Pressure–temperature paths of tectonic blocks in mélange: Recording thermal evolution of a subduction channel at an initial stage of subduction

IF 3.5 2区 地球科学 Q1 GEOLOGY
Toru Takeshita, Takeshi Imayama, Mizuho Ando, Yuto Kimura, Marie Python
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引用次数: 0

Abstract

We investigated the tectonic evolution of amphibolite and blueschist tectonic blocks in the serpentinite- or pelite-matrix mélange, which are distributed at the highest structural level of the high-P/T type Kamuikotan metamorphic rocks in northern Japan. The tectonic blocks in this study area are divided into six rock types: garnet-epidote amphibolite, epidote amphibolite, amphibolite, plagioclase-poor amphibolite, epidote blueschist and glaucophane-bearing quartz schist. Based on phase equilibrium modelling, garnet-epidote amphibolite and epidote amphibolite experienced peak metamorphism at pressure and temperature conditions of 1.1–1.25 GPa and 550–590°C, and 0.8–1.3 GPa and 475–550°C, respectively, (at an apparent thermal gradient ranging between 13 to 17°C/km). By contrast, although the peak-metamorphic conditions for each one of amphibolite, plagioclase-poor amphibolite, and glaucophane-bearing quartz schist are not well constrained, they may have been originally metamorphosed at amphibolite to epidote-amphibolite facies at thermal gradients exceeding 20°C/km, inferred from the core composition of amphibole (edenite/magnesiohornblende/barroisite). The epidote blueschist experienced peak metamorphism at pressure and temperature conditions of 0.8–1.6 GPa and 360–520°C (most probably 0.8–0.85 GPa and 360–480°C). Although different types of tectonic blocks experienced a variety of peak metamorphism under different P/T conditions, all of them underwent epidote blueschist facies metamorphism at the peak or retrograde stage (as shown by the glaucophane rims of amphibole with different core compositions). The overall P–T paths appear counter-clockwise, which could be interpreted to reflect the cooling history of the subduction channel from the beginning to the steady state of subduction. The geothermal gradient could have changed from 15–17° to ~10°C/km over ~20–25 Myr, as estimated by previously reported radiometric ages. The protoliths to the tectonic blocks could have begun to subduct into the subduction channel at different times (where the thermal structure evolved with time), acquiring different prograde P–T paths. Subsequently, these tectonic blocks were juxtaposed at a certain depth and incorporated into the overlying serpentinite during the subduction stage. Finally, the serpentinite- or pelite-matrix mélange, including these tectonic blocks, were exhumed together with the coherent accretionary units as the former was emplaced over the latter.

朗格构造块体的压力-温度路径:记录俯冲初始阶段俯冲通道的热演化
摘要研究了位于日本北部高P/T型Kamuikotan变质岩最高构造层位的蛇纹岩或泥质岩基质岩中角闪岩和蓝片岩构造块体的构造演化。研究区构造块体划分为石榴石-绿帘石角闪岩、绿帘石角闪岩、角闪岩、斜长石-贫角闪岩、绿帘石蓝片岩和含蓝帘石石英片岩6种岩石类型。基于相平衡模型,石榴石—绿帘石角闪岩和绿帘石角闪岩分别在1.1 ~ 1.25 GPa和550 ~ 590℃、0.8 ~ 1.3 GPa和475 ~ 550℃的压力和温度条件下(表观热梯度范围为13 ~ 17℃/km)经历了变质峰。相比之下,虽然角闪岩、斜长石-贫角闪岩和含蓝闪石的石英片岩的峰值变质条件没有得到很好的限制,但从角闪岩的岩心组成(铁长岩/镁角闪石/钡辉石)推断,它们可能在温度梯度超过20℃/km时,从角闪岩到绿长岩-角闪岩相最初变质。绿帘石蓝片岩在0.8 ~ 1.6 GPa和360 ~ 520℃的压力和温度条件下(最有可能是0.8 ~ 0.85 GPa和360 ~ 480℃)发生了变质峰。虽然不同类型的构造块体在不同的P/T条件下经历了不同的峰值变质作用,但它们都在峰值或逆行阶段经历了绿帘岩蓝片岩相变质作用(如不同岩心组成的角闪洞的蓝绢岩边缘)。整体P-T路径呈逆时针方向,反映了俯冲通道从开始到稳定状态的冷却历史。根据先前报道的辐射年龄估计,地热梯度可能在~ 20-25 Myr范围内从15-17°变化到~10°C/km。构造块体原岩可能在不同时期(热构造随时间演化)开始向俯冲通道俯冲,形成不同的进积P-T路径。随后,这些构造块体在一定深度处并置,并在俯冲阶段与上覆蛇纹岩合并。最后,包括这些构造块体在内的蛇纹岩或泥岩基质与相贯的增生单元一起被发掘出来,因为前者位于后者之上。
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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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