Late Cambrian N-MORB-Type Amphibolite in Central Jilin Province, NE China: Implications for Subduction Initiation in the Paleo-Asian Ocean

The Shitoukoumen area in central Jilin Province, NE China, preserves a Permian blocks-in-matrix assemblage containing pre-Permian mafic and sedimentary blocks that provides an important record of the Paleozoic evolution of the eastern Paleo-Asian Ocean. We present petrographic, zircon U-Pb geochronological and Lu-Hf isotopic, and whole-rock geochemical and Sr-Nd isotopic data for newly discovered amphibolites. Zircons from the amphibolites show metamorphic characteristics, and zircon U-Pb dating yielded an age of 503 ± 4 Ma, recording Late Cambrian metamorphism. The amphibolite is tholeiitic, with low SiO2 (48.46–52.22 wt.%) and K2O (0.07–0.17 wt.%) and relatively high MgO (4.48–6.31 wt.%), Fe2O3T (11.01–15.48 wt.%), and CaO (9.07–12.55 wt.%). It has low total REE abundances and slightly LREE-depleted patterns, with no negative Nb-Ta anomalies. Its εHf(t) and εNd(t) values are +14.9 to +20.4 and +7.2 to +8.4, respectively, while the initial 87Sr/86Sr ratios are slightly elevated, consistent with seawater alteration. These geochemical and isotopic characteristics are consistent with an N-MORB-like oceanic crustal protolith derived from partial melting of a depleted spinel-facies mantle source. Together with the spatial association with the Zhangjiatun–Toudaogou–Xiaosuihe SSZ ophiolite, the amphibolite may represent a component of a metamorphic sole related to Late Cambrian intra-oceanic subduction initiation of the Paleo-Asian Ocean, although this interpretation remains tentative.