Late Jurassic Early Cretaceous Research Articles

УСЛОВИЯ ФОРМИРОВАНИЯ И ВОЗРАСТ РЕДКОМЕТАЛЬНОГО ОРУДЕНЕНИЯ КУКУЛЬБЕЙСКОГО РУДНОГО РАЙОНА (ВОСТОЧНОЕ ЗАБАЙКАЛЬЕ)

Актуальность. Определение современными методами условий формирования редкометального оруденения на примере типовых месторождений Кукульбейского рудного района, в совокупности с изотопно-геохронологическими исследованиями являются ключевыми аспектами в решении одной из важнейших фундаментальных проблем рудной геологии и геохимии – выяснение условий, источников, механизмов и последовательности формирования рудных концентраций металлов в различных геологических обстановках. Цель: получение новых данных о физико-химических условиях формирования редкометального оруденения, а также установление возраста оруденения. Объекты: Букукинское, Белухинское и Антоновогорское месторождения. Методы. Флюидные включения были изучены в прозрачно-полированных пластинках методами крио-термометрии и Рамановской спектроскопии. Крио-термометрические исследования выполнялись в микротермокамере THMSG-600 фирмы Linkam. Образцы последовательно охлаждались до температуры –190 °С и нагревались до полной гомогенизации вещества включений. В процессе наблюдения замерялись температуры эвтектики, плавления льда и гомогенизации. Температуры плавления льда для двухфазных и температуры плавления кристалликов соли для трехфазных флюидных включений позволили установить концентрацию солей в пересчете на NaCl-эквивалент. Состав газовой и минеральной фаз флюидных включений изучался методом Рамановской спектроскопии – спектрометр Ramanor U-1000 и детектор Horiba DU420E-OE-323 фирмы Jobin Yvon, лазер Millennia Pro фирмы Spectra-Physics; Confocal Raman Microscope alpha 300R фирмы WITec. 40Ar/39Ar метод изотопно-геохронологического датирования по методике ступенчатого прогрева применялся для установления абсолютного возраста оруденения. Результаты. Рудные минеральные ассоциации рассматриваемых месторождений сформировались при участии хлоридных высокотемпературных флюидов, насыщенных углекислотой. Повсеместно рудоносные флюиды содержат восстановленные газы, из которых превалирует метан, а сероводород, азот и водород находятся в подчиненном количестве. В рамках кристаллизации отдельных минеральных ассоциаций прослеживается эволюция рудоносного флюида: снижение температуры и концентрации растворов. Катионный состав растворов также изменился от существенно натрового до существенно калиевого. Для объектов исследования получены умеренные оценки глубин: 6–8 км для Букукинского, 4,5–6 км для Белухинского и 3–3,5 км для Антоновогорского месторождения. Возраст редкометального оруденения составляет на Антоновогорском месторождении – 146,7±1,7 млн лет (мусковит из кварц-мусковитовых грейзенов с вольфрамитом); на Букукинском месторождении – 135±2,8 млн лет (серицит из кварц-вольфрамитовых жил); на Белухинском месторождении – 155,6±1,8млн лет (серицит из кварц-вольфрамитовых жил). Выводы. Редкометальная минерализация Кукульбейского рудного района, на примере типовых месторождений, формировалась из высокотемпературных гидротермальных насыщенных восстановленными газами (метан, сероводород, азот и водород) углекислотно-водно-хлоридных растворов магматического происхождения. По глубине становления рассматриваемые рудоносные системы можно отнести к гипабиссальным от 8 до 3 км. Проведенные изотопно-геохронологические исследования показали, что в пределах Кукульбейского рудного района формирования вольфрамового оруденения происходило в позднеюрское-раннемеловое время, основная часть которого связана со становлением магматических пород кукульбейского комплекса, не исключено что позднеюрское оруденение связано со становлением магматических пород шахтаминского комплекса.

Late Jurassic–Early Cretaceous climate change record in clay minerals of the Norwegian-Greenland Seaway

The humid climate of northwestern Europe during the Late Jurassic–Early Cretaceous was punctuated by a widespread arid phase. There are few well constrained and complete sections that document this climate shift from semi-humid to arid, and back to more humid conditions. Detrital clay minerals formed in soils and derived through weathering may be used as palaeoclimatic proxies, and here, clay mineral assemblages from several well-constrained sections from the Norwegian Continental Shelf are presented as a N-S transect of the Norwegian-Greenland Seaway. Palaeoclimate signals in most of the clay mineral sections are complicated by diagenetic overprinting. Yet, towards the Boreal realm of the Norwegian-Greenland Seaway there is a discernible palaeoclimatic signal in the clay mineralogy of Well 16/3–4. At ~30°N, kaolinite is known to have reappeared in the clay mineralogical record in the upper part of Calpionellid Zone B (early-mid Berriasian). However, we find an absence of kaolinite at ~40°N in Well 16/3–4 from the North Sea until the late Berriasian, suggesting that initiation of the first humid phase in the Early Cretaceous Norwegian-Greenland Seaway was diachronous.

