Phanerozoic Age Research Articles

Современное представление о промышленной классификации колчеданно-полиметаллических месторождений Рудного Алтая

There are many classifications of polymetallic deposits. Many researchers, based on the genetic relationship of the deposits, attribute the deposits of the Rudny Altai to the VMS Kuroko type. Other authors classify polymetallic deposits of the Rudny Altai, relying on geodynamic conditions and modes of their formation. Such a variety of classifications makes it difficult to understand and use these classifications for various kinds of research, and questions also arise when comparing with foreign analogues. Therefore, the authors of this article propose to consider the modern unified classification of deposits of the Ore Altai according to J.W. Lydon (1988), based on the division of polymetallic deposits of VMS by volume ratio Zn/(Zn+Pb) into 2 main industrial genetic types (Zn-Pb-Cu and Cu-Zn). The first type (Zn-Pb-Cu) has a mass ratio of Zn/(Zn+Pb) from 0.70 to 0.80; the second type (Cu-Zn) is more than 0.95. Practically all deposits of the Ore Altai can be attributed to the Zn Pb-Cu group, as having mass ratios of Zn/(Zn+Pb) from 0.64 to 0.86 and localized in medium to acidic calcareous-alkaline volcanic rocks mainly of Phanerozoic age

First Ce-Nd isotope measurements of middle and lower continental crust samples support massive lower crust recycling over Earth's history

Long-lived radioactive isotopes provide valuable information on the evolution of Earth's geological reservoirs. Coupled measurements of the 138La-138Ce and 147Sm-143Nd systems have attracted much interest, but some critical reservoirs, such as the deep continental crust, have yet to be investigated. To address this gap, we report Ce-Nd isotope measurements of 35 crustal samples from Canada, western Europe, and Siberia, with Archean to Phanerozoic ages. Most samples from western Europe and Siberia are interpreted to represent the deepest continental crust because of low SiO2 content (< 55 wt%) and generally mafic compositions. In contrast, Precambrian composites from the Canadian shield have highly felsic compositions (61–71 wt% SiO2) and represent the old upper continental crust. Xenoliths from the Massif Central (France) and samples from uplifted massifs in the Ivrea-Verbano zone (Italy) plot on the Ce-Nd mantle array. Precambrian Canadian composites and xenoliths from Udachnaya (Siberian Craton) plot to the left of the mantle array, with the Siberian samples farthest from the array. The La-Ce system yields a ca. 1.8 Ga age for the Siberian xenoliths, distinct from the ca. 2.7 Ga Sm-Nd errorchron age. The La-Ce system was most likely reset during a large-scale episode of delamination and rejuvenation of the Archean lower lithosphere established in Siberia. A change in the La/Ce ratio during this event explains an increasing deviation with time of the Ce-Nd isotopic composition of the Siberian deep crust from the mantle array. We introduce a new parameter (θ) to quantify the likelihood of any rock evolving away from the mantle array. It represents the angle between the evolution vector of a sample and the mantle array, and it is calculated from measured La/Ce and Sm/Nd ratios. Samples that plot to the right of the mantle array have θ > 0, whereas samples that plot to the left of the mantle array have θ < 0. The angle θ was used to compare our samples to a worldwide granulite compilation (1581 samples) because it is independent of the age and the initial isotopic composition of the samples. The majority of lower crust rocks have negative θ, suggesting they are prone to evolve to the left of the mantle array, but to a lesser extent than the Siberian samples that are characterized by the most negative θ values. Thus, the Siberian samples with the most unusual Ce-Nd isotopic compositions are the only group to plot close to the deep crust end-member calculated from classical mass-balance budget of the bulk silicate Earth. These results suggest that the lower crust sampled by Siberian xenoliths is only a minor component in the terrestrial deep crust. We use this database to estimate new parent/daughter ratios for the lower continental crust: 138La/142Ce = 0.00380 ±0.00007 and 147Sm/144Nd = 0.128 ±0.007. Measurements and models are finally reconciled when considering massive recycling of lower continental crust through Earth's history, totaling 3 to 4 present-day, continental crust masses.

