bluuebunny/crossref_metadata_2025 huggingface dataset:
Timing of Ordovician deformation in the Caledonian-Appalachian orogen
Authors: Hall Leo M. , Roberts David
Published: September 2007

Evidence for Ordovician deformation is variably present throughout the Caledonides. The Grampian (late Cambrian-early Ordovician) and Taconian (middle to late Ordovician) major orogenies occurred during this time period and involved plutonism and metamorphism as well as deformation. The Taconian is particularly important in N America, whereas the Grampian (Penebscot), although locally identified, seems less important. Conversely, the Grampian appears to be more important in Scotland and in Scandinavia, where it is termed the Finnmarkian and constituted a major orogenic phase. Subduction and oceanic closure seems to have been an important Ordovician process, as indicated by abundant ophiolites. In Scandinavia the principal phase of thrust emplacement of the ophiolites occurred in the early to middle Ordovician. Major deformation in central E Greenland is of late Ordovician age.
Arenig to Wenlock age metamorphism in the Appalachians
Authors: Laird Jo
Published: September 2007

Arenig to Wenlock age metamorphism is best defined in the western part of the Appalachian orogen but may well be preserved in some terranes to the E that are underlain by basement which is not N. American regional metamorphism is primarily medium pressure, but low- to medium-pressure facies series metamorphism also occurs in the central Appalachians, and high-pressure metamorphism is locally preserved in northern Vermont and adjacent Quebec. Metamorphic grade varies from prehnite-pumpellyite to granulite facies; the isograds are generally parallel to the regional NE structural trend. Delimitation of the isograds has been hampered by widespread polymetamorphism, uncertainties in interpretation of field and isotopic data, and the need for more detailed mapping and petrological studies.
A brief review of early Ordovician to Devonian plutonism in the N American Caledonides
Authors: Wones David R. , Sinha A. K.
Published: September 2007

The distribution of mafic and felsic plutonic rocks has been used to constrain tectonic and thermal events in portions of the N American Caledonides. In the central and southern Appalachians studies of plutonic rocks identify an early island-arc environment followed by an episode of melting during over-thrusting. Devonian plutonism can be associated with both decompressional melting and rifting. The northern Appalachians also show evidence of early island-arc activity followed by collision of the N American plate with a collection of oceanic arcs and continental fragments. Devonian plutons reflect a more static environment (rift?) where incremental melting of a protolith provides the observed variations in the composition of the magmas.
Timing of Silurian to middle Devonian deformation in the Caledonides of Scandinavia, Svalbard and E Greenland
Authors: Roberts David
Published: September 2007

Polyphase orogenic deformation in Scandinavia during the period from early Silurian to middle Devonian time is concentrated in the time-range late Llandovery to early Devonian—the Scandian orogenic event. Deformation and metamorphism began earliest in the western internal parts of the orogen and shifted progressively eastwards to southeastwards towards the external foreland. Early SE-directed high-temperature ductile thrusting was succeeded by imbrication in several areas. In Svalbard the principal tectonothermal event is of middle to late Silurian age. Stratigraphical and isotopic evidence from the E Greenland Caledonides favours a latest Ordovician deformation and metamorphism in central regions, whereas the main deformation and thrusting in the extreme NE occurred during the middle Silurian.
Scandian-Acadian-Caledonian
sensu strictu
metamorphism in the age range 430–360 Ma
Authors: Robinson Peter , Tracy R. J. , Santallier D. S. , Andreasson P.-G. , Gil-Ibarguchi J. I.
Published: September 2007

Metamorphism in the age range Wenlock to mid-Devonian, when it can be recognized, appears to have accompanied some of the most intense phases of convergent tectonics in the orogen. Very different styles of metamorphism in different regions suggest different tectonic settings. In Scandinavia deep subduction and imbrication of the Baltic continental margin beneath N America (?), with little or no plutonism, produced a regional pattern leading to high temperature eclogites and granulites. In the British Isles, Canada and Maine, broad greenschist, prehnite-pumpellyite and lower-grade metamorphism accompanied the closure of Iapetus, with local higher-grade punctuations associated with widespread plutonic activity. In France, locally in Newfoundland, in southern New England and in part of the southern Appalachians both plutonic heating and intense deformation were involved in metamorphism to the amphibolite and locally the granulite facies, complicated in France by eclogites and locally by glaucophane schists.
Silurian-Permian palaeocontinental reconstructions and circum-Atlantic tectonics
Authors: Kent D. V. , Keppie J. D.
Published: September 2007

On the basis of the palaeomagnetic record, supplemented by constraints provided by faunal and tectonic information, reconstructions involving Laurentia, Baltica, Gondwana and numerous continent-like fragments and terranes within the circum-Atlantic Palaeozoic orogenic belts are made for three critical time intervals, late Silurian, early Carboniferous and late Carboniferous-early Permian, all of which bracket important phases of tectonic activity. The late Caledonian-Scandian-Acadian-Ligerian orogenies are due to the predominantly E-W final closure of Iapetus. This phase was followed in mid-Devonian times by a major sinistral megashear along the orogen which eliminated latitudinal separations and resulted in a Pangea-like assembly by the latest Devonian. The final phase involved a rotation of Gondwana with respect to the assembled Euramerican landmass, expressed as late Variscan-Hercynian dextral shear from central Europe to northeastern N America and culminating in the late Carboniferous-Permian Alleghanian orogeny in the Appalachians from New England S and in the Mauritanides.
Caledonian folding in the Bohemian Massif
Authors: Chaloupský J.
Published: September 2007

