Orthopyroxenite cumulates throughout the Critical Zone of the Bushveld Complex commonly contain prominent euhedral crystals of bottle-green augite, typically around 1 cm in size and surrounded by “haloes” of nearly pure plagioclase. On close examination, the augite grains can be seen to be oikocrysts, containing extensively resorbed chadacrysts of orthopyroxene, indicating an effective peritectic reaction relationship between the two pyroxenes. A detailed textural study of one such layer from the UG3 Unit in the Eastern Bushveld reveals some distinctive grain-scale features, including the presence of extensively 3D-interconnected chains of thousands of cumulus chromite grains that surround the orthopyroxene grains and extend through the augite oikocrysts. The high degree of interconnectivity is remarkable in view of the relatively low (5 vol.%) modal proportion of chromite in the sample. Other noteworthy features include zoning of the oikocrysts towards higher incompatible element contents in the rims and the presence of the plagioclase haloes around the augite oikocrysts. These haloes are essentially narrow zones devoid of orthopyroxene inclusions developed within large plagioclase oikocrysts that overgrow all the other phases. We propose a mechanism whereby the plagioclase oikocrysts grow early in communication with the main magma body within an initial crystal mush of orthopyroxene and chain-textured chromite. The clinopyroxene oikocrysts grew in the remaining pore space in such a way that dissolution of orthopyroxene occurred within a narrow few-mm wide chemical boundary layer ahead of the advancing oikocryst margin. The relative rates of crystallisation and dissolution were controlled by limited chemical diffusion through the boundary layer such that orthopyroxene grains more than a few grain diameters away showed no reaction at all. The plagioclase haloes developed as a result of continuing growth of the plagioclase oikocrysts to overtake the growing pyroxene oikocrysts, locking in the orthopyroxene-depleted boundary layer, and preserving euhedral cumulus morphologies away from the pyroxene oikocrysts. This texture represents a circumstance whereby oikocrysts in the same rock develop at different overlapping stages: plagioclase oikocrysts forming early in diffusive connection with the magma column, and clinopyroxene forming peritectic poikilitic textures within residual liquid pockets chemically isolated from the overlying magma body.

中文翻译：

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Bushveld 复合体凝固前沿内的包晶反应产生的单晶辉石自形晶体

正辉石在整个 Bushveld Complex 的临界区堆积，通常包含突出的深绿色辉石自形晶体，通常大小约为 1 厘米，周围环绕着几乎纯斜长石的“光晕”。仔细观察，可以看到辉石晶粒是 oikocrysts，包含广泛吸收的斜辉石 chadacrysts，表明两种辉石之间存在有效的包晶反应关系。对来自东布什维尔德 UG3 单元的一个这样的层的详细纹理研究揭示了一些独特的颗粒尺度特征，包括存在广泛的 3D 互连链，这些链由数以千计的铬铁矿颗粒组成，这些颗粒围绕着斜方辉石颗粒并延伸穿过辉石晶体. 鉴于相对较低（第 5 卷），高度的互连性是显着的。%) 样品中铬铁矿的模态比例。其他值得注意的特征包括卵晶向边缘中较高不相容元素含量的分区以及辉石卵晶周围存在斜长石晕。这些晕圈基本上是狭窄的区域，没有在大型斜长石oikocrysts 内发育的斜方辉石包裹体，所有其他相都长满了。我们提出了一种机制，通过这种机制，斜长石晶体在与主要岩浆体连通的早期生长，并在斜方辉石和链状纹理铬铁矿的初始晶体糊状物中形成。斜辉石oikocrysts 以这样的方式在剩余的孔隙空间中生长，即在前进的oikocryst 边缘之前，在狭窄的几毫米宽的化学边界层内发生斜辉石的溶解。结晶和溶解的相对速率受通过边界层的有限化学扩散控制，使得比几个晶粒直径远的斜方辉石晶粒根本没有反应。斜长石晕的形成是由于斜长石卵晶持续生长以超过正在生长的辉石卵晶，锁定在正辉石耗尽的边界层，并保持自形积云形态远离辉石卵晶。这种纹理代表了同一岩石中的卵晶在不同重叠阶段发育的情况：斜长​​石卵晶在与岩浆柱的扩散连接中形成早期，单斜辉石在与上覆岩浆体化学隔离的残余液体袋中形成包晶质晶纹理。