Thermochronological constraints on the Meso-Cenozoic tectonic evolution of the Haiyuan-Liupanshan region, northeastern Tibetan Plateau

The Cenozoic outward growth of the Tibetan Plateau is traditionally considered to be responsible for the arcuate structures in the northeastern Tibetan Plateau. However, it is still unclear how pre-Cenozoic tectonics contributed to this unique geomorphic setting. To decipher the Meso-Cenozoic tectonic evolution, we systematically summarize the published thermochronological data and perform a low-temperature thermochronology study on Paleozoic-Mesozoic samples from the Haiyuan-Liupanshan region. Apatite fission-track ages from the new samples vary widely, ranging from 158.4 Ma to 34.4 Ma. Apatite fission-track and (U-Th-Sm)/He thermal history modeling, constrained by zircon fission-track data and geological evidence, indicate that this area experienced multiple stages of cooling from the Late Triassic to the Cenozoic. These cooling events include (i) Late Triassic-Early Jurassic, (ii) Late Jurassic-Early Cretaceous, (iii) Late Cretaceous-Early Cenozoic, and (iv) Late Cenozoic (~7 Ma). Late Triassic-Early Jurassic cooling was driven by the collision of the Nouth China Plate and the Sorth China Plate. The Late Jurassic-Early Cretaceous tectonothermal event is interpreted as a record of the intracontinental orogeny in East Asia. It is possible that Late Cretaceous-Early Cenozoic exhumation was the far-field response to the Neo-Tethyan subduction under the Lhasa terrane. In addition, Late Cenozoic (~7 Ma) rapid cooling in the Yueliangshan Range (southern part of the Haiyuan-Liupanshan region) was a result of the intense activation of the easternmost Haiyuan fault, in response to the outward growth of the Tibetan Plateau. These findings suggest that pre-Cenozoic tectonics played an important role in the growth of the present-day northeastern Tibetan Plateau edifice.

Geochronology, geochemistry, and Hf Isotope of the granites from the Mo deposits in Fengning region, China: implications for tectonic evolution and mineralization of the North China Craton

The Dacaoping and Sadaigoumen Mo deposits were recently discovered along the northern margin of the North China Craton. In this paper, we present new zircon U–Pb ages, Hf isotope data, and whole-rock major and trace element data for the monzogranite and granodiorite in the Dacaoping Mo deposit and the monzogranite in the Sabagaogoumen Mo deposit, and we use these data to constrain the metallogenic epoch and the tectonic background in the area. Combined with previous data, we suggest four phases of mineralization in the Dacaoping and Sabagaogoumen Mo deposits; these phases are 248 Ma, 236 Ma, 147 Ma, and 140 Ma. The Early Triassic monzogranite and Early Cretaceous granodiorite in the Dacaoping Mo deposit and the Middle Triassic monzogranite in the Sabagaogoumen Mo deposit are I-type granite with similar geochemical characteristics. The Triassic granites formed in a collisional–post-collisional setting between the North China plate and the Siberian plate, and the Cretaceous granodiorite formed from the rapid thinning of the lithosphere. Their source rock should mainly originate from the partial melting of ancient crustal material. Compared with other Mo deposits in the region, we can further divide these Mo deposits into three phases, namely (1) Triassic (ca. 248–223 Ma), (2) Early–Middle Jurassic (ca. 187–165 Ma), and (3) Late Jurassic–Early Cretaceous (ca. 155–130 Ma), which are consistent with the times of magmatic activity. These activities occurred during the collision and post-collision between the North China plate and the Siberian plate after the Paleo-Asian Ocean’s closure, intracontinental orogeny, and rapid thinning of the lithosphere, respectively.

