The Neoarchean ultramafic–mafic complex in the Yinshan Block, North China Craton: Magmatic monitor of development of Archean lithospheric mantle

Abstract

Neoarchean ultramafic–mafic complexes are critical for characterizing the embryonic continental lithosphere and hence for arbitrating the extrapolation of plate tectonics into the Archean geology. This geochronological and geochemical study documents the newly identified amphibolite- to granulite-facies Xiwulanbulang (XWLBL) ultramafic–mafic complex in the Yinshan Block, North China Craton (NCC). It is composed mainly of meta-gabbro, hornblendite and serpentinite. SIMS U–Pb zircon dating on the later granite constrains a minimum formation age of ∼2581Ma for the complex, whereas the metamorphic zircons from the hornblendite and coexisting kyanite-garnet gneiss yield a metamorphic age of ∼2.50Ga.The dominant meta-gabbros in the complex show an “MORB” composition, and characterized by an LREE-depleted to flat REE pattern, yet possessing prominent Nb, Zr and Ti depletions, which suggest dual geochemical features. Combined with their depleted ɛNd (t) isotope compositions (+1.77 to +2.23) and regional high temperature high Mg# basaltic magmatism, we suggest that they may have been produced by interaction of the upwelling asthenosphere with the metasomatized lithospheric mantle. Minor hornblendites/pyroxenites that occurred in the complex display high MgO (13.6–20.2wt.%), CaO (7.5–13.8wt.%), Cr (475–2049ppm) and Ni (154–859ppm) contents, reflecting they have originated either as a crystal mush magma or high-degree partial melts of a mantle source. They show moderately fractionated REE patterns ((La/Yb)N=2.9–10.7) but depleted ɛNd (t) (+ 1.48 to +3.53) and zircon ɛHf (t) (up to +7.3) values. Considering their negative anomalies for Nb, Zr and Ti relative to neighboring REE, we suggest that the mantle source of the hornblendites was refractory depleted peridotite that experienced prior basaltic magma extraction followed by metasomatism of subduction-related fluids. Systematic variations trend in both major and trace elements between the hornblendites and the associated serpentinites support their genetic linkage. High CaO (0.7–6.9wt.%) and Al2O3 (3.9–6.9wt.%) contents but low Mg# (79–85) imply that the serpentinites were likely the cumulates segregated from the coexisting hornblendite/pyroxenite magma instead of mantle peridotite. Taking into full account the regional tectono-thermal events, we conclude that the XWLBL ultramafic–mafic complex was produced in a Neoarchean subduction-related zone, which was more likely trigged by the upwelling asthenosphere mantle as a result of rollback of subducted oceanic lithosphere.