deposition and imbrication of a 2670 2629 ma supracrustal sequence in the indin lake area southwestern slave province canada new u pb age data from the southwestern slave province demonstrate that units of the indin lake supracrustal belt form an imbricated structural stack the oldest rocks of the belt are undated mafic volcanic flows of the hewitt lake group that are crosscut by a 2670 ma felsic sill itself coeval with mafic through felsic volcanic rocks of the 2668 2671 ma leta arm group the youngest rocks of the belt are 2647 2629 ma turbidites and felsic volcanic rocks of the unconformably overlying chalco lake group tonalite orthogneiss of the adjacent cotterill gneiss complex is 2680 ma suggesting that it does not represent in situ basement to the supracrustal belt intercalation of the older hewitt lake and leta arm groups with the younger chalco lake group is interpreted to result from d1 imbrication and folding between 2629 and 2609 ma the age of a crosscutting tonalite intrusion subsequent d2 folding and regional low pressure metamorphism occurred between 2609 ma and ca 2590 ma d3 normal faulting between the

strain patterns decollement and incipient sagducted greenstone terrains in the archaean dharwar craton south india the archaean dharwar craton is characterized by two greenstone successions the 3 ga sargur group and the 3 0 2 5 ga dharwar supergroup examples of both successions are described from the region of jayachamarajapura where they are also distinguished by different tectonic patterns the younger greenstones have undergone only minor deformation and are only slightly metamorphosed and so provide a good case study of the relative behavior of greenstones in relation to their granite gneiss country rocks a detailed structural analysis indicates two strain fields associated with two deformational episodes d1 and d2 the d1 episode produced dome and basin structures and affected merely the older greenstones and the gneisses the mapped strain field is compatible with the hypothesis that it is associated with the development of diapiric type gravitational instabilities the d2 episode affects only the younger greenstone belt which has the overall geometry of a complex syncline it is discordant over a complex of gneisses and older greenstones that was deformed during the d1 episode the base of the discordant cover sequence is tectonized and constitutes a decollement surface kinematic criteria at this surface have opposite sense and converge towards the belt axis these structural features are interpreted in terms of progressive deformation compatible with the incipient development of a sagducting trough these results are consistent with those obtained from other parts of the craton where the tectonic evolution appears to reflect mainly relative vertical displacements facilitated by the reheating of continental crust during two major archaean tectonometamorphic episodes copyright 1996 elsevier science ltd

tectonic denudation of the shuswap metamorphic terrane of southeastern british columbia large scale over thrusting and regional extension are both known to have been operative within the shuswap metamorphic terrane of the southern omineca belt the monashee core complex of the northern shuswap terrane is interpreted to be a crustal scale duplex that was developed during late mesozoic shortening and thickening of the crust uplift of the monashee complex caused by basement duplexing is estimated to have been in excess of 25 km late cretaceous overthrusting of high grade metamorphic and related plutonic rocks during later stages of uplift and basement duplexing led to weakening of the crust and to gravitationally driven extension of a regional crustal welt shuswap culmination denudation and spreading of this uplifted metamorphic terrane are attributed to displacements across listric normal shear zones and faults localized along the outward dipping flanks of the shuswap culmination total extension across the shuswap culmination at the northern end of the monashee complex is estimated to be no greater than 30 this model of denudation and gravity induced spreading is extrapolated southward into the valhalla and okanagan regions where extension may be far more significant

emplacement and deformation of granites during transpression magnetic fabrics of the archean sparrow pluton slave province canada abstract the sparrow pluton is part of the prosperous suite of two mica granites that crop out within amphibolite grade meta greywackes of the southern yellowknife domain in the slave province of the canadian shield the magnetic susceptibility k and the anisotropy of magnetic susceptibility ams were used to systematically map the structural patterns in the pluton and to establish the relationship between plutonism and late archean tectonics in the region paramagnetic fe phyllosilicates biotite chlorite and very fine grained magnetite contribute to k and to the ams the magnetic foliation and the magnetic lineation are predominantly controlled by the biotite fabrics and their orientations are consistent with the regional d 2 strain field the horizontal magnetic lineations in the sparrow pluton suggest a horizontal stretching component associated with the regional d 2 event the zonation defined by k values is compatible with a fold pattern trending parallel to the regional f 2 folds and s 2 foliation and to the magnetic fabric trends in the pluton the intensities and symmetries of the ams also define map patterns that are consistent with d 2 deformation microstructural study indicates the pluton recorded d 2 strain as it crystallized and cooled from the solidus demonstrating syn d 2 emplacement the results indicate the pervasive structural patterns in the sparrow pluton are an integral part of the regional strain field and that they are kinematically consistent with a transpressive d 2 strain regime mapping the fabric patterns within syntectonic plutons provides a useful approach to the kinematic analysis of synemplacement deformation events in multiply deformed metamorphic terranes

protracted fluid induced melting during barrovian metamorphism in the central alps the timing and dynamics of fluid induced melting in the typical barrovian sequence of the central alps has been investigated using zircon chronology and trace element composition multiple zircon domains in leucosomes and country rocks yield u pb ages spanning from 32 to 22 ma the zircon formed during alpine melting can be distinguished from the inherited and detrital cores on the basis of their age th u 0 1 and trace element composition ti in zircon thermometry indicates crystallization temperatures around 620 700 c their composition allows discriminating between 1 zircon formation in the presence of early garnet 2 zircon in equilibrium with abundant l mree rich accessory phases allanite titanite and apatite typical of metatonalites and 3 zircon formed during melting of metasediments in feldspar dominated assemblages the distribution of zircon overgrowths and ages indicate that repeated melting events occurred within a single barrovian metamorphic cycle at roughly constant temperature that in the country rocks zircon formation was limited to the initial stages of melting whereas further melting concentrated in the segregated leucosomes that melting occurred at different times in samples a few meters apart because of the local rock composition and localized influx of the fluids and that leucosomes were repeatedly melted when fluids became available the geochronological data force a revision of the temperature time path of the migmatite belt in the central alps protracted melting over 10 my followed the fast exhumation of alpine eclogites contained within the same region and preceded fast cooling in the order of 100 c ma to upper crustal levels

a transitional eclogite to high pressure granulite facies overprint on coesite eclogite at taohang in the sulu ultrahigh pressure terrane eastern china abstract a transitional eclogite to high pressure granulite facies paragenesis omp pl qtz grt after peak coesite eclogite facies metamorphism and predating the later amphibolite facies overprint is identified in coesite eclogite from the taohang area of the sulu ultrahigh pressure uhp terrane in eastern china these minerals were equilibrated at 17 kbar and 820 c this reveals that fluid infiltration might activate retrograde recrystallisation even at a deep level during the exhumation process of the uhp rocks the tectono metamorphic significance of the unusually high pressure overprint is also discussed

slab derived fluids and quartz vein formation in an accretionary prism otago schist new zealand regional quartz vein formation and the fluxes flow paths and sources of metamorphic fluids were investigated in the mesozoic accretionary prism of new zealand by using a new chemical mass balance analysis of outcrops samples were collected at meter or submeter intervals along outcrop length traverses and combined to obtain average chemical compositions of whole outcrops mass balance analysis used zr as an immobile reference frame and as a monitor of sedimentary sorting processes sio2 zr systematics produced by sedimentary processes differ greatly from those caused by metasomatic mass transfer of silica allowing evaluation of vein formation mechanisms relatively undeformed metasedimentary outcrops of low metamorphic grade mostly prehnite pumpellyite facies are nearly unveined and characterized by sedimentary compositional trends more deformed outcrops of higher metamorphic grade mostly greenschist facies contain 10 30 vol quartz veins these outcrops underwent mass addition of externally derived silica into quartz veins accompanied by addition of na and removal of k and w average silica additions suggest a time integrated fluid flux of 104 105 batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document m 3 fluid end document batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document m 2 rock end document for fluids ascending through the prism dehydration of spilitized oceanic crust subducting beneath the prism is the most probable source for this large fluid flux and could also have caused the na k metasomatism the w removed from deep levels of the prism may have been deposited in focused retrograde au w quartz veins at shallow levels by ascending fluids transfer of sio2 from subducting slabs into accretionary prisms is a plausible mechanism for long term bulk silica enrichment of the continents beyond that possible by magmatic differentiation

the central slave basement complex part ii age and tectonic significance of high strain zones along the basement cover contact the basement cover high strain zone enveloping parts of the sleepy dragon complex northeast of yellowknife slave province canada has been reinvestigated integrated stratigraphic structural and geochronological data show that the high strain zone is of regional extent and is best interpreted as a decollement between crystalline ca 2 9 3 3 ga rocks of the central slave basement complex and pre 2687 ma cover rocks three temporally distinct mafic dyke swarms occur within the high strain zone the two oldest of these constrain the timing of the high strain event to between 2734 2 and 2687 1 ma at the time of decollement development the cover stratigraphy consisted of i the central slave cover group a thin pre 2734 ma succession of mafic and ultramafic volcanic rocks conglomerates fuchsitic quartzites minor rhyolites and banded iron formation and ii an overlying sequence of tholeiitic pillow basalts the central slave cover group is considered to be autochthonous whereas a variety of

antler orogeny a mediterranean type orogeny the late cenozoic thrust belts of the apennine carpathian and hellenic systems of the mediterranean region provide modern analogues for the middle paleozoic antler orogeny each of these young or active mountain belts formed in convergent systems in which thrusting occurred behind a zone of trench retreat while a region of extension developed contemporaneously within the hanging wall of the subduction systems dynamic models that have been recently developed for these tectonic systems provide a basis for interpretation for the antler belt many if not all of the geologic relations observed in the antler belt including the lack of a collided arc and subsidence of a broad extensional region behind the thrust belt are also present within these young mediterranean thrust belt systems if this interpretation is correct then the entire antler belt may have been preserved with no need for other tectonic elements to have been present west of the antler thrust belt and the havallah basin

u pb age trace element and hf isotope compositions of zircon in a quartz vein from eclogite in the western dabie mountains constraints on fluid flow during early exhumation of ultrahigh pressure rocks quartz veins in high pressure hp to ultrahigh pressure uhp rocks are the products of fluid rock interaction and thus provide insight into fluid processes in subduction zones in this paper we report an integrated study of mineral inclusion trace element u pb age and lu hf isotope compositions of hydrothermal zircon grains from a quartz vein within an uhp eclogite outcrop from the hong an area western dabie mountains these data are used to decipher the age conditions of formation and source of fluid for zircon formation during the exhumation of uhp rocks zircon grains from the vein have perfect euhedral shape and show sector zoning or weak zoning indicating that they precipitated from the aqueous fluid responsible for the vein formation raman spectroscopy analysis reveals that the zircon grains contain inclusions of garnet omphacite rutile quartz and h2o implying that they crystallized from aqueous fluid under hp eclogite facies conditions the zircon grains show low th u and lu hf ratios nearly flat hree patterns absent eu anomalies and low lree contents these characteristics are consistent with their precipitation in the presence of garnet and epidote and absence of feldspar and thus suggest that trace element concentrations in hydrothermal zircon are controlled by co precipitation of mineral assemblages crystallization temperatures of 670 to 712 c which were calculated using the ti content of zircon are consistent with their formation under eclogite facies conditions and may correspond to the temperature of the infiltrating fluid the weighted mean 206pb 238u age of 224 7 1 3 ma is taken as the best estimate for the age of quartz vein formation and records aqueous fluid flow during the early exhumation stage of uhp rocks the zircon grains in the quartz vein have hf compositions similar to those in the host eclogite which demonstrates isotopic equilibrium between fluid and rocks and that the fluid rock ratio was likely low

tectonic denudation of the darwin metamorphic core complex in the andes of tierra del fuego southernmost chile implications for cordilleran orogenesis cordillera darwin in the southernmost chilean andes is a tectonically denuded metamorphic core complex similar to those widely developed in the north american cordillera detailed comparison can be drawn with the shuswap metamorphic core complex in british columbia and washington analysis of the geotectonic history of the darwin complex reinforces the view that the presence of tectonically thickened continental crust is critical for development of most if not all known core complexes and also suggests that inversion of a marginal basin during a compressional regime is one way of initiating such a crustal welt the cordillera darwin complex is unique in the andes and antarctandes this indicates that although low angle normal faulting occurs at high levels in convergent plate settings such as the central andes and the himalayas extensional denudation of deep crustal levels in active andean type orogens may require special tectonic circumstances uplift of the darwin complex began at about 70 ma in a localized extensional setting within the developing transform zone between the south american and antarctic plates this strike slip setting is analogous to some metamorphic core complexes in north america and possibly to a recently identified core complex in new zealand it is likely to be a common one for core complexes in convergent orogens

