Cretaceous Time Research Articles

Early Cretaceous Provenance, Sediment Dispersal, and Foreland Basin Development in Southwestern Montana, North American Cordillera

AbstractThe Lower Cretaceous Kootenai Formation in western Montana records the onset of foredeep sedimentation in the Idaho‐Montana retroforeland basin of the North American Cordillera. Contrasting models for sediment dispersal during Early Cretaceous time have been proposed. We use sandstone petrography, large‐n (n = 600) detrital zircon U‐Pb geochronology, and mixture modeling to determine the provenance of the Kootenai Formation. These new data combined with additional data from the literature suggest three discrete provenance signatures. Type I sandstones are quartz‐rich with U‐Pb age peaks at ca. 158, 238, 410, 600, and 1,035 Ma; they were sourced from exhumed Jurassic continental strata and transported northward by axial drainages. Type II sandstones are lithic‐rich with U‐Pb age peaks at ca. 154, 166, 1,840, 1,920, 2,080, and 2,700 Ma; they were deposited by transverse rivers that drained exhumed lower‐middle Paleozoic strata from the Sevier belt. Type III sandstones contain feldspars and minor volcanic lithics with U‐Pb age spectra that are dominated by ca. 112 and 162 Ma ages; they were transported by transverse drainages that connected the hinterland of the orogen with the western Montana foreland. Based on provenance analysis of the Kootenai Formation, subtle Early Cretaceous thick‐skinned deformation in southwestern Montana did not exhume basement nor did it partition the foreland into locally sourced depocenters. Sediment sources for the western Montana foreland are consistent with those to the north in Alberta and different than southern Idaho. These data also provide insight into the translational history of terranes in the hinterland of the orogen during this time.

Late Cretaceous–Recent Low‐Temperature Cooling History and Tectonic Analysis of the Zuni Mountains, West‐Central New Mexico

AbstractLaramide tectonic models are testable with spatiotemporal constraints. The Colorado Plateau portion of the orogen, however, lacks detailed information on deformation timing. Herein, we analyze the Zuni Mountains, a Colorado Plateau Laramide arch, with apatite thermochronology, stratigraphic analysis, and thermal calculations to constrain and interpret its low‐temperature and tectonic history. Results show 2.9–3.6 km of pre‐exhumation burial by Late Cretaceous time that was sufficient to reset apatite (U‐Th)/He and apatite fission track dates after Pennsylvanian–Permian Ancestral Rocky Mountains exhumation. Low thermal conductivity of fine‐grained sediments was instrumental in facilitating sufficient temperatures to reset dates. Cooling histories of samples were structurally controlled. Forward and inverse models suggest ca. 80–45 Ma moderate to rapid cooling on the southwest flank and range crest of the arch, while the northeast flank displays 80–55 Ma slow cooling, very slow 55–20 Ma cooling and/or reheating, and post‐20 Ma rapid cooling. Northeast flank protracted cooling likely resulted from burial during backlimb tilting of the arch. Onset of exhumation was ca. 80 Ma, similar to 80–75 Ma onset of localized subsidence in San Juan Basin and 80–77 Ma onset of exhumation on the Nacimiento arch, but somewhat earlier than commonly considered for Colorado Plateau Laramide arches. In the regional context, our results support an easterly sweep of deformation driven by Farallon flat‐slab processes, and argues against prior suggestions for synchronous onset of Colorado Plateau Laramide deformation. Timing of post‐20 Ma cooling is most consistent with mantle‐driven epeirogenic uplift rather than Oligocene geotherm relaxation.

Early Cretaceous crust–mantle interaction linked to rollback of the Palaeo-Pacific flat-subducting slab: constraints from the intermediate–felsic volcanic rocks of the northern Great Xing’an Range, NE China

