Ti–rich biotite in spinel and quartz–bearing paragneiss and related rocks from the Mogok metamorphic belt, central Myanmar

Abstract

The Paleogene Mogok metamorphic belt in central Myanmar is composed mainly of high–grade metamorphic rocks from upper–amphibolite to granulite facies and younger intrusions. Ti–rich biotite grains (up to 6.9 wt% TiO₂) from the Onzon and Shwe Myin Tin areas were systematically analyzed to examine the mechanisms of Ti–bearing substitutions. The Onzon and Shwe Myin Tin paragneisses are composed mainly of garnet, biotite, plagioclase, K–feldspar, quartz, ilmenite, and graphite. One of these contains cordierite porphyroblasts, which contain a spinel [Al/(Al + Fe³⁺ + Cr + V) = 0.97–0.99, Mg/(Mg + Fe²⁺) = 0.34–0.35, Zn = 0.04–0.05 atoms per formula unit (apfu) for O = 4], quartz, sillimanite, biotite, plagioclase, and ilmenite assemblage as inclusions. The Shwe Myin Tin paragneiss contains sillimanite as an inclusion in garnet. Using the garnet–biotite geothermometer and the garnet–biotite–plagioclase–quartz (GBPQ) geobarometer, the matrix assemblage indicates pressures (P) and temperatures (T) of 0.77–0.84 GPa and 780–850 °C, respectively. The coexisting spinel and quartz and the estimated P/T conditions imply a wide distribution of granulite facies metamorphic rocks in the northern part of the Mogok metamorphic belt. The high Ti content of biotite in the sillimanite–free Onzon samples is probably progressed result of the Ti□R₋₂ substitution, where R is the sum of divalent cations and □ represents vacancy in the octahedral sites. The biotite grains in the sillimanite–bearing Shwe Myin Tin sample showed a combination of Ti□R₋₂ and TiRAl₋₂ substitutions.