the origin of the dead sea rift abstract the dead sea rift is considered to be a plate boundary of the transform type several key questions regarding its structure and evolution are does sea floor spreading activity propagate from the red sea into the dead sea rift did rifting activity start simultaneously along the entire length of the dead sea rift or did it propagate from several centres why did the initial propagation of the red sea into the gulf of suez stop and an opening of the gulf of elat start using crustal structure data from north africa and the eastern mediterranean and approximating the deformation of the lithosphere by a deformation of a multilayer thin sheet that overlies an inviscid half space the regional stress field in this region was calculated using this approach it is possible to take into account variations of lithospheric thickness and the transition from a continental to an oceanic crust by application of a strain dependent visco elastic model of a solid with damage it is possible to describe the process of creation and evolution of narrow zones of strain rate localization corresponding to the high value of the damage parameter i e fault zones mathematical simulation of the plate motion and faulting process suggests that the dead sea rift was created as a result of a simultaneous propagation of two different transforms one propagated from the red sea through the gulf of elat to the north the other transform started at the collision zone in turkey and propagated to the south

zoisite and clinozoisite segregations in metabasites tauern window austria as evidence for high pressure fluid rock interaction high pressure zoisite and clinozoisite bearing segregations are common in garnet and albite bearing amphibolites of the palaeozoic part of the lower schieferhulle south central tauern window austria the zoisite segregations primary assemblage zo qtz cal formed during an early to pre hercynian high pressure event p 0 6 gpa t 500 550 c by hydrofracturing as a result of protolith dehydration zoisite is growth zoned from fe3 poor cores al 2 fe 9 mol to fe3 rich rims 17 mol and has high sr pb and ga contents and lree enriched ree patterns controlling the trace element budget of the segregations hercynian deformation at c 0 7 gpa 600 c kinked and cracked primary zoisite and enhanced breakdown into secondary zoisite 13 mol al 2 fe clinozoisite 40 55 mol al 2 fe albite an 20 calcite and white mica during an eoalpine high pressure event at 0 9 1 2 gpa 400 500 c the clinozoisite segregations primary assemblage czo qtz omp ttn chl cal are mm to cm wide vein like bodies cross cutting fabric elements of the host garnet amphibolite they formed during the eoalpine high pressure event at 0 9 1 2 gpa 400 500 c during alpine exhumation omphacite was pseudomorphed by amphibole albite quartz and clinozoisite oxygen isotope data suggest equilibrium between host metabasite and zoisite segregations and indicate an internal fluid source and fluid buvering by the protolith mobility of p nb and lree changed the protolith s trace element composition in the vicinity of the zoisite segregations mobilization of lree is evidenced by decreasing modal amounts of lree rich epidote and decreasing lree contents in lree rich epidote towards the segregations chang ing the ree patterns of the host metabasite from lree enriched to lree depleted tectonic discrimi nation diagrams based on the trace element content of metabasites should be applied with extreme caution

high pressure 2000 mpa kyanite and glaucophane bearing pelitic schist and eclogite from cordillera de la costa belt venezuela in the cretaceous melange of the cordillera de la costa belt introduction north central venezuela there are knockers of eclogite barroisitetwo belts of high pressure hp low temperature lt bearing eclogite and pelitic glaucophane schist these occur in a metamorphic rocks are exposed in the caribbean mounmetamorphic melange matrix that locally consists of marble sertain system of northern venezuela which is part of the pentinite amphibolite actinolite schist feldspathic schist and gneiss complex east west trending boundary zone between the graphitic schist chloritoid schist and garnet bearing mica schist caribbean and south american plates e g menendez the protoliths for these various rock types exhibit a wide age range 1966 1967 these two belts are the cordillera de la cambrian to early cretaceous recently discovered knockers of costa and villa de cura belts both of which were pelitic glaucophane schist contain mg glaucophane paragonite metamorphosed during the mid cretaceous presumably kyanite garnet talc graphite rutile quartz in a subduction zone related to the mid cretaceous the coexistence of kyanite and mg glaucophane suggests minimum leeward antilles volcanic arc e g pindell 1993 from p 2000 mpa at t 600 c eclogite knockers from the same jurassic to eocene time north and south america were outcrop contain garnet and clinopyroxene which yield 500 c for diverging and new proto caribbean sea floor was created cores 700 c for rims and p 1200 mpa the assemblage between them e g pindell 1993 therefore the leegarnet biotite phengite albite within schists of the melange matrix ward antilles arc as well as the entire caribbean mounof this locality indicates metamorphic conditions of t tain system are allochthonous and formed far to the 450 520 c at p 1800 mpa because all lithologies in this west as part of the farallon north south american plate outcrop record high p conditions this metamorphic melange formed boundary zone e g pindell 1993 before or during peak metamorphism in a mid cretaceous subduction although the two hp lt belts have similar metazone morphic ages they differ greatly in lithology and metamorphic history the cordillera de la costa belt consists of oceanic and passive continental margin rocks intermixed with paleozoic granites and granitic gneisses in this belt the eclogites and blueschists were retrograded to epidote amphiboliteand greenschist facies assemblages following an apparent p t path typical for

geochemical and geochronological constraints on the tectonic setting of the xinlin ophiolite northern great xing an range ne china abstract the xinlin ophiolite located in the great xing an range domain of the central asian orogenic belt caob is considered to represent a fragment of an oceanic plate the ophiolite consists of several discontinuous blocks composed dominantly of serpentinized ultramafic rocks with subordinate cumulate gabbros basalts and diabase dikes which are intruded by younger leucogranite dikes here we characterize the geochemical features and formation age of the ophiolite the ultramafic rocks are dominated by harzburgite with minor dunite and are enriched in large ion lithophile elements liles and light rare earth elements lrees consistent with remnants of oceanic mantle the mafic rocks show depletion in some high field strength elements hfses and have relatively low tio2 and mgo contents and likely originated from an e morb like source in a back arc setting this suggests that the ophiolite is of supra subduction zone ssz origin the cross cutting leucogranite dikes most of which are trondhjemitic have high al2o3 contents low k2o na2o ratios and negative zircon ehf t values 8 5 to 3 8 average 5 8 which are features distinct to other granites e g fractionation shear and subduction types within the ophiolite and to coeval granitoids in the northern great xing an range zircon u pb dating on samples of gabbro and a leucogranite dike yielded ages of ca 480 and 324 1 ma respectively suggesting that the ophiolite formed in the early ordovician and was emplaced at ca 320 ma together with the ages of ca 480 ma for the duobaoshan arc and ca 340 ma for subduction related high mg diorites we suggest that ophiolite formation was probably related to the opening of an ordovician back arc basin whereas formation of the younger leucogranite dikes was probably related to closure of the back arc basin

