bluuebunny/crossref_metadata_2025 huggingface dataset:
The San Nicolás batholith of coastal Peru: early Palaeozoic continental arc or continental rift magmatism?
Authors: MUKASA S. B. , HENRY D. J.
Published: December 2007

Major and trace element, U-Pb zircon geochronology and Pb-isotopic data are presented on plutons from the San Nicolás batholith of central and southern Peru.

The major oxides of 9 samples indicate a high-K calc-alkaline trend, and trace element discriminant diagrams reveal affinities to volcanic arc or late- to post-collision calc-alkaline intrusions. The apparent contradiction implied by the Rb-Hf-Ta triangular diagram (within-plate) strongly suggests loss of Rb, an interpretation supported by the extensive deuteric alteration and sericitization of feldspar. Ta/Zr, Ta/Hf and Ta/Nb ratios are considerably higher than those of volcanic arc intrusions, and favour the late or post-collision calc-alkaline classification.

U-Pb zircon geochronology shows that the San Nicolás batholith was emplaced in two short-lived magmatic events at approximately 425 Ma and 394-388 Ma ago. The presence of an inherited component of radiogenic Pb of Precambrian age in the zircons suggests xenocrystic cores, which were most likely scavenged from Precambrian basement rocks of the Arequipa massif.

Time-adjusted Pb isotopic compositions of feldspars have the following ranges:
206
Pb/
204
Pb = 17.798-18.272,
207
Pb/
204
Pb = 15.571-15.626, and
208
Pb/
204
Pb = 38.182-38.291, indicating that Precambrian basement was incorporated into the magmas of the batholith. Other components are believed to have originated in an ‘enriched’ upper mantle or lower crustal wedge pervasively metasomatized by fluids extracted largely from subducted sediments.

U-Pb zircon age distribution, trace element characteristics and the linear nature of the plutons firmly establish the San Nicolás batholith as the plutonic substructure of an early Palaeozoic continental arc, which may extend northwards in Peru and southwards
Ophiolites in Northeast and East Africa: implications for Proterozoic crustal growth
Authors: BERHE SEIFE M.
Published: December 2007

The presence of ophiolite complexes in NE and E Africa has been documented using Landsat, field and geochemical studies. The present work identifies five major ophiolitic sutures in NE Africa, while plate reconstruction of Africa and Madagascar suggests a possible sixth ophiolite belt to the east. The ophiolites are considered to be remnants of supra-subduction zones and back-arc basins. The ophiolites are dismembered, and their mode of occurrence varies widely resulting in different structural relationships. In Western Ethiopia, the Yubdo complex is formed of harzburgite which grades into a cumulate sequence of ultramafic and gabbroic rocks and metabasalts. In Kenya, the Baragoi complex is formed of tectonized harzburgite with dunite and chromite pods, a cumulate sequence of ultramafic and gabbroic units and a dyke unit. Trace element data from the Baragoi complex show a transitional mid-ocean ridge basaltic to island arc tholeiitic affinity, and the presence of boninites suggests a supra-subduction setting, while data from the Adola-Moyale belt (S. Ethiopia-NE Kenya) indicate an island-arc and MORB geochemistry, which developed in a back-arc tectonic setting. In Sudan, the Ingessana complex which has an island arc tholeiitic tectonic affinity indicates development in a supra-subduction setting, while the trace element suggest that the Sol Hamed complex developed in a back-arc basin.

These ophiolite belts represent sutures marking the position of island arcs extending to those in Saudi Arabia on a pre-Red Sea drift reconstruction and further south to Mozambique. The terrane delimited by a proposed Ingessana-Port Sudan suture and by the Adola-Moyale ophiolite belts as defined in this study encompasses an area in which the crust was derived from oceanic and island arc material. Relative age reconstruction across the Arabian-Nubian Shield in East Africa broadly indicates eastward younging of the ophiolite belts. Regional geological, tectonic and geochemical studies suggest rifting at
c.
1200 Ma and subsequent convergence led to the development of intra-oceanic arcs and associated marginal basins in the north and narrow basins within the sialic basement gneisses further south in Kenya and Tanzania. This was followed by continent-continent collision which led to accretion of island arcs by gentle collision from the northeast in Saudi Arabia and severe crustal shortening in S Sudan, Kenya and SE Ethiopia as compared to Saudi Arabia, NE Sudan and N and W Ethiopia owing to oblique collision from the southeast.
The Sunnfjord Melange, evidence of Silurian ophiolite accretion in the West Norwegian Caledonides
Authors: ANDERSEN T. B. , SKJERLIE K. P. , FURNES H.
Published: December 2007

A major composite terrane, the Sunnfjord Melange, has been identified in the West Norwegian Caledonides. The rocks of the melange provide a terrane-link between the allochthonous continental rocks of the Dalsfjord Suite with its cover of continental margin deposits and the oceanic terrane of the Solund-Stavfjord Ophiolite Complex. The melange was formed as the ophiolite was emplaced on the the fossiliferous Lower to Middle Silurian continental margin deposits of the Herland Group. This group unconformably overlies older metasedimentary rocks of the Høyvik Group and the crystalline basement of the Dalsfjord Suite. A structural style characteristic of thin-skinned thrust-foldbelts is locally preserved within the Herland Group on Atløy, and the thrust-foldbelt was developed in the foreland of the continental margin during the ophiolite accretion. A U-Pb zircon age of 443 ± 3 Ma from the ophiolite, the Silurian fossils in the Herland Group and the identification of the Sunnfjord Melange as an obduction melange provide the basis for a well constrained model of ophiolite accretion in the Scandinavian Caledonides.
The Moine Thrust in the BIRPS data set
Authors: SNYDER DAVID B.
Published: December 2007

