Ordovician Sedimentary Rocks Research Articles

The making of Mt Everest: channel flow and low-angle normal faults in the compressional Himalayan orogen

Mt Everest (8849 m) spans the Greater Himalayan Sequence metamorphic rocks and the base of the unmetamorphosed Tethyan sedimentary rocks in the Nepal–South Tibet Himalaya. Two north-dipping, low-angle normal faults cut the massif: the upper Qomolangma Detachment placing Ordovician sedimentary rocks above Everest Series greenschist–amphibolite facies rocks; and the lower Lhotse Detachment placing Everest Series schists above sillimanite gneisses, migmatites and leucogranites. The two faults merge northwards into one large ductile shear zone (the South Tibetan Detachment). Pressure–temperature constraints and structural restoration show that the South Tibetan Detachment acted as a passive roof fault during extrusion of the footwall. At least 120 km of southward flow of the footwall rocks occurred during the Miocene, resulting in the exhumation of rocks that were buried to 5.5 kbar ( c. 18–22 km depth) below the detachment, juxtaposing them against hanging wall rocks that are essentially unmetamorphosed. The low-angle normal faults were operative during north–south convergence and reflect the exhumation of a locked passive roof fault, unrelated to any crustal extensional processes. U–(Th)–Pb dating of peraluminous leucogranites exposed on Mt Everest (21–20 Ma), Nuptse ( c. 19–18 Ma) and along the Rongbuk valley (15.6–15.4 Ma) show that ductile extrusion occurred during the Early Miocene, with brittle faulting at <15.4 Ma during exhumation.

Provenance of early Permian Li-rich claystone from Central Yunnan, South China: Constrained by Sr-Nd-Pb isotopes, geochemistry, and zircon U–Pb ages

A new lithium-rich claystone from the Lower Permian has been detected in Central Yunnan, Southwestern China that could be great potential for Lithium (Li) exploration, yet the origin of the claystone is still being debated. To study it further, new geochronological, geochemical, and Sr–Nd–Pb isotopic analyses were systematically conducted on the Daoshitou Formation (P1d) claystone, as well as on the underlying strata. Through detrital zircon U–Pb dating of the claystone in the Daoshitou Formation, three major age peaks were revealed, at ∼960 Ma, ∼790 Ma, ∼550 Ma, respectively, along with three minor peaks of ∼2500 Ma, ∼1500 Ma, and ∼1200 Ma. Detrital zircons in the claystone have age spectra that closely match those of the Lower Ordovician sedimentary rocks in Central Yunnan, indicating these rocks are the major contributor of detrital zircons. However, the rock source determined by geochemical features decouples with the zircon source determined by detrital zircon data. The A–CN–K ternary diagram obviously reveals analogous predicted weathering trends between the Weining Formation (C2w) and Daoshitou Formation, suggesting a potential provenance link. Furthermore, the weathering trends of samples from three drill cores indicate that the transformation of clay mineral (smectite–illite–chlorite/kaolinite) may facilitate Li enrichment. Smectite, as the precursor mineral for Li enrichment, typically forms and preserves in a slightly alkaline environment, which suggests that the Li-rich claystone is likely to formed in this environment. Low Na/Li and K/Li ratios indicate low cation exchange selectivity in sedimentary environments, which promote the proliferation of adsorption sites for Li on smectite. The weathering products from the Weining Formation, composed of impure carbonates, could provide a slightly alkaline environment with low ion strength for the adsorption of Li. In addition, Sr–Nd–Pb isotopic compositions also reveal the significant similarity between the Daoshitou Formation and Weining Formation. Therefore, we suggest that the weathering of the Weining Formation provided the primary material to Daoshitou Formation claystone.

