Deciphering the tectonometamorphic history of subducted metapelites using quartz-in-garnet and Ti-in-quartz (QuiG–TiQ) geothermobarometry—A key for understanding burial in the Scandinavian Caledonides

IF 3.5 2区 地球科学 Q1 GEOLOGY
Pauline Jeanneret, Iwona Klonowska, Christopher Barnes, Jarosław Majka, Johanna Holmberg, Mattia Gilio, William Nachlas, Matteo Alvaro, Karolina Kośmińska, Henning Lorenz, Thomas Zack, Anna Ladenberger, Hemin Koyi
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引用次数: 1

Abstract

The Seve Nappe Complex is a subduction-related high-grade metamorphic unit that was emplaced onto the margin of Baltica during Caledonian orogenesis. In this paper, the tectonometamorphic evolution of the Lower Seve Nappe in the Scandinavian Caledonides was characterized with the help of the continuous Collisional Orogeny in the Scandinavian Caledonides (COSC-1) drill core, using a combination of various P–T estimation techniques based on garnet–quartz mineral pairs (quartz-in-garnet and Ti-in-quartz [QuiG–TiQ]), conventional thermobarometry and thermodynamic modelling of phase equilibria. This multi-method approach yields complementary results and delivers critical data to constrain a comprehensive pressure–temperature–deformation–time (P–T–D–t) evolutionary path for the metasedimentary rocks of the Lower Seve Nappe. In the garnetiferous metasedimentary rocks, quartz inclusions in garnet preserve the P–T conditions of three distinct garnet growth stages corresponding to three metamorphic stages Ms1 to Ms3, including prograde and peak metamorphic conditions. Ms1 and Ms2 stages were constrained via quartz inclusions in garnet core and mantle. They are relatively close in the P–T space and could be considered as one single continuous prograde event occurring at epidote–amphibolite facies conditions of 460–520°C and 0.6–0.85 GPa. The growth of the garnet outermost rim defines the Ms3 stage at amphibolite facies conditions of 590–610°C and 1.13–1.18 GPa and corresponds to the peak metamorphic conditions. The microstructural analysis shows that the finite ductile strain pattern of the Lower Seve Nappe results from the superposition of four deformation phases. The initial phase D1 is defined by the S1 foliation that is still preserved as a curved inclusion trail in the garnet core. The D2 phase initiated contemporaneously with garnet core growth and the development of muscovite–biotite–plagioclase S2 foliation. Garnet outermost rim growth marks the end of the prograde path and peak metamorphic conditions. This stage is overprinted by the D3 phase and Ms4 stage associated with the development of the main regional metamorphic and mylonitic fabric S3 associated with C′-type shear bands along the retrograde path. Ms4 stage, which was constrained using traditional thermobarometric techniques, corresponds to the chemical re-equilibration of the metasedimentary minerals and occurred under amphibolite facies conditions at ~570–610°C and 0.78–1.00 GPa. The D3 phase is then generally weakly to strongly overprinted by later lower grade deformation D4 phase at greenschist facies conditions (Ms5). 40Ar/39Ar ages of syn-kinematic white mica and biotite indicate that the final stage of the thrusting of the Lower Seve Nappe and thus the timing of its emplacement onto the Offerdal Nappe occurred at c. 423 Ma. Collectively, these results are consistent with previous estimates of the timing and conditions of metamorphism derived from the Lower Seve Nappe especially in west-central Jämtland. However, application of QuiG–TiQ thermobarometry demonstrated that quartz inclusions in garnet can preserve different aspects of garnet growth, which are not accessible by traditional methods especially in complex terranes, and therefore provided new significant insights into the Lower Seve prograde evolution.

Abstract Image

用石英-石榴石和钛-石英(QuiG-TiQ)地温计破译俯冲变长岩的构造变质史——了解斯堪的纳维亚加里东尼构造埋藏的关键
西弗推覆杂岩体是加里东造山运动时期位于波罗的海边缘的一个与俯冲有关的高变质单元。本文利用斯堪的纳维亚加里东尼德山脉连续碰撞造山运动(COSC‐1)岩心,结合基于石榴石-石英矿物对(石英-石榴石和钛-石英[QuiG-TiQ])的各种P-T估算技术、常规热压法和相平衡热力学模型,对斯堪的纳维亚加里东尼德山脉下西韦推覆体的构造变质演化进行了表征。这种多方法方法产生了互补的结果,并提供了关键数据,以约束下西弗推覆体变质沉积岩的综合压力-温度-变形-时间(P-T-D-t)演化路径。在石榴石变质沉积岩中,石榴石中的石英包裹体保存了三个不同的石榴石生长阶段的P-T条件,对应于Ms1 ~ Ms3三个变质阶段,包括进变质和峰变质条件。石榴石岩心和地幔中的石英包裹体限制了Ms1和Ms2阶段。它们在P-T空间上相对接近,可以认为是发生在460-520℃、0.6-0.85 GPa的绿帘石-角闪岩相条件下的一个连续渐进事件。石榴石最外缘生长确定了590 ~ 610℃、1.13 ~ 1.18 GPa角闪岩相条件下的Ms3期,对应于变质峰期。显微组织分析表明,下西弗推覆体的有限韧性应变模式是四个变形相叠加的结果。初始阶段D1由S1片理定义,该片理在石榴石岩心中仍以弯曲的包裹体痕迹保存。D2期与石榴石岩心生长和白云母-黑云母-斜长石S2片理发育同时开始。石榴石最外缘生长标志着演化路径的结束和变质条件的高峰。该阶段被D3阶段和Ms4阶段覆盖,该阶段与主要区域变质岩和糜棱岩结构S3的发育有关,并与沿逆行路径的C '‐型剪切带有关。Ms4阶段对应于变质沉积岩矿物的化学再平衡,发生在角闪岩相条件下,温度为~570 ~ 610℃,GPa为0.78 ~ 1.00 GPa。在绿片岩相条件下,D3相通常被较低程度的D4相弱至强叠加(Ms5)。同时运动的白色云母和黑云母的40Ar/39Ar年龄表明,下西弗推覆体的最后逆冲阶段,因此它的侵位时间发生在约423 Ma。总的来说,这些结果与先前对下西韦推覆体(特别是中西部Jämtland)的变质作用时间和条件的估计一致。然而,QuiG-TiQ热压测量的应用表明,石榴石中的石英包裹体可以保存石榴石生长的不同方面,这是传统方法无法获得的,特别是在复杂的地体中,因此为下西五统进积演化提供了新的重要见解。
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来源期刊
CiteScore
6.60
自引率
11.80%
发文量
57
审稿时长
6-12 weeks
期刊介绍: The journal, which is published nine times a year, encompasses the entire range of metamorphic studies, from the scale of the individual crystal to that of lithospheric plates, including regional studies of metamorphic terranes, modelling of metamorphic processes, microstructural and deformation studies in relation to metamorphism, geochronology and geochemistry in metamorphic systems, the experimental study of metamorphic reactions, properties of metamorphic minerals and rocks and the economic aspects of metamorphic terranes.
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