Triassic-Early Jurassic Research Articles

Modelo cinemático del control estructural 4D y metalogénesis del distrito auroargentífero Vetas-California (Santander, Colombia)

). The gold and silver deposits of the Vetas-California mining district are hosted in Paleozoic gneisses and Mesozoic granites of the Santander massif in the Eastern Cordillera of Colombia, far from the northern Andean volcanic arc. In the California area, there is ambiguity in defining the epithermal-type metallogenic model with high to intermediate sulfidation due to the presence of Late Triassic-Early Jurassic and Miocene magmatism. The best-known hydrothermal episodes comprise two early porphyritic-style phases: a) characterized by a propylitic alteration with molybdenite of the ~10.1 Ma-old La Mascota breccia, related to granodiorite porphyries and smaller-volume rhyodacitic dykes; and b) attributed to a magmatic-hydrothermal system due to phyllic alteration associated with quartz-pyrite veins with hydrothermal sericite dated at ~3.4 Ma. These two hydrothermal episodes were followed by four epithermal phases related to the development of multiphase hydrothermal breccias and quartz-alunite alteration, defined by bornite, covellite, and chalcopyrite, alongside wolframite, enargite, and sphalerite, with gold and silver mineralization between ~2.6 and ~ 1.6 Ma. To understand the relationship of the emplacement of the porphyries and the subsequent auro-argentiferous enrichments to the deformation front observed in the Pamplona buttress, to the east of the Santander Massif, a 4D structural model of the mineralization is formulated here. This model is based on the spatial analysis of the stresses responsible for the La Baja-Angostura dextral fault in terms of the regional kinematics of the Bucaramanga sinistral fault. It is considered that the SW sector of the deposit area corresponds to a well-defined, tabular, sub-vertical body of the La Mascota breccia. On the other hand, the NE sector in the La Baja-Angostura fault is expressed as a topographically elevated right horsetail splay, which controls the more disseminated distribution of the mineralization by means of veinlet swarms arranged in an ~E-W direction. The disseminated mineralization of Vetas-California was probably favored by late thermal expansion-contraction events in the gneissic sockets, which allowed the accumulation and precipitation of boiling solutions rich in volatiles and elements of economic interest.

Initial sedimentation after the closure of the Sumdo Paleo-Tethys Ocean: Implications for early Mesozoic tectonics in the East Tethys domain

The closure of the Paleo-Tethys Ocean and subsequent collisional orogenesis in the early Mesozoic played an important role in the formation of the Qinghai–Tibetan Plateau. However, the final closure of the southernmost Paleo-Tethys branching ocean basin in the Qinghai–Tibetan Plateau—known as Sumdo Paleo-Tethys Ocean—and collision between Central and Southern Lhasa terranes remain controversial. In particular, the tectonic significance of early Mesozoic volcanism and sedimentation in the Lhasa terrane is unknown, but crucial for understanding the final stages of ocean closure. Here, we investigate the depositional ages and provenance of detrital zircon in clastic sedimentary rocks, as well as the petrogenesis of the interbedded andesite from the volcano-sedimentary sequence of the Late Triassic to Early Jurassic Xionglai Formation in the Tangjia-Sumdo area. Detrital zircon dates from the Xionglai Formation define five age peaks at 180–200 Ma, 230–340 Ma, 500–620 Ma, 1000–1240 Ma, and 1600–1800 Ma. Detrital zircon trace elements and Hf isotopic characteristics are interpreted to represent derivation from an accretionary wedge and later magmatism related to oceanic slab break-off as well as a minor component from coeval magmatic rocks in the Nyainrong-Nang area in northern Lhasa terrane. Interbedded andesite has a magnesian andesite signature and yields ages of 192–184 Ma with zircon εHf(t) values that range from −3 to +1. We interpret these data as the andesite being sourced from sediment-derived melts that interacted with the overlying mantle wedge in a post-collision extensional setting. Our study suggests that the closure of the Sumdo Paleo-Tethys Ocean and subsequent collision between Central Lhasa and Southern Lhasa terrane may have happened in Late Triassic-Early Jurassic. The final closure of the multi-Paleo-Tethys Ocean and subsequent collision in the Qinghai–Tibetan Plateau may have induced initial subduction of the Meso-/Neo-Tethys Ocean, representing the tectonic transition from collision/post-collision to subduction/accretion in the Qinghai–Tibetan Plateau during the Late Triassic to Early Jurassic.

