塔里木盆地柯坪地区晚奥陶世凯迪晚期的沉积环境和构造背景

SEDIMENTARY SETTING AND TECTONIC BACKGROUND OF THE UPPER KATIAN(UPPER ORDOVICIAN)IN KALPIN,TARIM BASIN

奥陶系在塔里木盆地西北缘柯坪地区广泛出露。本研究对该地区任英沟剖面上奥陶统凯迪阶上部的阿日苏组和铁热克阿瓦提组进行了系统的主、微量元素地球化学分析,通过一系列地球化学指标(CIA、CIAcorr.、Sr/Ba、U/Th、Ni/Co、V/Cr、Al_(2)O_(3)/TiO_(2)、TiO_(2)/Zr等)和主、微量元素判别图解,得出以下几点认识:(1)阿日苏组和铁热克阿瓦提组记录了化学风化作用先减弱后增强的变化趋势,对应了区域内古温度先降低后升高的过程,这与全球范围内Boda事件中期的降温-升温事件吻合;(2)该套地层沉积水体盐度不高,海水呈间歇性漫入,这与上覆的志留系柯坪塔格组底部地层的开阔海沉积环境不同;(3)该套地层的物源以远源较古老的沉积岩地层为主,混入近源早奥陶世地层,其沉积物的搬运和沉积过程受到南天山洋洋壳消减的影响;(4)柯坪塔格山北麓印干组顶部的厚层生物灰岩代表了柯坪地区早古生代最后的碳酸盐岩沉积,之后转换为以阿日苏组、铁热克阿瓦提组、柯坪塔格组、塔塔埃尔塔格组和克兹尔塔格组为代表的巨厚碎屑岩沉积,对应于整个塔里木盆地在奥陶纪晚期发生的沉积相转化事件。这一沉积模式的转换,可能与晚奥陶世至志留纪发生的区域内南天山洋洋壳消减有密切关系。

The Upper Ordovician is widely exposed in the Kalpin region,northwest of Tarim Basin.In this study,the Arisu Formation(Katian of the Upper Ordovician)and the Terekawat Formation(Katian of the Upper Ordovician,including the Siltstone Member and the Sandstone Member)were sampled in ascending order at the Renyinggou Section for geochemical analysis.Adopted methods include a series of geochemical analyses(e.g.,CIA,CIAcorr.,Sr/Ba,U/Th,Ni/Co,V/Cr,Al_(2)O_(3)/TiO_(2),TiO_(2)/Zr)and plotting of geochemical discrimination diagrams,on which to reconstruct paleoclimate change and tectonic activities in the northwestern Tarim Palaeoplate during the Late Ordovician.The results reveal a cooling-warming cycle in the Paraorthograptus pacificus graptolite biozone of the latest Katian,corresponding to the mid-Boda cooling and the late-Boda warming events on a global scale.The results also show a low paleosalinity when these formations were deposited,indicating a brackish water condition,compared to the normal marine setting in the overlying Kalpintag Formation(lower Silurian).The clastics of these two formations are suggested to come from pre-existed stones,which are mainly pre-Cambrian rocks with a few Early Ordovician deposits.The depositional transition from the dominant carbonates(e.g.,the Yingshan,Kanling,and Yingan formations)to the siliciclastic rocks(e.g.,Terekawat and Kalpintag formations)coincides with the enhancement of regional magmatic activity and tectonic thermal events in Tarim during the late Katian,which is possibly related to the subduction of the Turkestan Ocean.