tectonic and metasomatic mixing in a high t subduction zone melange insights into the geochemical evolution of the slab mantle interface abstract the catalina schist california contains an amphibolite grade 0 8 1 1 gpa 640 750 c melange unit consisting of mafic and ultramafic blocks in high mg schistose melange matrix with varying modal proportions of talc chlorite anthophyllite calcic amphibole enstatite and minor phases including zircon rutile apatite spinel and fe ni sulfides this melange unit is interpreted as a kilometer scale zone of tectonic and metasomatic mixing formed within a juvenile subduction zone the study of which may yield insight into chemical mixing processes at greater depths in subduction zones relationships among the major and trace element compositions of the mafic and ultramafic blocks in the melange the rinds developed at the margins of these blocks and the surrounding melange matrix are compatible with the evolution of the melange matrix through a complex combination of infiltrative and diffusional metasomatism and a process resembling mechanical mixing simple linear mixing models are compatible with the development of the melange matrix primarily through simple mixture of the ultramafic and mafic rocks with cr al ratios serving as indicators of the approximate proportions of the two lithologies this conclusion regarding mafic ultramafic mixing is consistent with the field observations and chemical trends indicating strong resemblance of large parts of the melange matrix with rinds developed at the margins of mafic and ultramafic blocks the overall process involved development of metasomatic assemblages through complex fluid mediated mixing of the blocks and matrix concurrent with deformation of these relatively weak rind materials which are rich in layer silicates and amphibole this deformation was sufficiently intense to transpose fabrics progressively disaggregate more rigid block derived materials in weaker chorite and talc rich melange and in some particularly weak lithologies e g chlorite talc and amphibole rich materials intimately juxtapose adjacent lithologies at the sub cm scale approaching grain scale sampled by the whole rock geochemical analyses chemical systematics of various elements in the melange matrix can be delineated based on the cr al based mixing model simple mixing relationships exhibited by al cr mg ni fe and zr provide a geochemical reference frame for considerations of mass and volume loss and gain within the melange matrix the compositional patterns of many other elements are explained by either redistribution local stripping or enrichment at varying scales within the melange ca na k ba and sr or massive addition from external sources si and h2o the latter probably in infiltrating h2o rich fluids that produced the dramatic o and h isotopic shifts in the melange melange formation resulting in the production of high variance ultramafic assemblages with high volatile contents may aid retention of volatiles in this case h2o to greater depths in subduction zones than in original subducted mafic and sedimentary materials the presence of such assemblages i e containing minerals such as talc chlorite and mg rich amphiboles would impact the rheology of the slab mantle interface and perhaps contribute to the low velocity seismic structure observed at near the slab mantle interface in some subduction zones if operative along the slab mantle interface complex mixing processes such as these involving the interplay between fluid mediated metasomatism and deformation also could impact slab incompatible trace element and isotopic signatures ultimately observed in arc magmas producing fluids with geochemical signatures inherited from interactions with hybridized rock compositions

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

enrichment of trace elements in garnet amphibolites from a paleo subduction zone catalina schist southern california the abundance p t stability solubility and element partitioning behavior of minerals such as rutile garnet sphene apatite zircon zoisite and allanite are critical variables in models for mass transfer from the slab to the mantle wedge in deep regions of subduction zones the influence of these minerals on the composition of subduction related magmas has been inferred and disputed from inverse modelling of the geochemistry of island arc basalt or by experiment although direct samples of the dehydration partial melting region of a mature subduction zone have not been reported from subduction complexes garnet amphibolites from melanges of circumpacific and caribbean blueschist terranes reflect high t 600 c conditions in shallower regions such rocks record geochemical processes that affected deep seated high t portions of paleo subduction zones in the catalina schist a subduction zone metamorphic terrane of southern california metasomatized and migmatitic garnet amphibolites occur as blocks in a matrix of meta ultramafic rocks this mafic and ultramafic complex may represent either slab derived material accreted to the mantle wedge of a nascent subduction zone or a portion of a shear zone closely related to the slab mantle wedge contact or both the trace element geochemistry of the complex and the distribution of trace elements among the minerals of garnet amphibolites were studied by inaa xrf electron microprobe and sem in order of increasing alteration from a probable metabasalt protolith three common types of garnet amphibolite blocks in the catalina schist are 1 non migmatitic clinopyroxene bearing blocks which are compositionally similar to morb that has lost an albite component 2 garnet amphibolite blocks which have rinds that reflect local interaction between metabasite metaperidotite and fluid and 3 migmatites that are extremely enriched in th hfse lree and other trace elements these trace element enrichments are mineralogically controlled by rutile garnet sphene apatite zircon zoisite and allanite alkali and alkaline earth elements are much less enriched in the solid assemblage and thus appear to be decoupled from the other elements in the inferred metasomatic process es the compositions of migmatitic garnet amphibolite blocks seem to complement that of average island arc tholeiite trace element metasomatism reflects fluid solid rather than melt solid interaction the metasomatic effects indicate that h2o rich fluid perhaps with a significant component of na al silicate and alkalis carried th u sr ree and hfse fractionations of lree in migmatites resemble those of migmatitic metasedimentary rocks underlying the mafic and ultramafic complex exotic lree deposited in allanite in migmatites could have been derived from fluids in equilibrium with subducted sediment if the paleo subduction zone represented by the mafic and ultramafic complex of the catalina schist had continued its thermal and fluid evolution a selvage of similarly enriched rocks might have been generated along the slab mantle wedge contact between 30 and 85 km depth rocks affected by subduction zone metasomatism although rarely recognized at the surface could be volumetrically significant products of the initiation of subduction and may prove to be geochemical probes of convergent margins that approach the significance of xenoliths in the study of other magmatic environments

thermal evolution of convergent plate margins thermal modeling and reevaluation of isotopic ar ages for blueschists in the franciscan complex of california modeling of heat flow through a wedge shaped accretionary complex emplaced beneath the overriding plate at a convergent plate margin provides insight into the tectonic significance of ar isotopic ages and metamorphic progressions across blueschist belts if convergence rates are maintained at rates of a few centimeters per year or more material recrystallized at depth where the accretionary wedge is narrow can be rapidly heated to high temperatures and then rapidly cooled to the low temperatures where ar loss by diffusion and recrystallization effectively ceases maximum temperatures are lower and the periods of heating and cooling are longer at shallower levels where the wedge is wider because of this difference in thermal history k ar and 40ar 39ar ages from blueschists recrystallized in the upper part of the accretionary wedge can be younger by several tens of million years than ages from material initially recrystallized at the same time but at greater depth application of this thermal modeling to the franciscan subduction complex of northern california indicates that the coherent blueschists in the eastern belt typically with 115 to 125 m y ar isotopic ages and the high grade blueschist blocks in the mud matrix melanges of the western portion of the central belt typically with 140 to 150 m y ar isotopic ages could both have formed during a single prolonged period of continuous convergence

