Amalgamation of the Ryoke and Sanbagawa metamorphic belts at the subduction interface: New insights from the Kashio mylonite along the Median Tectonic Line, Nagano, Japan
Amalgamation of the Ryoke and Sanbagawa metamorphic belts at the subduction interface: New insights from the Kashio mylonite along the Median Tectonic Line, Nagano, Japan
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Ryoke 和 Sanbakawa 变质带在俯冲界面处的合并：日本长野中位构造线沿线的 Kashio 糜棱岩的新见解
DOI：
10.1111/jmg.12633
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发表时间：
2021
影响因子：
3.4
通讯作者：
Hirata Takafumi
中科院分区：
文献类型：
--
作者：
Nakamura Yoshihiro;Miyazaki Kazuhiro;Takahashi Yutaka;Iwano Hideki;Danhara Tohru;Hirata Takafumi
本文对日本中部构造线（MTL）上的花岗质糜棱岩（柏潮糜棱岩）和三八川变质岩（沿着）中的糜棱岩类龙屋变质岩进行了详细的岩石学、构造和年代学研究。柏尾糜棱岩位于中部地区的Oshika地区，是少数几个可以用来确定糜棱岩化过程中详细的压力-温度-时间-变形（P-T-t-D）路径的地质单元之一，因为它是许多糜棱岩变质沉积物的小构造块体。详细的岩石学分析结合传统的温压法和P-T假剖面模型，估计Ryoke变质岩的峰值P-T条件（M1a）为650-790°C，压力为4.6-5.6 kbar。片麻状Ryoke花岗岩类在峰期变质作用后，在约685-710°C和4.6-5.8 kbar的温度下近水平侵位。嘉潮剪切带是在最后一次火山活动后立即发展起来的。71 Ma时，可划分出两期糜棱岩化（D1和D2）。随着与MTL的距离逐渐减小，在5.2-2.6 kbar的压力下，柏潮糜棱岩中记录的退积P-T条件（M1 b）显示出温度从710°C到450°C的系统变化。相比之下，具有分区石榴石的高度变形的糜棱岩在低P/T型变质作用后，随着温度从590°C降低到450°C，压力从4.0 kbar显著增加到8.3 kbar。这种指示等温压缩的温度范围与由石英微结构和石英轴织物张角变形温度计确定的阶段D1的变形温度一致。此外，利用对寄主Ryoke花岗岩类的时间-温度关系的修正，估算了两次糜棱岩化事件发生的时间分别为69-67 Ma和66-64 Ma，冷却速率约为34°C/Ma。构造环境的快速变化与应变局部化发生在69和64 Ma之间的短暂时期。野外观察和岩石学研究表明，D_1糜棱岩带是由中地壳至俯冲界面的快速沉降（≥10 km）和上盘岩石冷却而形成的。研究认为，高P/T型变质岩的底侵作用导致上盘和下盘岩石之间进一步冷却，形成了一条狭窄的D2糜棱岩带，该糜棱岩带是古老的板块边界。因此，在折返之前，低P/T型和高P/T型变质带已经在俯冲带的脆-韧性转换下合并为成对的变质带。俯冲界面上盘岩石的快速冷却在折返的高P/T型变质岩的热叠加中起着重要作用。
We present a detailed petrological, structural and geochronological study of the mylonitic Ryoke metamorphic rocks within the granitic mylonite (Kashio mylonite) and Sanbagawa metamorphic rocks along the Median Tectonic Line (MTL), Japan. Located in the Oshika area of the Chubu District, the Kashio mylonite is one of the few geologic units that can be used to determine detailed pressure–temperature–time–deformation (P–T–t–D) paths during mylonitization because it occurs as many small tectonic blocks of mylonitic metasediment. Detailed petrological analysis coupled with conventional thermobarometry andP–Tpseudosection modelling give estimated peakP–Tconditions (M1a) of 650–790°C at 4.6–5.6 kbar for the Ryoke metamorphic rocks. The gneissose Ryoke granitoids were emplaced subhorizontally at around 685–710°C and 4.6–5.8 kbar, after peak metamorphism. The Kashio shear zone developed immediately after the last igneous activity at ca. 71 Ma, and two stages of mylonitization (stages D1 and D2) can be identified from microstructural observations. The retrogradeP–Tconditions (M1b) recorded in the Kashio mylonite exhibit a systematic change in temperature from 710°C to 450°C at 5.2–2.6 kbar with decreasing distance from the MTL. By contrast, highly deformed mylonites with zoned garnets demonstrate a striking increase in pressure from 4.0 to 8.3 kbar with decreasing temperature from 590°C to 450°C after low‐P/T‐type metamorphism. Such a temperature range indicating isothermal compression is consistent with deformation temperatures of stage D1 determined from quartz microstructures and quartzc‐axis fabric opening‐angle deformation thermometer. Moreover, the timing of the two mylonitization episodes during retrograde metamorphism are estimated to be 69–67 and 66–64 Ma, respectively, with a high cooling rate of ~34°C/Ma using the revised time–temperature relationship of the host Ryoke granitoids. The rapid change in tectonic setting with strain localization occurred during the brief period between 69 and 64 Ma. Our field and petrological observations imply that a thick D1 mylonite zone was formed by rapid subsidence (≥10 km) with cooling of the hangingwall rocks from the middle crust to the subduction interface. It is considered that the underplating of exhumed high‐P/T‐type metamorphic rocks led to further cooling between hangingwall and footwall rocks and the formation of a narrow D2 mylonite zone, which served as an old plate boundary. Thus, low and high‐P/T‐type metamorphic belts had already been amalgamated as paired metamorphic belts beneath the brittle–ductile transition of the subduction zone before exhumation. The rapid cooling of hangingwall rocks at the subduction interface is proposed to play an essential role in the thermal overprinting of exhumed high‐P/T‐type metamorphic rocks.