Carboniferous fusuline Foraminifera: taxonomy, regional biostratigraphy, and palaeobiogeographic faunal development

This paper proposes a synthesis of the taxonomy, phylogeny, palaeogeographic distribution, regional biostratigraphy, and palaeobiogeographic faunal development of Carboniferous fusuline foraminifers. They appeared in the latest Tournaisian and comprised a small-sized, morphologically conservative taxonomic group during the Mississippian. Fusulines became larger and prevailed in Pennsylvanian foraminiferal assemblages. Carboniferous fusulines consist of Ozawainellidae, Staffellidae, Schubertellidae, Fusulinidae, and Schwagerinidae, in which 95 genera are considered as valid taxonomically. Upsizing their shells throughout the Pennsylvanian is likely related to symbiosis with photosynthetic microorganisms, which was accelerated by the acquisition of a keriothecal wall in Late Pennsylvanian schwagerinids. Regional fusuline succession data from 40 provinces provide a refined biostratigraphy, enabling zonation and correlation with substage- or higher-resolution precision in the Pennsylvanian. Their spatio-temporal faunal characteristics show that fusulines had a cosmopolitan palaeobiogeographic signature in Mississippian time, suggesting unrestricted faunal exchange through the palaeoequatorial Rheic Ocean. After the formation of Pangea, Pennsylvanian fusulines started to show provincialism, and their distributions defined the Ural-Arctic Region in the Boreal Realm, Palaeotethys, Panthalassa, and North American Craton regions in the Palaeoequatorial Realm, and Western Gondwana and Eastern Peri-Gondwana regions in the Gondwana Realm. The Western Palaeotethys and East European Platform Subregions maintained higher generic diversity throughout the Pennsylvanian.

Lithospheric structure of the North American Craton constrained by full waveform inversion

<p>Cratonic lithosphere is believed to be rigid and less deformed during a long period of time. However, the detailed structure of Cratons may bring information of the complex formation and assemblage process of the continental lithosphere. Here, we present the seismic radial anisotropic structure of the North American Craton (NAC) constrained by a regional full-waveform inversion (FWI) with 465,422 high-quality frequency-dependent travel time misfit measurements with the shortest period of 15 s from both the body wave and surface wave recordings of 5,120 stations and 160 earthquakes located in the contiguous U.S and surrounding regions. Started from an initial model constructed by combining US.2016 and Crust1.0 in the crust and S40RTS (isotropic) in the mantle, we are able to have the optimized crustal structure in terms of initial waveform similarity and get rid of existing features from other radially anisotropic mantle models.</p><p>Our new model reveals the NAC lithosphere with about +2% voigt shear wave speed anomaly and an average thickness of 200–250 km beneath the Superior Craton, and becomes thinner towards the eastern, the southern, and the southwestern margins with a thickness decreased to 100–150 km. The radial anisotropy manifests a layer of higher horizontal shear wave speed V<sub>SH </sub>(ξ=V<sub>SH</sub><sup>2</sup>/V<sub>SV</sub><sup>2</sup>>1) beneath the core of Superior Craton down to around 160 km, where the higher vertical shear wave speed V<sub>SV </sub>(ξ<1) is observed beneath 160 km. Such radial anisotropy layering is also observed in the margin of continental lithosphere but with shallower depth. The radial anisotropic layer matches the receiver function results of mid-lithosphere discontinuities of the Craton cores, and the lithosphere conductivity result. The radial anisotropy layering observation confirms the two-layered lithosphere structure of the NAC, where the upper layer likely represents the original radial anisotropy fabric related to the cooling of the craton core, while the lower layer might be related to the tectonic processes more recently, e.g., accretion . The lithospheric thinning beneath the NAC margins indicates the deformation of the lithosphere and is likely controlled by the large-scale mantle convection, therefore relates to the further modification process of the NAC.</p>

