provenance of the greater himalayan sequence evidence from mafic granulites and amphibolites in nw bhutan abstract mafic granulites and amphibolites in the masang kang area of nw bhutan himalaya have been investigated for their geochemical and isotopic characteristics in order to determine their protolith history bulk rock major and trace element geochemistry indicate that the rocks were originally tholeiitic and alkali basalts with minor ultramafics u pb zircon sims data suggest an age of 1742 39 ma for mafic magmatism the age corrected e nd 1742 values of the rocks are highly variable ranging from high positive 8 4 to negative 3 3 the positive value suggests a primitive magma source similar to that of rift related tholeiites we suggest that the rocks of the masang kang suite were produced during a major late paleoproterozoic thermal event that caused the mobilization and enrichment of the sub continental lithospheric mantle beneath the north indian margin the geochemical signature of these rift related metabasic rocks may have been produced during an earlier episode of oceanic underplating or subduction from which the fluid required to mobilize and enrich the overlying sub lithospheric mantle may have been derived though their occurrence is rare paleoproterozoic igneous rocks within the greater himalayan sequence ghs in addition to sources identified throughout the lhs may have contributed to the detrital zircon population that form the 1 7 1 9 ga peak in the age spectra of the lesser himalayan sequence lhs in addition the coeval paleoproterozoic magmatism in both lhs and ghs suggests that the two lithotectonic units may have belonged to the same continental plate at that time period

geochemistry and u pb protolith ages of eclogitic rocks of the asis lithodeme piaxtla suite acatlan complex southern mexico tectonothermal activity along the southern margin of the rheic ocean recent data indicating that the piaxtla suite acatlan complex southern mexico underwent eclogite facies metamorphism and exhumation during the devono carboniferous suggest an origin within the rheic ocean rather than the iapetus ocean the asis lithodeme piaxtla suite consists of polydeformed metasediments and eclogitic amphibolites that are intruded by megacrystic granitoid rocks u pb zircon data indicate that the metasediments were deposited after c 700 ma and before intrusion of c 470 420 ma quartz augen granite the metasedimentary rocks contain abundant mesoproterozoic detrital zircons c 1050 1250 ma and a few zircons in the range of c 900 992 and c 1330 1662 ma their geochemical and sm nd isotopic signature is typical of rift related passive margin sediments derived from an ancient cratonic source which is interpreted to be the adjacent mesoproterozoic oaxacan complex megacrystic granites were derived by partial melting of a c 1 ga crustal source similar to the oaxacan complex amphibolitic layers exhibit a continental tholeiitic geochemistry with a c 0 8 1 1 ga source t dm age and are inferred to have originated in a rift related environment by melting of lithospheric mantle in the ordovician this rifting may be related to the early ordovician drift of peri gondwanan terranes e g avalonia from gondwana and the origin of the rheic ocean

the supra ophiolitic sedimentary cover of the asbestos ophiolite quebec canada first geochemical evidence of transition from oceanic to continental sediment flux abstract the 473 5 3 myr asbestos ophiolite complex of the quebec appalachians was formed in a forearc basin and obducted on a margin of laurentia 460 myr ago the complex together with its sedimentary cover is well exposed at burbank hill 130 km sw of quebec city where eight distinct lithologies have been identified 1 pyroxenites and wehrlites with minor dunitic layers 2 fractured gabbros 3 breccia containing gabbro and diabase fragments 4 polygenic conglomerates 5 red mudstone chert sandstone succession 6 tuffs with intercalated grey chert 7 greenish grey mudstones and 8 slates and sandstones of the saint daniel melange the gabbroic and mafic volcanic rocks show the characteristics of arc tholeiites tio2 0 8 0 85 wt and boninites tio2 45 la sm n 1 9 3 3 these rock types also occur as mafic fragments in the polygenic conglomerate the mafic fragments crystals and rocks within the conglomerate and the ophiolitic sandstones also show the same greenschist facies metamorphism as the mafic igneous rocks of the complex the conglomerate was probably formed after the forearc crust was fragmented by pre obduction normal faults these fault scarps would have promoted the erosion of the oceanic crust leading to the accumulation of diverse detritus in grabens the sedimentary lithologies which overlie the conglomerate exhibit a gradual increase in continent derived material la sm n 5 and la yb 20 and a decrease in ophiolitic material moving upward in the stratigraphic sequence the sedimentary rocks at the bottom of the sequence group 3 are mostly composed of ophiolite material with only 20 of crustal material they also have a high content of cr and mgo with values of 350 480 ppm and 5 7 8 respectively the rocks of the uppermost part of the pile group 1 which contain up to 80 crustal material have low abundances of cr and mgo 30 100 ppm and 2 7 4 5 wt continental detritus on the top of the asbestos ophiolite suggests the complex formed near the laurentian continental margin and that the eroded continental material had access to the basin where the ophiolite was formed this is consistent with the obduction of the asbestos and thetford mines ophiolites on the laurentian margin shortly after their formation the proximity of thick laurentian continental crust near the trench could explain why the subduction zone blocked up soon after the ophiolites formation

mid jurassic tectonothermal event superposed on a paleozoic geological record in the acatlan complex of southern mexico hotspot activity during the breakup of pangea u pb isotopic analyses of zircon from the lowest structural units of the acatlan complex of southern mexico indicate that paleozoic tectonothermal events are overprinted by mid jurassic 175 3 to 171 1 ma low pressure migmatization 5 6 kb polyphase deformation and intrusion of felsic and mafic magmas ensuing rapid cooling recorded by 40ar 39ar muscovite biotite and k feldspar ages is estimated to have taken place at 21 3 c my at exhumation rates of 0 6 mm yr such rapid exhumation requires a combination of erosion and tectonic unroofing that is recorded by top to the west kinematic data synchronous tectonic unroofing is also recorded 100 km to the east in the adjacent oaxaca terrane where top to the north extensional shear zones occur in paleozoic strata this pattern of extension suggests tectonic unroofing in response to domal uplift radius 100 km like that associated with core complexes slab windows and hotspots most tectonic analyses for the jurassic place the acatlan complex in the forearc region of an arc in colombia lying 600 800 km inboard of the subduction zone presumably in response to flat slab subduction modern analogues suggest that flat slab subduction reflects subduction of young buoyant oceanic lithosphere adjacent to either a mid oceanic ridge or a plume since core complexes are typical of arc backarc regions and slab windows generally produce metamorphic belts the forearc setting and associated domal uplift suggest a plume to be the most likely cause of this jurassic tectonothermal pulse in southern mexico this plume activity is synchronous with the opening of the gulf of mexico during the breakup of pangea to which it may have contributed

eocene melting of precambrian lithospheric mantle analcime bearing volcanic rocks from the challis kamloops belt of south central british columbia abstract potassic silica undersaturated mafic volcanic rocks form a minor portion of the predominantly calc alkaline eocene challis kamloops volcanic belt which extends from the northwestern united states into central british columbia canada their major occurrence is in the penticton group in south central british columbia where they reach a thickness of up to 500 m and form the northwestern edge of the montana alkaline province these analcime bearing rocks 53 52 ma old are typically rhomb porphyries of ternary feldspar an28ab52or20 additional phenocryst phases include clinopyroxene analcime phlogopite and rare olivine the rocks are characterized by high total alkalis particularly k2o 4 5 wt as well as by a distinct enrichment of large ion lithophile elements versus heavy rare earth elements and high field strength elements they have unusual isotopic compositions compared to most other rocks of the challis kamloops belt particularly high negative nd values and elevated but relatively uniform initial 87sr 86sr ratios 0 7065 the potassic silica undersaturated rocks overlie precambrian crust and lithosphere and were at least in part derived from ancient metasomatized subcontinental mantle lithosphere which was modified in a precambrian subduction setting the alkaline rocks of the challis kamloops belt are related to a slab window environment in particular they were formed above the southern edge of the kula plate adjacent to the kula farallon slab window whereas the montana alkaline province situated well to the southeast was formed directly above the kula farallon slab window upwelling of the hotter asthenospheric mantle may have been the thermal trigger necessary to induce melting of fertile and metasomatized lithospheric mantle

