late cretaceous exhumation of the oman blueschists and eclogites a two stage extensional mechanism the continental material of the saih hatat window has been affected by a late cretaceous obduction related hp lt metamorphism below the oman ophiolite a high rate exhumation process is witnessed by the maastrichtian palaeocene onlap onto the blueschist facies rocks drastic metamorphic omissions are documented between the lowest eclogitic units and the overlying blueschist facies ones widespread late metamorphic shear structures point to a top to the nne detachment in opposition to the sense of the late cretaceous obduction the inversion of the shearing sense occurred under similar low temperature conditions in both the blueschist and eclogite facies units admitting that these hp lt metamorphic rocks formed progressively at various depth in the subducting arabian margin a two stage extensional mechanism of exhumation is suggested i early uplift of the eclogitic rocks up to the blueschists depth by ductile thinning or squeezing of a blind extensional allochthon ii exhumation of the whole hp lt metamorphic core complex by inversion of the obduction sole thrust and isostatic rebound of the lower plate

radial cracks around α quartz inclusions in almandine constraints on the metamorphic history of the oman mountains radiating tensional cracks around quartz inclusions in almandine have been observed in metapelite samples from the southeastern saih hatat tectonic window northeastern oman mountains these almandines show an inclusion rich glaucophane epidote and strongly deformed core with inclusions of different mineral phases the rim of the same almandines is inclusion poor and shows only quartz apatite zircon rutile and baal phosphates as inclusions quartz and apatite inclusions in the rim are single crystals often surrounded by radial cracks these radial cracks developed during uplift by the dilation of quartz 4 5 vol without a phase transformation subsequently these cracks were filled with kaolinite phengite si content 3 4 per formula unit p f u chlorite and fe oxides we calculated the appearance of radial cracks without phase transformation using the mathematical procedure of van der molen and van roermund 1 this calculation involves terms for thermal expansion isothermal compressibility and shear modulus for the example of quartz and almandine for the same p and t interval during a retrograde path published geothermobarometric estimates give pressures of between 1 0 and 2 0 gpa and temperatures of between 450 and 600 c for the peak conditions for these rocks of the saih hatat tectonic window on the basis of these p t data we calculated different retrograde p t paths in the quartz domain initiation of garnet fracturing is dependent on the p t starting conditions and the component of isothermal compression of the retrograde path the calculations yield a set of smooth monotonic curves whose exact position on the p t plane between 0 1 and 0 6 gpa and 40 and 500 c depends on the initial p t conditions and the component of isothermal compressibility of the retrograde p t paths this model can be used in general terms to estimate pressure and temperature for the following cases 1 if independent evidence such as petrological data allow the determination of the final pressure at which radial cracks appeared the initial inclusion pressure can be recalculated 2 if the initial inclusion pressure is known e g from petrological data the conditions of radial cracking can be calculated and the pair initial pressure final pressure leads to an estimate of the shape of the retrograde p t path as a function of its component of isothermal decompression in the example from the northeastern saih hatat tectonic window the late syntectonic growth of albite phengite kaolinite suggests that the final pressure for fracturing ranged between 0 4 gpa and 0 5 gpa at temperatures of 300 c these values correspond to high initial pressures of at least 2 0 gpa at a temperature of 550 c the following geodynamic model is suggested a regionally extended metamorphism led to the growth of inclusion rich garnets in the rocks from the northeastern saih hatat tectonic window at depths of about 30 km 0 1 gpa about 450 c continuing prograde metamorphism at a depth of more than 60 km with p 2 0 gpa and t 550 c affected a metapelite unit that is only exposed immediately south of as sifah village in this area clear rims of almandine grew around the older garnets and entrapped mainly quartz and apatite during uplift along a retrograde p t path with a large component of isothermal decompression radial cracks around quartz inclusions developed in the rims of almandines at a depth of about 12 km 0 4 0 5 gpa 300 c

