The Spatio-temporal Evolution Of Nd And Hf Isotopes In Granite Belt Of Erenhot-Dongwuqi And The Composition Of Deep Crustal

The Erenhot-Dongwuqi area is located on the southeastern margin of the Central Asian Orogenic Belt.Volume granitoids,especially the Late Paleozoic and Mesozoic granites were exposed in this area.This thesis focuses on the study of rock formation,mineralogy,zircon U-Pb geochronology,geochemistry and Lu-Hf isotope analysis of the Hongol and Baiyinwula granites in the Sonid Zuoqi,and the Gil Galangtu granite in the northern Abagaqi to define the rock type and original characteristics.Based on a large amount of geochronology and isotopic data,the space-time distribution and isotope characters maps of granites in granitoids in Erenhot-Dongwuqi area were made to study the deep crust composition and crustal growth.New ziron U-Pb dating reveal two Early Carboniferous ages of 329±4 Ma and 328±4 Ma for the Hongol pluton and the Gil Galangtu pluton,respectively.These Early Carboniferous granitoids are I-type granites with weak peraluminous,high potassium,and calcium-alkaline characteristics,similar with that of arc magma.Hf isotope data show that the ?_Hf?t?values of the two are between +4.3 and +9.0,and the two-stage model age is 767¹047 Ma.One Permian age of 280±3 Ma also has been got for the Baiyinwulabei pluton.This Permian granites have weakly peraluminous and high-K calc-alkaline characteristics,were typical A-type granites.According the spatial and temporal distribution of granitoids in the Erenhot-Dongwuqi area,few Early Paleozoic granites outcrop,but volumethe Carboniferous-Early Permian granitoids distributed in this area,with a southward younger trend.More and more Mesozoic granitoids are recognized,but they are still not as much as the Late Paleozoic granites,and there is a 30 to 40 Ma magmatism gap between the Triassic and the Jurassic.Most granitoids in the study area have high positive ?_Nd?t?and ?_Hf?t?values.The ?_Nd?t?and ?_Hf?t?values from Early Carboniferous to Triassic keeps rising,but in the late Mesozoic,it shows a downward trend.Combining petrographic studies,geochemical characteristics,and isotopic composition,early Carboniferous granitoids exhibit geochemical characteristics of arc magma,late Carboniferous granitoids maybe formed in a subduction-collisional tectonic setting.After that,massive Late Carboniferous-Early Permian A-type granites were emplaced,indicating an the tectonic setting change to apost-orogenic setting.The Permian A-type granites are belong to A2-type,but the Triassic A-type granites have A1-type and A2-type,This transition of the nature of the Permian Triassic A-type granites present more evidents for Permian post-orogenic tectonic.