长江三角洲东西部晚新生代地层中的重矿物差异及其物源意义

HEAVY MINERAL DISTRIBUTION AND ITS PROVENANCE IMPLICATION IN LATE CENOZOIC SEDIMENTS IN WESTERN AND EASTERN AREA OF THE CHANGJIANG RIVER DELTA

主要针对长江三角洲东西部两个晚新生代钻孔的重矿物分布特征,探讨了近河口区和内陆三角洲平原区物源演化差异。研究结果表明,近河口区自下而上地层中可以划分出5个不同的重矿物组合带,内陆平原区可划分出4个重矿物组合带,这些组合带分别对应着上新世、早、中、晚更新世和全新世。两区在上新世时期重矿物特征十分相似,以金红石、锆石、电气石等酸性-中酸性矿物为主,伴随着许多强氧化自生矿物,如赤铁矿、褐铁矿、白钛石等,表明了以基底和西部天目山为主的物源,指示了强烈风化的沉积环境。进入第四纪,两区沉积物来源发生了变化,内陆平原区继承了西部山地物源,但不稳定矿物增加,并伴随出现许多还原性自生矿物,如黄铁矿、重晶石、碳酸盐等,指示还原环境占主导。相比之下,近河口区物源明显发生变化,尤其是中更新世后矿物种类丰富,金红石、榍石等酸性-中酸性矿物,紫苏辉石、钛铁矿等基性矿物,透辉石、蓝晶石、符山石、十字石等典型变质岩矿物都稳定出现,意味着随着沉积盆地下沉,河口区逐渐接受了长江中下游和上游沉积物的物源。

Two Cenozoic boreholes Pd and Sk10 were obtained from the estuary and inland plain regions respectively in the Changjiang River delta. 118 and 310 sediment samples were taken respectively from the boreholes Pd and Sk10 for heavy mineral analysis to discuss the provenance difference between the two regions. Five zones were identified from Pd and four from Sk10, and these zones upwards have a good relation with Pliocene, Early, Mid-, and Late Pleistocene, and Holocene. During Pliocene both regions were characterized of Rutile, Zircon, and Tourmaline derived from andesiticgranitic igneous rocks, accompanying with many oxidant autogenetic minerals such as Hematite, Limonite, and Leucoxene. This heavy mineral assemblage implied local and/or near west highlands sediments provenance and a strong chemical weathering environment. However there was a great difference in heavy mineral assemblage between the estuary and inland plain regions during Quaternary, which implied the different provenances between the two regions. Inland plain region kept the same characteristic minerals as those during Pliocene. Rutile, Zircon, and Tourmaline derived from andesitic-granitic origin still remained a certain proportion but unstable and reductive autogenetic minerals increased obviously, such as Amphibole, Pyrite, Barite and Carbonate, and few Metamorphic minerals occurred. These suggested the same provenance from west highlands as that during Pliocene although they had different depositional environment. By contrast, in the estuary region the input of Straurolite, Kyanite and Vesuvianite of metamorphic derivation, together with a few Tourmaline of andesitic-granitic origin in the sediments of Early Pleistocene represented the extension of sediment sources to the lower and middle Changjiang River basin which was covered by old metamorphic and andesitic-granitic rocks as the study area subsided. Few heavy minerals were remained in the sediments of Mid- Pleistocene, when mottled thicker stiff muds occurred as the lacustrine facies, suggesting a quasi-coastal floodplain with lower capability of sediment transport. Especially after Mid-Pleistocene diverse mineral types occurred continuously, such as Rutil and Sphene from andesitic-granitic origin, Hypersthene and Ilmenite from basaltic and Diopside, Kynite, Straurolite and Vesuvianite from metamorphic, which showed that the region received more sediments from the middle-lower and upper Changjiang River gradually with further subsidence of the delta region.