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Petrographic Discrimination of Low‐ and High‐Temperature I‐type Granites
Author(s) -
White Allan J. R.,
Chappell Bruce W.
Publication year - 2004
Publication title -
resource geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.597
H-Index - 43
eISSN - 1751-3928
pISSN - 1344-1698
DOI - 10.1111/j.1751-3928.2004.tb00203.x
Subject(s) - petrography , plagioclase , geology , felsic , geochemistry , mineralogy , petrology , quartz , mafic , paleontology
. The existence of two groups of I‐type granites, those formed at relatively low‐ and high‐temperatures, has been established using differences in zircon saturation and patterns of bulk compositional variation between those two groups. Granites of these two groups also show some distinctive petrographic differences. The clearest petrographic distinction between the low‐ and high‐temperature granites is seen in the crystals of plagioclase. In all but the most felsic low‐temperature granites these crystals are always complex and commonly have corroded cores, which may be as calcic as An 80 . Core compositions are fairly uniform in composition compared with the outer parts, which display normal and oscillatory zones of varying widths. In the high‐temperature granites the crystals of plagioclase are generally better‐shaped with a uniform composition and lack the prominent corroded cores of the low‐temperature granites. These differences are due to the fact that homogenization of plagioclase proceeds very slowly at low magmatic temperatures.