Melt must transfer through the lower crust, yet the field signatures and mechanisms involved in such transfer zones (excluding dykes) are still poorly understood. We report field and microstructural evidence of a deformation‐assisted melt transfer zone that developed in the lower crustal magmatic arc environment of Fiordland, New Zealand. A 30–40 m wide hornblende‐rich body comprising hornblende ± clinozoisite and/or garnet exhibits 'igneous‐like' features and is hosted within a metamorphic, two‐pyroxene–pargasite gabbroic gneiss (GG). Previous studies have interpreted the hornblende‐rich body as an igneous cumulate or a mass transfer zone. We present field and microstructural characteristics supporting the later and indicating the body has formed by deformation‐assisted, channelized, reactive porous melt flow. The host granulite facies GG contains distinctive rectilinear dykes and garnet reaction zones (GRZ) from earlier in the geological history; these form important reaction and strain markers. Field observations show that the mineral assemblages and microstructures of the GG and GRZ are progressively modified with proximity to the hornblende‐rich body. At the same time, GRZ bend systematically into the hornblende‐rich body on each side of the unit, showing apparent sinistral shearing. Within the hornblende‐rich body itself, microstructures and electron back‐scatter diffraction mapping show evidence of the former presence of melt including observations consistent with melt crystallization within pore spaces, elongate pseudomorphs of melt films along grain boundaries, minerals with low dihedral angles as small as <10° and up to <60°, and interconnected 3D melt pseudomorph networks. Reaction microstructures with highly irregular contact boundaries are observed at the field and thin‐section scale in remnant islands of original rock and replaced grains, respectively. We infer that the hornblende‐rich body was formed by modification of the host GG in situ due to reaction between an externally derived, reactive, hydrous gabbroic to intermediate melt percolating via porous melt flow through an actively deforming zone. Extensive melt–rock interaction and metasomatism occurred via coupled dissolution–precipitation, triggered by chemical disequilibrium between the host rock and the fluxing melt. As a result, the host plagioclase and pyroxene became unstable and were reacted and dissolved into the melt, while hornblende and to a lesser extent clinozoisite and garnet grew replacing the unstable phases. Our study shows that hornblendite rocks commonly observed within deep crustal sections, and attributed to cumulate fractionation processes, may instead delineate areas of deformation‐assisted, channelized reactive porous melt flow formed by melt‐mediated coupled dissolution–precipitation replacement reactions.

中文翻译：

---

变形辅助熔体转移的场和微观结构特征：来自新西兰岩浆弧下地壳的见解

熔体必须通过下地壳转移，但此类转移带（不包括岩脉）中涉及的场特征和机制仍知之甚少。我们报告了在新西兰峡湾下地壳岩浆弧环境中形成的变形辅助熔体转移带的现场和微观结构证据。一个 30-40 m 宽的富含角闪石的体，包含角闪石±斜长石和/或石榴石，具有“类火成岩”特征，位于变质的双辉石-帕格瓦石辉长片麻岩 (GG) 中。先前的研究已将富含角闪石的物体解释为火成岩堆积或传质区。我们展示了支持后者的场和微观结构特征，并表明主体是通过变形辅助的、通道化的、反应性多孔熔体流动形成的。主麻粒岩相GG包含地质历史早期独特的直线岩脉和石榴石反应带（GRZ）；这些形成重要的反应和应变标记。实地观察表明，GG 和 GRZ 的矿物组合和微观结构随着靠近富含角闪石的体而逐渐改变。同时，GRZ系统地弯曲到单元两侧富含角闪石的体中，显示出明显的左旋剪切。在富含角闪石的体内，微观结构和电子背散射衍射图显示了先前存在熔体的证据，包括与孔隙空间内熔体结晶一致的观察结果、沿晶界的熔体膜的拉长假晶、具有低二面角的矿物<10° 至 <60°，和相互连接的 3D 熔体伪形态网络。分别在原岩残余岛和置换颗粒的现场和薄片尺度上观察到具有高度不规则接触边界的反应微结构。我们推断富含角闪石的体是通过原位改性宿主 GG 形成的，这是由于外部衍生的、反应性的、含水的辉长岩与中间熔体之间的反应，通过多孔熔体流过主动变形区。广泛的熔体-岩石相互作用和交代通过溶解-沉淀耦合发生，由主岩和流动熔体之间的化学不平衡触发。结果，宿主斜长石和辉石变得不稳定，发生反应溶解到熔体中，而角闪石以及在较小程度上斜长石和石榴石的生长取代了不稳定相。我们的研究表明，通常在地壳深处观察到的角闪石岩石，归因于累积分馏过程，可能会描绘出由熔体介导的溶解-沉淀耦合置换反应形成的变形辅助、通道化反应多孔熔体流动区域。