Late Middle Jurassic Research Articles

Characteristics and genesis of reef-bank complexes in deep shelf facies: A case study of the Middle-Late Jurassic in the northern Amu Darya Basin, Central Asia

A large-rimmed carbonate platform was developed in the Amu Darya Basin during the Middle-Late Jurassic Callovian-Oxfordian period. What distinguishes it from typical carbonate platforms is that a series of reef-bank complexes had extensively developed in the deep shelf depositional zone of the basin. However, only a few studies have reported on the classification, characteristics, and genesis of these reef-bank complexes in relatively deep water, greatly limiting the development of deep-water carbonate sedimentology. To address this issue, the types and the genesis of reef-bank complexes in the deep shelf environment have been clarified based on the systematic petrography, seismic sedimentology, and geomorphology study of the Callovian-Oxfordian carbonate rocks in the northern Amu Darya Basin during the Middle-Late Jurassic period. The results find that the reef-bank complexes are widely distributed in the deep shelf environment in the study area with laminar, reticulated, and zonal distributions. The reef-bank complexes include barrier-bonding reef-bank complexes, lime-mud mounds (LMMs), and granular shoals (GSs). The deep shelf environment can be further divided into an inner shelf, shelf margin, and shelf slope. The inner shelf and shelf margin have a relatively shallow water body and a strong sedimentary hydrodynamic force, mainly developing reticulated reef-bank complexes and laminar GSs, whereas the shelf slope mostly develops zonal LMM deposits in strips. The scale of the reef-bank complexes is mainly controlled by basement paleogeomorphology and water energy. Relatively high-energy reef-bank complex bodies are developed on the seaward side of the paleouplift limb with relatively turbulent hydrodynamic conditions, whereas low-energy LMMs are mostly developed on the high position of paleouplift and landward side. The obtained findings can deepen our understanding of relatively deep-water carbonate sedimentation and enrich the carbonate sedimentation theory.

New palaeoclimatic constraints from paleosols on the Middle-Late Jurassic landscape, Western Colorado, U.S.A

ABSTRACT The Wanakah Formation and Tidwell Member of the Morrison Formation record intervals of paedogenesis in the Paradox Basin and Central Colorado trough, western Colorado during the Middle-Late Jurassic. Detailed field description of paleosols and paedogenic carbonates at different stratigraphic horizons from three localities document four main pedotypes: vertisols, gleysols, oxisols, and protosols. Generally, gleysols reflect reducing conditions, protosols and oxisols reflect oxidizing conditions, and vertisols reflect fluctuations in oxidizing/reducing conditions (seasonality). Major elemental geochemical ratios in samples from these paleosols suggest variable redox conditions and a sub-humid to humid paleoclimate with seasonal precipitations during paleosol development. Estimated mean annual temperature based on elemental analyses of paleosol B in paleosols from Ribbon Trail and Escalante Canyon in western Colorado suggest mesic – thermic paleoclimate. Mean annual precipitation indicates sub-humid to humid regional palaeoclimatic conditions marked by seasonal precipitation. Clay mineralogy of these paleosols, determined by X-ray diffraction, shows the abundance of illite most likely formed from smectite alteration due to episodic wetting and drying. δ13C and δ18O isotope analyses of carbonate veins in these paleosols suggest that they formed recently from meteoric water and therefore are not considered for palaeoclimatic reconstructions. Mean annual temperature and mean annual precipitation estimates in the Wanakah Formation and Tidwell Member of the Morrison Formation provide new information on the local and regional palaeoclimatic and paleoenvironmental conditions during the Middle Jurassic in western Colorado as well as data for comparison with the Upper Morrison paleoclimate from northern and southern localities in the western United States.

