物源—河流过程—化学风化对松花江水系沉积物重矿物组成的影响

松花江水系作为我国七大水系之一,对其沉积物组成的深入探究对揭示源区控制因素和沉积物的搬运—沉积过程具有重要意义。重矿物蕴含着源岩母岩的重要信息,是解开由源到汇过程和物源示踪的重要工具。为了评估物源、河流过程和化学风化对重矿物组成的影响,我们从松花江水系干流和支流的边滩以及阶地共取32个样品,进行分粒级(<63μm、63~125μm和125~250μm)的重矿物分析。结果表明,松花江水系源区母岩信息在嫩江各支流(诺敏河最为典型)的重矿物组成中得到很好的反映,但在嫩江干流中没有得到反映,这表明物源对重矿物组成的控制受到河流过程的影响。松花江水系的重矿物组成以角闪石、绿帘石、钛铁矿和榍石为主。河流沉积物的重矿物组成主要富集在63~125μm组分,同一河流的不同河段的重矿物组成存在显著差异(巴兰河尤为典型),表明了河流搬运—沉积过程对重矿物组成起到重要控制。哈尔滨松花江T2阶地沉积物(弱风化)的重矿物组成基本保留了现代河流砂特征,讷谟尔河T1阶地沉积物(中等风化)的重矿物受到一定程度的改造,而受到严重化学风化影响的通河松花江T3阶地的重矿物组成已遭受严重破坏,不稳定矿物(角闪石和辉石)以及蕴含的母岩信息已完全消失,表明了重矿物组成明显受到沉积物化学风化程度的影响。由于源—汇过程中重矿物的混合和由此导致的稀释作用,使得沉积物携带的物源信息经过长距离搬运之后逐渐变得模糊。因此,我们认为,在进行河流重矿物源—汇过程研究中,宽粒度分析窗口和足够多的样品数量需要考虑以充分获取源区完整的重矿物组成信息。同时,在利用河流阶地重矿物组成进行源—汇联系和古水系演化研究时,需要首先评估阶地沉积物的化学风化程度。

Influence of provenance-river process-chemical weathering on heavy mineral composition of the Songhua River sediment

The Songhua River system is one of the seven major river systems in China, the study of its sediment composition is of great significance to reveal the source control factors and sediment transport-sediment process. Heavy minerals contain important information about the source rock parent rock, and are an important tool for unraveling the process from source to sink and traceability of provenance. In order to assess the influence of provenance, river processes and chemical weathering on the composition of heavy minerals, we collected 32 samples from the sediments of the main stream and tributaries of the Songhua River system(<63 μm, 63~125 μm and 125~250 μm)heavy mineral analysis. The results show that the parent rock information of Songhua River system source region is well reflected in the heavy mineral composition of the tributaries of the Nenjiang River(the Nuomin River is the most typical), but it is not reflected in the main stream of the Nenjiang River. This indicates that the source control of heavy mineral composition is affected by the river process influence. The heavy mineral composition of the Songhua River system is dominated by hornblende, epidote, ilmenite and sphene. The heavy mineral composition of river sediments is mainly enriched in 63~125 μm. There are significant differences in the heavy mineral composition of different sections of the same river(the Balan River is particularly typical),indicating that the river transport-deposition process has an important control effect on the composition of heavy minerals. The heavy mineral composition of T2 terrace sediments(weakly weathered)in the Songhua River in Harbin basically retains the characteristics of modern river sand,and the heavy minerals of the T1 terrace sediments(medium weathering)of the Nemor River have been modified to a certain extent and are severely chemically weathered. The heavy mineral composition of the affected T3 terrace of Tonghe Songhua River has been severely damaged. The unstable minerals(hornblende and pyroxene)and the contained parent rock information have completely disappeared, indicating that the heavy mineral composition is obviously affected by the chemical weathering degree of sediment. Due to the mixing of heavy minerals in the source-sink process and the resulting dilution, the provenance information carried by the sediments gradually becomes blurred after long-distance transportation. Therefore, we believe that in the study of the source-sink process of heavy minerals in rivers, a wide particle size analysis window and a sufficient number of samples need to be considered to fully obtain complete heavy mineral composition information in the source area. At the same time, when using the heavy mineral composition of river terraces to study the source-sink relationship and the evolution of paleo-water systems, it is necessary to first evaluate the degree of chemical weathering of terrace sediments.