how does orogenic crust deform evidence of crustal scale competent behaviour within the partially molten middle crust during orogenic compression abstract granitic partial melts are generally thought to significantly weaken the orogenic crust and ultimately lead to the collapse of an orogen studies from different orogens have shown that the syn melt deformation behaviour at the orogenic scale is however more complex in addition once fully crystalline granitic material strengthens the crust linking the evolution from melt present to melt absent deformation at the scale of the orogen is a challenging but necessary task if one wishes to investigate the overall behaviour of the middle crust in this paper we make observations of orogen scale deformation developed over a period of 30 40 ma during crustal partial melting of the middle and lower crust of a palaeoproterozoic orogen the crust shows a globally common pattern where coeval partial melting compressional deformation and transtensional structures co exist we demonstrate this complex interaction through an integrated approach using multiple datasets the key observation is that of widespread regular orogen scale shortening by folding of sub horizontal anatectic granitic sheets and highly migmatized lithologies this shortening interacted with modest orogen parallel lateral stretch expressed as local extensional transtensional structures the pervasive and dominant deformation style is that of folding of the syn kinematic granite sheets and voluminous migmatites this demonstrates that the principal deformation style of deformation within the partially molten mid crust is competent rather than weak the observed evidence of weak behaviour such as strike slip or transtensional shear zones accommodating lateral escape are localized in comparison and not necessarily associated directly with the granite sheets or the highly migmatized volumes of the crust moreover the strain intensity fold wavelengths and amplitudes seems independent of the overall melt fraction the implications are that while widespread volumes of partial melt will weaken the crust overall compared to the brittle upper crust and while individual melt bodies can possibly persist for up to some millions of years in the middle crust i the relative strength between partially molten volumes is not primarily controlled by melt fraction and ii individual melt volumes may be too short lived at the time scale of orogenic deformation i e orogenic strain rates to significantly influence the overall deformation style

late orogenic svecofennian deformation in sw finland constrained by pegmatite emplacement ages late stages of the svecofennian orogeny in sw finland were related to a tranpressional stress field and dextral movements along crustal scale shear zones under decreasing temperature and pressure conditions in the kemio area a minimum estimate for the time span of movements along one of these shear zones is obtained from the ages of 1840 to 1830 myr old microcline granite sheets related to early ductile deformation and by dating structurally late rare mineral granite pegmatites related to brittle deformation one pegmatite was emplaced when the rheological conditions in the gabbro changed from ductile to semi ductile and brittle it has u pb ferrotapiolite ages ranging between 1807 0 2 9 myr 2 s and 1803 1 2 9 2 0 myr 2 s another pegmatite emplaced under brittle conditions has an u pb ferrotapiolite age of 1802 9 1 3 myr 2 s these pegmatites were emplaced preferentially in gabbroic rocks that showed brittle to semi ductile deformation at a time when their more felsic host rocks still showed ductile deformation the age range bracketed by the microcline granite sheets and the structurally late pegmatites indicates that ductile deformation related to the transpressional late svecofennian tectonic regime in southwestern finland persisted for at least 30 40 myr

cordierite orthopyroxene rocks the granulite facies equivalents of the orijarvi cordierite anthophyllite rocks in west uusimaa southwest finland abstract cordierite hypersthene rocks with a chemistry comparable to the cordierite anthophyllite rocks of orijarvi have been found in the high grade west uusimaa complex sw finland chemistry petrography and spatial relations indicate that the cordierite hypersthene rocks represent the high grade equivalents of the orijarvi cordierite anthophyllite rocks metamorphic conditions for the orijarvi and west uusimaa complex occurrences are estimated at respectively 550 650 c 3 5 kbar low h 2 o activity and 710 810 c 3 5 kbar low h 2 o high co 2 activity cordierte anthophyllite rocks are generally associated with massive cu pb zn deposits it is suggested that the cordierite hypersthene rocks might be potential guides to cu pb zn deposits in high grade regions

the early proterozoic orijarvi triangle southwest finland a key area on the tectonic evolution of the svecofennides abstract a consistent pattern of three successive deformation phases is present in the west uusimaa area in the south svecofennian schist belt of finland in which the orijarvi triangle bordered by shear zones forms a low strain area it is shown that the well preserved early folds and foliation in the orijarvi triangle correlate with the regionally oldest and completely transposed structures a model is given for the tectonic development of the west uusimaa area involving 1 an early phase of subhorizontal probably thrust related folding 2 a second phase of crustal shortening and 3 a late phase of folding and formation of vertical shear zones the first two phases took place during prograde metamorphism whereas the last one records retrogression shear zones were probably active during all phases subhorizontally thrusting during d 1 transversely wrenching during d 2 and vertically uplift during d 3 it is suggested that the d 1 d 2 sequence represents a progressive deformation in a plate collision system whereas d 3 is related to a 1533 23 ma old post svecofennian possibly gothian event a model is presented in which the development and preservation of the orijarvi triangle is explained by the local development of transversal shear zones along anatectic plutons during d 2 these shear zones were then reactivated during d 3 the model presented implies that the sigmoidal pattern present in southern finnish geology was caused during a late stage of the d 2 crustal shortening event along three broad parallel sinistral shear zones it is suggested that these shear zones represent deep seated parts of wrench faults in a complex plate tectonic system involving several island arcs and that they were caused by differential movement of the south svecofennian schist belt with respect to the tampere schist belt

zircon and monazite geochronology of deformation in the pielavesi shear zone finland multistage evolution of the archaean proterozoic boundary in the fennoscandian shield the raahe ladoga shear complex is a major crustal structure representing the archaean palaeoproterozoic boundary in the fennoscandian shield the complex developed during the svecofennian orogeny c 1 9 1 8 ga beginning with regional thrust tectonic phases d 1 and d 2 followed by large scale shearing events d 3 and d 4 the pielavesi shear zone is a vertical north south trending shear zone within the raahe ladoga shear complex formed during regional d 3 shearing and later reactivated during the regional d 4 phase three north south trending elongate granitoid intrusions were selected as representative of silicic melts that intruded the transtensional pielavesi shear zone during the regional d 3 phase the oriented magmatic fabric of the granitoids indicates that they intruded coeval to the deformation event the zircon u th pb secondary ionization mass spectrometry sims ages of these intrusions 1888 4 1884 6 and 1883 5 ma overlap within error and provide a direct age for the regional d 3 deformation e hf t 1 1 to 3 4 and e nd t 1 2 to 0 4 values from these granitoids are both consistent with a predominantly juvenile source affected by a minor archaean component u th pb sims analyses of metamorphic monazite formed within a crosscutting blastomylonite provide an age for the regional d 4 phase and associated fluid activity of 1793 3 ma supplementary material analytical methods and tabulated analytical data are available at https doi org 10 6084 m9 figshare c 3498501

xenoliths in a mafic dyke at popes harbour nova scotia implications for the basement to the meguma group a recently recognized series of northwest trending mafic dykes cuts the cambro ordovician meguma group along the eastern shore of nova scotia some of the dykes contain gneissic and meta plutonic xenoliths interpreted here as including fragments of the basement to the meguma group most of the gneissic xenoliths in the dyke exposed at popes harbour are sillimanite and or kyanite bearing quartz poor metapelites and garnet orthopyroxene bearing tonalitic rock other enclaves include amphibolite quartzite gabbro granite granodiorite and granitic pegmatite textural features suggest that the earliest paragneissic assemblage included poikiloblastic garnet xfe 0 55 0 65 xmg 0 05 0 30 xca 0 02 0 16 xmn 0 02 0 16 biotite kyanite and or sillimanite oligoclase and quartz subsequent recrystallization at higher temperature and or lower pressure conditions is suggested by the replacement of kyanite by sillimanite mineral core composition data indicate early metamorphic conditions of 45

