chalcophile element geochemistry and petrogenesis of high ti and low ti magmas in the permian emeishan large igneous province sw china sulfide poor mafic layered intrusions sills dykes and lava flows in the funing region sw china are part of the 260 ma emeishan large igneous province they belong to either a high ti group tio2 1 6 4 4 wt with elevated ti y ratios 351 1 018 or a low ti group tio2 1 low y pd pm ratios 0 2 7 1 and nearly constant cu pd pm ratios 1 5 3 8 the even and parallel chalcophile element patterns of the high pge low ti subgroup rocks are likely a result of olivine dominated fractionation under s undersaturated condition the low pge low ti group rocks have low mgo 4 5 8 9 wt and very poor pge pt pd 0 5 1 6 ppb ir 0 004 0 02 ppb with low cu zr ratios 0 1 0 5 high y pd pm 26 70 and variable cu pd pm ratios 2 8 14 the trough like chalcophile element patterns of the low pge low ti subgroup rocks indicate that the magmas were sulfide saturation and sulfide melts were extracted from the magmas the extracted sulfide melts might be potential ni cu sulfide ores at depth in the funing region

eaglebine play of the southwestern east texas basin stratigraphic and depositional framework of the upper cretaceous cenomanian turonian woodbine and eagle ford groups the woodbine and eagle ford groups of the southwestern east texas basin compose an emerging play which has generated considerable interest because of its potential for new hydrocarbon production from both sandstone and mudrock reservoirs however the play s stratigraphic and depositional relations are complex and directly relate to the play s exploration challenges productive woodbine and eagle ford sub clarksville sandstones intertongue with a poorly defined subregional mudrock dominated interval that thins southwestward toward the san marcos arch we propose dividing this succession into two intervals 1 the lower unit a high gamma ray unit at the base of this mudrock succession that is inferred to be equivalent to the maness shale of the washita group and to part of the lower eagle ford group on the san marcos arch and 2 an upper unit a basinward thickening zone of consistently lower gamma ray log facies inferred to be equivalent to the woodbine group pepper shale and the eagle ford group of the east texas basin because the cenomanian turonian boundary occurs within the eagle ford group of the east texas basin and the lower eagle ford section of the san marcos arch most of the maness through eagle ford succession exists as a much thinned section on the arch basinwide integration of the woodbine sequence stratigraphic framework shows that the number of fourth order sequences in the unit decreases westward from 14 in the basin axis to no more than 9 in the most active part of the eaglebine play because of their systematic depositional pinch out approaching the western basin margin the eagle ford group consists of three fourth order sequences capped by the sub clarksville sandstones that accumulated after the major late cenomanian early turonian flooding event recorded by a basinwide transgressive systems tract tst at the base of the unit depositional systems of the woodbine group vary within the study area even between stratigraphically adjacent systems on shelf siliciclastic systems include fluvial dominated delta incised valley fill fluvial and nearshore marine and wave dominated delta deposits

structural and geochronological constraints on the tectonic evolution of the dulong song chay tectonic dome in yunnan province sw china abstract the dulong song chay tectonic dome lies on the border of china se yunnan province and northern vietnam and consists of two tectonic and lithologic units a core complex and a cover sequence separated by an extensional detachment fault these two units are overlain unconformably by late triassic strata the core complex is composed of gneiss schist and amphibolite shrimp zircon u pb dating results for the orthogneiss yield an age of 799 10 ma which is considered to be the crystallization age of its igneous protolith formed in an arc related environment a granitic intrusion within the core complex occurred with an age of 436 402 ma which probably formed during partial closure of paleotethys within the core complex metamorphic grades change sharply from upper greenschist low amphibolite facies in the core to low greenschist facies in the cover sequence there are two arrays of foliation within the core complex detachment fault and the cover sequence s 1 and s 2 the pervasive s 1 is the axial plane of intrafolial s 0 folds d 1 deformation related to this foliation is characterized by extensional structures the strata were structurally thinned or selectively removed along the detachment faults indicating exhumation of the dulong song chay tectonic dome the major extension occurred at 237 ma determined by shrimp zircon u pb and 39 ar 40 ar isotopic dating techniques regionally simultaneous tectonic extension was associated with pre indosinian collision between the south china and indochina blocks the s 2 foliation appears as the axial plane of nw striking s 1 buckling folds formed during a compressional regime of d 2 d 2 is associated with collision between the south china and indochina blocks along the jinshajiang ailao shan suture zone and represents the indosinian deformation the dulong granites intruded the dulong song chay dome at 144 2 140 2 and 116 10 ma based on 39 ar 40 ar measurement on muscovite and biotite the dome was later overprinted by a conjugate strike slip fault and related thrust fault which formed a vortex structure contemporaneously with late cenozoic sinistral movement on the ailao shan red river fault

ndsr isotopic study of proterozoic to triassic sediments from southeastern british columbia abstract sedimentary rocks from the southeastern canadian cordillera varying in age from mid proterozoic to triassic have been analysed for their nd isotopic composition e nd t values range from 1 5 to 17 2 and nd model ages t dm nd vary from 1 00 to 2 32 ga much in excess over their stratigraphic ages the difference between the two ages ranges from 0 43 to 1 93 ga decreasing as the sediments become stratigraphically older nd isotopic compositions of sediments suggest that there are two distinct continental sources of sediments in the cordilleran miogeocline a an older source e nd 0 from 20 3 to 23 8 with apparent crustal residence ages of 2 26 2 32 ga and b a younger source average e nd 0 14 0 with the crustal residence age of about 1 8 ga the older residence ages are interpreted to be due to mixtures of materials from the archean and proterozoic crusts sediments belonging to the laib formation lower cambrian and lower clastic part of the milford group namurian appear to have derived from an archean continental segment with some admixture of sediments from the early proterozoic source the younger crustal source which seems to be the principal supplier of sediments for most of the stratigraphic units aldridge formation middle proterozoic horsethief creek group upper proterozoic hamill and lardeau groups lower cambrian nelway formation middle cambrian active formation ordovician and part of milford group is not the archean canadian shield immediately to the east recent subsurface mapping of the alberta basement beneath the phanerozoic sedimentary cover using u pb zircon dating of igneous lithologies 1 has established the extensive archean and early proterozoic crustal provinces it is proposed here that the early proterozoic province of west central alberta is the most likely source of the easterly derived sedimentary rocks of the cordilleran miogeocline with t cr ages from 1 94 to 1 50 ga on the basis of sedimentologic tectonic and isotopic evidence we suggest that the younger sediments could be to some extent recycled older sediments for the triassic slocan and ymir groups eastern part of quesnel terrane sedimentologic lithologic tectonic and isotopic evidence suggest the source was a western one with about 1 8 ga crustal residence age comparison of the crustal residence ages with stratigraphic ages shows the best fit nd evolution curve for the cordilleran miogeocline approximately coincides with the continental growth curve deduced for the north atlantic province and similar to the growth curve for gondwana during the phanerozoic this curve also passes through the north american composite the levelling off of crustal residence ages for the mid proterozoic to triassic sediments at about 1 8 ga supports the idea of negligible crustal growth since at least 1 7 ga the average crustal residence age of the modern miogeoclinal and cratonic sediments

stratal slicing of miocene pliocene sediments in vermilion block 50 tiger shoal area offshore louisiana many people have viewed modern land surfaces from commercial airplanes and marveled at the form and geometry of geomorphic features such as river channels deltas barrier islands and dune fields these views represent complete images of the modern time surfaces we can classify the depositional nature of features on these images by interpreting their planiform geometry and geographic context in fact modern 3 d seismic technology has made it possible for us to image similar but much older geomorphic or depositional features preserved in the rock record unfortunately although many reservoir scale well to well scale features can be detected in vertical seismic lines few such features can be resolved and interpreted in the vertical perspective because of the data s limited bandwidth only in the horizontal perspective are such depositional features large enough to be resolvable when displayed in map view on geologic time surfaces

controls on platinum group elemental distributions of podiform chromitites a case study of high cr and high al chromitites from chinese orogenic belts abstract a study of podiform chromite deposits from the asiatic orogenic belt and the qilian qiangling kunlun himalaya tectonic domain provides new insights into the geochemistry of the pges in podiform chromite deposits and the genesis of the deposits themselves the bulk of deposits which occur in mantle peridotites of ophiolites have typical ophiolitic pge patterns that are depleted in pt and pd relative to the average upper mantle and have negatively sloping distributions on mantle normalized diagrams type i high cr chromitites have higher os ir ru and rh contents than type ii high al chromitites although both have similar pd and pt most of the type i and ii chromite deposits have lower pd and pt contents than the upper mantle peridotites in which they occur podiform chromitites are essentially products of melt rock interaction in the upper mantle their cr and pges were contributed by not only the invading magmas but also by the upper mantle host the chromite deposits are in part metasomatic replacement bodies the type i high cr chromitite pge patterns were produced by interaction between s undersaturated boninitic magmas and depleted harzburgites whereas the type ii high al chromitite pge patterns were formed by interaction between initially s saturated tholeiitic magmas and depleted harzburgites the low to very low pd and pt contents of both type i and type ii chromitites require that the mantle assemblage in which the chromite deposits were formed had lost their sulfides and hence pd and pt prior to formation of the chromite deposits in addition no or little pd and pt were deposited by the invading magma which either remained s undersaturated boninite or became morb s undersaturated due to interaction with the s depleted harzburgitic mantle it is suggested that the very low ir os and ru contents of boninites in general might be due to loss of ir during the formation of podiform chromitites it is suggested that podiform chromitites with type i pge patterns were formed in an island arc environment whereas those with type ii pge patterns were formed in a back arc setting

miocene chronostratigraphy paleogeography and play framework of the burgos basin southern gulf of mexico this study characterizes miocene chronostratigraphy and plays in the burgos basin and adjacent south texas within an area of approximately 39 700 km2 15 300 mi2 onshore and offshore to the 500 m 1640 ft isobath using greater than 40 000 linear kilometers 25 000 mi of two dimensional seismic lines 115 onshore wells 9 offshore wells and paleontological data we established a correlation framework of 9 key surfaces upper oligocene to lower pliocene representing major probably third order sequence boundaries and maximum flooding surfaces throughout the basin five of the burgos miocene surfaces coincide with regional chronostratigraphic surfaces from the veracruz and laguna madre tuxpan basins thus establishing a consistent correlation framework throughout much of the mexican gulf coast basin twenty miocene plays are defined by four age divisions lower miocene middle miocene upper miocene 1 and upper miocene 2 and four paleogeographic settings unexpanded and expanded shelf proximal slope and distal slope because of proven high productivity in salt bounded basins in the northern gulf of mexico the onlap of strata onto diapirs in the eastern burgos salt province was evaluated as a fifth setting the paleogeographic provinces and onlap areas exhibit characteristic seismic facies stratal geometries and structural relations a characterization of each one of these being key to the overall play evaluation this play framework provides the means for continuing exploration of miocene strata and evaluation of key play elements reservoir presence and quality seal trap source and migration and timing in this structurally complex underexplored basin the relative importance of these play elements varies systematically for each play especially between the onshore shelf plays and the offshore deep water plays where fault complexity and stratigraphic variability are greater

on shelf lower miocene oakville sediment dispersal patterns within a three dimensional sequence stratigraphic architectural framework and implications for deep water reservoirs in the central coastal area of texas through integrating three dimensional seismic and wireline log pattern data we found that the sequence stratigraphic architecture of the lower miocene oakville formation along the central part of the texas gulf coast reveals four third order sequences the lower two containing lowstand lst transgressive tst and highstand systems tracts hst and the upper two appearing to contain only tst and hst these sequences average 1 2 m y in duration the lowstand incised valley fill sandstones are massive as thick as 420 ft 73 m on wireline logs seismic stratal slices demonstrate that the incised valleys are dip oriented and more than 20 mi 32 km wide strata reflections within the incised valleys onlap the sides of the valley the third order tst comprise backstepping high frequency sequences seismic stratal slices and wireline log patterns within the tst suggest deltaic sediment dispersal patterns that appear reworked third order hst comprise aggradational to progradational high frequency sequences and seismic stratal slices and wireline log patterns indicate a variety of depositional environments that we interpreted as deltaic shoreface coastal plain and interdistributary the hst is significantly truncated by the overlying sequence boundary sandstone rich incised valley fills suggest that large amounts of sediment may have been transported through these channels and may have supplied lowstand deposits and associated reservoirs seaward of the previous shelf edge

