geochemical characteristics of the permian basins and their provenances across the solonker suture zone assessment of net crustal growth during the closure of the palaeo asian ocean abstract the solonker suture zone is commonly recognised as the location of the late permian to early triassic closure of the palaeo asian ocean in the southeastern segment of the central asian orogenic belt however the absence of typical suture related features as a consequence of uncommon collisional geometries gave it a cryptic nature thus the tectonic setting which led to suturing still remains enigmatic a geochemical characterisation of permian sedimentary and volcanic rocks across the suture was carried out supplemented with hf and nd isotopic analyses this approach enables not only a better definition of such regional suture but also estimates on the long controversial issue of net crustal growth in accretionary tectonic environments the results indicate short sedimentary transport distances between the arc basins and their provenances of which the studied volcanic rocks were a major contributor similar enrichment and depletion patterns with respect to n morb and average continental crust further corroborate a close source sediment relationship immobile element provenance analyses indicate that the active continental northern margin of the north china craton was a major source for arc basins to the south of the solonker suture zone to its north arc basins are interpreted to be sourced by a more complex mixture of provenances e g the baolidao volcanic arc suite and the heterogenous precambrian basement of southern mongolia an overall collisional tectonic setting across the suture is recognised the geochemical signature of sedimentary rocks to the south of the suture points at an active continental arc setting whereas the bimodal geochemical distribution of the samples to the north shows a contemporaneous active oceanic island arc as well as a passive margin environment these features favour a double sided subduction of the palaeo asian ocean beneath the north china craton and the mongolian arcs throughout the palaeozoic including back arc basin opening north of the suture in the permian analysis of hf and nd isotopic compositions revealed that magmas to the south were produced involving significant crustal contamination thus having less radiogenic compositions north of the suture however isotopic compositions tend to be more radiogenic implying a more juvenile contribution however the bulk isotopic compositions are close to present day chur suggesting that crustal reworking appears to be equally balanced by juvenile addition during the palaeozoic closure of the palaeo asian ocean across the solonker suture zone

late carboniferous to early permian oceanic subduction in central inner mongolia and its correlation with the tectonic evolution of the southeastern central asian orogenic belt abstract the late paleozoic magmatism in central inner mongolia provides important insights on the tectonic evolution and crustal growth in the central asian orogenic belt caob which formed due to the closure of the paleo asian ocean pao this paper presents new zircon u pb ages and hf isotopic compositions as well as whole rock geochemical data on a suite of volcanic rocks from the late paleozoic baoligaomiao formation and coeval intrusions in the baiyinwula region of the mongolian arc this study revealed that the magmatic sequences evolution includes 1 early andesites 317 311 ma with enrichment in large ion lithophile elements liles depletion in high field strength elements hsfes and positive zircon ehf t values from 9 0 to 15 5 indicating a derivation from enriched mantle 2 felsic rocks emplaced from 306 ma to 292 ma with relatively lower ehf t values from 6 3 to 11 3 implying juvenile crust as the primary magma source and 3 a type igneous rocks 280 278 ma the comparison of palaeontological lithostratigraphical and magmatic evolution in late paleozoic between different tectonic units in the eastern part of caob has displayed that the subduction of paleo asian oceanic crust caused the opening of the hegenshan ocean along the southern margin of mongolian arc in devonian and the baoligaomiao formation volcanic rocks and coeval intrusions have recorded early northwards subduction and subsequent slab rollback of hegenshan oceanic crust

anatomy of two permian greenschist to blueschist facies tectonic melanges in the solonker suture zone inner mongolia northeastern china evidence for divergent double subduction and soft collision the timing and processes of the paleo asian ocean pao closure are still controversial issues due to ambiguous interpretation of the regional geological records we describe the structural and kinematic features of two permian greenschist blueschist facies tectonic melanges at zhurihe and erdaojing in inner mongolia northeastern china separated by the solonker suture the tectonic melanges have block in matrix fabrics the zhurihe tectonic melange in the south is dominated by ne trending folds and east west to ne trending reverse strike slip faults indicating a nw se compressive direction blocks in the zhurihe melange include ophiolitic fragments c 292 ma and ocean island sequences c 274 ma which show e morb and oib geochemical affinities respectively the matrix includes blueschist greenschist quartz schist and clastics of which the blueschist yielded a zircon u pb age of c 255 ma and is geochemically e morb like the erdaojing melange north of the solonker suture is dominated by east west trending folds and ne and nw trending reverse faults indicating a north south compressive orientation the erdaojing melange is composed of early permian c 281 ma ophiolite blocks and middle permian c 266 ma actinolite schist and clastics as the matrix the erdaojing ophiolitic rocks are geochemically n morb tholeiitic our results define two parallel permian morb type ophiolitic belts separated by the central solonker suture these observations are evidence for the permian divergent double subduction and early triassic soft collision model for the pao along the solonker suture supplementary material detailed analytical methods and results are available at https doi org 10 6084 m9 figshare c 5008910

final amalgamation of the tianshan and junggar orogenic collage in the southwestern central asian orogenic belt constraints on the closure of the paleo asian ocean abstract the tianshan and junggar orogenic collage occupies the southwestern part of the central asian orogenic belt and was assembled by collision accretion of several continental blocks and island arcs during late paleozoic early mesozoic time following the consumption of the junggar and south tianshan oceanic basins in the western segment of the paleo asian ocean considerable and continuing controversy has surrounded when where and how this orogenic collage was eventually amalgamated which plays a crucial role in understanding the formation of the central asian orogenic belt this review synthesizes the most recent data and geologic evidence that place critical constraints on the closure history of the oceanic domains a comprehensive data compilation indicates that the formation of ophiolites along major sutures in the tianshan and junggar region lasted to the early carboniferous for the kalamaili suture zone to the mid carboniferous for the west junggar region and the north tianshan suture zone and to the late carboniferous for the south tianshan suture zone ca 325 310 ma ultra high pressure metamorphism along the western part of the south tianshan suture occurred nearly synchronously with the collision between the tarim craton and the central tianshan yili block the oceanic closure and collision events were sequentially followed by widespread and intensive a type granitic and bimodal magmatism which commenced in the mid carboniferous along the kalamaili belt and in the latest carboniferous to earliest permian in other regions contemporaneously large scale strike slip shearing took place along major faults subparallel to main sutures with a dominant dextral sense and deformation time since the carboniferous permian transition available data and geological evidence support an eastward propagating scissor like closure of the western segment of the paleo asian ocean along the south tianshan and north tianshan suture zones over a period from the mid to end carboniferous with the south tianshan belt representing the final suturing site in the early permian the tarim mantle plume likely merely affected the southwestern part of the assembled tarim and tianshan region as indicated by regional discrepancy in type and geochemistry of early permian a type granitoids the amalgamation of the tianshan and junggar orogenic collage was associated with the tectonic bending of the kazakhstan orocline leaving a piece of trapped oceanic crust in its middle part beneath the present junggar basin

strike slip tectonics and granite petrogenesis transpression is a process that thickens the crust and therefore in obliquely convergent orogens as in normally convergent orogens there is the potential to generate granitic melts individual transcurrent shear zones may not only control the ascent paths siting and emplacement mechanisms of plutons but they may also cause the genesis of the granitoids themselves two contrasting situations are examined in the hercynian shear zones of iberia thickening together with hydrous fluxing along shear zones created intracrustal wet melting of fertile gondwanan sediments to produce syntectonic granites in the northern part of the british caledonides the association of compositionally expanded granitoids with a major mantle component and transcurrent shear zones may be explained by melting of continental crust at the lower limits of crustal transpressional faults detaching into the moho

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

late carboniferous to early permian magmatic pulses in the uliastai continental margin linked to slab rollback implications for evolution of the central asian orogenic belt abstract the paleo asian ocean underwent a protracted closure history during late paleozoic here we investigate the magmatic evolution during this process based on a detailed study in the baiyinwula region along the uliastai continental margin the major rock types in this area are late carboniferous early permian volcanic sequences and coeval intrusions we identified four stages of magmatic evolution based on the diverse assemblages and their precise isotopic ages the first stage is represented by andesites with a zircon 206pb 238u age of ca 326 12 ma these rocks are metaluminous to weakly peraluminous high k calc alkaline and possess high na2o k2o ratios in the range of 1 23 to 2 45 they also display enrichment of large ion lithophile elements lile and depletion of high field strength elements hfse with markedly positive zircon ehf t varying from 8 1 to 15 6 the geochemical features of these andesites are similar to those of typical arc volcanic rocks the second stage includes granodiorites emplaced at 318 6 1 8 ma the rocks are high k calc alkaline with a cnk values ranging from 0 95 to 1 06 and show enrichment in lile and depletion in hfse they show geochemical affinities to adakites with high sr and low y and yb contents indicating magma derivation from thickened lower crust zircon grains from these rocks display positive initial ehf t values ranging from 11 1 to 14 6 with corresponding two stage hf model ages tdm2 of 394 622 ma the third stage consists of syenogranite together with a volcanic suite ranging in composition from rhyolite todacite which formed during 303 4 1 2 to 285 1 2 2 ma they possess elevated silica and alkali contents high feot mgo and ga al ratios low al2o3 mgo and cao contents and high rb y nb ce zr y and ga contents strong negative ba sr and eu anomalies showing i to a type granitic affinities zircons in these rocks show elevated hf isotopic compositions ehf t 9 9 to 14 6 with tdm2 varying from 324 to 673 ma the fourth magmatic pulse is represented by k feldspar granite with zircon u pb ages from 283 2 1 9 ma to 280 0 1 4 ma and typical alkalic a type granite geochemistry these rocks possess positive ehf t values in the range of 9 7 to15 2 and a restricted range of hf model age from 327 to 684 ma the magmatic rocks from the four stages show comparable ehf t and t2dm suggesting that the magmas were derived from the same evolving mantle derived source we propose a tectonic model linking the evolution of the magmatism with the closure of the paleo asian ocean that involved the following stages the andesites were formed during the initial oceanic subduction stage with magma sourced from the metasomatized lithospheric mantle stage 2 adakite like rocks were derived from subduction induced thickened crust subsequent slab rollback resulted in asthenospheric upwelling and melting of residual juvenile crust to generate the i and a type syenogranite rhyolite and dacite suite finally followed by the a type k feldspar granite

nature and timing of the solonker suture of the central asian orogenic belt insights from geochronology and geochemistry of basic intrusions in the xilin gol complex inner mongolia china the solonker suture zone of the central asian orogenic belt caob records the final closure of the paleo asian ocean the nature and timing of final collision along the solonker suture has long been controversial partly because of an incomplete record of isotopic ages and differing interpretations of the geological environments of key tectonic units the xilin gol complex consisting of strongly deformed gneisses schists and amphibolites is such a key tectonic unit within the caob lenticular or quasi lamellar amphibolites are dispersed throughout the complex intercalated with biotite plagioclase gneiss both rock types experienced amphibolite facies metamorphism the protolith of the amphibolite is a basic rock that intruded into the biotite plagioclase gneiss at 319 4 ma based on la icpms zircon u pb dating the basic intrusion was sourced from a modified magma that experienced crystal fractionation and was admixed with slab derived fluids the slab derived fluids which formed during early paleozoic oceanic subduction along the north dipping sonidzuoqi xilinhot subduction zone mixed with the magma source and produced subduction related geochemical signatures superimposed on volcanic arc chemistry after early paleozoic oceanic subduction and arc continent collision a transient stage of extension occurred between 313 and 280 ma in the sonidzuoqi xilinhot area deformation and recrystallization during the switch from compression to extension and reheating by the later magmatic intrusions reset the isotope systems of minerals in the xilin gol complex recorded by a 312 2 1 5 ma biotite 40ar 39ar age from biotite plagioclase gneiss a 309 12 ma zircon intercept age and a 307 5 3 5 ma hornblende 40ar 39ar age from amphibolites in the complex there was an arc forearc related marine basin at the southern margin of the xilin gol complex during the permian the closure of the oceanic basin led to late paleozoic middle triassic north dipping subduction beneath the xilin gol complex and induced the amphibolite facies metamorphism of the complex the final suturing of the solonker zone occurred from 269 to 231 ma this latest amphibolite facies metamorphism with pressures of 0 31 0 39 gpa and temperatures of 620 660 c was recorded at 263 4 1 4 ma to the xilin gol complex as indicated by the hornblende 40ar 39ar age from the amphibolites as well as several zircon ages of 260 3 231 3 ma the xilin gol complex documented the progressive accretion of a single long lived subduction system at the southern margin of the south mongolian microcontinent from the early paleozoic 452 ma to middle triassic 231 ma the caob shows protracted collision prior to final suturing

caledonian plutonism and major lineaments in the sw scottish highlands within the sw grampian highlands several ne sw trending shear zones and faults related to the caledonian transpressional collision are recognized as being distinct from an intersect ing set of nw se trending pre caledonian crustal lineaments which were reactivated during caledonian orogenesis the geochemical and isotopic characteristics of the 425 400 ma granitoids of this region show that they were derived by anatexis of the lower crust associated with a mantle component which provided an influx of mantle heat and or metasomatic fluids their spatial distribution suggests a close genetic relationship with caledonian shear zones having made the fundamental distinction between ascent transport and emplacement arrival configurations possible for granitoid magmas new data are presented for these plutons which show that i a common modal sequence of intrusive phases can be recognized ii these phases are all sited at shear zone or lineament intersections where transtensional zones allowed and facilitated ascent iii emplacement was often by a process of localized in situ ballooning the existence of such lineaments in these orientations in the sw highlands may indicate that the structure of the lower crust can be regarded as a series of blocks bounded by intersecting ductile zones of high strain the establishment of such a pattern of structural interactions within the lower crust could provide a mechanistic framework within which the location of anatectic zones siting and ascent pathways and any subsequent emplacement phenomena can be explained in orogenic belts

