detrital and metamorphic 40 ar 39 ar ages from muscovite and whole rock samples meguma supergroup southern nova scotia 40ar 39ar dates from eight muscovite separates and two whole rock samples of slate siltstone and sandstone provide detrital metamorphic and post metamorphic overprinting ages for the low grade metamorphic rocks of the meguma supergroup in the mahone bay region of southern nova scotia variably reset detrital muscovite ages of 600 550 ma agree with previously obtained u pb dates from detrital zircon separates of foliation parallel muscovite yield precise 40ar 39ar ages of 395 388 ma petrographic and metamorphic data indicate that these are metamorphic mica growth ages the range overlaps with the younger limit of previously reported whole rock ages 410 380 ma but is more tightly constrained an apparent inconsistency between previous 40ar 39ar whole rock ages as old as 410 ma and regional deformation that affected sedimentary rocks as young as 385 ma has been resolved by the younger ages reported here and by recent adjustments to the early paleozoic time scale which suggest that the lochkovian to

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

a fluid inclusion and isotopic study of an intrusion related gold deposit irgd setting in the 380 ma south mountain batholith nova scotia canada evidence for multiple fluid reservoirs a set of sheeted quartz veins cutting 380 ma monzogranite at sandwich point nova scotia canada provide an opportunity to address issues regarding fluid reservoirs and genesis of intrusion related gold deposits the quartz veins locally with arsenopyrite 5 and elevated au bi sb cu zn occur within the reduced south mountain batholith which also has other zones of anomalous gold enrichment the host granite intruded p 3 5 kbars lower paleozoic metaturbiditic rocks of the meguma supergroup well known for orogenic vein gold mineralization relevant field observations include the following 1 the granite contains pegmatite segregations and is cut by aplitic dykes and zones 1 2 m of spaced fracture cleavage 2 sheeted veins containing coarse comb textured quartz extend into a pegmatite zone 3 arsenopyrite bearing greisens dominated by f rich muscovite occur adjacent the quartz veins and 4 vein and greisen formation is consistent with riedel shear geometry although these features suggest a magmatic origin for the vein forming fluids geochemical studies indicate a more complex origin vein quartz contains two types of aqueous fluid inclusion assemblages fia type 1 is a low salinity 3 wt equivalent nacl with minor co2 2 mol and has th 280 340 c in contrast type 2 is a high salinity 20 25 wt equivalent nacl ca rich fluid with th 160 200 c pressure corrected fluid inclusion data reflect expulsion of a magmatic fluid near the granite solidus 650 c that cooled and mixed with a lower temperature 400 c wall rock equilibrated ca rich fluid evidence for fluid unmixing an important process in some intrusion related gold deposit settings is lacking stable isotopic o d s analyses for quartz muscovite and arsenopyrite samples from vein and greisens indicate the following 1 18oqtz 11 7 to 17 8 and 18omusc 10 7 to 11 2 2 dmusc 44 to 54 and 3 34saspy 7 8 to 10 3 these data are interpreted in conjunction with fluid inclusion data to reflect contamination of a magmatic derived fluid delta 18 hbox o rm h 2 rm o 10 by an external fluid delta 18 hbox o rm h 2 rm o 15 the latter having equilibrated with the surrounding metasedimentary rocks the 34s data are inconsistent with a direct igneous source based on other studies for the host intrusion delta 18 hbox o rm h 2 rm o 5 and are instead consistent with an external reservoir for sulphur based on 34sh2s data for the surrounding metasedimentary rocks divergent fluid reservoirs are also supported by analyses of pb isotopes for pegmatitic k feldspar and vein arsenopyrite collectively the data indicate that the vein and greisen forming fluids had a complex origin and reflect both magmatic and non magmatic reservoirs thus although the geological setting suggests a magmatic origin the geochemical data indicate involvement of multiple reservoirs these results suggest multiple reservoirs for this intrusion related gold deposit setting and caution against interpreting the genesis of intrusion related gold deposit mineralization in somewhat analogous settings based on a limited geochemical data set

geochemical evolution of peraluminous plutons in southern nova scotia canada a pegmatite poor suite three southern nova scotia plutons crystallized rapidly at 375 ma from magma containing both mantle and crustal components isotopic and chemical data suggest that the crustal contribution included both lower crustal material and cambro ordovician turbidites of the host meguma group despite local evidence of mixing and mingling of magmas the bulk of the plutons evolved by assimilation and fractional crystallisation evolved portions of the plutons have compositions appropriate for development of rare metal pegmatite fields but pegmatites are relatively rare and little differentiated like parental plutons pegmatites fall into biotite plagioclase and muscovite potassium feldspar assemblages the latter locally contain mn rich garnet biotite giving calculated p t conditions of pegmatite crystallisation of 620 c 0 44 gpa under water saturated conditions host rocks at the time of emplacement experienced p t conditions varying from 500 to 620 c at 0 44 gpa beryl bearing pegmatites occur only where host rocks were below sillimanite grade and pegmatites of any kind are scarce in sillimanite grade host rocks rarity of pegmatites can be ascribed to a combination of insufficient amounts of f li and b in the magma with rapid cooling which prevented extensive fractionation and undercooling our data suggest rather that peraluminous suites produced by rapid heating due to incursions of lithospheric mantle into supracrustal rocks and typified by biotite rich cordierite bearing plutons are not favorable locales for major rare metal enriched pegmatite fields such fields appear to require relatively prolonged anatexis resulting from crustal thickening or an areally extensive mafic underplate

petrology chronology and sequence of vein systems systematic magmatic and hydrothermal history of a major intracontinental shear zone canadian appalachians abstract intra continental shear zones developed during continental collision may experience prolonged magmatism and mineralization the cobequid shear zone formed part of a ne sw trending orogen parallel shear system in the late devonian early carboniferous where syn tectonic granite gabbro plutons and volcanic rocks 4 km thick were progressively deformed in late carboniferous to permian alleghanian collision of africa with laurentia formed the e w trending minas fault zone reactivating parts of the cobequid shear zone the 50 ma history of hydrothermal mineralization following pluton emplacement is difficult to resolve from field relationships of veins but sem study of thin sections provides clear detail on the sequence of mineralization the general paragenesis is albite quartz chlorite monazite biotite calcite allanite pyrite fe carbonates fe oxides minor sulfides calcite and synchysite chronology was determined from literature reports and new u pb la icpms dating of monazite and allanite in veins vein mineralization was closely linked to magmatic events vein emplacement occurred preferentially during fault movement recognised from basin margin inversion as a result of fractures opening in the damage zone of master faults the sequence of mineralization from ca 355 ma riebeckite and albite veins to ca 327 305 ma siderite magnetite and sulfide mineralization resembles precambrian iron oxide copper gold iocg systems in the literature the abundant magmatic na halogens and co2 in veins and some magmatic bodies characteristic of iocg systems were derived from the deeply subducted rheic ocean slab with little terrigenous sediment regional extension of the magdalen basin caused asthenospheric upwelling and melting of the previously metasomatized sub continental lithospheric mantle crustal scale strike slip faulting facilitated the rise of magmas resulting in high heat flow driving an active hydrothermal system

slow spreading ridge axis tectonics evidence from the lizard complex uk abstract the east coast section of the lizard ophiolite complex cornwall u k preserves a series of interrelated extensional faults and dyke sets which indicate that the formation of this section of obducted oceanic lithosphere resulted from a combination of contemporaneous structural and magmatic events three dolerite dyke sets are evident the first two sets 1 and 2 are cut by and rotated on a series of ridge parallel extensional faults and shear zones later dykes set 3 cross cut the extensional faults these data indicate that both rotation and significant thinning in the region of 40 of this part of the oceanic lithosphere occurred prior to the final period of dyke injection the thinning of the lithosphere was the result of extension along the normal faults all of the dolerite dykes show morb characteristics although the late dykes set 3 are more primitive tending to be aphyric to sparse olivine phyric and show higher cr ni and lower zr values than the typically plagioclase phyric dolerite dykes sets 1 and 2 intruded prior to the ductile shearing event the more primitive dolerites post date the tectonism and indicate the lack of a significant magma chamber following a period of amagmatic extension the data are consistent with observations at present day slow spreading ridge axes and some mafic ultramafic ophiolite sequences and provide insight into the nature of ridge related structures and the interrelationships between tectonism and geochemical evolution of the dykes and record a mechanism listric faulting to account for the presence of peridotites and gabbros commonly dredged at slow spreading centres

