Désiré Alphonse Rakotondravaly^{1, 2,}, Roger Randrianja²

Granitic rocks of the Ibity, Tsarasaotra, and Ambatofinandrahana areas of the Itremo domain, in the Precambrian basement of Madagascar were characterized by using the naked eye, microscopic observations, and whole rock chemical analyses with the aim of understanding their petrogenesis, their interrelations, and their membership in the known magmatic suites in the Itremo domain. The granitic rocks of this study are mainly composed of monzonitic rocks for the Tsarasaotra and granite for the other areas. Major minerals identified are alkali feldspars, quartz, plagioclase, and accessory minerals are amphiboles, micas, pyroxene, iron-titanic minerals, titanite, and epidote. Major element compositions are intermediate for the Tsarasaotra monzonitic rocks and felsic for the other rocks. All the studied rocks are silica-saturated with enough aluminum, ferroan, calc-alkalic to alkalic, metaluminous, rarely peraluminous, I-type granitic rocks. Transition elements contents are generally low for the granites and high for the monzonitic rocks, while the compositions of compatible elements are in general high. Chondrite-normalized patterns of REE show high enrichment of LREE and low to moderate enrichment of HREE. The behaviors of major elements (especially Ti, Al, Fe, Mn, Mg, and Ca oxides) and trace elements (Eu anomaly, La/Sm) in the studied rocks show magmatic differentiation signatures in relation to crystal fractionation. These rocks' magmatic sources were most likely enriched-MORBs that evolved through within-plate enrichment for the Tsarasaotra monzonitic rocks and Ifasina granite, and through subduction for the Ibity granite, Tsarasaotra granite dyke, and Antsahakely granite. The succession of these two different processes identified in the studied rocks reveals their membership in the Imorona-Itsindro. Although some of the studied rocks are adakite-like, their characteristics as well as their interrelations indicate magmatic differentiation signatures rather than slab-melting origin. This work is a contribution to the promotion of scientific and geologic research in Madagascar, and it is in perspective of its extension to the geochemical characterization of laterites and mineralization potential identification related to the studied rocks in the Itremo domain as well as in the Antananarivo domain of Madagascar’s Precambrian basement.

geochemistry, petrology, rocks, mineral, granite, monzonitic, Itremo, Ibity, Tsarasaotra, Ambatofinandrahana, Ifasina, Antsahakely, Imorona-Itsindro, magmatic, adakitic