geochemistry of late proterozoic basaltic rocks from southeastern cape breton island nova scotia late proterozoic volcanic rocks of the fourchu group from the avalon zone in southeastern cape breton island nova scotia are composed predominantly of mafic and felsic types with subordinate inte

a grenvillian arc on the margin of amazonia evidence from the southern oaxacan complex southern mexico abstract the rocks in the southern oaxacan complex of mexico form part of the basement of oaxaquia a terrane that underlies most of eastern mexico they have undergone granulite facies metamorphism at 1 ga that was initially overprinted by lower amphibolite facies metamorphism accompanied by extensive hydration and then by greenschist facies metamorphism geochemical data indicate that protoliths include volcanic arc lavas and sediments intruded by a rift related granite the metavolcanic rocks have sio2 ranging between 55 and 72 are typically enriched in highly incompatible elements such as ba rb and light rare earth elements and are accompanied by relative depletion of nb and ti characteristic of subduction related magmas the granite has sio2 ranging between 67 and 70 is enriched in strongly incompatible elements and depleted in nb and ti indicative of either a crustal source or a subduction related tectonic setting concordant u pb analyses of single zircons from this granite yield an age of 1117 4 and 988 5 ma interpreted as the age of intrusion and the time of peak granulite facies metamorphism respectively combined with published information from other parts of mexico these data indicate that oaxaquia was a juvenile arc metamorphosed during a relatively young grenvillian metamorphism at 990 ma similar characteristics may also be found in the andean massifs of colombia and in the appalachian terranes carolina piedmont and avalon of amazonian provenance this suggests that oaxaquia may also have lain adjacent to northwestern amazonia at 1 ga a location that may allow neoproterozoic subduction as the origin of the hydrous lower amphibolite facies metamorphism it would also provide a proximal source for low grade to unmetamorphosed 1 ga and neoproterozoic igneous boulders in northern oaxaquia and 1000 900 ma detrital zircons in neoproterozoic and paleozoic sedimentary rocks in avalonia and iberia

isotope and trace element geochemistry of proterozoic natkusiak flood basalts from the northwestern canadian shield abstract the proterozoic continental tholeiites of natkusiak province 723 ma old of the northwestern canadian shield are part of the franklin magmatic event they resemble phanerozoic flood basalts in their incompatible trace element and sr nd pb isotopic variations as well as in their large volumes and a relatively short period of eruption the wide variations in sr nd pb isotopic and incompatible trace element ratios e g initial nd 5 6 to 5 7 in the natkusiak basalts together with the correlations between the isotopic and trace element ratios imply the involvement of a crustal component in their genesis the natkusiak basalts are inferred to be derived from the subcontinental upper mantle and contaminated by precambrian basement during their ascent

petrological and geochemical relationships between pyroxene megacrysts and associated alkali basalts from massif central france major trace element and ree analyses as well as sr isotopic ratios have been obtained on twelve clinopyene megacrysts and phenocrysts and their alkali basalt hosts from the french massif central equilibrium between crystals and host was examined based on petrographic and geochemical data two types of pyroxenes are recognized the acmite bearing clinopyroxenes rich in incompatible elements and the salitic clinopyroxenes poor in incompatible elements 87sr 86sr isotopic data reveal no significant difference between clinopyroxenes and host lavas they are in apparent isotopic equilibrium the sr isotopic ratios of the two types of pyroxenes are also quite similar however pyroxene crystals from the first group are not in equilibrium with their host they have crystallized at high pressure from differentiated alkali lavas and have been incorporated in a more primitive magma pyroxene crystals from the second group are in apparent equilibrium with their host lava they have crystallized at various pressures for the latter distribution coefficients are proposed for compatible elements trace elements and ree

geochemistry of low grade clastic rocks in the acatlan complex of southern mexico evidence for local provenance in felsic intermediate igneous rocks abstract geochemical data of sedimentary basins have been used to determine their plate tectonic setting here we test geochemical data for paired psammite pelite samples from two greywacke packages that underlie 60 of the paleozoic acatlan complex of southern mexico one an ordovician rift passive margin suite and the other a carboniferous active margin suite both units consist of low grade polydeformed psammites and pelites associated with rare tholeiitic flows and dikes analyses of paired psammite pelite samples from a number of different units in the eastern part of the acatlan complex indicate that they all have a similar geochemistry with their major components dependent on the proportion of sand and clay the chemical index of alteration yielded values between 55 and 70 recording a moderate degree of weathering of plagioclase to illite kaolinite in the source area the ree patterns suggest provenance in felsic intermediate igneous rocks and the wide range of tio2 zr ratios indicates that this was followed by extensive sorting the mantle normalized trace element patterns of the psammites generally display positive zr and hf anomalies reflecting elevated concentration of zircon in these rocks detrital zircon ages from some of these units show that the major source for these clastic rocks probably lay in the adjacent mesoproterozoic oaxacan complex and in the neoproterozoic basement underlying the acatlan complex and or the yucatan peninsula geological parameters point to a rift passive margin setting for the ordovician rocks and an active subduction related margin for the carboniferous units however the commonly used tectonic discrimination bi to tri variant diagrams cannot readily distinguish between the two suites on geochemical grounds their geochemistry probably reflects the general felsic intermediate compositions of the rocks in the precambrian source regions rather than the contemporaneous tectonic setting future work involving a more advanced statistical approach e g multivariant methods multidimensional scaling and classification trees may yield more refined results particularly in combination with detrital zircon geochronology

crustal evolution of the hercynian belt of western europe evidence from lower crustal granulitic xenoliths french massif central abstract granulite facies lower crustal xenoliths scavenged by the tertiary basaltic pipes of bournac and roche pointue massif central france have been analysed for major and trace elements and radiogenic sr nd isotope ratios they display wide variations in major elements rare earth elements and isotopes 87 sr 86 sr 0 7041 0 7216 nd 11 9 to 1 5 which tend to correlate with the three different rock types encountered basic and acidic meta igneous and metasediments the basic liquids are of calc alkaline affinity and cumulates are abundant there is a gap in silica content between the basic and acid meta igneous xenoliths the isotopic compositions suggest mixing of old pre existing continental crust represented by the metasediments and mantle derived basic melts which represent juvenile addition to the crust during the hercynian orogeny the basic meta igneous rocks have been contaminated during their emplacement into the crust whereas the surrounding metasediments have suffered partial fusion and contamination by the same liquid producing the hercynian granitoids which have nd and sr initial ratios identical to those of the lower crustal xenoliths the residue from melting may be represented by the acid meta igneous rocks some of which have positive eu anomalies these results indicate that an important basaltic underplating event occurred during the evolution of hercynian orogenic belt although the exact timing of the underplating requires further constraint