Multistage structural deformations of a superimposed basin system and its tectonic response to regional geological evolution: A case study from the Late Jurassic-Early Cretaceous Tanan depression, Hailar-Tamtsag basin

Abstract The spatial-temporal extent and influence of the Mongol-Okhotsk and Paleo-Pacific tectonic regimes in northeast (NE) China and adjacent areas during late Mesozoic times have long been controversial. As a superimposed rift system which formed due to extensive intracontinental extension during the Late Jurassic to Early Cretaceous periods, the Hailar-Tamtsag basin is a key area to study the relationship between the Mongol-Okhotsk and Paleo-Pacific tectonic regimes and their superposition. With the aim of constraining the evolution of those two regimes, in this paper, the Tanan depression, which is a second-order structural unit in the western part of the Hailar-Tamtsag basin, has been chosen as the research object. On the basis of structural analysis of three-dimensional (3D) seismic data in the Tanan depression, and taking into account the controlling effect and influence of the pre-existing facies on the basin evolution, the deformation events in the Early Cretaceous can be subdivided into four periods: 1) A NW-SE trending extension during the deposition of the Tongbomiao Formation to the upper part of the second Member of the Nantun Formation; 2) A NW-SE trending compression during the deposition of the upper part of the second Member of the Nantun Formation to the first Member of the Yimin Formation; 3) A near EW trending extension during the deposition of the second Member to the lower part of third Member of the Yimin Formation; 4) A near EW trending compression during the deposition of the upper part of the third Member of the Yimin Formation. Additionally, we identify compressional structures in the basement in the southeast part of the depression, which is a key study that resulted from the Mongol-Okhotsk collisional orogeny during the Late Jurassic. The above five major deformational events make up a relatively complete tectonic deformation sequence of the Hailar-Tamtsag basin. Moreover, the deformation sequence, together with other available information from the literature in NE China and adjacent areas, including petrology, geochemistry and geochronology data, also record the whole process from the orogenic stage after the closure of the Mongol-Okhotsk Ocean, over the post-orogenic collapse of the Mongol-Okhotsk orogen, to the weakening of the influence of the Mongol–Okhotsk regime, and eventually to be replaced by the Paleo-Pacific tectonic regime. Considering that the superposition and transformation of the two tectonic regimes involves almost all the geological process in NE China and adjacent area during the late Mesozoic, the complete geological record in the Tanan depression during this period provides new constraints on the research of regional tectonic evolution, and the model of multiphase deformation is also expected to have useful implications for future studies of complex basin systems.

Comprehensive study on the conventional petroleum system of the Masilah oilfields, Sayun-Masilah Basin, Yemen

The Masilah oilfields are rich-oil provinces in the Sayun-Masilah Basin. The petroleum system including essential elements and processes is a very important for understanding and development of oilfield to further explore hydrocarbons in the whole basin. Integrated geochemical, geological, petrological and petrophysical analyses were performed on the source and reservoir rocks in the Masilah oilfields to gives information about the complete petroleum system. The Masilah oilfields filled with syn- and post-rift sediments, including a self-contained source-reservoir system. The geochemical results indicate that the organic-rich shales of the Madbi Formation are considered as oil-source rocks, with high total organic carbon content of more than 5.0 wt% TOC and oil-prone kerogen Types II and I. The Madbi shales are currently characterized by thermally mature level, within the oil generation window. Geochemical biomarker correlations of oil-source rock indicate that there is a genetic link between the oils and the Late Jurassic Madbi shale source rock in the Masilah oilfields. Therefore, the geochemical characteristics of the Madbi source rock have been collaborated into basin models and illustrate that the Madbi source rock had passed the peak-oil generation window during the Late Cretaceous to present-day and that large amounts of oil were generated. The generated oil was expelled and migrated to the overlain Early Cretaceous Qishn clastic reservoir rocks through faults during the Oligocene-Middle Miocene. The oil was then accumulated and trapped into horst and tilted fault blocks that initial formed during the Late Jurassic-Early Cretaceous rifting.

Tectono‐morphological evolution of the Cauvery, Vaigai, and Thamirabarani River basins: Implications on timing, stratigraphic markers, relative roles of intrinsic and extrinsic factors, and transience of Southern Indian landscape

