geodynamic evolution of the sw europe variscides 1 the early evolution of sw europe variscides started by opening of the rheic ocean at 500 ma splitting avalonia from armorica iberia subduction on the se side of rheic generated the paleotethys back arc basin 430 390 ma splitting armorica from iberia with development of porto tomar ferreira do alentejo ptfa dextral transform defining the boundary between continental armorica and finisterra microplate to the w obduction of paleotethys was followed by armorica iberia collision and emplacement of nw iberian allochthonous units at 390 370 ma whereas toward the west of ptfa there was antithetic ophiolite obduction beja acebuches and rheic ophiolites plus finisterra continental slices on top of ossa morena zone with simultaneous development of eclogites and orogenic magmatism under a flake double wedge tectonic regime continued convergence 370 ma proceeded by intracontinental deformation with progressive tightening of the ibero armorican arc through dextral transpression on the cantabrian indentor from iberia to armorica the proposed model is discussed at the light of the driving mechanism of soft plate tectonics

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

ophiolite obduction pulses as a proxy indicator of superplume events abstract a major new synthesis of ophiolite geochronology and map of global phanerozoic distribution indicates that previously noted pulses of ophiolite obduction can be linked to superplume related tectonism a marked cyclicity is evident in obduction ages obtained from minerals in ophiolite metamorphic soles and obduction related minor intrusions this episodicity is in phase with periods of predominantly uniform polarity of the geomagnetic field formation of massive carbon rich deposits sea level highstands and formation of large flood basalt provinces all generally considered to be superplume proxy indicators a key to interpreting ophiolite obduction as a further proxy is the mid cretaceous superplume associated ocean margin compressional deformation during this event thermal rejuvenation and increased buoyancy of ocean lithosphere caused arc terrane collision marginal basin shoaling back arc basin closure and ophiolite obduction convergent margins were placed in compression with increased coupling between subducting and overriding plates resulting in major ocean margin deformation being directly datable and tectonically and petrologically distinctive makes ophiolite assemblages of considerable use for identifying superplume events

variscan vs alpine tectonothermal evolution of the southern carpathian orogen constraints from 40ar 39ar ages abstract 40 ar 39 ar hornblende and muscovite ages indicate that basement units exposed within all major alpine nappes of the southern carpathian orogen were penetratively deformed and metamorphosed during late variscan carboniferous tectonothermal events subsequent exhumation resulted in cooling from ca 500 c hornblende k ar retention temperature to ca 375 c muscovite k ar retention temperature between ca 320 and 295 ma late variscan exhumation of middle crustal levels is interpreted to have resulted from intracontinental contraction and regional uplift subsequent to variscan continental collision between gondwana derived tectonic elements e g africa and laurussia muscovite ages within mylonite zones suggest that late variscan shear zones were probably associated with a regional shear system that developed along the southern margins of laurussia alpine metamorphic effects within the southern carpathians are generally nonpenetrative and are only significant within narrow zones of retrogression that are interpreted to have formed during initial attenuation of variscan crust associated with formation of the jurassic severin rift

quaternary glacial stratigraphy and chronology of mexico the volcano iztaccihuatl in central mexico was glaciated twice during the middle pleistocene once probably in pre illinoian or pre bull lake time and once in late illinoian or bull lake time glaciation during the late pleistocene was restricted to the late wisconsin or pinedale a maximum advance and one readvance are recorded in the early part and one readvance in the latter part three or four small neoglacial advances occurred during the holocene two other volcanoes nearby ajusco and malinche have a partial record of late pleistocene and holocene glaciations three others popocatepetl pico de orizaba and nevado de toluca have a full holocene record of three to five glacial advances during neoglaciation

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

40ar 39ar mineral ages from the oki metamorphic complex oki dogo southwest japan implications for regional correlations abstract the oki metamorphic complex exposed in the oki dogo islands consists predominantly of psammitic and pelitic gneisses with subordinate amphibolite and rare calcareous gneiss the oki gneisses were regionally metamorphosed to general amphibolite facies conditions with local development of granulite facies assemblages peak metamorphic conditions of c 800 c have been suggested hornblende concentrates from amphibolites collected within the oki metamorphic complex record 40 ar 39 ar isotope correlation ages of 199 192 ma these are interpreted to date the post metamorphic cooling through temperatures required for intracrystalline retention of argon c 500 c muscovite concentrates record 40 ar 39 ar plateau ages of 167 168 ma these are interpreted to date post metamorphic cooling through appropriate closure temperature of muscovite c 400 375 c combined with the previously reported geochronological data the oki metamorphic complex appears to have experienced peak metamorphic conditions at c 250 ma subsequently it cooled and was exhumed at the earth s surface at c 90 ma with cooling rate of c 5 c ma the oki metamorphic complex records a similar prograde metamorphic event as the hida metamorphic complex exposed in central japan the cooling and exhumation rates of the hida metamorphic complex were significantly more rapid compared with the oki metamorphic complex and they were exhumed with extensively intruded jurassic granites funatsu granites

