seismic and magnetic anisotropy of serpentinized ophiolite implications for oceanic spreading rate dependent anisotropy compressional and shear wave anisotropy shear wave birefringence and the anisotropy of magnetic susceptibility were measured on a series of dunites sampled from the dinardic hellenic ophiolites the densities of these materials ranged from 3330 kg m 3 to 2620 kg m 3 and indicate degrees of serpentinization from 2 3 to 87 9 respectively magnetic susceptibility increases and the compressional and shear wave velocities decrease in proportion to the degree of serpentinization as has been observed by other workers in all cases the magnetic susceptibility tensor is described by an oblate spheroid whose minor axis is closely aligned to the pole of the foliation but the magnetic anisotropy is not related to the degree of serpentinization the compressional wave anisotropy e monotonically decays from 12 for nearly pure olivine dunite to less than 2 for the most serpentinized sample this observation strongly suggests that serpentinization progressively destroys the original anisotropy by consuming the preferentially aligned olivines and replacing them with randomly oriented serpentines typical serpentine mesh textures seen in microscopic thin section examinations support this suggestion this loss of anisotropy with serpentinization may partly explain the apparent relationship between seismic compressional wave anisotropy of the oceanic lithosphere the more complex geological structure of slow spreading ridges may admit more sea water via faults for deep circulation which increases serpentinization and consequence of which is decreased seismic anisotropy 2007 elsevier b v all rights reserved

passive and active seismological study of bending related faulting and mantle serpentinization at the middle america trench water transported within the subducting oceanic lithosphere into the earth s interior affects a wealth of subduction zone processes including intraslab earthquakes and arc magmatism in recent years growing evidence suggests that much of the hydration of oceanic plates occurs at the trench ocean slope right before subduction here normal faults are created while the rigid lithosphere bends into the trench offshore of middle america multi channel seismic reflection imaging suggests that bending related faults cut into the uppermost mantle providing a mechanism for hydration and transformation of mantle peridotites into serpentinites seismic wide angle reflection and refraction data were collected coincident with one of the seismic profiles where the faults have been imaged travel time inversion provides evidence that both crustal and uppermost mantle velocities are reduced with respect to the velocity structure found in mature oceanic crust away from deep sea trenches if mantle velocity reduction is solely produced by hydration velocities indicate 10 15 of serpentinization in the uppermost 3 km of the mantle where seismic data provide enough resolution a small network of ocean bottom hydrophones deployed for about a month detected 3 local micro earthquakes per day earthquake epicentres align with fault scarps at the seafloor and continuous earthquake activity might be an important process to facilitate the percolation of seawater into the upper mantle

origin of convex tetrads in rare earth element patterns of hydrothermally altered siliceous igneous rocks from the zinnwald sn w deposit germany the chondrite normalized rare earth element ree patterns of whole rock samples from evolved granitic systems hosting rare metal deposits sometimes show a split into four consecutive curved segments referred to as tetrads in the present contribution a rigorous statistical method is proposed that can be used to test whether geological significance should be attributed to tetrads that are only of limited size the method involves a detailed evaluation of element and sample specific random and systematic errors that are constrained on the basis of independent repeated preparations and analyses of sample and reference materials application of the proposed method to samples from the granite hosted zinnwald sn w deposit germany revealed that at least two tetrads in normalized whole rock ree patterns have to be analytically significant to rule out that fractional crystallization led to the unusual behavior of the rees based on the analysis of altered albite granite and greisen samples from the endocontact of the zinnwald granite massif it is demonstrated that the lanthanide tetrad effect is responsible for the formation of the convex tetrads geological and petrological evidence suggests that the tetrads in the samples developed prior to greisenization and related cassiterite precipitation in contrast to the endocontact samples the rhyolitic wall rocks are typified by normalized ree patterns having tetrads that are variable in size and frequently close to the limit of analytical significance the sizes of the tetrads apparently correlate with the intensity of albitization but show no relation to subsequent alteration processes including greisenization and low temperature argillization this observation proves that curved segments in normalized whole rock ree patterns can be introduced during hydrothermal fluid rock interaction

depletion of light rare earth elements in felsic magmas contrary to simple generalizations about their behavior light rare earth elements lree do not act as incompatible elements in very felsic magmas in fact lree concentrations typically decrease often drastically during differentiation of such magmas the simplest explanation for this depletion involves the separation of minute easily overlooked quantities of an extremely lree rich accessory mineral either monazite or allanite our data indicate that felsic liquids with 50 ppm lree may be saturated in either of these accessories and that the concentration required for saturation decreases in increasingly felsic liquids this accounts for incompatible behavior of lree even at high concentrations in mafic magmas in contrast to compatible behavior at low concentrations in felsic magmas partitioning of lree into solid rather than liquid has important implications for trace element and nd isotope modeling of crustal anatexis as well as for magma differentiation

high resolution palynological analysis in lake sapanca as a tool to detect recent earthquakes on the north anatolian fault high resolution palynological analysis of a 38 cm long core collected from lake sapanca northwest turkey reveals large earthquakes that occurred during the second half of the 20th century along the north anatolian fault zone four events have disturbed the lacustrine sedimentary sequence three of the four events are historical earthquakes in 1999 in izmit 1967 in mudurnu and 1957 in abant these events are recorded in the core by turbiditic deposits and reworked sediment and by low overall palynomorph concentrations but high values of thick exined pollen fern spores and fungal spores palynomorphs in the event beds have been grouped based on their associations in modern moss river and lake samples the inferred mechanisms of transport and sources for the palynomorphs are 1 lake sediment displaced by slump 2 collapsed shoreline sediment owing to seiche waves and sudden lake level changes 3 subsidence of deltas and 4 river transported soil and sediment from upland areas the 1999 izmit earthquake is only weakly recorded by palynomorphs probably due to recent engineering control on the rivers the 1967 mudurnu earthquake had the strongest effect on the lake introducing successive packages of sediment to the centre of the lake from underwater slopes the lakeshore and rivers

xenoliths in a mafic dyke at popes harbour nova scotia implications for the basement to the meguma group a recently recognized series of northwest trending mafic dykes cuts the cambro ordovician meguma group along the eastern shore of nova scotia some of the dykes contain gneissic and meta plutonic xenoliths interpreted here as including fragments of the basement to the meguma group most of the gneissic xenoliths in the dyke exposed at popes harbour are sillimanite and or kyanite bearing quartz poor metapelites and garnet orthopyroxene bearing tonalitic rock other enclaves include amphibolite quartzite gabbro granite granodiorite and granitic pegmatite textural features suggest that the earliest paragneissic assemblage included poikiloblastic garnet xfe 0 55 0 65 xmg 0 05 0 30 xca 0 02 0 16 xmn 0 02 0 16 biotite kyanite and or sillimanite oligoclase and quartz subsequent recrystallization at higher temperature and or lower pressure conditions is suggested by the replacement of kyanite by sillimanite mineral core composition data indicate early metamorphic conditions of 45

multiparameter two dimensional inversion of scattered teleseismic body waves 3 application to the cascadia 1993 data set this is the third paper in a three part series that examines formal inversion of the teleseismic p wave coda for discontinuous two dimensional 2 d variations in elastic properties beneath dense arrays of three component broadband seismometers in this paper the method is applied to data from the incorporated research institutions for seismology program for array seismic studies of the continental lithosphere iris passcal cascadia 1993 experiment undertaken across central oregon two major features are imaged in the resulting model the continental moho becomes evident 150 km from the coast beneath the western cascades and extends through the eastern end of the profile at 35 40 km depth in the western portion of the model oceanic crust of the subducting juan de fuca plate dips shallowly 12 at the coast and more steeply 27 below the willamette valley and is evident to depths of 100 km beneath the high cascades the abrupt increase in plate dip at 40 km depth coincides with an apparent thickening of the oceanic crust followed by a diminution in its signature building on previous work we argue that these results are consistent with the consequences of prograde metamorphic reactions occurring within the oceanic crust progressive dehydration at lower grade facies conditions culminates in the transformation to eclogite producing a pronounced increase in the seismic velocity density and dip of the subducting plate and structural complexity in the overlying wedge

geochemistry of the proterozoic wolframite bearing greisen veins and the associated granite at rostberget northern sweden at rostberget in northern sweden a swarm of thin undeformed wolframite bearing greisen veins occurs within the 1 78 ga old joran granite intrusion this is a microcline rich porphyritic granite

competitive hydration and dehydration at olivine quartz boundary revealed by hydrothermal experiments implications for silica metasomatism at the crust mantle boundary abstract serpentinization occurs via interactions between mantle peridotite and water that commonly passes through the crust given that such a fluid has a high silica activity compared with mantle peridotite it is thought that serpentinization and silica metasomatism occur simultaneously at the crust mantle boundary in this study we conducted hydrothermal experiments in the olivine ol quartz qtz h 2 o system at 250 c and vapor saturated pressure under highly alkaline conditions naoh aq ph 13 8 at 25 c to clarify the mechanism of silica metasomatism at the crust mantle boundary composite powders consisting of a qtz layer and an ol layer were set in tube in tube vessels after the experiments the extents of serpentinization and metasomatic reactions were evaluated as a function of distance from the ol qtz boundary the mineralogy of the reaction products in the ol hosted region changed with increasing distance from the ol qtz boundary from smectite serpentine smc zone to serpentine brucite magnetite brc zone olivine hydration proceeded in both zones but the total h 2 o content in the products was greater in the brc zone than in the smc zone mass balance calculations revealed that olivine hydration occurred without any supply of silica in the brucite zone in contrast the smc zone was formed by silica metasomatism via competitive hydration and dehydration reactions in the smc zone smectite formed via the simultaneous progress of olivine hydration and serpentine dehydration and around the boundary of the smc and brc zones serpentine formation occurred by olivine hydration and brucite dehydration the relative extent of hydration and dehydration reactions controlled the along tube variation in the rate of h 2 o production consumption and the rate of volume increase our findings suggest that the competitive progress of serpentinization and silica metasomatic reactions would cause fluctuations in pore fluid pressure possibly affecting the mechanical behavior of the crust mantle boundary

petrogenesis of the triassic bayan ulan alkaline granitic pluton in the north gobi rift of central mongolia implications for the evolution of early mesozoic granitoid magmatism in the central asian orogenic belt abstract the 221 ma bayan ulan bu granitic pluton occurs in the north gobi rift of central mongolia which was superimposed on the paleozoic central asian orogenic belt caob an area characterized by voluminous juvenile crust this large 1000 km 2 pluton is part of an early mesozoic granitoid province in the central part of the caob it consists mainly of alkali feldspar granites containing mesoperthite with interstitial quartz minor ca na and na amphiboles and rare fe rich biotite graphic texture is common the bu granites are fractionated leucocratic alkaline a type rocks that are enriched in incompatible elements particularly rb and th and depleted in ba sr eu and ti most have nd t values of 1 4 to 1 7 and t dm model ages 800 and 1200 ma and lie within the range of other a type granites of the caob granitoid province the granites are interpreted to have formed by the partial melting of underplated neoproterozoic mildly alkaline basalts in the lower crust followed by fractional crystallization dominated by feldspars the formation of the bu pluton and the coeval khentei batholith the largest late triassic intrusion in north central mongolia appears to be related either to a plume or to the closure of the mongol okhotsk ocean north gobi rift magmatism extends across the mongol okhotsk suture so a plume related origin is more consistent with available data this plume which would provide the heat source for magmatism in this area explains the eastward movement of large scale magmatism over time from the tarim traps 300 275 ma through khangai 270 240 ma to khentei 230 195 ma in central mongolia

