The controversy over the origin of massive chromitites in layered intrusions has recently become more contentious than ever before. At issue is whether they are produced via gravity settling/in situ crystallization of chromite directly on the chamber floor or by kinetic sieving, metasomatic replacement, or sill-like intrusions beneath the chamber floor, i.e., in deep parts of the cumulate pile. The latter group of models implies that massive chromitites have never been on the chamber floor. In this paper, we show that decisive clues to the ‘chamber floor dilemma’ come from field relations of massive chromitite with magmatic dropstones in the Bushveld Complex, South Africa - the largest fossilized magma chamber in the Earth's continental crust. The roof sequences in such basaltic magma chambers are known to be inherently unstable; therefore, crustal instability results in their breakdown and collapse as angular blocks (i.e., magmatic dropstones) through the resident melt column onto the upward-growing chamber floor. We have discovered such magmatic dropstones in the sequences that host massive UG1, UG2, and MG2 chromitites in the Critical Zone of the Bushveld Complex. The dropstones are easily recognizable in the field because they are composed of fine-grained melanorite/orthopyroxenite, which are texturally dissimilar from and have sharp contacts with adjacent cumulate rocks (chromitite, anorthosite, norite, etc.). The dropstones range in shape from angular to lenticular fragments a few centimetres in size to large tabular dropstones of ∼0.5–1.0 m across and ∼10 m long. The dropstones indent pre-existing layers of chromitite/anorthosite beneath them and are covered by subsequently deposited layers of the same rocks above them. Some dropstones appeared to have knifed into chromitite/anorthosite layers and cut them off, and a few dropstones appeared to have been driven into the floor cumulates and crumpled the layers outwards and upward. Also, some beheaded dropstones indicate the truncation of the floor cumulates by planar erosional surfaces. The physical relationships of dropstones with their host rocks indicate that: (1) there was almost invariably a sharp interface between the top of the inward-growing cumulate pile and the overlying resident melt; (2) the uppermost part of the cumulate pile was coherent and igneous layering, involving chromitites, was generally fully developed right up to the crystal-liquid interface; (3) by the time of dropstones's landing, the chromitite layers were already formed on the chamber floor, with the resident melt immediately overlying them; (4) the existence of chromitite layers at the crystal-liquid interface implies their formation through the deposition of chromite alone directly onto the chamber floor; and (5) such crystal deposition requires the resident melt in the chamber to be chromite-only saturated (i.e., no other liquidus phases in the chamber). The occurrence of magmatic dropstones thus rules out the formation of massive chromitites by processes that operate at some depth in the cumulate pile (e.g., kinetic sieving, metasomatic replacement, or sill-like intrusions) and rather indicate their deposition via in situ crystallization from a chromite-only-saturated magma directly on the upward-growing floor of a Bushveld magma chamber. A thorough review of this concept demonstrates its high potential in explaining most field, textural and chemical features of massive chromitites in the Bushveld Complex.

中文翻译：

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南非布什维尔德杂岩体的大量铬铁矿：对现有假设的批判性回顾


关于层状侵入体中块状铬铁矿起源的争论最近变得比以往任何时候都更加有争议。问题在于它们是否是通过铬铁矿直接在室底板上的重力沉降/原位结晶产生的，还是通过动力学筛分、交代置换或室底板下方（即堆积堆的深处）的窗台状侵入产生的。后一组模型表明，巨大的铬铁矿从未出现在房间地板上。在本文中，我们表明，“室底困境”的决定性线索来自南非布什维尔德杂岩中块状铬铁矿与岩浆落石的场关系，布什维尔德杂岩是地球大陆地壳中最大的化石岩浆室。众所周知，这种玄武岩岩浆房的顶部序列本质上是不稳定的。因此，地壳不稳定性导致它们作为角块（即岩浆落石）通过驻留的熔体柱破裂并塌陷到向上生长的室底板上。我们在 Bushveld 杂岩临界带中含有大量 UG1、UG2 和 MG2 铬铁矿的序列中发现了此类岩浆落石。落石在现场很容易识别，因为它们由细粒黑长石/斜方辉石岩组成，其结构与邻近的堆积岩（铬铁矿、钙长石、苏长岩等）不同，并且与相邻的堆积岩有明显的接触。落石的形状多种多样，从几厘米大小的角形碎片到透镜状碎片，再到宽约 0.5-1.0 m、长约 10 m 的大型板状落石。这些落石在其下方凹进了预先存在的铬铁矿/斜长石层，并被随后在其上方沉积的相同岩石层所覆盖。 一些落石似乎已切入铬铁矿/斜长石层并将其切断，而一些落石似乎已被推入地板堆积物中并将层向外和向上弄皱。此外，一些斩首的落石表明地面是由平面侵蚀表面累积而成的。落石与其主岩的物理关系表明：（1）向内生长的堆积物顶部与上覆的常驻熔体之间几乎总是存在尖锐的界面； (2)堆积堆的最上部是连贯的火成岩层状，涉及铬铁矿，一般完全发育到晶液界面； (3) 当落石落地时，铬铁矿层已经在室底板上形成，并且驻留的熔体立即覆盖在它们上面； (4) 晶液界面处铬铁矿层的存在意味着它们是通过直接将铬铁矿单独沉积到室底板上而形成的； (5)这种晶体沉积要求室中的驻留熔体仅是铬铁矿饱和的(即室中没有其他液相)。因此，岩浆落石的出现排除了通过在堆积堆中一定深度进行的过程（例如动力筛分、交代置换或基台状侵入）形成块状铬铁矿的可能性，而是表明它们是通过原位结晶而沉积的。仅含铬铁矿的饱和岩浆直接位于布什维尔德岩浆房向上生长的底部。对这一概念的彻底回顾表明，它在解释布什维尔德杂岩中块状铬铁矿的大多数现场、结构和化学特征方面具有巨大潜力。