pressure temperature and time indicators in mafic schist their application to reconstructing the polymetamorphic history of vermont mineral chemistry and overgrowth relationships in mafic schist from vermont are shown to be sensitive indicators of pressure temperature and relative time and to be good chronicles of the paleozoic history of this polymetamorphic terrane within the common assemblage amphibole chlorite epidote plagioclase quartz ti phase carbonate k mica fe 3 oxide electron microprobe analyses show that increasing metamorphic grade as defined by intercalated pelitic schist is recorded by an increase in the edenite glaucophane and tschermakite contents of amphibole in the anorthite content of plagioclase and in the substitution of al vi f 2 ti al iv for fe 2 mg mn si in biotite chlorite and muscovite with increasing pressure the glaucophane component of amphibole increases for medium pressure metamorphism the albite oligoclase gap is in the garnet zone where amphibole has between 1 2 and 1 8 formula proportion al iv and al vi fe 3 ti cr this gap is below the garnet isograd in low pressure mafic schist where al iv and al vi fe 3 ti cr in the amphibole are both less than 0 6 mineral growth periods observed are characterized by metamorphic grade and facies series and are assigned to two ordovician taconic and two devonian acadian events silurian devonian mafic schist in northeastern vermont records two periods of low pressure devonian metamorphism these events are expressed by medium pressure and low pressure metamorphism in the cambrian and ordovician rocks to the west and south in the pre silurian rocks high pressure and medium pressure metamorphism is assigned to the ordovician the observed high pressure metamorphism is confined to a 110 by 40 km area along the green mountain anticlinorium axis in north central vermont

dating deformation and cooling in the caledonian thrust nappes of north sutherland scotland insights from 40ar 39ar and rb sr chronology 40 ar 39 ar and rb sr mineral ages have been determined from various lithologies exposed in the caledonian foreland and structurally overlying thrust nappes of north sutherland scotland rb sr muscovite ages of c 428 c 421 and c 413 ma obtained from moine thrust zone mylonites are interpreted to date closely regional thrusting during the late silurian to early devonian 40 ar 39 ar muscovite ages within the lower parts of the moine nappe are mostly anomalously old with respect to rb sr analyses of muscovites from the same samples it is likely that this discrepancy results from a component of extraneous or excess argon 40 ar 39 ar hornblende ages and rb sr and 40 ar 39 ar muscovite ages obtained from structurally higher metamorphic units in the caledonian thrust nappes generally range between c 440 ma and c 410 ma these ages are interpreted to date cooling during and following d 2 regional thrusting and folding within internal sectors of the nappe sequence a possible tectonic model involves the silurian collision of baltica with scottish segments of laurentia resulting in the scandian orogeny and broadly coeval moine thrust zone d 2 structures were superimposed on structures and metamorphic fabrics formed during a regional mid ordovician tectonothermal event dated previously at c 470 460 ma syn d 2 temperatures were generally 600 c and sufficient to achieve more or less complete thermal rejuvenation of rb sr and 40 ar 39 ar isotopic systems in muscovite and hornblende even in areas of low d 2 strain

cadomian 40ar 39ar apparent age spectra of detrital muscovites from the eastern alps concentrates of detrital muscovite were prepared from the bischofalm quartzite late ordovician exposed in the southalpine carnic alps austria two size fractions 150 175 pm and 85 100 m from two samples display similarly discordant 40 ar 39 ar age spectra in which apparent ages systematically increase from c 300 ma to c 600 ma in low temperature portions of each analysis intermediate and high temperature increments are characterized by apparent ages of c 620 645 ma characteristics of the age spectra suggest minor late palaeozoic variscan or combined variscan alpine rejuvenations of argon isotopic systems within detrital grains which originally crystallized within a late precambrian source terrain the detrital muscovite ages provide evidence for a cadomian age source region for some basement sequences within the austroalpine southalpine sectors of the eastern alps the occurrence of cadomian age detritus in the eastern alps suggests links with armorica and other similarly aged tectonic elements within alpine chains initiating along northern margins of gondwana

