扬子地台前寒武纪－寒武纪界线地层的微体植物化石群

前寒武纪一寒武纪界线地层在扬子地台发育良好、分布广泛,并有包括微体植物群、海藻、瓶形原生物、具硬体动物及‘澄江动物群’等化石的发现。因而,扬子地台成为研究前寒武纪－寒武纪界线生物地层的代表地区之一。基于对扬子地台前寒武纪－寒武纪界线地层已知微体植物化石记录的回顾和论定,总结概括了下寒武统微体植物化石组合：寒武系最底部硅质磷块岩中的Ｍｉｃｒｈｙｓｔｒｉｄｉｕｍ－Ｐａｒａｃｙｍａｔｉｏｓｐｈａｅｒａ－Ｍｅｇａｔｈｒｉｘ组合和较高层位页岩的Ｓｋｉａｇｉａ－Ａｎｎｕｌｕｍ－Ａｒｃｈａｅｏｄｉｓｃｉｎａ组合。它们分别与Ａｎａｂａｒｉｔｅｓ－Ｃｉｒｃｏｔｈｅｃａ－Ｐｒｏｔｏｈｅｒｔｚｉｎａ小壳动物化石带和Ｐａｒａｂａｄｉｅｌｌａ及Ｅｏｒｅｄｌｉｃｈｉａ三叶虫带相对应。与东欧地台疑源类生物地层相对照,扬子地台Ｍｉｃｒｈｙｓｔｒｉｄｉｕｍ－Ｐａｒａｃｙｍａｔｉｏｓｐｈａｅｒａ－Ｍｅｇａｔｈｒｉｘ组合的纵向分布大体和东欧地台Ａｓｔｅｒｉｄｉｕｍｔｏｒｎａｔｕｍ－Ｃｏｍａｓｐｈａｅｒｉｄｉｕｍｖｅｌｖｅｔｕｍ组合带相一致,而以上两地台的化石组合产出层位可能都略高于纽芬兰东部剖面,依据Ｐｈｙｃｏｄｅｓｐｅｄｕｍ所厘定的寒武系底界?     

Phosphorite generative processes around the Precambrian-Cambrian boundary in South China: An integrated study of Mo and phosphate O isotopic compositions

Large phosphorite deposits in Central Guizhou,China,were formed around the Precambrian/Cambrian boundary(PC/C),including the Ediacaran(Doushantuo stage) and early Cambrian(Gezhongwu stage).Among them,Gezhongwu phosphorite from Zhijin are enriched in rare earth elements(REE) plus yttrium(REY),reaching 3.503 million tons.Although phosphorites have attracted great attention,the specific sources P and REY remained unclear.To determine the P and REY sources and establish a phosphogenic model of PC/C phosphorite,we present an integrated dataset of Mo and phosphate O isotopes for the first time,along with carbonate C and O isotopes,geology,petrology,and geochemistry.In all samples,δ~(18) Op,Y/Ho,and Zr/Hf decreased from the Ediacaran to the early Cambrian,indicating increased terrigenous weathering fluxes while decreased upwelling water input.Furthermore,terrigenous weathering delivery significantly elevated marine REY concentrations in the Cambrian in Zhijin.The Ceanom and δ~(98)/~(95) Mo suggest that seawater was oxidized in the later Ediacaran and became entirely oxic in the early Cambrian.The positive feedback between oxygen levels in atmosphere and primary productivity caused progressive oxygenation in ocean-atmosphere system and enable phosphorites to be formed by different mechanisms.Results show that the Lower Doushantuo consist of abiotic intraclasts and exhibited "seawaterlike" REY types,indicating abiological and mechanical reworking phosphogenesis.The Upper Doushantuo and Gezhongwu Formation contained mainly microbial debris and abiogenic intraclasts,and exhibit "hat-shaped" REY plots,suggesting microbially mediated phosphogenesis.Based on this data set,we developed a phosphogenic model illustrating formation of these two phosphorite deposits,wherein the Lower Doushantuo phosphorite formed through the reworking of pre-existing phosphatic sediments in anoxic and abiotic ocean,whereas the Upper Doushantuo and Gezhongwu phosphorite formed via microbial metabolisms in oxic and biotic conditions.Our study has implications on the PC/C phosphorite generative processes,as well as paleoenvironmental conditions.     

