Zonation of low grade regional metamorphic rocks, Nambucca slate belt, northeastern New South Wales
Titel:
Zonation of low grade regional metamorphic rocks, Nambucca slate belt, northeastern New South Wales
Auteur:
Leitch, Evan C.
Verschenen in:
Australian journal of earth sciences
Paginering:
Jaargang 22 (1975) nr. 4 pagina's 413-422
Jaar:
1975
Inhoud:
Crystallization of stilpnomelane and pumpellyite, disappearance of prehnite and development of assemblages containing the pair pumpellyite-actinolite, and disappearance of pumpellyite, define isograds separating four zones of progressive regional metamorphism in the Nambucca Slate Belt of northeastern New South Wales. Mineralogical changes are paralleled by textural changes in metaclastic rocks. Detrital grains are progressively modified, a preferred orientation of material becomes increasingly perfected, and a foliation develops as grade increases. The most important secondary phases are albite, white mica, chlorite, stilpnomelane, actinolite, pumpellyite, prehnite, epidote, clinozoisite, and calcite. White mica shows an increase in 'crystallinity' with grade, chlorite becomes more aluminous, and the ferric iron content of epidote-group minerals decreases. In metabasic rocks hydration reactions involving the anorthite component of plagioclase and calcium-poor pyroxene (or olivine) probably led to the formation of prehnite, pumpellyite, and chlorite. At higher grades prehnite possibly reacted with chlorite or calcic pyroxene, and pumpellyite may have disappeared by reaction with the 'greenalite molecule' of chlorite, yielding actinolite, clinozoisite, and a more aluminous chlorite. The rocks belong in the prehnite-pumpellyite, pumpellyite-actinolite, and green-schist facies; an intermediate facies series is indicated. Comparisons with experimental work suggests that a temperature of about 360-370°C and pressures in the range 4 to 7 kilobars (depending on the geothermal gradient) were operative at the pumpellyite-actinolite /greenschist boundary.
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