petrology geochemistry and u pb geochronology of the betic ophiolites inferences for pangaea break up and birth of the westernmost tethys ocean abstract the betic ophiolitic unit part of the mulhacen complex in the betic cordillera se spain is composed of several tectonic slices of eclogitized mafic and ultramafic rocks together with oceanic metasediments a comprehensive review of the petrological characteristics of the betic ophiolites shows notable analogies with other ophiolites from the alps apennines system indicating that they represent relics of oceanic lithosphere created at the westernmost end of the western tethys u pb dating by shrimp sensitive high resolution ion microprobe on zircon grains carefully investigated by cathodoluminescence images has been carried out on an eclogitized metagabbro lugros outcrop and a metadolerite dyke intruded in ultramafic rocks almirez outcrop giving consistent magmatic ages of around 185 3 ma this age marks the beginning of the betic oceanic magmatism transitional to enriched morb that followed pangaea break up and the earliest oceanic accretion of the western tethys ocean subsequently ultraslow mid ocean ridge spreading developed from the betic oceanic basin northeastward to the ligurian and alpine tethys domains mainly around 165 to 140 ma

late oligocene miocene syn late orogenic successions in western and central mediterranean chains from the betic cordillera to the southern apennines the identification of syn and late orogenic flysch deposits extending from the betic cordillera to the southern apennines assists in the reconstruction of the tectonic sedimentary evolution of the perimediterranean chains a microplate was located between the european and african plates during the late jurassic early cretaceous bordered northwards by the piemontese ocean and southwards by another north africa flysch basin or maghrebian ocean the piemontese ocean and the northern margin of the microplate were structured from the late cretaceous to the eocene to create an eo alpine chain the southern margin of the microplate was deformed in the aquitanian when the internal areas of the maghrebian ocean were characterized by syn orogenic flysch deposits this episode culminated with metamorphism 25 22 ma and nappe emplacement which destroyed the former palaeogeography and created an orogenic belt alkapeca afterwards a lower burdigalian late orogenic cycle started in the deformed area which as a result of the opening of the algero provencal basin caused the fragmentation of the alkapeca its thrusting on the flysch basin and the collision with the north africa and south iberia margins these latter were folded and thrusted the flysch units pushed over the external domain and also back thrusted langhian late orogenic deposits suture the new tectonic features finally the whole orogen was thrust onto the foredeep during the middle late miocene

two magmatic series with island arc affinities within the vourinos ophiolite within the vourinos ophiolite evidence of two magmatic series has been preserved in cognate cumulates and in effusive and hypabyssal rocks which constitute the earlier krapa sequence and the younger asprokambo sequence the asprokambo dyke basic magmas which are poor in incompatible elements and relatively ni and cr rich bear some resemblance to very low ti basalts transitional to boninites found in subduction related arcs or interarc basins krapa series magmas from sills massive and pillow lavas are best equated with low k tholeiites of island arc suites compositions of al and ti poor cpx in lavas from both series are comparable to those in island arc basalts the asprokambo cpx being richer in ca and cr than those from krapa the large volume of cumulates from the krapa sequence displays the following crystallization order ol sp cpx pl opx mt periodic influx of fresh magma batches into the magma chamber occurred mainly during the formation of the lower cumulates wehrlite ol clinopyroxenite and melagabbro the upper cumulates gabbronorite and leucogabbronorite with minor mt bearing gabbronorite crystallized in the upper levels of a magma chamber which became progressively smaller with time in the asprokambo sequence ol sp opx cpx pi and amph are the successively crystallizing phases the ortho to heteradcumulates consist of websterite pl websterite gabbronorite amphibole bearing leuconorite diorite and granophyre in cumulates especially in the lower krapa sequence significant subsolidus reaction was probably induced by the persistence of high geothermal gradients linked to continuous magmatism petrological features indicate that the evolution of the krapa series is more compatible with an intermediate fractional equilibrium crystallization history in an initially open system whereas nearly perfect fractional crystallization in closed system may have occurred in the small asprokambo magma chambers chemical variations in the lavas of both series can be explained in terms of crystallization of the observed cumulates significantly the asprokambo intrusives have igneous mg hornblende and vanadium bearing chromian aluminous titaniferous magnetite crystallization of which is responsible for the calcalkaline evolutionary trend of these rocks major and trace element modelling necessitates a two stage model for the petrogenesis of the vourinos parental melts involving high degree remelting of previously depleted mantle sources favoured by the influx of subduction derived hydrous fluids the primary magmas parental to the krapa and asprokambo series could have been derived respectively by 20 and 30 equilibrium partial fusion of variably depleted lherzolitic sources leaving residua having a harzburgitic to dunitic composition