the formation of modified zircons in f rich highly evolved granites an example from the shuangji granites in eastern tianshan china abstract two populations of zircons are present in the f rich highly evolved 300 ma shuangji granites xinjiang eastern tianshan china zircon i comprise a very small proportion of the population they show characteristics of primary igneous zircons and yield an age of ca 300 ma zircon ii are the dominant type and are weakly luminescent under cathodoluminescence cl but show a range of complex textures in back scattered electron bse images zircon ii are characterized by th u rich mineral inclusions not seen in zircon i and can be divided into zircon iia and zircon iib autometasomatism during the magmatic hydrothermal phase of the highly evolved shuangji granites fluids rich in f interacted with zircon i through a coupled dissolution reprecipitation process forming pores and cracks in the zircon grains the simultaneous alteration of u rich euxenite released high amounts of u and th into the fluids interaction of these fluids with porous zircon i resulted in the precipitation of u and th rich inclusions in the zircon i grains radiation damage resulted in the formation of metamict zircons later hydrothermal alteration affected the metamict zircons forming nanoscale baddeleyite in the amorphous zone subsequent radiation damage accumulated continuously resulting in the final textures of zircon ii

cretaceous ongonites topaz bearing albite rich microleucogranites from ongon khairkhan central mongolia products of extreme magmatic fractionation and pervasive metasomatic fluid rock interaction abstract ongonites were defined at their type locality at ongon khairkhan central mongolia as pristine magmatic topaz bearing albite quartz keratophyres with up to 4 wt f and containing phenocrysts of albite k feldspar quartz and rare mica and topaz hosted in a groundmass composed of the same minerals however detailed petrographic and sem eds studies indicate that these rocks underwent considerable subsolidus exchange with deuteric fluids as evidenced by the presence of albitic plagioclase ab 100 and end member orthoclase or 100 secondary li fe rich mica zinnwaldite enriched in rare metals sn w ta nb pitted feldspars containing fluid inclusions and disseminated fluorite the 120 ma old dyke rocks emplaced at a high structural level in the crust are strongly peraluminous leucogranites characterized by high al and alkalis that are also enriched in rb cs ga and ta depleted in mg ca zr ba sr and eu and have anomalous k rb rb sr zr hf and nb ta ratios compared to the average continental crust however the suite has nd isotopic ratios nd 120 1 similar to those of contemporaneous a type granites of the mongolian transbaikalian igneous province of the central asian orogenic belt the inferred primary 18 o 6 to 7 and pb isotopic values are consistent with a granitic parent magma and interaction with orthomagmatic fluids the ongonites and constituent minerals record 1 an extensive and protracted crystal fractionation history in part due to the presence of volatiles particularly f which depressed the solidus temperature of the felsic rocks and extended its duration of crystallization and 2 subsolidus exchange with fluids which includes late flux of heated meteoric water as indicated by modified whole rock 18 o values 0 5 to 2 7 the interaction of the ongonites with internally derived orthomagmatic fluids is considered to result in enrichment and or redistribution of several incompatible elements but not to have greatly modified their original major element chemistry which indicates that this suite represents the last stages of fractionation of a highly differentiated f rich granitic magma the escape of these evolved melts from the apical part of the underlying pluton is now represented by the ongonite dykes late stage magmatic water rich fluids enriched in incompatible elements including nb ta sn and w were responsible for the late to post magmatic alteration and associated w mineralization

granodiorites of the south mountain batholith nova scotia canada derived by partial melting of avalonia granulite rocks beneath the meguma terrane implications for the heat source of the late devonian granites of the northern appalachians abstract the late devonian south mountain batholith smb of nova scotia is the largest batholith of the northern appalachians the peraluminous granitic rocks range from biotite granodiorite to leucogranite samples collected from a drill core of the scrag lake granodioritic pluton of the western smb are chemically homogeneous from the surface to a depth of 1425 m the homogeneous composition implies that the granodiorite was derived from a relatively homogeneous source and that country rock assimilation was not an important source for the parental magma equilibrium partial melt modeling of underlying sub meguma granulite rocks indicates that they are the primary source rocks of the granodiorites we suggest that mantle derived magmas intruded the lower crust and induce large scale melting of the granulite basement rocks to produce the granodiorites fractional crystallization of the granodiorites plus assimilation of meguma supergroup metasediments likely produces the silica rich rocks of the smb the cause of mantle melting is uncertain however it may be related to the transitioning of the northern appalachians from a position above the deep mantle pacific large low shear velocity province llsvp to a higher shear velocity region of the mantle

chemical and isotopic pb sr zonation in a peraluminous granite pluton role of fluid fractionation the davis lake pluton dlp 800 km2 of southwestern nova scotia canada part of the large peraluminous south mountain batholith of ca 380 ma u pb zircon ar ar mica consists of granite and subordinate topaz muscovite leucogranite that hosts greisen tin base metal mineralization a new pb pb isochron age for leucogranite from the most evolved part of the dlp indicates a crystallization age of 378 3 6 ma coincident with other radiometric ages of the dlp rb sr re os pb pb the intrusion displays a compositional zonation defined by lead and strontium isotopic ratios as well as some major elements e g si f incompatible trace elements e g li rb ta u sn and elemental ratios e g k rb and nb ta the greisens and the leucogranites that host them are characterized by extreme radiogenic compositions for pb and sr and their chemical isotopic trends are extensions of the trends displayed by the less evolved granites the covariations of the isotopic ratios with several major and trace elements and elemental ratios as well as the pb pb and rb sr isochrones indicate that all phases of the intrusion originated from a homogeneous parental magma the granitoid magma underwent extensive fractional crystallization of feldspars minor biotite and accessory minerals monazite apatite and zircon in a compositionally zoned magma chamber that was subsequently accompanied by fluid fractionation during which time the internally derived fluorine rich fluids modified the rb sr u pb and th pb ratios leading to distinct variations of 87sr 86sr 206pb 204pb 238u 204pb and 232th 204pb isotopic ratios these data therefore document the evolution of a granitic magma through magmatic i e crystal fractionation orthomagmatic i e crystal fluid fractionation and hydrothermal i e fluid fractionation stages that culminated in the formation of a tin base metal deposit the pb isotope data also constrain the source region for the dlp as being avalonian basement that by inference must underlie much of the meguma terrane

p wave anisotropic tomography of the crust and upper mantle under hokkaido japan abstract we determined a 3 d anisotropic velocity structure beneath hokkaido in northern japan using a large number of first p wave arrival times the travel time inversion is parameterized with an isotropic component and two anisotropic parameters for each grid node by assuming p wave azimuthal anisotropy with hexagonal symmetry axis distributed horizontally the geometries of the conrad and moho discontinuities and the upper boundary of the subducting pacific plate are taken into account in the model our results show that low velocity zones exist beneath active arc volcanoes in the crust and in the central portion of mantle wedge above the high velocity pacific slab which are similar to the previous tomographic images p wave anisotropy is revealed in the crust mantle wedge and the subducting pacific slab in the upper crust the anisotropy is possibly caused by microcracks and cracks in the lower crust the anisotropic structure is different from that in the upper crust and it may be affected considerably by the plastic flow deformation in the mantle wedge the fast velocity direction fvd is generally trench normal in most of the study area while it becomes trench parallel in the pacific coast areas close to the trench which agrees with the shear wave splitting results beneath the volcanic front the fvd is also trench parallel which may suggest the existence of complex 3 d mantle flow in the mantle wedge the fvd in the subducting pacific slab is nearly north south which likely keeps the original fossil anisotropy of the pacific plate formed at the mid ocean ridge or is affected by the olivine fabric transition from a type to b type fabric due to the changes in water content stress and temperature

dating transcurrent terrane accretion an example from the meguma and avalon composite terranes in the northern appalachians the polygenetic minas geofracture separates the meguma and avalon composite terranes of the maritime appalachians dynamically recrystallized muscovite and biotite occur within d3 shear zones which developed during dextral movement along the geofracture these record 40ar 39ar plateau ages between circa 360 and 370 ma comparable plateau ages are recorded by micas within undeformed portions of granitic plutons and their low pressure contact metamorphic aureoles these are similar to u pb ages previously reported for igneous monazite therefore indicating relatively rapid postmagmatic cooling at shallow crustal levels the d3 fabrics were superimposed on earlier s1 structures which formed during initial amalgamation of the meguma and avalon composite terranes whole rock slate phyllite samples collected outside high temperature portions of the plutonic contact aureoles yield 40ar 39ar plateau ages which correspond to groupings of circa 400 395 ma 390 385 ma and 380 375 ma these are interpreted to date diachronous formation of the s1 fabrics and concomitant low grade regional metamorphism in the circa 25 ma between initial amalgamation and granitic plutonism it is inferred that there was a slow rise of isotherms through the crust as a result of overthrust tectonic thickening this led to crustal melting and the production of widespread granitic magmas which were emplaced immediately prior to d3 dextral shearing

an evaluation of crustal assimilation within the late devonian south mountain batholith sw nova scotia the late devonian south mountain batholith smb of southwestern nova scotia is the largest batholith in the appalachian orogen of eastern north america and contains economic deposits of u and sn the smb comprises at least 11 individual plutons which range in composition from granodiorite to biotite monzogranite leucomonzogranite and leucogranite previous studies have suggested that a combination of fractional crystallization assimilation of meguma supergroup country rocks and an influx of magmatic fluids contributed to the chemical evolution of the smb the amount of crustal assimilation is estimated to be as high as 33 melts modelling assuming a starting composition of granodiorite with h 2 o 4 wt pressure 4 kbar 12 km and f o 2 fmq can reproduce the chemical evolution observed in the smb however some leucogranites likely require an additional component e g hydrothermal fluids to explain their alkali metal enrichment e g na k rb zircon saturation thermometry estimates indicate the salmontail lake and scrag lake granodiorite plutons had high minimum initial temperatures of 823 6 c and 832 2 c respectively which are similar to low zircon inheritance granitoids the tio 2 al 2 o 3 and alkali lime ratios of the surrounding country rocks and the leucogranites indicate the amount of crustal assimilation is likely to be between 10 and 20 our findings suggest the granodiorites of the smb were likely produced by partial melting of the sub meguma supergroup e g avalon terrane lower crust caused by the contemporaneous injection of high temperature mafic to ultramafic magmas

sedimentary fractionations between al ti and zr and the genesis of strongly peraluminous granites sedimentary sorting produces alternating shales and sandstones with complementary chemistries the preferential partitioning of quartz and zircon in the coarse grained fraction of the sediments results in a strong fractionation between sio 2 zr and other components al 2 o 3 tio 2 as a consequence shales may be defined chemically as having higher al 2 o 3 sio 2 and tio 2 zr ratios than their source rocks sandstones have the opposite characteristics and vary in composition according to the maturity of their source and the intensity of sorting processes most sedimentary suites display a good correlation between al 2 o 3 sio 2 and tio 2 zr this result is used to evaluate an average bulk composition for recent clastic materials and to model their composition on a ternary al ti zr diagram because the al 2 o 3 tio 2 ratio of igneous rocks is highly sensitive to magmatic differentiation igneous and sedimentary trends crosscut in the al ti zr di

problems of the origin ore bearing and evolution of rare metal granitoids abstract on the basis of detailed geological petrographic geochemical including isotopic and partition coefficient evidence and experimental phase relationships in the systems granite h2o hf ongonite h2o hf quartz albite orthoclase f h2o data on melt and fluid inclusions in minerals studies it has been shown that rare metal li f granites and ongonites were formed at the latest stages of crystallization differentiation of a granitic magma with a somewhat increased fluorine content at the end of the process of crystallization differentiation as a result of the build up of water in residual magma emanation differentiation metamagmatism may play a significant role parental magma of rare metal li f granites with a somewhat increased fluorine content may have been formed either owing to crystallization differentiation of a common sialic granitic melt or owing to the anatexis of rare metal metasomatites in continental crust environments

