Age, geochemistry and tectonic setting of the Neoproterozoic (ca 830 Ma) gabbros on the southern margin of the North China Craton

Many studies have addressed the Paleozoic to Triassic collisional processes along the southern margins of the North China Craton (NCC), but little is known about the evolution of this margin during Precambrian time. This paper examines the geochronology and geochemistry of gabbros from the Luanchuan Group, which is located on the southern margin of the NCC. These gabbros are dated at ca 830Ma by SHRIMP and LA-ICP-MS zircon U-Pb methods. They are characterized by high contents of TiO₂ (2.21-4.45wt%) and light-REEs (rare earth elements) ((La/Yb)_N=9.54-7.71). The gabbros have overall OIB (ocean island basalts)-like trace element patterns, without the positive Pb and Sr anomalies and negative Nb-Ta anomalies. In addition, they have low La/Nb (<1.5) and La/Ta (<30) ratios, indicating an origin in the asthenospheric mantle. The depletions of HFSEs (high field strength elements) (e.g. Zr-Hf and Ti) probably suggest that their source has been metasomatized by carbonates. The gabbros show negative e{open}_Nd(t) (-1.5 to -3.0), suggesting crustal contamination or mixing with metasomatized lithospheric mantle. However, the low Th and U contents, constant incompatible-element ratios (e.g. Zr/Nb, La/Nb, La/Sm) and the absences of correlations of e{open}_Nd(t) with MgO, Nb/Nb*, SiO₂ or 1/Nd preclude significant crustal contamination. Alternatively, the wide range of Hf-isotope ratios in zircons (e{open}_Hf(t) from -3.6 to +6.3) may imply the interaction between asthenosphere-derived melts and the metasomatized lithospheric mantle. Petrological modeling suggests that the gabbros may have been generated from the low-degree (2-3%) partial melting of lherzolite with 2% garnet at depths greater than 85km. The gabbros may have been generated in a within-plate rift setting. The gabbros may best be correlated with the Neoproterozoic magmatic rocks in the NQB (North Qinling Belt), representing the relicts onto the southern margin of the NCC following the Neoproterozoic rifting between the NCC and the NQB. Therefore, the NCC and NQB may have been connected at ca 830Ma. Moreover, the occurrence of the gabbros implies that the present southern margin of the NCC may have been mixed with late Mesoproterozoic to Neoproterozoic crustal materials from the NQB. The new findings imply that the southern margin of the NCC has been the locus of at least three extension-convergence cycles, and the reactivation of such tectonic margins may be more common in the geological record than previously recognized.