WHOLE-ROCK ND (ID-TIMS) AND ZIRCON HF-U-PB (LAMS) ISOTOPE COMPOSITION OF MESOPROTEROZOIC A-TYPE GRANITES IN MANNEFALLKNAUSANE, WESTERN DRONNING MAUD LAND, ANTARCTICA

Crystalline bedrock in western Dronning Maud Land, Antarctica is composed of the Archean Grunehogna craton and the Mesoproterozoic Maud mobile belt. ~100 km south of Archean-Proterozoic transition, in the Proterozoic realm, nunataks of Mannefallknausane (~74.5^oS, 15^oW) are dominated by Precambrian granitoid rocks and rare paragneisses. Three principal granites can be identified: a white, garnet-bearing K-feldspar-megacrystic biotite granite; a red biotite-hornblende±clinopyroxene granite with and without plagioclase-mantled K-feldspar-megacrysts (rapakivi texture); and a dark green porphyritic charnockite with orthopyroxene and hornblende. For two varieties of the red granite (wiborgite, pyterlite), our new U-Pb data imply crystallization ages of 1073 ± 6 Ma and 1084 ± 8 Ma, respectively. These are compatible with a U-Pb zircon upper intercept age of 1073 ± 8 Ma of the charnockite (Arndt et al., 1991). The initial Nd isotope composition of these rocks is relatively radiogenic [ε_Nd (1075 Ma) value of the biotite granite -0.5; red granite +0.3, +0.5; charnockite +1.4], as is that of a country-rock gneiss from the surrounding bedrock (+1.0). Initial zircon ε_Hf values of the wiborgite and pyterlite are +5.2 ± 2.2 (2σ) and +6.3 ± 1.8 (2σ), respectively, and thus confirm the juvenile character of the red granite. The ε_Nd (1075 Ma) value of a Proterozoic mafic plutonic rock from the nearby Heimefrontfjella region is clearly more positive (+4.1) than those of the A-type association of Mannefallknausane, implying a depleted mantle source. The A-type granite-charnockite association in western Dronning Maud Land was emplaced into a comparatively juvenile crust at 1075 Ma and has Nd and Hf isotope systematics that are compatible with a major Mesoproterozoic crustal source component.