synorogenic melting of mafic lower crust constraints from geochronology petrology and sr nd pb and o isotope geochemistry of quartz diorites damara orogen namibia quartz diorites represent the earliest ca 540 ma and most primitive plutonic rocks in the pan african damara belt and they pre date the main phase of high t regional metamorphism two suites of synorogenic quartz diorites are unusual among damaran intrusive rocks in their elemental and isotopic features comparison of the diorite compositions with melts from amphibolite dehydration melting experiments points to a garnet bearing meta tholeiite probably enriched in k2o as a likely source rock partial melting processes generated mafic ca 50 wt sio2 quartz diorites in the deep crust at temperatures of between 1 000 and 1 100 c based on comparison with experimental results and similar temperature estimates based on p2o5 solubility in mafic rocks subsequently the quartz diorites evolved by multistage polybaric differentiation processes including fractional crystallization of mainly hornblende and plagioclase and assimilation of felsic basement gneisses although their chemical characteristics high lile low hfse resemble those of other quartz diorites with calc alkaline affinities they differ in their enriched sr initial 87sr 86sr 0 70943 0 71285 nd initial e nd 9 1 to 15 2 and o 18o 6 8 8 1 isotope compositions neodymium model ages tdm that range from 1 7 to 2 2 ga and large variation in 207pb 204pb relative to 206pb 204pb indicates involvement of ancient crustal material lead 206pb 204pb 17 08 17 23 207pb 204pb 15 53 15 62 208pb 204pb 37 71 38 16 isotope compositions are strongly retarded indicating that the source underwent a pre pan african u pb fractionation and u depletion it is proposed that the quartz diorites originated by synorogenic high temperature melting of mafic lower crust this contrasts with previous suggestions favouring an origin of these rocks by melting of an enriched mantle during pan african times with characteristics modified by subduction of oceanic crust and sedimentary rocks

calcic amphibole equilibria and a new amphibole plagioclase geothermometer there is currently a dearth of reliable thermobarometers for many hornblende and plagioclase bearing rocks such as granitoids and amphibolites a semi empirical thermodynamic evaluation of the available experimental data on amphibole plagioclase assemblages leads to a new thermometer based on the aliv content of amphibole coexisting with plagioclase in silica saturated rocks the principal exchange vector in amphiboles as a function of temperature in both the natural and experimental studies is left na square 1 right a left alsi 1 right t1 we have analysed the data using 3 different amphibole activity models to calibrate the thermometer reactions 1 edenite 4 quartz tremolite albite 1 2 pargasite 4 quartz hornblende albite 2 the equilibrium relation for both 1 and 2 leads to the proposed new thermometer t frac 0 677p 48 98 y 0 0429 0 008314 ln k and k left frac si 4 8 si right x ab plag where si is the number of atoms per formula unit in amphiboles with p in kbar and t in k the term y represents plagioclase non ideality rtln ab from darken s quadratic formalism dqf with y 0 for xab 0 5 and y 8 06 25 5 1 xab 2 for xab 0 5 the best fits to the data were obtained by assuming complete coupling between al on the t1 site and na in the a site of amphibole and the standard deviation of residuals in the fit is 38 c the thermometer is robust to ferric iron recalculation procedures from electron probe data and should yield temperatures of equilibration for hornblende plagioclase assemblages with uncertainties of around 75 c for rocks equilibrated at temperatures in the range 500 1100 c the thermometer should only be used in this temperature range and for assemblages with plagioclase less calcic than an92 and with amphiboles containing less than 7 8 si atoms pfu good results have been attained on natural examples from greenschist to granulite facies metamorphic rocks as well as from a variety of mafic to acid intrusive and extrusive igneous rocks our analysis shows that the pressure dependence is poorly constrained and the equilibria are not suitable for barometry

the evolution of the southern menderes massif in sw turkey as revealed by zircon dating this geochronological study concentrates on the evolution of the southern part of the menderes massif a metamorphic core complex in w turkey by dating single zircons with the pb pb evaporation and the u pb methods we have analysed zircons from granitic augen gneisses of the core as well as from quartzites of the metasedimentary envelope zircons from the granitic core gneisses are euhedral with typical igneous morphologies the single zircon ages of these gneisses indicate a time span for magmatic activity from 520 to 570 ma with a major event at about 550 ma some gneisses contain older zircons of c 660 ma which are interpreted as inherited grains zircons from the quartzites are detrital as indicated by their rounded shapes and pitted surfaces their ages vary from 526 to 2094 ma these data partially coincide with the ages of the gneisses but also indicate an older source region for the sediments neither a tertiary origin nor metamorphic overgrowth is documented in the pb ages of the zircons the results suggest that the southern part of the menderes massif developed during a major magmatic episode in latest proterozoic times thus suggesting a relation with other pan african regions in n africa

experimental calibration of the aluminum in hornblende geobarometer with application to long valley caldera california volcanic rocks a new geobarometer based on the al content of igneous hornblendes in equilibrium with melt fluid biotite quartz sanidine plagioclase sphene and magnetite or ilmenite has been calibrated experimentally the calibration was performed by equilibrating the required phase assemblage over the pressure range 2 8 kbar at 740 780c and then analyzing euhedral hornblendes in equilibrium with glass melt experiments were performed on natural samples of both volcanic and plutonic rocks earlier empirical calibrations of this geobarometer relied on analyzing natural hornblendes from plutons with the required phase assemblage and inferring pressure from nearby metamorphic country rocks the experimental calibration differs from the empirical calibrations especially above 5 kbar and shows that the al content of hornblendes in equilibrium with the required phase assemblage is greater for a given total pressure than previously thought the geobarometers uncertainty is dramatically reduced the derived equation is p plus minus 0 5 kbar 3 minus 3 46 plus minus 0 24 4 23 plus minus 0 13 al sup t the geobarometer is applied to post bishop tuff volcanic rocks from long valley caldera california and reveals that most rhyodacites in this complex erupted from depths of about 6 km these eruptions occurred over 500 000 yr suggesting that the rhyodacitic magma reservoir beneath longmore valley had reached a steady p depth t state less

proto tethyan remnants in northwest iran geochemistry of the gneisses and metapelitic rocks abstract the iran continental crust was metamorphosed intruded by granitoid magmas folded and faulted during the late precambrian by the pan african orogeny the basement complex in the takab complex northwest iran consists of gneisses amphibolites pelitic schists meta ultramafic and calc silicate rocks geochemically the protoliths of the takab gneisses are slightly peraluminous and medium to high potassic with calc alkaline affinity these gneisses may have been emplaced in volcanic arc tectonic setting furthermore the metapelitic protolith is shale deposited in an active continental margin setting all these characteristics and presence of paleo suture zone and ophiolitic rocks i e serpentines meta mafic and meta ultramafic rocks around the high grade metamorphic rocks suggest that a continental margin magmatic arc andean type formed the takab precambrian basement the basement complexes are extensively overprinted by the pan african orogeny and younger igneous events this supports the inference that early cambrian orogenesis in the takab complex region of northwest iran marks one of the fundamental lithospheric boundaries within gondwana which belonged to a greater late neoproterozoic early paleozoic orogenic system that was active along the proto tethyan margin of the gondwana supercontinent extending at least from its arabian margin to the himalayan margin of the indian subcontinent