monsoon induced errors in paleotemperature estimation based on leaf morphology analysis in central japan abstract models based on the relationship between leaf morphology and climate parameters of different regions have been used for paleoclimate estimation information from japanese floras has been included in some of these models however variation in the distribution pattern of leaf margin within japan caused by the effect of topography and east asian monsoon has not been reflected in these previously published leaf margin analysis models in this study we investigate the relationship between leaf characters and climate parameters under different climate conditions within a small geographical scale and in doing so improve the accuracy of leaf physiognomy based paleoclimate reconstruction we use the local flora in central japan as an example to determine the distribution pattern of leaf margin toothed and untoothed and habit evergreen and deciduous characteristics under monsoon climate conditions in areas along the pacific ocean p zone the leaf margin and habit types correlated with the temperature parameters and growing season precipitation the p zone includes more toothed species than in other regions of the world obtained with published models at a given mean annual temperature in the areas along the sea of japan j zone with deeper snow cover during the winter the percent of sub canopy evergreen shrubs increases especially at higher altitudes no significant relationship between leaf margin characters and climate parameters was observed in this area as heavy snow was a feature of this region since late early pleistocene times paleoclimate estimation based on leaf margin of fossil assemblages younger than early pleistocene from these areas might be less reliable in addition we found that the leaf margin of woody shrub species is less sensitive to the mean annual temperature than tree species

archaean orogenic ultrapotassic magmatism an example from the southern abitibi greenstone belt the clericy pluton in the southern abitibi greenstone belt of quebec is a syenitic intrusion cross cut by nordmarkitic dykes it is probably one of the oldest known ultrapotassic intrusions about 2 7 ga it crystallized in a crustal environment under low ph20 2 5 kbar and low fo2 conditions most trace elements behaved as compatible elements suggesting that sphene and apatite were involved together with clinopyroxene during low pressure crystal fractionation the large range of initial isotopic ratios calculated for 2 68 ga nd 1 9 to 2 3 as well as the trace element distribution suggest that the syenitic pluton is the product of differentiation from a parental magma with a composition similar to that of timiskaming type archaean shoshonites contaminated by a crustal component on the other hand the nordmarkitic dykes are probably the result of crustal anatexis of metasedimentary rocks and some addition of the syenitic magma the clericy peralkaline syenite differs chemically from typical intraplate syenites in being saturated with respect to quartz and displaying lower ti and hfse contents it is related to an ultrapotassic volcano plutonic event which characterized the last 20 ma of igneous activity in the abitibi greenstone belt it may represent an increased rate of convergence of small heterogeneous plates during a subduction driven accretion tectonic event which possibly marked the culmination of the kenoran orogeny in the superior province of the canadian shield

melting of heterogeneous mantle in a slab window environment examples from the middle tertiary masset basalts queen charlotte islands british columbia voluminous basalts dominate a middle tertiary bimodal volcanic assemblage in the queen charlotte islands british columbia canada these moderately enriched mid ocean ridge basalt morb like magmas were associated with significant crustal extension related to the subduction of the pacific farallon ridge and the initiation of a slab window environment which persisted for 35 ma these asthenospheric melts were derived from a low degree of melting of a heterogeneous peridotite source mainly in the spinel stability field as inferred from fractional melting inversion calculations the basaltic magmas in turn gave rise to both tholeiitic and calc alkaline volcanic strata up to 3 5 km thick in local grabens the tholeiitic basalts resemble morb with rare earth element ree patterns ranging from slightly depleted to enriched in light ree lree the calc alkaline basaltic andesites are enriched in lree but are not strongly depleted in nb the rocks have overlapping nd sr and pb isotopic compositions sim

old and juvenile source of paleozoic and mesozoic basaltic magmas in the acatlan and ayu complexes southern mexico nd isotopic constraints abstract the neoproterozoic paleozoic acatlan complex and the mesozoic ayu complex of southern mexico consist of clastic rocks and rift related igneous rocks inferred to have originated along a rifted passive margin in southwestern mexico either in an inactive neoproterozoic ordovician or an active devonian carboniferous and triassic early jurassic tectonic setting the latter formed on the inner margin of a backarc basin these passive margin rocks were partly underthrust beneath the acatlan complex into which they were subsequently extruded extrusion was synchronous with the backarc basin development thus i the neoproterozoic ordovician rocks underwent underthrusting high pressure metamorphism and extrusion during the late devonian carboniferous 365 330 ma ii the carboniferous rocks underwent underthrusting amphibolite facies metamorphism and extrusion during the permian and triassic and iii the triassic lower jurassic rocks underwent underthrusting amphibolite facies metamorphism and extrusion during the jurassic nd isotopic data from tholeiitic mafic rocks on either side of the hp extrusion zone reveal that both were underlain by similar peri rodinian subcontinental lithospheric mantle in the neoproterozoic ordovician which was supplemented in the devonian carboniferous and triassic early jurassic by a juvenile depleted mantle source the alternation of underthrusting and backarc rifting accompanied by extrusion may be related to flattening and steepening of the beniof zone respectively

correlation coefficient patterns and their interpretation in three basaltic suites single stage magmatic fractionation processes result in igneous rock suites in which correlation coefficients between pairs of elements are generally high the tertiary lavas of e iceland are used as an illustration of this type of behaviour when a second process such as fractionation at a different pressure wall rock assimilation or mixing with a different magma occurs correlation coefficients are altered in a predictable way if the process and the chemical characteristics of the participating rocks minerals and liquids are known a computer program has been devised to simulate complete correlation coefficient matrices for any desired two or multi stage process e g fractional crystallisation coupled with contamination basaltic suites from mahabaleshwar india and the rooi rand southern africa show correlation coefficient patterns characteristic of multi stage evolution and these are interpreted with the aid of computer simulation the mahabaleshwar suite shows clear evidence of contamination by a granitic component the rooi rand rocks however remain problematic

an archean oceanic felsic dyke swarm in a nascent arc the hunter mine group abitibi greenstone belt canada abstract the 2730 ma old hunter mine group hmg a dominantly felsic subaqueous volcanic sequence was formed during early arc construction in the abitibi greenstone belt quebec canada the western part of the hmg contains a felsic dyke swarm up to 1 5 km wide and traceable up section for 2 5 km five distinct generations were identified 1 aphanitic to feldspar phyric dykes 2 quartz feldspar phyric dykes with nd values suggest that an older sialic substrate was not involved in their genesis we infer that the felsic rocks were generated by melting of mafic oceanic crust the swarm was emplaced during nascent oceanic island arc development and was related to rifting of the arc the conformably overlying morb like basalts and basaltic komatiites of the stoughton roquemaure group used the same conduits and further indicate splitting of the arc hmg and associated parts of the abitibi greenstone belts bear a strong resemblance to modern rifted intraoceanic arcs of the western pacific

inception and demise of a neoproterozoic ocean basin evidence from the ougda complex western hoggar algeria the neoproterozoic ougda magmatic complex occurs within platformal carbonate rocks in the western part of the pan african fold belt of the tuareg shield nw africa it is composed of 800 ma old relatively high p t i e grt cpx bearing p 5 kbar t 900 q tholeiitic mafic ultramafic cumulates and related rocks intruded by intermediate to mafic calcalkali plutons e g cpx hbl bearing gabbro and dikes apparent contrasts in structural level of crystallization indicate that the calc alkali rocks are significantly younger than the tholeiites which temporally correlate with a period of regional extension in this part of africa intrusion of the calc alkali rocks may have occurred during the formation of an arc after the tholeiitic rocks had been diapirically emplaced within the shelf carbonates and prior to 630 ma the pan african orogeny data reported herein indicate that the ougda complex records the inception and demise of a neoproterozoic ocean basin similar crustal sections have been described from collisional e g aleutian islands and extensional e g ivrea verbano zone settings indicating that processes operating in both environments can generate nearly indistinguishable igneous suites the prevalence of shallow level calc alkali rocks in both settings may mask the presence of more mafic tholeiitic rocks at depth