Iron oxidation state in serpentines and magnesian chlorites of subduction-related rocks

Abstract. The ferric iron content in hydrothermally altered ultrabasic rocks and their major minerals, serpentines and Mg-chlorites, is important for establishing the oxidation state budget from oceanic ridges to subduction zones, in carbonaceous chondrites, and for modeling phase equilibria. A compilation of literature Mössbauer spectroscopic data on serpentines and magnesian chlorites from high-pressure ophiolites yields much lower ferric-to-total-iron ratios (Fe3+ / Fetotal) than those obtained on similar samples by X-ray absorption near-edge spectroscopy (XANES), leading to contradictory estimates of the ferric iron budget of subduction zones. New Mössbauer analysis of antigorite and Mg-chlorite samples from suites of high-pressure ophiolitic terrains of various Phanerozoic ages confirms the low and homogeneous values previously obtained by this technique, while lizardite inherited from oceanic hydrothermal alteration is ferric iron rich. We argue that XANES values may be biased by photo-oxidation when samples have a high Mg content, which is the case for serpentines and chlorites from subduction zones. Photo-oxidation is less important in Fe-poor phyllosilicates of the mica and talc families and does not affect the Fe-rich serpentines (greenalite, cronstedtite) of meteorites or Fe-rich terrestrial phyllosilicates. Mössbauer Fe3+ / Fetotal ratios of serpentine confirm the occurrence of a major redox change at the lizardite–antigorite transition near 300–400 ∘C rather than at the dehydration of antigorite at 500–650 ∘C in serpentinites from high-pressure ophiolites.

Tracing tectonic processes from Proto- to Paleo-Tethys in the East Kunlun Orogen by detrital zircons

The East Kunlun Orogen (EKO), a crucial Tethys composite accretionary belt in the northern Tibetan Plateau, experienced a polyphase tectonic history during an orogenic evolution from the Proto-Tethys to the Paleo-Tethys oceans. It is thus an ideal natural laboratory for tracing the Tethys orogenic cycles. Although numerous studies have been done on the geologic evolution of the East Kunlun Orogen, the timing of initial subduction, final closure, and the transition from the Proto-Tethys to the Paleo-Tethys still remain controversial. This study presents U-Pb-Hf isotopic analyses of 794 detrital zircons from three modern river drainage samples and two sedimentary samples in the EKO. The analysed zircons cluster in five age groups: 2900–2100 Ma, 1900–1300 Ma, 1200–730 Ma, 550–370 Ma, and 290–200 Ma, in which almost 67 % are of Phanerozoic age. The εHf(t) values of the 550–370 Ma detrital zircons have a broad range from −28.8 to + 15.9 and exhibit a trend towards positive values, suggesting that subduction of the Proto-Tethys Ocean initiated at ca. 550 Ma and accretion of the continental arc occurred with increasing addition of mantle material. The εHf(t) values (−8 to + 13) of 290–200 Ma detrital zircons have no obvious evolutionary trend, reflecting the continuous crust-mantle interaction during the evolution of Paleo-Tethys arc. Our results, combined with previous research, reveal two discontinuous and distinct orogenic cycles from the Proto-Tethys to the Paleo-Tethys in the EKO: the Proto-Tethys orogenic cycle from oceanic subduction to continental collision and orogenic collapse at ca. 550–370 Ma, and orogenesis of the Paleo-Tethys Ocean at ca. 290–200 Ma in the EKO, with a gap of ∼ 80 Myr, during which time there was no subduction, and the extensional setting resulted in rare identifiable zircon record.

Sediment provenance of the Lulehe Formation in the Qaidam basin: Insight to initial Cenozoic deposition and deformation in northern Tibetan plateau

AbstractUnravelling early Cenozoic basin development in northern Tibetan Plateau remains crucial to understanding continental deformation mechanisms and to assessing models of plateau growth. We target coarse‐grained red beds from the Cenozoic basal Lulehe Formation in the Qaidam basin by combining conglomerate clast compositions, paleocurrent determinations, sandstone petrography, heavy mineral analysis and detrital zircon U–Pb geochronology to characterize sediment provenance and the relationship between deformation and deposition. The red beds are dominated by matrix‐supported, poorly sorted clastic rocks, implying low compositional and textural maturity and short transport distances. Although most sandstones have high (meta)sedimentary lithic fragment contents and abundant heavy minerals of metamorphic origin (e.g., garnet, epidote and chlorite), spatiotemporal differences in detrital compositions are evident. Detrital zircon grains mainly have Phanerozoic ages (210–280 Ma and 390–480 Ma), but Proterozoic ages (750–1000 Ma, 1700–2000 Ma and 2300–2500 Ma) are also prominent in some samples. Analysed strata display dissimilar (including south‐, north‐ and west‐directed) paleocurrent orientations. These results demonstrate that the Cenozoic basal deposits were derived from localized, spatially diverse sources with small drainage networks. We advocate that initial sedimentary filling in the northern Qaidam basin was fed by parent‐rocks from the North Qaidam‐South Qilian belts and the pre‐Cenozoic basement within the Qaidam terrane interior, rather than southern distant Eastern Kunlun regions. Seismic and drilling well stratigraphic data indicate the presence of paleohighs and syn‐sedimentary reverse faults and noteworthy diversity in sediment thickness of the Lulehe Formation, revealing that the Qaidam terrane exhibited as several isolated depocenters, rather than a coherent basin, in the early stage of the Cenozoic deposition. We suggest the Cenozoic Qaidam basin to have developed in a contractional deformation regime, which supports models with synchronous deformation throughout most of Tibet shortly after the India‐Eurasia collision.