The Bohemian Massif was formed by metamorphic and tectonic processes during a long period which began about 800–900 Ma ago with the break-up of the ancient continental crust and Brioverian rift sedimentation and ended at the close of the Palaeozoic by transition to the platform-development stage. Caledonian folding has been evidenced in the NE part of the Massif.
The third synthesis: Wenlock to mid-Devonian (end of Acadian orogeny)
Authors: Hatcher Robert D.
Published: September 2007

Major deformational-thermal events occurring during the Wenlock to mid-Devonian interval include the Acadian (N America, France, Spain) and the Scandian. These events are possibly related to the closing of the Iapetos Ocean between Laurentia and Baltica and the suturing of the Avalon microcontinent and other smaller terranes to N America. Metamorphism, plutonism and deformation during this interval resulted from the convergence events. Metamorphic grade and intensity of deformation vary spatially throughout the orogen. Both felsic (I-, S- and A-type granites) and mafic plutonism were abundant in N America in the Acadian, but Scandian metamorphism produced relatively little plutonism, most of which was felsic (I-type granites). Volcanic activity was restricted during this time, occurring in Scotland with substantial amounts in E Greenland, the Canadian Maritimes and New England with minor marine tholeiitic volcanism in SW England, France and Germany.
Basement-cover relationships in the Appalachian-Caledonian-Variscan orogen: mid-Devonian (end of Acadian orogeny) to end of Permian
Authors: Hatcher Robert D.
Published: September 2007

The Alleghanian-Variscan orogen was, in part, superimposed upon continental crust produced by earlier Palaeozoic orogenies from continental platform and slope and rise sediments volcanic rocks, together with island arcs, rift volcanic rocks and sediments from diverse sources and older Precambrian basement. The effects of the Alleghanian-Variscan orogeny are particularly marked in the southern half of the Palaeozoic mountain chains of N American and western Europe. The Alleghanian-Variscan event(s) produced both ductile and brittle deformation involving folding and faulting (both thrust and strike-slip). Movement along many of these faults produced large displacements of the order of tens to hundreds of kilometres. Large-scale transport on thrusts in the southern and central Appalachians and NW and W Africa (Mauritanides) probably resulted from collision related to final closing of the Theic Ocean. Dominant thrust transport in the southern and central Appalachians contrasts with dominant large-scale strike-slip motion in the Maritime Appalachians of Canada and transpressive motion in the Variscides of western Europe. The character of Variscan deformation in Europe suggests that the orientation of boundary stresses there was somewhat different from those in N America. Late Palaeozoic thrusting in southeastern N America and northwestern Africa suggests that collision was normal there but it may have been oblique in western Europe. However, thrusting occurs along both the northern (Ireland, Wales, Belgium) and southern (France, Iberia) flanks of the Variscan orogen in western Europe. Large-scale over-thrusting of basement and cover rocks in the Rheinisches Schiefergebirge has also been suggested. The basement appears to have responded homogeneously in large domains with major differences in style related in part to stress orientation.
Late Palaeozoic sedimentation along the Appalachian orogen
Authors: Thomas William A. , Schenk Paul E.
Published: September 2007

The sedimentological framework of post-Early Devonian strata in the Appalachians includes two different domains. S of the New York recess in the foreland fold-thrust belt, clastic wedges of two different ages (Devonian-Early Mississippian and Late Mississippian-Pennsylvanian) constitute the sedimentological record of the Acadian and Alleghanian orogenies. The widespread semicircular clastic wedges prograded cratonward across broad foreland basins from orogenic provenances. Distribution of siliciclastic sediments suggests a genetic relation to large-scale curves (salients and recesses) of the orogenic belt and indicates variation in time of orogeny along strike. N of the New York recess in the more interior part of the orogen, Devonian to Permian siliciclastic sediments derived from local fault-block uplifts filled and overflowed pull-apart basins associated with mainly dextral wrench faults. The history of sediment supply and deposition varies from basin to basin. Parts of the northern domain are covered by more widespread siliciclastic sediments. Differences in the two domains reflect not only variation along strike in time of orogeny but also a range of structural style from large-scale cratonward thrusting in the S to wrench faulting in the N.
Devono-Carboniferous river systems and sediment dispersal from the orogenic belts and cratons of NW Europe
Authors: Leeder Michael Robert
Published: September 2007