AbstractLate Mesozoic igneous rocks are important for deciphering the Mesozoic tectonic setting of NE China. In this paper, we present whole-rock geochemical data, zircon U–Pb ages and Lu–Hf isotope data for Early Cretaceous volcanic rocks from the Tulihe area of the northern Great Xing’an Range (GXR), with the aim of evaluating the petrogenesis and genetic relationships of these rocks, inferring crust–mantle interactions and better constraining extension-related geodynamic processes in the GXR. Zircon U–Pb ages indicate that the rhyolites and trachytic volcanic rocks formed during late Early Cretaceous time (c. 130–126 Ma). Geochemically, the highly fractionated I-type rhyolites exhibit high-K calc-alkaline, metaluminous to weakly peraluminous characteristics. They are enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs) but depleted in high-field-strength elements (HFSEs), with their magmatic zircons ϵHf(t) values ranging from +4.1 to +9.0. These features suggest that the rhyolites were derived from the partial melting of a dominantly juvenile, K-rich basaltic lower crust. The trachytic volcanic rocks are high-K calc-alkaline series and exhibit metaluminous characteristics. They have a wide range of zircon ϵHf(t) values (−17.8 to +12.9), indicating that these trachytic volcanic rocks originated from a dominantly lithospheric-mantle source with the involvement of asthenospheric mantle materials, and subsequently underwent extensive assimilation and fractional crystallization processes. Combining our results and the spatiotemporal migration of the late Early Cretaceous magmatic events, we propose that intense Early Cretaceous crust–mantle interaction took place within the northern GXR, and possibly the whole of NE China, and that it was related to the upwelling of asthenospheric mantle induced by rollback of the Palaeo-Pacific flat-subducting slab.

Facies, sequence stratigraphy and depositional environment of the Lower Cretaceous Giumal Formation in Spiti (Tethyan Himalaya, India)

The Lower Cretaceous marine deposits of the Himachal Himalaya are assigned to the Giumal Formation and best exposed in Spiti Valley (Northern India). To date, three sections near the villages of Chikkim, Domal and Gete are studied concerning its general lithological character and age. However, only the biostratigraphically constrained section near Chikkim exposes a complete succession of approximately 300 m in thickness with both lithostratigraphic boundaries preserved. Thus, it is representative for the entire Spiti area. Although some few lithological details on the siliciclastic sequence of the Giumal Formation are known, a comprehensive study of the regional sedimentary development is missing. A detailed microfacies study is performed for the entire sequence of Lower Cretaceous outer neritic to slope environments of Spiti. The specific sedimentary character of the unit results in discrimination of eleven microfacies types. Furthermore, twenty-three sedimentary sequences are identified which correspond with four major regressive/transgressive cycles. Reconstruction of a relative sea-level curve clearly indicates 10 major and minor sea-level drops which correlate well with eustatically induced megacycles between the latest Tithonian to the late Albian. The depositional environment of the Giumal Formation in Spiti is discussed in view of 16 coeval sedimentary sequences framing the Indian Craton during the Late Jurassic and Cretaceous time slice.

Baseline evaluation of legacy hydrochemical data for the Republican River watershed, southcentral Nebraska, USA

The objective of this study was to comprehensively evaluate and interpret hydrogeochemically the extensive legacy surface-water data for the expansive Republican River watershed in southcentral Nebraska. Historical (1968–1991) hydrochemistry data [major cations and anions, total salinity [specific conductance (SC)] and pH supplied by the Nebraska Department of Environmental Quality and the US Geological Survey were hydrogeochemically and statistically analyzed. Interpretation of this large legacy dataset (315 sample analyses at 12 Republican River sampling stations) revealed that the Republican River’s range in SC is relatively low (200–1020 µS/cm), with a river-wide median of 587 µS/cm (~ 380 mg/L). The range in SC for 10 tributaries (204 sample analyses) is 190–2910 µS/cm with a median value of 550 µS/cm (~ 360 mg/L). The Ca–HCO3 hydrochemical facies dominates throughout the watershed followed by the Na–HCO3 facies. Hydrochemical equilibrium modeling reveals that most samples occurred in an aqueous environment characterized by gypsum undersaturation but slight calcite and dolomite supersaturation; low supersaturation values comprise a line of evidence for the thermodynamic equilibrium or near equilibrium of the studied surface waters. Scatter plots of meq/L concentrations for selected anions and cations reveal the impact of silicate mineral (e.g., feldspars) weathering on the aqueous hydrochemistry throughout the watershed. These ubiquitous feldspar minerals most likely originated along the eastern slope of the Front Range during Late Cretaceous time (Laramide Orogeny) and were subsequently transported into the study area. The relatively high concentrations of sulfate and sodium in numerous samples collected from the Arikaree River probably originate in the underlying Pierre Shale, rich in S-bearing minerals.

The closure of the Vardar Ocean (the western domain of the northern Neotethys) from the early Middle Jurassic to the Paleocene time, based on the surface geology of eastern Pelagonia and the Vardar zone, biostratigraphy, and seismic-tomographic images of the mantle below the Central Hellenides.

