REACTION PROGRESS AND TIMING OF AMPHIBOLITE-FACIES RETROGRESSION OF A HIGH-PRESSURE GARNET-CLINOPYROXENITE, NORTH-EAST GREENLAND ECLOGITE PROVINCE

Exhumation of HP/UHP terranes commonly follows an isothermal decompression path through the amphibolite-facies. Eclogites are often variably retrogressed allowing for the determination of reactions involved in and the timing of exhumation. Samples collected from a 40 m x 25 m garnet-clinopyroxenite pod at Danmarkshavn (76°46'N, 18°40'W) in the North-East Greenland Eclogite Province are from the pristine eclogitic core (0046), a partially amphibolitized fractured zone extending into the core (0051), and an amphibolitic rim (0048). Quantitatively determined reaction progress and the overall reactions from (1) 0046 to 0048, and (2) 0051 to 0048 show that the trigger in both cases is fluid availability:

2.935 Cpx + 1.085 Grt + 0.198 Plag + 1.356 H2O = 1.356 Amp + 0.513 Qtz (1)

2.572 Cpx + 1.436 Grt + 10.732 Qtz + 1.064 Rtl + 2.273 H2O = 2.273 Amp + 0.099 Plag + 1.064 Ilm (2)

Zircon growth in the amphibolitized garnet-pyroxenite is interpreted to record fluid influx during retrogression. U-Pb SHRIMP 206Pb/238U ages of metamorphic rims were determined on protolith zircons from the pristine HP rock (0046), and the rim amphibolite (0048). Ages from both samples define two populations at 370 Ma and 340 Ma. The older age is equivalent to the age of pegmatites in the neck of mafic boudins and attributed to the start of amphibolite-facies retrogression. The younger 340 Ma age corresponds to the age of a widespread suite of pegmatites that crosscut regional structures. The 370 Ma zircon growth at amphibolite facies conditions underscores the difficulty of linking zircon growth to specific P-T conditions. The common assumption that dating metamorphic zircon rims in apparently pristine HP rocks provides the age of HP metamorphism may not be valid.