multi channel seismic imaging of a crustal magma chamber along the east pacific rise a reflection observed on multi channel seismic profiles along and across the east pacific rise between 8 50 n and 13 30 n is interpreted to arise from the top of a crustal magma chamber located 1 2 2 4 km below the sea floor the magma chamber is quite narrow 4 6 km wide but can be traced as a nearly continuous feature for tens of kilometres along the rise axis

petrology age and tectonic setting of the white rock formation meguma terrane nova scotia evidence for silurian continental rifting the white rock formation in the yarmouth area of the meguma terrane of southern nova scotia consists mainly of mafic tuffaceous rocks with less abundant mafic flows epiclastic and clastic sedimentary rocks and minor intermediate and felsic crystal tuff it is divided into seven map units that appear to young from west to east inconsistent with a previously assumed synclinal structure the white rock formation is flanked on both northwest and southeast by mainly the cambrian to lower ordovician halifax formation the western contact is interpreted to be a sheared disconformity whereas the eastern contact appears to be a major brittle fault and shear zone that juxtaposes different crustal levels the granitic brenton pluton forms a faulted lens within the eastern shear zone a felsic tuff from the upper part of the white rock formation yielded a u pb zircon age of 438 3 2 ma identical within error to published ages for the brenton pluton and felsic volcanic rocks near the base of the white rock formati

ireteba pluton eldorado mountains nevada late deep source peraluminous magmatism in the cordilleran interior abstract the ireteba pluton is a 66 ma 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 mathrm biotite pm mathrm muscovite pm mathrm garnet end document granite emplaced at the eastern edge of the cordilleran plutonic belt in southeastern nevada in common with other cordilleran peraluminous granites its mineralogy major element chemistry isotopic composition and abundant proterozoic zircon inheritance document crustal origin distinctive trace element chemistry field relations and inherited components furth

granodiorites of the south mountain batholith nova scotia canada derived by partial melting of avalonia granulite rocks beneath the meguma terrane implications for the heat source of the late devonian granites of the northern appalachians abstract the late devonian south mountain batholith smb of nova scotia is the largest batholith of the northern appalachians the peraluminous granitic rocks range from biotite granodiorite to leucogranite samples collected from a drill core of the scrag lake granodioritic pluton of the western smb are chemically homogeneous from the surface to a depth of 1425 m the homogeneous composition implies that the granodiorite was derived from a relatively homogeneous source and that country rock assimilation was not an important source for the parental magma equilibrium partial melt modeling of underlying sub meguma granulite rocks indicates that they are the primary source rocks of the granodiorites we suggest that mantle derived magmas intruded the lower crust and induce large scale melting of the granulite basement rocks to produce the granodiorites fractional crystallization of the granodiorites plus assimilation of meguma supergroup metasediments likely produces the silica rich rocks of the smb the cause of mantle melting is uncertain however it may be related to the transitioning of the northern appalachians from a position above the deep mantle pacific large low shear velocity province llsvp to a higher shear velocity region of the mantle

the contact aureole of the south mountain batholith in halifax nova scotia geology mineral assemblages and isograds the contact aureole at the eastern margin of the south mountain batholith halifax pluton underlies most of the city of halifax nova scotia the halifax group in the study area includes two lithological units the cunard and bluestone formations before intrusion both had been affected by greenschist facies regional metamorphism and deformed into northeast southwest trending regional upright folds associated with a strong slaty cleavage contact metamorphic isograds trend obliquely across the halifax peninsula at a high angle to regional structural trends at 2 5 3 km from the intrusive contact sparse cordierite spots mark the outer limit of the contact aureole the biotite in isograd is marked by the development of biotite within chlorite muscovite stacks inherited from regional metamorphism pyrrhotite is the dominant sulphide mineral throughout the contact aureole with increasing metamorphic grade assemblages in both units are marked by increasing modal abundance of cordierite and biotite with k feldspar variably developed within ca 600 m of the contact however there is a marked difference in the distribution and appearance of andalusite between the two units in aluminous pelites of the cunard formation idioblastic chiastolite appears before biotite more than 1500 m from the contact in the less aluminous bluestone formation andalusite is present only within ca 500 m of the contact where it forms xenoblastic spongy crystals in both units the assemblage andalusite biotite k feldspar cordierite is developed near the contact with local fibrolite and evidence of incipient partial melting petrographic constraints suggest pressure temperature conditions at the contact of ca 2 5 3 0 kbar 1 kbar 100 mpa at ca 650 c

chemical and isotopic pb sr zonation in a peraluminous granite pluton role of fluid fractionation the davis lake pluton dlp 800 km2 of southwestern nova scotia canada part of the large peraluminous south mountain batholith of ca 380 ma u pb zircon ar ar mica consists of granite and subordinate topaz muscovite leucogranite that hosts greisen tin base metal mineralization a new pb pb isochron age for leucogranite from the most evolved part of the dlp indicates a crystallization age of 378 3 6 ma coincident with other radiometric ages of the dlp rb sr re os pb pb the intrusion displays a compositional zonation defined by lead and strontium isotopic ratios as well as some major elements e g si f incompatible trace elements e g li rb ta u sn and elemental ratios e g k rb and nb ta the greisens and the leucogranites that host them are characterized by extreme radiogenic compositions for pb and sr and their chemical isotopic trends are extensions of the trends displayed by the less evolved granites the covariations of the isotopic ratios with several major and trace elements and elemental ratios as well as the pb pb and rb sr isochrones indicate that all phases of the intrusion originated from a homogeneous parental magma the granitoid magma underwent extensive fractional crystallization of feldspars minor biotite and accessory minerals monazite apatite and zircon in a compositionally zoned magma chamber that was subsequently accompanied by fluid fractionation during which time the internally derived fluorine rich fluids modified the rb sr u pb and th pb ratios leading to distinct variations of 87sr 86sr 206pb 204pb 238u 204pb and 232th 204pb isotopic ratios these data therefore document the evolution of a granitic magma through magmatic i e crystal fractionation orthomagmatic i e crystal fluid fractionation and hydrothermal i e fluid fractionation stages that culminated in the formation of a tin base metal deposit the pb isotope data also constrain the source region for the dlp as being avalonian basement that by inference must underlie much of the meguma terrane

geochemical studies of abyssal lavas recovered by dsrv alvin from eastern galapagos rift inca transform and ecuador rift 2 phase chemistry and crystallization history a diverse suite of lavas recovered by dsrv alvin from the eastern galapagos rift and inca transform includes mid ocean ridge tholeiitic basalts morb iron and titanium enriched basalts feti basalts and abyssal andesites rock types transitional in character ferrobasalts and basaltic andesites were also recovered the most mafic glassy basalts contain plagioclase augite and olivine as near liquidus phases whereas in more fractionated basalts pigeonite replaces olivine and iron titanium oxides crystallize plagioclase crystallizes after pyroxenes and iron titanium oxides in andesites possibly due to increased water contents or cooling rates apatite phenocrysts are present in some andesitic glasses ovoid sulfide globules are also common in many lavas compositional variations of phenocrysts in glassy lavas reflect changes in magma chemistry temperature of crystallization and cooling rate the overall chemical variations parallel the chemical evolution of the lava suite and are similar to those in other fractionated tholeiitic complexes elemental partitioning between plagioclase pyroxene and olivine glass pairs suggests that equilibration occurred at low pressure in a rather restricted temperature range various geothermometers indicate that the most primitive morb began to crystallize between 1150 and 1200 c with fo2 10 7 atm coexisting iron titanium oxides in more evolved lavas yield temperatures 1025 c to as low as 910 c withfo2 from 10 8 to 10 12 atm ph 2 o could have been as high as 1 kbar during andesite crystallization compositions of the lavas from the galapagos rift follow the experimentally determined 1 atm qfm liquid line of descent least squares calculations for the major elements indicate that the entire suite of lavas can be produced by fractional crystallization of successive residual liquids from a morb parent magma feti basalts represent 30 65 cumulative weight percent crystallization of plagioclase augite and olivine an additional 30 50 fractionation of pyroxenes plagioclase titanomagnetite and possible apatite is required to generate andesite from feti basalt liquids the presence of partially resorbed mafic xenocrysts in some andesites feti basalt inclusions in these xenocrysts high silica glass inclusions in basaltic andesites and the transitional chemistry of basaltic andesites are evidence that some magma mixing occurred during crystal fractionation the diversity of lava types recovered at single dive sites suggests that low pressure fractional crystallization is a very efficient process beneath the eastern galapagos rift and that isolated magma bodies must be present at shallow levels beneath the accretionary locus voluminous feti basalts erupted at the rift transform intersection are genetically related to the rift lavas but their restricted chemistry reflects different thermal and tectonic controls on their petrogenesis

