amphibole sponge in arc crust pressure temperature time paths followed by arc magmas ascending through the lithosphere dictate the phase assemblage that crystallizes and hence the compositions of liquid fractionates here we use la yb and dy yb versus sio 2 relationships from selected volcanoes to show that amphibole is an important mineral during differentiation of arc magma production of intermediate and silicic arc magmas occurs as magmas stall and cool in the mid lower crust where amphibole is stable because amphibole is rarely a phenocryst phase we term this cryptic amphibole fractionation if this process is as widespread as our investigation suggests then 1 amphibole cumulates may act as an effective filter for water dissolved in mantle derived magmas 2 amphibole cumulates may act as a fertile source of intracrustal melts and fluids and 3 recycling of amphibole cumulates has the potential to return incompatible trace elements and water to the mantle

dehydration melting of amphibolite at 10 kbar the effects of temperature and time we have simulated the dehydration melting of a natural low k calcic amphibolite 67 4 hornblende 32 5 anorthite in piston cylinder experiments at 10 kbar and 750 1000 c for 1 9 days the solidus temperature is lower than 750 c garnet appears at 850 c the overall reaction is hb p l cpx al hb ca hb ga opx three stages of reaction are 1 melting dominated by the growth of clinopyroxene and garnet with little change in composition of liquid or garnet 2 a reversal of this reaction between 875 c and 900 c with decreases in the amounts of liquid and garnet and 3 a large increase in liquid along with the loss of hornblende and decrease of plagioclase while clinopyroxene and garnet increase garnet is enriched in pyrope and zoned from fe cores to mg edges range 3 mol pyrope liquid composition is enriched first in an to 950 c and then in ab the liquids are more calcic and aluminous than natural tonalites which is attributed to the plagioclase composition an90 the formation of peraluminous liquid from the metaluminous amphibolite is caused by anorthite not h2o saturated conditions the results are consistent with an amphibolite phase diagram with relatively high solidus temperatures in the garnet absent field 900 1000 c but with a solidus backbend at 7 9 kbar coincident with the garnet in boundary hornblende breakdown due to garnet formation in a closed system must make h2o available for h2o undersaturated melting right down to the h2o saturated solidus below 700 c which defines a large low temperature pt area where hydrous granitoid melts can be generated with residual garnet and hornblende

late ordovician to early devonian adakites and nb enriched basalts in the liuyuan area beishan nw china implications for early paleozoic slab melting and crustal growth in the southern altaids abstract we report newly defined nb enriched basalts adakites and dacites from the beishan nw china of the southern altaids based on field geochemical isotopic and geochronology studies two phases of adakites adakite i and adakite ii have been defined which are calc alkaline and characterized by high na 2 o k 2 o ratios 1 49 1 71 and 2 32 3 64 and sr contents 494 1213 ppm and 325 494 ppm negligible to positive eu anomalies strong depletion of hree e g yb 0 48 0 93 ppm and 0 50 0 99 ppm and y 6 87 9 80 ppm and 6 02 10 30 ppm and enriched in rb sr ba k and depleted nb and ti they are characterized by relatively low e nd t values 0 8 to 0 9 and 0 6 to 3 8 and relatively constant high 87 sr 86 sr i ratios 0 70635 0 70636 and 0 70583 0 70651 the zircons of adakite i have relatively low e hf t 0 8 to 2 7 the nb enriched basalts are sodium rich n 2 o k 2 o 1 31 4 44 with higher tio 2 p 2 o 5 zr and nb contents and nb th pm nb la pm and nb u ratios than typical arc basalts they are relatively enriched in rb ba u pb and k depleted in nb and minor negative to positive ba zr sr and ti they have low positive e nd t 0 9 to 2 3 and relatively high 87 sr 86 sr i 0 70556 0 70691 ratios the dacites are typical arc magmas with moderately enriched lile distinctly negative eu nb sr and ti anomalies they have positive e nd t 2 2 and relatively high 87 sr 86 sr i 0 70786 we argue that the liuyuan adakites were most probably related to the melting of young hot subducted crust of the paleo asian ocean which included tectonically subducted radiogenic crustal material and or inheritance from highly radiogenic oceanic crust e g oib the nb enriched basalts likely resulted from mantle wedge peridotites metasomatized by adakites and or further changed by components other than adakites e g minor slab derived fluids based on own zircon sims u pb dating of these key rock types we further propose that from the late ordovician to early devonian large volumes of magma consisting of late ordovician nb enriched basalts 451 ma and dacites 442 ma late silurian adakite i 424 ma early devonian adakite ii 374 ma and i s a type granites 436 ma 380 ma developed in the southern altaids together with other geochronological data from the literature we conclude that subducted oceanic slab melting was frequent from 470 ma to 370 ma our results suggest that frequent hot and or young oceanic crustal subduction and slab melting were important mechanisms in the accretionary growth of the southern altaids

adakites from continental collision zones melting of thickened lower crust beneath southern tibet adakites are geochemically distinct intermediate to felsic lavas found exclusively in subduction zones here we report the first example of such magmas from southern tibet in an active continental collision environment the tibetan adakites were emplaced from ca 26 to 10 ma and their overall geochemical characteristics suggest an origin by melting of eclogites and or garnet amphibolites in the lower part 50 km of thickened tibetan crust this lower crustal melting required a significantly elevated geotherm which we attribute to removal of the tectonically thickened lithospheric mantle in late oligocene time the identification of collision type adakites from southern tibet lends new constraints to not only the himalayan tibetan orogenesis how and when the indian lithosphere started underthrusting asia can be depicted but also the growth of the early continental crust on earth that consists dominantly of the tonalite trondhjemite granodiorite suites marked by adakitic geochemical affinities

reassessment of continental growth during the accretionary history of the central asian orogenic belt we argue that the production of mantle derived or juvenile continental crust during the accretionary history of the central asian orogenic belt caob has been grossly overestimated this is because previous assessments only considered the palaeozoic evolution of the belt whereas its accretionary history already began in the latest mesoproterozoic furthermore much of the juvenile growth in central asia occurred in late permian and mesozoic times after completion of caob evolution and perhaps related to major plume activity we demonstrate from zircon ages and nd hf isotopic systematics from selected terranes within the caob that many neoproterozoic to palaeozoic granitoids in the accreted terranes of the belt are derived from melting of heterogeneous precambrian crust or through mixing of old continental crust with juvenile or short lived material most likely in continental arc settings at the same time juvenile growth in the caob occurred during the latest neoproterozoic to palaeozoic in oceanic island arc settings and during accretion of oceanic island arc and precambrian terranes however taking together our data do not support unusually high crust production rates during evolution of the caob significant variations in zircon ehf values at a given magmatic age suggest that granitoid magmas were assembled from small batches of melt that seem to mirror the isotopic characteristics of compositionally and chronologically heterogeneous crustal sources we reiterate that the chemical characteristics of crustally derived granitoids are inherited from their source s and cannot be used to reconstruct tectonic settings and thus many tectonic models solely based on chemical data may need re evaluation crustal evolution in the caob involved both juvenile material and abundant reworking of older crust with varying proportions throughout its accretionary history and we see many similarities with the evolution of the sw pacific and the tasmanides of eastern australia

the structure evolution and symmetry of the magma poor rifted margins of the north and central atlantic a synthesis abstract magma poor rifted margins consistently show extreme crustal thinning accompanied by normal faulting the serpentinization of the mantle beneath crust thinned to less than 8 2 km and the unroofing of a broad zone of mantle within the continent ocean transition accompanied by the development of detachment and other large offset faults these observations are the logical result of the progressive extension of cool lithosphere away from thermal anomalies such as plumes although the paucity of magmatism may be explained by depth dependent extension of the lithosphere and by pre depleted sub crustal lithosphere rifting above initially cool sub lithospheric mantle also may explain the subsidence deficit observed at some margins synrift subsidence is buffered by the simultaneous influx of warmer oceanic asthenosphere as it extends thins and cools ductile creeping layers in the mid and deep crust become progressively more brittle resulting in increased coupling between the upper and lower crust and eventually the embrittlement of the entire crust faults cutting from the surface across the moho bringing water into the mantle and causing its serpentinization increased coupling and the development of serpentine detachments predict the development of late stage asymmetry once the entire crust is brittle such detachments are imaged on some margins and inferred on others analysis of the crustal structure across conjugate margins shows that these are approximately symmetric until this late stage when they become markedly asymmetric similar analyses show that depth dependent stretching of the crust is insufficient to explain the discrepancy between the amount of the visible extension along faults and the amount of crustal thinning instead this extension discrepancy may be related to the complex evolution of brittle deformation through multiple phases and styles of faulting related to the changes in the rheological character and strength of the lithosphere as it is thinned complex polyphase faulting continues after complete crustal separation resulting in the exhumation of broad expanses of peridotitic basement the top of which is everywhere marked by an exhumed slip surface similar to the corrugated surface observed at mid ocean ridges the similarity in processes between mantle unroofing and seafloor spreading makes the distinction between the cot and true oceanic crust difficult and possibly moot

end permian to mid triassic termination of the accretionary processes of the southern altaids implications for the geodynamic evolution phanerozoic continental growth and metallogeny of central asia the altaids is one of the largest accretionary orogenic collages in the world with the highest rate of phanerozoic continental growth and significant metallogenic importance it is widely accepted that subduction related orogenesis of the altaids started in the late precambrian and gradually migrated southward present coordinates however it is uncertain when and how the building of the altaids was finally completed based on structural geology geochemical geochronological and paleomagnetic data this paper presents late paleozoic to early mesozoic accretionary tectonics of two key areas north xinjiang in the west and inner mongolia in the east together with neighboring mongolia the late paleozoic tectonics of north xinjiang and adjacent areas were characterized by continuous southward accretion along the wide southern active margin of siberia and its final amalgamation with the passive margin of tarim which may have lasted to the end permian to early mid triassic in contrast in inner mongolia and adjacent areas two wide accretionary wedges developed along the southern active margin of siberia and the northern active margin of the north china craton which may have lasted to the mid triassic the final products of the long lived accretionary processes in the southern altaids include late paleozoic to permian arcs late paleozoic to mid triassic accretionary wedges composed of radiolarian cherts pillow lavas and ophiolitic fragments and high pressure ultrahigh pressure metamorphic rocks permian alaskan type zoned mafic ultramafic complexes intruded along some major faults of the tien shan we define a new tarim suture zone immediately north of the tarim craton that is probably now buried below the tien shan as a result of northward subduction of the tarim block in the cenozoic the docking of the tarim and north china cratons against the southern active margin of siberia in the end permian to mid triassic resulted in the final closure of the paleoasian ocean and terminated the accretionary orogenesis of the southern altaids in this part of central asia this complex geodynamic evolution led to formation of giant metal deposits in central asia and to substantial continental growth

a review of the western part of the altaids a key to understanding the architecture of accretionary orogens abstract the altaids is one of the largest accretionary collages in the world and the tectonic styles of the accretionary processes have been interpreted in several ways including as an amalgamation of multiple terranes as a result of oroclinal bending of a long single arc or as a caledonian continental collision based on recent tectonostratigraphic analyses together with paleomagnetic data the tectonic styles of the neoproterozoic to paleozoic accretionary processes of the altaids are discussed the western altaids is the main focus of the study which was mainly composed of several independent linear components such as arcs and microcontinents with proterozoic basement and cover rocks various kinds of arcs existed in the paleo asian ocean including a complicated type of arc alaskan type which is a combination of the japan and mariana type intra oceanic arcs and the cordillera type continental arcs these linear components rotated and collided with each other with multiple subduction polarities which could have been an important result of multiple linear element amalgamation and which has contributed greatly to the architecture of the eurasian continent the basic tectonic styles of the altaids can be summarized as arc arc collision oroclinal bending and large scale rotation and multiple subductions with a complicated archipelago paleogeography these basic features of accretionary orogens in general can be attributed to the amalgamation of complicated multiple linear elements some mesozoic to cenozoic accretionary orogens in the world are also characterized by processes of multiple linear element amalgamation more attention should be paid to the multiple linear element amalgamation of ancient accretionary orogens which will shed light on lateral and vertical continental growth

continuous marine radiocarbon reservoir calibration and the 13c suess effect in the irish sea results from the first multi centennial shell based marine master chronology abstract the identification in various proxy records of periods of rapid decadal scale climate change over recent millennia together with the possibility that feedback mechanisms may amplify climate system responses to increasing atmospheric co2 highlights the importance of a detailed understanding at high spatial and temporal resolutions of forcings and feedbacks within the system such an understanding has hitherto been limited because the temperate marine environment has lacked an absolute timescale of the kind provided by tree rings for the terrestrial environment and by corals for the tropical marine environment here we present the first annually resolved multi centennial 489 year absolutely dated shell based marine master chronology the chronology has been constructed by detrending and averaging annual growth increment widths in the shells of multiple specimens of the very long lived bivalve mollusc arctica islandica collected from sites to the south and west of the isle of man in the irish sea the strength of the common environmental signal expressed in the chronology is fully comparable with equivalent statistics for tree ring chronologies analysis of the 14c signal in the shells shows no trend in the marine radiocarbon reservoir correction r although it may be more variable before 1750 the 13c signal shows a very significant r2 0 456 p

