model for the evolution of the avalonian cadomian belt the inception and termination of the ca 670 550 ma avalonian cadomian orogenic cycle are interpreted in terms of the amalgamation and breakup of a late precambrian supercontinent synorogenic arc related volcano sedimentary and granitoid rocks are attributed to oblique southward directed subduction along the margin of the supercontinent after its amalgamation breakup of the supercontinent and inception of the iapetus cycle led to termination of subduction and its replacement by transform activity the resulting closure of late precambrian arc related basins and the development of early cambrian platform successions with acado baltic fauna accompanied the formation of iapetus but took place on the active margin of a preexisting ocean and do not record the evolution of its rifted european flank

revision of the marine chronology in the wanganui basin new zealand based on the isothermal plateau fission track dating of tephra horizons abstract the occurrence of tephra horizons in basins adjacent to volcanic arcs provide an excellent opportunity for establishing a reliable chronostratigraphic framework for detailed sedimentological studies in this study three widespread and stratigraphically important rhyolitic tephra horizons interbedded in plio pleistocene strata of the wanganui basin new zealand are dated by application of the isothermal plateau fission track itpft technique to hydrated glass shards all glass samples were corrected for annealing and consequently yield reliable ages rangitawa tephra yielded statistically indistinguishable ages from three localities that are in excellent agreement with recently determined zircon fission track age estimates of ca 0 35 ma itpft ages of 1 05 0 05 and 1 63 0 15 ma for potaka pumice and pakihikura pumice respectively are considerably older than previous ft estimates but consistent with new magnetostratigraphic data that places the potaka within the jaramillo subchron and pakihikura within the matuyama chron between the cobb mountain and olduvai subchrons combining our fission track ages with the magnetostratigraphy the true age of sediments within the wanganui basin is found to be significantly underestimated sedimentation rates of between ca 680 630 m ma from 1 63 ma to 0 35 ma are calculated in the eastern part of the basin and are much lower than those calculated using the previous ft chronology this new itpft age data demonstrates that the existing plio pleistocene marine chronology in new zealand will require age revision and has important implications when considering the evolution of several other sedimentary basins in southern north island that contain the same itpft dated tephra horizons

a precise late neoproterozoic u pb zircon age for the syntectonic perelle quartz diorite guernsey channel islands uk a precise u pb zircon age of 611 4 2 0 1 1 ma has been determined for the foliated quartz diorite at perelle bay guernsey uk channel islands the age is based on the analysis of single zircons half of a single grain and groups of two and three zircons the age is nearly 100 ma younger than that suggested by other workers based on 207 pb 206 pb dates of large multi grain fractions of zircon and thus rules out this intrusion as the earliest marker of the onset of cadomian magmatism in the north armorican massif a 611 ma crystallization age for the perelle quartz diorite is consistent with previous detailed fabric development studies that suggested magmatic fabrics within the pluton are largely overprinted by similarly oriented solid state deformation fabrics resulting from syntectonic emplacement during cadomian orogenesis the u pb age is approximately 8 15 ma older than 40 ar 39 ar hornblende dates for the pluton consistent with the emplacement of the intrusion followed by slow cooling in a region of moderate to high ambient crustal temperatures these new data emphasize the fact that periods of pluton emplacement are considerably briefer than periods of deformation and thus syntectonic intrusions do not date regional deformational events but only mark events within a period of regional tectonism the ability to analyse single zircons and selected portions of single zircons using ultra low blank u pb analytical techniques documents the feasibility of determining precise emplacement ages of other neoproterozoic granitoids that contain a significant percentage of xenocrystic zircon

old crow tephra a new late pleistocene stratigraphic marker across north central alaska and western yukon territory old crow tephra is the first extensive pleistocene tephra unit to be documented in the northwestern part of north america it has a calc alkaline dacitic composition with abundant pyroxene plagioclase and fe ti oxides and minor hornblende biotite apatite and zircon thin clear bubble wall fragments are the dominant type of glass shard this tephra can be recognized by its glass and phenocryst compositions as determined by x ray fluorescence microprobe and instrumental neutron activation techniques it has an age between the limits of 60 000 and 120 000 yr set by 14c and fission track measurements respectively old crow tephra has been recognized in the koyukuk basin and fairbanks region of alaska and in the old crow lowlands of the northern yukon territory some 600 km to the east northeast the source vent is unknown but these occurrences considered in relation to the distant locations of potential quaternary volcanic sources demonstrate the widespread distribution of this tephra and underscore its importance as a regional stratigraphic marker

multi method dating comparison for mid pleistocene rangitawa tephra new zealand abstract the rangitawa tephra a widespread mid pleistocene marker in the new zealand region is dated by fission track thermoluminescence tl infrared stimulated luminescence irsl k ar and single crystal laser fusion sclf 40 ar 39 ar methods as well as by matching to the astronomical timescale via oxygen isotope stratigraphy in deep sea cores fifty one independent numerical age estimates for rangitawa tephra range from 173 to 456 ka however only 15 of these results are considered to be reliable five zircon fission track results four isothermal plateau fission track itpft age estimates on glass shards four sclf 40 ar 39 ar results and two astronomically calibrated age estimates from deep sea cores the weighted mean of the nine reliable fission track age determinations is 345 12 ka in excellent agreement with the weighted mean of 340 7 ka for the two reliable astronomically calibrated age estimates the weighted mean of the four reliable 40 ar 39 ar results 302 8 ka is significantly younger and further ar ar dating is recommended to investigate this difference since the tl and irsl dating methods are experimental for sediments beyond 200 ka we have excluded these from the group of reliable age results tl and irsl results on dunesand appear to be systematically younger than tl results on loess and loess results depend upon analytical procedures

integrated tephrochronology and magnetostratigraphy for cyclothemic marine strata wanganui basin implications for the pliocene pleistocene boundary in new zealand abstract the middle pliocene to earliest pleistocene part of the rangitikei river section in wanganui basin contains at least six distal tephras five are rhyolitic kowhai eagle hill ohingaiti waipura and vinegar hill tephras and one is andesitic mangamako tephra all tephras occur as macroscopic layers and are enclosed within cyclothemic marine sediments that accumulated in shoreface to outer shelf water depths under conditions of oscillating sea level driven by glacio eustacy the glass and ferromagnesian compositions of the silicic tephras imply derivation from calc alkaline rhyolitic sources either or both of taupo volcanic zone and southern coromandel volcanic zone the waipuru and vinegar hill tephras have isothermal plateau fission track itpft ages of 1 87 0 15 ma and 1 75 0 13 ma respectively paleomagnetic results allow identification of i the gauss matuyama boundary located ca 70 m below the top of the mangaweka mudstone and ii the top of the olduvai subchron placed at the base of the waipuru shellbed the itpft ages of the tephras are consistent with the interpreted magnetostratigraphy and also with astronomically tuned timescales correlation of the rangitikei sequences to the odp site 846 18o record indicates that each sequence corresponds to a glacial interglacial isotope stage couplet and allows ages to be interpolated for tephras not dated directly integration of the new ages for the waipuru and vinegar hill tephras the magnetostratigraphy and the sequence correlations to the astronomically tuned oxygen isotope timescale lead us to place the pliocene pleistocene boundary as defined at the vrica stratotype at the base of the highstand systems tract siltstone of sequence 17 in the rangitikei section 60 m below the vinegar hill tephra