Meso‐Cenozoic Tectonic History of the Altai: New Insights From Apatite U‐Pb and Fission Track Thermochronology for the Fuyun Area (Xinjiang, China)

AbstractThe Altai is an enigmatic, relatively young mountain belt with sharp relief (up to 4,500 m high) that developed thousands of kilometers away from the nearest current plate margins. The Fuyun area, at the interface between the southern margin of the Chinese Altai and the Junggar Basin, represents a key locality for understanding the Meso‐Cenozoic deformation and exhumation history of the Altai. The complex structural architecture of the Fuyun area suggests that multiple deformation events affected the area, which ultimately led to the exhumation of the Altai. Furthermore, the area hosts orogenic‐type mineralization, suggesting a history of fluid alteration. However, in contrast to the well‐constrained Palaeozoic history, the timing of Meso‐Cenozoic deformation, metasomatism and exhumation has not been comprehensively studied. This study presents new apatite U‐Pb, trace element and fission track data for the Fuyun area and integrates these with previous studies for the Altai to shed more light on the Meso‐Cenozoic tectonic history. The apatite U‐Pb dates, associated with LREE‐depleted trace element profiles, suggest that a phase of Middle–Late Jurassic (∼170–160 Ma) metasomatism affected the Keketuohai area, which is potentially linked to the timing of rare‐metal mineralization. The apatite fission track results and thermal history models reveal rapid early Late Cretaceous (∼100–75 Ma) cooling linked to tectonic exhumation throughout the Chinese Altai, associated with distant plate‐margin processes. In addition, samples taken in vicinity to the frontal thrusts of the Altai record evidence for Cenozoic partial resetting of the apatite fission track system.

The diagenesis and evolution of conglomeratic reservoirs in the Wutonggou Formation in the Yingyeer area

The conglomeratic reservoirs of the Permian Wutonggou Formation in the Yingyeer area of the Turpan–Hami Basin are key petroliferous horizons, with enormous potential for hydrocarbon exploration. Existing studies on the conglomeratic reservoirs in this area mainly focus on the sedimentary environment and sand body distribution, while few of them concern the diagenesis and evolution of the conglomeratic reservoirs, leading to deficient knowledge about relevant mechanisms. By comprehensively utilizing the data from ordinary and casting thin section identification, the x-ray diffraction analysis of whole rock and clay, scanning electron microscopy, and other analyses and assays, this study analyzed and investigated the petrological characteristics, diagenetic types, and characteristics, and diagenetic evolutionary stages and mechanisms of the conglomeratic reservoirs of the Permian Wutonggou Formation in the Yingyeer area. The study results are as follows. Under the influence of sedimentary provenances, tectonic evolution, and paleogeomorphology, the conglomeratic reservoirs of the Permian Wutonggou formation have low clastic grain maturity and low structural maturity, suggesting proximal provenances. The reservoirs are subjected to various diagenesis, including carbonate cementation, which fills pores and throats; compaction as a major factor leading to poor reservoir quality, and dissolution, which significantly improves the reservoir properties. The conglomeratic reservoirs in the Wutonggou formation are generally in substage A of the middle diagenetic stage. The diagenetic evolution of the reservoirs generally underwent early mechanical compaction, calcite precipitation, secondary quartz overgrowth, the injection of organic fluids, the dissolution of feldspars and lithic grains, the transformation of clay minerals, and calcite cementation in sequence. Hydrocarbon charging mainly occurred during the late Middle Jurassic and was greatly influenced by early compaction and dissolution.

A novel gymnosperm reproductive organ from the Jurassic of China

A feature that distinguishes angiosperms from their gymnosperm peers is that the ovules of angiosperms are enclosed before pollination. Theoretically, the angiosperms were derived from fossil seed plants that have their ovules not enclosed before pollination (gymnosperms). The origin of angiosperms is essentially a process in which the former exposed ovules got enclosed. Although various fossil taxa with different extent of ovule-protection have been documented in the Mesozoic, the morphological gap between angiosperms and gymnosperms is still far from bridged. To narrow down such a gap, here we report a novel fossil gymnosperm, Paradoxa n. gen., demonstrating a mosaic feature comparable to angiosperms in certain aspects from the Jiulongshan Formation (Middle-Late Jurassic) of Inner Mongolia, China: pollen grains trapped inside the female unit imply that Paradoxa is a gymnosperm, while several characters (especially long apical process and partially covered ovule) make it more comparable to angiosperms. It is noteworthy that, different from Caytonia, Umkomasia and Petriellaea (all in the so-called seed ferns), Paradoxa’s Bau-plan is more comparable to that of the basalmost angiosperm Amborella, suggesting a possible provenance of angiosperm carpels.

