palaeoclimate perturbations before the sheinwoodian glaciation a trigger for extinctiond during the ireviken event telychian to sheinwoodian conodonts pterospathodus eopennatus ssp n 1 zone through the ozarkodina sagitta rhenana superzone were studied in detail for their oxygen isotope composition the upper kockelella ranuliformis and ozarkodina sagitta rhenana zones represent the peak interval of the sheinwoodian positive 18oapatite excursion reflecting the coolest conditions with marine sea water temperatures decreased by more than 6 c in the subtropics the 18oapatite data cover pre through post ireviken event strata the study includes also material from younger levels up to the late wenlock ctenognathodus murchisoni zone however this record is incomplete with one major hiatus reflecting the regression associated with the middle silurian mulde event the stratigraphic levels of the analyzed conodont samples from estonia can be correlated with high precision to the datum points of the ireviken event strata in the lower and upper visby formations on gotland sweden inferred short term climatic changes in the upper telychian part of the studied interval may have been caused by phases of major volcanic activity along the caledonian front across the ireviken event we observe variations in oxygen isotope ratios which allow a direct comparison to the stepwise extinctions and faunal reorganisations in different groups our data suggest that faunal extinctions are connected to time intervals of warming before the establishment of more stable and cooler conditions during the main sheinwoodian glacial the most severe extinctions and faunal turnovers at datum points 2 and 4 occurred during the early warmer interval of the event after the shift into the sheinwoodian icehouse in the lower kockelella ranuliformis zone reflected by rapidly decreasing sea water temperatures faunas restabilized and reefal communities started to flourish sedimentary evidence for the glaciation is discussed for different palaeolatitudes less

the sacta limestone member early wenlock cool water temperate carbonate deposition at the distal foreland of gondwana s active margin bolivia abstract the sacta limestone member is located towards the base of the silurian kirusillas formation in central bolivia and forms part of the sedimentary record of the peru bolivia palaeozoic basin the early wenlock sheinwoodian age of this thin carbonate unit has been established by conodont and foraminifer biostratigraphy during the silurian a thick siliciclastic wedge was deposited in the foredeep of the basin related to tectonism along the active margin of gondwana strong subsidence and downslope resedimentation slides slumps debris flows turbidites dominated towards the west of the basin during the llandovery and early wenlock in contrast a much thinner record was deposited at the distal margin of the foreland basin pinching out towards the northeast and onlapping the stable precambrian basement the sacta member represents cool water temperate carbonate deposition at mid to high latitude on the distal stable margin of this foreland basin its sedimentation took place towards the end of a phase of important tectonic instability and the carbonate bed was itself also partially resedimented late ashgill llandovery glaciation of nearby reliefs along the active margin of gondwana was followed by brief and local cool water temperate carbonate deposition in the early wenlock and subsequently by the development of a storm and wave dominated shallow clastic shelf during the rest of the silurian the sacta member records carbonate deposition in western gondwana during a brief and singular event with important palaeogeographic and palaeoclimatic implications

origin of the adakite high nb basalt association and its implications for postsubduction magmatism in baja california mexico constraining the origin of the adakite high nb basalt hnb association in baja california mexico is critical to a better understanding of global arc magmatism currently the preferred explanation for the close spatial and temporal association of the two rock suites is through melting of the basaltic portion of the subducted farallon cocos plate thus providing support for the slab melting origin of adakites elsewhere moreover a tectono magmatic model involving the production of both adakite and hnb from slab melts offers a comprehensive explanation for the origin of the atypical arc related postsubduction magmatism in baja california this paper proposes alternative models for the origin of hnb and postsubduction magmatism in baja california wherein the unusual geologic setting of western mexico and westward movement of north america permitted the influx of pacific asthenosphere beneath the adjacent gulf of california after the cessation of subduction unlike the previous tectono magmatic model the new models propose that the asthenosphere provided a direct source for postsubduction tholeiitic and rare alkali magmas that were erupted in baja california as tholeiites and hnb respectively fractional crystallization of some of the hnb magmas plus assimilation of tholeiitic materials produced nb enriched basalts neb the influx of pacific asthenosphere after the cessation of subduction also provided thermal energy to melt the mafic lower baja california crust producing adakite rocks and the preexist ing metasomatized mantle wedge producing bajaites and calc alkaline magmas

tectonic evolution of a composite collision orogen an overview on the qinling tongbai hong an dabie sulu orogenic belt in central china abstract the formation of collisional orogens is a prominent feature in convergent plate margins it is generally a complex process involving multistage tectonism of compression and extension due to continental subduction and collision the paleozoic convergence between the south china block scb and the north china block ncb is associated with a series of tectonic processes such as oceanic subduction terrane accretion and continental collision resulting in the qinling tongbai hong an dabie sulu orogenic belt while the arc continent collision orogeny is significant during the paleozoic in the qinling tongbai hong an orogens of central china the continent continent collision orogeny is prominent during the early mesozoic in the dabie sulu orogens of east central china this article presents an overview of regional geology geochronology and geochemistry for the composite orogenic belt the qinling tongbai hong an orogens exhibit the early paleozoic hp uhp metamorphism the carboniferous hp metamorphism and the paleozoic arc type magmatism but the three tectonothermal events are absent in the dabie sulu orogens the triassic uhp metamorphism is prominent in the dabie sulu orogens but it is absent in the qinling tongbai orogens the hong an orogen records both the hp and uhp metamorphism of triassic age and collided continental margins contain both the juvenile and ancient crustal rocks so do in the qinling and tongbai orogens in contrast only ancient crustal rocks were involved in the uhp metamorphism in the dabie sulu orogenic belt without involvement of the juvenile arc crust on the other hand the deformed and low grade metamorphosed accretionary wedge was developed on the passive continental margin during subduction in the late permian to early triassic along the northern margin of the dabie sulu orogenic belt and it was developed on the passive oceanic margin during subduction in the early paleozoic along the northern margin of the qinling orogen three episodes of arc continent collision are suggested to occur during the paleozoic continental convergence between the scb and ncb the first episode of arc continent collision is caused by northward subduction of the north qinling unit beneath the erlangping unit resulting in uhp metamorphism at ca 480 490 ma and the accretion of the north qinling unit to the ncb the second episode of arc continent collision is caused by northward subduction of the prototethyan oceanic crust beneath an andes type continental arc leading to granulite facies metamorphism at ca 420 430 ma and the accretion of the shangdan arc terrane to the ncb and reworking of the north qinling erlangping and kuanping units the third episode of arc continent collision is caused by northward subduction of the paleotethyan oceanic crust resulting in the hp eclogite facies metamorphism at ca 310 ma in the hong an orogen and low p metamorphism in the qinling tongbai orogens as well as crustal accretion to the ncb the closure of backarc basins is also associated with the arc continent collision processes with the possible cause for granulite facies metamorphism the massive continental subduction of the scb beneath the ncb took place in the triassic with the final continent continent collision and uhp metamorphism at ca 225 240 ma therefore the qinling tongbai hong an dabie sulu orogenic belt records the development of plate tectonics from oceanic subduction and arc type magmatism to arc continent and continent continent collision

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

anatexis of ultrahigh pressure eclogite during exhumation in the north qaidam ultrahigh pressure terrane constraints from petrology zircon u pb dating and geochemistry plagioclase rich leucosomes and k feldspar rich veins are widely distributed within retrogressed eclogite in the xitieshan area of the north qaidam ultrahigh pressure metamorphic terrane western china in this contribution a combined study of petrology zircon internal structure zircon u pb age zircon trace element composition whole rock geochemistry and sr nd isotopes in plagioclase rich leucosomes and k feldspar rich veins provides insight into the nature and timing of partial melting in these rocks petrological evidence for partial melting is provided by elongated highly cuspate feldspars quartz along grain boundaries e g muscovite garnet and clinopyroxene and felsic veinlets composed of quartz and feldspar along the boundaries of garnet clinopyroxene and clinopyroxene plagioclase symplectite in the metabasite whole rock geochemistry suggests that the plagioclase rich leucosomes have lower contents of both rare earth elements rees and high field strength elements hfses but higher large ion lithophile elements liles e g rb ba k sr and pb than the metabasite hosts the plagioclase rich leucosomes may be divided into two subgroups according to their distinct ree patterns 1 higher total ree content with or without weak negative eu anomalies and 2 lower total ree content with conspicuous positive eu anomalies the eu rich group also shows higher sr content than those in the eu poor group however the k feldspar rich veins show higher ree hfse and lile concentrations than the plagioclase rich leucosomes and metabasites the inherited cores of k feldspar rich veins are of typical magmatic origin oscillatory zoning with high th u ratios enriched heavy h rees and negative eu anomalies yielding 206 pb 238 u ages ranging from 904 8 ma to 915 28 ma the zircon cores of plagioclase rich leucosomes and mantles of k feldspar rich veins exhibit no zoning or weak zoning with flat hree patterns and no negative eu anomalies and they contain mineral inclusions of garnet and clinopyroxene implying that these zircon domains were formed during an episode of eclogite facies metamorphism with 206 pb 238 u ages ranging from 444 10 ma to 452 9 ma internal textures and mineral inclusions quartz plagioclase and k feldspar and trace element systematics steep hree patterns with negative eu anomalies and low th u ratios of the zircon rims from plagioclase rich leucosomes and k feldspar rich veins are similar to anatectic zircon these anatectic zircon domains yielded 206 pb 238 u ages ranging from 430 10 ma to 440 12 ma and 406 12 ma to 430 13 ma respectively an integrated study of petrology geochronology and geochemistry demonstrated that the metabasites of the north qaidam ultrahigh pressure terrane likely experienced initial partial melting with plagioclase rich leucosome formation under eclogite facies conditions and partial melt crystallization during the granulite facies stage triggered by dehydration melting involving zoisite and rare muscovite the k feldspar rich veins within the metabasites may be the result of injected melts that were derived from partial melting of nearby gneisses

