metamorphism at the base of the samail ophiolite southeastern oman mountains metamorphic rocks showing an inverted metamorphic zonation from upper amphibolite facies to greenschist facies occur beneath the samail ophiolite the metamorphic protolith was a section of marine basalts mn rich cherts and argillites garnet clinopyroxene amphibolites occur within 2 m of the contact hornblende clinopyroxene assemblages occur within 80 m of the contact and lower amphibolite facies rocks persist to 130 m from the contact the higher grade rocks show polyphase deformation cataclastic fabrics and partial retrogression to greenschist facies assemblages however lower grade phyllites show evidence of only one episode of metamorphic crystallization unmetamorphosed sediments in contact with the basal metamorphic sheet do not show evidence of polyphase deformation the distribution coefficient kd feo mgo gar feo mgo cpx ranges from 4 4 to 5 3 suggesting a t range of 755 865 c at 200 mpa low jadeite content in clinopyroxene and low glaucophane content of ca amphiboles are consistent with moderate pressures of crystallization the metamorphic sheet is inferred to be of composite origin the highest grade rocks were tectonically transported the greatest distance and the lower grade rocks were successively incorporated into the sheet as the peridotite was thrust over progressively cooler rocks residual heat from the ophiolite is the dominant heat source for the metamorphism and this could have been augmented by a limited amount of frictional heat generated during thrusting these thermal constraints suggest that the metamorphism occurred early in the tectonic transport history of the samail ophiolite

birth death and resurrection the life cycle of suprasubduction zone ophiolites 1 suprasubduction zone ssz ophiolites display a consistent sequence of events during their formation and evolution that suggests that they form in response to processes that are common to all such ophiolites this sequence includes the following 1 birth which entails the formation of the ophiolite above a nascent or reconfigured subduction zone this stage is typically characterized by the eruption of arc tholeiite lavas and the formation of layered gabbros and sheeted dike complex 2 youth during which is continued melting of refractory asthenosphere depleted during birth occurs in response to fluid flux from the subducting slab with extensional deformation of the older plutonic suite eruption of refractory lavas and the intrusion of wehrlite pyroxenite 3 maturity with the onset of semistable arc volcanism typically calc alkaline as the subduction zone matures and stabilizes and the intrusion of quartz diorite and eruption of silicic lavas and 4 death which is the sudden demise of active spreading and ophiolite related volcanism which in many cases is linked to collision with an active spreading center and the onset of shallow underthrusting of the buoyant spreading axis expressed as dikes and lavas with oceanic basalt compositions that crosscut or overlie rocks of the older suites 5 resurrection with emplacement by obduction onto a passive margin or accretionary uplift with continued subduction the early stages 1 3 may be diachronous and each stage may overlap in both time and space the existence of this consistent progression implies that ophiolite formation is not a stochastic event but is a natural consequence of the ssz tectonic setting

model for the origin of the troodos massif cyprus and other mideast ophiolites any comprehensive model for the origin of the troodos complex and other mideast ophiolite complexes must explain the geochemical evidence for subduction zone involvement the thin inferred oceanic crust the extensional environment indicated by sheeted dikes the existence of fault zones perpendicular to the inferred spreading axes and the discontinuous nature of ophiolite exposures around the arabian block the andaman sea region of the indian ocean may provide an actualistic model for the origin of these ophiolites there spreading takes place in short segments above a subduction zone in the region of active spreading the andaman sea model substantially accounts for the known geologic and geophysical data from the circum arabian ophiolite belt and it leads to some predictions about the structural and geophysical relationships to be expected in this important belt the model also predicts that andaman sea spreading center magmas may be similar in composition to those found in the mideast ophiolites

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

metamorphic p t paths from pelitic schists and greenstones from the south west tauern window eastern alps petrological data from intercalated pelitic schists and greenstones are used to construct a pressure temperature path followed by the upper schieferhulle ush series during progressive metamorphism and uplift in the south west tauern window italy pseudomorphs of al epidote fe epidote albite oligoclase chlorite after lawsonite and data on amphibole crystal chemistry indicate early metamorphism in the lawsonite albite chlorite subfacies of the blueschist facies at p 7 8 kbar geothermometry and geobarometry yield conditions of final equilibration of the matrix assemblage of 475 25 c 5 6 kbar calculations with plagioclase and phengite inclusions in garnet indicate early garnet growth at pressures of 7 5 kbar garnet zoning patterns are complex and reversals in zoning can be correlated between samples thermodynamic modelling of these zoning profiles implies garnet growth in response to four distinct phases of tectonic activity fluid inclusion data from coexisting immiscible h2o co2 nacl fluids constrain the uplift path to have passed through temperatures of 380 30 c at 1 3 0 2 kbar there is no evidence for metamorphism of ush at pressures greater than 7 5 kbar in this area of the tauern window this is in contrast to pressures of 10 kbar recorded in the lower schieferhulle only 2 3 km across strike a history of differential uplift and thinning of the intervening section during metamorphism is necessary to reconcile the p t data obtained from these adjacent tectonic units

