calcite textures microstructures and rheological properties of marble mylonites in the bancroft shear zone ontario canada calcite textures in several microstructural domains from the bancroft shear zone reveal a sequence of transitions which indicate contributions from a variety of deformation mechanisms at greenschist facies conditions the coarse marble protolith has undergone low strains accommodated by twinning and grain boundary migration porphyroclastic mylonite has a well developed crystallographic preferred orientation and microstructures indicative of dislocation creep and rotation recrystallization mechanisms the textures are consistent with high temperature deformation experiments where r f and c slip systems were active complete dynamic recrystallization produced equant medium sized grains 50 m with a texture that is nearly random secondary calcite growth observed under cathodoluminescence and variation in 18o value with microstructurc indicate large fluid fluxes during mylonitization fluid assisted grain boundary sliding and a minor component of dislocation creep are the inferred deformation mechanisms finer grained s c mylonite has a well developed shape fabric and crystallographic preferred orientation with a point maximum and great circle girdle attributed to attainment of an easy slip orientation for most grains ultramylonites have homogeneous textures with finc elongate grains 20 30 m mantled by disseminated secondary phases textures in ultramylonites are characterized by single point maxima and great circle girdles the most evolved ultramylonite has a point maximum oblique to the shear plane with a symmetry indicating rotation of the maximum toward the extension direction texture microstructure relations in these mylonites indicate that the competition between deformation mechanisms was highly sensitive to grain size strain and secondary phases in addition to temperature strain rate and differential stress the varied deformation mechanisms and application of experimentally derived constitutive equations indicate significant differences in differential stress and strain rate among microstructural domains these differences reflect strain softening associated with dynamic recrystallization and work hardening associated with dissemination of secondary phases and the transition from grain boundary sliding to dislocation creep

transtensional origin of multi order cross folds in a high grade gneiss complex southwestern grenville province formation during post peak gravitational collapse the well mapped western part of the ottawa river gneiss complex orgc new name a large metamorphic core complex hosts a system of gently plunging cross folds of outcrop to regional scale situated in the ductile detachment zone between the lower grade cover and high grade core of the complex the cross folds are buckle structures that deform the attenuated gneissic layering plunge parallel to the regional elongation lineation range from upright to recumbent and exhibit distinctive hinge parallel elongation lineations all features of extension dominated ductile transtension our l s fabric data are consistent both with kinematic modelling that predicts progressive constrictional strain in the hinge zones of transtensional folds and with dynamic modelling that predicts rotation of flattened fold limbs into moderately dipping attitudes on the basis of petrologic data we show that cross folding postdated the peak ottawan metamorphism and took place during retrogression and exhumation of the thrust sheet stack the cross folds form an inclined system that is principally developed in retrograde melt weakened amphibolite facies rocks with the transtensional origin implying that exhumation and retrogression of the high grade core of the complex took place in an oblique extensional setting a transtensional origin for the cross folds removes the need to appeal to orogen parallel regional shortening an implausible requirement of previous interpretations and is compatible with data indicating that much of the visible fabric and structure of amphibolite facies domains of the orgc developed during post peak exhumation retrogression and gravitational collapse of the thrust sheet stack

retrograde deformation within the carthage colton zone as recorded by fluid inclusions and feldspar compositions tectonic implications for the southern grenville province the carthage colton zone ccz located in the northwestern portion of the proterozoic adirondack terrane forms the boundary between the adirondack lowlands and highlands and is characterized by textures indicative of ductile deformation samples from three rock types have been collected from this zone 1 pyroxene bearing syenite gneisses of the diana igneous complex 2 paragneiss samples from near the northwestern boundary of the diana complex 3 a quartz rich metasediment feldspar geothermometry performed on the diana metasyenites shows that porphyroclasts are relict igneous grains that formed att 900 c while dynamic recrystallization occurred at temperatures as low as 470 to 550 c as shown by the compositions of feldspar neoblasts all samples examined in this study from the ccz contain a suite of co2 rich fluid inclusions that are distinctive both texturally and in their microthermometric behavior th 27 7 to 7 1 c as compared to co2 rich adirondack fluid inclusions that do not lie within this zone th 45 9 to 31 0 c the results from fluid inclusion microthermometry and feldspar geothermometry restrict the conditions of dynamic recrystallization to temperatures and pressures of 400 to 550 c and 3 to 5 kbar the retrograde pressure temperature path must pass through these conditions similar fluid inclusion results have been obtained from the parry sound shear zone pssz which is a grenvillian shear zone that is located in southern ontario lamb and moecher 1992 however the inferred retrogradep t paths for these two areas the ccz and the pssz are different and this difference may be a result of late deformation along shear zones that are located between the two areas

nature of the elzevir mazinaw domain boundary grenville orogen ontario the mooroton shear zone msz of the metasedimentary belt of the grenville orogen in ontario separates the mazinaw domain to the east from the elzevir domain the shear zone is composed of mafic metavolcanic metaclastic and marble mylonites and is characterized by a subvertical foliation which crosscuts the regional foliation and a near vertical stretching lineation microstructures indicate deformation occurred under crystal plastic conditions and shear sense indicators give movement of east mazinaw side up the geometry of the msz which follows the eastern margin of the elzevir tonalite implies that lithologic anisotropy guided its location garnet biotite and garnet hornblende peak temperatures are juxtaposed across the msz with temperatures ranging from 460 to 530 c in the southeastern elzevir and 490 to 620 c in the southwestern mazinaw conditions in amphibolite facies limiting barometers applied to rocks in both domains show no significant offset petrologic data within the shear zone

