Chapter 9. COMPOSITIONAL ZONING and INCLUSIONS in METAMORPHIC MINERALS

Pressure–temperature-deformation history for a part of the Mesoproterozoic fold belt in North Singhbhum, Eastern India

“…This is probably due to homogenization as a result of volume diffusion at the peak temperature (Tracy, 1982). The Fe/Mg ratio, as well as the spessartite content, decrease in the higher grade rocks.…”

Pan-African metamorphic evolution in the southern Yaounde Group (Oubanguide Complex, Cameroon) as revealed by EMP-monazite dating and thermobarometry of garnet metapelites

“…In each sample, the cores of zoned garnet and its plagioclase and biotite inclusions, as well as cores of zoned matrix plagioclase and relic mica of S 1 in microlithons should represent an early stage of the metamorphic evolution. A late stage of metamorphism is documented by the garnet rims (excluding the parts with retrogressive increase of Mn), large biotite which crystallised in the microlithons or in pressure shadows around the garnet porphyroblast, and the rims of zoned matrix plagioclase (Tracy, 1982;St-Onge, 1987;Triboulet and Audren, 1985;Schulz, 1990Schulz, , 1993. Such localised equilibria within low-variance assemblages allow to evaluate P-T changes for the consecutive stages of garnet growth by geothermobarometry.…”

Petrology and P–T path of high-pressure granulite from the Dulan area, North Qaidam Mountains, northwestern China

“…Mineral zoning and composition can be sensitive indicators of metamorphic reactions, and are thus of great importance in the interpretation of P-T evolution of metamorphic rocks (Tracy, 1982;Spear, 1993;O'Brien, 1997;Elvevold and Gilotti, 2000). In order to quantitatively constrain the metamorphic evolution history of HP granulite in Dulan, nearly all primary, secondary and accessory minerals were collected for chemical analyses, and compositional zoning profiles and X-ray maps of garnet were produced.…”

“…In general, increasing rate of intra-crystalline diffusion will lead to reduction of existing chemical profiles and homogenization of garnet with respect to Fe, Mg, Ca and Mn at temperatures in excess of 700°C (Tracy, 1982;Chakraborty and Ganguly, 1990;Spear et al, 1990;Robinson, 1991). Nevertheless, though the peak temperature reached 900°C, garnets from the Dulan HP granulites exhibit intact prograde zoning.…”