Facies, palaeocurrent and provenance analysis combine to give vital information concerning hinterland tectonic events, particularly when continental facies are used to assess orogenic and/or epeirogenic uplift. In Palaeo-Europa steady progress has been made in this area but much integrative tectono-sedimentary study is still needed. Six palaeogeographic maps are presented in the paper with continental/alluvial facies (‘molasse’) and turbiditic continental margin facies (‘flysch’) mapped out. Palaeocurrent data is ‘distilled’ to highlight impressions of terrestrial drainage trends. Clastic wedges are identified within basins caused by major tectonic events, and include the following: (a) Lower—Middle Devonian successor basins within the Caledonides (fore-arc and inter-arc types, and Pannonian types); (b) Lower Devonian-Lower Carboniferous back-arc and foreland tilt block basins related to lithospheric stretching caused by Acado-Ligerian subduction and collision; (c) Upper Devonian-Upper Carboniferous foreland basins formed by diachronous closure of the Rheno-Hercynian back-arc successions by thrust stacking; (d) Middle-Upper Carboniferous passive thermal cooling of the stretched Caledonian lithosphere; (e) final Upper Carboniferous collision of Gondwanaland and tightening of the Ibero-Armorican arc. No tectonic explanation is yet forthcoming for the growth and development of the huge ‘Pennine’ drainage system which dominated sedimentation over large areas of Ireland, Scotland, England and the North Sea during Middle-Upper Carboniferous times.
Middle Devonian to Permian plutonism and volcanism in the N American Appalachians
Authors: Don Hermes O. , Murray Daniel P.
Published: September 2007

Mid-Devonian to late Palaeozoic igneous activity in the N American Appalachians occurs in the eastern portions of the orogen. Represented radiometric ages are almost continuous from 250 to 370 Ma and older, thus overlapping Acadian and Alleghanian orogenic events. In many instances igneous activity is generally similar to that initiated earlier in the Palaeozoic. Plutonic rocks consist of granitoids with subordinate intermediate to mafic rocks. Plutonism is mainly of ‘Caledonian’ or alkalic variety and is dissimilar to subduction-related magmatism of the Andean type. Regional synthesis indicates that some general distinctions of plutonism occur across strike as well as along strike from S to N within the orogen in eastern N America.

In the southern Appalachians granites and minor contemporaneous gabbros occur in all lithotectonic zones between the Blue Ridge and the Coastal Plain. These plutons are commonly syn-kinematic in the eastern Piedmont where Alleghanian deformation and metamorphism are best exhibited, but are late- to post-kinematic to the W. The rocks include metaluminous biotite and megacrystic granites and lesser peraluminous two-mica granites. Late Palaeozoic igneous activity is sparse to absent in the central Appalachians.

In New England several kinds of mid-Devonian to Carboniferous igneous activity are represented. Biotite and biotite + muscovite granitoids dominate the central terranes, whereas the more easterly Avalon zone is intruded by alkalic-peralkalic plutonic rocks and contains minor bimodal volcanics. Final Permian plutonism includes intrusion of homogeneous metaluminous to peraluminous granite.

Within the Canadian Maritimes biotite and megacrystic granites followed by mildly alkalic varieties dominate the Devonian-Carboniferous plutons of New Brunswick and Newfoundland, whereas muscovite + biotite rocks are prominent in the Meguma terrane of Nova Scotia. Volcanism, which is bimodal, is best documented in New Brunswick and northern Nova Scotia. Igneous activity of Permian age is not recognized in the Canadian Maritimes.

The petrological diversity together with the considerable span of time represented by the late Palaeozoic igneous activity in the N American Appalachians indicates repeated and locally prolonged anatexis of a variety of heterogeneous crustal sources. The alkalic and bimodal suites are characteristic of extensional or anorogenic tectonics. The more aluminous rocks are generally compositionally similar to the Caledonian and, to a lesser extent, the Hercynian suites of Europe. Overall the igneous activity may reflect relatively independent rifting and accretion of small crustal plates of diverse thicknesses and compositions in which magmatism was not related to a simple subduction island-arc setting.
Late Palaeozoic metamorphism in the northwestern Iberian Peninsula, Brittany and related areas in SW Europe
Authors: Martínez Francisco J. , Rolet Joël
Published: September 2007

In the NW of the Iberian Peninsula elongated and coalescent thermal domes are centred in P
3
antiforms and associated with granite plutonism. After the Emsian and before the Namurian, assemblages with garnet, staurolite and kyanite changed into others with andalusite-sillimanite, biotite and cordierite giving evidence of heating and/or decompression related to syn-metamorphic granite intrusion. The Namurian is roughly the metamorphic peak of low-pressure conditions in the inner zones. In the central Armorican domain in Brittany the main metamorphic event has an age between 330 and 345 Ma. There is also a pressure decrease with time and the earlier zones are telescoped in the W by subautochthonous leucogranites associated with high-grade zones. Contact aureoles affecting the Dinantian basins in Brittany must be late Carboniferous. In the other areas, i.e. Catalonia, the Pyrenees, Montagne Noire and the French Massif Central, the history is essentially the same, with the low-pressure metamorphic peak being Namuro-Westphalian.
Porphyroblast-matrix microstructural relationships: recent approaches and problems
Authors: Vernon R. H.
Published: September 2007

Careful observation and interpretation of suitably oriented sections indicates that many porphyroblasts with curved inclusion trails grow during the development of a crenulation foliation. Inclusion trails that are approximately parallel in separate porphyroblasts, or that outline axial surfaces of crenulations that are parallel in separate porphyroblasts, suggest that many porphyroblasts do not rotate, with respect to geographic coordinates, during later deformation. Moreover, where such rotation has not occurred, inclusion trails in porphyroblasts may indicate former orientations of S-surfaces that have been obliterated from the matrix.

