Phanerozoic Time Research Articles

Petrogenesis and geochemical characteristics of Early Carboniferous sanukitic high‐Mg andesite from Atengtao Mountain, Yili Block: Implications for the tectonic setting during Late Palaeozoic in Chinese West Tianshan

As an important part of the Central Asian Orogenic Belt (CAOB), Chinese western Tianshan contains voluminous Late Palaeozoic volcanic rocks. Here, we present geochronological, geochemical, and in situ Lu–Hf isotopic data from the Atengtao sanukitic high‐Mg andesites (HMAs) of the western Yili Block. LA‐ICP‐MS zircon U–Pb analyses indicate that the Atengtao high‐Mg andesites were generated in the Early Carboniferous (346 ± 2 Ma). All the studied rock samples are characterized by high MgO (3.30–5.45 wt.%), SiO2 (53.25–60.10 wt.%), Cr (90.3–157 ppm), and Ni (31.0–46.7 ppm) contents, with geochemical characteristics analogous to those of sanukitoids of the Setouchi Volcanic Belt (SVB), SW Japan, and Bieluagaxi area, West Junggar. The Atengtao sanukitic HMAs are enriched in LILE and LREE but depleted in HFSE, similar to those of typical arc‐related magmatic rocks. They are also characterized by high Nb (11.4–15.8 ppm) concentrations, high Th/La (0.3) and Th/Yb (2.6–4.0) ratios, and varying zircon εHf(t) values (−2.3 to +4.3). These features are suggested to result from the partial melting of a relatively enriched mantle wedge metasomatized by subducting oceanic slab‐derived sediments. Considering the occurrence of sanukitic HMAs, combined with the contemporaneous special rock assemblages (such as slab‐derived adakite, Nb‐enrich basalt, A‐type granite/rhyolite, and bimodal volcanic rocks) found in the western part of Yili Block, we propose that the generation of the Carboniferous sanukitic HMAs in Atengtao Mountain were most likely related to the slab break‐off following the southern Tianshan Ocean slab northward subduction beneath the Yili–Central Tianshan Block. This mechanism not only produces intense magmatism with compositional diversity but also gives rise to a local extension in the western part of Yili Block. The slab break‐off model may play an important role in the crustal growth of the CAOB in Phanerozoic time.

Geodynamic Evolution and Metallogeny of Archaean Structural and Compositional Complexes in the Northwestern Russian Arctic

This paper highlights the geodynamic evolution of the early Precambrian rock associations in the northwestern part of the Russian Arctic where the rocks are exposed in the Kola region (northeastern Baltic Shield). The evolution is shown to predetermine the metallogenic potential of the area. It is emphasized that the Earth’s evolution is a non-linear process. Thus, we cannot draw direct analogies with Phanerozoic time or purely apply the principle of actualism, which is still widely used by experts in Precambrian geology to study the premetamorphic history of ancient deposits. In both cases, the principles should be adjusted. This article provides a novel technique for reconstructing geodynamic regimes of protolith formation in the early Precambrian. The technique identifies changing trends in geodynamic regimes during the formation of the Archean structural and compositional complexes in the Kola region. These trends fit into the earlier suggested general scheme of their formation, thus enhancing its reliability. The metallogeny of the ore areas is specified. The results of the geodynamic reconstructions explain most of the location patterns of minerals within the Kola region. Thus, the authors consider the metallogenic forecast based on geodynamic reconstructions to be a promising trend for further research.

Alternating coarse- and fine-grained sedimentation in Precambrian deep-water ramp (Apiúna Formation, SE of Brazil): Tectonic and climate control or sea level variations?

