PARTIAL MELTING OF ULTRAHIGH-PRESSURE ECLOGITES FROM THE NORTH-EAST GREENLAND CALEDONIDES

Melting of UHP rocks has been suggested as a trigger for exhumation. Kyanite eclogite from the ultrahigh-pressure (UHP) terrane in the North-East Greenland Caledonides shows evidence of incipient partial melting under HP conditions. The studied sample contains a peak mineral assemblage of large anhedral garnet and kyanite (Grt I and KyI, both up to 5 mm), omphacite (Cpx I), rutile and coesite in zircon. Evidence of partial melting is indicated by the retrograde mineral assemblage of garnet (Grt II), kyanite (Ky II), diopsidic clinopyroxene (Cpx II), amphibole, plagioclase, quartz, biotite and rare epidote/clinozoisite. Grt II and Ky II occur as small (< 300µm) grains with euhedral crystal faces in equilibrium with plagioclase. Grt II has a higher pyrope and grossular content than Grt I. Quartz and Ky I and II are surrounded by plagioclase, which forms cusps into surrounding phases such as omphacite. Graphic intergrowths of amphibole or Cpx II with poikioblastic plagioclase are common. Rare epidote/clinozoisite is in equilibrium with amphibole and plagioclase. Small amounts of biotite occur locally, but no K-feldspar or white mica was found. The textures and Ca-rich composition of the phases (i.e. plagioclase, Cpx II, Grt II, amphibole, epidote) crystallized in the presence of melt suggest that zoisite, rather than phengite, decomposed at initial melt formation. We constructed isochemical phase diagrams (IPD) using Perple_X for the whole-rock composition (IPD I) and a domain that contains Grt II, Cpx II, amphibole and plagioclase (IPD II). On IPD I, intersections of garnet isopleths for the Grt I core indicate a peak P-T condition at 3.4 GPa, 880 °C. IPD II shows zoisite-stable fields located at higher P and T regions than the observed equilibrium assemblage. Isopleths for Grt II, Cpx II, and plagioclase yield a P–T range of 1.6 GPa, 750 to 825 °C, and indicates a melt crystallized near 1.6 GPa to form the observed mineral assemblage. A near isothermal decompression path for the UHP eclogite with zoisite dehydration melting best explains the observed features in the rock, and agrees with a previously determined PT path for nearby anatectic metapelites. The North-East Greenland UHP terrane was partially melted on the decompression path in the quartz stability field; therefore, melting cannot be the trigger for exhumation.