In ideal situations, porphyroblast-matrix microstructural relationships can be used, with more confidence than before, to provide useful information on metamorphic/deformation histories and on the details of metamorphic reactions. Porphyroblast-matrix relationships are also useful in contact metamorphic situations and can provide evidence of deformation accompanying contact metamorphism. Though mostly applied to medium-grade metamorphic rocks, porphyroblast-matrix relationships can also be useful in some high-grade situations.

A model suggested by Bell & Rubenach and Bell
et al.
postulates that porphyroblasts grow in zones of relatively weak coaxial deformation between anastomosing zones of strong non-coaxial deformation during crenulation folding. Many observations appear to be consistent with this model, but it is difficult to verify, and much detailed observation and testing of critical aspects of the model are needed. Regardless of whether or not the model is accepted, curved inclusion trails may be used to relate growth of porphyroblasts to generations and stages of crenulation development.
Vertical movement in orogenic belts and the timing of metamorphism relative to deformation
Authors: Ridley John
Published: September 2007

Any phase of deformation in an orogenic belt that causes relative vertical movements of bodies of rock by a few kilometres or more should have a recognizable imprint in the metamorphic history. The effect seen in a
P-T-t
path will depend on whether the rock body examined moved relatively up or down in the orogenic pile. An upthrown block is predicted to cool soon after the onset of movement, but the resulting
P-T
path will be insignificantly different to that resulting from a cycle of thickening and erosion. A downthrown block will undergo renewed heating after movement, and evidence of the earlier metamorphic history will probably be overprinted. It is suggested that the recognition of phases of relative vertical movement in an orogenic belt is best made by combined studies of deformation-metamorphism textural relationships, and
P-T-t
paths. The Lukmanier Pass area of the Alps illustrates the textural patterns predicted: here, a phase of deformation immediately post-dates the metamorphic peak, suggesting uplift of the area relative to the Helvetic nappes to the north.
Deformation and metamorphism in convergent orogens: a model for uplift and exhumation of metamorphic terrains
Authors: Jamieson R. A. , Beaumont C.
Published: September 2007

A conceptual model is presented for deformation, uplift and exhumation in convergent orogens based on flexural models of foreland basin subsidence, the role of inherited rifted margins in convergent orogens, critical wedge theory and the use of
P-T-t
data to construct particle paths within the deforming orogen. Three states of overthrust orogens can be defined, based on the relative rates of mass loss by erosion and mass addition by accretion and underplating. Rapid exhumation of metamorphic terrains is most likely in the steady state, where erosion balances accretion; a constructive state, where mass addition outweighs erosion, initially results in deep burial and relatively little exhumation; a destructive state, where mass loss outweighs mass addition, results in slow exhumation or subsidence of metamorphic rocks, depending on whether mass is lost by erosion or redistributed during extension. In order to test these predictions, reliable
P-T-t
data are required on peak metamorphism, pluton emplacement, cooling, exhumation, and uplift, for a range of localities within an orogen.
The geochronology of metamorphic processes
Authors: Zeitler Peter K.
Published: September 2007

Metamorphic processes are complex, and the geochronology of these processes will be complex as well. A primary requirement of metamorphic geochronology is adequate time resolution, if metamorphism is to be viewed as more than just an event. Currently, geochronologists can achieve time resolution of 10
6
years or better for Phanerozoic and some Proterozoic rocks. A second requirement of metamorphic geochronology is that the systematics of dating schemes be understood in the context of the geochemical or geodynamic environment being examined. The approach that has been used most successfully, at least in young rocks, exploits the temperature-dependent nature of daughter-isotope diffusion to make measurements of the thermal history of metamorphic belts. Specifically, the fission-track and
40
Ar/
39
Ar methods can be used to trace the post-peak temperature
P-T
path of a metamorphic rock from 500 to 100°C. Some progress has also been made in using other systems such as U-Pb, Rb-Sr, and Sm-Nd to date minerals having higher closure temperatures. New technologies and techniques of interpretation, such as the use of
40
Ar/
39
Ar age spectra to determine diffusion profiles, or the ion microprobe to determine age zoning in zircon, are making available a great deal of additional geochronological information. In general, however, before tight constraints on the evolution of metamorphic belts can be obtained, many more diffusion data for systems of geochronological interest are needed, as are better constraints on the stability relationships under metamorphic conditions of accessory minerals such as zircon.
K-Ar and
40
Ar/
39
Ar dating of cleavage formed during very low-grade metamorphism: a review
Authors: Reuter A. , Dallmeyer R. D.
Published: September 2007

Dating cleavage formed during very low-grade (anchizonal) metamorphism of pelitic rocks by K-Ar and/or
40
Ar/
39
Ar whole-rock analysis is complicated bypolymineralogical sample characteristics and potential argon inheritance within detrital grains. Complex mineralogical effects may be resolved by careful resolution of both K/Ca and
40
Ar/
39
Ar age spectra. However, detrital influence can be maintained even within penetratively cleaved metapelite samples (whole-rock and constituent white-mica-rich size fractions down to 0.4–0.63 μm) until metamorphic conditions of the anchizone/epizone boundary. The
40
Ar/
39
and K-Ar whole-rock dating of cleaved metatuffs interbedded with anchizonal metapelites has yielded results consistent with biostratigrapic constraints on the age of cleavage formation, indicating a lack of detritai influence. Constituent grains within white-mica-rich size fractions separated from upper anchizone metatuffs are characterized by well-defined margins and record mutually concordant K-Ar and
40
Ar/
39
Ar total-gas ages which display no systematic variations with grain size. Middle anchizone metatuff size fractions are composed of white mica grains with poorly defined edges. These consistently record
40
Ar/
39
Ar total-gas ages which are 10–15% older than corresponding K-Ar dates (whole-rock and size fractions). This discordance is attributed to recoil-loss of
39
Ar during irradiation, and implies that morphology of grain edges (affecting surface/volume ratios) can be a major factor controlling
39
Ar recoil.
Discordant relationship between degree of very low-grade metamorphism and the development of slaty cleavage
Authors: Kisch Hanan J.
Published: September 2007

