Continental Suture Zone Research Articles

Revised Paleozoic depositional history of the central Rocky Mountains (Utah and Colorado)

Precise sedimentological, stratigraphic, paleontological, and geochronological data collected from Paleozoic strata of the Dinosaur National Monument (DNM) area (NW Colorado and NE Utah) allow for comprehensive revision of the depositional history of that part of the central Rocky Mountains.Recovery of multiple species of the trilobite genus Elrathiella from sandstone beds in the middle of the Lodore Formation assigns those deposits to the mid-Miaolingian Ehmaniella Zone near the top of the Wuliuan Stage. This is significantly older than the previous “Dresbachian” age estimate for those coarse clastics, which attributed them to deposition during the late Miaolingian (latest Drumian or Guzhangian). This age revision re-positions the southernmost boundary of widespread Miaolingian inner detrital belt deposits of Wyoming to within 90 km of an extensive area of central Colorado where strata of this age are absent, and middle Furongian successions rest directly on basement. The boundary between these two regions, one with and one without Miaolingian strata, is nearly coincident with the Cheyenne belt, an east–west oriented Precambrian continental suture zone at the northern margin of the Yavapai Terrane. This association supports the idea that deep-seated crustal-scale features controlled the Cambrian paleotopography of the Rocky Mountain region and, in turn, the regional pattern of Cambrian transgression across western Laurentia.Early cursory paleontological work suggested a Devonian age for a thick sandstone interval near the top of the Lodore Formation, based on phosphatic fossils. We demonstrate that these fossils are in fact Cambrian obolid brachiopod shells. However, we provide multiple lines of evidence, including carbonate δ13C chemostratigraphic data, distinctive lithofacies, and stratigraphic relationships to argue that a thin interval of shale and dolostone, as well as a previously unrecognized paleokarst and regolith interval at the top of what was mapped as Lodore Formation, are in fact part of the Upper Devonian Parting Formation. The thick sub-Mississippian paleoregolith is consistent with elevated rates of weathering during the Late Devonian–Early Mississippian. δ13C chemostratigraphic data for the Madison Limestone include a large double-peaked Kinderhookian positive isotopic excursion (up to ~6.5 ‰), providing a precise estimate of the timing of Mississippian transgression in this part of the Rocky Mountain region.Detrital zircon geochronologic data for Cambrian to Pennsylvanian strata show remarkably consistent patterns dominated by ~1100 Ma, ~1400 Ma, and ~1700 Ma peaks, reflecting relative igneous quiescence over much of the Paleozoic for this region, a span of ~200 m.y., despite some orogenic activity (e.g., Antler Orogeny, Ancestral Rockies) during that time interval.

Structural control of Cambrian paleotopography and patterns of transgression in western Laurentia

Abstract The Cambrian transgression across the Great Unconformity produced one of the largest expansions of shallow marine habitats and associated diversification of marine invertebrate faunas in Earth history. However, identification of the underlying controls on the pattern of transgression of Cambrian seas has been hampered by imprecise or inaccurate age assignments for many formations. Recovery of an Ehmaniella Zone trilobite fauna from the Lodore Formation in northwestern Colorado (United States) revises the age of this unit to be significantly older, specifically middle Miaolingian (upper Wuliuan). This expands the established distribution of thick Miaolingian deposits of the northern Rocky Mountains to within 90 km of a broad region of central Colorado where Miaolingian strata are missing and Furongian successions rest directly on basement. The boundary between these two regions marks the position of an ~200 km east-west offset within the generally north-south–trending Cambrian paleoshoreline of western North America. The offset is co-located with a Precambrian continental suture zone (Cheyenne belt) at the northern Yavapai terrane margin and is directly east of an offset of similar magnitude and latitude in the early to middle Paleozoic shelf edge in Nevada. We thus posit that Precambrian deep-seated crustal-scale features controlled the Cambrian paleotopography of western Laurentia, strongly influencing the patterns of Cambrian transgression and structure of the shallow marine ecosystem established during continental submergence. These continental-scale structural elements remained a major control on marine paleogeography for >200 m.y. into the late Paleozoic.

