bluuebunny/crossref_metadata_2025 huggingface dataset:
Genesis of, and exploration for, industrial and ore minerals
Authors: RICE C. M.
Published: December 2007

Report of the 11th AGM of the Mineral Deposits Studies Group held in the Department of Applied Geology, University of Strathclyde, Scotland, 19–21 December 1985.

The meeting titled 'Genesis of, and exploration for, industrial and ore minerals' was attended by 92 participants (from academia and industry) and included a field trip, 38 papers and several poster sessions. On the first day
M. J. Russell
and
A. J. Hall
led a field trip to Tyndrum where dumps, veins and wall rocks were examined in drizzle.

A keynote address on Black Smokers and a video taken by submersible were given by
John Edmond.
He described hydrothermal activity on intermediate and fast spreading ridges in the Eastern Pacific. The fluids are of sea-water origin, acid (pH about 3) and with temperatures around 350 °C. Chloride is the only anion and H
2
S concentrations reach 300 ppm whereas Cu and Zn are usually between 1 and 5 ppm. While the metalliferous sediments are analogues to ophiolite-type massive sulphide ore bodies no active example has been found that remotely compares to them in size. Most of the ore-forming metals are lost as 'black smoke' and the ore localization mechanism is a puzzle.

Hydrothermal fluids from the Gulf of California are alkaline, contain major ammonium and are depleted in ore metals possibly suggesting that a sediment-hosted massive sulphide deposit is forming at depths. Precipitation may be caused by reaction of the ore fluids with biogenic carbonate tests and with ammonia produced by the thermocatalytic breakdown of
Geology, geochronology and geodynamic implications of the Cenozoic magmatic province in W and SE Ethiopia
Authors: Berhe S. M. , Desta B. , Nicoletti M. , Teferra M.
Published: December 2007

New K-Ar dates are presented for areas in W and SE Ethiopia. In the west, the dates distinguish the Geba Basalts of 40 to 32 Ma (previously determined by Merla
et al.
1979) from the Welega Shield Volcanics which are shown to range from 11.2 ± 2.2 to 7.8 ± 1.6 Ma. In SE Ethiopia, the Lower Stratoid flood basalts range from 30 ± 4.5 to 23.5 ± 4.5 Ma and are unconformably overlain by the Reira-Sanete shield volcanics which range from c. 15 to
c.
2 Ma. The unconformity is marked by a palaeosol as are several of the intervals between the major volcanic stages of Ethiopia.

Using new field observations, together with previously published results and unpublished data from the Ethiopian Institute of Geological Surveys, it is suggested that the Ethiopian Flood Basalts were erupted in three major stages. Stage 1, which is mainly older than 40 Ma is separated from Stage 2, 34 to 30 Ma for NW Ethiopia and 40 to 30 Ma for SW Ethiopia, by erosional unconformities. Stage 3 spans 30 to 26 Ma in NW Ethiopia, and 30 to 21 Ma in SW Ethiopia and both are marked by the incoming of silicic volcanism. In W Ethiopia, Stages 1 and 3 are not developed, whilst in SE Ethiopia the Tertiary volcanism commences with Stage 3 flood basalts. The overlying shield volcanics; (25 to 13 Ma in NW Ethiopia and 15 to 7 Ma in W Ethiopia) represent a localized terminal episode built on the Plateau and are considered a fourth stage.

The earliest volcanism is restricted to two areas: in SW Ethiopia, where the Akobo Basalts give ages as old as 49.4 Ma (Davidson & Rex 1980), and in NW Ethiopia where the Ashange Basalts underlie the Aiba Basalts which are dated at 34 to 30 Ma. The Ashange volcanism may have begun in the Palaeocene—Eocene but radiometric dates are ambiguous and inconsistent. These two volcanic centres show lithological and geochemical differences, and can be related to two separate rift zones north of Lake Turkana and further north in the Tana Graben. Volcanism appears to have spread by the expansion of these areas, and to have migrated eastwards with time towards the Main Ethiopian Rift which begun to form about 14 Ma ago. There is a complex pattern of domal uplifts within these two volcanic centres with volcanism either preceding doming, or the two being contemporaneous.
The movement and entrapment of petroleum fluids in the subsurface
Authors: ENGLAND W. A. , MACKENZIE A. S. , MANN D. M. , QUIGLEY T. M.
Published: December 2007

This paper discusses the migration of petroleum from its formation in a source rock to its subsequent possible entrapment in a reservoir. The chemical and physical properties of petroleum gases and liquids are stressed, particularly their phase behaviour under subsurface conditions which is shown to be a very important factor in determining migration behaviour. Engineering correlations are presented for estimating the properties of petroleum fluids under geologically realistic conditions. The directions and magnitudes of the forces acting on migrating petroleum are deduced from the combined effects of buoyancy and water flow in compacting sediments. These forces are combined, using a fluid potential description. This procedure allows the direction of migration to be denned. The rate of migration is then estimated from the properties of the sediments involved, allowing a distinction to be made between ‘lateral’ and ‘vertical’ carrier beds. This simplified approach is suitable for rapid predictive calculations in petroleum exploration. It is compared with the more complex 3-D computer modelling approaches which are currently becoming available. Migration losses are related to the cumulative pore volume employed by the petroleum in establishing a migration pathway. The petroleum migration mechanism is shown to be predominantly by bulk flow, with a small diffusive contribution for light hydrocarbons over distances less than
c.
100 m. The loss factors involved in secondary migration are estimated from field evidence. The mechanism of reservoir filling is presented as a logical extension to those described for migration. This, together with the inefficiency of in-reservoir mixing by diffusion or convection, is shown to tend to cause significant lateral compostional gradients in reservoirs over and above the gravitationally induced vertical gradients described by other workers.
Arthropod cuticles in coal
Authors: BARTRAM K. M. , JERAM A. J. , SELDEN P. A.
Published: December 2007

