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武当群变质沉积-火山岩组合是南秦岭地体中重要的基底岩石,其形成时代和地球化学特征可以为理解秦岭造山带的构造演化提供重要的证据。本文报道豫西地区武当群上部沉积岩和下部中-酸性火山岩Sm-Nd同位素和锆石Lu-Hf同位素组成,探讨火山岩成因和沉积岩物源的同位素特征。上部沉积岩的碎屑锆石初始ε_(Hf)值变化在-30～+10之间,对应的模式年龄值t_(DM2)在1.0Ga至3.2Ga之间,初始ε_(Nd)值在-4.0至-6.0之间。沉积物源表现为主要与扬子陆块有亲缘关系的地壳物质和近源的下部火山岩混合的特征。火山岩的锆石初始ε_(Hf)值变化在-35～+15之间,对应的模式年龄值t_(DM2)在0.8Ga至3.5Ga之间,集中于1.5～1.8Ga和2.2～2.4Ga两个峰值。2个变质石英角斑岩样品初始ε_(Nd)值分别为-9.2和-10.7,而报道的湖北武当群的玄武-安山质熔岩的初始ε_(Nd)值以正值为主。因此,武当群不同类型的火山岩可能存在着成因差异。具有低初始ε_(Nd)值和ε_(Hf)值特征的火山岩可能由地壳物质的重熔而形成的;有些火山岩具有初始ε_(Hf)值变化范围较大(-35～+15)或正初始ε_(Nd)值的特点,可能是壳、幔物质混合成因,有显著的幔源或新生地壳物质的贡献。武当群Nd-Hf同位素组成和碎屑锆石年龄分布特征表明,与扬子陆块有亲缘关系的南秦岭地体在元古代期间可能经历多期地壳增生和再造作用。
The metamorphic sedimentary-volcanic assemblage of Wudang Group is an important basement rock in the South Qinling terrane. The formation age and geochemical characteristics of the Wudang Group provide important evidences for understanding the tectonic evolution of the Qinling orogenic belt. This paper reports the Sm-Nd isotopic composition of zircons and the Sm-Nd isotopic composition of the upper and lower middle-acidic volcanic rocks in western Wudang area, and discusses the origin of volcanic rocks and the isotopic characteristics of sedimentary rocks. The initial densities of detrital zircons in the upper sedimentary rocks varied from -30 to +10. The corresponding model ages t_ (DM2) ranged from 1.0Ga to 3.2Ga, and the initial ε_ (Nd) values ranged from -4.0 To -6.0 between. Sediment sources are characterized by a mixture of crustal material mainly associated with the Yangtze block and lower volcanic rocks of near origin. The initial ε_ (Hf) values of volcanic rocks vary from -35 to +15, and the corresponding model ages t_ (DM2) range from 0.8Ga to 3.5Ga, and are concentrated in 1.5 ~ 1.8Ga and 2.2 ~ 2.4Ga A peak. The initial ε_ (Nd) values of the two metamorphic quartz porphyry samples are -9.2 and -10.7, respectively, while the reported initial ε_ (Nd) values of the basaltic-andesitic lavas of the Wudang Group in Hubei Province are positive. Therefore, different types of volcanic rocks in the Wudang Group may have genetic differences. Volcanic rocks with low initial ε_ (Nd) and ε_ (Hf) values may be formed by the remelting of crustal material. Some volcanic rocks have a large range of initial ε_ (Hf) values (-35 ~ +15) or positive The characteristics of initial ε_ (Nd) values may be due to the mixed genesis of crust and mantle material with significant contributions from mantle or new crustal material. The Nd-Hf isotopic compositions and detrital zircon age distributions of the Wudang group indicate that the South Qinling terrain, which is related to the Yangtze block, may undergo multi-stage crustal hyperplasia and remolding during the Proterozoic.