the stable isotopic composition of modern soil carbonate and its relationship to climate the oxygen isotopic composition of modern soil carbonate is well correlated with the isotopic composition of local meteoric water the carbon isotopic cycle for co 2 in soils can be described in terms of the proportion of biomass using the c 4 photosynthetic pathway and the co 2 respiration rate of the soil at low soil respiration rates significant atmospheric co 2 mixing can occur in general the carbon isotopic composition of soil carbonate is related to the proportion of c 4 biomass present in soil but soils that freeze to the depth of carbonate formation often have a significant atmospheric component this suggests that freezing of the soil solution should be considered as another important mechanism for soil carbonate formation because of these relationships the isotopic composition of soil carbonate may be a paleoclimatic and paleoecologic indicator in cases in which diagenetic alteration has not occurred

origin of high potassium talc alkaline i type granitoids many i type granitoid magmas are generated through partial melting of older metaigneous rocks and the compositions of such melts are broadly calc alkaline and metaluminous these melts are granitic to tonalitic and result from thermal extremes in their lower crustal source regions data on the experimental partial melting of common crustal rocks suggest that high k i type granitoid magmas can be derived only from the partial melting of hydrous calc alkaline to high k calc alkaline mafic to intermediate metamorphic rocks in the crust because of their low k2o contents metabasaltic rocks of all kinds are unsuitable as sources and models that propose mixing of mantle derived basaltic magmas and crustal melts are also inadequate there is no requirement that i type calc alkaline magmatism be related in any way to subduction processes

emplacement setting of the granite sheeted pluton of esperanca brasiliano orogen northeastern brazil abstract anisotropy of magnetic susceptibility ams and isotopic u pb sm nd data were combined to study the emplacement setting of the granite sheets that constitute the esperanca pluton in the borborema province northeastern brazil the sheets dip moderately to the se along the contact zone between the paleoproterozoic basement rocks and early neoproterozoic orthogneisses and metasediments granite fabrics were determined mainly using ams in 136 sites distributed within the central and western part of the pluton the sheets normally have susceptibility lower than 0 35 msi but locally where a ti poor magnetite appears with titanite the susceptibility increases up to 5 msi comparison between the silicate fabric and ams showed inconsistencies between the shape of mineral and magnetic ellipsoids despite of their orientations that fit fairly well to each other ams indicated the deformation was partitioned between the lower tonalite syenogranite and upper leucogranite and coarse porphyritic granite sheets in the lower sheets the curvilinear lineation trajectory is attributed to a dominant heterogeneous pure shear event that flattened laterally the still molten tonalite and syenogranite into the regional foliation the associated microstructures are typically magmatic zircon u pb data of the syenogranite yielded a crystallization age of 592 5 ma in the upper sheets the fabric recorded a component of simple shear deformation that displaced the coarse porphyritic granite and the top gneissic host rocks to the southwest microstructures are mostly of post full crystallization type tdm model ages and nd t 0 values indicate that the magma contaminated by partial melting of the regional host rocks sheet propagation at the emplacement level would have exploited the contact zone between crustal blocks of different rheologies when the melt pressures would be able to tensionally fail the anisotropy of the host rocks

transformation of aragonite dominated lime muds to microcrystalline limestones scanning electron microscopy of textures in several well lithified pleistocene micrites mixed micrite and microspar from south florida and the bahamas shows significant diagenetic alteration of the lime mud precursors the dominance of aragonite in those precursor muds is supported by the surprising presence of abundant aragonite relics in the neomorphic calcite crystals confirmed by feigl stain x ray and electron diffraction polished very lightly etched surfaces of neomorphic calcite crystals even in the microspar show aragonite relics and pits both absent in coexisting void fill cements similarities between micrite and microspar crystals indicate their common origin through a one step neomorphic process of calcitization that is the mud did not first calcitize to micrite and then alter to microspar by aggrading neomorphism this conclusion has more general implications for the interpretation of the origins of other older microsparitic carbonates both early calcitization and concurrent early meteoric cementation were significant in producing the florida bahamas micrites studied which have low porosities similar to those found in most ancient fine grained limestones these low porosities despite the absence of significant overburden indicate that major porosity reduction without compaction can occur in calcitized aragonite dominated muds