evolution of late palaeozoic shearing in the cobequid highlands constraints on the fragmentation of the appalachian orogen in nova scotia along intra continental shear zones

interaction among upper crustal lower crustal and mantle materials in the port mouton pluton meguma lithotectonic zone southwest nova scotia the port mouton pluton is unique among the late devonian peraluminous granitoid bodies in the meguma lithotectonic zone of southwestern nova scotia in its lithological heterogeneity extensive physical and chemical interac tion with the country rocks clear evidence for mingling and mixing with mafic magmas and highly abundant pegmatites new u pb age determinations on monazite establish an intrusion age of 373 1 ma similar to the ages of other meguma lithotectonic zone granitoid plutons and mafic intrusions field relations petrology and geochemistry define three stages of intrusion of the port mouton pluton i early stage discontinuously exposed around the outer margin of the pluton dominated by coarse grained tonalite granodiorite and with rb sr 0 40 and gdn lun 0 55 eu eu 2 and iii late stage consisting of abundant minor sheets throughout the pluton dominated by fine grained tonalite granodiorite and leucogranite that are similar to rocks of the early and middle stages the port mouton pluton shows a wider range of 87 sr 86 sri 0 7036 0 7154 and a wider range and generally higher endi 3 72 to 2 12 than other granitoid rocks in the meguma lithotectonic zone potentially reflecting a com plex partially equilibrated interaction among mantle lower crust and upper crust field petrological and chemical evi dence for the involvement of mantle derived magmas and melting of upper crust permit modelling of the port mouton pluton granitoid compositions by three simultaneous mixing equations these mixing model results suggest that the early stage granitoid rocks can form from simple three component mixing relationships when the bulk distribution coefficients between residuum and melt for sr and nd range from 1 05 to 1 18 or two component mixing combined with fraction ation of material like the known felsic lower crust the middle stage granitoid rocks only yield solutions involving two component mixing and fractionation of material unlike the known felsic lower crust we conclude that the late devonian mafic magmas played a major role in the formation of granitoid magmas in the meguma lithotectonic zone by supplying heat and material to cause partial fusion of the avalon lower crust

ophiolitic mylonites in the lizard complex ductile extension in the lower oceanic crust ophiolitic rocks in the lizard complex southwestern england preserve fine examples of mylonitic schists within low angle amphibolite facies shear zones two prominent shear zones with type localities at carrick luz and porthoustock both have a footwall in the mafic ultramafic interface at the base of the oceanic crust the carrick luz shear zone shows mylonitized gabbro above peridotite mafic dikes both predate and postdate the shearing the porthoustock shear zone brings down higher oceanic crustal levels gabbro sheeted dike complex against ultramafic rocks and has an extensional geometry shear sense indicators show hanging wall movement directions different from those predicted by previous northward obduction models generation of these ophiolitic mylonites is interpreted instead as due to ductile extension at an oceanic spreading center

sm nd isotopic systematics as tectonic tracers an example from west avalonia in the canadian appalachians abstract the tectonothermal evolution of west avalonia in mainland nova scotia appalachian orogen provides an excellent example of the application of sm nd isotopic analyses to tectonic studies regional syntheses indicate that west avalonia originated in the neoproterozoic as one of a number of terranes collectively known as peri gondwanan terranes along the gondwanan margin these terranes are characterized by late neoproterozoic voluminous arc related magmatism west avalonia became detached from gondwana in the early ordovician was accreted to laurentia by the silurian and was deformed by post accretionary strike slip tectonics in the late devonian and carboniferous associated with the amalgamation of pangea all stages of this evolution can be recognized in the sm nd isotopic signature in various neoproterozoic and paleozoic volcanic and sedimentary successions in mainland nova scotia crustally derived felsic volcanic rocks of late neoproterozoic ca 615 ma early cambrian and early silurian ages define an envelope that provides gross constraints on the sm nd isotopic composition of the underlying basement source e nd values range from 0 78 to 2 92 in the neoproterozoic rocks from 2 0 to 5 0 in the cambrian rocks and from 0 1 to 2 5 in the silurian rocks t dm model ages for each of these suites are remarkably similar they typically range from 0 9 to 1 2 ga suggesting that the sm nd isotopic composition of these suites was profoundly influenced by repeated melting of the same basement source this sm nd envelope provides a reference by which other suites may be compared and interpreted for example turbidites in a late neoproterozoic volcanic arc basin have sm nd compositions that lie within this envelope suggesting that they are derived from the coeval volcanic rocks that flank the basin the genetic significance of the model ages can be investigated by the analysis of felsic and metasedimentary rocks that pre date the main arc phase two felsic complexes have compositions that lie within the envelope and have similar model ages the presence of this signature in early avalonian rocks suggests that the model ages represent a genuine tectonothermal event at ca 1 0 to 1 2 ga that formed juvenile proto avalonian crust by implication the main arc phase in avalonia recycled this crust sm nd isotopic analyses of the metasedimentary sequences are incompatible with previous interpretations of deposition in a passive margin setting that pre dated avalonian arc activity instead the data indicate derivation from two distinct sources ancient gondwanan basement and juvenile avalonian arc suggesting deposition in a western pacific type back arc environment this re interpretation has important implications for neoproterozoic reconstructions the sm nd isotopic composition of west avalonian basement contrasts with that of other peri gondwanan terranes for example similarly aged felsic rocks in the cadomian terrane of nw france are characterized by more negative e nd values and older t dm model ages 1 0 1 9 ga these data together with detrital u pb data suggest that west avalonia was developed along the amazonian margin whereas cadomia was developed along the west african margin of gondwana sm nd isotopic analyses of early silurian clastic rocks in mainland nova scotia show fundamental differences with unconformably underlying neoproterozoic and early paleozoic avalonian successions these data have e nd values ranging from 5 7 to 6 1 with t dm model ages of about 1 4 to 1 7 ga indicating that the clastic rocks cannot have been derived from underlying successions a variety of geologic data and regional constraints suggests that they may have been derived from the caledonide orogen of western europe thereby providing a minimum age for the accretion of west avalonia to laurentia baltica sm nd isotopic analyses from late devonian early carboniferous continental clastic rocks deposited along the southern flank of avalonia in mainland nova scotia also show little influence from avalonian rocks and are compatible with derivation from the meguma terrane immediately to the south these data are consistent with regional synthesis which indicate uplift of the meguma terrane relative to west avalonia during dextral transpression

microstructures of the cayman trough cabbros many different deformation textures are observed in submersible collected gabbros from the mid cayman rise spreading center indicating considerable variation in conditions of deformation in this slowly spreading ridge features suggesting flow range from mechanical twins and bent crystals low strain to complete recrystallization with a well developed foliation high strain textures suggesting fracture include zones 0 1 10 mm wide of extremely reduced grain size cracked grains with undulose extinction and crosscutting fractures these textures occur separately next to and overprinting each other of the variables controlling the deformation features temperature and water pressure are most easily estimated minimum temperature during deformation is suggested by minimum temperature during deformation of mineral assemblages unaffected by this deformation this temperature is above 500 550 c for the majority of the features observed observations suggest enhanced recrystallization where primary minera

