Reconstruction of the Neoproterozoic–Cambrian Orogenesis in Princess Elisabeth Land (East Antarctica) from a Study of Granitic Rocks

Abundant Cambrian granitic rocks in Princess Elizabeth Land and adjacent regions of Antarctica occupy various positions (from syn- to postkinematic) in the structure of the crust. Their mineral and isotopic composition reflects both the character and age of the parental substrate and geodynamic formation conditions of the region. The study of Cambrian processes is important for this region, because almost all known models of the formation of the Gondwana supercontinent suggest the Neoproterozoic–Cambrian amalgamation of two or three continental blocks, parts of which can be distinguished in this sector. The paper presents geological data on granitic rocks, as well as their mineral and Nd isotopic composition. Field observations and analytical data indicate a different structure and petrographic composition of Cambrian granitic rocks in Princess Elizabeth Land, which are related to their different formation conditions. Synkinematic biotite or garnet–biotite peraluminous S-type granites mostly occur in the eastern Princess Elizabeth Land. Mostly late and postkinematic biotite and amphibole–biotite (±orthopyroxene) granites or granodiorites are typical of its western part. Their compositions are similar to that of A-type granites. In comparison with coeval granites in adjacent areas, the character of Cambrian granites in Princess Elizabeth Land substantiates the presence of structural zones identified from geological data and indicates the presence of a Cambrian orogen probably with a collisional nature, as well as the location of its hinterland in intracontinental Antarctica.     

Tectonic Setting and Nature of the Provenance of Sedimentary Rocks in Lanping Mesozoic- Cenozoic Basin： Evidence from Geochemistry of Sandstones

The geochemical composition of sandstones in the sedimentary basin is controlled mainly by the tectonic setting of the provenance, and it is therefore possible to reveal the tectonic setting of the provenance and the nature of source rocks in terms of the geochemical composition of sandstones. The major elements, rare-earth dements and trace elements of the Mesozoic-Cenozoic sandstones in the Lanping Basin are studied in this paper, revealing that the tectonic settings of the provenance for Mesozoic-Cenozoic sedimentary rocks in the Lanping Basin belong to a passive continental margin and a continental island arc. Combined with the data on sedimentary facies and palaeogeography, it is referred that the eastern part of the basin is located mainly at the tectonic setting of the passive continental margin before Mesozoic, whereas the western part may be represented by a continental island arc. This is compatible with the regional geology data. The protoliths of sedimentary rocks should be derived from the upper continental crust, and are composed mainly of felsic rocks, mixed with some andesitic rocks and old sediment components. Therefore, the Lanping Mesozoic-Cenozoic Basin is a typical continental-type basin. This provides strong geochemical evidence for the evolution of the paleo-Tethys and theb asin-range transition.     

Geochemical Constraints of Sediments on the Provenance, Depositional Environment and Tectonic Setting of the Songliao Prototype Basin

The geochemistry of sediments is primarily controlled by their provenances, and different tectonic settings have distinctive provenance characteristics and sedimentary processes. So, it is possible to discriminate provenances, depositional environments and tectonic settings in the development of a sedimentary basin with the geochemistry of the clastic rocks. The analytical results of the present paper demonstrate that sediments in the Songliao prototype basin are enriched in silica (SiO2=66.48-80.51 %), and their ΣREE are 30-130 dmes of that of chondrite with remarkable Eu anomalies. In discriminating diagrams of Eu/Eu vs eeeeeREE and (La/Yb)N vs ΣREE, most samples locate above the line Eu/ Eu=l, on the right of the line Eu/Eu/ΣREE=1 and under the line La/Yb)N/eeeeeREE=1/8, which indicates that the depositional environment of sediments in the basin was oxidizing. In addition, variations of MgO, TiO2, A12O3, FeO+Fe2O3, Na2O and CaO vs SiO2 reflect a tendency of increasing mineral maturity of sediments     

