intraplate extension prior to continental extrusion along the ailao shan red river shear zone left lateral movement of the ailao shan red river shear zone lends support to the hypothesis of continental extrusion resulting from the collision of india with asia our new observations from northwestern yunnan china and northwestern vietnam on different sides of the shear zone demonstrate that the sinistral offset was 600 km according to correlations of permian triassic flood basalt successions and late paleogene highly potassic mafic magmas we conclude that the shear was propagating on the south china continental margin and does not correspond to a suture between south china and indochina furthermore the highly potassic magmas were emplaced from ca 40 to 30 ma before the shear movement which was caused by the late oligocene to early miocene ca 27 22 ma extrusion activity this suggests that a late eocene to early oligocene intraplate extension possibly induced by delamination of thickened continental lithosphere took place in northwestern yunnan or eastern tibet as a response to the india asia collision this extension and sea floor spreading of the south china sea that began ca 30 ma could have accounted for the initiation of the ailao shan red river shear zone

origin ore forming fluid evolution and timing of the logrosan sn w ore deposits central iberian zone spain the logrosan sn w ore deposits in the metallogenic sn w province of the european variscan belt consist of endo and exogranitic greisen type and quartz cassiterite veins associated with a s type granite mineral characterization fluid inclusion study isotope geochemistry and ar ar geochronology have been combined in order to reconstruct the conditions for sn w mineralization the endo and exogranitic mineralization must have been developed in a relatively long lived system 308 303 ma during or soon after the emplacement of the logrosan related granite at ca 308 ma the mineralizing fluids are characterized by complex aqueous and volatile h2o n2 co2 ch4 nacl fluid inclusions microthermometry and raman analyses indicate that fluid composition evolved from n2 ch4 to n2 rich followed by co2 rich fluids with varying amounts of h2o the presence of n2 and ch4 suggests the interaction with fluids derived from the nearby metasedimentary host rocks a model of host rock interaction assimilation and mixing of metamorphic and magmatic fluids resulting in change of the redox conditions is proposed for tin deposition later sulfide minerals were precipitated as a result of pressure and temperature release

the rootzone of an ancient hydrothermal system exposed in the troodos ophiolite cyprus abstract a well exposed oceanic hydrothermal rootzone in the troodos ophiolite cyprus is composed of the basal sheeted dikes the upper 50 100 m of the gabbroic sequence and a contact aureole interpreted as a fossil conductive boundary layer the basal sheeted dikes and upper gabbros were pervasively altered at high temperatures 400 775 c and cut by a series of broadly distributed mineralized faults that are oriented parallel to the strike of the sheeted dike complex fluid inclusion and mineralogical data for fault breccias show that circulating fluids were 350 400 c fairly oxidizing and relatively h2s rich with a range in salinity 1 5 7 8 wt nacl equivalent field and petrological evidence suggest that the rootzone migrated from the basal sheeted dikes into the upper gabbros and back again perhaps for several cycles these migrations and p t oscillations in the conductive boundary layer were likely caused by fluctuations in thermal conditions imposed by the magmatic system at a spreadin

metamorphism and deformation along the emplacement thrust of the samail ophiolite oman abstract the base of the samail ophiolite is a metamorphic shear zone that provides key information about ophiolite emplacement the classic exposure of this metamorphic sole at wadi tayin consists of a few meters of garnet clinopyroxene mafic gneiss and 80 m of upper amphibolite facies mafic and quartzose tectonite overlying 150 m of lower amphibolite facies mafic and sedimentary schist the sole is bounded by faults but is internally coherent peak metamorphic temperatures decrease exponentially from 825 c at the top of the sole to 500 c at its base as documented by changes in grain size hornblende composition the disappearance of chlorite and epidote garnet clinopyroxene element partitioning and garnet hornblende partitioning the inverted peak temperature gradient decreases from 4000 c km in the upper 50 m to 100 c km in the lower 150 m contrary to earlier reports greenschist facies tectonites are not present at wadi tayin although the entire sole underwent greenschist facies retrogression driven by the influx of fluids from the overlying peridotite or underlying carbonate calculations based on an amphibolite constitutive relation reveal that although the section represents a lithologic continuum the lower 200 m of the sole is a relatively undeformed block whereas the uppermost 25 m represent material initially separated by tens to a couple of hundreds of kilometers and now integrated into a single column

genesis of the vein type tungsten mineralization at nyakabingo rwanda in the karagwe ankole belt central africa the vein type tungsten deposit at nyakabingo in the central tungsten belt of rwanda is located in the eastern flank of the complex bumbogo anticlinal structure the host rock is composed of alternating sequences of sandstones quartzites and black pyritiferous metapelitic rocks two types of w mineralized quartz veins have been observed bedding parallel and quartz veins that are at high angle to the bedding which are termed crosscutting veins both vein types have been interpreted to have been formed in a late stage of a compressional deformation event both vein types are associated with small alteration zones comprising silicification tourmalinization and muscovitization dating of muscovite crystals at the border of the veins resulted in a maximum age of 992 4 1 5 ma this age is within error similar to the ages obtained for the specialized g4 granites i e 986 10 ma the w bearing minerals formed during two different phases the first phase is characterized by scheelite and massive wolframite while the second phase is formed by ferberite pseudomorphs after scheelite these minerals occur late in the evolution of the massive quartz veins sometimes even in fractures that crosscut the veins the ore minerals precipitated from a h2o co2 ch4 n2 nacl kcl fluid with low to moderate salinity 0 6 13 8 eq wt nacl and minimal trapping temperatures between 247 and 344 c the quartz veins have been crosscut by sulfide rich veins based on the similar setting mineralogy stable isotope and fluid composition it is considered that both types of w mineralized quartz veins formed during the same mineralizing event given the overlap in age between the g4 granites and the mineralized quartz veins and the typical association of the w deposits in rwanda but also worldwide with granite intrusions w originated from the geochemically specialized g4 granites intense water rock interaction and mixing with metamorphic fluids largely overprinted the original magmatic hydrothermal signature

comment and reply on pacific margins of laurentia and east antarctica australia as a conjugate rift pair evidence and implications for an eocambrian supercontinent