an evaluation of crustal assimilation within the late devonian south mountain batholith sw nova scotia the late devonian south mountain batholith smb of southwestern nova scotia is the largest batholith in the appalachian orogen of eastern north america and contains economic deposits of u and sn the smb comprises at least 11 individual plutons which range in composition from granodiorite to biotite monzogranite leucomonzogranite and leucogranite previous studies have suggested that a combination of fractional crystallization assimilation of meguma supergroup country rocks and an influx of magmatic fluids contributed to the chemical evolution of the smb the amount of crustal assimilation is estimated to be as high as 33 melts modelling assuming a starting composition of granodiorite with h 2 o 4 wt pressure 4 kbar 12 km and f o 2 fmq can reproduce the chemical evolution observed in the smb however some leucogranites likely require an additional component e g hydrothermal fluids to explain their alkali metal enrichment e g na k rb zircon saturation thermometry estimates indicate the salmontail lake and scrag lake granodiorite plutons had high minimum initial temperatures of 823 6 c and 832 2 c respectively which are similar to low zircon inheritance granitoids the tio 2 al 2 o 3 and alkali lime ratios of the surrounding country rocks and the leucogranites indicate the amount of crustal assimilation is likely to be between 10 and 20 our findings suggest the granodiorites of the smb were likely produced by partial melting of the sub meguma supergroup e g avalon terrane lower crust caused by the contemporaneous injection of high temperature mafic to ultramafic magmas

a comparison of the age and composition of the shelburne dyke nova scotia and the messejana dyke spain new 40ar 39ar age determinations on biotite from the shelburne dyke of nova scotia and from the messejana dyke of spain are identical at 203 2 ma in addition both dykes have similar chemical compositions and probably represent similar magma sources petrogenetic processes and tectonic settings related to an early phase of opening of the north atlantic ocean

a model for the tectonic evolution of south cornwall a stratigraphy for south cornwall is combined with an analysis of the chronology of thrust nappe emplacement the resulting model suggests that northward overthrusting of nappes of flysch and olistostrome ophiolite and continental basement is related to the closure of an oceanic basin in south cornwall involving southward subduction throughout the devonian the subsequent continental collision between the normannian high and part of laurentia occurred in the upper devonian with deformatian continuing until late carboniferous times

meguma terrane orocline u pb age and paleomagnetism of the silurian mavillette gabbro nova scotia canada

tectonics granitoids and mesozoic gold deposits in east shandong china abstract mesozoic granitoids in east shandong 1 contain some 30 moz gold making east shandong the largest granitoid hosted gold province in the world the gold province is located along the northern boundary of the suture zone between the north china and south china cratons according to their mineralogical geochemical and geochronological characteristics two groups of mesozoic granitoids are recognised in east shandong namely the middle yanshanian my ca 160 150 130 ma and the late yanshanian ly ca 117 85 ma granitoids the first group of granitoids only occur to the north of the suture zone whereas the second group of granitoids occur on both sides of the suture zone these characteristics suggest that the two groups of granitoids were associated with different tectonic events a tectonic analysis suggests that the my granitoids are the result of collision and suturing of the north and south china cratons during the triassic to jurassic whereas formation of the ly granitoids is attributed to the distal inland influence of subduction of the pacific izanagi plate underneath the eurasian continent during the cretaceous the major gold deposits are spatially associated with the my granitoids in east shandong most of the gold occurs either as quartz veins in the granitoids or disseminated in altered shear zones along the boundary of the granitoids and archaean metamorphic rocks the major gold mineralization and alteration generally display the characteristics of gold only provinces namely co 2 rich low salinity and reduced fluids and proximal mica and sulfide alteration these characteristics are different from those of phanerozoic granitoid related skarn porphyry systems accordingly the gold mineralization may be more tectonically and structurally controlled rather than have a direct genetic relationship with the granitoid itself according to the relationships between tectonics granitoids distribution of gold deposits and fluid geochemistry the major gold in east shandong may have been formed during the subsequent exhumation stage after the collision of the north and south china cratons

a petrological and geochronological study of a 360 ma metallogenic event in maritime canada with implications for lithophile metal mineralization in the canadian appalachians the occurrence of ca 360 ma greisen related molybdenite mineralization in the clayton hill biotite muscovite leucomonzongranite 1 km2 is unusual for the southwestern meguma terrane of the maritime appalachians where 380 ma granite related sn mineralization dominates the presence of extensive hornfelsing and occurrence of mineralized sn w cu zn skarns in the area suggests a larger intrusion occurs at depth the intrusion and mineralization are constrained to 361 ma based on concordant re os molybdenite and 40ar 39ar muscovite ages which equates to other ca 360 ma granites in the area hence magmatism of this age is more widespread than previously considered the intrusion is highly evolved 74 wt sio2 metaluminous a cnk 1 1 05 a cnk molar ratio al2o3 cao na2o k2o and depleted in cafemic and transition elements rare earth elements ree 106 23 ppm are strongly fractionated with light rare earth elements lree enriched la lun 10 and negative eu anomalies biotite is

derivation of the early carboniferous wedgeport pluton by crystal fractionation of a mafic parental magma a rare case of an a type granite within the meguma terrane nova scotia canada the cambrian ordovician metasedimentary rocks of the meguma terrane canadian appalachians were extensively intruded by silicic plutons during middle devonian to early carboniferous times syn plutonic but volumetrically minor mafic ultramafic intrusions were also emplaced in most localities the silicic plutons and mafic ultramafic intrusions do not appear to be petrogenetically related and are likely derived from different sources the attwoods brook gabbronorite of sw nova scotia yielded an in situ zircon weighted mean 206pb 238u age of 357 9 3 3 ma that is within the uncertainty of the age of the neighbouring wedgeport pluton 357 1 ma the wedgeport pluton is a rare example of a mantle derived peraluminous a type granite within the meguma terrane the similar ages and nd isotopes of the attwoods brook gabbronorite end t 1 1 to 4 0 and wedgeport pluton end t 2 1 to 3 3 suggest the two intrusions are petrogenetically related fractional crystallization modelling demonstrates that a parental magma similar to the attwoods brook gabbronorite can produce residual silicic liquids that resemble the granites of the wedgeport pluton indicating that they could be members of the same intrusive complex the emplacement of the gabbronorite and wedgeport pluton occurred during a period of tensional plate stress that was contemporaneous with rifting of the maritimes basin that produced the fountain lake continental flood basalts and a type granites of the cobequid highlands within the avalon terrane it is possible that the early carboniferous rocks of sw nova scotia are related to the rifted related magmatism within the maritimes basin

systematics of modal phenocryst assemblages in submarine basalts petrologic implications phenocryst assemblages in ocean ridge basalts generally show an increasing proportion of plagioclase as the total amount of phenocrysts increases the variations in phase assemblages as well as most crystal liquid kd s are similar to variations equimodal trends predicted by low pressure laboratory experiments suggesting that many of these basalts have experienced varying degrees of low pressure cyrstallization prior to quenching with little sorting of crystals and liquid important exceptions include moderately to highly phyric basalts enriched either in plagioclase or olivine which lie well off the experimental trends in these basalts megacrysts and xenocrysts usually cited as evidence for magma mixing commonly represent a small proportion of the total crystalline phase assemblage however phase proportions for many of these basalts lie well outside the range that could be produced by simple mixing selective gravitative sorting either prior or subsequent to mixing appears to be the likely explanation for these phyric basalts a relation between spreading rate and phase proportions is neither supported nor refuted by the data which as yet do not adequately represent fast spreading ridges

the provenance of jurassic and lower cretaceous clastic sediments offshore southwestern nova scotia jurassic and cretaceous sandstones in the shelburne subbasin and fundy basin offshore nova scotia are poorly known but are of current interest for petroleum exploration the goal of this study is to determine the provenance of sandstones and shales which will contribute to a better understanding of regional tectonics and paleogeography in the study area mineral and lithic clast chemistry was determined from samples from conventional cores and cuttings from exploration wells using scanning electron microscopy and an electron microprobe whole rock geochemical composition of shales was used to test the hypotheses regarding provenance of mesozoic clastic sedimentary rocks in the sw scotian basin lower jurassic clastic sedimentary rocks in the fundy basin contain magnetite biotite and chlorite suggesting local supply from the north mountain basalt and meguma terrane whereas pyrope and anthophyllite suggest small supply from distant sources in the sw scotian basin detrital minerals lithic clasts and shale geochemistry from middle jurassic to early cretaceous indicate a predominant meguma terrane source and transport by local rivers rare spinel and garnet grains of meta ultramafic rocks only in the middle jurassic at the mohawk b 93 well suggest minor supply from the rising labrador rift via the same river that transported distant sediments to the fundy basin lower cretaceous sandstones from the mohican i 100 well contain minor garnet spinel and tourmaline from meta ultramafic rocks characteristic of sediment supplied to the central scotian basin at that time the dominant meguma terrane provenance precludes thick deep water sandstones in the eastern part of the shelburne subbasin but the evidence of middle jurassic distant river supply through the fundy basin is encouraging for deep water reservoir quality in the western part

