Geoheritages in the Qinling Orogenic Belt of China: Features and comparative analyses

The Qinling Orogenic Belt is a typical composite continental orogenic belt in China. The Qinling Mountains is not only a natural demarcation that separates China into a southern part and a northern part, in terms of Qinling Orogenic Belt’s geography, climate, organisms, and river systems, but also forms a cultural boundary between the Yellow River Civilization to the north and the Yangtze River Civilization to the south. The Xi’an Qinling Zhongnanshan World Geopark is situated in the major portion of the collision zone between the South China and North China plates, which is also a typical section of the Qinling Orogenic Belt. It has been the focus of great attention for its long history of geological evolution, intensive tectonism, various rock types, unique strata, frequent magmatism, and abundant geological heritage sites. This paper clarifies the regional geological background of the Qinling Zhongnanshan World Geopark in 3 aspects, that is, the regional geological setting, regional evolution, and regional strata. With an integrated analysis and classification, the paper describes the main features of geological relics and geological remains of scientific significance in the geopark, on the basis of an investigation and systematic analysis for the geological relic resources of the world geopark. Compared with the geosites in other world geoparks, the sites in the Zhongnanshan area are particularly distinctive, even unique.

Read more

Origin and influence of a Late Mesozoic multistage I- and A-type granitic complex in northern Fujian Province, South China

The Late Mesozoic Pucheng granitic complex in northern Fujian Province is composed of the Chengbu and Shipi peralkaline A-type granitoids, the Pucheng A-type granitoid, and the Yongxing I-type granitoid. The Chengbu and Shipi peralkaline granitoids were dated at 160 and 130 Ma, respectively, and are explained by the melting of meta-igneous rocks with TDM2 of 1.70 Ga and 2.01 to 2.05 Ga. The Pucheng A-type granitoid was emplaced at 110 and 102 Ma and is divided into two groups. Group 1 comprises the extremely felsic A-type granite (SiO2 > 73 wt.%), high FeOT/MgO (>16), and low Ga/Al ratios and Zr + Nb + Ce + Y content. Group 2 contains a less-evolved A-type granite with high Zr + Nb + Ce + Y content. Both groups can be explained by dehydration melting of meta-igneous rocks at low pressure. The diversity between the two groups was caused by different physicochemical environments. The Yongxing I-type granite, which is abundant in mafic microgranular enclaves, was emplaced at 106 ± 1 Ma. It can be explained by the mingling of mantle-derived and crustal-derived magmas. Our data along with previously published data demonstrate that the northern Fujian Province was under an extensional environment from 160 to 100 Ma. This was caused by the NW-trending subduction of the palaeo-Pacific Plate. The Jurassic peralkaline granitoids were probably generated in a rift environment as a result of the reactivation of pre-existing faults caused by the initial subduction of the palaeo-Pacific Plate. The Early Cretaceous peralkaline granitoids may represent a rift environment as a tectonic response from low-angle subduction to an increasing subduction dip angle. The 100 Ma granitoids derive from an intraplate extensional environment caused by the roll-back of the palaeo-Pacific Plate.

Read more

Linking the Qinling Orogeny with the Chang 7 shale (Triassic Yanchang Formation) deposition: Evidence from major, trace, and rare earth element geochemistry

The lacustrine organic-rich shale of Triassic Chang 7 Member is an unconventional shale oil reservoir that is a major hydrocarbon target in the Ordos Basin, northern China. This study uses major, trace, and rare earth element (REE) abundance documented through a 115-m-thick core obtained from the western part of the Ordos Basin to assess dominant controls on temporal variations in total organic carbon in the Chang 7 shale. Our results suggest that total organic carbon trends in the Chang 7 can be directly linked with the tectonic and magmatic evolution of Qinling orogen. The Chang 73 and Chang 72 submembers of the lower part of the Chang 7 were likely deposited in association with heighted tectonic and magmatic activity of the Qinling Orogeny. Sediments derived from these submembers display light REE and trace element compositions similar to the Tianshui rhyolites suggesting deposition contemporaneous with magmatic and/or volcanic activity of the Qinling Orogeny. Higher nutrient concentrations introduced by volcanic eruptions would have stimulated primary productivity in the surface water enhancing the organic carbon flux to the lake bottom. In addition, higher rates of mountain building in the Qinling Orogeny may have been responsible for increasing subsidence rate of the southern Ordos Basin, also favourable for the establishment of anoxic bottom-water conditions. In contrast, lower light REE and trace element abundances of the Chang 71 submember at the top of the Chang 7 suggest deposition during a tectonically and magmatic quiescent period of the Qinling Orogeny. The diminished volcanic flux could have lowered the rate of primary productivity. Moreover, a reduced rate of mountain building would have decreased the rate of foredeep subsidence favouring shallower water and oxic/suboxic bottom-water conditions. Thus, the hydrography of the lake during accumulation of the Chang 71 was not conducive for organic carbon accumulation. Together, the correlation of the Qinling mountain building and magmatism and deposition of the Chang 7 shale provide evidence for the role of tectonism in the accumulation of organic carbon enriched sediment.

