experimental calibration of the roles of temperature and composition in the ca in olivine geothermometer at 0 1 mpa abstract the calcium content of olivine in equilibrium with a low ca pyroxene and clinopyroxene has previously been calibrated in several studies at elevated pressure as a geobarometer however the calcium content of olivine in such systems is also dependent on temperature and the mg 100 mg mg fe of the system which is represented here by the olivine forsterite fo content to isolate these variables a series of experiments has been performed at 0 1 mpa between 1170 c and 1322 c in both the simple cao mgo al2o3 sio2 cmas system and in fe na ti k bearing compositions in the cmas system the ca content of pure forsterite coexisting with low ca pyroxene and clinopyroxene is strongly temperature dependent decreasing by 0 1 wt cao between 1322 c and 1254 c addition of fe to the system leads to a substantial increase in the ca content of olivine coexisting with a low ca pyroxene and clinopyroxene at a given temperature empirical fitting of these data gives a geothermometer applicable at low pressure and in the compositional range fo70 fo100 t c 12368 2032 ln x mo 6 395 1 79 3 235 0 51 fo 273 where the values in parentheses are the standard error on the parameter this reproduces all experimental data within 20 c

petrologic determination of ascent rates for the 1995 1997 soufriere hills volcano andesitic magma the recent eruption of the soufriere hills volcano in montserrat july 1995 to present october 1997 has produced a hornblende bearing andesitic lava dome it is possible to petrologically estimate changes in ascent rates of amphibole bearing magmas for certain rates of decompression a breakdown rim of fine grained anhydrous reaction products forms where amphibole is in contact with melt the thickness of the rim varies with ascent rate most of the amphibole phenocrysts in the magma storage region lack breakdown rims about 10 have 200 400 m thick coarse grained breakdown rims that are interpreted to be relicts of a past heating event study of a time series of new dome andesites showed that ascent rate increased from december 1995 0 001 m s to july 1996 0 008 m s while eruptive style remained extrusive ascent rate increased to 0 012 m s in august 1996 and the first major explosive eruption occurred on 17 18 september 1996

ultra high temperature metamorphism of mafic granulites from south altyn orogen west china a result from the rapid exhumation of deeply subducted continental crust

p t t evolution of ultrahigh temperature granulites from the saxon granulite massif germany part ii geochronology the granulites of the saxon granulite massif equilibrated at high pressure and ultrahigh temperature and were exhumed in large part under near isothermal decompression this raises the question of whether p t t data on the peak metamorphism may still be retrieved with confidence felsic and mafic granulites with geochronologically useful major and accessory phases have provided a basis to relate p t estimates with isotopic ages presented in a companion paper the assemblage garnet clinopyroxene in mafic granulite records peak temperatures of 1010 1060 c consistent with minimum estimates of around 967 c and 22 3 kbar obtained from the assemblage garnet kyanite ternary feldspar quartz in felsic granulite multiple partial overprint of these assemblages reflects a clockwise p t evolution garnet and kyanite in the felsic granulite were successively overgrown by plagioclase spinel plagioclase sapphirine plagioclase and biotite plagioclase most of this overprinting occurred within the stability field of sillimanite garnet clinopyroxene in the mafic granulite were replaced by clinopyroxene amphibole plagioclase magnetite the high p t conditions and the absence of thermal relaxation features in these granulites require a short lived metamorphism with rapid exhumation the ages of peak metamorphism 342 ma and shallow level granitoid intrusions 333 ma constrain the time span for the exhumation of the saxon granulites to about 9 ma

high temperature overprint in u hpm rocks exhumed from subduction zones a product of isothermal decompression or a consequence of slab break off slab rollback abstract this paper presents and discusses petrological observations from high to ultrahigh pressure u hp metamorphic terrains in relation to existing geophysical and numerical models for subduction and exhumation processes in orogenic belts the interpretations are mostly based on observations from gneiss terrains bearing abundant bodies mafic meta eclogite and ultramafic garnet peridotite and pyroxenite exposed in collisional orogens the inclusions and compositional zoning of minerals are considered to be first order information that is needed to constrain pt paths of hp uhp rocks and reconstruct the related geodynamic models for subduction and exhumation of crustal and mantle rocks the bohemian massif of the european variscides is used as the basis for a model example to explain these processes but u hp rocks from various other terrains are taken into consideration to discuss available pt paths in relation to proposed subduction and exhumation rates of u hp rocks based on geophysical and geochronological data primarily information used in this respect include textural relations and preserved prograde zoning in minerals from many u hp rocks which reveal that a relatively cool geothermal gradient typical of subduction zones tended to prevail during the prograde and peak pressure segments of pt paths prior to initiation of exhumation and may have continued even with cooling if exhumation rates were rapid the commonly applied interpretation of isothermal decompression during exhumation is critically appraised considering whether a simple thermal relaxation and radiogenic heating during exhumation is responsible for formation of post peak pressure retrograde mineral assemblages and textures observed in u hp rocks we go on to consider whether this can satisfactorily explain the often pervasive medium pressure high temperature metamorphic re equilibration of u hp rocks or whether an additional external source of heat is a better explanation we conclude that the commonly observed high temperature metamorphic overprint exhibited by u hp rocks occurs mostly after rocks have been exhumed from the subduction channel and have reached normal crustal positions when mantle upwelling resulting from slab breakoff delamination or slab rollback takes place at the onset of continent continent collision we also explore contrasting pt trajectories for mantle rocks that have been entrained into crustal material during their subduction or exhumation pt paths of mantle and subducted crustal rocks tend to converge as mantle rocks impinge upon the cooler subduction zone and once entrained share a common evolution that depends on the exhumation mechanism and rate considering all of the data presented in this work we conclude that the diverse polyphase metamorphic evolution exhibited by u hp terrains embodied in the pt paths of hp and uhp rocks has important consequences for reconstructing their changing thermal regimes and provides important constraints for geodynamic models involving subduction and the transition to collision

the nature and occurrence of discrete zoning in plagioclase from recently erupted andesitic volcanic rocks montserrat abstract recently erupted rocks from montserrat lesser antilles were found to be coarsely porphyritic including plagioclase magnesio hornblende orthopyroxene titano magnetite and rare quartz phenocrysts zonations within plagioclase show three distinct periods within the evolution of the andesitic magmas initial steady state oscillatory overgrowths an49 54 represent an early period of growth within a confined andesitic magma chamber interaction with an intruding more mafic hotter magma resulted in thick zones of dissolution features that cut through these early growth layers the dissolution zones and crystal fragments are overgrown by thin calcic an86 plagioclase rims a plagioclase growth rate of 10 7cm s suggests that the calcic plagioclase rims were formed in approximately 10 h consistent with the idea that mineral growth may increase magma viscosity inducing conduit plugging and shallow pressurization resulting in cycles of dome inflation eruption and explosive activity thermodynamic modeling of heating and mixing of the andesitic magma with a basaltic liquid produces plagioclase of composition an79 not the observed composition of the late plagioclase an86 we conclude that the final plagioclase grew under non equilibrium conditions as a result of the mixing event

