Chronology,Geochemical Characteristics And Geological Significance Of Rhyolite From The East Of Hebei Proince Qian Xi Donghuangyu

In this paper,the rhyolite in Donghuangyu area,western Qianxi of eastern Hebei was selected as the research object.Based on field investigation and previous work in the area,the distribution and petrological characteristics of the Donghuangyu rhyolite were determined.The geochronological study of the Donghuangyu rhyolite indicates the formation age of the rhyolite.The petrogenesis and tectonic setting of the donghuangyu rhyolite are discussed through in-depth analysis of the geochemical characteristics and zircon Hf isotope.The Donghuangyu rhyolite,which is exposed in the central of eastern Hebei province,is distributed nearly east-westward.In the field,it is grayish-gray with porphyritic structure.The matrix is cryptocrystalline,and the phenocrysts are plagioclase(15%)and quartz(10%).Plagioclase has obvious Polysynthetic twin structure.The cryptocrystalline feldspar and quartz are oriented to form the fluidal structure.Zircon U-Pb dating results of donghuangyu rhyolites are 161.3±1.6 Ma and 161.72±2.0 Ma,which were formed in the Late Jurassic.It is consistent with the eruption time of the Tiaojishan Formation volcanic rocks in northern Hebei and western Liaoning,and with the intrusion time of yuerya granite in the region.They could be formed in the same geodynamic background.Therefore we suggest the donghuangyu rhyolite should be classified as the Tiaojishan Formation rather than Donglingtai Formation as previously assigned according to its age.Geochemical analysis shows that the donghuangyu rhyolite is characterized by high silicon(74.10-78.00%),alkali(5.11-8.35%)contents,low titanium(0.067-0.14%),magnesium(0.13-0.28%),phosphorus(0.01-0.029%),Sr and Yb contents.At the same time,they are enriched in Rb,Ba and LREE,and depleted in Nb,P,Ti,Ce and HREE.(La/Yb)N values are 4.86?23.88,indicating that REE fractionation is obvious,and they also have negative anomaly to slightly positive anomaly(?Eu=0.58?1.31).Their zirconium saturation temperatures are 724.66?777.24?,they have differentiation index of 93.34?97.27.They have the characteristics of high fractionated I-type granite and low Sr and Yb granite.Zircon Lu-Hf isotopic results show that the?Hf(t)values of the Donghuangyu rhyolite are-20.57?-8.76,and the two-stage model ages(T_DM2)are 1.64?2.38 Ga,indicating that the magma originated from the partial melting of the crustal material in the Archean and Paleoproterozoic,and was formed by the eruption of the separation and crystallization of amphibole and plagioclase in the later period.Based on the analysis of regional geological background and geochemical characteristics,we suggest the donghuangyu rhyolite was formed under the subduction of the Pacific plate.