Tectonic setting and mineralisation potential of the Cowley Ophiolite Complex, north Queensland

Northeast Queensland contains multiple slices of mafic–ultramafic units, strung out along regional faults that mark major tectonic boundaries. One such complex is the Cowley Ophiolite Complex, which is situated along the Russell-Mulgrave Fault. The Cowley Ophiolite Complex is a differentiated mafic–ultramafic complex composed of gabbro, chlorite schist, anthophyllite schist and serpentinite. We interpret that the alteration assemblages observed throughout the Cowley Ophiolite Complex reflect an amphibolite facies metasomatic overprint within a supra-subduction zone setting. This interpretation is consistent with geochemical discrimination of the gabbro, and chromite grains from the anthophyllite schist. The Cowley Ophiolite Complex records a higher metamorphic grade than the surrounding Hodgkinson Formation, and we interpret this to reflect the allochthonous structural setting of the complex, positioned along an ancient subduction margin. This subduction margin is represented today by the Russell-Mulgrave Fault. Metasomatism and emplacement of the complex probably pre-dated deposition of the Mossman Orogen’s active margin successions. Our interpretation of an active subduction complex, which pre-dated the formation of the Mossman Orogen, suggests that the Russell-Mulgrave Fault is a Paleozoic, continental suture zone. The Cowley Ophiolite Complex presents little indication of economic mineralisation; however, much of the complex remains unexplored. KEY POINTS The Cowley Ophiolite Complex is a differentiated, mafic–ultramafic complex formed within a supra-subduction zone setting. Emplacement of the Cowley Ophiolite Complex, along the Russell-Mulgrave Fault, pre-dated the formation of the Hodgkinson Province. The Russell-Mulgrave Fault represents an ancient subduction margin, and is interpreted as a Paleozoic, continental suture zone.

Rare Earth Element and Incompatible Trace Element Abundances in Emeralds Reveal Their Formation Environments

Emeralds require the unusual association of typically compatible elements (Cr, V), with incompatible Be to form, and occur in complex tectonic settings associated with sediments (type IIB; Colombia) or, more commonly, with magmatism and regional metamorphism (IA). Precise rare earth element (REE) and incompatible trace element abundances are reported for a global suite of emeralds, enabling the identification of the environments in which they formed. Type IIB emeralds have nearly flat continental crust normalized REE patterns (La/YbCC = ~2), consistent with a sedimentary source origin. Type IA emerald REE patterns have upturns in the heavy REE (La/YbCC = ~0.3), a feature also shared with South African emeralds occurring in Archaean host rocks. Modeling of type IA emerald compositions indicates that they form from magmatic fluids of sedimentary (S)-type granite melts interacting with Cr, V-rich mafic–ultramafic crustal protoliths. This geochemical signature links emerald formation with continental suture zones. Diamonds, rubies, and sapphires have been considered as ‘plate tectonic gemstones’ based on mineral inclusions within them, or associations with plate tectonic indicators. Emeralds are distinct plate tectonic gemstones, recording geochemical evidence for origin within their mineral structure, and indicating that plate tectonic processes have led to emerald deposit formation since at least the Archaean.

Mineralization proximal to the final Nuna suture in northeastern Australia

Mineralization along continental suture zones is facilitated through the frequent presence of pathways from fertile mantle source regions to crustal repositories. Due to their inherent rheological weakness, these suture zones are often concealed, which hinders surface-based observations. Here, we use zircon U-Pb and sericite 40Ar/39Ar dating, and whole-rock geochemical data, to investigate the crystallization and mineralization history from a sequence of granites (sensu lato), volcanic rocks and sedimentary rocks from the Au–Ag–Pb–Zn Empress Springs Project in northeast Australia, which are under >50 m of Phanerozoic cover and located near the interpreted ca. 1.6 Ga tectonic boundary between the North Australia Craton (Mount Isa Inlier) and Laurentia (Georgetown Inlier). Zircon U-Pb dating indicates that granite emplacement, volcanic eruptions and dolerite intrusions occurred between 1564 ± 6 and 1546 ± 13 Ma (2σ), corresponding to the 1560–1550 Ma Esmeralda Supersuite and the Croydon Volcanic Group exposed in the western Georgetown Inlier. U-Pb ages from detrital zircon grains in a sedimentary rock revealed a near unimodal ca. 1560 Ma population, likely sourced from the surrounding granitic and volcanic rocks. Sericite 40Ar/39Ar dating yielded disturbed spectra with evidence for incorporation of excess radiogenic Ar, but with a probable Carboniferous to Permian age, potentially dating the timing of gold mineralization. Geochemical similarities point towards an epithermal origin for the Empress Springs Project, consistent with post-orogenic mineralization at either ca. 1560–1550 Ma or 330–250 Ma. It is likely that a series of west-dipping lithospheric-scale faults below the Empress Springs Project, here termed the Empress Suture Zone, demarcate the final suture zone associated with the assembly of Nuna. Reactivation of the Empress Suture Zone ~1.3 b.y. after crystallization with potential contemporaneous gold mineralization attests to its longevity for fluid mobility.

