We examined Sm-Nd isotopes of samples from the Archean tonalitic-granodioritic orthogneiss mass of Mt. Riiser-Larsen in the Napier Complex, East Antarctica. Analytical data show εNd ≈ 0 of ∼ 3.27 and ∼ 3.07 Ga at the time of protolith formation, as determined by SHRIMP zircon analyses. This differs from previously reported Sm-Nd whole-rock isotope data from the oldest tonalitic orthogneisses of Mt. Sones in the Napier Complex, which show εNd = 0 at 3.87-3.80 Ga (T_CHUR), which is coincident with ∼ 3.8 Ga from SHRIMP zircon analyses. These data suggest that the voluminous and homogeneous tonalitic-granodioritic rocks retained the εNd ≈ 0 signal throughout the protolith-metamorphic process and that the source materials of the rocks showed εNd ≈ 0 at ∼ 3.80, 3.27, and 3.07 Ga. Tonalitic-granodioritic orthogneisses from the Napier Complex may contain genetic information regarding Nd isotopic evolution from the Eoarchean to Mesoarchean.

The Napier Complex in East Antarctica has a complex thermal history, including ultra-high-temperature (UHT) metamorphism. Geochronology, trace element, and isotope geochemistry of zircon, apatite, and monazite in three felsic gneisses collected from Harvey Nunatak were studied using a sensitive high-resolution ion microprobe (SHRIMP) for the first time. Most zircons showed nebulous to fir-tree zoning, which is a common feature of zircons in granulite facies rocks, regardless of the core or rim. The U-Pb dating and rare earth element abundance of zircon indicated that zircon crystallization by regional metamorphism continued from 2567-2460 Ma, consistent with the previously proposed timing of UHT metamorphism. The zircon grains contained a large amount of Li (59-668 ppm). Li was incorporated with Cl at the interstitial sites of the zircon structure, and the zircons crystallized in melts with an abundant supply of Li and Cl. Monazite crystallized from apatite after the UHT metamorphism events of 2071 and 1799 Ma. The U-Pb system of apatite was completely disturbed by the crystallization of monazite at 1785 Ma. In addition, the U-Pb systems of apatite and monazite were disturbed at approximately 500 Ma.

The Sør Rondane Mountains, East Antarctica have been thought to be situated in the collision zone between East and West Gondwana during the final stage of amalgamation of the Gondwana supercontinent. They are, therefore, recognized as a key region for understanding the geological phenomena during the collisions and for testing the proposed tectonic models. We identified metamorphic rocks with different pressure-temperature-time paths that are bounded by a ductile shear zone at Oyayubi ridge, Brattnipene, Sør Rondane Mountains. Based on field and microscopic observations, chemical analyses of minerals, and zircon U-Pb dating, the sillimanite-garnet-biotite gneisses (i.e., pelitic gneisses) from higher structural level show a peak metamorphism at ∼ 590 Ma that took place under conditions of ∼ 830-840 °C and 0.8-0.9 GPa, and these high-temperature conditions lasted until ∼ 550 Ma. These rocks underwent isothermal decompression and then retrograde hydration under lower pressure-temperature conditions than 530 °C and 0.4 GPa at ∼ 530 Ma. In contrast, the orthogneisses that consist of hornblende-biotite gneiss and garnet-clinopyroxene gneiss from lower structural levels did not undergo metamorphism at ∼ 600 Ma but underwent metamorphism at ∼ 570 Ma and reached peak conditions of 700-760 °C and 0.6-0.9 GPa at ∼ 560-550 Ma. These observations suggest thrusting of the pelitic gneiss over the orthogneiss at ∼ 570-550 Ma, causing a prograde metamorphism of the orthogneisses and a decompression of the pelitic gneisses as well as uplift and subsequent rapid denudation. The results indicate two stages of collision in the Sør Rondane Mountains and that the ductile shear zone bounding the pelitic gneiss and orthogneiss units may have been part of the continental plate collision boundary at ∼ 570-550 Ma.

The Sør Rondane Mountains (SRM) in East Antarctica, lie within the late Neoproterozoic-early Paleozoic collision zone between East and West Gondwana. Many studies have been carried out in the eastern SRM, whereas fundamental questions on the western SRM remain unanswered, for example, tectonic division and detailed metamorphic age. This paper describes zircon U-Pb ages from gneisses in the western SRM, and the tectonic division of this area is discussed.

The rocks of the study area are divided into units 1 to 3 based on their lithology and structural position. Units 1 and 2, which are divided by the Kanino-tume shear zone (KSZ), are mainly composed of greenschist/amphibolite-facies gneisses and amphibolite/granulite-facies gneisses, respectively. Unit 3 is mostly occupied by the Proterozoic GTTG. It is, in this study, revealed that the metamorphic ages of units 1 and 2 are, respectively, ∼ 600-550 and ∼ 670-600 Ma. The distribution of the KSZ has been inferred by mapping, but the present results on the distribution of metamorphic ages confirm this. The KSZ of the low-angle shear zone may be a critical tectonic boundary dividing the two gneiss units of ∼ 600-550 and ∼ 670-600 Ma within the East African-Antarctic orogen.

The Napier Complex in Enderby Land and western Kemp Land is a unique component of the East Antarctic Shield because it records a timeline of crustal growth from the Eo- to Neoarchean. It is mainly composed of enderbitic and charnockitic gneisses and granulites that were metamorphosed at ∼ 2.5 Ga, and locally at ∼ 2.8 Ga, under high- to ultra-high-temperature conditions. Despite generating scientific interest for several decades, the geological history of the complex has not been well constrained. In this study, samples from the Napier Mountains were selected for zircon imaging and U-Pb dating by Secondary Ion Mass Spectrometry. They record metamorphic growth, recrystallization, and modification of zircon at 2800-2770, 2740-2720, and 2490-2460 Ma. For the first time, fluid-related alteration at around 2730 Ma is evident in a granitic gneiss from Grimsley Peaks. At the similar time, dioritic gneiss was formed at Mount Marr. Tonalitic and granitic gneisses from Grimsley Peaks yield protolith crystallization ages of around 3210 and 2825 Ma, respectively. The generation of granitic gneiss was coeval with ∼ 2.8 Ga metamorphism in the area. These new data from this little-known part of the complex provide a better understanding of the crustal evolution of the Napier Complex.