a rendzina from the lower carboniferous of south wales a thin calcrete crust horizon from the lower carboniferous llanelly formation of south wales consists of two parts an upper laminated unit and a lower peloidal unit the former is interpreted as a subaerial stromatolite and the latter as an a horizon of a palaeosol comparisons are made with the a horizons of rendzinas and it is concluded that the calcrete crust represents a complete rendzina profile this fossil rendzina contains abundant evidence of a soil fauna in the form of fecal pellets and small burrows

questions about myrmekite in deformed rocks abstract several questions about the development of myrmekite in deformed felsic and metapelitic rocks remain unanswered although a general relationship between deformation and myrmekite appears to be well established the fact that myrmekite grows into relatively non deformed k feldspar while being simultaneously deformed and recrystallized at the rear of the growing lobes suggests that strain energy may not be a major contributor to the advance of the growth interface of the myrmekite colony furthermore because myrmekite typically nucleates on existing plagioclase rather than in k feldspar strain energy may not contribute directly to nucleation of myrmekite either at least in many instances however strain energy probably contributes indirectly by facilitating access of fluids to growth sites thereby altering the local chemical environment and so promoting development of myrmekite because myrmekite recrystallizes readily in deformed felsic rocks it is one of the main contributors to the development of folia

the hohonu batholith of north westland new zealand granitoid compositions controlled by source h 2 o contents and generated during tectonic transition geochemical studies on the hohonu batholith of the west coast south island new zealand have recognised two distinct but chemically related suites of mid cretaceous granitoids the suites are characterised by restricted radiogenic isotopic compositions sr i 0 7062 to 0 7085 nd i 4 4 to 6 1 and represent melting of a mafic lithosphere source followed by interaction with ordovician metasediments the two suites te kinga suite and deutgam suite are distinguished by contrasting contents of al2o3 na2o sr ba eu and hree attributable to different residual asssemblages controlled by differing h2o contents during melting of a metabasaltic source the relatively mafic metaluminous i type deutgam suite represents magmas derived by dehydration melting in equilibrium with an amphibolitic plagioclase amphibole residue in contrast the peraluminous high silica compositions of the te kinga suite were produced by melting at higher h2o contents reducing the stability of plagioclase and resulting in a melt in equilibrium with a plagioclase free eclogitic garnet amphibole residue residual plagioclase during generation of the deutgam suite resulted in lower al2o3 na2o sr ba and eu contents whereas residual garnet during generation of the te kinga suite resulted in depleted hree contents the mid cretaceous granitoids of the hohonu batholith were generated during a period of rapid tectonic transition from crustal thickening during collision to crustal thinning and core complex formation during extension

petrology of the anorogenic oxidised jamon and musa granites amazonian craton implications for the genesis of proterozoic a type granites the 1 88 ga jamon and musa granites are magnetite bearing anorogenic a type granites of paleoproterozoic age they intrude the archaean rocks of the rio maria granite greenstone terrain in the eastern part of the amazonian craton in northern brazil a suite of biotite amphibole monzogranite to syenogranite with associated dacite porphyry dp and granite porphyry gp dykes dominates in these subalkaline granites that vary from metaluminous to peraluminous and show high feo feo mgo and k2o na2o in spite of their broad geochemical similarities the jamon and musa granites show some significant differences in their ree patterns and in the behaviour of y the jamon granites are related by fractional crystallisation of plagioclase potassium feldspar quartz biotite magnetite amphibole apatite ilmenite geochemical modelling and nd isotopic data indicate that the archaean granodiorites trondhjemites and tonalites of the rio maria region are not the source of the jamon granite and associated dyke magmas archaean quartz diorites differentiated from the mantle at least 1000 m y before the emplacement of the granites have a composition adequate to generate dp and the hornblende biotite monzogranite magmas by different degrees of partial melting a larger extent of amphibole fractionation during the evolution of the musa pluton can explain some of the observed differences between it and the jamon pluton the studied granites crystallised at relatively high fo2 and are anorogenic magnetite series granites in this aspect as well as concerning geochemical characteristics they display many affinities with the proterozoic a type granites of south western united states the jamon and musa granites differ from the anorthosite mangerite charnockite rapakivi granite suites of north eastern canada and from the reduced rapakivi granites of the fennoscandian shield in several aspects probably because of different magmatic sources

