Revisiting the “Yanbian Terrane”: Implications for Neoproterozoic tectonic evolution of the western Yangtze Block, South China

Abstract

There is a consensus that the nature and evolution of the widespread Neoproterozoic igneous rocks along the western margin of the Yangtze Block are important for understanding the tectonic evolution of South China, but the petrogenesis and tectonic affiliations of these rocks are still highly debated. The Yanbian region in the southwestern Yangtze Block preserves Neoproterozoic igneous packages that provide a rare opportunity for unraveling the regional tectonomagmatic evolution. SHRIMP U–Pb zircon data show two major episodes of mafic–ultramafic magmatism in the region. The elder one is represented by a basaltic suite within the Yanbian Group that formed at ca. 920–900(?) Ma. Geochemical and Nd isotopic data suggest these basalts were derived from a highly depleted asthenospheric mantle in a continental back-arc basin, rather than being part of an ophiolite suite formed in an oceanic setting as previously suggested. A major fold and thrust event took place during the final stage of the Sibao orogenesis at ca. 900 Ma, forming the east-northeasterly trending structures within the Yanbian Group rocks (and regional pre-900 Ma rocks as well). The younger magmatic episode is represented by the non-deformed mafic–ultramafic intrusions dated at 820–800 Ma. Mineral compositions and whole-rock geochemistry suggest that they are affinitive with rift-related, rather than arc-related, magmatism, with some pre-existing arc components being assimilated during their genesis. These mafic–ultramafic intrusions belong to the ca. 830–740 Ma anorogenic igneous provinces along the north-south trending Kangdian Rift in the western Yangtze Block. The igneous rocks in the Yanbian region had thus recorded an important tectonic transition from a back-arc setting during the Sibao Orogeny at ≥900 Ma to a continental rift setting after ca. 825 Ma in the southwestern Yangtze Block. This tectonic transition most likely corresponded to the amalgamation and breakup of the Rodinia supercontinent. We see neither geological nor geochemical evidence for the existence of a mid-Neoproterozoic (830–740 Ma) magmatic arc or an ophiolitic complex in the Yanbian region as suggested by previous studies.