crustal structure and surface zonation of the canadian appalachians implications of deep seismic reflection data in 1986 1181 km of marine seismic reflection data was collected to 18 20 s of two way traveltime in the gulf of st lawrence area the seismic profiles sample all major surface tectono stratigraphic zones of the canadian appalachians they complement the 1984 deep reflection survey northeast of newfoundland together the seismic profiles reveal the regional three dimensional geometry of the orogen three lower crustal blocks are distinguished on the seismic data they are referred to as the grenville central and avalon blocks from west to east the grenville block is wedge shaped in section and its subsurface edge follows the form of the appalachian structural front the grenville block abuts the central block at mid crustal to mantle depths the avalon block meets the central block at a steep junction that penetrates the entire crust consistent differences in the seismic character of the moho help identify boundaries of the deep crustal blocks the moho signature varies from uniform over extended di

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

proto avalonia a 1 2 1 0 ga tectonothermal event and constraints for the evolution of rodinia the neoproterozoic evolution of avalonia is thought to have been geodynamically linked to the amalgamation and dispersal of rodinia similar sm nd isotopic signatures for different periods of arc activity suggest that avalonian basement or proto avalonia was generated in a series of primitive oceanic island arcs between 1 2 and 1 0 ga because this interval coincides with the amalgamation of rodinia proto avalonia is inferred to have been located in a panthalassa like peri rodinian ocean an early 760 660 ma phase of avalonian arc activity is attributed to renewed subduction in the peri rodinian ocean following the breakup of rodinia which caused the accretion of avalonian terranes to the gondwanan margin by ca 650 ma further subduction along the margin occurred outboard of these terranes and resulted in the onset of main phase avalonian volcanism at 630 ma the diachronous cessation of arc magmatism is attributed to ridge trench collision and the generation of a continental transform the geodynamic link between avalonia and rodinia is analogous to that between the mesozoic dispersal of pangea and the tectonothermal evolution of western north america this event also resulted in the accretion of outboard terranes and in arc related magmatism that is currently being terminated in a diachronous manner by ridge collision and the generation of the san andreas transform the model implies that the neoproterozoic evolution of avalonia and other peri gondwanan terranes provide important constraints on the tectonic history of a large portion of the rodinian continental margin

rheological stratification of the lithosphere abstract the rheology of the lithosphere is estimated on the basis of pore pressure dependent frictional failure in the brittle regime and power law steady state creep in the ductile regime different petrological models of the continental and oceanic lithosphere combined with different geotherms and thicknesses are used to generate models covering a variety of geodynamic situations results show that the depth dependence of lithospheric rheology varies with the tectonic province and that in many instances the continental lithosphere has one at the bottom of the crust or two the previous one plus another at mid crustal levels soft ductile layers sandwiched between brittle layers the depth distribution of geophysical parameters seismicity seismic wave velocity and electrical conductivity matches the theological predictions well the lithospheric rheological profiles are used to analyze observed variations in the structural style of deformed continental margin and oceanic terranes in the southeastern canadian cordillera the large scale characteristics of the system show a satisfactory agreement with the inferred rheological structure sub horizontal decollements should be the rule rather than the exception where the lithospheric rheology is strongly stratified

u pb geochronology of devonian granites in the meguma terrane of nova scotia canada evidence for hotspot melting of a neoproterozoic source abstract u pb isotopic analyses of monazite and zircon from six granitic plutons in the meguma terrane yield nearly concordant ages of documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 373 pm 3 end document ma interpreted as the time of intrusion u pb analyses of euhedral zircons with thick rims overgrowing cores which were abraded to remove all or most of the rim plot on chords between documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage p

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

sequence stratigraphy and proveance on gondwana s margin the meguma zone cambrian to devonian of nova scotia canada the meguma zone is the second largest terrane in the canadian appalachians three thick sandstones cambrian upper ordovician and lower devonian alternate with two thick shales lower ordovician and silurian the succession is a marginal assemblage shoaling upward from deep sea fan complexes to coastal facies in terms of sequence stratigraphy the succession consists of a basal type 1 sequence the meguma supergroup and three overlying type 2 sequences collectively the annapolis supergroup interpretations of sedimentary environments and stratigraphic relations agree with those of classical systems tracts the meguma zone is a sedimentary sink for an enormous amount of well sorted fine grained sand and silt this quantity and its westward dispersal indicate a gondwanan derivation stratigraphic units of the west african craton mimic those of the meguma zone in lithology provenance dispersal succession and age during neoproterozoic time continental ice sheets rivers and wind moved sediment southeastward down a cratonic paleoslope from what is now morocco through mauritania and mali remnants of widespread sand sheets extend across southern mali this sand reservoir became the source rock for the meguma zone in the early cambrian the paleoslope reversed perhaps due to birth of iapetus these same agents eroded the malian sand sheets episodically during relative sea level lows in cambrian late ordovician and early devonian times remnants of the resulting northwestward moving cratonic sands and silts occur today as mesas and buttes in northern mali mauritania southern morocco and algeria the ultimate destination of this detritus was the continental margin of north gondwana this sediment now forms the meguma zone and other terranes of southern europe northern africa and the middle east

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

field relationships petrology age and tectonic setting of the late cambrian ordovician west barneys river plutonic suite southern antigonish highlands nova scotia canada the west barneys river plutonic suite consists of gabbro syenite monzonite alkali feldspar syenite to quartz alkali feldspar syenite and alkali feldspar granite outcropping in an area of 100 km 2 in the southern antigonish highlands magma mixing and mingling textures indicate a comagmatic relationship between some of the mafic and intermediate felsic lithologies however nine u pb zircon ages three by thermal ionization mass spectrometry tims and six by laser ablation inductively coupled plasma mass spectrometry la icp ms from the west barneys river suite and the lithologically similar cape porcupine complex located 60 km to the east range from ca 495 to 460 ma indicating that emplacement occurred over a significant span of time intermediate to felsic rocks consist mainly of perthitic k feldspar and variable amounts of quartz interstitial granophyre is present in some samples consistent with shallow emplacement mafic phases are fe rich amphibole and clinopyroxene and in some units fayalite intermediate and felsic samples have chemical characteristics of within plate ferroan a type granitoid rocks gabbroic rocks consist of plagioclase oligoclase labradorite and augite diopside with less abundant orthopyroxene olivine biotite and ilmenite magnetite their chemical compositions are transitional from tholeiitic to alkalic and characteristic of continental within plate mafic rocks the e nd values are similar in gabbroic syenitic and granitic samples ranging between 0 9 and 4 9 consistent with a co genetic origin for the mafic and intermediate felsic components of the suite and derivation from avalonian subcontinental lithospheric mantle in an extensional environment

lu hf zircon and sm nd whole rock isotope constraints on the extent of juvenile arc crust in avalonia examples from newfoundland and nova scotia canada avalonia the largest accreted crustal block in the appalachian orogen consists of neoproterozoic magmatic arc sequences that represent protracted and episodic subduction related magmatism before deposition of an ediacaran ordovician cover sequence including siliciclastic rocks zircon crystals were obtained from arc related magmatic rocks and from clastic sedimentary sequences and analyzed in situ for their hf isotope composition the majority of magmatic and detrital zircons are dominated by initial 176hf 177hf values that are more radiogenic than chondritic uniform reservoir chur with calculated crust formation hf tdm model ages that range from 0 84 to 1 30 ga these results suggest formation by partial melting of juvenile mantle in a neoproterozoic continental arc some zircons have hf tdm model ages ca 1 39 3 09 ga with ehf values of 33 9 to 0 5 and more clearly indicate involvement of older lithosphere in their petrogenesis whole rock sm nd isotopic compositions from felsic volcanic rocks are