interlayered eclogites blueschists and epidote amphibolites from ne oman a record of protolith compositional control and limited fluid infiltration abbreviations ab albite ac acmite act actinolite alm aleclogites blueschists clinopyroxene rich rocks and epidote ammandine aln allanite ap apatite aug augite bar barroisite bt biotite phibolites occur interlayered on a millimetre to decimetre scale in a cc calcite chl chlorite cpx clinopyroxene crs crossite czo clinozoisite mafic unit interbedded with calcareous and quartz mica schists in dol dolomite ep epidote fe ox fe oxides gln glaucophane gross ne oman these rocks constitute part of the continental margin grossularite gt garnet hm haematite jd jadeite kln kaolinite kspar of oman subducted during the cretaceous petrological data indicate k feldspar mgt magnetite oxychl oxychlorite pg paragonite ph that they evolved through a clockwise p t path and attained peak phengite qz quartz pyrp pyrope rt rutile spess spessartine tc talc conditions at t 560 c p 12 kbar where the mineral ttn titanite win winchite zrn zircon assemblages of the blueschists eclogites and clinopyroxene rich rocks were all stable whole rock major trace and rare earth element data indicate that the mineralogical differences between these rock types resulted from the metamorphism of interlayered basaltic flows introduction and tuffs of variable compositions possibly reflecting different degrees interlayered blueschists eclogites and or greenschists are of magmatic differentiation the interlayered epidote amphibolites common in several areas such as cazadero california are chemically similar to the blueschists and appear to have formed e g oh et al 1991 the shuksan suite washington from the eclogites and blueschists during unroofing aided by fluid state dungan et al 1983 owen 1989 the tauern infiltration oxygen isotope data for minerals and whole rocks window eastern alps e g selverstone et al 1992 and indicate low fluid rock ratios during metamorphism and unroofing several cycladic islands e g okrusch brocker 1990 the data also suggest that different layers may have interacted with this interlayering has been attributed to 1 differences fluids of different isotopic signatures throughout their history these in the p t conditions of equilibration in the different results are similar to data reported for other areas of continent layers e g reinsch 1979 ridley 1984 2 original continent collision or continental crust subduction differences in the chemical compositions of the layers before metamorphism e g dungan et al 1983 oh et al 1991 or 3 differences in the extent of retrograde overprinting as a result of variable fluid infiltration e g schliestedt matthews 1987 brocker 1990 barrientos selverstone 1993

geologic section through the samail ophiolite and associated rocks along a muscat ibra transect southeastern oman mountains regional mapping at a 1 60 000 scale of a 30 km strip from the gulf of oman muscat across the oman mountains 130 km to the south provides the geologic setting for the 95 m y ibra section of the samail ophiolite where best preserved the ibra ophiolite section is an 8 km thick section of oceanic consisting of 0 5 km of pillow lavas 1 2 1 6 km of sheeted diabase dike complex 0 2 1 0 km of high level noncumulate gabbro and 3 0 5 0 km of cumulate gabbro that is underlain by tectonite peridotite 9 12 km thick the ibra section is found on the southward dipping limb of the sayah hatat antiform the tectonite peridotite represents uniformly depleted harzburgite and dunite that have been deformed by high temperature low stress asthenospheric flow discordant dunites within the tectonite peridotite appear to represent either flow crystallization products from primary picritic liquids or reaction products of these liquids with the harzburgite the structural base of the tectonite peridotite is overprinted by a high stress low temperature deformation that can be related to its oceanic detachment the layered gabbros are predominantly olivine clinopyroxene plagioclase cumulates and orthopyroxene does not occur as a cumulus phase occurrence of cumulate wehrlites and picrites at high stratigraphic levels within the layered gabbros is evidence that the gabbroic section crystallized predominantly from the bottom upward in a periodically replenished magma chamber high level gabbro represents remnants of crystallization at the roof of the magma chamber and intrudes most overlying diabase dikes both the diabase dike complex and pillow lavas are hydrothermally altered and alteration and metamorphism increase downward zeolite to epidote amphibolite facies in spite of pervasive alteration relict primary mineralogy and bulk chemistry suggest that the diabase dikes and pillow lavas are cogenetic with the underlying gabbros the present day samail thrust surface truncates ophiolite stratigraphy and puts the samail ophiolite on top of unmetamorphosed hawasina melange the last motion on this surface was probably no older than maestrichtian 70 65 m y garnet amphibolites exposed as remnants of earlier thrusting 90 m y record initial ophiolite detachment at a 14 20 km depth within the tethyan oceanic lithosphere the hawasina group which underlays the samail ophiolite is a block melange where exposed near muscat in the north and it grades into an imbricated broken formation at the southern limit of the map area the hawasina group is thrust over permian to late cretaceous shelf carbonates that represent autochthonous arabian continental shelf deposits recent post miocene collapse and dome structures such as the ibra dome have complicated ophiolite stratigraphy south of jabal dimh our geologic studies strongly indicate the samail ophiolite represents a large coherent slab of transported oceanic lithosphere formed at a late cretaceous spreading center in the tethyan sea

k ar ages of metamorphic rocks at the base of the samail ophiolite oman hornblendes from amphiobolities in the sheet of metamorphic rocks beneath the peridotite member of the samail ophiolite and phyllites farther from the peridotite contact have weighted mean sup 40 ar sup 39 ar total fusion ages of 90 0 3 0 m y and 79 5 3 0 m y respectively the amphibolities represent the first tectonic slice welded to the base of the samail ophiolite after it was detached from the tethyan oceanic crust formation of the amphiobolities occurred no more than 3 to 7 m y after crystallization of plagiogranite in the ophiolite the phyllites represent another tectonic slice of ocean floor sediments welded to the ophiolite as it was transported further from the tethyan spreading axis the k ar ages suggest assuming a half spreading rate of 2 to 5 cm yr that detachment of the samail ophiolite and formation of amphibolite facies rocks occurred no more than 60 to 350 km from spreading center using the same spreading rate one can calculate a minimum half width of 300 to 750 km for the tethyan ocean during the late cretaceous

petrology of fe mg carpholite bearing metasediments from ne oman abstract fe mg carpholite occurs in metasediments of tectonically disrupted basement shelf and foreland basin units that structurally underlie the semail ophiolite in ne oman in the lower grade structurally higher units fe rich carpholite coexists with paragonite quartz illite kaolinite and chlorite whereas in deeper units fe mg carpholite occurs with pyrophyllite sudoite phengite and or chloritoid mineral compositions in these units indicate that chlorite is more magnesian than coexisting fe mg carpholite at low temperatures and pressures but at higher metamorphic grades xmg decreases in the order sudoite carpholite chlorite chloritoid this suggests a reversal in fe mg partitioning between fe mg carpholite and chlorite at temperatures below or close to those of the breakdown of kaolinite quartz to pyrophyllite and at xmg 0 35 phase relations and mineral equilibria indicate that the p t conditions of formation of the fe mg carpholite bearing rocks of ne oman range from 280 315 c 3 6 kbar for the structurally highest units to 325 440 c 6 9 5 kbar for the deepest units indicating a systematic down section increase in metamorphic grade textural relations in these rocks interpreted in the context of pertinent equilibria are consistent with the clockwise p t paths previously constrained for these units from petrological studies of interlayered isofacial mafic rocks

