white mica crystallinity finite strain and cleavage development across a large variscan structure nw spain x ray diffraction analyses were performed on white micas formed under strain and low grade metamorphic conditions from sandstones and siltstones along a transect through the narcea antiform a large structure that separates the external from the internal zones of the variscan belt of nw spain the results indicate a close spatial relationship between the onset of metamorphism and the cleavage front at the cleavage front strain is characterized by oblate ellipsoids that have x y z ratios from 1 to 1 5 the x y z ratio increases westward up to 3 0 as does the grade of metamorphism and is accompanied by an increase in the 9crystallinity9 index of the white mica from very low to low metamorphic grade this increase corresponds qualitatively to an increase in penetrativeness of the axial planar cleavage in the western part of the narcea antiform the major deformation event was the development of kilometre scale reverse shear zones because of polyphase deformation these rocks are not suitable for quantitative strain analysis but qualitatively finite strain can be observed to be prolate and invariably larger in this region than in the eastern sector the metamorphic grade is higher in the western part of the narcea antiform with local maxima occurring next to the base of the shear zones the b 0 cell parameter a semi quantitative geobarometer measured in the white mica is chaotic in the eastern sector showing a clear detrital inheritance in the western sector it is rational due to complete re equilibration during thermal resetting the inverse correlation between finite strain and white mica 9crystallinity9 is close showing that the anchizone epizone limit can be related to the increase of finite strain above values of r s 1 5 the close relation between white mica 9crystallinity9 and strain is suitable for tracking strain variations in extensive fine grained siliciclastic rocks where no appropiate strain markers are found

alleghanian reactivation of the acadian fold belt meguma zone southwest nova scotia shear zones and northwest verging folds define a 30 km wide belt of deformation that overprints the acadian fold belt and telescopes isograds in the meguma zone in southwest nova scotia the shear zones appear to form a linked system that accommodated convergence dominated transpression of the meguma zone against an irregular avalon boundary available geochronological data indicate a mid carboniferous alleghanian variscan age for the overprinting deformation the mid carboniferous basement reactivation in southwest nova scotia is likely coeval with deformation of carboniferous strata and reactivation of basement structures meguma group in the northern meguma zone together these mid carboniferous structures may define a wide belt of alleghanian variscan deformation across the northwest cratonward margin of the meguma zone

a sand rich submarine fan in the lower mesozoic mineral king caldera complex sierra nevada california abstract a turbidite sequence in the lower mesozoic mineral king roof pendant southern sierra nevada california is interpreted to be a small submarine fan informally named the bullfrog fan which formed in a continental arc setting abundant sand sized sediment was supplied to the fan by rapid downslope reworking of rhyolitic pyroclastic debris from a silicic volcanic source that lay to the southeast in an area now occupied by younger plutons the bullfrog fan accumulated in a segment of an epicontinental silicic volcanic are in which caldera subsidence structures have been identified the bullfrog fan is a 480 m thick progradational sequence bed thickness average grain size and abundance of channelized mass flow deposits all increase toward the southeast where the most proximal part of the fan and the inferred source area have been intruded by a large cretaceous granitic pluton the bullfrog fan prograded onto starved basin plain sediments consisting of very thin bedded and laminated silt and pyritic mud fan fringe and lobe fringe deposits compose only the basal tenth of the section resulting in abrupt lateral pinch out of fan sandstones into starved basin plain deposits the overlying outer fan depositional lobes compose over one third of the section have high sandstone to shale ratios and commonly lack fine grained interlobe deposits suggesting that depo itionally the lobes laterally interfinger and are vertically stacked these are overlain by midfan sandstones with abundant channel deposits that form over half of the bullfrog fan the midfan deposits consist of a lower sequence of lobe apical channel cycles and an upper sequence of braided channel cycles each lobe apical channel cycle is characterized by an unchannelized upward coarsening and thickening sequence overlain by a channelized upward fining and thinning sequence channelized deposits form about 40 percent of the lobe apical channel cycles and each cycle records the extension of a channel over the depositional lobe that it fed pebbly sandstones characterize both the lobe apical channels and the braided channel cycles and interchannel material is rare pebbly mudstone debr s flow deposits are locally interstratified with the braided channel deposits and overlie the midfan in a nearly continuous sheet less than 8 m thick the debris flows locally pass upward into laminated and rippled shallow marine siltstones coquinites limestones and cross bedded sandstones bearing late triassic megafossils although faulting has obscured this contact in most areas nonetheless the shallow marine sedimentary rocks apparently directly overlie midfan deposits with no inner fan or slope deposits intervening without sufficient clay sized sediment highly mobile turbidity currents capable of depositing basin plain fan fringe lobe fringe and interchannel turbidites apparently could not develop instead high density turbidity currents and fluidized flows deposited sandand pebble sized pyroclastic debris on smooth and channeled lobes that filled a relatively shallow basin within a submarine caldera complex

