Illite crystallinity, b₀ and Si content of K‐white mica as indicators of metamorphic conditions in low‐grade metamorphic rocks at Cobar, New South Wales

Abstract

Illite crystallinity (IC), b₀ values and Si contents of K‐white mica are reported from Early Devonian metapelites at Cobar. The IC studies indicate metamorphic conditions near Cobar to have been transitional between the anchizone and epizone; farther north at the Elura mine they are anchizonal. The b₀ values vary from x=0.8995 nm at Elura to 0.9032 nm at the Peak mine south of Cobar, with most in the range 9.011–0.9021 nm, suggesting a change from low‐ to intermediate‐pressure metamorphism from north to south. Si contents of the K‐white micas at the CSA mine near Cobar suggest a pressure of 3 kbar.

Key words:

• b₀
• diagenesis
• facies
• geobarometry
• greenschist
• illite crystallinity
• metamorphism
• metapelites
• metasiltstones
• phengite
• prehnite‐pumpellyite facies