柴北缘大煤沟组七段页岩地球化学特征——对中侏罗世晚期物源及风化作用的指示及意义

Geochemistry of 7 Member Shale of the Dameigou Formation in the Northern Qaidam Basin,China:Significance and Implication for Provenance and Source Weathering in the Late Middle Jurassic

柴达木盆地北缘中侏罗世晚期沉积的大煤沟组七段页岩(J2d7)记录了该地质历史时期的物源、风化作用及对应的气候和构造特征信息。对钻井岩芯样品进行元素地球化学分析,结果显示,较高的Al2O3/TiO2,La/Sc,Th/Co,较低的TiO2/Zr,以及TiO2-Zr、La/Sc-Th/Co、La/Th-Hf、La-Th-Sc组成表明大煤沟组七段页岩物源主要为长英质花岗闪长岩。A-CN-K组成特征指示钾交代作用对页岩的影响有限,整体较高化学蚀变指数(CIA)、化学风化指数(CIW)、斜长石蚀变指数(PIA)表明物源区经历了中等—强烈的化学风化作用,且自下而上,风化作用逐渐减弱,结合逐步降低的古气候指数C值及增高的Sr/Cu值,可得出在中侏罗世晚期柴北缘地区的古气候条件由温暖潮湿向干旱炎热明显转变,为该时期我国西北地区“干热化”事件在柴北缘的具体表现,同期构造沉降幅度大于因气候变干导致的湖平面下降速率,为有机质的富集提供了有利条件。

The shale of the 7 Member(J2d7)in the late Middle Jurassic Dameigou Formation,northern Qaidam Ba-sin,records important information about the provenance and weathering,and the corresponding climatic and structur-al developments.An analysis of the element geochemistry in drill core samples showed high ratios of Al2O3/TiO2,La/Sc and Th/Co and a low ratio of TiO2/Zr.The TiO2-Zr,La/Sc-Th/Co,La/Th-Hf and La-Th-Sc diagrams all indicate that granodiorite was the main source of the J2d7 shales.The A-CN-K ternary diagram indicates only a limited influ-ence of potassium metasomatism on the shale.The overall high values of the chemical alteration index(CIA),chemi-cal weathering index(CIW)and plagioclase alteration index(PIA)are evidence that the provenance experienced moderate to intense chemical weathering,gradually weakening from bottom to top.Combined with the lowered paleo-climatic index C and increased Sr/Cu values,it is concluded that,during the late Middle Jurassic,the paleoclimate in the northern Qaidam Basin clearly changed from warm and humid to arid and hot,and is regarded as firm evidence of a hot,arid climate event in northwestern China.During that period,tectonic subsidence exceeded the rate of lake level decline that resulted from the changing climate and provided favorable conditions for organic matter enrichment.