新疆叶城柯克亚8～3.85Ma沉积地层中常量和微量元素分布对气候演化的响应

The Response of the Main and Trace Element Concentrations to Climate Evolvement in 8-3.85 Ma Sediment of the Kekeya Section at Yecheng,Xinjiang

柯克亚剖面位于青藏高原西北缘新疆叶城县柯克亚乡，出露的地层由老到新为乌恰群、阿图什组和西域组，息厚度为2217．9m。在野外地层剖面详细实测与沉积相研究的基础上，系统采集了柯克亚剖面上8～3．85Ma间沉积地层的地球化学样品并进行了常量和微量元素的测试和分析。剖面沉积物中元素平均值与上部陆壳（UCC）元素平均质量分数比揭示了研究区在8～3．85Ma期间，息体较为干旱寒冷的气候环境特征。在此基础上，结合具有较好指示作用的元素变化特征，根据CIA、w（CaO）／w（Sr）、w（Rb）／w（Sr）、w（CaO）／w（MgO）比值的变化将研究区的古气候变化分为3个阶段：剖面自下向上0～222m（乌恰群第1～7层，年龄为8～7．3Ma）为冷干气候；222～419m（乌恰群第8～15层，年龄为7．3～6．6Ma）为暖湿-冷干交替的气候；419～1665m（乌恰群第16层～阿图什组第24层，年龄为6．6～3．85Ma）气候更为冷干。根据古气候研究结果推测青藏高原在8Ma前已经隆升到了一定的高度，阻挡了来自印度洋的夏季风的北上，导致塔里木盆地干旱化。约6．6Ma前气候的进一步变干可能与青藏高原的进一步隆升和来自西伯利亚的冬季风的进一步加强有关。

The Kekeya section at Yecheng, lies in the northwestern margin of the Qinghai-Tibet Plateau. It is composed of three strata from Miocene to Pliocene with the thickness of 2 271.9 m. The lowest stratum is Wuqia Group, with Atushi Formation in the middle and the Xiyu Formation in the upper part. According to the detailed research of measured section and sedimentary facies, major and trace elements were analyzed for the ancient climate information from the deposition of the Kekeya section during 8--3.85 Ma. The element ratios between the deposit and the upper continental crust （UCC） indicate that the paleoclimate of the study area was characterized by cold-dry during 8--3.85 Ma. According to variations of CIA,CaO/Sr, Rb/Sr,CaO/MgO, the palaeoclimatic of the area can be divided into three stages： 0-222 m （1-7 beds in Wuqia Group, about 8. 3--8 Ma） was characterized by cold-dry; 222--419 m （8--18 beds in Wuqia Group, about 8.3--6.6 Ma） was marked by cold-dry alternating warm-wet; 419--1 665 m （16--24 beds in Wuqia Group to Atushi Formation , about 6.6--3.85 Ma） was characterized by more cold-dry than the above two stages. The results of palaeoclimate research lead to the conclusion that the Qinghai- Tibet Plateau might have uplifted to an important critical altitude since 8 Ma. The uplifted plateau may have obstacled the moisture current from the south, so the Tarim Basin turned dry. Before 6.6 Ma, the continuing uplifting of the plateau and the strengthening of winter monsoon from the Siberian induced the climate, causing more drought.