CONTINENTAL MAFIC DYKE SWARMS AS TECTONIC INDICATORS - AN EXAMPLE FROM THE VESTFOLD-HILLS, EAST ANTARCTICA

The two main types of magma that were emplaced as mafic dyke swarms in the Vestfold Hills are high-Mg tholeiite (HMT) and Fe-rich tholeiite (FRT). The former magma type is less dense than the Vestfold Hills fel sic crust, whereas the latter is more dense at deep to intermediate cr ustal levels. Therefore, FRT dyke emplacement requires a higher deviat oric extensional stress, as is reflected in a more constant orientatio n. Palaeoproterozoic HMT-FRT bimodal dyke swarms were emplaced around 2.4 Ga and 2240 Ma. The geometry and orientation of 2240 Ma HMT dykes imply a low deviatoric extensional stress field, inconsistent with a m antle plume origin for magmatism. Current petrogenetic models for HMT magmatism suggest generation at active (convergent) plate boundaries. Although this is broadly consistent with emplacement of the bimodal dy ke swarms subsequent to a crustal thickening event at 2.5 Ga, the exac t mechanisms for melting and the generation of crustal extensional str ess are unknown. A possible scenario involves the sinking of gravitati onally unstable oceanic crust. A major Mesoproterozoic magmatic-extens ional event in the Vestfold Hills started with normal faulting, follow ed by lamprophyric dyke emplacement and by two distinct episodes of FR T dyke emplacement at 1380 and 1245 Ma. A consistent direction of the stress field during this event suggests control of a linear belt of cr ustal uplift. By comparison with other mafic dyke swarms, this is inte rpreted to be due to a sheet of upwelling asthenosphere (elongated plu me) around 1.4 Ga, rather than a low-angle detachment. This upwelling led to thermal erosion of the base of the lithosphere that channelled a subsequent magma pulse at 1245 Ma. Erosion during a long period of u plift prior to 1380 Ma was followed by sedimentation during thermal su bsidence, as suggested by segmentation of 1245 Ma dykes. The lithosphe ric structure inherited from the Mesoproterozoic magmatic-extensional event was probably reactivated during two high-grade tectono-metamorph ic events around 1.0 and 0.5 Ga. The Vestfold Hills formed part of a r elatively stable craton, peripheral to the mobile belt that was formed during these events.