鄂西走马地区ZK701孔南华系大塘坡组主量元素地球化学对古气候的指示

Geochemistry characteristics of major elements and their indication to paleoclimate in the Datangpo formation of Nanhua system of ZK701 drillcore in Zouma area of western Hubei province

地球于新元古代后期的成冰纪发生了Sturtian和Marinoan 2次全球规模的冰川事件,被称为"雪球地球"期,该时期的古气候演化一直被地质界广泛关注。扬子地区南华系相当于成冰系,其大塘坡组为古城冰期(Sturtian)与南沱冰期(Marinoan)的间冰期沉积。本文对鄂西走马地区ZK701钻孔岩心大塘坡组细碎屑岩样品主量元素地球化学进行了研究,ICV(成分变异指数)值显示沉积物具有初次沉积的特征,计算了CIA(化学蚀变指数)、CIW(化学风化指数)、PIA(斜长石蚀变指数)、n(Al)/n(Ca)及n(Na)/n(K),探讨了该区南华纪间冰期的古气候演化过程,结合前人研究,认为大塘坡间冰期早期气候干冷(CIA值介于55.6~69.8,均值65.0),大塘坡中晚期气候恢复温暖湿润并趋于稳定(CIA介于71.0~77.7,均值73.4),CIW,PIA,n(Na)/n(K)等指标反映的古气候演化过程与CIA反映一致,n(Al)/n(Ca)的变化也反映了陆源碎屑输入量随着古气候波动。本研究为扬子地区南华纪间冰期古气候演化分析提供了重要基础资料。

There were two global-scale glacial events,Sturtian and Marinoan,during the Cryogenian in late Neoproterozoic.It is known as the "Snowball Earth"period,and the paleoclimate evolution during this period has been widely concerned by geologists.The Nanhua system in Yangtze region was equivalent to the Cryogenian,and the Datangpo formation was interglacial sedimentary deposits during the Gucheng(Sturtian)and the Nantuo(Marinoan)glacial period.In this paper,the elemental geochemistries of the fine clastic rock samples from Datangpo formation of ZK701 drillcore in Zouma area of western Hubei province were studied.ICV(component variation index)values showed that the sediments had the characteristic of primary sedimentation.CIA(chemical alteration index),CIW(chemical weathering index),PIA(plagioclase alteration index),n(Al)/n(Ca)and n(Na)/n(K)ratio were calculated,and the paleoclimatic evolution of the Nanhua interglacial period in this area was discussed.According to previous studies,the climate of Datangpo in the early interglacial period was dry and cold(CIA value ranged from 55.6 to 69.8,mean value 65.0),and the climate in middle and late interglacial period was warm,humid and stable(CIA value ranged from 71.0 to 77.7,mean value 73.4).The paleoclimate evolution process reflected by CIW,PIA,n(Na)/n(K),and other values were consistent with the CIA value,and the change of n(Al)/n(Ca)value also reflected the fluctuation of terrigenous debris input with the paleoclimate.This study provided basic data for the interglacial paleoclimate evolution of Nanhua period in Yangtze region.