boninitic and tholeiitic dykes in the lewis hills mantle section of the bay of islands ophiolite implications for magmatism adjacent to a fracture zone in a back arc spreading environment a detailed integrated field petrographic and geochemical study of the springers hill area of the bay of islands ophiolite exposed in the lewis hills was undertaken to explain the anomalously high abundance of veins and dykes of chromitite orthopyroxenite and clinopyroxenite and their associated dunites hosted by a refractory harzburgite dunite mixture a geodynamic situation is presented which is constrained by previous studies requiring formation of the springers hill mantle section at a ridge fracture zone intersection and the whole of the bay of islands ophiolite within a back arc spreading environment the veins and dykes formed during magmatism at the ridge fracture zone intersection and along the fracture zone as progressively hotter more fertile richer in clinopyroxene asthenosphere ascended and was channelled up and along the fracture zone wall shallow melting of refractory harzburgite in the presence of subduction derived hydrous fluids produced light rare earth element lree enric

boninitic magmatism in a continental margin setting yukon tanana terrane southeastern yukon canada mid paleozoic mafic rocks in the finlayson lake region of the yukon tanana terrane southeastern yukon canada have the diagnostic geochemical signatures of boninites high mgo cr ni and co contents intermediate sio 2 contents high mg 9s mgo mgo feo al 2 o 3 tio 2 and zr hf middle rare earth element ree ratios low tio 2 ree and high field strength element contents and u shaped primitive mantle normalized trace element patterns however unlike most modern and ancient boninitic rocks that are typically associated with intraoceanic realms those from the finlayson lake region are part of a mid paleozoic continental margin arc backarc magmatic system we propose a model in which the boninitic rocks from the finlayson lake region formed as a result of spreading ridge propagation into an arc built on composite basement of oceanic and continental crust in the oceanic segment upwelling asthenosphere induced melting of a subducted slab metasomatized refractory mantle source to form boninitic magmatism in the continental sector upwelling asthenospheric mantle and or the melts derived thereof induced crustal melting which explains the large volume of temporally equivalent felsic volcanic and intrusive rocks

seismicity normal faulting and the geomorphological development of the gulf of corinth greece the corinth earthquakes of february and march 1981 three major destructive earthquakes of m s 6 7 6 4 6 4 occurred in the eastern part of the gulf of corinth in february and march 1981 associated normal faulting was observed on both the north and the south sides of the gulf examination of teleseismic local seismic surface faulting and geomorpbological data suggests that the first and second of these shocks activated major north dipping normal faults these faults control the topography and bathymetry and are related to the recent uplift and subsidence of the coastline it is probable that the first earthquake occurred on a faultwhich outcrops underwater and the second on a fault which is visible on land it appears that the third shock activated an antithetic normal fault dipping southwards on the north side of the gulf and that the overall structure of the gulf of corinth graben is asymmetric the northern antithetic fault clearly demonstrates the geomorphological changes associated with developing young normal faulting in addition we suggest that observations of recent coastline movement provide a powerful tool for identifying major active fault systems and predicting their historical role in the paleogeography of an area

variscan intra orogenic extensional tectonics in the ossa morena zone evora aracena lora del rio metamorphic belt sw iberian massif shrimp zircon u th pb geochronology

evidence for transform faulting in the ionian sea the cephalonia island earthquake sequence of 1983 accurate locations of aftershocks of the january 17 1983 ms 7 0 main shock in the ionian islands have been determined as well as fault plane solutions for this main shock and its largest aftershock which are interpreted as a right lateral strike slip motion with a thrust component on a fault striking in about a ne sw direction

constructional features of the troodos ophiolite and implications for the distribution of orebodies and the generation of oceanic crust the map pattern of the troodos ophiolite in cyprus is an expression of a simple elongate anticlinal structure the cross sectional form of the anticline can be described by a sinusoidal dip model on the northern flank of the anticline the thickness of various units number of gossans etc were measured along profiles at 2 5 km intervals normal to strike using existing maps supplemented by new observations profiles are approximately perpendicular to the spreading direction during crustal construction and uncertainties in unit thickness are smaller than the range of unit thicknesses data on the profiles were analyzed quantitatively by harmonic analysis using the fourier transform method the harmonic analysis discloses cyclic variation in the thickness of major units number of gossans etc that provides a link with the regular spacing of five groups of cu fe zn orebodies groups of orebodies occur preferentially where sheeted dikes are at a shallow crustal depth sheet and massive flows and dike

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

temporal characteristics of some earthquake sequences in greece latoussakis j stavrakakis g drakopoulos j papanastassiou d and drakatos g 1991 temporal characteristics of some earthquake sequences in greece in m wyss editor earthquake prediction tectonophysics 193 299 310 temporal features of the aftershock activities following some moderate shallow earthquakes in greece have been studied by making use of the modified omori formula and akaike s information criterion aic although the main shocks occurred in different seismogenic regions in greece the aftershock activities show almost the same anomalous pattern in most cases the secondary aftershocks of the aftershock activity of the main event are preceded by seismic quiescence also in some cases the aftershock activity recovers to a normal level or increases beyond it prior to the occurrence of the large aftershock the observed temporal pattern may be useful in predicting a large aftershock which might be as large and destructive as the main shock if the aftershock activity is monitored immediately after the occurrence of the main shock

cambrian ensialic rift related magmatism in the ossa morena zone evora aracena metamorphic belt sw iberian massif sm nd isotopes and shrimp zircon u th pb geochronology abstract the late ediacaran c 560 550 ma serie negra sediments of the evora aracena metamorphic belt ossa morena zone sw iberian massif preserve a record of the erosion of an avalonian cadomian magmatic arc and subsequent related turbiditic sedimentation detrital zircon from the serie negra is characterized by predominantly ediacaran and cryogenian ages with few paleoproterozoic and archean cores and a marked lack of grenvillian ages these features when combined with the metasediments enrichment in lree la yb 14 negative eu anomalies low 147 sm 144 nd values 0 121 and negative e nd 550 5 5 indicate that the protolith serie negra sediments were derived from a continental magmatic arc a period of late cadomian ca 560 540 ma tectonism was followed by an extended episode of widespread bimodal magmatism related to cambrian ca 540 500 ma rifting this tectonic inversion is expressed in the geological record by a regional early cambrian unconformity shrimp zircon u th pb ages from four felsic orthogneisses from the evora massif record cambrian 527 10 ma 522 5 ma 517 6 ma and 505 5 ma crystallization ages for their igneous protoliths this confirms the existence of widespread lower paleozoic igneous activity in the ossa morena zone i a lower cambrian ca 535 515 ma igneous felsic dominated sedimentary complex with calc alkaline signature and associated carbonate and siliciclastic deposition and ii a middle cambrian ordovician ca 515 490 ma igneous bimodal sedimentary complex with calc alkaline and tholeiitic signatures and associated dominant siliciclastic deposition but also carbonate sediments the cambrian felsic magmatism was characterized by negative eu anomalies la lu n 0 8 11 147 sm 144 nd 0 1289 0 1447 and e nd 500 ranging from 1 5 to 0 8 a tendency towards peraluminous compositions suggests late fractionation low degrees of partial melting or the mixing of crustal and mantle derived material in the magma source region some felsic rocks possibly represent the last residual melts of high temperature zircon undersaturated mafic magmas later affected by crustal contamination while others indicate partial melting of crustal metasediments variably contaminated by basaltic liquids the transition from early felsic dominated to later more mafic magmatism suggests the gradual opening of the system to tholeiitic n e morb products th n ta n 147 sm 144 nd 0 1478 0 1797 and e nd 500 values ranging from 6 4 to 7 3 while the n morb amphibolites have 147 sm 144 nd 0 1818 0 1979 and e nd 500 values of 5 8 and 7 0 reaching a maximum of 9 1 in contrast other amphibolites have a negative ta anomaly 1 35 n ta n 147 sm 144 nd values ranging from 0 1639 to 0 1946 and e nd 500 values of 3 5 to 5 2 suggesting derivation by crustal assimilation processes the sub alkaline igneous precursors of the amphibolites were most likely generated in a rift setting by asthenospheric upwelling these results strengthen the proposed geodynamic scenarios for the sw iberian massif by which cadomian accretion gave rise to an ensialic rift that developed into a proto oceanic basin and incipient spreading opening of the rheic ocean a similar transition from a convergent to a divergent plate boundary during the ediacaran to cambrian ordovician has also been reported in other segments of the northern gondwana margin

