Phase relations, element distributions, and geochemistry of metamorphic rocks from the eastern Arunta Block, N.T.

The Jervois Range area, northeastern Arunta Block, N.T., underwent a single, prograde, low-pressure regional metamorphisn, accompanied by granite emplacement, dated at 1850 m.y. Subsequent emplacement of alaskitic and large pegmatitic bodies, dated at 1700 m.y., resulted in the retrograde hydration of cordierite. Metamorphic grades in the Jervois Range area range from the uppermost greenschist facies, characterized by the assemblage chlorite + andalusite + muscovite, to the middle of the amphibolite facies, characterized by the appearance of sillimanite. Characteristic metapelitic assemblages in the lower amphibolite facies include cordierite + chlorite + biotite, cordierite + andalusite + biotite, staurolite + almandine + andalusite, and almandine + chlorite + quartz. Using the compositions of naturally-occurring minerals, realistic metamorphic reactions have been inferred for the prograde breakdown of muscovite + chlorite, and the retrograde hydration of cordierite + biotite. Naturally-occurring reactions and assemblages may differ markedly from those proposed on theoretical grounds. Chlorite-quartz-garnet and other naturally-occurring assemblages in the system SiO^-A^O^-FeO-MgO-H^O may be derived by the isochemical metamorphism of parent-rocks of appropriate bulk-composition. These parent-rocks may have been formed originally either by the halmyrolytic alteration of volcanoclastic sediments, or by the deposition of sediments rich in authigenic chamosite. Because of their relatively simple bulk-composition, mineral compositions in chlorite-quartz-garnet schists may be used to infer realistic, multivariant garnet-forming reactions. P-T conditions of prograde metajiorphism in the Jervois Range area appear to have been 2-3 kb at 480-580°C. These values are consistent with metamorphic assemblages found in the northeastern Arunta Block to the west of the Jervois Range area. Metamorphic rocks in the Valley Bore area, eastern Harts Range, N.T., belong to the upper amphibolite and lower granulite facies, temperatures of metamorphism ranging from 620 to 820°C and pressures varying from 6 to 10 kb. These higher temperatures and relatively higher pressures are consistent with those inferred for rocks in the Strangways Range area, 100 km to the west of the Valley Bore area. These contrasting metamorphic conditions in the two areas are unlikely to have been formed under a single geothermal gradient, and the two areas may be parts of two distinct metamorphic "belts", each with its own geothermal gradient. Naturally-occurring assemblages indicate that the correct calibration of the cordierite-garnet geothermometer-beobarometer is of the form proposed by Hensen and Green (1971t 1972, 1973)* Calculations and natural parageneses indicate that there is no convincing evidence to support a modification of the cordierite-gedrite kyanitequartz divariant stability field experimentally derived by Green and Vernon (1972).