evidence for the granulite granite connection penecontemporaneous high grade metamorphism granitic magmatism and core complex development in the liscomb complex nova scotia canada abstract upper amphibolite granulite facies gneisses and granites of the liscomb complex nova scotia canada which are exposed in a core complex within the cambro ordovician meguma group of southern nova scotia yielded concordant u pb zircon monazite ages of 377 2 and 374 3 ma respectively geochronological and geochemical data suggest a single devonian high grade metamorphic event which generated the granitic magma by partial melting of the fertile liscomb gneisses at a depth of 30 km the melting was also synchronous with an extensional event during which the gneisses were uplifted in a core complex associated with the intrusion of granitoids to a depth of 10 km subsequently the gneisses and granites underwent rapid exhumation before the deposition of unconformably overlying late fammenian rocks at 364 ma these events took place during terminal stages of the acadian orogeny and the onset of extensional tectonics in atlantic canada during the middle late devonian the close temporal and spatial association of liscomb gneisses granulites and granites their major and trace element compositions and their overlapping isotopic characteristics confirm the hypothesis that high grade metamorphism and generation of granitic melt are complementary processes as the liscomb granites are of similar age mineralogy and chemistry to the voluminous granitoid plutons found throughout the meguma terrane a similar process is indicated for the rest of the terrane

permian volcanism in the central western carpathians slovakia basin and range type rifting in the southern laurussian margin the 1 500 to 2 000 m thick permian volcano sedimentary malu ina formation of the uppermost nappe of the central western carpathians a segment of the alpine carpathian orogenic belt occurs in several fault blocks distributed across slovakia this unit is a part of a post variscan overstep suite that followed accretion of the gothic terranes to laurussia it consists of three upward fining megacycles of semi arid arid fluvial lacustrine clastic redbeds and local dolomites and evaporites abundant intercalated volcanic rocks are predominantly mafic lava flows volcaniclastic rocks and dykes are subordinate felsic rocks are represented by rare volcaniclastics and dykes compositionally the mafic rocks are rift related continental tholeiites with enriched light ree patterns having la yb n ratios between 2 and 5 5 and with mantle normalized patterns characterized by negative nb ta anomalies the rocks were derived from subcontinental lithospheric mantle and were affected by crustal contamination it is inferred that the volcanism of the malu ina formation formed in a basin and range tectonic setting in which rifting followed collision of the palaeo tethys ridge with the trench bordering southern laurussia this model can be applied to other permian volcanic suites of rift basins in the eastern alps and carpathians over a strike length of about 1 000 km which indicates the width of the slab window

origin of the silicic volcanic rocks of the early permian panjal traps kashmir india abstract the panjal traps of kashmir india represent a significant outpouring of mafic and silicic volcanic rocks during the early permian and are synchronous with the opening of the neotethys ocean previous studies have suggested that the silicic volcanic rocks are derived by differentiation of mafic magmas dacites and rhyolites collected from the lower portions of the volcanic pile near pampore kashmir are peraluminous anck 1 0 in composition their calculated whole rock i sr values are variable i sr 0 69307 to 0 71825 and indicate open system behavior of either rb or sr or both whereas their nd isotope compositions end t 8 6 to 8 9 are nearly uniform the end t values and trace element th nb pm 4 nb u 8 ratios suggest the rocks are derived partially or wholly from the crust least squares and melts modeling indicate that the likely source of the silicic rocks is the middle crust rather than the lower or upper crust furthermore the silicic panjal traps have trace element compositions similar to some felsic volcanic rocks and a type granitic rocks from other large igneous provinces e g karoo parana and camp the paleoproterozoic t dm ages t dm 1836 to 1937 ma indicate that the rocks were likely derived from an ancient crustal source which experienced multiple episodes of recycling in contrast to silicic volcanic rocks from other lips the silicic panjal traps are unusual in that they did not involve mantle source material the heat required to melt the crust was likely due to the continuous injection of contemporaneous basaltic magmas which formed the majority of the mafic panjal traps

late paleozoic gabbroic rocks of the bridge river accretionary complex southwestern british columbia geology and geochemistry gabbroic bodies in the bralorne gold bridge area of southwestern british columbia are associated with the oceanic bridge river complex of the western canadian cordillera one of the suspect terranes accreted to north america in the jurassic the gabbros are locally cut by tonalites and are structurally interleaved with ultramafic rocks phyllites graphitic cherts and carbonate lenses that comprise the lower part of the bridge river complex their late carboniferous crystallization age overlaps the depositional age of affiliated supracrustal rocks mississippian jurassic some of which have been metamorphosed to blueschist facies compositionally the gabbros resemble mafic plutonic rocks of ophiolitic complexes and gabbroic rocks of the nearby shulaps range they display some affinity to oceanic island arc tholeiitic suites the bralorne and shulaps gabbros include cumulates and appear to have been derived from a single light ree depleted peridotitic source by melting and subsequent fractional crystallization accumulation of various combinations of plagioclase pyroxenes and olivine the tonalites are compositionally distinct from typical ophiolitic plagiogranites but might be related to the associated gabbros the gabbroic bodies occur within tectonic slivers derived from the oceanic crust that floored a deep ocean basin that existed during the late paleozoic and early mesozoic the bridge river complex comprises fragments of oceanic crust that were tectonically incorporated into an east verging accretionary prism during a middle late triassic to jurassic collisional event

geochemistry and petrogenesis of basaltic rocks from coppermine river area northwest territories proterozoic basaltic flows 2000 m thick and associated dykes and sills from the coppermine river area northwest territories have chemical compositions typical of continental tholeiites the low mg fe ratio and abundances of ni and cr indicate that the lavas were extensively fractionated prior to extrusion the variations of incompatible elements such as k rb ree y zr nb and th suggest that the rocks were affected by interaction with continental crust the samples least affected by contamination have trace element compositions very similar to those of p type mid ocean ridge basalts it is suggested that continental tholeiites have been generated from the same source as p type oceanic tholeiites and geochemical features such as the enrichment of some lithophile elements in many of these rocks may be related to crustal contamination the variations within the volcanic pile of the coppermine river area are related to those of an exposed part of the muskox layered intrusion

trace element mobility during low grade metamorphism and silicification of basaltic rocks from saint john new brunswick upper carboniferous basaltic lavas from saint john new brunswick were metamorphosed under prehnite pumpellyite conditions and some were also affected by extensive silicification silicification le

geology and geochemistry of huronian rhyolites and low ti continental tholeiites from the thessalon region central ontario bimodal volcanism associated with early phases of huronian rifting in central ontario dated about 2450 ma produced low ti tholeiitic basalts and two varieties of crustally derived calc alkaline rhyolite early tholeiites are characteristically highly evolved have mg values from 30 to 50 and display pronounced enrichment in large ion lithophile elements lile and light rare earth element lree in comparison with modern oceanic basalts fractionated heavy rare earth element hree patterns and low ti zr p nb ba and k abundances ti zr ratios rise progressively in early basalts and associated basaltic andesite fractionates from about 35 in early flows to 55 in central units late basalts also carry enriched lile and lree but in contrast to early types have average mg values greater than 50 and lower rare earth element ree abundances with flat hree patterns they also display negative ba nb and p anomalies on chondrite normalized distribution diagrams but lack low k zr and ti contents