high resolution quantitative imaging of plagioclase composition using accumulated backscattered electron images new constraints on oscillatory zoning oscillatory zoning in plagioclase is investigated at small scale 10 m using accumulated backscattered electron bse images as high resolution imaging method combined with electron microprobe quantitative analysis gray value profiles across these images can be calibrated for an content with a resolution of 0 5 mol an applied to oscillatory zoned crystals this new application of bse imaging allows better characterization of zoning patterns along a profile and quantification of wavelength amplitude and shape of the oscillations we also obtain high resolution information on the morphology of growth zones boundaries this approach allows us to better classify the different types of oscillations and concentric zoning dissolution is more frequent than usually recognized major resorption surfaces crosscut several growth zones irregular 5 10 m saw tooth zones are delimited by faint wavy dissolution surfaces and must be distinguished from small scale oscillations 1 3 m with straight boundaries this suggests at least two mechanisms for the formation of these zoning patterns faint oscillations are probably caused by local kinetic control whereas wavy dissolution surfaces involve magma chamber dynamics

exhumation mechanisms of melt bearing ultrahigh pressure crustal rocks during collision of spontaneously moving plates a series of 2d petrological thermomechanical numerical experiments was conducted to i characterize the variability of exhumation mechanisms of ultrahigh pressure metamorphic uhpm rocks during collision of spontaneously moving plates and ii study the possible geodynamic effects of melting at ultrahigh pressure conditions for the exhumation of high temperature ultrahigh pressure metamorphic ht uhpm rocks to this end the models include fluid and melt induced weakening of rocks five distinct modes of exhumation of u hpm rocks associated with changes in several parameters in the models of plate collision and continent subduction are identified as follows vertical crustal extrusion large scale crustal stacking shallow crustal delamination trans lithospheric diapirism and channel flow the variation in exhumation mechanisms for u hpm rocks in numerical models of collision driven by spontaneously moving plates contrasts with the domination of the channel flow mode of exhumation in a majority of the published results from numerical models of collision that used a prescribed plate convergence velocity and or did not include fluid and melt induced weakening of rocks this difference in the range of exhumation mechanisms suggests that the prescribed convergence velocity condition and the neglect of fluid and melt related weakening effects in the earlier models may inhibit development of several important collisional processes found in our experiments such as slab breakoff vertical crustal extrusion large scale stacking shallow crustal delamination and relamination and eduction of the continental plate consequently the significance of channel flow for the exhumation of uhpm rocks may have been overstated based on the results of the earlier numerical experiments in addition the results from this study extend over a larger proportion of the high temperature range of p t conditions documented from uhpm rocks including those retrieved from ht uhpm rocks than the results of experiments from previous numerical models in particular the highest peak metamorphic temperatures up to 1000 degrees c are recorded in the case of the vertical crustal extrusion model in which subducted continental crust is subjected to a period of prolonged heating by asthenospheric mantle abutting the continental side of the vertically hanging slab nonetheless some extreme temperature conditions which have been suggested for the kokchetav and bohemian massifs perhaps up to 1100 1200 degrees c are still to be achieved in experiments using numerical models

tumulus development on lava flows insights from observations of active tumuli and analysis of formation models here we use observations of active flows along with detailed morphometric field measurements of more than 70 tumuli on flows at mount etna italy kilauea and hualalai us volcanoes to constrain a previously published model that estimates the pressure needed to form tumuli in an attempt to discover the nature and magnitude of pressure variations within active lava flow interiors we then consider how tumuli differ from idealized circular plates we incorporate observations of active tumuli and find that they may grow asymmetrically yet produce a symmetrical tumulus and can form where the flow path significantly changes direction bending models of clamped edges provide the most reasonable head estimates for the tumuli in our study tumulus formation requires the proper combination of cooling and effusion rate if cooling is too extensive and effusion rate too low the crust will provide too much resistance to bending if cooling is too limited and effusion rates too high crusts will not develop or have insufficient strength to resist fracture and subsequent breakouts we do not find it surprising that tumuli are rarely found over well established lava tubes that typically have rigid walls overlying crusts that exceed 2 m in thickness and provide too much resistance to bending silicic flows lack tumuli because the viscosity gradients within the flow are insufficient to concentrate stress in a localized area

evidence of former stishovite in uhp eclogite from the south altyn tagh western china abstract we report here for the first time long prism needle shaped polycrystalline quartz aggregates and square parallelogram shaped single phase quartz inclusions in omphacite and garnet of ultrahigh pressure uhp eclogite from the yinggelisayi area in the south altyn uhp belt based on their morphology and mineral composition these quartz inclusions in omphacite and garnet are quartz paramorphs after stishovite the minimum peak pressure of the eclogite is estimated to be 8 9 gpa at 800 1000 c based on the stability field of stishovite this new evidence together with previous discovery of kyanite spineal exsolutions in former stishovite of the pelitic gneiss from the same region suggests a ultra deep subduction and exhumation of the south altyn continental rocks to from mantle depths in the stability field of stishovite evidence of ultra deep subduction of continental materials might be more common and diverse than previous thought

package fluids 1 computer programs for analysis of fluid inclusion data and for modelling bulk fluid properties the computer package fluids contains five sets of computer programs written in c for the calculation of fluid properties both model fluids and data from fluid inclusions can be analysed in the programs bulk density molar volume and composition of fluid inclusions are calculated with the program bulk which uses microthermometric data micro raman spectroscopic data cation ratios and volume fraction estimations as input the program offers a broad variety of equations of state which are selected according to the fluid system or the temperature and pressure conditions of interest the isochores that correspond to the output or test v x properties of fluid inclusions are calculated by the program isoc in calculating the isochores this program takes account of the compressibility and expansion of selected host minerals the program test allows different equations of state to be compared directly with experimental data thereby facilitating selection of the most adequate model in the programs bulk and isoc loner is a group of programs that handle individual equations of state eos for all fluid systems presented in the programs bulk and isoc aqso is a group of programs that allows purely empirical modelling of electrolyte bearing aqueous solutions henry s law for dilute gas bearing aqueous solutions and osmotic coefficients for dilute electrolyte bearing aqueous solutions the programs are available on the university of leoben web site at http www unileoben ac at buero62 minpet ronald

interaction between climate volcanism and isostatic rebound in southeast alaska during the last deglaciation abstract observations of enhanced volcanic frequency during the last deglaciation have led to the hypothesis that ice unloading in glaciated volcanic terrains can promote volcanism through decompression melting in the shallow mantle or a reduction in crustal magma storage time however a direct link between regional climate change isostatic adjustment and the initiation of volcanism remains to be demonstrated due to the difficulty of obtaining high resolution well dated records that capture short term climate and volcanic variability traced to a particular source region here we present an exceptionally resolved record of 19 tephra layers paired with foraminiferal oxygen isotopes and alkenone paleotemperatures from marine sediment cores along the southeast alaska margin spanning the last deglacial transition major element compositions of the tephras indicate a predominant source from the nearby mt edgecumbe volcanic field mevf we constrain the timing of this regional eruptive sequence to 14 6 13 1 ka the sudden increase in volcanic activity from the mevf coincides with the onset of bolling allerod interstadial warmth the disappearance of ice rafted detritus and rapid vertical land motion associated with modeled regional isostatic rebound in response to glacier retreat these data support the hypothesis that regional deglaciation can rapidly trigger volcanic activity rapid sea surface temperature fluctuations and an increase in local salinity i e 18osw variability are associated with the interval of intense volcanic activity consistent with a two way interaction between climate and volcanism in which rapid volcanic response to ice unloading may in turn enhance short term melting of the glaciers plausibly via albedo effects on glacier ablation zones

