| This paper researched early Cretaceous intrusive rocks in Genduohe area, Inner Mongolia, China. On the basis of previous research, such conclusions as the emplacement time of intrusive rocks, geochemical characteristics, the nature of magma source during corresponding epoch, petrogenesis and it’s tectonic background have been concludec trough detailed studies, which provide new data for the research of magma intrusive time and tectonic evolution history in early Cretaceous at Daxingan Range.The granodiorite(represented by the Xinlitun intrusive mass) is mainly concentrated on Xinlitunlinchang area. The NE and NNE fault were cut by intrusive rocks which mainly distributed in north and south orientation as a batholith that make up by 11 intrusive bodies and with an exposure area about 45.12 km2. The monzonitic granite exposes greatly in the study areas especially at Genduohelinchang area(represented by the Genduohe intrusive mass) but seldom exposure at Jiqinhelinchang area and east and northeast parts of study areas. Genduohe intrusive body mainly distributed northeast orientation as a batholith and make up by 11 intrusive bodies with an exposure area about 59.07km2.The zircon LA-ICPMS U-Pb ages of Xinlitun intrusive mass and Genduohe intrusive mass are 135.6±1.3Ma and 130.3±2.4Ma, respectively, which indicate the intrusive mass generation time is early cretaceous but not Indosinian or early Yanshanian stage as previous results.The Xinlitun intrusive mass, with 65.64~67.09 wt.%SiO2, Rittmann Index(σ)2.40~2.92, belong to Calc-alkaline series, A/CNK ratio range is 0.92 ~ 0.97, present a quasi-aluminous characteristic. Total Rare elements(ΣREEs) is 102.3~159.13×10-6, the mean content is 122.86×10-6, there is a deeply fractionafion of LREEs but weekly of HREEs. Tm show lightly positive anomalies, Er,Yb,Eu with a negative pattern, δEu also weekly negative with the content of 0.61 ~ 0.80 which imply the fractionafion crystallisation of plagioclase in the magma source. The granite at this region are high Sr low Yb type by the Sr and Yb content: Sr>462μg/g,Yb<2.04μg/g, respectively.The Genduohe intrusive mass, with 66.88~76.53%wt.%SiO2, Rittmann Index(σ)2.27~3.13, belong to Calc-alkaline series, A/CNK ratio range is 0.92~0.97, present a peraluminous characteristic. Total Rare elements(ΣREEs) is 141.53~194.03×10-6, there is a fully fractionafion between LREEs and HREEs and with obviously enrichment of LREEs. Ce,Gd show obviously positive anomalies, The value of δEu is 0.61~0.80 which imply the residual of plagioclase in the magma source. Trace elements have an characteristic that depleted in Ba, Nb, C, P, Ti, but enrichment in K, Rb, Th, Nd; Rb, Th obviously depleted in responds to Ba which is a signature that exist of plagioclase but lack of garnet and amphibolic in magma source also imply that the shallow magma source of this stage. The element Nb show negative anomalies which express the relationship with subduction.The granodiorite of Xinlitun mass is I type granodiorite with quasi-aluminous, High-K calc-alkaline series characteristics and show high Sr low Yb type, that indicate it formed in lower crust with high press. The rock associations characteristics of the mass similar to active continental margin which imply it may generated in the active continental margin background, that is have relationship with paleopacific subduction.The monzogranite in this region formed in extension background at post-oregen stage. Extension leading to crust reduction and as a result asthenosphere intrusive happen, high temperature form asthenosphere make the melt of crust and leading to the intrusive of monzogranite at this region. |
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