DEFORMATION REGIMES AND LOW-RANK PALEOTECTONIC STRESSES OF THE WESTERN TIAN SHAN IN THE MESOZOIC – CENOZOIC

There is a description of the results of studies of deformation regimes and reconstruction of stress fields of certain time intervals of the Mesozoic – Cenozoic history of the western part of the Tien Shan based on the geological and structural data and earthquake focal mechanisms. The relevance of research is determined by the existing difficulties in the age reference of paleostress fields, which is associated with the retrospective nature of the initial data and the features of geological evolution, as well as the existence of controversial, unresolved issues regarding the development of the western periclinal part of the Tien Shan in the Mesozoic – Cenozoic. The aim of the research is to reconstruct the rank components of paleostresses in the earth’s crust in the western part of the Tien Shan at separate stages of the Mesozoic – Cenozoic development. The time intervals for which the stress field diagrams were obtained represent cyclically manifested phases of the first-rank rhythms, which are characterized by alternation of different deformation modes. The method for reconstructing paleostresses was based on the concept of the formation of superimposed systems of cracks of various generations, which were sequentially manifested at various stages of the development of deformation processes. Analysis of the data showed that with increasing age of rocks the number of different fracture systems actually increases – the earlier systems are superimposed by the later. At the same time, the characteristic elements in Quaternary rocks manifest themselves in different ways in the older rocks. This allows them to be used for the reconstruction of the Late Cenozoic stress field based on mass measurements of fracturing in rocks of earlier Mesozoic – Cenozoic ages. Based on the step-by-step identification of the heterochronous cracks system and determination of a pair of maxima of the fracture density with asymmetric scattering, region-based diagrams of the first-rank stress fields were obtained for different stages of manifestation of Mesozoic – Cenozoic deformation regimes: 1 – from the Permian-Triassic to the Middle Jurassic; 2 – Middle-Late Jurassic; 3 – Late Jurassic – Early Cretaceous; 4 – Late Cretaceous – Paleocene – Eocene; 5 – Oligocene – Quaternary. The results obtained indicate the alternation of compression and extension phases, first-rank for the region, and give an idea of the main trends in its geodynamic evolution. They can be used in the development of a tectodynamic model of the region.

Natural gas characteristics and gas-source comparisons of the Lower Triassic Feixianguan Formation, Eastern Sichuan Basin, China

There is great controversy regarding the origin and source of natural gas in the Lower Triassic Feixianguan Formation in the Eastern Sichuan Basin. This seriously restricts the study of natural gas dynamics in the Feixianguan Formation and thus hampers natural gas exploration in the region, so further study is urgently required. Using experimental tests of natural gas composition, stable isotopes, and noble gas isotopes with gas chromatography (GC) and mass spectrometry (MS) studies of source rock and reservoir asphalt saturated hydrocarbons, the natural gas geochemical characteristics, the genetic identification and a gas-source comparison of the Feixianguan Formation were studied. Then, constrained by the thermal history, the histories of gas generation and expulsion were restored by basin simulation technology. Finally, a gas accumulation model was established for the Feixianguan Formation. The results showed that (1) the H2S-rich and H2S-poor gas reservoirs of the Feixianguan Formation are distributed on the east and west sides of the Kaijiang-Liangping trough in the Eastern Sichuan Basin, respectively. The carbon and hydrogen isotope compositions of the natural gas in the gas reservoirs are generally heavy and have typical characteristics of high-maturity dry gas reservoirs. (2) The natural gas of the Feixianguan Formation is organic thermogenic gas, which is mainly oil-type gas generated by the secondary cracking of crude oil. The gas-generating parent material is mainly type II kerogen. (3) The natural gas of the Feixianguan Formation in the Eastern Sichuan Basin was mainly generated by argillaceous source rocks of the Upper Permian Longtan Formation. (4) Natural gas accumulation occurred as follows: the paleo-structure heights were filled with crude oil in the Early Jurassic, and paleo-oil reservoirs were formed in the Feixianguan Formation; during the Middle-Late Jurassic, the paleo-oil reservoirs were cracked when the reservoir temperatures rose above 160 °C, and paleo-gas reservoirs were formed. Since the end of the Late Jurassic, the paleo-gas reservoirs have been adjusted and reformed to form the present-day natural gas reservoirs. These results provide a basis for studying natural gas accumulation dynamics of the Feixianguan Formation in the Eastern Sichuan Basin.

