The Geochronology,Geochemistry And Petrogenesis Of Granitoid In Qinggele Area,Northern Alax

The Alax block located in the west of North China platform, contains lots ofgranites with different epochs.Recent research have showed that there are somePermian granites in the northern margin of Alax, which is different from the previousperspective. This paper made detailed studies on zircon U-Pb chronology,petrology,geochemistry, Sr,Nd and Hf isotopic compositions of the granitoids in theQinggele area. The following achievements and conclusions have been made.LA-ICP-MS U-Pb dating of zircons from the Halinudeng pluton yields an age of284±2Ma,indicating that the formation of this plution is Early Permian.Zircon U-Pbdating yield a206Pb/208Pb age of275±1Ma of Hashatu granitoid and a206Pb/208Pb ageof321±1Ma of Deerhetongte granitoid.Combining other people’s research, the lateCarboniferous-middle Permian granites constituted main body of the northern Alaxmagmatic activity.The main rock types of the northern Alax magmatic rock include biotitemonzogranites, biotite-plagioclase granite and granodiorite. Geochemicalcharacteristics indicate that they are metaluminous-weakly peraluminous(A/CNK=0.98~1.08), calc-alkaline to high-K calc-alkaline rocks (Na~2O=2.5%~4.4%，K~2O=2.1%~7.3%),and the Derhetongte pluton is proved alkaline series.The mineralof the granites did not include cordierite and muscovite, and the main dark mineral isbiotite, as the characteristic of I-type granite.All the rocks have lower REE contents(12~198×10-6),and the Hashatu graniteshow relatively flat REE patterns, without significant fractionations of REEs(LREE/HREE=1.0~1.7, La/YbN=1.3~2.8).It has strength Eu deplete,chondrite-normalized REE pattern is―sea-gull‖type, which means plagioclase left inthe magma chamber.While the other granites is enriched in LREEs(La/YbN=4.1~52.8)， and the chondrite-normalized REE pattern show theright-inclined feature, with significantly positive Eu anomalies (δEu=0.8~1.9),whichmay reflect little plagioclase left in the magma chamber.Besides, most of the granitesare high-Sr (149.9×10-6~749.4×10-6) and low-Yb (Y≤18×10-6, Yb≤1.9×10-6), similarly to the adakites.The granites in Qinggele area has low initial Sr isotopic radios of0.70635~0.71770and variable εNd(t) of-8.7~-16.4,the two-stage Nd depleted-mantlemodel ages(TDM2) range from1.7Ga~2.3Ga.The measurement of zircon Hf isotopic inthe granite exhibits wide range of εHf(t) values (-20~-4),and the relatively old mantlemodel ages(1.5Ga~2.2Ga).The Sr-Nd-Hf isotopic compositions suggest that thegranites were derived from the partial melting of ancient crust probably with littleadditions of mantle material and the northern margin of Alax has old metamorphicbasement.The late Early Carboniferous to Early Permian intrusive rocks are interpreted assubduction related magmatic rocks emplaced in an Andean-style continental marginarc during the southward subduction of the Paleo-Asian oceanic plate beneath theAlax block. The arc-related granite occurred at the structural transition fromcollisional compression to extension tectonic.