early silurian glaciation along the western margin of gondwana peru bolivia and northern argentina palaeogeographic and geodynamic setting the record of the ordovician silurian transition within the southern peru bolivia palaeozoic basin extending across peru bolivia and northern argentina is characterized by a diamictite bearing unit which overlies several different ordovician units and underlies mid silurian shales we review the lithostratigraphy sedimentology and biostratigraphy of this unit revising stratigraphic relationships and several ideas and interpretations previously proposed analysis of facies and lithofacies associations allow for a reinterpretation of sedimentary processes and environments involved during its deposition turbidite and shale interbeds within the diamictites indicate a deep marine environment with interbedded mud flows debris flows slumps and large displaced slabs providing evidence for sediment instability and resedimentation chitinozoan biostratigraphy indicates a llandovery age in contrast with previous proposals of a hirnantian age and suggests the need for more detailed sedimentologic and biostratigraphic studies and a reassessment of this diamictite unit in relation to adjacent regions in south america the resedimented character of the deposit explains some of the ordovician fauna previously described which must be considered as recycled from underlying units in situ fauna needs to be reassessed as it may be endemic diachronic and or indicative of migrations within gondwana glacially faceted and striated clasts as well as large granitoid boulders within the resedimented materials provide evidence for glaciation of the source area and are interpreted as recycled from former glacigenic deposits the evidence found in the central andes indicates a glaciomarine origin for the diamictites and corroborates glaciation of a western source area prior to the late llandovery telychian the precise age of glaciation in this part of gondwana cannot be confirmed do to a lack of true tillites tectonic deformation and the resulting relief are respectively identified as the origin for sediment instability and for local glaciation along the active margin of western gondwana during the late ordovician and early silurian the new data imply that the ordovician silurian boundary was not preserved in some areas of the basin and that an erosional hiatus disconformity with variable regional significance is present between different ordovician units and the diamictite unit late ordovician late caradoc early ashgill euxinic black shales locally present in the central altiplano and eastern cordillera of bolivia may represent the youngest ordovician unit in the region and more research should be focused on them paleogeographic reconstruction of the early silurian peru bolivia basin depicts the extension of the diamictite unit southwards into northern argentina and western paraguay connecting with the parana basin through the asuncion arch to the north it continued into northern peru and ecuador connecting with the silurian record in colombia and venezuela and to the northeast into brazil solimoes and amazonas basins the sedimentary record of glaciation in the peru bolivia basin was due to the development of local ice fields to the west of the basin related with tectonism and resulting reliefs along the active margin of gondwana and probably did not coincide with the development of the north african late ashgillian ice cap

a reevaluation of tectonic discrimination diagrams and a new probabilistic approach using large geochemical databases moving beyond binary and ternary plots 1 statistically rigorous confidence intervals calculated from analyses of basaltic volcanic samples obtained from two large geochemical databases petdb and georoc demonstrate that published binary and ternary discrimination diagrams seldom correctly classify samples from mid ocean ridges island arcs and ocean islands with better than 60 accuracy the confidence intervals provide a measure of certainty that was previously absent from these diagrams allowing for a more robust analysis of results a new probabilistic method for geochemical discrimination is developed using the geochemical databases the new method is n dimensional and uses a priori data to construct probability distribution functions from which a posteriori probabilities are generated tests of the new method demonstrate single analysis classification success rates for volcanic rocks from island arcs to be 83 from ocean islands to be 75 and from mid ocean ridges to be 76

early palaeozoic cooling events peri gondwana and beyond the short lived end ordovician hirnantian glaciation allied to marine mass extinction is variously considered as a short lived event or as the peak of long drawn out climatic cooling through at least late ordovician early silurian times evidence from early palaeozoic facies faunas and stable isotope excursions used to interpret climatic cooling events ranges farther from late mid cambrian to late silurian times glacigenic sediments structures and geomorphology provide direct evidence of glacial episodes cool water carbonate deposition which is particularly widespread during the late ordovician boda event in high latitude peri gondwana gondwana and beyond into mid low palaeolatitudes is interpreted as indicating global cooling not warming as has been proposed such carbonates also characterize mid latitude continents widely at horizons earlier in the ordovician and more locally in the mid silurian in high latitude gondwana cool water carbonate mounds have distinctive facies controlled mound faunas across palaeocontinents other facies evidence for palaeoclimates includes black shale deposition including deglacial organic rich hot shales which indicate transgression in epeiric seas and sea level curves interpreted from facies and faunal successions correlation is shown between facies evidence and positive c isotope excursions from which cyclicities are apparent the possible interface of orbitally controlled rhythms is considered against evolving palaeobiogeography and changes in global sea level and in pco2 facies and faunal evidence from peri gondwanan terranes armorica central europe alborz is assessed with that from gondwana mostly north africa south america and correlatives in avalonia baltica and laurentia to establish a wider picture of early palaeozoic cooling events

first record and timing of uhp metamorphism from zircon in the xitieshan terrane implications for the evolution of the entire north qaidam metamorphic belt the xitieshan terrane is one of four metamorphic terranes in the north qaidam metamorphic belt which is an early paleozoic high pressure to ultrahigh pressure hp uhp metamorphic belt in nw china however conclusive evidence and precise timing of uhp metamorphism in the xitieshan terrane have not been well documented in this study we report an integrated study of zircon grains from an amphibolite in the xitieshan terrane in terms of mineral inclusions trace elements and u pb age systematics the first record of coesite is reported as an inclusion in a metamorphic zircon which provides unambiguous evidence for the uhp metamorphism of the xitieshan terrane the metamorphic zircon domains have weak or no zoning very low th u ratios insignificant eu anomalies and flat hree patterns zircon grains contain inclusions of garnet omphacite and rutile in addition to the coesite inclusion these inclusions indicate that the metamorphic zircon grains formed under uhp metamorphic conditions the metamorphic zircon grains were dated by the sims and la icpms methods and yield weighted mean ages of 432 14 and 441 9 ma respectively combined with previous results the hp uhp metamorphism of the xitieshan terrane may have lasted 460 440 ma with the peak uhp metamorphism at 441 9 ma a compilation of the reported geochronological data reveals that all four terranes of the north qaidam metamorphic belt might have experienced coeval uhp metamorphism during the early paleozoic 420 450 ma and thus may have suffered a coherent subduction uhp metamorphism and exhumation cycle

multistage anatexis during tectonic evolution from oceanic subduction to continental collision a review of the north qaidam uhp belt nw china abstract the north qaidam ultra high pressure uhp metamorphic belt in the northern tibetan plateau is considered as a typical alpine type uhp metamorphic belt due to the early paleozoic subduction of the qaidam block beneath the qilian block the well preserved paleozoic metatexite migmatite diatexite migmatite felsic sheet dyke and anatectic granite in the north qaidam uhp metamorphic belt provide us with an excellent opportunity to study the generation transport and final fate of crustal melt and geodynamic processes associated with orogenesis based on structural relationships petrology geochronology whole rock geochemistry and sr nd isotope data the early paleozoic anatexis during oceanic subduction to continental collision in the north qaidam uhp metamorphic belt can be further divided into three stages of development 470 ma 446 428 ma and 432 420 ma the first stage granitic leucosomes are rich in k poor in na with low sr y and enrichment of large ion lithophile elements liles which were probably derived from partial melting of ancient felsic gneiss in continental arc circumstances during oceanic subduction the second stage na rich leucosomes and tonalite plutons are characterized by high na sr sr y and la yb and low heavy rare earth elements hrees with positive end t values of 0 1 4 3 and zircon ehf t values of 8 3 10 2 similar to typical tonalite trondhjemite granodiorites ttgs these ttg like magmas were produced through partial melting of newly emplaced gabbroic rocks with arc affinity under high pressure hp granulite facies conditions in a thickened lower crust during continental collision the volumetrically significant migrated plutons evolved from ttg like melt will become segments of continental crust and contribute to crustal growth with partial residual products of hp granulite and or garnet pyroxenite to the mantle by delamination the third stage of anatexis was preserved in both migmatitic uhp gneiss and eclogite in the xitieshan and luliangshan terranes the kfs rich felsic leucosomes inside uhp gneisses in the third stage are characterized by high alkali contents and low mafic component with feot mgo tio2 2 0 2 high la yb and sr y and low y and hrees 3 high al2o3 and low mg values and 4 enriched in liles e g rb ba k sr pb and poor in high field strength elements hfses partial melting of uhp eclogite and felsic gneiss during the initial retrogression stage with pl rich and kfs rich leucosome formation was triggered by dehydration melting involving predominant zoisite and muscovite the anatectic melts from partial melting of deeply subducted uhp gneiss have accumulated and migrated outside the deeply subducted crustal slice in the form of syn collisional plutons the melt evolution from the leucosomes produced at the early exhumation stage to syn collisional granitoids produced at the late exhumation might contribute greatly to the exhumation of the north qaidam uhp metamorphic belt from mantle depths to the lower crustal levels

geochemistry and zircon u pb ages of adakitic rocks from the dulan area of the north qaidam uhp terrane north tibet constraints on the timing and nature of regional tectonothermal events associated with collisional orogeny abstract geochemistry and u pb ages of leucosomes and tonalites in a high pressure granulite unit of the dulan area have been determined to constrain the tectonothermal evolution related to collision and thickening of lower crust in the north qaidam mountains nqd leucosomes and tonalites show a marked chemical resemblance to adakites 1 high la yb and sr y and low y and hree 2 high al 2 o 3 and low mg values with obvious positive eu anomalies and 3 slightly positive e nd t values zircon u pb analysis of leucosomes and tonalites yielded 206 pb 238 u ages of 428 437 ma and 436 437 ma respectively which constrain the emplacement ages of the adakitic rocks petrological geochronological and geochemical characters indicate that the adakitic rocks may have been derived from partial melting of a thickened mafic lower crust 50 km suggesting a dominating source regime of synchronous high pressure mafic granulites contemporary magmatism in other units of the nqd shows evidence of a widespread tectonothermal event during early silurian 420 450 ma that includes metamorphism magmatism and anatexis related to collision and thickening of lower crust