chemical fractionation of brine in the mcmurdo ice shelf antarctica abstract during the austral summers of 1976 77 and 1978 79 several ice cores were taken from the mcmurdo ice shelf brine zone to investigate its thermal physical and chemical properties this brine zone consists of a series of superimposed brine layers waves that originate at the seaward edge of the ice shelf and migrate at various rates depending upon their age and position in the ice shelf the brine in these layers becomes increasingly concentrated as the waves migrate inland through the permeable ice shelf firn chemical analyses of brine samples from the youngest uppermost brine wave show that it contains sea salts in normal seawater proportions further inland deeper and older brine layers though slightly highly saline s200 are severely depleted in s04 2 na ratio being an order of magnitude less than that of normal seawater analyses of na k ca2 mg2 so4 2 and c1 together with solubility and temperature considerations show that the sulfate depletion is due to selective precipitation of mirabilite na2s041oh2o the location of the inland boundry of brine penetration is closely related to the depth at which the brine encounters the firn ice transition however a small but measurable migration of brine is still occurring in otherwise impermeable ice this is attributed to eutectic dissolution of the ice by concentrated brine as it moves into deeper and warmer parts of the mcmurdo ice shelf

late orogenic plastic to brittle extension along the robertson lake shear zone implications for the style of deep crustal extension in the grenville orogen canada rocks regionally metamorphosed and deformed at middle to lower crustal levels in the contraction dominated mesoproterozoic grenville orogen are exposed in southern ontario canada investigation of the robertson lake shear zone rlsz indicates that extension is a significant component in the late tectonic evolution of this deeply eroded orogen the rlsz is a discrete zone 0 5 1 0 km thick that can be traced for nearly 100 km cross cutting regional structures and lithologic units the zone is composed of a thick mylonite zone and a narrow overlying brecciated zone mylonite foliation shallowly dips east southeast and contains a down dip lineation mylonites are characterized by microstructures indicative of crystal plastic deformation and contain a variety of shear sense indicators including s c and c c composite fabrics sigma porphyroclasts mica fish and other shape fabrics oblique to shear planes consistently indicating a normal down to the east sense of shear a zone of cataclastic structures overlies the mylonite zone and brittle deformation overprints the mylonitic fabrics slickensides in the zone generally strike parallel to foliation in the mylonites but have steeper dips sense of slip indicators and brittle fault orientations also indicate down to the east displacement during east west extension juxtaposition of brittle and crystal plastic structures is resolved with a model of displacement during exhumation whereby localization of cataclasis occurred along a previously active mylonite zone metamorphic facies are upper greenschist in the hanging wall and upper amphibolite in the footwall and metamorphic grade increases from west to east in both the hanging wall and footwall the regional variations in metamorphic grade and the low angle shear zone geometry are a result of isostatic flexural rotations that accompanied extension combined with the crystal plastic to cataclastic nature of the rlsz new 40ar 39ar isotope data from biotite constrain the timing of shear zone displacement until at least 901 1 ma late in the evolution of the grenville orogen

coeval thrusting and extension during lower crustal ductile flow implications for exhumation of high grade metamorphic rocks numerical models of tectonic processes in large hot orogens offer insight into the styles controls and consequences of ductile flow of middle and lower orogenic crust we present results from crustal scale thermal mechanical models illustrating contrasting styles of syn and post convergent ductile flow crustal viscosity in the models is controlled by temperature dependent flow laws scaled to represent lithological strength variations an additional viscosity decrease at high temperature can be imposed to represent weakening during incipient partial melting two end member tectonic styles in which lower crustal flow is driven either by gravity or tectonics develop in response to the initial properties and evolving strength of the model crust homogeneous channel flow driven by the gravitational potential energy difference between a plateau and its foreland is characteristic of himalayan tibetan ht series models with relatively weak laterally homogeneous crust and moderately high heat production in contrast grenville orogen go series models with relatively strong laterally heterogeneous crust do not reach the low effective viscosity eff 1019 pa s required for gravity driven channel flow instead tectonically driven lateral flow and explusion of lower crust is triggered by underthrusting of a strong indentor the dominant structures are ductile fold nappes that may be transported hundreds of kilometres towards the foreland when tectonic convergence is turned off the model orogens undergo gravitational spreading in which ductile middle and lower crust flow laterally from the plateau towards the foreland driving thrusting on the orogenic flanks and ductile thinning in the orogenic core the resulting geometry can resemble that produced by syn convergent ductile flow except for the development of regional scale normal sense shear zones in the post convergent stage model feasibility is tested against data from the western grenville orogen in ontario the comparison suggests that high grade metamorphic rocks in the orogenic core record the effects of protracted syn and post convergent ductile flow whereas those on the flanks record mainly post convergent thrusting and extension the models suggest a number of ways to distinguish among different styles of syn and post convergent ductile flow metamorphic p t t paths alone are not diagnostic of tectonic process without additional information on crustal scale structure and timing in combination with erosion both types of flow may contribute to exhumation of regionally extensive high grade gneiss terranes

listric normal faulting during postorogenic extension revealed by 40ar 39ar thermochronology near the robertson lake shear zone grenville orogen canada the robertson lake shear zone is a major plastic to brittle extensional shear zone in the grenville orogen that bounds the mazinaw and sharbot lake domains and provides information on the style of late extension and the unroofing history of the orogen argon isotope data were collected from hornblende and micas to determine 40ar 39ar ages constrain the temperature time histories of these two domains and infer the unroofing history of the region hornblende cooling ages across the mazinaw domain footwall show little variation indicating uniform unroofing of the footwall since 950 ma phlogopite muscovite and biotite cooling ages of the footwall are 924 to 890 ma the cooling history of the mazinaw domain is characterized by slow cooling after peak metamorphism circa 1000 ma accelerated cooling 4 5 c m y from 950 ma to 890 ma and an average cooling rate of 1 c m y to circa 590 ma when these rocks were at or near the surface the cooling history of sharbot lake hanging wall domain is drastically different than that of the mazinaw domain hornblende and biotite cooling ages in the central portion of the domain are 1009 and 969 ma respectively indicating a cooling rate of 5 c m y after slow cooling from metamorphic temperatures biotite and phlogopite cooling ages determined from samples located at different distances from the shear zone do not lie along the same cooling curve indicating that the cooling history varied across the domain cooling rates in the hanging wall adjacent to the shear zone are low 2 c m y a biotite cooling age 1029 ma and preservation of an amphibole growth age 1205 ma in the hanging wall adjacent to the shear zone reflect shallow crustal levels for this sample since 1205 ma these data indicate that the hanging wall away from the shear zone was unroofed from deeper crustal levels faster and much later than the hanging wall adjacent to the shear zone the varied cooling histories across the region are resolved by listric normal faulting that lead to uniform unroofing of the footwall and differential unroofing across the hanging wall due to rotation during fault displacement

