新疆库车坳陷西段古近纪高溴岩盐与钾石盐成因联系

Genetic relations between Paleogene high bromine rock salt and sylvite in west of Kuqa depression in Xinjiang

新疆库车坳陷西段岩盐层中显示的南部高钾低溴,而北部高溴低钾的特征与卤水的正常演化过程相矛盾的问题尚不清楚。通过探讨库车坳陷西段古近纪高溴岩盐与钾石盐的成因联系,指导钾盐资源预测。通过地层对比、海侵过程分析及锶硫同位素特征研究,认为克深208井高溴岩盐层形成早于羊塔4井钾石盐层。对原生岩盐岩屑中石盐包裹体形态及均一温度、主量和微量元素、锶硫同位素、构造背景分析认为,羊塔4井钾石盐层形成于独立封闭演化的次级凹陷;克深208井高溴岩盐层形成阶段的卤水更接近海相端元,并在次级凹陷中经历长期演化;克深208井成盐晚期接受了大量陆源水混入,该阶段形成的岩盐层为库车坳陷西段海退时期的产物。总体来看,海水在古近纪自西向东进入库车坳陷后,先到达克深区所在的北部,后进入南部区域,而羊塔区处于海侵的末端,为卤水更浅、长期封闭的终端次级凹陷,成为最有利的成钾区域;海退阶段,残留卤水汇聚在海退通道内,此时通道内的局部凹陷处(如克深区晚期成盐阶段)是重要的成钾有利区。库车坳陷西段古近纪成钾总体遵循“海侵末端封闭成钾,海退通道凹陷成钾”模式,为库车坳陷寻找钾盐资源提供了重要的思路。

The characteristics of high potassium and low bromine in the south,and high bromine and low potassium in the north of the rock salt layers west of Kuqa depression,which is contradicted against the normal evolvement of brine.The purpose of this paper is to explore the genetic relationship between the high bromine rock salt and sylvite in the west Kuqa depression and to guide the prediction of potassium resources.In this paper,the study on stratigraphic correlation,the transgression process of Kuqa depression and the analysis of strontium and sulfur isotope show that the the formation of high bromine rock salt in drill Keshen-208 is earlier than sylvite in drill Yangta-4.Study on protogenetic rock salt cutting inclusion features,inclusion homogenization temperature,main and trace elements,strontium and sulfur isotope characteristics and tectonic background analysis shows that the evolution of drill Yangta-4 was independent and closed in the secondary depression and the brine in the formation stage of high bromine rock salt of drill Keshen-208 was closer to the seawater and evolved in the secondary depression for a long time.The late second salt forming stage of drill Keshen-208 accepted more land-based water,which was in response to the brine evolution stage in the regression period.In general,after entering the Kuqa depression from west to East in the Paleogene,the seawater first went to the north area and then to the south.Yangta area is located at the end of the transgression,which is a terminal secondary depression with shallower brine and long-term closure,becoming the most favorable potassium formation area.At the regression stage,the residual brine converges in the regression channel.At this time,the local depression in the channel(such as the late salt forming stage in Keshen area)is an important favorable area for potassium formation.The Paleogene potash formation in the western section of Kuqa depression generally follows the model of"forming potassium closing the transgressive end and forming potassium in the regression channel depression",which provides a new important idea for the search for potash resources in Kuqa depression.