origin and evolution of avalonia evidence from u pb and lu hf isotopes in zircon from the mira terrane canada and the stavelot venn massif belgium laser ablation inductively coupled plasma mass spectrometry u pb and lu hf isotope analyses of detrital zircon from neoproterozoic cambrian clastic sedimentary rocks in the mira terrane cape breton island nova scotia canada west avalonia and the stavelot venn massif east belgium east avalonia support deposition on an originally coherent microcontinent crustal evolution trends defined by hf t values varying with age reflect juvenile magma production in the source continent at 1 2 2 2 and 2 4 3 1 ga mixing of juvenile and recycled crust in continental magmatic arcs is recognized at 0 5 0 72 1 4 1 7 1 8 2 2 and 2 4 2 7 ga these results concur with the crustal evolution in amazonia the likely parent craton crustal evolution in avalonia is recorded in detrital and magmatic zircon from neoproterozoic arcs 680 550 ma positive hf t values suggest juvenile input and mixing with recycled crust most negative hf t values represent recycling of predominantly mesoproterozoic underlying crust avalonian arc magmatism was followed by late neoproterozoic early cambrian sedimentation in various belts in west avalonia these belts were juxtaposed by strike slip during late early cambrian deposition in a rift basin the youngest detrital zircon population c 517 ma probably represents synrift magmatism before break up of avalonia migmatization at 406 2 ma in a xenolith from the east avalonian crust reflects post collisional heating supplementary materials details of sample locations and analytical results are available at www geolsoc org uk sup18641

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

correlation between seismic reflectivity and rheology in extended lithosphere southern tuscany inner northern apennines italy abstract in southern tuscany inner zone of the northern apennines the lithosphere has been affected by extensional deformation since the middle miocene and is characterized by high heat flow 120 mw m 2 both local shallow and regional deep seismic reflection profiles clearly show the occurrence of a sharp reflector k horizon within the upper crust which is probably located at the top of a shear zone acting as the decollement horizon for the latest pliocene quaternary normal faults its depth varies between 3 and 6 km in geothermal areas and 9 and 12 km elsewhere borehole control in the geothermal areas shows that it occurs at a temperature of approximately 450 c the hypothesis that the k horizon represents the top of the brittle ductile transition agrees with two lines of evidence a it approximately follows the same isotherm over the whole region and b its depth coincides with rheological predictions of the brittle ductile transition for the range of high pore fluid pressures and strain rates estimated for southern tuscany it appears therefore that at least under specific conditions very high pore fluid pressures strain concentration the brittle ductile transition may have a seismic signature

dating transcurrent terrane accretion an example from the meguma and avalon composite terranes in the northern appalachians the polygenetic minas geofracture separates the meguma and avalon composite terranes of the maritime appalachians dynamically recrystallized muscovite and biotite occur within d3 shear zones which developed during dextral movement along the geofracture these record 40ar 39ar plateau ages between circa 360 and 370 ma comparable plateau ages are recorded by micas within undeformed portions of granitic plutons and their low pressure contact metamorphic aureoles these are similar to u pb ages previously reported for igneous monazite therefore indicating relatively rapid postmagmatic cooling at shallow crustal levels the d3 fabrics were superimposed on earlier s1 structures which formed during initial amalgamation of the meguma and avalon composite terranes whole rock slate phyllite samples collected outside high temperature portions of the plutonic contact aureoles yield 40ar 39ar plateau ages which correspond to groupings of circa 400 395 ma 390 385 ma and 380 375 ma these are interpreted to date diachronous formation of the s1 fabrics and concomitant low grade regional metamorphism in the circa 25 ma between initial amalgamation and granitic plutonism it is inferred that there was a slow rise of isotherms through the crust as a result of overthrust tectonic thickening this led to crustal melting and the production of widespread granitic magmas which were emplaced immediately prior to d3 dextral shearing

geochemical and petrological evidence of the subduction of delaminated adriatic continental lithosphere in the genesis of the neogene quaternary magmatism of central italy abstract serri g innocenti f and manetti p 1993 geochemical and petrological evidence of the subduction of delaminated adriatic continental lithosphere in the genesis of the neogene quaternary magmatism of central italy in m j r wortel u hansen and r sabadini editors relationships between mantle processes and geological processes at or near the earth s surface tectonophysics 223 117 147 the neogene quaternary magmatism of the northern apenninic arc took place in four phases separated in space and time which become progressively younger from west to east phase i 14 ma phase ii 7 3 6 0 ma phase iii 5 1 2 2 ma phase iv 1 3 0 1 ma this magmatism is the result of the activation of three physically separate sources 1 the adriatic continental crust extracted from the mantle in the late proterozoic 2 a strongly refractory recently k enriched harzburgitic mantle located in the mechanical boundary layer mbl of the lithosphere and 3 a recently metasomatized cpx rich mantle compositionally variable from iherzolite to wehrlite clinopyroxenite interpreted as an ephemerally k enriched asthenosphere the adriatic continental crust is the dominant source of the acid plutonic and volcanic rocks of the tuscan region the acid magmatism is mostly found inside an ellipsoidal area about 150 300 km centred on giglio island here defined as the tuscan crustal dome within this area mantle derived magmas unaffected by important crustal contamination processes and mixing with crustal anatectic melts have so far not been found pure crustal magmas are rare but are represented for example by some of the san vincenzo and roccastrada rhyolites virtually all the tuscan acid centres show evidence of mixing with potassic mantle derived magmas major and trace elements as well as 87 sr 86 sr and 143 nd 144 nd data on primitive rocks mg 65 reveal two groups of mantle derived magmas these define two distinct mantle enrichment trends both essentially due to the additions of k rich components which metasomatized separate compositionally diverse upper mantle sectors in both cases the most remarkable mineralogical effect of these enrichment processes is the production of variable amount of phlogopite through reaction between fluids and or melts with the mantle the rocks of group i ol hy and q normative lamproites ultrapotassic high mg latites ultrapotassic shoshonites and shoshonites saturated trend are considered to be derived by partial melting at low pressure 87 sr 86 sr 0 717 ce sr 0 3 and k 2 o na 2 o 6 7 and low ratios of 143 nd 144 nd 0 5121 0 5120 and ba la 87 sr 86 sr 0 712 and k 2 o na 2 o 8 9 values and low 143 nd 144 nd ba la 0 10 ratios these constraints do not allow to exclude a complete identity between the k rich components which metasomatized the mantle sources of the saturated and undersaturated trend magmas the geochemical and isotopic features of the components that metasomatized the mantle sources of the northern apenninic arc magmatism can be explained by a geodynamic process which causes a large amount of crustal materials to be incorporated within the upper mantle we propose that the delamination and subduction of the adriatic continental lithosphere related to the still ongoing northern apennine continental collision provide a viable mechanism to explain the genesis and eastward discontinuous migration of the magmatism in central italy the subduction of delaminated lithospheric mantle with lower crustal slivers would have exposed uppermost mantle adriatic mbl and crustal units previously imbricated in the apennine chain to the heating advected by the upwelling of a recently and ephemerally k enriched asthenospheric mantle wedge and by the underplating of magmas derived from it we consider that the diapiric uprising of a hot crustally contaminated asthenosphere occurs in the wake left above the sinking of the adriatic delaminated subducting continental lithosphere the delamination subduction process of the adriatic lithosphere has probably started in the early middle miocene but earlier than 15 14 ma ago as indicated by the age and petrologic characteristics of the first magmatic episode sisco lamproite of the northern apennine orogenesis