A database of detrital zircon U–Pb ages in the North China Craton from the Paleoproterozoic to the early Palaeozoic

AbstractU–Pb dating of detrital zircon is widely used in geology to identify the source of sediments and constrain palaeogeography and tectonic history. We collected the detrital zircon from the Paleoproterozoic to early Palaeozoic strata to understand the North China Craton's geological chronological characteristics. After screening and sorting out data information, we managed and analysed 84 papers from 2006 to 2022. Three hundred seventy‐nine samples comprising 29,431 pieces of U–Pb ages of detrital zircons gathered from the Proterozoic to Ordovician sedimentary rocks in North China Craton (NCC). Detrital zircons within five areas in the NCC, i. e., Xuhuai Basin, Zhaertai‐Bayan Obo‐Huade Basin, Yanliao Basin, Xiong'er Basin and Alxa Block, are discussed in detail. Meanwhile, we collected 2,588 pieces of Lu‐Hf detrital zircon data in the study. This Database can be helpful in understanding and explaining the tectonic evolution of the NCC from the Paleoproterozoic to the Early Palaeozoic.

Age and tectonic significance of the Benton pluton, Eel River area, west-central New Brunswick, Canada

The Benton pluton, located in the Eel River area of west-central New Brunswick, Canada, consists of two components—the Caldwell Brook quartz monzonite and Dugan Road monzogranite. Field relations suggest that the two are essentially coeval, the Dugan Road being slightly younger. The Benton pluton intrudes Cambrian to Early Ordovician sedimentary rocks of the Woodstock Group and overlying Early to Middle Ordovician calc-alkaline rhyolitic, andesitic, dacitic, and basaltic rocks of the lower Meductic Group. A new U–Pb (LA-ICP-MS) zircon age of 467 ± 2. Ma indicates that the Dugan Road monzogranite component of the Benton pluton was emplaced during the Middle Ordovician (late Dapingian to early Darriwilian). The Dugan Road monzogranite is ca. 7 million years younger than the nearby arc-related Connell Mountain tonalite and Gibson granodiorite, and ca. 13 million years younger than arc- related volcanic rocks of the Porten Road Formation, the oldest unit of the Meductic Group. The Benton pluton is interpreted to have been emplaced in an active extensional arc setting, coinciding with a shift in the focus of subduction-related volcanism from the Eel River area of New Brunswick to the Greenfield area of adjacent Maine, USA.

The Ordovician System in Australia and New Zealand

Abstract The stratigraphic overview presented in this chapter substantially updates and revises the last major review of the Ordovician rocks of Australia and New Zealand published 40 years ago. In the western two-thirds of the present-day continent of Australia, Ordovician sedimentary rocks are restricted to intracratonic basins. The Canning Basin (Western Australia) and Amadeus Basin (central Australia) contain the best known Lower and Middle Ordovician shallow marine successions. The eastern third of the continent, known as the Tasmanides, comprises multiple orogens (i.e. Delamerian, Lachlan, New England, Thomson, Mossman) that formed along the convergent East Gondwana Margin. As a result, volcanic and intrusive rocks are much more common in these orogens than in the intracratonic basins. Their deep-water depositional environments span 31 graptolite biozones. Slope and basinal siliceous sedimentary rocks are constrained by a newly defined set of 12 conodont biozones, complementing the conodont biostratigraphic scheme refined for shallow-water environments from the basal boundary of the Ordovician to the latest Katian. In some places, these conodont biozones are integrated with radiometric ages from tuff interbeds (e.g. Canning Basin). Ordovician graptolitic strata in the Buller Terrane of New Zealand share palaeogeographic links with those in the Bendigo Zone of the western Lachlan Orogen.

The amalgamation of Gondwana: calcite twinning and finite strains from the early–late Paleozoic Buzios, Ross, Kurgiakh and Gondwanide orogens

