KEEPING IT HOT BUT NOT TOO HEAVY: LOW PRESSURE ECLOGITE WITH A GRANULITE FACIES OVERPRINT IN LIVERPOOL LAND, EAST GREENLAND CALEDONIDES

Ultra-high pressure (UHP) and high pressure (HP) metamorphism have been reported from mafic eclogite masses (m-to-km scale) within migmatitic granitic gneisses in the footwall of the Gubbedalen shear zone (GSZ) in southern Liverpool Land (LL), East Greenland Caledonides. Although coesite has not been observed, pressure deduced in previous studies from jadeite (Jd) content of omphacite (Omp) in eclogite 3 km south of the GSZ is 1.7 to 2.5 GPa. However, ~7 km south of the GSZ (Rendeelv) primary Omp is much less sodic (max ~Jd20) and low-pressure (LP) eclogite facies conditions must have existed for part of the P-T history. Although eclogites in LL exhibit some evidence of compositional and textural re-equilibration driven by decompression, the compositional homogeneity of matrix Omp and Omp inclusions in garnet indicates that the low-Jd content is primary and not due to retrograde re-equilibration. Grt-Cpx thermometry (Krogh Ravna, 2000; Fe3+-corrected) on low Jd eclogites yield T = 650-750C for Omp inclusions in garnet and homogeneous matrix Omp. High-Jd eclogites yield T > 850C; this could result from inappropriate extrapolation of the Grt-Cpx thermometer to high-Na pyroxene. Symplectite textures abound in retrograde mafic and rare metasedimentary eclogite and reflect evolving P-T during exhumation. Earliest symplectites at garnet-omphacite grain boundaries in mafic eclogites are Opx+Pl and Cpx+Pl, followed by Hbl+Pl, then pervasive recrystallization of omphacite to Cpx+Pl pseudomorphs. Grt, Ky, and Qtz in metasedimentary eclogite are mantled by coronas of Anorthite + Hercynite symplectite or aluminous (high CaTs) Cpx. Complete retrogression of mafic assemblages to Hbl+Pl+Bt+Cpx is common at margins of large (100 m-scale) eclogite bodies and is pervasive in smaller (1-10 m-scale) mafic masses entrained in gneiss. Retrograde reaction is static, preserving delicate symplectite and symplectite-within-symplectite textures. Neo-formed garnet does not occur in any retrograde assemblage; minor (10 micron-scale) compositional re-equilibration of eclogitic garnet rims is evident in BSE and X-ray maps. These textures and mineral assemblages are broadly consistent with a ‘hot’ retrograde decompression path for the Liverpool Land eclogites. The peak LP conditions are consistent with the absence of coesite inclusions in garnet.