Stable isotopic fingerprint of a hyporheic–hypolentic boundary in a reservoir

Stable isotopes of H₂O are used to define the hyporheic–hypolentic boundary in Ledbetter Creek as it discharges to Kentucky Lake, a constructed reservoir in western Kentucky, USA. High-resolution (centimeter-scale) sample collection and analysis were utilized to determine one-dimensional variations in δ²H and δ¹⁸O of H₂O and chloride (Cl⁻) across the boundary. During reservoir low stand in winter, the hyporheic–hypolentic zone contains water from Ledbetter Creek and groundwater separated by an interface at ~10 cm below the channel bottom. Following reservoir-stage increase in spring and summer, water from Kentucky Lake infiltrates into the hyporheic–hypolentic zone to a depth of at least 18 cm below the channel bottom. Reservoir-stage decline in autumn causes source-water mixing, largely obscuring the hyporheic–hypolentic boundary. Stable isotopes provide an effective complement to conventional tracers for delineation of water masses within the hyporheic–hypolentic zone.

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Resumen Se han utilizado isótopos estables del agua para definir el límite hiporreico-hipoléntico en Ledbetter Creek, que constituye una zona de descarga del lago Kentucky, una presa construida al Oeste de Kentucky, USA. Se ha llevado a cabo una recogida de muestras de alta resolución (a escala centimétrica) y se utilizaron los resultados para determinar las variaciones unidimensionales en δ²H y δ¹⁸O del H₂O y los cloruros (Cl⁻) alrededor del límite. Durante la época de niveles bajos en invierno, la zona hiporreica-hipolentica tiene agua procedente de Ledbetter Creek y de agua subterránea separada por una interfase de ~10 cm debajo del límite del canal. Siguiendo el incremento de los niveles en la presa en primavera y verano, el agua del Lago Kentucky se infiltra en la zona hiporreica-hipoléntica hasta una profundidad de, al menos, 18 cm bajo el límite hiporreico-hipoléntico. Los isótopos estables aportan un complemento efectivo a los trazadores convencionales para la delimitación de masas de agua dentro de la zona hiporreica-hipoléntica.

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Résumé L’utilisation des isotopes stables de la molécule d’eau a permis de définir l’interface hyporhéique-hypolentique dans la Ledbetter Creek, au point de déversement dans le réservoir artificiel que constitue le Kentucky Lake (Ouest du Kentucky, Etats-Unis). Une campagne de prélèvements à haute résolution (échelle centimétrique) a contribué à déterminer les variations unidimensionnelles des valeurs de δ2H et de δ18O de la molécule d’eau et de la concentration en chlorures (Cl⁻) de part et d’autre de l’interface. En hiver, lorsque le niveau du réservoir est minimal, la zone hyporhéique-hypolentique contient de l’eau de la Ledbetter Creek et de l’eau souterraine, séparées par un interface situé environ 10 cm sous le fond du chenal. Suite aux recharges printanières et estivales, l’eau du Kentucky Lake envahit la zone hyporhéique-hypolentique sur plus de 18 cm sous le fond du chenal. En automne, la baisse de niveau dans le réservoir occasionne un mélange des eaux, rendant diffus l’interface hyporhéique-hypolentique. Les isotopes stables constituent ainsi un complément efficace aux traceurs conventionnels pour la délimitation des masses d’eau dans la zone hyporhéique-hypolentique.

     

Biostratigraphy of the Cretaceous-Paleogene boundary deposits in the Arctic region of West Siberia (Implications of Foraminifers)

Benthic foraminifers from borehole sections recovered by drilling in the Yamal Peninsula, West Siberia, characterize the Ceratobulimina cretacea Beds (the upper Campanian-lower Maastrichtian) and the Spiroplectammina variabilis-Gaudryina rugosa spinulosa and Spiroplectammina kasanzevi-Bulimina rosenkrantzi regional zones of the lower and upper Maastrichtian, respectively. The Danian Stage is missing from the sections, which include marine deposits of the Selandian Stage attributed to the Ceratolamarckina tuberculata Beds. Foraminiferal assemblages of the beds include the Siberian endemic species associated with Paleocene foraminifers of the Midway-type fauna of subglobal distribution range. Occurrence of the latter suggests that warm-water surface currents from the North Atlantic reached southern areas of the Kara Sea.     