tourmalines from appalachian caledonian massive sulfide deposits textural chemical and isotopic relationships tourmaline is a common gangue mineral in several strata bound massive sulfide deposits of the appalachian caledonian orogen most of the tourmaline rich deposits occur in dominantly metasedimentary terranes such as bleikvassli norway black hawk maine ore knob north carolina and elizabeth vermont minor amounts of tourmaline are also known in association with volcanic hosted deposits in the appalachians and in the trondheim district of norway the tourmaline forms isolated euhedral crystals or crystal aggregates in massive sulfide and adjacent wall rocks in a few areas it is found in conformable tourmaline rich layers tourmalinites associated minerals may include quartz plagioclase phlogopite and dolomite with pyrrhotite chalcopyrite galena and sphalerite many of the tourmaline crystals are optically zoned in which brown zones concentrically alternate with pale yellow or green zones a few specimens of tourmaline intergrown with massive sulfide e g ore knob display fine delicate growth lamellae similar to those commonly observed in hydrothermal vein sphalerites skarn garnets and sphalerites from the massive sulfide deposits of the kuroko district japan detailed geochemical studies indicate that the tourmalines have characteristic major and trace element compositions electron microprobe analyses show that with rare exception the tourmalines are mg rich dravites avg feo feo mgo 0 21 abrupt transitions between color zones within some tourmaline are accompanied by relative variations in feo feo mgo of as much as 30 percent yellow brown zones are more mg rich than blue or green zones in contrast the fine growth lamellae are not characterized by such variations in chemistry spectrographic analyses of pure mineral separates indicate that the tourmalines also contain high average contents of cr 78 ppm cu 77 ppm pb 114 ppm sr 98 ppm and v 195 ppm while uniformly low in li 7 ppm and mn 382 ppm in all analyzed samples fluorine is much more abundant than chlorine the average f cl ratio is 33 tourmalines from tourmalinites and massive sulfide deposits are typically characterized by heavier delta 18 o values 9 5 15 5ppm and less variable heavier delta d values generally 45 to 60ppm than tourmalines from unmetamorphosed igneous pegmatites the latter typically have delta 18 o 9 5 per mil and delta d or 60 per mil oxygen isotope fractionations between quartz and tourmaline delta 1 27 4 01 do not correlate directly with inferred grade of subsequent metamorphism although in areas of high metamorphic grade the delta values approach those of igneous quartz tourmaline pairs the delta 18 o values of quartz and tourmaline are believed to reflect variation in the oxygen isotope compositions of the host lithologies the similar narrow range in hydrogen isotope composition for tourmaline in all studied deposits is compatible with a modified seawater origin for the tourmaline and sulfide forming fluids the distinctive textural and chemical characteristics of the tourmalines reported here have both exploration and genetic significance though it may have undergone recrystallization and some accompanying chemical and isotopic exchange during regional metamorphism tourmaline in the massive sulfide deposits is an early formed perhaps diagenetic mineral in which fine lameliar zoning may be a preserved premetamorphic feature in any case the tourmalines from the massive sulfide deposits are mg rich dravites with trace element and stable isotope compositions substantially different from those of the fe rich schorls typical of most granitic rocks these geochemical and isotopic signatures are attributed to the original environment of formation of the associated sulfide deposits and suggest that mg rich tourmaline may be a valuable prospecting guide in the search for massive sulfide deposits

the volcanic stratigraphy and petrogenesis of the oman ophiolite complex the volcanic stratigraphy and trace element geochemistry of the oman ophiolite complex indicate a multistage magmatic origin comprising 1 magmatism due to sea floor spreading in a marginal basin 2 magmatism associated with discrete submarine volcanic centres or seamounts 3 magmatism associated with crustal uplift and rifting and 4 magmatism associated with continent arc collision trace element petrogenetic modelling is used to investigate the nature of the mantle source region and the partial melting and fractional crystallization history for each magmatic event the petrogenetic pathway for the sea floor spreading lavas requires a high degree of melting of a mantle that was depleted in incompatible elements prior to subduction but subsequently selectively enriched in certain elements mostly lil elements and h2o from an underlying subduction zone it also requires magma mixing in an open system magma chamber prior to eruption the seamount lavas were probably derived by a similar degree of partial melting of a similar source but fractional crystallization was restricted to smaller high level magma chambers the rifting lavas were derived from a mantle source that was more depleted than the seamount lavas prior to subduction but which was later modified by a larger subduction zone component the syn collision lavas were however derived from an enriched mantle source which probably underlay the passive continental margin rather than the marginal basin complex results such as these may provide considerable insight into the petrogenetic changes accompanying the transitions from spreading to arc volcanism in a supra subduction zone setting