Definitive comparisons of data compiled on the grade of incipient metamorphism associated with the appearance of slaty cleavage in different areas are impeded by the lack of uniform characterization of the slaty cleavage. However, the grade associated with the appearance of a well-developed, closely spaced, continuous slaty cleavage is not uniform: though low- to mid-anchimetamorphic in many areas, it ranges from high diagenetic to ‘epimetamorphic’.

This discordant relationship may in part reflect differences in lithology of the preexisting rocks, taking place at lower grades in pelites and marls than in greywackes. It may also possibly reflect differences of fabric and preservation of pre-deformational illite ‘crystallinities’ during subsequent regional deformation and cleavage formation, for instance due to lack of synkinematic recrystallization of phyllosilicates in the initial stages of formation of crenulation cleavage in pelites with a strong pre-existing layer-parallel fabric.

Another explanation for the discordance is believed to lie in the different time relations between deformation and metamorphism in the various terrains: in many cases the assumed syncrystalline nature of the deformation has been insufficiently substantiated. More information—specifically including TEM/SEM studies on the deformation of the newly formed phyllosilicates—are needed to interpret the nature of the discordance adequately.
The influence of fluids on the thermal history of a metamorphic terrain: New Hampshire, USA
Authors: Chamberlain C. P. , Rumble Douglas
Published: September 2007

The metamorphic terrain in central New Hampshire is characterized by over 9000 km
2
of primarily sillimanite — muscovite-bearing rocks. Superimposed on this large thermal high are a series of hot spots about 20 km
2
in area. Thermal models suggest that the large-scale thermal structure of central New Hampshire can be explained by conductive heat transfer processes. However, the hot spots can neither be explained by conductive heat transfer nor by intrusion of plutons. Isotopic and petrological studies demonstrate that the hot spots were formed by the focusing of hot metamorphic fluids into zones that had relatively high fracture permeability.

The infiltration of hot fluids has significantly affected the
P-T
paths of rocks in central New Hampshire. Within the region of abundant hot spots, the
P-T
history of the metamorphic rocks is different from that outside the hot spot region. The
P-T
paths in the central portion of New Hampshire, the area with the hot spots, are characterized by isobaric-heating paths. The rocks passed near aluminium silicate triple point pressures and temperatures and later equilibrated at similar pressures, around 4.0 kbars, and at higher temperatures, approximately 650–750°C. The absence of other mechanisms to produce the isobaric-heating paths and the evidence for intense hydrothermal activity suggest that fluids played an integral role in creating the thermal spikes along the
P-T
trajectory.

Analytical solutions to differential equations that describe focused fluid flow are used to estimate the duration of hydrothermal activity and the amount of fluid involved. The steep thermal gradients adjacent to the hot spots and the observed temperature increase within hot spots are consistent with large fluid fluxes, > 1 × 10
−9
m
3
fluid per m
2
s
−1
, over short periods of time, c. 100000 years. This indicates that
P-T
paths from rocks within hot spots record only small portions of the geological evolution of the metamorphic terrain.
Implications of thermal and baric structure for controls on metamorphism: northern New England, USA
Authors: Day Howard W. , Chamberlain C. Page
Published: September 2007

Isograds and bathograds define an apparent thermal and baric structure of the Appalachian metamorphic belt in northern New England. Peak metamorphism occurred primarily during the Devonian Acadian orogeny at about 390 Ma. Kyanite-sillimanite metamorphism took place along heating-decompression paths and andalusite-sillimanite metamorphism is consistent with isobaric heating or limited decompression during heating. Isograds were deformed by early nappes and late domes, and dip steeply in some areas. The aluminium silicate bathograd dips gently east towards lower preserved pressures, intersects the regional trends of isograds, and is deformed by late cross-folds. Argon ages of micas as young as 250 Ma in the highest grade rocks record a long postmetamorphic cooling history, whereas lower grade rocks record much older argon ages. Comparison of these observations with simple thermal models suggests that differences of uplift history did not dominate the prograde metamorphic history and that spatial variations of the pre-tectonic thermal regime or of thermophysical characteristics of the lithosphere were important influences on the observed metamorphism.
Thermal evolution of the Kisseynew sedimentary gneiss belt, Manitoba: metamorphism at an early Proterozoic accretionary margin
Authors: Gordon T. M.
Published: September 2007