Middle Devonian Picrites of the Southern Margin of Altay Orogenic Belt and Implications for the Tectonic Setting and Petrogenesis

INTRODUCTION Inrecentyears,greatprogressonthegeologic tec tonicevolutionandmineralresourcesofXinjianghas beenachieved.However,manyissuesarestilldebated, suchasancienttectonicpatternsandtheclosuretimeof theancientoceanicbasin(LiandXu,2004).Theseis sueshavelimitedourknowledgeoftheformationande volutionofAsiancontinents,aswellastheexploration anddevelopmentofmineralresources. Recently,theHilaketehalasuporphyrycopperde positwasdiscoveredinthestrataoftheMiddleDevoni anBeitashanFormatio…     

Geochemical and Sr–Nd–Pb-Isotope Characteristics of Ice-Rafted Sediments of the Arctic Ocean

The paper discusses the geochemical and Sr–Nd–Pb-isotope data on ice-rafted sediments (IRS) from different areas of the Arctic Ocean. Samples were collected during the Cruise of R/V Polarstern between Spitsbergen and North Pole, Yermak Plateau, as well as in Fram Strait. It is shown that the studied IRS samples in terms of La_N/Yb_N and εNd values are close to the composition of suspended particulate matter (SPM) from the mouth parts of large rivers and rivers transporting the sedimentary erosion products. This also follows from their Th/Sc, Th/Co, La/Sc, La/Sm, Sc/Th ratios and Sc content and from the position of their data points in the Sc–Th/Sc, La/Sc–Th/Co, and La/Sm–Sc/Th diagrams between the average SPM compositions of the Ob and Lena rivers. The values of ²⁰⁷Pb/²⁰⁶Pb and εNd in IRS samples give grounds to suggest that the rock complexes of the European, North American, and Asian continental margins could be potential sources for this sedimentary material. In the ²⁰⁷Pb/²⁰⁶Pb–εNd diagram, the IRS samples from all three studied areas define a compact cluster and are mainly confined to the triangle with corners represented by the Mackenzie River, Okhotsk–Chukotka volcanic area, and Lena River, being closer to the latter. In the Sm/Nd–εNd diagram, IRS points also form a compact field, being located almost in the middle between the average SPM compositions of the Yenisei and Ob rivers, on the one hand, and Lena River, on the other. In all diagrams, IRS samples from the different West Arctic areas show no significant scatter. With allowance for the fact that sediments are not subjected to significant homogenization during ice rafting, sediments from all three areas were obtained from a common source. As seen from the position of IRS data points in the ⁸⁷Sr/⁸⁶Sr–εNd diagram, this source was the Asian continental margin.     

Magnetostratigraphy of the Middle–Upper Cambrian Verkhnyaya Lena Group and Lower Ordovician Ust-Kut formation in the southern Siberian Platform

The available paleomagnetic data on the Verkhnyaya Lena Group from different areas of the southern Siberian Platform are revised. The group rests unconformably upon the Lower Cambrian strata and is overlain by Lower Ordovician rocks, which determines conditionally the age of its red-colored deposits. Paleomagnetic correlation of composite sections through the region using defined zones of normal and reversed magnetic polarity serves as a basis for development of the magnetostratigraphic scale for the Verkhnyaya Lena Group. The scale includes nine magnetic zones, which play the role of markers; seven of them are traceable in all the examined sections of the southern Siberian Platform. By the distribution of zones with normal (N) and reversed (R) polarity, the magnetostratigraphic scale is subdivided into three parts. Its lower part is represented by reversed polarity, which is characteristic of the second half of the Lower Cambrian. The middle part is characterized by frequently alternating zones with normal and reversed polarity corresponding to the Middle Cambrian. The upper part of the scale corresponds to the interval of reversed polarity characteristic of the Upper Cambrian and Lower Ordovician. The Middle–Upper Cambrian boundary is located near the last N–R reversal of the geomagnetic field in the Cambrian. The magnetostratigraphic scale includes nine orthozones united into three superzones, which are attributed to two hyperzones of magnetic polarity.     

