the origin and age of the metamorphic sole from the rogozna mts western vardar belt new evidence for the one ocean model for the balkan ophiolites abstract this study brings new geochronological and petrochemical data from the metamorphic sole beneath the rogozna mts western vardar ophiolite belt the rogozna metamorphic sole is located at the base of a serpentinite nappe and consists of amphibolites and talc chlorite schists the rogozna amphibolites are medium to fine grained rocks with nematoblastic texture and pronounced foliation they consist of green amphibole 70 vol with variable silica contents 6 4 to 7 8 si apfu as well as mg molmg mg fe tot 0 53 to 0 77 and variably albitized plagioclase 30 vol ab 24 ab 98 amphibolites are overprinted by a retrograde assemblage containing actinolite epidote clinoclore sericite chlorite and magnetite the amphibolites formed due to metamorphism of two basaltic suites subalkaline tholeiitic and alkaline subalkaline tholeiitic amphibolites possess low zr nb y th hf tio 2 and p 2 o 5 values and a lree depleted pattern typical for the n morb normal mid ocean ridge basalt to bab back arc basalt origin alkaline amphibolites show elevated concentrations of zr nb y th hf tio 2 and p 2 o 5 with a lree enriched pattern typically displayed by oib ocean island basalt amphibolites were crystallized during intra oceanic thrusting at temperatures between 685 c and 765 c and at a depth of 12 17 km 40 ar 39 ar cooling ages of amphibole ranging from 165 to 170 ma slightly postdate the sole formation talc chlorite schists are related to retrograde greenschist facies metamorphism they consist of fe rich talc and cr rich chlorite peninite diabantite pseudomorphs after amphibole and morb type cr al spinel surrounded by al and mg poor ferrit chromite the occurrence of ferrit chromite is related to earlier amphibolite facies metamorphism chlorite pseudomorphs after amphibole were formed at 300 410 c this study suggests that there is no essential difference in the emplacement age of the dinaric and west vardar ophiolite belts supporting the interpretation involving a single mesozoic ocean in the balkan sector

origin and emplacement of an inferred late jurassic subduction accretion complex euboea eastern greece in northern euboea central eastern greece an up to 3 km thick polygenetic melange pagondas complex is structurally interleaved between a triassic jurassic carbonate platform pelagonian zone and an overriding harzburgitic ophiolite the melange mainly comprises late triassic shallow water limestone and calciturbidites radiolarites triassic jurassic tholeiites alkaline basalts and minor andesites the units concerned range from kilometre sized thrust sheets and detached blocks to broken formation and structureless or bedded matrix supported conglomerates diamictite the melange includes remnants of neotethyan oceanic lithosphere overlain by radiolarites hemipelagic carbonates and distal calciturbidites derived from a mesozoic carbonate platform tholeiites were erupted at a triassic jurassic spreading axis whilst within plate type alkali basalts are interpreted mainly as seamounts kilometre scale detached blocks of shallow water coralline limestone are identified as collapsed atolls formed within an ocean and or along the rifted continental margin volcaniclastic sediments are locally interbedded with radiolarite and reflect post volcanic erosion of the ocean floor intra oceanic convergence began apparently in late early jurassic time giving rise to the euboea ophiolite above an inferred westwards dipping subduction zone the pagondas complex then developed as an accretionary prism the subduction trench later collided with the pelagonian passive margin driving the hot euobea ophiolite over the accretionary complex to produce amphibolites and greenschists of the metamorphic sole trench margin collision then drove the entire supra subduction zone complex apparently eastwards downflexing the pelagonian carbonate platform to form a foredeep in which late jurassic kimmeridgian tithonian radiolarian sediments accumulated during emplacement the accretionary complex was disrupted and partly resedimented as debris flows turbiditic volcaniclastic sandstone and shale in a foredeep or foreland basin setting

comparisons between tethyan anorthosite bearing ophiolites and archean anorthosite bearing layered intrusions implications for archean geodynamic processes

shock and annealing in aubrites implications for parent body history several aubrites e g lap 03719 bishopville khor temiki alh 83015 contain orthopyroxene grains that exhibit more pronounced shock effects than associated olivine grains the orthopyroxene grains in these samples have clinoenstatite lamellae on 100 and exhibit weak mosaic extinction characteristic of shock stage s4 the olivine grains exhibit either sharp optical extinction characteristic of shock stage s1 as in lap 03719 or undulose extinction shock stage s2 as in bishopville and alh 83015 the khor temiki regolith breccia contains s1 and s2 olivine grains because literature data show that diffusion is much slower in orthopyroxene than in olivine it seems likely that aubrites experienced postshock impact induced annealing after differentiation the aubrite parent asteroid suffered major collisions that caused extensive brecciation of near surface materials and damaged orthopyroxene and olivine crystal lattices as a result of these impact events some aubrites were shocked and buried within warm ejecta blankets or beneath fallback debris under the crater floor entombed olivine crystal lattices healed and became unstrained reaching shock stage s1 but orthopyroxene lattices retained their s4 level shock damaged features aubrites with s4 orthopyroxene and s2 olivine were probably very weakly shocked again after olivine was annealed to s1

size frequency distributions of chondrules and chondrule fragments in ll3 chondrites implications for parent body fragmentation of chondrules we measured the sizes and textural types of 719 intact chondrules and 1322 chondrule fragments in thin sections of semarkona ll3 0 bishunpur ll3 1 krymka ll3 1 piancaldoli ll3 4 and lewis cliff 88175 ll3 8 the mean apparent diameter of chondrules in these ll3 chondrites is 0 80 units or 570 um much smaller than the previous rough estimate of 900 m chondrule fragments in the five ll3 chondrites have a mean apparent cross section of 1 60 phi units or 330 m the smallest fragments are isolated olivine and pyroxene grains these are probably phenocrysts liberated from disrupted porphyritic chondrules all five ll3 chondrites have fragment chondrule number ratios exceeding unity suggesting that substantial numbers of the chondrules in these rocks were shattered most fragmentation probably occurred on the parent asteroid porphyritic chondrules porphyritic olivine porphyritic pyroxene porphyritic olivine pyroxene are more readily broken than droplet chondrules barred olivine radial pyroxene cryptocrystalline the porphyritic fragment chondrule number ratio 2 0 appreciably exceeds that of droplet textured objects 0 9 intact droplet chondrules have a larger mean size than intact porphyritic chondrules implying that large porphyritic chondrules are fragmented preferentially this is consistent with the relatively low percentage of porphyritic chondrules within the set of the largest chondrules 57 compared to that within the set of the smallest chondrules 81 differences in mean size among chondrule textural types may be due mainly to parent body chondrule fragmentation events and not to chondrule formation processes in the solar nebula

the liuyuan complex in the beishan nw china a carboniferous permian ophiolitic fore arc sliver in the southern altaids the tectonic history and time of closure of the palaeo asian ocean of the altaids are issues of lively current debate to address these issues this paper presents detailed geological petrological and geochemical data of the liuyuan complex lc in the beishan region in nw china located in the southernmost altaids in order to constrain its age origin and tectonic setting the lc mainly comprises massive basalts pillow basalts basaltic breccias gabbros and ultramafic rocks together with cherts and tuffs most prominent are gabbros and large volumes of basaltic lavas these mafic rocks have high tio 2 contents flat rare earth element ree patterns and show high field strength elements hfses similar to those of mid ocean ridge basalts morb the mafic rocks exhibit positive e nd t 6 6 9 0 values representing magmas derived from the mantle but these basic rocks are also enriched in th relative to rees and are systematically depleted in nb ta ti relative to rees there is also a large range in initial 87 sr 86 sr 0 7037 0 7093 all these variables indicate that mantle derived magma was contaminated by fluids and or melts from a subducting lithospheric slab and formed in a supra subduction zone ssz setting a gabbro intruded in the complex was dated by la icp ms on 20 zircons that yielded a 206 pb 238 u weighted average age of 286 2 ma considering the fact that all these basalts are imbricated against permian tuffaceous sediments and limestone we propose that the lc formed as an ophiolite in a fore arc in carboniferous permian time this indicates that the palaeo asian ocean still existed at 286 2 ma in early permian time and thus the time of closure of the palaeo asian ocean was in or after the late permian

mirdita zone ophiolites and associated sediments in albania reveal neotethys ocean origin the mirdita ophiolite zone in albania is associated with widespread melanges containing components of up to nappe size we dated matrix and components of the melange by radiolarians conodonts and other taxa the components consist of radiolarites pelagic limestones and shallow water limestones all of triassic age as well as ophiolites triassic radiolarite as a primary cover of ophiolite material proves middle triassic onset of mirdita ocean floor formation the melange contains a turbiditic radiolarite rich matrix radiolaritic flysch dated as late bajocian to early oxfordian it formed as a synorogenic sediment during west directed thrusting of ophiolite and sediment cover nappes representing ocean floor and underplated fragments of the western continental margin the tectonic structures formed during these orogenic events younger kimmeridian or eohellenic orogeny are sealed by late jurassic platform carbonates the geological history conforms with that of the inner dinarides and adjoining areas we therefore correlate the mirdita pindos ophiolite zone with the vardar zone and explain its present position by far distance west directed thrusting

structure and geochemistry of tethyan ophiolites and their petrogenesis in subduction rollback systems suprasubduction zone ssz ophiolites in orogenic belts represent oceanic crust generation in subduction rollback cycles during the closing stages of basins prior to terminal continental collisions mantle flow and slab rollback result in one or more episodes of arc splitting and basin opening producing a collage of protoarc and forearc oceanic lithosphere in suprasubduction zone settings the jurassic cretaceous ssz tethyan ophiolites in the eastern mediterranean region i e mirdita pindos troodos kizildag oman generally have penrose type oceanic crust and contain well developed sheeted dike complexes indicative of magmatic extension beneath narrow rift zones during their seafloor spreading evolution igneous accretion of these ssz tethyan ophiolites involved upper plate extension and advanced melting of previously depleted asthenosphere in host basins showing a progressive evolution from morb like to iat island arc tholeiite to boninitic extremely refractory protoarc assemblages however there are some distinct differences in the geochemical evolution of these tethyan ophiolites that appear to have resulted from variations in their subduction zone geodynamics whereas a major part of the kizildag and troodos lavas shows island arc affinity similar to their counterparts in the pindos and mirdita ophiolites a significant component of the oman lavas indicates morb affinity and the majority of the kizildag and oman data plot within the mantle array between n morb and e morb on the nb yb th yb discriminant diagram furthermore the troodos and oman lavas do not show any particular th enrichment in their multi element patterns suggesting that fluid melt input from subducted sediments was not that significant in generation of their magmas although all ophiolites exhibit geochemical features indicating increased subduction influence during the melt evolution of their younger extrusive sequences and dike intrusions as evidenced by their negative nd values their overall characteristic trace element patterns seem to have been strongly affected by the maturity of the subduction systems in which they developed

geological evolution of the albanian ophiolites and their platform periphery geological studies of the tectonic zones of eastern albania indicate that they represent the passive margin to a neotethyan mirdita ocean basin situated on the western margin of the korab pelagonian microcontinent

basaltic breccias of the clipperton fracture zone east pacific sedimentation and tectonics in a fast slipping oceanic transform investigations of the clipperton transform fault a fast slipping 11 cm yr transform plate boundary offsetting the east pacific rise indicate that the most common types of rocks occurring in this setting comprise a distinctive suite of sedimentary breccias and mudstones these rocks occur within a narrow 5 km wide domain of rugged terrain inferred to be the surface expression of strike slip faulting the sedimentary breccias are composed of angular basaltic clasts set in a fine grained matrix of very angular basaltic mineral and glass fragments and a subordinate amount of their altered equivalents the mudstones consist of material similar to the breccia matrix but also include a small amount of pelagic carbonate these sediments represent the deposits of small active debris fan systems located at the base of slopes flanking the elongate transform parallel basins of the region the active mass wasting and breccia formation occur in young crust as it passes through the transform fault domain the sediments are the products of physical and chemical disintegration and tectonic abrasion of basaltic crust initially formed at the ridge transform intersections most of the fine grained fraction consists of rock flour formed by frictional movement along fault surfaces and shear fractures its abundance reflects the high strain rates of this fast slipping transform fault as a result of differential vertical movements of crustal blocks associated with wrench tectonics these sediments undergo multiple cycles of erosion and redeposition as sections of the floor of the transform valley escape into the aseismic limbs of the fracture zone the breccias and mudstones an preserved in the basal sedimentary stratigraphy of the oceanic crust these distinctive assemblages may serve as indicators of oceanic crust from transform fault zones expose in some ophiolites