miocene magnesian andesites and dacites evia greece adakites associated with subducting slab detachment and extension abstract mid miocene volcanic rocks are rare in the aegean region although early miocene and late miocene quaternary volcanism is widespread at oxylithos island of evia 14 ma dacites form a dome or sub volcanic complex phreatomagmatic eruptions formed nearby rhyolitic pyroclastic surge deposits the calc alkaline dacites contain bronzite mg rich clinopyroxene phlogopite and plagioclase phenocrysts the magma results from mixing of a mg rich andesitic magma similar to that in the nearby island of skyros with more felsic magmas represented by the rhyolitic pyroclastics the dacites are geochemically similar to adakites which are derived by partial melting of eclogitic subducted oceanic crust and have low y and yb and high sr y ratio 87 sr 86 sr 0 7095 is found in both dacite and rhyolite lead isotopic composition from the high mg andesite from skyros with 207 pb 204 pb 15 70 and 208 pb 204 pb 38 90 forms a linear trend with evia dacite and rhyolite with 207 pb 204 pb 15 71 and 208 pb 204 pb 39 05 the high temperatures required to produce such magma resulted from decompression due to extension of the aegean basin at the same time as the initial intrusion of the detached subducted slab in the western aegean that has been imaged by seismic tomography the oxylithos rocks extend the known occurrences of adakite series rocks this series is not restricted to sites with subduction of young oceanic crust

fluid fractionation of tungsten during granite pegmatite differentiation and the metal source of peribatholitic w quartz veins evidence from the karagwe ankole belt rwanda abstract the identification of a magmatic source for granite associated rare metal w nb ta and sn mineralisation in metasediment hosted quartz veins is often obscured by intense fluid rock interactions which metamorphically overprinted most source signatures in the vein system in order to address this recurrent metal sourcing problem we have studied the metasediment hosted tungsten bearing quartz veins of the nyakabingo deposit of the karagwe ankole belt in central rwanda the vein system 992 2 ma is spatiotemporal related to the well characterised b rich f poor g4 leucogranite pegmatite suite 986 10 ma to 975 8 ma of the gatumba gitarama area which culminated in nb ta sn mineralisation muscovite in the nyakabingo veins is significantly enriched in granitophile elements rb cs w and sn and show alkali metal signatures equivalent to muscovite of less differentiated pegmatite zones of the gatumba gitarama area pegmatitic muscovite records a decrease in w content with increasing differentiation proxies rb and cs in contrast to the continuous enrichment of other high field strength elements nb and ta and sn this is an indication of a selective redistribution for w by fluid exsolution and fluid fractionation primary fluid inclusions in tourmaline of these less differentiated pegmatites demonstrate the presence of medium to low saline h 2 o nacl kcl mgcl 2 complex salt e g rb cs fluids which started to exsolve at the g4 granite pegmatite transition stage laser ablation inductively coupled plasma mass spectrometry shows significant tungsten enrichment in these fluid phases 5 500 ppm fractional crystallisation has been identified previously as the driving mechanism for the transition from g4 granites less differentiated biotite biotite muscovite towards muscovite pegmatites and eventually columbite tantalite mineralised pegmatites the general absence of tungsten mineralisation in this magmatic suite including the most differentiated columbite tantalite mineralised pegmatites of the gatumba gitarama area emphasises the efficiency of fluid saturation to extract crystal melt incompatible tungsten from the differentiating melt phase fluid melt crystal partitioning calculations support the concept of a magmatic hydrothermal fluid source for tungsten and constrain the range of permissible crystal melt and fluid melt partition coefficients together with realistic values for water solubility in the parental g4 granitic melt consequently we propose that for highly differentiated b rich f poor granite systems fluid saturation started prior to or at the granite pegmatite transition stage resulting in apical to peribatholitic tungsten veins systems that are paragenetically older than the final pegmatite stage

precise 40ar 39ar ages from the metamorphic sole of the mersin ophiolite southern turkey abstract the mersin ophiolite is located on the southern flank of the e w trending central tauride belt in turkey it is one of the late cretaceous neotethyan oceanic lithospheric remnants the mersin ophiolite formed in a suprasubduction zone tectonic setting in southern turkey at the beginning of the late cretaceous the mersin ophiolite is one of the best examples in turkey in order to study reconstruction of ophiolite emplacement along the alpine himalayan orogenic belt 40ar 39ar incremental heating measurements were performed on seven obduction related subophiolitic metamorphic rocks hornblende separates yielded isochron ages ranging from 96 0 0 7 ma to 91 6 0 3 ma all errors 1 five of the seven hornblende age determinations are indistinguishable at the 95 confidence level and have a weighted mean age of 92 6 0 2 2 ma we interpret these ages as the date of cooling below 500 c intraoceanic thrusting occurred 4 ma soon after formation of oceanic crust the sole was crosscut by microgabbro diabase dikes less than 3 m y later the final obduction onto the tauride platform occurred during the late cretaceous early paleocene our new high precision ages constrain intraoceanic thrusting for a single ophiolite mersin in the tauride belt

late orogenic gold mineralization in the western domain of the karagwe ankole belt central africa auriferous quartz veins from the byumba deposit rwanda abstract gold mineralization in the kab karagwe ankole belt of central africa dominantly occurs as quartz lode and ferruginous breccia systems primary gold mineralization in the kab is possibly linked to a deformational event in the early neoproterozoic which is accompanied by shear movements and post compressional magmatism the byumba deposit is chosen due to its position in the kab mesoproterozoic metasedimentary stratigraphy akanyaru supergroup interesting structural trend nw trending shear zone and its closeness to magmatic intrusions explorative drillings from the byumba deposit in rwanda were logged and sampled for further petrographic investigation in situ rb sr geochronology laser ablation inductively coupled plasma triple quadrupole mass spectrometry and micro x ray fluorescence xrf major and trace element mapping distinct phases of folding and shear deformation are present at byumba and three main phases of quartz veining are identified pre syn post folding the first quartz vein generation is layer parallel and folded along the bedding indicative of a pre folding origin v1 generation a second generation of quartz veins v2a is concentrated in fold hinges and related to the primary folding of the layers cleavage parallel and often boudinaged v2b quartz veins are emplaced during late stage folding post folding v3 massive or sigmoidal quartz veins crosscut the folds and cleavage element mapping of the gold by using xrf shows that chlorite rich quartz veins v3 host the primary gold mineralization in the form of sub micron gold and of small disseminated blebs cores additionally recorded a supergene enrichment phase of gold at reduction zone boundaries in situ rb sr geochronology on white mica of the crosscutting post folding quartz veins v3 point towards an early neoproterozoic age of 900 ma the gold mineralization at byumba is interpreted to be related to the regional early neoproterozoic compressional deformation event based on 1 the post folding timing of the auriferous veins 2 the observation of shear related textures both in the host rock and the veins and 3 the location of the byumba deposit within a regional shear structure this association with compressional shearing points towards an orogenic origin for gold mineralization however a magmatic hydrothermal influence from contemporaneous granitic magmatism cannot be excluded

