U-PB ZIRCON DATING OF MAFIC DYKES AND ITS APPLICATION TO THE PROTEROZOIC GEOLOGICAL HISTORY OF THE VESTFOLD HILLS, EAST ANTARCTICA

The Vestfold Hills, one of several Archaean cratonic blocks within the East Antarctic Shield, comprises a high-grade metamorphic basement co mplex intruded by at least nine generations of Early to Middle Protero zoic mafic dykes. Extensive U-Pb ion microprobe (SHRIMP) analyses of z ircons, derived predominantly from late-stage felsic differentiates of the mafic dykes, provide precise crystallisation ages for several dyk e generations. These new ages enable constraints to be placed on both the history of mafic magmatism in the Vestfold Hills and the timing of the various interspersed Proterozoic deformation events. In addition to demonstrating the utility of zircons derived from felsic late-stage differentiates for the dating of co-genetic mafic dykes, this study a lso places doubt on previous whole-rock Rb-Sr dating of mafic dyke sui tes in this and other areas of East Antarctica. The Pb-207/Pb-206 zirc on ages of 2241 +/- 4 Ma and 223 8 +/- 7 Ma for the Homogeneous and Mo ttled Norites, respectively, provide a younger emplacement age for ass ociated group 2 High-Mg tholeiite dykes than the whole-rock Rb-Sr date (2424 +/- 72 Ma) originally interpreted as the age of all high-Mg int rusives in the Vestfold Hills. Zircon ages of 1754 +/- 16 Ma and 1832 +/- 72 Ma confirm the previously defined Rb-Sr age of the group 2 Fe-r ich tholeiites. Two later dyke generations, the group 3 and 4 Fe-rich tholeiites, are distinguished on the basis of field orientations and c ross-cutting relationships, and yield zircon emplacement ages of 1380 +/- 7 Ma and 1241 +/- 5 Ma which also define minimum ages for two suit es of lamprophyre dykes. Xenocrystic zircons within both felsic segreg ations and mafic dykes yield zircon ages of 2478 +/- 5 Ma to approxima tely 2740 Ma, indicating the presence of Archaean crustal source rocks of this antiquity beneath the Vestfold Hills.