high pressure amphibolite granulite facies metamorphism in the paleoproterozoic rudall complex central western australia abstract the rudall complex forms the basement of the paleoproterozoic to neoproterozoic paterson orogen which was deformed during at least three major orogenies the complex comprises a sequence of metamorphosed clastic and chemical sediments and basalts deposited in a marginal basin environment prior to extensive felsic intrusion deformation of the sequence during the first yapungku orogeny between 2000 and 1760 ma included extensive thrust stacking from ene to wsw the thermal peak of the associated high pressure metamorphism was late to post tectonic with respect to this deformation thermobarometry on amphibolites and mafic granulites that contain various combinations of the minerals hornblende plagioclase quartz garnet othopyroxene and clinopyroxene indicates that temperatures peaked at around 800 c while pressures reached 1200 mpa the late timing of peak temperatures together with the presence in some amphibolites of amphibole plagioclase symplectitic coronas around garnet indicates a steeply decompressive clockwise p t t path peak metamorphic pressures indicate that the crust was locally thickened by at least 40 km during an event that showed similarities to phanerozoic collisional tectonics it is suggested that this deformational and metamorphic event records the collision between the pilbara craton and a continent to the northeast the paleoproterozoic evolution of australia has previously been attributed to ensialic processes characterised by low pressure metamorphism and anti clockwise p t t paths the metamorphic history of the rudall complex is not consistent with this view and the documented high pressure metamorphism is broadly synchronous with a medium pressure metamorphic event in central australia the strangways orogeny and possibly also in northwestern australia the capricorn orogeny collisional plate tectonic processes played a significant role in the paleoproterozoic evolution of northern australia

the amphibolite granulite facies transition in west uusimaa s w finland a fluid inclusion study this work presents the results of a fluid inclusion study of an amphibolite granulite facies transition in west uusimaa s w finland early fluid inclusions in the granulite facies area are characteristically carbonic co2 in contrast to predominantly aqueous early inclusions in the amphibolite facies area these early inclusions can be related to peak metamorphic conditions 750 820 c and 3 5 kbar for peak granulite facies metamorphism relatively young co2 inclusions with low densities 0 8g cm3 indicate that the first part of the cooling history of the rocks was characterized by a near isothermal uplift n2 ch4 inclusions with compositions ranging between pure ch4 and pure n2 raman spectral analysis were found in the whole area they are probably syn or even pre early inclusions only nearly critical homogenizing inclusions have been found low density pressure estimates based on densities of early fluid inclusions show that the rapid transition of amphibolite towards granulite facies metamorphism is virtually isobaric granulite facies metamorphism in west uusimaa is a thermal event probably induced by the influx of hot co2 bearing fluids

proterozoic tectonic evolution and late svecokarelian plate deformation of the central baltic shield the continental crust of the central baltic shield evolved by accretion towards the west during the svecokarelian orogeny 1700 2200 ma ago the following features are consistent with a plate tectonic mechanism involving subduction of oceanic crust below an archean craton in the east flysch sediments with serpentinite masses and pillow lavas linear high grade metamorphic zones island arc type volcanic belts and late tectonic batholiths with porphyry type cu mo deposits semi consolidated new crust was affected by late svecokarelian deformation dn after 1850 ma nne trending folds with crenulation cleavage were overprinted on older structures together with associated nw trending ductile transcurrent shear zones that curve the fn folds into gentle s and z shapes the late tectonic batholiths intruded partly at the same time as and partly after the dn deformation

thermal properties of granulite facies rocks in the precambrian basement of finland and estonia abstract we present results of heat production thermal conductivity and p wave velocity measurements of 252 rock samples from five granulite facies areas in finland and estonia these compositionally mainly intermediate archaean and palaeoproterozoic metamorphic rocks have relatively high heat production values mean values averaged by areas range from 0 57 to 2 24 w m 3 the lowest values are in the varpaisjarvi area which is the oldest and most mafic and where the highest metamorphic pressure occurred 8 11 kbar whereas the highest heat production is found in the turku granulite belt where the metamorphic pressure was 4 6 kbar the heat production decreases with the increase of the metamorphic pressure however a general numerical relationship cannot be presented because of considerable variations in heat production data there is no significant difference in the metamorphic temperatures of the studied granulites and therefore no dependence on temperature is found further our data suggest no relationship between heat production and p wave velocity the mean thermal conductivity of granulites varies from 3 0 to 3 5 w m 1 k 1 slightly elevated thermal conductivity values in the varpaisjarvi and lapland granulite areas can be attributed to higher sillimanite and quartz contents respectively

migmatization related to mafic underplating and intra or back arc spreading above a subduction boundary in a 2 0 1 8 ga accretionary orogen sweden abstract absolute ages of migmatization and protolith formation and constraints on the timing of ductile deformation in two major lithotectonic units in the south western part of a 2 0 1 8 ga orogenic belt in the fennoscandian shield south eastern sweden have been determined using u pb ion probe analysis of different generations of zircon in veined gneisses and leucocratic granite detrital and xenocrystic zircon in paragneiss and garnet bearing leucogranite respectively in the bergslagen lithotectonic unit show ages of 2 1 2 0 ga and 1 9 ga deposition of the sedimentary material occurred during or after a subduction related magmatic event at 1 91 1 87 ga two orthogneiss protoliths formed during this magmatic event around 1 88 ga while most zircon in the leucosome in a third migmatitic orthogneiss was inherited from a 1 85 ga igneous protolith a polyphase tectonothermal evolution with anatexis under low p metamorphic conditions around 1 86 ga m1 and 1 84 1 81 ga m2 is inferred for the migmatitic gneisses in the bergslagen unit garnet bearing leucogranite crystallised around 1 84 1 83 ga close in time to major folding of the m1 gneissic fabric a previously unrecognised 1 86 1 85 ga ductile deformational event under medium grade metamorphic conditions has been identified in the adjacent lithotectonic unit to the south smaland lithotectonic unit close in time to the m1 event in the bergslagen unit to the north by constraining the timing of anatexis and comparing with information bearing on crustal thickness excess mass at depth and the character and age of magmatic activity it is inferred that anatexis in the bergslagen lithotectonic unit is related to pulses of mafic underplating during the early stages of two separate subduction related magmatic episodes after the 1 91 1 87 ga magmatic event it is suggested that each pulse was related to intra or back arc spreading above a subduction boundary which had entered a retreating mode with separate long periods 20 50 ma of extension or transtension this study challenges the need to invoke crustal thickening related to plate collision at 1 9 1 8 ga as a mechanism to explain high grade metamorphism in the southern part of the 2 0 1 8 ga orogen instead a solely accretionary tectonic model involving an overriding plate along an active continental margin with significant extensional or transtensional crustal deformation is preferred