The Kisseynew sedimentary gneiss belt forms part of an early Proterozoic accretionary margin preserved between two Archaean cratons in the south-west Canadian Shield. The sediments are volcanogenic turbidites deposited from flanking volcanic belts at 1910–1875 Ma. Two compressional tectonic events preceded 1815 Ma metamorphism, which reached conditions of 750°C at about 5.5 kbar. Subsequent uplift took the rocks through to 280°C at about 1705 Ma. Thermal modelling suggests that, over this time interval, the average uplift rate was 0.06 km Ma
−1
. The K, U and Th analyses of the metasediments, combined with the estimated burial depth, show that in situ radiogenic heat production was insufficient to cause the high-temperature, moderate-pressure metamorphism. A modern analogy in southern Alaska suggests that elevated thermal flux due to magmatic intrusion may be a common phenomenon during the late stages of evolution of accretionary environments.
An empirical model for metamorphic evolution of the Archaean Slave Province and adjacent Thelon Tectonic Zone, north-western Canadian Shield
Authors: Thompson Peter H.
Published: September 2007

On the basis of
P-T
estimates derived from metamorphic assemblages and the geological setting before and after peak metamorphism, Archaean thermal evolution is attributed to overthickening of previously thinned sialic crust and overlying supracrustal basins. The extensional phase increases geothermal gradients and forms relatively ductile zones within which subsequent compression transports supracrustal rocks to metamorphic pressures, contributes to the rise of isotherms to high levels in the crust, and causes the moderate uplift and erosion required to bring low-
P
-high-
T
rocks to the surface of crust 35 km thick.

Proterozoic upper amphibolite- and granulite-facies metamorphism and plutonism in the western Thelon Tectonic Zone is attributed to a similar thermal evolution 500 Ma later. Continued convergence across a long linear belt of overthickened, partially melted crust caused hot upper and middle crustal-level rocks to be thrust on to the Slave Province and formation of a medium pressure overprint on the Archaean.

The empirical models derived for the region generate low-
P
-high-
T
metamorphism without excessive overthickening or major heat transfer by granitic magmatism. For both terrains, metamorphic data, considered in their geological context, are consistent with an ensialic intra-plate tectonic setting rather than an ensimatic, subduction-related interplate setting.
P-T
-time history and thermal modelling of an anorthosite-granulite interface, Eastern Ghats metamorphic belt, India
Authors: Mukherjee Amalbikash
Published: September 2007

A pressure-temperature-time synthesis and a thermal model are presented for the anorthosite-massif-granulite interface near Bolangir, Orissa, India: a type area of the Eastern Ghats metamorphic belt. While the anorthosite pluton can be shown to have been forcefully intruded as a schlieren dome into the granulite cover, the calculated mineral equilibria of the interface suggest a
P-T
convergence at 620–740°C and 4.7–7 kbar and a nearly isobaric cooling in this pressure range under a cratonic overburden between 15 and 25 km. Parameterized model calculations suggest that a noritic anorthosite pluton of 7.5 and 10 km radius with 50% initial crystallinity and a lower crustal contact temperature in the region of 800°C would cool from the initial temperature of 1200°C to 850–900°C in about 20 Ma and then the cooling rate would fall dramatically to the order of 0.75°C Ma
−1
. Finite difference calculations of the heat conduction from the equilibrated anorthosite-granulite interface suggest that for some combinations of the thermal properties of the medium and the heat generation and uplift rates, boundary conditions based on an initial transient elevated geotherm produce a nearly isobaric cooling path. The initial prograde metamorphism of the granulites, the anorthosite invasion and the final cooling and
P-T
convergence of the anorthosite- granulite interface have resulted in a composite overturned (anticlockwise) Y-shaped
P-T-t
configuration. It is suggested that this configuration is characteristic of the Eastern Ghats type metamorphic belt, representatives of which seem to be widespread in the Archaean to Proterozoic continental segments of North America, Scandinavia and parts of Africa, Western Australia and Antarctica. In contrast to the Alpine type belts, the Eastern Ghats type belts are much wider and thickly cratonized with low isostatic resilience (low uplift/exhumation rate) and have anorthosite intrusive cores acting as ‘exotic’ heat sources, resulting in characteristic anticlockwise overturned and forked Y-shaped
P-T-t
loops.
P-T
-fluid regimes of metamorphism and related magmatism with specific reference to the granulite-facies Sharyzhalgay complex of Lake Baikal
Authors: Perchuk Leonid L.
Published: September 2007

Petrological studies, including geothermobarometry, of metamorphic mineral parageneses together with isotopic data, reveal differences in the
P-T
-fluid regimes of Precambrian shields and surrounding fold belts. Typical features of shields are CO
2
-rich fluids and preservation of retrograde portions of
P-T
trajectories, while in fold belts, prograde as well as retrograde stages of metamorphism are recorded in the minerals and the CO
2
contents of the fluids vary widely. Geothermobarometric estimates of prograde and retrograde stages in several metamorphosed fold-belt complexes are in accord with the distribution of Al
2
SiO
5
polymorphs.