Seismicity in the equatorial margin of Brazil reactivates the Precambrian basement fabric

The continental margin of Brazil in intraplate South America exhibits a seismicity rate 70% higher than the average stable continental regions. This margin encompasses Northeastern Brazil, where most of the seismicity and active faults concentrate along the coast. The current study presents an integration of high-resolution aeromagnetic, seismological, and geological data to investigate the relationship between the seismicity and basement fabric in two areas in the northern part of the Precambrian Borborema Province. Our investigation indicates a relation between the metamorphic basement foliation and two seismogenic faults. In the first area adjacent to the Transbrasiliano Lineament, the largest continental suture zone in the South American platform, the magnetic anomaly patterns indicate that the regional basement grain and structural elements are E-W-oriented, which coincides with the strike of the E-W-striking, strike-slip Riacho Fundo Seismogenic Fault. In the second area, the magnetic anomaly patterns also coincide with the NE-SW-striking, strike-slip Samambaia Fault, and a swarm of chalcedony-quartz veins. Euler deconvolution used to estimate the source depth of the magnetic anomalies indicates that the faults in both cases could be longer and deeper than segments illuminated by the present-day seismicity. Although these findings are difficult to use as predictive tools, they indicate that the basement fabric could be reactivated by seismicity in continental interiors, as previously discussed in several cases in Brazil.

Tectonic evolution of the Grenville Orogen in the central Appalachians

Remnants of the Grenville Orogeny—a globally pervasive tectonothermal event resulting from the amalgamation of the supercontinent Rodinia between ca. 1.3 and 1.0 Ga—are exposed throughout eastern North America for ca. 3000 km from Labrador to Alabama. Although tectonic models for the development of the Grenville have historically been based on observations from the large and well-exposed Canadian Grenville Province, recent investigations suggest that initial continental collision occurred proximal to the Appalachian Blue Ridge. Mesoproterozoic rocks within this portion of the orogen are exposed within a series of allochthonous terranes that were extensively reworked during Paleozoic orogenesis. To better understand the tectonic history of this critical region, we utilized garnet and monazite geochronology to reconstruct the metamorphic history of eleven samples of ortho- and paragneiss from the Appalachian Blue Ridge Province.The resultant dates, together with new results from trace element thermobarometry, define a prolonged record of high temperature metamorphism coincident with the timing of continent–continent collision as recognized in the northern portion of the orogen. Initial garnet and monazite growth commenced between 1100 and 1065 Ma with peak granulite-facies conditions (750–900 °C, 0.55–0.85 GPa) occurring from 1050 to 1010 Ma. Burial and heating of the paragneisses progressed rapidly with peak metamorphism occurring ∼20–30 Myr after deposition of the sedimentary protoliths. Local hydrothermal alteration followed peak metamorphism and occurred at 1000–990 Ma with later cooling through ∼660 °C between 980 and 935 Ma (3 ± 2 °C/Myr). The metamorphic history outlined here, together with insights from previous studies, is consistent with formation of the Blue Ridge rocks within a continental suture zone that had an orogenic architecture comparable to that of the Himalayan-Tibetan orogen.

The Upper Devonian Aley carbonatite, NE British Columbia: a product of Antler orogenesis in the western Foreland Belt of the Canadian Cordillera

Paleozoic continental margin strata in the western Foreland Belt of the Canadian Cordillera are characterized in part by alkaline volcanic sequences, carbonatite intrusions, coarse clastic sedimentary units, and erosional unconformities. These strata also contain a record of mid-Paleozoic contractional deformation unseen in coeval passive margin strata in the eastern Foreland Belt. In order to test potential genetic links between Paleozoic alkaline igneous activity, active margin sedimentation, and deformation in the western Foreland Belt, and better understand their implications for the evolution of the Foreland Belt as a whole, we have undertaken a detailed mapping and structural study of the Aley carbonatite intrusion and its host strata in the western Foreland Belt of NE British Columbia. Our work demonstrates that carbonatite emplacement was coeval with a Late Devonian contractional nappe-forming tectonic event. Interpreting tectonism as associated with continental collision along a long-lived active margin provides the best explanation for our structural and stratigraphic observations, and suggests that the western Foreland Belt is far-travelled and exotic relative to coeval passive margin strata in the eastern Foreland Belt. Deformed alkaline-carbonatite intrusions that characterize continental suture zones in Africa may provide an analogue for the Aley carbonatite and correlative alkaline-carbonatite complexes in the western Foreland Belt.