minas fault zone late paleozoic history of an intra continental orogenic transform fault in the canadian appalachians abstract the minas fault zone mfz defines the boundary between the avalon and meguma terranes in the canadian appalachians and is exposed in mainland nova scotia and southern new brunswick these terranes originated along the gondwanan margin but had accreted to laurentia by the middle devonian the surface trace of the mfz is adjacent to the southern margin of the latedevonian permian maritimes basin the late devonian late carboniferous evolution of the mfz involves several episodes of oblique dextral shear that resulted in basin formation and inversion and at various times the zone was the focus of magmatism regional fluid flow and mineralization in the late devonian early carboniferous asymmetric rifting accompanied by dextral shear produced two coeval sequences the horton group which is dominated by continental clastic strata and the fountain lake group which consists predominantly of bimodal volcanic rocks that overlie high level plutons emplaced along active shear zones the overall tectonic environment may have been dominated by dextral transtension along the southern margin of laurentia which corresponded with the northern flank of the rheic ocean a major change in the evolution of the minas fault zone occurred in the late mississippian early pennsylvanian and produced the e w chedabucto fault clockwise rotation of pre existing structures local zones of transtension and transpression as well as regional fluid flow and extensive mineralization this major change may reflect the onset of laurentia gondwana oblique collision the effects of which continued into the latest carboniferous with coeval development of flower structures and pull apart basins in zones of local transpression and transtension

tectonic evolution of nw iberia during the paleozoic inferred from the geochemical record of detrital rocks in the cantabrian zone the cantabrian zone of nw iberia preserves a voluminous almost continuous sedimentary sequence that ranges in age from ediacaran to early permian its tectonic setting is controversial and recent hypotheses include i passive margin deposition along the northern margin of gondwana ii an active continental margin or iii the margin of a drifting ribbon continent and iv a combination of the three aforementioned possibilities in this paper we present geochemical data from 21 samples taken in detrital rocks in the cantabrian zone from ediacaran to early permian age the results together with previously published detrital zircon ages from these strata allow a comprehensive analysis of changing tectonic scenarios and provenance through time collectively these data indicate that this portion of iberia was an active margin during the ediacaran that evolved to become part of the passive margin of gondwana at least from ordovician to late devonian times changes in geochemistry zircon age distribution and sm nd isotopes during the carboniferous are attributed to the far field effects of the onset of the collision between gondwana and laurussia and related processes such as recycling of older sedimentary sequences as well as the involvement of the rocks formed during the variscan orogeny and oroclinal buckling latest carboniferous and permian show a more juvenile sm nd and higher values of illite cristallinity that may support the hypothesis of lithospheric foundering after oroclinal buckling

orogenesis and basin development u pb detrital zircon age constraints on evolution of the late paleozoic st marys basin central mainland nova scotia abstract the st marys basin along the southern flank of the composite late paleozoic magdalen basin in the canadian appalachians and along the avalon meguma terrane boundary contains late devonian early carboniferous continental clastic rocks of the horton group that were deposited in fluvial and lacustrine environments after the peak of the acadian orogeny shrimp ii geological survey of canada data on approximately 100 detrital zircons from three samples of horton group rocks from the st marys basin show that most of the zircons have been involved in a multistage history recycled from clastic rocks in the adjacent meguma and avalonian terranes although there is a minor contribution from early silurian 411 ma and late devonian suites ca 380 370 ma neoproterozoic ca 700 550 ma and paleoproterozoic ca 2 0 2 2 ga zircon populations predominate with a minor contribution from ca 1 0 1 2 and 1 8 ga zircons published u pb single zircon analyses on clastic sedimentary rocks indicate th

hydroxide in pyroxene variations in the natural environment a suite of 51 pyroxenes from a wide range of geological environments was examined with infrared spectroscopy small amounts of oh occur in nearly all the samples suggesting that oh groups are a minor component of most pyroxenes the oh concentrations vary 0 002 0 12 wt oh as a function of geological setting with the greatest amounts occurring in mantle derived samples the intensities of certain oh absorption bands are somewhat correlated with pyroxene composition in particular with the presence of trivalent cations this suggests that crystal chemical or compositional factors also control the oh incorporation the oh incorporated in the pyroxene structure is easily distinguishable from oh owing to amphibole lamellae which usually occur in diopsides and often in orthopyroxenes oh in pyroxene is probably a function of the activity of hydrous components during crystallization but may also depend on postcrystallization changes in the geological environment

brunswick subduction complex in the canadian appalachians record of the late ordovician to late silurian collision between laurentia and the gander margin of avalon the brunswick subduction complex in the new brunswick part of the canadian appalachians records the late ordovician to late silurian collision between laurentia and the gander margin of avalon the brunswick complex is anomalously well preserved compared with equivalent rocks and structures elsewhere owing to its unique position in the deepest part of the quebec reentrant of the laurentian margin this part of the margin experienced less underthrusting and exhumation and overprinting by orogen parallel faulting than the adjacent promontories where collision started earlier the early southeast to east vergent thrust related structures represent a progressive d1 deformation that formed in response to northwestward subduction of the previously extended gander margin and subsequent tectonic unroofing of the subduction complex the original shallow northwestward dipping envelope to s1 was deformed in the late silurian by d2 upright folds and associated shear zones into a steep belt during terminal collision the d2 structures probably formed in response to sinistral transpression together d1 and d2 indicate that convergence was oblique and sinistral most rocks incorporated in the subduction complex formed in the tetagouche back arc basin that evolved from rifting of an arenig magmatic arc built on the gander margin into a wide marginal basin subduction was initiated in the late ordovician 455 ma in the back arc basin following collision of the middle ordovician popelogan arc with laurentia in the caradoc shortly after its opening in the late arenig 473 ma the closure history of the iapetus ocean involved more than one subduction zone and arc continent collision and rivals the southwestern pacific ocean in its complexities