secular changes in the oxygen isotope ratios of mollusc shells during the holocene of central japan in order to estimate changes in the relative water temperature during the holocene oxygen isotopes ratios were measured in mollusc shells collected from archaeological sites and natural shell beds in central japan analyses were made on the clam meretrix lamarckii which lives on the open coast at depths of about 5 10 m serial samples were analyzed for each shell to obtain isotopic profiles daily growth lines were examined to determine the dates or season a living specimen was analyzed to compare with the results of the holocene specimens the oxygen isotope fluctuation of the specimen shows a clear correspondence with the growth rate pattern indicating that the fluctuation reflects the annual variation of temperature twenty specimens of various ages ranging from 9000 to 1500 yr b p were analyzed the isotopic values indicate that the water temperature was lowest at 9000 yr b p highest at 7000 yr b p and again became low during 4500 4000 yr b p then it slightly increased after 4000 yr b p with an indistinct low temperature episode at about 2000 yr b p the change accords well with the climatic and sea level changes known in central japan

early cretaceous adakitic granites in the northern dabie complex central china implications for partial melting and delamination of thickened lower crust abstract to date few adakitic rocks have been reported in direct association with contemporary intra continental extensional structures which has cast doubt on genetic models involving partial melting of the lower crust this study presents early cretaceous 143 129 ma new sensitive high resolution ion microprobe shrimp zircon u pb ages adakitic granites which are directly associated with a contemporary metamorphic core complex i e the northern dabie complex in the dabie area these granites exhibit relatively high sr contents negligible to positive eu and sr anomalies high la yb and sr y ratios but very low yb and y contents similar to subducted oceanic crust derived adakites they are also characterized however by very low mgo or mg and ni values and nd sr isotope compositions end t 14 6 to 19 4 and 87sr 86sr i 0 7067 0 7087 similar to triassic continent derived eclogites subducted in the dabie sulu orogen additionally late granitic dikes in the adakitic intrusions exhibit low sr contents clearly negative eu and sr anomalies low la yb and sr y ratios but relatively high yb and y contents similar to 118 105 ma granites in the northern dabie complex based on composition and geochronology data of neoproterozoic amphibolites and orthogneisses triassic high to ultra high pressure metamorphic rocks and early cretaceous mafic ultramafic intrusive rocks and the constraints provided by experimental melt data for tonalites metabasaltic rocks and eclogites we suggest that the adakitic granites were most probably generated by partial melting of thickened amphibole or rutile bearing eclogitic lower crust as a consequence of triassic middle jurassic subduction and thrusting the late dikes probably originated from plagioclase bearing intermediate granulites moreover we suggest that late mesozoic delamination or foundering of thickened eclogitic lower crust is also a more plausible mechanism for the petrogenesis of early cretaceous mafic ultramafic intrusive rocks in the dabie area and probably involved partial melting of a mixed source comprising eclogitic lower crust that had delaminated or foundered into upper lithospheric or asthenospheric mantle peridotite asthenospheric upwelling in response to post collisional delamination of lithospheric mantle was likely to have provided the heat source for the cretaceous magmatism

cenozoic k rich adakitic volcanic rocks in the hohxil area northern tibet lower crustal melting in an intracontinental setting it is generally accepted that the cenozoic potassic volcanic rocks of northern tibet were derived from a lithospheric mantle source here we report new chronological geochemical and isotopic data for the miocene ca 18 15 ma k rich adakitic volcanic rocks from the hohxil area of the songpan ganzi block in northern tibet we contend that these rocks were generated by partial melting of the mafic lower crust in an intracontinental setting unrelated to subduction of oceanic crust the hohxil rocks exhibit high sr y and la yb ratios high sr and la contents but low yb and y concentrations similar to adakites formed by slab melting associated with subduction however their relatively low e n d values 2 09 to 4 58 high 8 7 sr 8 6 sr 0 7072 0 7075 th u th ba and rb ba ratios and distinctive potassium enrichments k 2 o na 2 o are very different from the composition of typical adakites in addition those k rich adakitic rocks with the highest sio 2 contents 61 wt exhibit the lowest 8 7 sr 8 6 sr ratios and highest e n d values and are the oldest cenozoic volcanic rocks exposed in the songpan ganzi block suggesting that they were derived neither directly from a mantle source nor by differentiation of a shoshonitic magma taking into account the composition of lower crustal mafic xenoliths in tibet as well as the tectonic and geophysical evidence we conclude that the hohxil adakitic magmas were produced by partial melting of amphibole bearing eclogites with a k rich mafic bulk composition in the lower part 55 km of the thickened northern tibetan crust partial melting of the lower crust may have been triggered by dehydration release of fluids from sedimentary materials in the southward subducted continental lithosphere

cambrian to early silurian ophiolite and accretionary processes in the beishan collage nw china implications for the architecture of the southern altaids the mechanism of continental growth of the altaids is currently under debate between models invoking continuous subduction accretion or punctuated accretion by closure of multiple ocean basins we use the yueyashan xichangjing ophiolite belt of the beishan collage southern altaids to constrain the earliest oceanic crust in the southern palaeo asian ocean five lithotectonic units were identified from s to n the huaniushan block a sedimentary passive margin the structurally incoherent yueyashan xichangjing ophiolite complex a coherent sedimentary package and the mazongshan island arc with granitic rocks we present a structural analysis of the accretionary complex which is composed of the incoherent ophiolitic melange and coherent sedimentary rocks to work out the tectonic polarity a new weighted mean 206 pb 238 u age of 533 1 7 ma from a plagiogranite in the yueyashan xichangjing ophiolite indicates that the ocean floor formed in early cambrian time furthermore we present new geochemical data to constrain the tectonic setting of the yueyashan xichangjing ophiolite the yueyashan xichangjing ophiolite was emplaced as a result of northward subduction of an oceanic plate beneath the mazongshan island arc to the north in late ordovician to early silurian time together with data from the literature our work demonstrates that multiple overlapping periods of accretion existed in the palaeozoic in the northern and southern altaids therefore a model of multiple accretion by closure of several ocean basins is most viable

the genesis of early carboniferous adakitic rocks at the southern margin of the alxa block north china abstract adakite is a highly debated petrologic term that was originally proposed to describe igneous rocks formed by slab melting subsequent studies reported other ways to generate adakitic signatures such as the melting of mafic lower continental crust and fractional crystallization of basaltic magma we studied adakitic rocks from the taohuala mountain at the southern margin of the alxa block north china these rocks are characterized by high sr concentrations 300 450 ppm high sr y 20 75 ppm and la yb n 25 67 ppm ratios and low y

evidence for temporal fluctuations in marine radiocarbon reservoir ages in the santa barbara channel southern california abstract differences in the 14 c ages of closely associated marine shell and carbonized plant material from stratified archaeological deposits on san miguel island california suggest holocene 10 000 present fluctuations in marine 14 c reservoir ages these fluctuations coincide with 18 o and 13 c shifts measured in mytilus californianus shells from the same stratigraphic contexts and general atmospheric oceanic circulation models for the region based on these data we make three primary observations 1 significant changes appear to have occurred in the radiocarbon reservoir during the holocene 2 these fluctuations appear to correlate with regional oceanographic changes and 3 high resolution 14 c dating of marine shells may require different r values for different periods of time

field geology geochronology and geochemistry of mafic ultramafic rocks from alxa china implications for late permian accretionary tectonics in the southern altaids abstract the time of termination of orogenesis for the southern altaids has been controversial systematic investigations of field geology geochronology and geochemistry on newly discriminated mafic ultramafic rocks from northern alxa in the southern altaids were conducted to address the termination problem the mafic ultramafic rocks are located in the bijiertai honggueryulin and qinggele areas stretching from west to east for about 100 km all rocks occur high grade gneisses as tectonic lenses that are composed of peridotite pyroxenite gabbro and serpentinite most of which have undergone pronounced alteration i e serpentinization and chloritization geochemically the rocks are characterized by uniform compositional trends i e with low sio2 contents 42 51 52 21 wt and alkalinity na2o k2o 0 01 5 45 wt mostly less than 0 8 wt and enrichments in mgo 7 37 43 36 wt with mg 52 75 91 87 as the rocks have been strongly altered and have a wide range of loss on ignition loi 0 44 14 07 wt values they may have been subjected to considerable alteration by either seawater or metamorphic fluids the ree and trace element patterns show a relatively fractionated trend with lile enrichment and hfse depletion similar to that of t morb between n morb and e morb indicating that the parental melt resulted from the partial melting of oceanic lithospheric mantle overprinted by fluid alteration of island arc origin the ultramafic rocks are relics derived from the magma after a large degree of partial melting of oceanic lithospheric mantle with superposed island arc processes under the influence of mid ocean ridge magmatism la icp ms u pb zircon ages of gabbros from three spots are 274 3 ma mswd 0 35 306 3 ma mswd 0 49 262 5 ma mswd 1 2 respectively representing the formation ages of the mafic ultramafic rocks therefore considering other previously published data we suggest that the mafic ultramafic rocks were products of south dipping subduction most probably with a slab window caused by ridge subduction of the paleo asian ocean plate beneath the alxa block in the late carboniferous to late permian before the ocean completely closed this sheds light on the controversial tectonic history of the southern altaids and supports the concept that the termination of orogenesis was in the end permian to triassic

high sr y and la yb ratios the meaning of the adakitic signature abstract the name of adakite is used to describe a far too large group of rocks whose sole common feature is high sr y and la yb ratios however such a signature can be achieved via different processes melting of a high sr y and la yb source deep melting with abundant residual garnet fractional crystallization or afc or interactions of felsic melts with the mantle causing selective enrichment in lree and sr over hree a database of the compositions of adakitic rocks including high silica and low silica adakites martin h smithies r h rapp r p moyen j f champion d c 2005 an overview of adakite tonalite trondhjemite granodiorite ttg and sanukitoid relationships and some implications for crustal evolution lithos 79 1 2 1 24 continental adakites and archaean adakites was assembled geochemical modeling of the potential processes is used to interpret it and reveals that 1 the classical model of slab melting provides the best explanation for the genesis of high silica adakites 2 low silica adakites are explained by garnet present melting of an adakite metasomatized mantle i e at depths greater than 2 5 gpa 3 continental adakites is a term encompassing a huge range of rocks with a corresponding diversity of petrogenetic processes and most of them are different from both low and high silica adakites 4 archaean adakites show a bimodal composition range with some very high sr y examples similar to part of the ttg suite reflecting deep melting 2 0 gpa of a basaltic source with a relatively high sr y while lower sr y rocks formed by shallower 1 0 gpa melting of similar sources comparison with the archaean ttg suite highlights the heterogeneity of the ttgs whose composition spreads the whole combined range of hsa and archaean adakites pointing to a diversity of sources and processes contributing to the ttg suite

stone age midden deposition assessed by bivalve sclerochronology and radiocarbon wiggle matching of arctica islandica shell increments abstract sclerochronology the study of the skeletal diaries of mollusks and corals is a high resolution geochronological tool of versatile usage in archaeology and paleontology with increasingly growing opportunities much of the recent efforts have concentrated on building multi centennial bivalve growth records using annually deposited increments in the holocene shells comparable to tree ring chronologies in the context of geoarchaeology the hitherto unachieved potential includes the application of sclerochronology to reconstruct long term settlement histories here we contribute to both of these critical issues by presenting the first multi shell sclerochronology constrained by methods originally developed in tree ring research using anthropogenically deposited bivalve shells of arctica islandica excavated from a stone age midden in north norway our systematic chronological approach to shell growth histories lays the foundation for a multi dimensional dating framework that interacts between the incremental radioisotopic and stratigraphic evidence we show how the crossdating within and between the single shell records yields a 155 year multi shell sclerochronology supported by the 14 c ams dates that demonstrates minimum midden accumulation of 82 years and a depositional rate of 0 3 cm yr sclerochronology paves the way for radiocarbon wiggle matching which narrows the probabilistic 2 uncertainty range for the oldest and youngest 14 c ams dates to 3150 2980 and 3060 2890 bc respectively we attribute the spectral characteristics of the chronology primarily to the north atlantic oscillation suggesting essentially similar influences of climate variability on the stone age culture and our own society

