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

partial transformation of gabbro to coesite bearing eclogite from yangkou the sulu terrane eastern china an ultra high pressure uhp metamorphic slab at yangkou beach near qingdao in the sulu region of china consists of blocks of eclogite facies metagabbro metagranitoid ultramafic rock and mylonitic orthogneisses enclosed in granitic gneiss a gradational sequence from incipiently metamorphosed gabbro to completely recrystallized coesite eclogite formed at ultra high pressures was identified in a single 30 m block metagabbro is preserved in the core whereas coesite eclogite occurs along the block margins the metagabbro contains an igneous assemblage of pl aug opx qtz bt ilm ti mag it shows relict magmatic textures and reaction coronas fine grained garnet developed along boundaries between plagioclase and other phases primary plagioclase broke down to ab ky ms zo grt amp augite is rimmed by sodic augite or omphacite whereas orthopyroxene is rimmed by a corona of cum act and omp qtz layers or only omp qtz in transitional rocks augite and orthopyroxene are totally replaced by omphacite and the lower pressure assemblage ab ky phn zo grt coexists with domains of omp jd70 73 ky phn in pseudomorphs after plagioclase both massive and weakly deformed coesite bearing eclogites contain omp ky grt phn coe qtz rt and preserve a faint gabbroic texture coesite inclusions in garnet and omphacite exhibit limited conversion to palisade quartz some intergranular coesite and quartz pseudomorphs after coesite also occur assemblages of the coronal stage transitional and uhp peak occurred at about 540 50 c at c 13 kbar 600 800 c at 15 25 kbar and 800 850 c at 30 kbar respectively garnet from the coronal through the transitional to the eclogite stage rocks show a decrease in almandine and an increase in grossular pyrope components garnet in low grade rocks contains higher mno and lower pyrope components the growth textures of garnet within pseudomorphs after plagioclase or along grain boundaries between plagioclase and other phases are complex the application of garnet zoning to estimate p t should be carried out with caution some garnet enclosing quartz aggregates as inclusions shows radial growth boundaries these quartz aggregates as well as other primary and low p phases persisted metastably at uhp conditions due to sluggish reactions resulting from the lack of fluid during prograde and retrograde p t evolution

the pyrope coesite rocks and their country rocks at parigi dora maira massif western alps detailed petrography mineral chemistry and pt path both the coarse and fine grained varieties of the partly coesite bearing pyrope quartzites their interlayered jadeite kyanite rocks and the biotite phengite gneiss country rock common to all of them were subjected to detailed petrographic and textural studies in order to determine the sequence of crystallisation of their mineral constituents which were also studied analytically by microprobe prior to pyrope and coesite growth the mg rich metapelites were talc kyanite chlorite rutile ellenbergerite schists which upon continued prograde metamorphism developed first pyrope megacrysts in silica deficient local environments at the expense of chlorite talc kyanite and subsequently the smaller pyrope crystals with coesite inclusions from reacting talc kyanite based on geobarometrically useful mineral inclusions as well as on experimentally determined phase relations a prograde pt path simplified for water activity 1 is constructed which passes through the approximate pt conditions 16 kbar and 560 c 29 kbar and 720 c and finally up to 37 kbar at about 800 c where the mg rich metapelite was a pyrope coesite rock with phengite kyanite and talc still present during the retrograde path pyrope was altered metasomatically either into phlogopite kyanite quartz or at a later stage to chlorite muscovite quartz both assemblages yield pt constraints the latter about 7 9 kbar 500 600 c the country rock gneisses have also endured high pressures of at least 15 kbar but they provide mostly constraints on the lowest portion of the uplift conditions within the greenschist facies about 5 kbar 450 c microprobe data are presented for the following minerals pyrope ellenbergerite dumortierite unusually mgti rich jadeite vermiculite formed after na phlogopite paragonite and for several generations of phengite chlorite talc phlogopite dravite and glaucophane in the high pressure rocks as well as for biotite chlorite phengites epidote garnet albite and k feldspar in the country rock gneisses an outstanding open problem identified in this study is the preservation of minerals as inclusions within kyanite and pyrope beyond their pt stability limits

coesite and pure pyrope in high grade blueschists of the western alps a first record and some consequences a pyrope quartzite originally described by vialon 1966 from the dora maira massif was resampled and reinvestigated garnet up to 25 cm in size phengite kyanite talc and rutile are in textural equilibrium in an undeformed matrix of polygonal quartz the garnet is a pyrope almandine solid solution with 90 to 98 mol mg end member it contains inclusions of coesite which has partially inverted to quartz resulting in a typical radial cracking of the host garnet around the inclusions several lines of evidence show that coesite crystallised under nearly static pressure conditions and that the whole matrix has once been coesite

alpe arami a peridotite massif from depths of more than 300 kilometers the abundance of fetio 3 and chromite precipitates in olivine of the alpe arami peridotite massif switzerland requires a much higher solubility for highly charged cations than is found in mantle xenoliths from depths of 250 kilometers three previously unknown crystal structures of fetio 3 were identified that indicate that the originally exsolved phase was the high pressure perovskite polymorph of ilmenite implying a minimum depth of origin of 300 kilometers these observations coupled with the unique lattice preferred orientation of olivine further suggest that the original composition of olivine may reflect that of a precursor phase perhaps wadsleyite that is stable only at depths greater than 400 kilometers

the effect of fluid and deformation on zoning and inclusion patterns in poly metamorphic garnets within the bergen arcs of w norway caledonian eclogite facies assemblages t 650 c p 15 kbar have formed from grenvillian granulites t 800 900 c p 10 kbar along shear zones and fluid pathways garnets in the granulites grti pyr56 40 alm45 25gro19 14 are unzoned or display a weak ca 1 wt feo over 1000 m zoning the eclogite facies rocks contain garnets inherited from their granulite facies protoliths these relict garnets have certain areas with compositions identical to the garnets in their nearby granulite but can be crosscut by bands of a more almrich composition grtii pyr31 41alm40 47gro17 21 formed during the eclogite facies event these bands orientated preferentially parallel or perpendicular to the eclogite foliation may contain mineral filled veins or trails of eclogite facies minerals omphacite amphibole white mica kyanite quartz and dolomite steep compositional gradients up to 9 wt feo over 40 m separate the two generations of garnets indicating limited volume diffusion the bands are interpreted as fluid rich channels where element mobility must have been infinitely greater than it was for the temperature controlled volume diffusion at mineral interfaces in the granulites the re equilibration of granulite facies garnets during the eclogite facies event must therefore be a function of fracture density deformation and fluid availability the results cast doubts on modern petrological and geochronological methods that assume pure temperature controlled chemical re equilibration of garnets

beyond threshold hillslopes channel adjustment to base level fall in tectonically active mountain ranges numerous empirical and model based studies argue that in general hillslopes and river channels increase their gradients to accommodate high rates of base level fall to date however few data sets show the dynamic range of both these relationships needed to test theoretical models of hillslope evolution and river incision here we utilize concentrations of 10 be in quartz extracted from river sand on the eastern margin of the tibetan plateau to explore relationships among short term 10 2 10 5 a erosion rate hillslope gradient and channel steepness our data illustrate nonlinear behavior and a threshold in the relationship between erosion rate and mean hillslope gradient confi rming the generalization that hillslopes around the world are limited by slope stability and cease to provide a metric for erosion at high rates 0 2 mm a the relationship between channel steepness index and erosion rate is also nonlinear but channels continue to steepen beyond the point where threshold hillslopes emerge up to at least 0 6 mm a demonstrating that channel steepness is a more reliable topographic metric than mean hillslope gradient for erosion rate and that channels ultimately drive landscape adjustment to increasing rates of base level fall in tectonically active settings

reaction enhanced ductility the role of solid solid univariant reactions in deformation of the crust and mantle abstract in collision belts and subduction zones penetrative deformation of thrust sheets may be catalyzed by metamorphic reactions if the reactions have a significant effect on the mechanical properties of rocks reaction enhanced ductility may result if a reaction produces a significant decrease in grain size thus allowing the rock to deform superplastically the product grain size of a reaction is a function of the ratio of nucleation and growth rates this ratio varies along any pressure temperature time ptt path and only attains a significant value at a certain distance from an equilibrium boundary a comparison of rates of change of p and t along typical segments of ptt paths for crustal rocks leads to the conclusion that a high nucleation rate a small grain size and resulting superplastic deformation are more likely to be achieved when reaction boundaries are crossed during underthrusting rather than during the thermal readjustments of a perturbed geotherm following crustal thickening or during uplift an example of reaction enhanced ductility occurs in eclogite facies metagranitoids in the sesia lanzo zone western alps where medium grained plagioclase reacted to fine grained jadeite quartz during early alpine underthrusting microstructural evidence from shear zones shows that the jadeite quartz aggregates were highly ductile and it is inferred that the small grain size allowed superplastic deformation further evidence suggests that this process operated during regional deformation of metagranitoids rubie 1982 the reaction of olivine to spinel or the beta phase by metastable overstepping of the two phase stability fields in a subducting plate ringwood 1975 may result in fine grained products and thus may be analogous to the albite jadeite quartz reaction the consequent decrease in effective viscosity is likely to have important consequences for plate dynamics at and below the depths of major phase transitions

heat flow heat production and lithospheric thermal regime in the iberian peninsula abstract the first heat flow and heat production maps of the iberian peninsula and its margins using previously acquired data are presented the surface heat flow map includes 553 determinations carried out on water and mining exploration wells oil exploration wells and on the seafloor the surface heat flow varies noticeably from the iberian mainland 65 10 mw m 2 to the atlantic and the mediterranean margins where the heat flow reaches values of about 40 50 mw m 2 and 80 100 mw m 2 respectively the heat production map consists of 664 determinations carried out on rock samples from the variscan iberian massif and the betics the higher values are obtained for granitic rocks 2 5 3 5 w m 3 whereas metasediments and basic rocks reach values of 1 2 5 w m 3 and nearly zero respectively the lithospheric structure deduced for the iberian peninsula by combining heat flow heat production and elevation data indicates that most of the iberian mainland is characterized by a lithospheric thickness of 110 5 km this value is maintained across the west atlantic margin whereas towards the mediterranean margin the lithospheric thickness decreases down to 60 40 km this study also suggests that the heat production in the southern variscan iberian massif must be noticeably higher 3 7 0 5 w m 3 than in the rest of the areas in order to fit the measured heat flow crustal thickness and elevation data

