bluuebunny/crossref_metadata_2025 huggingface dataset:
Preliminary comparison of diagenetic stratiform copper mineralization from Redstone, NW Territories, Canada,and Kamoto, Shaban copperbelt, Zaire
Authors: Chartrand F. M. , Brown A. C.
Published: December 2007

Recent studies of stratiform copper mineralization in the Redstone copperbelt of the NW Territories, Canada, indicate analogies with sediments of modern intertidal and supratidal environments. Textural analyses show that mineralization occurred in microbial-laminated carbonate units during diagenesis following initial enrichment of the carbonates in iron sulphides and evaporitic sulphates.

Similar features have been described for the dolomitic beds hosting the well-known Kamoto ores of the Shaban copperbelt. The present work gives a brief comparison of the Redstone and Shaban host-rocks and their mineralization. Similarities include the interpreted shallow marine/red-bed sequence and the presence of stromatolitic and microbial-laminated structures in the carbonates. However, the Shaban host-rocks are more magnesian than those at Redstone, and silicification in Shaba surpasses the more normal diagenetic silification seen at Redstone. The copper, copper-iron and iron sulphides of Redstone and Shaba exhibit similar peneconcordant arrays across their respective basal carbonate horizons, but cobalt is important only in the Shaban mineralization. A more detailed comparison of host-rocks and mineralization in these two copperbelts is currently in progress.
Metallogenesis of an early Proterozoic foreland sequence, eastern Hudson Bay, Canada
Authors: Chandler F. W.
Published: December 2007

On the E side of Hudson Bay, the NW margin of the Archaean Superior Structural Province is overlain by an unmetamorphosed, little deformed, Early Proterozoic supracrustal sequence. The sequence is divisible into three parts, possible tectosomes, each characterized by a distinct style of mineralization. The arkosic, alluvial Richmond Gulf Group was shed eastward from a thermal uplift and has been preserved only in a later aulacogen, the Richmond Gulf Graben. It contains an apatite-cemented redbed. Secondly, a widespread post-aulacogen, carbonate-rich, W-facing miogeocline blanketed the region. A pyritic zone developed at its basal contact is particularly thick and hosts disseminated copper mineralization where formed on the arkose in the graben. A pyritic, brecciated, stromatolite-rich horizon within the carbonate contains galena and sphalerite. Thirdly, widespread iron formation and a local 60 cm thick manganese carbonate bed, overlying W-derived basalt flows, flysch and arkosic molasse, all of uncertain tectonic affinity, terminate the sequence.
Structural evolution of extensional basin margins
Authors: Gibbs A. D.
Published: December 2007

A new model for the structural development of extensional basins is outlined. The model suggests a close similarity in geometry between faults in high extension basins such as the North Sea and those in contractional zones. The geometric analogues are discussed and where necessary new terms are introduced to describe the structural style in the context of basin formation. Parallels are noted between contractional and extensional systems in deep crustal control on ramp and flat deformation zones which are now being interpreted from deep reflection work, e.g. MOIST and COCORP. The model is based on interpretation of industry seismic records particularly in the North Sea, and on field analogues of extensional faulting in the Basin and Range province of Nevada.
Long-lived fault zones and their influence on the tectonic development of the southwestern Barents Sea
Authors: Gabrielsen R. H.
Published: December 2007

The southwestern Barents Sea shows a complicated network of fault complexes and systems with various geometrical and genetic characteristics: trend, relation to basement, reactivation, fault plane geometry, and regional significance. Using these parameters, a classification of the fault complexes has been made, and a correlation to onshore faults has been attempted.

The area is subdivided into several fault blocks separated by deep-seated zones of weakness (first and second class fault systems). These are reactivated in connection with later (Mesozoic) movements in the area, and have acted as foci of strain so leaving the central area of the blocks relatively strain free. A third class of fault systems is of local significance, and reflects deformation limited to the interior of the fault blocks.
Rb-Sr whole-rock isochron ages,
δ
18
O values and geochemical data for the Sarn Igneous Complex and the Parwyd gneisses of the Mona Complex of Llŷn, N Wales
Authors: Beckinsale R. D. , Evans J. A. , Thorpe R. S. , Gibbons W. , Harmon R. S.
Published: December 2007

The Mona Complex exposed on the Llŷn peninsula includes the Sarn Complex, which is a suite of variably-deformed, plutonic igneous rocks (originally mapped partly as gneisses) ranging from gabbro to granite, and an inlier of high-grade paragneisses. Geochemical data for samples from the Sarn Complex may be divided into two distinct populations. The Rb-Sr data for samples from these geochemical groups define significantly different ages. The older age, 549 ± 19 Ma, initial
87
Sr/
86
Sr 0.7096 ± 3, is identified as the age of intrusion in late Precambrian to Cambrian times, while a younger age of 458 ± 16 Ma may approximate to the age of a resetting event. The Parwyd paragneisses yield an Rb-Sr whole-rock isochron age of 542 ± 17 Ma, concordant with the age of intrusion of the Sarn Complex, with an initial
87
Sr/
86
Sr of 0.7069 ± 4. For the Sarn Complex, δ
18
O values range from +9.5 to +12.0 per mil (SMOW), and in the Parwyd metasediments from +9.4 to +13.3 per mil. We conclude that the earliest magmatism and metamorphism in the Mona Complex of Llŷn occurred during late Precambrian to Cambrian times, broadly contemporaneous with similar events in Anglesey. The high initial
87
Sr/
86
Sr and high δ
18
O values for the Sarn Complex are most simply interpreted as reflecting either a crustal source-region or incorporation of a significant proportion of crustal material (such as the Parwyd gneisses) into a mantle-derived magma.
Regional O-, Sr-, and Pb-isotope relationships in late Cenozoic calc-alkaline lavas of the Andean Cordillera
Authors: Harmon R. S. , Barreiro B. A. , Moorbath S. , Hoefs J. , Francis P. W. , Thorpe R. S. , Déruelle B. , McHugh J. , Viglino J. A.
Published: December 2007