origin of the rheic ocean rifting along a neoproterozoic suture the rheic ocean is widely believed to have formed in the late cambrian early ordovician as a result of the drift of peri gondwanan terranes such as avalonia and carolina from the northern margin of gondwana and to have been consumed in the devonian carboniferous by continent continent collision during the formation of pangea other peri gondwanan terranes e g armorica ossa morena northwest iberia saxo thuringia moldanubia remained along the gondwanan margin at the time of rheic ocean formation differences in the neoproterozoic histories of these peri gondwanan terranes suggest the location of the rheic ocean rift may have been inherited from neoproterozoic lithospheric structures formed by the accretion and dispersal of peri gondwanan terranes along the northern gondwanan margin prior to rheic ocean opening avalonia and carolina have sm nd isotopic characteristics indicative of recycling of a juvenile ca 1 ga source and they were accreted to the northern gondwanan margin prior to voluminous late neoproterozoic arc magmatism in contrast sm nd isotopic characteristics of most other peri gondwanan terranes closely match those of eburnian basement suggesting they reflect recycling of ancient 2 ga west african crust the basements of terranes initially rifted from gondwana to form the rheic ocean were those that had previously accreted during neoproterozoic orogenesis suggesting the rift was located near the suture between the accreted terranes and cratonic northern gondwana opening of the rheic ocean coincided with the onset of subduction beneath the laurentian margin in its predecessor the iapetus ocean suggesting geodynamic linkages between the destruction of the iapetus ocean and the creation of the rheic ocean

new u pb zircon ages and the duration and division of devonian time newly determined u pb zircon ages of volcanic ashes closely tied to biostratigraphic zones are used to revise the devonian time scale they are 1 417 6 1 0 ma for an ash within the conodont zone of icriodus woschmidti i w hesperius early lochkovian 2 408 3 1 9 ma for an ash of early emsian age correlated with the conodont zones of po dehiscens lower po inversus 3 391 4 1 8 ma for an ash within the po c costatus zone and probably within the upper half of the zone eifelian and 4 381 1 1 3 ma for an ash within the range of the frasnian conodont palmatolepis punctata pa punctata zone to upper pa hassi zone u pb zircon ages for two rhyolites bracketing a palyniferous bed of the pusillites lepidophyta spore zone are dated at 363 8 2 2 ma and 363 4 1 8 ma respectively suggesting an age of363 ma for a level within the late famennian pa g expansa zone these data together with other published zircon ages suggest that the base and top of the devonian lie close to 418 ma and 362 ma respectively thus lengthening the period by20 over current estimates we suggest that the duration of the middle devonian eifelian and givetian is rather brief perhaps no longer than 11 5 myr 394 ma 382 5 ma and that the emsian and famennian are the longest stages in the period with estimated durations of15 5 myr and 14 5 myr respectively 1998 elsevier science b v all rights reserved

the influence of water on the petrogenesis of subductionrelated igneous rocks the presence of dissolved water can significantly change the sequence in which different minerals crystallize from a silicate magma1 3 this will alter the compositional path followed by residual liquids the liquid line of descent and modify the types and compositions of crystals that accumulate at the site of cooling the sharp decrease in the solubility of water in silicate melts with decreasing pressure makes direct measurements of pre eruptive concentrations in magmas difficult because much of the dissolved water boils off before eruption here we report experimental results on the crystallization of basalts that contain substantial dissolved water up to 6 wt and show how the results can be used to infer the amount of dissolved h2o involved in the petrogenesis of lavas that preserve a record of their liquid lines of descent or the accumulated minerals left behind after solidification the presence of abundant magmatic water is a signature of subduction zone volcanism4 and can be used to associate a suite of magmas or igneous cumulates with a convergent margin back arc or interarc tectonic setting

synthesis of palaeozoic deformational events and terrane accretion in the canadian appalachians plate tectonic theory predicts that most deformation is associated with subduction and terrane accretion with some deformation associated with transform transcurrent movements deformation associated with subduction varies between two end members 1 where the tectonic regime is dominated by subduction of oceanic lithosphere containing small terranes a narrow surface zone of accretionary deformation along the subduction zone starts diachronously on the subducting plate at the trench as material is transferred from the subducting plate to the over riding plate and 2 where continent continent collision is occurring a wide surface zone of accretionary deformation starts synchronously or with limited diachronism palaeozoic deformational events in the canadian appalachians correspond to narrow diachronous events in the ordovician and silurian whereas devonian carboniferous and permian deformational events are widespread and broadly synchronous along the western side of the canadian appalachians the taconian deformational event starts diachronously throughout the ordovician and corresponds to the north north west accretion of the notre dame ascot weedon st victor and various ophiolitic massifs volcanic arc and peri arc terranes over cratonic north america within the eastern half of the central mobile belt the late cambrian early ordovician penobscotian deformational event corresponds to the south easterly accretion of the exploits subzone various volcanic are and peri arc terranes over the gander zone continental rise in the centre of the orogen the late ordovician silurian beothukan deformational event corresponds to the south easterly accretion of the notre dame over the exploits gander subzones along the south eastern side of the central mobile belt the silurian ganderian deformational event corresponds to the north north east sinistral transcurrent accretion of the avalon composite terrane microcontinent over the gander exploits zones along the south eastern half of the orogen the late silurian middle devonian acadian deformation event corresponds to the westerly accretion of the meguma terrane intradeep or continental rise over the avalon composite terrane affecting the entire orogen the late devonian carboniferous and permian acadian alleghanian deformational events correspond to the east west convergence between laurentia and gondwana continent continent collision

