柴达木盆地北缘中—下侏罗统碎屑岩地球化学特征及其地质意义

Geochemistry Characteristics of Middle-Lower Jurassic Clastic Rocks in the Northern Margin of Qaidam Basin and Their Geological Significance

柴达木盆地北缘中—下侏罗统是柴达木周缘中小盆地油气勘探重点关注的层段。为了给柴达木盆地北缘侏罗系油气勘探提供基础地质事实的支撑,从碎屑岩地球化学特征角度探讨柴达木盆地北缘早—中侏罗世古地理、古环境特征。柴达木盆地北缘中—下侏罗统碎屑岩主量元素与大陆上地壳整体一致,MgO、Na_2O、K_2O含量轻微亏损;稀土元素含量整体中等偏高,但与大陆上地壳具有极为类似的配分模式;微量元素中大离子亲石元素整体中轻微亏损,相容元素中等亏损,高场强元素无亏损—弱亏损。中—下侏罗统细碎屑岩样品化学风化程度中等偏高,意味着早—中侏罗世柴达木盆地北缘处于温暖潮湿的气候背景,与前人古生物研究结论以及研究区位于特提斯洋北部中纬度、临海位置吻合。样品沉积再旋回程度不强,说明物源区岩石不存在强烈的沉积再旋回,样品微量元素分析可以客观反映源区岩石微量元素特征。岩石中碎屑物质主要来源于由中酸性岩浆岩组成的后太古代上陆壳。古湖盆水体环境判别结果表明,柴达木盆地北缘早—中侏罗世属于贫氧—还原环境,向上有减弱趋势,这与不同阶段盆地类型有关。早侏罗世断陷盆地湖盆规模不大,但水体深度较大,湖盆较为封闭;中侏罗世属坳陷盆地,湖盆面积大,但水深相对变浅。

Middle-Lower Jurassic in the northern margin of Qaidam Basin is the focus of oil and gas exploration in the medium-small basins around Qaidam Basin.In order to provide a basic geological support for Jurassic oil and gas exploration in the northern margin of Qaidam Basin,the Middle-Lower Jurassic palaeogeography and paleoenvironment were discussed according to the geochemistry characteristics of clastic rocks.These rocks are similar in major oxide compositions to the upper continental crust（UCC）,but have slightly lower contents of MgO,Na_2O and K_2O.They have moderate abundance of REE with patterns,which are identical to that of the upper continental crust.The trace elements are characterized by medium depletion of large ion lithophileelement（LILE）,medium depletion of compatible element and weak-rare depletion of high field strength element（HFSE）.The Middle-Lower Jurassic clastic rocks display medium to high weathering,suggesting a warm and humid palaeoclimate for the northern margin of Qaidam Basin during Early-Middle Jurassic,which is consistent with previous palaeontology studies and the location of the study area in the mid-latitude of the northern Tethyan ocean.The insignificant sedimentary cycle in the clastic rocks suggests the inexistence of intensive sedimentary cycle for their provenance.The characteristics of trace elements can objectively reflect those of the provenance,and the detritus of the rocks are predominantly derived from Post-Archean upper continental crust mainly consisting of intermediate-acid magmatic rock.The water environment discrimination of ancient lake basin manifests a hypoxic-reducing environment for the northern margin of Qaidam Basin during Early-Middle Jurassic,and the reducing condition is gradually weakened from Early Jurassic to Middle Jurassic.It may be related to the different prototype basins of the different stages;the graben basin formed in Early Jurassic is small and deep water,and is characterized by enclosed lake basin;whereas,the graben basin formed in Middle Jurassic is large and shallow water.