Compaction of igneous cumulates part II: Compaction and the development of igneous foliations1
Quantitative petrofabric analysis of 66 samples from olivine-bearing zones III and IV of the Middle Banded series of the Stillwater complex, Montana, was conducted to allow direct comparison among textural and chemical variations. Systematic variations in foliations with stratigraphie height are observed in both zones. Statistical analysis of the orientations of inequant grains was used to calculate an "Alignment Factor" (AF), corresponding to the extent of the development of the foliation. Both petrofabric and pétrographie evidence indicates that the foliation is best explained by compaction of the crystal pile. Comparison of AFs with estimates of the amount of compaction, based on wholerock compositions, shows approximate agreement. Variations in mole fraction Cl of interstitial apatite mimic the observed trends in the AF, indicating that local concentrations of fluids exsolved from interstitial liquid were important in the development of strong foliations. We'conclude that: (1) differing amounts of compaction led to variations in the igneous foliations; (2) local volatile enrichment enhanced the development of the foliation,- and (3) variations in plagioclase morphology (from tabular to blocky) were produced by growth and/or recrystallization under differing amounts of uniaxial stress. © 1998 by The University of Chicago. All rights reserved.
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