Radiometric age constraints on mineral growth,metamorphism, and tectonism of the Gummfluh Klippe,Briançonnais domain of the Préalpes,Switzerland

A combined ⁴⁰Ar/³⁹Ar, K/Ar, Rb/Sr and stable isotope study has been made of white micas from the Gummfluh klippe (Briançonnais domain of the Préalpes), Switzerland. The klippe consists mainly of Mesozoic to early Tertiary carbonate rocks metamorphosed from anchizonal to epizonal conditions. At the base of the klippe is a 10–50 m thick, ductilely deformed marble mylonite containing deformed authigenic quartz segregations. Stable isotope measurements of the coexisting calcite (¹⁸O_SMOW=24.5) and quartz (¹⁸O_SMOW=28.4) from the mylonite indicate relatively low temperatures (<300°C) during mylonitization. Analyses of white mica separates of varying size fractions from the mylonitic rocks by K/Ar and Rb/Sr techniques yield ages between 57 and 103 Ma. This variation is correlated with two parameters, the size of the mineral fraction, and the proportion of 2M₁ (more phengitic) to 1M (more muscovitic) polytype in the sample. The K/Ar and Rb/Sr ages are generally younger in the smaller size fractions, which also containless 2M₁ phengite. High precision ⁴⁰Ar/³⁹Ar age spectra from different size fractions of these micas record three distinct components, a small Hercynian component (ca. 200–300 Ma), a significant Eoalpine component (64–80 Ma) forming ⁴⁰Ar/³⁹Ar age plateaus, and a very minor Tertiary component (ca. 20–40 Ma). Characterization of the samples by SEM indicates the presence of two white mica populations, a coarser grained, deformed, detrital mica that probably corresponds to the 2M₁ phengite and a finer grained neoformed 1M mica. Collectively these observations suggest that the Gummfluh samples contain a mixture of detrital phengites of Hercynian age together with neocrystallized muscovites grown during the late Eoalpine metamorphic event followed by minor argon loss during the Tertiary. The main geologic episode recorded in the ⁴⁰Ar/³⁹Ar age spectra of white micas in the mylonite is of Late Cretaceous/Early Tertiary age (64–80 Ma), representing the first reliable Eoalpine ages ever to be reported from the Préalpes. Contrary to tectonic models, the marble mylonite at the base of the Gummfluh klippe appears to be a Cretaceous thrust plane and not the thrust surface formed during transport of the klippe into its present position from the Penninic Alps during the Tertiary. The late Cretaceous thrust developed during marine sedimentation at a depth of 800 m below the seafloor at temperatures of 280°C, facilitated by warm fluids along the tectonic discontinuity.     

Research on the growth orientation of pyrite grains in the colloform textures in Baiyunpu Pb–Zn polymetallic deposit,Hunan, China

Mineralogy and Petrology - A large number of colloform-textured pyrites were found in Baiyunpu Pb–Zn ore bodies in Xinshao County, Hunan, China. This study investigates the growth orientation...     

Tube growth patterns and microbial mat-related lifestyles in the Ediacaran fossil Cloudina,Gaojiashan Lagerstätte,South China