optical determination of topotactic aragonite calcite growth kinetics metamorphic implications textures produced by the partial recrystallization of aragonite to calcite in the blueschist facies show that heterogeneous topotactic nucleation is rapid enough to permit growth kinetics to dominate the overall geologic rate of transformation both grain boundary velocities measured microscopically between 375 c and 455 c and analysis of topotactic relations predict a growth mechanism involving only small atomic displacements near a highly coherent boundary between the two phases the kinetic equation of turnbull for interface controlled growth based on that mechanism accurately describes the experimental results when calculated rates are integrated over geologically plausible pressure temperature time paths for convergent plate margins even the most extreme values lead to the conclusion that aragonite bearing rocks enroute to the surface must enter the calcite stability field between 125 c and 175 c the widespread occurrence of aragonite along the pacific margin of north america and by contrast

fluid rock interaction is decisive for the formation of tungsten deposits tungsten mineralization is typically associated with reduced granitic magmas of crustal origin while this type of magmatism is widespread economic tungsten deposits are highly localized with 90 produced from only three countries worldwide therefore the occurrence of reduced magmatism while necessary for tungsten enrichment seems to be insufficient to form such rare deposits here we explore the mechanisms that lead to wolframite precipitation and evaluate whether they may exert a decisive control on tungsten global distribution tungsten differs from other rare metals enriched in magmatic hydrothermal ore deposits because it is transported as an anionic species precipitation of the main tungstate minerals scheelite cawo 4 and wolframite fe mn wo 4 thus depends on the availability of calcium iron or manganese we demonstrate quantitatively that magmatic fluids at panasqueira portugal provide tungsten in solution whereas the host rock contributes the iron required to precipitate wolframite the combination of special source conditions with specific reactive host rocks explains why major wolframite deposits are rare and confined to a few ore provinces globally

a recalibration of the chlorite biotite muscovite geobarometer massonne 1981 reported experiments on the composition of muscovite in the assemblage phlogopite muscovite k feldspar quart the experimental data have been used to recalculate the equilibrium conditions for the chlorite muscovite biotite quartz assemblage which has been proposed as a new geobarometer by powell and evans 1983 the recalculated equilibrium shows much steeperdp dt slopes than those calculated by powell and evans 1983 nevertheless with the necessary precautions it is suggested that the barometer may provide the critical user with valuable pressure estimates

experimental investigations of blueschist greenschist transition equilibria pressure dependence ofal2o3 contents in sodic amphiboles a new geobarometer

transpressional folding and associated cross fold jointing controlling the geometry of post orogenic vein type w sn mineralization examples from minas da panasqueira portugal the world class w sn panasqueira deposit consists of an extensive subhorizontal vein swarm peripheral to a late orogenic greisen cupola the vein swarm consists of hundreds of co planar quartz veins that are overlapping and connected laterally over large distances various segmentation structures a local zigzag geometry and the occurrence of straight propagation paths indicate that they exploited a regional joint system a detailed orientation analysis of the systematic joints reveals a geometrical relationship with the subvertical f2 fold generation reflecting late variscan transpression the joints are consistently orthogonal to the steeply plunging s0 s2 intersection lineation both on the regional and the outcrop scale and are thus defined as cross fold or ac joints the joint system developed during the waning stages of the variscan orogeny when already uplifted to an upper crustal level veining reactivated these cross fold joints under the conditions of hydraulic overpressures and low differential stress the consistent subperpendicular orientation of the veins relative to the non cylindrical f2 hinge lines also when having an inclined attitude demonstrates that veining did not occur during far field horizontal compression vein orientation is determined by local stress states variable on a meter scale but with the minimum principal stress consistently subparallel to fold hinge lines the conspicuous subhorizontal attitude of the panasqueira vein swarm is thus dictated by the geometry of late orogenic folds which developed synchronous with oroclinal buckling of the ibero armorican arc

structural evolution of an a type subduction zone lofoten rombak area northern scandinavian caledonides the climactic phase of the caledonian orogeny in scandinavia involved collision of the baltic and greenland cratons in late ordovician to silurian time we infer that the collision resulted in large scale subduction of the leading edge of the baltic craton beneath an andean type greenland continental margin deep erosional levels in the lofoten rombak area norway sweden 69 n permit direct examination and evaluation of the structural evolution of a continental or a type subduction zone the early stages of collision involved en bloc underthrusting of the continental margin to depths of at least 30 km during this process deformational effects in the lower plate were restricted to its upper few hundred meters ultimately en bloc a type subduction should be limted by buoyancy effects as convergence continued the lower plate became involved in a series of imbricate thrusts with the same vergence as the intial subduction zone the temporal and spatial relationships between these basement involved thrusts are similar to those observed in foreland fold and thrust terrains field and geophysical data bearing on lower plate behavior elsewhere in the caledonldes and in other collisional orogens suggest that the structural features observed in the lofoten rombak area may be generally characteristic of a type subduction in continent continent collisional settings

tectonics magmatism hydrothermalism and sets of flat joints locally filled by snw aplite pegmatite and quartz veins southeastern border of the serra de estrela granitic massif beira baixa portugal abstract sets of flat joints controlling barren or sn w mineralized veins with aplite pegmatite and or quartz fillings form an interesting but difficult and controversial problem in fact at least three more or less independent parameters influence the formation of these vein type deposits 1 the creation of flat sets 2 the origin of opening s and 3 their filling s relatively few studies have been made of these domains and even if their authors did determine similar successions of events i e creation opening and filling of the joints they disagreed and even came to drastically contrary conclusions concerning the mode of the origin and the opening of flat joints these different hypotheses are summarized and compared with our own results the latter were obtained from observations of flat joint sets and of mineralized or unmineralized quartz and or aplite pegmatite veins occupying parts of these sets on the southern flank of the serra de estrela and by comparison with already known flat sets locally with veins of the iberian variscan belt the following conclusions are drawn 1 1 sets of flat joints are very common structures in the iberian variscan chain 2 2 the sets are locally opened and filled by veins either with or without mineralization which is present only in close proximity of granitic intrusions especially near late well differentiated leucogranitic cupolas elsewhere the joints remain closed and barren 3 3 the genetic history of the flat joints is clearly independent of their opening s and filling s these joints originated long before the opening and infilling the later having been two sub synchronous or penecontemporaneous mechanisms namely the consequence of shrinkage of the granites and hydraulic over pressuring of different fluids clearly several genetic stages were involved 4 4 the origin of flat joint systems can be related to shearing movements associated with weak to very weak tectonics which developed when the main structuration of the belt by the first syn metamorphic phase of folding was already achieved and when the belt was partially uplifted and eroded 5 5 the formation of the flat joints terminated just before the intrusion of the latest granites stephanian leucogranites and thus before the injection of the residual magmatic liquids aplites pegmatites or of hydrothermal sn w mineralizing fluids in fact some late biotitic leucogranites filled some scarcely opened joints

