摘要:
台湾山溪性小河流每年向边缘海输入巨量沉积物,对东海陆架的沉积过程产生了显著的影响。本文分析台湾两条典型河流(兰阳溪和浊水溪)沉积物的全粒级碎屑重矿物组成,共鉴定出20种重矿物,但重量百分含量较低,为0.004%~0.116%。兰阳溪的主要重矿物组合为:锆石—菱镁矿—赤褐铁矿—锐钛矿—黄铁矿,浊水溪为:锆石—石榴石—赤褐铁矿—钛铁矿—锐钛矿—白钛石。研究流域的碎屑重矿物组成存在明显的沿程不均一性,指示出多数沉积物主要受到近源影响,从上游至下游重矿物组成的继承性较差。因此,基岩性质是河流碎屑重矿物组成的主要控制因素:兰阳溪和浊水溪的下游地区主要受各自流域内第四纪碎屑沉积物的贡献,上游地区则主要受到中央山脉庐山组的贡献。基岩控制起主导作用也使得重矿物指数如ATi、GZi和ZTR等难以恰当地应用于台湾山溪性小河流中。并且,大陆东部典型入海河流的重矿物组成与台湾河流存在明显区别:大陆入海河流中主要重矿物为磁铁矿和绿帘石;与之不同的是,兰阳溪河口富集锆石、菱镁矿和黄铁矿,浊水溪河口富集锆石和石榴石。这种差异主要反映了流域内基岩性质的不同。
关键词:
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台湾 /
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浊水溪 /
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兰阳溪 /
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重矿物 /
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物源分析
Abstract:
A total of 384 Mt/yr of suspended sediment is supplied to ocean by small mountainous rivers in Taiwan (1970—1999), equivalent to the sum of sediment discharge from three major rivers (the Yangtze River, the Yellow River and the Pearl River) in China. Hence, it has an appreciable impact on the sedimentary process of the epicontinental shelf. There are altogether 151 rivers in Taiwan, most of which originate from the Central Range. For our study river basins, the Zhuoshui River and the Lanyang River feed into the Taiwan Strait and the Pacific Ocean, respectively. The Zhuoshui River flows through the western Central Range, the Hsuehshan Range, the Western Foothills and the Coastal Plain in turn. And sequences of the Hsuehshan Range and the western Central Range outcrop in the Lanyang River basin. Heavy mineral types and contents of eight sediment samples from the Zhuoshui River (ZS1~ZS5) and the Lanyang River (LY1~LY3) are analyzed in whole size fraction. Twenty kinds of heavy minerals are identified. The weight percentage of heavy minerals varies from 0.039% to 0.116% in the Zhuoshui River, and from 0.004% to 0.040% in the Lanyang River. On the whole, the weight percentage declines from upstream to downstream.
There is an irregular spatial distribution of heavy mineral assemblages along both rivers. For the Zhuoshui River, the dominated heavy mineral assemblage is pyrite-limonite-magnetite in the upper reaches, zircon-limonite-magnetite-pyrite in the middle reaches, zircon-garnet-limonite-ilmenite-anatase-leucoxene in the lower reaches. For the Lanyang River, the dominated heavy mineral assemblage is zircon-epidote-pyrite in the upper reaches, zircon-magnesite-limonite-leucoxene in the middle reaches, zircon-magnesite-limonite-anatase-pyrite in the lower reaches. Variations of heavy mineral types and contents along the two rivers indicate that most sediments are mainly supplied by proximal source rocks, and signatures of heavy minerals from upstream are masked. Also, there exists difference in contributions to sediments between potential provenances: heavy minerals in the lower reaches of both rivers are mostly derived from Quaternary sediments, and in the upper reaches mainly supplied by Lushan Formation of the Central Range. In addition, because the distribution of heavy mineral assemblages is primarily controlled by source rocks, some heavy mineral indexes (ATi, GZi and ZTR) cannot be used properly in these small mountainous rivers.
The differences in bedrocks between mainland and Taiwan make the heavy mineral assemblages different between both sides of the strait. The typical heavy minerals in the rivers of Chinese mainland, like magnetite and epidote, are not common in the Taiwanese Rivers. However, the enrichment of zircon, magnesite and pyrite in the mouth of the Lanyang River, and the enrichment of zircon and garnet in the mouth of the Zhuoshui River, are there respective characteristics. All these features may offer some reference value for tracing the provenances of sediments in the epicontinental shelf.
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