ISOTOPIC CONSTRAINTS ON THE AGE AND ORIGIN OF THE BRATTSKARVET INTRUSIVE SUITE, DRONNING-MAUD-LAND, ANTARCTICA

The alkali granitoids of the Brattskarvet intrusive suite constitute t he largest intrusion (approximately 100 km2) post-dating Middle Proter ozoic orogeny in the Sverdrupfjella region of the Maudheim tectonic pr ovince, western Dronning Maud Land. Rb-Sr whole-rock data indicate an intrusive age of 518 +/- 15 Ma (R0 = 0.7081), and Sm-Nd whole-rock dat a an age of 522 +/- 120 Ma (R0 = 0.5111). Rb-Sr mineralogical data (bi otite, alkali feldspar, sphene) yield ages of 482-465 Ma. The intrusiv e age of the suite is strongly indicated to be approximately 519 Ma, w ith subsequent cooling at approximately 476 Ma. Pb-Pb whole-rock data yield no age information, but indicate a lower-crustal source with low U/Pb ratios. These data support field evidence that intrusion post-da ted the early orogeny (Kibaran, 1100-1000 Ma), but predated the less i ntense deformation synchronous with the Ross or Pan-African event in C ambrian times. The country rocks, paragneisses of the Sverdrupfjella G roup, retain a Kibaran whole-rock Rb-Sr overprint of approximately 117 0 Ma (R0 = 0.7036), as do gneissic xenoliths enclosed in the intrusion , approximately 1046 Ma (R0 = 0.7105). Sm-Nd and Pb-Pb whole-rock data from these rocks are equivocal; Pb-Pb whole-rock data indicate a sour ce similar to upper-crustal rocks. U-Pb data from two zircon fractions extracted from the paragneiss confirm the dominance of Kibaran ages. Two hornfelsic xenoliths have undergone complete Rb-Sr resetting durin g the magmatism, but have retained Sm-Nd characteristics of the countr y rocks. Although biotite and feldspar from the country rocks underwen t Rb-Sr resetting during the Ross event, muscovite from one sample has retained a Kibaran age, and thus suggests a narrow range for the crus tal temperatures during the younger event. Combined Sr and Nd data ind icate that the intrusion is isotopically discrete from the country roc k, precluding an origin through local anatexis or contamination/mixing of an externally derived magma with the paragneisses. Derivation from an antique Sr- and Nd-depleted granulitic terrane, possibly attenuate d continental crust predating the accretionary Kibaran orogeny, is fav oured. The A-type geochemical character of the suite supports this int erpretation.