Characteristics And Tectonic Environment Of Huluhe Volcanic Rocks In The Xiangong Of Baoji

The study area is located at the junction of the North Qinling-Qilian orogenic belt and the Ordos Basin.The research in this area has important geological significance for studying the formation and evolution process of the Chinese mainland,and it is a research hotspot in recent years.There is a set of volcanic rocks exposed in the south of Fengjiashan Reservoir in Xiangong,Baoji County.Following the previous perspective of view,Shaanxi Geological Survey?2003?classified the volcanic rocks as the Xiong'er Group,but different from the classi-cal bear ear group in the formation age,rock assemblage and formation environment.But it is similar to Huluhe Group.Therefore,this paper takes the volcanic rocks as the main research object,studies them from their thin section identification,typical profile description,geoche-mistry and U-Pb geochronology,closely links with the results of regional geological survey,and combines plate tectonic theory to discuss the properties of the magma source area and the tectonic environment of its format-Ion.The Huluhe volcanic rock in the Xiangong of Baoji is mainly a set of marine volcanic eruption rock series.It is a set of shallow metamorphic volcanic rocks with quartz veins.The exposed thickness is about 1500 meters and the exposed area is about 12km².The SiO2content of the rock is 47.42-50.12wt.%,which is a basic volcanic rock with low K,low Mg,high Na,low Al and high Ti.It can be observed from the normalized distribution pattern of rare earth element chondrite that the curve is generally flat and inclined to tilt slightly to the right,similar to that of light rare earth enriched E-MORB,with a?La/Yb?N value of 1.67².30.The?La/Sm?N value is 0.91¹.18,and the fractional distillation between light and heavy rare earths is not obvious;the elemental spheroidal meteorite standardized spider web diagram is also similar to E-MORB,with relative loss of Nb,Ti,Y,P and other high field strength elements,while rich in large ion lithophile elements such as Th?U and Rb.Its original rock is basalt,belonging to the series of continental basalt.The LA-ICP-MS zircon U-Pb dating results show that the main crystallization age of the Huluhe volcanic rocks in Xiangong of Baoji is?436.9±5.7?Ma^401.1±4.0Ma,and the age is Early Silurian-Early Devonian.The trace elements of the Huluhe volcanic rocks are characterized by IAB,MORB and WPB,which may be the back-arc intraplate rift-small ocean basin formed at the edge of the converging plate in the middle and late Caledonian period.Trace and rare earth elements indicate that 10%to 15%of the partially melted products of the pomegranate+spinel dimeric olivine whose Magma is the subduction fluid,and a small amount of crust mixing.This indicates that in the early Siluk-Early Devonian,Baoji County Gong area may be in the tectonic environment of the back-arc small ocean basin at the edge of the convergence plate.The formation of volcanic rocks in the Xiangong of Baoji is quite different from that of the typical Xiong'er Group in the southern margin of the North China Block.It should be disintegrated from the original Xiong'er group.It may be the Huluhe group.