basin thermal history favours an accretionary origin for the southern uplands terrane scottish caledonides there is general agreement that the siluro ordovician southern uplands suspect terrane is an imbricate thrust belt formed adjacent to the laurentian continental margin during closure of the iapetus ocean however there are conflicting models for its tectonosedimentary evolution especially for the initial depositional environment was it a convergent forearc trench with subsequent incorporation of strata into a suprasubduction accretionary prism or was it a back arc or continental margin extensional basin subsequently caught up in a foreland fold and thrust belt to address this issue the different thermal histories of the contrasting convergent and extensional sedimentary basin models have been investigated palaeogeothermal conditions in the southern uplands terrane assessed from regional metamorphic patterns clay mineral assemblages and k white mica composition rule out extension and support deposition in a low heat flow plate convergent setting there is no evidence for a change from extensional to convergent basin regimes from north to south as required in some models instead throughout the terrane there is a close temporal and spatial relationship between deposition burial metamorphism and tectonism that is elsewhere typical of accretion

40ar 39ar white mica ages reveal neoproterozoic paleozoic provenance and an alleghanian overprint in coeval upper ordovician lower devonian rocks of meguma and avalonia abstract 40ar 39ar analyses ir single grain total fusion of white micas from upper ordovician lower devonian sedimentary strata in the meguma annapolis section and avalon arisaig section terranes of the canadian appalachians complement recent detrital zircon studies by providing constraints on local source areas and identifying the age of low temperature a remarkable outcome of the study is that many muscovite ages are significantly younger than the depositional age of the strata they do not record the middle late devonian age of acadian folding or adjacent intrusions but instead preserve a precise age of 322 6 2 1 ma 16 analyses although the sections sampled display no discernable evidence for carboniferous tectonism these ages are coeval with documented alleghanian deformation in both avalonian and meguma terrane rocks and we interpret them to reflect distributed fluid flow coeval with dextral shear along the avalon meguma terrane boundary

diagenesis to metamorphism transition in an episutural basin the late paleozoic st mary s basin nova scotia canada the late devonian early carboniferous st mary s basin in the canadian appalachians consists of horton group fluviatile and lacustrine clastic rocks the basin occurs along the boundary between the avalon and meguma ter ranes and developed during coeval dextral shear along that boundary x ray diffraction reveals that the rocks contain ubiq uitous quartz k white mica and albite illite smectite mixed layers and chlorite are very common and na k mica kaolinite chlorite smectite mixed layers k feldspar berthierine and rutile occur in some samples crystal chemical pa rameters of white mica indicate the pressure and temperature of mineral growth and discriminate between diagenetic an chizone and low grade metamorphic processes kubler index values measured in the 5 a u peak and the presence of chlorite mica stacks are indicative of high anchizone epizone grades with a crystallinity crystal size and number of de fects that increases towards the chedabucto fault which defines the northern margin of the basin kubler index values measured in the 10 a u peak indicate that a late fluid rich event could have produced the observed illite smectite mixed layers the overall clay mineral content and the b cell dimension of the k white micas are typical of postdepositional evo lution in extensional sedimentary basins with high heat flow 35 8c km taken together our data record two superposed events related to deformation along the basin margins and coeval regional fluid flow in which retrograde reactions at tem perature t 300 8c regional syntheses indicate that this fluid flow may have occurred during epi sodes of late carboniferous dextral shear along the avalon meguma terrane boundary

melting experiments on lherzolite klb 1 under hydrous conditions and generation of high magnesian andesitic melts the partial melting of natural lherzolite klb 1 at 1 gpa was studied for both h2o undersaturated and saturated conditions the quenched glasses obtained at 1000 and 1050 c are andesitic in composition having 54 4 60 3 wt sio2 whereas partial melts formed at temperatures above 1100 c have basaltic compositions this suggests that high magnesian andesites can be formed by direct partial melting of the hydrous upper mantle only at relatively low temperatures these andesitic melts together with those obtained at 1100 c contain more than 3 wt h2o and have compositions characteristic of hydrous melts high sio2 and al2o3 and low feo mgo and cao the results of melting studies on klb 1 show a strong increase in the degree of melting when h2o is added to lherzolite at a given temperature

precollision and postcollision thermal events in the himalaya illite crystallinity and b o spacing analyses of rocks from the lesser himalaya show that the upper paleocene lower eocene subathu formation deposited immediately prior to or early in the india asia collision shows distinctly lower metamorphic grades than the anchiepizone upper precambrian lower paleozoic rocks the latter have been affected by a precollision metamorphic event the probable age of which is paleozoic although a mesozoic age is not ruled out thermal effects of the main central thrust are restricted to a section 1300 m below the thrust therefore the le fort model for metamorphic inversion and granite formation needs modifying because the exposed main central thrust footwall rocks could not have generated the fluids responsible for anatectic melting