Seismic tomographic images of the mantle below the Hellenides indicate that the Vardar Ocean probably had a composite width of over 3000 km. From surface geology we know that this ocean was initially located between two passive margins: Pelagonian Adria in the west and Serbo-Macedonian-Eurasia in the east. Pelagonia was covered by a carbonate platform that accumulated, during Late Triassic to Early Cretaceous time, where highly diversified carbonate sedimentary environments evolved and reacted to the adjacent, converging Vardar Ocean plate. We conceive that on the east side of the Vardar Ocean, a Cretaceous carbonate platform evolved from the Aptian to the Maastrichtian time in the forearc basin of the Vardar supra-subduction volcanic arc complex. The closure of the Vardar Ocean occurred in one episode of ophiolite obduction and in two episodes of intra-oceanic subduction. 1. During the Middle Jurassic time a 1200-km slab of west Vardar lithosphere subducted beneath the supra-subduction, 'Eohellenic', arc, while a 200-km-wide slab obducted onto Pelagonia between the Callovian and Valanginian times. 2. During the Late Jurassic through to the Cretaceous time a 1700-km-wide slab subducted beneath the evolving east Vardar-zone arc-complex. Pelagonia, the trailing edge of the subducting east-Vardar Ocean slab, crashed and underthrust the Vardar arc complex during the Paleocene time and ultimately crashed with Serbo-Macedonia. Since the late Early Jurassic time, the Hellenides have moved about 3000 km toward the northeast while the Atlantic Ocean spread.

A major change in magma sources in late Mesozoic active margin of the circum‐Sea of Japan domain: Geochemical constraints from late Paleozoic to Paleogene mafic dykes in the Sergeevka belt, southern Primorye, Russia

AbstractMore than 30 mafic dykes crop out in the Sergeevka belt in the coastal South Primorye, Far East Russia, of which geologic settings have been unclear for years. This study conducted major‐ and trace elements characterization, Sr–Nd isotope analyses, and Ar–Ar amphibole and U–Pb zircon datings for these rocks in order to identify their origin. The results demonstrated that all dykes are characterized by high Ba/Yb and low Nb/Y, Zr/Y, and Th/Yb ratios, which suggest their origin from arc melts derived from thin wedge mantle and shallow‐dipping slab. These dykes are clearly separated into two distinct age/geochemistry suites; that is, the Paleogene and Early Cretaceous one with dolerites/basalts and adakitic rocks, and the Permian–Triassic one with high‐Mg and high‐Al gabbro‐dolerite varieties. Their geochemistry suggests that the older suite was sourced from a primitive depleted MORB mantle (DMM)‐type mantle, whereas the younger suite from an enriched mantle II (EM2)‐type mantle domain. The transition in source type from DMM to EM2 occurred during the Jurassic‐earliest Cretaceous time, probably by a strong influence of a mantle plume onto the long‐continuing subduction‐related magmatism. The plume influence reached the maximum when the unique meimechite‐picrite complex formed in the region.

Early Cretaceous magmatism of the Langxian complex in the eastern Gangdese batholith, southern Tibet: Neo-Tethys ocean subduction re-initiation