ultra high pressure metamorphism and orogenesis an alpine perspective article i nfo the pressure temperature time deformation evolution of the high to ultra high pressure units of the western alps has been progressively refined in the last 40 years leading to several paradigm shifts in the understanding of orogenesis this set of information combined with cretaceous tertiary plate kinematic reconstructions and mesozoic palaeogeography indicates that the western alps represent the amalgamation of a cretaceous and an eocene orogen which developed at the expense of the adriatic and european rifted margins respectively in the cretaceous nne directed drift of the adriatic plate parallel to the inherited jurassic structural trends led to the development of a highly oblique subduction boundary in this setting the canavese zone and parts of the sesia zone derived from the hyper extended adriatic margin underwent deformation and metamorphism at conditions ranging from sub greenschist to eclogite facies a lower eocene switch to nnw directed motion perpendicular to the rift related jurassic structural trends culminated in the collision between the proto alps and the brianconnais block at ca 44 ma as a result the brianconnais domain and parts of its hyper extended margin preserved in the u hp piemonte units were accreted to the orogen continued convergence was subsequently accommodated by subduction of the more external valaisan basin and of the thinned european margin the arrival of thick european continental crust at the subduction zone at ca 35 30 ma marked the onset of the final continental collision to a first order the orogen grew through the progressive episodic accretion of units located towards north westerly positions accretion of subducting units at the front of the orogen was coeval with kinematic reworking of tectonic contacts in the hangingwall units locally resulting in renewed deformation metamorphism

the san nicolas batholith of coastal peru early palaeozoic continental arc or continental rift magmatism major and trace element u pb zircon geochronology and pb isotopic data are presented on plutons from the san nicolas batholith of central and southern peru the major oxides of 9 samples indicate a high k calc alkaline trend and trace element discriminant diagrams reveal affinities to volcanic arc or late to post collision calc alkaline intrusions the apparent contradiction implied by the rb hf ta triangular diagram within plate strongly suggests loss of rb an interpretation supported by the extensive deuteric alteration and sericitization of feldspar ta zr ta hf and ta nb ratios are considerably higher than those of volcanic arc intrusions and favour the late or post collision calc alkaline classification u pb zircon geochronology shows that the san nicolas batholith was emplaced in two short lived magmatic events at approximately 425 ma and 394 388 ma ago the presence of an inherited component of radiogenic pb of precambrian age in the zircons suggests xenocrystic cores which were most likely scavenged from precambrian basement rocks of the arequipa massif time adjusted pb isotopic compositions of feldspars have the following ranges 206 pb 204 pb 17 798 18 272 207 pb 204 pb 15 571 15 626 and 208 pb 204 pb 38 182 38 291 indicating that precambrian basement was incorporated into the magmas of the batholith other components are believed to have originated in an enriched upper mantle or lower crustal wedge pervasively metasomatized by fluids extracted largely from subducted sediments u pb zircon age distribution trace element characteristics and the linear nature of the plutons firmly establish the san nicolas batholith as the plutonic substructure of an early palaeozoic continental arc which may extend northwards in peru and southwards

the tectonic transition from oceanic subduction to continental subduction zirconological constraints from two types of eclogites in the north qaidam orogen northern tibet abstract in the plate tectonics theory continental subduction is pulled by subduction of dense oceanic crust in practice however it is not easy to demonstrate that preceding oceanic crust exposes as oceanic type eclogite together with continental type eclogite in collisional orogens the north qaidam orogen in northern tibet is an ultrahigh pressure uhp metamorphic belt that contains the two types of eclogites providing us with an excellent opportunity to study the tectonic transition from oceanic subduction to continental subduction in order to constrain the protolith nature and metamorphic evolution of eclogites we performed a combined study of zircon u pb ages trace elements mineral inclusions and o hf isotopes for various eclogites from the orogen we discriminate the two types of eclogites by their differences in zircon u pb ages and o hf isotopes cl dark zircon domains exhibit high th u ratios steep hree patterns and significantly negative eu anomalies indicating that they are protolith zircons of magmatic origin with different extents of metamorphic recrystallization relict magmatic zircon domains in type i eclogites yield neoproterozoic protolith ages of 830 ma and hf model ages of 850 1100 ma whereas those in type ii eclogites yield cambrian protolith u pb ages of 489 ma and hf model ages of 500 650 ma most of the cl bright zircon domains show low th u ratios flat hree patterns and no negative eu anomalies and contain mineral inclusions of garnet omphacite and rutile indicating their growth under eclogite facies metamorphic conditions these metamorphic domains have consistent eclogite facies metamorphic ages of 433 440 ma throughout the north qaidam orogen regardless of the eclogite types and locations the metamorphic zircon domains in type i eclogites mostly exhibit 18o values higher than normal mantle values whereas type ii eclogites mostly have 18o values lower than the normal mantle values the difference in the 18o values indicates that their protoliths underwent different temperatures of hydrothermal alteration at different tectonic settings combining zircon u pb ages and o hf isotope compositions with local tectonics it is inferred that type i eclogites were metamorphosed from neoproterozoic continental mafic rocks whereas type ii eclogites were metamorphosed from oceanic mafic rocks that were subducted prior to the continental subduction the consistent eclogite facies metamorphic ages for the two types of eclogites indicate that the exhumed oceanic type eclogite was detached from the subducted oceanic crust and then entrained by the exhuming continental crust therefore the coexistence of oceanic and continental type eclogites in the north qaidam orogen demonstrates the tectonic transition from oceanic subduction to continental collision in the early paleozoic

late miocene adakites and nb enriched basalts from vizcaino peninsula mexico indicators of east pacific rise subduction below southern baja california a typical slab melt association was emplaced from 11 to 8 ma in the santa clara volcanic field vizcaino peninsula baja california sur it includes adakitic domes and associated pyroclastic flow deposits together with lava flows of niobium enriched basalts the trace element and isotopic sr nd pb signatures of adakites are consistent with melting of altered mid ocean ridge basalts and the sources of the nb enriched basalts contain an enriched mantle wedge component such associations commonly form at depths of 70 80 km during low dip subduction of very young oceanic crust however the santa clara field is relatively close 100 km to the paleotrench which suggests that the genesis of its adakites and nb enriched basalts occurred in a very high thermal regime linked to the subduction of the then active guadalupe spreading center of the east pacific rise our data suggest that the asthenospheric window documented below northern baja california also developed beneath the south of the peninsula during the neogene this hypothesis is consistent with the spatial distribution and the ages of adakites and magnesian andesites from this region

age and nature of eclogites in the huwan shear zone and the multi stage evolution of the qinling dabie sulu orogen central china abstract in situ la icpms u pb trace element and hf isotope data in zircon demonstrate a carboniferous age for eclogite facies metamorphism in siluro devonian protoliths in the huwan shear zone dabie mountains central china this age contrasts with the more prevailing triassic age for high to ultrahigh pressure hp to uhp metamorphism in the qinling dabie sulu orogen metamorphic zircon in two eclogite samples from sujiahe is characterized by low th u ratios small negative eu anomalies flat hree patterns and low 176 lu 177 hf ratios these geochemical signatures suggest that the zircon crystallized in the presence of garnet and in the absence of plagioclase feldspar furthermore temperatures of 655 and 638 c calculated using the ti content of zircon are consistent with their formation during eclogite facies metamorphism the weighted mean 206 pb 238 u age of 309 4 ma 2 for this zircon improves previous age estimates for eclogite facies metamorphism in the huwan shear zone ranging from 420 to 220 ma metamorphic zircon from one eclogite sample from hujiawan most likely formed during prograde metamorphism yields an equivalent age estimate of 312 11 ma magmatic zircon cores in the three samples yield ages for the magmatic protoliths of the eclogites ranging from 420 7 to 406 5 ma and post dating the middle paleozoic collision of the north china and the qinling terrain the zircon crystals in the three eclogite samples display a large variation of e hf t values of 4 9 to 21 3 the metamorphic zircon overgrowths show the same range of e hf t values as those of the inherited magmatic crystal interiors this suggests that the metamorphic zircon overgrowths may have formed by dissolution reprecipitation of pre existing magmatic zircon thereby preserving their original hf isotopic composition the high e hf t values suggest that the protoliths were derived from depleted mantle sources most likely paleotethyan oceanic crust while the low e hf t values are attributed to crustal contamination some eclogites in the huwan shear zone have a distinctive signature of continental crust most probably derived from the yangtze craton the coexistence of paleozoic oceanic crust and neoproterozoic continental crust with similar metamorphic ages in the huwan shear zone implies that paleozoic paleotethyan oceanic crust was produced within a marginal basin of the northern yangtze craton the opening of the paleo tethyan ocean was slightly younger than the collision of the north china craton and the qinling terrain during the late paleozoic in the qinling dabie sulu orogen subduction of the paleo tethyan oceanic crust and associated continental basement resulted in the 309 2 ma 2 eclogite facies metamorphism in the huwan shear zone the subsequent triassic continent continent collision led to the final coalescence of the yangtze and sino korean cratons amalgamation of the yangtze and north china cratons was therefore a multistage process extending over at least 200 ma