diamond from the dabie shan metamorphic rocks and its implication for tectonic setting diamond occurs in ultrahigh pressure metamorphic rocks from dabie shan anhui province eastern china diamond bearing rocks include eclogite gamet pyroxenite and jadeitite diamond occurs in a mineral assemblage with coesite and jadeite the diamonds and diamondiferous rocks of dabie shan are interpreted to be the products of ultrahigh pressure metamorphism in the underthrust basement of the yangtze continental plate during the early mesozoic at greater than 4 0 gigapascals and 900 c this interpretation is based on the distribution of rock units the stability field of diamond and isotopic data indicating a crustal origin for the rocks most diamonds occur as euhedral inclusions in garnets and are 10 to 60 micrometers across although some are up to 700 micrometers across

radiogenic heat production in the variscan crust new determinations and distribution models in corsica northwestern mediterranean abstract new heat production values of variscan volcanic rocks cropping out in the southern part of the corsican batholith were determined in the laboratory by analysing uranium thorium and potassium concentrations with gamma ray spectrometry they vary from a minimum of 0 2 for basalts to a maximum of 4 2 w m 3 for calc alkaline granites the th u ratio in variscan granitoids is slightly higher than normal generally our measurements yield values higher than those previously determined these data together with those available for the variscan massifs of the northwestern mediterranean were used for constructing models of distribution of radiogenic heat production of the crust the lithologic composition was deduced from available p wave velocity data by taking into account the pressure and temperature effects depending on the rate of decrease with depth of the radiogenic heat the crustal contribution in terms of heat flux ranges from 26 to 39 mw m 2 and on average is 33 mw m 2 the mean heat production of the crust is 1 0 w m 3 both average crustal heat source and crustal heat contribution are quite similar to those estimated with the heat production seismic velocity relationship

ultradeep metamorphic rocks the retrospective viewpoint ultradeep or ultra high pressure uhp metamorphic rocks formed from crustal protoliths within the stability field of coesite at pressures 2 5 3 0 gpa corresponding to depths 80 120 km occur locally though regionally distributed in at least five continental areas their recognition is solely based on characteristic minerals and mineral assemblages calibrated by experimental high pressure studies detailed petrographic and microprobe work especially on mineral inclusions in favorable cases allows the derivation of prograde pt paths during subduction and of retrograde ones during exhumation commonly the gneisses adjacent to the uhp rocks do not exhibit signs of ultradeep metamorphism apparently because the kinetics of their mineral reactions are sufficiently fast to allow complete reequilibration to shallower pt conditions during the retrograde path it is also possible however that uhp equilibria were not attained throughout the rock volumes subducted but only along zones of shearing and fluid introduction if it is true that not all uhp metamorphic rocks return to the crustal orogenic belts but some continue to be subducted to greater mantle depths the classical geochemical pattern of a one way mass transfer from mantle to crust throughout the earth s history is at stake the assumed gradual growth of continents may have had a counterpart of continent destruction during collision events most recent experimental studies at high pressures and relatively low temperatures show that at least three new hydrous mg a1 silicates exist that were not found in nature thus far but may be characteristic minerals in the cold portions of old subduction zones thus extending the water retentivity of subducting slabs to greater and hitherto unexpected depths

comment on paleomagnetic constraints on the geodynamic history of the major blocks of china from the permian to the present by r j enkin et al

clinoenstatite in alpe arami peridotite additional evidence of very high pressure observations by transmission electron microscopy show that lamellae of clinoenstatite are present in diopside grains of the alpe arami garnet lherzolite of the swiss alps the simplest interpretation of the orientation crystallography and microstructures of the lamellae and the phase relationships in this system is that the lamellae originally exsolved as the high pressure c centered form of clinoenstatite these results imply that the rocks were exhumed from a minimum depth of 250 kilometers before or during continental collision

metamorphic and tectonic domains of china abstract ten metamorphic domains can be distinguished in china comprising four cratonic three intracratonic and three intercratonic domains each domain contains one or more metamorphic belts each of which in turn contains a characteristic metamorphic facies or facies series that was formed during a distinct metamorphic epoch the metamorphic domains reflect the tectonic domains and tectonic evolution of china ancient continental nucleii in the north china and tarim alxa cratons were probably unified with the yangtze craton during the early proterozoic to form the china platform widespread greenschist facies metamorphism during the middle and late proterozoic accompanied by glaucophane greenschist facies metamorphism represents a rifting and closure event in the china platform a second rifting and closure event in the china platform occurred during the caledonian the china and siberian platforms were closed during the hercynian to form the eurasian continent closure of the ancient tethys ocean occurred in the indosinian epoch and subduction and collision within xizang tibet and taiwan occurred during mesozoic cenozoic time the distribution in time of types of metamorphism in china suggests cyclical changes of metamorphism known as the archaean proterozoic and phanerozoic megacycles each megacycle since the archaean consists of a change from progressive low to intermediate grade metamorphism to lower grade greenschist metamorphism that was superimposed on a general trend in which high grade metamorphism became progressively less important with time the change in metamorphic megacycles shows a general secular decrease in regional heat supply during metamorphism punctuated by episodic high grade progressive metamorphism within orogenic belts

holocene paleotemperatures deduced from geothermal measurements abstract detailed measurements of temperature and rock thermal properties were carried out in a 1000 m deep borehole drilled in massive granite using a suitable reduction these measurements reveal a coherent variation of apparent heat flow with depth which is attributed to past variations of the ground temperature during the last 10 000 years two inversion techniques of the data are described in order to reconstruct these paleotemperatures a classical inversion method allows a discussion of the information content of the data but leads to unreasonable temperature variations another method using bounds on paleotemperature variations provides more acceptable temperature models the main features of these models are the presence of a cold event identified as the little ice age and of warm episodes around 1500 bc and at the beginning of the present century the amplitude of the inferred variations would be at least 1 09 c

talc magnesite and ti clinohumite bearing ultrahigh pressure meta mafic and ultramafic complex in the dabie mountains china

heat flow heat production and crustal structure in peri atlantic regions abstract a linear relationship is commonly observed between heat flow and heat generation it provides information about the distribution of radioelements with depth over large regions the linear relationship is no more tenable but points still cluster within an elongated trend in which a regression analysis provides the bulk reduced heat flow and a gross estimate of the heat production distribution with depth over the whole considered region when geological provinces share an identical heat flow heat generation relation they form a geothermal province data measurements in western europe suggest that at least two provinces should be considered one can be related to the hercynian crust and the second to the older caledonian crust we suggest that the depth obtained from thermal data corresponds to the depth of the mafic granulite layer western france south and central england share identical geothermal parameters which can be correlated with the depth of the granulitic layer down to 17 18 km data from scotland do not fit the relationship of south and central u k and present a lower slope indicating that the radiogenic enriched layer is shallow 8 10 km this correlates with the seismic structure of the caledonian crust where the granulite layer is observed at this depth data from scotland can be compared to those from norway newfoundland new hampshire and eastern appalachian crust where the same crustal composition is observed

thermochronologic constraints on deformation and cooling history of high and ultrahigh pressure rocks in the qinling dabie orogen eastern china the hong an block is the best place to study the exhumation of high pressure hp and ultrahigh pressure uhp metamorphic rocks in the qinling dabie orogen of eastern china because it lacks the extensive cretaceous tectonic and thermal overprint observed in the dabie shan we measured timing of deformation and rate of cooling of the hp uhp rocks by 40ar 39ar analyses of synkinematic minerals from tectonites of key structural zones in the hong an and tongbai shan normal sense shear along the north dipping huwan detachment at the northern edge of the hong an block occurred between 237 and 231 ma this detachment facilitated the bulk of the exhumation of the hp uhp rocks our new 40ar 39ar ages combined with u pb zircon and sm nd ages of 245 240 ma suggest that exhumation of uhp rocks from mantle depths occurred at rates of 5 25 mm yr from 245 to 230 ma the mountain range is a warped extensional footwall within which white mica cooled from 225 to 205 ma locally younger extension is recorded by white mica recrystallization at 198 194 ma after which the entire block had cooled to below 300 c early cretaceous 40ar 39ar ages from the tongbai shear zone indicate that dextral shear along the southwest boundary of the orogen was contemporaneous with normal to sinistral oblique slip along the xiaotian mozitan fault along the northern boundary coeval dextral and sinistral shear zones along the northern and southwestern margins of the hong an and dabie shan would have caused eastward lateral extrusion of these two blocks perhaps driven by collision of the lhasa block with eurasia

upper crustal recycling in southern britain evidence from nd and sr isotopes analyses of representative rocks from southern britain are used to examine in detail the evolution of this segment of continental crust the vast majority of late precambrian basement rocks have unradiogenic 87sr 86sr ratios and model tdmnd ages of 1 3 ga although locally the rosslare complex and rushton schists have model tdmnd ages 1 7 ga identifying for the first time an old component in the basement of southern britain following the closure of the iapetus ocean sediments exhibit a progressive increase in model tdmnd ages 1 35 ga in the lower palaeozoic to 1 75 ga in the jurassic this variation demonstrates the southerly transport of detritus tdmnd 2 0 ga from the mixed archaean proterozoic terrains of northern europe and america the granitoids have significantly higher end than those of contemporaneous sediments and thus yield a different nd isotope evolution curve when considered in isolation the majority of rocks analysed have model tdmsr ages in the range 1 2 to 0 46 ga with youngest sediments generally recording the lowest model ages therefore with time the sediments record a striking increase in the ratio of model nd to model sr age the data suggest that the rb sr ratios of the crustal rocks is increased by a factor of approximately 2 every 900 ma implying that on average the british upper crust was reworked by erosion and sedimentation every 600 ma

precadomian relicts in the armorican massif their age and role in the evolution of the western and central european cadomian hercynian belt abstract well dated precambrian is mostly developed in the north of the armorican massif in which area voluminous cadomian magmatism is dated at between 650 and 550 ma much older relicts occur at cap de la hague in guernsey and in the tregor and are also found in the northern continental margin of the iberian peninsula in all these occurrences whole rock systems have been opened so that the true ages cannot be determined by the rb sr whole rock isochron method four u pb zircon ages are between 1 8 and 2 ga in guernsey icart orthogneisses in tregor port beni trebeurden morguignen orthogneisses there is no evidence from strontium isotopes that these isolated and scattered relicts have a wide extension or that such ancient continental crust played an important role in magma genesis from 650 ma to 270 ma ago on the contrary the evolution of initial 87sr 86sr ratios with time shows that the observed mid and west european continental crust is probably not older than 700 ma the increase of the initial 87sr 86sr ratios of the magmas with time suggests that after its formation in cadomian times this segment of continental crust evolved virtually as a closed system and hercynian magmatism arose principally from re melting of relatively young sialic components