新特提斯洋长期俯冲消减作用在早白垩世可能经历二次俯冲启动或板片俯冲几何形态的重大转换。确定西藏南部冈底斯岩基早白垩世岩浆作用的岩石地球化学特征和作用方式是甄别上述过程的关键，对理解新特提斯洋的俯冲演化过程至关重要。本文就冈底斯岩基东段朗县杂岩中保存的各类早白垩世岩浆岩，开展了锆石U-Pb地质年代学和Hf同位素、全岩元素和同位素（Sr-Nd）组成分析。数据结果表明：1）基性岩侵位时代为早白垩世晚期（103.6~100.8Ma），为高钾钙碱性偏铝质岩石，锆石ε_Hf（t）=+0.3~+5.7，全岩ε_Nd（t）=-0.8和-0.3，暗示其岩浆源区具有大量俯冲沉积物或流体的混入，为沉积物熔体和流体交代的地幔楔物质部分熔融的产物，经历了一定程度的角闪石分离结晶作用；2）中性岩形成于99.8~97.6Ma，略晚于基性岩，其主量元素与基性岩具有较好的线性关系，全岩ε_Nd（t）=+1.1，具有较多的地幔物质参与，为基性岩浆进一步演化形成；3）酸性岩（脉体）记录了多阶段岩浆作用（124.1~95.3Ma），根据同位素组成不同进一步划分为两类，第一类具有较低的全岩ε_Nd（t）值（-8.3~-6.0），其岩浆源区显示富集特征，t_DM2=1385~1586Ma，由古老地壳物质的再熔融形成；第二类的锆石ε_Hf（t）值（-2.8~+3.2）变化较大，岩脉的锆石ε_Hf（t）=+0.4~+8.1，t_DM=428~906Ma，全岩ε_Nd（t）=+0.1和+0.8，表明岩浆源区具有不均一性，为古老地壳物质被富流体地幔岩浆改造形成；和4）镁铁质包体的主量元素与寄主花岗岩具有较好的线性关系，锆石的Hf同位素组成变化较大（ε_Hf（t）=-9.3~+4.1），变化范围可达13个ε单位，为岩浆混合成因。寄主花岗岩和角闪辉长岩分别作为酸性和基性端元，是基性岩浆与其诱发古老地壳熔融形成的花岗质岩浆经混合形成。结合冈底斯岩基早白垩世岩浆岩的研究结果，朗县杂岩在早白垩世（124~97Ma）的岩浆作用具有明显的岩浆混合现象，锆石Hf和全岩Sr-Nd同位素组成变化较大，可达13个ε单位，其岩浆源区复杂且富含流体，代表了新特提斯洋在早期（240~144Ma）经历漫长的俯冲之后，在早白垩世时期（~120Ma）俯冲带发生跃迁或俯冲角度达到临界点，导致大量俯冲沉积物和流体沿俯冲带俯冲下去，与发生部分熔融的地幔楔物质混合，底侵导致上覆古老地壳物质的再熔融，形成早白垩世复杂的岩浆岩组合，很可能是新特提斯洋二次俯冲开始的标志。;The subduction of the Neo-Tethyan oceanic lithosphere might experienced a major shift in the subdcution geometry in the Early Cretaceous time during its long-lasting subducion. Knowing the geochemical nature and the mode of the Early Cretaceous magmatism in the Gangdese batholith is critical to deccriminate the subdcution processes in this time. Data from zircon U-Pb geochronology and geochemical (whole-rock element and isotope, zircon Hf) analyses on various rock types preserved in the Langxian complex show that: 1) the mafic rocks formed at 103.6~100.8Ma are of high-K calc-alkaline aluminous in composition. They are characterized by relatively low zircon ε_Hf(t) (+0.3~+5.7) and bulk ε_Nd(t) (-0.8 and -0.3), indicating that they were drived from partial melting of depeled mantle wedge metasomatized by fluids from subducted sediment; 2) the intermediate rocks formed at 99.8~97.6Ma and have slightly higher Nd isotope compositions with ε_Nd(t)=+1.1. They display a good linear relationship in major oxides with those in the mafic ones, suggesting that they are derivative products from the mafic suite; 3) the felsic rock (pluton and dike) crystallized at 124.1~95.3Ma. They could be subdivided into two types based on their isotopic compositions. The first type has a lower ε_Nd(t) (-8.3~6.0) but higher t_DM2 (1385~1586Ma). They formed by remelting of ancient crustal materials. In contrast, ε_Hf(t) and ε_Nd(t) in the second type granitic rocks vary widely with ε_Hf(t) ranging from -2.8 to +3.2 for the plutonic granite and from +0.4 to +8.1 for the dike, respectively. Together with relatively low Nd isotope (ε_Nd(t)=+0.1 and +0.8) and t_DM(428~906Ma), these characteristics suggest a relative young source region due to modification of ancient crustal materials by melts derived from fluid-enriched mantle; 4)enclaves hosted by the granitic rocks show good linear relationship in major oxides with the host rocks. They display a rather large shift of ~13 epsilon units in zircon Hf isotope compositions (ε_Hf(t)=-9.3~+4.1) and might represent the melts formed by various degrees of mixing an acidic end member of the hosted granite wih a mafic end member of hornblende gabbro. Combined with literature data of the Gangdese batholith, magmatic rocks from the Langxian complex preserved a key record of a major magmatic process during the Early Cretaceous (122~97Ma). Melting and mixing of various source components (ancient crustal material, fluid-enriched mantle, and depleted mantle) is one of the important features of the Early Cretaceous magmatism due to subduction of the Neo-Tethyan ocean and resulted in large shifts (up to ~13 epsilon units) in zircon Hf as well as in bulk Sr-Nd isotope compositions. To introduce various components into and achieve a large isotopic heterogeneity in the source region, it requires a major change in the subduction geometry in the long subduction of the Neo-Tethyan subdction system. Since the initation of the subduction at ~240Ma, subdcition of the Neo-Tethyan Ocean might experinecd a critical reorganization enabling the introduction of an increased amount of sediments and fluids into the subduting system and the modification of the mantle wedge in the Early Cretaceous (~120Ma). The mafic magmas caused remelting of the overlying ancient crustal material and finally formed the complex magmatic activity in the Early Cretaceous, which is likely to be a sign of the beginning of the second subduction of the Neo-Tethyan Ocean.