uplift of deep crust during orogenic extensional collapse a model based on field studies in the sogn sunnfjord region of western norway the western gneiss region wgr in norway experienced high pressure metamorphism during silurian devonian continent continent collision the eclogite bearing lower crust is separated from the middle and upper crust by major detachment zones formed during extensional collapse of the orogen formation of the devonian basins is related to the extension the footwall of the detachment zones comprises three structural and metamorphic zones the upper zone zone 1 is characterized by penetrative homogeneous down to the west simple shear developed under retrograde greenschist facies metamorphism zone 2 suffered inhomogeneous simple shear of the same polarity petrography and mineral chemistry data from the lower zone zone 3 show a record of initial eclogite facies metamorphism at 600 c and 16 kbar which was decompressed almost isothermally to amphibolite facies conditions at 550 c and 10 12 kbar both the eclogite and amphibolite facies metamorphism developed in a regime of pure shear with vertical shortening the rapid decompression records an approach of approximately 20 km to the surface related to uplift that was probably the result of the removal of a thickened thermal boundary layer in the mantle lithosphere the pure shear regime which developed initially in the lower crust was truncated by zones of simple shear as the lower crust was uplifted to middle and upper crustal levels extension by simple shear in the upper crust was rooted in the lower crust where extension occurred by pure shear the shear zones in zones 1 and 2 did not penetrate the pure shear regime of the lower crust a considerable amount of tectonic stripping of the orogenic welt predates deposition of the devonian sedimentary basins

geochemistry of caledonian metabasalts from some norwegian ophiolite fragments major and trace element analyses of metabasalts from five of the lower palaeozoic caledonian ophiolite fragments along a strike distance of some 800 km of the west norwegian coast are presented characteristically the ree patterns show depletion of both lree and hree relative to intermediate ree giving an upward convex shape but nearly flat lree depleted or enriched patterns occur the ree patterns of the metabasalts from three of these sequences i e the lykling solund and stavfjorden ophiolite fragments show little within sequence but large between sequence variation the ree patterns of the metabasalts from the gullfjellet major bergen arc and skalvaer ophiolite fragments however show significant within sequence variations indicative of local source heterogeneties

structural setting and geochemistry of archaean i type granites in the bardoc coolgardie area of the norseman wiluna belt western australia abstract regional structures in the bardoc coolgardie area of the archaean norseman wiluna greenstone belt in western australia developed during early large scale thrusting followed by a transpressional regime characterized by enewsw shortening and large scale sinistral strike slip faults structural relationships between granites and greenstone successions indicate emplacement of granitic rocks throughout this deformation history emplacement mechanisms include solid or liquid state diapirism forceful dilation of country rocks and passive stoping i type granites are common and include early and late tectonic intrusions the widespread occurrence of i type granodiorite and monzogranite probably derived by partial melting of lower continental crust or subducted oceanic crust suggests a continental margin setting for the belt alkalic quartz diorite occurs as rounded enclaves in granodiorite and is enriched in ba rb sr ce la p and f which suggests a mantle derived volatile component these quartz diorite melts may have formed by partial melting of metasomatized mantle or alternatively mantle degassing may have lowered solidus temperatures in the source region and thus induced melting emplacement of hot volatilerich quartz diorite melts into the lower crust may have lowered solidus temperatures in the source rocks for granodiorite and monzogranite recent models for archaean gold mineralization suggest that auriferous fluids contained co 2 derived from lower crustal or mantle sources in the bardoc coolgardie area co 2 may have been flushed out during partial melting of the lower continental crust however the origin of the gold in the mineralizing fluids requires some other explanation because gold mineralization is generally a late stage event and took place over a considerably shorter time interval than the formation and emplacement of i type granites

magmatic development and tectonomagmatic models for the solund stavfjord ophiolite complex west norwegian caledonides abstract geochemical variation and different extrusion rates of the late ordovician solund stavfjord ophiolite complex ssoc suggest both open and closed magma chambers closed chambers existed off axis and within a transform fault and are indicated by dykes and pillow lavas that evolved from high mgo basalt to ferrobasalt by fractional crystallization of olivine plagioclase and clinopyroxene hybrid magmas with ferrobasaltic composition represent open magmatic systems and are characterized by frequent eruptions of sheet flows the ssoc is interpreted as representing either a normal or a propagating ridge transform intersection rti the propagating rti model is favoured as it explains both the high frequency of ferrobasalt and uplift of ultramafic rocks to above the sea level which is suggested by detrital ultramafics occurring in an ophiolitic melange

the behaviour of platinum group elements during partial melting crystal fractionation and sulphide segregation an example from the cape smith fold belt northern quebec abstract the behaviour of the pt group elements pge during both normal igneous processes and during the formation of pge deposits is poorly understood this is in part because of the limited data set available for nonmineralized rocks accordingly pge concentrations have been determined in komatiitic basalts associated with ni cu sulphide deposits rich in pge from the proterozoic cape smith fold belt of northern quebec the lavas have been divided into olivine phyric pyroxene phyric and plagioclase phyric on the basis of the phenocrysts present olivine phyric lavas have mg s greater than 0 66 and are thought to be primary partial melts the bulk partition coefficients of the pge during partial melting calculated from the average of olivine phyric basalts are 6 2 0 6 and 0 2 for ir rh pt and pd respectively which indicates a decrease in compatibility of the pge in this order the pyroxene and plagioclase phyric lavas are thought to have formed from the olivine phyric lavas by crystal fractionation of olivine and chromite in the pyroxene phyric lavas and olivine chromite and pyroxene in the plagioclase phyric lavas ir shows a strong positive correlation with mg cr and ni and this is attributed to ir having partitioned into olivine or chromite if olivine chromite and clinopyroxene were the only phases to have crystallized then pd should have behaved as an incompatible element however it does not correlate with the lithophile incompatible elements it does however correlate with rh and pt this is interpreted to suggest that rh pt and pd were controlled by sulphide segregation the removal of a small amount of sulphide along with the olivine and chromite during crystal fractionation could have resulted in the mildly compatible behavior exhibited by rh pt and pd the composition of the sulphides in the sulphide deposits may be modelled by assuming that the silicate magma from which they segregated was similar in composition to the spinifex textured komatiites containing 15 mgo

zircon u pb geochronology and elemental and sr nd isotope geochemistry of permian mafic rocks in the funing area sw china mafic layered intrusions and sills and spatially associated andesitic basalts are well preserved in the funing area sw china the 258 3 ma layered intrusions are composed of fine grained gabbro gabbro and diorite the 260 3 ma sills consist of undifferentiated diabases both the layered intrusions and volcanic rocks belong to a low ti group whereas the diabases belong to a high ti group rocks of the high ti group have feo tio2 and p2o5 higher but mgo and th nb ratios lower than those of the low ti group they have initial 87sr 86sr ratios 0 706 0 707 lower and nd 1 5 to 0 6 higher than the low ti equivalents 0 710 0 715 and 9 6 to 4 0 respectively the high ti group was formed from relatively primitive high ti magmas generated by low degrees 7 3 9 5 of partial melting of an enriched oib type asthenospheric mantle source the low ti group may have formed from melts derived from an em2 like lithospheric mantle source the mafic rocks at funing are part of the emeishan large igneous province formed by a mantle plume at 260 ma

smnd isotopic systematics of a gabbro eclogite transition abstract sm nd isotopic systems have been studied in 1 a sequence of coronitic olivine gabbros showing different degrees of transition to eclogite 2 coronitic leuco gabbro norite and 3 country rock eclogite all from the nordoyane brattvag area western norway isochrons defined by combinations of whole rock samples and relict igneous phases give ages of 1198 56 and 1289 48 ma olivine gabbro and 926 70 ma leuco gabbro norite and have been interpreted in terms of two different episodes of igneous intrusion in gabbro with advanced corona formation relict augite tends to retain its original nd isotopic composition while plagioclase may show selective disturbance related to recrystallization and partial replacement by garnet in completely eclogitized samples there is a correlation between the degree of isotopic equilibrium and microstructural equilibrium whole rock eclogites occurring within gabbro are characterized by pseudomorphs after igneous phases and preserve original igneous nd isotopic composition results for eclogite minerals suggest caledonian ages but these phases have not achieved complete isotopic equilibrium in contrast nd isotopic systems in texturally equilibrated and strongly deformed eclogite within adjacent gneisses have been disturbed on a whole rock scale minerals from an external eclogite define a sm nd isochron with an age of 400 16 ma which is interpreted to date synkinematic eclogite equilibration at high p and t conditions

siderophile and chalcophile metals as tracers of the evolution of the siberian trap in the noril sk region russia abstract in this study cu ni and platinum group elements pge were determined in a sequence of basaltic and picritic lavas from the siberian trap in the noril sk area of russia to constrain genetic relationships between the basalts and the petrogenesis of ni cu pge sulfide deposits associated with the talnakh and noril sk intrusions in the most primitive basalts 8 19 wt mgo of the tuklonsky tk suite pt and pd concentrations range from 4 13 ppb increasing with decreasing mgo content whereas ir contents decrease with mgo from 0 8 0 05 ppb the contrasting behavior of these elements which all have very high sulfide silicate partition coefficients as well as the primitive mantle like ratios of cu y and pd y suggests that these magmas were not sulfide saturated the high pge abundances imply that their parental magmas were also not sulfide saturated during partial melting in the mantle due to sulfide segregation the overlying basalts of the nadezhdinsky nd series are low in cu and ni 52 and 38 ppm respectively highly depleted in all pge and have very low cu y pd y and pd cu ratios however in stratigraphically higher levels cu ni and pge concentrations increase systematically through the morongovsky mr suite to reach a concentration plateau in the uppermost mokulaevsky mk suite pt 8 ppb pd 9 ppb ir 0 12 ppb rh 0 4 ppb at the same time ratios such as cu y increase and approach primitive mantle values however ratios involving pge such as pd y remain low suggesting the removal of small amounts of sulfide 0 01 0 03 the compositional variations in the basalts and the sulfide liquids can be quantitatively described by fractional segregation of a sulfide liquid in an open or closed system magma chamber the latter model suggests that the basalts represent the eruption products of a zoned magma chamber in which light magma with crustal components contaminated overlies less contaminated denser magma crustal contamination caused sulfide saturation and the resulting sulfide liquids settled through a magma column and accumulated at the bottom of the chamber in this model the sulfide liquid is not in equilibrium with the whole magma mass and sulfide segregation is compared with the zone refining process of metallurgy the sulfides become more enriched as they move through the magma and although the magma left behind is depleted in pge cu and ni their concentrations also increase with depth eventually the magma chamber is emptied from the top to the bottom producing the flood basalt sequence and the associated intrusions and ore deposits in the open system model sulfide saturation was initially caused by assimilation of crustal material by the tuklonsky magma continuous and simultaneous replenishment assimilation and crystallization processes formed the lower nd lavas the concurrent removal of 0 5 1 sulfide strongly depleted these magmas in chalcophile and siderophile metals due to the continuous replenishment of the magma chamber with uncontaminated pge rich magma succeeding lavas mr mk show diminishing signs of crustal contamination and become less sulfide saturated as indicated by the increasing ni cu and pge abundances during the evolution of the chamber the magma remained sulfur saturated and sulfides accumulated at the base the composition of the sulfide ores could be regarded as a mixture consisting of low ni cu and pge sulfides derived with a low silicate sulfide ratio 100 from the tk nd magma and high ni cu and pge sulfides formed with a high ratio 10 000 from the mr mk magma

