Showing papers on "Permian published in 2014"

Triassic tectonics and mineral systems in the Qinling Orogen, central China

Abstract: The Qinling Orogen in central China preserves the records of a complex geological and tectonic history, and also carries abundant mineral resources. Previous investigations on the Qinling Orogen have identified that the final transition from the Palaeo-Tethys Ocean to a continental orogen occurred in the Triassic, although the timing of the oceanic closure and the tectonic affinity of the Triassic Qinling belt have been poorly constrained. In this contribution, we compile the palaeomagnetic data and geological evidence related to sedimentation, magmatism and mineral systems. The palaeomagnetic data show that the Palaeo-Tethys Ocean in the Qinling Orogen gradually closed westward during the Triassic, terminating in the beginning of the Jurassic. Triassic strata are widely developed in the Qinling Orogen and conformably overlie the Permian strata. They are composed of carbonates and siliciclastic rocks that were strongly deformed and weakly metamorphosed during the Late Triassic to Late Jurassic. The Triassic magmatic rocks are dominated by I-type granitoids, with subordinate S-type and alkaline granites and minor carbonatite dykes, with a clear northward transition in lithology and geochemical features from the Mian-Lue Suture. Different types of Triassic mineral systems have been reported from this region, including sedimentary-hosted epizonogenic hydrothermal Hg–Sb, Pb–Zn ± Ag, Au deposits, metamorphic hydrothermal lodes of Au, Ag–Pb–Zn, Mo–Au and Mo, magmatic hydrothermal systems including porphyry Mo, breccia pipe Au, and carbonatite-hosted Mo deposits. We infer that the Triassic Qinling Orogen witnessed synchronous oceanic plate subduction in the west and continental linkage in the east, with a transition from oceanic basin to continental orogen through gradual subduction-related consumption of the oceanic plate. The Palaeo-Tethys Ocean closed in a westward scissor-like fashion in the Triassic and continental collision and extensive crustal shortening in the Jurassic, following which post-collisional extension occurred in the Early Cretaceous. Copyright © 2014 John Wiley & Sons, Ltd.

The Origin and Early Evolution of Sauria: Reassessing the Permian Saurian Fossil Record and the Timing of the Crocodile-Lizard Divergence

Abstract: Sauria is the crown-group of Diapsida and is subdivided into Lepidosauromorpha and Archosauromorpha, comprising a high percentage of the diversity of living and fossil tetrapods. The split between lepidosauromorphs and archosauromorphs (the crocodile-lizard, or bird-lizard, divergence) is considered one of the key calibration points for molecular analyses of tetrapod phylogeny. Saurians have a very rich Mesozoic and Cenozoic fossil record, but their late Paleozoic (Permian) record is problematic. Several Permian specimens have been referred to Sauria, but the phylogenetic affinity of some of these records remains questionable. We reexamine and review all of these specimens here, providing new data on early saurian evolution including osteohistology, and present a new morphological phylogenetic dataset. We support previous studies that find that no valid Permian record for Lepidosauromorpha, and we also reject some of the previous referrals of Permian specimens to Archosauromorpha. The most informative Permian archosauromorph is Protorosaurus speneri from the middle Late Permian of Western Europe. A historically problematic specimen from the Late Permian of Tanzania is redescribed and reidentified as a new genus and species of basal archosauromorph: Aenigmastropheus parringtoni. The supposed protorosaur Eorasaurus olsoni from the Late Permian of Russia is recovered among Archosauriformes and may be the oldest known member of the group but the phylogenetic support for this position is low. The assignment of Archosaurus rossicus from the latest Permian of Russia to the archosauromorph clade Proterosuchidae is supported. Our revision suggests a minimum fossil calibration date for the crocodile-lizard split of 254.7 Ma. The occurrences of basal archosauromorphs in the northern (30°N) and southern (55°S) parts of Pangea imply a wider paleobiogeographic distribution for the group during the Late Permian than previously appreciated. Early archosauromorph growth strategies appear to be more diverse than previously suggested based on new data on the osteohistology of Aenigmastropheus.