reconstructing estuarine conditions oyster shells as recorders of environmental change southwest florida abstract live collected shells of the oyster crassostrea virginica contain geochemical records of modern temperature and salinity so records of prehistoric conditions may be obtained from subfossil shells restoration of channelized watersheds in florida is receiving much attention and plans for targeted watersheds require information about estuarine conditions before channelization lack of historical records necessitates alternative methods to understand pre disturbance conditions a 14c calibrated amino acid geochronology based on racemization of glutamic acid yielded ages ranging from 190 1220 ad and from 1270 1860 ad for subfossil oysters from blackwater river near natural watershed and for faka union bay channelized watershed respectively 18o and 13c values of subfossil shells from blackwater river indicate salinity and summer temperatures similar to present winter temperatures recorded in shells from 190 590 720 and 1050 ad appear 1 5 c colder than present winter temperatures whereas the shell from 1220 ad records winter temperatures similar to modern winter temperatures these temperature shifts may indicate change in climate or natural seasonal variation of winter temperature from year to year subfossils from faka union bay may reflect a complicated hydrology which cannot be evaluated by isotopic compositions alone and demonstrates the need for development of independent elemental proxies for temperature and salinity decreases in 13c from subfossil to modern shells may in part result from co2 added to the atmosphere from fossil fuel burning the suess effect subfossil 13c that is 1 more positive than modern shells suggest a change in the dominant carbon sources from terrestrial c4 or aquatic plants to c3 plants mangroves

generation of sodium rich magmas from newly underplated basaltic crust sodium rich rocks of trondhjemite tonalite dacite ttd or granodiorite ttg suites form much of precambrian continental crust1 they are thought to have formed by partial melting of subducted oceanic crust2 3 a process that would have been much more widespread early in earth history than at present owing to the higher thermal gradients prevailing at that time4 phanerozoic ttd suites do exist however and seem also to relate to subduction zones5 defant and drummond6 proposed that these suites form where young 25 myr hot oceanic lithosphere is subducted and melts thus locally simulating the conditions that led to widespread crustal growth in the archaean here we describe plutonic and volcanic rocks from the cordillera blanca complex in peru which have characteristics of the high al ttd suite but which were produced above a subduction zone containing a 60 myr old slab we present evidence that the complex formed by partial melting of newly underplated basaltic crust and argue that this mechanism should be considered more generally as an additional way of generating sodium rich arc magmas

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

the dynamic evolution of the palaeozoic geography of eastern asia abstract new palaeogeographical reconstructions are presented for eleven time intervals through the palaeozoic of the eastern asia region from the middle cambrian at 510 ma to the end of the permian at 250 ma they centre on the continental blocks of north china south china and annamia indochina and their relationships with northeastern gondwana which was united to form part of pangea from the late carboniferous onwards also shown is the continent of tarim during the lower palaeozoic as well as the hutag uul songliao and khanka jiamasu bureya terranes both of which straddle the russian mongolian and chinese borders today from silurian times onwards we conclude that annamia and south china were united as a single continent throughout the lower palaeozoic and early devonian and were translocated by major strike slip faulting along the northeastern gondwana margin during that period from off afghanistan to outboard of the sibumasu and australian sectors of the superterrane they left the gondwana marginal area together during the lower devonian opening of the palaeotethys ocean but very shortly afterwards they themselves divided into the two separate continental blocks that we recognise today not to reunite until the triassic the various cambrian to permian rocks found in japan largely represent active volcanic arcs which originally lay to the southeast of south china although the carboniferous was more quiescent there the neotethys ocean opened during the permian dividing sibumasu and the tibetan terranes from gondwana and the palaeotethys ocean started to close progressively in the upper palaeozoic as most of the east asian continents and smaller terranes moved towards siberia the positions of the various continents and terranes have been deduced from a mixture of palaeomagnetic and faunal data the positions of large igneous provinces and kimberlites and the need to provide kinematic continuity between maps of successive ages however many uncertainties remain

evidence in cerro pampa volcanic rocks for slab melting prior to ridge trench collision in southern south america late miocene approx 12 ma hornblende bearing andesitic to dacitic documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage wasysym pagestyle empty declaremathsizes 10 9 7 6 begin document mathrm 63 to 68 sio 2 1 2 to 1 9 k 2 o end document adakite flows at the small cerro pampa center in patagonia 47 55 s 71 25 have some of the clearest slab melt geochemical signatures yet seen in a phanerozoic center on continental crust these magmas formed in response to melting of the hot thin slab that was subducting beneath south america prior to the collision of the chile rise at approx 6 ma or at approx 10 ma their n morb like 87 sr 86 sr 0 70285 0 70309 and low 206 pb 204 pb 18 44 18 59 ratios show that they could have been generated by

shell gathering from mangroves and the seasonality of the southeast asian monsoon using high resolution stable isotopic analysis of the tropical estuarine bivalve geloina erosa from the great cave of niah sarawak methods and reconnaissance of molluscs of early holocene and modern times this paper describes a reconnaissance high resolution study of stable isotopes 18o and 13c from modern shells of the estuarine bivalve geloina erosa and those dated to the early holocene that were harvested by people from mangroves near the great cave of niah on the north coast of borneo this reconnaissance study provides high resolution palaeoclimatic palaeohydrological information concerning early human activity in the region and the past character of the southeast asian monsoon laser ablation continuous flow isotope ratio mass spectrometry la cf irms on modern shells of geloina erosa revealed saw tooth stable isotopic profiles that bear a close resemblance to peaks and troughs of trends in recent local rainfall including the 1998 el nino drought highlighting the potential of geloina erosa for reconstructing seasonality of the southeast asian palaeomonsoon la cf irms analysis of prehistoric shells of geloina erosa held in the harrisson archives of the sarawak museum revealed cyclical shifts in 18o with similar amplitudes of variation as found in modern shells of geloina erosa as at the present day this probably reflects the changing seasonality of the monsoon rains with shifts to negative 18o values during periods of high runoff lighter mean 18o values of the prehistoric shells however may indicate a greater annual surplus of rainfall and possibly consistent with the early holocene strengthening of the summer monsoon at that time the similarity of the last growth increment 18o of the prehistoric shells to their mean 18o profile values suggests that gathering took place during times of moderate runoff

accretionary growth and crust formation in the central asian orogenic belt and comparison with the arabian nubian shield kroner a windley b f badarch g tomurtogoo o hegner e jahn b m gruschka s khain e v demoux a and wingate m t d 2007 accretionary growth and crust formation in the central asian orogenic belt and comparison with the arabian nubian shield in hatcher r d jr carlson m p mcbride j h and martinez catalan j r eds 4 d framework of continental crust geological society of america memoir 200 p 181 209 doi 10 1130 2007 1200 11 for permission to copy contact editing geosociety org 2007 the geological society of america all rights reserved kroner kroener mail uni mainz de windley brian windley btinternet com tomurtogoo igmr magicnet mn hegner hegner lmu de jahn jahn earth sinica edu tw khain khain ginras ru demoux demoux uni mainz de wingate mwingate tsrc uwa edu au

a model for monazite melt equilibrium and application to the generation of granitic magmas abstract in order to model the behaviour of zr hf ti p u th y and the rare earth elements ree in the continental crust the parameters which control the equilibria between accessory minerals and granitic melts must be determined this paper proposes equations for the equilibrium between monazite and ca poor felsic melts in a first step all the light ree are considered as a single component and an equation describing monazite solubility is deduced from the available experimental data in a second step the fractionation of ree by monazite is investigated on the basis of a natural example of coexisting monazite and obsidian from macusani peru the solubility of monazite can be used as a thermometer based on the total ree content of natural magmatic rocks application of ree thermometry to various peraluminous rock series suggests that it is a reliable tool for determining temperatures of magmatic processes the equation governing ree fractionation by monazite is used to discuss genetic relationships between a melt and a presumed parental rock using the composition of the monazite in the parental rocks and the ree pattern of the daughter rock this method has been applied successfully to high himalayan manaslu and tibetan slab granites it is suggested that detailed petrographical work on monazite including electron microprobe analyses and sem imaging of its internal structures combined with the model presented here can be a useful tool for deciphering the genesis and the differentiation of peraluminous granitic magmas in the continental crust

tectonic evolution of the south tianshan orogen and adjacent regions nw china geochemical and age constraints of granitoid rocks geochemical and geochronological evidence was obtained from granitoids of the south tianshan orogen and adjacent regions which consist of three individual tectonic domains the kazakhstan yili plate the central tianshan terrane and the tarim plate from north to south the central tianshan terrane is structurally bounded by the early paleozoic nikolaev line north nalati fault and late paleozoic atbashy inyl chek south nalati qawabulak fault zones against the kazakhstan yili and tarim plates respectively the meta aluminous to weakly peraluminous granitic rocks which are exposed along the kekesu river and the bikai river across the central tianshan terrane have a tholeiitic calc alkaline or high potassium calc alkaline composition i type geochemical trace element characteristics and the y versus rb nb or y versus nb discrimination diagrams favor a continental arc setting for these granitoid rocks shrimp u pb and la icp ms u pb zircon age data indicate that the magmatism started at about 480 ma continued from 460 to 330 ma and ended at about 275 ma the earlier magmatism 470 ma is considered to be the result of a simultaneous southward and northward subduction of the terskey ocean beneath the northern margin of the tarim plate and the kazakhstan yili plate respectively the later magmatism 460 330 ma is related to the northward subduction of the south tianshan ocean beneath the southern margin of the kazakhstan yili central tianshan plate the dataset presented here in conjunction with previously published data support a late paleozoic tectonic evolution of the south tianshan orogen not a triassic one as recently suggested by shrimp u pb zircon dating for eclogites

petrogenesis of cretaceous adakitic and shoshonitic igneous rocks in the luzong area anhui province eastern china implications for geodynamics and cu au mineralization both adakitic and shoshonitic igneous rocks in the luzong area anhui province eastern china are associated with cretaceous cu au mineralization the shaxi quartz diorite porphyrites exhibit adakite like geochemical features such as light rare earth element lree enrichment heavy ree hree depletion high al2o3 mgo sr sr y and la yb values and low y and yb contents they have low end t values 3 46 to 6 28 and high 87sr 86sr i ratios 0 7051 0 7057 sensitive high resolution ion microprobe shrimp zircon analyses indicate a crystallization age of 136 3 ma for the adakitic rocks most volcanic rocks and the majority of monzonites and syenites in the luzong area are k rich or shoshonitic and were also produced during the cretaceous 140 125 ma they are enriched in lree and large ion lithophile elements and depleted in ti and nb and ba and exhibit relatively lower end t values ranging from 4 65 to 7 03 and relatively higher 87sr 86sr i ratios varying between 0 7057 and 0 7062 the shoshonitic and adakitic rocks in the luzong area have similar pb isotopic compositions 206pb 204pb 17 90 18 83 207pb 204pb 15 45 15 62 and 208pb 204pb 38 07 38 80 geological data from the luzong area suggest that the cretaceous igneous rocks are distributed along ne fault zones e g tanlu and yangtze river fault zones in eastern china and were likely formed in an extensional setting within the yangtze block the shaxi adakitic rocks were probably derived by the partial melting of delaminated lower crust at pressures equivalent to crustal thickness of 50 km i e 1 5 gpa possibly leaving rutile bearing eclogitic residue the shoshonitic magmas in contrast originated mainly from an enriched mantle metasomatized by subducted oceanic sediments they underwent early high pressure 1 5 gpa fractional crystallization at the boundary between thickened 50 km lower crust and lithospheric mantle and late low pressure 1 5 gpa fractional crystallization in the shallow 50 km crust the adakitic and shoshonitic rocks appear to be linked to an intra continental extensional setting where partial melting of enriched mantle and delaminated lower crust was probably controlled by lithospheric thinning and upwelling of hot asthenosphere along ne fault zones e g tanlu and yangtze river fault zones in eastern china both the shoshonitic and adakitic magmas were fertile with respect to cu au mineralization

amphibolite dehydration melting sorting out the solidus amphibolite melts under vapour absent conditions by dehydration melting during which h 2o released from hornblende is transferred directly into h 2o undersaturated silicate liquid five independent 1991 studies on amphibolite dehydration melting differ from each other and from earlier conceptual treatments five solidi between 7 and 10 kb vary from 975 c to 740 c two solidi above 10 kb are near vertical near 975 c and 850 c some differences are due to different amphibolite compositions and mineralogy we use the results of earlier experiments involving garnet supported by our own new results to construct a closed system phase diagram for a simple fully hydra ted amphibolite hornblende plagioclase showing the solidus and the melting interval for hornblende garnet other minerals liquid the amphibolite vapour absent solidus is defined by the beginnings of two multivariant reactions 1 a near vertical curve large positive d p dt where the formation of augite dominates and 2 a near horizontal curve a t higher pressures where the formation of garnet dominates the solidus curve bends back estimated near 900 c 9 kb with slope changing to low positive dp dt the new phase diagram expands the field for liquid generation with garnet amphibolite residues to much lower temperatures and pressures than the other recent experimental results increasing the range from which small amounts of hydrous granitoid melts may be segregated by compaction the reversed near horizontal slope of the solidus near 10 kb has interesting petrological consequences