relative geobarometry from high pressure rocks of quartzofeldspathic composition from the sesia zone western alps italy in the sesia zone of the western alps italy early alpine blueschist to eclogite facies metamorphism of rocks of quartzofeldspathic composition has produced the same high pressure assemblage of quartz na pyroxene na amphibole paragonite phengite zoisite garnet magnetite sphene and fe sulphide the qfs assemblage over an area 150 km2 relative gradients in pressure and temperature over this region are reflected in the variations in mineral chemistries of the individual phases of the quartzofeldspathic assemblage through continuous reactions mineralogical discontinuities do not occur in the qfs assemblage of this region increases in the jadeite content of the pyroxenes xjd 0 48 to xjd 0 93 and in the glaucophane content of the amphiboles xgl 0 89 to xgl 0 96 occur from the southwest to the northeast of the region studied analysis of coexisting garnets and pyroxenes indicate that the compositional variation of amphiboles and pyroxenes is associated with a decrease in the grossular component of the coexisting garnet zoned pyroxenes and garnets together with the regional trends in mineral chemistries suggest that the evolution of the qfs assemblage with increasing pressure may be modelled by pressure sensitive continuous reactions in which amphibole zoisite and the more jadeitic pyroxene constitute the high pressure assemblage chemographic constraints permit the positioning in pressure temperature space of the compositional isopleths of those model continuous reactions involving these phases which meet the textural and chemical criteria observed in the natural assemblages the low dp dt slope 20 bars c of these isopleths causes the continuous reactions to be useful for geobarometric calculations at pressures above the absolute breakdown of albite to jadeite plus quartz in addition the pseudobinary loops for the other continuous reactions which are potentially useful geobarometers and involve either the naalca 1mg 1 exchange or the mgca 1 exchange are calculated comparison of mineral chemistries with the isopleths yields a relative barometric scheme for the localities studied with these barometric observations it is possible to show that the p t path which the sesia body travelled towards the final recorded state was one of increasing pressure other blueschist and eclogite occurrences from syros and sifnos which contain rocks of quartzofeldspathic composition are also examined

surface rupture and slip distribution of the 1940 imperial valley earthquake imperial fault southern california implications for rupture segmentation and dynamics we analyzed high resolution aerial photography taken soon after the 1940 imperial valley earthquake to provide a higher resolution distribution of displacement with which to test variation in lateral slip to compare the difference in making slip measurements parallel to local or regional fault strike and to compare to the seismo logical properties of the 1940 earthquake we performed 648 new measurements of displacement along the 15 km section of rupture for which imagery exists nearly 7 m of maximum displacement occurred within 2 km of the border which was higher than previous estimates with an average slip of about 5 5 m for this section there is considerable variation along the strike on the order of 1 m of variability across hundreds of meters of lateral distance in the section of high slip areas of larger offset tend to have greater variations in displacement from these observations we conclude that lateral variability of displacement typically varies by as much as 30 along a rupture section the difference in measured displacement at a specific point along the fault varied considerably when using local fault strikes versus that of regional or average fault however the overall average difference between measurements for different azimuths at a specific point is only 0 01 0 19 m indicating that this is not a major issue if one is consistent finally abrupt changes in displacement along a strike are apparently related to segmentation and the rupture process and mimic the subevents as documented through seismological methods the largest moment pulse corresponds to the section south of the border whereas the greatest displacements occurred north of the border where the average moment release is inferred to be less we explain this by inferring a strong shallow as perity in the border region consistent with 1979 rupture event beneath it online material high resolution imagery of the 1940 rupture with locations of offset measurements and tabulated offset values

kar and 40ar39ar study of metamorphic rocks associated with the oman ophiolite tectonic implications abstract k ar analyses on extracted minerals are reported for a variety of metamorphic rocks associated with the sumail ophiolite amphibolites lying at the sole of the ophiolite yield ages of 95 100 ma which are viewed as reflecting times of crystallization high pressure metamorphics of the saih hatat reveal complex results white micas range in age from 80 to 131 ma whereas blue amphiboles indicate ages that are systematically lower than those of coexisting white micas investigation of a few white micas by the 40 ar 39 ar step heating method yields rather intricate age spectra featuring low apparent ages in the first and the last stages of gas release and high apparent ages in between two explanations can be equally envisaged for these convex upward age spectra the first is the mixing of two generations of micas corresponding to two main metamorphisms the first one m1 is a low to medium temperature high pressure event which conceivably occurred between 130 and 114 m y ago the second m2 overprints m1 and has produced rocks typical of the greenschist facies it took place 80 m y ago and also affected the sole of the ophiolites the second is the presence of excess argon in mica mixtures with complex degassing properties thus the two metamorphic phases identified by microscopic inspection are not significantly different in age they occurred in the 70 80 ma interval moreover k ar dates on amphibole from gabbroic dikes intersecting the peridotites suggest that they are genetically linked to the mafic part of the ophiolites assuming that metamorphism is a tracer of tectonic events we view the infraophiolitic amphibolites as the result of an intraoceanic thrusting which took place near a spreading center nevertheless the uncertainty as to the age of the blueschist metamorphism precludes the possibility of indicating a timetable based on metamorphic ages for the motion shift of africa relative to eurasia during the late cretaceous a tentative model of the obduction of the sumail ophiolite is proposed assuming an age of about 130 ma for m1 we stress differences with respect to earlier reconstructions which advocated an 80 m y old blueschist metamorphism