crustal xenoliths in triassic lamprophyre dykes in western morocco tectonic implications for the rheic ocean suture dykes of calc alkaline lamprophyre cutting granite of the hercynian jebilet massif of the moroccan meseta western morocco contain crustal xenoliths the xenoliths range in composition from mafic cognate cumulates and upper crustal granitic rocks through gneisses to middle crustal felsic granulites shrimp u th pb zircon analyses of these rocks indicate that the dykes were likely intruded during middle triassic times similar 235 ma whereas the xenoliths contain zircons with concordant carboniferous early permian neoproterozoic and palaeoproterozoic ages 280 328 ma c 540 615 ma 700 ma and similar 2000 ma the 280 328 ma ages appear to record synchronous intrusive and high grade up to granulite facies variscan metamorphic events suggesting that high grade metamorphism may have facilitated the s type granitic magmatism on the other hand the similar 540 615 ma 700 ma and 2000 ma ages correspond with pan african and eburnian orogenic events recorded in the west african craton in a triassic reconstruction morocco is juxtaposed against nova scotia canada and some have proposed that the basement of the easternmost terrane meguma terrane is a piece of the west african craton however lower crustal xenoliths from devonian dykes similar 370 ma cutting the meguma terrane have yielded late devonian neo and mesoproterozoic ages 378 ma 575 629 ma similar 880 1050 ma and similar 1530 ma the presence of similar 1 ga ages suggests that the basement of the meguma terrane is avalonian rather than west african implying that in a pangean reconstruction the rheic ocean suture between nw africa and maritime canada coincides with the atlantic ocean

geochemistry of late proterozoic basaltic rocks from southeastern new brunswick canada late proterozoic volcanic rocks of the coldbrook group 630 600 ma from the avalon zone of the appalachians in southeastern new brunswick canada are predominantly bimodal and occur in three belts fault blocks eastern central and western the basaltic rocks were emplaced in a convergent plate margin setting and range from incompatible element depleted island arc tholeiites in the eastern belt to calc alkalic types of the western belt the distinct compositional zonation of the basaltic rocks resembles the across arc variation observed in recent volcanic arc systems the trends include an increase of the lile and hfse abundances and the zr sio2 th sio2 la yb th nb th hf and zr y ratios from the eastern belt in the southeast through the central belt to the western belt in the northwest the zonation of the coldbrook group is attributed to a northwest dipping subduction zone during late proterozoic time it is suggested that some of the late proterozoic basins in the avalon zone of the northern appalachians reached a stage of oceanization

550 580 ma magmatism in cape breton island nova scotia canada the product of nw dipping subduction during the final stage of amalgamation of gondwana abstract cape breton island is host to widespread magmatic activity dated between 550 and 580 ma in the south its products include the capelin cove pluton and host coastal volcanic belt both are composed of bimodal tholeiitic calc alkaline suites that were emplaced on a thin sialic crust in a volcanic arc system their trace element and nd isotope variations are consistent with fractional crystallization accompanied by lower crustal contamination in a compositionally zoned magma chamber in contrast the indian brook and kerrs brook plutons and price point volcanic rocks in central cape breton island have higher contents of strongly incompatible trace elements and resemble modern arc related calc alkaline rocks emplaced on a moderately thick continental crust comparison with available data for other 550 580 ma igneous rocks across cape breton island indicates that they were produced above a nw dipping subduction zone this convergence was synchronous with the final stage of globalscale orogenic activity that culminated with the final amalgamation of gondwana available detrital zircon ages in late neoproterozoic 650 545 ma sediments in the antigonish highlands avalon composite terrane suggest derivation from the amazon craton it is proposed that the avalon composite terrane formed off northwestern south america during the late neoproterozoic

petrogenesis of the 723 ma coronation sills amundsen basin arctic canada implications for the break up of rodinia the coronation mafic sills and associated natkusiak flood basalts in arctic canada are part of the 723 ma franklin igneous events a flood basalt and dyke swarm province that extends across the southern arctic of north america and greenland two representative sills 70 and 25 m thick respectively outcrop near coronation gulf in nunavut they are composed of clinopyroxene plagioclase olivine and fe ti oxides and have tholeiitic composition similar to the natkusiak basalts their end values range between 4 2 and 5 9 and have la yb n ratios of 1 5 2 these rocks are mantle derived and were affected by minor crustal contamination compositional variations indicate that the coronation sills have experienced extensive fractionation including flow differentiation during emplacement and show increased differentiation from west to east across the province a feature similar to the mackenzie dyke swarm however the mackenzie dyke swarm and the franklin igneous events have distinct chemical composition the franklin rocks are related to a mantle plume located near victoria island at the center of the dyke swarm and could represent an offshoot from the superplume related to the break up of rodinia the absence of such dykes and sills in siberia argues against locating siberia against northern laurentia in a rodinia reconstruction 2003 elsevier b v all rights reserved

geological setting and fluid inclusion characteristics of a lead copper barium occurrence hosted in a neoproterozoic mafic sill at kiatak northumberland island northwestern greenland abstract we describe a pb cu ba occurrence in northumberland island northwestern greenland mineralization occupies the brecciated upper contact domain of a neoproterozoic diabase sill belonging to the franklin thule dike swarm 720 716 ma with pyrite bearing black shales of the dundas group upper thule supergroup the host tholeiitic diabase sill is of different composition low tio 2 and p 2 o 5 than the locally crosscutting dikes high tio 2 and p 2 o 5 chloritization carbonatization and silicification are intense in proximity to sulfides coarse grained open space filling galena and minor chalcopyrite are accompanied by 2 generations of calcite and 2 of barite galena contains significant amounts of ag av 400 ppm sb av 700 ppm se av 20 ppm traces of bi cd and sn fluid inclusions in the gangue of the kiatak occurrence indicate two fluid types prior to galena precipitation a cacl 2 nacl rich aqueous brine 20 wt eq nacl cooled from temperatures 300 c and was trapped first in early calcite and with further cooling in barite together with solid bitumen inclusions following galena crystallization secondary inclusions containing a similar brine but of lower salinity higher ca na ratio and lower temperature were trapped in calcite corrosion of galena was followed by precipitation of lower temperature 100 c barite from a second fluid comprising immiscible water and ch 4 despite its location in the contact between shale and large mafic sill the low temperature mineralization postdates the cooling of the sill and may be related to basinal fluid circulation controlled by regional extensional faults parallel to diabase dikes although uneconomic the kiatak occurrence may be witness to a larger metallogenic process that could have formed significant sedex type metal concentrations in strata within the thule supergroup

geochemistry of the neoproterozoic metasedimentary gamble brook formation avalon terrane nova scotia evidence for a rifted arc environment along the west gondwanan margin of rodinia abstract the neoproterozoic gamble brook formation in the avalon terrane of nova scotia consists of quartzitic pelitic and psammitic metasedimentary rocks the formation is generally interpreted to have been deposited in an atlantic type passive margin setting along the amazonian margin of rodinia before the main ca 650 550 ma phase of avalonian subduction related magmatism the geochemical and sm nd isotopic signature of these metasedimentary rocks however shows that although the quartzites are predominantly derived from ancient cratonic basement the pelites and psammites were derived from moderately fractionated mafic to intermediate proto avalonian crust the two contrasting sources for the gamble brook rocks are incompatible with a passive margin setting and are instead interpreted to reflect deposition along a basin margin within a rifted arc environment taken as a whole the amazonian margin of gondwana preserves evidence of episodic subduction in a western pacific like setting between 1000 a

