New results of structural and lithostratigraphic investigations performed in the Urtier Valley, located south of Aosta Valley at the northern flank of the Gran Paradiso Massif, shed new light on the nappe stack architecture of the Western Alps. Two distinct Piemonte–Ligurian units were recognized in the area: (a) the blueschist facies Broillot unit, with a well-preserved lithostratigraphic succession that includes ophiolites and their metasedimentary cover and (2) the eclogitic Bardonney unit with clasts and/or slices of ophiolites embedded in a carbonate/quarzitic and/or metabasic matrix. Three major deformation stages have been recognized (D1, D2, and D3). D1 phase occurred during the peak metamorphic conditions (i.e., eclogite or blueschist facies). D2 and D3 phases are related to multistage exhumation processes, from early stacking to late refolding of former nappe contacts. Despite the intensity of deformation and metamorphism, the original lithostratigraphic succession of the Broillot unit allowed us to use a lithostratigraphic approach to reconstruct the structures in the study area. For the first time, differences in the nappe stack exposed south and north of the Aosta Valley are reported. In fact, contrary to what is observed in the north, the eclogitic units exposed in the Urtier Valley lay above and not below the blueschist units. A new model for the tectonic evolution, which ranges from stages of subduction to large-scale nappe refolding, is proposed. It is able to explain the difference in the architecture of the north and south of the Aosta Valley.
The Sanshilipu gabbro complex is located along the Shangdan Suture, which is important to understanding the evolution of the Qinling Orogen, Central China. The complex is mainly composed of norite, gabbro, and gabbro–diorite. Laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) zircon dating yields a weighted mean 206Pb/238U age of 414 ± 2.8 Ma, interpreted to represent the crystallization age of the gabbro complex. The complex has SiO2 contents ranging from 48.6 to 51.3 wt% with enrichment of Na relative to K, showing calc-alkaline feature. The studied samples are characterized by an enrichment of large-ion lithophile elements (Ba, Sr, and Pb) and depletion of HFSE (Nb, Ta, Zr, Hf, and Ti) and a large range of Nb/Ta ratios (3.7 to 11.8). They also have early flattened-LREE distribution patterns. These geochemical characteristics indicate that the Sanshilipu gabbro complex was formed in an island-arc environment. They have slightly enriched Sr–Nd isotope compositions with (87Sr/86Sr)i = 0.7047–0.7063 and ɛNd(t) from −0.44 to −4.63, indicating that the formation of the complex was involved in some subducted sediments and oceanic crust and fluid released from the subducted slab. The identification of island-arc-originated mafic rocks suggests that the onset time of the collision between the Yangtze Craton and the North China Craton should be younger than 415 Ma.
Sedimentological, stratigraphical, palaeontological, and geochemical evidence from outcrops of the Early Aptian (Cretaceous) succession in Mount Pagasarri (northern Spain) dated by ammonites is described to elucidate the origin of organic carbon-rich sediments and carbonate platforms in the Early Aptian of the northern margin of the Tethyan Ocean. A 1,600-m-thick section spans the latest Barremian to the end of the Early Aptian and is divided into 3 units: a lower marine nearshore, a middle rudist-coral shallow-water carbonate platform, and a deeper-water outer shelf. Deshayesites oglanlensis, D. forbesi, D. deshayesi and Dufrenoyia furcata ammonite zones are represented. Three organic carbon-rich intervals are recognised. The oldest (top Barremian) is correlated with the Taxy Level of France/Germany. A second organic layer in the lower part of the Deshayesites deshayesi Zone sediments correlates with the Selli Event of Italy. A third organic-carbon enriched layer in the D. furcata Zone sediments post-dates the Aparein Level of the Aralar area in Spain. Carbon isotope curves reveal a brief negative excursion followed by a more prolonged positive trend during the Early Aptian interpreted as oceanic anoxic event 1a (OAE1a). Three carbonate platform growth phases are recognised in the Basque-Cantabrian Basin with OAE1a being developed between carbonate platform Phases 1 and 2. The Peñascal Limestones (late Early Aptian) represent carbonate platform Phase 3 of the Basque-Cantabrian Basin succeeded by ammonite-bearing marls of the latest Early Aptian, reflecting platform demise and subsequent marine flooding. The Mount Pagasarri area is a key locality for study of the oceanic anoxic events in the Greenhouse Earth of the Early Aptian.
