北祁连边麻沟粉砂岩碎屑锆石U-Pb年龄及Hf同位素特征

北祁连走廊南山边麻沟-大岔大坂一带保存了记录新元古代-晚古生代地质演化的众多构造-岩石单元。在边麻沟柴达诺岛弧花岗岩体之上, 不整合覆盖了一套由硅泥质岩、粉砂岩、杂砂岩、砾岩共同组成的浊流沉积组合, 其沉积时代缺乏可靠证据, 沉积物源区和构造属性也缺乏研究。粉砂岩中碎屑锆石具有典型岩浆锆石结构特征, 主要集中于529~484 Ma和466~442 Ma两个年龄群, 最年轻碎屑锆石为425 Ma。结合锆石Hf同位素特征和区域已有同位素年代学资料, 认为这些粉砂岩中的碎屑锆石主要来自柴达诺花岗岩体(516~505 Ma), 其次来自北祁连奥陶-志留纪岛弧中酸性岩浆岩, 少数来自寒武纪蛇绿岩及中元古代基底岩石。综合研究分析认为, 边麻沟浊积岩可能形成于志留纪弧前残余海盆环境。

U-Pb ages and Hf isotopes of detrital zircons from siltstone,Bianmagou, North Qilian Mountains

Multiple rock units, including the Neo-Proterozoic basement, Early Paleozoic ophiolites, arc granites and volcanics, and Late Paleozoic sediments, are preserved in the Bianmagou-Dachadaban area of Zoulang Nanshan, North Qilian, providing important records of the Neo-Proterozoic to Late Paleozoic geological evolution of the orogen. In Bianmagou a suite of gray-green turbiditic rocks composed of silicious mudstone, siltstone, greywacke, and conglomerate are covered unconformably on the Chaidanuo granite batholith, while the age, sedimentary sources and tectonic significance of the tuibidites are still unclear. Most detrital zircons from the siltstones are magmatic and lack of abrading to roundness by transport, and were formed during the time periods of 529~484 Ma and 466~442 Ma, respectively. The youngest zircon grain was formed at 425 Ma or in Meso-Proterozoic. It is likely that the detrital zircons were derived mainly from the Chaidanuo granitoids(516~505 Ma), subordinately from the Ordovician-Silurian intermediate-felsic magmatic rocks, and rarely from the Paleozoic ophiolites and Meso-Proterozoic basement rocks. Based on rock association of the turbidites, age distribution of detrital zircons from the siltstone, as well as evidences from former studies, it is proposed that the Bianmagou turbidites were mostly likely formed in a relic forearc basin of Silurian.