Geochronological And Geochemical Characteristics Of Alkali Feldspar Granite From Hongshanzi Granite Pluton, Chifeng

Hongshanzi granite rocks tectonic position is lies in the Liaoyuan block between the Xar Moron-Changchun suture zone and Chifeng-Kaiyuan suture zone.It is a Complex pluton,which marginal facies is the Alkali feldspar granite,and it is a important composition of Guyuan-Hongshanzi volcano rock Uranium.The area of Hongshanzi granite pluton about 290km2,and it extends in NE direction.Based on the collection of previous research,this paper take Alkali feldspar granite as the subject to researck it's Chronology,by the LA-ICP-MS Zircon U-Pb isotopic dating.Besides,based on petrological and geochemical analysis,combined with the existing geological data and geological background,we discussed the causes and the provenance of Alkali feldspar granite.The LA-ICP-MS Zircon U-Pb isotopic dating presents that the Alkali feldspar graniteyield weighted an average is 151.4Ma±1.1Ma,MSWD=0.57.According to the latest international geological year,it belongs to the product of Late JurassicThe whole-rock geochemistry of the rhyolites show an A-type magmatic affinit y,with a typical enrichment in SiO2(SiO2=74.63~75.27%),total alkali(ALK=8.75~9.15%)butlowcontentsof FeOT/MgO(FeOT/MgO =12.27~14.66,an average of 13.03),Al2O3(Al2O3=12.47~12.73%,an average of 12.63%),MgO(Mg O=0.17~0.20,an average of 0.19),P2O5(P2O5=0.02~0.03%,an average of 0.02%),K2O/Na2O=1.29~1.35,DI rang from 92.33~92.94,belonging to metaluminous high-K calc-alkaline serie s rocks.A/CNK=0.931~0.956,in the A/CNK-A/NK diagram,those samples fall into the quasi aluminum range.The total amount of REE(excluding Y)is relatively high,REEs fractionate evidently.The fractionated REE patterns showing strong negat ive Euanomaly,and there is an obviously fractionation between LREE and HREE.The trace elements show that the Alkali feldspar granite enrich in Th,U,Zr,Hf,Rb,K,La,Ce,and depleteBa,Sr,Ta,Nb,P.The ratio of Alkali feldspar granits 10000Ga/Al range from 3.27~3.96,above of2.6,the value of Zr+Nb+Ce+Y range from 512.6×10-6~642.9×10-6,above of 350×10-6.In the diagram of Nb,Zr,(K2O+Na2O)/Ca O-10000Ga/A,(K2O+Na2O)/Ca O,Fe OT/Mg OZr+Nb+Ce+Y,All the samples were put into the A granite area.And the calculate zi rcon saturation temperature range from 814?~837?(average 827?),match the Atype magmatic affinity with high temperature.It has low Ba,Sr,belongs to low Ba-S r granites,with high ratio of Rb/Sr,range from 17.56~19.90(>0.5),high ratios of Ti/Y,Ti/Zr,range from 10.40~11.60,2.14~2.77(<20)respectively.The ratio of Nb/Ta=12.58~12.94,averge 12.76;low Mg#(averge 12.23)and Cr(averge 2.33×10-6)?Ni(averge 0.60×10-6),Co(averge 0.57×10-6),V(averge 5.74×10-6).It should be t he product of the partial melting of the upper and middle crust resulting from the underplating of magma from the enriched mantle and mixing with a small amount of mantle material.On the major elements discrimination diagrams of the tectonic settings diagram MgO-FeOT,all of samples drop into the margin of POG.On the diagram of SiO2-Al2O3?FeOT/(Fe OT+ MgO)-SiO2,FeOT+ MgO-CaO,Alkali feldspargranits drop into POG,in the R1-R2 diagram,all the samples fall into the post orogenic environment,and in the Lg(CaO/(K2O+Na2O))-SiO2 diagram,the sample fall into the extensional environment.On the Y+Nb-Rb and Y-Nb diagrams,all the samples are lacated in the WPG area,all of results above indicate that the formation of Alkali feldspar granite is related to the tectonic environment of plate internal post-tectonic stretching tension.The largly scatters of late Jurassic A type granites(Rhyolite)indicate this area has swich into the regional extensional environment during the late Jurassic.Therefor Mongol-khotsk sea has closed before the late Jurassic.