Protoliths of the Napier Complex in Enderby Land, East Antarctica; an Overview and Implication for Crustal Formation of Archaean Continents

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) metamorphic rocks of the Napier Complex in Enderby Land, East Antarctica, consist of mantle protolith (serpentinite 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 protoliths 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 formation 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 lherzolitic 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 pyroxene 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 peridotite, 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) indicated 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) metamorphism (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. (1999) divided the ultramafic rocks ex- komatiitic basalts and komatiites in the Napier Complex cept for pyroxenite into phlogopite-bearing and -free va- as described above. rieties. The former exhibits a magmatic differentiation The U-Pb SHRIMP or SIMS dating of zircons sepa- controlled by olivine fractionation, and also has the chem- rated from TTG protoliths (described as orthogneiss or ical affinity with komatiitic rocks such as the Munro-type felsic gneiss or rarely charnockite) shows four clusters of komatiite. The field occurrence of this type of ultramafic formation ages (Fig. 1); ~ 3.8 Gyr ago from Fyfe Hills, rocks such as the layer intercalated with the orthopyrox- Mt. Sones and Gage Ridge (Black et al., 1986a; Harley ene felsic gneiss is consistent with magmatic lithology. As and Black, 1997; Kelly and Harley, 2005), 3.28-3.23 Gyr compared with the former, the latter is enriched in MgO, ago from Tonagh Island and Mt. Riiser-Larsen (Shiraishi

Ni and Cr, and depleted in Al2O3, CaO and V. Suzuki et et al., 1997; Hokada et al., 2003), ~ 3.0 Gyr ago from Mt. al. (1999) interpreted the latter, phlogopite-free type, as a Riiser-Larsen, Proclamation Island and Dallwitz Nunatak depleted or residual mantle peridotite formed by partial (Harley and Black, 1997; Hokada et al., 2003; Kelly and melting and extraction of 40-60% komatiitic magma. Harley, 2005), and 2.68-2.63 Gyr ago from Tonagh Island From Tonagh Island, Owada et al. (1999, 2000) also (Carson et al., 2002; Crowe et al., 2002). These U-Pb data showed that the ultramafic rocks (pyroxenite, websteritic are usually obtained from zircons with oscillatory zoning peridotite and hornblende-bearing lherzolitic peridotite) and/or high Th/U ratio, which have been interpreted as are enriched in MgO (~ 31 wt%) and LREE, and compa- the ages of magmatic episodes (Carson et al., 2002; Hoka rable with komatiitic basalts to komatiites. da et al., 2004; Kelly and Harley, 2005). As to the very 2. The mafic gneisses are mostly derived from old age (~ 3.8 Gyr ago), Owada et al. (1994) and Asami et non-alkaline and tholeiitic basalts, and have the geochem- al. (1998) reported the Sm-Nd bulk rock age of 3.71 Gyr ical affinities with MORBs (Sheraton et al., 1980, 1987) ago for mafic gneisses from Tonagh Island and CHIME or within-plate basalts (Owada et al., 1999, 2000). Fur- zircon age of 3.65 Gyr ago for one felsic gneiss from Mt. thermore, Suzuki et al. (1999) have classified the mafic Cronus, respectively. Further, in relation to 3.28-3.23 and gneisses into the quartz-free, light rare earth element ~ 3.0 Gyr ago, radiometric data determined by other (LREE)-depleted and N-MORB type, and the quartz- methods such as conventional zircon data, Rb-Sr bulk bearing, LREE-enriched and T- or E-MORB variety in rock data and Sm-Nd bulk rock data showed the ages of the Mt. Riiser-Larsen area, and they suggested that these protoliths to be 3.2-3.0 Gyr ago from Proclamation Island two types of the mafic gneisses were derived from a dif- (Black et al., 1986b; Sheraton and Black, 1983) and Fyfe ferent source material from each other. Hills (Black et al., 1983; Black et al., 1984). The 3.3 Gyr 3. The orthopyroxene felsic gneiss is chemically event as reported from Tonagh Island and Mt. Riiser- comparative with CIPW normative tonalite to granodio- Larsen has been also reported as protolith ages from other rite, and depleted in Y and heavy rare earth element Archaean terrains such as eastern Australia (Kemp et al., (HREE) (Sheraton et al., 1987; Suzuki et al., 1999). These 2006) and South Africa (Shirey et al., 2003). The young- are quite similar to geochemical features of the Archaean est episode (2.6 Gyr ago) that has been reported only from TTG (tonalite-trondhjemite-granodiorite) suite (Luais and Tonagh Island is consistent with the peak age of forma- Hawkesworth, 1994). The garnet-poor felsic gneiss be- tion of juvenile (mantle-derived) crust in Earth’s history longs to CIPW normative trondhjemite and granite, but (Condie, 1997). does not show the Y and HREE depletion. On the other hand, the paragneiss from Zircon Point 4. The pelitic and psammitic gneisses, which are contains zircons that exhibit oscillatory zoning and steep

- geochemically defined by high Al2O3 and low C compo- “magmatic” REE pattern, and yield 2.8 Gyr ago of SIMS nent in the ACF diagram, are characteristically enriched inherited and core age (Kelly and Harley, 2005). This age in MgO, Cr, Ni and Co (Table 1). The enrichments of has been, however, not reported as a protolith age from