radiocarbon variability in the western equatorial pacific inferred from a high resolution coral record from nauru island we have generated a high resolution coral delta sup 14 c record spanning the last 50 years to document the seasonal and interannual redistribution of surface waters in the western tropical pacific prebomb 1947 endash 1956 delta sup 14 c values average minus 63 per thousand and have a total range of 30 per thousand values begin to increase in 1957 reaching a maximum of 137 per thousand in mid 1983 large interannual variability of up to 80 per thousand closely follows the el ni tilde n o southern oscillation enso during each enso warm phase delta sup 14 c values begin to increase reflecting the reduction of low sup 14 c water upwelling in the east and the invasion of subtropical water into the western equatorial tropical pacific maximum delta sup 14 c values are in phase or lag the corresponding sea surface temperature maxima in the eastern tropical pacific whereas the rapid return to more negative delta sup 14 c is in phase with eastern pacific enso indices the highest amplitude excursions occur during the 1965 1966 and 1972 1973 events when the sup 14 c contrast is highest between the eastern pacific and subtropics the 1982 1983 el ni tilde n o although a larger enso event has a lower delta sup 14 c amplitude reflecting the penetration of bomb radiocarbon into the equatorial undercurrent and the reduced contrast in delta sup 14 c betweenmore thermocline and subtropical surface waters at that time this coral record demonstrates the potential for using similar radiocarbon time series for documenting variability in pacific shallow circulation over interannual and decadal timescales copyright 1998 american geophysical union less

isotopic evidence for climate change during the vandal minimum from ariopsis felis otoliths and mercenaria campechiensis shells southwest florida usa archaeological evidence from coastal southwest florida suggests this region and its local inhabitants the calusa were affected by drought and cooling during the vandal minimum climate episode ad 500 800 to test this hypothesis we reconstructed seasonal scale climate conditions using stable oxygen and carbon isotope ratios 18o and 13c preserved in ariopsis felis otoliths and mercenaria campechiensis shells comparing 18o records from both species distinguishes between cool versus warm and wet versus dry conditions 18o values from four otoliths indicate cooling of winter temperatures occurred in the early vandal minimum ad 500 600 and late half of the middle vandal minimum ad 650 700 persistent dry summers punctuated by occasional years with wet summers throughout the vandal minimum were detected from 18o values of eight archaeological shells our climate reconstructions are in good agreement with archaeological observations and with the cool and dry conditions documented in europe

amphibole out phase boundary in partially melted metabasalt its control over liquid fraction and composition and source permeability a number of experimental studies in recent years have demonstrated that partial melting of hydrated basalt at lower crustal to upper mantle pressures is capable of generating up to 40 high sio2 liquids closely comparable in many geochemical respects to tonalitic trondhjemitic granitoids ttg from these studies it is apparent that the initiation of melting coincides with the beginning of amphibole dehydration but that a broad reaction interval over which amphibole coexists with high sio2 liquids is defined by the wet basalt solidus on one side and the amphibole out phase boundary on the other the phase relations of an alkali rich tholeiitic metabasalt have been examined in the outer half of this region up to and beyond the amphibole out phase boundary results indicate that amphibole exerts a strong control over the amount and composition of coexisting liquid over this interval melt fraction increases slowly and gradually from 0 up to 20 30 between the wet basalt solidus and up to amphibole out but increases substantially from 30 to 50 across and immediately beyond it liquids are strongly to moderately peraluminous molar al2o3 cao na2o k2o a cnk 1 0 up to amphibole out neutral at the boundary a cnk 1 0 and increasingly metaluminous a cnk 1 0 beyond it up to amphibole out potassium is strongly partitioned into liquid relative to amphibole i e mineral melt kd 0 2 0 3 whereas titanium is strongly partitioned into liquid kd 2 0 6 0 physical and chemical criteria suggest that efficient melt segregation leading to ttg plutonism requires more than 20 30 batch melting of a garnet bearing basaltic protolith near or beyond the amphibole out phase boundary leaving dry residues of eclogite or garnet granulite and producing metaluminous liquids that are strongly depleted in heavy rare earth elements hrees and yttrium y lower degrees of melting may involve critical melting where melt is squeezed out of the residue by deformation of the crystalline matrix resulting in disequilibrium melt compositions

evolution of a permian intraoceanic arc trench system in the solonker suture zone central asian orogenic belt china and mongolia abstract the identification of a fossil arc trench system from the ophiolite decorated solonker suture zone in the southernmost central asian orogenic belt caob enables us to constrain the timing of pre subduction extension ca 299 290 ma subduction initiation ca 294 280 ma ridge trench collision ca 281 273 ma and slab break off ca 255 248 ma in the permian a fraction of proto arc crust ca 45 km long up to 8 km wide is preserved as a volcanic plutonic sequence and is juxtaposed against a wide ca 30 80 km forearc melange this proto arc crust comprises two distinct magma series island arc tholeiite iat and mid ocean ridge basalt morb both of which have strong supra subduction zone ssz geochemical signatures zircons from a gabbro and a plagiogranite yielded weighted mean 206pb 238u ages of 284 0 4 0 and 288 0 6 0 ma the forearc melange consists of numerous ophiolite fragments and continental margin derived olistoliths blocks that predate the ophiolite the olistoliths are best represented by a gabbroic block 291 8 2 3 ma that contains granite xenoliths 312 6 1 8 and 313 6 3 1 ma other dated blocks include a trondhjemite 323 9 2 7 ma a gabbro 296 6 1 7 ma and a tonalite 294 9 2 4 ma small bodies of diabase andesite and diorite in the forearc melange exhibit a wide variety of geochemical signatures we dated zircons from an n morb like diabase 274 4 2 5 ma an e morb like diabase 252 5 2 3 ma a transitional sanukitoid adakite andesite 250 2 2 4 ma a sanukitoid high mg diorite 251 8 1 1 ma and an anorthosite 252 2 1 7 ma the n morb like diabase contains ca 301 394 ma zircon xenocrysts suggesting assimilation of trench sediments when a spreading ridge intersected a trench the other dated rocks simultaneously formed near the permian triassic boundary and captured abundant zircon xenocrysts ca 269 295 ma ca 301 495 ma and ca 923 2501 ma our new formation ages constrain a magmatic episode in response to slab break off beneath a fossil forearc in a young post collisional setting and the youngest xenocryst ages ca 269 273 ma may define the maximum depositional age of trench sediments

deep sea coral evidence for rapid change in ventilation of the deep north atlantic 15 400 years ago coupled radiocarbon and thorium 230 dates from benthic coral species reveal that the ventilation rate of the north atlantic upper deep water varied greatly during the last deglaciation radiocarbon ages in several corals of the same age 15 41 0 17 thousand years and nearly the same depth 1800 meters in the western north atlantic ocean increased by as much as 670 years during the 30 to 160 year life spans of the samples cadmium calcium ratios in one coral imply that the nutrient content of these deep waters also increased our data show that the deep ocean changed on decadal centennial time scales during rapid changes in the surface ocean and the atmosphere

a review of the relationships between granitoid types their origins and their geodynamic environments abstract granitoids are divided into several types according to their mineral assemblages their field and petrographical features and their chemical and isotopic characteristics this typology complements most of the recent classifications because it is not based solely on chemical and isotopic criteria but also on the field petrographical and mineralogical criteria it thus has the advantage of distinguishing the various granitoid types in the field in most cases the proposed classification shares many similarities with the twenty most used genetic classifications of granitoids both types of peraluminous granitoids are of crustal origin the tholeiitic alkaline and peralkaline granitoids are of mantle origin and both types of calc alkaline granitoids are of mixed origin and involve both crustal and mantle materials each granitoid type is generated and emplaced in a very specific tectonic setting each stage of the wilson cycle is characterised by typical associations of granitoids well typed and precisely dated granitoids can then complement structural approaches and indicate on the geodynamic environment with reference to some case studies the use of granitoids as tracers of the geodynamic evolution is also proposed and discussed

geochronology and petrology of migmatites from the proterozoic damara belt importance of episodic fluid present disequilibrium melting and consequences for granite petrology abstract the oetmoed granite migmatite complex ogmc central damara orogen namibia consists of grt and crd bearing s type granites and hbl and titanite bearing a type granites that intruded into crd sil kfs bearing metasedimentary rocks stromatic migmatites and nebulites stromatic migmatites formed by limited in situ partial melting of metapelites under h2o saturated conditions at 700 c and 4 5 kbar this partial melting event took place close to the peak of regional metamorphism at 510 ma as revealed by sm nd garnet whole rock ages as well as u pb monazite ages the newly formed melt remained more or less at the site of origin melanosomes of the stromatic migmatites do not resemble true residues instead they represent reaction zones between in situ melt and the metasedimentary host rock leucosomes from the stromatic migmatites are lree and hfse depleted which is typical for the low melt fractions generally observed in migmatite terranes mass balance calculations suggest that these leucosomes may be disequilibrium melts similar 18 o whole rock values between 13 and 14 observed in leucosomes and corresponding melanosomes suggest homogenization of oxygen isotopes between the different rock types via the melt and an internal fluid phase during melting other leucosomes with a major element chemistry similar to the leucosomes from the stromatic migmatites but higher ree contents suggest significant incorporation of lree enriched phases monazite from the country rocks for these leucosomes sm nd garnet whole rock ages are 473 ma indicating a second phase of melting nebulites mainly result from injection of granitic melts into the country rocks but their residual chemistry indicates that partial melting and limited melt removal must have occurred monazites from these nebulites record concordant u pb ages between 540 ma and 470 ma indicating episodic migmatization during high grade regional metamorphism intrusive peraluminous granites are likely generated by partial melting of pelitic sources in the lower crust and different source rocks are probably not the controlling factor for the different chemistry of leucosomes and granites therefore different melting conditions fluid absent vs fluid present and different modes of entrainment and solubilities of accessory phases control the elemental budgets of the leucosomes large scale migmatite granite complexes are a substantial part of some high grade terrains but the distinctive geochemical composition of some leucosomes make these rocks unlikely precursors of large scale granitic bodies

middle cambrian to permian subduction related accretionary orogenesis of northern xinjiang nw china implications for the tectonic evolution of central asia abstract middle cambrian to permian subduction related records are widely distributed in northern xinjiang which can be grouped into the chinese altay east junggar eastern tien shan west junggar yili and tarim domains by integrating paleogeographic and geological data we suppose that the chinese altay east junggar eastern tien shan domain was more closely located to siberia while the west junggar and yili domains occupied an intermediate position near the kazakhstan block in the early paleozoic paleoasian ocean distribution of andean type magmatic arcs island arcs accretionary wedges ophiolitic slices and or microcontinents shows an archipelago paleogeography forming a huge accretionary active margin sequences the tarim domain was on the opposite side of the early paleozoic paleoasian ocean remaining passive margin these tectonic units drifted northwards and approached the southern active margin of the siberian craton in the late paleozoic leading to termination of the paleoasian ocean and formation of a complicated orogenic collage between siberian craton and the tarim block between the end permian and triassic these multiple accretion processes significantly contributed to the lateral growth of central asia

origin and assembly of the tethyside orogenic collage at the expense of gondwana land abstract the tethysides are a superorogenic complex flanking the eurasian continent to the south and consisting of the cimmerides and alpides products of palaeo and neo tethys respectively we here review their evolution mainly on the basis of new maps showing the distribution of sutures magmatic rocks certain palaeobiogeographically and palaeoclimatologically significant taxa and facies and fragments of pan african 900 450 ma orogenic system forming the basement of many tethyside blocks these are supplemented by palaeomagnetic data reported in the literature a fundamental tenet of this paper is that major sutures which contain ophiolite fragments represent tectonic sections between continental blocks where oceanic crust has been subducted palaeo tethys came into existence largely in late carboniferous time coevally it began to be consumed by both internal and peripheral subduction zones which continued into the permian some of these had been inherited from pre tethyan times in the later permian rifting subparallel with the northern margin of gondwana land began between the zagros and malaysia separating a cimmerian continent from n gondwana land and thus heralding the opening of neo tethys and other smaller oceans that were back arc basins of palaeo tethys this rifting possibly also extended farther west into crete and mainland greece however the north china block yangtze block huanan block the eastern moity of the qangtang block north tibet and annamia all originally pieces of the end proterozoic early palaeozoic gondwana land had already separated from it in pre late carboniferous times possibly during the devonian all of these blocks and the cimmerian continent were characterized by cathaysian floral elements in late palaeozoic time palaeomagnetic and palaeontological data showing the original gondwana land affinity of these continental blocks are supplemented by correlating late proterozoic early palaeozoic pan african sutures orogenic belts and sedimentary basin fragments across tethyside sutures late permian foraminiferal provinces are related to this palaeogeographical interpretation by triassic times most cimmeride subduction zones were already in existence the cimmerian continent accelerated its separation from gondwana land and locally in the late permian began disintegrating internally along the waser rushan pshart banggong co nu jiang mandalay ocean by late triassic time all of the chinese blocks except lhasa and annamia had collided with each other and with laurasia the resulting enormous orogenic collage had a soft cushion between itself and laurasia in the form of the enormous accretionary complex of the songpan ganzi this connection enabled laurasian land vertebrates to reach south east asia by late triassic time in late triassic to middle jurassic times most major cimmeride collisions were completed widespread aridity in central asia occurred in late jurassic time probably in the rain shadow of the newly formed cimmeride mountain wall neo tethyan subduction systems formed along the s margin of the cimmerides or within neo tethyan oceanic lithosphere during the jurassic most if not all were north or east dipping they continued the northerly migration of the tethyside blocks evolution of the tethysides influenced the distribution of marine and terrestrial organisms and affected sea level changes and patterns of atmospheric circulation during much of the mesozoic and cainozoic it is likely to have reflected the surface expression of a persistent trend in the large scale convective circulation in the mantle that continuously transported material northward into the tethyan domain

