Abstract

Quartz pseudomorphic after tridymite has recently been recognized in the micropegmatite member of the Nickel Irruptive of the Sudbury Basin, Ontario. The "tridymite" occurs as blades radiating from quartz xenocrysts or adjacent to quartzite xenoliths, where the micropegmatite has brecciated and assimilated the overlying quartzite. A plume-like, almost spherulitic intergrowth of quartz and orthoclase is commonly associated with the "tridymite." These features indicate a high intrusion temperature in the 800 degrees C. to 900 degrees C. range, and rapid cooling of the upper phase of the micropegmatite.

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