cr bearing tourmaline associated with emerald deposits from swat nw pakistan genesis and its exploration significance carbonate talc altered ultramafic and mafic rocks of the obducted oceanic lithosphere along the main mantle thrust in the swat valley host emerald deposits and are invaded by quartz veins and stockworks both the quartz veins and carbonate talc altered rocks contain discrete grains and clusters of tourmaline crystals which may be associated with emerald and or chromian muscovite tourmalines from the mingora and gujar kili emerald mines and the spin obo kuh area are ni bearing cr rich dravites and oxy dravites that exhibit substantial chemical variability with 2 0 2 5 atoms per formula unit apfu mg relatively low al 4 8 5 8 apfu high cr 0 5 1 8 apfu high cr cr al ratios of 0 08 0 28 significant amounts of ni up to 0 11 apfu and variable calculated w site o 2 0 2 0 8 apfu the mechanisms that appear to control the tourmaline chemistry at these localites are combinations of the dominant substitution al cr with lesser amounts of the mg oh 1 al cr o 2 and al 3 fe 3 substitutions relative to the mingora gujar kili and spin obo kuh areas the charbagh tourmalines are oxy dravites that have comparable mg contents 2 2 2 3 apfu with variable calculated w site o 2 0 6 0 9 apfu but are more fe rich 0 5 apfu and al rich 6 0 6 3 apfu with low cr 0 00 0 01 apfu and ni contents 0 01 0 02 apfu the mechanism that appears to control the variable chemistry of the charbagh tourmalines is primarily mg oh 1 al o 2 the chemical characteristics of tourmalines from the various localities relate most clearly to differences in host rock chemistry tourmalines from the carbonate talc altered ultramafic mafic rocks and the invasive quartz veins have a common hydrothermal origin associated with emplacement and crystallization of younger granitoids furthermore the occurrence of swat emeralds in quartz tourmaline chromian muscovite veins suggests that the tourmaline is cogenetic with emerald formation the quality and quantity of swat emeralds depend on the 1 host rock composition and 2 nature of the hydrothermal fluids large deposits of gem quality emerald occur in carbonate altered ultramafic rocks which also contain cr rich dravites and oxy dravites this suggests that the presence of high cr magnesian tourmalines particularly in magnesite talc altered ultramafic rocks can represent a criterion for further emerald exploration in the lower swat region of pakistan and comparable ultramafic hosted emerald producing regions

detrital chromites in metasediments of the east arabian continental margin in the saih hatat area constraints for the palaeogeographic setting of the hawasina and semail basins oman mountains in oman the convergence between arabia and eurasia resulted in the late cretaceous overthrusting of oceanic crust and mantle lithosphere onto the arabian continental margin during this compressional event a part of the continental plate was subducted to a depth of more than 60 km 0 5 gpa 250 350 c to more than 2 0 gpa 550 c resulting in progressive metamorphism of the continental margin sediments well exposed in the saih hatat tectonic window northeastern oman mountains we attempt to constrain the possibility of one continuous history of extension starting along the east arabian continental margin in the permian that was followed by one continuous history of convergence starting at 90 ma near a dead oceanic ridge this compression resulted in the observed progressive metamorphism by ophiolite overthrusting onto the continental margin constraining arguments are the palaeogeographic setting before ophiolite obduction of the as sifah units and the hawasina complex near ghurba detrital chromites in the triassic cretaceous metasediments of the hawasina complex are compared with magmatic semail chromites and the whole rock chemistry of these metasediments and associated metabasites are investigated in contrast to former hypotheses differences in the chemical composition between detrital and magmatic chromites and the probable origin of all detrital chromites in the hawasina basin from permian age oceanic rocks suggest that the high pressure metamorphic sediments of as sifah can be considered as part of the basal deposits of the hawasina basin