heat flow and heat production in brittany western france new heat flow determinations in brittany western france allow mapping with a good coverage 1 point per 1000 km2 over that region a high heat flow zone oblique to the present geological structures trends from northern central brittany to the south and then turns to the ese towards the massif central heat production a and heat flow q are tested against a linear relationship the data do not fit a strict linear relationship but the points are distributed within an elongated cluster in which a linear regression can be approximated yielding the following equation q 26 6 3 5 mw m2 15 6 0 6 km a several tests of significance corroborate the linear cluster and do not alter the computed parameters the mapping of the residual values shows a geographical distribution which can be correlated with filtered gravimetric and magnetic data those residuals reflect local variations in heat production distribution or in the heat transport mechanism the high value of the slope 15 6 km computed during the linear regression reflects the influence of the upper to middle crust this value is too elevated to represent the thickness of the granitic intrusions which are known to be shallow rooted 4 6 km from gravity data interpretation compared to adjacent hercynian countries the data lie within the range of those of central and southwestern england

crustal temperature and mantle heat flow in europe abstract the map of the surface heat flow in europe was supplemented by the crustal thickness pattern and data on crustal radioactivity and transformed to the mantle moho heat flow distribution using a model of radioactivity decreasing exponentially with depth a z a 0 exp z d with a0 2 wm 3 and d 10km we computed generalized one dimensional steady state conductive temperature depth curves for the major tectonic units of europe the available information on the heat flow tectonic age heat flow heat generation and heat flow crustal thickness relationships were summarized and discussed in terms of crustal structure the mohorovicic discontinuity is clearly not an isothermal surface neither is the outflow of heat from the upper mantle constant but both parameters may vary within broad limits the moho temperature can be as low as 280 c in the shield areas however it can exceed 900 c in some young intrarnontane depressions or rift structures regional variations of the moho heat flow can be as large as 30 40 mwm 2 the value of the moho temperature reflects primarily the tectonic stabilization of the crust as a result of the deep seated processes in the upper mantle

geochronology and isotopic character of ultrahigh pressure metamorphism with implications for collision of the sino korean and yangtze cratons central china the collision zone between the sino korean and yangtze cratons in central china is marked by subduction related metamorphic rocks composed of crustal protoliths these range in metamorphic grade from ultrahigh pressure eclogite to low greenschist facies the paragenesis of the ultrahigh pressure rocks coesite and diamond bearing indicates subduction to depths greater than 120 km as a result of continental collision despite extreme metamorphic conditions zircons have not undergone pb loss but rather illustrate two periods of growth crystallization between 700 and 800 ma and ultrahigh pressure metamorphism at 218 2 5 ma previous studies have predicted a time transgressive collision but age data presented in this study indicate that the collision was approximately coeval along the length of the suture u pb zircon ages of these ultrahigh pressure metamorphic rocks are 218 4 2 5 ma and 217 1 8 7 ma for the dabie mountains and shandong peninsula respectively the similar ages imply that the tan lu fault which offsets the ultrahigh pressure metamorphic rocks is a secondary feature not related to collision or subduction whole rock isotopic data show that both rb sr and u pb were open systems during metamorphism but that sm nd probably remained closed nd initial isotopes display crustal signatures indicating that the crust subducted as a coherent slab rather than becoming intercalated with mantle material at depth sm nd data in conjunction with u pb zircon dating indicate that the ultrahigh pressure rocks are not typical yangtze craton basement 2 9 ga but that they originally crystallized in a rift environment between 700 and 800 ma discrepancies between 40ar 39ar plateaus sm nd isochrons and u pb zircon ages render calculation of a p t t path unrealistic

migmatites granites and orogeny flow modes of partially molten rocks and magmas associated with melt solid segregation in orogenic belts abstract this paper presents a model for the genesis of migmatites and granites during orogenic evolution based on the analysis of several phanerozoic crustal segments and a review of the physical properties of partially molten rocks and magmas this model inventories the modes of bulk flow of partially molten rocks and magmas and the mechanisms of melt solid segregation for each mode partial melting of rocks is associated with a strength decrease of two to three orders of magnitude leading to strain partitioning expressed by channel flow driven either by forces related to plate tectonics vertical channel flow or by the gravity force associated with lateral variations of crustal thicknesses horizontal channel flow although the structural characteristics of migmatites indicate that deformation plays a role in melt migration the emplacement of laccoliths of leucogranites above migmatites attests to the efficiency of the buoyancy force another decrease in apparent strength of about ten orders of magnitude is associated with the loss of continuity of the solid framework marking the transition from partially molten rocks to magmas the breakdown of the solid framework also allows for settling of the solid in suspension increasing the buoyancy of the remaining magma accordingly domes cored by diatexites former heterogeneous magmas and mantled by metatexites former partially molten rocks are interpreted as gravitational instabilities driven by the relative buoyancy of the magma and permitted by the weakness of the partially molten rocks this model provides a first order framework to elucidate the development of a crustal scale horizontal layering during the evolution of orogenic belts that are affected by partial melting in this case the middle crust is dominated by migmatites with domes cored by diatexites and mantled by metatexites that correspond to a partially molten and magmatic zone respectively the granitic dikes and sills and the associated laccoliths of leucogranites correspond to an intrusive zone overlying the partially molten zone the refractory lower crust is potentially in part formed by accumulation of solids segregated from the overlying heterogeneous magmas

jurassic paleomagnetic constraints on the collision of the north and south china blocks this paper reports a paleomagnetic study of jurassic rocks from the ordos basin in the north china block ncb a recent remagnetization and a high temperature and or high coercivity component with dual polarities have been isolated the middle jurassic pole 74 n 233 e a95 5 is roughly consistent with previous results and the lower jurassic pole 82 n 286 e a95 7 is located between the late triassic and middle jurassic poles these results combined with paleomagnetic results from adjacent blocks suggest that 1 the pole for the ncb suffered an angular displacement of 47 8 between the late triassic and middle jurassic 2 the scb underwent final accretion to the ncb in the middle jurassic and 3 accretion of the ncb to siberia was not complete until the late jurassic and possibly even until the early cretaceous

the high pressure stability of hydrous phases in orogenic belts an experimental approach on eclogite forming processes abstract metamorphic evolution of orogenic belts is characterized by reactions involving hydrous phases stability relations of hydrous phases depend on h 2 o activity availability during metamorphism however hydrous phases do occur in synthetic and natural systems also at water undersaturated conditions h 2 o undersaturated experiments in the cao al 2 o 3 sio 2 h 2 o model system were performed in order to predict the role of hydrous phases in h 2 o undersaturated compositions at constant pressure and temperature these experiments yielded different assemblages of hydrous anhydrous phases depending on the experimental bulk h 2 o content in natural eclogites of mafic to intermediate bulk composition common hydrous phases include amphibole lawsonite zoisite mg chloritoid paragonite and talc complex continuous reactions delimit the stability fields of the different assemblages involving these hydrous phases such continuous reactions can be used as powerful petrogenetic tools to derive p t conditions of natural eclogites derivation of h 2 o undersaturated phase relationships for natural and synthetic systems show that hydrous phases also occur in h 2 o undersaturated bulk compositions at eclogite facies conditions comparison of experimentally determined phase relationships in mafic rocks with pressure temperature evolution of orogenic terrains demonstrates that at subsolidus conditions a trend toward h 2 o saturation is necessarily maintained during prograde p t paths in orogenic belts as far as dehydration reactions occur any prograde p t path necessarily leads to a maximized occurrence of hydrous phases and thus a maximized h 2 o preservation is achieved during metamorphism

coesite bearing eclogite from the dabie mountains in central china coesite and coesite pseudomorphs are recognized in eclogite from the dabie mountains a collision zone between the sino korean and yangtze cratons in central china this third known occurrence of coesite in deep crustal rocks is within an archean gneiss terrane and is the first locality where coesite has been identified as an inclusion in both omphacite and garnet crystals at the same location coesite bearing eclogite is estimated to form at pressures of 28 kbar and temperatures from 600 to 710 c the dabie mountains eclogite underwent retrograde metamorphism to amphibolite facies after its formation the preservation of coesite in eclogite is found to be strongly controlled by the pressure temperature time path that coesite bearing eclogite followed during its uplift and retrograde metamorphism

u pb zircon rb sr and sm nd geochronology of high to very high pressure meta acidic rocks from the western alps three meta acidic rocks from the western italian alps a magnesiochloritoid bearing metapelite from the monte rosa massif a coesite pyrope quartzite from the dora maira massif and the monte mucrone granite in the sesia zone have been studied by u pb zircon rb sr on whole rock apatite and phengite and sm nd wholerock methods the mineral parageneses of the investigated rocks indicate high to very high pressure and medium to high temperature metamorphism this combined isotopic study has enabled us to constrain the ages of magmatic and metamorphic events and also to compare the behaviour of u pb zircon systems in three intensely metamorphosed areas of the pennine domain the u pb zircon data have yielded a magmatic age for the monte mucrone granite at 286 2 ma this result confirms the occurence of late hercynian magmatism in the sesia zone as in other austro alpine units and in other areas of the european crystalline basement in the monte rosa massif a geologically meaningless lower intercept age of 192 2 ma has been interpreted as an artefact due to a complex evolution of the u pb zircon system the magmatic shape of the zircons implies a magmatic or volcano sedimentary protolith for this rock originally considered as a metasediment the very high pressure metamorphism in the dora maira quartzite has produced an opening of the u pb zircon system at 121 12 29 ma the rb sr data support the occurence of high grade metamorphism during cretaceous times phengites model ages are slightly younger than the u pb zircon lower intercept ages due to cooling phenomena or possible response of the phengites to a later deformation the nd model ages from the whole rock samples as well as the u pb upper intercept ages from zircons of all three investigated rocks indicate the presence of proterozoic crustal components inherited from the precursors of these meta acidic rocks the studied zircon populations and their u pb systems apparently showed open system behaviour only when affected by extreme metamorphic conditions 700 750 c 28 kbar whereas eclogite facies conditions of 500 550 c and 14 16 kbar were not enough to disturb significantly the u pb zircon evolution it is also probable that the sedimentary or magmatic origin of the protoliths of these meta acidic rocks which involved different characteristics such as grain size and fluid phase concentration and composition could be another important factor controlling the u pb zircon system behaviour during metamorphic events