An abundance of scorpion cuticles from Westphalian (Upper Carboniferous) coals of Yorkshire is described, and other records of arthropod cuticles in coals are reviewed. The absence of cuticles assignable to arthropod groups other than scorpions is thought to be due to preferential preservation of the unique exocuticle of scorpions; it alone is preserved and appears to retain an organic nature. The cuticle is recovered from all the lithotypes of humic bituminous coals although it is most common in coals rich in inertinite macerals. From the present study it is uncertain whether the scorpions were aquatic or terrestrial. The recognition of arthropod cuticle as a coal maceral could aid environmental interpretations. The abundance of arthropod cuticle in the coals studied indicates its potential use in correlation and in determining the thermal maturity of sediments.
Metamorphic studies: research in progress
Authors: BRODIE KATE
Published: December 2007

Report of a meeting of the Metamorphic Studies Group held at Burlington House, on 5 March, 1986. The meeting was organized by K. H. Brodie.

A Metamorphic Studies Group meeting on
Metamorphic studies: research in progress
and the fifth Annual General Meeting of the Group were held on 5 March 1986. Fifteen papers were presented, including four from overseas speakers. For the first time this year the talks included one invited overseas speaker, Professor W. L. Griffin from the Mineralogisk-Geologisk Museum in Oslo, who presented a review of high pressure metamorphism in Western Norway.

The morning session was chaired by Stephen Daly, and concentrated on the topical theme of the role of fluids in metamorphism.
B. Yardley
started the session with a joint paper with
S. Bottrell
considering the important effects that fluid immiscibility may have in natural metamorphism, particularly at low grades. In particular, in a two-phase system the relative permeability may become very low for a phase of low proportions. This may have the effect concentrating, for example, NaCl in the fluid phase.
K. W. Burton
then considered the effects of fluid composition on surface energy, using metamorphic sector zoning as an example. Increasing surface energy increases the overstepping of an equilibrium boundary, leading to fewer nuclei which will tend to grow relatively large. In the garnets investigated, inclusions, were found to be rich in CO
2
, H
2
O (plus
N
+Ar), and it was suggested that quartz intergrowths commonly develop as a result of low
Rapid deposition of sand and gravel mixtures downstream of a negative step: the role of matrix-infilling and particle-overpassing in the process of bar-front accretion
Authors: CARLING P. A. , GLAISTER M. S.
Published: December 2007

An experimental flow-channel was used to investigate the rapid deposition of sand and gravel mixtures downstream of a negative step. Hydraulic processes give rise to the quasi-simultaneous accumulation of openwork gravels, matrix-filled contact framework and dilated framework gravels. The resultant structure is analogous to a bar-front in gravel-bed streams. Flow conditions were non-uniform but steady in time, whereas open and closed gravel fabrics are usually interpreted as representing deposition from unsteady flows—for example a varying discharge regime.

Consideration of the percentage of sand in the deposits indicated that, in terms of packing, the sediments may be considered as a simple binary mixture with partial framework dilation occurring with sand percentages in excess of
c.
32%. Despite dilation, for sand percentages less than 50%, structural stability of the foreset gravels was maintained so that the bar-front exhibited a distinct brinkpoint, and a planar slip-face (at the angle of initial yield) which made an angular contact with the base of the flume. For sand percentages in excess of 80%, individual gravel clasts were completely supported by sand. The original structural stability was lost so that the bar-front, eroded by reverse currents, assumed a tangential contact with the flume base below the angle of initial yield. The brinkpoint was rounded and indistinct. In addition to altering the bar-front slope, increasing the quantity of sand in the bed sediments had a major influence on the dip of gravel particles on the foresets.

In all experimental runs a surface sand-seal (with openwork gravels beneath) developed upstream of the brinkpoint. The mechanics of sand-seal formation require further investigation. However, an overpassing mechanism was noted to be very important in sorting gravel particles rolling over the brinkpoint. A published mathematical model describing the overpassing mechanism was found to be appropriate to the present experimental observations.
14
C deformation chronologies in coastal Iran, Greece and Jordan
Authors: VITA-FINZI C.
Published: December 2007

Shoreline chronologies calibrated by
14
C dating can help to disentangle the history of continental deformation. Holocene uplift rates in the SE Zagros of Iran may be explained by serial folding which is partly aseismic and currently in abeyance. The geometry and age of raised beach platforms in the western Makran are consistent with structural models which postulate imbricate thrusting powered by low-angle subduction. The Late Quaternary fossil beaches bordering the eastern Gulf of Corinth indicate repeated shifts in the locus of seismic activity and owe their present elevation to uplift of the footwalls of major normal faults. On the Gulf of Aqaba, coastal emergence during the last 30 000 years appears to represent the early stages of spreading on the Red Sea axis. Coastal sequences thus contribute to structural analysis by revealing changes in the pattern as well as the rate of deformation over the last 50 000 years.
Improved discrimination of rock units using Landsat Thematic Mapper imagery of the Oman ophiolite
Authors: ROTHERY D. A.
Published: December 2007