experimental evidence for diamond facies metamorphism in the dora maira massif the peak metamorphic conditions of subducted continental crust in the dora maira massif western alps have been revised by combining experimental results in the kcmash system with petrologic information from whiteschists textural observations in whiteschists suggest that the peak metamorphic assemblage garnet phengite kyanite coesite talc originates from the reaction kyanite talc garnet coesite liquid in the experimentally determined petrogenetic grid this reaction occurs above 45 kbar at 730 c at lower pressures talc reacts either to orthopyroxene and coesite or together with phengite to biotite coesite and kyanite the liberated liquid contains probably similar amounts of h2o and dissolved granitic components the composition of the liquid in the whiteschists at peak metamorphic conditions a major unknown in earlier studies was probably very similar to the liquid composition produced in the experiments therefore the experimentally determined petrogenetic grid represents a good model for the estimation of the peak metamorphic conditions in whiteschists experimentally determined si isopleths for phengite further constrain peak pressures to 43 kbar for the measured si 3 60 of phengite in the natural whiteschists all these data provide evidence that the whiteschists reached diamond facies conditions the fluid absent equilibrium 4 kyanite 3 celadonite 4 coesite 3 muscovite pyrope has been calibrated on the basis of garnet and phengite compositions in the experiments and serves as a geothermobarometer for ultra high pressure uhp metapelites for graphite bearing metapelites and kyanite phengite eclogites forming the country rocks of the whiteschists peak metamorphic pressures of about 44 3 kbar were calculated from this barometer for temperatures of 750 c estimated from garnet phengite thermometry therefore the whole ultra high pressure unit of the dora maira massif most likely experienced peak metamorphic conditions in the diamond stability field while graphite is common in the metapelites diamond has not been found so far the absence of metamorphic microdiamonds might be explained by the low temperature of metamorphism the absence of a free fluid phase in the metapelites and a short residence time in diamond facies conditions resulting in kinetic problems in the conversion of graphite to diamond

ultrahigh pressure minerals and metamorphic terranes the view from china abstract ultrahigh pressure uhp metamorphism refers to mineralogical modifications of continental and oceanic crustal protoliths associated mafic ultramafic rocks initially formed or emplaced in shallow levels of the lithosphere but which subsequently have experienced p t conditions within or above the coesite stability field 2 7 gpa 700 c typical products include eclogite garnet peridotite and uhp varieties of metapelite quartzite marble paragneiss and orthogneiss uhp metamorphic assemblages require relatively cold lithospheric subduction to mantle depths some recrystallization even occurs under forbidden p t conditions characterized by a geotherm of the current explosion of research on continental uhp terranes reflects their significance for mantle dynamics and the tectonics of continental subduction collision exhumation mantle slab interactions and geochemical recycling a further characterization of uhp phases and positive identification of uhp minerals requires new experimental studies coupled with state of the art analyses for example the very rare occurrence of microdiamond inclusions in zircons from dabie sulu uhp rocks may reflect high f o 2 attending recrystallization inasmuch as epidote is rather common rutile needles within garnets from sulu uhp eclogitic rocks may not be the result of exsolution so in such cases the apparent uhp pressure may have been over estimated hadean igneous microdiamond inclusions in jack hills detrital zircons could have originated from mantle xenoliths whereas abundant detrital phanerozoic diamonds containing inclusions of coesite and other eclogitic minerals from new south wales might have been derived from unexposed uhp metamorphic terranes micro mineral intergrowth and nano size minerals may hold important key to deciphering the actual p t paths of subduction and mantle return flow although most exhumed terranes have returned surfaceward relatively rapidly after short time of uhp condition the long duration of storage at great depth and slow exhumation for the largest uhp terranes remain as major problems

eclogite facies quartz veins within metabasites of the dabie shan eastern china pressure temperature time deformation path composition of the fluid phase and fluid flow during exhumation of high pressure rocks

u pb zircon geochronology of the yellowknife volcanic belt nwt canada new constraints on the timing and duration of greenstone belt magmatism volcanic and sedimentary rocks in the slave province collectively termed the yellowknife supergroup have long been considered to represent laterally continuous and correlative stratigraphically coherent homoclinal sequences correlation based on the lithologic analysis of certain distinctive units has been most intensely applied to the yellowknife volcanic belt in the southern slave province u pb zircon data for seven samples collected from within the belt are reported here the distinctiveness of the townsite formation a term applied to three lenticles of rhyodacitic to dacitic tuffs breccias and pillowed flows in comparison to the overlying and underlying tholeiitic basalts kam group has led to its use as a key marker unit for stratigraphic correlations and palinspastic restorations of offsets on proterozoic faults in the area a sample from each of the three segments was collected for u pb zircon geochronology in addition numerous thinner tuffaceous beds from the kam group were also dated

eocene structural development of the valhalla complex southeastern british columbia the valhalla complex a cordilleran metamorphic core complex is a 100 km by 30 km structural culmination within the omineca belt of southeastern british columbia it comprises sheets of granitic orthogneiss ranging in age from 100 to 59 ma with intervening paragneiss of uncertain age and stratigraphic correlation the complex is roofed by the ductile valkyr shear zone and the ductile brittle slocan lake fault zone the upper plate comprises lower grade metasedimentary rocks intruded by middle jurassic plutons the valkyr shear zone and the slocan lake fault zone deform 62 and 59 ma granitic sheets in their footwalls the easterly directed valkyr shear zone is a 2 to 3 km thick zone of distributed ductile strain which is arched over the complex and is exposed around the periphery on the northern western and southern margins the shear zone was active between 59 and 54 ma under amphibolite facies conditions the juxtaposition of upper and lower plates with different structural and metamorphic histories indicates that the valkyr shear zone is a significant structure with large displacement there is evidence to support an easterly rooting direction consistent with an extensional origin its surface breakaway is suggested to be west of the valhalla complex the slocan lake fault zone on the eastern side of the complex is a gently 30 easterly dipping ductile brittle normal fault which roots to the east it was active between 54 and approximately 45 ma and truncates the valkyr shear zone timing and structural relationships indicate that the valkyr shear zone and the slocan lake fault zone are genetically related movement on the ductile valkyr shear zone arching of the complex and displacement on the slocan lake fault zone occurred as a continuum in early to middle eocene time this paper documents the presence of significant eocene ductile strain in the valhalla complex and suggests that the role of extension in this region is more profound than had been previously recognized

fluid flow during exhumation of deeply subducted continental crust zircon u pb age and o isotope studies of a quartz vein within ultrahigh pressure eclogite quartz veins in high pressure to ultrahigh pressure metamorphic rocks witness channelized fluid flow that transports both mass and heat during collisional orogenesis this flow can occur in the direction of changing temperature pressure during subduction or exhumation shrimp u pb dating of zircon from a kyanite quartz vein within ultrahigh pressure eclogite in the dabie continental collision orogen yields two age groups at 212 7 and 181 13 ma which are similar to two groups of la icpms age at 210 4 and 180 5 ma for the same sample these ages are significantly younger than zircon u pb ages of 224 2 ma from the host eclogite thus the two age groups from the vein date two episodes of fluid flow involving zircon growth the first due to decompression dehydration during exhumation and the second due to heating dehydration in response to a cryptic thermal event after continental collision laser fluorination o isotope analyses gave similar 18o values for minerals from both vein and eclogite indicating that the vein forming fluid was internally derived synchronous cooling between the vein and eclogite is suggested by almost the same quartz mineral fractionation values with regularly decreasing temperatures that are in concordance with rates of o diffusion in the minerals while the quartz veining was caused by decompression dehydration at 700 650 c in a transition from ultrahigh pressure to high pressure eclogite facies retrogression the postcollisional fluid flow was retriggered by heating dehydration at 500 c without corresponding metamorphism in either case the kyanite quartz vein formed later than the peak ultrahigh pressure metamorphic event at the middle triassic pointing to focused fluid flow during exhumation rather than subduction the growth of metamorphic zircon in the eclogite appears to have depended on fluid availability so that their occurrence is a type of geohygrometer besides geochronological applicability to dating of metamorphic events in orogenic cycles

an archean metamorphic core complex in the southern slave province basement cover structural relations between the sleepy dragon complex and the yellowknife supergroup archean rocks in the fenton lake brown lake area southern slave province are subdivided into two lithotectonic domains a supracrustal domain which consists mainly of the archean yellowknife supergroup and a gneiss granite domain the latter is composed of gneissic and metaigneous rocks of the sleepy dragon complex determined to be basement to the yellowknife supergroup and granite plutons including the 2641 3 5 ma suse lake granite and the 2583 5 1 ma morose granite volcanic rocks of the cameron river belt and greywacke mudstone turbiditic metasedimentary rocks of the burwash formation constitute the supracrustal domain a late archean amphibolite to greenschist facies ductile to local brittle high strain zone separates the domains kinematic indicators demonstrate that the zone experienced two kinematically opposed episodes of displacement the older episode involved pre to synthermal peak thrusting of the supracrustal rocks over the gneiss granite domain thrusting is kinematically an

linking deformation migmatite formation and zircon u pb geochronology in polymetamorphic orthogneisses sveconorwegian province sweden absolute ages of migmatization in the polymetamorphic parautochthonous basement of the sveconorwegian province sweden have been determined using u pb ion probe analysis of zircon domains that formed in leucosome of migmatitic orthogneisses migmatite zircon was formed by recrystallization whereas dissolution reprecipitation and neocrystallization were subordinate the recrystallized migmatite zircon was identified by comparison of zircon in mesosomes and leucosomes it is backscatter electron bright u rich 800 4400 ppm with low th u ratios generally 0 01 0 1 unzoned or oscillatory ghost zoned and occurs as up to 100 m thick rims with transitional contacts to cores of protolith zircon protolith ages of 1686 12 and 1668 11 ma were obtained from moderately resorbed igneous zircon crystals generally th u 0 5 1 5 u 300 ppm in mesosomes protolith zircon is also present as resorbed cores in the leucosomes linkage of folding synchronous migmatization and formation of recrystallized zircon rims allowed direct dating of south vergent folding at 976 7 ma at a second locality similar recrystallized zircon rims in leucosome date pre sveconorwegian migmatization at 1425 7 ma an upper age bracket of 1394 12 ma for two overprinting phases of deformation upright folding along gently ssw plunging axes and stretching in ese was set by zircon in a folded metagranitic dyke lower age brackets for these events were set at 952 7 and 946 8 ma by zircon in two crosscutting and undeformed granite pegmatite dykes together with previously published data the present results demonstrate i tectonometamorphic reworking during the hallandian orogenesis at 1 44 1 42 ga resulting in migmatization and formation of a coarse gneissic layering ii sveconorwegian continent continent collision at 0 98 0 96 ga involving a emplacement of an eclogite unit b regional high pressure granulite facies metamorphism c southvergent folding subhorizontal east west stretching and migmatization all of which caused overprint or transposition of older mesoproterozoic and sveconorwegian structures the sveconorwegian migmatization and folding took place during or shortly after the emplacement of sveconorwegian eclogite and is interpreted as a result of north south shortening synchronous with east west extension and unroofing during late stages of the continent continent collision

