摘要
晚白垩世,地球处于极端高温状态,被称为“热室地球”。此时期,新特提斯洋俯冲带与环太平洋俯冲带正值构造活跃高峰,被认为与热室地球的形成及终结紧密相关。然而,俯冲带活动如何影响全球气候的具体机制尚不明确,其中一个主要原因是缺乏对地表过程速率的定量约束。冈底斯弧是新特提斯洋向北俯冲过程中在中国西藏南部形成的典型安第斯型大陆弧,其岩浆活动在约100~80Ma期间进入高强度爆发阶段。同时,冈底斯弧剥蚀所产出的碎屑大量沉积于临近的日喀则弧前盆地,在深海环境中形成浊积水道系统。这些水道沉积主要由块状至中厚层砾岩与中厚层粗砂岩组成,呈透镜状沿东西走向大规模发育,反映了源区的快速剥蚀过程,可为重建热室背景下大陆弧造山和剥蚀-风化过程提供重要信息。本研究系统调查了日喀则弧前盆地东部的水道沉积。结果显示,这些沉积的形成集中于100~90Ma。水道沉积中,岩浆岩砾石和岩屑占比达56%~83%,且随沉积时代变新,侵入岩组分相较于火山岩逐渐增加。砂岩中碎屑锆石U-Pb年龄多<200Ma,呈现~108Ma的主峰,以及~124Ma和~156Ma的次峰,具有正的εHf(t)值,仅<90Ma的样品中出现大量>200Ma、εHf(t)为负值的锆石。综合前人研究表明,90Ma之前的水道沉积记录了冈底斯弧持续剥蚀及侵入岩逐渐出露的过程;而90Ma之后的沉积则反映了岩浆弧被切穿、物源区向北迁移的演化趋势。通过比较碎屑锆石年龄峰值与最大沉积年龄之间的差值,估算源区岩浆岩从侵位到出露并搬运至弧前盆地所经历的时间为8~34Myr。结合角闪石全铝压力计推算的中酸性岩浆岩中侵位深度(约12~35km),推算晚白垩世冈底斯弧的剥蚀速率达0.4~4.3km/Myr。在高温气候背景下,如此高强度的剥蚀可能促进了硅酸盐岩的加速化学风化,从而加速了大气CO_(2)的消耗。90Ma后,日喀则弧前盆地中反映源区快速剥蚀的粗粒水道沉积显著减少,全球气候也进入降温阶段。这种构造与气候同步变化的趋势,支持了构造剥蚀在热室气候下可能驱动全球气候转变的假说。
The Late Cretaceous was marked by prolonged extreme greenhouse climate conditions,often referred to as a"Hothouse Earth"climate state.This interval coincided with peak tectonic activity along the Neo-Tethyan and circum-Pacific subduction systems,which has been linked to both the initiation and eventual termination of the hothouse climate.However,the mechanisms by which subduction zones may have influenced global climate remain poorly understood,largely due to limited quantitative constraints on surface processes.The Gangdese arc in southern Xizang,China,developed during northward subduction of the Neo-Tethyan oceanic lithosphere beneath the Eurasian continental lithosphere,with a major magmatic flare-up taking place between 100Ma and 80Ma.Large volumes of detritus derived from the Gangdese arc were deposited in the adjacent Xigaze forearc basin,forming deep-marine turbidite channel systems.These channel deposits develop extensively along an east-west trend,composing of predominantly thick-bedded conglomerates and coarse sandstones.They reflect rapid erosion of the source region,thus could provide important constraints on erosional and weathering processes in continental arcs under warm climate conditions.Maximum depositional ages derived from detrital zircon U-Pb geochronology indicate most of these channel deposits developed between 100Ma and 90Ma.Conglomerate and sandstone compositions indicate that 56%~83%of the detritus were derived from igneous sources,and the proportion of plutonic to volcanic components increases upward in the stratigraphy.Detrital zircon U-Pb ages are mostly<200Ma with positiveεHf(t)values,with prominent age peaks at~108Ma,and subsidiary peaks at~124Ma and~156Ma.Pre-Mesozoic zircons with negativeεHf(t)values occur mostly in the<90Ma samples.These results suggest that the>90Ma channel deposits record sustained erosion and progressive exhumation of plutonic rocks in the Gangdese arc,whereas the<90Ma deposits reflect a shift in sediment provenance and northward migration of the source area.By comparing the depositional ages with detrital zircon U-Pb age peaks,we estimate the lag time between arc intrusion and final deposition to be 8~34Myr.Combined with estimated emplacement depths of 12~35km for Gangdese intermediate-felsic intrusions(based on Al-in-hornblende barometry),we estimate erosion rates of 0.4~4.3km/Myr during 100~90Ma.Under the prevailing warm climate background,such intense erosion likely accelerated silicate weathering,thereby enhancing atmospheric CO_(2) consumption.After 90Ma,development of coarse-grained channel deposits declined in the Xigaze forearc basin,coinciding with a global cooling trend.This synchronous change between tectonic activity and climate supports the hypothesis that arc-related tectonic erosion under hothouse conditions may have played a role in driving global climate transition.
作者
葛子逸
姜禾禾
安慰
吴福元
GE ZiYi;JIANG HeHe;AN Wei;WU FuYuan(State Key Laboratory of Lithospheric and Environmental Coevolution,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China;College of Earth and Planetary Sciences,University of Chinese Academy of Sciences,Beijing 100049,China;School of Resources and Environmental Engineering,Hefei University of Technology,Hefei 230009,China)
出处
《岩石学报》
2025年第10期3591-3604,共14页
Acta Petrologica Sinica
基金
国家重点研发计划项目(2022YFF0800800)
中国科学院先导项目(XDB0710000)
中国科学院引才计划青年项目(E2515104)联合资助
中科院地质与地球物理研究所重点部署项目(IGGCAS-202201)
基金委科学中心项目(42488201)。
关键词
冈底斯弧
日喀则弧前盆地
水道砾岩
剥蚀速率
西藏
Gangdese arc
Xigaze forearc basin
Channel conglomerate
Denuation rates
Xizang
作者简介
第一作者:葛子逸,男,2001年生,硕士生,矿物学、岩石学、矿床学专业,E-mail:geziyi23@mails.ucas.ac.cn;通讯作者:姜禾禾,女,1988年生,博士,副研究员,主要研究方向为俯冲带碳循环,E-mail:jiang.hehe@mail.iggcas.ac.cn。