the effects of sub blocking temperature metamorphism on the k ar systematics of hornblendes 40ar 39ar dating of polymetamorphic garnet amphibolite from the franciscan complex california the effects of sub blocking temperature metamorphism on the k ar system in hornblende as revealed by 40ar 39ar release spectra have been studied in a polymetamorphic knocker from the franciscan complex fc california a primary amphibolite assemblage of horn blende rutile epidote apatite garnet sphene is variably overprinted by a blueschist facies assemblage of blue amphibole lawsonite chlorite white mica pumpellyite sphene the secondary assemblage formed at a temperature of 370 c below that at which rapid ar diffusion is expected in hornblende hornblendes from three variably altered samples of garnet amphibolite yield total gas ages of 147 to 161 ma but the corresponding plateau ages of 163 0 2 8 160 6 2 2 and 161 8 2 2 ma are identical within error hornblende separates with lower total gas ages come from more highly overprinted rocks have excess k compared to that expected on the basis of electron probe analyses and exhibit anomalously high k ca ratios in the low temperature fractions of their incremental heating spectra the reduced total gas ages result from the presence of thin 2 m sheets of younger white mica in hornblendes from the moderately and highly altered amphibolites the secondary micas are difficult to detect because of their small size and low abundance 2 but because their k content is 50 to 100 times that of the host hornblende they contribute significantly to the k and ar budgets of the sample the mica intergrowths are not removed by normal sample preparation but because the mica inclusions degas at lower temperatures than hornblende during vacuum extraction incremental heating analyses can provide precise cooling ages for the hornblendes as well as useful estimates of the age of the mica inclusions the hornblende separate from the most altered sample contained 20 10 younger blue amphibole replacing hornblende but its plateau age was not significantly affected this is consistent with a replacement process in which k and ar loss from the hornblende are coupled leaving the k ar system undisturbed in relict primary grains the k and ar budgets of the sample are not strongly affected by the blue amphibole because of its very low k content because partial replacement of primary amphibole by high k phyllosilicates occurs in many geological environments effects like those described here could be widespread

allanite and other ree rich epidote group minerals epidote group minerals rich in rare earth elements ree in particular allanite are common accessory phases in igneous metamorphic metasomatic and sedimentary rocks small amounts of ree are present in most epidote group minerals but in allanite and the related minerals dissakisite ferriallanite dollaseite khristovite and androsite the ree are essential structural constituents an important characteristic of ree rich epidote group minerals is that their octahedrally coordinated m sites contain major amounts of divalent cations this paper summarizes literature data for these minerals and discusses their chemistry occurrence phase relations and petrologic and geologic significance the chapter emphasizes allanite because it is the most common and best studied of the ree rich epidote group minerals epidote group minerals contain isolated silicon tetrahedra and corner sharing groups of two tetrahedra and are thus assigned to the disilicate or sorosilicate structural family for a detailed description of the structure see franz and liebscher 2004 the epidote group structural formula is a2m3 sio4 si2o7 o f oh or in a simplified form a2m3si3o11 o f oh in which a ca sr pb2 mn2 th ree3 and u and m al fe3 fe2 mn3 mn2 mg cr3 and v3 deer et al 1986 there are two structurally different a sites a 1 and a 2 with different coordination numbers and there are three different m sites m 1 m 2 and m 3 which are all octahedrally coordinated ueda 1955 dollase 1971 in epidote group minerals trivalent ree are accommodated in the a sites which in endmember epidote both contain ca the incorporation of ree3 is commonly charge balanced by a divalent cation fe2 mn2 mg substituted for a trivalent one in the m sites table 1

petrology of amphibolite facies mafic and ultramafic rocks from the catalina schist southern california metasomatism and migmatization in a subduction zone metamorphic setting the catalina schist of southern california is a subduction zone metamorphic terrane it consists of three tectonic units of amphibolite high p greenschist and blueschist facies rocks that are structurally juxtaposed across faults forming an apparent inverted metamorphic gradient migmatitic and non migmatitic metabasite blocks surrounded by a meta ultramafic matrix comprise the upper part of the catalina amphibolite unit fluid rock interaction at high p high t conditions caused partial melting of migmatitic blocks metasomatic exchange between metabasite blocks and ultramafic rocks infiltration of silica into ultramafic rocks and loss of an albitic component from nonmigmatitic clinopyroxene bearing metabasite blocks partial melting took place at an estimated p 8 11 kbar and t 640 750 c at high h2o activity the melting reaction probably involved plagioclase quartz trondhjemitic melts were produced and are preserved as leucocratic regions in migmatitic blocks and as pegmatitic dikes that cut ultramafic rocks the metasomatic and melting processes reflected in these rocks could be analogous to those proposed for fluid and melt transfer of components from a subducting slab to the mantle wedge aqueous fluids rather than melts seem to have accomplished the bulk of mass transfer within the mafic and ultramafic complex

a rheological model of a fractured solid abstract experiments to study the behavior of various materials point to the relation that exists between elastic properties and the type of stress the influence of the state of stress on the elasticity of a fractured material will be discussed for a physically non linear model of an elastic solid the strain dependent moduli model of material presented in this paper makes it possible to describe this feature of a solid it also permits to simulate a dilatancy of rocks a damage parameter introduced into the model using a thermodynamical approach allows to describe a rheological transition from the ductile regime to the brittle one and to simulate the rock s memory narrow fracture zone creation and strain rate localization additionally the model enables the investigation of the final geometry of fracture zones and also to simulate their creation process taking into account pre existing fracture zones the process of narrow fracture zone creation and strain rate localization was simulated numerically for single axis compression and shear flow

routing of western canadian plains runoff during the 8 2 ka cold event 1 the collapse of the laurentide ice sheet over hudson bay 8 47 ka allowed the rapid drainage of glacial lake agassiz into the labrador sea an event identified as causing a reduction in atlantic meridional overturning circulation amoc and the 8 2 ka cold event atmosphere ocean models simulations based on this forcing however fail to reproduce several characteristics of this event particularly its duration here we use planktonic foraminifera u ca records to document the routing of western canadian plains runoff that accompanied ice sheet collapse geochemical modeling of the 7 nmol mol increase in u ca at the opening of hudson bay indicates an increase in freshwater discharge of 0 13 0 03 sverdrups 106 m3 s 1 from routing a sufficient magnitude to cause an amoc reduction we suggest that this routing event suppressed amoc strength for several centuries after the drainage of lake agassiz explaining multi centennial climate anomalies associated with the 8 2 ka cold event

thermal effects of metamorphic fluids in subduction zones in subduction zones large volumes of fluid are released from the subducting slab during prograde metamorphism of hydrated oceanic crust once released metamorphic fluids tend to migrate upward owing to buoyancy fluid migration can affect the thermal structure of a subduction zone by 1 advecting heat and 2 causing retrograde hydration carbonation reactions in the hanging wall under most p t conditions the volumetric heat capacities of h 2 o and rocks are similar therefore the thermal effects of fluid migration and uplift are comparable calculations suggest that advective heat transport by fluid flow causes relatively minor thermal effects pervasive upward fluid flow results in thermal effects analogous to regional uplift rates on the order of 0 2 mm yr fluid flow channelized along the subduction shear zone translates the local thermal structure of the shear zone updip at 1 mm yr a more important thermal effect results from exothermic retrograde reactions in the hanging wall that release 30 kj mol of h 2 o consumed retrograde reactions significantly retard cooling of the hanging wall just as prograde reactions retard warming of the subducting slab