revised model calculations for the thermal histories of ordinary chondrite parent bodies recent measurements of ordinary chondrite physical and thermal properties along with new geothermometry studies have provided the necessary parameters for updating a previously proposed model miyamoto et al 1981 for the thermal evolution and internal structure of ordinary chondrite parent bodies model calculations assumed a heat source term derived from the decay of 26al justification is provided differences from the previous model include varying the thermal diffusivity parameter with increasing temperature and decreasing porosity using variable physical and thermal parameters to provide end member models and incorporating a shortened thermal history of 60 ma obtained from new pb pb chronology of phosphates rather than 100 ma times of isotopic closure in chondrite phosphates overlap the thermal model estimates and postmetamorphic cooling rates from the model approximately coincide in both trend and magnitude with metallographic and fission track cooling rate data model calculations attempt to match peak metamorphic conditions in the central portions of these bodies and yield accretion ages between 1 4 to 3 1 ma after calcium aluminum inclusion cai formation model calculations also predict that both the h and the l chondrite parent asteroids consisted of 80 equilibrated and 20 unequilibrated chondritic material and that their original radii ranged from 80 to 95 km

origin of a unique impact melt rock the l chondrite ramsdorf we have studied a unique impact melt rock the ramsdorf l chondrite using optical and scanning microscopy and electron microprobe analysis ramsdorf contains not only clast poor impact melt begemann and wlotzka 1969 but also a chondritic portion 60 g with what appears at low magnification to be a normal well defined chondritic texture however detailed studies at high magnification show that 90 vol of the crystals in the chondritic portion were largely melted by the impact the chondrules lack normal microtextures and are ghosts of the original features the only relics from the precursor chondrules are olivine crystals which have the highest melting temperature 1620 c pyroxene rich chondrules were so extensively melted that no phenocrysts were preserved and the melt crystallized in situ before significant mixing with exterior olivine rich melts fine grained pyroxene chondrule ghosts have sharper boundaries with the matrix than porphyritic olivine and pyroxene chondrule ghosts probably because pyroxene rich melts are significantly more viscous complex textures that formed by injection of melt along cracks and fractures in relic olivines suggest that the chondritic portion of ramsdorf formed directly from petrologic type 3 4 material by strong shock we infer that ramsdorf was largely melted by shock pressures of 75 90 gpa and that chondrule ghosts and relic olivine phenocrysts were locally preserved by rapid cooling quenching was not due to the addition of cold clasts into the melt but to heterogeneous shock heating that only caused internal melting of large olivines and pyroxenes ramsdorf appears to be one of the most heavily shocked meteorites that has retained some trace of its original texture

post shock annealing of miller range 99301 ll6 implications for impact heating of ordinary chondrites mil 99301 is an ll chondrite that has experienced successive episodes of thermal metamorphism shock metamorphism and annealing the first recognizable petrogenetic episode resulted in thermal metamorphism of the rock to petrologic type 6 as indicated by homogeneous olivine compositions significant textural recrystallization and the presence of coarse grains of plagioclase metallic fe ni and troilite the source of heat for this thermal episode is not identified the rock also experienced shock metamorphism to shock stage s4 as indicated by extensive silicate darkening caused by the dispersion within silicate grains of thin chromite melt veins and trails of metallic fe ni and troilite blebs polycrystalline troilite myrmekitic plessite a relatively high occurrence abundance oa of metallic cu 3 6 the presence of numerous chromite plagioclase assemblages and coarse grains of low ca clinopyroxene with polysynthetic twinning the shock event responsible for these effects must have occurred after the epoch of thermal metamorphism to type 6 levels otherwise the polycrystallinity of the troilite would have disappeared and the low ca clinopyroxene would have transformed into orthopyroxene despite abundant evidence of strong shock olivine and plagioclase in mil 99301 exhibit sharp optical extinction consistent with shock stage s1 and characteristic of an unshocked rock this implies that an episode of post shock annealing healed the damaged olivine and plagioclase crystal lattices and thereby changed undulose extinction into sharp extinction the rock was probably annealed to metamorphic levels approximating petrologic type 4 more significant heating would have transformed the low ca clinopyroxene into orthopyroxene it is not plausible that an episode of annealing occurring after the epoch of thermal metamorphism could have been caused by the decay of 26al because this isotope would have decayed away by that time impact heating is a more plausible source of post metamorphic annealing of rocks in the vicinity of impact craters on low density high porosity asteroids with rubble pile structures

possible melting produced chondrule destruction in nwa 6604 ck4 chondrite in analyzing a thin section of the nwa 6604 ck4 meteorite only altered chondrules and various components that are probably left behind the destruction of former chondrules can be observed we suggest that melting grain size decrease resorption of the original chondrules and crystallization of opaque minerals were the main processes that destroyed the chondrules four different events could be identified as having occurred during this alteration first opaques crystallized along former fractures producing chains of separated grains later opaques and ca rich minerals crystallized together in veins and large melt pockets this was the strongest recrystallization phase involving the largest volume of melt this occurred along different fractures than the first phase above during the third phase only ca rich plagioclase crystallized along thin veins and in a fourth phase fractures formed again partly along those formed during the second phases but without substantial mineral infill two simple possible case models should be considered for this meteorite alteration by purely impact driven processes or mainly by melt driven processes although for ck4 chondrites the shock produced alteration driven by impact is the more accepted and widespread approach melting is also compatible with the observed textural characteristics of chondrule destruction during melting recrystallization took place producing iron rich minerals earlier and ca si rich ones later the penetration of melts into veins contributed in the chondrule destruction the stress directions also changed during these alterations and minerals that formed later filled differently oriented fractures than the earlier ones from our observations we favor a view where heat driven melting and recrystallization produced the destruction and uniform mineralogy in the sample

39ar40ar age and petrology of chico large scale impact melting on the l chondrite parent body our studies of the 105 kg chico l chondrite show that it contains 60 impact melt and the largest volume of impact melt recognized in stony meteorites we suggest that it is part of a much larger dike complex that formed when chondritic impact melt was intruded into host chondrite during a large if not catastrophic impact on the l chondrite parent body at about 0 5 ga petrologic and 39ar 40ar dating studies were made on several lithologies including the massive melt zone host chondrite and melt chondrite boundaries for the purpose of studying the melting and thermal histories associated with impacts on small bodies and their effects on the k ar chronometer the chondritic host is shocked to stage s6 and contains pockets and veins of melt there are no unmelted clasts in the interior of the melt coalesced metal troilite nodules reach up to 2 cm in size melt near the contact with the host chondrite contains numerous clasts and quenched more rapidly metal troilite textures suggest cooling rates of 0 1 c s in the interior of the melt dike during crystallization secondary kamacite rims indicate cooling at 0 01 1 c y over the range of 700 500 c consistent with an impact heated volume of up to a kilometer in thickness compositions of olivines and pyroxenes are generally similar in melt and chondritic host reflecting rapid crystallization not metamorphic equilibration the interior melt shows an overall depletion in k whereas the melt near the boundary is enriched in k the 39ar 40ar release spectra during stepwise heating of both melt and chondrite samples can be divided into two parts based on ar diffusion properties and k ca ratios the low temperature high k ca phase of both melt and host chondrite show ages of 0 54 0 78 ga ages of the high temperature low k ca phase of the melt are comparable or higher 0 61 1 35 ga whereas those of the host chondrite are even higher 0 87 1 86 ga due to lesser degrees of degassing isochron plots for several melt samples suggest an age of 0 53 ga and the presence of variable amounts of excess 40ar not completely degassed by the impact even this age however is significantly higher than the previously reported rb sr isochron age of 0 467 015 ga the apparent retention of radiogenic 40ar in the chico impact melt in spite of its relatively large size absence of clasts and moderately slow cooling rate below 700 c raises questions as to the reliability of using melts for 39ar 40ar dating of meteoritic impact events

contribution of early impact events to metal silicate separation thermal annealing and volatile redistribution evidence in the pultusk h chondrite three dimensional x ray tomographic reconstructions and petrologic studies reveal voluminous accumulations of metal in pultusk h chondrite at the contact of these accumulations the chondritic rock is enriched in troilite the rock contains plagioclase rich bands with textures suggesting crystallization from melt unusually large phosphates are associated with the plagioclase and consist of assemblages of merrillite and fluorapatite and chlorapatite the metal accumulations were formed by impact melting rapid segregation of metal sulfide melt and the incorporation of this melt into the fractured crater basement the impact most likely occurred in the early evolution of the h chondrite parent body when post impact heat overlapped with radiogenic heat this enabled slow cooling and separation of the metallic melt into metal rich and sulfide rich fractions this led to recrystallization of chondritic rock in contact with the metal accumulations and the crystallization of shock melts phosphorus was liberated from the metal and subsumed by the silicate shock melt owing to oxidative conditions upon slow cooling the melt was also a host for volatiles upon further cooling phosphorus reacted with silicates leading to the formation of merrillite while volatiles partitioned into the residual halogen rich dry fluid in the late stages the fluid altered merrillite to patchy cl f apatite the above sequence of alterations demonstrates that impact during the early evolution of chondritic parent bodies might have contributed to local metal segregation and silicate melting in addition postshock conditions supported secondary processes compositional textural equilibration redistribution of volatiles and fluid alterations

oceanic crust generation in an island arc tectonic setting se anatolian orogenic belt turkey a number of late cretaceous ophiolitic bodies are located between the metamorphic massifs of the southeast anatolian orogenic system one of them the goksun ophiolite northern kahramanmaras which crops out in a tectonic window bounded by the malatya metamorphic units on both the north and south is located in the ew trending nappe zone of the southeast anatolian orogenic belt between goksunand afsin northern kahramanmaras it consists of ultramafic mafic cumulates isotropic gabbro a sheeted dyke complex plagiogranite volcanic rocks and associated volcanosedimentary units the ophiolitic rocks and the tectonically overlying malatya keban metamorphic units were intruded by syn collisional granitoids 85 ma the volcanic units are characterized by a wide spectrum of rocks ranging in composition from basalt to rhyolite the sheeted dykes consist of diabase and microdiorite whereas the isotropic gabbros consist of gabbro diorite and quartzdiorite the magmatic rocks in the goksunophiolite are part of a co magmatic differentiated series of subalkaline tholeiites selective enrichment of some lil elements rb ba k sr and th and depletion of the hfs elements nb ta ti zr relative to n morb are the main features of the upper crustal rocks the presence of negative anomalies for ta nb ti the ratios of selected trace elements nb th th yb ta yb and normalized ree patterns all are indicative of a subduction related environment all the geochemical evidence both from the volcanic rocks and the deeper levels sheeted dykes and isotropic gabbro show that the goksun ophiolite formed during the mature stage of a suprasubduction zone ssz tectonic setting in the southern branch of the neotethyan ocean between the malatya keban platform to the north and the arabian platform to the south during late cretaceous times geological geochronological and petrological data on the goksun ophiolite and the baskil magmatic arc suggest that there were two subduction zones the first one dipping beneath the malatya keban platform generating the baskil magmatic arc and the second one further south within the ocean basin generating the goksun ophiolite in a suprasubduction zone environment

abee and related eh chondrite impact melt breccias abstract the abee eh chondrite is an impact melt breccia about 80 90 of the chondrules were melted many of those that remain have been partly resorbed euhedral enstatite phenocrysts are abundant many have nucleated on the surfaces of relict chondrules and now protrude into kamacite globules euhedral laths of graphite essentially identical to those produced magmatically on earth occur throughout the rock abee experienced two episodes of impact melting with an intervening period of brecciation the first impact melting event produced a relatively homogeneous rock with abundant melt upon cooling the rock was shattered by bombarding meteoroids igneous oldhamite rich dark inclusions were introduced at this time a second impact event produced melt which engulfed the abee fragments and dark inclusions primarily digesting the small ones heat lost to the surrounding clastic debris caused the groundmass melt to quench previously described diamonds in abee containing typical solar system isotopic compositions of n and xe were probably produced from graphite by shock several other eh chondrites may also be impact melt breccias the most extensively melted of these is adhi kot which contains shocked but largely unmelted chondrule rich clasts as well as chondrule free sulfide and silica rich clasts that also appear to be impact products