extension in the central metasedimentary belt of the ontario grenville timing and tectonic significance a laterally extensive zone of southeast dipping marble mylonites in the central metasedimentary belt cmb of the southern ontario grenville the bancroft shear zone records southeast directed extension and a displacement of about 10 12 km calcite graphite carbon isotope thermometry indicates that the maximum temperature at which shearing occurred was ca 450 c in contrast carbon isotope fractionations 13 c cc gr within the marble protolith consistently give temperatures of 650 700 c the bancroft shear zone postdates 1060 ma and older high grade shear zones in the central gneiss belt and the cmb that are associated with northwest directed thrusting comparison of the temperature data from the retrograde marble mylonites with cooling curves for the cmb gives a late grenvillian age range of 935 1010 ma for the extensional event the geometric relation between thrusting and normal faulting in the cmb is comparable to that in parts of the himalayas and southern tibet and similarly we interpret late normal faulting in the cmb to be a result of gravitational collapse of a thrust thickened crust in contrast to the himalaya tibet region however accretion in the grenville becomes older in the direction of regional thrusting

calcite graphite thermometry of the franklin marble new jersey highlands abstract we present new stable isotope data for the mesoproterozoic franklin marble from outcrops along an 80 km traverse parallel to and across strike of the structural grain of the western new jersey highlands calcite and dolomite from marble have an average 13c of documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 0 35 mbox textperthousand pm 0 73 mbox textperthousand end document pdb documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepacka

differential unroofing within the central metasedimentary belt of the grenville orogen constraints from 40ar 39ar thermochronology an 40ar 39ar thermochronological investigation of upper greenschist to granulite facies gneiss amphibolite and marble was conducted in the central metasedimentary belt cmb ontario to constrain its cooling history incremental 40ar 39ar release spectra indicate that substantial differential unroofing occurred in the cmb between 1000 and 600 ma a consistent pattern of significantly older hornblende and phlogopite 40ar 3ar cooling ages on the southeast sides of major northeast striking shear zones is interpreted to reflect late displacement due to extensional deformation variations in hornblende 40ar 39ar age plateaus exceeding 200 ma occur over distances less than 50 km with major age discontinuities occurring across the robertson lake shear zone and the sharbot lake mylonite zone which separate the sharbot lake terrane from the elzevir and frontenac terranes extensional displacements of up to 14 km are inferred between the frontenac and elzevir terranes of the cmb no evidence for significant post argon closure vertical displacement is indicated in the vicinity of the perth road mylonite within the frontenac terrane variations of nearly 100 ma in phlogopite 40ar 39ar plateau ages occur in undeformed marble on either side of the bancroft shear zone phlogopites from sheared and mylonitized marble within the shear zone yield 40ar 39ar diffusional loss profiles but have older geologically meaningless ages thought to reflect incorporation of excess argon by 900 ma southeast directed extension was occurring throughout the cmb possibly initiated along previous zones of compressional shearing an easterly migration of active zones of extension is inferred possibly related to an earlier overall easterly migration of active zones of regional thrusting and easterly migration of an ancient subduction zone the duration of extensional shearing is not well constrained but must have ceased before 600 ma as required by the deposition of overlying undeformed cambrian and or ordovician sedimentary rocks

co2 rich fluid inclusions in the whitestone anorthosite implications for the retrograde history of the parry sound shear zone grenville province canada the whitestone anorthosite wsa located in southern ontario underwent granulite facies metamorphism during the grenville orogeny at 1 16 ga during the waning stages of metamorphism fluids infiltrated the outer portions of the anorthosite and promoted the formation of an envelope comprised of upper amphibolite facies mineral assemblages also this envelope corresponds to portions of the anorthosite that underwent deformation related to movement along a high grade ductile shear zone samples from this portion of the anorthosite the margin contain co2 rich inclusions in plagioclase porphyroclasts relict igneous phenocrysts matrix plagioclase and garnet these inclusions have features which normally are interpreted as indicating that they are texturally primary but they have relatively low co2 densities 0 61 0 95 g cm 3 plagioclase from the anorthosite interior contains texturally secondary inclusions with relatively high co2 densities generally from 0 99 to 1 10 g cm 3 the high co2 densities suggest that the inclusions in the plagioclase of the anorthosite core formed prior to inclusions in porphyroclast minerals of the outer portions of the anorthosite an interpretation that is apparently inconsistent with inclusion textures this apparent paradox indicates that most fluid inclusions from the anorthosite margin were formed during or were modified by the dynamic recrystallization that affected this portion of the wsa in either case late formation or modification the texturally primary fluid inclusions do not contain pristine samples of the peak metamorphic fluid furthermore because shear related deformation is apparently associated with entrapment of the lowest fluid densities some strain localization persisted to relatively low temperatures e g less than approximately 500 c these results constrain a part of the retrograde p t path for this portion of the grenville orogen to temperatures of approximately 400 500 c at pressures of approximately 1 2 kbar