Petrogenesis of pleistocene basalts from the Western snake river plain, Idaho

We present new geochemical, Sr, Nd, and Pb isotope, and 40Ar/39Ar data from Pleistocene basalts of the Western Snake River Plain (WSRP), Idaho, USA to explore their petrogenesis and to investigate the nature of the lithosphere at the western boundary of the North American craton. The basalts are divided into three groups based on their geochemical and isotopic characteristics. Prior to ∼1 Ma, volcanoes in the WSRP erupted iron-rich tholeiites (FeB1), but subsequent volcanism was dominated by concurrent eruptions of mildly alkaline, alumina-rich lavas (AlB) and iron-rich tholeiites (FeB2) with isotopic signatures similar to the AlB lavas. New 40Ar/39Ar dates of AlB and FeB2 basalts range from 0.920 ± 0.049 Ma to 0.287 ± 0.014 Ma. MELTS models of FeB1 differentiation trends indicate that the range of compositions in this suite can be produced by 10–15% crystallization of olivine and plagioclase at low pressure using the least evolved FeB1 composition as a parental magma; isotopic ratios can be produced via combined assimilation of a Miocene rhyolite and fractional crystallization. Additional modeling suggests that parental magmas at AlB centers were produced by 3–12% equilibrium melting of a garnet-spinel enriched mantle source, slightly different to that proposed for the youngest mildly alkaline lavas of the eastern and central Snake River Plain. Our new geochemical, isotopic, and geochronologic data of the FeB2 basalts suggests they are related to AlB-type magmas via a combination of fractional crystallization and assimilation of evolved mafic crust. MELTS models suggest that crystallization of an AlB parental melt at a depth of 6–8 km (2.5 kb) could produce residual liquids having many of the major oxide characteristics of FeB2 ferrobasalts. Sr-Nd-Pb isotopic signatures of these three suites indicate a dominant contribution from an enriched plume source. FeB1 lavas are likely products of mixing between melts of an enriched plume mantle source (represented by Imnaha and Steens Basalts of the Columbia River Basalt Group) and isotopically heterogeneous sub-continental lithospheric mantle (SCLM) that has been isolated from the convecting mantle since the Archean. Isotopic ratios of FeB2 and AlB lavas capture mixing between enriched plume mantle and a more isotopically homogeneous ancient SCLM domain characteristic of the eastern and central Snake River Plain, with a coupled decrease in lithospheric contribution and degree of partial melting through time to the present. Mixtures of enriched asthenospheric reservoirs with lithospheric mantle have been proposed for neighboring volcanic fields to the east along the strike of the Yellowstone-SRP hotspot track, and to the west due to differences in the mantle underlying the boundary of the North American craton and accreted terranes. Our petrogenetic model for the Pleistocene WSRP basalts suggests that there is also a lateral, across strike gradient in the geometry and interaction of enriched plume mantle and ancient lithosphere. We reiterate suggestions that the WSRP is a lithosphere-scale conduit connecting initial plume head impingement in east-central Oregon with the subsequent Yellowstone-SRP hotspot plume tail track.

The Newly Discovered Neoproterozoic Aillikite Occurrence in Vinoren (Southern Norway): Age, Geodynamic Position and Mineralogical Evidence of Diamond-Bearing Mantle Source

During the period 750–600 Ma ago, prior to the final break-up of the supercontinent Rodinia, the crust of both the North American Craton and Baltica was intruded by significant amounts of rift-related magmas originating from the mantle. In the Proterozoic crust of Southern Norway, the 580 Ma old Fen carbonatite-ultramafic complex is a representative of this type of rocks. In this paper, we report the occurrence of an ultramafic lamprophyre dyke which possibly is linked to the Fen complex, although 40Ar/39Ar data from phenocrystic phlogopite from the dyke gave an age of 686 ± 9 Ma. The lamprophyre dyke was recently discovered in one of the Kongsberg silver mines at Vinoren, Norway. Whole rock geochemistry, geochronological and mineralogical data from the ultramafic lamprophyre dyke are presented aiming to elucidate its origin and possible geodynamic setting. From the whole-rock composition of the Vinoren dyke, the rock could be recognized as transitional between carbonatite and kimberlite-II (orangeite). From its diagnostic mineralogy, the rock is classified as aillikite. The compositions and xenocrystic nature of several of the major and accessory minerals from the Vinoren aillikite are characteristic for diamondiferous rocks (kimberlites/lamproites/UML): Phlogopite with kinoshitalite-rich rims, chromite-spinel-ulvöspinel series, Mg- and Mn-rich ilmenites, rutile and lucasite-(Ce). We suggest that the aillikite melt formed during partial melting of a MARID (mica-amphibole-rutile-ilmenite-diopside)-like source under CO2 fluxing. The pre-rifting geodynamic setting of the Vinoren aillikite before the Rodinia supercontinent breakup suggests a relatively thick SCLM (Subcontinental Lithospheric Mantle) during this stage and might indicate a diamond-bearing source for the parental melt. This is in contrast to the about 100 Ma younger Fen complex, which were derived from a thin SCLM.

Nailed to the craton: Stratigraphic continuity across the southeastern Canadian Cordillera with tectonic implications for ribbon continent models

Cambrian and Upper Devonian to Mississippian strata can be confidently traced westward, without strike-slip offset, from the autochthonous section above North American basement into the southeastern Canadian Cordillera, and are thus “nailed” to the craton. These strata are in turn stratigraphically pinned to older (Mesoproterozoic Belt-Purcell Supergroup, Neoproterozoic Windermere Supergroup, and Ediacaran), intermediate-aged (Ordovician–Silurian), and younger (Permian to Middle Jurassic) strata found only in the mountains, thus linking them to the adjacent autochthonous craton. The overlapping distribution of linking successions, regionally traceable unique stratigraphic horizons in the Belt-Purcell and Windermere Supergroups, and across-strike stratigraphic features show that the entire Cariboo, northern Selkirk, Purcell, and Rocky Mountains are directly tied to the adjacent North American craton without discernible strike-slip or oblique displacement, or substantial purely convergent plate-scale (>400 km) horizontal displacement. They link the entire width of the Belt-Purcell and Windermere basins in the southeastern Canadian Cordillera to the adjacent craton and show that any proposed Cretaceous ribbon continent suture, with its thousands of kilometers of proposed displacement, cannot run through the southeastern Canadian Cordillera.

