petrogenesis of the early cretaceous adakite like porphyries and associated basaltic andesites in the eastern jiangnan orogen southern china abstract early cretaceous quartz porphyries are associated with mantle derived mafic rocks basaltic andesites in the eastern segment of the neoproterozoic jiangnan orogen southern china zircons from two quartz porphyries yield identical u pb ages at 142 1 ma by the la icp ms method hornblende and augite are the dominant phenocrysts in the basaltic andesites and the groundmass plagioclase has a high anorthite content 80 both of which indicate high h 2 o 6 wt contents in the magma the basaltic andesites have low tio 2 0 68 0 78 wt and nb la 0 23 0 28 nb and ta depletions high initial 87 sr 86 sr ratios 0 710 and unradiogenic nd isotopes e nd t 3 4 to 3 7 implying that they were generated from the partial melting of an isotopically enriched lithospheric mantle source that had been metasomatized by subduction related fluids the quartz porphyries sio 2 65 6 70 5 wt show adakite like geochemical features with high sr y 18 1 53 4 and la yb 18 6 39 4 ratios but low y 9 44 15 2 ppm and yb 0 84 1 43 ppm contents they have low mgo 0 72 1 46 wt ni 1 04 14 7 ppm and cr 2 34 36 7 ppm contents and lower nd isotopic ratios e nd t 6 02 to 7 64 than the basaltic andesites suggesting that they may have been generated from the partial melting of the lower part of a thickened continental crust without interaction with the underlying lithospheric mantle the mafic magmas ascended from the metasomatized and hydrated lithospheric mantle and heated the lower part of thickened continental crust triggering partial melting to form the adakite like porphyries the moderate levels of sr 204 546 ppm and negative eu anomalies eu eu 0 69 0 86 in the porphyries require the presence of minor plagioclase as residual phase in the magma source indicating the continental crust may be not as thick 50 km as previously suggested the new recognition of adakite like porphyries and associated mafic rocks in the eastern jiangnan orogen reveals the significant role of neoproterozoic orogen related crust and oceanic sediments in the formation of mesozoic adakite like rocks of central eastern china

geochemistry and petrogenesis of intrusive and extrusive ophiolitic plagiogranites central anatolian crystalline complex turkey plagiogranites associated with the sarikaraman ophiolite of the central anatolian crystalline complex turkey closely resemble other plagiogranites from supra subduction zone type ophiolites of neotethys the ophiolite is remarkable in displaying a higher proportion of the plagiogranite suite ca 10 by volume than is usually associated with such bodies the sarikaraman plagiogranites are represented by intrusive sheets and netvein trondhjemites largely developed at the top of the upper gabbros and as multiphase dykes within the sheeted dyke complex the plagiogranite dykes are considered to feed extrusive silicified rhyolites associated with the basaltic lavas in the volcanic section of the ophiolite field relations suggest that the trondhjemites were probably generated from the roof section of a dynamic and evolving gabbroic magma chamber both the deep seated trondhjemites and the volcanic rhyolites constitute the sarikaraman plagiogranite suite geochemically there is complete overlap between the intrusive trondhjemites and extrusive rhyolites which are characterised by morb normalized low hfs element contents with small negative nb ta anomalies and variably enhanced lil element abundances unlike other plagiogranites however the sarikaraman suite is not characterized by consistently low k2o contents a feature that reflects the variable mobilization of the lil elements under lower greenschist facies conditions the ree are uniformly enriched relative to the basic components of the complex but have similar normalized patterns exhibiting mild light ree depletion in terms of their origin the initial or most primitive plagiogranite melts could have been generated by either fractional crystallization 70 85 of clinopyroxene feldspar amphibole or partial melting 5 15 batch melting of a gabbroic source material although only the first process can produce most of the range of the plagiogranite compositions as a group the plagiogranites exhibit some degree of internal variation which can be generated by further fractionation largely dominated by feldspar with minor apatite and amphibole

thermal gradient and geochronology of a paleozoic high grade terrane in the northeastern cathaysia block south china abstract the chencai terrane in the northeastern cathaysia block forms part of the northeast extension of the paleozoic orogeny in south china recent field investigations have revealed the occurrence of high grade rocks in this terrane including granulite facies metapelites khondalites and orthopyroxene bearing granulites however the thermal gradients and metamorphic history of this region have not been well constrained thus impeding a better understanding of the geodynamic history of the paleozoic orogeny in south china here we report results from a systematic study on the metamorphic pt conditions and geochronology of the orthopyroxene granulites and metasedimentary rocks metapelites from the chencai area the orthopyroxene granulites preserve granulite facies peak assemblage of opx grt pl kfs qtz and a retrograde p t trajectory peak pt conditions are 845 860 c and 0 83 0 86 gpa corresponding to a thermal gradient of 30 c km which is much hotter than the normal thermal gradient of continental crust mineral compositions of the metasedimentary rocks re equilibrated during retrograde metamorphism so that the peak pt conditions recorded by these rocks are much lower than those obtained from the orthopyroxene granulite peak pt conditions are 710 720 c 0 57 0 58 gpa for garnet biotite gneiss and 760 780 c 0 60 0 62 gpa for biotite garnetite zircon u pb dating shows that this episode of granulite facies metamorphism occurred at 430 450 ma and the detrital population shows minor or no contribution from the basement rocks of the cathaysia block the neoproterozoic rocks occurring along the jiangnan belt are considered to have provided the sedimentary source as indicated by the two main age peaks of 0 79 ga and 0 83 ga from the detrital zircon cores lu hf isotope data on the zircons indicates extensive alteration and little new zircon growth during the paleozoic metamorphism the ehf t values and model ages tdm1 of zircons suggest that the source rocks involved juvenile crustal components with hf model ages of 1 1 1 5 ga sedimentation of such a continental shelf sequence as well as the high thermal gradient of 30 c km confirm the previously proposed continental arc formation and maturation model in the chencai region

simulations of permian climate and comparisons with climate sensitive sediments abstract we use a climate model to simulate two intervals of permian climate the sakmarian ca 280 ma at the end of the major permo carboniferous glaciation and the wordian ca 265 ma we explore the climate sensitivity to various levels of atmospheric co2 concentration and to changes in geography and topography between the two periods the model simulates large seasonality and high aridity in the continental interiors of both hemispheres for both periods the northern summer monsoon weakens and the southern monsoon strengthens between the sakmarian and the wordian owing to changes in geography and topography the northern middle and high latitudes cool in winter between the sakmarian and wordian associated with northward shift of the continents this high latitude cooling strengthens the winter westerlies and shifts the maximum storm track precipitation south in the southern hemisphere the winter westerlies weaken from the sakmarian to the wordian starting the simulations with no permanent i

enrichment of trace elements in garnet amphibolites from a paleo subduction zone catalina schist southern california the abundance p t stability solubility and element partitioning behavior of minerals such as rutile garnet sphene apatite zircon zoisite and allanite are critical variables in models for mass transfer from the slab to the mantle wedge in deep regions of subduction zones the influence of these minerals on the composition of subduction related magmas has been inferred and disputed from inverse modelling of the geochemistry of island arc basalt or by experiment although direct samples of the dehydration partial melting region of a mature subduction zone have not been reported from subduction complexes garnet amphibolites from melanges of circumpacific and caribbean blueschist terranes reflect high t 600 c conditions in shallower regions such rocks record geochemical processes that affected deep seated high t portions of paleo subduction zones in the catalina schist a subduction zone metamorphic terrane of southern california metasomatized and migmatitic garnet amphibolites occur as blocks in a matrix of meta ultramafic rocks this mafic and ultramafic complex may represent either slab derived material accreted to the mantle wedge of a nascent subduction zone or a portion of a shear zone closely related to the slab mantle wedge contact or both the trace element geochemistry of the complex and the distribution of trace elements among the minerals of garnet amphibolites were studied by inaa xrf electron microprobe and sem in order of increasing alteration from a probable metabasalt protolith three common types of garnet amphibolite blocks in the catalina schist are 1 non migmatitic clinopyroxene bearing blocks which are compositionally similar to morb that has lost an albite component 2 garnet amphibolite blocks which have rinds that reflect local interaction between metabasite metaperidotite and fluid and 3 migmatites that are extremely enriched in th hfse lree and other trace elements these trace element enrichments are mineralogically controlled by rutile garnet sphene apatite zircon zoisite and allanite alkali and alkaline earth elements are much less enriched in the solid assemblage and thus appear to be decoupled from the other elements in the inferred metasomatic process es the compositions of migmatitic garnet amphibolite blocks seem to complement that of average island arc tholeiite trace element metasomatism reflects fluid solid rather than melt solid interaction the metasomatic effects indicate that h2o rich fluid perhaps with a significant component of na al silicate and alkalis carried th u sr ree and hfse fractionations of lree in migmatites resemble those of migmatitic metasedimentary rocks underlying the mafic and ultramafic complex exotic lree deposited in allanite in migmatites could have been derived from fluids in equilibrium with subducted sediment if the paleo subduction zone represented by the mafic and ultramafic complex of the catalina schist had continued its thermal and fluid evolution a selvage of similarly enriched rocks might have been generated along the slab mantle wedge contact between 30 and 85 km depth rocks affected by subduction zone metasomatism although rarely recognized at the surface could be volumetrically significant products of the initiation of subduction and may prove to be geochemical probes of convergent margins that approach the significance of xenoliths in the study of other magmatic environments

lithospheric structures and possible processes in phanerozoic eastern australia from deep seismic investigations abstract the eromanga basin in eastern australia developed during four major episodes on the rifted eastern margin of the australian precambrian craton seismic reflection profiling together with coincident wide angle reflection refraction profiling indicates structures and compositions in the crust which are dominant in various depth ranges some structural features seem to be limited to the upper crust other deep reflection events and high crustal velocities predominate in the lower crust with mafic intrusion from upper mantle sources being one preferred interpretation these features emphasize the difference in structures and composition between the upper and lower parts of the crust yet other structural and deep reflection features are interpreted as being truly lithospheric in scale because they seem to have seismic expression right from the surface to the upper mantle similar interpretations are made for other continental regions indicating that lithospheric processes affect different depth ranges of the crust and upper mantle in a variety of ways any modelling of such processes in the lithosphere must lead to these quite different end results at appropriate depth ranges

sedimentology taphonomy and palaeoecology of a laminated plattenkalk from the kimmeridgian of the northern franconian alb southern germany abstract at wattendorf in the northern franconian alb southern germany centimetre to decimetre thick packages of finely laminated limestones plattenkalk occur intercalated between well bedded graded grainstones and rudstones that blanket a relief produced by now dolomitized microbialite sponge reefs these beds reach their greatest thickness in depressions between topographic highs and thin towards and finally disappear on the crests the early late kimmeridgian graded packstone bindstone alternations represent the earliest plattenkalk occurrence in southern germany the undisturbed lamination of the sediment strongly points to oxygen free conditions on the seafloor and within the sediment inimical to higher forms of life the plattenkalk contains a diverse biota of benthic and nektonic organisms excavation of a 13 cm thick plattenkalk unit across an area of 80 m2 produced 3500 fossils which with the exception of the bivalve aulacomyella exhibit a random stratigraphic distribution two thirds of the individuals had a benthic mode of life attached to hard substrate this seems to contradict the evidence of oxygen free conditions on the sea floor such as undisturbed lamination presence of articulated skeletons and preservation of soft parts however palaeoecological and taphonomic analyses indicate that the benthic faunal elements are allochthonous having settled out of suspension and thus must have been derived from hard substrate areas of neighbouring topographic highs solely the bivalve aulacomyella which occurs concentrated on a single bedding plane may have colonised the plattenkalk depression during brief periods of oxygenation alternatively a pseudoplanktonic mode of life attached to floating sea weed is envisaged for the bivalve the formation of the plattenkalk is related to local and global factors a drop in relative sea level caused the death of the microbialite sponge reefs restricted circulation in depressions between the crests of the former reefs led to anoxic conditions and the formation of finely laminated sediments stabilised by microbial mats the small depressions received sediment and skeletal elements of benthic organisms that were swept off neighbouring topographic highs by storms and that entered the depressions partly as turbidity currents partly as suspension clouds a rise in the relative sea level finally terminated the restricted circulation in the depressions and closed the taphonomic window that led to the preservation of the plattenkalk biota

adakitic signature formed by fractional crystallization an interpretation for the neo proterozoic meta plagiogranites of the ne jiangxi ophiolitic melange belt south china article i nfo the low temperature high pressure metamorphic rocks which are exposed as exotic blocks within the serpentinite matrix of the ne jiangxi ophiolitic melange belt consist of enclave bearing and free meta plagiogranites both rock types are characterized by a metamorphic mineral assemblage consisting of ab rich plagioclase quartz clinopyroxene amphibole while rutile titanite and zircon occur as accessories the former primary magmatic mineral parageneses for the meta mafic enclaves and the meta plagiogranites are similar and consist of an rich plagioclase hornblende ilmenite monazite apatite however the ab rich plagioclase abundance of the meta mafic enclave is much lower and the hornblende and accessory mineral abundances are higher compared with those of the meta plagiogranite host rocks meta plagiogranites with mafic enclaves have a chemical composition similar to that of typical adakites with elevated al2o3 15 86 wt mg 53 5 and sr contents 485 ppm and a positive eu anomaly the rare earth element patterns are highly fractionated with la yb nn12 and low y b4 ppm and yb b0 4 ppm the mafic enclave has a basaltic composition with a negative eu anomaly and has ca 10 times higher hree and y concentrations than the plagiogranite host rocks enclave free meta plagiogranites also have high al2o3 16 08 wt na2o 8 88 wt and mg 52 3 values without obvious eu enrichment or depletion the yand yb concentrations 13 and 1 33 ppm respectively are higher than those of the enclave bearing meta plagiogranites and slightly lower than those of the mafic enclave zircon of the enclave bearing meta plagiogranites gives a weighted mean 206 pb 238 u crystallization age of 970 21 ma these meta plagiogranites have low 87

