ZIRCON U-PB AND HORNBLENDE AR-40-AR-39 AGES FOR THE DUFEK LAYERED MAFIC INTRUSION, ANTARCTICA - IMPLICATIONS FOR THE AGE OF THE FERRAR LARGE IGNEOUS PROVINCE

New high-precision zircon U-Pb ages for the Dufek intrusion establish the crystallization age for the only layered mafic intrusion (sensu st ricto) known in the Ferrar igneous province: Antarctica, one of the la rgest igneous provinces in the world. Three concordant zircon fraction s from the capping Lexington Granophyre yield an age of 183.9 +/- 0.3 Ma (2 sigma), which we interpret as the crystallization age for the Du fek intrusion. Concordant zircon fractions from a silicic dike crosscu tting the upper layered gabbroic sequence yield a slightly younger cry stallization age of 182.7 +/- 0.4 Ma (2 sigma). For both samples, agre ement within errors between the two U-Pb and (207)pb/(206)pb* ages fo r three fractions each with a unique set of physical and chemical char acteristics demonstrates that inheritance of older zircon components f rom the country rocks was imperceptible. It also suggests that assimil ation or anatexis of the lower Paleozoic country rocks was not the dom inant mechanism in the formation of the granophyre. These new U-Pb dat es agree, within analytical errors, with recent zircon and baddeleyite U-Pb ages for dolerite sills in both the Ferrar and Karoo igneous pro vinces, which demonstrates the Vastness of the rift axis along which s imultaneous magmatic activity occurred. Contemporaneity between the Du fek intrusion and other Ferrar Group rocks supports the possibility th at this pluton acted as a conduit for magmas of the widely distributed Ferrar sills and flows: Hornblende Ar-40-Ar-39 ages for the Lexington Granophyre (175 +/- 2 Ma) barely overlap with those for a silicic dik e (179 +/- 2 Ma) in the layered gabbro section just below the granophy re. The younger of the two hornblende ages may be explained by degassi ng of inclusions or by protracted hydrothermal alteration preventing t he Ar-Ar geochronometer form becoming a closed-system in the higher le vel granophyre until after it had done so in the more deeply positione d silicic dike. The 179 +/- 2 Ma age of the Dufek silicic dike is inte rpreted to represent cooling through the Ar retention temperature for hornblende of about 500 degrees C. Combining this cooling age with the U-Pb crystallization age of 182.7 +/- 0.4 Ma yields a cooling rate fo r the dike of approximately 100 degrees C/m.y. Inasmuch as this rate i s not rapid, it can be inferred that the bulk of the Dufek intrusion w as still fairly hot when the dated silicic dike was emplaced. Copyrigh t (C) 1997 Elsevier Science Ltd.