comparison of garnet biotite calcite graphite and calcite dolomite thermometry in the grenville orogen ontario canada the elzevir terrane of the grenville orogen in southern ontario contains metapelites and abundant graphitic marbles that were regionally metamorphosed from the upper greenschist to upper amphibolite facies comparative thermometry was undertaken with widely used calibrations for the systems garnet biotite calcite dolomite and calcite graphite temperatures that are obtained from matrix biotites paired with prograde garnet near rim analyses are usually consistent with those determined using calcite graphite thermometry however calcite graphite thermometry occasionally yields low temperatures due to lack of equilibration of anomalously light graphite application of calcite graphite and garnet biotite systems may yield temperatures up to 70 c higher than calcite dolomite in amphibolite facies rocks calcite dolomite temperatures most closely approach those from calcite graphite and garnet biotite when the samples contain a single generation of dolomite and calcite grains contain no visible dolomite exsolution lamellae however some of these samples yield temperatures considerably lower than temperatures calculated from calcite graphite and garnet biotite thermometry indicating that the calcite dolomite thermometer may have been partially reset during retrogression estimated peak metamorphic temperatures of regional metamorphism between madoc upper greenschist facies and bancroft upper amphibolite facies range from 500 to 650 c these results place the chlorite staurolite isograd at 540 c the kyanite sillimanite isograd at 590 c and the sillimanite k feldspar isograd at 650 c although each thermometer may have an absolute uncertainty of as much as 50 c the 50 to 60 c temperature differences between the isograds are probably accurate to 10 to 20 c an incomplete picture of the thermal gradients can result from the application of only one thermometer in a given area simultaneous application of several systems allows one to recognize and overcome the inherent limitations of each thermometer

suturing and extensional reactivation in the grenville orogen canada sutures are zones of weakness within orogenic belts that have the potential to become reactivated during orogenic evolution the robertson lake shear zone marks a major tectonic boundary in the southeastern grenville orogen of canada that has been intermittently active for at least 130 m y the shear zone played a major role in the compressional stage of the orogenic cycle as well as during postorogenic collapse the zone separates the elzevir terrane to the west and the frontenac terrane to the east sphene ages u pb indicate that these two terranes have distinct tectonothermal histories and that the shear zone represents a cryptic suture in its current state the shear zone is a low angle 30 ese dip plastic to brittle extensional shear zone that separates the mazinaw footwall and sharbot lake hanging wall domains integration of structural metamorphic and chronologic data leads to a model that describes the complete evolution of this fundamental tectonic boundary that evolved from an early compressional zone ca 1030 ma to a late extensional zone until at least 900 ma

calcite graphite isotope thermometry a test for polymetamorphism in marble tudor gabbro aureole ontario canada graphitization and coarsening of organic material in carbonate bearing metasedimentary rocks is accompanied by carbon isotope exchange which is the basis of a refractory pressure independent geothermometer comparison of observed isotopic fractionations between calcite and graphite a13ccal gr with independent petrological thermometers provides the following empirical calibration over the range 400 800 c a13ccal gr 5 81 x lo6 x t k 2 61 this system has its greatest potential in marbles where calcite graphite is a common assemblage and other geothermometers are often unavailable the temperature dependency of this empirical calibration differs from theoretical calibrations reasons for this are unclear but the new empirical calibration yields temperature estimates in better agreement with independent thermometry from several terranes and is preferred for geological applications both calcite graphite isotopic thermometry and calcite dolomite solvus thermometry are applied to marble adjacent to the tudor gabbro in the grenville province of ontario canada the marble has undergone two metamorphic episodes early contact metamorphism and later regional metamorphism values of a13cca gr decrease regularly from c 8 0 in samples over 2 km from the pluton to values of 3 4 within 200m of the contact these samples appear to preserve fractionations from the early thermal aureole with the empirical geothermometer and indicate temperatures of 450 500 c away from the intrusion and 700 750 c near the gabbro this thermal profile around the gabbro is consistent with conductive heat flow models in contrast the distribution of mg between calcite and dolomite has been completely reset during later regional metamorphism and yields uniform temperatures of c 500 c even at the contact graphite textures are important for interpreting the results of the calcite graphite thermometer coarsening of graphite approaching the tudor gabbro correlates with the decrease in isotopic fractionations and provides textural evidence that graphite crystallization took place at the time of intrusion in contrast to isotopic exchange during prograde metamorphism which is facilitated by graphitization retrogressive carbon isotopic exchange appears to require recrystallization of graphite which is sluggish and easily recognized texturally resistance of the calcite graphite system to resetting permits thermometry in polymetamorphic settings to see through later events that have disturbed other systems

marble mylonites in the bancroft shear zone ontario canada microstructures and deformation mechanisms abstract mylonitization of medium grade marbles in the bancroft shear zone ontario canada is characterized by decreasing grain size of both calcite and graphite and a variety of textures calcite grain sizes vary from several millimeters in the protolith to 50 200 m in mylonite to m in ultramylonite corresponding calcite grain shapes are equant in the protolith elongate in protomylonite first developed dimensional preferred orientation equant in coarse mylonite elongate in fine mylonite second developed dimensional preferred orientation and generally equant in ultramylonite which suggests that external energy applied stress that tends to elongate grains competed with internal energy sources e g distortional strain that favor equant shapes graphite grain size changes from several millimeters to centimeters in the protolith to submicroscopic in ultramylonite in the mylonitic stages graphite is present as dark bands while in the ultramylonitic stage it is preserved as a fine coating on calcite grains based on textural evidence twinning exponential creep regime i dynamic recrystallization power law creep regime ii and possibly grain boundary sliding superplasticity regime iii are considered the dominant deformation mechanisms with increasing intensity of mylonitization their activity is largely controlled by calcite grain size calcite grain size reduction occurred predominantly by the process of rotation recrystallization during the early stages of mylonitization as indicated by the occurrence of core and mantle or mortar structures and by the grain size of subgrains and recrystallized grains grain elongation in s c structures indicates the activity of migration recrystallization these structures are not the result of flattening of originally equant grains differential stress estimates in coarse mylonites and ultramylonites based on recrystallized grain size are 2 5 and 14 38 mpa respectively initial grain size reduction of graphite occurred by progressive separation along basal planes analogous to mica fish formation in quartzo feldspathic mylonites calcite graphite thermometry on mylonitic and ultramylonitic samples shows that the metamorphic conditions during mylonitization were 475 50 c which combined with a differential stress value of 26 mpa gives a strain rate of 1 2 x 10 10 s 1 based on constitutive equations corresponding displacement rates are mm yr 1