multistage hydrothermal alteration of gabbroic rocks from the failed mathematician ridge oxygen isotopic mineralogical and fluid inclusion data are presented for fresh and hydrothermally altered gabbroic rocks from the failed mathematician ridge the mathematician ridge though initially a fast spreading ridge presently has a deep central rift valley more typical of slow spreading rifts the inner rift walls expose rare plutonic rocks uncomplicated by proximity to deep large offset fracture zones undeformed mathematician ridge gabbros preserve most of the original texture and phase compositions and have been only slightly mineralogically or isotopically modified by post crystallization hydrothermal alteration metagabbros have been subjected to a pervasive ductile deformation and recrystallization to foliated rocks composed of aluminous amphibole low calcium plagioclase and ilmenite oxygen isotope and phase chemical data suggest this event began at temperatures of up to 700 c water rock ratios of less than one and produced modified seawater fluids with isotopic compositions of 3 5 6 0 the amphibolites are cross cut by multiple generation quartz epidote veins forming a network replacement of the amphibole grade mineralogies by greenschist grade mineralogies fluid oxygen isotopic compositions of 2 4 and a two phase separation in the fluid captured in conjugate vapor fluid inclusions are all associated with the formation of this network fluid inclusion data for the quartz indicate initial temperatures of over 500 c these samples may represent rocks from the 1 2 km deep plumbing system of black smoker type venting

metamorphism in oceanic layer 3 gorringe bank eastern atlantic gorringe bank is an anomalously high structure of the eastern part of the north atlantic which was known to be composed of mantle derived peridotites layer 4 and gabbros layer 3 during the submersible cruise cyagor ii in 1981 the contact between layer 4 and layer 3 was observed on mount gettysburg and interpreted as tectonic the overlying series of gabbro was extensively sampled on both mounts composing the bank gettysburg and ormonde coarse grained to pegmatoid clinopyroxene gabbros predominate and are associated with differentiated rocks ferrogabbros and diorites cumulate gabbros are missing the gabbroic section sampled is therefore interpreted as the upper part of the plutonic section most samples were strongly recrystallized during two distinct events metamorphism occurred close to the ridge axis from interaction of a seawater derived fluid with still hot gabbros high temperature shear zones favoured fluid circulation but the water rock ratio estimated from the sodium input was very small in undeformed rocks 1 the low w r ratio explains the strong evolution of the fluid phase and therefore some particular compositions of secondary minerals low temperature alteration occurred when the gabbros were tectonically emplaced close to the sea bottom

provenance and paleodrainage patterns of upper jurassic and lower cretaceous synrift sandstones in the flemish pass basin offshore newfoundland east coast of canada upper jurassic to lower cretaceous synrift sandstones from two industry exploratory wells in the flemish pass basin were studied for provenance using heavy mineral proxies these sandstones were deposited during the north atlantic rifting stage at which time rifting intensified between iberia and the grand banks several heavy mineral methods were used including in situ u pb geochronology and morphological and chemical analysis of detrital zircons chemical discrimination of detrital tourmalines and heavy mineral ratios clastic bed load material entering the northern flemish pass basin during the tithonian to berriasian was connected to transport systems reaching west as far as 400 to 500 km 249 311 mi source areas to the west included igneous rocks and associated cover sequences of the avalon zone as well as the magmatic rocks metasedimentary rocks and associated cover sequences present in the central mobile belt the eastward directed input of bed load material into the basin is considered to have resulted in the concentration of sandstone units along its western margin with deteriorating reservoir grade toward the east given the regional extent of drainage systems it is likely that similar constraints apply to equivalent sandstones in the adjacent east orphan basin by the valanginian barremian paleodrainage orientations had switched and bed load material began to enter the flemish pass basin from more proximal areas to the southeast with the avalon zone basement overall no evidence exists to support sediment sourcing from the iberian margin or the flemish cap galicia bank continental fragment to the east and material from these areas is instead interpreted to have been shed into the incipient atlantic ocean or bay of biscay also no evidence exists to support sourcing from grenville basement farther to the west implying that a drainage divide may have existed somewhere between central and western newfoundland west of which material was presumably being transported to the south or southwest

birth and demise of the rheic ocean magmatic arc s combined u pb and hf isotope analyses in detrital zircon from sw iberia siliciclastic strata paleozoic continental reconstructions indicate that subduction of rheic oceanic lithosphere led to collision between laurussia and gondwana which was a major event in the formation of the ouachita appalachian variscan orogenic belt and the amalgamation of pangea however arc systems which record rheic ocean subduction are poorly preserved the preservation of devonian detrital zircon in late devonian early carboniferous siliciclastic rocks of sw iberia rather than arc related igneous rocks indicates that direct evidence of the arc system may have been largely destroyed by erosion here we report in situ detrital zircon u pb isotopic analyses of late devonian early carboniferous siliciclastic rocks from the pulo do lobo zone which is a reworked late paleozoic suture zone located between laurussia and gondwana detrital zircon age spectra from the pulo do lobo zone frasnian formations show striking similarities revealing a wide range of ages dominated by neoproterozoic and paleoproterozoic grains sourced from rocks typical of peri gondwanan terranes such as avalonia the meguma terrane and the ossa morena zone pulo do lobo rocks also include representative populations of mesoproterozoic and early silurian zircons that are typical of avalonia and the meguma terrane which are absent in the ossa morena zone the famennian tournaisian formations from the pulo do lobo zone however contain more abundant middle late devonian zircon indicating the contribution from a previously unrecognized source probably related to the rheic ocean magmatic arc s the middle late devonian to early carboniferous zircon ages from the siliciclastic rocks of sw iberia south portuguese pulo do lobo and ossa morena zones have a wide range in ehft values 8 2 to 8 3 indicating the likely crystallization from magmas formed in a convergent setting the missing rheic ocean arc was probably built on a meguma avalonia type basement we propose for the pulo do lobo zone that the frasnian sedimentation occurred through the opening of a back arc basin formed along the laurussian active margin during rheic ocean subduction as has been recently proposed for the rhenohercynian zone in central europe detrital zircon ages in the frasnian siliciclastic rocks indicate provenance in the meguma terrane avalonia and devonian rheic ocean arc s as a result of back arc basin inversion the frasnian formations underwent deformation metamorphism and denudation and were unconformably overlain by famennian to visean siliciclastic strata including the phyllite quartzite formation of the south portuguese zone the latest devonian early carboniferous detritus were probably shed to the pulo do lobo zone represa and santa iria formations by recycling of devonian siliciclastic rocks from the south portuguese zone meguma terrane and from a new distinct source with baltica laurentia derivation preserved in the horta da torre formation and alajar melange

evidence for the granulite granite connection penecontemporaneous high grade metamorphism granitic magmatism and core complex development in the liscomb complex nova scotia canada abstract upper amphibolite granulite facies gneisses and granites of the liscomb complex nova scotia canada which are exposed in a core complex within the cambro ordovician meguma group of southern nova scotia yielded concordant u pb zircon monazite ages of 377 2 and 374 3 ma respectively geochronological and geochemical data suggest a single devonian high grade metamorphic event which generated the granitic magma by partial melting of the fertile liscomb gneisses at a depth of 30 km the melting was also synchronous with an extensional event during which the gneisses were uplifted in a core complex associated with the intrusion of granitoids to a depth of 10 km subsequently the gneisses and granites underwent rapid exhumation before the deposition of unconformably overlying late fammenian rocks at 364 ma these events took place during terminal stages of the acadian orogeny and the onset of extensional tectonics in atlantic canada during the middle late devonian the close temporal and spatial association of liscomb gneisses granulites and granites their major and trace element compositions and their overlapping isotopic characteristics confirm the hypothesis that high grade metamorphism and generation of granitic melt are complementary processes as the liscomb granites are of similar age mineralogy and chemistry to the voluminous granitoid plutons found throughout the meguma terrane a similar process is indicated for the rest of the terrane