first finding of microdiamond coesite and other uhp phases in felsic granulites in the moldanubian zone implications for deep subduction and a revised geodynamic model for variscan orogeny in the bohemian massif abstract heavy mineral fractions and polished thin sections from felsic granulites from the moldanubian zone of the bohemian massif were thoroughly studied by means of raman microspectroscopy combined with optical microscopy and scanning electron microscopy the following phases were identified among others as inclusions in robust minerals such as garnet and zircon diamond characterized by an intense narrow peak at 1332 cm 1 was found in two inclusions in zircon they have a size of 5 m coesite identified by its very characteristic peak at 520 cm 1 was found in an inclusion in garnet together with quartz coesite has been almost completely transformed into quartz only minor coesite nano domains remain kumdykolite the orthorhombic polymorph of naalsi 3 o 8 characterized by strong peaks at 220 456 and 492 cm 1 occurs either as single crystals or as a part of multiphase inclusions in garnet and in zircon along with other mineral phases such as k feldspar phengite rutile moissanite sic exhibiting the characteristic raman bands at 767 788 and 969 cm 1 occurs as inclusions in garnet diamond and coesite are considered to have formed at the peak ultrahigh pressure metamorphic uhpm conditions kumdykolite has been proposed to be a metastable phase formed during rapid cooling from high temperature moissanite points to extremely reduced conditions during subduction to great depths the finding of uhp phases in felsic granulites in the moldanubian zone is clear evidence for subduction of crustal materials to mantle depths the garnet hosting the uhp phase inclusions usually preserves prograde compositional zoning this in combination with the uhpm mineral inclusions suggests that the felsic material should have passed uhp metamorphism at a low temperature gradient isothermal decompression the commonly accepted model at temperatures of 850 950 c would have substantially modified and homogenized the garnet composition eliminating any compositional zoning therefore the uhpm felsic rocks could have experienced only a short term granulite facies metamorphism after they were exhumed to crustal levels

unusual magma storage conditions at mt etna southern italy as evidenced by plagioclase megacryst bearing lavas implications for the plumbing system geometry and summit caldera collapse at mt etna volcano the emission of plagioclase megacryst bearing lavas known locally as cicirara has occurred rarely and generally in association with unusual volcanological phenomena in this work we interpret the magma chamber processes and the structural features of the plumbing system that led to the production of these peculiar volcanic rocks based on a detailed study of plagioclase megacrysts including their oscillatory zoning sieve textures and fluid inclusions patchy zoning suggests limited ascent in the deep levels of the plumbing system based on the plagioclase nucleation threshold and the volatile saturation depth at intermediate water undersaturated levels of the plumbing system ascent is faster as indicated by crystals with coarse sieve textures storage at shallow water saturated levels less than 6 km deep is associated with oscillatory zoning with very small changes in an slightly larger an variations coupled with different wavelengths provide evidence of convection of crystals across distinct zones of the chamber stripes of melt inclusions formed at steps of magma ascent and volatile loss whereas layers of fluid inclusions may be related to episodes of volatile flushing into the magma chamber in contrast strongly sieve textured envelopes with an increase and constant feo may be related to mixing with more volatile rich magmas of similar composition we interpret the repeated occurrence of cicirara lavas as evidence that the shallow portion of the plumbing system underwent a progressive coalescence of a complex network of dykes and sills in response to increasing rates of magma supply from depth major magma withdrawals from this larger reservoir may be linked to episodes of summit instability associated with major caldera collapses

uht metamorphism peaking above 1100 c with slow cooling insights from pelitic granulites in the jining complex north china craton

the application of zircon cathodoluminescence imaging th u pb chemistry and u pb ages in interpreting discrete magmatic and high grade metamorphic events in the north china craton at the archean proterozoic boundary abstract two distinct late neoarchean to early paleoproterozoic magmatic events are recognized at the northern margin of the eastern block of the north china craton based on interpretation of zircon structure cathodoluminescence signature th u pb chemistry and age the first event is represented by emplacement of the chaihe trondhjemite and xianjinchang tonalite with magmatic zircon core ages ranging from 2559 2553 7 ma respectively these rocks experienced a subsequent high grade metamorphic event constrained by partial recrystallization of primary magmatic zircon and growth of new zircon rims between 2517 and 2490 ma a second magmatic episode generated by partial melting of crustal rocks during this metamorphic event is represented by the hongshilazi syenogranite with an age of 2502 11 ma the short time interval between initial magmatism and high grade regional metamorphism signifies they were related to the same major tectonothermal event when considered along with evidence of a similar sequence of events at this time 800 km to the south in eastern hebei and the previously established 2500 ma age of granulite facies metamorphism in mafic rocks across the craton it is proposed that a large mantle plume developed beneath the north china craton at the close of the archean

textural evolution of polyhedral olivine experiencing rapid cooling rates dynamic crystallization experiments in the cao mgo al2o3 sio2 cmas system have been used to investigate the change in crystal shape when pre existing polyhedral olivine crystals are cooled rapidly 1 639 2 182 c h polyhedral olivines are crystallized initially in a first step using a slow cooling rate 2 c h then skeletal and dendritic overgrowths develop on the polyhedral crystals during a subsequent fast cooling event during this second episode small dendritic olivines also nucleate within the liquid phase observation of the experimental sample by optical microscopy shows that the polyhedral olivine shape progressively changes to a skeletal and then to a dendritic morphology in the following sequence polyhedral hopper polyhedral dendritic polyhedral this evolutional sequence is discussed in terms of changes in the crystal growth conditions during cooling and a general relation between these olivine dynamic crystallization experiments and the integrated model of crystal growth by sunagawa bull miner 104 81 87 1981 morphology of crystals terra scientific publishing company 1987 is proposed

a multi methodological re evaluation of the volcanic events during the 1580 ce and 1808 eruptions at sao jorge island azores archipelago portugal abstract among all eruptions that took place on the azores archipelago since the portuguese settlement in the xvi century those of 1580 and 1808 in the island of sao jorge were unusual the magma erupted in 1580 ascended from at least 23 km and ponded at a minimum depth of 16 km here residual magma fractionated mafic phases and plagioclase during 228 years before erupted again in 1808 magma emission occurred from numerous fissures active at the same time and opening for several kilometres following a local tectonic trend hydro volcanic outbursts and hot rock fall over steep slopes caused casualties and injuries and lava flows and heavy tephra fallout affected the local economy despite of all these dramatic consequences the memory of these eruptions is almost vanished representative geosites and morphological landforms if not hindered by lush vegetation have been reshaped by diffuse quarries and reworked to recover croplands and pastures aiming at a better comprehension of the multiple hazards associated with volcanic eruptions at sao jorge island we retraced the volcanic events of these two eruptions by matching our re interpretation of historic reports with field data and photogrammetry and with fluid inclusions petrography and geochemical data of erupted products

petrology and geochronology of eclogites from the western segment of the altyn tagh northwestern china abstract the eclogites from the western segment of the altyn tagh occur as lens or boundins within quartzofeldspathic gneisses or pelitic gneisses characterized by amphibolite facies parageneses the petrographic features and reaction textures testify to three main metamorphic stages 1 the peak eclogite facies stage p 15 kbar t 665 880 c 2 the granulitic facies stage p 11 14 kbar t 670 800 c and 3 the amphibolite facies stage p 6 3 9 5 kbar t 619 738 c during decompression the trace element rare earth element and sm nd isotopic data suggest that most of the eclogites have protolith features resembling transitional type t type mid ocean ridge basalt morb the well preserved eclogite was selected for sm nd and u pb isotopic dating the sm nd isotopic data yield a whole rock garnet omphacite isochron of 500 10 ma age the u pb isotopic measurement of zircons from the same eclogite shows that the four grain populations are near concordant and are well plotted on concordia giving a weighted mean age of 503 9 5 3 ma the two different geochronological methods yield a similar age which is interpreted as the peak metamorphic age of eclogites and reflects the existence of a mountain root related to continental subduction during early paleozoic moreover based on the comparison with similar rocks in the northern margin of the qaidam basin a displacement of 350 400 km for the altyn tagh fault have been estimated