magmatic epidote and its petrologic significance epidote is a major magmatic mineral in tonalite and granodiorite in a belt coextensive with the mesozoic accreted terranes between northern california and southeastern alaska textural and chemical evidence indicates that epidote crystallized as a relatively late but magmatic mineral that formed through reaction with hornblende in the presence of a melt phase the observed relations concur with experimental data on crystallization of epidote from synthetic granodiorite at 8 kbar total pressure plutonic rocks bearing magmatic epidote must have formed under moderately high pressures corresponding to lower crustal depths under fairly oxidizing conditions

panafrican basement and mesozoic gabbro in the zagros orogenic belt in the dorud azna region nw iran laser ablation icp ms zircon ages and geochemistry abstract the dorud azna region of the sanandaj sirjan metamorphic zone ssmz in nw iran is part of the zagros orogenic belt and exposes an amphibolite grade metamorphic succession of mixed continental e g the granitic galeh doz orthogneiss and oceanic or rift e g various amphibolites origin which is intruded by the darijune gabbro laser ablation icp ms u pb zircon ages of 608 18 ma and 588 41 ma of the granitic galeh doz orthogneiss indicate a panafrican basement similar to that of the yazd block of central iran we therefore interpret this part of the ssmz to represent a previously subducted part of the panafrican basement of the iranian microcontinent based on geochemistry amphibolites and metagabbros from the amphibolite unit representially potentially the cover on the panafrican galeh doz orthogneiss are interpreted to have their origin in a likely upper paleozoic to mesozoic rift or e morb ophiolite succession one of the metagabbros here termed dare hedavand metagabbro yield a 206pb 238u mean age of 314 6 3 7 ma talc bearing greenschists to epidote amphibolites of the triassic june complex formed in a similar but younger environment furthermore the metamorphic complex is intruded by the darijune gabbro which yields a mean icp ms u pb zircon age of 170 2 3 1 ma the final stage of likely jurassic greenschist facies grade metamorphism postdates the gabbro intrusion we interpret the darijune gabbro to have resulted from the initial subduction of neotethyan oceanic lithosphere in a continental arc setting the new data combined with data from literature indicate that the ssmz represents a tectonic element of a mixed continental and oceanic origin likely accreted to central iran during a jurassic tectonic process the new data also constrains thick skinned nappe stacking within the sanandaj sirjan zone

origins and paleoceanographic significance of laminated muds from the ulleung basin east sea sea of japan abstract a detailed facies analysis of long piston core sediments from the ulleung basin reveals four mud facies laminated mud crudely laminated mud bioturbated mud and homogenous mud the laminated and homogeneous muds were most likely deposited from turbidity currents whereas the bioturbated and crudely laminated muds were formed by hemipelagic pelagic sedimentation under well oxygenated and poorly oxygenated bottom water conditions respectively a detailed analysis of vertical distribution of the mud facies together with a chronostratigraphic framework derived from correlative tephra layers with known eruption ages reveals paleoenvironmental changes during the late quaternary during the period between 49 and 23 ka eustatic lowering of sea level facilitated frequent slope failure resulting in repeated deposition of fine grained turbidites on the basin floor normal hemipelagic sediments were intensively bioturbated under well oxygenated bottom water conditions during the period between 23 and 13 to 11 ka deposition from turbidity currents continued but hemipelagic sediments were non bioturbated under poorly oxygenated bottom water conditions the bottom water deoxygenation is attributed to further lowering of sea level during the last glacial period and consequent intensified stratification in the water column by freshwater supply to the nearly isolated sea primary laminae preserved in the crudely laminated mud generally exhibit low compositional bimodality in backscattered electron images which suggests low surface primary productivity and presence of superficial micro bioturbation after rapid rise of sea level at 13 to 11 ka the slopes were stabilized and pelagic sedimentation has prevailed under well oxygenated bottom water conditions

persistent late pleistocene holocene seasonal upwelling and varves off the coast of california abstract seasonal production of siliceous phytoplankton and a seasonal flux of terrigenous clastic material produced varved sediments along the continental slope off california during the late pleistocene to mid holocene light colored sediment within laminations and in sediment trap samples was deposited during summer upwelling and contains an abundance of the diatoms skeletonema costatum and chaetoceros spp resting spores dark colored sediment deposited in the fall and winter contains abundant thalassiosira pacifica and has more terrigenous material distribution of diatoms in varves shows that seasonal upwelling has persisted along the california coast and has remained strongly seasonal since the late pleistocene

eastern turkish high plateau as a small turkic type orogen implications for post collisional crust forming processes in turkic type orogens abstract post collisional magmatism may be generated by extensive crustal melting in tibet type collisional environments or by falling out of slabs from under giant subduction accretion complexes in turkic type collisional orogens giving rise to decompression melting of the asthenospheric mantle replacing the removed oceanic lithosphere in turkic type post collisional magmatism the magmatic products are dominantly alkalic to peralkalic and greatly resemble those of extensional regions giving rise to much confusion especially in interpreting old collisional orogenic belts such magmatic regions are also host to a variety of economically valuable ore deposits including gold one place in the world where today active turkic type post collisional magmatism is present is the eastern anatolian high plateau produced after the terminal arabia eurasia collision in the late miocene the plateau is mostly underlain by the late cretaceous to oligocene east anatolian accretionary complex which formed south of the rhodope pontide magmatic arc this subduction accretion complex has been further shortening since the collision but it has also since been domed and became almost entirely covered by at least 15 000 km3 of volcanic rocks the volcanic rocks are calc alkalic in the north transitional in the middle and alkalic in the south of the plateau where the crust is thinnest today less than 38 km the volcanics are derived almost entirely from an enriched mantle the ages of the volcanics also become younger from north to south from about 11 ma to possibly 17th century ad we interpret the origin of the magmatic rocks as the result of decompression melting of the asthenospheric mantle sucked towards the exposed base of the east anatolian accretionary complex as the oceanic lithosphere beneath it fell out the lower density of the hot asthospheric material was the cause of the doming we believe that similar processes dominated the post collisional tectonics of such vast turkic type orogens as the altaids of central asia the late devonian early carboniferous lachlan belt in southeastern australia and the pan african collage of northeast afro arabia it is likely that archaean collisions were dominated by turkic type post collisional events rather than tibetan ones that only became common in the proterozoic

clinopyroxene composition of ophiolite basalts as petrogenetic indicator abstract a statistical microprobe study has been carried out on augitic clinopyroxene in high ti to low ti metabasalts from various phanerozoic ophiolites a comparison has been made with clinopyroxenes from mid ocean ridge basalts morb oceanic intraplate basalts island arc tholeiites and boninites leaving aside the effect of cooling rate and of fractional crystallization as major factors influencing pyroxene chemistry their composition appears to be essentially related to the type of host magma both the tio2 and the tio 2 al 2 o 3 in clinopyroxenes are positively correlated with the zr y ratios of the host basalts which is a diagnostic parameter insensitive to alteration increasing regularly from low ti to high ti ophiolitic basalts ti also shows a positive correlation and average d ti cpx rock partition coefficients varying from 0 3 0 6 to 1 from low ti to high ti basalts which compare favorably with those from tongan southwest pacific basaltic andesites 0 3 0 6 and morb 0 75 respectively likewise the average k fe mg calculated for clinopyroxene rock pairs varies from 0 20 in low ti basalts to 0 26 in high ti basalts up to 0 33 in the highest ti rocks suggesting a significant dependence of this parameter on magma composition the compositional correspondence of clinopyroxenes from various ophiolitic basalts with those from oceanic actualistic equivalents is shown in various diagrams concomitantly with increasing tio2 and na2o and decreasing sio2 contents clinopyroxenes from very low ti to high ti ophiolitic basalts are best equated with those from boninites basaltic andesites and tholeiites from island arcs and morb respectively accordingly a parallel increase is shown by the substitutional molecules catial2o6 and natisialo6 within each basalt type fractional crystallization processes are reflected in the natisialo6 and fetial2o6 increase and mgcrsialo6 decrease from early to late crystallized clinopyroxenes

