Geochronology,geochemistry And Geological Significances Of The Late Paleozoic Mafic Dykes From Beishan In Gansu Province

As an important part of Central Asian Orogenic Belt(CAOB),Beishan area in Gansu province is located in the southern margin of CAOB,and is a key area for understanding the Paleozoic tectonic evolution and magmatic activity of CAOB.Several Late Paleozoic mafic dykes were formed and preserved in the Beishan area in Gansu province,and mafic dykes was selected as the research object of this paper.Geological survey,petrography,LA-ICP-MS zircon U-Pb dating,elemental and isotopic geochemistry studies for mafic dykes have been carried out.Combined with the data form literatures,the tectonic setting,magmatic evolution and deep dynamic mechanism in research area are dicussed.The Late Paleozoic mafic dykes from Beishan in Gansu province intruded into the Ordovican and Carboniferous strata in northwest and northeast directions,and petrographic analysis show that its composition lithology was diabase,gabbro and allgovite.LA-ICP-MS U-Pb zircon dating analysis yield an age of 285±4.8Ma from diabase.Compared with the geochronology data form literature and surrounding rock emplaced by mafic dykes,the emplacement time of mafic dykes from Beishan in Gansu Province should be Early Permian.Elemental and isotopic geochemistry analysis shows that the mafic dykes have high Ca O(5.61%?11.74%),Al₂O₃(13.05%?20.45%),Mg O(4.13%?11.19%),and TFe₂O₃(7.31%?13.61%)contents,and the content of ALK(K₂O+Na₂O),Na O and K₂O is2.30%?11.88%,1.80%?9.61%and 0.11%?2.72%,respectively.This mafic dykes display light rare earth element enrichment and deleption.They are enriched in large ion lithophile element such as Rb,U and K,delepted in high field strength element such as Nb and Ta,exhibit positive?_Nd(t)value of 3.8?9.3 and have a large range initial ⁸⁷Sr/⁸⁶Sr ratios of0.703072?0.709367.Mafic dykes from Beishan in Gansu Province are fallen into subalkaline series,and belong to tholeiitic basalts.Some samples were composed of positive Nd isotopes,enriched LILEs and delepted HFSEs,and showed a decoupling relationship between isotopes and trace elements,indicating that the magma may generated by the interaction between depleted asthenospheric mantle and lithospheric mantle.Although zircon is captured in the rocks,the sample has a high Mg content,a positive?_Nd(t)values and depleted LREEs,which reflects the low degree of contamination(1%),and the contamination does not singnificantly affect the magmatic material composition.The Mg^#value(37.89-69.4)of mafic dykes slightly lower the than parental magma,indicating that parental magma has undergone a certain degree of evolution,and the occurrence of fractional crystallization of Clinopyroxene plagioclase and olivine were confirmed by harker diagram.Combined with the related diagram of tectonic setting discrimination and regional research data from literature,it is believe that Late Paleozoic mafic dykes in Beishan area was emplaced in intraplate extensional setting.By sorting out the Paleozoic magmatic rocks and sedimentary strata in the Beishan area,and according to the special properties of the mafic dykes,it is proved that the Paleo-Asian ocean(Beishan Ocean)has been closure before Late Permian.The source area of mafic dykes from Beishan in Gansu has two end member features:depleted asthenospheric mantle and metasomatized lithospheric mantle.Based on the history of regional tectonic evolution,it is believed that this may be generated by interaction between plate subduction and lithospheric delaminaiton.