fluid inclusion and stable isotope constraints on the genesis of the world class zhuxi w cu skarn deposit in south china abstract scheelite mineralization accompanied by traces of chalcopyrite occurs in skarn systems in the giant zhuxi deposit in the taqian area northeastern jiangxi province south china three paragenetic stages of skarn with ore deposition have been recognized the prograde skarn retrograde skarn and hydrothermal quartz sulfide stages microthermometer data showed that the prograde skarn were formed at relatively high temperatures 450 500 c and pressures 0 97 1 38 kbars and low oxidation to weak reduction conditions the raman microprobe data demonstrated that the fluid of the stage was h2o nacl kcl cacl2 ch4 c2h4 system the retrograde skarn alteration formed at 1 24 kbars and relatively medium temperatures 280 320 c and the ore forming fluid was h2o nacl kcl cacl2 system during the hydrothermal quartz sulfide stage the fluids were methane bearing aqueous low salinity 0 9 8 0 wt nacleqv and low homogeneous temperature 160 240 c correspondingly the minimum fluid pressures were evaluated between 75 and 252 bars the h and o isotopic values from the prograde and retrograde stages implied that the mineralizing fluids were derived principally from magmatic water the mineralizing fluids in the quartz sulfide hydrothermal stages were mingled with magmatic water with some meteoric water as well as minor contributions of wall rocks the sulfur and lead isotopes indicated that ore forming materials including sulfur lead copper zinc and iron in the zhuxi deposit were derived from local magmatic and sedimentary sources geological fluid inclusion and isotopic data supported that the zhuxi deposit is a typical skarn type w cu deposit related to ilmenite series granitic rocks and formed in a compressional tectonic environment

counterclockwise p t t trajectory from the metamorphic sole of a neo tethyan ophiolite turkey abstract the kiziltepe ophiolitic thrust sheet in the bolkar mountains of turkey occurs between two subparallel ophiolite belts bounding the tauride carbonate platform and represents a remnant of the cretaceous neo tethyan oceanic lithosphere it is underlain by foliated amphibolite that represents a metamorphic sole developed at the inception of an intra oceanic subduction zone in the neo tethys 92 90 ma blueschist facies overprinting of the amphibolite indicates that the metamorphic sole was dragged deeper into the subduction zone where it experienced increasing p t with cooling regional tectonic constraints suggest a maastrichtian age for the timing of this blueschist facies metamorphism sodic amphibole rich veins and crossite mg riebeckite rims on hornblende suggest that growth of blueschist facies minerals was facilitated by infiltration of fluid along fractures and grain boundaries we infer a counterclockwise p t t trajectory during which metamorphism was accompanied succeeded by rapid uplift along the northern edge of the tauride belt in late cretaceous early tertiary time

phase relations of epidote blueschists phase diagrams have been computed for minerals in the system ncmash using the optimised thermodynamic data base of berman 1988 augmented by data on glaucophane and pumpellyite to illustrate the relationships between rocks containing epidote and sodic amphibole and those containing assemblages characteristic of neighbouring metamorphic facies the stoichiometries of stable bounding equilibria yield a simple list of mineral parageneses that serves to define an epidote blueschist facies and surrounding metamorphic facies the calculations suggest that when iron free the pairs zoisite glaucophane and clinozoisite glaucophane do not possess a stability field the natural fe bearing paragenesis of epidote sodic amphibole occupies a closed field in the pt diagram whose size increases dramatically as the ratio fe3 al in the minerals is increased where garnet is involved in bounding reactions such as with respect to eclogite and garnet bearing albite epidote amphibolite the epidote blueschist field decreases in size as the ratio fe2 mg is increased the classic blueschist to greenschist transition reaction is best developed in rocks with large fe3 and fe2 contents and is possibly metastable in mgal rich rocks in the latter a wedge of albite epidote actinolite barroisite amphibolite separates the epidote blueschist from the greenschits paragenesis field a first occurrence of garnet divides the epidote blueschist field into a lower temperature garnet free zone and a higher temperature garnet bearing zone although the temperatures calculated for this garnet isograd seem high the locations of all the facies boundaries are influenced by h2o activity however even minor amounts of co2 in the fluid phase x co2 0 005 render the epidote sodic amphibole paragenesis metastable so that facies boundaries cannot be shifted by varying the co2 content of the fluid phase below 600 c no mutual boundaries exist between the amphibolite facies and the epidote blueschist and eclogite facies respectively the calculated low pressure limit for the eclogite paragenesis pyralspite omphacite jd 50 quartz typical metabasite compositions is 12 to 14 kbars with h2o activity 0 9 or 1 5 to 2 kbar lower for jd 30

the origin of kibaran late mid proterozoic tin tungsten and gold quartz vein deposits in central africa a fluid inclusions study composition and evolution of fluids depositing tin and tungsten ores in kibaran quartz vein deposits allow the modelling of devolatizing evolved granites as their source at depth fluids forming gold quartz veins and breccias are different from the first especially by showing characteristics of a high pressure environment all deposits are controlled by compressional deformation whose fading phases affect earlier formed veins these findings lead to the conclusion that both anatectic melting resulting in intrusion of fertile granites and the generation of fluids forming gold deposits are the final consequence of deep crustal metamorphism the latter was caused by crustal thickening immediately preceding the metallogenetic climax

narrow subducting slabs and the origin of backarc basins abstract both tectonic and seismic data suggest that horizontally wide in the direction parallel to the axis of the trench dipping subducting slabs typically move stably at fixed angles to the horizontal and that their associated trenches migrate very slowly or not at all in contrast where backarc basins babs exist the associated trenches typically migrate at rates of several cm year this behavior may be related to the finite width of the slabs under babs analysis shows that hydrodynamic suction created by the downdip motion of a wide slab is sufficient to balance the pull of gravity on the heavy slab however for a narrow slab sideways asthenospheric flow into the slab overriding plate corner effectively eliminates the suction which results in the rapid sinking of the slab hydrodynamic modeling is used to quantify this hypothesis modeling is used also to show that wide slabs are stable in their inclined position not because of the gravitational wing like sliding but due to the lifting force produced by the negative pressure suction in the slab overriding plate corner finally the model results confirm that for relatively narrow slabs this lifting force is small and cannot counterbalance gravity therefore when bending moments in a narrow slab exceed the plastic yield limit the slab becomes unstable this instability can cause the rapid migration of the slab s hinge and the associated arc at rates of 10 20 cm year and thus form the bab

discovery of epidosites in a modern oceanic setting the tonga forearc the first suite of oceanic epidosites has been recovered from the tonga forearc epidosites are metal depleted rocks composed of subequal proportions of epidote and quartz that lack any original igneous texture epidosites have been interpreted to form in upflow zones at the base of ore forming oceanic hydrothermal systems that vent as black smokers on the sea floor tongan epidosites metasomatically replaced basaltic and plagiogranite protoliths and formed under similar conditions to epidosites hosted in many ophiolites interpreted to have formed in a suprasubduction zone setting the similarities shared by the tongan and suprasubduction zone ophiolite epidosites along with the absence of epidosites in collections from modern mid ocean ridges suggest that tectonic setting plays a role in their formation