central virginia volcanic plutonic belt as a host for massive sulfide deposits strata bound massive sulfide deposits of the central virginia piedmont occur in a volcanic plutonic belt composed of lower cambrian metamorphosed volcanic rocks and locally of pre upper ordovician low potassium granitoid rocks the belt interpreted as an ancient island arc begins about 50 km south of washington d c and extends some 175 km south westward the volcanic rocks within the belt are located along the flanks of the quantico columbia synclinorium and the arvonia syncline the chopawamsic formation a western facies of the belt consists of volcanic rocks that have the geochemical features rare earth and immobile elements of a tholeiitic island arc suite it also contains calc alkaline components tholeiitic amphibolites of the ta river metamorphic suite on the eastern side of the belt form an oceanward facies of the island arc coeval with the chopawamsic this distribution suggests an underlying westward dipping cambrian subduction zone contrasts in stratigraphy geochemistry and magnetic and gravity features indicate that the central virginia volcanic plutonic belt is not related to the carolina slate belt as has been proposed in the past known sulfide bodies in the chopawamsic and its coeval units total about 13 5 million tons the mineral district on the northwest flank of the quantico columbia synclinorium has the largest deposits and contains about 10 million tons sulfide bodies in the belt consist largely of pyrite with minor sphalerite and chalcopyrite zn and pb are important in a few of the smaller deposits there are local concentrations of sphalerite and other base metal minerals within the major pyritic deposits zn 1 12 5 cu 1 2 pb as much as 5 the dominantly sphaleritic deposits are found in and northward for about 15 km from the mineral district sulfide mineralization is found in all types of volcanic rock but the large sulfide bodies on the northwest flank of the quantico columbia synclinorium are in felsite amphibolite containing local beds of iron formation is abundant on the southeast flank of the arvonia syncline however most sulfide mineralization there took place in felsitic to intermediate layers mineralization attributed to submarine volcanic exhalation took place during formation of the island arc

formation and exhumation of blueschists and eclogites from ne oman new perspectives from rb sr and 40ar 39ar dating seven eclogite facies samples from lithologically different units which structurally underlie the semail ophiolite were dated by the 40 ar 39 ar and rb sr methods despite extensive efforts phengite dated by the 40 ar 39 ar method yielded saddle hump or irregularly shaped spectra with uninterpretable isochrons the total gas ages for the phengite ranged from 136 to 85 ma clinopyroxene phengite epidote phengite and whole rock phengite rb sr isochrons for the same samples yielded ages of 78i2 ma we therefore conclude that the eclogite facies rocks cooled through 500 u ca tc 7 8i2 ma and that the 40 ar 39 ar dates can only constrain maximum ages due to the occurrence of excess ar inhomogeneously distributed in different sites our new results lead us to conclude that high pressure metamorphism of the oman margin took place in the late cretaceous contemporaneous with ophiolite emplacement previously published structural and petrological data lead us to suggest that this metamorphism resulted from intracontinental subduction and crustal thickening along a ne dipping zone choking of this subduction zone followed by ductile thinning of a crustal mass wedged between deeply subducted continental material and overthrust shelf and slope units facilitated the exhumation of the eclogite facies rocks from depths of c 50 km to 10 15 km within c 10 ma and led to their juxtaposition against overlying lower grade rocks final exhumation of all high pressure rocks was driven primarily by erosion and assisted by normal faulting in the upper plate

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

provenance of the lower paleozoic balcarce formation tandilia system buenos aires province argentina implications for paleogeographic reconstructions of sw gondwana abstract lower paleozoic moderately sorted quartz arenites from the balcarce formation deposited in eastern argentina tandilia system comprise mainly detrital material derived from old upper crustal material the sources were magmatic sedimentary and subordinated felsic metamorphic terranes high concentrations of tourmaline and ti rich heavy minerals including zircon and nearly euhedral chromite are common trace element concentrations nb cr on rutile indicate pelitic and metabasaltic sources respectively major element analyses on chromites indicate a basic volcanic protolith of mid oceanic ridge origin which was exposed close to the depositional basin the delivery of chromite may be associated with convergent tectonics causing the consumption and obduction of oceanic crust during pre upper ordovician times the oblique orthogonal collision of the precordillera terrane with the western border of the rio de la plata craton west of the balcarce basin or source further to the east from a lower palaeozoic extensional basin are possibilities geochemical and petrographic data exclude the underlying precambrian and cambrian sedimentary rocks as dominant sources and favour the basement of the rio de la plata craton including cambrian rift related granites of south africa and the sierras australes eastern argentina as main suppliers of detritus trace element geochemistry of recycled pyroclastic material associated with the quartz arenites also suggests volcanic arc sources the provenance of the pyroclastic material may either be the puna famatina arc located in north and central argentina or a hypothetical active margin further to the south these ash layers are equivalent in age to volcanic zircons found in the devonian bokkeveld group in western south africa the deposition of a glacial diamictite of hirnantian age sierra del volcan diamictite is interpreted as a member of the balcarce formation based on the stratigraphic re location of the glacial diamictite and trace fossils the balcarce formation is considered here to be ordovician to silurian in age the balcarce formation can be correlated with similar rocks in south africa the peninsula formation and the upper table mountain group windhoek and nardouw subgroups including the hirnantian glacial deposit of the pakhuis formation