petrology and origin of primitive lavas from the troodos ophiolite cyprus parental magmas to the troodos ophiolite are characterised by low tio2 and al2o3 and high sio2 extremely fresh and chemically primitive high mgo rocks are found within the upper pillow lavas and along the arakapas fault belt of cyprus and contain forsteritic olivine enstatite and groundmass clinopyroxene set in glass or plagioclase with accessory magnesiochromite and sometimes hornblende they are quartz normative and may have originally contained up to 3 wt h2o geochemically there are three distinct groups of primitive lavas based on tio2 and zr contents but also reflected by cao na2o and ree abundances these groups cannot be related by crystal fractionation and are considered to have been generated by incremental melting of a variably depleted source region the parental magma to the least depleted group group i was that of the major portion of the troodos plutonic complex and is similar to those postulated for other low ti ophiolites chemically it has close affinities with komatiitic basalts the most depleted lavas group iii all have u shaped ree profiles and variable 143nd 144nd ratios interpreted in terms of metasomatism of the source region by an incompatible element enriched component which was probably derived from a subducted slab these lavas represent an intermediate step in the development of boninite series rocks

tectono magmatic evolution of the west philippine mariana region and the origin of boninites re examination of published data on the tectomagmatic evolution of the west philippine mariana region indicate that arc magmatism and back arc extensional pulses are not synchronous but are largely asynchronous arc volcanism ceases within a few million years of the development of a back arc basin and recommences oceanward on a new arc during the final stages in the development of the back arc basin following karig s model we believe that ascent beneath the arc axial chain of a line of diapirs of morb source mantle interferes with processes of arc magma generation and arc magmatism wanes partial melting of the diapirs produces voluminous morb type tholeiites which split the arc and form the crust of a widening back arc basin we show for both the west philippine mariana region and occurrences of boninite like lavas in ophiolites that boninites appear to be erupted after arc magmatism and immediately before eruption of morb type lavas passage of a diapir of hot morb source mantle through refractory sub arc peridotite which has been enriched in lile and h2o will result in limited partial melting of the hydrous peridotite and the restricted production of boninitic magmas these are more likely to erupt modified or only slightly modified by crystal fractionation through the thin forearc crust continued ascent of the morb source mantle diapir will result in it partially melting to yield morb tholeiites which erupt shortly after and in considerably more voluminous amounts than the boninites finally we suggest that forearcs regions are likely to be incorporated into foldbelts and that boninite or low ti lava bearing ophiolites may characterize such regions

geochemical evidence for sundering of the west mariana arc in miocene ash from the parece vela basin abstract glass and mineral fragments from discrete volcanic ash layers were sampled from dsdp ipod site 450 in the parece vela basin philippine sea and analyzed by electron microprobe the ashes are interpreted as eruptive products of the adjacent west mariana arc system between 25 and 14 ma b p and have compositions between basaltic andesite and rhyolite and rarely boninite continuous chemical trends appear to reflect mixing of mafic and silicic magmas discontinuous trends between these end members are relatively few and are consistent with liquid lines produced by fractional crystallization andesitic tephra become progressively richer in mgo and cao through the middle miocene while boninite appears towards the end of the sequence between 14 and 15 ma b p coeval rhyolitic glasses become richer in k 2 o and na 2 o with maximum concentrations at about 15 ma b p chronologic changes in fractionation type and composition of parent magmas are interpreted to reflect the subaerial volcanic evolution of the west mariana arc the appearance of boninite is believed to signal early stages of arc sundering and corresponds temporally with regional uplift of the sea floor above the carbonate compensation depth precursor to a new pulse of back arc spreading

the ca 350 ma beja igneous complex a record of transcurrent slab break off in the southern iberia variscan belt abstract we report the results of an isotopic study of the large gabbro dioritic beja igneous complex bic in the boundary between the highly contrasting ossa morena om and south portuguese sp zones of the southern iberian variscan orogen this boundary is interpreted as a major suture zone materialized by discontinuous scattered strips of mafic ultramafic rocks the so called beja acebuches ophiolite complex baoc and by melange deposits of middle to late devonian age in the pulo do lobo accretionary prism plap the beja gabbro was interpreted either as part of the ophiolite like units or as a broadly arc related massif reflecting the northward subduction of oceanic lithosphere u pb zircon id tims dating of two diorites and a granodiorite points to igneous emplacement ages of 350 2 ma serpa 352 2 ma torrao and 353 4 ma sao pedro respectively whereas a felsic dyke yields a slightly younger age of 345 2 ma these results show that published ar ar dates do not represent igneous crystallization ages but merely reflect regional cooling below ca 500 c at least 10 ma after the major intrusive event probably as a result of uplift of the omz side of the suture zone relative to the subsiding spz 87sr 86sr350 and end350 display a large range of values from 0 7041 to 0 7093 and from 4 0 to 6 1 respectively which documents a rather complex petrogenetic history with an important role played by crustal contamination processes the more primitive sr and nd isotope signatures are measured in the mafic cumulates while radiogenic sr and unradiogenic nd isotope compositions occur in the more evolved rock types the broad trend of decreasing end350 with decreasing sm nd and increasing sio2 concentration is reminiscent of crustal assimilation combined with fractional assimilation afc end values of flasergabbros and associated cumulates ascribed to the ophiolite like unit in the guadiana valley are close to zero or even slightly negative irrespective of the age chosen for the correction for in situ radioactive decay of 147sm in the range 350 500 ma assumed to encompass the true geological age this observation conflicts with a truly oceanic derivation of these rocks which could rather represent early stage deformed and metamorphosed equivalents of the beja gabbro most metabasalts of the same area have positive albeit variable end consistent with mildly depleted mantle sources and or significant crustal assimilation only a few samples reach highly positive values end350 500 ca 8 similar to those typical for n morbs as do the acebuches amphibolites near aracena end350 500 ca 9 these data suggest that the baoc is not only disrupted but also composite since the gabbroic rocks are not cogenetic with the basalts based on the late devonian age of the collision between the omz and spz the emplacement of the beja gabbro cannot be related to an active subduction of oceanic lithosphere instead the generation in the mantle and emplacement in the overlying continental crust of copious volumes of mafic magmas with concomitant lp ht metamorphism is tentatively ascribed to a process of transcurrent slab break off that followed oblique subduction and collision

precursory seismic quiescence seventeen cases of precursory seismic quiescence to mainshocks with magnitudes fromml 4 7 toms 8 0 are summarized the amount of rate decrease ranges from 45 to 90 the significance of these changes varies between 90 and 99 99 the assumption that the background rate is approximately constant is fulfilled in most crustal volumes studied all quiescence anomalies seem to have abrupt beginnings and the rate during the anomalous period is fairly constant the duration of the precursors ranges from 15 to 75 months and it is not clear what factors determine that time at least three successful predictions have been based on seismic quiescence these cases have shown that mainshocks can be predicted based on quiescence but they have also shown that the interpretation of the data in real time is difficult and nonunique if a false alarm is defined as a period of quiescence with a significance level larger than a precursory quiescence in the same tectonic area then we estimate based on searches in four areas that the false alarm rate may be on the order of 50 failure to predict may be expected in perhaps 50 of mainshocks even in carefully monitored areas quiescence cannot be used as a precursor in tectonic environments with low seismic activity most characteristics of the phenomenon are still poorly defined but data exist which probably permit at least a doubling of the presently available data on case histories

87 sr 86 sr applied to age discrimination of the palaeozoic carbonates of the ossa morena zone sw iberia variscides four distinct palaeozoic episodes of marine carbonate sedimentation are reported in the ossa morena zone sw iberian variscides among them the cambrian and the devonian episodes are those that are well represented in the portuguese domains of the ossa morena zone in this zone the strong variscan deformation and metamorphic recrystallization obliterate the biostratigraphic contents of some carbonate local successions and their ages have been based on lithostratigraphic correlations the 87sr 86sr values for the ossa morena zone carbonate rocks allow to make an isotopic lithostratigraphic correlation based on the comparison between strontium ratios of marine carbonate rocks without chronological data and carbonate rocks with a good age control moreover the established 87sr 86sr fingerprints of the marine carbonate episodes from ossa morena zone should be simultaneously correlated with the worldwide seawater 87sr 86sr curve despite the interaction of the marine carbonate sedimentation with late secondary dolomitization processes or with high temperature metamorphic metasomatic fluids which increases the primary 87sr 86sr ratios this methodology allows to define two main distinct clusters of 87sr 86sr for the ossa morena zone marine carbonates a devonian cluster with strontium ratios lower than 0 708000 and a cambrian cluster ranging from 0 708777 to 0 708299 0 708575 0 000161 that can reach 0 709227 0 708866 0 709087 0 000118 if incipient late secondary dolomitization or high temperature metamorphism metasomatism operated the obtained 87sr 86sr isotopic ratios corroborate the lithostratigraphic correlations between the carbonate rocks of abrantes assumar estremoz viana alvito ficalho and escoural successions with the ovetian marianian limestones of alter do chao elvas succession moreover this methodology allows to establish the isotopic discrimination between the upper silurian devonian limestones of ferrarias barrancos and bencatel successions and the marbles of estremoz succession