Subsurface structures and nature of seafloor mounds in the northern South China Sea margin: Implications for Mesozoic hydrocarbon exploration

The Chaoshan Depression in the northern South China Sea (SCS) margin hosts thick (ca. 4–9 km) Mesozoic strata including two sets of hydrocarbon source rocks (the Late Triassic-Early Jurassic and Late Jurassic marine facies sequences). It was predicted to be petroliferous but unproven. Numerous submarine mounds are distributed on its seafloor, but whether they are mud volcanoes or magmatic volcanoes remains controversial due to poor knowledge of the subsurface structures. To reveal the nature of these mounds and the Mesozoic hydrocarbon potential in the Chaoshan Depression, multi-channel seismic (MCS) and ocean bottom seismometer (OBS) data along line CS-L1 across three seafloor mounds are processed and analyzed. The MCS profile reveals the existence of fuzzy reflection zones (FRZs) within the Mesozoic strata beneath these mounds. The overlying sedimentary sequences appear uplifted and deformed. The OBS velocity results exhibit that these FRZs feature significantly lower velocities than their host strata with velocity differences ranging from ∼0.25 km/s to 0.71 km/s, likely resulting from vigorous fluid charging rather than magma intrusion. Thus these FRZs are identified as mud-fluid diapirs. Between these diapirs and the overlying seafloor mounds, numerous high-angle faults exist, serving as vertical fluid feeder conduits. Additionally, several bright spots, acting as indicators of gas reservoirs, are observed near these faults. Moreover, bottom simulating reflectors with reverse polarity are imaged within the shallow strata around the seafloor mounds on an adjacent MCS profile, indicating rich gas trapping and/or gas hydrate accumulation. Therefore, we propose that these seafloor mounds are mud volcanoes with fluids coming from the underlying diapirs. As the diapirs extend downward into the Jurassic and Triassic strata, the hydrocarbon gases feeding the mud volcanoes and diapirs are reasoned to originate from the deep Mesozoic source rocks, implying significant Mesozoic hydrocarbon potential in the Chaoshan Depression of the SCS.

Whitehorse Trough records Late Triassic–Cretaceous accretionary orogenesis in the Northern Canadian Cordillera via detrital mineral thermochronometry

The Whitehorse Trough formed during early Mesozoic accretion of the Intermontane terranes to northwestern North America. Here we investigate its thermal history using detrital mineral thermochronology, including 171 single-crystal (U–Th)/He zircon (ZHe) ages from 35 samples, 158 single-crystal (U–Th)/He apatite (AHe) ages from 33 samples, and apatite fission track (AFT) ages from 12 samples. ZHe single crystal ages range from 222 to 42 Ma and define Triassic–Early Jurassic, Late Jurassic, and Cretaceous–Paleogene age groups. AFT central ages range from 95 to 30 Ma with a dominant age peak at ∼50 Ma, and AHe single crystal ages range from 228 to 13 Ma with a dominant age peak between 50 and 40 Ma. Forward and inverse models of thermochronological data are compatible with two regional burial/heating stages that variably reset He in zircon. Maximum temperatures of the Whitehorse Trough strata locally exceeded 150 °C during Early Jurassic burial and shortening into a fold and thrust belt. Following Middle to Late Jurassic exhumation-related cooling and development of a prominent unconformity, Whitehorse Trough strata were buried again. Temperatures locally exceeded 150 °C during the Cretaceous, suggesting maximum burial of between ∼4 and 7.5 km. Heating and cooling rates during the Early–Middle Jurassic were ∼10 °C/myr, coinciding with deposition, fold and thrust belt development, and regional crustal thickening during the final stages of Intermontane terrane accretion. Maximum heating rates during the Cretaceous were ∼4–7 °C/myr and likely correspond to regional crustal thickening of the northern Cordillera hinterland and establishment of an outboard, Andean-type continental arc system.

Geological and chronostratigraphic overview of the Upper Triassic and Jurassic successions of the Junggar Basin, NW China

Abstract The vast, widely exposed terrestrial (lacustrine to fluvial) Upper Triassic–Jurassic (except Tithonian) successions of the Junggar Basin not only record most of the stratigraphic boundaries of the Upper Triassic and Jurassic, including the Triassic–Jurassic boundary and the Hettangian–Sinemurian, Sinemurian–Pliensbachian, Pliensbachian–Toarcian, Lower–Middle Jurassic, Middle–Upper Jurassic and Oxfordian–Kimmeridgian boundaries, but also record a range of geological, organic, palaeogeographic and palaeoclimatic events known to have happened globally in the Late Triassic and Jurassic. The Triassic–Jurassic boundary is placed in the stratigraphic interval of the first occurrence of Retitriletes austroclavatidites and Callialasporites dampieri and the last occurrence of Lunatisporites rhaeticus . The end-Triassic mass extinction is characterized by the disappearance of most of the sporomorph and macro-plant taxa. The end-Triassic mass extinction occurred before the first occurrence of the sporomorph Cerebropollenites thiergartii , and ended after its appearance when life began to revive. The Junggar Basin was situated at a high latitude during the Late Triassic–Early Jurassic Pliensbachian ‘hothouse’ and ‘greenhouse’ periods. The Late Triassic–Mid Jurassic Bajocian was humid and warm, and rich in coal swamps, except the Toarcian, which yields little coal because it was relatively warmer and drier. It became arid from the early Late Jurassic Oxfordian.