the genesis of early carboniferous adakitic rocks at the southern margin of the alxa block north china abstract adakite is a highly debated petrologic term that was originally proposed to describe igneous rocks formed by slab melting subsequent studies reported other ways to generate adakitic signatures such as the melting of mafic lower continental crust and fractional crystallization of basaltic magma we studied adakitic rocks from the taohuala mountain at the southern margin of the alxa block north china these rocks are characterized by high sr concentrations 300 450 ppm high sr y 20 75 ppm and la yb n 25 67 ppm ratios and low y

rare earth element mobility in the roffna gneiss switzerland the roffna gneiss a deformed hercynian granite porphyry within the penninic nappes of eastern switzerland underwent extreme cataclasis with the progressive development of phengite towards the margins of the nappe under conditions of the glaucophane schist to greenschist facies this resulted in the selective mobilization of major and trace elements over distances of 10 s to 100 s of meters and the resetting of the rb sr whole rock isotopic systems some 100 my ago the component ratios and compositionvolume relationships of progressively deformed gneiss samples studied here suggest that this process was essentially isovolumetric the mineralogy of the deformation sequence appears to have been controlled by a reaction involving the breakdown of microcline albite and biotite and the formation of phengite and quartz the fluids introduced mg and h2o promoting the development of phengite and removed the na being released by the breakdown of albite the fluids were most probably derived from the surrounding triassic carbonates and quartzites these relatively high fo2 and carbonate rich fluids also introduced rare earth elements ree into the gneiss the gneiss was progressively enriched in eu up to 60 y up to 40 and yb up to 100 these enrichments are associated with the development of epitaxial xenotime around zircon in the most phengite rich sample while the ree were mobile uranium and thorium were essentially immobile the formation of xenotime was suggested to explain the observed heavy ree enrichment when large differences in the ree contents were found for replicate analyses using hf and then lithium metaborate for dissolution these differences arose because xenotime like monazite can be difficult if not impossible to dissolve in hydrofluoric acid due to the possibility of incomplete sample dissolution we now recommend fusion with lithium metaborate for all ree lu hf or sm nd studies

amphibole compositions and metamorphic history of the rand schist and the greenschist unit of the catalina schist southern california the catalina schist and rand schist are two high p t terranes in southern california the catalina schist is correlated with the franciscan complex and occurs in the continental borderland it consists of a blueschist facies melange tectonically overlain by a greenschist unit which in turn is overthrust by an amphibolite unit the greenschist unit itself is inversely zoned from epidote amphibolite fades at the top through greenschist facies in the center to transitional blueschist greenschist facies at the base the rand schist is part of the eugeoclinal pelona orocopia schist terrane which lies interior to the present continental margin structurally beneath precambrian to mesozoic sialic basement the rand schist is inversely zoned from epidote amphibolite facies to transitional blueschist greenschist facies similar to the greenschist unit of the catalina schist

mineralogy geochemistry and genesis of the ferromanganese ores from hazara area nw himalayas northern pakistan abstract ferromanganese ores occur in hazara area near abbottabad within the hazira formation of the kalachitta margala thrust belt of the nw himalayas of the indo pakistan plate in pakistan the hazira formation of cambrian age is a relatively thin unit up to 150 m thick which consists of either thin ferruginous siltstone with variable amounts of clay shale ferromanganese ores phosphorite and barite or thicker evenly bedded reddish brown siltstone it has a conformable lower contact with the abbottabad formation cambrian and an unconformable upper contact with the samana suk limestone jurassic the ferromanganese ores of the hazara area were studied in terms of their mineral and geochemical composition mineralogically a variety of mn and fe phases have been noticed in kakul galdanian and chura gali ferromanganese ores bixbyite is the principal mn phase while pyrolusite hollandite partridgeite and braunite occur in lesser amounts hematite however is the only fe phase in these ores gangue minerals include carbonates cryptocrystalline quartz iron clay and variable amounts of apatite plumbogummite and barite glauconite occurs in traces textural features suggest remobilization and subsequent precipitation of early diagenetic mn and fe phases along the fractures and interstices of the gangue minerals during later metamorphism chemical analyses show that the mn fe ratio in the ores is highly variable and ranges from 0 46 to 5 25 these ores exhibit a line of descent from lree to hree with a small positive ce anomaly their ree is higher than the hydrothermal mn deposits and lower than the hydrogenous crust the petrochemical characteristics suggest a mixed hydrothermal hydrogenetic source for the ferromanganese ores which have formed in shallow water or continental shelf environment due to the upwelling of anoxic deep seated water

sr bearing zoisite and epidote in ultra high pressure uhp metamorphic rocks from the su lu province eastern china an important sr reservoir under uhp conditions sr bearing zoisite and epidote are common constituents of eclogites and associated paraschists throughout the su lu ultra high pressure uhp province eastern china the sro content of prograde zoisite and epidote reaches 3 2 wt in crystal cores and generally decreases toward crystal margins retrograde epidote is poorer in sro 0 1 wt preliminary rare earth element ree analyses of epidote give la2o3 up to 2 9 wt ce2o3 5 9 wt and nd2o3 3 0 wt ree contents of zoisite are distinctly lower la 2o3 up to 0 16 wt ce2o3 up to 0 26 wt and nd2o3 up to 0 16 wt than coexisting epidote apatite is always more depleted in sro 0 10 0 59 wt on average than coexisting zoisite and epidote and sr ca partition coefficients for zoisite and epidote and apatite sr ca zo ep ap range from 5 to 20 sro content of k white mica 0 012 0 044 wt is an order of magnitude lower than that of apatite an evaluation of the sro content in zoisite and epidote and their modal abundances in seven samples indicates that 70 of the whole rock sro is contained in these minerals apatite and k white mica are only minor reservoirs for sro in these rocks zoisite and epidote are thus regarded as the most important sr reservoirs at uhp conditions where calcic plagioclase and titanite are unstable

petrologic structural and age relations of serpentinite amphibolite and blueschist in the shuksan suite of the iron mountain gee point area north cascades washington part of the shuksan blueschist terrane near iron mountain and gee point north cascades washington has associated serpentinite amphibolite barroisite schist blueschist rare eclogite and blackwall type metasomatic rock field petrographic and microprobe observations indicate that the amphibolite and barroisite schists were metamorphosed in contact with peridotite and suggest that the peridotite may have been a heat source the serpentinite and associated rocks are structurally concordant with the regional blueschists and have been overprinted by blueschist metamorphism isotopic dating gives metamorphic ages of 148 5 to 164 6 m y for the amphibolite and barroisite schist and 129 5 m y for nearby regional shuksan blueschists the origin of the serpentinite amphibolite blueschist assemblage is interpreted to be the result of sequential events in a subduction zone as subduction began oceanic crustal materials underwent high temperature metamorphism along the hot ultramafic hanging wall and were converted to amphibolites materials that were subducted later came in contact with a cooler hanging wall and recrystallized as blueschist this hypothesis may be applicable to the origin of similar rock associations in the franciscan terrane and other orogenic belts