upper paleozoic mafic and intermediate volcanic rocks of the mount pleasant caldera associated with the sn w deposit in southwestern new brunswick canada petrogenesis and metallogenic implications abstract upper paleozoic 365 ma mafic and intermediate volcanic rocks of the piskahegan group constitute a subordinate part of the mount pleasant caldera which is associated with a significant polymetallic deposit tungsten molybdenum bismuth zones 33 mt ore with 0 21 w 0 1 mo and 0 08 bi and tin indium zones 4 8 mt with 0 82 sn and 129 g t in in southwestern new brunswick canada the epicontinental caldera complex formed during the opening of the late paleozoic maritimes basin in the northern appalachians the mafic and intermediate rocks make up two compositionally distinct associations the first association includes evolved rift related continental tholeiitic basalts and the second association comprises calc alkaline andesites although both associations were emplaced penecontemporaneously the basalts have low mg 0 34 0 40 smooth chondrite normalized ree patterns with la yb n 5 6 primitive mantle normalized trace element patterns without noticeable negative nb ta anomalies and their nd t ranges from 2 5 to 2 2 the basalts were generated by partial melting of a transition zone between spinel and garnet mantle peridotite at a depth of 70 90 km the calc alkaline andesites of the second association have chondrite normalized ree patterns that are more fractionated with la yb n 7 8 5 but without significant negative eu anomalies compared to the basaltic rocks they have lower nd t values ranging from 0 5 to 1 9 and their mantle normalized trace element plots show negative nb ta anomalies the nd t values display negative correlations with indicators of crustal contamination such as th la th nb and sio2 the andesitic rocks are interpreted to have formed by assimilation fractional crystallization processes which resulted in the contamination of a precursor basaltic magma with crustal material the parent basaltic magma for both suites underwent a different evolution the tholeiitic basalts experienced shallow seated fractional crystallization and evolved along a tholeiitic trend of early iron enrichment non oxic conditions the contaminated magma of the second association followed a calc alkaline fractionation trend of no iron enrichment oxidizing conditions characterized by a high ph2o and po2 environment at the mid crust levels the piskahegan group which is associated with an important polymetallic mineral deposit differs from the numerous non mineralized rift related volcanic suites of the regional upper devonian to lower carboniferous successions in the maritimes basin by the presence of a significant amount of coeval calc alkaline andesite which may be an indicator of potential mineralization

mueilha rare metals granite eastern desert of egypt an example of a magmatic hydrothermal system in the arabian nubian shield abstract the mueilha granite pluton is one of the rare metals bearing peraluminous granitic plutons in the arabian nubian shield it represents the apical part of a highly evolved magma chamber emplaced at a shallow level subsequent to the post pan african orogeny the pluton can be seen as a highly leucocratic medium grained albite oligoclase framework infilled with quartz k feldspar and muscovite that are variably overgrown by k feldspar muscovite quartz and topaz megacrysts the increasing number and size of the k feldspar megacrysts at the expense of the whitened albite oligoclase framework imparts variably red color to the mueilha granite contacts between the milky white and red granites are usually gradational but may be locally sharp or may form narrow transition zones resulting from abrupt variations in texture and lithology textural relations indicate an initial stage of hydrothermal albitization of magmatic plagioclase and crystallization of topaz megacrysts resulting from infiltration of na rich fluorine bearing fluids a subsequent stage of metasomatic enrichment is characterized by extensive growth of large k feldspar quartz and muscovite megacrysts at the expense of the albite oligoclase crystals as a result of infiltration of k si rich hydrous fluids post magmatic infiltration of hydrous fluids along the fault planes is shown by the intense replacement of alkali feldspar megacrysts by quartz development of myrmekitic intergrowth pockets along the k feldspar megacrysts and sealing of the micro fractures by cryptocrystalline mixtures of clay minerals iron oxides sericite and chlorite compositionally the red granitic rocks have higher sio 2 fe 2 o 3total k 2 o na 2 o ree y th u zr and zn and lower al 2 o 3 ga ta nb and mo compared to the milky white granites lile and sn do not show clear variation trends throughout the mueilha granite pluton suggesting their immobility during hydrothermal alteration microthermometric measurements indicate that the interactions with the hydrothermal fluids started at a minimum temperature 400 c most likely during the late stage crystallization of the mueilha granite and continued after their complete solidification i e subsolidus conditions at a temperature as low as 120 c the high fertility of mueilha granite is most plausibly the result of partial melting within the undepleted juvenile crust of the arabian nubian shield that has formed during the pan african orogeny

a petrological and geochronological study of a 360 ma metallogenic event in maritime canada with implications for lithophile metal mineralization in the canadian appalachians the occurrence of ca 360 ma greisen related molybdenite mineralization in the clayton hill biotite muscovite leucomonzongranite 1 km2 is unusual for the southwestern meguma terrane of the maritime appalachians where 380 ma granite related sn mineralization dominates the presence of extensive hornfelsing and occurrence of mineralized sn w cu zn skarns in the area suggests a larger intrusion occurs at depth the intrusion and mineralization are constrained to 361 ma based on concordant re os molybdenite and 40ar 39ar muscovite ages which equates to other ca 360 ma granites in the area hence magmatism of this age is more widespread than previously considered the intrusion is highly evolved 74 wt sio2 metaluminous a cnk 1 1 05 a cnk molar ratio al2o3 cao na2o k2o and depleted in cafemic and transition elements rare earth elements ree 106 23 ppm are strongly fractionated with light rare earth elements lree enriched la lun 10 and negative eu anomalies biotite is

internal differentiation of rare element pegmatites effects of boron phosphorus and fluorine lithium rich rare element pegmatites are characterized by high concentrations of b p and f the interactions of these components with h2o and rare alkalis lower liquidas and solidus temperatures enhance silicate liquid h2o miscibility and control partitioning and concentration of group i elements and higher field strength cations boron f and perhaps p may form peralkaline na and li species that promote early saturation in mica quartz activities of f p and especially b are largely unbuffered throughout crystallization concentration of these components through fractional crystallization involving muscovite generates a peralkaline na aluminosilicate rich melt or vapor from which albitites rich in tourmaline phosphates f rich micas beryl zircon and nb ta sn oxides crystallize phase equilibrium experiments with peraluminous b p f rich rhyolite obsidian macusanite simulate many features of rare element pegmatites especially at h2o undersaturated conditions

experimental determination of the solubility of ce monazite in sio2 al2o3 k2o na2o melts at 800 c 2 kbar under h2o saturated conditions the solubility of cerium monazite cepo4 in sio2 al2o3 k2o na2o melts was determined at 800 c 2 kbar under water saturated conditions dissolution experiments using synthetic ce monazite crystals and crystallization experiments using ce and po4 bearing glasses yield results in good agreement microprobe analyses of glasses using a sensitive method detection limit to 130 ppm show that the solubility of monazite is dependent on the melt composition and is especially low for peraluminous compositions application of these data to various geologic occurrences is in good agreement with petrographic and geochemical observations and shows that monazite fractionation is probably a very common phenomenon in two mica granites this provides a good explanation for the light ree and th depletion commonly observed in such rocks

petrogenesis of tourmaline granites and topaz granites the contribution of experimental data abstract fluorine and boron are generally only minor components of granitic rocks but may be found highly concentrated in certain tourmaline granites and topaz granites up to 1 wt b2o3 and 3 2 wt f respectively studies of various examples of these granite types which occur as small bodies emplaced at shallow levels p lithostatic textural and petrographic data indicate that the tourmaline and topaz granite magmas crystallized at low to moderate temperatures below 700 c and that a fluid phase may have been present during the crystallization interval both granites varieties are peraluminous co up to 5 characterized by the presence of white micas lithium micas or muscovite aluminium silicates and other al rich minerals garnet cordierite boron and fluorine are hosted by tourmaline schorl rich member of the schorl dravite series topaz and mica respectively these granites have low ca fe mg and ti contents each the crystallization of tourmaline granite and topaz granite magmas is discussed using experimental data derived from studies of granitic systems with added boron and fluorine both elements act as a flux enabling boron and fluorine enriched melts to persist to temperatures as low as 650 c at 1 kbar in addition the effect of increasing fluorine content displaces the minimum in q ab or at ph2o 1 kbar toward the albite apex plots of fluorine rich topaz granites coincide with this trend suggesting that the role of fluorine in magmatic processes may have affected the rock composition as well as crystallization temperatures boron rich granites plot near the low ph2o minima in qz ab or suggesting that boron has less of an influence on minimum melt compositions the low temperatures of crystallization permits phases such as muscovite or andalusite topaz and two alkali feldspars to crystallize directly from f and b enriched melts at low pressures in addition the limited available experimental data are used to compare the effects of fluorine and boron on major and trace element distribution during melt vapour interaction

intraplate seismicity and related mantle hydration at the nicaraguan trench outer rise summary we examine micro earthquake records from a dense temporary array of ocean bottom seismometers obs and hydrophones that has been installed from september to november 2005 at the trench outer rise offshore nicaragua approximately 1 5 locatable earthquakes per day within the array of 110 120 km show the high seismic activity in this region seismicity is restricted to the upper 15 km of the mantle and hence where temperatures reach 350 400 c which is smaller than values observed for large mantle intraplate events 650 c determination of moment tensor solutions suggest a change of the stress region from tensional in the upper layers of the oceanic plate to compressional beneath the neutral plane between both regimes is located at 6 9 km beneath moho and thus very shallow fluids which are thought to travel through the tensional fault system into the upper mantle may not be able to penetrate any deeper the earthquake catalogue which seems to be complete for magnitudes above mw 1 6 1 8 suggests a strong change of the lithospheric rheology when approaching the trench and b factors that is the ratio between small and large earthquakes increase significantly in the closest 20 km to the trench axis implying that the crust and upper mantle is massively weakened and hence ruptures more frequently but under less release of stress we explain this with a partly serpentinized upper mantle

a model for monazite melt equilibrium and application to the generation of granitic magmas abstract in order to model the behaviour of zr hf ti p u th y and the rare earth elements ree in the continental crust the parameters which control the equilibria between accessory minerals and granitic melts must be determined this paper proposes equations for the equilibrium between monazite and ca poor felsic melts in a first step all the light ree are considered as a single component and an equation describing monazite solubility is deduced from the available experimental data in a second step the fractionation of ree by monazite is investigated on the basis of a natural example of coexisting monazite and obsidian from macusani peru the solubility of monazite can be used as a thermometer based on the total ree content of natural magmatic rocks application of ree thermometry to various peraluminous rock series suggests that it is a reliable tool for determining temperatures of magmatic processes the equation governing ree fractionation by monazite is used to discuss genetic relationships between a melt and a presumed parental rock using the composition of the monazite in the parental rocks and the ree pattern of the daughter rock this method has been applied successfully to high himalayan manaslu and tibetan slab granites it is suggested that detailed petrographical work on monazite including electron microprobe analyses and sem imaging of its internal structures combined with the model presented here can be a useful tool for deciphering the genesis and the differentiation of peraluminous granitic magmas in the continental crust

resolving the fine scale velocity structure of continental hyperextension at the deep galicia margin using full waveform inversion continental hyperextension during magma poor rifting at the deep galicia margin is characterised by a complex pattern of faulting thin continental fault blocks and the serpentinisation with local exhumation of mantle peridotites along the s reflector interpreted as a detachment surface in order to understand fully the evolution of these features it is important to image seismically the structure and to model the velocity structure to the greatest resolution possible travel time tomography models have revealed the long wavelength velocity structure of this hyperextended domain but are often insufficient to match accurately the short wavelength structure observed in reflection seismic imaging here we demonstrate the application of two dimensional 2d time domain acoustic full waveform inversion to deep water seismic data collected at the deep galicia margin in order to attain a high resolution velocity model of continental hyperextension we have used several quality assurance procedures to assess the velocity model including comparison of the observed and modelled waveforms checkerboard tests testing of parameter and inversion strategy and comparison with the migrated reflection image our final model exhibits an increase in the resolution of subsurface velocities with particular improvement observed in the westernmost continental fault blocks with a clear rotation of the velocity field to match steeply dipping reflectors across the s reflector there is a sharpening in the velocity contrast with lower velocities beneath s indicative of preferential mantle serpentinisation this study supports the hypothesis that normal faulting acts to hydrate the upper mantle peridotite observed as a systematic decrease in seismic velocities consistent with increased serpentinisation our results confirm the feasibility of applying the full waveform inversion method to sparse deep water crustal datasets