early middle paleozoic intraplate orogeny in the ogcheon belt south korea a new insight on the paleozoic buildup of east asia the polycyclic ogcheon belt of south korea represents the boundary between two precambrian blocks investigation on the timing and kinematics of the block agglomeration is of considerable bearing on the understanding of the constitution of the asian continent we report here structural evidence showing that the earlier tectonic event dates back to early middle paleozoic the ogcheon tectonism or orogeny the ductile piling up of nappes of the ogcheon belt corresponds to an intracontinental orogeny involving the opening and the subsequent closure of an aborted rift and not of a wide oceanic area the nappes contain unequivocal evidence for ductile shearing with a northwest over southeast sense of movement field evidence includes the presence of s1 and s1 2 foliations bearing an extensive stretching lineation perpendicular to the belt associated with a strongly noncoaxial deformation regime the f1 isoclinal folds are commonly a type and sheath folds whose axes are parallel to the stretching lineation whereas the southeast facing f2 recumbent folds have subhorizontal axes parallel to the belt all lines of evidence unconformity superposed deformations age of the metamorphism etc imply that the d1 2 ogcheon tectonism is pre middle carboniferous late silurian early devonian the subsequent indosinian middle triassic f3 upright synfolial folds that trend nne ssw and later structures clearly overprint the middle paleozoic ductile thrusts and isoclinal folds the early paleozoic ogcheon supergroup is formed of a thick volcanosedimentary pile deposited above an early platform sequence in a rift basin on the basis of the geochemical characteristics of the metavolcanics it appears that oceanization did not occur and ogcheon rift aborted soon after it was formed we suggest that middle paleozoic ductile nappe structures have been formed by the structural inversion of the rift extensional features during the ductile stacking episode the early formed foliation and isoclinal folds have been folded in a continuum during the same tectonic event middle carboniferous terrigenous formations have been deposited unconformably upon folded and thrust older rocks a tentative correlation with early paleozoic imjingang qinling and cathaysian belts within the asian continent reveals that ogcheon rift was emplaced within the south china plate and that imjingang belt should represent the limit between north and south china blocks in the korean peninsula

an outline of the plate tectonics of china china is principally a part of the eurasian plate but the margins of the indian and philippine sea plates are involved in the himalayas and in the coastal range of taiwan respectively within the eurasian plate the cathaysian paleoplate is separated from the angaraian paleoplate by the junggar hegen suture which contains paleozoic ophiolites and rare blueschists the three microplates of the cathaysian paleoplate consist of precambrian cratons and or phanerozoic accretionary fold belts these coalesced precambrian cratons record at least six stages of intense orogeny before cratonization the paleozoic to cenozoic accretionary fold belts of china can be correlated with similar events now found in west pacific andean and atlantic type active continental margins ophiolites occupying many of these tectonic zones provide evidence for the age and igneous history of oceanic crust formed during the paleozoic to cenozoic the presence of blueschist in some of these chinese sutures reveals evidence of large scale subduction and tectonic exhumation during consolidation of the eurasian plate cenozoic collision of the eurasian and indian plates produced deformation and uplift of the himalayas strongly influencing the tectonics of western china in contrast mesozoic tertiary evolution of eastern china is typical basin range geology similar to that of the western united states which included development of deep sedimentary basins along with calc alkaline plutonic and volcanic activity associated with crustal thinning and high heat flow the complicated tectonic evolution of china is greatly illuminated by the presence of ophiolites and blueschists in proterozoic to tertiary convergent boundaries these petrotectonic assemblages provide evidence of an extremely mobile history of plate movement in china

