Detrital zircon geochronology of the Jurassic strata in the western Ordos Basin, North China: Constraints on the provenance and its tectonic implication

U–Pb detrital zircon data from the Jurassic strata in the western Ordos Basin (WOB) show significant age differences between formations, from lower to upper including the Yan’an, Zhiluo, Anding, and Fenfanghe formations. The main peaks in the detrital zircon age spectra of the Yan’an Formation are at ca. 245, 444, 1813, and 2,387 Ma, which resemble those of the underlying Triassic strata in the WOB. Combined with the occurrence of reworked sporopollen and carbonate debris, the sediments of the Yan’an Formation are interpreted to be potentially recycled from the residual Triassic highlands. The overlying Zhiluo Formation contains detrital zircons mostly in the 260–290 Ma age range with only one peak at 269 Ma, reflecting primary derivations from the Permian granites in the Yinshan Belt to the north and/or the Alxa Block to the northwest. The detrital zircon age compositions of the Anding Formation, however, are most varied, with one prominent age peak at 274 Ma and several other minor peaks at 348 Ma, 447, 932, 1641, 2143, and 2514 Ma, reflecting complex origins. The uppermost syntectonic Fenfanghe Formation, contains detrital zircons with similar age compositions with the Yan’an Formation, once again interpreted as the result of recycled sedimentation. These two phases of multicycle sedimentation in the WOB reveals two intense tectonic events in the Late Triassic and Late Jurassic, whereas the provenance transition between the Yan’an and Zhiluo formations implies a less intense but extensive event in the Middle Jurassic. The complex age composition of the Anding Formation, however, are possibly as the result of climate change corresponding to the change of petrology and sediment colour. These Early‐Middle Mesozoic tectonic events evidenced by the U–Pb ages record the tectonic transition of the North China Block from the Tethys to the paleo‐Pacific tectonic domains.

Read more

New emission-reducing stoves not much better for environment than old stoves, study finds

By Madeleine Jepsen Cookstoves meant to curb carbon emissions and reduce pollutants may not be as climate-friendly as previously thought, a new study finds. Traditional bio-fueled cookstoves, used by one-third of the global population, burn wood and other organic material to cook food. A new study measuring the types of carbon in fine particulate matter emitted from these types of cookstoves in India found newer stoves designed to reduce emissions …

Read more

Migmatization and intrusion of “S‐type” granites in the trans‐Himalayan Ladakh Magmatic Arc of north India and their bearing on Indo‐Eurasian collisional tectonics

The Ladakh Magmatic Arc (LMA) of trans‐Himalaya was formed due to subduction of the Tethyan Ocean beneath the Eurasian continental plate. This was followed by the Indo‐Eurasian continental collision that gave rise to the Himalayan orogen. In this work, we present results from U–Pb geochronology of zircon of 2 samples from a previously unknown migmatite body within the LMA and one two‐mica granite body that lies adjacent to this migmatite and intrudes the LMA. One sample of mesosome from this migmatite gives a crystallization age of 66.9 ± 1.5 Ma (n = 11 zircon) along with a younger population of zircon (n = 24) giving an age of 62.1 ± 1.3 Ma. This sample also shows the presence of a younger cluster of zircon (n = 7) giving a concordant age of 50.0 ± 2.9 Ma. The other sample of one leucosome contains a cluster of inherited zircon (n = 5) giving a concordant age of 72.86 ± 0.83 Ma. However, the majority of the population (n = 10) gives a concordant crystallization age of 55.31 ± 1.5 Ma. Limited Hf isotopic analysis of the leucosome shows positive εHft values ranging from 7 to 15, comparable to that of the LMA. High and positive εHft values and absence of any Palaeozoic xenocrystic zircon help us infer that the Indian continental crust was not involved in the partial melting and leucosome generation process. Therefore, this event of migmatization predates Indo‐Eurasian collision. By comparing Th/U ratio versus age for both the mesosome and leucosome and their respective age spectra, we infer that this part of the LMA underwent pronounced partial melting between 55 and 60 Ma and generated leucosome till ~50 Ma. The two‐mica granite contains muscovite, indicating its crustal origin. It gives 2 closely spaced age spectra of 38.1 ± 1.1 Ma (n = 7) and 34.31 ± 0.48 Ma (n = 4). This two‐mica granite also contains xenocrysts of Palaeozoic ages. By integrating our studies with earlier works, we infer that this migmatization of the LMA indicates its collision with the Indian plate, prior to Indo‐Eurasian continental accretion, at 55–50 Ma. The two‐mica granite contains xenocrystic material from both the LMA and the Indian continental crust and indicates S‐type granite magmatism related to India–Eurasia continental collision at 35–40 Ma.