Are the depositional processes and architecture of Precambrian deep-water deposits the same of the Phanerozoic time? Few examples of Precambrian deep-water deposits exist in the literature. This paper describes the Apiúna Formation, a Neoproterozoic deep-water succession, c. 4500 m thick, located in SE Brazil. Detailed facies analysis, six large logs, 2000–3150 m thick, and a geological map have been applied to define types and geometrical distribution of three fine-grained and four coarse-grained architectural elements. Fine-grained elements (mudstone and fine-grained sandstone) constitute more than 70% of the thickness of the Apiúna Formation; these enclose coarse-grained elements, constituted of lenticular sandstone or pebbly sandstone bodies. Isolated coarse-grained elements display parallel palaeoflow directions suggesting multi-point source of the sediment. Due to this last aspect and the mud-dominance, the Apiúna Formation is interpreted as fine-grained ramp depositional system characterised by episodes of coarse-grained sedimentation. Nevertheless, this unit diverges from the Phanerozoic models for having (1) a wide range of grain-size, (2) small channel-levee systems, (3) immature clastic material and (4) irregular distribution of the architectural elements. In Apiúna sedimentary succession, five phases of coarse-grained sedimentation alternate to six phases of fine-grained sedimentation. In unvegetated earth, different rates of chemical weathering and mechanical erosion of the soils, controlled by climate and tectonic activity, can explain these variations of the clastic input. In rainy and warm climate conditions, Precambrian soils on silicate rocks formed thick clayey profiles. During quiescent or low tectonic activity, a dynamic equilibrium between weathering and erosion by runoffs on unprotected by rooted-vegetation soils produced high quantity of fine-grained material. During high tectonic activity and drier climate conditions, clayey soils did not form or were very reduced and mechanical erosion of the silicate rocks produced coarse-grained sediment. Thereby, high input of fine-grained deposits can be considered a peculiar characteristic of Precambrian deep-water systems.

Zircon Hf-O isotopic constraints on the origin of Late Mesozoic felsic volcanic rocks from the Great Xing'an Range, NE China

The voluminous Late Mesozoic magmatism was related to extensive re-melting of juvenile materials that were added to the Central East Asia continent in Phanerozoic time. The most favoured magma generation mechanism of Late Mesozoic magmas is partial melting of underplated lower crust that had radiogenic Hf-Nd isotopic characteristics, but this mechanism faces difficulties when interpreting other isotopic data. The tectonic environment controlling the generation of the Late Mesozoic felsic magmas is also in dispute. In this study, we obtained new U-Pb ages, and geochemical and isotopic data of representative Jurassic (154.4 ± 1.5 Ma) and Cretaceous (140.2 ± 1.5 Ma) felsic volcanic samples. The Jurassic sample has inherited zircon cores of Permian age, with depleted mantle-like εHf(t) of +7.4 − +8.5, which is in contrast with those of the magmatic zircons (εHf(t) = +2.4 ± 0.7). Whereas the inherited cores and the magmatic zircons have identical mantle-like δ18O composition ranges (4.25–5.29‰ and 4.69–5.54‰, respectively). These Hf-O isotopic characteristics suggest a mixed source of enriched mantle materials rather than ancient crustal components and a depleted mantle source represented by the inherited Permian zircon core. This mechanism is manifested by the eruption of Jurassic alkaline basalts originated from an enriched mantle source. The Cretaceous sample has high εHf(t) of +7.0 − +10.5, suggesting re-melting of a mafic magma derived from a depleted mantle-source. However, the sub-mantle zircon δ18O values (3.70–4.58‰) suggest the depleted mantle-derived mafic source rocks had experienced high temperature hydrothermal alteration at upper crustal level. Therefore, the Cretaceous felsic magma, if not all, could be generated by re-melting of down-dropped supracrustal volcanic rocks that experienced high temperature oxygen isotope alteration. The two processes, enriched mantle-contribution and supracrustal juvenile material re-melting, are new generation mechanisms of the Late Mesozoic magmas from Central East Asia. Rift settings may have controlled these processes throughout crustal and mantle levels.

