Experimental high-pressure granulites: Some applications to natural mafic xenolith suites and Archean granulite terranes

Abstract

High-pressure granulite assemblages have been produced as residues in partial melting experiments on a natural (alkalic basalt) amphibolite between pressures of 12 and 18 kbar. In particular, at 18 kbar, partial melting of the hornblende + plagioclase ± quartz assemblage under fluid-absent conditions produces garnet + clinopyroxene + new albitic plagioclase + melt. Seismic velocities ( V P ) are estimated from the modal data for the experimental assemblages and range from 6.90 km/s for hornblende-bearing residues to 7.62 for the dominantly garnet- clinopyroxene residues. These values are typical for rock types in the lowermost crust, transitional to mantle. The experimental results help place additional pressure-temperature- a (H 2 O) constraints on the source region for the natural high-pressure granulites. The experimental residue assemblage formed at 18 kbar has been described in natural xenolith suites from the Delegate Pipes in Australia, where the pipes intrude Upper Ordovician and Lower Devonian continental crust, and in the Bearpaw Mountains, Montana, where the xenolith- bearing magmas intrude older crust of Archean age. The combination of these data shows that the xenoliths may indeed represent lowermost continental crust and furthermore helps interpret the nature of the crust-mantle boundary in these areas. The Delegate Pipes xenoliths suggest that the crust-mantle boundary may be the site for partial melting and assimilation, whereas the Bearpaw Mountains samples indicate that magmatic underplating may have been a major process in generating thick continental Archean crust. The experimental data and Bearpaw Mountains xenoliths suggest that the large range of rock types found in Archean granulite terranes may not be representative of the lowermost continental crust.