40 ar 39 ar ages of detrital white mica from upper austroalpine units in the eastern alps austria evidence for cadomian and contrasting variscan sources five detrital white mica concentrates from very low grade metaclastic sequences within pre variscan basement and post variscan cover units of the upper austroalpine nappe complex eastern alps have been dated with 40ar 39ar incremental heating techniques to constrain the age of tectonothermal events in their respective source areas two samples from early palaeozoic sandstone exposed within the same alpine nappe record slightly discordant age spectra the maximum age recorded in one is 562 2 0 7 ma whereas the other yielded a 40ar 39ar plateau age of 607 3 0 3 ma these results indicate a source area affected by cadomian tectonothermal activity three detrital muscovite concentrates from post variscan late carboniferous and permian cover sequences exposed within three different alpine nappes yielded 40ar 39ar plateau ages of 359 6 1 1 ma 310 5 1 2 ma and 303 3 0 2 ma the contrasting detrital white mica ages are interpreted to reflect different source areas detrital muscovite from a post variscan carboniferous molasse type sequence and from a permian verrucano type sequence record ages which indicate late variscan e g 330 300 ma metamorphic sources by contrast detrital white mica from another permian verrucano type sequence suggests a source area affected by early variscan e g 400 360 ma metamorphism these results help clarify palinspastic relationships and tectonic correlations between pre late carboniferous metamorphic basement sequences and carboniferous to permian cover sequences

40ar 39ar muscovite dates from the nappe region of southwestern norway dating extensional deformation in the scandinavian caledonides abstract the caledonian thrust nappes and basal decollement zone above the baltic shield record penetrative caledonian deformation related to ese directed nappe transport and subsequent wnw directed ductile shearing during extensional collapse of the orogen muscovite within proterozoic quartzite conglomerates in the lower bergsdalen nappe that record only ese vergent caledonian deformation display internally concordant 40ar 39ar age spectra which yield plateau ages of 403 ma and 398 ma similar plateau ages 402 and 399 ma were obtained from muscovite from mica schists which record additional penetrative wnw directed shearing suggesting that muscovite intracrystalline argon systems did not cool below appropriate closure temperatures until during or after extensional wnw vergent movements quartz microfabrics indicate that temperatures were higher than those required for argon retention in muscovite until the end of the wnw directed nappe translation extension and that lower temperatures were maintained during subsequent movements along w and nw dipping extensional ductile shear zones the 40ar 39ar muscovite ages are therefore interpreted to post date the extensional wnw directed shearing along the decollement zone and closely date the initiation of w and nw dipping extensional shear zones when compared to the age of contractional deformation the results confirm that extension followed orogenic contraction very shortly after the main scandian orogenic phase

tectonic setting of the sandia pluton an orogenic 1 4 ga granite in new mexico structural studies of the circa 1 42 ga sandia pluton and its aureole document significant deformation synchronous with pluton emplacement and call into question the anorogenic label associated with this and other 1 4 ga granites in the southwestern united states the se margin of the pluton is a 1 to 2 km wide nw dipping ductile shear zone field and microstructural observations melt filled shear bands high temperature dynamic recrystallization of k feldspar megacrysts and crosscutting pegmatite dikes indicate that top to the nw normal movement in the shear zone took place in the presence of melt subparallel magmatic fabrics north of and structurally above the shear zone contain kinematic indicators consistent with top to the nw shear sense suggesting that over large regions of the pluton magmatic flow mimicked solid state strain in the northern aureole contact metamorphic aluminosilicate porphyroblasts grew during the formation of a ne striking crenulation cleavage s3 and related folds of late stage pegmatite dikes these features document the synchroneity of magma emplacement shortening and metamorphism and indicate that the sandia pluton is syntectonic not anorogenic we interpret the kinematic consistency of structural elements from the base of the pluton the interior of the pluton and the northern aureole to reflect a regional larger than the pluton strain field and suggest that the orogeny recorded in and around the sandia pluton involved a three dimensional strain field with subhorizontal extension n s and contraction e w directions n s extension is documented by the orientation of mineral lineations and movement directions in the basal shear zone and in high strain zones in the northern aureole and by the orientations of tabular pegmatite and aplite dikes in the pluton and aureole east to se shortening is documented in the northern aureole by orientations of folded pegmatite dikes and associated s3 crenulation cleavage and east to se shortening or least extension directions in the pluton proper are documented by the intersections of orthogonal dikes thus emplacement of the sandia pluton is interpreted to record a snapshot of regional strains inboard of an active plate margin rather than local strains generated by emplacement

