豫西地区秦岭造山带武当群Nd-Hf同位素组成及其物源特征

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εHf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.

Zircon Hf isotopic composition and source characteristics of the Wudang Group in the Qinling orogenic belt, western Henan province

The metamorphosed volcanic-sedimentary sequences of the Wudang Group belong to an important rock unit of the basement in the South Qinling block.Formation ages and geochemical features of the Wuclang Group can provide essential information for understanding of the tectonic evolution of the Qinling orogenic belt.In this study,we report whole-rock Sm-Nd and zircon Lu-Hf isotopic composition of sedimentary rocks in the upper part and volcanic rocks in the lower part of the Wudang Group,and provide discussion on petrogenesis of the volcanic rocks and source characteristics of the sedimentary rocks.Zircon grains from the Wudang sedimentary rocks give initial δ_(Hf) values ranging from-30 to+10,corresponding to model age values(t_(DM2))between about 1.0Ga and 3.2Ga,initial δ_(Nd) values of whole-rocks vary from-4.0 to-6.0.The sedimentary sources originated from basement rocks of a Yangtze-affinity and volcanic rocks of the lower part of the Wudang Group.Initial δ_(Hf) values of zircon grains from the volcanic rocks vary from-35 to+15,corresponding to t_(DM2) values of about 0.8Ga to 3.5Ga,but clustering around 1.5～1.8Ga and 2.2～2.4Ga.Initial ε_(Nd) values of two quartz-keratophyre samples of volcanic rock are-9.2 and-10.7,different from the reported values for the Wudang basaltic-andesitic rocks exposed in Hubei Province,where most of the volcanic rocks have positive εNd values.Therefore,different origins can be proposed for different volcanic rock types of the Wudang Group.Those volcanic rocks characterized by low initial ε_(Nd) and ε_(Hf) values originated mainly from re-melting crustal material,while others have a broad range of initiai ε_(Hf) values(-35 to+15)or positive initial ε_(Nd) values,which indicate significant contribution of mantle material or juvenile crustal rocks together with old crustal rocks to the magma(s)of the Wudang volcanic rocks.Evidence of Nd-Hf isotopic compositions and detrital zircon age patterns of the Wudang Group suggests that the South Qinling terrane of Yangtze-affinity probably underwent multi-stage history of crustal accretion and reworking during the Proterozoie time.