the role of komatiitic and picritic magmatism and s saturation in the formation of ore deposits abstract the introduction of high temperature high magnesium komatiitic and picritic magmas into the earth s upper crust has given rise either directly or indirectly to many of the world s major ore deposits there is a direct association between komatiitic magmatism and kambalda style ni cu pge deposits the link between magma type and mineralization is not as clear cut in the case of the noril sk cu ni pge deposits but nevertheless there is an association between picritic magmatism and ore deposit development komatiitic and or picritic magmatism may also have contributed to the formation of many types of hydrothermal ore deposits recent nd isotope data generated by kent et al 1994 provide strong evidence for the hypothesis proposed by keays in 1983 that the au in archaean au deposits was derived either directly or indirectly from komatiites through the action of metamorphic fluids the importance of komatiitic and picritic magmas in ore forming processes arises from the fact that they are among the few mafic ultramafic magma types that are s undersaturated at the time of magma formation s saturation of magmas leads to marked depletion in the chalcophile metals for example czamanske and moore 1977 estimated that a third of the original cu and ni contents of morb were lost from the magmas due to extraction into magmatic sulphides the depletion in au and the pge is much greater due to their much larger partition coefficients keays and scott 1976 komatiitic and picritic magmas are s undersaturated because they are high temperature magmas produced by large degrees of partial melting of upper mantle sourc regions that were already depleted in s through earlier partial melting events most mafic magmas produced by less than 25 partial melting are s saturated and hence depleted in the ore forming chalcophile elements in contrast komatiitic and picritic magmas have their full complement of chalcophile metals when they do become s saturated they form sulphides that are strongly enriched in ni cu au pge and other strongly chalcophile metals these may accumulate to form ore deposits directly e g magmatic ni cu pge sulphides or be dispersed in komatiitic picritic rocks or their fractionation products making these source rocks for hydrothermal au or vms deposits

depositional settings and history of the lower miocene fleming group refugio county texas as defined using seismic geomorphology abstract a study of the lower miocene fleming group in refugio county texas from 3 d seismic and wireline log data documents continuous strandplain and associated lower coastal plain fluvial systems along a wave dominated shoreline this study delineates and characterizes through the interpretation of seismic attributes and seismic geomorphology maps from 15 horizons three principal styles of stratal patterns 1 parallel and subparallel strike continuous sets of beach ridge and swale deposits within a strandplain system that prograded at least 12 km 2 sinuous channel belts in a coeval mixed load meanderbelt system and 3 acoustically dim nearly featureless seismic attribute patterns that record homogenous and topographically subdued shallow marine mudstones associated with transgressive marine deposits and flooding surfaces holocene depositional analogs for these lower miocene sandy strandplain deposits include the nayarit coastline in mexico whereas the lower coastal plain between galveston and corpus christi texas serves as an appropriate analog for mixed load meanderbelt systems the lower miocene coastline in refugio county consists of a regressive phase that was punctuated by two transgressive phases these episodes of marine transgression correspond to the anahuac formation and the marginulina a shale respectively succeeding events record complex amalgamated beach ridge deposits and coeval fluvial channel systems that exhibit patterns of channel migration erosion incision and downdip fusion of channel belts this study concludes that potential reservoir geometries and architectures in the lower miocene succession are much more complex than previously inferred the result of rapidly fluctuating and evolving strike fed shoreline systems and contemporaneous fluvial dip dispersal systems some manifested by shore parallel trends confined in swales improved reservoir development strategies in this prolific hydrocarbon bearing play should be guided by distribution of sandy depositional elements and the paleogeographic setting

siderophile and chalcophile metal variations in tertiary picrites and basalts from west greenland with implications for the sulphide saturation history of continental flood basalt magmas sixty five million year old continental flood basalts crop out on qeqertarssuaq island and the nuussuaq peninsula in west greenland and they include 1 000 m of picritic lavas and discrete 10 to 50 m thick members of highly contaminated basalts on qeqertarssuaq the lavas are allocated to the vaigat and malig t formations of which the former includes the naujanguit member which consists of picrites with 7 29 wt mgo 80 1 400 ppm ni 5 7 9 4 ppb pt and 4 2 12 9 ppb pd the naujanguit member contains two horizons of contaminated basalts the asuk and kuganguaq which have elevated sio2 52 58 wt and low to moderate mgo 7 5 12 8 wt these lavas are broadly characterized by low cu and ni abundances average 40 ppm ni and 45 ppm cu and very low pt 0 16 0 63 ppb and pd 0 13 0 68 ppb abundances and in the case of the asuk they contain shale xenoliths and droplets of native iron and troilite the contaminated basalts from nuussuaq the b0 to b4 members are also usually ni cu and platinum group elements pge depleted the geochemical signatures especially the ratios of incompatible trace elements such as th nb of all of the contaminated basalts from qeqertarssuaq and some of those from nuussuaq record what appears to be a chemical contribution from deltaic shales that lie immediately below the lavas this suggests that the contamination of the magmas occurred during the migration of the magmas through plumbing systems developed in sedimentary rocks and hence at a high crustal level nickel cu and pge depletion together with geochemical signatures produced by crustal contamination are also a feature of siberian trap basalts from the noril sk region these basalts belong to the 0 to 500 m thick 5 000 to 10 000 km3 nadezhdinsky formation which is centered in the noril sk region a major difference between siberia and west greenland is that pge depletion in the nadezhdinsky formation samples with the lowest cu and ni contents is much more severe than that of the west greenland contaminated basalts moreover the volumes of the contaminated and metal depleted volcanic rocks in west greenland pale is significant when compared to the nadezhdinsky formation local centers rarely contain more than 15 thin flows with a combined thickness of 50 m and more typically 10 20 m so the volume of the eruptive portions of each system is probably two orders of magnitude smaller than the nadezhdinsky edifice the west greenland centres are juxtaposed along fault zones that appear to be linked to the subsidence of the tertiary delta and so emplacement along n s structures appears to be a principal control on the distribution of lavas and feeder intrusions this leads us to suggest that the greenland system is small and segregation of sulphide took place at high levels in the crust whereas at noril sk the saturation event took place at depth with subsequent emplacement of sulphide bearing magmas into high levels of the crust as a consequence it may be unreasonable to expect that the west greenland flood basalts experienced mineralizing processes on the scale of the noril sk system

mesozoic cenozoic unroofing of the southern appalachian basin apatite fission track evidence from middle pennsylvanian sandstones apatite fission track ages 137 pm 8 to 92 pm 7 ma and mean etchable track lengths 12 1 pm 1 8 to 12 9 pm 1 3 mu m are reported for 10 sandstones from the breathitt formation middle pennsylvanian at the northern end of the pine mountain overthrust in kentucky and virginia because all of the apatite fission track ages are 150 200 m y younger than the depositional age fission tracks that formed in the detrital apatite prior to deposition have been totally annealed thus maximum temperatures attained by these samples exceeded 110 pm 20 circ c the length distribution of etchable horizontally confined fission tracks precludes rapid 10 c m y cooling through the zone of partial annealing 60 125 c assuming a geothermal gradient of 25 pm 5 circ c km and a mean surface temperature of 15 c sandstones at the northern end of the pine mountain overthrust underwent slow erosional unroofing of 4 pm 1 km since the early cretaceous average rate of 28 pm 6 m m y the age t

high frequency 4th order sequence stratigraphy of early miocene deltaic shorelines offshore texas and louisiana abstract well logs and 3d seismic reflection data are integrated to image lower miocene depocenters of the upper texas and westernmost louisiana coastal and offshore areas although previously interpreted at a large scale 3rd order cycles the detailed stratigraphy and depositional history of the early miocene succession has not been fully developed the upper lower miocene interval from robulus l mfs10 to amphistegina b mfs9 has been further subdivided into five 4th order cycles each 200 300 m thick to provide finer scale stratigraphic detail and interpretation of depositional environments results of the finer scale correlations are presented in a series of sandstone percent maps overall the maps display a strike elongate sub parallel to the present day coastline orientation for the sandstone bodies which thin to the southeast basinward net sandstone thickness is relatively low in older cycles and increases in younger cycles suggesting an overall progradation of the deltaic system into the mapped area through time continued marine regression and southward movement of the shoreline are also confirmed by seismic amplitude maps after a major transgression associated with robulus l deltaic progradation occurred under rising sea level conditions deltaic sediments prograded southward and sandstone brought to the upper texas coast was distributed laterally by longshore currents to form strandplain coast lines maximum regression occurred during the interval mfs9 2 to mfs9 3 when a deltaic depocenter formed offshore texas in east high island area however sandstone thickness progressively decreased laterally to the east offshore louisiana where more shale was present and marine processes reworked deltaic derived sandstones into shore parallel bars there was a clear retreat of the shoreline during the youngest cycle marking the beginning of transgression associated with amphistegina b detailed stratigraphic interpretation at a 4th order scale shows more variability in the dominant shoreline processes during early miocene than previously thought recognition of sedimentary systems variability at a 4th order scale is critical for improved hydrocarbon exploration and in understanding potential future co2 storage in the area

tectonic controls on the late miocene holocene volcanic eruptions of the tengchong volcanic field along the southeastern margin of the tibetan plateau abstract the tengchong volcanic field located along the southeastern margin of the tibetan plateau experienced multiple eruption stages since the late miocene including time intervals of 5 5 4 0 ma 3 9 0 9 ma 0 8 0 01 ma and younger than 0 01 ma these eruption stages produced different volcanic rocks principally basaltic and basaltic andesite series at the same time or prior to volcanic eruptions in the tengchong volcanic field ne nne trending rift basins and ns striking normal faults formed during the late miocene pliocene in addition rapid exhumation of the neighboring mountains occurred at 6 5 ma constrained by apatite fission track dating and its thermochronological modeling at present the state of stress in the tengchong volcanic field and its surroundings is nne ne compression and wnw nw extension based on seismic foci mechanisms petrologic and geochemical data indicate that the source of the tengchong volcanic rocks belongs to an intracontinental tectonic setting but not a subduction or collision zone between indian and eurasian plates since the late miocene the dextral strike slip motion of the sagaing fault induced e w extension the sagaing dextral strike slip motion might disturb the lower crust upper mantle of the tengchong block resulting in the partial melting of the upper mantle which in turn induced volcanic eruptions characterized by mature island arc features

autometasomatic rb enrichments in highly evolved granites causing lowered rbsr isochron intercepts abstract whole rock samples of the younger members of a suite of the erzgebirge granites yield rb sr isochrons with remarkably low intercepts 0 689 0 702 whereas the older granites yield initial sr isotope ratios which are higher and rather uniform 0 70677 22 a model is proposed to explain this observation by a two stage evolution of the younger granites after a time span t 1 following the intrusion autometasomatic processes caused by residual magmatic solutions increase the rb contents of the younger granites by more or less the same amount this rb addition lowers the intercepts of the isochrons but it leaves their slopes undisturbed the rb amounts added during autometasomatosis can be derived by a petrogenetic model of fractional crystallization the time t 1 may be calculated approximately on the basis that the true initial ratio is at least as high as those of the older granite suite t 1 is in the order of 10 ma and agrees well with the difference in age between whole rock data and the rb sr mineral ages of metasomatically formed dark micas in the rocks 10 20 ma and with k ar data it is concluded that decreased granite isochron intercepts may be more common than hitherto expected

