TITLE:
Mineral Chemistry, Geobarometry and Oxygen Fugacity of the Granitic Rocks from the Itremo Domain, Central Madagascar
AUTHORS:
Désiré Alphonse Rakotondravaly, Roger Randrianja
KEYWORDS:
Itremo, Granitic, Monzonitic, Mineral Chemistry, Geobarometer, Fugacity
JOURNAL NAME:
Journal of Geoscience and Environment Protection,
Vol.10 No.4,
April
21,
2022
ABSTRACT: Major and accessory minerals from the Ibity granite, Tsarasaotra
monzonitic and granite dykes, and Antsahakely granite of the Itremo domain in
the Precambrian basement of Madagascar were characterized by using microscopic observations and chemical analyses with the aim of
understanding their chemical characteristics and estimating the
crystallization pressure and oxygen fugacity of their host rocks. Plagioclases
in these rocks are albite and oligoclase, while alkali feldspars are
orthoclase. For the phlogopite-micas, Fe-biotite and Li-phengite are common for
the Ibity and Antsahakely granites, Mg-biotite is common for the Ibity granite and the Tsarasaotra monzonitic and granite
dykes, and siderophyllite and
Zinnwaldite are specific to the Ibity granite. Phlogopite-micas in the
studied rocks are mainly primary, accessorily re-equilibrated, and rarely
secondary. Calcic amphiboles distributed in the Magnesio-and Ferro-hornblende
are identified in the Tsarasaotra monzonitic, whereas amphibole is rare and
absent in the other rocks. Igneous titanite is observed in the Ibity granite and in the Tsarasaotra monzonitic rocks,
which have similar compositions to some REE oxide-rich titanites.
Concerning the Fe-Ti oxide phases, the rhombohedral and spinel/trifer tetroxide
phases are found in both the Tsarasaotra monzonitic and the Tsarasaotra granite
dyke, the trifer tetroxide and spinel + wüstite
phases are found only in the Ibity granite, and the pseudobrookite + rhombohedral
phase is found only in the Tsarasaotra
granite dyke. The epidote mineral, rarely found in the Antsahakely
granite, could be an indicator of metamorphism or hydrothermal activity involved during the emplacement of this
rock. Aluminum in hornblende geobarometer gave pressure ranges of around
5 kbar for the Tsarasaotra monzonitic rocks. The Titanite geobarometer gave
pressures of 2.5 - 3.2 kbar for the Ibity granite, 2.9 kbar for the Tsarasaotra monzonitic,
and 7.1 kbar for the Antsahakely granite. Both amphibole and Fe-Ti oxide-base
oxygen fugacity reveal high oxygen fugacity
conditions for the Tsarasaotra monzonitic and granite dyke emplacements,
which might have a relationship with a porphyritic environment.