reply to d pană s discussion on the eastern carpathians ophiolites romania remnants of a triassic ocean lithos 108 2009 151 171

mesozoic tertiary tectonic evolution of albania in its regional eastern mediterranean context abstract research carried out since the opening of albania to the western geological community in the 1990s allows the geology and tectonics of this small balkan country to be integrated into an overall interpretation of neotethys in the eastern mediterranean region specifically greece and the former yugoslavia region albanian geology is most famous for the occurrence of contrasting ophiolites of jurassic age the country is sub divided into three main geotectonic units those in the west i e sazani ionian kruja krasta cukali and the albanian alps relate to the western margin of apulia effectively part of north africa from early mesozoic time onwards in the east is the korabi zone interpreted as a microcontinent located further northeast within neotethys between the two is the mirdita zone dominated by two contrasting jurassic ophiolites the western type ophiolite and the eastern type ophiolite taking account of evidence from greece and former yugoslavia the following tectonic evolution is proposed from early mesozoic time onwards neotethys in albania existed as a northward continuation of the pindos oceanic basin in greece pindos mirdita ocean a korabi microcontinent was rifted off apulia in the early mid triassic followed by passive margin subsidence until late middle jurassic time a rifted slow spreading mor type ophiolite formed within the western pindos mirdita oceanic basin in late triassic early late jurassic time using present co ordinates the shkoder peje scutari pec lineament in the north of albania is interpreted as an important transform fault zone that influenced the regional tectonic evolution during the entire mesozoic early tertiary rift drift emplacement history of the pindos mirdita ocean taking account of evidence from the greek and former yugoslavia ophiolites the easterly albanian ophiolites formed within the pindos mirdita ocean above a westward dipping intra oceanic subduction zone in the early late jurassic metamorphic soles were formed during initial intra oceanic displacement ca 162 174 m y and underlying melanges originated as subduction accretion complexes the mor western type ophiolite evolved into the iat boninitic eastern type ophiolite after subduction began during tithonian berriasian time trench margin collision emplaced the ophiolites eastwards over the korabi margin coupled with redeposition of ophiolitic and continental margin material as exotic blocks debris flow deposits and turbidites these were emplaced onto the upper surface of the ophiolitic thrust sheet during the latest stages of emplacement onto the korabi microcontinent following collision in the east oceanic crust remained within a small remnant pindos mirdita oceanic basin passive conditions were restored in the mid late cretaceous with shallow water carbonate deposition on microcontinental units and both pelagic and redeposited carbonates in basinal settings from the maastrichtian onwards africa eurasia convergence resulted in large scale westward thrusting deforming the apulian foreland in oligo miocene time the suture zone was re activated in oligo miocene time as the albanie thassalie piggy back type basin in central and southern albania albania as a whole was rotated 45 clockwise as part of the western aegean arc in post eocene time coastal and offshore albania experienced strong subsidence after early miocene time forming the periadriatic basin including the tirana depression interpreted as a foredeep related to continuing regional convergence southern albania experienced extensional tectonics in plio quaternary time related to southward roll back of the aegean active margin

experimental partial melting of the st severin ll and lost city h chondrites we have performed melting experiments between 1170 1325 c and at an oxygen fugacity of iw 1 on the ordinary chondrites lost city h and st severin ll in order to investigate the origin of diogenites and their possible relationship to eucrites low ca pyroxene disappears from the lost city residuum just above 1220 c at a mg near 66 accordingly lost city does not have a sufficiently large stability field for orthopyroxene to be a likely parent composition for melts which could crystallize diogenites in contrast orthopyroxene in st severin persists up to about 1325 c at which point it has a mg similar to that of typical diogenites 75 this is only marginally compatible with diogenite origins as the most magnesian pyroxenes from diogenites mg 82 and howardites mg 85 are more mgo rich than any pyroxene formed from a st severin melt accordingly if diogenites formed from a source having the bulk composition of an ordinary chondrite than either the fo2 had to be lower than that inferred for eucrite formation or more likely the parent body had a lower fesi ratio than that of an ll chondrite also low temperature melts of st severin are depleted in elements compatible with pyroxene and so while broadly eucritic do not closely match the compositions of eucrites that are thought to be primary partial melts hence it doesn t appear possible to produce both diogenites and eucrites from the same source region composition if eucrites are primary partial melts reconnaiss mce experiments at higher oxygen fugacity iw 2 produce broadly angritic melts for lost city as was observed for murchison and allende in contrast our na2o poor st severin charge produced a broadly eucritic melt at this higher fo2 this difference in melting behavior is probably because ll chondrites have a much lower bulk fesi ratio than the h cm or cv chondrites

disequilibrium partial melting experiments on the leedey l6 chondrite textural controls on melting processes a series of experiments were designed to investigate the textural and compositional changes that take place during disequilibrium partial melting of chondritic material chips of the l6 chondrite leedey were heated at 1200 degreesc and log fo 2 iw 1 for durations of 1 h to 21 days we observed a progression of kinetically controlled textural changes in melt and restite minerals and changes in the liquidus mineralogy in response to factors such as volatile loss during the course of the experiments both olivine and orthopyroxene recrystallized at different times rare relic chondrules could still be identified after 21 days the silicate melts that form are very heterogeneous in terms of both major and trace element chemistry reflecting heterogeneity of the localized mineral assemblage particularly with respect to phosphates and clinopyroxene metal sulfide melts formed in short duration runs are also heterogeneous the experimental data are relevant to aspects of the genesis of primitive achondrites such as the acapulcoites the observed textures are consistent with a model for acapulcoite petrogenesis in which silicate melting was limited to only a few volume percent of the chondritic source rock the experiments are also relevant to the behavior of chondritic material that has been partially melted in an impact environment

slow spreading accretion and mantle denudation in the mirdita ophiolite albania the mirdita ophiolite in albania occupies a n s corridor which escaped most alpine and cenozoic deformation possibly due to a thick ophiolitic basement the sheeted dike complex strikes ns and dips steeply indicating that the ridge was oriented parallel to the ns corridor and that the ophiolite has not been significantly tilted although differential motion between individual massifs cannot be excluded classically the western massifs have been considered lherzolitic and the eastern massifs harzburgitic detailed structural mapping reveals that the deep mantle section was harzburgitic and that the major differences between the two are restricted to the uppermost mantle and lower crustal section these are typically ophiolitic in the eastern massifs being composed of a thick dunitic transition zone rich in basaltic impregnations and chromite deposits overlain by a lower crust of layered gabbros in contrast in the western massifs the uppermost mantle is composed of highly strained to mylonitic lherzolites which originate from more depleted harzburgites by impregnation and tectonic dispersion of melt during deformation occurring at 1000 800 c layered gabbros are locally absent and the crust can be reduced to diabase dikes or sills and extrusives the diabase intrusions are locally sheared together with peridotites and metamorphosed to amphibolites the contrast between the eastern and western massifs is ascribed to successive episodes of magmatic and amagmatic spreading in a slow spreading environment the low t high strain deformation of the western massifs is localized in the dome shaped envelopes of these massifs this structure and even the present day topography of the western massifs evoke the turtleback domes described along the mid atlantic ridge mar and explained by mantle denudation tucholke et al 1998

reconstructing plate boundaries in the jurassic neo tethys from the east and west vardar ophiolites greece and serbia jurassic subduction initiation in the neo tethys ocean eventually led to the collision of the adria africa and eurasia continents and the formation of an 6 000 km long alpine orogen spanning from iberia to iran reconstructing the location and geometry of the plate boundaries of the now disappeared neo tethys during the initial moments of its closure is instrumental to perform more realistic plate reconstructions of this region of ancient ocean basins in general and on the process of subduction initiation neo tethyan relics are preserved in an ophiolite belt distributed above the dinaric hellenic fold thrust belt here we provide the first quantitative constraints on the geometry of the spreading ridges and trenches active in the jurassic neo tethys using a paleomagnetically based net tectonic rotation analysis of sheeted dykes and dykes from the west and east vardar ophiolites of serbia maljen and ibar and greece othris pindos vourinos and guevgueli based on our results and existing geological evidence we show that initial middle jurassic 175 ma closure of the western neo tethys was accommodated at a n s trending west dipping subduction zone initiated near and parallel to the spreading ridge the west vardar ophiolites formed in the forearc parallel to this new trench simultaneously the east vardar ophiolites formed above a second n s to nw se trending subduction zone located close to the european passive margin we tentatively propose that this second subduction zone had been active since at least the middle triassic simultaneously accommodating the closure of the paleo tethys and the back arc opening of neo tethys

the behavior of actinides phosphorus and rare earth elements during chondrite metamorphism new data on the u pu and p distributions in less metamorphosed h chondrites type 3 5 coupled with literature results permit a provisional picture to be assembled of the chemistry of these elements and for the rare earth elements in ordinary chondrites and the changes brought about by chondritic metamorphism preferential associations of phosphates with metals and or sulndes in all chondrites strongly indicate an initially siderophile or conceivably chalcophile character for p in ordinary chondrite precursor materials with phosphate subsequently formed by oxidation this oxidation occurred prior to or during chondritic metal silicate fractionation uranium is initially concentrated in chondrule glass at 100 ppb levels with phosphates primarily merrillite in h 3 chondrites being essentially u free 20 ppb as chondrule glass devitrified during metamorphism u migrated into phosphates reaching 50 ppb in nadiabondi h 5 merrillite and 200 300 ppb in merrillite from equilibrated chondrites but froze out before total concentration in phosphates occurred relative 244 pu fission track densities in the outer 5 m of olivine and pyroxene grains in contact with merrillite and with chondrule mesostasis in bremervorde h 3 give pu mesostasis pu merrillite 0 01 implying total concentration of pu in phosphates similarly no detectable pu 0 1 ppb was found in chondrule mesostasis in tieschitz and sharps whereas direct measurements of tracks in phosphates in h 3 chondrites are consistent with high 10 ppb pu concentrations thus a strong pu p correlation is indicated for ordinary chondrites there is variable pu u fractionation in all chondritic phosphates reaching an extreme degree in the unequilibrated chondrites therefore the pu u ratio in phosphates appears relatively useless for relative meteorite chronology literature data indicate that the ree are located in chondrules in unequilibrated chondrites most likely in glass thus there may also be strong pu nd fractionation within these meteorites like u the ree migrate into phosphates during metamorphism but unlike u appear to be quantitatively concentrated in phosphates in equilibrated chondrites thus relative ages based on pu nd may be possible for equilibrated chondrites but the same chronological conclusions are probably obtainable from pu concentrations in phosphates i e on the pu p ratio however pu p chronology is possible only for ordinary chondrites so there appears to be no universal reference element to cancel the effects of pu chemical fractionation in all meteorites available data are consistent with but certainly do not prove that variations in pu p represent age differences but if these age differences do not exist then it is conceivable that the solar system 244 pu 238 u ratio important for cosmochronology is still lower than the presently accepted value of 0 007

the portales valley meteorite breccia evidence for impact induced melting and metamorphism of an ordinary chondrite abstract the portales valley h chondrite fall is an annealed impact melt breccia with coarse metal interstitial to angular and subrounded silicate clasts the large metal rich regions exhibit a widmanstatten structure and contain very little troilite we were able to examine a 16 5 kg metal rich specimen of portales valley silicates contain numerous flecks of metallic cu and curvilinear trails of tiny metallic fe ni blebs characteristic of shocked and annealed chondrites one silicate clast appears to have experienced little

evolution of boninites and island arc tholeiites in the pindos ophiolite greece whole rock geochemical and sm nd isotope data are presented for a representative suite of crustal rocks from the pindos ophiolite in order to resolve the origin of the geochemical signature of the boninites comparison is made with triassic morb from the avdella melange and with other ophiolitic rocks of northwestern greece hydrothermal alteration results in large scatter in sr and k and some variability in ba th and u the pindos boninites contain high zr and hf with respect to ree characteristic of many boninites pb la sm and nb decrease with decreasing tio 2 from morb to iat but then increase in the boninites nd isotopic values expressed as e nd decrease systematically with decreasing tio 2 from 7 8 in iat to 0 6 3 0 in boninites as mantle wedge harzburgite became increasingly depleted another magma source contributed significant amounts of pb ree and probably nb the pb and other large ion lithophile elements may have been transported in aqueous solutions from the subducting slab but the ree and nb imply an ocean island basalt oib type source within the mantle wedge this oib source is a consequence of mantle plume activity during late triassic rifting