Do the deformed alkaline rocks always serve as a marker of continental suture zone? A case study from parts of the Chotanagpur Granite Gneissic complex, India

In Santuri- Kankarkiari area of Purulia, West Bengal, India, a nepheline syenite body cuts across the regional Grenvillian (∼1000–950 Ma) fabric (S2) of the host Chotanagpur Granite Gneissic Complex (CGGC). Subsequently, the nepheline syenite of the studied area and its basement are metamorphosed and develop a pervasive regional fabrics (S3). Isoclinal folding of S2 fabric of the host gneissic rocks resulted in axial planar S3 fabric. Field relations, ‘frozen-in’ textural feature, petrology and geochemistry of the studied rock and the published geological and geochronological information from the adjoining areas are consistent with (a) derivation of its magmatic protolith from the then sub-continental lithospheric mantle and emplacement of the melt in a rift basin (b) the rifting of the Grenvillian basement seems to have occurred during (∼950–900 Ma) and (c) burial, deformation and metamorphism (peaked at 9–10 kbar, 690–770 °C) of the studied alkaline rocks and its Grenvillian basement seem to have occurred in a continent-continent collisional tectonic setting during ∼900–780Ma. The commonly perceived notion that the deformed alkaline rocks (DARs) are invariably linked to continental suture along which a former ocean basin floor was consumed is not supported in this study. Integrating all the information, we propose that the nepheline syenite of the studied area and also from parts of the Granulite Terrane of South India represent metamorphosed and deformed aulacogen rock ensemble that should not be associated with opening and closure of an oceanic crust. Combining the information of this study and the published works, it is demonstrated that the deformed alkaline rocks (DARs) can also form in a coherent crustal block without any connection with a continental suture zone. A plausible tectonic model for the evolution of the CGGC has also been presented.

Lithospheric density structure beneath the Tarim basin and surroundings, northwestern China, from the joint inversion of gravity and topography

Intraplate strain generally focuses in discrete zones, but despite the profound impact of this partitioning on global tectonics, geodynamics, and seismic hazard, the processes by which deformation becomes localized are not well understood. Such heterogeneous intraplate strain is exemplified in central Asia, where the Indo-Eurasian collision has caused widespread deformation while the Tarim block has experienced minimal Cenozoic shortening. The apparent stability of Tarim may arise either because strain is dominantly accommodated by pre-existing faults in the continental suture zones that bound it—essentially discretizing Eurasia into microplates—or because the lithospheric-scale strength (i.e., viscosity) of the Tarim block is greater than its surroundings. Here, we jointly analyze seismic velocity, gravity, topography, and temperature to develop a 3-D density model of the crust and upper mantle in this region. The Tarim crust is characterized by high density, vs, vp, and vp/vs, consistent with a dominantly mafic composition and with the presence of an oceanic plateau beneath Tarim. Low-density but high-velocity mantle lithosphere beneath southern (southwestern) Tarim underlies a suite of Permian plume-related mafic intrusions and A-type granites sourced in previously depleted mantle lithosphere; we posit that this region was further depleted, dehydrated, and strengthened by Permian plume magmatism. The actively deforming western and southern margins of Tarim—the Tien Shan, Kunlun Shan, and Altyn Tagh fault—are underlain by buoyant upper mantle with low velocity; we hypothesize that this material has been hydrated by mantle-derived fluids that have preferentially migrated along Paleozoic continental sutures. Such hydrous material should be weak, and herein strain focuses there because of lithospheric-scale variations in rheology rather than the pre-existence of faults in the brittle crust. Thus this world-class example of strain partitioning arises not simply from the pre-existence of brittle faults but from the thermo-chemical and therefore rheological variations inherited from prior tectonism.

Structural framework of the Jaibaras Rift, Brazil, based on geophysical data

The Cambro-Ordovician Jaibaras Rift is a NE–SW trending elongated feature, controlled by the Transbrasiliano lineament, locally known as Sobral-Pedro II shear zone (SPIISZ). An integrated study of geophysical data (gammaspectrometry, magnetometry and gravimetry) was undertaken in the Jaibaras Rift area, between Ceará Central (CCD) and Médio Coreaú domains (MCD), northwest Borborema Province. Geophysical data were interpreted qualitatively and quantitatively in order to understand the tectono-magmatic relations and rift formation based on the main geophysical lineaments, source geometry and depth, and separation of geophysical domains. In addition, a 2D gravity model was generated. The results show a structural partition characterized by NE–SW lineaments and E–W inflexions, where CCD presents a relatively mild magnetic field, whilst the MCD field is more disturbed. The Jaibaras Rift is characterized by positive magnetic and gravity anomalies. The SPIISZ, which corresponds to the SE fault edge of the Jaibaras Rift, is marked by strong magnetic dipoles and strong gravity gradients in the profile, showing the deep character of the Transbrasiliano lineament in the region. The Café-Ipueiras fault, at the NW edge of the rift, is well marked in gravity profiles, but displays low contrast of the magnetic field. Interpretation of the gravimetric anomaly map allowed to recognizing the main NE–SW axis, with alternation of maxima and minima in MCD. A regional gravity gradient reveals significant lateral density variation between the MCD and CCD perpendicular to the SPIISZ, emphasizing it as a main continental suture zone between crustal blocks.

