Metamorphic record of the Asemi‐gawa eclogite unit in the Sanbagawa belt, southwest Japan: Constraints from inclusions study in garnet porphyroblasts
Metamorphic record of the Asemi‐gawa eclogite unit in the Sanbagawa belt, southwest Japan: Constraints from inclusions study in garnet porphyroblasts
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日本西南部三波川带阿塞美川榴辉岩单元的变质记录：石榴石成斑细胞包裹体研究的限制
DOI：
10.1111/jmg.12456
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发表时间：
2018
影响因子：
3.4
通讯作者：
Kouketsu Yui
中科院分区：
文献类型：
--
作者：
Taguchi Tomoki;Enami Masaki;Kouketsu Yui
三八川带是世界上著名的俯冲高压变质带之一。然而，榴辉岩的空间分布在带尚未令人满意的建立，除了在别子地区，四国中部，日本西南部，因为大多数榴辉岩的岩石在折返过程中受到较低的压力叠加。为了更好地确定榴辉岩单元的区域分布及其变质特征，采用电子探针显微分析仪和拉曼光谱法对四国中部Asemi-gawa地区的泥质和镁铁质片岩进行了石榴石变斑岩包体岩相学研究。所有泥质片岩样品都是高度退化的，并且在基质中不包括指标HP矿物如硬玉、绿辉石、钠云母或蓝闪石。泥质片岩中的石榴子石变斑晶呈半自形或他形晶体，具有不规则形状的内部段和过度生长的外部段的成分分带，其边界以spessartine的不连续变化为标志。这一特征表明，内节的吸收过程发生在外节形成之前，表明两个节段之间的不连续结晶。某些复合带石榴石颗粒的内段显示Mn振荡，意味着在最初折返阶段变质流体的渗透。早期榴辉岩相事件的证据是从矿物包裹体（例如，硬玉、钠长岩、蓝闪石）。基性片岩在基质中不含指标HP矿物，这与泥质片岩不同。镁铁质片岩中的石榴子石颗粒显示出简单的正常环带，在晶体形成过程中没有不连续的生长。石榴石中没有指示HP矿物包裹体，因此没有证据表明榴辉岩相条件。泥质和镁铁质片岩中石榴石中的石英包裹体显示残余压力值（ε ω1）分别>8.5 cm− 1和<8.5 cm− 1。结合拉曼地质压力测量和常规热力学计算，给出泥质片岩在460-520°C时的峰值P-T条件为1.6-2.1 GPa。根据拉曼测温结果，将镁铁质片岩中石英包裹体的δ ω 1值换算为466-549°C时的变质压力为1.2-1.4 GPa。这些结果表明，在镁铁质片岩和几乎相邻的泥质片岩之间确实存在压力差，它们经历了不同的变质历史。此外，Asemi-gawa榴辉岩单元的峰值P-T值与四国中部Besshi地区的Sanbagawa榴辉岩单元的峰值P-T值一致，表明这些榴辉岩单元具有相似的P-T轨迹。Asemi-gawa榴辉岩单元存在于有限的区域内，主要由泥质片岩组成。我们推断，这些丰富的泥质片岩通过降低大块岩石密度和强度，在浮力驱动的折返中发挥了关键作用。
The Sanbagawa belt is one of the famous subduction‐related high‐pressure (HP) metamorphic belts in the world. However, spatial distributions of eclogite units in the belt have not yet satisfactorily established, except within the Besshi region, central Shikoku, southwest Japan because most eclogitic rocks were affected by lower pressure overprinting during exhumation. In order to better determine the areal distribution of the eclogite units and their metamorphic features, inclusion petrography of garnet porphyroblasts using a combination of electron probe microanalyser and Raman spectroscopy was applied to pelitic and mafic schists from the Asemi‐gawa region, central Shikoku. All pelitic schist samples are highly retrogressed, and include no index HP minerals such as jadeite, omphacite, paragonite, or glaucophane in the matrix. Garnet porphyroblasts in pelitic schists occur as subhedral or anhedral crystals, and show compositional zoning with irregular‐shaped inner segments and overgrown outer segments, the boundary of which is marked by discontinuous changes in spessartine. This feature suggests that a resorption process of the inner segment occurred prior to the formation of the outer segment, indicating discontinuous crystallization between the two segments. The inner segment of some composite‐zoned garnet grains displays Mn oscillations, implying infiltration of metamorphic fluid during the initial exhumation stage. Evidence for an early eclogite facies event was determined from mineral inclusions (e.g., jadeite, paragonite, glaucophane) in the garnet inner segments. Mafic schists include no index HP minerals in the matrix as with pelitic schists. Garnet grains in mafic schists show simple normal zoning, recording no discontinuous growth during crystal formation. There are no index HP mineral inclusions in the garnet, and thus no evidence suggesting eclogite facies conditions. Quartz inclusions in garnet of the pelitic and mafic schists show residual pressure values (∆ω1) of >8.5 cm−1and <8.5 cm−1respectively. The combination of Raman geobarometry and conventional thermodynamic calculations gives peakP–Tconditions of 1.6–2.1 GPa at 460–520°C for the pelitic schists. The ∆ω1values of quartz inclusions in mafic schists are converted to a metamorphic pressure of 1.2–1.4 GPa at 466–549°C based on Raman geothermometry results. These results indicate that a pressure gap definitely exists between the mafic schists and the almost adjacent pelitic schists, which have experienced a different metamorphic history. Furthermore, the peakP–Tvalues of the Asemi‐gawa eclogite unit are compatible with those of Sanbagawa eclogite unit in the Besshi region of central Shikoku, suggesting that these eclogite units share a similarP–Ttrajectory. The Asemi‐gawa eclogite unit exists in a limited area and is composed of mostly pelitic schists. We infer that these abundant pelitic schists played a key role in buoyancy‐driven exhumation by reducing bulk rock density and strength.