A polygenetic origin for the Sikhoran ultramafic-mafic complex in South Iran

The nature and geodynamic setting of the Sikhoran ultramafic–mafic complex, southwest Iran, is debated, with multiple competing theories having been proposed in recent years. The Sikhoran ultramafic–mafic complex is located in the southern part of the Sanandaj-Sirjan metamorphic-magmatic zone (SSMMZ) and is comprised of porphyroclastic, transitional and layered ultramafic-mafic sequences of Carboniferous age. These units are cut by isotropic gabbros, diabasic and pegmatoid gabbroic dikes with ages ranging from Permian to the Cretaceous. These bodies also intrude into Upper Paleozoic metamorphic rocks, where they induced high-temperature contact metamorphism during the Late Carboniferous. Here we present new textural descriptions, mineral and whole-rock geochemical analyses, and the results of zircon UPb dating of magmatic mafic rocks and metamorphic host rocks from this region in order to reconstruct the petrogenesis of the Sikhoran ultramafic–mafic complex. Zircon UPb ages from layered gabbros indicate Late Carboniferous (320.8 ± 6.4 Ma) crystallization, which is similar to the age of the Upper Paleozoic host gneisses (334.6 ± 4.9 Ma). By contrast, isotropic gabbros that cross-cut the complex have a crystallization age of 178.3 ± 2.3 Ma, and anatectic quartz diorite/plagiogranite produced from partial melting of hosting amphibolite in contact with the gabbros yielded Late Triassic-Early Jurassic ages (187.2 ± 2.6 Ma). These data suggest a polygenetic origin and a range of tectonic settings for various parts of the ultramafic-mafic complex. Geochemical modeling shows that mantle plume-related melting (∼5–40%) of metasomatized mantle could have formed the ultramafic-mafic layered cumulate complex during the Late Carboniferous. A geochemical transition from island arc tholeiite (IAT) isotropic gabbros to cross-cutting enriched mid-ocean ridge basalts (E-MORB)–diabasic dikes occurred between the Late Triassic-Early Jurassic to the Late Cretaceous. The mantle-derived ultramafic parts of the Sikhoran ultramafic-mafic complex have similarities with other known ultramafic-mafic mantle plumes (e.g., Tinaqulllo, Lherz, and Ronda), and its layered ultramafic-mafic segment has similarity with layered intrusions (e.g., Pulur complex in Turkey). The highest observed crystallization temperature in porphyroclastic, transitional and layered ultramafic mantle sections of ∼1140 °C is considered as the mantle potential temperature, since the highest crystallization temperatures are observed in the most forsteritic olivine crystals. Based on amphibole chemistry, the crystallization conditions of transitional to layered ultramafic rocks at 900–1005 °C occurred at high H2O contents in a melt between nickel–nickel oxide (NNO) −8.5 and NNO −9.0, which lies along the NNO−2 buffer. These results are consistent with a model in which the opening of the Zagros Neotethys Oceanic basin in the Esfandagheh region initiated in the Late Carboniferous, alongside the formation of an extensional environment that allowed ascent of a subcontinental mantle plume. Subduction initiation of oceanic lithosphere began during the Late Triassic-Early Jurassic and ended in the Late Cretaceous.

Detrital geochronology of the Cunningham Lake formation: an overlap succession linking Cache Creek terrane to Stikinia at ∼205 Ma

Detrital zircon from three coarse-grained marine siliciclastic rocks was analyzed for U–Pb, Lu–Hf, and trace element compositions to constrain the timing of deposition and sediment provenance of the Cunningham Lake formation (formerly siliciclastic unit of the Sitlika assemblage) in north-central British Columbia. This strategy tests previously proposed sedimentary linkages between the Cache Creek terrane and the westerly rocks of the Stikine terrane. All three samples indicate maximum depositional ages at ca. 205–202 Ma (Rhaetian). The samples contain a predominant ca. 225–215 Ma detrital population, sourced from proximal contemporaneous volcanic arcs, and minor Permian to Middle Triassic and Carboniferous arc-derived detrital populations. The absence of Precambrian grains is consistent with the strongly suprachondritic zircon compositions (εHf(t) = +7 to +20), and indicates exclusively juvenile sources for the Cunningham Lake formation. Late Triassic sources of zircon are not known in the Cache Creek terrane and, except within western Stikine terrane, are uncommon among the Intermontane terranes that amalgamated with the Cache Creek terrane during Late Triassic–Early Jurassic. The Stikine suite (ca. 230–214 Ma) and coeval volcanic rocks in western Stikinia are the most probable sources of Late Triassic detritus for the Cunningham Lake formation. Stikinia’s Paleozoic basement is the probable source of Carboniferous detrital zircon. Volcanic arc–backarc complexes in the Cache Creek terrane are the most likely sources of Permian to Middle Triassic detritus in the Intermontane terranes. Accordingly, the siliciclastic rocks of the Cunningham Lake formation represent an overlap sedimentary succession that links Stikinia to the Cache Creek terrane by the latest Triassic.

