基于人工和TIMA自动化方法的松花江水系重矿物组成:对源-汇物源示踪的指示

Heavy mineral composition of the Songhua River system identified by manual and TIMA automatic methods and implications for provenance tracing

  • 摘要: 松花江水系作为中国东北地区最主要的河流系统,其重矿物组成对于揭示其沉积环境、沉积物源-汇过程和水系演化等地表过程具有重要意义。但目前对松花江水系重矿物组成了解有限,对影响其重矿物组成的控制因素尚不明晰。为此,在松花江干流和10个主要支流的边滩采集河流沙样品,在通河获取2个松花江T3阶地样品,分别采用TESCAN Integrated Mineral Analyzer(TIMA)自动化矿物识别技术和人工方法,对63~250 μm粒级组分进行重矿物分析,旨在给出松花江水系的重矿物组成,并分析物源、河流过程和化学风化等因素对重矿物组成的影响和控制,以及评估人工和TIMA方法在重矿物鉴定中的应用情况。结果表明,发源于不同山岭(大兴安岭、小兴安岭和长白山)的各支流具有显著不同的重矿物组成,角闪石、榍石、绿帘石、白钛石、辉石、钛铁矿和赤、褐铁矿是区分各物源的指示性矿物。源自大兴安岭的嫩江支流具有显著高含量的辉石(诺敏河)、钛铁矿(甘河、多布库尔河和阿伦河)和赤、褐铁矿(甘河、多布库尔河和雅鲁河),显示了基性火成岩物质对重矿物组成的控制,这在现代河床砾石的岩性中得到证明。然而,在松花江干流中,这些重矿物含量显著降低,显示了极弱的大兴安岭源区信息,表明支流物源对重矿物组成的控制受到干流所携带沉积物的削弱,干流物源占据了主导地位。值得指出的是,松花江干流的重矿物组成显示出为拉林河和第二松花江物源的混合,表明干流更多地继承了松嫩平原东南山岭(长白山余脉)的重矿物信息。受到严重化学风化影响的通河T3阶地样品的重矿物组成简单,辉石和角闪石等抗风化能力弱的不稳定重矿物已完全消失,而稳定矿物高度富集,反映了化学风化对重矿物组成起到主要控制作用。相对于传统人工鉴定方法得到的重矿物组成(通常只能鉴定出20余种矿物相),TIMA方法能够给出更多的矿物种类信息(通常能识别出40余种矿物相),但在识别同质异相和多晶型矿物方面存在不足。

     

    Abstract: The Songhua River is the most important fluvial system in northeast China and its heavy mineral composition is of great significance to the revealing of depositional environment, source-sink relationship and drainage evolution. However, the current study concerning the heavy mineral composition of the Songhua River system is very limited, and the factors affecting the composition of heavy minerals are not clear up to date. In order to solve the problem, sand samples are collected along the banks of the main stream and 10 major tributaries of the river system, in addition to the two from the T3 terraces of the Tonghe River. Using the new automated mineral identification technology TESCAN Integrated Mineral Analyzer (TIMA) and the manual method respectively, heavy mineral analysis of the sample components in depth of 63~250 m is carried out for the samples. The influences of sediment sources, fluvial processes and chemical weathering on the heavy mineral composition of the Songhua River system are discussed. The results show that the tributaries sediments are sourced from different mountains, such as the Great Hinggan Mountains, the Lesser Hinggan Mountains and the Changbai Mountains. These mountains are significantly different in heavy mineral composition, and the heavy minerals of hornblende, titanite, epidote, leucoxene, augite, ilmenite and hematite+limonite are effective indicators to distinguish the water systems of different sources. The Nenjiang tributaries are originated from the Great Hinggan Mountains, of which sediments are rich in augite (the Nuomin River), ilmenite (the Ganhe River, Duobukuer River and Alun River) and hematite+limonite (the Ganhe River, Duobukuer River and Yalu River), showing the control of the basal source over heavy mineral composition, which can be observed in the modern riverbed gravels. However, in the main stream of the Songhua River, those heavy minerals are significantly reduced, suggesting that the influence of the Great Hinggan Mountains, as a source area, is extremely weak and the source control of heavy mineral composition is heavily influenced by river process. It is worth to point out that the heavy mineral composition of the Songhua River main stream is the mixture of the Lalin River and the Second Songhua River, indicating that the main stream of the Songhua River has inherited more heavy mineral information from the southeastern mountains of the Songnen Plain (the remnants of Changbai Mountain). The composition of heavy minerals in T3 terrace samples of Tonghe, which was severely affected by chemical weathering, is extremely simple, the unstable heavy minerals such as augite and hornblende have completely disappeared, and stable minerals are highly enriched, reflecting the control of chemical weathering over heavy mineral composition. Compared with the traditional manual identification of monotonous heavy mineral composition (usually only more than 20 mineral species can be identified), the TIMA method can reveal more mineral information (usually more than 40 mineral species can be identified), but there are shortcomings in identifying homogenous and polymorphic minerals.

     

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