shock features in iron nickel metal and troilite of l group ordinary chondrites shock metamorphic features in opaque minerals feni metal and troilite of 22 l chondrites have been studied petrographically and geochemically in an attempt to establish a connection between the present silicate based shock classification scheme stoffler et al 1991 and the peak shock and postshock thermal history recorded in these minerals unshocked to weakly shocked s1 s3 l chondrites contain feni metal and troilite that display textures related to normal slow cooling they may also contain rare disequilibrium shock features which suggest localized departures from equilibrium shock conditions above shock stage s3 selected melting of feni metal and troilite produces melt droplets whose composition and abundance correspond to the maximum equilibrium shock state achieved by the sample at these higher shock levels the abundance of other shock induced features such as polycrystalline kamacite sheared and fizzed troilite coarse grained pearlitic plessite polycrystalline troilite and polymineralic melt veins serve as textural criteria that can be used to establish peak shock conditions minimum postshock temperatures obtained from analyses of plessite components show a systematic increase in temperature with an increase in shock stage thereby providing additional information about the postshock thermal histories of l chondrites at the highest shock levels recorded in l chondrites s6 and above melting and chemical homogenization of feni metal produces flattened ni profiles that may partially to completely obscure any evidence for an earlier slow cooling history all of these features serve as aids for shock classifying l chondrites as well as for quantifying minimum peak temperatures that resulted during shock metamorphism

shock metamorphism of ordinary chondrites a revised petrographic classification of progressive stages of shock metamorphism of ordinary chondrites is proposed six stages of shock s1 to s6 are defined based on shock effects in olivine and plagioclase as recognized by thin section microscopy the characteristic shock effects of each shock stage are s1 unshocked sharp optical extinction of olivine s2 very weakly shocked undulatory extinction of olivine s3 weakly shocked planar fractures in olivine s4 moderately shocked mosaicism in olivine s5 strongly shocked isotropization of plagioclase maskelynite and planar deformation features in olivine and s6 very strongly shocked recrystallization of olivine sometimes combined with phase transformations ringwoodite and or phases produced by dissociation reactions s6 effects are always restricted to regions adjacent to melted portions of a sample which is otherwise only strongly shocked in stages s3 to s6 localized melting results from stress and temperature peaks which locally deviate from the equilibration shock pressure due to differences in shock impedance these melting effects are a opaque melt veins shock veins b melt pockets with interconnecting melt veins c melt dikes and d troilite metal deposits in fractures based on a critical evaluation of data from shock recovery experiments a shock pressure calibration for the six shock stages is proposed which defines the s1 s2 s2 s3 s3 s4 s4 s5 and s5 s6 transitions at 5 5 10 15 20 30 35 and 45 55 gpa respectively whole rock melting and formation of impact melt rocks or melt breccias occurs at about 75 90 gpa the symbol for the shock stage may be used in combination with the symbol for the petrologic type to abbreviate the complete classification of a chondrite e g h5 s3 we propose this new shock classification and pressure calibration system to replace previous systems which are out of date with respect to the pressure calibration nominally restricted to l chondrites and based on incomplete and in part illdefined sets of shock effects we have classified seventy six ordinary chondrites using the new classification system and conclude the following 1 1 shock effects and the sequence of progressively increasing degrees of shock metamorphism are very similar in h l and ll groups differences in the frequency distribution of shock stages are relatively minor e g l chondrites appear to have the largest fraction with stages s5 and s6 this suggests that the collisional histories of the h l and ll parent bodies were similar 2 2 petrologic type 3 chondrites are deficient in stages s4 to s6 and with increasing petrologic type the frequency of stages s4 to s6 increases we suggest that the more porous and volatile rich type 3 chondrites are subject to melting at a lower shock pressure than the nonporous chondrites of higher petrologic type volatiles trapped in pores cause shock induced dispersal of the shocked and melted material into small particles which are not expected to survive as meteorites 3 3 stage s3 is the most abundant in nearly all petrologic types 4 4 at shock pressures in excess of about 35 gpa s5 and s6 4he and 40ar are almost completely lost pressures below 10 gpa s1 and s2 do not cause noble gas losses

accretion metamorphism and brecciation of ordinary chondrites evidence from petrologic studies of meteorites from roosevelt county new mexico we have made petrologic studies of a suite of 29 newly discovered ordinary chondrites from roosevelt county new mexico including an h3 chondrite that is rich in solar wind gases and contains clasts of melt rock and graphite magnetite rich material and an l5 chondrite with a unique igneous inclusion our data and published work show that mean cao concentrations of low ca pyroxene can be used to distinguish petrologic types in h l and ll groups mean cao concentrations in olivine show significant variations among equilibrated chondrites but these are not correlated with petrologic type varied cooling rates could be responsible the degree of heterogeneity of feo concentrations in olivines of types 4 6 is also not correlated with degree of metamorphism it may be controlled more by postmetamorphic brecciation and mixing of material than by metamorphism mean feo concentrations of silicates show average increases of 3 5 from type 4 to type 6 in each group this increase and the accompanying increase in bulk concentrations of siderophile elements appear to predate parent body formation correlation of nebula controlled bulk chemistry and planetary metamorphic effects could only have been produced under special conditions of accretion one possible origin is runaway growth which allowed parent bodies to form by accretion kilometer sized planetesimals to larger bodies accompanied by differential movement of chemically distinct large and small bodies to produce radially zoned parent bodies that were subsequently metamorphosed by internal heating alternatively ordinary chondrites may have been metamorphosed in a variety of planetesimals each of which had a uniform chemical composition and metamorphic temperature correlations between composition and temperature may have arisen if both were correlated with another parameter such as heliocentric distance

h l chondrite lapaz icefield 031047 a feather of icarus abstract antarctic meteorite lap 031047 is an ordinary chondrite composed of loosely consolidated chondritic fragments its petrography oxygen isotopic composition and geochemical inventory are ambiguous and indicate an intermediate character between h and l chondrites petrographic indicators suggest lap 031047 suffered a shock metamorphic overprint below 10 gpa which did not destroy its unusually high porosity of 27 vol metallographic textures in lap 031047 indicate heating above 700 c and subsequent cooling which caused massive transformation of taenite to kamacite the depletion of thermally labile trace elements the crystallization of chondritic glass to microcrystalline plagioclase of unusual composition and the occurrence of coarsely crystallized chondrule fragments is further evidence for post metamorphic heating to 700 750 c however this heating event had a transient character because olivine and low ca pyroxene did not equilibrate nearly complete degassing up to very high temperatures is indicated by the thorough resetting of lap 031047 s ar ar reservoir 100 55 ma ago a noble gas cosmic ray exposure age indicates it was reduced to a meter size fragment at

the bangong lake ophiolite nw tibet and its bearing on the tectonic evolution of the bangong nujiang suture zone abstract the jurassic bangong lake ophiolite nw tibet is a key element within the western part of the bangong nujiang suture zone which marks the boundary between the lhasa and qiangtang blocks it is a tectonic melange consisting of numerous blocks of peridotite mafic lavas and dikes the mantle peridotites include both clinopyroxene bearing and clinopyroxene free harzburgites the cpx bearing harzburgite contains al rich spinel with low cr s 20 25 resembling peridotites formed in mid ocean ridge settings on the other hand the cpx free harzburgite is highly depleted with cr rich spinel cr 69 73 typical of peridotites formed in subduction zone environments mafic rocks include lavas of n morb and e morb affinity and boninites the n morb rocks consist of pillow lavas and mafic dikes whereas the e morb rocks are brecciated basalts the boninites have high sio 2 53 2 57 9 wt mgo 6 5 12 5 wt cr 166 752 ppm and ni 63 213 ppm and low tio 2 0 22 0 37 wt and y 5 34 8 10 ppm and are characterized by having u shaped chondrite normalized ree patterns the n morb and e morb lavas probably formed by different degrees of partial melting of primitive mantle whereas the boninites reflect partial melting of depleted peridotite in a suprasubduction zone environment the geochemistry of the ophiolite suggests that it is a fragment of oceanic lithosphere formed originally at a mid ocean ridge mor and then trapped above an intraoceanic subduction zone ssz where the mantle peridotites were modified by boninitic melts the bangong nujiang suture zone is believed to mark the boundary between two blocks within gondwanaland rather than to separate gondwanaland from eurasia

preservation of ancient impact ages on the r chondrite parent body 40ar 39ar age of hornblende bearing r chondrite lap 04840 the hornblende and biotite bearing r chondrite lap 04840 is a rare kind of meteorite possibly containing outer solar system water stored during metamorphism or postshock annealing deep within an asteroid because little is known regarding its age and origin we determined 40ar 39ar ages on hornblende rich separates of the meteorite and obtained plateau ages of 4340 40 to 4380 30 ma these well defined plateau ages coupled with evidence for postshock annealing indicate this meteorite records an ancient shock event and subsequent annealing the age of 4340 4380 ma or 4 34 4 38 ga for this and other previously dated r chondrites is much older than most impact events recorded by ordinary chondrites and points to an ancient event or events that predated the late heavy bombardment that is recorded in so many meteorites and lunar samples

cat mountain a meteoritic sample of an impact melted asteroid regolith cat mountain is a new ordinary chondrite impact melt breccia that contains several shocked chondrule bearing clasts of l5 material these clasts are surrounded by a total impact melt of similar composition material which appears to have cooled over a period of a few thousand years probably within a melt breccia lens in the bottom of a large 1 km diameter crater on an l chondrite asteroid noble gas isotopes indicate that the sample was involved in at least two different impact events approximately 880 and 20 myr ago following the 4 55 ga accretion of primitive chondritic material the 880 ma event is responsible for the impact breccia texture of the sample and the 20 ma event reduced the sample to a meter sized object we also infer that another impact occurred between 880 and 20 ma possibly the 500 ma event recorded in many other l chondrites to jettison the material from the asteroid belt into an orbit that evolved into an earth crossing trajectory the shock metamorphic processes that occurred at 880 ma redistributed the opaque phases in the meteorite and altered the crystalline characteristics of silicate phases this reduced the reflectance of the l5 material and decreased the amplitude of its spectral absorption features these characteristics are consistent with the spectral characteristics of some c class asteroids and suggest that some dark asteroids that appear to belong to the c class could be covered with shocked ordinary chondrite material if one assumes that cat mountain came from the same asteroid as other l chondrites with the same cosmic ray exposure age then the juxtaposition of these different materials suggests asteroids are rubble piles which are heterogeneous on a scale less than 100 m furthermore the structural integrity of cat mountain and other l chondrites suggests the strengths of asteroid rubble piles are limited by fractures and contrasting material properties and are thus inherently weak in a ram pressure regime produced when they enter a planetary atmosphere however in a regime where the asteroid is the target of impact fragmentation rather than the projectile the added porosity of a rubble pile structure will compensate for the presence of fractures and absorb a large amount of the impact energy in this case the structural integrity of the asteroid may appear to be the same as a previously unshocked chondritic material

tectonic significance of the geochemistry and petrology of ophiolites in southeast anatolia turkey abstract the southeast anatolian ophiolites form discontinuous linear belts of oceanic fragments immediately north of the bitlis zagros suture which marks a continental collision zone between the arabian platform to the south and the taurides to the north the late cretaceous ophiolites in the southeast anatolia are represented by the kizildag in the hatay area the goksun and the berit to the north of kahramanmaras ispendere in the malatya area and komurhan guleman in the elazig area the goksun berit ispendere komurhan and guleman ophiolites were attached to the base of the tauride platform i e the malatya keban along its northern active margin and intruded by the i type calc alkaline granitoids of late cretaceous age in contrast the kizildag hatay ophiolite was thrust over the arabian passive margin in the south the cumulate rocks in the ophiolites are represented by dunite wehrlite lherzolite olivine clinopyroxenite olivine gabbronorite olivine gabbro gabbronorite and gabbro highly magnesian olivines fo88 to 74 and pyroxenes mg 95 to 60 as well as highly calcic plagioclases an95 to 68 from the cumulate rocks differ from oceanic equivalents which mainly formed from a mor basaltic melt the order of crystallization in mineral phases whole rock and mineral chemistry data from the cumulates suggest that the primary magma is compositionaly similar to that observed in modern island arc tholeiitic sequences the volcanic and subvolcanic rocks exhibit tholeiitic compositions chondrite normalized ree n morb normalized multi element patterns and tectonic discrimination diagrams suggest the existence of two main types of parental basic magmas in the crustal rocks of the se anatolian ophiolites these are i iat series that characterize the kizildag goksun ispendere komurhan and guleman ophiolites and ii low ti boninitic series known only in the kizildag ophiolite the geochemistry of the crustal rocks suggests that they formed in a suprasubduction zone tectonic setting including arc forearc environments in the southern neotethys these well preserved plutonic oceanic crustal remnants mainly exhibit an intact ophiolite pseudostratigraphy and are overlain by a volcanic sedimentary unit made up of alternations of basic to acidic extrusive rocks debris flows volcaniclastic sandstones and pelagic limestones interpreted as a tholeiitic ensimatic island arc assemblage built on the suprasubduction zone type crust the metamorphism of some of the ophiolites i e berit in the se anatolia might be related to later stages of intraoceanic subduction during the evolution of the southern neotethys