Quaternary-Recent volcanism in the Andes has occurred in three regions: 45–33°S, 28–16°S and 2°S–5°N, each of which has a distinct plate tectonic setting and contains volcanic suites with different chemical and isotopic characteristics. Isotope ratios of O and Sr are lowest and those of Pb least variable in the southern volcanic zone (SVZ) where medium-K lavas have isotopic characteristics equivalent to volcanics from intraoceanic arcs where continental crust is absent. The SVZ lavas were probably derived from an asthenospheric mantle source above a shallow Benioff zone. Parental basaltic magmas rose largely unmodified through thin continental crust, where differentiation occurred by low-pressure crystal fractionation without concurrent modification of isotopic composition. The slight enrichment of
206
Pb in the northern volcanic zone (NVZ), where the crust is thin and the Benioff zone deep, suggests a greater subduction-zone component for parental medium-K magmas. The slight
18
O enrichment suggests a small amount of lower crustal interaction. Isotope ratios of O and Sr are highest and those of Pb most variable in the central volcanic zone (CVZ). Parental magmas in the CVZ were probably generated within the same mantle source region as those in the SVZ and NVZ. Subsequently, during transit through the exceptionally thick continental crust of the central Andes, high-K magmas were produced by a combination of bulk contamination in the lower crust and later low-pressure fractional crystallization-assimilation processes in the upper crust which altered both chemical and isotopic compositions.
New geological and geochronological data from the Colombian Andes: continental growth by multiple accretion
Authors: McCourt W. J. , Aspden J. A. , Brook M.
Published: December 2007

The regional geology of Colombia consists of a composite Palaeozoic to Cenozoic sedimentary basin overlying Precambrian shield rocks, which are exposed in the Eastern Cordillera, Magdalena Valley and at least part of the extensive Llanos Plains. A Palaeozoic metamorphic schist belt with abundant post-metamorphic igneous rocks comprises the Central Cordillera. A Mesozoic volcanic suite extends from the western flank of the Central Cordillera and accounts for the bulk of the Western Cordillera. A Cenozoic sedimentary sequence, probably overlying a Late Cretaceous oceanic crust and fore-arc complex, forms the Pacific Coastal Plain. The continental edge of central and southern Colombia is interpreted as a composite margin made up of successively-accreted oceanic island arc related sequences of Palaeozoic, Early Mesozoic and Late Cretaceous ages. The present-day cycle, which began in the Miocene, is not associated with island arc volcanism.
Cretaceous tholeiitic volcanic rocks from the Western Cordillera of Colombia
Authors: Millward D. , Marriner G. F. , Saunders A. D.
Published: December 2007

The Western Cordillera of Colombia, underlain by crust of probable oceanic character, is constructed almost entirely of Cretaceous sedimentary and volcanic rocks. The Diabase Group consists of tholeiitic basalts, dolerites and gabbros, with thin intercalations of chert and shale. Two major sedimentary sequences, one predominantly arenaceous, the other argillaceous, are conformable with the volcanic rocks. Diabase Group rocks possess a limited compositional range and show only minor magmatic evolution. The main geochemical characteristics include low concentrations of LIL elements, light-depleted to slightly light-element-enriched chondrite-normalized REE patterns, and high Nb and Ta contents relative to N-type MORB. Detailed studies of minor and trace elements indicate that the rocks have a transitional nature, similar to T-type MORB. The Western Cordilleran sequence probably formed in an extensive oceanic flood basalt province, similar to the Caribbean or Nauru Basin, adjacent to the margin of southAmerica
Palaeomagnetic studies of a Cretaceous molasse sequence in the central Andes (Coloso Formation, Northern Chile)
Authors: Turner P. , Clemmey H. , Flint S.
Published: December 2007

The Coloso Formation is an early Cretaceous conglomerate-dominated molasse sequence in northern Chile. The sequence was deposited rapidly and records the unroofing of granodioritic and andesitic source rocks. Thin sand units throughout the sequence have been studied palaeomagnetically; the results are complex but can be related to the diagenesis of arid-zone alluvium. Natural remanent magnetization (NRM) may be normal or reversed but is usually a combination of the two. Isolation of magnetic components by vector analysis of thermal and AF demagnetization data yields both normal and reversed components which are approximately antiparallel and yield an average pole position at 70°S, 192°E. This is consistent with late Cretaceous data from northern Chile but not with the established apparent polar wander (APW) path for South America. It may indicate late Cretaceous remagnetization but is considered to be more likely an indication that the present APW path assumes no tectonic rotation and that important relative movements have taken place between northern Chile and Argentina.
The Andean orogenic period and its major structures in central and northern Peru
Authors: Mégard F.
Published: December 2007

