It has been well-proposed that continental crust might subduct into mantle depths for over 100 km, resulting in ultrahigh-pressure metamorphism in continental crust featured by minerals such as coesite and diamond (Zheng 2012 and therein). However, ultrahigh-pressure rocks can hardly be exhumed in the Precambrian orogens. In addition to ultrahigh-pressure metamorphism in continent, magmatism from high-pressure melting of continental crust may be another angle of perspective to look into. The North China Craton (NCC) is known to be amalgamated by two blocks along the Trans-North China Orogen (TNCO) at ca. 1.9 Ga, producing retro-grade eclogite with unclear oceanic or continental crust origin. Here we identify a nepheline syenite (Aohu pluton) in the Yunzhong Mts in the inlier of the TNCO. It comprises of main nepheline, albite, K-feldspar, biotite, with minor calcite and opaque minerals. No associated mafic or felsic rocks are outcropped in the pluton. SIMS zircon U-Pb analyses yield a crystallization age of 1893 ± 13 Ma and a metamorphic overprinting age of 1871 ± 17 Ma; and this ca. 1890 Ma age is contemporaneous with the regional high-grade metamorphism. The rocks have high SiO2 and Al2O3 contents, with low MgO and Mg#, and extremely high K2O+Na2O. Their trace elements display an arc-affinitive pattern. They show high whole-rock (87Sr/86Sr)i ratios, low whole-rock εNd(t) values; and varied zircon in situ εHf(t) values. All these features agree with a continental crust-originated hypothesis for the nepheline syenite. Specifically, their origination is consistent with low-degrees melting of continental crust with the presence CO2-rich fluids under a high pressure. Based on the regional geology, we propose that the continental crust in the TNCO has likely penetrated the Moho during the Late Paleoproterozoic. 

Reference(s)

 Zheng, YF, 2012, Metamorphic chemical geodynamics in continental subduction zones: Chemical Geology, 3, 495-519.