comment on baltic provenance of top famennian siliciclastic material of the northern rhenish massif rhenohercynian zone of the variscan orogen by koltonik et al international journal of earth sciences 2018 107 2645 2669 koltonik et al int j earth sci 107 2645 2669 2018 evidence that the late devonian siliciclastic rocks from the rheno hercynian zone in germany derived from baltica and scandinavian caledonides this finding together with what is known about the provenance of the pulo do lobo and south portuguese zones in portugal and spain reinforces the probability that late devonian basins may have been sourced from distinct terranes placed along the variscan suture our comment intends to underline changes in the provenance of the late devonian basins along the active margin of laurussia and also to improve the correlation model for the variscan tectonic units from sw iberia and germany

a comparative 40ar 39ar conventional and laserprobe study of muscovite from the port mouton pluton southwest nova scotia we report 40ar 39ar age spectrum and laserprobe data for primary magmatic or fluido magmatic muscovite minerals from the port mouton pluton one of several weakly peraluminous peripheral plutons in the meguma terrane southwestern nova scotia laserprobe data from the cores of thin grain fragments suggest that this pluton cooled rapidly following intrusion at 373 1 ma the u pb monazite age the rims of thicker more complete grains record ages of 315 325 ma even in cases where there have been no apparent changes in grain rim chemistry and where deformation is minimal the observed age gradients may be the result of prolonged reheating during the late carboniferous alleghanian orogeny or alternatively the result of rapid cooling at this time to temperatures below the closure value for muscovite rims conventional age spectra obtained from muscovite separates record neither the older intrusion age nor the younger reset cooling age instead these intermediate ages appear to reflect the averaging of int

petrology and mineralogy of dredged gabbro from gettysburg bank eastern atlantic gabbros have been dredged from gettysburg bank 110 km west of portugal on the azores gibraltar fracture zone primary minerals in olivine pyroxene and brown hornblende gabbros are partially replaced by metamorphic minerals igneous textures are inhomogeneously overprinted by a granular polyhedral deformation and a cataclastic deformation amphiboles show characteristics which indicate a transition from crystallisation in a magma chamber to formation of amphibole in solid gabbro under metasomatic conditions of the amphiboles analysed chlorine was present in the green amphiboles but below 0 05 in the brown suggesting the penetration of sea water after the formation of the brown amphibole but during the formation of the green

metabasalts from the mid atlantic ridge new insights into hydrothermal systems in slow spreading crust an extensive suite of hydrothermally altered rocks were recovered byalvin and dredging along the mark mid atlantic ridge south of the kane fracture zone 23 24 n where detachment faulting has provided a window into the crustal component of hydrothermal systems rocks of basaltic composition are altered to two assemblages with these characteristics i type i albitic plagioclase an02 10 mixed layer smectite chlorite or chlorite actinolite quartz sphene 20 of the clinopyroxene is altered and cu and zn are leached the geochemical signature of these alteration types reflects the relative proportion and composition of secondary minerals and the degree of alteration of primary phases and does not show simple predictive relationships element mobilities indicate that both alteration types formed at low water rock ratios the mark assemblages are typical of the greenschist and transition to the amphibolite facies and represent two distinct albeit overlapping temperature regimes type i 180 to 300 c and type ii 250 to 450 c by analogy with dsdp odp hole 504b and many ophiolites the mark metabasalts were altered within the downwelling limb of a hydrothermal cell and type i and ii samples formed in the upper and lower portions of the sheeted like complex respectively episodic magmatic and hydrothermal events at slow spreading ridges suggest that these observed mineral assemblages represent the cumulative effects of more than one hydrothermal event groundmass and vein assemblages in the mark metabasalts indicate either that alteration conditions did not change during successive hydrothermal events or that these assemblages record only the highest temperature event lack of retrograde reactions or overprinting of lower temperature assemblages e g zeolites suggests that there is a continuum in alteration conditions while crustal segments remain in the ridge axis environment the type ii samples may be representative of thereaction zone where compositions of hydrothermal fluids actively venting at the seafloor today become fixed this prediction necessitates interaction between hydrothermal fluids and intersertal glass and or mafic phases in addition to plagioclase in order to produce the observed range in vented fluid ph

petrology age and tectonic setting of the seal island pluton offshore southwestern nova scotia the seal island pluton outcrops only on small islands located on the continental shelf 45 km south of nova scotia although geophysical data indicate that the pluton is part of large granitoid units that cover thousands of square kilometres farther offshore based on the island outcrops the seal island pluton consists of biotite monzogranite and muscovite biotite monzogranite of uncertain relative age metasedimentary xenoliths combined with characteristic magnetic patterns indicate that the pluton intruded the cambrian ordovician meguma group compared with the biotite monzogranite the muscovite biotite monzogranite is higher in sio2 more peraluminous and more depleted in heavy rare earth elements and also has lower end 1 39 versus 0 82 possibly the result of more contamination by meguma group sedimentary rocks the biotite monzogranite yielded a late devonian u pb zircon age of 362 8 0 7 ma although the relatively minor petrological differences between the two units do not preclude a co m

the port mouton shear zone intersection of a regional fault with a crystallizing granitoid pluton abstract the peraluminous tonalite monzogranite port mouton pluton is a petrological geochemical structural and geochronological anomaly among the many late devonian granitoid intrusions of the meguma lithotectonic zone of southern nova scotia the most remarkable structural feature of this pluton is a 4 km wide zone of strongly foliated 040 subvertical monzogranites culminating in a narrow 10 30 m straight zone of compositionally banded rocks that extends for at least 3 km along strike the banded monzogranites consist of alternating melanocratic and leucocratic compositions that are complementary to the overall composition of that part of the pluton suggesting an origin by mineral melt and mineral mineral sorting biotite and feldspar are strongly foliated in the plane of the compositional bands these compositional variations and foliations originated by a process of segregation flow during shearing of the main magma with a crystallinity of 55 75 subsequent minor brittle fracturing of feldspars twinning of microcline development of blocky sub grains in quartz and kinking of micas demonstrate overprinting by a high temperature deformation straddling the monzogranite solidus small folds and late sigmoidal dykes indicate dextral movement on the shear zone this port mouton shear zone pmsz is approximately co linear with the only outcrops of late devonian mafic intrusions in the area two of which are syn plutonic with well developed mingling textures in the marginal tonalite of the port mouton pluton also closely co linear with the mafic intrusions are a granitoid dyke that extends well beyond the outer contact of the port mouton pluton a swarm of large aligned angular xenolithic slabs a zone of thin wispy schlieren banding a large be bearing pegmatite and a breccia pipe with abundant garnetiferous metapelitic xenoliths in various ways the shear zone may control all of these features the port mouton shear zone is parallel to many other ne trending faults and shear zones in the northern appalachians probably related to the docking of the meguma zone along the cobequid chedabucto fault system

late palaeozoic formation and development of the st marys basin mainland nova scotia canada a prolonged record of intracontinental strike slip deformation during the assembly of pangaea

u pb zircon ages and provenance of the white rock formation of the rockville notch group meguma terrane nova scotia canada evidence for the sardian gap and west african origin the white rock formation is the lowermost formation of the rockville notch group an assemblage of silurian devonian rocks preserved in five areas along the northwestern margin of the meguma terrane of nova scotia the formation consists mainly of mafic and felsic metavolcanic rocks interlayered with and overlain by marine metasedimentary rocks felsic metatuff has now been dated from four locations near both the bottom and top of the volcanic pile and yielded a narrow age range with errors of about 446 434 ma these dates confirm a 30 ma hiatus after deposition of the early ordovician hellgate formation in the underlying halifax group this hiatus is coeval with the sardian gap in the lower palaeozoic of peri gondwanan europe the metavolcanic metasedimentary assemblage is overlain by mainly metasiltstone with abundant quartzite and metaconglomerate lenses some of the latter were previously interpreted to be ordovician tillite an interpretation no longer viable new detrital zircon data from metas