postshock annealing and postannealing shock in equilibrated ordinary chondrites implications for the thermal and shock histories of chondritic asteroids abstract in addition to shock effects in olivine plagioclase orthopyroxene and ca pyroxene petrographic shock indicators in equilibrated ordinary chondrites oc include chromite veinlets chromite plagioclase assemblages polycrystalline troilite metallic cu irregularly shaped troilite grains within metallic fe ni rapidly solidified metal sulfide intergrowths martensite and various types of plessite metal sulfide veins large metal and or sulfide nodules silicate melt veins silicate darkening low ca clinopyroxene silicate melt pockets and large regions of silicate melt the presence of some of these indicators in every petrologic type 4 to 6 ordinary chondrite demonstrates that collisional events caused all equilibrated oc to reach shock stages s3 s6 those type 4 to 6 oc that are classified as shock stage s1 on the basis of sharp optical extinction in olivine underwent postshock annealing due to burial beneath materials heated by the impact event those type 4 to 6 oc that are classified s2 on the basis of undulose extinction and lack of planar fractures in olivine were shocked to stage s3 s6 annealed to stage s1 and then shocked again to stage s2 some oc were probably shocked to stage s3 after annealing it seems likely that many oc experienced multiple episodes of shock and annealing because 40ar 39ar chronological data indicate that mil 99301 ll6 s1 was annealed 4 26 ga ago presumably as a consequence of a major impact it seems reasonable to suggest that other equilibrated s1 and s2 oc which contain relict shock features were also annealed by impacts because some type 6 s1 oc e g guarena kernouve portales valley all of which contain relict shock features were annealed 4 44 4 45 ga ago during a period when impacts were prevalent and most oc were thermally metamorphosed it follows that impact induced annealing could have contributed significantly to oc thermal metamorphism

origin of rapidly solidified metal troilite grains in chondrites and iron meteorites inclusions of troilite and metallic fe ni 0 2 4 mm in size with a dendritic or cellular texture were observed in 12 ordinary chondrites cooling rates in the interval 1400 950 c calculated from the spacing of secondary dendrite arms or cell widths and published experimental data range from 10 7 to 104 c sec in 8 of these chondrites which are breccias containing some normal slow cooled metal grains the inclusions solidified before they were incorporated into the breccias their cooling rates of 1 300 c sec indicate cooling by radiation or by conduction in contact with cold silicate or hot silicate volumes only 6 40 mm in size this is quantitative evidence that these inclusions and their associated clasts were melted on the surface of a parent body by impact and were not formed at depth from an internally derived melt in ramsdorf rose city and shaw which show extensive reheating to 1000 c fe fes textures in melted areas are coarser and indicate cooling rates of 10 1 to 10 4 c sec during solidification this metal may have solidified inside hot silicate volumes that were 10 300 cm in size as shaw and rose city are breccias of unmelted and melted material their melted metal did not necessarily cool through 1000 c within a few m of the surface shock melted fine grained irregular intergrowths of metal and troilite formed in situ in many irons and some chondrites by rapid solidification at cooling rates of 105 c sec their kamacite and taenite compositions may result from annealing at 250 c of metallic glass or exceedingly fine grained quench products

emplacement of the jurassic mirdita ophiolites southern albania evidence from associated clastic and carbonate sediments sedimentology can shed light on the emplacement of oceanic lithosphere i e ophiolites onto continental crust and post emplacement settings an example chosen here is the well exposed jurassic mirdita ophiolite in southern albania successions studied in five different ophiolitic massifs voskopoja luniku shpati rehove and morava document variable depositional processes and palaeoenvironments in the light of evidence from comparable settings elsewhere e g n albania n greece ophiolitic extrusive rocks pillow basalts and lava breccias locally retain an intact cover of oceanic radiolarian chert in the shpati massif elsewhere ophiolite derived clastics typically overlie basaltic extrusives or ultramafic rocks directly the oldest dated sediments are calpionellid and ammonite bearing pelagic carbonates of latest jurassic berrasian age similar calpionellid limestones elsewhere n albania n greece post date the regional ophiolite emplacement at one locality in s albania voskopoja calpionellid limestones are gradationally underlain by thick ophiolite derived breccias containing both ultramafic and mafic clasts that were derived by mass wasting of subaqueous fault scarps during or soon after the latest stages of ophiolite emplacement an intercalation of serpentinite rich debris flows at this locality is indicative of mobilisation of hydrated oceanic ultramafic rocks some of the ophiolite derived conglomerates e g shpati massif include well rounded serpentinite and basalt clasts suggestive of a high energy shallow water origin the berriasian pelagic limestones at voskopoja experienced reworking and slumping probably related to shallowing and a switch to neritic deposition mixed ophiolite derived clastic and neritic carbonate sediments accumulated later during the early cretaceous mainly barremian aptian in variable deltaic lagoonal and shallow marine settings these sediments were influenced by local tectonics or eustatic sea level change terrigenous sediment gradually encroached from neighbouring landmasses as the ophiolite was faulted or eroded an aptian transgression was followed by regression creating a local unconformity e g at boboshtica a turonian marine transgression initiated widespread upper cretaceous shelf carbonate deposition in the regional context the southern albania ophiolites appear to have been rapidly emplaced onto a continental margin in a subaqueous setting during the late jurassic late oxfordian late tithonian this was followed by gradual emergence probably in response to thinning of the ophiolite by erosion and or exhumation the sedimentary cover of the south albanian ophiolites is consistent with rapid relatively short distance emplacement of a regional scale ophiolite over a local pelagonian korabi microcontinent

relationships among physical properties as indicators of high temperature deformation or post shock thermal annealing in ordinary chondrites collisions and attendant shock compaction must have been important for the accretion and lithification of planetesimals including the parent bodies of chondrites but the conditions under which these occurred are not well constrained a simple model for the compaction of chondrites predicts that shock intensity as recorded by shock stage should be related to porosity and grain fabric to test this model we studied sixteen ordinary chondrites of different groups h l ll using x ray computed microtomography ct to measure porosity and metal fabric ideal gas pycnometry and 3d laser scanning to determine porosity and optical microscopy om to determine shock stage these included a subsample of six chondrites previously studied using transmission electron microscopy tem to characterize microstructures in olivine combining with previous data results support the simple model in general but not for chondrites with low shock porosity foliation low spf chondrites these include kernouve h6 portales valley h6 7 butsura h6 park l6 gro 85209 l6 estacado h6 mil 99301 ll6 spade h6 and queen s mercy h6 among others the data for these meteorites are best explained by high ambient heat during or after shock low spf chondrites tend to have older 40ar 39ar ages 4435 4526 ma than other non low spf type 6 chondrites in this study we conclude that the h l and ll asteroids all were shock compacted at an early stage while warm with collisions occurring during metamorphic heating of the parent bodies results ultimately bear on whether chondrite parent bodies have internal structures more akin to a metamorphosed onion shell or metamorphosed rubble pile and on the nature of accretion and lithification processes for planetesimals

equilibration temperatures of the ordinary chondrites a new evaluation abstract kretz 1982 and lindsley 1983 have each calibrated the ca pyroxene thermometer these new calibrations are applied to 24 analyzed pyroxenes from 24 h6 l6 and ll6 chondrites both thermometers agree that the h group equilibrated to a significantly lower temperature 820 830 c than the l and ll groups the two thermometers disagree on the value of the higher temperature of equilibration of the l and ll groups giving values of 860 c lindsley to 930 c kretz

an re os date for molybdenite bearing quartz vein mineralization within the kangerlussuaq alkaline complex east greenland implications for the timing of regional metallogenesis the kangerlussuaq alkaline complex east greenland is one of the largest alkaline complexes in the world it is known to host a number of occurrences of base and precious metal mineralization including the flammefjeld porphyry mo deposits and a suite of epithermal style base and precious metal veins in an area around the sondre syenitgletscher here we describe and date a previously unknown occurrence of molybdenite pyrite scheelite bearing quartz veins associated with alkali basalt dikes that intrude the main syenites of the complex the veins appear to be cogenetic with the dikes which crosscut and brecciate granodiorites of the cirque 1320 complex sulfur isotope signatures of molybdenite and pyrite in the veins give a tight range with mean 34 s of 2 2 0 7 consistent with a magmatic s source related to the intrusion of the dikes molybdenite from the veins gave an re os age of 52 74 0 26 ma some 13 m y older than molybdenite at the flammefjeld porphyry deposit thus distinguishing this metallogenic episode as a distinct event temporally unrelated to the known mo mineralization at flammefjeld significantly in terms of the timing of regional metallogenesis and magmatism our date suggests that the age of the kangerlussuaq alkaline complex itself may be slightly older than the 50 ma currently accepted based on 40 ar 39 ar dating from biotites in the main syenites given the common discrepancy in ore systems of 40 ar 39 ar which produce slightly younger dates than those by re os and u pb we suggest the actual age of the kangerlussuaq alkaline complex may be closer to 53 to 52 ma

mesoscale and microscale shock effects in the ll6 s4 chondrites saint severin and elbert a tale of two breccias

metamorphic soles from the albanian ophiolites petrology 40ar 39ar geochronology and geodynamic evolution in the eastern mediterranean region the albanian ophiolites are remnants of one branch of the tethyan oceanic basin obducted on carbonate margins during the mesozoic pressure temperature time and structural data in the metamorphic soles of these ophiolites place new constraints on the closure of this basin and the emplacement history of oceanic rocks the petrological and structural work documents an apparently inverted metamorphic gradient with granulite facies at the top decreasing downward through amphibolite then greenschist facies until unmetamorphosed rocks thermobarometry indicates that the granulite facies formed at peak temperatures of 800 860 c for pressures close to 1 gpa these conditions point to the subduction of the ophiolites beneath young oceanic lithosphere to a depth of 30 40 km a large set of 40ar 39ar data has been obtained on the metamorphic soles and some ophiolite intrusives using single grains and bulk mineral populations ages range from 160 to 174 ma corresponding to the middle jurassic bajocian bathonian within the same sole the convergence of results using single grains and mineral populations of amphibole and muscovite and the lack of internal age gradient within muscovite are interpreted to result from fast cooling of metamorphic soles the main result documented by this study is a systematic younging of metamorphic sole ages from south to north with a difference of 14 ma along the 140 km length of the belt the age of igneous rocks such as plagiogranites and mafic dikes in the ophiolites is equivalent to that of metamorphic soles which indicates that the ophiolite was still hot and young at the time of metamorphic sole formation the emplacement of the albanian ophiolites is the consequence of a complex tectonic evolution continental rifting slow mid ocean spreading followed by west dipping intraoceanic subduction initiated at the ridge axis symmetric intraoceanic thrusting with opposite vergence and at last emplacement onto the continental margin the geochronological and stratigraphic data suggest that this evolution lasted only a few million years moreover they indicate that this evolution stopped earlier to the south with the closure of a narrow basin than to the north where the basin was wider due to faster oceanic spreading

