bluuebunny/crossref_metadata_2025 huggingface dataset:
Kinematics of Basin and Range intraplate extension
Authors: Eddington Paul K. , Smith Robert B. , Renggli C.
Published: April 2008

Strain rates assessed from brittle fracture (associated with historic earthquakes) and total brittle-ductile deformation measured from geodetic data have been compared to estimates of palaeostrain, from Quaternary geology for the intraplate Great Basin part of the Basin and Range Province, western United States. These data provide an assessment of the kinematics and mode of lithospheric extension that the western U S Cordillera has experienced from the past few million years to the present. Strain and deformation rates were determined by the seismic-moment-tensor method using historic seismicity and fault-plane solutions for sub-regions of homogeneous strain. Contemporary deformation (with maximum-deformation rates) in the Great Basin occurs principally along the active seismic zones: (i) the southern Intermountain Seismic Belt (ISB), 4.7 mm a
−1
; (ii) along the western boundary, the Sierra Nevada front, 1.6 mm a
1
(28.0 mm a
−1
if the
M
8.3 1872 Owen Valley, California earthquake is included); and (iii) along the W-central Nevada seismic belt, 7.5 mm a
−1
. The integrated opening rate across the entire Great Basin is accommodated by E-W extension at 8–10 mm a
−1
to the N which diminishes to 3.5 mm a
−1
to the S. These results show 8–10 mm a
−1
contemporary extension across the entire Great Basin associated with earthquakes that compares to the ≤9 mm a
−1
value determined from intraplate tectonic models (constrained by satellite geodesy) implying that contemporary strain is generally released by earthquakes. Zones of maximum lithospheric extension correspond to belts of thin crust, high heat flow, and Quaternary basaltic volcanism, suggesting that these parameters are related through the mechanism of extension such as a stress relaxation, allowing buoyant uplift and ascension of magmas.

Contemporary strain and deformation rates have been determined from geodetic measurements yielding maximum deformation of 11.2 mm a
−1
in the Hebgen Lake, Montana portion of the Intermountain Seismic Belt; 3.6 mm a
−1
in the Walker Lane, Nevada area; and 2.5 mm a
−1
in the Owens Valley, California area adjacent to the Sierra Nevada front. Palaeostrain and deformation rates principally from Quaternary fault-displacement rates gave deformation rates of 7.4 mm a
−1
along the southern ISB. Geodetically determined deformation rates compare well with rates determined from seismic moments, whereas poorly constrained palaeo-strain rates from Quaternary geology are ∼10 times smaller than contemporary rates except in parts of central and southern California, Wyoming, parts of Utah, and along the Idaho-Wyoming border.
Footwall structural evolution of the Tucki Mountain detachment system, Death Valley region, southeastern California
Authors: Hodges K. V. , Walker J. D. , Wernicke B. P.
Published: April 2008

Tucki Mountain lies within the central portion of the Death Valley extended terrain. Its structure is dominated by the Cenozoic Tucki Mountain detachment system, a complex and long-lived extensional feature which places weakly metamorphosed-unmetamorphosed strata on middle to upper greenschist facies metamorphic rocks. Detailed analysis of tectonites in the footwall of the detachment system has led to the identification of seven phases of ductile structures. The earliest are latest Mesozoic-earliest Tertiary(?) isoclinal folds and associated schistosity which may have been related to compressional deformation. All subsequent generations of structures developed during Cenozoic extension at intermediate-upper crustal levels. Structures of
D
3
and
D
5
age are markedly asymmetric and record simple-shear deformation along the detachment system. Kinematic analysis of these structures indicates consistent NNW movement of the hanging wall relative to the footwall throughout the history of the fault zone. This extension direction is inconsistent with the overall N45W-S45E direction indicated for the Death Valley region by the orientation of major transfer faults like the Death Valley and Furnace Creek fault zones. The discrepancy may reflect late-stage changes in the regional extension direction from N-S to NW-SE, or a partitioning of overall NW-SE extension into domains of variably orientated local extension.
A compilation and regional interpretation of the northern North Sea gravity map
Authors: Zervos F.
Published: April 2008

A Bouguer anomaly map of the northern North Sea between the Shetland Islands and Norway has been compiled based on four different data sources. The gravity effect of the Devonian to recent sediments was calculated along six published seismic profiles crossing the area and a regional gravity anomaly map produced on the assumption that the sedimentary basin structure is two dimensional. Regional consideration of the gravity field revealed the presence of a mass excess beneath the low-density sediments within the Viking graben. This high-amplitude, long-wavelength anomaly is explained by thinning of the crust from 30 km beneath the flanks to 10 km beneath the centre of the graben, excluding the thickness of the sediments. Crustal thickness along the profiles was derived using the regional gravity field and from this a contour map of the Moho was produced. Predictions obtained from the gravity data confirm the crustal structure derived so far from seismic experiments in the North Sea. For comparison a three-dimensional model of crustal thickness was constructed and a second contour map of the Moho was produced. The Moho contour maps derived from the two-dimensional and three-dimensional models show close agreement, demonstrating that a two-dimensional approach is justified in this case.
Kinematics of the Sinai triple junction and a two-phase model of Arabia-Africa rifting
Authors: Courtillot Vincent , Armijo Rolando , Tapponnier Paul
Published: April 2008

