Geochronology,Geochemistry And Its Geological Significance Of Dacite In The Gujiaying Basin,South Jiangxi Province

Gujiaying Basin is located in the east of the Nanling Tectonic Belt and the east of the middle section of the Qinhang Junction Belt.Since the rocks in the basin were not exposed well and had suffered strong weathering,they have long been recognized as part of the Mesozoic volcanic rock series.Predecessors have done research on the volcanic rocks in this area before.However,because of the lack of field geological evidence and the incomplete rock sample analysis data,the type of rock has been controversial and those disputes limit the further study on the Early Paleozoic tectonic properties in southern Jiangxi and northern Guangdong.Based on previous studies,this paper focuses the volcanic rocks in the Gujiaying area.By way of field geological surveys,measured profiles,SHRIMP Zircon U-Pb Chronology research and geochemical characteristics research,this article analyzes its age,stratigraphic ownership,petrogenesis and tectonic setting.It provides references for further discussions on whether the Caledonian orogeny in South China belongs to intracontinental orogeny or subduction-collisional orogeny.The following content is the understanding obtained:In the basin,the mainly exposed are gray-white dacite and volcanic clastic rock interbedded with carbonaceous slate.It is the combination of intermediate-acid volcanic rock and shallow metamorphic rock.The SHRIMP zircon ²⁰⁶Pb/²³⁸U age(439.0±4.0Ma)and the measured profile of the dacite in the Gujiaying Basin indicate that it is the product of Caledonian volcanic activity.It was integrated on the pale metamorphic rock of the Huangzhudong Formation of the Late Ordovician and belongs to Nanjing Formation.The color of Gujiaying dacite is gray-white and it is mainly composed of quartz and plagioclase,the volcanic rock is rich in silicon and potassium but lack of sodium and magnesium.The content of Si O₂is high at around 67.79%?71.84%with the average content of 70.1%;the content of Al₂O₃is 13.55?14.29%with the average content of13.9%;the content of Fe is relatively low and the content of TFe is 3.87%?4.59%with the average content of 4.3%;the whole content of Na₂O is 1%?2.82%with the average content of 2.1%;the content of K₂O is 2.96%?3.87%with the average content of 3.4%,the Na₂O+K₂O is between 4%?6.42% with the average content of 5.6%.A/NK=1.59?2.82,A/CNK=1.03?2.72,K₂O/Na₂O=1.09?3.The content of CIPW standard mineral corundum(C)ranges from 0.68%to 9.23%with an average value of 3.1%;The large ion lithophile elements like Rb,Th,and K are relatively enriched,the high field strength elements like Ta and Nb are depleted and the negative Eu is abnormally obvious.Those characteristics are similar to volcanic arc.The value of(⁸⁷Sr/⁸⁶Sr)_iis quite uniform,ranging from 0.70624 to 0.70704 with an average value of 0.70664.The range of?_Nd(t)is from-6.5 to-6.3 with a small fluctuation range.The value of T_DM2is from 1685Ma to 1703Ma and it contains high radiogenic Pb,the(²⁰⁶Pb/²⁰⁴Pb)_ivalue is 17.853?17.847,the(²⁰⁷Pb/²⁰⁴Pb)_ivalue is 15.925?15.933 and the(²⁰⁸Pb/²⁰⁴Pb)_ivalue is 38.513?38.524.In the Yb-Th/Ta,Ta/Yb-Th/Ta and Rb/30-Hf-3Ta structural environment discrimination diagrams,the projection points are all fall within the active margin of the continent.Based on the main trace,isotopic geochemical analysis and the chronological studies of the dacite,it may be the product of crystallization differentiation of crust-mantle mixed magma,since the mantle-derived magmatic underplating provides heat and mixed with the Mesoproterozoic lower crustal sandy sedimentary rock.Combine this article and previous research results,there may be ocean basins in the middle section of the Qinhang junction belt during the Early Paleozoic period,the dacite was formed in the tectonic environment of the continental active margin and was the geological record of the oceanic crust subduction-collision in the middle section of the Qinhang junction belt.