age of amphibolitic metamorphism in the ophiolitic unit of the morais allochthon portugal implications for early hercynian orogenesis in the iberian massif structurally lower portions of the allochthonous morais complex are comprised of an ophiolitic unit exposed in two contrasting structural units separated by a ductile thrust fault the lower unit is dominated by variably foliated amphibolite whereas the upper unit is largely comprised of metaperidotite and metagabbro most of the ophiolitic unit has recrystallized synkinematically under amphibolite facies metamorphic conditions c 500 c and 5 kbar amphiboles formed during this metamorphism are typically magnesio hornblende an amphibole concentrate from the lower structural unit records a 39 ar 40 ar plateau age of 397 3 4 ma an amphibole concentrate from the upper structural unit displays an internally discordant 39 ar 40 ar age spectrum corresponding to a total gas age of 431 1 27 3 ma gas fractions evolved from each amphibole concentrate at intermediate and high experimental temperatures are characterized by similar apparent k ca ratios and yield well defined 36 ar 40 ar v 39 ar 40 ar isotope correlations which result in similar ages of 384 2 5 2 and 392 4 7 3 ma these are considered to be more reliable than the plateau ages and are interpreted as dating the last cooling through those temperatures required for intracrystalline retention of argon within constituent amphibole grains the associated tectonothermal event represented a first stage of hercynian orogenesis in the iberian massif it was probably related to oceanic closure and associated obduction of granulitic eclogitic massifs which at present structurally overlie the ophiolitic unit

the application of icp ms methods to tephrochronological problems pearce nicholas westgate j a perkins w t preece s j 2004 the application of icp ms methods to tephrochronological problems applied gechemistry 19 3 pp 289 322 rae2008

anti atlas morocco role in neoproterozoic western gondwana reconstruction abstract reconstruction of latest neoproterozoic gondwana hinges on the interpretation of the subduction and collision kinematics of pan african orogenic belts that rim the west african craton the anti atlas suture zone of southern morocco has presented an enigma in this reconstruction as the inferred subduction zone polarity and age of suturing appear to be incongruous with better known west african orogens to the west mauretanian bassaride and rokelide and transaharan orogens to the east ougarta tuareg gourma and dahomeyan contrary to previous interpretations new data from the anti atlas indicate a history of late neoproterozoic 750 600 north dipping subduction culminating in the 600 ma collision of the saghro magmatic arc to the north with the north facing rifted margin of the west african craton thus the anti atlas suture links a 6000 km long chain of pan african suture zones that essentially encircle the west african craton the suture zones demarcate the former position of subduction zones that in all cases dipped away from the west african craton the anti atlas suture links the western and eastern segments of the pan african orogenic belts associated with the amalgamation of western gondwana

sm nd isotope characteristics of late cadomian granite magmatism in northern france and the channel islands sm nd isotopic studies of granites within the late precambrian cadomian orogenic belt of the north armorican massif northwestern france and channel islands reveal differences between arc related granite magmatism in outboard terranes and intracrustal granite magmatism in inboard terranes late cadomian c 570 ma arc related granitoids exhibit a range of e nd 2 to 6 and nd model ages t dm 1 0 1 3 ga reflecting variable contamination between late precambrian mantle derived magmas and ancient c 2 0 ga continental crust the contamination did not involve exposed granitic icartian basement to anygreat degree a more likely contaminant being unexposed lower crust of intermediate to acidic granulitic composition or early cadomian plutons which were themselves contaminated by lower crust voluminous granites of the mancellian region c 550 540 ma share common isotopic characteristics e nd 4 to 7 t dm 1 5 1 7 ga with migmatites and anatectic granites produced by partial melting of metasedimentary sequences within the st malo region consistent with a common source

the cadomian orogeny in the north armorican massif a brief review the north armorican massif nam of nw france is the type locality of the late precambrian cadomian orogenic belt the nam largely escaped reworking during the upper palaeozoic variscan cycle with the result that earlier geological relationships are commonly preserved intact a large part of the cadomian belt of the nam comprises variably deformed and metamorphosed supracrustal rocks of the brioverian succession barrois 1895 1896 1908 graindor 1957 cogne 1962 le corre 1977 rabu et al 1982 1983 chantraine et al 1982 the cadomian orogeny which takes its name from cadomus caen in normandy was defined by bertrand 1921 as the late precambrian orogeny which in the nam resulted in the folding and uplift of the brioverian succession prior to the deposition of lower palaeozoic red bed sequences events which have been ascribed to the cadomian orogeny span the period c 700 425 ma during which time the brioverian was heterogeneously deformed and metamorphosed and intruded by calc alkaline igneous complexes the evolution of the belt has been interpreted in terms of the gradual cratonization of calc alkaline arc complexes and syn orogenic sediments in an andean type continental margin brown et al 1990 dupret et al 1990 this contribution reviews briefly the geology of the cadomian belt of the nam fig 1 and outlines some of the more controversial debates concerning the distribution and status of precadomian basement the stratigraphy of the brioverian succession the nature of cadomian magmatism the kinematic significance of major structures and the geotectonic setting of the component parts of the belt

