Summary of results. Joint GSQ-GA geochronology project: Monto and Maryborough 1:250 000 Sheet areas

This report presents new SHRIMP U-Pb zircon results for twelve samples from central, south-eastern Queensland (Figure 1 and Table 1). The work was carried out under the auspices of the National Geoscience Agreement (NGA) between Geoscience Australia and the Geological Survey of Queensland. The data and age interpretations contained in this report are available in Geoscience Australia’s Geochronology Delivery database (http://www.ga.gov.au/geochron-sapub-web/).
Ten of the samples reported here are a part of an extensive (&gt;500km) north-north-west trending belt of volcanic and plutonic rocks that stretches from south of Rockhampton to the Brisbane area. These rocks have been interpreted as recording a major transition from continental convergent magmatism during the Early to Mid Triassic (Hunter-Bowen Orogeny) to extension-related magmatism and associated basin formation in the Late Triassic (Holcombe &amp; others, 1997; Gust &amp; others, 1993; Stephens, 1991). Volcanic rocks in the belt range from basalts to rhyolites with associated pyroclastic and epiclastic sedimentary rocks. Plutonic rocks are dominated by high-level felsic granites with granodiorite, tonalite, diorites and gabbro (occasionally in close netvein association with felsic rocks) less abundant. Six of the Mid to Late Triassic magmatic units reported here have the same crystallisation age within their analytical uncertainties of 228 + 1.3Ma (MSWD = 2.4). This age is very similar to previous SHRIMP U-Pb zircon age determinations of magmatic units within this belt reported by Cross &amp; others, (2009), Carson &amp; others, (2006) and mentioned in Purdy (2010) and confirms the very restricted temporal distribution of this event.
Three new SHRIMP U-Pb zircon ages reported here have been used to redefine the Castletower Granite. Prior to this study this unit was mapped as a single, extensive (~45 x 10km) unit comprising several plutons in the Many Peaks Range, between Lake Awonga in the north and the Boyne and Kolan River headwaters in the south. No previous geochronology had been undertaken on this unit and it had been assigned a Permian to Triassic age based on its relationship with other units. The new geochronology reported here has allowed a subdivision of this unit. The northernmost pluton has retained the name Castletower Granite and is Late Triassic in age (221.8 + 1.3Ma, see 2003616). Granite and diorite/gabbro in central areas are contemporaneous and Late Permian in age. These are now mapped as the Many Peaks Granite (268.1 + 1.8Ma, see 1999101) and unnamed unit PRg/b (268.3 + 1.7Ma, see 2003617). Based on field relations (Purdy, 2010), granite in southern regions is considered Late Triassic in age and is mapped as the Bulburin Granite.
Bibliographic Reference: Cross, A.J., Purdy, D.J.&amp; Bultitude, R.J., 2012: Summary of results – joint GSQGA geochronology project: Monto and Maryborough 1:250 000 Sheet areas. Queensland Geological Record 2012/02.

Tags: Age determination methods, EARTH SCIENCES, Geochronology, Published_External, SHRIMP
Organization: Geoscience Australia Data