Abstract Orientated carbonate (calcite twinning strains; n = 78 with 2414 twin measurements) and quartzites (finite strains; n = 15) were collected around Gondwana to study the deformational history associated with the amalgamation of the supercontinent. The Buzios orogen (545–500 Ma), within interior Gondwana, records the high-grade collisional orogen between the São Francisco Craton (Brazil) and the Congo–Angola Craton (Angola and Namibia), and twinning strains in calc-silicates record a SE–NW shortening fabric parallel to the thrust transport. Along Gondwana's southern margin, the Saldanian–Ross–Delamerian orogen (590–480 Ma) is marked by a regional unconformity that cuts into deformed Neoproterozoic–Ordovician sedimentary rocks and associated intrusions. Cambrian carbonate is preserved in the central part of the southern Gondwana margin, namely in the Kango Inlier of the Cape Fold Belt and the Ellsworth, Pensacola and Transantarctic mountains. Paleozoic carbonate is not preserved in the Ventana Mountains in Argentina, in the Falkland Islands/Islas Malvinas or in Tasmania. Twinning strains in these Cambrian carbonate strata and synorogenic veins record a complex, overprinted deformation history with no stable foreland strain reference. The Kurgiakh orogen (490 Ma) along Gondwana's northern margin is also defined by a regional Ordovician unconformity throughout the Himalaya; these rocks record a mix of layer-parallel and layer-normal twinning strains with a likely Himalayan (40 Ma) strain overprint and no autochthonous foreland strain site. Conversely, the Gondwanide orogen (250 Ma) along Gondwana's southern margin has three foreland (autochthonous) sites for comparison with 59 allochthonous thrust-belt strain analyses. From west to east, these include: finite strains from Devonian quartzite preserve a layer-parallel shortening (LPS) strain rotated clockwise in the Ventana Mountains of Argentina; frontal (calcite twins) and internal (quartzite strains) samples in the Cape Fold Belt preserve a LPS fabric that is rotated clockwise from the autochthonous north–south horizontal shortening in the foreland strain site; Falkland Devonian quartzite shows the same clockwise rotation of the LPS fabric; and Permian limestone and veins in Tasmania record a thrust transport-parallel LPS fabric. Early amalgamation of Gondwana (Ordovician) is preserved by local layer-parallel and layer-normal strain without evidence of far-field deformation, whereas the Gondwanide orogen (Permian) is dominated by layer-parallel shortening, locally rotated by dextral shear along the margin, that propagated across the supercontinent.

Ordovician stratigraphy and biota of Mexico

Abstract In Mexico, Ordovician sedimentary rocks are exposed in the states of Baja California, Sonora, Chihuahua and Oaxaca, comprising approximately 30 stratigraphic successions ranging from Lower to Upper Ordovician. The ages of the sequences have been established primarily by utilizing conodonts and graptolites, which have also allowed us to differentiate between platform and oceanic basin environments. The State of Sonora has the most complete Ordovician stratigraphic sequences, ranging from Tremadocian to Hirnantian. The deposits in Baja California are Floian in age, while the sequences of Chihuahua range from Sandbian to Katian, and the deposits in Oaxaca are Tremadocian. The Ordovician deposits of northern Mexico (Baja California, Sonora, and Chihuahua) present a palaeogeographic relationship to the North American craton, mainly owing to faunal interspecific affinities, while the southern deposits (Oaxaca) are controversial owing to the high degree of endemism of the faunas; however, they show affinity with Gondwana, Baltica and Avalonia, with a possible insular origin. The biotic assemblages of the Ordovician of Mexico include a variety of taxa, including algae, poriferans, corals, bryozoans, brachiopods, molluscs, trilobites, echinoderms, graptolites and conodonts as predominant elements. Despite many years of field studies in Mexican Ordovician localities, biostratigraphic correlations are as yet insufficient and incomplete or are based on limited interpretations. Thus, the Ordovician biostratigraphic data from Mexico compiled in the present paper have great potential and significant value. The advancement in the knowledge of the Ordovician biostratigraphy of Mexico will contribute to a major understanding of the relationships with the Ordovician System to a continental scale. Future advances will come mainly through increasing the amount and quality of data as well as improving biocorrelations among the Ordovician sequences of Mexico.