First Findings of Trace Fossils from the Tavda Formation (Middle–Upper Eocene) in the Southwestern Part of Western Siberia

Trace fossils have been found for the first time in the Tavda Formation shallow marine facies, and their possible indicative value has been evaluated. The results of the ichnofossil study suggest that the Tavda Basin had low hydrodynamics. Under these conditions boreal and thetic waters of different characteristics were blended, which caused periodic formation of thermohaline water stratification and anoxic conditions in bottom water levels. Such periodic events conditioned mass extinctions of a wide variety of the Eocene marine biota thus preconditioning the development of freshwater fauna in the upper levels of the Tavda Sea along with saturation of the index beds of the Tavda Formation with micro- and macrofossils.     

Sorption of Impurity Elements by the Components of Lower–Middle Miocene Lignite from the Sergeevskoe Deposit (Upper Amur Region)

Doklady Earth Sciences - Based on model experiments on the sorption of chemical elements from multicomponent solutions with fungi biomass, lignite, and humic acids, it was assumed that qualitative...     

Geochronology, Geochemistry and Tectonic Setting of the Bairiqiete Granodiorite Intrusion (Rock Mass) from the Buqingshan Tectonic Mélange Belt in the Southern Margin of East Kunlun

This study focuses on the zircon U–Pb geochronology and geochemistry of the Bairiqiete granodiorite intrusion(rock mass) from the Buqingshan tectonic mélange belt in the southern margin of East Kunlun. The results show that the zircons are characterized by internal oscillatory zoning and high Th/U(0.14–0.80), indicative of an igneous origin. LA–ICP–MS U–Pb dating of zircons from the Bairiqiete granodiorite yielded an age of 439.0 ± 1.9 Ma(MSWD = 0.34), implying that the Bairiqiete granodiorite formed in the early Silurian. Geochemical analyses show that the rocks are medium-K calc-alkaline, relatively high in Al2O3(14.57–18.34 wt%) and metaluminous to weakly peraluminous. Rare-earth elements have low concentrations(45.49–168.31 ppm) and incline rightward with weak negative to weak positive Eu anomalies(δEu = 0.64–1.34). Trace-element geochemistry is characterized by negative anomalies of Nb, Ta, Zr, Hf and Ti and positive anomalies of Rb, Th and Ba. Moreover, the rocks have similar geochemical features with adakites. The Bairiqiete granodiorite appears to have a continental crust source and formed in a subduction-related island-arc setting. The Bairiqiete granodiorite was formed due to partial melting of the lower crust and suggests subduction in the Buqingshan area of the Proto-Tethys Ocean.     

Volcanic Tuffs,Red- and Usual-Colored Clayey Rocks in Upper Riphean–Vendian Deposits in the Middle and Southern Urals: Comparison of Lithochemical Characteristics

Lithology and Mineral Resources - In the lower part of Upper Vendian deposits on the East European Platform, packs of chocolate-brown clayey rocks/tuff-argillites containing intercalations of...     

A Reconstruction of a Vendian–Cambrian Active Continental Margin within the Southern Urals: Results of Detrital Zircons Studying from Ordovician Terrigenous Rocks

Geotectonics - Detrital zircons of Ordovician terrigenous sequences are studied in various Southern Uralian tectonic units. The age of detrital zircons of the West Uralian and Transuralian...     