Cloudina are important Ediacaran index fossils, as they have a consistent terminal Neoproterozoic age and a worldwide distribution. The morphology of Cloudina has been reconstructed primarily on the basis of three-dimensionally preserved Chinese and Spanish specimens, but questions remain surrounding their phylogenetic placement. Understanding Cloudina paleoecology is an important aspect for the interpretation of these enigmatic organisms. Although their paleoecological dynamics remain unresolved, a possible “mat sticker” life mode has found support from recent taphonomic investigation of Paraguayan Cloudina specimens. Here, from analysis of fossil material from the upper part of the Ediacaran Dengying Formation (Gaojiashan and Lijiagou sections, Ningqiang area, southern Shaanxi Province, South China), we document complex tube growth patterns of Cloudina in addition to a close association of these fossils with preserved microbially induced sedimentary structures (specifically matgrounds). Sedimentological and taphonomic data of Cloudina-bearing carbonate sequences at the Gaojiashan section show a relationship between Cloudina and microbial mats, and further document erectly preserved Cloudina tubes. In conjunction with observations from the Paraguay material, the Dengying fossils described here provide supportive evidence for the paleoecological interpretation of Cloudina as inhabitants of a microbially bound carbonate substrate. The apical-most part of the tube (or cone) may have served as an attachment apparatus, situated within or beneath a microbial mat. Three-dimensionally phosphatized tubes of Cloudina from the Lijiagou section reveal that single tubes often have multiple growth orientations, which is demonstrated to have resulted from the curved growth of the tube walls (both cones and funnels). As these organisms exhibited a sessile, epibenthic life mode, the observed plasticity in growth direction and orientation may have been a competition-driven adaptation to ensure wider access to available food sources and potentially greater feeding efficiency. Although Cloudina have been found to co-occur in some deposits with biostromes and bioherms (e.g., stromatolites and thrombolites), sedimentological and taphonomic features of the Ningqiang Cloudina suggest that they inhabited a stratified carbonate substrate exhibiting matground features, but where biostromes and bioherms were absent.     

Early Cretaceous K-rich rhyolites from the Duolong Cu–Au deposit,southern Qiangtang,China: evidence for crustal growth

We report new zircon U–Pb age, Hf isotopic, and major and trace element data for rhyolites from the Duolong Ore Concentration Area of the Southern Qiangtang Terrane. Building on previous studies, we constrain the tectonic setting and propose a model to explain the geodynamics and crustal growth during regional magmatism in the Early Cretaceous. The analysed rhyolites yield laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb ages of 115 and 118 Ma. The rocks are K-rich (K₂O = 6.66–9.93 wt.%; K₂O/Na₂O = 8.2–19.7 wt.%), alkaline and peraluminous (A/CNK = 1.02–1.46), and are characterized by high SiO₂ contents (72.8–78.8 wt.%) similar to highly fractionated I-type granites. Fractionation of Fe–Ti oxides, plagioclase, hornblende, Ti-bearing phases, apatite, monazite, allanite and zircon contributed to the variations in major and trace element chemistry. High K₂O contents are likely due to partial melting of the continental crust. The samples have positive zircon εHf(t) values ranging from +7.1 to +11.2. These features, together with young zircon Hf crustal model ages of 489–721 Ma, indicate that the K-rich rhyolites were derived from juvenile lower crust with an input of a mantle-derived component. We suggest that the Early Cretaceous K-rich rhyolites formed in a continental arc setting during northward subduction of Bangong Co–Nujiang oceanic lithosphere. Basaltic magma underplating was responsible for vertical crustal growth, triggered by slab roll-back in the Duolong Ore Concentration Area in the Early Cretaceous.     

The boring billion? – Lid tectonics, continental growth and environmental change associated with the Columbia supercontinent

The evolution of Earth＇s biosphere,atmosphere and hydrosphere is tied to the formation of continental crust and its subsequent movements on tectonic plates.The supercontinent cycle posits that the continental crust is periodically amalgamated into a single landmass,subsequently breaking up and dispersing into various continental fragments.Columbia is possibly the first true supercontinent,it amalgamated during the 2.0-1.7 Ga period,and collisional orogenesis resulting from its formation peaked at 1.95-1.85 Ga.Geological and palaeomagnetic evidence indicate that Columbia remained as a quasi-integral continental lid until at least 1.3 Ga.Numerous break-up attempts are evidenced by dyke swarms with a large temporal and spatial range; however,palaeomagnetic and geologic evidence suggest these attempts remained unsuccessful.Rather than dispersing into continental fragments,the Columbia supercontinent underwent only minor modifications to form the next supercontinent （Rodinia） at 1.1 -0.9 Ga; these included the transformation of external accretionary belts into the internal Grenville and equivalent collisional belts.Although Columbia provides evidence for a form of ‘lid tectonics’,modern style plate tectonics occurred on its periphery in the form of accretionary orogens.The detrital zircon and preserved geological record are compatible with an increase in the volume of continental crust during Columbia＇s lifespan; this is a consequence of the continuous accretionary processes along its margins.The quiescence in plate tectonic movements during Columbia＇s lifespan is correlative with a long period of stability in Earth＇s atmospheric and oceanic chemistry.Increased variability starting at 1.3 Ga in the environmental record coincides with the transformation of Columbia to Rodinia; thus,the link between plate tectonics and environmental change is strengthened with this interpretation of supercontinent history.     