Ordovician strata of the Indian subcontinent

Abstract Ordovician rocks of the Indian Tethyan Himalaya contain a conspicuous angular unconformity between mostly marine Cambrian and overlying terrestrial Ordovician strata, which is a record of the Kurgiakh Orogeny. This tectonic event is traceable across the Tethyan Himalaya from Pakistan to Bhutan. The Pin Formation in the Spiti Valley provides a high-resolution account of the marine depositional history, palaeontology and isotope geochemistry of Late Ordovician events. The middle (Takche) member is late Katian and the upper Mikkim Member is lower Silurian (Llandovery), based on an ozarkodinid conodont fauna. The Pin Formation records the Boda event, the last warming interval prior to Hirnantian glaciation. The δ 13 C carb chemostratigraphic data allow precise global correlation, and recognition of the Paroveja positive excursion, the last major excursion of the Katian. The Mikkim Member records a ‘lower HICE’ (Hirnantian isotopic carbon excursion) of the Katian–Hirnantian boundary interval. Palaeontological data indicate that there are no known fossils diagnostic of any Ordovician ages older than the Katian Stage in India. Evidence of Ordovician sedimentary rocks in the Lesser Himalaya is intriguing, but presently equivocal. The widespread absence of pre-Katian strata on the Indian subcontinent is due to erosion associated with the Kurghiak orogeny and delayed onlap onto topographically high areas.

Subduction initiation of the Proto-Tethys Ocean that facilitated climate change and biodiversification

The effect of plate interactions on surface processes, environmental changes of the earth and their contributions to biodiversification during Gondwana assembly has been hotly debated in the last decades. The late Neoproterozoic-early Paleozoic was a key period in the tectonic assembly of Gondwana, global climate change and metazoan evolution. During this interval, the amalgamation of Greater Gondwana was succeeded by the subduction initiation of oceanic slabs in the Proto-Tethys Ocean. Here, we use whole-rock geochemical data, detrital zircon U-Pb and Hf isotopic of Cambrian and Ordovician sedimentary rocks from the Baoshan block in the Southeastern Tibetan Plateau combined with existing data in the literature to constrain the timing of subduction initiation of the Proto-Tethys Ocean at 540-500 Ma. The results show that the Baoshan block, adjacent to the Pinjarra Orogen on the northern margin of India, witnessed the transition from passive to active continental margin in East Gondwana. After the subduction initiation, the source of the sedimentary rocks experienced more intense weathering, causing a higher sediment flux. Contemporaneously, the Cambrian Explosion and stepwise increase in atmospheric oxygen accompanied the Greater Gondwana assembly at the late Neoproterozoic-early Paleozoic boundary. Subduction of the Proto-Tethys Ocean and occurrence of peripheral orogens further increased atmospheric O2 content, transported nutrients to the ocean and reduced the greenhouse effect, which provided major propulsion for biological evolution.

Provenance of Devonian–Carboniferous strata of Colorado: The influence of the Cambrian and the Proterozoic

ABSTRACTWe report new LA-ICP-MS U–Pb detrital zircon ages and sedimentary petrology of silty to sandy limestones and dolostones, as well as calcareous to dolomitic sandstones of the Devonian–Carboniferous (Mississippian) Chaffee Group. We also report new detrital zircon ages from the late Cambrian Sawatch Quartzite, and a U–Pb zircon crystallization age on a late Mesoproterozoic (1087.9 ± 13.5 Ma) granitoid of underlying basement from the Eagle Basin of northwest Colorado. Grain populations in the Chaffee Group are mostly bimodal. More than 84% of zircons centered around a Paleoproterozoic (ca. 1.78 Ga) mode typical of the Yavapai province that forms much of the basement of Colorado and an early Mesoproterozoic (ca. 1.42 Ga) mode typical of A-type granites that intrude this region. A notable late Mesoproterozoic (ca. 1.08 Ga) mode exists in some Chaffee samples, giving those samples a trimodal detrital zircon age distribution. These bipartite or tripartite detrital zircon age modes exist in Cambrian, Devonian, and Carboniferous strata from paleogeographically adjacent successions, but the correlation between the Chaffee zircons is highest with the region’s basal Cambrian sandstones of the Sawatch Quartzite, Flathead Sandstone, and Ignacio Quartzite, which have similar (ca. 1.08 Ga, 1.43 Ga, 1.70 Ga, respectively) zircon populations, and a paucity of > 1.8 Ga grains. This similarity suggests that most grains in the Chaffee Group derive from recycling of these basal sandstones, and that little sediment was derived directly from thenexposed Precambrian basement highs, from the Wyoming craton to the north, or from Paleoproterozoic arcs and orogens to the west and northeast. Minor Mesoarchean to early Paleoproterozoic (ca. 3.00 to 2.40 Ga) grains exist in the Chaffee Group, an attribute shared by the Late Ordovician Harding Sandstone of Colorado’s Front Range, but that is absent from the region’s underlying Cambrian sandstones—suggesting some recycled mixture of Cambrian and Ordovician sedimentary rocks. No near-depositional age grains are present in the Chaffee Group. The youngest grain is Early Devonian (~417 Ma), > 45 million years (m.y.) older than these strata. Additionally, Paleozoic grains are extremely uncommon (< 0.1%; n = 2,927 grains).

