天水地区新近纪沉积物地球化学特征及其意义

GEOCHEMISTRY OF NEOGENE SEDIMENTARY ROCKS IN TIANSHUI BASIN AND TECTONIC-WEATHERING PROCESS OF ITS SOURCE

通过对天水盆地新近纪沉积物地化特征的研究表明:(1)天水盆地下山剖面泥岩富MgO、CaO、Nb、Sr、Th,贫SiO2、Al2O3、Ba、Cr、Ni、Rb,高Al2O3/TiO2、Cr/Zr、Cr/Th、Th/Sc,结合盆地的平面展布特征和地层厚度变化规律及沉积相的横向变化,初步认为盆地南区西秦岭北缘各时代花岗闪长岩和偏中性岩主要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K2O的含量随时间增长而增高,说明物源区在溯源侵蚀的过程中富钾值岩大量出现;(2)CIA和ICV值指示天水盆地碎屑沉积物的物源区古风化程度较强,沉积物成熟度较低,源区沉积再循环的物质很少或几乎不存在,在上新世到更新世之间存在化学风化作用减弱的趋势;(3)天水盆地从西秦岭山麓到华家岭地层沉积厚度越来越薄,从湖相逐渐过渡为湖泛平原相,结合主、微量元素构造判别图解,认为下山剖面沉积物是在压扭作用下的山前凹陷盆地构造环境中形成。

This research paper presents the geochemical characteristics of neogene clastic sedimentary rocks in the Tianhui Basin. Systematic research leads to the following conclusion： （1） MgO, CaO, Nb, Sr, Th are rich and SiO2, Al2O3, Ba, Cr, Ni, Rb are poor in the samples, the ratios of Al2O3/TiO2, Cr/Zr, Cr/Th and Th/Sc in the samples are also very high. These characteristics indicate that granitic diorite generated in all epoches, which is located in the south of the Tianshui Basin （western margin of Qinling mountain）, controlling the geochemical composition of the main provenance of the Neogene sedimentary rocks. Another minor parent rock is basic-ultra basic igneous rock generated when the ancient Qinling paleoocean closed. The ratio of K2O increased in the Xiashan mud stone at Tianshui with time. It probably resulted from the ancient river that discharged into the Tianshui Basin and K-rich magmatic rocks appeared gradually. （2） CIA and ICV values indicate moderate chemical weathering in the source region and low maturity of sedimentary rocks. Other evidence indicated that the deposition rate is also low and sedimentary recycle products are lack. （3） The strata become more and more thin from the south to the north of the basin, and the sedimentary facies also change from deep lake facies into lacustrine plain. In combination with geochemistry tectonograms, we can infer that the Xiashan mudstone at Tianshui was deposited in a foreland tectonic setting.