creation and preservation of subduction related inverted metamorphic gradients peak metamorphic temperatures and recrystallization increase structurally upward in the lower plate of several paleosubduction zones in the westernmost u s cordillera inverted metamorphic gradients are preserved in the pelona orocopia schist southern california the catalina schist california borderland the central metamorphic belt klamath province the south fork mountain schist northern california coast ranges and possibly the shuksan suite northern washington with the exception of the south fork mountain schist peak metamorphic temperatures increase structurally upward from 400 c to 650 c over 1 2 km indicating metamorphic gradients in excess of 100 c km 1 estimates of metamorphic gradients in some terranes such as the catalina schist depend critically on the effects of postmetamorphic faulting the tectonic settings and high metamorphic pressures 800 mpa suggest that the inverted metamorphic gradients formed in paleosubduction zones four out of five inverted metamorphic gradients lie structurally beneath ultramafic hanging walls whereas the pelona orocopia schist formed beneath granitic crystalline rocks the high temperatures recorded by the metamorphic rocks strongly suggest that inverted metamorphic gradients form during the early stages of subduction and thus provide insight into the early thermal history of subduction zones two separate two dimensional numerical models were constructed in order to constrain the thermal conditions under which inverted metamorphic gradients are created and preserved rapid subduction 10 cm yr 1 beneath young oceanic lithosphere 10 ma results in rapid heat conduction downward from the hanging wall and the creation of inverted thermal gradients in excess of 100 c km 1 in the top portion of the downgoing slab in order to be preserved in the geologic record the inverted metamorphic gradient must be accreted to the base of the hanging wall accretion of the upper portion of the subducting slab may occur because of declining temperatures that cause downward migration of the viscous slip zone and or greenschist facies devolatilization reactions that weaken the descending slab alternatively inverted metamorphic gradients may represent the continuous accretion of material at rates of 1 10 mm yr 1 under conditions of declining temperature continuously accreted inverted metamorphic gradients could form in oceanic lithosphere as old as 30 ma continued subduction of 1000 km of oceanic lithosphere effectively refrigerates the early accreted inverted metamorphic gradient synsubduction uplift and or moderately fast postsubduction uplift rates of 1 mm yr 1 are required to preserve the inverted metamorphic gradient the models predict that 1 the high grade metamorphic rocks in inverted metamorphic gradients form early in the subduction process and therefore effectively date the initiation of subduction and 2 inverted metamorphic gradients mark sites of major plate convergence in excess of 1000 km

significance of slab derived partial melts and aqueous fluids for the genesis of tholeiitic and calc alkaline island arc basalts evidence from mt arayat philippines abstract mt arayat an inactive stratovolcano is located behind the volcanic front of the bataan segment of the taiwan luzon arc in the central valley of luzon the philippines volcanism in this arc segment results from subduction of south china sea basin crust along the manila trench eruption products of mt arayat comprise pyroclastics and porphyritic basalt and andesite lavas of high k calc alkaline affinity compared to typical oceanic island arc volcanics the basalts display slightly elevated abundances of ta and nb relative to zr and hf and of ta nb zr hf and light rare earth elements lree s relative to y and heavy rare earths hree s their ree distribution suggests mixing between high hree low lree hree and low hree high lree hree components the latter suggesting the presence of residual garnet in the source region of mt arayat basalts enrichment of lree s relative to hree s is typical of calc alkaline and high k calc alkaline island arc basalts but is usually missing or small in island arc tholeiites lree enrichment is often attributed to a metasomatizing aqueous fluid generated during dehydration of the subducted slab for this model to be applicable extremely low abundances in the fluid of agents like f oh and or co2 3 which form hard complexes with the ree s are a prerequisite due to the temperature dependence of the eu 3 eu 2 redox potential divalent eu predominates in aqueous fluids of low complex forming capacity in the subduction zone environment whereas the other ree s with the possible exception of yb are still trivalent a lree enriched aqueous fluid generated by slab dehydration should therfore display a positive eu anomaly however lree enriched island arc basalts generally do not show positive eu anomalies implying that this lree enrichment is unlikely to result from ree transfer by an aqueous fluid instead a model is favoured which assumes contributions from both slab derived aqueous fluids and partial melts in the source region for island arc tholeiites the mantle wedge is only affected by a lile enriched aqueous fluid whereas the source region for calc alkaline and high k calc alkaline basalts is additionally metasomatized by small volumes of melt generated by small degrees of partial melting of the subducted slab with garnet as a residual phase these lree enriched partial melts may react with the depleted peridotite of the mantle wedge to produce domains of refertilized garnet peridotite subsequent partial melting produces calc alkaline melts whose trace element compositions display signatures of both depleted peridotite and more fertile garnet peridotite

primary fluids in low temperature eclogites evidence from two subduction complexes dominican republic and california usa eclogites occur as isolated blocks in melanges of both the samana peninsula dominican republic and the franciscan complex california usa in some of these eclogites fluid inclusions were found in omphacite and sodic calcic amphibole grains textures show that non planar populations of fluid inclusions formed during growth of clinopyroxene and amphibole in addition planar arrays of secondary fluid inclusions are found along healed cracks homogenization temperatures to liquid were used to calculate isochores for the fluid inclusions these data were compared with petrologic geothermobarometry temperature conditions of 500 700 c were estimated from garnetclinopyroxene geothermometry the jadeite contents of omphacite indicate minimum pressures of 8 11 kbar in this temperature range the p t estimates agree well with calculated isochores for primary fluid inclusions from the samana peninsula and show some overlap for both primary and secondary fluid inclusions from the franciscan complex salinities of 1 2 5 3 wt nacl equiv were estimated for both primary and secondary fluid inclusions from samana and franciscan eclogites these data suggest that low salinity aqueous fluids attended eclogite facies metamorphism and perhaps retrograde metamorphism in both subduction complexes the salinities and densities of fluid inclusions in eclogites from the samana peninsula and the franciscan complex resemble those of counterparts from garnet amphibolites of the catalina schist southern california an external source for such fluids is suggested by their homogeneous populations coupled with their low salinities geologic evidence suggests that the samana and franciscan eclogites may have been derived from a catalina like source terrane the catalina rocks are inferred to have interacted with large volumes of sediment derived fluid during subduction zone metamorphism at similar p but higher t conditions than those determined for samana and franciscan eclogite blocks these results contrast with data for fluid inclusions from eclogites of the monviso area western alps the monviso eclogites yield similar estimates for metamorphic p t to those obtained in this study but contain fluid inclusions of brine and of other saline aqueous fluids all of which are less dense than expected for incorporation at the reported eclogite facies conditions the differences between the properties of fluid inclusions from the ecologites and garnet amphibolites of the samana franciscan catalina subduction complexes and those of monviso probably reflect differences between fluid flow regimes during metamorphism