spatial and temporal variation in late cenozoic back arc volcanic rocks aegean sea region abstract the aegean sea and adjacent areas comprise thinned continental crust forming an extensional back arc basin behind the south aegean arc in this back arc region are many small volcanic centres with a wide range of volcanic products this late miocene to quaternary volcanism is volumetrically minor in contrast to the voluminous igneous activity of the south aegean arc the age and major and trace element geochemistry of these back arc volcanic rocks are reviewed from the literature where necessary new geochemical analysis have been made five magma types are recognised continental alkaline basalts sodic basalts with incompatible element enrichment and nb depletion trachytes shoshonites and calc alkaline andesites all but the continental alkaline basalts appear geochemically related to subduction processes there is a regular distribution of rock types in time and space the subduction related rocks occur in a broad arc between the modern south aegean arc and the early to middle miocene volcanicity of the central aegean region sodic basalts occur in the most inboard regions and are later than the trachytes shoshonites and calc alkaline magmas occur in the most outboard regions this back arc volcanism occurs only in areas of abundant shallow seismicity however areas of extension and seismicity in the northeast aegean distant from the subducting slab lack volcanoes with the exception of the continental alkaline basalts the magmas are derived only from mantle influenced by the aseismic part of the hellenic subduction slab between 200 and 400 km depth back arc extension associated with rotation of the aegean microplate and strike slip motion on the north anatolian fault system has created a fault system allowing these magmas to reach the surface analogous processes may be common in the final stages of closure of ancient orogens

petrogenesis of the concordia granite gneiss and its relation to wmo mineralization in western namaqualand south africa abstract the 1 1 ga concordia granite gneiss cgg is part of the late to postorogenic spektakel suite in the western namaqualand metamorphic complex south africa it intruded synkinematically with respect to the main d2 deformation event into lower to middle crustal rocks and granite emplacement was more or less coeval with the peak of granulite facies metamorphism 800 c 5 kbar several genetically related rock types megacrystic garnet bearing granite minor aplitic leucogranites and pegmatites are distinguished all varieties are sio 2 rich 69 79 wt peraluminous granites and show subalkaline monzonitic magma characteristics geochemical differences in whole rock chemistry between megacrystic granite and aplitic leucogranites e g lower al 2 o 3 mgo cao ba zr higher k 2 o rb nb w rb sr ga al and the decrease of e g cao mgo fe 2 o 3 ba zr th u with increasing sio 2 in the megacrystic granites as well as the variation in fe mn of magmatic garnets are best explained with crystal fractionation processes fractional crystallization of plagioclase produced potassium and silica rich residual melts characterised by very high rb sr rb ba u th mn fe ratios and higher concentration of w cu and zn crystal fractionation processes also resulted in a relative lree depletion and hree enrichment megacrystic granite la lu cn 8 87 31 67 aplitic leucogranite and pegmatites la lu cn 0 71 1 44 and evolution of pronounced negative eu anomalies the crystallization sequence near solidus crystallization of biotite prior to alkali feldspar suggests that the cgg magmas were h 2 o undersaturated over a long period of their evolution water saturation during late stage crystallization is however indicated by coarse late stage eutectic mineral textures pegmatites and w mo bearing siliceous rocks furthermore the economic potential of the cgg is supported by its geochemical signature e g high u th contents which is similar to evolved high heat production hhp granites the granitic magmas are attributed to partial melting of peraluminous crustal source rocks and are tentatively interpreted as fractionated s type granites the w mo deposits represent vein type and pegmatitic deposits genetically related to a deep seated granitic system

vein type tungsten deposits in rwanda rutsiro area of the karagwe ankole belt central africa abstract the rutsiro w sn nb ta deposit consists of vein hosted metal ores occurring within a black schist dominated formation of the rwandan proterozoic metasediments supergroup the collected samples revealed that the mineralised veins are mostly composed of quartz and ferberite the ore microscopy studies of the polished sections of the mineralised vein material confirmed ferberite as the main w mineral in the quartz veins ilmenite occurs within the ferberite crystals as inclusions in addition to quartz the concentrate samples contain a variety of minerals including cassiterite crystals tourmaline and free ilmenite and rutile crystals which came from the hosting black schists the sem eds analysis showed that free crystals of ree minerals such as monazite and xenotime are common in the concentrate samples electron probe microanalysis using wavelength dispersive spectrometry wds confirmed the composition of w and sn minerals to correspond to ferberite as the main w ore mineral together with some scheelite and cassiterite for tin ore mineral the microprobe analysis also revealed the occurrence of columbite tantalite and the association of indium and molybdenum traces with scheelite the analysed monazites fall into two groups monazite ce and monazite nd the proposed mineralisation history suggests that there is a replacement stage of scheelite ca2 by hydrothermal fe2 to produce reinite which is a ferberite polycrystalline pseudomorph the occurrence of sulphides such as arsenopyrite pyrite and pyrrhotite implies at least one sulphide phase in later stages of mineralisation the occurrence of tourmaline as inclusions within cassiterite crystals and the tourmalinisation of the country rocks close to the mineralised veins confirm tourmaline as an important pathfinder for w sn mineralisation the rutsiro w sn mineralisation is classified as a vein type deposit however two granitic cupolas underneath the deposit which are probably greisenised are suggested as source of the metal rich mineralising fluids geophysical works are required to investigate the presence of these cupolas

fundamental parameters for the formation of granite related tin deposits five principal parameters are considered fundamental for the formation of a granite related tin deposit 1 tin deposits occur associated with the latest phase of post kinematic granites in extensional and or shear tectonic regimes 2 the source granites are geochemically anomalous 3 source granites were h2o poor melts of low viscosity which ascended deep fractures passively to a high crustal level 4 inward crystallization from a pressure quenched carapace enabled mulitphase granite crystallization subsolidus alteration and encapsulation of exsolved fluid within cupolas or upward projection from the flattest upper contact of the granite 5 extremely late stage fluid release generally along pre intrusion weakness occurs after the bulk of the granite has crystallized sudden release of large fluid overpressures forms breccias whereas sudden release of slight fluid overpressures creates a sheeted vein system or a replacement deposit if porous permeable reactive rocks are encountered these hydrothermal systems have a long history of repeated hydrofracturing fluid circulation alteration precipitation and sealing in contrast to sheeted greisen systems wherein fluid overpressures were never excessive and a long fluid residence time created a protracted period of post magmatic alteration