thin lithosphere beneath the central appalachian mountains a combined seismic and magnetotelluric study abstract a joint analysis of magnetotelluric and sp receiver function data collected along a profile across the central appalachians highlights variations in regional lithospheric structure while the interpretation of each data set by itself is non unique we identify three distinct features that are consistent with both the resistivity model and the receiver function image 1 thin lithosphere beneath the appalachian mountains 2 somewhat thicker lithosphere to the east of the mountains beneath the coastal plain and 3 a lithosphere asthenosphere boundary that deepens to the west of the mountains in some regions the correspondence between seismic velocity discontinuities and resistivity mark the base of the lithosphere while in other locations we see seismic discontinuities that are contained within the lithosphere at the western end of our profile a transition from highly resistive lithosphere to more conductive mantle represents the transition across the grenville front the thickness of lithosphere beneath the grenville terrain is 140 km lithosphere at the eastern end of the profile has a thickness that is not well constrained by our coverage but is at least 110 km thick this lithosphere can be associated with a broader region of high resistivity material seen to extend further south directly beneath the appalachian mountains lithospheric thickness is inferred to be as thin as 80 km based on observations of elevated mantle conductivities and a westward dipping seismic converter electrical conductivities in the uppermost asthenospheric mantle are sufficiently high 0 1 s m to require the presence of a small volume of partial melt the location of these elevated conductivities is close offset 50 km to the west to eocene volcanic outcrops in and around harrisonburg va our observations speak to mechanisms of intraplate volcanism where there is no divergent or convergent plate motion to trigger mantle upwelling or obvious fluid release either of which can facilitate melting instead we suggest that small scale mantle convection related either to pre existing lithospheric thickness variations or to lithospheric loss through delamination coupled with relative plate motion with respect to the underlying asthenosphere can trigger small amounts of melting this melt migrates upslope along the base of the lithosphere potentially thermally eroding the lithosphere resulting in further thinning

new high precision p t estimates for oman blueschists implications for obduction nappe stacking and exhumation processes oman blueschists and eclogites lie below the obduction nappe of the semail ophiolite in one of the key areas on earth for the study of plate convergence here new metamorphic and tectonic constraints are provided for the central yet poorly constrained hulw unit sandwiched between the low grade units 10 kbar 300 c and the as sifah eclogites pmax 23 kbar tmax 600 c tweequ multi equilibrium thermobarometry using both compositional mapping and spot analyses and raman spectroscopy of carbonaceous material yield a high precision p t path for the hulw and as sheikh units and reveal that they shared a common p t history in four stages i a pressure decrease from 10 12 kbar 250 300 c to 7 9 kbar 300 350 c ii almost isobaric heating at 8 10 kbar from 300 350 c to 450 500 c iii a pressure decrease at moderate temperatures 450 500 c and iv isobaric cooling at 5 6 kbar from 450 500 to 300 c no significant pressure or temperature gap is observed across the upper plate lower plate discontinuity to the north and west of the hulw unit the combination of tectonic and p t data constrains the stacking chronology of the three main metamorphic units comprising the saih hatat window i e the ruwi quryat the hulw as sheikh and the diqdah as sifah units these results strengthen the view that the tectonic and metamorphic data are conveniently accounted for by a simple n vergent continental subduction of the passive arabian margin below the obduction nappe along a cold p t gradient