1 88 1 87 ga post kinematic intrusions of the central finland granitoid complex a shift from c type to a type magmatism during lithospheric convergence abstract the evolution of the svecofennian orogen spans from 1 95 ga to 1 80 ga with the main crust forming activity having taken place 1 90 1 87 ga ago subsequent deformation was largely concentrated to southern finland where transpressional structures indicate lithospheric convergence until 1 80 ga much of the svecofennian orogen in finland is occupied by the central finland granitoid complex cfgc that mainly consists of i type granodiorite and granite these 1 89 1 88 ga intrusive rocks are typically foliated and considered synkinematic a distinct suite of post kinematic undeformed or slightly foliated plutons crosscut the synkinematic rocks they are predominantly quartz monzonites and monzogranites and have been divided into three types according to their petrographic mineral and chemical characteristics compared to the calc alkaline synkinematic granitoids the post kinematic suite generally has an alkaline affinity with higher fe ti k ba zr and nb and lower mg ca and sr at a given sio 2 content the type 1 post kinematic plutons are peraluminous and contained a prominent sedimentary component in their source the types 2 and 3 plutons are marginally metaluminous to peraluminous the type 2 granites especially those in the western cfgc approach the classic 1 65 1 54 ga rapakivi granites of southern finland in their petrographic characteristics magmatic association tholeiitic mafic rocks and elevated contents of incompatible elements and hence have a type characteristics the type 3 plutons contain a pyroxene bearing margin type 3a or contain pyroxene throughout type 3b the type 3b rocks have c type charnockite magma type affinities and some plutons of this subgroup are alkaline the type 3a plutons appear to be transitional between the type 2 and type 3b plutons the synkinematic magmatic episode in the cfgc area probably involved partial melting of intermediate high k rocks in the lower crust with a magmatic addition from a mafic underplate this magmatic episode left a hornblende and biotite poor granulitic residue in the lower crust heat from the mafic underplate triggered partial melting of the granulite to produce the post kinematic magmatism and also introduced mafic material into the post kinematic magmas the post kinematic magmatism took place over quite a short time period ca 15 ma during overall lithospheric convergence it extended beyond the cfgc area and registers a temporal shift from the northeast toward the west and a general change in the character of magmatism from c type to a type the post kinematic magmatism probably resulted from extensional or transtensional events modifying the tectonically thickened crust partial melting of the mafic lower crust produced c type magmatism soon after the first melting episode the crust was already rather rigid and allowed the mafic magmas to follow a tholeiitic trend when the bimodal a type magmas were formed in the western cfgc

crystallographic orientation patterns of myrmekitic quartz a fabric memory in quartz ribbon bearing gneisses abstract crystallographic orientation patterns are found in myrmekitic quartz inclusions in plagioclase crystals in medium grade gneisses from a svecofennian fold belt in sw finland the development of crystallographic fabrics in myrmekite is studied in progressively deformed granitic veins deformation of these veins is accompanied by partial transformation of k feldspar to myrmekite which is a common synkinematic reaction in feldspar bearing rocks that are deformed in amphibolite facies conditions microstructures suggest that myrmekitic quartz nucleated epitaxially on a pre existing independent quartz crystal adjacent to the feldspar host an important consequence is that the orientation pattern of myrmekitic quartz reflects that of the preexisting crystals in the host rock at the time of nucleation as such the pattern represents a memory of an old fabric in annealed ribbon mylonites

prograde metamorphism of metapelites garnet biotite thermometry and prograde changes of biotite chemistry in high grade rocks of west uusimaa southwest finland abstract the metamorphic grade in west uusimaa sw finland increases from low to intermediate amphibolite facies in the west island of kemio up to granulite facies in the middle and eastern parts mineral assemblages in metapelitic rocks indicate metamorphic conditions of 700 825 c 3 5 kbar for the high amphibolite and granulite facies region which is in agreement with independent estimates these metamorphic conditions are used to evaluate garnet biotite thermometry zoning in garnet fe enrichment and mg depletion towards rims bordered by biotite or cordierite and a distinct fe depletion mg enrichment of biotites in contact with garnet are caused by fe mg exchange of these minerals during retrogression 550 c the general prograde metamorphic trend from medium to high grade in west uusimaa is reflected by a systematic change in the composition of biotites thermometry and biotite chemistry are used to indicate that the granulite facies metamorphism in west uusimaa is probably caused by a thermal event

high grade reworking of central australian granulites part 1 structural evolution abstract four distinct deformational events d2 d5 accompanying a granulite facies metamorphic cycle m2 m5 are shown to structurally overprint rework pre existing granulites m1 of the northeast strangways range in the central arunta block the first metamorphic cycle m1 at 1800 ma involved an anticlockwise p t path peaking at 850 950 c at 8 9 kbar m1 metamorphism involved widespread partial melting that produced stromitic migmatites and map scale concordant granitic gneisses isobaric cooling after the peak and accompanying hydration gave rise to a wide variety of coronitic and symplectitic reaction textures that enclose and replace m1 mineral parageneses no kinematic structuring is associated with m1 and the period encompassing the thermal peak and immediately subsequent are thought to have been absent of deviatoric stress the second metamorphic cycle m2 m5 possibly at 1400 1500 ma involved a clockwise p t path with a maximum p of 9 10 kbar and a thermal peak of approximately 800 c this metamorphic cycle accompanied the major ductile deformations recognized d2 d5 and has been labelled the proterozoic reworking the proterozoic reworking has been divided into two thermo barometric and structurally distinct periods d2 d3 involved regionally inclined ene wsw non coaxial shear of high bulk shear strains giving rise to an intense often mylonitic pervasive fabric s2 l2 and isoclinal and sheath folds on all scales f2 f3 s2 l2 is the first recognized kinematic fabric and does enclose and partially recrystallize m1 mineral parageneses including late m1 metamorphic reaction textures d2 d3 deformation was due to crustal shortening and gave rise to crustal over thickening loading accompanying prograde metamorphism m2 d4 d5 involved upright open and asymmetrical folding f4 and e w trending shear zone development s5 within a regionally extensive system of inclined oblique sinistral transpression both d2 d3 and d4 d5 episodes occurred under the same approximately e w directed compressive stress and are considered sequential episodes in the one tectonothermal cycle f4 folding accompanied significant melt formation and the peak of metamorphism of the m2 m5 metamorphic cycle s5 shear zones accompanied 3 4 kbar of decompression with cooling presumably during uplift in isostatic response to crustal over thickening in d2 d3 progression from inclined non coaxial shear to transpression occurred in response to an increase in the relative buoyancy forces limiting crustal thickening as is typical of many compressional orogens however the proterozoic reworking is atypical of simple linear or arcuate mountain belts because late stage shearing s5 is aligned sub parallel not at a high angle to the transport vector of the earlier ductile deformations d2 d3 consequently this region is modelled in terms of crustal shortening directed along the length of an e w trending orogen confined to the north and south by relatively stable crustal blocks when e w shortening could no longer be accommodated by crustal thickening strain was partitioned by sinistral transpression between the north and south bounding crustal blocks

very high co2 cordierite from norwegian lapland mineralogy petrology and carbon isotopes the most co2 rich cordierite thus far encountered in nature with about 2 2 wt co2 and 0 3 wt h2o occurs as large poikiloblasts in a strange non foliated reaction rock that dissects well foliated granulites being part of the classical lapland granulite area described by eskola the cordierite is optically positive with the highest optic angle 2vx 106 and birefringence 0 017 ever measured on natural cordierites but it is also optically very heterogeneous due to secondary loss of co2 along fractures and zones paralleling the fluid bearing channels based on the optical properties of the degassed lapland cordierite and on literature data a ternary diagram is given which shows the variations of this cordierite in 2vx and birefringence as a function of channel filling with both co2 and h2o

geothermometry geobarometry and the al2sio5 triple point at mt moosilauke new hampshire the distribution of aluminum silicate minerals in pelitic schists constrains pressure and temperature conditions during the acadian orogeny to have been approximately those of the al 2 sio 5 invariant point thus providing an excellent opportunity to evaluate the internal consistency of a variety of published geothermometers and geobarometers with the al 2 sio 5 invariant point of holdaway 1971 four fluid independent equilibria yield p t estimates consistent with holdaway 1971 two fluid dependent equilibria yield internally consistent but unrealistically high temperatures our data are consistent with the conclusions that 1 only pyrope grossular mixing in the quaternary garnet system is significantly non ideal and 2 the activity coefficient for the anorthite component in plagioclase is approximately 2 at triple point p t conditions modified journal abstract