Read more

Detrital zircon U–Pb ages, Hf isotopic constraints, and trace element analysis of Upper Cretaceous–Neogene sedimentary units in the Western Nepal Himalaya: Implications for provenance changes and India–Asia collision

The present study integrates detrital zircon U–Pb–Hf isotopic analysis from 13 sandstone samples from an Upper Cretaceous–Miocene sedimentary sequence in Nepal Himalaya to determine their provenance. These sequences constrain a shifting of provenance from south to north. The U–Pb ages from Upper Cretaceous–Palaeocene strata (Amile Formation) mainly cluster between ~1,860 and 1,400 Ma with a peak at ~1,630 Ma and an absence of grains younger than the Palaeoproterozoic (1,400 Ma) age. The detritus yielded positive detrital zircon ƐHf (t) values (as high as +10). However, the detrital zircon U–Pb ages from Eocene–Miocene sequence cluster at ~500–650, ~700–900, ~1,600–1,850, and ~2,500 Ma, and in addition, they have both positive and negative ƐHf (t) values (+11 to −25). This finding further elucidates that the detritus in the Amile Formation was entirely sourced from India, which changed following the time of the Bhainskati Formation deposition, to a mixture of both Asian and Indian affinities (the Himalayan region). This change in source region marks the possible time of the India–Asia collision during this transition phase, that is, Late Palaeocene–Earliest Eocene.

Read more

Enigmatic nodule-bearing rocks in the mafic to ultramafic Diana Mills pluton, Piedmont Province, Virginia

The Silurian Diana Mills pluton is a metamorphosed mafic–ultramafic body in the Piedmont Province of Virginia. Major rock types in the pluton include metadiorite and chlorite-amphibole rock, with lesser amounts of hornblendite. However, the most visually striking rocks, found at 4 locations, consist of tan-weathering nodules in a dark green matrix. The purpose of this study was to investigate the origin of these nodular rocks. All rocks show evidence of greenschist-facies metamorphism, but relict igneous textures are preserved locally. Apart from the nodules, Ca-amphibole is ubiquitous in all rock types. Nodules are dominated by serpentine or talc (+magnetite ± chromite), and many show a thin (≤5 mm) radially oriented shell of serpentine against adjacent matrix. The nodules are ultramafic (Mg# ~80–90, up to7,563 ppm Cr, up to 2,038 ppm Ni), and their normative mineralogy is dominated by olivine and orthopyroxene. Thus, they represent metamorphosed harzburgites and pyroxenites. Matrix minerals are dominantly amphibole + chlorite, along with variable amounts of talc + magnetite. Some matrix samples are chlorite-rich, probably reflecting metasomatic reaction with nodules (i.e., they are small-scale “blackwalls”). Matrix samples are also ultramafic (high Mg#, Cr, and Ni). The matrix of the nodule-bearing outcrops is essentially the same as other chlorite-amphibole rocks elsewhere in the pluton. We consider these rocks to represent emplacement of an original hornblende peridotite crystal mush (a mixture of crystals and hydrous melt), which locally carried harzburgitic nodules. The nodules most likely represent the earliest-formed cumulates from the Diana Mills parent magma.

Read more

Genesis of the Xishadegai Mo deposit in Inner Mongolia, North China: Constraints from geology, geochronology, fluid inclusion, and isotopic compositions

The Xishadegai Mo deposit is a medium-sized deposit located in the northern margin of the North China Craton. The Mo mineralization is structurally controlled, and spatially and temporally related to the Xishadegai felsic intrusive rocks. Ore bodies mainly occur as quartz veins/veinlets in altered granitic rocks associated with potassic, phyllic, argillic, and fluorite alterations. The ore-forming process can be divided into 3 stages: Stage I K-feldspar-quartz ± molybdenite, Stage II quartz-pyrite-molybdenite-muscovite ± fluorite, and Stage III quartz-fluorite ± muscovite. Four types of fluid inclusions were distinguished in smoky grey and dark grey quartz of the main-ore stage (II), including two-phase aqueous inclusions, CO2-H2O inclusions, daughter mineral-bearing multiphase inclusions, and minor vapour aqueous inclusions. The fluid inclusions in smoky grey and dark grey quartz are homogenized at temperatures of 195–350 °C and 191–291 °C, respectively, with calculated salinities of 3.9–11.1% NaCleq and 31.5–33.0% NaCleq, respectively. The ore-forming fluids belong to a H2O-CO2-NaCl system characterized by abundant CO2, moderate to high temperature, and low to high salinity. The δ18OH2O and δD values of ore-stage quartz vary from −0.2‰ to 0.9‰ and from −120‰ to −104‰, respectively, indicating that the ore-forming fluids were evolved from magmatic water and gradually mixed with significant amounts of meteoric water. Sulphur and lead isotopic compositions indicate that the ore materials were mainly derived from magmatic sources. Zircon LA-ICP-MS U–Pb dating on the mineralized porphyritic moyite yielded a weighted mean age of 235.1 ± 2.0 Ma, corresponding to the Triassic postcollisional setting following the closure of the Paleo-Asian Ocean between the Siberian Plate and the North China Craton. The εHf(t) values and TDM2 ages range from −15.0 to −12.8 and from 2.2 to 2.1 Ga, respectively, suggesting that the Xishadegai granite was mainly generated by melting of Paleoproterozoic crustal components. Collectively, evidence from geology, fluid inclusion, H-O-S-Pb isotopes, and geochronology suggests that the Xishadegai deposit could be classified as a magmatic–hydrothermal vein Mo deposit. Phase separation (immiscibility and boiling) was the most likely mechanism for ore deposition.

