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Título

INVESTIGATING THE MINERAL ASSEMBLAGE AND COMPOSITION OF THE MATRIX AND INCLUSIONS IN TITANITE OF AN IMPURE MARBLE TO RECONSTRUCT ITS METAMORPHIC EVOLUTION

Texto do resumo

Impure marbles may provide valuable information for the reconstruction of the geodynamic evolution of orogens as their mineral assemblage allows more diverse characterizations to understand their metamorphic evolution. The Gourma basin in northern Mali is the south Saharan segment of the Pan-African belt that formed in the amalgamation of Gondwana and represents the passive margin of the West African Craton. Coesite relics have been described as inclusions in omphacite in the impure marble of the Gourma area. However, few studies have been performed to investigate the metamorphic evolution of this rock. In this study, we employed petrography, Raman spectroscopy, mineral chemistry and a compositional map obtained with EPMA to compare the matrix assemblage with the mineralogy of inclusions in titanite to understand the relative timing and P-T conditions of titanite formation and its correlation with the rest of the rock. The mineral assemblage of the matrix Cal + Qz + Omp + Phg + Dol + Ttn + Grt + Zo + Amp + Gr allows the outline of four metamorphic stages: 1) Ca-carbonate, Mg-calcite and dolomite; 2) omphacite, phengite, quartz/coesite and garnet core; 3) quartz and garnet rim; and 4) amphibole-bearing symplectite. The texture relationship suggests that titanite formed during stage 3 and continued in stage 4. The assemblage is divided into two domains, one carbonate-rich and the other rich in silicates, containing lesser amounts of carbonate. In the carbonate-rich domains, calcite crystals occur as aggregates, presenting a strong zoning with a Fe- and Mg-richer core (Cal85-93Mgs03-10Sd03-06) and a Ca-richer rim (Cal91-98Mgs01-03Sd02-03). In this domain, carbonate often presents dolomite exsolutions. Dolomite in these aggregates is homogeneous Cal54-57Mgs30-33Sd11-13. In the silicate-rich domain, omphacite is homogeneous Jd35-41, and is being replaced by a symplectite with amphibole composition. Garnet shows a Ca-rich core (Grs44-48Alm33-37Prp14-17Sps03-04) and a thin Fe-richer rim (Grs37-41Alm38-43Prp12-16Sps04-05). The carbonate is a Ca-carbonate that displays smaller variations from the core (Cal90-94Mgs02-04Sd02-05) to the rim (Cal96-100Mgs01-02Sd02). Raman spectroscopy reveals that homogeneous Ca-carbonate cores present in both domains are calcite, though their pure composition and sharp contacts suggest they can represent aragonite pseudomorphs. Phengite presents a strong variation in Si composition varying from 3.32 to 3.52 (a.p.f.u.). Garnet, omphacite, amphibole, phengite, quartz and calcite occur as inclusions in titanite. Omphacite inclusions range from Jd38 to Jd45, while garnet composition is Alm32-39Grs40-45Prp15-17Sps03-06. The Si content in phengite hosted in titanite varies from 3.32 to 3.45. Rare inclusions with melt-like composition (i.e., carbonatitic melt) were also found suggesting that temperatures higher than 700 ºC were reached during crystallization. Since the mineral assemblage and compositions of the matrix are the same as that found as inclusions in titanite, except for carbonate other than calcite, titanite likely grew during peak to retrograde metamorphism. We envisage that such high-temperature conditions above the solidus may have affected the U/Pb isotopic system in titanite. As the chemical composition observed in these inclusions can help reconstruct the conditions of the metamorphism and the understanding of titanite formation, further investigation is necessary to define a P-T-t path for this rock.

Palavras Chave

UHP metamorphism; Compositional mapping; impure marble; XMapTools