Grenville Foreland Deformation and Sedimentation in Southwest Ohio Indicated by Reprocessed Seismic Reflection Profiles near Middletown, Ohio, USA

The late Mesoproterozoic to early Neoproterozoic Middle Run Formation contains vital information about the crustal evolution of the North American Craton. Four reprocessed seismic reflection lines in the vicinity of the AK Steel facility in Middletown, Ohio, provide new insights into the structural and depositional setting of the Middle Run Formation in this region. A residual statics solution improved the resolution and coherency of reflections in these profiles that underlie the Cambrian Mount Simon Sandstone. Reprocessing revealed gently inclined, west-southwest-dipping reflectors and the occurrence of an angular unconformity between the Middle Run Formation and the overlying Paleozoic strata. The weak and discontinuous seismic reflection character of the Middle Run Formation in these seismic lines overlies a sequence of stronger parallel reflections that are like those observed on the eastward ODNR-1-88 seismic line located near core hole DGS 2627, the stratotype of the Middle Run Formation. This inferred thickness indicates that the basin in which the Middle Run Formation was deposited ranges from at least 670 to 1,128 m (2,200 to 3,700 ft) deep at the AK Steel area and dips gently west-southwest, which is in contrast with the moderate easterly dip observed on the ODNR-1-88 seismic line to the northeast. Correlation of these features across the 10 km (approximately 6 mi) cross-strike gap between the AK Steel lines and the ODNR-1-88 seismic line suggests the presence of a reverse fault with approximately 792 m (2,600 ft) of estimated vertical displacement. A regional cross section—including the WSU 1990 seismic line eastward of the ODNR-1-88 line—exhibits a faulted west-verging asymmetric syncline in near proximity to the Grenville Front. This cross section also shows that deformation of the Middle Run Formation and the underlying layered sequence exhibits a consistent tectonic style of reverse faulting and folding that developed in response to Grenville Front tectonism.

Linking metamorphism, magma generation, and synorogenic sedimentation to crustal thickening during Southern Appalachian mountain building, USA

The nature of metamorphism, magma compositions, the spatial distribution of plutons, and foreland sediments reflect, in part, the character and thickness of continental crust. We utilized metamorphic pressure-temperature-time (P-T-t) paths, garnet Sm-Nd ages, zircon U-Pb ages, and pluton compositions to estimate paleocrustal thickness and temporal changes in crustal magma sources in the Blue Ridge of the southernmost Appalachians. Garnet Sm-Nd ages for amphibolite-facies metamorphic rocks range from 331 ± 4 to 320 ± 3 Ma. Low- and high-Sr/Y plutons that intruded these metamorphic rocks have zircon U-Pb ages of 390 ± 1 to 365 ± 1 Ma and 349 ± 2 to 335 ± 1 Ma, respectively. Therefore, garnet growth began during regional metamorphism synchronous with or shortly after intrusion of the youngest high-Sr/Y trondhjemite plutons. Phase diagram sections and thermobarometry indicate that garnet growth initiated at ∼5.8 kbar and 540 °C and grew during temperature increases of 60–100 °C and pressure increases of 2–3 kbar. The older, low-Sr/Y magmas are inferred to have been sourced in the crust at depths <∼30 km, insufficient for garnet to be stable. However, the younger, high-Sr/Y magmas are inferred to have been sourced at >30 km depths where garnet was stable. Hafnium isotopic compositions for all the plutons, but one, exhibit a range from negative initial εHf(i) to weakly positive initial εHf(i), indicating incomplete mixing of dominantly crustal sources. Our data require minimum crustal thicknesses of ∼33 km at 331 Ma; however, Alleghanian crustal thicknesses must have locally reached 39 km, based on crustal reconstruction adding the Alleghanian thrust sheet beneath the eastern Blue Ridge. We infer the presence of hot, tectonically thickened crust during intrusion of the early Alleghanian high-Sr/Y plutons and conclude that garnet growth and plutonism reflect a progressive increase in crustal thickness and depth of magma generation. The crustal thickening was synchronous with deposition of Mississippian to early Pennsylvanian sediments in the foreland basin of the Appalachian orogen between 350 and 320 Ma. This crustal thickening may have preceded emplacement of the Alleghanian thrust sheets onto the North American craton.