The Andean orogenic period began in the Albian and consisted of at least 6 discrete compressional phases which are Albian to late Miocene in age. Deformation migrated progressively toward the Amazonian foredeep. The major Andean structures in central and northern Peru are related to the Eocene (Incaic), early Miocene (Quechua 1) and late Miocene (Quechua 3) phases. The Maranon thrust and fold belt (MTFB) which bounds the western Cordillera to the NE rose in the Eocene. It was reactivated partly by the Quechua 1 phase, which also folded the Altiplano farther E and possibly part of the eastern Cordillera. The sub-Andean thrust and fold belt (STFB) is related to the Quechua 3 phase. In both MTFB and STFB the imbricate thrusts merge at depth in large décollement faults. Gravity sliding does not play a dominant role, for these faults have to be rooted westward in the pre-Andean basement. The corresponding crustal shortening played a significant part in the creation of the sialic root of the Andes.
Reflectances of ophiolite rocks in the Landsat MSS bands: relevance to lithological mapping by remote sensing
Authors: Rothery D. A.
Published: December 2007

Radiometry performed
in situ
on natural rock surfaces in the Oman ophiolite demonstrates the muting of spectral distinctions between rock types, due to metamorphism, alteration and weathering. Within class variation and between class overlaps can be minimized by intensity normalization. The use of intensity normalized spectral bands enables maximum-likelihood classification to discriminate some lithologies more effectively than by using ordinary MSS bands. However there is a trade-off between reduction of overlapping reflectances between different rock types and loss of albedo information which is diagnostically useful in some cases.
The Pleistocene succession of the Severn Estuary: a revised model based upon amino acid racemization studies
Authors: Andrews J. T. , Gilbertson D. D. , Hawkins A. B.
Published: December 2007

This paper reports the results and stratigraphic implications of amino acid racemization studies of fresh- and/or inter-tidal molluscs from: (i) the important Pleistocene sequences of Kenn Church, New Blind Yeo Drain, Yew Tree Farm and Kenn Pier (Avon County); (ii) the previously undescribed Pleistocene beach deposits beneath the Llanwern steel mill, South Wales; and (iii) re-prepared and re-analysed shells from the Th/U-series dated beach deposits in Belle Houge Cave, Jersey, Channel Islands.

These studies indicate the complexity of sea level change in the area. In amino acid Group II (Oxygen isotope stage 5e; 120,000–130,000 years BP) sea level reached 4–5 m OD and possibly 12 m OD. This episode is correlated with similar Group II beach deposits on the Gower coast, and freshwater deposits in SW and SE Britain. The data suggest these Group II deposits are different in age from the Ipswichian interglacial, inter-tidal deposits at Selsey.

A Group IV age is attributed to freshwater deposits at Yew Tree Farm and Kenn
Pier
which collected just above a contemporary sea level or included re-worked earlier inter-tidal rocky shore faunas. These are provisionally correlated with the Group IV deposits in the Thames Estuary at Purfleet (
c.
400,000–600,000 years BP). Since these deposits are thought at post-date glacial deposits previously identified at Kenn and Court Hill, Avon, this glacial episode is now regarded as being much older than the Wolstonian glacial stage with which it had been correlated previously.
Nannoplankton zonation and the Palaeocene/Eocene boundary beds of NW Europe: an indirect correlation by means of volcanic ash layers
Authors: Knox R. W. O’B.
Published: December 2007

Correlation of early Palaeogene ash layers in DSDP Hole 550 (Leg 80, Goban Spur) with those of the North Sea Basin indicates that the base of nannoplankton zone NP10 approximates the base of the Sele Formation of the North Sea area, the bases of the Danish ‘ash-series’, the German ash-bearing Untereozän 1, and the Paris Basin Sparnacian, and the top of the Woolwich & Reading Bottom Bed of the London Basin. If, as is now common practice, the Palaeocene/Eocene boundary is placed at the NP9/NP10 boundary, then the units listed above would constitute the basal units of the Eocene in their respective countries. The cessation of large-scale pyroclastic activity took place at about the middle of the Zone NP10 interval.
The Pontesford Lineament, Welsh Borderland
Authors: Woodcock N. H.
Published: December 2007

The NE-trending Pontesford Lineament centres on the Pontesford-Linley and adjacent faults in the Shelve/Longmynd inlier. To the NE it is usually drawn along the SE edge of the Cheshire Basin and to the SW along a variety of lines parallel to or converging with the Church Stretton Fault. New mapping and reassessment of published data suggest that SW of the Pontesford-Linley Fault the lineament is logically drawn along the Clun Forest Disturbance, a post-Pridoli (late Silurian) fold/fault belt, then across the NW side of the Llandrindod inlier to flank the Tywi Lineament.

The lineament was intermittently active at least from mid-Ordovician through to Triassic time, with both strike-slip and dip-slip displacements that involved the upper part of the Precambrian basement. Late Ordovician and early Silurian strike-slip was particularly important. The lineament controlled the sites of several Silurian sedimentary basins. Ordovician contrasts across the lineament can be explained either by contemporary fault control or by subsequent large displacements juxtaposing unlike terranes.