the hellenic ophiolites eastward or westward obduction of the maliac ocean a discussion ophiolitic bodies in the dinaro hellenic mountain belt are among the most important ones in the peri mediterranean alpine chains the characteristic feature of this ophiolitic belt is its middle to late jurassic age of obduction the ophiolitic bodies form two major belts on each side of the pelagonian zone an east pelagonian belt in the vardarian domain and a supra pelagonian ophiolitic belt spo to the west the different hypotheses relative to the origin of the spo present geodynamic evolution models accounting for most of the available data a mid triassic episode of rifting a ladinian jurassic episode of sea floor spreading forming notably the maliac ocean a middle to late jurassic convergent period with subduction and obduction episodes and finally a late episode of tertiary compressional deformation responsible for the westward thrusting of the jurassic ophiolitic nappes over the external zones despite many studies dating from the early 1970s the eastern or western pelagonian origin of these ophiolites especially the spo is still under dispute whatever the adopted hypothesis we consider that the main spo bodies n pindos vourinos othris evia argolis have the same origin because of their geographic continuity and of the similarities in their geological characteristics we propose that this ocean corresponds everywhere to the maliac ocean defined in othris from the well preserved sedimentary oceanic margin and ophiolitic nappes thrust during the late jurassic obduction onto the pelagonian platform there is strong evidence for the existence of two deep basins on both sides of the pelagonian continental ridge during triassic jurassic times they correspond respectively to the vardar area to the east and the pindos domain to the west one of these domains being at the origin of the spo the hypothesis of an eastward emplacement of the spo from the pindos domain is based mainly on sedimentological data from the margin series and on structural analyses of ophiolitic bodies however we conclude the westward obduction of the maliac ocean originating from the vardar area to be the best fitting model this westward model is supported by paleogeographic and structural constraints on regional scale notably the absence of obducted ophiolites in the jurassic series of the koziakas units units attributed to the western pelagonian margin and of the parnassus domain on the eastern side of the pindos basin is difficult to reconcile with an eastward obduction from the pindos domain other observations such as the distribution of ophiolitic detritus in the internal and external zones also promote the westward late jurassic obduction of the maliac ocean our preferred model offers a consistent explanation for the mechanism and timing of the emplacement of the spo as well as providing insight on the origin and emplacement of the vardarian ophiolites following this hypothesis there is no need for a clear boundary between the spo and the west vardarian ophiolitic bodies as they were obducted from the same oceanic basin and during the same jurassic tectonic event in this paper we develop evidence in favor of the eastern pelagonian origin for the spo our adopted model and provide discussion on the data supporting the main alternative hypothesis western origin for the spo

geodynamic evolution of ophiolites from albania and greece dinaric hellenic belt one two or more oceanic basins all the geological constraints for an exhaustive reconstruction of the triassic to tertiary tectonic history of the southern dinaric hellenic belt can be found in albania and greece this article aims to schematically reconstruct this long tectonic evolution primarily based on a detailed analysis of the tectonic setting the stratigraphy the geochemistry and the age of the ophiolites in contrast to what was previously reported in the literature we propose a new subdivision on a regional scale of the ophiolite complexes cropping out in albania and greece this new subdivision includes six types of ophiolite occurrences each corresponding to different tectonic units derived from a single obducted sheet these units are represented by 1 sub ophiolite melange 2 triassic ocean floor ophiolites 3 metamorphic soles 4 jurassic fore arc ophiolites 5 jurassic intra oceanic arc ophiolites and 6 jurassic back arc basin ophiolites the overall features of these ophiolites are coherent with the existence of a single though composite oceanic basin located east of the adria pelagonian continental margin this oceanic basin was originated during the middle triassic and was subsequently early jurassic affected by an east dipping intra oceanic subduction this subduction was responsible for the birth of intra oceanic arc and back arc oceanic basins separated by a continental volcanic arc during the early to middle jurassic from the uppermost middle jurassic to the early cretaceous an obduction developed during which the ophiolites were thrust westwards firstly onto the neighboring oceanic lithosphere and then onto the adria margin

spade an h chondrite impact melt breccia that experienced post shock annealing the low modal abundances of relict chondrules 1 8 vol and of coarse i e 200 m size isolated mafic silicate grains 1 8 vol in spade relative to mean h6 chondrites 11 4 and 9 8 vol respectively show spade to be a rock that has experienced a significant degree of melting various petrographic features e g chromite plagioclase assemblages chromite veinlets silicate darkening indicate that melting was caused by shock plagioclase was melted during the shock event and flowed so that it partially to completely surrounded nearby mafic silicate grains during crystallization plagioclase developed igneous zoning low ca pyroxene that crystallized from the melt or equilibrated with the melt at high temperatures acquired relatively high amounts of cao metallic fe ni cooled rapidly below the fe ni solvus and transformed into martensite subsequent reheating of the rock caused transformation of martensite into abundant duplex plessite ambiguities exist in the shock stage assignment of spade the extensive silicate darkening the occurrence of chromite plagioclase assemblages and the impact melted characteristics of spade are consistent with shock stage s6 low shock stage s2 is indicated by the undulose extinction and lack of planar fractures in olivine this suggests that spade reached a maximum prior shock level equivalent to stage s6 and then experienced post shock annealing probably to stage s1 these events were followed by a less intense impact that produced the undulose extinction in the olivine characteristic of shock stage s2 annealing could have occurred if spade were emplaced near impact melts beneath the crater floor or deposited in close proximity to hot debris within an ejecta blanket spade firmly establishes the case for post shock annealing this may have been a common process on oc asteroids

shock and annealing in the amphibole and mica bearing r chondrites mil 11207 r6 and lap 04840 r6 contain hornblende and phlogopite mil 07440 r6 contains accessory titan phlogopite and no hornblende all three meteorites have been shocked mil 11207 contains extensive sulfide veins pyroxene that formed from dehydrated hornblende and an extensive network of plagioclase glass mil 07440 contains chromite plagioclase assemblages chromite veinlets and blebs pincer shaped plagioclase patches but no sulfide veins lap 04840 contains olivine grains with chromite bleb laden cores and opaque free rims rare grains of pyroxene that formed from dehydrated hornblende and no sulfide veins these meteorites appear to have been heated to maximum temperatures of approximately 700 900 c under conditions of moderately high ph2o perhaps 250 500 bars all three samples underwent postshock annealing during this process olivine crystal lattices healed giving the rocks the appearance of shock stage s1 and diffusion of fe and s from thin sulfide veins to coarse sulfide grains caused the veins to disappear in mil 07440 and lap 04840 this latter process apparently also occurred in most s1 s2 ordinary chondrites of high petrologic type the pressure temperature conditions responsible for forming the amphibole and mica in these rocks may have been present at depths of a few tens of kilometers as suggested in the literature a giant impact or a series of smaller impacts would then have been required to excavate the hornblende and biotite bearing rocks and bring them closer to the surface it was in that latter location where the samples were shocked deposited in a hot ejecta blanket of low thermal diffusivity and annealed

shock and thermal history of northwest africa 4859 an annealed impact melt breccia of ll chondrite parentage containing unusual igneous features and pentlandite abstract northwest africa 4859 nwa 4859 is a meteorite of ll chondrite parentage that shows unusual igneous features and contains widely distributed pentlandite the most obvious unusual feature is a high proportion of large 3 cm diameter igneous textured enclaves lites interpreted as shock melts that were intruded into an ll chondrite host one such lite appears to have been produced by whole rock melting of ll chondrite initial rapid partial crystallization and subsequent slow cooling of the residual melt in the host to produce a differentiated object other unusual features include mm sized overgrowth objects fine grained plagioclase rich bands and coarse troilite 7 mm across grains all these features are interpreted as having crystallized from melts produced by a single transient shock event followed by slow cooling a subsequent shock event of moderate s3 intensity produced veining and transformed some of the pyroxene into the clinoenstatite polytype pentlandite together with associated troilite in nwa 4859 probably formed by the breakdown of a monosulfide precursor phase at low temperature 230 c following the second shock event nwa 4859 is interpreted to be an unusual impact melt breccia that contains shock melt which crystallized in different forms at depth within the parent body

chromite plagioclase assemblages as a new shock indicator implications for the shock and thermal histories of ordinary chondrites abstract chromite in ordinary chondrites oc can be used as a shock indicator a survey of 76 equilibrated h l and ll chondrites shows that unshocked chromite grains occur in equant subhedral and rounded morphologies surrounded by silicate or intergrown with metallic fe ni and or troilite some unmelted chromite grains are fractured or crushed during whole rock brecciation others are transected by opaque veins the veins form when impacts cause localized heating of metal troilite intergrowths above the fe fes eutectic 988 c mobilization of metal troilite melts and penetration of the melt into fractures in chromite grains chromite plagioclase assemblages occur in nearly every shock stage s3 s6 oc the assemblages range in size from 20 300 m and consist of 0 2 20 m size euhedral subhedral anhedral and rounded chromite grains surrounded by plagioclase or glass of plagioclase composition plagioclase has a low impedance to shock compression heat from shock melted plagioclase caused adjacent chromite grains to melt chromite grains crystallized from this melt those chromite grains in the assemblages that are completely surrounded by plagioclase are generally richer in al 2 o 3 than unmelted matrix chromite grains in the same meteorite chromite veinlets typically 0 5 2 m thick and 10 300 m long occur typically in the vicinity of chromite plagioclase assemblages the veinlets formed from chromite plagioclase melts that were injected into fractures in neighboring silicate grains chromite crystallized in the fractures and the residual plagioclase rich melt continued to flow eventually pooling to form plagioclase rich melt pockets chromite rich chondrules consisting mainly of olivine plagioclase normative mesostasis and 5 15 vol chromite occur in many shocked oc and oc regolith breccias but they are absent from primitive type 3 oc they may have formed by impact melting chromite plagioclase and adjacent mafic silicates during higher energy shock events the melt was jetted from the impact site and formed droplets due to surface tension crystallization of these droplets may have commenced in flight prior to landing on the parent body surface chromite plagioclase assemblages and chromite veinlets occur in 25 out of 25 shock stage s1 oc of petrologic type 5 and 6 that i examined although these rocks contain unstrained olivine with sharp optical extinction most possess other shock indicators such as extensive silicate darkening numerous occurrences of metallic cu polycrystalline troilite and opaque veins it seems likely that these rocks were shocked to levels at least as high as shock stage s3 and then annealed by heat generated during the shock event during annealing the olivine crystal lattices healed but other shock indicators survived published ar ar age data for some s1 oc indicate that many shock and annealing events occurred very early in the history of the parent asteroids the common occurrence of shocked and annealed oc is consistent with collisions being a major mechanism responsible for metamorphosing oc

petrogenesis of acapulcoites and lodranites a shock melting model abstract acapulcoites are modeled as having formed by shock melting cr like carbonaceous chondrite precursors the degree of melting of some acapulcoites was low enough to allow the preservation of 3 6 vol relict chondrules shock effects in acapulcoites include veins of metallic fe ni and troilite polycrystalline kamacite fine grained metal troilite assemblages metallic cu and irregularly shaped troilite grains within metallic fe ni while at elevated temperatures acapulcoites experienced appreciable reduction because graphite is present in some acapulcoites and lodranites it seems likely that carbon was the principal reducing agent reduction is responsible for the low contents of olivine fa 4 14 mol and low ca pyroxene fs 3 13 mol in the acapulcoites the observation that in more than two thirds of the acapulcoites the fa value is lower than the fs value in contrast to the case for equilibrated ordinary chondrites the low feo mno ratios in acapulcoite olivine 16 18 compared to 32 38 in equilibrated h chondrites the relatively high modal orthopyroxene olivine ratios e g 1 7 in monument draw compared to 0 74 in h chondrites and reverse zoning in some mafic silicate grains lodranites formed in a similar manner to acapulcoites but suffered more extensive heating loss of plagioclase and loss of an fe ni s melt acapulcoites and lodranites experienced moderate post shock annealing presumably resulting from burial beneath material of low thermal diffusivity the annealing process repaired damaged olivine crystal lattices lending acapulcoites and lodranites the appearance of unshocked i e shock stage s1 rocks any high pressure phases that may have formed during initial shock reverted to their low pressure polymorphs during annealing some samples were subsequently shocked again several acapulcoites reached shock stage s2 levels alh 84190 reached s3 and the lodranite mac 88177 reached s5