87sr 86sr ratios in permo carboniferous sea water from the analyses of well preserved brachiopod shells abstract sr isotopic analyses of well preserved portions of permo carboniferous brachiopods distributed globally confirm the general shape of the sr isotope age curve established by previous workers for this time interval there is little variation between the sr isotopic composition of unaltered portions of brachiopods and that of portions of the same shell interpreted to be diagenetically altered based on cathodoluminescence elemental and stable isotopic data however the sr isotopic composition in diagenetically altered micritic matrix adjacent to the shell is more radiogenic the sr isotopic composition in the unaltered portions of calcitic megafossils has potential as a stratigraphie tool

post brasiliano pan african high k granitic magmatism in central brazil the role of late precambrian early palaeozoic extension abstract in western goias brazil the emplacement of large high k postorogenic granites and associated small gabbro dioritic intrusions followed immediately after the last deformational events of the brasiliano pan african orogeny at 600 ma well fitted whole rock rb sr isochrons indicate ages which suggest two discrete intrusive events the older between 588 and 560 ma and the younger between 508 and 485 ma the older granites display general petrographic and geochemical characteristics of highly differentiated calc alkaline i type granitoids whereas the younger intrusions are more alkaline similar to a type granites initial 87 sr 86 sr ratios vary from 0 703 to 0 710 and initial nd isotope ratios yield nd t values in the range between 4 0 and 3 0 there are no major differences in initial isotopic compositions between the two granite groups suggesting that the parental magmas for both groups of rocks mostly originated by refusion of crustal sources isotopically similar to the 940 640 ma basement arc type metatonalites metagranodiorites and associated arc metavolcanics the major and trace element compositional differences between the two granite groups is explained in terms of modifications in the melting conditions within the crust with younger melts being produced by the refusion of anhydrous depleted crustal sources left after the extraction of previous batches of more hydrated calc alkaline magmas the heat input required to promote extensive remelting of the continental crust was most likely provided by mantle derived mafic magmas that invaded and probably underplated the crust during uplift and extension the two intrusive events are bimodal in nature and are interpreted as shallow level extension related events associated with regional uplift and denudation occurring just after two orogenic pulses at the end of the proterozoic and early palaeozoic the older at 600 ma and the younger between 550 and 510 ma the onset of the granite magmatism at 590 ma shortly post dating the brasiliano orogeny 600 ma is broadly coeval with the initial stages of sedimentation of the terrigenous and carbonatic rocks of the ensialic paraguay belt in brazil and its correlative in bolivia the tucavaca belt which probably correspond to rift deposits related to the break up of laurentia from gondwana at the end of the proterozoic and beginning of the palaeozoic granites of the younger group cut the deformational structures of the paraguay belt metasediments and pre date by between 40 and 20 ma the initial stages of subsidence and sedimentation of the parana basin

