AMALGAMATION OF COATS LAND/WESTERN DRONNING MAUD LAND WITH THE EAST ANTARCTIC CRATON: EVIDENCE FROM HEIMEFRONTFJELLA, DRONNING MAUD LAND, ANTARCTICA
The oldest rocks in the transect are pre-1.10 Ga granitic and supracrustal gneisses that underwent early Grenville deformation and migmatisation (D1/M1) prior to the intrusion of ca. 1.09-1.08 Ga granitoids. These rocks underwent granulite metamorphism and further deformation at ca. 1.08-1.02 Ga (D2/M2), prior to the intrusion of a mafic dyke swarm of inferred post-Grenville age. Pan-African orogenesis caused amphibolite metamorphism and deformation (D3/M3) at ca. 515-500 Ma, as determined by monazite (U-Pb) in metapelites, and titanite (U-Pb) and amphibole (K-Ar) in mafic gneisses and dykes. Both titanite and amphibole define strong D3 fabric elements. D3 deformation is focused in a kilometer-scale, vertical, ductile mylonite zone, the Heimefront shear zone (HSZ), where all rock types are transposed. As documented by other workers, the HSZ has a complex history, where D3 overprints earlier D2 fabrics. Whereas sinistral motion has been reported, D3 fabric elements (20° NNE-plunging lineation, asymmetric porphyroclasts, shear bands, and S/C fabrics) throughout the HSZ indicate southeast-side-up, oblique dextral shear.
Pan-African orogenesis caused widespread (but incomplete) retrogression of D2/M2 Grenvillian granulite assemblages, formation of locally penetrative mineral fabrics and ductile mylonitization. These new data are consistent with existing tectonic models, but we see no obvious need for previously defined terranes or crustal boundaries. Rather, we envision inhomogeneous Pan-African reworking of a complex, but potentially coherent, Grenvillian basement.