Preservation of partially completed metamorphic reactions in the form of partial pseudomorphs is very important as it provides direct insight onto the reaction mechanism and the phases involved in the reaction. The staurolite and andalusite grade rocks in western Maine, USA, contain cordierite porphyroblasts partly pseudomorphed by coarse‐grained muscovite and biotite. The pseudomorphs consist of a cordierite core surrounded by a reaction rim. Modal mineralogy, calculated using the ImageJ processing software based on backscatter images and X‐ray compositional maps, reveals that the core consists of cordierite (53.5%), muscovite (22.8%), biotite (9.1%), quartz (1 0.4%), plagioclase (3.1%) and ilmenite/pyrrhotite and apatite (1.1%) whereas the reaction rim consists of cordierite (1.8%), muscovite (51.6%), biotite (30.4%), quartz (4.3%), plagioclase (10%), garnet (1.2%), ilmenite/pyrrhotite and apatite (0.8%). The net effect of the cordierite breakdown reaction is an increase of 226% in muscovite, 334% in biotite and 323% in plagioclase content and a decrease of 97% in cordierite. The reaction involved exchange of components with the matrix requiring addition of H2O, K+, Na+ and Ti4+ and removal of SiO2, Mg2+ and PO43‐ from the reaction site. PT estimates using the garnet–biotite, Ti‐in‐biotite, Na‐in‐cordierite thermometers and the garnet–biotite–muscovite–plagioclase barometer indicate that cordierite breakdown occurred at ~550°C and 3.5 kbar. thermocalc modelling using the bulk rock composition suggests that cordierite is not stable at these conditions, whereas modelling using a thin section‐derived bulk composition indicates that cordierite stability extends to higher pressures, and most likely that the cordierite breakdown was not PT dependent. The incorporation of Na (up to 0.18 a.f.u.) into the cordierite structure has the effect of stabilizing the cordierite under a variety of H2O activity and limiting the role of fluids into destabilizing it. The cordierite cores contain evidence of plastic and brittle deformation in the form of subgrains and microcracks, which facilitated the infiltration of fluids that destabilized cordierite at constant PT conditions by leaching Na and introducing K. New mica growth along these structural heterogeneities suggests that deformation played an important role promoting breakdown of cordierite to muscovite and biotite.

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变形引起的堇青石分解——以美国缅因州西部为例

以部分假晶型的形式保存部分完成的变质反应非常重要，因为它可以直接了解反应机理和反应中涉及的相。美国缅因州西部的十字石和红柱石级岩石含有堇青石斑状体，部分由粗粒白云母和黑云母假形。假晶质由被反应边缘包围的堇青石核心组成。模态矿物学，使用基于背散射图像和 X 射线成分图的 ImageJ 处理软件计算，显示核心由堇青石 (53.5%)、白云母 (22.8%)、黑云母 (9.1%)、石英 (1 0.4%) 组成、斜长石 (3.1%) 和钛铁矿/磁黄铁矿和磷灰石 (1.1%) 而反应边缘由堇青石 (1.8%)、白云母 (51.6%)、黑云母 (30.4%)、石英 (4.3%) 组成，斜长石 (10%)、石榴石 (1.2%)、钛铁矿/磁黄铁矿和磷灰石 (0.8%)。堇青石分解反应的净效应是白云母增加 226%，黑云母增加 334%，斜长石增加 323%，堇青石减少 97%。该反应涉及组分与基质的交换，需要添加 H2O、K+、Na+ 和 Ti4+，并从反应位点去除 SiO2、Mg2+ 和 PO43-。使用石榴石-黑云母、Ti-in-黑云、Na-in-堇青石温度计和石榴石-黑云母-白云母-斜长石气压计的 PT 估计表明，堇青石分解发生在 ~550°C 和 3.5 kbar。使用大块岩石成分的热计算模型表明堇青石在这些条件下不稳定，而使用薄截面衍生的整体成分建模表明堇青石稳定性扩展到更高的压力，并且很可能堇青石分解不依赖于 PT。将 Na（高达 0.18 afu）掺入堇青石结构具有在各种 H2O 活性下稳定堇青石的作用，并限制流体对其不稳定的作用。堇青石核包含亚晶粒和微裂纹形式的塑性和脆性变形的证据，这促进了流体的渗透，通过浸出 Na 和引入 K 使堇青石在恒定的 PT 条件下不稳定。沿着这些结构异质性的新云母生长表明变形起到了促进作用。促进堇青石分解为白云母和黑云母的重要作用。并且很可能堇青石分解不依赖于 PT。将 Na（高达 0.18 afu）掺入堇青石结构具有在各种 H2O 活性下稳定堇青石的作用，并限制流体对其不稳定的作用。堇青石核包含亚晶粒和微裂纹形式的塑性和脆性变形的证据，这促进了流体的渗透，通过浸出 Na 和引入 K 使堇青石在恒定的 PT 条件下不稳定。沿着这些结构异质性的新云母生长表明变形起到了促进作用。促进堇青石分解为白云母和黑云母的重要作用。并且很可能堇青石分解不依赖于 PT。将 Na（高达 0.18 afu）掺入堇青石结构具有在各种 H2O 活性下稳定堇青石的作用，并限制流体对其不稳定的作用。堇青石核包含亚晶粒和微裂纹形式的塑性和脆性变形的证据，这促进了流体的渗透，通过浸出 Na 和引入 K 使堇青石在恒定的 PT 条件下不稳定。沿着这些结构异质性的新云母生长表明变形起到了促进作用。促进堇青石分解为白云母和黑云母的重要作用。) 进入堇青石结构具有在各种 H2O 活性下稳定堇青石并限制流体作用使其不稳定的作用。堇青石核包含亚晶粒和微裂纹形式的塑性和脆性变形的证据，这促进了流体的渗透，通过浸出 Na 和引入 K 使堇青石在恒定的 PT 条件下不稳定。沿着这些结构异质性的新云母生长表明变形起到了促进作用。促进堇青石分解为白云母和黑云母的重要作用。) 进入堇青石结构具有在各种 H2O 活性下稳定堇青石并限制流体作用使其不稳定的作用。堇青石核包含亚晶粒和微裂纹形式的塑性和脆性变形的证据，这促进了流体的渗透，通过浸出 Na 和引入 K 使堇青石在恒定的 PT 条件下不稳定。沿着这些结构异质性的新云母生长表明变形起到了促进作用。促进堇青石分解为白云母和黑云母的重要作用。