Metamorphism in supracrustal and ultramafic rocks in southern West Greenland and South-West Greenland 64 – 61.5°N, Geological Survey of Denmark and Greenland Report 2011/6

Mafic rocks with roughly basaltic bulk composition are useful relative indicators of metamorphic grade in southern West Greenland and South-West Greenland. Ideally, these rocks are mafic granulites within the granulite-facies conditions and amphibolite at amphibolite-facies conditions. In the field, good evidence for prevailing amphibolite-facies conditions is the presence of amphibolite ± clinopyroxene ± garnet ± epidote that displays the typical L to L-S fabric. In low-Ca mafic rocks the equilibrium coexistence with hornblende + Fe-Mg amphibole + plagioclase is another indication of amphibolite-facies conditions. Granulite-facies conditions in these same rock compositions would show clinopyroxene + orthopyroxene with plagioclase ± hornblende at pressures (below about 6-7 kbar) over a range of temperatures starting at about 700°C. Above these pressures, mafic bulk compositions would contain clinopyroxene + plagioclase + hornblende + garnet in the granulite facies, but this is not a diagnostic mineral assemblage, because clinopyroxene-garnet amphibolite is stable in the upper amphibolite facies. Aluminous gneisses, which on older maps are mapped as metasediments ("mica schist and gneiss" on new maps), are good indicators of metamorphic grade and P-T trajectory. Cordierite reaction rims around aluminosilicate are indicators uplift (P-decrease) at high temperature. Staurolite is part of the equilibrium mineral assemblage at some metasediment/ mica schist and gneiss localities, which indicates metamorphic conditions within amphibolite facies. The extent of hydration and subsequent metamorphism of ultramafic rocks is highly variable. Primary assemblages are olivine + orthopyroxene ± clinopyroxene ± spinel ± plagioclase. Metamorphic assemblages commonly contain anthophyllite + talc ± actinolite ± chlorite ± serpentine. Talc-bearing metamorphic assemblages limit the metamorphism to amphibolite facies; anthophyllite-bearing assemblages are not definitive and could be upper amphibolite- to lower granulite-facies. In places, cross-cutting pegmatite dykes have interacted extensively with the ultramafic rocks. Fluids from the dykes enhance the reactions, which gives coarse-grained reaction products. Plagioclase, K-feldspar, and quartz (pegmatite) react with the ultramafic minerals to give calcium amphibole, biotite, and anthophyllite/talc. This depletes the remaining fluid in K, Si, Ca, and Na. Where large amounts of calcic plagioclase and fluid are involved in producing calcium amphibole, these reactions can produce excess Al, which forms aluminous minerals like gedrite, sapphirine, kornerupine, cordierite, and pink corundum.