Zircon UPb and hornblende 40Ar 39Ar ages for the Dufek layered mafic intrusion, Antarctica: Implications for the age of the Ferrar large igneous province

Abstract

New high-precision zircon UPb ages for the Dufek intrusion establish the crystallization age for the only layered mafic intrusion (sensu stricto) known in the Ferrar igneous province, Antarctica, one of the largest igneous provinces in the world. Three concordant zircon fractions from the capping Lexington Granophyre yield an age of 183.9 ± 0.3 Ma (2σ), which we interpret as the crystallization age for the Dufek intrusion. Concordant zircon fractions from a silicic dike crosscutting the upper layered gabbroic sequence yield a slightly younger crystallization age of 182.7 ± 0.4 Ma (2σ) . For both samples, agreement within errors between the two UPb and 207pb∗/ 206Pb∗ ages for three fractions each with a unique set of physical and chemical characteristics demonstrates that inheritance of older zircon components from the country rocks was imperceptible. It also suggests that assimilation or anatexis of the lower Paleozoic country rocks was not the dominant mechanism in the formation of the granophyre. These new UPb dates agree, within analytical errors, with recent zircon and baddeleyite UPb ages for dolerite sills in both the Ferrar and Karoo igneous provinces, which demonstrates the vastness of the rift axis along which simultaneous magmatic activity occurred. Contemporaneity between the Dufek intrusion and other Ferrar Group rocks supports the possibility that this pluton acted as a conduit for magmas of the widely distributed Ferrar sills and flows. Hornblende 40Ar 39Ar ages for the Lexington Granophyre (175 ± 2 Ma) barely overlap with those for a silicic dike (179 ± 2 Ma) in the layered gabbro section just below the granophyre. The younger of the two hornblende ages may be explained by degassing of inclusions or by protracted hydrothermal alteration preventing the ArAr geochronometer form becoming a closed-system in the higher level granophyre until after it had done so in the more deeply positioned silicic dike. The 179 ± 2 Ma age of the Dufek silicic dike is interpreted to represent cooling through the Ar retention temperature for hornblende of about 500°C. Combining this cooling age with the UPb crystallization age of 182.7 ± 0.4 Ma yields a cooling rate for the dike of approximately 100°C/m.y. Inasmuch as this rate is not rapid, it can be inferred that the bulk of the Dufek intrusion was still fairly hot when the dated silicic dike was emplaced.