palaeomagnetic constraints on the collision and rotation of north and south china the eastern half of china is dominated by the north china block ncb and south china block scb separated by a complex fold system that trends roughly east west interpretation of the geology of this fold belt has led to a variety of estimates of the age of amalgamation of these two blocks1 7 palaeomagnetic results are few and have also been subject to conflicting interpretations8 11 here we present our own palaeomagnetic results for late permian rocks for both the scb and ncb and summarize other relevant palaeomagnetic data by reinterpreting the polarity given in some previous studies we are able to synthesize the results for the widespread emeishan basalts of the scb and to suggest a new model for the collision and relative rotation of the two blocks

retrograde formation of nacl scapolite in high pressure metaevaporites from the cordilleras beticas spain a permo triassic pelite carbonate rock series with interacalated metabasitic rocks in the cordilleras beticas spain was metamorphosed during the alpine metamorphism at high pressures pmin near 18 kbar the rocks show well preserved sedimentary features of evaporites such as pseudomorphs of talc of kyanite phengitetalc biotite and of quartz after sulfate minerals and relicts of baryte anhydrite nacl and kcl indicating a salt clay mixture of illite chlorite talc and halite as the original rock the evaporitic metapelites have a whole rock composition characterized by high mg mg ca ratios 0 7 variable alkaline and sr ba contents but are mostly k2o rich 8 8 wt the f 2600 ppm cl 3600 ppm and p2o5 0 24 wt contents are also high the pelitic member of this series is a fine grained biotite rock kyanite phengite talc biotite aggregates in pseudomorphs developed in the high pressure stage albite rich plagioclase was formed when the rocks crossed the albite stability curve in the early stages of the uplift scapolite rich in nacl ca ca na mol 24 40 and poor in so4 with cl cl co3 ratios between 0 6 and 0 8 formed as porphyroblasts sometimes replacing up to 60 of the rock in a late stage of metamorphism between 10 and 5 kbar near 600 c no reaction with albite is observed and the scapolite formed from biotite by begin gathered al biotite caco 3 nacl sio 2 hfill al poor biotite scapolite mgco 3 kcl hfill mgcl 2 h 2 o hfill end gathered

from adria to africa driven orogenesis evidence from porphyroblasts in the betic cordillera spain abstract in order to unravel the kinematic evolution of the betic rif orogen we investigated the geometry of porphyroblast inclusion trails in the nevado filabride complex a high pressure unit situated at the base of the betic nappe stack a large number of quantitative orientation data for these microstructures and their mutual overprinting relationships reveal two age groups of inclusion trails in the study area with distinctive geographic trends coupled to a comprehensive compilation of field data in the study area these microstructures are shown to coherently witness the development of early ne sw trending structures and fabrics followed by younger nw se trending ones whereby the former experienced clockwise reorientations a major change in the direction of bulk crustal shortening from nw se to ne sw is inferred from these data and tentatively linked to the plate tectonic history of the mediterranean region paleomagnetic data indicate that prior to about 35 ma nw se convergence between adria eurasia and iberia produced orogenesis along a belt bordering greater iberia including sardinian and corsica and probably continuous in the alps brianconnais domain the roughly ne sw trend of this precursor appears to be preserved within porphyroblasts that grew during this period presumably due to the partitioning of deformation around them africa iberia relative motion had been mainly transcurrent until 35 ma azores transform but then rapidly changed to head on nne ssw convergence this gave rise to a second set of nw se trending structures which accompanied deformation of the precursor orogen into several oroclines western alps gibraltar and calabria arcs orogenic collapse and crustal extension initiated shortly after this transition in tectonic regime suggesting a causal relationship we propose that the waning of compressive stresses transmitted by adria and their substitution by a near perpendicular maximum stress direction resulting from africa eurasia convergence could have enhanced the sinking of a thick lithospheric root or roll back of a subducted slab which in turn would have provoked crustal extension and basin formation