mantle and slab contributions in arc magmas destructive plate margins are major sites of terrestrial magmatism that have long had a key role in models for the generation of continental crust and the development of chemical heterogeneities in the upper mantle in these models it is important to evaluate the nature and size of contributions from preexisting crust in the generation of magmas above subduction zones and to constrain the size of the fluxes of different elements from the subducted crust and the overlying mantle wedge in arc magmas kay 1 980 hawkesworth ellam 1 989 hawkesworth et al 1 99 1 a mcculloch gamble 1 99 1 the more mobile components of the subducted oceanic crust are the products of element fractionation processes in the upper continental crust the ocean basins and hydrothermal systems on the ocean floor they therefore have distinctive isotope and even trace element ratios that in principle should be readily distinguished from those in peridotite in the mantle wedge prior to subduction in practice however element ratios are almost certainly fractionated during shallow level dehydration of sub ducted material and at greater depths where material from the slab appears to contribute in the generation of arc magmas moreover the isotope signal of slab derived material may be heavily diluted by exchange with

sr nd os s isotope and pge geochemistry of the xiarihamu magmatic sulfide deposit in the qinghai tibet plateau china the newly discovered xiarihamu ni cu deposit is located in the eastern kunlun orogenic belt in the northern part of the qinghai tibet plateau western china it is the largest magmatic ni cu sulfide deposit found thus far in an arc setting worldwide and ranks second in china in terms of total ni resources fe ni cu sulfide mineralization occurs in a small ultramafic body that is part of a larger mafic ultramafic complex formed by protracted silurian early devonian basaltic magmatism the mineralized ultramafic body is composed predominantly of lherzolite and olivine websterite with minor dunite websterite and orthopyroxenite here we report new pge platinum group element data and the results of a new integrated sr nd os s isotope study the initial concentrations of rh and pd in the parental magma are estimated to be 0 014 ppb and 0 24 ppb respectively which are more than one order of magnitude lower than those in undepleted mantle derived magmas such as many continental picrites the observed pge depletions in the xiarihamu parental magma are attributed to sulfide retention in the source mantle because the degree of partial melting required to generate the xiarihamu primary magma was not high enough for a magma of that composition to dissolve all sulfides in the source the 87sr 86sr i ratios and end t of the xiarihamu host rocks range from 0 7062 to 0 7105 and from 1 97 to 5 74 respectively indicating 5 30 wt crustal contamination in the xiarihamu magma these data also reveal that the source mantle for the xiarihamu magma is isotopically sr nd more enriched than that for the average cenozoic arc basalt the os t and 34s values of sulfide ores from the xiarihamu deposit range from 78 to 1393 and from 2 to 6 respectively these values clearly indicate addition of crustal os and s to the xiarihamu parental magma metal tenors such as ni and rh are inversely correlated with os t and 34s values this indicates that mixing between different pulses of magma with different isotope compositions took place during magma emplacement at xiarihamu and that external s played a critical role in the formation of the xiarihamu magmatic ni cu sulfide deposit

metamorphic evolution of the coesite bearing ultrahigh pressure terrane in the north qaidam northern tibet nw china widespread evidence for ultrahigh pressure uhp metamorphism is reported in the dulan eclogite bearing terrane the north qaidam altun hp uhp belt northern tibet this includes 1 coesite and associated uhp mineral inclusions in zircon separates from paragneiss and eclogite identified by laser raman spectroscopy 2 inclusions of quartz pseudomorphs after coesite and polycrystalline k feldspar quartz in eclogitic garnet and omphacite and 3 densely oriented sio2 lamellae in omphacitic clinopyroxene these lines of evidence demonstrate that the dulan region is a uhp metamorphic terrane in the north dulan belt ndb eclogites are characterized by the peak assemblage grt omp rt phn coe pseudomorph and retrograde symplectites of cpx ab and hbl pl the peak conditions of the ndb eclogites are p 2 9 3 2 gpa and t 631 687 c the eclogite shows a near isothermal decompression p t path suggesting a fast exhumation in the south dulan belt sdb three metamorphic stages are recognized in eclogites 1 a peak eclogite facies stage with the assemblage grt omp ky rt phn at p 2 9 3 3 gpa and t 729 746 c 2 a high pressure granulite facies stage with grt cpx jd 30 pl an24 29 scp at p 1 9 2 0 gpa t 873 948 c and 3 an amphibolite facies stage with the assemblage hbl pl ep czo at p 0 7 0 9 gpa and t 660 695 c the clockwise p t path of the sdb eclogites is different from the near isothermal decompression p t path from the ndb eclogites which suggests that the sdb was exhumed to a stable crustal depth at a slower rate in essence these two sub belts formed in different tectonic settings they both subducted to mantle depths of around 100 km but were exhumed to the earth s surface separately along different paths this uhp terrane plays an important role in understanding continental collision in north western china

eclogite facies metamorphism in impure marble from north qaidam orogenic belt geodynamic implications for early paleozoic continental arc collision abstract in the north qaidam ultrahigh pressure uhp metamorphic belt impure marble and interbedded eclogite represent a particular sedimentary provenance and tectonic setting which have important implications for a controversial problem the dynamic evolution of early paleozoic subduction collision complexes in this contribution detailed field work mineral chemistry and whole rock geochemistry are presented for impure marble to provide the first direct evidence for the recycling of carbonate sediments under ultrahigh pressures during subduction and collision in the yuka terrane in the north qaidam uhp metamorphic belt according to conventional geothermobarometry pre peak subduction to 0 8 1 3 gpa 485 569 c was followed by peak uhp metamorphism at 2 5 3 3 gpa 567 754 c and cooling to amphibolite facies conditions at 0 6 0 7 gpa 571 589 c u pb dating of zircons from impure marble reveals a large group with ages ranging from 441 to 458 ma peak at 450 ma a smaller group ranging from 770 to 1000 ma peak at 780 ma and minor 1 8 ga zircon aged ca 430 ma uhp metamorphism the youngest detrital zircons suggest a maximum depositional age of ca 442 ma and a burial rate of ca 1 0 1 1 cm yr when combined with p t conditions and uhp metamorphic age the ree and trace element patterns of impure marble with positive sr and u anomalies negative high field strength elements nb ta zr hf and ti and ce anomalies imply that the marble had a marine limestone precursor impure marble intercalated with micaschist and eclogite was similar to limestone and siltstone protoliths deposited in continental fore arc or arc setting with basic volcanic activity therefore the yuka terrane most likely evolved in a continental island arc setting during the paleozoic these data suggest that metasediments were derived from a mixture of proterozoic continental crust and juvenile early paleozoic oceanic and or island arc crust in addition their protoliths were likely deposited in a terrigenous dominated forearc marine basin rather than an intracontinental basin environment further evidence that some continental arc volcanic rock may have been the source of eclogite in the north qaidam these sediments formed in a forearc basin close to the qaidam block to the north were transported in the subduction zone to 100 110 km depth with uhp metamorphism prior to exhumation meanwhile the new results suggest that subduction erosion occurred along the active continental margin during the qaidam block with north dipping subduction indicating that the north qaidam uhp metamorphic belt may have formed during continental arc collision

geochemistry and trace element behaviors of eclogite during its exhumation in the xitieshan terrane north qaidam uhp belt nw china abstract the xitieshan terrane located in the central part of the north qaidam ultrahigh pressure uhp metamorphic belt china is mainly composed of orthogneiss and paragneiss and a few intercalated eclogite layers and boudins based on their bulk rock tio 2 contents the eclogites can be subdivided into a high ti group tio 2 2 and a low ti group tio 2 e nd t and restricted i sr values and therefore provide further evidence for the formation of the protoliths of the eclogites in an oceanic environment on the other hand the sr nd isotopes of high ti eclogites show mainly positive but also some negative e nd t values and relatively broadly distributed i sr values indicating minor crustal contamination of the ocean floor basalts considering available 750 877 ma protolith ages preserved in zircon cores it is inferred that some of the eclogites derived from neoproterozoic protoliths were emplaced onto the crust far ahead of the paleozoic deep subduction while the other eclogites originate from a different oceanic crust e g the paleo qilian ocean indicating multiple orogenies in the geological history of the xitieshan terrane china whole rock and in situ la icp ms mineral trace element analyses of eclogites revealed two stages of fluid behavior during retrogression that correspond to the two exhumation stages uncovered by phase equilibrium calculations the mineral scale trace element distributions and trace element inheritance of newly formed amphibole from its precursors indicate that at the peak metamorphic stage m1 and at the earlier eclogite facies overprint m2 the fluid was internally controlled by the rock itself within a mafic lens the amount of water soluble elements e g rb sr ba u pb and lree observed in the whole rock compositions as well as in amphiboles increases from the core phengite bearing eclogite to the rim amphibolite and implies an external fluid source for the amphibolite facies retrogression m3 which should be helpful for the final exhumation of uhp eclogite

from magmatic generation to uhp metamorphic overprint and subsequent exhumation a rapid cycle of plate movement recorded by the supra subduction zone ophiolite from the north qaidam orogen abstract the rapid subduction and exhumation of high pressure hp and ultrahigh pressure uhp rocks exposed in collisional belts play a critical role in understanding the evolution of geodynamics and plate tectonics in subduction zone environments herein ophiolitic fragments eclogites and amphibolites from a subduction channel exposed in the dulan terrane north qaidam orogen were investigated garnet compositional zoning from the studied eclogites records only a single cycle of subduction and exhumation new uranium lead u pb zircon ages of the current study combined with previous rutile u pb ages from eclogites indicate a rapid cycle of events starting with the generation of the magmatic protolith at ca 446 ma its subsequent subduction reaching a depth of ca 100 110 km at ca 435 ma and its exhumation into shallow portions of the crust which occurred at ca 416 ma thus a rapid 30 million years myr protolith generation subduction exhumation cycle could be recorded according to an ophiolitic rock association with island arc basalt iab like and mid ocean ridge basalt morb to ocean island basalt oib like trace element compositions the slab derived components determined by sr nd hf isotopes and the short time span of only about 11 myr between photolith generation and uhp metamorphism in eclogites the protolith of eclogite is best explained as a supra subduction zone ophiolite this finding likely records the closure of the south qilian ocean and the initiation of a continent arc collision at ca 446 ma furthermore starting with subduction erosion during continent arc collision in the early palaeozoic ophiolitic and arc related materials become subducted in the north qaidam it was inferred that such a rapid succession of events from protolith formation to uhp overprint and final exhumation could be explained by movements of rock material in a subduction channel thus this study provides a new view in understanding the mechanics of different episodes of subduction systems in north qaidam uhp metamorphic belt