calcrete in quaternary coastal dunes in southwestern australia a capillary rise phenomenon associated with plants abstract non pedogenic calcrete is forming as a thin sheet 10 50 cm thick just above the water table in the subsurface of a barrier dune terrain in south western australia the calcrete is forming in response to the regional evapo transpirative regime the annual rainfall is high but of short duration and it infiltrates rapidly into the sandy substrates relatively high air temperatures and evaporation coupled with wind stress combine to produce a water vapor deficit across the dune terrain the vegetation responds by drawing upon phreatic water and pellicular water of the vadose zone thereby precipitating caco3 where plants are not drawing phreatic water calcrete is developing by evaporation of water at the top of the zone of capillary rise various structure types of calcrete are developed in the study area namely rhizoconcretionary mottled massive and laminar forms these occur in a definite stratigraphic sequence related to maturity of profile vegetation cover and thickness of the vadose zone rhizoconcretionary calcrete forms in the vadose zone mottled and massive calcrete frequently capped by laminar calcrete develop in the zone of capillary rise these latter calcrete types form a sheet that is ubiquitous and parallel to the water table since it is a feature of the water table the calcrete sheet may potentially transect regional stratigraphic boundaries and unconformities at the local level the various structural types of calcrete may he used as paleowater table indicators

from arc accretion to continental collision in the eastern jiangnan orogen evidence from two phases of s type granites abstract the ne jiangxi ophiolitic melange separates the huaiyu and jiuling terranes in the eastern jiangnan orogen of the south china block two phases of s type granites which are geochemically and mineralogically similar to the himalayan leucogranites are newly recognized in mantle peridotites of the ne jiangxi ophiolite the first phase a tourmaline bearing granite porphyry has a sims zircon u pb age of 904 4 4 8 ma and is strongly enriched in k2o 4 00 4 26 wt and strongly peraluminous a cnk 2 94 3 24 it further shows high rb sr ratios and highly fractionated ree patterns indicating an origin due to fluid absent melting of metapelites at relatively high temperatures the granite porphyry exhibits highly evolved zircon hf 6 0 to 1 0 and whole rock nd 4 2 to 5 1 isotopes which is in contrast to juvenile derived magmatic rocks in the huaiyu terrane we thus propose that the granite porphyry was formed by melting of the metapelites sourced from the yangtze block subsequent to the accretion of the huaiyu arc to the jiuling terrane the continental margin of the yangtze block the second phase a muscovite bearing quartz porphyry is strongly peraluminous a cnk 1 29 1 34 with low cao na2o ratios 0 11 0 14 and low feot mgo tio2 contents 2 39 2 75 wt similar to melts derived from fluid fluxed melting of metapelites zircon grains from the quartz porphyry yielded u pb ages of ca 803 801 ma with mainly positive ehf t values 1 9 to 8 7 which are younger than the depositional ages of the basement sequences and nearly identical to the accumulation of the overlying unmetamorphosed cover in the jiangnan orogen therefore it is suggested that the quartz porphyry was formed in a post collisional extensional setting subsequent to the final assembly of the yangtze and cathaysia blocks in conjunction with published data we conclude that multiple subduction accretion events may have been involved in the formation of the south china block during neoproterozoic times

contrasting zircon hf and o isotopes in the two episodes of neoproterozoic granitoids in south china implications for growth and reworking of continental crust abstract the genetic links among rift magmatism crustal growth and water rock interaction are an important issue about mass and heat transfer between mantle and crust during supercontinent breakup a combined study of hf and o isotopes in zircons from neoproterozoic granitoids in south china provides evidence for growth and reworking of juvenile and ancient crusts with different styles of water rock interactions along rift tectonic zones two generations of the granitoids show contrasting features in both zircon hf and o isotope compositions indicating their distinct petrogenesis the 825 ma granitoids exhibit negative ehf t values of 3 4 0 8 to 1 6 0 8 with old model hf ages of 1 81 0 07 to 1 92 0 10 ga and high 18o values of 8 7 to 10 4 these indicate that the source material of granitoid magmas was derived from melting of paleoproterozoic basement that has the hf isotope signature similar to the enriched mantle but experienced chemical weathering process before anatexis reworking of ancient crust is demonstrated to occur at 825 ma in the orogenic collapse zone with overprinting of subsolidus hydrothermal alteration during magma emplacement in contrast the 760 750 ma bimodal intrusives are characterized by positive ehf t values of 3 5 0 8 to 9 9 0 8 with young model hf ages of 0 94 0 06 to 1 18 0 06 ga and both low and high 18o values of 4 2 to 6 2 relative to 5 3 0 3 for the normal mantle zircon prompt reworking of juvenile crust is suggested to occur at 750 ma in the rifted tectonic zone with occurrence of supersolidus hydrothermal alteration and local low 18o magmatism during supercontinent breakup contributions of the depleted mantle to their magma sources are contrasting in the two episodes of magmatism in association with breakup of the supercontinent rodinia while the change in melt source from the crust to the mantle keeps pace with the advance from continental rifting to supercontinent breakup significant transport of both heat and material from the depleted mantle to the continental crust only occurred along the active rifting zone in either case the growth and reworking of continental crust are episodically associated with rift magmatism

the permian moradi formation of northern niger paleosol morphology petrography and mineralogy abstract three basic paleosol morphologies named type a type b and type c are described from the middle upper permian strata of the moradi formation tim mersoi basin northern niger the moradi formation is a typical alluvial redbed succession dominated by red mudrocks with fine to coarse grained pebbly channel sandstones and matrix breccias type a paleosols are hosted by well sorted fine to medium grained trough cross bedded and massive sandstones and preserve abundant vertical to horizontal micritic and microspar calcite tubules interpreted as rhizoliths lateral variability of rhizoliths in type a paleosols and their close association with fluvial channel fill sediments suggests they are the roots of grove stands of phreatophytic vegetation that grew within unstable anabranching stream systems type b paleosols are hosted by mudrocks and preserve well developed ped structure abundant micritic calcite nodules and vertically stacked micritic calcite nodular bodies as well as rare calcite with satin spar texture interpreted as a pseudomorphic replacement of pedogenic gypsum the morphology of type b paleosols suggests they were formed in well drained floodplain deposits on stable landforms type c paleosols are similar to type b but preserve pedogenic structures indicative of soil volume expansion and contraction as well as more abundant stage ii pedogenic carbonate nodules the morphology of type c paleosols suggests that they developed periodically rather than seasonally in poorly drained deposits that nevertheless occupied a relatively stable part of the landscape such as the plains flanking ephemeral lakes or sabkhas x ray diffraction analysis of the the range of morphologies petrographic textures and mineralogy of the paleosol profiles indicates semi arid to hyper arid climatic setting this paleoclimatic reconstruction is in agreement with middle and late permian conceptual paleoclimate models and quantitative general circulation models nevertheless and in spite of an arid climate moradi paleosols and their host strata also indicate a relatively shallow groundwater table importantly this shallow groundwater resource undoubtedly helped to support the moderately diverse fossil vertebrate assemblage and large stature macrophytes preserved in the moradi formation

the role of vegetation in the formation of anabranching channels in an ephemeral river northern plains arid central australia as the distribution and abundance of vegetation in drylands is often controlled by the greater availability of water along river channels riparian vegetation has the potential to influence significantly dryland river form process and behaviour this paper demonstrates how a small indigenous shrub the inland teatree melaleuca glomerata influences the formation and maintenance of anabranching channels in a reach of the ephemeral marshall river northern plains arid central australia here the marshall is characterized by ridge form anabranching where water and sediment are routed through subparallel multiple channels of variable size which occur within a typically straight channel train channels are separated by channel train ridges narrow flow aligned vegetated features or by wider islands by providing a substantial element of boundary roughness dense stands of teatrees growing on channel beds or atop the ridges and islands influence flow velocities flow depths and sediment transport resulting in flow diversion bank and floodplain erosion and especially sediment deposition ridges and islands represent a continuum of forms and their formation and development can be divided into a three stage sequence involving teatree growth and alluvial sedimentation 1 teatrees colonize a flat sandy channel bed initiating the formation of ridges by lee side accretion individual ridges grow laterally vertically and longitudinally and maintain a geometrically similar streamlined lemniscate form that presents minimum drag 2 individual ridges grow in size and interact with neighbouring ridges causing the lemniscate forms to become distorted ridges in the lee of other ridges tend to be protected from the erosive effects of floods and survive whereas individual teatrees or small ridges exposed to flow concentrated between larger ridges tend to be removed 3 ridges lengthen and coalesce with downstream ridges eventually subdividing the channel train into well defined anabranches this sequence turns a channel initially obstructed with dense and chaotic stands of teatrees into a well organized system of ridge form anabranches in the moderate to low gradient marshall river which is colonized by an abundance of within channel vegetation and subject to declining downstream discharges this helps to minimize flow resistance thereby maintaining an efficient water and sediment flux copyright 2000 john wiley sons ltd

cretaceous pacific plate movement beneath se china evidence from episodic volcanism and related intrusions abstract extensive late mesozoic igneous rocks in se china have been widely considered to be generated under the paleo pacific tectonic regime the plate subduction model however remains controversial this study focuses on the cretaceous volcanic rocks in northwestern zhejiang province zircon u pb age determination indicates that cretaceous volcanism in northwestern zhejiang took place at three episodes of 140 130 130 127 and 123 118 ma in good agreement with the coeval lower volcanic series in southeastern zhejiang but lacking the episode at 110 88 ma corresponding to the upper volcanic series the cretaceous volcanic rocks in zhejiang therefore show an oceanward younging trend in situ zircon hf isotope analyses of three episodes of volcanics yield e hf t values of 11 2 to 8 7 4 8 to 2 4 and 4 4 to 2 2 respectively the entire sequences display typical isotopic features of magma mixing implying progressive involvement of juvenile component based on systematical researches on the cretaceous volcanic rocks and a series of granitoid plutons in zhejiang it is also identified that the juvenile component involvement gradually occurred from the inland to the coast under an enhanced lithospheric extensional tectonic setting all the observations in this study indicate the northwestward paleo pacific plate subduction with episodic slab rollback which triggered the arc system to retreat towards the pacific ocean rather than the southwestward subduction related to the ridge subduction along the lower yangtze river belt

permian phytogeographic patterns and climate data model comparisons abstract the most recent global icehouse hothouse climate transition in earth history began during the permian warmer polar conditions relative to today then persisted through the mesozoic and into the cenozoic we focus here on two permian stages the sakmarian 285 280 ma and the wordian 267 264 ma also known as the kazanian integrating floral with lithological data to determine their climates globally these stages postdate the permo carboniferous glaciation but retain a moderately steep equator to pole gradient judging by the level of floral and faunal differentiation floral data provide a particularly useful means of interpreting terrestrial paleoclimates often revealing information about climate gradations between dry and wet end member lithological indicators such as evaporites and coals we applied multivariate statistical analyses to the permian floral data to calibrate the nature of floral and geographical transitions as an aid to climate interpretation we then classified sakma

a brief review of ophiolites in china abstract ophiolites are widely distributed in western southwestern and northern china where they fall into four principal age groups neoproterozoic early paleozoic late paleozoic and mesozoic cenozoic neoproterozoic ophiolites are known only in the north qinling orogenic belt in ne jiangxi province and in western sichuan province phanerozoic ophiolites are grouped into the paleo asian tethyan and circum pacific series the paleo asian ophiolites crop out in the western and northern parts of china between the siberian and north china blocks and range in age from early to late paleozoic all of these ophiolites are of the franciscan formerly cordilleran type and many are superimposed on one another suggesting repeated accretion of arc assemblages in an environment similar to the present day western pacific ocean mediterranean type formerly tethyan type ophiolites are confined to sw china particularly tibet and yunnan province paleo tethyan ophiolites are characterized by morb type lavas and are typically bounded by old continental lithosphere suggesting that they formed in small intercontinental ocean basins neo tethyan ophiolites contain a range of lava types including morb iat and boninite indicating formation in a variety of suprasubduction zone ssz environments circum pacific ophiolites occur sporadically in taiwan and ne china where they form tectonic melanges composed mainly of metaperidotite gabbro and basalt