p t t path of garnetites in south altyn tagh west china a complete record of the ultradeep subduction and exhumation of continental crust

plagioclase zoning as an indicator of magma processes at bezymianny volcano kamchatka back scattered electron bse derived zoning patterns of plagioclase phenocrysts are used to identify magma processes at bezymianny volcano kamchatka based on the 2000 2007 sequence of eruptive products the erupted magmas are two pyroxene andesites which last equilibrated at 915 c temperature 77 87 mpa pressure and a water content of 1 4 wt textural and compositional zoning of individual plagioclase phenocrysts typically includes a repeated core to rim sequence of oscillatory zoning an50 60 truncated by a dissolution surface followed by an abrupt increase in an content up to an85 which then gradually decreases rimward this zoning pattern is interpreted to be the result of frequent replenishments of the magma chamber which cause both thermal and chemical interaction between resident and recharge magmas the outermost 70 to 150 m wide zoning patterns of plagioclase phenocrysts are composed of dissolution surface with a subsequent increase in an and fe contents zoning patterns of the rims exhibit correlation among plagioclase phenocrysts within one eruption rims are interpreted as a result of crystallization of a batch of magma in the conduit after recharge event

paleoproterozoic uht metamorphism with isobaric cooling ibc followed by decompression heating in the khondalite belt north china craton new evidence from two sapphirine formation processes

metamorphic evolution mineral chemistry and thermobarometry of schists and orthogneisses hosting ultra high pressure eclogites in the dabieshan of central china as is typical of ultra high pressure uhp terrains the regional extent of the uhp terrain in the dabieshan of central china is highly speculative since the volume of eclogites and paragneisses preserving unequivocal evidence of coesite and or diamond stability is very small by contrast the common garnet xmn 0 18 0 45 phengite si 3 2 3 35 zoned epidote ps38 97 biotite titanite two feldspars quartz assemblages in the more extensive orthogneisses have been previously thought to have formed under low p t conditions of ca 400 50 c at 4 kbar however certain orthogneiss samples preserve garnets with xca up to 0 50 rutile inclusions within titanite or epidote and relict phengite inclusions within epidote with si contents p f u of up to 3 49 overlapping with the highest values 3 49 3 62 recorded for phengites in samples of undoubted uhp schists these and other mineral composition features such as a site deficiencies in the highest si phengites na in garnets linked to y yb substitution and al f ti 1 o 1 substitution in titanites are taken to be pointers towards the orthogneisses having experienced a similar metamorphic evolution to the associated uhp schists and eclogites re evaluated garnet phengite and garnet biotite fe mg exchange thermometry and calculated 5 rutile 3 grossular 2sio2 h2o 5 titanite 2 zoisite equilibria indicate that the orthogneisses may indeed have followed a common subduction related clockwise p t path with the uhp paragneisses and eclogites through conditions of pmax at ca 690 c 715 c and 36 kbar to tmax at ca 710 c 755 c and 18 kbar prior to extensive re crystallisation and re equilibration of these ductile orthogneisses at ca 400 c 450 c and 6 kbar the consequential conclusion that it is no longer necessary to resort to models of tectonic juxtapositioning to explain the spatial association of these dabieshan orthogneisses with undoubted uhp lithologies has far reaching implications for the interpretation of controversial gneiss eclogite relationships in other uhp metamorphic terrains

a comparison of the evolution of diamondiferous quartz rich rocks from the saxonian erzgebirge and the kokchetav massif are so called diamondiferous gneisses magmatic rocks abstract the petrography and chemical composition of minerals of quartz rich diamondiferous rocks from the kokchetav massif especially the zonation of garnet were studied and compared with diamondiferous quartzofeldspathic rocks from the saxonian erzgebirge many compositional and textural features were found to be similar for instance microdiamonds are enclosed systematically in a specific intermediate growth zone of garnet in these rocks on the basis of experimental data a magmatic scenario was constructed to check if the quartz rich diamondiferous rocks are of magmatic origin by this the p t paths derived here for the kokchetav rocks and the textural observations it is concluded that the minerals of the diamondiferous rocks have crystallized from silicate melts these melts originated by anatexis of deeply submerged metasediments erzgebirge at t as high as 1200 c kokchetav massif at 50 100 c lower t and ascended from at least 200 km depth relics of the pre anatectic evolution are still present for instance as garnet cores after ascent and emplacement of the magma in deep portions of thickened continental crust kokchetav massif 45 50 km close to 800 c erzgebirge 55 60 km at 30 50 c lower t considerable quantities of white and or dark micas formed by peritectic reactions from melt for instance garnets could be resorbed at this stage and biotite grew instead after the magmatic stage retrogression took place much stronger in the kokchetav massif this was accompanied by deformation transforming broadly the magmatic texture of quartz rich diamondiferous rocks from the kokchetav massif to a gneissic texture

lava tubes terraces and megatumuli on the 1614 24 pahoehoe lava flow field mount etna sicily the 1614 1624 lava flow of mt etna was formed during a long duration flank eruption involving predominantly pahoehoe flows which produced unusual surface features including mega tumuli here defined and terraces detailed mapping of the flow units surface features and associated tubes reveals a complex sequence of emplacement for the field the stair stepped terraces appear to have been formed as a consequence of self damming of tube fed flows which developed perched ponds of lava surges of lava through tubes elevated sections of crusted lava at the distal ends of the flow to generate tumuli some as high as 130 m as a consequence of pressure via hydrostatic head conditions within the tube although pahoehoe lavas and the related features described here are atypical of mt etna they may reflect styles of eruption and lava emplacement found on volcanoes elsewhere

diamond former coesite and supersilicic garnet in metasedimentary rocks from the greek rhodope a new ultrahigh pressure metamorphic province established abstract we report here the first discovery of ultrahigh pressure uhp indicator minerals and textures from the rhodope metamorphic province rmp northern greece in particular we document the exsolution of quartz rutile and apatite in sodic garnet from metapelites garnet biotite kyanite gneisses which attests to the presence of a si ti na p rich precursor garnet phase similar textures in garnet have been reported in the literature only once before for eclogites from the su lu uhp metamorphic province china we also document the presence of microdiamonds and multicrystalline quartz pseudomorphs after coesite included in garnet from both eclogites and metapelites we then argue that these rocks had once been transported to depths exceeding 220 km well into the upper mantle thus establishing the rmp as another important uhp metamorphic belt in the world

thermobarometry and geotectonic significance of high pressure granulites examples from the moldanubian zone of the bohemian massif in lower austria