Peninsular India is an amalgam of transient landscapes evolved from the interactions between tectonic and climatic forcings. In order to appraise the tectono‐geomorphic evolution of South India, it is essential to understand the relationships between intrinsic and extrinsic processes and their geomorphic expressions at a river basin scale. Seven geomorphometric parameters, namely, longitudinal profile (Lp), asymmetry factor (Af), hypsometric curve and hypsometric integral (HI), mountain front sinuosity (Smf), river sinuosity (R), stream length gradient (Sl), and shape factor (Shp) were calculated for selected drainage basins of South India. Spatial analyses of these parameters in the light of systematic field mapping were attempted in this study. The results show the occurrences of southern sub‐basins with convex hypsometric curves indicating a youth stage and significant tectonic influence. The low and moderate Smf values within the basins are indicative of mountain fronts witnessing a high level of tectonic activity. The net effects of tectonic and geomorphometric processes are inferred to have been the responses of anti‐clockwise rotation of the Indian Plate and ridge uplift‐push from west. Directional change in Thamirabarani River Basin and drainage channel reorganizations in the Cauvery and Vaigai River basins stand testimony to these, among others. Amidst these, occurrences of tectonically quiescent regions and multiple incisions by river channels and flow of modern river channels in inherited palaeovalleys but on tangent directions of ancient flows are also observed. Five stages of landscape development are envisaged: First, inheritance of river basins from palaeodrainage systems; second, reversal of river flow directions during early part of Cenozoic and inception of evolution of modern river systems; third, during Miocene–Pliocene; fourth, during Pleistocene–Early Holocene; and fifth, during the Holocene–Anthropocene. The recent resurgence of tectonics is not only reflected in the shifting of axial rivers, but is also evidenced by seismicity and landslides/faulting. Together, transient nature of the Southern Indian Plate, first‐order control of tectonics, followed by climate and lithology over landscape evolution, inheritance of river valleys, synonymy of river basin responses to intrinsic and extrinsic geomorphic processes, however with unique signatures and basin‐scale responses, are inferred. Detailed morphometric studies and supplementation with precise age data may fine‐tune the proposed model. According to the model, inheritance of Mesozoic valley/structures during Late Jurassic–Early Cretaceous, drainage reversal and initiation of Cenozoic–Recent river basin evolution, intense peneplanation during Miocene–Pliocene, intense incision during Pleistocene, periodic climatic extremes during Early Cenozoic, Palaeocene–Eocene, Oligocene and corresponding pedogenesis, and terrace formation and sedimentation have been recognized. Based on the numerical ages and documentation of basin‐scale distribution of specific types of lithologies/weathering profiles/palaeosols, provisionally we recognize the laterite/ferricrete–bauxite–calcrete 53–50 and 37–23 Ma to be the oldest weathering profiles, palaeosols (I generation palaeosols) of alluvial fans with texturally immature parent rock clasts to be of 23–9 Ma, palaeosols with intensive calcitized rootlets that are found to occur in fluvial terraces of palaeo‐floodplain/natural levee (II generation palaeosols) to be of 3.5–2.5 Ma, and relict alluvium that are documented in inland river terraces and older deltaic plains and calcareous sandstones that are documented in coastal terraces to be of ~1 Ma old and attribute them to be the results of significant climatic events and Early Cenozoic uplift of Western Ghat that proceeded from south–north and its intensive tilt toward east since 25 Ma until today at a rate of 0.1 mm/year to be tectonic events that have significantly contributed toward landscape development on basin‐regional scales and concur with the evolutionary stages proposed in our model.

Detrital zircon U–Pb geochronology of Pennsylvanian–Permian sandstones from the Turnaicum and Meliaticum (Western Carpathians, Slovakia): provenance and tectonic implications

The blueschist-bearing Borka Nappe regarded as a part of the Meliaticum Unit s. l. and the Turnaicum Unit in its hanging-wall were thrusted together to the north on the Inner Western Carpathians basement during the orogenic events of the Late Jurassic–Early Cretaceous. A new U–Pb sensitive high-resolution ion microprobe (SHRIMP) magmatic, as well as detrital-zircon ages were used to constrain provenance and paleogeography. The detrital-zircon age spectrum of the studied metasediments contains mainly ages of the Ediacaran–Cryogenian (550–720 Ma) and Tonian–Stenian (0.9–1.1 Ga), which correspond to the Pan-African Belt and Saharan Metacraton. Subordinate Archean ages were likely derived from the Saharan Metacraton of North Gondwana (1.7–2.2 and 2.5–2.7 Ga). The 273 ± 2 Ma zircon age of a metarhyolite, along with the youngest detrital zircon of 265 ± 4 Ma from the associated metasediments, specify the maximum depositional age of the Bucina Fm. from the Borka Nappe to the Guadalupian. The rhyolite olistoliths from the Middle/Late Jurassic Meliaticum olistostrome yielded the latest Cambrian/earliest Ordovician (488 ± 6 Ma) and Middle Ordovician (~ 465 Ma) zircon ages. This is consistent with the U–Pb zircon ages from the foot-walling Southern Gemericum basement metavolcanites, which are, therefore, regarded as a potential source. The obtained zircon ages confirm the provenance from the Southern Gemericum basement and its Permian envelope for the Meliaticum-Borka Nappe and Turnaicum sedimentary embayment.