the svecofennian orogen a tectonic model abstract recent geological studies have provided new concepts concerning the timing of the final break up of the archaean craton age data on detrital zircons from sedimentary rocks in sweden and finland show marked similarities and indicate pervasive homogenization of accumulated material geochemical and isotopic studies in sweden and finland suggest the existence of two volcanic belts separated by an area dominated by sedimentary rocks however the ages of metamorphism and plutonism differ in particular the late post 1 85 ga granitoid magmatism was more voluminous in sweden than in finland the svecofennian crust in finland is anomalously thick up to 65 km and is characterized by a high velocity lower crust of variable thickness the evolution of the svecofennian orogen is modelled by opening of the pre svecofennian ocean at 1 95 ga and progressive accretion of two arc complexes to the archaean craton between 1 91 1 87 ga initial accretion at 1 91 ga led to thrusting of the slices of an island arc complex and proterozoic marine strata onto the thickening craton margin subduction reversal with simultaneous anti clockwise rotation of the converging plate or convergence of another plate resulted in subduction below the newly accreted arc complex the tectonic style at the craton margin changed from thrusting to lateral displacement in the east in finland in the west in sweden the subduction zone retreated along a transform fault a large sedimentary basin closed in the east 1 89 ga ago when another arc complex collided against the accreted one collision resulted in lithospheric thickening subsequent delamination of the thickened lithosphere and compensation by a hot mantle underplate and large scale granitoid magmatism as a result of mixing between mantle magma and remelted arc complex material the thick dense layer that formed in the lower crust as the result of underplating kept the thickened crust in isostatic balance thus the exhumation rate in this area was similar to the rate in areas of thinner crust subduction and sediment accumulation continued in the west until at least 1 87 ga when large areas in the east were already stabilized an intracratonic transpressional zone developed as the result of continued convergence subsequent extensional collapse along the transpressional zone caused melting of svecofennian sedimentary rocks migmatization and generation of the 1 84 1 82 ga anatectic granites of southern finland and central sweden the voluminous post 1 85 ga granitoid magmatism in sweden is tentatively associated with an east west extension that is not related to the extensional collapse

evaluation of olivine spinel geothermometry as an indicator of thermal history for peridotites olivine and spinel in peridotites from the miyamori ultramafic complex and the ichinogemata crater of northeast japan show a systematic variation in the mg mg fe ratio which is correlated mainly with the grain size of spinel this correlation can be explained by a diffusion model assuming a semi infinite composite sphere under cooling or heating conditions in order to obtain absolute temperatures of thermal events the olivine spinel geothermometer is applied to pairs of spinel core and olivine core average composition the calculated temperatures range over two hundred degrees and have a systematic relationship with the grain size of spinel in the miyamori complex the calculated temperatures decrease monotonically with decrease in grain size of spinel whereas in the ichinomegata lherzolite nodule those of spinel smaller than 0 2 mm increase as the grain size decreases and those of spinel larger than 0 2 mm remain constant regardless of further increase in grain size these observations in the light of the diffusion model suggest that the miyamori complex may have cooled from higher than 800 c to lower than 600 c and that the lherzolite nodule from the ichinomegata crater may have been in equilibrium at 900 c before it was heated above 1 100 c for less than a few days these two examples indicate that olivine spinel pairs of peridotites do not always indicate an appropriate equilibration temperature we cannot interpret the supposed equilibration temperatures until the existence of isothermal stages in the thermal history of peridotites is established by carefully checking the chemical heterogeneity

petrology of biotite cordierite garnet gneiss of the mccullough range nevada i evidence for proterozoic low pressure fluid absent granulite grade metamorphism in the southern cordillera proterozoic migmatitic paragneisses exposed in the mccullough range southern nevada consist of cordierite almanditic garnet biotite sillimanite plagioclase k feldspar quartz ilmenite hercynite this assemblage is indicative of a low pressure facies series at hornblende gr anulite grade textures record a single metamorphic event involving crystallization of cordierite at the expense of biotite and sillimanite thermobarometry utilizing cation exchange between garnet biotite cordierite hercynite and plagioclase yields a preferred temperature range of 590 750 c and a pressure range of 3 4 kb equilibrium among biotite sillimanite quartz garnet and k feldspar records ahlo between 0 03 and 0 26 the low ah o together with low fol qfm and optical properties of cordierite indicate metamorphism under fluid absent conditions preserved mineral compositions are not consistent with equilibrium with a melt phase earlier limited partial melting was apparently extensive enough to cause desiccation of the pelitic assemblage the relatively low pressures attending high grade metamorphism of the mccullough range paragneisses allies this terrane with biotite cordierite garnet granulites in other orogenic belts closure pressures and temperatures require a transient apparent thermal gradient of at least 50 c km during part of this proterozoic event in the southern cordillera

anorogenic granites magmatic underplating and the origin of intracratonic basins in a non extensional setting abstract we present an active rift model for cratonic basin formation with crustal kinematics governed by basaltic underplating a case study of a deeply eroded continental rift belt in south finland shows close association of anorogenic intrusives and remnants of cratonic basins lower crustal thinning took place during anorogenic magmatism whereas the upper crust experienced vertical block faulting in a laterally neutral deformation facies combined analyses of field data petrological constraints and geophysical evidence allow the reconstruction of the kinematic scenario of crustal thinning this model involves the formation of an incipient crustal dome by asthenospheric upwelling associated with basaltic underplating and migration of induced lower crustal partial melts to upper crustal levels in this model light lower crustal diorite is replaced by a denser mafic layer the consequences for the post magmatic crustal history are evaluated by numerical modelling of the effects of crustal densification subsidence occurs in the absence of crustal extension suggesting that the magmatic underplate is the key element for the formation of an intracratonic basin subsidence curves for this model show an incipient nearly linear fast downwarp related to crystallization of gabbro at the moho followed by slow contraction induced by solid state cooling the predicted subsidence is similar to the pattern of the vertical movement induced by passive extension in the proposed model of continental rifting the effects of extension and underplating largely cancel out concurrent crustal extension will cause the removal of a magmatic underplate by promoting basaltic extrusion the model provides a consistent explanation for a number of key aspects of cratonic basins including their circular shape crustal configuration and evolution and their spatial distribution on continents

timing of magmatism and migmatization in the 2 0 1 8 ga accretionary svecokarelian orogen south central sweden the palaeoproterozoic 2 0 1 8 ga svecokarelian orogen in central sweden consists of a low pressure predominantly medium grade metamorphic domain central part of bergslagen lithotectonic unit enclosed to the north and south by low pressure migmatite belts two periods of metamorphism 1 87 1 85 and 1 83 1 79 ga are known in the migmatite belts in this study new u th pb ion microprobe data on zircon and monazite from twelve samples of locally migmatized gneisses and felsic intrusive bodies determine both protolith and metamorphic ages in four sample areas north of stockholm inside or immediately adjacent to the medium grade metamorphic domain two orthogneiss samples from the rimbo area yield unusually old protolith ages of 1909 4 and 1908 4 ma while three orthogneisses from the skutskar and forsmark areas yield more typical protolith ages between 1901 3 and 1888 3 ma migmatized paragneiss samples from this and two earlier studies contain a significant detrital component sourced from this 1 9 ga magmatic suite they are interpreted to be deposited contemporaneously with or shortly after this magmatism migmatization of the paragneiss at rimbo was followed by intrusion of leucogranite at 1846 3 ma even in the other sample areas to the north hedesunda tierp skutskar and forsmark metamorphism including migmatization is constrained to the 1 87 1 85 ga interval and penetrative ductile deformation is limited by earlier studies in the forsmark area to 1 87 1 86 ga however apart from a metamorphic monazite age of 1863 1 ma precise ages were not possible to obtain due to the presence of only partially reset recrystallized domains in zircon or highly discordant u rich metamict and altered metamorphic rims migmatization was contemporaneous with magmatic activity at 1 87 1 84 ga in the bergslagen lithotectonic unit involving a mantle derived component and there is a spatial connection between migmatization and this magmatic phase in the hedesunda tierp sample area the close spatial and temporal interplay between ductile deformation magmatism and migmatization the p t metamorphic conditions and the continuation of similar magmatic activity around and after 1 8 ga support solely accretionary rather than combined accretionary and collisional orogenic processes as an explanation for the metamorphism the generally lower metamorphic grade and restricted influence of the younger metamorphic episode at least at the ground surface level distinguishes the central part of the bergslagen lithotectonic unit from the migmatite belts further north and south