The breakup of Arabia from Africa started at 40–50 Ma in the East African Rift system and concentrated from about 20 Ma around the Red Sea. Presently, the most active system centres along the Gulf of Aden, Red Sea and Dead Sea shear. Two less active features, the Gulf of Suez and East African Rift meet the main boundary at what have often been considered to be active triple junctions (the southern tip of Sinai and Afar). This paper focuses on the continental Sinai triple junction near 27°30′N. Almost no reliable kinematic data can be found in the Red Sea. Only total motion based on a fit of coastlines is available. Yet, the mechanical basis for this is now known to be wrong. We propose a mechanical interpretation that retains the same pole of ARA-NUB motion, with a reduced amount of total opening. The best data come from the Dead Sea shear: 100 km of left-lateral displacement occurred over the last 20, or more likely 15 Ma (subsidence event in the Gulf of Suez). The pull-apart basins in the Gulf of Elat provide the best constraints on the azimuth of SIN-ARA motion. Four models of SIN-ARA-NUB kinematics are proposed depending on whether the Gulf of Suez is; (i) now extinct, (ii) still opening in the original direction, (iii) in a state of left-lateral shear, (iv) in a state of right-lateral shear. Total SIN-NUB motion is unlikely to exceed 1–2 mm yr
−1
. We propose a two-phase history of break-up, first along the East African Rift/Red Sea/Gulf of Suez, superseded at ∼ 15 Ma by the present Gulf of Aden/Red Sea/Dead Sea shear. Distributed extension now occurs in the Afar and northern Red Sea deforming zones towards which three localized rifts propagate. The Sinai and Afar ‘triple junctions’ are actually kinks where plate boundaries that were not active at the same time are superimposed.
Slump strain in the Tertiary of Cyprus and the Spanish Pyrenees. Definition of palaeoslopes and models of soft-sediment deformation
Authors: Farrell S. G. , Eaton S.
Published: April 2008

Slumps exposed in Tertiary sediments in Cyprus and the Spanish Pyrenees vary in inferred translation distance from metres to hundreds and possibly thousands of metres. Contractional strain initiates open to tight upright to inclined folds with hinges that are sub-parallel to the strike of the palaeoslope. This strain is produced mainly by pure shear layer parallel shortening as the slump detachment propagates, but may involve a component of simple shear strain. Downslope translation of the failed unit commonly imparts a simple shear strain which tends to tighten folds and rotate fold hinges and fold axial surfaces. Fold hinges rotate towards the downslope direction, and fold axial surfaces rotate towards parallelism with the upper and lower surfaces of the slump. Coaxially refolded folds and sheath folds may also develop. Steeply plunging folds may initiate in slumps, due to the development of steeply dipping shear zones with margins sub-parallel to the downslope direction which accommodate differential downslope movements of segments of the slump. Fold axial surface attitude and fold profile may be used as a rough estimate of the degree of fold rotation.
Mass transfer in unmetamorphosed carbonates and during low-grade metamorphism of arenites
Authors: Clifford P. M. , Rice M. C. , Pryer L. L. , Fueten F.
Published: April 2008

Study of stylolites in Silurian carbonates of southern Ontario, and of spaced cleavage in Lower Palaeozoic meta-arenites of Nova Scotia, shows that diffusional or infiltrational mass transfer has been the dominant mechanism for dimensional change.

The carbonates show a quite specific pattern of stylolite growth in, and normal to, the mean plane of the stylolite, and there is always a low-porosity halo about each stylolite. Implied shortening usually exceeds the amplitude of any teeth in stylolites and may reach 5% of the ‘local’ sequence.

In the meta-arenites, increased inequancy, decreased long-axis fluctuation of quartz grains, lithon to cleavage, coupled with virtually constant grain dimension parallel to cleavage, suggest gross loss of volume. Modal and chemical analyses suggest that an average cleavage zone represents a dilation of −50%, expressed as shortening normal to cleavage. Bulk loss is about 10%, almost all SiO
2
.

In both cases, mass removal begins at zero or low levels of overall deformation. For the carbonates, total deformation is near zero; the process shuts down by self-suffocation. In the meta-arenites the process was probably partly driven by the conditions associated with greenschist-level metamorphism. In both cases, collapse across the surfaces of dissolution is nicely adjusted to keep the gross shortening approximately homogeneous.
The Kleszczów Graben (central Poland): reconstruction of the deformational history and inventory of the resulting soft-sediment deformational structures
Authors: Brodzikowski Krzysztof , Gotowała Roman , Kasza Ludwik , Van Loon Antonius J.
Published: April 2008

There is a deep tectonic graben near the village of Kleszczów, some 50 km south of Łódź. A Mesozoic substratum has subsided in the graben. The infilling consists of unconsolidated Tertiary and Quaternary deposits that reach thicknesses of up to 300 m; locally they may be even more than 400 m thick. The equivalent deposits outside the graben area are 5–7 (or more) times thinner.

The lowermost part of the graben contains the oldest infilling, probably indicating the very first moment of subsidence, and consists of late Oligocene deposits. The overlying sediments form an almost complete succession up to the Holocene and represent the best developed and most intensively studied Cenozoic section in Poland. The sections show a unique degree of exposure in continuous horizontal and vertical sections, well developed deformational structures and an exceptional thickness of the Cenozoic succession. There has been much interest in the geological history of the intensely deformed infilling of the graben. The reconstruction of the sedimentological and deformational history is the main topic of this contribution.
Endogenic processes as a cause of penecontemporaneous soft-sediment deformations in the fluviolacustrine Czyżów Series (Kleszczów Graben, central Poland)
Authors: Brodzikowski Krzysztof , Krzyszkowski Dariusz , Van Loon Antonius J.
Published: April 2008