relative elemental mobility during hydrothermal alteration of a basic sill isle of skye n w scotland in sconser quarry isle of skye a thin tertiary basic sill is cut by vertical veins which formed fluid conduits in a major meteoric hydrothermal system in order to study the relative mobility of different elements during hydrothermal metamorphism sill material adjacent to a large 6 mm wide vein was cut into slices and subjected to geochemical and isotopic analysis

metagabbro and associated eclogites in the lubrin area nevado filabride complex spain abstract several small bodies of metabasite maximum dimensions of 1000 m x 500 m are included in the metamorphic rocks of the nevado filabride complex in the betic cordilleras almeria region the body of 400 m x 100 m located 200 m due west of the lubrin village contains troctolitic gabbro with well preserved igneous textures and mineral compositions wholly amphibolitized gabbro garnet bearing metagabbro eclogite along with the textural and mineral changes sensible and regular geochemical variations can be observed where the content of mgo decreases from 24 to 11 while that of cao and na2o increases from 7 to 11 and from 2 to 3 respectively in addition the content of some minor elements such as sr y nb zr and sc increases while that of ni and cr decreases from troctolitic gabbro to the eclogite the amphibolitized gabbro shows values scattered around those of the troctolitic gabbro these geochemical variations are ascribed to inherited differences in the pre metamorphic protolith i e a fractionated gabbro which varies from olivine rich to clinopyroxene rich gabbro nevertheless some metasomatism affected the lubrin body without changing the main chemical trends as documented by the significantly different 87sr 86sr ratios of each rock type this points to a metasomatism which involved the introduction of crustal radiogenic strontium the petrographical and mineral chemical features are interpreted to be the result of syn metamorphic fluid circulation possibly combined with deformation by shearing the igneous texture and mineral chemistry have been retained wherever both fluid circulation and shearing were ineffective on the contrary where both events were effective the formation of eclogite occurred later the entire body underwent a retrogressive amphi bolitic stage under greenschist facies conditions which was probably responsible for the formation of the amphibolitized gabbro portion and for the retrogression of the eclogite

geochemistry and origin of basaltic lavas from marquesas archipelago french polynesia the marquesas archipelago a volcanic chain in french polynesia south central pacific ocean is predominantly composed of alkalic transitional and tholeiitic basalts the variation trends in these intraplate basaltic rocks imply that the magmas were derived from different upper mantle sources model calculations using the total inverse method show that the peridotite source of most marquesas basalts was enriched in incompatible elements compared to a primordial mantle and had higher than chondritic ratios of several elements such as la yb ti v and p ce a metasomatic enrichment event is suggested by the sequence of element enrichment in the source relative to the primordial mantle ba nb la ce sr sm eu zr hf ti y yb on the other hand some lavas including tholeiites of ua pou and alkalic basalts of hiva oa were probably derived from relatively depleted upper mantle in some islands such as hatutu the different types of basalts were generated from sources with rather similar compositions the residual phases of the marquesas magmas included garnet the sources of these magmas were similar in trace element chemistry to the oceanic mantle below hawaii

triassic paleomagnetic data from south china and their bearing on the tectonic evolution of the western circum pacific region abstract we report early and middle triassic paleomagnetic data from the south chinese provinces of fujian and guangxi the characteristic magnetization of the rocks in each case is concluded to be a primary remanence that passes the fold test the triassic pole from western guangxi and four other triassic and late permian poles from three provinces that lie on undisputed parts of the yangtze craton are well clustered this suggests that guangxi except the southeast part was also a part of the yangtze craton at least since the triassic and probably since the late permian with respect to the yangtze craton between the early triassic and the late cretaceous fujian may have been rotated 121 9 counterclockwise and displaced 22 9 north or rotated 59 9 clockwise and displaced 3 9 south depending on the hemisphere in which the magnetization was acquired based on consistency with other paleomagnetic results and in accordance with the geologic data from the area the former interpretation is preferred a remarkable coincidence of mesozoic poles for south china and south korea is observed which if true implies that the south china block except for the displaced terranes in the coastal provinces and korea may have been part of the same continental landmass from the triassic onwards this is consistent with some geologic observations suggesting affinities between the two places it also implies that the major fault zones in north china e g the tan lu fault are unrelated to major fault zones in southeast china e g the changle nanao fault with the former probably associated with the suturing of north and south china and the latter influenced by proto pacific plate motion