A model involving vertical redistribution is proposed for the granulite-facies Sharyzhalgay complex of Lake Baikal, which is interpreted to be the metamorphosed equivalent of the Karelian greenstone belt. Increased heat flow and burial leads to gravitational redistribution due to density differences, so that dome structures cored by granulite-facies gneiss occur within envelopes of down-sinking metabasites and meta-ultrabasites. The model is supported by the geochronology and by the
P-T
-fluid regimes deduced from geothermobarometry and fluid-inclusion studies of the polymetamorphic Sharyzhalgay complex. This model may apply to other granulite-facies complexes which appear to be the result of high-grade metamorphism of Archaean greenstone belts made up of tholeiitic and komatiitic volcanics. These rocks were originally underlain by a complex of acid volcanics (dacites and rhyolites).
The Precambrian mobile belt of southern Madagascar
Authors: Ackermand D. , Windley B. F. , Razafiniparany A.
Published: September 2007

The Precambrian of southern Madagascar consists largely of medium- and high-grade paragneisses and metasediments that have a predominantly north-south regional trend, which is parallel to that of the Mozambique belt of eastern Africa. The Precambrian geology of Madagascar is synthesized by Besairie (1971) and Hottin (1976). Much mineralogical information of relevance to metamorphic petrology is given by Lacroix (1922–23). Nicollet (1985, 1986) gives good metamorphic and
P-T
data from Ihosy and Vohibory (Fig. 1).

Figure 1 shows the distribution of the main metamorphic belts in the south of the island. Included in the amphibolite-facies belts are low-, medium- and high-grade types, and the granulite-facies belt includes pyroxene- and hornblende-facies types as well as some high-grade amphibolite facies (Vachette 1979). Most abundant are gneisses with various combinations of cordierite, sillimanite, garnet and biotite. The widespread development of cordierite reaches a peak in cordieritites which form layers up to several tens of metres thick in the gneisses. In the gneisses, sillimanite is locally concentrated in sillimanitite layers up to several metres thick, and marble and quartzite layers reach a few metres in thickness. Diopsidites occur in the gneisses as conformable layers up to several metres thick and have pegmatitic pockets with phlogopite, diopside, scapolite and anhydrite; some phlogopite crystals are over 1 metre long and phlogopite is mined locally (Lacroix 1941). Pegmatites occur in most rocks.

Foliation and layering are predominantly parallel and steep to vertical. Isoclinal folds have axes which plunge shallowly north-south, but in general there is a notable paucity
Multiple granulite-facies events in the southern Appalachians, USA
Authors: McLellan Eileen , Linder Daniel , Thomas Jenny
Published: September 2007

This paper summarizes present knowledge of an area in the southern Appalachians in which two granulite-grade events can be recognized in a complexly deformed suite of rocks. Metamorphic conditions in the first event exceeded 800°C, whereas those in the second attained only 700°C. Both events are shown to involve prograde dehydration; the rocks must therefore have experienced multiple
P-T-t
loops. Structurally interleaved with these rocks is a suite of less-deformed rocks which only experienced the second of these events and may represent cover lithologies to the polymetamorphic basement.
Inverted metamorphic zonation and Variscan thrust tectonics in the rouergue area (Massif Central, France):
P-T-t
record from mineral to regional scale
Authors: Burg J. P. , Leyreloup A. F. , Romney F. , Delor C. P.
Published: September 2007

Field evidence together with microstructural analysis define three types of tectonically produced inverted metamorphic zonation in the Variscan terrains of Rouergue (southern French Massif Central). They are associated with large-scale thrusting of rock slabs metamorphosed prior to their southward tectonic emplacement. Mineral equilibria, garnet-biotite geothermometry and garnet-plagioclase geobarometry reveal that post-, syn- and pre-metamorphic deformations result in three different
P-T
patterns. Post-metamorphic deformations are responsible for inverted and contrasted zonations where metamorphic discontinuities are critical evidence for nappe translations. Synmetamorphic thrusting results in thrust-related metamorphic zones with inverted thermal profiles and normal baric profiles. Apparent pressure differences are artifacts of different
P-T
paths and variable uplift rates. Pressures at peak metamorphic conditions are lowest in zones of rapid uplift, such as the thrust sequences. Metamorphism may outlast all important structural events apart from late folding in cases where syntectonic intrusions are emplaced within thrust contacts. They are responsible for sharp inverted thermal profiles and they strongly modify the isograd distribution in the rocks underlying metamorphic thrust units.
Magmatism, deformation and high-
T
, low-
P
regional metamorphism in the Nabitah mobile belt, southern Arabian Shield
Authors: Droop G. T. R. , Al-Filali I. Y.
Published: September 2007

This paper examines the temporal and causal relationships between late Proterozoic deformation, plutonism and high-
T
, low-
P
regional metamorphism in a small area in the Nabitah mobile belt (Stoeser et al. 1984). The rocks have island-arc affinities and record the following sequence of events spanning c. 100–150 Ma: (i) calc-alkaline volcanism, (ii) calc-alkaline plutonism, (iii) intense ductile deformation, (iv) thermal peak of metamorphism and formation of mantled gneiss domes, (v) late-stage plutonism.

The
P-T
conditions at the peak of metamorphism are estimated from mineral assemblages in marbles and calc-silicate rocks to have been 620 ± 30°C, 3.2 ± 2 kbar, indicating a transient upper crustal mean thermal gradient of 54 + 64/-19°C km
−1
. No information is yet available on the
P-T
path, but a history of isothermal decompression is unlikely.

