鄂西咸丰地区五峰组—龙马溪组黑色页岩古气候及物源特征分析

Analysis of Paleoclimate and Provenance Characteristics of Black Shale in Wufeng-Longmaxi Formation in Xianfeng Area,Western Hubei Province

晚奥陶世—早志留世之交是地球历史时期中的关键时段,该时段伴有全球生物大灭绝、冈瓦纳冰川以及火山事件。晚奥陶世末期的冰川事件历时相对较短,在中国尚未找到冰川存在的直接证据。为了更好地研究奥陶纪—志留纪交替时期是否存在古气候的变化,选取了鄂西咸丰地区鄂咸页1井岩心中新鲜的五峰组—龙马溪组泥页岩样品,通过其主量、微量元素特征分析,采用多种化学风化指数判定源区风化作用强度及古气候条件。鄂咸页1井五峰组—龙马溪组岩石样品成分变异指数(ICV)均大于1,表明源岩成分成熟度低,属构造活动时期的首次沉积。A-CN-K图解、主量元素Al_(2)O_(3)/TiO_(2)值也体现以长英质成分为主的物源特征。化学蚀变指数(CIA)、化学风化指数(CIW)和斜长石蚀变指数(PIA)等多种化学风化作用指标均指示:晚奥陶世—早志留世古风化作用强度经历中等—低等—中等、古气候温暖湿润—寒冷干燥—温暖湿润的变化过程。CIA指数的低值指示的五峰组下段(CIA平均值65.45)—五峰组顶部(CIA平均值61.93)—观音桥组(CIA平均值57.89)—龙马溪组底部(CIA平均值58.86)—龙马溪组上段(CIA平均值66.09),即赫南特期存在短暂的寒冷气候,间接证实了冈瓦纳冰川作用在中国华南地区存在相应的地球化学记录。

The Late Ordovician-Early Silurian transition was a critical period in the earth’s history,this period is accompanied by global mass extinctions,Gondwana glacier and volcanic events.The glacial events at the end of the Late Ordovician were relatively short-lived,there are no direct evidence of glaciers has been found in China.In order to better study whether there were paleoclimate changes during the Ordovician-Silurian transition,fresh shale samples from Wufeng-Longmaxi formation in the core of well Exianye-1 in Xianfeng area of western Hubei province were selected,through the analysis of its main and trace element characteristics,the intensity of weathering and paleoclimate conditions in the source region were determined by various chemical weathering indices.The compositional variation index(ICV)of all rock samples in wufeng-longmaxi formation of well Exianye-1 is greater than 1,it shows that the composition maturity of the source rock is low.it is the first sedimentation in the period of tectonic activity.The diagram of A-CN-K and the Al_(2)O_(3)/TiO_(2) ratios of major elements also reflect the provenance characteristics of felsic of principal component.Various chemical weathering indexes such as chemical alteration index(CIA),chemical weathering index(CIW)and plagioclase alteration index(PIA)all indicate:The intensity of weathering from Late Ordovician to Early Silurian underwent a process of moderate-low-moderate,warm-humid and cold-dry and warm-humid in paleoclimate.A low value of the CIA index indicates the lower segment of Wufeng formation(CIA=65.45)-top of Wufeng formation(CIA=61.93)-Guanyinqiao formation(CIA=57.89)-bottom of Longmaxi(CIA=58.86)-upper segment of Longmaxi formation(CIA=66.09).The short-lived cold climate in Hirnantian Age indirectly confirmed the existence of a corresponding geochemistry record of the Gondwana glaciation in southern China.