Geology of an unusual Precambrian high-grade metamorphic terrane : Enderby Land and western Kemp Land, Antarctica

The high-grade metamorphic rocks of Enderby Land and western Kemp Land, which form part of the East Antarctic Precambrian Shield, have been divided into two major metamorphic complexes: the Archaean Napier Complex and the Proterozoic Rayner Complex. The Napier Complex consists predominantly of pyroxene-quartz-feldspar gneiss and garnet-quartz-feldspar gneiss with subordinate mafic granulite, pyroxenite, and a variety of siliceous, aluminous, and ferruginous metasedirnents. Peak metamorphic conditions, accompanied by intense deformation ( M r D , ) , and granitoid emplacement, took place about 3070 Ma ago, although there is isotopic evidence for felsic crust as old as 3930 Ma. Very high temperatures of 900-1000°C at 7-11 kb (intermediate-pressure granulite facies) were reached in much of the Napier Complex and resulted in the formation of the rare associations of sapphirine + quartz, orthopyroxene + sillimanite, and osumilite within aluminous metasedirnents. The presence of these associations, as well as the common occurrence of calcic mesoperthite, is also consistent with very low P H 2 O (less than 500 bars) during metamorphism. Subsequent to this event, the Complex underwent near-isobaric cooling, suggesting that it was in approximate isostatic equilibrium with the underlying mantle. This implies that metamorphism either did not occur in association with extensive crustal thickening, or such thickening and consequent erosion and uplift preceded the thermal peak (M,). Further deformation took place about 2900 Ma (D2) and 2450 Ma (D3 ) ago, under slightly lower-grade (upper amphibolite to granulite facies) conditions, after which the Complex was essentially cratonised. Several suites of mafic dykes are present in the Napier Complex.Metatholeiites and high-Mg tholeiites were emplaced at about 2350-2400 Ma during the waning stages of metamorphism at considerable depths in the crust. Additionally, an abundant suite of dolerites (the Amundsen Dykes) was emplaced about 1200 Ma ago. Rare alkali melasyenites (lamproites) constitute the youngest group (482 Ma).The Rayner Complex was metamorphosed about 960 Ma ago at the same time as the Proterozoic rocks of the MacRobertson Land coast and northern Prince Charles Mountains. Limited reworking (and rehydration) of the margins of the Archaean craton is indicated in areas where mafic dykes occur as metamorphosed relics, but the relative abundance of metapelites and calc-silicate gneisses probably reflects Proterozoic sedimentation. The metamorphic grade of the Rayner Complex is somewhat lower (upper amphibolite to granulite facies) than that of the Napier Complex, although high-pressure (garnet +clinopyroxene) granulites are present locally near the contact with the Napier Complex. Water pressures were also higher, as evidenced by the relative abundance of migmatitic gneisses and hydrous minerals (biotite and hornblende) and the absence of mesoperthite. In contrast to the Napier Complex, metamorphism was followed by near-isothermal uplift consistent with major crustal thickening. The effects of the late Proterozoic metamorphism within the Napier Complex are generally confined to localised shear and mylonite zones (D4 , D5 ) and areas of retrogression. Granitic and pegmatitic dykes were emplaced into both the Rayner Complex and adjacent parts of the Napier Complex about 770 and possibly 500-550 Ma ago.

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