cretaceous and tertiary paleomagnetic results from southeast china and their tectonic implications abstract we report cretaceous and tertiary paleomagnetic data from fujian guangxi and guangdong provinces in south china in guangxi province we sampled early cretaceous redbeds of the xinlong formation at eight localities separated by up to 100 km the high temperature component obtained by thermal demagnetization is of dual polarity and passes the fold test and implies 18 8 counterclockwise rotation and insignificant poleward displacement with respect to cratonic localities in sichuan province a tertiary basin lying in a southeast trending fault zone is also rotated counterclockwise by 40 8 both results can be explained very well by a simple block rotation model for the cenozoic tectonic evolution of western guangxi province that is driven by left lateral shear on southeast trending faults the timing and sense of shear are the same as for india asia collision models involving the extrusion of indochina along the red river fault zone which lies several hundred kilometers to the southwest but the magnitude of fault displacement in guangxi is much less in the coastal provinces of fujian and guangdong we find late cretaceous directions that are rotated clockwise with respect to sichuan province combined with similar clockwise rotations found by two other studies of coeval rocks from the same region the mean rotation for the coastal provinces is 12 8 this could indicate a coherent block rotation of the region driven by extrusion tectonics however the limited number of sampling localities also allows the possibility that the directions reflect local rotations due to dextral shearing driven by circum pacific tectonics

the ghomarides nappes rif coastal range morocco a variscan chip in the alpine belt the ghomarides nappes are the uppermost basement nappes in the rifan alpine belt paleozoic rocks constitute the prominent part of these nappes together with triassic and younger cover sediments four nappes are distinguished each one with a distinct stratigraphical sequence especially for the devonian and carboniferous periods sedimentological and structural arguments plead for an initial location on the northern margin of the african continent the main folding and metamorphic events are shown to be eo variscan late acadian tectonic vergence was to the north present coordinates late variscan hercynian shortening gave upright nw se trending folds alpine ductile overprints are scarce except at the base of the lowest nappe where k ar phengite ages are about 25 ma the ghomarides nappes correlate with the malaguides nappes betic cordilleras southern spain the kabylian northern algeria and calabrian southern italy paleozoic nappes the tuscan south alpine basement and the upper austro alpine nappes the proposed correlations allow us to restaure the southern margin of the hercynian belt along northwestern africa

rare earth and trace element characteristics of ophiolitic metabasalts from the alpine apennine belt abstract the western mediterranean jurassic ophiolites contain abundant volcanic rocks that resemble modern ocean floor basalts in this paper we report analyses of rare earth elements ree transition elements ni co cr v sc and high field strength elements ti p y zr ta hf for metabasalts from representative ophiolite outcrops in the eastern alps corsica and the northern apennines liguria the chemical characteristics of the metabasalts range from normal to transitional mid ocean ridge basalt morb most chemical variation in the metabasalts from the different areas can be explained by low pressure fractional crystallization by differences in degree of partial melting and by minor chemical heterogeneities of the source but the transitional morb characteristics of some metabasalts from corsica balagne might reflect formation from a source with different mineral and chemical composition the estimated ree pattern of the source of the liguria type metabasalts corresponds to the pattern for certain ligurian ultramafic rocks which might therefore represent the residue from extraction of some ophiolitic lavas while the liguria type ophiolitic metabasalts might represent products of a normal for instance oceanic ridge the transitional metabasalts might be lavas erupted during the early stages of opening of a small ocean basin or along the continental margin of a larger ocean basin