Cassiterite U-Pb dating and micro-XRF analysis constraint on the formation of Xinlu Sn-Zn deposit, South China

This study presents rock slab and thin section reflection mapping, element mapping by Micro-XRF (μ-XRF) analysis, and in situ cassiterite U-Pb laser ablation-inductively coupled plasma mass spectrometer (LA-ICP-MS) ages of the Xinlu Sn-Zn deposit, Nanling Sn-W belt, South China. The new results provided constraints on the age and processes of Sn mineralization and thus further discussed the role of the Paleo-Pacific plate in the formation of Sn-W mineralization in the Nanling Sn-W belt. Based on mineralogy and μ-XRF analysis, four mineralization episodes are distinguished in the Baimianshan segment, Xinlu deposit, including 1) prograde skarn stage characterized by the mineral assemblages of garnet, pyroxene, marble, hornfels; 2) retrograde skarn-sulfide composed of massive pyrrhotite, sphalerite with parts of epidote, cassiterite; 3) quartz sulfide vein stage consisting of arsenopyrite, pyrite, quartz, and 4) barren calcite crosscutting the former episodes. The time of retrograde skarn-sulfide mineralization is dated by LA-ICP-MS cassiterite U-Pb at 168.7 ± 1.3 Ma. These U-Pb ages, combined with previous geochronological studies, support one Sn mineralization event associated with the 159.5–168.7 Ma magmatism in the Guposhan-Huashan district. The Xinlu Sn-Zn mineralization and the Middle-Late Jurassic Sn-W mineralization in the Nanling belt, South China is genetically related to asthenospheric upwelling and crust-mantle interaction caused by the subduction of the Paleo-Pacific plate. Our results demonstrate that thin section reflection and μ-XRF element mapping are powerful tools for determining paragenesis in ore deposits and are highly effective to guide the selection of analytical positions for cassiterite U-Pb dating. This method may also be applicable to in-situ geochronology of other minerals.

Re-assessment of the Late Jurassic eusauropod Mamenchisaurus sinocanadorum Russell and Zheng, 1993, and the evolution of exceptionally long necks in mamenchisaurids

The sauropod genus Mamenchisaurus, from the Late Jurassic–Early Cretaceous of East Asia, has a convoluted taxonomic history. Although included in the first cladistic analysis of sauropods, only recently has the monophyly of Mamenchisaurus, and the anatomical diversity of the many penecontemporaneous East Asian eusauropods, been evaluated critically. Here, we re-describe the holotype and only specimen of M. sinocanadorum. Although the original diagnosis is no longer adequate, we identify several autapomorphies that support the validity of this species, including an elongate external mandibular fenestra and distinctive pneumatic structures on the cervical centra. We incorporate new data into a phylogenetic character matrix that also includes Bellusaurus and Daanosaurus, both of which are known only from juvenile material and are often hypothesized to be neosauropods (or close relatives thereof). We recover all species of Mamenchisaurus as part of a radiation of predominantly Middle–Late Jurassic East Asian eusauropods, but the genus is non-monophyletic, underscoring the need for further systematic revision of mamenchisaurid taxonomy. Analyses that score ontogenetically variable characters ambiguously recover Bellusaurus and Daanosaurus as juvenile mamenchisaurids, a hypothesis supported by several features that are unique to mamenchisaurids or exhibit little homoplasy, including anteriorly bifurcate cervical ribs. Finally, computed-tomography reveals extensive vertebral pneumaticity in M. sinocanadorum that is comparable to that of the largest sauropods, and updated scaling analyses imply a neck over 14 m long, rivalling estimates for other exceptionally long-necked sauropods. Previous work has suggested that the elongated cervical ribs of particularly long-necked sauropods such as M. sinocanadorum stabilized the neck by limiting its mobility. Given that extent of pneumaticity responds dynamically to a bone’s habitual loading, we propose that long cervical ribs – and other structural modifications that limited flexibility – promoted the evolution of increasingly long necks by producing a more predictable biomechanical milieu amenable to increased pneumatization.