geochronology of the larderello geothermal field new data and the closure temperature issue the larderello geothermal field is generally accepted to have been produced by a granite intrusion at 4 9 km depth hydrothermal parageneses and fluid inclusions always formed at temperatures greater than or equal to the current ones which implies that the field has always undergone a roughly monotonic cooling history fluctuations 40 k since intrusion of the granite at 4 ma the heat required to maintain the thermal anomaly over such a long period is supplied by a seismically anomalous body of 32000 km3 rooted in the mantle borehole minerals from larderello are thus a unique well calibrated natural example of thermally induced ar and sr loss under geological conditions and time spans the observations biotites retain ar above 450 c agree well with other albeit less precise geological determinations but contrast with laboratory determinations of diffusivity from the literature we therefore performed a hydrothermal experiment on two larderello biotites and derived a diffusivity dlab 370 c 5 3 10 18 cm2s 1 in agreement with published estimates of diffusivity in annite from dlab and the rejuvenation of the k ar ages we calculate maximum survival times at the present in hole temperatures they trend smoothly over almost two orders of magnitude from 352 ka to 5 3 ka anticorrelating with depth laboratory diffusivities are inconsistent not only with geological facts but also among themselves from the geologically constrained lifetime of the thermal anomaly we derive a diffusivity dg 370 c 3 81 1021 cm2s 1 3 1 orders of magnitude lower than dlab the cause of these discrepancies must be sought among various laboratory artefacts overstepping of a critical temperature t enhanced diffusivities in wet experiments presence of fast pathway dislocation and pipe diffusion and of dissolution reprecipitation reactions which we imaged by scanning electron microscopy these phenomena are minor in geological settings in the absence of mineral transformation reactions complete or near complete resetting is achieved only by volume diffusion therefore laboratory determinations will necessarily result in apparent diffusivities that are too high compared to those actually effecting the resetting of natural geochronometers

440 ma igneous activity in the meguma terrane nova scotia canada part of the appalachian overstep sequence abraded zircons from the basal rhyolitic tuff member of the white rock formation which disconformably overlies the cambrian early ordovician meguma group have yielded a nearly concordant u pb age of 442 or 4 ma interpreted as the age of extrusion this age straddles the approximately 443 ma ordovician silurian boundary abraded zircons from the brenton granite lie on a chord with an upper intercept age of 439 4 3 ma which is interpreted to be the age of intrusion thus supporting its inferred subvolcanic nature on the other hand monazite from the brenton pluton yielded a nearly concordant analysis with a 207 pb 206 pb age of 380 or 3 ma which is interpreted to be the time of the low pressure high temperature metamorphism using these new data the following observations suggest that the meguma and avalon terranes were neighbors during the silurian early devonian 1 both have latest ordovician earliest silurian bimodal subaerial alkalic tholeiitic rift related volcanic rocks with nd signatures indicating a similar continental basement source 2 both show a similar progression of depositional environments subaerial in the earliest silurian progressively deeper water marine strata in the llandovery and wenlock switching to gradually shallowing marine environments in the ludlow and reverting to subaerial in the pragian and 3 both contain rhenish bohemian early devonian fauna furthermore the southeast to northwest transition from an offshore sandbar to a beach sand in the silurian white rock formation suggests the presence of land to the north now recognized as avalonia these conclusions support published suggestions that the siluro devonian successions in the meguma and avalon terranes form part of the overstep sequence that extends across most of the northern appalachians published data indicate that avalonia was adjacent to gondwana in the neoproterozoic that it separated from gondwana in the early ordovician and was accreted to eastern laurentia in the late ordovician early silurian on the basis of the data and correlations presented here we suggest that the meguma terrane travelled with avalonia this is consistent with the absence of a phase of deformation between the cambro ordovician meguma group and the silurian white rock formation

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

crustal models of the geothermal areas of southern tuscany italy abstract geological and geophysical data gathered in the geothermal areas of southern tuscany located in the inner part of the northern apennines indicate the presence of granite intrusions the most relevant data are 1 regional asthenospheric ascent to a depth of 40 50 km 2 granitic dykes as old as 3 8 2 9 ma and a widespread contact aureole found in several deep geothermal wells 3 very high heat flow values interpreted as the result of convective circulation of hot fluids above shallow magmatic bodies 4 the presence of low density bodies inferred from low gravity values often correlated with positive heat flow anomalies 5 the occurrence of a highly reflective horizon named k in a depth range of 3 12 km in the geothermal areas as shown by seismic reflection data 6 a 30 40 km wide low velocity body below the k horizon whose existence is supported by local earthquake tomography and teleseismic studies 7 the presence of a conductive body which correlates well with the anomalous tomographic and teleseismic body as revealed by magnetotelluric surveys the goal of the paper is to test different alternative crustal models by using these data the models are 1 granite intrusion 2 geopressurized fluids below impervious layers 3 contact metamorphic aureole 4 mylonite layers at the upper lower crust transition and 5 some combination of the four the occurrence of shallow intrusive bodies still partially molten and associated with saline brines is considered the most tenable model

upper crustal structure of the larderello geothermal field as a feature of post collisional extensional tectonics southern tuscany italy abstract reflection seismic profiles crossing the larderello geothermal field permit the distinction of three different seismic facies units in the upper continental crust the upper 3 4 km of the crust is characterized by low reflectivity its base corresponding to a reflector referred to as the k horizon and recognized in many other areas of southern tuscany beneath this reflector a seismic facies of 0 8 1 0 twt thickness is characterized by strong convergent and divergent reflections forming a lozange shaped geometry the lowermost seismic facies is characterized by almost horizontal reflections based on the distribution of the hypocentral microearthquakes temperature data recorded in deep boreholes and the geometry of normal faults the k horizon is explained here as a kinematically active rheological boundary which separates a brittle upper part from a ductile lower part according to this hypothesis the k horizon and the lozange reflections could originate from a mylonite band located at the contact between two geological bodies characterized by different rheological behavior it is inferred that this horizon acts as a decoupling plane in the upper continental crust it developed as a consequence of the extensional tectonics and the high heat flow which characterize southern tuscany