a tectonic model for the metamorphic evolution of the basal gneiss complex western south norway a review of currently available information relevant to the basal gneiss complex bgc of western south norway combined with the authors own observations leads to the following conclusions 1 most of the bgc consists of proterozoic crystalline rocks and probably subordinate lower palaeozoic cover 2 the last major deformation of these rocks was during the caledonian orogeny and involved large scale thrusting recumbent folding and doming the structural development of the bgc is closely tied in with that of the caledonian allochthon 3 the whole eclogite bearing part of the bgc has suffered a high pressure metamorphism with conditions of between 550 c 12 5 kbar sunnfjord and about 750 c 20 kbar more og romsdal at the metamorphic climax 4 this metamorphism was of caledonian age probably rather early in the caledonian tectonic history of the bgc and is considered to have been a rather transient event by setting these conclusions in a framework provided by geophysical evidence for the deep structure of the crust in southern norway we have constructed a geotectonic model to explain the recorded metamorphic history of the bgc it is suggested that considerable crustal thickening was caused by imbrication of the baltic plate margin during continental collision with the greenland plate this resulted in high pressure metamorphism in the resulting nappe stack progradation of the suture caused underthrusting of the baltic foreland below the eclogite bearing terrain causing it to emerge at the earth s surface aided by tectonic stripping and erosion application of isostacy equations to the model shows that eclogites can be formed by in situ metamorphism in crustal rocks and reappear at the land surface above a normal thickness of crust in a single orogenic episode of approximately 65 70 ma duration

relationships between the petrophysical properties density seismic velocity heat generation and mineralogical constitution the petrophysical properties density p compressional wave velocity v p and heat generation a exhibit considerable variation with rock type to characterize a given rock quantitatively a single parameter the cation fraction expressed by the k value has been found to be suited best 100 crystalline rocks have been sampled to cover the widest possible range in k value from silicic types like granophyres and granites through granodiorites tonalites diorites gabbros amphibolites hornblendites pyroxenites to ultrabasic rocks p k a and v p were determined experimentally the latter at 0 5 kbar 50 mpa the interrelations between these properties were established by regression analysis the relation k and v p k show a linear dependence whereas a k a and a v p follow exponential laws in a w m 3 12 20 230 5 k mole cm 3 in a 22 5 8 15 g cm 3 and in a 16 5 2 74 v p km s all correlation coefficients are better than 0 9

contrasting plate tectonic styles of the qinling dabie sulu and franciscan metamorphic belts the dabie mountains are part of the 2000 km long qinling dabie sulu suture zone juxtaposing the sino korean and yangtze cratons an eastern extension apparently crosses korea and lies along the japan sea side of honshu as the imjingang and sangun terranes respectively a northeastern segment may be present in sikhote alin russian far east this orogenic belt records late paleozoic ocean floor consumption and the triassic collision of two precambrian continental massifs in east central china coesite and microdiamond inclusions in strong refractory minerals of eclogite facies ultrahigh pressure uhp metamorphic rocks in the dabie sulu area attest to profound subduction of a leading salient of the old cold yangtze craton now recovered through tectonic exhumation and erosion northward increase in intensity of subsolidus recrystallization of the suture complex is analogous to the internal progression in grade of high pressure hp and uhp metamorphism documented in the western alps in both regions subduction of narrow prongs of continental material uhp metamorphism and return toward midcrustal levels of relatively lower density buoyant microcontinental blocks resulted from delamination of these rocks from the descending higher density oceanic crust capped lithospheric plate such salients of continental crust represent an integral structural part of the downgoing slab remain intact and may be dragged to great depths before disengaging and rising differentially as coherent blocks uhp parageneses include trace mineralogic relics requiring peak metamorphic conditions of 700 900 c and 28 35 kbar or more in contrast pacific type hp metamorphic belts as represented by the franciscan complex of western california recrystallized under physical conditions up to 200 500 c 10 3 kbar in this setting voluminous quartzo feldspathic and graywacke debris was carried downward on oceanic crust capped lithosphere choking the subduction zone with incompetent material sited between both plates and strongly adhering to neither this buoyant largely sedimentary complex decoupled at 25 30 km depth and ascended toward the surface in both alpine type intracontinental collision and pacific type underflow light sialic material displaced dense mantle thus the return to midcrustal levels was propelled dominantly by body forces

regional ultrahigh pressure coesite bearing eclogitic terrane in central china evidence from country rocks gneiss marble and metapelite the dabie mountains represent a fault bounded coesite bearing eclogite gneissic terrane within the collisional belt between the sino korean and yangtze cratons the following geologic evidence indicates that this terrane has been subjected to regional ultrahigh pressure metamorphism 1 the country rocks have a metamorphic pressure temperature evolution compatible with that of the eclogites on the basis of the occurrence of quartz pseudomorphs after coesite calcite pseudomorphs after aragonite relict high pressure assemblages in different rock types of the country rocks and consistent comparable compositional zonations of garnets in both country rocks and eclogites 2 peak metamorphic pressure temperature conditions consistently decrease both from east to west in the collisional belt and from north to south in the dabie mountains and clockwise pressure temperature paths are common for eclogites over the region 3 layered eclogites interbedded with gneisses and amphibolites contain omphacite kyanite and garnet indicating a minimum pressure of 20 kbar at 600 c for formation of these rocks

coesite in clinopyroxene in the caledonides and its implications for geodynamics coesite is reported for the first time from the caledonide orogen it occurs as inclusions in clinopyroxene in the dolomite eclogite at grytting norway and provides valuable new evidence to support the hypothesis of extremely high pressure 30 kbar in certain norwegian eclogites this finding has implications for the geodynamics and kinetics of the concepts of microscale intracrystalline tectonic overpressure and of macroscale local or regional tectonic overpressure and for the hypotheses of subduction and obduction involving a more than 100km vertical component of transport

high pressure blueschists and eclogites in hong an a framework for addressing the evolution of high and ultrahigh pressure rocks in central china the hong an region in the qinling dabie collisional zone in eastern china hosts a series of metamorphic rocks exposing a south to north distribution from blueschist blueschist greenschist amphibolite eclogite kyanite free and kyanite eclogite to coesite eclogite facies rocks that represent progressively deeper levels of the mesozoic subduction collision complex the hong an area is interesting for three reasons 1 it escaped the thermal and structural overprint imparted on much of the dabie mountains during early cretaceous intrusion of voluminous granites and granodiorites 2 the high pressure hp hong an eclogites are widely distributed often preserve prograde crystallization histories and can be directly linked in time and space to the blueschist blueschist greenschist rocks exposed to the south 3 the blueschist blueschist greenschist facies rocks are generally better exposed than their equivalents in the southeastern dabie mountains and offer some opportunity for simultaneous structural and metamorphic analysis the hong an area hp rocks offer perhaps the closest approximation to a preserved snapshot of mesozoic pressure temperature p t conditions attending early subduction exhumation in the region and are thus essential to generating a coherent picture of the dynamics attending both metamorphism and exposure of the coeval ultrahigh pressure uhp rocks the purpose of this contribution is twofold 1 to document previously unpublished metamorphic and structural data characterizing these hp sequences and their relative continuity in hong an 2 to incorporate these data with recent geochronologic structural and paleomagnetic information in the context of protracted late paleozoic through mesozoic subduction collision and exhumation metamorphism and exhumation of some of the southern hong an hp sequences appear to have occurred concomitant with oceanic subduction immediately to the west and thus may have preceded widespread continental subduction collision moreover all of the hp uhp sequences in the region were exhumed before the end of collision between the sino korean and yangtze cratons at ca 160 ma exhumation of hp uhp rocks both before and during continental plate collision is neither novel for central china nor for other hp uhp zones but is important to take into account when reconstructing the evolution of such orogens

geochemistry and age of ultrahigh pressure metamorphic rocks from the kokchetav massif northern kazakhstan isotopic and geochemical data of the zerenda series metamorphic rocks from the kokchetav massif are reported some of these rocks contain microdiamond inclusions in garnets and other indicators of ultrahigh pressure metamorphism p 40 kbar t 900 1000 c the diamond bearing rocks exhibit distinctive geochemical characteristics compared to typical crustal rocks the ree patterns range from lree depleted to slightly lree enriched chondrite normalized la yb n 0 1 5 4 with a negative eu anomaly they are depleted in incompatible elements e g sr ba u th with respect to the upper crust in contrast non diamondiferous rocks of the zerenda series exhibit normal crustal geochemistry all rocks of the zerenda series have very radiogenic lead isotopes the measured values 238u 204pb compared with those calculated for the interval between crust formation and ultrahigh pressure uhp metamorphism suggest a decrease by factors of up to 200 during the uhp metamorphism the sm nd mineral isochrons from the diamond bearing rocks and other rock types of the zerenda series give a middle cambrian 524 535 ma age of metamorphism the nd model ages show that crust formation occurred about 2 3 ga ago significant fractionation of sm and nd and loss of incompatible elements may be due to partial melting of the protoliths the ar ar age determinations of secondary biotite and muscovite from the diamond bearing rocks yield an age of 517 5 ma this cooling age requires a short time interval between uhp metamorphism and uplift to a crustal level ultrahigh pressure metamorphism might be a significant source of pb for the mantle we propose that the radiogenic pb of the oceanic array is the contamination traces of numerous uhp events beside the geological aspect we demonstrate a method of dating a high grade metamorphic terrain using nd isotopes we compare whole rock isochrons and mineral isochrons and in this way get some insight into the behaviour of the sm nd system during very high grade metamorphic events

unusually low δ18o ultra high pressure metamorphic rocks from the sulu terrain eastern china eclogite and quartzite interlayers containing ultra high pressure uhp metamorphic mineral assemblages occur at qinglongshan in the sulu terrain eastern china these rocks exhibit some of the most negative oxygen isotope compositions that have ever been reported for metamorphic rocks the 18 value of minerals in eclogite ranges from 10 4 to 9 0 o and in quartzite ranges from 10 2 to 7 3 near isotopic equilibrium between minerals and rocks suggests that these rocks must have acquired such low oxygen isotope compositions before the uhp metamorphism through near surface meteoric water rock interactions the rocks were then subducted to a great depth during continent continent collision forming uhp minerals with concomitant local oxygen isotope redistribution it is significant that the rocks have retained such negative oxygen isotope compositions despite subduction into the upper mantle

microdiamond in high grade metamorphic rocks of the western gneiss region norway three grains of microdiamond were recovered from high grade gneiss exposed in the western gneiss region norway identification and characterization of the diamond grains by raman and infrared spectroscopy indicate the presence of substitutional impurities of h and n primary fluid inclusions in garnet and quartz in the diamond bearing rock demonstrate the evolution of metamorphic volatile fluids from reduced n 2 co 2 compositions during the peak phase of metamorphism to n 2 ch 4 h 2 o bearing compositions during retrograde metamorphism compatible geologic petrologic and fluid composition data imply a metamorphic origin for the microdiamonds if so the metamorphic and fluid conditions recorded by the microdiamonds and gneissic host may be applicable to microdiamond investigations in other high pressure regionally metamorphosed orogens

