Basement evolution of the Sulu orogenic belt: Constraints on zircon U–Pb ages and trace elements from the Weihai gneisses

Abstract

Early Paleoproterozoic rocks occur sporadically in the Sulu ultrahigh‐pressure (UHP) belt. The tectonic affinity and evolution of these rocks are still disputed. Two magmatic events (Early Paleoproterozoic and Neoproterozoic) and two metamorphic events (Late Paleoproterozoic and Triassic) were revealed by the zircon U–Pb ages and trace elements from the Weihai gneisses, which provide a key clue to understand the evolution of the Sulu orogen. The Early Paleoproterozoic zircons occur as residual zircons or zircon‐inherited cores in the two gneisses. They generally show oscillation zoning and enriched HREE patterns. The weighted mean age of the upper intercept age of these zircons is ~2.46 Ga, suggesting that the protolith of the ancient gneisses formed at ~2.46 Ga. The Late Paleoproterozoic zircons occur as metamorphic zircon core or mantle domains in the two gneisses and a quartzite. They generally show planar zoning or no zoning, with relative lower Th/U ratios. These zircons yield metamorphic ages of 1.8–1.84 Ga, which are consistent with the age of the Paleoproterozoic ultrahigh‐temperature metamorphism of the northern Sulu orogenic belt. The Neoproterozoic zircons occur as magmatic cores or mantles in the zircon grains of the two gneisses and their wall rocks. They show typical oscillatory zoning and enriched HREE patterns, yielding magmatic ages of ca. 740–787 Ma. The Neoproterozoic igneous rocks are the dominant basement rocks in the Sulu orogenic belt. Triassic UHP metamorphism has been well studied in the Sulu orogenic belt. Two Triassic age groups were also recognized here: 228–229 and 208 Ma, respectively. The Triassic zircons occur as metamorphic rims of the two ancient gneisses and their wall rocks. These two age ranges correspond to the time of subduction and exhumation of the the South China Craton (SCC) during the Triassic orogeny. This study provides solid evidence that the Weihai ancient gneisses have a strong affinity with the Yangtze Block. The protoliths of them are Paleoproterozoic (ca. 2.46 Ga) magmatic rocks. They were subsequently suffered from ca. 1.8‐ to 1.84‐Ga metamorphism, new crustal materials overgrowth during ca. 740‐ to 787‐Ma magmatism, and then experienced Triassic continental deep subduction and exhumation process to build the Sulu orogenic belt.