a quantitative study of some aftershock sequences in greece temporal features of the aftershock activities following twelve moderate shallow earthquakes in greece have been studied quantitatively by making use of the modified omori s formula and akaike s information criterion aic

k ar dating on volcanic rocks in the bonin islands and its tectonic implication abstract the k ar dating method was applied to the volcanic rocks the boninite and related rocks from the bonin islands four fresh samples gave k ar ages of about 40 ma these ages suggest that the main volcanism of the bonin islands occurred about 40 ma ago as a result of the initiation of subduction of the pacific plate at its western margin results of the k ar dating on other rocks suggest that some volcanism or thermal disturbance may have occurred 5 10 ma ago at least in chichijima

magmatic origin of low ca olivine in subduction related magmas co existence of contrasting magmas unravelling the origin of different components contributing to subduction related magmas is a prerequisite to understanding the sources and processes involved in their origins mafic high ca subduction related magmas from geographically diverse areas indonesia solomon islands kamchatka valu fa ridge contain two populations of olivine crystals of which only the high ca population cao 0 3 0 5 wt crystallized from the melt that dominantly contributed to the whole rock composition forsterite rich fo90 94 low ca cao 0 3 wt olivine crystals which constitute 16 37 vol of total olivine population are generally interpreted as mantle or lithospheric xenocrysts however in these samples the olivine shape and chemical zoning the composition of included minerals orthopyroxene clinoenstatite and cr spinel and presence of melt inclusions are indications that these crystals are phenocrysts from a mafic magma with high silica and low calcium contents the coexistence of contrasting magmas mafic high ca silica poor versus low ca silica rich within a number of arc systems and their mixing may not be a rare event and should be taken into account when developing models of arc petrogenesis

the petrochemistry of ophiolite gabbroic complexes a key for the classification of ophiolites into low ti and high ti types ophiolites have been divided into two groups high ti and low ti types these can be discriminated by studying the fractionation trends of both gabbroic complexes this work and lavas and dykes 16 particularly in the tio2 m i diagram the first type typically shows morb like magmas whereas in the second the magma types have a spectrum of composition from mid ocean ridge basalts to island arc tholeiites and boninite like magmas often occur high ti ophiolites are petrologically and geochemically similar to major oceanic and ensialic back arc basin crusts as well as oceanic crust generated during the intermediate and late stage opening of intraoceanic back arc basins parental magmas and fractionation processes of low ti ophiolites fit with an hypothesis of their formation in the early stage of opening of intraoceanic back arc basins

olivine xenocrysts in picritic magmas experimental partial melting of plagioclase lherzolite at 0 7 mpa confining pressure has produced euhedral olivine crystals by corrosion and overgrowth during cooling the particular conditions of this experiment allow observation of both processes recorded in the crystal shape corrosion boundaries are rounded or straight when parallel to 001 intersection of 110 planes 010 and 110 facets are developed by fast overgrowth during quenching these observations support the contention that phenocrysts in basaltic or picritic magmas are in part xenocrysts the possible mantle origin of olivine crystals in two natural occurrences of ultramafic magmas the picritic pillow lavas of the troodos and a wehrlitic intrusion of the oman ophiolite is investigated in both cases discriminant characteristics are deduced from detailed microstructural study the mantle origin of olivine megacrysts in the investigated picrites raises the question about the existence of picritic magmas in the mantle

superposed hercynian and cadomian orogenic cycles in the ossa morena zone and related areas of the iberian massif polyphase ductile deformation and metamorphism in the ossa morena zone of the iberian massif are related to a complex evolution including two major tectonothermal episodes of cadomian late proterozoic to early cambrian and hercynian age middle to late paleozoic petrological structural and geochronological data attest to a number of tectonic episodes in the 620 480 ma interval the model presented for this orogeny comprises stages of volcanic arc generation backarc extension tectonic inversion crustal thickening and cratonization in an andean type continental margin the hercynian cycle began in the early paleozoic with episodes of continental rifting orogenic events occurred in the 390 300 ma interval and affected both the cadomian basement which was heterogeneously reworked and overprinted and the paleozoic cover hercynian regional metamorphism was generally of low grade high pressure assemblages and thermal domes developed locally and were related to subduction and late extensional events the geodynamic model proposed for the cadomian and hercynian orogenies and the correlation with comparable areas from pre mesozoic massifs elsewhere in western europe account for the geodynamic scenarios that drove the fate of cadomian terranes

significance of graphite occurrences in the aracena metamorphic belt iberian massif the significance of syngenetic and epigenetic graphite occurrences from the variscan high temperature low pressure aracena metamorphic belt is discussed in the framework of the tectono thermal evolution of this southern zone of the iberian massif syngenetic graphite is associated with both low to medium grade metamorphic rocks la umbria series precambrian in age and high grade granulite facies rocks the precambrian fuente del oro series and a cambrian calc silicate series epigenetic fluid deposited occurrences correspond to overgrowths on existing metamorphic graphite grains and vein type mineralization two types of graphitized particles with remarkable differences in reflectance anisotropy and size can be distinguished in the precambrian metapelites of the la umbria series large 150 m length platy crystals with high reflectance and anisotropy are interpreted as detrital and are considered indirect evidence of an old orogenic cycle prior to the cadomian orogeny during which metamorphism exceeded greenschist facies the coexistence of two types of particles explains the scattering of values of the c parameter of graphite determined by xrd c 6 72 6 74 a and the anomalously high temperatures of the dta exothermic peak close to 600 c of graphite with respect to that inferred from mineral assemblages in these rocks the presence of graphite rich quartzites and gneisses within the fuente del oro series and the calc silicate series is evidence of sedimentation under reducing conditions in a continental shelf the characteristics of graphite reflect the high grade metamorphic conditions attained in the southern area of the aracena metamorphic belt pervasive flow of fluids related to a major variscan extensional event resulted in overgrowths on the pre existing graphite in the gneisses and quartzites of the calc silicate series as evidenced by the heterogeneous isotopic composition of graphite single crystals in these rocks a later stage of graphite precipitation is represented by scarce vein type occurrences in mafic granulites that document channelled flow of fluids

tectonics of the akamas and mamonia ophiolites western cyprus magnetic petrofabrics and paleomagnetism abstract the akamas ophiolite is shown to be a distal off axis extension of the main outcrop of cretaceous ophiolite in the troodos complex of cyprus mantle sequence harzburgites of both ophiolites share similarly oriented mantle flow fabrics and the same tertiary magnetizations acquired during exhumation however compared with the troodos mantle sequence rocks the akamas ferromagnetic mineralogy is more oxidized and remanences with lower blocking temperatures were acquired chemically paleopoles calculated from published vectors and our own new data define an apparent polar wander path apwp for the troodos microplate the apwp shows that between 88 and 50 ma the troodos microplate was equatorial and the vertical axis for its 60 anticlockwise rotation was located within the microplate subsequently the microplate drifted northward to 34 n with minor anticlockwise rotation at a reduced rate that requires microplate rotation about a vertical axis located to the west of cyprus in the last 50 ma the allochthonous triassic mamonia terrane docked with the cretaceous troodos terrane in sw cyprus within it disrupted tectonized ophiolite has been regarded as part of a triassic ocean floor or as sheared fragments of cretaceous troodos ophiolite incorporated into the mamonia terrane when it docked with the troodos terrane whatever their provenance their paleomagnetic signals postdate their penetrative deformation and metamorphism and their paleopoles may still be used to track their post strain motion our calculations of paleopoles from published vectors for the mamonia terrane smear along an extension of the apwp for the troodos microplate that is moreover concentric with the troodos microplate this suggests that the paleopole dispersion of the triassic mamonia rocks and their post magnetization disruption occurred during their accretion onto the anticlockwise spinning troodos microplate

volcanic glass compositions of the troodos ophiolite cyprus fresh volcanic glass is preserved throughout the extrusive section of the troodos ophiolite which indicates that the lavas have not been pervasively metamorphosed glass compositions reveal the existence of two major magma suites apparently corresponding to distinct stratigraphic intervals the basal 400 500 m of the sequence consists of an andesite dacite rhyolite assemblage containing abundant hyaloclastites the remainder of the section comprises a basalt basaltic andesite assemblage with high mgo and low tio 2 and total iron the lower sequence is interpreted as an evolved arc tholeiite suite the upper has some similarities to boninitic lavas the close association in time and space of these two suites is similar to that observed in the mariana and bonin arcs and suggests that all of the troodos lavas were erupted in a subduction zone environment most probably in an incipient arc or fore arc