oceanographic control of diatom abundances and species distributions in surface sediments of the tropical and southeast atlantic abstract this paper is a descriptive account of the diatomaceous component of pelagic sediments from the tropical and southeast atlantic ocean geographic patterns of diatom abundances whole and fragment forms per gram major species relative abundances and diatom assemblages resolved by q mode factor analysis are presented comparisons drawn to sea surface temperature salinity and productivity distributions suggest that i diatom abundances primarily reflect surface productivity with no apparent dissolution effects due to water depth ii relative abundances of major species vary in their ecologic dependence but do reflect major oceanographic features of the surface waters and iii of the five diatom assemblages resolved four finely delineate the complexity of the low latitude oceanography these assemblages in decreasing order of statistical importance are associated with 1 warm waters of moderate to high primary productivity 2 the oceanward margin of the benguela current a region of high primary production with a maximum in austral summer 3 low salinity waters off the mouths of the zaire and niger rivers and 4 subtropical to tropical waters of lower productivity mostly in the western equatorial atlantic the fifth factor represents the equatorward advection of subantarctic to antarctic diatoms by deep ocean currents

the origin of brown hornblende in the artfjallet gabbro and dolerites brown hornblende occurs in minor amounts in the artfjallet gabbro and dolerites except in quartz dolerites where a pale green hornblende occurs in the gabbro brown hornblende is mostly ti bearing pargasite or kaersutite it occurs along veins of orthopyroxene as rims around and blebs in pyroxenes with orthopyroxene in coronas between olivine and plagioclase and in coronas between ilmenite and plagioclase in the olivine dolerites and orthopyroxene dolerites brown hornblende is ferroan titanian pargasite or ferroan kaersutite the pale green hornblende in the quartz dolerites is a magnesio hornblende the hornblendes in the dolerites are interstitial or granular in some dolerites occurring as coarse oikocrysts it is proposed that under certain conditions the ti content of hornblende can be used as a thermometer derived from experimental data of helz 1973 microstructures compositions and formation temperatures 1 040 c show that the brown hornblende in the gabbro is not magmatic but of subsolidus origin probably it formed as a result of the introduction of water into the gabbro during a deformation event that occurred early in the cooling history of the gabbro least squares modelling of hornblende formation indicates that all magmatic minerals must have participated in the reaction and that the reaction probably was not isochemical microstructures compositions and formation temperatures 1 030 965 c of brown hornblende in the dolerites are consistent with late stage crystallization from the magma for the pale green hornblende in the quartz dolerites a magmatic origin is likely but cannot be proven

boundary lines within petrologic diagrams which use oxides of major and minor elements use of some petrologic diagrams applied to analyses of volcanic rocks is unnecessarily difficult due to lack of data for construction of discriminant lines between rock series coordinates are provided for sufficient points to enable accurate plotting of the boundary lines within seven diagrams viz 1 tas total alkalies na2o k2o vs sio2 2 k2o vs sio2 3 afm 4 jensen 5 ktp k2o tio2 p2o5 6 fms feo mgo vs sio2 and 7 taktip k2o na2o k2o vs tio2p2o5 different versions of these boundaries are collated to indicate their variable position and it is demonstrated that inter laboratory analytical precision suffices to account for almost all of their spread on the tas and k2o vs sio2 diagrams

cenozoic evolution of neotethys and implications for the causes of plate motions africa north america eurasia plate circuit rotations combined with red sea rotations and new estimates of crustal shortening in iran define the cenozoic history of the neotethyan ocean between arabia and eurasia the new constraints indicate that arabia eurasia convergence has been fairly constant at 2 to 3 cm yr since 56 ma with slowing of africa eurasia motion to 1 cm yr near 25 ma coeval with the opening of the red sea ocean closure occurred no later than 10 ma and could have occurred prior to this time only if a large amount of continental lithosphere was subducted suggesting that slowing of africa significantly predated the arabia eurasia collision these kinematics imply that africa s disconnection with the negative buoyancy of the downgoing slab of lithosphere beneath southern eurasia slowed its motion the slow steady rate of northward subduction since 56 ma contrasts with strongly variable rates of magma production in the urumieh dokhtar arc implying magma production rate in continental arcs is not linked to subduction rate

biogenic sedimentation beneath the california current system for the past 30 kyr and its paleoceanographic significance a north south transect of 17 cores was constructed along the eastern boundary of the california current system from 33 to 42 n to investigate the changes in biogenic sedimentation over the past 30 kyr percentages and mass accumulation rates of caco3 corg and biogenic opal were assembled at 500 to 1000 years sample to provide relatively high resolution time space maps reveal a complex pattern of changes that do not follow a simple glacial interglacial two mode model biogenic sedimentation shows responses that are sometimes time transgressive and sometimes coeval and most of the responses show more consistency within a limited geographic area than any temporal consistency reconstructed conditions during late oxygen isotope stage 3 were more like early holocene conditions than any other time during the last 30 kyr coastal upwelling and productivity during oxygen isotope stage 3 were relatively strong along the central california margin but were weak along the northern california margin precipitation increased during the last glacial interval in the central california region and the waters of the southern california margin had relatively low productivity productivity on the southern oregon margin was relatively low at the beginning of the last glacial interval but by about 20 ka productivity in this area significantly increased this change suggests that the center of the divergence of the west wind drift shifted south at this time the end of the last glacial interval was characterized by increased productivity in the southern california margin and increased upwelling along the central california margin but upwelling remained weak along the northern california margin a sudden 300 years decrease in caco3 corg and biogenic opal occurred at 13 ka the changes suggest a major reorientation of the atmospheric circulation in the north pacific and western north america and the establishment of a strong seasonality in the central california region a carbonate preservation event occurred at 10 ka that appears to reflect the uptake of co2 by the terrestrial biosphere as the northern latitudes were reforested following retreat of the glaciers the holocene has been a period of relatively high productivity in the southern california margin relatively strong coastal upwelling along the central california margin relatively weak upwelling along the northern california margin and the northward migration of the divergence zone of the west wind drift

