Holness Marian B, Cawthorn R Grant, Roberts James
1Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ UK.
2School of Geosciences, University of the Witwatersrand, PO Wits, Johannesburg, 2050 South Africa.
Contrib Mineral Petrol. 2017;172(11):102. doi: 10.1007/s00410-017-1423-4. Epub 2017 Nov 16.
The thickness of the crystal mush on magma chamber floors can be constrained using the offset between the step-change in the median value of dihedral angles formed at the junctions between two grains of plagioclase and a grain of another phase (typically clinopyroxene, but also orthopyroxene and olivine) and the first appearance or disappearance of the liquidus phase associated with the step-change in median dihedral angle. We determined the mush thickness in the Rustenburg Layered Suite of the Bushveld Complex at clinopyroxene-in (in Lower Main Zone) and magnetite-in (in Upper Zone). We also examined an intermittent appearance of cumulus apatite in Upper Zone, using both the appearance and disappearance of cumulus apatite. In all cases, the mush thickness does not exceed 4 m. These values are consistent with field observations of a mechanically rigid mush at the bases of both magnetitite and chromitite layers overlying anorthosite. Mush thickness of the order of a few metres suggests that neither gravitationally-driven compaction nor compositional convection within the mush layer is likely to have been important processes during solidification: adcumulates in the Bushveld are most likely to have formed at the top of the mush during primary crystallisation. Similarly, it is unlikely either that migration of reactive liquids occurs through large stretches of stratigraphy, or that layering is formed by mechanisms other than primary accumulation.
利用斜长石的两个晶粒与另一相（通常是单斜辉石，但也有斜方辉石和橄榄石）晶粒之间形成的二面角中值的阶跃变化与该中值二面角阶跃变化相关的液相线相首次出现或消失之间的偏移，可以确定岩浆房底部晶体糊的厚度。我们确定了布什维尔德杂岩体的鲁斯滕堡层状岩体中，在单斜辉石进入（在下主带）和磁铁矿进入（在上带）时的糊层厚度。我们还利用堆积磷灰石的出现和消失，研究了上带堆积磷灰石的间歇性出现情况。在所有情况下，糊层厚度均不超过4米。这些数值与野外观察结果一致，即在斜长岩之上的磁铁矿层和铬铁矿层底部存在机械刚性的糊层。几米厚的糊层表明，在凝固过程中，重力驱动的压实作用和糊层内的成分对流作用都不太可能是重要过程：布什维尔德的堆晶最有可能在初次结晶期间在糊层顶部形成。同样，反应性液体不太可能通过大片地层迁移，分层也不太可能由初次堆积以外的机制形成。
