Continental Subduction in Paleoproterozoic: Insights from ∼ 1.9 Ga Nepheline Syenite in the North China Craton

Subduction of continent is known as a feature of modern plate tectonics, and it is still unclear whether such a process also occurred in the early Precambrian. Despite of ultra-high pressure metamorphism of terrestrial rocks as a proxy of continental subduction, magmatism produced during continental subduction is also a potential alternative. The North China craton is well-known for the occurrence of 1.9 Ga retrograde eclogite, which was likely originated from the subducting oceanic crust along the Trans-North China Orogen (TNCO). Here a nepheline syenite (the Aohu pluton), the first of the kind from the TNCO, is discovered in Yunzhong Mts. It comprises nepheline (-20 vol%), albite (50-55 vol%), K-feldspar (5-10 vol%), biotite (-10 vol%), and calcite (-5 vol%), with minor accessory minerals. No associated mafic or silicic rocks are outcropped in the pluton. SHRIMP zircon U-Pb analyses yield a crystallization age of 1893 +/- 13 Ma and a metamorphic overprinting of 1871 +/- 17 Ma. The rocks have medium SiO2 contents (51.9-58.9 wt%), low MgO (0.6-1.6 wt%) and Mg# (24-34), but high Al2O3 (20.0-22.9 wt%) and extremely high K2O + Na2O contents (10.0-13.6 wt%; Na2O/K2O = 1.3-2.6). They are enriched in light rare earth elements (REEs) and large ion lithophile elements, but relatively depleted in heavy REEs and high field strengthen elements. They have extremely low compatible elements (e.g., Ni and Cr). They show high (87Sr/86Sr)i ratios (0.706-0.707) but low epsilon Nd(t) values (-5.3 to -6.0; Nd T2DM = ca. 2.7 Ga). Their zircon in-situ epsilon Hf(t) values have a wide range (0 to -5.7; Hf T2DM = 2.5-2.85 Ga). The absence of mafic component and the extremely low-Mg feature, as well as their trace element patterns and initial Sr-Nd isotopes being compatible with melts derived from the Archean mafic crustal enclaves in the region, support a continental crust origin for the Aohu nepheline syenite. Based on comparison and modeling, we suggest that its generation was most likely under ultra-high pressure condition. It is thus proposed that continental subduction (into the mantle) might have occurred in the TNCO during the Paleoproterozoic.