Petrogenesis and Tectonic Implications of Middle Ordovician Ocean Island Basalts from the Chagantaolegai Ophiolitic Mélange in Junggar, NW China

Seamount accretion is one of the most significant accretionary orogenic processes in the Central Asian Orogenic Belt, but there are few paleo-seamounts reported from and debate on the tectonic evolution of the Junggar Ocean still exists. In this study, we present geochronological, mineralogical, geochemical and isotopic data for basalts from the Chagantaolegai ophiolitic mélanges in Junggar. Zircon U-Pb dating on one basalt yielded a weighted mean 206 Pb/238 U age of 469 ± 7 Ma, which suggests that it formed in the Middle Ordovician. All rock samples belong to alkaline basalt and show similar geochemical characteristics, displaying high TiO2(～3 wt%),(La/Yb)N(17.6–19.0), ΣREE(232–289 ppm) and enrichment in Nb and Ta, which implies an ocean island basalt(OIB) affinity. Based upon positive εN d(t)(+4.16 to +4.23), ΔNb(0.20–0.22) and low initial 87 Sr/86 Sr(0.70425 to 0.70452) and Zr/Nb(3.35–3.57), we suggest that the Chagantaolegai OIB samples were likely derived from a fertile mantle source related to plume. The OIB rock assemblage, chert and marble in the southern part of the Chagantaolegai ophiolitic mélange indicates that a Middle Ordovician seamount was accreted to the Boshchekul-Chingiz arc due to the northward subduction of the Junggar-Balkhash Ocean.     

Quick Evaluation of Present-Day Low-Total Organic Carbon Carbonate Source Rocks from Rock-Eval Data: Middle–Upper Ordovician in the Tabei Uplift, Tarim Basin

Previous studies have postulated the contribution of present-day low-total organic carbon(TOC) marine carbonate source rocks to oil accumulations in the Tabei Uplift, Tarim Basin, China. However, not all present-day low-TOC carbonates have generated and expelled hydrocarbons; therefore, to distinguish the source rocks that have already expelled sufficient hydrocarbons from those not expelled hydrocarbons, is crucial in source rock evaluation and resource assessment in the Tabei Uplift. Mass balance can be used to identify modern low-TOC carbonates resulting from hydrocarbon expulsion. However, the process is quite complicated, requiring many parameters and coefficients and thus also a massive data source. In this paper, we provide a quick and cost effective method for identifying carbonate source rock with present-day low TOC, using widely available Rock-Eval data. First, we identify present-day low-TOC carbonate source rocks in typical wells according to the mass balance approach. Second, we build an optimal model to evaluate source rocks from the analysis of the rocks' characteristics and their influencing factors, reported as positive or negative values of a dimensionless index of Rock-Eval data(IR). Positive IR corresponds to those samples which have expelled hydrocarbons. The optimal model optimizes complicated calculations and simulation processes; thus it could be widely applicable and competitive in the evaluation of present-day low TOC carbonates. By applying the model to the Rock-Eval dataset of the Tabei Uplift, we identify present-day low-TOC carbonate source rocks and primarily evaluate the contribution equivalent of 11.87×10~9 t oil.     

Formation and Development Forecast of the Western Arctic as a Segment of the Atlantic–Arctic Rift System

Geotectonics - The article overviews the existing data and ideas about the structure of the Atlantic–Arctic rift system, the youngest segment of which is the Western Arctic. The main...     

Tectonic and sedimentary basin evolution of the eastern Bangong–Nujiang zone (Tibet): a Reading cycle

The ophiolite-bearing Bangong-Nujiang zone (BNZ) traversing central Tibet from east to west separates the Qiangtang block in the north from the Lhasa block in the south. Their stratigraphic development indicates that both blocks once formed a continuous continental platform until the Late Triassic. Following Late Paleozoic-Triassic rifting, ocean crust formed between both blocks during the Late Triassic creating the Dongqiao-Naqu basin (DNB) among other basins (Yu et al. 1991). The analysis of the rift flank sequences reveals that rifting was dominated by transtension. The basin was shortened by post-Mid-Cretaceous transpression. Thus, the overall basin evolution represents a Reading cycle despite some active margin processes which gave this cycle a special imprint. Major basin parts were preserved despite transpressional shortening suggesting that the eastern BNZ represents a remnant basin. Our understanding of the DNB solves the prior problem of viewing the BNZ as a Mid-Late Jurassic collisional suture although typical collision-related deformation, thickening, mountain building, as well as related molasse formation are lacking. Our model also explains the scattered linear ophiolite distribution by local transpression of remnant oceanic basin floor without having to consider problematic long range ophiolite thrusting.     