uranium lead isotopic ages of the samail ophiolite oman with applications to tethyan ocean ridge tectonics plagiogranites are a minor but widespread component of the samail ophiolite plutonic member they crystallized from the most fractionated melts generated by magmatic crystallization and differentiation of a steady state magma chamber beneath the tethyan spreading ocean ridge and their ages are thought to mark the time of ocean crust formation isotopic u pb ages of zircons from 13 plagiogranites collected along a 270 km segment of the samail ophiolite subparallel to the regional trend of the sheeted dike complex the former spreading ridge axis direction define a narrow time interval of 93 5 97 9 m y with a pronounced clustering about 95 m y the zircon ages of the plagiogranites agree remarkably well with the early cenomanian to early turonian biostratigraphic ages of sediments that are intercalated within the ophiolite pillow lavas and that lie just above them tippit et al 1981 the agreement of radiometric and biostratigraphic ages provides strong support for the conclusion that the plagiogranite u pb ages closely date the time span of ocean crust formation no step changes in age patterns are observed along the ridge axis sheeted dike direction suggesting that there are no major internal offsets of the ophiolite by transform or other faults along most of the traverse one possible exception occurs at the southeastern end of the sampled interval ibra area where a 3 m y discontinuity might be caused by an unmapped fault assuming that the regional trend of the sheeted dikes n10 25 w marks the direction of the former spreading ridge axis the present array of sample localities spans a distance of 130 to 195 km normal to that axis i e in the spreading direction the data as a whole do not define a clear cut age trend normal to the spreading axis but by eliminating samples that may be aberrant due to faulting the data array suggests a pattern of increasing ages from east to west this would indicate that the samail ophiolite is derived from the western southwestern side of the tethyan spreading system a conclusion reached independently from geological evidence also the gentle age gradient in the spreading direction i e between 130 and 195 km of ocean crust were formed within an age span of no more than 2 m y suggests a very fast spreading rate this conclusion also is supported by independent geologic evidence

subduction zone polarity in the oman mountains implications for ophiolite emplacement two end member models have been proposed to account for the structure and metamorphism of rocks beneath the semail ophiolite in the oman mountains model a involves a single continuous ne directed subduction away from the continental margin during the late cretaceous the ophiolite and underlying thrust sheets of distal to proximal oceanic sediments were emplaced a minimum of 250 km sw onto the continental margin subduction of triassic jurassic oceanic basalts to c 10 kbar c 39 km depth led to the accretion of amphibolite facies rocks to the base of the ophiolite thrusting propagated towards the continental margin and ended with subduction of the thinned continental crust to c 20 kbar c 78 km depth choking the subduction zone buoyancy forces caused the rapid exhumation of eclogites blueschists and carpholite grade hp rocks along the ne margin of the continental plate during the later phase of foreland propagating thin skinned thrusting in the sw ne facing backfolding and backthrusting occurred in the hinterland with the final exhumation of the hp rocks model b follows recent suggestions that a nascent sw dipping subduction zone dipping beneath the continental margin existed between 130 and 95 ma prior to formation and emplacement of the ophiolite a major ne facing fold nappe structure in the pre permian basement rocks of saih hatat is interpreted as reflecting subduction beneath the margin two high pressure metamorphic events have been suggested the first predating ophiolite emplacement the second caused by ophiolite loading this model is untenable being based on a misinterpretation of the ne facing structures in northern saih hatat and on some dubious older 40ar 39ar cooling ages from the eclogite facies rocks of as sifah we conclude that all structures in northern oman and all the reliable geochronology point to a single emplacement obduction event lasting from cenomanian turonian time c 95 ma when amphibolites were accreted along the metamorphic sole of the ophiolite to campanian time when the continental margin was subducted to the ne producing blueschists and eclogites to the final thin skinned emplacement of all thrust sheets which ended before the late maastrichtian at c 68 ma

the biostratigraphy of sediments in the volcanic unit of the samail ophiolite sedimentary rocks of pelagic and ocean ridge affinity interbedded with or overlying the volcanic sequence at the top of the oman ophiolite contain datable radiolarian faunas that indicate 1 that the lavas were erupted in an oceanic environment during a narrow time interval ranging from the early cenomanian to the early turonian and 2 that the age progression within this volcanic sequence matches its development from a ridge axis to an off axis setting emplacement of the ophiolite has been dated as late campanian early maestrichtian glennie et al 1974 thus the time gap between formation and emplacement was very narrow

comment on structural evolution metamorphism and restoration of the arabian continental margin saih hatat region oman mountains by m p searle et al

chemical and boron isotopic composition of hydrothermal tourmaline from the panasqueira w sn cu deposit portugal abstract tourmaline is a locally abundant hydrothermal mineral in the wallrocks surrounding the w sn cu mineralized veins in the panasqueira deposit portugal and a minor phase within the veins themselves tourmaline chemical and boron isotope compositions have been determined from three settings 1 pervasive fine grained tourmalinization zones in wallrocks within 10 cm of the veins 2 coarser tourmaline in wallrock hosted fault zones and 3 needle shaped tourmaline from late stage vugs in the quartz veins all tourmalines from panasqueira have ferromagnesian compositions with significant octahedral aluminum contents and variable x site vacancies tourmaline compositions show significant chemical variations at the grain and sample scale but are homogeneous on the deposit scale tourmaline is typically optically and chemically zoned showing significant increases in fe and f and decrease in mg ca and al from core to rim moreover as a general trend al increases and mg decreases with proximity to the vein contact the observed chemical variation seems to have been controlled mainly by the vectors femg 1 x alna 1 r2 1 x al oh na 1 r2 1f 1 r2 oh f al 1o 1 and x al2o na 1 r2 2 oh f 1 the total range in tourmaline 11b values from the panasqueira w sn cu deposit is from 3 7 to 12 7 including tourmaline from the three settings the calculated b isotope composition of the hydrothermal fluid based on the average tourmaline composition of 9 and an estimated alteration temperature in the wallrocks based on ti in quartz thermometry of 500 c is 7 1 this isotopic composition is permissive of a boron source from local metasediments or an s type granite but the high boron concentration in the wallrocks with 50 vol tourmaline along with the typical element association of panasqueira w sn f nb ta rb makes a granitic source more likely late stage tourmaline needles in vugs within the mineralized veins show the same patterns and ranges in both chemical and boron isotope composition suggesting that exsolved magmatic fluids were injected in at least two pulses the variations in chemical and boron isotopic compositions of tourmaline combined with estimates from ti in quartz thermometry suggest that hydrothermal ore formation is associated with fluid cooling and geochemical fluid rock interactions

