Early stages of Gondwana breakup: The Ar-40/Ar-39 geochronology of Jurassic basaltic rocks from western Dronning Maud Land, Antarctica, and implications for the timing of magmatic and hydrothermal events

[1] The timing of magmatic events forming Jurassic basaltic rocks in Dronning Maud Land (DML), Antarctica, and hydrothermal activity that affected them is addressed with detailed Ar-40/Ar-39 incremental heating dating of feldspars. Plagioclase from an Utpostane gabbro and from a Kirwanveggen dolerite dike yield indistinguishable plateau ages at 177 +/- 1.8 Ma. Because of geologic controls, this establishes the age of tholeiites in Vestfjella. These plateau ages demonstrate synchroneity of tholeiitic magmatism of both DML and the well-documented Ferrar Province of Antarctica, which together with the closely temporal Karoo constitute the Gondwana breakup magmatism. Vug-filling microclines from Vestfjella yield plateau ages of 150 and 139 Ma which are interpreted to give the ages of secondary mineralization events. These results suggest a lengthy period of extension, possibly accompanied by pervasive low-temperature hydrothermal activity, between flood basalt magmatism and inception of seafloor spreading (circa 160-165 Ma), with younger moderate temperature events recorded by K-feldspars and related to early stages of spreading and/or changes in plate motions. The majority of plagioclase samples yield discordant age spectra that reflect primarily overprinting by younger events and incorporation of excess Ar, and they illustrate the complexities that can be produced