Read more

Provenance and tectonic setting of the Quemoco sandstones in the North Qiangtang Basin, North Tibet: Evidence from geochemistry and detrital zircon geochronology

The geochemistry and detrital zircon geochronology of the Quemoco sandstones from the Woruo Mountain area, North Qiangtang Basin, are analysed to discriminate their provenance and tectonic setting. The sandstones were classified as litharenite according to their geochemical data. The index of chemical variability and SiO2/Al2O3 ratio values suggest that the compositional maturity and recycling are weak to moderate. The weathering indices, such as chemical index of alteration, plagioclase index of alteration, and chemical index of weathering, and A–CN–K (Al2O3–(CaO* + Na2O)–K2O) ternary plot reflect that the source areas have undergone a weak degree of weathering. The TiO2 versus Zr, La/Th versus Hf, and Co/Th versus La/Sc bivariate diagrams and multimajor elements discrimination diagram of Quemoco sandstones indicate that the provenances were primarily derived from felsic igneous rocks and may also have less intermediate igneous source rocks. The paleocurrent data and low composition and textural maturity of the Quemoco sandstones suggested that the provenances are mainly from the southwest of the study area and with proximal source features. The U–Pb ages studied here could be divided into 8 groups: 206–219 Ma, 268–283 Ma, 446–630 Ma, 700–970 Ma, 1006–1346 Ma, 1457–1843 Ma, 2087–2214 Ma, and 2331–2587 Ma. The provenances are mainly from the Central Uplift Belt and surrounding of the study area according to the detrital zircon ages and rare earth element parameters, which are composed of Late Triassic tuff, Permian basalt, Early Ordovician granite, Cambrian granite gneiss, and Precambrian crystalline basement. Two multidimensional tectonic discrimination diagrams based on major elements show a complicated tectonic setting composed of a collision, rift, and minor arc setting for the Quemoco sandstones, which is consistent with the nature of a rifted margin during the late Late Triassic.

Read more

Fractal characteristics of fault systems and their geological significance in the Hutouya poly‐metallic orefield of Qimantage, East Kunlun, China

The fault fractal dimension is the integrated reflection of the quantity, scale, degree of development, combination mode, and dynamic mechanism of a fault and can be used as a quantitative indicator of fault structure complexity. In Hutouya polymetallic orefield of Qinghai province, China, the faults are developed, and many ore belts are located in the fault zone, which indicated that the faults have an important role in controlling mineralization. The two‐dimensional horizontal distribution of fault systems in the IV, V, VI, and VII ore zones of Hutouya orefield was studied based on fractal geometry. The fault distribution has fractal dimension values for zones IV, V, VI, and VII of 1.05, 1.157, 1.311, and 1.05, respectively. From the perspective of favourable structure, the potential for metallogenesis in the four ore zones is VI > VII > V > IV. The main controlling factor on ore formation in the Qimantage region is fault structure, and the results are consistent with the actual results. This work provides new insights into the relationship between fault systems and metallogenesis.

Read more

Metavolcanics from Capo Malfatano, SW Sardinia, Italy: New insight on the age and nature of Ordovician volcanism in the Variscan foreland zone

The tectonic/stratigraphic interpretation of the Cambro–Ordovician rocks exposed in the south‐westernmost sectors of the Sardinia island (i.e., belonging to the Bithia Formation or Unit) is still a hot point of discussion. A debated question is also the age of the metavolcanics interbedded in these rocks. In order to provide a clear answer to this problem, in this paper, we provide new U–Pb zircon dating and a petrographic–geochemical comparison of metavolcanics interbedded within the Bithia Formation with similar Sardinian Ordovician calc‐alkaline rocks. The Bithia metavolcanics plot in the rhyodacite/rhyolite field. They show negative troughs for Nb and Ta, negative anomalies for Sr, P, and Ti, and positive peaks for Cs, Rb, K, Th, U, and Pb in the multielement diagram normalized to primitive mantle and significant LREE enrichment, marked negative Eu anomaly, and flat HREE patterns in the chondrite‐normalized REE diagram. Owing to these geochemical features, the Bithia metavolcanics plot in the volcanic arc and active continental margin fields closely resembling Middle‐Late Ordovician felsic metavolcanics from the Sarrabus, Gerrei, Goceano, and Mt. Grighini units. A new U–Pb zircon age of 462.1 ± 4.3 Ma yielded by a metavolcanic layer within the Bithia Formation at Capo Malfatano must be interpreted as the emplacement age. The CL and BSE images of zircon grains do not show any evidence of interaction with late hydrothermal fluids that could have caused Pb loss and rejuvenated ages, confirming the Middle‐Late Ordovician age of the metavolcanics.

Read more

Age of crustal melting in Higher Himalayan Crystalline Sequence (Darjeeling, Eastern Himalaya): Constraints from SHRIMP U–Pb geochronology

The paper presents the petrological and geochronological studies of migmatite from an important section of Higher Himalayan Crystalline Sequence of Eastern Himalaya. Migmatites collected from the area characteristically contain biotite‐garnet‐sillimanite‐plagioclase‐K‐feldspar‐quartz as an important mineral assemblage, which has experienced extensive partial melting through dehydration melting reaction involving biotite. In this study, P–T evolution of these migmatites has been constrained through the use of multiequilibrium thermobarometry program winTWQ, conventional thermobarmetry, and pseudosection modelling in the MnNCKFMASHTO model system using Perple_X software. The unification of these three calculations demonstrates that the migmatite experienced peak pressure and temperature at 7.2 ± 0.5 kbar and 775 ± 20 °C, respectively. SHRIMP U–Pb chronological results yield the timing of crustal melting (21 Ma) in the migmatite.