polygenetic history of paleosols in middle upper pennsylvanian cyclothems of the illinois basin u s a part i characterization of paleosol types and interpretation of pedogenic processes abstract the morphology mineralogy and stratigraphic and lateral distribution of paleosols in pennsylvanian moscovian gzhelian atokan virgilian mixed marine and terrestrial coal bearing strata i e cyclothems in the illinois basin ib are presented in order to determine the dominant pedogenic processes within a cyclic sedimentary depositional framework paleosol morphologies range from weakly developed horizons with evidence of rooting and little to no pedogenic structure to horizons with well developed pedogenic slickensides angular blocky structure and carbonate nodules and tubules the majority of paleosols in strata from the basin interior preserve 1 low chroma gley colors 2 identifiable fossil plant organic matter that increases in abundance upward in the profiles toward the interpreted paleo surface and 3 calcite sphaerosiderite and pyrite cements and nodules in contrast paleosols in strata from the northern margin of the basin display high chroma colors and calcite cements and less fossil organic matter sphaerosiderite and pyrite cements these observations indicate that paleosols from the basin interior underwent a multi phase pedogenic and shallow burial history characterized by an initial better drained oxygenated stage under seasonal precipitation followed by a poorly drained reducing stage while paleosols from the northern margin of the basin experienced overall more oxygenated conditions throughout their pedogenic development x ray diffraction analysis of the 2 m size fraction of paleosol matrix in ib paleosols indicates a mineralogical composition dominated by kaolinite randomly interstratified r0 and ordered r1 illite smectite i s and subordinate amounts of chlorite these combined observational and mineralogical datasets indicate that significant changes in paleohydrology were largely responsible for the development of pennsylvanian soil profiles in the ib

magmatic relationships and ages between adakites magnesian andesites and nb enriched basalt andesites from hispaniola record of a major change in the caribbean island arc magma sources abstract located in the cordillera central of the dominican republic the late cretaceous tireo fm tf records a major change of the magma sources in the caribbean island arc it comprises a 3 km thick sequence of arc related volcanic and volcano sedimentary rocks with variable geochemical characteristics combined detailed mapping stratigraphy geochemistry and u pb ar ar geochronology show that the volcanic rocks of the tireo fm include two main volcanic sequences the lower volcanic sequence is dominated by monotonous submarine vitric lithic tuffs and volcanic breccias of andesite to basaltic andesite with minor interbedded flows of basalts and andesites fossil and u pb and 40ar 39ar geochronological data show that arc magmatism in the lower sequence began to accumulate before 90 ma from the aptian to turonian these rocks constitute an island arc tholeiitic suite derived from melting by fluxing of a mantle wedge with subduction related hydrous fluids the upper volcanic sequence is characterized by a spatial and temporal association of adakites high mg andesites and nb enriched basalts which collectivelly define a shift in the composition of the subduction related erupted lavas a dacitic to rhyolitic explosive volcanism with subaerial and episodic aerial eruptions and sub volcanic emplacements of domes characterize mainly this stratigraphic interval the onset of this volcanism took place at turonian coniacian boundary and continued in the santonian to lower campanian with minor events in the late campanian adakites represent melts of the subducting slab magnesian andesites the product of hybridization of adakite liquids with mantle peridotite and nb enriched basalts melts of the residue from hybridization we propose a model of oblique ridge subduction at 90 ma and possibly subsequent slab window formation as principal cause of magmatic variations recorded in the caribbean island arc above a southwestern dipping subduction zone

paleozoic terranes of eastern australia and the drift history of gondwana abstract critical assessment of paleozoic paleomagnetic results from australia shows that paleopoles from locations on the main craton and in the various terranes of the tasman fold belt of eastern australia follow the same path since 400 ma for the lachlan and thomson superterranes but not until 250 ma or younger for the new england superterrane most of the paleopoles from the tasman fold belt are derived from the lolworth ravenswood terrane of the thomson superterrane and the molong monaro terrane of the lachlan superterrane consideration of the paleomagnetic data and geological constraints suggests that these terranes were amalgamated with cratonic australia by the late early devonian the lolworth ravenswood terrane is interpreted to have undergone a 90 clockwise rotation between 425 and 380 ma although the tamworth terrane of the western new england superterrane is thought to have amalgamated with the lachlan superterrane by the late carboniferous geological syntheses suggest that movements between these regions may have persisted until the middle triassic this view is supported by the available paleomagnetic data with these constraints an apparent polar wander path for gondwana during the paleozoic has been constructed after review of the gondwana paleomagnetic data the drift history of gondwana with respect to laurentia and baltica during the paleozoic is shown in a series of paleogeographic maps

early paleozoic polyphase metamorphism in northern tibet china abstract the altun qilian kunlun orogenic system in the northern tibetan plateau is considered as the northernmost orogenic collage of the proto tethyan domain early paleozoic polyphase tectonothermal events related to the subduction of the proto tethyan ocean and subsequent collisional orogeny produced two dominant metamorphic belts the north altun nat north qilian nql hp lt metamorphic belt and south altun sat north qaidam nqd uhp metamorphic belt the nat nql hp lt metamorphic belt mainly consists of blueschist eclogite and hp metasediments eclogites record metamorphic conditions of 420 550 c and 21 25 kbar at 510 460 ma the hp lt metamorphic belt is associated with ophiolite subduction accretion complex and arc magmatic rocks suggesting that the nat nql is a typical early paleozoic accretionary orogenic belt in contrast the sat nqd uhp metamorphic belt is characterized by eclogite and garnet peridotite enclosed within continental orthogneiss and paragneiss these rocks record uhp metamorphism at t 700 c and p 27 kbar geochronology suggests that the uhp metamorphism occurred at 500 423 ma and eclogite protolith ages of 850 750 ma suggest that protoliths are related to neoproterozic continental rifting field relationships petrology and geochronology suggest that the sat nqd uhp metamorphic belt resulted from the deep subduction of continental crust moreover in the sat nqd uhp metamorphic belt two types of hp granulites have been recognized the first type of hp granulite was formed by overprinting of eclogites during exhumation the second type of hp granulite has a single metamorphic history and similar age to that of uhp eclogite penecontemporaneous metamorphic ages but different geothermal gradients between hp granulites and related uhp eclogite define a possible paired metamorphic belt which may be considered to be a diagnostic characteristic of collisional orogens high grade metamorphic rocks in the sat nqd commonly record barrovian type metamorphism although it remains unclear whether or not all of these rocks have an earlier u hp metamorphic history the low medium p barrovian metamorphism and associated magmatism are about 20 50 ma later than uhp metamorphism newly recognized eclogites in the northern part of the east kunlun mountains to the south of qaidam basin have a similar field relationship enclosed in continental gneiss and mineral assemblage to the nqd eclogites and a similar or slightly younger metamorphic age 410 430 ma the occurrence of similar continental type eclogites on two sides of the qaidam basin makes it necessary to reconsider the nature of the qaidam block the polyphase metamorphism in northern tibet can be linked to accretion and collisional orogenesis related to the evolution of prototethys through the early paleozoic

origin of high field strength element enrichment in the sulu arc southern philippines revisited the enrichment of high field strength elements hfse in sulu arc lavas has been proposed as a product of metasomatism of the mantle wedge it is postulated that a dacitic melt derived from melting of subducted sulu sea basaltic crust stabilizes in the mantle wedge amphibole which later breaks down and releases hfses into the source of basaltic arc lavas new data for primitive high k calc alkalic basalts that contain the highest hfses among sulu arc lavas and seafloor basalts subducting along the sulu trench have contrasting chemical and isotopic characteristics this makes it unlikely that the source of hfse enrichment in sulu arc lavas is melt derived from the subducted sulu sea basaltic crust or amphibole formed during metasomatism of the mantle wedge by such melt we propose that hfse enrichment in sulu arc lavas results from melting of a geochemically enriched component in the mantle wedge

the multi stage tectonic evolution of the xitieshan terrane north qaidam orogen western china from grenville age orogeny to early paleozoic ultrahigh pressure metamorphism abstract the geodynamic evolution of the early paleozoic ultrahigh pressure metamorphic belt in north qaidam western china is controversial due to ambiguous interpretations concerning the nature and ages of the eclogitic protoliths within this framework we present new la icp ms u pb zircon ages from eclogites and their country rock gneisses from the xitieshan terrane located in the central part of the north qaidam uhp metamorphic belt xitieshan terrane contains clearly different protolith characteristics of eclogites and as such provides a natural laboratory to investigate the geodynamic evolution of the north qaidam uhp metamorphic terrane la icp ms u pb zircon dating of three phengite bearing eclogites and two country rock gneiss samples from the xitieshan terrane yielded 424 427 ma and 917 920 ma ages respectively the age of 424 427 ma from eclogite probably reflects continental lithosphere subduction post dating oceanic lithosphere subduction at 440 460 ma the 0 91 0 92 ga metamorphic ages from gneiss and associated metamorphic mineral assemblages are interpreted as evidence for the occurrence of a grenville age orogeny in the north qaidam uhpm belt using internal microstructure geochemistry and u pb ages of zircon in this study combined with the petrological and geochemical investigations on the eclogites of previous literature s data three types of eclogitic protoliths are identified in the xitieshan terrane i e 1 subducted early paleozoic oceanic crust 440 460 ma 2 neoproterozoic oceanic crust material emplaced onto micro continental fragments ahead of the main early paleozoic collision event 440 420 ma and 3 neoproterozoic mafic dikes intruded in continental fragments rifted away from the former supercontinent rodinia these results demonstrate that the basement rocks of the north qaidam terrane formed part of the former supercontinent rodinia attached to the yangtze craton and or the qinling microcontinent and recorded a complex tectono metamorphic evolution that involved neoproterozoic and early paleozoic orogenies

