Protoliths of the Napier Complex in Enderby Land, East Antarctica ; an overview and implication for crustal formation of Archaean continents

概要

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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.
日本鉱物科学会の論文
2008-08-01