late holocene climate change for the eastern interior united states evidence from high resolution δ 18 o values of sagittal otoliths abstract stable oxygen isotope values were determined for freshwater drum aplodinotus grunniens sagittal otoliths recovered from the eastman rockshelter archaeological site in northeast tennessee to evaluate climate change for the eastern interior united states from 5500 calendar years ago to the present micromilled samples representing less than six days of otolith growth were extracted to acquire high resolution intra otolith variation in 18 o values freshwater drum sagittal otoliths form annual bands due to thermally induced growth cessation below 10 c 18 o h 2 o values can be calculated once a high resolution carbonate sample representing the beginning of the growing season is isolated maximum summer temperature can be calculated using the seasonal minimum 18 o caco 3 value and the 18 o h 2 o value maximum summer temperatures calculated from the freshwater drum sagittae suggest that summer temperatures generally decrease from 29 c at 5 5 ka to 22 c at 0 3 ka however warmer climates at 2 9 1 7 1 6 and 1 2 1 0 ka punctuate this trend a more complete picture of the climate is reconstructed because 18 o h 2 o values which are a function of the ratio of summer to winter precipitation are also calculated a relatively low average 18 o h 2 o value of 8 1 vsmow was calculated at 1 0 ka suggesting cold winters dry summers and or wet winters may have prevailed during part of the medieval warm period in tennessee contrary to studies suggesting that the holocene was extremely stable and the hypsithermal was invariably warm and dry additional evidence suggesting both significant climate variability and evidence for a wetter mid holocene is presented

water fluxed melting of the continental crust a review abstract water fluxed melting also known as fluid or water present melting is a fundamental process in the differentiation of continents but its importance has been underestimated in the past 20 years during which research efforts focused mostly on dehydration melting reactions involving hydrate phases in the absence of a separate aqueous phase the presence of a free aqueous phase in anatectic terranes influences all major physical and chemical aspects of the melting process from melt volumes viscosity and ability to segregate from rock pores to melt chemical and isotopic composition a review of the literature shows that melting due to the fluxing of aqueous fluids is a widespread process that can take place in diverse tectonic environments active tectono magmatic processes create conditions for the release of aqueous fluids and deformation driven transient high permeability channels capable of fluxing high temperature regions of the crust where they trigger voluminous melting water fluxed melting can be either congruent in regions at the water saturated solidus or incongruent at suprasolidus p t conditions incongruent melting reactions can give rise to peritectic hornblende or to nominally anhydrous minerals such as garnet sillimanite or orthopyroxene in this case the presence of an aqueous phase is indicated by a mismatch between the large melt fraction generated and the much smaller fractions predicted in its absence the relatively small volumes of aqueous fluids compared to that of rocks imply that melting reactions are generally rock buffered fluids tend to move upwards and down temperature however there are cases in which pressure gradients drive fluids up temperature potentially fluxing suprasolidus terranes crustal regions at conditions equivalent to the water saturated solidus represent a natural impediment to the up temperature migration of aqueous fluids because they are consumed in melting reactions in this case continued migration into supra solidus terranes take place through the migration of water rich melts thus melts become the transport agent of water into supra solidus terranes and responsible for water fluxed melting other processes such as the relatively rapid fluid migration through fractures also allow regional aqueous fluids to by pass the water saturated solidus fluid trap and trigger melting above solidus conditions when aqueous fluids or hydrous melts flux rocks at supra solidus conditions they equilibrate with the surroundings through further melting decreasing water activity and giving rise to undersaturated melts it is in these conditions that hornblende or anhydrous peritectic phases are stabilized unlike dehydration melting the melt fraction generated in this case is not limited by the water contained in hydrous minerals but by the volume of water added to the system unlike melting at the water saturated solidus these melts are capable of rising without freezing and do give rise to upper crustal granitic bodies

the most ancient ophiolite of the central asian fold belt u pb and pb pb zircon ages for the dunzhugur complex eastern sayan siberia and geodynamic implications abstract ophiolitic rocks with a zircon age of 1020 ma occur in the dunzhugur complex of east sayan siberia and are part of a neoproterozoic to early palaeozoic segment of the central asian fold belt the most spectacular suite is exposed along the oka and bokson rivers where a complete ophiolite sequence with mantle tectonites a layered sequence composed of dunite wehrlite and pyroxenite a gabbro section a sheeted diabase dyke complex and basaltic pillow lavas are exposed petrologic and geochemical data suggest that all members of the ophiolite originally belonged to the same cogenetic mafic ultramafic crustal section and support a supra subduction zone setting in a fore arc rifting environment for its origin two multigrain zircon size fractions from a plagiogranite are both slightly discordant but yielded a combined mean 207pb 206pb age of 1020 10 ma evaporation of three additional fractions of three to four grains each from the same sample produced a mean 207pb 207pb age of 1019 9 0 7 ma that we consider to most closely reflect the time of igneous crystallization of the plagiogranite this is the oldest ophiolite so far dated from the central asian fold belt the southern margin of the siberian craton and the palaeo asian ocean were established at the end of the mesoproterozoic at least 1000 ma ago during the time interval 1000 570 ma one or several large ocean basins existed between baltica siberia kazakhstan tarim and northern china and these blocks are therefore unlikely to have been part of the supercontinent rodinia rifting initiation of subduction and marginal basin formation began prior to 1000 ma and continued through 570 ma the dunzhugur ophiolite of eastern sayan provides evidence for the early opening of the palaeo asian ocean not later than 1000 ma ago

paleozoic accretionary orogenesis in the eastern beishan orogen constraints from zircon u pb and 40ar 39ar geochronology abstract the continental growth mechanism of the altaids in central asia is still in controversy between models of continuous subduction accretion versus punctuated accretion by closure of multiple oceanic basins the beishan orogenic belt located in the southern altaids is a natural laboratory to address this controversy key questions that are heavily debated are the closure time and subduction polarity of former oceans the emplacement time of ophiolites and the styles of accretion and collision this paper reports new structural data u pb and ar ar ages from the eastern beishan orogen that provide information on the accretion process and tectonic affiliation of various terranes our geochronological and structural results show that the younging direction of accretion was northwards and the subduction zone dipped southwards under the northern margin of the shuangyingshan micro continent this long lived and continuous accretion process formed the hanshan accretionary prism our field investigations show that the emplacement of the xiaohuangshan ophiolite was controlled by oceanic crust subduction beneath the forearc accretionary prism of the shuangyingshan mazongshan composite arc to the south moreover we address the age and terrane affiliation of lithologies in the eastern beishan orogen through detrital zircon geochronology of meta sedimentary rocks we provide new information on the ages subduction polarities and affiliation of constituent structural units as well as a new model of tectonic evolution of the eastern beishan orogen the accretionary processes and crustal growth of central asia were the result of multiple sequences of accretion and collision of manifold terranes

early cretaceous granitoids and their implications for the collapse of the dabie orogen eastern china shrimp zircon u pb dating and geochemistry abstract two stages of early cretaceous post orogenic granitoids are recognized in the dabie orogen eastern china which recorded processes of extensional collapse of the orogen the early stage granitoids 132 ma are foliated hornblende quartz monzonites and porphyritic monzogranites they are of high k calc alkaline series and metaluminous to weakly peraluminous with high k 2 o and low mgo contents mg values 32 0 46 0 they contain high sr low y and heavy rare earth elements hree and have high sr y and la yb n ratios without clear negative eu sr and ti anomalies the early stage deformed granitoids have adakitic geochemical compositions and are equilibrated with residues rich in garnet and poor in anorthite rich plagioclase and thus indicate the existence of an over thickened 50 km crustal root beneath the orogen at 132 ma the later stage granitoids 128 ma are undeformed fine grained monzogranites fine grained k feldspar granites and coarse grained k feldspar granite porphyry they belong to a peraluminous and high k calc alkaline to shoshonite series and display a flat hree pattern and have strong negative eu sr and ti anomalies with low sr y and la yb n ratios the late stage granitoids are equilibrated with residues rich in anorthite rich plagioclase hornblende ilmenite titanite and poor in garnet indicating that the crust of the dabie orogen became thinner

formation and evolution of granite magmas during crustal reworking the significance of diatexites taylor mclennan 1985 the physical processes ina continuous section through reworked archaean crust records the volved particularly how granite magma is formed remain generation of granitic magma and its subsequent development in the controversial miller et al 1988 white chappell opatica subprovince in the canadian shield there the transition 1990 a partially melted source can form a granitic from palaeosome to granite was a closed system process through magma only if large volumes of melt are separated from intermediate stages of patch migmatite and diatexite the average most of their residuum understanding this process is degree of partial melting was less than 30 but the melt crucial to determining how crustal recycling and differfraction was redistributed within individual diatexite layers during entiation occurs for example melt residuum separation deformation regions that lost melt became residual diatexites could be a nearly perfect single stage process at the enriched in tio2 feot mgo cao sc cr co sr rare earth site of partial melting as described for leucosomes in elements ree and high field strength elements hsfe melt metatexite migmatites wickham 1987a sawyer 1991 accumulated to create diatexite magmas enriched in large ion 1994 brown et al 1995 alternatively the separation lithophile elements lile but contaminated with residuum macould be an imperfect multi stage process that yields a terial such diatexite magmas are parental to granites found at magma with a large residual component as is observed higher crustal levels in the terrane flow of the diatexite magma in in diatexite migmatites bea 1991 greenfield et al response to deformation separated some of its residuum into schlieren 1996 sawyer 1996 the metatexite model necessarily parautochthonous plutons were created where ascending granitic produces leucocratic residuum free granites such as magma locally ponded below impermeable layers and structures those described by le fort 1981 and montel et al magma left the anatectic region in dykes and lost its remaining residuum as it crystallized consequently the allochthonous granite 1991 and maximizes the geochemical signature of magmas that rose through 20 km of crust to feed the highest level crustal differentiation in contrast the diatexite model plutons in this region are highly fractionated and essentially free of can produce residuum rich granites and so reduce the residuum geochemical effect of crustal differentiation as chappell 1996 noted how anatectic magmas formed in the deeper crust evolve to the granitic magmas emplaced in the upper crust is also poorly known because few crustal sections

the wuluma granite arunta block central australia an example of in situ near isochemical granite formation in a granulite facies terrane abstract the wuluma granite is a small elongate relatively undeformed pluton in the proterozoic strangways metamorphic complex central australia the complex constitutes a supracrustal assemblage that underwent granulite facies metamorphism 1800 ma ago metamorphism was associated with at least three phases of folding that ultimately produced upright regional doubly plunging f3 folds and isobaric cooling ensued generation of the wuluma granite occurred at 1750 ma based on rb sr isotopic data during syn d3 regional retrogression and rehydration of the terrane contacts between the granite and gneisses are invariably gradational at the pluton margin banded gneisses grade along strike into granite containing abundant biotite schlieren that parallel regional structures granite and pegmatite dykes cut these rocks inwards from the contact the granite is more homogeneous containing diffuse parallel schlieren and small aligned rectangular feldspar crystals indicating flow of magma rafts of unmelted granofels form a ghost layering they mimic macroscopic f3 folds and show only minor retrogressive metamorphic effects at the pluton core the granite is homogeneous and structurally isotropic containing some subrounded granofelsic inclusions very diffuse schlieren and disaggregated pegmatite dykes thus it appears that an isoclinally folded vertical body of quartzofeldspathic gneiss was melted in situ to form the pluton which did not break away from the source the body resembles a tapered diapir and we term this type of pluton a regional migmatite terrane granite geochemical data are consistent with the granite forming by anatexis of quartzofeldspathic migmatitic gneisses with appropriate composition the chemical similarity of both rock types implies derivation of the granite by either partial melting and retention of residual material in the magma or more complete melting followed by solidification virtually in situ the latter interpretation is preferred although biotite melanosome has been incorporated into the magmas migmatitic gneisses with feldspar proportions corresponding to ternary minima compositions are argued to have melted during influx of an externally derived aqueous fluid associated with pegmatites field evidence shows that slightly different gneisses melted to produce slightly different granite compositions even on outcrop scale the compositions of migmatites and derivative granite both range from 67 to 77 sio2 therefore crystal fractionation did not produce the geochemical variation rather in situ crystallization of the granite resulted in a composition near isochemical to that of the precursors but with a coarser obviously granitic texture

