Petrology and geochemistry of differentiated teschenite intrusions from the Hunter Valley,New South Wales,Australia |
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| Authors: | J A Gamble |
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| Institution: | (1) Department of Geology, Victoria University of Wellington, Private Bag, Wellington, New Zealand |
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| Abstract: | In the upper Hunter Valley of New South Wales a high level teschenitic sill complex emplaced into Permian coal measures derives from parent magmas which were themselves crystal fractionation products of alkali basaltic melts. The sills crystallised in situ and produced a spectrum of rock types ranging from olivine teschenite to teschenite, syenoteschenite and, ultimately, syenite. The olivine teschenites are also enriched in biotite and are crudely interlayered with teschenite and syenoteschenite. The lineage from olivine teschenite to syenoteschenite is characterised by a progressive decrease in olivine and a build-up of alkaline mesostasis which is accompanied by strong chemical zonation in abutting silicate minerals. The alkaline mesostasis and syenites are identical mineralogically. Primary crystallisation of olivine in the olivine teschenite-teschenite-syenoteschenite continuum and data from coexisting iron-titanium oxide pairs suggest that oxygen fugacity was constrained to a path parallel to the QFM buffer curve. Absence of olivine from the alkaline mesostasis and syenite veins, together with the appearance of sphene, indicates buffering of oxygen fugacity by other assemblages (probably annite-alkali feldspar-magnetite) and generally higher fO
2 in the residual liquids. Here, a build-up of CO2, F and Cl, in addition to H2O, influenced the relative stabilities of the pyroxene and amphibole minerals.Major and trace element data support an in situ, progressive congelation model for crystallisation. Mass balance solutions require participation of all phases to produce acceptable residuals. Simple mixing calculations suggest that syenoteschenite consists of teschenite plus approximately 40% syenite, in close agreement with the observed modal mineralogy. Segregation of syenite from syenoteschenite probably occurred when the residual liquid, as represented by mesostasis, reached a critical volume of around 40%. |
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