天水盆地新近纪沉积演化与地化分析

1、天水盆地新近纪沉积演化与地化分析        【中文摘要】天水盆地为青躲高原东北缘山前盆地,是陇中盆地的东南组成部分,其西部边沿为祁连-秦岭褶皱带,东部边沿为海源断裂-六盘山褶皱带,北部边沿为华家岭,基底为祁连山加里东褶皱带,为一近NNW向挤压盆地。西秦岭北缘断裂系穿盆而过,控制着天水盆地的形成和演化。本文根据沉积物岩性、粒度、沉积结构和构造特点,结合沉积相分析原理,对甘泉和尧店剖面新近系地层进行初步沉积相分析,主要划分为河流沉积体系、扇三角洲沉积体系、湖泊沉积体系。并根据盆地沉积特征、岩层接触关系及岩浆活动为标

2、志,揭示了天水盆地构造沉积演化的6个阶段:古近纪盆地受挤压在低洼的地方沉积了麦积山组;22Ma左右盆地再次受到挤压抬升和断陷使早期麦积山组地层发生变形剥蚀,此后沉积了区内广泛分布的甘泉组地层;12Ma左右构造活动使盆地持续凹陷,在整个陇中盆地沉积了“斑马层”尧店组沉积;7.4Ma左右盆地开始萎缩,在区内沉积了广泛分布的湖滩泥坪相的杨集寨组;4.07-3.6Ma构造活动使盆地开始消亡,大部分地区开始遭受剥蚀,3.6Ma以来盆地再次断陷接受喇嘛山组的沉积,第四纪构造抬升形成现在的地貌格式;另一方面,通过新近系碎屑沉积物地化特征研究表明:下山剖面泥岩富MgO、CaO、Nb、Sr、Th,贫SiO_2、

3、Al_2O_3、Ba、Cr、Ni、Rb,高Al_2O_3/TiO_2、Cr/Zr、Cr/Th、Th/Sc,根据稀土元素和特征元素的含量,初步以为盆地南区西秦岭北缘各时代花岗质闪长岩和偏中性岩控制着该区的沉积物地球化学组成,而古秦岭洋闭合期蛇绿岩岩套中的基性超基性岩有很大的影响,其结果是它们高度混合的物质给盆地提供物源,泥岩样品中K_2O的含量随时间增长而增高,说明物源区在溯源腐蚀的过程中富钾值岩大量出现;CIA和ICV值指示天水盆地碎屑沉积物的物源区中新世风化程度较强,源区沉积再循环的物质很少或几乎不存在,而上新世到更新世之间存在风化作用减弱的趋势。这些研究不仅天水盆地新生代沉积演化的研究有一

4、定的参考价值,而且对青躲高原东北部构造隆升过程以及内陆干旱化和东亚季风形成和演化研究提供基础资料。');【Abstract】 The Northeast margin of Qinghai-Tibetan plateau is consists of a series of mountain chains such as Qilian mountain,west Qinling,Xiangshan, Liupanshan and lots of basins such as Lanzhou,Linxia,Xining,Guide,Tianshui basin.In this * we

5、choise the tianshui basin to study the uplift of the Qinghai-Tibetan plateau.Tianshui basin was surround by three mountain chains which are qilian,west of qinling,liupanshan.In a more large scale it is the jont part of the huabei plate,Yangtze plate and qiangtang geological mass. The north of the ti

6、anshui basin is part of the Ordus and alashan mass,south of the basin is Yangtze palte and southwest is qiangtang mass.Tianshui baisin is also part of the Longzhong basin,Some main fault that across the north margin of west qinling control the basis structure of the basin.According to the rock prope

7、rty,rock granularity,sedimentary structure and texture,we have analysised the profile of the Ganquan,and demarcate the sedimentary face into 3 category,they are fluvial deposit system,alluvial fan deposit system and lacustrin deposit syetem.They present three main steps that Qinghai-Tibetan plateau

8、was deformed by the process of India-Eurasia collision happened during 35-30Ma,15-10Ma,8-6Ma.In orde to fully understand the structure of the basin we have checked 22 profiles across the basin,From the south to north of the basin,the stratm is thinning,it is accordance to the model of forland basin.

9、And the sediments record the uplift of the mountain adjacent the basin,According to the mountain -basin coupling hypothesis,it is the best way to reconstruct the uplift of the Tibetan plateau.This * presents geochemistry data of neogene clastic sedimentary rocks in Tianhui basin, systematic research

10、 leads to the following conclusion:(1)MgO、CaO、Nb、Sr、Th are rich and SiO2、Al2O3、Ba、Cr、Ni、Rb are deplete in the samples,the ratio Al2O3/TiO2、Cr/Zr、Cr/Th、Th/Sc in samples are also very high.These characters indicate that granitic diorite generated in all epoch located at south of Tianshui basin(west ma

11、rgin of Qinling mountain)control the geochemical composition of main provenance of the neogene sedimentary rocks,and another minor parent rock is basic-super basic ignenous rock generated as the ancient Qinling paleoocean closed.The ratio K2O have been increasing in Tianshui Xiashan mud stone as tim

12、e goby is another apparent character,we deem it result from the river that discharge into the basin exhumation K-rich magma rock gradually.(2)Low CIA and high ICV value indicate that weak chemical weathering in sourc region and low maturity of sedimentary rocks, then,with other evidence we can infer

13、 that the deposit rate is also low and Lack of sedimentary recycled product.(3)The thickness of strata become more and more thin from the south to north of the basin,and the sedimentary faces is also change from deep lake faces into lacustrine plain,combine with geochemistry tectonogram,we can infer

14、 Tianshui Xiashan mud stone are deposited in foreland tectonic setting.The geochemical signature of sedimentary rocks is mainly controlled by the parent petrology which generate in different tectonic setting.The early authors erect lots of element tectonic setting discrimination diagrams to infer th

15、e past tectonic activity of surrounding mountains under which sediments formed.Those diagrams have been widely used for twenty years.Studies have been carried to test the reliability of those diagrams.In this *,we surveyed 173 samples from the same sedimentary basin,but in the twelve plot(Al2O3/SiO2、TiO2、K2O/Na2O、Al2O3/(CaO+Na2O)-(Fe2O3+MgO)diagram,K2O/Na2O-SiO2、SiO2/Al2O3-K2O/Na2O、Ti/ZrLa/Sc and Th-Co-Zr/10、La-Th、La/Y-Sc/Cr、Th-Sc-Zr/10、La-Th-Sc diagram)173 samples from the same sedimentary basin fell in different tectonic field.They do not work properly.So we can infer tha