Divergent growth trajectories in China’s chemical industry: the case of the newly developed industrial parks in Shanghai,Nanjing and Ningbo

In the late 1990’s, the “new-economy” industries in China proved to be relatively vulnerable and were strongly hit by the financial crisis in Asia. As a result, a new economic support policy was introduced in China’s Yangtze Delta region, which put greater emphasis on the support of traditional industrial sectors, including the chemical industry. This paper investigates the effects of the growth of this industry, as well as the potential and current problems emerging from new growth paths. It compares the growth of three newly developed chemical industry parks in Shanghai, Nanjing and Ningbo. The paper is based on an institutional perspective of clustering processes arguing that regional industrialization is subject to formal and informal institutions which shape the growth paths and contribute to divergent regional trajectories. Although these industrial parks all benefit from the general economic upswing in China, their development is influenced by different business models, economic contexts, goals and strategies, leaving room for divergence and specialization. Due to the existing structure of operations, these parks have a great deal of potential but also face substantial challenges, such as the establishment of internal networks and close customer linkages. It is argued that this might limit their innovative capability in the future. Furthermore, their growth prospects differ depending on future government policies.     

Trace element zoning in garnet from the Kwoiek Area,British Columbia: disequilibrium partitioning during garnet growth?

Trace element zoning in garnets from two contact-metamorphosed rocks from the Kwoiek area, British Columbia (Hollister 1969a), was measured with an ion microprobe. Zoning profiles have three distinct parts with chemical breaks defined by co-variation of major and trace elements. Important features of the trace element zoning profiles are: (1) roughly bell-shaped zoning profiles for Y and the HREEs, (2) an abrupt increase in Ti at a point midway through each garnet with inflections in the zoning profiles of other elements (Li, Na, Cr, V, Y, Zr, and the HREE), and (3) irregular Cr and V profiles. Unlike Mn zoning, the zoning profiles of most other trace elements cannot be easily modeled using simple Rayleigh fractionation models. Ti activity in the two samples is buffered by phase relations with ilmenite. Garnets from a continuously heated contact metamorphic environment should display continuous Ti zoning profiles if equilibrium was maintained and provided the Ti buffering assemblage did not change during garnet growth. The irregular Ti profiles suggest disequilibrium behavior. Several elements (Cr, V) may indicate breakdown of a phase enriched in trace elements during metamorphism. The source for the mass excess of these elements is probably the refractory cores of ilmenite grains. Either differing matrix transport rates of trace lements or interface kinetic controlled segregation could explain the unusual trace element behavior at the element inflection point. The preferred explanation involves segregation of elements at the interface of the garnet that were trapped during episodes of rapid garnet growth.     

Yellow diamonds with colourless cores – evidence for episodic diamond growth beneath Chidliak and the Ekati Mine,Canada

Mineralogy and Petrology - Yellow diamonds from the CH-7 (Chidliak) and the Misery (Ekati Mine) kimberlites in northern Canada are characterised for their nitrogen characteristics, visible light...     

