Skeletal tourmaline, undercooling, and crystallization history of the Stone Mountain granite, Georgia, U.S.A.
Skeletal tourmaline, undercooling, and crystallization history of the Stone Mountain granite, Georgia, U.S.A.
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DOI：
10.3749/canmin.49.1.341
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发表时间：
2010-05
影响因子：
0.9
通讯作者：
Kristen Michelle Longfellow;S. E. Swanson
中科院分区：
文献类型：
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作者：
Kristen Michelle Longfellow;S. E. Swanson
位于格鲁吉亚的石山岩体由含电气石的黑云母-白云母花岗岩组成。岩体侵入于中地壳深度，并被晚期岩脉切割。电气石以大的骨架晶体形式出现在与晚期细晶岩和伟晶岩-细晶岩脉相同的区域的花岗岩中。在伟晶岩-细晶岩脉的细晶岩带中也有Skelet-tourism。伟晶岩和长石-石英-电气石细脉中含有自形电气石。花岗岩和含电气石岩脉中孤立的电气石骨架晶体被长石和石英的细粒浅色贫铁晕所包围。电子探针分析表明，所有的电气石都由黑云母组成。Skeleton tourlets显示出早棕色到晚蓝色的多色性。大多数自形电气石是均匀的棕色，但少数伟晶岩颗粒有蓝色的核心和棕色的边缘。棕色电气石的Ti含量高（>0.1 apfu），而蓝色电气石的Ti含量较低（apfu）。在不同质地的火山岩中，成分变化不大。这些纹理表明在40-100°C的过冷度下出现初始结晶。富硼流体与花岗岩发生反应，形成具有贫铁晕的骨架电气石。在伟晶岩-细晶岩脉，富硼流体（熔体）隔离沿着外缘的堤防和结晶骨架电气石从过冷熔体。从熔体中去除B提高了固相线温度，并导致包围骨架电气石的细晶形成。结晶的剩余熔体在较低的过冷度产生自形电气石的伟晶岩和静脉。石山花岗岩骨架晶体的发育表明，在某些中地壳花岗岩系统中，过冷是一个重要的过程。
The Stone Mountain pluton, in Georgia, is composed of tourmaline-bearing biotite–muscovite granite. The pluton was intruded at midcrustal depths and is cut by late-stage dikes. Tourmaline occurs as large skeletal crystals in the granite in the same areas as late-stage aplite and pegmatite–aplite dikes. Skeletal tourmaline also occurs in the aplite zone of pegmatite–aplite dikes. Pegmatites and thin feldspar – quartz – tourmaline veins contain euhedral tourmaline. Isolated skeletal crystals of tourmaline in the granite and tourmaline-bearing dikes are surrounded by a fine-grained leucocratic, Fe-depleted halo of feldspar and quartz. Electron-microprobe analyses show that all of the tourmaline consists of schorl. Skeletal tourmaline shows an early brown to late blue pleochroism. Most euhedral tourmaline is uniformly brown, but a few grains from pegmatite have a blue core and a brown rim. Brown tourmaline is high in Ti (>0.1 apfu ), whereas the blue tourmaline is lower in Ti ( apfu ). There is little compositional variation among the different textures of the tourmaline. These textures indicate an initial crystallization at undercoolings of 40–100°C. A boron-enriched fluid reacted with the existing granite to form skeletal tourmaline with a Fe-depleted halo. In the pegmatite–aplite dikes, the B-enriched fluid (melt) segregated along the outer margins of the dikes and crystallized skeletal tourmaline from an undercooled melt. Removal of B from the melt raised the solidus temperature and caused the aplite that encloses the skeletal tourmaline to form. Crystallization of the remaining melt at lower undercoolings produced euhedral tourmaline of the pegmatite and veins. The development of skeletal crystals of tourmaline at Stone Mountain indicates that undercooling is an important process in some midcrustal granite systems.