The long history of reactivation and basement control on the Pontesford Lineament is typical of lineaments throughout Wales and the Borderland. It suggests that both Caledonian and Variscan structural trends N of the Variscan Front may be inherited from older, pre-Silurian or even Precambrian, basement structures.
Geological evolution of the trachytic caldera volcano Menengai, Kenya Rift Valley
Authors: Leat P. T.
Published: December 2007

Menengai is a trachytic central volcano underlain by a high-level magma chamber. Activity started shortly before 0.18 Ma, with the growth of a low-angle trachyte lava shield having a volume of about 30 km³. Krakatau-style collapse to form a 77 km² caldera within an embayed ring-fracture was accompanied by the eruption of two ash-flow tuffs representing a combined magma volume of about 50 km³. The eruption of both tuffs was preceded by an air-fall phase. Both were emplaced as single flow units, and have intermediate aspect ratios of
c.
1:4000. The tuffs cover an area of
c.
1350 km², including outcrops higher than the eruptive vents. During post-caldera times, some 25 km³ of magma was erupted, mostly as lava flows which now cover the caldera floor, but also as cinder cones and plinian-type tephra sheets. The chronology of syn- and post-caldera events is based on correlation with dated fluctuations in the levels of nearby lakes, suggesting that the two ash-flows may have been erupted at about 29 000 and before 12 850 years ago. Lake sediments inside the caldera provide evidence for a late intra-caldera lake from about 10 300 to 8300 years B.P.
Comparative geobarometry of pelitic hornfelses associated with the Newer Gabbros: a preliminary study
Authors: Droop G. T. R. , Charnley N. R.
Published: December 2007

Physical conditions of contact metamorphism have been estimated for pelitic hornfelses collected from the inner aureoles of the Morven–Cabrach, Knock, Huntly and Belhelvie ‘Newer Gabbro’ intrusions using a few well-calibrated mineral equilibria.

Inner aureole temperature estimates range from 700°C to 850°C. Calculated pressures all fall between 4 kbar and 5 kbar, implying the existence of between 15 km and 18.5 km of overburden at each locality at the time of intrusion of the Newer Gabbros.

The results indicate that the four Newer Gabbros mentioned above were all emplaced at a similar crustal level, and that there has been no net regional tilting of the NE Dalradian since then. The results are not incompatible with some large-scale, late-stage (D
3
) gentle folding as implied by the regional isograd pattern.
Metamorphic conditions in central Connemara, Ireland
Authors: Treloar P. J.
Published: December 2007

Metamorphic conditions have been estimated for an area in central Connemara that includes Cashel and Lissoughter Hill. Pressures lie in the range 3.0–6.0 kbar. Temperatures increase from 625–675°C on the N slopes of Lissoughter Hill to 650–700°C on the S slopes, to 700–750°C in the migmatized metasediments N of the Cashel ultrabasic intrusion before increasing sharply towards the contact. Away from the area adjacent to the intrusion a horizontal thermal gradient of between 30°C and 40°C/km may be inferred. Differences in the relative spatial position with increasing temperature of the staurolite-out, muscovite and quartz-out and beginning of partial melting isograds imply that the rocks exposed at the eastern end of the steep belt at Maam Cross were metamorphosed at a lower pressure than those farther W at Recess. Similarly, the stable co-existence of muscovite and quartz in the Recess area may indicate that the second sillimanite isograd dips towards the S, a condition that would be consistent with sub-vertical isotherms associated with a thermal anticline centred on the Cashel volcanic arc.
Geothermometry and geobarometry of inverted metamorphic zones in the W central Scottish Dalradian
Authors: Watkins Keith P.
Published: December 2007

The attitude of the garnet isograd surface in the Balquhidder region of the Scottish Dalradian is such that the metamophic zones must be inverted, higher-grade rocks overlying lower-grade rocks. Textural and structural information indicates that peak Barrovian metamorphism post-dates emplacement of the Tay Nappe. As post-metamorphic folds are open and upright, no tectonic means of overturning the isograd surface can be invoked. Metamorphism must have occurred in a negative thermal gradient.

The metamorphic temperature at the garnet isograd is about 500°C and increases along a negative metamorphic gradient, perpendicular to the isograd surface, of about 27°C/km. Metamorphic pressure is about 9 kbar.

The initial temperature inversion was probably caused by emplacement of the Tay Nappe. It is unlikely that subsequent conductive thermal relaxation alone would have caused inverted metamorphic zonation. However, the relaxative process may have been sufficiently modified, by convective transfer of heat by fluid evolved from metamorphic reactions, for this to occur.
Northern Scotland as an Atlantic–North Sea divide
Authors: Watson Janet
Published: December 2007

President's anniversary address 1984

From the end of the Caledonian orogeny until the present, northern Scotland has acted as a structural high separating the North Sea and Atlantic basins or their forerunners. The geological history of this positive massif is discussed in relation to that of the adjacent cratonic basins, with the objective of determining the extent to which it adapted to regional tectonic developments and the manner in which its responses were modified by the pre-existing basement structure.

From Devonian to early Palaeocene times, the massif was generally low-lying and its behaviour with respect to vertical movements was essentially neutral. The land surface carved in the crystalline basement during the Devonian period was preserved with relatively small modifications and little metamorphic detritus was supplied to the adjacent basins. Tectonic events of major importance in the basins, such as the formation of the North Sea and Hebridean rift systems, produced little change of conditions in the intervening massif. It is inferred that the underlying keel of lithospheric mantle was protected from the effects of lithospheric stretching.

