黔东南华纪铁丝坳期-大塘坡期古气候的演变:来自CIA的证据

Paleoclimate Evolution of the Cryogenian Tiesi′ao FormationDatangpo Formation in Eastern Guizhou Province:Evidence from the Chemical Index of Alteration

新元古代晚期是全球气候急剧变化的时期,目前可区分出至少2个全球性冰期事件。黔东地区南华纪地层中存在上下2个具有明显冰碛岩特征的地层：下部的两界河组-铁丝坳组与上部的南沱组,可分别与720 Ma的Sturtian冰期、650 Ma的Marinoan冰期事件相对应,而铁丝坳组与南沱组之间的大塘坡组则代表了间冰期的沉积。通过对铜仁松桃2口钻孔岩心铁丝坳组-大塘坡组样品主量元素的研究,探讨了Sturtian冰期晚期至其冰后期的古气候演变过程,结合化学地层学与岩石学证据,发现Sturtian冰期结束后,气候的回暖并非是突然出现,而是在主冰期结束之后尚存在短暂的小冰期。由下至上,钻孔样品的化学蚀变指数（CIA）平均值从Sturtian冰期末铁丝坳组顶部的51升至大塘坡组底部的60。大塘坡组底部CIA值波动较大,在52~68之间呈锯齿状震荡上升,代表古气候环境所发生的动荡变化,结合在大塘坡组底部所发现的组内冰碛岩层位,指示存在着短暂的寒冷事件。大塘坡组一段黑色炭质页岩的CIA值稳定在66左右,反映气候稳定在相对温暖的状态;大塘坡组二、三段粉砂质页岩CIA指数平均值为67,向上CIA值表现出逐渐下降的趋势,可能与Marinoan冰期启动所导致的气候转冷有关。通过A-CN-K三角图解判断沉积气候环境的稳定性与源岩成分,进一步证明了Sturtian冰期结束后气候的不稳定,以及间冰期稳定的温暖气候。冰期-间冰期波动的古气候演化可能与＂大塘坡＂型锰矿的成矿作用相关。

Presently two episodes of global glacial events could be distinguished in Neoproterozoic,when dramatic changes in climate happened globally.Two formations of strata distributed in Nanhua System（Cryogenian）around eastern Guizhou Province with typical characteristics of glacial sediments.The lower named Tiesi＇ao Formation and the upper named Nantuo Formation correspond with Sturtian glaciation in720 Ma and Marinoan glaciation in 650 Ma respectively,and the strata between the two formations are of Datangpo Formation representing deposition during the interglaciation.This paper discusses paleoclimate evolution during the late Sturtian glacial period to interglacial period with the main elements dated from samples in Tiesi′ao Formation-Datangpo Formation,Songtao county and Tongren city,combined with evidence from chemistry and petrology.This illustrates that transient glacial events occurred following the end of Sturtian age.From bottom to top,the chemical index of alteration（CIA）raises from 51 in late Tiesi＇ao period to 60 in early Datangpo period.However,the shape of curve drawn with date from 52 to 68is not linear but a serrated upward trend,showing the occurrence of ephemeral glacial event.The average value of CIA from Carbonaceous shales in upper Datangpo Formation and silty shales in middle,lower Datangpo Formation are 66 and 67,both supporting a stable warm condition.However,the mean of CIA decreases suddenly at the top of the latter which may due to the beginning of the Marinoan glaciation.On the other hand,it is further confirmed with evaluations on stability of sedimentary condition and composition of source rocks by A-CN-K triangle graph that the instability of climate follows the Sturtian glaciation and the stability of warm climate during the interglaciation.Thus,the fluctuations of glaciation-interglaciation paleoclimate evolution may be related to ＂Datangpo＂ type manganese mineralization.