A paired apatite and calcite clumped isotope thermometry approach to estimating Cambro-Ordovician seawater temperatures and isotopic composition

The secular increase in δ18O values of both calcitic and phosphatic marine fossils through early Phanerozoic time suggests either that (1) early Paleozoic surface temperatures were high, in excess of 40 °C (tropical MAT), (2) the δ18O value of seawater has increased by 7–8‰ VSMOW through Paleozoic time, or (3) diagenesis has altered secular trends in early Paleozoic samples. Carbonate clumped isotope analysis, in combination with petrographic and elemental analysis, can deconvolve fluid composition from temperature effects and therefore determine which of these hypotheses best explain the secular δ18O increase. Clumped isotope measurements of a suite of calcitic and phosphatic marine fossils from late Cambrian- to Middle-late Ordovician-aged strata–the first paired fossil study of its kind–document tropical sea surface temperatures near modern temperatures (26–38 °C) and seawater oxygen isotope ratios similar to today’s ratios.

Effect of paleoseawater composition on hydrothermal exchange in midocean ridges

Variations in the Mg, Ca, Sr, and SO4 concentrations of paleoseawater can affect the chemical exchange between seawater and oceanic basalt in hydrothermal systems at midocean ridges (MOR). We present a model for evaluating the nature and magnitude of these previously unappreciated effects, using available estimates of paleoseawater composition over Phanerozoic time as inputs and 87Sr/86Sr of ophiolite epidosites and epidote-quartz veins as constraints. The results suggest that modern hydrothermal fluids are not typical due to low Ca and Sr relative to Mg and SO4 in modern seawater. At other times during the last 500 million years, particularly during the Cretaceous and Ordovician, hydrothermal fluids had more seawater-derived Sr and Ca, a prediction that is supported by Sr isotope data. The predicted 87Sr/86Sr of vent fluids varies cyclically in concert with ocean chemistry, with some values much higher than the modern value of ∼0.7037. The seawater chemistry effects can be expressed in terms of the transfer efficiency of basaltic Ca and Sr to seawater in hydrothermal systems, which varies by a factor of ∼1.6 over the Phanerozoic, with minima when seawater Mg and SO4 are low. This effect provides a modest negative feedback on seawater composition and 87Sr/86Sr changes. For the mid-Cretaceous, the low 87Sr/86Sr of seawater requires either exceptionally large amounts of low-temperature exchange with oceanic crust or that the weathering flux of continentally derived Sr was especially small. The model also has implications for MOR hydrothermal systems in the Precambrian, when low-seawater SO4 could help explain low seawater 87Sr/86Sr.

Carbon isotope perturbations and faunal changeovers during the Guadalupian mass extinction in the middle Yangtze Platform, South China

AbstractThe Guadalupian mass extinction took place during the major global environmental changes during Phanerozoic time. Large-scale sea-level fluctuations and a negative shift ofδ13C were associated with this crisis. However, the diagenetic or primary origin of the decreasedδ13C across the Guadalupian–Lopingian (G–L) boundary and the potential causes for this biotic crisis are still being intensely debated. Integrated analyses, including detailed petrographic examination, identification of foraminifer and fusulinid genera, and analysis of carbonateδ13Ccarband bulkδ13Corgacross the G–L boundary were therefore carried out at Tianfengping, Hubei Province, South China. Our results show that: (1) some foraminifer and most fusulinid genera disappear in the upper Maokou Formation (upper Guadalupian); (2) the negative shift ofδ13Ccarbin the uppermost Maokou Formation is of diagenetic origin, but the values ofδ13Ccarbin the remainder of the Maokou Formation and in the Wuchiaping Formation represent a primary signal of coeval seawater; and (3) the bulkδ13Corgperturbation across the G–L boundary at Tianfengping is mainly controlled by organic matter (OM) source, that is, terrestrial OM contribution. We suggest that theδ13Ccarbnegative shift in the lower Wuchiaping Formation (Wuchiapingian) compared to that in the lower–middle Maokou Formation (Capitanian) were probably caused by the re-oxidization of12C-rich OM during regression. Global regression resulted in the negative shift ofδ13Ccarbat the G–L boundary in South China and led to the loss of shallow-marine benthic habitat. Large-scale global regression is probably one of the main causes for this bio-crisis.