Middle Ordovician (Whiterockian) gastropods from central Sonora, Mexico: affinities with Laurentia and the Precordillera

AbstractThe Lower–Middle Ordovician (Ibexian, Whiterockian) sedimentary rocks exposed at Rancho Las Norias includes the informally named Las Norias formation, which consists of an intercalation of carbonate and clastic sediments with abundant marine fauna. These deposits occur as the most austral sedimentary rocks of Ordovician age for Laurentia, providing a critical link to understand the distribution of Ordovician marine faunas of North America. An investigation of the gastropod fauna from the upper portion of the Las Norias formation, Sonora, Mexico, is undertaken for the first time. The gastropod assemblage includes Maclurites acuminatus, ?Monitorella sp., Lecanospira sp., Malayaspira aff. M. rugosa, Lophospira perangulata, and Hormotoma? sp. This assemblage indicates a paleogeographic relationship with Laurentia, including the USA (Nevada), Canada (British Columbia, Newfoundland), Greenland, and the Argentine Precordillera.

Provenance and ore-forming process of Permian lithium-rich bauxite in central Yunnan, SW China

Permian lithium-rich karstic bauxites are newly discovered in central Yunnan province, western margin of the South China Block, with a reserve of 16 million tonnes, which has a great potential for aluminum and lithium exploitation. Provenance and ore-forming processes are still debatable. Mineralogical, geochemical, and geochronological analyses were conducted on two typical bauxite profiles to explore the genetic process of the Permian bauxites in central Yunnan province. Profile A contains three layers from bottom to top: clayey bauxite, claystone, pisolitic bauxite. Profile B contains five layers from bottom to top: claystone, pisolitic bauxite, claystone, pisolitic bauxite, and mottled claystone. Pisolitic bauxite mainly consists of diaspore and chamosite, with small amounts of goethite, anatase, and boehmite. Clayey bauxite dominantly consists of boehmite and kaolinite, with small amounts of diaspore and anatase. Claystone predominantly consists of kaolinite or illite. Three stages of bauxite formation are identified: (1) during weathering of regional sedimentary strata, boehmite, kaolinite, and illite were formed; (2) during bauxite deposition, diaspore, chamosite, anatase, pyrite, with small amounts of illite, kaolinite, and barite were formed; (3) during second exposure caused by paleo-uplift, goethite was formed. Correlation analyses demonstrate that kaolinite is the most likely host mineral of Li in the studied bauxites and the leaching process after bauxite deposition can significantly reduce the lithium contents. Two profiles’ differences in sequence can be attributed to their different locations in the paleokarst terrain. Topography of karst terrain controls the ore-forming conditions, thus affecting the sequence and mineralogy of bauxite profiles. In our case, during the bauxite deposition, the uplifted area was not submerged at first, then as the groundwater rose, the uplifted area was immersed. Therefore, the ore-forming conditions in uplifted areas changed from oxidizing to reducing and became more alkaline. In contrast, the karstic depression was submerged throughout the bauxite deposition. All layers in the karstic depression were formed under reducing and alkaline conditions. Data from U-Pb isotope measurements of detrital zircons from bauxites in central Yunnan province provides 3 main ages, which are 500–650 Ma, 800 Ma, and 980 Ma. There are several minor peak ages in the age spectrum, which are 288–310 Ma, 1500 Ma, 1800 Ma, 2000 Ma, and 2500 Ma. Age distribution pattern of bauxites is more similar to that of the Ordovician sedimentary units than other regional rocks in central Yunnan province, which indicates the major contribution of the eroded Ordovician sedimentary rocks to bauxites.