proterozoic history of the borborema province ne brazil correlations with neighboring cratons and pan african belts and implications for the evolution of western gondwana 1 geological and geochronological correlations between borborema province ne brazil and neighboring cratons and brasiliano pan african belts indicate that the amazonian west african and sao francisco congo cratons and the basement of the araguaia borborema nigerian and cameroon provinces were part of the atlantica supercontinent this continent was established at the end of the transamazonian eburnean cycle 2 0 ga and apart from ubiquitous taphrogenesis in the 1 8 1 7 ga interval remained largely unaffected for the following 1 ga around 1 ga an important magmatic event in borborema province correlates with rifting episodes and anorogenic magmatism in the sao francisco congo and amazonian cratons these events are interpreted as failed attempts to break up atlantica which at this time may have been part of the larger rodinia supercontinent renewed extensional conditions in borborema province during the middle and late neoproterozoic are attributed to far field stresses transmitted to the interior of atlantica by outwardly dipping subduction zones that encircled its northern present day coordinates portion the rarity of petrotectonic assemblages typical of subduction zone environments indicates that extension did not evolve enough to form large oceans basins and thus that the borborema province essentially includes reworked intracontinental domains regional deformation and metamorphism starting at 650 640 ma and shear zone development beginning at 590 595 ma were continuously developed through time and were synchronous throughout most of the borborema araguaia cameroon and nigerian provinces postorogenic conditions were reached 540 530 myr ago while active deformation was still occurring in other belts that accreted around atlantica to form western gondwana

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

revisiting early middle jurassic igneous activity in the nanling mountains south china geochemistry and implications for regional geodynamics abstract early middle jurassic igneous rocks 190 170 ma are distributed in an e w trending band within the nanling tectonic belt and have a wide range of compositions but are only present in limited volumes this scenario contrasts with the uniform but voluminous middle late jurassic igneous rocks 165 150 ma in this area the early middle jurassic rocks include oceanic island basalt oib type alkali basalts tholeiitic basalts and gabbros bimodal volcanic rocks syenites a type granites and high k calc alkaline granodiorites geochemical and isotopic data indicate that alkaline and tholeiitic basalts and syenites were derived from melting of the asthenospheric mantle with asthenosphere derived magmas mixing with variable amounts of magmas derived from melting of metasomatized lithospheric mantle in comparison a type granites in the study area were probably generated by shallow dehydration related melting of hornblende bearing continental crustal rocks that were heated by contemporaneous intrusion of mantle derived basaltic magmas and high k calc alkaline granodiorites resulted from the interaction between melts from upwelling asthenospheric mantle and the lower crust the early middle jurassic magmatic event is spatially variable in terms of lithology geochemistry and isotopic systematics this indicates that the deep mantle sources of the magmas that formed these igneous rocks were significantly heterogeneous and magmatism had a gradual decrease in the involvement of the asthenospheric mantle from west to east these variations in composition and sourcing of magmas in addition to the spatial distribution and the thermal structure of the crust mantle boundary during this magmatic event indicates that these igneous rocks formed during a period of rifting after the indosinian orogeny rather than during subduction of the paleo pacific oceanic crust

the oxygen and carbon isotope composition of carboniferous fossil components sea water effects the aragonitic molluscs and lime mud of the pennsylvanian buckhorn asphalt deese group of southern oklahoma precipitated calcium carbonate in oxygen and carbon isotopic equilibrium with ambient sea water in addition 18o values indicate that the pelecypods precipitated their shells during the warmer months of the year the coiled nautiloids probably precipitated their shells in the warm surface water and throughout the year for the orthocone nautiloids the 18o values suggest that they precipitated their shells in deeper cooler water the low mg calcite brachiopods of the mississippian lake valley formation of new mexico precipitated shells in oxygen and carbon isotopic equilibrium with ambient sea water the 18o and 13c values of the buckhorn and lake valley faunas in conjunction with other published results suggest that carboniferous sea water was on a average depleted in 18o by 1 5 2 pdb relative to recent sea water however the 13c value of 2 6 2 pdb for average carboniferous sea water is similar to that of recent ocean water early diagenetic alteration of metastable carbonates probably occurs in a meteoric sea water mixing zone in this zone the oxygen and carbon isotopic compositions of these components are increased by about 2 4 pdb over their marine composition