tectonics of an ultrahigh pressure metamorphic terrane the dabie shan tongbai shan orogen china ultrahigh pressure metamorphic rocks with coesite and diamond form a tectonic slice over 20 km thick called the eclogite zone within the dabie shan complex in the qinling orogen in central china the orogen separates the sino korean block in the north from the yangtze block in the south the dabie shan complex is a composite terrane made up of eclogite facies and amphibolite facies gneiss slices and represents fragments of the lower continental crust of the yangtze block the dabie shan complex is bounded in the south by a triassic foreland fold thrust belt and in the north by a greenschist facies metaclastic unit the foziling group which probably represents the passive continental apron deposits of the yangtze block farther north is a granulite facies gneiss complex the qinling group which has ultramafic slivers and includes the remnants of an island arc with two bounding suture zones north of the qinling group are early paleozoic active margin deposits of the sino korean block the eclogite zone in the dabie shan complex is sandwiched between amphibolite facies gneiss slices dating by sm nd rb sr and ar ar of two eclogite samples from the eclogite zone gives early to middle triassic ages 236 246 ma the initial end values indicate reworking of a 2 11 and 1 55 ga continental crust a himalayan type tectonic evolution is envisaged for the qinling orogen with the creation of a 100 km thick crustal thrust wedge through continuous underplating during the subduction of the yangtze continental lithosphere exhumation of the ultrahigh pressure metamorphic rocks was chiefly achieved by the southward propagation of the thrust planes thereby isostatically uplifting and eroding the earlier deeply subducted parts of the orogen a total of 680 km of southward thrusting in front of dabie shan is inferred based on the abrupt termination of the tanlu fault normal faulting possibly caused by gravitational collapse probably also had a role in the exhumation process

coesite inclusions and prograde compositional zonation of garnet in whiteschist of the hp uhpm kokchetav massif kazakhstan a record of progressive uhp metamorphism coarse grained whiteschist containing the assemblage garnet kyanite phengite talc quartz coesite is an abundant constituent of the ultrahigh pressure metamorphic uhpm belt in the kulet region of the kokchetav massif of kazakhstan garnet displays prograde compositional zonation with decreasing spessartine and increasing pyrope components from core to rim cores were recrystallized at t 380 c inner to 580 c outer at p 10 kbar garnet ilmenite geothermometry margarite quartz stability and mantles at t 720 760 c and ph20 34 36 kbar coesite graphite stability phengite geobarometer kfmash system reaction equilibria textural evidence indicates that rims grew during decompression and cooling within the qtz stability field silica inclusions quartz and or coesite of various textural types within garnets display a systematic zonal distribution cores contain abundant inclusions of euhedral quartz type 1 inclusions inner mantle regions contain inclusions of polycrystalline quartz pseudomorphs after coesite type 2 with minute dusty micro inclusions of chlorite and more rarely talc and kyanite in their cores intense radial and concentric fractures are well developed in the garnet intermediate mantle regions contain bimineralic inclusions with coesite cores and palisade quartz rims type 3 which are also surrounded by radial fractures subhedral inclusions of pure coesite without quartz overgrowths or radial fractures type 4 occur in the outer part of the mantle garnet rims are silica inclusion free type 1 inclusions in garnet cores represent the low p low t precursor stage to uhpm recrystallization and attest to the persistence of low p assemblages in the coesite stability field coesites in inclusion types 2 3 and 4 are interpreted to have sequentially crystallized by net transfer reaction kyanite talc garnet coesite h2o and were sequestered within the garnet with progressively decreasing amounts of intragranular aqueous fluid during the retrograde evolution of the rock all three inclusion types diverged from the host garnet p t path at the coesite quartz equilibrium and followed a trajectory parallel to the equilibrium boundary resulting in inclusion overpressure coesite in type 2 inclusions suffered rapid intragranular h2o catalysed transformation to quartz and ruptured the host garnet at about 600 c when inclusion p 27 kbar garnet host p 9 kbar instantaneous decompression to the host garnet p t path passed through the kyanite talc chlorite quartz reaction equilibrium resulting in the dusty micro assemblage in inclusion cores type 3 inclusions suffered a lower volumetric proportion transformation to quartz at the coesite quartz equilibrium and finally underwent rupture and decompression when t 400 c facilitating coesite preservation type 4 coesite inclusions are interpreted to have suffered minimal transformation to quartz and proceeded to surface temperature conditions along or near the coesite quartz equilibrium boundary

present day ultrahigh pressure conditions of coesite inclusions in zircon and garnet evidence from laser raman microspectroscopy coesite the dense high pressure polymorph of quartz occurs as inclusions in mechanically strong minerals in deeply subducted metamorphosed crustal rocks in a number of eurasian collisional orogens it is the primary indicator mineral of ultrahigh pressure p metamorphism whereas some coesite inclusions are untransformed most exhibit partial transformation to palisade quartz and a concomitant increase in volume resulting in rupture and radial fracturing of the host grain coesite can be identified by its diagnostic raman spectrum the strongest band at atmospheric pressure room temperature is at 521 cm 1 laser raman microspectroscopic analyses of coesite inclusions within garnet and zircon in ultrahigh p metamorphic rocks from kazakhstan indonesia and china reveal consistent differences in the raman spectra of 1 partially transformed coesite quartz main raman band at 521 cm 1 and 2 untransformed monomineralic coesite grains main band at 525 526 cm 1 applying the room temperature calibration of pressure dependence of the coesite raman spectrum we conclude that the latter coesite inclusions are subject to a remarkable pressure differential of 19 23 kbar with the host grains and are still undergoing pressure temperature conditions on or close to the quartz coesite equilibrium boundary

occurrence and field relationships of ultrahigh pressure metagranitoid and coesite eclogite in the su lu terrane eastern china coesite eclogite is associated with metagranitoid in a 50 100 m 2 outcrop within the regionally developed amphibolite facies su lu orthogneiss primary intrusive relationships between the metagranitoid and basic rocks and bulk chemistry analyses show that together they represent a composite igneous body that has subsequently been strongly deformed and metamorphosed the presence of rutile sodie pyroxene corona garnet and possible pseudomorphs after coesite all suggest very high pressures of metamorphism in the metagranitoid this is the first documented occurrence of ultrahigh pressure uhp metagranitoid outside of the european alps the existence of uhp metagranitoid shows that low density of rocks does not necessarily prevent subduction to mantle depths even at peak metamorphic conditions the uhp composite igneous body reported here would have a bulk density less than the mantle buoyancy forces may therefore have been important in the early exhumation of this unit other outcrops of coesite eclogite in the su lu region may also have been originally metamorphosed along with low density granitoid rocks

zircon response to diamond pressure metamorphism in the kokchetav massif ussr ultrahigh pressure metamorphism of the diamond bearing gneisses in the kokchetav massif ussr has been dated at 530 7 ma 2 sgr by shrimp ion microprobe analysis of metamorphic zircons zircon xenocrysts as old as 2000 ma have retained original isotopic compositions through the extreme metamorphic conditions and indicate that the protolith that was buried to diamond forming pressures included early proterozoic components highly variable reset ting of u pb isotopic systems in zircon xenocrysts at the time of metamorphism suggests that diffusive lead loss had commenced but was incomplete a disequilibrium observation consistent with relatively transient exposure of the rocks to extreme crustal depths

reconnaissance isotopic investigations on rocks of an undeformed granite contact within the coesite bearing unit of the dora maira massif abstract recent research on ultrahigh pressure rocks in val gilba and the brossasco area has revealed the complete textural preservation of large volumes of a premetamorphic granite together with its former contact aureole that was subducted to depths of 100 km as indicated by coesite pseudomorphs petrographic studies indicate that although most minerals have been affected by metamorphism biotite and potassium feldspars in the rock are largely preserved except for thin peripheral rimming of biotite by phengite and garnet rb sr analyses on seven biotite samples fail to produce a valid isochron indicating either incomplete resetting or inheritance of radiogenic sr during metamorphism ar ar data from two biotites likewise yield very irregular patterns that are consistent with an episode of severe ar loss from a pre alpine parent rock followed later by addition of inherited radiogenic ar the biotite rb sr and k ar systems have therefore been extensively disturbed during alpine metamorphism in spite of the largely unaltered appearance of the crystals in thin sections attempts to measure u pb ages with apatite also failed although the geochronological studies yielded no reliable ages pb and sr ratios from four potassium feldspar samples are within the ranges found in known hercynian rocks providing new evidence that hercynian rocks played a prominent role as source materials for the dora maira rocks sr ratios from two apatite samples agree closely with the potassium feldspar values the ratios are also substantially lower than those found in the whiteschists in completely recrystallized rocks at parigi ca 3 km north of the present rocks these results indicate that the undeformed rocks in val gilba brossasco area are less disturbed by alpine ultrahigh pressure metamorphism thereby preserving the record of the protoliths somewhat better than do the completely recrystallized rocks at parigi to the north the pb and sr isotope data further show that the minerals of the undeformed rocks failed to reach isotopic equilibrium despite subduction to 100 km and heating to 700 c

blueschists and eclogites a possible plate tectonic mechanism for their emplacement from the upper mantle the most significant problem of blueschists and eclogite origin is explaining the mechanism by which they were transported from great depths to the surface three aspects of this problem are discussed 1 1 prograde and retrograde trajectories in blueschist and eclogite bearing belts and the preservation conditions for such minerals as aragonite lawsonite and coesite 2 2 the mechanism by which low density crustal rocks with coesite and diamond descend to a depth of 100 150 km and the high rate of return from these depths to the surface 3 3 the origin of a tectonic mixture of rocks from different depths in melange zones using estimations of p t conditions and constructed p t t trajectories four types of blueschist belt are proposed these include two of the ernst types the alpine and franciscan and two additional types the dora maira and sanbagawa types key examples of geologically and mineralogically well studied complexes in the soviet union are discussed the kokchetav massif maksutov complex syumkeu belt borus belt the various models for the origin of different types of high pressure metamorphic belts are considered 1 1 slow uplift or uplifting of the thickened crust after cessation of subduction 2 2 more rapid tectonic pushing out 3 3 the same as in case 2 but with an additional champagne cork effect 4 4 the most complicated process with repeated deformation and metamorphism

