摘要:
通过阿拉善北部杭乌拉地区圆包山组碎屑锆石峰值年龄(420±3 Ma和419.6±3.6 Ma)以及圆包山组底部层位发育的笔石、头足类化石,认为圆包山组形成时代并非早志留世,而是泥盆纪.圆包山组中发育的大量沉积构造及其岩性特征,表明了圆包山组为典型的半深海—深海浊积岩沉积,但鲍马序列发育不完整,主要有AE、ABE、ADE和ABDE等序列组合,并划分了内扇、中扇和外扇三个沉积亚相.研究区位于阿拉善地区北部,属于西伯利亚板块南缘部分,是研究古亚洲洋闭合时间的关键部位.早古生代硅质岩岩石特征表明,研究区在早古生代处于一个被动大陆边缘的稳定大地构造背景.自晚古生代开始,研究区的大地构造活动性开始增强,逐渐由被动大陆边缘转变为活动大陆边缘,并形成了巨厚的浊积岩.根据浊积岩中槽模、沟模等沉积构造确定了物源区为研究区北部的西伯利亚板块,并根据浊积岩岩性、沉积特征以及砂岩地球化学特征的研究,认为研究区晚古生代早期的浊积岩形成于西伯利亚板块南缘的活动大陆边缘,并同时受岛弧作用的影响,导致浊积岩具有岛弧物源区属性.结合圆包山组中碎屑锆石特征及其年龄可以进一步确定圆包山组物源区位于研究区北部Gobi-Altai构造带.
Abstract:
Through the peak age of detrital zircons(420±3 Ma and 419.6±3.6 Ma) and the fossils of graptolites and cephalopods in the Yuanbaoshan Formation in Hangwula area, the north of Alxa, the conclusion is that the age of Yuanbaoshan Formation is Devonian, not Early Silurian. The information of lithology, sediment structures of Yuanbaoshan Formation shows that the Yuanbaoshan Formation is bathyal- pelagic deposit, but the Bouma sequence is not complete, including the ae, abe, ade, abde and other combinations, and the Formation is divided into three sedimentary subfacies which are inner fan, middle fan and outer fan. The study area, located in the north of Alxa region which belongs to the southern margin of the Siberian plate, is the key part of the study to the closing time of Paleo-Asian Ocean. The characteristics of Early Paleozoic cherts indicate that the tectonic setting of the study area is a stable passive continental margin in the early Paleozoic. Since Late Paleozoic, the tectonic activity in the study area began to increase, and gradually the tectonic setting transformed from a passive continental margin to an active continental margin, to form a thick turbidite. Source area is the Siberian plate in the north of the study area, according to the flute cast and gloove cast in the turbidite. And according to the study of lithology, sedimentary characteristics and geochemical characteristics of the turbidite, the tubidite in the study area formed in the continental margin in the southern margin of the Siberian plate, and also is affected by the island arc, resulting in the turbidite with island arc attributes. It should be noted that the peak age of detrital zircons of the turbidite is in accord with the age of island arc diorite in the Sunite Left Banner, therefore, the subduction from Tarim plate(or the oceanic crust of Paleo-Asian Ocean) to the Siberia plate began in 420 Ma.
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