miocene post collisional shoshonites and their crustal xenoliths yarlung zangbo suture zone southern tibet geodynamic implications the convergence between the indian plate and the southern margin of the eurasian continent created an active continental margin from late jurassic until about 40 ma ago which then evolved to form the himalaya and the tibetan plateau during the continental collision stage post collisional magmatism in southern tibet north of the yarlung zangbo suture zone yzsz has been active since 45 ma and is related to normal faulting and extensional tectonism to date no such magmatism was reported within the yzsz itself this paper reports on the discovery of miocene shoshonites within the yzsz they are significant because the magma traveled at least in part through oceanic crust thus limiting interaction with the continental crust to the mid crustal level and which affected the post collisional magmatic rocks occurring in the northern part of the subduction system in addition xenoliths and xenocrysts of crustal origin in these rocks constrain the nature of metamorphic rocks underlying the yzsz at mid crustal level the geochemical signatures of the shoshonitic rocks including nd and sr isotope systematics indicate derivation from a garnet bearing middle continental crustal source crustal imprint complicates modeling of the petrogenetic processes which occurred prior to mid crustal ponding of the magma which took place between 11 and 17 ma at depths of 40 to 50 km the significant role of crustal contamination raises serious concerns about models proposed for similar magmatic activity elsewhere in the himalaya and the tibetan plateau

hybridization between felsic and mafic magmas in calc alkaline granitoids a case study in northern sardinia italy abstract two examples of felsic mafic association resulting from a magma mingling mechanism have been studied in carboniferous calc alkaline granitoids from sardinia using mineralogical and geochemical techniques 1 the centimetric to decimetric magmatic mafic enclaves from the reversely zoned tonalitic santa restituta pluton this type of enclave which is common in calc alkaline granitoids represents blobs of mafic magma incorporated into a silicic host magma 2 the osidda mafic complex about one kilometre in diameter enclosed in the benettuti granodiorite this study intends to compare these two types of felsic mafic association in order 1 to clarify the nature of the interaction processes which occurred between the two magmatic components 2 to test the influence of the degree of fragmentation of the mafic component into the felsic matrix on the intensity of such processes the santa restituta enclaves sre and the osidda mafic complex omc exhibit the same general mineral assemblage plagioclase biotite hornblende quartz this assemblage is similar to that of the corresponding host granitoids however the sre and the omc are characterized by a greater abundance of biotite and hornblende the mineralogical study reveals the presence of two types of crystals the first type corresponds to minerals typical of the mafic or the felsic component in the mafic magma they are represented by remnants of an rich plagioclase by hornblende clots presumably pyroxene relicts and by orthopyroxene observed only in one facies of the omc these crystals represent remnants of phenocrysts present in the mafic component prior to the mingling event in the felsic component the first type of crystals is represented by quartz ocelli which are sometimes incorporated in the mafic magma as xenocrysts surrounded by a reaction rim of biotite and hornblende the second type comprises minerals which are found indiscriminantly in both magmatic components and which formed during the mingling event mingling minerals they include plagioclase an52 35 biotite hornblende quartz mineralogical data indicate that thermal equilibrium occurred between the mafic magma and the felsic matrix that equilibration was reached in the sre is shown by the compositional similarity of the mingling minerals of the enclaves and their host rocks however due to a lesser degree of fragmentation thermal equilibration was not reached in the omc geochemical data however do reveal close relationships between the two magmas which appear therefore to have both been hybridized by mutual chemical interaction a chemical comparison between the sre and the omc shows that in both cases the same qualitative interaction mechanisms operated between the host granitoids and either a voluminous mafic body osidda or a highly fragmented mafic component santa restituta enclaves however the intensity of hybridization is minor in the omc as compared with the sre it is suggested that hybridization of the host granitoids involves 1 mass to mass mixing due to disaggregation and dispersal of the mafic magma in the host granitoid resulting from mechanical stirring during the pluton s genesis from the initial meeting of the two magmas to the pluton s final emplacement 2 loss of certain elements si k rb ba li zr p nb ree which migrated towards the mafic magma through diffusion due to compositional gradients fluid transfer as a consequence the composition of the host granitoids is linked to that of their magmatic enclaves the enclaves with the lowest feo tot mgo ratios are enclosed in the least felsic granitoids and vice versa hybridization of the mafic component involves 1 mechanical transfer of crystals quartz from the host granitoid 2 enrichment in numerous elements migrating from the felsic component this is expressed mineralogically by the crystallization of numerous anhedral phases such as quartz sio2 biotite k rb h2o hornblende h2o apatite p and zircon zr 3 loss of ca and sr by the mafic component the nature of the processes of chemical migration are discussed the importance of the continuous renewal of the contact surface between the two components is emphasized the original compositions of the interacting mafic and granitic components become therefore highly modified this phenomenon can probably be generalized to apply to calc alkaline granitoids as a whole which are in fact characterized by the common occurrence of magma mingling

palaeozoic within plate volcanic rocks in nova scotia canada reinterpreted isotopic constraints on magmatic source and palaeocontinental reconstructions palaeozoic volcanism in the avalon terrane of northern nova scotia occurred during three time intervals cambrian early ordovician late ordovician early silurian and middle late devonian in the meguma terrane of southern nova scotia palaeozoic volcanism is limited to the middle ordovician geochemical data show that most of these volcanic rocks are bimodal within plate suites initial e nd signa tures range from 5 4 to 1 9 in the rhyolites and 6 8 to 2 7 in the basalts a difference attributable to the absence or presence respectively of a significant crustal component the data and regional tectonic settings of the avalon and meguma terranes suggest that the volcanism was generated in three different within plate settings 1 cambrian early ordovician volcanism related to thermal decay of late proterozoic arc magma tism during transtensional deformation 2 middle ordovician early silurian volcanism during sinistral tele scoping between laurentia and gondwana where extensional bends in the appalachians produced rifting and 3 devonian volcanism resulting from lithospheric delamination during dextral transpression and tele scoping in each setting active faults served as conduits for the magmas nd isotopic data indicate that the source of the palaeozoic felsic volcanic rocks is isotopically indistinguishable beneath southern and northern nova scotia and did not substantially change with time this crustal source appears to have separated from the mantle during the proterozoic a conclusion consistent with the hypothesis that the palaeozoic rocks in nova scotia were deposited upon a late proterozoic oceanic cratonic volcanic arc terrane the nd data when combined with published faunal palaeomagnetic and u pb isotopic data suggest that the avalon terrane was peripheral to gondwana off northwestern south america during neoproterozoic and early palaeozoic times

the sedimentary record of mantle plume uplift preceding eruption of the neoproterozoic natkusiak flood basalt strata of the neoproterozoic shaler group on victoria island in nw canada reveal stratigraphic and sedimentological features that are interpreted to be a response to differential crustal uplift immediately preceding emplacement of the 723 ma franklin gabbro sills and their extrusive equivalents the natkusiak formation flood basalts volcanism was coeval with sedimentation as indicated by loading features developed on the kuujjua formation a fluvial braidplain quartzarenite that underlies the lowermost basalt flow of the natkusiak formation in the sw part of minto inlier the kuujjua formation represents the top of an emergent cycle that began with deposition of intertidal carbonates followed by supratidal sabkha carbonates and sulphate evaporites of the underlying kilian formation the kuujjua formation thins and pinches out toward the northeast and there is erosional beveling of the upper kilian beneath it in the ne part of minto inlier the kuujjua and upper kilian are absent and the natkusiak form