petrology of very high pressure eclogitic rocks from the brossasco isasca complex dora maira massif italian western alps petrographical and mineral chemical data are given for the eclogites which occur in the garnet kyanite micaschists of the penninic dora maira massif between brossasco isasca and martiniana italian western alps and for a sodic whiteschist associated with the pyrope coesite whiteschists of martiniana the brossasco isasca bi eclogites are fine grained foliated and often mica rich rocks with a strong preferred orientation of omphacite crystals and white micas porphyroblasts of hornblende are common in some varieties whilst zoisite and kyanite occur occasionally in pale green varieties associated with leucocratic layers with quartz jadeite and garnet these features differentiate the bi eclogites from the eclogites that occur in other continental units of the western alps which all belong to type c garnet sodic pyroxene and glaucophane are the major minerals in the sodic whiteschist sodic pyroxene in the eclogites is an omphacite often close to jd50di50 with very little acmite and virtually no aliv and impure jadeite in the leucocratic layers and in the sodic whiteschist garnet is almandine with 20 30 mol for each of the pyrope and grossular components in the eclogites and a pyrope rich variety in the sodic whiteschist white mica is a variably substituted phengite and paragonite apparently only occurs as a replacement product of kyanite amphibole is hornblende in the eclogites but the most magnesian glaucophane yet described in the sodic whiteschist quartz pseudomorphs of coesite were found occasionally in a few pyroxenes and garnets the p t conditions during the vhp event are constrained in the eclogites by reactions which define a field ranging from 27 28 kbar to 35 kbar and from 680 to 750 c these temperatures are consistent with the results of garnet pyroxene and garnet phengite geothermometry which suggest that the eclogites may have equilibrated at around 700 c in the sodic whiteschist pressures ranging from 29 to 35 kbar can be deduced from the stability of the jadeite pyrope garnet glaucophane compatibility as in the eclogites water activity must have been low such conditions are close to the p t values estimated for the early alpine recrystallization of the pyrope coesite rock and like petrographical and mineralogical features set aside the bi eclogites from the other eclogites of the western alps instead indicating a close similarity to some of the eclogite bodies occurring in the adula nappe of the central alps an important corollary is that glaucophane stability at least in na and mg rich compositions and under very high pressures may extend up to 700 c in agreement with the ht stability limit suggested by experimental studies

kinetics of the coesite to quartz transformation abstract the survival of coesite in ultrahigh pressure uhp rocks has important implications for the exhumation of subducted crustal rocks we have conducted experiments to study the mechanism and rate of the coesite quartz transformation using polycrystalline coesite aggregates fabricated by devitrifying silica glass cylinders containing 2850 h 10 6 si at 1000 c and 3 6 gpa for 24 h conditions were adjusted following synthesis to transform the samples at 700 1000 c at pressures 190 410 mpa below the quartz coesite equilibrium boundary reaction proceeds via grain boundary nucleation and interface controlled growth with characteristic reaction textures remarkably similar to those seen in natural uhp rocks we infer that the experimental reaction mechanism is identical to that in nature a prerequisite for reliable extrapolation of the rate data growth rates obtained by direct measurement differ by up to two orders of magnitude from those estimated by fitting a rate equation to the transformation time data fitting the rates to turnbull s equation for growth therefore yields two distinct sets of parameters with similar activation energies 242 or 269 kj mol but significantly different pre exponential constants extrapolation based on either set of growth rates suggests that coesite should not be preserved on geologic time scales if it reaches the quartz stability field at temperatures above 375 400 c the survival of coesite has previously been linked to its inclusion in strong phases such as garnet that can sustain a high internal pressure during decompression other factors that may play a crucial role in preservation are low fluid availability possibly even less than that of our nominally dry experiments and the development of transformation stress which inhibits nucleation and growth these issues are discussed in the context of our experiments as well as recent observations from natural rocks

geology and structure of diamond bearing rocks of the kokchetav massif kazakhstan abstract two crustal settings for microdiamond formation have been described from eclogite bearing metamorphic areas 1 the economic concentration of microdiamonds in metasedimentary gneisses and calc silicate rocks northern kazakhstan and 2 microdiamonds recently found in eclogite garnet pyroxenite and jadeitite from dabie shan mountain eastern china the latter occurrence is interpreted to be the product of ultra high pressure metamorphism in a mesozoic collision zone there are however a number of discrepancies between the geological structural and geochemical data for kazakhstan microdiamond deposits and an interpretation in terms of a deep subduction zone model the geodynamic setting of the kokchetav massif can be defined as a continental rise prism environment related to a passive continental margin where rifting predominated during early palaeozoic orogeny the kumdikol microdiamond province is closely associated with a tectonic melange zone involved in ductile to semi ductile shearing abundant in graphite microdiamonds of the kumdikol area have a dual setting in the rocks they appear to be included not only in refractory garnet and zircon but also in almost all rock forming minerals as biotite phlogopite diopside quartz and secondary sericite chlorite and sericite chlorite calcite aggregates after garnet pyroxene and plagioclase and in spite of softness of the host phases microdiamonds are well preserved on the other hand the ore body extends along the shear zone and high concentrations of the microdiamonds within it are distributed without any lithological control along the local s c surfaces of the main kumdikol strike slip shear zone this duality of microdiamond settings in absence of practically all ultra high pressure minerals except diamond itself weakens the interpretation of this occurrence in terms of very deep subduction and very fast uplift and exhumation during 15 10 ma according to recent geochronological data these microdiamonds could have been formed metastably under anisotropic stress conditions in fact the available data from the kumdikol province provides certain support for either metastable and stable hypothesis

tectonic history of subduction zones inferred from retrograde blueschist p t paths many phanerozoic convergent plate junctions are marked by discontinuous blueschist belts reflecting relatively high pressure p prograde trajectories common blueschist paragneisses such as those of the western alps exhibit widespread overprinting by greenschist and or epidote amphibolite facies assemblages for this type of high p belt retrograde metamorphism involved fairly rapid nearly isothermal decompression some terranes underwent continued heating during early stages of pressure release uplift probably occurred as a consequence of the entrance of an island arc oceanic plateau or segment of continental crust into the subduction zone collision resulting in marked deceleration or cessation of underflow and buoyant approximately adiabatic rise of the stranded recrystallized subduction complex other high p belts such as the franciscan of western california preserve metamorphic aragonite and lack a low p overprint retrogression approximately retraced the prograde p t temperature path or for early formed high grade blocks occurred at even higher p t ratios parts of this type of metamorphic belt evidently migrated slowly back up the subduction zone in response to isostatic forces during continued plate descent and refrigeration upward motion took place as tectonically imbricated slices as laminar return flow in melange zones and perhaps partly a lateral spreading extension of the underplated accretionary prism retrograde p t trajectories of high pmore belts therefore provide important constraints on the tectonic evolution of convergent plate junctions less

u pb zircon ages constrain the architecture of the ultrahigh pressure qinling dabie orogen china new shrimp and tims zircon ages 40ar 39ar ages and eclogite locations contribute significantly to our understanding of the ultrahigh pressure dabie shan 1 the geographic extent of the yangtze craton that was subducted to ultrahigh pressure extends to the northern edge of the dabie shan 2 the northern half of the dabie shan is a magmatic complex intruded over a 10 myr interval between 137 and 126 ma that accommodated 100 n s stretching of the pre existing collisional architecture 3 granitic orthogneisses and enclosing ultrahigh pressure paragneisses have indistinguishable zircon populations the population of triassic zircon ages ranges from 219 to 245 ma leading us to question the prevailing assumption that 219 ma zircons formed at ultrahigh pressure and to propose instead that they reflect late retrogression at crustal pressures following the bulk of exhumation

the granulite eclogite facies transition a comparison of experimental work and a natural occurrence in the bergen arcs western norway a unique segment of the deeper parts of the caledonian continent continent collision zone is exposed in one of the thrust sheets in the bergen arcs of western norway it demonstrates that precambrian granulite facies rocks underwent fluid controlled eclogitization on a regional scale the transition from granulite to eclogite facies is sharp it occurs over distances of centimetres to metres and follows shear zones and infiltration fronts major anastomosing shear zones typically 50 to 100 m thick with well banded eclogite facies rocks crosscut the lithological boundaries they can be followed continuously for several kilometres breccias where metre size blocks of granulite float in an eclogite matrix outcrop over areas of 0 5 km2 areas comprising more than 80 eclogite occupy several square kilometres this mixture of metamorphic facies indicates the metastable occurrence of granulite facies rocks in the lower crust of modern and ancient orogenic belts in contrast to earlier experimental work such field relations imply that the granulite eclogite facies boundary in nature can be sharp and independent of lithological boundaries a partly eclogitized lower crust of the kind exposed in the bergen arcs can explain the 7 2 7 8 km s vp layer present in many crust mantle transition zones anastomosing shear zones and areas of almost complete eclogitization represent potential lower crustal reflectors

lithospheric thermal regimes in europe abstract the heat flow pattern of europe derived from more than 8000 observations is dominated by a general northeast to southwest increase in geothermal activity which is the consequence of the tectonic evolution of the whole continent this paper reviews the history of the geothermal mapping of europe and describes the recently compiled set of geothermal maps of europe surface heat flow is correlated with the age of the last tectonothermal event with the near surface heat production and with the crustal thickness deep temperature distributions within the crust lithosphere are calculated for 1 d 2 d and simple 3 d models characteristic temperature depth curves are given for some of the major tectonic provinces and the results of various techniques are compared the value of the crustal heat contribution is assessed and a regional moho heat flow pattern is proposed the mohorovicic discontinuity is clearly not an isothermal surface neither is the heat flow from below the crust constant

coesite in subducted continental crust p t history deduced from an elastic model abstract coesite has been found in a metasedimentary rock in a continental unit of the western alps dora maira massif the rock preserves the high pressure low temperature assemblage of nearly pure pyrope talc phengite kyanite in a quartz matrix coesite exclusively occurs as inclusions in pyrope and is partly inverted to quartz textural observations and mechanical considerations suggest that the rock crystallised in the coesite stability field a simple elastic model for a coesite inclusion in pyrope explains why coesite has been exclusively preserved in the inclusions and not in the matrix combined with petrological data the model constrains the crystallisation conditions of the pyrope coesite rock p 28kbar 650 this finding has an important bearing on the significance of p t conditions estimated from minerals occurring as inclusions and on the actual behaviour of continental crust in collision zones

