Mafic and ultramafic pods with eclogitic relics from the Proterozoic Nagssugtoqidian mobile belt of East Greenland

Abstract

The cores of amphibolitic and ultramafic pods within the basement complex of the Nagssugtoqidian mobile belt of East Greenland preserve metastable relics of older high-pressure assemblages. The bulkrock geochemistry of amphibolites indicates that their protoliths derive from low-pressure crystal fractionation (mainly controlled by plagioclase, pyroxene and olivine) of tholeiitic melts of probable MORB affinity. Amphibolites from the cores of the pods are corona-textured and provide microtextural evidence of a polyphase retrogression subsequent to a high- T eclogitic event, producing omphacite-garnet assemblages. Symplectitic intergrowths of Ca-clinopyroxene and plagioclase are the result of the unmixing of the older omphacite, whilst garnet is partly replaced by a fine-grained pseudomorph of orthopyroxene, anorthitic plagioclase and magnetite. This suggests subsequent partial re-equilibration in the intermediate-pressure granulite facies, producing clinopyroxene-orthopyroxene-plagioclase-magnetite assemblages. The final event in the amphibolite facies was controlled by the influx of dominantly hydrous fluids. According to kinetic constraints, the amphibolites developed coronitic or granoblastic textures. The coronas consist of plagioclase and hornblende between remaining garnet and unmixed clinopyroxene. Re-crystallized granoblastic amphibolites do not retain relics; according to the bulk rock chemistry, plagioclase and hornblende coexist with either garnet or clinopyroxene. Adjacent ultramafic rocks conform with this metamorphic evolution, being retrogressed from the spinel ±amphibole lherzolite facies to the tremolite-chlorite peridotite facies. Estimated equilibrium conditions for the final amphibolite facies event are T = 600 + 70°C and P = 5.2 ± 1 kbar. Mafic and ultramafic pods from the basement complex of the Nagssugtoqidian mobile belt of East Greenland probably represent fragments of an oceanic crust which was subjected to a subduction event of Proterozoic age.