Geochemical constraints on the tectonic setting of Paleoproterozoic A-type granites in the southern margin of the North China Craton

Abstract

The amalgamation of the Eastern and Western Blocks of the North China Craton (NCC) at ∼1.85 Ga resulted in the formation of the Central Zone between the two. Post-orogenic A-type granites in the NCC provide constraints on the timing of this collisional event. The Shicheng and Motianzhai granitic plutons in the Songshan district of the southern NCC have SHRIMP zircon U-Pb ages of 1743 ± 14 Ma and 1797 ± 14 Ma, respectively. Both plutons consist of biotite granite and granodiorite with total alkalis (K2O + Na2O) ranging from 7.65 to 9.12 wt.%. High Fe*(FeOt/FeOt + MgO) (0.79-0.90) and 10,000×Ga/Al ratios (2.19-3.26), indicate that these are A-type granites formed in a post-collisional environment. Whole-rock εNd(t) values range from -5.7 to -7.8 and TDM model ages from 2.60 to 2.93 Ga. Zircons from these granites have εHf(t) values ranging from -5.3 to -13.5, corresponding to two stage model ages (TDM C) of 2.85-3.29 Ga. These isotopic features suggest that the rocks were derived from the late Archean basement complexes. The A-type granites in Songshan are different from typical granitic rocks formed in anorogenic intra-continental rifts, but resemble those emplaced in post-orogenic extensional tectonic settings. Based on the criteria of Eby (1992), the Songshan A-type granites belong to the A2 (post-orogenic) type. This post-orogenic extensional event occurred after a major orogeny that formed the Central Zone at ca. 1.85 Ga and lasted for more than 50 myr. © 2009 Elsevier Ltd. All rights reserved.