北秦岭灵官庙盆地砂岩碎屑锆石LA-ICP-MSU-Pb年龄

秦岭造山带在印支造山作用主造山期之后,发育一系列晚三叠世—早侏罗世伸展断陷盆地。选择陕西丹凤北秦岭灵官庙盆地中低绿片岩相的灰绿色中细粒砂岩,进行碎屑锆石LA-ICP-MS U-Pb同位素分析。结果表明,85个以岩浆成因为主的碎屑锆石给出5个年龄组:426~605Ma、661~1174Ma、1494~1789Ma、2113~2261Ma和2826~3298Ma。其中,最年轻的锆石年龄为426Ma±5Ma,最老的锆石年龄为3298Ma±70Ma。主要的年龄峰期为426~605Ma年龄组,其次为661~1174Ma年龄组。这一年龄结构反映出物源区加里东造山期和Grenvillian期岩浆活动的重要影响。年龄谱中缺少中志留世—晚三叠世的锆石,说明普遍发育的印支期岩浆岩体并不构成物源区。通过与现今周缘地体年龄结构对比并结合其他地质证据认为,灵官庙盆地的物源可能主要来自加里东期侵位于二郎坪群、秦岭群中的花岗岩和新元古代宽坪群,少量来自二郎坪群和秦岭群。灵官庙盆地形成之后,盆地内沉积岩遭受了变质变形作用,记录了秦岭造山带在板块构造驱动的造山作用结束后进入了新的陆内造山作用阶段。

LA-ICP-MS U-Pb dating of detrital zircon from Lingguanmiao basin in North Qinling

A number of T3-J1 rift basins were developed in the Qinling orogen after the main orogenic episode of the Indosinian orogeny. Selecting grayish green and moderate-fine grained sandstone metamorphosed to low-greenschist facies from the Lingguanmiao basin of the North Qinling orogenic belt in Danfeng County of Shaanxi Province and conducting LA-ICP-MS detrital zircon U-Pb isotope analysis, the authors attempted to establish the age structure of the sedimentary rocks in the basin and infer the provenance and its variation. The result reveals that the 85 analytical points of the magma-genetic detrital zircons yielded five age groups: 426~605Ma, 661~1174Ma, 1494~1789Ma, 2113~2261Ma and 2826~3298Ma. Among the ages, the youngest age is 426Ma±5Ma, and the oldest is 3298Ma±70Ma. The major age groups are 426~605Ma and 661~1174Ma. The age structure reflects the tectono-magmatic events occurring in the source region during the Caledonian and Grenvillian orogenic periods, respectively. In addition, the lack of the ages from the Middle Silurian to Late Triassic can be attributed to the assumption that the widespread Indosinian magmatic bodies failed to make contribution to the source due to their poor development in North Qinling. In comparison with the age data from the surrounding terranes and in conjuction with other geological evidence, the authors hold that the provenance of the Lingguanmiao basin might mainly have come from the Neoproterozoic Kuanping Group and the Caledonian granites which had intruded into Erlangping and Qinling Groups, with the addition of a small amount of material from the Paleozoic Erlangping and Proterozoic Qinling complex. After the formation of Lingguanmiao basin, the sedimentary rocks in the basin suffered strong metamorphism and deformation, which might record the new intracontinental orogeny that Qinling experienced since the ending of the Indosinian orogeny set forth by plate tectonics.