processing of crust in the root of the caledonian continental collision zone the role of eclogitization segments of the root to the caledonian collision zone are exposed along the west coast of norway allowing the study of processes and petrophysical conditions at the deepest crustal levels p t conditions prevailing in these crustal root zones correspond to eclogite facies eclogitization is associated with marked changes in petrophysical properties notably a density 10 15 and vp increase which may give a mantle signature to an eclogitized crust the observed ductility enhancement may be caused by transformation plasticity reduction in grain size formation of new mineral assemblages and presence of fluid in the western gneiss region and the bergen arcs eclogites form along shear zones interpreted as fluid pathways with sharp boundaries to their metastable protolith this makes eclogite shear zones potential deep crustal reflectors dry crust subducted into roots of continental collision zones will undergo eclogitization only if hydrous fluids are available timing of metamorphic reactions does not depend on crossing of equilibrium boundaries in the p t space but rather on the introduction of fluids into the system eclogites are not only passive recorders of p t t paths illustrating very deep subduction of crust 100 km rapid changes in petrophysical properties make fluid induced eclogitization a dynamic process that influences geodynamics stresses set up by the volume changes or by tectonic forces are released by ductile deformation along shear zones and by seismic faulting as evidenced by syn eclogitic pseudotachylytes in the metastable protolith deformation including seismicity enhances fluid infiltration and further eclogitization dense and rheologically weak eclogites may delaminate and sink into the mantle in the absence of fluids crustal doubling may occur without metamorphic re equilibration the evolution of a collision zone depends on the fluid regime which should be considered in geodynamic modelling in addition to p and t

heat flow heat production and thermo tectonic setting in mainland uk new heat flow data for the united kingdom together with additional heat flow and heat production determinations for caledonian age granites have led to a revision of the uk heat flow map and a re examination of the relationship between heat flow q 0 and heat production a 0 for granites and basement rocks previously recognized broad belts of above average heat flow are now resolved into separate zones which reflect to a greater extent the geological structure and tectonic history of the uk the zones of highest heat flow are spatially associated with voluminous high heat production granitoid batholiths in sw england northern england and the eastern highlands of scotland a single linear correlation between q 0 and a 0 is no longer tenable and an analysis in terms of broad heat flow provinces each with a characteristic upper crustal heat production distribution and deep heat flow contribution is also considered to be an oversimplification on the q 0 a 0 plot the data form four separate clusters three corresponding to the granite batholiths in sw england northern england and the eastern highlands of scotland and the fourth to the basement rocks of central england and wales an explanation of the q 0 a 0 data is proposed in terms of the crustal structure and thermo tectonic setting of each area in the case of the granite batholiths the data reflect the contrasting depth extent and radioelement depth functions of the intrusions these parameters in turn are related to the magmatic evolution and emplacement history of each batholith and the nature of the crust into which they were emplaced

delamination in collisional orogens during continental collision the weight of the downgoing slab may lead to extensional shearing in the descending plate permitting separation of the downdip dense oceanic lithosphere from the adjacent lighter continental lithospher separation may occur by means of a lithosphere penetrating normal sense shear zone by means of pure shear necking of the lithosphere or by some combination of these mechanisms during separation the continental crust attached to the lower plate may undergo an episode of extensional tectonics the proximity of asthenosphere to crust may result in syntectonic regional metamorphism and plutonism if the continents continue to converge following delamination a compressive tectonic regime will be reestablished in the crust the subsequent cooling of asthenosphere at the base of the crust may lead to the establishment of a continuous flat moho the likely imbrication of crustal and mantle lithologies during delamination and during the subsequent compressive tectonic regime as well as underplating associated with syntectonic plutonism may account for the seismic complexities observed at moho depths beneath some mountain belts

a thermal tectonic model for high pressure rocks in the basal gneiss complex of western norway abstract the basal gneiss complex bgc of western norway is a segment of continental crust that was subjected to eclogite facies metamorphism during the caledonian orogeny and then was overprinted by amphibolite facies conditions numerical methods have been used to construct a model for thermal evolution of the bgc the calculated temperature depth time tzt paths for the bgc are in good agreement with the sequence of mineral assemblages that occur in metamorphosed mafic and ultramafic rocks however the thermal model indicates that retrograde zoning of minerals in the garnet bearing assemblages of eclogite and ultramafite may have developed metastably at pressures below 10 kbar numerical modeling of iron diffusion in garnet grains adjacent to olivine inclusions was used to calculate zoning profiles based upon the thermal model the calculated zoning profiles have shorter decay distances than the observed zoning profiles which may be due to uncertainties in the diffusion coefficients also discrepancies occur between the tzt paths and specific pt values calculated for eclogite and garnet ultramafite due either to the preservation of pre caledonian mineral assemblages or to utilization of geobarometers and thermometers which are based on different elements in different phases such as a1 in orthopyroxene and fe mg in olivine and garnet

metallogenesis in the french part of the variscan orogen part i u preconcentrations in pre variscan and variscan formations a comparison with sn w and au abstract in the french part of the variscan orogen the pre variscan metamorphic rocks and the variscan granites resulting from pure crustal recycling are enriched in uranium and tin data on tungsten and gold are too scarce for a reasonable estimation of the average concentrations of these elements to be obtained the spatial distribution of uranium preconcentrations is transverse to the main variscan structural domains and the distribution of the sn w mineralizations prompts the same conclusions u sn w and au metal enrichment in crustal domains in europe seems to result from late proterozoic to early palaeozoic basic and acid magmatism for uranium at least two main stages of partial melting early palaeozoic and late carboniferous lead to enrichment of the metal in peraluminous leucogranites these leucogranites represent the main source of late variscan uranium and tin within the peraluminous granitic complexes the main metal source is related to the late intrusion of small granitic cupolas strongly enriched in metals in the case of uranium the efficiency of the hydrothermal remobilization depends on the proportion of metal located in uraninite which is easily leachable by hydrothermal solutions the emplacement of these cupolas is structurally controlled in well defined areas by large shear zones these shear zones are continuously active during the successive emplacement of the granitic magmas most of the hydrothermal uranium mineralization is related to the permian reactivation of the shear zones in a brittle stage these zones then channel the hydrothermal fluids involved in the last metal concentration step leading to the formation of the orebodies the efficiency of metal remobilization is dependent on a close spatial relationship between magmatic structures enriched in uranium and brittle structures channeling the hydrothermal fluids

heat flow heat production and granite depth in western france newly measured heat flow and full heat production determinations over the armorican massif western france are tested against a linear relationship as proposed through birch s law the data do not fit a strict linear relationship but are within an elongated cluster which can be approximated in mw m and km by these parameters are compared with those of surrounding hercynian regions data from the central massif and previously failed to fit any linear relationship data for scotland and southern england could not be fitted to a single line however the present data and those from southern and central england do fit a single line suggesting that the three regions form a single geothermal province the depth parameter is greater than the root depths of the granitic plutons indicating that heat sources can exist at depths below the granitic intrusions the deep crustal structure revealed from seismic studies agrees with the radiogenic elements distribution deduced from thermal data

what brought them up exhumation of the dabie shan ultrahigh pressure rocks metamorphic coesite and diamond in the dabie shan eastern china testify to subduction of continental crust to 100 km depth exhumation of these ultrahigh pressure rocks through the crust encompassed two stages 1 south dipping foliation southeast plunging stretching lineation lineation parallel isoclinal folds and boudins indicate extreme subhorizontal shortening and subvertical extension during top to northwest shearing at 200 180 ma syntectonic recrystallization occurred at eclogite and amphibolite facies temperatures and at pressures below coesite stability 2 northwest southeast subhorizontal extension from 133 to 122 ma was concentrated within an asymmetric structural dome in a magmatic complex that forms the northern half of the dabie shan pluton cores at deep structural levels have weak hypersolidus fabrics and pluton carapaces are mylonitic gneisses formed at upper amphibolite facies conditions deformation is concentrated in greenschist facies mylonites and ultramylonites along the xiaotian mozitang detachment fault at the northern topographic limit of the dabie shan our preferred exhumation model involves two stages triassic indentation vertical extrusion and erosion followed by cretaceous plate margin transtension

35 ma old ultrahigh pressure metamorphism and evidence for very rapid exhumation in the dora maira massif western alps abstract ion microprobe shrimp data on zircons from various rock types of an ultrahigh pressure uhp metamorphic whiteschist type pyrope quartzite lens of the dora maira massif dmm consistently show domains giving a late eocene age of 35 4 1 0 ma which is taken as the age of uhp metamorphism these domains partially replace the older oscillatory zoning pattern of the zircons formed during primary magmatic crystallization at about 275 ma zircons of a metagranitic country rock next to the uhp metamorphic lens have these primary features best preserved all zircons measured also yield intermediate ages between 275 and 35 ma with a statistical concentration between 260 and 210 ma thus the uniformity of the initial zircon population both in the lens and the country rock evidences a common protolith that is a granite intruded during the late herynian while the intermediate ages are at least partly due to incomplete resetting of the zircons during uhp metamorphism those in the 260 210 ma range may be related to rifting episodes in the permotriassic the mg rich chemistry of the whiteschist lenses is due to local metasomatic alterations of the granite perhaps by fluids derived from evaporitic sediments dating as early as the permotriassic as well the more pervasive resetting of zircon ages during uhp metamorphism in the pyrope quartzite lenses is explained by the ubiquity of fluids and or melts produced during subduction by a series of dehydration reactions that occurred only in the more hydrous mg rich protoliths and not in their drier granitic neighborhood fission track ages determined partly on the same zircon samples yielding 29 9 1 4 ma mark the time when the uhpm unit had reached about 290 c at a shallow location within the crust thus exhumation over a vertical distance of about 120 km must have occurred within about 5 6 ma indicating an average uplift rate of about 20 24 km ma and an average cooling rate of about 85 100 c ma the radiometric data obtained do not lend any support to an eo alpine cretaceous subduction event so that deep subduction and immediately following exhumation all took place during early to mid tertiary time this scenario seems to apply to large portions of the western and central alps as well calling for drastic geodynamic reinterpretations of these parts of the alps