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

paleozoic accretionary and collisional tectonics of the eastern tianshan china implications for the continental growth of central asia this paper deals with the various tectonic units in the chinese eastern tianshan orogenic collage in the central asian orogenic belt and discusses the paleozoic geological history of the several periods of accretion and collision of archipelago systems lying between the tarim and southern angaran continental margins the late ordovician silurian to early devonian eastern tianshan archipelago was characterized by a the harlik dananhu subduction system with a s dipping polarity in the north b a southerly n dipping subduction system beneath the central tianshan arc in the middle and c the south tianshan ocean against tarim in the south during the devonian to early carboniferous n dipping subduction led to the harlik dananhu arc and the kanggurtag forearc basin accretionary complex in the early to mid carboniferous the magmatic front associated with the n dipping subduction beneath the dananhu harlik arc migrated southwards forming the yamansu arc constructed upon the kanggurtag accretionary forearc by the late carboniferous the dananhu harlik arc was attached northwards to the angaran margin resulting in lateral enlargement of the angaran continent in the latest carboniferous to early permian a multiple soft collision left wide suture zones in the south that include the ophiolite strewn aqikkuduk shaquanzi and kumishi accretion collision complexes which were stitched by early permian post collisional plutons by re defining and re interpreting the various tectonic terranes this paper presents a new improved model for the paleozoic evolution of this part of central asia

an early permian ca 280 ma silicic igneous province in the alxa block nw china a magmatic flare up triggered by a mantle plume abstract silicic igneous provinces have important implications for geodynamic processes in this study we show that the predominantly early permian ca 280 ma granitoids in the alxa block correspond to an important silicic igneous province with areal distribution in northwest nw china we present zircon u pb age and hf o isotopic and whole rock geochemical and sr nd isotopic data for the newly identified i and a type granites and high sr y granodiorites in the bayannuoergong batholith and gabbros diorites and quartz diorites in the bijiertai complex alxa block the i type monzogranites granodiorites and a type granites of the bayannuoergong batholith were probably generated by mixing between crust and mantle derived magmas partial melting of a thickened eclogitic lower crust and underplated mantle derived mafic rocks respectively the gabbros quartz diorites and diorites of the bijiertai complex were likely generated by partial melting of the lithospheric mantle differentiation from basaltic magmas and partial melting of the middle crust respectively despite pronounced geochemical and isotopic heterogeneity of these rocks new sims and la icp ms u pb zircon dating indicates that they were formed over a very short time interval at ca 280 ma thus the mantle lower crust and middle crust underwent widespread melting at this time given that these magmatic rocks have no spatial or temporal zonation and were probably formed in an extensional setting we suggest that the ca 280 ma tarim mantle plume in nw china may have triggered the magmatic flare up of the alxa silicic igneous province

how does the continental crust thin in a hyperextended rifted margin insights from the iberia margin the discovery of hyperthinned continental crust and exhumed mantle on present day deep water rifted margins leads to two fundamental questions 1 in detail how does the crust thin in extension and 2 what controls extreme crustal thinning and mantle exhumation reflection and refraction seismic lines across the iberian margin show decoupling levels in the crust cut by structures that eventually transfer deformation to mantle levels the region of decoupled extension appears to be more broadly distributed on the northern iberian margin and more localized on the southern margin based on drill hole data the transition from decoupled to coupled deformation occurred during tithonian time ca 145 ma this evolution from decoupled to coupled deformation may help explain the crustal architecture of the iberian margin an apparent delay of subsidence across the hyperextended coupled zone of the iberian margin may indicate that crustal thinning had to occur simultaneously with lithospheric necking and the advection of heat related to lithospheric mantle thinning

leucogranites in lhozag southern tibet implications for the tectonic evolution of the eastern himalaya abstract petrogenesis of the himalayan leucogranite is strongly influenced by conditions which are associated with the tectonic evolution of himalayan orogen in this article we present petrological geochronological and geochemical results of the lhozag leucogranites that crop out alongside the south tibetan detachment system stds in the east of himalaya zircon u pb dating revealed three episodes of leucogranitic magmatism in lhozag at 17 8 0 1 ma 15 1 0 1 ma and 12 0 0 1 ma respectively the lhozag leucogranites show relatively low end t low zircon ehf t and high initial 87sr 86sr ratios which are similar to the high himalayan crystalline series hhcs indicating that they were derived from the hhcs the characteristics of relatively high na2o and rb contents high rb sr ratios and low cao mgo tfe2o3 tio2 and sr contents indicate that both the ca 18 ma lhozag tourmaline leucogranites and the ca 15 ma lhozag two mica granites were derived from fluid absent muscovite dehydration melting of metasediments the opposite geochemistry characteristics of the ca 12 ma khula kangri two mica granites imply that these granites are derived from fluid present melting of metasediments four khula kangri two mica granite samples with relatively lower tio2 tfe2o3 mgo and cao contents higher rb concentrations and rb sr ratios could be evolved from the khula kangri two mica granites with relatively lower rb sr ratios the melting behaviors of the lhozag leucogranites varied from fluid absent melting to fluid present melting implying that there were p t xh2o variations in the deep crust the tectonic evolution would give rise to variation of p t xh2o variation and subsequent transformation of melting behavior our new results display the transformation of melting behavior of the lhozag leucogranites which implies the tectonic evolution from earlier n s extension to later e w extension in the eastern himalaya at ca 12 ma

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

terminal pleistocene through mid holocene archaeological sites as paleoclimatic archives for the peruvian coast abstract along the peruvian coast standard paleoclimatic archives such as pollen cores or corals are absent or undeveloped however anthropogenic deposits offer paleoclimatic information for the last 13 000 years at temporal scales ranging from centennial for long term records to seasonal for short term records this paper summarizes archaeologically based paleoclimatic data for the peruvian coast from the terminal pleistocene through the middle holocene south of 12 s coastal waters have been cool temperate and terrestrial conditions have been hyperarid for the entire period between 8000 and 3600 cal bp the highland precipitation supplying runoff to the south coast may have diminished sufficiently to restrict human occupation of ephemeral streams north of 10 s sea surface temperatures were warmer and seasonal precipitation may have been greater from prior to 13 000 to 5800 cal bp from 9000 to 5800 cal bp el nino was absent or very infrequent after 5800 cal bp the northern coastal waters cooled and el nino was present but with a lower frequency than seen in the historical and instrumental record el nino frequencies in the modern and historical range were established 3000 cal bp cultural changes correlate temporally with these major climatic transitions

oxygen isotopic evidence for greater seasonality in holocene shells of donax variabilis from florida abstract donax variabilis the variable coquina clam has been a common inhabitant of exposed sandy beach intertidal and shallow subtidal zones in the southeastern united states throughout the pleistocene and holocene it is ideally suited for paleotemperature studies because it is restricted to environments of well mixed normal marine seawater with a fairly uniform isotopic composition as a result oxygen isotopic variability in d variabilis shells is largely explained by temperature variation although d variabilis is small and short lived its shell represents an important paleoclimate archive because of its unique habitat preference high resolution sampling of individual d variabilis shells and comparison of oxygen isotopic temperature profiles with historical seawater temperatures from the northeastern florida coast indicate rapid shell growth over a brief life span of three to five or six months analysis of two modern shells reveals a close correspondence between isotopically determined water temperatures and historical water temperatures during the spring summer growing season paleotemperature profiles from four archaeological shells however suggest a longer growth interval spanning summer autumn two preceramic archaic period shells ca 4240 and 5570 14c yr bp and two orange period archaic shells ca 3600 and 3760 14c yr bp from four different archaeological sites yield paleotemperatures that average 3 5 c higher than present summer autumn water temperatures these warm paleotemperatures highlight seasonality differences associated with the mid holocene hypsithermal climatic interval in this region

paleoproterozoic evolution of the eastern alxa block westernmost north china evidence from in situ zircon u pb dating and hf o isotopes abstract the alxa block is the westernmost part of the north china craton ncc in the past it has been considered to be part of the archean ncc however formation and evolution of this block remain poorly understood and this has hampered a broader understanding of the ncc in this study we analyzed the in situ zircon u pb age and hf o isotopes for the two oldest rock units in the eastern alxa block namely the bayanwulashan and diebusige complexes the bayanwulashan complex consists mainly of metamorphic rocks with mafic and felsic protoliths sims u pb zircon dating results indicate that the primary magmatic ages of the mafic and felsic igneous rocks are ca 2 34 ga and ca 2 32 2 30 ga respectively and both sets of rocks were overprinted by two metamorphic events at ca 1 89 ga and ca 1 79 ga geochemical and zircon hf o isotopic data suggest that the ca 2 34 ga amphibolites within the bayanwulashan complex are characteristically high in tio 2 up to 3 2 zr up to 394 ppm and ti v 30 resembling the basaltic rocks formed within continental rifts the ca 2 32 2 30 ga felsic rocks were generated by remelting of dominant meta igneous rocks that have zircon hf model ages of 2 92 2 81 ga the diebusige complex is composed of amphibolites mafic gneisses and paragneisses it was intruded by granites dated at ca 1 97 1 98 ga and subjected to high grade metamorphic events at ca 1 89 ga and ca 1 79 ga re examination of u pb ages for detrital and metamorphic zircons indicate that the depositional ages of protoliths of the diebusige paragneisses may be considered to be between ca 2 45 and 2 0 ga these u pb zircon age data do not support the existence of exposed archean rocks in the eastern alxa block although the possibility of archean rocks at deeper crustal levels cannot be ruled out entirely the eastern alxa block contrasts to the neighboring yinshan block and khondlite belt as well as the trans north china orogen in terms of the timing and evolution of magmatism and metamorphism thus we conclude that the alxa block is a separated paleoproterozoic terrane from the western block of the ncc rather than the western extension of the yinshan block or part of the khondalite belt as previously proposed

sclerochronology and geochemical variation in limpet shells patella vulgata a new archive to reconstruct coastal sea surface temperature 1 climate archives contained in shells of the european limpet patella vulgata accumulated in archaeological deposits can potentially provide much needed information about holocene environmental change in midlatitude coastal areas before reconstructing climate information preserved in these zooarchaeological records we studied the controls on oxygen and carbon isotope ratios 18o and 13c respectively in modern specimens we tested the hypothesis that p vulgata precipitates its shell in isotopic equilibrium with the ambient water by comparing 18oshell with predicted values predicted 18oshell was constructed using observed sea surface temperature sst records and the equilibrium fractionation equation for calcite and water we assumed a constant 18owater value of 0 10 vsmow based on published regional measurements comparison of 18oshell with predicted values revealed that 18oshell values were higher than expected by 1 01 0 21 consequently estimated sst calculated from 18oshell was 4 2 2 3 c lower than observed sst however because of the relatively uniform offset between observed and expected 18o an adjustment can be made to account for this predictable vital effect thus past climate can be reliably reconstructed using this temperature proxy once the offset is taken into account 13c values have a similar cyclicity to the 18o variation and therefore vary seasonally however 13c is slightly out of phase relative to 18o an overall negative shift in 13cshell over the lifetime of the individual indicates a vital effect associated with ontogeny further study of environmental and ecological factors that influence shell 13c is required to evaluate fully the potential of carbon isotope ratios as a useful environmental proxy

fluid enhanced melting during prograde metamorphism anatexis is a commonly recognized feature of high grade metamorphism but segregated melts are generally ascribed to anatexis during peak metamorphic conditions and little is known about melting along the prograde path a suite of small volume deformed two mica leucogranites has been recognized within the high himalayan crystalline series of the garhwal himalaya these granites are consistently more siliceous than minimum melt granite compositions and are characterized by low rb sr ratios high ba low abundances of hfs elements and positive eu anomalies such trace element characteristics contrast strongly with the geochemistry of the well studied early miocene leucogranites of the high himalaya derived from fluid absent melting sm nd garnet dating of one deformed granite indicates a crystallization age of 39 3 ma c 15 ma before the emplacement of the more voluminous high himalayan leucogranites whilst some entrainment of restitic phases cannot be excluded trace element signatures suggest a low temperature 2 o activity positive eu anomalies and unusually low rb sr ratios are indicative of rapid disequilibrium melting fluid enhanced melting may be a common feature of prograde upper amphibolite facies metamorphism of orogenic belts predating peak metamorphism by at least 15 ma these melts will only crystallize within this period if they segregate from their protoliths subsequent dating of long lived melts would indicate erroneously young ages for the prograde melting events however melts formed in this way may be recognized by their distinctive trace element chemistry the persistence of early formed melts within an orogen provides insights into the prograde heating path and may be critical in controlling the rheology of the middle crust and hence its deformational history