multiple zircon growth during fast exhumation of diamondiferous deeply subducted continental crust kokchetav massif kazakhstan diamondiferous rocks from the kokchetav massif kazakhstan represent deeply subducted continental crust in order to constrain the age of ultra high pressure uhp metamorphism and subsequent retrogression during exhumation zircons from diamondiferous gneisses and metacarbonates have been investigated by a combined petrological and isotopic study four different zircon domains were distinguished on the basis of transmitted light microscopy cathodoluminescence trace element contents and mineral inclusions mineral inclusions and trace element characteristics of the zircon domains permit us to relate zircon growth to metamorphic conditions domain 1 consists of rounded cores and lacks evidence of uhp metamorphism domain 2 contains diamond coesite omphacite and titanian phengite inclusions providing evidence that it formed at uhp metamorphic conditions p 43 kbar t 950 c domain 3 is characterised by low pressure mineral inclusions such as garnet biotite and plagioclase which are common minerals in the granulite facies overprint of the gneisses p 10 kbar t 800 c this multi stage zircon growth during cooling and exhumation of the diamondiferous rocks can be best explained by zircon growth from zr saturated partial melts present in the gneisses domain 4 forms idiomorphic overgrowths and the rare earth element pattern indicates that it formed without coexisting garnet most probably at amphibolite facies conditions p 5 kbar t 600 c the metamorphic zircon domains were dated by shrimp ion microprobe and yielded ages of 527 5 528 8 and 526 5 ma for domains 2 3 and 4 respectively these indistinguishable ages provide evidence for a fast exhumation beyond the resolution of shrimp dating the mean age of all zircons formed between uhp metamorphic conditions and granulite facies metamorphism is 528 3 ma indicating that decompression took place in less than 6 ma hence the deeply subducted continental crust was exhumed from mantle depth to the base of the crust at rates higher than 1 8 cm year we propose a two stage exhumation model to explain the obtained p t t path fast exhumation on top of the subducted slab from depth 140 to 35 km was driven by buoyancy and facilitated by the presence of partial melts a period of near isobaric cooling was followed by a second decompression event probably related to extension in a late stage of continental collision

geology and u pb geochronology of the neoarchean snare river terrane tracking evolving tectonic regimes and crustal growth mechanisms u pb zircon crystallization ages determined by isotope dilution thermal ionization mass spectrometry id tims and laser ablation microprobe inductively coupled plasma mass spectrometry lam icp ms for 13 intrusive units in the neoarchean snare river terrane srt provide tight constraints on the timing of crust formation and orogenic evolution seven metaluminous plutons were emplaced over 80 ma from ca 2674 to 2589 ma whereas six peraluminous bodies were emplaced in a 15 ma interval from ca 2598 to 2585 ma a detrital zircon study yielded an age spectrum with peaks correlative with known magmatic events in the slave province with the ca 2635 ma age of the youngest detrital zircon population providing a maximum estimate for the onset of sedimentation this age contrasts with evidence for pre 2635 ma sedimentation elsewhere in the srt indicating that sedimentation was protracted and diachronous evolution of the srt can be subdivided into four stages i 2674 2635 ma formation of a metal

fluid regime in continental subduction zones petrological insights from ultrahigh pressure metamorphic rocks abstract intensive studies of ultrahigh pressure uhp metamorphic rocks in continental collision zones have provided petrological insights into the fluid regime during subduction and exhumation recent studies in the dabie sulu orogen of china have made important contributions to understanding fluid action in such systems hence these rocks are the main focus of this review sources of metamorphic fluid can be deduced from study of hydrous minerals structural hydroxyl and molecular water in nominally anhydrous minerals fluid and solid inclusions of uhp metamorphic minerals are also a record of fluid action in deep subduction zones the presence or absence of metamorphic fluid during subduction and exhumation dictates zircon growth and recrystallization making available materials for u pb dating isotopic dating demonstrates that after peak uhp metamorphism retrograde fluid with deuteric origin became active the decomposition of hydrous minerals and the exsolution of molecular water and structural hydroxyl during exhumation can provide sufficient amounts of aqueous fluid for quartz veining and amphibolite facies retrogression within uhp slabs aqueous fluid released by the exsolution of molecular water and structural hydroxyl from uhp nominally anhydrous minerals is characterized by low salinity and light hydrogen and oxygen isotope compositions therefore one part of the uhp slab is a source of aqueous fluid whereas another part is a sink in both cases the fluid action during subduction and exhumation is episodic rather than continuous supplementary material a description of the dabie sulu uhp metamorphic rocks analytical methods la icpms zircon u pb isotope data and rare earth element compositions are available at http www geolsoc org uk sup18355

shell size variation of the planktonic foraminifer neogloboquadrina pachyderma sin in the norwegian greenland sea during the last 1 3 myrs implications for paleoceanographic reconstructions we present measurements of the maximum diameter of the planktonic foraminifer neogloboquadrina pachyderma sin from six sediment cores ocean drilling program sites 643 644 907 909 985 and 987 from the norwegian greenland sea our data show a distinct net increase in mean shell size of n pachyderma sin at all sites during the last 1 3 ma with largest shell sizes reached after 0 4 ma external factors such as glacial interglacial variability and carbonate dissolution alone cannot account for the observed variation in mean shell size of n pachyderma sin we consider the observed shell size increase to mirror an evolutionary trend towards better adaptation of n pachyderma sin to the cold water environment after 1 1 1 0 ma probably the mid pleistocene climate shift and the associated change of amplitude and frequency of glacial interglacial fluctuations have triggered the evolution of this planktonic foraminifer oxygen and carbon stable isotope analyses of diverent shell size classes indicate that the observed shell size increase could not be explained by the functional concept that larger shells promote increasing sinking velocities during gametogenesis for paleoceanographic reconstructions the evolutionary adaptation of neogloboquadrina pachyderma sin to the cold water habitat has significant implications carbonate sedimentation in highest latitudes is highly dependent on the presence of this species in the norwegian greenland sea carbonate poor intervals before 1 1 ma are therefore not necessarily related to severe glacial conditions they are probably attributed to the absence of this not yet polar adapted species further transfer function and modern analog techniques used for the reconstruction of surface water conditions in high latitudes could therefore contain a large range of errors if they were applied to samples older than 1 1 1 0 myrs 2000 elsevier science b v all rights reserved

late archean tectono magmatic evolution of the central slave province northwest territories an early tectono stratigraphic and late plutonic assemblage are distinguished in the central part of the archean slave province contwoyto lake area on the basis of their relationship to deformation and metamorphic episodes and by geochemical characteristics the older assemblage which predates the onset of intense deformation and the thermal peak of metamorphism comprises 2 68 2 65 ga calc alkaline volcanic and plutonic rocks as well as extensive turbidites the younger assemblage comprises ca 2 61 2 58 ga plutonic units that were emplaced during and after deformation d d3 and the thermal peak of metamorphism compositions of the late plutonic units vary systematically with time of intrusion from an early calc alkaline diorite tonalite suite to late peraluminous granites the early assemblage is interpreted to represent remnants of an island arc or arcs overlain by turbidites and accreted during d1 and d2 the later assemblage is interpreted as a product of late to post accretion suprasu

thrusting magmatic intraplating and metamorphic core complex development in the archaean belleterre angliers greenstone belt superior province quebec canada abstract the belleterre angliers greenstone belt in the pontiac subprovince is the youngest and southernmost of the greenstone belts in the archaean superior province of the canadian shield the greenstone belt has been disrupted into three fragments from north to south the baby belleterre and lac des bois groups respectively the belleterre angliers greenstone belt has sheared contacts with younger pontiac metasediments and older tonalites the pontiac metasediments dip beneath the baby group and are tectonically interlayered with the tonalites thus the belleterre angliers belt is interpreted to be a thrust sheet 6 km thick in the north thinner in the south above a mid crustal duplex consisting of metasedimentary and tonalite imbricates the thrust sheet was transported from the north but age and geochemical constraints rule out derivation from the abitibi subprovince it probably roots in a band of ultramafic rocks that mark a major south vergent thrust in the northern pontiac subprovince after emplacement the belleterre angliers sheet and the adjacent pontiac metasediments were deformed by a system of linked extensional shears and transfer faults that border dome or antiformal areas in a style similar to phanerozoic metamorphic core complexes the orientation of shear zones and foliations indicates that the regional far field stresses were compressional both before and after extensional shearing since the system of extensional shears is intruded by a suite of monzodiorite granodiorite syenodiorite and syenite plutons with high mg number it is concluded that crustal inflation caused by the injection intraplating of large volumes of magma perturbed the regional compressive stress field and facilitated extensional faulting

extensional structures during deposition of the 3460 ma warrawoona group in the eastern pilbara craton western australia abstract structures in the granite greenstone terrain of the eastern pilbara were mapped in both the coongan belt greenstones and shaw batholith granitoids important structures that are coeval with an early magmatic event in both the granitoids and the greenstones were identified in the coongan greenstones a brittle extensional fault array is syn kinematic with felsic volcanics of the duffer formation dated at 3452 16 ma pidgeon 1978a in the underlying shaw batholith a major ductile shear zone was mapped at the contact with the coongan belt although the foliation in the shear zone forms part of a domal structure the stretching lineations constantly plunge to the ene the sense of shear as recorded in the shear zone is consistently east up with subsidiary sinistral strike slip movement on the e w trending part and dextral movement on the n s trending part a wedge shaped granitoid intruded into the shear zone during deformation this granitoid is part of the north shaw suite which has been dated at 3467 6 ma mcnaughton et al 1988 the structures in the greenstones and in the batholith have therefore formed during the same event at different crustal levels the structural kinematic magmatic metamorphic and geochronological data are consistent with a model in which these structures represent the downdip part of a dome with a core complex type of geometry in turn this implies that the domal geometry of the shaw batholith was initiated at mid crustal levels at a very early stage namely during deposition of the warrawoona group in the evolution of the pilbara craton as extension occurred during intrusion and extrusion of large volumes of magma the original distribution and thickness of granitoid bodies and greenstones were probably largely determined by the extensional geometry the initial domal shape of the shaw batholith has been enhanced during later deformation phases which caused steepening of n s trending structures

large areal extent of ultrahigh pressure metamorphism in the sulu ultrahigh pressure terrane of east china new implications from coesite and omphacite inclusions in zircon of granitic gneiss abstract coesite and omphacite inclusions have been identified for the first time as minute inclusions in zircon from amphibolite facies granitic orthogneiss in the sulu ultrahigh pressure uhp metamorphic terrane of eastern china by raman spectroscopy and microprobe analyses the occurrences of these minerals in the voluminous granitic gneiss of sulu support a regional and pervasive uhp metamorphic event that predated regional amphibolitic retrogression taking into account the widespread discoveries of coesite in other lithologies we thus conclude that a substantial crustal component in the sulu uhp metamorphic terrane appears to have shared a common history of triassic subduction to mantle depths and later exhumation

a new look at the stratigraphy of the yellowknife supergroup at yellowknife n w t implications for the age of gold bearing shear zones and archean basin evolution based on recent detailed mapping a revised stratigraphic column is proposed for the rocks of the archean yellowknife supergroup in the yellowknife greenstone belt the mafic volcanic rocks of the kam formation previously thought to represent the oldest supracrustal rocks of the belt overlap remnants of an earlier volcanic sedimentary sequence here referred to as the octopus formation as its enormous thickness makes it too unwieldy to be described as a single formation the kam formation is raised to group status and subdivided into four formations it is proposed that the kam group should replace the beaulieu group in the yellowknife area the chan formation at the base of the kam group consists of multiple gabbroic intrusions that were emplaced into a carapace of pillowed flows the intrusions locally resemble sheeted mafic dyke complexes in phanerozoic ophiolites thought to represent evidence for sea floor spreading the crestaurum formation which overlies the chan formation is characterized b

role of partial melting in the evolution of the sulu eastern china ultrahigh pressure terrane strongly deformed potassium feldspar rich dikes are widely distributed in the northern part of the sulu ultrahigh pressure uhp metamorphic terrane eastern china the fact that the crystallization ages of these dikes overlap with the age of peak uhp metamorphic conditions implies the presence of melt during metamorphism sr isotopic ratios of the dikes are compatible with their origin as partial melts of the dominant felsic sulu gneiss partial melting may be the key to solving several unusual features of the sulu and other uhp terranes such as the almost complete lack of mineralogical evidence for uhp conditions and the limited growth of zircon during uhp conditions in the dominant felsic gneiss in addition because partial melting will cause a drastic reduction in the strength of the uhp gneisses the most likely exhumation mechanism is diapiric rise of a low viscosity partially molten mass containing entrained blocks of eclogite and not a thin sheet as usually proposed