Terahertz Dating of Sedimentary Rocks

The depositional products of sedimentary rocks provide vital references for investigating the paleoenvironment, paleogeography, and tectonic evolution history. However, the detection methods of geological evolution are still relatively complicated, and how to combine geological age and geological evolution is confusing. Based on the sensitivity of terahertz waves to organic matter, a THz dating method was introduced for characterizing the geological age of sedimentary rocks. In this study, the geological evolution of Liujiang Basin was analyzed by using terahertz time domain spectroscopy (THz-TDS). According to the close relation between organic matter content and sedimentary environment, it can be inferred that the geological deposition in this area is affected by Marine cover. In addition, the refractive index of Ordovician sedimentary rocks is significantly higher than that of other sedimentary rocks. Based on these results, it is inferred that the sedimentary environment of the Liujiang Basin gradually changed from continental deposition to deep-water marine deposition from the Neoproterozoic to the Ordovician, and the sea water gradually retreated due to the crustal movement, resulting in a transition from deep-water deposition to continental inshore ocean facies deposition. These findings are highly consistent with the geological history of the study area. Combined with principal component analysis (PCA) technology, the relative geological age of sedimentary rocks can be divided. Our study con-firmed the reliability of this THz dating technique, which provides an effective way to study the geological evolution history of sedimentary rocks.

Detrital zircon ages, provenance and tectonic evolution in the early Paleozoic of Tasmania and Waratah Bay, Victoria

The provenance of the upper Cambrian to Upper Ordovician sedimentary rocks of Tasmania and Waratah Bay in southern Victoria provides information about the complex and dynamic tectonic environment present during their deposition. This paper uses U–Pb detrital zircon data to constrain stratigraphic comparisons and tectonic reconstructions of these rock sequences. Multivariate statistics are used to investigate the similarity between the U–Pb ages and quantify the disparity among different samples from various locations. In western and central Tasmania, the Tyennan region supplied most detrital zircons during the late Cambrian and Early Ordovician. The overlying Middle Ordovician Pioneer Sandstone records a switch in provenance with zircons derived from the Mount Read Volcanics (MRV) mixed with zircons similar to those from continental-derived Paleozoic sedimentary rocks deposited throughout east Gondwana. The Middle to Upper Ordovician Gordon Group in western and central Tasmania lacks detrital zircons younger than 1.2 Ga, which indicates a return to a local provenance from Precambrian rocks. In southern Tasmania, the switch to zircons derived from the MRV and east Gondwana-like sources occurred earlier within the Cambrian Deadmans Bay Formation, which is dominated by the east Gondwana Paleozoic zircon age signature. In the East Tasmania Terrane, Ordovician sedimentary rocks from Lefroy have detrital zircon populations dominated by Neoproterozoic and earliest Paleozoic sources similar to the Ordovician sedimentary rocks in the Lachlan Orogen. In southern Victoria, the Bear Gully Chert from Waratah Bay exhibit both Tyennan and distal Gondwana detrital sources. The switching of detrital zircon sources in the west Tasmanian sedimentary sequences implies the docking of Tasmania with mainland Australia during the Cambrian Tyennan Orogeny. The arrival of the distal zircons into these basins occurred at different times in the different areas, reflecting a complex local topography and paleogeography. Key Points Paleozoic sedimentary rocks in Tasmanian exhibit multisource detrital U–Pb age signatures that change over time, implying tectonic activity during their deposition. Paleozoic sedimentary rocks in northeastern Tasmania show Gondwana-wide detrital signatures similar to Lachlan Orogen. The Ordovician Bear Gully Chert at Waratah Bay in southern Victoria shows mixed Tasmanian and distal Gondwana detrital populations. The change in detrital zircon signature in western Tasmania suggests that VanDieland docked with the Australian continent during the Cambrian Tyennan Orogeny.

