Geochemistry, origin, and tectonic setting of granitic rocks of the Prince Charles Mountains, Antarctica

Granitic rocks in different terranes and of different ages in the Prince Charles Mountains (PCM) show systematic compositional differences. Archaean granitic basement rocks of the southern PCM have compositions unlike those of typical Archaean tonalite- tronhjemite-granodiorite (TTG) terranes and consist mainly of within-plate types, which probably post-date crust formation and early metamorphic events. Unusually HFSE-rich (Zr, Nb, and Y) hornblende-biotite granite gneiss with A-type (anorogenic) affinities was probably derived by fractionation of mafic magma, but other granites represent intracrustal melts. Orthopyroxene-bearing tonalitic to granitic orthogneiss of the c. 1000 Ma high-grade terrane in the northern PCM and adjacent areas includes a large proportion of Y-depleted, Sr-undepleted volcanic arc granitoids, probably derived by melting of a plagioclase-poor mafic source (e.g. amphibolite or eclogite) in a Palaeo- or Mesoproterozoic Andean-type plate margin. Tonalite-granodiorite and mafic to felsic metavolcanic rocks at Fisher Massif also formed in an active continental margin, with an associated island arc, about 1300 million years ago. Most c. 1000 Ma granitoids also have volcanic arc characteristics, but there are significant syn-collision and within-plate types, indicating a polygenetic origin in a high-grade terrane formed at a convergent plate margin. Syn to late-metamorphic orthopyroxene granitoids (charnockites) include HFSE-rich quartz monzonitic varieties, which probably formed by fractionation of mantle-derived magma, and more siliceous granites, which represent high-temperature, predominantly intracrustal melts of dry granulite-facies orthogneiss. These granites are mainly Y-depleted, implying high-pressure melting with residual garnet in crust thickened by continental collision (between Archaean cratons in India and Antarctica) and heated by magmatic underplating. Major Cambrian plutons have A-type features, consistent with melting of dry granulite-facies rocks caused by mafic underplating. Emplacement near the present Lambert Glacier graben suggests an association with internal fracturing that preceded eventual break-up of Gondwana.