Thick fluvial and lacustrine deposits of Quaternary age are rare in Europe. One such succession is found in the Kleszczów Graben. The high content of organogenic material is a specific characteristic. One of the major fluviolacustrine units in the graben is the Czyżów Series, formed during a Pleistocene interstadial. The series is characterized by abundant penecontemporaneous deformation structures whose formation was favoured by both the grain-size distribution (with a relatively high silt content) and repeated tectonic displacements that served as a trigger mechanism. The analysis of these structures forms the main topic of the present contribution. Endogenic activity within the graben emerged as the main cause not only of ‘normal’ tectonic deformations, but also of a large number of deformations that are usually considered to have a sedimentary or non-tectonic diagenetic origin.
Flame structures and associated deformations in Quaternary glaciolacustrine and glaciodeltaic deposits: examples from central Poland
Authors: Brodzikowski Krzysztof , Hałuszczak Andrzej
Published: April 2008

Glaciolacustrine and glaciodeltaic sediments in the Quaternary of the Kleszczów Graben show many types of penecontemporaneous deformations. Their formation was due to the existence of important vertical anisotropies of the mechanical properties. Flame structures and associated types of deformations occur frequently. These structures are described in this contribution and their origins analysed.
Alkaline rocks and their inclusions: a window on the Earth’s interior
Authors: Menzies Martin
Published: April 2008

The majority of inclusion-bearing alkaline rocks have a source region identical with that of ocean island basalts regardless of whether they were erupted through oceanic or continental crust. Models have been proposed where oceanic alkaline rocks are derived from the asthenosphere and during passage to the surface there have occurred variable amounts of interaction with lithosphere of the mid-ocean ridge basalt type. A similar scenario would account for most inclusion-bearing continental alkaline rocks. Genesis of micaceous kimberlites and lamproites, however, may require that asthenospheric plumes have impinged on ‘nuggets’ of aged heterogeneous enriched sub-continental lithosphere. This would lead to mixing of isotopically distinct asthenospheric and lithospheric melts.
The kimberlite clan: relationship with olivine and leucite lamproites, and inferences for upper-mantle metasomatism
Authors: Dawson J. B.
Published: April 2008

Kimberlites are rare ultrabasic potassic low-volume melts that originate in the diamond stability field of the upper mantle. They are petrographically complex, and wide mineralogical and chemical variations suggest that they should be regarded as a group or clan rather than a single narrowly-defined rock type. In S African kimberlites there is a major division between highly-micaceous (Group II) kimberlites that are mineralogically, chemically and isotopically distinct from poorly-micaceous (Group I) kimberlites.

Nd and Sr isotope studies of diamondiferous olivine and leucite lamproites from NW Australia indicate that they are isotopically similar to Group II kimberlites but are distinct from Group I. New bulk rock analyses of Group II kimberlites indicate closer chemical similarities with olivine lamproites than previously supposed.
Geochemistry of ocean island basalts from the South Atlantic: Ascension, Bouvet, St. Helena, Gough and Tristan da Cunha
Authors: Weaver Barry L. , Wood David A. , Tarney John , Joron Jean Louis
Published: April 2008

Basaltic and hawaiitic lavas from the S Atlantic Ocean islands of Ascension, Bouvet, St. Helena, Gough and Tristan da Cunha have been analysed for major elements and a wide range of trace elements. There is marked chemical (particularly trace-element) diversity between these islands which parallels observed Pb, Sr and Nd isotopic variations, and implies considerable large-scale heterogeneity in the source regions for ocean island volcanism. Trace-element and isotopic variation within individual islands suggests significant small-scale source heterogeneity.

Abundance ratios between highly-incompatible trace elements (Rb, Ba, Th, U, K, Ta, Nb and La) appear not to be fractionated during partial melting (except at low degrees of melting in the production of Tristan da Cunha lavas) and can be used to infer source characteristics. Lavas from the islands of Ascension, Bouvet and St. Helena have comparable highly-incompatible trace-element ratios (e.g. La/Nb, Th/Ta, La/Th, Th/U, Ba/La and Ba/Nb), with the exception of the apparent depletion of Rb and K relative to other highly-incompatible elements in St. Helena lavas. Tristan da Cunha and Gough lavas have similar La/Th ratios but much higher La/Nb, Th/Ta, Th/U, Ba/La and Ba/Nb ratios than do lavas from the other islands. These differences reflect depletion in Nb and Ta and enrichment in Ba relative to other highly-incompatible elements in Gough lavas.

The anomalous behaviour (enrichment) of Nb and Ta compared with other highly-incompatible trace elements, and specific consideration of relations between Ba, La and Nb, suggest that ancient subducted ocean crust, rather than primitive or depleted mantle, is the main component of the mantle source for ocean island basalts. Gough, Tristan da Cunha and Walvis Ridge lavas seem to be derived from such a source which has been contaminated by a component with high Ba/Nb, La/Nb and Ba/La ratios. Correlation between Ba/Nb and La/Nb in Gough basalts and hawaiites implies that these lavas contain variable proportions of this component. Pelagic sediment has the high Ba/Nb, La/Nb and Ba/La ratios required of this contaminant, and a few per cent of such material in the mantle source for Gough, Tristan da Cunha and Walvis Ridge lavas would account for their trace-element chemistry.
Outline of the petrology of the Kenya rift alkaline province
Authors: Baker B.H.
Published: April 2008

The Kenyan and N Tanzanian volcanic province contains sodic alkaline rocks ranging from melilitites and melanephelinites to transitional alkali basalts and their differentiates. Individual volcanoes display three principal magmatic suites: (i) nephelinitic; (ii) alkali basaltic; (iii) transitional basaltic. However, some large volcanoes contain more than one of these suites implying that parental magmas of variable alkalinity were available at certain times and places. A general decrease in alkalinity with time is detectable in the rift zone and for any time period there was a tendency for the least alkaline magmas to be erupted within the central and deepest part of the rift zone.