Read more

Formation age and geodynamic setting of the Neoproterozoic Shalong iron formation in the Central Tianshan, NW China: Constraints from zircon U–Pb dating, geochemistry, and Hf–Nd isotopes of the host rocks

The recently discovered Shalong iron deposit in the Central Tianshan with a reserve of 14 Mt of iron at 25–30 wt.% Fe shows the typical iron formation geological characteristics: The host rocks are mainly metamorphosed volcanic rocks including quartz hornblende schist, mica quartz schist, and plagioclase amphibolite; the iron orebodies are stratiform and bedded, indicative of strata-controlled deposits; metallic minerals are dominated by magnetite with haematite and ilmenite in minor quantities, whereas gangue minerals are quartz, amphibole, chloritoid, and plagioclase; and ore textures are banded and laminated. Here, we present LA-ICP-MS zircon U–Pb ages, whole-rock geochemistry, Sm–Nd isotope, and zircon Lu–Hf isotope analyses on the host rocks in order to constrain the timing and geodynamic setting of the deposit. The geochemical characteristics, Sm–Nd isotope, and zircon Lu–Hf isotope compositions of the host rocks indicate that their protoliths were likely basic and felsic volcanic rocks. The bimodal kind rock types, the juvenile Nd–Hf isotopic characters, and the within-plate trace element geochemical features of the host rocks together indicate that these rocks formed in a rift-like extensional setting. Zircon U–Pb dating of the host rock interlayer suggests that the Shalong iron formation was formed in the Neoproterozoic (ca. 760 Ma). Combined with geological characteristics, close association with meta-volcanic rocks, absence of glacial deposits, and within-plate geochemical and juvenile isotopic signatures of the host rocks, the Shalong iron deposit is interpreted to be an Algoma-type Neoproterozoic iron formation. The formation of this deposit can be linked to rift-like volcanic activities related to the breakup of the Rodinia supercontinent. Our results in conjunction with previous studies suggest that the Central Tianshan should be related to the breakup of Rodinia, as indicated by the Neoproterozoic rift-related igneous events of the Central Tianshan.

Read more

Whole-rock geochemistry and zircon Hf isotope of Late Carboniferous–Triassic sediments in the Bogda region, NW China: Clues for provenance and tectonic setting

Detailed petrological, whole-rock geochemical, and zircon Hf isotopic researches were conducted on the Late Carboniferous–Triassic sedimentary rocks in the Bogda region to evaluate the effects of weathering, sorting, and alteration, as well as to understand their provenance and tectonic setting. These rocks show variable major element compositions, but most are generally similar to post-Archean Australian shales. The relatively narrow range of TiO2–Zr variation and good correlation between Th/Sc and Zr/Sc signify no obvious sorting and recycling of the sedimentary rocks. The relatively high index of compositional variability (0.8–3.1) and low chemical index of alteration values (44.4–76.4) of the sedimentary rocks indicate that they are immature and probably undergo weak to moderate chemical weathering. In the chondrite-normalized diagrams, almost all the samples are distinguished by moderately enriched light rare earth element and flat heavy rare earth element patterns with negative Eu anomalies (Eu/Eu* = 0.5–0.8). In conjunction with their Th/Sc, Zr/Sc, La/Th, Zr/Al2O3, TiO2/Zr, Co/Th, and La/Sc ratios, we infer that the major sources of these sedimentary rocks were the intermediate–acidic igneous rocks from the North Tianshan and Yili–Central Tianshan. Combining the orientation of the Bogda region in the Palaeozoic that was roughly perpendicular to the Tianshan and/or East Junggar orogens with the sedimentologic, petrologic, and tectonic researches, we suggest that the Bogda region was a rift basin that occurs at high angles to the orogenic belt. On the other hand, the zircons from these sediments have minor Precambrian grains (<0.2%), positive εHf(t) values (mostly +7.1–+15.0) and young 2-stage Hf model ages (major peak <1,000 Ma), suggesting that a juvenile continental crust exists as the basement of the Junggar Basin.

