GEMOC - Department of EPS - Faculty of Science

CRUSTAL AGES DETERMINED BY SHRIMP DATING OF ZIRCON FROM GRANULITE XENOLITHS, NEW ENGLAND OROGEN, CENTRAL COASTAL QUEENSLAND

Charlotte M. Allen¹ and Ian S. Williams²

1. GEMOC ANU,
2. Research School of Earth Sciences ANU

Measuring the ages of zircons from the lower crust is the most direct means of determining the age structure of the continental crust. Somewhat less direct information can be obtained by studying the ages of inherited zircons in plutonic rocks. Granites are imperfect sampling media in that inherited zircon can be dissolved in some compositions. In an effort to understand the crustal structure in central coastal Queensland, we have done a reconnaissance study of zircons from three granulite xenoliths collected from Miocene mafic volcanic rocks originally studied by Griffin et al. (1987). These lower crustal xenoliths have been interpreted to be residues of melting events. Our results are compared to what we know about crustal ages from granites.

Two garnet-clinopyroxene granulite xenoliths from the northern tip of the Bowen Basin (Redcliff Vale and Mt St Martin) yielded only a few zircons. The Redcliff Vale sample yielded 9 zircons, and core ages of: ~1.84 Ga, ~1.40 Ga, Late Proterozoic (~600 Ma), Early Palaeozoic (~475 and 420 Ma), mid Palaeozoic (~300-260 Ma), and Early Cretaceous (~110 Ma). The size of the zircons made distinct rim analyses impossible. The Mt St Martin sample yielded only 4 zircons: three cores yielded ages of ~280-270 Ma, and an Early Cretaceous one (110 Ma). In contrast, a two-pyroxene granulite from 150 km west of Noosa Head (Brigooda Basalt) yielded more than 100 zircons, but only two age populations. Seven rim analyses yielded one age population with a mean of 221.0+6.6 Ma (95% confidence). Core ages (n=9) varied from that of the rim to a statistically distinct older one. The distinctly older cores (n=5) yielded a mean age of 241.6+5.5 Ma (95% confidence).

The xenolith from the Brigooda Basalt is from an area dominated by Triassic plutonism (K-Ar mineral ages; Gust et al., 1993). Three periods of igneous activity were distinguished: 250-230, 230-220, and 220-210 Ma. This granulite may be melt residue formed at 240 Ma (zircon cores) that was metamorphosed at 220 Ma (zircon rims) and brought to the surface in the Miocene. The two garnet-clinopyroxene xenoliths record a more complicated crustal history but one consistent with known geology. I-type granites and related volcanic rocks east and west of the northern tip of the Bowen Basin range in crystallization age (SHRIMP, zircon) from 308 to 278 Ma (Allen et al., in press; Black, 1994). Ten inheritance ages have been identified but only in volcanic rocks west of the Basin (2434 to 691 Ma; Black, 1994). East of the Basin, Triassic (~240 Ma) and Early Cretaceous igneous rocks are known ranging in age from 145-100 Ma (Allen et al., in press). The Early Palaeozoic and Late Proterozoic ages are more difficult to tie to plutonic events; they may be mixed ages. On the other hand, Siluro-Devonian intrusions are known in the New England Orogen, and in the Georgetown Inlier. Proterozoic rocks are common north and west of the Bowen Basin but this is the first direct evidence that lower crust of Proterozoic age underlaid the northern tip of the Basin Basin (New England Orogen) in the Miocene.

References

Allen, C.M., Williams, I.S., Stephens, C.J., and Fielding, C.R. (in press) Granite Genesis and Basin Formation in an Extensional Setting: the magmatic history of the northernmost New England Orogen. Australian Journal of Earth Sciences.

Black, L. (1994) U-Pb zircon ion-microprobe ages from the northern Drummond Basin, northeastern Queensland. AGSO Record 1994/34.

Griffin, W.L., Sutherland, F.L., and Hollis, J.D. (1987) Geothermal profile and crust-mantle transition beneath east-central Queensland: volcanology, xenolith petrology and seismic data. Journal of Volcanology and Geothermal Research 31, 177-203.

Gust, D. A., Stephens, C.J., and Grenfell (1993) Granitoids of the northern NEO: their distribution in time and space and their tectonic implications. NEO Conference Proceedings, UNE, Armidale, p. 565-572.