Magmatic differentiation by means of segregation and diapiric ascent of anorthositic crystal mush - the Murotomisaki Gabbroic Complex, Shikoku, Japan

Abstract

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.