Compositional variation within the suites was largely controlled by low-pressure crystalliquid fractionation. Extended fractionation produced salic differentiates. Liquid fractionation caused upward segregation of phonolitic and trachytic magmas which were erupted in preference to more mafic magmas. Isotopic data suggest that crustal contamination did not occur on a large scale.
Alkaline magmatism subsequent to collision in the Pan-African belt of the Adrar des Iforas (Mali)
Authors: Liégeois J. P. , Black R.
Published: April 2008

The Pan-African Trans-Saharan belt in the Iforas displays a rapid switch from subduction and collision-related calc-alkaline to typical A-type magmatism, which is accompanied by transcurrent movements along major shear zones inducing weak distension. Detailed Rb-Sr geochronology and geochemical data point to different mantle sources for orogenic (lithospheric depleted mantle + oceanic crust) and within-plate magmatism (more primitive asthenospheric mantle). Both groups suffer lower-crustal contamination. A model is proposed whereby asthenospheric mantle originally underlying the subducted plate has risen to shallow depth beneath the continental lithosphere after the rupture of the cold plunging plate. This source, which is often proposed for alkaline rocks, explains the great similarity of oversaturated alkaline ring-complexes whatever their environment. The peculiarities of the alkaline province, for example the lack of Sn mineralization when compared with the Niger-Nigerian province, may be related to the nature and composition of the basement.
The Monteregian Hills and White Mountain alkaline igneous provinces, eastern North America
Authors: Eby G. Nelson
Published: April 2008

The Monteregian Hills and White Mountain provinces consist of stocks, plugs, ring-dyke complexes and several large granite bodies emplaced into Precambrian gneisses, flat-lying Cambro-Ordovician sediments and the deformed Lower Palaeozoic section of the Appalachian fold belt. Felsic rocks dominate in the Appalachian fold belt, while elsewhere mafic and ultramafic rocks are significant components of the plutons. Igneous activity extended from 240 to 90 Ma ago with two major periods of magmatism, correlated with events in the opening of the N Atlantic Ocean, occurring between 200–165 Ma and 140–110 Ma ago.

Five major rock series have been identified: (1) undersaturated CO
2
-rich rocks, carbonatite and alnöite; (2) moderately to strongly undersaturated diorites-nepheline syenites; (3) slightly undersaturated to slightly oversaturated pyroxenites-gabbros-diorites-syenites; (4) alkali syenite-quartz syenite-granite; (5) metaluminous biotite granite. Series (1), (2) and (3) magmas were drawn from an isotopically depleted mantle which was enriched in incompatible elements shortly before or synchronous with melting. These magmas were produced by variable degrees of melting of garnet or spinel lherzolite. Series (4) and (5) magmas represent partial melts of a heterogeneous crustal section consisting of both meta-sedimentary and meta-igneous rocks of either Grenville (Precambrian) or Lower Palaeozoic age.
Areal sedimentation rate patterns of the southern Nares Abyssal Plain, western N Atlantic
Authors: Kuijpers A. , Duin E. J. Th. , de Lange G. J.
Published: April 2008

The Nares Abyssal Plain (NAP) consists of pelagic and turbiditic sediments, which are difficult to date owing to a lack of microfossils. Lithological, geochemical and seismic records indicate that relative accumulation rates of late Quaternary turbiditic sediments in the NAP decrease eastward by more than a factor of 2. Significant lateral variations of the sedimentation rate also occur; a maximum average sedimentation rate of several decimetres per 1000 years has been deduced. The high accumulation rates result from deposition from distal turbidity flows that were entrained via a high-energy abyssal-current system from the N American continental slope or rise into the southern NAP.
Influence of bottom currents on sediment texture and sea-floor morphology in the Argentine Basin
Authors: Ledbetter Michael T. , Klaus Adam
Published: April 2008

The relative bottom-current speed below 4000 m in the Argentine Basin was inferred from the regional pattern of mean particle size of the non-biogenic silt fraction of about 300 sea-floor samples. The pattern reveals a strong deep western boundary current (DWBC) entering the Argentine Basin from the Georgia Basin through a gap in the Falkland Fracture Zone. The DWBC turns W and flows as a contour current along the Falkland Escarpment and Argentine continental margin. At the southern end of the Rio Grande Rise, the DWBC is deflected to the E and SE where it flows along the lower flank of the Mid-Atlantic Ridge as a weak basin-wide return flow. Several smaller return-flow gyres in the southern and western basin are delineated in the regional pattern. The DWBC is strongest along the western margin of the Argentine Basin where 3.5 kHz echograms reveal very prolonged echoes with no sub-bottom reflectors. That reflector pattern is indicative of coarse turbidites, some of which have been winnowed to produce coarse-grained lag deposits under the axis of flow.

