The Formation And Evolution Of The Lower Crust In Western Liaoning

The North China craton,as one of the most ancient craton,retained materials recording the early evolution of crust and mantle.In the Mesozoic,the North China craton experienced destruction events and widespreadly formed volcanic rocks.Two sets of the Mesozoic volcanic rock stratum,Xinglonggou Formation and Lanqi Formation,in the western Liaoning region,outcropped on the surface.Previous studies have shown that these two strata were formed in early Jurassic and late Jurassic,respectively,of which the volcanic rocks associated with the partial melting of lower crust.We collected andesitic volcanic rocks and volcanic breccia samples from Xinglonggou Formation and Lanqi Formation in Chaoyang region and Beipiao region,western Liaoning,saparating a lot old zircons which crystallized before these two strata formation.Cl images and data of trace elements show that these zircons dominantly have characteristics of magmatic origin in their cores,of which the U-Pb age desitributes three periods in the Paleozoic(250?350 Ma),Paleoproterozoic(?1.8 Ga)and the late Archean to Paleoproterozoic(?2.5 Ga),representing three-stages of magmatic or metamorphic events.The former two stages were consistent with crustal growth or modification events of?2.5 Ga and orogenic events of?1.8 Ga,which is shown in many areas in the North China craton.Zircons with U-Pb ages of?2.5 Ga and?1.8 Ga correspond to Hf model ages of 2.6 Ga and 2.2-2.9 Ga,respectively,indicating that two magmatic or metamorphic events was reworking events of older lower crust.Meanwhile,In the western Liaoning region,the lower crustal growth event started at 2.9 Ga,hit the top in 2.6?2.8 Ga,which is consistent with other previous studies in North China craton.Many zircons show high oxygen isotopes(?18O=6?12‰),suggesting those zircons were not formed in crust growth caused by mantle materials,but produced during the crust reworking.The Palaeozoic zircons have variable?Hf(t)(-27.6?12.9)and ?18O,while some of the zircon oxygen isotope composition(5.3±0.3‰)is same to the original mantle.This result indicates that the zircon formed in mixing of lower crust and mantle,which is caused by mantle underplating.In a word,the geochronological and geochemical characteristics of zircon xenocrysts in Mesozoic volcanic rocks in western Liaoning region reveals that the ancient lower crust of underwent a process of 2.9?2.6 Ga mantle-crust differentiation,magmatic events of?2.5 Ga and?1.8 Ga lower crust metamorphism events.The?1.8 Ga lower crust metamorphism events was related to the formation of the Trans-North China Orogen,which is the mark of the final formation of the North China craton.In addition,the lower crust of western Liaoning experienced multiperiod lower crustal reworking which is caused by mantle underplating.Zircons formed at different stages were brought to the surface by Mesozoic magmatism,which is Xinglonggou Formatiom and Lanqi Formatiom andesitic volcanic rocks we collected.The andesitic magma almost did not form new zircon.