Stable isotopic information for hydrological investigation in Hailuogou watershed on the eastern slope of Mount Gongga, China

Hydrological systems have been seriously affected by changes of glaciations and snow covers in Mount Gongga, Sichuan, China, but the relevance of ice-snow melt for alpine river basin hydrology is so far not well known. To better understand hydrological features of the Hailuogou River, changes of δ¹⁸O and δ²H were investigated by analyzing 117 water samples collected from May 2008 to November 2009. Our results show that the stream water contains a relatively intermediate magnitude of isotopic variations, with δ¹⁸O ranging from ?18.09 to ?13.08 ‰ and δ²H from ?126.5 to ?88.8 ‰. The average values are both higher than those of ice-snow meltwater, but lower than those of meteoric water. These data also show a gradually increasing trend from upstream to downstream, and these changes might document the fingerprint of ice-snow melt in the headwater region and indicate the increasing recharge of heavy isotope-enriched waters with flow distance. The similarity in slopes of δ²H and δ¹⁸O relationship for meteoric waters and stream waters suggests that the isotopic signature of precipitation is well preserved in stream flow, and during the rainfall and stream flow the evaporation is only minor. Based on δ¹⁸O model, the results suggested that the fraction of ice-snow meltwater input over the total stream flow ranged from 84.50 to 86.52 % in the headwater region, but the fraction of ice-snow meltwater input from upper basin downward was significantly decreased. The study demonstrates that ice-snow meltwater is a substantially important water source in alpine regions on the edge of Tibetan Plateau.     

Stable isotopic investigations of early development in extant and fossil chambered cephalopods I. Oxygen isotopic composition of eggwater and carbon isotopic composition of siphuncle organic matter in Nautilus

Eggwaters from the chambered cephalopod Nautilus are depleted in both ¹⁸O and deuterium relative to ambient seawater. Eggwaters from six other species, including the related chambered cephalopod Sepia, do not show such depletion. These observations indicate that the previously observed step towards more positive $\text{δ}{}^{18}\text{O}$ values in calcium carbonate laid down after Nautilus hatches, relative to carbonate precipitated prior to hatching, can be explained by equilibration of the carbonate with water in the egg before hatching and with seawater after hatching. The presence of an oxygen isotope difference between eggwater and seawater for Nautilus and its absence for Sepia suggest that hatching will be recorded in the $\text{δ}{}^{18}\text{O}$ values of shell carbonates for some but not all extinct and extant chambered cephalopods.The $\text{δ}{}^{13}\text{C}$ values of the organic fraction of the siphuncle in Nautilus do not show any consistent pattern with regard to the time of formation before or after hatching. This observation suggests that the minimum in $\text{δ}{}^{13}\text{C}$ values previously observed for calcium carbonate precipitated after Nautilus hatches is not caused by a change in food sources once the animal becomes free-swimming, as has been suggested.     

To the problem of refining the western and eastern boundaries of the Achimov clinoform complex (West Siberia)

A brief summary of different viewpoints on the sedimentation model for the Achimov rock unit in West Siberia is presented. The differences in the concepts of the rock unit structure have led to ambiguous approaches to mapping of the boundaries of its area. Our study of the clinoform structural model for Neocomian section yielded geological and geophysical data substantiating the positions of the western and eastern boundaries of the Achimov rock unit.     

The eastern boundary of the Baikal collisional belt: Geological,geochronological, and Nd isotopic evidence