the ibero armorican arc a collision effect against an irregular continent the ibero armorican arc is the main variscan macrostructure in western europe although it was recognized in the 1920s its genesis is still debatable and the proposed models for its generation questionable however even if some doubts persist it seems that most agree on the general geodynamic evolution of this virgation in both branches in the middle to late devonian iberia was deformed by a sinistral transpressive regime while in the northern branch thrusting events were predominant in the carboniferous a dextral transpression begins to predominate in the armorican branch while the southern branch was deformed by southward thrusting in an attempt to correlate these events we propose that during the late devonian a cantabrian indentor moved northward producing the oblique closure of the southern part of the rheic ocean and an almost orthogonal closure in central europe in the carboniferous the collision with the irregular margin of laurasia induced a rotation of the indentor the intracontinental deformation was then achieved by dextral transpression in the northern branch and thrusting in the southern one

high pressure micro inclusions and development of an inverted metamorphic gradient in the santiago schists ordenes complex nw iberian massif spain evidence of subduction and syncollisional decompression the santiago schists are located in the basal unit of the ordenes complex one of the allochthonous complexes outcropping in the inner part of the hercynian belt in the north west of the iberian peninsula their tectonothermal evolution is characterized by the development of an eo hercynian metamorphic episode c 374 ma of high p low to intermediate t the mineral assemblage of the high p episode is preserved as a very thin si s1 foliation included in albite porphyroblasts being composed of albite garnet i white mica 1 chlorite 1 epidote quartz rutile ilmenite the equilibrium conditions for this mineral assemblage have been estimated by means of different thermobarometers at 495 10 c and 14 7 0 7 kbar probably minimum pressure the later evolution syn d2 of the schists defines a decompressive and slightly prograde p t path which reached its thermal peak at c 525 10 c and 7 kbar decompression of the unit occurred contemporaneously with an inversion of the metamorphic gradient so that the zones of garnet ii biotite with an upper subzone with chloritoid and staurolite developed from bottom to top of the formation the estimated p t path for the santiago schists suggests that the basal unit probably a fragment of the gondwana continental margin was uplifted immediately after its subduction at the beginning of the hercynian orogeny it also suggests that the greater part of the unroofing history of the unit took place in a context of ductile extension probably related to the continued subduction of the gondwana continental margin and the contemporaneous development of compensatory extension above it the inverted metamorphic gradient seems related to conductive heat transferred from a zone of the mantle wedge above the subducted continental margin when it came into contact with the upper parts of the schists along a detachment probably of extensional character the general metamorphic evolution of the santiago schists with the development of high p assemblages with garnet prior to decompressive and prograde parageneses with biotite is unusual in the context of the european hercynian belt and shows a close similarity to the tectonothermal evolution of several high p low to intermediate t circum pacific belts

late archaean early proterozoic source ages of zircons in rocks from the paleozoic orogen of western galicia nw spain abstract u pb data are reported for nine suites of zircons and three monazites from the paleozoic orogen in western galicia nw spain one paragneiss and six orthogneisses from the early paleozoic basement and two carboniferous ca 310 ma old intrusions of two mica granite new whole rock rb sr analyses along with earlier data indicate an age of ca 470 440 ma ordovician for the emplacement of the granitic precursors of the orthogneisses monazite from the paragneiss also yields an u pb age of ca 470 ma for all nine investigated suites of zircons the u pb systematics signal the presence of a minor proportion of precambrian zircon the zircon data from two orthogneisses and the paragneiss display roughly linear arrays with upper intercepts between about 3 0 ga and 2 0 ga and lower intercepts between ca 480 and 460 ma the former are interpreted as approaching the age of the old zircon component and the latter as reflecting either the time of crystallization of new zircons from the magma orthogneisses or the time of radiogenic lead loss from the old zircons paragneiss the suites of zircons from all other investigated orthogneisses suffered isotopic disturbance posterior to the granite emplacement 470 440 ma ago in most cases leading to false discordias without geochronological significance similarly the zircons of the two investigated two mica granites do not produce meaningful discordias because of post paleozoic disturbance the monazite u pb systems of the latter granites indicate sub recent lead loss from the high initial 87 sr 86 sr ratios an involvement of precambrian continental crust material is evident in the generation of the early paleozoic suite of granites while the zircon u pb data give evidence of the presence of about 3 0 2 0 ga old late archaean early proterozoic components in the source material zircons from the oldest sedimentary rocks in the area now present as catazonal paragneisses and a likely source for the granites likewise reveal a provenance age of 3 0 2 0 ga the late archaean early proterozoic source rocks in western galicia probably formed part of an archaean continental crust underlying much of western europe but mostly modified beyond recognition by younger events