Read more

Sedimentary budget of the Southwest Sub-basin, South China Sea: Controlling factors and geological implications

Calculation of the sediment budget in the South China Sea abyssal basin lacks constraints from drilling data and reinterpretation of seismic data. On the basis of six multichannel seismic profiles across the Southwest Sub-basin (SWSB) and the drilling data from International Ocean Discovery Program Expedition 349, we divided the Cenozoic sediments into four sedimentary units. The sedimentary budget of the abyssal basin at different geological times was calculated. The previous works in the Mekong continental shelf and slope areas were integrated to calculate the sediment budget of the whole SWSB. Our researches show that the sediment budget in the SWSB increased during the Palaeogene and reached its first peak because of intensified erosion, which might have resulted from the continued uplift of the Tibetan Plateau and accelerated southeastward extrusion of the Indo-China Peninsula. Since the Late Miocene, the sediment budget was mainly influenced by the East Asia monsoon, that is, the intensified winter monsoon decreased the sediment budget of the entire area during the Late Miocene, whereas the strengthened summer monsoon increased the sediment budget during the Pliocene and reached a peak during the Pleistocene. The sediment budgets of the Mekong continental shelf, the Mekong continental slope, and the abyssal basin have distinct characteristics, relating to the infilling sequence of the terrestrial sediments in different regions. The sediment provenance of the SWSB was mainly from the Indo-China Peninsula, the Nansha area, and the Palawan before the Late Miocene. After that time, sediments were mainly transported from the modern Mekong River.

Read more

An ichthyosaur from the UK Triassic–Jurassic boundary: A second specimen of the leptonectid ichthyosaur Wahlisaurus massarae Lomax 2016

The holotype of Wahlisaurus massarae is known only from a partial skull and postcranial skeleton from the Lower Jurassic, collected near Normanton on Soar, Nottinghamshire, UK. It is diagnosed relative to other ichthyosaurs on the basis of autapomorphies of the coracoid and a unique combination of characters. Here, we report a second specimen of W. massarae. The new specimen comprises a practically complete right coracoid that clearly shows the unique morphology of W. massarae. This specimen was collected in situ from a quarry in Somerset, from the base of the Blue Lias Formation, which corresponds to the Triassic–Jurassic boundary (uppermost Rhaetian or lowermost Hettangian), extending the geographic and stratigraphic range of the species. Furthermore, the coracoid of the new specimen is 20% anteroposteriorly longer than in the holotype, representing a larger individual. In light of preparation and the identification of additional fragments of the holotype, a redescription of the skull morphology of the holotype, as well as comparison with the skull roof of Leptonectes tenuirostris, is included.

Read more

At the southern limits of the Devonian reef zone: Palaeoecology of the Aferdou el Mrakib reef (Givetian, eastern Anti-Atlas, Morocco)

Devonian reefs of north-western Gondwana represent the southernmost record of shallow-water coral reefs in the Palaeozoic. However, few studies have attempted to date palaeoecological reconstructions of these high-latitude reefal buildups. This study provides the first detailed palaeoecological analysis of Aferdou el Mrakib, an isolated, Givetian coral-stromatoporoid reef, which developed in a semirestricted basin in the south-eastern part of the Rheic Ocean. The study documents spatial facies variability and succession of faunal replacements accompanying progressive reef accretion towards the sea surface. The investigations included both autochthonous communities found at the base of the reef and, partially, within the reef core, and allochthonous deposits of reef-derived skeletal debris that accumulated in the fore-reef setting. Contrary to some previous suggestions, the study shows that the Aferdou reef shared many characteristics of classical Middle Devonian coral-stromatoporoid buildups, including the ecological succession, limited role of calcareous algae, and development within the range of the euphotic zone, but likely below the zone of regular water agitation. Critical factors in the facies development and temporal changes in the character of reef building were the palaeobathymetry, dominant sedimentary and circulation regimes, level of wave energy, and, possibly, light availability. Distinctive features of the palaeoecology of Aferdou el Mrakib are the dominance of massive colonies of heliolitid tabulates and a subordinate role of massive stromatoporoids, both explained here primarily as a result of increased water turbidity in the high-latitude sedimentary basin. The growth of the high-latitude coral-stromatoporoid reefs in the south-eastern Rheic Ocean was favoured by a combination of the exceptionally warm climate and plate tectonic configuration typifying the Devonian. Of critical importance appears the palaeogeographic position of the Rheic, which resulted in the seawater circulation in the ocean being dominated by tropical water masses, with restricted inflow of cold water from the circumpolar oceanic circulation.

Read more

Identifying Early Carboniferous bimodal volcanic rocks and geochemical characteristics in the Atengtao Mountain, Yili Block (Chinese western Tianshan)

The Early Carboniferous bimodal volcanic rocks are firstly recognized in the Atengtao Mountain, Yili Block (Chinese Western Tianshan). The bimodal volcanic rocks are composed of dominant rhyolite and subordinate basaltic andesite. The SiO2 contents of basaltic andesites span a range of 52.08 to 60.44 wt.%, whereas those of the rhyolites range from 72.50 to 76.78 wt.%, showing a sharp gap between 60.44 and 72.50 wt.% for SiO2 content. Zircon U–Pb dating of rhyolitic sample yielded crystallization age of 340 ± 6 Ma, which is interpreted as the extrusive age of the bimodal volcanic rocks. Basaltic andesite samples belong to the calc-alkaline series, whereas rhyolites are peraluminous. The basaltic andesite samples display enrichment in LREE ((La/Yb)N = 3.16–6.27), and nearly no obvious Eu anomalies (δEu = 0.86–1.21), with relative enrichment in large ion lithophile elements (Rb, K, Sr, U) and depletion in high field strength elements (Nb, Ta). Compared to the basaltic andesites, the rhyolites show enriched LREE patterns ((La/Yb)N = 7.05–9.27) and significantly negative Eu anomalies (δEu = 0.36–0.62), with remarkably negative Nb, Ta, P, Ti, and Sr anomalies, which is consistent with A-type granites. The rhyolites have positive εHf(t) values from 1.5 to 4.8 and TDM2(Hf) values from 1,036 to 1,247 Ma. The basaltic andesites are interpreted as resulting from partial melting of an enriched lithospheric mantle source that was metasomatized by subduction-related components, whereas the rhyolites were derived from basaltic melt-induced reworking of Proterozoic juvenile crustal material. Based on our data, and taking into account the regional geology, a model of back-arc setting was proposed for the Early Carboniferous formation of the bimodal volcanic rocks in the Atengtao Mountain. Therefore, we suggest that the volcanic rocks were likely induced by the northward subduction of the South Tianshan Ocean. The subduction resulted in opening of a back-arc basin in Atengtao Mountain, leading to upwelling of mantle that had produced the basaltic andesites. Moreover, this process provided heat to remelt the crustal materials and form the rhyolites.

