Lake Boga Granite,northwestern Victoria: mineralogy,geochemistry and geochronology |
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| Authors: | S J Mills W D Birch R Maas D Phillips I R Plimer |
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| Institution: | 1. School of Earth Sciences, University of Melbourne , Vic, 3010, Australia;2. Geosciences , Museum Victoria , GPO Box 666E, Melbourne, Vic, 3001, Australia smills@museum.vic.gov.au;4. Geosciences , Museum Victoria , GPO Box 666E, Melbourne, Vic, 3001, Australia;5. School of Earth Sciences, University of Melbourne , Vic, 3010, Australia |
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| Abstract: | The Devonian Lake Boga Granite in northern Victoria, while almost entirely under thin Murray Basin cover, is one of the largest plutons in the western Lachlan Fold Belt. Its only exposure is a quarry penetrating the Cenozoic sediments. In the quarry, prominent pod pegmatites and miarolitic cavities suggest a high level of emplacement. The granite, a non-magnetic, fractionated S-type, contains a large range of accessory minerals, including primary uranium- and REE-bearing phosphates and oxides, and primary copper sulfides. Monazite-series minerals show an exceptionally wide range of compositions, from normal monazite-(Ce) through cheralite (Ca – Th-rich) to rare huttonitic monazite (Th-rich) and brabantite; U contents in monazite also vary widely (0 – 7.9 wt%). Primary low-Ca uraninites are well preserved and are unusual in having low Th/U and LREE. Late-stage cavity fluorapatite crystals up to several centimetres across show intricate elemental zoning patterns with extreme U gradients (<10 – 6900 ppm) in some crystals. New 40Ar – 39Ar ages for magmatic biotite, muscovite and K-rich feldspar average 365 ± 3 Ma, which approximates the emplacement age of the granite. This is supported by a 377 ± 12 Ma U – Th – Pb (CHIME) age for primary uraninite. New whole-rock geochemical data support earlier observations: the granite is strongly fractionated (SiO2 70.7 – 76.0 wt%; 4.2 – 0.6 wt% FeOt) and peraluminous (ASI = 1.23 – 1.45), and has slightly elevated Na2O and P2O5 (0.30 wt%) contents compared with other fractionated S-type granites from the Lachlan Fold Belt. Trace-element abundances are typical of fractionated granites, although U and Cu concentrations vary strongly and reach >60 and ≈1400 ppm, respectively. REE patterns also vary strongly, from LREE-enriched with moderate Eu depletion, to flat with strong Eu depletion. The flattest of the REE patterns, in samples with FeOtotal < 1%, are characterised by M-type tetrad effects. These and other samples also show low (subcrustal average) and variable Zr/Hf (35 – 16) and Nb/Ta (8 – 4) ratios; these and other unusual elemental fractionations are related to changes in elemental partitioning during the late magmatic stage, when felsic peraluminous magma and high-temperature magmatic fluid coexisted. |
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| Keywords: | argon – argon dating fluorapatite geochronology Lachlan Fold Belt Lake Boga Granite monazite REE-patterns S-type intrusion tetrad effect uraninite Victoria |
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