paleomagnetism of the central iberian curve s putative hinge too many oroclines in the iberian variscides abstract the variscan mountain belt in iberia defines a large s shape with the cantabrian orocline in the north and the central iberian curve an alleged orocline belt of opposite curvature to the south the cantabrian orocline is kinematically well constrained but the geometry and kinematics of the central iberian curve are still controversial here we investigate the kinematics of the central iberian curve which plays an important role in the amalgamation of pangea since it may have accommodated much of the post collisional deformation we have performed a paleomagnetic study on carboniferous granitoids and cambrian limestones within the hinge of the curve our paleomagnetic and rock magnetic results show a primary magnetization in the granitoids and a widespread carboniferous remagnetization of the limestones syn kinematic granitoids show ca 70 counter clockwise rotations consistent with the southern limb of the cantabrian orocline post kinematic granitoids and cambrian limestones show consistent inclinations but very scattered declinations suggesting that they were magnetized coevally to and after the 70 rotation our results show no differential rotations between northern southern limb and the hinge zone therefore we discard a late carboniferous oroclinal origin for the central iberian curve

geodynamic evolution of the continental allochthonous terrane cat of the braganca nappe complex ne portugal the braganca massif in particular its continental allochthonous terrane cat is a keystone in the comprehension of the pre variscan and variscan geodynamic evolution of the eurasian plate in particular of the high grade metamorphic evolution the braganca cat comprises three types of granulites recording quite distinct metamorphic histories and paragneisses with enclosed eclogite boudins it has undergone five characterizable main deformation phases during two orogenic cycles separated by the extensive intrusion of mafic ultramafic layered bodies types 2 and 3 granulites into a pervasively deformed granulitic lower continental crust type 1 granulites geochronological data obtained on high grade metamorphic rocks from braganca and other similar rocks in the variscan fold belt have been typically interpreted in two different ways polycyclical evolution precambrian and paleozoic variscan or monocyclical evolution variscan this second hypothesis is in clear contradiction with field evidence with tectonometamorphic data and even with some geochronological data as shown in this article thus we suggest that the tectonometamorphic evolution of the braganca cat and by comparison of other similar cat in the variscan fold belt took place during two orogenic cycles one precambrian grenvillian pan african cadomian and another one paleozoic variscan

iberian late variscan granitoids some considerations on crustal sources and the significance of mantle extraction ages a suite of post tectonic granitoids mostly peraluminous broadly i type granodiorites and monzogranites and mafic rocks from nwiberia with crystallization ages between ca 309 and 290 ma has been investigated for sm nd isotopes and inherited zircon content in order to constrain the nature of their source rocks end values at 300 ma vary from 0 2 to 5 9 and tdm values range from 1 01 to 1 58 ga inherited xenocrystic zircons yielded ages ranging from 458 to 676 ma with 90 of data between 490 and 646 ma corresponding to neoproterozoic mostly ediacaran cambrian andordovician ages only three highlydiscordant analyses yielded ages older than 650 ma based on the data reported herein and relevant data fromthe literaturewe contend that post tectonic granitoids of the iberian variscan belt with exception of the scarce anatectic s type granitoids were derived mostly from metaigneous lower crustal sources which in turn were ultimately derived from a subcontinental lithospheric mantle enriched between ca 0 9 and 1 1 ga i type granitoids and mantle derived mafic rocks both underwent varying degrees of contamination by ametasedimentary lower crust depleted in pre 650 ma zircon through previousmelting episodes with a time integrated sm nd evolution different to that of the metaigneous lower crust participation of this metasedimentary crust in the genesis of these granitoids may account for nd isotopic variability and nd model ages well in excess of 1 2 ga

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

palaeomagnetism and 36ar40ar vs 39ar40ar isotope correlation ages of dyke swarms in central kerala india tectonic implications palaeomagnetic results and whole rock 36ar40ar vs 39ar40ar isotope correlation ages are presented for dyke swarms occurring within the granulite terrain in central kerala west coast of india palaeomagnetic data show that the nnw sse leucogabbro dykes sampled are normally magnetised d 307 and i 57 whereas most nw se dolerite dykes are reversely magnetised d 163 and i 61 one dolerite dyke shows normal polarity the present 40ar39ar isotope ages for dolerite dykes and previously reported k ar ages for leucogabbros suggest 81 2 ma for acquisition of the leucogabbro directions and 69 1 ma for acquisition of the reverse magnetic directions within the dolerites based on palaeomagnetic and isotopic age data the dolerite dykes appears to represent feeder systems for the deccan volcanic sequences we suggest that the deccan eruptions were widespread and probably extended southward at least into central kerala the entire magmatic episode may have occurred between ca 70 and 65 ma during the 31r period probably corresponding to the cretaceous tertiary boundary the leucogabbros may correspond to the 34n superchron and represent a plume related magmatic episode precursory to the main volcanic event these data suggest that an early intraplate plume tectonic setting since ca 80 ma preceded lithospheric thinning asthenospheric upwelling and consequent massive flood volcanism during the period 70 65 ma this culminated in ocean floor spreading between the seychelles mascarene and continental india

