The field occurrence and origin of Xinglonggou adakites and adakitic rocks in Beipiao, western Liaoning Province and their geological implication

• 1、Division of Petrology and Mineralogy, China University of Geosciences, Beijing

Abstract：Two kinds of igneous rocks are identified in the “Xinglonggou Formation” exposed in Beipiao area, western Liaoning Province, according to field occurrence, lithology, crystallization age and geochemical characteristics of these rocks. The 144 Ma age high-magnesium dacitic plugs belong to the first group, and they intrude into the deformed volcanic rocks of 159 Ma age. The first group igneous rocks exhibit characteristics of low potassium (K2O < 2%), high magnesium (Mg# > 60) and calcium (CaO > 5%) content, low Rb content, but high Sr/Y ratio (> 60). Among these rocks, a lot of them have high Cs but low Li content. The second group igneous rocks are volcanics and had strongly deformed. Compared with the rocks of first group, they have high potassium (K2O > 2%) and Rb content, but low magnesium (Mg# < 60) and calcium (most samples CaO > 4%) content; meanwhile, they have lower Sr/Y ratios (in general < 60), and low Cs and high Li contents. The first group high-Mg dacites are typical adakite. They might result from partial melting of the mafic lower crust of Xing’an-Mongolian orogenic belt, and assimilated with mantle peridotite during ascending. The partial melting might be related to the subduction of Xing’an-Mongolian orogenic belt beneath North China Craton, or to the delamination of thickened lower crust of Xing’an-Mongolian orogenic belt, but not to the delamination of thickened lower crust of North China Craton. The second group belongs to high-potassium calc-alkaline volcanic rocks, and may not be related to the delamination of lower continental crust.

Keywords： adakite, dacite, field occurrence, petrogenesis, Beipiao, North China Craton