The short wavelength infrared channels recorded by the Landsat Thematic Mapper make this instrument more suitable than the Landsat Multispectral Scanner for lithological mapping using enhanced imagery. The weathering mineralogy of igneous rocks causes absorption features in their reflectance spectra which are particularly characteristic in the near infrared. A Landsat Thematic Mapper band 7,5,4 red, green, blue false colour composite of part of the Oman ophiolite confirms much of the previous mapping and indicates certain hitherto unrecorded features. These include a new mafic-ultramafic late-intrusive complex and differences between deeper and near-crustal levels in the mantle sequence, which may be related to the environment in which serpentinization took place. Decorrelation stretching is an excellent means of accentuating colour variations within an image as an aid to visual interpretation.
An occurrence of shoshonites near Kilmelford in the Scottish Caledonides and its tectonic implications
Authors: ZHOU J.-X.
Published: December 2007

The Kilmelford intrusives were emplaced in Dalradian metasediments and eruptives during the late stages of the Caledonian orogeny. The suite of rocks includes biotite–hornblende diorite and, to a minor extent, biotite–pyroxene and biotite–hornblende–pyroxene diorites, together with hornblende–biotite–quartz diorite, granodiorite, porphyrites of dioritic and dacitic composition and volcanic breccia. The major element geochemistry shows that the Kilmelford rock suite is alkali-calcic having petrogenetic affinities with the igneous rocks of the New Guinea 'continental arc'. The high contents of K, Sr, Ba, Rb, Zr and Ce, together with a high K
2
O/Na
2
O ratio, give these rocks their shoshonitic signature, which is of interest as an unusual example of the occurrence of shoshonites in the Scottish Caledonides. The rocks have very steep REE patterns (Ce
N
/Yb
N
= 10−28) and do not show a Eu anomaly. The shoshonitic signature of the Kilmelford rocks is interpreted as due to the late orogenic nature of the magmatism which took place when subduction had abated following the collision of the two continents.
Peralkaline volcanicity in the Arctic Basin—the Kap Washington Volcanics, petrology and palaeotectonics
Authors: BROWN P. E. , PARSONS I. , BECKER S. M.
Published: December 2007

The Kap Washington Group volcanic rocks occur on the north coast of Greenland and were extruded at or just before the Cretaceous–Tertiary boundary. The suite is strongly bimodal with basaltic lavas, comendites and comenditic ash-flow tuffs predominating. The basic lavas are of transitional basalt type but have characteristically alkaline minor-element contents. The riebeckite-bearing comendites have strong light REE enrichment and large negative Eu anomalies as commonly found in continental peralkaline suites. As well as the comendites there are other rhyolite lavas and minor intrusions of distinctly different chemistry which are thought to have originated by crustal refusion. The peralkaline nature of the siliceous rocks is in keeping with the extensional tectonic environment suggested by an intense alkali dolerite dyke swarm which immediately preceded the extrusive activity. Association with the development of the Makarov Basin in Cretaceous times is suggested.
A sequential back-arc and foreland basin thrust duplex model for the Southern Uplands of Scotland
Authors: STONE P. , FLOYD J. D. , BARNES R. P. , LINTERN B. C.
Published: December 2007

Thrust imbrication of Ordovician and Silurian submarine fan sequences overlapping pelagic deposits in the Southern Uplands has been interpreted in terms of an accretionary prism formed above a NW-directed subduction zone. Structural features invoked to support accretion are not definitive and could be explained in terms of a thin-skinned thrusting model. New palaeocurrent and compositional evidence from Llandeilo to mid-Llandovery age turbidites in the northern part of the Southern Uplands proves interdigitation of sediments with strongly contrasting petrography. Turbidites derived from the south contain significant quantities of fresh andesitic detritus whereas those from the north form more mature quartz-rich formations. This implies a back-arc situation; the turbidites being deposited in a basin with a relatively mature continental landmass to the north and a rifted continental fragment containing an active volcanic arc to the south. Oblique collision of the opposing continental margins of the Iapetus Ocean during the Llandovery caused the cessation of subduction. Underthrusting of the southern margin initiated a SE-propagating thrust stack which deformed the back-arc basin sequence and may eventually have ramped over the eroded and faulted remains of the volcanic arc.

A southward-migrating foreland basin formed ahead of the rising thrust stack and is now represented by the late Llandovery Hawick Group and Wenlock sequences. Mid- to end-Silurian sinistral strike-slip resulted from oblique collision and produced a transpressional regime during which reactivation of deep-seated structures allowed the intrusion of lamprophyre dykes and granites.
The onset and timing of Caledonian sinistral shear in County Down
Authors: ANDERSON T. B.
Published: December 2007

In the south-westerly Irish continuation of the Southern Uplands ample evidence of sinistral transpression includes: (a) the clockwise transection of F
1
folds by S
1
cleavage combined with sub-horizontal extension in the cleavage plane, (b) local development of vein arrays deformed by sinistral simple shear and (c) an abundance of sinistral wrench faults. The sinistral movements are dated relative to the local history of Caledonian deformation and the intrusion of two suites of Caledonian lamprophyre dykes.