Read more

Depositional environments and sources for the middle Eocene Fulra Limestone Formation, Kachchh Basin, western India: Evidences from facies analysis, mineralogy, and geochemistry

The middle Eocene Fulra Limestone Formation of the Kachchh Basin, western India was investigated to interpret depositional environments and sources of limestones based on facies analysis, mineralogy, and major, trace, and rare earth elements (REEs) geochemistry. Facies stacking pattern of the Fulra Limestone Formation exhibits an overall shallowing-upward sedimentary succession developed in the middle to inner ramp to lagoonal set-up. Staining of the thin sections and X-ray diffraction patterns show dominance of calcite mineral. The CaO, ranging from 40.45% to 49.69%, is the dominant oxide among the major oxides followed by SiO2 (4.04–10.3%). The dominance of CaO over MgO suggests that the major mineral phase has been calcite and no dolomitization occurred during the deposition of the Fulra limestone. The low proportion of SiO2 and Al2O3 suggests low siliciclastic input during the precipitation of the Fulra limestone that is confirmed by the positive correlation of ∑REE with Al2O3, Fe2O3, Ni, Th, Sc, and Y and the negative correlation of SiO2, Al2O3, and Sr with CaO. The average value of REE contents in the Fulra Limestone Formation are lower (*16.02 ppm) than the average value of typical marine carbonates (*28 ppm). The Post-Archean Australian Shale-normalized REE + Y patterns of the Fulra limestone exhibit seawater like shale-normalized REE + Y pattern, whereas slightly lower average value of Y/Ho ratio (34.60) than the typical seawater value (~44–74) suggests modification of the seawater, to some extent, by input of freshwater in a coastal environment. The REEs of the Fulra Limestone Formation have similarity with the carbonates of the Arabian Sea and/or shallow marine carbonates with exception of a few elements. It is envisaged from this study that the Fulra limestone was deposited in a shallow marine to coastal set-up where mixing of the continental material in seawater was feasible.

Read more

Radiometric dating of late Quaternary loess in the northern piedmont of South Tianshan Mountains: Implications for reliable dating

Reliable chronologies are prerequisites when interpreting proxy records in terrestrial archives of Quaternary climate and environmental change. Optically stimulated luminescence (OSL) dating and accelerator mass spectrometry radiocarbon dating (AMS 14C) are commonly used to date late Quaternary loess deposits in the Chinese Loess Plateau, but the range and reliability of the two methods in Central Asia are still debated. In this study, we investigate both fine-grained quartz OSL and AMS 14C dating of a late Quaternary loess section located at the northern piedmont of the South Tianshan Mountains in Central Asia and discuss the reliability of the two radiometric dating methods. The results show that the OSL and AMS 14C ages are generally consistent with the stratigraphic sequence when the ages are younger than 25 cal ka BP, which means that both can be used to establish a reliable chronology in the Ili Basin. But beyond this age, the OSL dating method seems to be a more reliable approach. The results also supported previous dates based on medium-grained quartz OSL dating of the Ili loess in the southern piedmont of the North Tianshan Mountains. Radiocarbon ages older than 25 cal ka BP should be treated with caution, and attention must be paid to the influence of pedoturbation on OSL signals in the Central Asian loess. Multiple dating approaches for mutual authentication and exploring new dating materials are suggested in further loess chronological research. These findings will be helpful in establishing a reliable timescale and in reconstructing high-resolution environmental change in Central Asia.

Read more

Late Devonian benthic ostracods from western Junggar, NW China: Implications for palaeoenvironmental reconstruction

This study focuses on the palaeoecology of the Late Devonian ostracods based on more than 4,500 carapaces and valves collected from stratal sections in western Junggar, NW China. According to the ostracod faunas from the Hongguleleng Formation, 3 ecological assemblages are proposed for open oceanic island arc context, that is, Ostracod Assemblage-1, Ostracod Assemblage-2, and Ostracod Assemblage-3. They characterize the foreshore, nearshore, and offshore environments, respectively. The water energy should be the first environmental factor controlling the composition of the ostracod assemblages from the Lower Member of the Hongguleleng Formation, and salinity fluctuations could be also involved.