crustal structure of the mid atlantic ridge at 23 n from seismic refraction studies an explosive seismic refraction experiment using four ocean bottom hydrophone receivers was carried out along a 120 km long section of the mid atlantic ridge mar median valley at latitude 23 n immediately south of its intersection with the kane fracture zone the data are interpreted in terms of laterally homogeneous horizontally layered structures using travel time inversion and synthetic seismogram modeling techniques these solutions are combined using two dimensional ray tracing to produce a single model defining the major along axis structural changes occurring beneath this section of the median valley with the exception of slightly low layer 3 velocities the crust beneath several tens of kilometers of the mar median valley has all the characteristics of simple mature ocean crust including a well defined moho transition zone 8 km s upper mantle velocities and a total crustal thickness of 6 7 km no evidence is found for the presence of a steady state axial magma chamber in the crust or upper mantle and the excellent propagation characteristics and orderly amplitude versus range relationships imply the existence of a considerable degree of lateral homogeneity beneath the median valley over distances of several tens of kilometers along axis we infer from these observations that the basic seismic characteristics of the ocean crust are frozen in in a time period less than that separating the major volcanic injection events 10 000 50 000 years by hydrothermal circulation penetrating to the base of the crust along the 100 km long ridge segment studied here two major along axis structural changes occur the first is a 10 to 15 km wide along axis zone of lowered velocities in the lower crust centered beneath a major along axis topographic high this is interpreted to be the remnants of the most recent phase of injection that has temporarily left behind it a region of elevated temperatures and pervasive cracking and thus reduced velocities the second occurs abruptly at latitude 23 15 n and is accompanied by both a major change in rift valley and crestal mountain morphology and the apparent noncoincidence of the median valley and central magnetic anomaly north of this boundary the 30 km long ridge segment that abuts the kane fracture zone has a crustal thickness of 4 5 km and no distinctive layering to produce the characteristic amplitude patterns that typify mature oceanic crust interpretation of the cause of this structural change is uncertain but may be related to local tectonic events perhaps a recent 10 to 20 km ridge jump to the east

hydrothermal clinopyroxenes of the skaergaard intrusion magmatic augites reacted with high temperature aqueous solutions to form secondary calcic pyroxenes during the subsolidus cooling of the skaergaard intrusion secondary hydrothermal clinopyroxenes replace wall rock igneous augites at the margins of veins filled with calcic amphibole these veins are up to several millimeters wide and tens of meters in length hydrothermal clinopyroxenes are a ubiquitous and characteristic phase in the earliest veins throughout the layered series of the intrusion and occur rarely in late veins that in some places crosscut the early veins associated secondary phases in early veins include amphiboles ranging in composition from actinolite to hornblende together with biotite fe ti oxides and calcic plagioclase hydrothermal clinopyroxenes in late veins may be associated with actinolite hornblende biotite magnetite and albite hydrothermal clinopyroxenes are depleted in fe mg and minor elements and enriched in ca and si relative to igneous augites in the layered series gabbros secondary vein pyroxenes are similar in composition to calcic pyroxenes from amphibolite facies metamorphic rocks clinopyroxene solvus thermometry suggests minimum temperatures of equilibration of between 500 and 750 c these temperatures combined with numerical transport models of the intrusion suggest that vein clinopyroxenes could have formed during 20 000 to 60 000 year time intervals associated with a maximum in the fluid flux through fractures in the layered series

significance of gneissic rocks in the liscomb complex nova scotia the liscomb complex comprises late devonian intrusive rocks principally peraluminous granite and medium to high grade metamorphic rocks gneisses that collectively are hosted by low grade greenschist facies metasediments of the cambro ordovician meguma group the conventional view that these gneisses contain high grade mineral assemblages and represent basement rocks has recently been challenged and indeed some of the rocks previously mapped as gneisses particularly metapelites have isotopic compositions resembling the meguma group amphibole bearing enclaves in the liscomb plutons however are isotopically distinct and in this regard resemble xenoliths of basement gneisses in the popes harbour lamprophyre dyke south of the liscomb area metasedimentary enclaves with meguma isotopic signatures can contain garnets with unzoned cores implying high temperatures that host high grade minerals prismatic sillimanite spinel and or corundum and are enclosed by retrograde zoned rims these f

building the crust at the mid atlantic ridge the morphology of the sea floor at mid ocean ridge spreading centres provides a key to understanding how ocean crust is constructed images of the axial zone of the slow spreading mid atlantic ridge obtained at a range of spatial scales show that crustal construction is complex and highly variable reflecting an underlying three dimensional variability in magmatic tectonic and hydrothermal processes

fe ti oxides in metamorphic basites from the eastern alps austria a contribution to the formation of solid solutions of natural fe ti oxide assemblages the development of fe ti oxide assemblages in basic rocks from the penninic series of the southern venediger rea austria during polyphase alpine metamorphism has been studied textural and compositional relations indicate thorough reequilibration of the opaque mineral assemblages during late barrovian metamorphism at essentially static conditions of lower amphibolite to greenschist facies in contrast silicate mineralogy of the preceeding blueschist to eclogite facies metamorphism might still be preserved to a large extent

importance of iron rich tholeiitic magmas at divergent plate margins a reappraisal when a tholeiitic liquid differentiates it may give rise to either iron poor silica rich rhyolitic differentiates or to iron rich silica poor differentiates as found in the skaergaard intrusion iron rich differentiated liquids are rare among erupted rocks but are found in small quantities from several localities at divergent plate margins among the tertiary basalts and intrusions in east greenland normal erupted basalts may be mixtures of primitive liquid and differentiated iron rich liquid which exists at depth but normally does not reach the surface because of its high density the evolved liquids of the skaergaard intrusion were of this kind data from mid oceanic ridges confirm this view we believe that iron rich differentiated liquids despite their scarcity on the surface are much more voluminous at depth as picrites at the other side of the density minimum attained during liquid evolution are thought to be the trend toward iron enrichment develops when the tholeiitic magma differentiates in a closed system at a relatively low oxidation state whereas the trend toward silica enrichment and iron depletion occurs when the magma has interacted with the oxidized and hydrated surroundings in the crust

occurrence origin and significance of melagranites in the south mountain batholith nova scotia melagranites colour index 20 with biotite garnet cordierite constitute 0 1 of the area of the 7300 km2 peraluminous south mountain batholith smb nova scotia the melagranites occur as small bodies showing sharp to gradational contacts against the meguma supergroup country rocks and coeval mingling contacts against other facies of the batholith they also occur as elliptical or blocky metre scale enclaves elsewhere in the smb characteristic petrological features of the melagranites include high modal abundances of sulphide minerals strongly reacted metasedimentary xenoliths mafic mineral rich clots apparent porphyritic textures with highly variable proportions of alkali feldspar megacrysts and allotriomorphic granular textures chemically and isotopically melagranite rocks have wide compositional variations in most major element trace element and isotopic variation diagrams the melagranites lie on mixing lines between the more abundant granodioritic and monzogranitic phases of the

evolution of oceanic gabbros from dsdp leg 82 influence of the fluid phase on metamorphic crystallizations abstract gabbro breccias were recovered from an anomalously shallow level of the ocean crust during dsdp leg 82 the rocks display evidence of metamorphic crystallization related either to localized deformation or to hydrothermal circulation of a seawater derived fluid under static conditions secondary phases consist of plagioclase amphibole and minor clinopyroxene ilmenite sphene and chlorite petrological study indicates that deformation took place at high temperature under anhydrous conditions and was followed by hydrothermal circulation the compositions of secondary minerals i e strong zonations presence of chlorine in amphiboles varying compositions of secondary plagioclase indicate that reactions of the gabbros with the fluids occurred at a low water rock ratio relations between cl na and k in amphiboles suggest penetration of at least two distinct fluids of different compositions metamorphic crystallization stopped when greenschist facies conditions were reached 350 c probably because hydrothermal circulation faded out

ridge axis deformation and coeval melt migration within layer 3 gabbros evidence from the lizard complex u k ridge parallel extensional shear zones within the layer 3 gabbros of the lizard ophiolite have recorded varied and numerous chemical and mineralogical changes under a variety of p t conditions these changes accompany tectonic exhumation of lower crustal levels by a listric fault mechanism during amagmatic extension at a slow spreading centre deformation within the mid crustal level of layer three resulted in the migration and impregnation of an evolved iron titanium rich silica poor melt along the shear zones this syntectonic infiltration occurred along the lowermost portions of listric faults which rooted in a partially molten zone beneath the ridge axis residual melt was tapped from this underlying zone and tectonically redistributed about deforming gabbros situated immediately above the partially molten zone the occurrence of sub solidus synkinematic assemblages within the shear zones which were generated at various temperatures and water rock ratios is explained in terms of their simultaneous development on listric fault planes in the presence of a hydrothermal fluid phase the sequential overprinting of pre existing assemblages arises from repeated block rotation during amagmatic episodes of extension within an oceanic slow spreading environment