Late Mesozoic sediment provenance on Georges Bank: Enlargement of river drainages to the Atlantic Ocean in the Late Jurassic–Early Cretaceous

The detrital mineralogy of Upper Jurassic to Lower Cretaceous sandstones from the Continental Offshore Stratigraphic Test G-2 well in the Georges Bank Basin was studied to better understand the varying supply of clastic sediment to the basin. Heavy minerals separated from cuttings were identified by chemical analyses, zircons were dated, and Raman spectroscopy was applied to identify titania polymorphs (rutile, anatase, and brookite). Upper Jurassic (Oxfordian) sands contain tourmaline, fluorapatite, Mn-almandine and grossular garnets (derived from the central Maine and coastal Maine belts), and pumice and trachytes reworked from volcanics in the Long Island platform. Zircon geochronology (300–450 and 550–700 Ma) supports sources from peri-Gondwanan Appalachian terranes. Lower Cretaceous heavy minerals show that Oxfordian supply was diluted to 30%, mostly by additional ilmenite, zircon, tourmaline, and staurolite, probably by components from central and northern Maine and New Brunswick. Rutile and anatase from metapelites are more common than in the Oxfordian, where mafic igneous sources predominate. Lower Cretaceous zircons include a few Mesoproterozoic grains that might be reworked from middle Paleozoic sedimentary rocks in the Gander terrane of northern Maine and New Brunswick, although subhedral grains together with rare chromite may indicate minor or intermittent supply from the Sable River draining Labrador. Heavy minerals provided important provenance information not available from zircon geochronology alone. Progressive enlargement of the catchment area resulted from an increasingly humid climate in the latest Jurassic and active mostly northeast-trending faults in the Early Cretaceous. This resulted in greater supply of petrographically more-mature sand to the basin.

Clay mineral dating of displacement on the Sronlairig Fault: implications for Mesozoic and Cenozoic tectonic evolution in northern Scotland

AbstractTemporary excavations during the construction of the Glendoe Hydro Scheme above Loch Ness in the Highlands of Scotland exposed a clay-rich fault gouge in Dalradian Supergroup psammite. The gouge coincides with the mapped trace of the subvertical Sronlairig Fault, a feature related in part to the Great Glen and Ericht–Laidon faults, which had been interpreted to result from brittle deformation during the Caledonian orogeny (c. 420–390 Ma). Exposure of this mica-rich gouge represented an exceptional opportunity to constrain the timing of the gouge-producing movement on the Sronlairig Fault using isotopic analysis to date the growth of authigenic (essentially synkinematic) clay mineralization. A series of fine-size separates was isolated prior to K–Ar analysis. Novel, capillary-encapsulated X-ray diffraction analysis was employed to ensure nearly perfect, random orientation and to facilitate the identification and quantification of mica polytypes. Coarser size fractions are composed of greater proportions of the 2M1 illite polytype. Finer size fractions show increasing proportions of the 1M illite polytype, with no evidence of 2M1 illite in the finest fractions. A series of Illite Age Analysis plots produced excellent R2 values with calculated mean ages of 296 ± 7 Ma (Late Carboniferous–Early Permian) for the oldest (2M1) illite and 145 ± 7 Ma (Late Jurassic–Early Cretaceous) for the youngest (1M) illite. The Late Carboniferous–Early Permian (Faulting event 1) age may represent resetting of earlier-formed micas or authigenesis during dextral displacement of the Great Glen Fault Zone (GGFZ). Contemporaneous WNW(NW)–ESE(SE) extension was important for basin development and hydrocarbon migration in the Pentland Firth and Moray Firth regions. The Late Jurassic–Early Cretaceous (Faulting event 2) age corresponds with Moray Firth Basin development and indicates that the GGFZ and related structures may have acted to partition the active extension in the Moray Firth region from relative inactivity in the Pentland Firth area at this time. These new age dates demonstrate the long-lived geological activity on the GGFZ, particularly so in post-Caledonian times where other isotopic evidence for younger tectonic overprints is lacking.

Origin and geodynamic significance of the Siuna Serpentinite Mélange, Northeast Nicaragua: Insights from the large-scale structure, petrology and geochemistry of the ultramafic blocks

