Geochronology And Geochemistry Of Xiaoling Formation Volcanic Rocks In Xiuyan Area,Eastern Liaoning Province

The Mesozoic Xiaoling Formation volcanic rocks are widely exposed in eastern Liaoning Province.There are still many uncertainties regarding rock combination,formation age,petrogenesis and structural background of these volcanic rocks at present.This paper selects the volcanic rocks of the Xiaoling Formation in the Xiuyan area of eastern Liaoning Province as object for research.Based on detailed geological survey and microlithography analysis indoors,with LA-ICP-MS zircon U-Pb dating,analysis of mass and trace elements and Hf isotopes,rock assemblage and formation age of these volcanic rocks is determined,their source area properties and tectonic background is discussed.The volcanic rocks of the Xiaoling Formation in the Xiuyan area in eastern Liaoning Province are mainly composed of volcanic lavas such as trachyandesite and rhyolite,and volcanic clastic rocks of similar composition,with some basalt trachyandesites and few dacites.These different types of volcanic rocks are all conformable contact with one and another.The zircons in the 3 dating samples of rhyolite,dacite,and trachy-andesite are mostly euhedralsemi-euhedral columnar,with a well-developed oscillating zonal structure,and relatively high Th/U values(0.63?1.49),indicating that they are of magma origin.The dating results obtained are 134 Ma,132Ma and 131 Ma,respectively,representing the magma crystallization age(134?131 Ma).That is,the volcanic rocks of the Xiaoling Formation in the Xiuyan area of eastern Liaoning Province were formed in the early Early Cretaceous.In terms of mass elements characteristics,the SiO₂ content of rhyolites is80.2?80.9 wt.%,the A/CNK value(aluminum saturation index)is greater than 1.1,and the SiO₂ content of trachyandesites and basaltic trachyandesites is 51.2?61.4 wt.%,well the A/CNK value is equal to or slightly less than 1.1.There is a Daly discontinuity in the composition of the two,and the total alkali content(K₂O+Na₂O(wt.%))of rhyolites does not positively correlated to their SiO₂ content,or even slightly lower than that of trachyandesites.The combination of volcanic rocks formed by the series does not match.In terms of the characteristics of trace elements,the volcanic rocks of the Xiaoling Formation all have a right-dipping rare-earth distribution mode,which are poor in Sr,Ba,Eu,Ti and P,and are enriched in high-field-strength elements(Th,U)and large ion philic elements(Rb,K).showing the characteristics of arc volcanic rock in general.However,the ratios of Nb/Ta,Ce/Pb,Hf/Sm,and Zr/Hf are significantly different from the anomalies of Ce and Eu.In terms of Hf isotopic composition,the ratio of ¹⁷⁶Hf/¹⁷⁷Hf of rhyolite are between 0.282116 and 0.282242 and the value of?_Hf(t)are-16.0?-20.4,while the ratio of ¹⁷⁶Hf/¹⁷⁷Hf of trachy-andesite is between 0.281948 and 0.282432,and?_Hf(t)is-9.8?-14.7.Results show that the acidic volcanic rock dominated by rhyolite and the intermediate-basic volcanic rock composed of trachyandesites and basalt trachyandesites have different magma sources.The former magma originated from the partial melting of the ancient crust,while the latter originated from the enriched mantle replaced by subduction fluids.Together they constitute a bimodal volcanic rock combination.Based on previous research results,it is believed that the Early Cretaceous Xiaoling Formation volcanic rocks in the Xiuyan area were formed in extensional tectonic setting,and their geodynamic mechanism is related to the previous subduction retreat of the subduction zone of Paleo-Pacific plate in Early Cretaceous.