Provenance and metamorphic records of forearc sediments from the Heilongjiang Complex, NE China: Implications for subduction erosion associated with the Mudanjiang Ocean

Subduction erosion is widely thought to play a significant role in crustal recycling at modern convergent plate margins. However, identifying subduction erosion in fossil accretionary margins is difficult due to multiple episodes of tectonic superimposition and an absence of direct geophysical evidence. By assessing provenance and metamorphic records in this study, we are the first to document the record of subduction erosion in Late Triassic to Middle Jurassic meta-sedimentary rocks from the Luobei Heilongjiang Complex, NE China. Most detrital zircons from four meta-sedimentary samples have typical core-rim textures. Approximately 90% of the detrital zircon cores yielded Phanerozoic ages with εHf(t) values of +12.8 to −17.6. Geochronological and isotopic comparisons indicate that the meta-sedimentary rocks have a provenance in the eastern Songnen Block and were deposited in a forearc basin. The youngest ages of detrital zircon cores span from 229 to 176 Ma, while the metamorphic ages obtained from detrital zircon rims range from 209 to 185 Ma. Combined with the 186−165 Ma phengite 40Ar-39Ar ages of the meta-sedimentary rocks, this suggests that their protoliths were deposited during the Late Triassic−Middle Jurassic. Mineral inclusions define peak blueschist-facies metamorphism at pressure-temperature conditions of 0.9−1.0 GPa and 359−365 °C; this indicates that forearc sediment from the overriding plate was abraded into the subduction channel and underwent high-pressure metamorphism. Metamorphic ages (209−165 Ma) obtained from phengite and detrital zircon rims suggest that a period of subduction erosion occurred during slab subduction. These results reveal rapid progression from early forearc sedimentation (229−165 Ma) to subduction erosion (209−165 Ma) during westward subduction of the Mudanjiang Ocean.

Syn-rift to post-rift tectonic transition and drainage reorganization in continental rifting basins: Detrital zircon analysis from the Songliao Basin, NE China

Tectonic transition from a syn-rift stage to subsequent post-rift stage is an important mechanism in the evolution of extensional basins. The sedimentary infill records the crustal response to this process. We have obtained new detrital zircon U-Pb and Lu-Hf signatures from the Lower Cretaceous stratigraphic successions encompassing the commonly accepted syn- to post-rift transition boundary, the T4 unconformity, in the Songliao Basin, NE China. These constrain the Songliao Basin’s evolution from its center to distal margins, providing insights into the sediment provenance and dispersal pattern over the tectonic transition. Analysis of zircons from the syn-rift (the Shahezi and Yingcheng formations) and immediate post-rift (the Lower and Middle Denglouku Formation) stages reveals Phanerozoic age populations with positive ƐHf(t) values, which were derived from the proximal juvenile mantle-derived melt origin bedrocks of the Songliao Block. In contrast, the overlying samples from the Upper Denglouku Formation deposited in the subsequent post-rift stage contain exotic and ancient zircon populations with ages of 2.5 Ga & 1.8 Ga and complex hafnium signatures, characteristic of a mixed origin. These are interpreted to be transported from distant cratonic terranes via larger drainage networks. It is obvious that the sediment dispersal pattern switched from being a local and hydrologically closed “intraregional” pattern to a “transcontinental” pattern during the transition. The time lag between the development of the T4 unconformity and the drainage reorganization also ensures a distinguishable ∼3 Myr (106103 Ma, Late Albian) transition period of regional extent. During this transition stage, syn-rift faulting was replaced by post-rift thermal subsidence, exhibiting a uniform sag configuration. Our new findings are important for understanding other continental rift basins during syn- to post-rift transition, which often demonstrates a complex interaction between the linkage and integration of sub-basins, and the reorganization of fluvial drainages and catchment systems.

