bluuebunny/crossref_metadata_2025_split huggingface dataset:
The kimberlite clan: relationship with olivine and leucite lamproites, and inferences for upper-mantle metasomatism
Authors: Dawson J. B. 
Published: April 2008

Summary
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

Summary
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

Summary
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

Summary
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

Summary
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

Summary
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

Summary
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

Summary
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

Summary

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

Summary
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

Summary

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

Summary
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

Summary

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

Summary
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

Summary
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.