A second generation of BIRPS profiles combine with older data to form a loose (10 by 40 km spacing) grid over the offshore projection of the Moine Thrust. These profiles show clear reflections that arguably represent fault plane reflections from the Moine and related Caledonian thrust structures in the location suggested by some earlier workers. Reflectors predominate at two structural levels: 8-10 km (2.5-3.5 sTWT) and 13-16 km (4.0-5.5 sTWT) depths. The deeper set appears to define an antiformal stack of thrusts or duplex with possible continuation to deeper levels. By analogy with onshore sections and after subsequent extension is restored, this deeper set correlates with the Moine Thrust onshore. The shallower set aligns with normal faults that bound half grabens filled with Devonian and Permo-Triassic rocks. This set correlates with the Naver Thrust after similar restoration. Normal faulting appears to have reactivated some of the thrusts, increasing their reflectivity. Detailed stacking- and interval-velocity studies indicate that an increase of about 1-2km s
−1
occurs across the reflections here associated with the two fault zones. The overall reflection pattern is consistent with a complex stack of thrust sheets, but lack of resolution prevents a complete structural analysis.
The Devonian lacustrine sediments of Orkney, Scotland; implications for climate cyclicity, basin structure and maturation history
Authors: ASTIN T. R.
Published: December 2007

The Orcadian Basin of northern Scotland (largely of Middle Devonian age) is a major lacustrine rift basin, with the widely correlateable, Achanarras/Sandwick lake bed extending for at least 800 km along the rift. Above this lake bed, there are about 540 m of cyclic lacustrine sediments, with lake sedimentation terminated by alluvial fan progadation. The Upper Stromness Flagstone Formation and the Rousay Flagstone Formation making up this lacustrine sequence are given revised and precise definitions within four reference sections from the Orkney Islands. The sequence consists of about 45 first-order cycles averaging about 12 m in thickness, which resulted from long-term climatic fluctuations in rainfall with a cycle time-scale of about 25 000 years. Longer climatic fluctuations of about 100000 and 400000 years are also present. These periods correspond to the precession and two eccentricity orbital cycles.

Detailed knowledge of the stratigraphy constrains the structural and maturation history of the basin. The basin formed in a series of extensional half graben, and the Orkney area suffered later inversion. The lake sediments first reached the oil window at relatively shallow depths (
c
. 2000 m), probably in the early Carboniferous, implying very high geothermal gradients during the early part of the basin history as corroborated by the presence of Devonian volcanics in the region. In the Orkney area, uplift probably started in the late Carboniferous.
Basin evolution and fault reactivation in the western Cameros Basin, Northern Spain
Authors: PLATT NIGEL H.
Published: December 2007

Mesozoic-Cenozoic sedimentation in the Cameros Basin area strongly reflects tectonic factors. The tectonic history is dominated by two principal extensional phases (Triassic, Late Jurassic-Early Cretaceous) and strong Pyrenean compression (Oligocene). Late Variscan NW-SE and NE-SW fault sets were repeatedly reactivated as dip-slip and wrench faults respectively throughout extension and compression. E-W Variscan thrust planes showed relatively minor reactivation in this area.

Mesozoic fault control on sedimentation resulted in significant lateral variations in stratigraphic and sedimentological development. Variation in deformation style and thrust architecture reflects the differing responses of the laterally heterogeneous sequence to Oligocene compression.
Structural and stratigraphic evolution of the West Jaca thrust-top basin, Spanish Pyrenees
Authors: TURNER JONATHAN P.
Published: December 2007

Thrust-top basins form and fill during their detachment from the foreland margin by active thrust sheets. Strike-normal (N–S) sections from the West Jaca thrust-top basin (Southwest Pyrenees) to the Ebro basin show that the Pena flexure, the local expression of the thrust front, can be interpreted as a passive-roof duplex. The duplex accommodated up to 7 km bulk shortening at the tip of the Gavarnie thrust sheet. Unravelling the timing of uplift and sedimentation in the West Jaca basin has revealed that development of the Pena flexure represented the climactic phase of deformation. Lateral (E–W) diachroneity of gross stratigraphy and thrust emergence throughout the southwest Pyrenees suggests that the thrust front propagated both toward the foreland and westward, parallel to the orogen axis.
Footwall uplift associated with Late Jurassic normal faulting in the northern North Sea
Authors: YIELDING GRAHAM
Published: December 2007

Tilted fault-blocks in the North Sea show varying degrees of erosion. Such footwall erosion is predicted by the domino model of extension, in which rigid fault-blocks rotate as extension proceeds. Profiles of three North Sea fields (Snorre, Brent and Heather) are analysed to estimate the amount of uplift that accompanied Late Jurassic normal faulting, and these estimates are compared with the footwall uplift predicted by the domino model. Agreement between observed and predicted uplift is excellent, varying from >1 km at Snorre to
c
. 100 m at Heather. For a given amount of extension, large fault-blocks tend to be bounded by large faults and to show a greater degree of erosion.