lithostratigraphic and tectonic setting of gold mineralization in the archean cratons of tanzania and uganda east africa abstract the archean cratons of tanzania and uganda are reviewed in terms of their lithostratigraphy and tectonometamorphic events the earliest of which is dated at 2900 ma set in granitoid cratonic terrane the greenstone belts of the nyanzian system 2700 2500 ma form the main host rocks to syngenetic and epigenetic types of gold mineralization stratabound gold mineralizations occurring in the banded ironstone formation of the greenstone belts may have been remobilized during major metamorphic magmatic events at 2500 ma and partly redistributed in the form of hydrothermal auriferous quartz veins subsequently metasomatic gold mineralization in the form of impregnations occurred the location of gold mineralization is controlled by lithology metamorphism and tectonics and is discussed in the context of crustal evolution

lead isotopic compositions of neogene volcanic rocks from the aegean extensional area abstract lead isotope composition has been determined for a diverse suite of cenozoic igneous rocks in the central and north aegean sea these include voluminous miocene shoshonitic and calc alkali volcanic rocks and correlative plutonic phases middle miocene adakitic high mg andesite and associated rhyolite and middle miocene to quaternary ne normative basalt trachyte and k rich andesite the trace element contents of most of the studied rocks suggests that pb isotopic composition is not a result of upper crustal contamination but rather is a feature of the mantle source for k rich rocks this mantle source is represented by primitive mafic to intermediate volcanic rocks with 87 sr 86 sr 0 7095 high 207 pb 204 pb and 208 pb 204 pb 1000 ppm sr and ba and 35 ppm pb the high 207 pb 204 pb requires a metasomatic origin that lowered u pb and increased th pb followed by a second stage residence time of 1 ga this k rich magma was generated as a result of late tertiary extension of the aegean in the south aegean arc there is more evidence for crustal contamination either from present crust or from subduction of nile derived sediment

fluid inclusion and isotopic c h o s and pb constraints on the origin of late mesozoic vein type w mineralization in northern guangdong south china abstract northern guangdong of south china occurs many large tungsten deposits including shirenzhang meiziwo and yaoling quartz vein type wolframite deposits in this study we carried out a detailed study on fluid inclusions and c h o s pb isotopic analyses of mineral separates for these three deposits in order to resolve the origin and evolution of ore forming fluids and the ore deposition mechanism the ore veins in these deposits present a similar mineral paragenesis that could be divided into three stages i pre ore stage ii syn ore stage and iii post ore stage the pre ore stage is the magmatic hydrothermal transition stage which is marked by fluid exsolution from the highly fractionated granites accompanied with potassic alteration and greisenization the syn ore stage can be further divided into two stages silicate oxide stage stage ii 1 that is characterized by significant tin tungsten mineral deposition and sulfide stage stage ii 2 that is characterized by abundant sulfide minerals deposition the post ore stage is dominated by quartz and fluorite the three deposits show similar temperature and salinity variations for syn ore and post ore stages in the shirenzhang deposit the fluid temperature and salinity range from 239 to 301 c and 1 4 to 8 7 wt nacl equiv in stage ii 1 from 206 to 256 c and 1 4 to 7 0 wt nacl equiv in stage ii 2 and from 186 to 236 c and 1 4 to 8 6 wt nacl equiv in stage iii in the meiziwo deposit the fluid temperature and salinity range from 242 to 310 c and 1 1 to 8 6 wt nacl equiv in stage ii 1 from 196 to 252 c and 1 4 to 8 1 wt nacl equiv in stage ii 2 and from 188 to 234 c and 1 2 to 7 3 wt nacl equiv in stage iii in the yaoling deposit the fluid temperature and salinity range from 230 to 304 c and 1 2 to 9 0 wt nacl equiv in stage ii 1 from 203 to 258 c and 1 4 to 6 5 wt nacl equiv in stage ii 2 and from 184 to 231 c and 1 6 to 8 3 wt nacl equiv in stage iii the ore forming fluids in the three deposits belong to a medium temperature low salinity h2o nacl system with trace amounts of volatile components including co2 ch4 and n2 the c h o isotope data 13cco2 18 9 to 6 7 18owater 1 1 to 6 3 dh2o 78 to 55 indicate that the ore forming fluids were mainly magmatic water which might be modified by fluid rock interaction and mixing with meteoric water sulfur 34s 7 0 to 0 3 and pb isotope data 206pb 204pb 18 531 18 693 207pb 204pb 15 728 15 753 208pb 204pb 38 902 39 081 suggest that the ore metal and sulfur are of magmatic origin the involvement of both organic matter in metasedimentary rocks during fluid rock interaction and oxidized meteoric water may have been responsible for the negative 34s and 13c values fluid boiling fluid rock interaction and minor input of meteoric water might have been effective factors for the precipitation of wolframite cassiterite and scheelite the sulfide precipitation may have resulted from cooling and dilution of magmatic fluids by mixing with meteoric water

multiple fluids involved in granite related w sn deposits from the world class jiangxi province china abstract this paper aims at providing new insights into w sn ore forming processes within one of the largest granitic provinces in the world nanling range south china which was emplaced during the jurassic cretaceous period the origin composition and pressure temperature conditions of fluids involved in the w sn ore forming processes have been investigated by microthermometry raman spectroscopy la icpms hydrogen isotope analyses of fluid inclusions and oxygen isotope analyses of minerals from the maoping and piaotang w sn deposits for each deposit pre quartz syn wolframite and cassiterite and post ore quartz topaz and fluorite minerals were studied in both deposits the vast majority of fluid inclusions are aqueous with salinities between 0 0 and 12 6 wt equiv nacl and homogenization temperatures between 136 and 349 c a minor proportion 5 of inclusions observed in the ore stage quartz from maoping have aquo carbonic compositions for both deposits four compositional groups are defined early quartz fluid inclusions are characterized by salinities between 0 4 and 9 0 wt equiv nacl trapping temperatures between 150 and 350 c and pressures between 20 and 150 mpa la icpms analyses of these fluid inclusions reveal a wide range of na k and li concentrations as well as relatively low metal contents w at piaotang fluid inclusions in wolframite as well as in post ore quartz and fluorite have salinities of 5 6 to 12 6 wt eq nacl trapping temperatures between 150 and 400 c and pressures of 20 to 150 mpa la icpms analyses of these fluid inclusions reveal similar compositions to early quartz fluid inclusions values of 18o in wolframite and quartz crystals range from 0 8 to 5 2 vsmow while the d values of fluid inclusions range from 66 to 62 vsmow collectively the data suggest the involvement of four aqueous fluid end members mixed episodically in the mineralization process a a low salinity low temperature metal poor low 18o low d fluid derived from meteoric water b a high salinity high temperature metal poor high 18o low d fluid derived from a differentiated peraluminous granitic magma c a high salinity high temperature metal rich lower d magmatic fluid derived from a more differentiated peraluminous granitic magma and d a high salinity high temperature metal rich high 18o low d magmatic fluid this study shows that multiple fluids with distinct magmatic and meteoric origins were involved in the formation of these w sn deposits and that the dilution of metal bearing magmatic fluids by meteoric fluids was probably the main driver for ore deposition the common fluid history of the two deposits studied as well as similarities with other deposits in the jiangxi province points toward common ore forming processes at the regional scale