3d crustal and lithospheric model of the arabia eurasia collision zone three dimensional joined inversion of topography gravity and geoid data was carried out in the collision zone between the eurasian and arabian plates comprising central and eastern anatolia the black sea basin and the caucasus region the 3d algorithm first introduced by motavalli anbaran et al 2013 is based on a bayesian approach with gaussian density functions in which the targeted area is divided into vertical columns each comprised of four layers namely water of known thickness i e bathymetry if present crust lithospheric mantle and asthenosphere the inversion results are moho depth average crustal density and depth to the lithosphere asthenosphere boundary defined here as the 1350 c isotherm existing seismic moho depths were used as a priori information in order to constrain crustal thicknesses thickest crust up to 50 km was found underneath the great caucasus mountains thinnest crust 22 25 km underneath the black sea basin the east anatolian plateau is underlain by relatively thick crust up to 45 km thinning to less than 40 km towards the central anatolian plateau eastern anatolia and the eastern part of central anatolia are underlain by strongly thinned lithosphere up to 100 km thickest lithosphere up to 200 km underlies the caspian basin and the russian platform

the queensborough mafic ultramafic complex a fragment of a meso proterozoic ophiolite grenville province canada abstract the queensborough mafic ultramafic complex occurs as a fault bounded block up to 10 km wide and having an area 220 km2 it lies in the grimsthorpe domain of the bancroft elzevir mazinaw sharbot lake terrane in the central metasedimentary belt of the grenville province it has been suggested without adequate supporting data that the complex may represent oceanic crust a fragment of an ophiolite or even a metavolcanic sequence made up of basaltic and komatiitic flows the geological and tectonic significance of the complex is assessed using field relationships petrography and geochemistry structurally the lowest part of the complex comprises a series of ultramafic rocks characterized by metre scale compositional layering represented by several different metamorphic assemblages of talc chlorite carbonate anthophyllite and actinolite tremolite these assemblages indicate that the original rocks were cumulate peridotites and pyroxenites the ultramafic rocks are overlain structurally by a series of mafic rocks predominantly massive to highly sheared gabbros the gabbros are penetrated by a series of mafic dykes and include a few small enclaves of pillowed mafic volcanics major and trace element chemistry shows that the mafic rocks represent a fractionally crystallized sequence of tholeiitic gabbros lavas and mafic dykes and that the ultramafic cumulates are co genetic the regional geological setting and the trace element signatures of the mafic rocks suggest that they were formed in a back arc basin comparison of the queensborough complex with proterozoic and phanerozoic igneous complexes suggests that it represents a partially preserved crustal section of a mesoproterozoic ophiolite in addition the rocks of the queensborough complex are petrographically and geochemically similar to those of the vavilov basin which occurs in the deepest part of the tyrrhenian sea by analogy with this neogene back arc basin we suggest that the central metasedimentary belt which contains the queensborough complex formed by attenuation of the continental crust the complex is preserved along a suture and is interpreted as having formed during the collision of a continental island arc with the continental margin of laurentia the characteristics and preservation of the queensborough mafic ultramafic complex are very similar to those of many phanerozoic ophiolites and suggest that mesoproterozoic thermotectonic processes are also similar to those presently operating

empirical tests of carbon isotope thermometry in granulites from southern california abstract peak metamorphic carbon isotope fractionations between calcite and graphite cal gr in marbles and calc silicates from the cucamonga granulite terrane san gabriel mountains california range from 3 48 to 2 90 the data are used to test three previously published calibrations of the calcite graphite carbon isotope thermometer an empirical calibration of the calcite graphite carbon isotope thermometer gives temperatures of 700 750 c a theoretical experimental calibration of the system gives temperatures of 760 870 c an experimental calibration gives temperatures of 870 1300 c temperatures calculated using the empirical calibration are in agreement with those calculated from garnet based cation exchange thermometry when uncertainty is considered temperatures calculated using the theoretical experimental calibration overlap the upper range of cation exchange thermometry temperatures and range to 50 c higher the experimental calibration yields temperatures from 50 to 480 c higher than those from cation exchange thermometry moreover temperatures from the experimental calibration are also inconsistent with mineral and melt equilibria in the granulite phase assemblage despite the better agreement between cation exchange thermometry and the empirical calibration of the calcite graphite system temperatures calculated using the theoretical experimental calibration may be real peak metamorphic temperatures if retrograde diffusion partially reset garnet based cation exchange thermometers by c 50 c then the cation exchange temperatures are consistent with those from the theoretical empirical calibration thermometric evidence from biotite dehydration melting equilibria is consistent with either the empirical calibration if melting was fluid present or the theoretical experimental calibration if melting was fluid absent