post orogenic exhumation of an auriferous terrane the paleoplacer potential of the early carboniferous st marys basin canadian appalachians the st marys basin of mainland nova scotia canada consists of late devonian early carboniferous clastic rocks of the horton group deposited in the waning stages of the ca 420 360 ma acadian orogeny in the canadian appalachians clast lithologies and lithogeochemical analyses indicate that the detritus was predominantly derived from the meguma terrane that occurs to the south of the basin the meguma terrane contains abundant mesothermal gold deposits that are coeval with peak magmatic activity from ca 380 to 370 ma and underwent rapid uplift and erosion between ca 370 and 360 ma within the st marys basin the contact between the lacustrine little stewiacke river formation and the fluviatile barrens hills formation is interpreted to represent a shoreline and a potentially favorable environment for depositing paleoplacer gold geochemical analyses of lithologies adjacent to this contact indicate that the siltstones are predominantly derived from meguma terrane metasedimentary rocks whereas the sandstones and conglomerates are predominantly derived from meguma terrane granitoids geochemical and mineralogical analysis indicate the accumulation of heavy minerals including zircon and gold micron scale 150 m gold identified in the matrix of the conglomerates has a nuggety appearance and flakey microtexture indicative of a detrital origin this observation indicates that the mesothermal deposits were exhumed by the latest devonian consistent with regional tectonic syntheses that invoke rapid uplift of the meguma terrane following peak orogenic activity this study suggests that favorable depositional environments for paleoplacer deposits may occur in horton group rocks that are derived from the meguma terrane

water in the lower crustal granulite xenoliths from nushan eastern china 1 nominally anhydrous clinopyroxene cpx orthopyroxene opx and plagioclase pl from 10 lower crustal granulite two pyroxene granulite and hypersthene granulite xenoliths in cenozoic basalts from the nushan volcano eastern china have been analyzed for their hydrogen content by microscopic fourier transform infrared spectroscopy micro ftir the results demonstrate that hydrogen was incorporated in all these minerals in the manner of oh and that the content h2o weight is up to 2360 ppm for cpx 1170 ppm for opx and 880 ppm for pl on the basis of the water content of constitutive minerals and their proportions whole rock water contents of the nushan granulites were estimated to be 150 950 ppm estimated equilibrium temperatures of the nushan granulites are in the range of 810 892 c corresponding to the lowermost crust at nushan about 25 30 km therefore this study provides direct evidence that the lower continental crust even the lowermost part devoid of hydrous minerals can contain a certain amount of water in nominally anhydrous minerals

collision along an irregular margin a regional plate tectonic interpretation of the canadian appalachians an idealized plate tectonic model for the pre carboniferous development of the canadian appalachians explains the 400 km dextral offset of tectonostratigraphic zones from quebec and northern new brunswick to newfoundland and the up to 600 km offset of oppositely verging belts of acadian deformation from the gaspe peninsula to eastern newfoundland it is proposed that these offsets which occur at the st lawrence promontory result from the collision of an irregular north american passive continental margin with island arc and continental crust to the east along an east dipping subduction zone the line of subduction is assumed to have been linear and the subducting slab to have maintained its mechanical integrity during collision a jigsaw fit of the opposite sides of the iapetus ocean is made unnecessary by invoking lithospheric delamination and tectonic wedging during the acadian orogeny in newfoundland the model is consistent with surface geology and recent deep seismic reflection observations fro

interlaboratory standardization and calibration of day mineral crystallinity and crystallite size data clay mineral crystallinity and crystallite domain size data determined by x ray diffraction xrd are methods extensively used in the characterization of very low grade metamorphic conditions however the lack of sufficient interlaboratory standardization has made comparisons between different research groups unreliable due to significant variations in numerical results obtained a consequence of the different machine conditions measurement methods and sample preparations used during analysis a calibration approach to the standardization of data using rock chip standards is presented which allows data sets produced by different research groups to be directly and quantitatively compared a standardized scale the crystallinity index standard cis is proposed with illite crystallinity anchizonal boundary limits of 0 25d 2 and 0 42d 2 and equivalent illite crystallite sizes of 52 and 23 nm respectively determined by the warren averbach method calibrating both old and new data will enable more reliable comparisons between similar and contrasting geological environments and should improve the accuracy and reliability of correlations made between xrd data and other indicators of very low grade metamorphism hence increasing the value of such clay mineral studies

a survey of hydrous species and concentrations in igneous feldspars the hydrous components in 85 feldspars from various igneous environments spanning the range of naturally occurring compositions were examined with infrared spectroscopy the feldspars contain structural oh 0 512 ppm h 2o h 2o 0 1350 ppm h 2o and nh 4 0 1500 ppm nh 4 groups as well as fluid inclusions and alteration products although composition and structure do influence the type of hydrous species that can be incorporated into a particular feldspar mineral the concentration of these species is not controlled by major element composition coarse perthitic microclines have a heterogeneous distribution of hydrous species and contain h 2o or nh 4 in k rich lamellae and fluid inclusions in na rich areas the structural oh in plagioclase feldspars is not associated with twin boundaries or exsolution lamellae all of the possible structural hydrous species are found in pegmatite feldspars whereas volcanic feldspars contain only structural oh the variation in oh concentration within a given feldspar composition suggests that fluids in the geologic environment play a role in determining the hydrogen concentration of each sample the vast majority of plutonic feldspars have undergone partial or total equilibration with meteoric fluids during low temperature 400 150 c hydrothermal exchange obliterating any structural hydrogen and creating substantial concentrations of fluid inclusions up to 4000 ppm h 2o in the exchanged regions the amount of water stored as fluid inclusions within feldspars in the upper crust 1 10 19 kg is small compared to the 1 35 10 21 kg of water in the oceans but is roughly equivalent to the reservoir of water stored in hydrous minerals in the upper crust

