermanite - forsterite, fixes a Quartz at 0.06, a value compared to those, significantly higher, for the nepheline-albite balances (-0.47) and forsterite-enstatite (-0.57). Highly resorbed orthopyroxene xenocrysts, resembling those crystallizing in lower crustal granulites, indicate that the melilitites may have also entrained lower crustal or underplated subalkaline rocks.