The Brent Province of the northern North Sea is an area of prolific hydrocarbon discoveries. Almost all of the major oil-fields occur in tilted fault-blocks which were formed by Late Jurassic extension and subsequently buried by Cretaceous-Tertiary thermal subsidence (

Badley
et al.
1988

;

Marsden
et al.
1990

). The crests of the tilted blocks are typically eroded or degraded, with varying amounts of Middle-Lower Jurassic and Upper Triassic section removed. This footwall erosion is commonly ascribed in the literature to pre-rifting doming (
Ziegler 1982
), heterogeneous stretching (
Coward 1986
), local transpression (
Beach 1985
) or even regional compression (
Frost 1987
). The purpose of this paper is to show that the uplift and degradation is a direct consequence of the Late Jurassic normal faulting and fault-block rotation. The degree of uplift is related in a simple way to the size (across strike) of
Geochemistry and origin of the Archaean Beit Bridge complex, Limpopo Belt, South Africa
Authors: BORYTA MARK , CONDIE KENT C.
Published: December 2007

The Beit Bridge complex (BBC) in the Central Zone of the Limpopo Belt in Southern Africa includes metasedimentary rocks of Archaean age (chiefly metapelites, quartzites and carbonates), amphibolites (chiefly mafic dykes and sills) and granitoids which comprise ≥50% of the complex. The amphibolites are tholeiites with relatively unfractionated REE patterns and pronounced subduction-zone geochemical signatures in terms of relatively immobile incompatible elements. Tonalites and granites, which also exhibit such a signature, can be formed by batch melting of a mafic source with residual garnet or amphibole. Most granites have negative Eu anomalies and appear to represent residual liquids.

BBC metapelites have negative Eu anomalies and other immobile element distributions similar to Phanerozoic shales. One group of metapelites that shows significant depletion in Ca, Na and Sr appears to have been derived from intensely weathered sources. The negative Eu anomalies in BBC metapelites suggest intracrustal melting of their sources and thus the presence of evolved continental crust in this part of Southern Africa by about 3000 Ma. BBC quartzites are quartz-rich sandstones that are depleted in HFS elements, Ba and Sr, relative to Phanerozoic counterparts; a feature that probably reflects less recycling during the early Archaean. BBC banded iron formation (BIF) is similar to other Archaean BIF and shows large positive Eu anomalies. BBC carbonates are dolostones depleted in REE, HFS elements, U, Th, Ba, Sr, Ni, Sc, V and Y and enriched in Fe, Mn, Na, Rb and Cs relative to Phanerozoic marine carbonates.
Structural trends in central Britain from images of gravity and aeromagnetic fields
Authors: LEE M. K. , PHARAOH T. C. , SOPER N. J.
Published: December 2007

Digital image processing techniques have been used to analyse the regional gravity and aeromagnetic datasets in central Britain. Colour and shaded-relief images have been generated which convey information on both anomaly amplitude (as colour) and anomaly gradient (as relief) and highlight structural trends, lineaments and textural contrasts not easily discernible on standard contour maps. We have identified some of the more important features and have tried to relate these to the evolution of the crust in each region. On a broad scale the images emphasize Caledonoid (ENE) trends to the north of the Solway line and both Tornquist (SE) and Caledonoid (NE to ENE) trends to the south. The pattern of magnetic anomalies over central England seems to define the extent of the shallow late Precambrian–early Palaeozoic basement of the Midlands Microcraton and delineates crustal elements and characteristic trends within it. Magnetic lineaments and the grain of gravity anomalies, trending in northeasterly and northwesterly directions on either side of the microcraton, seem to relate to structures which originated during the evolution of the Welsh and eastern English Caledonides respectively. Where Lower Palaeozoic and Precambrian rocks form the concealed ‘basement’, as in most of England, the gravity lineaments tend to reflect the influence of basement fractures on the subsequent pattern of sedimentation whereas many of the features on the magnetic images are more directly related to structures within the basement itself.
Do fossil plants give a climatic signal?
Authors: CHALONER WILLIAM G. , CREBER GEOFFREY T.
Published: December 2007

The agreement in the pattern of major biomes with that of climatic zonation of the Earth gives a strong indication that climate is the overriding influence controlling the distribution of plant communities. Three aspects of plants which may be preserved in the fossil state, give a signal of the climatic conditions under which they grew: (1) the present climatic association of their 'nearest living relative'; (2) leaf physiognomy of arborescent plants; (3) the character of their secondary xylem ('wood' of ordinary usage) reflecting, by the presence or absence of growth rings, the seasonality (or lack of it) in their environment and the potential for tree growth that it offered. The significance and limitations of these 'palaeoclimatic signals' as they may be read from the fossil plant record are reviewed and evaluated. The recent demonstration that stomatal frequency of leaves is responsive to changes in ambient carbon dioxide partial pressure offers promise for direct palaeobotanical evidence for past changes in the level of this climatically significant atmospheric constituent.
Is the Colonsay-west Islay block of SW Scotland an allochthonous terrane? Evidence from Dalradian tillite clasts
Authors: FITCHES W. R. , MUIR R. J. , MALTMAN A. J. , BENTLEY M. R.
Published: December 2007