mylonite to megabreccia tracking fault events within a transcurrent terrane boundary in nova scotia canada transcurrent terrane boundaries commonly evolve into long lived faults that preserve little evidence for early docking events a remarkable exception is exposed at clarke head along the appalachian meguma terrane boundary in nova scotia canada where a late carboniferous fault megabreccia contains devonian 369 ma u pb zircon granulite grade mylonite fractured by veins filled with visean amphibole ca 335 ma ar ar this fractured mylonite was later mixed with early carboniferous sedimentary rocks during megabrecciation ca 315 310 ma these three fault events are reflected in the tectonostratigraphic record devonian ca 370 360 ma transpressional terrane docking ramped meguma up against avalonia and shed clastic detritus across the fault system the visean brittle deformation recorded by the amphibole veins was coeval with marine regression at surface the late namurian megabrecciation event similarly produced unconformity followed by renewed nonmarine clastic sedimentation the clarke head megabreccia therefore preserves an episodic late paleozoic fault history spanning some 55 m y during convergence between laurentia and gondwana and the assembly of the pangean supercontinent

effect of water on melting phase relations and melt composition in the system mg2sio4mgsio3h2o up to 15 gpa melting experiments in the system mg2sio4 mgsio3 h2o up to pressures of 15 gpa show that the presence of h2o drastically changes melting temperature liquidus phases and the coexisting melt composition the system mg2sio4 h2o shows incongruent melting characterized by a high mgsi ratio of the eutectic composition exceeding 2 0 above 12 gpa whereas it exhibits congruent melting up to at least 7 7 gpa in the mgsio3 h2o system incongruent melting is observed up to 3 gpa which switches to congruent melting above 5 5 gpa and persists to at least 12 gpa the variation of the eutectic composition in the system mg2sio4 mgsio3 h2o is consistent with the variations of the melting temperatures in the systems mg2sio4 h2o and mgsio3 h2o the melting temperature solidus in the system mg2sio4 h2o decreases from 1800 c at atmospheric pressure to less than 1200 c at 10 gpa while in the system mgsio3 h2o it is reduced to a minimum value of 1250 c at approximately 5 5 gpa and then increases with pressure the composition of the initial melt in this system drastically changes at around 5 5 gpa the mgsi ratio being about 3 at 15 gpa variations of the ternary phase diagrams mgo sio2 h2o with pressure are compiled and compared with previous work on the relevant systems a modified spinel like hydrous phase is synthesized at 15 5 gpa the maximum amount of h2o accommodated in this phase is estimated to be 3 3 wt from observed chemical composition and crystal chemical considerations implying that a significant amount of h2o can be accommodated in the mantle transition zone the overall effect of h2o on ultrabasic magma genesis in the mantle is discussed a possibility is presented for the formation of komatiite magma involving the effect of h2o in the mantle of the early earth mg rich komatiite magma was possibly generated at a depth of only 200 km at temperatures much lower than that having been assumed previously

diastathermal extensional metamorphism at very low grades and possible high grade analogues abstract a genetic link between some areas of very low grade of metamorphism and an extensional setting is argued a model is presented in which the metamorphism develops in response to enhanced thermal gradients generated by extensional processes and the pressure temperature time paths are anticipated as being anticlockwise in form examples of granulite facies metamorphism attributed by sandiford and powell 1 to extensional settings may represent the lower crustal analogue of the very low grade examples examined here extensional processes may thus provide the mechanisms by which metamorphism in a variety of crustal levels can develop the traditional association of regional metamorphism with convergent processes is no longer tenable and the termdiastathermal metamorphism proposed by robinson 2 is appropriate to demonstrate the fundamental difference between these two types of regional metamorphism

40ar 39ar age of shear zones in the southwest meguma zone between yarmouth and meteghan nova scotia the cape st marys cranberry point and chebogue point shear zones lie within a zone of d2 deformation that overprinted d1 fabrics and reactivated f1 folds between the shear zones d2 was accompanied by greenschist facies metamorphism that variably overprinted earlier metamorphism of varying grade a collection of single muscovite grains from pressure shadows defining l2 in a sample from the cape st marys shear zone yields an average apparent age of 316 5 ma the age obtained for these grains from an isotope correlation diagram is 320 5 ma ages for muscovite separates for two samples from the cranberry point shear zone are 325 4 and 319 4 ma an amphibolite from this zone yields a discordant spectrum interpreted to reflect the combined effects of acadian metamorphism and excess argon the muscovite ages approximate the time of syntectonic deformation because muscovite closure temperatures approximate the temperature of metamorphism during d2 thus together with published data from the chebogue

evidence of multi stage faulting by clay mineral analysis example in a normal fault zone affecting arkosic sandstones annot sandstones abstract fault affecting silicoclastic sediments are commonly enriched in clay minerals clays are sensitive to fluid rock interactions and deformation mechanisms in this paper they are used as proxy for fault activity and behavior the present study focuses on clay mineral assemblages from the point vert normal fault zone located in the annot sandstones a priabonian rupelian turbidite succession of the alpine foredeep in se france in this area the annot sandstones were buried around 6 8 km below the front of alpine nappes soon after their deposition and exhumed during the middle late miocene the fault affects arkosic sandstone beds alternating with pelitic layers and displays throw of about thirty meters the fault core zone comprises intensely foliated sandstones bounding a corridor of gouge about 20 cm thick the foliated sandstones display clay concentration along s c structures characterized by dissolution of k feldspar and their replacement by mica associated with quartz pressure solution intense microfracturation and quartz vein precipitation the gouge is formed by a clayey matrix containing fragments of foliated sandstones and pelites however a detailed petrographical investigation suggests complex polyphase deformation processes optical and sem observations show that the clay minerals fraction of all studied rocks pelites and sandstones from the damage and core zones of the fault is dominated by white micas and chlorite these minerals have two different origins detrital and newly formed detrital micas are identified by their larger shape and their chemical composition with a lower fe mg content than the newly formed white micas in the foliated sandstones newly formed white micas are concentrated along s c structures or replace k feldspar both types of newly formed micas display the same chemical composition confirmed microstructural observations suggesting that they formed in the same conditions they have the following structural formulas na0 05 k0 86 al 1 77 fe0 08 mg0 15 si3 22 al0 78 o10 oh 2 they are enriched in fe and mg compared to the detrital micas newly formed chlorites are associated with micas along the shear planes according to microprobe analyses they present the following structural formula al1 48 fe2 50 mg1 84 si2 82 al1 18 o10 oh 8 all these data suggest that these clay minerals are synkinematic and registered the fault activity in the gouge samples illite and chlorite are the major clay minerals smectite is locally present in some samples in the foliated sandstones kubler index ki 001 xrd peak width at half height data and thermodynamic calculations from synkinematic chlorite chemistry suggest that the main fault deformation occurred under temperatures around 220 c diagenesis to anchizone boundary ki measured on pelites and sandstones from the hanging and footwall display similar values coherent with the maximal burial temperature of the annot sandstones in this area the gouge samples have a higher ki index which could be explained by a reactivation of the fault at lower temperatures during the exhumation of the annot sandstones formation