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

late cenozoic tectonism collapse caldera and plateau formation in the central andes abstract the evolution of andean volcanism including the formation of late miocene to recent collapse calderas on the puna plateau is generally interpreted in terms of the kinematic framework of the nazca and south american plates we present evidence that caldera dynamics and associated ignimbrite volcanism are genetically linked to the activity of first order nw se striking zones of left lateral transtension on the local and regional scales consequently ages of collapse calderas indicate activity of these fault zones which initiated at about 10 ma on the puna plateau the onset of such faulting points to a change in the deformation regime from dominantly vertical thickening to orogen parallel stretching upon reaching maximum crustal thickness and critical surface elevation horizontal magma sheets that formed at mid crustal level possibly due to heat advection by volume increase of asthenospheric mantle below thickened crust were tapped by sub vertical faults this accounts well for the observed tectono magmatic phenomena at surface it follows that formation of collapse calderas and eruption of voluminous ignimbrites appear to be related to the mechanical evolution of the andean plateau rather than to changes in the geometry of the wadati benioff zone or plate boundary kinematics

ordovician marine volcanic and sedimentary record of rifting and volcanotectonism snowdon wales united kingdom wales was part of a continental convergent plate margin for 50 m y during ordovician time the crustal stress regime was one of sinistral transtension due largely to oblique subduction early uplift and subsequent growth of subaerial arc volcanoes were followed by development of a broad fault bound marine basin 150 km wide in an infra arc or back arc setting within the basin relatively narrow grabens became sites of pronounced subsidence and bimodal basalt rhyolite volcanism the grabens developed in fault splays above steep crust penetrating discontinuities that focused extension and plumbing of magmas at these sites at snowdon the bedded pyroclastic formation records submarine and island basaltic volcanism with associated sedimentation at the site of an active rhyolitic caldera volcano the volcanoes formed on the axis of the snowdon graben and the basaltic succession which contains several unconformities due to marine abrasion at sea level shows a complex history of alternate uplift and subsidence the uplift is attributable to resurgence of rhyolitic magma and was 336 m in total subsidence due mainly to crustal extension was 500 m at a rate of 500 m m y the bedded pyroclastic formation shows clear evidence of tectonic influence in the timing location and style of basaltic magmatism repeated reactivation of magmatic plumbing systems demonstrates fault or fault intersection controlled channeling of magmas in the upper crust episodes of relatively vigorous subsidence preceded copious magmatism and both intrusive and extrusive basalt magmas exploited contemporaneously active normal faults pronounced topographic ponding of extrusive products and highly variable trends of fault block subsidence and rotation evident from the volcanogenic sedimentary deposits testify to chaotic piecemeal displacements on faults spaced commonly at

crustal structures revealed from a deep seismic reflection profile across the solonker suture zone of the central asian orogenic belt northern china an integrated interpretation abstract the solonker suture zone is one of the most important tectonic boundaries in the southeastern part of the central asian orogenic belt caob an 630 km long reflection seismic profile across this suture was recently completed by the chinese sinoprobe project the processed seismic data show clear crustal structures and provide new constraints on the tectonic and crustal evolution models the moho is delineated as a relatively flat boundary between a strongly reflective lower crust and a transparent mantle at a depth of 40 45 km 14 5 s two way travel time which is in agreement with the refraction data recorded along the same profile in a broad view the profile images an orogen that appears bivergent with and approximately centered on the solonker suture zone the southern portion of this profile is dominated by a crustal scale cratonward propagating fold and thrust system that formed during the late permian and triassic through collision and subsequent convergence in a post collisional stage the major thrust faults are truncated by mesozoic granitoid plutons in the upper crust and by the moho at the base of the crust this geometry suggests that the moho was formed after the thrusting event the northern portion of the profile although partially obliterated by post collisional magmatic bodies shows major south dipping folding and thrusting bands of layered reflectors immediately overlying the moho are interpreted as basaltic sills derived from the mantle episodic mafic underplating may have occurred in this region giving rise to post collisional magmatic events and renewal of the moho a few mantle reflectors are also visible the overall geometry of these mantle reflectors supports the tectonic models that the southern orogen manchurides experienced south directed subduction and the northern orogen altaids underwent north directed subduction prior to collision along the solonker suture zone

magmatic evidence for middle late permian tectonic evolution on the northern margin of the north china craton abstract late paleozoic tectonic evolution of the northern margin of the north china craton ncc remains controversial especially for the mechanism and time during the late stage oceanic consumption of the paleo asian ocean pao we address this issue through geochronological geochemical and lu hf isotopic studies on felsic mafic volcanic rock suites and granite plutons from the damao area in the western segment of the ncc s northern margin zircon u pb dating reveals that the volcanic suites with eruptive unconformities in between erupted primarily at 275 260 ma while the granites emplaced later at 255 ma the volcanic rocks contain rhyolites and dacites having an a type affinity with high feot mgo k2o na2o cao and ga al ratios enriched th u zr hf and depleted ba sr and eu contents the basaltic rocks display low mgo and tio2 contents negative nb ta ti anomalies as well as negative hf t values we suggest that the 275 260 ma volcanic suites were generated in a post collisional setting with the dominant rhyolites and dacites derived from the mixing between mantle and crustal magmas and subordinate basalts from the partial melting of enriched sub continental lithospheric mantle the sudden and intensive occurrence of these 275 260 ma felsic dominant bimodal post collisional magmatism and their sudden increase of zircon hf t values imply a significant mantle contribution to their sources possibly due to the slab breakoff of the subducting pao plate the 255 ma granites also have an a type affinity and a large spread of negative hf t values 3 6 to 30 5 indicating a prominent involvement of continental crust materials that was likely related to crustal reworking after the slab breakoff overall these 275 255 ma magmatic suites document a significant post collisional crustal extension at the western segment of the northern ncc in this region the southward subduction of the pao likely ceased prior to 275 ma on the basis of a regional comparison

solonker suture in east asia and its bearing on the final closure of the eastern segment of the palaeo asian ocean abstract the location and tectonic nature of the solonker suture in east asia and hence the late palaeozoic to early mesozoic closure of the palaeo asian ocean have notoriously been enigmatic in the past decades due to limited rock exposure and the absence of unambiguous collision related regional features several tectonic models have been proposed since but in many cases these models were derived only from single key exposures or methodologies often being over simplified or interpreted leading to questionable extrapolations on regional and global scales despite the complexity of the palaeozoic accretionary tectonic framework now the regionally consistent availability of geochronological geochemical stratigraphic and palaeo geographic data enables us to integrate these into a highly detailed and coherent palaeozoic tectonic review of the suture the region across the solonker suture can generally be subdivided into three major palaeozoic tectonic provinces e g i a sino cratonic province that resembles the active palaeozoic northern margin of the north china craton to the south of the suture ii a mongolian province that comprises the south eastern margin of the mongolian terrane to the north of the suture and iii an east asian pre pacific province in north east china that accreted with the late permian initiation of pacific plate subduction the sino cratonic province experienced episodic tectonic activity that includes the accretion of the initially ensialic bainaimiao arc onto the passive northern margin of the north china craton at 437 453 ma subsequent southward directed subduction activity beneath the amalgamated margin until at least 400 ma followed by a temporary cessation of magmatic activity accompanied by a switch from north to southward directed continental drift and finally concluded by renewed subduction activity until the final closure of the palaeo asian ocean the mongolian province is suggested to have been more closely associated with the siberian craton before the presumably late devonian opening of the mongol okhotsk ocean the east asian pre pacific province originally rifted away from the eastern margin of gondwana at 600 750 ma recorded early palaeozoic 500 ma orogenic processes along the then northern margin of the siberian craton the baolidao arc system successively developed along the present day south eastern margin of the mongolian province due to continued subduction activity throughout the palaeozoic after the middle carboniferous opening of the hegenshan back arc basin renewed subduction beneath a matured baolidao arc during carboniferous and permian times led to the obduction of the hegenshan ophiolite not earlier than 270 ma shortly after the palaeo asian ocean diachronously closed from west to east along the solonker suture in the late permian to early triassic the east asian pre pacific province re attached itself by the mid mesozoic followed by a change in the regional stress regime controlled by palaeo pacific plate subduction a divergent double sided subduction model for the closure of the palaeo asian ocean provides based on the reviewed data a tectonic model that is not only able to reasonably well explain the absence of typical collision related features in the region but also the abundance of mesozoic a type granitic magmatism in north east china

structural and kinematic analysis of the early paleozoic ondor sum hongqi melange belt eastern part of the altaids caob in inner mongolia china we present a structural and kinematic study of an early paleozoic subduction melange and a magmatic arc that form the main elements of the southern orogen belt of inner mongolia which lies in the eastearn part of the altaids or central asia orogenic belt the structural analysis of the melange conducted in the hongqi and ondor sum areas western inner mongolia shows two phrases of ductile deformation the d1 event is responsible for the pervasive s1 foliation nw se striking l1 stretching lineation and f1 intrafolial folds these microstructures are coeval with a greenschist facies metamorphism during d2 nw verging f2 folds associated with a s2 axial planar cleavage deformed s1 and l1 the d1 kinematic criteria indicate a top to the nw sense of shear d1 and d2 developed before the unconformable deposition of the early devonian shallow water sandstone a lithosphere scale geodynamic model involving an early paleozoic southeast directed subduction beneath the north china craton and late silurian collision of the north china craton with an hypothetical microcontinent is proposed to account for the microstructural evolution

provenance and tectonic setting of the upper palaeozoic sandstones in western inner mongolia the shalazhashan and solonker belts china insights from detrital zircon u pb ages and hf isotopes the solonker and shalazhashan belts are hotly debated tectonic units of the central asian orogenic belt caob because they may either represent a permian or triassic suture zone of the caob or a rifting zone overprinted on an early palaeozoic orogen provenance analysis of the upper palaeozoic sandstones in these belts may provide useful constraints on this issue this study collected six sandstone samples from three study areas the mandula area of the solonker belt the quagan qulu area of the shalazhashan belt but close to the alxa block and the enger us area of the shalazhashan belt for framework petrography zircon morphology u pb and lu hf isotopic analyses framework petrography reveals that the mandula and enger us area samples contain high proportions of volcanic fragments whereas the samples from the quagan qulu area include not only volcanic fragments but also significant amounts of biotite and muscovite the detrital zircons of the mandula area and the enger us area yield two main age groups i 260 330 ma with dominant e hf t values of 5 to 12 and ii 420 550 ma with dominant e hf t values of 9 to 9 suggesting that early palaeozoic arc related magmatic rocks and late palaeozoic syn depositional volcanic rocks are the main source rocks the detrital zircons of the quagan qulu area have one main age group of 420 500 ma and some grains of 0 9 1 1 ga 1 4 1 5 ga 1 8 1 9 ga and 2 5 ga which derive from the northern margin of the alxa block the lithological and fossil assemblages of the upper palaeozoic sandstones suggest shallow marine to deep water depositional environments and a northward deepening transition based on the zircon spectra sedimentary environment analysis and previous studies we argue that the solonker belt and the shalazhashan belt of the caob are in extensional basins of a fore arc or rifting setting