39ar recoil artifacts in chloritized biotite biotite separates from the metamorphosed granitic rocks of eastern taiwan yield disturbed 40ar39ar age spectra their disturbed profiles become more pronounced with increasing degree of chloritization and are attributed to mixed release of argon from sub micron scale biotite and chlorite intercalations 39ar internal recoil best explains the discordance of the age spectra chlorite is randomly intercalated with biotite at scales ranging from 10 a to several tens of microns both 40ar and k were evidently lost during the chloritization reaction during the neutron irradiation 39ar atoms recoil into the chlorite from the adjacent biotite intercalations the recoil and chloritization combine to produce a variable 40ar39ar spatial distribution in the intercalated biotite chlorite during 40ar39ar step heating analyses argon in biotite is released in two pulses centered at 650 c and 950 1050 c which represent two stages of dehydroxylation of the biotite chlorite out gasses in two pulses centered at 550 c and 700 900 c during dehydroxylation and then decomposes this differential thermal release of argon from each mineral phase with variable 40ar39ar ratios yields discordant incremental heating age spectra

fission track ages of late cenozoic distal tephra beds in the yukon territory and alaska six distal tephra beds from the yukon territory and alaska have been dated by the fission track method zircon and glass ages were determined for the fort selkirk and lost chicken tephra beds but only glass ages for the others assuming that no track fading has occurred in the glass old crow and dawson tephra beds are younger than 120 000 and 52 000 years bp respectively mosquito gulch tephra is 1 22 ma old fort selkirk tephra is about 1 ma old the ester ash bed is 0 45 ma old and the best estimate of the age of lost chicken tephra is the range 1 7 2 6 ma it is evident from these results and from the known abundance of tephra beds within late cenozoic deposits of the yukon territory and alaska that application of the fission track method to distal tephra in conjunction with detailed characterization studies offers great potential for elucidation of the late cenozoic geologic history of alaska and the yukon territory

do faults trigger folding in the lithosphere a number of observations reveal large periodic undulations within the oceanic and continental lithospheres the question if these observations are the result of large scale compressive instabilities i e buckling remains open in this study we support the buckling hypothesis by direct numerical modeling we compare our results with the data on three most proeminent cases of the oceanic and continental folding like deformation indian ocean western gobi central asia and central australia we demonstrate that under reasonable tectonic stresses folds can develop from brittle faults cutting through the brittle parts of a lithosphere the predicted wavelengths and finite growth rates are in agreement with observations we also show that within a continental lithosphere with thermal age greater than 400 my either a bi harmonic mode two superimposed wavelengths crustal and mantle one or a coupled mode mono layer deformation of inelastic folding can develop depending on the strength and thickness of the lower crust

variable variscan thermal rejuvenation in the st malo region cadomian orogen france evidence from 40ar 39ar mineral ages abstract the st malo region in north west france contains migmatites and anatectic granites derived by partial melting of metasedimentary protoliths during cadomian orogenesis at c 540 ma previously reported rb sr model ages for muscovite and biotite range from c 550 to c 300 ma and suggest variable resetting of mineral isotopic systems these rocks display microscopic evidence for variably intense cadomian intracrystalline plastic strain but record no obvious evidence of penetrative palaeozoic regional deformation 40ar 39ar mineral ages have been determined to evaluate better the extent timing and significance of palaeozoic overprinting eleven muscovite concentrates and one whole rock phyllite have been prepared from various units exposed in the st malo and adjacent mancellian regions in the mancellian region muscovite from two facies of the bonnemain granite complex record 40ar 39ar plateau ages of c 527 and 521 ma an internally discordant 40ar 39ar release spectrum characterizes muscovite from protomylonitic granite within the cadomian alexain deux evailles ize granite complex and probably records the effects of variscan displacement along the north armorican shear zone muscovite concentrates from anatectic granite and from cadomian mylonites along ductile shear zones within the north western sector of the st malo region exhibit internally discordant 40ar 39ar release spectra which suggest variable and partial late palaeozoic rejuvenation by contrast muscovite concentrates from samples of variably mylonitic brioverian metasedimentary rocks exposed within the south eastern sector of the st malo region display internally concordant apparent age spectra which define plateaux of 326 320 ma a whole rock phyllite sample from brioverian metasedimentary rocks exposed along the eastern boundary of the st malo region displays an internally discordant argon release pattern which is interpreted to reflect the effects of a partial late palaeozoic thermal overprint muscovite from the plelan granite part of the variscan plelan bobital granite complex yields a 40ar 39ar plateau age of c 307 ma the 40ar 39ar results indicate that cadomian rocks of the st malo region have undergone a widespread and variable palaeozoic carboniferous rejuvenation of intracrystalline argon systems which apparently did not affect the mancellian region this rejuvenation was not accompanied by penetrative regional deformation and was probably of a static thermal hydrothermal origin the heat source for rejuvenation was probably either the result of heating during variscan extension or advection from variscan granites which are argued to underlie the st malo region

toward a comprehensive upper quaternary tephra and ignimbrite stratigraphy in new zealand using electron microprobe analysis of glass shards abstract new zealand quaternary marine and terrestrial sequences contain numerous tephras or volcanic ash horizons that are the distal correlatives of voluminous welded ignimbrite sheets erupted from central north island electron microprobe analyses of glass shards from the distal tephras demonstrate their homogeneity and are shown to identify each tephra examined by matching tephras from stratigraphically controlled sequences the first comprehensive tephra stratigraphy spanning from 50 000 to 700 000 yr ago and covering the new zealand region is advanced analyses on glass shards from the unwelded base of ignimbrite sheets are comparable to the distal tephra analyses and allow correlation between ignimbrites and to the distal tephras the better exposed tephra record constrains the number of separate eruptive events and the stratigraphy of the ignimbrites both of which were previously confused by lack of outcrop samples from pumiceous marine sediments were found to contain two or more chemically distinct populations of glass the pumice is in cross bedded sands or sand lenses within conglomerate attesting to a shallow high energy environment where reworking could occur however each glass population could be matched to older known tephras

correspondence between glass ft and 14 c ages of silicic pyroclastic flow deposits sourced from maninjau caldera west central sumatra abstract paroxysmal pyroclastic flow deposits sourced from maninjau caldera in west central sumatra are dated at 50 3 ka n 3 using the isothermal plateau and diameter corrected fission track itpft and dcft respectively techniques on glass shard constituents in addition charcoal obtained from tall trees in position of growth within the paroxysmal flow deposit on the upper flanks for the caldera are also dated at 52 3 2 14c ka n 2 and 51 1 3 2 14c ka n 1 using an acid base wet oxidation stepped combustion abox sc and standard acid base acid aba 14c techniques respectively the close correspondence in 14c ages of charcoal sample splits analysed at two laboratories australian national university australia and waikato university new zealand verifies the reliability of these 14c techniques up to at least 50 ka based on concordant ages derived from glass ft and 14c techniques an age of 52 3 ka is assigned to the latest silicic eruptive activity at maninjau caldera this chronology is further confirmed by the occurrence of a silicic tephra bed that closely underlies paroxysmal maninjau deposits at two sections and is correlated with youngest 75 ka toba tephra ytt erupted from toba caldera in north central sumatra this study not only provides a much needed regional chronological reference point for quaternary deposits in west central sumatra but also extends the minimum age range of the glass ft technique from 75 ka down to c 50 ka that is now for the first time within the extended maximum age range of the 14c technique

