WAGNERITE, APATITE AND BIOTITE IN GRANULITE FACIES ROCKS OF THE NAPIER COMPLEX, ENDERBY LAND, ANTARCTICA; THE ROLE OF HALOGENS

Wagnerite, (Mg,Fe)₂PO₄(F,OH), has been discovered in Late Archean pegmatite pods and veinlets at four localities in the ultrahigh-temperature Napier complex and in a phlogopite-enstatite-sapphirine-cordierite granulite reworked in the Early Cambrian (“Rayner” Complex), all in coastal exposures around Amundsen and Casey Bays, western Enderby Land, Antarctica. Associated minerals in the pegmatites include biotite, apatite, sillimanite, quartz, garnet, perthite, beryllian sapphirine-khmaralite, surinamite, cordierite, ilmenite-hematite intergrowths, and rutile. Textural relationships suggest that wagnerite is a primary phase that formed at temperatures of ~900°C (granulite) and possibly ~1000°C (pegmatite) at ~1GPa. Apatite shows complex textures, in most cases it armors or rims the wagnerite grains. Three generations of apatite are evident (1) a primary chlorapatite to Cl-rich fluorapatite that appears to be coeval with wagnerite (“Christmas Point” only), (2) rims of chlor-fluorapatite that surround wagnerite in most samples and (3) late F-rich apatite (possibly carbonato-apatite) that has partially replaced the apatite rims. Electron microprobe analysis shows wagnerite Fe/(Fe+Mg) ratio ranging from 0.026 to 0.073, which is roughly correlated with Fe/(Fe+Mg) ratio in contiguous biotite grains. F and Fe contents in the biotite vary inversely; which is attributed to F-Fe avoidance. Biotite F/(F+Cl) ratios do not vary systematically with apatite F/(F+Cl) ratios, instead there is an increase of apatite F+Cl with increasing wagnerite F amounts. Since OH is assumed to be (1-F) in wagnerite and (1-F-Cl) in apatite, this suggests a possible regular distribution of OH between the two minerals. Overall, the distribution of the halogens and Mg-Fe among biotite, apatite and wagnerite is not particularly regular, and so there is little evidence for chemical equilibrium, possibly as result of multiple events. This disequilibrium is also reflected in the complex apatite textures and variable chemical compositions of biotite. The presence of significant Cl in pegmatitic apatite and biotite, and the near absence of Cl in these minerals within the granulite supports evidence obtained from bulk analyses of other Napier complex pegmatites for Cl enrichment in the pegmatites relative to their host rocks.