geochemistry and ar ar muscovite ages of the daraban leucogranite mawat ophiolite northeastern iraq implications for arabia eurasia continental collision abstract daraban leucogranite dykes intruded discordantly into the basal serpentinized harzburgite of the mawat ophiolite kurdistan region ne iraq these coarse grained muscovite tourmaline leucogranites are the first leucogranite dykes identified within the mawat ophiolite they are mainly composed of quartz k feldspar plagioclase tourmaline muscovite and secondary phologopite while zircon xenotime corundum mangano ilemnite and cassiterite occur as accessories the a cnk value of the granite dyke samples varies from 1 10 to 1 22 indicating a strongly peraluminous composition cao na2o ranges from 0 11 to 0 15 and al2o3 tio2 from 264 to 463 similar to the strongly peraluminous sp granites exposed in high pressure collision zones such as the himalayas ar ar muscovite step heating dating yields 37 57 0 25 and 38 02 0 53 ma plateau ages for two samples which are thought to reflect either their magmatic emplacement or resetting during collision related metamorphism mineral chemistry shows evidence of both primary and secondary types of muscovite with cores favouring the magmatic interpretation and slight effects of a late syn serpentinization fluid seen at the rims geochemical features of daraban leucogranite dykes favour a syn collisional tectonic setting they probably formed in response to the continental collision between eurasia and arabia during the initial stage of the opening of the gulf of aden at 37 ma the muscovite ages and geochemical features of daraban leucogranite are strong evidence for the timing of the continental collision between northeastern arabia and eurasia in kurdistan region of iraq

the boundary between the simao and yangtze blocks and their locations in gondwana and rodinia constraints from detrital and inherited zircons abstract the simao block which is the northern extension of the indochina block and the yangtze block were amalgamated by the closure of a paleo tethys branch ocean commonly referred to as the ailaoshan ocean the exact location of the suture between these two continental blocks has been contested some researchers believed that it is located within the cenozoic ailaoshan shear zone whereas other researchers thought that it is present 50 km farther to the west the detrital and inherited xenocrystic zircons from paleozoic sedimentary rocks and a triassic granite dyke respectively in the laowangzhai mojiang suspect terrane which occurs between the two suggested locations for the same suture show that neither of these previously suggested locations is accurate the age distribution pattern of detrital and inherited xenocrystic zircons from the laowangzhai mojiang suspect terrane is similar to that of detrital zircons from the simao indochina block but different from that of those from the jianshui area in the western margin of the yangtze block this indicates that the laowangzhai mojiang suspect terrane belongs to the simao indochina block the actual location of the suture between the simao indochina and yangtze blocks is suggested to superimpose the ailaoshan late devonian to early carboniferous ophiolite belt within the suspect terrane the new interpretation is also supported by hf isotopes of detrital zircons from the related continental blocks the age distribution pattern of precambrian detrital zircons from the western part of the yangtze block is different from that of those from australia or tethyan himalaya the northern part of the indian continent indicating that the yangtze block was not an integral part of australia or india in rodinia the age distribution pattern of pre carboniferous detrital zircons from the simao indochina block is remarkably similar to that of those from the tethyan himalaya indicating that the simao indochina block was derived from indian gondwana

morphology and distribution of fossil soils in the permo pennsylvanian wichita and bowie groups north central texas usa implications for western equatorial pangean palaeoclimate during icehouse greenhouse transition analysis of stacked permo pennsylvanian palaeosols from north central texas documents the influence of palaeolandscape position on pedogenesis in aggradational depositional settings palaeosols of the eastern shelf of the midland basin exhibit stratigraphic trends in the distribution of soil horizons structure rooting density clay mineralogy and colour that record long term changes in soil forming conditions driven by both local processes and regional climate palaeosols similar to modern histosols ultisols vertisols inceptisols and entisols all bearing morphological mineralogical and chemical characteristics consistent with a tropical humid climate represent the late pennsylvanian suite of palaeosol orders palaeosols similar to modern alfisols vertisols inceptisols aridisols and entisols preserve characteristics indicative of a drier and seasonal tropical climate throughout the lower permian strata the changes in palaeosol morphology are interpreted as being a result of an overall climatic trend from relatively humid and tropical moist conditions characterized by high rainfall in the late pennsylvanian to progressively drier semi arid to arid tropical climate characterized by seasonal rainfall in early permian time based on known late palaeozoic palaeogeography and current hypotheses for atmospheric circulation over western equatorial pangea the pennsylvanian palaeosols in this study may be recording a climate that is the result of an orographic control over regionalscale atmospheric circulation the trend towards a drier climate interpreted from the permian palaeosols may be recording the breakdown of this preexisting orographic effect and the onset of a monsoonal atmospheric circulation system over this region

a seismic refraction study of the quilpie trough and adjacent basement highs eromanga basin eastern australia abstract the jurassic cretaceous eromanga basin in eastern australia is a relatively undisturbed sedimentary sequence covering the devonian quilpie trough and the early palaeozoic canaway ridge and cheepie shelf seismic refraction recording across these three features with additional reflection gravity and well information has been used to determine the velocity structure to a depth of 10 km the thickness of the eromanga basin sequence along the traverse varies between 1 1 and 1 4 km with p wave velocities ranging from 2 0 to 3 8 km s the maximum thickness of the underlying quilpie trough is about 3 8 km but the base of the trough is hard to define the velocities in this devonian sequence range from 4 0 to 5 0 5 5 km s two small bodies with a velocity of 4 0 4 5 km s occur immediately below the eromanga basin sediments on the canaway ridge and the cheepie shelf they may be remnants of devonian sediments on an erosion surface the velocities within the ordovician basement are less below the canaway ridge than under the cheepie shelf and quilpie trough they reach 6 0 km s at 7 km depth below the ridge at 4 km depth under the shelf and at an intermediate depth under the trough from gravity modelling the density of the basement to the quilpie trough increases at a depth of about 7 km this may be related to volcanics found in the basement further north the canaway ridge has a lower density than the cheepie shelf or the basement of the quilpie trough relative vertical movement between these three features may in part be related to these density differences

fluid inclusion and stable isotope constraints on the genesis of the world class zhuxi w cu skarn deposit in south china abstract scheelite mineralization accompanied by traces of chalcopyrite occurs in skarn systems in the giant zhuxi deposit in the taqian area northeastern jiangxi province south china three paragenetic stages of skarn with ore deposition have been recognized the prograde skarn retrograde skarn and hydrothermal quartz sulfide stages microthermometer data showed that the prograde skarn were formed at relatively high temperatures 450 500 c and pressures 0 97 1 38 kbars and low oxidation to weak reduction conditions the raman microprobe data demonstrated that the fluid of the stage was h2o nacl kcl cacl2 ch4 c2h4 system the retrograde skarn alteration formed at 1 24 kbars and relatively medium temperatures 280 320 c and the ore forming fluid was h2o nacl kcl cacl2 system during the hydrothermal quartz sulfide stage the fluids were methane bearing aqueous low salinity 0 9 8 0 wt nacleqv and low homogeneous temperature 160 240 c correspondingly the minimum fluid pressures were evaluated between 75 and 252 bars the h and o isotopic values from the prograde and retrograde stages implied that the mineralizing fluids were derived principally from magmatic water the mineralizing fluids in the quartz sulfide hydrothermal stages were mingled with magmatic water with some meteoric water as well as minor contributions of wall rocks the sulfur and lead isotopes indicated that ore forming materials including sulfur lead copper zinc and iron in the zhuxi deposit were derived from local magmatic and sedimentary sources geological fluid inclusion and isotopic data supported that the zhuxi deposit is a typical skarn type w cu deposit related to ilmenite series granitic rocks and formed in a compressional tectonic environment

shrimp zircon u pb geochronological and whole rock geochemical evidence for an early neoproterozoic sibaoan magmatic arc along the southeastern margin of the yangtze block abstract it has been generally accepted that the south china block was formed through amalgamation of the yangtze and cathaysia blocks during the proterozoic sibaoan orogenesis but the timing and kinematics of the sibao orogeny are still not well constrained we report here shrimp u pb zircon geochronological and geochemical data for the taohong and xiqiu tonalite granodiorite stocks from northeastern zhejiang southeastern margin of the yangtze block our data demonstrate that these rocks dated at 913 15 ma and 905 14 ma are typical amphibole rich calc alkaline granitoids formed in an active continental margin combined with previously reported isotopic dates for the 1 0 ga ophiolites and 0 97 ga adakitic rocks from northeastern jiangxi the timing of the sibao orogenesis is thus believed to be between 1 0 and 0 9 ga in its eastern segment it is noted that the sibao orogeny in south china is in general contemporaneous with some other early neoproterozoic 1 0 0 9 ga orogenic belts such as the eastern ghats belt of india and the rayner province in east antarctica indicating that the assembly of rodinia was not finally completed until 0 9 ga

p wave velocity features of the lithosphere under the eromanga basin eastern australia including a prominent mid crustal conrad discontinuity abstract the crustal structure of the central eromanga basin in the northern part of the australian tasman geosyncline revealed by coincident seismic reflection and refraction shooting contrasts with some neighbouring regions of the continent the depth to the crust mantle boundary moho of 36 41 km is much less than that under the north australian craton to the northwest 50 55 km and the lachlan fold belt to the southeast 43 51 km but is similar to that under the drummond and bowen basins to the east the seismic velocity boundaries within the crust are sharp compared with the transitional nature of the boundaries under the north australian and lachlan provinces in particular there is a sharp velocity increase at mid crustal depths 21 24 km which has not been observed with such clarity elsewhere in australia the conrad discontinuity in the lower crust the many discontinuous sub horizontal reflections are in marked contrast to lack of reflecting horizons in the upper crust further emphasising the differences between the upper and lower crust the crust mantle boundary moho is characterised by an increase in velocity from 7 1 7 7 km s to a value of 8 15 0 04 km s the depth to the moho under the canaway ridge a prominent basement high is shallower by about 5 km than the regional moho depth there is also no mid crustal horizon under the canaway ridge but there is a very sharp velocity increase at the moho depth of 34 km the ridge could be interpreted as a horst structure extending to at least moho depths but it could also have a different intra crustal structure from the surrounding area the sub crustal lithosphere has features which have been interpreted from limited data as being caused by a velocity gradient at 56 57 km depth with a low velocity zone above it because of the contrasting crustal thicknesses and velocity gradients the lithosphere of the central eromanga basin cannot be considered as an extension of the exposed lachlan fold belt or the north australian craton the lack of seismic reflections from the upper crust indicates no coherent accoustic impedance pattern at wavelengths greater than 100 m consistent with an upper crustal basement of tightly folded meta sedimentary and meta volcanic rocks the crustal structure is consistent with a pericratonic or arc back arc basin being cratonised in an episode of convergent tectonics in the early palaeozoic the seismic reflections from the lower crust indicate that it could have developed in a different tectonic environment

magmatism along the southeast margin of the yangtze block precambrian collision of the yangtze and cathysia blocks of china the time and the environment of formation of igneous rocks along the southern margin of the yangtze continental block in southern anhui province and northeastern jiangxi province china are discussed sm nd dating for samples from two ophiolite suites give ages of about 1 ga late proterozoic granodiorites in the study area have ages of about 0 9 ga and have normal intrusive relations against banxi group metamorphic country rocks trace element abundances and initial nd isotope ratios for ophiolite and proterozoic granodiorites are consistent with formation in an island arc environment in contrast sr and nd isotopes for mesozoic granitoids of the region suggest that they are products of remelting of crustal sources similar to the basement of the yangtze block these relations do not support a recently proposed mesozoic overthrust tectonic model for southeast china but rather favor a late precambrian collision history for the yangtze and cathysia blocks

tectonic setting for the genesis of oceanic plagiogranites evidence from a paleo spreading structure in the oman ophiolite abstract in the oman ophiolite detailed structural and lithological mapping of the 60 45 km maqsad area has revealed several plagiogranite intrusions ranging from metre scale dykes to hectometre scale bodies these plagiogranites are spatially related to kilometre scale mafic plutons displaying generally pegmatitic textures and locally deformed in amphibolite facies conditions along shear zones up to a few hundred metres thick isotope data demonstrate the mantle origin of these pegmatites and plagiogranites nd ranges from 6 3 to 9 9 and has an average value 26 samples of 8 1 within the field of present day morbs and similar to the average nd 7 8 of the oman ophiolite primary igneous sequence these intrusions are not restricted to a given structural level their paleovertical extent exceeds 5 6 km from 2 km below the moho up to the base of the sheeted dyke complex their modal composition evolves up section from predominantly pyroxenites and gabbro norites in the mantle harzburgites to gabbro norites diorites and quartz diorites at crustal level petrography whole rock and mineral chemistry show that this vertical succession of lithologies can be simply explained by low pressure fractional crystallization of a hydrated basaltic melt plagiogranites although more abundant at upper crustal level are not restricted to this horizon plagioclase composition in plagiogranites presents a remarkable evolution with paleodepth from highly calcic 70 95 an in the mantle section intermediate and highly scattered in the lower crust 10 85 an to quite sodic at upper crustal level 5 20 an highly calcic plagioclases in quartz bearing rocks is a puzzling character inconsistent with fractional crystallization even if high degrees of fractional crystallization of a hydrated basaltic melt remain the simplest way to account for the chemistry of the upper crustal plagiogranites processes such as fluid induced remelting or assimilation of country rocks must be invoked to account for the chemistry and field characteristics of the deeper plagiogranites mantle and crustal structures in the maqsad area indicate that the pegmatites and plagiogranites emplaced in an ocean ridge setting when a mantle diapir soaked with basaltic melt intruded a cool hydrothermally altered lithosphere they were eventually transposed in an off axis setting as the ascension of the diapir progressed up to moho level it is concluded that the formation of large volumes of buoyant leucocratic rocks is possible at ocean ridges provided periods of amagmatic spreading are long enough to induce the growth and alteration of an axial lithospheric lid this is likely more frequent at slow spreading centres where mantle upwelling is discontinuous in space and time