Late Paleozoic pre- and syn-kinematic plutons of the Kangguer–Huangshan Shear zone: Inference on the tectonic evolution of the eastern Chinese north Tianshan

Abstract: Permian large-scale transcurrent tectonics and massive magmatism are prominent features of the Tianshan belt and neighboring regions of the Central Asian Orogenic Belt. Structural, geochronological and geochemical analyses of Carboniferous and Permian intrusive rocks associated with the Kangguer–Huangshan Shear Zone (eastern Chinese North Tianshan) provide constraints on their tectonic setting and the tectonic evolution of the Tianshan belt as well. Carboniferous granitic rocks were emplaced at 338 ± 4 Ma and 347 ± 2 Ma, respectively, and show geochemical features typical of the calc-alkaline series. These arc-type granites do not display ductile deformation, probably because they were completely cooled at the time of shearing tectonics, and are only offset by brittle strike-slip faults. In contrast, Permian granitoids display pervasive ductile tectonic features diagnostic of synkinematic emplacement. Four gabbro and diorite samples from the East Huangshan intrusive complex yielded zircon U-Pb ages of 267 to 275 Ma, and a granitic dike is dated at 290 ± 1 Ma. The granitic dike is cut by en-echelon right-lateral strike-slip faults, and the mafic intrusive complex displays a sigmoidal shape with mylonitic foliation localized at its margins. Other specific pluton shapes (such as tongue and tadpole-like) and syn-magmatic deformation can be observed in intrusions of the same age, showing similar fabrics and kinematics consistent with that of the Kangguer–Huangshan Shear Zone. Numerous mafic to felsic dikes occur within and off the shear zone with a dominant SE-NW orientation and minor varieties in N-S or NNE-SSW directions. One gabbro dike that intrudes the early Carboniferous granite of the East Kanggurtag area yielded a magmatic age of 274 ± 4 Ma, and contains older zircons (∼340 Ma, ∼390 Ma, ∼450 Ma, and 1.3-2.2 Ga) probably inherited from intruded rocks. The Permian intrusive rocks have variable chemical compositions suggesting derivation of these rocks from depleted and undepleted (or enriched) mantle sources with involvement of subduction-related components. We conclude from our integrated analysis of the geological, structural, geochemical and geochronological data that the Permian magmatic rocks were formed in a post-collisional/post-orogenic setting from multiple sources, and were emplaced under the control of large-scale dextral transcurrent tectonics.

Atmospheric CO 2 and O 2 During the Phanerozoic: Tools, Patterns, and Impacts

Abstract: The amount of CO 2 and O 2 in the atmosphere over long timescales (> 10 5 years) is largely controlled by several key processes. Reconstruction of atmospheric CO 2 and O 2 in the geologic past can be accomplished either with proxies or by modeling the long-term carbon and sulfur cycles. Application of these two independent approaches yields similar results. CO 2 was high during the early Paleozoic (> 2000 ppm) and parts of the Mesozoic (~ 1000 ppm) but low during the Carboniferous, Permian, and late Cenozoic ( 2 patterns are strongly coupled to independent evidence for global temperature. O 2 records show oscillating values (15–25%) with a distinct peak (> 30%) during the Permian. There is a compelling link between this Phanerozoic peak in atmospheric O 2 and a concomitant interval of insect gigantism.

Triassic sedimentation and postaccretionary crustal evolution along the Solonker suture zone in Inner Mongolia, China

Abstract: Detrital zircon U-Pb dating of the Xingfuzhilu Formation in southern Inner Mongolia yields a maximum depositional age of around 220 Ma. The predominantly Permian and Triassic zircons are characterized by oscillatory zoning and euhedral shapes, with mostly positive zircon eHf(t) values (+2.0 to +16.4), indicating that they were derived from a proximal magmatic source. Early-Middle Paleozoic zircons have variable zircon eHf(t) values from −6.2 to +11.2 and are characterized by weak oscillatory zoning and subhedral-subrounded shapes, suggesting that the sources are a proximal magmatic arc, possibly mixed with components of the Ondor Sum magmatic arc and the magmatic arc at the northern margin of the North China Craton. The remnants of Precambrian blocks in the southeastern Central Asian Orogenic Belt (CAOB), and the North China Craton may also have been a minor source region for the Xingfuzhilu succession. These results, combined with regional data, indicate that a closing remnant ocean basin or narrow seaway possibly existed in the Middle Permian (Guadalupian) immediately prior to final collision of the CAOB and closure of the Paleo-Asian Ocean. Subsequent collision resulted in the crustal uplift and thickening along the Solonker suture zone, accompanied by possible slab break-off and lithospheric delamination during the Latest Permian to Middle Triassic. The resultant orogen in the Late Triassic underwent exhumation and denudation of rocks in response to the postorogenic collapse and regional extension. Vertical crustal growth in the Triassic is documented by detrital zircons from the Xingfuzhilu Formation and appears to have been widespread across entire eastern CAOB.