i type plutonism in a continental back arc setting miocene granitoids and monzonites from the central aegean sea greece miocene extension in the aegean sea was accompanied by the intrusion of granitoid and monzonitic magmas at upper crustal levels with increasing distance from the trench the metaluminous to slightly peraluminous i type intrusives show a systematic regional variation in their compositions medium to high k granodiorites in the wsw high k low silica granites in the central region and high k high silica granites monzonites monzogranites and composite lamprophyric monzonitic dikes in the ene the granodioritic and low silica granitic plutons comprise similar ranges of sio2 63 5 71 6 wt and initial nd sr and o isotopic values end i 7 0 to 9 8 87sr 86sr i 0 7092 0 7115 18o 9 7 11 5 but display significant differences in their chemical compositions compared with the granodiorites the low silica granites have higher abundances of large ion lithophile light rare earth and some high field strength elements initial isotopic signatures of the high silica granites in the ene end i 9 0 to 9 6 87sr 86sr i 0 7128 0 7144 18o 10 8 12 0 are slightly more crust like than those of the low silica granites in chemical variation diagrams samples from the high silica granites continue the trends defined by those from the low silica granites it is suggested that all granitoid magmas originated by dehydration melting of metaluminous older crustal sources most probably metagreywackes u pb systematics of zircon populations in conjunction with zr sio2 systematics and zircon saturation temperatures suggest a high proportion of restitic or assimilated crustal material in these magmas monzonitic to monzogranitic rocks show chemical and isotopic characteristics end i 0 8 to 3 6 87sr 86sr i 0 7059 0 7071 18o 9 4 10 6 that are significantly different from those of the granitoids a small portion of an older crustal component in these magmas is indicated by u pb systematics of zircon populations lamprophyric dikes display chemical and isotopic signatures end i 0 5 87sr 86sr i 0 7044 18o 8 9 that are compatible with an origin from either a mantle source previously enriched by slab derived hydrous fluids melts as suggested by negative nb ta anomalies or a new k rich basaltic underplate abundant composite dikes indicate the coexistence of and mingling mixing between lamprophyric melts and monzonitic magmas

geology and thermochronology of tertiary cordilleran style metamorphic core complexes in the saghand region of central iran an 100 km long north south belt of metamorphic core complexes is localized along the boundary between the yazd and tabas tectonic blocks of the central iranian micro continent between the towns of saghand and posht e badam amphibolite facies mylonitic gneisses are structurally overlain by east tilted supracrustal rocks including thick 1 km steeply dipping nonmarine siliciclastic and volcanic strata near the detachment the neybaz chatak fault the gneisses are generally overprinted by chlorite brecciation crosscutting relationships along with u pb zircon and 40 ar 39 ar age data indicate that migmatization mylonitic deformation volcanism and sedimentation all occurred in the middle eocene between ca 49 and 41 ma the westernmost portion of the tabas block immediately east of the complexes is an east tilted crustal section of neoproterozoic cambrian crystalline rocks and metasedi mentary strata 10 km thick the 40 ar 39 ar biotite ages of 150 160 ma from structurally deep parts of the section contrast with ages of 218 295 ma from shallower parts and suggest late jurassic tilting of the crustal section these results define three events 1 a late jurassic period of upper crustal cooling of the western tabas block that corresponds to regional jurassic cretaceous tectonism and erosion recorded by a strong angular unconformity below mid cretaceous strata throughout central iran 2 profound approximately east west middle eocene crustal extension plutonism and volcanism ca 44 40 ma and 3 2 3 km of early miocene ca 20 ma erosional exhumation of both core complex and tabas block assemblages at uppermost crustal levels resulting from significant north south shortening the discovery of these and other complexes within the mid tertiary magmatic arcs of iran demonstrates that cordilleran style core complexes are an important tectonic element in all major segments of the alpine himalayan orogenic system

sediment geochemical records of productivity and oxygen depletion along the margin of western north america during the past 60 000 years teleconnections with greenland ice and the cariaco basin abstract many sediment records from the margins of the californias alta and baja collected in water depths between 60 and 1200 m contain anoxic intervals laminated sediments that can be correlated with interstadial intervals as defined by the oxygen isotope composition of greenland ice dansgaard oeschger d o cycles these intervals include all or parts of oxygen isotope stage 3 ois3 60 24 cal ka the bolling allerod warm interval b a 15 13 cal ka and the holocene this study uses organic carbon corg and trace element proxies for anoxia and productivity namely elevated concentrations and accumulation rates of molybdenum and cadmium in these laminated sediments to suggest that productivity may be more important than ventilation in producing changes in bottom water oxygen bwo conditions on open highly productive continental margins the main conclusion from these proxies is that during the last glacial interval lgi 24 15 cal ka and the younger dryas cold interval yd 13 11 6 cal ka productivity was lower and bwo levels were higher than during ois3 the b a and the holocene on all margins of the californias the corg and trace element profiles in the lgi b a holocene transition in the cariaco basin on the margin of northern venezuela are remarkably similar to those in the transition on the northern california margin correlation between d o cycles in greenland ice with gray scale measurements in varved sediments in the cariaco basin also is well established synchronous climate driven changes as recorded in the sediments on the margins of the californias sediments from the cariaco basin and in the gisp 2 greenland ice core support the hypothesis that changes in atmospheric dynamics played a major role in abrupt climate change during the last 60 ka millennial scale cycles in productivity and oxygen depletion on the margins of the californias demonstrate that the california current system was poised at a threshold whereby perturbations of atmospheric circulation produced rapid changes in circulation in the eastern north pacific ocean it is likely that the pacific and atlantic oceans were linked through the atmosphere warmer air temperatures during interstadials would have strengthened hadley and walker circulations which in turn would have strengthened the subtropical high pressure systems in both the north pacific and the north atlantic producing increased rainfall over the cariaco basin and increased upwelling along the margins of the californias

mineral chemistry and phase relations of co ni arsenides and sulfarsenides from the baycheh bagh deposit zanjan province iran abstract the polymetallic hydrothermal veins of baycheh bagh deposit in nw iran occur in quartz carbonate veins which are hosted by oligocene to miocene volcanic mainly andesite and volcaniclastic agglomerate tuff rocks unconformably overlying metamorphic basement rocks three transitional paragentic stages of hypogene mineralization have been identified stage 1 includes base metal cu zn pb sulfides and gold and is succeeded by co ni fe as minerals and bismuth stage 2 and cu bi sulfides sulfosalts stage 3 stage 2 ni co fe mineralization is complex and is subdivided into three substages starting with sulfarsenides arsenopyrite alloclasite i ii followed by native bismuth triarsenides skutterudite i ii iii and diarsenides rammelsbergite safflorite and terminated by sulfarsenides gersdorffite cobaltite solid solution alloclacite iii skutterudite and alloclasite are present in several generations and preserve delicate growth zoning reflecting a multi stage episodic growth the sulfarsenides and arsenides reveal significant substitution of ni co fe as well as as and s reflecting compositional changes of the ore forming fluid e g activities of metals as s as well as changes in ph and oxidation state the mineral assemblages and comparison with similar as dominated vein type ni co mineralization elsewhere collectively referred to as five element veins suggest that the complex arsenide sulfarsenide assemblages formed at temperatures below 300 c baycheh bagh can be regarded as a member of this heterogeneous group of bi co ni as deposits a peculiarity of hydrothermal vein type mineralization at baycheh bagh is that initial epithermal cu au zn pb mineralization evolved into bi co ni as mineralization with time