New geological. geochronological, and Nd isotopic data are reported for the rocks occurring at the interfluve of the Barguzin, Nomama, and Katera rivers, where the main structural elements of the Early Paleozoic collisional system have been established. The strike-slip and thrust Tompuda-Nomama and Barguzin boundary sutures separate the Svetlaya and the Katera zones of the Baikal-Muya Belt from the Barguzin terrigenous-carbonate terrane. The age estimates of syntectonic (prebatholithic) gneissic granite and gabbrodiorite intrusive bodies (469 ± 4 and 468 ± 8 Ma, respectively) coincide with the age of collisional events in the Ol’khon, Southwest Baikal, and Sayan regions (480–470 Ma). A linear zone with zonal metamorphism and granite-gneiss domes dated at 470 Ma is revealed in the allochthonous fold-nappe packet of the Upper Riphean Barguzin Formation. This zone of Caledonian remobilization marks the collisional front between the Riphean structural units of the Barguzin Terrane consolidated 0.60–0.55 Ga ago and the Baikal-Muya Belt. New data allow us to recognize this zone as the northeastern flank of the Baikal Collisional Belt. The Nd isotopic data for the reference igneous complexes of the collisional zone indicate that the Late Riphean juvenile crust was involved in the Ordovician remobilization in the zone of conjugation of the consolidated Baikalian structural elements at the northeastern flank of the Baikal Belt and likely was a basement of the entire Barguzin Terrane or, at least, its frontal portion. The lateral displacements of the terranes to the northeast during the Early Ordovician collision were constrained by the rigid structural framework of the Baikalides in the Muya segment of the Baikal-Muya Belt, where the Riphean blocks were involved in strike-slip faulting and the Vendian-Cambrian superimposed basin underwent deformation. Finally, it may be concluded that the Early Ordovician was an epoch of collision, complex in kinematics, between heterogeneous blocks of the continental crust: the Baikalides of the Baikal-Muya Belt and polycyclic Barguzin-Vitim Superterrane.     

Mid-Wisconsin seasonal temperatures reconstructed from fossil beetle assemblages in eastern North America: comparisons with other proxy records from the Northern Hemisphere

Mutual climatic range (MCR) analysis was applied to 15 North American beetle assemblages spanning the interval from > 52 000 to 17 200 yr BP, bracketing a Mid-Wisconsin interstadial interval. The analyses yielded estimates of mean July (TMAX) and mean January (TMIN) temperatures. The oldest assemblage (> 52 ka) yielded TMAX values 7.5–8°C lower than present and TMIN values 15–18°C lower than present. A Mid-Wisconsin interstadial warming dating from 43.5–39 ka was rapid and intense. At the peak of the warming event, about 42 ka, TMAX values were only 1–2°C lower than modern. This level of amelioration apparently lasted only about 2000–3000 yr. By 23.7 ka, TMAX values declined to 11.5–10°C lower than modern, but another, small-scale amelioration is indicated by assemblages dating from 20.5 to 19.7 ka. The interstadial event recorded from the site at Titusville, Pennsylvania closely matches the timing and intensity of the climate change estimated from British beetle faunas in the Upton Warren interstadial. Another warm interval (ca. 31–32.5 ka) has been documented from fossil beetle assemblages in Europe and North America. Copyright © 1999 John Wiley & Sons, Ltd.     

A lithofacies model of upper jurassic productive deposits in the eastern part of the Shirotnoye Priobie region (Western Siberia)

On the basis of an integrated interpretation of well data, core analysis, geophysical well logging, and seismic data a new geological model of the JV₁¹⁻² stratum is suggested for one of the Shirotnoye Priobie areas in Western Siberia. The results of acoustic inverse and the rock elastic property research were used for the validation of lithofacies zonation. Supplementary exploration of the area and production drilling are recommended.     

Interglacial Chironomidae (Diptera) from Thule, Northwest Greenland: matching modern analogues to fossil assemblages

An analysis of subfossil insect remains (Diptera, Chironomidae) from an interglacial site at Narsaarsuk near Thule Air Base, NW Greenland, was undertaken to complement our understanding of last interglacial environments in the Arctic by analogue matching to modern chironomid assemblages. The subfossil larval midge head capsules were well preserved and 82% of the chironomid remains were identified as eight different extant chironomid taxa. The assemblage was dominated by the lotic Diamesa (43.8%), a number of lentic taxa (Hydrobaenus, Psectrocladius, Cricotopus/Orthocladius) and a few semi-aquatic taxa (Smittia, Chaetocladius). A single black fly head capsule (Diptera, Simuliidae) was registered. The interglacial sample was compared to subfossil chironomid assemblages from 42 lakes in West Greenland, two glacier lakes (with and without river influence) and a quantitative zoobenthos study from Narsaq Elv. Similarity analysis, analogue matching and multidimensional scaling suggest a lotic, cold, glacier-fed interglacial palaeo-biotope. Quantitative temperature reconstruction was not possible owing to a high dissimilarity to modern lentic chironomid assemblages from West Greenland. However, the simple numerical methods convincingly managed to reflect an interglacial river and stream environment, which can be difficult to document from other palaeoecological data.     