argon retentivity of hornblendes a field experiment in a slowly cooled metamorphic terrane hornblende from samples of amphibolite and granitic gneiss collected within a single outcrop in the central adirondacks yield significantly different sup 40 ar sup 39 ark sub k dates of 948 plus minus 5 and 907 plus minus 5 ma assuming that this terrane cooled slowly following high grade metamorphism and that the samples have experienced the same thermal history the difference in dates apparently reflects a corresponding difference in blocking temperature for diffusion of radiogenic argon in these hornblende samples the fe fe mg mn of the hornblende samples are 0 8 and 0 6 the higher ratio corresponding to the younger sup 40 ar sup 39 ar sub k date transmission electron microscopy observations indicate that both hornblende samples are homogeneous and devoid of any exsolution but contain zones of fibrous phyllosilicates approximately 0 1 to 2 mu m wide parallelling 100 and 110 these alteration zones probably formed during post metamorphic cooling as a result of the migration of fluids through the hornblendes and are obvious pathways for argon escape from hornblende as these features are more abundant in the hornblende sample with the younger sup 40 ar sup 39 ar date and higher fe fe mg mn they may influence the argon blocking temperature by effectively partitioning the hornblende grains intomore diffusion domains of varying size biotite from the granitic gneiss yields an sup 40 ar sup 39 ar sub k date of 853 plus minus 2 ma with a mildly discordant step heating spectrum that in part reflects the degassing of submicroscopic inclusions precipitated during alteration of the host biotite plagioclase from the amphibolite yields a sup 40 ar sup 39 ar sub k integrated date of 734 plus minus 3 ma less

exsolution in hornblende and its consequences for 40ar39ar age spectra and closure temperature abstract step heating studies of hornblende and biotite which coexist in an amphibolite near north walpole vermont reveal dramatically different 40 ar 39 ar age spectra the biotite mg 59 is characterized by a plateau over 98 percent 39ar release at 343 5 ma in strong contrast to the amphibole which exhibits both a complex age spectrum and a k ar age significantly younger than the biotite the amphibole release pattern contains three complete age cycles which range between extremes of 199 ma and 408 ma interestingly the k ca ratio calculated from reactor produced isotopes also varies with ar release but follows a trend which is antipathetic to the 40 ar 39 ar age pattern detailed microstructural examination including transmission electron microscopy tem reveals a complex exsolution structure that is consistent with both the complexity of the release pattern as well as the low degree of radiogenic 40ar 40ar retentivity in the amphibole this result underscores the variability of closure temperature tc for 40ar in metamorphic amphiboles and should warn against accepting a single universal value of tc

late precambrian tectonothermal evolution of the malverns complex the malverns complex is a variably deformed and metamorphosed late precambrian calc alkaline plutonic suite exposed within the avalon terrane of central england published u pb zircon and monazite ages indicate emplacement at c 680 670 ma 40ar 39ar mineral cooling ages presented here place constraints on the timing of 1 upper greenschist to low amphibolite facies metamorphism and associated ductile deformation and 2 late hydrothermal activity and variable thermal reactivation hornblende from meta diorites records 36ar 40ar v 39ar 40ar isotope correlation ages of c 649 ma and c 652 ma these are significantly younger than the crystallization age of the complex and are interpreted to date cooling following metamorphism they further provide maximum age constraints for the development of mylonitic fabrics that overprint metamorphic textures hornblende from a diorite net veined by late granite pegmatites that are discordant to the mylonitic fabrics records a significantly younger isotope correlation age of c 610 ma this is interpreted to date a static thermal rejuvenation associated with pegmatite emplacement and places minimum age constraints on the timing of mylonitization a plateau age of c 597 ma recorded by muscovite from a greisen vein within a granite is interpreted to date closely the hydrothermal alteration of the complex detrital muscovite of probable metamorphic origin within a lower cambrian sandstone records a plateau age of c 598 ma and may have been derived from local units which were first metamorphosed at c 650 ma and later reheated at c 610 600 ma the 40ar 39ar ages are consistent with episodic tectonothermal activity within the malverns complex during the interval c 650 600 ma and indicate a more complex precambrian history than previously recognized for the avalon terrane of southern britain

