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

摘要

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.