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Mafic schlieren, crystal accumulation and differentiation in granitic magmas: an integrated case study
Contributions to Mineralogy and Petrology ( IF 3.5 ) Pub Date : 2020-04-30 , DOI: 10.1007/s00410-020-01689-x
J. D. Clemens , G. Stevens , S. le Roux , G. L. Wallis

Observations in the S-type granites (s.l.) of the Wilson’s Promontory batholith demonstrate that one type of schlieren in granitic rocks represent accumulations of mainly mafic magmatic minerals, with internal layering formed through pulsed magma flow. Loss of interstitial magmatic liquid played, at most, a minor role in shaping the preserved compositions of the schlieren; filter pressing was not involved, and simple gravity settling of crystals was also insignificant. Through dissolution-reprecipitation and reactions with residual magmatic liquids in the Wilsons Promontory schlieren, the original accumulated crystals of mafic minerals were largely supplanted by later generations or completely new phases. In the present case, the original accumulating minerals were garnet and orthopyroxene, with minor biotite and accessory minerals. The schlieren retain some of the early, euhedral, compositionally distinct, accumulated biotite, but most biotite formed through reaction of accumulated orthopyroxene and garnet with residual liquid. Some early, accumulated garnet remains, but this is not the peritectic garnet that was originally entrained into the magmas, at source depths. Rather, these are magmatic crystals formed, at mid-crustal depths, through dissolution–reprecipitation of the original peritectic garnet. At emplacement level, another episode of garnet dissolution–reprecipitation occurred, close to the solidus, extensively reorganising the grain-scale igneous textures. Although schlieren preserve structural, chemical and some textural features that can be used to infer their origins, their present microtextures do not fully reflect their initial formation mechanisms. Also, the physical mechanisms of schlieren formation most commonly have little similarity with the processes that were responsible for the main chemical variations in the batholith. Nevertheless, the presence of schlieren indicates that the granitic magmas were flowing in sheet-like laminae, and they provide information on the high-temperature igneous minerals that were suspended in their parent magmas, prior to emplacement.

中文翻译:

花岗质岩浆中的镁铁质纹影、晶体积累和分异:综合案例研究

对威尔逊岬基岩 S 型花岗岩 (sl) 的观察表明,花岗岩中的一种纹影代表主要是基性岩浆矿物的堆积,其内部层状结构是通过脉冲岩浆流形成的。间隙岩浆液的损失在塑造保存的纹影组成方面最多起次要作用;不涉及压滤,晶体的简单重力沉降也无关紧要。通过溶解-再沉淀以及与威尔逊岬纹影岩浆液的残余反应,原有的基性矿物堆积晶体在很大程度上被后世或全新的相所取代。在本案中,原始堆积矿物为石榴石和斜方辉石,少量黑云母和副矿物。纹影保留了一些早期的、自形的、成分不同的、堆积的黑云母,但大多数黑云母是通过堆积的斜方辉石和石榴石与残余液体反应形成的。一些早期堆积的石榴石仍然存在,但这不是最初夹带在源深度处的岩浆中的包晶石榴石。相反,这些是岩浆晶体,在地壳中部深度,通过原始包晶石榴石的溶解-再沉淀形成。在就位水平,在固相线附近发生了另一场石榴石溶解-再沉淀事件,广泛地重组了颗粒尺度的火成岩结构。尽管纹影保留了可用于推断其起源的结构、化学和一些纹理特征,但它们目前的微观纹理并不能完全反映其初始形成机制。还,纹影形成的物理机制通常与造成基岩中主要化学变化的过程几乎没有相似之处。尽管如此,纹影的存在表明花岗质岩浆在片状层状结构中流动,它们提供了关于在侵位前悬浮在母岩浆中的高温火成岩矿物的信息。
更新日期:2020-04-30
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