mesozoic granitic magmatism in extensional tectonics near the mongolian border in china and its implications for crustal growth abstract the yagan area of the southernmost sino mongolian border is characterized by an extensional structure where a large metamorphic core complex yagan onch hayrhan and voluminous granitoids are exposed new isotopic age data indicate that the granitoids which were previously regarded as paleozoic in age were emplaced in early and late mesozoic times the early mesozoic granitoids have 228 7 ma u pb zircon age and consist of linear mylonitic quartz monzonites and biotite monzogranites their chemical compositions are similar to those of potassic granites and shoshonitic series and show an intraplate and post collisional environment in tectonic discrimination diagrams their fabrics reveal that they experienced syn emplacement extensional deformation all these characteristics suggest that the adjustment thinning and extensional deformation at middle to lower crustal levels might have occurred in the early mesozoic the late mesozoic granitoids have a u pb zircon age of 135 2 ma and are made up of large elliptical granitic plutons they are high k calc alkaline and were forcefully emplaced in the dome extensional setting both the early and late mesozoic granitoids have end t values of 2 3 to 5 in strong contrast with the negative end t values 11 of the precambrian host rocks this suggests that juvenile mantle derived components were involved in the formation of the granitoids the similar situation is omnipresent in central asia this study demonstrates that tectonic extension magmatism and crustal growth are closely related and that post collisional and intraplate magmatism was probably a significant process for continental growth in the phanerozoic

the solund stavfjord ophiolite complex and associated rocks west norwegian caledonides geology geochemistry and tectonic environment

dehydration melting and water saturated melting of basaltic and andesitic greenstones and amphibolites at 1 3 and 6 9 kb

the annieopsquotch ophiolite belt of southwest newfoundland geology and tectonic significance the annieopsquotch ophiolite belt consists of ophiolite fragments exposed from buchans in central newfoundland to the southwest tip of the island the largest remnants belong to the annieopsquotch complex in the northeast and the long range mafic ultramafic complex in the southwest the arenigian annieopsquotch complex exposes an ophiolite stratigraphy from layered cumulate gabbro moho sequence through high level gabbro sheeted dikes pillow lava and chert the long range mafic ultramafic complex consists largely of cumulate and high level metagabbro with subordinate ultramafic rock and metabasalt all engulfed as massif to xenolith size fragments in ordovician tonalitic and younger mafic to felsic granitoid intrusions the latter rocks are intersected by a wide locally gently inclined high temperature ductile thrust zone which also encompasses amphibolite semipelitic schist and marble between buchans and the southwest tip of newfoundland the tonalite terrane encloses many smaller ophiolitic fragments the ophiolite fragments of the northern half of the belt have similar dike trends have n type morb chemistry and likely formed one major allochthon of iapetus oceanic crust this was disrupted by the intrusion of tonalite and other magmas in an arc or arc trench environment and by subsequent faulting an attempt is made to assess the significance of the annieopsquotch belt in relation to ether newfoundland ophiolites

genetic stratigraphic sequences in basin analysis ii application to northwest gulf of mexico cenozoic basin the northwest gulf of mexico cenozoic sedimentary wedge illustrates the application of genetic stratigraphic sequence analysis and documents several general conclusions 1 sequences defined by regional marine flooding are the principal genetic stratigraphic units of the basin fill continental margins are characterized by repetitive episodes of basin margin offlap punctuated by periods of transgression and marine flooding of the depositional platform 2 continental margin outbuilding is concentrated at one or more shelf edge deltaic depocenters separated by interdeltaic bights depocenters remain fixed during a depositional episode but commonly relocate during transgression and flooding 3 a distinct syndepositional structural style in prograding continental margins results in sporadic uplift of a basin fringing peripheral bulge and accentuates preservation of shelf margin facies along zones of extensional normal faulting and enhanced subsidence 4 genetic stratigraphic sequences commonly reflect an evolving interplay among two or even three variables for example early cenozoic gulf sequences are most closely related to tectonic events of the intraplate source terrane which in turn affect rate and location of sediment supply and basin margin response to loading late cenozoic sequences more closely reflect proposed eustatic cycles

role of sea level forced sedimentary processes on the distribution of organic carbon rich marine sediments a review of the late quaternary sapropels in the mediterranean sea abstract the 124 ka to present sapropel s5 s1 series in the eastern mediterranean sea correspond to the last glacial interglacial period and permit detailed investigation of 5th order sedimentary frequencies the sapropel layers are clustered in high frequency milankovian cycle that occur within sequence stratigraphic 4th order cycles of late pleistocene holocene sea level oscillations the sea level changes had an amplitude exceeding 120 m and have been correlated to the glacial interglacial cycles modulated by the 100 ka eccentricity using published data we examined occurrences of the complete s5 s1 series in the mediterranean sea and then we focussed on two stratigraphic sequences in the central adriatic shelf and middle adriatic depression where sapropels s5 and s1 were deposited during periods of sea level rise recorded by the transgressive systems tract marine isotopic stage 5 and 1 respectively sapropels s4 and s3 were deposited during the highstand systems tract that developed throughout mis 5 of the late pleistocene sequence sapropel s2 should have been deposited during the warm mis 3 3 and followed the mis 4 sea level drop when sediment supply was enhanced during the late highstand and falling stage of sea level of the late pleistocene sequence the pronounced mis 2 cold stage led to a lowstand and to the regional erosional surface sequence boundary we also compared the adriatic sapropel s5 s2 sequence 4th order to the mid pliocene cluster o sequence 3rd order in both cases sapropels are well formed before the increase of sediment supply due to progradation of the shelf wedge that occurs in the late high stand and initial falling stage in sea level finally we considered the influence of sea level lowering which could cause a weakening of north adriatic deep water formation therefore influencing the mediterranean deep water circulation this condition would favour oxygen depletion but it does not correspond to a well developed sapropel as in the case of s2 based on the assumption that productivity increases related to insolation play a dominant role in their sedimentation sapropel s2 is the result of low amplitude insolation maxima that were unable to generate a productivity increase leading to the burial of conspicuous amount of organic matter therefore in the adriatic sea the absence of the s2 sapropel is correlated to the high sedimentation rate diluting the signal whereas in the eastern mediterranean basin this sapropel is virtually absent because of a weak orbital forcing combined with oxidation that erased its record

miocene sequence biostratigraphy of the northern gulf of mexico abstract calcareous nannofossil abundance and diversity curves have been successfully used for biostratigraphic sequence modeling in the gulf of mexico miocene well data from the trend reveal that floral pulses characterized by high abundance and diversity correspond to regional foraminiferal tops these pulses vary greatly in magnitude in the studied wells and when plotted on a time track show a sinuous pattern imaging the third order cycles of haq et al 1988 results indicate that floral pulses of regional significance in the miocene of the gulf of mexico are a counterpart of transgressive systems tracts of fourth order sequences correlations of these pulses foraminiferal tops and eustatic cycle curves first proposed by jiang and watkins 1992 are updated in this study the results link biostratigraphy with sequence stratigraphy and will assist in integrating micropaleontologic data in sequence stratigraphic modeling

role of strike slip faulting in the evolution of allochthonous terranes in the philippines concepts of allochthonous terrane transport and emplacement are dominated by the assumption that most terranes originate on the subducting plate collide with the upper plate and are emplaced there movement of terranes along the convergent margin is recognized but is generally attributed to postcollision slip in the northern philippines allochthonous terranes originate primarily within the arc system have been translated along it by strike slip faults and were emplaced by cessation of that slip the authors suggest that in the philippines some originally vertical strike slip boundaries may have evolved into shallow dipping sutures marked by fold and thrust systems this mode of terrane evolution may be more common than generally appreciated particularly in orogenic belts developed in response to oblique convergence

lower palaeozoic convergent plate margin volcanism on bomlo sw norway and its bearing on the tectonic environments of the norwegian caledonides the island of bomlo presents a geological section through a caledonian convergent plate margin the lykling ophiolite and the overlying ensimatic are sequence of the geitung unit were emplaced onto the baltic shield probably in arenig times after a period of deep erosion the basin and range type volcanics of the siggjo complex were deposited followed by a hiatus in the magmatism and a weak but widespread phase of vertical tectonism possibly due to the successful subduction of a spreading ridge at the top of the mainly sedimentary ashgill llandovery vikafjord group the basin and range type volcanism resumed this was probably preceded immediately by the calc alkaline subaerial volcanism at the base of the langevag group which passes stratigraphically upwards into basin fill lithologies such as submarine volcaniclastics cherts turbidites and submarine greenstones of chemistry compatible with the early stages of ensialic back arc basin development the stratigraphy and volcanism are discussed in context with recent views on the rest of the norwegian caledonides

impact of regional variation in detrital mineral composition on reservoir quality in deep to ultradeep lower miocene sandstones western gulf of mexico abstract future exploration in lower miocene sandstones in the gulf of mexico will focus increasingly at depths greater than 4 5 km and reservoir quality will be a critical risk factor in these deep to ultradeep reservoirs the goal of this study was to understand the variation in reservoir quality of lower miocene sandstones across the western gulf of mexico to do this we examined regional variation in detrital mineral composition diagenesis and reservoir quality in five areas 1 louisiana 2 upper texas coast 3 lower texas coast 4 burgos basin mexico and 5 veracruz basin mexico using petrographic and petrophysical data from depths of 0 9 7 2 km there are strong variations in mineralogical composition within the study area lower miocene sandstones from offshore louisiana have an average composition of quartz 86 feldspar 12 and rock fragments 2 q 86 f 12 r 2 feldspar and rock fragment content increase southward as source areas shifted to include volcanic and carbonate rocks composition of samples from offshore texas ranges from q 67 f 24 r 9 in the upper texas coast to q 58 f 24 r 19 in the lower texas coast lower miocene sandstones from the onshore burgos basin northern mexico have an average composition of q 54 f 22 r 23 whereas sandstones from the veracruz basin southern mexico contain the highest proportion of rock fragments q 33 f 12 r 55 main diagenetic events in quartz rich lower miocene sandstones in louisiana were mechanical compaction and precipitation of quartz cement compactional porosity loss increased to the south with increasing rock fragment content calcite is the most abundant cement in the south and is strongly related to reservoir quality loss at moderate burial depths the best reservoir quality occurs in quartz rich sandstones in louisiana and decreases with increasing lithic content in texas and mexico porosity is higher in louisiana and upper texas than in lower texas and mexico at all depths and temperatures but at depths 5 km and temperatures 175 c porosity differences are lessened the lower miocene sandstone trend in the western gulf of mexico from louisiana to mexico is an example of the importance of variation in detrital mineralogy as a control on diagenesis and reservoir quality

combined sm nd and rb sr isotope systematics in the donegal granitoids and their petrogenetic implications the metaluminous to peraluminous granitoids of the donegal batholith northwest ireland were emplaced at c 400 ma into greenschist amphibolite facies metasediments of the dalradian supergroup sm nd and rb sr isotopic data are provided for eleven granitoid samples from six of the plutons and one specimen from the northeast granodiorite pluton of the newry complex co down the donegal results reveal essentially similar initial sr isotope ratios 0 7051 0 7068 but highly variable initial e nd values 1 2 to 8 3 and 0 5 for newry certain granitoids have distinctive nd isotopic compositions characteristic of the involvement of old lree enriched continental crust in some cases or young crust and or mantle derived magmas in others the nd and sr isotopic variations can be explained by a variety of mixing hypotheses

pt and pd mobility in hydrothermal fluids evidence from komatiites and from thermodynamic modelling abstract komatiite host rocks and ores provide a natural laboratory for the study of pt and pd during hydration and carbonation of ultramafic rocks the degree of correlation between pt and pd and of those elements with lithophile elements such as ti in komatiitic rocks and ores is strongly dependent on the magmatic sulfide content of the rock pt and pd are typically very strongly correlated within s poor komatiites and in disseminated ores containing 5 volume or less magmatic sulfides in contrast they are almost completely decoupled in sulfide rich ore samples thermodynamic modelling based on the existing body of published experimental data on pt and pd solubilities suggests that pd and pt can be readily transported as bisulfide complexes in acidic neutral solutions under reduced and moderate oxidation condition at 300 c and chloride complexes are only important under extremely acidic oxidised conditions pd is much more soluble than pt in the case of fluids in sulfide rich environments in equilibrium with the characteristic assemblages found within altered and metamorphosed komatiites solubilities of both elements are much lower in sulfide poor environments especially where ph values are constrained to neutral values by the near ubiquitous presence of carbonate minerals in the alteration assemblage this accounts for the evident near immobility of pt and pd in even the most altered silicate host rocks as compared with their wide dispersion in sulfide rich ores