timing petrogenesis and tectonic setting of the late paleozoic gabbro granodiorite granite intrusions in the shalazhashan of northern alxa constraints on the southernmost boundary of the central asian orogenic belt abstract the late paleozoic tectonic setting and location of the southernmost boundary of the central asian orogenic belt caob with respect to the alxa block or alxa north china craton ancc are debated this paper presents new geochronological petrological geochemical and zircon hf isotopic data of the late paleozoic intrusions from the shalazhashan in northern alxa and discusses the tectonic setting and boundary between the caob and ancc using zircon u pb dating intrusions can be broadly grouped as late carboniferous granodiorites 301 ma middle permian gabbros 264 ma and granites 266 ma and late permian granodiorites monzogranites and quartz monzodiorites 254 250 ma the late carboniferous granodiorites are slightly peraluminous and calcic the remarkably high zircon hf isotopes e hf t 6 10 and characteristics of high silica adakites suggest that these granodiorites were mainly derived from hot basaltic slab melts of the subducted oceanic crust the middle permian gabbros exhibited typical cumulate textures and were derived from the partial melting of depleted mantle the middle permian granites are slightly peraluminous with high k calc alkaline and low e hf t values from 0 9 to 2 9 these granites were most likely derived from juvenile materials mixed with old crustal materials the late permian granodiorites monzogranites and quartz monzodiorites are characterized as metaluminous to slightly peraluminous with variable peacock alkali lime index values from calc alkalic to alkali calcic these rocks were mainly derived from juvenile crustal materials as evidenced by their high e hf t values 3 3 to 8 9 the juvenile sources of the above intrusions in the shalazhashan are similar to those of the granitoids from the caob but distinct from the granitoids within the alxa block these findings suggest that the shalazhashan zone belongs to the caob rather than the alxa block and that its boundary with the alxa block can be regarded as the southernmost boundary of the caob the recognition of late carboniferous typical adakite magmatism in the region provides evidence for the subduction of the oceanic crust of the caob the middle late permian magmatisms 266 250 ma display a bimodal association with high k calc alkaline features and are interpreted as forming in a post collision setting these studies by interaction of regional geology provide new constraints on the tectonic evolution of the southern caob during the late paleozoic and the location of the southernmost margin of the caob

origin of mesozoic adakitic intrusive rocks in the ningzhen area of east china partial melting of delaminated lower continental crust to the best of our knowledge modern adakites have not been documented in a nonarc environment we report geochemical and isotopic data for early cretaceous anjishan adakitic intrusive rocks that are in a continental setting unrelated to subduction the anjishan adakitic intrusive rocks which are exposed in the ningzhen area of east china have high sr y and la yb ratios coupled with low yb and y as well as relatively high mgo contents and mg numbers mg 0 4 0 6 similar to products from slab melting however low n d t values 6 8 to 9 7 and high 8 7 sr 8 6 sr i 0 7053 0 7066 are inconsistent with an origin by slab melting the tectonics and geochemistry lead us to conclude that adakitic magmas were most likely derived from partial melting of mafic material at the base of the continental crust high sr y and la yb ratios of the adakitic intrusive rocks suggest that garnet was stable as a residual phase during partial melting implying that the crustal thickness exceeded 40 km in the early cretaceous the present thickness of the crust in the ningzhen area is only 30 km and therefore the crust appears to have been thinned by at least 10 km since the early cretaceous the relatively high mgo contents and mg of the anjishan intrusive rocks suggest that adakitic magmas interacted with mantle rocks possibly coinciding with lower crustal delamination which would also account for the observed thinning

petrogenesis of carboniferous adakites and nb enriched arc basalts in the alataw area northern tianshan range western china implications for phanerozoic crustal growth in the central asia orogenic belt abstract carboniferous volcanic rocks in the alataw area northern tianshan range xinjiang consist of early carboniferous ca 320 ma adakites and nb enriched arc basalts and basaltic andesites nebs and late carboniferous ca 306 310 ma mainly high k calc alkaline andesites dacites and rhyolites the adakites are calc alkaline and characterized by high na2o k2o 1 52 3 32 ratios negligible to positive eu anomalies strong depletion of heavy rare earth elements e g yb 0 74 1 47 ppm and y 6 7 14 9 ppm positive sr and ba but negative nb and ti anomalies and relatively constant end t values 3 4 6 6 and 87sr 86sr i ratios 0 7035 0 7042 some andesitic and dacitic adakite samples exhibit high mgo contents similar to magnesian andesites the nebs are sodium rich na2o k2o 2 03 8 06 and differ from the vast majority of arc basalts in their higher nb zr tio2 and p2o5 contents and nb th nb la and nb u ratios and minor negative to positive anomalies in ba nb sr zr and ti they have the highest end t values 6 4 11 6 but varying 87sr 86sr i ratios 0 7007 0 7063 the high k calc alkaline suite is similar to typical normal arc volcanic rocks in terms of moderately fractionated rare earth abundance and distinctly negative eu nb sr and ti anomalies they have end t values 1 2 6 4 and 87sr 86sr i ratios 0 7018 0 7059 geochemically they are similar to coeval i type granitoids in the alataw area given the presence of early carboniferous ophiolites in the northern tianshan range and the isotopically inappropriate compositions of proterozoic metamorphic basement in the alataw area we argue that the alataw adakites were most probably related to the melting of young subducted crust of the northern tianshan ocean the nebs likely originated from mantle wedge peridotites metasomatized by adakites and minor slab derived fluids the later high k calc alkaline suite was generated by afc processes that acted on melts derived from a mantle wedge metasomatized by hydrous fluids the larger range of isotopic compositions exhibited by both the neb and high k suite relative to the adakites suggests that the mantle wedge was heterogeneous prior to slab or fluid mediated metasomatism continental crustal growth of the central asian orogenic belt was dominated by contributions of the juvenile materials from the depleted mantle prior to 270 ma and possibly afterwards the results of this study suggest that other carboniferous nb enriched basalts in the tianshan range were generated by subduction processes rather than by intraplate tectonics as previously proposed

neoarchean paleoproterozoic multiple tectonothermal events in the western alxa block north china craton and their geological implication evidence from zircon u pb ages and hf isotopic composition abstract the alxa block was traditionally considered to be part of the north china craton but its metamorphic basement has been poorly studied here we present a systematic zircon u pb and hf isotopic investigation on four orthogneiss samples in the beidashan area of the western alxa block the petrographic and geochemical data show that these rocks are granodioritic and trondhjemitic gneisses with ttg tonalite trondhjemite granodiorite characteristics zircons from the ttg gneisses display typical core rim or core mantle rim structures u pb datings and hf isotopic analyses reveal two distinct age populations the latest neoarchean 2 5 ga and the late palaeoproterozoic 1 85 ga the magmatic zircon cores and metamorphic mantles rims of the ttg gneisses were dated at similar ages around 2 5 ga supporting the existence of archean rocks in the western alxa block the short time interval between the latest neoarchean magmatism and the subsequent metamorphism suggests that they were related to the same latest neoarchean tectonothermal event the 2 5 ga zircons have hf t mainly between 0 8 and 5 0 t dm hf model ages mainly between 2 6 and 2 8 ga with a peak at 2 7 ga and t dmc hf model ages mainly between 2 7 and 3 0 ga with a peak at 2 8 ga the age of 1 85 ga obtained from two trondhjemitic gneisses is interpreted as the age of the late paleoproterzoic high grade metamorphism our combined datasets show that the ttg gneisses in the baidashan area of the western alxa block experienced a main 2 7 2 8 ga crust growth a 2 5 ga magmatic metamorphic event and a 1 85 ga high grade metamorphic event the sequence of events is very similar to that of the other north china craton a combination of the data of paleoproterozoic metamorphic rocks in the alxa block suggests that the alxa block is the western extension of the khondalite belt rather than the yinshan block

evolution of the south china sea revised ages for breakup and seafloor spreading abstract the continental breakup which gave way to the formation of the oceanic south china sea scs basin began in the latest cretaceous in the northeastern scs and propagated in southern and western direction over a long period of time possibly more than 40 m y the seafloor spreading history of the south china sea has been interpreted in different ways in the past and the debate over the correct timing of the major tectonic events continues we review the different models that have been published and present a revised interpretation of seafloor spreading anomalies based on three datasets with documented high quality which cover all of the scs but the northernmost and southernmost parts we can precisely date the onset of seafloor spreading in the central part of the scs at 32 ma after a ridge jump at 25 ma spreading also began in the southwestern sub basin and spreading ended at 20 5 ma in the entire basin followed by a phase of magmatic seamount formation mainly along the abandoned spreading ridge spreading rates vary from 56 mm yr in the early stages to 72 mm yr after the ridge jump to 80 mm yr in the southwestern sub basin we find indications for a stepwise propagation of the seafloor spreading from northeast to southwest in segments bounded by major fracture zones seafloor spreading ended abruptly probably because the subduction zone along the eastern and southern boundary of the scs of which today the manila trench remains was blocked by collision with a continental fragment possibly the northern part of palawan or a part of the dangerous grounds

origin of high sr y type granitic magmatism in the southwestern of the alxa block northwest china abstract the petrogenesis of high sr y type magmas is still open to debate usually such magmas could result from melting under high pressure settings 12 kbar in this paper we gave an example that some high sr y type magmas could originate from melting of crustal materials at pressure of 10 12 kbar we carried out a study of petrogenesis for devonian high sr y granites from the beidashan batholith 397 411 ma southwestern alxa block northwest china the beidashan granites have sio 2 69 21 74 60 wt and al 2 o 3 14 01 16 20 wt with a cnk ratios of 0 99 1 08 according to their trace element compositions and whole rock zirconium saturation temperatures t zr the beidashan granites can be divided into two groups group i dy yb n 1 2 3 0 eu eu 0 77 1 3 t zr 761 856 c resulted from fluid absent partial melting of mafic to intermediate crustal materials leaving garnet residuum at pressure of 12 kbar and group ii dy yb n 0 76 2 16 eu eu 1 7 5 3 t zr 651 785 c formed by fluid present melting of mafic to intermediate crustal materials with residual amphibole in the source at pressure of 10 kbar both group i and group ii show high sr y and la yb n features they show i sr 0 7134 0 7180 e nd t 6 61 to 9 71 t 2dm 1 7 1 9 ga e hf t 5 6 to 9 9 and t dm c 1 7 2 0 ga indicating that the beidashan high sr y granites were derived from melting of crustal basement materials our results suggest that some high sr y type granites formed under relatively lower pressure conditions 10 12 kbar and they could not be an indicative of partial melting of thickened crust

alteration of otolith aragonite effects of prehistoric cooking methods on otolith chemistry geochemical analyses of fish otoliths from archaeological sites are a valuable source of paleoclimate data a common assumption in such research is that the otolith chemistry is unaltered from the time of capture until excavation although post depositional diagenesis will vary between sites according to local burial conditions the effects of common pre depositional treatment such as cooking or trash disposal can be evaluated experimentally six atlantic croaker micropogonius undulatus and six gafftopsail catfish bagre marinus were treated using five different methods to simulate cooking and trash disposal the left sagittal otolith was removed as an experimental control prior to cooking or burning each fish after cooking or burning the remaining otolith was removed and each otolith pair was tested to measure changes in crystallography carbon and oxygen isotope content and elemental chemistry recrystallization and changes in isotope chemistry only occurred in burned otoliths in contrast elemental composition was altered during each treatment method these experiments validate the use of carbon and oxygen isotope analyses of unburned otoliths in paleoclimate research but suggest caution in the interpretation of elemental data

stable isotopes and sclerochronology of the bivalve mesodesma donacium potential application to peruvian paleoceanographic reconstructions reconstructing the past enso el nino southern oscillation interannual variability requires very high resolution sea surface temperature sst proxies in order to compensate for the lack of coral archive in the eastern pacific cold tongue we studied the potentiality of the bivalve species mesodesma donacium as a sst tracer for the peruvian coast biomineralisation cycles and growth line formation in the shells are shown to be controlled by tides the sclerochronology based on fortnightly cycles allowed us to compare local sst data with high resolution shell records as a result of the high correlation between sst and aragonite delta 18 o we calculated a linear sst fractionation relationship for the 14 20 degrees c range t degrees c 17 41 1 15 3 66 0 16 delta 18 o arag pdb delta 18 o wat smow the difference from the commonly used grossman and ku 1986 grossman e l ku t l 1986 oxygen and carbon fractionation in biogenic aragonite temperature effect chemical geology 59 59 74 equation suggests the potential existence of a species dependence of vital effect within bivalves this preliminary equation allows monthly scale sst reconstructions with a 1 5 degrees c precision using m donacium archaeological shells from southern peru giving potential insights into the problem of past enso behaviour c 2005 elsevier b v all rights reserved