Provenance of Late Ordovician sedimentary rocks in the SE Yangtze block: Implications for deposition in an active continental margin

The Jiangnan Orogen separates the Yangtze block from the Cathaysia block to the southeast. Several competing models have been proposed for the evolution of both blocks and the intervening orogen, which mainly focus on aspects of igneous and metamorphic rocks. Limited research has been conducted on the sedimentary rocks in this region. Provenance of the Late Ordovician sedimentary rocks in the northeastern Jiangnan Orogen has been constrained based on systemic analyses of petrology, heavy mineral assemblages, geochemistry, and detrital zircon U-Pb ages, which provide vital clues for reconstructing the evolution of this belt during the early Paleozoic. Late Ordovician deposits in the northeastern Jiangnan Orogen consist of conglomerates, sandstones, siltstones, shales, calcareous mudstones, and limestones. These strata were deposited in a shallow to deep sea environment, showing water depth deepening to the northwest. The sandstone contains lithic graywackes and feldspar-lithic graywackes, with low compositional and textural maturities. They consist, on average, of quartz clasts (27%), feldspar clasts (18%), and lithic fragments (55%). The lithic fragments are angular, including andesites, rhyolites, tuffs, granites, slates, phyllites, quartzites, siltstones, and chert, which also occur in conglomerate form. Minor pyroxene, chromite, magnetite, and garnet grains occur in the sandstone. Geochemical analyses show that the sandstone and siltstone have widely variable Fe2O3T + MgO (4.43–12.05%), Cs, V, Ni, and Ti contents. They also exhibit a distinct negative Nb-Ta trough compared to the average upper continental crust and have light rare earth element (LREE) enrichment patterns with obvious Eu anomalies, similar to subduction-related rocks in an arc setting. Detrital zircon U-Pb ages demonstrate a mixed source from ca. 784 Ma and ca. 455 Ma. Paleocurrent indicators point to a southeastward origin. These results demonstrate that Late Ordovician sedimentary deposition occurred in a northwestward-facing active continental margin along the southeastern Yangtze block.

Magnetic Anisotropy of Rocks: A New Classification of Inverse Magnetic Fabrics to Help Geological Interpretations

AbstractInverse magnetic fabrics defined by anisotropy of magnetic susceptibility with the longest principal axis perpendicular to bedding were identified in the studied samples of Ordovician sedimentary rocks from the Barrandian area, Czech Republic. These fabrics are related to ankerite fibers in association with cone‐in‐cone structures. To understand the meaning of such inverse magnetic fabrics for geological interpretations, a new, more detailed classification of inverse magnetic fabrics' types is formulated in this paper. Distinguishing the structural (i.e., shape) and magnetocrystalline anisotropies in rocks, three new (sub)types of magnetic fabrics were defined in this study based on the combination of structural normal/inverse and magnetocrystalline normal/inverse anisotropies. As a result, normal magnetic fabric is extended by new fabric types associated with inverse anisotropy: simple inverse, pure inverse, and pseudo‐normal magnetic fabrics. The studied fibrous ankerite shows simple inverse magnetic fabric, which results from the combination of normal magnetocrystalline and inverse structural anisotropies.

Large mass-independent sulphur isotope anomalies link stratospheric volcanism to the Late Ordovician mass extinction

Volcanic eruptions are thought to be a key driver of rapid climate perturbations over geological time, such as global cooling, global warming, and changes in ocean chemistry. However, identification of stratospheric volcanic eruptions in the geological record and their causal link to the mass extinction events during the past 540 million years remains challenging. Here we report unexpected, large mass-independent sulphur isotopic compositions of pyrite with Δ33S of up to 0.91‰ in Late Ordovician sedimentary rocks from South China. The magnitude of the Δ33S is similar to that discovered in ice core sulphate originating from stratospheric volcanism. The coincidence between the large Δ33S and the first pulse of the Late Ordovician mass extinction about 445 million years ago suggests that stratospheric volcanic eruptions may have contributed to synergetic environmental deteriorations such as prolonged climatic perturbations and oceanic anoxia, related to the mass extinction.