the origins of granulites a metamorphic perspective although many recent reviews emphasize a uniformity in granulite pressure temperature p t conditions and paths granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation a thorough survey of over 90 granulite terranes or occurrences reveals that over 50 of them record p t conditions outside the 7 5 1 kbar and 800 50 c average granulite regime preferred by many authors in particular an increasing number of very high temperature 900 1000 c terranes are being recognized both on the basis of distinctive mineral assemblages and geothermobarometry petrogenetic grid and geothermobarometric approaches to the determination and interpretation of p t histories are both evaluated within the context of reaction textures to demonstrate that the large range in p t conditions is indeed real and that both near isothermal decompression itd and near isobaric cooling ibc p t paths are important amphibolite granulite transitions promoted by the passage of co 2 rich fluids as observed in southern india and sri lanka are exceptional and not representative of fluid related processes in the majority of terranes it is considered on the contrary that fluid absent conditions are typical of most granulites at or near the time of their recorded thermal maxima itd granulites are interpreted to have formed in crust thickened by collision with magmatic additions being an important extra heat source erosion alone is not however considered to be the dominant post collisional thinning process instead the itd paths are generated during more rapid thinning 1 2 mm yr exposure related to tectonic exhumation during moderate rate or waning extension ibc granulites may have formed in a variety of settings those which show anticlockwise p t histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion with or without additional crustal extension ibc granulites at shallow levels very rapid 5 mm yr extensional thinning subsequent to collision it is suggested that the preservation of ibc paths rather than itd paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust and that hybrid itd to ibc paths should also be observed most ibc granulites and probably many itd granulites have not been exposed at the earth s surface as a result of the tectonic episodes which produced them but have resided in the middle and lower crust for long periods of time 100 2000 ma following these events the eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation

fluid and deformation induced metamorphic processes around moho beneath continent collision zones examples from the exposed root zone of the caledonian mountain belt w norway abstract exposed high pressure hp and ultra high pressure uhp metamorphic terrains have been studied in order to assess the metamorphic processes and their role in changing petrophysical properties near moho depth in continental root zones the investigation points to a critical role of fluid and deformation in metamorphic transformation in the deep crust and upper mantle this applies to a formation of granulite facies areas b transformation of granulites to eclogites c retrogression of eclogite facies rocks to amphibolite and green schist facies rocks and d the spinel lherzolite to garnet lherzolite transition dry rocks both feldspar bearing and ultramafic remain with their pre hp and uhp structures and anhydrous mineralogy preserved while reactions occur where fluid has been introduced along deformation zones a mixture of metamorphic facies formed at widely variable times on metre to km scale will be present throughout the crust and upper mantle pseudotachylytes frictional melts or ultracomminuted material are observed in both ultramafic and feldspar bearing lithologies spatially associated with hp and uhp rocks suggesting that rock properties at moho depth allow earthquakes seismicity enhances the metamorphic and metasomatic transitions through fragmentation and by opening the rock for fluid influx ductile eclogite facies shear zones nucleate along the brittle structures these observations point to moho as a rock processing zone with the following facets 1 a metastable dry and strong lower crust and upper mantle 2 earthquakes and tremors result in fluid flow and hp metamorphism 3 a pronounced weakening of the hydrated and transformed rocks allows flow of material and the development of new fabrics lpo in the transformed rocks deep tremor and earthquakes at moho depth may record ongoing metamorphic transitions

dead delta s former productivity two trillion shells at the mouth of the colorado river the diversion of the colorado river by dams and irrigation projects started in the 1930s triggered the collapse of the colorado delta ecosystem paleontological ecological geochronological stable isotope field and satellite image data provide estimates of the delta s benthic productivity during the 1 k y directly preceding the artificial shutdown of the river at least 2 1012 shells of bivalve mollusks make up the current beaches and islands of the delta the 125 individual valves dated using 14c calibrated amino acid racemization indicate that these shells range in age from a d 950 to 1950 seasonal intrashell cycles in 180 values indicate that average sized bivalves lived at least 3 yr the most conservative calculation based on these numbers indicates that during the time of natural river flow an average standing population of 6 109 bivalve mollusks population density 50 m2 thrived on the delta in contrast the present abundance of shelly benthic macroinvertebrates is 94 lower 3 m2 in 1999 2000 the dramatic decrease in abundance testifies to the severe loss of benthic productivity resulting from diversion of the river s flow and the inadequacy of its partial resumption 1981 to present an integration of paleontological records with geomorphological geochemical and geochronological data can provide quantitative insights into human impact on coastal ecosystems

preservation of palaeoproterozoic early svecofennian structures in the orijarvi area sw finland evidence for polyphase strain partitioning abstract the predominantly migmatitic palaeoproterozoic uusimaa belt preserves early lower grade svecofennian structures in the orijarvi area in sw finland this study aims at explaining the deformational history responsible for its preservation and also at defining the age of the early svecofennian deformation detailed structural analysis reveals that the preservation was enabled by polyphase strain partitioning which initiated during the early svecofennian d2 deformation 1875 ma ago as revealed by ion microprobe u pb data on zircons from granodioritic and intermediate syn d2 intrusive dykes the d2 structures were low strain upright folds at high crustal levels and sub horizontal high strain folds at deeper crustal levels the sub horizontal d2 structures were refolded into upright folds during the subsequent late svecofennian d3 deformation whereas the upright d2 structures behaved as almost rigid blocks that caused strain partitioning into high strain zones along the block margins this accounts for the low cumulative strain in specific parts of the orijarvi area further strain partitioning during d4 caused reverse dip slip movements along regional scale shear zones crustal depth controlled the metamorphic grade during d2 when local migmatisation took place at deep crustal levels later metamorphic overprint during d3 deformation is evident from post d2 growth of sillimanite and a second generation of andalusite similarities in the structural patterns between the orijarvi area and the tampere vammala area 100 km to the north suggest that irrespective of the age of the later overprint subsequent deformation was localised along the margins of the early formed upright domains while the low grade rocks within the domains were preserved

the thermotectonic evolution of a proterozoic low pressure granulite dome west uusimaa sw finland three fold generations have been recognized in svecofennian rocks 1 800 ma from west uusimaa sw finland the first one f1 might be related to thrusting and imbrication tectonics at plate collision contacts the main generation f2 is due to a n s horizontal crustal shortening which created at first e w trending upright folds in the whole region and later tightened these f2 folds in the western part of the belt whereas conjugate shear zones and tectonic lenses of competent rock bodies developed in the eastern part simultaneously the metamorphic conditions rose from amphibolite to granulite facies in this eastern part which is known as the west uusimaa complex the amphibolite to granulite facies transition zone along the western boundary of the granulite facies complex is studied in detail a number of prograde mineral reactions are telescoped in this transition zone the breakdown of biotite and amphibole to ortho clino pyroxene in metaigneous rocks the appearance of garnet in cordierite bearing metapelites and the appearance of scapolite in calcareous rocks distinct mineralogical changes also occur in this zone which cross cuts all major structures and rock units and are only affected by late f3 folding open disharmonic folds with approximately n s trending axial planes and young shear zones associated with pseudotachylite generation the absence of any evidence of block faulting and tilting of the crust that could be associated with the granulite complex suggests that the whole region represents one crustal level