ЛАТЕРАЛЬНАЯ ЗОНАЛЬНОСТЬ АНЮЙСКОГО СЕГМЕНТА ЧУКОТСКОГО ПЛУТОНИЧЕСКОГО ПОЯСА (ЗАПАДНАЯ ЧУКОТКА)

Early Cretaceous granitoids of the Anyui segment of the Chukchi plutonic belt are represented by associations: gabbro-granite, monzodiorite-granite and granite-leucogranite. For the rocks of gabbro-granite and monzonite-granite associations with respect to the primitive mantle, enrichment of Cs, Rb, K was established, with depletion of Nb, Hf, Ti. Granitoids of the granite-leucogranite association are characterized by the most contrasting ratios of enriched and depleted elements, with deep lows of Ba, Nb, Sr, P, Ti against the background of high Cs, Rb, K, Th, U, as well as the absence of a minimum of Hf. Along with the presence of lateral zonality in the placement of granitoids in relation to the southwestern border of the Anyuy Terrain, these data suggest the subductive nature of the formation of gold-bearing gabbro-granite and monzonite-granite associations in the conditions of an active continental margin that arose in the southwestern part of the Chukchi Terrain in Early Cretaceous time due to subduction.

Chapter 3.1a Antarctic Peninsula and South Shetland Islands: volcanology

Abstract The voluminous continental margin volcanic arc of the Antarctic Peninsula is one of the major tectonic features of West Antarctica. It extends from the Trinity Peninsula and the South Shetland Islands in the north to Alexander Island and Palmer Land in the south, a distance of c. 1300 km, and was related to east-directed subduction beneath the continental margin. Thicknesses of exposed volcanic rocks are up to c. 1.5 km, and the terrain is highly dissected by erosion and heavily glacierized. The arc was active from Late Jurassic or Early Cretaceous times until the Early Miocene, a period of climate cooling from subtropical to glacial. The migration of the volcanic axis was towards the trench over time along most of the length of the arc. Early volcanism was commonly submarine but most of the volcanism was subaerial. Basaltic–andesitic stratocones and large silicic composite volcanoes with calderas can be identified. Other rock associations include volcaniclastic fans, distal tuff accumulations, coastal wetlands and glacio-marine eruptions. Other groups of volcanic rocks of Jurassic age in Alexander Island comprise accreted oceanic basalts within an accretionary complex and volcanic rocks erupted within a rift basin along the continental margin that apparently predate subduction.

Chapter 3.1b Antarctic Peninsula and South Shetland Islands: petrology

Abstract The Antarctic Peninsula contains a record of continental-margin volcanism extending from Jurassic to Recent times. Subduction of the Pacific oceanic lithosphere beneath the continental margin developed after Late Jurassic volcanism in Alexander Island that was related to extension of the continental margin. Mesozoic ocean-floor basalts emplaced within the Alexander Island accretionary complex have compositions derived from Pacific mantle. The Antarctic Peninsula volcanic arc was active from about Early Cretaceous times until the Early Miocene. It was affected by hydrothermal alteration, and by regional and contact metamorphism generally of zeolite to prehnite–pumpellyite facies. Distinct geochemical groups recognized within the volcanic rocks suggest varied magma generation processes related to changes in subduction dynamics. The four groups are: calc-alkaline, high-Mg andesitic, adakitic and high-Zr, the last two being described in this arc for the first time. The dominant calc-alkaline group ranges from primitive mafic magmas to rhyolite, and from low- to high-K in composition, and was generated from a mantle wedge with variable depletion. The high-Mg and adakitic rocks indicate periods of melting of the subducting slab and variable equilibration of the melts with mantle. The high-Zr group is interpreted as peralkaline and may have been related to extension of the arc.

ROLE OF THE CRUSTAL DETACHEMENTS DURING RIFTING, INSIGHTS FROM LABRADOR SEA

The Labrador Sea is part of the Atlantic rift system which opens during Early Cretaceous times in the intracontinental domain. The rifting stage continues and generates an expansion center, with the establishments of an oceanic ridge during Late Cretaceous-Paleocene times. The rifting was asymmetric with a gradual propagation northward. The Cretaceous stretching and thinning of the continental crust led to the development of the transitional domain, with a controversial origin either thinned continental or oceanic. With this study, we bring new evidences for the presence of exhumed upper mantle in the transition domain based on geometry of the detachment faults, typical “S” shape, and we emphasize the role of thedetachments in the rifting of continental crust and its ultimate break-up. We observed the lateral change, from South to North, of the continental to oceanic crust transition, from a step-up to a step-down. Based on, we suggest that it might reflect the changes in the upper versus lower plate position of the Labrador Sea, a change accommodated across the Snorri Fracture Zone.

