Abstract

Adamant pluton is a zoned body of igneous origin, with a core of pyroxene monzonite transitional outward to hornblende quartz monzonite and biotite-hornblende granodiorite. On chemical and textural evidence, the lithologic zoning resulted from reaction with introduced water and oxygen during regional metamorphism to upper amphibolite facies. Feldspars in the monzonite are orthoclase and andesine of intermediate structural state. During transformation of the pluton, systematic changes in the feldspars comprised: (1) ordering of alkali feldspars toward intermediate microcline, with increase of 2V, decrease of Na and Ca content, and appearance of grid twins; (2) ordering and increase in Na content of plagioclase. The most ordered feldspars, with highest Na distribution coefficient, are found in peripheral granodiorite and in pegmatite cutting the quartz monzonite. The feldspar data support the model proposed for the evolution of the pluton, indicating approach to equilibrium in the peripheral rocks at lower temperatures than that attendant to the formation of the core.

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