obduction related planar and linear fabrics in oman under the oman ophiolites three different types of alpine synmetamorphic fabrics are seen each of them is taken as the signature of a particular stage of the obduction process the complex high grade fabric in the basal metamorphic sheet is linked to the intraoceanic detachment stage pre obduction sensu stricto the l s fabric of the muscat nappes and hatat internal border associated with low temperature high pressure metamorphism is regarded as the effect of an early obduction that is the epicontinental thrusting of the matrah peridotites the much simpler fabric of the anchizonal hawasina and autochthonous cover resulted from the gravity sliding of the semail ophiolites during the final obduction phase

lithogeochemical studies of aureoles around the panasqueira tin tungsten deposit portugal multi element lithogeochemical surveys of aureoles surrounding tin tungsten bearing quartz vein systems have been utilized infrequently in exploring for these deposits at panasqueira beira beixa portugal a wrich quartz vein mineralization occurs in the beira schists surrounding a greisenized cupola and offers a suitable test case for such lithogeochemical studies therefore shale samples were collected at a density of about 6 samples per km2 pronounced anomalies of tin and tungsten have been detected in these samples together with those of other elements coeval with the mineralizing processes

parageneses and compositions of amphiboles from franciscan jadeite glaucophane type facies series metabasites at cazadero california sodic amphiboles are common in franciscan type ii and type iii metabasites from cazadero california they occur as 1 vein fillings 2 overgrowths on relict augites 3 discrete tiny crystals in the groundmass and 4 composite crystals with metamorphic ca na pyroxenes in low grade rocks they become coarse grained and show strong preferred orientation in schistose high grade rocks in the lowest grade only riebeckite to crossite appears with increasing grade sodic amphibole becomes first enriched in glaucophane component later coexists with actinolite and finally at even higher grade becomes winchite actinolite first appears in foliated blueschists of the upper pumpellyite zone it occurs 1 interlayered on a millimetre scale with glaucophane prisms and 2 as segments of composite amphibole crystals actinolite is considered to be in equilibrium with other high pressure phases on the basis of its restricted occurrence in higher grade rocks textural and compositional characteristics and fe mg distribution coefficient between actinolite and chlorite detailed analyses delineate a compositional gap for coexisting sodic and calcic amphiboles at the highest grade winchite appears at the expense of the actinolite glaucophane pair compositional characteristics of franciscan amphiboles from ward creek are compared with those of other high p t facies series the amphibole trend in terms of major components is very sensitive to the metamorphic field gradient na amphibole appears at lower grade than actinolite along the higher p t facies series e g franciscan and new caledonia whereas reverse relations occur in the lower p t facies series e g sanbagawa and new zealand available data also indicate that at low temperature conditions such as those of the blueschist and pumpellyite actinolite facies large compositional gaps exist between ca and na amphiboles and between actinolite and hornblende whereas at higher temperatures such as in the epidote amphibolite greenschist and eclogite facies the gaps become very restricted common occurrence of both sodic and calcic amphiboles and ca na pyroxene together with albite quartz in the ward creek metabasites and their compositional trends are characteristic of the jadeite glaucophane type facies series in new caledonia blueschists ca na pyroxenes are also common na amphiboles do not appear alone at low grade in metabasites instead na amphiboles coexist with ca amphiboles throughout the progressive sequence however for metabasites of the intermediate pressure facies series such as those of the sanbagawa belt japan and south island new zealand ca na pyroxene and glaucophane are not common sodic amphiboles are restricted to crossite and riebeckite in composition and clinopyroxenes to acmite and sodic augite and occur only in fe2o3 rich metabasites the glaucophane component of na amphibole systematically decreases from ward creek new caledonia through sanbagawa to new zealand this relation is consistent with estimated pressure decrease employing the geobarometer of maruyama et al 1986 similarly the decrease in tschermakite content and increase in nam4 of ca amphiboles from new zealand through sanbagawa to new caledonia is consistent with the geobarometry of brown 1977b therefore the difference in compositional trends of amphiboles can be used as a guide for p t detail within the metamorphic facies series

evolution and paragenetic context of low δd hydrothermal fluids from the panasqueira w sn deposit portugal new evidence from microthermometric stable isotope noble gas and halogen analyses of primary fluid inclusions abstract the evolution paragenetic context and origin of remarkably low d hydrothermal fluids from the hercynian panasqueira w sn cu ag deposit have been investigated through a combined microthermometric stable isotope halogen and noble gas fluid inclusion study large variations in d between 60 and 134 have been observed in primary fluid inclusions from growth zones in a quartz crystal pa66 paragenetically constrained to the main sulfide stage mss of ore formation the same fluids exhibited relatively constant fluid inclusion homogenisation temperatures 254 to 260 c salinities 7 4 to 8 7 wt nacl equivalent and calculated fluid 18o 3 8 to 4 4 all the fluids exhibited 40ar excesses mean molar br cl and i cl ratios varied from 2 3 10 3 4 2 10 3 and 8 10 4 10 10 4 respectively with the more br rich fluids being associated with the more deuterium depleted fluids the low palaeolatitude of panasqueira throughout the main stages of ore formation and the overwhelming predominance of meteoric fluids in the main late and post ore mineralising fluids with d of 40 to 65 lends little support for any models involving low d palaeometeoric water the limited range of 40ar cl ratios 1 1 10 5 1 6 10 5 also precludes boiling during the mss or mixing of meteoric water with highly fractionated deuterium depleted magmatic waters as viable mechanisms for producing the low d fluids the high br cl and i cl of the mss fluids indicate significant fluid interactions with organic rich sediments or metasediments in particular regionally abundant carboniferous coals or coaly sediments that are inferred to be depleted in deuterium by around 85 compared to palaeometeoric water the large range of d observed in the mss fluids may be explained by isotopic exchange of palaeometeoric water with these coals at varying water rock wt wt ratios between 0 02 and 0 002 with fluid oxygen isotopic compositions controlled by exchange with the beira schists such a model is consistent with the requirements that any satisfactory model must take into account viz i the relative constancy of temperature salinity 40ar and fluid 18o during excursions to low d ii the high i cl and br cl ratios observed and iii the coincidence of these low d fluids with the onset of precipitation of polymetallic sulfides and phosphates under reducing conditions the model indicates that meta sedimentary sources of ore forming components became increasingly important during the evolution of the panasqueira sn w deposit and accounts in part for the observed progression from oxide dominated to base metal sulfide dominated parageneses