u pb isotopic behaviour of zircon during upper amphibolite facies fluid infiltration in the napier complex east antarctica understanding the factors that contribute to u pb discordance in zircon is essential for interpreting isotopic data and for assessing the validity of concordia intercept ages modification caused by interaction with metamorphic fluids is often cited as a primary means by which significant or even complete isotopic resetting of u pb systematics in zircon might be achieved under subsolidus conditions we present a field example from the napier complex east antarctica in which a palaeoproterozoic 2450 70 ma zircon population interacted locally with an early palaeozoic 498 1 7 ma aqueous fluid at upper amphibolite facies conditions conventional ion microprobe analysis of sectioned and polished grain surfaces indicates that fluid interaction resulted in minor disturbance of u and pb in zircons both normal and reverse discordance with limited displacement along a chord with a lower intercept that coincides with the timing of fluid infiltration in contrast ion probe drilling or depth profiling into unpolished natural zircon crystal surfaces revealed extensive disturbance of u pb systematics to depths of 0 2 m with near surface ages consistent with the timing of fluid influx at 498 ma although zircon underwent some radiogenic pb redistribution during fluid interaction infiltrating fluids resulted in minimal grain scale isotopic modification of zircon based on ion probe depth profiling results we propose that limited normal discordance observed in the conventional ion microprobe zircon analyses in this case is controlled by an analytical mixture of reset and or recrystallised zircon along penetrative micro fracture networks with that of adjacent unaffected zircon we also suggest that the observed reverse discordance is genuine resulting from localised intra grain net accumulations of radiogenic pb we conclude that the isotopic response of zircon in this case is controlled by the interaction of an aqueous metamorphic fluid of low to moderate salinity resulting in sub micrometre depth scale isotopic modification at natural crystal faces and along penetrative micro fracture networks and that grain scale isotopic modification was negligible therefore we urge caution when considering regional chronological interpretations that appeal to significant zircon isotopic resetting caused exclusively by metamorphic fluid interaction at upper amphibolite facies conditions

crustal thickness under the gulf of st lawrence northern appalachians from gravity and deep seismic data we have determined crustal thickness in the gulf of st lawrence an area that corresponds to an offset of the main northern appalachians units a complete bouguer anomaly was calculated from recent depth to basement compilations and sediment densities from well data the moho surface was obtained by inverting the bouguer anomaly assuming a single density contrast at depth and using an average depth provided by deep reflection seismic data the resulting crustal model shows a moho depth of 42 44 km beneath the grenville craton north of the appalachian deformation front south of this front the depth to moho displays a pronounced thinning of the crust beneath the carboniferous magdalen basin this is in striking contrast to the deep seismic data which give a moho depth of about 43 km the modelling of the bouguer anomaly in the magdalen basin taking into account the seismic reflection and refraction data reconciles these different results and suggests that a 43 km deep moho beneath the basin is as

interpretation of heat flow density of the apennine chain italy the apennine chain is a consequence of the continental collision between europe and africa characterized by several compressive phases with events of intense shortening during one of these phases from early oligocene to early miocene the continental crust broke into various segments which became packed one over the other forming an adriatic verging structure of imbricated listric wedges this event is thermally modeled in the southern apennines by the successive overthrust of two slabs of 7 km total thickness and explains the low heat flow density hfd 40 mw m 2 observed there in the northwestern apennines modeling is on the basis of the overthrusting of a slab 10 km thick which reduced the hfd of the steady lithosphere 63 mw m 2 observed on the iblean platform to 50 4 mw m 2 from the late miocene onwards tensional tectonism developed in the internal areas of the chain application of mckenzie s simple stretching model shows that the high regional heat flow 100 mw m 2 obtaining by filtering the observed data is explained by a stretching of the lithosphere of a factor 3 1

extension uplift of rocks and cooling ages in thinned crustal provinces the larderello geothermal area inner northern apennines italy in extensional tectonic settings vertical movements of the earth s surface are largely controlled by rates of strain extensional strain rates within the larderello geothermal area located in inner parts of the northern apennines have been evaluated together with uplift rates and 40 ar 39 ar cooling ages results indicate that extensional strain rates have been relatively low 5 8 10 17 s 1 since the middle pliocene differential uplift rates are estimated to have been high c 0 2 mm year strain and uplift rates within the larderello region are consistent with the regional tectonic framework the low strain rate rapid uplifting and high heat flow within southern tuscany can be explained by an eastward dipping lithospheric shear maintained since at least middle pliocene times

on ferroan a type granitoids their compositional variability and modes of origin we recognize eight types of ferroan granitoid that can be distinguished on the basis of major element chemistry these include alkalic granitoids that may be metaluminous or peralkaline alkali calcic granitoids that may be metaluminous peraluminous or peralkaline calc alkalic granitoids that may be metaluminous or peraluminous and rare calcic ferroan granitoids these granitoids may form by two distinct end member processes extreme differentiation of basaltic melts results in ferroan granitoids that are either peralkaline alkalic and alkali calcic or metaluminous alkalic alkali calcic and calc alkalic with alkalinity increasing with increasing pressure of differentiation partial melting of tonalitic to granodioritic crust produces alkali calcic to calc alkalic granitoids that are metaluminous at low pressures and peraluminous at high pressures it is likely that a combination of these two processes plays some role in the formation of most ferroan granitoids most granitoids that are referred to as ferroan however the term has become more confusing than edifying because it has been applied to a broad spectrum of granitoid compositions with varying petrogenesis for this reason we recommend that the term be discontinued and ferroan used in its stead

tectonic significance of late ordovician silicic magmatism avalon terrane northern antigonish highlands nova scotia12 1 this article is one of a series of papers published in cjes special issue in honour of ward neale on the theme of appalachian and grenvillian geology 2 contribution to international geological correlation programme igcp project 497 avalonia was a microcontinent during most of the ordovician separating the iapetus ocean to the north from the rheic ocean to the south in the northern antigonish highlands nova scotia volcanic rocks dunn point and mcgillivray brook formations underlie early silurian early devonian strata arisaig group and were thought to represent extension that heralded the development of the basin into which arisaig group strata were deposited however recent u pb zircon thermal ionization mass spectrometry data from rhyolite in the dunn point formation dpf yielded an age of 460 0 3 4 ma and here we report a concordant age of 454 5 0 7 ma for an ignimbrite in the overlying mcgillivray brook formation mbf these data confirm a ca 10 million year gap between volcanism and onset of arisaig group deposition which occurred after accretion of avalonia to baltica the dpf and mbf both resemble a type sio2 rich magmas the mbf has very high concentrations of zr 745 1965 ppm y 65 213 ppm nb 57

chemical and isotopic composition of the lower crust beneath the meguma lithotectonic zone nova scotia evidence from granulite facies xenoliths granulite facies xenoliths from an upper devonian lamprophyre dyke near tangier nova scotia provide new information about the lower crust in the meguma zone two mineralogically and chemically distinet groups of xenoliths occur both groups contain quartz feldspar biotite fe ti oxides sulfides in addition group a contains garnet graphite aluminosilicates spinel sapphirine hight al2o3 subgroups a1 and a2 or clinopyroxene sphene apatite high cao sub group a3 group b has highly variable proportions of orthopyroxene b1 clinopyroxene b2 and amphibole b3 trace element contents of the highly aluminous xenoliths compare closely with average to upper crustal model compositions and are similar in many aspects to other undepleted granulite facies rocks lowp t sedimentary assemblages of quartz chlorite clay minerals calcite albite or paragonite can account for the compositions of group a xenoliths within group b a high mgo mgo 13 wt subgroup with high transition metal contents and low mgo mgo 9 wt sub groups with higher lil large ion lithophile element contents exist although the rare earth and high field strength elements indicate a tholeiitic or low k calc alkaline chemistry the lil elements are as high as those from high k calc alkaline volcanic are rocks isotopically group a ranges from endt 2 56 to 0 80 and87sr 86srt 0 7046 to 0 7182 fort 370 ma for group b these values are 1 45 to 5 33 and 0 7028 to 0 7048 respectively model ages tchur are correspondingly low and are tightly constrained 544 52 ma these young ages contrast with the middle proterozoic nd model ages of the overlying meguma zone low grade flysch this time inverted stratigraphy appears to be the product of a tectonic break between a younger autochthonous tangier lower crust avalon and an older allochthonous meguma group upper crust