In the Scottish Caledonides context, the Colonsay-west Islay terrane has seemed anomalous and therefore likely to be allochthonous. New studies suggest, however, that its basement is not necessarily exotic but represents the missing early Proterozoic link between Greenland and Scandinavia. Recent interpretations of the Scottish mainland deformation history imply that the age of the Grampian events there resembles that of the late Proterozoic deformation previously identified in the Colonsay Group cover succession. Reconnaisance studies of Dalradian tillite provenance suggest that the Colonsay-west Islay basement was a source of Dalradian sediment and perhaps floored that basin. There is, therefore, less reason now to regard the Colonsay-west Islay terrane as allochthonous, and recent hypotheses of its tectonic emplacement being the cause of the Grampian orogeny should be viewed with caution.
The Caledonian Heilhornet Pluton, north-central Norway: geological setting, radiometric age and implications for the Scandinavian Caledonides
Authors: NORDGULEN Ø. , SCHOUENBORG B.
Published: December 2007

The Heilhornet Pluton is a medium-sized (
c
. 50 km
2
), sub-circular, discordant intrusion which penetrates the thrust zone separating the Helgeland Nappe Complex from paragneisses in the Vestranden region, north-central Norway. The intrusion consists of biotite ± hornblende granite and ranges in composition from 61.5 to 72.5 wt.% Si0
2
The initial Sr-ratio (
c
. 0.7070) shows that crustal sources have been involved in the petrogenesis of the granite. A U-Pb zircon age of 444 ± 11 Ma for the pluton indicates that juxtaposition of the nappe complex with metasedimentary rocks in Vestranden took place prior to or during the late Ordovician. A consequence of this is that the general correlation of the metasedimentary rocks in Vestranden with the Seve Nappes must be revised. Final eastward thrusting of the complex, together with a yet unknown part of the Vestranden supracrustal sequence, probably took place during the middle Silurian to early Devonian, Scandian, orogenic event.
Plate reorganization: a cause of rapid late Neogene subsidence and sedimentation around the North Atlantic?
Authors: CLOETINGH S. , GRADSTEIN F. M. , KOOI H. , GRANT A. C. , KAMINSKI M.
Published: December 2007

Subsidence analysis of wells in the central North Sea and Labrador-Grand Banks and oft the West Greenland, Scotian shelf and United States Atlantic margin shows distinct quantitative stratigraphic correlation patterns of circum North Atlantic sites. A significant departure from the overall decrease in subsidence for the Pliocene occurs in many wells, when the rate is found to have increased one or more orders of magnitude from Oligocence/Miocene rates. Wells were selected along transects from shore to basin to find if relative basin position is influenced by differential basin subsidence. Although stratigraphic resolution is not detailed, more basinward sites experienced up to four times larger subsidence rates in the late Neogene than in the Oligocene/Miocene, with a peak in the Pliocene. Wells at the basin edge experienced much less subsidence or showed uplift. The observations are consistent with a rapid change of intraplate stress at the cause for this observed transition in Neogene subsidence. Major reorganizations of spreading direction and rate occurred during the Pliocene along the entire Atlantic spreading system, possibly in conjunction with more global changes in plate motions. We propose that the associated changes in intraplate stress caused the excess margin subsidence. Relative uplift along basin edges is consistent with this mechanism of relative movement and may explain apparent eustatic changes in sea level.
Sequence stratigraphy of the Palaeozoic Welsh Basin
Authors: WOODCOCK N. H.
Published: December 2007

The timing and extent of unconformities in the Welsh Basin are investigated using ‘rock preservation curves’ derived from outcrop stratigraphic logs. Four basin-wide unconformities occur, focussed in late Precambrian, late Tremadoc, Pusgillian (early Ashgill) and mid-Devonian times. These bound three megasequences, equating with newly defined lithostratigraphic units, the Dyfed, Gwynedd and Powys Supergroups. Less extensive unconformities bound 18 component sequences.

The majority of the sequence boundaries reflect a component of tectonic or volcanotectonic activity rather than a pure eustatic sea-level change. The megasequence boundaries are attributed to late Precambrian to early Cambrian onset of rifting to form a passive margin, Tremadoc onset of subduction with intra-arc then back-arc extension, late Caradoc end to subduction, and late Early Devonian collisional deformation. The megasequences and controlling events can be tentatively matched with other basins on the Avalonian margin.

More generally, this study shows that sequence analysis is feasible in onshore basins lacking well and seismic data, and that the global eustatic interpretation of sequence stratigraphy is only partially applicable to active margin basins.
Age of crystallization and cooling of the K2 gneiss in the Baltoro Karakoram
Authors: SEARLE M. P. , PARRISH R. R. , TIRRUL R. , REX D. C.
Published: December 2007

The mountains of K2 (8611 m) and Broad Peak (8047 m) in the Baltoro Karakoram (northern Pakistan) are composed of plagioclase–homblende and biotite–hornblende-K-feldspar orthogneisses and amphibolite–facies paragneisses, intruded by garnet–biotite–muscovite–tourmaline leucogranitic veins. A U-Pb zircon age of 115–120 Ma was obtained on an orthogneiss from the south face of K2.
40
Ar-
39
Ar analysis on hornblende yields a plateau age of 90.6 ± 1.8 Ma, consistent with a mid-Cretaceous phase of magmatism, concomitant with early subduction-related components of the Karakoram batholith (Muztagh Tower unit, Hunza plutonic unit). We interpret the K2 gneiss as representing a culmination of mid-crustal rocks along a discontinuous but wide zone north of the Karakoram batholith.
Ordovician graptolite biostratigraphy in the Welsh Basin: a review
Authors: RUSHTON A. W. A.
Published: December 2007