Phanerozoic magma underplating and crustal growth beneath the North China Craton

AbstractEvidence for post‐Archaean crustal growth via magma underplating is largely based on U–Pb dating of zircons from granulite‐facies xenoliths. However, whether the young zircons from such xenoliths are genetically related to magma underplating or to anatexis remains controversial. The lower‐crustal xenoliths carried by igneous rocks in the Chifeng and Ningcheng (North China Craton) have low SiO2 and high MgO, indicating that parental melts of their protoliths were of unambiguous mantle origin. The xenoliths contain abundant magmatic zircons with late‐Palaeozoic ages, and have more radiogenic zircon Hf‐isotope compositions and hence younger model ages than ancient crustal magmas and the “reworking array” of the basement rocks. Our data suggest that the granulites represent episodic magmatic underplating to the lower crust of this craton in Phanerozoic time. Considering the observation that regional lowermost crust (~5 km) is mafic and characterized by Phanerozoic zircons, this work reports an example of post‐Archaean crustal growth via magma underplating.

Kinematics and significance of a poly-deformed crustal-scale shear zone in central to south-eastern Madagascar: the Itremo–Ikalamavony thrust

Across the crystalline basement of Madagascar, late Archaean rocks of the Antananarivo Block are tectonically overlain by Proterozoic, predominantly metasedimentary units of the Ikalamavony and Itremo Groups of the Southwest Madagascar Block. The generally west-dipping tectonic contact can be traced for more than 750 km from NW to SE and is referred to here as the Itremo–Ikalamavony thrust. The basal units of the SW Madagascar Block comprise metasedimentary quartzites with the potential to preserve a multistage deformation history in their microstructures. Previous studies suggest contrasting structural evolutions for this contact, including eastward thrusting, top-to-the-west directed extension and right-lateral strike-slip deformation during the late Neoproterozoic/Ediacaran. In this study, we integrate remote sensing analyses, structural and petrological fieldwork, as well as microstructural investigations of predominantly quartz mylonites from the central southern segment of the contact between Ankaramena and Maropaika. In this area, two major phases of ductile deformation under high-grade metamorphic conditions occurred in latest Neoproterozoic/early Phanerozoic times. A first (Andreaba) phase produces a penetrative foliation, which is parallel to the contact between the two blocks and contemporaneous with widespread magmatism. A second (Ihosy) phase of deformation folds Andreaba-related structures. The investigated (micro-)structures indicate that (a) juxtaposition of both blocks possibly already occurred prior to the Andreaba phase, (b) (re-)activation with top-to-the-east thrusting took place during the latest stages of the Andreaba phase, (c) the Ihosy phase resulted in regional-scale open folding of the tectonic contact and (d) reactivation of parts of the contact took place at distinctively lower temperatures post-dating the major ductile deformations.

Fold belts and sedimentary basins of the Eurasian Arctic

The vast Eurasian Arctic epicontinental shelf and adjoining mainland has a very complex structure and tectonic history as a result of a series of continent–continent collisions, accretion of terranes and crustal extension phases during Neoproterozoic and Phanerozoic times. Significant parts of major Eurasian fold belts extend far north into the Arctic below thick infill of post-orogenic sedimentary basins, where their architecture remains highly disputed. Large Eurasian Arctic sedimentary basins formed as a result of orogenic collapse, back-arc extension, or intracontinental extension associated with the breakup of the Laurussia, Laurasia, Pangea and Eurasia supercontinents. There are over 40 sedimentary basins of variable age and genesis which are thought to bear significant undiscovered hydrocarbon resources in the region. This article reviews the current state-of-knowledge of Eurasian Arctic tectonics and highlights questions that remain to be addressed. The overall focus is on the Russian sector of the Arctic being less known to a broad geoscience community.

The influence of the biological pump on ocean chemistry: implications for long-term trends in marine redox chemistry, the global carbon cycle, and marine animal ecosystems.