proterozoic extensional tectonics of the central fennoscandian shield results from the baltic and bothnian echoes from the lithosphere experiment a three dimensional crustal structure of the bothnian sea is deduced from the near vertical reflection data of the crosscutting lines 1 6 7 and c of the babel baltic and bothnian echoes from the lithosphere experiment for this study the common depth point stacked sections have been migrated with a velocity 6 5 km s and displayed as stacked envelope sections to show their enhanced large scale crustal structures the seismic sections show listric shear zones which dip toward se and flatten at major detachment zones 35 40 km and 48 km in depth the detachment zones are associated with changes in p wave velocity from 6 8 km s to 7 0 km s and from 7 4 km s to 8 2 km s and coincide with the middle and lower crustal boundary and the moho boundary respectively the geometry and crosscutting relationship of the detachment zone are well developed on the e w directed line 7 from where the moho detachment zone can be interpreted as to be younger different shear zone systems operated in the parallel n s directed lines 1 and 6 and therefore it is not easy to correlate crustal structures between lines 1 and 6 in the thinned southern part of study area the lower crust has a highly reflective lamellar structure and a strongly reflective subhorizontal moho increased reflectivity of the lower crust is attributed to stretching and mafic underplating of the crust associated with the subjotnian diabase dykes and rapakivi magmatism in the upper crust the rapakivi granite and diabase association is characterized by a rather transparent area within which strong reflections mimic graben and horst structures the distribution of the bimodal association seems to be controlled by the low angle listric shear zones the spatial distribution of outcropping lithologies combined with deeply penetrating shear zones suggests a simple shear extensional deformation the first stage of deformation which concentrated in the upper and middle crust records the collapse of the overthickened svecofennian crust the second stage involving the entire crust may record either the continuation of the postcollisional collapse or a separate anorogenic extensional period the seismic structures predict major post svecofennian deformation faulting and tilting in the central fennoscandian shield the extensional tectonics operating in the fennoscandian shield in the proterozoic is comparable to modern extensional tectonics

methane in fluid inclusions from granulites a product of hydrogen diffusion many fluid inclusion studies of granulite grade rocks reveal the presence of co2 rich inclusions that appear to have been trapped near the peak of metamorphism final melting temperatures of co2 tm co2 reported for these inclusions are often below the co2 triple point of 56 6 c and some are below 60 c this freezing point lowering is usually attributed to the presence of a second volatile component such as ch4 and the presence of ch4 has been confirmed in some cases by raman or mass spectroscopy a co2 melting temperature near 56 6 c is commonly offered as evidence that the inclusions contain nearly pure co2 however significant amounts of ch4 may be present but cause seemingly insignificant freezing point depressions c o h fluid speciation calculations for conditions representative of granulite facies metamorphism indicate that ch4 may comprise a significant portion of peak metamorphic fluids when graphite is present but it is never a significant species in co2 rich fluids in most cases the amount of ch4 should be virtually undetectable with microthermometric or spectroscopic techniques only in aqueous fluid inclusions can co2 and ch4 both be significant species post trapping speciation changes to fluid inclusions at constant mass cannot account for the reported compositions unless graphite precipitates in the inclusions thus the observed compositions require post trapping compositional changes due to loss or gain of components we have modeled variations in the fugacities of molecular fluid species in inclusion and matrix fluids during uplift from 6 kb and 800 c assuming hypothetical uplift pressure temperature paths which are concave or convex toward the temperature axis t concave and t convex respectively our results suggest that for rocks buffered at o2 within one log unit of the fayalite magnetite quartz equilibrium fmq 1 most uplift paths result in external h2 overpressures of bars to tens of bars at temperatures 400 c the highest overpressures are generated during t concave uplift compositional changes resulting from equilibration of such gradients via diffusive addition of hydrogen to peak metamorphic fluid inclusions and concomitant reduction of co2 by conversion to ch4 and h2o are consistent with the compositions of fluid inclusions reported from granulite terranes previous workers have postulated that co2 rich fluid inclusions in granulites could originate from post trapping diffusive loss of h2o from h2o co2 inclusions in response to an h2o gradient between the inclusion and matrix fluids the results of the present study suggest that for fluids buffered by fmq 1 this is possible only if 1 1 uplift is t convex and the matrix fluid composition remains nearly constant or 2 2 the matrix fluid evolves toward relatively h2o poor compositions the latter could occur if influx of co2 rich fluids occurs during uplift

fluid inclusions in rocks from the amphibolite facies gneiss to charnockite progression in southern karnataka india direct evidence concerning the fluids of granulite metamorphism fluid inclusion studies of rocks from the late archaean amphibolite facies to granulite facies transition zone of southern india provide support for the hypothesis that co2 rich h2o poor fluids were a major factor in the origin of the high grade terrain charnockites closely associated leucogranites and quartzo feldspathic veins contain vast numbers of large co2 rich inclusions in planar arrays in quartz and feldspar whereas amphibole bearing gray gneisses of essentially the same compositions as adjacent charnockites in mixed facies quarries contain no large fluid inclusions inclusions in the northernmost incipient charnockites as at kabbal karnataka occasionally contain about 25 mol of immiscible h2o lining cavity walls whereas inclusions from the charnockite massif terrane farther south do not have visibile h2o microthermometry of co2 inclusions shows that miscible ch4 and n2 must be small probably less than 10mol combined densities of co2 increase steadily from north to south across the transitional terrane entrapment pressures calculated from the co2 equation of state range from 5 kbar in the north to 7 5 kbar in the south at the mineralogically inferred average metamorphic temperature of 750 c in quantitative agreement with mineralogic geobarometry this agreement leads to the inference that the fluid inclusions were trapped at or near peak metamorphic conditions calculations on the stability of the charnockite assemblage biotite orthopyroxene k feldspar quartz show that an associated fluid phase must have less than 0 35 h2o activity at the inferred p and t conditions which agrees with the petrographic observations high tio2 content of biotite stabilizes it to lower h2o activities and the steady increase of biotite tio2 southward in the area suggests progressive decrease of ah2o with increasing grade oxygen fugacities calculated from orthopyroxene magnetite quartz are considerably higher than the graphite co2 o2 buffer which explains the absence of graphite in the charnockites the present study quantifies the nature of the vapours in the southern india granulite metamorphism it remains to be determined whether co2 flushing of the crust can by itself create large terranes of largeion lithophile depleted granulites or whether removal of h2o bearing anatectic melts is essential

the link between migmatites and s type granites in the turku area southern finland abstract metamorphism and compositions of metasedimentary migmatites and related small scale granites are investigated in the turku area sw finland phase assemblages and mineral compositions suggest peak p t conditions of 800 c and 6 kbar the portion of melt produced in the migmatites by dehydration melting is estimated to be 20 wt the partial melt is k 2 o rich and water undersaturated 4 wt h 2 o the leucosomes and small scale granites have almost the same proportions of felsic minerals but the mafic minerals grt crd bt are enriched in the granites along with the contents of tio 2 feo tot mgo and hfse the leucosomes have low total ree values and strongly positive eu anomalies the ree patterns of the granites are characterized by relatively high lree low hree and distinct negative eu anomalies the distribution of major elements and trace element signatures suggests that leucosomes and granites are related as cumulates and evolved residual melts respectively the decimeter to meter scale leucosomes are interpreted to be the product of crystal fractionation which occurred during near isobaric cooling and the granites are the crystallized residual melts which could move further on and form larger bodies within and outside of the migmatite complex