Neotethyan Ankara Melange, central Turkey: Formation by accretion of seamounts and supra-subduction zone ophiolites in an oceanic fore-arc setting

The Neotethyan Ankara Melange is a global reference for oceanic accretionary processes preserved in a collisional orogen. Here we show, based on eight representative outcrops around Ankara, that the melange encompasses stratigraphically coherent volcanic-sedimentary successions of mainly Late Jurassic-Early Cretaceous age. Our results indicate that the melange has a relatively organised tectonostratigraphy, in contrast to previous interpretations that emphasised its chaotic nature. Coherent volcanic-sedimentary successions of Late Jurassic-Early Cretaceous age are interpreted as flank facies of a large oceanic seamount (probably plume related) and its capping carbonate platform (atoll), including reef-and back-reef facies. Remnants of other seamounts are dated as Late Triassic-Early Jurassic and Late Cretaceous. Variably dismembered supra-subduction zone ophiolites of latest Early Jurassic-Middle Jurassic age, including boninites, formed in an oceanic fore-arc setting, similar to that of the Cenozoic Izu-Bonin fore arc, NW Pacific. The melange in the Ankara region partly accreted during the Early Cretaceous (pre-Albian) in response to collision of the inferred large seamount with the oceanic (ophiolitic) fore arc. During the collision which involved subduction-erosion slices of proximal–distal seamount flank lithologies accreted sequentially, whereas the seamount core subducted. Also during the collision, the distal outer edge of the ophiolitic fore arc, mainly mantle harzburgite, detached from the over-riding lithosphere and was incorporated into the melange. Structural evidence emphasises the importance of tectonic processes (rather than olistostromes) in the melange accretion. The accretionary prism in the outcrops studied was covered by arc-related sediments and volcanics during the Late Cretaceous and deformed by collision-related processes during latest Cretaceous-Paleogene.

Ghost-arc geochemical anomaly at a spreading ridge caused by supersized flat subduction

The Southern Atlantic-Southwest Indian ridges (SASWIR) host mid-ocean ridge basalts with a residual subduction-related geochemical fingerprint (i.e., a ghost-arc signature) of unclear origin. Here, we show through an analysis of plate kinematic reconstructions and seismic tomography models that the SASWIR subduction-modified mantle source formed in the Jurassic close to the Georgia Islands slab (GI) and remained near-stationary in the mantle reference frame. In this analysis, the GI lies far inboard the Jurassic Patagonian-Antarctic Peninsula active margin. This was formerly attributed to a large-scale flat subduction event in the Late Triassic-Early Jurassic. We propose that during this flat slab stage, the subduction-modified mantle areas beneath the Mesozoic active margin and surrounding sutures zones may have been bulldozed inland by >2280 km. After the demise of the flat slab, this mantle anomaly remained near-stationary and was sampled by the Karoo mantle plume 183 Million years (Myr) ago and again since 55 Myr ago by the SASWIR. We refer to this process as asthenospheric anomaly telescoping. This study provides a hitherto unrecognized geodynamic effect of flat subduction, the viability of which we support through numerical modeling.

Geology, geochemistry, isotope geochemistry and fluid inclusions of the Early Carboniferous granitic rocks from Bajo de La Leona, Deseado Massif (Santa Cruz, Argentina) and geological relationships with the Triassic-Jurassic magmatism