a type post collisional granites in the borborema province ne brazil the queimadas pluton abstract the queimadas pluton constitutes an e w elongated tabular intrusion exposed over an area of 50 km 2 it is intruded in paleoproterozoic gneiss migmatite of the alto pajeu terrane in the central tectonic domain of the borborema province and has a zircon u pb age of 570 20 ma it is cut by later shear zones with nne direction which provoked necking and disruption in the body a suite of biotite amphibole monzogranites associated with quartz diorite and later leucogranite and diabase dominates in these alkaline granites that are metaluminous to slightly peraluminous and show high feot feot mgo and k 2 o na 2 o ratios their ree patterns are moderately fractionated with ce yb n ratios of 10 16 and significant negative eu anomalies eu 0 40 0 67 geochemical and nd isotopic data indicate that the queimadas pluton magma originated by partial melting of a crustal fertile granodioritic rocks the studied granites crystallized under relatively low f o 2 and are a type post collisional granitoids

petrography and geochemistry of the belle roche breccia lower visean belgium evidence for brecciation by evaporite dissolution the lower visean belle roche breccia east belgium displays a number of features that indicate brecciation by evaporite dissolution collapse the sharp lower contact of the breccia the gradual transition into the overlying strata the presence of semi continuous beds within the breccia giving it a crude stratification and the existence of several types of calcite dolomite and silica evaporite pseudomorphs furthermore the majority of the breccia fragments indicates hypersaline to lagoonal sedimentation conditions most of these fragments display an interlocking fabric the interpretation is also supported by the existence of continuous evaporite beds replaced by carbonates calcite and dolomite both under and overlying the breccia the brecciation history is characterized by gradual subsidence multiple brecciation episodes are recognized and are best seen in the lower breccia which underwent at least two major brecciation episodes here brecciated and veined breccia fragments occur within a microsparite neomorphosed mud matrix brecciation of these strata was due to the dissolution of interlayered evaporites the second brecciation event relates to infiltration of meteoric water and to the dissolution of the remaining evaporites this infiltration was probably triggered by the orogenic event at the end of the visean sudetic orogenic phase the whole breccia was finally cemented by a blocky calcite the different lithologies and cements were characterized by their trace element mg sr na fe mn k insoluble residue and organic matter content carbon oxygen isotope data of the cements and replaced evaporite layers helped to place the multiple collapse episodes within a general diagenetic model solution reprecipitation processes within the original aragonite dominated mud fragments as well as in the early diagenetic dolomite fragments have been recognized the geochemical data show that these transformation processes occurred in equilibrium with the same fluid these processes may have occurred within a freshwater lens very early in the diagenetic evolution or under shallow burial conditions cementation of blocky calcite occurred in a meteoric realm under burial conditions

magnesium isotope fractionation during low mg calcite precipitation in a limestone cave field study and experiments abstract the chemical and isotopic composition of speleothem calcite and particularly that of stalagmites and flowstones is increasingly exploited as an archive of past environmental change in continental settings despite intensive research including modelling and novel approaches speleothem data remain difficult to interpret a possible way foreword is to apply a multi proxy approach including non conventional isotope systems for the first time we here present a complete analytical dataset of magnesium isotopes 26 mg from a monitored cave in nw germany bunker cave the data set includes 26 mg values of loess derived soil above the cave 1 0 0 5 soil water 1 2 0 5 the carbonate hostrock 3 8 0 5 dripwater in the cave 1 8 0 2 speleothem low mg calcite stalactites stalagmites 4 3 0 6 cave loam 0 6 0 1 and runoff water 1 8 0 1 in the cave respectively magnesium isotope fractionation processes during weathering and interaction between soil cover hostrock and solute bearing soil water are non trivial and depend on a number of variables including solution residence times dissolution rates adsorption effects and potential neo formation of solids in the regolith and the carbonate aquifer apparent mg isotope fractionation between dripwater and speleothem low mg calcite is about 1000ln mg cc mg aq 2 4 a similar mg isotope fractionation 1000ln mg cc mg aq 2 1 is obtained by abiogenic precipitation experiments carried out at aqueous mg ca ratios and temperatures close to cave conditions accordingly 26 mg discrimination during low mg calcite formation in caves is highly related to inorganic fractionation effects which may comprise dehydration of mg 2 prior to incorporation into calcite surface entrapment of light isotopes and reaction kinetics relevance of kinetics is supported by a significant negative correlation of mg isotope fractionation with the precipitation rate for inorganic precipitation experiments

