Subduction-related mantle metasomatism and partial melting in the northern North China Craton: Insights from amphibolite enclaves, Siziwangqi, Inner Mongolia

The Siziwangqi region of Inner Mongolia lies along the northern margin of the North China Craton. It consists of diverse suites of Precambrian and Phanerozoic rock units including relicts of the Paleo-Asian Ocean, arc magmatic rocks, amphibolites, and TTG (tonalite–trondhjemite–granodiorite). The amphibolite rocks in the Siziwangqi area occur as enclaves embedded in granodiorite. Geochemical, Lu–Hf isotopic, and zircon U–Pb geochronological analyses on amphibolite enclaves from Siziwangqi indicate end Archean subduction-related mantle metasomatism accompanied by partial melting. U-Pb zircon dating of amphibolite reveals a protolith age of 2503 ± 6 Ma with an inherited zircon age of ~ 2549 Ma. Geochemical data indicate that the amphibolites range in composition from subalkaline basalt through basaltic–andesite and andesite, all with island arc affinities. The εHf(t) values for the amphibolites range from ＋2.7 to ＋11.4 and their T_DM1 model ages range from 2681 to 2517 Ma and peak at 2625 Ma, suggesting dominant late Neoarchean mantle input. This study shows that the amphibolite protoliths likely formed by partial melting of spinel-bearing lherzolites from a depleted mantle, which had been metasomatized by subduction-related fluids.