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

摘要

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 conti-nental 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 (65.86wt.%-78.08wt.%), K2O (3.29 wt.%-7.62wt.%) and low P2O5 (0.01 wt.%-0.27wt.%). 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 εHf(t)(+2.5 to +6.6) and whole-rock εHf(t)(+0.7 to +4.5) with two-stage model ages (TDM2^Hf=2.87-2.64 Ga;TDM2^Nd=2.77-2.50 Ga) similar to those of the Archean TTG-ty pe 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 tempera-tures (694-889℃) 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.