petrogenesis of the late jurassic peraluminous biotite granites and muscovite bearing granites in se china geochronological elemental and sr nd o hf isotopic constraints biotite granites and muscovite bearing granites are dominant rock types of the widespread granites in se china however their petrogenesis has been enigmatic a combined study of zircon u pb dating and lu hf isotopes whole rock element geochemistry and sr nd o isotopes was performed for three late mesozoic granitic plutons xinfengjie jiangbei and dabu in central jiangxi province se china all the plutons are composed of biotite granites and muscovite bearing granites that have been poorly investigated previously the new data not only allow us to assess their sources and magma evolution processes but also helps us to better understand the genetic link to the large scale polymetallic mineralization in se china la icp ms zircon u pb dating shows that three plutons were emplaced in the late jurassic 159 148 ma and that the muscovite bearing granites are almost contemporaneous with the biotite granites the biotite granites have sio2 contents of 70 3 74 4 wt and are weakly to strongly peraluminous with asi from 1 00 to 1 26 and show a general decrease in asi with increasing sio2 they have relatively high zircon saturation temperatures t zr 707 817 c most 745 c and show a general decrease in t zr with increasing sio2 they have high initial 87sr 86sr ratios 0 7136 to 0 7166 and high 18o values 9 1 12 8 most 9 5 and clearly negative e nd t 9 5 to 11 8 and e hf t in situ zircon 13 1 to 13 5 the muscovite bearing granites have high sio2 contents 74 7 78 2 wt they are also weakly to strongly peraluminous with asi of 1 04 1 18 but show a general increase in asi with increasing sio2 they have relatively low t zr 671 764 c most 9 5 and e hf t in situ zircon of 11 3 to 13 1 geochemical data suggest the origin of the biotite granites and muscovite bearing granites as follows partial melting of precambrian metasedimentary rocks mainly two mica schist in the lower crust at temperatures of ca 820 c generated the melts of the less felsic biotite granites such primary crustal melts underwent biotite dominant fractionation crystallization forming the felsic biotite granites progressive plagioclase dominant fractionation crystallization from the evolved biotite granites produced the more felsic muscovite bearing granites thus the biotite granites belong to the s type whereas the muscovite bearing granites are highly fractionated s type granites we further suggest that during the formation of the muscovite bearing granites the fractional crystallization was accompanied by fluid fractionation and most likely the addition of internally derived mineralizing fluids that is why the large scale polymetallic mineralization is closely related to the muscovite bearing granites rather than biotite granites in se china this is important to further understand the source and origin of biotite granites and muscovite bearing granites in se china even worldwide

geochemistry of the highly evolved sn w mo bearing mount douglas granite new brunswick canada implications for origin and mineralization abstract the late devonian 368 1 ma post orogenic peraluminous mount douglas granite located in southwestern new brunswick canada forms the eastern part of the saint george batholith the batholith was emplaced following the accretion of the gander and avalon zones of the northern appalachians during the neoacadian orogeny the mount douglas granite is divided into three units dmd1 dmd2 and dmd3 that formed by progressive high degrees of fractional crystallization the two most fractionated units i e dmd2 and dmd3 are locally associated with greisen sheeted veins related endogranitic sn w mo mineralization whole rock 18o values of 6 0 to 7 3 high initial 87sr 86sr ratios of 0 70550 to 0 71665 slightly positive end 368 ma values 0 3 to 1 1 and pb isotopic data indicate that the granite was derived dominantly from partial melting of juvenile avalonian crust contaminated by supracrustal rocks petrochemical features support the idea of multiple periods of fractional crystallization at lower temperatures producing compositionally more evolved magmas this is supported by estimated zircon saturation temperatures that decrease from dmd1 747 826 c dmd2 733 817 c dmd3 729 816 c the estimated crystallization pressure based on normative quartz and feldspar albite orthoclase contents also decreases with increasing silica content from 2 3 kbar 7 7 km in dmd1 to 0 3 kbar 1 km in dmd3 whole rock geochemical characteristics of this system show that unit dmd3 is the most highly evolved unit it has the highest sio2 avg 76 4 wt lile contents e g li rb cs rb sr mean 42 certain hfse ta th u including y 138 ppm and ree and has the most pronounced negative eu anomalies avg eu eu 0 08 and the lowest k rb 70 127 nb ta mean 4 9 and zr hf mean 23 5 therefore unit dmd3 with the highest content of incompatible elements is the most prospective for sn w zn bi and u mineralization and then dmd2

tectonic record magmatic history and hydrothermal alteration in the hercynian guerande leucogranite armorican massif france abstract the guerande peraluminous leucogranite was emplaced at the end of the carboniferous in the southern part of the armorican massif at the scale of the intrusion this granite displays structural heterogeneities with a weak deformation in the southwestern part whereas the northwestern part is marked by the occurrence of s c and mylonitic extensional fabrics quartz veins and pegmatite dykes orientations as well as lineations directions in the granite and its country rocks demonstrate both e w and n s stretching therefore during its emplacement in an extensional tectonic regime the syntectonic guerande granite has probably experienced some partitioning of the deformation the southwestern part is characterized by a muscovite biotite assemblage the presence of restites and migmatitic enclaves and a low abundance of quartz veins compared to pegmatite dykes in contrast the northwestern part is characterized by a muscovite tourmaline assemblage evidence of albitization and gresenization and a larger amount of quartz veins the southwestern part is thus interpreted as the feeding zone of the intrusion whereas the northwestern part corresponds to its apical zone the granite samples display continuous compositional evolutions in the range of 69 8 75 3 wt sio 2 high initial 87 sr 86 sr ratios and low e nd t values suggest that the peraluminous guerande granite a cnk 1 1 was formed by partial melting of metasedimentary formations magmatic evolution was controlled primarily by fractional crystallization of k feldspar biotite and plagioclase an20 the samples from the apical zone show evidence of secondary muscovitization they are also characterized by a high content in incompatible elements such as cs and sn as well as low nb ta and k rb ratios the apical zone of the guerande granite underwent a pervasive hydrothermal alteration during or soon after its emplacement u th pb dating on zircon and monazite revealed that the guerande granite was emplaced 309 7 1 3 ma ago and that a late magmatic activity synchronous with hydrothermal circulation occurred at ca 303 ma these new structural petrological and geochronological data presented for the guerande leucogranite highlight the interplay between the emplacement in an extensional tectonic regime magmatic differentiation and hydrothermal alteration and provide a general background for the understanding of the processes controlling some mineralization in the western european hercynian belt

transition from amagmatic to volcanic margin mantle exhumation in the voring basin before the icelandic plume influence abstract the outer voring basin norwegian sea is an abnormally deep and distal part of the norwegian passive margin known as magma rich yet the lastest borehole drilled in its northernmost part led to a drastic change in the interpretation of seismic reflection data this update motivates a new model for the entire mesozoic eocene rifting period here i evaluate the link between tectonics serpentinization and magmatism in the norwegian margin from the ryazanian valanginian to the palaeocene eocene magmatic breakup maps of the regional nise sandstones and main regional unconformities specifically highlight the fundamental role of the nyk vema structure to understand the basin and to question the deformation timing related to deep structures the top of partially serpentinized mantle is supported by high amplitude reflectors clearly visible on certain seismic lines known as t reflector covered with gravity driven pre exhumation sediments and further deposits a new structural scheme based on the interpretation of 2d and 3d seismic data shows the complex organization of fault networks and constrains a major deformation period between the late campanian and the late palaeocene ocean scale complementary arguments are in good agreement with lower cretaceous mantle exhumation before becoming a volcanic margin in the palaeocene this segment of the norwegian margin appears as amagmatic until the late campanian at least i propose an integrated model since the onset of hyperextension in the end of the jurassic until the cessation of activity of the aegir ridge in the miocene this model has implications regarding the context dependent interpretation of lower crustal bodies the ubiquity of serpentinization and mantle exhumation worldwide and on the hydrocarbon system

k ar and 40ar 39ar geochronology of granitoid rocks from southern nova scotia its bearing on the geological evolution of the meguma zone of the appalachians k ar and or 40ar 39ar apparent ages are reported for 28 samples of biotite and muscovite from nova scotia s south mountain batholith and associated northern satellite stocks in addition 14 mica samples from the southern satellite stocks were dated previous workers using primarily k ar and rb sr techniques have reported a spread of apparent ages ranging from 240 420 ma for these intrusive rocks the present data set is internally consistent and has been calibrated against accepted interlaboratory standards a mean age of 367 ma was obtained for the south mountain batholith and the northern stocks a value consistent with results recently obtained in an rb sr study by clarke and halliday no differences in apparent age among the various phases or clear geographic age distribution patterns were evident satellite stocks in the southern part of the province have in general younger apparent ages clustered in the range 300 320 ma we suggest that in southern areas mica argon systems record the effect

dating mineralization using several isotopic methods an example from the south mountain batholith nova scotia canada abstract granitic plutons in the meguma terrane may be broadly divided into two suites 1 an areally predominant cogenetic suite of peraluminous granitoid rocks tonalite leucogranite and 2 an areally subordinate paraintrusive suite specialized in sn w mo cu f u p zn as which appears to have been produced by fluid interaction with either residual magma or crystallized rock granite related mineralization is generally considered to have developed during terminal stages of magmatic crystallization that lead to metasomatism and hydrothermal alteration previous geochronological data have suggested that the main cogenetic suite was emplaced at 350 375 ma whereas some of the paraintrusive suite is markedly younger 250 350 ma the apparently large time interval between the intrusion of the cogenetic suite and some of the mineralization suggests that they are not coeval and has led to the conclusions that the younger mineralization may be related to later magmatism and or tectonothermal events rocks associated with two mineral occurrences in the south mountain batholith that had previously yielded anomalously young isotopic ages millet brook and westfield have been analyzed for u pb and ar isotopes in an attempt to resolve this dilemma new geochronological data from the millet brook area indicate that intrusion of granodiorite at 377 5 3 ma nearly concordant monazite age was closely followed by intrusion of most other phases all of which had cooled through 375 c closure temperature for ar in muscovite by 377 373 6 ma 40 ar 39 ar plateau ages new data from westfield indicate that a paraintrusive porphyry was intruded at 385 2 ma upper intercept of a chord through four nearly concordant zircon fractions and that greisen muscovite cooled through the closure temperature by 368 2 6 ma 40 ar 39 ar plateau age these data indicate that a previously reported rb sr whole rock isochron age of 270 ma at westfield reflect isotopic resetting that occurred below the blocking temperature for 40 ar 39 ar in muscovite 375 c clearly further rb sr analysis is required to document this process other 250 300 ma ages in the meguma terrane are recorded along the tobeatic fault zone away from the fault ages increase upto 370 ma suggesting that the fault may have acted as an important pathway for fluids and heat that facilitated isotopic resetting rb sr whole rock and mineral systems may be affected after formation of an alteration assemblage associated with mineralization by the migration of rb and its concentration in secondary minerals

rb sr age and geochemical distinctions between the carboniferous tin bearing davis lake complex and the devonian south mountain batholith meguma terrane nova scotia the davis lake complex dlc composed of biotite monzogranite leucomonzogranite and cassiterite topaz greisen hosts the east kemptville tin mine in southwestern nova scotia the dlc monzogranite contains glomeroporphyritic biotite with ilmenite and many rare earth element ree bearing accessory minerals zircon bearing quartz phenocrysts and xenoliths of biotite granite primary muscovite is rare major and trace element geochemical trends indicate well defined but limited magmatic differentiation trends ree patterns of the least evolved granites are flat and show a ce yb ratio of 10 the dlc was previously considered cogenetic with the devonian south mountain batholith smb on the basis of its location lithologies and similarities in major and trace element geochemistry however new rb sr whole rock isotopic data indicate an rb sr date of 330 7 ma mean square of weighted deviates mswd 2 8 for the dlc implying that it is at least 35 ma younger than the smb the initial 87sr 86sr rat