The net export of organic matter from the surface ocean and its respiration at depth create vertical gradients in nutrient and oxygen availability that play a primary role in structuring marine ecosystems. Changes in the properties of this ‘biological pump’ have been hypothesized to account for important shifts in marine ecosystem structure, including the Cambrian explosion. However, the influence of variation in the behavior of the biological pump on ocean biogeochemistry remains poorly quantified, preventing any detailed exploration of how changes in the biological pump over geological time may have shaped long‐term shifts in ocean chemistry, biogeochemical cycling, and ecosystem structure. Here, we use a 3‐dimensional Earth system model of intermediate complexity to quantitatively explore the effects of the biological pump on marine chemistry. We find that when respiration of sinking organic matter is efficient, due to slower sinking or higher respiration rates, anoxia tends to be more prevalent and to occur in shallower waters. Consequently, the Phanerozoic trend toward less bottom‐water anoxia in continental shelf settings can potentially be explained by a change in the spatial dynamics of nutrient cycling rather than by any change in the ocean phosphate inventory. The model results further suggest that the Phanerozoic decline in the prevalence ocean anoxia is, in part, a consequence of the evolution of larger phytoplankton, many of which produce mineralized tests. We hypothesize that the Phanerozoic trend toward greater animal abundance and metabolic demand was driven more by increased oxygen concentrations in shelf environments than by greater food (nutrient) availability. In fact, a lower‐than‐modern ocean phosphate inventory in our closed system model is unable to account for the Paleozoic prevalence of bottom‐water anoxia. Overall, these model simulations suggest that the changing spatial distribution of photosynthesis and respiration in the oceans has exerted a first‐order control on Earth system evolution across Phanerozoic time.

Frequency of Proterozoic geomagnetic superchrons

Long-term geodynamo evolution is expected to respond to inner core growth and changing patterns of mantle convection. Three geomagnetic superchrons, during which Earth's magnetic field maintained a near-constant polarity state through tens of Myr, are known from the bio/magnetostratigraphic record of Phanerozoic time, perhaps timed according to supercontinental episodicity. Some geodynamo simulations incorporating a much smaller inner core, as would have characterized Proterozoic time, produce field reversals at a much lower rate. Here we compile polarity ratios of site means within a quality-filtered global Proterozoic paleomagnetic database, according to recent plate kinematic models. Various smoothing parameters, optimized to successfully identify the known Phanerozoic superchrons, indicate 3–10 possible Proterozoic superchrons during the 1300 Myr interval studied. Proterozoic geodynamo evolution thus appears to indicate a relatively narrow range of reversal behavior through the last two billion years, implying either remarkable stability of core dynamics over this time or insensitivity of reversal rate to core evolution.

Impact of atmospheric pCO2, seawater temperature, and calcification rate on the δ18O and δ13C composition of echinoid calcite (Echinometra viridis)

The tropical echinoid Echinometra viridis was reared in controlled laboratory experiments at temperatures of approximately 20°C and 30°C to mimic winter and summer temperatures and at carbon dioxide (CO2) partial pressures of approximately 487ppm-v and 805ppm-v to simulate current and predicted-end-of-century levels. Spine material produced during the experimental period and dissolved inorganic carbon (DIC) of the corresponding culture solutions were then analyzed for stable oxygen (δ18Oe, δ18ODIC) and carbon (δ13Ce, δ13CDIC) isotopic composition. Fractionation of oxygen stable isotopes between the echinoid spines and DIC of their corresponding culture solutions (Δ18O=δ18Oe−δ18ODIC) was significantly inversely correlated with seawater temperature but not significantly correlated with atmospheric pCO2. Fractionation of carbon stable isotopes between the echinoid spines and DIC of their corresponding culture solutions (Δ13C=δ13Ce−δ13CDIC) was significantly positively correlated with pCO2 and significantly inversely correlated with temperature, with pCO2 functioning as the primary factor and temperature moderating the pCO2–Δ13C relationship. Echinoid calcification rate was significantly inversely correlated with both Δ18O and Δ13C across treatments, with effects of pCO2 and temperature controlled for through ANOVA. Therefore, calcification rate and potentially the rate of co-occurring dissolution appear to be important drivers of the kinetic isotope effects observed in the echinoid spines. Study results suggest that echinoid Δ18O monitors seawater temperature, but not atmospheric pCO2, and that echinoid Δ13C monitors atmospheric pCO2, with temperature moderating this relationship. These findings, coupled with echinoids' long and generally high-quality fossil record, support prior assertions that fossil echinoid Δ18O is a viable archive of paleo-seawater temperature throughout Phanerozoic time, and that Δ13C merits further investigation as a potential proxy of paleo-atmospheric pCO2. However, the apparent impact of calcification rate on echinoid Δ18O and Δ13C suggests that paleoceanographic reconstructions derived from these proxies in fossil echinoids could be improved by incorporating the effects of growth rate.

