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天水-秦安地区保存有多种成因类型的新生代沉积物,其物质来源及其变化对于研究青藏高原东北部的隆升剥蚀历史、构造变形方式以及与之相关的地貌演化过程具有重要意义.本文通过碎屑锆石U-Pb年龄物源示踪方法,对该区的古近纪洪积砾岩、新近纪河流和湖相沉积进行了分析,并与中新世以来的风尘堆积进行了对比.结果显示:1)古近纪洪积砾岩中包含大量亲扬子地块的560~1100Ma的锆石颗粒,但这一年龄的锆石颗粒在早中新世河流相沉积中显著减少,同时出现了大量200~360Ma的锆石颗粒,指示着古近纪洪积砾岩主要来自西秦岭的中部和/或南部,而早中新世河流相沉积主要来自西秦岭北部;2)约11.5Ma以来,该区河流相沉积以380~450Ma的锆石颗粒占主导,与六盘山南部岩体的年龄一致,指示着六盘山南部的初始隆升;3)该区晚中新世湖相沉积的锆石年龄分布明显不同于同时期的河流沉积物,但与秦安中新世风成红土、晚中新世-上新世三趾马红土及第四纪黄土十分相似,指示着该区中新世的细颗粒水成沉积物很可能主要为风尘物质.本文的研究揭示出天水-秦安地区新生代沉积物的物源转变和地貌演化均与青藏高原东北部的阶段性隆升密切相关,特别是晚渐新世-早中新世青藏高原北部的隆升,可能既为中新世风成红土的出现创造了物源和风动力条件,也为其堆积创造了地貌条件.
The Cenozoic sediments with many genetic types preserved in Tianshui-Qin’an area are of great significance for studying the history of uplifting and denudation, the tectonic deformation and the related evolution processes in northeastern Qinghai-Tibet Plateau. Based on the U-Pb zircon dating from detrital zircons, the sediments of the late Cretaceous, the Cretaceous, and the lacustrine sediments in the Paleogene have been analyzed and compared with the dust accumulation since the Miocene The results show that: (1) There are 560 ~ 1100 Ma zircons in the Paleogene Hongji conglomerate with a large amount of 560 ~ 1100 Ma zircons. However, this age of zircon grains significantly decreased during the deposition of the Early Miocene river facies. At the same time, a large number of 200 ~ 360Ma zircons, indicating that the Paleogene Hongji conglomerate mainly came from the central and / or southern part of the West Qinling, while the Early Miocene river sediments mainly came from the northern part of the West Qinling. 2) Since about 11.5Ma, the river facies The zircons dominated at 380-450 Ma and were consistent with the age of the southern Liupanshan rock, indicating the initial uplifting in southern Liupanshan. 3) The age distribution of the late Miocene lacustrine sediments was significantly different In the same period The river sediments, however, are similar to the Qinan apomitic boulders, the Late Miocene-Pliocene marblite and Quaternary loess, indicating that the Miocene fine-grained sediments of the area are likely to be predominant Which is dusty matter.The research of this paper reveals that the source and the evolution of the Cenozoic sediments in the Tianshui-Qin’an area are closely related to the stage uplift of the northeastern Qinghai-Xizang Plateau, especially the Late Oligocene-Early Miocene The uplift of the northern plateau may have created both the provenance and the wind-driven conditions for the advent of Miocene-derived laterites, as well as the geomorphological conditions for their accumulation.