Read more

Petrogenesis of basalt from the middle Okinawa Trough: New insights from olivine-hosted melt inclusions

Silicate melt inclusions (SMIs) in basalt from the middle Okinawa Trough are investigated in detail to understand the source geochemical characteristics and petrogenesis of the basalt. The middle Okinawa Trough basalts contain abundant SMIs within olivine phenocrysts. The major element compositions of the SMIs are analysed using an electron microprobe. After correcting for olivine postentrapment crystallization, the major element compositions of the SMIs are more variable in terms of SiO2 (49.7–54.8 wt%) and MgO (5.1–7.2 wt%) than those of the host basalt (SiO2, 52.4 wt%; MgO, 5.7 wt%), which could indicate the existence of more primitive melts than erupted bulk rock. The major element compositions of the SMIs indicate that the SMI-recorded melt evolution process was mainly dominated by the fractional crystallization of olivine. According to laser ablation inductively coupled mass spectrometer analyses, the trace element compositions of the SMIs are similar to those of the host bulk rock and feature obvious enrichment in Pb and large-ion lithophile elements relative to high field strength elements, rare earth elements, and highly negative Nb and Ta anomalies. The low Ce/Pb ratios, high Pb contents, and trace element distribution patterns in the SMIs are related to contributions from subducted sedimentary components in the magma source. Based on the electron microprobe analyses, the chemical composition of the olivine suggests that the lithology of the basalt source may be pyroxenite. The basalts may have been generated by the partial melting of pyroxenite during interactions between mantle-wedge peridotite and Si-rich melts that were released from the subducted slab.

Read more

Accumulation conditions and prospective areas of shale gas in the Middle Jurassic Dameigou Formation, northern Qaidam Basin, Northwest China

The Qaidam Basin is one of the most resource-rich onshore petroliferous basins in Northwest China. The Middle Jurassic Dameigou Formation crops out along the northern margin of the basin and consists of a sequence of lacustrine sedimentary units, including sets of thick, black shale unit. The shale gas exploration well CY1 had shown significant logging anomalies that have been interpreted to indicate the presence of shale gas. This gas signature well provides valuable shale rock and shale gas parameters that enabled the identification of three areas for future shale gas exploration in this region, namely, to the south-east of Lenghu, in the area around Yuqia, and to the south-east of Huaitoutala. This study presents new data for organic-rich shale samples of the Dameigou Formation that were collected from boreholes and outcrops that enable the determination of some of the conditions of shale gas accumulation. The black shale unit within seventh interval of the Dameigou Formation is about 20 to 80 m thick. Its total organic carbon content is high (average of 2.62%), kerogen types are II1 and II2, and thermal maturity is mid to high (average Ro of 0.89%). It contains low abundances of brittle minerals and high abundances of clay minerals, micrometre to nanometre pore sizes, and high gas contents (average of 1.11 m3/t). These shale properties favour both shale gas generation and accumulation within the northern Qaidam Basin and are comparable to the properties of shales within shale gas fields in North America and elsewhere in China.

Read more

Exploring the end-Ordovician extinctions in Hirnantian near-shore carbonate rocks of northern Guizhou, SW China: A refined stratigraphy and regional correlation

Richly fossiliferous Hirnantian shelly strata of near-shore facies in northern Guizhou, South China, known as the Kuanyinchiao Formation, superbly record glacioeustatic sea-level fluctuations and benthic faunal turnover. Recent studies of the temporal and spatial distribution of these carbonates and shelly fossils permit a critical stratigraphic revision and establishment of a robust regional stratigraphic correlation. The formation is revised to include three informal subdivisions, that is, units A, B, and C, in ascending order. Unit B of the formation typically is dominated by peloidal or oolitic grainstones, and unit C is composed of skeletal wackestone and calcareous mudstone, both units sharing distinctive coral and brachiopod faunas. This contrasts sharply with unit A of the formation, consisting of mudstone, silty mudstone, or calcareous mudstone, that yields the cool-water Hirnantia fauna and associated coral fauna. In view of the presence of carbonate ooids and peloids, rugose corals, and a distinctive brachiopod assemblage, all indicative of warm-water conditions, unit B, as well as unit C yielding the same shelly fauna, is interpreted as representing postglacial sedimentation immediately following the major Hirnantian glaciation, thus marking a significant climatic shift. Similar warm-water carbonate rocks have been recognized in a number of regions along the margin of the Qianzhong Oldland, including Bijie, Renhuai, Tongzi, and Fenggang of northern Guizhou. Such a vast distribution area of these rocks indicates that postglacial carbonates are more widespread on the Yangtze Platform of South China than previously thought, providing a rare window into rocks and fossils of the survival interval immediately following the extinction event associated with the Hirnantian glacial episodes.