neoproterozoic early devonian magmatism in the antigonish highlands avalon terrane nova scotia tracking the evolution of the mantle and crustal sources during the evolution of the rheic ocean abstract the antigonish highlands lies in the avalon terrane avalonia in the canadian appalachians and records four episodes of continental rift related bimodal magmatism ranging in age from late neoproterozoic to middle devonian the oldest episode ca 615 ma clydesdale formation was emplaced in a local rift setting in an andean style arc when avalonia resided along the northern gondwanan margin early cambrian magmatism arbuckle brook formation occurred in a local transtensional setting after arc magmatism had given way to a san andreas type transform environment middle ordovician dunn point and bears brook formations magmatism occurred in a local rift in an ensialic island arc setting analogous to the modern taupo volcanic zone in northern new zealand after avalonia had separated from gondwana middle devonian mcarras brook formation and correlatives occurred after avalonia had collided with laurentia in each episode mafic rocks are characterized by high feot feot mgo tio2 zr y and ti y typical of differentiated within plate mafic magmas multi element spider diagrams indicate that all four episodes were generated in an enriched sub continental lithospheric mantle lil hfs and ree patterns indicate that the clydesdale dunn point bears brook and mcarras brook formations were sourced in the shallow spinel lherzolite mantle and that the arbuckle brook formation was generated in the deeper garnet lherzolite mantle in each episode coeval felsic rocks were generated by crustal anatexis taken together the sm nd isotopic data for the mafic rocks form an envelope that defines the evolution of the mantle source beneath the antigonish highlands that was enriched between 0 8 and 1 1 ga and has an average sm nd ratio of ca 0 24 a value that is typical of an enriched mantle source sm nd isotopic data for the felsic rocks are characterized by a recurrence of tdm ages between 0 95 and 1 0 ga interpreted to reflect repeated melting of the lower crust that was itself derived from a depleted mantle source between 0 95 and 1 1 ga the remarkable geochemical and isotopic similarity of the neoproterozoic to middle devonian mafic and felsic magmas indicate that the crust and sub continental lithospheric mantle beneath the antigonish highlands were coupled during the rift and drift of avalonia from the gondwanan margin during the formation of the rheic ocean in the ordovician its accretion to baltica and to laurentia in the silurian and post accretionary strike slip movement of avalon along the laurentian margin although the sm nd isotopic data suggest that avalonian basement and lithospheric mantle formed over the same time interval 0 8 to 1 1 ga the divergence of their respective envelopes with time reflects their differing end values and sm nd ratios we suggest that avalonian crustal basement and its enriched mantle were both formed between 0 8 and 1 1 ga in a panthalassa type ocean that surrounded the supercontinent rodinia and were accreted to the northern gondwanan margin at about 650 ma prior to the oldest magmatic event in the antigonish highlands

an autochthonous avalonian basement source for the latest ordovician brenton pluton in the meguma terrane of nova scotia u pb hf isotopic constraints and paleogeographic implications the ediacaran ordovician meguma supergroup was thrust over avalonia basement prior to the intrusion of post acadian ca 370 ma s type granitic batholiths this has led to two main hypotheses regarding the original location of the meguma terrane a continental rise prism bordering either nw africa or avalonia on the other hand the pre acadian ca 440 ma brenton pluton has yielded the following u pb la icp ms zircon data 1 448 3 ma population peak inferred to be the intrusive age and 2 ca 550 and 700 ma inherited ages common to both avalonia and nw africa in contrast hf isotopic analyses of zircon yielded model ages ranging from 814 to 1127 ma with most between 940 and 1040 ma such ages are typical of avalonia and not nw africa the ages of the inherited zircons found within the brenton pluton suggest that it was probably derived by partial melting of sub meguma mid crustal avalonian rocks upon which the meguma supergroup was deposited although avalonia is commonly included in the peri gondwanan terranes off nw africa or amazonia paleomagnetic data faunal provinciality and hf data suggest that during the ediacaran early cambrian it was an island chain lying near the tropics ca 20 30 s and was possibly a continuation of the bolshezemel volcanic arc accreted to northern baltica during the ediacaran timanide orogenesis this is consistent with the similar derital zircon population in the ediacaran cambrian meguma supergroup and the dividal group in northeastern baltica

truncation and translation of appalachian promontories mid paleozoic strike slip tectonics and basin initiation accreted terranes of the appalachian iapetan and peri gondwanan realms display structural trends that are mainly concordant with promontories and embayments in the laurentian margin indicating that during accretion the shape of the continental margin acted as a template around which accreted terranes were molded in north carolina and new foundland post accretion transcurrent motion appears to be recorded by displaced outboard portions of promontories no longer concordant with those in laurentia a bend in structural trends confined to the peri gondwanan realm at the north carolina south carolina state line is interpreted to represent the dextrally displaced outboard portion of the virginia promontory in newfoundland the hermitage flexure is interpreted as a dextrally displaced laurentian promontory that originated along strike to the northeast in both places promontories were truncated and dextrally translated for 220 250 km by a devonian mississippian orogen parallel transcurrent system which may well have extended for the length of the eastern laurentian margin south of the nearby st lawrence promontory extreme thinning of appalachian crust beneath the maritimes basin is consistent with extension at a releasing stepover estimates of extension are consistent with those obtained from promontory offsets

tectonics of the punta bianca promontory insights for the evolution of the northern apennines northern tyrrhenian sea basin the structural setting of the eastern headland of the la spezia gulf punta bianca promontory provides some constraints on the contractional and extensional framework of the northern apennines northern tyrrhenian sea basin geodynamic system and allows a detailed reconstruction of a complex evolutionary path out of sequence thrusting is supposed in the inner side of the chain two different extensional events are identified the older early middle miocene may be related to the imbrication of the deeper metamorphic units of the apenninic thrust wedge younger extension fits very well into an eastward asymmetrical faulting framework related to the opening of the northern tyrrhenian sea basin from the late miocene to the pliocene pleistocene well exposed low angle normal faults developed during this event extensional structures were then overprinted and reactivated by a mainly strike slip regime in the quaternary

postorogenic alkali feldspar granite and associated pegmatites in west avalonia the petrology of the neoproterozoic georgeville pluton antigonish highlands nova scotia the 579 8 2 2 ma 40ar 39ar muscovite georgeville pluton in mainland nova scotia is an epizonal body consisting of alkali feldspar granite and related pegmatite the pluton intrudes the ca 619

palaeozoic within plate volcanic rocks in nova scotia canada reinterpreted isotopic constraints on magmatic source and palaeocontinental reconstructions palaeozoic volcanism in the avalon terrane of northern nova scotia occurred during three time intervals cambrian early ordovician late ordovician early silurian and middle late devonian in the meguma terrane of southern nova scotia palaeozoic volcanism is limited to the middle ordovician geochemical data show that most of these volcanic rocks are bimodal within plate suites initial e nd signa tures range from 5 4 to 1 9 in the rhyolites and 6 8 to 2 7 in the basalts a difference attributable to the absence or presence respectively of a significant crustal component the data and regional tectonic settings of the avalon and meguma terranes suggest that the volcanism was generated in three different within plate settings 1 cambrian early ordovician volcanism related to thermal decay of late proterozoic arc magma tism during transtensional deformation 2 middle ordovician early silurian volcanism during sinistral tele scoping between laurentia and gondwana where extensional bends in the appalachians produced rifting and 3 devonian volcanism resulting from lithospheric delamination during dextral transpression and tele scoping in each setting active faults served as conduits for the magmas nd isotopic data indicate that the source of the palaeozoic felsic volcanic rocks is isotopically indistinguishable beneath southern and northern nova scotia and did not substantially change with time this crustal source appears to have separated from the mantle during the proterozoic a conclusion consistent with the hypothesis that the palaeozoic rocks in nova scotia were deposited upon a late proterozoic oceanic cratonic volcanic arc terrane the nd data when combined with published faunal palaeomagnetic and u pb isotopic data suggest that the avalon terrane was peripheral to gondwana off northwestern south america during neoproterozoic and early palaeozoic times