Concepts and revised models for Phanerozoic orogenic gold deposits

Abstract Existing published models for orogenic gold deposits (OGDs) do not adequately describe or explain most deposits of Phanerozoic age, and there are numerous reasons why Phanerozoic OGDs might differ significantly from older deposits. We subdivide Phanerozoic OGDs into four main subtypes, based on a number of descriptive criteria, including tectonic setting, lithological siting, and characteristics of the mineralization in each subtype. The four subtypes are: (1) crustal-scale fault-associated (CSF) subtype, (2) sediment-hosted orogenic gold (SHOG) subtype, (3) forearc (FA) subtype, and (4) syn- and late tectonic dispersed (SLTD) subtype. Lead isotopic studies suggest that Pb and other metals in all but the FA subtype were likely derived from relatively small source reservoirs in the middle or upper crust. OGDs formed in large, lithologically and structurally homogeneous regions will tend to be of the same subtype; however, in geologically complex orogenic belts it is common to find two or more subtypes that formed at approximately the same time. Based on the synthesis of global OGDs of Phanerozoic age, districts containing CSF or SHOG subtype deposits appear to have the best potential for hosting multiple large deposits. FA subtype deposits form in a relatively uncommon tectonic setting (accretionary forearc, possibly overlying a subducting spreading ridge) and are likely to be rare. SLTD subtype OGDs are the most common, but most are small and uneconomic, although they commonly generate substantial alluvial gold deposits.

Archean versus Phanerozoic oceanic crust formation and tectonics: Ophiolites through time

• Global compilation of the salient features of ophiolites. • Lithological differences between Archean and Proterozoic/Phanerozoic ophiolites. • Subduction-dominant Archean oceanic crust with accretionary cycle plate tectonics. • For Proterozoic and Phanerozoic a combination of both accretionary cycle and Wilson cycles. A global compilation of structural, lithological, and geochemical data on a selection of Archean, Proterozoic and Phanerozoic magmatic complexes, interpreted as ophiolites, is presented. Ophiolites, based on Phanerozoic examples, can be classified into subduction-related and subduction unrelated categories. These categories can be further subdivided into several subtypes depending on the proximity to subduction zones, and on sequential development from rifting – drifting to seafloor spreading for the subduction-unrelated category. From bottom to top ophiolites exhibit a magmatic sequence of ultramafic rocks (upper mantle units), gabbros (layered and isotropic), basaltic dikes and lavas, as well as boninites and felsic dikes and lavas in the subduction-related types. Archean greenstone belts show large variations in their construction, but Eoarchean examples display identical structure and lithology to the Phanerozoic ophiolites, attesting to the operation of seafloor spreading and subduction zone processes in the early Earth's history. Lithological differences between the Archean and Proterozoic/Phanerozoic ophiolites are demonstrated in the common occurrence of komatiites and felsic rocks, and scarcity of sheeted dike complexes in the former, compared to the opposite situation in those ophiolites that are younger than ca. 2 Ga. Geochemically there is a concomitant decrease in the content of incompatible elements (e.g., Sr, Zr, Y, Nb) and an increase in the content of compatible elements (e.g., Mg, Cr, Ni). In terms of tectonic environment analyzes, the Archean ophiolites are more abundantly subduction-related than those of Proterozoic and Phanerozoic ages. The subduction-dominant Archean oceanic crust (ophiolites) was characterized by accretionary cycle plate tectonics, whereas for those of Proterozoic and Phanerozoic ages, a combination of both accretionary cycle and Wilson cycles plate tectonic processes was operative.

Establishing Formation Features of Limnogenic Minerals

This article considers the formation conditions of nonmetallic and ore minerals in limnogenic structures. Lakes naturally enrich a wide range of useful components – silicate, carbonate, water-soluble substances, ores, and organominerals. The most significant resources of modern lakes, apart from water, include sapropels, diatomites, lime, and mineral salts. Deposits of sand, clay, oil shale, oil and gas, coal, phosphorites, zeolites, evaporites, bauxites, ferromanganese, copper ores, placer minerals, and some rare and dispersed elements are associated with limnogenic complexes of different ages. The authors show that a greater variety of minerals of ancient lacustrine complexes compared to modern ones is associated both with variations of lacustrine lithogenesis in the past and with post-sedimentation transformations of sedimentary matter. A particular note is taken about the epigenetic enrichment of ancient lacustrine complexes with ore components. Hydrocarbons and diatomites of lacustrine genesis are of a higher quality compared to similar minerals of marine genesis. The evolutionary changes in the processes of accumulation of limnogenic minerals have affected biogenic and chemogenic components to a large extent. An example of this is the progressive accumulation of caustobiolites and the sulfate evolution of evaporites in lacustrine structures during the Phanerozoic age. An analysis of the genetic characteristics of lacustrine minerals makes it possible to develop new exploratory traits of some sedimentary deposits.