四川宝兴灵关─金台山飞来峰泥盆纪地层古生物

首次详细地描述了灵关一金台山飞来峰泥盆纪地层剖面，将泥盆系划分为平驿铺组、甘溪组、养马坝组和观雾山组４个岩石地层单位，建立了４个腕足类组合带和２个珊瑚类组合带，并与邻区进行了对比。通过年代地层界线的讨论，初步认为该飞来峰内仅有下泥盆统和中泥盆统中下部地层。     

Hydrogen isotopic composition of water from fossil micrometeorites in howardites

We have measured the hydrogen isotopic composition (D/H ratios) of the water from 13 carbonaceous chondritic microclasts (CCMs, size <1 mm) trapped in two howardites (Kapoeta and Yamato-793497) early in the evolution of Solar System. The division into tochilinite-rich; magnetite-rich, olivine-poor; magnetite-rich, olivine-rich CCM types is corroborated by the hydrogen isotopic compositions. Both mineralogy and hydrogen isotopic compositions demonstrate that tochilinite-rich CCMs represent CM2 chondritic matter. In contrast, there is no good match between the isotopic and mineralogical properties of the magnetite-rich CCMs and the known groups of carbonaceous chondrites, suggesting that magnetite-rich CCMs represent a new kind of chondritic matter, not yet sampled in meteorite collections. This demonstrates that the view of the asteroid belt revealed by the collection of meteorites is incomplete. The study of (micro)clasts offers a unique opportunity to better decipher the nature and relative abundance of asteroids.The average hydrogen isotopic composition of water belonging to CCMs, D/H = (152.0 ± 4.8) × 10⁻⁶ (1σ_m), is similar to that of Antarctic micrometeorites (AMMs), D/H = (161.2 ± 3.8) × 10⁻⁶ (1σ_m). The similarity, in terms of mineralogy and hydrogen isotopic composition, between CCMs and AMMs demonstrates that the composition of the micrometeorites has not been modified over the whole history of the Solar System. It indicates that the composition of the micrometeorite flux onto Earth has been, and is, dominated by a mixture of CM2-like; magnetite-rich, olivine-poor; magnetite-rich, olivine-rich carbonaceous chondritic matter exemplified by CCMs found in howardites. Because CCMs have not suffered atmospheric entry, they provide an abundant source of pristine micrometeorites.The average D/H ratio of the whole population of CCMs is identical within errors to that of the Earth (149 ± 3 × 10⁻⁶). The match between the CCMs D/H ratio and that of the Earth is especially remarkable because 1) three different populations of CCMs are needed to make the D/H ratio of the Earth; 2) there is no single carbonaceous chondrite group for which a similar match exists. This observation suggests that CCMs population might be representative of the late veneer agent(s) that delivered water to the Earth.     

The Jurassic/Cretaceous boundary in northern Siberia and Boreal-Tethyan correlation of the boundary beds

There is no international consensus regarding the GSSP for the Berriasian, the basal stage of the Cretaceous System. Any of the events discussed by the international expert community can be regarded as a marker of the Jurassic/Cretaceous boundary: a phylogenetic change of taxa, paleomagnetic reversal, or isotopic excursion. However, the problem of identification of this level in Boreal sections can be solved only using a combination of data obtained by paleontological and nonpaleontological methods of stratigraphy (bio-, chemo-, magnetostratigraphy, etc.). With any of the accepted markers, the Jurassic/Cretaceous boundary in Siberian sections will be within the upper part of the regional Bazhenovo Horizon. The least interval of the uncertainty of the position of this boundary in Siberian sections will be ensured by the selection of one of two markers: biostratigraphic (base of the Pseudosubplanites grandis Subzone) or magnetostratigraphic (base of the M18r magnetozone).     