evolution of ptygmatic folds in migmatites from the type area s finland abstract it is proposed that ptygmatic folds in migmatites form during leucosome melanosome differentiation volume loss by depletion of quartz and feldspar takes place in the interlimb domains of folds leaving a residual melanosome buckling leucosomes increase in volume due to this process and are protected from substantial thinning concurrent with tectonic segregation albeit at a lower rate the leucosome expands by replacing the melanosome this process is induced by fluid activity at the leucosome boundary as suggested by the occurrence of secondary offshoot veinlets segregation and growth causes the most typical aspect of ptygmatic folds preservation of an overall buckle shape during the stage when the folds become isoclinal and devoid of interlimb matrix

the tampere schist belt structural style within an early proterozoic volcanic arc system in southern finland abstract the central part of the narrow e w trending tampere schist belt comprises volcanic and sedimentary rocks of low metamorphic grade surrounded by intrusive rocks and migmatites the first folding event with subvertical axial surfaces and originally subhorizontal fold axes dominates the structure of the belt well preserved primary structures indicate a major e w trending syncline in the northern part of the belt with minor upright open to tight synclines and anticlines in the limb areas f 1 fold axes in the limbs have been rotated towards the steeply to vertically plunging stretching lineation dextral f 2 folds with subvertical e w to ne sw striking s 2 crenulation cleavage and steeply plunging fold axes overprint penetrative s 1 schistosity subhorizontal dextral shear is inferred from the d 2 structures d 3 kinking and fracturing is restricted to narrow nw se to n s trending zones the shear zone between the schists and the migmatites to the south is a south dipping thrust or reverse fault with uplift of the higher grade rocks in the northern part of the tampere schist belt another major fault is postulated both shear zones were formed at a late stage of progressive d 1 deformation on the basis of these structures and published sedimentological geochemical and isotopic data the tampere schist belt is interpreted as an upper part of a thrust sheet within a forearc of an ancient volcanic arc system the vergence of structures suggests subduction toward the present south

age and tectonic framework of structurally controlled palaeoproterozoic gold mineralization in the hame belt of southern finland abstract a number of gold occurrences in the palaeoproterozoic svecofennian domain of southern finland show a clear structural control on mineralization and are classified as orogenic gold samples of pre syn and late to post mineralization rocks taken from two gold prospects in the somero tammela gold zone of the hame belt have been studied to gain absolute ages of mineralization by using sims u pb of zircon and titanite and ar ar single grain step heating of hornblende and biotite gold mineralization in the somero tammela gold zone is closely related to quartz veins and controlled by wsw ene to sw ne trending shear zones and faults formed during prolonged dextral oblique contraction in the late stages of svecofennian orogenic evolution zircons from a felsic volcanic host rock give an age of 1881 3 ma interpreted as the crystallization age of the rock and providing a maximum age for mineralization inherited zircons in a gold bearing quartz vein with the youngest zircon rim of 1832 15 ma give a maximum age of quartz vein emplacement a 1786 7 ma mean 207pb 206pb age of a pegmatitic quartz feldspar dyke providing a minimum age for mineralization is interpreted as reflecting lead loss whereas an 1815 4 ma zircon age is probably closer to the crystallization age of the dyke this age overlaps with 1805 4 5 4 ma 39ar 40ar age of hornblende from a quartz vein 1 78 and 1 76 ga 39ar 40ar biotite cooling ages mark the minimum age for mineralization in summary the age of mineralization can be estimated at ca 1 82 1 79 ga which is in good agreement with the age of dextral shearing in southern finland the controlling structures wsw ene to sw ne dextral shear zones and nw se oriented faults formed during approximately wnw ese oriented sub horizontal shortening during crossing ductile brittle boundary conditions the steep shear zones and faults and the intersection lines of the two sets impart a directional permeability allowing channelling of the mineralizing fluids in these structures this was supported by prolonged dextral oblique contraction enabling continuous or episodic slip along the faults and shear zones the late svecofennian tectonic episode could be described as an extension contraction cycle in a convergent plate margin setting above a retreating subduction zone oblique extension 1 86 1 83 ga caused by slab rollback was followed by a rapid switch to oblique contraction at ca 1 82 ga induced by arrival of positively buoyant crustal fragments e g oceanic ridge oceanic plateau or continental sliver at the subduction zone the contractional period characterized by dextral transpression provided a favourable setting for orogenic gold mineralization the termination of the svecofennian orogenic evolution in finland after 1 79 ga might be caused by lock up of the subduction zone and slab break off the striking nw se alignment of orogenic type gold occurrences in the svecofennian domain of southern finland suggests that the late svecofennian 1 82 1 79 ga gold mineralizing event may be regionally more widespread and of greater importance than hitherto expected

a cleavage triple point and its meso scopic structures the mustio sink svecofennides of sw finland abstract a cleavage triple point ctp structure is analyzed located at the west side of the mustio gneiss dome in the svecofennides of southwest finland the presence of the ctp and the pattern of mesoscopic fold structures exclude the origin of the mustio dome by successive interference of fold phases the highly variable deformation structures are explained in a single phase deformation model by using the theoretical specific strain environments of a ctp these environments are 1 horizontal oblation on top of a dome 2 transition from horizontal to vertical oblation on the flanks of a dome and 3 vertical constriction in the center of the ctp it is shown that each strain environment is associated with specific development of foliation folds mesoscopic fold interference and strain intensity the theoretical strain environments are confirmed by strain analysis

the mode of deformation in the orogenic mid crust revealed by seismic attribute analysis 1 the processes and the structures within the middle and the lower crust are intimately related to the evolution of orogenies but more detailed knowledge is needed of these deep crustal processes seismic reflection data chiefly displayed in amplitude are commonly used to interpret deep crustal structures the strongest amplitude events are probably related to lithological changes and do not necessarily correspond to geological structures tectonic boundaries or deformation fabrics furthermore the detailed structures and the strain distribution between the interpreted structural boundaries remain obscure here we show an example of how seismic attributes combined with the seismic facies interpretation technique can be used to enhance 2d seismic reflection data from the palaeoproterozoic svecofennian crust of southern finland to reveal unprecedentedly detailed information about the deformation fabrics within the mid crust of a collisional orogen the images are plausibly interpreted to show that the extension lateral flow of the orogenic middle and lower crust was mainly accommodated by kilometer scale s c structures the structures form penetrative deformation fabrics which are correlated with outcrop observations the successful enhancement of the seismic data confirms the ductile extension affecting hot orogenic crust and gives new information about the strain distribution of the regional syn to late orogenic deformation the seismic attribute method and the seismic facies interpretational approach described in this paper should be applicable to other seismic datasets from the crystalline basement

upper cretaceous ferron sandstone major coalbed methane play in central utah recent drilling for coalbed gas in the upper cretaceous ferron sandstone member of central utah has resulted in one of the most successful plays of this kind exploration to date has resulted in three fields and a potential fairway 6 10 mi 10 16 km wide and 20 60 mi 32 96 km long corresponding to shallow coal occurrence at depths of about 1800 3500 ft 545 1060 m in the ferron a sequence of interbedded fluvial deltaic sandstone shale and coal in the lower part of the cretaceous mancos shale coalbed methane cbm reservoirs in this interval consist of thin to moderately thick 3 10 ft 1 3 m coal beds of relatively low rank high volatile b bituminous and variable gas content ranging from 100 scf ton or less in the south to as high as 500 600 scf ton in the north productive wells have averaged more than 500 mcf day and after several years continue to typically show negative production declines in the major productive area drunkards wash unit the first 33 producers averaged 974 mcf and 85 bbl of water per day after five years of continuous production estimated ultimate recoverable reserves for individual wells in this unit range from 1 5 to 4 bcf based on several criteria including gas content thermal maturity and chronostratigraphy the play is divided into northern and southern parts the northern part is characterized by coals that have the following characteristics 1 high gas contents 2 moderate thermal maturity e g vitrinite reflectance ro values of 0 6 0 8 3 good permeabilities 5 20 md 4 lack of exposure and 5 overpressuring due to artesian conditions southern coals have much lower average gas contents 100 scf ton and lower thermal maturity ro 0 4 0 6 and they are exposed along an extensive 35 mi 56 km outcrop belt that may have allowed a degree of flushing these coals however are also thicker and more extensive than those to the north and thus may retain significant begin page 200 potential northern coals appear to contain a mixture of gas from three sources in situ thermogenic methane migrated thermogenic methane from more mature sources and late stage biogenic gas current development is focused on the northern part of the stated fairway where well control and an existing infrastructure are present indications are that cbm exploration in the ferron will expand considerably in the near future