Ordovician graptolite biostratigraphy in Wales is crude compared with that of the Silurian and is in need of refinement, especially by detailed collecting from suitable sections and consequent taxonomic work. This has been done in parts of the Arenig, Llandeilo and lower Caradoc successions. There is particular potential for subdivision of and improvement in the correlation of the Llanvirn Series and the
gracilis-linearis
biozones, but there is little prospect of much new and detailed information from the upper Tremadoc or Ashgill Series.
Dinantian dolomites from East Fife: hydrothermal overprinting of early mixing-zone stable isotopic and Fe/Mn compositions
Authors: SEARL A. , FALLICK A. E.
Published: December 2007

Thin marine limestones from the Dinantian of the St Monans syncline, East Fife, are extensively dolomitized. They occur within a deltaic sequence cut by Upper Carboniferous volcanic rocks. The volcanic rocks have been extensively modified by CO
2
-enriched hydrothermal fluids. The dolomite is ferroan and nonluminescent. Stable isotope compositions for groundmass dolomite range from δ
18
O = −4.76‰ to −0.47‰ and δ
13
C = −3.29‰ to 0.60‰. Later, void-filling dolomite falls into two groups; the first is
18
O depleted, with δ
18
O as low as −10.6‰, while the second is
18
O-enriched, δ
18
O = −2.8‰ to −1.2‰. Despite almost homogeneous staining characteristics, backscattered SEM imaging shows the dolomites to be chemically extremely heterogeneous. Dolomitization was a multistage process in which an early ‘mixed water’ component was variably remobilized to form the ‘matrix dolomite’. Biomolds and other vuggy post-early-dolomite porosity are filled by dolomite precipitated within a volcanically driven hydrothermal system. The chemistry of the later dolomite reflects the mixing of upward moving hydrothermal fluids, modified by interaction with volcanic material, with downward moving seawater feeding the hydrothermal system. In addition, material released during normal processes of siliciclastic diagenesis added a further component to the late stage dolomitization. Only two of the limestones escaped complete dolomitization and appear to have undergone early post-depositional diagenesis within meteoric porefluids. They contain early meteoric calcite cements and later post-compaction calcite cements, dolomite and siderite. One of the implications of this study is that the interpretation of dolomite geochemical data can be vastly enhanced by the use of back scattered electron microscopy to reveal otherwise hidden compositional heterogeneity.
The scientific ancestry and historiography of
The Silurian System
Authors: TORRENS H. S.
Published: December 2007

The historical background to Murchison's massive work of synthesis of 1839,
The Silurian System,
is considered with special reference to Shropshire, one of its most critical areas of investigation. The question of why there has always seemed to be such a great chronological gap between the onsets of industrialization and of geological investigations (subjects often thought to be intricately linked) is posed. Evidence is given for the considerable amount of pre-Murchisonian investigation of the geology in the area. It proves to be often of a notable quality, particularly after 1810 with new focus on the ordering of stratigraphical sequences. It becomes clear that Murchison was often unwilling to acknowledge properly his debts, whether factual or conceptual, to those who had gone before him. Murchison's work as a 'Silurian' historian proves of noticeably lower quality than his work as a stratigrapher.
The diagenetic and economic significance of composite grains of monazite and hydrocarbon in Western Australian arenites
Authors: RASMUSSEN BIRGER , GLOVER J. E.
Published: December 2007

Composite grains consisting of detrital monazite and solid hydrocarbon, ranging up to 0.5 mm in diameter, have been found in Phanerozoic sedimentary rocks in three Western Australian basins. The most common form is a monazite core surrounded by a hydrocarbon envelope. The envelopes, which have conchoidal fracture and radial cracks, apparently congealed when hydrocarbons were polymerized and crosslinked by irradiation from the monazite. Some composite bodies contain fractured monazite grains within solid hydrocarbon and a few consist of a microbreccia of hydrocarbon and disseminated monazite fragments. The breakup of the monazite was probably a diagenetic chemical process that began when crosslinked hydrocarbons within cracks and open cleavages swelled as they were infiltrated by fluid hydrocarbons of low molecular weight.

Five microprobe analyses of the monazite reveal the following ranges in percent composition: La
2
O
3
, 10.88–13.66; Ce
2
O
3
, 25.58–28.78; Pr
2
O
3
, 3.03–5.38; Nd
2
O
3
, 9.15–12.11; ThO
2
, 7.0–11.49; U
3
O
8
, 0–1.44. 2V
γ
extends from 12.5–15.5°.

In the northern Perth Basin, the composite bodies, which contain cores of monazite derived from local gneissic basement rocks, formed after the growth of siderite cement but before extensive silicification. In the central Perth Basin, the composite bodies seem to have formed after the growth of kaolinite and before silicification. Textures associated with the bodies allow them to be slotted into the diagenetic sequence and they therefore show when the oil was present. Analysis of the bodies has indicated a contribution of oil from the Lower Triassic Kockatea Shale, but some may have grown from more than one generation of petroleum.