The Chicxulub impact structure reveals the first in-situ Jurassic magmatic intrusions of the Yucatán Peninsula, Mexico

Impact events that create complex craters excavate mid- to lower-crustal rocks, offering a unique perspective on the interior composition and internal dynamics of planetary bodies. On the Yucatán Peninsula, Mexico, the surface geology mainly consists of ∼3 km thick sedimentary rocks, with a lack of exposure of crystalline basement in many areas. Consequently, current understanding of the Yucatán subsurface is largely based on impact ejecta and drill cores recovered from the 180–200-km-diameter Chicxulub impact structure. In this study, we present the first apatite and titanite UPb ages for pre-impact dacitic, doleritic, and felsitic magmatic dikes preserved in Chicxulub's peak ring sampled during the 2016 IODP-ICDP Expedition 364. Dating yielded two age groups, with Carboniferous dacites (328–318 Ma) and a felsite (330± 9 Ma) overlapping in age with most of the granitoid basement sampled in the Expedition 364 drill core, as well as Jurassic dolerites (169–159 Ma) and a felsite (158 ± 19 Ma) that represent the first in situ sampling of Jurassic-age magmatic intrusions for the Yucatán Peninsula. Further investigation of the Nd, Sr, and Hf isotopic compositions of these pre-impact lithologies and impact melt rocks from the peak ring structure suggest that dolerites generally contributed up to ∼10 vol% of the Chicxulub impact melt rock sampled in the peak ring. This percentage implies that the dolerites comprised a large part of the Yucatán subsurface by volume, representing a hitherto unsampled pervasive Jurassic magmatic phase. We interpret this magmatic phase to be related to the opening of the Gulf of Mexico, representing the first physical sampling of lithologies associated with the southern extension of the opening of the Gulf of Mexico and likely constraining its onset to the Late Middle Jurassic.

Radiolarian assemblage from radiolarites of the Krs Gradac section (SW Serbia): On the way to a better reconstruction of the Middle-Late Jurassic geodynamic history of the Inner Dinarides

The type section of the Gonje Formation (Krs Gradac, Sjenica area) is characterized by an abundance of radiolarian tests, which, however, are gene rally recrystallized and poorly preserved. This paper focuses on the age of radiolarite sequence below the series with intercalated turbidites and mass transport deposits. The Callovian/Oxfordian (?early Oxfordian) moderately preserved radiolarian assemblage was determined from the higher part of the radiolaritic sequence without mass transport deposits. The new biostratigraphic age is consistent with the previously published data of the authors and reveals that the radiolaritic sequence without mass transport deposits of the Gonje Formation was deposited between the Bathonian and Oxfordian. The biostratigraphic age of this part of the Gonje Formation evidence that the Ljubis Basin underwent in Late Oxfordian to earliest Tithonian times a phase of starvation with reduced sediment supply or production. The reasons for this starvation are discussed.

СТРУКТУРНАЯ ПОЗИЦИЯ, ПЕТРОГЕОХИМИЧЕСКАЯ И ПЕТРОПАЛЕОМАГНИТНАЯ ХАРАКТЕРИСТИКА ВУЛКАНИТОВ НИЛАНСКОГО ТЕРРЕЙНА (ЗОНА СОЧЛЕНЕНИЯ МОНГОЛО-ОХОТСКОГО И СИХОТЭ-АЛИНЬСКОГО ОРОГЕНОВ) И ГЕОДИНАМИЧЕСКИЕ ВЫВОДЫ

The paper presents the results of structural, geochemical, magnetic and paleomagnetic studies on Mesozoic rocks of the Nilan terrane, which from the north flank on the Paleozoic rocks of the Paukan block (52.0°N, 135.6°E). Jurassic and Triassic (?) multiple disturbance deposits of the terrane were found to be fragments of the Middle-Late Jurassic accretionary complex with accumulations of older oceanic rocks varying in thickness and the oceanic plate with completely destroyed stratigraphy. Basic and intermediate volcanic rocks hosted within Jurassic and Triassic (?) sedimentary rocks of the terrane are of oceanic origin. Based on geochemical and paleomagnetic data, it is assumed that the volcanic rocks formed near the equator in geodynamic settings of mid-ocean ridge and intraplate ocean islands.

Parelateriformius from the Middle–Late Jurassic of China reinterpreted as the earliest Dascillidae (Coleoptera: Dascilloidea)

The elateriform genus Parelateriformius Yan & Wang from the Middle–Late Jurassic Daohugou beds has been placed previously in the extinct Lasiosynidae or the extant Eulichadidae. Our reinvestigation of the type specimens and additional materials suggests that the character combination of Parelateriformius (e.g., tarsomeres 2–4 distinctly lobed, posterior pronotal margin crenulate, radial cell short and with truncate base, and metakatepisternal suture complete) is not accordant with either Lasiosynidae or Eulichadidae. Instead, it should belong to the family Dascillidae and appears closely related to the extant Petalon. A new species of Parelateriformius is also described, as P. grimaldii Li & Cai sp. nov. Our discovery highlights the antiquity of Dascillidae, which is consistent with the recent fossil-calibrated molecular dating of beetles.