chemical transects across intra oceanic arcs implications for the tectonic setting of ophiolites systematic spatial variations in ti y and zr are found in volcanic sequences sampled as transects from back arc through arc to trench within the western pacific intra oceanic arc systems these correlations occur regardless of eruption age reflecting the retention of relative positions between individual tectonic units concentrations of these incompatible elements are taken to reflect the nature of the underlying mantle at a particular trench volcano distance during the subduction process volcanics become more depleted trenchwards implying that progressively shallower levels melts were extracted from more refractory or harzburgitic mantle a model is proposed whereby a subduction system is superimposed on a compositionally stratified mantle which is a product of an earlier spreading event on the basis of this evidence the trench distance variation model is applied to the troodos ophiolite complex similar spatial changes in lava chemistry are found across the ophiolite providing evidence for the relative position of the troodos trench and direction of subduction beneath the troodos basin these findings have implications for studies in other orogenic terrains where structure and exposure permit their application

some features of aftershock patterns in greece summary a quantitative study of 12 aftershock sequences which occurred in greece and the surrounding regions from 1971 to 1980 has been made the aim of the study was to investigate the temporal pattern of each aftershock sequence and to examine the possibility of predicting the largest aftershock the magnitudes of the main shocks were ml 5 0 the minimum and maximum numbers of aftershocks were 25 and 191 respectively the temporal analysis was performed using the modified omori formula and akaike information criterion first we examine the omori law to see if holds for the whole aftershock sequence second we examine the same data set in order to find out if there is any possibility of distinguishing two aftershock sequences as follows 1 the first must include all the shocks since the occurrence of the main event until the occurrence of the largest aftershock and 2 the second must include all events following the occurrence of the largest aftershock until the end of the aftershock activity in 10 cases out of 12 the model of two different aftershock sequences provides the best fit to the data the aftershock activity prior to the large aftershocks was then examined although the main shocks occurred in different seismogenic regions an almost identical behaviour of aftershock activity was observed the activity shows an appreciable decrease from the level expected from the modified omori formula also in some cases the aftershock activity recovers to a normal level or increases beyond it prior to the occurrence of the main aftershock these results seem to be independent of the magnitude of the large aftershock if the aftershock activity is monitored in real time this observed temporal pattern may be useful in predicting a large aftershock

radiometric discrimination of pre variscan amphibolites in the ediacaran serie negra ossa morena zone sw iberia new results on the geochronology of metabasites hosted by the neoproterozoic late ediacaran serie negra of the ossa morena zone iberian massif are presented these rocks record a protracted subduction at least since the latest cryogenian followed in a continuum by early cambrian rifting the serie negra country rocks are continental crust forearc segments of the thinned continental margin of gondwana the metabasite suite exhibits e and n morb as well as volcanic arc signatures and discloses the existence of a diverse late ediacaran early cambrian magmatism across the ossa morena zone protoliths of the e morb metabasites are clearly ediacaran they exhibit geochemical characteristics typical of younger island arcs notably of their forearc zones affected by extension soon after the initiation of subduction radiometric dating of n morb metabasites shows that these rocks are significatively younger than their country rocks whose age has been stratigraphically and paleontologically constrained as ediacaran they record early cambrian rifting developed in a continuum after a protracted subduction metabasite petrological zonation permits to reconstruct a single ediacaran subduction zone located to the s of the current omz and dipping towards the n in present day geographical coordinates

age constraints to the relationships between magmatism metamorphism and tectonism in the aracena metamorphic belt southern spain the aracena metamorphic belt amb southwest iberian peninsula is characterized by the following geological elements a a high temperature low pressure ht lp metamorphic belt a few kilometres wide and more than 200 km long b a linear belt of oceanic amphibolites with a low pressure inverted metamorphic gradient c crustal scale ductile shear zones and d mafic noritic intrusions of high mg andesite boninite composition the relationships between these elements led to the proposal of a model of ridge subduction for this sector of the hercynian belt of europe this interpretation is supported by the age relationships displayed between the main rock units considered representative of the main tectonic and petrological processes responsible for the geological elements mentioned previously the results of a geochronological study ar ar rb sr and sm nd clearly support a late paleozoic tectonic evolution at an active continental margin the time evolution of the metamorphism in the oceanic domain ranging from 342 6 0 6 ma in the west to 328 4 1 2 ma in the east over a distance of 70 km along the metamorphic belt support a tectonic model of triple junction migration responsible for the creation at depth of a slab free window with decisive consequences for the thermal evolution of the region the origin of the linear metamorphic belt of ht lp regime may be explained by the migration along a continental margin of a punctual thermal anomaly induced by the creation of a triple junction at the continental margin

distribution and origin of igneous rocks from the landward slopes of the mariana trench implications for its structure and evolution the landward slope of the mariana trench is composed largely of igneous rocks serpentinites and serpentinized ultramafic rocks occur at nearly all structural levels on the slope from depths of 8000 to 1200 m seamountlike features on the trench slope break are the surface expression of serpentinite diapirs cumulate and massive gabbros are found several varieties of volcanic rocks are common including boninites altered and metamorphosed basalts andesites and dacites the chemical characteristics of the volcanic rocks indicate that nearly all are products of island arc volcanism together with the gabbros these volcanic rocks represent what is probably a late eocene arc complex these rocks were probably the first volcanic products to result from the subduction of the pacific plate beneath the phillippine sea plate their exposure on the trench slope today implies a significant amount of tectonic erosion of the landward slope since eocene time most of this removal of material appears to have occurred during the early stages of subduction there are isolated occurrences on the landward slope of rock assemblages including alkalic basalts chert hyaloclastites upper cretaceous siliceous sediments and shallow water limestones these assemblages are very similar to rocks dredged from seamounts on the offshore flankmore of the trench and their presence on the landward slope suggests that since the cessation of vigorous tectonic erosion there has been episodic accretion of seamount fragments to the landward slope less

p t paths from garnet zoning a new technique for deciphering tectonic processes in crystalline terranes a procedure to calculate quantitative rock pressure temperature p t paths based on chemical zoning profiles in garnet combined with mineral chemistry from the other minerals in the assemblage has been applied to samples from diverse tectonic settings with the result that the calculated p t path is shown to be a sensitive monitor of tectonic processes terranes where metamorphic recrystallization paths are controlled by uplift and erosion show heating during decompression in the prograde p t path and cooling during decompression in the retrograde path in nappe terranes where hot rocks are emplaced over cool rocks the upper plate shows cooling during decompression whereas the lower plate shows heating during compression in nappe terranes where cool rocks are emplaced over hot the lower plate shows cooling during compression calculation of p t paths from mineral zoning should provide a powerful new tool for deciphering tectonic processes in crystalline terranes

tectonometamorphic evolution of the aracena metamorphic belt sw spain resulting from ridge trench interaction during variscan plate convergence 1 the aracena metamorphic belt is a high temperature low pressure ht lp band located at the southern end of the european variscan chain it marks a suture between armorica and avalonia this belt is characterized by 1 the presence of mid ocean ridge basalt derived metabasites that were affected by an inverted ht lp metamorphism related to sw verging thrusting whose thermal peak diachronously migrated eastward 2 the occurrence of a ht lp metamorphism related to an extensional event affecting continental rocks belonging to the ossa morena zone with peak temperatures that are 150 c higher than that recorded in the oceanic metabasites and 3 the presence of near trench magmatism with high mg andesite composition these and other characteristics can be interpreted in terms of a ridge trench trench triple junction that migrated along the armorican margin during variscan oblique convergence

geochemistry of cumulus peridotites and gabbros from the marum ophiolite complex northern papua new guinea the layered cumulus rocks of the marum ophiolite complex in northern papua new guinea range from highly magnesian dunite wehrlite and lherzolite through pyroxenite to norite gabbro with minor anorthosite and ferronorite gabbro near the top of the sequence most of the cumulates particularly the gabbroic rocks are characterised by recrystallised adcumulus textures and all intercumulus melt mesostasis has been expelled trends in the cumulate sequence from mg rich to more fe ca and al rich compositions are consistent with the formation of the layered sequence by magmatic accumulation from mafic tholeiitic magmas with varying degrees of differentiation the cumulates are characterised by extremely low levels of incompatible elements k ba rb p zr nb hf y and ree at all levels of differentiation ree patterns are strongly depleted in lree hree abundances range from 0 3 chondrites in peridotite to 3 x chondrites in the norite gabbros

correlation between seismic anisotropy and major geological structures in sw iberia a case study on continental lithosphere deformation in this paper we postulate a correlation between the lithospheric structure and seismic anisotropy inferred from geophysical data and the major structures and tectonic history past tectonothermal features of the outcropping basement rocks in sw iberia oblique plate convergence orogenesis and terrane accretion have been documented for pre mesozoic times in this area however the deviations and areal variations observed between the azimuthal anisotropy of seismic waves and the structural trends of the accessible crust indicate that the tectonic model of orogen parallel lithospheric deformation does not appear to succeed in relating hercynian surface geology and the structural geology of the subcontinental lithospheric mantle the model of polyorogenic lithosphere deformation proposed accounts for this divergence and is supported from additional perspectives including the manifestation of thermal effects induced in the crust by lithospheric mantle shear zones the incidence of crust mantle coupling and or decoupling and the scales of subcrustal heterogeneity

