长江口表层沉积物重矿物在不同粒级中的分布与研究意义

Patterns of heavy mineral combination in different grain-size categories and their sedimentary significance:A case study for surfical sediments in the Changjiang River Estuary

在粒度分析基础上,以0.5Φ为间距对长江口表层沉积物的不同粒级分样进行碎屑重矿物分析,并对各粒级重矿物特征与广粒级(1.5Φ~6Φ)进行相似度分析。结果表明,长江口沉积物粒度跨度大,从粗砂到泥(黏土)均有分布,粉砂含量高。样内不同粒级分样重矿物含量、种类和组合均有一定变化,样间表现出较一致的规律性。长江口出现碎屑重矿物30种左右,粗粒级分样中矿物种数为10余种,细粒级分样中重矿物种类增至20多种;3.5Φ~4.5Φ是重矿物种类多、矿物成分复杂的粒级区段。粗粒级(粒径大于3Φ)云母族富集,其重矿物组合为云母+角闪石,随粒级变细(粒径小于4Φ)云母急剧减少;闪石族分布粒级广泛,在3Φ~5Φ粒级含量相对较高;帘石族、稳定矿物尤其是金属矿物在粒径小于3.5Φ粒级逐渐增多。长江口重矿物整体组合为普通角闪石+绿帘石+褐铁矿,榍石为特征矿物。相似度分析表明,长江口沉积物样品中主要粒级与广粒级重矿物特征相近,能代表沉积物整体重矿物特征。长江口碎屑重矿物特征深受粒度分布的影响,其矿物种类、矿物组合和矿物指数及其所蕴含的水动力和物源意义都要结合粒度特征来综合分析。

On the basis of particle size analysis,heavy mineral analysis was carried out in different size fractions with a spacing of 0.5Φof the Changjiang River Estuary surface sediment.Heavy mineral similarity value calculated between the each grain group and wide window grain size group(1.5Φto 6Φ).The results show that the surface sediments of the Changjiang River Estuary have a large grain size span,ranging from coarse sand to mud,and the silt content is high.Intrasample variability of heavy mineral contents,species and compositions is ubiquitous,intersample variability has certain regularity.There are about 30 kinds of heavy minerals in the Changjiang River Estuary,about ten kinds of minerals in the coarse-grained fractions,the number of heavy minerals in fine-grained fractions has increased to more than twenty.3.5Φto 4.5Φis a fraction with numerous and complex heavy minerals.Mica is rich in the coarse-grained grade(<3Φ),and the heavy mineral combination is mica+hornblende,but almost absent in fine grains of>4Φ.The amphibole group almost exist all range of grain sizes and the content is high in grain of 3Φto 5Φ.Epidote group,stable minerals,and especially Fe-metallic minerals increase at fine grain of>3.5Φ.The heavy minerals combination is hornblende+epidote+limonite in the Changjiang River Estuary,sphene is the characteristic mineral.Similarity analysis shows that the main grain size and the wide grain size of the sediment have similarities in the characteristics of heavy minerals,which is the most suitable size for replacing the heavy minerals of the whole sediment.The heavy mineral characteristics are deeply affected by the particle size distribution in the Changjiang River Estuary.Mineral type,mineral combination and indice,and the hydrodynamic and provenance implications must be combined with the grain size analysis.