Stratigraphic Completeness of Carbonate-Dominated Records From Continental Interiors Versus Continental Margins: Stratigraphic Thinning Occurs Via Condensation and Omission At Multiple Scales

Abstract Over Phanerozoic time scales, stratigraphic records from continental interiors, or cratons, are dramatically thinner and are generally assumed to be relatively incomplete, with more numerous and longer-duration hiatuses, compared to records from more rapidly subsiding continental margins. However, this assumption need not be true for the shorter time scales (i.e., several 106 years and shorter) over which accumulation of the preserved stratigraphic record on the continental interior actually takes place. This study compares carbonate-dominated sedimentary records from the Devonian continental interior (Iowa) and continental margin (Nevada) to evaluate whether the continental-interior record is (1) miniaturized, i.e., thinner, but equally complete; (2) comparable in thickness and quality where the sedimentary record is preserved, but certain portions of the record are notably absent, i.e., omitted or truncated; or (3) so invariably preserved that equivalent stratigraphic packages cannot be recognized compared to the continental-margin record. This study finds that fourth- to fifth-order cycles in Iowa are, on average, approximately half as thick and half as numerous compared to the continental margin of Nevada. Moreover, the stratigraphic record on the continental margin of Nevada is dominated by cycles that are composed of tidal-flat and shallow subtidal lithofacies, capped by simple flooding surfaces, and exhibit catch-up or keep-up depositional styles. The continental-interior record in Iowa is dominated by cycles that are composed of subtidal lithofacies, capped by a range of bounding surfaces, and exhibit catch-down or give-up depositional styles. These findings suggest that the thin record in Iowa is largely a result of low sedimentation rates associated with suppressed carbonate production in this epeiric-sea setting, rather than minimal subsidence rates and an increased potential for subaerial exposure and associated loss of record.

A link in the chain of the Cambrian zooplankton: bradoriid arthropods invade the water column

AbstractBradoriids are small bivalved arthropods that had global distribution for about 20 million years beginning at Cambrian Epoch 2 (c. 521 Ma). The majority of bradoriids are considered to be benthic, favouring oxygenated waters, as suggested by their anatomy, lithofacies distribution, faunal associates and provinciality. Most bradoriids were extinct by the end of the Drumian Age (middle of Cambrian Epoch 3). The post-Drumian is characterized by widespread dysoxic shelf lithofacies in southern Britain and Scandinavia and by the abundance of phosphatocopid arthropods. This interval is also associated with two bradoriid species with wide intercontinental distribution:Anabarochilina primordialis, which had a geographical range from the palaeo-tropics to high southern palaeo-latitude, andAnabarochilina australis, which extended through the palaeo-tropics from Laurentia to Gondwana. The wide environmental and geographical range of these species, coupled with a carapace anatomy that suggests an active lifestyle, is used to infer a zooplanktonic lifestyle. A possible driver of this widespread Cambrian bradoriid zooplankton was sea-level rise coupled to the periodic spread of low oxygen conditions onto continental shelves, acting in tandem with anatomical pre-adaptations for swimming. Parallels exist with the myodocope ostracod colonization of the water column during Silurian time, which may also have been influenced by extrinsic environmental controls acting on anatomical pre-adaptations for swimming. Similar biological and environmental mechanisms may have facilitated arthropod zooplankton colonizations across Phanerozoic time.

