Greenschist facies metamorphism of a rift basin sequence, Breadalbane, NSW
Titel:
Greenschist facies metamorphism of a rift basin sequence, Breadalbane, NSW
Auteur:
Farrell, T. R. Offler, R.
Verschenen in:
Australian journal of earth sciences
Paginering:
Jaargang 36 (1989) nr. 3 pagina's 337-349
Jaar:
1989-09
Inhoud:
The Siluro-Devonian, volcano-sedimentary rift basin sequence at Breadalbane, NSW, has undergone three phases of deformation, with biotite grade metamorphism occurring during the first two events. A peak metamorphic temperature of 425 ± 30°C is indicated by calcite-dolomite geothermometry, and a pressure of 4.0 ±0.5 kbar is estimated from the Si content of white mica. These P/T estimates give a geothermal gradient of 30°C/km, consistent with the inferred tectonic environment in the Lachlan Fold Belt during the Devonian. Biotite is present in two distinct parageneses: brown biotite is associated with white mica-chlorite-quartz (± epidote, albite) in a variety of rock types, whereas green biotite occurs only in magnetite ore in association with chlorite, stilpnomelane and ferroan-talc. Three biotite-forming mechanisms have been identified: (1) the breakdown of the assemblage K-feldspar + chlorite in acid volcaniclastics and psammites; (2) a continuous reaction between phengitic muscovite and chlorite in acid to intermediate volcaniclastics of suitable bulk composition; and (3) a reaction between chlorite and stilpnomelane which produced green biotite with interlayered chlorite + stilpnomelane.
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