contact zone interaction of metabasites with metapelites amphibolite facies mineral assemblages chemical profiles and their origin the puolankajarvi formation finland abstract the amphibolite facies iron rich metapelites garnet staurolite mica schists of the early proterozoic puolankajarvi formation are intercalated with metamorphosed basic rocks which vary from iron rich amphibolites to magnesium rich layered sills and serpentinites garnet cummingtonite hornblende assemblages are abundant at the contacts of the metapelites with amphibolites while garnet chlorite garnet gedrite chlorite garnet gedrite cummingtonite chlorite and chlorite cummingtonite hornblende assemblages occur at their contacts with the layered magnesium rich sill the bulk composition profiles of these contacts normalised with respect to chromium and or aluminium show conspicuous enrichment in fe s ga and co and depletion in si k ba and rb some elements e g mg mn ni p zr and hf are depleted or enriched in only small amounts while others e g ca na sr and the rare earth elements may show both enrichment and depletion in the course of a profile although early infiltration during the hydration of metabasites and tectonic reworking may have produced some interaction of the basites with the pelites the major metasomatic event is considered to have bile elements the composition of the metapelites for example approached that of magnesium rich ultramafites through potassium depletion while the ultramafites became markedly enriched in iron to conserve the volume silica was depleted from the contact which underwent iron enrichment

crystallographic preferred orientation of talc determined by an improved ebsd procedure for sheet silicates implications for anisotropy at the slab mantle interface due to si metasomatism

crystal chemical controls on rare earth element partitioning between epidote group minerals and melts an experimental and theoretical study we have experimentally determined the partitioning of ree rare earth elements between zoisite and hydrous silicate melt at 1 100 c and 3 gpa all ree behave moderately compatible in zoisite with respect to the melt and all d ree zo melt show a smooth parabolic dependence on ionic radius the partitioning parabola peaks at nd left d nd zo melt 4 9 right and the compatibility slightly decreases towards la left d la zo melt 3 9 right and decreases by half an order of magnitude towards yb left d yb zo melt 1 1 right application of the elastic strain model of blundy and wood 1994 to the available zoisite and allanite ree mineral melt partitioning data and comparison with partitioning pattern calculated from a combination of structural and physical data taken from the literature with the elastic strain model suggest that in zoisite ree prefer the a1 site and that only la and ce are incorporated into the a2 site in significant amounts in contrast in allanite all ree are preferentially incorporated into the large and highly co ordinated a2 site as a result zoisite fractionates the mree effectively from the hree and moderately from the lree while allanite fractionates the lree very effectively from the mree and hree consequently the presence of either zoisite or allanite during slab melting will lead to quite different ree pattern in the produced melt

ree mobilization during small scale high pressure fluid rock interaction and zoisite fluid partitioning of la to eu a zoisite quartz segregation and its adjacent host metabasite from the tauern window austria have been investigated in detail to study the geochemical behaviour of ree during high pressure fluid rock interaction and the role of zoisite the segregation formed at t 500 to 550 c p 0 6 gpa because of decompressional dehydration of the host metabasite during the formation of the segregation the ree la to tb and dy to lu were geochemically decoupled in the metabasite la to tb are incorporated in allanite and were leached from the host rock by the fluid phase this decoupling and the leaching of la to tb significantly alter the ree patterns of the host rock in the immediate vicinity of the segregation from lree enriched to lree depleted based on a mass balance calculation for la ce nd sm and eu the composition of the fluid from which the segregation was precipitated is calculated the fluid is lree enriched with la sm n 3 7 individual ree contents range from eu 2 4 1 1 ppm up to ce 90 20 ppm ree 190 50 ppm calculated partition coefficients dreezoisite fluid are generally 1 and increase from dlazoisite fluid 0 08 0 02 to deuzoisite fluid 0 8 0 4 indicating a stronger preference of zoisite for mree than for lree to describe the fractionation of la relative to sm between zoisite and fluid the la sm exchange coefficient between zoisite and fluid is calculated the derived exchange coefficient kd la sm zoisite fluid 0 2 0 1 suggests that a fluid in equilibrium with zoisite will have a three to ten times higher la sm ratio than coexisting zoisite

distributed damage faulting and friction we present a formulation for mechanical modeling of geological processes in the seismogenic crust using damage rheology the seismogenic layer is treated as an elastic medium where distributed damage modifying the elastic stiffness evolves as a function of the deformation history the model damage rheology is based on thermodynamic principles and fundamental observations of rock deformation the theoretical analysis leads to a kinetic equation for damage evolution having two principal coefficients the first is a criterion for the transition between strength degradation and recovering healing and is related to friction the second is a rate coefficient of damage evolution which can have different values or functional forms for positive degradation and negative healing evolution we constrain these coefficients by fitting model predictions to laboratory data including coefficient of friction in sawcut setting intact strength in fracture experiments first yielding in faulting experiments under three dimensional strain onset and evolution of acoustic emission and dynamic instability the model damage rheology accounts for many realistic features of three dimensional deformation fields associated with an earthquake cycle these include aseismic deformation gradual strength degradation development of process zones and branching faults around high damage areas strain localization brittle failure and state dependent friction some properties of the model damage rheology e g cyclic stick slip behavior with possible accompanying creep are illustrated with simplified analytical results the developments of the paper provide an internally consistent framework for simulating long histories of crustal deformation and studying the coupled evolution of regional earthquakes and faults this is done in a follow up work

the fate of subducted oceanic slabs in the shallow mantle insights from boron isotopes and light element composition of metasomatized blueschists from the mariana forearc abstract serpentine muds from south chamorro seamount scs drilled during odp leg 195 at site 1200 contain metamafic clasts that experienced blueschist facies metamorphism including the critical mineral assemblage pumpellyite na amphibole epidote these schists represent fragments from the actual slab mantle interface at 27 km depth their heterogeneous lithology with a metasomatic character indicates significant mobility of major elements in the mariana forearc a region of melange formation as it can also be observed in onland exposures such as the catalina schist as the mariana forearc blueschists show no late stage alteration they permit the direct study of material transfer during the subduction processes at an active convergent margin this study presents the first data of detailed b isotope 11 b and light element variations in blueschist facies minerals from the mariana arc system the primary foci are b and li concentrations and 11 b values analyzed by sims and tof sims techniques minerals such as na rich amphibole phengite and chlorite are found to be strongly enriched in li up to 70 g g be up to 8 g g and b up to 35 g g and with 11 b values of 6 4 these new data are consistent with isotopically heavy b being released into the mariana forearc mantle wedge serpentinization of dry mantle peridotite after interaction with b rich slab released high ph fluids and confirm models of significant b loss and b isotope fractionation during forearc shallow slab dehydration the elevated li be and b concentrations in minerals that comprise the bulk of the rocks namely amphibole phengite and chlorite bear a strong potential to further transport li and b as well as the isotopically light component of b to greater depths in the mantle where ongoing metamorphism is responsible for further isotope and elemental fractionation and the formation of distinct mantle reservoirs e g volcanic arc and oceanic intra plate oib magmas

titanium mobility in metamorphic systems a review abstract titanium is a dispersed lithophile element commonly regarded as chemically immobile at sub magmatic temperatures however a growing number of field studies demonstrate that ti is somewhat mobile in certain environments this review examines the current state of knowledge concerning the mobility of ti during alteration and metamorphism of crustal rocks also considered are relevant ti oxide solubility data from experiments and determinations of ti concentrations in natural waters where ti mobility is well documented its length scale is 10 m however ti may move greater distances in deep crustal shear zones the solubility of ti oxides in geologic fluids is strongly temperature dependent under low to medium grade metamorphic conditions ti mobility is limited by the low solubility of ti oxides in the associated fluids under eclogite conditions ti mobility is probably limited not by ti oxide solubility which reaches 1 ti but by restricted fluid flow in low permeability rocks