The current study is carried out the bulk mineralogical and geochemical investigation of the Early Eocene Sohnari coal-bearing sedimentary rocks from the southern Indus Basin in Pakistan. The sediments of the Sohnari Member of Laki Formation are composed of shale and sandstone. Their provenance and tectonic setting can well constrain the evolution of the Indus Basin and help solve the controversy regarding the timing of the India–Eurasia collision. X-ray diffraction results confirm the existence of abundant quartz, zircon, and hematite. The assemblages of hematite, calcite, halite, and gypsum indicate the depositional history of Early Eocene time of southern Indus Basin had with dry, low temperature, and high salinity conditions. The geochemical results demonstrate that the Sohnari sediments are dominated by mature sublitharenites derived predominantly from quartz sedimentary and felsic rocks with minor arc-related mafic inputs. The chemical index of alteration and plagioclase index of alteration of the Sohnari Member indicate the sediments experienced low to high weathering conditions in the southern Indus Basin during Early Eocene time. The provenance characteristics suggest that the Early Eocene sediments were deposited in a passive margin, instead of a foreland basin. Therefore, we propose that India might still have not collided with Eurasia in the Ypresian of Early Eocene (56–47.8 Ma).
Unconventional tight oil reservoirs have recently emerged as a significant source of oil worldwide, and juxtaposed to organic-rich source rocks, tight mixed siliciclastic–carbonate sedimentary reservoirs are widely found. In China, such reservoirs are mainly found in saline lacustrine basins. In this paper, we conduct a systematic study on advantageous source–reservoir combinations (SRCs) in the Cretaceous Xiagou Formation in the Qingxi Sag of the Jiuquan Basin, a tight saline lacustrine and mixed siliciclastic–carbonate sedimentary oil reservoir. Mixed sediments result in multiple SRCs as follows: self-contained SRC, thick interbedded-type SRC, thin interbedded-type SRC, and down generated-up stored SRC. Geochemical characteristics of organic matter in the Cretaceous Xiagou Formation are established on the basis of Rock-Eval pyrolysis and total organic carbon (TOC) and vitrinite reflectance analyses. TOC (0.5–4.12%, av. 1.62%) and S2 (0.36–26.63 mg/g, av. 5.62 mg/g) indicate a good hydrocarbon generative potential. The organic matters contain predominantly Type II kerogen with minor contributions from Type I kerogen. The samples analysed have vitrinite reflectance in the range of 0.65–1.0%Ro, and pyrolysis Tmax in the range of 430–450 °C indicate that the source rocks are early mature to mature and within the main stage of oil generation. The tight oil resources are divided into 4 classifications according to TOC versus S1 diagram, and dolomitic mudstone in lower K1g3 and K1g1 is primarily considered to be potentially generative and productive. Reservoir characterization such as porosity, permeability, and pore-size distribution derived from nuclear magnetic resonance indicates that nano-meter-sized pores dominate pore size distribution, regardless of the lithology. Fractures contribute to the creation of relatively higher productivity reservoirs. The statistics of oil testing results shows that the daily oil productions increased basically from the down generated-up stored SRCs to thin interbedded-type SRCs to thick interbedded-type SRCs to self-contained SRCs. The movable oil saturations derived from nuclear magnetic resonance have the same changes as daily oil production. The self-contained SRCs and the thick interbedded-type SRCs are identified as advantageous SRCs that are important for tight oil prediction.
The tectonic affinity of the Ama Drime Massif in central Himalaya remains a subject of debate. We provide evidences of the metamorphic conditions and new geochronological data for orthogneiss and paragneiss in the Riwu area, eastern side of Ama Drime Massif. The peak P–T conditions for garnet gneiss is 13–14 kbar and 750–765 °C using phase equilibria modelling. The zircon U–Pb dating results reveal that the orthogneiss has a crystallization age of 1,848 ± 12 Ma and zircon rims age of 1,767 ± 18 Ma. Three major age peaks characterize all detrital zircons from the paragneisses: 975, 1,161, and 1,749 Ma. These new data integrated with previous literature data indicate that (a) the Ama Drime Massif might be ascribed to the Greater Himalayan Series; and (b) the Himalayan terrane may have been located closer to both the India and Lhasa terranes during the amalgamation of the supercontinent Columbia.
The Middle Triassic (Anisian) Otter Sandstone Formation of the Wessex Basin (UK) has long been considered a single undivided geological formation, but building on previous work, it is shown here to have a remarkably well-defined internal stratigraphy that is evident not only in the fluvial sedimentary architecture but also in the colour, grain size, sorting, and whole-rock geochemistry of the sandstones. The Otter Sandstone Formation is here divided into four named members of very different character (West Down Member, Otterton Ledge Member, Chiselbury Bay Member, and Pennington Point Member). The study highlights the importance of outcrop-to-subsurface correlation in stratigraphic studies, which suggests several major unconformities within the Otter Sandstone Formation. The Otter Sandstone Formation provides evidence for an increase in humidity throughout the early Middle Triassic, with an upward decrease in the proportion of calcrete and an apparent increase in channel size. Whole-rock major and trace geochemistry (chemostratigraphy) is widely used in the subdivision and correlation of red-bed sheet sandstones, and it is shown here that data, even when used in a very raw and unprocessed way, can be a useful stratigraphic discriminant. Fluvial sheet sandstones are often regarded as relatively homogeneous aquifer units, but this study shows that changes in fluvial style and background climate can introduce major changes to the geometry and physical properties of the aquifer or reservoir. The recognition of thin marker beds such as palaeosols can be particularly critical for the correlation of thick fluvial sheet sandstones.
