Yanshan Period Volcanism And The Relationship Between It And The Abyssal Magmatic Activity Within The Chicheng County, North Hebei Province

Volcanics in Chicheng area includes volcanic lava and volcaniclastic rock, the main is volcanic lava, and the second is the volcaniclastic rock, and it mainly distributed in north,east and south of Zhangjiakou area. This article analysed the rock types, geochemical characteristics, the source of magma, the tectonic setting and the formation of the times for the lava in this area using the method of petrography, mineralogy, geochemistry and zircon LA-ICP-MS U-Pb dating. On this basis, contrast with granite magmatism and deduce the tectonic evolution process of Chicheng area.According to the rock major element data, Volcanic rock samples are mainly divided into rhyolites, trachyte, trachyandesite by using volcanic rock type diagrams. According to the A/CNK value, volcano rocks in Chicheng area belongs to peraluminous granite, and the same to the granite. The rhyolite’s characteristics is enrichment in LREE and loss in HREE and a high degree of magmatic differentiation, calc alkaline trend. The division of genetic types of volcanic rocks, due to the rock of Si O2, Na2 O and K2 O content is high and the values of Cao, Mg O, Ti O2 and P2O5 is low, the figure of Sr,Rb is low, the figure of Zr, Nb, Ce is high, and it is consistent with the characteristics of A granite, and it is also proved in K2O-Na2 O diagram. The tectonic environment map shows that the volcanic rocks in Chicheng are all formed in post collision environment. The εHf（t） values of volcanic rock are negative, indicating that volcanic rocks in Chicheng area source rocks from the lower crust. According to the two stage of volcanic rocks, the age of the source rocks of the volcanic rocks in Chicheng area is 1896 Ma and 1763 Ma, and it belongs to ancient yuan. The reason of the difference of source rock age is different source of the volcanic magma. According to zircon LA-ICP-MS U-Pb dating, the age of the volcanic rocks is between（137.91±0.95）Ma and（141.03±0.95）Ma, and it belongs to yanshanian period.Through the comparison of the volcanic rocks and the granite in the period of yanshanian period in Chicheng area. there are some connections and differences between them. The granite in Yanshanian are mainly I type granite, and different from characteristics of volcanic rocks. The εHf（t） values of volcanic rock and granite are negative, and it means both them are from the crust. According to the two stage of volcanic rocks, the average of them is 1838Ma¹763Ma, 1940 Ma. The Material source of volcanic rocks and the granite in Chicheng area are from Lower crust. The volcanic rock belongs to Post-collision granites, and the granite belongs to Syn-collision granite had the tectonic conversion event, the time limit was（143.9±2.2） Ma^141.03±0.95 Ma, and it lasts about 2.87 Ma.