北祁连东段景泰地区下古生界两套砂岩微量元素和稀土元素特征及其构造意义

Two suits of sandstones in the Lower Paleozoic at Jingtai,eastern North Qilian:Trace and rare earth elements characteristics and tectonic setting

早奥陶世和早志留世是北祁连加里东造山带构造演化和盆地转变的关键时期。在造山带东段景泰地区,下奥陶统阴沟组和下志留统肮脏沟组两套砂岩的微量元素和稀土元素特征显示,阴沟组杂砂岩样品(Cj-1和Cj-3)具有最小的Eu/Eu*及最大的Th/Sc和∑REE,肮脏沟组杂砂岩具有较小的Eu/Eu*和较大的Th/Sc及∑REE;阴沟组岩屑砂岩样品(Cj-13、Cj-15和Cj-18)具有最大的Eu/Eu*及最小的Th/Sc、∑REE和La/Yb。多个物源、构造背景判别图解和多元素蛛网图分析表明,阴沟组杂砂岩样品具大陆边缘的构造背景,主要物源为大陆上地壳再旋回沉积物和长英质岩石;岩屑砂岩样品为岛弧构造背景,以中基性安山质岩石为主要物源,可能受陆源物质的微弱影响。肮脏沟组杂砂岩构造背景复杂,表现出大陆岛弧、活动陆缘和被动陆缘三种环境共存的特点,受中基性火山弧物质、长英质岩石和再旋回沉积岩的混合物源的影响。两套砂岩的元素特征表明二者可能具有相似的源区。阴沟组杂砂岩源区可能为阿拉善地块南缘海原群变沉积岩或其他相似的陆源再旋回沉积物,砂岩碎屑以来自初始火山弧物质为主,以石灰沟岛弧型中基性火山岩作为其源岩最合适。阴沟组形成于初始弧后盆地环境,是岛弧活动的直接记录。肮脏沟组可能的源岩为阿拉善地块南缘海原群变沉积岩和中高等成熟度的石灰沟岛弧型火山岩及海原群岛弧型变火山岩,沉积于弧后前陆盆地,对构造环境的反映存在滞后性。

Early Ordovician and Early basin in the North Qilian Caledonian orogenic Silurian are the key periods for the evolution of tectonics and belts. The characteristics of trace and rare earth elements for the sandstones from the Lower Ordovician Yingou Formation and Lower Silurian Angzanggou Formation in Jingtai area were presented. The greywacke samples （Cj-1 and Cj-3） from the Yingou Formation are characterized by minimal Eu/Eu^＊ and maximal values of Th/Sc and ∑REE. In contrast, the lithic sandstone samples （Cj-13, Cj-15 and Cj-18） from the Yingou Formation, possess maximal Eu/Eu^＊ , minimal values of Th/Sc, ∑REE and La/Yb, and the greywacke samples, of Angzanggou Formation have intermediate values of Eu/Eu^＊ , Th/Sc and ∑REE. Employing diverse plots of provenance and tectonic setting discrimination and multi-element diagrams, this study shows that （1） the lower part of the Yingou Formation, represented by the greywacke samples of Cj-1 and Cj-3, was mainly derived from the recycled deposits and felsic sources in the upper continental crust with the tectonic setting being the continental margin and the middle-upper part of the Yingou Formation, represented by the lithic sandstone samples of Cj-13, Cj-15 and Cj-18, from the mafic-intermediate andesitic sources in an island arc environment, with a probable tiny supply from continental margin materials; （2） with regard to the Angzanggou Formation, it was deposited in a complex tectonic setting where the continental island arc, active continental margin and passive continental margin coexisted. The sediments were derived from mixed sources of mafic-intermediate volcanic arc, felsic rocks and recycled sedimentary rocks. The two suits of sandstones may be sourced from similar sources. The constraining on the sources of sandstones shows that （1） the potential source for furthermore, the greywacke samples were probably derived from the metasedimentary rocks of the Haiyuan Group lying in the southern margin of Alashan Block and other recycled terrigenous sediments with a comparable property to the former. The lithic fragments in the lithic sandstone samples are dominated by volcanic island arc materials and they are comparable with the mafic-intermediate volcanic rocks in the early stage of the Shihuigou Arc, which suggests that the Yingou Formation was deposited in an incipient back-arc basin mirroring the action within the island arc; （2） the sediments in the Angzanggou Formation were probably sourced from the metasedimentary rocks of Haiyuan Group in the south margin of Alashan Block, the island arc volcanic rocks with moderate-high maturity in Shihuigou area and the island arc metavolcanic rocks of the Haiyuan Group. The Angzanggou Formation was deposited in a retro-arc basin, reflecting the tectonic setting with a hysteresis.