pb sr nd isotopic behavior of deeply subducted crustal rocks from the dora maira massif western alps italy ii what is the age of the ultrahigh pressure metamorphism pb nd and sr isotope data are reported from two localities on mineral separates from mg rich metapelites and associated rocks that have been subducted to depths of at least 100 km for which metamorphic conditions are estimated at 28 33 kilobars pressure and 700 800 c and then returned to the surface initial isotope ratio data from the granitoid country rock are similar to those found in the metapelites the initial ratios indicate predominantly recycled aged granitic crustal materials for the sources of all of the samples five zircon samples 4 from pyrope megacrysts and 1 from fine grained pyrope quartzite lenses in the metapelites accurately define a chord yielding intercept ages of 304 10 and 38 0 1 4 ma in a concordia diagram zircon from the country rock also plots along the chord the zircon data together with initial nd and sr data indicate that the sedimentary sources of the rocks were derived mainly or entirely from sialic hercynian rocks ellenbergerite from pyrope megacrysts and monazite from the fine grained ground mass yield slightly younger ages of 30 34 ma apparently reflecting lower blocking temperatures than that of zircon sm nd data from a pyrope megacryst give an errorchron corresponding to an age of 38 ma in agreement with the zircon date a major question concerns the timing of the ultrahigh pressure metamorphism experimental data suggest that pyrope and quartz coesite as well as ellenbergerite formed by various metamorphic reactions if as generally assumed the ultrahigh pressure metamorphism occurred ca 100 ma ago our data require that the zircon did not experience measurable lead loss at that time but lost major amounts of lead 38 ma ago during late alpine metamorphism estimates of diffusion rates for nd in pyrope further suggest that the apparent sm nd age of 38 ma for the megacryst is not consistent with that model those problems are resolved if the ultrahigh pressure metamorphism occurred 38 40 ma ago but problems remain from ar ar dates of 100 ma on phengite an inferred 120 ma age for zircon lead loss from another study and possibly by the very rapid uplift required if the metamorphism is that young

diamond inclusions in garnets from metamorphic rocks a new environment for diamond formation diamonds commonly occur in kimberlites lamproites and alluvial sediments derived from these rocks more recently diamonds or their graphite pseudomorphs have been discovered in ultramafic massifs1 and picrites2 here we report the occurrence of diamonds in situ in crustal rocks highly retrograded high pressure metamorphic garnet pyroxene and pyroxene carbonate garnet rocks biotite gneisses and schists from the kokchetav massif northern kazakhstan ussr the diamonds are cubo octahedral averaging 12 m in size and occur in zircons and with euhedral graphite as inclusions in unzoned garnets we believe that the zircon and garnet matrices protected these diamonds from retrogressive transformation to graphite mica rutile titanite clinopyroxene kyanite and zircon also occur as inclusions in garnet often intergrown with the diamonds equilibration relations of inclusions and host garnets indicate that both diamonds and graphite crystallized from a fluid phase under static conditions at pressures of 40 kbar and temperatures 900 1 000 c

exhumation of high pressure rocks a review of concepts and processes the exhumation of high pressure metamorphic rocks requires either the removal of the overburden that caused the high pressures or the transport of the metamorphic rocks through the overburden exhumation cannot be achieved simply by thrusting or strike slip faulting it may be caused by erosion of shortened and thickened crust but this is unlikely to be the only mechanism for exhuming rocks from depths greater than about 20 km one or more of the following additional mechanisms may be involved 1 corner flow of low viscosity material trapped between the upper and lower plates in a subduction zone can cause upward flow of deeply buried rock and may explain some occurrences of high pressure tectonic blocks in melange this process does not however appear to be adequate to explain the exhumation of regional high pressure terrains 2 buoyancy forces acting directly on metamorphic rock bodies may cause them to rise relative to more dense surroundings this is likely to be the most important mechanism of exhumation of crustal rocks subducted into the mantle but cannot explain the emplacement of coherent tracts of high density metamorphic rock into shallow crustal levels some high pressure blocks emplaced at shallow levels in accretionary terrains may have been entrained in diapiric intrusions of low density mud or serpentinite 3 extension driven by the forces associated with contrasts in surface elevation may explain the exhumation and structural setting of many high pressure terrains extension may occur in the upper part of an accretionary wedge thickened by underplating or it may affect the whole lithosphere in a region of intracontinental convergence if surface elevation has been increased by the removal of a lithospheric root in the second case extension may be accompanied by magmatism and an evolution towards higher temperature during decompression of the metamorphic terrain

ages of ultrahigh pressure metamorphism and protolith orthogneisses from the eastern dabie shan u pb zircon geochronology the dabie shan contains two rare geologic features one the occurrence of ultra high pressure metamorphic rocks peak p 2 7 gpa uhp in the east central part of this belt and two some of these uhp rocks are characterized by anomalously 18o depleted oxygen isotopic compositions geochronological data are presented that constrain the age of the uhp metamorphism the ages of protoliths and potentially the age of hydrothermal alteration believed to be responsible for the anomalous oxygen isotopic compositions the age of uhp metamorphism in the eastern dabie shan is dated at 218 5 1 7 ma using u pb in zircon separated from host gneisses this age is identical within uncertainty with a less precise age using u pb in zircon separated from uhp eclogite 225 5 3 6 ma and with previously published ages from uhp eclogites of the dabie shan map relationships strongly support an in situ as opposed to exotic orgin of the uhp eclogites in this region the identical ages of uhp metamorphism in the gneisses and eclogites provides additional support for this interpretation the age of the protolith gneisses is dated as 772 5 9 5 ma based on the upper intercept of the zircons which accords well with other dates of granitic gneisses in the qinling belt the association of mafic and felsic magmatism is interpreted to reflect rifting along the northern margin of the yangtze block at this time these upper intercept ages may date the hydrothermal circulation of 18o depleted meteoric water which gave rise to the anomalously 18o depleted uhp protoliths of the dabie shan and sulu region the coincidence of these protolith ages with the early sinian glaciation of south china is consonant with the suggestion that the highly anomalous oxygen isotopic compositions in this region date from this time the upper intercept age of the eclogite is imprecisely determined at 447 82 79 ma but is younger than the host gneisses and is not characterized by anomalous oxygen isotopic compositions

coesite in the straumen kyanite eclogite pod norway the occurrence of coesite in the straumen kyaniteeclogite pod in the western gneiss region wgr norway is described although the microtextures of coesite and of its partial or complete transformation into quartz are similar to those described at the first coesite occurrence in norway the grytting dolomite eclogite pod there are some significant differences at this second locality these include the host mineral of the sio2 inclusions being garnet instead of clinopyroxene and also occasionally kyanite an initial coesite eclogite sub facies paragenesis was followed by a quartz eclogite sub facies paragenesis which in addition to rutile and dolomite or calcite contains the highest number 7 of silicate phases known in any norwegian eclogite garnet clinopyroxene kyanite quartz phengite zoisite and clinoamphibole this second occurrence of definite coesite i consolidates the very high pressury estimates about 30 kbar minimum previously deduced for certain eclogite pods occurring in the wgr and ii justifies the recognition of the norwegian coesite eclogite province constituted of a nappe terrane of about 30 kbar parageneses now residing in the wgr

dynamics of sediment subduction accretion at convergent margins short term modes long term deformation and tectonic implications a conceptual model is proposed for the short term operation of subduction accretion systems at convergent margins the operation is described in terms of the mass fluxes among the four system components namely the accretionary or prowedge wedge p an uplifted plug u a retrowedge r and the subduction zone the latter consists of a conduit c of slowly moving and deforming material and an underlying subduction channel where material moves and deforms more rapidly a broad range of convergent margin types can be accommodated within the conceptual model framework with differing short term modes that depend on which of the p u r c components that are active finite element geodynamical model experiments are used to investigate whether these modes are dynamically feasible these experiments determine excitation of modes under controls that include model entry and exit mass fluxes flexural loading by accreted material and slab pull forces boundary velocities corresponding to subduction and subduction zone advance retreat distribution thickness and density of accreted sediment and internal frictional properties of the coulomb sediments that all modes with the exception of p in which only a prowedge is created are seen in the models provides support for the concepts and allows conditions that favor particular modes to be recognized in addition the finite deformation predicted by the models is used to infer the long term tectonic styles associated with both single mode evolutions and those that exhibit mode switching a steady state mass balance can occur for modes 0 pure subduction p c i e where components p and c are active or other modes when the surface mass denudation plus the subduction flux balances accretion plus tectonic underplating the p c mode is favored by subduction zone retreat and or an increasing subduction load and creates an apparent landward dipping backstop p u c is predicted to be the most common mode and it creates an apparent seaward dipping backstop p u r c is an inefficient mode but is favored by easily detached retrocrust or sediment and or a decreasing subduction load

exhumation of coesite bearing rocks in the dora maira massif western alps italy coesite bearing very high pressure rocks in the southern dora maira massif western alps italy emerged from a depth of 100 km during cretaceous time these rocks are now covered only by a thin fraction of the synmetamorphic overburden tectonic contacts above the coesite bearing rocks are retrograde ductile shear zones that operated with a normal sense of motion they cut out thick crustal sections from within the tectonometamorphic pile and caused the condensation of the eoalpine baric and thermal structure the result of this was the lateral dispersion of the eoalpine orogenic wedge and the facilitation of the unroofing of the very high p rocks extensional unroofing is consistent with the absence of deeply cutting cretaceous erosion and with pressure temperature paths that show that the very high p rocks cooled during decompression the very high p rocks reached relatively shallow crustal levels by ca 40 ma and the attenuated sequence overthrust a more external unit these indicate that the exhumation of the very high p rocks took place within an overall plate onvergence setting prior to the final suturing of europe and adria

the pressure path of solid inclusions in minerals the retention of coesite inclusions during uplift abstract an inclusion model allows calculation of the stresses in and around minerals included in other minerals during changes in pressure and temperature the equations are applied to illustrate cooling and uplift histories of quarzo feldspathic rocks from 500 c and various pressures to ambient conditions even in the absence of pore fluid pressure microfractures may open at external pressures of 200 400 mpa and temperatures of 200 400 c due to differential volume changes of the constituent minerals coesite included in garnet cannot have formed during progressive metamorphism from quartz at lithostatic pressures below the coesite stability field because of differential volume changes coesite inclusions are captured by their host minerals at ultra high pressures and they persist to lower pressures because of the large volume increase occurring at the coesite to quartz transition the pi t path followed by the sio2 inclusion traces the boundary between the stability fields of coesite and quartz until radial fractures develop in the host at low external pressure

seismic consequences of warm versus cool subduction metamorphism examples from southwest and northeast japan warm and cool subduction zones exhibit differences in seismicity seismic structure and arc magmatism which reflect differences in metamorphic reactions occurring in subducting oceanic crust in southwest japan arc volcanism is sparse and intraslab earthquakes extend to 65 kilometers depth in northeast japan arc volcanism is more common and intraslab earthquakes reach 200 kilometers depth thermal petrologic models predict that oceanic crust subducting beneath southwest japan is 300 to 500 c warmer than beneath northeast japan resulting in shallower eclogite transformation and slab dehydration reactions and possible slab melting