Tectonic Switching of the Trans-North China Orogen in the Middle Paleoproterozoic: Insights From Mafic Magmatism in the Luliang Complex

Recognition of tectonic switching is of great significance for understanding the formation and evolution of ancient accretionary orogens. In this study, whole-rock geochemical and Sm-Nd isotopic and zircon U-Pb, Lu-Hf isotopic, and trace element data are reported for two generations of metamorphosed mafic rocks from the Luliang Complex, middle Trans-North China Orogen. Although both episodes of mafic magmatism were derived from variable degrees of interaction between a subcontinental lithospheric mantle and slab-derived hydrous fluids and melts, the younger ones (2.09-2.06 Ga) had higher whole-rock epsilon(Nd)(t) and zircon epsilon(Hf)(t) values, higher (Ta/La)(N) ratios, and weaker Nb, Ta, and Ti anomalies in the primitive mantle-normalized trace element diagram compared with the older ones (2.18-2.15 Ga), implying a less-enriched mantle source and subdued subduction features. More importantly, a decrease of U/Yb and Ce/Ce* ratios of magmatic zircons and whole-rocks V/Ti ratios from the older to younger ones implied less slab-derived fluid addition, and a decrease of whole-rock (Sm/Yb)(N) and Hf/Lu ratios suggests decreasing pressure of melt extraction and so lithosphere thickness. In consideration of other tectonothermal activity and paleogeography observed from the middle Trans-North China Orogen, we propose a tectonic switching model to explain the middle to late Paleoproterozoic tectonic evolution caused by early subduction hinge retreat to later advance and ultimately continental collision.