晚第四纪钱塘江下切河谷充填物物源特征

Provenance for the late Quaternary Qiantang River incised-valley fill

对晚第四纪钱塘江下切河谷内SE2孔沉积物重矿物和稀土—微量元素进行系统分析,并与邻区长江沉积物相应特征进行对比。结果显示:(1)晚第四纪钱塘江下切河谷充填物中河床(U5)、河漫滩(U4)和古河口湾(U3)沉积物的重矿物组合为赤褐铁矿—磁铁矿—锆石—白钛石—锐钛矿;微量元素中的铁族元素、大离子亲石元素和高场强元素与平均大陆上地壳(UCC)相近,且大离子亲石元素和大部分高场强元素与北美页岩(NASC)相近,在Th/Co-Cr/Th投点图中靠近钱塘江物源区,远离现代长江沉积物投点区;稀土元素球粒陨石和NASC标准化曲线与钱塘江沉积物特征相似,不同于现代长江沉积物。(2)近岸浅海(U2)和现代河口湾(U1)沉积物的重矿物组合为绿帘石—辉石—角闪石—石榴石—榍石—金红石—电气石;微量元素具有与UCC较为相近或富集的铁族元素,大离子亲石元素和高场强元素相对亏损,且与NASC相比,铁族和高场强元素明显亏损,大离子亲石元素亏损较弱,在Th/Co-Cr/Th投点图中主要落于长江和钱塘江来源沉积物之间;稀土元素在球粒陨石和NASC标准化曲线中,配分模式与现代长江沉积物极为相似。(3)U5、U4和U3单元具有与U1和U2单元不同的沉积物来源:前者母岩主要为长英质中酸性火成岩,为古钱塘江提供;而后者沉积物主要来源于中基性物源区,由长江提供,同时上游钱塘江也提供了部分沉积物。可见,长江沉积物自近岸浅海沉积时期开始大量进入到钱塘江下切河谷内,为钱塘江下切河谷的沉积环境演化提供了重要物质基础。

In this paper,the heavy-mineral assemblages and the trace-rare earth elements of Borehole SE2 sediments in the late Quaternary Qiantang River incised valley were analyzed systematically in order to investigate their provenance and evaluate the coupling relationship between the Qiantang River and Changjiang incised valleys.Results indicate that:(1)The sediments of the river bed(U5),flood plain(U4)and ancient estuary(U3)are characterized by a heavy-mineral assemblage of hematite-limonite,magnetite,zircon,leucoxene and anatase;show a similar composition of siderophile,large-ion-lithophile,and high field-strength elements with those of the UCC,and a similarity for large-ion-lithophile elements and most high field-strength elements with those of NASC;plot towards the felsic end members in the Th/Co vs.Cr/Th plot;and bear a different distribution pattern in comparison with the modern Changjiang sediments in the Chondrile-and NASC-normalized rare-earth elements diagram.(2)The sediments of the nearshore shallow sea(U2)and modern estuary(U1)are featured by a heavy-mineral assemblage of epidote,pyroxenite,hornblendite,garnet,sphere,rutile and tourmaline;show a similar composition of siderophile elements,and depleted large-ion-lithophile and high field-strength elements compared to the UCC,and a depleted composition of siderophile,high field-strength and large-ion-lithophile elements in comparison with the NASC;plot between the Changjiang River and Qiantang River end members in the Th/Co vs.Cr/Th plot;and resemble the distribution patterns of the modern Changjiang sediments in the Chondrile-and NASC-normalized rare-earth elements diagrams.(3)The deposits of the U5,U4 and U3 units bear a different provenance with those of the U2 and U1 units,with the former derived from the intermediate-felsic source rocks,i.e.from the Qiantang River drainage area,whereas,the latter mostly originated from intermediate-mafic source rocks,namely from the Changjiang catchment,and secondly from the Qiantang River drainage.The Changjiang-derived sediments began to significantly provide sediments for the evolution of the late Quaternary Qiantang River incised-valley fill since the formation of the nearshore shallow sea(U2).