calderas and caldera structures a review abstract calderas are important features in all volcanic environments and are commonly the sites of geothermal activity and mineralisation yet it is only in the last 25 years that a thorough three dimensional study of calderas has been carried out utilising studies of eroded calderas geophysical analysis of their structures and analogue modelling of caldera formation as more data has become available on calderas their individuality has become apparent a distinction between caldera caldera complex cauldron and ring structure is necessary and new definitions are given in this paper descriptions of calderas based on dominant composition of eruptives basaltic peralkaline andesitic dacitic rhyolitic can be used and characteristics of each broad group are given styles of eruption may be effusive or explosive with the former dominant in basaltic calderas and the latter dominant in andesitic dacitic rhyolitic and peralkaline calderas four end member collapse styles occur plate or piston piecemeal trapdoor and downsag but many calderas have multiple styles features of so called funnel and chaotic calderas proposed in the literature can be explained by other collapse styles and the terms are considered unnecessary ground deformation comprises subsidence or collapse essential characteristics of a caldera and uplifting doming and fracturing due to tumescence and or resurgence frequent but not essential collapse may occur on pre existing structures such as regional faults or on faults created during the caldera formation and the shape of the collapse area will be influenced by depth size and shape of the magma chamber the final morphology of a caldera will depend on how the caldera floor breaks up whether collapse takes place in one event or multiple events whether vertical movement is spasmodic or continuous throughout the eruptive sequence and whether blocks subside uniformly or chaotically at one or more collapse centres a meaningful description of any caldera should therefore include number of collapse events presence or absence of resurgence caldera floor coherency caldera floor collapse geometry and dominant composition of eruptives

late permian triassic metallogeny in the chinese altay orogen constraints from mica 40ar 39ar dating on ore deposits abstract the central asian orogenic belt caob constitutes the largest phanerozoic accretionary orogen on earth it extends over 5000 km and hosting numerous metal deposits the chinese altay orogen an important element of the caob hosts abundant devonian ca 410 370 ma deposits the 40 ar 39 ar dating of seven mica separates from the representative samples syngenetic with orogenic type mineralization is summarized to record a poorly studied permian to triassic metallogenic episode in the chinese altay orogen the kelan and maizi basins in the chinese altay orogen which likely represent an arc accretionary complex contain a series of polymetallic lode deposits hosted in low grade metamorphic volcano sedimentary rocks two muscovite and five biotite separates were obtained from the ore forming veins paragenetically associated with au bearing polymetallic sulfides in the keketale pb zn wulasigou cu tiemurt pb zn dadonggou pb zn and sarekuobu au deposits these separates yielded 40 ar 39 ar plateau ages ranging from 260 ma to 205 ma integration of these results with other published geological and geochronological data indicates that the au cu pb zn mineralization post dated the final caob assembly with fluid movement and mineralization possibly driven by regional metamorphism and deformation it is herein proposed for a metallogenic model that the metamorphic fluid migration following final assembly of the caob results into the formation of the deposits

geochronological data of igneous and metamorphic rocks from the xing an mongolia orogenic belt of the eastern central asian orogenic belt implications for the final closure of the paleo asian ocean in order to better constrain the evolution of the xing an mongolia orogenic belt and the resulting closure of the paleo asian ocean we conducted an integrated investigation involving u pb dating of igneous and detrital zircon and a synthesis of existing work across the erdaojing and ondor sum complexes in the eastern segment of the central asian orogenic belt geochronological analysis revealed a different tectonic setting between the ondor sum complex tulinkai area and the erdaojing accretion complex as detrital zircon u pb analysis identified different major age provenances 1 the precambrian basement of the north china craton and paleozoic bainaimiao arc along the northern margin of north china to the south are the provenance rocks of the schists in the tulinkai area and 2 the precambrian basement of the xilinhot complex and paleozoic baolidao arc to the north are the provenance rocks of the schists in the erdaojing area we infer that the existence of the ocean between the above two regions in the late of early paleozoic in accordance with the result of this study furthermore a comparison of the youngest age population in the intruded diorite rock and metasedimentary rocks with u pb ages implies that the final closure of the paleo asian ocean along the solonker suture zone most likely occurred in the period between 239 and 222 ma late paleozoic triassic

structural analysis of syn depositional hydrothermal veins of the 3 48 ga dresser formation pilbara craton australia abstract the lower chert unit of the 3 48 ga dresser formation pilbara craton western australia hosts earth s oldest convincing evidence of life in the form of stromatolites in a depositional setting that has been interpreted as either a shallow marine lagoon an active volcanic caldera or a basin deposited during regional extension understanding the environment where early life thrived is important not only for early life studies on earth but in the search for life on mars and for understanding the setting for the origin of life structural analysis of the extensive network of syn depositional hydrothermal chert barite veins that immediately underlies the dresser was undertaken in order to test if deposition occurred within an evolving caldera system or during regional extension over 190 hydrothermal veins were mapped for their geometry and relative age relationships bedding orientations were recorded within nine fault bounded structural sub blocks and used for back rotating the measured veins to their original orientation the results show that 1 there are four main vein systems including three that were emplaced during deposition of the dresser formation lower chert unit and a fourth that was emplaced within syn to post depositional listric growth faults 2 the majority of veins were sub vertical prior to regional tilting 3 the first three vein systems show mutually cross cutting relationships 4 vein densities and the number of vein systems vary between sub blocks 5 conjugate vein networks ne sw and nw se dominate two middle and northern sub blocks these results show that the first three vein systems were emplaced through fracturing under a stress regime with a vertical 1 vertical conjugate fractures are consistent with sandbox models of overburden fracturing during emplacement and discharge of a magma chamber in a volcanic caldera areas of single vein orientations reflect marginal areas of the caldera system dominated by radial or by ring faults the veins in late listric growth faults represent the final period of caldera collapse which controlled the depositional thicknesses of overlying strata possibly during subsequent regional extension these results document an unique setting for earliest life on earth within shallow water and terrestrial deposits of an active low eruptive caldera setting

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

age and provenance constraints on seismically determined crustal layers beneath the paleozoic southern central asian orogen inner mongolia china abstract we present 110 ages and 51 in situ 18o values for zircon xenocrysts from a post 99 ma intraplate basaltic rock suite hosted in a subduction accretion complex of the southern central asian orogenic belt in order to constrain a seismic profile across the paleozoic southern orogen of inner mongolia and the northern margin of the north china craton two zircon populations are recognized namely a phanerozoic group of 70 zircons comprising granitoid derived ca 431 99 ma n 31 peak at 256 ma meta granitoid derived ca 449 113 ma n 24 peak at 251 ma and gabbro derived 436 242 ma n 15 peaks at 264 and 244 ma grains each textural type is characterized by a distinct zircon oxygen isotope composition and is thus endowed with a genetic connotation the precambrian population 2605 741 ma n 40 exhibits a prominent age peak at 2520 ma granulite facies metamorphism and four small peaks at ca 1900 1600 and 800 ma our new data together with literature zircon ages significantly constrain models of three seismically determined deep crustal layers beneath the fossil subduction zone forearc along the active northern margin of the north china craton namely 1 an upper arc crust of early to mid paleozoic age intruded by a major permian triassic composite granitoid gabbroic pluton 8 20 km depth 2 a middle crust predominantly consisting of mid meso to neoproterozoic felsic and mafic gneisses and 3 a lower crust composed predominantly of late archean granulite facies rocks we conclude that the paleozoic orogenic crust is limited to the upper crustal level and the middle to lower crust has a north china craton affinity furthermore integrating our data with surface geological petrological and geochronological constraints we present a new conceptual model of orogenic uplift lithospheric delamination and crustal underthrusting for this key ocean continent convergent margin

tectonic controls of ordovician arc and marginal basin volcanism in wales ensialic destructive plate margin volcanism persisted in wales from late tremadoc to caradoc times late tremadoc uplift and erosion in nw wales is attributable to onset of subduction and development of a frontal ridge system fore arc south east of this in the vicinity of the harlech dome and in sw wales more localized uplift was associated with the development of arc volcanoes at a volcanic front in north wales the arc volcanism occurred in a regime of e w crustal extension and was centred on a deep crustal discontinuity during arenig times the arc terrane was transgressed subsequently marine conditions with local and temporary emergence persisted through arenig to caradoc episodes of marginal basin volcanism while there was widespread and persistent subsidence in wales during development of the marginal basin the volcanism occurred mainly along complex and relatively narrow grabens which were sites of repeated channelling of magmas and pronounced subsidence the grabens mark the sites of deep and steep crustal discontinuities fractures one of which controlled the earlier arc volcanism changing sites of volcanism in wales can now be seen as due to transfer of active extension from one fracture to another there is evidence that the fractures were present by tremadoc times and they almost certainly formed before development of the cambrian marine rift basin they are most likely to have been strike slip faults or shear zones and their pattern in wales indicates that they may represent part of a complicated deep crustal strike slip duplex system the volcanically active grabens of the marginal basin are considered to reflect upward propagation of splay faults flower structure which channelled magmas through the cover during extension across the fractures the fractures and associated grabens strongly influenced the location and orientation of zones of relatively high strain that developed during late caledonian compression and transpression in north wales the volcanically active grabens trended broadly n s oblique to the exposed ne sw basement tectonic grain and dominantly e w extension seems to have persisted throughout the ordovician volcanism while there is considerable uncertainty concerning ordovician plate tectonic configurations and displacements the extension is best explained by invoking a n s convergence vector between the oceanic and over riding microcontinental plates with resultant sinistral shear stress in the latter and sinistral transtensional displacements across the arc and marginal basin the model highlights the suspect nature of the relationship between the welsh ordovician terrane and others to the north

tectonic significance of the xilin gol complex inner mongolia china petrological geochemical and u pb zircon age constraints abstract the xilin gol complex consisting of strongly deformed and metamorphosed rocks is exposed as a large tectonic unit within the central asian orogenic belt caob it is located on the xilinhot sonidzuoqi north dipping thrust belt and near the solonker suture zone that is widely regarded to record the terminal evolution of the caob its genesis and tectonic origin are a key to test the existing tectonic models as to whether there was a single arc chain by successive accretion or multiple amalgamation involving some microcontinents this study reports petrological geochemical and zircon u pb age data of biotite plagioclase gneisses from the complex as migmatization is common in the gneiss restitic and magmatic components are identified in the gneiss by their petrographic features mineral chemistry indicates that the xilin gol complex was subjected to amphibolite facies metamorphism three groups of zircons are identified by cathodoluminescence cl images and th u ratios 1 the first and most abundant group is of magmatic origin and yields a weighted mean 206 pb 238 u age of 452 5 ma 2 the second scarcest group is of metamorphic origin and yields a weighted mean 206 pb 238 u age of 339 4 ma 3 the third group is minor of apparent detrital origin and yields 207 pb 206 pb ages from 1902 11 to 886 54 ma for the cores with a peak at 1150 to 900 ma whole rock major and trace element contents are representative for the upper continental crust with rb pb u th k lree enrichment and nb ta sr depletion this suggests the gneisses are sourced in the upper continental crust with some old sediment contamination the protolith of the biotite plagioclase gneiss is a sedimentary rock with detrital source ages between 1902 11 ma and 886 54 ma that was deposited at the southern active continental margin of the siberia craton during the amalgamation of the rodinia supercontinent partial melting and migmatization occurred at 452 5 ma in response to north dipping oceanic subduction in the sonidzuoqi xilinhot area many new magmatic materials formed in the xilin gol complex during this time while it resided in a continental marginal arc tectonic setting oceanic subduction started at 490 8 ma and lasted at least to 440 4 ma in the early paleozoic the caob was an advancing accretionary orogen the complex was mildly affected by a 320 310 ma magmatic event in the xilinhot sonidzuoqi area therefore the xilin gol complex recorded a long lived successive accretion along the southern margin of the siberia craton

permian stratigraphy and correlation of northeast china a review palaeontological lithostratigraphical data from the permian strata and correlation of the permian successions for different tectonic units in northeast china are reviewed and summarized in this paper permian strata in northeast china are dominated by brachiopods fusulinoideans and land plants with limited ammonoids conodonts and bivalves the cisuralian early permian in the northern margin of the north china block and in the manchuride belt is composed mostly of marine massive limestone with the characteristic pseudoschwagerina zone in the asselian and sakmarian and the misellina claudiae zone in the kungurian the cisuralian in the xing an block and the northeastern part of inner mongolia is dominated by huge terrestrial deposits with fossil plants the guadalupian middle permian in the manchuride altaid and yanbian belts are characterized by bi temperate roadian or early wordian monodiexodina fauna and the late wordian capitanian codonofusiella schwagerina or neoschwagerina yabeina faunas the mixed brachiopod faunas between the boreal antitropical and the palaeoequatorial cathaysian forms the roadian or early wordian solitary coral faunas and the late wordian capitanian compound waagenophyllum wentzelella fauna the nadanhada terrane contains some exotic limestone blocks with a typical cathaysian neoschwagerina yabeina fauna in a late jurassic early cretaceous melange which is related to mesozoic subduction in the western circum pacific region the lopingian late permian in northeast china is mostly characterized by terrestrial molasse deposits with a mixed flora between the boreal angaran and the palaeoequatorial cathaysian provinces indicating the final closure of the palaeo asian ocean c 2005 elsevier ltd all rights reserved

