Precise U–Pb baddeleyite dating of the Derim Derim Dolerite, McArthur Basin, Northern Territory: old and new SHRIMP and ID-TIMS constraints

Abstract

The Mesoproterozoic Roper Group of the McArthur Basin has excellent petroleum potential, but exploration has been hampered by poor constraints on its post-depositional history that has compromised understanding of the tectonostratigraphic evolution of the basin. The Derim Derim Dolerite occupies an important position in the event chronology of the McArthur Basin, having intruded the Roper Group prior to post-Roper basin inversion, and it is also a major component of Mesoproterozoic intraplate mafic magmatism in northern Australia. Since 1997, the Derim Derim Dolerite has been assigned a magmatic crystallisation age of 1324 ± 4 Ma (all uncertainties are 95% confidence), based on unpublished sensitive high-resolution ion micro probe (SHRIMP) U–Pb analyses on baddeleyite attributed to a dolerite sample from Bureau of Mineral Resources drill-hole Urapunga 5. Herein, we establish that the SHRIMP sample originated from the type locality of the Derim Derim Dolerite in outcrop 90 km northwest of Urapunga 5 and document the 207Pb/206Pb date interpreted from the 1997 dataset. New U–Pb SHRIMP reanalysis of the same grain-mounts yielded a mean 207Pb/206Pb date of 1320.1 ± 5.3 Ma, confirming the 1997 result, and isotope dilution-thermal ionisation mass spectrometry (ID-TIMS) analysis of baddeleyites plucked from the mounts yielded a precise mean 207Pb/206Pb date of 1327.5 ± 0.6 Ma. This date is significantly older than a baddeleyite U–Pb ID-TIMS date of ca 1313 Ma recently reported elsewhere from dolerite in the Beetaloo Sub-basin 200 km to the south, indicating that magmatism attributed to the Derim Derim Dolerite spanned at least 10–15 Ma. Previously documented geochemical variation in Mesoproterozoic intraplate mafic rocks across the Northern Territory (such as the 1325 ± 36 Ma Galiwinku Dolerite in the McArthur Basin, 1316 ± 40 Ma phonolites in the Nimbuwah Domain of the eastern Pine Creek Orogen, and 1295 ± 14 Ma gabbro in the Tomkinson Province) may reflect episodic pulses of magmatism hitherto obscured by the low precision of the available isotopic dates. The timing and geochemistry of Derim Derim-Galiwinku mafic igneous activity is strikingly similar to that of the Yanliao Large Igneous Province (LIP) in the northern North China Craton, and the global paucity of 1330–1300 Ma LIPs suggests that the North Australian Craton and the North China Craton were in relatively close proximity at that time. KEY POINTS We document a previously unpublished U–Pb baddeleyite date of 1324 ± 4 Ma for the Derim Derim Dolerite in the McArthur Basin, obtained via sensitive high-resolution ion micro probe (SHRIMP), and substantiate it with an isotope dilution-thermal ionisation mass spectrometry (ID-TIMS) date of 1327.5 ± 0.6 Ma from the same sample. Our dates are significantly older than a U–Pb baddeleyite ID-TIMS date of ca 1313 Ma from dolerite in the Beetaloo Sub-basin 200 km to the south, indicating that magmatism attributed to the Derim Derim Dolerite spanned at least 10–15 Ma. Contemporaneous intraplate mafic magmatism extended hundreds of kilometres across the Northern Territory, spanning the Tomkinson Province, the McArthur Basin, and the Nimbuwah Domain of the eastern Pine Creek Orogen. The timing and geochemistry of Derim Derim Dolerite magmatism strongly resembles the Yanliao Large Igneous Province (LIP) in the northern North China Craton, which suggests that the North Australian Craton and the North China Craton were in close proximity during the Mesoproterozoic.