buoyancy driven rapid exhumation of ultrahigh pressure metamorphosed continental crust abstract preservation of ultrahigh pressure uhp minerals formed at depths of 90 125 km require unusual conditions our subduction model involves underflow of a salient 250 150 km wide 90 125 km long of continental crust embedded in cold largely oceanic crust capped lithosphere loss of leading portions of the high density oceanic lithosphere by slab break off as increasing volumes of microcontinental material enter the subduction zone buoyancy driven return toward midcrustal levels of a thin 2 15 km thick low density slice finally uplift backfolding normal faulting and exposure of the uhp terrane sustained over 20 million years rapid 5 mm year exhumation of the thin aspect ratio uhp sialic sheet caught between cooler hanging wall plate and refrigerating downgoing lithosphere allows withdrawal of heat along both its upper and lower surfaces the intracratonal position of most uhp complexes reflects consumption of an intervening ocean basin and introduction of a sialic promontory into the subduction zone uhp metamorphic terranes consist chiefly of transformed yet relatively low density continental crust compared with displaced mantle material otherwise such complexes could not return to shallow depths relatively rare metabasaltic metagabbroic and metacherty lithologies retain traces of phases characteristic of uhp conditions because they are massive virtually impervious to fluids and nearly anhydrous in contrast h2o rich quartzofeldspathic gneissose schistose more permeable metasedimentary and metagranitic units have backreacted thoroughly so coesite and other uhp silicates are exceedingly rare because of the initial presence of biogenic carbon and its especially sluggish transformation rate uhp paragneisses contain the most abundantly preserved crustal diamonds

diamond and coesite discovered in saxony type granulite solution to the variscan garnet peridotite enigma diamond and coesite were discovered in situ as inclusions in garnet kyanite and zircon in high pressure granulites from northern bohemian massif these continental crustal rocks were therefore subducted to depths of c 140 km which also explains their common association with mantle garnet bearing peridotites models involving crustal thickening for these high pressure granulites need to be significantly modified whole variscan belt with numerous hp granulite occurrences can represent a large ultrahigh pressure terrain

eclogites of the snowbird tectonic zone petrological and u pb geochronological evidence for paleoproterozoic high pressure metamorphism in the western canadian shield eclogite occurs within the southern domain of the east athabasca mylonite triangle in northern saskatchewan situated at the boundary between the archean rae and hearne provinces of the western canadian shield the east athabasca mylonite triangle is a fundamental exposure of the 3 000 km long snowbird tectonic zone the eclogite occurs in association with a variety of lower crustal high pressure granulites that record a complex metamorphic history from 2 6 to 1 9 ga temperatures of the eclogite facies metamorphism are constrained by garnet clinopyroxene exchange thermometry at 920 1 000 c minimum pressure conditions are recorded by the jadeite quartz albite geobarometer at 1 8 2 0 gpa a near isothermal decompression path to granulite facies conditions is inferred from retrograde reaction textures involving the formation of granulite facies assemblages such as orthopyroxene plagioclase and pargasite plagioclase u pb idtims zircon geochronology of the eclogite yields a weighted mean 207pb 206pb date of 1 904 0 0 3 ma which we interpret as the time of peak eclogite facies metamorphism shrimp in situ analyses of metamorphic zircons included within omphacitic clinopyroxene support this interpretation with a weighted mean 207pb 206pb date of 1 905 19 ma inclusion suites of high pressure phases and the petrographic setting of zircon are a direct link between zircon growth and eclogite facies metamorphism zircon from one eclogite sample has older cores that are 2 54 ga which is a minimum age for the emplacement or earliest metamorphism of the gabbroic protolith u pb rutile data indicate slow cooling at 1 c ma below 500 c from 1 88 to 1 85 ga the formation and exhumation of the eclogites at ca 1 9 ga has important implications for the tectonic significance of the snowbird tectonic zone during the paleoproterozoic the eclogites described here are consistent with transport of continental crust to mantle depths during the paleoproterozoic followed by rapid buoyancy driven exhumation to normal lower crustal depths

titanium solubility in coexisting garnet and clinopyroxene at very high pressure the significance of exsolved rutile in garnet exsolution microstructures including ilmenite garnet in clinopyroxene and rutile in garnet are common in clinopyroxenite and eclogite from the sulu ultrahigh pressure uhp terrane in order to understand the phase relations and ti solubility in both garnet and clinopyroxene in a natural tio2 bearing system several experiments at 5 15 gpa 1000 1400 c were carried out using the multianvil high pressure apparatus the hujianlin ilmenite rich garnet clinopyroxenite showing exsolution microstructure was selected as starting material because it closely approaches a composition lying in the tio2 cao mgo feo al2o3 sio2 system except for minor melt in one experiment at 1400 c and 5 gpa other run products contain majoritic garnet clinopyroxene ilmenite or rutile and exhibit neoblastic texture with increasing pressure ti and ca mg and si contents of neoblastic garnet increase with decreasing al the principal coupled substitutions are ca2 ti4 2al3 and si4 mg2 2al3 responding to majorite component increase titanium solubility 0 8 4 5 wt as tio2 in garnet and grtti cpxti ratio have a pronounced positive correlation with pressure between 5 and 15 gpa on the other hand the coexisting clinopyroxene contains low ti 0 17 0 53 wt as tio2 and shows no significant pressure effect rutile exsolution in garnet is coupled to that of pyroxene exsolution both are exsolved from majoritic garnet on decompression therefore the amount of such exsolved lamellae is a potential indicator of high pressure metamorphism in exhumed rocks whereas the tio2 content of clinopyroxene coexisting with garnet is not sensitive to pressure change

temperatures of granulite facies metamorphism constraints from experimental phase equilibria and thermobarometry corrected for retrograde exchange this study assesses temperatures of formation of common granulites by combining experimental constraints on the p t stability of granulite facies mineral associations with a garnet orthopyroxene grt opx thermobarometry scheme based on al solubility in opx corrected for late fe mg exchange we applied this scheme to 414 granulites of mafic intermediate and aluminous bulk compositions our findings suggest that granulites are much hotter than traditionally assumed and that the p t conditions of the amphibolite granulite transition portrayed in current petrology textbooks are significant underestimates by over 100 c for aluminous and intermediate granulites mean corrected temperatures based on our method are 890 17 and 841 11 c respectively uncertainties reported as 95 confidence limits on the mean consistent with minimum temperatures for orthopyroxene production by fluid absent partial melting in these bulk compositions in contrast mean temperatures based on grt opx fe mg exchange equilibria using the same thermodynamic data are 732 22 and 723 11 c respectively well below the minimum temperatures for opx stability for mafic granulites the mean corrected temperature using our method is 816 12 c similar to the mean temperature of 793 13 c from fe mg exchange reasons for the differences between the mafic granulites and aluminous intermediate granulites are unclear but may be due to the lower al concentrations in opx in the mafic rocks and possible deficiencies in the thermodynamic modelling of these low concentrations we discuss a number of well known granulite terrains in the context of our findings including the adirondacks the acadian granulites of new england the incipient charnockites of southern india and sri lanka and the kerala khondalite belt our findings carry implications for thermotectonic models of granulite formation a computer program to perform our thermobarometry calculations rclc is available from the journal of petrology website at http www petrology oupjournals org or from the authors at http www geo ucalgary ca pattison drm pattison rclc htm