mineralogy geochemistry and geological significance of tourmaline rich rocks from the paleozoic cinco villas massif western pyrenees spain tourmaline rich rocks associated with clastic metasedimentary rocks of carboniferous age occur in the cinco villas massif western pyrenees three types of tourmaline rich rocks were distinguished 1 fine grained stratiform tourmaline rich rocks which are associated with carbonaceous metapelites tr1 2 stratabound tourmaline rich rocks associated with metapelites in the contact aureole of the aya granitoid pluton tr2 3 stratabound to massive tourmaline rich rocks associated with psammopelites in contact with granites and pegmatites tr3 tourmalines belong to the schorl dravite solid solution series and have a wide compositional range from nearly end member dravite for tr1 tourmalines to schorl for tr3 tourmalines tr2 tourmalines have intermediate compositions the fe fe mg typically varies between 0 02 and 0 55 increasing from tr1 to tr3 the tr1 tourmalines commonly display a discontinuous chemical zoning with fe rich green cores 8 8 5 feo and mg rich colorless rims 10 11 mgo in contrast crystals that exhibit fine growth lamellae appear to lack significant chemical zoning oxygen and hydrogen isotope compositions also reveal major differences between tr1 and tr3 tourmalines the former displaying heavier 18o values 17 7 19 and d values 35 to 42 than tr3 tourmalines 11 to 13 and 47 to 76 respectively the tr2 tourmalines show intermediate values of 11 3 to 14 6 for 18o and 40 to 55 for d linear and continuous chemical variations obtained for major and trace elements of the whole rocks reflect mixing between clay rich and quartz rich end members indicative that some tourmaline rich rocks contain a significant detrital component chondrite normalized ree rare earth element patterns of tourmaline rich rocks are similar to those of surrounding unaltered clastic metasediments except for some tr1 rocks which are characterized by low contents of ree mass balance calculations show that tourmaline forming processes plus metamorphism led to mass and volume changes at mesoscopic scales 10 for the tr1 tourmalinites silicon fe mn and ree elements were partially lost from sedimentary rocks whereas mg and particularly b were added to pelitic sediments available data nevertheless do not allow an assessment of the boron source formation of the tr1 tourmaline rich rocks probably was the net result of several processes including direct precipitation from b rich hydrothermal fluids or colloids early diagenetic reactions of carbonaceous pelitic sediments with these fluids and subsequent recrystallization during regional metamorphism the tr2 tourmaline rich rocks mainly developed by metamorphic recrystallization of tr1 tourmaline rich rocks and veins adjacent to pegmatites and granitic rocks tr3 are the result of boron metasomatism the primary boron having been recycled from stratiform tourmalinites during regional metamorphism and magmatism

use of tourmaline in stream sediments to detect submarine exhalative sulfide deposits example from central virginia u s a abstract the detection of several unmined submarine exhalative sulfide deposits most of which are concealed with no surface expression was investigated using the distribution grain counts of tourmaline in stream sediment concentrates the number of sulfide zones located using this method is comparable to that achieved using trace metal nalyses of fine grained stream sediment andmnz sbnd fe oxide coatings on stream pebbles because both dravite associated with a sulfide zone and schorl from a pegmatite are known to occur in the field area it is likely that both will occur in stream sediment and will be included in tourmaline grain counts of sediment concentrates attempts to differentiate dravite and schorl in the sediment concentrates based on microscopically observable physical properties were unsuccessful but semi quantitative chemical analyses should readily accomplish this because of large differences in mg and fe concentrations

olivine spinel geospeedometry analysis of diffusion controlled mg fe2 exchange subsolidus mg fe2 exchange between olivine and spinel is governed by mg fe2 interdiffusion incomplete exchange results in mg fe2 heterogeneity in both olivine and spinel which provides information on the thermal histories of the host rocks a composite sphere model has been developed to obtain quantitative cooling rates or heating duration from the mg fe2 heterogeneity the model assumes that a spherical core of spinel and a surrounding semi infinite spherical shell of olivine interact by diffusion controlled exchange of mg and fe2 the differential equations describing the model are solved numerically by finite difference approximations the numerical solution reveals that cooling rates or heating duration can be estimated from the relationship between the grain size of spinel and temperature calculated from the chemical compositions of the core of a spinel grain and of olivine far away from it the calculated temperature is employed in place of mg mg fe2 at the center of spinel to obtain the absolute temperature of thermal events this olivine spinel geospeedometer has been applied to peridotites gabbro and picrites from some ophiolite complexes in japan to estimate their cooling rates the estimated cooling rates for the peridotites range from 10 4 to 10 1 c yr and those for the picrites from 103 to 104 c yr the geospeedometer has been extended to estimate the heating duration of lherzolite xenoliths in basalt from the ichinomegata crater northeast japan the estimated heating duration of the xenoliths is less than one day