A SUMMARY OVERVIEW OF DEPOSITS AND LARGE OCCURENCES OF NOBLE PRECAMBRIAN CORUNDUM IN THE WORLD

The paper is a summary review of the deposits and occurrences of noble corundum in Precambrian ore regions of the world. The ore bodies of ruby and sapphire deposits are divided into two groups. The first group comprises magmatic, pegmatite, and metasomatic deposits, mainly of Phanerozoic age. The second group is mainly represented by Precambrian formations. The article considers the formation of noble corundum associated with Precambrian complexes within the borders of provinces, belts and ore regions, and provides a brief description of corundum-bearing complexes in the Precambrian from Neoarchean to Neoproterozoic, with references to the respective deposits or occurrences. We indicate the P-T setup for metamorphism during the formation of corundum deposits from Precambrian corundum-bearing metamorphic complexes aged from 2.7–2.8 Ga to 610 Ma.

Saqez–Sardasht Goldfield, North Sanandaj‐Sirjan Zone, Iran: A Tectono‐Metallogenic Synthesis

AbstractThe Sanandaj–Sirjan Zone (SSZ), as the metamorphic‐magmatic core of the Zagros Orogen in southwestern Iran, contains several styles of gold deposit of Phanerozoic age. The northern SSZ includes an ENE‐trending goldfield belt. This area that encompasses the main orogenic gold deposits, e.g., Qolqoleh, Kervian, Qabaqhlujeh, and the Barika VMS goldfield, was chosen for this research to study the spatial and temporal relationships between gold mineralization and orogenic phases. Regarding the rock unit variations, metamorphism, magmatism and the settings of the structures, the study area is divided into four distinct tectonic blocks, separated by three main NW‐trending thrust faults (suture lines) including, from NE to SW, the Tamugheh, the Ebrahim Hesar and the Zagros main thrust (ZMT) faults. The area between the Tamugheh and Ebrahim Hesar faults is a tectonized/uplifted basement of accretionary wedge‐originated thrust slivers, hosting the above orogenic gold mineralizations. The other area between the here termed Ebrahim Hesar fault and the ZMT is an island‐arc basin, proposed here as the Sardasht–Barika zone, including the only recognized massive sulfide gold district all over the SSZ, named Barika. The Barika goldfield was metamorphosed, deformed and enriched due to the island‐ arc collision to the Arabian continent, before the closure of Neotethys on the eastern flank.

Petrogenesis of a rare Ediacaran tonalite–trondhjemite–granodiorite suite, Egypt, and implications for Neoproterozoic Gondwana assembly

AbstractMost tonalite–trondhjemite–granodiorite (TTG) suites are Archean–Palaeoproterozoic in age, but those of Neoproterozoic–Phanerozoic age are scarce. A rare Ediacaran high-Al TTG suite has been identified at the Fannani Igneous Complex (FIC) in the northern Arabian–Nubian Shield, which is essentially composed of amalgamated Neoproterozoic island-arc Pan-African composite terranes that contain several ophiolitic sutures. The FIC exhibits a wide range of SiO2, Al2O3, Sr and Zr, shows moderate rare earth element (REE) enrichment, and K, Ti, Nb, Y and heavy REE depletion. It is a subsolvus suite with clear orogenic affinities and strong arc-geochemical signatures. The precise U–Pb zircon thermal ionization mass spectrometry age obtained (607.4 ± 1.95 Ma) indicates oceanic subduction extended to late stages of the East African Orogeny. The FIC exhibits87Sr/86Sr compositions of 0.70346–0.71091 (Sr(i)ratio, 0.70284), and143Nd/144Nd of 0.51254–0.51270 (ϵNd(t) = +5.12 to +7.16), typical of modern oceanic-arc rocks (as Japan-arc basalts), and suggestive of mantle sources and island-arc settings. The FIC possesses low values of Yb (1.55 ppm), Nb (14 ppm) and Y (24 ppm), and high ratios of Sr/Y (27), Zr/Sm (46) and Nb/Ta (11.8), typical of magmas produced by anatexis of a basaltic slab. Partial melting models show that the FIC magma was generated by melting (F= 0.25–0.50) of a subducted oceanic crust transformed into eclogite, leaving 10–25% garnet in the residue. The FIC and similar complexes produced via slab melting during the closure of the Mozambique Ocean formed large juvenile belts along the East African Orogen that sutured East and West Gondwana together into a united supercontinent.