short paper cadomian terrane tectonics and magmatism in the armorican massif the north armorican composite terrane nw france is a collage of displaced terranes which result from the amalgamation of cadomian continental arcs and marginal basin complexes by sinistral transpression along a continental margin above a subduction zone early cadomian arc activity occurred at c 700 650 ma but terrane accretion did not occur until c 540 ma and post tectonic magmatism persisted well into the palaeozoic cadomian events thus span a considerably greater period than previously supposed

tectonothermal chronology of early cadomian arc development in guernsey and sark channel islands the cadomian orogen exposed in nw france and the channel islands comprises a collage of variably displaced terranes which record the late proterozoic evolution of a continental margin within a complex subduction zone on the islands of guernsey and sark calc alkaline quartz diorite stocks were emplaced during early stages of the cadomian orogeny zircons from two stocks of deformed quartz diorite display evidence of inheritance in larger and more magnetic fractions upper intercept ages for these discordant concentrates are c 1300 ma sm nd whole rock model ages t dm suggest older ages of c 2000 ma conversely the smallest and least magnetic zircons exhibit a distinct lead loss pattern suggesting that the 207 pb 206 pb dates of c 700 ma correspond to the time of crystallization of the stocks and provide a lower age constraint for initiation of subduction related magmatism in the cadomian orogen hornblende concentrates from deformed quartz diorite stocks exposed on guernsey and sark display discordant 40 ar 39 ar age spectra five samples record well defined isotope correlation ages ranging between 596 ma and 606 ma a hornblende concentrate from host orthogneiss sark yields a plateau age of 608 ma the hornblende ages are interpreted as dating post metamorphic cooling following late proterozoic deformation and metamorphism within the arc system both magmatic and cooling ages from this early cadomian arc contrast with those of younger initial tectonothermal activity c 570 ma and c 540 ma in other cadomian terrane elements and suggest palinspastic separation until very late stages of the orogeny the polyphase nature of this late proterozoic activity is similar to that of related orogens which probably occupied similar tectonic settings along the margin of gondwana e g sw iberia west africa

secondary convection and the growth of the oceanic lithosphere abstract this paper presents a study of the evolution of the oceanic lithosphere from a thermomechanical approach we have investigated the finite amplitude development of secondary convection cells beneath the oceanic plates by means of the single mode mean field equations and the fully 2 dimensional convection equations using finite element techniques both newtonian and non newtonian rheologies with highly temperature and pressure dependences and an activation energy of 100 kcal mol 1 have been employed the temperature at the base of the convecting medium governs the final thickness of the lithosphere the mean interior temperature varies only slightly during the temporal evolution non newtonian rheology has the tendency to induce oscillatory time dependent behavior of the flow heat flow topography and gravity are influenced by secondary convection in two ways small scale perturbations with wavelengths of around 600 km arise from the lateral thermal differences between the uprising and descending convective limbs large scale features are also produced as a consequence of lithospheric growth the calculated quantities of the heat flow topography and gravity associated with small scale convection are typically in the range of o hfu o 102m and o 10 mgal the horizontal mean temperature profiles from the convection model are used to calculate long wavelength geophysical observables as a function of age convective processes are found to reduce the rate of lithospheric thickening predictions from our model can fit well the observed data of heat flow ocean floor topography and geoid offsets along fracture zones the last data base exhibiting the most sensitivity to thermal perturbations below the lithosphere our calculations show that the oceanic lithosphere is able to grow continuously up to o 109 y long past the flattening of the seafloor we report here that the thickness of a thermally equilibrated lithosphere could reach 250 km which lies within the appropriate range of values for the continental lithosphere as inferred from studies in seismology flexure observations and secular polar motions

full glacial paleosols in perennially frozen loess sequences klondike goldfields yukon territory canada perennially frozen loess deposits in the klondike goldfields include paleosols formed in full glacial environments correlated by alaskan distal tephra with marine isotope stages mis 2 and 4 patterns of organic and inorganic carbon and clay distribution microstructures and profile morphologies indicate that soil formation occurred in a base rich environment in which organic matter accreted predominantly as root detritus at sites approximately 20 km apart the expression of cryoturbation and ice wedge development decreases in strength upward in loess paleosol sequences correlated with mis 4 suggesting increasing aridity configurations of cryoturbation features and ice wedge thaw unconformities the presence of numerous ground squirrel burrows and an absence of peat accumulation suggest that these substrates were predominantly welldrained with active layers of equal or greater thickness than in modern soils on similar sites in the west central yukon some characteristics of these paleosols are similar to those of modern steppe and tundra soils consistent with plant macrofossil evidence for local ecological diversity during full glacial conditions in eastern beringia

behaviour of rb sr whole rock and u pb zircon systems during partial melting as shown in migmatitic gneisses from the st malo massif ne brittany france the age of regional metamorphism and partial melting in the late proterozoic succession of the st malo area has been established at around 540 ma whole rock homogenization of the sr isotope systems has been achieved only in the anatectic granites whereas the associated metatexitic and diatexitic gneisses remain highly heterogeneous at lower grade than the sillimanite isograd metasedimentary gneisses contain detrital zircons which record the regional metamorphic event as a u pb lower intercept age 536 14 ma in the sillimanite zone u pb zircon lower intercept ages are around 500 10 ma and suggest a recent loss of radiogenic pb from the observed overgrowths zircons from the anatectic granites do not yield u pb intercept ages due to the presence of inherited radiogenic pb but the finer fractions define a discordant array whose upper intercept on concordia yields 555 13 ma an age significantly older than the partial melting event indicated by the lower grade zircon ages the lower intercept defined by the fine grained anatectic zircons yields a date in the future negative ordinate value in their regional context the st malo migmatites can be considered as resulting from late stage crustal melting in the cadomian orogenic belt following genesis of calc alkaline magma at an active margin around 600 ma ago an age of 540 ma for partial melting in the st malo migmatites places the end of the cadomian orogeny near the precambrian cambrian boundary

u pb geochronological constraints on the timing of plutonism volcanism and sedimentation jersey channel islands uk uranium lead dates for single and small multiple grain zircon separates are presented for the main geological units from jersey british channel islands a turbiditic sandstone from the jersey shale formation brioverian supergroup contains detrital zircons of a variety of ages ranging from c 2 6 to 0 6 ga the youngest zircons yielded nearly concordant u pb dates of 586 7 3 0 ma and 587 1 2 7 ma providing a maximum depositional age for the unit zircons from overlying volcanic rocks anne port rhyolite give an upper intercept date of 582 8 3 2 7 ma considered to be the age of eruption the undeformed sw jersey granite which intrudes and locally thermally metamorphoses the folded jersey shale formation is dated at 580 2 3 1 6 ma these dates constrain tightly the timing of deposition of the jersey shale formation and the age of deformation on jersey and indicate a dynamic arc environment during the late neoproterozoic zircons from the nw jersey granite define an upper intercept date of 482 7 1 6 ma similar to the previously reported 40 ar 39 ar hornblende date of c 473 ma confirming a renewed pulse of magmatism during the ordovician in this sector of cadomia

