辽北德仁组火山岩LA-ICP-MS锆石U-Pb年龄及岩石地球化学特征

LA-ICP-MS zircon U-Pb dating and geochemical characteristics of the volcanic rocks in Deren Formation, northern Liaoning Province

通过对辽北平岗盆地中德仁组火山岩的岩石学、地球化学和年代学的研究,探讨了其岩石成因、形成时代、构造环境,以及华北克拉通东部岩石圈减薄时间和减薄机制。用LA-ICP-MS技术测定了平岗盆地中德仁组中部英安质角砾晶屑岩屑凝灰岩和上部流纹岩中的锆石U-Th-Pb同位素组成。获得前者的锆石^(206)Pb/^(238)U年龄为246.8±3.5Ma和169.6±4.6Ma,代表了2期岩浆活动的年龄,是平岗盆地基底早三叠世和中侏罗世岩浆活动的表现;后者的锆石^(206)Pb/^(238)U年龄为117.0±0.9Ma,表明流纹岩形成于早白垩世。德仁组火山岩富集大离子亲石元素K、Rb、Ba,亏损高场强元素Nb、Ta、Zr、Hf、Y、Yb;富集轻稀土元素,亏损重稀土元素,稀土元素配分曲线呈右倾型,表现出壳源特征。德仁组火山岩形成于活动大陆边缘火山弧环境,原始岩浆来源于盆地基底岩浆岩重熔。研究区在117~135Ma期间,发生了由正地形向负地形的重大转变,区域上发育伸展盆地和变质核杂岩,并在基底岩石重熔过程中受到地幔混染作用,表明研究区在早白垩世发生了岩石圈减薄,且减薄的起始时间早于117.0±0.9Ma,减薄机制为拆沉作用。

This paper presents detailed petrologic, geochemical and geochronologic data of the volcanic rocks in Deren Formation of Pinggang Basin, northern Liaoning Province. The authors investigated their petrogenesis, formation age and tectonic settings and discussed the thinning time and mechanism of North China Craton. The LA-ICP-MS zircon U-Pb dating was conducted on dacitic breccia-crystal-lithic tuff in Deren Formation and the upper rhyolite. The former yielded^(206)Pb/^(238)U ages of 246.8±3.5 Ma and 169.6±4.6 Ma which are supposed to represent two stages of magmatism, reflecting the Early Triassic and Middle Cretaceous magma activities in the basement of Pinggang Basin. The latter yielded^(206)Pb/^(238)U age of 117.0±0.9 Ma, which indicates that the rhyolites were formed in the Early Cretaceous. Volcanic rocks in Deren Formation are enriched in large ion lithophile elements(e.g., K, Rb, Ba)but are depleted in high field strength elements(e.g., Nb, Ta, Zr, Hf, Y, Yb). Besides, due to enrichment of LREEs and depletion ofHREEs, the REE patterns exhibit right-inclined feature. These geochemical characteristics show that the source of the primitive magma was derived from the crust, and they were probably formed at the setting of volcanic arc along the active continental margins,resulting from remelting of the basement magma rocks. Major changes from positive to negative landform in the study area occurred during 117~135 Ma. In combination with wide distribution of extensional basins and metamorphic core complexes, the authors hold that the magma might have been affected by mantle contamination during remelting of these basement rocks, and the thinning of lithosphere might have occurred during Early Cretaceous(earlier than 117±1.2 Ma), which resulted in the delamination.