The cause of the metamorphism was probably the convective transfer of heat by the emplacement of high-level intrusions in an island-arc complex. The observation that the ductile deformation and the thermal maximum were almost coeval argues against a causal relationship, except in that whatever caused the deformation (an arc-continent collision?) may also have led to a reduction in magmatic activity and hence regional heat flow, thus terminating the metamorphism.
Exploration of the Brent Province
Authors: Bowen J. M.
Published: September 2007

The Brent Province is situated in the central part of the northern North Sea between 60°30′ and 62° North. A few licences were issued in 1965 (Round 2) and again in 1969 (Round 3). Seismic exploration from the mid-1960s onwards showed a thick Tertiary/Cretaceous sequence draped unconformably over tilted fault blocks of unknown age. In 1971, coincident with the opening of the 4th Round, Shell’s 211/29–1 pioneered drilling in the area with a major oil discovery, the Brent Field in Middle Jurassic deltaic sandstones. The 4th Round resulted in allocation of almost all the remaining blocks. Between 1972 and 1978, 32 discoveries were made in the UK sector of which 17 are currently in production and a further 7 in the pre-development stage; thereafter only minor reserves have been added, the area now being relatively mature. The exploration history in Norway has been similar although delayed by a few years; it is also less mature as several blocks have still to be explored. Total reserves for the province now approximate to 15 billion barrels of oil equivalent.
An introduction to the Brent Group: a literature review
Authors: Richards P. C.
Published: September 2007

There is a vast volume of published literature on the Brent Group, and consequently many controversies or differing points of view regarding the age, nature and palaeogeographic evolution of the Group. Brent Group lithostratigraphy is, however, relatively uncontroversial. A five-fold subdivision of the Brent Group into the Broom, Rannoch, Etive, Ness and Tarbert Formations is used by most authors. The Oseberg Formation is used to describe Broom Formation equivalent strata in Norwegian waters. There are, however, problems of applying this simple lithostratigraphy in some parts of the basin, where the artificial subdivision may unduly constrain depositional system correlation. Many companies use their own, field-specific, stratigraphic schemes for reservoir subdivision purposes, and some authors have applied a sequence stratigraphic framework to the Brent Group in preference to the lithostratigraphic scheme. There is, however, discord over the actual stratigraphic position of sequence boundaries and the time equivalence of strata. Brent Group biostratigraphy is less well documented in the literature than lithostratigraphy, and represents the area where most new work is needed if the evolution of the depositional system is to be better understood.

Analyses of the depositional environment of the Brent formations are numerous but more or less similar. The origin of the Broom Formation is the most controversial aspect, with a range of shallow to deep water interpretations; a fan delta interpretation is preferred in recent publications. The Rannoch and Etive Formations are generally interpreted as shoreface to foreshore/channel deposits, and the Ness Formation as delta top sediments. The Tarbert Formation represents an essentially transgressive unit. There are two main published models to explain the regional evolution of the Brent Group, and this is possibly the most active of the controversies in Brent Group geology. The first model considers the system as a northwards prograding delta with a southerly source and a concentric arrangement of facies belts across the basin. The second model envisages a dominantly transverse sediment supply to the basin, with subsequent localized northwards progradation within the basin.
A regional reassessment of the Brent Group, UK sector, North Sea
Authors: Cannon S. J. C. , Giles M. R. , Whitaker M. F. , Please P. M. , Martin S. V.
Published: September 2007

The deltaic Brent Group sequence consists of three formations. The Rannoch is a shoreface section; the Etive is a barrier facies. The Ness contains three members: the Enrick, representing a marginal marine back barrier environment; the Oich, an organic-rich open lagoonal shale and the Foyers, a largely fluvially-dominated delta-top or coastal plain deposit. During the Bajocian, facies belts rapidly prograded northwards until the northern part of the area was reached. Here progradation stalled and thick delta front sediments were deposited. Shortly before the delta began to retreat, a temporary relative sea-level rise pushed the delta front southwards resulting in the deposition of an intra-Ness barrier. The progradational sequence is underlain by the Aalenian to lowermost Bajocian Broom Formation fan delta and overlain by the transgressive Tarbert sequence of largely Bathonian age.
Advance and retreat of the Brent delta: recent contributions to the depositional model
Authors: Helland-Hansen W. , Ashton M. , Lømo L. , Steel R.
Published: September 2007

The behaviour and the development of the Brent Group in the Northern North Sea is documented by cross-sections and paleogeographic maps. Although this work is based on a previously published Norsk Hydro study, we present recent progress in the understanding of the Brent Group which has been achieved by a better timing of depositional events, by integrating new well information, and by generating new models based on the new information.

The early (Aalenian) lateral infill of the Oseberg Formation was deposited as a response to a relative fall of sea-level, probably generated by a tectonic uplift. Fan deltaic sediments built out towards the west and northwest and backfilled the previously emergent areas during the subsequent relative sea-level rise.

The progradation of the Brent Delta (Rannoch, Etive and lower Ness Formations) took place in Late Aalenian to Early Bajocian. The thickness of the Etive relative to the Rannoch Formation indicates thickening due to bathymetric deepening in the area south of the maximum regression whereas subsidence controlled the thickening in the area of the maximum extent of the delta.

During the retrogradational part of the delta development (the Tarbert and upper Ness Formations, Early Bajocian to Early Bathonian), increasing tectonic activity can be documented, both in terms of enhanced differential subsidence across faults and by early rotational uplift on some fault blocks.