the influence of source rock type chemical weathering and sorting on the geochemistry of clastic sediments from the quetico metasedimentary belt superior province canada abstract the geochemistry of clastic sediments is controlled by at least four factors source rock composition weathering of the source rocks sorting of the weathered detritus and post depositional alteration during diagenesis and metamorphism in order to evaluate the relative importance of these factors major trace and rare earth element contents were determined for distal and proximal clastic turbidites from the archaean quetico metasedimentary belt the degree of metamorphism of the samples ranges from middle greenschist to upper amphibolite facies simple mixing of archaean mafic and acidic rocks of similar composition to those found in the adjacent granite greenstone terrains will account for the tio2 feot mgo cr sc y and hree contents of the clastic turbidites the metasediments are enriched in lree relative to the mafic acidic rock mixing line indicating that the acidic source was enriched in the lree compared to archaean terrains from other parts of the world some quetico turbidites have an additional fe rich source component possibly a chemical sediment incorporated at the time of turbidite slumping cao na2o k2o rb and sr form trends on bivariant plots that are oblique to the mafic acidic rock mixing line and hence cannot be accounted for by simple mixing of the source rocks chemical weathering of feldspars prior to deposition can account for cao na2o k2o rb and sr trends since cao na2o and sr are lost to the weathering solutions and k2o and rb are retained on the product clays the higher na 2 o cao k 2 o na 2 o rb k 2 o and sr cao ratios but lower cao content of the distal samples suggest that they were derived from a bulk sediment that had experienced a greater degree of chemical weathering than the proximal sediments sorting is the factor that most strongly influences the composition of individual samples within the proximal sample group the coarse grained metagraywackes with their higher quartz feldspar and heavy mineral contents are enriched in sio2 cao na2o sr lree zr and hf relative to the fine grained clay rich metapelites metapelites are enriched in tio2 al2o3 feot mgo k2o cr zn sc y hree cs and rb as a result of their higher clay contents thus the use of clastic sediment compositions in determining average crustal compositions e g for the ree can only give satisfactory results when all grain size fractions are sampled in their correct relative proportions the chemical differences observed between the high and low grade samples can be accounted for by source rock type chemical weathering of the source rocks or sorting post depositional element mobility does not appear to be an important factor in determining the composition of the quetico metasediments

late paleozoic to early triassic multiple roll back and oroclinal bending of the mongolia collage in central asia abstract the architecture and mechanics of an orogen can be understood in terms of a system of collages that are characterized by a complex assemblage of multiple components but the fundamental paleogeographic framework and the tectonic relationships between the different components are often insufficiently defined because of unavailable data the central asian orogenic belt caob provides an ideal opportunity to address the fundamental framework of paleogeography and tectonic relationships between the diverse and many components in this huge collage in this paper we review several lines of available evidence which enable us to propose a new tectonic model of huge roll back in the formation of the accretionary tectonics of the mongolian collage in central asia in the early paleozoic the mongolia collage comprised the southern siberian and the tuva mongol oroclines the siberia craton and the mongolia collage jointly formed a giant tadpole shape within the paleo asian and panthanlassic oceans its head siberia was to the south and the tail tuva mongol to the northwest the structures and tectonic zonation of the mongolia collage are characteristic of multiple arcs which have been separately described in detail in different segments southwards from the southern siberia east sayan west sayan gorny altai chara via the lake zone junggar tianshan gobi altai beishan alxa to the manlay hegenshan baolidao solonker segments almost all segments underwent early paleozoic to permian or even triassic frontal subduction and accretion while rifting in the late carboniferous to permian or triassic occurred in the outward oceanward westward advancing mongolian collage therefore we suggest that a huge complex roll back active from the carboniferous to permian or even to late triassic facilitated the formation of the mongolian collage the outward multiple roll back process was compatible and almost coeval with the start of the tuva mongol orocline and rotation of the siberian craton as confirmed by paleomagnetic and structural data during the roll back processes an archipelago paleogeography was formed behind the frontal subduction and accretion in which independent arcs or terranes were amalgamated or collided to form composite arcs or terranes either simultaneously or at slightly different times the roll back process was affected by the collision of the kazakhstan collage along the chara and karamay zones in the early permian the collision of the tarim craton along the south tianshan zone in the early permian the collision of the dunhuang block along the liuyuan zone in the early permian triassic the collision of the alxa block along the qugan qulu zone in the permian and the collision of the north china craton along the solonker zone in the middle late triassic the tectonic styles and architecture of accretionary orogenic belts like the caob are characterized both by the amalgamation of multiple terranes and by oroclinal bending the systematic anatomy of the multiple roll back processes and their interactions with the adjacent collages shed light on the evolving orogenic architecture and the crustal accretionary history of orogens

detrital zircons from phanerozoic rocks of the songliao block ne china evidence and tectonic implications abstract rocks that crop out in the northern part of the songliao block are mainly consist of high grade metamorphic gneiss paleozoic strata and mesozoic granites they are essentially similar to rocks reported from beneath the songliao basin that occupies the majority of the songliao block four samples of paleozoic metasedimentary rocks from tieli in the north eastern part of the songliao block yield detrital zircon u pb ages ranging from 2690 to 501 ma with four age populations at 2071 2690 ma with a peak at 2585 ma 1776 1997 ma with a peak at 1890 ma 719 991 ma with a peak at 800 ma and 501 592 ma with a peak at 518 ma these are similar to age populations in other parts of the central asian orogenic belt caob although sediments from the songliao block contain more abundant archean and proterozoic detrital zircons than the neighboring jiamusi khanka block to the east and xing an block to the west this may indicate that rocks of this age comprise a minor component of the songliao block the pan african zircon ages from the songliao block taken together with 500 ma magmatic and high grade metamorphic zircons obtained from the nearby erguna xing an and jiamusi khanka blocks indicate that pan african events affected all blocks of the caob in ne china this suggests that these blocks not only share a common basement but that they had a common history an extensive late pan african 500 ma orogenic terrane thus occupies much of the caob in ne china

discovery of a late devonian magmatic arc in the southern lancangjiang zone western yunnan geochemical and zircon u pb geochronological constraints on the evolution of tethyan ocean basins in sw china abstract the tibetan plateau and western yunnan are known to have formed by the amalgamation of gondwana derived continental blocks and arc terranes as a result of tethyan subduction followed by continental collisions during the paleozoic mesozoic and cenozoic the devonian and the southern lancangjiang zone western yunnan is a critical period and key region for studying the transformation between the proto tethyan and paleo tethyan oceanic systems new geochemical data and la icpms u pb zircon ages of the late devonian volcanogenic sediments from the southern lancangjiang zone in western yunnan sw china are presented the studied sedimentary rocks of the nanguang formation are volcaniclastic rocks with high volcanic lithic content 55 65 mostly andesite dacite with some rhyolite and tuffs whole rock geochemistry zircon trace elements and detrital modal analyses indicate derivation from a subduction related magmatic arc three tuff samples yield late devonian weighted mean 206 pb 238 u ages of 378 4 ma 366 5 ma and 382 8 ma suggesting a late devonian depositional age 104 zircon u pb analyses on 104 zircon grains from two sandstone samples present extremely tight age clusters mostly ranging from 380 ma to 360 ma this indicates a single late devonian igneous source a short transport distance and a high rate of denudation and deposition within an arc related basin are considered likely for the tuffs and volcaniclastic rocks in this study this implies the presence of an as yet unidentified late devonian magmatic arc in the southern lancangjiang zone the cryptic late devonian arc is likely to represent either a continuation of late ordovician late silurian proto tethyan subduction or the initial stage of the paleo tethyan lincang arc and indicates that subduction of the changning menglian ocean beneath the simao indochina block occurred in the late devonian

downsag and extension at calderas new perspectives on collapse geometries from ice melt mining and volcanic subsidence structures at calderas may form as a result of precursory tumescence subsidence due withdrawal of magmatic support resurgence and regional tectonism structural reactivation and overprinting are common to explore which types of structures may derive directly from subsidence without other factors evidence is reviewed from pits caused by the melting of buried ice blocks mining subsidence scaled subsidence models and from over 50 calderas this review suggests that complex patterns of peripheral deformation with multiple ring and arcuate fractures both inside and outside caldera rims topographic embayments arcuate graben and concentric zones of extension and compression may form as a direct result of subsidence and do not require a complex subsidence and inflation history downsag is a feature of many calderas and it does not indicate subsidence on an inward dipping ring fault as has been inferred previously where magmatic inflation is absent or slight initial arcuate faults formed during collapse are likely to be multiple and dip outwards to vertical associated downsag causes the peripheries of calderas undergo radial centripetal extension and this accounts for some of the complex peripheral fractures arcuate crevasses graben and some topographic moats the structural boundary of a caldera defined here as the outermost limits of subsidence and related deformation including downsag commonly lies outside ring faults and outside the embayed topographic wall it is likely to be funnel shaped i e inward dipping even though ring and arcuate fractures within it may dip outward inward dipping arcuate normal faults at shallow levels and steep inward dipping contacts between a caldera s fill and walls may both occur at a caldera that has initially subsided on outward dipping ring faults they arise due to peripheral surficial extension gravitational spreading and scarp collapse topographic enlargement at some calderas and the formation of embayments may reflect general progressive downsag and localized downsag respectively these processes may occur in addition to surficial degradation of oversteep ring fault scarps

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

geochronological and hf isotopic variability of detrital zircons in paleozoic strata across the accretionary collision zone between the north china craton and mongolian arcs and tectonic implications the central asian orogenic belt formed by accretion subsequent to the contraction of the paleo asian ocean and its southeastern segment terminated along the solonker suture zone amalgamating the mongolian arcs and the north china craton by the end of the early triassic since typical regional collisional features are absent its tectonic evolution remains speculative an investigation into the variability of detrital zircon u pb ages and hf ratios in permian strata along a southeast northwest transect from the mongolian arcs to the north china craton reveals distinct differences northern basins carry a broad mesoproterozoic to latest precambrian age signature and their sedimentary source terranes are of mixed juvenile to crustal magmatic origin in contrast southern basins contain detritus from the north china craton and their sources are of dominantly crustal contaminated magmatic origin provenance analysis suggests that in the early paleozoic ca 429 ma the paleo asian ocean was consumed along the uliastai arc and the north china craton initiating the formation of the northern and southern accretionary orogens respectively by the end of the middle carboniferous the mongolian arcs were consolidated after the accretion of the uliastai arc in the late carboniferous ca 314 ma the hegenshan back arc basin opened detaching the northern accretionary orogen while subduction continued there it may have temporarily ceased in the south after the accretion of a microcontinent ca 300 ma by the end of the middle permian ca 269 ma back arc basin closure led to the formation and obduction of the suprasubduction zone hegenshan ophiolite eventually the paleo asian ocean closed by double sided subduction such accretionary wedge wedge collision would not involve continental deep subduction unlike other continental collisions the early stages of the sequence likely resembled a pacific type scenario dominated by the subduction of oceanic lithosphere including a japanese sea type back arc basin opening the late stages involved the accretion of large scale continental blocks ultimately leading to final ocean closure which may be similar to the archipelago type setting of present day southeast asia

zircon u pb ages and hf isotopes of paleozoic metasedimentary rocks from the habahe group in the qinghe area chinese altai and their tectonic implications abstract as one of the largest accretionary orogens in earth s history the central asian orogenic belt has attracted much attention in the past two decades but there are still many unresolved issues regarding its tectonic nature and evolution this is the case with the chinese altai whose tectonic provenance remains controversial especially for the paleozoic era as the most widely exposed meta sedimentary rocks in the chinese altai the habahe group was previously considered as representing a component of precambrian micro continent forming at a passive continental margin our new u pb data for detrital zircons from the habahe group in the qinghe region of the eastern chinese altai indicate that the strata were deposited from the late silurian to early devonian 427 405 ma and experienced regional metamorphism in the middle devonian 383 9 ma as constrained by the age of youngest detrital zircons 427 13 ma the age 405 3 ma of the tonalite that intrudes the habahe group and the metamorphic age of 383 9 ma additionally the majority of zircons from the habahe group have positive ehf t values 0 22 15 02 these data combined with geochemical features and maturity of the sedimentary rocks indicate an immature provenance for the habahe group with the source materials mainly derived from a juvenile crust with some old continental components the juvenile crust was most likely formed in a subduction zone developing in the chinese altai during early middle paleozoic time new geochemical data presented in this study suggest that the chinese altai was a transitional arc during early paleozoic time similar to the japan type arc with involvements of old continental components therefore it is proposed that the chinese altai was likely rifted from the south margin of the siberia craton probably related to the break up of the rodinia supercontinent and then experienced accretion and reworking with the subduction of the paleo asian ocean during early paleozoic time