The uneventful history of the massif was interrupted by two phases of mafic igneous activity—in late Carboniferous and Palaeocene times—which record responses to regional plate motions. The formation of the Palaeocene igneous province was followed by a unique phase of uplift as a result of which the ancient sub-Devonian land surface was elevated and dissected and abundant crystalline detritus was supplied to the North Sea. The style of these events and the anomalous character of the continental margin off NW Britain may have been determined by the refractory nature of the Precambrian basement which inhibited stretching prior to sea floor spreading.
U-Pb zircon dating of granitoid plutons from the West Coast Province of Peninsular Malaysia
Authors: Liew T. C. , Page R. W.
Published: December 2007

U-Pb zircon dating of granitoid plutons from the West Coast Province of Peninsular Malaysia yields precise and unambiguous ages of 198–220 Ma (late Triassic). These ages are considerably older than previously reported K-Ar mica ages which have been largely reset, but agree with or are younger than Rb-Sr whole rock apparent ages. The U-Pb zircon ages are considered to represent the best estimate of the intrusive ages of the plutons. Some of the problems associated with the interpretations of the pre-Triassic Rb-Sr whole rock apparent ages are discussed here and alternative ways to explain these ages in the light of recent work are considered. Reversely discordant patterns displayed by zircon populations from a number of plutons yield consistent upper intersection ages of ~1500–1700 Ma that approximately date source-derived, inherited zircon components. The Precambrian ages of these inherited zircon components suggest that the West Coast Province S-type granitoids are derived from sedimentary sources recycled, at least in part, from mid-Proterozoic crystalline basement sources.
The North Wales earthquake of 19 July 1984
Authors: Turbitt T. , Barker E. J. , Browitt C. W. A. , Howells M. , Marrow P. C. , Musson R. M. W. , Newmark R. H. , Redmayne D. W. , Walker A. B. , Jacob A. W. B. , Ryan E. , Ward V.
Published: December 2007

The intra-plate earthquake on the Lleyn Peninsula, North Wales afforded a rare opportunity to obtain accurate focal parameters for the main shock and the aftershock sequence. The focal mechanism does not correlate with movement on any of the large faults showing surface expressions in the area but does indicate a NW–SE oriented maximum compressional stress direction, in agreement with measurements made in other parts of Britain. Aftershock activity was concentrated around the focus of the main shock at depths of between 21 and 24 km. The depth indicates an unusually deep transition between brittle and ductile zones that is possibly a consequence of low heat flow in the region.
Structural setting and geochronological significance of the West Highland Granitic Gneiss, a deformed early granite within Proterozoic, Moine rocks of NW Scotland
Authors: Barr D. , Roberts A. M. , Highton A. J. , Parson L. M. , Harrisg A. L.
Published: December 2007

A re-investigation of the six major outcrops of granitic gneiss within the Glenfinnan and Loch Eil divisions of the Moine has shown that they are a suite of deformed and metamorphosed granite intrusions. They record all the tectonic events recognized in their host metasediments, while the absence of thermal aureoles suggests that their emplacement was synmetamorphic, i.e. syn-D
1
rather than pre-D
1
. Their dominant gneissic fabric is of S
2
age, but in low-strain augen an earlier S
1
fabric is preserved which is cut by MP
1
leucocratic segregations contemporaneous with, but compositionally distinct from, those in the pelitic regional migmatites of the Moine. The deformed granites underwent local MP
2
remelting and have been widely intruded by later, Caledonian pegmatites. The gneiss has a uniformly granitic
(sensu stricto)
bulk composition, regardless of country-rock lithology or location within the Glenfinnan and Loch Eil divisions. It has strongly S-type chemical affinities, being reduced and peraluminous with a high initial
87
Sr/
86
Sr ratio. It was probably produced at least in part by partial melting of Moine-like rocks at depth. A published Rb/Sr whole-rock isochron of
c.
1028 Ma probably dates intrusion of the Ardgour body and confirms the reality of a 'Grenvillian' event in the Glenfinnan and Loch Eil divisions of the Moine.
Alkaline igneous rocks: a review symposium
Authors: Fitton J. G. , Upton B. G. J.
Published: December 2007

Report of a meeting of the Volcanic Studies Group of the Geological Society held at Heriot-Watt University, Edinburgh, on 13–16 September 1984. The organizers were Dr J. G. Fitton and Professor B. G. J. Upton. The papers presented at the symposium will be published as a Special Publication of the Geological Society.

The symposium was held to mark the passage of 10 years since the publication of the review volume The Alkaline Rocks, edited by Henning Sørensen, who opened the meeting and presented a summary of the proceedings at the end. Two and a half days were devoted to the presentation of papers and one and a half days to local field excursions. The meeting was well attended with about 150 delegates, 60 of whom were from overseas. In addition to the 30 papers presented there were several poster displays.

Presentations included specific case histories of individual alkaline volcanoes and regional reviews of alkaline rock occurrences in both continental and oceanic settings. Reviews were presented of the occurrence of various alkaline associations, including kimberlite, lamproite, lamprophyre, nephelinite, and carbonatite, and the impact of isotopic and experimental petrological studies on the subject was emphasized.