the acadian fold belt in the meguma terrane nova scotia cross sections fold mechanisms and tectonic implications 1 structural data and cross sections from the eastern part of the devonian carboniferous fold belt of the meguma terrane give insight into its structural development and crustal tectonics the sandy goldenville formation the active competent member during buckling formed a multilayer several kilometers thick that lay beneath the denser softer muds and silts of the halifax formation cross sections reveal 2 orders of folds 11 18 km wavelength folds are interpreted as resulting from buckling under the influence of gravity 4 6 km wavelength folds reflect the thickness of the multilayer with buckle shortening in the range 32 44 depth to detachment derived from the sections 11 13 5 km below present erosion surface is such that given the depth of the detachment below the paleosurface during the main phase pre devonian granite of folding it would likely have been a relatively soft and thick shear zone that would have influenced profile geometry of the main phase folds at this stage of fold belt development granite production that may have been triggered by mantle delamination during continuing convergence accompanying terminal pangean assembly and ocean basin closure was followed by rapid exhumation later localized deformation late devonian carboniferous in the fold belt took place as the meguma terrane was displaced along the terrane bounding cobequid chedabucto fault system the characteristic asymmetric cross sections of structures of this phase are attributed to cooling and strengthening of the detachment that was a consequence of the postgranite uplift and exhumation

zircon u pb and hf isotopic constraints from eastern transhimalayan batholiths on the precollisional magmatic and tectonic evolution in southern tibet abstract before the indian collision with asia northward subduction of the neo tethyan oceanic lithosphere produced an andean type convergent margin in south asia characterized by arc magmatism starting from the early jurassic and lasting until the eocene the magmatic arc is now represented by widespread intrusive bodies or the so called transhimalayan batholiths in the lhasa terrane of southern tibet that have been divided into two main magmatic suites i e the northern plutonic belt and the southern gangdese batholith their temporal distribution concerning how exactly the magmatic suites correlate eastwards and then southeastwards around the eastern himalayan syntaxis however remains poorly constrained here we report the first combined zircon u pb and hf isotopic study of the transhimalayan batholiths from the eastern part of the lhasa terrane 95 97 5 e and 28 5 30 n se tibet zircon u pb dating results of 24 granitoids indicate that the rocks were emplaced principally in the early cretaceous 133 110 ma and subordinately in the paleocene 66 57 ma long after an older granite intrusion in the earliest jurassic 198 ma the zircon ehf t values range from 5 to 20 yielding hf crustal model ages tdmc between 0 8 and 2 4 ga that peak at 1 7 ga suggesting a major episode of crustal growth in the proterozoic and a predominantly or even exclusively crustal source for the granitoid petrogenesis these zircon u pb and hf isotopic constraints together with whole rock geochemical characteristics allow us to correlate them to s type granitoids in the northern plutonic belt rather than to the i type gangdese batholith from the central part of the southern lhasa terrane in contrast to the gangdese magmatism that formed with a significant juvenile mantle input related to the neo tethyan subduction these s type granitoids show geochemical similarities to the north american cordilleran interior batholiths that did not form directly from a subduction setting thus we attribute the petrogenesis of the widespread early cretaceous granitoids in the northern belt and eastern himalayan batholiths to a postcollisional regime due to the late jurassic early cretaceous continental collision between the lhasa and qiangtang terranes under this framework the s type magmatism was generated in response to collision induced crustal thickening which may have capabilities of causing crustal anatexis by itself in the region however continued interplay with the neo tethyan subduction zone processes such as back arc extension could also have played a crucial and long lasting role in the magma generation

extensional shear zones as imaged by reflection seismic lines the larderello geothermal field central italy the larderello geothermal field is located in the inner northern apennines in an area which has been subject to extension since the early miocene the latest extensional episode pliocene present has resulted in the formation of nw trending nedipping listric normal faults whose geometry is controlled down to f3 km by borehole data in this paper we integrate a new interpretation of seismic reflection lines with existing seismic field and borehole data to analyse the relations among listric normal faults the top of the brittle ductile transition and the migration of geothermal fluids in accordance with previous interpretations we consider the strong reflector k horizon marking the top of the reflective mid lower crust and located at a depth of 3 5 km in the geothermal area to represent the top of the brittle ductile transition its reflectivity most probably derives from the presence of overpressured fluids we identify three main nw trending nedipping extensional brittle shear zones showing listric geometry and soling out in the vicinity of the k horizon the latter appears to be dislocated in correspondence of the soling out of the shear zones these shear zones because of the associated intense fracturing represent the most natural channels of upward migration of geothermal fluids from the magmatic sources located below the k horizon we suggest that these two conclusions that listric normal faults root at or near the brittle ductile transition and that they act as preferential upward migration paths for magmatic fluids may be of general validity for geothermal fields located in extensional settings d 2002 elsevier science b v all rights reserved

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

seismic imaging of extended crust with emphasis on the western united states understanding of the crust has improved dramatically following the application of seismic reflection and refraction techniques to studies of the deep crust this is particularly true in areas where the last tectonic event was extensional such as the basin and range province of the western united states and much of western europe in these regions a characteristic reflective pattern has emerged whereby the lower crust is highly reflective and the upper crust and upper mantle are either poorly reflective or strikingly nonreflective in the metamorphic core complex belt in the western united states where extension can be as much as an order of magnitude greater than in the more classic continental rift zones the lower crustal reflectivity thickens and rises yielding a picture of a crust that is reflective throughout synthetic seismic studies have documented that the reflectivity in these regions can be modeled by numerous laminae tens of meters thick and hundreds of meters across characterized by inter layered high and low velocities two geologic factors are interpreted as contributing to this layered character ductile strain responding to stress in the thermally weakened middle and lower crust and intrusive layering corresponding to injection of subhorizontal sheets of mantle derived magmas these two processes yield a variety of geologic structures including transposed compositional layering mylonitic ductile shear zones and intrusive mafic sheets all of which occur at the proper scales to cause the prominent reflectivity observed if metamorphic core complexes are representative of extended continental crust world wide then these results suggest that magmatism and ductile flow have also contributed to the evolution of the middle and lower crust in many other areas around the world

hydraulic characteristics of gabion stepped weirs experimental studies on the hydraulics of flow through and over gabion stepped weirs are presented two flow components were observed i e base flow through the void between filled stones and overflow on the gabions the energy loss ratios in the gabion stepped weirs are greater than those in the corresponding horizontal stepped weirs by approximately 7 10 and 14 for weir slopes of 30 45 and 60 respectively as a result the velocity at the outlet is lower stone size and shape have little influence on the energy loss and flow velocity as compared to the increasing effect of the weir slope the pressure acting on the step face for the gabion stepped weirs is less than that of the horizontal steps owing to the cushioning effect generated by filled stones to reduce pressure on a step face at a fixed discharge for different weir slopes the void ratio of the filled stones should be low to allow a small amount of base flow the pressure distribution pattern on the horizontal face of a step is provided

