Zircon U-Pb Geochronology And Geochemistry Of Volcanic Rocks From The Dashizhai Formation In Eastern Inner Mongolia

This paper selects the Dashizhai formation in eastern Inner Mongolia asthe research object, through the system of petrology, geochronology,geochemistry, Hf isotopic studies and regional geological data, determine thevolcano rock formation era, explores the magma source and properties,clarifies the volcano rock tectonic environment.1, Dashizhai formation in volcano rock distribution characteristicsDashizhai formation in volcano rocks are widely distributed in InnerMongolia, East, West, East, Nenjiang. On the space, from the west to the Eastrock assemblage differences exist, the West in Sonid Zuoqi, by ANN hill hightuff with andesite and small amounts of basaltic lava transition to andesite,dacite, rhyolite rock intermediate-acid extrusive rocks; in the middle of yellowhillock region of beam and the Linxi region, consists of a lower angle porphyry-quartz keratophyre-rhyolite combination transition to middle Pillow Basaltspilite-as well as the combination of the upper part of the basalt and andesitemainly in basic lava; in the eastern Ulanhot area, mainly for the rhyolite anddacite rock and other acidic lava and a few basic lava composition of thevolcano rock combination, Dashizhai formation in volcano rock rockassemblage is mainly rhyolite dacite rock with continuous basalt basalticandesite, evolution trend. 2, Dashizhai formation in volcano rock formation ageThe Dashizhai area of the Dashizhai formation in volcano rockconstruction group profiles of actual measurement and detailed petrographicaland chronology of the show,17pieces dating sample zircon cathodeluminescence （CL） image display zircon is automorphic crystal, and theinternal development of oscillatory zoning, Th/U ratio was high, display formagmatic zircon, LA-ICP MS U-Pb zircon dating results of276-298Ma,indicating Dashizhai formation in volcano rock belongs to the early Permianmagma eruption products; another deformation granite porphyry in samples ofzircon dating results for148Ma, the formed during late Jurassic, and graniteporphyry and part of Dashizhai formation in volcano rock occurred thedeformation effects, suggesting that the tectonic deformation effects shouldoccur in the late Jurassic after.3, Dashizhai area Dashizhai formation in volcano rock geochemistrySiO₂content ranged from49.68%to88.43%, Dashizhai formation rocksvolcano rocks with high Al₂O₃, TiO₂, MgO, CaO, total alkali content, and lowK₂O feature; neutral rocks with high Al₂O₃content, K₂O, CaO, Na₂O, MgOcontent changes in the larger, lower TiO₂content characteristics of acid rocks;with higher SiO₂, Al₂O₃, Na2O content, MgO, lower TiO₂content, K₂O and CaOcontent change. Through the TAS projection diagram and Zr/TiO₂-Nb/Yprojection map, Dashizhai formation in volcano rock assemblages of rhyolite,dacite,andesite,basalt and basaltic andesite. In the FAM diagram, showingcalcium alkaline evolution trend, in the SiO₂-K₂O map volcano rock bodybelongs to high potassium calc alkaline series and potassium calc-alkalineseries.REE diagrams show enrichment of light rare earth elements, loss of heavyrare earth elements, showing a" rightist" trend. Trace element spider diagram,each volcano rock generally with the enrichment of Rb, Ba, Th, and other largeion lithophile elements （LILE） and light rare earth element （LREE）, Nb, Ta, Ti,loss of high strength element （HSFE）. Dashizhai formation in volcano rockREE and trace element diagrams and typical Andean arc volcano rock geochemical diagrams similar to. Rare earth element abundance than basalt,rhyolite rare earth element abundance, show rhyolite and not by basalticmagma evolution.4, Dashizhai formation in volcano rock magma source properties and magmaevolution characteristicsThe basalt in REE diagrams, basaltic rocks rich in LREEs, and the loss ofHREEs; in the trace element spider diagram, loss of Nb, Ta, Ti high strengthelement, and the enrichment of large ion lithophile elements, and basalt Zr（139.3ppm-230.3ppm） and Y（20.51ppm-27ppm） content is higher, may saidbasalt originated in the loss of the lithospheric mantle. Low Th/La （0.04-0.08）and Th/Nb ratio （0.09-0.16）, and Guo Feng presented in the first group ofbasalt, basaltic magma source area should be long-term loss of mantle sourceregion, according to the Dashizhai formation in volcano rock tectonicenvironment, think of Dashizhai basalts should be subducted oceanic slab.Loss of lithosphere mantle formed by partial melting.Acidic rocks with high silicon （SiO₂=68.70%⁸8.43%）, the content ofMgO subjects ranged from0.05%-0.34%, peraluminous （A/CNK=1.06-2.05）,but also has a very low Ni, Sr, Rb, Cr, Th content. The distribution patterns ofrare earth elements with minor Eu abnormalities and the trace element spiderdiagrams marked Sr abnormalities are described the source region for thestable region of plagioclase. Rhyolite and enrichment of light rare earthelements and Rb, Th, Ba, K and other large ionic elements, loss of heavy rareearth elements and Nb, Ta, Eu, Ti and other elements of the character, so theDashizhai formation in rhyolite is likely to be the products of partial melting ofcrustal material. Isotope analysis results show that, with the Hf（t）=+3.90⁺12.87, TDM1=407Ma⁷96Ma, TDM₂=473Ma¹295Ma, and the predecessorto the Central Asian orogenic belt in granite results are similar, suggesting theDashizhai formation in rhyolite originated in the new students in lower crustpartial melting.In this paper the main calcium alkaline rhyolite for aluminum, A/CNK index>1.1, but due to the fluid or melt function, the height difference of I typegranite A/CNK index also would be greater than1.1. Dashizhai formation inrhyolite originated in the new students in lower crust partial melting, and I typegranite of the same origin, and the Dashizhai formation in rhyolite overall lossof Nb, Ta high strength element, but Rb, Th, high content of U, Ba, Sr, Eu withdifferent degrees of negative anomaly and CaO content most of low（0.03-0.78）, so the Dashizhai formation in rhyolite and differentiation of higherdegree of I type granite similar.5, Dashizhai formation in volcano rock tectonic backgroundDashizhai area Dashizhai formation in volcano rocks combination ofrhyolite andesite basalt continuous evolution calcium alkaline rock series, andactive continental margin in the rock assemblage features similar;geochemistry enrichment of light rare earth elements and Rb, Ba, Th and otherlarge ionic elements, whereas the loss of heavy rare earth elements and Nb,Ta, Ti high strength element, similar to the active continental margin volcanorocks. Rhyolite in tectonic discrimination diagrams. In arc volcano rock area, atthe same time basalt in the tectonic discrimination diagrams into calciumalkaline region. From the rock assemblage features and geochemicalproperties, combined with previous petrology, geochemistry, paleontologicaldata, suggesting the Dashizhai formation in volcano rocks formed in the activecontinental margin arc subduction environment, rather than previous thoughtrift environment or aulacogen environment.