Petrogenesis and metallogenesis of an extraordinary deeply hidden granite pluton overlain by W-Zn-Pb-Ag-mineralized roof: Example from Xidamingshan district, South China

Common understanding of pluton-related W-polymetallic mineralization is mainly based on studies in areas with exposed granitic plutons or plutons buried at shallow levels. However, there exist a large number of ore deposits that are considered to be associated with deeply hidden granitic plutons (deeper than 500 m). Their metallogenesis is not well documented due to lack of deep drilling and sampling. In this paper, we present a deeply hidden granitic pluton recently discovered by a kilometer-scale drilling project in the Xidamingshan Mountains area in western South China. The pluton, here refereed to Xidamingshan granitic pluton, underlies the Xidamingshan W-Zn-Pb-Ag-mineralized district. Petrographic examinations reveal that the hidden granite is biotite granite with partly hydrothermal alteration and that the upper part of the pluton is subjected to greisenization. Geochronological analyses indicate that the granite pluton was emplaced at the Late Cretaceous time with zircon U-Pb ages of 98.0 ± 1.0 Ma and 99.8 ± 0.9 Ma. The emplacement was subsequently followed by hydrothermal activity as indicated by muscovite 40Ar-39Ar ages of 93 ± 3 Ma and 92 ± 3 Ma. Re-Os dating of molybdenite yields an isochronal age of 95.9 ± 1.7 Ma (MSWD = 2.2). These ages indicate that the Xidamingshan mineralization is temporally related to the hidden granite. The granite is strongly peraluminous (A/CNK = 1.50 – 1.61) with high-K calc-alkaline affinity (ASI = 1.46 – 1.61) and shows enrichment in Rb, Ta, U, and K and depletion in Nb, P, and Ti. It contains muscovite and biotite but is lack of hornblende. It shows a steady correlation between P2O5 and increasing SiO2, high Rb/Sr ratios (1.33 – 3.12), and high zirconium saturation temperature (average of 767 °C). The Xidamingshan granite is thus a typical S-type granite. Geochemical signature and negative εHf(t) values (-4.9 – −12.6) indicate that the hidden granite might have been generated by melting of crustal materials of argillaceous-depleted sedimentary rocks. Granites with such geochemical characteristics are known as having W-polymetallic mineralization specialization based on comparison with other ore-forming granites in South China. Significant W-polymetallic mineralization has been found within the thick Cambrian strata overlying the hidden granite pluton. Distribution of the related deposits demonstrates a typical zoning pattern from skarn-type W deposits, through vein-type Pb-Zn deposits, to vein-type Ag deposits. Given the combined evidences of metallogenic specialization of the source rocks for granitic magmas, geochronological coupling of the magmatism and hydrothermal activity, and the hydrothermal alteration and mineralization zoning, it is suggested that there exists a hydrothermal W-Zn-Pb-Ag mineralization system driven by the emplacement of the Late Cretaceous S-type hidden granite pluton. The findings of this study provide insights into future deep drilling and related ore exploration in regions with hidden granite in South China and the world.

Subduction zone processes and crustal growth mechanisms at Pacific Rim convergent margins: modern and ancient analogues

AbstractContinents grow mainly through magmatism, relamination, accretionary prism development, sediment underplating, tectonic accretion of seamounts, oceanic plateaus and oceanic lithosphere, and collisions of island arcs at convergent margins. The modern Pacific–Rim subduction zone environments present a natural laboratory to examine the nature of these processes. The papers in this special issue focus on the: (1) modern and ancient accretionary margins of Japan; (2) arc–continent collision zone in the Taiwan orogenic belt; (3) accreting versus non-accreting convergent margins of the Americas; and (4) several examples of ancient convergent margins of East Asia. Subduction erosion and sediment underplating are important processes, affecting the melt evolution of arc magmas by giving them special crustal isotopic characteristics. Oblique arc–continent collisions cause strong deformation partitioning that results in orogen-parallel extension, crustal exhumation and wrench faulting in the hinterland, and thrust faulting–folding in the foreland. Trench-parallel widths of subducting slabs exert major control on slab geometries, the degree of coupling–decoupling between the lower and upper plates, and subduction velocity partitioning. An initially large width of the subducting Palaeo-Pacific Plate against East Asia caused flat subduction and resistance to slab rollback during the Triassic Period. These conditions resulted in shortening across SE China. Foundering and delamination of the flat slab during the Early Jurassic Epoch led to slab segmentation and reduced slab widths, followed by slab steepening and rollback. This pull-away tectonics induced lithospheric extension and magmatism in SE China during Late Jurassic – Cretaceous time. Melting of subducted carbonaceous sediments commonly produces networks of silicate veins in CLM that may subsequently undergo partial melting, producing ultrapotassic magmas.