Given the ubiquity of detrital monazite, similar bodies are likely to be useful diagenetic and economic indicators elsewhere.
Seismic images of continental lithosphere
Authors: BLUNDELL D. J.
Published: December 2007

Seismic reflection profiles in USA and West Germany over thirty years ago showed strong reflections from the lower crust but technical advances through the 1970s were needed for the method to image the continental lithosphere. The British Institutions Reflection Profiling Syndicate, BIRPS, found advantage in working offshore and has obtained over 12,000 km of profiles around Britain during the past decade, giving the densest regional coverage anywhere. Reflections are observed from strata in sedimentary basins, from low angle faults that can be followed from surface to mid-crust, from a laterally variable reflective zone in the lower crust, from the Moho and from fault-like features in the upper mantle. There is evidence around Britain that faults bounding sedimentary basins are older structures which have been reactivated. It is argued that lower crustal reflectivity around Britain is mainly due to anastomosing ductile shears which accommodate extension in the lower crust commensurate with extension in the upper crust by faulting and block rotation. There is also evidence of reflections preserved from older orogenic features, so that reflections interpreted as extensional ductile shears in the lower crust may also derive from older features that have been reactivated. Most reflections in the lower crust arise from rocks that have been highly strained. Evidence of lateral variability in the reflective character of the Moho suggests that the Moho has been actively involved in the structural evolution of the lithosphere. Upper mantle reflections are interpreted as shear zones which transfer crustal deformation through the lithosphere.
A multiple fluid history recorded in Alpine ophiolites
Authors: FRÜH-GREEN GRETCHEN L. , WEISSERT HELMUT , BERNOULLI DANIEL
Published: December 2007

Petrographic and cathode luminescence studies of Alpine ophicalcites are combined with stable isotope data of pelagic sediments and associated ophiolite relicts, to document multiple phases of fluid-rock interaction related to sea-floor processes and Alpine orogeny. Internal sediments and primary carbonate microstructures in ophicalcites provide evidence for early sea-floor fragmentation, cementation and fluid activity in the Jurassic Tethys. Ca-carbonate cementation is comparable to cementation in modern transform setting analogues, where alkaline and Ca-enriched fluids are associated with serpentinites. Local mineralization and isotopic compositions of serpentine in ophicalcites are interpreted as signs of hydrothermal activity at temperatures of 100–150 °C.

Differences in the degree of oxygen isotope re-equilibration reflect differences in fluid/rock interaction during Alpine accretionary tectonics and continent-continent collision. Regional-scale homogenization of oxygen isotopes in carbonates and cherts indicate the presence of pervasive metamorphic fluids in the pelagic sediments during early Alpine recrystallization. In contrast, a lack of oxygen isotope re-equilibration in serpentine may reflect lower permeabilities and more limited fluid flow in serpentinites during Alpine metamorphism. Cathode luminescence and preliminary oxygen isotope data of late vein-forming phases suggest that late metamorphic fluids were channelled along discrete brittle fractures in a closed, rock-dominated system.
The structure and stratigraphy of SE Zanskar, Ladakh Himalaya
Authors: McELROY R. , CATER J. , ROBERTS I. , PECKHAM A. , BOND M.
Published: December 2007

A field study of over 4 km thickness of Cambro-Ordovician to Cretaceous sediments deposited on the passive margin of northern India has provided significant new data on the thickness, age and depositional environments of these deposits. The first detailed structural map of the Phuctal area and a regional map of Eastern Zanskar are presented, together with sequentially restored structural cross sections. An imbricate thrust duplex and a lateral ramp stack, which formed in the Phuctal area during SW-directed thrust propagation, were later deformed by NE-directed structures probably produced during gravitational collapse of the thrust stack. Collapse occurred along the previously documented Zanskar shear zone, and also by reactivation of the basal detachment of the Zangla nappe. This dorsal collapse of the orogen was probably related to an increase in uplift rate during the Neogene, and may have coincided with the initiation of the Main Boundary Thrust farther south.
Geodynamic evolution of the northern and central Andes during early to middle Mesozoic times: a Tethyan model
Authors: JAILLARD ETIENNE , SOLER PIERRE , CARLIER GABRIEL , MOURIER THOMAS
Published: December 2007

The early to middle Mesozoic sedimentary and magmatic history of the northern and central Andes (i.e. the NNE-trending Colombian-Ecuadorian segment, and the NW-trending Peruvian segment) exhibits a spatially contrasted evolution involving several successive tectonic and geodynamic settings.

The late Triassic to late Liassic period began with a widespread marine transgression. From the latest Triassic, the Colombian marine shelf was progressively destroyed by southward propagating extensional tectonic activity but marine sedimentation continued in Peru. During this time, no significant magmatic activity is recorded except in the emerging Colombian area. This period is interpreted as the result of rifting of a Tethyan oceanic arm which separated the Colombian and palaeo-Mexican margins.

From latest Liassic, an important calc-alkaline magmatic arc developed along the emergent Colombian segment. Further south, the north-Peruvian shelf probably emerged, but marine sedimentation continued in southern Peru. During middle and early Late Jurassic times, the Colombian segment was characterized by important magmatic activity and by coarse clastic continental sedimentation. Along the Peruvian segment, a turbiditic trough, emergent areas, and continental basins were created, and the scarcity of calc-alkaline magmatism suggests that only very local subduction took place. This period is regarded as one of the southeastward subduction beneath the Colombian segment, and of Tethyan oceanic crust originating in the newly formed ‘Colombian’ oceanic arm. This pattern would have induced a chiefly left-lateral transform motion along the Peruvian segment.