products of 2 65 2 58 ga orogenesis in the slave province correlated with slave northern cordillera lithospheric evolution snorcle seismic reflection patterns reflection patterns along lithoprobe slave northern cordillera lithospheric evolution snorcle seismic reflection line 1 in the southwestern slave province are interpreted as products of tectonic wedging during late archean lithospheric convergence the interpretation is aided by application of new seismic imaging techniques by correlation of upper crustal reflection patterns to known geology and by a comparison of lithospheric reflection patterns to similar convergent zones elsewhere in the yellowknife area reflection patterns consist of 1 east dipping reflections at 12 14 s that project into the upper mantle 2 a wedge shaped body in the lower crust with an east dipping reflection fabric that is truncated on the west by 3 a series of west dipping reflections that outline thrust and fold structures in the upper crust the similarity of these reflection patterns to those of the proterozoic fort simpson hottah collision zone 300 km to the west provides support for the interpretation that reflection patterns beneath the slave province are also products of collisional tectonics rocks within the slave province preserve evidence of a 2 65 2 58 ga pan slave orogenic event in which the 2 9 ga central slave basement collided with the 2 7 ga juvenile eastern slave province their suture is interpreted to be a west dipping surface at 4 5 s 12 15 km beneath yellowknife and to project to the surface east of the profile in the lower crust and upper mantle east dipping reflections are interpreted to delineate a coeval subduction zone and accretionary wedge the upper crustal thrust and fold structures are likely linked to gold bearing shear zones at yellowknife these results provide tantalizing evidence that processes similar to those of modern convergent zones were operational at 2 65 2 58 ga

pb u and th diffusion in natural zircon zircon zrsio4 is one of the most widely used minerals for determining the age origin and thermal history of rocks by u th pb geochronology but the parameters describing the solid state volume diffusion rates of these elements in natural zircon which are themselves important for establishing the limits on the applicability of zircon for geochronological studies have remained poorly quantified1 this is because of the measurement difficulties associated with the low p p m concentrations and low diffusion rates of these elements in natural zircon and the chemical and physical heterogeneity present in most crystals here we present direct measurements of the uranium thorium and lead loss from a thermally treated gem quality natural zircon and show that lead diffuses much faster than uranium or thorium we find that the u th pb isotopic system in natural zircon will typically have a closure temperature greater than 900 c which explains why zircon is apparently such a robust geochrometer and is capable of remaining isotopically closed through extended periods of high grade metamorphism and partial melting of the host rock

in situ u pb dating and trace element analysis of zircons in thin sections of eclogite refining constraints on the ultra high pressure metamorphism of the sulu terrane china abstract in situ u pb dating and trace element analysis of zircons combined with a textural relationship investigation in thin section is a powerful tool to constrain the ultra high pressure stage of high grade metamorphism two types of zircon grains have been identified in thin sections of a retrograde eclogite from the main hole of the chinese continental scientific drill project in the sulu uhp terrane type 1 zircon grains occur as inclusions in fresh garnet and omphacite and type 2 zircon grains were found in symplectite around omphacite the fresh rims of type 1 zircons and mantles of a few type 2 zircons exhibit remarkably lower ree y nb and ta contents than the inherited zircon cores suggesting coeval growth with garnet rutile and apatite during uhp metamorphism these may have formed in the uhp metamorphism and survived retrograde metamorphism the weighted average 206pb 238u age of these zircon domains 230 4 ma 2 agrees well with the published age of coesite bearing zircon separates 230 1 ma 2 suggesting that the peak uhp metamorphism in the sulu terrane may have occurred at 230 ma zircon domains surrounded or cut across by symplectite could have been altered by retrograde metamorphism together they provide a younger weighted average 206pb 238u age of 209 4 ma 2 these retrograde zircon domains have similar ree compositions to the 230 ma uhp zircon domains these observations imply that the 209 ma zircon domains could have formed by fluid activity associated alterations in the amphibolite facies metamorphism which could have resulted in the complete loss of pb but not rees in these domains

timing of plutonism deformation and metamorphism in the yellowknife domain slave province canada u pb zircon titanite and monazite ages for plutonic rocks are used to constrain the timing of post 2660 ma deformational and metamorphic events in the yellowknife domain southern slave province diorite and tonalite plutons of the defeat suite yield ages of 2628 3 and 2624 3 ma older than previous less precise estimates for this suite of ca 2618 2620 ma the defeat suite plutons crosscut and postdate upright f1 folds in turbidites of the ca 2680 2660 ma burwash formation d1 deformation is therefore bracketed between 2660 and 2630 ma the d1 event significantly predates deposition of polymict conglomerates of the jackson lake formation a second folding event f2 represented by northwest to north trending steeply plunging folds of bedding and d1 structures affected the area after intrusion of the hidden lake pluton at 2606 2 ma and synchronously with intrusion of the 2596 2 ma sparrow lake pluton a member of the two mica granite prosperous suite the time of initiation and the duratio

neodymium isotopic evidence for the tectonic assembly of late archean crust in the slave province northwest canada the ca 2 7 2 5 ga slave province is a granitegreenstone terrane comprising deformed sedimentary and subordinate volcanic belts extensively intruded by granitoid rocks the nd isotopic data are reported for 58 samples of supracrustal and granitoid rocks exposed along a 400 km east west transect at 65 n across the structural grain of the province initial nd values reveal distinctly different crustal sources in the eastern compared to the western parts of the province as expected from tectonic assembly of the province through accretion of juvenile crust to older continental crust supracrustal sequences ca 2 71 2 65 ga from the central and eastern parts of the province have positive nd 1 values 0 3 to 3 6 consistent with juvenile sources and formation remote from significantly older crust syn to late deformation ca 2 63 2 60 ga mantle derived diorites and related tonalites type i from the central and eastern parts of the province have similar initial nd values 0 1 to 2 7 in contrast samples from the westernmost plutons which intrude exposed pre 3 1 ga crust have much lower nd 1 values 1 0 to4 6 suggesting contamination of these magmas by older crust the nd 1 values of post deformation granites s s type ii also vary systematically across the province values for granites west of longitude 110 30 w range from 0 2 to 5 3 those to the east range from 0 6 to 3 7 these data suggest mixed crustal sources dominated by mid to early archean material nd 2 6 to 17 at 2 6 ga for the western granitoid rocks and juvenile sources for the eastern granites the nd isotopic data are consistent with the geology of the province in that exposures of mid to early archean crustal rocks predating the principal 2 7 2 5 ga orogenic event are restricted to the western part of the province the asymmetric pattern defined by the nd isotopic data indicates the presence of distinct crustal rocks beneath the slave province similar isotopic variations observed across phanerozoic collisional orogens have been interpreted to reflect tectonic assembly of crust by accretion of juvenile crustal terranes to an older continental margin this process may also have been an important mechanism in the cratonization of the slave province

evolution of an archean basement complex and its autochthonous cover southern slave province canada abstract the sleepy dragon complex sdc is one of several antiformal exposures of a 4 0 2 85 ga basement block underlying the west central region of the archean slave province northwestern canada this basement block the central slave basement complex csbc is overlain by an autochthonous to locally parautochthonous dominantly quartz rich clastic sequence the 2 85 2 80 ga central slave cover group together these units comprised a fundamental building block during neoarchean growth and assembly of the slave craton the well exposed patterson lake morose lake area represents a type locality for study of the csbc central slave cover group and overlying rocks of the yellowknife supergroup field mapping and u pb geochronology employing both thermal ionisation mass spectrometry tims and sensitive high resolution ion microprobe shrimp document a prolonged crustal history oldest intact basement units consist of foliated to gneissic tonalite dated in two places at 2955 12 ma and 2944 9 ma indirect evidence for a ca 3150 ma basement component is obtained both from xenocrystic zircon in tonalite and detrital zircons from the overlying central slave cover group which here consists of the patterson lake formation a granite boulder from this formation is dated at 2934 3 ma and may be locally derived a sheared mafic volcanic unit along the basement cover contact contains 2942 3 ma metamorphic titanite which provides a minimum deposition age this volcanic unit is therefore also a part of the basement complex younger events in the sdc mainly reflect tonalite trondhjemite granodiorite plutonism during construction of the overlying cameron river and beaulieu river greenstone belts of the yellowknife supergroup a unit of k feldspar megacrystic granodiorite is dated at 2726 3 ma and tonalite and granodiorite bodies located toward the centre of the sdc have primary crystallization ages of 2683 3 2 ma 2677 2 ma and 2672 7 6 ma the core of the southern sdc is occupied by a late tectonic to post tectonic granite pluton previously dated at 2586 2 ma an intravolcanic unconformity along the southern margin of the sdc developed in part on 2 7 ga granodiorite is constrained by existing u pb data to have formed sometime between ca 2685 2660 ma an abundance of synvolcanic plutons of similar age and the presence of a 3 km thick mafic volcanic sequence overlying a less dense magmatically active felsic crust suggest that gravitational instability and magmatic diapirism may have played important roles in doming and uplift of the sdc prior to regional d 1 deformation the younger toward centre zonation of plutonic rocks in the sdc is documented in other domal granite gneiss complexes of archean age e g pilbara craton western australia where it is attributed to upward and outward displacement of older crust during repeated magma channeling into a growing mid crustal dome in the west central slave craton such a process might account for the regional presence of local unconformities of apparently similar age the domal basement plutonic complexes would have been further amplified after 2660 ma during the development of regional f 1 and f 2 fold belts

melting induced fluid flow during exhumation of gneisses of the sulu ultrahigh pressure terrane abstract hydrothermally altered rocks are products of fluid rock interactions and typically preserve numerous quartz veins that formed as chemical precipitates from fluids that fill up cracks thus quartz veins are the record of the fluid system that involved fracture flow in the direction of changing temperature or pressure in order to decipher the fluid activity in the sulu ultrahigh pressure uhp terrane in eastern china quartz veins together with an adjacent eclogite lens and the host gneiss were studied in one location a deformed quartz vein is located at the boundary between the host gneiss and the eclogite lens the amphibolite facies overprinting of the eclogite lens decreases from the rim to the core of the lens with fresh eclogite preserved in the core the foliated biotite gneiss contains felsic veins and residual phengites zircon rims from the gneiss are characterized by melt related signatures with steep hree patterns high hf contents and negative eu anomalies and a pool of weighted average 206 pb 238 u analyses reveal an age of 219 3 ma 2 which is younger than the uhp metamorphic age 236 2 ma 2 recorded by zircons from the eclogite lens this suggests that the gneiss in the sulu uhp terrane could have suffered from partial melting due to phengite dehydration during the hot exhumation stage the formation age of the quartz vein 219 2 ma 2 defined by zircon rims agrees well with the partial melting time 219 3 ma 2 of the host gneiss the initial 176 hf 177 hf ratios of zircon rims from the quartz vein are obviously lower than zircons from the eclogite lens but overlap with the coeval zircon domains from the nearby granite dikes produced by partial melting of orthogneiss these observations suggest that the quartz vein and corresponding fluid flow could be associated with partial melting of the host gneiss on the other hand amphibole bearing and hree rich zircon rims from the amphibolite pool an amphibolite facies metamorphic age of 217 5 ma 2 overlap with the formation age of the quartz vein this implies that retrogression of the eclogite lens could have been caused by melting induced fluid flow based on the above observations we speculate that partial melting of the gneiss in the continental subduction related uhp belt could have induced a significant fluid flow during the exhumation stage and thus contributed significantly to the extensive retrogression of eclogites in the sulu uhp terrane