tectonic evolution from subduction to arc continent collision of the junggar ocean constraints from u pb dating and hf isotopes of detrital zircons from the north tianshan belt nw china the junggar ocean once situated north present coordinate of the yili and central tianshan blocks during early paleozoic to late carboniferous time was a major southern branch of the paleo asian ocean the opening expansion and final closure of which led to the development of the central asian orogenic belt between eastern europe siberia and tarim north china however the detailed evolution of the junggar ocean has not been well constrained this paper reports u pb and lu hf isotopic data of detrital zircons from sandstones in the north tianshan belt which can provide new insights into understanding the paleozoic development of the junggar ocean most detrital zircons exhibit oscillatory zoning and high th u ratios typical of igneous origin the predominant paleozoic zircons yield major age populations at ca 294 313 327 338 375 440 455 and 474 502 ma and are interpreted to have been derived from the long lived volcanic and island arc systems formed by the southward subduction of the junggar ocean and subsequent collisional and postcollisional magmatism the minor precambrian zircons yield ages scattering at ca 550 680 765 890 970 990 1160 1250 1500 1690 1750 1840 1970 2440 2500 and 2615 2700 ma which are nearly but not fully congruent with those from the adjacent beishan and kuluketage precambrian terranes therefore our results indicate that the central tianshan block was once part of the tarim block during precambrian time most of the 541 440 ma zircons possess low negative ehf t values while the 440 300 ma zircons exhibit dominantly positive ehf t values which can be linked to the junggar oceanic slab rollback since ca 440 ma this event subsequently gave rise to the opening of the south tianshan back arc basin ocean between the central tianshan and tarim blocks exhumation of high pressure granulites and formation of a series of island arcs in the junggar ocean combined with previous studies we suggest that the junggar ocean was probably closed at ca 300 ma in association with arc continent collision followed by postcollisional magmatism it deserves mentioning that the ca 0 5 and 1 4 1 5 ga detrital zircons and contemporaneous magmatic rocks only occur in the central tianshan block not in the tarim block therefore more detailed investigations are needed to better elucidate the origin and precambrian evolution of the central tianshan block

geochemistry and geochronology of the delinggou intrusion implications for the subduction of the paleo asian ocean beneath the north china craton abstract the formation and evolution of the central asian orogenic belt caob are intimately related to the opening and closure of the paleo asian ocean the final closure led to amalgamation or collision between the caob and north china craton however the issue about the subduction polarity and final closure of the paleo asian ocean has long been debated in order to resolve this issue we carried out a detailed geochemical and age study on the delinggou intrusion from the siziwangqi area of central inner mongolia at the northern margin of the north china craton the delinggou intrusion consists predominantly of diorite and quartz diorite with minor gabbroic diorite u pb zircon dating indicates that the intrusion was emplaced during early carboniferous 330 to 343 ma geochemically the rocks are metaluminous and belong to the calc alkaline series they are enriched in large ion lithophile elements lile e g rb sr and ba and depleted in high field strength elements hfse e g zr nb hf ta and ti the delinggou rocks were probably generated by partial melting of an enriched subcontinental lithospheric mantle further contaminated by crustal material during the magmatic differentiation in a continental arc setting we conclude that the paleo asian ocean underwent southward subduction beneath the north china craton and the subduction initiated in the carboniferous 343 330 ma but the final ocean closure time is not yet constrained

geochronology of late caledonian magmatism in northern britain rb sr k ar ar ar and sm nd isotopic data are presented for mineral separates and whole rock samples from old red sandstone ors lavas associated minor intrusions and 9newer granite9 intrusive complexes from the cheviot hills north to shetland these data yield 16 internally consistent rb sr mineral ages and one each of ar ar sm nd and k ar ages when considered with precise and reliable published data it is clear that there are systematic differences in the age of late caledonian magmatism through the geological regions of scotland in the grampian highlands and the scottish midland valley magmatism was concentrated during the period 423 to 410 ma ors volcanism in the highlands occurred around 420 ma with plutonism continuing to about 408 ma volcanism and plutonism in the midland valley took place between 415 and 410 ma in the southern uplands magmatism occurred earlier in the north with all magmatism close to the southern uplands fault dated around 408 ma and that close to the iapetus suture dated at around 394 ma as no evidence exists for ocean closure prior to c 400 ma the age of igneous activity north of the southern uplands fault is consistent with chemical evidence for a relationship to active nw directed subduction the younger magmatism of the s part of the southern uplands cannot be subduction related and the c 408 ma plutons nearer the southern uplands fault can only be subduction related if tectonic construction of the southern uplands was a two stage process

petrology of the hegenshan ophiolite and its implication for the tectonic evolution of northern china abstract petrographic observations and mineralogical and geochemical analyses have revealed that the hegenshan ophiolite is of mid ocean ridge origin and has been subjected to dynamothermal metamorphism at medium p t conditions the metamorphism is characterized by a prograde change in paragenesis from the greenschist to epidote amphibolite facies with peak temperature conditions of 570 640 c at pressures of 4 10 kbar the amphiboles formed by this metamorphism show k ar ages of 110 130 ma the metamorphic conditions and k ar ages suggest that the hegenshan ophiolite is located at the suture between the siberian and north china continental blocks where the continental collision in this area took place in middle mesozoic time given the temporal and spatial distribution of the igneous activity around the da hinggan ling mountains it is suggested that the extensive yanshanian magmatism in this region resulted predominantly from a southward subduction of an oceanic plate prior to collision alternatively it may possibly have resulted from the collision itself at the final stage

metallogenic and tectonic implications of detrital zircon u pb hf isotopes and detrital rutile geochemistry of late carboniferous karstic bauxite on the southern margin of the north china craton abstract late carboniferous bauxite is widely developed on the north china craton ncc and is referred to as the g layer bauxite the bauxite ores of the southern ncc are closely related to the tectonic evolution of the ncc and its neighboring north qinling belt nqb to the south in this study based on the geological and mineralogical observations we used the u pb and hf isotopes of detrital zircons and chemical compositions of detrital rutiles from the gunziying and shibanhe bauxite deposits along the southern margin of the ncc to study the metallogenic process of bauxite ores and their tectonic implications one and two bauxite ore layers were observed in the gunziying and shibanhe bauixte deposits respectively the bauxite ore is dominated by diaspore illite anatase kaolinite pyrite zircon and rutile analyses of detrital zircons revealed that the largest group 432 515 ma at a peak age of 450 ma is spread throughout the late carboniferous bauxite deposits with ehf t values of 5 7 21 4 similar to the zircons from the nqb similarly the detrital zircons with peak ages of 950 ma and age clusters at 1 1 ga 1 3 ga and 1 8 ga as well as those ranging in age from 635 to 797 ma also exhibited ehf t values similar to the zircons from the nqb in addition the results suggested that 62 73 and 75 88 2 of detrital rutiles showed metapelitic protoliths and amphibolite eclogite facies temperatures respectively while 23 27 and 10 11 8 exhibited metamafic protoliths and granulite facies temperatures respectively based on the results above we propose that the provenance of the bauxite ores on the southern margin of the ncc is primarily from the nqb the dominant contributors were the neoproterozoic kuanping group and the meso neoproterozoic qinling group the archean proterozoic metamorphic rocks in the paleo uplifts of the ncc were also likely involved in the bauxite formation a probable outline of the bauxite formation process on the southern ncc former weathered residuals and subsequent deposition is proposed and a reasonable interpretation that the nqb was accreted to the ncc in the late carboniferous and formed a northward lying slope basin within the ncc is supported

stratigraphy and geochronology of permo carboniferous strata in the western north china craton insights into the tectonic evolution of the southern paleo asian ocean abstract the northwestern ordos terrane not in the western north china craton w ncc comprises the northwestern ordos basin in the east and the eastern alxa massif in the west bound by the helanshan tectonic belt htb the key position makes the not crucial for understanding the evolutionary processes of the w ncc and particularly the tectonic relation of the alxa massif with the w ncc in this study petrologic stratigraphic and geochronologic studies were conducted on permo carboniferous successions in the not stratigraphic correlation reveals that carboniferous marine successions display a transgressive sequence with a slight westward deepening facies variation evidenced by the continuous onlap of tidal flat layers toward the east the permian nonmarine strata in the htb and the ordos basin have no substantial facies variation defining an upward regressive sequence from deltaic to fluvial associations while time equivalent units in the eastern alxa massif have been eroded the generally ssw directed paleocurrents suggest that permo carboniferous siliciclastic materials were derived from a highland to the northeast the unified sedimentary system in the not constrains the alxa massif to be part of the w ncc the lower carboniferous sandstone contain zircons with a concentrated age cluster of 1700 2700 ma comparable to archean to paleoproterozoic crystalline basement in the northern w ncc by contrast in addition to zircons of 1700 2700 ma late carboniferous and permian sandstones all contain abundant paleozoic zircons with two age clusters around 300 ma and 420 ma which are similar to age patterns of paleozoic magmatism in the northern w ncc zircon age profile and sandstone modal composition indicate the origin from an andean type continental arc the permo carboniferous tectono sedimentary processes of the not should occur in a marginal basin behind the continental arc along the northern w ncc in response to the southward subduction of solonker ocean southern branch of paleo asian ocean

deformed continental arc sequences in the south tianshan new constraints on the early paleozoic accretionary tectonics of the central asian orogenic belt abstract the tectonic affinity and structural evolution of the south tianshan are key issues to understand the paleozoic accretionary or collisional orogenesis in the southwestern central asian orogenic belt but remain controversial geological and structural investigations on the early paleozoic low grade metamorphic volcanic sedimentary sequences around the bayinbuluk basin demonstrate that they were prevalently reworked by north vergent ductile deformation and overlain unconformably by devonian to carboniferous carbonates zircon u pb dating indicates that the volcanic rocks erupted during the middle ordovician to early devonian time 460 410 ma and contain neoarchean to neoproterozoic zircon xenocrysts 2 75 ga 2 46 ga 1 60 ga 997 963 ma and 827 ma their whole rock elemental sr nd isotopic end t 5 0 to 3 6 and zircon lu hf isotopic ehf t 13 5 to 11 6 features resemble those of andean type continental arc volcanic rocks the coexisting tuffaceous sedimentary rocks indicate a depositional environment amidst a continental volcanic arc at 421 to 404 ma the newly obtained and available geochronological geochemical and structural data suggest that 1 a wide active continental margin covering the south and central tianshan and northern tarim was formed due to early mid paleozoic southward subduction of the paleo tianshan ocean beneath the tarim craton and 2 a mid paleozoic transition from advancing to retreating accretionary orogeny was followed by opening of the south tianshan back arc basins and break up of the northern tarim continental margin the south tianshan was subsequently subjected to regional deformation during the successive diachronous closure of the major and back arc oceanic basins and locally reworked by the ductile strike slip faulting 307 255 ma

youngest radiocarbon age for jefferson s ground sloth megalonyx jeffersonii xenarthra megalonychidae abstract a partial skeleton of the extinct ground sloth megalonyx jeffersonii recovered from a farm near millersburg ohio in 1890 was radiocarbon dated for the first time the ungual dated is part of a skeleton mounted for exhibit at the orton geological museum at ohio state university and was the first mounted skeleton of this animal from its initial discovery the bones were treated with multiple organic compounds that had the potential to compromise the radiocarbon age and the specimen required special treatments in order to obtain a valid radiocarbon age the 14 c measurement on the ungual from this skeleton 11 235 40 14 c yr bp 13 180 13 034 cal yr bp is the youngest 14 c age presently determined for m jeffersonii

reconstructions of east asian blocks in pangea preface abstract pangea is the youngest supercontinent in earth s history forming about 300 250 ma ago as supported by large amounts of geological paleomagnetic and paleontonlogical data there is little debate on the positions of major continental blocks in pangea however controversy has long surrounded the relations of east asian blocks with pangea most pangea reconstructions assume that the east asian blocks were not the components of pangea but separated from pangea by the paleo tethys ocean these reconstructions were mainly based on geological and paleomagnetic data before the 1990 s but did not fully consider recent data especially for major orogenic belts that formed through the subduction and closure of the proto tethys paleo asian and paleo tethys oceans in east asia to determine where when and how these oceans were closed and whether or not the east asian blocks were added to pangea the national natural science foundation of china nsfc set up a nsfc major program entitled reconstructions of east asian continental blocks in pangea which started in 2012 and was completed in 2016 in the past five years the researchers of this project carried out extensive field based geological and paleomagnetic investigations on the east asian blocks and their intervening mountain belts and produced large amounts of data and competing interpretations which provide constraints on the paleogeographic evolution of the east asian blocks from the breakup of rodinia to the assembly of pangea this forms the justification for organizing this special issue in which we collected ten papers that timely summarize these new data and interpretations and present new models for the reconstructions of the east asian continental blocks in pangea