Evolution of the 3.65–2.58 Ga Mairi Gneiss Complex,Brazil: Implications for growth of the continental crust in the São Francisco Craton

The composition and formation of the Earth’s primitive continental crust and mantle differentiation are key issues to understand and reconstruct the geodynamic terrestrial evolution, especially during the Archean. However, the scarcity of exposure to these rocks, the complexity of lithological relationships, and the high degree of superimposed deformation, especially with long-lived magmatism, make it difficult to study ancient rocks. Despite this complexity, exposures of the Archean Mairi Gneiss Complex basement unit in the São Francisco Craton offer important information about the evolution of South America’s primitive crust. Therefore, here we present field relationships, LA-ICP-SFMS zircon U-Pb ages, and LA-ICP-MCMS Lu-Hf isotope data for the recently identified Eoarchean to Neoarchean gneisses of the Mairi Complex. The Complex is composed of massive and banded gneisses with mafic members ranging from dioritic to tonalitic, and felsic members ranging from TTG (Tonalite-Trondhjemite-Granodiorite) to granitic composition. Our new data point to several magmatic episodes in the formation of the Mairi Gneiss Complex: Eoarchean (ca. 3.65–3.60 Ga), early Paleoarchean (ca. 3.55–3.52 Ga), middle-late Paleoarchean (ca. 3.49–3.33 Ga) and Neoarchean (ca. 2.74–2.58 Ga), with no records of Mesoarchean rocks. Lu-Hf data unveiled a progressive evolution of mantle differentiation and crustal recycling over time. In the Eoarchean, rocks are probably formed by the interaction between the pre-existing crust and juvenile contribution from chondritic to weakly depleted mantle sources, whereas mantle depletion played a role in the Paleoarchean, followed by greater differentiation of the crust with thickening and recycling in the middle–late Paleoarchean. A different stage of crustal growth and recycling dominated the Neoarchean, probably owing to the thickening of the continental crust by collision, continental arc growth, and mantle differentiation.     

Detrital zircon U–Pb ages and Hf-isotope systematics from the Gadag Greenstone Belt: Archean crustal growth in the western Dharwar Craton,India

The Western Dharwar Craton in peninsular India comprises a typical Meso- to Neo-Archean granite-greenstone terrain. Detrital zircons from two metagreywackes in a late basin from the Gadag Greenstone Belt preserve at least eight age populations ranging in age from ca 3.34 to 2.55 Ga, and grains as old as ca 3.54 Ga. The zircon provenances for the two samples appear to be the same up to ca 3.25 Ga, with relatively juvenile ε_Hf values largely between zero and depleted mantle values. After 3.25 Ga, one sample has similar ε_Hf values whereas the other has only negative values indicative of Hf-evolution in a crustal environment. After ca 3.25 Ga the source regions for the two samples were distinctly different.The detrital zircons reflect the age and evolution of the upper crust of the Western Dharwar Craton. Modeling of Hf isotopic evolution of the detrital zircons suggests two major crust-forming events at ca. 3.6 and 3.36 Ga, and some indication of juvenile addition to the crust at ca 2.6 Ga. The maximum sedimentation age of the greywackes is constrained by the youngest detrital zircon population at 2547 ± 5 Ma. Gold mineralization in the belt is dated at 2522 ± 6 Ma and constrains greywacke sedimentation, deformation and metamorphism to a ca 25 my interval.     

Geochronology and Sr–Nd–Hf isotopic composition of the granites, enclaves, and dikes in the Karamay area, NW China: Insights into late Carboniferous crustal growth of West Junggar

New whole-rock major and trace elements,and zircon U-Pb and Hf-Nd isotope compositions are reported for the Karamay dikes,enclaves,and host granites in the West Junggar,NW China.Zircon U-Pb dating of the Karamay pluton yields an age of 300.7 ± 2.3 Ma for the enclave and 300.0 ± 2.6 Ma for the host granite,which was intruded by dike with an age of 298 Ma.The host granites exhibit relatively low SiO_2 contents and A/CNK and Ga/Al ratios,low initial ~(87)Sr/~(86)Sr ratios(0.703421—0.703526) and positive εHf(t)(5.5—14.1) and εNd(t)(7.3—8.1) values with a young model age,suggesting that they are I-type granites and were mainly derived from a juvenile lower crustal source.The enclaves and dikes belong to an andesitic calc-alkaline series and have high MgO concentrations at low silica content and positive εNf(t)(7.6-13.2,14.2-14.9) and εNd(t)(6.8-8.3,~6.9) values.They are enriched in LILEs(Rb,Ba and U) and LREE and depleted in HFSEs(Nb and Ta) with insignificant negative Eu anomalies,indicating that the melts were derived from an enriched lithospheric mantle modified by subducted oceanic crust-derived melts and minor fluids,followed by fractional crystallization.The Karamay host granites and enclaves are of mixed origin and are most probably formed by the interaction between the lower crust- and lithospheric mantle-derived magmas,and were intruded by the unmixed dikes subsequently.The upwelling mantle through a slab window in an island arc environment might have triggered partial melting of the lithospheric mantle and its subsequent interaction with the granitic magma,further suggesting that the ridge subduction played an important role in the crustal growth of West Junggar.     