p t t history of the lower austroalpine nappe complex in the tarntaler berge nw of the tauern window implications for the geotectonic evolution of the central eastern alps new petrologic and 40ar 39ar geochronologic data constrain conditions of alpine metamorphism along the northwestern border of the tauern window the p t estimations based on phengite barometry were determined for samples from units of the lower austroalpine nappe complex exposed above the southpenninic interior of the tauern window and from upper parts of the southpenninic bundner schiefer sequence results suggest that both mesozoic metasedimentary nappe units reckner and hippold nappes and an ophiolitic nappe reckner complex of the lower austroalpine nappe complex have been metamorphosed at pressures between 8 and 10 5 kbar and temperatures around 350 c the structurally highest lower austroalpine unit quartzphyllite nappe was not affected by high pressure metamorphism and records maximum p t conditions of approximately 4 kbar and 400 c highest parts of the structurally underlying southpenninic bundner schiefer sequence were metamorphosed at intermediate pressures 6 7 kbar temperatures increased in all structural units during decompression whole rock 40ar 39ar plateau ages of silicic phyllites and cherts with abundant high si phengites record ages around 50 ma in the reckner nappe and 44 37 ma in the hippold nappe and southpenninic bundner schiefer sequence these ages are interpreted to date closely the high pressure metamorphism

variscan exhumation of a subducted paleozoic continental margin the basal units of the ordenes complex galicia nw spain a structural and metamorphic study was carried out in the basal units of the ordenes complex in spain thought to represent a subducted part of the paleozoic margin of gondwana according to their metamorphic evolution this part of the margin was subducted at the onset of the variscan orogeny becoming part of an accretionary complex developed below a colliding element built previously variations in the pt conditions of the first high pressure metamorphic event along the units indicate a polarity of the subduction to the west subsequent underthrusting of more continental material blocked the subduction and triggered the ascent and exhumation of the basal units whereas the convergence continued recumbent folds and thrusts developed along with successive normal detachments compressional and extensional structures were synchronous or alternated in time and together induced the thinning and tapering of the orogenic wedge and its lateral spreading the unroofing took place locally under an inverted temperature gradient caused by a detachment which carried a part of the hot mantle wedge above the subduction zone over the subducted units

climatic variability over the last 30 000 years recorded in la piscina de yuriria a central mexican crater lake the trans mexican volcanic belt provides an excellent setting for reconstruction of late quaternary climate from different natural archives moreover human impact on the landscape since the mid holocene provides a good opportunity to investigate the complex interplay of natural and anthropogenic forcing of landscape change however despite the wealth of records understanding of the environmental history of the region and its wider significance for climate change across the northern neotropics remains incomplete we present a radiocarbon dated multiple proxy sedimentology sedimentary geochemistry ostracods diatoms stable isotopes record of climatic and environmental change based on the lacustrine sediments from la piscina de yuriria a hydrologically closed volcanic crater in the northern tmvb much of the last glacial interval was characterised by low effective moisture associated with a weakened north american monsoon nam although the interval from 30 000 to 27 500 abp experienced abrupt changes in rainfall the period corresponding to the late glacial stadial was also dry and the lake may have dried out at this time there was a change to wetter but variable conditions during the early holocene as the nam strengthened progressive drying during the later holocene was accompanied by phases of catchment disturbance which were partly the result of human impact

polyphase variscan emplacement of exotic terranes morais and braganca massifs onto iberian successions evidence from 40ar 39ar mineral ages abstract the braganca and morais nappe complexes comprise internally imbricated continental tectonic units including schists deformed metagranite granulite and local eclogite and a structurally underlying ophiolite nappe high pressure assemblages in some of the continental sequences were variably retrogressed during an amphibolite facies metamorphic overprint by contrast rocks within the ophiolite nappe display prograde amphibolite facies assemblages hornblende concentrates from both tectonic units exposed in the braganca nappe complex record 40 ar 39 ar plateau and isotope correlation ages of c 385 390 ma a similar date is recorded by hornblende within mylonitic amphibolite ophiolite protolith along a ductile thrust which separates the two major nappe units muscovite within blastomylonitic metagranite lagoa augen gneiss in upper structural levels of the morais massif records 370 375 ma 40 ar 39 ar plateau ages muscovite within phyllinite developed along a ductile thrust separating the gneiss from the structurally underlying ophiolite nappe records a 375 ma age together the 40 ar 39 ar data suggest that a regionally significant early variscan amphibolite facies metamorphism occurred prior to c 385 390 ma early devonian this was associated with ductile imbrication of continental and ophiolite structural units with resultant formation of a composite nappe complex the composite nappe complex must have been maintained at relatively high structural levels throughout the remaining variscan evolution because there is no evidence for post 370 ma rejuvenation of intracrystalline argon systems in any of crystalline structural units examined this is in marked contrast to a significant late devonian c 330 ma locally high pressure metamorphism which is recorded in structurally underlying elements of the lower allochthon part of a foreland thrust complex composed of metasedimentary units with iberian protolith affinities together available chronologic constraints indicate emplacement of the previously assembled composite nappe complex into its present relative structural position at c 330 ma with resultant deformation and metamorphism of structurally underlying components of the iberian thrust complex