A glimpse of the lost Upper Triassic to Middle Jurassic architecture of the Dinaric Carbonate Platform margin and slope

In the southernmost outcrops of the Slovenian Basin the Middle Jurassic coarse-grained limestone breccia (mega)beds are interstratified within a succession that is otherwise dominated by hemipelagites and distal turbidites. In this paper, these beds are described as the Ponikve Breccia Member of the Tolmin Formation. We provide descriptions of the studied sections with detailed geological maps and analysis of the breccia lithoclasts. From the latter, a non-outcropping margin of the Dinaric Carbonate Platform is reconstructed. In the Late Triassic the platform margin was characterized by a Dachstein-type marginal reef. After the end-Triassic extinction event, the platform architecture remained, but the reefs were replaced by sand shoals characterized by ooids. In the late Early Jurassic and/or early Middle Jurassic a slope area might have been dissected by normal faults and a step-like paleotopography was formed. In the Bajocian, during a period of major regional geodynamic perturbations, extensional or transtensional tectonic activity intensified and triggered the large-scale collapses of the Dinaric Carbonate Platform margin producing the limestone breccias described herein. This may in turn have caused a backstepping of the platform margin, as is evident from the occurrence of Late Jurassic marginal reefs that are installed directly above the Upper Triassic and Lower Jurassic inner platform successions.

Complex Late Triassic-Middle Jurassic Subduction-Related Magmatic History from Detritus of Nominal Middle Jurassic Brooks Range Ophiolite, Northern Alaska

Abstract In northern Alaska, the Early Cretaceous sedimentary Yukon-Koyukuk basin documents the progressive unroofing of the adjacent Brooks Range orogen. Igneous clasts in the lower conglomerate are believed to originate from ophiolitic rocks of the two uppermost allochthons in the Brooks Range, the Brooks Range ophiolite and the Angayucham terrane. The emplacement of these oceanic terranes onto the continental margin of the Arctic Alaska terrane documents the initiation of Brookian orogenesis. While most agree that the Angayucham terrane represents a widespread distribution of Late Devonian oceanic crust and Triassic-Early Jurassic oceanic plateau(s)/island(s), the age and origin of the Brooks Range ophiolite remains controversial. We present new age, whole-rock chemistry, and isotopic data from igneous clasts as well as a few Angayucham terrane outcrop samples from the NE Yukon-Koyukuk basin. Our results show that the igneous clasts are mostly subduction-related and more likely to represent eroded material from the Brooks Range ophiolite rather than the Angayucham terrane. Our Late Triassic, and Early and Middle Jurassic zircon crystallization ages for the igneous clasts, combined with their immobile trace element compositions documenting various stages of oceanic subduction (mature arc and later rifting), suggest a long-lived subduction system that was active in the Late Triassic and throughout the Middle Jurassic. Radiogenic lead and neodymium isotopic results yield juvenile signatures for both the igneous clasts and the Angayucham terrane, pointing to their formation in an intraoceanic setting distal from the continental rocks and sediments of the Arctic Alaska terrane. These new data, combined with the published data of others, allow us to propose a revised tectonic model that integrates Late Triassic island arc formation with the tectonic development and emplacement of the Brooks Range ophiolite.

Middle-Late Jurassic chronology and source-to-sink system of the Sikeshu section in the southern margin of the Junggar Basin, Xinjiang

