ORIGIN OF THE PHENOCRYST ASSEMBLAGE IN MAFIC ALKALINE DIKES OF THE MIOCENE MT. TSARANTANANA VOLCANIC FIELD, NORTHERN MADAGASCAR

The petrography, mineral chemistry, and geochemistry of a suite of 15 Ma old mafic dikes (41-49 wt% SiO₂) have been investigated to characterize the evolution of Massif du Tsarantanana and to obtain information about the subcontinental mantle beneath northern Madagascar. Aphyric and porphyritic dikes occur in which olivine (Fo_70-80) and clinopyroxene comprise the phenocryst phases. In some samples, “megacrysts” of amphibole also occur. Most of the pyroxenes (phenocrysts and groundmass) are Ti-augites with oscillatory and/or sector zoning, but resorbed cores with distinct overgrowths also occur. Three pyroxene textural types can be distinguished. Type I (most common) display continuous zonation from pinkish-brown cores to purplish-brown rims. These Ti-augites show variable TiO₂ (1.8-3.4 wt%), Al₂O₃ (3.7-7.1), Cr₂O₃ (<0.1), and Na₂O (0.3-0.6), which increase with decreasing X_Mg (0.82-0.72). Type II have pale-green (resorbed) cores surrounded by zoned Ti-augite rims. These cores contain higher Na₂O (0.5-0.8 vs. 0.32-0.40), Al₂O₃ (6.4-9.0 vs. 4.7-5.5), and Cr₂O₃ (0.45-0.62 vs. 0.09-0.31), but lower TiO₂ (1.7-2.1 vs. 2.5-4.3) than the rims, with higher X_Mg than cores (0.89-0.92 vs. 0.85-0.90). Type III pyroxenes are pale yellow-green, with X_Mg (0.80-0.85) intermediate to Types I and II, and low TiO₂ (0.9-1.8 wt%), Cr₂O₃ (~0.1), and Al₂O₃ (2.8-6.3), and high Na₂O (0.5-2.3). Amphiboles consist of pale yellow-brown titanian pargasite cores surrounded by reaction rims of small lath-like grains of clinopyroxene (with compositions like Type III phenocrysts), ilmenite, and blades of dark-brown kaersutite.

P-T calculations based on Type II pyroxene cores, and the pargasitic compositions of amphibole cores, indicate that these phases formed under upper-mantle conditions at considerable depth (P ~ 15 Kb). We interpret the pargasites as xenocrysts incorporated into the ascending mafic melt, and the green augite cores as high-P phenocrysts; the other pyroxene crystals (and kaersutite) crystallized at shallower depths. The presence of xenocrystic amphibole lends possible clues to understanding contamination processes occurring during ascent and crystallization of the mafic melts. As an example, the light-REE in an amphibole-bearing dike are lower by a factor of ~4 compared with amphibole-free mafic dikes.