Clockwise transection of folds by cleavage is not a feature of the Northern Belt or of the northern tracts of the Central Belt in County Down. Its geographic distribution suggests that sinistral transpression was first effective during the accretion of Llandovery-age tracts mid-way south in the Central Belt. Sinistral movements continued episodically during the deformation of early pre- and post-S
1
vein sets and was finally expressed in widespread wrench faulting which continued beyond the date of intrusion of the younger dykes, at about 400 Ma.
Constraints on the age of movement of the Montsech and Cotiella Thrusts, south central Pyrenees, Spain
Authors: FARRELL S. G. , WILLIAMS G. D. , ATKINSON C. D.
Published: December 2007

Palaeocene to Oligocene thrusting in the Pyrenees was synchronous with the development of the South Pyrenean foreland basin (collectively the Tremp–Graus, Ainsa and Jaca basins). Sedimentological and stratigraphical evidence from the foreland basin is used to constrain the age of movement of the Montsech, Cotiella and External Sierras thrusts. Movement on the emergent imbricate fan at the Montsech front began in the Early Eocene (Illerdian) during an early phase of thrusting that detached in the Triassic evaporites in that region. This phase of thrusting defined the Tremp–Graus basin as a thrust-sheet-top basin. Following the early thrusting, Cuisian age tectonics in the Tremp–Graus and Ainsa basins was dominated by minor normal fault movements during flexural subsidence associated with thrust sheet loading. Growth of the Boltaña anticline to the west of the Ainsa basin indicates that the Ainsa basin had become detached as a thrust-sheet-top basin in the Lutetian. Movement on the Peña Montañesa thrust, and by inference on the Cotiella thrust, began in post-Lutetian times during the propagation of lower thrusts that involved pre-Triassic (Variscan) basement. Thrusting in the External Sierras caused folding and faulting of Oligocene sediments, indicating the continued southward propagation of basement-involved thrusts in the Upper Eocene and Oligocene. Many faults in the Southern Pyrenees have evidence of early normal and later reverse senses of movement indicating reactivation of normal faults during thrusting.
Sedimentary evidence of the Neogene evolution of SE Spain
Authors: CATER J. M. L.
Published: December 2007

The Neogene sediments of SE Spain are dominated by gravity-flow deposits, ranging from millimetre-scale dilute turbidites to kilometre-scale olistholiths. Details of the Neogene sequences at Finestrat, Alcoy and Fontanars provide examples of responses to periodic tectonic events in a variety of background settings. The main control on sedimentation in each area was the episodic rise of Triassic gypsiferous mudstone diapirs. This probably resulted from periodic regional extension and compression due to contraction and spreading of the Alboran Sea marginal basin.
Geochemical evolution of the Juan Fernandez Islands, SE Pacific
Authors: BAKER P. E. , GLEDHILL A. , HARVEY P. K. , HAWKESWORTH C. J.
Published: December 2007

The two main islands of the Juan Fernandez group, some 180 km apart are intra-plate volcanoes related to a single hot spot. K-Ar determinations suggest an age difference of
c.
3 Ma which is in accord with a spreading rate of
c.
6 cm a
−1
in this part of the Nazca plate. The younger island of Alexander Selkirk is a relatively recent volcano (<1 Ma) composed of olivine tholeiites, many of them picrites, together with occasional quartz tholeiites and rare trachytes. Robinson Crusoe is an older, deeply dissected island with several separate volcanic centres. The older lavas are predominantly olivine tholeiites, again often picritic, with an occasional quartz tholeiite flow. Alkali basalt flows, never abundant, become more common in the higher part of the succession. Basanite flows are restricted to younger parasitic centres, but basanite dykes and sills cut the older formations. The tholeiites of Alexander Selkirk have a low pressure phenocryst assemblage and their low Ba contents, low Nb/Zr ratios and REE patterns suggest higher degrees of mantle partial melting than their closest counterparts on Robinson Crusoe. The basanites of the latter island have a higher pressure phenocryst assemblage and have higher La/Sm ratios than the tholeiitic lavas. The basanites are not always distinct from the Robinson Crusoe tholeiites on the basis of incompatible trace elements.
86
Sr/
87
Sr ratios are fairly uniform (
c.
0.7035) in all the rock types but
143
Nd/
144
Nd ratios are more variable, attaining their lowest values in some of the basanites. Trace element and isotopic data suggest remelting of source regions modified by migration of small degree partial melts over a long period.
Composite lava flows of Ordovician age in the English Lake District
Authors: ALLEN P. M. , COOPER D. C. , FORTEY N. J.
Published: December 2007