Oxygen isotopes in authigenic quartz from massive salt deposits

We describe a new geochemical tool that could provide temperatures of ancient basins in which massive evaporites were deposited: the oxygen isotope composition of fine crystalline quartz found in large halite bodies. Such quartz is well preserved from post-depositional alterations and it can be relatively easily separated. For the purpose of this study, five halite samples were selected from four various evaporite basins, spanning in age from the Early Cambrian to the Late Jurassic. The obtained isotope temperatures refer to locations where evaporites were deposited, i.e. in subtropical zones of the Earth, as it may be estimated from continental distribution during Phanerozoic times. Reasonable temperatures are obtained, with an assumption for the δ18O of brines ranging from −1‰ to 0‰ during halite deposition. The assumption of higher δ18O values leads to unrealistic temperatures, higher than the homogenization temperatures of fluid inclusions in halite.

Lithological and age structure of the lower crust beneath the northern edge of the North China Craton: Xenolith evidence

Abstract Deep-seated xenoliths in volcanic rocks offer direct glimpses into the nature and evolution of the lower continental crust. In this contribution, new data on the U–Pb ages and Hf isotopes of zircons in six felsic granulite xenoliths and one pyroxenite xenolith from the Hannuoba Cenozoic basalts, combined with published data from mafic to felsic xenoliths, are used to constrain the lithological and age structure of the lower crust beneath the northern edge of the North China Craton. Two newly-reported felsic granulites contain Precambrian zircons with positive (+ 7.5–+ 10.6) and negative e Hf values (− 10.1 to − 3.7) corresponding to upper intercept ages of 2449 ± 62 Ma and 1880 ± 54 Ma, respectively, indicating crustal accretion in the late Archean and reworking in Paleoproterozoic time. Zircons in another four felsic xenoliths give Phanerozoic ages from 142 Ma to 73 Ma and zircons from one pyroxenite xenolith give a concordant age of 158 Ma. The zircon e Hf values of these four felsic xenoliths range between − 23.3 and − 19.1, reflecting re-melting of the pre-existing lower crust. Integration of geothermobarometric, and geochronological data on the Hannuoba xenoliths with seismic refraction studies shows that the lower crust beneath the northern edge of the North China Craton is temporally and compositionally zoned: the upper lower crust (24–33 km) consists dominantly of Archean (~ 2.5 Ga with minor 2.7 Ga) felsic granulites with subordinate felsic granulites that reworked at 140–120 Ma; both Precambrian and late Mesozoic mafic granulites are important constituents of the middle lower crust (33–38 km); major late Mesozoic (140–120 Ma) and less Cenozoic (45–47 Ma) granulites and pyroxenites are presented in the lowermost crust (38–42 km). The zoned architecture of the lower crust beneath Hannuoba suggests a complex evolution beneath the northern margin of the craton, including late Neoarchean (~ 2.5 Ga) accretion and subsequent episodic accretion and/or reworking during Paleoproterozoic (~ 1.8–1.9 Ga) and Phanerozoic (~ 220 Ma, 120–140 Ma, 45–47 Ma) times, with possible links to circum-craton subduction/collision events and the destruction of the North China Craton.

Earth history. Low mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals.

The oxygenation of Earth's surface fundamentally altered global biogeochemical cycles and ultimately paved the way for the rise of metazoans at the end of the Proterozoic. However, current estimates for atmospheric oxygen (O2) levels during the billion years leading up to this time vary widely. On the basis of chromium (Cr) isotope data from a suite of Proterozoic sediments from China, Australia, and North America, interpreted in the context of data from similar depositional environments from Phanerozoic time, we find evidence for inhibited oxidation of Cr at Earth's surface in the mid-Proterozoic (1.8 to 0.8 billion years ago). These data suggest that atmospheric O2 levels were at most 0.1% of present atmospheric levels. Direct evidence for such low O2 concentrations in the Proterozoic helps explain the late emergence and diversification of metazoans.