shell midden sclerochronology abstract sclerochronological analysis of shell midden faunal remains offers a unique opportunity to study climate and environmental change in relation to human activity middens exist near most aquatic habitats almost worldwide for much of the late pleistocene and holocene they afford good preservation of accretionary calcium carbonate remains such as mollusk shells and fish otoliths in comparatively well dated contexts as such midden samples are a source of sequential proxy records of past climate and environmental conditions and may provide detailed pictures of human environment interaction however midden samples present several challenges to the development and application of sclerochronological proxies including unique taphonomic concerns here these challenges are outlined within descriptions of the major applications of midden sclerochronology archeological concerns including season of capture reconstructing the history of anthropogenic environmental impacts and paleoclimate environment proxy reconstructions

migmatization and secondary granitic magmas effects of emplacement and crystallization of primary granitoids in southern bohemia austria the sauwald and muhl zones of the prebatholithic moldanubian middle crust in northern austria contain metapelites and metaluminous to weakly peraluminous metagreywackes respectively both zones were affected by low pressure high temperature metamorphism and anatexis the metapelites of the sauwald zone became in situ diatexites probably by fluid absent reactions involving the breakdown of muscovite and the partial breakdown of biotite the biotite plagioclase quartz gneisses of the muhl zone experienced only slight melting following this event and while the mid crust was still hot additional heat was locally advected into the muhl zone by the intrusion of the weinsberg granite this brought about fluid present partial melting of the biotite plagioclasequartz gneisses producing relatively large volumes of metaluminous to weakly peraluminous i type schlieren granite this cool wet restite rich magma remained close to its site of generation thus infracrustal i type granitoids may be formed anywhere in the crust and not always at high t under special circumstances the heat and fluids from granitic magmas can spawn secondary granites also relatively low initial 87sr 86sr values of around 0 707 in i type rocks do not necessarily indicate either lower crustal magma sources or mixing with mantle derived magma the weinsberg granite magma came from the lower crust p probably 700 mpa where widespread fluid absent breakdown of biotite plagioclase quartz assemblages occurred the necessary high heat flow was probably provided by newly underplated mafic magmas however these seem not to have mixed or mingled with the crustally derived weinsberg magmas deep equivalents of the muhl zone metagreywackes may have formed the weinsberg protolith fluid absent experiments show that the melting temperature probably exceeded 850 c and that a garnet bearing orthopyroxene rich residue should be present in the lower crust fluid present experiments demonstrate that the availability of free h2o can radically alter the characteristics of the partial melts from apparent s type mineralogy with fluid absent melting to i type mineralogy with wet melting

magma migration folding and disaggregation of migmatites in the karakoram shear zone ladakh nw india effi cient extraction of granitic magma from crustal sources requires the development of an extensive permeable network of melt bearing channels during deformation we investigate rocks that have undergone deformation and melting within the karakoram shear zone of ladakh nw india in which leucosome distribution is inferred to record the permeable network for magma extraction delicate structures preserved in these rocks record the development of this permeable magma network and its subsequent destruction to form a mobile mass of melt and solids resulting from the interplay between folding and magma migration during folding magma migrated from rock pores into layer parallel and axial planar sheets forming a stromatic migmatite or metatexite with two communicating sets of sheets intersecting parallel to the fold axis once the network was developed folding and stretching was eased by magma migration and slip along axial planar magma sheets folding and magma migration led to layer disaggregation transposition and the formation of a diatexite where rock coherency and banding were destroyed a number of structures developed during this process such as cuspate fold hinges disharmonic folds truncated layering shear along axial planar leucosomes and fl ow drag and disruption of melanosomes in this system magma migration was an integral part of deformation and assisted the folding and stretching of metatexites while folding gave rise to a magma sheet network now preserved as leucosomes as well as the pressure gradients that drove magma migration and the breakup of the metatexite thus metatexite folding increased melt interconnectivity while magma mobility increased strain rate and released differential stresses

late holocene sea temperatures along the central california coast mussel shells from central california coastal archaeological sites record changes in sea surface temperatures in the past 2000 years water temperatures inferred from oxygen isotopes in the shells were about 1 c cooler than present and stable between 2000 and 700 yr ago between about 700 and 500 yr ago seasonal variation was greater than present with extremes above and below historic levels water temperatures were 2 3 c cooler than today 500 300 yr ago the interval of variable sea temperatures 700 500 yr ago partially coincided with an interval of drought throughout central california a coincident disruption in human settlement along the coast suggests movements of people related to declining water sources quantities of fish bone in central coast middens dating to this same period are high relative to other periods and the remains of northern anchovies a species sensitive to changing oceanographic conditions are also abundant the continued use of local fisheries suggests that changes in settlement and diet were influenced more by drought than by a decrease in marine productivity as fish provided a staple during an interval of low terrestrial productivity

geochronology and geochemistry of early permian mafic ultramafic complexes in the beishan area xinjiang nw china implications for late paleozoic tectonic evolution of the southern altaids abstract the hongshishan and pobei mafic ultramafic complexes outcrop in the beishan area in the southernmost altaids they consist of olivine gabbros dunites and pyroxenites zircons from olivine gabbros from the hongshishan complex yield a u pb age for emplacement of 281 8 2 6 ma the olivine gabbros have low sio 2 47 08 48 66 tio 2 0 14 0 29 mno 0 07 0 09 and k 2 o 0 06 0 09 but high mg values 81 t fe 2 o 3 4 93 6 29 the hongshishan and pobei mafic ultramafic complexes are characterized by negative anomalies in high field strength elements zr hf nb ta which we interpret as arc related signatures they also have spikes in large ion lithophile fluid mobile elements ba u pb sr and high mobile immobile element ratios i e primitive mantle normalized sr nd ba la 1 which are attributed to an aqueous fluid component in the source zircon hf isotopes indicate that the source magma was derived either from a depleted mantle with some crustal contamination or from an enriched mantle these data are interpreted to indicate that the hongshishan and pobei mafic ultramafic complexes were emplaced in the early permian subduction related environment which suggests that the paleo asian ocean in the southernmost part of altaids did not close until after early permian

palaeozoic arc magmatism in the central asian orogenic belt of kazakhstan shrimp zircon ages and whole rock nd isotopic systematics abstract early palaeozoic tonalite to granodiorite intrusions in northern kazakhstan are associated with lode gold mineralization and have shrimp zircon ages of 457 3 6 6 ma aksu 452 9 5 6 ma and 447 4 5 4 ma both zholymbet the stepnyak intrusion contains large xenoliths with an age of 480 6 5 0 ma one early palaeozoic zircon from a porphyritic diorite at stepnyak has a core with a near concordant 207pb 206pb age of 3888 1 5 ma whereas other xenocrystic grains are between 983 and 2698 ma old the early archaean age is probably inherited from unexposed basement of the kokchetav massif and represents the oldest crustal material so far known from the asian continent it appears that the aksu zholimbet and stepnyak granitoids were emplaced in the late ordovician in an andean or japan type continental arc environment on the margin of the kokchetav massif late palaeozoic granitoids in central kazakhstan have devonian zircon ages of 407 1 3 9 381 1 3 1 and 369 2 4 9 ma whereas the youngest sample from the topar massif has a late carboniferous emplacement age of 314 1 5 1 ma initial end values range from 5 to 1 corresponding to nd model ages of 1 1 0 6 ga the isotopic data are similar to those of other phanerozoic granitoids of the central asian orogenic belt and corroborate melting of predominantly juvenile crustal protoliths our data also confirm that devonian and carboniferous arc magmatism in central kazakhstan assimilated older and already accreted island arc systems yet is derived from primitive sources probably in a subduction setting the range of end values with corresponding nd mean crustal residence ages in the range of 0 6 1 1 ga for most samples indicates variable recycling of late proterozoic largely juvenile continental crust in the formation of the palaeozoic magmatic arcs of central kazakhstan the orogenic evolution terminated with permian anorogenic rift related alkaline granite magmatism end 2 to 8 which reflects melting of juvenile mantle derived lower crust

lithospheric delamination in post collisional setting evidence from intrusive magmatism from the north qilian orogen to southern margin of the alxa block nw china abstract post collisional granitoids are widespread in the north qilian and southern margin of the alxa block and their petrogenesis can provide important insights into the lithospheric processes in a post collisional setting this paper carries out an integrated study of u pb zircon dating geochemical and sr nd hf isotopic compositions for five early paleozoic intrusive plutons from the north qilian to southern margin of the alxa block the geochronological and geochemical results show that their magmatism can be divided into three periods with distinct geochemical features the early period intrusive rocks 440 ma include the lianhuashan lhs and mengjiadawan mjdw granodiorites both of them display high sr y ratios 52 91 coupled with low y and hree contents implying that they were derived from partial melting of thickened lower crust with garnet in the residue the middle period intrusive rocks 430 ma including the mjdw quartz diorites and yangqiandashan yqds granodiorites are high k calc alkaline with low sr y values the geochemical and isotopic data suggest that they are generated from partial melting of lower crust without garnet in the residue the late period intrusive rocks 414 422 ma represented by the shengrongsi srs and xinkaigou xkg plutons are a type or alkali feldspar granites they are possibly derived from partial melting of felsic crustal material under lower pressure condition our data show decreasing magma crystallization ages from mjdw pluton in the north and lhs pluton in the south to the srs and xkg plutons in the central part of the study area we suggest that such spatial and temporal variations of magmatic suites were caused by lithospheric delamination after the collision between the central qilian and the alxa block a more plausible explanation is that the delamination propagated from the margin part of the thickened lithosphere to inward beneath the north qilian and southern margin of the alxa block

marine radiocarbon reservoir effect along the northern chile southern peru coast 14 24 s throughout the holocene abstract through an extensive sampling and dating of pairs of associated shells and charcoal fragments combined with reanalysis of all the available previous data we reconstruct the evolution throughout the holocene of the regional marine radiocarbon reservoir effect r values along the northern chile southern peru area 14 24 s after elimination of the cases in which the terrestrial component yielded older ages than the marine shells to which they were associated the study is based upon data from 47 pairs of associated marine and terrestrial material our results suggest major changes in both the magnitude and variability range of r during the whole holocene period 1 between 10 400 and 6840 cal yr bp high values 511 278 yr probably result from a strengthened se pacific subtropical anticyclone and shoaling of equatorial subsurface waters during intensified upwelling events 2 between 5180 and 1160 cal yr bp lower values 226 98 yr may reflect a major influence of subtropical water and diminished coastal upwelling processes 3 during the last thousand years high values between 355 105 and 253 207 yr indicate an increased influence of 14 c depleted water masses and of enso for the early twentieth century a r value of 253 207 yr was calculated

leucogranite geochronological constraints on the termination of the south tibetan detachment in eastern himalaya abstract the south tibetan detachment system stds developed between the very low grade tethyan himalayan sequence ths and the high grade metamorphic greater himalayan sequence ghs over an along strike distance exceeding 2000 km within the himalayan orogen motion along the stds has been associated with the exhumation of the ghs and the uplift of the himalayan mountains geological field mapping and u th pb geochronological analyses were performed to trace and date the stds in the yadong region of southern tibet where it is a top to the north shear zone juxtaposing the yadong klippe of the ths with the high grade ghs this shear zone is intruded by multiple generations of leucogranites emplaced mainly at ca 23 ma ca 20 18 ma and ca 17 16 ma the undeformed textures of the leucogranites and their cross cutting relationships with the shear zone indicate that ductile motion along the stds terminated by ca 20 ma this is largely consistent with the deformation history of the stds in central and western himalaya the termination of shearing along the stds during the early miocene occurred well before the rapid cooling of the ghs during the middle miocene suggesting that exhumation and decompression of the ghs was still ongoing after the cessation of motion along the stds

carboniferous to early triassic magmatism and accretion in alxa nw china implications for accretionary orogenesis of the southern altaids the alxa occupies a critical position between the tianshan beishan to the west and the solonker suture to the east and is significant for understanding the accretionary orogenesis of the southern altaids to unravel the tectonic history of the alxa we undertook an integrated study of field geology geochemistry and geochronology of magmatic rocks and an accretionary complex six granites and one rhyolite from the zhusileng hangwula arc zha show peraluminous high k calc alkaline composition and one gabbro is tholeiitic they show incompatible element rich patterns and negative nb ta anomalies on the primitive mantle normalized trace element spider diagrams la icp ms zircon u pb dating reveals 348 329 and 267 250 ma magmatic events these data indicate that the zha was a japan type island arc from late paleozoic to early triassic the engger us melange comprises n morb type pillow basalt tuff chert siliceous mudstone limestone and turbidite which are tectonically juxtaposed by block in matrix structure this melange is interpreted as a carboniferous permian accretionary complex these new data combined with previous studies confirm that the paleo asian ocean subducted bidirectionally in the alxa in carboniferous early triassic a consistent divergent subduction system existed from alxa to solonker before the terminal closure of the paleo asian ocean in the mid late triassic supplementary material https doi org 10 6084 m9 figshare c 4962230