Abstract

To investigate the metamorphism of zircons, the authors studied the aureole of a moderate-sized Mesozoic pluton, a large Mesozoic batholith, and progressive zones of Paleozoic regional metamorphism. The study confirmed the following: (1) metadetrital zircons refacet only at near-melting conditions and even then are distinguishable from typical igneous crystals; (2) uranium-plus-thorium content decreases with increased grain size; (3) lead loss is closely related to uranium-thorium content; and (4) independent of uranium-thorium content, zircons show increased lead loss with increased grade of metamorphism, the degree of loss appearing to increase with the duration of heating. It further confirmed that granitic rocks may incorporate metadetrital zircons containing older radiogenic lead and that the zircon “radioactivity clock” is rarely if ever entirely reset by metamorphism.

The study showed that hyacinth zircons lose their color progressively with increasing metamorphism and that the color loss is a function of maximum temperature rather than thermal duration.

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