740 Ma vase-shaped microfossils from Yukon, Canada: Implications for Neoproterozoic chronology and biostratigraphy

Research Article| August 01, 2014 740 Ma vase-shaped microfossils from Yukon, Canada: Implications for Neoproterozoic chronology and biostratigraphy Justin V. Strauss; Justin V. Strauss 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA Search for other works by this author on: GSW Google Scholar Alan D. Rooney; Alan D. Rooney 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA Search for other works by this author on: GSW Google Scholar Francis A. Macdonald; Francis A. Macdonald 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA Search for other works by this author on: GSW Google Scholar Alan D. Brandon; Alan D. Brandon 2Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas, 77204, USA Search for other works by this author on: GSW Google Scholar Andrew H. Knoll Andrew H. Knoll 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA Search for other works by this author on: GSW Google Scholar Geology (2014) 42 (8): 659–662. https://doi.org/10.1130/G35736.1 Article history received: 01 Apr 2014 rev-recd: 13 May 2014 accepted: 14 May 2014 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Justin V. Strauss, Alan D. Rooney, Francis A. Macdonald, Alan D. Brandon, Andrew H. Knoll; 740 Ma vase-shaped microfossils from Yukon, Canada: Implications for Neoproterozoic chronology and biostratigraphy. Geology 2014;; 42 (8): 659–662. doi: https://doi.org/10.1130/G35736.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Biostratigraphy underpins the Phanerozoic time scale, but its application to pre-Ediacaran strata has remained limited because Proterozoic taxa commonly have long or unknown stratigraphic ranges, poorly understood taphonomic constraints, and/or inadequate geochronological context. Here we report the discovery of abundant vase-shaped microfossils from the Callison Lake dolostone of the Coal Creek inlier (Yukon, Canada) that highlight the potential for biostratigraphic correlation of Neoproterozoic successions using species-level assemblage zones of limited duration. The fossiliferous horizon, dated here by Re-Os geochronology at 739.9 ± 6.1 Ma, shares multiple species-level taxa with a well-characterized assemblage from the Chuar Group of the Grand Canyon (Arizona, USA), dated by U-Pb on zircon from an interbedded tuff at 742 ± 6 Ma. The overlapping age and species assemblages from these two deposits suggest biostratigraphic utility, at least within Neoproterozoic basins of Laurentia, and perhaps globally. The new Re-Os age also confirms the timing of the Islay δ13Ccarbonate anomaly in northwestern Canada, which predates the onset of the Sturtian glaciation by >15 m.y. Together these data provide global calibration of sedimentary, paleontological, and geochemical records on the eve of profound environmental and evolutionary change. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geochronology and geochemistry of Early Cretaceous volcanic rocks from the Baiyingaolao Formation in the central Great Xing'an Range, NE China, and its tectonic implications

Abstract We undertook zircon U–Pb dating and geochemical analyses of volcanic rocks from the Baiyingaolao Formation in the central Great Xing'an Range, northeastern China, with an aim to determine their age, petrogenesis and sources, which are important for understanding the Late Mesozoic tectonic evolution of the eastern section of the Central Asian Orogenic Belt. Lithologically, the Baiyingaolao Formation is composed mainly of rhyolites and rhyolitic tuffs, with minor trachy dacites. The zircons from three rhyolitic tuffs and two rhyolites are euhedral–subhedral in shape, display fine-scale oscillatory growth zoning and have high Th/U ratios (0.72–2.60), indicating a magmatic origin. The results of LA–ICP-MS zircon U–Pb dating indicate that the volcanic rocks from the Baiyingaolao Formation in the study area formed during the Early Cretaceous time with ages of 134–130 Ma. Petrological and geochemical characteristics of these volcanic rocks suggest that they are all highly fractionated I-type igneous rocks, and their parental magmas were likely derived from the partial melting of lower crustal materials with plagioclase, hornblende and apatite as the residual phases. In addition, the volcanics sampled in this paper, tectonically located in the Xing'an terrain, have high initial 176Hf/177Hf ratios (0.282770–0.283035) and positive eHf(t) values (2.42–11.96), combined with young Hf two-stage model ages of 1134–541 Ma, reflecting that the crustal growth of the Xing'an terrain occurred during Neoproterozoic and Phanerozoic times. These data, combined with previous studies on the contemporaneous magma-tectonic activities in NE China, suggest that the generation of the Early Cretaceous volcanic rocks in the central Great Xing'an Range was related to the lithospheric delamination caused by the subduction of the Paleo-Pacific plate.