Alkaline magmas are characterized by high concentrations of large-ion lithophile elements (LILE). The causes of this have been the subject of debate over the past decade, with metasomatic enrichment of the mantle source before melting being a currently favoured hypothesis. Studies of mantle enrichment and melting processes are clearly essential to an understanding of alkaline magma
Placer formation by fluvial degradation of an alluvial fan sequence: the Proterozoic Carbon Leader placer, Witwatersrand Supergroup, South Africa
Authors: Buck S. G. , Minter W. E. L.
Published: December 2007

The Carbon Leader placer is one of a sequence of placers formed in the Carletonville goldfield, South Africa, in response to periodic fluvial degradation of a series of alluvial (fluvial) fans spreading southward into the Witwatersrand Basin. The Carbon Leader placer consists of broad, interconnecting, low-sinuosity ribbon bodies of conglomeratic and arenaceous sediments, which occur in two palaeodrainage systems, initially centred some 12 km apart. Ribbon bodies range to 500 m in width and 2 m in thickness, and accumulated from extensive migration of belts of small braided streams. Lithofacies and placer mineral distributions imply that fluvial current velocities were generally greater in the central parts of the palaeodrainage systems, decreasing towards the margins.
The Cae Coch volcanogenic massive sulphide deposit, Trefriw, North Wales
Authors: Ball T. K. , Bland D. J.
Published: December 2007

The Cae Coch massive pyrite deposit is described and re-interpreted as a sedimentary volcanogenic orebody of about 200 000 tons. It is located at the contact between the Ordovician Dolgarrog Volcanic Formation (basic tuffs and lavas) and the overlying Llanrychwyn Slates (pyritic black mudstone) and close to a major intrusive rhyolite. The minerals are dominantly pyrite and quartz with minor molybdenite and the deposit shows zonal variation in trace Ba, Cu, Zn, Ni, and Co and detectable concentrations of Sn, TI, Hg and Ag. The footwall rocks have been highly altered and the basic rocks show increases in K, Ba, S and carbonate and losses of Cu, V and Co and probably light rare earth elements (LREE). A progressive gain in K, Ba and Pb and a depletion of Ga, Sr, Sn, Zn and REE is observed with increasing alteration of the rhyolite. The deposit is concluded to be the result of accumulation in a brine pool but with a component of debris flow of uncertain provenance. The deposit resembles Kuroko style mineralization and seems to be unique in the immediate district.
Graptolites as indicators of the temperature histories of rocks
Authors: Goodarzi Fariborz , Norford B. S.
Published: December 2007

A pilot study of the optical properties of graptolite skeletal materials shows progressive changes of reflectance, bireflectance, refractive index and absorptive index, produced by temperature changes related to depth of burial. Samples of Lower Ordovician to Upper Silurian graptolites were selected from across Canada, from locations where the conodont alteration indices of adjacent conodont collections provide a control index of temperature. The reaction to temperature of graptolite skeletal material appears to be similar to that of bitumen, but somewhat different to that of the vitrinite and liptinite maceral components of coal. Maximum reflectance in oil and bireflectance appear to be the characteristics most diagnostic of maximum temperature, and both show progressively higher rates of increase at higher temperatures. These factors show potential for more sensitive discrimination of temperatures at high levels of conodont alteration indices. The optical properties of graptolites may perhaps provide a useful tool for unravelling the post-depositional histories of Lower Palaeozoic rocks in deformed belts.
Chronology of granite magmatism and associated mineralization, SW England
Authors: Darbyshire D. P. F. , Shepherd T. J.
Published: December 2007

An Rb-Sr study of Hercynian magmatism in SW England has established that the major granites were emplaced
c.
290–280 Ma and that the inferred sequence of intrusion shows no geographic pattern. High initial
87
Sr/
86
Sr ratios (0.710–0.716) are compatible with the ‘S–type’ and peraluminous mineralogical characteristics of the granites. REE distributions are consistent with those for normal leucogranites, in that they have a high LREE/HREE ratio and negative europium anomaly. However, the REE profiles for Bodmin and Carnmenellis contrast markedly with those of Land’s End and Dartmoor, implying differences both in the chemistry of the source region and in conditions of crystallization. Ages obtained for the Brannel and Wherry elvans (quartz porphyry dykes) suggest more than one episode of injection during the interval
c.
280–270 Ma. An independent fluid inclusion Rb-Sr age for the South Crofty Sn–W deposit (269 ± 4 Ma) has shown that in the Camborne-Redruth area, the main stage Polymetallic mineralization is significantly younger than the Carnmenellis host granite. There is little evidence to support the idea of an older Westphalian event, and thus in the broader European Hercynian context the granites show greatest affinity with the younger Sn–W granites of Portugal.
The Palaeogene time-scale: radiometric dating versus magnetostratigraphic approach
Authors: Odin G. S. , Curry D.
Published: December 2007

Numerical geological time-scales rest firmly on the analysis of radiometric data. Since the discovery of oceanic magnetic stripes and their signification, alternative time-scales have been constructed whose numerical predictions are based on the hypothesis of a constant rate of sea-floor spreading. The most recent such scale uses only five tie-points, plus present day, to subdivide a span of over 80 million years. The present paper demonstrates that the rate of ocean-floor spreading has varied markedly, and without obvious pattern both in time and in space. Thus, magnetostratigraphic scales based on large interpolations are fundamentally flawed. An alternative scale, based on a consideration of all radiometric data and using magnetostratigraphic data only for small interpolations is commended.
U-Pb Isotopic evidence for the accretion of a continental microplate in the Zalm region of the Saudi Arabian Shield
Authors: Stacey J. S. , Agar R. A.
Published: December 2007

The area covered by this work includes three of the main tectonic units of the Arabian Shield: the Afif continental terrain, the Nabitah suture with its associated mobile belt, and the Asir ensimatic arc terrain. The geology of the Zalm area is well understood, and this geochronologic and isotopic study confirms that the southern Afif terrain was a continental microplate in the late Proterozoic. The study also provides a time frame for the crustal evolution of this part of the Shield.