paleoenvironmental reconstruction from chemical and isotopic compositions of permo pennsylvanian pedogenic minerals mineralogical and chemical analysis of late pennsylvanian and early permian paleosols from the eastern shelf of the midland basin north central texas usa are used to test hypothesized climate change in late paleozoic western equatorial pangea previously defined independently on the bases of sedimentologic and paleontologic proxies and climate models the 0 2 m size phyllosilicate fraction in the studied paleosols exhibits down profile trends in mineralogy and chemical composition that are consistent with modern weathering profiles suggesting a dominantly pedogenic origin a stratigraphic trend from kaolinite dominated profiles in upper pennsylvanian paleosols to profiles dominated by smectite and hydroxy interlayered 2 1 phyllosilicates in lower permian paleosols indicates a relatively rapid decrease in soil weathering and leaching in the latest pennsylvanian followed by a more gradual decrease in leaching throughout the early permian the chemical composition cation ratios and exchange capacity of these phyllosilicates further corroborates this shift toward less intensive leaching presumably in response to climate change from humid to progressively more arid conditions the phyllosilicates in the 0 2 m size fraction and contemporaneous pedogenic calcites from the permo pennsylvanian paleosols exhibit a long term stratigraphic increase in their 18o values of as much as 3 2 and 5 2 respectively this long term trend is consistent with a transition throughout the latest pennsylvanian through early permian toward progressively more evaporatively enriched soil waters superimposed on the long term trend is an apparent rapid enrichment 1 5 to 2 in phyllosilicate 18o values immediately above the pennsylvanian permian boundary observed oxygen isotope fractionation between the phyllosilicates and calcites within individual paleosols indicate isotopic disequilibrium between mineral pairs this is attributed to a minor detrital component in the pedogenic clay dominated phyllosilicate fraction coupled with the effects of seasonality of mineral formation inferred 18o compositions of late paleozoic meteoric water 2 to 4 are compatible with less intensive soil leaching under conditions of increasing aridity possibly coupled with a shift in local precipitation from a continental source to a marine source

early neoproterozoic multiple arc back arc system formation during subduction accretion processes between the yangtze and cathaysia blocks new constraints from the supra subduction zone ne jiangxi ophiolite south china the ne jiangxi ophiolite in the eastern jiangnan orogen is a tectonic melange that mainly consists of individual tectonic blocks comprising pyroxenite gabbro basalt diorite granite and chert in a matrix of serpentinite or tuffaceous greywacke a combined geochemical and geochronological study of the ne jiangxi ophiolite was undertaken to constrain the timing and tectonic setting of its formation the basalts were geochemically subdivided into three groups with different feot and tio2 contents group 1 basalts have the lowest feot 12 17 13 07 wt and tio2 1 48 1 62 wt contents and the lowest nb yb 0 80 0 88 and th nb 0 02 0 03 ratios furthermore they have normal mid ocean ridge basalt n morb like trace element patterns suggesting derivation from an n morb type mantle source without subduction input group 2 fe ti basalts have the highest feot 15 52 16 30 wt and tio2 3 06 3 23 wt contents nb yb and th nb ratios from 1 75 to 1 89 and from 0 11 to 0 15 respectively and trace element patterns similar to those of back arc basin basalts the geochemical characteristics suggest that group 2 basalts were derived from a slightly enriched morb type mantle source with a minor subduction contribution in contrast group 3 fe ti basalts have moderate feot 12 98 13 40 wt and tio2 2 37 2 71 wt contents and nb yb and th nb ratios from 1 28 to 1 45 and from 0 27 to 0 30 respectively these basalts further display markedly negative nb ta anomalies and show a geochemical affinity to island arc basalts iab indicating a slightly enriched morb type mantle source that was significantly influenced by subduction derived fluids and or melts sims zircon u pb dating on gabbros gave ages of 995 22 ma and 993 12 ma which are interpreted as the formation age of the ne jiangxi ophiolite positive zircon ehf t 8 8 to 13 8 values for the gabbros and whole rock end t 5 5 to 6 6 values for the basalts indicate that the ne jiangxi ophiolite originated from an isotopically homogeneous depleted mantle source the diversity of morb to iab like basalts and the presence of fe ti basalts favor a formation of the ne jiangxi ophiolite during the initial rifting phase of an intra oceanic back arc basin between an oceanic arc huaiyu terrane and the continental margin of the yangtze block jiuling terrane at ca 990 ma both the present and previous studies imply that multiple arc back arc systems formed during long lasting subduction accretion processes between the yangtze and cathaysia blocks during the early neoproterozoic

the case for archaean boninites rare archaean light rare earth element lree enriched mafic rocks derived from a strongly refractory mantle source show a range of features in common with modern boninites these archaean second stage melts are divided into at least two distinct groups whundo type and whitney type whundo type rocks are most like modern boninites in terms of their composition and association with tholeiitic to calc alkaline mafic to intermediate volcanics small compositional differences compared to modern boninites including higher al2o3 and heavy ree hree probably reflect secular changes in mantle temperatures and a more garnet rich residual source whundo type rocks are known from 3 12 and 2 8 ga assemblages and are true archaean analogues of modern boninites whitney type rocks occur throughout the archaean as far back as ca 3 8 ga and are closely associated with ultramafic magmatism including komatiites in an affiliation unlike that of modern subduction zones they are characterised by very high al2o3 and hree concentrations and their extremely depleted compositions require a source which at some stage was more garnet rich than the source for either modern boninites or whundo type second stage melts low la yb and la gd ratios compared to whundo type rocks and modern boninites either reflect very weak subduction related metasomatism of the mantle source or very limited crustal assimilation by a refractory mantle derived melt regardless the petrogenesis of the whitney type rocks appears either directly or indirectly related to plume magmatism if whitney type rocks have a boninitic petrogenesis then a plume related model similar to that proposed for the modern tongan high ca boninites might apply but with uniquely archaean source compositions and source enrichment processes second stage melts from barberton s africa 3 5 ga and ca 3 0 ga rocks from the central pilbara australia have features in common with both whundo and whitney types but appear more closely related to the whitney type subduction zone processes essentially the same as those that produce modern boninites have operated since at least 3 12 ga while a uniquely archaean boninite forming process involving more buoyant oceanic plates and very inefficient mantle source enrichment may have occurred before then

unusual secondary ca mg carbonate kerolite deposits in basaltic caves kauai hawaii abstract aragonite calcite monohydrocalcite dolomite magnesite hydromagnesite and talc like kerolite have been identified in speleothems from basaltic caves on the island of kauai hawaii these secondary minerals have crystallized in the presence of freshwater seeping from the basaltic host rock and the mineralization process is ongoing at some locations the deposits are extremely variable in macroscopic character ranging from millimeter thin powdery coatings to hard crusts several centimeters thick many deposits are associated with actively forming microbial mats and stromatolitic structures are common in the thicker deposits formation of the mineral deposits is a result of a combination of direct precipitation from solution by evaporation and co2 degassing alteration of precursor minerals and or microbial processes and physicochemical conditions within the mats these unusual deposits provide important insights into the mineralogy and geochemistry of low temperature alteration and carb

velocity depth modelling using reflection and refraction data recorded in the central eromanga basin queensland australia abstract seismic refraction recordings along a line extending from mt howitt no 1 well to eromanga in southwest queensland over the eromanga and cooper basin sediments revealed two intra basement refractors not recorded by coincident reflection profiling these refractors were recorded at 3 5 km and 5 0 km depth between the basin basement unconformity at 2 4 km depth and a low velocity zone at about 8 0 km depth precise modelling of the refracted first arrival times showed that the basin structure and faults seen on the reflection profile extended into basement to at least 5 5 km depth with increasing displacement on the faults with depth causing lateral variation of structure and velocity a feature of the seismic refraction data in this region is a series of up to five multiples which can be observed from 10 km to at least 40 km distance travel time characteristics indicate that these multiples are waves refracted within basement and multiply reflected at the surface travel time modelling of the refracted first arrivals multiples and two way reflection times provides a tight constraint on velocity and structure to a depth of about 5 5 km

fossil trees in ancient fluvial channel deposits evidence of seasonal and longer term climatic variability it has been established that large numbers of certain trees can survive in the beds of rivers of northeastern australia where a strongly seasonal distribution of precipitation causes extreme variations in flow on both a yearly and longer term basis in these rivers minimal flow occurs throughout much of any year and for periods of up to several years allowing the trees to become established and to adapt their form in order to facilitate their survival in environments that experience periodic inundation by fast flowing debris laden water such trees notably paperbark trees of the angiosperm genus melaleuca adopt a reclined to prostrate downstream trailing habit have a multiple stemmed form modified crown with weeping foliage development of thick spongy bark anchoring of roots into firm to lithified substrates beneath the channel floor root regeneration and develop in flow parallel linear groves individuals from within flow parallel linear groves are preserved in situ within the alluvial deposit of the river following burial and death four examples of in situ tree fossils within alluvial channel deposits in the permian of eastern australia demonstrate that specialised riverbed plant communities also existed at times in the geological past these examples from the lower permian carmila beds upper permian moranbah coal measures and baralaba coal measures of central queensland and the upper permian newcastle coal measures of central new south wales show several of the characteristics of trees described from modern rivers in northeastern australia including preservation in closely spaced groups these properties together with independent sedimentological evidence suggest that the permian trees were adapted to an environment affected by highly variable runoff albeit in a more temperate climatic situation than the modem australian examples it is proposed that occurrences of fossil trees preserved in situ within alluvial channel deposits may be diagnostic of environments controlled by seasonal and longer term variability in fluvial runoff and hence may have value in interpreting aspects of palaeoclimate from ancient alluvial successions c 2001 elsevier science b v all rights reserved

the australian central eromanga basin project an introduction abstract the australian bureau of mineral resources is carrying out a major multidisciplinary program of geological and geophysical studies in southwestern queensland in cooperation with the geological survey of queensland the project is aimed at providing information on the regional structure and depositional history of the central eromanga basin and the underlying adavale cooper and galilee basins the information being obtained is particularly relevant to a better understanding of the petroleum prospectivity of the area the program includes geophysical surveys involving 1400 km of new six fold cdp seismic reflection coverage on regional traverses up to 400 km long crossing the main structural elements of the area gravity measurements along all new seismic reflection traverses refraction surveys along two major east west and north south traverses and magnetotelluric soundings along the same major east west traverse landsat imagery studies are providing new perspective on many regional structures when used in conjunction with seismic and gravity information wireline logs and synthetic seismograms are being used with the new seismic data to re examine stratigraphic correlations palynologic and lithologic studies are underway to assist in determining depositional environments source rock maturation hydrological and geochemical studies are providing information on the generation and migration of hydrocarbons a significant feature of the program is the extension of the recording time of all new reflection data to 20 s to obtain good quality deep crustal reflection information comparable to that obtained on cocorp programs in the united states the reflection data is being interpreted with the refraction gravity and magnetotelluric data to investigate the relationship of deep crustal and upper mantle features to the sedimentary basins in the central eromanga basin area

climate of the supercontinent pangea numerous climate models predict that the geography of the supercontinent pangea was conducive to the establishment of a megamonsoonal circulation in general geologic evidence supports the hypothesis of a megamonsoon that reached maximum strength in the triassic pangea in the late carboniferous had widespread peat formation in what is now central and eastern north america and europe and relatively dry conditions on the colorado plateau the equatorial region of the continent became drier through the end of the carboniferous by the permian the equatorial region of pangea was dry and indicators of aridity and rainfall seasonality became more widespread wind directions from colorado plateau eolian sandstones are consistent with an increasing influence of monsoonal circulation at this time in the triassic climate in the colorado plateau region became relatively wet though still seasonal and the few eolian sandstones indicate a major shift in wind direction at that time in addition sedimentation

the petrogenesis of leucogranitic dykes intruding the northern semail ophiolite united arab emirates field relationships geochemistry and sr nd isotope systematics the khawr fakkan block of the semail ophiolite united arab emirates exhibits a suite of 10 100 m scale metaluminous to peraluminous granitic intrusions ranging from cordierite andalusite biotite monzogranites to garnet tourmaline leucogranites which intrude mantle sequence harzburgites and lower crustal sequence cumulate gabbros structural constraints suggest that the subduction of continental sedimentary material beneath the hot proto ophiolite in an intra oceanic arc environment led to granulite facies metamorphism at the subduction front and the generation of granitic melts which were emplaced up to the level of the ophiolite moho compositions indicate the analysed granitoids were largely minimum melts that crystallised at variable a h2o and pressures of 3 to 5 kbar the lile sr rb and ba covariation modelling suggests that the granitoids formed largely by the dehydration melting of muscovite rich metasediments initial 87sr 86sr ratios of analysed dykes vary between 0 710 and 0 706 at initial nd values of between 6 3 and 0 5 cogenetic units of a composite sill from ra s dadnah yield a sm nd isochron age of 98 8 9 5 ma mswd 1 18 geochemical and isotopic characteristics of the analysed granitic intrusions indicate that the subducted continental material was derived from oceanic trench fill haybi complex sediments preserved as greenschist asimah area to granulite facies bani hamid area ophiolitic metamorphic sole terranes the sr nd isotope systematics suggest that hybrid granitic melts were derived from pre magmatic mixing of two contrasting subduction zone sources

classification of paleosols despite increased interest in paleosols during the past decade no satisfactory classification is in current use presented here is a largely descriptive classification system that utilizes those pedogenic features that have the highest preservation potential in the rock record emphasized in the classification are morphological and mineralogical features that are easily recognizable in the field and through the petrographic microscope the classification is based on evaluation of the relative prominence in a paleosol of six pedogenic features or processes organic matter content horizonation redox conditions in situ mineral alteration illuviation of insoluble minerals compounds and accumulation of soluble minerals the most prominent of these six features processes provides the key to classifying a paleosol into one of nine orders four of the order names are borrowed from soil taxonomy histosol spodosol oxisol vertisol whereas the other five order names are presented here for the first time calcisol gypsisol gleysol argillisol protosol the orders may be preceded by one or more subordinate modifiers that describe other important features of the paleosol the classification is relatively easy to apply to the rock record and should enhance communication and aid in the standardization of terminology