u mo and ree mineralization in late tectonic granitic pegmatites southwestern grenville province canada abstract late tectonic granitic pegmatites occur throughout most of the southwestern grenville province of ontario and quebec these vary from mineralogically simple 3 4 major phases to complex 4 major phases pegmatites the latter are variably enriched in u th mo and rare earth elements ree and zr and nb the mineralogical and whole rock geochemical data obtained from simple pegmatites suggest crystallization from low temperature h 2 o undersaturated partial melts generated in amphibolite to granulite grade felsic paragneisses and orthogneisses that have undergone an earlier stage of melting these magmas coalesced and fractionated during ascent to mid crustal levels the mineralogical complexity and increase in abundance of ferromagnesian silicates in individual pegmatites arises from interaction with the host rocks hybridization skarnification previous research has shown that there is an empirical relationship between ferromagnesian silicate content and u th and ree enrichment although this phenomena has not been investigated petrogenetically interaction with the host rocks hybridization produced compositional changes in the pegmatites through bimetasomatic two way exchange processes that modified normal fractional crystallization processes high concentrations of ca fe mg and possibly ti depending on the host rock are interpreted to indicate bimetasomatic exchange resulting in a mineralogical modification of the pegmatitic magma for instance the introduction of ca into the pegmatite magma may lead to local saturation of titanite and or allanite high concentrations of u th ree and nb in these titanites suggests that elemental distribution coefficients are affected within the pegmatite melts by the appearance of hybrid titanite the role of a fluid phase in the hybridization skarnification process is two fold 1 the fluid phase will enhance bimetasomatic diffusive exchange of constituents between the host rocks e g ca fe mg ti and the pegmatite si al k na fe u th ree nb zr h 2 o f cl over that of normal melt solid diffusivities and 2 the fluid may be enriched in certain constituents that become saturated during metasomatic interaction between the pegmatite and host rocks

formation crystallization and migration of melt in the mid orogenic crust muskoka domain migmatites grenville province ontario migmatitic orthogneisses in the muskoka domain southwestern grenville province ontario formed during the ottawan stage c 1080 1050ma of the grenvillian orogeny stromatic migmatites are volumetrically dominant comprising granodioritic gneisses with 2 5cm thick granitic leucosomes locally rimmed by thin melanosomes that constitute 20 30 vol and locally 40 50 vol of the outcrops patch migmatites in dioritic gneisses form large 10m pinch and swell structures within the stromatic migmatites and consist of decimetre scale irregular patches of granitic leucosome surrounded by medium grained hornblende plagioclase melanosomes interpreted as restite the patches connect to larger networks of zoned pegmatite dykes petrographic and geochemical evidence suggests that the patch leucosomes formed by 20 40 fluid present equilibrium melting of the dioritic gneiss followed by feldspar dominated crystallization the dyke networks may have resulted from hydraulic fracturing probably when the melts reached water saturation during crystallization field and geochemical data from the stromatic migmatites suggest a similar petrogenesis to the patch migmatites but with significant additions of externally derived melts indicating that they acted as conduits for melts derived from deeper structural levels within the orogen we hypothesize that the muskoka domain represents a transfer zone for melts migrating to higher structural levels during grenvillian deformation

seismic images of a tectonic subdivision of the grenville orogen beneath lakes ontario and erie new seismic data from marine air gun and vibroseis profiles in lake ontario and lake erie provide images of subhorizontal phanerozoic sediments underlain by a remarkable series of easterly dipping reflections that extends from the crystalline basement to the lower crust these reflections are interpreted as structural features of crustal scale subdivisions within the grenville orogen broadly deformed imbricated and overlapping thrust sheets within the western central metasedimentary belt are succeeded to the west by a complex zone of easterly dipping apparent thrust faults that are interpreted as a southwest subsurface extension of the boundary zone between the central metasedimentary belt and the central gneiss belt the interpreted central metasedimentary belt boundary zone has a characteristic magnetic anomaly that provides a link from the adjacent ends of lakes ontario and erie to structures exposed 150 km to the north less reflective west dipping events are interpreted as structures within the

evolution of deep crustal normal faults constraints from thermobarometry in the grenville orogen ontario canada abstract the robertson lake shear zone rlsz is a low angle 30 dip plastic to brittle extensional shear zone located in the southeastern grenville orogen ontario canada thermobarometric data have been collected to evaluate the effects of extension on regional metamorphic field gradients upper greenschist facies metamorphic conditions are preserved in the footwall and upper amphibolite facies metamorphic conditions in the hanging wall however steep gradients in metamorphic pressure are present adjacent to the robertson lake shear zone in the footwall pressures increase from 550 mpa to 800 mpa within 8 km of the rlsz and in the hanging wall pressures decrease from 900 mpa to 500 mpa within 12 km of the rlsz the pressure gradients are interpreted to be a product of isostatic flexural rotations in response to extension along the zone the degree of rotation indicated by the baric gradients in the hanging wall and footwall are compatible and require an initial shear zone orientation of 60 90 which suggests reactivation of an initially high angle terrane boundary previous studies provide constraints of a depth extent of 40 km for the rlsz and crustal thickness of 60 km during extension thus extensional rotations were accommodated by decoupling and regional scale flow in the lower third of the crust

cooling and inferred uplift erosion history of the grenville orogen ontario constraints from 40ar 39ar thermochronology stepwise 40ar 39ar degassing experiments of 57 mineral separates of hornblende muscovite biotite and perthitic microcline have been used in conjunction with petrologic observations to place regional constraints on the postmetamorphic cooling and the inferred uplift and erosion history of the grenville orogen in ontario the 40ar 39ar data support an interpretation of slow nearly uniform cooling 1 4 c m y from temperatures of 500 c to below 150 c in the central gneiss belt cgb hornblendes cooled through ar closure between 930 and 1025 ma whereas in the central metasedimentary belt cmb hornblendes record the following range in 40ar 39ar cooling ages 1104 ma in the frontenac terrane 1007 1067 ma in the sharbot lake terrane 919 1026 ma in the elzevir terrane and 972 ma in the central metasedimentary belt boundary zone regional uplift erosion rates of 0 03 0 14 km m y have been estimated for the grenville orogen in ontario based on the 40ar 39ar data a model retrograde p t path for rocks of the cgb and an upper time constraint provided by flat overlying cambrian and ordovician sediments these erosion rates are consistent with rates estimated for other proterozoic or archean granulite terranes but are an order of magnitude slower than active orogens such as the alps and himalayas a regular variation in hornblende 40ar 39ar cooling ages is observed in rocks that traverse highly strained often mylonitic shear zones that separate the four major terranes of the cmb the pattern of 40ar 39ar ages is interpreted to reflect late tectonic extension consistent with field observations in the central metasedimentary belt boundary zone and elsewhere in the cmb up to 13 km of vertical displacement is inferred for some rocks in the cmb between the time they cooled below closure to argon diffusion in hornblende 500 c and their exposure at the surface 25 c