metamorphic growth and recrystallization of zircon distinction by simultaneous in situ analyses of trace elements u th pb and lu hf isotopes in zircons from eclogite facies rocks in the sulu orogen simultaneous in situ analyses of trace elements u th pb and lu hf isotopes were carried out on distinct domains of zircons in ultrahigh pressure uhp eclogite facies metamorphic rocks from the main hole of the chinese continental scientific drilling ccsd in the sulu orogen for the first time trace elements are directly linked to lu hf isotopes in metamorphic zircons with reference to their u pb dates this enables methodological integration to distinguish four types of metamorphic zircon solid state replacement and dissolution recrystallizations of protolith zircons and new growth from the aqueous fluid metamorphically grown zircons are characterized by concordant u pb ages for the metamorphism flat hree patterns typical of the garnet effect low contents of ree especially hree y nb ta and th u high contents of hf low lu gd n lu hf and th u 0 1 ratios and elevated 176hf 177hf ratios relative to solid state recrystallized zircons this suggests the effects of both garnet and fluid on the growth of metamorphic zircons in contrast metamorphic recrystallization has reset the u th pb isotope system of protolith zircons to different extents depending on the extents of fluid action during metamorphism solid state recrystallized zircons exhibit the lowest degrees of resetting and thus almost inherit all geochemical features from the protolith zircons which are characterized by discordant u pb ages close to or below the protolith age steep mree hree patterns typical of magmatic origin high contents of trace elements and their ratios and low 176hf 177hf ratios on the other hand dissolution recrystallized zircons show the highest degrees of reworking and thus have concordant or nearly concordant u pb ages for the metamorphism steep mree hree patterns lowered contents of trace elements such as ree th u y nb ta and ti relative to the protolith zircons and almost unchanged hf isotope ratios replacement recrystallized zircons display intermediate degrees of reworking and thus have their many features of elements and isotopes in between while the metamorphic growth in the presence of both garnet and fluid is characterized by both depletion of hree with flat pattern and the low contents of trace elements the metamorphic recrystallization in the presence of aqueous fluid is indicated by gradual decreases of mree to hree without the flat hree pattern therefore the simultaneous in situ analyses of metamorphic zircons have the advantage over single term analyses in making distinction between the new growth and the different types of recrystallization

geochemistry zircon upb geochronology and luhf isotopic composition of eclogites and their host gneisses in the dulan area north qaidam uhp terrane new evidence for deep continental subduction abstract in this study we link zircon u pb shrimp and la icp ms geochronology and the lu hf isotopic composition of eclogites and their host gneisses schists with whole rock geochemistry of eclogites in the dulan area to constrain their protoliths and metamorphic relationships u pb dating suggests that the protolith of one of the eclogites was a neoproterozoic mafic intrusive rock 828 58 ma and the protolith of enclosing orthogneiss was an early neoproterozoic granitoid 923 12 ma detrital zircons from grt bearing mica schists yield ages of 0 9 2 5 ga with a dominant range of 1 0 1 8 ga indicating sedimentary sources from neoproterozoic to neoarchean crust and a depositional age 0 9 ga the matching metamorphic ages of eclogites 438 5 ma 436 4 ma and their country rocks grt bearing mica schists 438 4 ma 439 8 ma orthogneiss 427 8 ma indicate that all studied samples experienced coeval early paleozoic hp uhp metamorphism the u pb ages and hf isotopic compositions of the inherited magmatic zircon cores of an eclogite sample e hf 800 2 6 9 2 t dm1 1 0 1 3 ga t dm2 1 1 1 4 ga suggest that the protolith may be derived from neoproterozoic depleted mantle with variable proportions of an older crustal component the magmatic zircon cores of the orthogneiss e hf 900 7 3 to 0 2 t dm2 1 8 2 1 ga suggest that the parental magma was derived from a paleoproterozoic crustal source hf isotopic compositions of the detrital zircons from the metasediments e hf t 19 4 to 10 6 suggest three crust formation and reworking events 1 archean t dm2 2 7 2 9 ga juvenile crust reworked at 2 5 ga 2 early paleoproterozoic t dm2 2 3 2 5 ga juvenile crust reworked at 1 8 ga and 3 late paleoproterozoic t dm2 1 5 1 9 ga juvenile crust reworked in the neoproterozoic whole rock geochemical data suggest that the protoliths of the dulan eclogites were probably derived from a continental rift or an incipient oceanic basin rather than a large long lived ocean basin thus combined with field relationships petrology geochemistry zircon u pb dating and the lu hf isotopic analysis presented in this paper and reported from previous studies we suggest that the dulan eclogites and their country rocks experienced a common uhp metamorphism during late ordovician deep continental subduction

protolith of eclogites in the north qaidam and altun uhp terrane nw china earlier oceanic crust an early paleozoic ultrahigh pressure metamorphic belt occurs in the north qaidam altun mountains and was offset about 400 km southwestward by the altyn tagh strike slip fault eclogites in the belt consist of major basaltic and minor picritic rock types and can be subdivided into three groups high tio2 2 5 wt medium tio2 1 2 wt and low tio2 1 wt geochemical evidence shows that the protoliths of the eclogite were basaltic rocks from different tectonic environments including mid ocean ridge basalt island arc basalt and ocean island basalt the nd isotopes of these rocks are similar to those of morb characterized by mainly positive and minor negative end 0 values providing further evidence that the eclogite protoliths were ocean floor basalts to which minor crustal components were added during subduction geochronological data indicate that the uhpm occurred about 500 440 ma ago and that there were two eclogite protoliths with ages of 800 750 and 1000 ma respectively geochemically and isotopically the eclogites are similar to basaltic rocks in luliangshan north qaidam mountains which have sr and nd ages of 768 39 and 780 22 ma the age and composition of these volcanic rocks suggest that they were one type of the protoliths for the eclogites a later series of calc alkaline island arc volcanic rocks was formed at about 500 ma ago essentially contemporaneous with the uhp metamorphism we propose the following tectonic model for the evolution of the north qaidam uhp belt at about 1000 ma ago a number of continents were amalgamated to form the rodinian continent in this general area which contained some oceanic volcanic rocks possibly as ophiolitic fragments this part of rodinia was then rifted at about 800 750 ma ago to form an oceanic basin with a variety of morb and ocean island basalts closure of this ocean basin produced the neoproterozoic ophiolites and granitic gneisses in the north qaidam mountains in the early paleozoic another qilian ocean basin formed and subduction of this oceanic lithosphere formed the island arc volcanic rocks at about 500 ma ago and the subduction related granites 470 450 ma once the oceanic lithosphere was consumed the subsequent continent continent collision led to deep subduction of continental crust which was tectonically mixed with the eclogites to cause the ultrahigh pressure metamorphism

petrology geochemistry zircon u pb dating and lu hf isotope of granitic leucosomes within felsic gneiss from the north qaidam uhp terrane constraints on the timing and nature of partial melting abstract granitic leucosomes are widely distributed within felsic gneiss in the north qaidam ultrahigh pressure uhp metamorphic terrane in western china which is crucial to understanding the relationships between partial melting metamorphic evolution and orogenic processes we have applied petrology whole rock geochemistry and sr nd isotope zircon u pb geochronology trace element composition and lu hf isotope of these granitic leucosomes to determine the nature and timing of partial melting of these rocks anatexis of the felsic gneiss is evidenced by 1 highly cuspate elongated feldspar grains along quartz quartz and quartz feldspar boundaries 2 cuspate wedge shaped pockets of k feldspar quartz plagioclase muscovite along the boundaries of quartz and or plagioclase and 3 felsic veinlets of k feldspar quartz plagioclase muscovite along grain boundaries major elements feo t mno mgo and tio 2 as well as lrees hrees and hfses are mainly retained in the melanosomes whereas the large ion lithophile elements liles e g rb ba k sr pb are preferentially partitioned into the granitic leucosomes three discrete u pb ages are recorded in the zoned zircons from the melanosomes and granitic leucosomes the inherited magmatic pre metamorphic zircon cores from melanosomes and granitic leucosomes contain quartz feldspar inclusions and record a neoproterozoic protolith age of approximately 950 ma the unzoned zircon mantles in the melanosomes and granitic leucosomes show characteristics similar to metamorphic zircons in terms of such as remarkably flat heavy rare earth element hree patterns an absence of obviously negative eu anomalies and low th u ratios these zircon mantles record an eclogite facies metamorphic age of 444 449 ma the last discrete age at 433 435 ma is preserved in anatectic zircon rims which display pronounced oscillatory zoning and contain felsic mineral inclusions of k feldspar plagioclase quartz the anatectic zircons have negative eu anomalies and pronounced positive ce anomalies and their low th u ratios and lu hf isotope features that are indistinguishable from those of the metamorphic domains the similarity in lu hf signatures indicates a closed system whereby no hf was added either from the outside via fluids or surprisingly from the pre metamorphic cores during partial melting muscovite relicts coexist with cuspate feldspars in the granitic leucosomes suggesting that the anatectic melts originate from dehydration melting due to breakdown of the muscovite in the felsic gneiss in conclusion an integrated study of the petrology geochronology and geochemistry demonstrates that partial melting of felsic gneiss in the nqd uhp terrane could mainly occurred during the initial retrogression stage and anatectic melt crystallized probably under granulite facies conditions