Paleozoic plutonic rocks from the eastern Deseado Massif, southern Patagonia Argentina, generally appear as stocks and sills of variable lithological composition from gabbro-diorite to leucogranite, with pegmatite-aplite differentiations. Their ages vary from Early-Middle Ordovician up to Early Carboniferous.The Early Carboniferous Laguna Turbia Leucogranite (LTL) crops out in the Bajo de La Leona area as a small stock located between the Permian conglomerates of La Golondrina Formation and the Late Triassic-Early Jurassic plutonic rocks of La Leona Formation. It shows a peneplanized landscape due to erosion processes and has microscopic evidence of solid-state deformation features. The geochemical signature is calc-alkaline, typical of magmatic arc environments. The high K contents and FI data are consistent with a crustal provenance.Four kilometers to the SSW of the LTL appears a biotitic granite whose petrographic, geochemical and isotopic characteristics indicate that it belongs to a different magmatic cycle, of possible Late Devonian-Early Carboniferous age. It has been named Laguna Larga Granite (LLG).The Late Triassic-Early Jurassic intrusive rocks (La Leona Formation) occur as several plutons in close spatial association with pre-Permian igneous-metamorphic basement rocks in the eastern part of the Deseado Massif. These granitoid suites crop out in erosional windows through the widespread Middle to Late Jurassic Bahía Laura Complex and younger cover.The main isotopic feature of the granitic rocks of Bajo de La Leona is their lack of equilibrium, which is especially evident in the case of the LLG, given its extraordinarily low values of δ18O.Fluid inclusion data obtained allowed the identification of different types of fluid formation environments: (a) a porphyry type, related to the Early Carboniferous leucogranite suggesting an origin from cooling magmatic solutions; (b) late magmatic/high salinity fluids related to Mesozoic quartz monzonite; (c) solutions of hydrothermal/mesothermal origin that generated the quartz gold veins (LSV and LTV); and (d) solutions associated with an epithermal system superimposed on quartz monzonite of La Leona Formation, which gave rise to Cu mineralizations in quartz-veins with adularia of La Leona and Schultz mines. The FI and stable isotope data suggest that ore fluids related to quartz veins were product of mixing igneous and meteoric solutions.The emplacement of the LTL extends the known Paleozoic magmatic history of the eastern Deseado Massif into the Early Carboniferous. Magmatism of that age is scarcely known in southern Patagonia.

Lithofacies and Diagenetic Controls on Tight Silty and Sandy Upper Triassic Reservoirs of the Heshui Oil Field (Ordos Basin, North China)

Summary Tight oil, present in reservoirs of low porosity and permeability, can be regarded as a kind of unconventional resource. The tightening process in this kind of reservoir is controlled by the lithology and diagenetic history of the host formation. Upper Triassic Yanchang Formation siltstones and sandstones are the main reservoirs for hydrocarbon accumulation in the Heshui Oil Field (HOF), southwestern Ordos Basin. The reservoirs exhibit low porosity, low permeability, and strong heterogeneity. In recent years, numerous drillcores have been recovered from these units, but the porosity-permeability characteristics and burial history of these silty and sandy reservoirs have not yet been reported in detail. In this study, an integrated analysis of the lithofacies, diagenesis, and reservoir characteristics of the siltstones and sandstones was achieved using a combination of core and thin section, grain size, scanning electron microscopy (SEM), X-ray diffraction (XRD), δ13C and δ18O, mercury intrusion capillary pressure (MICP), and porosity and permeability data. Our primary goals were to quantify the porosity-permeability characteristics of these silty and sandy reservoirs, restore their diagenetic histories, and examine the paragenetic relationship of reservoir tightness to hydrocarbon accumulation. The silty and sandy reservoirs represent braided river delta facies consisting of compositionally and texturally immature sediments. In the burial environment, they underwent complex diagenetic processes that reduced porosity from an initial average of ~38% to the present ~8%. Porosity-destructive processes included compaction (~ –12.5%) and cementation (~ –21%), with increases in porosity related to grain dissolution (~ +2.2%) and tectonic fractures (~ +1.1%). The reservoirs underwent four diagenetic stages: (1) Penesyngenetic and Eogenetic A Stage (Late Triassic-Early Jurassic); (2) Eogenetic B Stage (Late Jurassic); (3) Early Mesogenetic A Stage (Early Cretaceous); and (4) Late Mesogenetic A Stage (Late Cretaceous to recent). Hydrocarbon charging of these reservoirs occurred in three pulses. Existing pore space was partly filled by hydrocarbons during the Eogenetic B Stage. A second hydrocarbon charging event occurred during the Early Mesogenetic A Stage, when residual primary intergranular pores and secondary dissolution pores were filled. A third hydrocarbon charging event occurred during the Late Mesogenetic A Stage, when the reservoirs were tight. Siltstone beds deposited in delta front environments are the main future exploration targets in the Chang 6 to 8 members. The results of this study provide a useful reference framework for future exploration of hydrocarbon resources in the Upper Triassic Yanchang Formation of the HOF, as well as potential insights into the evolution of similarly tight reservoirs in other basins.

Late Jurassic back‐arc extension in the Neuquén Basin (37°S): Insights from structural, sedimentological and provenance analyses

AbstractThe Middle Jurassic–Early Cretaceous evolution of the Neuquén Basin is traditionally attributed to a long phase of thermal subsidence. However, recent works have challenged this model. In view of this, we study the Late Jurassic Tordillo Formation, a non‐marine depositional unit that marks a shift to regional regression across the basin. Previous studies propose different causes for this regression, including the growth of the magmatic arc in the west, uplift in the south or extension in the north. We studied the Tordillo Formation in sections located at an intermediate position in the Neuquén Basin, in order to understand the tectonic processes active during sedimentation. We present evidence of normal faulting within the Tordillo Formation and the base of the overlying Vaca Muerta Formation. Some of these faults can be attributed as syndepositional. We characterize the Tordillo Formation as part of a distal fan‐playa lake depositional system with a contemporaneous western magmatic arc as the main source of sediment. When compared to the Late Triassic–Early Jurassic NE to NNE‐oriented rifting, which marks the opening of the Neuquén Basin, the Late Jurassic extension shows a switch in stress orientation; the latter is orthogonal to the north‐trending subduction zone. We interpret this change as a renewed phase of back‐arc extension induced by slab rollback along with minor distributed intraplate extension prior to opening of the South Atlantic Ocean.

