USE OF CALCULATED ISOCHEMICAL PRESSURE-TEMPERATURE, PT-N AND PT-X SECTIONS TO CONSTRAIN THE P-T DECOMPRESSION PATH AND FOLLOW CHANGES IN MODES AND COMPOSITIONS OF MINERALS AND MELT IN PARTIALLY MELTED METAPELITES OF THE UHP TERRANE, N.E. GREENLAND CALEDONIDES

Peak conditions of ultrahigh pressure (UHP) metamorphism on Rabbit Ears Island (REI) in N.E. Greenland have been estimated at 972°C and 3.6 GPa in kyanite eclogite (Gilotti & Ravna, 2002, Geology, 30, 551-554). We have examined rare UHP metapelites on REI and demonstrated that they partially melted on their decompression path. Calculated isochemical phase diagram sections for the REI metapelites provide information about the sequence of changes in mineral assemblage and mineral compositions at pressure-temperature conditions along the path. The rocks contain abundant garnet megacrysts whose cores are restitic from peak UHP conditions. Their grossular-rich rims grew in equilibrium with melt whose crystallization products form much of the leucocratic rock matrix. Contours of garnet volume and grossular content provide good constraints on the steep decompression path. The mineral assemblage that is common to all leucosome patches contains garnet, phengite, biotite, kyanite, rutile, but no sillimanite or ilmenite, in a matrix of two antiperthitic feldspars and quartz. This solidus assemblage constrains the low pressure, cooling portion of the path. In the mid-pressure region, the P-T path traverses a large field where garnet, liquid, two feldspars, quartz, kyanite and rutile coexist. Proportions and compositions of garnet, plagioclase, alkali feldspar and liquid vary throughout this field, but many compositional contours are nearly parallel, and mineral components have different susceptibility to diffusion, so constraints on the location of the bend in the P-T path are limited. The preferred P-T path has a near isothermal decompression segment with a bend at ~1.5 GPa followed by a near isobaric cooling path to approximately 1.2 GPa at 700°C.

We constructed composite P-n (moles of phases) for the isothermal decompression segment and T-n for the isobaric cooling segment (referred to as PT-n sections) and composite P-X (mole fractions of components in solution phases) and T-X diagrams (referred to as PT-X sections) to display changes along the P-T path of the REI metapelites. These diagrams show that major changes in assemblage occur in the pressure range from 2.4 to 2.2 GPa (-Cpx +Liq +Pl +Grt) on the isothermal decompression segment and from 850° to 800°C (cross solidus) on the isobaric cooling segment.