a 20 year varve record of siliceous phytoplankton variability in the central gulf of california abstract year to year variability in the structure of the preserved siliceous phytoplankton assemblage is reconstructed from a box core retrieved from the varved sediments along the guaymas slope of the gulf of california a 20 year record from 1953 through 1972 is obtained from interpretation of the varve stratigraphy supported by 210 pb and 228 th dating ecological time series derived from microfossil analysis show a strong coherence with interannual sea level anomalies indicating a response to large scale climatic variability associated with the el nino southern oscillation phenomenon el nino periods are generally marked by increases in preserved abundances of the total siliceous assemblage and in particular by greater numbers of individuals within species whose distribution is limited to tropical and subtropical waters

late cretaceous and early miocene andean type plutonic activity in northern makran and central iran two calc alkaline plutonic complexes bazman and natanz intruded through the south eastern active and south western ancient continental margins of central iran have been dated by the rb sr whole rock isochron method at 74 2 and 24 4 5 ma respectively detailed trace element studies together with low 87 sr 86 sr initial ratios 87 sr 86 sr ratios and the gabbros at least were intruded some 10 ma earlier

tectonics of the zagros orogenic belt of iran new data and interpretations abstract the zagros orogenic belt of iran is the result of the opening and closure of the neo tethys oceanic realm and consists from northeast to southwest of three parallel tectonic subdivisions 1 1 the urumieh dokhtar magmatic assemblage 2 2 the sanandaj sirjan zone and 3 3 the zagros simply folded belt recent detailed and reconnaissance mapping and stratigraphie structural analyses in various parts of the belt indicate that the tectonics of the sanandaj sirjan zone is dominated by either large scale composite duplex structures or low and high angle ne dipping imbricate systems these thrust systems have transported numerous horses and sheets nappes of metamorphosed and non metamorphosed phanerozoic rocks for tens or possibly hundreds of kilometers stratigraphic evidence and synorogenic conglomerates indicate that thrusting was initiated in the late cretaceous and has continued episodically since that time shear sense indicators in various parts of the zone reveal a ne sw direction of transport thrust systems generated in each episode are superimposed upon the previous structures cutting across and displacing them and producing complicated stacks of thrust sheets southwestward transport and stacking of the thrust sheets has resulted in an increase of about 10 15 km in the thickness of the continental crust in the southwestern part of this zone the so called z main zagros thrust which has been traditionally considered as the boundary between the sanandaj sirjan zone and the zagros simply folded belt is by no means a single high angle reverse fault nor is it a narrow zone of crushed rocks but a series of low angle thrusts locally cut by normal faults these thrusts are indistinguishable from those mapped in the sanandaj sirjan zone and are geometrically part of the sanandaj sirjan zone structures there is no single thrust among them that can be singled out and referred to as the main one the boundary of the sanandaj sirjan zone with the urumieh dokhtar magmatic assemblage to the northeast is characterized by a series of structural depressions formed by compression rock sequences including ophiolite complexes exposed along this boundary are strongly dismembered sheared and mylonitized shearing planes and foliations are either subvertical or dipping at high angles to the northeast this zone is suggested to be the suture between the afro arabian and the iranian plates overall the intensity of deformation increases northeastward from the persian gulf mesopotamian basin where strata are undeformed and horizontal through the zagros simply folded belt which is characterized by en echelon doubly plunging flexural slip folds gentle to open in the southwest and close to locally sw overturned in the northeast the intensity of deformation continues to increase further northeastward through the sanandaj sirjan zone where thrust faulting is dominant this pattern suggests that the deformational front has migrated through time from the northeastern part of the sanandaj sirjan zone i e the suture zone to its present position in the central portion of the persian gulf mesopotamian foreland basin

amphibole out phase boundary in partially melted metabasalt its control over liquid fraction and composition and source permeability a number of experimental studies in recent years have demonstrated that partial melting of hydrated basalt at lower crustal to upper mantle pressures is capable of generating up to 40 high sio2 liquids closely comparable in many geochemical respects to tonalitic trondhjemitic granitoids ttg from these studies it is apparent that the initiation of melting coincides with the beginning of amphibole dehydration but that a broad reaction interval over which amphibole coexists with high sio2 liquids is defined by the wet basalt solidus on one side and the amphibole out phase boundary on the other the phase relations of an alkali rich tholeiitic metabasalt have been examined in the outer half of this region up to and beyond the amphibole out phase boundary results indicate that amphibole exerts a strong control over the amount and composition of coexisting liquid over this interval melt fraction increases slowly and gradually from 0 up to 20 30 between the wet basalt solidus and up to amphibole out but increases substantially from 30 to 50 across and immediately beyond it liquids are strongly to moderately peraluminous molar al2o3 cao na2o k2o a cnk 1 0 up to amphibole out neutral at the boundary a cnk 1 0 and increasingly metaluminous a cnk 1 0 beyond it up to amphibole out potassium is strongly partitioned into liquid relative to amphibole i e mineral melt kd 0 2 0 3 whereas titanium is strongly partitioned into liquid kd 2 0 6 0 physical and chemical criteria suggest that efficient melt segregation leading to ttg plutonism requires more than 20 30 batch melting of a garnet bearing basaltic protolith near or beyond the amphibole out phase boundary leaving dry residues of eclogite or garnet granulite and producing metaluminous liquids that are strongly depleted in heavy rare earth elements hrees and yttrium y lower degrees of melting may involve critical melting where melt is squeezed out of the residue by deformation of the crystalline matrix resulting in disequilibrium melt compositions

marble hosted sulfide ores in the angouran zn pb ag deposit nw iran interaction of sedimentary brines with a metamorphic core complex the angouran zn pb ag deposit zanjan province nw iran is located within the central sanandaj sirjan zone of the zagros orogenic belt the deposit has proven and estimated resources of 4 7 mt of sulfide ore at 27 7 zn 2 4 pb and 110 g t ag and 14 6 mt of oxidized carbonate ores at 22 zn and 4 6 pb it is hosted by a metamorphic core complex that is unconformably overlain by a neogene volcanic and evaporite bearing marine to continental sedimentary sequence the sulfide orebody precursor to the significant nonsulfide ores is located at the crest of an open anticline at the contact between neoproterozoic to cambrian footwall micaschists and hanging wall marbles 40ar 39ar data on muscovite from mineralized and unaltered footwall micaschists suggest a rapid mid miocene exhumation of the metamorphic basement 20 ma and yield an upper age constraint for mineralization the fine grained sulfide ore is massive replacive often brecciated clearly postmetamorphic and dominated by fe poor sphalerite with minor galena pyrite anhydrite quartz muscovite dolomite and rare calcite sphalerite contains na ca cl brine inclusions 23 25 mass total dissolved solids with homogenization temperatures of 180 70 c fluid inclusion chemistry na k li ca mg cl br ore geochemistry s and pb isotope data suggest that the angouran sulfide ore formed by the interaction of modified strongly evaporated miocene seawater and the lithotypes of an exhumed metamorphic core complex minor contributions of metals from miocene igneous rocks cannot be excluded mineralization occurred in a collisional intra arc setting with high heat flow probably during the transition from an extensional to a compressional regime the angouran deposit may represent a new type of low temperature carbonate hosted zn pb ore that is distinct from mississippi valley type and sedimentary exhalative deposits