Rheological control on the tectonic evolution of a continental suture zone: the Variscan example from NW Iberia (Spain)

The Variscan continental suture zone exposed in NW Iberia is examined to uncover the long-lived rheological control exerted by the strata deposited over the external parts of Gondwana on its geodynamic evolution. The suture occurs within a set of allochthonous terranes whose limits were taken as domain boundaries to interpret the Variscan stacking of Paleozoic continental domains and retrodeform the resulting nappe pile. The suture zone formed due to closure of ocean basins located between Gondwana and Laurussia during the Late Paleozoic and consists of relics of oceanic and transitional crust. The suture zone exhibits a tabular to lens shape due to repeated tectonic events dominated by non-coaxial deformation (thrusts and low-angle normal faults). Thrusting and normal faulting also involved the margins of the continents bounding the suture. The structure of the continental blocks, however, is dominated by folds, particularly large nappe folds with pronounced superimposed flattening. The upper part of the basal allochthonous units comprises a rheologically incompetent domain below the suture zone. This domain is typified by the carbonaceous-rich strata, which are probably Ordovician–Silurian sediments based on U–Pb detrital zircon populations. The rheology of this layer determined the location of the first accretionary thrust that initiated the Late Devonian subduction of the Gondwana margin below the suture zone. By favoring fault development, the upper sequence of the basal allochthonous units as a whole influenced the exhumation of deep-seated continental crust, the transference of the suture zone over Gondwana, and the re-equilibration of the resulting overthickened crust.

Lithospheric and asthenospheric contributions to shear-wave splitting observations in the southeastern United States

We present observations of both null and non-null SKS splitting from temporary deployments across the southeastern United States in order to evaluate the relative contributions of lithospheric deformation and asthenospheric flow to regional anisotropy. Data for this study come from four temporary broadband seismic deployments: the Appalachian Seismic Transect (AST), the Test Experiment for Eastern North America (TEENA), the South Carolina Earth Physics Project (SCEPP), and the Florida to Edmonton Array (FLED). In general, we find fast directions aligned roughly parallel to absolute plate motion of the North American plate (APM) within and west of the Southern Appalachians, whereas to the southeast, we find a broad area dominated by complex splitting patterns consisting of well-constrained null splitting measurements over a range of backazimuths along with a very small number of resolved non-null measurements. This change in splitting patterns is consistent with a transition from drag induced asthenospheric flow beneath the older sections of the North American continent to vertical or incoherent mantle flow, likely in combination with complex lithospheric anisotropy, beneath the younger accreted terranes to the southeast. In addition to these general patterns, we find a number of non-null splitting measurements that are not aligned with APM, but are instead aligned with prominent magnetic anomalies that may correspond to ancient continental suture zones or faults. This would suggest that in these areas, a strongly anisotropic (but localized) lithospheric fabric dominates over any ambient asthenospheric anisotropic signature. In areas with generally strong APM parallel splitting, this would imply a thick sheared mantle lithosphere whose deformation-induced anisotropy is strong enough to overprint the anisotropy induced by APM, and is aligned with the shallower crustal structures responsible for generating the observed magnetic anomalies. In the southeastern areas dominated by null splitting measurements, there may be no strong signature from asthenospheric anisotropy to override, but a substantial lithospheric thickness is still required to generate the magnitude of the observed SKS splitting (∼1s). More data are required to verify these results, but future datasets including data from USArray may be able to exploit the correlations between null and non-null SKS splitting measurements and magnetic lineaments to better constrain the provenance of the regional anisotropic signature.

The Moho: Boundary above upper mantle peridotites or lower crustal eclogites? A global review and new interpretations for passive margins

Abstract We have performed a global study of 2D crustal scale wide-angle profiles across passive margins, with regard to local elevations in the Moho which could possibly be interpreted as indicative of lower crustal eclogites. A total of 16 candidates have been found, mainly in the North Atlantic and around Australia. These cases make up c. 6% only of the total profile length studied, confirming the interpretation of the Moho generally representing the top of the mantle. The interpreted candidates for lower crustal eclogites indicate that there may be a link between eclogite bodies and continental suture zones in the Barents Sea, off mid-Norway, in the Newfoundland Appalachians and in the Yilgarn Craton, western Australia. It is also possible that there is a genetic link between the formation of Caledonian eclogites and the Jan Mayen Fracture Zone, which is the only major fracture zone in the North Atlantic. Several of the inferred lower crustal eclogite bodies are located close to lines of major changes in strain, such as coastlines and shelf edges, indicating that lower crustal eclogite bodies may be important in guiding the evolution of basin architecture. Interpreting the Moho beneath the Bedout High on the northwest shelf of Australia as the top of a body of lower crustal eclogites, may imply that the northern termination of the Lambert Shelf represents a paleo suture zone and that the western termination of the Broome Platform acted as a major transfer zone. The significant increase in crustal thickness implied by the eclogite model has important implications for estimates of stretching history, subsidence and hydrocarbon maturation modelling.