anomalously high sc contents in ore minerals from sn w deposits possible economic significance and genetic implications abstract an investigation on sn w and w deposits and associated granites in the erzgebirge region central kazakhstan mongolian altai and central mongolia reveals anomalously high sc contents up to several thousands of parts per million in wolframite and cassiterite from sn w deposits in the eastern erzgebirge even though whole rock samples of granites from these localities have low sc contents typical of high silica granitic rocks it is possible to trace the distribution of sc in rocks and veins using data on accessory and ore minerals enriched in sc columbite rutile cassiterite wolframite zircon scheelite and mica there are no indications for an enrichment of sc during the magmatic evolution of the granite intrusions magmatic fractionation causes rather low sc contents in evolved granites on the other hand sc enrichment in zircon found in these rocks is clearly related to processes of secondary alteration leading to high u ca and fe besides sc enrichment in the mineral enhanced sc contents were also found in whole rock samples from the greisen the occurrence of sc rich wolframite and cassiterite is mainly restricted to endocontact orebodies greisen and veins outside the rare metal bearing granites the sc content in wolframite drops off if host rocks are formed by schist and gneiss correlation between sc and nb concentrations in wolframite found in endocontact orebodies also disappears in the exocontact veins except if the vein system provides a focused fluid flow sc enrichment in sn w mineralisation is accompanied by strong y sc y yb and th u fractionation in sc bearing minerals as indicated by trace element contents in wolframite and scheelite y sc ratios in wolframite below 0 1 are typical of sn deposits it is suggested that certain conditions of complexation ph and eh in f rich fluids are necessary prerequisites for sc enrichment and deposition our results hint for a sc input into the ore forming system by an external f rich fluid carrying also u and heavy ree and possibly derived from a mantle related source although whole rock samples of rare metal bearing topaz granite have low sc contents the related sn w ores may represent an interesting source for sc extraction because of their enrichment in sc

a new assessment of the abundance of serpentinite in the oceanic crust the gabbro model of the composition of the oceanic crust has prevailed for more than two decades but serpentinites are common along much of the mid atlantic ridge and elsewhere leading to renewed debate over the abundance of serpentinites in crust created at slow spreading ridges based on data from both oceanic and ophiolite samples it has also been suggested that serpentinites are not distinguishable from meta gabbros and diabases by their seismic properties we have made new statistical analyses of the relationship between vs and vp in oceanic gabbro and diabase samples and serpentinized peridotites and compared them to in situ s wave and p wave velocities derived from expanding spread profiles esps acquired in the atlantic our findings demonstrate 1 that there is some overlap of the properties of gabbroic samples taken from ophiolites with the properties of serpentinites but 2 over the range of typical layer 3 seismic velocities the seismic properties of oceanic gabbros and diabases are distinctly different from the properties of serpentinites and 3 that in situ seismic velocities are in excellent agreement with the properties of the gabbros while the shear velocities in serpentinites are significantly lower partially serpentinized peridotite is certainly present but not abundant in normal oceanic crust

evidence for the granulite granite connection penecontemporaneous high grade metamorphism granitic magmatism and core complex development in the liscomb complex nova scotia canada abstract upper amphibolite granulite facies gneisses and granites of the liscomb complex nova scotia canada which are exposed in a core complex within the cambro ordovician meguma group of southern nova scotia yielded concordant u pb zircon monazite ages of 377 2 and 374 3 ma respectively geochronological and geochemical data suggest a single devonian high grade metamorphic event which generated the granitic magma by partial melting of the fertile liscomb gneisses at a depth of 30 km the melting was also synchronous with an extensional event during which the gneisses were uplifted in a core complex associated with the intrusion of granitoids to a depth of 10 km subsequently the gneisses and granites underwent rapid exhumation before the deposition of unconformably overlying late fammenian rocks at 364 ma these events took place during terminal stages of the acadian orogeny and the onset of extensional tectonics in atlantic canada during the middle late devonian the close temporal and spatial association of liscomb gneisses granulites and granites their major and trace element compositions and their overlapping isotopic characteristics confirm the hypothesis that high grade metamorphism and generation of granitic melt are complementary processes as the liscomb granites are of similar age mineralogy and chemistry to the voluminous granitoid plutons found throughout the meguma terrane a similar process is indicated for the rest of the terrane

the beauvoir topaz lepidolite albite granite massif central france the disseminated magmatic sn li ta nb be mineralization the beauvoir topaz lepidolite albite granite french massif central is the latest intrusion in a variscan peraluminous granitic complex composed of three units successively emplaced the concealed la bosse granite the colettes two mica granite and the beauvoir granite the 900 m deep drill hole of the geologie profonde de la france deep drilling program of the continental crust has provided a continuously cored section of the beauvoir granite the exposed section of the beauvoir granite is presently mined for kaolin in the upper 200 m it also represents a huge subeconomic disseminated li sn ta be deposit sharing many characteristics with rare metal bearing pegmatites compared to similar li f rich igneous bodies the beauvoir granite is strongly enriched in sn 200 1 400 ppm ta 20 400 ppm and be 20 300 ppm li is located in lepidolite and amblygonite ta and nb mainly in columbo tantalite and uranium rich microlite and sn in cassiterite be is located in lepidolite crystals and in unidentified mineral s the major element composition of the beauvoir granite is similar to that of macusani volcanic glasses of peru and ongonite subvolcanic rocks of mongolia the main structural textural mineralogical and geochemical characteristics of the beauvoir granite can be explained by extreme fractionation of f and li rich magmas and associated fluid phases most trace elements show a strong upward enrichment the high f sub o 2 low solidus temperature and high fluorine content of the beauvoir magma have considerably reduced sn and w fractionation in the expelled magmatic fluid despite its high chlorine content 25 30 wt nacl equiv however f and li enriched geochemical halos have developed in the enclosing mica schist in response to the boiling of the magmatic fluid the relatively deep emplacement of the granite 3 km has limited the development of hydraulic fracturing and subsequent hydrothermal circulation and mineralization thus despite the very high specialization of the beauvoir granite no significant vein type mineralization is directly related to it the la bosse quartz ferberite stockwork was developed before the emplacement of the colettes and beauvoir granites and is related to the earlier concealed la bosse granite

melt composition control of zr hf fractionation in magmatic processes zircon zrsio4 and hafnon hfsio4 solubilities in water saturated granitic melts have been determined as a function of melt composition at 800 and 1035 c at 200 mpa the solubilities of zircon and hafnon in metaluminous or peraluminous melts are orders of magnitude lower than in strongly peralkaline melt moreover the molar ratio of zircon and hafnon solubility is a function of melt composition although the solubilities are nearly identical in peralkaline melts zircon on a molar basis is up to five times more soluble than hafnon in peraluminous melts accordingly calculated partition coefficients of zr and hf between zircon and melt are nearly equal for the peralkaline melts whereas for metaluminous and peraluminous melts dhf dzr for zircon is 0 5 to 0 2 consequently zircon fractionation will strongly decrease zr hf in some granites whereas it has little effect on the zr hf ratio in alkaline melts or similar depolymerized melt compositions the ratio of the molar solubilities of zircon and hafnon for a given melt composition temperature and pressure is proportional to the hf zr activity coefficient ratio in the melt the data imply that this ratio is nearly constant and probably close to unity for a wide range of peralkaline and similar depolymerized melts however it changes by a factor of two to five over a relatively small interval of melt compositions when a nearly fully polymerized melt structure is approached for most ferromagnesian minerals in equilibrium with a depolymerized melt dhf dzr typical values of dhf dzr range from 1 5 to 2 5 for clinopyroxene amphibole and titanite because of the change in the hf zr activity ratio in the melt the relative fractionation of zr and hf by these minerals will disappear or even be reversed when the melt composition approaches that of a metaluminous or peraluminous granite it is thus not surprising that fractional crystallization of such granitic magmas leads to a decrease in zr hf whereas fractional crystallization of depolymerized melts tends to increase zr hf there is no need to invoke fluid metasomatism to explain these effects results demonstrate that for ions with identical charge and nearly identical radius crystal chemistry does not alone determine relative compatibilities rather the effect of changing activity coefficients in the melt may be comparable to or even larger than elastic strain effects in the crystal lattice

cascadia subducting plate fluids channelled to fore arc mantle corner ets and silica deposition in this study we first summarize the constraints that on the cascadia subduction thrust there is a 70 km gap downdip between the megathrust seismogenic zone and the episodic tremor and slip ets that lies further landward there is not a continuous transition from unstable to conditionally stable sliding seismic rupture occurs mainly offshore for this hot subduction zone ets lies onshore we then suggest what does control the downdip position of ets we conclude that fluids from dehydration of the downgoing plate focused to rise above the fore arc mantle corner are responsible for ets there is a remarkable correspondence between the position of ets and this corner along the whole margin hydrated mineral assemblages in the subducting oceanic crust and uppermost mantle are dehydrated with downdip increasing temperature and seismic tomography data indicate that these fluids have strongly serpentinized the overlying fore arc mantle laboratory data show that such fore arc mantle serpentinite has low permeability and likely blocks vertical expulsion and restricts flow updip within the underlying permeable oceanic crust and subduction shear zone at the fore arc mantle corner these fluids are released upward into the more permeable overlying fore arc crust an indication of this fluid flux comes from low poisson s ratios and vp vs found above the corner that may be explained by a concentration of quartz which has exceptionally low poisson s ratio the rising fluids should be silica saturated and precipitate quartz with decreasing temperature and pressure as they rise above the corner

petrogenetic evolution of late paleozoic rhyolites of the harvey group southwestern new brunswick canada hosting uranium mineralization the 360 ma subaerial felsic volcanic and volcaniclastic rocks of the harvey group form a belt about 15 km long and 3 km wide in southwestern new brunswick canada that has been correlated with parts of the mount pleasant caldera complex the site of a significant polymetallic tin tungsten molybdenum indium and bismuth deposit the harvey volcanic rocks are highly fractionated peraluminous within plate f rich rhyolites which host uranium mineralization the rocks were modified by late magmatic and post magmatic processes a comparison of the composition of whole rocks and melt inclusions in the quartz phenocrysts shows that some trace elements including u were affected by the post magmatic processes their flat ree patterns accompanied by distinct negative eu anomalies are typical of highly evolved f rich leucogranites and rhyolites nd isotopic ratios nd 360 0 6 to 1 0 are similar to those of the felsic rocks of the mount pleasant complex the harvey rhyolites were generated by extensive fractional crystallization of andesites of the mount pleasant caldera the melt evolved at the apex of the magma chamber where volatile elements become concentrated the harvey rhyolite with melt inclusions containing 20 ppm u had the potential to develop a significant u mineralization the erupted glassy rhyolite is a favorable u source rock amendable to leaching by post magmatic hydrothermal and meteoric water the high th u ratios in the harvey volcanic rocks compared to the low ratios in the u rich melt inclusions is indicative of such a process

anatomy of a megathrust the 2010 m8 8 maule chile earthquake rupture zone imaged using seismic tomography abstract knowledge of seismic velocities in the seismogenic part of subduction zones can reveal how material properties may influence large ruptures observations of aftershocks that followed the 2010 m w 8 8 maule chile earthquake provide an exceptional dataset to examine the physical properties of a megathrust rupture zone we manually analysed aftershocks from onshore seismic stations and ocean bottom seismometers to derive a 3 d velocity model of the rupture zone using local earthquake tomography from the trench to the magmatic arc our velocity model illuminates the main features within the subduction zone we interpret an east dipping high p wave velocity anomaly 6 9 km s as the subducting oceanic crust and a low p wave velocity we assess the role played by physical heterogeneity in governing megathrust behaviour greatest slip during the maule earthquake occurred in areas of moderate p wave velocity 6 5 7 5 km s where the interface is structurally more uniform at shallow depths high fluid pressure likely influenced the up dip limit of seismic activity the high velocity bodies lie above portions of the plate interface where there was reduced coseismic slip and minimal postseismic activity the northern velocity anomaly may have acted as a structural discontinuity within the forearc influencing the pronounced crustal seismicity in the pichilemu region our work provides evidence for how the ancient geological structure of the forearc may influence the seismic behaviour of subduction megathrusts