Three composite andesitic lava flows, within the Ordovician Borrowdale Volcanic Group, are described. They are 35-250 m thick and in their simplest form consist of a massive, flow-jointed andesite lower part and a flow-laminated to blocky basaltic andesite upper part with a zone of interlayering between them. One flow has a very thin, discontinuous, basal unit of xenolithic, flow-laminated basaltic andesite. The interlayered zone comprises layers and lenses of andesite, basaltic andesite and mixed rocks consisting of closely packed inclusions of pale coloured andesite in a chlorite-rich matrix. The rocks have undergone several stages of alteration: (a) deuteric alteration, including silicification associated with autobrecciation of the upper component and inter-magma reactions in the interlayered zone; (b) soda-metasomatism, preferentially of basaltic andesite; (c) metamorphism, attributed to the Lake District granite batholith, giving an actinolite-sericite assemblage; (d) regional metamorphism to give a chlorite-epidote-actinolite-sphene assemblage with rare prehnite and pumpellyite; (e) calcite veining. These alterations have modified the bulk chemistry of the rocks, but the concentrations of Al, P, Ti, V, Cr, Co, Ni, Ga, Y, Zr, Nb, La, Ce and Th are largely unaffected on the scale of a 2 kg sample. Using these elements the composition of the lavas compares with high-K orogenic andesites emplaced through continental crust. Each of the flows shows distinct chemical characteristics which may be explained by variations of phenocryst content or the assimilation of cognate xenoliths. Variations within flows can be explained by magma mixing and fractional crystallization of plagioclase, minor pyroxene, ilmenite and zircon. A mechanism for eruption is proposed in which a volatile-rich basic magma intruded de-gassed cooler, less dense andesite magma in a high level sub-volcanic chamber. The basic magma formed the lower layer in the chamber. On cooling and crystallization of plagioclase near the interface the volatile content of the residual basic liquid rose and the density decreased allowing the liquids to form an emulsion-like layer at the interface. This separated and rose into the andesite magma and the process was repeated to form an interlayered zone at the junction. Before mixing was complete the magma was erupted as a series of composite flows.
Basement–cover relationships and structure within the Moine rocks of central and southeast Sutherland
Authors: STRACHAN R. A. , HOLDSWORTH R. E.
Published: December 2007

Inliers of Archaean to early Proterozoic Lewisian basement crop out within the Proterozoic Moine Succession of central and SE Sutherland. Both the Lewisian basement and the Moine cover have been affected by two major tectonic events. The earliest event probably occurred during the late Proterozoic and involved minor folding and upper greenschist to upper amphibolite facies metamorphism. Intense deformation largely obliterated the unconformable relationship between the Lewisian and Moine rocks, and imposed a planar banding on the basement lithologies. Subsequent reworking during the Caledonian orogeny (500–400 Ma) resulted in ductile thrusting and folding, and was associated with a widespread low- to mid-amphibolite facies metamorphic overprint. Within this context the Lewisian inliers of central and SE Sutherland occur in two distinct structural settings: (i) as allochthonous sheets which rest on major Caledonian ductile thrusts; (ii) in the cores of NW-verging intensely curvilinear sheath folds of Caledonian age.
The distribution of ammonium in granites from South-West England
Authors: HALL A.
Published: December 2007

The ammonium contents of granites, pegmatites and hydrothermally altered rocks from SW England have been measured. Ammonium levels in the granites are generally high compared with those from other regions, averaging 36ppm, and they differ markedly between intrusions. The pegmatites show higher ammonium contents than any other igneous rocks which have yet been investigated. Ammonium contents are strongly enriched in the hydrothermally altered rocks, including greisens and kaolinized granites. There is a good correlation between the average ammonium content of the intrusions in SW England and their initial
87
Sr/
86
Sr ratios and peraluminosity. This relationship supports the hypothesis that the ammonium in the granites is derived from a sedimentary source, either in the magmatic source region or via contamination of the magma.
The tectonic evolution of the Zoumi Sandstone, western Moroccan Rif
Authors: MORLEY C. K.
Published: December 2007

The Zoumi Sandstone is an Oligo-Miocene syn-tectonic flysch sequence, deposited on the back of the Mesorif thrust sheet, whose sediment source was from the foreland to the south. Variations within the Zoumi Sandstone from thick to thin bedded turbidite lobes are explained as the result of deposition in small, confined basins or larger, more extensive basins, respectively. Formation of small basins is related to contractional deformation in the underlying Mesorif thrust sheet. Propagation of the Zoumi thrust through the developed basin with areas of relative uplift and subsidence produced complex hanging-wall cutoff geometry. A complicated footwall geometry exists because deformation in the more external Mesorif thrust sheet preceded the final displacement of the more internal, Intrarif thrust sheet. The latter incorporated the Zoumi Sandstone into its leading edge and thrust it over the Mesorif during the Miocene. This deformation sequence of two or more simultaneously active thrust sheets is described as synchronous thrusting.
Basement rocks of the Halmahera region, eastern Indonesia: a Late Cretaceous–early Tertiary arc and fore-arc
Authors: HALL R. , AUDLEY-CHARLES M. G. , BANNER F. T. , HIDAYAT S. , TOBING S. L.
Published: December 2007

Halmahera is a K-shaped island located at the junction of several major arc-trench systems of the western Pacific–eastern Indonesia region. Western Halmahera is an active volcanic arc above a zone of intense seismicity which characterizes the north Molucca Sea. Eastern Halmahera has a basement of dismembered ophiolitic rocks with slices of Mesozoic and Eocene sediments overlain unconformably by Middle Oligocene and younger sedimentary and volcanic rocks. The Mesozoic and Eocene sediments reveal notable stratigraphical and petrological similarities to the Marianas fore-arc and the eastern Halmahera Basement Complex is interpreted as a pre-Oligocene fore-arc lacking an accretionary complex. There is some evidence that the pre-Oligocene volcanic arc behind this fore-arc now forms part of the basement of western Halmahera. The Mesozoic and Tertiary sediments were imbricated together with igneous and metamorphic rocks representing the deeper parts of the fore-arc during the Late Eocene plate reorganization event recognized throughout the western Pacific margins. The eastern Halmahera Basement Complex can be traced into eastern Mindanao, probably further northwards in the eastern Philippines and may be related to similar terranes within and around the present Philippine Sea Plate. In contrast, the southern part of the island of Bacan at the southwestern end of the Halmahera group has a basement of continental metamorphic rocks associated with a deformed ophiolitic complex quite different to the basement of eastern Halmahera. The metamorphic rocks are interpreted to be part of the north Australian continental margin basement which is separated from the Halmahera Basement Complex by a splay of the Sorong Fault system and the deformed ophiolite complex of Bacan is suggested to represent magmatism in the fault zone.
Stratigraphy, geochronology and volcano-tectonic evolution of the Kedong–Naivasha–Kinangop region, Gregory Rift Valley, Kenya
Authors: BAKER B. H. , MITCHELL J. G. , WILLIAMS L. A. J.
Published: December 2007