The Kabid formation is the oldest in the region and comprises pelitic, arkosic and felsic high-grade paragneisses. U-Pb zircon data from a pelitic garnet-sillimanite gneiss show that this part of the continental basement in the southern Afif terrain may be as old as 1770 Ma. Isotopic nalyses indicate that lead from the Kabid gneiss resided in the upper continental crust for a long period before 1770 Ma, and consequently Archaean source rocks may be present within the southern Afif terrain.

Pb and Rb-Sr isotopic data in the Zalm region reveal a change in the nature of the underlying crust, from continental basement in the northeast, to less radiogenic marginal arc rocks in the southwest. This change is coincident with both aeromagnetic data, and a facies change within a pre-collision marginal basin. Miogeosynclinal continental shelf facies of the Siham group lie unconformably over the Kabid formation, and are in the area of continental lead signatures. Eugeosynclinal deep water sediments and volcanics, in association with ultramafic rocks, occur in the area of marginal arc signatures.

U-Pb zircon age determinations show that this 'Andean' continental margin developed before about 720 Ma, and emplacement of calc-alkaline plutonic rocks continued until about 690 Ma. During the period 685-640 Ma, the continental Afif microplate collided with the Asir terrain as part of the Nabitah orogeny. At approximately 640 Ma ago, the Najd strike-slip faulting commenced with a dextral phase that controlled the emplacement of granitic plutons as well as the development of a series of large pull-apart grabens. Some of these grabens were floored by new oceanic crust and were filled with volcanosedimentary rocks of the Bani Ghayy group. Subsequently, the Najd fault system changed to sinistral strike-slip motion at about 620 Ma ago.
The geochemistry of late Upper Proterozoic Volcanic Groups in the Red Sea Hills of NE Sudan—evolution of a late Proterozoic volcanic arc system
Authors: Klemenic P. M. , Poole S. , Ali S. E. M.
Published: December 2007

The Kadawçb and Homogar Volcanic Groups occur in the Sudanese portion of the Nubian Shield and are late Proterozoic in age (723 Ma and 671 Ma, respectively). The lavas of both groups display a calc-alkaline character, are broadly similar in terms of their lithology, phenocryst mineralogy and major element composition, and geochemically they are similar to modern arc lavas associated with converging plate margins. However, a clear distinction can be made between the two groups on the basis of the trace element geochemistry of the basaltic components of the suites. The Homogar basalts are chemically more evolved and are enriched in large ion lithophile elements (LILE) and rare earth elements (REE) having high La/Nb, Ce/Yb
N
and
87
Sr/
86
Sr ratios compared with those of the Kadawçb group. These chemical differences possibly follow an evolutionary trend as the less developed Kadawçb lavas are some 50 Ma older than those of the evolved Homogar group. It is suggested that the older Kadawçb lavas were erupted along an intra-oceanic island arc system, while the younger Homogar lavas were extruded along a more evolved transitional arc or possibly a continental margin. The differences in light/heavy REE and LIL/high-field strength element ratios may reflect variation in the degree of alteration of an overlying mantle wedge, possibly due to the introduction of LILE- and REE-enriched fluids derived from dehydrating subducted oceanic lithosphere.
The significance of continental derivation of the Proterozoic Mahanid Formation, southeastern Arabian Shield
Authors: White D. L.
Published: December 2007

The Mahanid Formation is a quartzo-feldspathic meta-sedimentary unit exposed in the southeastern part of the Arabian shield. Detritus was derived from a continental landmass located to the southeast. Erosional products were deposited in an adjacent marginal basin which existed from middle to late Proterozoic time. The Mahanid Formation is correlated with other formations in the eastern shield area, but differs from meta-sedimentary units in the western part of the Arabian shield.
Metamorphic studies: research in progress
Authors: Brodie Kate
Published: December 2007

Report of a meeting of the Metamorphic Studies Group held at Burlington House, 6 March 1985. The meeting was organized by Dr K. H. Brodie.

A Metamorphic Studies Group Meeting on ‘Metamorphic studies: research in progress’ and the fourth Annual General Meeting of the Group were held on 6 March 1985. Fifteen papers were presented, on a wide range of metamorphic subjects, ranging from very low-grade metamorphism to high-grade granulites and migmatites. Half the papers were presented by research students, and each contribution was followed by lively discussion.