The Siuna Serpentinite Mélange (SSM) is located in the boundary zone between continental Chortis block and oceanic Mesquito Composite Terrane in NE Nicaragua. In this paper, we present new large-scale structure, meso-scale kinematic analysis, 40Ar39Ar geochronology, mineral chemistry, in situ trace element composition of clinopyroxene, and bulk rock geochemical data for representative ultramafic rocks preserved as blocks in the SSM. Structurally, the mélange is a > 2.5 km-thick tectonic unit characterized by a disrupted block-in-matrix fabric with occurrence of exotic blocks, which have igneous and sedimentary protoliths and were variably metamorphosed to high-pressure (P) conditions. There are also blocks of ultramafic rocks, which consist mainly of harzburgite and clinopyroxene (Cpx)-rich harzburgite, with minor lherzolite and pyroxenite, and rare dunite and chromitite. In these rocks, relics of bastite and mesh textures of lizardite are replaced by antigorite blades and overprinted by antigorite schistosity.The presented data give rise to argue the following points. (1) The serpentinite matrix of the mélange resulted from hydration, ductile deformation and metamorphism at high-P conditions of abyssal and supra-subduction zone (SSZ) mantle peridotites in a subduction channel. (2) The geochemical characteristics of the ultramafic protoliths can be explained by at least two-stages of melting and refertilization processes: the first-stage occurred in a mid-ocean ridge (MOR) environment and produced Cpx-rich harzburgite and the second-stage took place in an SSZ setting and resulted in harzburgite, dunite and chromitite. (3) The second-stage magmatic processes produced melts that range in composition between island-arc tholeiites (IAT) and boninites, similar to arc-like protoliths of some mafic blocks in the mélange, which establishes a genetic link between blocks and matrix. (4) The presence of ultramafic and mafic blocks in the SSM, derived from both an island-arc upper plate and a MOR-like lower plate sources, suggests its position in an intra-oceanic subduction zone during the late Jurassic-early Cretaceous. (5) By the Hauterivian, the SSM became exhumed and tectonically emplaced onto the Chortis block as a consequence of the collision between the intra-oceanic arc and the southern continental margin of North America.

Zircon U-Pb-Hf Isotopic and Trace-Element Geochemistry Constraints on the Late Jurassic–Early Cretaceous Magmatic Evolution of Southeastern Zhejiang, South China

AbstractThe Late Jurassic–Early Cretaceous magmatism at the margin of the South China Block (SCB) provide ideal targets to investigate magma petrogenesis and tectonic evolution of the SCB during th...

Neotectonic Structures of the Heraklion Peninsula (Southwestern Crimea)

The results of the structural–geomorphic analysis of the Heraklion peninsula in southwestern Crimea are presented. The Western, Central, and Southeastern segments, which differ in the density and orientation of fracturing zones and faults, drainage system characteristics, and coastal zone structure, were recognized. It was demonstrated that the neotectonic movements in the upper structural unit are manifested most distinctly above the Late Jurassic–Early Cretaceous collisional suture that was formed during the closure of the back-arc basin with oceanic crust.

The newest structures of the Heraclion peninsula (South-Western Crimea)

The article presents the results of the structural-geomorphological analysis of the Heraklion peninsula of the South-Western Crimea. The Western, Central and South-Eastern segments are distinguished, differing in density and direction of fracture zones, faults, the nature of the ravine-gulch net, and the structure of the coastal zone. It is shown that neotectonic movements in the upper structural floor are most intensively manifested over the Late Jurassic-Early Cretaceous collisional suture formed when the back-arc basin with the oceanic crust was closed.

Petrography and source rock potential of Chia Gara Formation (Late Jurassic\u2013Early Cretaceous) in Northern Iraq and Kurdistan Region

The petrographic characteristics and petroleum potential of Chia Gara Formation (Late Jurassic–Early Cretaceous; Tithonian–Berriasian) in Northern Iraq are studied. In order to find out the disparity of Chia Gara Formation in different tectonic zones, two outcrops (Banik and Barsarin sections) and well-Aj-12 were used for this study. In general, the Chia Gara Formation is lithologically composed of black shale, thin-bedded brown argillaceous limestone, thin to medium yellowish limestone. Petrographic study shows that the matrix is mainly composed of micrite, and various diagenetic processes had influenced limestones of Chia Gara Formation, such as neomorphism, dissolution, calcification, cementation and compaction. Fracture, vuggy and moldic are common types of porosity that were observed in this study. However, porosity was later destroyed by some diagenetic processes. The destruction of porosity resulted in considerably low permeability. In addition to petrography, the source rock potential of Chia Gara Formation was also studied. The average values for Total Organic Carbon in percentage (TOC%) of the formation were 1.70, 3.40 and 0.89 for Barsarin, Banik and Aj-12 sections, showing good, very good, and fair organic carbon content, respectively. The pyrolysis executed for studied samples revealed that kerogen type of Chia Gara Formation in Banik section and well Aj-12 belongs to type-II and type-III kerogens, whereas Barsarin section belongs to type-III and type-IV kerogens. The plot of TOC% versus generation potential (S1 + S2) indicated poor to very good potential of hydrocarbon generation for studied samples. The values of the Tmax (the temperature that results in the maximum release of hydrocarbons (S2) during pyrolysis program) suggested that the organic matter in Banik and Barsarin sections is in mature stage, whereas it is thermally immature (or late immature) in well Aj-12. Vitrinite reflectance (R%) also indicated Oil Window to Early Gas Window in Barsarin, Oil Window in Banik, and Early Oil Window in Aj-12. This study revealed that the studied sections from different tectonic zones show slightly different stages of thermal maturity. The high thermal maturity in Barsarin area can be resulted in different type of kerogen compared to the other sections.