the jiashan syenite in northern hebei a record of lithospheric thinning in the yanshan intracontinental orogenic belt abstract the early cretaceous jiashan syenite is located in a region of late jurassic crustal thickening the jiashan syenite can be divided into three concentrically arranged units the jiangjiawan longtangou and longtannangou units which were intruded sequentially geochemically the jiashan syenite has a high ga al ratio 3 is enriched in silica alkalis fe ree th ga nb zr and hf is depleted in mg ba sr and ti and in transition elements such as cr co ni and v the three units of the jiashan syenite have ce pb ratios ranging from 6 12 to 13 41 and are enriched in light ree lree with a moderate eu negative anomaly the 87sr 86sr initial ratios range from 0 701409 to 0 707405 with a mean of 0 70379 the end t values of 2 27 to 5 58 indicate that the magma was probably derived from enriched mantle the jiashan syenite is a post orogenic intrusion and can be considered to be an a type granite it was emplaced in an environment of lithospheric extension during asthenospheric mantle upwelling this suggests that the regional compressional shortening and crustal thickening tectonic regime in the yanshan orogenic belt during the late jurassic ca 135 ma changed to lithospheric extension and thinning in the early cretaceous

brasiliano pan african granitic magmatism in the pajeu paraiba belt northeast brazil an isotopic and geochronological approach the neoproterozoic brasiliano pan african orogeny in the borborema province bp ne brazil is characterized by intense granitic magmatism spatially associated with continental scale shear zones and metamorphism under high temperature conditions u pb zircon and rb sr whole rock data for 14 granite intrusions from the paje u parabelt have ages that suggest more than 100 ma of intrusive magmatism which can be divided in four events the oldest granites between 620 and 600 ma are medium to slightly high k calc alkaline i type granitoids intruded into metagreywackes and gneiss granites of 0 9 1 0 ga they are related to the peak of metamorphism and to the development of a flat lying foliation the youngest intrusions 540 520 ma have geochemical signature of a type post orogenic extension related granites and are associated with sub volcanic bimodal magmatism probably contemporaneous with deposition of small sedimentary basins in the north iara jaibaras graben and sa ori and central fatima bet ania and carnatectonic domains of the borborema province they reflect post tectonic relaxation of the brasiliano orogeny between these episodes two other intrusive events were identified 1 high k calc alkaline granitoids and shoshonitic granitoids associated with mafic syenites meladiorites and hornblende biotite diorites intruded between 590 and 581 ma associated with a transcurrent deformation event and 2 alkaline post collisional granitoids having u pb zircon ages of ca 570 ma marking the final stage of the brasiliano orogeny in the south domain of the borborema province migmatization took place between 610 and 600 ma although similar ages were not found in the granitoids of the central tectonic domain field evidence suggest that migmatization followed the intrusions of the oldest 620 600 ma granitoids 2004 elsevier b v all rights reserved