early silurian glaciations in brazil abstract the opening of the brazilian paleozoic basins during the late ordovician was followed by a tectonic stabilization in the silurian at the time of the early silurian glaciations deposition of continental sandstones prevailed in the basins offshore environments were present in the amazonas basin and to a smaller extent in the parana basin the ice sheets advanced over fluviatile and littoral sediments except for the south margin of the amazonas basin where the glacial diamictites immediately overlie marine facies the ice produced much meltwater even when advancing and this was probably climatically controlled the ice movements were from the northwest at the northern margin of the amazonas basin and towards the northeast at the southern margin in the parnaiba basin and its outliers the jatoba basin and cariri valley the ice came from southeast and probably also from east further to the south in the parana basin it came from the north and northeast four glaciations have been dated to the early silurian the oldest is dated as latest ashgill and or earliest llandovery in the parana basin as evidenced from tillites in the upper rio ivai and lower vila maria formations including the former iapo formation it is contemporaneous with a glaciation in the cape basin south africa the next glaciation is dated as early middle llandovery gregarius graptolite zone and is present in the amazonas basin and the andean belt and probably also in the parnaiba basin and its outliers the two youngest tillites were widespread in the northern brazilian basins amazonas parnaiba and jatoba basins and cariri valley as well as in the andean belt and are dated as early late llandovery possibly starting already in the late middle llandovery and latest llandovery to earliest wenlock the latter tillite is the most widespread one assuming these glaciations are related to polar positions the apparent migration of the south pole ice cap must have been quite rapid from central north africa in late ashgill towards central brazil in the middle late llandovery the glaciation in the parana basin took place up to 45 from the late ashgillian south pole the cool climate continued during the silurian but no new glaciations are known before latest devonian

petrology and geochemistry of high niobium eclogite in the north qaidam orogen western china implications for an eclogite facies metamorphosed island arc slice abstract to better understand the evolution of the north qaidam orogen eclogite that forms lens like bodies in granitic gneiss was studied in the luliangshan terrane north qaidam ultrahigh pressure uhp metamorphic belt petrographic and mineralogical studies as well as pressure temperature calculations showed that the eclogite underwent at least three metamorphic stages peak eclogite facies metamorphism at 647 768 c and 3 0 gpa was followed by high pressure hp granulite facies metamorphism at 815 929 c 1 61 1 75 gpa and cooling to amphibolite facies metamorphism at 606 720 c 0 41 0 56 gpa three age groups can be identified via la icp ms zircon u pb eclogite dating 1 an eclogite facies metamorphic age of ca 432 428 ma 2 an hp granulite facies retrograde age of ca 424 420 ma and 3 ages of 415 382 ma which constrained the timing of amphibolite facies metamorphism the studied eclogite is characterized by high field strength element enrichment particularly in nb content 57 8 124 ppm and flat rare earth element patterns similar to high nb island arc basalts described in many modern arcs that feature subduction of oceanic slabs the samples have positive end t values 2 1 to 5 1 and positive ehf t values 4 9 to 10 0 indicating that the protolith of the high nb eclogite was directly derived from the depleted mantle with little crustal contamination these island arc basalts have been interpreted to have arisen from the partial melting of mantle wedge garnet amphibole or garnet peridotite metasomatized by slab dehydrated melts of a subducted oceanic slab they were emplaced onto the continental block by arc magmatism prior to the main collision event then the island arc materials were dragged into the subduction channel with eclogite facies metamorphism via subduction of the qaidam block

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

differential exhumation and cooling history of north qaidam uhp metamorphic rocks nw china constraints from zircon and rutile thermometry and u pb geochronology abstract we analyze zircon and rutile u pb ages and trace elements using la icp ms and sims of eclogites from the north qaidam ultra high pressure metamorphic belt in nw china zr in rutile and ti in zircon thermometries give consistent peak metamorphic temperatures for two fresh eclogite samples zircon and rutile u pb dating from the yuka eclogite yielded ages of 430 2 3 7 ma and 407 16 ma respectively zircons from north dulan eclogite record two distinct eclogite facies metamorphic events at 446 6 4 1 and 430 5 4 7 ma with t ti in zircon 684 16 c and 649 26 c respectively while rutile u pb dating for the same north dulan eclogite sample yielded an age of 416 10 ma these rutile u pb ages are interpreted to record the time when these rocks cooled down to the closure temperature of pb in rutile here taken to be ca 570 c thus we obtain a slow cooling rate of 3 4 c ma for yuka and north dulan terranes for one granulite facies overprinted south dulan eclogite sample rutile u pb dating gave a younger age of 383 10 ma combined with literature chronological and temperature data for south dulan the data imply later exhumation than the other samples with a faster cooling rate of 6 6 c ma after the granulite facies event the t t paths obtained show that the north qaidam eclogites experienced considerably slower cooling history than previously assumed and also that while the thermal histories of the yuka and north dulan eclogites resemble one another the south dulan eclogite is quite distinct so a differential exhumation and emplacement of uhp metamorphic rocks occurred in the north qaidam

fluctuations in epeiric sea carbonate production during silurian positive carbon isotope excursions a review of proposed paleoceanographic models positive excursions in the carbon isotopic ratio of marine carbonates 13ccarb are commonly attributed to an increase in the burial ratio of organic versus carbonate carbon in the global oceans regardless of whether this increased deposition of organic carbon was oceanographically or biologically induced the fact that high carbon isotope values are most often attributed to the sequestration and burial of organic carbon remains the notion that high 13ccarb values should be the result of organic rich deposition in epeiric sea settings is not surprising considering that the overwhelming majority of organic carbon burial in the modern oceans occurs in continental shelf and margin settings what is surprising is that major positive carbon isotope excursions during the silurian are recorded in clean carbonate successions with little to no appreciable black shale deposition in epeiric seas in fact silurian 13ccarb excursions occurred during intervals of prolific reef development and an overall expansion of carbonate platform environments globally this discrepancy has sparked considerable debate among silurian carbon isotope specialists but has offered a valuable opportunity to re examine the intricate connections among nutrients primary productivity carbonate sedimentology and the global carbon cycle in the lower paleozoic here we consider several sedimentological and geochemical features of one of these silurian positive carbon isotope 13ccarb excursions and discuss the broader implications of current silurian 13ccarb models to the interpretation of lower paleozoic carbonate sequences

volcanism mineralization and metamorphism at the xitieshan pb zn deposit nw china insights from zircon geochronology and geochemistry abstract the footwall volcanic rocks of the ordovician tanjianshan group in the world class xitieshan pb zn deposit have experienced prolonged arc volcanism followed by strong metamorphism and deformation this has resulted in a complex thermal history and led to ambiguity in interpretation of zircon geochronological results an integrated study involving textural characterization cl imaging trace element analysis ti in zircon thermometry and la icpms u pb dating has provided tight constraints on the age and genesis of the zircon groups in the volcanic rocks the temperature of metamorphism and deformation indicated by metacryst minerals and micro structures in the volcanic rocks ranges from 550 to 650 c which partially overlaps with the lower temperature range of zircon crystallization 600 750 c calculated using the ti in zircon thermometer cathodoluminescence images and trace element compositions confirm a magmatic origin for the zircons which have also been variably altered by metamorphic fluids two ranges of u pb ages 475 470 ma and 460 450 ma have been obtained on typical magmatic zircons and are interpreted to represent pre mineralization arc volcanism in the xitieshan deposit a younger age group of 440 430 ma for the fluid modified zircons is considered to record post ore metamorphism during the north qadaim orogeny thus we propose that the original exhalative ores at the xitieshan pb zn deposit formed at 450 440 ma

paleoenvironmental changes in the silurian indicated by stable isotopes in brachiopod shells from gotland sweden ratios of stable carbon and oxygen isotopes in brachiopod shells more than 370 specimens esp atrypa reticularis from the silurian of gotland sweden have been analysed preservation of biological skeletal ultrastructures observed in sem micrographs and cathodoluminescence analyses indicate that usually no diagenetical alteration occurs the silurian of gotland consists of 440 m carbonate deposits spanning the late llandovery to late ludlow epochs 431 411 m y repeatedly uniform sequences of micritic limestones and marks are interrupted by complex structured reefs and adjacent platform sediments previously the alteration of facies is interpreted as the result of sea level fluctuations caused by a gradual regression with superimposed minor transgressive pulses the silurian sequence of gotland exhibits principally parallel carbon and oxygen isotope records corresponding closely to the topostratigraphic units lower values occur in periods dominated by deposition of marly sequences higher values are observed in periods dominated by reefs and extended carbonate platforms the isotope ratios are influenced by local as well as global factors the oxygen isotope ratios are interpreted to reflect paleosalinity changes due to varying freshwater input rather than changes in paleotemperature consequently the facies distribution of gotland is interpreted as resulting from changes in terrigenous input caused by different rates of continental weathering and freshwater runoff rather than by sea level fluctuations periods of arid climate and therefore anti estuarine downwelling of oxygenated surface water appear as short episodes of reef growing 1 5 m y in an epoche characterized by a tropic humid climate which causes an estuarine circulation and the upwelling of co2 rich deep water carbon isotope ratios are obviously connected to these changes in circulation by the advection of 13c rich surface water arid episodes or upwelling of 13c depleted deep water humid episodes of a silurian ocean which itself reveals generally euxinic deep water conditions due to the burial of organic carbon in black shales

