CAMBRIAN GRANITES OF PRYDZ BAY, EAST ANTARCTICA: EVIDENCE FOR MELTING OF A TWO-COMPONENT GRANULITE-FACIES CRUST?

Neoproterozoic granulitic gneisses along the Prydz Bay coast are intruded by Cambrian granites (500-520 Ma). Occurring over ~130 km, the granites are structurally, texturally and mineralogically diverse, ranging from veins (<1 m) of fine-grained, foliated biotite granite to bodies > 1 km across of unfoliated, K-feldspar phyric biotite-hornblende granite. Whole rock compositions are characterized by high total alkalis, high K/Na, low Mg, Cr, Ni, high Zr (up to 1625 ppm), LREE (up to 2101 ppm Ce) and Th (up to 878 ppm Th; Th/U = 28-142), as well as elevated Ga ([Ga/Al]*10⁴ = 2.9-11.0) and halogens (726-3953 ppm F, 27-1195 ppm Cl), all characteristics of A-type granites. REE patterns are characterized by variable HREE fractionation and Eu depletion; La/Yb ratios show regional differences, being highest in 3 of the 4 eastern intrusions (La_N/Yb_N = 217-454) and lowest in the western intrusions (La_N/Yb_N = 36-67). Sr-Nd-Hf isotope ratios (⁸⁷Sr/⁸⁶Sr 0.708- 0.720, ε_Nd -8 to –14, ε_Hf ~-9) are consistent with magma sources in equivalents of the exposed para- and orthogneisses. A metasedimentary source component could explain the mildly peraluminous (ASI up to 1.13) nature of some of the granites. Initial Pb isotope compositions, measured on feldspars, are characterized by high ²⁰⁷Pb/²⁰⁴Pb (15.71-15.77) and low ²⁰⁶Pb/²⁰⁴Pb (17.7-18.16) compared to global Pb isotope growth curves. This “high-207/low-206” signature requires Pb isotope evolution in a high-U/Pb reservoir since about 2.5 Ga, followed by a low-U/Pb (μ<<10) stage for several 100 Ma prior to granite formation. In the present case, lowering of U/Pb may be related to U removal from older (up to 2.5 Ga) crustal protoliths during the ca. 1 Ga granulite facies event recorded in Prydz Bay gneisses. Co-variation of REE-Y and Sr-Nd isotopes in the granites suggests the presence of two crustal components in the granite source. Component 1 (ε_Nd below -14, ⁸⁷Sr/⁸⁶Sr >0.72) produced melts with strong HREE fractionation and Y depletion (major residual garnet), represented by 3 of the 4 eastern intrusions. Component 2 (ε_Nd higher than -7, ⁸⁷Sr/⁸⁶Sr<0.708) produced melts with lower Gd_N/Yb_N (less residual garnet), represented by the western intrusions. Both components contain the high-207/low-206 isotope signature inferred to be linked to the regional granulite gneisses.