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.

Petrogenesis and Metallogenesis of Granitoids in the Yangla Cu-W Polymetallic Deposit, Southwest China: Evidence from Zircon Trace Elements and Hf Isotope

Magmatic zircon tends to present characteristic trends in trace element compositions in response to magma petrogenesis and metallogenesis, such that zircon may provide a window into melt evolution not accessible by whole rock chemistry. The Yangla large Cu deposit is located in the central part of the Jinshajiang Suture Zone, southwest China, constrained between the Jinshajiang and Yangla Faults. In this study, the trace elements and hafnium isotopic compositions of zircons from quartz diorite were studied. Previous published relevant data of Yangla granitoid plutons (i.e., dioritic enclave, granodiorite, and quartz monzonite porphyry) also have been systematically cited and discussed. The result shows that the crystallization temperature and two-stage Hf mode ages (tCDM; the age of the source rocks for the magmas) gradually increased while the oxygen fugacity (fO2) and εHf(t) values gradually decreased, corresponding to the diorite enclave (~232 Ma), through granodiorite (~208 Ma) and quartz monzonite porphyry (~202 Ma), and to quartz diorite (~195 Ma). It is suggested that four plutons were from the same three-component mixing of upper crust + lower crust + mantle magmas, while the upper crustal metasediments ratios were gradually increased from the early to late stage. The increasing upper crust inputs resulted in higher melting temperatures and compositions of the initial magma. All melts experienced distinct fractional crystallization of apatite, titanite, and amphibole, and the later granite melts experienced higher assimilation and fractional crystallization degrees than the early ones in the evolution processes four stages of intrusive rocks. These Yangla granitoids are the products of large-scale acid magmatic emplacement activities in the Triassic-early Jurassic and have a good metallogenic potential of the Cu-W polymetallic deposit.

Mesozoic–Cenozoic multistage tectonic evolution of the Pamir: Detrital fission‐track constraints from the Tajik Basin

AbstractKnowledge of the tectonic history of the Pamir contributes to our understanding of both the evolution of collisional orogenic belts as well as factors controlling Central Asian aridification. It is, however, not easy to decipher the Mesozoic–Cenozoic tectonics of the Pamir due to extensive Neogene deformation in an orogen that remains largely understudied. This study reports detrital apatite and zircon fission‐track (FT) ages from both the eastern Tajik Basin sedimentary rocks and Pamir modern river sands. These FT data, supported by vitrinite reflectance and zircon and apatite U–Pb double dating, suggest that the majority of the FT ages are unreset and record exhumation stages of the Pamir, which has served as the source terrane of the Tajik Basin since the Cretaceous. Furthermore, we combine the new data with a compilation of published detrital apatite and zircon FT data from both the Tajik Basin sedimentary rocks and Pamir modern river sands, to explore the Mesozoic–Cenozoic tectonic history of Pamir. Deconvolved FT Peak Ages document two major Mesozoic exhumation events associated with the Late Triassic–Early Jurassic Cimmerian orogeny that reflects accretion of the Pamir terranes, as well as the Early–early Late Cretaceous deformation associated with the northward subduction of the Neo‐Tethys Ocean beneath Pamir. The compiled data also show significant Late Eocene–Neogene exhumation associated with the ongoing formation of the Pamir, which peaks at ca. 36, 25, 14 and 7 Ma.

Contrasting origins of A-type granites in the Late Triassic-Early Jurassic Pitou complex, southern Jiangxi province: Implications for Mesozoic tectonic evolution in South China

