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)(Al1.35Mg0.40Fe2+0.11Fe3+0.13Ti0.02)Σ=2.02[Si3.44Al0.56]O10(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)(Mg1.48Fe2+0.39Fe3+0.45Al0.30Ti0.06)Σ=2.68[Si2.98Al1.02]O10((OH)1.77F0.23); Phl-1M: (K0.85Ca0.01)(Mg1.43Fe2+0.45Fe3+0.54Al0.21Ti0.06)Σ=2.69[Si2.88Al1.12]O10((OH)1.99F0.01)). This assemblage is rarely observed in UHP eclogite-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 Phl-1M allowed 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 °C thermal range. The intersection of the calculated P-T assemblage equilibrium curve with the country rock P-T metamorphic path occurs at P ~ 2.4 ± 0.2 GPa and T ~ 700 °C, thus reflecting an early stage of exhumation, close to the metamorphic peak of the Sulu UHP terrane.