Summary of results. Joint NTGS-GA geochronology project: central and western Aileron Province

Abstract

This Record presents six previously unpublished U-Pb SHRIMP zircon geochronological results from the Aileron Province in the Northern Territory. The data were collected to investigate the timing of localised and poorly documented granulite facies high-T, low-P metamorphism across isolated outcrops in the central and western Aileron Province. The study was also designed to test the maximum deposition ages of the metasedimentary rocks across this large area, and to determine whether the samples analysed are high-grade equivalents of the Lander Rock Formation.

Migmatitic psammitic metasedimentary rock of the Lander Rock Formation (NTGS MD03IRS33, GA 1936427), from an isolated hill 7 km south of the Andrew Young Hills and 3.5 km south of Black Hill Bore in southwestern MOUNT DOREEN, yielded a detrital zircon age spectra with major provenance components at ca 1860 Ma, ca 1878 Ma and ca 1892 Ma; and smaller high frequency maxima at ca 1832 Ma, ca 1953 Ma, ca 1975 Ma, ca 2023 Ma, ca 2193 Ma, and ca 2688 Ma. A weighted mean 207Pb/206Pb age of 1832 ± 10 Ma is interpreted as a maximum depositional age for the sedimentary protolith. Zircon rims record high-grade metamorphism at 1769 ± 14 Ma.

Two kilometres to the northeast of MD03IRS33, mafic granulite (NTGS MD03IRS29, GA 1936426) intrudes the psammitic and pelitic metasedimentary sequence, and is interpreted to be a metamorphosed sill-like body. It yielded a main, low Th/U ratio zircon population with a weighted mean 207Pb/206Pb age of 1768 ± 4 Ma, interpreted as the timing of granulite-facies metamorphism. Zircon rims record a thermal event at ca 1630 Ma, coeval with intrusion of the unmetamorphosed Andrew Young Igneous Complex, 4 km to the north.

Massive cordierite-rich rock (NTGS MT03IRS015, GA 1936424) was collected from an isolated hill of granulitefacies metasedimentary rocks mapped as high grade Lander Rock Formation, 25 km east-northeast of Mount Singleton in southwestern MOUNT THEO. This sample yielded a weighted mean 207Pb/206Pb age of 1761 ± 4 Ma from a distinct oscillatory zoned population of zircons, interpreted to record zircon crystallisation associated with high-grade metamorphism and partial melting. Older zircons have dates in the range 2809-1834 Ma, with a dominant provenance component at 1865-1855 Ma, and a youngest population at 1841 ± 8 Ma, reflecting the maximum depositional age of the sedimentary precursor.

An unnamed metapsammite (NTGS NP03IRS010, GA 1936422) was collected from a low hill of granulite-facies migmatitic metasedimentary rocks, 10 km north-northwest of Mount Denison homestead in NAPPERBY. It yielded zircons with dates between ca 2671 Ma and ca 1844 Ma, with dominant provenance components at ca 1854 Ma and ca 1875 Ma. A weighted mean 207Pb/206Pb age of 1853 ± 6 Ma is interpreted as the maximum depositional age of the sedimentary precursor.

Approximately 200 m west-northwest of NP03IRS010, a cordierite-biotite-K-feldspar-quartz-bearing granitic rock (NTGS NP03IRS011, GA 1936421), interpreted to be locally derived from partial melting of the metasedimentary rocks, has transitional contacts with migmatitic metapsammite. Zircon needles and rims yield a weighted mean 207Pb/206Pb age of 1801 ± 2 Ma, interpreted to record the magmatic crystallisation age of this granite. Inheritance (ca 2460-1827 Ma) suggests local derivation of the S-type granite during low-pressure granulite-facies metamorphism of the Lander Rock Formation or equivalents at ca 1801 Ma.

Quartz-rich metapsammitic rock (NTGS HE03IRS006, GA 1936420), collected 30 km north-northwest of Mount Chapple in HERMANNSBURG, was originally assigned to the Narwietooma Metamorphic Complex; it yielded a detrital zircon age spectra with a major provenance component at ca 1855 Ma, and smaller high frequency maxima at ca 1845 Ma, ca 1920 Ma, ca 1950 Ma, ca 2084 Ma, ca 2306 Ma and ca 2484 Ma. Coupled with a maximum depositional age of 1847 ± 5 Ma, these data are consistent with a Lander Rock Formation protolith.

These results support the interpretation that most, if not all, of the isolated outcrops of high-grade metasedimentary rock in the central and western Aileron Province belong to Lander Rock Formation, with sampled units having maximum ages for deposition spanning 1853 ± 6 Ma to 1832 ± 10 Ma. The timing of zircon crystallisation associated with high-grade metamorphism is variable across the region, with interpreted metamorphic ages of ca 1800 Ma in northwestern NAPPERBY, 1770-1765 Ma in southwestern MOUNT DOREEN, and ca 1760 Ma in southwestern MOUNT THEO.