The use of garnet porphyroblasts to resolve the metamorphic pressure-temperature-deformation (P-T-d) path: An example from the Imjingang belts, South Korea

Abstract

Quantitative P-T-deformation path determination for metamorphic rocks can be revealed by an approach combined isopleths intersection of zoned garnet compositions with foliation intersection/inflexion axes (FIAs) preserved within the garnet porphyroblast. The method presented here is applied to metapelites in the Jingok Unit of the Imjingang belt, South Korea. Garnet and staurolite FIA trends are E-W to WNW-ESE (90–120°) and NE-SW to ENE-WSW (50–80°), respectively. The FIA data indicate that garnet porphyroblasts have grown under N-S to NNE-SSW bulk shortening. P-T paths for garnet growth in Grt, St and Ky zone in the Jingok Unit, calculated from geothermobarometers and the intersection of isopleths on P-T pseudosections in the MnO-Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O (MnNCKFMASH) system constructed using program THERMOCALC, are compressional heating from 545 °C at 5.3 kbar to 610–640°C at 7.9–8.8 kbar. All compositional and microtextural data and thermodynamic modeling indicate that garnet porphyroblasts have formed during compressional heating under N-S crustal shortening, whereas staurolite porphyroblasts have formed dominantly during sub-isothermal decompression with overgrowing NE/ENE FIA. Consequently, the metapelites in the Jingok Unit have experienced clockwise P-T path derived from Barrovian-type metamorphism belong in amphibolites facies.