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 Ritscherflya Supergroup, Antarctica, is intruded by numerous mafic to intermediate sills and small layered bodies. Contact relations are well exposed where the Grunehogna and Kullen sills intruded argillaceous and arenaceous sedimentary rocks of the Schumacherfjellet and Hogfonna Formations of the Ahlmannryggen Group. A 3 m thick peripheral zone above the upper contact of the Grunehogna sill consists of reconstituted sedimentary rocks which have been fluidized and recrystallized due to the intrusion. Local melting and crystallization of the sediments, under hydrous conditions and pressures below 312 bars, have produced zones of granite and granophyre, which grade into arenites where primary sedimentary structures are preserved. The sedimentary rocks have been deformed into large disharmonic folds where the sill transgresses the Schumacherfjellet Formation. The contact relations suggest that emplacement of the mafic sills and sheets at Grunehogna occurred when the sediments were still water-saturated and only partially lithified.
Geologists from many countries have worked in Antarctica since the turn of the twentieth century and during the past thirty years the level of research activity on the continent has increased annually. As a result there has been a proliferation of stratigraphical names and lithostratigraphical schemes (Thomson, 1990). Furthermore, geologists from different nations are familiar with different standards and codes of nomenclature, which has resulted in a number of inconsistencies in Antarctic stratigraphical names. The SCAR Working Groups on Geology and Solid Earth Geophysics therefore recommended at their meeting in 1990 with SCAR XXII in São Paulo that, for sedimentary rocks, geologists working in Antarctica should adhere to the stratigraphical principles and recommendations proposed by Hedberg (1976). It was recognized, however, that igneous and metamorphic rocks present special problems of nomenclature. An ad hoc group of the SCAR Working Group on Geology for naming igneous and metamorphic rock units, constituted by the authors of this note, was therefore established to consider and discuss these problems and to formulate recommendations for suitable schemes that may be used internationally for Antarctica.
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