evidence for forearc seafloor spreading from the betts cove ophiolite newfoundland oceanic crust of boninitic affinity abstract the ordovician betts cove ophiolite of newfoundland has a well developed cumulate sequence in which is rooted a sheeted dyke complex that grades up into pillow lavas dominant chromite olivine orthopyroxene cumulate peridotites and orthopyroxenites have phase assemblages and mineral chemistries consistent with crystallization from boninitic magmas dykes and lavas have phenocrysts of olivine high cr al chromite orthopyroxene low tio2 clinopyroxene they have high sio2 and mgo contents and depleted u shaped trace element profiles indistinguishable from those of bonin islands boninites field data imply that cumulates dykes and lavas all are comagmatic while geochemical and mineralogical data indicate that all are of boninitic affinity since boninites are only found in forearcs this implies that the betts cove ophiolitic crust formed in a forearc since the entire oceanic crustal section at betts cove is of boninitic affinity then this implies that true seafloor spreading can initiate in forearc

nd sr isotopic study of primitive lavas from the troodos ophiolite cyprus evidence for a subduction related setting primitive lavas from the troodos ophiolite have a large variation in initial 143 nd 144 nd ratios from nd 7 9 to 0 8 which precludes a mid ocean ridge origin initial 87 sr 86 sr ratios vary from 0 70415 to 0 70705 the range of nd values may be accounted for by a two component mixing model whereby variable amounts of a light rare earth element ree enriched component with nd between 5 and 7 has interacted with a depleted iherzolite source with nd 8 this is consistent with an arc related setting where fluids derived from a subducting slab induce further melting in an overlying iherzolite source

the lyngen gabbro the lower crust of an ordovician incipient arc we present evidence for the origin of the lyngen gabbro of the ordovician lyngen magmatic complex in troms northern norway the two magmatic suites of the lyngen gabbro strike parallel nne ssw and have distinct magmatic signatures we define these signatures by using major and trace element analyses together with selected major and trace element mineral analyses and 143nd 144nd isotope whole rock analyses of gabbroic to tonalitic plutonic rocks from seven detailed cross sections from this large gabbro complex the western suite of the lyngen gabbro precipitated from magma that may have been derived from the same system as the associated volcanic rocks the gabbros have high an content an 90 of their plagioclases relative to co existing mafic minerals together with somewhat high nd t values 6 this implies that the parental magmas were hydrous tholeiites similar to those found in back arc basins today the eastern suite on the other hand consist of cumulates that were precipitated from melts resembling those of ultra depleted high ca boninitic magmas found in fore arcs extremely high an plagioclases an 95 co exist with evolved mafic minerals and oxides and the nd t values are lower 4 than in the western suite the eastern suite has no volcanic counterpart but dikes intersecting the suites have compositions that possibly represent its parental magma the oceanic rypdalen shear zone generally separates the two suites in the north but several non tectonic transitions from boninitic to tholeiitic signatures southwards advocate that the magmatism happened concurrently the magmatic proximity between the suites the hydrous magmatism and the absence of a silicic or calc alkaline mature arc section suggests that the lyngen gabbro formed in the iapetus ocean under conditions presently found in incipient arcs later emplaced as outer arc highs

rare earth element geochemistry of the betts cove ophiolite newfoundland complexities in ophiolite formation the betts cove ophiolite includes the components of typical ocean crust pillow lavas sheeted dikes gabbros and ultramafics however the trace element geochemistry of basaltic rocks is unusual three geochemical units are recognized within the lava and dike members within the pillow lavas the geochemical units correspond to stratigraphic units upper lavas have normal i e typical for ocean floor basalts tio2 contents 0 75 to 2 0 wt heavy rare earth elements hree values in the range 6 20 chondrites and chondrite normalized ree patterns with relative lree depletion intermediate lavas have tio2 contents between 0 30 and 0 50 wt hree contents from 4 7 chondrites and extreme relative lree depletion lower lavas have anomalously low tio2 contents 0 30 wt and unusual convex downwards ree patterns with ree abundances around 2 5 chondrite these geochemical differences can be explained if the three groups were derived from different mantle sources independent mantle sources for the three units are consistent with their different 143nd144nd ratios varying at 480 m y b p from 0 51222 in a lower lava to 0 51238 in an upper lava the upper lavas may be partial melts of a source similar in composition to that of modern morb the intermediate lavas may be from a very depleted oceanic mantle second stage melt and the lower lavas may have formed by melting an extremely depleted mantle that had been invaded by a lree enriched fluid a possible tectonic environment where these different sources could be juxtaposed is a back arc or inter arc basin

variscan shear zone deformation of late precambrian basement in sw iberia implications for circum atlantic pre mesozoic tectonics abstract polyphase deformation and metamorphism in the badajoz cordoba shear belt sw spain relate to a poly orogenic evolution involving two major tectonothermal episodes the accretion of mid and lower crust tectonic slices in an orogenic wedge complex during pre variscan times and the development of an intracontinental ductile shear zone during the late palaeozoic structural analysis of zones with different intensities of shear dominated variscan deformation allows a recognition of the poly deformed nature of most units of the belt identification of varying deformation mechanisms and a description of the geometry of early structures the implications for the regional tectonics of this area of the southern iberian massif are discussed in the light of the geodynamic evolution of the ibero armorican arc during the variscan orogeny and the reconstruction of the late precambrian cadomian avalonian panafrican orogen of the circum atlantic region

structural and metamorphic evolution of the almaden de la plata core seville spain in relation to syn metamorphic shear between the ossa morena and south portuguese zones of the iberian variscan fold belt abstract the almaden de la plata core is a tectonic unit situated in the southern branch of the iberian variscan fold belt it has traditionally been regarded as an eastern extension of the aracena metamorphic belt which is located at the south southeast boundary of the ossa morena with the south portguese zone low pressure high temperature parageneses gt crd sill are found in the highest grade areas while a metamorphic gradient of medium pressure type gt sta bearing schists and gneisses is observed for the middle stages of the prograding metamorphic evolution later processes indicate an evolution to the very low grade locally retrograding previous parageneses five main tectonic events are observed d 1 oriented inclusions within cordierite and garnet and d 2 are not present in the surrounding area d 2 and d 3 have a blastomylonitic high grade metamorphism and mylonitic low grade character respectively d 4 is responsible for the present boundaries of the almaden de la plata core while d 5 corresponds to late hercynian fractures events 1 4 book place under high to low and very low metamorphic grades the metamorphic climax was reached pre to syn d 2 the metabasic rocks acebuches amphibolites show medium to high grade associations hbl plag cpx with a well developed n120 e mineral lineation these features change towards the contact with the south portuguese materials of the pulo do lobo group where metabasites are transformed into very fine grained amphibolitic schists greenschist associations with actinolite and relicts of hornblende and clinopyroxene the later are mylonitic rocks with n150 e stretching lineations and s c structures as well as sigmoidal pressure shadows indicating a southward vergence the low grade metabasites are interpreted as the bottom of a d 3 main thrust which put the ossa morena and south portuguese zones into contact these results support the hypothesis that the acebuches amphibolites of the aracena belt do not pass gradually into the pulo do lobo group but are thrust upon that unit

mid paleozoic initiation of the northern cordilleran marginal backarc basin geologic geochemical and neodymium isotope evidence from the oldest mafic magmatic rocks in the yukon tanana terrane finlayson lake district southeast yukon canada the fire lake formation of the yukon tanana terrane in the finlayson lake region yukon canada consists primarily of late devonian ca 365 360 ma mafic metavolcanic rocks and smaller volumes of mafic and ultramafic subvolcanic metamorphosed intrusions in this paper field geochemical and nd isotope attributes of these rocks are presented in an attempt to understand their tectonic setting the magmatic processes involved in their formation and their roles in cordil leran crustal growth the mafic rocks of the fire lake formation exhibit a wide diversity of geochemical signatures and are classified into seven chemically defined suites 1 back arc basin basalt 2 enriched mid oceanic ridge basalt e morb 3 oceanic island basalt oib 4 th rich oib 5 boninite 6 island arc tholeiite and 7 light rare earth element lree enriched island arc tholeiite the diversity of geochemical signatures is interpreted to represent variable mixtures of asthenospheric morb type mantle subarc mantle wedge and lithospheric oib type mantle with or without elemental contributions from the subducted slab and or continental crust these suites of rocks are also associated with fine grained basinal sedimentary facies variations in metavolcanic and metasedimentary unit thickness extensional synvolcanic faults and apparent extensional fault controlled emplacement of mafic intrusive rocks and hydrothermal volcanic hosted massive sulfide mineralization the suites also exhibit a broad spatial distribution those with arc signatures nb thmn 1 mn normalized to primitive mantle values are located primarily in the western parts of the formation and suites with nonarc signatures nb thmn 1 are located primarily within the eastern parts of the formation collectively these geologic and geochemical attributes are interpreted to stem from the transition from arc magmatism to the initiation of an extensional backarc basinal environment associated with an east dipping subduction zone the initiation of backarc basin magmatism recorded in the fire lake formation was part of a much larger late devonian backarc basinal system forming along the western edge of the margin of north america the fire lake formation is interpreted to represent 1 the commencement of yukon tanana arc rifting and separation from the north american cratonic margin and 2 the initiation of a marginal backarc basin now the slide mountain terrane inboard of the yukon tanana arc system this tectonic evolution likely occurred either as a result of slab rollback toward the west within the convergent margin responsible for yukon tanana arc activity or as a result of the propagation of the slide mountain backarc basin spreading ridges into the yukon tanana arc system this yukon tanana arc rifting episode was also broadly coincident with rifting and hydrothermal activity within rocks of the north american craton the geochemical and isotopic signatures of magmatic rocks in the fire lake formation have some features similar to intraoceanic arc rocks e g boninites island arc tholei ites and many have juvenile nd isotope signatures i e nd t 0 most have nd t 5 suggesting that the pericratonic terranes of the northern cordillera have a significant juvenile component if this is the case throughout the yukon tanana terrane then the pericratonic terranes may have contributed much more juvenile material to cordil leran crustal growth in the phanerozoic than has previously been considered