evidence for a stronger oxygen minimum zone off central california during late pleistocene to early holocene of 31 deep sea cores collected along the central california continental slope 18 have distinctly laminated sediment at depth but none have laminations in the top few centimetres the cores with laminated facies are restricted to water depths between 508 and 1508 m but not all cores taken from this depth interval have laminated facies 14 c dates yield an extrapolated age of 4700 b p for the top of the uppermost laminated unit comparisons of the diatom flora in the laminated couplets with diatom floras in a 13 month sediment trap record suggest that the laminations are varvelike couplets of seasonal sedimentation the laminated facies represent a period from the last global deglaciation to early holocene when the oxygen minimum zone along the northeastern pacific ocean was stronger than at present a stronger oxygen minimum zone during this time is inferred to be the result of intensified upwelling

origin and assembly of the tethyside orogenic collage at the expense of gondwana land abstract the tethysides are a superorogenic complex flanking the eurasian continent to the south and consisting of the cimmerides and alpides products of palaeo and neo tethys respectively we here review their evolution mainly on the basis of new maps showing the distribution of sutures magmatic rocks certain palaeobiogeographically and palaeoclimatologically significant taxa and facies and fragments of pan african 900 450 ma orogenic system forming the basement of many tethyside blocks these are supplemented by palaeomagnetic data reported in the literature a fundamental tenet of this paper is that major sutures which contain ophiolite fragments represent tectonic sections between continental blocks where oceanic crust has been subducted palaeo tethys came into existence largely in late carboniferous time coevally it began to be consumed by both internal and peripheral subduction zones which continued into the permian some of these had been inherited from pre tethyan times in the later permian rifting subparallel with the northern margin of gondwana land began between the zagros and malaysia separating a cimmerian continent from n gondwana land and thus heralding the opening of neo tethys and other smaller oceans that were back arc basins of palaeo tethys this rifting possibly also extended farther west into crete and mainland greece however the north china block yangtze block huanan block the eastern moity of the qangtang block north tibet and annamia all originally pieces of the end proterozoic early palaeozoic gondwana land had already separated from it in pre late carboniferous times possibly during the devonian all of these blocks and the cimmerian continent were characterized by cathaysian floral elements in late palaeozoic time palaeomagnetic and palaeontological data showing the original gondwana land affinity of these continental blocks are supplemented by correlating late proterozoic early palaeozoic pan african sutures orogenic belts and sedimentary basin fragments across tethyside sutures late permian foraminiferal provinces are related to this palaeogeographical interpretation by triassic times most cimmeride subduction zones were already in existence the cimmerian continent accelerated its separation from gondwana land and locally in the late permian began disintegrating internally along the waser rushan pshart banggong co nu jiang mandalay ocean by late triassic time all of the chinese blocks except lhasa and annamia had collided with each other and with laurasia the resulting enormous orogenic collage had a soft cushion between itself and laurasia in the form of the enormous accretionary complex of the songpan ganzi this connection enabled laurasian land vertebrates to reach south east asia by late triassic time in late triassic to middle jurassic times most major cimmeride collisions were completed widespread aridity in central asia occurred in late jurassic time probably in the rain shadow of the newly formed cimmeride mountain wall neo tethyan subduction systems formed along the s margin of the cimmerides or within neo tethyan oceanic lithosphere during the jurassic most if not all were north or east dipping they continued the northerly migration of the tethyside blocks evolution of the tethysides influenced the distribution of marine and terrestrial organisms and affected sea level changes and patterns of atmospheric circulation during much of the mesozoic and cainozoic it is likely to have reflected the surface expression of a persistent trend in the large scale convective circulation in the mantle that continuously transported material northward into the tethyan domain

inorganic geochemical indicators of glacial interglacial changes in productivity and anoxia on the california continental margin abstract evidence from sediments in cores collected from within the present oxygen minimum zone omz 600 1200 m on the central and northern california margins record several episodes during the last interstadial ois 3 ca 60 24 ka of deposition of laminated sediments containing elevated concentrations of several trace elements indicative of anoxic conditions e g mo ni zn and cu the presence of abundant well preserved organic matter as well as lack of bioturbation and the presence of elevated concentrations of mo and other trace elements all support the theory that the omz in the northeastern pacific ocean was more intense possibly anoxic at several times during the late pleistocene sediments of all ages in cores from the southern california margin contain elevated concentrations of mo suggesting that this area has always had higher rates of sulfate reduction than either the central or northern california areas most of the ba in sediments in all cores collected on the upper continental slope 200 2700 m off california and southern oregon is derived from detrital clastic material and this source did not change much in time however the amount of biogenic ba did vary with time and these variations closely follow the temporal variations in organic c corg mass accumulation rate using ba and corg mass accumulation rates as proxy variables for productivity all cores show that organic productivity under the california current upwelling system was highest during ois 3 and the holocene and lowest during the last glacial interval lgi ca 24 10 ka all paleoproductivity proxy variables indicate that the southern california area has always experienced higher productivity than other areas under the california current at least over the last 50 ky

coastal upwelling in the california current system abstract coastal upwelling in the california current system has been the subject of large scale studies off california and baja california and of small scale studies off oregon recent studies of the winds along the entire coast from 25 n to 50 n indicate that there are significant along shore variations in the strength of coastal upwelling which are reflected in the observed temperature distribution active upwelling appears to be restricted to a narrow coastal band about 10 25 km wide along the entire coast but the region influenced by coastal upwelling may be much wider intensive observations of the upwelling zone during summer off oregon show the presence of a southward coastal jet at the surface a mean vertical shear a poleward undercurrent along the bottom and persistently sloping isopycnals over the continental shelf most of the upwelling there occurs during relatively short periods several days long of upwelling favorable winds during the upwelling season off oregon the offshore ekman transport is carried by the surface ekman layer and the onshore return flow occurs through a quasi geostrophic interior it is not known whether the structure and dynamics observed off oregon are typical of the upwelling zone along the entire coast though some of the same features have been observed off baja california current and future research will eventually show whether the oregon results are also applicable in the region of persistently strong upwelling favorable winds off northern california and in the region of complex bathymetry off central and southern california

empirical calibration of the silica ca tschermak s anorthite scan geobarometer the pressure sensitive equilibrium among anorthite quartz and the ca tschermak component in clinopyroxene caal2sio6 cats caal2siocpx6 sioqtz2 caal2si2opl8 scan can be used as a geobarometer in granulites with the proper assemblage and has been calibrated using mineral composition data from partial melting experiments of natural assemblages and from phase equilibrium experiments on the end member cmas system the experimental data cover the p t range 4 32 kbar and 900 1400 c linear least squares regression analysis of the experimental data resulted in the following empirical expressions for pressure in terms of composition and temperature p 5 066 0 760 1300 800 t lnk276 16 t 2 5 kbar orp 6 330 0 116 lnk301 9 t 1 0 kbar where k aplan acpxcats the first equation incorporates an enthalpy term but is less accurate than the second equation in which the enthalpy of reaction is ignored application of these expressions to natural and experimental equilibrium mineral assemblages demonstrates that the empirical barometers are applicable over a wide range of pressures 4 kbar temperatures 700 c and bulk compositions mg 32 5

