Geochemical study of granulites from Mt. Riiser-Larsen, Enderby Land, East Antarctica: Implication for protoliths of the Archaean Napier Complex

The Mt. Riiser-Larsen area of the Napier Complex, East Antarctica, is underlain by various kinds of ultrahigh-temperature (UHT) metamorphic rocks such as orthopyroxene felsic gneiss, garnet felsic gneiss, garnet-sillimanite gneiss, mafic gneiss and meta-ultramafic rocks. On the basis of the mode of occurrence and petrography, the garnet felsic gneisses are further divided into garnet felsic gneiss I (garnet-poor type) and II (garnet-rich type), the mafic gneisses into quartz-free and -bearing types, and the metaultramafic rocks into phlogopite-free and -bearing types. Bulk rock chemical analyses for the major, minor and rare earth elements (REEs) demonstrate that (1) both the garnetsillimanite gneiss and garnet felsic gneiss II are of sedimentary origin such as mudstone and sandstone, respectively, and (2) other rocks are of igneous origin, as follows: i) the orthopyroxene felsic gneiss and garnet felsic gneiss I are chemically comparable with CIPW normative tonalite to granodiorite and granite, respectively, of which the former also exhibits a calc-alkaline compositional variation with a REE signature comparable to the Archaean TTG (tonalite-trondhjemite-granodiorite); ii) the quartz-free and quartz-bearing mafic gneisses have been derived from flat or slightly LREE-depleted and LREE-enriched tholeiitic basalts, respectively; iii) phlogopite-free and phlogopite-bearing meta-ultramafic rocks have been derived from depleted mantle peridotites and komatiitic rocks, respectively, of which the latter shows a magmatic compositional variation controlled by addition or subtraction of only olivine. These chemical features reveal that the various kinds of original rocks are intermingled in the Riiser-Larsen area, which is important for understanding the site of development of original rocks in the Napier Complex