neoproterozoic high mg basalts formed by melting of ambient mantle in south china abstract the southeastern yangtze block south china contains voluminous late neoproterozoic felsic plutonic rocks and minor mafic ultramafic volcanic rocks some ca 830 ma lavas hosted in the neoproterozoic strata have pillow structures and micro spinifex textures and thus previously were considered as komatiitic basalts derived from a mantle plume these rocks have high mgo 10 1 16 3 wt variable cao 6 57 11 2 wt restricted al2o3 11 2 13 4 wt and low tio2 0 50 0 61 wt p2o5 0 03 0 04 wt and incompatible trace elements they also have negative nd 5 4 to 2 9 and high 207pb 204pb 15 71 15 76 and initial 187os 188os ratios 0 1100 0 1931 indicating crustal contamination the most primitive samples show arc like trace elemental features which are characterized by enrichment of lree and lile relative to hfse in the primitive mantle normalized trace element diagrams suggesting derivation from enriched mantle sources primelt2 modeling shows that their primary magmas were produced by about 30 partial melting of a fertile peridotite source having a mantle potential temperature of 1440 1500 c such a potential temperature is in agreement with the estimates for ambient mantle during the neoproterozoic these evidence suggest that the high mg basalts are similar to boninites and probably be part of an ophiolite assemblage they were generated by melting of a hot ambient mantle source in relation to slab subduction during the amalgamation of the yangtze and cathaysia blocks

geochemistry and paleotectonic setting of ediacaran metabasites from the ossa morena zone sw iberia new results on the geochemistry of neoproterozoic late ediacaran metabasites of the ossa morena zone omz iberian massif are presented the metabasite suite exhibits n and e morb signatures as well as volcanic arc signatures the three amphibolite groups are discerned on the basis of major and trace element contents and the nd isotope relations help unravel the existence of a diverse magmatism during late ediacaran times across the omz n morb serie negra metabasites are distributed in the southern and mostly the central omz monesterio antiform calc alkaline metabasites are exclusively constrained to the coimbra cordoba sector of the northern omz finally e morb type metabasites are widespread across the entire omz in present day active geodynamic settings such magma types are typical of younger island arcs notably of forearc zones that are affected by extension soon after the initiation of subduction the new geochemical data permit us to postulate a petrological zonation congruent with a n dipping subduction zone located to the s of the current omz during the late ediacaran we argue that the metabasite host units represent continental shallow crustal forearc segments of a convergent margin the volcanic arc edifice would have developed during the latest ediacaran to early cambrian times and has been preserved often without major orogenic reworking in the central and northern omz

relative seismic quiescence before large aftershocks in terms of the modified omori s formula and akaike s information criterion thirteen aftershock sequences have been analysed quantitatively to check if any anomalous change of seismic activity took place before the occurrence of the largest aftershock the analysis follows the method proposed by the epicentre distribution of the main shocks covers the aegean sea greece area and the surrounding regions the earthquakes occurred from 1988 to 1998 although difficulties existed for a quantitative treatment of seismic quiescence the results of the temporal analysis of the aftershock activities seem to be promising in eight of thirteen cases the observed temporal pattern is similar the aftershock activity decreases significantly before the occurrence of the largest aftershock with respect to the level expected according to the modified omori s formula thereafter the activity recovers to its previous level just before or immediately after the occurrence of the main aftershock moreover an estimation of the reliability of the method is made based on the results of this study as well as of previous similar investigations from a total of 41 aftershock sequences which have been analysed following matsu ura s method 29 of them 71 show a remarkable relative quiescence before the occurrence of the main shock although it is difficult to construct a model which explains the precursory pattern the real time monitoring and check for seismic quiescence could at least contribute to a qualitative prediction of the largest aftershock which could at times be as large and disastrous as the main shock

amphibole plagioclase equilibria an empirical model for the relation albite tremolite edenite 4 quartz equilibria between plagioclase calcic amphibole and quartz can be described in part by the relation among mineral components naalsi3o8 ca2mg5si8o22 oh 2 naca2mg5alsi7o22 oh 2 4sio2 this relation governs the partitioning of na between plagioclase and the a site of coexisting amphibole data from natural amphibolites reveal that this partitioning is systematic and sensitive to metamorphic grade the ideal portion of the equilibrium constant kid xna a x a xab derived from natural samples is sensitive to bulk composition inasmuch as both plagioclase and amphibole are highly non ideal samples from a single outcrop have values ranging from 0 5 xab 0 74 to 4 1 xab 0 10 the continuous reaction naalsi3o8 ca2mg5si8o22 oh 2 naca2mg5alsi7o22 oh 2 4sio2 proceeds to the right with increasing grade of metamorphism and for a given bulk composition kid increases with increasing temperature two related discontinuous reactions actinolite albite hornblende oligoclase quartz and actinolite oligoclase hornblende anorthite quartz also proceed to the right with increasing metamorphic grade and result in changes in the topology of a phase diagram that describes the partitioning of na between plagioclase and amphibole a site a schreinemakers net is presented that is consistent with natural occurrences the results of this study should aid in the delineation of metamorphic facies within amphibolites

spheroidal textures in igneous rocks textural consequences of h2o saturation in basaltic melts abstract when melts reach h 2 o saturation before they are emplaced at the earth s surface they exsolve a hydrous fluid phase at elevated pressure and high temperature a hydrous fluid is not pure h 2 o but can contain some quantities of silicate and metal oxide solute notably if fluid saturation occurs at high pressure chemically and physically fluid saturated basalts behave like any other system with liquid immiscibility textures that can result after solidification are globules of former fluid dispersed in a former silicate melt or silicate melt orbs embedded in a former fluid matrix the troodos ophiolite on the island of cyprus generated in a back arc basin above a wet intra oceanic subduction zone carries in its magmatic stratigraphy lithologies distinctly orbicular and globular in shape many melt compositions parental to the ophiolite were h 2 o saturated near their liquidus temperatures before they reached their depths of emplacement within the ophiolite sequence spheroids are not uncommon in magmatic rocks but still debated as to how they form we propose that globular and orbicular textures in igneous worldwide may result when melts reach h 2 o saturation and exsolve an immiscible solute bearing h 2 o dominated fluid phase at elevated temperature and pressure before or while they are emplaced

background seismicity rates and precursory seismic quiescence imperial valley california the problem of recognizing precursory seismicity rate changes consists of two steps first a background or normal rate must be determined and second anomalous rates must be recognized in this paper we address the first of these steps determining a background seismicity rate two problems must be dealt with if a meaningful background rate is to be determined first dependent events must be recognized and removed from consideration and second detection or reporting changes must be accounted for a number of techniques are available for recognizing dependent events we used the one developed by mcnally 1976 in this work the technique we used for recgonizing detection and reporting changes was developed by habermann 1983 we demonstrate these techniques on seismicity from the imperial valley region of california after a reasonable background rate is determined we examine seismicity rates in this region prior to the imperial valley earthquake of 15 october 1979 the source area of the earthquake shows a possible precursory quiescence which started during july 1979 the time of this rate decrease is close to the time of a change in the seismic instrumentation in the imperial valley which makes it difficult to determine whether the decrease is real or related to detection several other seismicity rate changes observed in the imperial valley appear to be related to changes in the operation of the network there these include quiescent periods which occur only if magnitude cutoffs are applied to the data and periods of low reporting of events with m 2 5 these observations suggest that magnitudes were not reported for some events during several time periods all of these observations indicate that determination of detection histories is crucial for meaningful seismicity studies