ordovician earliest silurian rift tholeiites in the acatlan complex southern mexico evidence of rifting on the southern margin of the rheic ocean abstract the acatlan complex of southern mexico is a vestige of a paleozoic ocean inferred to be either the cambro ordovician iapetus and or the ordovician carboniferous rheic oceans ordovician granitoids in the complex have been interpreted as either the products of dehydration melting arc or rift magmatism however the geochemistry of felsic rocks is inconclusive the geochemistry of a recently discovered major ordovician earliest silurian mafic igneous suite associated with these granitoids is critical to defining the tectonic setting of the igneous event and to paleogeographic reconstructions such data from three areas in the acatlan complex xayacatlan patlanoaya and cuaulote document tholeiitic within plate characteristics with a source in either primitive mantle or mantle previously modified by subduction related magmatism possibly combined with crustal contamination this combined with their occurrence as a dike swarm intruding rift passive margin clastic sedimentary rocks indicates rifting of a continental margin mafic dikes at xayacatlan yielded a concordant u pb tims zircon age of 442 1 ma and a 40 ar 39 ar hornblende plateau age of 434 3 ma the age of mafic magmatism at other localities is defined by the ages of associated granitoids intruded at ca 461 ma and by the age of the youngest detrital zircon in the host rocks 496 25 ma at patlanoaya previously published age data suggest that this igneous event may have started earlier at 478 5 ma early ordovician although the life spans of the iapetus and rheic oceans overlap during the ordovician subduction and collision characterize the former whereas the latter was in a rift drift mode thus this ordovician earliest silurian magmatism is more consistent with rifting along the southern margin of the rheic ocean initiation of rifting at ca 480 ma is contemporaneous with separation of peri gondwanan terranes such as avalonia and carolinia from amazonia oaxaquia subsequent rifting may be analogous to mesozoic cenozoic western north america where transtensional deformation is related to dispersal of terranes e g baja california and cretaceous baja british columbia leading to a gradual collapse of the inboard continental margin

the eocene oligocene magmatic hiatus in the south central canadian cordillera a capture of the kula plate by the pacific plate the tertiary paleogene and neogene geological record in south central canadian cordillera is dominated by the 350 400 km wide lower eocene volcanic arc and the overlying miocene recent back arc lavas that are separated by a hiatus in magmatic activity between 48 and 24 ma in the black dome area 240 km north of vancouver the eocene volcanic rocks are mainly continental margin calc alkaline andesite and dacite resulting from the melting of a juvenile mafic source at the base of the crust in contrast the miocene volcanic rocks resemble continental flood basalts both eocene and miocene rocks from the black dome volcanic complex have high positive end values 7 2 to 7 4 and 6 4 to 7 6 respectively and low initial sr isotopic ratios 0 702 516 0 703 528 and 0 703 376 0 703 392 respectively comparable to modern oceanic basalts the onset of the hiatus in magmatism at 48 ma coincides with capture of the kula plate by the pacific plate resulting in a change in convergence direction with the

evolution of the ligurian tethys in the western alps sm nd and u pb geochronology and rare earth element geochemistry of the montgenevre ophiolite france abstract we provide geochemical and geochronological data for gabbro diorite and albitite samples from the montgenevre ophiolite in the western alps this well preserved remnant of the piemont ligurian oceanic basin shows evidence of intra oceanic deformation and metamorphism but has suffered minor ductile deformation and metamorphism during the alpine orogeny the gabbros have geochemical features and initial nd isotopic signatures similar to that of mid oceanic ridge gabbros indicating that they were extracted from a depleted mantle source with no evidence of continental contamination end t 8 cumulation played an important role in the genesis of these gabbros and their rare earth element ree patterns are controlled by the major cumulus phases modelling the ree data for the gabbros and diorites provides support for the hypothesis that the dioritic magmas have been derived from small degree 5 partial melting of the surrounding gabbros during shearing at high temperature high t zircons from a leucodioritic vein within sheared gabbros are poorly discordant and cluster the concordia at 156 3 ma while zircons from an albitite lens within the mantle rocks display concordant ages at 148 2 ma in both cases the zircons show no evidence of inheritance and these values are interpreted as crystallisation ages the 160 150 ma age bracket very likely records a late stage in the magmatic history of the montgenevre ophiolite since the emplacement of diorite clearly post dates the layered gabbros these radiometric ages correlate well with the late bathonian to early kimmeridgian stratigraphic age 160 140 ma of the earliest post ophiolitic radiolarian sediments the whole rock sm nd system of the gabbros yielded an isochron age of 198 22 ma end t of 8 8 this value is significantly older than the 165 ma age of western alps ophiolitic gabbros published elsewhere but is similar within error to the 180 ma age of eclogitised gabbros from the ligurides and corsica this early jurassic age is interpreted as the emplacement age of the gabbroic melts into the mantle lherzolites and could be associated with the early stage of rifting between the european and austro alpine continental margins these ages and their interpretation are consistent with the model of asymmetric mantle denudation by an oblique detachment fault proposed by lemoine et al 1987 this model implies that the newly formed lherzolite gabbro oceanic domain probably remained close to the spreading centre and therefore experienced slow cooling and low spreading rates this appears to be correlated by our radiometric results that suggest a life span of at least 30 ma for the formation of the oceanic crust in this part of the piemont ligurian ocean

probing crustal and mantle lithosphere origin through ordovician volcanic rocks along the iberian passive margin of gondwana abstract northwestern iberia preserves one of the most complete paleozoic sequences that document the origin and development of a passive margin along the southern gondwanan flank of the rheic ocean in addition to a well preserved sedimentary record there is widespread ordovician volcanic activity that can be used to probe the nature of the lower crust and mantle lithosphere that sourced the volcanic rocks during the rheic ocean opening the ordovician rift related volcanic sequences provide first order constraints on the early evolution of the rheic ocean in addition to published and new lithogeochemical data we provide sm nd isotopic data which together constrain the mantle or crustal source and allow an assessment on the role of the basement in rheic ocean magmatism the data imply that the mafic rocks are derived from a variety of sources including juvenile mantle that was contaminated by subduction coeval with early ordovician magmatism suggesting the importance of arc activity in northwest iberia during the opening of the rheic ocean other basalts were derived from a subcontinental lithospheric mantle that was enriched at about 1 0 ga basalts derived from a mantle enriched at ca 1 0 ga occur along other parts of the gondwanan margin avalonia oaxaquia and so the iberian basalts may be a local representation of a regionally significant enriched mantle the sm nd isotopic characteristics permit a genetic connection between this mantle source and the basement rocks recently identified in northwest iberia felsic magmas are predominantly intra crustal magmas derived from melting a mesoproterozoic crust lending support to other lines of data that the gondwanan margin of northwest iberia was predominantly underlain by a south american rio negro source

eocene volcanism in the buck creek basin central british columbia canada transition from arc to extensional volcanism abstract the eocene buck creek volcanic complex central british columbia canada is a part of the challis kamloops volcanic province that extends from yukon across british columbia to the northern united states running approximately parallel to the margin of the north american continent the complex is associated with the formation of a fault bounded basin controlled by strike slip faults the volcanic rocks were emplaced during two short lived

isotope and trace element heterogeneities in high grade basic metamorphic rocks of marvejols tectonic implications for the hercynian suture zone of the french massif central abstract the leptyno amphibolite group which occurs at the major suture of the variscan orogen in the french massif central contains abundant meta basic rocks in the form of granulites amphibolites and eclogites mafic granulite boudins from the marvejols area generally have geochemical characteristics similar to oceanic tholeiites of morb affinity they have flat or slightly lree depleted patterns high nd i and low sr i they lie very close the mantle array at the probable time of their formation 480 ma one granulitic boudin is lree enriched with low hree and hfse and appears to be a low ti lava with island arc tholeiite affinities similar material also forms much of the amphibolites surrounding the boudins the close association of low ti lavas and morb tholeiites suggests formation in a marginal basin or back arc environment