Diamond growth from oxidized carbon sources beneath the Northern Slave Craton, Canada: A δC-N study of eclogite-hosted diamonds from the Jericho kimberlite

Diamonds from high- and low-MgO groups of eclogite xenoliths from the Jericho kimberlite, Slave Craton, Canada were analyzed for carbon isotope compositions and nitrogen contents. Diamonds extracted from the two groups show remarkably different nitrogen abundances and δ¹³C values. While diamonds from high-MgO eclogites have low nitrogen contents (5-82 ppm) and extremely low δ¹³C values clustering at ∼−40‰, diamonds from the low-MgO eclogites have high nitrogen contents (>1200 ppm) and δ¹³C values from −3.5‰ to −5.3‰.Coupled cathodoluminescence (CL) imaging and SIMS analysis of the Jericho diamonds provides insight into diamond growth processes. Diamonds from the high-MgO eclogites display little CL structure and generally have constant δ¹³C values and nitrogen contents. Some of these diamonds have secondary rims with increasing δ¹³C values from −40‰ to ∼−34‰, which suggests secondary diamond growth occurred from an oxidized growth medium. The extreme negative δ¹³C values of the high-MgO eclogite diamonds cannot be produced by Rayleigh isotopic fractionation of average mantle-derived carbon (−5‰) or carbon derived from typical organic matter (∼−25‰). However, excursions in δ¹³C values to −60‰ are known in the organic sedimentary record at ca. 2.7 and 2.0 Ga, such that diamonds from the high-MgO eclogites could have formed from similar organic matter brought into the Slave lithospheric mantle by subduction.SIMS analyses of a diamond from a low-MgO eclogite show an outer core with systematic rimwards increases in δ¹³C values coupled with decreases in nitrogen contents, and a rim with pronounced alternating growth zones. The coupled δ¹³C-nitrogen data suggest that the diamond precipitated during fractional crystallization from an oxidized fluid/melt from which nitrogen was progressively depleted during growth. Model calculations of the co-variation of δ¹³C-N yielded a partition coefficient (K_N) value of 5, indicating that nitrogen is strongly compatible in diamond relative to the growth medium. δ¹³C values of diamond cores (−4‰) dictate the growth medium had higher δ¹³C values than primary mantle-derived carbon. Therefore, possible carbon sources for the low-MgO eclogite diamonds include oxidized mantle-derived (e.g. protokimberlite or carbonatite) fluids/melts that underwent some fractionation during migration or, devolatilized subducted carbonates.     

Significance of river–aquifer interactions for reach-scale thermal patterns and trout growth potential in the Motueka River, New Zealand

To assess whether reaches of the Motueka River (New Zealand) that gain water from groundwater were likely to represent significant cold-water refugia for brown trout during periods of high water temperatures, water temperature was monitored for more than 18 months in two gaining reaches of the Motueka River and three reaches that were predicted to be losing water to groundwater. These data were used to predict brown trout (Salmo trutta) growth in gaining and losing reaches. Groundwater inputs had a small effect on water temperature at the reach-scale and modelling suggests that the differences observed were unlikely to result in appreciable differences in trout growth. Several coldwater patches were identified within the study reach that were up to 3.5°C cooler than the mainstem, but these were generally shallow and were unlikely to provide refuge for adult trout. The exception was Hinetai Spring, which had a mean water temperature of close to 16°C during the period January–March, when temperatures in the mainstem regularly exceeded 19°C. Trout were observed within the cold-water plume at the mouth of Hinetai Stream, which would allow them to thermoregulate when mainstem temperatures are unfavourable while still being able to capitalise on food resources available in the mainstem.     

