Phengite megacryst quasi-exsolving phlogopite, from Sulu ultra-high pressure metamorphic terrane, Qinglongshan, Donghai County (eastern China): New data for P-T-X conditions during exhumation

A large crystal of trigonal phengite (Phe-3T; K0.96Na0.02Ba0.01)(Al1.35Mg0.40Fe0.112+Fe0.133+Ti0.02)(Sigma=2.02) [Si3.44Al0.56)O-10(OH)(2)), sampled from a phengite+quartz metamorphic vein in the eclogites-bearing rocks of Sulu UHP metamorphic terrane, exhibits unusual thin lamellae of ferrian-aluminian trigonal/monoclinic phlogopite polytypes (Phl-3T: (K0.78Na0.01)(Mgt.48Fe0.392+Fe0.453+Al0.30Ti0.06)(Sigma=2.68)[Si2.98Al1.02]O-10((OH)(1.77)F-0.23): Phl-1M: (K0.85Ca0.01)(Mg1.43Fe0.452+Fe0.543+Al0.2Ti0.06)(Sigma=2.69)[Si2.88Al1.12]O-10((OH)(1.99)F-0.01)). This assemblage is rarely observed in UHP edogite-bearing rocks, and has never been reported before in the Sulu UHP metamorphic terrane. A detailed crystal-chemical characterisation of Phe-3T, Phl-3T and Phi-Wallowed the development of a thermodynamic model that estimates the P-T conditions of formation of such an assemblage and helps to understand the relationship between mica polytypes and a quartz dominated (silica oversaturated) system, which characterised the Sulu vein. Phe-3T varies in composition upon approaching Phl-3T/1M, showing a decrease of Si and Al and an increase of Fe and Mg contents. Observed phlogopites cannot be ascribed to bare exsolving processes, and require the involvement of a non-isochemical reaction, dominated by exsolution (quasi-exsolution process) in combination with a contribution from an "external chemical supplier" (most probably provided by circulating fluids). Mass balance calculations between the mineralogical assemblage of Sulu vein and the solute components of alkali-alumino-silicate high pressure fluids demonstrate that a silicic-type major element composition accounts for a phengite-quartz based rock. The occurrence of phlogopite micas can be explained as an effect of disproportion in combination with supply components, i.e. Mg, Al and K, from alkali-alumino-silicate high pressure fluids. On the basis of energy modelling of the reaction process from phengite to phlogopite in presence of quartz and a matter exchanger, we determined that such a transformation is weakly dependent on T over the 500-930 degrees C thermal range. The intersection of the calculated P-T assemblage equilibrium curve with the country rock P-T metamorphic path occurs at P similar to 2.4 +/- 0.2 GPa and T similar to 700 degrees C, thus reflecting an early stage of exhumation, close to the metamorphic peak of the Sulu UHP terrane. (C) 2018 Elsevier B.V. All rights reserved.