University Of Tasmania
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Geochemistry and geochronology of palaeoproterozoic Fe-rich tholeiites and metasediments from the Georgetown Inlier, North Queensland : their petrogenesis, metamorphic history, tectonic setting, and implications for relationship with the Broken Hill and Mt Isa sequences

posted on 2023-05-27, 10:57 authored by Michael BakerMichael Baker
Fe-rich tholeiites intruded the lower Etheridge Group of the Georgetown Inlier of north Queensland during the late-Palaeoproterozoic (ca. 1680-1640 Ma), during a prolonged period of rift-related sedimentation associated with extension in the Georgetown basin. The tholeiites comprise two lithologically distinct units: the extrusive Dead Horse Metabasalt (DHM), a conformable 1000m-thick series of fine- to coarse-grained basalts and pillow lavas; and the intrusive Cobbold Metadolerite (CMD), a multitude of sills and lesser dykes that range from a few metres to over 500 m in thickness. U-Pb LA-ICPMS dating of igneous zircons indicate the tholeiites were intruded in at least two pulses. The first coincided with the extrusion of the DHM and associated CMD sills at ca. 1670 Ma, while a younger pulse occurred during the deposition of the overlying Lane Creek Formation at ca. 1640 Ma. The DHM and CMD are co-magmatic, and have compositions typical of relatively evolved, low-K, Fe-rich continental tholeiites and tholeiites generated in plume-triggered volcanic passive margins leading up to continental breakup. Immobile major and trace element patterns suggest that the tholeiites were derived from partial melting of the convecting depleted upper mantle depleted mantle melt, and melts evolved along a trend of increasing Fe-enrichment to strongly Fe-enriched (21.11 wt% total Fe as Fe\\(_2\\)O\\(_3\\)) ferro-tholeiites. Initial ˜í¬µ\\(_{Nd}\\) values range between +2.6 and +5.3 (average +4.0), confirming a depleted mantle source for the tholeiites. T\\(_{DM}\\) ages range between 1.85 Ga and 3.39 Ga (average 2.5 Ga). Several samples show slight LILE- and LREE-enrichment and have low initial ˜í¬µ\\(_{Nd}\\) values (+2.6-2.9) consistent with limited crustal contamination involving local host sedimentary rocks of the lower Etheridge Group. Between ca. 1600 and 1500 Ma, the lower Etheridge Group was deformed and metamorphosed by at least two regionally significant orogenic events. The first event (D\\(_1\\)) was the most intense, producing a pervasive foliation and metamorphism (M\\(_1\\)) that varied in intensity from upper amphibolite facies in the east, through to lower greenschist facies in the south-west. Metasediments in the south of the inlier record P-T conditions of up to 650¬¨‚àûC and 7.5 kbar for M\\(_1\\). D\\(_1\\) was followed by a period of retrogressive metamorphism that lasted until ca. 1550 Ma, when voluminous granitoids of the Forsayth Batholith were intruded into the lower Etheridge Group. High-temperature, low-pressure metamorphism (M\\(_2\\)) was associated with this event. Initial P-T conditions for M\\(_2\\) are inferred from a retrogressive mineral assemblage that returned conditions of 550¬¨‚àûC and 3 kbar. EMPA monazite dating returned a possible ca. 1585 Ma age for M\\(_1\\) and a ca. 1550-1530 Ma age for M\\(_2\\). A younger monazite population (ca. 1500 Ma) suggests either M\\(_2\\) continued for a further 30-50 million years, or an additional weak thermal event affected the Etheridge Group at this time. The DHM and CMD are broadly correlated temporally and geochemically to basic gneisses of the Broken Hill Group in the Willyama Inlier, and amphibolites of the Soldiers Cap Group in the Mt Isa Eastern Succession. In particular, immobile trace element ratios and REE patterns between these tholeiitic suites are virtually indistinguishable. The crystallisation ages of the DHM and CMD, as determined by U-Pb LA-ICPMS zircon dating also closely resembles established ages for the Broken Hill (ca. 1685 Ma) and Mt Isa Eastern Succession (ca. 1670 Ma) tholeiitic rocks. This relationship is of regional significance, as it suggests the terranes were spatially associated during the Palaeoproterozoic, and places the Georgetown Inlier within the Proterozoic framework of northern Australia, and likely a part of the North Australian Craton in the Palaeoproterozoic.


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