With the improvement of isotopic testing technology, high-precision dating methods such as SHRIMP or LA-ICP-MS of detrital zircon have been used to analyze and infer the formation age of sedimentary basins, the source of detrital materials, and the tectonic background of the formation and widely used. The LA-ICP-MS zircon U-Pb dating method was used to study the detrital zircon in the Middle-Late Jurassic sandstone in the Sikeshu section of the southern margin of the Junggar Basin, Xinjiang. The results show that the detrital zircon ages are mainly 270~447Ma,240~440Ma,245~432Ma, and 286~360Ma. There are also several ancient zircon ages, mainly 945~1080Ma. Combined with zircon mineralogical characteristics, CL images, Th/U ratio, sandstone petrographic characteristics, and regional chronological data, the primary provenance of the Middle-Late Jurassic sandstone is mainly from the Yilinghabilga Mountains in the North Tianshan, and Carboniferous-Permian volcanic rocks, pyroclastic rocks and Carboniferous granites exposed in the Borokonu Mountains of the Central Tianshan Mountains. By comparing the age changes between the four sampling points, the change process of the North Tianshan-Middle Tianshan source-sink system is obtained. From west to east, the upper Devonian glutamate, the Middle Devonian carbonate, and the large granite bodies of the Carboniferous series constituted the early watershed. From the middle to late period of the Toutunhe Formation to the Xishanyao Formation, from east to west, the Cambrian and Silurian carbonate rocks and the large granites and other rocks formed the late water line.

Timing of the Kaiyuan-Jiapigou shear zone in the northern margin of the North China Craton: Implications for closure of the Mongol-Okhotsk Ocean

The ductile Kaiyuan–Jiapigou Shear Zone (KJSZ) is located within the eastern part of the northern margin of the North China Craton (NCC). The foliation within the shear zone generally dips at 55° towards N20° and contains a lineation that plunges at 40° towards N30°. Field and thin section observations and quartz c-axis fabrics of the KJSZ indicate that this zone records top-to-the-SSW shearing associated with a NNE–SSW regional compression event. Muscovite, biotite, and hornblende 40Ar/39Ar step-heating ages constrain the timing of shearing and the tectonothermal history of the KJSZ. Quartz c-axis fabrics and mineral deformation features indicate the deformation within the KJSZ occurred at temperatures of 400°C–450°C, which are lower than the Ar–Ar closure temperature of hornblende but higher than the closure temperatures of muscovite and biotite. Samples from the KJSZ yield 40Ar/39Ar step-heating ages from 164.6 ± 0.5 to 165.2 ± 0.6 Ma for hornblende samples and from 161.4 ± 0.4 to 163.1 ± 0.4 Ma for mica samples. Combining these ages with the temperature of deformation within the KJSZ indicates that this shear zone most likely formed between 165.2 ± 0.6 and 161.4 ± 0.4 Ma. Linking these new data with the tectonics and geochronology of the eastern part of the northern margin of the NCC suggests that the deformation within this region was most likely controlled by a nearly N–S compressional event associated with the far field effect of the Mongol–Okhotsk Ocean closure during the Middle–Late Jurassic.

Subduction initiation of the Bangong–Nujiang Tethys Ocean, Tibetan Plateau

• The northward subduction initiation of the meso -Tethys Ocean was diachronous from the east (∼240–230 Ma) to the west (213–194 Ma). • The collision and slab breakoff during the closure of the Paleo-Tethys Ocean were vital to the northward subduction initiation. • The southward subduction of the meso -Tethys Ocean was initiated no later than the Middle Triassic . • The southward subduction was attributed to the closure of the Paleo-Tethys Ocean and the spreading of the NeoTethys Ocean. The initiation of subduction is widely recognized as a critical process associated with the evolution of the meso -Tethys Ocean. Here, we present zircon U–Pb, Lu–Hf isotope, and whole-rock geochemical data for a suite of Triassic–Jurassic magmatic rocks from the central Tibetan Plateau, with the aim of gaining insights into the subduction initiation of the Bangong–Nujiang Tethys Ocean (BNTO). Early Triassic trachyandesite (248 Ma) from the South Qiangtang terrane resemble intraplate magma and probably resulted from magmatic activity at the passive continental margin during the BNTO opening. Late Triassic S-type granodiorite (220 Ma) and Middle Jurassic SSZ-type gabbro (∼165 Ma) from the South Qiangtang terrane may result from the northward subduction of the BNTO oceanic lithosphere. Jurassic granitoids (I and S type; 193–177 Ma) and metamorphic rocks (179–174 Ma) from the Amdo and Jiayuqiao subterranes are related to the northward subduction of the BNTO oceanic lithosphere. Middle–Late Jurassic diorites (164–162 Ma) from the Central Lhasa subterrane have geochemical features of continental margin arc magma with a sediment component, and these are derived from the southward subduction of the BNTO oceanic lithosphere. Finally, this Triassic–Jurassic magmatic suite of various geochemical features was combined with the results of previous studies, and we suggest that the initiation of northward BNTO subduction was probably diachronous from the east (∼240–230 Ma) to the west (213–194 Ma). This initiation resulted both from collision and slab breakoff during the closure of the Longmuco–Shuanghu–Lancangjiang Paleo-Tethys Ocean and considerable density and rheology contrasts between the thick oceanic lithosphere and continental lithosphere. The southward subduction was initiated no later than the Middle Triassic and was attributed to the closure of the Sumdo Paleo-Tethys Ocean and the spreading of the Indus–Yarlung Zangbo NeoTethys Ocean.

