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The Ahlmannryggen-Borgmassivet area of western Dronning Maud Land comprises a relatively undeformed, unmetamorphosed sequence of sedimentary-volcanogenic rocks, the Ritscherflya Supergroup, intruded by a suite of continental tholeiites, the Borgmassivet Intrusions. New Rb-Sr and Sm-Nd whole rock data from the Högfonna Formation at Grunehogna indicate a depositional age of ≈1080 Ma, the first reported direct dating of any member of the Ritscherflya Supergroup. These rocks are interpreted as a molasse-type deposit following the Kibaran orogeny at 1200–1100 Ma, and correlation is made with the Umkondo and Koras groups of southern Africa. The Ritscherflya Supergroup is intruded by the Grunehogna and Kullen sills; the ≈1000 Ma Grunehogna sill intruded unconsolidated sediments, causing partial melting of the sediments. Rb-Sr data from the Kullen sill yield an age of 1429 Ma, clearly inconsistent with these data. Combined Sr and Nd data are compatible with crustal contamination of this sill, producing a Rb-Sr pseudo-isochron with no geological age significance. By comparison with other outcrops of the Borgmassivet Intrusions at Robertskollen and Annandagstoppane, it is concluded that contamination and pseudo-isochrons may be responsible for the wide range in reported ages older than 1000 Ma. Thus the intrusive age of the Borgmassivet Intrusions is concluded to be ≈1000 Ma old. Nd model age data indicate that all rock types were ultimately derived from material separated from a depleted mantle source at ≈2200 Ma.

The Mid-Proterozoic-Early Palaeozoic tectonic development of western Dronning Maud Land is examined in the light of recent proposals that suggest geological continuity between Laurentia and Gondwana during the Middle Proterozoic (the SWEAT hypothesis). Gondwana amalgamation from the remaining constituents followed fragmentation and rifting of Laurentia from this supercontinent, culminating in a Late Cambrian orogeny at c . 500 Ma. The Mid-Proterozoic-Early Palaeozoic tectonic development of western Dronning Maud Land was significantly different to that observed in the Transantarctic Mountains, and the c. 500 Ma event in particular is characterized by: (1) a regional isotopic resetting event at c. 470 Ma, (2) scarce coeval magmatism, comprising only one major intrusion of alkaline affinity, and volumetrically small ‘S-type’ granitic sheets derived from local anatexis of gneisses, (3) structural deformation considerably less intense than during the preceding Kibaran (1100 Ma) orogeny, comprising relatively open to gentle folding, (4) reactivation of thrusting initiated during the Kibaran orogeny, with associated metasomatism. It is concluded that the effects of the Late Cambrian ( c . 500 Ma) event are consistent with crustal thickening through large-scale underplating, rather than subduction-related processes as seen in the Ross orogeny of the Transantarctic Mountains. It is also concluded that Gondwana amalgamation occurred along suture lines not currently exposed in outcrop in Dronning Maud Land. These data neither contradict nor support the SWEAT hypothesis as currently argued.

Summary The Orange River Group (ORG) consists of a thick volcano-sedimentary pile (> 8 km) preserved as a subprovince within the Namaqua Province of southern Africa. Volcanic activity during deposition of the ORG was predominantly subaerial, and involved the eruption of an andesite dominated calcalkaline suite ranging from basalt through to rhyolite. Agreement between Rb-Sr, Th-Pb, Pb-Pb and Sm-Nd isochron ages indicate extrusion of the ORG around 2000 Ma ago. Compared with modern arc suites, the ORG is characterized by high K and Mg, together with related trace elements (Rb, Sr, Ba, U, Th, LREE; Ni, Cr). Basic lavas differ from many modern calcalkaline types in having lower A1 contents reminiscent of tholeiites. When normalized to N-type MORB, the ORG basic lavas show enrichments in K, Rb, Ba, Th, and the LREE, and depletions in Nb, Ta, Zr, Hf, Ti, Y and the HREE). Metamorphic overprinting associated with the ∼ 1000 Ma Namaqua tectonothermal event are evident to varying degrees throughout the subprovince, and precludes precise monitoring of initial isotope ratios at time of eruption. Evidence from the rocks least affected by metamorphic overprinting suggests common initial Sr, Pb and Nd isotopic compositions for the entire basalt to rhyolite suite in this particular area, which would support a fractional crystallization model and rule out major involvement of old Archaean crust to produce the voluminous andesites. Initial isotopic data translate to ɛ sr (T) + 10), ɛNd(T)∼0, and an apparent high μ value, which, when taken together with the geochemical features described previously, indicate that the source region to the mafic parental magmas of the ORG shows patterns similar to those in the mantle wedge overlying subduction zones. The calcalkaline ORG volcanic suite may therefore represent the remnants of a 2000 Ma old Proterozoic arc complex. The high K character, the intimate association of comagmatic granitoids, the development of copper porphyries, and the lack of any direct or indirect evidence for underlying Archaean crust all point to a mature island arc environment. Active continental margins such as the Andes show similar geochemical and isotopic features, but it is argued that the high elevation of such margins will cause them to be preferentially eroded and lack representation in ancient terrains.

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