By Kimmeridgian-Tithonian times, the palaeogeographic framework had drastically changed. Along the Colombian segment, magmatic activity ceased, and continental accretions occurred along dextral strike-slip sutures. In Peru, tectonic activity was recorded by the creation of a new turbiditic trough and by the resumption of detrital sedimentation. In the coastal area, arc-related volcanism indicated that subduction took place beneath this segment. This geodynamic change is interpreted as the result of a sharp decrease in the spreading activity of the Tethyan ridges and replacement by Pacific spreading centres inducing a roughly northeastward convergence direction.
Geochemical character of Silurian volcanism in SW Ireland
Authors: SLOAN R. J. , BENNETT M. C.
Published: December 2007

The Dunquin Group (mid-Silurian) of Dingle, southwest Ireland is a sequence of basaltic to rhyolitic lavas and pyroclastic rocks interbedded with shallow- to non-marine sediments that records one of only four known outbreaks of Silurian volcanicity south of the Iapetus Suture in the British Isles. New geochemical, petrographic and field evidence indicate that the volcanic sequences exposed in the Dunquin inlier, and the islands of Beginish and Inishvickillane could have formed by fractionational crystallization in a layered, repeatedly replenished basaltic magma chamber, the site of which is probably coincident with an intense aeromagnetic anomaly to the southwest of Inishvickillane. The volcanic suite is subalkaline, transitional between calc-alkaline and tholeiitic and is non-bimodal. High LILE/HFSE ratios and high LREE concentrations suggest that the parent basaltic magmas were generated at a destructive continental margin, although subduction may have ceased before eruption.
Silurian and Early Devonian sinistral deformation of the Ratagain granite, Scotland: constraints on the age of Caledonian movements on the Great Glen fault system
Authors: HUTTON D. H. W. , McERLEAN M.
Published: December 2007

The Ratagain granite complex (425 ± 3 Ma) was emplaced adjacent to the Strathconan Fault, an important member of the Great Glen fault system, and was deformed by sinistrpl shear associated with this fault just prior to its complete crystallization. It was deformed again by low temperature sinistrral mylonite and breccia zones penecontemporaneously with the regional minette dyke swam (410-395 Ma). Very little evidence is seen in the complex of deformation in the high temperature/post-crystsllization intend, and it is concluded that Caledonian sinistral motion on the Strathconan Fault occurred in two distinct phases: Mid-Siurian (Wenlock) and Early Devonian.
Geochronological evidence from discordant plutons for a late Proterozoic orogen in the Caledonides of Finnmark, northern Norway
Authors: DALY J. S. , AITCHESON S. J. , CLIFF R. A. , GAYER R. A. , RICE A. H. N.
Published: December 2007

Rb-Sr whole-rock, Sm-Nd mineral and U-Pb zircon dates are reported for minor 0granites and Seiland Igneous Province gabbros in the Kalak Nappe complex (Middle Allochthon), northern Norwegian Caledonides. The Litlefjord (Rb-Sr WR 813 ± 62 Ma, MSWD 2.7; U-Pb zircon 804 ± 19 Ma, MSWD 5.5) and Repvig (Rb-Sr WR 851± 130 Ma, MSWD 5.1) granites and the Hasvik gabbro (Sm-Nd WR/minerals 700 ± 33 Ma, MSWD 0.7) cut previously-deformed metasediments of the Scbrcby Succession while the Kvalfjord (Sm-Nd WR/minerals 612 f ± 33 Ma, MSWD 0.1) and Storvik (Sm-Nd WR/minerals 604 ± 44 Ma, MSWD 0.2) gabbros cut ‘basement' paragneisses.

The main (Dl–D2) deformation affecting the Sbrcby Succession in the Kalak Nappe complex, which was previously attributed to the early Ordovician Finnmarkian phase of the Caledonian orogeny, took place before c. 800 Ma in the pre-Caledonian 'Porsanger Orogeny'. Early ‘Sørøy’ plutons of the Seiland Igneous Province were apparently emplaced during this event. However, the later, more voluminous ‘Stjernøy’ intrusions are
c
. 700–520 Ma old and post-date the Porsanger orogeny. These pre-Caledonian intrusions must be inter-, rather than syn-orogenic as previously claimed. This obviates the problem of accomodating the voluminous Seiland magmatism during orogenic compression and is consistent with an early Caledonian rift origin for these rocks, probably related to the opening of Iapetus.
Exhumed lower crust in NW Ireland, and a model for crustal conductivity
Authors: SANDERS I. S.
Published: December 2007

Granulite-facies gneisses within the Ox Mountains inlier, NW Ireland, are interpreted as an exhumed segment of sub-Dalradian lower crust, formed at 605 ± 37 Ma and later wedged upwards by crustal imbrication along the N-dipping Highland Boundary Fault-Clew Bay terrane boundary. A lower crustal provenance for the gneisses is suggested by evidence for eclogite decompression and for the later replacement of sillimanite by kyanite in response to slow cooling at depth.

Granulite-facies rocks are believed to be present throughout much of the lower continental crust. Granulites are electrically resistant in contrast to the lower crust, which is usually electrically conducting. This paradox may be resolved by inferring that the granulite-facies rocks form dry, kilometre-scale lenses (mega-augen) wrapped around by anastomosing, retrograded, brine-soaked, electrically-conducting shear zones.

Retrogressive hydration is believed (a) to seal the margins of the dry mega-augen with expanded reaction products, and (b) to concentrate the residual brine. Amphibolite-facies rocks associated with the lower crustal shear zones (above about 350°C) would equilibrate with strong, unsaturated brine and become electrically conducting. In the upper crust, similar rocks would concentrate the brine to the point of saturation, whereupon halite would precipitate, all water be consumed, and the rock would become an insulator.
Symposium on ophiolite genesis and evolution of oceanic lithosphere
Authors: SEARLE M. P.
Published: December 2007

The recently published Special Publication No 49 of the Geological Society of London The Geology and Tectorlics of the Oman Region edited by A. H. F. Robertson, M. P. Searle and A. C. Ries (Feb. 1990) resulted largely from a six year AMOCO-funded project concentrating mainly on the structure, strat atigraphy and sedimentology of the platform margin and basin sediments beneath the Semail ophiolite. This study, and more particularly the BRGM-French project (1982–1990) formed the basis of the ophiolite conference. It was held at the Sultan Qaboos University, Sultanate of Oman, 7–18 January 1990, and was the first, major, international geological conference to be held in Oman. Over 300participants from 27 countries attended.

