Amphibole‐olivine‐spinel, cordierite‐anthophyllite and related hornfelses associated with metamorphosed serpentinites in the goobarragandra district, near Tumut, New South Wales

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Abstract
A large variety of hornfelses is developed in and around the Coolac Serpentinite, in the aureole of the Bogong Granite. Essentially isochemical metamorphism of partly serpentinized harzburgites has produced metaserpentinites containing variable amounts of talc, tremolite, olivine, orthopryoxene, and other phases: Chromian spinel, partly changed to magnetite and/or ferrichromite, remains as a relict primary phase very near the granite contact. Alumino‐ferromagnesian hornfelses containing amphiboles, spinel, and magnetite, with or without olivine, chlorite, orthopyroxene, and sulphides, as well as cordierite‐antho‐phyllite hornfelses, various calc‐silicate hornfelses and mafic hornfelses, are intimately associated with the metaserpentinites. The chemical compositions of the amphibole‐olivine‐spinel hornfelses can be matched with those of chlorite‐rich and chlorite‐tremolite‐rich rocks outside the thermal aureole produced by reaction‐zone metasomatism of tectonic inclusions within the Coolac Serpentinite and of rocks adjacent to it. The relatively high contents of Cr and Ni and the presence of relict chromian spinel in most of the amphibole‐olivine‐spinel hornfelses strongly suggest that they were serpentinites; but their very high bulk Al2O3 contents indicate that they are not simply isochemically metamorphosed ultramafic rocks. Other Cr‐Ni‐poor amphibole‐spinel‐bearing hornfelses, as well as the cordierite‐anthophyllite and calc‐silicate hornfelses, are thought to be high‐grade equivalents of mafic materials altered in reaction zones. Temperatures near the Bogong Granite were in the range 55O°‐700°C assuming pressures (Ph2o = Ptotal) of 1–2 kb. Chemical data for aluminoferromagnesian metamorphic rocks from other localities suggest their derivation by essentially isochemical metamorphism of mafic to ultramafic rocks which had previously undergone either low‐grade reaction‐zone metasomatism in a serpentinite environment or low‐grade regional‐burial metasomatism.