Twenty-two new K-Ar dates allow reorganization of the volcano-stratigraphy of the Kedong–Kinangop region on the east side of the central Kenya rift for the most recent 4 Ma, during which time most of the structural features evolved. Four principal phases of volcanic eruption were: widespread Kinangop tuff ash flows (3.7–3.4 Ma), Limuru flood trachytes (2.0–1.8 Ma), basalts and flood trachytes of the rift floor (1.65–0.9 Ma), Quaternary salic caldera volcanoes (0.4–0 Ma). K-Ar dating allows recognition of the Cobb Mountain normal polarity event at about 1.18 Ma, and dating of the Kedong flood at about 0.1 Ma. Major faulting occurred at 4–3 Ma, converting an early half-graben to a graben. Fault zones then migrated inwards progressively forming step-fault platforms at 3–2 Ma and 1.7 Ma, such that the graben narrowed with time. Extensive minor faulting of the rift floor occurred during the most recent 0.8 Ma. The average extension rate across the central rift was 3 mm a
−1
for the last 7 Ma, and is close to plate tectonic estimates. Individual fault strain rates are estimated at 0.1–0.4 mm a
−1
. Faulting was episodic rather than continuous, and some episodes affected both sides of the rift. Volumes of magma erupted during 4–0 Ma require generation of at least 1.2 × 10
5
km
3
of primary magma in the 200 km length of the south–central rift if fractional crystallization is the dominant differentiation mechanism. This volume implies mantle upflow either as an asthenosphere ‘plume’, or as the result of lithosphere extension. The tectonic and magmatic sequence of events is proposed to result from lithosphere thinning, emplacement of large magma bodies in or near the lower crust, regional uplifts and superimposed limited lateral separation of the lithosphere as a result of plate motions.
Neogene bottom water-masses and benthic foraminifera in the NE Atlantic Ocean
Authors: MURRAY J. W.
Published: December 2007

Modern benthic foraminiferal assemblages show a correlation with oceanic bottom water-masses. A study of the Neogene benthic foraminifera from seven sites to the south and two sites to the north of the Azores Ridge shows patterns of change which can be interpreted in terms of past water-masses. An Antarctic bottom water (AABW) influence has been present to the south since the mid-Miocene but to the north of the Azores Ridge, AABW was present only up to the late Pliocene, since when it has been absent. Overlying AABW is North Atlantic deep water (NADW) and North East Atlantic deep water (NEADW). The latter shows a spectacular increase in importance from the late Pliocene to Recent. The faunal changes can be correlated with changes in circulation in the late Pliocene which heralded the incoming of major glacial events from
c.
2.4 Ma.
Fluid inclusion studies
Authors: FOSTER R. P.
Published: December 2007

A thematic session on fluid inclusions was held on 15–16 December 1986, at the University of Southampton as part of the annual meeting of the Mineral Deposits Studies Group and was supported by the Applied Mineralogy Group of the Mineralogical Society. Four of the papers are published in this volume (referees: D. H. M. Alderton, A. V. Bromley, R. P. Foster, A. P. Gize, R. A. Ixer, N. J. Jackson, D. A. C. Manning and J. J. Wilkinson), and abstracts are presented for five other contributions.

Providing as always an annual forum for the reporting and discussion of mineral deposits research, the 1986 meeting focused in part on fluid inclusion studies in metallogenesis. Opening the meeting, keynote speaker Roedder (Roedder and Howard) referred to a recent study of the Taolin Zn–Pb–fluorite deposit (People's Republic of China) in which homogenization temperatures of 120–200°C and salinity determinations of 0–14 wt% NaCl equiv. were more indicative of low-temperature epithermal mineralization than a Mississippi Valley-type deposit. Some of the data, particularly the homogenization data, differed significantly from the results of an earlier study and were used to emphasize the problems that can be encountered in fluid-inclusion studies. Particular reference was made to the stretching or decrepitation of earlier, lower-Th inclusions by later hotter fluids.later hotter fluids.

In the more typical MW-type environment of the Illinois Fluorite District, petroleum-rich and aqueous inclusions were the subject of investigation by
Gize and Richardson
who observed that the petroleum phase was a strongly
Fluid inclusions related to uranium deposits: a review
Authors: POTY B. , PAGEL M.
Published: December 2007

Fluid inclusions have been observed and studied in rocks of most types of uranium deposits, except surflcial ones such as calcretes and the roll-type in sandstone. Temperatures of formation range up to nearly 400 °C but most deposits were formed at more moderate temperatures of 90–250 °C. It is evident from the relatively small number of studies, however, that the ore-forming process occurred at deeper levels than previously thought.