gps and tectonic evidence for a diffuse plate boundary at the azores triple junction abstract we use gps bathymetric structural and seismic data to define the pattern of present deformation along the northern half of the azores plateau where the nubia eurasia plate boundary terminates at the axis of the mid atlantic ridge mar new and existing campaign gps velocities from the azores islands reveal extension oblique to a series of en echelon volcanic ridges occupied by terceira s jorge pico and faial islands in a frame of reference defined by 69 continuous gps stations on the eurasia plate terceira island moves 2 1 mm yr away from eurasia consistent with the island s location within the terceira rift and plate boundary structure the volcanic ridges south of the terceira rift move toward wsw at progressively faster rates reaching a maximum of 3 5 0 5 mm yr 2 for the pico faial volcanic ridge the hypothesis that the terceira rift accommodates all nubia eurasia plate motion is rejected at high confidence level based on the motions of sites on s jorge island just west of terceira rift all of the islands move relative to the nubia plate with pico island exhibiting the slowest motion only 1 0 5 mm yr 2 detailed bathymetry from the interior of the hypothesized azores microplate reveals faults that crosscut young mar seafloor fabric these observations and the gps evidence for distributed deformation described above argue against the existence of a rigid or semi rigid azores microplate and instead suggest that nubia eurasia plate motion is accommodated by extension across a 140 km wide zone east of the mar axis most likely bounded to the north by the northern shoulder of the terceira rift the mar spreading rate along the western end of the azores deformation zone 38 5 n 39 5 n is intermediate between the eurasia north america rate measured at 39 5 n and the nubia north america rate measured at 38 5 n consistent with the joint conclusions that the nubia eurasia boundary is broad where it intersects the mar and the azores triple junction is diffuse rather than discrete

diapiric exhumation of earth s youngest uhp eclogites in the gneiss domes of the d entrecasteaux islands papua new guinea abstract the woodlark rift in papua new guinea hosts the world s youngest 2 8 ma eclogite facies rocks and extensional deformation has played a key role in exhuming these u hp rocks at rates of 20 mm yr during the eocene papuan arc continent collision australian plate derived continental rocks were subducted to u hp depths there they remained for up to 30 m y until the pliocene when asthenospheric circulation ahead of the west propagating woodlark spreading ridge introduced heat and fluids this caused rocks to break away from the paleosubduction channel recrystallize in the eclogite facies and rise as rayleigh taylor instabilities the diapirs ascended adiabatically undergoing partial melting which lowered their viscosity and increased buoyancy u hp crust ponded near the moho at 2 4 ma thickening the crust to 40 km 11 kb domal uplifts emerged above sea level and these are still underlain by an unusually thick crust 26 km for a rift that has stretched by factor of 3 since 6 ma after ponding they acquired a flat lying foliation during amphibolite facies retrogression vertical shortening accompanied the gravitationally driven outflow of ponded lower crust the weak material was extended parallel to the rift margin thinning ductilely by w k 0 2 and was mechanically decoupled from and orthogonal to plate motion in the rift top e shear fabrics suggest that this flow was westward perhaps driven by isostatic stresses towards a strongly thinned rift corridor ahead of the woodlark spreading ridge at 100 c per m y and were mechanically incorporated into the rift s upper crust final exposure was by normal faulting and minor erosion such exhumation may also apply to other u hp terranes where less evidence for moho ponding is preserved

the fate of zircon during uht uhp metamorphism isotopic u pb δ18o hf and trace element constraints garnet clinopyroxene ultra high pressure uhp rocks from the northern bohemian massif contain zircon with micro diamond inclusions trace element concentrations oxygen and hafnium isotopic composition and u pb age of distinct textural domains in zircon characterize their growth conditions and temporal evolution diamond bearing zircon mantle domains with relicts of oscillatory zoning have uniform th u ratios 0 1 0 2 high ti contents 110 190 ppm corresponding to temperatures of at least 1100 c and some two of 17 mantle analyses preserve steep heavy rare earth element hree patterns with ybn gdn 10 11 with a weak negative eu anomaly these signatures are consistent with crystallization from a melt under uhp ultra high temperature uht conditions some of the bright cathodoluminscence cl rims preserve th u and ti values characteristic of the zircon mantles but others show elevated th u ratios of 0 3 0 4 and lower ti contents 20 40 ppm only 13 ppm in a rare low cl outer rim as they feature flat hree patterns and negative eu anomalies and commonly make embayments and truncate the mantle zoning we suggest that they have formed through recrystallization in the solid state during exhumation of the rock when both garnet and plagioclase were stable the three zircon domains that is cores mantles and rims yield u pb concordia ages of 340 9 1 5 340 3 1 5 and 341 2 3 4 ma respectively when linked to the previously reconstructed p t path of the rock the error limits of the zircon mantle and rim ages constrain the exhumation of the rocks from depth of 140 km uhp to 80 km hp to a minimum rate of 1 5 cm yr 1 the zircon cores are heterogeneous in terms of th u ratio below 0 1 but also above 0 2 and ree characteristics and their ehf values scatter between 15 7 and 4 8 with similar values for individual domains within a single zircon grain suggesting a very localized control on hafnium isotope composition on a grain scale the non equilibrated ehf values as well as a large range of the hf depleted mantle model ages possibly reflect the presence of a heterogeneous population of old zircon consequently the uniform and young 238u 206pb ages may represent near complete resetting of the u pb geochronometer during the uhp uht event at c 340 ma through dissolution reprecipitation process in contrast to hf the oxygen isotope composition of zircon is homogeneous ranging between 7 8 and 9 6 vsmow reflecting a source containing upper crustal material and homogenization at uhp uht conditions our study documents that continental crust was subducted to mantle depths at c 340 ma during the variscan orogeny and was subsequently very rapidly exhumed implying that the sequence of events was faster than can be resolved by the secondary ion mass spectrometry technique

detrital zircon geochronology of the northeastern tibetan plateau u pb geochronologic analyses have been conducted on 413 detrital zircon grains collected from 16 samples in the altun shan nan shan and qilian shan the samples come primarily from quartz arenites and metaturbidites of middle to late proterozoic age and from feldspathic and volcanic clast rich sandstones of early paleozoic age zircon grains in proterozoic strata resting on tarim basement yielded mainly 2 0 1 9 ga ages whereas proterozoic strata of the qaidam and qilian terranes yielded mainly ca 930 820 ma and ca 1 9 1 1 ga ages the younger grains were apparently shed from local igneous rocks whereas the grains older than 1 1 ga were shed from an undetermined continental source grains in the lower paleozoic strata are mainly ca 500 430 ma and were shed from nearby plutonic and possibly volcanic rocks that formed in a magmatic arc setting our detrital zircon ages are consistent with a model first proposed by e r sobel and n arnaud in which early paleozoic magmatism occurred within a single northeast facing magmatic arc that was constructed across an assemblage of middle to late proterozoic accretionary complexes remnants of magmatic arcs and shallow marine strata this arc system was accreted to the tarim and sino korean cratons during silurian devonian time the resulting suture has been reactivated as tertiary thrust faults that currently define the structural and topographic margin of the tibetan plateau our data also provide two new estimates for the offset along the eastern altyn tagh fault a belt of middle proterozoic shallow marine strata is offset by 400 km whereas a belt of 490 480 ma magmatic arc rocks is offset by 370 km these values are generally similar to the 350 400 km offset reported in most previous studies

