Tatsuro Adachi

Contrasting metamorphic records and their implications for tectonic process in the central Sør Rondane Mountains, eastern Dronning Maud Land, East Antarctica

Abstract Metamorphic rocks in the central part of Sør Rondane Mountains, eastern Dronning Maud Land, East Antarctica, are classified into three types based on petrological characteristics. (i) The Austkampane area preserves c. 800 °C and 0.5–0.6 GPa peak metamorphic conditions followed by decompression and subsequent isobaric cooling and later hydration (A-type). (ii) The Brattnipene and eastern Menipa area preserve peak P–T conditions of c. 800 °C and 0.7–0.8 GPa with subsequent isobaric cooling and later hydration (B-type). (iii) The area including Lunckeryggen, southern Walnumfjella and western Menipa preserves an amphibolite–facies peak metamorphic condition with signatures of prograde metamorphism (L-type), which are typically unaffected by the retrograde hydration event. Peak granulite–facies metamorphism of A- and B-type rocks are contemporaneous at c. 640–600 Ma, but a difference in the P–T paths between these rocks can be explained by thrusting of the A-type rock unit onto the B-type rock unit. By contrast, the timing of the metamorphism of the L-type rocks is significantly younger at c. 550 Ma, possibly related to the intrusion of pegmatites and granitoids. These metamorphic records in the central part of the Sør Rondane Mountains can be a test ground for the regional tectonic processes proposed for the orogeny related to Gondwana formation. Supplementary material: Representative mineral compositions are listed at www.geolsoc.org.uk/SUP18623

Paleozoic Subduction-Accretion-Closure Histories in the West Mongolian Segment of the Paleo-Asian Ocean: Evidence from Pressure-Temperature-Time-Protolith Evolution of High-Mg and -Al Gneisses in the Altai Mountains

High-Mg, high-Al metasedimentary gneisses from the Altai Mountains, Mongolia, belonging to a subduction-accretion complex within the Central Asian Orogenic Belt can be divided into five rock types on the basis of mineral assemblages. Most rock types have high MgO and Al2O3 content and low CaO, Na2O, Rb, and Sr content. All rock types experienced a similar medium-pressure metamorphism characterized by a “hairpin”-shaped counterclockwise pressure-temperature path. U-Pb zircon and U-Th-Pb monazite ages indicated metamorphism at ca. 356 Ma and 277 Ma and inherited ages of 510–379 Ma, suggesting possible provenance to granitoids comparable to those in the Altai Mountains, China. The zircons that newly nucleated at ca. 356 Ma are characterized by high concentrations of light rare earth elements without a Ce anomaly—features common in zircons from hydrothermally altered rocks and a reducing environment. Petrological and geochronological results in this study suggest the following tectonic evolution: (1) continuous subduction and accretion of paleo-Asian oceanic crust during the Early Paleozoic, resulting in periodic granitoid magmatism in the period 510–380 Ma and a continuous supply of granite-derived sediments providing detrital zircon and monazite grains to the accretionary prism; (2) ridge subduction during the Late Devonian–Early Carboniferous (ca. 356 Ma), resulting in hydrothermal metamorphism of the accretionary prism and interaction with seawater that produced rocks with unusual whole-rock chemistry; and (3) closure of the ocean leading to continental collision in the Early Permian (ca. 277 Ma), with part of the accretionary prism squeezed into lower crustal levels to form medium-pressure metamorphic rocks.

Multiple Collisions in the East African–Antarctica Orogen: Constraints from Timing of Metamorphism in the Filchnerfjella and Hochlinfjellet Terranes in Central Dronning Maud Land

Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon age dating was applied to several types of cordierite-bearing gneisses and orthopyroxene-bearing gneiss from the basement rocks in central Dronning Maud Land, East Antarctica, to clarify the timing of tectonothermal events during the amalgamation of East and West Gondwana. The zircon ages of ca. 522–525 and 598–599 Ma were obtained in Filchnerfjella and Hochlinfjellet, respectively, and the cathodoluminescence images of analyzed zircons suggest that these were formed during high-temperature metamorphism. These two sets of ages are interpreted to represent periods that immediately followed the peak metamorphism. The dating results reveal an age gap of 80 Myr between the two areas, indicating different collisional events. In the cordierite-bearing samples in Filchnerfjella, zircon cores with oscillatory zoning have concordia ages that range from 1800 to 650 Ma. These are considered to represent the ages of detrital zircons derived over a long time from different source materials. In Hochlinfjellet, the inherited detrital zircons with oscillatory zoning have concordia ages of ca. 930, 780, and 730 Ma, indicating sedimentary protoliths derived from differently aged igneous source rocks, possibly formed in a subduction zone as an accretionary prism before the collision. According to previous age results, previously documented shear zones are now interpreted as representing a major tectonic boundary. The obtained age results imply that different allochthonous metamorphic terranes were assembled along the collisional zone of the East African–Antarctica Orogen during the period from 650 to 500 Ma.