the geochemical nature of the igneous rocks of the sharbot lake domain central metasedimentary belt ontario two bimodal mafic silicic suites of igneous rocks the sharbot lake volcanic rocks and the lavant igneous complex are identified geochemically in the sharbot lake domain of the central metasedimentary belt in ontario and their genesis and thermotectonic environment are evaluated the sharbot lake volcanic rocks comprise a series of basalts characterized by light rare earth element lree depletion and relatively high concentrations of fe2o3 tio2 mno v and y together with rhyolites and silicic pyroclastic rocks they are intruded by rocks of the lavant igneous complex which comprises tholeiitic gabbros characterized by lree enrichment and low concentrations of fe2o3 tio2 mno v and y and granitoid rocks the trace element signatures of the mafic rocks of the sharbot lake volcanic sequences are most like those of back arc tholeiitic basalts and those of the lavant igneous complex are comparable to those of low k tholeiitic basalt suites the trace element signatures of the silicic rocks ass

constraints on the duration of tectonic processes protracted extension and deep crustal rotation in the grenville orogen geochronological data on metamorphic minerals within and outside the bancroft shearzone alarge scaleproterozoicshearzone provideevidenceforextensionaldisplacement over a period of 150 m y this is the longest time interval yet demonstrated for any faultrespondingtooneorogenicphase extensionaldisplacementratesalongthebancroft shear zone averaged over 150 m y from 1040 to 893 ma are 0 06 to 0 13 km m y during extension unroofingofthefootwallwasaccompaniedbyrotationas5to15kmofoverlying crustwasunloaded similarlower platerotationhasbeeninferredformanyofthetertiary metamorphic core complexes exposed in the western united states the protracted displacement and rotation history of the proterozoic bancroft shear zone contrasts with the muchfasterextensionalunroofingoftheyoungermetamorphiccorecomplexes suggesting inherent differences in the tectonic processes operating in these two geologic settings similarities however indicate that both settings may share common aspects of tectonic deformation these findings underscore the geologic importance of large deep crustal faults and indicate that present day zones of active extension such as in the himalayas could remain active many millions of years into the future

u pb geochronology of the grenville orogen of ontario and new york constraints on ancient crustal tectonics based on lithological structural and geophysi cal characteristics the proterozoic grenville orogen of southern ontario and new york has been divided into domains that are separated from each other by ductile shear zones in order to constrain the timing of meta morphism u pb ages were determined on metamorphic and igneous sphenes from marbles calc silicate gneisses amphibolites granitoids skarns and pegmatites in addi tion u pb ages were obtained for monazites from meta pelites and for a rutile from an amphibolite these mineral ages constrain the timing of mineral growth the duration of metamorphism and the cooling history of the different domains that make up the southern part of the exposed grenville orogen based on the ages from metamorphic minerals regional and contact metamorphism occurred in the following intervals central granulite terrane

erratum the high pressure belt in the grenville province architecture timing and exhumation we propose that the grenvillian allochthonous terranes may be grouped into high pressure hp and low pressure lp belts and examine the hp belt in detail in the western and central grenville province the hp belt is developed in paleo and mesoproterozoic rocks of the pre grenvillian laurentian margin and characterized by grenvillian eclogite and co facial hp granulite in mafic rocks pressure temperature p t estimates for eclogite facies conditions in well preserved assemblages are about 1800 mpa and 850 c in the central grenville province hp rocks formed at 1060 1040 ma and underwent a single stage of unroofing with transport into the upper crust by 1020 ma whereas farther west they underwent two stages of unroofing separated by penetrative mid crustal recrystallization before transport to the upper crust at 1020 ma unroofing processes were comparable in the two areas involving both thrusting and extensional faulting in an orogen propagating into its foreland by understacking in detail thrusting episodes preceded extension in the western grenville province whereas in the central grenville province they were coeval resulting in unroofing by tectonic extrusion in the central grenville province the footwall ramp is well preserved but any former ramp in the western grenville province was obliterated by later lower crustal extensional flow continuation of the hp belt into the eastern grenville province is not established but likely on geological grounds however the pattern of deep crustal seismic reflection in the lithoprobe eastern canadian shield onshore offshore transect ecsoot line contrasts with that father west suggesting that if present the hp rocks were exhumed by a different mechanism

harvey cardiff domain and its relationship to the composite arc belt grenville province insights from u pb geochronology and geochemistrythis article is one of a series of papers published in this special issue on the theme of geochronology in honour of tom krogh combined mapping u pb isotope dilution thermal ionization mass spectrometry id tims geochronology and geochemical studies in harvey cardiff domain in the western composite arc belt reveal the presence of a long lived magmatic metamorphic history between 1290 and 1030 ma following intrusion of gneissic tonalite of the anstruther and burleigh gneiss complexes at ca 1290 ma diorite of the salmon burn intrusive complex was emplaced at 1242 1 1 1 ma a potassium feldspar megacrystic monzogranite in the salerno creek deformation zone that forms the boundary between harvey cardiff domain and bancroft terrane was emplaced at 1211 3 1 5 ma similar to the 1220 1 6 ma old junction pluton and a previously reported age of ca 1229 ma from another methuen suite granite in the domain all three ages are 20 to 30 million years younger than methuen suite ages elsewhere in the composite arc belt 1250 1240 ma deformation along the salerno creek deformation zone is constrained between 1211 ma the age of th