Permian-Triassic conodonts from Dajiang (Guizhou, South China) and their implication for the age of microbialite deposition in the aftermath of the End-Permian mass extinction

Abstract: The widespread microbialites deposition that followed the End-Permian mass extinction in the Tethyan realm have been intensively studied because of the evidence they provide on the nature of this crisis and its aftermath. However, the age of the microbialite event remains controversial. New conodont collection across the Permian-Triassic (P-T) transition from Dajiang (Guizhou Province, South China) in this study enable us to discriminate four conodont zones, in ascending order, they are: Hindeodus parvus zone, Isarcicella lobata zone, Isarcicella isarcica zone and Hindeodus sosioensis zone. The age of microbialite in the P-T transition at the Dajiang Section is considered to be within the Hindeodus parvus zone and thus to clearly post-date the main extinction crisis. Reviewing the age of onset of microbialites throughout the Tethyan regions reveals two different ages: a Hindeodus changxingensis zone age is dominant in south-western and westernmost Tethys, whilst most other regions show microbialite deposition began in the Hindeodus parvus zone. Our investigation also indicates that two conodont changes occur at this time: an increase of hindeodid species immediately following a sequence boundary and the mass extinction, and a phase of extinction losses in the earliest Triassic Isarcicella isarcica zone during highstand development.

The plant fossil record reflects just two great extinction events

Abstract: Five great taxonomic extinctions (the so-called ‘Big Five Mass Extinctions’) are widely recognized in life history, at the end of the Ordovician, Frasnian (Late Devonian), Permian, Triassic and Cretaceous. All of them were originally identified in the marine fossil record and have been interpreted as the result of abrupt global environmental changes. Previous evidence has suggested that terrestrial biota were also subjected to ecological disruption during these events, but it is unknown whether they suffered the same level of large-scale taxonomic disruption as marine organisms. Here we show that the plant fossil record in fact only provides evidence of two mass taxonomic extinction events, one through the Carboniferous-Permian transition, the other during middle-late Permian times. This differs significantly from the mass extinctions recognized in the marine realm and suggests that vascular plants have a special capacity for surviving abrupt environmental changes.

Global continental weathering trends across the Early Permian glacial to postglacial transition: Correlating high- and low-paleolatitude sedimentary records

Abstract: Time-equivalent Early Permian sedimentary successions from high-latitude Gondwana basins and from equatorial accumulations in North China, covering the glacial to postglacial transition, display correlatable trends in continental weathering intensity based on chemical index of alteration (CIA) values. The successions display a pattern of CIA values that varies with latitude, similar to modern estuarine suspended sediments. Based on the modern-day CIA-temperature correlation, the glacial Early Permian low- to high-latitude land surface temperature gradient is estimated as ∼20 °C, slightly higher than the calculated values for the contemporaneous sea-surface temperature gradient.

Post-accretionary permian granitoids in the Chinese Altai orogen: Geochronology, petrogenesis and tectonic implications

Abstract: The Altai orogen is an important constituent of the Central Asian Orogenic Belt (CAOB). The main orogenic processes occurred mainly in the early to middle Paleozoic and involved a series of northward subduction and terrane accretion. However, termination of the accretionary and post-accretionary processes remains poorly defined. The Chinese Altai is located in the southern part of this orogeny, which is widely intruded by Permian granitic plutons. These plutons are approximately circular in shape, free of deformation and generally cutting pre-Permian structures, suggesting a post-tectonic formation. We report the results of geochronological and geochemical data from five specific granitic plutons (Buerjin, Xibodu, Daqiaonan, Aweitan, and Adenbluk), which all yielded magmatic zircon U-Pb ages of about 270 Ma. These plutons are composed of high-K calc-alkaline rocks, including K-feldspar megaphyric granite, biotite granite and monzogranite that have metaluminous to weakly LREE-enriched, coupled with negative Eu anomalies. Significant negative anomalies of Ba, Sr, P, and Ti are also observed in the primitive-mantle normalized diagram. They have positive whole-rock eNd(t) (+1.3 to +7.2) and zircon eHf(t) values (+5.6 to +12.9), yielding Sm-Nd model ages of ≤0.9 Ga. Therefore, these granitoids are proposed to have been generated by differentiation of mantle-derived magmas with variable crustal contamination. In view of the field occurrence, structural analysis, regional tectonics and geochemical characteristics, these Permian plutons are concluded to be post-accretionary or post-collisional. Asthenospheric upwelling after the collision and amalgamation of the Altai and Junggar blocks could have caused the mantle-derived magmas that evolved to form the granitoids. We note that Permian intrusions are not only widespread but also voluminous in the CAOB. They mostly are post-collisional products, and some of them might have been related to the large igneous province activity in the Tarim Block.