Evolution of the Archean and Paleoproterozoic lower crust beneath the Trans-North China Orogen and the Western Block of the North China Craton

We report U–Pb ages and Hf isotopes of zircons in granulites from two continental suture zones in the North China Craton (NCC)—from granulite xenoliths entrained in the Cenozoic Hannuoba basalts within the Trans-North China Orogen (TNCO) that amalgamates the Western and Eastern Blocks, and from a mafic granulite associated with ultrahigh-temperature (UHT) granulites within the Inner Mongolia Suture Zone (IMSZ) that welds the Yinshan and Ordos Blocks. The zircons in these rocks, together with the inherited zircons from Mesozoic subalkalic intrusives of these regions, show several distinct U–Pb age populations and provide constraints on the evolution of the lower crust beneath the NCC. Parts of the older zircons from the TNCO yield discordant data that project to ages of about 2.5 to 2.7Ga. These old zircons show Hf TDM model ages of 2.6–2.9Ga suggesting the existence of Archean lower crust beneath the TNCO, which is coupled with the Neoarchean metamorphic basement and greenstone belt. A major zircon population grew at 1.75–2.0Ga, consistent with the timing of the Paleoproterozoic collision between the various crustal blocks within the NCC, suggesting that this event had a significant effect on the Archean lower crust. The older inherited zircons from the Western Block also yield discordant data that project to younger ages of about 1.8 to 1.9Ga. This age is consistent with the timing of intrusion of the mafic magmatic protoliths of the granulites associated with the UHT rocks in the khondalite belt within the IMSZ. These older zircons yield Hf TDM model ages of 2.0–2.5Ga suggesting that the lower crust beneath the Western Block in the NCC to be dominantly Paleoproterozoic, and decoupled with the Archean metamorphic basement of the region. Yet another population of zircons from the Hannuoba banded granulite xenoliths, garnet-bearing pyroxenite cumulate and dark pyroxenite xenoliths yield a large spread of Phanerozoic concordant ages ranging from 470Ma to 40Ma with peaks at 315Ma, 220–230Ma, 120Ma and 46Ma. The Late Paleozoic zircons predominantly occur in the garnet-bearing pyroxenite xenoliths and yield variable εHf(t) values of −24 to +18. Triassic–Cretaceous zircons mainly occur in the banded granulite xenoliths and yield highly variable εHf(t) values, ranging from −48 to +16. The Tertiary zircons occur only in the dark pyroxenite xenoliths and show a restricted εHf(t) with a dominant range from 0 to +18. These data suggest episodic magmatic underplating in the ancient lower crust of the NCC, lasting continuously throughout Phanerozoic, producing zircons from the underplated magmas or providing the heat source for the recrystallization of zircons from the ancient crust. Our data clearly demonstrate the important events of Phanerozoic magmatic underplating beneath the Neoarchean–Paleoproterozoic crust of the NCC.

Tectonometamorphic evolution of the Rhodope orogen

[1] This study combines new data on tectonostratigraphy, macrostructures and microstructures, petrology, and geochronology to propose a comprehensive model for the tectonometamorphic evolution of the Rhodope orogen from the Jurassic to the early Paleogene. Rocks from two study areas in the central and eastern Greek Rhodope represent a continental suture zone (Rhodope Suture Zone), with the included material most likely forming an extensional allochthon south of the European continent during Permo/Triassic times that was subsequently subducted beneath Europe in the Early Jurassic (≥180 Ma). On the basis of comparable metamorphic ages and coherent structures but differences in metamorphic conditions and lithologies, the rocks of the Rhodope Suture Zone are subdivided into an upper and a lower part. The prograde history is linked with subduction-related structures in the lower part (uniaxial stretching, deformation stage D1). In metapelites, the earliest stage of metamorphism recorded at circa 180 Ma occurred at least under ultrahigh-pressure metamorphic conditions. The rocks of the upper part experienced isothermal decompression with partial anatexis. Exhumation paths of both parts differ in temperature because of the relative tectonic position within the exhuming wedge. Exhumation was forced by the Nestos Shear Zone that controlled the early phase of normal displacement by SW shearing at the base (lower part) and NE shearing on top (upper part) from the Late Jurassic to the Late Cretaceous (deformation stage D2). An intervening stage of mineral recrystallization and thermal reequilibration in the upper part was followed by a common exhumation history of both parts at pressures lower than about 12 kbar (35–40 km depth). During this stage, exhumation was controlled by southwest directed shearing and folding (deformation stage D3). Slab retreat to the south led to subsequent extension (deformation stage D4) and final exhumation coeval with the formation of basement domes, thus making the Rhodope a classic core complex juxtaposed with former structurally higher units.

