Paleoproterozoic ophiolite remnants in the northern margin of the North China Craton: Evidence from the Chicheng peridotite massif

Abstract

Precambrian ophiolites provide important clues to the onset of modern-style plate tectonics. Peridotite massifs exposed in Chicheng area in the northern margin of the North China Craton occur as discrete tectonic lenses or boudins within Archean-Paleoproterozoic meta-volcanic and meta-sedimentary rocks of the Hongqiyingzi Group, along with retrograde eclogites. These serpentinized peridotites are composed dominantly of harzburgites and dunites, with minor ultramafic cumulates. Here we report the petrography, whole-rock chemical compositions, and mineral chemistry of the Chicheng peridotites with a view to evaluate their genesis and tectonic significance. The high Mg-number (91.2–93.5), low Al2O3 and CaO contents of harzburgites and dunites, as well as the high Fo values of olivine and Cr-number of spinel from these rocks suggest that the rocks represent highly refractory residues of partial melting. The ultramafic cumulates on the top of the peridotite massif might represent the early ultramafic cumulates from partial melt, as indicated by their relatively low Mg-number (87.0 and 91.8), and slightly higher Al2O3 and CaO contents. We infer that the Chicheng peridotites represent fragments of ancient sub-oceanic lithospheric mantle. The peridotites and the coexisting retrograded eclogites together with the gneisses in Chicheng might form part of a tectonically dismembered Paleoproterozoic ophiolite. The ophiolite was emplaced along the continental margin and subducted to the depth of eclogites facies during the formation of the Trans-North China Orogen. The metasomatized orthopyroxene in the Chicheng peridotites and flat REE pattern of whole rocks indicate melt/fuild metasomatism on residual peridotites during the post-melting stage. Amphiboles mostly show tremolite composition and were formed by retrograde metamorphism as displayed by their low Na, K, Ti and high Si contents. The ubiquitous existence of metamorphic amphiboles in Chicheng peridotites suggests that the ophiolite was subjected to amphibole-facies metamorphism before their exhumation in the Phanerozoic. The recognition of the ophiolite and its evolution history suggest that modern-style plate tectonics had begun at least in the Paleoproterozoic.