THE AGE OF THE RITSCHERFLYA SUPERGROUP AND BORGMASSIVET INTRUSIONS, DRONNING-MAUD-LAND, ANTARCTICA

The Ahlmannryggen-Borgmassivet area of western Dronning Maud Land comp rises 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 Hogfonna Format ion at Grunehogna indic ate a depositional age of approximate to 1080 Ma, the first reported d irect dating of any member of the Ritscherflya Supergroup. These rocks are interpreted as a molasse-type deposit following the Kibaran oroge ny 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 approximate to 1000 Ma Grunehogn a sill intruded unconsolidated sediments, causing partial melting of t he 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 com patible with crustal contamination of this sill, producing a Rb-Sr pse udo-isochron with no geological age significance. By comparison with o ther outcrops of the Borgmassivet Intrusions at Robertskollen and Anna ndagstoppane, 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 conclude d to be approximate to 1000 Ma old. Nd model age data indicate that al l rock types were ultimately derived from material separated from a de pleted mantle source at approximate to 2200 Ma.