218 Journal of Mineralogical and PetrologicalH. Ishizuka Sciences, Volume 103, page 218─ 225, 2008 REVIEW Protoliths of the Napier Complex in Enderby Land, East Antarctica; an overview and implication for crustal formation of Archaean continents Hideo ISHIZUKA Department of Geology, Kochi University, Kochi 780-8520, Japan Field and geochemical studies overviewed here reveal that the precursors of ultrahigh-temperature (UHT) met- amorphic rocks of the Napier Complex in Enderby Land, East Antarctica, consist of mantle protolith (serpenti- nite and depleted peridotite), magmatic protolith (tonalite-trondhjemite-granodiorite (TTG) suite, basaltic to komatiitic rocks and anorthosite), and sedimentary protolith (MgO-, Ni-, Cr- and Co-rich sediments, impure quartzite, banded iron formation (BIF) and calc-silicate rock). This assemblage of protoliths, especially the komatiite-TTG association with minor anorthosite, is reminiscent of the Archaean greenstone-granite belts. The U-Pb SHRIMP or SIMS dating of zircons with oscillatory zoning and magmatic Th/U ratio from the TTG pro- toliths shows four age clusters such as ~ 3.8, 3.3, 3.0 and 2.6 Gyr ago, suggesting the multi-stages of protolith formation. The modern analogy for the genesis of TTG suite suggests that the tectonic setting of the protolith formation and emplacement can be considered in the framework of the present-day intra-oceanic island-arc and related subduction regime. Subsequently, the Napier Complex was stabilized during the UHT metamorphism at ~ 2.59-2.55 or ~ 2.50-2.45 Gyr ago. Keywords: Greenstone-granite belt, Napier Complex, Subduction regime, TTG suite, UHT metamorphism INTRODUCTION Complex offers an excellent opportunity to study the for- mation and evolution of the Archaean continental crust. The Archaean continental crust is the first granitic materi- In the following, the protoliths of the Napier Com- al to appear on Earth. Neither other rocky planets nor me- plex will be overviewed, and its implication for crustal teorites have been known to possess any such granitic formation of Archaean continents will be discussed in the materials. Therefore, the formation of Archaean continen- framework of present-day tectonic regime. tal crust plays an important role in understanding the Earth’s own activities. LITHOLOGY AND METAMORPHISM The Napier Complex occurs in Enderby Land, East Antarctica (Fig. 1). After the reconnaissance investigation The Napier Complex comprises two lithologic units; lay- of the Soviet pioneers (Ravich and Kamenev, 1975), the ered gneiss unit and massive gneiss unit (Sheraton et al., extensive geological studies of the Napier Complex were 1987; Ishizuka et al., 1998; Osanai et al., 1999). The done by the Australian and Japanese expeditions. The re- boundary between the two units is transitional in the Mt. sults of these studies as summarized by Grew (1982), Riiser-Larsen area (Fig. 1), but in Tonagh Island (Fig. 1) Sheraton et al. (1987), Harley and Hensen (1990), Moto- it is closely associated with the shear zone. On the basis yoshi (1998) and Ishizuka (2008) have revealed that 1) the of the regional geologic map of Sheraton et al. (1987), it protoliths of the Napier Complex were formed at a very is estimated that the layered gneiss unit occupies about old geologic age, going back to the early Archaean, and 2) 40% and the massive one about 55%, and others about they were subsequently stabilized during the ultrahigh- 5% of the outcrops, although the vast area is covered by temperature (UHT) metamorphism (>1130 °C) at latest snow or ice. Archaean era. It is, therefore, most likely that the Napier The layered gneiss unit is characterized by layering doi:10.2465/jmps.080328 structure composed of pelitic to rarely psammitic gneisses H. Ishizuka, [email protected] Corresponding author (now represented by garnet-sillimanite and garnet-rich Protoliths of the Napier Complex in Enderby Land 219 Figure 1. (A) Far-distance view of layered gneiss unit in the Mt. Riiser-Larsen area (the width of photograph is approximately 2.0 km). (B) Lo- cality map of the Napier Complex, in which some localities are given along with protolith age determined by U-Pb SHRIMP or SIMS zircon dating (data sources, see text). felsic gneisses, respectively), garnet-poor felsic gneiss, near the contact with the neighboring gneiss to the coarse- orthopyroxene felsic gneiss, mafic gneiss, and impure grained in the center of the mafic gneiss layer. These ob- quartzite. Of these, the layers of the mafic gneiss are servations suggest the mafic gneiss to be intrusive in ori- sometimes oblique against the layers or foliation of the gin. Small but significant amount of banded iron for‑ neighboring pelitic gneiss, and the grain size of the mafic mation (BIF) that consists mainly of quartz and magnetite gneiss sometimes changes gradually from the fine-grained with minor inverted pigeonite, garnet and amphibole 220 H. Ishizuka Table 1. Compositions of the Napier ultrahigh-temperature (UHT) metamorphic rocks * - - - - Fe2O3 represents total iron as Fe2O3. SB 1, High Mg mafic gneiss (Sheraton and Black, 1981); SZ 1, Average of 3 phlogopite free ultramafic rocks (Suzuki et al., 1999); SZ-2, Average of 5 phlogopite-bearing ultramafic rocks (Suzuki et al., 1999); OW-1, Hornblende-bearing lher- zolitic peridotite (Owada et al., 2000; SH-1, Average of 10 mafic gneisses (Sheraton et al., 1987); SZ-3, Average of 5 quartz-free mafic gneisses (Suzuki et al., 1999); SZ-4, Average of 3 quartz-bearing mafic gneisses (Suzuki et al., 1999); OW-2, Average of 4 brown bornblende two py- roxene gneisses (Owada et al., 2000); OW-3, Average of 3 two pyroxene gneiises (Owada et al., 2000); OW-4, Average of 2 biotite-bearing two pyroxene gneiises (Owada et al., 2000); SH-2, Average of 21 depleted orthopyroxene felsic gneisses (Sheraton et al., 1987); SH-3, Average of 6 undepleted granitic gneisses (Sheraton et al., 1987); SZ-5, Average of 6 orthopyroxene felsic gneisses (Suzuki et al., 1999); SZ-6, Average of 7 garnet-poor felsic gneisses (Suzuki et al., 1999); SH-4, Sillimanite-sapphirine-osumilite-garnet granulite (Sheraton et al., 1987); SZ-7, Av- erage of 3 garnet-sillimanite gneisses (Suzuki et al., 1999); SH-5, Sillimanite-garnet-osumilite(-spinel) quartzite (Sheraton et al., 1987); SZ-8, Average of 4 garnet-rich felsic gneisses (Suzuki et al., 1999). Protoliths of the Napier Complex in Enderby Land 221 Table 1. (Continued) so developed as the layered gneiss unit, and its lithology is rather monotonous. As suggested by Sheraton et al. (1987), this occurrence of the orthopyroxene felsic gneiss is reminiscent of its derivation from igneous protoliths. Ultramafic rocks, ranging from pyroxenite to perido- tite, are very small in amount but widespread in distribu- tion, and occur as lenses, pods and layers. In the Mt. Riis- er-Larsen area, the blocks of ultramafic rocks are chara‑ cteristically present in the boundary between the layered and massive gneiss units, and also a layer of ultramafic rock occurs in the massive gneiss unit. Another type of ultramafic rock called high-Mg rock (Sheraton et al., 1987) is present as dikes in the layered and massive gneiss units. Anorthosite sometimes occurs as thin layer associated with leuconorite and leucogabbro at Bunt Is- land, Fyfe Hills, Mount Hardy and Wyers Ice Shelf, and as blocks at the Mt. Riiser-Larsen area. After the formation of protoliths, the Napier Com- plex underwent the metamorphism characterized by the presence of such diagnostic minerals and mineral assem- blages as osumilite, inverted pigeonite, sapphirine + quartz, spinel + quartz, and orthopyroxene + sillimanite + quartz (Sheraton et al., 1987; Harley and Hensen, 1990; Motoyoshi, 1998). Recent studies on alumina content of orthopyroxene (Harley and Motoyoshi, 2000), feldspar thermometry (Hokada, 2001; Hokada and Suzuki, 2006) and pyroxene thermometry (Ishizuka et al., 2002) indicat- ed the peak metamorphic temperature to be up to 1130 °C. These facts imply that the Napier Complex suffered the extremely high temperature crustal metamorphism as presently referred to ultrahigh-temperature (UHT) meta- morphism (Spear, 1993). On the basis of U-Pb SHRIMP or SIMS zircon dating, there are two interpretations for the peak age of the Napier UHT metamorphism such as ~ 2.59-2.55 Gyr ago (Harley et al., 2001; Crowe et al., 2002; Kelly and Harley, 2005) and ~ 2.50-2.45 Gyr ago (Grew, 1998; Carson et al., 2002; Hokada et al., 2004; Su- zuki et al., 2006). PROTOLITH AND AGE sometimes occurs as alternating with the pelitic gneiss. The dominant protoliths of the Napier UHT metamorphic BIF occurrence closely associated with pelitic gneiss and rocks have been extensively studied mainly by geochemi- total - its unique composition such as Fe2O3 = 43 49 wt% and cal approaches (Sheraton et al., 1980, 1987; Sheraton and - SiO2 = 42 54 wt% (Sandiford and Wilson, 1986) suggest Black, 1981; Suzuki et al., 1999; Owada et al., 1999, its origin to be sedimentary rocks. Calc-silicate rocks 2000) (Table 1), and the results are summarized below. with the mineral assemblage of diopside + plagioclase ± 1. The ultramafic rocks comprise pyroxenite, peri- scapolite ± wollastonite ± grossular ± quartz ± calcite are dotite and high-Mg rock (dikes). The high-Mg rock is restricted to Mt. Bergin, McLeod Nunataks, and the areas characterized by high MgO (~ 17.93 wt%), Cr (~ 2220 around Khmara Bay (Fig. 1). ppm) and Ni (~ 628 ppm) contents, but rather low TiO2, The massive gneiss unit comprises mainly orthopy- Na2O, P2O5, Zr, Nb and Y contents (Table 1), and in these roxene felsic gneiss, in which the layering structure is not respects they are interpreted as having some chemical af- 222 H. Ishizuka finities with basaltic komatiites (Sheraton and Black, to reflect the existence of komatiite-high Mg basalt sourc- 1981; Sheraton et al., 1987). In the Mt. Riiser-Larsen es. This suggestion is consistent with the occurrence of area, Suzuki et al.