TTG and potassic granitoids in the Eastern North China Craton : making Neoarchean upper continental crust during micro-continental collision and post-collisional extension.

摘要

As the major component, Archean granitoids provide us with an insight into the formation of the early continental crust. We report the study of a series of Neoarchean granitoids, including tonalite–trondhjemite–granodiorite (TTG) and potassic granitoids, in the Xingcheng region of the eastern North China Craton. Zircon U–Pb dating shows that the TTG granitoids were emplaced in the Neoarchean within a 75 Myr period (2595–2520 Ma), with coeval mafic magmatic enclaves, followed by intrusion of potassic granitoids. The geochemistry of the TTG granitoids is consistent with partial melting of Mesoarchean enriched mafic crustal sources at different depths (up to 10–12 kbar equivalent pressure) during a continental collision event. The potassic granitoids are derived from either low-degree melting of Mesoarchean enriched mafic crustal sources or remelting of Mesoarchean TTGs in response to post-collisional extension, and were hybridized with Neoarchean mantle-derived mafic melts to various degrees. The TTG and potassic granitoids in the Xingcheng region record the evolution from collision of micro-continental blocks to post-collisional extension, consistent with other studies, suggesting that the amalgamation of micro-continental blocks is what gave rise to the cratonization of the North China Craton at the end of the Archean. The rock assemblage of these granitoids resembles those of syn- and post-collisional magmatism in Phanerozoic orogenic belts, and the estimated average composition is similar to that of the present-day upper continental crust, suggesting that a prototype upper continental crust might have been developed at the end of the Archean from a mixture of TTG and potassic granitoids. Together with concurrent high-grade metamorphism in the North China Craton, we conclude that collisional orogenesis is responsible for continental cratonization at the end of the Archean in the North China Craton.