high mg andesites from the southern termination of the new hebrides island arc sw pacific abstract in the southern new hebrides arc magmatic and tectonic processes are closely linked between 21 s and 22 s a normal broadly arc tholeiitic magmatic suite is essentially similar to those occurring in the main part of the arc whereas south of 22 s at the southern termination of the arc a high mg andesite suite appears with the more mafic endmembers having mineralogical and compositional affinities to high ca boninites during the last 2 ma this southern termination propagated southwards and played a continuing role in the transient transform junction between the southern tip of the trench and the n s spreading axis of the north fiji basin in this region which has been dominated for several million years by fast transform motions the low magma production rates in this section of the arc and the unusual boninite related affinities of the arc volcanics may be due to a combination of a subducting slab torn by hinge zones abnormally small depth to slab distances beneath volcanoes and an unusually hot ambient geotherm due to rising diapirs supplying the intersecting back arc spreading axis the on going collision between the loyalty ridge and the arc may also contribute to these unusual magmatic characteristics as presently only a very small amount of convergence occurs along the southernmost segment of the trench in such a complex tectonic setting with a spreading axis propagating into an intra oceanic arc and major transform plate motions generation of high ca boninitic magmas occurs by melting of a refractory hydrated mantle at a shallow level under the hunter ridge the necessary extra heat being supplied by the rising diapirs supplying magmas to the intersecting spreading ridge axis the high mg andesite suite probably results via a combination of fractional crystallization from such high ca boninitic parental magmas and subsequent magma mixing processes operative all along the southern termination of the arc

the role of deformation in the formation of metamorphic gradients ridge subduction beneath the oman ophiolite two tectonic scenarios have been proposed for genesis and emplacement of the oman ophiolite one suggests that the ophiolite was generated at a spreading center the other suggests generation within an intraoceanic arc an integrated thermal and kinematic model of the temperature stress rock type and displacement fields during early stages of the emplacement of the oman ophiolite was developed to test these two possibilities the thermal evolution was calculated by a finite difference algorithm for heat conduction considering heats of metamorphic reactions deformational heating heat advection by flowing rock mantle heat flow and radioactive heating the stress and displacement fields were calculated by an analytical model using a velocity boundary condition power law constitutive relations and a brittle frictional sliding relationship field observations in oman can be satisfied with the spreading center model but not with the arc model moreover simulations indicate that the ophiolite was probably 2 m y old at the time of intraoceanic thrusting and that shear stresses of 100 mpa were attained during thrusting in addition to satisfying currently available field constraints these simulations also indicate specific further work that will help resolve the controversy over thevolution of this unique orogen future work should be directed toward 1 using thermobarometry and thermochronology to constrain the spatial variation in ptt paths of the metamorphic sole 2 dating the igneous genesis of the ophiolite and 3 using quartzite and dunite recrystallized grain size paleopiezometers to infer stresses during thrusting these simulations point out the important effect synmetamorphic deformation can have on the formation of metamorphic field gradients in contractional fault zones metamorphic field gradients may be normal or inverted and contracted or extended relative to the initial thermobarometric gradient and adjacent fault zone rocks may display crossing pt paths unusually contracted or extended metamorphic field gradients may also develop in the footwalls of normal fault zones where portions of the heating hangingwall accrete to the uplifting cooling footwall

petrogenesis of a tholeiite boninite sequence from ayios mamas troodos ophiolite evidence for splitting of a volcanic arc lavas and intrusives from ayios mamas at the western end of the limassol forest complex south of the arakapas fault belt troodos comprise three groups a a basal sequence of pillow lavas intruded by dikes and sills with the phenocryst assemblage plag cpx mt ol b a middle sequence of pillows and massive flows cpx plag ol and c an upper sequence mostly of pillows and breccia ol opx cpx sp the lower group is tholeiitic and the upper two groups boninitic group c whole rock and glass mg numbers exceed 72 and range up to 78 with corresponding olivine compositions of fo92 they are strongly depleted in high field strength elements and on the basis of cr y variation and major element mass balances are interpreted to reflect partial melting of a basalt depleted cpx harzburgite source group a tholeiites in contrast are interpreted to derive from a more fertile lherzolite source magmaphile and compatible trace element variation is best modelled after an open magma system involving periodic replenishment tapping and fractionation although neither tholeiites nor boninites represent steady state compositions the boninites belong to a cao rich sio2 poor variant resembling those from guam but contrasting with those from bonin and cape vogel morb normalized enrichment of low field strength elements k sr ba and rb in all groups indicates subduction derived source contamination it is proposed that the ayios mamas section reflects splitting of a volcanic arc prior to back arc spreading in an environment analogous to that of the present day andaman sea

geological setting and timing of the world class sn nb ta and li mineralization of manono kitotolo katanga democratic republic of congo the central african mesoproterozoic kibara belt in katanga drcongo forms a metallogenic province that hosts a variety of granite related mineralization rich in cassiterite columbite tantalite wolframite ferberite spodumene and beryl this mineralization is mainly present in pegmatites and quartz veins that are thought to be associated with the youngest granite generation in the kibara belt i e so called e group granite generation manono kitotolo is one of the world s largest sn nb ta and li mineralized pegmatites with a large resource of spodumene columbite tantalite and cassiterite still remaining mineralized pegmatites have intruded along the foliation in the mesoproterozoic metasedimentary rocks and dolerites of the manono kitotolo area the pegmatites have been emplaced very late during the climax of the kibaran orogeny probably during the transition from orogenic collapse to extensional tectonics based on their structural position the emplacement of the pegmatites resulted in an intense alteration of the doleritic and metasedimentary host rocks resulting in muscovitization tourmalinization and silicification a mineralogical and geochemical zonation typical for granitic pegmatites has been identified that was affected by metasomatic hydrothermal alteration mainly albitization and greisenization the latter alteration is associated with the main phase of cassiterite mineralization a first stage with nb ta li and minor sn however already formed pre alteration directly associated with pegmatite crystallization 40ar 39ar muscovite dating of unaltered pegmatites resulted in ages of 938 8 5 1 ma and 934 0 5 9 ma these ages are overlapping with the u pb age of the nb ta mineralization 940 5 1 ma also giving new temporal constraints on the fertile e group granite generation in the kib the 40ar 39ar muscovite age of a mineralized greisen is 923 3 8 3 ma which is younger but still partly within error of the 40ar 39ar ages of the unaltered pegmatites based on the partial overlap in age but also on field and paragenetic relationships and the stable isotope composition of the fluids this greisenization and associated cassiterite mineralization can still be linked to the pegmatite crystallization