chemical and isotopic composition of the lower crust beneath the meguma lithotectonic zone nova scotia evidence from granulite facies xenoliths granulite facies xenoliths from an upper devonian lamprophyre dyke near tangier nova scotia provide new information about the lower crust in the meguma zone two mineralogically and chemically distinet groups of xenoliths occur both groups contain quartz feldspar biotite fe ti oxides sulfides in addition group a contains garnet graphite aluminosilicates spinel sapphirine hight al2o3 subgroups a1 and a2 or clinopyroxene sphene apatite high cao sub group a3 group b has highly variable proportions of orthopyroxene b1 clinopyroxene b2 and amphibole b3 trace element contents of the highly aluminous xenoliths compare closely with average to upper crustal model compositions and are similar in many aspects to other undepleted granulite facies rocks lowp t sedimentary assemblages of quartz chlorite clay minerals calcite albite or paragonite can account for the compositions of group a xenoliths within group b a high mgo mgo 13 wt subgroup with high transition metal contents and low mgo mgo 9 wt sub groups with higher lil large ion lithophile element contents exist although the rare earth and high field strength elements indicate a tholeiitic or low k calc alkaline chemistry the lil elements are as high as those from high k calc alkaline volcanic are rocks isotopically group a ranges from endt 2 56 to 0 80 and87sr 86srt 0 7046 to 0 7182 fort 370 ma for group b these values are 1 45 to 5 33 and 0 7028 to 0 7048 respectively model ages tchur are correspondingly low and are tightly constrained 544 52 ma these young ages contrast with the middle proterozoic nd model ages of the overlying meguma zone low grade flysch this time inverted stratigraphy appears to be the product of a tectonic break between a younger autochthonous tangier lower crust avalon and an older allochthonous meguma group upper crust

three dimensional models of p wave velocity and p to s velocity ratio in the southern central andes by simultaneous inversion of local earthquake data the pisco 94 proyecto de investigation sismologica de la cordillera occidental 1994 seismological network of 31 digital broad band and short period three component seismometers was deployed in northern chile between the coastal cordillera and the western cordillera more than 5300 local seismic events were observed in a 100 day period a subset of high quality p and s arrival time data was used to invert simultaneously for hypocenters and velocity structure additional data from two other networks in the region could be included the velocity models show a number of prominent anomalies outlining an extremely thickened crust about 70 km beneath the forearc region an anomalous crustal structure beneath the recent magmatic arc western cordillera characterized by very low velocities and a high velocity slab a region of an increased vp vs ratio has been found directly above the wadati benioff zone which might be caused by hydration processes a zone of lower than average velocities and a high vp vs ratio might correspond to the asthenospheric wedge the upper edge of the wadati benioff zone is sharply defined by intermediate depth hypocenters while evidence for a double seismic zone can hardly be seen crustal events between the precordillera and the western cordillera have been observed for the first time and are mainly located in the vicinity of the salar de atacama down to depths of about 40 km

an autochthonous avalonian basement source for the latest ordovician brenton pluton in the meguma terrane of nova scotia u pb hf isotopic constraints and paleogeographic implications the ediacaran ordovician meguma supergroup was thrust over avalonia basement prior to the intrusion of post acadian ca 370 ma s type granitic batholiths this has led to two main hypotheses regarding the original location of the meguma terrane a continental rise prism bordering either nw africa or avalonia on the other hand the pre acadian ca 440 ma brenton pluton has yielded the following u pb la icp ms zircon data 1 448 3 ma population peak inferred to be the intrusive age and 2 ca 550 and 700 ma inherited ages common to both avalonia and nw africa in contrast hf isotopic analyses of zircon yielded model ages ranging from 814 to 1127 ma with most between 940 and 1040 ma such ages are typical of avalonia and not nw africa the ages of the inherited zircons found within the brenton pluton suggest that it was probably derived by partial melting of sub meguma mid crustal avalonian rocks upon which the meguma supergroup was deposited although avalonia is commonly included in the peri gondwanan terranes off nw africa or amazonia paleomagnetic data faunal provinciality and hf data suggest that during the ediacaran early cambrian it was an island chain lying near the tropics ca 20 30 s and was possibly a continuation of the bolshezemel volcanic arc accreted to northern baltica during the ediacaran timanide orogenesis this is consistent with the similar derital zircon population in the ediacaran cambrian meguma supergroup and the dividal group in northeastern baltica

seismic waves velocities and anisotropy in serpentinized peridotites from xigaze ophiolite abundance of serpentine in slow spreading ridge the effect of serpentinization on seismic wave velocity and anisotropy has been analysed in 6 peridotite samples of the xigaze ophiolite having harzburgitic composition and a degree of serpentinization ranging from 3 to 70 we found i p and s wave velocities are linearly correlated with serpentine content ii anisotropy of p and s waves decreases with increasing serpentinization while in fresh peridotites anisotropy is of the order of 6 for 70 of serpentinization it drops to 1 for p waves and to 2 for s waves comparison of published laboratory velocity data in oceanic lithologies like gabbros and dolerites with serpentinized peridotite data shows the difficulty of distinguishing between gabbros and peridotites having a percentage of serpentine between 10 and 40 by seismic waves velocities only this comparison suggests that seismic velocity data are consistent with the existence of partialy serpentinized peridotites up to 40 of serpentinization in the oceanic crust formed at slow spreading ridges in accordance with geological observation

geochronology geochemistry and sr nd pb hf isotopes of the zhunsujihua granitoid intrusions associated with the molybdenum deposit northern inner mongolia china implications for petrogenesis and tectonic setting the zhunsujihua porphyry molybdenum deposit located in northern inner mongolia of china that belongs to central asian orogenic belt caob is the only mo deposit formed in the late carboniferous in this area so far its mineralization is mainly restricted to the zhunsujihua granitoid intrusions which are composed of the main granodiorite gd and crosscutting virtually coeval minor syn ore leucogranite lg and diorite porphyry dp dykes la icp ms zircon u pb dating yields crystallization ages of 300 0 2 0 299 3 2 0 and 299 0 2 6 ma for the gd lg and dp respectively the major and trace element lithogeochemical data show that the gd and lg are metaluminous to weakly peraluminous high k calc alkaline series with i type granite characteristics strongly oxidized with low concentrations of ba nb sr p and ti and elevated k and rb contents indicating typical arc magmatic features the lg is a product derived by extensive fractional crystallization of a parental magma similar to the gd as evident from the lower eu eu nb ta zr hf and t zr the moderately altered dp exhibits high concentrations of k rb cs lree y and low sr y with a positive e nd 300 ma which indicates a mantle or juvenile source associated with an arc setting the sr nd hf isotope data show low i sr 0 70406 0 70461 and moderate e nd 300 ma 0 9 to 1 5 for the gd and lg and relatively high e hf 300 ma values 3 6 to 11 2 for the gd suggesting the magma mainly originated from the juvenile lower crust that was derived from depleted mantle with a minor component of ancient continental crust lead isotope data have characteristics of a lower crust source with minor contamination by upper crustal material combined with previous research the zhunsujihua granitoid intrusions developed in an intracontinental volcanic arc uliastai associated with northward subduction of the paleo asian ocean plate during late carboniferous to early permian this suggests that the subduction of paleo asian ocean may have continued to late carboniferous and the hengenshan basin is probably closed during the early permian fractional crystallization is the main evolutionary process of the felsic magma which has played an important role in the mo mineralization the coeval dp may have provided additional heat for the extended evolution of the crystallizing felsic magma chamber

element geochemistry mineralogy geochronology and zircon hf isotope of the luxi and xiazhuang granites in guangdong province china implications for u mineralization abstract the luxi and xiazhuang granitic plutons are parts of the guidong granite belt in northern guangdong province china they occur next to each other and have similar crystallization ages however the xiazhuang pluton hosts several important uranium u ore deposits whereas the luxi pluton does not this study compares the two plutons using whole rock geochemistry biotite compositions zircon u pb ages by la icpms and in situ zircon hf isotope data by la mc icpms to understand the fundamental controls results show that the two plutons have similar ages 237 ma for the luxi pluton and 234 ma for the xiazhuang pluton they also have similar geochemical features that are characteristic of s type granites zircons from both plutons have negative e hf t values 9 to 12 and a paleoproterozoic two stage hf model age t dm2 1838 ma to 2025 ma suggesting that their parental magmas were all derived from paleoproterozoic metasedimentary rocks the geochemical and petrological data suggest that these two plutons crystallized from magmas derived from the same parent at different stages the parental magma of the xiazhuang pluton is more evolved than that of the luxi pluton this factor plus different physical chemical conditions such as temperature volatile components and oxidation state resulted in higher u content and favorable u bearing species in the xiazhuang magmatic hydrothermal system u occurs mainly as uraninite in the xiazhuang pluton which facilitates u transfer from granite into co 2 rich hydrothermal fluid to form u ore deposits in contrast u occurs mainly as ree bearing minerals in the luxi magmatic system preventing u transfer to associated hydrothermal fluids as a result the xiazhuang pluton hosts several important uranium ore deposits whereas the luxi pluton is ore barren

columbite solubility in granitic melts consequences for the enrichment and fractionation of nb and ta in the earth s crust the behaviour of niobium and tantalum in magmatic processes has been investigated by conducting mnnb2o6 and mnta2o6 solubility experiments in nominally dry to water saturated peralkaline aluminium saturation index a s i 0 64 to peraluminous a s i 1 22 granitic melts at 800 to 1035 c and 800 to 5000 bars the attainment of equilibrium is demonstrated by the concurrence of the solubility products from dissolution crystallization mn doped and nb or ta doped experiments at the same pressure and temperature the solubility products of mnnb2o6 ksp nb and mnta2o6 ksp ta at 800 c and 2 kbar both increase dramatically with alkali contents in water saturated peralkaline melts they range from 1 2 10 4 and 2 6 10 4 mol2 kg2 respectively in subaluminous melt a s i 1 02 to 202 10 4 and 255 10 4 mol2 kg2 respectively in peralkaline melt a s i 0 64 this increase from the subaluminous composition can be explained by five non bridging oxygens being required for each excess atom of nb5 or ta5 that is dissolved into the melt the ksp nb and ksp ta also increase weakly with al content in peraluminous melts ranging up to 1 7 10 4 and 4 6 10 4 mol2 kg2 respectively in the a s i 1 22 composition columbite tantalite solubilities in subaluminous and peraluminous melts a s i 1 02 and 1 22 are strongly temperature dependent increasing by a factor of 10 to 20 from 800 to 1035 c by contrast columbite tantalite solubility in the peralkaline composition a s i 0 64 is only weakly temperature dependent increasing by a factor of less than 3 over the same temperature range similarly ksp nb and ksp ta increase by more than two orders of magnitude with the first 3 wt h2o added to the a s i 1 02 and 1 22 compositions whereas there is no detectable change in solubility for the a s i 0 64 composition over the same range of water contents solubilities are only slightly dependent on pressure over the range 800 to 5000 bars the data for water saturated sub and peraluminous granites have been extrapolated to 600 c conditions at which pegmatites and highly evolved granites may crystallize using a melt concentration of 0 05 wt mno 70 to 100 ppm nb or 500 to 1400 ppm ta are required for manganocolumbite and manganotantalite saturation respectively the solubility data are also used to model the fractionation of nb and ta between rutile and silicate melts predicted rutile melt partition coefficients increase by about two orders of magnitude from peralkaline to peraluminous granitic compositions it is demonstrated that the nb2o5 ta2o5 activity coefficient ratio in the melt phase depends on melt composition this ratio is estimated to decrease by a factor of 4 to 5 from andesitic to peraluminous granitic melt compositions accordingly all the relevant accessory phases in subaluminous to peraluminous granites are predicted to incorporate nb preferentially over ta this explains the enrichment of ta over nb observed in highly fractionated granitic rocks and in the continental crust in general