subduction products according to experimental prediction knowledge of the thermal structure and processes in subduction zones is so uncertain that our more extensive knowledge of source materials and phase relationships remains insufficient for prediction of magmatic products the phase relationships do provide important constraints for testing geophysical and petrogenetic models especially when considered along with geochemical constraints physical questions such as the behavior of partially molten rocks are intimately related to the compositions of magmas from source to eruption site tentative predictions and conclusions include 1 andesite is not a primary magma from oceanic crust 2 it is unlikely that liquids from subducted oceanic crust yield andesites by fractionation except by melting of amphibolite in an especially warm subducted crust 3 it is more likely that hydrous siliceous magmas from the crust leak into the overlying mantle 4 andesite magmas can be generated in peridotite only under exceptional circumstances 5 h2o undersaturated basic magmas from peridotite modified by aqueous fluids or hydrous magma may yield andesite by fractionation 6 phlogopite peridotite could escape melting and yield alkalic magmas at deeper levels 7 andesite magmas could be produced in continental crust only by extreme heating through underplating of basalts or by mixing of basalt and rhyolite derived from crust most of these conclusions need revision if convection in the asthenosphere wedge raises the temperature of the subducted slab to 1250 c at 100 km as proposed by marsh 1979a this demonstrates the dependence of experimental predictions on geophysical models

partitioning of sr between coexisting minerals of the hollandite and piemontite groups in a quartz rich schist from the sanbagawa metamorphic belt japan coexisting minerals of the hollandite cryptomelane series piemontite group and abswurmbachite braunite series occur as a small nodule in a quartzose schist metachert from the epidote amphibolite facies area of the sanbagawa metamorphic belt central shikoku japan strontium ba k and pb contents of the hollandite cryptomelane minerals are 0 00 0 42 0 07 0 99 0 00 0 75 and 0 00 0 05 apfu atoms per formula unit o 16 respectively the piemontite group minerals can be divided into common piemontite ca sr in the ten coordinated a2 site and strontiopiemontite sr ca strontium ba and pb contents of the strontiopiemontite reach 0 71 0 26 and 0 20 apfu o 12 5 respectively suggesting the existence of two possible new end members for epidote group minerals ba piemontite baca mn3 fe3 al 3si3o12 oh and pb piemontite pbca mn3 fe3 al 3si3o12 oh the abswurmbachite braunite series minerals cu 0 30 0 53 apfu o 12 contain up to 0 34 apfu mg indicating the exchange mg mn2 cu 1 distribution coefficients for sr between the hollandite cryptomelane series and the piemontite group minerals are up to 20 implying that the hollandite type structure with its large open tunnel is more suitable for incorporation of sr than the a2 site of the piemontite group minerals a synthetic aluminosilicate analogue of the hollandite cryptomelane compound is stable under p 8 10 gpa and suggests the hollandite type aluminosilicate may be a reservoir of sr and other large ion lithophile elements in the upper mantle

anomalous intra plate high mg andesites in the choshi area chiba central japan produced during early stages of japan sea opening abstract geochemical and isotopic data are reported for 20 ma high magnesian andesite hma lavas erupted on the forearc side of central japan in the choshi coastal area chiba rarely olivine phyric 345 ppm high sr 520 ppm high sr y ca 30 and high la yb n 8 conforming to the definition of low silica adakites transitional in character to hma however despite slight negative nb and ta troughs in a few samples their incompatible element distribution patterns appear closer to those of intra plate basalts apparently confirming previous suggestions that terms such as hma and adakite cover a diverse range of compositions enriched in radiogenic sr and nd ca 0 7040 to 0 7043 and 0 5128 to 0 51275 respectively the choshi hma shows relatively low 206pb 204pb 18 25 to 18 35 and 208pb 204pb 38 37 to 38 48 ratios resembling those of late miocene intra plate basalts in the japan sea and sw japan in this regard the choshi hma differs from those of the classic middle miocene 14 to 12 ma hma described from the setouchi volcanic belt e g tatsumi 1981 a short distance to the west which also shows relative high k contents although hmas have been generally attributed to subduction related processes the lack of high field strength element hfse depletions cf large ionic lithophile elements lile in most choshi hma suggests an absence of so called subduction signatures accordingly we suggest that the choshi hma probably tapped a fertile asthenospheric region of the convecting mantle wedge primitive melt products having acquired hma like character as a result of shallow level modification by interaction with lithospheric components aided by thermal input resulting from mantle extrusion during the early stages of japan sea opening 23 15 ma

on the relation between seismic wave velocity and stress in a solid summary the paper considers a model comprised of variable moduli elastic solids incorporating fracturing caused by tectonic stress the model gives us a possibility to calculate the seismic velocities and crack distribution around density variations the results of numerical simulations are shown

the geochemistry of young volcanism throughout western panama and southeastern costa rica an overview oblique aseismic subduction below western panama and southeastern costa rica has produced recent arc related volcanism the aseismicity is probably related to the subduction of relatively hot oceanic lithosphere the volcanism throughout this region over the past 2 ma has been quite distinct consisting of felsic magmas andesites to rhyolites but mainly dacites with geochemical signatures suggesting a metamorphosed basaltic source it is believed that the subduction of young oceanic crust sets up conditions under which the slab melts rather than the overlying mantle wedge rocks with slab melt geochemistries and associated with young subducted crust have been termed adakites elsewhere the young adakite melts are sometimes associated with a few rare young high nb basalts but there is no obvious genetic link between them through differentiation high nb basalts may also be derived from the partial melting of the subducted oceanic crust high nb basalt migmatites have been found with pegmatites of adakite compositions in the exposed subduction terrain of the catalina schist california alternatively the high nb basalts may be partial melts of phlogopite rich mantle that has previously reacted with adakite magmas eruption of adakites and high nb basalts was preceded by a 2 3 ma period of relative quiescence prior to this there was a 7 ma period of calc alkaline volcanism typical of the present day magmatism associated with a distinct benioff zone found throughout the central american arc the abrupt transition in volcanism with time from an early calc alkaline sequence to a later adakite high nb basalt sequence may record a change in the tectonic setting of western panama and southeastern costa rica over the past 12 ma

rare earth element distributions among minerals in a granodiorite and their petrogenetic implications a study of the distribution of lanthanide rare earths in a granodiorite from the eastern peninsular ranges batholith southern california reveals that a large fraction of the ree in this rock resides in the accessory phases sphene and allanite the minerals plagioclase alkali feldspar biotite epidote and apatite each contribute approximately 1 or less of each ree to the whole rock with the exception of eu for which plagioclase contributes 7 sphene and allanite together contain 80 to 95 of each ree each of these phases is zoned in ree concentration with substantial decreases from core to margin textural observations argue for relatively early saturation and precipitation of sphene and allanite in the magma ree zoning trends in sphene and allanite and unexpectedly low ree concentrations in largely later crystallizing minerals such as feldspar indicate that the precipitation of sphene and allanite significantly reduced ree concentrations in residual melts these results illustrate the potential that sphene and allanite have for controlling the behavior of ree in granitic magmas available information collectively suggest that the sampled granodiorite existed as a complete melt that the ree contained in the assemblage of phases were derived by direct crystallization from the melt and that the melt behaved essentially as a closed system once crystallization of the phases now present began close correspondences between the major and trace element chemistries of the granodiorite and phenocryst poor lavas from similar tectonic settings support these conclusions the ree pattern of the granodiorite melt appears to have originated at depth and is characteristic of its source regions and derivation mechanism the high liquidus temperature of a granodiorite melt 1000 c indicates the importance of mantle derived components within the sources of batholithic magmas in the peninsular ranges