Read more

Mineralogy, zircon U–Pb–Hf isotopes, and whole‐rock geochemistry of Late Cretaceous–Eocene granites from the Tengchong terrane, western Yunnan, China: Record of the closure of the Neo‐Tethyan Ocean

New data from mineralogy, geochemistry, zircon U–Pb dating, and Hf isotopes have revealed Late Cretaceous to Eocene magmatic intrusions in the Tengchong terrane and constrained the origin, tectonic setting, and characteristics of tin‐bearing and barren granitoids. We divide the studied granitoids into two groups: (a) the Late Cretaceous Zhinaxiang (70 Ma) and Early Paleocene Dazhupeng (65 Ma) S‐type barren granites and (b) the Eocene Lailishan A2‐type tin‐bearing granite (50 Ma). All these granitoids display Si‐ and K‐rich and calc‐alkaline characteristics and have similar chondrite‐normalized rare earth element patterns. However, geochemical differences do exist between the two groups of granites. The Zhinaxiang and Dazhupeng S‐type granites are slightly peraluminous, enriched in large‐ion lithophile elements (e.g., Rb and K), and depleted in high‐field‐strength elements (e.g., Nb, Ta, Zr, and Hf), whereas the Lailishan A2‐type granite has higher TFe2O3, high‐field‐strength element content, and Ga/Al ratios. The geochemical and Hf isotopic data indicate that the granites in this study were generated by partial melting of Paleoproterozoic metasedimentary rocks. Due to the subduction of the Neo‐Tethyan Ocean beneath the Eurasian plate, the Zhinaxiang and Dazhupeng S‐type granites were formed in a thickened‐crustal environment, whereas the Lailishan A2‐type granite was emplaced in a post‐collisional extensional setting attributed to slab break‐off. By comparison between tin‐bearing and barren granites, we propose that the Sn mineralization could be related to relatively high‐temperature and low‐pressure crystallization conditions.

Read more

Migmatization and intrusion of “S-type” granites in the trans-Himalayan Ladakh Magmatic Arc of north India and their bearing on Indo-Eurasian collisional tectonics

The Ladakh Magmatic Arc (LMA) of trans-Himalaya was formed due to subduction of the Tethyan Ocean beneath the Eurasian continental plate. This was followed by the Indo-Eurasian continental collision that gave rise to the Himalayan orogen. In this work, we present results from U–Pb geochronology of zircon of 2 samples from a previously unknown migmatite body within the LMA and one two-mica granite body that lies adjacent to this migmatite and intrudes the LMA. One sample of mesosome from this migmatite gives a crystallization age of 66.9 ± 1.5 Ma (n = 11 zircon) along with a younger population of zircon (n = 24) giving an age of 62.1 ± 1.3 Ma. This sample also shows the presence of a younger cluster of zircon (n = 7) giving a concordant age of 50.0 ± 2.9 Ma. The other sample of one leucosome contains a cluster of inherited zircon (n = 5) giving a concordant age of 72.86 ± 0.83 Ma. However, the majority of the population (n = 10) gives a concordant crystallization age of 55.31 ± 1.5 Ma. Limited Hf isotopic analysis of the leucosome shows positive εHft values ranging from 7 to 15, comparable to that of the LMA. High and positive εHft values and absence of any Palaeozoic xenocrystic zircon help us infer that the Indian continental crust was not involved in the partial melting and leucosome generation process. Therefore, this event of migmatization predates Indo-Eurasian collision. By comparing Th/U ratio versus age for both the mesosome and leucosome and their respective age spectra, we infer that this part of the LMA underwent pronounced partial melting between 55 and 60 Ma and generated leucosome till ~50 Ma. The two-mica granite contains muscovite, indicating its crustal origin. It gives 2 closely spaced age spectra of 38.1 ± 1.1 Ma (n = 7) and 34.31 ± 0.48 Ma (n = 4). This two-mica granite also contains xenocrysts of Palaeozoic ages. By integrating our studies with earlier works, we infer that this migmatization of the LMA indicates its collision with the Indian plate, prior to Indo-Eurasian continental accretion, at 55–50 Ma. The two-mica granite contains xenocrystic material from both the LMA and the Indian continental crust and indicates S-type granite magmatism related to India–Eurasia continental collision at 35–40 Ma.

Read more

Innovation mitigates cloud problem in global climate and weather forecast models

CIRES scientist’s new framework promises to improve cloud representation, forecast accuracy Anyone with a cell phone camera and kids or dogs knows that resolution is “expensive”: taking lots of very high-resolution photographs and video clips can quickly fill a device. An analogous resolution challenge in weather and climate modeling has dogged modelers for years: Computationally, it’s just too expensive to represent certain clouds in the detail needed to make them …

Read more