the geochemistry of young volcanism throughout western panama and southeastern costa rica an overview oblique aseismic subduction below western panama and southeastern costa rica has produced recent arc related volcanism the aseismicity is probably related to the subduction of relatively hot oceanic lithosphere the volcanism throughout this region over the past 2 ma has been quite distinct consisting of felsic magmas andesites to rhyolites but mainly dacites with geochemical signatures suggesting a metamorphosed basaltic source it is believed that the subduction of young oceanic crust sets up conditions under which the slab melts rather than the overlying mantle wedge rocks with slab melt geochemistries and associated with young subducted crust have been termed adakites elsewhere the young adakite melts are sometimes associated with a few rare young high nb basalts but there is no obvious genetic link between them through differentiation high nb basalts may also be derived from the partial melting of the subducted oceanic crust high nb basalt migmatites have been found with pegmatites of adakite compositions in the exposed subduction terrain of the catalina schist california alternatively the high nb basalts may be partial melts of phlogopite rich mantle that has previously reacted with adakite magmas eruption of adakites and high nb basalts was preceded by a 2 3 ma period of relative quiescence prior to this there was a 7 ma period of calc alkaline volcanism typical of the present day magmatism associated with a distinct benioff zone found throughout the central american arc the abrupt transition in volcanism with time from an early calc alkaline sequence to a later adakite high nb basalt sequence may record a change in the tectonic setting of western panama and southeastern costa rica over the past 12 ma

magnesian andesites nb enriched basalt andesites and adakites from late archean 2 7 ga wawa greenstone belts superior province canada implications for late archean subduction zone petrogenetic processes magnesian andesites ma occur with normal tholeiitic to calc alkaline basalt andesite suites in four greenstone belts of the 2 7 ga wawa subprovince canada collectively the magnesian andesites span ranges of sio2 56 64 wt mg number 0 64 0 50 with cr and ni contents of 531 106 and 230 21 ppm respectively relative to normal andesites the magnesian andesites form distinct trends on variation diagrams with relatively high th and lree contents uniform yb over a range of mgo more fractionated hree and lower nb thpm and nb lapm ratios niobium enriched basalts and andesites neba nb 7 16 ppm and an al enriched rhyolite adakite suite are associated in space and time with the magnesian andesites nb enriched basalts and andesites are characterized by high tio2 p2o5 th and zr contents variably high zr hf 36 44 ratios and more fractionated hree gd ybcn 1 3 4 1 compared to the normal tholeiitic to calc alkaline basalt andesite suites the adakite suite has the high al al2o3 16 18 wt high la ybcn 21 43 and low yb 0 4 1 2 ppm of archean tonalite trondhjemite granodiorite ttg suites and cenozoic adakites indicative of liquids derived mainly from slab melting the basalt andesite suites are not characterized by normal tholeiitic or calc alkaline fractionation trends of major or trace elements rather compositional trends can be accounted for by some combination of fractional crystallization and variable degrees of metasomatism of the source of basalt and or andesites by adakitic liquids the occurrence of magnesian andesites nb enriched basalts andesites and adakites has been described from certain phanerozoic arcs featuring shallow subduction of young and or hot oceanic lithosphere adakites likely represent slab melts magnesian andesites the product of hybridization of adakite liquids with mantle peridotite and nb enriched basalts andesites melts of the residue from hybridization geological similarities between the late archean wawa greenstone belts and certain cenozoic transpressional orogens with the ma neba adakite association suggest that subduction of young hot oceanic lithosphere may have played an important role in the production of this arc related association in the late archean

early palaeozoic north qaidam uhp metamorphic belt on the north eastern tibetan plateau and a paired subduction model an early palaeozoic uhp metamorphic belt was recently discovered in the north qaidam mountains in the north eastern part of the tibet plateau the belt contains abundant eclogite with lesser amounts of garnet peridotite and gneiss the paragneiss contains abundant zircon with coesite inclusions whereas the eclogite has garnet with coesite pseudomorphs and omphacite with exsolved quartz the calculated peak metamorphic conditions for the eclogite are t 730 c and p 2 8 gpa the garnet peridotites are magnesium rich with mg s 100 mg mg fe up to 92 olivine in the peridotites is highly magnesian fo 91 5 and tio2 poor and is typical of low t garnet peridotites calculated peak metamorphic conditions for the peridotites are t 837 c and p 2 5 gpa u pb and sm nd isotope dating indicate that continental subduction started during the early ordovician 495 ma the north qaidam uhp together with the north qilian hp belt about 300 km to the north constitute a pair of genetically related early palaeozoic subduction zones along the northern margin of the tibet plateau

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

adakites without slab melting high pressure differentiation of island arc magma mindanao the philippines abstract new geochemical data for pleistocene magmatic rocks from the surigao peninsula eastern mindanao the philippines demonstrate typical adakitic traits including elevation of sr y and depletion of the heavy rare earth elements 87sr 86sr and 143nd 144nd ratios of the adakites do not support melting of the subducted philippine sea plate but resemble pliocene arc lavas generated in the same subduction zone excepting the heavy rare earth elements the adakites and arc lavas also possess similar ratios of incompatible elements suggesting that the adakites were ultimately derived from melting of the mantle wedge the wide range of sio2 in the adakites and its strong correlation with trace element concentrations and ratios indicate two possible mechanisms for generating the adakitic signature 1 adakitic melt was produced from basaltic arc magma by fractional crystallisation of a garnet bearing assemblage 2 solidified basaltic rock containing garnet melted to yield adakitic magma in either case the basaltic precursor was generated from fluid modified mantle then differentiated within the garnet stability field in surigao this requires differentiation within mantle the surigao example suggests that any subduction zone has the potential to produce adakitic magma if basalt crystallises at sufficient depth this has important implications for the geodynamics of modern and ancient subduction zones that have generated similar rocks

the ireviken event in the lower silurian of gotland sweden relation to similar palaeozoic and proterozoic events abstract for a long time the silurian was thought to represent a time of stable environmental conditions in the greenhouse period that followed the late ordovician glaciation during the past decade knowledge about the silurian has increased markedly and today it is known that the conditions in the silurian were much more variable than previously assumed detailed isotopic investigations have revealed several distinct positive excursions in both carbon and oxygen isotope values in low latitudes these periods of high c and o isotope values are in many cases characterised by the growth of reefs and the formation of extended carbonate platforms the sediments deposited during these excursions contain impoverished fossil assemblages especially with respect to conodonts graptolites and trilobites a conspicuous isotope excursion coincident with facies changes and a marked mass extinction is observed near the llandovery wenlock boundary this event is called the ireviken event after its type locality on the island of gotland sweden jeppsson l 1987 in palaeobiology of conodonts ellis horwood ltd chichester pp 129 145 here isotope data from nine sections at the nw coast of gotland are presented that cover the time interval of the ireviken event the 13c mean values rise from 1 4 to 4 5 and the 18o values increase from 5 6 to 5 0 the relative timing of stable isotope development extinctions and facies development is discussed it is shown that first extinctions precede the isotope excursion this indicates that extinction events and stable isotope development are only indirectly connected but might reflect the same causes other events characterised by similar relationships between positive isotope excursions mass extinctions and facies development are found in younger parts of the silurian late wenlock and ludlow in the late ordovician the late cambrian and with some reservations in the proterozoic the similarities between these events indicate analogous controlling mechanisms for the silurian climatic changes between humid and arid conditions in low latitudes were postulated in an earlier study of the authors a palaeoceanographic climatic model was postulated which is consistent with most of the sedimentological palaeontological and geochemical data here we demonstrate that this model may be applicable also for the older events

early silurian carbonate platform ostracods from iran a peri gondwanan fauna with strong laurentian affinities abstract early silurian aeronian carbonate platform deposits of the lower niur formation of east central iran yield an ostracod fauna of fourteen species dominated by podocopes with rare palaeocopes and binodicopes the podocope rich fauna is unusual in that early palaeozoic carbonate platforms are typically dominated by palaeocopes despite its mid palaeolatitude peri gondwanan setting and isolation from low palaeolatitudes the ostracod assemblage of the lower niur formation shows striking affinities to species level with late ordovician and early silurian ostracod faunas of palaeo equatorial laurentia biogeographically the lower niur formation ostracod fauna also shows species and genus links with late ordovician and early silurian faunas of low palaeolatitude 0 30 s baltica and mid palaeolatitude peri gondwanan uzbekistan dispelling the notion of strict endemism for early silurian ostracods two new species bulbosclerites jamshidis sp nov and steusloffina fravashi sp nov are described

u pb zircon geochronology of coesite bearing eclogites from the southern dulan area of the north qaidam uhp terrane northwestern china spatially and temporally extensive uhp metamorphism during continental subduction coesite bearing eclogites from 100 km 2 in the southern dulan area north qaidam mountains nqm of western china contain zircon that records protolith crystallization and ultra high pressure uhp metamorphism sensitive high resolution ion microprobe mass spectrometer and laser ablation inductively coupled plasma mass spectrometry u pb analyses from cathodoluminescence cl dark zircon cores in a coesite bearing eclogite yield an upper intercept age of 838 50 ma and oscillatory zoned cores in a kyanite bearing eclogite gave a weighted mean 206 pb 238 u age of 832 20 ma these zircon cores yield steep heavy rare earth element hree slopes and negative eu anomalies that suggest a magmatic origin thus c 835 ma is interpreted as the eclogite protolith age unzoned cl grey or bright zircon and zircon rims from four samples yield weighted mean ages of 430 4 438 2 446 10 and 446 3 ma flat hree patterns without eu anomalies and contain inclusions of garnet omphacite rutile phengite and rare coesite these ages are interpreted to record 16 5 myr of uhp metamorphism these new uhp ages overlap the age range of both eclogite and paragneiss from the northern dulan area suggesting that all uhp rock types in the dulan area belong to the same tectonic unit our results are consistent with slow continental subduction but do not match oceanic subduction and diapiric exhumation uhp model predictions these new data suggest that similar to eclogites in other hp uhp units of the nqm and south altyn tagh protoliths of the eclogites in the dulan area formed in a continental setting during the neoproterozoic and then subducted to mantle depth together with continental materials during the early palaeozoic