A photomicrographic essay illustrating replacement textures
نویسندگان
چکیده
A sequence of sixteen photomicrographs of thin sections of unaltered quartz diorite through a zone of deformation to myrmekite-bearing granite near Temecula, California, shows the textural and mineralogical changes that occurred in a quartz diorite as (1) K-metasomatism altered the primary plagioclase crystals to form microcline, myrmekite, quartz-bleb clusters, and recrystallized sodic plagioclase and as (2) Si-metasomatism converted microfractured biotite and hornblende into quartz. Cathodoluminescence, electron microprobe, and scanning electron studies confirm the chemical changes that occurred in the altered and recrystallized minerals. These photomicrographs and others (a) provide clues to the origin of metasomatic granitic in other terranes, (b) show that massive granite lacking gneissic fabric or cataclasis can be formed by K-metasomatism, (c) indicate that sharp contacts between mafic and felsic igneous rocks are not always caused by injection of magma into fractured rock, (d) reveal that complete cataclasis of all normally zoned plagioclase crystals in a mafic rock followed by K-metasomatism results in metasomatic granite lacking both normally zoned plagioclase and Kfeldspar megacrysts, and (e) suggest that selective deformation of a few normally zoned plagioclase crystals in a primary rock can result in K-feldspar megacrysts coexisting with normally zoned plagioclase in granodiorite or quartz monzonite.
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