palaeozoic terrane amalgamation in central europe a ree and sm nd isotope study of the pre variscan basement ne bohemian massif

isothermal plateau fission track ages of hydrated glass shards from silicic tephra beds abstract the age of silicic tephra beds that have experienced a simple thermal history can be accurately and precisely determined by application of the isothermal plateau fission track technique to hydrated glass shards age estimates obtained by this means compare very well with those based on co existing mineral phases using the k ar 40ar 39ar external detector fission track and thermoluminescence techniques this concordance contradicts the popular belief that hydrated glass shards give unreliable ages distal tephra beds many of which are difficult to date because of their fine grainsize low abundance of crystals and common presence of detrital grains are readily dated by the isothermal plateau glass fission track method this new advance should greatly facilitate the development of detailed chronologies of tephra bearing sedimentary sequences located far from volcanic centres

onshore offshore correlation of pleistocene rhyolitic eruptions from new zealand implications for tvz eruptive history and paleoenvironmental construction abstract taupo volcanic zone tvz in the north island new zealand is arguably the most active quaternary rhyolitic system in the world numerous and widespread rhyolitic tephra layers sourced from the tvz form valuable chronostratigraphic markers in onshore and offshore sedimentary sequences in deep sea cores from ocean drilling program odp leg 181 sites 1125 1124 1123 and 1122 located east of new zealand ca 100 tephra beds are recognised post dating the plio pleistocene boundary at 1 81 ma these tephras have been dated by a combination of magnetostratigraphy orbitally tuned stable isotope data and isothermal plateau fission track ages the widespread occurrence of ash offshore to the east of new zealand is favoured by the small size of new zealand the explosivity of the mainly plinian and ignimbritic eruptions and the prevailing westerly wind field although some tephras can be directly attributed to known tvz eruptions there are many more tephras represented within odp cores that have yet to be recognised in near source on land sequences this is due to proximal source area erosion and or deep burial as well as the adverse effect of vapour phase alteration and devitrification within near source welded ignimbrites despite these difficulties a number of key deep sea tephras can be reliably correlated to equivalent aged tephra exposed in uplifted marine back arc successions of wanganui basin where an excellent chronology has been developed based on magnetostratigraphy orbitally calibrated sedimentary cycles and isothermal plateau fission track ages on tephra significant pleistocene tephra markers include the kawakawa omataroa rangitawa onepuhi kaukatea kidnappers b potaka unit d ahuroa ongatiti rewa sub rewa pakihikura ototoka and table flat tephras six other tephra layers are correlated between odp core sites but have yet to be recognised within onshore records the identification of pleistocene tvz sourced tephras within the odp record and their correlation to wanganui basin and other onshore sites is a significant advance as it provides 1 an even more detailed history of the tvz than can be currently achieved from the near source record 2 a high resolution tephrochronologic framework for future onshore offshore paleoenvironmental reconstructions and 3 well dated tephra beds correlated from the offshore odp sites with astronomically tuned timescales provide an opportunity to critically evaluate the chronostratigraphic framework for onshore plio pleistocene sedimentary sequences e g wanganui basin cf naish et al 1998 quaternary science reviews 17 695 710

polyphase late paleozoic tectonothermal evolution of the southwestern meguma terrane nova scotia evidence from 40ar 39ar mineral ages 40ar 39ar incremental release ages of hornblende muscovite and biotite from a variety of granitic stocks and host metamorphic rocks suggest a complex late paleozoic tectonothermal evolution for the southwestern meguma terrane regional d1 folding with cleavage formation under greenschist lower amphibolite facies m1 metamorphic conditions occurred at ca 400 410 ma and was followed by emplacement of a series of granitic stocks ranging in age between ca 375 and 315 ma these were emplaced at relatively shallow crustal levels and developed contact metamorphic aureoles of variable grade these are locally superposed on m1 regional metamorphic assemblages and result in a complex isograd pattern 40ar 39ar mineral ages suggest episodes of contact metamorphism occurred at 1 360 375 ma possibly related to emplacement of the south mountain batholith or temporal equivalents 2 350 356 ma around the port mouton pluton and northeastern shelburne pluton 3 ca 315 325 ma near the wedgeport pluton and in

developments in the analysis of volcanic glass shards by laser ablation icp ms quantitative and single internal standard multielement methods abstract laser ablation inductively coupled plasma mass spectrometry la icp ms has been applied to the analysis of fine grained 125 250 m volcanic glass shards separated from tephra deposits our earlier calibration strategies used ce as an internal standard which required the use of another trace element analytical technique to determine ce followed by off line data correction by comparison with well characterised glass separates we have progressed from these earlier studies to use a minor isotope of a major element routinely determined by epma in tephrochronological studies as an internal standard for la icp ms analysis internal standards used during this study include 29si 30si or 44ca for fully quantitative determinations or 57fe for a single internal standard multi element calibration so called semiquantitative analysis no subsequent off line data correction is required the differences between each technique are minimal and choice of technique depends largely upon instrument sensitivity as little as 80 g of material is consumed during a single determination which takes approximately two minutes and detection limits are below 1 part per million accuracy is generally 15 when compared with other analytical techniques and precision may vary between 5 and 20 the worse precisions reflecting within sample variability rare earth and other trace element data can be used to correlate or distinguish individual tephra deposits with confidence and specific examples are illustrated with quaternary tephra deposits from alaska and india and cretaceous tephra deposits from banks island in arctic canada

tephrochronology of late cenozoic loess at fairbanks central alaska nineteen distinctive distal tephra beds from the gold hill loess in the fairbanks region central alaska are divided into two groups based on their color mineralogy glass shard morphology and the major and trace element composition of their glass shards this dichotomy in tephra composition is believed to reflect the two source regions namely the eastern aleutian arc alaska peninsula region and the wrangell volcanic field despite the fragmentary nature of the preserved loess record which contains many local unconformities these tephra beds can be used to link the numerous loess exposures in the fairbanks region in their proper time sequence establishing thereby a comprehensive reliable time stratigraphic framework because these distal tephra beds are related to large magnitude volcanic eruptions given the long distance to the nearest possible source vent they will also enable correlation of the late cenozoic sedimentary sequence at fairbanks to those of adjacent regions of alaska and the yukon tephra beds with bubble wall or chunky glass shards have been dated by the isothermal plateau fission track method results demonstrate that the gold hill loess spans a long interval of time from late pliocene time to the last interglacial about 125 ka however the age of the basal part of this formation is not well constrained due to the apparent absence of tephra beds paleomagnetic studies suggest that loess deposition in the fairbanks region began about 3 ma