origin of topaz bearing and related peraluminous granites of the late devonian davis lake pluton nova scotia canada crystal versus fluid fractionation abstract the davis lake pluton in the late devonian south mountain batholith in southwestern nova scotia canada is composed of peraluminous leucomonzogranites and subordinate topaz bearing leucogranites that host greisens and a primary tin deposit the concentrations of incompatible trace elements e g rb li u f ta and sn and element e g k rb nb ta and la sm and isotopic ratios e g 206 pb 204 pb and 87 sr 86 sr display a spatial zonation the granitic rocks were generated by extensive fractional crystallization of major phases feldspars and biotite and accessory minerals monazite apatite and zircon in a compositionally zoned magma chamber fractional crystallization of the evolving f rich peraluminous granitic magma was accompanied particularly in later stages by fluid fractionation involving fluorine complexing fluid fractionation was a major differentiation process during the terminal stages of the pluton s evolution it played an important role in the genesis of the evolved rocks which are characterized by low k rb 24 150 nb ta 1 6 la sm 1 5 4 and eu eu 0 01 0 3 ratios these ratios as well as the enhanced concentrations of li rb u ta and sn in these rocks correlate with f fluorine rich fluids modified the rb sr and u pb ratios which produced the distinct variations of 87 sr 86 sr and 206 pb 204 pb isotopic ratios the parent magma was formed by dehydration melting of lower crustal metasedimentary rocks the melting was related to crustal thickening in association with terrane and collision along the eastern margin of the appalachians

contiguous rather than discrete paleozoic histories for the avalon and meguma terranes based on detrital zircon data upper ordovician lower devonian strata of the meguma terrane in the canadian appalachians contain zircon populations including an important mesoproterozoic zircon population 1 0 1 4 ga similar to those in coeval strata of avalonia and strongly suggest contiguous rather than discrete histories for these terranes throughout the paleozoic that these terranes were juxtaposed throughout the early paleozoic is indicated by the absence of a cambrian ordovician accretionary event the lack of intervening suture zone ophio litic units and the similarity of avalonian and meguma basement nd isotope signatures in early paleozoic igneous suites as avalonia had accreted to laurentia baltica by the early silurian these data suggest that the meguma terrane like avalonia resided along the same northern margin of the rheic ocean at that time these conclusions have implications for reconstructions of the northern gondwanan margin in the early paleozoic and imply that the silurian devonian acadian orogeny in maritime canada occurred in an andean type setting and was not related to collision of the meguma terrane with the laurentian margin

the deep crust of the southern rhine graben reflectivity and seismicity as images of dynamic processes abstract the rhine graben part of the european cenozoic rift system deserves special attention because of its location in the foreland of the alpine orogen the phanerozoic evolution of the lithosphere in this region is defined by a set of major geodynamic events ranging from the variscan orogeny late orogenic crustal re equilibration to the interference of rifting and alpine orogeny in tertiary times the rhine graben is one of the most detailedly studied continental grabens prospecting for hydrocarbons in its sedimentary fill and intensive pre site studies for the continental deep drilling program ktb on its eastern crystalline flank black forest provided a comprehensive set of geoscience data seismic investigations ranging from a deep seismic network of near vertical reflection to concurrent wide angle refraction experiments were accompanied by seismological and geological surveys therefore a direct observational comparison between the two depth definitions of crustal subdivisions elastic vs strength rheological was possible in large parts of western europe deep seismic sounding dss investigation distinguishes between a crystalline upper crust of comparatively low p wave velocities 5 9 6 0 km s with little and discrete reflectivity and a strongly reflective lower crust with higher p wave velocities about 6 4 6 8 km s both are separated by the conrad discontinuity in contrast strength and rheological considerations define a brittle upper crust and a ductile lower crust the boundary between these two realms is usually referred to as the brittle ductile transition zone which in seismically active regions is imaged by the deepest crustal hypocentres the present brittle ductile transition in the rhine graben region as defined by the deepest hypocentres does not necessarily coincide with the structural separation in an upper and lower crust as seen in deep seismic studies this suggests that the laminated lower crust developed during the crustal re equilibration phase at the end of the variscan orogeny the second part of this paper concentrates on evidence of recent tectonic activities related to rifting processes impacting on the variscan lower crust a remarkable concentration of deep crustal earthquakes reaching to the top of an asymmetrical thinned lower crust is observed at or near the eastern border fault of the southern rhine graben and is possibly induced by the young rifting process we suggest that this asymmetry could be a consequence of the uppermost mantle detaching from the upper crust in collision with the alpine orogen this allows us to propose the distinction between the influence of compression extension continental subduction and intra crustal detachment during graben formation the dynamic interference of rifting and foreland compression shown by structural and seismological observations leads to the notion of small scale collisional escape tectonics associated with an intra continental transform fault at the southern end of the rhine graben the rhine sao ne transform zone

crustal extension in the northern apennines the transition from compression to extension in the alpi apuane core complex an interpretation of northern apennine geology is presented which relates the temporal and spatial occurrence of both compressional and extensional deformation features in terms of the changing dynamic evolution within an accretionary wedge after a model proposed by platt 1986 followed by the initiation and development of continental rifting during cretaceous to eocene time an accretionary wedge formed as remnant tethyan oceanic crust subducted beneath the rotating corsica sardinia microplate microplate collision during the oligocene was characterized by the rapid imbrication of buoyant continental crust of the italian continental margin the record of which is preserved within the duplex structure geometry of the alpi apuane region the overthickened wedge geometry returned to a more stable configuration by developing extensional features during the miocene both listric normal faults at upper crustal levels and shear zones indicating evidence of distributed ductile extensional strain at mid crustal levels are recorded it is proposed that large scale regional extension with associated volcanism beginning in the messinian was represented by the intrusion of asthenospheric material from the subducted plate into the already attenuated accretionary complex further rifting perhaps aided by subduction and back arc processes in the southern apennines led to the formation of the tyrrhenian sea as an oceanic basin both the apennines and north american core complexes record evidence of crustal thickening followed by crustal thinning and finally of continental rifting this suggests that the similar histories of these regions with vastly different plate tectonic settings may both be explained by processes linked to the changing internal dynamics of accretionary wedges

middle to late paleozoic acadian orogeny in the northern appalachians a laramide style plume modified orogeny the laramide orogeny of the western united states is proposed as a modern analogue for the silurian devonian acadian orogeny and subsequent diachronous voluminous short lived magmatism and basin formation in the northern appalachians shallowing of the benioff zone accounts for several enigmatic features associated with plate convergence in the northern appalachians including 1 wenlockian ludlovian termination of arc related magmatism in the avalon terrane followed by a period of relative magmatic quiescence from 395 to 380 ma and 2 diachronous migration of the acadian deformation front from ca 415 ma in the southeast to ca 370 ma in the northwest extending more than 600 km into the continental interior the flattening of the subduction zone is attributed to overriding of a plume by the convergent margin which may explain 1 the abrupt termination of magmatic quiescence by 380 370 ma and voluminous felsic magmatism and production of plume related lamprophyres in the southeast meguma terrane as the plume thermally eroded the oceanic lithosphere causing melting of the lower crust 2 late devonian regional high temperature low pressure metamorphism in the meguma terrane related to the thermal anomalies above a plume 3 synchronous devonian emplacement of meguma gold deposits and associated siderophile elements possibly derived from fluid circulation above an ascending plume 4 rapid late devonian uplift and erosion of as much as 10 km due to dynamic uplift over a plume 5 migration of magmatism to the north avalon terrane cobequid highlands so that plume related carboniferous magmatism occurred in and around the carboniferous permian maritimes basin 6 the high density lens at the base of the crust beneath the maritimes basin the product of plume derived underplated mafic rocks and 7 a subsidence mechanism for formation of the maritimes basin by cooling of a decapitated plume head