geochemical and geochronological cross section of the deep variscan crust the cabo ortegal high pressure nappe northwestern spain the high pressure rocks of the cabo ortegal nappe and related massifs are represented by several tectonometamorphic units which underwent separate histories before regional overthrusting and amphibolite facies recrystallization at 380 ma the occurrence of final cooling is recorded by 40ar 39ar and rb sr mica ages of up to 350 ma u pb zircon dating and sm nd and rb sr isotope data together with ree and major and trace element geochemistry provide constraints on the timing of earlier tectonothermal events and the nature of the protoliths the earliest tectonothermal event occurred at 490 480 ma and corresponds to the emplacement of lree enriched magmas of probable calc alkaline affinity and their metamorphism in the granulite facies this occurred at approximately the same time as the eclogite facies metamorphism of morb like protholiths associated with basic to intermediate granulites together this suggests formation of syn accretion granulites in an active margin setting a second high grade metamorphic event is recorded at 420 ma in metasedimentary formations associated with eclogite like assemblages garnet omphacite plagioclase this event is possibly related to a collisional plate tectonic setting in a back arc basin the age and origin of the associated ultrabasic rocks remains unclear but a late pyroxenite vein has yielded an emplacement age of 390 ma

the origin of the variscan upper allochthons in the ortegal complex northwestern iberia sm nd isotopic constraints on the closure of the rheic ocean northwestern iberia preserves a stack of allochthons in which the vestiges of a suture zone generated during the variscan orogeny by the late paleozoic collision between laurussia and gondwana are exposed lower allochthons contain ordovician ophiolite known as the lower ophiolite and are structurally overlain by devonian ophiolite upper ophiolite which are in turn structurally overlain by allochthons containing high grade metamorphic rocks with continental affinities and late cambrian early ordovician protolith ages known as the upper units geochemical and sm nd isotopic data from the upper ophiolite and the structurally overlying upper units exposed in the ortegal complex of galicia show that these allochthons are derived from a variety of mantle and crustal sources and indicate that the suture zone juxtaposes a variety of oceanic assemblages the general isotopic characteristics of each assemblage are similar to allochthons in other variscan complexes in nw iberia suggesting that the allocht

anisotropy of magnetic susceptibility constraints on variscan obduction processes in the braganca massif ne portugal abstract in this paper we present an anisotropy of magnetic susceptibility ams and structural study of high grade metamorphic and ophiolitic rocks from the braganca massif trying to contribute to the debate concerning the identification of transport directions during the closure of the variscan ocean the study area comprises high grade metamorphic rocks of the continental allochthonous terrane cat and the northern ophiolite terrane not in the braganca massif mineral parageneses and fabrics were characterized through conventional petrographic and microstructural studies electron microprobe and high field and thermomagnetic analysis with very few exceptions the magnetic lineations and foliations were coincident within limited error with the observed mesoscopic mylonitic and metamorphic foliations and with the mineral and stretching lineations in places of the studied major tectonic contacts where the mineral and stretching lineations were obliterated by post kinematic recrystallization ams data revealed a magnetic lineation ams results confirmed the nnw sse to n s amphibolite facies lineation common to cat and not and revealed an e w greenschist facies lineation in not ams results combined with detailed structural and metamorphic data from previous work showed that there are two e w lineations separated in time 1 an e w mesoscopic and magnetic lineation in competent granulites that is older than the n s magnetic lineation in more ductile gneisses of a major shear zone within the cat 2 an e w magnetic lineation in incompetent greenschists of the not younger than the nnw sse to n s mesoscopic and magnetic lineations found in more competent amphibolites of cat and not the nnw sse to n s lineations can be interpreted as the result of the uppermost allochthonous terranes transport to the nnw over the iberian terrane dated with 40 ar 39 ar at ca 390 ma