Petrogenesis and tectonic implications of two types of Liaoji granitoid in the Jiao-Liao-Ji Belt, North China Craton

Two types of granitoid (monzogranite and granodiorite) are recognized within the Paleoproterozoic Liaoji granitoids of the Jiao-Liao-Ji Belt (JLJB) in the Hupiyu and Simenzi areas, North China Craton. We conducted zircon U-Pb geochronological, geochemical, and Hf isotopic analyses of these granitoids. Zircon U-Pb dating indicates an emplacement age of 2180 +/- 14 Ma for the monzogranite (Hupiyu granitoids) and ages of 2130 +/- 24 and 2173 +/- 11 Ma for the Fangjiaweizi and Dadingzi granodiorites at Simenzi, respectively. The Hupiyu monzogranite samples are enriched in Zr, Ga, and Y, and depleted in Nb and Ti. Low Nb/Y ratios and high Zr saturation temperatures (849-884 degrees C) indicate that the Hupiyu monzogranites have an affinity with A(2)-subtype granitoids. The Fangjiaweizi and Dadingzi granodiorites have high Sr/Y and low K2O/Na2O ratios, indicating an affinity with adakitic rocks. Geochemical compositions and wide variation in epsilon(Hf)(t) values suggest that the monzogranites and granodiorites were derived from thinned and thickened lower crust, respectively, and that magmatism was accompanied by input of mantle material. This study represents the first identification of pre-tectonic adakitic rocks in the JLJB. The assemblage of pre-tectonic A(2)-type granitoids and adakitic rocks indicates a continental back-arc basin as the initial tectonic setting of the JLJB. These new data, combined with results of previous studies, support an arc-continent collision model for the JLJB.