0.44 Ma以来南方风尘加积型红土物源分析:重矿物和碎屑锆石年代学证据  

Provenance Analysis of Eolian Red Earth Aggradation in Southern China since 0.44 Ma:Heavy minerals and detrital zircon geochronology

作　　者：张晓 朱丽东[1] 李凤全[1] 马桢桢 熊文婷 贾佳 王琳怡 ZHANG Xiao;ZHU LiDong;LI FengQuan;MA ZhenZhen;XIONG WenTing;JIA Jia;WANG LinYi(College of Geography and Environmental Sciences,Zhejiang Normal University,Jinhua,Zhejiang 321004,China)

机构地区：[1]浙江师范大学地理与环境科学学院,浙江金华321004

出　　处：《沉积学报》2022年第2期494-507,共14页Acta Sedimentologica Sinica

基　　金：国家自然科学基金(41572345,41971111)。

摘　　要：中亚热带地区的风尘加积型红土是中国南方第四纪环境研究的重要陆相沉积载体。许多典型剖面自0.4 Ma以来红化和网纹化程度明显减弱,气候趋于干凉,物源尚有近源、远源、远近混合源等说法。以九江地区JL剖面为研究对象,基于重矿物组成及碎屑锆石U-Pb年代学证据,重点探讨该沉积剖面第⑥~⑧层加积型红土(底界年龄0.44 Ma)物源问题,初步得出以下结论:1)JL剖面第⑥~⑧层属黄棕色土沉积,黄棕色土、网纹黄棕色土两类样品共鉴定出19种重矿物。22个样品均以不透明矿物为主,含量>50%;其次是锆石、金红石、锐钛矿、电气石、绿帘石、辉石等常见矿物。2)黄棕色土和网纹黄棕色土的极稳定、稳定矿物含量以及矿物成熟度ZTR和石榴子石指数GZi值分别为52.49%和52.88%,32.22%和41.49%,47.53和72.73,0.10和0.06,与可能的远程物源北方黄土(17.77%、19.21%、8.42、0.47)存在明显差异,而与长江沿岸风成砂样品(55.93%、20.15%、43.65、0.02)相似性更大。两类样品物源指数PI、与长江沉积物的相似度指数cosθ值分别为0.73和0.70、0.76和0.74,均支持长江源的结论。3)典型样品年龄峰多且分布范围广,70~130 Ma、200~300 Ma、390~570 Ma、680~870 Ma、870~3013 Ma等年龄峰与长江沉积物相应年龄峰吻合度高,而<65Ma、150~200 Ma年龄段锆石多出现在黄棕色土样品中,且可能与秦岭造山带的贡献有关,但贡献不显著。JL剖面0.44 Ma以来加积型红土近源物质贡献为主,粉尘主要源自长江谷地裸露的河漫滩。4)与JL剖面粉砂粒级稳定元素地球化学物源示踪结果混合源并不完全一致,可能意味加积型红土的物源有粒级效应。重矿物及碎屑锆石年龄更好地示踪了>20μm组分的物质来源及贡献,不排除北方物源贡献的可能性,其贡献量可能富集于4~20μm组分。Aggradated red earth is a typical eolian deposit in the mid-subtropical zone,where it is characterized by simultaneous deposition and weathering.The complete stratigraphy includes four soil units(upwards from the base):a reticular gravel unit,a reticular red soil unit,a homogeneous red soil unit and a yellowish-brown soil unit.Previous studies have shown that the degree of reddening and reticulation has clearly been weakened since 0.4 Ma,when the climate tended to be dry and cool.There are still some disputes on provenance,such as proximal source,distal source and mixed source.To determine it,we analyzed 22 heavy mineral samples and two detrital zircon U-Pb age spectra of the yellowish-brown soil deposit in the JL section(29°42′N,116°02′E)at the northern piedmont of Lushan Mountain,which is 1846 cm thick.Moreover,chronological studies of this section by others indicate that the slightly reticular yellowish-brown soil and yellowish-brown soil was formed in the middle and late Pleistocene since 0.44 Ma.The following conclusions are drawn:(1)There are 19 kinds of heavy minerals in the yellowish-brown soil and reticular yellowish-brown soil,predominantly opaque minerals(more than 50%).Common minerals include zir⁃con,rutile,anatase,tourmaline,epidote,pyroxene,etc.(2)The content of extremely stable and stable minerals,and the values of ZTR(zircon-tourmaline-rutile)reflecting mineral maturity and GZi(garnet:zircon index)of these soils are respectively 52.49%and 52.88%,32.22%and 41.49%,47.53 and 72.73,0.10 and 0.06,which are signifi⁃cantly different from the northern loess values(17.77%,19.21%,8.42,0.47).However,they are more closely simi⁃lar to the values for eolian sand samples along the Yangtze River(55.93%,20.15%,43.65,0.02).These results show that the PI values of the yellowish-brown and reticular yellowish-brown soils are 0.73 and 0.70 respectively,which are inclined to a Yangtze River source.The similarity index(cosθ)of these two types of soil and the eolian sand samples along the Yangtze River(0.76 a

关 键 词：晚中更新世 加积型红土 重矿物 锆石U-PB年龄 物源 

分 类 号：P534.63[天文地球—第四纪地质学]