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

neoproterozoic post collisional extension of the central jiangnan orogen geochemical geochronological and lu hf isotopic constraints from the ca 820 800 ma magmatic rocks abstract neoproterozoic magmatic rocks in south china contain important information on evaluating crustal reworking during the yangtze cathaysia collision and post collisional extension we present here geochemical geochronological and lu hf isotopic data of a suite of granitoids and diorites from the jiuling and meixian plutons of the central jiangnan orogen the granitoids display intermediate geochemical features between s type and i type granites they exhibit arc like patterns of ree and trace element partition reflecting inheritance of juvenile arc derived crust igneous zircons from the jiuling and meixian granitoids yield u pb ages of 822 819 ma and 816 815 ma respectively inherited zircons from the granitoids commonly yield a major age population of 980 860 ma consistent with the time of the shuangxiwu volcanic arc the igneous and inherited zircons have highly variable ehf t values ranging from 9 to 12 2 we interpret the granitoids to have been generated by remelting and binary mixing of 1 juvenile arc derived crustal material and 2 supracrustal material of the shuangqiaoshan meta sedimentary rocks the previous crustal thickening achieved by the yangtze cathaysia collision is a prerequisite for their generation the two granitoids together with regional 825 815 ma s type granites indicate incipient post collisional collapse of thickened crust heralding tectonic switch from compression to extension the meixian diorites display arc like geochemical characteristics and yield zircon u pb ages of 805 804 ma and neutral to positive zircon ehf t values 0 1 7 5 the diorites were generated by decompression melting of arc derived lower crust associated with asthenospheric mantle upwelling during continental rifting such rifting may be part of global neoproterozoic rifting events that finally led to breakup of the rodinia supercontinent

rates of weathering and chemical depletion in soils across a chronosequence of lake michigan sand dunes abstract accumulation of soil organic matter and depletion of base cations and trace metals were measured in surface soils i e o horizon and upper 15 cm of mineral soil across a 4150 year soil chronosequence of dune capped beach ridges bordering northern lake michigan within 400 years of beach ridge formation coniferous forest develops soil c and n accumulate to steady state levels of 40 and 1 3 mg ha 1 respectively and the ph of the upper mineral soil decreases from 8 5 to 4 3 as carbonate minerals dissolve after substantial proportions of ca and mg have been removed an incipient spodic horizon can be observed the upper surface of which progressively deepens with increasing soil age analysis of soil strain of the upper mineral soil showed gradual dilation reaching 121 after several centuries of soil development after accounting for soil dilation quantities of ca mg p zn mn fe and v in the upper 15 cm of mineral soil decreased exponentially to low asymptotic levels representing quantities contained in weathering resistant minerals mass balance modeling indicated weathering and elemental losses in the order of decreasing decay constant as ca mg mn zn p v fe al k na ti this sequence is consistent with weathering of carbonates then ferromagnesian minerals then feldspars p levels are tightly coupled with ca losses in soils older than 200 years suggesting that ca rather than fe or al stabilizes p high concentrations of heavy metals in the organic horizon relative to the upper mineral soil indicate atmospheric inputs of industrial pollutants

petrology and u pb geochronology of the telohat migmatites aleksod central hoggar algeria the aleksod region is composed of metasedimentary rocks and large areas of biotite and hornblende bearing migmatites anatexis associated with the main deformation stages occurred under high pressure and temperature conditions estimated at 13 2 kbar and 750 50 c the bulk mineralogical composition of the telohat migmatites shows that their protolith was granodioritic internal structures of zircons and u pb data suggest a polyphased evolution with a 2131 12 ma age for the protolith and a 609 17 ma age for the pan african tectono metamorphic evolution thus precluding any kibaran event in the aleksod area leucosomes are richer in sr and display lower rb zr nb y th u and ree contents than melanosomes wherein accessory phases are stored eu contents are also lower in the leucosomes but in lesser proportion than the other rare earth s leading to a significant positive anomaly petrogenetic modelling accounting for accessory mineral phases clearly shows that the trace element contents of leucosomes and melanosomes follow a distribution law consistent neither with equilibrium nor fractional melting their trace element patterns are best explained by the model of disequilibrium melting with mixing of a few residual phases the present results and previous sr isotopic data as well raise the question of disequilibrium melting in anatexis of crustal material

morphology and isotope heterogeneity of late quaternary pedogenic carbonates implications for paleosol carbonates as paleoenvironmental proxies abstract stable isotope values and radiocarbon ages were determined for pedogenic carbonate microsamples from relict soils and palaeosols from the rio grande rift region of new mexico carbonate nodules and clusters were sampled from fluvial and piedmont soils and palaeosols with widely varying exposure durations 7 900 ky pedogenic carbonates from individual soils exhibited irregular relationships between age and depth ranging in radiocarbon age by 13 ky at individual depths 18 o values and to a lesser extent 13 c values likewise had wide ranges within soils however comparing the stable isotope values of carbonates with similar radiocarbon ages within and between soils revealed consistent trends in stable isotope values over time 18 o values of pedogenic carbonates tended to decrease with decreasing age 25 17 radiocarbon ka and increased with decreasing age 13 c values with either depth or age although the oldest palaeosol piedmont carbonates radiocarbon ages of 24 5 19 2 ka are 1 more negative than the younger piedmont palaeosol carbonates pedogenic carbonates in the relict fluvial soil which has by far the longest exposure period showed the greatest range in 13 c values comparing the 13 c values of samples of similar radiocarbon age from the relict fluvial soil indicates a transition from a mixed c 3 c 4 ecosystem at 11 ka to an effectively pure c 3 ecosystem at 9 ka followed by a return to mixed vegetation by 5 ka thus although c 4 vegetation persisted in southern new mexico through the last glaciation drought resistant c 3 vegetation became dominant at 9 ka prior to significant strengthening of the summer monsoon this study illustrates that the stable isotope values of carbonate nodules and clusters from relict soils and or slowly buried palaeosols can be used for palaeoenvironmental reconstructions in the context of a detailed chronologic framework

subduction related origin of the 750 ma xuelongbao adakitic complex sichuan province china implications for the tectonic setting of the giant neoproterozoic magmatic event in south china abstract the xuelongbao plutonic complex in the eastern margin of the tibetan plateau is dated at 748 7 ma using the shrimp zircon u pb method and represents part of the neoproterozoic igneous assemblage of south china rocks in the complex include tonalite and granodiorite and have sio2 ranging from 62 0 to 74 8 wt and al2o3 from 14 3 to 20 9 wt their na2o contents range from 4 2 to 6 7 wt and k2o from 0 47 to 1 96 wt indicating that they belong to the na series these rocks show chondrite normalized ree patterns depleted in hree and variably enriched in lree they have positive sr and negative nb and ti anomalies in the primitive mantle normalized trace elemental spider diagram their sr contents range from 320 to 780 ppm and y contents are lower than 10 ppm resulting in high sr y ratios 52 320 characteristic of typical adakites their end t and initial sr isotopic compositions range from 0 36 to 2 88 and from 0 7033 to 0 7054 respectively the geochemical features of the xuelongbao plutonic complex are consistent with an origin from adakitic magmas that were likely derived from partial melting of a subducted oceanic slab together with arc signatures of other granites and mafic intrusions in the region the 750 ma xuelongbao adakitic complex provides evidence for a major subduction related neoproterozoic magmatic event of south china

zircon ages sr nd hf isotopic compositions and geochemistry of granitoids associated with the northern ophiolite melange of central cuba tectonic implication for late cretaceous magmatism in the northwestern caribbean the geology of cuba is representative of the orogenic belt fringing the northwesternmost margin of the caribbean plate it is characterized by a tectonic collage made up of a continental margin volcanic arc and ophiolitic and subduction complexes formed as a result of collision between the leading edge of the caribbean plate and the caribeana terrane in the latest cretaceous to early tertiary and by oblique convergence with the continental margin of the north american plate during tertiary times the northern ophiolite belt of central cuba consists of a melange containing blocks of eclogite garnet amphibolite amphibolite blueschist greenschist quartzite metapelite antigoritite and various types of intrusive rocks that occur as blocks or intrusive gabbro and diabase of the ophiolitic sequence we present the first combined shrimp zircon u pb ages sr nd hf isotopic compositions as well as major and trace element data for nine arc related granitoid rocks from the northern ophiolite melange of central cuba a trondhjemite from the cerro el chivo area south of santa clara provided a concordant zircon age of 86 ma whereas a granite and a tonalite from the las bocas body north of placetas yielded ages of 75 ma two peraluminous trondhjemitic samples taken from the tres guanos area s of perea yielded zircon crystallization ages of 72 and 74 ma rare cambrian 536 ma and permian 268 ma inherited zircons were found in these samples hf isotope ratios were measured in five of the dated zircon grains from the tres guanos samples the three youngest zircons with ages of 72 and 74 ma show derivation from a depleted source region as can be expected for arc granitoids as well as the oldest xenocryst 536 ma which is less depleted but still from a juvenile pan african source a permian xenocryst 268 ma has a negative hf t value 2 35 implying that the zircon is derived from a continental source the initial nd isotopic composition nd t of four of the dated granitoids is almost identical at 7 3 to 7 7 and implies a depleted source whereas one of the tres guanos trondhjemite samples containing an inherited cambrian zircon has a lower nd t of 4 9 possibly reflecting input from an older crustal component the initial sr isotopic compositions of all five analyzed samples vary between 0 7031 and

evidence for late palaeozoic glaciation in southern tanzania abstract new observations from basal karoo beds of the ruhuhu basin in southern tanzania prove the existence of the late palaeozoic glaciation in the region bordering lake nyasa evidence for the glaciation includes the identification of tillites and varved lacustrine siltstones with dropstones the tillites contain abundant striated and facetted pebbles and cobbles floating in a rock flour matrix consisting of finely ground quartz and feldspar features supporting subglacial transport of the rudaceous sediments are microstriations on clasts pressure flaked edges of grains and cleavage breakage observed under the sem the topography of the pre karoo surface suggests continental glaciation the onset of the glaciation was concomitant with late carboniferous to early permian taphrogenic movements which established the large ne trending karoo trough of tanzania

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

genesis of the shangjinshan w mo polymetallic deposit in the eastern jiangnan tungsten belt evidences from geochemistry geochronology and zircon hf isotope analyses abstract the newly discovered shangjinshan w mo polymetallic deposit located at the southern anhui southeast china is a large scale skarn type tungsten polymetallic deposit according to mineral assemblages ore textures structures and superposition relationships of the mineralization three ore types have been recognized veinlet disseminated skarn the primary type quartz vein type and porphyry type the mineralized event of this deposit could be divided into four stages i the prograde skarn stage ii retrograde skarn stage iii quartz sulfide vein stage and iv quartz calcite vein stage the intrusion in the tungsten rich district of the shangjinshan is mainly composed of the granodiorite with zircon u pb ages of 138 4 1 2 ma to 136 7 1 3 ma meanwhile re os isotopic dating of five molybdenite samples yields an isochron age of 141 9 3 1 ma and a weighted average age of 139 3 1 7 ma respectively the ore forming ages are consistent with the rock forming ages within errors indicating that the w mo mineralization is genetically related to the shangjinshan granodiorite the granodiorite is i type granites characterized by metaluminous to slightly peraluminous and high k cal alkaline compositions they are also enriched in large ion lithophile elements liles and light rare earth elements lrees and depleted in high field strength elements hfses and heavy rare earth element hrees with obviously negative eu anomalies eu eu 0 50 0 84 the zircon ehf t values and two stage hf model ages tdm2 fall into the range 6 18 to 2 04 and 1 5 1 3 ga respectively the relatively low rhenium contents in molybdenite and high radioactive pb isotopes of the pyrites from stage ii indicate that the ore forming material was derived from shangjinshan granitic magmatism together with previously published data we propose that the w related granitoids in the jnb were likely generated by partial melting of the shangxi group with some additional mantle materials the granitic magmatism was caused by the asthenosphere upwelling triggered by the roll back of subduced paleo pacific plate during late jurassic to early cretaceous these granitic stocks predominately contributed to the production of skarn alteration and mineralization resulting in the giant tungsten polymetallic district in the jiangnan tungsten belt

