Abstract
Measurements of dihedral angles at three-grain junctions in gabbros, involving two grains of plagioclase and one grain of another mineral, demonstrate that the median dihedral angle is generally the same for all minerals in any sample. The few exceptions to this can be attributed to reaction or to the cessation of growth of plagioclase during the last stages of solidification of highly evolved liquids that do not crystallize volumetrically important amounts of plagioclase. The dihedral angle is therefore primarily controlled by the growth behavior of plagioclase in the last remaining liquid. The final value of the dihedral angle is controlled by the extent to which plagioclase growth is accommodated on the (010) faces: low angles form when growth on the (010) faces is minor compared with that on the other growth faces, and high angles form when the (010) faces accommodate significant growth. The response of dihedral angles to changes in crystallization time is therefore explained by the changing response of plagioclase growth to cooling rate, with limited growth on (010) faces during rapid cooling (leading to a low dihedral angle) and more significant growth at slow cooling (leading to high dihedral angle). The correspondence between dihedral angle and plagioclase grain shape (as quantified by the average apparent aspect ratio observed in thin section) is clearly evident for non-fractionated bodies such as dolerite sills. Although the stratigraphic variation of the overall plagioclase grain shape in the floor cumulates of the Skaergaard Intrusion is broadly similar to that observed in sills, there is no correspondence to observed augite�plagioclase�plagioclase dihedral angles, which show a step-wise stratigraphic variation, corresponding to changes in the liquidus assemblage. This decoupling occurs because plagioclase growth in layered intrusions occurs in two stages, the first at, or close to, the magma�mush interface and the second within the mush. Chemical maps of samples on either side of the augite-in dihedral angle step demonstrate a step-wise change in the aspect ratio of the plagioclase grown during the second stage, with the aspect ratio of this stage corresponding to that predicted from the dihedral angles. Plagioclase shape in layered intrusions thus records two separate thermal regimes, with the overall shape controlled by the global cooling rate of the intrusion, and the second (minor) stage within the mushy layer reflecting local thermal buffering controlled by the liquidus assemblage of the bulk magma. Dihedral angles in layered intrusions record the second thermal regime.
Highlights
Recent work has expanded the potential of microstructure to provide quantitative constraints on the cooling and crystallization history of basaltic intrusions by the addition of two new parameters to the more usually deployed crystal size distribution
Because dihedral angles are formed during the last stages of solidification, the position of the step-wise change closely approximates the stratigraphic horizon corresponding to the base of the mushy layer at the moment the bulk liquid became saturated in augite—the two samples are some distance below the actual Lower Zone a (LZa)–b boundary and contain the same liquidus assemblage of olivine þ plagioclase
In the Skaergaard and Rum intrusions, and in the lower parts of the Bushveld Upper Zone, the dihedral angle at junctions formed by two grain of plagioclase and one grain of another mineral is the same for all minerals in the rock, regardless of the nature of the mafic mineral forming the three-grain junction, and regardless of whether that phase is interstitial or cumulus
Summary
Recent work has expanded the potential of microstructure to provide quantitative constraints on the cooling and crystallization history of basaltic intrusions by the addition of two new parameters to the more usually deployed crystal size distribution. Because dihedral angles are formed during the last stages of solidification, the position of the step-wise change closely approximates the stratigraphic horizon corresponding to the base of the mushy layer at the moment the bulk liquid became saturated in augite—the two samples are some distance below the actual LZa–b boundary (the precise position of which is difficult to determine in the drill core; Holness et al, 2015) and contain the same liquidus assemblage of olivine þ plagioclase (with interstitial augite). The three-grain junctions formed by the growth of interstitial biotite are not dominated by the biotite (001) faces, creating a biotite–plagioclase–plagioclase dihedral angle population with a median value of $86 6 4 in sample 118676 (290 m stratigraphic height) (Table 4). The most highly evolved rocks sampled by the Bierkraal drill cores are ferrodiorites typified by a cumulus assemblage of plagioclase (An43), fayalitic olivine, ferro-augite, Fe-Ti oxides and apatite, with abundant oikocrystic amphibole (Fig. 16a) and biotite [see Tegner
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