Constraints on crustal structure and composition within a continental suture zone in the Irish Caledonides from shear wave wide-angle reflection data and lower crustal xenoliths

SUMMARY Shear wave seismic velocities (Vs) when used together with compressional wave velocities (Vp) are a powerful diagnostic of the chemical composition and mineralogy of the continental lithosphere. In this paper, we present whole crustal models of Vs and Vp velocities from two wide-angle seismic profiles from southwest Ireland, which straddle the Eastern Avalonian Terrane within the Iapetus Suture Zone. These models are based on traveltime interpretations of primary and reflected P- and S-wave seismic phases generated by explosive underwater sources. The quality of the shear wave data is exceptional when compared to similar data sets recorded elsewhere and allows the distribution of Poisson's ratio (σ) within the entire crust to be accurately determined. Models of Poisson's ratio, Vp and Vs are then used to constrain crustal composition at various depths down to the Moho (ca. 32 km depth) using published laboratory velocity measurements from rocks of different metamorphic grades and chemistry. The results indicate that the Irish crust in this region of the Caledonian orogenic belt is unusually felsic in bulk composition with a σ value of 0.247, which is below the global average value of 0.265. Variations in σ within the upper ca. 5 km of crust are greatest (0.20–0.28) and can be correlated with siliciclastic and carbonate sediments in Devonian and Carboniferous extensional sedimentary basins that were formed during the early part of the Variscan orogenic cycle. The typically more uniform variation in the mid crust down to 15 km depth is consistent with a granite/granodiorite or metagreywacke composition of greenschist to amphibolite facies mineralogy. Within the lower crust the value of σ (0.257–0.265) is significantly less than the global mean value of 0.281 predicted from an average crustal petrology model. This suggests a bulk silica content (%SiO2) of ∼64 per cent for the lower crust, based on an empirical relationship between σ and %SiO2, determined from compilations of laboratory data. These results are consistent with petrophysical, geochemical and petrological data from an unusually well-preserved lower crustal xenolith suite from the Irish Midlands, hosted in early Carboniferous volcanic rocks. The xenoliths (mainly partially melted granulite facies metapelites) evidently represent the bulk composition of the lower crust, which was largely derived by accretion of sedimentary material, derived from oceanic, island arc and continental margin sources, during oblique Caledonian collision.

Detrital sources and sediment delivery to the early post-rift (Albian-Cenomanian) Newfoundland Basin east of the Grand Banks: results from ODP Leg 210

Abstract The young narrow arm of the Atlantic Ocean that separated Newfoundland from Iberia in the Albian–Cenomanian accumulated a thick succession of muddy gravity flow deposits that were recovered by drilling at Ocean Drilling Program Site 1276 off the Grand Banks of Newfoundland. The sediment thickness and sand content are somewhat surprising given the number of contemporaneous shelf and slope basins that would have acted as traps for detritus shed from the Newfoundland margin. A set of individual studies was undertaken to trace the source of this detritus and to test hypotheses that the material might have been derived, in part, from the east (Iberia), the north (Flemish Cap and northern rift extensions), and/or from various sources to the west (Grand Banks, Interior Newfoundland, Avalon Uplift). By synthesizing seismic data, paleocurrent data from gravity flow deposits, sediment composition (sand detrital modes, clay mineralogy) and detrital mica ages we conclude that the likely source of the Albian–Cenomanian sediment was the Avalon Uplift, situated in the southern Grand Banks. Key pieces of evidence that lead us to this result are: thinning of the sediment package towards the north and thickening towards the southwest, the paleocurrent directions apparently suggesting a southwesterly source, the low proportion of potassium feldspar in the sandstones, and the lack of palygorskite in the shales as compared to Iberian-sourced successions. The apparent inconsistency of detrital mica ages from these deposits, suggesting an Iberian (Hercynian) source with age 270–340 Ma, is attributed to the localization of rifting along a Paleozoic continental suture zone, the Hercynian Front. This tectonic front separates distinctly different geologic units (domains) on the conjugate margins. Such a suture would have generated a significant peripheral foreland basin across the now-submerged shelf regions of Newfoundland allowing transfer of sediment from the Iberian to the Newfoundland domains. Upon Mesozoic rifting and uplift, these mica-bearing sediments from the foreland basin succession could have been reworked and shed eastward into the nascent Atlantic Ocean basin at ODP Site 1276. Alternatively, Hercynian metamorphism and deformation might have extended onto what is now the southeastern Grand Banks, so that young detrital micas could have been derived directly from the Avalon Uplift. Results from this work are relevant to an assessment of sandstone distribution in parts of the Salar Basin and the expected diagenetic overprint for sandy reservoir facies.