the german bank pluton offshore sw nova scotia age petrology and regional significance for alleghanian plutonism permian plutonism is widespread in the u s appalachians but not in the canadian sector of the appalachians the german bank pluton sampled in two boreholes offshore southwestern nova scotia gave a u pb age of 300 1 ma determined by isotope dilution thermal ionization mass spectrometry id tims on thermally annealed and chemically etched zircon grains electron microprobe dating of monazite yielded an age of 308 13 ma late paleozoic plutonism in the canadian appalachians was dominated by northwestward subduction of the rheic ocean whereas the rheic ocean in the u s appalachians closed by southeasterly subduction a slab tear therefore developed at the southwestern margin of the rheic ocean slab beneath the canadian appalachians the high sr and ba metaluminous magnetite granite of the german bank pluton resulted from melting of underplated lower crust and or lithospheric mantle by a rising asthenospheric diapir related to this slab tear geochemically the german bank pluton is similar to most 300 265 ma plutons of the southern appalachians except in its strongly negative nd isotopic composition that results from the underthrusting of a ca 2 ga lower crustal block of saharan craton

late paleozoic strike slip faults in maritime canada and their role in the reconfiguration of the northern appalachian orogen major late paleozoic faults many with documented strike slip motion have dissected the ordovician devonian appalachian orogen in the maritime provinces of atlantic canada activity alternated between east west faults minas trend and ne sw faults appalachian trend nw se faults canso trend were probably conjugate to minas trend faults major dextral movement on faults with appalachian trend in total between 200 and 300 km began in the late devonian this movement initiated the maritimes basin in a transtensional environment at a releasing bend formed around a promontory in the laurentian margin and thinned the crust accounting for the major subsidence of the basin appalachian trend strike slip continued in the mississippian but was accompanied by major movement on e w minas trend faults culminating around the mississippian pennsylvanian boundary juxtaposing the meguma and avalon terranes of the appalachians close to their present day configuration however strike slip continued during the pennsylvanian permian interval resulting in transpressional deformation that reactivated and inverted earlier extensional faults a final major episode of transtension mainly sinistral occurred during the mesozoic opening of the atlantic ocean restoration of movements on these faults amounting to several hundred kilometers of slip explains anomalies in the present day distribution of terranes amalgamated during early paleozoic appalachian tectonism in the restored geometry the nashoba and ellsworth terranes of ganderia are adjacent to one another and the meguma terrane lies clearly outboard of avalonia a restored post acadian paleogeography not the present day geometry of the orogen should be used as a basis for reconstructions of its earlier paleozoic history

a new petrogenetic grid for low grade metabasites abstract we have used internally consistent thermodynamic data to present calculated phase equilibria for the system na2o cao mgo al2o3 sio2 h2o ncmash in the range 0 500 c and 0 1 10 kbar involving the phases anorthite glaucophane grossular heulandite jadeite laumontite lawsonite paragonite prehnite pumpellyite stilbite tremolite wairakite zoisite with excess albite clinochlore quartz and pure water average activity terms derived from published mineral chemical data were included for clinochlore glaucophane prehnite pumpellyite tremolite and zoisite the new petrogenetic grid delineates stability fields and parageneses of common index minerals in zeolite prehniteactinolite prehnite pumpellyite pumpellyite actinolite blueschist and greenschist facies metabasites the stability fields of mineral assemblages containing prehnite pumpellyite epidote actinolite albite chlorite quartz were analysed in some detail using activity data calculated from five specific samples for example the prehnite actinolite facies covers a p t field ranging from about 220 to 320 c at pressures below 4 5 kbar the transition from the prehnite actinolite and pumpellyite actinolite to greenschist facies occurs at about 250 300 c at 1 3 kbar and at about 250 350 c at 3 8 kbar p t fields of individual facies overlap considerably due to variations in chemical composition

systematic mineralogical diversity in a type granitic intrusions control of magmatic source and geological processes the origin and causes of mineralogical diversity of a type granites are debated the series of a type granite plutons with distinct mineralogical differences emplaced along an upper paleozoic crustal scale shear zone in the cobequid highlands nova scotia provide an opportunity to examine the origin of different a type plutons in a similar tectonic setting based on the ferromagnesian minerals present the plutons are classified into sodic granites with sodic amphibole calcic granites with calcic amphibole and biotite granites sodic and calcic granites occur exclusively in complex intrusions with subequal amounts of gabbro in the eastern shear zone whereas plutons in the western shear zone with lesser gabbro are solely biotite granites trace elements and radiogenic isotopes show that the three granite types have different sources intensive parameters including temperature pressure and water in melt contents were estimated from mineralogical and geochemical data modeling of these geochemical data suggests that the biotite and calcic granites were derived by 20 40 partial melting of intracrustal feldspathic rocks whereas the sodic granites are extreme fractionates 90 of coeval mafic magma we propose that supply of upper paleozoic mafic magma probably related to regional extension and decompression melting beneath the magdalen basin created a deep crustal hot zone in the eastern cobequid highlands and extreme fractionation of underplated mafic sills produced the sodic granites heat transfer from crystallizing mafic magma induced partial melting of the surrounding crust creating batches of biotite and calcic granitic melts in different depths fractionated and crustally derived melts segregated along crustal scale faults constructing the complex plutons in the east melting of the crust was further facilitated by the release of water from the crustal rocks upon heating in the eastern shear zone water was released predominantly by magmatic rocks and in lesser amounts compared to the west where neoproterozoic sedimentary and volcaniclastic rocks are more abundant the volatile rich granitic melts in the western part of the shear zone were crystallized rapidly stabilizing only biotite this study demonstrates that the mineralogical variations in a type granites arise from rather similar magma compositions but they are important petrogenetic indicators of varying sources specific magmatic processes and emplacement conditions

rhenium osmium geochronology of arsenopyrite in meguma group gold deposits meguma terrane nova scotia canada evidence for multiple gold mineralizing events rhenium osmium geochronology using arsenopyrite was undertaken for three gold deposits in the meguma terrane nova scotia canada in order to better constrain their age of formation and to assess the utility of arsenopyrite for dating similar deposits globally analyses of arsenopyrite from bedding concordant veins from the ovens locality southwest meguma terrane yield a precise re os isochron age of 409 5 ma arsenopyrite analyses from a bedding discordant vein at this locality indicate an identical re os age of 407 4 ma saddle reef veins from the dufferin deposit northeast meguma terrane contain arsenopyrite with a precise re os isochron age of 380 3 ma the lack of common os in some arsenopyrite samples from both the ovens and dufferin permit calculation of single mineral model ages for each deposit which are identical to those determined using the isochron method initial os compositions for the two vein types at the ovens suggest a predominately crustal source of os in the mineralizing fluids whereas a less radiogenic initial os composition for arsenopyrite from dufferin does not as clearly define a crustal metal source at a third locality the touquoy deposit the re os systematics of arsenopyrite associated with disseminated gold mineralization do not define a precise formation age possibly as a result of mixing of re and os derived from the mineralizing fluid and the shale host rock the re os ages of arsenopyrite indicate that there were at least two distinct periods of gold deposition in the meguma terrane coinciding with widespread tectonothermal events regional deformation and metamorphism associated with acadian orogenesis and widespread generation of meta and peraluminous granites and high grade metamorphism within the basement rocks under the meguma terrane the ca 407 ma age for the ovens arsenopyrite provides the best estimate for the timing of regional acadian deformation in the meguma terrane and is slightly older than previous estimates based solely on 40ar 39ar dating

the structure of sw cornwall and its bearing on the emplacement of the lizard complex six phases of deformation can be recognized in the devonian metasediments to the n of the lizard complex the first two phases d 1 d 2 show a variety of orientations and styles but with a general n or nw vergence these phases while respresenting regional deformation events are considerably modified by the emplacement of the lizard complex there is a general increase in intensity of both d 1 and d 2 structures towards the lizard boundary this is accompanied by variation in the orientation of both early phases of deformation leading to a variety of interference patterns domains of oblique f 1 folds with axes trending n s are developed in the rocks immediately to the ne and nw of the lizard and are considered to have formed by differential nnw directed thrusting of slabs of the complex during the early regional deformation d 2 produced discrete shear zones ahead and below the lizard rocks and may have resulted from the gravitational collapse of the d 1 nappes the d 3 deformation post dates the lizard emplacement being related to the cornubian batholith subsequent phases represent essentially post batholith and hence post lizard emplacement deformation