tectonic evolution of the alpujarride complex betic cordillera southern spain abstract the alpujarride complex belonging to the internal zones of the betic cordillera is an e w elongated domain of metamorphic nappes which extends over more than 400 km in southern spain the alpujarride complex suffered two shearing episodes defined by different kinematic and physical conditions the first one involved ductile thrusting from the south west towards the north east related to the regional metamorphism of the alpujarride nappes this process led to crustal thickening related to which the hot thrusting of the ronda peridotites occurred the second shearing episode directed northwards occurred after the metamorphism it corresponds to a deformation in the brittle ductile transition that led to the thinning of the crust thickened in the previous episode previous tectonic models of the betic cordillera have only considered the northwards shearing which reflect the n s convergence of europe and africa from the oligocene onwards the present work clearly points to the existence of previous episodes of ductile thrusting towards the north east that reflect an earlier process of continental collision and that in future should be considered in tectonic models of the betic orogen

tectonics of south china key to understanding west pacific geology abstract south china is not a post caledonian platform but a composite of orogenic belts this late proterozoic mesozoic orogen is a collage of three continental fragments the three blocks are yangzi huanan and dongnanya and they are separated by the banxi nanpanjiang formerly xianggangzhe and gunanhai suture zones yangzi was separated from gondwanaland during late precambrian when an open ocean called banxi was present between the two continents tectonic processes at an active margin during sinian and early paleozoic time led to the genesis of an accretionary wedge complex the banxi melange and the huanan flysch nappes on the northern margin of huanan which was then the northern margin of the gondwana continent reorganization of plates during the devonian suspended subduction at the huanan active margin huanan was uplifted and unconformably overlain by transgressive deposits of devonian and or carboniferous age a remnant ocean the nanpanjiang sea still existed between yangzi and huanan and deep water sedimentation continued both at the southern margin of yangzi and at the northwestern margin of huanan the latter again became an active margin during late paleozoic when permian and triassic flysch sediments were deposited in foredeeps ahead of advancing nappes huanan and yangzi collided during the triassic and resulted in the deformation of the passive margin of the latter to form the yangzi deformed belt huanan was separated from a more southerly continent dongnanya in the devonian by seafloor spreading which created a late paleozoic and early mesozoic ocean gunanhai ocean a continuously deposited sequence ranging in age from devonian to triassic was laid down on the southern passive margin of huanan on the other side of the ocean the dongnanya permian strata include glacial marine deposits of gondwanaland affinity dongnanya became separated from gondwanaland during the late permian when it marched northward to be reunited with huanan the huanan passive margin sequence was deformed by folding and overthrusting after the late mesozoic collision of huanan and dongnanya scattered outcrops of the suture melange resulting from this collision occur at a few localities in coastal fujian dongnanya in our model was the mesozoic continent of southeast asia peripheral to mainland asia this may or may not have been the eastern continuation of the microcontinent sibumasu siam thailand burma malaysia sumatra the collision of dongnanya and asia gave rise not only to the gunanhai melange of coastal fujian but also to the ophiolite melanges parts of which are present in taiwan tailuko the philippines and western malaysia

geochemistry of metabasites from the nevado filabride complex betic cordilleras spain relics of a dismembered ophiolitic sequence abstract major and trace element distribution including ree data in the metabasites of the nevado filabride complex betic cordilleras spain suggests that the whole mafic ultramafic association represents a dismembered ophiolitic sequence all the analyzed samples have been affected by sea floor alteration yet only the fine grained rocks have suffered some significant mobilization of the lree life the other western mediterranean ophiolites this sequence is related to the opening of the liguro piemontese basin during mesozoic times however the metabasites from betic cordilleras differ from the ophiolitic basalts by higher values of the more incompatible over less incompatible element ratios the entire variation of these ratios in the liguro piemontese basin is related to a dynamic melting process coupled with a diapiric uprise of the mantle source from the stability field of garnet peridotites possibly up to the spinel plagioclase facies transition in this process the ophiolites of the betic cordilleras would record the deepest stages and the lowest degrees of melting this feature is related to the peculiar location of these ophiolites at the western end of the liguro piemontese basin