geochemistry and geochronology of the banxi sb deposit implications for fluid origin and the evolution of sb mineralization in central western hunan south china abstract central western hunan in south china hosts the largest antimony belt in the world with two types of sb deposits identified sb au woxi type and sb only xikuangshan type banxi is the most representative deposit in the region with vein type sb mineralization hosted in neoproterozoic clastic rocks stibnite developing in ores and arsenopyrite mainly occurring in altered country rocks trace element contents including rare earth elements ree and isotopic ratios s pb sr nd he and ar in stibnite and or arsenopyrite were analyzed to determine the timing of ore formation and to elucidate fluid origin and evolution processes most sulfides have high ree contents 45 103 ppm moderate fractionation between lree and hree lree hree 6 38 11 56 and slightly negative eu anomalies eu eu 0 53 0 72 the 34 s values are 3 88 5 81 for stibnite and 9 25 11 82 for arsenopyrite the 206 pb 204 pb 207 pb 204 pb and 208 pb 204 pb ratios range from 18 617 to 18 635 15 695 to 15 739 and 38 965 to 38 981 for stibnite and from 18 594 to 18 609 from 15 587 to 15 698 and from 38 893 to 38 926 for arsenopyrite the 87 rb 86 sr and 87 sr 86 sr ratios are 0 3016 3 538 and 0 711463 0 717591 respectively for stibnite and 0 2251 2 214 and 0 711244 0 711565 respectively for arsenopyrite the 147 sm 144 nd and 143 nd 144 nd ratios are 0 1174 0 9816 and 0 511942 0 512768 for stibnite with end t values t 130 ma ranging from 12 4 to 6 6 and two stage model ages t 2dm ranging from 1457 to 1932 ma the 40 ar 36 ar and r ra ratios where ra is the atmospheric 3 he 4 he ratio of 1 4 10 6 are 409 545 and 0 0088 0 0348 respectively for stibnite the rb sr and sm nd isotopic analyses of the sulfides yield isochron ages of 129 4 2 4 ma 2 mswd 1 3 and 130 4 1 9 ma 2 mswd 1 6 respectively the combined trace element and isotopic data set indicates that the ore forming fluids were a mixture of dominantly deep basement derived solute rich parent water and a small amount of dilute heated meteoric water the decreases in ree 34 s 206 pb 204 pb 207 pb 204 pb 208 pb 204 pb 87 rb 86 sr 87 sr 86 sr 147 sm 144 nd and 143 nd 144 nd and increases in 40 ar 36 ar and 4 he contents in the stibnite from the earlier quartz stibnite mineralization stage stage ii through to the later stibnite mineralization stage stage iii indicate that the influx of meteoric water increased during sb mineralization the deep sourced relatively high temperature rock reacted fluid mixed with increasing amounts of more oxidized low temperature meteoric fluids causing the precipitation of arsenopyrite and stibnite sequentially at the banxi deposit a two period genetic model has been proposed for the world class sb mineralization in the central western hunan region the woxi type sb au deposits were derived from orogenic fluids generated from folding and shearing during the caledonian and indosinian orogeny whereas the xikuangshan type sb deposits were formed by mixing heated basement derived hydrothermal fluid circulating during the flare up of magmatism in south china with lower temperature meteoric water

early mesozoic thrust tectonics of the northwest zhejiang region southeast china the nw zhejiang region of south china occupies a key tectonic position near the suture zone of the yangtze and cathaysian blocks and is of critical importance for the assembly of east asia sedimentological and tectonic analyses indicate that the region had a se dipping paleoslope in the late paleozoic to early triassic a transitional sedimentary environment from deep sea to continental molasse is documented in the early triassic late triassic interval associated structures are nw vergent folds and thrusts that root southeastward beneath the high grade chencai metamorphic complex the structural styles of this foreland fold and thrust belt are characterized by multifold duplexes and individual folds that are together zoned from se to nw as follows 1 core zone characterized by shear folds and ductile thrusts 2 se belt with out of sequence thrusting of multifold duplexes and an average shortening of 50 3 central belt with duplexes imbricate fans and an average shortening of 40 and 4 nw belt with jura type folds and a shortening of 10 a tectonic model for the foreland fold and thrust belt is discussed in relation to the early mesozoic archipelago paleogeography of south china

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

tectonic evolution of the southeastern margin of the yangtze block constraints from shrimp u pb and la icp ms hf isotopic studies of zircon from the eastern jiangnan orogenic belt and implications for the tectonic interpretation of south china abstract the jiangnan orogenic belt is a ne sw trending fold belt in the southeastern yangtze block of south china the eastern segment of the belt located in northeastern jiangxi and southern anhui provinces is predominated by greenschist facies metamorphosed volcanic sedimentary strata with minor ophiolite and their protoliths are considered to have formed in a back arc basin zircon shrimp u pb dating reveals that these rocks were deposited emplaced in the period 842 825 ma zircon la icp ms hf analyses show that most igneous zircons from the volcanic sedimentary rocks possess positive hf t values of 0 09 12 74 corresponding to depleted source model ages of 0 9 1 4 ga this suggests a significant neoproterozoic juvenile crustal growth event in the southeastern part of the yangtze block which is also documented by gabbro 833 ma from ophiolite with positive hf t values of 5 24 13 25 and hafnium model ages of 0 87 1 20 ga these new data combined with previous results indicate the presence of an arc and back arc basin system along the southeastern margin of the yangtze block at 842 825 ma collision of the arc with the proposed shuangxiwu composite terrane which had previously formed away from the southeastern margin of the yangtze block through possible subduction and collision led to the closure of the back arc basin and the formation of the jiangnan belt in the period 825 815 ma we suggest that south china is an accretionary orogen and that orogen parallel motion along terrane bounding shear zones may have played a significant role in its amalgamation

venezuelan paleoflora of the pennsylvanian early permian paleobiogeographical relationships to central and western equatorial pangea abstract the flora of northwestern venezuela shows close links with the pennsylvanian flora of the northern hemisphere and northern africa in the early permian it also closely matches the flora reported in the southwestern and central united states the permian fossils from venezuela have various species and genera in common with that of this part of the usa not only flora but also warm water marine fauna the floristic data studied here provide evidence of a close relationship of the plants of the central portion of pangea with those of gondwanaland based on these similarities in the flora it is suggested that during the pennsylvanian early permian the northeastern part of gondwanaland which was one of the regions most affected by the formation of pangea had a progressively drier climate with the vegetation characteristic of such conditions moreover the relationship between the vegetation of this equatorial area and that of the cathaysian province during the early permian is discussed both showed the presence of gigantopterid genera although there were climatic differences furthermore the differences in the species of the group suggest that the two regions may have had quite different vegetation rather than the shared one traditionally proposed

integrated ediacaran sinian chronostratigraphy of south china abstract the ediacaran period was a critical time in the history of life and earth understanding the complex interactions between geological and biological events during this interval requires high resolution chronostratigraphy thanks to its uniquely favorable palaeogeographic setting south china is an ideal region for constructing a framework for global chronostratigraphic correlation of ediacaran strata here continuous ediacaran successions contain both siliciclastic and carbonate rocks that range from shallow to deep marine and contain rich fossil assemblages showing a variety of preservation styles as part of the present study 62 reference sections distributed throughout the yangtze platform were investigated analyses of the sedimentary facies and sequence stratigraphy of more than 40 sections revealed four major sequences in the shallow marine facies three major sequence boundaries occur respectively in the middle and upper parts of the doushantuo formation and in the middle part of the dengying formation subdivision and correlation of the ediacaran successions of the yangtze platform based on sequence stratigraphy is congruent with the carbon isotope chemostratigraphy of 12 reference sections 1075 samples the sequence boundary sb2 in the middle part of the doushantuo formation is suggested here as the lower xiadongian upper yangtzean series boundary of the ediacaran which coincides with several globally recognizable events namely 1 the end of the neoproterozoic glaciations 2 the first appearance of animals 3 the sudden increase in the diversity of acanthomorphic acritarchs and 4 a negative 13 c excursion the ediacaran system of south china may be subdivided into five stages furthermore the criteria for defining these stages may be applicable throughout the world also the composite carbon isotope profile of the ediacaran of south china can be used as a global reference for chronostratigraphic subdivision and correlation in addition to the globally recognizable negative 13 c excursion in the cap carbonate at the base of the ediacaran the negative 13 c excursion in the uppermost doushantuo wonoka shuram interval represents another globally correlatable major excursion the positive 13 c excursion immediately following the uppermost doushantuo excursion and ending at 548 ma is also useful for global correlation current ediacaran chronostratigraphy and global correlations indicate that the upper doushantuo wonoka shuram negative 13 c excursion ranges from ca 560 ma to 551 ma and therefore coincides with the evolutionary renovation of life feedback between evolutionary innovation and geochemical cycling was the key mechanism for this unique and most strongly negative 13 c excursion in earth history our chronostratigraphic correlations also imply that the earliest known metazoan biotas namely the weng an and characteristic ediacaran biotas do not constitute two temporally separate evolutionary lineages as proposed previously but represent two parallel evolutionary pathways preceding the cambrian explosion of animal life

pangaean climates megamonsoons of the megacontinent the climate for a highly idealized geographic representation of the megacontinent of pangaea corresponding to the period 250 200 m y ago is explored using a low resolution atmospheric general circulation model that is coupled to a static mixed layer ocean of constant depth with a provision for sea ice formation and decay soil moisture is included as a predicted variable on the continent because there are large uncertainties about key factors that might influence the pangaean climate sensitivity experiments are used to examine the climate for two different mean elevations 0 and 1000 m two different snowcover prescriptions and two different levels of implied greenhouse heating and solar luminosity modern co2 concentration and modern solar luminosity fivefold increase in modern co2 concentration and a 1 decrease in modern solar luminosity in all of the experiments the large bipolar land masses of laurasia and gondwanaland joined near the equator in the west and separated by the tethys sea in the east exhibit extreme continentality with hot summers cold winters and large scale summer and winter monsoon circulations year round or seasonal aridity is simulated for all but the eastern coastal regions the tropical western coast and the regions poleward of 40 latitude the simulated climate of the idealized pangaean continent has many features that correspond qualitatively with geologic evidence for tropical warmth and aridity in the late permian and triassic however the model results serve mainly to indicate the importance of various climate forming factors geography topography heating detailed comparisons will require new experiments with better models and more accurate geologic boundary conditions

generation of plagiogranite by amphibolite anatexis in oceanic shear zones field geochronological and rare earth element ree evidence obtained from the fournier oceanic fragment of the canadian appalachians indicates that its plagiogranite component was generated by the anatexis of amphibolite and not by the fractionation of basic magma we propose that the process occurred in two stages first gabbro was plastically deformed at high temperature to form dry low angle shear zones that subsequently evolved to amphibolite via the addition of light ree enriched hydrothermal solutions second the amphibolite underwent partial melting during shear to yield a migmatite comprising bands of plagiogranite alternating with amphibolite restite the plagiogranite locally coalesced to form pods dikes and lenses that injected the surrounding undeformed gabbro we attribute the development of the fournier plagiogranite to dynamothermal processes occurring in proximity to a spreading center due to asthenosphere induced shear within ocean layer 3 this serves as an important illustration of the dynamic nature of metamorphism and melting that can occur in the ocean crust

u pb geochronology and origin of granitoid rocks in the thetford mines ophiolite canadian appalachians we integrate new and existing age data from ophiolite and subophiolite rocks in the thetford mines region quebec appalachians to determine the timing of 1 oceanic crust formation 2 oceanic decoupling 3 ophiolite emplacement onto the continent and 4 postemplacement taconic orogenesis few ophiolites possess high precision temporal data for all these events this allows us to develop models to explain the origin of emplacement related granitoids that have intruded the ophiolite u pb zircon ages have been determined for four samples from the thetford mines ophiolite oceanic plagiogranite from the lac de l9est region of the thetford massif yields an age of 480 2 ma this age is identical to that of the adjacent mont ham massif thrust sheet 478 3 2 ma suggesting a close temporal and spatial association of these two units in the source oceanic domain two peridotite hosted granitoid samples yield crystallization ages of 470 5 3 ma and 469 4 ma a precambrian inherited zircon component is also present in one sample these granitoids are strongly peraluminous contain abundant inherited zircon exhibit variable but high 87 sr 86 sr initial ratios and low 208 pb 206 pb ratios in igneous zircon which suggest that they were derived by partial melting of laurentian margin sedimentary rocks the ages indicate that the ophiolite was thrust over the laurentian margin within 1 17 m y of oceanic crustal crystallization and that these obduction related granites were generated during a period of shear heating calculations indicate that shearing can generate the peridotite hosted granitoid melts and their observed contact zone mineralogies under specific conditions although margin sedimentary rock melting in response to west dipping subduction related arc magmatism cannot be excluded

reply to the comment by zhou et al on revisiting the yanbian terrane implications for neoproterozoic tectonic evolution of the western yangtze block south china precambrian res 151 2006 14 30 precambrian res 154 2007 153 157

