Zircon U-Pb Age and Hf Isotope of Paleoproterozoic Pelitic Granulites at Weihai, Sulu Orogen: Implications for Tectonic Affinity

Paleoproterozoic pelitic granulites have been reported in Weihai region, the tectonic affinity of which is still under controversy. The pelitic granulite occur as a lens in the granitic gneisses and the lithologies of the lens gradually changes from the core to the margin, from undeformed coarse-grained granulite, foliated fine-grained granulite, garnet-biotite-gneiss to migmatized granulite. The coarse-grained pelitic granulite exhibits coarse-grained porphyroblastic texture. The fine-grained pelitic granulite shows a weakly foliated layer with a fine-grained porphyroblastic texture. The garnet-biotite-gneiss displays porphyroblastic texture. The three samples described above mainly comprise of garnet + biotite + plagioclase (antiperthite) + quartz + sillimanite. The migmatite is made up of melanosomes and leucosomes, in which the former mainly consists of garnet + plagioclase + biotite + quartz + sillimanite and the latter has mineral assemblage of quartz + plagioclase + K-feldpsar. All of the four samples contain the accessory mineral assemblage of rutile + zircon + monazite. Zircons from the coarse-grained pelitic granulite are round or elliptical in shape and show homogenous cloudy zoning or patchy zoning in internal structure, and they have low Th/U ratios (0.01-0.30, mostly < 0.1), flat HREE patterns with positive Ce and negative Eu anomalies and high Hf/Y ratios (19-537, mostly > 100). These characteristics are consistent with the granulite-facies zircon growth. Ti-in-zircon thermometers give temperature of 788-892℃ (mean=837±24℃). These metamorphic zircons yield an upper intercept age of 1 863±18 Ma and a concordant ²⁰⁶Pb/²³⁸U age of 1 832±23 Ma, ε_Hf(t) values ranging from -3.4 to -4.9 (mean=-4.23±0.35) and corresponding T_DM2are between 2 716±107 Ma and 2 807±93 Ma (mean=2 767±44 Ma). The morphology and trace element characteristics of the zircon grains from fine-grained pelitic granulite also suggest a typical granulite-facies metamorphic zircon. The calculated metamorphic temperature is 804-909℃ (mean=845±23℃). Metamorphic zircons define an upper intercept age of 1 823±14 Ma with a tight cluster concordant ²⁰⁶Pb/²³⁸U age of 1 812±13 Ma. The ε_Hf(t) values of them range from -3.7 to -5.7 (mean=-4.67±0.37). Calculated T_DM2 are from 2 705±133 Ma to 2 826±116 Ma (mean=2 766±46 Ma). Zircon grains from the garnet-biotite-gneiss also grew under granulite-facies condition according to the CL and trace element features. The metamorphic temperature calculated based on the Ti contents in zircon is from 785℃ to 923℃ (mean=820±32℃). U-Pb dating of zircons gives a discordant upper intercept age of 1 807±22 Ma. The ε_Hf(t) values range from -4.5 to -9.0 (mean=-6.07±0.48). T_DM2 range from 2 742±90 Ma to 3 020±92 Ma (mean=2839±41 Ma). Most of the zircons from the migmatite exhibit a core-rim texture comprising of granulite-facies metamorphic core and a narrow anatectic rim. The metamorphic cores give temperature ranging from 754 ℃ to 875℃ (mean=818±30℃) and yield an upper intercept age of 1 822±19 Ma. The ε_Hf(t) values are of -4.3 to -6.3 (mean=-5.47±0.35) and corresponding T_DM2 are between 2 742±82 Ma and 2 864±91 Ma (mean=2 814±43 Ma). In conclusion, the pelitic granulite, garnet-biotite-gneiss and migmatite have experienced the same granulite-facies metamorphism at about~1.8 Ga and they share a common protolith which was derived from Neoarchean (2.7-2.8 Ga) crust. The changed lithologies from core to the margin of the pelitic granulite lens may be caused by the collisional orogeny during the late Triassic. Thus, the pelitic granulites in Weihai region have the tectonic affinity to NCC and formed when involved in the evolution of supercontinent Columbia, then they incorporated into the Sulu ultrahigh-pressure metamorphic belt during the collision between the Yangtze and the North China cratons and subsequently exhumed in Triassic.