tectonically controlled piecemeal caldera collapse a case study of glencoe volcano scotland caldera collapse at glencoe scotland was incremental and involved complex movements of numerous fault blocks before formation of the ring fault for which the volcano is renowned intracaldera depocenters had the form of grabens or half grabens locally with downsag and bounding monoclinal flexures and these changed form and location for each of the five major ignimbrite eruptions that represent the early volcanic history this piecemeal caldera collapse is in contrast to previous interpretations in which all subsidence was thought to have involved a coherent crustal block moving on the ring fault collapse and magmatic plumbing were profoundly influenced by preexisting tectonic faults trending northwest and northeast a dominant northwest trending graben controlled the general location and form of major caldera depocenters and repeatedly channeled a major river through glencoe the main graben was transected orthogonally by two cross grabens each of the first three caldera eruption cycles involved initial phreatomagmatic explosivity which built tuff cones followed by magmatic fountaining that produced lava like silicic ignimbrites the three lava like ignimbrites total more than 300 m thick two later eruptions produced eutaxitic silicic ignimbrites together more than 300 m thick and contemporaneous progressive downsag was associated with the formation of extensional fractures crevasses hundreds of meters deep sills up to 100 m thick of mingled andesite and rhyolite between the intracaldera ignimbrites were accommodated by increments of subsidence unconformities and sedimentary layers within the stratigraphic succession record fluvial erosion and abrupt switches to alluvial and or lacustrine deposition between each ignimbrite eruption the changes in drainage and sedimentation and the development of coarse debris avalanche breccias reflect tectonic faulting during the periods between major eruptions the duration of the period including the five caldera forming eruptions was probably 0 5 m y and the magnitude of the tectonic faulting that occurred prior to and during the caldera developments 0 5 km m y normal displacement is similar to that of the most actively subsiding sedimentary basins glencoe shows that the piecemeal nature of calderas may be plainly evident only in deeply dissected systems which with knowledge that caldera volcanoes commonly overlie faults suggests that piecemeal calderas may be more common than previously recognized the research also shows that the emplacement of granites s l was episodic rather than synchronous throughout the magmatic province and it suggests that numerous plutons in the province e g in donegal ireland may have formed at sites of central volcanoes that are no longer preserved

zircon u pb ages and geochemistry of newly discovered neoproterozoic orthogneisses in the mishan region ne china constraints on the high grade metamorphism and tectonic affinity of the jiamusi khanka block abstract the jiamusi khanka block located in the easternmost segment of the central asian orogenic belt caob is one of the least understood blocks in northeastern china the age of its basement the timing of the high grade metamorphism and the tectonic affinity of this block have all been subjects of controversy here we describe newly discovered neoproterozoic orthogneisses from the mishan region in the central jiamusi khanka block sims and la icp ms u pb dating of the magmatic cores of zircons from three of these high grade orthogneisses yielded weighted mean ages that range from 898 4 to 891 13 ma indicating that early neoproterozoic magmatism did occur in the jiamusi khanka block twenty two analyses of metamorphic zircons rims as well as some grains gave two groups of concordant ages one at ca 563 ma interpreted as the timing of the high grade metamorphism and the other around 518 496 ma interpreted as representing a subsequent retrograde metamorphism the orthogneisses represent metamorphosed peraluminous syn collisional syenogranites with sio2 71 29 78 08 wt a cnk 1 06 1 99 and na2o k2o 0 03 1 49 and they belong chemically to the high potassium calc alkaline series they have low sr 32 134 ppm contents and sr y ratios 1 6 16 9 and display strongly fractionated to nearly flat ree patterns with negative eu anomalies eu 0 22 0 95 and depletions in nb ta these geochemical characteristics suggest that the syenogranitic rocks were derived under low pressures from a crustal source with a high proportion of sedimentary rocks perhaps in a subduction related setting that was undergoing transition from compression to extension in the context of the reconstruction of gondwana the ca 898 891 ma magmatic activity and the ca 563 ma high grade metamorphism in the mishan region provide constraints on the possible linkages between the jiamusi khanka block and east gondwana during the neoproterozoic to early paleozoic

explosive rhyolitic volcanism in the jemez mountains vent locations caldera development and relation to regional structure the jemez volcanic field straddles the western margin of the rio grande rift where the rift is intersected by the jemez lineament in north central new mexico the field has a record of volcanism extending back to before 13 ma initial basaltic activity was related to active rifting with minor rhyolitic eruptions occurring along n s rift bounding faults between 10 and 7 ma voluminous andesitic volcanism took place in the central jemez mountains area overwhelming contemporaneous basaltic and rhyolitic magmatism an apparent tectonic lull took place from 7 to 4 ma accompanied by lower eruption rates during this interval dacitic magmas were erupted to form the tschicoma volcanic center but mafic and rhyolitic volcanism virtually ceased since 4 ma accompanying resumption of rifting a growing silicic magma system has been present under the central part of the jemez mountains ultimately evolving to the magma body that produced the voluminous rhyolitic bandelier tuffs explosive rhyolitic eruptions from this large magma body have occurred many times since 3 to 4 ma early eruptions 3 6 2 8 ma produced high silica rhyolite ignimbrites restricted to the southwest part of the jemez mountains formation of these units may have been accompanied by caldera collapse these events were followed by the two caldera forming bandelier tuff ignimbrite eruptions 1 45 and 1 12 ma post caldera explosive and effusive rhyolite eruptions have also tapped the magma body from vents generally located along ring fractures after both bandelier events vent and caldera locations for the rhyolitic eruptions during 3 4 ma have been inferred from grain size characteristics dispersal patterns and facies variations in the plinian deposits and ignimbrites lithic breccia zones of the pre bandelier ignimbrites indicate possible caldera sources in the southwest part of the present valles caldera during the eruption of both bandelier tuffs initial plinian falls and early pyroclastic flows emanated from vents centrally located in the jemez mountains in the lower bandelier tuff eruption a transition to ring fracture vents occurred before the emission of later pyroclastic flows but there is no strong evidence to suggest such a transition occurred during the upper bandelier eruption calderas associated with the lower and upper bandelier tuffs toledo and valles respectively are almost identical in location as are the plinian vent sites for these two large eruptions the toledo embayment northeast of the valles caldera contains lava domes from up to 3 6 ma and may be a caldera or crater associated with early explosive dacitic volcanism in the tschicoma volcanic center post 1 4 ma lava domes also fill this depression the main volcanic features of the jemez mountains field including the valles caldera complex eruption vents and the apical graben of the post valles redondo resurgent block appear to be aligned along the ne sw trending jemez fault zone this zone the local expression of a precambrian basement feature the jemez lineament has exerted strong control on the location and style of eruptions from the bandelier rhyolitic magma system

did the paleo asian ocean between north china block and mongolia block exist during the late paleozoic first paleomagnetic evidence from central eastern inner mongolia china 1 the tectonic evolution of the paleo asian ocean between the north china block ncb and the mongolia block mob is a contentious issue and geodynamic models remain speculative in an effort to puzzle out this controversy a paleomagnetic study was carried out on the silurian to permian formations in central eastern inner mongolia china more than 680 sedimentary and volcanic samples were collected from 86 sites we have established titanium poor magnetite and hematite as the principal magnetic carriers anisotropy of the magnetic susceptibility measurements demonstrate negligible deformation of the majority of study rocks with sedimentary fabrics from primary magnetizations a late devonian and a permian pole are calculated for inner mongolia block imb at 46 8 n 349 1 e dp 14 6 dm 27 3 with n 3 and 48 7 n 3 7 e dp 5 2 dm 9 1 with n 6 respectively two stages of secondary magnetization are also identified probably due to early permian and early cretaceous magmatic events as preliminary results the comparison of our new paleomagnetic poles with available data from ncb mob and siberia indicates that 1 the paleolatitudes of imb ncb and mob are consistent between late devonian and permian suggesting pre late devonian closure of the paleo asian ocean and further evaluation of these three blocks as a single entity and 2 post permian intracontinental deformation was significant and characterized by block rotations which are due to strike slip faulting within the welded ncb imb mob block

experimental study of caldera formation scaled experiments have been carried out on caldera collapse mechanisms using silicone as analogue magma and dry sand as analogue rock experiments were carried out in two and three dimensions using a range of roof aspect ratios thickness width 0 2 to 4 5 appropriate for caldera collapse they reveal a general mechanism of collapse only weakly dependent on the shape of the reservoir for low roof aspect ratios 1 subsidence starts by flexure of the roof and the formation of outward dipping reverse ring faults which in turn trigger formation of peripheral inward dipping normal ring faults the subsidence always occurs asymmetrically in cross section the reverse faults delimit a coherent piston bounded on each side by an annular zone of inwardly tilted strata located between the reverse and normal ring fault sets the surface depression consists of a nondeformed area piston surrounded by an annular extensional zone tilted strata for high aspect ratios 1 multiple reverse faults break up the roof into large pieces and subsidence occurred as a series of nested wedges 2 d or cones 3 d the extensional zone dominates the surface depression in the case where preexisting regional faults do not play a major role the collapse mechanics of calderas probably depends strongly on the roof aspect ratio calderas with low roof aspect ratios are predicted to collapse as coherent pistons along reverse faults the annular extensional zone might be the source of the large landslides that generate intracaldera megabreccias collapse into magma reservoirs with high roof aspect ratios may be the origin of some funnel calderas where explosive reaming is not dominant

assembly of the lhasa and qiangtang terranes in central tibet by divergent double subduction abstract integration of lithostratigraphic magmatic and metamorphic data from the lhasa qiangtang collision zone in central tibet including the bangong suture zone and adjacent regions of the lhasa and qiangtang terranes indicates assembly through divergent double sided subduction this collision zone is characterized by the absence of early cretaceous high grade metamorphic rocks and the presence of extensive magmatism with enhanced mantle contributions at ca 120 110 ma two jurassic cretaceous magmatic arcs are identified from the caima duobuza rongma kangqiong amdo magmatic belt in the western qiangtang terrane and from the along tso yanhu daguo baingoin daru tso magmatic belt in the northern lhasa terrane these two magmatic arcs reflect northward and southward subduction of the bangong ocean lithosphere respectively available multidisciplinary data reconcile that the bangong ocean may have closed during the late jurassic early cretaceous most likely ca 140 130 ma through arc arc soft collision rather than continent continent hard collision subduction zone retreat associated with convergence beneath the lhasa terrane may have driven its rifting and separation from the northern margin of gondwana leading to its accretion within asia

late paleozoic subduction and collision processes during the amalgamation of the central asian orogenic belt along the south tianshan suture zone abstract the provenance of late paleozoic siliciclastic sedimentary strata on the northern margin of the tarim craton and the south tianshan orogenic belt provides important insights into subduction and collision processes during the formation of the southern central asian orogenic belt detrital zircons from carboniferous and permian sedimentary rocks in the south tianshan belt show two predominant age populations of 500 400 and 305 270 ma and three subordinate clusters around 2 5 ga 2 0 1 7 ga and 1 2 0 6 ma such age patterns are similar to major magmatic episodes in the tarim craton but are distinct from those in the central tianshan yili block implying that the carboniferous permian strata in the south tianshan belt were deposited on the northern margin of the tarim craton these data in combination with carboniferous passive margin deposition along the south tianshan and northern tarim regions and intense arc magmatism in the central tianshan area support the northward subduction of the south tianshan oceanic crust the abrupt decrease of zircon e hf t values at 310 ma indicates the tectonic transformation from oceanic subduction to continental collision however syn and post collisional sediments in the south tianshan and northern tarim regions did not contain detritus from the central tianshan yili block as evidenced by the absence paucity of 380 310 ma detrital zircons in the late paleozoic strata and by the stratigraphic record in the regions insignificant surface erosion and uplifting during the collision and ultra high pressure rock exhumation require divergence in the frontal wedge we propose that the switch from a convergent to a divergent regime was triggered by the arrival of the tarim mantle plume in the latest carboniferous which possibly had profound effects on regional sedimentation and exhumation of ultra high pressure rocks in the orogenic belt

fluidization of wet sediments during the emplacement and cooling of various igneous bodies during the emplacement and cooling of igneous bodies fluidization of wet sediment by water vapour may occur by heating or by pressure relief during the opening of fractures momentary fluidization causes sediment reconstitution and local transport continuous fluidization can result in substantial sediment displacement theoretical considerations show that fluidization due to heating is not likely to occur at depths where pressure is much greater than 312 bars studies of the emplacement of sills of andesite ayrshire fluid rhyolite ramsey island and viscous rhyolite moelwyn hills recognize a wide range of phenomena attributable to wet sediment fluidization and in places modify existing interpretations the transgressive bases of the subaqueously welded ash flow tuffs of the capel curig volcanic formation are consistent with emplacement attended by fluidization of the subjacent sediment evidently fluidization is a means by which large volumes of wet sediment can be replaced by igneous material with minimal disturbance of the remaining host also fluidization can result in exceptionally complex contact relationships between sedimentary host and intrusive rocks and can cause fractures formed during cooling to be thoroughly pervaded by sediment fluidization is commonly associated with the formation of peperites