Al2O3 and MgO are in favor of the occurrence of sapphi- any other areas in the Napier Complex. rine, osumilite, sillimanite, garnet and cordierite in the pelitic and psammitic gneisses under the UHT metamor- DISCUSSION AND CONCLUSION phic conditions. Such enrichments of MgO, Cr and Ni have been reported from the sedimentary rocks in other The protoliths of the Napier UHT metamorphic rocks Archaean terrains (Condie, 1997) such as the Kaapvaal overviewed above can be divided into three types, that is, craton in southern Africa (Condie and Wronkiewicz, mantle protolith (serpentinite and peridotite), magmatic 1990) where enrichment in these components is suggested protolith (TTG suite with minor granitic rocks, basalts, Protoliths of the Napier Complex in Enderby Land 223 basaltic komatiite, komatiite and anorthosite), and sedi- Also, the U-Pb SHRIMP or SIMS zircon dating of mentary protolith (MgO-, Ni-, Cr- and Co-rich sedi- the Napier TTG protoliths that show four age clusters ments, impure quartzite, BIF, and calc-silicate rock). (~ 3.8, 3.3, 3.0, and 2.6 Gyr ago) seems to need further Of particular interest among these protoliths is the consideration. These ages are generally interpreted as the presence of the komatiite-TTG association with small ages of magmatic episodes, by which the TTG protoliths amount of anorthosite, which has been commonly de- were produced. It is, however, noted that the U-Pb zircon scribed in other Archaean greenstone-granite belts (e.g., ages do not always indicate the formation ages of juvenile Condie, 1994). This implies that the formation and em- crust, and there is a possibility that the TTG protoliths placement of the Napier protoliths should be considered were derived from pre-existing materials such as sedi- in the framework of the origin and evolution of the Ar- ments by re-melting processes. The discrimination be- chaean greenstone-granite belts. In this respect, the recent tween the juvenile crust and re-melting crust that is not study of seismic experiments demonstrates that TTG vari- examined by means of only the U-Pb isotope method is eties (tonalitic rocks) could exist in the middle to lower essential to evaluate the crustal growth during Earth’s his- crust of the Mariana intra-oceanic island arc (Takahashi tory (Condie, 1997). In this respect, recent progress of ox- et al., 2007). This is consistent with the occurrence of ob- ygen and Lu-Hf zircon isotope systematics in combined ducted tonalitic rocks at the northern tip (the Tanzawa with U-Pb zircon isotope geochemistry (Cavosie et al., Mountain) of the Izu arc where the arc collides with the 2005; Kemp et al., 2006; Hawkesworth and Kemp 2006) Japan arc (Kawate and Arima, 1998). This is also consis- shows that 1) the high δ18O value (such as more than 18 tent with the petrological aspect, that is, the modern TTG 6.5‰) reflects a component of O-enriched and supra- suite could derived from the anatexis of the differentiated crustal materials such as recycled sedimentary rocks (re- basaltic lower crust (Tatsumi, 2000) or it has been pro- melting crust), while the low δ18O value (such as less than duced as the products of magmatic processes such as 6.5‰) represents a component of the mantle-derived melting of basaltic source rocks (low-Mg amphibolites) magma (juvenile crust), 2) the model age of Lu-Hf iso- that occur in subduction zone (Foley et al., 2002). These tope is useful to examine the formation age of juvenile facts are indicative of the “modern intra-oceanic island crust, while the U-Pb isotope records multi-stages of arc-like” tectonic setting that may have played an impor- thermal events including magmatic episodes during re- tant role in the formation and emplacement of the Napier melting process, by which zircon with oscillatory zoning TTG protolith. Some of the associated mafic gneiss or can be crystallized, and 3) the in-situ analyses of these mafic granulite with the Napier TTG protolith has an af- isotopes can be available by using laser-ablation multi- finity with low-Ti MORBs that can be presently defined collector inductively coupled plasma-mass spectrometer to be island-arc basalts (Table 1). This is also suggestive (LA-MC-ICP-MS). It follows that the protolith ages of of a tectonic setting of subduction-related regime. the Napier Complex must be re-examined by using the It has been, however, indicated that the Archaean in-situ analysis of U-Pb, oxygen, and Lu-Hf zircon iso- TTG suite are generally depleted in HREE, that is, gener- topes, which may provide important information in simu- ally high in chondrite normalized (La/Yb)N ratio and low lating the crustal formation of the Napier continent. in YbN content, as compared with the modern TTG vari- In conclusion, the present study reveals that the pro- ety (Martin, 1994; Luais and Hawkesworth, 1994). These tolith association of the Napier Complex is very similar to geochemical features require a source material containing those of the Archaean greenstone-granite belts, and it is the residual amphibole and/or garnet for the Archaean suggested that the tectonic setting of its formation and TTG suite, because of strong partitioning of HREE into emplacement can be considered in the framework of the these minerals (e.g., Martin, 1994). This difference is con- present-day intra-oceanic island-arc and related subduc- sidered to result in a difference of geothermal gradient at tion regime. subduction zone from the high gradient at Archaean to the low variety at present, that is, at similar depths the high ACKNOWLEDGMENTS gradient favors the stability of garnet and amphibole (e.g., Condie, 1997). This consideration is largely based on the I would gratefully acknowledge all members of the Japa- assumption that the Archaean subducted plate was rela- nese Antarctic Research Expedition (JARE) geology tively young (<30 Myr) and warm (Condie, 1997). There group, especially the Nishi-Higashi group, with whom I is, however, no field evidence for this model, and it is still enjoyed frank discussions during the course of the study. I in dispute. Thus, the field occurrence of the TTG protolith sincerely thank M. Owada and M. Yuhara for careful and and its field relation with surrounding rocks need to be in- constructive reviews of this manuscript. vestigated in more detail in the Napier Complex. 224 H. Ishizuka

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