Metamorphic Pressure–Temperature Path for Ultrahigh-Pressure Eclogites from North-East Greenland Determined from Isochemical Phase Equilibrium Diagrams, Conventional and Zr-in-rutile Thermometry
Monday, 15 December 2014
A kyanite eclogite from the ultrahigh-pressure (UHP) terrane in the North-East Greenland Caledonides was studied in order to establish a detailed metamorphic pressure–temperature (P–T) path. The peak mineral assemblage is garnet + omphacite + kyanite + phengite + rutile ± coesite. Garnets are large, up to 6 mm, and contain inclusions of phengite, quartz, clinopyroxene and rutile. A few quartz inclusions in garnet are surrounded by clinopyroxene. Rutile is common both as inclusions in garnet and a matrix mineral. Coesite has been identified in 5 samples from the UHP terrane; although no coesite has been found in the studied sample, radial fractures around quartz inclusions in garnet point to former coesite. The garnets are pyrope-rich (>38 mol %) with homogeneous cores and slightly zoned rims. Electron microprobe analysis indicates that clinopyroxene and phengite are also compositionally zoned. Symplectites composed of diopsidic clinopyroxene, plagioclase, amphibole and quartz replace omphacite. Isochemical phase equilibrium diagrams (i.e. pseudosections) have been modeled for the bulk composition of the sample and a symplectite within the system MnO-TiO2-Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-O2 using PERPLE_X. A P-X(O2) pseudosection at 800 °C indicates that the maximum pressure of the peak mineral assemblage strongly depends on the O2 content. An optimized O2 content related to 5% of the total Fe being trivalent, was thus utilized for modeling a P–T pseudosection. Intersections of garnet isopleths for core, inner rim and outer rim compositions on the P-T pseudosection show a P–T path from UHP conditions at 3.5 GPa and 740 °C to decompression at 1.9 GPa and 840 °C to 1.8 GPa, 750 °C. Pseudosections for the symplectite give conditions of 1.3 GPa and 775 °C. The equilibrium temperature for clinopyroxene inclusions in garnet is about 800 °C estimated with the garnet–clinopyroxene Fe2+–Mg exchange thermometry (Ravna 2000). Zr-in-rutile thermometer (Tomkins et al. 2007) yielded rutile crystallization temperatures around 790 °C at 3.5 GPa, with rutile inclusion temperatures slightly higher. The clockwise metamorphic P–T path is consistent with UHP metamorphism in an intracontinental subduction zone within the overriding plate of the Greenland Caledonides.