evidence for the location and cause of large crustal earthquakes in japan large crustal earthquakes magnitude 5 7 8 0 depth 0 20 km during a period of 115 years from 1885 to 1999 in japan occurred in or around zones of low seismic velocity revealed by seismic tomography the low velocity zones may represent weak sections of the seismogenic crust the crustal weakening is considered to be closely related to the subduction of the oceanic pacific and philippine sea plates in this region along the volcanic front and in back arc areas the crustal weakening may be caused by active volcanoes and magma chambers in the forearc areas fluids are detected in the earthquake source zones which may have contributed to the crustal weakening and the rupture nucleation the fluids may be related to the dehydration of the subducting philippine sea slab these results indicate that large crustal earthquakes do not strike anywhere but only anomalous areas that may be detected with geophysical methods

the east kemptville tin deposit yarmouth county nova scotia a pb isotope study of the leucogranite and mineralized greisens evidence for a 366 ma metallogenic event pb isotopic data for whole rock leucogranite and mineral separates variably mineralized greisen and galena from the east kemptville sn cu zn ag deposit yarmouth county southwestern nova scoti

the early jurassic bokan mountain peralkaline granitic complex southeastern alaska geochemistry petrogenesis and rare metal mineralization abstract the early jurassic ca 177 ma bokan mountain granitic complex located on southern prince of wales island southernmost alaska cross cuts paleozoic igneous and metasedimentary rocks of the alexander terrane of the north american cordillera and was emplaced during a rifting event the complex is a circular body 3 km in diameter of peralkaline granitic composition that has a core of arfvedsonite granite surrounded by aegirine granite all the rock forming minerals typically record a two stage growth history and aegirine and arfvedsonite were the last major phases to crystallize from the magma the bokan granites and related dikes have sio2 from 72 to 78 wt high iron feo tot 3 4 5 wt and alkali 8 10 wt concentrations with high feo tot feo tot mgo ratios typically 0 95 and the molar al2o3 na2o k2o ratio

accessory minerals and the geochemical evolution of crustal magmatic systems a summary and prospectus of experimental approaches it has recently been recognized that accessory minerals in continental rocks may contain a substantial fraction of the whole rock complement of trace elements and geochemically important isotopes this observation is in itself interesting but its importance is magnified many fold when one considers the possible consequences to the geochemical rearrangement that accompanies crustal melting if for example an accessory phase is insoluble in early formed partial melts it may effectively retain in the residue many incompatible elements e g ree and daughter isotopes e g radiogenic pb that would otherwise be partitioned into the melt in addition should an unmelted accessory mineral e g zircon be entrained in a departing melt fraction it may carry with it radiogenic components pb nd143 hf176 characteristic of the source region the effectiveness of phases such as zircon apatite sphene allanite epidote and monazite in the roles noted above can only be gauged from knowledge of the three fundamental accessory phase parameters i e 1 the solubilities of the accessory minerals in crustal melts 2 the equilibrium mineral liquid partition coefficients for the trace element and isotopes of interest and 3 the diffusivities that govern the rates at which equilibrium will be approached in general these parameters can only be measured in laboratory experiments and relatively few data have been accumulated to date it does seem clear however that apatite generally is residual insoluble during crustal melting and zircon can be depending on melting conditions and abundance of zircon in the source petrographic and geochemical studies indicate that monazite allanite and epidote may also be highly insoluble in some cases but the specific circumstances that promote insolubility remain unknown the few experimentally determined partition coefficients now available for apatite and zircon agree qualitatively with natural rock data on mineral separates however preliminary diffusion measurements and inferences from natural systems suggest that trace element and isotopic equilibrium between undissolved accessories and crustal melts is in fact rarely approached ironically it is this very disequilibrium in the form of preserved diffusion profiles that may afford the most information on time temperature histories of melting events continued experimental interest in the fundamental accessory phase parameters will eventually provide the tools that will enable rigorous assessment not only of the timing and duration of crustal melting episodes but also of the extent of melting and re distribution of trace elements and radiogenic isotopes

anatexis and metamorphism in tectonically thickened continental crust exemplified by the sevier hinterland western north america the generation of granitoid magmas by partial melting of crustal rocks during continental thickening events is well documented in many geological provinces throughout the world including the late mesozoic sevier belt of western north america we present a thermal and petrologic model of anatexis and metamorphism in regions of crustal thickening where the only mantle contribution is the normal conductive supply of heat through the base of the lithosphere i e advection of mass and energy are excluded we distinguish between formation of migmatites and generation of mobile granitoid magmas and examine the temporal and spatial relationships between these two distinct anatectic processes between anatexis and regional deformation and between anatexis and metamorphism a fundamental conclusion is that if protoliths rich in hydrous minerals are present regional anatexis is the end product of classical barrovian metamorphism in thickened continental crust even in the absence of a free water rich fluid phase barrovian metamorphic facies series are predicted with thickening ratios maximum crustal thickness attained initial crustal thickness as low as 1 3 but mobile granitoid magmas are not formed if this ratio is less than approximately 1 5 above these lower bounds barrovian metamorphism and anatectic granitoid magmatism occur independently of the magnitude of thickening and of the way in which thickening is accomplished both processes are sensitive to a diminished heat supply lowering either moho heat flow or crustal radioactive heat production results in blueschist eclogite metamorphism and inhibits the formation of mobile granitoid magmas we model anatexis under fluid absent conditions and show that with such a constraint migmatization is always a syn kinematic process relative to the crustal thickening event whereas generation of mobile granitoid magmas is in most cases post kinematic relative to crustal thickening but can be syn kinematic if thickening takes more than approximately 50 myr the typical time intervals for melting are consistent with geological observations mobile granitoid magmas are predicted by most of our models within approximately 10 myr of the end of the crustal thickening event this incubation period results primarily from the temperature increase required for the dehydration melting reactions capable of producing large melt fractions to occur the energetic requirements of anatexis are relatively minor compared to conductive crustal thermal budgets as shown by the fact that once the necessary p t conditions are attained melting reactions are completed within time intervals on the order of 1 myr i e 1 2 orders of magnitude smaller than the characteristic time scales of the tectonic processes involved in crustal thickening

seismic constraints on the water flux delivered to the deep earth by subduction for the first time large quantities of water are identified throughout the length of a subducting plate and an efficient pathway is defined for injecting water into the deep 600 km earth by the incorporation of water into the mantle of an oceanic plate as serpentine water is thought to facilitate plate tectonics but the quantities of water involved and thus its effect are poorly known taking advantage of an anomalous seismic arrival recorded in the tonga fiji subduction zone the presence volume water content and flux of serpentine and water associated with the subducting oceanic mantle at tonga is quantified at 2 10 8 tg ma h 2 o many times larger than previous inferences based on surface observations water fluxes of this magnitude into the deep earth will substantially increase the role of water in hypotheses of plate tectonic movements mantle plumes and flood basalt activity

significance of gneissic rocks in the liscomb complex nova scotia the liscomb complex comprises late devonian intrusive rocks principally peraluminous granite and medium to high grade metamorphic rocks gneisses that collectively are hosted by low grade greenschist facies metasediments of the cambro ordovician meguma group the conventional view that these gneisses contain high grade mineral assemblages and represent basement rocks has recently been challenged and indeed some of the rocks previously mapped as gneisses particularly metapelites have isotopic compositions resembling the meguma group amphibole bearing enclaves in the liscomb plutons however are isotopically distinct and in this regard resemble xenoliths of basement gneisses in the popes harbour lamprophyre dyke south of the liscomb area metasedimentary enclaves with meguma isotopic signatures can contain garnets with unzoned cores implying high temperatures that host high grade minerals prismatic sillimanite spinel and or corundum and are enclosed by retrograde zoned rims these f

vapor absent melting at 10 kbar of a biotite and amphibole bearing tonalitic gneiss implications for the generation of a type granites vapor absent melting experiments on a biotite and amphibole bearing archean tonalitic gneiss agc150 at 10 kbar and 875 to 1050 c show that amphibole breaks down from 900 to 950 c producing garnet orthopyroxene and granitic melt biotite dehydration melting produces 10 wt melt up to 950 c via incongruent melting reactions that produce garnet orthopyroxene and titanomagnetite widespread biotite dehydration melting occurs between 950 and 975 c and produces orthopyroxene magnetite titanomagnetite and 20 wt fluorine rich melt up to 0 31 wt f minor f rich 2 7 wt biotite is present even at 1000 c our experiments show that under vapor absent conditions intrusion of hot mantle derived magmas into the lower crust is necessary to initiate widespread biotite dehydration melting in rocks with compositions like agc150 we propose that the high thermal stability of biotite in agc150 suggests that this rock is residual after a previous episode of partial dehydroxylation that left behind somewhat f enriched biotite we show that dehydration melting of such f enriched biotite produces f rich granitic liquids with compositions within the range of a type granite and leaves behind a granulitic residue consisting of orthopyroxene plagioclase quartz titanomagnetite and magnetite

are the lower planes of double seismic zones caused by serpentine dehydration in subducting oceanic mantle in a number of subduction zones earthquakes at 50 200 km depth define two dipping planes separated by 20 40 km that appear to merge downdip upper plane earthquakes are inferred to occur within the subducting oceanic crust whereas lower plane earthquakes occur in the subducting oceanic mantle beneath northeast japan the results of a new finite element heat transfer model suggest that the lower seismic plane cuts across isotherms at a shallow angle lower plane earthquakes occur at 550 800 c at 100 km depth and at 350 600 c at 160 km depth these conditions coincide with the dehydration reaction antigorite serpentine forsterite enstatite h 2 o which suggests that lower plane earthquakes may be triggered by dehydration embrittlement which in turn suggests that the subducting oceanic mantle is partially hydrated serpentinization may occur in the trench outer rise region where faulting may promote infiltration of seawater several tens of kilometers into the oceanic lithosphere if this hypothesis is correct current subduction zone h 2 o budgets may significantly underestimate the amount of bound h 2 o entering the subduction factory

lower carboniferous post orogenic granites in central eastern sierra de velasco sierras pampeanas argentina u pb monazite geochronology geochemistry and sr nd isotopes the central eastern part of the sierra de velasco sierras pampeanas nw argentina is formed by the large huaco 40 30 km and sanagasta 25 15 km granite massifs and the small la chinchilla stock 2 2 km the larger granites intrude into ordovician metagranitoids and crosscut devonian mylonitic shear zones whereas the small stock sharply intrudes into the huaco granite the two voluminous granites are biotitic muscovitic and biotitic porphyritic syeno to monzogranites they contain small and rounded tonalitic and quartz dioritic mafic microgranular enclaves the small stock is an equigranular zinnwaldite and fluorite bearing monzogranite the studied granites are silica rich sio2 70 potassium rich k2o 4 ferroan alkali calcic to slightly calk alkalic and moderately to weakly peraluminous a cnk 1 06 1 18 huaco granite 1 01 1 09 sanagasta granite 1 05 1 06 la chinchilla stock they have moderate to strong enrichments in several lil li rb cs and hfs nb ta y th u elements and low sr ba and eu contents u pb monazite age determinations indicate lower carboniferous crystallization ages 350 358 ma for the huaco granite 352 7 1 4 ma for the sanagasta granite and 344 5 1 4 ma for the la chinchilla stock the larger granites have similar nd values between 2 1 and 4 3 whereas the younger stock has higher nd of 0 6 to 1 4 roughly comparable to the values obtained for the carboniferous san blas granite 1 4 to 1 7 located in the north of the sierra the huaco and sanagasta granites have a mainly crustal source but with some participation of a more primitive possibly mantle derived component the main crustal component can be attributed to ordovician peraluminous metagranitoids the la chinchilla stock derives from a more primitive source suggesting an increase with time in the participation of the primitive component during magma genesis the studied granites were generated during a post orogenic period in a within plate setting possibly as a response to the collapse of the previous famatinian orogen extension of the crust and mantle upwelling they are part of the group of middle devonian lower carboniferous granites of the sierras pampeanas the distribution and u pb ages of these granites suggests a northward arc parallel migration of this mainly post orogenic magmatism with time