late paleozoic collision delamination short lived magmatism and rapid denudation in the meguma terrane nova scotia canada constraints from 40ar 39ar isotopic data the meguma terrane consists of 8 15 km of early paleozoic stratified rocks deposited on a continental basement that were deformed and metamorphosed during the late paleozoic as a result of lithospheric plate collision the oldest cleavage previously published whole rock 40ar 39ar plateau ages of 415 395 ma dates the onset of crustal thickening which was followed by voluminous but short lived late devonian granitic and minor mafic magmatism 380 370 ma this magmatism may have been the product of delamination of the lower lithosphere and upwelling of asthenosphere which effected melting above the new moho and resulted in intrusion at depths of 5 12 km 40ar 39ar plateau ages of hornblende muscovite and biotite indicate that at the present erosion level most of the meguma terrane cooled through 300 c by 368 360 ma slightly earlier than for the southwestern meguma terrane ca 345 ma the present erosion level was exhumed by the latest devonian early carboniferous visean the age of the

tectonic influence on sedimentation along the southern flank of the late paleozoic magdalen basin in the canadian appalachians geochemical and isotopic constraints on the horton group in the st marys basin nova scotia the late devonian early carboniferous st marys basin in mainland nova scotia canada consists of horton group intracontinental clastic rocks that were deposited along the avalon meguma terrane boundary during the waning stages of the acadian orogeny and prior to terminal collision in the appalachian orogen these rocks represent the earliest stages of deposition along the southern flank of the composite magdalen basin late devonian to permian that extends over much of maritime canada and oversteps terrane boundaries in the canadian appalachians as such the horton group represents deposition in an episutural basin the horton group in the st marys basin predominantly consists of 3000 4000 m of clastic sedimentary rocks deposited in fluviatile and lacustrine nearshore environments geochemical analyses of clastic rocks reveal a range of sio2 content 60 94 wt high total rare earth elements and initial 87sr 86sr and strongly negative nd t t 360 ma the signature of the passive margin to active continental margin implied by standard discrimination diagrams probably is inherited largely from rocks in the source region and is not an indication of the setting in which the basin formed although the data indicate that some lithologies contain a minor avalonian basement component the geochemical and isotopic signatures of the sedimentary rocks reveal mixed sources that can be attributed largely to uplift and erosion of meguma terrane metasedimentary and granitoid rocks immediately to the south of the st marys basin regional syntheses indicate that this uplift occurred before and during horton group deposition and was coeval with dextral ramping of the meguma terrane over the avalon terrane along the southern flank of the magdalen basin

tem and aem constraints on the origin and significance of chlorite mica stacks in slates an example from central wales u k chlorite mica stacks grains of intergrown chlorite and white mica in a matrix of fine grained white mica and chlorite have been studied using xrd sem empa tem and aem methods the stacks occur in a weakly cleaved llandoverian mudstone central wales which has a white mica illite crystallinity index of 0 35 a20 corresponding to the lower anchizone white mica occurs as packets 100 to 8 m thick interleaved with dominant chlorite packets in stacks with both apparent coherent interfaces or cross cutting small angle boundaries with chlorite layers it is well crystallized 2m polytype with phengitic composition and low paragonite component na na k 0 0 07 chlorite in stacks is fe rich and relatively homogeneous tio2 crystals surround stacks and occur within chlorite and white mica is ti rich compared to matrix white mica fine grained white mica and chlorite in the matrix have two different orientations one parallel to bedding and one parallel to cleavage which is approximately 30 50 to bedding matrix white mica is predominantly a 2m1 polytype but some cleavage parallel white mica is 3 t and some bedding parallel white mica is 1md it is na rich na na k 0 14 0 42 and relatively heterogeneous some discrete paragonite and phengitic muscovite are observed to coexist in the cleavage orientation matrix white mica and chlorite each contain less fe than corresponding white mica and chlorite in stacks both matrix and stack chlorite are one layer polytypes the data imply that chlorite in the stacks is largely derived from the replacement of volcanogenic biotite and other ferromagnesian minerals probably via intermediate expandable trioctahedral phyllosilicates most intergrowths of chlorite and mica in stacks formed by mica replacement of chlorite and altered biotite along cleavage fissures subsequent deformation caused further modification of pre existing chlorite mica stacks whereas partial dissolution of stacks and bedding parallel matrix phyllosilicates resulted in the formation of cleavage parallel phyllosilicates

