Progress of actinolite-forming reactions in mafic schists during retrograde metamorphism: an example from the Sanbagawa metamorphic belt in central Shikoku, Japan
Progress of actinolite-forming reactions in mafic schists during retrograde metamorphism: an example from the Sanbagawa metamorphic belt in central Shikoku, Japan
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DOI：
10.1111/j.1525-1314.2005.00580.x
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发表时间：
2005-06-01
影响因子：
3.4
通讯作者：
Toriumi, M
中科院分区：
文献类型：
--
作者：
Okamoto, A;Toriumi, M
水化反应是区域变质作用期间流体-岩石相互作用的直接证据。在本研究中，研究了镁铁质片岩中产生逆生阳起石的水合反应，以评估反应进程的控制因素。三八川带镁铁质片岩含有角闪石，并与绿帘石、绿泥石、斜长石和石英共存。角闪石从核心到边缘通常显示出两种类型的成分带：高品位区为斜角闪石 -> 角闪石 -> 阳起石，低品位区为绞石 -> 阳起石。两种类型都表明角闪石是在变质带折返阶段生长的。角闪石分带和质量平衡关系的微观结构表明：（1）阳起石形成反应以预先存在的角闪石为代价进行； (2)当一摩尔预先存在的角闪石发生反应时，角闪石的分解反应比角闪石消耗更多的H2O流体。反应进程以阳起石占总角闪石的体积分数Y-act表示，具体如下：（1）反应在各镁铁质层中均匀进行； (2) 角闪石分解反应的程度一般较低(Y-act < 0.5)，但在石榴石带的高品位部分急剧增加(Y-act > 0.7)； (3) winchite 分解反应的程度通常较高 (Y-act > 0.7)。在角闪石内观察到许多微裂纹，角闪石分解反应的程度与角闪石核心尺寸的减小相关。角闪石的脆性断裂可能通过增加H2O的流入和角闪石的表面积来增强逆反应进程。与高品位岩石相比，低品位岩石中的绞岩分解反应进展顺利，其中反应进展与绞岩的脆性断裂无关。低品位岩石中的高反应程度可能是由于反应前的温奇石尺寸较小。
Hydration reactions are direct evidence of fluid-rock interaction during regional metamorphism. In this study, hydration reactions to produce retrograde actinolite in mafic schists are investigated to evaluate the controlling factors on the reaction progress. Mafic schists in the Sanbagawa belt contain amphibole coexisting with epidote, chlorite, plagioclase and quartz. Amphibole typically shows two types of compositional zoning from core to rim: barroisite -> hornblende -> actinolite in the high-grade zone, and winchite -> actinolite in the low-grade zone. Both types indicate that amphibole grew during the exhumation stage of the metamorphic belt. Microstructures of amphibole zoning and mass-balance relations suggest that: (1) the actinolite-forming reactions proceeded at the expense of the preexisting amphibole; and (2) the breakdown reaction of hornblende consumed more H2O fluid than that of winchite, when one mole of preexisting amphibole was reacted. Reaction progress is indicated by the volume fraction of actinolite to total amphibole, Y-act, with the following details: (1) reaction proceeded homogeneously in each mafic layer; (2) the extent of the hornblende breakdown reaction is commonly low (Y-act < 0.5), but it increases drastically in the high-grade part of the garnet zone (Y-act > 0.7); and (3) the extent of the winchite breakdown reaction is commonly high (Y-act > 0.7). Many microcracks are observed within hornblende, and the extent of hornblende breakdown reaction is correlated with the size reduction of the hornblende core. Brittle fracturing of hornblende may have enhanced retrograde reaction progress by increasing of influx of H2O and the surface area of hornblende. In contrast to high-grade rocks, the winchite breakdown reaction is well advanced in the low-grade rocks, where reaction progress is not associated with brittle fracturing of winchite. The high extent of the reaction in the low-grade rocks may be due to small size of winchite before the reaction.