High-Al garnet-sillimanite-graphite gneisses (khondalites) from the Matale area in central Sri Lanka show evidence for open system formation of symplectites. Three types of khondalite (type A, B, and C) were collected from two localities (Lo1 and Lo2) in this area. In Lo1, khondalite type A shows a stable mineral assemblage with garnet (Grt), ribbon quartz (Qtz), prismatic sillimanite (Sil), alkali feldspar (Akfs) and minor graphite. The abundant leucosome is interpreted as (modified) crystallised melt (Liq). Qtz is invariably rimmed by Akfs double corona. In type B, Grt and Sil are partially rimmed by spinel (Spl)-Akfs symplectites. Type C samples show the extensive development of Spl-Akfs symplectites, locally with Bt, as well as corundum (Crn)-Akfs symplectites after Sil in domains relatively remote from Grt. Mineral inclusions in Grt suggest advanced biotite dehydration melting producing Grt ± Spl + Liq, similar to other Sri Lankan metapelitic granulites. Peak P-T conditions are ~8–9 kbar and ~ 850 °C. The symplectites developed on the retrograde path, with Ti in Bt thermometry indicating 700–720 °C. The chemical potential calculations indicated that the chemical potential gradient from garnet to sillimanite could well have produced the AKfs+Spl symplectite. Hence the most critical point is the initiation of Grt breakdown reactions which can facilitate the formation of Spl + Akfs and Crn + Akfs symplectites after sillimanite via the chemical potential gradient. Textural observations coupled with mass balance of symplectitic reaction textures indicate that addition of external Na and K to reaction sites is needed to trigger the symplectite formation. Excess Fe and Mg from Grt breakdown probably diffused away from garnet along a chemical potential gradient. As a result, Sil in the vicinity of Grt has broken down to Spl-Akfs symplectites, whereas Sil further away from Grt produced Crn-Akfs symplectites. Spl-Akfs and Crn-Akfs symplectites after Grt and Sil in khondalites are restricted to the boundary zone between the Wanni and Highland Complexes. The formation of symplectites is inferred to have taken place during or just after a decompression stage with minor cooling, under the influence of an alkaline fluid derived from nearby pegmatites, when the assemblage Grt + Sil was already metastable. The combination of fluids and symplectites indicates a very fast process rate.

中文翻译：

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在碱性流体存在下的辛盐生长：来自斯里兰卡高地复合体高铝变质沉积物的证据

来自斯里兰卡中部马塔莱地区的高铝石榴石-硅线石-石墨片麻岩（孔达岩）显示了辛长岩形成开放系统的证据。从该地区的两个地点（Lo1 和 Lo2）采集了三种类型的孔达石（A、B 和 C 型）。在 Lo1 中，孔达石 A 型显示出稳定的矿物组合，包括石榴石 (Grt)、带状石英 (Qtz)、棱柱状硅线石 (Sil)、碱性长石 (Akfs) 和少量石墨。丰富的白质体被解释为（修饰的）结晶熔体（Liq）。Qtz 总是被 Akfs 双电晕包围。在 B 型中，Grt 和 Sil 部分被尖晶石 (Spl)-Akfs 辛晶石包围。C 型样品显示了 Spl-Akfs 辛聚合体的广泛发展，局部有 Bt，以及在相对远离 Grt 的域中 Sil 之后的刚玉 (Crn)-Akfs 辛聚合体。Grt 中的矿物包裹体表明高级黑云母脱水熔融产生 Grt ± Spl + Liq，类似于其他斯里兰卡变泥质麻粒岩。峰值 PT 条件为 ~8–9 kbar 和 ~ 850 °C。辛晶在逆行路径上发展，Bt 温度测量中的 Ti 指示 700-720 °C。化学势计算表明，从石榴石到硅线石的化学势梯度很可能产生了 AKfs+Spl 辛钙石。因此，最关键的一点是 Grt 分解反应的开始，它可以通过化学势梯度促进 Spl + Akfs 和 Crn + Akfs 辛晶石在硅线石之后的形成。纹理观察加上辛晶反应结构的质量平衡表明需要向反应位点添加外部 Na 和 K 才能触发辛晶形成。Grt 分解产生的过量 Fe 和 Mg 可能沿着化学势梯度从石榴石中扩散出去。结果，Grt 附近的 Sil 分解为 Spl-Akfs 辛聚合体，而远离 Grt 的 Sil 产生 Crn-Akfs 辛聚合体。孔达岩中 Grt 和 Sil 之后的 Spl-Akfs 和 Crn-Akfs 辛合岩仅限于万尼和高地杂岩之间的边界带。在来自附近伟晶岩的碱性流体的影响下，当组合 Grt + Sil 已经处于亚稳态时，推测在减压阶段期间或刚好在减压阶段之后形成辛晶岩。流体和辛形体的组合表明处理速度非常快。Grt 附近的 Sil 已分解为 Spl-Akfs 辛聚合体，而远离 Grt 的 Sil 产生 Crn-Akfs 辛聚合体。孔达岩中 Grt 和 Sil 之后的 Spl-Akfs 和 Crn-Akfs 辛合岩仅限于万尼和高地杂岩之间的边界带。在来自附近伟晶岩的碱性流体的影响下，当组合 Grt + Sil 已经处于亚稳态时，推测在减压阶段期间或刚好在减压阶段之后发生了辛晶岩的形成。流体和辛晶石的组合表明处理速度非常快。Grt 附近的 Sil 已分解为 Spl-Akfs 辛聚合体，而远离 Grt 的 Sil 产生 Crn-Akfs 辛聚合体。孔达岩中 Grt 和 Sil 之后的 Spl-Akfs 和 Crn-Akfs 辛合岩仅限于万尼和高地杂岩之间的边界带。在来自附近伟晶岩的碱性流体的影响下，当组合 Grt + Sil 已经处于亚稳态时，推测在减压阶段期间或刚好在减压阶段之后发生了辛晶岩的形成。流体和辛晶石的组合表明处理速度非常快。孔达岩中 Grt 和 Sil 之后的 Spl-Akfs 和 Crn-Akfs 辛合岩仅限于万尼和高地杂岩之间的边界带。在来自附近伟晶岩的碱性流体的影响下，当组合 Grt + Sil 已经处于亚稳态时，推测在减压阶段期间或刚好在减压阶段之后发生了辛晶岩的形成。流体和辛晶石的组合表明处理速度非常快。孔达岩中 Grt 和 Sil 之后的 Spl-Akfs 和 Crn-Akfs 辛合岩仅限于万尼和高地杂岩之间的边界带。在来自附近伟晶岩的碱性流体的影响下，当组合 Grt + Sil 已经处于亚稳态时，推测在减压阶段期间或刚好在减压阶段之后发生了辛晶岩的形成。流体和辛晶石的组合表明处理速度非常快。