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| Titel |
Siderite and zirconian pyroxene in the Mt Gibraltar microsyenite, New South Wales, Australia: Late magmatic mineralogy reflecting uncommon crystallization conditions and magma composition. |
| VerfasserIn |
Tom Andersen, Paul Carr, Muriel Erambert |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250048526
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| Zusammenfassung |
| The Mt Gibraltar intrusion near Mittagong and Bowral in New South Wales, Australia (lat.
34º37’42”S, long. 150º24’12”) is a small intrusive body of hypersolvus alkaline microsyenite
emplaced into the Triassic Hawkesbury Sandstone of the Sydney Basin in Jurassic time,
possibly related to extensional faulting. The rock itself consists of intermediate alkali feldspar
with minor titanomagnetite and interstitial pyroxene ranging from nearly pure hedenbergite to
-Hd50Aeg50 in composition. It is crosscut by an irregular system of veins consisting of
homogeneous alkali feldspar (-Ab50Or50), clinopyroxene evolving from sodic hedenbergite
to zirconium-rich aegirine, arfvedsonite and siderite, reflecting highly peralkaline residual
liquid compositions formed during in-situ crystallization of the magma. During the final,
post-magmatic stage of evolution of the veins, microcrystalline or amorphous silica
precipitated together with calcite as fillings in miarolitic cavities. The microsyenite
contains up to 3500 ppm Zr, but lacks primary zircon, baddeleyite and alkaline
zirconium silicate minerals; zirconium is hosted by pyroxene and to a lesser extent,
amphibole.
The late magmatic mineralogy of the rock is a consequence of anomalous physicochemical
conditions of crystallization. At a pressure of 700 bars (estimated from geological
constraints), a phase assemblage with Ab50Or50 + aegirine + arfvedsonite + siderite +
hydrous silicate melt would be stable at T=650-670 ºC and log fO2= -22, which is very close
to the magnetite-wustite buffer curve. High-Zr pyroxene has a restricted stability field in the
system SiO2-ZrO2-FeO-FeO1.5-NaO0.5-HO0.5, combining moderately elevated peralkalinity,
intermediate silica activity and low oxygen fugacity. Under such conditions, Zr will be
incorporated in pyroxene and amphibole, whereas zircon or more exotic Zr silicates will not
crystallize.
The Mt Gibraltar microsyenite provides an example of a separate family of alkaline
igneous rocks, which are neither “miaskitic” with zircon nor “agpaitic” (s.l.) with alkaline
zirconium silicate minerals. The reason for this is the uncommon combination of
crystallization conditions and magma composition. Also because of the highly reducing
conditions of late magmatic crystallization, the Mt Gibraltar microsyenite provides a rare,
possibly unique example of siderite occurring as a member of a magmatic mineral
assemblage.
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