time span of plutonism fabric development and cooling in a neoproterozoic magmatic arc segment u pb age constraints from syn tectonic plutons sark channel islands uk abstract new u pb zircon and titanite dates from syn tectonic plutons on the british channel island of sark constrain the time span of plutonism fabric development and cooling in this part of the neoproterozoic cadomian magmatic arc the tintageu leucogneiss is a mylonitic unit that was dated previously at 615 6 4 2 2 3 ma the port du moulin quartz diorite which intruded the tintageu unit contains a high strain solid state deformation fabric that is less intense than but parallel to fabrics in the leucogneiss and yields a u pb zircon date of 613 5 2 3 1 5 ma the little sark quartz diorite also displays solid state deformation fabrics in addition to relict magmatic textures and yields a u pb zircon date of 611 4 2 1 1 3 ma the north sark granodiorite is largely penetratively undeformed exhibits mainly magmatic fabrics and textures and has a u pb zircon date of 608 7 1 1 1 0 ma two fractions of titanite from each intrusion are essentially concordant and are identical within error with mean dates of 606 5 0 4 ma port du moulin quartz diorite 606 2 0 6 ma little sark quartz diorite 606 4 0 6 ma north sark granodiorite the new u pb data in combination with previous u pb and 40 ar 39 ar data and previous field studies confirm the syn tectonic nature of the sark plutons and quantify the time span ca 7 m y required for intrusion and sufficient crystallization of each body to record incremental strain during waning deformation titanite u pb and hornblende 40 ar 39 ar dates mark final cooling about 2 m y after intrusion of the last pluton

significance of a hiatus in down temperature fabric development within syn tectonic quartz diorite complexes channel islands uk detailed fabric development histories have been deduced for neoproterozoic foliated quartz diorite complexes of the channel islands uk the plutons were emplaced during regional d2 deformation magmatic fabrics formed at low crystal contents pre rheologically critical melt percent are only locally preserved due to overprinting by solid state deformation which occurred at moderate temperatures 400 550 c this is demonstrated by brittle microcracking of plagioclase and amphibole with only local ductile bending and dynamic recrystallization marginal recrystallization and myrmekite development on alkali feldspar ductile deformation of quartz to form types 2 3 ribbons quartz recrystallization by sub grain rotation associated with a slip but not c slip biotite recrystallization within micro shears and variable amphibole recrystallization the deformation histories display a distinct division between magmatic state and moderate temperature solid state deformation features with little evidence for intervening high temperature solid state fabrics this contrasts with criteria widely considered diagnostic of syn tectonic plutons which envisage development of a continuum of magmatic through high temperature solid state deformation fabrics however we consider the plutons syn tectonic and attribute the hiatus to contrasting strain and pluton cooling rates length and nature of deformation events and host rock ambient temperatures intermediate magmas emplaced at moderate to shallow levels in the crust may be more prone to develop such apparently punctuated deformation histories than sio 2 richer granitoids

timing of post tectonic cadomian magmatism on guernsey channel islands evidence from 40ar 39ar mineral ages the channel island of guernsey exposes pre and post tectonic calc alkaline plutonic rocks attributed to late precambrian cadomian subduction activity 40 ar 39 ar mineral cooling ages presented here clarify the age and time span of post tectonic intrusions amphibole from the bon repos meladiorite yields an isotope correlation age of 605 3 4 6 ma indicating emplacement during late stages of the tectonothermal event recorded within its host rocks igneous hornblendes from the northern igneous complex record consistent isotope correlation ages st peter port gabbro 570 2 1 1 ma bordeaux diorite complex 569 0 1 1 and 565 3 2 2 ma and a comagmatic enclave within cobo granite 570 2 1 5 ma hornblende and biotite from contact metamorphosed host rocks adjacent to the cobo granite yield isotope correlation ages of 569 6 1 1 and 563 8 3 3 ma respectively these ages together with field data demonstrate that the various members of the northern igneous complex were emplaced contemporaneously at c 570 ma significantly earlier than a c 500 ma rb sr whole rock isochron age previously reported for the cobo granite the 40 ar 39 ar ages indicate that post tectonic magmatism on guernsey was broadly coeval with post metamorphic cooling around the baie de st brieuc on the adjacent french mainland and occurred c 30 ma prior to peak cadomian deformation and metamorphism in the st malo area

chronology of cadomian tectonothermal activity in the baie de saint brieuc north brittany france evidence from 40ar 39ar mineral ages the late precambrian cadomian orogen exposed in the north armorican massif northwest france is a collage of displaced terranes that in part developed during amalgamation of continental arc and marginal basin complexes 40ar 39ar mineral ages reported here place new constraints on the timing of cadomian tectonothermal activity in the southern part of the st brieuc terrane in the baie de saint brieuc area brioverian supracrustal units were deformed metamorphosed and intruded by calc alkaline plutonic complexes metamorphic hornblende from a metabasic amphibolite sheet within brioverian rocks records an isotope correlation age of 568 4 2 6 ma interpreted to date postmetamorphic cooling through appropriate argon closure temperatures similar isotope correlation cooling ages are recorded by metamorphic hornblende within both an amphibolite sheet intrusive into the penthievre complex 567 5 1 2 ma and the la croix gibat amphibolite 574 8 2 1 ma igneous hornblende from the late tectonic to pos

neoproterozoic shear zone tectonics within the icartian basement of guernsey and sark channel islands the channel islands of guernsey and sark are amongst the few localities within the neoproterozoic cadomian orogenic belt where palaeoproterozoic basement is exposed basement units collectively referred to as icartian comprise orthogneisses e g the c 2000 ma icart granite gneiss metasediments and amphibolites on guernsey the protolith to the icart granite gneiss was intruded into metasediments already deformed during a d 1 deformation event both were variably deformed during a d 2 event within an approximately north south trending steeply dipping oblique dextral shear zone on sark metasediments and amphibolites carry a d 1 fabric not necessarily correlatable with that on guernsey which was deformed during d 2 to form recumbent gently inclined folds and tectonic fabrics which locally carry top to the south kinematic indicators early cadomian quartz diorites which intruded the icartian basement carry variably developed magmatic and moderate temperature 400 550 c ls solid state fabrics which are similarly oriented to d 2 basement structures this indicates that the quartz diorites were emplaced and deformed during a protracted d 2 event published 40 ar 39 ar amphibole cooling ages of c 600 ma from guernsey and sark quartz diorites and a host icartian amphibolite are consistent with reworking of basement units during the cadomian orogeny the contrasting kinematic pattern during cadomian deformation implies that a flat lying shear zone on sark may have accommodated oblique dextral strike slip movement on guernsey with an overall southerly direction of tectonic transport deformation most likely occurred in the hanging wall of a major south dipping neoproterozoic subduction zone