In the interior of the basin, where bottom-current flow is weakest, sediment consists of clay and silt which is deposited in large migrating mud waves with wavelengths of 3–10 km and heights of up to 137 m (average 26 m). The mud waves consist of material swept into the basin by the DWBC and delivered to the basin margin by down-slope processes. Thus material is winnowed by the DWBC and deposited as a fine-grained chaff in abyssal antidunes which migrate to the centre of the basin and form thick drift deposits.
Regional setting and geophysical characterization of the Great Meteor East area in the Madeira Abyssal Plain
Authors: Searle R. C.
Published: April 2008

A detailed geophysical study of the Great Meteor East area centred near 31.5°N, 24.5°W in the Madeira Abyssal Plain is described in this paper. The area is some 200 km × 300 km in extent, and stretches from the lowermost NW African continental rise to the lower flank of the Mid-Atlantic Ridge on sea-floor 80–100 Ma old. It has been studied using long-range side-scan sonar (GLORIA), seismic reflection and 3.5 kHz profiling, bathymetry and magnetometry. The flat abyssal plain is bounded to the E by an abrupt change of slope at the base of the rise. The line of this slope change is also approximately the distal limit of debris flows and coarse-sand-based turbidites derived from the upper part of the continental slope. Side-scan sonar reveals diminishing sediment surface variation in progressively more distal parts of the plain. Abyssal hills some tens of kilometres across and a few hundred metres high stand up above the flat surface of the abyssal plain which is 5400 m deep. These hills, which are aligned NE-SW perpendicular to the palaeo-spreading direction, were produced by normal faulting at the axis of the Cretaceous Mid-Atlantic Ridge. They are covered mostly by pelagic sediment which can be traced below the abyssal plain as a drape over igneous basement. Consideration of seismic reflection profiles in combination with core data suggests that Neogene and Quaternary turbidites compose most of the uppermost 200–500 m of the abyssal-plain sediments.
Sedimentation on the Madeira Abyssal Plain over the last 300 000 years
Authors: Weaver P. P. E. , Rothwell R. G.
Published: April 2008

The sediments of the Madeira Abyssal Plain consist of thick turbidites (up to 5 m) separated by thin pelagic layers of clay, marl or ooze. Over most of the area the turbidites consist of ungraded massive silts and silty clays, sometimes with coarser bases. The mineralogy of these sediments suggests that they derive from the NW African margin. Isopach maps of individual turbidites show several of them thickening towards the W; their silty bases, however, coarsen towards the E suggesting transport from the NW African margin, followed by ponding on the plain. Some of the smaller turbidities are concentrated in particular areas of the plain. One group is concentrated in the N and NE, whereas a group of pure CaCO
3
turbidites are limited to the SW, suggesting an origin from the nearby seamounts.
Geotechnical characteristics of fine-grained turbidite sequences from the Nares Abyssal Plain
Authors: Shephard L. E. , Rutledge A. K. , Bryant W. R. , Moran K. M.
Published: April 2008

Geotechnical property analyses of 21 piston cores obtained from the Nares Abyssal Plain show variations that correspond to facies changes within fine-grained turbidite sequences. Variations in index properties within the turbidite sequences are gradational from the turbidite bases, often containing silt beds or laminae, upwards through the chemically laminated turbiditic clays and into the overlying pelagic and hemipelagic clays. Variations in index properties between sequences can be substantial, with water contents varying by more than 70% over sub-bottom depths of less than a few centimetres.

Consolidation test results indicate that most sediments appear to be normally consolidated to over-consolidated. The state of consolidation may vary over short depth intervals within turbidite sequences as a result of the pronounced changes in lithological type.

Clay-fabric analyses show significant differences in particle type and degree of sorting between the turbiditic and non-turbiditic clay facies. Particle-to-particle orientations appear quite variable within both clay facies, possibly as a result of modification of the primary fabric by post-depositional changes.
Proterozoic Mantle Heterogeneity: Geochemical Evidence from Contrasting Basic Dykes
Authors: Hall R. P. , Hughes D. J. , Friend C. R. L. , Snyder G. L.
Published: April 2008

Two swarms of early Proterozoic basic dykes occur in the Archaean craton of SW Greenland. The MD dykes are tholeiitic dolerites, while the BN dykes are norites. Virtually identical norites occur with tholeiitic dolerites in the Scourie swarm of NW Scotland and in the Laramie, Bighorn and Beartooth ranges of Wyoming, USA. Close geochemical similarities between typical coarse-grained norites and quenched varieties in SW Greenland and Wyoming suggest that the former type has not been strongly modified by crystal accumulation. The mineralogy and geochemistry of the quenched norites closely resemble those of boninites. Most of the norites have high MgO (
c.
13–18 wt%) and correspondingly high Cr and Ni, but they also have relatively high silica, alkali and large ion lithophile (LIL) element abundances. They are all light rare-earth element (REE) enriched (La = 40 times average CI chondrite, La
N
/Lu
N
c.
7). In contrast, most of the tholeiitic MD dolerites have fiat, unfractionated REE distributions. The tholeiites were probably derived from a fertile mantle source. However, the norites originated from a depleted harzburgitic mantle which had earlier borne Archaean komatiitic and tholeiitic magmas but which had been metasomatically replenished in LIL elements by fluids derived from Archaean oceanic and lower continental crustal material returned to the mantle during late-Archaean crustal thickening. Some major layered basic igneous intrusions, such as Bushveld and Stillwater, were derived by the mixing of two different basic magmas, which were equivalent to the tholeiitic and noritic (boninitic) dyke magmas of SW Greenland, NW Scotland and Wyoming. The apparently ubiquitous occurrence of tholeiite-norite associations suggests that similar large-scale mantle heterogeneities, due to the voluminous extraction of Archaean komatiitic and tholeiitic magmas, had become well established by the early Proterozoic.
Proterozoic Volcanogenic Mineralization Styles
Authors: Rickard D.
Published: April 2008

The Proterozoic contrasts with the Archaean and Phanerozoic in the apparent relative lack of volcanogenic mineral deposits. However, analogues of the three distinct volcanogenic mineralization styles recognized in the Recent record can be traced back into the Proterozoic and probably into the Archaean. These are: (1) massive sulphide deposits, (2) iron ores and (3) epithermal deposits. The Proterozoic is characterized by sedimentary basins and stable cratons, and Proterozoic ore styles tend to reflect these trends. However, volcanogenic mineralization appears to be at least as abundant in suitable successions in the Proterozoic as in other eras.

