U-Pb dating, trace element and Hf isotopic characteristics of zircons from meta-gabbro in Yangkou area, north Sulu UHP terrane

Meta-gabrro blocks are commonly distributed in Yangkou area, north Sulu ultrahigh-pressure (UHP) metamorphic bell. Laser Raman spectroscopy and cathodoluminescence (CL) image reveal that zircons separated from the meta-gabbro Y1 and Y2 preserve complex mineral inclusions in different domains. Obviously Zoned, dark -luminescent zircon cores preserve mineral inclusions of clinopyroxene (Cpx) + orthopyroxene (Opx) + plagioclase (Pl) + quartz (Qtz) + biotite (Bt) + ilmenite (Ilm) + apatite (Ap), and weakly zoned, grey-white luminescent zircon rims contain mineral inclusions of Cpx + Opx + Pl + Ap indicative of a mafic igneous protolith. Both zircon cores and rims have similar rare earth element (BEE) patterns. The chondrite-normalized REE patterns of the cores and rims are characterized by higher contents of the (heavy) HREE, and particularly lower contents of the (light) LREE with (La/Yb)(N) of 0.00015 to 0.00039. Their strongly negative Eu anomaly (mean EU/Eu-center dot = 0. 21 similar to - 0. 26), and high Ce/Ce-center dot ratio (mean Ce/Ce (center dot) = 71.5 similar to 147.4) and Th/U ratio (mean Th/U = 1.97 similar to 2.69) is a common feature of magmatic zircon. Magmatic zircon core and recrystallized zircon rims from both samples Y1 and Y2 yielded (206)pb/U-238 ages of 809 similar to 380Ma with an upper intercept age of 785 +/- 15Ma (2 sigma) and 784 +/- 12Ma (2 sigma) respectively, interpreted as the age of a mafic igneous protolith, and a lower intercept age of 226 +/- 24Ma (2 sigma) and 228 +/- 26Ma (2 sigma) respectively, interpreted as the UHP metamorphic age. Magmatic zircon cores show higher Lu-176/(127) Hf ratios of 0.00044 similar to 0.00291 and (176)yb/(177) Hf ratios of 0.0165 similar to 0.1168. The Hf-176/(177) Hf(t) ratios range from 0. 281956 to 0. 282048, corresponding to epsilon(Hf)(t) of -8.5 to - 14.0 and t(DM2) ages of 2.03 to 2.32Ga. In contrast, recrystallized zircon rims are characterized by lower Lu-176/Hf-177 (0.00029 similar to 0.00060) and Yb-176/Hf-177 (0.0112 similar to 0.0200) ratios. The Hf-176/(177) Hf(t) ratios range from 0.281953 to 0.282005, with epsilon(Hf)(t) of - 10.2 to - 11.9 t(DM2) ages of 2.12 to 2.21 Ga. The Hf contents of the studied meta-gabbro samples imply a significant input from the enriched mantle to the Sulu UHP terrane during paleoproterozoic times. Whereas lower Lu-176/(177) Hf and Yb-176/(177) Hf ratios for the recrystallized zircon rims and similar Hf-176/(177) Hf(t), epsilon(Hf)(t) and model age (t(DM2)) to the magmatic cores indicate the Lu-Hf isotope of recrystallized rims was equilibrated with the whole-rock during the UHP metamorphism.