correlation and characterisation of individual glass shards from tephra deposits using trace element laser ablation icp ms analyses current status and future potential pearce nicholas denton j s perkins w t westgate j a 2007 correlation and characterisation of individual glass shards from tephra deposits using trace element laser ablation icp ms analyses current status and future potential journal of quaternary science 22 7 pp 721 736 rae2008

discovery of a large magnitude late pleistocene volcanic eruption in alaska an extensive blanket of dacitic tephra was deposited across alaska and the western yukon territory by a plinian eruption about 80 000 years ago the volume of tephra erupted probably exceeded 50 cubic kilometers and the source was probably in the wrangell mountains this widespread layer of tephra serves as an excellent late pleistocene stratigraphic marker

paleoecology of beringian packrat middens from central yukon territory canada rodent middens from ice rich loess deposits are important new paleoenvironmental archives for eastern beringia plant macrofossils recovered from three middens associated with dawson tephra ca 24 000 14 c yr b p at two sites in yukon territory include diverse graminoids forbs and mosses these data suggest substantial local scale floristic and habitat diversity in valley settings including steppetundra on well drained soils moist streamside meadows and hydric habitats fossil arctic ground squirrel burrows and nesting sites indicate that permafrost active layers were thicker during pleistocene glacial periods than at present on north facing slopes d 2004 university of washington all rights reserved

the correlation between reduction in fission track diameter and areal track density in volcanic glass shards and its application in dating tephra beds abstract the relationship between the reduction in fission track diameter and the reduction in areal track density in hydrated glass shards has been investigated under varying annealing and etching conditions a near 1 1 relation is observed for adequately etched tracks following laboratory heating of induced tracks the partially reduced areal track density was renormalized with error

geochemical evidence for triassic rifting in southwestern florida abstract mesozoic rhyolites and basalts from three drillholes penetrating the pre cretaceous basement of southwestern florida were analyzed for their trace element and sr and nd isotopic compositions major and trace element abundances suggest the suite was erupted in an extensional environment and isotopic data indicate derivation primarily from aged enriched precambrian continental lithosphere combined to varying degrees with an asthenospheric component nd model ages show no significant difference in the age of the lithosphere from which these rocks and volcanic rocks of paleozoic age from north florida were derived arguing against major displacement across the peninsula including along proposed extensions of the jay fault and or bahamas fracture zone we suggest that the geophysically inferred boundary across the peninsula if a fault may have had a strong normal component consistent with the greater depth to basement south of the boundary in this view the southwestern florida mesozoic volcanic province sfmv may represent a part of a major triassic rift basin associated with the breakup of pangaea

thermoluminescence age of old crow tephra in alaska thermoluminescence dates for the loess deposits above and below the old crow tephra at the halfway house site near fairbanks limit the age of the tephra to 86 8 ka this is in excellent agreement with other estimates already in the literature the dating was performed using additive dose procedures and optical selection of the potassium feldspar signal from a polymineral mixture of fine grains

dating early and middle reid pleistocene glaciations in central yukon by tephrochronology abstract the late cenozoic deposits of central yukon contain numerous distal tephra beds derived from vents in the wrangell mountains and aleutian arc alaska peninsula region we use a few of these tephra beds to gain a better understanding on the timing of extensive pleistocene glaciations that affected this area exposures at fort selkirk show that the cordilleran ice sheet advanced close to the outer limit of glaciation about 1 5 myr ago at the midnight dome terrace near dawson city exposed outwash gravel aeolian sand and loess related to valley glaciers in the adjacent ogilvie mountains are of the same age reid glacial deposits at ash bend on the stewart river are older than oxygen isotope stage ois 6 and likely of ois 8 age that is about 250 000 yr b p supporting evidence for this chronology comes from major peaks in the rates of terrigeneous sediment input into the gulf of alaska at 1 5 and 0 25 myr b p

a 3 m y record of pliocene pleistocene loess in interior alaska many distal tephra beds exist in the thick fossiliferous loess deposits near fairbanks interior alaska isothermal plateau fission track ages determined on glass shards from tephra beds in conjunction with tephrostratigraphic and magnetostratigraphic techniques indicate that loess deposition began in the late pliocene an antiquity previously unsuspected hence there is the opportunity now to reconstruct a detailed well dated record of environmental changes in interior alaska during the past 3 m y

precise u pb zircon ages from alderney channel islands growing evidence for discrete neoproterozoic magmatic episodes in northern cadomia the northernmost exposures of rocks formed during the late neoproterozoic cadomian orogeny in the channel islands northern france region occur on alderney the island mainly comprises foliated quartz diorite once considered to be 2 ga pre cadomian basement and an undeformed basic to intermediate plutonic complex a precise age of 610 2 ma based on u pb analyses of single and small groups of zircons for the foliated fort tourgis quartz diorite demonstrates that the oldest rocks were emplaced and deformed during a cadomian magmatic event the age is virtually identical to ages from similar foliated syntectonic quartz diorite bodies on the islands of guernsey and sark and at la hague north normandy indicating that this magmatic and deformational event was regional in extent discordant zircon xenocrysts define an upper intercept age of c 2 ga indicating the presence of palaeoproterozoic basement at depth single zircons from the undeformed bibette head granodiorite give a precise u pb age of 572 1 ma this age is closely similar to that for the emplacement of the northern igneous complex of guernsey the emerging data indicate that cadomian magmatism in the northern channel islands region was not a protracted continuum but occurred during two distinct short lived events separated by c 30 40 my

abyssal hills formed by stretching oceanic lithosphere tectonic plates are formed and move apart at mid ocean ridges some portion of this plate separation process can occur by stretching of the crust resulting in a complex pattern of extensional faults abyssal hills the most ubiquitous topographic features on earth1 are thought to be a product of this faulting2 3 here we report the results of a self consistent numerical model of lithospheric formation and stretching that includes spontaneous fault creation in this model an axial valley develops where the fault activity is most concentrated the frozen fault generated topography rafted out of the axial valley is visually and statistically similar to observed abyssal hills formed at many slower spreading ridges faults appear to be replaced by new faults because their offset changes the local stress field we accordingly need no temporal variation in magmatism as required by some previous models4 5 6 to control the spacing or offset of faults our model results suggest instead that the irregularity of abyssal hill relief may result from a self organized critical stress state at spreading centres