overview of ophiolites and related units in the late palaeozoic early cenozoic magmatic and tectonic development of tethys in the northern part of the balkan region abstract the northern balkan peninsula including serbia montenegro bosnia croatia and the former yugoslavian republic of macedonia represents an excellent region for the study of tectonic processes related to mesozoic tethyan ophiolite genesis and emplacement we first summarise the main tectonic units of the northern balkan peninsula and then use this information to discuss tectonic processes including rifting sea floor spreading ophiolite genesis and emplacement melange accretion ocean basin closure and collision we then discuss alternative models of ophiolite genesis and emplacement for the region and suggest that multi ocean basin interpretations fit the data better than single ocean basin interpretations rifting of adria gondwana during the triassic created a rift in the south budva zone and opened a triassic oceanic basin further north dinaride ocean occurrences of inferred sub continental mantle lithosphere in the dinaride ophiolite belt e g zlatibor may record extensional exhumation within an ocean continent transition zone bordering the adria dinaride continent this was followed by emplacement together with ophiolites and melange during upper jurassic early cretaceous time upper triassic radiolarites and mid ocean ridge type basalts formed at a spreading ridge after continental break up the oceanic lithosphere of the dinaride ophiolite belt was partly generated above a subduction zone the metamorphic soles of the dinaride ophiolites formed during mid late jurassic mainly based on k ar dating widespread melange that is associated with the ophiolites represents a subduction complex controlled by tectonic accretion and sedimentary reworking in trench and fore arc basin settings a possible cause of jurassic dinaride ophiolite emplacement was collision of a subduction trench with a continental margin further north mesozoic oceanic lithosphere subducted northeastwards present coordinates opening a late jurassic marginal basin in the main vardar zone the dinaride ocean in the south closed during late jurassic early cretaceous time tithonian berriasian deformed oceanic crust melange and magmatic arc rocks further north main vardar zone were transgressed by mainly clastic sediments during the early cretaceous however part of the vardar ocean vardar zone western belt or sava zone remained partially open until latest cretaceous time generally northward subduction within this remnant ocean triggered further supra subduction zone ophiolite genesis during the late cretaceous the ocean closed by the maastrichtian followed by early cenozoic regional scale southward thrusting that locally intercalated older and younger mesozoic ophiolites and melanges future progress particularly depends on determining the crystallisation ages of the ophiolites obtaining better structural data on the direction of initial ophiolite emplacement and unravelling the palaeozoic tectonic development of the eurasian continental margin

tectono stratigraphy and evolution of the mesozoic pindos ophiolite and related units northwestern greece the northwest pindos mountains expose mesozoic and tertiary thrust sheets which include the jurassic pindos ophiolite composed of ultramafic and mafic oceanic crustal rocks the tectono stratigraphy of these units is i mainly ultramafic ophiolitic thrust sheet and basal metamorphic sole dramala complex and loumnitsa unit ii late cretaceous platformal limestones orliakas group iii dismembered intrusive and extrusive ophiolitic rocks aspropotamos complex iv tectonic melange and olistostromes dominated by triassic jurassic volcanic and sedimtntary rocks avdella melange v coherent thrust sheets of late jurassic late cretaceous deep water sediments dio dendra group vi early tertiary flysch pindos flysch immobile trace element studies indicate that triassic and jurassic extrusives of the volcanic sedimentary melange formed mainly at within plate and or mid ocean ridge settings by contrast structurally overlying ophiolitic extrusives include boninitic volcanics and island arc tholeiites of supra subduction zone origin initial ophiolite displacement c 165 ma is recorded in metamorphic sole formation including basal peridotite mylonite amphibolites and greenschists the sole rocks have morb and wpb chemical affinities in our favoured model the pindos ophiolite formed above a mid jurassic westerly dipping intra oceanic subduction zone accompanied by accretion to form melange followed by collision and emplacement with the pelagonian margin final suturing of the pindos ocean however did not take place until early tertiary times

the jurassic south albanian ophiolites mor vs ssz type ophiolites abstract within the western belt of the southern albanian ophiolites the voskopoja ophiolite consists of three subunits voskopoja morava and rehove these are predominantly lherzolites with minor harzburgites and dunites in the mantle section above come ultramafic and mafic cumulates including wehrlites troctolites and olivine gabbros gabbronorites are restricted to the morava subunit isotropic clinopyroxene gabbros extrusives and sediments are present only in rehove and voskopoja the volcanic section is dominated by basaltic breccias including megablocks with sheeted dykes pillow lavas and isolated dykes the basaltic breccias grade upwards into sandstones in turn interlayered with argillites and cherts of jurassic age the basalts are predominantly clinopyroxene plagioclase basalts either aphyric or plagioclase phyric geochemically they are divided into four groups 1 an intermediate ti and zr group with low ni hereafter called low ni group 2 an intermediate ti zr group with high ni hereafter called high ni group 3 a high ti zr group and 4 a low ti zr group the high ni content in group 2 is interpreted as originating from olivine and spinel xenocrysts apart from the high ni content groups 1 and 2 are comparable with the volcanics of the low to high ti intermediate ophiolites by contrast group 3 is more comparable to the high ti ophiolitic extrusives in the western ophiolite belt of northern albania group 4 consists ssz type basalts and is widespread in the volcanics of the eastern ophiolite belt comparison of the ultramafic mafic cumulates and the basaltic volcanics with those in the northern part of the western belt in albania and the pindos ophiolite indicates that there is a systematic variation in petrography and geochemistry from north to south in the western belt with an increasingly distinct ssz signature towards the south ultramafic and mafic cumulates as well as basalts from the shebenik massif in the eastern belt are similar to those of voskopoja implying a genetic relationship

geochemistry of lau basin volcanic rocks influence of ridge segmentation and arc proximity side scan sonar imaging of the central lau basin sw pacific has revealed a central lau spreading centre clsc propagating southwards at the expense of an eastern lau spreading centre elsc with a small intermediate lau spreading centre ilsc forming a relay between the two volcanic rocks sampled along these spreading centres and from two adjacent seamounts are glassy to fine grained pillow lavas and sheet flows of basalts ferrobasalts and andesites the evolved rocks are mostly confined to the propagating tip of the clsc and can be explained by a high rate of cooling relative to magma supply as invoked for magma genesis at propagating ridges elsewhere compared with equivalent rocks from the eastern pacific the most evolved members of the clsc suite require similarly high degrees 90 of fractional crystallization from their basaltic parents fractional crystallisation cannot however account for the compositional differences between clsc and elsc lavas whereas the composition of the clsc lavas lies just within the compositional spectrum of typical n morb the elsc lavas are distinctly enriched in alkali and alkaline earth elements reach oxide and apatite saturation at lower fe ti and p concentrations and generally show greater vesicularity despite slightly greater depths of eruption all indicative of a water rich subduction component they also have lower contents of ni sc na and fe and higher contents of si at a given mgo concentration that indicate a more depleted and more hydrous mantle beneath the elsc compared with the clsc these results provide further evidence that beneath the central lau basin the source composition changes progressively from morb type to island arc tholeiite type as the subduction zone is approached both eastwards from the clsc to elsc and southwards along the elsc to the valu fa ridge they also indicate that the composition of the subduction component may vary systematically away from the arc with th lree ba rb and h as h2o all present close to the arc only ba rb and h2o present at intermediate distances and just h2o perceptible at the furthest distances

shocked meteorites argon 40 argon 39 evidence for multiple impacts to contribute to the understanding of the impact history of asteroids we performed a high resolution 40ar 39ar study of ten moderately to highly shocked chondrites which we selected according to the shock classification given by stoffler et al 1991 two recent shocked chondrite falls and two highly shocked eucrites completed our sample suite when possible we separated impact melt from host rock for separate analysis in total we studied 28 samples from 14 meteorites in some cases atmospheric ar that we associate with terrestrial weathering was identified and corrected for the ages we obtained range between 100 ma and 4 1 ga and are clearly distinct from primordial ages that correspond to solar system formation we reproduced the previously reported cluster of l chondrite ages 500 ma the most prominent result of our study is that in the case of chondrites melts generally are older than host rocks or melt embedded unmolten rocks to solve this apparent paradox we propose that the melt forming event which was the most severe shock episode in the history of these meteorites has not been the only occasion affecting their k ar systems at least one later impact metamorphism must have occured the response of the k ar clock to this second event was more severe in the host rock than in the previously in the first event generated melt veins and pockets because of different ar retention rates hence impact metamorphism on meteorite parent bodies indeed was a multistage process extending in time over billions of years

ordinary chondrite metallography part 2 formation of zoned and unzoned metal particles in relatively unshocked h l and ll chondrites we studied the metallography of fe ni metal particles in 17 relatively unshocked ordinary chondrites and interpreted their microstructures using the results of p free fe ni alloy cooling experiments described in reisener and goldstein 2003 two types of fe ni metal particles were observed in the chondrites zoned taenite kamacite particles and zoneless plessite particles which lack systematic ni zoning and consist of tetrataenite in a kamacite matrix both types of metal particles formed during metamorphism in a parent body from homogeneous p poor taenite grains the phase transformations during cooling from peak metamorphic temperatures were controlled by the presence or absence of grain boundaries in the taenite particles polycrystalline taenite particles transformed to zoned taenite kamacite particles by kamacite nucleation at taenite taenite grain boundaries during cooling monocrystalline taenite particles transformed to zoneless plessite particles by martensite formation and subsequent martensite decomposition to tetrataenite and kamacite during the same cooling process the varying proportions of zoned taenite kamacite particles and zoneless plessite particles in types 46 ordinary chondrites can be attributed to the conversion of polycrystalline taenite to monocrystalline taenite during metamorphism type 4 chondrites have no zoneless plessite particles because metamorphism was not intense enough to form monocrystalline taenite particles type 6 chondrites have larger and more abundant zoneless plessite particles than type 5 chondrites because intense metamorphism in type 6 chondrites generated more monocrystalline taenite particles the distribution of zoneless plessite particles in ordinary chondrites is entirely consistent with our understanding of fe ni alloy phase transformations during cooling the distribution cannot be explained by hot accretion autometamorphism post metamorphic brecciation or shock processing

zircon ages of metamorphic and magmatic rocks within peridotite bearing melanges crucial time constraints on early carboniferous extensional tectonics in the chinese tianshan abstract we dated and geochemically characterized peridotite bearing melanges in the chinese south tianshan and within the main tianshan shear zone the yushugou tonghuashan melange in the chinese south tianshan exposes a tectonic juxtaposition of a diapirically emplaced metaperidotite predominantly lherzolite massif with a high grade metamorphic terrane ca 10 km long protolith age ca 445 466 ma metamorphic zircons of a mafic granulite end t 5 0 yielded a weighted mean 206pb 238u age of 341 8 ma that we interpret as the time of granulite facies metamorphism the youngest zircon rims of an intermediate granulite end t 4 3 have a mean age of 332 13 ma that records a retrogressive metamorphic event these ages determine the timing ca 341 332 ma of mantle diapirism through continental crust a dolerite dike end t 2 3 emplaced into metaperidote has a crystallization age of 335 5 ma that on the basis of geochemistry we interpret as representing e morb oib magmatism that accompanied mantle diapirism an undeformed pink granite end t 3 6 intrudes the melange matrix and has an emplacement age of 324 5 ma thus providing an upper time limit for a tectonic movement that led to melange formation the older time limit of deformation ca 362 352 ma is constrained by the youngest ages of thermo tectonically modified zircons in a mylonitized metagabbro and a foliated meta andesite magmatic zircons in the meta andesite end t 3 3 and a dacite end t 5 6 have eruption ages of 433 4 ma and 435 3 ma that date formation of the melange matrix a microgabbro dike end t 10 0 cutting metasediments i e melange matrix has an emplacement age of 279 3 ma and contains abundant zircon xenocrysts ranging in age from archean to late paleozoic similarly a dolerite dike a meta andesite an intermediate granulite and a metagabbro all contain inherited precambrian to paleozoic zircons we accordingly conclude that the melange was formed by mantle diapirism in the early carboniferous and was subsequently affected by an early permian volumetrically minor mafic episode two foliated garnet granites end t 7 0 and 6 9 in direct contact with metaperidotite within the main tianshan shear zone have emplacement ages of 341 2 ma and 329 4 ma that reflect an early carboniferous episode of crustal anatexis they also contain neoproterozoic zircon xenocrysts also within the main tianshan shear zone a gabbro end t 8 7 has an emplacement age of 297 3 ma and contains abundant neoproterozoic to late paleozoic zircon xenocrysts based on the regional geological relationships we associate early carboniferous mantle diapirism and crustal anatexis with lower crustal delamination induced asthenospheric upwelling the numerous precambrian zircon xenocrysts support a chinese tianshan tarim connection the early permian mafic episode coincided in time with the development of an early permian unconformity in the chinese north tianshan

