Characteristics And Tectonic Setting Of The Volcanic Rocks From Mesozoic Baiyin’gaolao Formation In Wuchagou Area, Middle Of Daxing’anling

The Early Mesozoic volcanic rocks from the Baiyin′gaolao Formation are well developed in the area of Wuchagou, middle of Daxing′anling, eastern of Inner Mongolia. It can provide a reliable basis for tectonic environments of the volcanic rocks and Early Mesozoic regional tectonic evolution to find out the lithological features and the spatial distribution characteristics of the volcanic rocks. The writer chooses the volcanic rocks in this area as an analyzing object. This paper identifies the petrology characteristics of the Baiyingaolao Formation volcanic rocks,discusses the magmatic source and property and illustrates tectonic setting of the volcanic rocks, based on the research of systemic lithology and geochemistry.The Baiyingaolao Formation volcanic rocks mainly consist of crystal-lithic welded tuff, rhyolite and felsite. Analysis based on 19 Petrochemical data of series of the volcanic rocks, rock association and chemical composition indicates that the Baiyingaolao Formation volcanic rocks in this area are characterized by high silicon, high alkali, rich potassium, high aluminum, low calcium and low magnesium.The trace element spider diagrams shows the curves are of right inclinnation on the whole. The diagrams show pronounced negative anomalies of high field-strength elment (HSFE) ,such as Nb, Ta, Ti, ect, which indicates the volcanic rocks in this area were formed under the compression action. Negative P anomaly shows magma may be contaminated by the crust. Positive anomalies of K and Pb reflect participation of continental crustal material.The curves in the diagrams of REE patterns are roughly parallel to one another, which suggests LREE-enriched patterns, weak REE fractionation and pronounced negative Eu anomaly. Negative Eu anomaly widely existing in the volcanic rocks in this area, the curves in the diagrams of REE patterns are of“V”form, which suggests fractional crystallization in feldspars had taken away a mass of Eu. It is similar to the characteristics of the crustal granite, which is formed through remelting of sialic layerof continental crust. It is concluded that the Baiyingaolao Formation volcanic rocks in this area have a character of crust-source.On Rittmann-Gottiry diagram, it explains further in detail that the volcanic rocks in this area were formed in volcanic arc, which suggests the volcanic rocks were formed in the tectonic setting of compressive orogenic belts. A striking feature is their low value of ISr(≈0.704) and high value ofεNd (t) (positive), which indicates a close relationship to depleted mantle.Zircon U-Pb dating of the Baiyingaolao Formation gives an age of about 125 Ma, indicating a Early Cretaceous age.