Archean and Early Proterozoic Events Along the Snowbird Tectonic Zone in Northern Saskatchewan, Canada

The Snowbird tectonic zone is a major boundary structure in the Canadian shield, separating rocks of the Rae and Hearne lithostructural domains. Field investigations along portions of the Snowbird tectonic zone exposed in Northern Saskatchewan have revealed a major granulite facies mylonite belt that records deformation and metamorphism at several intervals in the Archean and early Proterozoic. Early assemblages are dominated by granulite grade gneisses that were deformed in a major left-lateral strike-slip shear zone at ca. 3200 Ma. The major episode of deformation along this portion of the Snowbird tectonic zone occurred at ca. 2600 Ma, again at granulite facies in a dominantly right-lateral strike-slip shear zone. Little evidence of Proterozoic deformation is recorded within this region of the Snowbird tectonic zone itself, but neighboring rocks in the Rae province to the west preserve granulite grade right-lateral fabrics dated at ca. 1850 Ma. These constraints suggest that the Snowbird tectonic zone was a focus of major regional deformation throughout the middle and late Archean, but that regional deformation was partitioned outward to the surrounding Rae (and Hearne?) province rocks during the early Proterozoic. These data suggest that the Snowbird tectonic zone was not a major continental suture zone in the early Proterozoic.

The Nain-Churchill Boundary and the Nain Plutonic Suite: A Regional Perspective on the Geologic Setting of the Voisey’s Bay Ni-Cu-Co Deposit

Northern Labrador is the site of two major lithotectonic elements of eastern Canadian Shield geology. First, it encompasses a Paleoproterozoic continental suture zone: Archean rocks of the Nain province tectonically juxtaposed against Paleoproterozoic and reworked Archean rocks of the Churchill province. The Nain-Churchill suture evolved between 1860 and 1740 Ma, impressing upon the Archean and Paleoproterozoic rocks a regional metamorphic and tectonic signature referred to as the Torngat orogen. An important component of the Torngat orogen is the Tasiuyak gneiss, a Paleoproterozoic metasedimentary unit that occupies most of the eastern margin of the Churchill province. Second, northern Labrador is the site of a world-renowned terrane of Mesoproterozoic anorogenic igneous rocks, the Nain Plutonic Suite, that sits astride the suture and acts as a stitching batholith between the Nain and Churchill provinces. The Nain Plutonic Suite, comprising amalgamated granitic, anorthositic, dioritic, and troctolitic intrusions, largely obscures the suture for nearly half of its total length. The Suite evolved between 1350 and 1290 Ma and is interpreted as a product of magmatism associated with limited intracontinental rifting of the conjoined Nain-Churchill crust above a mantle plume. The Voisey’s Bay Ni-Cu-Co deposit, serendipitously discovered in 1993, resides within one of the oldest troctolitic intrusions of the Nain Plutonic Suite, a 1334 Ma intrusion that transgresses a contact between Archean and Paleoproterozoic gneisses. Genetic controls on the setting of mineralization have been built upon the Nain-Churchill boundary as a zone of weakness that was exploited by mantle magmas and have implicated the Tasiuyak gneiss as a crustal sulfur source. Although the Archean-Paleoproterozoic junction at Voisey’s Bay is presently interpreted as representing the Nain-Churchill boundary, in reality the provincial affinity of the Archean gneisses is equivocal. This uncertainty dictates caution in assigning the Nain-Churchill boundary as the convenient transcrustal conduit that allowed the Voisey’s Bay mafic intrusions rapid transit from their mantle staging area. Modeling the sulfide deposit as being solely a result of sulfur saturation of mafic magma by proximal Paleoproterozoic Tasiuyak paragneisses is also questionable because other genetic processes may have been responsible; for example, desilication of gneiss is strongly indicated by aluminous refractory minerals in vestiges of (fused?) Tasiuyak gneiss. The spatial distribution of the mineralized rock at Voisey’s Bay, apparently within the subchamber plumbing system feeding a larger intrusion and within remnants of the larger intrusion itself, may serve as a paradigm for similar deposits in the Nain area, but the preerosional comparative morphology of that larger intrusion relative to the better preserved ones may never be known. A thorough understanding of the setting and genesis of the Voisey’s Bay deposit will better facilitate establishing exploration targets in other parts of the Nain region, in Labrador as a whole, and in other geologically similar regions of the world.