a case of obduction related high pressure low temperature metamorphism in upper crustal nappes arabian continental margin oman p t paths and kinematic interpretation in the southeastern oman mountains the saih hatat tectonic window reveals arabian continental material beneath the oceanic sensu lato nappes i e the hawasina sedimentary units and the huge sumail ophiolite the northeastern internal half of the window suffered an intense deformation associated with a high pressure low temperature hp lt metamorphism approximately dated at 80 75 ma according to the available stratigraphie data after the deformation due to the absence of collision the obduction inherited structural and metamorphic pattern was virtually unmodified a metamorphic map is established based on mineral assemblages in metapelites and metabasites low grade assemblages occur in the highest units muscat nappes quryat unit characteristic minerals are femg carpholite lawsonite associated with kaolinite or pyrophyllite and locally sudoite in the metapelites while lawsonite and blue amphiboles are found in the metabasites high grade rocks outcrop in the deepest and easternmost units as sifah antiform and are typified by an epidote glaucophane bearing eclogite assemblage intermediate units hulw show assemblages of intermediate grade chloritoid schists and glaucophane bearing metabasites higher grade assemblages such as talc chloritoid were not found inverted gradients are observed locally in the muscat nappes pyrophyllitebearing units above kaolinite quartz bearing units and these are ascribed to late metamorphic thrust displacements which are demonstrated by microstructures a complex p t evolution is fairly well documented by the mineral assemblages prograde evolution in the subophiolitic continental rocks began with a rather hot gradient of about 30 35 c km with chloritoid growth in the higher units thereafter pressure increased with the temperature remaining constant or even decreasing chloritoid replaced by kaolinite oxides and carpholite growth in synkinematic conditions the peak p t conditions are close to 8 kbar 270 c for the muscat nappes and 11 kbar 400 c for the as sifah unit the corresponding surface related gradient is low 10 15 c km during the retrograde evolution pressure and temperature generally decreased concomitantly but in some of the upper units temperature remained constant or increased slightly for some time at which point sudoite growth from carpholite took place the metamorphic pattern and mineral evolution are interpreted in the framework of the late cretaceous obduction the already cold oceanic lithosphere was thrust upon a relatively hot thinned passive margin with coeval basaltic volcanism pressure progressively increased in the passive margin due to the wedge shape of the advancing ophiolite the maximum thickness of which was about 25 km in the as sifah area in this area the overburden was also attributable to the accumulation of continental material up to 5 km thick by means of a complex set of synmetamorphic often conjugated folds and thrusts the basal peridotites of the obducted slab were probably not hotter than 300 c during its thrusting upon the continental margin the temperature of the tectonic pile below the ophiolite remained low because it was thrust above more external and still colder continental areas late thrusting in the same southerly direction controlled the retrograde evolution together with erosional and gravity driven tectonic unloading

metamorphic evolution of norwegian country rock eclogites as deduced from mineral inclusions and compositional zoning in garnets abstract detailed studies of pre early syn and post eclogite mineral associations in some west norwegian country rock eclogites have made it possible to reconstruct their metamorphic evolution pre eclogite stages are represented by various amphibolite facies associations found in the central parts of prograde zoned garnets early and syn eclogite stages are represented by various eclogitic assemblages containing low and high jd omphacites respectively the former located in the intermediate zones and the latter in the outermost rims of prograde zoned garnets and in the matrix post eclogite stages include granulite amphibolite and greenschist facies associations representing replacement phases after higher grade assemblages p t estimates indicate a prograde evolution along an extremely steep p t gradient 7 9 kbar over 100 200 c while the initial post eclogite evolution involved a nearly adiabatic decompression on the order of 10 kbar

ductile thrusting recorded by the garnet isograd from blueschist facies metapelites of the ile de groix armorican massif france

characterization and timing of the different types of fluids present in the barren and ore veins of the w sn deposit of panasqueira central portugal the panasqueira w sn deposit is the largest quartz vein type deposit of the iberian peninsula and the most important wolframite deposit in western europe the ore veins are almost exclusively sub horizontal besides ore bearing sub horizontal veins the panasqueira mine also contains barren quartz veins there are essentially two generations of barren quartz quartz contemporaneous with the earliest regional metamorphism qi and recrystallized quartz contemporaneous with the thermal metamorphism related to the granite intrusion qii fluid inclusion studies microthermometry and raman were undertaken in order to distinguish fluids contemporaneous with the barren quartz from those contemporaneous with the ore bearing quartz qiii fluid inclusion data indicate that the barren and ore bearing quartz fluids are dominantly aqueous 93 to 98 mol h2o with a nearly constant bulk salinity 8 to 12 wt eq nacl with the quantity of volatile component determined by raman spectrometry higher in qiii but never greater than 5 mol however the co2 ch4 n2 ratio is different for each type of quartz volatiles are dominated by ch4 10 to 96 mol zch4 and or n2 3 to 87 mol zn2 in the barren quartz and by co2 60 to 73 mol zco2 in ore bearing quartz the bulk chemical composition of the fluids in qiii is comparable to that found commonly in hydrothermal fluids associated with wolframite mineralization where na k ca and hco3 cl so4 a dispersion in th 226 to 350 c found in qiii together with a variation in the degree of filling 0 5 to 0 7 and with the consequent variation of fluid densities 0 70 to 0 79 may result from changes in the fluid pressure regime below lithostatic pressure suggesting vein filling related to tectonic events