spatial and temporal variability in crystallization of celadonites within the troodos ophiolite cyprus implications for low temperature alteration of the oceanic crust celadonite a low temperature 30 c hydrothermal alteration mineral is a predominant fracture and void filling phase within the volcanic rocks of the troodos ophiolite the combined chemical and structural properties of celadonite i e high k2o and ar retentive along with its common occurrence provide a valuable tracer for studying the temporal and spatial variability of low temperature hydrothermal fluid circulation and alteration within this ancient oceanic crust some 54 new k ar age determinations of celadonites from various geographic and stratigraphic locations within the extrusive rocks of troodos yield crystallization ages ranging from 90 9 1 0 to 49 8 0 5 ma with the oldest in close agreement with the estimated 91 92 ma crystallization age of troodos igneous rocks the youngest age indicates that low temperature water rock chemical exchange continued for at least 40 m y after crustal formation this represents a 100 increase over previous estimates based on limited numbers of k ar dates of celadonite of the duration of low temperature mineral precipitation in troodos correlation between celadonite ages and field relationships suggests the following 1 there is no apparent relationship between celadonite crystallization and stratigraphic depth 2 celadonite crystallization occurs homogeneously on an outcrop scale with adjacent samples from a single flow unit yielding equivalent k ar ages 3 flow units with relatively high primary permeabilities e g pillows and breccias yield celadonites with younger k ar ages 4 celadonite precipitation at distinct locations occurred rapidly with little or no age difference between rim and core sections of single large deposits the above correlations suggest that low temperature hydrothermal fluid circulation and secondary mineral precipitation are controlled by the local alteration conditions such as intrinsic permeability degree of fracturing and water rock ratios and are independent of time and space integration of these local heterogeneities over the entire extrusive sequence suggests that the upper oceanic crust undergoes relatively homogeneous closure to hydrothermal fluid circulation no evidence is found to support a progressive upward sealing of the oceanic crust

dynamic multifractality in earthquake time series insights from the corinth rift greece earthquake time series are widely used to characterize the main features of seismicity and to provide useful insights into the dynamics of the seismogenic system properties such as intermittency and non stationary clustering are common in earthquake time series such that multifractal concepts seem essential to describe the temporal clustering variability here we use a multifractal approach to study the time dynamics of the recent earthquake activity in the corinth rift the results indicate the degree of heterogeneous clustering and correlations acting at all time scales that suggest non poissonian behavior additionally the multifractal analysis in different time periods showed that the degree of multifractality exhibits strong variations with time which are associated with the dynamic evolution of the earthquake activity in the rift and the transition between periods of high and low seismicity

transition from aftershock to normal activity the 1965 rat islands earthquake aftershock sequence a transition from aftershock to normal activity is identified 2200 days after the occurrence of the 1965 rat islands earthquake of mw 8 7 after this time we observe a significant departure of the seismicity in the focal region from the extrapolated trend of the aftershock activity predicted by the modified omori formula proposed by utsu in 1961 the maximum likelihood estimate of the parameters for the formula and the selection of a statistical model based on the akaike information criterion show that the whole aftershock sequence is best modeled by the combination of an ordinary aftershock sequence and secondary aftershock activity associated with the largest aftershock fitting of the activity after the transition period to the modified omori formula results in an unusually low value of the decay constant we interpret that this activity represents normal or background seismicity and not another secondary aftershock sequence defining background activity in a focal region of a large earthquake is essential for detection of seismic quiescence prior to the next major event

source processes of the 1981 gulf of corinth earthquake sequence from body wave analysis abstract teleseismic long period body waves from the 24 february 1981 gulf of corinth earthquake and its two principal aftershocks of 25 february 02h35m and 4 march 21h58m 1981 are studied to determine source characteristics focal mechanisms along with observed surface fault breaks suggest that the corinth earthquake sequence represents normal faulting due to the n s trending extension depths of the three shocks estimated by matching synthetic seismograms to observations are found to lie between 4 and 12 km the azimuthal variation of observed body wave duration indicates that the main shock is a multiple event and that the main rupture occurred about 3 to 4 sec after a relatively small foreshock and propagated toward the w nw seismic moments deduced from the body wave synthetics are 8 1 10 25 2 7 10 25 and 2 2 10 25 dyne cm for the main 25 february and 4 march shocks respectively average final displacements and stress drops are estimated to be 37 cm and 10 bars for the main shock for a circular fault of radius 15 km 22 cm and 8 bars for the 25 february shock and 18 cm and 7 bars for the 4 march shock for circular faults of radius 11 km the striking features of the earthquake sequence are the low stress drops of the main shock and its two principal aftershocks and the clear eastward migration of aftershock activities the unusually long source time function rise times 4 sec for the main shock 2 5 sec for both aftershocks and low stress drops suggest an overall slow energy release during the earthquake sequence

seismic quiescence precursor to the 1983 kaoiki ms 6 6 hawaii earthquake a remarkable seismicity rate decrease of 65 per cent occurred in most of the aftershock volume 2 4 yr before the 1983 kaoiki ms 6 6 main shock if the background seismicity rate 1978 to mid 1981 is extrapolated to the time of the main shock more than 300 earthquakes of ml 1 8 are missing because of the quiescence subvolumes measuring 5 5 km on a side and located within the aftershock zone showed seismicity rate decreases ranging from 0 to 90 per cent the volume of no change had dimensions of approximately 10 3 km it contained the main shock hypocenter and was located near the center of the 10 km radius aftershock area the seismic quiescence in different subvolumes of the aftershock volume surrounding this central nonquiet zone varied somewhat in starting time amount of rate decrease and statistical significance according to the standard deviate z test the reported rate decrease is the most pronounced example of precursory quiescence defined to date the seismicity pattern before the 1983 kaoiki shock conformed to the quiescence hypothesis proposed on the basis of the precursors to the 1975 hawaii main shock no quiescence was observed within the immediate surrounding of the hypocenter while strong rate decreases occurred in most of the rest of the main shock source volume it is thus hypothesized that major asperities which contain high stress levels and from which main ruptures can emanate may be recognizable as volumes of constant seismicity rate within surrounding volumes of quiescence provided that decreases of seismicity rate as a function of time can be defined quantitatively

ftir spectrum of phenocryst olivine as an indicator of silica saturation in magmas fourier transform infrared ftir absorption spectra of hydroxyl were measured on olivine phenocrysts from hydrous basaltic melts that originated in island arc tectonic settings the basaltic melts encompass a wide range of silica activities from orthopyroxene saturated hypersthene normative to nepheline normative compositions the intensities and wavenumber placement of hydroxyl absorption bands correlate with the degree of silica saturation of the parent melt from which the olivine crystallized olivines from silica undersaturated nepheline normative melts absorb ir radiation in the wavenumber range 3430 3590 cm 1 group 1 in contrast olivines from orthopyroxene saturated boninitic melts exhibit hydroxyl absorption bands in the wavenumber range 3285 3380 cm 1 group 2 olivines crystallized at intermediate silica activities exhibit a combination of the two groups of hydroxyl ir bands where the proportion of group 2 bands increases with increasing silica saturation of the parent melt the positions of hydroxyl absorption peaks observed here for natural samples are consistent with previous measurements on experimentally annealed olivines thus protonation experiments can be employed to make spectroscopically dry olivine structures visible by ir yielding information on the silica saturation of the parental magmas hydroxyl concentrations in the studied olivines were estimated to be 1 2 ppm corresponding to an olivine melt partition coefficient of similar to 1 0 0 3 x 10 4

boninite petrogenesis and alteration history constraints from stable isotope compositions of boninites from cape vogel new caledonia and cyprus 18o values of unaltered olivine and pyroxene phenocrysts in boninites from several areas range from 5 8 to 7 4 and indicate that the source for most boninites is more 18o rich than morbs and other oceanic basalts the source for oxygen and other major elements is most likely a refractory portion of the mantle having a 18o value of up to 7 0 to which must be added a small amount of h2o rich fluid to induce partial melting this fluid which is derived from subducted crust is the vehicle for lrees including nd the variable normally low nd values typical of boninites do not correlate with the 18o values post eruptive exchange of oxygen in the glass of boninites with that of sea water at low temperatures 150 c produces 18o values of 10 in optically fresh glass hydration of the glass has increased the water contents of most boninites from estimated magmatic values of 1 2 wt to 2 4 wt and produced d values of 80 which may be lower than the original magmatic d values in contrast to most submarine pillow basalts the magmatic volatile composition of boninite lavas has been extensively modified as a result of post eruptive interaction with seawater

