Leucogranite bodies are ubiquitous in the upper structural levels of the Himalayan metamorphic slab. Their formation has ramifications for myriad processes including the generation of crustal melts and orogenic heat budgets. One particularly enigmatic variety of the Himalayan leucogranites, abundant in the Langtang region of Nepal, are the banded tourmaline leucogranites; typified by centimetre-scale compositional banding between tourmaline-rich and quartzo-feldspathic domains. Here, we use in-situ Rb–Sr isotopic chemistry and microstructural analysis to show these banded tourmaline leucogranites do not represent a direct product of melt crystallization but instead were formed by metasomatism of psammitic country-rock. This metasomatism was likely driven by the release of boron-rich volatiles during the crystallization of neighboring muscovite–biotite leucogranite bodies. Rb–Sr isochrons based on biotite–plagioclase ± white mica and K-feldspar data define overlapping dates of ca. 17.5 Ma from both the banded tourmaline leucogranite and its paired muscovite–biotite leucogranite. The characteristic banding appearance of these rocks is a product of heteroepitaxial nucleation of tourmaline on biotite folia and the replacement of pre-existing biotite and plagioclase with tourmaline and K-feldspar. The heteroepitaxy relationship of tourmaline on biotite is characterized by the {10–10} face of tourmaline parallel to biotite (001), with the tourmaline c-axis parallel to either the biotite [110] or [010] direction. One of the broader implications of our findings is that field estimates based on the volume of coarse-grained leucocratic outcrop overestimates the amount of melt generated at the top of the Himalayan slab.

淡色花岗岩体普遍存在于喜马拉雅变质板的上层构造层中。它们的形成对无数过程都有影响，包括地壳熔体的产生和造山热平衡。喜马拉雅淡色花岗岩中一种特别神秘的品种是带状碧玺淡色花岗岩，它在尼泊尔的 Langtang 地区盛产。以富含碧玺和石英长石域之间的厘米级成分带为代表。在这里，我们使用原位Rb-Sr 同位素化学和显微结构分析表明，这些带状碧玺隐色花岗岩并非熔融结晶的直接产物，而是由砂砾岩围岩交代作用形成的。这种交代作用可能是由邻近的白云母-黑云母淡色花岗岩体结晶过程中富硼挥发物的释放所驱动的。基于黑云母斜长石 ± 白云母和钾长石数据的 Rb-Sr 等时线定义了 ca 的重叠日期。17.5 Ma 来自带状碧玺隐色花岗岩及其成对的白云母-黑云母隐色花岗岩。这些岩石的特征带状外观是碧玺在黑云母叶上的异质外延成核以及用碧玺和钾长石替代先前存在的黑云母和斜长石的产物。c轴平行于黑云母 [110] 或 [010] 方向。我们研究结果的更广泛意义之一是，基于粗粒白层露头体积的现场估计高估了喜马拉雅板块顶部产生的熔体量。