岩芯沉积物化学元素及重矿物含量变化对腾格里地区碎屑物源的指示

PROVENANCE SHIFT DURING QUATERNARY PERIOD EVIDENCED BY CHANGES IN GEOCHEMICAL ELEMENTS AND HEAVY MINERAL CONTENTS IN CORE SEDIMENTS IN TENGGER AREA

腾格里地区地处青藏高原与戈壁阿尔泰山之间的山间盆地的西南隅,该地区的沉积物对构造及气候变化非常敏感。BJ14钻孔位于腾格里沙漠腹地白碱湖一带,钻孔岩芯长度为104 m,覆盖了整个第四纪以来的沉积。分别采用高分辨率的X射线衍射(XRD)及X射线荧光光谱(XRF)岩芯扫描技术系统地分析了第四纪BJ14钻孔岩芯沉积物中的重矿物及化学元素组成,并通过部分样品的重矿物镜下鉴定检验了利用XRD获得的重矿物分析结果。结果显示,沉积物中多种化学元素(如Si、Al、Cl和S)的相对强度同步变化,且与多种重矿物(如锆石、金红石、白钛石、石榴石、绿帘石、电气石)的含量在约1.8 Ma、1.2~0.6 Ma时段同步变化。这种变化与根据碎屑锆石U-Pb年龄谱获得的源区变化信息基本一致,共同指示在约1.8 Ma、1.2~0.6 Ma时段腾格里地区源自青藏高原东北缘的碎屑物质增加。同时,腾格里地区的ZTR指数在1.8 Ma及0.7 Ma前后明显降低,指示腾格里碎屑物源区在该时段构造活动加强。因此,腾格里地区钻孔岩芯沉积物物源的变化敏感地响应了青藏高原在第四纪期间的阶段性隆升。

Tengger is located in a basin(37°30'~40°00'N,102°30'~106°00'E)northeastern of the Tibetan Plateau and south of the Gobi Altai Mountains.The 104 m long core BJ14(39°05'23.89"N,104°11'50.82"E)was drilled from hinterland of the Tengger Desert with chronology spanning the entire Quaternary period.Here we report variations of geochemical elements and heavy minerals percentage with ages.The relative content(intensity)of geochemical elements of entire core was measured with high resolution(10 mm interval)X-ray fluorescence spectroscopy(XRF)equipped with the core scanner,and heavy mineral contents of forty samples were measured through X-ray diffraction(XRD)and further tested under microscope.The results support the following recognitions:(1)Heavy minerals percentage changed significantly at the period of 1.9~1.8 Ma,0.9 Ma and 0.6 Ma.In details,since about 1.8 Ma,the percentage of zircon and rutile decreased abruptly,while percentages of epidote,garnet and amphibole increased synchronously.During 1.2~0.6 Ma,the content of epidote,garnet and amphibole increased to a high level,especially the average value of the epidote increased from<7%to about 11%.(2)During the interval of 0.9~0.7 Ma,the relative intensity of elements Si and Al decreased abruptly,and the relative intensity of elements S and Cl increased synchronously.(3)The increase of epidote,garnet and amphibole percentage in the Tengger area at intervals of 1.8 Ma and 1.2~0.6 Ma might indicate increase of detrital materials which were originated from the northern Tibetan Plateau.As is consistent with the provenance variation revealed from U-Pb age spectrum of detrital zircon grains.While the abrupt decrease of the relative intensity of elements Si and Al,and synchronously step increase of elements S and Cl at the interval of 0.9~0.7 Ma may be a response to the shrinkage of paleolake in the Tengger which was supported by changes of sedimentary lithology.In addition,the ZTR index significantly decreased at around 1.8 Ma and 0.7 Ma,indicating that the tectonic movements obviously enhanced in the prominent provenance area of Tengger,northeastern Tibetan Plateau.Combined with previous studies,this study therefore suggests that the provenance variation in the Tengger area at around 1.8 Ma and at the interval of 1.2~0.6 Ma was a response to the"Qing-Tibetan movement episode C"and the"Kunhuang Movement".