kinematic link between episodic trapdoor collapse of the negra muerta caldera and motion on the olacapato el toro fault zone southern central andes a combined geochronological and structural analysis of the miocene negra muerta caldera was designed to better understand caldera formation associated with prominent faults on the central andean plateau rb sr ages of the caldera outflow facies indicate that caldera formation occurred in two volcano tectonic episodes the first episode commenced with explosive eruption of the 9 0 0 1 ma andesitic acay ignimbrite followed by a period of volcanic quiescence and moderate tectonic activity dominant volcanic and tectonic activity occurred during the second episode which is bracketed by eruption of the 7 6 0 1 ma rhyolitic toba 1 ignimbrite and effusive discharge of the 7 3 0 1 ma rhyodacitic to andesitic lava flows structural relationships between rocks of the negra muerta volcanic complex and collapse induced normal faults notably ne striking normal faults agree with simultaneous volcanic activity and floor subsidence of the caldera during the second episode floor subsidence was achieved by tilting on an outward dipping reverse fault to the northwest of the caldera floor around a hinge zone located south of the caldera floor this induced horizontal extension of the caldera floor and was accomplished by fragmentation of and intrusion of dikes into the floor collapse induced and post collapse fault populations of the caldera do not differ significantly in the directions of their axes of maximum extension and are in this respect kinematically compatible with left lateral slip on the nearby olacapato el toro fault zone this furnishes evidence for a kinematic control by prominent faults on the formation of collapse calderas in the central andes the structural analysis of the negra muerta caldera shows that collapse calderas can serve as deformation markers that contribute in elucidating the regional kinematic regime and the time of activity of prominent dislocations genetically related to collapse calderas

geochemistry geochronology and sr nd isotopic compositions of permian volcanic rocks in the northern margin of the north china block implications for the tectonic setting of the southeastern central asian orogenic belt the southeastern part of the central asian orogenic belt caob which records the collision of the north china block ncb with the south mongolian microcontinent is a key area for reconstructing the tectonic history of the caob controversy persists regarding the timing of the final structural amalgamation of the region therefore it remains unclear whether the late paleozoic thick volcanic successions were generated in a subduction or post orogenic environment redefining the age of the formation and analyzing the geochemical compositions of these volcanic rocks can provide clues regarding the regional tectonic evolution during the late paleozoic and place constraints on the closure time of the paleo asian ocean in this study we present geochemical geochronologic and sr nd isotopic data for 29 volcanic rock samples from the elitu formation in xianghuangqi central inner mongolia the elitu volcanic rocks have latest early to middle permian ages between 272 and 268 ma most of the mafic intermediate and felsic rocks show k normal and high k calc alkaline characteristics melting is considered to be due to large scale upwelling of the metasomatic lithospheric mantle and different degrees of melting of the thickened lower crust the northern margin of the ncb which represents the southeastern boundary of the caob records transtensional and subsequently extensional tectonics associated with late carboniferous to middle permian volcanic activity

geochemical discrimination of clastic sedimentary rock sources abstract factors controlling the geochemistry of a clastic sedimentary rock can include 1 composition of source terrain 2 chemical weathering 3 hydraulic sorting 4 diagenesis 5 metamorphism and 6 hydrothermal alteration a linear solution inferring source terrain composition from geochemistry of the sediment is impossible in this multivariate system as several unknowns will commonly be present the use of graphical analysis of element pairs circumvents the problem chemically immobile elements will maintain invariant ratios during rock mass change caused by either addition or depletion of mobile elements this results in chemically immobile element scattergrams exhibiting linear trends along radians extending from the origin if the major mineral phases of the immobile elements have behaved in a hydrodynamically similar manner as chemically mobile element plots will produce a scatter of points this relationship can be used to test chemical immobility and similarity in hydrodynamic sorting history the constraint on analysis is that the source area must be compositionally uniform or the sediment well mixed prior to delivery to the basin a second technique using sio2 plots has also been developed to investigate element mobility and hydrodynamic behaviour of the mineral phases containing the elements sio2 immobile element plots result in a linear arrangement of points extending towards either 0 or 100 sio2 the 0 intercept position occurs for elements with major mineral phases concentrated in sand the 100 intercept for those concentrated in clays plotting chemically mobile elements produces different patterns and this can be used to gain information on alteration and sorting history elemental ratios for chemically immobile elements with similar hydrodynamic behaviour will be the same as those for the weighted average composition of the source material this provides a powerful tool for deducing source terrain from sediment geochemistry techniques outlined above were tested on archaean metasandstones from superior province canada immobile element ratio diagrams for nb al ti and zr al ti indicate that a calc alkaline extrusive intrusive suite lying to the north of the study area was the source and not five other volcanic suites in the region this conclusion agrees with previous clast lithology studies and accentuates the applicability of the geochemical techniques

the cretaceous messum igneous complex s w etendeka namibia reinterpretation in terms of a downsag cauldron subsidence model abstract the messum igneous complex mic lies within the ene trending zone of lower cretaceous 132 ma damaraland intrusive complexes in namibia intruded into pan african damara basement it is defined by a roughly circular structure 18 km in diameter the bounding ring fault exposed along the eastern sector encircling messum are the volcanic sequences of the goboboseb mountains comprising a lower basalt series tafelkop and tafelberg types followed with intervening basalts by four voluminous quartz latite ql eruptions goboboseb and springbok ql units inferred stages of development are a an initial very broad basaltic lava shield comprising the tafelberg and tafelkop basalts and messum crater basalts mcb possibly ponded in near vent lava lakes embedded within the lower basaltic sequence is a localised rhyolite dominated eruptive centre ca 5 km in diameter interpreted as a funnel caldera located towards the centre of the mic b downsagging extending northwards from messum following the goboboseb ql eruptions 2300 km3 ponding of overlying basaltic units c climactic springbok ql eruption 6300 km3 producing further downsag together with the inward radial dip of all volcanic units towards the mic ring fault initiation d cauldron subsidence emplacement of a granitoid suite forming the mic moat area between the ring fault and the core region e intrusion of gabbroic cone sheets into incompletely solidified granitic melts within the southeastern moat resulting hybridisation and magma mingling produced extensive development of heterogeneous granitoid and hybrid dioritic lithologies f cone sheet intrusions of the eastern gabbros into more highly solidified granitoids of the southeastern moat g intrusion of thick 1 2 km western gabbro cone sheets exhibiting local fine scale layering into solidified granitoids mainly within the western moat minor late stage granitic intrusions h 2 3 ma quiescent period followed by quartz and ne syenite intrusions and finally basanite dykes emplaced within the mic core accompanying differential uplift of the core uplift resurgence within the mic has accompanied intrusion of the moat granitoids and mafic cone sheets thereby juxtaposing volcanic and intrusive sequences phases of both subsidence and uplift have characterised the mic the nw scotland tertiary central igneous complexes and messum show evidence of a number of parallel developments but also important differences the mic differs markedly from caldera systems within the western usa and circum pacific messum is therefore suggested to represent a distinct class of intrusive extrusive central complex although probably common in large igneous provinces

evolution of the solonker suture zone constraints from zircon u pb ages hf isotopic ratios and whole rock nd sr isotope compositions of subduction and collision related magmas and forearc sediments abstract the solonker zone in northern inner mongolia china is considered as the suture between the north china craton and the south mongolian microcontinent two magmatic belts are recognized along the suture zone a subduction related magmatic belt represented by the baolidao arc rocks and a younger collision related granite belt represented by the halatu granites we use zircon u pb ages zircon in situ hf isotopic analyses and whole rock nd sr isotopic data of the two magmatic belts and related forearc sediments the xilinhot metamorphic complex to constrain timing of the suturing and to discuss the petrogenesis of the magmatic rocks a gabbroic diorite bld 1 of the baolidao arc was dated at 310 5 ma by shrimp this sample shows an e nd t value of 2 5 and i sr of 0 7052 hf isotopic analyses on 25 zircons from the same sample show e hf t 5 4 to 11 5 another diorite sample xh 2 of the same arc from south of xilinhot displays even more depleted isotopic compositions with e nd t 5 6 and i sr 0 7037 the main population of zircons from this sample have highly variable and depleted hf isotopic compositions e hf t 0 18 3 the large variation in hf isotopic composition of zircons with largely the same crystallization age from a single pluton is explained by a mixing process between depleted mantle derived magma and continental crust in an active continental arc setting the halatu granite hlt 2 was dated at 234 7 ma by shrimp zircons from the granite also show a large variation of e hf t values 9 1 to 26 despite most samples having whole rock e hf t 2 the large variation in e hf t values suggests that the granite formed probably by partial melting of two source regions a dominant juvenile crust and a subordinate old continental crust most zircons from the xilinhot metamorphic complex show ages comparable with those of the baolidao arc rocks suggesting that the protolith of the metamorphic complex was probably deposited during or after arc magmatism some zircons however show precambrian ages that fall into two groups one with ages of 780 900 ma resembling those from the south mongolian microcontinent and the other with ages of 1524 2900 ma similar to those of the north china craton thus the protolith of the metamorphic complex probably formed in a forearc basin during convergence of the two continents and metamorphosed subsequently during collision in the late paleozoic our zircon age data thus constrain timing of collision between the south mongolian microcontinent and the north china craton to have been between 296 and 234 ma

volcanic and structural evolution of the okataina volcanic centre dominantly silicic volcanism associated with the taupo rift new zealand article the okataina volcanic centre ovc contains the northeasternmost caldera complex in onshore taupo volcanic zone tvz new zealand sited largely within the taupo rift pyroclastic fall deposits and ignimbrites deposited in the bay of plenty coast area between 420 and 625 ka were probably erupted from ovc and these provide a maximum age for the centre the earliest ignimbrite for which there is good evidence for eruption from ovc is the c 550 ka quartz biotite ignimbrite exposed to the west of lake okataina and to se of ovc this ignimbrite probably correlates with one of the early ignimbrites found in the kawerau geothermal wells and is large enough to have been accompanied by caldera collapse it was followed by extensive rhyolitic explosive eruptions of the murupara subgroup culminating with eruption of the matahina ignimbrite at c 325 ka this c 160 km 3 magma volume eruption was accompanied by caldera collapse to form the southern part of the present day okataina caldera complex a long duration sequence of rhyolite lavas and pyroclastics was then erupted on the southern and western sides of ovc before eruption of the 100 km

evolution source and tectonic significance of early mesozoic granitoid magmatism in the central asian orogenic belt central segment abstract numerous early mesozoic granitoids have been recognized from the central segment of the central asian orogenic belt caob they can be broadly classified into two groups according to zircon u pb ages an early stage group covering the time span from early to middle triassic 250 230 ma and a late stage group emplaced during late triassic to early jurassic ca 230 190 ma early stage 250 230 ma granitoids are mainly distributed in the western central mongolia erguna belt cmeb the western altai belt ab the south mongolia xing an belt smxb and the beishan inner mongolia jilin belt bijb they consist mainly of quartz diorites granodiorites and monzogranites mostly of i type with minor mafic intrusions with some of them showing adakite like signatures and some with s type features late stage 230 190 ma granitoids mainly occur in the north mongolia transbaikalia belt nmtb the eastern cmeb erguna massif and the eastern altai belt ab they are predominately syenogranites monzogranites and syenites associated with many alkaline granites and mafic intrusions and are a type and transitional i a type or highly fractionated i type granites whole rock sr nd and zircon hf isotopic data have been compiled for regional isotopic mapping the end t values show large variations from 7 0 to 7 4 and nd model ages tdm from 0 46 ga to 1 43 ga as well as the initial sr isotopic ratios sri from 0 7023 to 0 7174 the zircon ehf t values vary from 4 6 to 15 3 and give two stage hf model ages tdm2 from 0 30 ga to 2 09 ga the extremely large variations of whole rock sr nd and zircon hf isotopes demonstrate strong isotopic heterogeneity of the source regions that are mainly dominated by juvenile components with significant old crustal participation furthermore the late stage granitoids in the nmtb the cmeb and the ab generally have more negative end t values and more variable zircon ehf t values than those of the early stage granitoids in the same belt implying an increasing crustal signature from early to late stage in the assumed heterogeneous source regions within the same belt which probably results from melting of shallower crust in parallel with a shift to more alkaline chemistry of the late granitoid magmas by contrast most late stage granitoids in the smxb and the bijb have more positive end t values and homogeneous zircon ehf t values than those of the early stage granitoids in the same belt indicating more juvenile contribution to the source of the these granitoids the generation of the early mesozoic granitoid magmas in the nmtb and the cmeb was dominated by the ongoing closure of the mongol okhotsk ocean and some was probably related to a mantle plume process they were possibly derived from subducted materials melting or juvenile components with some probable contributions from ancient continental crust early mesozoic granitoid magmas in the smxb the ab and the bijb were generated in a post non orogenic setting after the closure of the paleo asian ocean and were the results of partial melting of crustal components in response to underplating of mantle derived magmas most likely linked to lithospheric thickening and delamination and asthenospheric upwelling early mesozoic granitoid magmatism provides critical information on mesozoic post accretionary tectonic evolution of the paleo asian ocean and transitional tectonic regimes from early mesozoic subduction to late mesozoic closure of the mongol okhotsk ocean as well as post accretionary continental growth