secular trends in the thermal evolution of metamorphic terrains the thermal evolution of convergent orogens is sensitive to the deformation history of the mantle part of the lithosphere if the dense mantle lithosphere thickens as a consequence of convergent deformation and remains attached during subsequent denudation of the thickened crust then high pressure low to intermediate temperature glaucophane schist assemblages will form during both thickening and denudation the actual form of the p t paths for a given deformation depending primarily on the initial thermal state of the lithosphere and particularly the ratio of the thickness of the crust and mantle lithosphere however if mantle lithosphere is thinned during convergence by processes such as thermal boundary layer detachment then any early formed high pressure low to intermediate temperature glaucophane schist assemblages will be obliterated during subsequent high temperature overprints since the stability of the thermal boundary layer during convergent deformations is controlled by the rate of convergence and the convective vigour of the subjacent mantle any change in the convective vigour of the mantle through geologic time in response to for example changing global heat production may therefore be reflected in the thermal evolution of metamorphic terrains formed during convergent deformations models of the thermal evolution of deforming lithosphere appropriate to an earth in which the effect of declining global heat production and convective vigour is mediated by the process of thermal boundary layer detachment suggest that the preservation of high pressure low to intermediate temperature glaucophane schist assemblages in convergent orogens has become increasingly probable through geological time a suggestion in accord with the observed secular trend in the character of metamorphic terrains preserved in the geological record

complex evolution of folds and cleavages in archean rocks yellowknife n w t overturned folds and later cleavages in a metasedimentary terrain with marginal volcanics near yellowknife form a complex succession in which structures repeatedly trend eastward and northward early major east west folds are overturned away from a southern granodioritic complex and terminate westward against a major syncline with limbs overturned inward trending north south along the western granitoid boundary of the terrain later smaller folds limbs 1 km which die out away from granitoid plutons include a progression from northerly to easterly trending structures several sets of cleavage foliations mostly steeply inclined transgress the folds an east west set succeeded by a regionally dominant set striking northwest to north and a later north south set confined to near the western margin of the terrain the easterly and northerly near orthogonal trends followed by successive structures could reflect the influence of fault bounded crustal blocks during emplacement of fault controlled grani

crustal shortening and thickening in neoarchean granite greenstone belts a case study from the link between the 2 7 ga elu and hope bay belts northeast slave craton canada abstract the elu link between the 2 7 ga hope bay and elu belts in the northeast bathurst block of the slave craton comprises supracrustal and intrusive rocks variably deformed by three tectono metamorphic events d1 d3 the geometry of d1 structures formed during prograde metamorphism is uncertain because of subsequent overprint d2 occurred in two stages predating d2a and postdating d2b peak metamorphism d1 and d2a were thrusting events inferred from peak metamorphic pressures of 6 7 kbar 670 mpa retained by a garnet orthogneiss the latter is diagnostic of thrust tectonism in archean granite greenstone belts with no characteristic thrust faults unlike d2a d2b was a vertical general flattening event prevailing during the formation of magmatic domes and interdomal folds that form the main strain patterns of the belts this was followed by the formation of buckled f3 folds associated with d3 vertical constriction the switch from thrust to vertical tectonics during peak metamorphism and subsequent deformation resulted in intense recrystallization that explains the poor preservation and scarcity of early formed shears including thrust zones a tectonic process combining d1 d2a thrust stacking sagduction and vertical stretching during d2b and d3 is suggested to explain crustal thickening in the elu link and terrains of similar ages

the central slave basement complex part i its structural topology and autochthonous cover new field and geochronological data are used to define the distribution of mesoarchean basement rocks in the south central slave province this distribution reflects a single contiguous basement terrane that we propose to call the central slave basement complex it shows a structural topology that is internally consistent and compatible with known regional folding and faulting events a sample of a proposed basement gneiss below the courageous lake greenstone belt central slave province has been dated by u pb methods and yields an age of 3325 8 ma consistent with the new basement distribution this sample also contains 2723 3 ma metamorphic zircon and ca 2680 ma titanite the central slave basement complex is overlain by a thin discontinuous but distinctive cover sequence that includes minor volcanic rocks clastic sedimentary rocks and banded iron formation all previously known and some new occurrences of this distinctive cover sequence occur in the immediate stratigraphic hanging wall of the

multiple dikes in lower kam group yellowknife greenstone belt evidence for archean sea floor spreading a mafic intrusive extrusive complex at the base of the kam group in the archean yellowknife supergroup at yellowknife northwest territories canada grades from gabbros through a zone of multiple and sheeted dikes into massive and pillowed flows with thin beds of interflow sediments although rock types and primary structures resemble those in upper parts of phanerozoic ophiolites the kam group lacks an ultramafic base and is much thicker than typical ophiolites because similar mafic sections occur in certain atypical ophiolites and in plateau areas of modern ocean basins however the kam group is viewed as a remnant of archean oceanic crust rather than as a product of intracontinental rift volcanism the dikes are analogous to sheeted dikes of ophiolites and are suggestive of sea floor spreading in an archean marginal basin setting

structural geometry of an exhumed uhp terrane in the eastern sulu orogen china implications for continental collisional processes high precision 1 200 1 1000 mapping of yangkou bay eastern sulu orogen defines the structural geometry and history of one of the world s most significant uhp ultrahigh pressure belts at least four stages of folds are recognized in uhp eclogites and associated quartzo feldspathic gneiss uhp eclogite facies rootless f1 and isoclinal f2 folds are preserved locally in coesite eclogite mylonitic to ultramylonitic quartzo feldspathic and coesite eclogite shear zones separate small scale 5 10 m thick nappes of ultramafic mafic uhp rocks from banded quartzo feldspathic gneiss these shear zones are folded and progressively overprinted by amphibolite greenschist facies shear zones the prograde to retrograde d1 d5 deformation sequence is explained by deep subduction of offscraped thrust slices of lower continental or oceanic crust from the down going plate caught between the colliding north and south china cratons in the mesozoic after these slices were structurally isolated along the plate interface they were rolled in the subduction channel during exhumation and structural juxtaposition with quartzo feldspathic gneisses forming several generations of folds sequentially lower grade foliations and lineations and intruded by in situ and exotically derived melts shear zones formed during different deformation generations are wider with lower grades suggesting that deep crustal upper mantle deformation operates more efficiently perhaps with more active crystallographic slip systems than deformation at mid upper crustal levels

tectonomagmatic evolution of cenozoic extension in the north american cordillera the spatial and temporal distributions of cenozoic extension and magmatism in the cordillera suggest that the onset of major crustal extension at a particular latitude was confined to a relatively narrow belt 100 km pre extension and followed the onset of intermediate and silicic magmatism by no more than a few million years extension began in early eocene time in southern british columbia northern washington idaho and montana farther s extension began at about the eocene oligocene boundary in the great basin and slightly later in the mojave sonora desert region the intervening area at the latitude of las vegas remained quiescent until mid miocene time compositional and isotopic characteristics of most pre miocene magmas are consistent with their containing major components of melted continental crust in mid miocene time two major changes occurred widening of the area of extension and the widespread appearance of basaltic magmas the area affected by extension from southwestern montana to the lake mead region widened to several hundred kilometres by this time extension in southern british columbia northern washington and northern montana had ceased probably before the end of the eocene and extension s of lake mead except in the gulf of california had waned regions affected by the broader belt of extension during late miocene pliocene and quaternary time experienced basaltic magmatism which began along a central rift zone in the northern part of the region and which had within a few million years spread to include most of the region later basaltic activity has tended to concentrate in restricted zones especially near the margins of the extended area we recognize a correlation between the amount of earlier crustal thickening and cenozoic extension and between the length of time after shortening but before extension and the degree to which a given region was intruded by late cretaceous plutons the localization of extension in areas of previous crustal thickening and the dependence of the timing of extension on the thermal state of the overthickened crust is consistent with a simple thermal mechanical model developed in a companion paper sonder et al this raises the possibility that stresses inherent in the north american plate dominated over plate interaction forces as controls of the cenozoic tectonomagmatic evolution of the north american cordillera especially in its earlier stages

complete miscibility between silicate melts and hydrous fluids in the upper mantle experimental evidence and geochemical implications the phase relationships between silicate melts and hydrous fluids were studied by direct visual observation in an externally heated diamond anvil cell complete miscibility of silicate melt and water was observed for a wide range of melt compositions including nepheline jadeite dacite haplogranite and ca bearing granite some evidence for complete miscibility was also observed in the system basalt h2o the critical temperatures in all systems decrease rapidly with pressure at 15 kbar the critical temperature for nepheline is around 550 c for jadeite around 800 c and for granitic compositions it is close to 900 c in general the critical temperatures appear to increase with silica content in the system our results suggest that there is complete miscibility between silicate melts and water in most of the upper mantle except at very shallow depths this means that a water saturated solidus cannot be defined any more in the deeper parts of the upper mantle very silica rich melt inclusions found in spinel lherzolites associated with fluid inclusion are probably the result of the unmixing of a supercritical fluid containing comparable amounts of water and silicate components the decomposition of amphibole in a subducted slab occurs at conditions where the miscibility gap between fluid and silicate melt is not yet closed while the decomposition of phengite and lawsonite occurs far beyond the critical curve accordingly the fluids released by the breakdown of these minerals should have very different properties highly mobile hydrous fluids containing little dissolved silicate should be produced by amphibole breakdown while the decomposition of lawsonite and phengite will lead to much more silicate rich and less mobile fluid phases

garnet spinel corundum quartz bearing titanohematite veins in eclogite from the sulu ultrahigh pressure terrane imprint of a short lived high temperature metamorphic stage abstract eclogite hosted garnet spinel corundum quartz bearing titanohematite veins and lenses 10 20 cm in width are described for the first time in the sulu ultrahigh pressure metamorphic terrane some of the lenses were aligned parallel to the foliation of the host eclogite suggesting that they were the product of ductile deformation of one titanohematite layer a vein composed of titanohematite ilmenite hematite spinel garnet corundum quartz k feldspar albite was studied in detail ti fe oxides account for up to 80 and al rich phases for 15 of the total volume of this vein electron microprobe analyses show that the titanohematite solid solution was made up of 0 75 hematite 0 25 ilmenite the unusual mineral assemblage of garnet spinel corundum quartz implies that this vein could have experienced high temperatures 900 c although the garnets showed well defined mg and mn diffusion zoning in the rim as a result of the high temperature event slight mg and mn growth zoning was preserved in the core thus we suggest that the sulu uhp terrane could have experienced a short lived high temperature 900 c stage during exhumation garnets in the titanohematite vein were characterized by extremely low trace element contents petrological and geochemical features of the veins suggest that they could be metamorphic products of igneous cumulates composed of magnetite plagioclase clinopyroxene

structural development of angular volcanic belts in the archean slave province narrow belts of metavolcanics lying between metasediments and granitoid rocks in the southwestern part of the archean slave province display a distinctive angular map pattern steep homoclinal segm

oxygen isotope geochemistry of ultrahigh pressure metamorphic rocks from 200 4000 m core samples of the chinese continental scientific drilling abstract oxygen isotope studies were carried out for ultrahigh pressure metamorphic rocks from the main hole of the chinese continental scientific drilling in the sulu orogen the samples of interest include various types of lithology mainly eclogite and gneiss in a depth of 200 to 4000 m containing five continuous core segments between contrasting lithologies the results show a large variation in 18o value from 10 41 to 9 63 for constituent minerals distinct 18o depletions are observed in the frequently alternated layers of eclogite and gneiss in which the variations of 18o values are gradual regardless of lithologies both equilibrium and disequilibrium o isotope fractionations are observed between quartz and the other minerals special attention was paid to the relationship between distance petrography and 18o value of adjacent samples the results show o isotope heterogeneities between the different and same lithologies on scales of about 20 to 50 cm corresponding to the maximum scales of fluid mobility during the continental collision amphibolite facies retrograde metamorphism during exhumation caused mineral reactions and o isotope disequilibria between some of the minerals considerable changes occur in 18o and petrography at the contact between eclogite and gneiss suggesting that the contact between different lithologies is the most favorable place for fluid activity despite the widespread retrogression retrograde fluid was internally buffered in the stable isotope compositions the retrograde fluid is of deuteric origin and thus was derived from the decompression exsolution of structural hydroxyl although locally external fluids became available along fault zones and lithological layers it still acted within the exhumed slabs with internal origins fluid flow also took place after the amphibolite facies retrogression but it only affects the o isotope compositions of feldspar and mica premetamorphic protoliths are deduced to have heterogeneous 18o values due to varying degrees of meteoric water rock interaction before continental collision minimum depth of the 18o depletion is up to 3300 m together with areal 18o depletion in surface outcropped rocks along the dabie sulu orogenic belt at least 66 000 km3 of supracrustal rocks were interacted with meteoric water along the northern margin of the south china block during the neoproterozoic