Possible armalcolite pseudomorph-bearing garnet–sillimanite gneiss from Skallevikshalsen, Lützow-Holm Complex, East Antarctica: Implications for ultrahigh-temperature metamorphism

Abstract A possible armalcolite pseudomorph has been identified in garnet–sillimanite gneiss from Skallevikshalsen, located c. 30 km NE of Rundvågshetta, in a terrane with the highest metamorphic grade in the Lützow-Holm Complex, East Antarctica. It occurs as an Fe–Mg–Ti compositional domain consisting of ilmenite, rutile and pseudorutile, partially mantled by rutile within ilmenite. The domain yields an average XMg of 0.171±0.036 exceeding by 3 wt% TiO2 from armalcolite stoichiometry, while the analysis closest to armalcolite stoichiometry has an XMg value close to 0.202. Host ilmenite with 0.4 mol% hematite is in contact with prismatic sillimanite, quartz, plagioclase and K-feldspar. In run products of annealing experiments performed to investigate the origin of the pseudomorph, armalcolite–ilmenite reaction coronae were developed around relict rutile in rock fragments of quartz eclogite from the Higashi-Akaishi mass of the Sanbagawa belt, central Shikoku, Japan. The experiments were carried out at 1 atm and 960–1050 °C with wüstite–magnetite buffer and imply a minimum temperature of 1290 °C for armalcolite stability when extrapolated to Skallevikshalsen pressures of 1.0 GPa. Mineral chemistry thermobarometry for Skallevikshalsen yields a metamorphic path with P–T peak conditions of 0.88–1.1 GPa and 970–1050 °C, followed by retrograde metamorphism at 0.6 GPa and 780 °C, and finally metasomatic alteration at c. 630 °C. This P–T path matches that for similar ultrahigh-temperature metamorphic rocks from Rundvågshetta and Sri Lanka, and is markedly lower in temperature than the unreasonable estimates based on armalcolite stability. This discrepancy is inferred to reflect chemical impurities in armalcolite that lower its minimum temperature stability by more than 200 °C.

Ediacaran mineralized microfossils from the basinal facies of the Doushantuo Formation in northwestern Hunan Province, South China

Abstract. The Ediacaran sediments in the Yangtze Block preserve valuable records of biological evolution and drastic climate changes. However, most of the Ediacaran fossil occurrences have been reported from the shallow platformal facies, and few fossils have been described from the deep basinal facies in the Yangtze Block. Here, we report mineralized microfossils from the basinal facies of the Ediacaran Doushantuo Formation at Fengtan in northwestern Hunan Province, China. The submillimeter-sized microfossils with oval-discoidal shapes have apatite walls of variable thickness. The lack of morphological regularity, ornamentation, and distinct apertures indicates that they are not protozoan shells or tests. The apatite walls have more likely replaced the recalcitrant organic walls of prasinophyte phycomata and/or algal resting cysts. These fell from shallow water and were quickly preserved in a deeper setting, associated with a local decline in pH caused by bacterial decomposition and elevated concentration of phosphate in seawater. This discovery provides new insight into the stratigraphic correlation between the shallow and basinal facies of the Chinese Ediacaran.

An overview of metamorphic geology from central Indonesia : Importance of South Sulawesi, Central Java and South-West Kalimantan metamorphic terranes

Various metamorphic rocks expose in the central part of Indonesia including Java, Kalimantan and Sulawesi islands. This paper explains results of the compilation of all relevant data regarding to the study of petrological, geochemical and geochronological investigations of metamorphic rocks from Bantimala and Barru Complexes in South Sulawesi, Luk Ulo Complex in Central Java, Meratus Complex in South Kalimantan, and Nangapinoh area of Schwaner Mountains in West Kalimantan. High-pressure and very high-pressure metamorphic rocks crop out in the Bantimala Complex and experienced peak metamorphic condition to eclogite-facies. High-pressure metamorphic rocks also expose in the Luk Ulo Complex in Central Java with peak metamorphic condition in the eclogite-facies. From the South Kalimantan, the P-T condition of metamorphic rocks were estimated from Mg-rich chloritoid that recrystallized at pressure of ~1 .8 GPa. The K-Ar dating of metamorphic rocks from South Sulawesi, Central Java and South Kalimantan were on the same range of Early Cretaceous that have a possibility of separated parts derived from a single subduction complex. Metamorphic rocks exposed in Schwaner Mountains from West Kalimantan are metatonalite and local contact metamorphism with the variety of hornfelsic rocks. No reliable P-T condition has been estimated from these metamorphic rocks. The LA-ICP-MS U-Pb zircon dating of metatonalite from Schwaner Mountains yields magmatic ages of Late Triassic that are older than the K-Ar granitoids ages from Schwaner Mountains (Late Jurassic to Cretaceous) but still in range of K-Ar granitoids ages from northwest Kalimantan (Carboniferous to Triassic). This evidence might indicate that magmatism of granitoids of the several parts in Schwaner Mountains were contemporaneous with northwest Kalimantan domain. On the basis of whole rock chemistry, the protolith of basic metamorphic rocks from South Sulawesi and Central Java are basaltic to andesitic igneous rocks derived from tectonic environment generating MORB or within-plate signature. From the overview, it is confirmed that South Sulawesi, South Kalimantan and Central Java have possibility to be formed a part of a single subduction complex. From West Kalimantan, whole-rock chemistry analyses suggested that the metatonalites and granitoids are derived from a volcanic arc tectonic environment.