Modification of a magmatic tonalite to produce a megacrystal granodiorite by K-metasomatism, Monterey peninsula and northern Santa Lucia Mountains, California, USA
A megacrystal granodiorite, occurring in a pluton in the Monterey peninsula of California, is gradational south to quartz monzonite and then to tonalite and trondhjemite and southeast to granodiorite and then to tonalite. A primary magmatic origin for the pluton is indicated by normally zoned plagioclase crystals and by dikes that extend into biotite-hornblende quartz diorite southeast of Point Lobos and into biotite schists south of Carmel Valley Village. Plagioclase in the tonalite lacks zonation, but toward the northwest, the plagioclase becomes increasingly zoned in the megacrystal granodiorite. Therefore, the shallowest part of the original pluton must have been near Monterey. The tonalite is undeformed but progressively becomes cataclastically broken from the southeast toward the northwest. In less than one kilometer, broken plagioclase crystals are increasingly penetrated and gradually replaced by microcline, converting the tonalite into granodiorite. K-metasomatism first creates tiny microcline islands and veins in interiors of broken plagioclase crystals. Then, the microcline grows and engulfs plagioclase remnants and other groundmass minerals. Eventually, the microcline becomes megacrysts, some as large as 4 cm long. Wartlike myrmekite commonly borders the microcline. In formerly strongly-deformed zones, the microcline megacrysts locally have parallel alignment. Some megacrysts show concentric zonation and alignments of tiny plagioclase inclusions parallel to possible crystal faces of the microcline. Thus, the four facies in the pluton, tonalite, quartz monzonite, granodiorite, and megacrystal granodiorite, which seem to have formed by magmatic differentiation, are a result of cataclasis of a former trondhjemite/tonalite pluton in which some of the plagioclase has been replaced progressively by microcline.
Overlooked experimental evidence for K-replacements of plagioclase and origin of microcline in granite plutons
Experimental studies show that K replacement of Na in triclinic plagioclase under hydrothermal conditions results directly in triclinic microcline without forming intermediate monoclinic orthoclase. These experiments bring laboratory and field evidence into agreement for the K-metasomatic origin of granites which contain exclusively microcline as the potash feldspar. Such replacements of a more mafic rock can be on a plutonic scale and range from barely perceptible to being so extensive that almost no trace of an original relatively-mafic rock remains. Microcline, as the exclusive K-feldspar, and myrmekite may be the only clues to the metasomatism.
Methods of determining the desired frequency characteristics of the automatic control systems
Considered the methods of determining the desired frequency characteristics of the automatic control systems while the synthesis of their corrective devices.