Provenance changes across the mid-Cretaceous unconformity in basins of northeastern China: Evidence for an integrated paleolake system and tectonic transformation

Abstract Detrital zircon U-Pb dating and whole-rock Nd isotopic analyses were carried out on selected stratigraphic horizons across a major unconformity between synrift and postrift stages in the Songliao Basin and Dasanjiang basin group of NE China to constrain the crustal evolution of the source area providing detritus into these basins. The strata underlying the mid-Cretaceous unconformity in the Songliao Basin show regionally distinct detrital zircon age populations and Nd isotopic compositions, which generally are characterized by Phanerozoic age peaks and relatively depleted Nd isotopic compositions, indicating derivation from nearby highlands. In contrast, the overlying strata are dominated by Proterozoic zircon ages and enriched Nd isotopic compositions, which imply that the provenance source region shifted to the northern part of the North China craton. A coeval provenance change also affected the sedimentary architecture in the eastern Dasanjiang basin group, marking the migration of erosion centers from west to east. The contribution from Lesser Xing’an–Zhangguangcai ranges was pronounced during deposition of synrift strata but became negligible afterward in the Songliao and Dasanjiang areas, which is consistent with both basin complexes temporarily forming an extensive lake system during the early Late Cretaceous. This paleolake was likely responsible for transgressive events recorded in the Late Cretaceous strata of basins in NE China. Combining observations from seismic reflection profiles and the stratigraphic record of neighboring intracontinental sedimentary basins as well as widespread contemporaneous exhumation and denudation events, we suggest that the provenance variation in basin strata was controlled by large-scale tectonic transitions in East Asia. The switch from extension to contraction during the mid-Cretaceous is attributed to the docking of the Okhotomorsk block along the East Asian continental margin. The resulting lithospheric buckling might have been responsible for reshaping the basin-and-range configuration in NE Asia.

Constraints on the early Mesozoic denudation of the Qinling orogen from Upper Triassic-Lower Jurassic successions in the Zigui Basin, central China

The Qinling mountains are situated in central China and mark the collisional assembly between the North and South China blocks. To understand the early Mesozoic evolution of the Qinling orogenic belt, we present new paleocurrent data, sandstone petrology and detrital zircon U-Pb ages for Upper Triassic-Lower Jurassic sedimentary successions in the Zigui Basin in the northern margin of the South China block. The deltaic sandstones in the Upper Triassic Jiuligang Formation have high concentrations of quartz grains and the basal alluvial conglomerates of the Lower Jurassic Tongzhuyuan Formation contain abundant chert gravels with sandy matrixs rich in quartz grains. Both the Upper Triassic sandstones and Lower Jurassic conglomerate have two major detrital zircon U-Pb age groups of 2100–1700 Ma and 950–700 Ma with very few Phanerozoic ages. The overlying fluvial sandstones of the Tongzhuyuan Formation display relative enrichment in volcanic and metamorphic rock fragments. They are characterized by primary zircon U-Pb age groups of 250–200 Ma and 950–700 Ma. Consistent with the northwestward paleocurrents, zircon grains of 2100–1700 Ma and 900–700 Ma were likely derived directly or recycled from the Paleoproterozoic and Neoproterozoic basement of the South China Block, with the latter also possibly sourced from the Qinling orogen. The 250–200 Ma age group is understood to be associated with volcanic rock fragments and, considering the southward paleocurrents, indicates a volcanic source within the Qinling orogen. Along with the increased low-grade metamorphic rock fragments in sandstones, this significant provenance change in the Early Jurassic indicates the initial massive erosion and rapid southward propagation of the south Qinling orogen.

High-temperature kinetic isotope fractionation of calcium in epidosites from modern and ancient seafloor hydrothermal systems

