New Sensitive High Resolution Ion Microprobe (SHRIMP) U–Pb geochronological results for three samples are presented in this Record. The samples are from cores of basement units intersected in drill holes that penetrated overlying rocks of the Karumba (Cenozoic) and Carpentaria (Mesozoic) basins. One of the samples is from the Proterozoic Keer Weer Province, one is from the adjoining Proterozoic Kowanyama Province, and the third is from the Carboniferous–Permian Townsville–Mornington Island Belt (Kennedy Igneous Association). Extensively altered gneiss (sample BB6204/11044786) of the Keer Weer Complex was intersected at a depth of ca 885.7 m in bore hole Z.C.L. Weipa 1 located ca 11 km southeast of Weipa. The gneiss is interpreted to represent a deformed and recrystallised granitoid and is in an area of high magnetisation on geophysical images. Zircons analysed from sample BB6204/11044786 collected from the ca 974.8–976.6 m interval yielded a 207Pb/206Pb magmatic crystallisation age of 1539.7 ± 2.8 Ma. An almost identical 207Pb/206Pb magmatic crystallisation age of 1538.4 ± 3.3 Ma was obtained for sample BB6200/11044787, a diorite representing the basement in F.B.H. Wyaaba 1 located ca 415 km farther south. The diorite is also extensively altered but is only slightly deformed and retains some relict primary igneous grains. The diorite has a low to moderate response on aeromagnetic images in contrast to the relative high magnetisation shown by the granitic gneiss forming the basement in drill hole Z.C.L. Weipa 1. It mainly intrudes rocks of the Kowanyama Province, which are characterised by an overall relatively uniform low magnetic response. These granitoids were emplaced during the waning of a major late Paleoproterozoic–early Mesoproterozoic tectonothermal event that affected north Queensland. The event was accompanied by greenschist–granulite facies metamorphism and, according to Blewett & Black (1998) it marked the termination of the first major crust-forming period following the ca 1880–1850 Ma Barramundi Orogeny of Etheridge et al. (1987). Zircons analysed from the third sample, a carbonaceous siltstone from the concealed Gamboola Basin yielded a 238U/206Pb maximum depositional age of 279.9 ± 2.4 Ma, thereby supporting the early Permian age of deposition proposed by some previous investigators. Interestingly, although the basin is located in the Townsville–Mornington Island Belt, the detrital zircon population is dominated by ca 1572 Ma grains implying the local Etheridge and Croydon provinces were the main sources of the detritus in the siltstone rather than the surrounding Kennedy Igneous Association.
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