Magmatic differentiation by means of segregation and diapiric ascent of anorthositic crystal mush - the Murotomisaki Gabbroic Complex, Shikoku, Japan
Magmatic differentiation by means of segregation and diapiric ascent of anorthositic crystal mush - the Murotomisaki Gabbroic Complex, Shikoku, Japan
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通过斜长岩晶糊的分离和底辟上升进行的岩浆分异 - 日本四国的室富岬辉长岩杂岩体
DOI：
10.2465/jmps.060617b
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发表时间：
2006
影响因子：
0.7
通讯作者：
T. Akatsuka
中科院分区：
文献类型：
--
作者：
T. Hoshide;M. Obata;T. Akatsuka
室富崎辉长岩是一个含有多个橄榄石富集层的层状岩床状火成岩杂岩。在我们之前的论文（Hoshide et al.，2006），我们已经确定了橄榄石富集的两个区域：“晶体积累区（AC区100”，由橄榄石晶体的重力沉降和积累形成，和“晶体生长区（GR区100”，其中模态橄榄石的增加是由橄榄石的晶体生长而不是晶体积累引起的。 根据全岩成分数据，我们发现AC带岩石定义了一个线性的成分趋势（称为“AC趋势”），这是符合晶体沉降和积累假说。然而，GR区数据定义了另一个线性趋势，其斜率与AC趋势的斜率不同。此外，粗辉长岩和上部橄榄辉长岩的GR带和斜长岩脉的GR带上方发生的成分大致上位于同一趋势，但在GR带成分的相反侧，定义的“GR趋势”作为一个整体。某些斜长岩脉和波状伟晶岩脉具有羽状构造，表明这些脉是岩浆分异过程中被底辟运动和上升的结晶浆的残余。 考虑到所观察到的成分关系和斜长岩和波状伟晶岩脉的产状模式，我们得出结论，半凝固结晶边界层的斜长岩材料的偏析和分离负责GR区和GR趋势的形成。相平衡计算表明，假设的斜长岩材料是分馏的熔体和从熔体中沉淀出来的斜长石晶体的混合物。GR带代表斜长岩晶体泥和粗辉长岩分离的残留物，上部橄榄石辉长岩部分代表晶体泥和初始熔体的混合物。
The Murotomisaki Gabbro is a sill-like layered igneous complex that contains several layers of olivine enrichment. In our previous paper (Hoshide et al., 2006), we have identified two zones of olivine enrichment: ‘the crystal accumulation zone (AC zone)’, formed by gravity settling and accumulation of olivine crystals, and ‘the crystal growth zone (GR zone)’, in which increase of modal olivine was caused by crystal growth of olivine and not by crystal accumulation. Based on whole rock compositional data, we have found that the AC zone rocks define a linear compositional trend (termed as ‘AC trend’) which is consistent with the crystal settling and accumulation hypothesis. However the GR zone data define another linear trend with a slope different from that of the AC trend. Moreover, the compositions of the coarse gabbros and the upper olivine gabbros that occur above the GR zone and an anorthosite vein from the GR zone roughly lie on the same trend, but on the opposite side of the GR zone composition, defining the ‘GR trend’ as a whole. Some anorthositic veins and wavy pegmatitic veins have plume-like structures, suggesting that these veins are remnant of crystal mushes that have been mobilized and ascended diapirically during magmatic differentiation. Considering the observed compositional relationships and the mode of occurrences of the anorthosite and wavy pegmatitic veins, we conclude that the segregation and separation of anorthositic material out of semi-solidified crystallization boundary layers was responsible for the formation of the GR zone and the GR trend. Phase equilibrium calculations reveal that the hypothetical anorthosite material was a mixture of fractionated melt and plagioclase crystals that precipitated from the melt. The GR zone represents a residue from the separation of anorthositic crystal mushes and the coarse gabbros and the upper olivine gabbro parts represent mixtures of the crystal mush and the initial melt.