The Rum Layered Suite, NW Scotland, hosts Cr-spinel seams at the bases of peridotite–troctolite macro-rhythmic units in the eastern portion of the intrusion. Here, we present detailed field observations together with microstructural and mineral chemical analyses for the Unit 7–8 Cr-spinel seam and associated cumulates in the Eastern Layered Intrusion. Detailed mapping and sampling reveal significant lateral variations in the structural characteristics and mineral compositions of the Unit 7–8 boundary zone rocks. Although the Cr-spinel seam is laterally continuous over ~ 3 km, it is absent towards the centre and the margins of the intrusion. The compositional characteristics of Cr-spinel and plagioclase vary systematically along strike, exhibiting a chemical evolution towards more differentiated compositions with increasing distance from the main feeder conduit of the Rum intrusion; the Long Loch Fault. On the basis of our combined datasets, we propose that the upper part of the troctolite, the anorthosite layer underlying the Cr-spinel seam and the seam itself formed during a multi-stage magma replenishment event. The stages can be summarised as follows: (1) peridotite schlieren and anorthosite autoliths formed following melt infiltration and cumulate assimilation in the crystal mush of the Unit 7 troctolite. (2) The anorthosite layer then formed from the Unit 7 troctolite crystal mush by thermal erosion and dissolution due to infiltrating magma. (3) Subsequent dissolution of the anorthosite layer by new replenishing magma led to peritectic in situ crystallisation of the Unit 7–8 Cr-spinel seam, with (4) continued magma input eventually producing the overlying Unit 8 peridotite. In the central part of the Rum Layered Suite, the aforementioned assimilation of the troctolitic footwall formed the anorthosite layer. However, the absence of anorthosite in close proximity to the Long Loch Fault can be explained by enhanced thermochemical erosion close to the feeder zone, and its absence close to the margins of the intrusion, at maximum distance from the Long Loch Fault, may be due to cooling of the magma and loss of erosion potential. In line with other recent studies on PGE-bearing chromitites in layered intrusions, we highlight the importance of multi-stage intrusive magma replenishment to the formation of spatially coupled anorthosite and Cr-spinel seams, as well as the lateral mineral chemical variations observed in the Unit 7–8 boundary zone cumulates.

中文翻译：

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苏格兰西北部朗姆酒东部层状侵入体 7-8 单元边界区的横向变化：对铬尖晶石接缝形成的起源和时间的影响

位于苏格兰西北部的 Rum Layered Suite 在侵入体东部的橄榄岩 - troctolite 宏观韵律单元的底部拥有 Cr 尖晶石接缝。在这里，我们对东部层状侵入体中的 Unit 7-8 Cr-尖晶石层和相关堆积物进行了详细的实地观察以及微观结构和矿物化学分析。详细的绘图和采样揭示了 Unit 7-8 边界带岩石的结构特征和矿物组成的显着横向变化。虽然铬尖晶石接缝横向连续超过 3 公里，但在侵入体的中心和边缘不存在。Cr-尖晶石和斜长石的成分特征沿走向有系统的变化，随着与朗姆酒入侵的主要进料管道的距离增加，表现出向更差异化成分的化学演化；长湖断层。基于我们的组合数据集，我们提出在多阶段岩浆补充事件期间形成的梭长岩上部、铬尖晶石层下方的斜长岩层和煤层本身。这些阶段可概括如下： (1) 橄榄岩纹影和斜长岩自溶岩在熔体渗透并在 7 号单元滑石的结晶糊中积累同化后形成。(2) 斜长岩层是由 7 单元的 troctolite 晶体糊状物由于岩浆浸润而热侵蚀和溶解形成的。(3) 新补充的岩浆对斜长岩层的后续溶解导致 7-8 单元 Cr 尖晶石层的包晶原位结晶，(4) 持续的岩浆输入最终产生了上覆的 8 单元橄榄岩。在朗姆酒分层组曲的中央部分，上述对褶皱岩下盘的同化作用形成了斜长岩层。然而，在靠近长湖断层的地方没有斜长岩可以用靠近补给带的热化学侵蚀增强来解释，而在距离长湖断层最大距离的侵入体边缘附近没有斜长岩可能是由于岩浆冷却和侵蚀潜力丧失。与最近对层状侵入体中含 PGE 的铬铁矿的其他研究一致，