Structural architecture of the Western Alpine Ophiolites, and the Jurassic seafloor spreading tectonics of the Alpine Tethys

We present a regional synthesis of the structural architecture and tectonic evolution of the Western Alpine Ophiolites (WAO), exposed in NW Italy. The WAO represent the remnants of Alpine Tethys (Ligurian–Piedmont Ocean) that opened between Europe and Adria, and developed in four stages from the Middle Jurassic to Early Cretaceous. Emplacement of gabbroic intrusions into the extending lithospheric mantle of Europe–Adria marked the main magmatic event (Stage 1). Coalescent shear zones in the fossil upper mantle form lithospheric-scale detachment faults, which led to the exhumation of upper mantle peridotites and gabbros on the seafloor, and extensive serpentinization (Stage 2). Detachment faults, and serpentinized peridotites–gabbros in their footwalls, represent preserved fossil oceanic core complexes within the WAO. Emplacement of ophiolitic breccias and basaltic lava flows marked the syn-extensional phase (Stage 3). Radiolarian chert and limestone were deposited unconformably on this syn-extensional volcanic–sedimentary sequence, marking the post-extensional phase (Stage 4). Magmatic ages of gabbroic intrusions and mafic–felsic dykes, and depositional ages of post-extensional sequences in the WAO constrain the timing of the opening of the Ligurian–Piedmont Ocean to the Middle Jurassic ( c. 170–168 Ma), followed by a tectonic quiescence stage in the Late Jurassic–Early Cretaceous. Thematic collection: This article is part of the ‘Tethyan ophiolites and Tethyan seaways collection' available at: https://www.lyellcollection.org/cc/tethyan-ophiolites-and-tethyan-seaways

Southward subduction of the Mongolia–Okhostk Ocean: Insights from Early–Middle Triassic intrusive rocks from the Jiawula–Tsagenbulagen area in NE China

New zircon U–Pb age data, Hf isotopes, whole‐rock major and trace elements, and Sr–Nd isotopic data for Early–Middle Triassic intrusive rocks from the Jiawula and Tsagenbulagen area in the southern part of the Erguna Massif provide valuable constraints on the southward subduction of the Mongolia–Okhostk Ocean lithosphere. The Jiawula–Tsagenbulagen granitoids mainly consist of granodiorite, quartz monzonite, and biotite granite. These granitoids, long believed to be mainly Late Jurassic–Early Cretaceous (ca. 150–139 Ma) and minor Permian (ca. 278 Ma and 254–253 Ma), are newly found to contain a latest Early Triassic to Middle Triassic generation (zircon U–Pb age: ca. 248–244 Ma). Geochemically, the Early to Middle Triassic granitoids are characterized by high SiO2 (74–77 wt.%) and Al2O3 (12.2–15.95 wt.%), low Mg# values (12.98–37.32), and variable Na2O/K2O ratios (0.62–1.75). They are metaluminous to peraluminous and belong to the high‐K calc‐alkaline series. Moreover, they show enrichment in light rare earth elements and large‐ion lithophile elements and depletion in heavy rare earth elements and high‐field‐strength elements (e.g., Nb, Ta, and Ti). They have an increased amount of SiO2/Al2O3 but lower V/Th ratios and negative correlations between V/Th and SiO2/Al2O3 ratios, indicating that the magmas experienced various degrees of biotite and/or plagioclase fractional crystallization. Their whole‐rock εNd(t) and zircon εHf(t) values ranged from +1.07 to +4.15 and from +0.5 to +2.9, respectively, indicating that the primary magmas were probably generated by partial melting of juvenile crustal material with the help of mantle‐derived mafic magmas. These magmas subsequently underwent fractional crystallization, magma mixing, and a small amount of crustal contamination during their emplacement. The geochemical characteristics of these Early–Middle Triassic intrusive rocks have affinities with intrusive rocks from active continental margin settings. Therefore, we conclude that the Early–Middle Triassic magmatism in the Erguna Massif was generated within an Andean‐type arc setting related to the southwards subduction of the Mongol–Okhotsk oceanic plate beneath the Erguna Massif. We summarized previous age data of the Early to Late Triassic magmatic rocks from Erguna and Central Mongolia and identified arc setting granites and post‐orogenic granites. Thus, a tectonic evolutionary model was proposed for the geological observations in the Erguna and Central Mongolia massifs, involving the Triassic continuous southward subduction in the Erguna Massif and the Late Triassic slab‐rollback and back‐arc basin extension in the south‐eastern Central Mongolia Massif.