Nová data k výzkumu tvrdýitu z Krušné hory u Berouna (Česká republika)

During the revision of beraunite like minerals from old localities represented by historical samples in the mineralogical collection of the National Museum in Prague, the second occurrence of tvrdýite was confirmed in the iron ores from abandoned iron deposit Krušná hora in the Central Bohemia (Czech Republic). Krušná hora is situated about 12 km NW of Beroun (30 km WSW of Prague) in an area of the Ordovician sedimentary rocks of the Teplá-Barrandian unit. Tvrdýite forms brown-green, yellow-green to light-green radial aggregates up to 3 mm in size. Tvrdýite is monoclinic, space group C2/c with following unit-cell parameters refined from the X-ray powder diffraction data: a 20.529(9), b 5.105(2), c 18.869(8) Å, β 92.8(4)° and V 1975.1(8) Å3; its empirical formula is (Na+0.13Fe2+0.86Mg2+0.01)Σ1.00(Al3+2.39Fe3+0.57)Σ2.96(Fe3+1.93Al3+0.03)Σ1.96[(PO4)3.99(VO4)0.01]Σ4.00(OH)4.61(OH2)4.00·2H2O. The mineral was found in association with smaller bluish radial aggregates of unidentified Fe-Al phosphate (probably Al-rich dufrénite or different generation of tvrdýite).

Provenance investigation for the Cambrian–Ordovician strata from the northern margin of the western Yangtze Block: implications for locating the South China Block in Gondwana

AbstractWe report new U–Pb isotopic data for detrital zircons from Cambrian–Ordovician strata on the northern margin of the western Yangtze Block, which together with published U–Pb isotopic data for coeval strata in the South China Block, provide critical constraints on the provenance of these sediments and further shed light on the early Palaeozoic position of the South China Block in the context of Gondwana. Detrital zircons in this study yield four major age peaks in the early Palaeoproterozoic, early Neoproterozoic, middle Neoproterozoic and late Neoproterozoic – early Palaeozoic. The dominant age population of 900–700 Ma matches well with magmatic ages from the nearby Panxi–Hannan Belt, which indicates that Cambrian–Ordovician sedimentary rocks in the western Yangtze Block were mainly of local derivation. However, compilations of detrital zircon ages for the Cambrian–Ordovician strata from the Cathaysia Block and the eastern Yangtze Block show that both blocks are dominated by late Mesoproterozoic- and early Neoproterozoic-aged detrital zircons, which suggests a remarkable exotic input with typical Gondwana signatures. According to the integrated detrital zircon age spectra of the Cambrian–Ordovician sedimentary rocks from the entire South China Block and palaeocurrent data, the South China Block should have been linked with North India and Western Australia within East Gondwana. Specifically, the Cathaysia Block was located adjacent to Western Australia, while the Yangtze Block was connected with North India.

Interpretation and significance of combined trace element and U–Pb isotopic data of detrital rutile: a case study from late Ordovician sedimentary rocks of Saxo-Thuringia, Germany

The U–Pb age and trace element composition of detrital rutile provide information about the metamorphic history of the source region that cannot be constrained by traditional U–Pb dating of detrital zircon. Previous provenance investigations focussed on only one of these methods. Based on a large LA-ICP-MS trace element and U–Pb isotopic dataset of detrital rutile and U–Pb isotopic data of detrital zircon from late Ordovician sedimentary rocks of Saxo-Thuringia, Germany, this paper discusses the application and significance of combining these methods in provenance investigations. U–Pb age spectra from the detrital zircons analysed show multiple age components (multimodal age spectra) in all samples. This is in contrast with the detrital rutile data, as only one sample yielded a multimodal U–Pb age distribution. Multimodal age spectra of detrital rutile are most likely preserved in sediments from (large) catchment areas with complex geological histories. They may also be related to specific sedimentation events, such as glacial washout during the retreat of large ice shields (e.g. the Hirnantian glaciation of Gondwana). Unimodal age spectra are however not restricted to small catchment areas, if the provenance region is characterized by a pervasive thermal overprint such as the Pan-African orogeny throughout Gondwana. Unimodal age distributions may further consist of overlapping age cluster detectable by different trace element composition of the detrital rutile grains. The combined U–Pb age and trace element data from detrital rutile grains demonstrate that rutile sourced from metapelitic rocks yield reliable and precise U–Pb ages. In contrast, detrital rutile classified to be of metamafic origin generally has too low uranium concentrations to be dated reliably by LA-ICP-MS. Detrital rutile records low- to medium-grade metamorphic events in the source region and therefore has the potential to better constrain the maximum depositional age of sedimentary rocks in comparison to U–Pb dating of detrital zircon.