depositional styles from miocene through pleistocene in the north central gulf of mexico an historical reconstruction abstract the minerals management service u s department of the interior has recently classified the producible sands from the approximately 1 100 fields across the northern gulf of mexico into groups of genetically related plays defined by production chronostratigraphy lithostratigraphy and structure each field was assigned a structural code and each producible sand was classified by depositional environment correlation of the type logs with biostratigraphic and seismic data established 13 chronozones from lower miocene to upper pleistocene sands within each chronozone were classified as transgressive aggradational progradational or submarine fan facies hydrocarbon plays encompassing all discovered resources were mapped for each chronozone this paper describes the distribution and continuity of hydrocarbon plays in the louisiana ocs comprised of discovered resources in sands deposited by the ancestral mississippi river delta pronounced changes in depositional styles and hydrocarbon prone areas are observed from the lower miocene through the pleistocene as the ancestral mississippi river delta migrated basinward and across offshore louisiana lower miocene hydrocarbon plays are restricted to the western portion of the louisiana shelf a marked eastward migration of the depocenters occurred in middle miocene during the upper miocene the depocenter began migrating to the west and extended significantly basinward throughout the pliocene and pleistocene the depocenter migrated basinward and westward the distributions of known hydrocarbon plays provide an organized framework for exploration analogs

complete remagnetization of some permian dykes from western norway induced during burial uplift abstract stable paleomagnetic directions in four basaltic dykes and in some associated caledonian metamorphic rocks define high latitude mesozoic paleomagnetic pole positions which are not compatible with the k ar age of 250 my determined for the dykes a monotonic increase of 40 occurs in the potassium content of samples taken across a dyke 32 cm wide this is accompanied by only a 1 2 variation in the k ar age of the samples suggesting the absence of any significant level of initial argon titanomagnetite grains having bulk compositions around x 0 6 have suffered extensive low temperature alteration forming assemblages of ferri rutile granules in a matrix of pure magnetite the complete remagnetisation of both the dykes and the associated country rocks is probably an expression of a vrm acquired at elevated temperatures 150 500 c at the emplacement depth of the dykes a stable remanent magnetisation was locked in during uplift of the area probably related to the kimmerian basin development in the adjacent north sea and the epeirogenic uplift of western fennoscandia

contrasting provenance of triassic jurassic sediments in north sea rift a single zircon sims sm nd and trace element study abstract an ion microprobe sims u pb zircon study has been performed on a selection of five samples in order to identify the various provenances of sands sandstones from the lunde and statfjord formations the gullfaks oilfield the data are integrated with sr element analyses and new and published sm nd whole rock data which give consistent and additional information in this geologically complex area previous work has shown that the upper parts of the lunde formation have relatively low nd model ages typically 1 5 to 1 6 ga reflecting stable tectonic conditions the overlying raude and eirikson members of the statfjord formation record interfingering units recording relatively low 1 5 ga and substantially higher nd model ages this may reflect an early stage of rifting in the northern north sea rift zone while the upper nansen member represents a return to relatively low uniform nd ages and quiet tectonic conditions the present data reveal a more complex interplay between sediment provenance terranes than previously assumed the sample of the lunde fm is characterised by a high number of zircon age peaks at 0 39 to 1 75 ga it was transported from the south and has an ultimate provenance in the sw margin of norway after the mid devonian the eirikson 1 sample with t dm of 2 4 ga represents the income of units with substantially higher nd model ages a prominent peak of archaean zircons at 2 56 to 3 00 ga reflects a sediment source in archaean gneisses which were located in the north smaller peaks at 0 38 to 0 54 ga and 1 10 to 1 90 ga represent a separate provenance component in the east shetland platform the raude and eirikson 2 samples have t dm of 1 7 and 1 8 ga and show a cumulative zircon peak at 0 38 to 0 46 ga and a high number of small peaks in the age ranges 0 70 to 2 23 ga and 2 40 to 3 00 ga these units where transported from the south and have an ultimate provenance in the east shetland platform the sample of the nansen fm records a peak of zircon ages at 1 58 to 1 74 ga was derived from the south and represents re cycled devonian sediments from sw norway the four samples derived from the south may have experienced sediment redeposition of these the nansen sample is the best example of a sediment sequence which has been preserved and transported as an isolated unit through erosion sediment transport and re deposition there is a perfect match between the geochemistry of this sample and devonian sediments from sw norway suggesting subordinate contamination from other provenance terranes during sedimentation and sediment re deposition this suggests that provenance studies of devonian sediments from the north sea area are important for an understanding of the ultimate source areas and processes of sediment re deposition in the younger rift

from seismic stratigraphy to seismic sedimentology a sensible transition abstract the concept of seismic stratigraphy was developed during the 1970 s based on 2 d seismic technology utilizing reflection terminations configurations and external forms interpreted from multiple seismic profiles geologists could analyze seismic facies within depositional sequence boundaries and infer regional scale depositional environments and depositional history three dimensional seismic data provide spatially dense high resolution horizontal reflection patterns for improving seismic facies analysis and make seismic sedimentology a reality a step beyond seismic stratigraphy seismic sedimentology is the study of depositional facies lithology and architecture by using the relationships between spatial reflection patterns and the morphology of depositional systems a major obstacle facing seismic interpreters used to be their inability to pick reservoir scale depositional surfaces on vertical seismic profiles this problem has been largely resolved by a technique called stratal slicing stratal slicing tries to pick depositional surfaces by proportionally slicing between two reference seismic events stratal slices from two 3 d seismic volumes in louisiana illustrate that seismic sedimentology can significantly improve seismic facies analysis by 1 reducing ambiguity in seismic facies and lithologic interpretation 2 increasing vertical resolution from the third order sequence 100 m to the reservoir 10 m level and 3 helping identify reservoir scale depositional architecture

volatile phase fluxed anatexis of metasediments during late caledonian ophiolite obduction evidence from the sogneskollen granitic complex west norway in the west norwegian caledonides minor granitic sheets were intruded during obduction of the c 443 ma solund stavfjord ophiolite complex the sogneskollen granitic complex is the largest intrusive complex and consists of several minor lenses and one larger lens of granodiorite the sogneskollen granitic complex samples generally have high ba sr and na 2 o very low y values nd 420 ma values c 7 to 10 1 and 87 sr 86 sr 420 ma ratios of 0 70437 0 70705 the ree patterns are strongly hree depleted and la yb n varies from 0 3 to 36 most samples display small positive eu anomalies but those that are most fractionated have small negative anomalies the main part of the sogneskollen granitic complex is a white composite granodiorite which is intruded by a suite of grey biotite granite dykes it is suggested that this and other similar granitic sheets in the area most probably formed by melting of sediments in the presence of a h 2 o bearing volatile phase during obduction of the ophiolite the quartzofeldspathic source sediments most probably contained detritus from long term rb depleted lower continental crust and from an island arc the sediments were deposited in large amounts in the marginal basin now represented by the solund stavfjord ophiolite complex during obduction the sediments were buried beneath the hot overriding ophiolite complex and the associated island arc and yielded granitic magmas during partial melting

high frequency miocene sequence stratigraphy offshore louisiana cycle framework and influence on production distribution in a mature shelf province the regressive miocene succession of offshore louisiana comprises 10 third order sequences and no fewer than 58 fourth order sequences which average approximately 1 1 and 0 19 m y in duration respectively comparable to durations measured in the gulf coast basin and basins worldwide upper lower to middle miocene distal third order sequences comprise mostly lowstand prograding wedge slope fan and basin floor fan deposits in contrast middle to upper miocene medial sequences record progressively more landward systems tracts 1 the lateral transition between on shelf incised valley fills and the proximal parts of basinward thickening lowstand prograding wedges and 2 cyclic on shelf highstand and transgressive systems tracts upper miocene inner shelf and marginal marine systems tracts and more abundant incised valleys dominate the thinner proximal third order sequences this genetic framework has a major influence on hydrocarbon distribution although a strong structural trapping component is present in the fields more than 90 of cumulative production originates where fourth order systems tracts stack to form third order lowstand systems tracts in all 10 third order sequences the development of a high frequency sequence framework for the prolific miocene succession and the discovery that hydrocarbons are pooled within the miocene third order lowstand systems tracts yield a focused model for the development of abundant undiscovered miocene reserves in the mature northern gulf of mexico shelf province

platinum group elements pge in basalts of the seaward dipping reflector sequence se greenland coast the rift related seaward dipping reflector sequence sdrs se of introduction greenland consists of basaltic lavas that exhibit variable degrees of seaward dipping reflector sequences sdrss are charmagmatic differentiation derived from a heterogeneous mantle source acteristic features of volcanic rifted margins the sdrs platinum group elements pge are used to provide insights into se of greenland formed during the initial break up of the petrogenetic evolution of the sdrs and to characterize the the north atlantic in the late paleocene early eocene magma sources noble metal concentrations correlate well with duncan et al 1996 and spans the whole period from indicators for magmatic differentiation mg number mgo excontinental break up to seafloor spreading brooks hibiting two distinct trends concentrations of ir ru and rh tend nielsen 1982 saunders et al 1997 large volumes of to decrease with progressive differentiation indicating compatible lava erupted in a short period of time under subaerial behaviour of these elements during fractional crystallization processes or shallow water conditions along the continental margins the variation of pt and pd shows segmented trends in primitive of norway and greenland the high magma production magmas pt and pd are incompatible and become enriched in the rates which led to the formation of magmatic sequences melt the primitive magma is s undersaturated despite derivation up to 6 km thick are inferred to require the presence of from a depleted mid ocean ridge basalt source at a moderate degree a mantle plume white mckenzie 1989 of melting reflecting enhanced s solubility in the melt caused by the aims of ocean drilling program odp legs 152 high fe content and elevated temperature in the more evolved lavas and 163 were to investigate the origin and nature of the pt and pd decrease with decreasing mgo and mg number this sdrs with regard to the influence of a mantle plume indicates that s saturation had occurred with pt and pd being larsen et al 1994a duncan et al 1996 nine sites were incorporated in sulphides which probably segregated during ascent drilled in two transects distal to the ancient plume track bulk partition coefficients for the pge during partial melting are at 63 n and 66 n fig 1 calculated based on data from a primitive basaltic unit with mgo it has been shown that platinum group element pge 20 wt representing a near primary magma composition the systematics can provide important information about determined bulk partition coefficients for an s undersaturated melt the petrogenesis of basaltic magmas such as magmatic are about 2 ir 4 ru 1 2 rh 0 5 pt and 0 4 pd this differentiation sulphide segregation and mantle source indicates that ir ru and rh are compatible during partial melting characteristics e g brugmann et al 1987 greenough whereas pt and pd are incompatible owen 1992 barnes picard 1993 seitz keays

geochronology of the buchans roberts arm and victoria lake groups and mansfield cove complex newfoundland volcanic groups of the central mobile belt of the newfoundland appalachians have previously been subdivided into early and late arc sequences separated in time by a quiescent caradocian stage defined in some areas by fossil bearing black shales new u pb zircon ages of and 473 2 ma for rhyolites of the buchans and roberts arm groups respectively show them to be correlative early ordovician sequences these ages serve to refute both the previous rb sr whole rock isochron ages of 447 ma and the idea that these groups were late arc sequences these new ages corroborate evidence from late arenig early llanvirn conodonts in the buchans group and calibrate this fossil occurrence a new u pb zircon age of 479 3 ma from plagiogranite of the mansfield cove complex immediately west of the roberts arm group shows that this plutonic body is only slightly older than the adjacent volcanic rocks and not hadrynian as previously supposed local field relationships suggest this body may represent part of a dis