challenges in high pressure granulite metamorphism in the era of pseudosections reaction textures compositional zoning and tectonic interpretation with examples from the bohemian massif high pressure granulites are an important record of geodynamic processes in overthickened or subducted continental crust orthopyroxene free assemblages in granitic ternary feldspar s quartz garnet kyanite rutile intermediate ternary feldspar s quartz garnet clinopyroxene kyanite rutile titanite and basic garnet clinopyroxene plagioclase quartz rutile compositions indicate formation conditions at mantle depths clinopyroxene compositions in variscan high pressure granulites are unusual in that they include omphacite in plagioclase bearing rocks thus not eclogite and al rich diopside i e indicating high ca tschermak content with both yielding temperatures above 900 c problems such as compositional zoning multiple generations of key phases in reaction domains and unmixing of high temperature solid solution phases during cooling ternary feldspars omphacite clearly indicate disequilibrium and require very careful interpretation as to which phases and compositions possibly represent a former equilibrium association pressure temperature p t determination by the pseudosection method although allowing prediction of mineral assemblages compositions and molar proportions for a fixed bulk composition for modelled p t conditions still requires reliable activity composition information for the key phases feldspar and clinopyroxene as well as an interpretation of former equilibrium compositions in the investigated samples i e the same restrictions applying to conventional thermobarometry the interpretations of some recently determined pseudosections for the composition of variscan clinopyroxene bearing high pressure granulites contradict numerous published p t paths however quantitative information from thermobarometry or pseudosections must be integrated with key petrographic observations in the case of the variscan example it is argued that petrographic observations and published p t paths are consistent with mineral assemblages predicted in pseudosections and support existing tectonometamorphic models

high pressure granulites formation recovery of peak conditions and implications for tectonics high pressure granulites are characterised by the key associations garnet clinopyroxene plagioclase quartz in basic rocks and kyanite k feldspar metapelites and felsic rocks and are typically orthopyroxene free in both basic and felsic bulk compositions in regional metamorphic areas two essential varieties exist a high to ultrahigh temperature group and a group representing overprinted eclogites the high to ultrahigh temperature type formerly contained high temperature ternary feldspar now mesoperthite coexisting with kyanite is associated with garnet peridotites and formed at conditions above 900 c and 1 5 gpa clinopyroxene in subordinate basic rocks is al rich and textural evidence points to a high pressure high temperature melting history the second variety contains symplectite like or poikilitic clinopyroxene plagioclase intergrowths indicating former plagioclase free i e eclogite facies assemblages this type of rock formed at conditions straddling the high pressure amphibolite high pressure granulite field at around 700 850 c 1 0 1 4 gpa importantly in the majority of high pressure granulites orthopyroxene is secondary and is a product of reactions at pressures lower than the peak recorded pressure in contrast to low and medium pressure granulites which form at conditions attainable in the mid to lower levels of normal continental crust high pressure granulites of nonxenolith origin mostly represent rocks formed as a result of short lived tectonic events that led to crustal thickening or subduction of the crust into the mantle short times at high temperature conditions are reflected in the preservation of prograde zoning in garnet and pyroxene high pressure granulites of both regional types although rare are known from both old and young metamorphic terranes e g c 45 ma namche barwa e himalaya 400 340 ma european variscides 1 8 ga hengshan china 1 9 ga snowbird saskatchewan and 2 5 ga jianping china this spread of ages supports proposals suggesting that thermal and tectonic processes in the lithosphere have not changed significantly since at least the end of the archean

the 1998 faial earthquake azores evidence for a transform fault associated with the nubia eurasia plate boundary abstract with very few exceptions m 4 tectonic earthquakes in the azores show normal fault solution and occur away from the islands exceptionally the 1998 shock was pure strike slip and occurred within the northern edge of the pico faial ridge fault plane solutions show two possible planes of rupture striking ene wsw dextral and nnw sse sinistral the former has not been recognised in the azores but is parallel to the transform direction related to the relative motion between the eurasia and nubia plates therefore the main question we address in the present study is do transform faults related to the eurasia nubia plate boundary exist in the azores knowing that the main source of strain is related to plate kinematics we conclude that the sinistral strike slip nnw sse fault plane solution is not consistent with either the fault dip ca 65 which is typical of a normal fault or the ca ene wsw direction of maximum extension both are consistent with a normal fault as observed in most major earthquakes on faults striking around nnw sse in the azores in contrast the dextral strike slip ene wsw fault plane solution is consistent with the transform direction related to the anticlockwise rotation of nubia relative to eurasia altogether tectonic data measured ground motion observed destruction and modelling are consistent with a dextral strike slip source fault striking ene wsw furthermore the bulk clockwise rotation measured by gps is typical of bookshelf block rotations observed at the termination of such master strike slip faults therefore we suggest that the 1998 earthquake can be related to the wsw termination of a transform ene wsw fault plane solution associated with the nubia eurasia diffuse plate boundary

an exceptional case of endogenous lava dome growth spawning pyroclastic avalanches the 1999 bocca nuova eruption of mt etna italy abstract during an eruption at the bocca nuova one of the summit craters of mt etna in october november 1999 a part of the crater floor near its wnw rim was uplifted to form a dome shaped feature that consisted of older lava and pyroclastics filling the crater this endogenous dome grew rapidly over the crater rim thus being perched precariously over the steep outer slope of the bocca nuova and near continuous collapse of its steep flanks generated swiftly moving pyroclastic avalanches over a period of several hours these avalanches advanced at speeds of 10 20 m s and extended up to 0 7 km from their source on top of lavas emplaced immediately before their deposits were subsequently covered by lava flows that issued from vents below the front of the dome and from the bocca nuova itself growth of the dome was caused by the vertical intrusion of magma in the marginal western part of the crater which deformed and uplifted previously emplaced still hot and plastically deformable eruptive products filling the crater the resulting avalanches had all the characteristics of pyroclastic flows spawned by collapse of unstable flanks of lava domes but in this case the magma involved was of mafic hawaiitic composition and would under normal circumstances have produced fluid lava flows the formation of the dome and the generation of the pyroclastic avalanches owe their occurrence to the rheological properties of the eruptive products filling the crater which were transformed into the dome and to the morphological configuration of the bocca nuova and its surroundings the density contrast between successive erupted products may also have played a role although events of this type are to be considered exceptional at etna their recurrence might represent a serious hazard to visitors to the summit area

age petrogenesis and tectonic implications of early devonian bimodal volcanic rocks in the south altyn nw china abstract in this paper we report zircon u pb dating hf isotopes geochemical and sr nd isotopic data with the aim the petrogenesis and regional tectonic evolution of early devonian bimodal volcanic rock in the south altyn nw china new la icpms zircon u pb isotopic data constrained them at ca 406 ma the mafic samples are characterized by high fe cr and ni contents low ti and mg contents slightly enriched lree patterns and low la yb n la nb and la ta ratios and positive e nd t values 3 3 to 3 4 indicating that they were likely derived from strong batch melting of the asthenosphere in the spinel facies field the felsic rocks show an a type affinity with high alkalis fe ga zr nb ce and y contents low mg sr content high rb sr and ga al ratios enrichment in lile e g rb k th u and lree and depletion in ba sr nb ta p and ti and fractionated ree patterns with very strong negative eu anomalies these features along with distinct e nd t values 0 5 to 2 3 and mostly positive e hf t 0 29 to 5 18 indicate that the felsic rocks were mainly generated by partial melting of the crust in low pressure and high temperature conditions and simultaneously underwent slight magma mixing of such melts with mantle magma according to the petrogenetic schemes and geological background of the early devonian bimodal volcanic rocks tholeiite and a type dacite rhyolite they should have formed in a post collisional extensional setting moreover on the basis of spatial and temporal distribution and formation mechanism the tectonic magmatic evolution of the early paleozoic south altyn tagh could be divided into three stages 505 472 ma continental collision the magmatite formed under high pressure conditions due to the deep subduction and initial tearing of continental slab 467 450 ma continental slab break off the magmatite formed at high temperature and low pressure in virtue of felsic upper crust uplifting and mantle magma underplating and 432 385 ma post collisional extension the magmatite consists of a type granites and bimodal volcanic rocks which are the products of the interaction between mantle and crust and asthenosphere upwelling