The chemistry of the fluids ranges from nearly pure water to very dense brines and CO
2
is a common component. In some cases gas analyses allow determination of
f

O
2

of the fluid, a key parameter for uranium transport and deposition.
Basin formation at the end of a strike-slip fault: the Cerdanya Basin (eastern Pyrenees)
Authors: CABRERA L. , ROCA E. , SANTANACH P.
Published: December 2007

The Cerdanya basin was formed during dextral slip along the NE–SW La Tet fault (eastern Pyrenees) which formed a horsetail structure at its SW end. The resulting basin is clearly asymmetrical, formed by several basement blocks tilted towards the La Tet fault and limited by E–W faults. These faults absorbed the movement along the major strike-slip fault. The early basin evolution (Middle–Late Miocene) resulted from strike-slip, while the later evolution (Early Pliocene) involved a generally extensional regime.

A variety of depositional environments developed in the basin during its early evolution: spatially restricted and widespread alluvial fan, distributary fluvial, marginal and open lacustrine. The structural features of the basin that developed during this evolutionary stage differ from those described from most other basins related to strike-slip faulting. Despite this, the structural and sedimentary features of the Cerdanya basin fit the general model of strike-slip related basins.
Chemical characters of volcanic rocks from Andaman ophiolite, India
Authors: RAY K. K. , SENGUPTA S. , VAN DEN HUL H. J.
Published: December 2007

Two parallel belts of late Mesozoic ophiolitic rocks occur along the outer arc ridge and inner volcanic line of the Indonesian arc system. The northernmost occurrence along the outer arc ridge is in the Andaman group of islands where ophiolitic rocks occur as dismembered slices emplaced over Eocene to Oligocene turbidites.

Volcanic members of the ophiolitic suite occur in two thrust slices, one consisting only of pillow basalts while the other is made up of basalts, basaltic andesites and acid differentiates. Certain chemical characters of the basaltic components are comparable to MAR basalts at 45° N. On the basis of comparable REE patterns and the presence of appreciable volumes of acid differentiates, they are further suggested to represent marginal basin crust. Variable (La/Ce)
N
and (La/Sm)
N
ratios in these basalts are interpreted in terms of a heterogeneous source.

The basaltic melts have undergone extensive fractional crystallization of clinopyroxene, olivine and plagioclase to produce basaltic andesite and acid volcanic rocks. Between the parent and the differentiates, there is no appreciable change in (Ce/Yb)
N
ratio or HREE abundance level, suggesting low pressure fractionation in a shallow magma chamber, possibly beneath a slow spreading centre.
Possible Variscan thrusting beneath the Somerton Anticline, Somerset
Authors: DONATO J. A.
Published: December 2007

Vibroseis seismic data were acquired by Goal Petroleum plc during 1982 in the vicinity of the Somerton Anticline, Somerset. These data show the simple form of the anticline but in addition show a southward-dipping, discontinuous seismic event beneath. Gravity modelling demonstrates that this event may be correlated with an E–W-trending gravity gradient which can be followed for approximately 150 km. The dipping seismic event may represent a Variscan thrust plane which dips to the SSW in contrast to similar planes seen to the east which dip to the SSE. The variation in dip seen between these postulated thrust planes may explain the curvature of structural trends seen within, and to the east of, the Mendip Hills. The Polden Hills Fault runs sub-parallel to the proposed thrust plane beneath the Somerton Anticline and may be linked to a Mesozoic reactivation of the postulated thrust.
A carbonatitic diatreme from Umberatana, South Australia
Authors: LOTTERMOSER B. G.
Published: December 2007

The breccia body at Umberatana, previously mapped as a diapir, intrudes Late Proterozoic metasediments and envelops Palaeozoic alkaline to peralkaline granites. Lineament-controlled emplacement led to limited structural disruption of the adjacent country rock and resulted in a phlogopite-rich breccia which contains albitite fragments, metasomatized Late Proterozoic clasts, altered alkaline basalts and dismembered metasomatized pelites of the granite thermal aureole. Breccia formation occurred after peralkaline granite emplacement (418 ± 2 Ma) and is thus unrelated to the Cambro-Ordovician Delamerian Orogeny. The breccia body is re-interpreted as a diatreme which resulted from multiple, possibly CO
2
-rich, halogen-bearing pulses of mantle-derived fluids rich in Ca, Mg,Ti,Fe
3+
,P, Sr, Na and Ce. Fluid contributions from a sedimentary source cannot be excluded, because the fluids traversed Late Proterozoic metasediments. The close association of alkaline to peralkaline igneous lithologies and mantle-derived fluids is related to the deep fracturing of the lithosphere and mantle-degassing.
Mapping the volcanic hazards from Soufriere Hills Volcano, Montserrat, West Indies using an image processor
Authors: WADGE G. , ISAACS M. C.
Published: December 2007

