关键词: 黄河, 钾长石, Pb同位素, 物源示踪

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