petrological and geochronological constraints on high pressure high temperature metamorphism in the snowbird tectonic zone canada the upper deck of the east athabasca mylonite triangle eamt northern saskatchewan canada contains mafic granulites that have undergone high p t metamorphism at conditions ranging from 1 3 to 1 9 gpa 890 960 c coronitic textures in these mafic granulites indicate a near isothermal decompression path to 0 9 gpa 800 c the godfrey granite occurs to the north adjacent to the upper deck high p t domain well preserved corona textures in the godfrey granite constrain igneous crystallization and early metamorphism in the intermediate pressure granulite field opx pl at 1 0 gpa 775 c followed by metamorphism in the high pressure granulite field grt cpx pl at 1 2 gpa 860 c u pb geochronology of zircon in upper deck mafic granulite yields evidence for events at both c 2 5 ga and c 1 9 ga the oldest zircon dates are interpreted to constrain a minimum age for crystallization or early metamorphism of the protolith a population of 1 9 ga zircon in one mafic granulite is interpreted to constrain the timing of high p t metamorphism titanite from the mafic granulites yields dates ranging from 1900 to 1894 ma and is interpreted to have grown along the decompression path but still above its closure temperature indicating cooling following the high p t metamorphism from c 960 650 c in 4 10 myr zircon dates from the godfrey granite indicate a minimum crystallization age of 2 61ga without any evidence for 1 9 ga overgrowths the data indicate that an early granulite facies event occurred at c 2 55 2 52 ga in the lower crust c 1 0 gpa but at 1 9 ga the upper deck underwent high p t metamorphism then decompressed to 0 9 1 0 gpa juxtaposition of the upper deck and godfrey granite would have occurred after or been related to this decompression in this model the high p t rocks are exhumed quickly following the high pressure metamorphism this type of metamorphism is typically associated with collisional orogenesis which has important implications for the snowbird tectonic zone as a fundamental boundary in the canadian shield

combined rutile zircon thermometry and u pb geochronology new constraints on early paleozoic hp uht granulite in the south altyn tagh north tibet china abstract hp granulite and garnet peridotite constitute a hp metamorphic unit in the south altyn tagh northern tibet previous data suggest that hp granulites experienced high pressure ultrahigh temperature metamorphism 870 1050 c 19 25 kbar followed by a medium pressure granulite facies overprint 780 820 c 9 5 12 kbar previous zircon u pb geochronology of hp granulites gave an age of ca 500 ma but it is unclear if this age reflects the metamorphic peak or the post peak overprint in this contribution new shrimp analyses indicate that the zircons associated with the peak metamorphic assemblage in hp felsic granulite are characterized by flat heavy ree patterns and 30 62 ppm ti yielding a temperature of 892 1002 c by ti in zircon thermometry suggesting that the growth of the zircons occurred under or close to peak ultrahigh temperature conditions rather than under post peak conditions these zircons gave a shrimp u pb age of 495 4 ma representing the age of peak metamorphism in contrast oscillatory zoned zircon cores yield magmatic ree patterns ti in zircon temperatures of 720 820 c and neoproterozoic u pb ages 936 22 ma this indicates that the protoliths of the felsic granulites were formed in the early neoproterozoic and were taken to extreme temperatures and pressures during the early paleozoic orogenic event in situ la icpms and emp analyses indicate that rutiles are characterized by broad scatter and bimodal distribution in both zr concentration and u pb age which are interpreted as a result of recrystallization or growth rather than volume diffusion the first rutile u pb age cluster is at ca 485 ma with a zr concentration of ca 3000 ppm yielding ca 950 c at 20 kbar implying that they grew at or near the peak metamorphic conditions the second rutile u pb age cluster is at ca 450 ma with a zr concentration of ca 1000 ppm corresponding to a temperature around 780 c at 10 kbar which is related to post peak reheating under medium pressure medium temperature granulite facies conditions these results demonstrate that hp granulites in the south altyn tagh experienced a peak ultrahigh temperature and high pressure metamorphism at ca 495 ma followed by medium pressure medium temperature granulite facies overprint before ca 450 ma

model of syn convergent extrusion of orogenic lower crust in the core of the variscan belt implications for exhumation of high pressure rocks in large hot orogens reflection seismic section field structural analysis and gravimetric modelling of orogenic lower crust in the core of a carboniferous orogenic root reveal details of the polyphase process of exhumation subvertical amphibolite facies fabrics strike parallel to former plate margins that collided in the nw the fabrics are developed in both mid crustal and lower crustal high pressure granulite units as a result of intensive nw se intraroot horizontal shortening driven probably by the west directed collision in granulites the steep fabrics originated as a result of extrusion of orogenic lower crust in a 20 km wide vertical ascent channel from lower crustal depths at 350 340 ma the large granulite bodies preserve older granulite facies fabrics documenting a two stage evolution during the exhumation process surface exposures of granulites coincide with the absence of subhorizontal seismic reflectors at depth suggesting preservation of the 20 km wide subvertical tabular structure reaching moho depths horizontal seismic reflectors surrounding the vertical channel structure corroborate a dominant flat migmatitic fabric developed in all tectonic units this structural pattern is interpreted in terms of subhorizontal spreading of partially molten orogenic lower crust in mid crustal levels 765 c and 0 76 gpa at 342 337 ma large massifs of extruded and progressively dismembered felsic granulites disturbed mid crustal fabrics in the surrounding horizontally flowing partially molten crust the horizontal mid crustal flow resulted in collapse of the supra crustal tepla barrandian unit interpreted as the orogenic lid along a large scale crustal detachment above the extruded lower crustal dome the presence of felsic granulites at the bottom of the orogenic root is considered to be a key factor controlling the exhumation of orogenic lower crust in large hot orogens

garnet zoning and reaction textures in overprinted eclogites bohemian massif european variscides a record of their thermal history during exhumation eclogites from different parts of the variscan basement of the bohemian massif show a variation in temperatures recorded at peak pressures as well as during subsequent exhumation these differences are reflected in the extent of modification of pre syn and post eclogite facies garnet compositions and by the types of reactions taking place at garnet rims munchberg massif eclogites formed at conditions of 600 650 c 20 kbar were mainly overprinted in the amphibolite facies field and contain garnets with an old homogeneous core and often well preserved idiomorphic growth zoning to mg rich rims in the marianske lazn complex the eclogite facies stage 640 715 c 15 kbar many eclogites show well developed granulite facies overprints with widespread orthopyroxene growth in garnet and omphacite domains spinel and or sapphirine after kyanite and in some cases even a low pressure high temperature overprint that produced fe rich olivine the latter rocks clearly endured higher temperatures during breakdown but remarkably strong growth zoning and sharp overgrowth features are preserved in some cases diffusion modeling shows that the preservation of zoning in garnet of rocks that have experienced granulite facies temperatures is only possible if the time spent at these conditions is short 1 ma this time scale is inconsistent with a smooth p t t path as expected from thermal modeling for erosion or extension driven exhumation and implies that these variscan rocks experienced short lived thermal events during their exhumation history