early history of the carthage colton shear zone grenville province northwest adirondacks new york u s a abstract the adirondack mountains expose two distinct tectonic elements of the proterozoic grenville province of northeastern north america the adirondack lowlands and highlands the lowlands are located along the eastern edge of the metasedimentary belt and the highlands form the western portion of the granulite terrane the two are separated by the carthage colton shear zone ccsz u pb titanite and 40ar 39ar hornblende ages in the lowlands are ca 100 m yr older than in the highlands across the ccsz while both the lowlands and the highlands record a history of metamorphism during the elzevirian orogeny ca 1150 ma only the highlands record evidence of a major phase of grenvillian thermotectonic activity ottawan orogeny at ca 1090 1030 ma proposed tectonic models require that the lowlands were either laterally separated from the highlands or were at structurally higher levels during this metamorphism new u pb and 40ar 39ar ages from titanite and hornblende bearing mylonites in the dana hil

u pb zircon geochronology of mesoproterozoic postorogenic rocks and implications for post ottawan magmatism and metallogenesis new jersey highlands and contiguous areas usa abstract postorogenic rocks are widespread in grenville terranes of the north central appalachians where they form small discordant largely pegmatitic felsic intrusive bodies veins and dikes and also metasomatic calcic skarns that are unfoliated and postdate the regional 1090 to 1030 ma upper amphibolite to granulite facies metamorphism related to the grenville ottawan orogeny zircons from magmatic and nonmagmatic rocks from northern new jersey and southern new york were dated to provide information on the regional tectonomagmatic and metallogenic history following ottawan orogenesis we obtained u th pb zircon ages of 1004 3 ma for pegmatite associated with the 1020 4 ma mount eve granite near big island new york 986 4 ma for unfoliated discordant pegmatite that intrudes supracrustal marble at the buckwheat open cut franklin new jersey 990 ma for a silicate borate skarn layer in the franklin marble at rudeville new jersey and 940 2 ma for a calc silicate skarn layer at lower twin lake new york this new data together with previously published ages of 1020 4 to 965 10 ma for postorogenic rocks from new jersey and southern new york provide evidence of magmatic activity that lasted for up to 60 ma past the peak of high grade metamorphism postorogenic magmatism was almost exclusively felsic and involved relatively small volumes of metaluminous to mildly peraluminous melt that fractionated from an a type granite parent source field relationships suggest the melts were emplaced along lithosphere scale fault zones in the highlands that were undergoing extension and that emplacement followed orogenic collapse by least 30 ma postorogenic felsic intrusions correspond to the niobium yttrium fluorine nyf class of pegmatites of cern 1992a geochronologic data provide a temporal constraint on late stage hydrothermal activity and a metallogenic event in new jersey at 990 to 940 ma that mineralized pegmatites with subeconomic to economic deposits of magnetite u th rare earth element ree and formed metasomatic calcic skarn bodies in marble and reactive carbonate rocks mineralization associated with this event overlaps the timing of pegmatite emplacement suggesting a petrogenetic relationship coeval metallogeny at 975 to 950 ma in the new york hudson highlands and 980 to 937 ma in the canadian grenville province implies that this event was widespread following the ottawan phase of the grenville orogeny

stable isotope thermometry at high temperatures determination of accurate temperatures for geological events has been the grail of stable isotope geochemistry since the seminal 1947 paper by urey in theory this should be simple calibrated mineral pairs are common analysis is rapid and there is no significant pressure correction however in spite of widespread application the promise of reliable thermometry has been elusive stable isotope temperatures in metamorphic and igneous rocks are often controversial is the fractionation between two phases a thermometer speedometer rate dependant hygrometer p h2o dependant or chimera a number of factors have contributed to this uncertainty including incomplete and sometimes conflicting calibration of isotope fractionation factors limited understanding of the kinetics of mineral exchange and the lack of microanalytical techniques this situation has improved dramatically over the past decade permitting reliable and detailed thermal histories to be inferred in a landmark paper bottinga and javoy 1975 empirically calibrated the oxygen isotope fractionation factors among ten rock forming minerals at t 500 c their compilation shows that 84 of analyzed samples contain at least three minerals meeting a concordance test for equilibrium they concluded the great majority of igneous and metamorphic rocks has conserved a state of oxygen isotope exchange equilibrium if correct this bodes well for thermometry however this conclusion was contested by deines 1977 who critically examined the existing data and concluded that less than half of the rocks analyzed to date would yield concordant 18o derived temperatures while seeming irreconcilable both conclusions are based on fact all rocks contain some degree of disequilibrium but many also preserve a thermometric record it is the purpose of this review to discuss the new strategies for reliable isotope thermometry and to explore disequilibrium processes so they may be recognized and properly interpreted the organization of this paper will be to discuss the assumptions

palaeomagnetic dating of fault rocks evidence for permian and mesozoic movements and brittle deformation along the extensional dalsfjord fault western norway summary palaeomagnetic and structural studies of the dalsfjord nappe western norway show that the basal ow angle detachment dalsfjord fault is a long lived fault zone and that the most important phase of faulting was of devonian extension probably nucleated on an earlier silurian scandian thrust fault rocks produced during subsequent movements indicate that the dalsfjord fault underwent periods of brittle low angle extensional reactivation during the permian 250 260 ma and upper jurassic lower cretaceous c 150 ma corresponding with stages of major extensional movements on the continental shelf palaeomagnetic studies may be of great importance for dating faults and major movement stages in long lived fault systems the particular importance of the results is that they show that low angle normal faults can operate in a brittle upper crustal regime

synorogenic collapse a perspective from the middle crust the proterozoic grenville orogen structural petrological and geochronological studies of the middle to late proterozoic grenville orogen in ontario canada indicate that a major extensional fault developed synchronously with late thrusting this fault zone was initiated during peak metamorphism and extended into the crust to depths of at least 25 kilometers the temporal and spatial relations among faulting metamorphism and regional compression indicate that synorogenic collapse initiated because the crust exceeded the maximum physiographic height and thickness that could be supported by its rheology comparison of grenville with recent himalayan orogenic activity suggests that during proterozoic times physiographic height crustal thickness and crustal strength were similar to modern conditions in orogenic belts