Petrology of the Nandewar Volcano,N.S.W., Australia

The Nandewar Mountains, N.S.W., Australia, are the remains of a Miocene continental alkaline volcano whose products range from olivine basalts to comendites and alkali rhyolites. Intermediate hawaiites, mugearites and benmoreites predominate in the shield, in which olivine basalts are rare, and the trachytic rocks form many intrusions into the shield. The Nandewar alkaline series shows extreme fractionation of a relatively differentiated alkali olivine basalt magma, saturated with silica, to yield extremely oversaturated peralkaline comendites and peraluminous alkali rhyolites. The nature of the ferromagnesian phases forming was controlled by low oxygen fugacities. Throughout the series clinopyroxenes range from diopsidic augite, through sodic ferrohedenbergites to hedenbergite-acmite solid solutions. Riebeckite-arfvedsonite solid solutions appear in the trachytes and comendites, and aenigmatite appears in some of the peralkaline rocks. The feldspars in the series fractionate from calcic labradorite through potash oligoclase and calcic anothoclase towards the minimum melting alkali feldspar composition, Ab₆₅Or₃₅. The compositions of the alkali rhyolites approach the minimum in the system SiO₂-KAlSi₃O₈-NaAlSi₃O₈. All the mineralogical and chemical evidence points to the development of the Nandewar series by the processes of extreme crystallization differentiation of an alkali olivine basalt parent magma. No significant contamination occurred, xenoliths and xenocrysts are absent, and volatile transfer and metasomatism played a minor role.     

Bassett,K.N., Gordon,H.W., Nobes,D.C., & Jacomb,C., “Gardening at the edge: Documenting the limits of tropical Polynesian kumara horticulture in southern New Zealand.” Geoarchaeology 19(3) 2004, 185–218

The original article to which this Erratum refers was published in Geoarchaeology 19(3), 2004, 185–218. Due to an oversight, authorship of the paper by Bassett, K.N., Gordon, H.W., Nobes, D.C., & Jacomb, C. (2004), “Gardening at the edge: Documenting the limits of tropical Polynesian kumara horticulture in southern New Zealand” (Geoarchaeology 19, No. 3, pp. 185–218), was assigned incorrectly. The correct authorship is Hamish W. Gordon, Kari N. Bassett, David C. Nobes, and Chris Jacomb. The authors apologize for this error.     

Polymetamorphism of the sulphide ores of Broken Hill,N. S. W., Australia

Two main periods of metamorphism have effected the Broken Hill base metal deposit. The first, at granulite grade, occurred at 1,700 m.y., the second, at lower amphibilite grade, occurred at 500 m.y. The earlier metamorphism correlates with two stages of intense regional folding; the latter occurs as narrow shears across the orebody. The prograde metamorphism caused intense brecciation, development of an ore mush with ore movement, formation of ore-bearing parapegmatites and boudins and much recrystallization of ore and gangue minerals. The orebody parallels an axial plane schistosity in the wall rocks with numerous ore piercement structures causing local discordancies. Ore in retrograde zones is again brecciated with galena further recrystallizing after destruction of prograde recrystallization. Gangue minerals remain essentially as brecciated fragments without further recrystallization. Secondary hydrothermal veins with rare silver minerals derived from the orebody transect the retrograde zones. Galena is plastically injected into fractures in the retrograde wall rock schists. Prograde ore shows co-recrystallization of various sulphides and gangue minerals yielding characteristic annealed textures. Quartz, garnet, hedenbergite, roepperite and apatite co-recrystallize with galena, sphalerite and chalcopyrite with balanced surface tensions. Retrograde ore shows mainly fragments of gangue and sphalerite set in a matrix of further recrystallized galena or schistose galena with a superimposed sub-grain structure. The significance of the ubiquitous sub-structures within galena is considered in terms of retrograde effects upon high grade metamorphic textures.

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Zusammenfassung Das Erzvorkommen in Broken Hill, N.S.W., Australien, unterlag zwei Hauptphasen regionaler Metamorphose. Die erste Phase, die eine Granulitfacies erreichte, fand vor 1700 Millionen Jahren statt; die zweite Phase, vor 500 Millionen Jahren, zeigt einen Amphibolit-Grad. Die frühere Metamorphose korreliert mit zwei Stufen intensiver Faltungen, die spätere trat in Form enger Scherungen quer durch den Erzkörper auf. Die prograde Metamorphose verursachte intensive Breccienbildung, die Entwicklung eines Erzbreies mit begleitender Erzbewegung, die Bildung erzführender Parapegmatite und Boudinagen sowie reichliche Rekristallisation des Erzes und der Gangmineralien. Der Erzkörper liegt parallel eine axial-plane-schistocity im Nebengestein mit zahlreichen Erzdurchdringungen, die zu örtlicher Diskordanz führen. In retrograden Zonen wurde das Erz ein zweites Mal brecciert, was von einer weiteren Rekristallisation des Bleiglanzes unter Zerstörung der prograden Rekristallisation begleitet wurde. Die Gangmineralien bleiben in der Hauptsache im breccierten Zustand ohne weitere Rekristallisation. Sekundäre hydrothermale Erzgängchen mit seltenen Silbermineralien, die vom Erzkörper stammen, durchschneiden die retrograde Zone. Bleiglanz ist in plastischem Zustand in die Spalten des retrograden Nebengesteines (Schist) hineingedrückt worden. Progrades Erz zeigt Co-rekristallisation der verschiedenen Sulphide und Gangmineralien mit charakteristischen Temperungs gefügen. Quarz, Granat, Hedenbergit, Roepperit und Apatit co-rekristallisieren mit Bleiglanz, Zinkblende und Kupferkies mit ausgeglichenen Oberflächenspannungen. Retrogrades Erz zeigt hauptsächlich zerstückelte Gangmineralien und Zinkblende in einer Grundmasse weiter rekristallisierten oder schiefrigen Bleiglanzes mit einer überprägten Sub-grain Textur. Die Bedeutung der allgegenwärtigen Sub-Textur im Bleiglanz wird im Sinne eines retrograden Effektes auf hochgradige metamorphe Strukturen gedeutet.

     

Rare earth element investigation of the Cliefden Outcrop,N.S.W., Australia

The effect of low grade hydrous burial metamorphism (prehnite-pumpellyite facies) upon the rare earth elements (REE) has been studied by using samples from the Cliefden Outcrop, New South Wales. The REE, together with other reputedly immobile elements, have been mobilised during the metamorphism. Although mobile, the REE have behaved remarkably coherently with little light rare earth (LREE) fractionation. This is reflected in the chondrite normalised patterns which are sub-parallel to parallel in shape. High correlations of REE with other elements can be used to predict the maximum likely variation of these elements in the studied outcrop. The high correlations do not necessarily mean that, for similarly metamorphosed terrains, crystallisation-differentiation processes have operated but may rather have resulted from strong geochemical coherence during post-crystallisation elemental redistribution. The REE do not appear to be strongly domain controlled within the Cliefden Outcrop.