the sunnfjord melange evidence of silurian ophiolite accretion in the west norwegian caledonides a major composite terrane the sunnfjord melange has been identified in the west norwegian caledonides the rocks of the melange provide a terrane link between the allochthonous continental rocks of the dalsfjord suite with its cover of continental margin deposits and the oceanic terrane of the solund stavfjord ophiolite complex the melange was formed as the ophiolite was emplaced on the the fossiliferous lower to middle silurian continental margin deposits of the herland group this group unconformably overlies older metasedimentary rocks of the hoyvik group and the crystalline basement of the dalsfjord suite a structural style characteristic of thin skinned thrust foldbelts is locally preserved within the herland group on atloy and the thrust foldbelt was developed in the foreland of the continental margin during the ophiolite accretion a u pb zircon age of 443 3 ma from the ophiolite the silurian fossils in the herland group and the identification of the sunnfjord melange as an obduction melange provide the basis for a well constrained model of ophiolite accretion in the scandinavian caledonides

floral pulses foraminiferal peaks and inferred sequences in the miocene of the northern gulf of mexico abstract investigation of miocene calcareous nannofloral pulses in 10 deep wells from the northern part of the west cameron and neighbouring ocs areas offshore south western louisiana gulf of mexico shows that nannoplankton pulses closely match previously determined foraminiferal peaks a framework of correlation has been constructed for these pulses and foraminiferal peaks and they have been successfully tied to the third order eustatic cycles although essentially confirming the cycles of haq et al 1988 these data suggest the following conclusions 1 the tb1 tb2 supercycle boundary coincides with the top of the anahuac in the basal miocene downdip the boundary is recognized at the lower limit of the lowstand correlated with cristellaria r 2 of the six third order cycles in tb2 tb2 4 is the most prominent it persists from 17 4 to 14 0 m y bp and the condensed interval is correlated with robulus 43 tb2 3 is relatively brief as it persists for only about 1 3 million years 18 7 to 17 4 m y bp deposits of this cycle correlated with robulus chambersi and discorbis b are often missing or incomplete in the study area the subjacent cycle tb2 2 is stratigraphically above siphonina davisi and its condensed interval is correlated with the second marginulina a peak 3 the condensed interval of tb2 5 corresponds to cristellaria i and that of tb2 6 to textularia w the tb2 tb3 supercycle boundary coincides with the upper limit of globorotalia fohsi robusta that berggren et al 1985 dated 11 5 m y bp 4 the most significant sea level drop during miocene time occurred in cycle tb3 1 this drop resulted in the absence of most of nanno zone nn8 in the study area downdip sediments of this cycle are successionally characterized by from top to bottom bolivina thalmani discorbis 12b textularia l cibicides inflata bigenerina 2 and cibicides carstensi 5 the transgressive systems tract of tb3 2 is successionally characterized by cristellaria k amphistegina e and cyclammina 3 other upper miocene markers bigenerina a robulus e textularia 6 and textularia x occurred during the highstand of this cycle 6 sediments of the youngest miocene cycle tb3 3 are missing in the study area due to deep erosion during lowstand of tb3 4 in the early pliocene the lower pliocene encountered in five of the 10 studied wells was also analysed the data suggest that the condensed interval of the lower pliocene cycle tb3 4 is correlated with the buliminella 1 peak and that of tb3 5 with globigerina nepenthes

interpretive advantages of 90 phase wavelets part 2 seismic applications we examine field seismic data to test the benefits of 90 phase wavelets in thin bed interpretation that are predicted by seismic modeling in part 1 of this paper in an interbedded sandstone shale miocene succession in the gulf of mexico basin a 90 phase shift of nearly zero phase seismic data significantly improves lithologic and stratigraphic interpretation a match between seismic and acoustic impedance ai profiles results in a better tie between seismic amplitude traces and lithology indicative logs better geometric imaging of ai units that does not use dual polarity seismic events results in easier and more accurate reservoir delineation less amplitude distortion and the stratigraphy independent nature of thin bed interference significantly improves stratigraphic resolution and seismic stratigraphic profiling for a ricker like wavelet having small side lobes stratigraphic resolution of 90 phase data is considerably higher than that of zero phase data in this specific case stratigraphic re

platinum group element geochemistry of andesite intrusions of the kelian region east kalimantan indonesia implications of gold depletion in the intrusions associated with the kelian gold deposit nineteen andesite samples from the kelian gold mine and two adjacent prospects magerang imang and nakan in east kalimantan have been analyzed for cu re au and the platinum group elements pge the aim of the study was to use variations in pge concentrations to test whether the chalcophile elements became enriched or depleted with fractionation in the igneous suites associated with the kelian deposit when the data are plotted on mantle normalized chalcophile element diagrams the patterns for the magerang imang and nakan suites are subparallel over a range of pge concentrations of about 20 times pd ir ratios for magerang imang hornblende andesites and nakan pyroxene andesites are 15 to 54 and 60 to 129 respectively the lower pd ir ratios for the magerang imang suite are due to a reversal in the slope of the mantle normalized pattern between ir and os that is not seen in the nakan suite all pge concentrations from both the magerang imang and nakan suites decrease with increased fractionation furthermore all samples which have not been subjected to strong alteration are strongly depleted in au relative to adjacent elements on mantle normalized chalcophile element plots crosscutting relationships show that the gold mineralization at kelian is younger than the associated central and eastern andesite intrusions so they cannot be the source of the gold in the deposit the gold was probably derived from slightly younger intrusions similar in age to the adjacent magerang imang hornblende andesites which appear to be coeval with the mineralization and which are also depleted in gold the subparallel pge patterns preclude the depletion of cu au and pge by sulfide fractionation which would fractionate highly chalcophile pge from less chalcophile cu and re we suggest that the parallel cu au pge patterns are due to mixing between a relatively mafic pge rich magma and a more felsic magma with lower pge concentrations

u pb ages of ophiolites and arc related plutons of the norwegian caledonides implications for the development of iapetus u pb zircon ages are reported for four ophiolites and three crosscutting arc related plutons from the norwegian caledonides plagiogranite differentiated from gabbro of the karmoy ophiolite is dated at 493 7 4 ma whereas arc related trondhjemite cutting this ophiolite crystallized at 485 2 ma a crosscutting clinopyroxene phyric gabbro intrusion is dated at 470 9 5 ma by near concordant magmatic titanite sphene and discordant u rich 2903 6677 ppm zircon lower intercepts of 247 68 and 191 70 ma defined by the plagiogranite and clinopyroxene phyric gabbro best fit lines may reflect a real low t alteration rift related event

petrogenesis and significance of late caledonian granitoid magmatism in western norway obduction of the late ordovician solund stavfjord ophiolite complex 443 3 ma west norwegian caledonides involved generation and high level emplacement of granitic and granodioritic dikes and plutons initial 87sr 86sr ratios in the granites are low 0 7042 0 7059 suggesting either a mantle component or a rb poor crustal source initial end end t ranges from 0 8 to 8 8 indicating that the granites represent recycling of old crustal rocks which is supported by precambrian inheritance in zircons from two of the studied granites i argue that the rb sr and the sm nd isotope systems are decoupled in the sense that the sr and the nd isotopes derive their dominant signals from two different sources a mantle source and a crustal source respectively the granites are metaluminous to peraluminous and typically have high sr ba and na2o k2o ratios sio2 contents range from 66 to 74 wt ree abundances are highly variable the la contents range from 80 to 200 times chondrite and are inversely correlated with the contents of sio2 the concentration of nd in the granites decreases asymptotically with decreasing end t suggesting fractional crystallization of accessory phases and assimilation of continental crust this argument is supported by the presence of partly dismembered xenoliths in the granites with end t values that are significantly lower than end t values in the host granite the following models are suggested for the granites when the ophiolite complex obducted an outboard subduction zone approached the continental margin and subduction related magmas accumulated beneath the continental margin and probably intruded the overlying eugeosynclinal deposits the mantle derived magmas most likely evolved to granitoid composition by assimilation of these eugeosynclinal sediments and by fractional crystallization of amphibole feldspar sphene and allanite alternatively but less likely the heat content of the mantle derived magmas caused extensive melting of immature graywackes and calc alkaline volcaniclastic rocks in the deepest portions of the eugeosyncline either way during ascent the compositions of the granitic melts were modified by fractional crystallization of lree rich phases and by assimilation of continental metasediments

constraints on melting and magma production in the crust major intrusions of granitic rocks are found in several tectonic settings and in all cases crustal melts may contribute to the volumes of granitic magma high grade metamorphism and partial melting of the crust take place predominantly under fluid absent conditions we present a model for calculating the amounts of melt that may be formed by fluid absent breakdown of micas and amphiboles in common crustal rock types pelitic quartzofeldspathic intermediate and mafic melt proportions depend mainly on the kind of source rock the pressure at which melting takes place the temperature and the hydrous mineral h2o content of the source as a consequence of the pressure dependence of water solubility in silicate melts any given source rock will produce more melt by a given fluid absent reaction at lower pressure at a given pressure higher temperature reactions can produce more melt from a given source rock based on a survey of the compositions of common rock types we show that the amounts of melt can vary from 50vol thus crustal rocks vary widely in their fertility as magma sources depending on the types and amounts of hydrous phases they contain in general muscovite breakdown in pelites will yield only small quantities of melt and lead to migmatite formation biotite breakdown in pelites occurs at higher temperature and because most high grade pelites below granulite grade are biotite rich can yield up to about 50 vol of granitoid melt rocks of intermediate composition and hornblende rich mafic rocks are potentially highly fertile magma sources also provided that the high temperatures necessary for biotite and hornblende breakdown are realized pyroxene rich mafic rocks and quartzofeldspathic rocks are much less potentially fertile data suggest that mechanisms exist for the efficient segregation of melt and restite in systems with 30and probably 20vol melt the pressure temperature history of a region can greatly influence crustal source fertility through its control over the occurrence of subsolidus dehydration and melting equilibria

magma mixing recorded by sr isotopes of plagioclase from dacites of the quaternary tengchong volcanic field se tibetan plateau abstract the tengchong volcanic field east of the burma arc in sw china comprises numerous quaternary volcanoes the volcanic rocks can be grouped into four units numbered 1 4 from oldest to youngest units 1 3 and 4 are composed of trachybasalt basaltic trachyandesite and trachyandesite respectively whereas unit 2 consists of hornblende dacite primary minerals in the dacite include plagioclase clinopyroxene amphibole and magnetite all set in a glassy groundmass some of the dacites are moderately to highly weathered and show effects of low temperature alteration with loss on ignition loi up to 9 6 wt and positive correlations of loi with al2o3 but negative correlations with sio2 cao and na2o fresh dacites loi plagioclase in the fresh dacite has relatively high sr contents and 87sr 86sr ratios ranging from 0 7050 to 07125 except for one value of 0 7516 whereas plagioclase in the altered dacite has low sr contents and highly variable sr isotopic compositions 0 7039 0 7138 indistinguishable from those of the fresh rocks both an values and sr isotopic compositions of the plagioclase indicate mixing of mantle and crustally derived magmas we therefore propose that mantle derived basaltic magma caused partial melting of the lower middle crust and that mixing of the mafic and felsic magmas produced the dacite in staging magma chambers the dacites contain minerals crystallized from both the mafic and felsic magmas as well as a few xenocrysts plucked from the country rocks

the origin of oceanic plagiogranites from the karmoy ophiolite western norway both field relationships and geochemical characteristics indicate two suites of plagiogranitic and related rocks coexisting in the higher parts of the karmoy ophiolite of western norway the plutonic zone of this ophiolite can be subdivided into three complexes the east karmoy igneous complex the visnes high level complex and the veavagen igneous complex and plagiogranitic rocks are well developed in the first two of these within the east karmoy igneous complex plagiogranites are associated with high temperature pre basic dyke shear zones rare earth element modelling indicates that these plagiogranites were derived by anatexis of amphibolite hydrated diabase assuming a starting material consisting of 40 hornblende and 60 plagioclase and that batch melting occurred within the stability field of hornblende in comparison plagiogranite occurs in a number of bodies in the upper part of the visnes high level complex and forms a sandwich horizon together with biotite diorites and epidosites between a roof assemblage of dykes microgabbros and magnetite gabbros and a floor assemblage of layered and non layered gabbros the r e e modelling of the petrogenesis of this series of plagiogranites indicates that they were derived by filter pressing of a differentiated interstitial liquid to the vari textured gabbros although the distribution of highly hygromagmatophile elements such as k rb ba etc cannot be explained satisfactorily by this model alone depletion in these elements appears to be an autometasomatic effect

