Geochemical Characteristics Of Volcanic Rocks In Early Cretaceous Baiyingaolao Formation At Haoqin Of Xiwuqi,Inner-Mongolia

This paper based on "Inner Mongolia 1:50000 Haoqin four pieces of regional geological survey" project. The article studied the geology, geochemistry and ziron U-Pb age of volcanic rocks in Baiyingaolao Formation from Haoqin area Inner Mongolia. The author determined the formation age, summarized the petrological and geochemical characteristics, discussed the volcanic magmatic and analyzed the tectonic setting of the volcanic rocks.The tectonic location of the study area in the northern margin of North China plate,located between the He Genshan fracture and Xar Moron fault. It belonged to the Central Asian orogenic belt when in Neoproterozoic-Paleozoic, while in Mesozoic and Cenozoic,it belonged to circum Pacific tectonic domain in Da Hinggan Mountains Mesozoic volcanic rock belts. The new generation is influenced by the subduction of the pacific plate,and the magmatic activity is strong. The volcanic rocks from Baiyingaolao Formation are mainly distributed in North Diehaerzhai Baolegesi, NE to banded distribution, unconformity with Meiletu volcanic rocks. The volcanic rocks include Rhyolitic tuff, Rhyolitic,Rhyolitic ignimbrite breccia and Rhyolitic breccia agglomerate. The zircon U-Pb dating result of volcanic rocks is 143.3 ± 1.9Ma, indicating that the acid volcanic rocks of Baiyingaolao Formation were formed in Early Cretaceous.The geochemistry of major elements indicate that the volcamic rocks from Baiyingaolao Formation are characterized by high-SiO2, alkali, lower Calcium and Magnesium, belongs to high-K calc-alkaline series rocks, near to the average of world A-type granite and alkaline rhyolite. The average FeOT/MgO ratio of volcanic value is 7.05(varying from 2.30 to 16.20), significantly higher than the M-type(average 2.37), I-type(average 2.27) and S-type(average 2.38) granite. In Harker diagram, the magjor elements of volcanic rocks share a well linearity similar,indicating that these volcanic rocks have similar magma source.The rare earth element distribution pattern of the volcanic rocks from Baiyingaolao formation present a asymmetric seagulls curve that the REE are rich and strong negative Eu anoalies(?Eu=0.01~0.37); The trace element geochemistry are characterized evidently by enrichment K,Rb and LREE, mediate depletion of HSFE like Nb,Ta,Ti,P, strong depletion of Ba,Sr, similar to continental crust and trace element spidergram, showed that partial melting of the volcano rocks may originate from crustal material; The average Rb/Sr ratio of volcanic is 12.01, significantly different from I-type(0.61),S-type(1.81)and M-type(0.06) granite.The geochemical characteristics of the volcanic rocks are similar to A-type granite,low Ba-Sr rhyolite from Da Hinggan Mountains defined by Zhang et al. In classification illustration of granite, the volcanic rocks of Baiyingaolao Formation show an affinty with the A2-type granite. Combining with the regional researching results, we suggest that the volcamic rocks in Baiyingaolao Formation from Haoqin area were derived from partial melting of crustal rocks. The volcamic rocks were impacted by the subduction of Paleo-Pacific Plate and formed in the extensional setting after the compressive orogenic.