geochronology and geochemistry of granitoids related to the giant dahutang tungsten deposit middle yangtze river region china implications for petrogenesis geodynamic setting and mineralization abstract porphyry and skarn deposits in the middle yangtze valley within the northern yangtze craton have a combined tungsten resource of 3 million tonnes mt and represent one of the most important tungsten regions in the world the dahutang porphyry tungsten deposit with reserves of 1 mt is one of the largest deposits uranium pb analyses for the ore related granitoids yield ages of 147 4 0 58 ma 148 3 1 9 ma for porphyritic biotite granite 144 7 0 47 ma 146 1 0 64 ma for fine grained granite and 143 0 0 76 ma 143 1 1 2 ma for granite porphyry a progressive youngling of ages that is consistent with field observations geochemical data show that the three types of granite are characterized by enrichments in rb pb and u and depletion in ba nb p and ti with asi molar al 2 o 3 cao na 2 o k 2 o 1 1 that is characteristic of a peraluminous melt the p 2 o 5 contents of the granites are 0 13 0 37 and have a positive correlation with sio 2 and they are thus s type intrusions they exhibit initial 87 sr 86 sr of 0 721 to 0 731 and e nd t of 5 06 to 7 99 for porphyritic biotite granite 0 7196 to 0 7289 and 6 29 to 6 74 for fine grained granite and 0 7153 to 0 7365 and 5 09 to 7 64 for granite porphyry chondrite normalized rare earth element ree patterns for the granites are characterized by enrichment in the light ree and a strong negative eu anomaly indicating that they were derived from the proterozoic pelitic and psammitic basement strata and experienced strong fractional crystallization of plagioclase our ca 150 140 ma age for the dahutang s type magmatism and w mineralization is identical to that of the i type magmatism related to cu au mo fe bearing porphyry and skarn deposits along the middle to lower yangtze river valley we propose that the latest jurassic to earliest cretaceous granitoids and ores formed during a tearing of the subducting izanagi slab which caused the upwelling of asthenosphere and resulting mantle crust interaction the s type granitoids and related w ore systems resulted from the re melting of the proterozoic crust whereas the i type granitic rocks and related ores are attributed to the partial melting of the subducted slab

vertisol carbonate properties in relation to mean annual precipitation implications for paleoprecipitation estimates abstract previous publications combining the properties of multiple soil orders show that depth to carbonate dtc increases systematically between 350 and 1000 mm of mean annual precipitation map we hypothesize that carbonate in vertisols clay rich shrink swell soils respond differently to water flux than other soil orders because of lower permeability to test this hypothesis we compiled soil description and characterization data from multiple published sources across a late pleistocene climosequence of the coast prairie of texas to assess the relationship between map 700 1400 mm and dtc the dtc of carbonate nodules represents an index of accumulation and the dtc of calcium carbonate equivalent total carbonate 2 0 mm diam an index of leaching the dtc for 1 2 and 5 abundances were assessed using regression analysis the r2 values were highest for the dtc of 2 nodules and of 1 calcium carbonate equivalent in vertisol microlows surprisingly relatively high r2 values were calculated

structural styles and basin evolution in eromanga region eastern australia the eromanga region of eastern australia has undergone at least four major tectonic cycles the results of which are recognized in the structures seen on seismic sections the late proterozoic early phanerozoic cycle ended with the formation of the thomson foldbelt which forms basement in the region the subsequent reactivations of the fault systems within this basement and deeper lithosphere have produced the structural styles now important as exploration targets most reactivations are interpreted as resulting from crustal shortening events affecting sedimentary strata deposited in an epicontinental shallow water environment with small strike slip adjustments affecting some structures the dominant style of deformation evident from the seismic sections is that seen across high angle thrust faults in other parts of the world with the amount of overthrusting being comparatively small generally the maximum throw on faults in the devonian strata can be measured in thousands of meters whereas those in the permian triassic and jurassic cretaceous strata are a few hundred meters at most the devonian sequences of the adavale basin and associated troughs were deformed during two carboniferous events the first with crustal shortening in a north south direction and the second in an east west direction associated small strike slip movements were accommodated along a number of faults including the warrego grenfield warbreccan and canaway faults southwest of our study area in the permian triassic cooper basin other workers have interpreted a northwest southeast crustal shortening event during the late permian and another wrench induced northeast southwest crustal shortening during the triassic during the middle tertiary a crustal shortening event regionally affected the jurassic cretaceous eromanga basin sequence and older sedimentary rocks this event is seen pro inently in a deformation zone between the canaway and cunnavalla faults with the direction of crustal shortening being northeast southwest small strike slip movements are interpreted as partially decoupling events from neighboring provinces the adavale basin and cheepie shelf to the east and southeast and the cooper basin and warbreccan dome to the southwest and northwest respectively in this partial decoupling the canaway cunnavalla warrego and warbreccan faults are interpreted as playing an important role the basin modifying tectonic episodes within the lithosphere control structural style these relatively small scale events have resulted in structures that provide attractive petroleum prospects

permian climate development in the northern peri tethys area the lodeve basin french massif central compared in a european and global context abstract the lodeve basin which contains an exceptional long and nearly uninterrupted profile of early to late permian continental red beds is well correlated to the other european permocarboniferous basins together they cover the time span from the late pennsylvanian to the permian triassic boundary although these basins were placed within the equatorial belt for the whole permian several climatic changes are recorded in the sediments terrestrial sedimentation is governed by the interaction of climate and tectonics as well as by volcanism to a variable degree therefore small scale climatic variations of the milankovitch band are hardly discernable and if so they could not be correlated between different basins indeed larger scale climatic shifts could be identified it is shown that the shift from the carboniferous icehouse to the mesozoic warmhouse was a multi stage process this global warming resulted in a general aridisation of northern pangea indicated by outspreading playa and sabkha red beds aeolianites and evaporites several short phases of increased humidity interrupted this trend indicated by wet red beds lake sediments and coal deposits in central and southern europe these wet periods are traceable within the late stephanian lower rotliegend gzhelian asselian topmost lower rotliegend asselian sakmarian upper rotliegend i late artinskian early kungurian and upper rotliegend ii zechstein wuchiapingian the aridisation of the northern pangea which accelerated near the sakmarian artinskian transition is attributed to a reorganisation of the major ocean currents cold water currents along the western flank of the pangea indicated by the permian chert event blocks moisture transport from west onto the continent during roadian and wordian the onset of an additional cold water current along the cimmerian continents blocks moisture transport from the east both phenomena together are responsible for the maximum of aridity at the early to middle permian transition additionally the equatorial climate has been affected by an increasing seasonality within the late pennsylvanian and permian caused by the wide shifting of the zone of inter tropical convergence itc over the equator based on a new reconstruction of the pangean assembly in this paper climatic changes in the permian are interpreted and causes of the arid humid arid shifts are examined phases of higher humidity are often related to events of global impact like the waxing and waning of the gondwanan glaciation obtained in the south african karoo basin dwyka group but also regional events had strong climatic influence the zechstein transgression in the northern and southern permian basin as well as the bellerophon transgression of the southern alps caused the wet phase in the wuchiapingian

geochronology and geochemistry of the granites from the zhuxi w cu ore deposit in south china implication for petrogenesis geodynamical setting and mineralization abstract the giant zhuxi tungsten deposit is located in the taqian fuchun ore belt in northeastern jiangxi province and genetically associated with the zhuxi granitic stocks and dykes three mineralization related granites including granite porphyry dykes gp biotite granitic stocks bg and white granitic dykes wg were identified in the zhuxi deposit shrimp zircon u pb analysis for the three granitic rocks present ages ranging from 153 5 1 0 ma to 150 4 1 0 ma the bg mainly contains quartz microcline albite biotite and muscovite with minor accessory minerals including zircon apatite monazite ti fe oxides and dolerite however the wg is mainly composed of quartz microcline and albite with minor muscovite and accessory minerals the gp is a medium grained porphyritic granite and its phenocrysts include quartz alkali feldspar muscovite and plagioclase all the zhuxi granites have high sio2 content 71 97 wt 81 19 wt and total alkali 3 25 wt 9 42 wt and their valid aluminum saturation index asi values show a wide range of 1 03 to 2 49 high rb sr ratios low sr content

paleozoic tectonothermal event in mt dongbatu dunhuang terrane southernmost central asian orogenic belt caob implications for petrogenesis and geological evolution abstract in this contribution systematic petrographical whole rock geochemical geochronological and zircon lu hf isotopic investigations were conducted on paleozoic ca 410 400 ma metamorphic magmatic rocks from mt dongbatu in the central dunhuang terrane the metamorphic rocks in the shangkouzi area are of meta calcsilicate rocks and granitoid rocks occurring within these rocks are geochemically of trondhjemite whereas metamorphic rocks in the gangou area are metabasaltic rocks la icp ms zircon u pb dating results reveal that these metamorphic magmatic rocks recorded ca 1 6 ga magmatic event and metamorphic events of ca 410 400 ma and ca 370 ma field occurrence whole rock geochemistry mineral composition and zircon hf isotopes suggest that the 1 6 ga mantle derived basaltic magma together with iron rich and ca rich components and mica quartz schist of paleoproterozoic supracrustal rocks probably contributed to protolith of meta calcsilicate rocks in the shangkouzi area and granitoid rocks within meta calcsilicate rocks were possibly generated by partial melting of calcsilicate rocks combined with previously published data it is suggested that the protoliths of paleozoic high grade metamafic rocks in the dunhuang terrane are predominantly composed at least of basaltic rocks produced by magmatic events of 2 5 1 8 ga ca 1 8 ga and ca 1 6 ga the paleozoic granitoid rocks in the dunhuang terrane might have been derived from different magma sources as partial melting of subducted plate or thickened lower continental crust or pre existing basaltic rocks produced by 1 6 ga and 1 8 ga magmatic activities and calcsilicate rocks genetically associated basaltic magmatism at ca 1 6 ga considering regional tectonic evolution it can be concluded that the dunhuang terrane was probably in a post collisional extension setting at ca 1 6 ga producing certain amount of basaltic and calcsilicate rocks from the beginning of silurian ca 440 ma the dunhuang terrane was involved in the orogenic processes related to the closure of the southern paleo asian ocean becoming the active continental margin of the southernmost caob

comment on revisiting the yanbian terrane implications for neoproterozoic tectonic evolution of the western yangtze block south china precambrian res 151 2006 14 30

new chronological constraints for cryogenian to cambrian rocks in the three gorges weng an and chengjiang areas south china abstract the neoproterozoic and cambrian were two of the most dramatic periods in the history of the earth because large multi cellular animals first appeared then in the so called cambrian explosion to better understand this event many paleontological and geochemical studies now focus on rocks in south china because of the fossiliferous succession and good exposure since the recognition of the yangtze gorges and chengjiang area as type localities of the sinian ediacaran and meishucunian early cambrian systems both sections have been intensively investigated in order to decipher the relationships between the evolution of life and surface environmental changes it is necessary to understand their paleontological geochemical and geo chronological constraints this study presents new chronological constraints for the cryogenian to cambrian rocks in the three gorges weng an and chengjiang areas south china we discovered two tuff layers one at the base of the shuijingtuo fm at three gorges and the other at the bottom of the dahai member in chengjiang in addition we collected sandstones from neoproterozoic strata in the three gorges chengjiang and weng an areas for provenance analysis zircons separated from the tuff layers provide new nano sims u pb ages of 526 4 5 4 ma at the base of the shuijingtuo fm and 523 9 6 7 ma at the bottom of the dahai member the tuffaceous beds occur at an unconformity and nodules are common in the three gorges meishucun and taoying sections indicating that major and relatively wide scale volcanic and sedimentological events occurred at ca 525 ma moreover carbonate carbon isotope chemostratigraphies at morocco siberia three gorges and meishucun display different characteristics during the tommotian one possibility is that the south china ocean was separated from an outer ocean at that time detrital zircons in sandstones have age populations at ca 2 7 1 8 1 6 1 0 and from 0 9 to 0 75 ga indicating that the paleo hinterland of the nanhua and kangdian rift basins was geologically complex despite the lack of ca 1 6 ga rocks in the yangtze and cathaysia blocks these data are nevertheless interpreted to indicate derivation of the zircons from basement rocks in the yangtze craton

contrasting rift and subduction related plagiogranites in the jinshajiang ophiolitic melange southwest china and implications for the paleo tethys 1 the jinshajiang ophiolitic melange zone in southwest china represents a remnant of the eastern paleo tethys ocean field geochronological and geochemical studies have identified two distinct suites of plagiogranites within the melange the dongzhulin trondhjemite and jiyidu tonalite which represent rift and subduction settings respectively related to opening and closing of the ocean shrimp u pb analysis on zircons extracted from the dongzhulin trondhjemite yields a mean 206pb 238u age of 347 7 ma ree and isotopic characteristics suggest an origin from low pressure partial melting of an amphibolitic protolith highly variable hf isotopic compositions for zircons from this body may indicate a heterogenous source involving both depleted mantle and enriched continental components this together with geologic relations suggests formation near an embryonic spreading center in a continent ocean transition setting the jiyidu tonalite has a u pb zircon age of 283 3 ma and geochemical data indicates high sr y la yb n nb ta and low y and marked heavy ree depletion these signatures suggest derivation from low degree partial melting of subducted slab at pressure high enough to stabilize garnet and rutile a slab melt origin is also supported by in situ hf and o data for zircon that show isotopic compositions comparable with typical altered oceanic crust thus the crystallization age of the jiyidu high sr y tonalite provides a constraint for the subduction of the jinshajiang ocean floor the rift related dongzhulin trondhjemite and subduction related jiyidu high sr y tonalite constrain the timing and setting of opening and closing of this segment of the paleo tethys ocean