Dinosaur Tracks from the Caucasian Mineral Waters Country in the Context of the Study of Lower Cretaceous Paleosols of This Region

This article examines the geological and stratigraphic context of several types dinosaur tracks discovered in Lower Cretaceous deposits exposed in the vicinity of city of Kislovodsk (Caucasian Spa Country; Stavropol Krai). The tracks occur on the bedding surfaces of Lower Cretaceous (Valanginian) limestones, dolostones and marls. Two track morphotypes are described. Some ideas are proposed on the paleoclimatic and environmental conditions in the region during Early Cretaceous times. We suggest that the tracks were produced in a coastal plain in the littoral zone of a marine basin (possibly, a brackish lagoon) during a warm, seasonally dry climate.

The Barreirinhas Basin internal architecture and the evidence of transform movements along the Romanche Fracture Zone, Brazilian Equatorial Margin

The Barreirinhas Basin, located in the central part of the Brazilian Equatorial Margin, was originated by Pangea breakup during Early Cretaceous time. The Romanche Fracture Zone played an essential role during the entire tectono-stratigraphic evolution of the Barreirinhas Basin. This fracture zone divided the basin in two sub-basins to the south and north with distinct structural style printed on the basin internal geometry. This study is aimed to investigate how the tectonic and sedimentary evolution in the onshore, shallow and deep-water Barreirinhas Basin, were influenced by the Romanche Fracture Zone. Eight 2D seismic sections, subjected to seismic attributes to enhance the seismic stratigraphic horizons and structural features, and supported by five well profiles, allowed analyzing the basin infill, major faults, and the spatial relationship with the fracture zone. The basin evolution and the tectono-stratigraphic architecture are marked by structures from rift to drift stages. The rift sequences are composed of four seismic units (U1-U4) delimited on top by the horizon R4. The sag and drift sequences are composed of six seismic units (U5-U10) delimited between horizons R4-R10. The structural architectures are dominated by transtensional regimes, such as negative flower faults, tilted blocks, and wipeout zones, whereas transpressional tectonics is also present by reverse faults and positive flowers, which are fingerprints of Romanche Fracture Zone. Especially in the southern sub-basin, the transform tectonic activity deformed the entire basin from Aptian to Holocene. These structures correspond to those in Africa conjugate basins, where recurrent dextral strike-slip plate motions caused transtensional and transpressional deformations in the sedimentary pile since separation of South America and Africa. Asymmetric basin architecture, intensity and asynchrony of tectonic regimes distinguish the Atlantic Equatorial basins.

Magma Loading in the Southern Coast Plutonic Complex, British Columbia and Washington

Abstract The southen end of the 1800 km long Coast Plutonic Complex (CPC), exposed in the Harrison Lake area of British Columbia and in the North Cascades of Washington, bears a record of great crustal thickening -20 to 40 km in localized zones during Late Cretaceous times. During this period, the CPC was positioned at the continental margin during collision/subduction of the Farallon plate. Arc magmatism and regional orogenic contraction were both active as potential crustal thickening processes. Magmatism is favored in this report as the dominant factor based on the delineation of four spatially and temporally separate loading events, the close association of the loaded areas with emplacement of large plutons, and a paucity of evidence of deep regional tectonic contraction. The timing and spatial location of crustal loading events are documented by the following: zircon ages in plutons; an early event of low pressure in pluton aureoles evidenced by andalusite, now pseudomorphed by high-pressure minerals; high pressures in country rock in pluton aureoles measured by mineral compositions in the assemblages garnet-biotite-muscovite-plagioclase and garnet-aluminum silicate-plagioclase; high pressures recorded in plutons by Al-in-hornblende barometry; and uplift ages of plutons derived from K-Ar and Ar-Ar ages of micas and hornblende in plutons.