Calcium isotope ratios in epidote from epidosites in ophiolites of varying Phanerozoic ages have 44Ca/40Ca ratios that are lower by 0.1 to 0.6‰ relative to typical mid-ocean ridge hydrothermal fluids. Epidosites are inferred to form in high-temperature parts of seafloor hydrothermal systems at temperatures above 300°C and where fluid fluxes are high, so the Ca isotopic composition of the epidote is likely to reflect fractionation during growth of crystals from aqueous solution. Available Ca isotope data from MOR hydrothermal vent fluids and mantle peridotites constrain the δ44Ca of likely modern hydrothermal fluids to a narrow range at δ44Ca=−0.05±0.1. A reactive-transport model is used to evaluate whether the δ44Ca of hydrothermal fluids might have been higher during the Cretaceous and Late Cambrian, the ages of the Troodos, Oman, and Betts Cove ophiolites from which we have data. For these calculations we use the epidosite 87Sr/86Sr as a guide to the extent of Ca isotopic exchange that affected the ancient hydrothermal fluids, which were derived from seawater with higher Ca and Sr, and lower sulfate concentration, than modern seawater. The calculations suggest that the ancient hydrothermal fluid δ44Ca values were not much different from modern values, with the possible exception of the Late Cambrian example. We infer that the epidote-fluid Ca isotope fractionation averaging Δ44Ca=−0.2 to −0.6, is most likely due to kinetic isotope fractionation during mineral precipitation. There is evidence from the literature that hydrothermal epidote may commonly form from oversaturated solutions, which makes the kinetic isotope interpretation plausible. The equilibrium epidote-fluid Ca isotope fractionation is estimated to be Δ44Caeq≈0 based on recently reported DFT calculations. Our results suggest that kinetic calcium isotope fractionation can affect hydrothermal silicate minerals, and may be only slightly smaller in magnitude than the effects observed in Ca-bearing minerals at low temperature. Kinetic isotope effects during mineral growth could provide new insights into the formation mechanisms of hydrothermal silicate minerals.

Evidence for Late Triassic crustal suturing of the Central and Southern Pamir

The timing of closure of the Paleotethys and Rushan ocean basins and suturing of Gondwanan crustal fragments in the Pamir is not well resolved. Whereas the Central Pamir terrane is generally interpreted to have collided with the Northern Pamir terrane at the end of the Triassic, closure of the Rushan ocean and collision of the Southern Pamir terrane has been interpreted to be either broadly coeval (i.e., Late Triassic–Early Jurassic ~200 Ma) or have occurred significantly after closure of the Paleotethys in the Late Jurassic. New petrographic analyses and detrital zircon U-Pb data of the Lokzun Group Flysch and the terrigenous Darbasatash Group of the Southern Pamir terrane advocate for coeval terrane accretion of the Central and Southern Pamir terranes by the Latest Triassic. The Rhaetian Lokzun Group Flysch and the Early Jurassic Darbasatash Group have well-constrained stratigraphic ages and lie directly below and above a post-Cimmerian orogeny angular unconformity, respectively. Petrographic results for both successions indicate a recycled orogenic belt source, and detrital zircon age spectrum for both contain two prominent Phanerozoic age populations: an Early Silurian–Early Devonian (~440–408 Ma) and a Carboniferous Late Triassic (~350–222 Ma). These results suggest sediment was sourced from the Karakul-Mazar terrane, located in the northern Pamir, which requires closure of the Paleotethys and Rushan ocean basins and initial collision with the Central and Southern Pamir terranes by the end of the Late Triassic. Furthermore, these results suggest that the Northern Pamir terrane was topographically elevated relative to the surrounding terranes throughout the early Mesozoic.

Kimberlite Genesis According to Data on Hot Accretion of the Earth

Analysis of modern geological and planetological data indicates hot accretion of the Earth. Impact melting during its formation led to origin of layered global magma ocean. Kimberlites were from residual melts of its lower peridotite latyer. This explains high content of meltphylic (lithophylic) components in kimberlites. Prolonged cooling and crystallization of magma ocean from top to bottom in the Earth’s history is the cause of mainly the Phanerozoic age of kimberlites. Moving apart of lithosphere and products of magma ocean fractionation contained therein when oceanic areas are formed, explains absence of kimberlites in these areas and Clifford rule nature. Low temperature of kimberlite magmas and high content of volatile component in them was caused by formation from residual melts. Increased temperature of crystallization of volatile-poor melts and emission of volatile components from magmas at shallow stages of upwelling led to solidification of upper parts of magmatic columns and their explosion during further upwelling under impact of high pressure of gas phase preserved by solidification. This explains the fact that, kimberlites formed mostly explosion pipes, and that kimberlite lavas are lacking. Diamonds were crystallized as a result of carbon accumulation in residual melt. Evolution of its composition during crystallization from peridotite through eclogite to kimberlite was responsible for existence of xenoliths of the same composition in kimberlites and inclusions in diamonds, and decrease of their average isotope age in this sequence. Degree of the saturation of the melt with carbon during its fractionation increased. Due to that, layer growth was followed by radial one, and morphology of originating diamond crystals was evolved from octahedrons to dodecahedrons and then to cubes and aggregates, and sculptures were formed on them. Accumulation of meltphylic components in residual melts is causes increase of nitrogen, light carbon isotope and other admixtures content in later diamonds.