a jurassic garnet bearing granitic pluton from ne china showing tetrad ree patterns abstract ne china is characterized by the massive distribution of phanerozoic granitoids most of them are of i and a type granites whereas s type granites are rarely documented the present work deals with the dongqing pluton a small granitic body emplaced in the southern zhangguangcai range the pluton comprises a two mica garnet granite and a garnet bearing muscovite granite the latter occurs as veins in the former the pluton shows a gradational contact with the surrounding host granites rb sr and sm nd isotope analyses on whole rocks and minerals reveal that the two types of granites were emplaced synchronously at about 160 ma the pluton was emplaced coeval with the surrounding i type granitic pluton and had a rapid cooling history it is characterized by an initial sr isotopic ratio of 0 706 slightly negative end t values 0 5 to 1 9 and young depleted mantle model ages 970 1090 ma this suggests that the parent magma originated from partial melting of relatively juvenile crust which is largely compatible with the general scenario for much of the phanerozoic granitoids emplaced in the central asian orogenic belt geochemically the granites of the dongqing pluton are peraluminous with a shand index molar ratio a cnk of 1 0 1 1 for the two mica granites and 1 2 1 3 for the garnet bearing granites all the garnet bearing granites and some of the two mica granites show tetrad ree patterns tetrad group whereas most two mica granites show normal granitic ree patterns normal group the normal group granites exhibit depletion in nb ta p and ti in spidergrams and generally weak positive eu anomalies in ree patterns by contrast the tetrad group granites manifest depletion in ba nb ta sr p and ti and significant negative eu anomalies the trace element data constrain the parental magmas to having undergone extensive magmatic differentiation during their late stage magmatic evolution intense interaction of residual melts with aqueous hydrothermal fluids resulted in the non charac charge and radius controlled trace element behavior and the tetrad effect in ree distribution patterns this in turn leads to the invalidation of the commonly used tectonic discrimination criteria derived from trace element abundances of normal granites in view of this and previous studies we conclude that there were probably no s type granites produced in ne china during the phanerozoic consequently weathered sedimentary material did not play an important role in the genesis of the strongly peraluminous granites in the zhangguangcai range

geochemical and sr nd pb isotopic data bearing on the origin of pan africangranitoids in the kid area southeast sinai egypt the kid plutonic suite in southeast sinai egypt is composed of 1 a gabbro diorite complex 2 quartz monzodiorite 3 granodiorite monzogranite association 4 syenogranite major and traceelement analyses and radiogenic isotope data rb sr sm nd and pb pb are presented for these rocks rb sr isochrons indicate ages of 581 11 ma 576 11 ma and 570 4 ma for the quartz monzodiorite granodiorite monzogranite association and syenogranite respectively the gabbro diorite complex is a layered intrusion varying in sio2 from 42 to 59 wt it shows a calc alkaline trend and the trace element characteristics of arc lavas subsequently this magma fractionated to give quartz monzodiorite andsyenogranite magmas the quartz monzodiorite has low initial 87 sr 86 sr ratios 0 703 and high nd values 4 4 0 3 with trace element characteristics of volcanic arc granites the granodiorite monzograniteassociation has higher initial sr ratio 0 7044 lower nd 1 5 2 7 and relatively high 207 pb 204 pb and 208 pb 204 pb significantly different from those of the older rocks isotopic data and ree modelling suggestthat it formed by partial melting of an early pan african amphibolitic lower crust the syenogranite isa typical highly fractionated post orogenic granite with a deep negative eu anomaly hfse and reeenrichment a 87 sr 86 sr initial ratio of 0 7033 nd of 4 5 0 5 and distinct variations in the 206 pb 204 pbratio the isotopic compositions of sr nd and pb indicate that the syenogranite and the quartzmonzodiorite are comagmatic increasing fluorine concentration from the quartz monzodiorite to thesyenogranite suggests that fluid fractionation involving fluorine complexing played an important role inthe genesis of the syenogranite this process led to the enrichment of hfse and hree to extensivefractional crystallization to changes in the u pb ratios and a distinct variation in 206 pb 204 pb

origin of topaz bearing and related peraluminous granites of the late devonian davis lake pluton nova scotia canada crystal versus fluid fractionation abstract the davis lake pluton in the late devonian south mountain batholith in southwestern nova scotia canada is composed of peraluminous leucomonzogranites and subordinate topaz bearing leucogranites that host greisens and a primary tin deposit the concentrations of incompatible trace elements e g rb li u f ta and sn and element e g k rb nb ta and la sm and isotopic ratios e g 206 pb 204 pb and 87 sr 86 sr display a spatial zonation the granitic rocks were generated by extensive fractional crystallization of major phases feldspars and biotite and accessory minerals monazite apatite and zircon in a compositionally zoned magma chamber fractional crystallization of the evolving f rich peraluminous granitic magma was accompanied particularly in later stages by fluid fractionation involving fluorine complexing fluid fractionation was a major differentiation process during the terminal stages of the pluton s evolution it played an important role in the genesis of the evolved rocks which are characterized by low k rb 24 150 nb ta 1 6 la sm 1 5 4 and eu eu 0 01 0 3 ratios these ratios as well as the enhanced concentrations of li rb u ta and sn in these rocks correlate with f fluorine rich fluids modified the rb sr and u pb ratios which produced the distinct variations of 87 sr 86 sr and 206 pb 204 pb isotopic ratios the parent magma was formed by dehydration melting of lower crustal metasedimentary rocks the melting was related to crustal thickening in association with terrane and collision along the eastern margin of the appalachians

an inverted continental moho and serpentinization of the forearc mantle volatiles that are transported by subducting lithospheric plates to depths greater than 100 km are thought to induce partial melting in the overlying mantle wedge resulting in arc magmatism and the addition of significant quantities of material to the overlying lithosphere asthenospheric flow and upwelling within the wedge produce increased lithospheric temperatures in this back arc region but the forearc mantle in the corner of the wedge is thought to be significantly cooler here we explore the structure of the mantle wedge in the southern cascadia subduction zone using scattered teleseismic waves recorded on a dense portable array of broadband seismometers we find very low shear wave velocities in the cold forearc mantle indicated by the exceptional occurrence of an inverted continental moho which reverts to normal polarity seaward of the cascade arc this observation provides compelling evidence for a highly hydrated and serpentinized forearc region consistent with thermal and petrological models of the forearc mantle wedge this serpentinized material is thought to have low strength and may therefore control the down dip rupture limit of great thrust earthquakes as well as the nature of large scale flow in the mantle wedge

seismological evidence for the existence of serpentinized wedge mantle serpentinized peridotite is detected seismologically by mapping poisson s ratio beneath the kanto district central japan because serpentinite has a higher poisson s ratio than other rocks in the crust and upper mantle we determine three dimensional p and s wave velocity models using travel time tomography and then construct a three dimensional map of poisson s ratio the models show low velocity anomalies near the upper boundary of the slab in a depth range from 20 to 70 km in the low velocity region there is an area with poisson s ratio greater than 0 3 at depths of 20 45 km which we attribute to serpentinized peridotite the rest of the low velocity region is interpreted as descending oceanic crust with a poisson s ratio of about 0 25 the high poisson s ratio area is associated with low seismicity and weak interplate coupling which is consistent with the ductility of serpentinite

petrogenesis of the natkusiak continental basalts victoria island northwest territories canada the natkusiak flows 1100 m thick sills and dykes of victoria island northwest territories are part of an extensive late hadrynian magmatic event in the northern canadian shield the flows are subaerially erupted plateau basalts and together with associated doleritic sills and dykes display many geochemical features typical of continental tholeiites in addition to fractional crystallization the rocks were affected by crustal contamination but to a smaller degree than usually encountered in continental tholeiites elsewhere the basalts were derived from a depleted upper mantle source very similar to that of n type mid ocean ridge basalts although it is suggested that the melting residue contained small amounts of garnet 2

large scale hydration of the lithosphere above subducting slabs during subduction oceanic lithosphere capped by variably hydrated uppermost mantle oceanic crust and sediments undergoes progressive metamorphism and devolatilization the petrologic evolution of subducting oceanic lithosphere and the overriding continental oceanic lithosphere can be predicted by integrating thermal models with petrologic phase equilibria in mature subduction zones that have achieved thermal steady state the subducting slab lies at subsolidus conditions for depths 100 150 km most subducting oceanic crust passes through the blueschist to eclogite facies transition where large amounts of h2o are released by the breakdown of na amphibole lawsonite or clinozoisite and chlorite the large amounts of h2o released by continuous blueschist eclogite reactions can cause large scale hydration of the overriding lithosphere and can trigger partial melting of the mantle wedge lowermost lithosphere and asthenosphere upward migration of slab derived fluids add h2o co2 sio2 and incompatible elements to the overlying lithospheric plate that result in the formation of new phases e g serpentine amphibole and mica and chemical modification of existing phases e g trace element enrichment of clinopyroxene evidence for large scale lithospheric hydration above subducting slabs includes serpentinite diapirs observed in the marianas forearc low p wave velocities observed in the hanging wall of the alaskan and japanese subduction zones and studies of high pressure metamorphic terrains

petrology and geochemistry of paleoproterozoic a type granite at kanigiri in the nellore khammam schist belt andhra pradesh india abstract the kanigiri pluton 6 2 2 1 km 2 995 ma occurs close to a major fault and is intrusive into the meta volcanics in the western margin of the nellore khammam schist belt in andhra pradesh india the dominantly metaluminous pluton displays geochemical characteristics of a type granites including high abundances of nb zr y ta and ree except eu and low concentrations of sc ba sr and eu displaying distinct variation trends with increasing silica content it has less fractionated ree patterns and large negative eu anomalies the cluster analyses of major and trace elements indicate three main associations a a primary magmatic association characterized by the presence of la ce fe ti mn k ba and sr b ore related association consisting of zr hf nb y and c a volatile association with rb ta li and cs systematic variation in sr rb sr rb ba ratios along with and low k rb ratios reflects fractionation of feldspars variations in sc and ta result from fractionation of ferromagnesian silicates removal of zircon and allanite affected the hf ta and ce yb ratios limited variation in source sensitive y nb and yb ta ratios is consistent with the results of melting experiments and indicates that metaluminous granitoids of this pluton were probably derived through melting of lower crustal sources geochemical field and petrological data for the kanigiri granite suggest that the granite is a partial melting product of mafic rocks like gabbros that are also present in the study region

mantle wedge water contents estimated from seismic velocities in partially serpentinized peridotites 1 water carried into subduction zones with the down going plate and subsequently released by dehydration reactions at depth affects the composition of the mantle wedge triggers partial melting and affects subduction zone seismicity partially serpentinized peridotite may be a significant reservoir for water in the subducted plate the mantle wedge and the overriding plate here we develop a model that relates the degree of serpentinization and water content of partially serpentinized peridotites to their seismic p wave velocities in partially serpentinized ultramafic rocks a 1 decrease in p wave velocity corresponds to a 2 4 increase in serpentine content and a 0 3 0 18 moles m3 increase in h2o content up to a maximum of 13 where there is evidence of serpentinization mantle serpentine content is typically 15 corresponding to 4 5 wt h2o 6 10 moles m3