cambrian successions of the meguma terrane nova scotia and harlech dome north wales dispersed fragments of a peri gondwanan basin abstract the meguma terrane of nova scotia and the harlech dome of north wales preserve similar sedimentary successions of cambrian age both successions comprise a thick succession of early cambrian sandstone turbidites overlain by early to middle cambrian alternating mud rich and sand rich units in which manganese is concentrated in two stratigraphic intervals above these both successions comprise anoxic organic rich turbidites shallowing upward into paler more bioturbated tremadocian mudstone with rhabdinopora within the limited constraints of the available biostratigraphic and geochronological data major changes in environment occurred synchronously in the two successions both successions show much greater similarity to each other than to adjacent successions on avalonia9 a detrital zircon analysis from the rhinog formation low in the harlech dome succession reveals distinct clusters of ages around 537 ma and 2 0 2 1 ga a close similarity to analyses from the meguma terrane suggests proximity between the two terranes on the margin of gondwana during the cambrian period we suggest the term megumia for the palaeogeographical domain that included the two successions which was dispersed during subsequent appalachian and caledonian movements these observations suggest that megumia may have separated terranes previously regarded as parts of avalonia supplementary material u pb detrital zircon analytical data are available at http www geolsoc org uk sup18431

oxidized magnetite series rapakivi type granites of carajas brazil implications for classification and petrogenesis of a type granites abstract the varying geochemical and petrogenetic nature of a type granites is a controversial issue the oxidized magnetite series a type granites defined by anderson and bender anderson j l bender e e 1989 nature and origin of proterozoic a type granitic magmatism in the southwestern united states of america lithos 23 19 52 are the most problematic as they do not strictly follow the original definition of a type granites and approach calc alkaline and i type granites in some aspects the oxidized jamon suite a type granites of the carajas province of the amazonian craton are compared with the magnetite series granites of laurentia and other representative a type granites including finnish rapakivi and lachlan fold belt a type granites as well as with calc alkaline i type orogenic granites the geochemistry and petrogenesis of different groups of a types granites are discussed with an emphasis on oxidized a type granites in order to define their geochemical signatures and to clarify the processes involved in their petrogenesis oxidized a type granites are clearly distinguished from calc alkaline cordilleran granites not only regarding trace element composition as previously demonstrated but also in their major element geochemistry oxidized a type granites have high whole rock feo t feo t mgo tio 2 mgo and k 2 o na 2 o and low al 2 o 3 and cao compared to calc alkaline granites the contrast of al 2 o 3 contents in these two granite groups is remarkable the cao feo t mgo tio 2 vs cao al 2 o 3 and cao feo t mgo tio 2 vs al 2 o 3 diagrams are proposed to distinguish a type and calc alkaline granites whole rock feo t feo t mgo and the feo t feo t mgo vs al 2 o 3 and feo t feo t mgo vs al 2 o 3 k 2 o na 2 o diagrams are suggested for discrimination of oxidized and reduced a type granites experimental data indicate that besides pressure the nature of a type granites is dependent of o 2 conditions and the water content of magma sources oxidized a type magmas are considered to be derived from melts with appreciable water contents 4 wt originating from lower crustal quartz feldspathic igneous sources under oxidizing conditions and which had clinopyroxene as an important residual phase reduced a type granites may be derived from quartz feldspathic igneous sources with a metasedimentary component or alternatively from differentiated tholeiitic sources the imprint of the different magma sources is largely responsible for the geochemical and petrological contrasts between distinct a type granite groups assuming conditions near the nno buffer as a minimum for oxidized granites magnetite bearing granites formed near fmq buffer conditions are not stricto sensu oxidized granites and a correspondence between oxidized and reduced a type granites and respectively magnetite series and ilmenite series granites is not always observed

deep seismic reflection data from the bay of fundy and gulf of maine tectonic implications for the northern appalachians three deep penetration seismic reflection profiles were collected off southwest nova scotia to determine the crustal structure and geometry beneath the avalon and meguma zones of the appalachian orogen in canada onshore geological features have been traced seawards using new gravity and magnetic anomaly maps the seismic data can also be correlated with the previous united states geological survey profile in the central gulf of maine two seismically distinct lower crustal blocks are identified the avalon and sable lower crustal blocks separated by a major north dipping reflection zone that cuts the entire crust the recognition of the sable block adds a fourth block to the three already identified in the canadian appalachians the sable block is overlain by the meguma zone the avalon zone overlies at least the northern part of the avalon lower crustal block although offshore extension of geological features is not unequivocal it appears that a north dipping reflection zone southwest of nova scotia m

age of detrital zircon and titanite in the meguma group southern nova scotia canada clues to the origin of the meguma terrane abstract precise u pb isotopic analysis of seventeen selected high quality laboratory abraded zircon single grains seven multigrain fractions of similar material and four paramagnetic titanite fractions record the ages of the various detrital grain types from two turbidite samples collected from the top and about 5 km below the top of the goldenville formation of the meguma group the results show that the source of both sediments contained 600 ma and 2000 ma old volcanic plutonic components whereas a third 3000 ma old component occurs exclusively in the upper sample and the youngest zircons and titanite are found in the lower sample lead lead ages of bulk fractions at 2300 ma and two nearly concordant single grains at 2040 ma and 2140 ma upper sample together with 2157 ma bulk fraction and 2060 ma and 2080 ma single grain fractions lower sample indicate that the 2000 ma component dominates the pink purple euhedral grain types detrital titanite gives ages of 605 ma upper sample and 552 ma lower sample with the latter sample containing a minor 1975 ma component colourless zircons record this same age difference the lower sample contained the youngest concordant single zircon grain 566 ma and 580 605 ma bulk grain fraction even though two concordant single grains have ages of 632 and 643 ma estimated bulk fraction ages for certain zircon grain types from the upper sample fall in the range of 605 to 675 ma these data suggest that while the youngest rocks in the source region were removed by erosion during the early stages of deposition of the goldenville formation the oldest source rocks were not unroofed until later in the depositional history the 566 8 ma age of the youngest concordant zircon from the stratigraphically lower sample provides a maximum age for the deposition of this grain with the base of the cambrian placed between 545 and 530 ma the goldenville formation may straddle the precambrian cambrian boundary graptolites from the immediately overlying halifax formation are of early ordovician tremadocian age the lower sample contains spherical zircons that may have been derived from mafic granulite with an estimated age of 2000 2200 ma with this one possible exception all of the zircons analyzed appear to be igneous in origin and all are relatively euhedral indicating that rocks of the ages referred to above probably outcrop in the source area anomalous discordance patterns imply that part of the 2960 ma area was metamorphosed by the 2000 ma event and that both the 2960 and 2000 ma regions were metamorphosed during the 600 ma event the complete lack of detritus with ages between 680 and 2040 ma virtually eliminates a northern north american or baltic shield provenance however the available data are not sufficient to distinguish between a west african or guyana shield source