Geochemistry and Petrogenesis of the ca. 2.5 Ga High-K Granitoids in the Southern North China Craton

Abstract

Archean high-K granitoids, generally formed after tonalite-trondhjemite-granodiorite (TTGs), are important for understanding crustal reworking of ancient cratons. The Linshan Archean high-K granitoids from the southern Trans-North China Orogen (TNCO) provide a window into the continental crustal evolution of the North China Craton (NCC). They mainly consist of monzogranite and granodiorite which were formed during 2 542–2 503 Ma. The high-K granitoids have high SiO2 (65.86 wt.%–78.08 wt.%), K2O (3.29 wt.%–7.62 wt.%) and low P2O5 (0.01 wt.%–0.27 wt.%). They display right inclined REE patterns with negative Eu anomalies (Eu/Eu*=0.20–0.81). Their spider diagram is characterized by enrichment of Rb, K, Th, U and depletion of Nb, Ta, Zr, Ti. The rocks have positive and variable zircon eHf(t) (+2.5 to +6.6) and whole-rock eNd(t)(+0.7 to +4.5) with two-stage model ages (TDM2Hf=2.87–2.64 Ga; TDM2Nd=2.77–2.50 Ga) similar to those of the Archean TTG-type rocks, amphibolites and diorites in the area. These evidences suggest that the high-K granitoids were produced by partial melting of juvenile crustal rocks. The Linshan high-K granitoids show relatively high whole-rock zircon saturation temperatures (694–889 oC) and low Sr/Y ratios (0.27–21.1), indicating low pressure partial melting. Combined with other geological evidences, the Linshan high-K granitoids are suggested to have been produced by partial melting of the continental crust in a post-collision extensional environment after an arc-continent collision. Thus, the NCC did not amalgamate together until ca. 2.5 Ga. Compiled zircon U-Pb ages and Hf isotopes reveal that the ca. 2.5 Ga magmatism represents reworking of the continental crust.