permian geodynamic setting of northeast china and adjacent regions closure of the paleo asian ocean and subduction of the paleo pacific plate abstract northeast china and adjacent regions are located in the central east asian continent and consist tectonically of both the paleo asian and paleo pacific orogens between the siberian platform and sino korean north china block this paper discusses some hotly debated issues concerning the permian geodynamic setting of these regions based on a comprehensive analysis of available geological geochemical paleobiogeographical and paleomagnetic data spatial and temporal distribution of ophiolites and associated continental marginal sequences permian sedimentary sequences spatial distribution and geochemistry of permian magmatic rocks and the evolution of paleobiogeographical realms imply 1 that the permian marine basins in northeast china and adjacent regions include remnants of the paleo asian ocean in southeastern inner mongolia and central jilin province and active continental margins of the paleo pacific ocean 2 that the suture between the siberian and sino korean paleoplates was finally emplaced in the permian and is located in areas from suolunshan solonker eastwards through regions north to the xar moron river in southeastern inner mongolia and then central jilin province to the yanji area and 3 that the permian crustal evolution of northeast china and adjacent regions as well as parts of the siberian paleoplate was influenced by subduction of the paleo pacific oceanic plate finally the permian tectonic framework and paleogeography of northeast china and adjacent regions in central east asia are discussed briefly and early and late permian palinspastic reconstruction maps are provided

imaging the bracciano caldera system by aeromagnetic data inversion sabatini volcanic district central italy abstract bracciano lake located in central italy 30 km northwest of rome is a volcano tectonic depression developed in pleistocene time representing the sabatini volcanic district svd western eruptive center little is known about bracciano caldera deep structure and potential field methods are appropriate to probe through a volcanic pile in this paper we present a 3d magnetic inversion model of the bracciano caldera system based upon new aeromagnetic data and paleomagnetic measurements carried out by istituto nazionale di geofisica e vulcanologia ingv magnetic modelling provides a reconstruction of bracciano caldera morphology whose extension is about 20 km north south and 15 km east west the maximum collapse depth is up to 1 km and the depocentre area appears localised to the north of the present bracciano lake shoreline considering the geological records of the main bracciano caldera forming events we have interpreted the resulting magnetic model as an overlap of two collapse structures generating a nested system whose location was strictly influenced by tectonic processes our 3d magnetic model confirms and consolidates the validity of the magnetic method to highlight deep structures in volcanic environments

syn eruptive inter eruptive relations in the syn rift deposits of the precuyano cycle sierra de chacaico neuquen basin argentina abstract the syn rift volcanic successions of the upper triassic lower jurassic precuyano cycle i e the lapa formation from the sierra de chacaico in the neuquen basin argentina were studied in order to address the distinctive characteristics of accumulation during syn eruptive and inter eruptive periods in a depocentre associated with active volcanism and extensional tectonics in particular the syn rift fill in this area comprises a wide range of compositions as well as of transport and depositional processes lava flows coexist with pyroclastic and epiclastic deposits in the same accumulation space in order to analyse the complexities inherent in a volcanic environment subjected to extension five different accumulation units were identified in the area lava flow shallow intrusion units pyroclastic units volcaniclastic alluvial units polymictic alluvial units and lacustrine units the analysis of each of these units and of the relationship between them provided meaningful insights into the evolution of the syn rift sedimentary environments and the identification of different stages of effusive activity explosive activity and relative quiescence determining syn eruptive and inter eruptive rock units the relationship between these units was examined and two accumulation stages were defined the underfilled stage originates when the material supplied to the depocentre during the eruptive events is not enough to level the existent topography allowing the development of high gradient alluvial systems during the next inter eruptive period the overfilled stage occurs when extensive pyroclastic density current deposits choke the accumulation space during syn eruptive periods causing low gradient sedimentary systems to develop during the subsequent inter eruptive periods

superimposed deformation of the solonker belt and nearby regions in western inner mongolia chinag shi and otherssuperimposed deformation of the solonker belt

ordovician 40ar 39ar phengite ages from the blueschist facies ondor sum subduction accretion complex inner mongolia and implications for the early paleozoic history of continental blocks in china and adjacent areas we obtained 453 21 8 ma and 449 41 8 ma 2 laser step heating 40 ar 39 ar plateau ages for phengite from quartzite mylonites from the blueschist facies ondor sum subduction accretion complex in inner mongolia northern china these ages are within error of the inverse isochron ages calculated using the plateau steps and the weighted mean ages of total fusion of single grains the compositional change from glaucophane in the cores to crossite in the rims of blue amphiboles as revealed by electron microprobe analysis points to decompression probably caused by progressive exhumation of the subducted material the late ordovician ages were not affected by excess argon incorporation because in all likelihood the oceanic sediments were wet on arrival at the trench and free of older detrital mica the ca 450 ma ages are hence interpreted as the time of crystallization during mylonitization under high fluid activity at fairly low temperatures this means that accretion of the quartzite mylonite unit occured about 200 ma before final closure of the paleo asian ocean amalgamation of the siberian tarim and north china cratons and formation of the end permian solonker suture zone we argue that the early paleozoic evolution of the ondor sum complex occurred along the northeastern cimmerian margin of gondwana which was composed of micro continents fringed by subduction accretion complexes and island arcs the later evolution took place during the building of the eurasian continent following middle devonian and younger rifting along the east gondwanan margin and northward drift of the detached north china craton an extensive review shows that this type of two stage scenario probably also applies to the geodynamic evolution of other micro continents like south china tarim a number of kazakh terranes alashan qaidam and kunlun as well as south kitakami and correlatives in japan and probably indochina like the north china craton these were bordered by early paleozoic subduction accretion complexes island arcs or contained calc alkaline volcanic mar gins like for example the central tienshan north qinling north qaidam altun north qilian and kunlun belts in china as well as the oeyama and miyamori ophiolites and matsugadaira motai blueschist belt of japan and the dismembered sergeevka ophiolite of the southern part of the russian far east this implies that a vast orogenic system comprising an archipelago of micro continents seems to have existed along the cimmerian margin of east gondwana in early paleozoic time in which the ultrahigh pressure metamorphism that characterizes the early evolution of many of the asian micro continents occurred introduction and aim

paleozoic magmatism and metamorphism in the central tianshan block revealed by u pb and lu hf isotope studies of detrital zircons from the south tianshan belt nw china as a major precambrian microcontinent in the southernmost central asian orogenic belt caob the central tianshan block cts in the chinese tianshan is essential for understanding the final assembly of the southern caob it experienced multistage paleozoic magmatism and metamorphism but the detailed processes are still controversial and far from being completely understood this paper reports coupled u pb and lu hf isotopic data of detrital zircons from late paleozoic meta sedimentary strata in the south tianshan belt which can provide new insight into deciphering the paleozoic evolution of the eastern segment of the cts block characterized by typical oscillatory zoning and high th u ratios 0 2 detrital zircons in the permian sedimentary samples yield dominant age populations at ca 505 490 ma 475 440 ma 430 400 ma and 340 250 ma pinpointing the development of four episodes of magmatism in the eastern cts block particularly ordovician silurian 475 440 ma zircons possessing low negative ehf t values predominate in sedimentary strata in and surrounding the cts block indicating that the 475 440 ma magmatic rocks probably constitute the main body of the cts block the origin of this early paleozoic episode of magmatism was most likely related to the southward subduction of the junggar ocean beneath the cts block carboniferous triassic 340 250 ma zircons have dominantly positive ehf t values probably derived from the post collisional juvenile rocks in the cts block combined with previous studies our data suggest that the single source terrane for the sampled strata was the cts block which had been a topographic high providing substantial detritus to the surrounding areas at least since the early permian in the metasedimentary sample detrital zircons mostly show partially fully recrystallized internal textures and low th u ratios 0 2 probably sourced from the amphibolite to granulite facies metamorphosed rocks in the eastern cts block the youngest zircons suggest that the sampled metasedimentary strata were carboniferous 341 305 ma in age rather than neoproterozoic or early sinian in chinese literatures as previously interpreted based on u pb hf isotopes and ti in zircon thermometer of metamorphic zircons in this study as well as previous investigations we suggest that the eastern cts block has experienced a a 400 370 ma collisional event related to the closure of the eastern segment of the south tianshan ocean and b a 360 340 ma extensional event as a result of post collisional mantle upwelling therefore the final assembly of the eastern tianshan took place during late carboniferous time

subduction related metasomatic mantle source in the eastern central asian orogenic belt evidence from amphibolites in the xilingol complex inner mongolia china abstract the central asian orogenic belt caob formed mainly in the paleozoic due to the closure of the paleo asian oceanic basins and accompanying prolonged accretion of pelagic sediments oceanic crust magmatic arcs and precambrian terranes the timing of subduction accretion processes and closure of the paleo asian ocean has long been controversial and is addressed in a geochemical and isotopic investigation of mafic rocks which can yield important insight into the geodynamics of subduction zone environments the xilingol complex located on the northern subduction accretion zone of the caob mainly comprises strongly deformed quartzo feldspathic gneisses with intercalated lenticular or quasi lamellar amphibolite bodies an integrated study of the petrology geochemistry and geochronology of a suite of amphibolites from the complex constrains the nature of the mantle source and the tectono metamorphic events in the belt the protoliths of these amphibolites are gabbros and gabbroic diorites that intruded at ca 340 321 ma with positive ehf t values ranging from 2 89 to 12 98 their t dm1 model ages range from 455 to 855 ma and peak at 617 ma suggesting that these mafic rocks are derived from a depleted continental lithospheric mantle the primitive magma was generated by variable degrees of partial melting of spinel bearing peridotites fractionation of olivine clinopyroxene and hornblende has played a dominant role during magma differentiation with little or no crustal contamination the mafic rocks are derived from a late neoproterozoic depleted mantle source that was subsequently enriched by melts affected by slab derived fluids and sediments or melts with a sedimentary source rock the carboniferous mafic rocks in the northern accretionary zone of the caob record a regional extensional event after the early paleozoic subduction of the paleo asian ocean both addition of mantle derived magmas and recycling of oceanic crust played key roles in significant late carboniferous ca 340 309 ma vertical crustal growth in the caob amphibolite facies metamorphism p 0 34 0 52 gpa t 675 708 c affected these mafic rocks in the xilingol complex at ca 306 296 ma which may be related to the crustal thickening by northward subduction of a forearc oceanic crust beneath the southern margin of the south mongolian microcontinent the final formation of the solonker zone may have lasted until ca 228 ma

tectonic influences in piecemeal caldera collapse at glencoe volcano scotland glencoe volcano is renowned as the archetype for caldera collapse that results from piston like subsidence of a coherent crustal block on a ring fault however the caldera floor rocks and the geometry of seven intracaldera units that record large volume explosive eruptions show that the collapse involved incremental and haphazard subsidence of numerous crustal blocks before development of the ring fault some subsidence involved flexure downsag with associated development of extensional crevasses hundreds of metres deep a system of orthogonal faults and related grabens records the influence of two intersecting basement discontinuities one of which is inferred to have linked with the active great glen fault substantial extensional and or transtensional faulting with distinct related sedimentary responses occurred without eruptions the tectonic framework substantially controlled the magmatic plumbing and the locations of vents caldera depocentres and through going rivers the related sediments influenced contrasting eruption styles e g transitions between phreatomagmatic and magmatic and the shallow level emplacement of sills

precambrian crystalline basement in southern mongolia as revealed by shrimp zircon dating single zircon ages determined by ion microprobe shrimp ii for granitoid gneisses from the southern slope of the baga bogd massif gobi altai southern mongolia reveal several episodes of zircon growth ranging from late palaeoproterozoic to late cambrian the oldest events are documented by a zircon crystallization age for a gneiss protolith at 1519 11 ma and by a xenocrystic zircon from a dark grey augen gneiss yielding an age of c 1701 ma discrete igneous events are recorded in granite gneisses with protolith emplacement ages of 983 6 956 3 and 954 8 ma these ages provide the first record of early neoproterozoic magmatic activity in this region a much younger and discrete magmatic event is recorded by several dioritic to granitic orthogneisses which are tectonically interlayered with the older gneisses and have protolith emplacement ages between 502 and 498 ma these late cambrian granitoids of calc alkaline affinity are likely to have been emplaced along an active continental margin and suggest that the baga bogd precambrian crustal fragment was either docked against the southward present day coordinates growing margin of the caob or was a large enough crustal entity to develop an arc along its margin we speculate that the precambrian gneisses of this massif may be part of a crustal fragment rifted off the tarim craton

a middle permian ophiolitic melange belt in the solonker suture zone western inner mongolia china implications for the evolution of the paleo asian ocean

phanerozoic evolution of the inner mongolia daxinganling orogenic belt in north china constraints from geochronology of ophiolites and associated formations