the distribution of hydroxyl in garnets from the subcontinental mantle of southern africa 166 garnets of dominantly mantle origin were analyzed for oh content by infrared ir spectroscopy ir spectra in the 3400 3700 cm 1 region display consistent absorption patterns attributable to oh structurally bound within the garnet crystal occasionally contaminated by low intensity oh absorptions from microscopic inclusions the principal structural oh absorption occurs near 3570 cm 1 with the appearance of additional absorptions near 3512 cm 1 and 3650 cm 1 dependent on garnet composition or paragenesis samples derive from a wide variety of rock types occurring as xenoliths in kimberlites of southern africa oh abundances using the best currently available calibration range from less than 1 up to 135 ppm h 2o and increase in the general order as follows on craton eclogites group 1 basaltic kimberlite alnoite alkali basalt the oh contents of common lithospheric granets from coarse peridotites including several phlogopite bearing samples are typically less than 20 ppm h 2o for tectonic settings of kimberlites both on and off the archaean kaapvaal craton ti rich garnets from deformed peridotites are richer in oh supporting previous suggestions of association of these xenoliths with putative megacryst magmas subcalcic cr rich xenocrysts diamond inclusion garnets and garnets from diamondiferous eclogites have very low oh contents similar to eclogites and depleted peridotites without macroscopic diamonds the oh content of southern african peridotite and eclogite garnets are significantly lower on average than those previously examined from the colorado plateau diatremes while details of emplacement related h mobility in garnets remain to be established our results suggest that garnets record useful information on the role of water or other hydrous volatile species in petrological processes at their source regions in the mantle although garnets do not appear to constitute a large reservoir of mantle hydrogen the large stability range of oh bearing garnet in the crust and mantle implies wide applicability as a qualitative hydrobarometer

geochemical and isotopic characteristics of early silurian clastic sequences in antigonish highlands nova scotia canada constraints on the accretion of avalonia in the appalachian caledonlde orogen avalonia is a terrane that accreted to laurentia baltica during the development of the appalachian caledonide orogen interpretations of the timing of accretion have been constrained by comparing faunal affinities overstep sequences age and kinematics of inferred accretionary deformational events and controversial paleomagnetic data we show that the time of accretion of avalonia may also be constrained by contrasts in the geochemical and isotopic signatures of its igneous rocks which reflect the characteristics of the underlying continental basement and mantle and sedimentary rocks which reflect provenance early silurian clastic sedimentary rocks of the beechill cove formation antigonish highlands nova scotia were deposited on avalonian crust the formation predominantly consists of approximately 80 m of siltstones and shales deposited in a nearshore environment and derived from the northeast their age is constrained by paleontological data and by directly underlying late ordovician early s

the structure and the phyllosilicates chemistry crystallinity and texture of talas ala tau tien shan kyrgyz republic comparison with more recent subduction complexes abstract geological mapping and structural analysis of the talas ala tau tien shan kyrgyz republic have revealed a complex structure composed of folds with axial plane cleavage and thrust faults verging towards the ne the main structures of the range correspond to minor tertiary and carboniferous permian deformation superimposed on the main deformation event that took place during the baikalian orogeny the pervasive axial plane cleavage diminishes in penetrativity from the hinterland to the foreland in both the uzunakhmat and karagoin sheets the main thrusts developed phyllonitic shear related rocks on the hangingwall immediately above the thrust planes a crystal chemical study of the phyllosilicates growth during the baikalian deformation event along a cross section revealed changes in the crystallinity composition and lattice parameters of them the phyllosilicates present in the talas ala tau rocks were crystallized in very low grade metamorphic conditions that is below 300 c as indicated by their kubler index ki which decreases from sw towards the ne detailed tem study of the phyllosilicates reveals a clear textural difference at the lattice level between samples with higher or lower ki parameters there is also a clear difference in crystal chemical parameters ki and b and composition between the phyllosilicates growth in relation to the axial plane cleavage and the ones belonging to the thrust related phyllonites the first ones are more affected by the ferrimuscovitic vector than the phyllosilicates of phyllonites closer to the theoretical phengitic component huge ranges of values of phengitic content of micas at sample level are interpreted as the result of a decompression path from at least 8 kbar we propose a subduction geodynamic environment for the regional deformation and the origin of the phyllosilicates as they are similar to those obtained in more recent accretionary complexes

40ar 39ar dating in the lochaber mulgrave area northern mainland nova scotia implications for timing of regional metamorphism and sediment provenance in the late devonian early carboniferous horton group 40ar 39ar dating of whole rock samples and muscovite separates using age spectrum analysis and of single muscovite grains using total fusion analysis yields new insights into the timing of regional metamorphism and sediment provenance in the late devonian early carboniferous horton group in the lochaber mulgrave area of nova scotia the time of regional metamorphism is constrained to ca 340 335 ma by whole rock spectra from well cleaved slate and shale samples from the lowermost clam harbour river and overlying tracadie road formations of the horton group this ca 340 335 ma event may have been the result of burial and deformation of the horton group by older volcanic and sedimentary rocks of the guysborough group which were overthrust from the south as the result of development of a positive flower structure at a restraining bend along the cobequid chedabucto fault system the boundary between the meguma and avalon terranes detrital muscovite ages of ca 410 380 and ca 500 ma were obtained from