We have used a digital model of the topography of Montserrat, a simple mathematical model of gravitational flow and some assumptions of the way in which the next eruption will develop to create a map of the volcanic hazards from Soufriere Hills Volcano. This has been done using an image processing computer to simulate the deposits produced by pyroclastic flows. This technique has the advantages over more traditional cartographic methods of spatial precision, rapid computation of multiple eruption models and the explicit nature of the physical model used. Soufriere Hills Volcano is a small andesitic volcano characterized by a cluster of summit domes and an apron of pyroclastic flow deposits and mudflows upon which several thousand people now live. Most of the flanks were covered by deposits from a series of eruptions from 24000 to 16000 a BP, though there is some evidence that dome growth and small pyroclastic flows have occurred since. The modelling is constrained by field evidence from the deposits of previous eruptions. Although the evidence is not good enough to model individual flow units, the cumulative deposits can be used. From the eruption deposit models we have created a new type of map specifically for emergency planning. This sequential hazard zone map attempts to portray the regions that would be at hazard from pyroclastic flows during successive stages from the start of an eruption whose energy release was increasing with time.
Late Palaeogene–Quaternary geology of Halmahera, Eastern Indonesia: initiation of a volcanic island arc
Authors: HALL R. , AUDLEY-CHARLES M. G. , BANNER F. T. , HIDAYAT S. , TOBING S. L.
Published: December 2007

The Late Palaeogene–Quaternary stratigraphy of Halmahera is described, and new formation names are proposed, based on recent field investigations of the NE and central part of the island. This stratigraphic information provides new insights into the Neogene history of Halmahera and the development of the present island arc. The Late Palaeogene and younger rocks rest unconformably on an ophiolitic Basement Complex which formed part of a Late Cretaceous–Early Tertiary fore-arc. After volcanic arc activity ceased in the Eocene the former fore-arc terrane was uplifted and deeply eroded in the Late Palaeogene. Some of the Late Palaeogene–Early Miocene river valleys are currently being re-excavated by the present rivers. Slow subsidence began in the mid–late Oligocene and by the end of the Miocene all eastern Halmahera was the site of shallow-water carbonate deposition. There is no evidence for arc volcanism in central Halmahera at this time and the reported Oligo-Miocene volcanism in nearby regions is interpreted as volcanism related to the Sorong Fault system. The Miocene shallow water region subsided rapidly in the Early Pliocene and the sedimentary basin formed was filled with marls succeeded by siliciclastic turbidites, with increasing amounts of calc-alkaline volcanic debris from a Pliocene volcanic arc built on the western arms of Halmahera, probably on the eroded Early Tertiary arc. This phase of rapid subsidence in the Pliocene back-arc region resulted from the initiation of subduction of the Molucca Sea lithosphere eastwards beneath Halmahera. Differential subsidence on NW–SE and NE–SW sets of faults in the region immediately behind the active arc led to the formation of deep sediment-filled basins adjacent to the eastern arms. A major deformation event in the Pleistocene resulted in folding and local thrusting at the junction between eastern and western Halmahera and volcanism ceased in the Pliocene arc. The third Halmahera arc, the Quaternary arc, currently active in the northern part of the islands, began activity within the last 1 Ma and is built upon the deformed and partly eroded Pliocene arc. The Pleistocene deformation event and shift in position of the arc are interpreted as the result of the interaction of the eastward-dipping Molucca Sea plate with adjacent plates, either with a fragment of the Australian continent in the Sorong Fault zone and/or with the Philippine Sea plate beneath northern Halmahera.
Early Tertiary structural inversion in the Stord Basin, Norwegian North Sea
Authors: BIDDLE K. T. , RUDOLPH K. W.
Published: December 2007

The Stord Basin is an east-sloping extensional basin that lies to the east of the Viking Graben in Norwegian waters of the northern North Sea. The basin formed by extension with normal faulting beginning in Triassic time and continuing episodically into the Middle and Late Jurassic. Seismic reflection profiles show that the normal faults which affect the sedimentary section are steeply dipping and approximately planar. Some of these faults were reactivated in the early Tertiary. This reactivation reversed earlier-accumulated normal displacement and created relative structural highs out of previous structural lows. The most obvious evidence of fault reactivation and structural inversion is the presence of hanging-wall folds that parallel the strike of reactivated faults. The inverted structures of the Stord Basin are part of a widespread field of early Tertiary inverted structures in north-west Europe. We suggest that these structures formed in response to horizontal compression that resulted from the interaction of stress fields set up by Alpine continent-continent collision and the early opening history of the North Atlantic Ocean.
The deep structure of northern England and the Iapetus Suture zone from BIRPS deep seismic reflection profiles
Authors: FREEMAN B. , KLEMPERER S. L. , HOBBS R. W.
Published: December 2007

The NEC deep seismic reflection profile images structures in the lower crust which represent the Iapetus Suture zone beneath southern Scotland and northern England. On the basis of reflectivity characteristics of the lower crust and reflection Moho we recognize four different crustal zones which correspond to four crustal/terrane types: Midland Valley (zone
A),
a sub-continental subduction complex (zone
B
), Lake District (zone
C
) and Midland Platform (zone
D).
The junctions between each of these zones are interpreted as being tectonic.
A
and
B
originate from the northern continental margin of the Iapetus Ocean and are separated from
C
and
D
(which are derived from a southern continent) by strong northerly dipping reflections between approximately 15 and 30 km deep in the crust. The structure responsible for these reflections does not displace the reflection Moho. North of the
AB/CD
junction, the base of zone
B
(and base of the reflective crust) is marked by a pair of parallel reflectors persistent for a horizontal length of about 55 km. These reflectors are interpreted as the top and bottom of a slice of remnant oceanic crust. At their northerly limit the deepest of the two parallel reflectors transects the Moho to a depth of 6-7 km into the uppermost mantle. We suggest that their termination delimits the northerly extent of a décollement or shear zone at the present continental Moho. The shear zone was active during the late stages of Caledonian collision when mantle, originally from beneath the southern continent, underthrust old and newly created crust of the collision zone.