constraining the prograde and retrograde p t t path of eocene hp rocks by shrimp dating of different zircon domains inferred rates of heating burial cooling and exhumation for central rhodope northern greece ion microprobe shrimp dating was carried out on different zircon domains from metamorphic rocks of the hp ht terrane of central rhodope northern greece to constrain the timing of prograde and retrograde stages within a single tectono metamorphic cycle a well determined p t t relative path for the metamorphic rocks of this terrane was used as a petrological basis for the geochronological investigations ion microprobe work was assisted by cathodoluminescence cl images of the zircon crystals the geochronological results revealed that hercynian continental crust was subducted during the eocene several stages of the eocene tectono metamorphic cycle including both the prograde and retrograde parts of the p t path above ca 300 c 0 3 gpa were dated using zircons from the following rock types 1 a deformed quartz vein probably formed at ca 300 c 0 3 gpa zircons in this vein precipitated from a hydrothermal fluid they yielded an age of 45 3 0 9 ma which corresponds to the time of a low t prograde stage of metamorphism 2 in kyanite eclogites zircons were entirely reset during eclogite facies metamorphism resetting was very probably enhanced by the presence of fluids derived by h2o liberating reactions close to the p peak they yielded an age of 42 2 0 9 ma 3 orthogneisses surrounding the kyanite eclogites contained zircons with magmatic oscillatory zoned cores which yielded hercynian ages of 294 8 ma age of granitic protolith formation whereas cl bright metamorphic rims yielded like the eclogite zircons ages of 42 0 1 1 ma therefore both the eclogites and orthogneisses are interpreted to have approached maximum depth at around 42 ma 4 in a leucosome of a migmatized orthogneiss oscillatory zoned zircons yielded an age of 40 0 1 0 ma at this time the rocks reached maximum temperatures during early decompression 5 a late pegmatite crosscutting the schistosity of amphibolites contained oscillatory zoned zircons that yielded a crystallization age of 36 1 1 2 ma thus the whole tectono metamorphic cycle above ca 300 c 0 3 gpa lasted from 45 3 0 9 ma to 36 1 1 2 ma that is 9 2 ma with an extreme error value of 2 1 ma based on combined shrimp and petrological data the average rates of heating and burial during subduction above ca 300 c 0 3 gpa are 94 c ma and 15 mm a respectively rates of cooling and exhumation also above 300 c 0 3 gpa are calculated to be 128 c ma and 7 7 mm a the eocene age of metamorphism in central rhodope implies that the terrane of at least central rhodope and the cyclades very probably was part of the same continental crust

multiple partial melting events in the sulu uhp terrane zircon u pb dating of granitic leucosomes within amphibolite and gneiss thin layers and lenses of granitic leucosome are widely distributed within amphibolites paragneisses and orthogneisses of the sulu uhp terrane they are parallel to or cross cut foliations in the host rocks at different scales and show evidence of coalescence and migration to form centimetre to decimetre scale segregations variously migmatized rocks extend at least 350 km from sw sulu maobei to ne sulu weihai in a band at least 50 km wide a combined study of mineral inclusions cathoduluminescence cl images u pb la icp ms dates and in situ trace element compositions of zircon provide clear evidence on the nature and timing of partial melting in these uhp rocks most zircon from the granitic leucosomes occurs as distinct overgrowths around inherited igneous or metamorphic cores or as new euhedral crystals the overgrowths and new crystals commonly show perfectly euhedral shapes have pronounced oscillatory zoning and contain felsic mineral inclusions such as kfs pl qtz ilm monazite mon in contrast the inherited igneous or metamorphic cores are rounded or irregular contain low p or uhp mineral inclusions and show clear dissolution textures these data suggest that the new zircon is anatectic in origin and that it grew during partial melting of the uhp rocks the ree patterns of the anatectic zircon show steep slopes from the hree to lree with strongly to moderately negative eu anomalies eu eu 0 31 0 72 and pronounced positive ce anomalies ce ce 6 8 26 5 abundant u pb spot analyses of the anatectic zircon reveal two discrete and meaningful ages of partial melting within the sulu uhp terrane anatectic zircon from 12 granitic leucosomes within amphibolites paragneisses and orthogneisses from sulu uhp slices ii and iii yields consistent mean u pb ages of 219 0 1 2 to 218 3 1 6 ma 218 8 2 0 to 217 3 1 7 ma and 218 2 1 4 to 215 0 1 5 ma respectively in contrast anatectic zircon from six granitic leucosomes within paragneisses and orthogneisses from sulu uhp slice iii records younger mean u pb ages of 151 9 1 3 to 151 1 1 8 ma and 155 9 1 8 to 153 7 1 7 ma respectively these data imply that the sulu uhp terrane experienced two mesozoic partial melting events the first partial melting event 219 215 ma was probably associated with a late triassic granulite facies stage of hot exhumation whereas the second 156 151 ma is interpreted as the result of middle late jurassic extension and thinning of the previously thickened crust of the sulu uhp terrane both partial melting events induced extensive retrograde metamorphism of the eclogites and their country rocks

transition from back arc to foreland basin development in the southernmost andes stratigraphic record from the ultima esperanza district chile during the mesozoic evolution of the southernmost andes back arc basin formation in an extensional setting was followed by foreland basin development in a compressional setting a stratigraphic record of this tectonic transition is described from jurassic and cretaceous rock units exposed in the patagonian fold thrust belt at 51 s latitude initiation of an extensional deep marine trough in late jurassic time is documented by interstratified rhyolites and marine mudstones in the tobifera formation development of the early cretaceous rocas verdes back arc basin is recorded by zapata formation submarine slope deposits which draped the west facing passively subsiding cratonic margin of the basin the onset of andean compressional orogenesis and creation of the magallanes foreland basin is marked by the influx of coarse grained sandstone turbidites of the albian cenomanian punta barrosa formation during late cretaceous time the foreland basin configuration consisted of a narrow foredeep trough bounded by a gently sloping foreland ramp on the craton to the east this geometry reflects greater flexural subsidence of the stretched and thermally weakened passive back arc basin margin beneath the load of the obducted rocas verdes basin floor changes in depositional regime and sediment dispersal patterns in the latest cretaceous together with an eastward shift in the magallanes basin depocenter record cratonward migration of deformation in the patagonian fold thrust belt loading of the thicker cratonic lithosphere caused subsidence of the foreland ramp to form a wide flexural basin

postcollision extension in arc continent collision zones eastern indonesia postcollisional extension superimposed on arc continent collision complexes is a common feature in the orogenic belts of eastern indonesia in the southern banda arc the arc continent collision event is essentially a pliocene and younger feature but already an element of extension is being superimposed on the compressional structures of the collision complex it is thus likely that the extension commences within a very short period 5 m y after the initiation of collision related compressional deformation a similar history of fold belt development immediately followed by extension can be inferred for the lengguru fold belt and the adjacent wandamen wondiwoi terranes of irian jaya and for the gulf of bone in sulawesi it is suggested that the extension results from decoupling of the subducting oceanic lithosphere from the unsubductable continental lithosphere and that the superimposition of extension is a virtually unavoidable consequence of the arc continent collision

basement controlled structural fronts forming an apparent major refold pattern in the yellowknife domain slave province trend lines of tight folds f0 and f1 in archean metasediments curve across the yellowknife supracrustal domain to form an apparent major refold pattern hinge traces marking pronounced changes in trend extend up to 50 km more locally f1 folds are discontinuous and markedly discordant in trend suggesting that the major hinge traces rather than simply following axial surfaces denote structural fronts where fold sets with distinctive initial trends have interacted cleavages grouped as s2 and s3 each with subsets postdate the f1 folds a regional s3 cleavage strikes parallel to major hinge traces whereas discontinuous s3 subsets are oblique some terminating along well defined cleavage fronts for example one subset replaces another along a front generally parallel to a hinge trace or fold front but offset 1 2 km the development of the apparent refold pattern and the parallelism of various generations of structural fronts and cleavages can be explained by the response of cover rocks to recur

u pb zircon ages and field relationships of archean basement and proterozoic intrusions south central wopmay orogen nwt implications for tectonic assignments new field and u pb zircon data from the south central wopmay orogen south of 65 n establish crystallization ages of basement and plutonic phases and bring to the forefront questions on the architecture of the orogen the complex and extensive 3300 2575 ma basement domain was derived from the adjacent slave craton paleoproterozoic supracrustal rocks dominated by an extensive pelitic psammitic sequence with basal arenite conglomerate and carbonate unconformably overlie this basement pre to post kinematic paleoproterozoic plutons intrude both basement and overlying strata and crystallized at 1877 1867 1862 and 1858 1850 ma the first pulse of plutonism constrains an early generation of metamorphism and deformation to younger than 1877 ma and is corroborated by metamorphic zircon in archean basement the interval from 1867 to 1862 ma brackets ductile deformation in granite and granodiorite intrusions although corresponding structure in the metasedimentary strata is equivocal post kinematic ca 1

accretion of the archean slave province continental rift models have long been applied to the archean slave province of northwestern canada a reassessment of these models shows them to be incompatible with observed geological relations and suggests that contractional tectonic models may be more appropriate than extensional ones regions composed of different rock suites e g orthogneisses vs mafic volcanics are separated by high strain zones recording large displacements it is proposed that the high strain zones separate four distinct terranes that have been juxtaposed during collisional orogenesis from west to east these include the anton terrane interpreted as an archean microcontinent the sleepy dragon terrane possibly an exhumed more eastern part of the anton terrane the contwoyto terrane a westward verging fold and thrust belt containing tectonic slivers of greenstone volcanics and the hackett river volcanic terrane interpreted as an archean island arc the contwoyto and hackett river terranes represent a paired accretionary prism and island arc system that formed above an east dipping subduction zone these collided with the anton microcontinent producing a basement nappe expressed as the sleepy dragon terrane during the main accretion event within the slave province the whole tectonic assemblage was intruded by late kinematic to postkinematic granitoids

an extended episode of early mesoproterozoic metamorphic fluid flow in the reynolds range central australia the products of metamorphic fluid flow are preserved in zones within the marbles and metamorphosed semipelites of the upper calcsilicate unit in the granulite portion of the late palaeoproterozoic reynolds range group northern arunta block central australia the zones of retrogression characterized by minerals such as wollastonite grossular and clinohumite local resetting of oxygen isotopic compositions and local major element metasomatism were channelways for water rich fluids derived from granulite facies metapelites u th pb isotopic ages measured by the shrimp ion microprobe on zircon and monazite from a granulite facies semipelite an early semiconcordant aluminous quartz rich fluid flow segregation and a late discordant quartz rich segregation record some of the extended thermal history of the area zircon cores from the semipelite show its likely protolith to be an igneous rock 1812 11 ma old itself derived from a source containing zircon as old as 2 2 ga low th u overgrowths on the zircon grew during granulite facies metamorphism at 1594 6 ma monazite cooled to its blocking temperature at 1576 8 ma zircon cores from the semiconcordant segregation are dominantly 2 3 ga old indicating that the source of the fluids was not the particular metamorphosed semipelite studied two generations of low th u overgrowths on the zircon give indistinguishable ages for the older and younger of 1589 8 and 1582 8 ma respectively the monazite age is the same 1576 12 ma zircon from the late discordant segregation gave 1568 4 ma fluid flow occurred for at least 18 3 ma and ended 26 3 ma after the peak of metamorphism suggesting a very slow cooling rate of 3 c ma 1 the last regional high grade metamorphism in the reynolds range occurred at 1 6 ga not 1 78 ga as previously thought the high grade event at 1 78 ga is a separate event that affected only the basement to the reynolds range group