Paleomagnetic study of ferromanganese crusts recovered from the northwest Pacific — Testing the applicability of the magnetostratigraphic method to estimate growth rate

Previous studies have shown that ferromanganese crusts in the Pacific Ocean commonly record paleomagnetic reversals and that the reversal patterns can be used to estimate growth rates. In order to investigate the applicability of the magnetostratigraphic method, we conducted paleomagnetic measurements of crust samples recovered from five locations in the northwest Pacific. A series of thin slices, with thicknesses of 0.5–1.0 mm, was prepared for each sample, and a paleomagnetic polarity was determined for each slice. In all five samples, we found a consistent reversal pattern of N1–R1–N2–R2–N3 from the surface to the inner part of the crust. In three samples, another polarity interval (R3) was recognized below the N3 section of the crust. These data suggest that ferromanganese crusts in the northwest Pacific recorded paleomagnetic reversals and that reversal patterns can be used for ocean-scale correlations. The magnetostratigraphic method suggests constant growth rates of 1.49, 2.54, 3.56 and 3.67 mm/Ma for four samples, three of which are consistent with those estimated using ¹⁰Be/⁹Be dating at the 2σ (standard deviation) level.     

Neoproterozoic crustal growth at the margin of the East Gondwana continent – age and isotopic constraints from the easternmost inliers of Oman

ABSTRACT

The Jebel Ja’alan and Qalhat inliers of Oman represent the easternmost exposures in the Arabian peninsula of the Neoproterozoic basement associated with the East African Orogen (EAO) and the assembly of East and West Gondwana. These inliers expose tonalitic gneisses and metasediments intruded by granodiorites and granites of the Ja’alan batholith. Zircons from the gneisses yield U–Pb SIMS ages of ca. 900–880 Ma, which are interpreted as crystallization ages. These represent the oldest magmatic events associated with the closure of the Mozambique Ocean reported to date. Zircon of this age is also the dominant component in the metasediments. The Ja’alan batholith yields ages of ca. 840–825 Ma. Nd isotopes indicate that both the gneisses and the batholith range from juvenile to slightly more evolved, with ε_Nd(t) of +6 to +1.5 interpreted to reflect variable contamination by older, evolved continental material; this is also indicated by >900 Ma detrital zircon from the metasediments. The Nd data also contrast with the uniformly juvenile signature of younger, ca. 840 Ma, rocks of the Marbat region of southern Oman that lie structurally to the west. The Ja’alan and Qalhat inliers thus document eastward increasing age and continental influence, consistent with the progressive development of arc rocks onto the western margin of East Gondwana, although the location and nature of the eastern continental block remain elusive.     

Post-orogenic shoshonitic magmas of the Yzerfontein pluton,South Africa: the ‘smoking gun’ of mantle melting and crustal growth during Cape granite genesis?

The post-orogenic Yzerfontein pluton, in the Saldania Belt of South Africa was constructed through numerous injections of shoshonitic magmas. Most magma compositions are adequately modelled as products of fractionation, but the monzogranites and syenogranites may have a separate origin. A separate high-Mg mafic series has a less radiogenic mantle source. Fine-grained magmatic enclaves in the intermediate shoshonitic rocks are autoliths. The pluton was emplaced between 533 ± 3 and 537 ± 3 Ma (LA-SF-ICP-MS U–Pb zircon), essentially synchronously with many granitic magmas of the Cape Granite Suite (CGS). Yzerfontein may represent a high-level expression of the mantle heat source that initiated partial melting of the local crust and produced the CGS granitic magmas, late in the Saldanian Orogeny. However, magma mixing is not evident at emplacement level and there are no magmatic kinships with the I-type granitic rocks of the CGS. The mantle wedge is inferred to have been enriched during subduction along the active continental margin. In the late- to post-orogenic phase, the enriched mantle partially melted to produce heterogeneous magma batches, exemplified by those that formed the Yzerfontein pluton, which was further hybridised through minor assimilation of crustal materials. Like Yzerfontein, the small volumes of mafic rocks associated with many batholiths, worldwide, are probably also low-volume, high-level expressions of crustal growth through the emplacement of major amounts of mafic magma into the deep crust.     