late cretaceous early tertiary convergence between the indian and arabian plates recorded in ophiolites and related sediments abstract remnants of ocean floor forming the eastern ophiolite belt in oman and the western ophiolite belt in pakistan have a common plate tectonic history culminating in emplacement at the cretaceous tertiary boundary fragments of ocean floor in these two belts have ages between 150 and 65 ma and recorded tectonic events in the early indian ocean at 150 ma 130 120 ma 110 100 ma and 70 65 ma new radiometric and chronostratigraphic ages and paleomagnetic and sedimentary information are used to relocate these ophiolites in the frame of the evolving indian ocean which was characterised by the stepwise breakup of gondwana at 158 ma east and west gondwana 130 ma southern atlantic breakup of east gondwana 95 84 ma madagascar and india seychelles 65 ma india and seychelles and finally at 40 ma rifting between africa and arabia the 150 ma old oceanic rocks of masirah island in oman originally formed the extension of the basins now preserved along the eastern edge of the afro arabian plate masirah drifted together with india seychelles when a new ridge formed at approximately 130 ma separating microplates e g kabul block from the northern edge of greater india parts of this new ocean were later emplaced to form the western ophiolite belt in pakistan consumption of oceanic crust and emplacement of ophiolites during the closure of the neotethys occurred not only north of india seychelles but also along two or three subduction zones between the afro arabian plate and india seychelles documented by the formation of metamorphic soles beneath ophiolites relics of ocean floor formed approximately 65 70 ma ago now form the upper units in the western ophiolite belt of pakistan where they overlie an accretionary prism of pillow lavas of dominantly aptian albian age and sedimentary rocks the breakup of india and the seychelles at 65 ma was associated with the eruption of flood basalts deccan and seychelles and enhanced counter clockwise movement of india this breakup possibly caused the late jurassic early cretaceous fragments of ocean floor e g masirah to be thrust onto the eastern edge of oman the ophiolites which were emplaced onto the western edge of india drifted as part of this plate farther north where collision with eurasia and the accreted microplates occurred in the eocene 55 ma

ecological and paleoenvironmental information using stable isotope profiles from living and fossil molluscs stable oxygen and carbon isotopic profiles from recent specimens of spisula solidissima and placopecten magellanicus collected off the virginia coast are used to calibrate annual temperature controlled cycles in the 18o profile and shell growth characteristics the amplitude of the annual 18o cycles decreases with increasing water depth and may be used as a relative depth indicator the isotopic of profiles of two fossil specimens of pleistocene spisula are very similar in amplitude and character to modern analogues and verify the formation of annual growth increments in these fossil specimens offsets between the 18o profiles of the fossil and modern specimens are related to known temperature and ice volume effects during the late pleistocene and support the utility of mollusc isotope records as a stratigraphic tool carbon isotopic profiles from all specimens show light values in the spring and or fall associated with seasonal phytoplankton productivity offsets in the carbon isotopic composition between infaunal and epifaunal species may be related to sediment porewater geochemistry and oxidation of organic matter trends toward lighter 13c values through ontogeny suggest the effects of metabolic changes from a fast growing juvenile into a slower growing sexually mature adult anomalies in both the oxygen and carbon isotopic profiles identify the formation of minor shell growth increments as corresponding to hydrographic perturbations and to winter cold temperatures

boninitic volcanism in the oman ophiolite implications for thermal condition during transition from spreading ridge to arc the discovery of boninite a typical high mgo andesite in the oman ophiolite is reported the boninites in the oman ophiolite occur as lavas and dikes of the alley volcanic sequence that overlie or crosscut the spreading ridge derived lavas geotimes volcanic sequence and sheeted dikes the phenocryst mineral assemblage and the major and trace element compositions observed for these boninites resemble those of the izu mariana forearc boninites indicating that the alley boninites represent primitive melt generated by partial melting of hydrous peridotite the occurrence of boninite provides strong thermal and chemical constraints on the formation of the oman ophiolite that require hot hydrous shallow mantle 1250 c at 30 km depth to have underlain the proto oman ophiolite at the time of boninite generation the initiation of subduction of the young hot oceanic lithosphere and obduction of the future oman ophiolite near the spreading ridge and the resultant melting of the highly depleted shallow mantle wedge metasomatized by slab derived fluid represent the most favorable mechanism for the genesis of the alley boninites

metal source and wolframite precipitation process at the xihuashan tungsten deposit south china insights from mineralogy fluid inclusion and stable isotope abstract the xihuashan tungsten deposit hosted in the late jurassic granitic pluton in the nanling range of south china has a total resource of about 81 300 tonnes of wo 3 with an average ore grade of 1 08 wo 3 wolframite is the dominant ore mineral and intergrown with quartz in the main mineralization stage ore forming fluids trapped in wolframite have d and 18 o values from 82 to 64 and 7 4 to 8 8 respectively those in quartz have similar d 72 to 58 and 18 o 6 8 to 8 0 values indicative of a magmatic fluids simultaneously trapped by quartz and wolframite la icp ms analyses for individual fluid inclusion show that this mineralizing fluid contains measurable li rb cs k na ti cu zn as and w 1 to 125 ppm with average of 19 ppm while depleted in fe and mn the wolframite from the xihuashan tungsten deposit contains high feo 10 9 to 17 7 wt and mno 5 9 to 12 7 wt contents with fe fe mn atomic ratio of 0 46 to 0 75 thus requires the availability of external fe and mn we detect that the fe and mn contents in mica from the greisen are remarkably lower than primary mica from granite some magmatic micas were observed in greisen and were subjected to hydrothermal alteration compared to the core the rim of these micas depleted in fe mn f and na the siderite and pyrophanite are formed along cleavage planes of altered magmatic mica that are evidence to be due to fe and mn release during granite alteration thus we demonstrate quantitatively that magmatic fluids at xihuashan provide w in solution whereas the hosted granite alteration contributes fe and mn to precipitate wolframite it is also supported by wolframites have trace and rare earth elements characteristics similar to those of granite and some characteristics similar to the greisen therefore the ore forming fluids has components derived from the last highly evolved residual granitic melt and components acquired by releasing through the hosted granite alteration fluid rock interaction exert a principle control on wolframite precipitation based on mineralogy fluid inclusion and stable isotope we proposed three stage process to illustrate the genetical link between tungsten mineralization and granite