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

stratiform tourmalinites in metamorphic terranes and their geologic significance stratiform tourmalinites are significant minor rock types in many regional metamorphic terranes of the world tourmalinites are more widespread than previously recognized and are especially common in proterozoic and early paleozoic sequences dominated by clastic metasedimentary rocks they consist of conformable layers made up primarily of quartz and abundant tourmaline the latter typically exceeding 15 to 20 of rock volumes a few tourmalinites display striking sedimentary structures such as graded bedding cross laminations slump and flame structures and rip up clasts these and other geologic features provide important constraints for assessing the origin of these rocks and they suggest that tourmalinites form by the early diagenetic modification of a primary boron rich chemical precipitate tourmalinites are significant in preserving a valuable record of unusual chemical and paleogeographic conditions in clastic sedimentary basins and in their close association with a variety of stratabound mineral deposits

the chemistry of detrital chromian spinels and its implications for the geodynamic evolution of the eastern alps detrital chromian spinel is a key mineral in paleogeographic reconstructions the chemical compositions of the spinels provide even more specific information about their host ophiolites because their chemistry is related to the degree of melting of the mantle peridotite and thus to the geotectonic environment of ocean crust formation

debris fan reworking during low magnitude floods in the green river canyons of the eastern uinta mountains colorado and utah the magnitude and frequency of tributary debris flows and the historical range of main stem river discharges are the main factors that create and modify rapids in the colorado river system monitoring of two recently aggraded debris fans in the green river canyons of the eastern uinta mountains shows that main stem floods with magnitudes between 40 and 75 of the predam 2 yr flood cause significant reworking of fan deposits cutbanks formed at fan margins during both small and large flows indicating that lateral bank erosion is an important reworking mechanism armoring of the debris fan surface limited the degree of reworking by successive floods even when subsequent flood magnitudes were similar to those that caused significant reworking peak discharges increased the width of the reworked zone decreased fan constrictions and lowered the water surface elevation of the ponded backwater contrary to predam geomorphic evidence monitoring indicated that eroded material from recently aggraded debris fans was deposited in bars adjacent to the downstream parts of both fans this change in the organization of the fan eddy complex has the potential to alter the location of recirculating eddies and associated areas of fine grained sediment deposition and storage

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

chromian spinels and olivines in a contact metamorphosed peridotite sediment system from nagasawa sw japan implications for mobility of elements in a hydrothermal condition system abstract we describe some unusual metaperidotites that form a mega block in thermally metamorphosed sediments adjacent to a granitic intrusion in the nagasawa area sw japan to investigate mobility of elements in a peridotite sediment fluid system the critical mineral assemblage of the metaperidotite is olivine talc antigorite indicating a univariant reaction at 500 c the metaperidotites contain chromian spinels as well as up to 5 of sulfides pyrrhotite and pentlandite with limited solid solutions that form composite grains with magnetite the host sediments contain detrital chromian spinels the olivines in the metaperidotites have low contents of nio 0 1 0 2 wt almost irrespective of variable fo contents 85 92 the chromian spinels in the metaperidotites have high values of cr 0 9 1 0 cr cr cr al and low values of mg 0 2 mg mg mg fe 2 and y fe fe fe 3 fe 3 cr al they contain appreciable amounts of zno 1 wt and low contents of coo and nio the detrital chromian spinels in the metasediments have intermediate values of cr 0 4 0 8 and low values of mg 0 and y fe 0 but they have high contents of zno up to 10 wt the olivines in the peridotites contain inclusions of high cr chromian spinel rather than magnetite the peculiar characteristics of the nagasawa metaperidotites and hornfelses resulted from a contact metamorphic event at 500 c that was accompanied by interactions between peridotite and sediment that were facilitated by circulating hydrothermal fluids the mobile zinc in these fluids was possibly derived from the sediments