0.44Ma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

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...

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

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 1 846 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 zircon， 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 significantly different from the northern loess values （17.77%， 19.21%， 8.42， 0.47）. However， they are more closely similar 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 and 0.74） also support the conclusion of a Yangtze River source. （3） The analysis of detrital zircon U-Pb geochronology showed numerous and widely distributed age peaks for typical samples. The age peaks of 70?130， 200?300， 390?570， 680?870 and 870?3 013 Ma are consistent with the age peaks of Yangtze River sediments， while those of <65 Ma and 150?200 Ma in the yellowish-brown soil samples are related to the Qinling orogenic belt， but the contribution is not significant. Proximal sources are predominant in the aggradated red earth in the JL profile since 0.44 Ma， and the dust has mainly come from the exposed floodplain of the Yangtze valley. （4） There is not complete consistency with the geochemical source tracing results of particle size stable elements in the JL profile （supporting the mixed-source conclusion）， which may indicate that the provenance of the red earth aggradation was affected by grain size. The grain size of the heavy minerals and detrital zircons is usually more than 20 μm， and the contribution from the north loess is enriched in the 4?20 μm components.