alkali metal and rare earth element evolution of rock forming minerals from the gatumba area pegmatites rwanda quantitative assessment of crystal melt fractionation in the regional zonation of pegmatite groups abstract this study presents a general model for the evaluation of rayleigh fractional crystallisation as the principal differentiation mechanism in the formation of regionally zoned common and rare element pegmatites the magmatic evolution of these systems from a granitic source is reconstructed by means of alkali element and rare earth element ree analyses of rock forming minerals feldspars micas and tourmaline which represent a whole sequence of regional pegmatite zonation the gatumba pegmatite field rwanda central africa is chosen as case study area because of its well developed regional zonation sequence the pegmatites are spatially and temporally related to peraluminous g4 granites 986 10 ma the regional zonation is developed around a g4 granite and the proximal pegmatites grade outwardly into biotite two mica and muscovite pegmatites rare element nb ta sn pegmatites occur most distal from the granite alkali metal fractionation trends in pegmatitic k feldspar rb 350 6000 ppm cs 2 160 ppm and muscovite rb 670 7000 ppm cs 10 150 ppm define a single and continuous trend which is modelled by rayleigh fractional crystallisation starting from a parental granite composition g4 composition k 3 1 wt rb 222 ppm cs 11 ppm the fractionation model shows moreover that the pegmatites adjacent to the parental pluton are the least fractionated and the distal pegmatites are the most fractionated biotite pegmatites form from 0 to 69 crystallisation two mica pegmatites from 69 to 92 and muscovite pegmatites from 92 crystallisation onwards the extreme rb and cs enrichment in rare element pegmatites requires at least 98 fractionation of the initial g4 granite composition mathematical derivation of the k rb versus cs relationship in k feldspars confirms rayleigh fractional crystallisation as the main differentiation process in the development of regional pegmatite zonation moreover 1 it demonstrates the continuity of the fractionation process from biotite pegmatites to rare element pegmatite and indicates a genetic link among them and 2 it allows a general evaluation of pegmatite fields in terms of system parameters i e the initial element concentration in the granitic melt and partition coefficients ree patterns of the rock forming minerals feldspars muscovite biotite and schorl show a distinct transition along the regional zonation they evolve from sloping fractionated to more flat patterns starting from the biotite up to the muscovite pegmatites minerals from the rare element pegmatites feldspars muscovite and elbaite show again more fractionated heavy ree depleted patterns the observed evolution in the ree patterns corresponds to early crystallisation of light ree enriched monazite and a late crystallisation of mainly cogenetic heavy ree enriched phases such as apatite columbite group minerals and beryl modelling of rayleigh fractionation starting from an initial parental granite g4 composition lan ybn 10 and ree 83 shows that this evolution in mineral ree pattern is the result of fractional crystallisation of the pegmatitic melt and precipitation of ree incorporating minerals such as monazite and apatite consequently trace element modelling indicates that rayleigh fractional crystallisation governs the mineralogical and geochemical evolution from a granite source to common and eventually rare element pegmatites this mechanism shows that granitic pegmatites are extremely fractionated residual melts which are genetically and directly connected to a main granite body

structure and metamorphism of blueschist eclogite facies rocks from the northeastern oman mountains the northern part of the saih hatat window oman shows high pressure metamorphic rocks derived from shelf sediments and pre premian continental basement and is atypical of sub ophiolite metamorphism elsewhere the high pressure rocks are divided into structural units originally bounded by foreland propagating thrusts formed during ophiolite obduction although now many contacts are backthrusts normal faults or extensional shear zones metamorphic breaks exist across many unit boundaries the deepest unit as sifah has eclogite facies assemblages in metabasites and metapelites which record evolution of p t conditions along a clockwise path culminating al 23 2 5kbar 540 75 c contrasting markedly with overlying units 5 10 kbar 200 500 c although separated by 40 ar 39 ar apparent ages our tectonic model relates all the high p units to a single convergent event in the late cretaceous the as sifah eclogites were exhumed in two stages i tectonic emplacement against other units at c 20 25 km depth and ii exhumation of the entire high p zone by culmination collapse after obduction

rapid emplacement of the oman ophiolite thermal and geochronologic constraints in understanding of ophiolite emplacement requires knowledge of the elapsed time between igneous crystallization and intraoceanic thrusting and the rate and duration of that thrusting hornblende 4oar 39ar ages demonstrate that the igneous oceanic crust in oman crystallized and cooled to 825 k in 1 2 m y hornblende ages from metamorphic rocks and from cross cutting dikes require that the basal metamorphic thrust fault beneath the ophiolite also cooled below 825 k in 1 2 m y motion along the sole thrust accounted for 200 km of displacement at a rate of 100 200 mm yr on the basis of age relationships and thermal considerations we favor a two stage model for the initial stages of samall ophiolite emplacement intraoceanic thrusting over 2 m y old lithosphere at 150 km m y parallel to a spreading ridge for 1 2 m y followed by equally rapid and brief thrusting over cold and old lithosphere preservation of the samail ophiolite is a direct result of its young age and positive buoyancy at the time of ocean closure and we propose that all ophiolites that originated near spreading centers and were emplaced onto continents were young at the time of intraoceanic thrusting

styles of back arc extension in the central mediterranean in the central mediterranean two back arc basins the liguro provencal lpb and the tyrrhenian basin tb opened progressively and consecutively from the late eocene oligocene to the present evolution in space and time of rifting and drifting processes along three different transects across these basins shows differences in the style of extension lpb opened with the formation of a narrow single rift while in the tb deformation and magmatism is spread over a wide area moreover at the northern end of the tb the locus of extension progressively migrated towards the east whereas in the southern tb the locus of extension and magmatism migrated inside the basin inducing continental break up and drifting of the previously formed older conjugate basins we propose that these different styles of back arc extension depend upon internal conditions such as prerift rheology linked with its geological heritage and external conditions e g the style of subduction

hydrothermal alteration in a modern suprasubduction zone the tonga forearc crust an extensive suite of hydrothermally altered basalts gabbros and plagiogranites was recovered from the trench facing slope of the tonga forearc the tectonic setting lithology and geochemistry of these samples make them a unique collection for comparison with suprasubduction zone ssz ophiolites petrography mineral chemistry and geothermometry are used to constrain the metamorphic evolution of ocean crust formed in a modern ssz setting seawater derived hydrothermal fluids first penetrated the lower crust along grain boundaries and microscopic fracture networks at temperatures 800 c as the plutonic sequence cooled amphibole progressively replaced the mafic phases followed by chlorite and epidote below 550 c basalts record peak alteration temperatures up to 773 c however most were altered at lower temperatures typical of mid ocean ridge mor volcanic sequences kelman 1998 epidosites formed by pervasive alteration of basalt and plagiogranite at greenschist facies conditions and at high water rock ratios pervasively altered gabbros and basalts display evidence of late cataclastic deformation and or contain veins that is likely related to a later tectonic event such as trench rollback the range of alteration temperatures and mineral assemblages in basalts and gabbros are similar to those described from both ssz ophiolites and mors however the degree of alteration in basalts and the presence of epidosites in the tonga collection are most similar to alteration characteristics in ssz ophiolites the initiation of high temperature brittle deformation in the absence of ductile deformation suggests that the tonga forearc crust was constructed in a magma rich environment similar to fast spreading mors