The origin of A-type granites is purely circumstantial. Granitic complexes are quite common and generally composed of different type granites. I- and A-type granite association in a granitic complex is often described in the literature. Here we present a granitic complex, i.e. the Pitou complex in southern Jiangxi province, South China that is composed of different varieties of A-type granites. Detailed zircon LA-ICP-MS U Pb chronology and in situ Hf isotope, mineral chemistry and whole-rock element and Sr Nd isotope data of this granitic complex are used to explore their origin and to further constrain the geodynamics of Mesozoic magmatism in South China. Our new data indicate that the Pitou complex was emplaced during the Late Triassic (237 Ma) (central to northern part) and Early Jurassic (188–185 Ma) (southern part), respectively and that both the Late Triassic and Early Jurassic granites belong to A-type but show contrasting origins. The Late Triassic granites show an association of alkali-feldspar granite-syenogranite-monzogranite and the rock type of the Early Jurassic granites is alkali-feldspar granite. All the granites are composed mainly of K-feldspar, quartz, plagioclase and biotite (ferri-biotite or annite) with biotite occurring as anhedral crystal, interstitial to quartz and feldspar. Both the Late Triassic and Early Jurassic granites have high SiO 2 (>70 wt%) and total alkalis (K 2 O + Na 2 O > 8 wt%) with high FeO T /(FeO T + MgO) and low Mg#. They are enriched in rare earth elements (except Eu) and depleted in Sr and Ba. They display Zr + Y + Ce + Nb >350 ppm, 10,000 × Ga/Al >2.6 and have high zircon saturation temperatures (>800 °C). Biotites in the granites are Fe-rich with very low Fe 3+ /Fe 2+ , indicating fairly reducing conditions. However, the Late Triassic and Early Jurassic granites also show some geochemical differences. The Late Triassic granites have lower FeO T /(FeO T + MgO) and higher Mg# than the Early Jurassic granites, with the Mg# of the former consistent with pure crustal melts and the Mg# of the latter much lower than pure crustal melts. In addition, the Late Triassic granites have lower zircon saturation temperatures (up to 843 °C, average 823 °C) than the Early Jurassic granites (up to 946 °C, average 875 °C). Furthermore, the Late Triassic granites have high 87 Sr/ 86 Sr (t) (0.7132–0.7226) and negative ε Nd (t) (−10.3 to −10.9) and ε Hf (t) (in situ zircon) (−11.5 to −11.6) that are similar to those of the early Paleozoic granitoids in the region whereas the Early Jurassic granites have low 87 Sr/ 86 Sr (t) (down to 0.7034) and positive ε Nd (t) (up to +3.0) and ε Hf (t) (in situ zircon) (up to +8.3) that are similar to those of the Early Jurassic gabbroic rocks in the region. Geochemical data and major element modeling suggest that the Late Triassic A-type granites were formed by shallow (ca. 15–20 km depth) dehydration melting of the early Paleozoic granitoids triggered by intraplating of basaltic magmas and the Early Jurassic A-type granites were produced by extensive fractionation (ca. 90% to 95%) from gabbroic magmas by removal of plagioclase, amphibole, K-feldspar, clinopyroxene and magnetite. We further suggest the origin of the Late Triassic A-type granites was related to the transtension of the regional NE–trending strike-slip faults caused by the early Indosinian continental collisions and the Early Jurassic A-type granites were emplaced in a back-arc extensional setting coupled with the slab rollback and subsequent slab break-off of the subducted Palaeo-Pacific plate. Our new data confirm that the early Mesozoic magmatism in South China was controlled by continental collisions during the Late Permian to Triassic amalgamation in Southeast Asia whereas the late Mesozoic magmatism was related to the northwestward subduction of the Palaeo-Pacific plate. A transition in the dominant tectonic regime from the Tethyan to the Pacific tectonic domain most likely took place during the Rhaetian. • The Pitou granitic complex emplaced during the Late Triassic and Early Jurassic. • Both Late Triassic and Early Jurassic granites are A-type but show contrasting origin. • Late Triassic granites formed by shallow dehydration melting of Paleozoic granitoids. • Early Jurassic granites formed by extensive fractionation from gabbroic magmas. • Origin of both A-type granites suggests different geodynamic mechanisms.

Why are the Appalachians high? New insights from detrital apatite laser ablation (U-Th-Sm)/He dating

The timing of surface uplift across the Appalachian Mountains, Eastern USA, remains a controversial topic. Thermal history modelling, geomorphological analysis, stratigraphic analysis, and river profile analysis have all suggested the region experienced surface uplift during the late Cenozoic. This is in contrast with many low-temperature thermochronology studies, which consistently present Mesozoic dates and typically infer the landscape to be the remnant of a much older mountain belt that has been slowly exhumed following the end of rifting in the Late Triassic-Early Jurassic. Resolving a regional exhumation history of the Appalachians through traditional low-temperature thermochronology methods is difficult due to the spatial constraints of bedrock sampling, while traditional detrital river sampling has limitations underpinned by analytical issues and grain selection bias. We aim to resolve the exhumation histories of large areas across the Appalachians using detrital laser ablation apatite (U-Th-Sm)/He dating (LA-AHe). LA-AHe is a new dating approach that produces a high number of apatite (U-Th-Sm)/He ages from modern river sands to infer the low-temperature (70–40°C) exhumation history of the bedrock across a catchment. Supplementary apatite and zircon U-Pb dates were obtained to study sediment transport in each catchment and their respective bedrock geological histories. LA-AHe results show most dates to be Cretaceous (69%), with only a small Cenozoic fraction (6%), while thermo-kinematic modelling of data from four catchments can be explained by protracted Mesozoic-Cenozoic exhumation with rates ∼31–20 m/Myr. The lack of Cenozoic dates does imply the magnitude of exhumation has not been significant enough to greatly affect the apatite (U-Th-Sm)/He system, though models incorporating enhanced late Cenozoic exhumation between 2.1–1.4 km are consistent with data from three catchments. These results have implications for the study of elevated topography across passive margins and ancient orogens and highlight how laser ablation apatite (U-Th-Sm)/He dating can be effectively used to study the change of landscapes over geological timescales.

