Partial melting of zoisite eclogite from the Sanddal area, North-East Greenland Caledonides

Zoisite eclogite from the North-East Greenland eclogite province was studied to reveal its partial melting history. Large anhedral garnet (Grt I), omphacite, kyanite, zoisite, phengite, quartz, and accessory rutile and zircon represent the mineral assemblage at peak pressure. Grt I consists of an inclusion-rich core and an inclusion-poor rim. Common inclusions are single grains of plagioclase, quartz, K-feldspar, and multiphase solid inclusions (MSIs) of K-feldspar + albite and phengite + albite + apatite. A second generation of garnet (Grt II), small and euhedral, shares straight boundaries with plagioclase. Matrix plagioclase displays cusps into omphacite, and shows low dihedral angles at interfaces with garnet and omphacite. Rare occurrences of albite + K-feldspar + kyanite display offshoots into plagioclase in matrix, indicating a potential melt loss event. The Na- and K-rich inclusions and matrix assemblages could be interpreted as crystallized melt derived from paragonite and phengite melting with Grt II as a peritectic phase. Graphic intergrowths of amphibole and plagioclase form in the matrix adjacent to relic zoisite which surrounds kyanite, indicating melting of zoisite and melt crystallization. To test the interpretation, pseudosections were constructed with PERPLE_X for the bulk rock (PS I) and melt re-integrated (PS II) compositions. Isopleths of Si-in-phengite and XNa-in-omphacite, contouring PS I, yield maximum pressure of 2.2 GPa at 770 ºC. A pressure-temperature (P-T) path towards 900 ºC, 1.2 GPa, constrained by intersecting isopleths for the garnet rim and Ca-in-amphibole, crosses the solidus at high-pressure (HP) with successive zoisite growth at the expense of kyanite. On the P-T path, PS I predicts trondhjemitic melts with modal melt up to 7% at HP and 2% at the maximum T. However, the melt composition differs from the granitic and tonalitic compositions indicated by the crystallized assemblages, revealing uncertainties in the modeling. PS II indicates a prograde assemblage with paragonite present at 1.4 - 2.0 GPa, 600 - 690 ºC before reaching peak P. Therefore, a clockwise P-T path is delineated to account for formation of paragonite and phengite, the subsequent melting of the phases and zoisite in the eclogite and the crystallization of mineral assemblages (e.g. MSIs) as melt products.