Geochronology and geochemistry of 2.5 to 2.4 Ga granitic plutons from the southern margin of the North China Craton: Implications for a tectonic transition from arc to post-collisional setting

The Xutai and Lujiagou plutons, exposed in the Songshan-Jishan area in the southern margin of the North China Craton (NCC), are studied to constrain the tectonic regime of the NCC between 2.5 and 2.4 Ga. The Xutai pluton (dated at 2509 +/- 33 Ma) mainly consists of tonalite and monzogranite with a calc-alkaline affinity (Rittmann Index (delta)<3.3). Low 10,000 Ga/Al (1.15 to 1.38) and zircon saturation temperature (T-Zr = 612 to 779 degrees C) and a negative correlation between P2O5 and SiO2 classify them as I-type granite. Enriched LREE, low Y and Yb with high Sr/Y ratios are attributed to garnet in the residue, indicating a deep source for the pluton. Zircon grains have positive epsilon(Hf)(t) values (+ 4.6 to + 9.4) with T-DM(C) ages ranging from 2708 to 2435 Ma (average of 2502 Ma), close to the crystallization age of the pluton, indicative of a juvenile crust source. Negative Nb, Ta and Ti anomalies of the pluton and its intrusive contact relationship with older subduction-related amphibolites are supportive of an arc setting. The Lujiagou pluton (dated at 2399 +/- 24 Ma) mainly consists of syenogranite with a calc-alkaline affinity (delta<3.3). High SiO2 (73.25 to 75.21 wt.%) and K2O (5.38 to 8.03 wt.%) and low P2O5 (0.01 to 0.04 wt.%) indicate that the primitive magmas of the rocks have experienced fractionation. The rocks have 10,000 Ga/Al (0.71 to 2.55) and T-Zr (614 to 796 degrees C) lower than those for A-type granite and negative correlation between P2O5 and SiO2, consistent with highly fractionated I-type character. Zircon epsilon(Hf)(t) values (-0.2 to +4.4) with T-DM(C) from 2909 to 2652 Ma are indicative of mantle-derived components, contaminated by older crust. However, the rocks have high SiO2, K2O and low Mg-#, precluding their derivation from a juvenile source. Considering the LREE enriched nature, low Y, negative Eu anomalies and zircon Hf and whole rock Nd isotopic components identical to TTGs, we propose that the Lujiagou pluton inherited from preexisting Songshan TTGs, consistent with formation in an extensional setting. We thus propose that the Xutai and Lujiagou plutons record a tectonic transition from compressive to extensional setting in the southern margin of the NCC during 2.5 to 2.4 Ga. (C) 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.