Softening the steps to gigantism in sauropod dinosaurs through the evolution of a pedal pad.

How sauropod dinosaurs were able to withstand the forces associated with their immense size represents one of the most challenging biomechanical scenarios in the evolution of terrestrial tetrapods, but also one lacking robust biomechanical testing. Here, we use finite element analyses to quantify the biomechanical effects of foot skeletal postures with and without the presence of a soft tissue pad in sauropodomorphs. We find that none of the models can maintain bone stresses that fall within optimal bone safety factors in the absence of a soft tissue pad. Our findings suggest that a soft tissue pad in sauropods would have reduced bone stresses by combining the mechanical advantages of a functionally plantigrade foot with the plesiomorphic skeletally digitigrade saurischian condition. The acquisition of a developed soft tissue pad by the Late Triassic–Early Jurassic may represent one of the key adaptations for the evolution of gigantism that has become emblematic of these dinosaurs.

The first identified oceanic core complex in the Bangong–Nujiang suture zone, central Tibet: New insights into the early Mesozoic tectonic evolution of the Meso-Tethys Ocean

In this paper, we report the first identified oceanic core complex (OCC) in the Saibucuo area of the Bangong-Nujiang suture zone, describe the geology and petrology of this OCC, and present new zircon U-Pb ages and Lu-Hf isotopes, and whole-rock major- and trace-elements and Sr-Nd isotopes for the diabase dikes in this OCC. The Saibucuo OCC consists of serpentinites, serpentinized peridotites, diabase dikes, tectonic breccias, sedimentary breccias, and limestones. The serpentinized peridotites cross-cut by carbonate veins, the sedimentary breccias with simple gravel compositions cut by normal faults, together with the limestones in the cover sequence lead to the conclusion that the Saibucuo OCC is similar to the OCCs of mid-ocean ridge settings. This conclusion is consistent with the E-MORB features of diabase dikes, which are formed by the interaction of the mid-ocean ridge and mantle plume, and the abyssal peridotite features of serpentinized peridotites. Zircon U–Pb ages of the diabase dikes indicate that the Saibucuo OCC was formed at the ca. 190 Ma. Based on these data and regional geological records, the early Mesozoic tectonic evolution of the Meso-Tethys Ocean was restored. The Meso-Tethys Ocean was a broad and mature oceanic basin during the Late Triassic-Early Jurassic. Coeval collision of the Northern and Southern Qiangtang terranes during the Late Triassic initiated the northward subduction of the Meso-Tethys Ocean, and the northward migration of the mid-ocean ridge. Continuous northward subduction of the Meso-Tethys Ocean caused the mid-ocean ridge to gradually approach the northward subduction zone, and eventually to subduct and collide with the northern trench during the Early-Middle Jurassic. The mid-ocean ridge within the Meso-Tethys Ocean was slow spreading during the Early Jurassic.

Discovery of Early Mesozoic Magmatism in the Northern Muglad Basin (Sudan): Assessment of Its Impacts on Basement Reservoir

The crystalline basement in the Central African Rift System (CARS) has been confirmed as a good unconventional reservoir in the surrounding basins of the Muglad area, Sudan. This study has recognized at least five Early Mesozoic intrusions for the first time at the northern margin of the Muglad Basin. Field and petrographic investigations show that they are mainly granite, syenite, diorite, and monzonite, with dominant alkali feldspar, plagioclase, quartz, and minor hornblende, biotite, and magnetite. Zircon internal structures, Th/U ratios, and rare earth element (REE) patterns indicate their magmatic origin. A total of 210 zircons from five samples were analyzed using LA-ICP-MS and they yielded U-Pb ages from 195.0 ± 6 to 225.0 ± 3 Ma, belonging to the Late Triassic-Early Jurassic. These results suggest that the Muglad Basin basement is not universally Precambrian as previously considered. The older basement was intruded and modified by Early Mesozoic magmas as evidenced by field contacts and our dating results. These magmatic events produced diverse Mesozoic igneous rocks prior to the formation of the Muglad Basin (Early Cretaceous), becoming new members of the crystalline basement. This composite basement then experienced prolonged weathering and fracturing for 65–95 Ma, resulting in porous and permeable weathered layers or fractured zones, making the basement a good unconventional reservoir for oil and gas.