mineral oxygen isotope and hydroxyl content changes in ultrahigh pressure eclogite gneiss contacts from chinese continental scientific drilling project cores a combined study of mineral o isotopes and hydroxyl contents was carried out for the contacts between ultrahigh pressure eclogite and gneiss from main hole of the chinese continental scientific drilling project in the sulu orogen while there is a large d 18 o variation from 8 3 to 7 3 for all minerals different styles of mineral pair fractionation occur at the boundaries of different lithologies both equilibrium and disequilibrium o isotope fractionations are observed between quartz and the other minerals with reversed fractionations between omphacite and garnet in some samples of eclogite this suggests that both eclogite and gneiss acquired their negative d 18 o values by meteoric hydrothermal alteration of their protoliths at high temperatures before subduction and that fluid assisted o isotope exchange did take place across the boundary of different lithologies at local scales during amphibolite facies retrogression fourier transform infrared spectroscopy analysis yielded h2o concentrations of 50 to 1144 p p m by weight for garnet and 139 to 751 p p m for omphacite the state of equilibrium or disequilibrium o isotope fractionations between omphacite and garnet are correlated with variations in their water content at local scales indicating that the internally derived fluid plays a critical role in retrograde metamorphism during exhumation the retrograde metamorphism results in mineral reactions and o isotope disequilibria between some of the minerals but the fluid for retrogression was derived from the decompression exsolution of structural hydroxyl and thus internally buffered in the o isotope composition a quantitative estimate suggests that a hand specimen 3 6 9 cm of eclogite composed of 70 garnet and 30 omphacite can release 0 316 g water by the decompression exsolution of structural hydroxyl which can form 14 4 g amphibole during exhumation this provides sufficient amounts of water for the amphibolite facies retrogression

crust mantle coupling revisited the archean slave craton nwt canada abstract lithospheric domains in the slave province inferred from regional variations in chemistry of mantle derived garnets are parallel with northeasterly structural trends in neoarchean surface rocks and were used previously to infer late neoarchean crust mantle coupling and argue against the presence of diamondiferous mesoarchean lithosphere under the western slave craton prior to its late neoarchean orogenic climax a review of more recent evidence does not support this view but concludes that a mesoarchean lithospheric root was coupled to mesoarchean crust of the central slave superterrane csst prior to accretion of adjacent juvenile neoarchean terranes a significant part of this root survived the neoarchean break up of the csst neoarchean terrane accretion and a complex sequence of post accretion deformation granitoid plutonism and metamorphism it was least modified in the ultra depleted harzburgitic layer udl under the central part of the contwoyto terrane where it is trapped above the deepest part of an underplated paleoproterozoic slab which stabilized the remnants of the archean root and caused downward younging of the lithosphere upper mantle domains recognized on the basis of the distribution of peridotitic garnets are not primary neoarchean features but were either inherited from the original mesoarchean root or resulted from neoarchean and paleoproterozoic tectonic erosion and or metasomatism of this root the slave province resembles other diamondiferous cratons in that its diamond endowment consists of both mesoarchean and paleoproterozoic stones

the bell lake group and anton complex a basement cover sequence beneath the archean yellowknife greenstone belt revealed and implicated in greenstone belt formation u pb zircon and monazite geochronology indicates the presence of a 2 93 ga basement anton complex and 2 8 ga cover sequence bell lake group beneath the 2 70 to 2 72 ga kam group of the yellowknife greenstone belt in the southern slave province the bell lake group comprises remnants of a quartzite rhyolite banded iron formation succession the ages of detrital zircons from the quartzite unit constrain its deposition to be younger than 2 92 and range up to 3 7 ga u pb ages for gneisses beneath the bell lake group are in excess of 2 93 ga and they are locally overlain by the quartzite the contact between the tholeiitic to calc alkaline kam group and the bell lake group is poorly exposed and equivocal approximately 6 km higher in the section the ranney chert a felsic volcaniclastic layer separates 2 70 2 72 ga tholeiitic to calc alkaline upper kam group rocks from a lower tholeiitic section containing sheeted dike complexes zircons from the ranney chert yield pb pb ages ranging from 2 72

experimental constraints on phase relations in subducted continental crust synthesis piston cylinder experiments were carried out in the range 2 0 4 5 gpa and 680 1 050 c to investigate phase relations in subducted continental crust a model composition kcmash has been used because all major ultrahigh pressure uhp minerals of the whole range of rock types typical for continental crust can be reproduced within this system the combination of experimental results with phase petrologic constraints permits construction of a uhp petrogenetic grid the phase relations demonstrate that the most important uhp paragenesis consists of coesite kyanite phengite clinopyroxene and garnet in subducted continental crust below 700 c talc is stable instead of garnet as most of these minerals are also stable at much lower pressure and temperature conditions it is thus not easy to recognize uhp metamorphism in subducted crust a general feature however is the absence of feldspars at h2o saturated conditions plagioclase is never stable at uhp conditions but k feldspar can occur in h2o undersaturated rocks mineral compositions in the experiments are fully buffered by coexisting phases the si content of phengite and biotite increase with increasing pressure at 4 0 gpa 780 c biotite contains 3 28 si per formula unit which is most probably caused by solid solution of biotite with talc above 800 c the caal2sio6 component in clinopyroxene buffered with kyanite coesite and a mg phase increases with increasing temperature providing a tool to distinguish between cold and hot eclogites up to 10 ca eskolaite ca0 5 0 5alsi2o6 in clinopyroxene has been found at the highest temperature and pressure investigated 900 c 4 5 gpa garnet buffered with coesite kyanite and clinopyroxene displays an increase of grossular component with increasing pressure for a given temperature although the investigated system represents a simplification with respect to natural rocks it helps to constrain general features of subducted continental crust the observed phase relations and phase compositions demonstrate that at pressures 3 0 gpa and temperatures 800 c continental crust can retain significant amounts of h2o 1 wt whereas k free mafic or ultramafic rocks are dry at these conditions uhp parageneses are only preserved if the whole exhumation path is situated within the stability field of phengite i e if there is cooling during exhumation or if the whole exhumation occurred at t 700 c in contrast break down of phengite and concomitant partial melting in terranes that show isothermal decompression may lead to a complete recrystallization of the subducted crust during exhumation the density of uhp rocks can be estimated on the basis of the established phase relations pelitic rocks are likely to have a density close to mantle rocks 3 3 g cm3 because of significant amounts of dense garnet and kyanite whereas granitic rocks are less dense 3 0 g cm3 hence subducted average continental crust is most probably buoyant with respect to mantle rocks and tends to get exhumed as soon as it is detached from the down going slab electronic supplementary material to this paper can be obtained by using the springer link server located at http dx doi org 10 1007 s00410 001 0336 3

evidence for archean ocean opening and closing in the southern slave province archean mafic volcanic and plutonic rocks of the cameron and beaulieu river greenstone belts outcrop in the southern slave structural province of the canadian shield lower portions of the greenstone belts have ophiolitic characteristics but young felsic units may represent superimposed arc magmatism the greenstone belts are separated from older circa 2 7 3 2 ga quartzo feldspathic gneisses of the sleepy dragon complex by a regional decollement zone patterson lake structural complex consisting of deformed mafic volcanic rocks thin slivers of gneiss ultramafic rocks and banded iron formation mixed in a highly deformed phyllonitic and phacoidally cleaved metapelitic matrix high synmetamorphic strains are indicated by abundant intrafolial folds transposed layering rare sheath folds and by the parallelism of elongated primary features and metamorphic mineral lineations the patterson lake structural complex is similar to melanges of phanerozoic mountain belts adjacent quartzo feldspathic gneisses are strongly mylonitized the melange and deeper mylonites formed when the greenstone belts were emplaced as thrust sheets wildflysch shed from tips of emergent thrusts was in part incorporated into the basal melange as the allochthon overrode its own detritus continued compression involved the basement in the deformation uplifting the sleepy dragon complex in a structural culmination mantled by the imbricated greenstone stack late granodioritic to granitic intrusions formed regional thermal aureoles which overprint earlier metamorphic fabrics

structural patterns and tectonic evolution of supracrustal domains in the archean slave province canada a new plate tectonic model accounts for lithological relations and regional structural patterns in late archean supracrustal domains of the slave province multiple dykes and pillowed mafic flows most common in the western part of the province suggest sea floor spreading the mafic volcanics lying in narrow homoclinal belts stratigraphically below more extensive turbidites are viewed as megaxenolithic remnants of oceanic crust preserved on the periphery of granitoid plutons and blocks of sialic crust closure of an oceanic basin was marked by emplacement of the granitoid plutons and coeval felsic volcanics the latter predominating over mafic volcanics in northeastern domains the felsic calc alkaline magmas may have risen from a shallow dipping subduction zone westerly verging folds westerly convex fold arcs and inclinations of later foliations particularly in lower level rocks of higher metamorphic grade are in accord with underthrusting to the east the zone of underthrusting shifted progressi

laser ablation icp ms u pb and 40 ar 39 ar age constraints on neoarchean to paleoproterozoic magmatic and tectono metamorphic evolution of the link between hope bay and elu greenstone belts northeast slave craton nwt canada abstract the elu link connecting the neoarchean hope bay and elu granite greenstone belts in the bathurst block of the northeast slave craton consists of volcano sedimentary and gabbro granite rocks laser ablation icp ms zircon and titanite u pb dating was combined with mineral 40 ar 39 ar age data to date the elu link rocks and d 1 d 3 tectono metamorphic events as well as characterize the response to the thelon orogeny in the area the volcanic rocks are correlated with the ca 2716 ma flake lake suite in the adjacent hope bay belt whereas the sedimentary units have a minimum depositional age of 2684 11 ma nearly coinciding with the onset of d 1 deformation most gabbro granite bodies were emplaced between 2651 14 ma and 2577 13 ma coeval with the d 2 deformation whose accompanying metamorphism culminated at 2632 7 ma older ages up to 3042 22 ma retained by either the gabbro granite or their hosts are indicative of crustal contamination a few gabbro granites were also emplaced during the 2577 13 ma to 2494 21 ma d 3 deformation event however the 2494 21 ma intrusions are unusual and mark the archean paleoproterozoic transition the d 3 deformation was followed by erosional exhumation and localized thermal pulses associated with the emplacement of diabase dikes at 2228 8 ma 2128 11 ma and 802 75 ma the thelon orogeny imprint inferred from the 2054 11 ma to 1919 5 ma biotite ages corresponds to nearly isobaric low temperature

anton terrane revisited late archean exhumation of a moderate pressure granulite terrane in the western slave province the anton terrane of the western slave province has been interpreted as a remnant of a meso archean 3500 2800 ma microcontinent the largest coherent entity of the terrane is a previously unrecognized 50 by 250 km granulite complex herein termed the snare domain the granulite facies mineral assemblages give the domain a characteristic magnetic signature that is distinct from areas of exposed meso archean basement in the slave province the domain consists of interlayered 3100 2624 ma orthogneisses and paragneisses and 2600 2585 ma granitoid rocks its structure was largely established during 2610 2590 ma orogeny throughout the province that culminated in ca 2590 ma granulite facies metamorphism pressure temperature p t determinations yield peak conditions of 775 875 c at 6 7 kbar shortly thereafter 2 4 kbar of decompression occurred before the domain cooled below 450 c decompression and retrogression textures are widespread but are most pervasive adjacent to late extensional detachment faults u pb zircon age data suggest that the snare domain was partially exhumed from mid crustal levels ca 2585 2580 ma we attribute exhumation to tectonic denudation of an overthickened crustal welt in the western slave province localization of the crustal welt in this part of the slave province may be related to the presence there of meso archean crust

phase relationships of s type granite with h2o to 35 kbar muscovite granite from harney peak south dakota muscovite granite 13 8 muscovite 4 8 normative corundum was reacted with varying percentages of h 2o in cold seal vessels at 2 kbar and in piston cylinder apparatus between 10 and 35 kbar the diagrams illustrating melting crystallization relationships are p t sections with both excess h 2o and with no h 2o added 0 66 h 2o in rock t x h 2o sections at 15 kbar and 25 kbar showing h 2o undersaturated conditions the h 2o undersaturated surface for the crystallization of quartz coesite small amounts of aluminosilicate minerals persist to higher temperatures glass compositions measured by electron microprobe from samples with 5 h 2o at 15 kbar confirm that liquids are syenitic through at least 100 c above the solidus as predicted from the effect of pressure on the residua system results are explained successfully by phase relationships involving muscovite quartz and orthoclase in k 2o al 2o 3 sio 2 h 2o with reactions depicted in a p total p e h 2o t model with special reference to the divariant surfaces in the region p e h 2o p total with reduced p e h 2o or a h 2o produced either by small amounts of h 2o and h 2o undersaturation or by co 2 h 2o mixtures subsolidus dehydration reaction temperatures decrease vapor present solidus temperatures increase and muscovite stability in presence of liquid increases in general muscovite biotite and amphibole can be precipitated from magmas containing only a few tenths per cent h 2o although the h 2o undersaturated liquids coexisting with crystals may contain 3 or more dissolved h 2o this particular granite cannot be a primary magma from mantle or subducted oceanic crust it is a possible product of partial fusion of pelitic rocks between about 20 km and 40 km depth given sufficient h2o and xenocrystal muscovite or sillimanite from the source rocks the phase relationships are consistent with the idea of s type granites but not sufficient to prove the origin of this rock additional tests require phase relationships of other associated granitic rocks and details of geochemistry geophysics and field relationships

