Zircon U–Pb age and geochemical constraints on the tectonic affinity of the Jiaodong terrane in the Sulu orogen, China

Because of tectonic mixture by subduction erosion during Triassic continental collision between the North and South China Blocks, identifying the tectonic affinity of the Jiaodong terrane in the Sulu orogen is a key to recognition of the suture location in east-central China. In this study, zircon U–Pb ages, whole-rock major and trace elements as well as Rb–Sr and Sm–Nd isotopes, and mineral O isotopes were determined for ultrahigh-pressure (UHP) granitic gneiss and enclosed eclogite in the Jiaodong terrane, a petrotectonic unit southeast of the Wulian-Yantai fault. The granitic gneiss is characterized mostly by: (1) the presence of major Neoproterozoic igneous zircons and minor Triassic metamorphic zircons, which yielded the U–Pb ages of 741 ± 6 Ma interpreted as the emplacement age of the protoliths and 234 ± 5 Ma as the time of UHP metamorphism, respectively; (2) negative ɛNd(t) values of −7.4 to −3.8 at t = 750 Ma with Nd model ages of 1.73–2.03 Ga; and (3) low δ18O zircons of −0.4 to 4.1‰. Zircons from the eclogite also yield concordant metamorphic ages ranging from 229 ± 3 to 239 ± 7 Ma, with low δ18O values of 0.2–5.1‰. Thus, both granitic gneiss and eclogites share the same 18O depletion in their Neoproterozoic protoliths and experienced the same metamorphic event in the Triassic. Zircon U–Pb ages of 1866 ± 34 to 2057 ± 31 Ma were obtained for a few samples, consistent with the presence of Paleoproterozoic crust in the South China Block. In particular, the widespread occurrence of Neoproterozoic igneous protoliths with low δ18O values is evident in this region, with an outcrop area of over 10,000 km2. Such Neoproterozoic igneous rocks with low δ18O are not found in the North China Block, but are considered to be characteristic products of Neoproterozoic rift magmatism along the northern margin of the South China Block. Therefore, the protoliths of UHP metamorphic rocks in the Jiaodong terrane have a tectonic affinity to the South China Block, like those rocks elsewhere along the Dabie–Sulu orogenic belt. Their petrogenesis is explained by the plate-rift model within the framework of tectonic evolution from supercontinental rift to breakup during the middle Neoproterozoic. Despite crustal detachment and tectonic accretion during the Triassic continental collision, the presence of 1.8–2.0 Ga protoliths in the UHP metamorphic zone does not mean that these lithotectonic slices were derived from the continental margin of the North China Block.