Chronology, Geochemistry And Petrogenesis Of Mesozoic Volcanic Rocks From Qinglong Area Of Hebei And Jianchang Area Of Liaoning

The petrology, geochronology and geochemistry of the Mesozoic volcanic rocks in the Qinglong and Jianchang areashas been studied in this paper, and the following conclusions can be obtained:The Mesozoic volcanic rocks in the Qinglong and Jianchang areas are mainly composed of the Tiaojishan and Yixian formations. The Tiaojishan formation consist primarily of trachyandesite andandesite, the Yixian formation is dominated by trachyandesite and rhyolite.Zircon U-Pb ages indicate that they were formed in 163±3 Ma and 121±1Ma, respectively.The Tiaojishan formation is mainlytrachyandesite and trachyteincomposition,and is rich in silicon and sodium,poor in magnesium.They havelow total rare earth elements(ΣREE=110. 93×10-6~169.79×10-6),relatively significant fractionation of HREE and LREE [(LaN /YbN)=13.52~24.46], and weak negative Eu anomaly or positive Eu anomaly(δEu=0.96-1.15). They are enriched in LILE(Ba, K, Sr) and depleted in HSFE(Nb, Ta, P, Ti), have high ratios of Sr/Y(49~74) and Sr content(682×10-6~1065×10-6), low Y(9.1×10-6~17.9×10-6) and Yb content(0.79×10-6~1.74×10-6), characteristics typical of adakite. The εHf(t) values range from-12.0 to-15.6,and the corresponding tDM2 vary from 2.7Ga to 3.0Ga, indicating that the magma could be derived from partial melting of the Mesoarchean lower crust.The Yixian trachyandesites have similar REE and trace element distribution with the Tiaojishan trachyandesites, but relativelylow Sr, high Y and Yb contents, do not display thecharacteristics of adakite. The felsic volcanic rocks of the Yixian Formationare rich in silicon and alkalis,poor in calcium,ferrum and magnesium. The strong depletion of Sr, P, Eu is results of fractionation crystallization of plagioclase and apatite. The felsic volcanic rocks are derived from the differentiation of the trachyandesite magma. The εHf(t) values range from-9.0 to-12.7,and the corresponding tDM2 vary from 2.6 Ga to 2.8Ga;The geochemical data suggest that the magma could be derived from partial melting of the Neoarchean lower crust.Combined with regional tectonic evolution, it is concluded that the Tiaojishan volcanic rocks were formed in the crustal thickening under the background of plate tectonic environment and the Yixianvolcanic rocks were formed in the extensional background of intraplateorogenic tectonic environment, which has nothing to do with the subduction of the ancient Pacific plate and the Mongolia-Okhotsk oceanic crust to the Eurasian plate.