Although analogues can be found in the Proterozoic for the Phanerozoic volcanogenic ore styles, no satisfactory Recent analogue of the Broken Hill-type has been identified. Similar processes in the mid-Proterozoic and Phanerozoic record appear to have resulted in SEDEX Zn-Pb deposits of Mt Isa and Irish types. The Broken Hill-type was formed in a shallow aqueous/sub-aerial environment and, as with the coeval sedimentary ores, may have been modified by non-actualistic, early Proterozoic surface environmental conditions. In contrast, the major metal geochemistry of Proterozoic massive sulphide ores differs little from their Archaean or Phanerozoic counterparts, possibly reflecting their deep ocean genesis.

Formation of Proterozoic volcanogenic massive sulphide deposits was concentrated around 1.9 Ga. The ore districts are restricted to a linear zone, about 1000 km wide and some 8000 km long, on the eastern margin of the reconstructed Proterozoic supercontinent. It is suggested that the massive sulphide ore districts trace the scar of an early Proterozoic Wilson cycle. In contrast, Proterozoic Broken Hill-type ores and their sub-varieties display a wide geographic spread, consistent with their essentially continental character.
Geochemistry, Petrogenesis and Tectonic Environment of Circum-Superior Belt Basalts, Canada
Authors: Arndt N. T. , Brügmann G. E. , Lehnert K. , Chauvel C. , Chappell B. W.
Published: April 2008

The Circum-Superior Belt of Proterozoic basalts and sedimentary rocks surrounds, and in some places, unconformably overlies the Archaean Superior Craton in northern Quebec, Canada. Four main groups of volcanic rocks are recognized. The oldest unit, the Eskimo Formation on the Belcher Islands, consists of subaerial massive continental tholeiites with 4–7% MgO, relatively high SiO
2
and K
2
O, enriched LREE and other incompatible elements, negative Nb and Ta anomalies, and ɛ
Nd
(T = 1.8 Ga) of −7 to −8. The overlying Flaherty Formation contains largely subaqueous, massive or pillowed, mildly alkaline (transitional) basalts with relatively low SiO
2
, high alkalis and TiO
2
, enriched incompatible trace elements (including Nb and Ta), and ɛ
Nd
of + 3. Stratigraphically higher units (on the Ottawa Islands and the Chukotat Formation, Cape Smith Belt) contain subaqueous, massive or pillowed flows, and layered gabbro-peridotite flows with compositions ranging from MgO-rich ol-phyric basalt and evolved cpx-plag-phyric basalt to ol-cpx cumulate. Maximum MgO in noncumulate samples reaches 17 wt%, and trace element characteristics are MORB-like: LREE and other incompatible elements are depleted, and ɛ
Nd
is + 4 to + 5. The highest units, exposed on the Ottawa Islands, consist of komatiitic basalts and include unusual spinifex-textured layered flows. Although maximum noncumulate MgO contents are about 16 wt%, olivine xenocrysts in cumulate layers have the composition Fo
92
, and provide evidence of a komatiite (MgO = ∼ 22%) precursor magma. Komatiitic basalts are less depleted than the underlying Mg-basalts, with fiat to enriched LREE, negative Nb and Ta anomalies, and ɛ
Nd
= 0.

The chemical features of Eskimo Formation basalts and spinifex basalts can be explained by crustal contamination of parental magmas derived from depleted mantle with ɛ
Nd
= + 5: the continental tholeiites by 20–30% contamination of parental basalt and the spinifex basalts by about 7% contamination of parental komatiite. The Mg-basalt series results from partial melting of depleted mantle followed by low pressure fractional crystallization, while Flaherty Formation basalts come from a more enriched mantle source.

The stratigraphic sequence—(1) early contaminated continental basalt, (2) transitional basalt, (3) MORB-like basalt, (4) crustally-contaminated komatiite—is considered to have arisen during continental rifting, spreading, and then closure of an ocean basin surrounding the Superior Craton.
Metavolcanic Rocks of the La Ronge Domain in the Churchill Province, Saskatchewan: Geochemical Evidence for a Volcanic Arc Origin
Authors: Watters B. R. , Pearce J. A.
Published: April 2008

The La Ronge Domain in the Churchill Province of the Canadian Shield is a volcanic-sedimentary-plutonic belt of early Proterozoic age. Geochemical data, supported by geological evidence, strongly suggest that the La Ronge metavolcanics were emplaced in an island arc setting. The metavolcanic rocks range from basalt through andesite to dacite and rhyolite. Calcalkaline and tholeiitic types are dominant, with minor alkaline varieties. Classification based on the major elements produces ambiguous results because of mobility of these elements during metamorphism. However, relatively immobile, minor and trace elements provide reliable characterization, of the basalts in particular. The latter display distinctive trace element properties and MORB-normalized geochemical patterns similar to those of recent volcanic arc basalts, being enriched in incompatible elements of low ionic potential and depleted in those of high ionic potential. The calcalkaline basalts are, in addition, enriched in Th, P, Ce and Sm. A minor within-plate component is also displayed by some metabasalts, suggesting that magma genesis in parts of the La Ronge ‘arc’ involved sub-continental lithosphere. Rare earth element patterns show enrichment in the LREE with little or no Eu anomaly. Interpretation by means of trace element discrimination diagrams (incorporating elements such as Ti, Y, Nb, Zr, Cr, Th, Yb and Ce) provides clear evidence for emplacement in a volcanic arc tectonic setting.
Geochemistry, Petrogenesis and Tectonic Significance of the Early Proterozoic Loch Maree Group Amphibolites of the Lewisian Complex, NW Scotland
Authors: Johnson Y. A. , Park R. G. , Winchester J. A.
Published: April 2008

