Tectonic Significance And Volcanism Of Late Jurassic And Early Cretaceous In Xuanhua Area,North Of Hebei Province

The Xuanhua area is located in the western part of the Yanshan tectonic belt,where have a large number of late Mesozoic volcanic rocks.On the basis of study of past geological data,the author conducted detailed field geological work,combined the geochemical characteristics of major and trace elements carried out research on the distribution characteristics and petrology of the Late Mesozoic volcanic rock strata in Xuanhua area,to discuss the origin of volcanic rocks and magma Sources and evolution,and the resulting tectonic settings and dynamics.Mainly achieved the following aspects:The Tiaojishan Formation volcanic rocks formed in the Middle-Late Jurassic,and the lithology mainly consisted of trachyte,trachy dacite,and a small amount of rhyolite and its corresponding pyroclastics,forming multiple eruption-overflow cycles;The Houcheng Formation volcanic rocks are mainly composed of basaltic andesite,trachyte,and granitic pyroclastic rocks and rhyolitic volcaniclastic rocks,forming overflow-eruption-overflow volcanic cycles;the volcanic rocks of the Zhangjiakou Formation are dominated by clastic rocks.As the main melody,it is characterized by the overflow of the rock.The Tiaojishan Formation volcanic rocks are characterized by high silicon,high aluminum,rich in potassium and alkali,and iron-depleted magnesium.They belong to alkaline and aluminum-peraluminous volcanic rocks.The Basaltic Andesites erupted early in the Houcheng Formation are characterized by low silicon and iron-richmagnesium relatively.The trachyte,trachy dacite volcanic rocks erupted in the middle and late stages are high in silicon,rich in potassium and alkalis,belonging to the calcium alkali-alkali and quasi-aluminum-weak peraluminous series.Zhangjiakou Formation are characterized by the high silicon,high aluminum,potassium-rich alkali,and magnesium-depleted,belonging to the calcium-alkaline quasi-aluminum-weak peraluminous series volcanic rocks.In the late Mesozoic volcanic rocks,enriched large-scale ionic lithophilic elements such as K,Rb,and Pb,depleted relatively high field strength elements such as Nb,Ta,P,Ce,and Ti.The content of heavy rare earth elements in the volcanic rocks of the Tiaojishan Formation is relatively high,followed by the Houcheng Group volcanic rocks,the volcanic rocks in the Zhangjiakou Formation are the lowest.The distribution pattern of transitional group elements in the original mantle standardization shows a deep “W” type.Ni and Cr are strongly depleted and relatively enriched in Ti and V elements,indicated olivine,pyroxene,or garnet is left in the volcanic rock magma source region.The volcanic rocks of the Tiaojishan Formation and Houcheng Formation belong to A2 subgrain granitoid volcanic rocks,while the Zhangjiakou Formation volcanic rocks belong to C-type adakitic volcanic rocks.The volcanic magma of the Tiaojishan Formation and Houcheng Formation originates from mixing of the partial melting of the upper mantle and the potassium-rich crust source magma to varying degrees;the volcanic magma of the Zhangjiakou Formation originates from the The part melts of thickened crust.The volcanic rocks of the Tiaojishan Formation are products of magmatic processes that are squeezed into extensional structures.Volcanic rocks of the Houcheng Formation and the Zhangjiakou Formation are formed in a regional extensional tectonic setting.Their formation is controlled by the remoted effect of the close of Okhotsk Ocean and Izanagi plate subduction.