Details Geological Mapping and Petrological Characteristics of EL Shereik Study Area, River Nile State, North Sudan

The present study focused on details geology of the lithostratigraphy, phenomena and petrogenesis, with full classification of rock units and correlate by the regional geology in the study area. The previous studies, focused on regional geology, which can be divided in Bayuda Terrane, represented by Dem El Tor Shear Zone (DTSZ), its Precambrian to Tertiary ages (isotopic signatures of later cooling 590 to 550 Ma), with dominated by metamorphic and intrusive rocks belonging to the Pre-Nubian basement complex. The study area lies at El Shereik, River Nile State, Sudan. It is characterized by low-lying bed plains, covered with superficial deposits, with an arid climate. The techniques of this work, represented in an official, field works and laboratory test, with used Land Sat Images ETM. And Nubian Arabian Shield, represented by the Keraf Shear Zone (KSZ), its Precambrian to Phanerozoic ages (Neoproterozoic, Pb isotope ages dating, of forms is -730 -710Ma and ended in -565Ma), with dominated by metamorphic, ophiolitic mélange, volcano-sedimentary sequences, and Phanerozoic sediments. But through this detailed surveyed, we discovered different types of rock units; they were found as accumulated and highly deformed, affected by various thrusting faults. collected more than 40 samples, through six traverses, for classification and petrographic studies and classified more than 9 types of rooks units didn’t mentioned before in previous study, all of them well exposed as following: metamorphic rocks, included, migmatites, calc-silicates, wollastonite, talc-schist, carbonaceous (dolomitic marble), amphibolites, graphitic schist, mica schist, quartzo-feldspathic schist, and grey gneiss, While the Igneous rocks, consisted of dykes, as dolerite, trachy-basaltic, Rhyolite porphyry, pegmatites, diorite, and quartz veins, whereas the superficial deposits, included, of aeolian, fluvial, and collovial. These rocks extend and spread to the outside of the limits of the study area and most of them oriented parallel with KSZ SE - NW region and a few of them are oriented E-W, This is maybe due to the collusion of the contact boundary between KSZ and DTSZ Bayuda Tehran. Talc-schist, Wollastonite and graphitic schist, represent a strategic stockpile besides gold mining. Studies conducted with DTSZ, its old age, occurred before Neoproterozoic compared to KSZ. Because it has the first deformation of the folding of pre- KSZ proportion to the presence of folding in the west and east of the Nile zone, according to the border between Bayuda Terrane and Nubian Arabian Shield, as the suggest result. And the previous studies of the ages dating confirm it.

New insights on the early Mesozoic evolution of multiple tectonic regimes in the northeastern North China Craton from the detrital zircon provenance of sedimentary strata

Abstract. To investigate the timing of deposition and provenance of early Mesozoic strata in the northeastern North China Craton (NCC) and to understand the early Mesozoic paleotectonic evolution of the region, we combine stratigraphy, U–Pb zircon geochronology, and Hf isotopic analyses. Early Mesozoic strata include the Early Triassic Heisonggou, Late Triassic Changbai and Xiaoyingzi, and Early Jurassic Yihe formations. Detrital zircons in the Heisonggou Formation yield ∼ 58 % Neoarchean to Paleoproterozoic ages and ∼ 42 % Phanerozoic ages and were sourced from areas to the south and north of the basins within the NCC, respectively. This indicates that Early Triassic deposition was controlled primarily by the southward subduction of the Paleo-Asian oceanic plate beneath the NCC and collision between the NCC and the Yangtze Craton (YC). Approximately 88 % of the sediments within the Late Triassic Xiaoyingzi Formation were sourced from the NCC to the south, with the remaining ∼ 12 % from the Xing'an–Mongolia Orogenic Belt (XMOB) to the north. This implies that Late Triassic deposition was related to the final closure of the Paleo-Asian Ocean during the Middle Triassic and the rapid exhumation of the Su–Lu Orogenic Belt between the NCC and YC. In contrast, ∼ 88 % of sediments within the Early Jurassic Yihe Formation were sourced from the XMOB to the north, with the remaining ∼ 12 % from the NCC to the south. We therefore infer that rapid uplift of the XMOB and the onset of the subduction of the Paleo-Pacific Plate beneath Eurasia occurred in the Early Jurassic.