oblique lineations in orthogneisses and supracrustal rocks vertical partitioning of strain in a hot crust eastern borborema province ne brazil detailed structural work conducted at the eastern area of the neoproterozoic brasiliano zpan african borborema province northeastern brazil has shown two orientations of stretching lineations with ese trend in supracrustal rocks and ne trend in underlying orthogneisses in the metasedimentary sequence numerous kinematic indicators showing a top to the northwest sense of shear denote a welldeveloped non coaxial deformation in the orthogneisses lineations formed dominantly during coaxial deformation although a component of ne directed shear is locally observed the two lineations were produced under similar high temperature metamorphic conditions and are interpreted as the result of a protracted nw se contractional strain regime where i subhorizontal non coaxial shear with superimposed flattening led to an initial phase of nw directed thrusting ii flattening strains mainly accumulated in the orthogneisses with progressive deformation leading to a lineation oblique to the transport direction iii the subhorizontal fabric in basement and cover rocks was refolded by overturned folds and then iv cross cut by conjugate ene striking dextral and nne striking sinistral shear zones contemporaneous with ne trending upright folds it is proposed that vertical partitioning of strain between basement and cover may explain the presence of oblique lineations in this orogenic belt that did not go through a final stage of extensional collapse q 2005 elsevier ltd all rights reserved

facies sequences on a carbonate ramp the carboniferous limestone of south wales during early carboniferous times a major sea level rise led to the development of an extensive carbonate ramp over what is now south wales differential subsidence and sea level changes resulted in distinctive facies sequences in the ramp succession and a model is offered which recognizes three distinct geomorpho tectonic settings inner mid and outer ramp the inner ramp zone occurs in the more landward part of the province and was an area undergoing little or no subsidence the sequence is dominated by oolitic grainstones and peritidal limestones representing shoal and back shoal environments the peritidal units are transgressive deposits consisting of stacked asymmetrical shallowing up cycles the sequence contains many subaerial breaks and tectonic uplift resulted in base level changes and fluvial incision the mid ramp zone sequence is intermediate in thickness between the inner and outer ramp successions and consists mainly of bioclastic limestones deposited below fairweather wave base sedimentation periodically exceeded sea level rise and subsidence and regressive progradational oolitic sand bodies developed the thickest of which are stacked units with up to four individual sand bodies storm processes were of major importance in this setting the outer ramp zone is represented by a thick sequence of muddy bioclastic limestones deposited below storm wave base and major waulsortian reef mounds also developed none of the shallowing phases seen in the other ramp zones can be detected in this sequence subsidence and eustatic sea level rise seem to have been the major controls on deposition but the recognition of eustatic sea level falls is difficult the detailed facies model for ramp carbonates presented here may be applicable elsewhere in the geological record

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

trans alkaline magmatism in the serrinha pedro velho complex borborema province ne brazil and its correlations with the magmatism in eastern nigeria abstract the sol nea complex 572 8 ma comprises various intrusions that total 200 km 2 it cuts palaeoproterozoic migmatized orthogneisses of the serrinha pedro velho complex which is part of the sao jose do campestre massif north tectonic domain of the borborema province ne brazil it is a post collisional intrusion constituted of coarse grained biotite amphibole monzogranites fine grained leucogranites and diorites major trace and rare earth elements data used to constrain the origin and nature of these rocks indicate that the monzogranites display chemical features of weakly peraluminous ferro potassic trans alkaline granitoids similar to coeval occurrences described in eastern nigerian sm nd data together with the whole rock geochemical data suggest that i the monzogranites were generated by partial melting of tonalitic gneiss from the lower crust leaving behind a granulitic residue ii the diorites originated by partial melting of an enriched lithospheric mantle iii the evolution of the sol nea complex involved magma mixing between the monzogranites and diorites k feldspar and amphibole fractionation appear to have occurred during the evolution of the monzogranites leucogranites constituting an elliptical shaped intrusion 80 km 2 the dona ines pluton are intruded to the north of the sol nea complex they also occur as small intrusions cross cutting the border of the sol nea complex geochemical and isotopic data suggest that the leucogranites were generated by melting of crustal palaeoproterozoic rocks rb sr data gave an age of 544 16 ma trans alkaline magmatism is associated with ductile brittle deformation in the serrinha pedro velho complex of the sao jose do campestre massif this event follows the peak of an extensional related high temperature metamorphism upper amphibolite to granulite facies with estimated age of 578 to 574 ma if a pre gondwana reconstruction is considered this event is seen to extend to the african side of gondwana in eastern nigeria