Mullite-corundum-spinel-cordierite-plagioclase xenoliths in the Skaergaard Marginal Border Group: multi-stage interaction between metasediments and basaltic magma

摘要

Metapelitic country rocks were contact- and pyro-metamorphosed by the Tertiary Skaergaard Intrusion, East Greenland. In an initial stage of heating, while they were probably still in place within the host rock contact aureole, they overstepped a range of equilibrium and disequilibrium melting reactions and produced both a granitic melt and very refractory spinel + cordierite + plagioclase + - corundum residuals. Parts of these refractory rocks were then subjected to another melting event after being entrained as xenoliths into the Skaergaard Marginal Border Group, where they experienced a temperature of about l,000 deg C at a pressure of about 650 bars and at an oxygen fugacity about 0.2-0.5 log units below the FMQ buffer. Here, they underwent bulk melting, but did not mix with the Skaergaard magma, presumably because of the high viscosity contrast. The Al-rich melts crystallized to an assemblage of corundum + mullite + sillimanite + plagioclase + spinel + rutile + - tridymite + - cordierite and they reacted with the surrounding basalt producing a strongly cryptically zoned rim of plagioclase (An_(55) close to the basalt to An_(90) close to the Al-rich melt). The assemblage in the inner parts of the xenoliths provides textural evidence for disequilibrium growth due to slow diffusivities in the highly viscous, probably water-free Al-rich melt. Later interaction of lower temperature ferrobasaltic to granophyric melts with the xenoliths along their margins and along cracks led to consumption of corundum and mullite and to the stable assemblage of spinel + cordierite + plagioclase + quartz + K-feldspar + magnetite + ilmenite at about 800 deg C.