zircon u pb ages and hf o isotopic composition of migmatites from the zanjan takab complex nw iran constraints on partial melting of metasediments abstract we study migmatites and other metamorphic rocks in the zanjan takab region of nw iran and use these results to report the first evidence of oligocene core complex formation in iran four samples of migmatites associated with paragneisses including leucosomes and associated para amphibolite melanosomes were selected for u pb dating and hf o isotopic analysis zircon cores interpreted as originally detrital zircons have variable ages that peak at ca 100 110 ma but their sedimentation age indicated by the youngest 206 pb 238 u ages is ca 35 40 ma new zircons associated with incipient melting occur as overgrowths around zircon cores and or as newly grown grains morphologies and internal structures suggest that rim growth and formation of new zircons were associated with partial melting all four samples contain zircons with rims that yield 206 pb 238 u ages of 28 25 ma indicating that partial melting occurred in late oligocene time 18 o values for zircon rims vary between 8 2 and 12 3 significantly higher than expected for mantle inputs 18 o 6 and consistent with equilibrium with surface materials zircon rims yield ehf t between 2 2 and 12 4 and two stage hf model ages of 448 562 ma indicating that the region is underlain by cadomian caledonian crust according to the hf o isotopic values the main mechanism forming zircon rims was dissolution of pre existing detrital zircons with reprecipitation of new zircon shortly thereafter oligocene ages indicate that partial melting accompanied core complex formation in the zanjan takab region extension melting and core complex formation in south central iran are eocene in age but younger ages of oligocene miocene in nw iran and turkey indicate that extension was distributed throughout the region during cenozoic time

the eocene bimodal piranshahr massif of the sanandaj sirjan zone nw iran a marker of the end of the collision in the zagros orogen abstract the bimodal piranshahr massif is composed of coeval but geochemically unrelated mafic 40 7 0 2 ma zircon u pb sensitive high resolution ion microprobe shrimp age and a type felsic rocks 41 0 5 ma rb sr and 41 3 0 8 ma zircon u pb shrimp age the mafic rocks consist of two geochemical types of gabbros that derived from different magmas the more abundant gabbros are moderately alkaline have ratios of large ion lithophile elements to ree and high field strength elements to ree similar to those of intraplate mantle magmas 87 sr 86 sr 41 ma 0 7036 and e nd 41 ma 7 2 the less abundant gabbros have calc alkaline affinities 87 sr 86 sr 41 ma 0 7043 and e nd 41 ma 4 78 felsic rocks are metaluminous a2 type annite fayalite hedenbergite hypersolvus leucocratic coarse grained agpaitic syenites pulaskites and granites with 87 sr 86 sr 41 ma 0 7048 and e nd 41 ma 3 6 4 5 syenites pulaskites and granites are genetically related pulaskites probably represent alkali enriched water rich residual melts from which an f rich vapour phase was later separated granites were probably generated during open system processes in which f rich hydrous alkaline fluids released from the syenites acted upon pre existing felsic rocks the c 41 ma age of the post collisional piranshahr massif indicates that the related collision probably occurred at 50 60 ma i e late palaeocene or early eocene thus resolving a much debated question

interplay between geochemistry and magma dynamics during magma interaction an example from the sithonia plutonic complex ne greece orogenic granitoids often display mineralogical and geochemical features suggesting that open system magmatic processes played a key role in their evolution this is testified by the presence of enclaves of more mafic magmas dispersed into the granitoid mass the occurrence of strong disequilibrium textures in mineralogical phases and or extreme geochemical and isotopic variability in this contribution intrusive rocks constituting the sithonia plutonic complex northern greece are studied on the basis of mineral chemistry whole rock major trace element geochemistry and sr and nd isotopic composition sithonia rocks can be divided into a basic group bearing macroscopic mafic enclaves microscopic disequilibrium textures geochemical and isotopic evidence of magma interaction and an acid group in which most geochemical and isotopic features are consistent with a magma mixing process but macroscopic and microscopic features are lacking a two step mixing plus fractional crystallization mfc process is considered responsible for the evolution of the basic group the firststepexplainsthechemicalvariationinthemaficenclavegroup abasicmagma representedbytheleastevolvedenclaves interacted

partial melting of two amphibolites contrasting experimental results under fluid absent conditions a large portion of the lower continental crust may be amphibolitic in composition and without a free fluid phase as a consequence h2o undersaturated or fluid absent melting of amphibolites may be responsible for the formation of some granites and migmatites produced during major orogenic events in an attempt to determine the systematics of melting under fluid absent conditions a series of piston cylinder experiments was conducted on two natural amphibolites one a meta alkali basalt aba with a total water content of wt contained in hornblende and the other a meta island arc tholeiite iat which has 1 1 3 wt water contained in hornblende cummingtonite and biotite the experimentally determined melting ranges of the two amphibolites showed that the solidus temperatures and sta temperature interval over which amphibole was stable were controlled by the amphibolites different bulk compositions and their resulting metamorphic assemblages the volume of melt produced by melting of the two amphibolites were compared with estimated amounts based on burnham s 1979 water melt solubility model and the fluid absent melting model presented by clemens and vielzeuf 1987 the observed melt volumes were greater than estimated as the water content of melt largely detemines the volume of melt produced independent measurements of the water content of the glass formed during partial melting in the aba were made by thermogravimetric analyses the water content of the aba glass is 2 wt which is less than the assumed melt water content water content of the melt used in previous modeling of fluidabsent anatexis in mafic lithologies as a consequence more melt can be expected during fluid absent partial melting of mafic lower crust as is observed in the experiments a modification of the clemens and vielzeuf 1987 fluid absent melting model for mafic compositions has been made using the experimental data available on melting in basaltic systems and is presented here for pressures of 5 8 and 10 kbar tectonic scenarios in which the crust is thickened i e by collision then undergoes extension or where a previously thinned crust is later rethickened provide enough heat so that amphibolite melting under fluid absent conditions can become importan and hence responsible for some melts produced during post collisional magmatism the results may also have applications to melting in hydrated oceanic crust in subduction zones and in island arc terains

application of geoelectric methods using buried electrodes in exploration and mining1 various geoelectric methods which have been developed and applied in the last 10 20 years in elgi are discussed these methods which use buried electrodes are hole to surface gradient mapping to detect bauxite deposits in sinkholes below a resistive screening layer in mine gradient profiling to map the basement topography below galleries and the hole to surface version of geoelectric layer tracing to find outcrops of mineralized zones penetrated by drillings data processing procedures have been developed on the basis of common concepts and hypotheses to link theoretical models with geological structures the objects investigated are determined as the difference between the theoretical models and geological structures the predominant part of the real electric field measured above the geological structures is the theoretical field related to the theoretical model the effects of the objects the anomaly are superposed on the theoretical field but their extent is small compared with the values of the latter the theoretical field and the anomaly depend strongly on the separation from the sources for this reason the anomalies are difficult to recognize therefore the ratio of the theoretical field to the measured one is computed since a the apparent specific conductivity is proportional to this ratio it is demonstrated that since the changes in the a curve depending on the location of the observation point are small the anomalies can easily be recognized on the curve the a curve computed in the above way reflects the objects better than the originally measured electric fields examples illustrate the solution of the above mentioned geological problems by the practical application of adequate geoelectric methods using buried electrodes

