May 19th, 2008
Geochemistry and tectonic setting of mafic rocks in western Dronning Maud Land, East Antarctica: implications for the geodynamic evolution of the Proterozoic Maud Belt
On the basis of new bulk major and trace element (including REE) as well as Sm-Nd and Rb-Sr isotope data, used in conjunction with available geochronological data, a post-tectonic mafic igneous province and four groups of pre- to syntectonic amphibolite are distinguished in the polymetamorphic Maud Belt of western Dronning Maud Land, East Antarctica. Protoliths of the Group 1 amphibolites are interpreted as volcanic arc mafic intrusions with Archaean to Palaeoproterozoic Nd model ages and depletion in Nb and Ta. Isotopic and lithogeochemical characteristics of this earliest group of amphibolite indicate that the Maud Belt was once an active continental volcanic arc. The most likely position of this arc, for which a late Mesoproterozoic age (c. 1140 Ma) is indicated by available U-Pb single-zircon age data, was on the southeastern margin of the Kaapvaal-Grunehogna Craton. The protoliths of Group 2 amphibolites are attributed to the 1110 Ma Borgmassivet-Umkondo thermal event on the basis of comparable Nd model ages and trace element distributions. Group 3 amphibolite protoliths are characterized by mid-ocean ridge basalt-type REE patterns and low Th/Yb ratios, and they are related to Neoproterozoic extension. Group 4 amphibolite protoliths are distinguished by high Dy/Yb ratios and are attributed to a phase of syntectonic Pan-African magmatism as indicated by Rb-Sr isotope data.
The high-grade metamorphic Maud Belt of western Dronning Maud Land, East Antarctica, has been interpreted as a juvenile island arc that was tectonically juxtaposed along the margin of an Archaean cratonic block, known as the Grunehogna Craton, at the end of the Mesoproterozoic (Arndt et al. 1991; Jacobs et al. 1993; Grantham et al. 1995; Groenewald et al. 1995; Golynsky & Jacobs 2001; Bauer et al. 2003b; Paulsson & Austrheim 2003; Basson et al. 2004). In contrast to the high-grade ortho- and paragneisses of the Maud Belt, a low-grade volcano-sedimentary Mesoproterozoic succession (Ritscherflya Supergroup) overlies Archaean basement on the Grunehogna Craton (Fig. 1). A major geophysical boundary, known as the Pencksokket-Jutulstraumen Discontinuity, separates the Grunehogna Craton from the Maud Belt and has been interpreted as a Mesozoic continental rift with an orientation that was structurally controlled by major ‘Grenvillian’ (about 1.1 Ga) and/or Pan-African (about 550 Ma) thust shear zones (Ferraccioli et al. 2005; Fig. 1). Extensive mafic sills of the 1107Ma Borgmassivet Suite (Krynauw et al. 1991; M. Knoper, unpubl. data reported by Frimmel 2004) intruded the volcano-sedimentary rocks of the Ritscherflya Supergroup, whereas numerous mafic bodies occur as pre-, syn- and posttectonic amphibolite dykes, sills and boudins in the Maud Belt. Geochemical data for these mafic rocks are scarce in the literature and largely confined to samples from the southwesternmost part of the belt (Bauer et al. 2003a). To better assess the likely tectonic setting of the various stages of magmatism recorded in the Proterozoic to Cambrian rocks of Dronning Maud Land, new major and trace element (including REE) as well as Sr and Nd isotopic data for a range of metabasites of different age from across the Maud Belt and the adjacent Ritscherflya Basin were collected in this study.
General agreement exists on the Grunehogna Craton having formed part of the Kaapvaal Craton prior to the break-up of Gondwana. Its pre-Gondwana geodynamic evolution remains, however, highly speculative. Three tectonic models have been suggested for the evolution of the southern margin of the Kaapvaal-Grunehogna Craton, including western Dronning Maud Land: (1) closure of a complex ‘Tugela Ocean’ with one or more intra-oceanic arcs and generally southward-directed subduction zones (Arima et al. 2001); (2) a southward-directed subduction, followed by a northward-directed subduction in western Dronning Maud Land (Bauer et al. 2003Ë); (3) two southward-directed subduction zones, with the northern one separating the Grunehogna Province from the Kaapvaal Craton (Basson et al. 2004). All of these models consider the Maud Belt to represent a continuation of the late Mesoproterozoic Namaqua-Natal Belt of southern Africa into East Antarctica and they involve deposition of at least the lower parts of the Ritscherflya Supergroup in a peripheral forearc cratonic basin in response to accretion and continental collision of an island arc (Maud Belt) to the margin of the Grunehogna Craton (e.g. Groenewald et al. 1995; Moyes & Harris 1996; Basson et al. 2004). The polarity of subduction is, in most cases, assumed to be directed away from the craton toward allegedly juvenile island arc/s that form the Namaqua-Natal and Maud Belts (e.g. Jacobs et al. 1993; Grantham et al. 1995; Groenewald et al. 1995; Golynsky & Jacobs 2001; Basson et al. 2004). Reconstruction of a late Mesoproterozoic P-T-1 path for the Maud Belt and direct comparison with that recorded in the Namaqua-Natal Belt was recently shown to be complicated because of major tectonic reworking during Pan-African times at c. 540 Ma (Board et al. 2005). Consequently, the various mafic bodies that occur as deformed dykes, sills and/or boudins must predate or be syntectonic with the Pan-African orogeny and thus provide potential information on the late Meso- to Neoproterozoic history of the Maud Belt.