shear deformation and eclogite formation within granulite facies anorthosites of the bergen arcs western norway the anorthosites of the bergen arcs contain two distinct metamorphic mineral assemblages which are stable under different p t conditions a granulite facies assemblage which equilibrated at t 800 900 c p 10 kbar is defined by the minerals plagioclase al rich diopside cpx i garnet gnt i scapolite orthopyroxene dark brown hornblende this assemblage is linked by corona structures to a primary magmatic mineralogy where coexistence of olivine and plagioclase limits the maximum crystallization pressure of the anorthosites to 8 kbar at 1200 c the anorthosites are transected by a series of fine grained shear zones in which the granulite facies assemblage is replaced by an eclogite mineralogy defined by omphacite cpx ii jd40 50 garnet kyanite epidote clinozoisite phengite quartz amphibole with plagioclase sometimes remaining in the marginal parts mineral compositions indicate equilibration conditions for the eclogite paragenesis of t 700 800 c p 16 19 kbar a series of textural and mineralogical changes which can be observed as the strain increases into the shear zones demonstrates that the eclogite mineralogy is formed at the expense of the granulite facies minerals during the early stages of shear zone development the eclogite mineralogy defines a strong mylonitic fabric which again is truncated by centimetres thick secondary shear zones containing clasts of both granulite and eclogite minerals these latter fine grained ultramylonites may be connected to uplift of this deep seated complex the eclogite shear zones are highly oblique to the granulite structures indicating that the eclogite shear zones formed under a new stress regime rb sr dating shows that the granulite facies assemblage formed in sveconorwegian time or earlier while minerals from shear zones define ages around 400 ma indicating a caledonian formation of the eclogites this work demonstrates the critical role of deformation in triggering metamorphic reactions whether the shear stress also was important in stabilizing the eclogite assemblages is not clear from the available data

plagiogranites from the ocean crust and ophiolites rock types similar to those found in ophiolites have been recovered from the floors of ocean basins1 3 this is critical evidence for the theory that ophiolites represent the ocean crust4 5 aumento6 has described diorites quartz diorites and trondhjemites dredged from the mid atlantic ridge mar near 45 n and noted that these were the first rocks analogous to the low k acid differentiates in ophiolites collectively referred to as oceanic plagiogranites 7 8 to be recognized amongst those recovered from the modern ocean floor only a few plagiogranites from the ocean floor ocean crust plagiogranites have since been described these were dredged from widely spaced localities on the mar9 11 the amani plateau and the kyushu palau ridge north west pacific 12 however the kyushu palau ridge could be the product of destructive plate margin magmatism13 as are the potassic granites from the aves ridge caribbean 14 plagiogranite xenoliths also occur in extrusives on surtsey and askja iceland 15 that only some of the petrological compositions of ophiolite plagiogranites are represented in the ocean crust is not surprising in view of the small sample of ocean crust plagiogranites it is less obvious however if ophiolites represent the ocean crust why certain acidic rock types from the oceans have not been found in ophiolites i show here that the difference between the two groups of plagiogranites is evidence that ophiolites do not represent all kinds of ocean crust and that a greater range of magma compositions and or conditions of magmatic differentiation is represented by the ocean crust

crustal melting in the lower parts of island arcs an example from the bremanger granitoid complex west norwegian caledonides the 440 5 ma bremanger granitoid complex bgc west norwegian caledonides comprises tonalites granodiorites and minor granites the bgc is coeval with a nearby subduction related gabbronoritic dioritic intrusion the 443 4 ma gasoy intrusion and an ophiolite complex generated in a back arc basin the 443 3 ma solund stavfjord ophiolite complex the bgc has a restricted isotopic variation nd440ma 0 04 to 2 21 87sr 86sr440 ma 0 7045 to 0 7054 indicating a substantial mantle component but it is enriched in crustal material relative to the primitive parts of the gabbronoritic rocks of the gasoy intrusion nd440 ma 1 67 87sr 86sr440 ma 0 7048 the chondrite normalised ree patterns are fractionated la yb n 8 to 16 smooth or with negative or positive eu anomalies geochemical modelling suggests that the bgc does not represent residual melts produced by fractional crystallisation of basic magma but is compatible with partial melting of mafic source rocks with various amounts of h2o residual garnet is suggested by hree depletion indicating a minimum pressure of 9 10 kbar during melting

vapour absent melting from 10 to 20 kbar of crustal rocks that contain multiple hydrous phases implications for anatexis in the deep to very deep continental crust and active continental margins

petrogenesis and tectonic significance of the late permian middle triassic calc alkaline granites in the balong region eastern kunlun orogen china numerous calc alkaline granitoid intrusions in the eastern kunlun orogen provide a valuable opportunity to constrain the evolution of the orogen the age and genesis of these intrusions however remain poorly understood the granitoid intrusions near the balong region eastern kunlun orogen consist of granodiorite diorite and syenogranite the granodiorite contains crystallized segregations abundant mafic microgranular enclaves mmes and small quartz diorite stocks in situ zircon u pb dating reveals that the granodiorites and quartz diorites were emplaced between 263 and 241 ma whereas the syenogranite was produced at c 231 ma the granodiorite and quartz diorite have a calc alkaline affinity and are metaluminous and na rich with slightly enriched sr nd isotope compositions the granodiorite is characterized by fractionated ree patterns whereas the quartz diorite displays a relatively flat ree pattern the mmes are consistent with the granodiorite in terms of incompatible elements and sr nd isotope composition compared to the granodiorite and diorite the syenogranite has higher sio 2 k rb th and sr contents and a lower rb sr ratio the results presented here when combined with regional geological data indicate that the granodiorite and quartz diorite were derived from dehydration melting of mafic lower crustal rocks during the n directed subduction of the anyemaqen ocean lithosphere in late permian middle triassic times whereas the syenogranite was produced at a higher crustal level in a syn collisional setting compared to the granodiorite

the behaviour of platinum group elements in basalts from the east greenland rifted margin the continental flood basalts of the east greenland volcanic rifted margin were extruded during continental breakup above the ancestral iceland mantle plume at 55 ma three distinct magma types the low ti high ti and very high ti series lts hts and vhts respectively are found intercalated in the 6 km thick plateau lava sequence incompatible trace elements indicate that the lts are derived from a more depleted mantle source compared to hts and vhts the lts is characterised by increasing cu 105 to 248 ppm and pd 7 to 24 ppb constant cu pd ratio 10 000 and decreasing ir 1 1 to 0 05 ppb and ru 1 8 to 0 3 ppb concentrations during magmatic differentiation 16 to 7 wt mgo the constant cu pd ratio reflects silicate and chromite dominated fractionation without concurrent segregation of sulphide s undersaturated differentiation is also indicated in the hts which also displays increasing pd 6 16 ppb and decreasing ir concentrations 1 to 0 05 ppb during differentiation and the cu pd ratios for the entire series average 21 000 however some hts samples have elevated cu pd ratios up to 33 000 cu pd ratios in the hts do not correlate with mgo and this is interpreted to reflect varying cu pd ratios of hts parental magmas rather than s saturated differentiation during s undersaturated differentiation of the lts and hts pt pd ratios decrease from 1 3 to 0 11 and 1 1 to 0 2 respectively which indicates that pd is much more incompatible than pt during s undersaturated differentiation the vhts consists exclusively of highly evolved samples with low mgo 6 6 6 1 wt and pd ir ratios 98 228 here cu pd ratios increase from 17 500 to 35 000 with decreasing cr concentrations which indicate that these magmas experienced silicate fractionation with concurrent segregation of sulphide the lts represent melting of a depleted source and show high pge concentrations and constant cu pd ratios during s undersaturated differentiation melting of a normal depleted upper mantle source generates s saturated melts morb and the depleted lts source is therefore considered an extraordinary s poor component within the ancestral iceland plume of the three series the vhts contain the largest contribution from enriched mantle portions the vhts have similar pge but much higher nb concentrations for instance compared to the most evolved lts and hts samples indicating that the enriched source contributes nb but not pge

vapor absent melting at 10 kbar of a biotite and amphibole bearing tonalitic gneiss implications for the generation of a type granites vapor absent melting experiments on a biotite and amphibole bearing archean tonalitic gneiss agc150 at 10 kbar and 875 to 1050 c show that amphibole breaks down from 900 to 950 c producing garnet orthopyroxene and granitic melt biotite dehydration melting produces 10 wt melt up to 950 c via incongruent melting reactions that produce garnet orthopyroxene and titanomagnetite widespread biotite dehydration melting occurs between 950 and 975 c and produces orthopyroxene magnetite titanomagnetite and 20 wt fluorine rich melt up to 0 31 wt f minor f rich 2 7 wt biotite is present even at 1000 c our experiments show that under vapor absent conditions intrusion of hot mantle derived magmas into the lower crust is necessary to initiate widespread biotite dehydration melting in rocks with compositions like agc150 we propose that the high thermal stability of biotite in agc150 suggests that this rock is residual after a previous episode of partial dehydroxylation that left behind somewhat f enriched biotite we show that dehydration melting of such f enriched biotite produces f rich granitic liquids with compositions within the range of a type granite and leaves behind a granulitic residue consisting of orthopyroxene plagioclase quartz titanomagnetite and magnetite

rare earth element distributions among minerals in a granodiorite and their petrogenetic implications a study of the distribution of lanthanide rare earths in a granodiorite from the eastern peninsular ranges batholith southern california reveals that a large fraction of the ree in this rock resides in the accessory phases sphene and allanite the minerals plagioclase alkali feldspar biotite epidote and apatite each contribute approximately 1 or less of each ree to the whole rock with the exception of eu for which plagioclase contributes 7 sphene and allanite together contain 80 to 95 of each ree each of these phases is zoned in ree concentration with substantial decreases from core to margin textural observations argue for relatively early saturation and precipitation of sphene and allanite in the magma ree zoning trends in sphene and allanite and unexpectedly low ree concentrations in largely later crystallizing minerals such as feldspar indicate that the precipitation of sphene and allanite significantly reduced ree concentrations in residual melts these results illustrate the potential that sphene and allanite have for controlling the behavior of ree in granitic magmas available information collectively suggest that the sampled granodiorite existed as a complete melt that the ree contained in the assemblage of phases were derived by direct crystallization from the melt and that the melt behaved essentially as a closed system once crystallization of the phases now present began close correspondences between the major and trace element chemistries of the granodiorite and phenocryst poor lavas from similar tectonic settings support these conclusions the ree pattern of the granodiorite melt appears to have originated at depth and is characteristic of its source regions and derivation mechanism the high liquidus temperature of a granodiorite melt 1000 c indicates the importance of mantle derived components within the sources of batholithic magmas in the peninsular ranges

implications from zircon saturation temperatures and lithological assemblages for early permian thermal anomaly in northwest china despite considerable efforts the paleozoic tectonic setting subduction vs post collision vs mantle plume in the central asian orogenic belt caob remains controversial with aims of solving this issue we examined zircon saturation temperatures of granitoids emplaced in different settings and demonstrated that zircon saturation thermometry of felsic igneous rocks especially when integrated with their lithology and geochemistry can be a simple and powerful tool for tectonic discrimination this method is then applied in northern xinjiang where the early permian granitoids have higher saturation temperatures 800 c than the silurian carboniferous granitoids mainly 800 c moreover the latter are mainly of i type and are characterized by relatively narrow range of uniformly higher end consistent with high ehf in zircons from contemporaneous ultramafic mafic complexes they were most likely derived from subduction related processes in contrast the high temperature early permian granitoids and associated ultramafic mafic rocks show a wide range in end and ehf in zircons consistent with the involvement of a great variety of crustal and mantle materials in magma genesis the thermal anomaly and diversity of permian magmatic rocks in the caob can be ascribed to a mantle plume activity in agreement with widespread contemporaneous flood basalts and associated mafic dykes and ultramafic complex in tarim therefore the temporal variation in temperature and geochemistry of paleozoic granitoids in northern xinjiang mirrors a significant change in tectonic setting from a subduction collision setting during silurian carboniferous to an intra plate setting since early permian