new seismic reflection results in the central eromanga basin queensland australia the key to understanding its tectonic evolution abstract regional seismic traverses were recorded from 1980 to 1982 by the bureau of mineral resources geology and geophysics as part of a major multidisciplinary program aimed at studying the geological evolution of the central eromanga basin area and its petroleum potential six fold cdp reflection data were obtained to 20 s reflection time on 1400 km of traverses up to 400 km long crossing the main structural features of the area additional seismic reflection information to complement the study was obtained from 2300 km of older mainly single coverage analogue data which was transcribed to digital format and reprocessed the central eromanga basin area has a complex history and contains four phanerozoic potentially hydrocarbon bearing basins each separated by unconformities i e the devonian adavale basin the late carboniferous to triassic cooper and galilee basins and the jurassic cretaceous eromanga basin the seismic results are being interpreted with the aid of synthetic seismograms from key wells to provide information on the extent nature and relationship of these basins the quality of the reflection data from the devonian sequence is generally very good and these data are proving to be most useful in the study thick devonian sediments were deposited over a wide area well beyond the present confines of the adavale basin and its associated troughs intensive folding and faulting accompanied by major basement uplift including the canaway ridge in the centre of the area took place during the mid carboniferous kanimblan orogeny this was followed by a period of erosion which truncated the devonian sequence and resulted in separation of the adavale basin from the other devonian troughs in the area the preliminary seismic results give some indication also of the extent and nature of the cooper and galilee basins sediments these are generally thin in the central area and thicken to the west and east respectively where coal measures produce good lithologic and seismic markers gentle folding and minor faulting apparent in the seismic reflection sections from the overlying eromanga basin sequence have resulted mainly from basement uplift and minor movements along pre existing faults continuing into the late mid tertiary and possibly from some compaction of the thick sedimentary section the seismic information is also assisting in studies of the petroleum potential of the area providing information particularly on structuring timing and depth of burial of the prospective sediments

mount st helens potential example of the partial melting of the subducted lithosphere in a volcanic arc mount st helens 50 km to the west of mount adams and the main cascade volcanic chain is only 80 km above the subducting oceanic lithosphere the elevated temperatures off the subducting slab because of the close proximity of the juan de fuca ridge to the trench may induce slab melting at a depth of 80 km dacites from mount st helens have geochemical compositions off magmas that are derived by direct partial melting of metamorphosed basalts at high pressure i e relatively high ai al2o3 15 at 70 sio2 low y and yb because of garnet and amphibole stability in the source low sc and high sr and eu trace element modeling of the partial melting of mid oceanic ridge basalt morb from the juan de fuca ridge that yields a hornblende eclogite residue can reproduce the mount st helens data results off the model are quite distinct from data derived from the mount adams volcanic rocks in contrast mount adams is 135 km above the subducting slab and is associated with normal arc magmatism believed to be derived from the mantle above the subducting plate the cascade are has been active in its present locality because of oblique subduction for the past 7 m y the major volcanoes along the arc have existed for at least 500 ka but mount st helens has existed for 40 ka we suggest that the subducting plate may have reached elevated temperatures because of the approach of north america to the juan de fuca ridge at 40 ka which initiated melting of the slab

mio pliocene adakite generation related to flat subduction in southern ecuador the quimsacocha volcanic center abstract new geochemical and geochronological data on the miocene pliocene quimsacocha volcanic center qvc have led to the recognition of adakitic lavas generated by slab melting related to the flat slab subduction in southern ecuador and northern peru the qvc located in the presently inactive southern part of the ecuadorian arc was built up during three distinctive volcanic phases the first phase generated a basal edifice with mainly andesitic lava flows while the second phase is characterized by the emplacement of cryptodomes domes and related outflow breccias comprised of andesites and some dacites the last phase released rhyolitic ignimbrites associated with the formation of a large caldera which was later partly filled by dacitic rhyolitic domes geochemical data for the qvc indicate higher al2o3 tio2 na2o zr and sr contents and lower fe2o3 mgo y mree and hree abundances compared to other eruptive rocks of the plio quaternary volcanic front of ecuador such geochemical features as well as the frequent presence of an associated epithermal gold deposit are characteristic of the involvement of slab melts also known as adakites 1 2 in the generation of these magmas after a calc alkaline arc magmatism phase slab horizontalization in response to the subduction of a buoyant oceanic plateau results in increased involvement of a slab melting component in the magmas produced however pristine adakites were generated and emplaced during a relatively short period as indicated by zircon fission track ages then volcanic activity stopped and a volcanic gap formed the identification of these adakites their location and age support a model of slab melting associated with flat slab subduction m a gutscher et al geology 28 2000 535 538

imprints of post glacial climates and palaeogeography in the detrital clay mineral assemblages of an upper permian fluviolacustrine gondwana deposit from northern malawi abstract detailed quantitative clay mineral studies on an upper permian fluviolacustrine gondwana deposit 55 s from northern malawi 9 s 32 e in central southern africa show imprints of late permian climates on the clay mineral assemblages the clay mineral assemblage in the bottom 75 m is characterised by abundant illite chlorite regular smectite chlorite corrensite regular illite smectite and vermiculite zone i between 75 240 m illite and chlorite are the principal clay minerals whereas corrensite and irregular illite smectite mixed layers disappear and vermiculite is rarely present zone ii the assemblage between 240 305 m is similar to that in zone i but is distinguished by the absence of irregular illite smectite and a decrease in corrensite zone iii kerogen fluorescence and chemistry vitrinite reflectance and palynomorph thermal alteration index indicate insignificant thermal diagenesis 50 70 c the absence of depth temperature related variation in mixed layer mineral composition or in illite crystallinity indicate that the original clay mineral composition has not been overprinted by diagenetic modifications sandstone provenance and palaeocurrent directions suggest that detritus throughout deposition was derived from the slowly uplifting nyika plateau basement and sedimentary rocks southwest and west of the basin furthermore compositional and textural immaturity of the sandstones as well as the fresh nature of the feldspars strongly suggest that the detritus was little altered the clay mineral assemblages in the three zones can thus be interpreted as representing the signatures of the climate during the periods of deposition zones i and iii represent warmer and more seasonal climates than zone ii which had generally more humid but less distinct seasons sedimentological features such as the presence of non glacial rhythmites sub millimetric to millimetric laminated mudrocks elevated caco 3 and toc contents in claystones and mudstones in zones i and iii whereas organic lean coarse siltstones and sandstones are common in zone ii the three assemblage zones reflect changes perhaps associated with global late permian climate cycles the quantitative clay mineral composition of the upper permian deposit is similar to those from modern temperate lake sediments from equivalent geographic and latitudinal settings and source rocks in the drainage basins in the northern hemisphere the upper permian clay mineral assemblage contains substantial quantities of corrensite which to date is documented from hypersaline or metamorphosed sedimentary environments we report here its first occurrence from a shallow buried freshwater lacustrine deposit and present evidence for syngenetic aggradation of detrital mixed layers in a magnesium enriched lacustrine depositional environment

vertebrate extinction across permian triassic boundary in karoo basin south africa distinct assemblages of paleosols above and below the permian triassic boundary in the karoo basin of south africa are evidence for reorganization of ecosystems following this greatest of all mass extinctions the permian triassic boundary is recognized from the last appearance of dicynodon and from a series of negative excursions in the isotopic composition of carbon within therapsid tusks pedogenic carbonate nodules and organic matter the boundary is also marked by laminated beds with very weakly developed paleosols a change from purple 10r to brownish red 2 5yr paleosols and a thin 10 cm claystone breccia of reworked soil clasts paleoclimatic changes include a shift from arid and highly seasonal paleoclimate inferred from diffuse and shallow calcareous nodules in permian paleosols to semiarid and less seasonal paleoclimate inferred from deep and well focused calcic horizons in triassic paleosols an earliest triassic shift to warmer and wetter paleoclimate is also indicated by increased chemical weathering abundance of lycopsids and diversity of labyrinthodonts permian paleosols have root traces comparable to those of open shrubland and riparian woodland whereas triassic paleosols have root traces and profile forms like soils of open woodland this is a significant paleoenvironmental change but not as dramatic a change as would be expected from the devastating extinctions of 88 of fossil vertebrate genera latest permian therapsid reptiles were diverse and ecologically specialized in contrast the principal earliest triassic therapsid lystrosaurus was a burrower with no specific habitat preference its short internal nares barrel chest and high neural spines would have given it greater aerobic scope than preexisting therapsids and may have been an advantage under conditions of hypercapnia and hypoxia these adaptations and associated ecosystem changes are compatible with widespread vertebrate mortality by acidosis and pulmonary edema in a post apocalyptic greenhouse created by the voluminous release of methane from shallow marine and permafrost clathrates

late paleozoic tropical climate response to gondwanan deglaciation coupled climate biome model simulations of the late paleozoic were developed to determine the response of pangean tropical climate to gondwanan deglaciation the model simulations predict substantial changes over equatorial pangea including continental drying a reversal of equatorial winds warming heavier 18 o values of meteoric precipitation and the expansion of deserts and the contraction of forests the magnitude of these tropical responses is sensi tive to the extent of gondwana continental ice and the deglacial rise in atmospheric pco 2 boundary conditions that are not well known for the late paleozoic nonetheless the model predictions are consistent with climatic and environmental trends determined from terrestrial proxy data implying that the deglaciation of gondwana was a transformational climate event in tropical pangea

zircon u pb and hf isotopic study of mesozoic felsic rocks from eastern zhejiang south china geochemical contrast between the yangtze and cathaysia blocks abstract the jiangshan shaoxing fault zone jsfz in zhejiang province has been proposed to represent a suture between the yangtze and cathaysia blocks in south china in this study in situ zircon u pb and hf isotopic analysis and whole rock major and trace element measurement of early to middle cretaceous felsic rocks across the fault zone were conducted to constrain the nature of the fault zone twelve cretaceous granitoid bodies were sampled from the nw and se sides of the fault zone respectively with composition ranging from diorite to granite sio2 56 2 76 6 wt these granitoids yielded u pb ages ranging from 135 100 ma with a systematic variation in zircon hf isotopic compositions ehf t 6 9 to 7 0 in the nw side vs 1 9 to 12 9 in the se side the tdm2 values for the granitoids from the nw side are 0 34 to 1 33 ga with two peaks at ca 876 and 1170 ma respectively whereas those from the se side are 0 70 to 1 62 ga with a single peak at ca 1126 ma the hf isotopic disparity for the two sides may indicate a fundamental difference in the lower crustal compositions of the yangtze and cathaysia blocks supporting that the jsfz is possibly a suture zone between the two blocks our results together with the available geological data suggest that the mesoproterozoic materials are important for both the yangtze and cathaysia basement and the neoproterozoic magmatic activities were important in the yangtze block possibly related to the break up of the rodinia supercontinent but less significant in the cathaysia block this may imply that the two blocks have not completely juxtaposed in the neoproterozoic

neoproterozoic to early paleozoic polyorogenic deformation in the southeastern margin of the yangtze block constraints from structural analysis and 40ar 39ar geochronology abstract the southeastern margin of the yangtze block is a suitable region to decipher neoproterozoic to early paleozoic polyorogenic deformation of the south china block based on 1 50 000 scale geological mapping four deformational phases were identified through structural analysis and 40 ar 39 ar dating the earliest event d 1 corresponds to nearly e w striking folding and top to the south thrusting developed during neoproterozoic orogeny caused by the collision between the yangtze block and the cathaysia block the d 2 event is characterized by the nearly e w striking top to the north thrusting the d 3 deformation is marked by the nne striking folding thrusting and dextral shearing the d 3 structures constitute a positive flower pattern with opposing kinematics rooted in the jiangwan dextral shear zone the d 4 event is the nearly e w striking ductile dextral shearing 40 ar 39 ar geochronology shows that the nne trending dextral shearing and the nearly e w striking dextral shearing occurred at 449 4 ma and 429 3 ma respectively combining with previous geochronology all the d 2 d 3 d 4 events are related to the early paleozoic orogeny both the d 2 top to the north thrusts and d 3 nne striking positive flower structures are syn orogenic products these incompatible syn orogenic structures indicate that the orientation of the principal stress changed from the n s striking to e w striking at 450 ma the nearly e w striking ductile dextral shearing d 4 is the outcome of post orogeny

adakitic granites within the ne jiangxi ophiolites south china geochemical and nd isotopic evidence geochemical and nd isotopic data are reported for the ca 970 ma albite granites within the neoproterozoic ne jiangxi ophiolite south china these granites are high in al2o3 15 17 na2o 7 3 8 5 mg 43 62 sr 383 920 ppm sr y 166 444 and y yb 12 14 but low in k2o 0 2 0 6 tio2 0 3 p2o5 0 1 furthermore they display consistently high morb like initial end values of 4 6 to 6 7 at ca 970 ma these geochemcial and isotopic characters are similar to those of the modern adakites compared with the modern adakites the xiwan albite granites are depleted in middle to heavy ree with y yb ratio of 12 14 which can be attributed to residual amphibole and garnet in the source geological and geochemical features suggested that the granites were generated by low degrees of partial melting of subducted spilitized oceanic crust at pressures high enough to stabilize garnet and amphibole these are the first adakitic granites identified within the ne jiangxi ophiolite their presence shows that granitic rocks that intrude ophiolites can be generated not only during the processes of sea floor spreading oceanic crust shearing and obduction of fragmented oceanic lithosphere as previously thought but also during the process of subduction of oceanic lithosphere the shrimp u pb zircon age of 968 23 ma for the xiwan adakitic granite geochimica 23 1994 117 should be re interpreted as the timing of subduction of oceanic crust rather than as the age of oceanic crustal formation as previously interpreted