u pb geochronology and sm nd isotopic composition of proterozoic gneisses channel islands uk gneissic units of the channel islands united kingdom have traditionally been considered to be palaeoproterozoic basement to the late neoproterozoic cadomia terrane based on lithological correlations and imprecise or ambiguous isotopic data a new precise u pb date of 2061 2 ma based on analyses of single zircons for the icart granitic orthogneiss of guernsey confirms a palaeoproterozoic age of its igneous protolith the nd depleted mantle model age t dm of this gneiss is 2220 ma only slightly older than its crystallization age indicating that it represents juvenile crust other gneisses from guernsey have similar t dm ages ranging from 2210 to 2370 ma suggesting that they are also exposures of juvenile palaeoproterozoic crust a component from a penetratively deformed orthogneiss from sark previously correlated with icartian gneisses on guernsey yields a u pb zircon date of 616 4 2 ma this crystallization age demonstrates that the protolith was a cadomian intrusion and that penetrative deformation and amphibolite facies metamorphism in the northern channel islands were cadomian in age the use of gneissic features as a means of invoking the antiquity of many other undated gneissic units in the region is thus considered unreliable the presence of 2170 7 ma zircon xenocrysts within the sark orthogneiss combined with its initial e nd 615 9 5 are strong evidence that pre cadomian basement was a significant source component and that documenting the extent and age of pre cadomian basement may be possible by indirect geochemical methods

early to middle pleistocene tephrochronology of north island new zealand implications for volcanism tectonism and paleoenvironments thick volcaniclastic sequences of early to middle pleistocene age in southern north island new zealand contain rhyolitic tephra beds that record the early history of the taupo volcanic zone tvz at least 54 different tephra beds are recorded and their chronology is defined by glass fission track ages and paleomagnetism the tephra beds span the interval ca 2 0 0 6 ma and provide an event frequency of 1 19 k y significantly higher than the frequency of sheet forming ignimbrites preserved in the tvz at this time 1 100 k y the distal tephra beds thus provide a record of volcanism not revealed in the proximal volcanic region and suggest a major period of explosive activity at ca 1 79 1 60 ma several important marker horizons are identified pakihikura tephra 1 63 ma mangatewaiiti tephra 1 24 ma potaka tephra 1 00 ma kidnappers b tephra 1 00 ma and kaukatea tephra 0 88 ma these tephra beds allow direct correlation between 1 marine and nonmarine facies and 2 the fore arc and back arc regions of new zealand the tephra beds provide a framework for a paleoenvironmental reconstruction of the southern north island volcaniclastic transport routes from the tvz to basins in the south and southeast and through the site of present mountain ranges supplied material to a terrestrial lowland fore arc area in the interval 1 64 0 7 ma uplift and deformation since 0 7 ma have disrupted paleodrainage routes diverting them to the north and southwest

the cadomian active margin north armorican massif france a segment of the north atlantic panafrican belt abstract within the european variscan belt the armorican massif northwestern france contains an isolated fragment of the north atlantic panafrican belt the cadomian block mostly unaffected by variscan deformation this block is composed of neoproterozoic rocks ranging in age from 620 to 540 ma brioverian locally overlying a composite basement of icartian 2000 ma or pentevrian 750 ma age the cadomian tectonics the last magmatic manifestations of which are dated at around 540 ma were sealed by a cover of early cambrian rocks in normandy and early ordovician rocks in brittany five units are distinguished in this cadomian block tregor saint brieuc guingamp saint malo and fougeres although the units are bound by major faults the overall coherence of their different protolithic characteristics suggests that they can be fitted into a simple geodynamic model the tregor unit represented by the north tregor batholith of crustal affinity and its icartian host rock probably reflects an early cadomian evolution the saint brieuc unit essentially made up of magmatic rocks of juvenile affinity includes two assemblages juxtaposed by the cadomian tectonics 1 a volcanosedimentary succession formed in a typical island arc setting reflecting subduction around 610 ma and 2 a volcanic plutonic complex emplaced around 590 ma in an extensional active continental margin of pentevrian age the guingamp unit predominantly migmatitic includes a tectonic slice of deep origin the extrusion of which accompanied the exhumation of the migmatites it may represent the trace of a possible suture the saint malo unit is composed of brioverian detrital rocks the significance of which is uncertain passive continental margin or accretionary prism it is structured around a late migmatitic dome 540 ma the fougeres unit corresponds to the outer domain of the cadomian orogeny it was only weakly deformed by cadomian tectonics and is intruded by the large mancellian batholith of crustal origin dated at 540 ma together the structural units are interpreted in the framework of an evolving active margin juxtaposing continental margin marginal basin and volcanic arc before being shortened during the cadomian convergent tectonics

tectonomagmatism in continental arcs evidence from the sark arc complex abstract the island of sark channel islands uk exposes syntectonic plutons and country rock gneisses within a precambrian cadomian continental arc this sark arc complex records sequential pulses of magmatism over a period of 7 ma ca 616 609 ma the earliest intrusion ca 616 ma was a composite sill that shows an ultramafic base overlain by a magma mingled net vein complex subsequently deformed at near solidus temperatures into the amphibolitic and tonalitic tintageu banded gneisses the deformation was synchronous with d 2 deformation of the paragneissic envelope with both intrusion and country rock showing flat top to the south ls fabrics later plutonism injected three homogeneous quartz diorite granodiorite sheets the creux moulin pluton 150 250 m ca 614 ma the little sark pluton 700 m 611 ma and the northern pluton 500 m 609 ma similar but thinner sheets in the south derrible hogsback dixcart and west port es saies brecqhou are interpreted as offshoots from the creux moulin pluton and little sark pluton respectively all these plutons show the same ls fabric seen in the older gneisses with rare magmatic fabrics and common solid state fabrics recording syntectonic crystallisation and cooling the cooling rate increased rapidly with decreasing crystallisation age 9 ma for the oldest intrusion to cool to lower amphibolite conditions 7 8 ma for the creux moulin pluton 5 6 ma for the little sark pluton and 2 the initiation of the d 2 event is suggested to have been a response to the intrusion of the tintageu magma which promoted a rapid increase in strain rate 10 14 s 1 that focussed extensional deformation into the sark area the increased rates of extension allowed ingress of the subsequent quartz diorite granodiorite sheets although strain rate slowly declined as the whole complex cooled during exhumation the regional architecture of syntectonic cadomian arc complexes includes flat lying sark type and steep guernsey type domains produced synchronously in shear zone networks induced by oblique subduction a pattern seen in other continental arcs such as that running from alaska to california