碰撞造山运动中白云质和镁质岩石的变质作用及其对造山运动二氧化碳脱气的影响

IF 3.5 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
Tamang Shashi, Groppo Chiara, Girault Frédéric, Perrier Frédéric, Rolfo Franco
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引用次数: 0

摘要

含碳酸盐沉积物的主要碳酸盐成分为方解石、白云石或菱镁矿,是地球历史上被动边缘沉积序列的重要组成部分。在参与碰撞造山过程时,这些沉积物在不同的温度和压力下发生变质,并发生脱碳反应。虽然对其中一些岩性(即不纯灰岩和白云石、普通泥灰岩和钙质辉长岩)的造山变质作用了解得相对较多,但对以白云石或菱镁矿为主要碳酸盐成分的碳酸盐-硅酸盐混合岩的变质演化和脱碳历史却知之甚少。本文介绍了对尼泊尔中部变质岩代表性样本的岩石学研究结果,这些样本来自喜马拉雅造山运动期间变质的新生代白云岩和菱镁矿原岩。详细描述了最初含有不同数量白云石或菱镁矿的沉积物中形成的主要变质组合。通过对七个样本进行前向热力学建模,可以对以下方面进行限制:(i) 主要的脱碳反应,(ii) 发生这些反应的 P-T 条件,(iii) 流体的成分,以及 (iv) 释放的二氧化碳量。我们得出的结论是,白云质和镁质辉长岩和泥灰岩原本含有 15-40% 的碳酸盐,其二氧化碳生产率很高(>5.5 ±1.0 CO2 wt%和高达 10.5 ±1.5 CO2 wt%),而当碳酸盐含量超过 60-70% 时,除非水性流体从外部渗入并引发脱碳反应,否则二氧化碳生产率可以忽略不计。由于白云石和菱镁矿原岩在喜马拉雅造山带仍处于活动状态的沉积序列中非常丰富,这里描述的脱碳过程可能是目前在地表观测到的弥漫性二氧化碳脱气的原因。此外,我们首次提出,本研究调查的奇特富镁集合体可能来自蒸发态原岩,因此整个上小喜马拉雅序列可能代表了新生代序列的变质产物,该序列由碳酸盐岩、蒸发态和辉绿岩沉积层交替组成。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Metamorphism of dolomitic and magnesitic rocks in collisional orogens and implications for orogenic CO2 degassing
Carbonate-bearing sediments, containing calcite, dolomite or magnesite as major carbonate components, are important components of sedimentary sequences deposited on passive margins through Earth’s history. When involved in collisional orogenic processes, these sediments are metamorphosed at variable temperatures and pressures, and undergo decarbonation reactions. While the orogenic metamorphism of some of these lithologies (i.e., impure limestones and dolostones, marls sensu strictu and calcareous pelites) is relatively well understood, very little is known about the metamorphic evolution and decarbonation history of mixed carbonate-silicate rocks in which either dolomite or magnesite is the dominant carbonate component. Here we present the results of a petrologic study of representative samples of metasediments from Central Nepal, derived from Proterozoic dolomitic and magnesitic protoliths metamorphosed during the Himalayan orogeny. The main metamorphic assemblages developed in sediments originally containing different amounts of dolomite or magnesite are characterised in detail. Forward thermodynamic modelling applied to seven samples allows constraints to be placed on: (i) the main decarbonation reactions, (ii) the P-T conditions under which these reactions took place, (iii) the composition of the fluids, and (iv) the amounts of CO2 released. We conclude that the CO2 productivity of dolomitic and magnesitic pelites and marls originally containing 15-40% carbonate is significant (>5.5 ±1.0 CO2 wt% and up to 10.5 ±1.5 CO2 wt%), whereas for carbonate contents above 60-70%, CO2 productivity is negligible unless aqueous fluids infiltrate from the outside and trigger decarbonation reactions. Since the dolomitic and magnesitic protoliths are significantly abundant in the sedimentary sequences involved in the still active Himalayan orogen, the decarbonation processes described here could contribute to the diffuse CO2 degassing currently observed at the surface. Furthermore, we propose for the first time that the peculiar magnesium-rich assemblages investigated in this study may derive from evaporitic protoliths, and that the whole Upper Lesser Himalayan Sequence may therefore represent the metamorphic product of a Proterozoic sequence consisting of alternating layers of carbonatic, evaporitic and pelitic sediments.
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来源期刊
Journal of Petrology
Journal of Petrology 地学-地球化学与地球物理
CiteScore
6.90
自引率
12.80%
发文量
117
审稿时长
12 months
期刊介绍: The Journal of Petrology provides an international forum for the publication of high quality research in the broad field of igneous and metamorphic petrology and petrogenesis. Papers published cover a vast range of topics in areas such as major element, trace element and isotope geochemistry and geochronology applied to petrogenesis; experimental petrology; processes of magma generation, differentiation and emplacement; quantitative studies of rock-forming minerals and their paragenesis; regional studies of igneous and meta morphic rocks which contribute to the solution of fundamental petrological problems; theoretical modelling of petrogenetic processes.
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