Geochemistry of the High-Mg Andesites at Zhangwu, Western Liaoning: Implication for Delamination of Newly Formed Lower Crust

Ten volcanic samples at Zhangwu, western Liaoning Province, North China were selected for a systematic geochemical, mineralogical and geochronological study, which provides an opportunity to explore the interaction between the continental crust and mantle beneath the north margin of the North China craton. Except one basalt sample (SiO2= 50.23%), the other nine samples are andesitic with SiO2 contents ranging from 53% to 59%. They have relatively high MgO (3.4%-6.1%, Mg# =50-64) and Ni and Cr contents (Ni 27ppm- 197ppm, Cr 51ppm-478ppm). Other geochemical characteristics of Zhangwu high-Mg andesites (HMAs) include strong fractionation of light rare earth elements (LREE) from heavy rare earth elements (HREE), and Sr from Y, with La/Yb greater than 15, and high Sr/Y (34-115). Zircons of andesite YX270 yield three age groups with no Precambrian age, which precludes origin of the Zhangwu HMAs from the partial melting of the Precambrian crust. The oldest age group peaking at 253 Ma is interpreted to represent the collision of the Siberia block and the North China block, resulting in formation of the Central Asian orogenic belt by closure of the Mongol-Okhotsk Ocean. The intermediate age group corresponds to the basalt underplating which caused the widespread coeval granitoids in the North China craton with a peak 206Pb/ 238U age of 172 Ma. The youngest age group gives a 206Pb/ 238U age of 126 Ma, which is interpreted as the eruption age of the Zhangwu HMAs. The high 87Sr/86Sri(126Ma)>0.706 and low ɛ Ndi(126Ma) = -6.36~-13.99 of the Zhangwu HMAs are distinct from slab melts. The common presence of reversely zoned clinopyroxene phenocrysts in the Zhangwu HMAs argues against the origin of the Zhangwu HMAs either from melting of the water saturated mantle or melting of the lower crust. In light of the evidence mentioned above, the envisaged scenario for the formation of the Zhangwu HMAs is related to the basaltic underplating at the base of the crust, which led to the thickening of the lower crust and formation of lower crustal eclogite, followed by foundering of the eclogitic lower crust into the asthenosphere. The foundered eclogite then melted and the resultant melts interacted with surrounding peridotite during their upward transport, which finally produced the high-Mg andesites. This well explains the high-Mg adakitic characters and absence of ancient inherited zircon in the Zhangwu lavas.