川西南地区五峰—龙马溪组黑色页岩古气候及物源特征: 来自新地2井地球化学记录*

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—志留纪交替时期是否存在古气候的变化,选取了川西南地区新地2井岩心中新鲜的上奥陶统五峰组—下志留统龙马溪组泥页岩样品,通过其主量、微量及稀土元素特征分析,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。新地2井五峰—龙马溪组所有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。稀土元素表现为轻稀土富集、重稀土平坦并伴有明显的负Eu 异常,配分模式与花岗岩相似,表明源岩以亲花岗岩、长英质为主;A-CN-K图解、主量元素Al₂O₃/TiO₂值、微量元素Cr/Zr值和Th/Sc值也体现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顶部(平均值64.14)、观音桥组(平均值61.7)和龙马溪组底部(平均值64.61)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。

Palaeoclimatology and provenance of black shales from Wufeng-Longmaxi Formations in southwestern Sichuan Province:From geochemical records of Well Xindi-2

The Late Ordovician-Early Silurian transition was a critical interval in Earth’s history,marked by mass extinction,the Gondwana Glaciation and volcanic events. The end of Ordovician glaciations had a short-lived duration and there is no direct evidence for the glaciation in China. In order to confirm whether the palaeoclimatic changes ever existed during the end of the Ordovician-Early Silurian, we carried out major and trace elemental analyses on the core mudstone samples from the Wufeng-Longmaxi Formations of Well Xindi-2 in southwestern Sichuan Province,and reconstructed palaeoweathering conditions and palaeoclimatology through various chemical weathering indices. The ICVs(index of chemical varition)of all samples are greater than 1,indicating that they are compositionally inmature and first cycle deposits in tectonically active areas. The REE patterns are similar to the granite’s,which are characterized by slight LREE enrichments and accompanied by flat HREE trends with weakly negative Eu anomalies,suggesting that the source rocks are mainly pro-granite and felsic. The A-CN-K triangular diagram,Al₂O₃/TiO₂, Cr/Zr, and Th/Sc ratios indicate that the provenance of the rocks is also primarily felsic. Indicated by the CIA(Chemical Index of Alteration),CIW(Chemical Index of Weathering)and PIA(Plagioclase Index of Alteration),the sediments experienced from moderate to weak then to moderate chemical weathering in the source area,and palaeoclimate shifted from warm to cold and then to warm again during the Late Ordovician-Early Silurian. The low CIA values of the Upper Wufeng Formation(CIA_aver=64.14),Guanyinqiao Formation(CIA_aver=61.7)and the bottom of Longmaxi Formation(CIA_aver=64.61)reflect that there was a short cold climate time in Hirnantian Age,and represent indirect geochemical records in the South China of Gondwana Glaciation.