Read more

Using dated molecular phylogenies to help reconstruct geological, climatic, and biological history: Examples from Colombia

Attempts at historical reconstruction are based on limited data. We are more likely to produce accurate historical reconstructions by utilizing information from diverse sources and pooling data within the relevant research communities which will allow us to build up a moving picture of the geological, climatic, and biological evolution of our planet. We suggest that dated phylogenies of plants can contribute greatly to a better understanding of Earth history. Timing of phylogenetic splits of lowland restricted lineages on either side of the Andes could provide information on the timing of montane uplift and associated climatic changes. The timing of the arrival and diversification of organisms restricted to specific climatic regimes at a particular altitude can provide information on the age at which mountains reached a height adequate for that climate once corrected for global climate changes. As a model for study, we discuss how dated phylogenies in biome rich Colombia may contribute to an understanding of geological and climatic change in north-western South America. Lowland wet forest restricted lineages separated from the mid-Miocene, whereas lineages primarily restricted to mid-altitude cloud forests began to diversify from the mid- to late-Miocene and the majority of high-altitude Páramo lineages began to diversify during the Pliocene. The age of diversification of altitudinally restricted lineages therefore gives an indication of the age at which particular altitudes may have been reached.

Read more

The evaluation of soil stability in loess hilly and gully region of Northern Shaanxi based on GIS

Based on the previous research in loess hilly region of Northern Shaanxi, this paper takes the soil erosion degree as the main measure of soil stability and the soil utility, annual average rainfall in flood season (from June to September), and topography, as the main measure indexes of soil stability. After that, the evaluation system of soil stability in loess hilly region of Northern Shaanxi can be constructed, which can be done by special analysis of GIS. The results illustrate that the soil stability showed a trend of high south-east and low north-west. Soil with good stability is mainly distributed in Ganquan County, and Yanchuan County, where vegetation coverage is high, vegetation types are mostly forest land and grassland, ecological environment is good, and precipitation erosion effect is not significant. Soil with intermediate stability is mainly distributed in Baota district and its surrounding areas, where the main vegetation types are bush fallow and grassland, and the terrain is flat and gently rolling. Soil with the worst stability is mainly distributed in Suide County, and Wuqi County. The area is mostly sandy and desert, the terrain is fragmented, soil is loose, vegetation cover is not high, making the soil the worst soil stability, and strong rainfall conditions are prone to soil erosion. The prerequisite of the implementation of soil consolidation projects is having evaluation on soil stability. The research results can be the theoretical evidence, and implement guarantee of regional soil exploitation and reorganization, and the reference to enhancing the assurance of ecological safety.

Read more

3D imaging of the crust and upper mantle beneath the Arctic Ocean from Rayleigh-wave group-velocity analysis

A 3D S-velocity model for the crust and upper mantle beneath the Arctic Ocean is determined from the analysis of fundamental-mode Rayleigh-wave group velocity. This new model allows the characterization of the principal features present beneath the Arctic Ocean, in terms of S-velocity, for a depth range from 0 to 400 km. In the depth range from 0 to 5 km, the basins with thicker sediments are imaged by the lowest S-velocity values (2.2–2.3 km/s), whereas the highest S-velocity values (3.2–3.3 km/s) are associated with a thin sedimentary cover. In the depth range from 5 to 10 km, the lowest S-velocity values (2.6–2.7 km/s) are also shown in areas with a thick sedimentary cover (>10 km). The higher S-velocity values (4.0–4.2 km/s) are associated with the areas in which the Moho depth is overcome. In the depth range from 10 to 35 km, also the S-velocity values are higher (>4 km/s) for the zones in which the Moho depth is overcome. The S-velocities determined in this study allows to consider the Alpha-Mendeleev Ridge (formed by the Alpha Ridge and the Mendeleev Ridge) as a continuous feature, because the Alpha Ridge and the Mendeleev Ridge are characterized by similar S-velocities (e.g., ~3.6 km/s for depths from 10 to 15 km). This region also can be considered as an oceanic large igneous province, in terms of S-velocity, because the determined S-velocity pattern is characteristic of these oceanic regions. In general, for the lithosphere (from 35 to 180 km depth), the S-velocity can be easily correlated with the different age of the Arctic Ocean regions and with the surface tectonic features, whereas for the asthenosphere and the mantle below it (from 180 to 400 km depth), this correlation is not clear. In general, a small S-velocity contrast (0.1–0.2 km/s) is found between the lithosphere and asthenosphere, which makes it difficult to determine this boundary. Opposite to this, the high S-velocity contrast (0.4–0.5 km/s) found at the asthenosphere base allows the easy determination of this boundary.

Read more

Controls of a Triassic fan-delta system, Junggar Basin, NW China

Both sedimentology and sequence stratigraphy variability in the deltaic area are distinguished in the fan-delta system of the Northwestern Junggar Basin. Sediment supply plays a significant role in controlling the fan-delta petrology, evolution, and depositional processes and the delivery of sediment into deeper parts of the basin. The results of this study show that variations in the lithology of the sediment supply control the fan-delta petrology and spatial and temporal variations in sediment supply influence the location and shifting of depocentres. A fining-upward fan-delta succession developed as accommodation rates outpaced the sediment supply rate. Frequent flood events were mostly recorded by sediment gravity flow deposits, which in turn influenced the geometry of the sandstone bodies and their reservoir quality. Hyperpycnal flow was a significant delta front process and an important mechanism for sediment delivery into deeper waters, favoured by the combination of denudation of coastal ranges and the fresh waters of the receiving basin. This study highlights that a detailed understanding of variations in sediment supply is essential to accurately predict sediment partitions in a delta system and the delivery of coarse-grained sediments into deeper parts of lacustrine basins.