The early Proterozoic Loch Maree Group (LMG) is a supracrustal sequence occupying two belts, 5 km and 3.5 km wide respectively, at Gairloch and Loch Maree in the southern region of the Scottish Lewisian Complex. The LMG at Gairloch consists of about one-third metasediments, mainly metagreywackes, and two-thirds amphibolite. The LMG amphibolites are of igneous origin, possess a tholeiitic chemistry, and can be divided into two petrogenetic groups. Group A consists of the main amphibolite sheets, believed to represent lavas, which show little or no LREE enrichment and flat incompatible element profiles, with some LILE depletion. Their chemical signatures are similar to those of ocean-floor (MORB) basalts. Group B consists of thin amphibolites and biotite-hornblende schists, believed to be sills, which show LREE and LILE enrichment trends quite different from group A and more similar to those of the well-documented Lewisian Scourie dykes.

It is suggested that the voluminous group A amphibolites represent primitive basalts extruded rapidly, directly from the mantle, after a period of extensional rifting had caused significant crustal thinning. Subsequently, after the cessation of extension and the filling of the rift basin, later magmas, represented by the group B amphibolites, became subject to crustal contamination before being emplaced as sills within the supracrustal pile. An early, pre-rifting stage of extension is represented by the Scourie dykes, which possess normal continental basalt geochemistry.
Geochemistry and Petrology of Metavolcanic Rocks of the Early Proterozoic Mazaruni Greenstone Belt, Northern Guyana
Authors: Renner R. , Gibbs A. K.
Published: April 2008

The dominantly metavolcanic Issineru Formation (IF) forms the lower 5–7 km of the stratotype for the early Proterozoic Mazaruni greenstone belt, N Guyana. Basalts and gabbros comprise the lower part of the section. The upper half includes some basalts, but is predominantly intermediate and felsic rocks. Most of the Issineru Formation volcanics form a tholeiitic series but minor calcalkaline hornblende andesites and two-pyroxene basaltic andesites with alkaline affinities also occur. Issineru Formation tholeiitic basalts have flat, and slightly LREE-depleted patterns with overall abundances of about 10 × chondrites. Some porphyritic basalts have LREE-enriched patterns and may be calcalkaline. The basalts have similar high field strength element (HFS) patterns on multielement MORB-normalized diagrams, with fairly flat patterns and depletion in P and Ti. Their large ion lithophile element (LIL) contents are more variable, due in part to the LIL’s susceptibility to alteration, but range from slightly less than MORB to about 8 × MORB. These patterns are transitional between MORB and island arc tholeiites. Tholeiitic basaltic andesites and andesites have higher REE levels than the basalts and slightly LREE-enriched patterns (Ce/ Yb = 1.3–2). Calcalkaline hornblende basaltic-andesites and andesites have REE patterns with steeper slopes (Ce/Yb=4.2). All of the Issineru felsic rocks have high REE levels (La = 60) and relatively flat patterns with pronounced negative Eu anomalies. Elsewhere in N Guyana there are rhyolites with steeply-sloping REE patterns, which are considered part of the calcalkaline series. The two-pyroxene basaltic andesites with alkaline affinity have very steep REE patterns (Ce/Yb= 10). They may be sills associated with post Trans-Amazonian continental volcanics, rather than coeval members of the greenstone belt volcanism. The IF tholeiitic series can be qualitatively explained by fractional crystallization from a mafic parent with a flat REE pattern.
Alteration of Late Middle Proterozoic Volcanics and its Relation to Stratabound Copper-Silver-Gold Mineralization Along the Margin of the Kalahari Craton in Swa/Namibia and Botswana
Authors: Borg G. , Maiden K. J.
Published: April 2008

Metavolcanic and metasedimentary rocks of late Middle Proterozoic age are preserved in basins along the northern margin of the Kalahari Craton. Three broad stratigraphic divisions are recognized: a lower acid extrusive unit, a basic extrusive/red bed unit and a shallow marine unit.

The basalts of the basic extrusive/red bed unit are interpreted to be continental within-plate basalts. Copper mineralization occurs as native copper in amygdaloidal altered basalt, and as sediment-hosted, stratiform sulphide deposits at the base of the shallow marine unit.

The rocks have been strongly altered during regional metamorphism which reached temperatures up to 350°C. Massive domains were metamorphosed to oligoclase-chlorite-epidote-haematite rocks. In permeable zones such as flow top breccias, amygdaloidal zones, and regions of secondary, tectonically induced fracturing, the basalts have been altered to epidote-quartz-chlorite-haematite-muscovite rocks. During this epidote alteration copper, zinc and cobalt were leached from the basalts, thus providing a possible metal source for the sediment-hosted deposits.