petrofabric of forsterite marbles and related rocks from a low pressure metamorphic terrain almaden de la plata massif ossa morena zone sw spain and its kinematic interpretation abstract a fabric study of forsterite marble and calc silicate rock paragenetic minerals is presented for samples collected in the almaden de la plata massif of sw spain that experienced variscan low pressure metamorphism related to the tectonic evolution of the suture between the ossa morena and south portuguese zones of the iberian massif two tectonically stacked metamorphic units contain marble metacarbonate and variably impure dolomitic rocks that record contrasting thermal conditions of the upper greenschist structuraly upper unit and the upper amphibolite granulite facies structuraly lower unit different reactional paths lead to formation of forsterite marbles in both units dolomite and calcite petrofabrics disclose their contrasting rheological behaviour during syn metamorphic ductile deformation under temperatures between 500 and 700 c dolomite formed harder porphyroclast systems and boudinaged bands whereas weaker calcite underwent dynamic recrystallization in both cases deformation was facilitated by activation of various intracrystalline slip systems complemented by other deformation mechanisms the fabrics of other paragenetic metamorphic minerals also denote crystal plasticity and exhibit close geometrical relationships with respect to the macroscopic foliation and mineral stretching lineation including a systematic asymmetry suggestive of non coaxial deformation components foliations and lineations are interpreted as solid state flow planes and directions bearing a kinematic significance in particular regional sinistral shear at mid deep crustal realms coeval with low pressure metamorphism explains the imbrication of metamorphic units and their exhumation concomitant with the overall wrench tectonic framework ruling the tectonic evolution of the ossa morena south portuguese suture between the late devonian 360 ma and the latest carboniferous 300 ma

origin of podiform chromite deposits by magma mingling abstract i have oversaturated the immiscible system sio2 al2o3 feo k2o with fecr2o4 to study the fractionation of chromite between conjugate siliceous and fayalitic melts although both melts are oversaturated with chromite chromite only nucleates in the fayalite rich melt where the crystal melt interfacial energy sl is lowest in less than three days at 1150 c all excess chromium in the bulk composition is concentrated as cumulus chromite in the fayalitic melt pools producing nodular cumulate textures as observed in podiform chromite ore when these results are applied to the lithospheric mantle podiform chromite deposits may form when droplets of olivine normative high pressure low viscosity melt mingle with siliceous low pressure more viscous melt in shallow magma conduits it is inferred from the experimental analogue that chromite only nucleates in the melt where sl is lowest which in the mantle would be the high pressure melt the low pressure melt only serves as the diffusive chromium reservoir the richest chromite ores form when the volume ratio of the melt nucleating chromite is small relative to the ambient melt

geochemistry of high mg andesites from cape vogel papua new guinea abstract a detailed geochemical investigation of high mg andesites from cape vogel papua new guinea shows that these rocks are chemically as well as mineralogically unusual they are nonetheless important to our understanding of the earth s mantle since they represent primary magmas possibly derived in an island arc environment two groups of high mg andesites are recognized at cape vogel type c characterized by a concave ree pattern la yb n and zr nb 35 and type e with la yb n of 3 7 and zr nb 19 the origin of the two groups and their unique trace element patterns is attributed to a complex series of enrichment events superimposed on a previously depleted mantle source in general the enrichment events do not seem similar to those represented in island arc volcanics while some similarities are noted for oceanic ridge and island settings a marked depletion in ti relative to zr and y amongst high mg andesites is a unique facet of their geochemistry trends in the major and trace element geochemistry of type e high mg andesites were modelled orthopyroxene fractionation and or accumulation can explain much of the variation although decoupling of major and selected trace elements is noted source heterogeneity may also be a factor the origin of these rocks and their tectonic setting remains enigmatic however enough geochemical and petrographic evidence is available to suggest they are distinct from most low ti basalts associated with ophiolites

petrogenesis of the early silurian dashuang high mg basalt andesite dacite in eastern south china origin from a palaeosubduction modified mantle the early paleozoic kwangsian event the first compressive orogeny after the rodinia assemblage in the south china block scb is characterized by the angular unconformity between the devonian and pre devonian sequences and widespread granitoids and small volume mafic rocks however the mechanism triggering this event is highly debated and the associated mantle source is poorly known our recent investigations identified the occurrence of early silurian high mg basalt andesite dacite in the dashuang area along the northwestern yunkai domain in the scb this study presents a set of new zircon u pb geochronological whole rock geochemical and sr nd isotopic data for these volcanic rocks they yield weighted mean 206 pb 238 u ages of c 430 443 ma similar to the age peaks of early middle paleozoic magmatism and metamorphism in the scb these volcanic rocks have high mgo cr and ni contents and mg numbers they also exhibit arc like geochemical affinities with crustal like sr nd isotopic compositions similar to the neoproterozoic high mg basalt andesite in the eastern scb such signatures combined with other geological observations suggest that the dashuang high mg rocks were derived from an ancient probably early neoproterozoic mantle wedge source the early paleozoic intra continental reactivation in response to late orogenic extension triggered the melting of this palaeosubduction modified mantle which led to the formation of these volcanic rocks

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

kinematic history of a reactivated oceanic suture the mamonia complex suture zone sw cyprus the mamonia complex suture zone in sw cyprus marks an arcuate tectonic suture between an upper cretaceous oceanic microplate of supra subduction zone origin troodos complex and a deformed triassic passive margin sequence mamonia complex mapping of the serpentinite filled fault zones that define the suture zone combined with microstructural studies of its constituent fault rocks has revealed evidence of three main tectonic events of upper cretaceous age 1 high temperature c 600 c transform related dextral strike slip and localized metamorphism of mamonia complex units c 90 83 ma 2 retrograde hydration c 83 73 ma 3 regional contractional reactivation at low temperatures which ceased by the latest maastrichtian c 65 ma the present day structural geometries are interpreted as the result of maastrichtian contractional reactivation of a pre existing steep and irregular structural architecture that formed during transtension the resultant orientation of finite strain axes and thrust displacements display a radial distribution documenting near fault normal contraction along the whole length of the arcuate suture zone this pattern of displacements suggests a model whereby the troodos microplate impinged on the mamonia complex during westerly to southwesterly directed plate motion contractionally reactivating the mechanically weak suture zone

analysis of relaxation temporal patterns in greece through the retas model approach the temporal decay of eight aftershock sequences in the area of greece after 1975 was examined with main shocks magnitudes of mw 6 6 the analysis was done through the restricted epidemic type aftershock sequence retas stochastic model which enables the possibility to recognize the prevailing clustering pattern of the relaxation process in the examined areas in four of the cases the analysis selected the epidemic type aftershock sequence etas model to offer the most appropriate depiction of the aftershock temporal distribution which presumes that all shocks to the smallest ones in the sample can cause secondary aftershocks while for the rest of the sequences triggering potential seems to have aftershocks above a certain magnitude threshold retas model and they are expected to induce secondary activity the models developed on aftershock data were also applied to forecast real seismicity after the conclusion of the aftershock sequences for four out of eight cases we obtained promising estimations of ensuing seismicity after the end of the sequences with models based only on aftershock data some features of the retas model simulation were also studied like simulating magnitudes revealing that it is reasonable to consider in the model the temporal behavior of the aftershocks magnitudes as well stochastic modeling was also applied to estimate the duration of the relaxation process assuming that the end of each sequence is marked by the divergence of real seismicity from the modified omori formula mof model the latter known to represent pure aftershock activity the obtained results give an indication that possibly low stressing rate results in longer duration of the relaxation process in a region 2007 elsevier b v all rights reserved

spinel compositions of mantle hosted chromitite from the eastern anatolian ophiolite body turkey implications for deep and shallow magmatic processes abstract the eastern anatolian accretionary complex includes several ophiolitic megablocks and or tectonic slivers within a 150 180 km long east west trending complex formed during the late cretaceous tertiary in eastern turkey the alabayir mollatopuz yukaribalcikli and mehmetalan ophiolites are megablocks or tectonic slivers containing locally massive nodular or schlieren banded chromitite layers these podiform chromitites formed in mantle sequences and are classified as high cr chromitites cr 0 63 0 88 mg 0 50 0 64 0 01 0 5 wt tio 2 5 7 18 8 wt al 2 o 3 calculated parental melt compositions of these chromitites indicate boninite magma characteristics 8 2 13 4 wt al 2 o 3 0 64 1 50 feo mgo the pge patterns also support the view that they crystallized from a boninitic melt the total pge contents of chromitites vary between 79 and 390 ppb pd ir ratios of chromitites vary from 0 07 to 0 28 and are consistent with an ipge fractionated nature the examined chromitites were divided into two groups in terms of their mineral chemistry group i alabayir mollatopuz yukaribalcikli and mehmetalan i chromitites group ii mehmetalan ii chromitites mineral chemistry of these chromitites and their parental melt composition indicate that group i chromitites were probably formed at shallow mantle depths and that group ii chromitites formed in deeper parts of the mantle mineral chemistry of these eastern anatolia high cr chromitites indicate that they formed not only in the deeper mantle but also in shallower parts of the moho transition zone