四川宝兴灵关─金台山飞来峰泥盆纪地层古生物

首次详细地描述了灵关一金台山飞来峰泥盆纪地层剖面，将泥盆系划分为平驿铺组、甘溪组、养马坝组和观雾山组４个岩石地层单位，建立了４个腕足类组合带和２个珊瑚类组合带，并与邻区进行了对比。通过年代地层界线的讨论，初步认为该飞来峰内仅有下泥盆统和中泥盆统中下部地层。     

Stratigraphic setting and age of the complex Tappania-bearing Proterozoic fossil biota of Siberia

A microfossil assemblage has been found in the widely spaced (300 km apart) Baikit and Katanga areas (inner Siberian Platform) in stratigraphically coeval strata in the lower part of the petroliferous Kamo Group (Riphean). It comprises complex protists Tappania plana Yin; acritarchs Valeria lophostriata Jank., Satka sp., Lophosphaeridium sp., Pulvinosphaeridium sp., and Miroedichia sp.; porous vesicles of Tasmanites sp. and Osculosphaera sp.; vesicles with anastomosing processes, and spiral filaments of Obruchevella and Glomovertella. Siberian fossil microbiota is considerably more diverse than biota with Tappania protists known in Australia and China. A taxonomically similar fossil biota found 1500 km northeast of the well-studied Riphean section in the Olenek uplift suggests that the host rocks, too, are stratigraphically coeval. This permits refining the age of remains of the first complex eukaryotes from the inner platform and is an additional correlation criterion for the lower Kamo Group.     

The roads of the Sayan Mountains: Theorizing remoteness in eastern Siberia

The Sayan crossroads is a distinct cultural and economic region in the mountains of eastern Siberia. It spans three federal units in the Russian Federation: the national republics of Tyva (Todzha kozhuun) and Buryatia (Okinskii raion), as well as Irkutsk oblast (Tofalarskoe municipality). Attempts at integration by the state during the Soviet period and afterwards have privileged the construction of roads in a variety of forms to connect these areas economically with regional centers and, in turn, the rest of the country. Yet this process has been uneven and led to divergences in the economic regimes in each of the three regions that make up the crossroads. The evolution of subsistence economies, exploitation by extractive industries, and the development of tourism as an alternative source of income all differ across the three federal subunits. In turn, these divergences within the crossroads as a region point to variation in the condition of remoteness. Remoteness is an instance of relative immobility, determined by physical geography, environment, and ethnicity. And remoteness influences the function that roads play in integrating state spaces both economically and politically. In turn, this article argues for the foregrounding of the remote in the literature on mobilities in human geography, considering what the condition of remoteness allows for and forecloses in the articulation of state power and the integration of hard-to-reach areas.     

Precambrian granulites of the Aldan shield, eastern Siberia, USSR

Abstract Precambrian granulites of the Aldan shield in southern Yakutia, USSR, form a massif of 200,000 km² bounded by younger fold-belts to the south, west and east. The massif consists of several blocks that reflect a primary heterogeneity of composition and differences in structural and thermodynamic evolution of different parts of the area. According to structural and petrological data the massif can be divided into two megablocks: eastern Aldan and western Aldan. They are separated by a narrow meridional fold-belt. Structural evolution of this central zone was determined by the geodynamics of the mega-blocks and was completed in the late Archaean. Towards the south, this central zone is ‘transformed’into the relatively small Sutam block adjoining the Stanovoy fold-belt that bounds the Aldan shield on the south. The Sutam block is separated from the other structural units of the Aldan shield by a system of north trending grabens filled by post-Archaean sediments. The Aldan shield is composed of Archaean high-grade granulites, while the Stanovoy fold-belt, to the south, consists of highly foliated Proterozoic rocks metamorphosed under relatively lower-grade conditions. However, relics of the granulites are mapped within the fold-belt. They contain high-grade assemblages (e.g. Opx + Sil + Qz, Sap + Qz, Opx + Gr + Sil, etc.). One of the relics, the Tokskii block, which is only slightly touched by diaphthoresis, is located in the southeastern part of the Stanovoy fold-belt. Metamorphic conditions of the Tokskii block are compared with those of the Sutam block and a similar evolution of the units is revealed. Mineral assemblages and mineral compositions do not vary within each unit, but they change in a north-south direction. The Opx + Sil + Qz assemblage has been found only in Sutam and Tok, but not in eastern Aldan and western Aldan. The Sap + Qz assemblage has been found in the Tokskii block but has not yet been found in the Sutam block. The pyrope content in garnets, from metapelites of both blocks, is significantly higher than that from the Aldan (eastern and western blocks) rocks to the north. The most important assemblages from different units of the Aldan shield have been studied using the electron microprobe in order to unravel the metamorphic evolution of the granulites and thus to deduce the thermodynamic regime of this evolution. A geodynamic model for the Aldan shield is discussed in terms of Archaean island arc development.