a shock metamorphic model for silicate darkening and compositionally variable plagioclase in ck and ordinary chondrites abstract silicate darkening in ordinary chondrites oc is caused by tiny grains of metallic fe ni and troilite occurring mainly within curvilinear trails that traverse silicate interiors and decorate or in some cases cut across silicate grain boundaries highly shocked oc characterized by olivine grains with undulose to mosaic extinction tend to have greater degrees of silicate darkening than lightly shocked oc this indicates that silicate darkening is probably a result of shock metamorphism the low fe fes eutectic temperature 988 c renders metal and troilite susceptible to melting and mobilization during shock heating a few oc also contain thin melt veins of chromite this implies that localized shock temperatures reached 1635 c in these meteorites silicate darkening is also evident in ck carbonaceous chondrites where magnetite and pentlandite grains form analogous curvilinear trails it is possible that magnetite and pentlandite in ck chondrites were mobilized during shock metamorphism and dispersed through silicate interiors alternatively metal and troilite may have been dispersed and then transformed into magnetite and pentlandite during subsequent oxidation of these chondrites unshocked oc tend to have plagioclase with uniform compositions shocked oc tend to have plagioclase with more variable albeit still stoichiometric compositions the low impedance of plagioclase to shock compression makes it particularly susceptible to melting and mobilization this is consistent with the molten appearance of plagioclase in highly shocked oc e g rose city and paragould ck chondrites also have compositionally variable plagioclase the common association of silicate darkening with compositionally variable plagioclase is consistent with the hypothesis that both are products of shock metamorphism some ck and oc chondrites exhibit light shock effects in olivine that are consistent with equilibrium peak shock pressures that are too low to account for the silicate darkening or opaque shock veins in these meteorites therefore the olivine in these chondrites may have been annealed after intense shock produced these effects a few ck chondrites that contain olivine with undulose or mosaic extinction e g lew87009 and eet83311 may have been shocked again after annealing

petrologic geochemical and experimental constraints on models of chondrule formation abstract the petrologic and geochemical properties of chondrules as well as results of experimental studies provide strong constraints on chondrule formation models nebular formation is indicated by the non mass fractionated oxygen isotopic compositions of bulk chondrules chondrule formation from a melt is required by the prototypical spheroidal shapes and the presence of euhedral phenocrysts and glassy mesostases incomplete melting is indicated by the abundance of porphyritic chondrules which experiments demonstrate require relict nuclei and coarse relict grains the length of time that chondrules were hot is constrained by their retention of relict grains and moderately volatile elements rapid cooling of chondrules after formation is supported by the presence of zoned phenocrysts isotopic anomalies and dynamic crystallization experiments it is clear from the presence of relict grains enveloping compound chondrules and igneous rims that many chondrules were heated again after cooling the heating mechanism responsible for chondrule formation seems to have operated at varying intensities over large regions of the inner solar nebula for at least the time it took ambient nebular temperatures to cool from above 900 to below 600 k the chondrule formation mechanism provided a repeatable source of energy capable of highly localized melting characteristic of flash heating the occurrence of ferroan microchondrules with low melting temperatures within some chondrule rims indicates that chondrule formation did not occur exclusively in high temperature regions near the sun as required in bipolar outflow models mechanisms for forming chondrules that are consistent with the constraints include various flash heating models nebular lightning magnetic reconnection flares gas dynamic shock waves and radiative heating

mid ocean ridge and supra subduction affinities in the pindos ophiolites greece implications for magma genesis in a forearc setting abstract the pindos ophiolitic massif is considered an important key area within the albanide hellenide ophiolitic belt and is represented by two tectonically distinct ophiolitic units 1 a lower unit including an intrusive and a volcanic section and 2 an upper ophiolitic unit mainly including mantle harzburgites both units share similar metamorphic soles and tectono sedimentary melanges at their bases the intrusive section of the lower unit is composed by an alternation of troctolites with various ultramafic rock types including dunites lherzolites olivine websterites olivine gabbros anorthositic gabbros gabbros and rare gabbronorites the volcanic and subvolcanic sequence of the lower unit can geochemically be subdivided into three groups of rocks 1 basalts and basaltic andesites of the lower pillow section showing a clear high ti affinity 2 basaltic andesites of the upper pillow section with high ti affinity but showing many geochemical differences with respect to the first group 3 very low ti boninitic basaltic and basaltic andesitic lava flows separating the lower and upper pillow sections and dykes widespread throughout the pindos ophiolites these different magmatic groups originated from fractional crystallization from different primary magmas which were generated in turn from partial melting of mantle sources progressively depleted by previous melt extractions group 1 volcanics may have derived from partial melting ca 20 of an undepleted lherzolitic source while group 2 basaltic rocks may have derived from partial melting ca 10 of a mantle that had previously experienced mid ocean ridge basalt morb extraction finally the group 3 boninites may have derived from partial melting ca 12 17 of a mantle peridotite previously depleted by primary melt extraction of groups 1 and 2 primary melts in order to explain the coexistence of these geochemically different magma groups two petrogenetic models formerly proposed for the albanian ophiolites are discussed

mineralogy petrology chronology and exposure history of the chelyabinsk meteorite and parent body three masses of the chelyabinsk meteorite have been studied with a wide range of analytical techniques to understand the mineralogical variation and thermal history of the chelyabinsk parent body the samples exhibit little to no postentry oxidation via mossbauer and raman spectroscopy indicating their fresh character but despite the rapid collection and care of handling some low levels of terrestrial contamination did nonetheless result detailed studies show three distinct lithologies indicative of a genomict breccia a light colored lithology is ll5 material that has experienced thermal metamorphism and subsequent shock at levels near s4 the second lithology is a shock darkened ll5 material in which the darkening is caused by melt and metal troilite veins along grain boundaries the third lithology is an impact melt breccia that formed at high temperatures 1600 c and it experienced rapid cooling and degassing of s2 gas portions of light and dark lithologies from chel 101 and the impact melt breccias chel 102 and chel 103 were prepared and analyzed for rb sr sm nd and ar ar dating when combined with results from other studies and chronometers at least eight impact events e g 4 53 ga 4 45 ga 3 73 ga 2 81 ga 1 46 ga 852 ma 312 ma and 27 ma are clearly identified for chelyabinsk indicating a complex history of impacts and heating events finally noble gases yield young cosmic ray exposure ages near 1 ma these young ages together with the absence of measurable cosmogenic derived sm and cr indicate that chelyabinsk may have been derived from a recent breakup event on an neo of ll chondrite composition

petrogenesis and tectonic significance of jurassic iat magma types in the hellenide ophiolites as deduced from the rhodiani ophiolites pelagonian zone greece abstract the rhodiani ophiolites are represented by two tectonically superimposed ophiolitic units the lower ultramafic unit and the upper volcanic unit both bearing calcareous sedimentary covers the ultramafic unit consists of mantle harzburgites with dunite pods and chromitite ores and represents the typical mantle section of supra subduction zone ssz settings the volcanic unit is represented by a sheeted dyke complex overlain by a pillow and massive lava sequence both including basalts basaltic andesites andesites and dacites chemically the volcanic unit displays low ti affinity typical of island arc tholeiite iat ophiolitic series from ssz settings having as most distinctive chemical features low ti v ratios the rare earth element and incompatible element composition of the more primitive basaltic andesites from the rhodiani ophiolites can be successfully reproduced with about 15 non modal fractional melting of depleted lherzolites which are very common in the hellenide ophiolites the calculated residua correspond to the depleted harzburgites found in the rhodiani and othrys ophiolites both field and chemical evidence suggest that the whole sequence of the rhodiani volcanic unit from basalt to dacite originated by low pressure fractional crystallization under partially open system conditions the modelling of mantle source melt generation and mantle residua carried out in this paper provides new constraints for the tectono magmatic evolution of the mirdita pindos oceanic basin

clinopyroxene composition of volcanics from the manipur ophiolite northeastern india implications to geodynamic setting the volcanic section of the manipur ophiolite mo representing the crustal portion of the neo tethyan oceanic lithosphere occurs as basalt basaltic trachyandesite and dacite in the gamnom phangrei sector manipur at 25 01 n 25 09 n and 94 24 e 94 27 e they associate with cherts and ultramafics the clinopyroxene compositions of basalt and basaltic trachyandesite obtained through electron microprobe analyzer were used as a petrogenetic indicator to identify the parent magma types and their tectonic settings based on the variable content of major oxides they are classified as high and low ti clinopyroxenes high ti and al contents with relatively lower silica saturation are observed in the former group and vice versa in the latter the tidcpx rock values in low and high ti clinopyroxene are comparable with island arc basaltic andesite and morb respectively which confirms that the clinopyroxene composition is primarily related to the host magma type and its tectonic setting clinopyroxene thermometry ranging 1150 605 c suggests progressive differentiation of the parent magmas several bivariate and tectonic discrimination diagrams depict morb non orogenic setting and island arc boninitic magma affinities orogenic setting for the high and low ti clinopyroxenes respectively the coexistence of both morb and island arc boninitic magma types in the volcanic section of manipur ophiolite as characterized by their varying ti al and si contents may indicate either juxtaposition of rocks formed in diverse tectonic settings i e due to transformation of tectonic setting from mid ocean ridge to supra subduction zone or a change in magma composition in a subduction zone setting however field relationships coupled with the mineral chemical signatures implies a supra subduction zone setting for the evolution of the crustal section of mo

petrology and geochemistry of patuxent range 91501 a clast poor impact melt from the l chondrite parent body and lewis cliff 88663 an l7 chondrite we have performed petrologic and geochemical studies of patuxent range pat 91501 and lewis cliff lew 88663 pat 91501 originally classified as an l7 chondrite is rather a unique near total impact melt from the l chondrite parent body lewis cliff 88663 was originally classified as an achondrite but we find that it is a very weakly shocked l7 chondrite pat 91501 is an unshocked homogeneous igneous textured ultramafic rock composed of euhedral to subhedral olivine low ca pyroxene augite and chrome rich spinels with interstitial albitic plagioclase and minor silica alumina alkali rich glass only 10 relic chondritic material is present olivine grains are homogeneous fa25 2 26 8 low ca pyroxene wo1 9 7 2en71 9 78 2fs19 9 20 9 and augite wo29 8 39 0en49 2 55 3fs11 8 14 9 display a strong linear tio2 al2o3 correlation resulting from igneous fractionation plagioclase is variable in composition or3 0 7 7ab79 8 84 1an8 2 17 2 chrome rich spinels are variable in composition and zoned from cr rich cores to ti al rich rims some have evolved compositions with up to 7 9 wt tio2 pat 91501 bulk silicate has an l chondrite lithophile element composition except for depletions in zn and br siderophile and chalcophile elements are highly depleted due to sequestration in centimeter size metal troilite nodules the minerals in lew 88663 are more uniform in composition than those in pat 91501 olivine grains have low cao and cr2o3 contents similar to those in l5 6 chondrites pyroxenes have high tio2 contents with only a diffuse tio2 al2o3 correlation low ca pyroxenes are less calcic wo1 6 3 1en76 5 77 0fs20 4 21 4 while augites wo39 5 45 6en46 8 51 1fs7 6 9 4 and plagioclases or2 6 5 7ab74 1 83 1an11 2 23 3 are more calcic spinels are homogeneous and compositionally similar to those in l6 chondrites lew 88663 has an l chondrite bulk composition for lithophile elements and only slight depletions in siderophile and chalcophile elements that are plausibly due to weathering and or sample heterogeneity

first finding of impact melt in the iie netschaevo meteorite about half of the iie nonmagmatic iron meteorites contain silicate inclusions with a primitive to differentiated nature the presence of preserved chondrules has been reported for two iie meteorites so far netschaevo and mont dieu which represent the most primitive silicate material within this group in this study silicate inclusions from two samples of netschaevo were examined both silicate inclusions are characterized by a porphyritic texture dominated by clusters of coarse grained olivine and pyroxene set in a fine grained groundmass that consists of new crystals of olivine and a glassy appearing matrix this texture does not correspond to the description of the previously examined pieces of netschaevo which consist of primitive chondrule bearing angular clasts detailed petrographic observations and geochemical analyses suggest that the investigated samples of netschaevo consist of quenched impact melt this implies that netschaevo is a breccia containing metamorphosed and impact melt rock imr clasts and that collisions played a major role in the formation of the iie group