Read more

Geochemical characteristics of Late Carboniferous volcanic rocks in South-eastern Heishanling, Beishan area, Xinjiang, China

The Palaeozoic Beishan Rift Valley zone in Xinjiang, China, is located between the Dunhuang and Kuluketage blocks, in the north-eastern margin of the Tarim Plate. Carboniferous–Permian volcanism is evident in this area. The Ganquan cycle, the main eruptive event of the Late Carboniferous in the Beishan region, produced large volumes of lava and was related to Carboniferous–Permian volcanism in the rift valley. Major element, rare earth element (REE), and trace element compositions of volcanic rocks were determined, with results indicating that volcanic rocks of the Ganquan cycle are mainly an association of sodic calc-alkaline rocks, with some alkaline compositions. The REE characteristics of all rock types indicate a continuous and progressive evolution, with a gradual increase in light REE enrichment, and positive to negative Eu anomalies reflecting crystallization of plagioclase. Trace element compositions show obvious depletion in Nb and Ta, and high abundances of large-ion lithophile elements, indicative of a subduction-related origin. It is concluded that the Late Carboniferous Beishan Rift Valley of the Ganquan cycle formed in a compressional tectonic environment, which generated a set of volcanic rock assemblages and sedimentary formations. Subsequent Early Permian volcanic activity was caused by large-scale delamination of the orogenic belt and thickened lithosphere, which resulted in large-scale ascent of partial melt from the mantle.

Read more

Depositional processes and sedimentation pattern in an intermontane basin: Insights from the Imphal Basin, Indo-Myanmar Range, NE India

Intermontane basins make a distinct category of sedimentary basins, but nevertheless, the depositional processes and sedimentation patterns therein are not well understood. Quaternary sediment-filled Imphal Basin in the Indo-Myanmar Range provides us with an excellent opportunity of lending insight into these sedimentological aspects of an intermontane basin. Detailed facies analysis brings out that five distinct facies associations (“FAA”–“FAE”) have developed in different parts of the basin, each one of which indicates a specific subenvironment of deposition. Facies characters and spatial distribution of facies associations reveal that the basin fill records interlinked lacustrine, fluvial, and alluvial fan processes. Comprising mud and peat, the facies association “FAA” represents deposits of a lake and lakeshore swamp. The facies association “FAB” comprises mud interfingered with crudely bedded gravel and represents suspension fallout in a lake with intervening gravel-rich, hyperconcentrated flows that diffusively plunged into the lakebed. The sand-dominated facies association “FAC” and gravel-dominated facies association “FAD” were deposited in fluvial domains of meandering and braided streams, respectively. Rimming the basin margin, the gravel-dominated facies association “FAE” was deposited in the alluvial fan–interfan areas. Widespread development of the lacustrine facies (“FAA” and “FAB”) around the Loktak Lake along with extensive peat development over the lacustrine mud suggests lakeshore regression and development of vast swamps under humid conditions. The regression under humid conditions could be a result of fast and enhanced influx of finer sediments into the lake that transcended the creation of accommodation space and ultimately led to faster infilling of the lake margins favouring development of vast swamps in due course of time.

Read more

The Central Canyon depositional patterns and filling process in east of Lingshui Depression, Qiongdongnan Basin northern South China Sea

Central canyons are considered to be important conduits for the transfer of sediment to the abyssal plains. High-resolution 3D seismic data and logging data from the east of Lingshui Depression are used to investigate the depositional elements, patterns, and filling processes of central canyon in the Qiongdongnan Basin, northern South China Sea. The Central Canyon in this segment is 30 km wide and more than 1,000 m deep. It was found to be characterized by being subparallel to the continental shelf. In this study, 6 types of deep-water gravity flow depositional elements were identified, including erosion surfaces, basal lags, turbidite channel complexes, mass transport deposits, lobe complex, and pelagic deposits. Each type of these depositional elements was found to have distinct external features, internal structures, and lateral characteristics in the seismic profiles. Meanwhile, these different assemblages of depositional elements could be composed of 5 depositional units. And in a vertical direction, the evolutionary history could be divided into 5 stages as follows: (a) a repeated cut and fill stage; (b) high net-to-gross channel complex dominated stage; (c) low high net-to-gross channel and pelagic shale dominated stage; (d) lobe complex deposition stage; and (e) mass transport deposit dominated stage. During Stage 3, the development of small-scale turbidite channels is the indicators of northern sediment supply. During Stage 4, 7 lobes have been identified in the lobe complexes. The evolution and depositional processes in the study area were most likely controlled by the negative relief induced by the palaeo-seafloor morphology, as well as the structural inversions of the Red River Fault and the basement faults. Additionally, the sediment supply, sea-level fluctuations, and tectonic activities also controlled and influenced the depositional processes, along with the internal architecture of the canyon.
The results of this study have potentially important implications for the improved understanding the vertical evolution of submarine canyons and also shed a new light on the reservoir potential of the Lingshui section of the Central Canyon.

Read more