Abstract:

The collision of the Indian plate with Eurasia in the early Cenozoic era drove the emergence of the Tibetan plateau. At the same time, the subduction of the western Pacific plate towards Eurasia resulted in the stretching and thinning of the lithosphere in eastern Asia, leading to a series of faulted basins and marginal seas. The macro-geomorphic pattern of East Asia was finally established under the control of these two tectonic domains. In this case, the Yellow River, which originated from the Tibetan plateau and flowed through the Loess Plateau and the North China Plain, carried a huge amount of detrital material into the Bohai Sea, which played an important role in the regional geochemical cycle, environmental change, sedimentary flux and the diffusion of detrital material in the shelf sea. Therefore, tracing sediment sources in the Yellow River Basin is of great importance for understanding the coupling relationship between uplift and denudation in the northeastern Tibetan plateau, East Asian monsoon evolution, and detrital material accumulation. However, the Yellow River Basin spans multiple climatic and tectonic zones with different provenance areas, so it is particularly critical to select appropriate provenance tracing methods.

Although K-feldspar is more vulnerable to chemical weathering than zircon, it is a widely distributed rock forming mineral and can best represent the provenance characteristics of a certain area. The non-clay minerals in the Yellow River Basin are mainly composed of quartz and feldspar. At the same time, the Pb isotope ratios(²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb)of K-feldspar in different blocks are much different from those of Nd and Sr isotope systems and are often used to construct regional Pb isotope geochemistry province, continental crust evolution, and reconstruct paleocurrent direction, etc. In recent years, detrital K-feldspar Pb isotopic composition has been successfully used to trace the provenance of the Indus, Yangtze and Mississippi Rivers. But this method has not been carried out in the Yellow River Basin. Therefore, we systematically analyzed the detailed K-feldspar Pb isotopic compositions from the Yellow River Basin, and compared the results with the potential source areas to determine the specific source areas. It also can provide basic comparative data for future studies on the formation age of the Yellow River and material source areas of the Loess Plateau and deserts in the northwestern China.

We analyzed 15 samples from the Yellow River Basin and obtained 967 in-situ Pb isotopic results of K-feldspar grains by laser erosion inductively coupled plasma mass spectrometer(LA-MC-ICP-MS). K-feldspar grains in the samples from the Yellow River are angular, subangular and subcircular, with diameters ranging from 20μm to 300μm. The ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios of K-feldspar grains from the source of the Yellow River to Lanzhou city range from 20 to 16 and 42 to 36. However, some ratios of ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb of K-feldspar grains from the Lanzhou city range from 23 to 19 and 40 to 37, respectively. The ²⁰⁶Pb/²⁰⁴Pb ratio of most K-feldspar samples in Bayannur city is greater than 19, and the maximum value is 24.79, while this ratio from Hequ and Hancheng cities located in the middle reaches of the Yellow River is less than 18.5. The ²⁰⁶Pb/²⁰⁴Pb ratios of the Mesozoic sandstone near the Hequ city range from 16 to 15. The ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios of K-feldspar grains from the Weihe River, which is the largest tributary of the Yellow River, range from 19 to 17 and 40 to 37. The ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios of K-feldspar grains in the Fenhe River, Yiluohe River, Kaifeng and Lijin cities range from 21 to 14 and 42 to 33. The comparison results of ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios show that the Pb isotopic compositions of K-feldspar grains in the upper Yellow River, Daxiahe River and Huangshui River are significantly different from those in the Lanzhou city. The Pb isotopic composition of K-feldspar grains from the Yellow River from the Lanzhou city is consistent with that in the Bayannur city, which is influenced by similar eolian provenance. K-feldspar grains from the Yellow River and Fen River in the Jinshan Gorge are mainly from the Loess Plateau. By contrast, the K-feldspar grains in the Weihe River are mainly derived from the Qinling Mountains. The Pb isotopic compositions of K-feldspar grains in the Kaifeng and Lijin cities of the lower Yellow River are different to those in the upper Yellow River and the North China Plate, but similar to those in the middle reaches of the Yellow River. The Loess Plateau plays a leading role in the source of K-feldspar gains in the middle and lower reaches of the Yellow River.

Key words: Yellow River, K-feldspar, Pb isotope, provenance

摘要：

示踪黄河流域的泥沙来源, 对于认识和理解青藏高原隆升剥蚀和西太平洋边缘海沉积之间的耦合关系至关重要。钾长石是河流沉积物中常见的造岩矿物之一, 其铅(Pb)同位素比值在应用于大河物源示踪研究时效果良好, 但这一研究在黄河流域还未开展。文中利用激光剥蚀电感耦合等离子质谱仪(LA-MC-ICP-MS)对黄河流域的15件样品进行分析, 获得了967颗钾长石的原位Pb同位素结果。²⁰⁶Pb/²⁰⁴Pb和 ²⁰⁸Pb/²⁰⁴Pb比值二维散点图和多维判别图(MDS)结果表明, 黄河玛多-同德段、 大夏河和湟水的钾长石Pb同位素组成与黄河兰州段存在明显差异; 黄河兰州段的钾长石Pb同位素组成与黄河巴彦淖尔段一致, 二者受相似的风成物源区的影响; 黄河晋陕峡谷段、 汾河的钾长石主要来自黄土高原; 渭河的钾长石主要来自秦岭。黄河开封和利津段的钾长石Pb同位素组成一致, 都与黄河上游和华北板块明显不同, 但与黄河中游相似。黄土高原对黄河中、 下游的钾长石来源起主导作用。

关键词: 黄河, 钾长石, Pb同位素, 物源示踪