Sediment provenances of a Mesozoic intracontinental basin enclosed by multiple orogenic belts, Junggar Basin, NW China: insights from detrital ilmenite, Cr-spinel geochemistry, and zircon U–Pb geochronology

ABSTRACT The Junggar Basin was an intracontinental basin enclosed by multiple orogenies in the Mesozoic. Unravelling the provenances development in the western Junggar Basin enclosed by the West Junggar Orogenic Belt (WJOB) and Tianshan Orogenic Belt (TSOB) would elucidate the exhumation history of these orogenic belts during the post-orogenic period. The TSOB includes Central Tianshan which includes a continental block with the Precambrian to early Palaeozoic metamorphic basements, while the WJOB includes Palaeozoic accretionary complexes. Given their contrasting geological histories, we use detrital garnets, metamorphic ilmenites, epidotes, and Precambrian zircons to distinguish the TSOB and detrital Cr-spinel to distinguish the WJOB. Combined with sedimentary analysis, palaeocurrents, heavy mineral data, seismic profiles, and zircon U-Pb geochronology, here we reconstruct a four-phase source-to-sink model. During the syn-rift phase, the southern basin was stably fed by the TSOB in the Triassic, evidenced by high Garnet-zircon index and stable single-peak Carboniferous detrital zircon age spectra. During the post-rift sag basin phase, the source-to-sink system experienced a significant change, because detrital zircon age spectra change into multiple peaks. During the transpressional basin phase, the WJOB replaced the TSOB as the dominant provenance in the late Middle Jurassic and the TSOB became the dominant provenance again in the Late Jurassic. Syndepositional volcanic materials provided a large amount of sediments. During the sag basin phase, lake transgression happened and the TSOB provenance could not reach the northern basin. We concluded that the WJOB experienced transpression and obvious denudation in the late Middle Jurassic, and the TSOB uplifted under transpression in the Late Jurassic. The transpression controlled the change of major provenances. The aridification in the Late Jurassic was recorded by the decrease in the Zircon-Tourmaline-Rutile index and the increase in epidotes and magnetites. The humidification in the Early Cretaceous was responded to by lake transgression and sediment recycling.

First estimates for Jurassic seawater temperatures based on oxygen isotope analyses of calcitic fossils from Central Iran: Evidence for a major plate tectonic shift?

• First absolute seawater temperatures for the Jurassic of Iran are estimated. • Oxygen isotope analyses of calcitic fossils indicate a significant long-term warming. • Warming is explained by plate tectonic shift of Central Iran into lower latitudes. • Warming also caused changes in sedimentation and ecosystem composition. • Plate tectonics drive regional long-term climate patterns in the Middle-Late Jurassic. Stable oxygen isotope values of calcitic marine fossils (δ 18 O shell ) were used to calculate the first absolute seawater temperature estimates for the Jurassic of Central Iran. Results indicate a significant long-term warming in the study area through the Middle to Late Jurassic of around 6 °C or more (0.3 to 0.4 °C per million years). Following recent methodological advances in temperature reconstructions, the δ 18 O shell values of bivalves translate into water temperatures slightly warmer than in today’s oceans at comparable latitudes with averages of 26.6 °C in the Middle Jurassic and 32.1 °C in the Late Jurassic. Belemnites would indicate still warmer temperatures, but these values might be less reliable and partly overestimated due to potentially altered specimens. The recorded long-term warming can be explained by a concurrent major plate tectonic shift of Central Iran from temperate to tropical latitudes. The gradual influence of this southward drift is also seen by changes in sedimentation and ecosystem composition. This way, the stable oxygen isotope record of Central Iran is further evidence for the strong impact of plate tectonics on regional long-term climate patterns in the Middle and Late Jurassic.