the 1991 1996 natmap slave province project introduction the 11 papers contributed to this issue on the geology of the slave province are summarized and where appropriate links between data from the various contributions are emphasized in the discussion we provide a synthesis of the present knowledge base on pre yellowknife supergroup basement rocks throughout the slave province all available age data on mesoarchean and paleoarchean basement rocks are compiled and in a first attempt to seek regional correlations 10 events of crustal growth and or reworking are recognized spanning well over 1 billion years of earth s history the spatial distributions of these events across the craton are discussed finally we pose a series of important questions that remain and that may help guide future research

origin of retrograde fluid in ultrahigh pressure metamorphic rocks constraints from mineral hydrogen isotope and water content changes in eclogite gneiss transitions in the sulu orogen abstract by taking advantage of having depth profiles between contrasting lithologies from core samples of the chinese continental scientific drilling ccsd project a combined study was carried out to examine changes in mineral h isotope total water and hydroxyl contents in garnet and omphacite across the contacts between ultrahigh pressure uhp eclogite and gneiss in the sulu orogen east central china the samples of interest were from two continuous core segments from the ccsd main hole at depths of 734 21 737 16 and 929 67 932 86 m respectively the results show d values of 116 to 64 for garnet and 104 to 82 for omphacite consistent with incorporation of meteoric water into protoliths of uhp metamorphic rocks by high t alteration both equilibrium and disequilibrium h isotope fractionations were observed between garnet and omphacite suggesting fluid assisted h isotope exchange at local scales during amphibolite facies retrogression while bulk water analysis gave total h2o concentrations of 522 1584 ppm for garnet and 1170 20745 ppm for omphacite structural hydroxyl analysis yielded h2o contents of 80 413 ppm for garnet and 228 412 ppm for omphacite it appears that significant amounts of molecular h2o are present in the minerals pointing to enhanced capacity of water storage in the uhp eclogite minerals hydrogen isotope variations in the transition between eclogite and gneiss show correlations with variations in their water contents petrographically the degree of retrograde metamorphism generally increases with decreasing distance from the eclogite gneiss boundary thus retrograde metamorphism results in mineral reactions and h isotope variation because hydroxyl solubility in nominally anhydrous minerals decreases with dropping pressure significant amounts of water are expected to be released from the minerals during decompression exhumation decompression exsolution of structural hydroxyl from 1 m3 volume of eclogite composed of only garnet and omphacite results in release of a quantitative estimate of 3 07 3 44 kg water that can form 140 156 kg amphibole during exhumation therefore it is concluded that fluid for retrogression of the eclogites away from the eclogite gneiss boundary was derived from the decompression exsolution of structural hydroxyl and molecular h2o in nominally anhydrous minerals for the eclogites adjacent to gneiss in contrast the retrograde metamorphism was principally caused by aqueous fluid from the gneiss which is relatively rich in water consequently both the origin and availability of metamorphic fluid during exhumation of deeply subducted continental crust are deciphered by this combined study focusing on the transitions and the retrograde processes between the felsic and mafic uhp rocks

post tectonic uplift of the cordillera darwin orogenic core complex evidence from fission track geochronology and closing temperature time relationships the cordillera darwin orogenic core complex is a structural and topographic culmination in the southern andes which underwent mid cretaceous penetrative deformation and metamorphism associated with the deformation and incipient obduction of a partly ophiolitic back are basin rocas verdes block along its southern boundary whereas structural evidence suggests uplift of the rocas verdes terrane relative to the cordillera darwin during this event metamorphic assemblages reflect a greater net uplift of the opposite sense new fission track ages together with existing k ar and rb sr ages were used to estimate cooling and uplift histories and their relationships to foreland sedimentation and present day topographic relief the results suggest that whereas initiation of uplift in the cordillera darwin and rocas verdes blocks may have accompanied mid cretaceous deformation the major rapid uplift around 0 5 1 5 mm y occurred after the deformation also uplift occurred earlier in the rocas verdes block than in the cordillera darwin block the latter having experienced a greater net uplift since the mid cretaceous the major post tectonic uplift of the cordillera darwin occurred at a slower rate 0 05 0 20 mm y and coincides with the deposition of thick flysch and conglomerates in the foreland basin

laser 40 ar 39 ar thermochronology of archean rocks in yellowknife domain southwestern slave province insights into the cooling history of an archean granite greenstone terrane laser 40 ar 39 ar ages for hornblende muscovite and biotite from archean plutonic rocks of yellowknife domain southwestern slave province complement u pb ages from minerals with higher closure temperatures zircon monazite titanite together the data indicate that the plutonic suites of different ages defeat prosperous morose coring three major thermal culminations in the domain have distinct archean cooling histories until below approximately 250 c biotite closure the cooling trajectories for successive suites appear to document a decrease in cooling rate over the course of orogeny possibly reflecting the build up of heat within the orogen coupled with a transition to lower rates of exhumation and or erosion greater structural relief over the sleepy dragon basement culmination may be responsible for slower cooling of the morose granite situated in its core at or below the closure temperature of biotite the cooling curves converge with one another documenting a transition from localized to regional cooling the fact that cooling was locality specific until at least the biotite closure temperature suggests that just as intrusion and related peak metamorphism were diachronous across the region so was cooling to approximately 250 c uniform regional isotherms thus were not established until 200 ma after the youngest archean plutonism the variable disturbance of biotite ar spectra but generally not muscovite or hornblende indicates that the paleoproterozoic thermal disturbance of archean rocks in yellowknife domain did not exceed 200 300 c

a synmetamorphic lateral fluid flow model for gold mineralization in the archean southern kalgoorlie and norseman terranes western australia gold deposits are widely distributed in the archean southern kalgoorlie and norseman greenstone terranes eastern goldfields superterrane western australia which have collectively produced more than 1 850 metric tons t of gold gold deposits occur in a variety of host rocks and structural settings but all are associated with quartz veins and or shear zones that formed during the latest stages of regional compression available data indicate a similar h 2 o co 2 ch 4 ore fluid at each deposit alteration assemblages vary with metamorphic grade in a manner which indicates that late tectonic regional metamorphic gradients were still in place when gold was deposited from hydrothermal fluids these late tectonic thermal gradients were related to differential uplift of deep hot crust at the margin of the greenstone belt accompanied by intrusion of voluminous monzogranite magma at this time the western margin of the greenstone belt was a zone of high heat flow as were intrusive centers in the northern part of the southern kalgoorlie terrane these spatial and temporal relationships in combination with the apparent absence of a viable local source for the ore fluid at most deposits suggest that gold was deposited from a small number of large synmetamorphic hydrothermal systems published thermal models predict up temperature lateral fluid flow toward zones of high heat flow under these circumstances a model involving up temperature lateral flow of metamorphic fluids overcomes some of the problems associated with deeply sourced and conventional metamorphic models for the ore fluids and is consistent with most of the features of gold deposits in the southern kalgoorlie and norseman terranes potassium metasomatism and radiogenic isotope signatures associated with gold mineralization are ascribed to a metamorphic fluid that was modified by exchange with biotite bearing felsic rocks granitoids felsic volcanic and volcaniclastic rocks in the course of lateral flow toward zones of high heat flow

subduction ultrahigh pressure metamorphism and regurgitation of buoyant crustal slices implications for arcs and continental growth abstract intracontinental collisional belts that retain mineralogic relics of phanerozoic ultrahigh pressure uhp metamorphism are increasingly being recognized adjacent regions generally lack evidence of coeval calcalkaline volcanism plutonism following consumption of intervening oceanic lithosphere each orogen marks the site of a deeply subducted microcontinental promontory fragments of which later returned to midcrustal levels mafic and ultramafic rocks are volumetrically minor pressures in uhp complexes approached or exceeded 2 8 gpa at 600 900 c subduction zones involve low t prograde gradients and constitute the only plate tectonic realm where such conditions exist although internal portions of descending plates may have even lower geothermal gradients crustal upper margins are typified by p t trajectories of 5 10 c km attending underflow clinoamphibole rich metabasaltic rocks dehydrate at pressures 2 2 2 5 gpa this accounts for the kinetically favorable solid state production of mafic eclogites and partial fusion of morbs to form hydrous andesitic melts devolatilization of partly serpentinized mantle lithosphere also introduces h 2 o to the magmagenic zone as large masses of continental crust enter deeper portions of subduction zones mineralogic transformations slow or cease because micas the major oh bearing phases in such rocks retain volatile contents to pressures 4 gpa persistence of hydroxyl bearing layer silicates may explain the metastable preservation of low density assemblages in continental crust subjected to uhp conditions and the dearth of calcalkaline magmatism in most collision belts exhumed uhp terranes are intensely deformed and disaggregated all consist dominantly of quartzofeldspathic compositions mafic eclogites garnet peridotites constitute only a few volume percent eventual decoupling of the relatively low density subducted crust promotes buoyancy propelled transport to shallow depths along the subduction channel with the hanging wall lithosphere acting as a stress guide extensional collapse and erosional unroofing of rising masses result in the continued ascent of deeply subducted but buoyant crustal material although equidimensional massifs must return surfaceward large volume to surface areas render the contained uhp phase assemblages virtually unquenchable surviving uhp terranes thus consist of relatively thin slabs of continental crust allochthons that retain uhp relics apparently lost heat by conduction across both upper normal fault and lower reverse fault contacts based on present exposures estimated thicknesses of uhp sheets are qinling dabie sulu belt 5 km kokchetav complex 1 3 km dora maira massif 1 2 km western gneiss region 1 km slices evidently rose to midcrustal levels at exhumation rates approaching 10 mm per year back reaction attending decompression in all cases was nearly complete where uhp mineral inclusions persisted retrogression apparently was limited by the relative impermeability of the rocks to catalytic aqueous fluids and by declining temperatures the coarse grain size of host phases also disfavored complete retrogression

ultrahigh ϵnd eclogites from an ultrahigh pressure metamorphic terrane of china the su lu ultrahigh pressure uhp metamorphic terrane is part of the qinling dabie suture zone formed by the collision between the sino korean and yangtze cratonic blocks geochemical and isotopic investigation of the uhp metamorphic rocks has revealed a spectacular history for some eclogitic rocks retrograded coesite bearing eclogites from the weihai area in the eastern end of the su lu terrane have recorded an extraordinary nd isotopic growth with a world record high nd value of 264 range 170 to 264 such isotopic characteristics are consistent with the observed extreme lree fractionation and very high sm nd ratios the sm nd isotopic data allow us to calculate the minimum age of protolith formation at 1 7 ga assuming their evolution from a depleted or a chondritic mantle geochemical data indicate that the eclogite protoliths were of basaltic compositions originally but underwent metasomatism water rock interaction at high pressures and temperatures leading to the present apparent andesitic composition and extreme lree depletion this process took place in the proterozoic 1 7 ga and was not related to the mesozoic continental collision and ultrahigh pressure metamorphism at 210 220 ma ago because of a protracted retrograde metamorphism garnet sm nd systems were not closed until 160 ma ago 50 ma after the peak eclogite metamorphism this implies that the exhumation history is significantly different in different parts of the dabie and su lu uhp metamorphic terranes from geochemical analyses and isotopic age determinations we argue that at least two ancient tectonic events in the middle and late proterozoic can be identified prior to the last mesozoic continental collision and accompanying uhp metamorphism in the su lu and dabie terrane