Sr,Nd, and Hf Isotope Composition of Rocks of the Reft Gabbro–Diorite–Tonalite Complex (Eastern Slope of the Middle Urals): Petrological and Geological Implications

This paper reports Rb–Sr and Sm–Nd isotope data on the gabbro–diorite–tonalite rock association of the Reft massif (eastern margin of the Middle Urals) and Lu–Hf isotope data on zircon populations from these rocks. In terms of Nd and Hf isotope composition, the rocks of the studied association are subdivided into two distinctly different groups. The first group consists of gabbros and diorites, as well as plagioclase granites from thin dikes and veins cutting across the gabbros. In terms of ⁴³Nd/¹⁴⁴Nd _i = 0.512518–0.512573 (ε_Nd(T) = +8.6...+9.7) and ¹⁷⁶Hf/¹⁷⁷Hf _i = 0.282961–0.283019 (ε_Hf(T) = +15.9...+17.9), these rocks are practically identical to depleted mantle. Their Nd and Hf model ages show wide variations, but in general are close to their crystallization time. The second group is represented by tonalites and quartz diorites, which compose a large body occupying over half of the massif area. These rocks are characterized by the lower values of ¹⁴³Nd/¹⁴⁴Nd _i = 0.512265–0.512388 (ε_Nd(T) = +3.7...+6.0) and ¹⁷⁶Hf/¹⁷⁷Hf _i = 0.282826–0.282870 (ε_Hf(T) = +11.1...+12.7). The T_DM values of the second group are much (two–three times) higher than their geological age (crystallization time), which indicates sufficiently long crustal residence time of their source. The initial ⁸⁷Sr/⁸⁶Sr in the rocks of both the groups varies from 0.70348 to 0.70495. This is likely explained by the different saturation of melts with fluid enriched in radiogenic Sr. The source of this fluid could be seawater that was buried in a subduction zone with oceanic sediments and released during slab dehydration. Obtained data make it possible to conclude that the formation of the studied gabbro–diorite–tonalite association is a result of spatially and temporally close magma formation processes in the crust and mantle, with insignificant contribution of differentiation of mantle basite magma.     

Sources of Magmatic Rocks from the Deep-Sea Floor of the Arctic Ocean and the Central Atlantic: Evidence from Data on the U–Pb Age,Hf Isotopes,and REE Geochemistry of Zircons

The results of geochronological (U–Pb), isotope–geochemical (Lu–Hf), and geochemical (REEs) studies of young (MZ, KZ) and xenogenic (AR, PR) zircons from magmatic rocks of the Central Arctic rises of the Arctic Ocean (AO) and the crest zone of the Mid-Atlantic Ridge (MAR) are presented. The data obtained show that the depleted mantle could be a source of young (KZ) zircons of the MAR, whereas young (MZ) zircons of the MAR and all xenogenic (AR, PR) zircons of the AO and MAR are from crustal rocks of the continental lithosphere.     

New Genus and Species of Rhyacophilidae (Insecta: Trichoptera) from the Middle Jurassic of China

One new genus with a new fossil species,Declinimodus setulosus gen.et sp.nov.which was found in the Middle Jurassic Jiulongshan Formation of Daohugou,Inner Mongolia,China,are described and illustrated.The new species is assigned to the Rhyacophilidae according to buttonshaped of the 2nd segment of maxillary palp,and the forked R1 （in the forewing,located near apex）.These new discovery are the earliest fossil records of Rhyacophilidae,which providing important evident for the study of the origin and early evolution of Trichoptera.