the tectonic framework of the sumatran subduction zone the great aceh andaman earthquake of december 26 2004 and its tragic consequences brought the sumatran region and its active tectonics into the world s focus the plate tectonic setting of sumatra has been as it is today for tens of millions of years and catastrophic geologic events have likely been plentiful the immaturity of our understanding of great earthquakes and other types of geologic hazards contributed to the surprise regarding the location of the 2004 earthquake the timing however is probably best understood simply in terms of the inevitability of the infrequent events that shape the course of geologic progress our best hope is to improve understanding of the processes involved and decrease our vulnerability to them

cenozoic deformation in western yunnan china myanmar border in the present paper we describe ductile and brittle deformation styles in western yunnan and ne myanmar using field data and landsat 7 imagery we show that this complex area located at the northern termination of sunda plate three rivers area was wedged during the tertiary between the left lateral ailao shan chong shan metamorphic belts to the east and the right lateral shan scarp gaoligong metamorphic belt in west this triangular region therefore underwent the effects of these continental size ductile strike slip faults separating major blocks with a dominant ew to ene compression since the late miocene date of the reversal of motion along the rrf the incipient eastward motion of the sunda block and the persisting right lateral motion along its western boundary sagaing fault created n s compression and e w to wnw extension underlined by left lateral transtension along the wanding nanting fault zones at the same time the diangcan shan situated along strike the ailao shan metamorphic belt was slightly impinged by the blocks extruded from the syntaxis and exhumed again from the early pliocene in accordance with this late and still active state of stress q 2004 elsevier ltd all rights reserved

petrography stable isotope geochemistry and diagenesis of miocene sandstones vermilion block 31 offshore louisiana abstract sandstones from three conventional cores between depths of 11 140 and 14 910 feet currently have an average composition of q86f10r4 but have lost approximately 5 percent feldspar to replacement by carbonates and to dissolution like other deeply buried gulf coast sandtones plagioclase is being albitized and k feldspar is dissolving patterns of feldspar distsribution in the deeper cores suggest that tight cementation by calcite retarded feldspar diagenesis and that waters of different composition or temperature must have invaded different sandstone beds in close proximity calcite quartz and chlorite are most responsible for causing deterioration of reservoir quality calcite volume averages 4 8 percent and occludes all pores in samples containing more than 5 percent calcite textural relations and isotopic data indicate that calcite precipitated over a broad temperature range and from fluids of variable composition during burial the 87sr 86sr ratio of cements is consistent with the derivation of the sr from coeval or only very slightly older skeletal material prior to extensive silicate reactions the volume of quartz cement ranges from 0 to 14 percent mean 5 and is selective to coarser cleaner sandstones that had the highest permeability 18o values of quartz overgrowths 34 2 smow suggest that quartz cement precipitated between temperatures of 35 to 50 c from water having a 18o of about 3 1 5 silica for quartz cement was largely imported because there is negligible pressure solution of silicate grains and the amount of silica released from feldspar alteration is insufficient to account for the present quartz volume chlorite is not abundant and its diagenesis is complex including two stages of precipitation and a later stage of dissolution the first episode of precipitation formed thin grain coats that rarely inhibited quartz cementation a later episode partly filled both primary and secondary pores and strongly influenced permeability total thin section porosity ranges from 0 to 31 percent and averages 16 percent of total pores 75 percent are intergranular 17 percent are oversize pores inferred to have formed by feldspar dissolution 6 percent are intragranular pores 1 percent are oversize pores developed by fossil dissolution and a trace are micropores within clay clasts glauconite rocks fragments and clay cements

ordovician volcanism in the welsh borderland basaltic andesitic and rhyodacitic rhyolitic volcanism was widespread during ordovician time in the welsh basin new chemical data are presented for llanvirn to caradoc lavas and tuffs from the welsh borderland which during ordovician time formed the southeastern margin of the welsh basin in view of the observed chemical alteration immobile elements are used in the interpretation of the original geochemical character the data indicate that the llanvirn stapeley volcanic group of the shelve inlier was a bimodal basalt basaltic andesite rhyodacite rhyolite association the basalts have trace element contents of tholeiitic associations with a subduction related character the caradoc whittery and hagley volcanic groups of the shelve inlier comprise lavas and tuffs of calc alkaline andesite blocks sampled from the breidden hills show that these were also derived from a calc alkaline volcano associated caradoc pumice and ash flow deposits from the breidden hills are probably of altered calc alkaline rhyodacite rhyolite composition the sibdon carwood basalt flow the only known example of ordovician volcanism east of the pontesford linley and church stretton lineaments has transitional tholeiitic to alkaline character with trace element contents influenced by subduction related processes the overall tholeiitic to calc alkaline nature of the magmatism is consistent with the view that during llanvirn to caradoc time the welsh basin was an ensialic marginal basin

chalcophile element constraints on magma differentiation of quaternary volcanoes in tengchong sw china abstract the tengchong volcanic field comprises numerous quaternary volcanoes in sw china the volcanic rocks are grouped into units 1 4 from the oldest to youngest units 1 3 and 4 are composed of trachybasalt basaltic trachyandesite and trachyandesite respectively and unit 2 consists of hornblende bearing dacite this rock assemblage resembles those of arc volcanic sequences related to oceanic slab subduction rocks of units 1 and 3 contain olivine phenocrysts with fo contents ranging from 65 to 85 mole indicating early fractionation of olivine and chromite prior to the eruption of magma all the rocks from units 1 3 and 4 have very low pge concentrations with

uranium lead zircon and titanite ages from the northern portion of the western gneiss region south central norway abstract the geochronologic history of the northern portion of the western gneiss region south central norway has been investigated by u pb analyses of zircon and titanite from various basement units and one supracrustal gneiss a six point four zircon and two titanite discordia line from a migmatite melt pod and its host gneiss astfjord migmatite gneiss defines upper and lower intercept ages of 1659 1 1 8 ma and 393 6 3 6 ma respectively the upper intercept age is interpreted as a time of tonalite emplacement and migmatite formation in the region a seven point three zircon and four titanite discordia line from the ingdal granite gneiss has similar upper and lower intercept ages 1652 9 1 7 ma and 396 1 4 9 ma respectively and the upper intercept age is interpreted as the time of granite crystallization the lower intercept age from both theastfjord migmatite gneiss and the ingdal granite gneiss is interpreted as the time of regional metamorphic resetting and is evidence for caledonian influence in the region titanite from these two gneiss units as well as from other tonalite granite and supracrustal gneisses throughout the western gneiss region displays a remarkably uniform pattern of discordance titanite discordance is variable from 6 to 100 and in general the degree of discordance is proportional to the caledonian metamorphic grade of nearby supracrustal schists episodic diffusional lead loss from titanite during caledonian metamorphism probably caused the discordance pattern but a combined mechanism of mixing and diffusional lead loss cannot be ruled out in any case the seventeen point titanite and zircon discordia line clearly indicates that 1 the bulk of the granitoid terrane in this portion of the western gneiss region was emplaced migmatized and cooled in a short time interval about 1657 ma ago 2 a second short lived thermal event that exceeded the blocking temperature of titanite occurred about 395 ma ago and 3 titanite and zircon in the region were not isotopically disturbed by comparable geologic events in the period from 1657 to 395 ma or at any time after 395 ma

u pb geochronology of the coney head complex newfoundland the coney head complex white bay newfoundland comprises allochthonous plutonic rocks and deformed equivalents emplaced over the continental margin during the ordovician taconic orogeny a u pb zircon age of 474 2 ma for tonalite considered as having formed by partial melting in an island arc indicates that it crystallized in arenig time a microgranite dated by u pb zircon titanite at 432 2 ma cuts the tonalite inherited proterozoic zircon in the microgranite indicates that the rock formed from magma generated in part by melting of continental crust or sediments derived from such crust these relationships indicate that the microgranite was emplaced in silurian time after ordovician transport of the tonalite above a continental substrate

geochemistry of the emeishan flood basalts at yangliuping sichuan sw china implications for sulfide segregation a well developed 1 000 m thick basaltic sequence in the yangliuping region northern part of the emeishan basalt province includes the lower and middle units of tholeiitic basalts and an upper unit of both tholeiites and subalkalic basalts the basalts contain 42 55 wt sio2 and 4 1 8 3 wt mgo most of these lavas have gd yb 2 0 zr nb 12 and nd 260 ma values from 2 5 to 4 7 the platinum group elements pge are very mildly depleted in most of the basalts which contain 8 19 ppb pt and 7 27 ppb pd however a significant proportion of the middle unit basalts are strongly depleted in pge with some samples having concentrations lower than detection limits they have extremely high zr nb ratios up to 14 5 and low nd 260 ma values 3 21 to 0 65 features of extensive lower crustal contamination some samples in this unit have high ni pd 3 965 61 198 and low pd cr 410 000 3 930 000 ratios indicating sulfide segregation and pge depletion prior to eruption the primary magmas were s undersaturated and derived from partial melting at variable depths in the upper mantle the early and late stage magmas as represented by the lower and upper units underwent afc processes which induced mild s saturation and pge depletion in some of the basalts whereas the magmas represented by the middle unit experienced more extensive crustal contamination resulting in stronger s saturation and in most cases significant pge depletion

meta igneous origin of hercynian peraluminous granites in n w french massif central implications for crustal history reconstructions seventy samples of hercynian peraluminous granites gueret millevaches and saint sylvestre massifs and metamorphic units of the limousin area were analysed for rb sr and sm nd the source rocks of the peraluminous granites can be found in the metamorphic rocks of limousin among them meta igneous rocks were largely predominent over meta sedimentary rocks in the source of the three granites millevaches and gueret granites were generated by the partial melting of rocks comprising meta volcanics and meta sediments whereas the saint sylvestre granite was produced exclusively by the melting of late precambrian granites this leads to confusing t dm nd values the confusion being amplified by the segregation of monazite during the petrogenetic evolution of the peraluminous granites which leads to dramatic fractionation in sm nd ratios the data of the present study tend to demonstrate that peraluminous granites do not give a good representation of isotopic mean crustal estimates late precambrian time seems however to have been a period of extensive crustal generation in western europe

cenozoic basalts in se china chalcophile element geochemistry sulfide saturation history and source heterogeneity abstract cenozoic basalts in se china may be derived from a mixture of depleted morb mantle dmm and enriched mantle 2 em2 sources but whether these basalts share a common mantle source or magmatic history remains unknown to investigate these unresolved issues this study sampled basalts from niutoushan and mingxi fujian province xilong zhejiang province and penghu taiwan for geochemical analysis the basalt samples show oib like trace element patterns and have low pge contents with 0 02 0 7 ppb ir and pd 0 05 1 4 ppb ru 0 01 0 2 ppb rh and 0 06 1 1 ppb pt all samples have high cu pd ratios ranging from 69 000 to 3 500 000 and low cu zr ratios ranging from 0 1 to 0 8 suggesting sulfur saturated fractionation model calculations indicate that the basalts are depleted in pge due to the retention of 0 001 to 0 1 sulfide in the mantle and the removal of up to 0 0022 sulfide during magma ascent the crystallization of olivine and spinel and partial melting are insufficient to account for the observed pge variation in these basalts thus the distinct pge patterns in basalts with different ages may reflect the heterogeneity of the mantle source beneath se china the source heterogeneity may be due to compositional heterogeneity particularly variations in oxygen fugacity and pge mineral phases or due to variable fluid melt metasomatic agents in the sub continental lithospheric mantle this heterogeneity is possibly related to the westward subduction of the paleo pacific plate