A model for multiphase ore genesis is presented in which basalt alteration is the main metal source for epigenetic mineralization.
The Geochemistry of Upper Proterozoic Lavas From the Red Sea Hills, NE Sudan
Authors: Klemenic P. M.
Published: April 2008

The geochemistry of the Kadewēb and Homogar Volcanic Groups from the Red Sea Hills of north-east Sudan is discussed. Rb-Sr studies show that these lavas are Upper Proterozoic in age (720 Ma and 670 Ma respectively). The lavas of both groups display a calcalkaline character and are broadly similar in terms of their lithology, phenocryst mineralogy and major element chemistry. Geochemically the Kadawēb and Homogar volcanic rocks are similar to modern arc lavas associated with converging plate margins, although they tend to have higher concentrations of some HFS elements. Despite overall similarity, a distinction can be made between the two groups on the basis of the geochemistry of the basaltic components of the suites. The Homogar basalts are enriched in LIL and REE, having high La/Nb, (Ce/Yb)

n

and Ta/Yb ratios compared with those of the Kadawēb group. It is suggested that the older Kadawēb lavas were erupted along an intra-oceanic island arc system, while the younger Homogar lavas were extruded along a more evolved, possibly continental, arc margin. Differences between these Upper Proterozoic rocks and modern arc lavas may be explained as resulting from differences in the nature of the Proterozoic mantle.
Geochemical and Geochronological Signatures in Australian Proterozoic Igneous Rocks
Authors: Wyborn L. A. I. , Page R. W. , Parker A. J.
Published: April 2008

Most igneous activity in Australian Proterozoic domains is related to major orogenic events, or else to crustal extensional events in which there is an earlier volcanogenic rift phase overlain by a non-volcanic thermal subsidence phase. The igneous suites are either predominantly mafic, felsic or bimodal, and there are no major suites of intermediate igneous rocks. With time, the amount of igneous material decreases volumetrically in the Proterozoic sedimentary sequences and there are also distinct compositional changes. Depleted tholeiites are more common in the early Proterozoic sequences, whereas enriched tholeiites and alkaline rocks are dominant after 1800 Ma. Felsic igneous rocks are rare in the earliest Proterozoic sequences, but dominate in the period 1880-1400 Ma. They are also compositionally time controlled, and certain major magmatic felsic suites at
c
. 1860, 1790, 1740, 1670, 1500 Ma have distinct chemical parameters which are common to most domains. All the major suites of felsic igneous rocks show enrichment in K, Rb, Th, U, La, and Ce relative to Phanerozoic analogues. The felsic rocks are predominantly I-types, derived from Proterozoic infracrustal rather than Archaean precursors; rare S-types also appear to have Proterozoic sources. Our preferred model has the felsic igneous rocks derived from underplated sources which are progressively accreted to the base of the crust, as a result of a series of extensional episodes. There are no direct compositional equivalents of modern subduction-related igneous sequences suggesting that subduction as it is known today was not a dominant process in the production of the igneous suites. There are also no recognizable ophiolites, ocean-floor crust, or other evidence of a Wilson cycle.
Geochemical and Tectonic Evolution of the Proterozoic Telemark Supracrustals, Southern Norway
Authors: Brewer T. S. , Atkin B. P.
Published: April 2008

The Telemark supracrustals represent the only low-grade metamorphic lithologies within the Sveconorwegian Province of southern Norway. These supracrustals outcrop in five regions and are composed of metabasalts, metarhyolites and metasediments.

In each of the supracrustal regions three metamorphic events can be identified—burial, regional and thermal metamorphism. All of these metamorphic events have had variable effects on the major and trace element compositions of the acid and basic volcanics. Thus, in each region a ‘screened’ data set of relatively unaltered samples has been produced.

In all of the regions the volcanics are strongly bimodal and all possess geochemical signatures indicative of destructive plate margin volcanism. Thus, by combining the new chemical, lithological and structural data a new tectonic model is proposed, which suggests that the supracrustals were formed at a destructive plate margin. This model also satisfies the data for the Middle Proterozoic volcanic suites from Canada, which supports the hypothesis that the Sveconorwegian-Grenville orogen was developed along a continental margin and driven by plate-tectonic processes.
Upper Proterozoic Basaltic Volcanism in the Northern Massif Armoricain, France
Authors: Lees G. J. , Roach R. A. , Shufflebotham M. M. , Griffiths N. H.
Published: April 2008

A 2 km thick sequence of subaqueous basaltic rocks—the Hillion-Erquy Volcanic Formation (HEVF)—forms the lower part of the late Proterozoic Brioverian supracrustal sequence around the Baie de St. Brieuc, N Brittany. Basic dykes cutting the adjacent Pre-Brioverian crystalline basement (the Pentevrian) are interpreted as feeders to the volcanic sequence. As a result of the end Precambrian-early Palaeozoic Cadomian orogenic event, the basic volcanics and feeder dykes experienced a regional metamorphism varying from prehnite-pumpellyite facies in the east to upper greenschist (or amphibolite) in the west of the Baie de St. Brieuc region.

The compositions of the basic rocks are evolved, ranging from basalts to basaltic andesites. The suite is subalkaline and tholeiitic in character. Both major and trace element discriminants indicate the suite to be of continental flood basalt affinity. Incompatible element diagrams show evidence for a certain degree of crustal contamination during the evolution of the suite.

It is proposed that the HEVF was erupted in a rapidly subsiding ensialic basin, formed during a phase of crustal extension. Volcanism ceased abruptly and the basin rapidly filled with sediment.

A brief review is given of several other broadly similar basic sequences occurring in other parts of N Brittany. Their chemistry is compared with that of the HEVF and the implications for a geotectonic reconstruction of the region in late Proterozoic times discussed.