Paleoproterozoic and Triassic metamorphic events in the Jiaobei Terrane, Jiao-Liao-Ji Belt, China: Hidden clues on multiple metamorphism and new insights into complex tectonic evolution

Abstract

The Laixi Complex located on the margin between the Jiao-Liao-Ji orogenic belt (JLJB) and the Sulu ultra-high pressure (UHP) belt preserves the records of complex tectonic evolution of both belts from the Paleoproterozoic to the Early-Middle Triassic. With an outcrop area of ca. 240 km2, the complex is dominantly composed of metasedimentary rocks including Grt-Ky-St-schists, (Grt)-Mus-schists, Mus-Qtz-schists, quartzites and marbles intruded by amphibolite dykes and Mus-bearing granitic gneisses. Here we present an integrated study of petrography, metamorphic paragenesis, mineral inclusions and cathodoluminescence (CL) features of zircon grains together with U-Pb ages, and muscovite Ar-Ar geochronology. Our results constrain the depositional age of the metasedimentary rocks as ca. 2.1–1.96 Ga. Although Paleoproterozoic magmatic protoliths including ca. 1.87–1.85 Ga amphibolites and 1.87–1.86 Ga granitic gneisses are identified, Neoproterozoic (ca. 800–650 Ma) protolith ages that are common in the Sulu UHP rocks is absent in this complex. Two discrete metamorphic events are distinguished for the first time from the metasedimentary rocks. The first episode of granulite facies metamorphism was characterized by a mineral assemblage of Grt + Sil + Bt + Kfs ± Pl + Qtz as inclusions preserved in the metamorphic zircons. In contrast, the second phase of lower amphibolite facies metamorphism is recorded by Grt ± Ky ± St + Mus ± Bt + Kfs ± Pl + Qtz in the matrix of the host rocks. Metamorphic zircon U-Pb and muscovite Ar-Ar dating constrains the two metamorphic events in the Laixi Complex as Paleoproterozoic (ca. 1.94–1.88 Ga) and Early-Middle Triassic (ca. 252–244 Ma). The metamorphic units in the Laixi Complex show similar features of lithology, geochronologic and metamorphic evolution to that of the Paleoproterozoic Jiaobei Terrane (JBT) in the southwestern JLJB, and are clearly distinguished from the Sulu UHP rocks. We therefore consider the Laixi Complex to be an integral component of the JBT during the collisional orogeny between Longgang and Langrim blocks in Paleoproterozoic. The Early-Middle Triassic lower amphibolite facies metamorphic event identified for the first time in the Laixi Complex coincides with that (249–230 Ma) of the adjacent Jiaodong Terrane in the northeastern Sulu UHP belt. These findings suggest that the Laixi Complex was also involved in Triassic collision-subduction between the North China and South China blocks, and experienced low-grade metamorphism at the middle crustal level. Combined with the observations in previous studies, we propose a multiphase metamorphic belt extending at least 300 km from Yantai through Laixi to Wulian, parallel to the Yantai-Qingdao-Wulian Fault.