petrogenesis of the natkusiak continental basalts victoria island northwest territories canada the natkusiak flows 1100 m thick sills and dykes of victoria island northwest territories are part of an extensive late hadrynian magmatic event in the northern canadian shield the flows are subaerially erupted plateau basalts and together with associated doleritic sills and dykes display many geochemical features typical of continental tholeiites in addition to fractional crystallization the rocks were affected by crustal contamination but to a smaller degree than usually encountered in continental tholeiites elsewhere the basalts were derived from a depleted upper mantle source very similar to that of n type mid ocean ridge basalts although it is suggested that the melting residue contained small amounts of garnet 2

circa 546 ma plume related dykes in the 1 ga novillo gneiss east central mexico evidence for the initial separation of avalonia abstract the 1 ga novillo gneiss exposed in an inlier in the front ranges of the laramide belt in east central mexico is cut by an unmetamorphosed ne trending mafic dyke swarm the gneiss represents the northernmost inlier of the oaxaquia terrane that underlies the backbone of mexico titanite and biotite from metasedimentary rocks yielded ages of 928 2 ma concordant u pb age and duplicate 40 ar 39 ar plateau ages of 697 7 and 697 10 ma which are inferred to represent cooling through 660 and 300 350 c respectively on the other hand hornblende from cross cutting ne trending mafic dykes yielded a 40 ar 39 ar plateau age of 546 5 ma which is inferred to closely post date the time of intrusion combined with published ages u pb zircon sm nd garnet k ar hornblende the data suggest that following granulite facies metamorphism at 982 6 ma 8 9 9 7 kbar and 730 775 c the novillo gneiss cooled at a rate of 1 45 c my if this latter rate is extrapolated it would place the rocks at the surface 30 c by 497 ma about 50 my after dyke emplacement geochemistry of the dykes indicates that they are fe rich tholeiites with i low mgo but high tio 2 and other hfse ii smooth mantle normalized incompatible trace element patterns peaking at nb and la and iii intraplate tectonic affinities these features are typical of enriched tholeiites associated with plumes current precambrian paleogeographic reconstructions place avalonia against oaxaquia and subsequently separate avalonia by way of ridge trench collision at 550 ma giving rise to the unique avalonian cambrian fauna and a rapid phase of early cambrian subsidence these events are contemporaneous with emplacement of the novillo mafic dykes suggesting that the plume related magmatism and separation of avalonia from oaxaquia are linked

upper triassic takla group volcanic rocks stikine terrane north central british columbia geochemistry petrogenesis and tectonic implications the upper triassic takla group volcano sedimentary assemblage is part of the stikine terrane of the intermontane belt in the canadian cordillera and covers an area of more than 30 000 km2 in a belt up to 50 km wide and more than 800 km long in the mcconnell creek area of north central british columbia the assemblage consists of plagioclase clinopyroxene phyric dominantly basaltic to andesitic flows and pyroclastic rocks interlayered with volcanogenic sedimentary rocks compositionally the volcanic rocks are intermediate between tholeiitic and calc alkaline their mantle normalized trace element patterns are characterized by a moderate large ion lithophile element enrichment and nb and ti depletion suggesting that magmatism occurred in a volcanic arc setting flat heavy rare earth element chondrite normalized patterns with la yb n ratios from 2 to 4 5 suggest that the parent magma was produced by mantle melting in the spinel stability field the low sr isotopic ratios 87sr 86sri approximately equa

geochemistry and petrogenesis of upper cretaceous basaltic rocks from southern malagasy abstract upper cretaceous basaltic rocks of southern malagasy consist of tholeiitic flows and dykes and alkali basaltic dykes continental tholeiites include relatively primitive lava types with mg numbers 0 6 0 5 and voluminous evolved basaltic lavas and dykes with mg number 0 4 the evolved tholeiites have compositions similar to the high ti or enriched basalts from the karoo and parana flood basalt provinces they are characterized by a high content of strongly and moderately incompatible trace elements and a relative depletion of nb and sr the evolved tholeiites were probably derived from subcontinental lithosphere the primitive tholeiites resemble enriched morb and are not directly related to the evolved types alkali basalts 1 15 normative nepheline possess compositional characteristics of oceanic island basalts and show some geochemical similarities to the primitive tholeiites both rock types were probably derived from a mantle plume the upwelling plume induced lithospheric melting which produced the parental magmas of the evolved tholeiites the mesozoic volcanism of malagasy displays many similarities to other flood basalt provinces including its large volume rock composition and emplacement near plate margins

the origin of the variscan upper allochthons in the ortegal complex northwestern iberia sm nd isotopic constraints on the closure of the rheic ocean northwestern iberia preserves a stack of allochthons in which the vestiges of a suture zone generated during the variscan orogeny by the late paleozoic collision between laurussia and gondwana are exposed lower allochthons contain ordovician ophiolite known as the lower ophiolite and are structurally overlain by devonian ophiolite upper ophiolite which are in turn structurally overlain by allochthons containing high grade metamorphic rocks with continental affinities and late cambrian early ordovician protolith ages known as the upper units geochemical and sm nd isotopic data from the upper ophiolite and the structurally overlying upper units exposed in the ortegal complex of galicia show that these allochthons are derived from a variety of mantle and crustal sources and indicate that the suture zone juxtaposes a variety of oceanic assemblages the general isotopic characteristics of each assemblage are similar to allochthons in other variscan complexes in nw iberia suggesting that the allocht

geochemistry of early mesozoic tholeiites from morocco abstract mesozoic dolerites from two areas of morocco the high atlas fold belt between marrakech and demnat and the anti atlas belt in the area of foum zquid are most high ti quartz normative tholeiites whichi in many respects resemble mesozoic dolerite dikes from eastern north america the dolerites display a wide range of major and trace element compositions some of which are due to fractional crystallization the doleritic sequences from high atlas also show vertical stratigraphic zonation which is characterized by a progressive depletion of lithophile elements toward the top this trend together with regularities of trace element ratio variations are indicative of a dynamic melting of an initially homogeneous source it is suggested that the continental upper mantle source for dolerites of morocco was enriched in several incompatible elements in comparison with the upper mantle source for ocean floor tholeiites

diagenetic history of norphlet formation upper jurassic rankin county mississippi abstract abstract most of the norphlet sandstone examined in four cores 15 770 to 22 500 feet is eolian in origin as shown by thick high angle crossbeds bimodal texture with well sorted laminae and relict hematite grain coatings typical of desert sand norphlet sandstones average 77 percent quartz 16 percent feldspar and 7 percent rock fragments partial to total loss of halite cement from cores during coring and slabbing operations hampers the interpretation of diagenetic history however the inferred sequence of diagenetic events is cementation by illite k feldspar quartz calcite anhydrite and halite development of secondary porosity by dissolution of halite plagioclase vrf s and carbonate cement hydrocarbon migration and pyrite generation dolomitization and cementation by late stage illite and quartz halite and anhydrite were derived from the underlying louann pyrite is present in most samples and formed when sour gas passed through the sands and reduced hematite grain coatings to pyrite the scarcity of quartz cement is attributed to the lack of shale beneath the norphlet illite cement of two ages is present early formed illite is leaflike flakes that coat quartz grains that inhibited the development of quartz overgrowths it is not a seriously deleterious mineral late illite is thread like and grows in narrow secondary pores where it seriously reduces permeability illite cement and sericitic alteration of plagioclase are more abundant deeper than 19 900 feet halite was introduced after moderate burial because prior to its development the sandstones underwent modest compaction as shown by sutured quartz grains and pre cement porosity values of 22 to 30 percent indicating compactional losses of 15 to 23 percent porosity before cementation the original texture of the sands strongly influenced diagenetic events