Genesis and paleoenvironmental analysis of the ironstone facies of the Maastrichtian Patti Formation, Bida Basin, Nigeria

The recently constructed Lokoja-Abuja highway and a quarry which cut through the Patti Formation, provided opportunity for the field study and sampling of the exposed ironstone interbeds within the carbonaceous shales and clays in Ahoko-Idu axis of the southern Bida Basin, Nigeria. The origin and paleoenvironmental significance of the ferruginous intervals were examined in this paper. Investigation reveals that the ferruginous bands occur as three lithofacies comprising of laminated, bioturbated and spheroidal concretionary facies. The primary components and depositional features of the host shales (detrital minerals, wood and herbaceous fragments, laminations) are largely preserved in the ironstones indicating compositional similarity and therefore a diagenetic origin for the ironstones. Results of XRD reveal substantial amount of siderite and illite but minor amounts of kaolinite, hematite, calcite and magnetite. Major elements such as Fe2O3, SiO2, Al2O3, CaO, MgO and P2O5 range from 41.50 to 54.60%, 6.10–23.30%, 3.64–7.00%, 1.40–13.60%, 0.30–1.20% and 0.10–8.80% respectively. The current data is suggestive that kaolinite was diagenetically replaced by siderite at early stage of burial in a reducing environment. Migrating pore water, probably enriched with alkali metals like Ca and Mg, sourced from sea water in the depositional basin and abundance of organic matter in the primary sediments are interpreted to be part of the factors responsible for the siderite precipitation. Both the host shale and the ferruginous beds contain significant amount of MgO, CaO, relics of dissolved bioclasts and body fossils. The geochemical proxies combined with the presence of magnesian siderite in the ironstones indicate that the host shales were deposited in marine environment with occasional freshwater influence. The study concludes that the ironstones are marine to non-marine, sideritic and diagenetic. The laminated and fossiliferous ironstone facies was deposited at the peak of marine transgression during the Upper Cretaceous time and represents maximum flooding surface. Shallowing up and regression into fresh water swamp condition was marked by the occurrence of freshwater sideritic ironstone.

Sedimentology and provenance of newly identified Upper Cretaceous trench basin strata, Dênggar, southern Tibet: Implications for development of the Eurasian margin prior to India–Asia collision

AbstractTrench basins preserved along the southern margin of the Lhasa Terrane, Tibet, are sedimentologic records of convergent margin processes preceding Cenozoic India–Asia collision. We present new sedimentologic, petrographic and geochronologic data from the Rongmawa Formation and surrounding strata near Dênggar, Tibet, to determine depositional environment, provenance and age. Depositional ages range from ca. 92 to 87 Ma and lithofacies are consistent with deposition by low‐ and high‐density turbidity currents and suspension settling of pelagic detritus in a deep‐marine, trench basin setting. Sandstone modal analyses and U–Pb geochronology indicate that trench basin detritus in this region was derived from the Lhasa Terrane. We interpret that the Cretaceous subduction trench received detritus from an axial sediment dispersal system that transported sediment from headwaters in the central‐southern Lhasa terrane near Lhasa City directly to the trench and then flowed westwards parallel to the trench. The preservation of trench basin strata deposited during Late Cretaceous time compared with the lack of trench deposits prior to ca. 92 Ma and after ca. 80 Ma suggests the margin experienced a period of significant accretion during this interval. In addition, deposition of trench basin strata occurred during Late Cretaceous adakitic magmatism and high‐temperature metamorphism, which are hypothesized to be explained by subduction of an oceanic ridge or subduction zone retreat and related upper plate extension along the southern margin of the Lhasa terrane. Subduction of an oceanic ridge may provide a mechanism to potentially erode forearc basin strata and promote increased sediment delivery directly to the trench.

Episodic hydrocarbon charge in tight Mississippian reservoirs of Central Oklahoma, USA: Insights from oil inclusion geochemistry

The unforeseen discovery of substantial hydrocarbons in ultra-tight Mississippian reservoirs in Oklahoma testifies to the complexity of the hydrocarbon charge across the flanks of the Nemaha Uplift. Based on crude oil and fluid-inclusion chemistry, the hydrocarbons at the western side of the Nemaha Uplift consist of highly-mature, thermally-cracked and less-mature uncracked hydrocarbons (0.50–1.4% calculated vitrinite reflectance, Rc). The oils are less mature on the eastern flank of the structure (0.5–0.7% Rc). Based on distributions of biomarkers and higher diamondoid (composed of three adamantane cages or more) distributions, the hydrocarbons entrapped within reservoir fluid-inclusions are sourced from the Woodford Shale and also from Mississippian carbonates. To our knowledge, this is the first work to describe oil correlations using diamondoids inside fluid inclusions. Petroleum compositions in the ultra-tight Mississippian reservoirs are highly variable, which highlights the need for a thorough assessment of mechanisms controlling charge. By integrating oil and fluid inclusion geochemistry and fluid-inclusion microthermometry with basin modeling, we show that the primary controls are: (1) proximity to the source-rock kitchen(s), (2) episodic charging from Permian through Cretaceous time, and (3) mixing.