Coupled fission-track and U–Pb analyses of detrital zircons are presented. A Cretaceous intracratonic basin formed between 93 to 60 Ma, the Bauru Basin, is investigated to distinguish its provenance and information about the main orogenetic events of the South American Platform. The results of U–Pb zircon dating suggest that Rhyacian (Transamazonian Cycle, 2.3–2.05 Ga), Statherian (1.8–1 Ga), Sunsás (1.2–0.9 Ga), Tonian (Early Brasiliano, 1 to 0.85 Ga) and Brasiliano (0.65 to 0.50 Ga) terranes and units are the main source areas. The fission-track data provide important evidence of cooling during the Early Brasiliano and Brasiliano periods and yields 2 main Paleozoic age populations, one between 500 and 360 Ma and the other between 345 and 230 Ma. These populations were interpreted as resulting from different tectonic events along the western margin of the South American Platform that affected the platform’s internal composition by uplift and exhumation.
This study presents zircon U–Pb geochronology with major, trace, rare earth element and Sr–Nd isotope geochemistry for plagiogranite dyke swarms occurring within the gabbro unit of the lower part of the Kuerti ophiolite in southern Chinese Altay, Northwest China. These intrusive plagiogranites cut across the metamorphic zone consisting of amphibolites that are related to hydrothermal alteration and shearing of gabbros. LA-ICP-MS zircon U–Pb dating of 2 plagiogranite samples yields approximately 390 Ma age of origin. They are geochemically characterized by relatively high SiO2 and Na2O, low K2O, TiO2, and Al2O3 contents, with marked enrichment in light rare earth elements and depletion in Nb, Ta, and Ti. They have slightly higher initial 87Sr/86Sr ratios (0.7040 to 0.7049), lower εNd(t) values (+3.6 to +6.3), and higher Th contents (2.84–11.9 ppm) than those (0.7034 to 0.7048, +7.2 to +10.3, and 0.04 to 1.79) of closely associated amphibolites. Geological and geochemical attributes suggest that the plagiogranites were generated by the anatexis of hydrated and hydrothermally altered amphibolites and minor sediments from oceanic crust during shearing at low pressure (<0.1 GPa) and temperature (<850 °C) conditions in a suprasubduction zone setting. The ridge subduction at approximately 390 Ma caused the upwelling of the hot asthenosphere and triggered the spreading of the Kuerti back-arc basin, followed by the formation of the plagiogranites in the Kuerti ophiolite.
The Great Xing’an Range in north-eastern China hosts numerous super-large Ag–Pb–Zn deposits and some Fe–Sn deposits. The Mesozoic Haobugao Fe–Zn polymetallic skarn deposit in the southern Great Xing’an Range is contemporaneous with the regional Ag–Pb–Zn mineralization. Numerous ore bodies are hosted in the Lower Permian carbonate strata or along the contact with the Early Cretaceous granite. According to the field and systematic petrography and mineralography research, the Haobugao mineralization phases are divided into 3 paragenetic stages: prograde stage, retrograde stage, and sulphide stage. Magnetite mainly occurred in the retrograde stage and replaced the anhydrous skarn minerals (e.g., garnet and diopside). Two types of magnetite (Mag1 and Mag2), including 6 subtypes, can be distinguished based on the scanning electron microscopy and back scattered electron images. Electron probe microanalysis and laser ablation inductively coupled plasma mass spectrometer analysis were used to determine major and trace elements in different types of magnetite. Mag1 has higher Ti and V concentrations than Mag2, indicating a relatively higher depositional temperature. Mag1 also contains relatively higher Mg and Mn concentrations, coupled with much lower Si and Al concentrations, which reflects a low fluid/rock ratio at the site of Mag1 deposition. Element variation features of Mag1 and Mag2 reveal that the Haobugao mineralization fluids gradually evolved from high-temperature and low fluid/rock ratio fluids to relatively low-temperature and high fluid/rock ratio fluids. However, electron probe microanalysis data of Mag2 display significantly higher Sn concentrations (up to 2.82 wt.%) than that in Mag1, which indicates that Sn can be incorporated into magnetite crystal lattice. We propose a possible substitution mechanism of Sn4+ + Mn2+ = 2Fe3+, supported by the strongly positive correlation between Sn4+ and Mn2+, whereby a substitution of Sn4+ for Fe3+ in octahedral sites of magnetite requires a compensatory substitution of Mn2+ for Fe3+ to maintain the charge balance.