convergence history across zagros iran constraints from collisional and earlier deformation the zagros orogen provides a unique opportunity within the alpine system to evaluate the interplay between a young tertiary collision and earlier subduction obduction processes within the crush zone and the sanandaj sirjan internal zone separating the zagros fold belt from central iran we document several major tectonic events taking place at the end of the cretaceous of the eocene and from the mio pliocene onwards ca 20 15 ma contrary to recent interpretations our data cross sections and description of the overall deformation style strongly suggest that the main zagros thrust mzt is deeply rooted possibly to moho depths and that the suture zone effectively runs along the mzt field observations show that the final resorption of the oceanic domain took place slightly after 35 ma and that collision must have started before ca 23 25 ma in northern zagros the shortening rate across the crush zone since the mid miocene 20 15 ma is estimated at a minimum 3 4 mm year shear movements in the crush zone during the eocene oligocene period and extensional strike slip movements in the internal zones during the late cretaceous point to an oblique setting early in the convergence history a geotectonic scenario for convergence from the time of obduction to the present is finally proposed

a new tectonic scenario for the sanandaj sirjan zone iran abstract recent geochemical studies of volcanic rocks forming part of the ophiolites within the zagros and naien baft orogen indicate that most of them were developed as supra subduction ophiolites in intra oceanic island arc environments intra oceanic island arcs and ophiolites now forming the naien baft zone were emplaced southwestward onto the northeastern margin of the south sanandaj sirjan zone while those now in the high zagros were emplaced southwestward onto the northern margin of arabia thereafter subduction continued on opposite sides of the remnant oceans the floor of neo tethys ocean was subducted at a low angle beneath the entire sanandaj sirjan zone and the floor of the naien baft ocean was subducted beneath the central iranian micro continent the naien baft ocean extended into north west iran only temporarily this failed ocean arm between the urumieh dokhtar magmatic assemblage and the main zagros thrust was filled by thick upper triassic upper jurassic sediments the naien baft ocean finally closed in the paleocene and neo tethys closed in the early to middle eocene after arabia was sutured to iran the urumieh dokhtar magmatic assemblage recorded slab break off in the middle eocene

varves in marine sediments a review the global compilation of reported marine varved sedimentary records throughout the quaternary contains 52 sites marine varve deposition and preservation typically depend on environmental and sedimentological conditions such as a sufficiently high sedimentation rate severe depletion of dissolved oxygen in bottom water to exclude bioturbation by macrobenthos and a seasonally varying sedimentary input to yield a recognizable rhythmic varve pattern additional oceanographic factors include the strength and depth range of the oxygen minimum zone omz and regional anthropogenic eutrophication from point sources such as large polluted rivers quaternary marine varves are not only found in those parts of the open ocean that comply with these conditions but also in fjords embayments and estuaries with thermohaline density stratification and nearshore marine lakes with strong hydrologic connections to ocean water this review critically describes settings and sedimentological characteristics of marine sites where varves have been reported broader applications of marine varve records are discussed for example in terms of radiocarbon calibration on high resolution time scales constraining paleoceanographic variability and global change teleconnections diagnosing factors affecting modern fish population dynamics based on past performance detailing biogeochemical cycles and deciphering complex factors influencing marine productivity varves in saline water bodies without hydrologic connection to the global ocean are not included in this review and instead pertain to the companion review on lacustrine varves where also general and fundamental principles of varve deposition classification and dating are covered zolitschka et al 2015 the postulated occurrence of varves in pre quaternary rocks is critically discussed in this review with representative examples in the case of non evaporitic laminations in fine grained ancient marine rocks such as banded iron formations and black shales laminations may not be varves but instead may have multiple alternative origins such as event beds or formation via bottom currents that transport and sort silt sized particles clay floccules and organic mineral aggregates in the form of migrating bedload ripples

petrogenesis and tectonic evolution of metaluminous sub alkaline granitoids from the takab complex nw iran the takab complex is composed of a variety of metamorphic rocks including amphibolites metapelites mafic granulites migmatites and meta ultramafics which are intruded by the granitoid the granitoid magmatic activity occurred in relation to the subduction of the neo tethys oceanic crust beneath the iranian crust during tertiary times the granitoids are mainly granodiorite quartz monzodiorite monzonite and quartz diorite chemically the magmatic rocks are characterized by asi 1 04 ai 0 87 and high contents of cao up to 14 5 wt which are consistent with the i type magmatic series low feot feot mgo values 0 75 as well as low nb y and k2o contents of the investigated rocks resemble the calc alkaline series low sio2 k2o na2o and al2o3 accompanied by high cao and feo contents indicate melting of metabasites as an appropriate source for the intrusions negative ti and nb anomalies verify a metaluminous crustal origin for the protoliths of the investigated igneous rocks these are comparable with compositions of the associated mafic migmatites in the takab metamorphic complex which originated from the partial melting of amphibolites therefore crustal melting and a collision related origin for the takab calc alkaline intrusions are proposed here on the basis of mineralogy and geochemical characteristics the p t evolution during magmatic crystallization and subsolidus cooling stages is determined by the study of mineral chemistry of the granodiorite and the quartz diorite magmatic crystallization pressure and temperature for the quartz diorite and the granodiorite are estimated to be p 7 8 2 5 kbar t 760 75 c and p 5 1 kbar t 700 c respectively subsolidus conditions are consistent with temperatures of 620 c and 600 c and pressures of 5 kbar and 3 5 kbar for the quartz diorite and the granodiorite respectively

cretaceous tertiary convergence and continental collision sanandaj sirjan zone western iran abstract the sanandaj sirjan zone contains the metamorphic core of the zagros continental collision zone in western iran the zone has been subdivided into the following from southwest to northeast an outer belt of imbricate thrust slices radiolarite bisotun ophiolite and marginal sub zones which consist of mesozoic deep marine sediments shallow marine carbonates oceanic crust and volcanic arc respectively and an inner complexly deformed sub zone late palaeozoic mesozoic passive margin succession rifting and sea floor spreading of tethys occurred in the permian to triassic but in the sanandaj sirjan zone extension related successions are mainly of late triassic age subduction of tethyan sea floor in the late jurassic to cretaceous produced deformation metamorphism and unconformities in the marginal and complexly deformed sub zones deformation climaxed in the late cretaceous when a major southwest vergent fold belt formed associated with greenschist facies metamorphism and post dated by abundant palaeogene granitic plutons in the southwest of the zone a late cretaceous island arc passive margin collision occurred with ophiolite emplacement onto the northern arabian margin similar to that in oman final closure of tethys was not completed until the miocene when central iran collided with the northeast arabian margin

biogenic and abiogenic components of laminated hemipelagic sediments in the central gulf of california abstract alternating light and dark submillimeter to millimeter thick layered sediments rhythmites are found along the slopes of the central and southern gulf of california where a strong oxygen minimum zone intersects the sediment water interface two short intervals of distinct alternations were sampled lamina by lamina from two cores in the central gulf both cores are from the slope of the guaymas basin one core is from the mainland side the other core is from the baja side each lamina sample was analyzed for its opal micro floral assemblage textural character trace metal and bulk chemical cu mn fe ni zn ca k al sio2 and c org content and mineralogy on both sides of the gulf the light laminae contain more diatom frustules up to 90 and less terrigenous and organic material than the dark laminae the sediment is predominantly silt sized the dark laminae contain more terrigenous material which is primarily clay size on both sides of the gulf the light laminae apparently represent sedimentation during the dry winter season of northwesterly winds whereas the dark laminae represent the wet summer season of southeasterly winds