Linear growth rate and sectorial growth dynamics of diamond crystals grown by the temperature-gradient techniques (Fe–Ni–C system)

The paper reports data on the linear growth rates of synthetic diamond single crystals grown at high P–T parameters by the temperature-gradient technique in the Fe–Ni–C system. Techniques of stepwise temperature changes and generation of growth microzoning were applied to evaluate the growth rates of various octahedral and cubic growth sectors and variations in these rates with growth time. The maximum linear growth rates of the order of 100–300 µm/h were detected at the initial activation of crystal growth, after which the growth rates nonlinearly decreased throughout the whole growth time to 5–20 µm/h. The fact that the linear growth rates can broadly vary indicates that the inner structure and growth dynamics of single diamond crystals grown by the temperature-gradient technique should be taken into account when applied in mineral–geochemical studies (capture of inclusions, accommodation of admixture components, changes of the defective structure, etc.).     

Microfossils from the Wumishan Formation of the Jixian System in the Ming Tombs,Beijing,China

Members Ⅰ-Ⅳ of the Wumishan Formation of the Jixian System are exposed respectively in Mt. Cuihua and Mt. Hushan in the Ming Tombs, Beijing. Black banded cherts which are extensively developed in this formation contain an abundance ofmicrofossils that are well preserved and variable in form. A study of thin sections shows that the microfossils include 10 genera, 12 species, 3 of. species, 2 indeterminate species and 1 unnamed form, of which 1 genus and 7 species are newly erected. The present assemblage is morphologically comparable to those from the Wumishan Formation in the Western Hills of Beijing, Jixian County of Tianjin and Kuancheng of Hebei Province.     

Late Permian–Triassic siliciclastic provenance,palaeogeography, and crustal growth of the Songpan terrane,eastern Tibetan Plateau: evidence from U–Pb ages,trace elements,and Hf isotopes of detrital zircons

In order to constrain the detrital provenance of the siliciclastic rocks, palaeogeographic variations, and crustal growth history of central China, we carried out simultaneously in situ U–Pb dating and trace element and Hf isotope analyses on 368 detrital zircons obtained from upper Permian–Triassic sandstones of the Songpan terrane, eastern Tibetan Plateau. Two groups of detrital zircons, i.e. magmatic and metamorphic in origin, have been identified based on cathodoluminescence images, zircon Ti-temperatures, and Th/U ratios. Our data suggest that the derivation of siliciclastic rocks in the Songpan terrane was mainly from the Qinling, Qilian, and Kunlun orogens, whereas the Yangtze and North China Cratons served as minor source areas during late Permian–Triassic times. The detrital zircons from Middle–Late Triassic siliciclastic rocks exhibit wide age spectra with two dominant populations of 230–600 Ma and >1600 Ma, peaking at ~1.8–1.9 Ga and ~2.4–2.5 Ga, suggestive of a derivation from the Qinling, Qilian, and Kunlun orogens and the Yangtze Craton being the minor source area. The proportions of detrital zircon populations from the northern Qinling, Qilian, and Kunlun orogens distinctly decreased during Middle–Late Triassic time, demonstrating that the initial uplift of the western Qinling occurred then and it could have blocked most of the detritus from the Qilian–northern Qinling orogens and North China Cratons into the main Songpan–Ganzi depositional basin. The relatively detrital zircon proportions of the Yangtze Craton source decreased during Early-Middle Late Triassic time, indicating that the Longmenshan orogen was probably being elevated, since the early Late Triassic and gradually formed a barrier between the Yangtze Craton and the Songpan terrane. In addition, our Lu–Hf isotopic results also reveal that the Phanerozoic magmatic rocks in central China had been the primary products of crustal reworking with insignificant formation of a juvenile crust.