1、天水盆地新近紀沉積演化與地化分析        【中文摘要】天水盆地為青躲高原東北緣山前盆地,是隴中盆地的東南組成部分,其西部邊沿為祁連-秦嶺褶皺帶,東部邊沿為海源斷裂-六盤山褶皺帶,北部邊沿為華家嶺,基底為祁連山加里東褶皺帶,為一近NNW向擠壓盆地。西秦嶺北緣斷裂系穿盆而過,控制著天水盆地的形成和演化。本文根據沉積物巖性、粒度、沉積結構和構造特點,結合沉積相分析原理,對甘泉和堯店剖面新近系地層進行初步沉積相分析,主要劃分為河流沉積體系、扇三角洲沉積體系、湖泊沉積體系。并根據盆地沉積特征、巖層接觸關系及巖漿活動為標

2、志,揭示了天水盆地構造沉積演化的6個階段:古近紀盆地受擠壓在低洼的地方沉積了麥積山組;22Ma左右盆地再次受到擠壓抬升和斷陷使早期麥積山組地層發生變形剝蝕,此后沉積了區內廣泛分布的甘泉組地層;12Ma左右構造活動使盆地持續凹陷,在整個隴中盆地沉積了“斑馬層”堯店組沉積;7.4Ma左右盆地開始萎縮,在區內沉積了廣泛分布的湖灘泥坪相的楊集寨組;4.07-3.6Ma構造活動使盆地開始消亡,大部分地區開始遭受剝蝕,3.6Ma以來盆地再次斷陷接受喇嘛山組的沉積,第四紀構造抬升形成現在的地貌格式;另一方面,通過新近系碎屑沉積物地化特征研究表明:下山剖面泥巖富MgO、CaO、Nb、Sr、Th,貧SiO_2、

3、Al_2O_3、Ba、Cr、Ni、Rb,高Al_2O_3/TiO_2、Cr/Zr、Cr/Th、Th/Sc,根據稀土元素和特征元素的含量,初步以為盆地南區西秦嶺北緣各時代花崗質閃長巖和偏中性巖控制著該區的沉積物地球化學組成,而古秦嶺洋閉合期蛇綠巖巖套中的基性超基性巖有很大的影響,其結果是它們高度混合的物質給盆地提供物源,泥巖樣品中K_2O的含量隨時間增長而增高,說明物源區在溯源腐蝕的過程中富鉀值巖大量出現;CIA和ICV值指示天水盆地碎屑沉積物的物源區中新世風化程度較強,源區沉積再循環的物質很少或幾乎不存在,而上新世到更新世之間存在風化作用減弱的趨勢。這些研究不僅天水盆地新生代沉積演化的研究有一

4、定的參考價值,而且對青躲高原東北部構造隆升過程以及內陸干旱化和東亞季風形成和演化研究提供基礎資料。');【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