Repeated turnovers in Late Jurassic faunal assemblages of the Gulf of Mexico: Correlation with cold ocean water

Our research on Late Jurassic–earliest Cretaceous (Kimmeridgian–earliest Berriasian) marine shelf to bathyal sediments in northeastern Mexico reveals significant repeated oceanic and biotic turnovers in the northwestern Proto-Gulf of Mexico. We here document these environmental changes based on alternating cold and warm water mollusk assemblages, element concentrations and organofacies, as well as associated changes in sea level. We hypothesize that oscillations in the oceanic current system of the Gulf of Mexico were responsible for appearance of cold-water assemblages of Boreal and possibly Austral origins, situated in specific horizons which were previously described as poor in fossil content or even as fossil-free. In addition, these specific horizons show significant changes in element concentrations and organofacies and correlate to relative sea-level changes.

Late Jurassic–Early Cretaceous tectonic evolution of the Great Xing’an Range: geochronological and geochemical evidence from granitoids and volcanic rocks in the Erguna Block, NE China

ABSTRACTThe late Mesozoic magmatic record within the Erguna Block is critical to evaluate the tectonic history and geodynamic evolution of the Great Xing’an Range, NE China. Here, we provide geochronological and geochemical data on Late Jurassic–Early Cretaceous plutonic-volcanic rocks in the northern Erguna Block and discuss their origin within a regional tectonic framework. Late Mesozoic magmatism in the Erguna Block can be divided into two major periods: Late Jurassic (162–150 Ma) and Early Cretaceous (140–125 Ma). Late Jurassic quartz monzonite and dacite show adakite characteristics such as high Al2O3, high Sr, and steeply fractionated REE patterns. Contemporary granitoids and rhyolites are also characterized by strong enrichment of light rare earth elements (LREE) and significant depletion in heavy rare earth elements (HREE), but with more pronounced negative Eu anomalies. Early Cretaceous trachytes and monzoporphyries exhibit moderate LREE enrichment and relatively flat HREE distributions. Coeval granites and rhyolites have transitional signatures between A-type and fractionated I-type felsic rocks. Both Late Jurassic and Early Cretaceous rocks have distinctive negative Nb, Ta, and Ti anomalies, and positive zircon εHf(t) values, suggesting that these magmas were derived from partial melting of Meso-Neoproterozoic accreted lower crust, although melting occurred at a variety of crustal levels. The transition from adakite to non-adakite magmatism reflects continued crustal thinning from Late Jurassic to Early Cretaceous. Our data, together with recently reported isotopic data for plutonic and volcanic rocks, as well as geochemical data, in NE China, suggest that Late Jurassic–Early Cretaceous magmatism in the Erguna Block was possibly induced by post-collisional extension after closure of the Mongol-Okhotsk Ocean.

Late Jurassic-Early Cretaceous Erg Deposits in the Mengyin Basin, Western Shandong Province, China: Inferences about the Wind Regime and Paleogeography

The Late Jurassic-Early Cretaceous Santai Formation, sporadically distributed in western Shandong Province, comprises terrestrial alluvial-eolian successions, which records regional wind patterns and paleogeography in eastern North China. This paper conducts an analysis of eolian stratification, bounding surfaces, facies architecture and paleowind direction of the Santai Formation in the east of the Mengyin Basin, western Shandong Province. Three basic types of eolian stratification are recognized in the Santai Formation, including grainflow strata, wind ripple strata and adhesion strata, and have been grouped into eolian dune and interdune facies associations. The occurrences of reactivation surfaces and superimposition surfaces within eolian dune deposits indicate active compound dunes or draas. The association of adhesion strata with grainflow or windripple strata is the development of a wet eolian system. Cross-strata dip direction indicates different paleowind directions from the lower to the upper part of the Santai paleoerg. The lower part of the paleoerg was characterized by paleowinds blowing from northwest to northeast, whereas the upper part was under the influence of paleowinds consistently towards east to northeast. The identified changes in wind directions possibly suggest wind regime shifts from monsoon circulation to westlies of planetary wind system, which may be related with the collapse of the East China Plateau during the Late Jurassic to Early Cretaceous.