二叠纪-三叠纪之交重大地质转折期,海相地质记录指示全球发生了一系列显著的生物和环境事件,但是,该时期陆相古气候、古风化作用等方面的研究还很薄弱,争议较大.为了恢复新疆准噶尔盆地玛湖凹陷上二叠统乌尔禾组至中三叠统克拉玛依组的古气候、物源特征等,本研究对玛湖凹陷钻井岩心中的泥岩样品开展了全岩主量和微量元素测试,采用多种化学风化指数判定源区风化程度及古气候条件,通过多种地球化学比值及图解来恢复源岩岩性及其构造背景.化学蚀变指数(Chemical Index of Alteration,CIA)、化学风化指数(Chemical Index of Weathering,CIW)、Parker风化指数(Weathering Index of Parker,WIP)和斜长石蚀变指数(Plagioclase Index of Alteration,PIA)等多种化学风化作用指标均指示,玛湖凹陷自晚二叠世至早三叠世发生显著的风化作用变化,由低等程度的化学风化作用转变为中等程度的化学风化作用,某些季节可能会较为温暖湿润,且在整个早三叠世保持大致稳定,在早三叠世晚期稍减弱,这与锶同位素反映的全球风化作用变化趋势一致.中三叠世时的化学风化作用与早三叠世相比,并未降低,反而稍有增加,这可能代表了地区性事件.早三叠世化学风化作用显著增强的原因可能在于全球变暖、植被破坏及季节性降雨增加等.此外,上二叠统乌尔禾组至中三叠统克拉玛依组的物源岩性主要为长英质火成岩,源岩形成时的构造背景可能为大洋岛弧环境.
英文摘要:
A series of distinct biological and environmental events have occurred during the Permian-Triassic transitional period, which are mainly indicated by marine geological record.However,little study has been done on palaeoclimate and palaeoweathering on land in Early Triassic.In order to reconstruct the palaeoclimatological information,provenance and tectonic setting from Late Permian to Middle Triassic in Mahu sag,Junggar basin,bulk-rock major and trace elemental analyses were carried out on the core mudstone samples.Palaeoweathering condition and palaeoclimatology were reconstructed through various chemical weathering indices,while the provenance and tectonic setting were inferred from various discrimination diagrams.The chemical weathering switched from lower level to middle level from Late Permian to Early Triassic,and remained steady during the Early Triassic with minor weakened trend in the late Early Triassic,indicated by the CIA(Chemical Index of Alteration), CIW (Chemical Index of Weathering),WIP(Weathering Index of Parker)and PIA (Plagioclase Index of Alteration),which agrees with the global weathering evolutionary trend shown by Sr isotopes.The palaeoclimatology from Early to Middle Triassic was warm and wet seasonally,in comparison with the cool and dry environments during the Late Permian.However,the chemical weathering of Middle Triassic in Mahu sag did not weaken simultaneously with the global trend,but increased slightly, which might be a local event.In addition,the provenance of the Upper Permian Wuerhe Formation,Early Triassic Baikouquan Formation and Middle Triassic Kelamayi Formation should be felsic igneous rocks,and the tectonic setting during the formation of source rocks probably was the oceanic island arc setting.