Other
Scientific paper
Nov 1959
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1959gecoa..17..272w&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 17, Issue 3-4, pp.272-285
Other
Scientific paper
Plots of modal percentages of mineral pairs in granitic rocks show negative correlation between the abundance of potassium feldspar and the abundances of plagioclase and biotite, and the percentage anorthite in plagioclase. A positive correlation is found between plagioclase and biotite, plagioclase and percentage anorthite in plagioclase, and biotite and percentage anorthite in plagioclase. The distribution of quartz is erratic as shown by the absence of correlation between quartz and other mineral abundances. Study of modal compositions of groups of granitic rocks subdivided on the basis of various textural properties leads to characterization of granitic rocks in terms of two ideal "end members". One type of granite consists of subhedral zoned plagioclase laths with interstitial quartz and potassium feldspar; abundances of plagioclase, biotite and hornblende are comparatively high in this type, whereas potassium feldspar is scarce. The other end member granite contains roughly equal amounts of quartz, potassium feldspar and plagioclase, with minor biotite; all minerals are anhedral, and the plagioclase is twinned and unzoned. Complete gradation exists between these two types of granite. The complete gradation between the two granitic textural and compositional types indicates that most granites have evolved by one general process. In view of the definite correlations, both positive and negative, among the abundances of potassium feldspar, plagioclase, biotite and percentage anorthite in plagioclase, it is proposed that this process is one of magmatic differentiation. The relationships can be explained by differentiation but would appear fortuitous if metasomatism were a major factor.
McEwen Michael C.
Rogers James W.
Whitfield J. M.
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