The morning session was chaired by Ben Harte, the retiring Chairman of the Group, and concentrated on high-grade metamorphic rocks, and regional deformation and metamorphism. I. Cartwright presented a joint paper with A. C. Barnicoat which considered reactions within partially melted corundum-bearing metapelites from the Lewisian Complex. They demonstrated that silica-undersaturated restites can co-exist with silica-saturated partial melts under appropriate conditions. This was followed by a contribution from T. F. Emmett on the magmatic and metamorphic evolution of high-grade rocks from the Jotun Nappe in Norway. The Group was then pleased to welcome J. C. Schumacher from the Mineralogisches Institut in Kiel, F. R. Germany, who presented a paper describing microstructural evidence for reactions producing a range of Al-rich assemblages and discussing the relative roles of diffusion versus reaction in controlling the reactions. P. J. Treloar then gave a paper describing some unusual and colourful mineral assemblages from a Cu-Zn massive sulphide body in Finland, where high concentrations of Cr in the
Does the golden spike still glitter?
Authors: HOLLAND CHARLES HEPWORTH
Published: December 2007

A
modus operandi
for Precambrian stratigraphy is suggested which combines the geochronometrical scale and late Proterozoic chronostratigraphy. The development of Phanerozoic stratigraphical classification is followed from the pioneering days of Murchison, through the growing understanding of fossils in relation to time and the later distinction of rock and time-rock, to the advent of the golden spike. Criticisms of chronostratigraphy (global standard stratigraphy) are assessed and its value upheld. Stratigraphical procedures such as those of event stratigraphy are useful in particular situations, but correlative methods in the Phanerozoic are dominated by biostratigraphy. Examples are taken of its use and potential in North American Cordilleran Palaeozoic stratigraphy, in the Triassic System as a whole, and in the Caledonides of Britain and Ireland.
Sedimentation and evolution of the Cretaceous Gyeongsang Basin, southeastern Korea
Authors: CHOI H. I.
Published: December 2007

The sedimentation and evolutionary history of the Gyeongsang Basin, and their implications for plate tectonics are reviewed based mainly on sedimentological work on the Gyeongsang strata, and the regional geology of the Japanese Islands. During the Cretaceous, E Asia was characterized by an Andean-type continental margin formed by subduction of the Kula plate and Kula–Pacific ridge. The marginal zone was subjected to precursory extensional tectonism of the magmatic arc so that fault-bounded continental depressions developed, including the proto-Gyeongsang Basin.

The Gyeongsang strata (about 9,000 m thick) consist in ascending order of the Sindong, the Hayang with volcanic' stics, and volcanic Yuchon Groups. During the Sindong period extensional tectonism prevailed a the proto-Gyeongsang Basin was a graben occupied by alluvial fan, fluvial plain and lake environments from the margins to the centre of the basin. Streams flowed from the upthrown lip areas to the centre. Alluvial fans expanded periodically with rejuvenation of fault activity, while the fluvial plaid/lake boundary shifted in response to climatic changes.

During the Hayang period the faults were no longer active and consequently the basin extended beyond them (probably by back faulting). The water level of the lake(s) at the basin centre fluctuated with changes of the evaporation/precipitation ratio. During parts of the period, the marginal fluvial plain was drained by ephemeral streams which disappeared before reaching the lake(s), depositing fluvial sequences lacking channel sandstones. Localized volcanism formed volcanic terrains and supplied pyroclastics: this volcanism was climactic during the Yuchon period and terminated the sedimentation. Granitic plutons intruded all the Gyeongsang strata at about the end of the cretaceous.
Deep seismic reflection profiling between England, France and Ireland
Authors:   BRITISH INSTITUTIONS REFLECTION PROFILING SYNDICATE (BIRPS),   ETUDE DE LA CROÛTE CONTINENTALE ET OCÉANIQUE PAR RÉFLEXION ET RÉFRACTION SISMIQUE (ECORS)
Published: December 2007

The South West Approaches Traverse comprises 1600 km of profiling to 15 seconds two-way time. The profiles were planned to investigate Variscan structures in the crust and upper mantle. In Ireland, SW England and NW France, Variscan deformation and granite intrusion extend from Middle Devonian to Early Permian time, about 370 to 270 Ma ago. We have imaged a normal fault dipping gently south under the Celtic Sea that corresponds at surface, in position and trend, with the Variscan 'front' and can be traced down to at least 20 km depth. As under Caledonian Britain to the north, the lower crust is everywhere strongly reflective from about 6 seconds two-way time down to the Moho at about 10 seconds. The Haig Fras granite and Cornubian batholith, crossed west of the Isles of Scilly, appear only as unusually shallow portions of the lower crustal reflectors.
The Bristol-Mendip foreland thrust belt
Authors: WILLIAMS G. D. , CHAPMAN T. J.
Published: December 2007

The thrusts and associated fold structures of the Bristol–Mendip area are interpreted in terms of a thin skinned, piggy-back foreland thrust belt of Variscan age with dominant northward transport. Strike-normal balanced sections and compatible strike-parallel sections have been constructed to illustrate the 3-D geometry of the linked thrust system. North–south shortening of c. 20 km (40%) is calculated from deformed and restored strike-normal sections. A thick and extensive thrust wedge of pre-Old Red Sandstone basement in the west of the area is responsible for culminations expressed at the surface as the Mendip periclines. Uplifted culminations permit erosion to lower stratigraphic levels in the west and south of the area. Strike-parallel sections and a hanging wall cut-off map indicate that the majority of thrusts in the area are mainly oblique ramps rather than simple frontal and lateral ramps, or flats. The en-echelon pattern of the Mendip periclines may be related to the oblique NE climb of two of these thrusts. The angle of frontal ramp climb through the Dinantian stratigraphy is generally low, and this explains the presence of numerous klippen of Lower on Upper Carboniferous rocks, whilst suggesting that certain thrust sheets were much more extensive than present outcrops indicate. Early, pre-thrusting movement on basement lineaments in the Variscan foreland created uplifts, angular unconformities and a predominance of N–S structures in the north of the area.