formation of paleoproterozoic eclogitic mantle slave province canada insights from in situ hf and u pb isotopic analyses of mantle zircons abstract in situ hf isotope analyses and u pb dates were obtained by laser ablation mc icp ms for a zircon bearing mantle eclogite xenolith from the diamondiferous jericho kimberlite located within the archean slave province nunavut canada the u pb zircon results yield a wide range of ages 2 0 to 0 8 ga indicating a complex geological history of importance one zircon yields a u pb upper intercept date of 1989 67 ma providing a new minimum age constraint for zircon crystallization and eclogite formation in contrast hf isotope systematics for the same zircons display an intriguing uniformity and corresponding hf depleted mantle model ages range between 2 1 0 1 and 2 3 0 1 ga the youngest hf model age is within error to the oldest u pb date the jericho eclogites have previously been interpreted as representing remnants of metamorphosed oceanic crust and their formation related to paleoproterozoic subduction regimes along the western margin of the archean slave craton during the wopmay orogeny hf isotope compositions and u pb results for the jericho zircons reported here are in good agreement with a paleoproterozoic subduction model suggesting that generation of oceanic crust and eclogite formation occurred between 2 0 and 2 1 ga the slightly older hf depleted mantle model ages 2 1 to 2 3 ga may be reconciled with this model by invoking mixing between crustal derived hf from sediments and more radiogenic hf associated with the oceanic crust during the 2 ga subduction event this results in intermediate hf isotope compositions for the jericho zircons that yield fictitiously older hf model ages

brittle deformation and damage induced seismic wave anisotropy in rocks summary we study the relations between rock fracturing non linear deformation and damage and stress induced anisotropy of seismic waves by comparing theoretical predictions of a damage rheology model to results of laboratory experiments with granite samples the employed damage model provides a generalization of hookean elasticity to a non linear continuum mechanics framework of cracked media incorporating degradation and recovery of the effective elastic properties along with gradual accumulation of irreversible deformation beyond the elastic regime the model assumes isotropic distribution of local microcracks expressed in terms of a scalar damage variable but the non linear elastic response caused by the opening closure and evolution of the internal cracks is predicted to lead to seismic wave anisotropy we develop relations between the seismic wave anisotropy internal rock damage and stress field and test the viscoelastic damage rheology against sets of laboratory experiments with cylindrical granite samples the observed data include measurements of stress and strain in three loading cycles culminating in a final macroscopic failure together with measured wave velocities along and perpendicular to the axis of the cylinder using a single set of parameters the model fits well the overall evolution of the axial and transversal stress strain relations as well as the anisotropic elastic wave velocities during all cycles from the onset of fracturing in the first cycle until the macroscopic failure in the final cycle

mount st helens potential example of the partial melting of the subducted lithosphere in a volcanic arc mount st helens 50 km to the west of mount adams and the main cascade volcanic chain is only 80 km above the subducting oceanic lithosphere the elevated temperatures off the subducting slab because of the close proximity of the juan de fuca ridge to the trench may induce slab melting at a depth of 80 km dacites from mount st helens have geochemical compositions off magmas that are derived by direct partial melting of metamorphosed basalts at high pressure i e relatively high ai al2o3 15 at 70 sio2 low y and yb because of garnet and amphibole stability in the source low sc and high sr and eu trace element modeling of the partial melting of mid oceanic ridge basalt morb from the juan de fuca ridge that yields a hornblende eclogite residue can reproduce the mount st helens data results off the model are quite distinct from data derived from the mount adams volcanic rocks in contrast mount adams is 135 km above the subducting slab and is associated with normal arc magmatism believed to be derived from the mantle above the subducting plate the cascade are has been active in its present locality because of oblique subduction for the past 7 m y the major volcanoes along the arc have existed for at least 500 ka but mount st helens has existed for 40 ka we suggest that the subducting plate may have reached elevated temperatures because of the approach of north america to the juan de fuca ridge at 40 ka which initiated melting of the slab

metasomatism and partial melting in a subduction complex catalina schist southern california partial melting accompanied high pressure amphibolitization of eclogitic blocks and metasomatism of blocks and their peridotite matrix in a unit of the catalina schist a subduction zone metamorphic terrane migmatitic blocks contain leucocratic zones veins and pods with albitic plagioclase quartz muscovite mineralogically similar dikes and veins cut the ultra mafic matrix a few veins can be traced back into migmatitic blocks uniform phase proportions and igneous textures indicate that the dikes and veins crystallized from silicate melts field relations mineral and bulk compositions and comparisons with experimental data suggest that the migmatites and dikes are partial melts of variably metasomatized amphibolites element partitioning phase equilibrium and fluid inclusion data are compatible with partial melting of the amphibolites at p 8 11 kbar and t 640 750 c in the presence of a low salinity aqueous fluid although the catalina metamorphism occurred at much shallower levels than those inferred for subduction related magmatism the process envisioned is one of multistage slab derived metasomatic contamination of the mantle in the hanging wall of a subduction zone

adakitic granites within the ne jiangxi ophiolites south china geochemical and nd isotopic evidence geochemical and nd isotopic data are reported for the ca 970 ma albite granites within the neoproterozoic ne jiangxi ophiolite south china these granites are high in al2o3 15 17 na2o 7 3 8 5 mg 43 62 sr 383 920 ppm sr y 166 444 and y yb 12 14 but low in k2o 0 2 0 6 tio2 0 3 p2o5 0 1 furthermore they display consistently high morb like initial end values of 4 6 to 6 7 at ca 970 ma these geochemcial and isotopic characters are similar to those of the modern adakites compared with the modern adakites the xiwan albite granites are depleted in middle to heavy ree with y yb ratio of 12 14 which can be attributed to residual amphibole and garnet in the source geological and geochemical features suggested that the granites were generated by low degrees of partial melting of subducted spilitized oceanic crust at pressures high enough to stabilize garnet and amphibole these are the first adakitic granites identified within the ne jiangxi ophiolite their presence shows that granitic rocks that intrude ophiolites can be generated not only during the processes of sea floor spreading oceanic crust shearing and obduction of fragmented oceanic lithosphere as previously thought but also during the process of subduction of oceanic lithosphere the shrimp u pb zircon age of 968 23 ma for the xiwan adakitic granite geochimica 23 1994 117 should be re interpreted as the timing of subduction of oceanic crust rather than as the age of oceanic crustal formation as previously interpreted