Since the classic work of
Glennie et al. (1974)
and the Shell team, the Semail ophiolite in the Oman Mountains has been widely regarded as the largest, best exposed and most spectacular example of an obducted ophiolite complex anywhere in the world. Subsequent field projects, including the Open University Oman Ophiolite project (1975–1985) summarized by Lippard et al. (1986) and the University of California-USGS project (1977–1979) published in a special issue of Journal of Geophysical Research (1981) vol.
86
, concentrated mainly on the Semail ophiolite complex.

Since 1982, the Bureau de Recherches Geologiques et Minieres (BRGM) have remapped large areas of the central and southern mountains and 13 maps at 1: 100,OOO scale have been published. Together with new detailed studies of the Semail ophiolite, some of the French teams’ results have been published in a
The relationship between organic maturation, the widths of thermal aureoles and the thicknesses of sills in the Midland Valley of Scotland and Northern England
Authors: RAYMOND A. C. , MURCHISON D. G.
Published: December 2007

The contrasting nature of thermal aureoles that surround mid-Carboniferous alkaline-dolerite sills and early Permian quartz- dolerite silk of the Midland Valley of Scotland and northern England is described. The development of the aureoles is influenced by the degree of consolidation of the sediments, their volume of pore water when invaded by the magma, the time period between deposition of the invaded sediments and emplacement of the intrusion and the degree of organic maturation at the time of intrusion. A correlation exists between the logarithms of the thicknesses of sills and thermal aureoles of the quartz dolerites. Equivalent data for the alkaline dolerites lie more than two standard errors from the regression line for the quartz-dolerite plots.
40
Ar/
39
Ar geochronology of Alpine tectonism in the Betic Cordilleras (southern Spain)
Authors: MONIE PATRICK , GALINDO-ZALDIVAR JESUS , LODEIRO FRANCISCO GONZALEZ , GOFFE BRUNO , JABALOY ANTONIO
Published: December 2007

The
40
Ar/
39
Ar dating method has been applied to metamorphic rocks of the Alpujarride and Nevado-Filabride nappes (Alboran domain, SE Spain) in a first attempt to discriminate individual phases of deformation and metamorphism. The upper Nevado-Filabride nappes experienced an early eclogitic and blueschist metamorphism for which a barroisitic amphibole indicates a minimum age of 48 Ma. An Early Miocene age is attributed to the subsequent amphibolite facies metamorphism. Deformation associated with this metamorphic evolution is of unknown direction. The Alpujarride nappes record a plurifacial metamorphic evolution with the superimposition of low-pressure assemblages upon high-pressure ones with variable
P
–
T
ratios. Phengite from a carpholite-bearing high pressure-low temperature (
HP
/
LT
) assemblage gives an age of 25 Ma interpreted to reflect the end of the high-pressure evolution. Biotite and muscovite from high-grade metamorphic rocks overprinted under low-pressure conditions yield similar closure ages of 19 Ma dating cooling after the main episode of
E
–
NE
directed ductile deformation. This deformation was followed by W–SW directed extensional events producing brittle structures in the Alpujarride nappes and ductile-brittle shearing in the Nevado-Filabride nappes. Biotite and muscovite from ductile sheared rocks in the detachment zone between the two complexes have concordant ages of 16–17 Ma related to the end of the extensional ductile deformation. Therefore, a correlation of metamorphic and tectonic events between the two nappe complexes seems possible only since the Early Miocene and later.
Tectonics and Sedimentation
Authors: PICKERING K. T.
Published: December 2007

The papers in this thematic set arose from the meeting, sponsored by The Geological Society at Burlington House on Thursday l February 1990, on ‘Tectonics and Sedimentation ’. It was a joint meeting of the British Sedimentological Research Group and the Tectonics Studies Group, with oral presentation of 20 papers and several posters. About 160 people attended this lively and successful meeting. This introduction also serves (1s a report on the meeting. Of the 20 presentations, eight papers are published in this thematic set.

After an introduction by the convenor of the meeting and editor of the resulting thematic set, K. T. Pickering, the first paper was delivered by
T. Elliott
(Liverpool Univ.) on the controls on sediment deposition in foreland basins, and gave the resulting implications for basin modelling. This paper emphasized the importance of pre-existing structures in a thrust belt as a major control on sedimentation, together with ‘external’ factors such as climate change and global changes in sea level. These themes were to recur throughout the meeting.

The nature of collision-accretion by arcs and ridges against continental-arc CNSt was explored in presentations on forearc settings by
S. Flint
(Liverpool Univ.),
E. J. Jolley
(BP London), P. Turner (Birmingham Univ.),
G. Williams
&
T. Buddin
(Keele Univ.) in relation to the subduction of aseismic ridges at the Andean margin (also covered in a poster by
T. Buddin
on neo-tectonics of the continental forearc in northern Chile), and by
W. Soh
(Tokyo Univ.),
K. T. Pickering
(Leicester Univ.),
A.