alteration of the oceanic crust implications for geochemical cycles of lithium and boron abstract fresh tholeiitic basalt glass has been reacted with seawater at 150 c water rock mass ratio of 10 and fresh diabase has been reacted with a na k ca cl fluid at 375 c water rock mass ratios of 1 2 and 5 to understand better the role of temperature basalt composition and water rock mass ratio on the direction and magnitude of b and li exchange during basalt alteration at 150 c slight but nevertheless significant amounts of b and li were removed from seawater and incorporated into a dominantly smectite alteration phase at 375 c however b and li were leached from basalt b behaved as a soluble element and attained concentrations in solution limited only by the b concentration in basalt and the water rock mass ratio li however was less mobile for example at water rock mass ratios of 1 2 and 5 the percent of li leached from basalt was 58 70 and 92 respectively this suggests some mineralogic control on li mobility during hydrothermal alteration of basalt especially at low water rock mass ratios in general these results as well as those for b are consistent with the temperature dependent chemistry of altered seafloor basalt and the chemistry of ridge crest hydrothermal fluids based on the distribution and chemistry of products of seafloor weathering low 150 c and high temperature hydrothermal alteration of basalt and the chemistry of ridge crest hydrothermal fluids it was estimated that alteration of the oceanic crust is a li source for seawater this is not true for b however since the hot spring flux estimated for b is balanced by low temperature basalt alteration these data coupled with b and li flux estimates for other processes e g continental weathering clay mineral adsorption authigenic silicate formation and formation of siliceous skeletal material yield new insight into the b and li geochemical cycles calculations performed here indicate relatively good agreement between the magnitude of b and li sources and sinks the geochemical cycle of b however may be affected by serpentinization of mantle derived peridotite in oceanic fracture zones serpentinites are conspicuously enriched in b and if the b source for these rocks is seawater then an additional b sink exists which must be integrated into the b geochemical cycle however until more data are available in terms of areal extent of serpentinization serpentite chemistry and isotopic composition the importance of b in these rocks with respect to the b geochemical cycle remains speculative at best

a revised lithostratigraphy of the lower middle ordovician beekmantown group st lawrence lowlands quebec and ontario the lower lower middle ordovician beekmantown group of the st lawrence lowlands is a variably thick pertitidal succession of dolostone limestone quartzose carbonate and subordinate siltstone and shale that is gradationally bound by the potsdam group below and unconformably to conformably by the chazy group above it is here considered to include three regionally extensive formations a basal theresa a middle beauharnois and an upper redefined carillon a principal reference section is established in the subsurface the theresa formation is a transgressive succession above potsdam group siliciclastics and below beauharnois formation carbonates it is dominated by quartz arenite and quartzose dolostone skeletal fossils are rare usually gastropods whereas trace fossils are abundant the beauharnois formation is divided into two members a basal ogdensburg and an upper huntingdon the ogdensburg member is sandy especially in its lower part and relatively more fossiliferous than the huntingdon

the eclogitized pillows of the betic ophiolitic association relics of the tethys ocean floor incorporated in the alpine chain after subduction in the volcanic sequence of the betic ophiolitic association boa volcanic structures and textures are preserved in spite of being metamorphosed to the edogite facies the original quenched glassy margins of the pillows and lava flows are still recognizable by the darker colour fine grained textures and scarcity of phenocrysts the boa eclogitized pillows have chemical compositions very similar to the basalts enriched in lil elements erupted nowadays at the mid oceanic ridges magmatism which generated the boa most likely began under continental rift conditions at the triassic jurassic boundary and continued under ocean floor extensional conditions during the lower and middle jurassic in age and petrological characteristics this magmatism is equivalent to that of the western tethys ophiolites during the late cretaceous due to the collision of african and iberian plates the boa ophiolites were subducted and underwent a metamorphism in the eclogite fades whose climax in the lugros outcrop can be estimated at 650 700 c and about 20 kb