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MULTISTAGE HISTORY RECORDED BY ZIRCONS FROM A KYANITE ELCOGITE, NORTH QAIDAM UHP TERRANE, CHINA
A kyanite eclogite sample exhibits a peak mineral assemblage of garnet, omphacite, kyanite , quartz and rutile that is replaced by the retrograde assemblage of amphibole and zoisite. Garnet are zoned with increasing Ca and Mg content from core to rim, as well as a decreasing content of Fe and Mn from core to rim.. Omphacite is partially replaced by fine grained symplectite along grain boundaries. Xenomorphic kyanite is also partially replaced by symplectite of fine grained margarite and zoisite.
Dark CL cores of zircons yield 1181-3634 ppm U, 43.3-1012 ppm Th and Th/U= 0.030-0.337. Seventeen analysis of dark zircon cores yield a weighted mean age of 453.6 ± 4.9 (MSWD=3.2). REE patterns are characterized by a strong negative Eu anomaly (Eu/Eu*=0.20-0.94), and a steep HREE slope (Yb/Gd=52.8-628). Ti-in-zircon thermometry yields an average temperature of 744± 100 °C. Grey CL zircon zones yield 108-470 ppm U, 0.86-64.5 ppm Th and Th/U= 0.008-0.137. Seven analyses of grey zircon areas yield a weighted mean age of 438 ± 3.7 (MSWD=0.72). REE patterns are characterized by the absence of a negative Eu anomaly (Eu/Eu*=0.51-3.0), and a steep HREE slope (Yb/Gd=122-427). Ti-in-zircon thermometry shows an average temperature of 942± 29 °C. Bright CL rims of zircons yield 5-73 ppm U, 0.32-49.8 ppm Th and Th/U= 0.009-0.682. Fourteen analysis of bright zircon rims yield a weighted mean age of 437 ± 10 (MSWD=7.1). REE patterns are characterized by no negative Eu anomaly (Eu/Eu*=0.27-2.17), and a flat HREE slope (Yb/Gd=2.9-41). Ti-in-zircon thermometry yields an average temperature of 760± 67 °C.
It is interpreted that dark zircons cores are representative of the protolith. These zircons represent protolith zircons that experienced Pb-loss, but have still largely preserved their trace element information. Grey zircon areas represent early stages of zircon growth before the full volume of garnet has developed. Bright rims of zircons represent the eclogite facies metamorphism, or zircons growth after more extensive garnet growth has occurred.