reactive monazite and robust zircon growth in diatexites and leucogranites from a hot slowly cooled orogen implications for the palaeoproterozoic tectonic evolution of the central fennoscandian shield sweden monazite in melt producing poly metamorphic terranes can grow dissolve or reprecipitate at different stages during orogenic evolution particularly in hot slowly cooling orogens such as the svecofennian owing to the high heat flow in such orogens small variations in pressure temperature or deformation intensity may promote a mineral reaction monazite in diatexites and leucogranites from two svecofennian domains yields older coeval and younger u pb sims and emp ages than zircon from the same rock as zircon precipitated during the melt bearing stage its u pb ages reflect the timing of peak metamorphism which is associated with partial melting and leucogranite formation in one of the domains the granite and diatexite belt zircon ages range between 1 87 and 1 86 ga whereas monazite yields two distinct double peaks at 1 87 1 86 and 1 82 1 80 ga the younger double peak is related to monazite growth or reprecipitation during subsolidus conditions associated with deformation along late orogenic shear zones magmatic monazite in leucogranite records systematic variations in composition and age during growth that can be directly linked to th u ratios and preferential growth sites of zircon reflecting the transition from melt to melt crystallisation of the magma in the adjacent ljusdal domain peak metamorphism in amphibolite facies occurred at 1 83 1 82 ga as given by both zircon and monazite chronology pre partial melting 1 85 ga contact metamorphic monazite is preserved in spite of the high grade overprint by combining structural analysis petrography and monazite and zircon geochronology a metamorphic terrane boundary has been identified it is concluded that the boundary formed by crustal shortening accommodated by major thrusting

evolution of p t conditions during a high grade metamorphic event in the maniwaki area grenville province pelitic and basic rocks of the grenville supergroup in the maniwaki area 100 km north of ottawa were selected for a detailed investigation of the metamorphic history of the central metasedimentary belt of the grenville province mineral assemblages suggest metamorphic conditions of the granulite facies with local and irregular persistence of the amphibolite facies chemical analysis of minerals reveals a lack of equilibrium on the thin section scale systematic variation of mineral compositions is explained as the net result of several mineral equilibria established at different stages during the metamorphic evolution of the area peak temperatures obtained by biotite garnet thermometry cores of isolated grains are between 760 and 860 c whereas clinopyroxene garnet thermometry gives 740 820 c retrograde temperatures as estimated by various thermometers range between 550 and 750 c biotite garnet 498 and 670 c cordierite garnet and 690 and 720 c clinopyroxene garnet peak pressures were 6

svecofennian magmatic and metamorphic evolution in southwestern finland as revealed by u pb zircon sims geochronology abstract zircons from six samples collected from igneous and metamorphic rocks were dated using the nordsim ion microprobe in order to investigate the tectonic evolution of the palaeoproterozoic svecofennian orogen in southwestern finland these rocks represent pre collisional collisional and post collisional stages of the orogeny the ion microprobe results reveal two age groups of granodioritic tonalitic rocks the intrusions have different tectonic settings the orijarvi granodiorite represents pre collisional 1 91 1 88 ga island arc related magmatism and yielded an age of 1898 9 ma whereas the collision related masku tonalite was dated at 1854 18 ma the latter age accords with more accurate previous conventional zircon age data and constrains the emplacement age of collisional granitoids to 1 87 ga this is interpreted to reflect the collision between the southern svecofennian arc complex with the central svecofennian arc complex and the formation of a suture zone between them during d2 deformation granulite facies metamorphism in the turku area was dated at 1824 5 ma using zircons from leucosome in the lemu metapelite this age constrains d3 folding related to post collisional crustal shortening in this area crustal melting continued until 1 81 ga as indicated by the youngest leucosome zircons and metamorphic rims of enderbite zircons new metamorphic zircon growth took place in older granitoids at granulite facies but not at amphibolite facies detrital zircons with ages between 2 91 and 1 97 ga were found in the mesosome of the lemu metapelite and 2 64 1 93 ga inherited cores were found in the 1 87 ga masku tonalite

evolution of a svecofennian mantled gneiss dome in sw finland with evidence for thrusting abstract the structure and petrology of a dome in svecofennian migmatitic gneisses schists amphibolites and granitoids on kemlo island is described the structure resembles classical mantled gneiss domes in that it comprises a core and mantle but differs in that intrusive granitoids occur in the mantle rather than the core four generations of folds are recognised and structural and petrological evidence is presented for an f 2 related thrusting origin for the discontinuity that separates core and mantle the metamorphic evidence suggests that the core was subject to low pressure amphibolite facies conditions at a time prior to thrusting when the mantle was subject to higher pressure amphibolite facies conditions it is pointed out that metamorphic jumps elsewhere on kemio can be better explained by thrusting than by normal faulting as previously suggested the structure is shown to be adequately explained as an interference structure resulting from superposition of f 3 and f 4 folding the possibility of diapirism cannot be eliminated but there is no necessity to invoke such a process and no evidence for it it is suggested that the interference pattern interpretation plus a thrusting origin for the core mantle contact and the folded foliation may have more general applicability the structural and metamorphic history is very similar to that of phanerozoic mountain belts that are interpreted as a product of plate tectonics it is suggested that the same model can be applied to the svecofennian

p t t evolution of orogenic belts and the causes of regional metamorphism barrow 1893 introduced three important ideas that furthered understanding of metamorphic processes i the use of critical index minerals in argillaceous rocks to define metamorphic zones and elucidate spatial features of regional metamorphism ii the concept of progressive metamorphism and iii the concept of magmatic advection of heat as a possible cause of regional metamorphism this article expands upon these themes by reviewing our understanding of the dynamic evolution of orogenic belts as interpreted from the p t t paths of metamorphic rocks and by considering the likely causes of the different kinds of regional metamorphism that we observe within orogenic belts understanding metamorphic rocks allows the distinction of two fundamentally different types of orogenic belt defined by relative timing of maximum t and maximum p orogenic belts characterized by clockwise p t paths achieved maximum p before maximum t the metamorphic peak normally post dated early deformation within the belt and additional heating above the normal conductive flux has been related to the amount of overthickening by contrast orogenic belts characterized by counterclockwise p t paths achieved maximum t before maximum p the metamorphic peak normally pre dated or was synchronous with early deformation within the belt and additional heating above the normal conductive flux has been related to the emplacement of plutons techniques used to constrain portions of p t t paths include the use of mineral inclusion suites in porphyroblasts and reaction textures thermobarometry the use of fluid inclusions thermodynamic approaches such as the gibbs method radiogenic isotope dating fission track studies and numerical modelling we can utilize specific mineral parageneses in suitable rocks to determine individual p t t paths and a set of p t t paths from one orogenic belt allows us to interpret the spatial variation in dynamic evolution of the metamorphism recent advances are reviewed with reference to collision metamorphism high temperature low pressure metamorphism granulite metamorphism and subduction zone metamorphism and some important directions for future work are indicated