coexisting amphiboles in an eclogite from the western alps new constraints on the miscibility gap between sodic and calcic amphiboles crystal chemical relationships between coexisting sodic and calcic amphiboles have been studied in eclogitic metagabbros from the aosta valley western alps textural analysis gives evidence of three successive high pressure parageneses 1 pre kinematic high grade blueschist assemblages preserved as polymineralic inclusions in garnet cores and made of glaucophane and actinolite stage a 2 synkinematic eclogite assemblages composed of garnet omphacite glaucophane actinolite white mica clinozoisite quartz rutile stage b 3 post kinematic epitactic overgrowths of barroisitic amphibole on glaucophane and actinolite stage c p t conditions of the eclogitic metamorphism have been estimated at around 500 550 c 16 kbar glaucophane and actinolite coexist as discrete grains in stage a and b assemblages this texture and the chemistry of the amphiboles unambiguously denotes the existence of a miscibility gap between sodic and calcic amphiboles from nam4 0 80 in actinolite to nam4 1 70 in glaucophane at t 500 550 c a comparison with published analyses allows a new solvus along the glaucophane actinolite join to be drawn the later barroisitic amphibole stage c exhibits strong chemical zonation indicating disequilibrium growth this amphibole cannot either be used to define a miscibility gap with glaucophane or actinolite or be considered as an intermediate stage between these two end members

upper cretaceous muti formation transition of a mesozoic nate platform to a foreland basin in the oman mountains the upper cretaceous turonian campanian muti formation sayja member documents the transition from a passive continental margin to a foreland basin related to overthrusting of continental margin and ophiolitic nappes derived from the tethys ocean upper cretaceous northeastward subduction culminated in collision of a trench with the arabian margin as the trench docked with the margin the lithosphere was flexed forming a peripheral bulge that migrated cratonward with time the platform edge was initially uplifted turonian and deeply eroded creating the wasia aruma break after passage of the peripheral bulge subsidence began with accumulation first of ferruginous crusts on hardgrounds lime muds were then deposited on a deepening unstable sea floor along with phosphatic nodules and crusts turonian coniacian passage of the overthrust load over the arabian continental margin edge downflexed the lithosphere santonian campanian resulting in drastic foundering of the old shelf edge to form a foredeep upper platform horizons collapsed as slump sheets and debris flows limestone blocks and lithoclastic debris flows were shed by mass wasting of the already deeply eroded old platform edge mud and silt were derived from the uplifted arabian continent and deposited by mainly gravitational processes in a foredeep below the c c d subsidence of the arabian platform edge allowed the semail ophiolite nappe finally to override the muti basin late campanian with little internal deformation submarine emplacement is suggested by the absence of ophiolitic detritus in the muti formation the stratigraphic evolution of the muti formation is in good general agreement with a model of the transition of an old thermally mature passive continental margin to a foreland basin where the emplaced load is submerged

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

a new geobarometer for the assemblage biotite muscovite chlorite quartz the equilibrium thermodynamics of the reaction 4kmga1si40 o oh mg5a1 si30 o oh muscovlte chlorite muscovite biotite ka13si 0 oh 3kmg3aisi 0 0 oh 7si0 4h o quartz fluid can be formulated as a geobarometer thermo dynamic data for the mg a1 celadonite end member of muscovite is derived from the reversed mineral equilibria experiments of velde i 965 using the calculation method of powell 1978 giving afh 1298 5823 5 kj s 1298 0 3060kjk v 1298 13 8kjkbar

sequence of thrusting and origin of culminations in the northern and central oman mountains abstract detailed mapping and structural analysis of three large scale culminations sumeini and asjudi half windows and haybi hawasina window in the oman mountains shows a considerably more complex history of deformation than a simple foreland or downward sequence of thrust development early thrusting processes tended to create a regular stacking order of imbricate slices and major thrust sheets complying with the rules of thrust propagation assembled progressively downwards and forwards in the direction of translation out of sequence thrusts can also be demonstrated in places by truncation of footwall structures folds imbricate slices etc gross strain differences between thrust sheets downward facing structures in footwall units and elimination of thrust sheets beneath late stage thrusts frequently cut up section through the previously assembled stack putting previously younger lower thrust sheets over previously older higher ones many of the culminations in the northern and central oman mountains were formed by ramping associated with this late stage leap frog rethrusting event

cretaceous high pressure metamorphism in ne oman and its relationship to subduction and ophiolite nappe emplacement an occurrence of glaucophane bearing schists is reported from near muscat in ne oman in rocks which underlie the upper cretaceous semail ophiolite nappe these blueschists formed by the metamorphism of quartz sandstones shales and basic igneous rocks belong to a pre permian basement in the area metamorphic micas phengites from these rocks have been dated at between 101 and 80 ma k ar ages indicating a mid to late cretaceous age for the high pressure event a syn to late f 1 assemblage of almandine glaucophane phengite in the pelites shows that the peak of metamorphism reached about 400 450 c and 4 8 kbars pressure this was followed by a greenschist facies overprint with the formation of epidote and albite porphyroblasts in the meta igneous rocks and a widespread retrogression to chlorite the high pressure metamorphism is ascribed to the effects of partial subduction of the ne oman continental margin during the initial stages of ophiolite obduction and the subsequent greenschist facies event to uplift of the area during the later stages of emplacement of the nappes

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

petrochemistry and origin of sub ophiolitic metamorphic and related rocks in the oman mountains the semail ophiolite complex of the northern oman mountains was obducted during cenomanian to campanian upper cretaceous times onto the arabian continental margin during this time span schistose metamorphic rocks of amphibolite and greenschist facies exhibiting polyphase deformation were formed and accreted below the ophiolite slice whole rock and mineral analyses confirm that the amphibolites were originally tholeiitic basalts comparable to present day occurrences at mid ocean ridges a comparison of trace element geochemistries however suggests that the protoliths were not equivalent to the haybi alkaline and transitional tholeiites found in the underlying structural slice or to the semail ophiolite volcanic rocks which lie structurally higher in the sequence it is therefore suggested that the amphibolites may have been derived from jurassic or lower cretaceous oceanic basalts not recognized elsewhere in the oman mountains structurally beneath the amphibolites within the metamorphic sheet greenschist facies metasedimentary rocks including quartzites and marbles are interbanded with metabasite horizons containing actinolite and less commonly blue crossitic amphiboles it is suggested that the sedimentary and volcanic protoliths were metamorphosed in a subduction zone environment during initial ophiolite displacement and were accreted to the base of the ophiolite during progressive underthrusting