post collisional southeastern beishan granites geochemistry geochronology sr nd hf isotopes and their implications for tectonic evolution abstract bandaoshan granites are exposed in the southeastern beishan area in the central part of the central asia orogenic belt caob la icp ms u pb zircon dating indicates that bandaoshan granites were emplaced into precambrian basement at 285 4 ma and their geochemistry indicates that they are alkali rich potassium high granites initial end values 4 3 to 2 7 and ehf values 2 7 to 0 7 suggest that bandaoshan granites were derived from mantle derived melt and an upper continental crustal or sedimentary component the latter plays a significant role in their genesis in combination with regional geology the early permian bandaoshan pluton is interpreted to form in a post collisional environment in the southeastern beishan area late carboniferous qiaowan granites early permian yin aoxia granites and middle permian xijianquanzi granites are also considered as post collisional granites and together with bandaoshan granites indicate that the region was in a post collisional stage from the end of late carboniferous to middle permian

geochemistry and u pb detrital zircon dating of paleozoic graywackes in east junggar nw china insights into subduction accretion processes in the southern central asian orogenic belt abstract the southern central asian orogenic belt caob is characterized by multiple and linear accretionary orogenic collages including paleozoic arcs ophiolites and accretionay wedges a complex history of subduction accretion processes makes it difficult to distinguish the origin of these various terranes and reconstruct the tectonic evolution of the southern caob in order to provide constraints on the accretionary history we analyzed major and trace element compositions of paleozoic graywackes from the huangcaopo group hg and kubusu group kg in east junggar the hg graywackes have relatively low chemical index of alteration cia values 50 to 66 suggesting a source that underwent relatively weak chemical weathering the identical average index of compositional variability icv values 1 1 for both the kg and hg samples point to an immature source for the paleozoic graywackes in east junggar which is consistent with an andesitic felsic igneous source characterized by low la th ratios and relatively high hf contents these graywackes are geochemically similar to continental island arc sediments and therefore were probably deposited at an active continental margin u pb dating of detrital zircons from the lower subgroup of the hg yielded a young age peak at 440 ma indicating a post early silurian depositional age however the youngest populations of detrital zircons from the kg graywackes and the upper subgroup of the hg yielded 206 pb 238 u ages of 346 ma and 355 ma respectively which suggest a post early carboniferous depositional age because of similarities of rock assemblages these two units should be incorporated into the early carboniferous nanmingshui formation the detrital zircon age spectrum of the early paleozoic hg graywackes resembles that of the habahe sediments in the chinese altai which suggests that the ocean between east junggar and the chinese altai was closed before the deposition of the sediments and that the armantai ophiolite was emplaced prior to the early devonian the differences in age spectra for detrital zircons from the post early carboniferous graywackes in east junggar and the harlik arc indicate that the emplacement of the kalamaili ophiolite postdates the early carboniferous therefore a long lasting northward subduction accretion process is suggested for the formation of east junggar and the reconstruction of the early paleozoic evolution of the southern caob

association of permian gabbro and granite in the langshan southern central asian orogenic belt age origin and tectonic implications abstract the permian magmatism in the alxa region provides critical geological evidence to constrain the geodynamic processes in the southern central asian orogenic belt the shouji batholith is located in the southern langshan eastern alxa region and contains monzogranites k feldspar granites and gabbros new zircon u pb age data show that the shouji batholith formed at 282 268 ma the shouji granites have high na2o k2o nb and zr contents and high 10000 ga al feot feot mgo yb nb and yb ta ratios similar to the values of a2 type granites the shouij granites exhibit enrichment in light rare earth elements lree and large ion lithophile elements lile depletion in nb ti and sr and enriched isotopic compositions these shouji granites were generated by partial melting of dehydrated tonalite trondhjemite granodiorite rocks of the diebusige complex in the lower crust the shouji gabbros are sodium rich and low k calc alkaline rocks they are enriched in lree and lile depleted in high field strength elements and have enriched isotopic compositions these shouji gabbros were generated by partial melting of a lithospheric mantle source metasomatized by melts derived from an ancient continental crust in an active continental margin roll back of the enger us subducting slab accounts for generations of the shouji a2 type granites and gabbros the zircon u pb ages of carboniferous permian magmatism show a younging trend toward the northwest in the southern alxa region evidence from late paleozoic magmatism and arc derived sediments indicates that the southern alxa switched from an active continental margin to an intra oceanic arc during the early carboniferous late permian as a result of retreat of the subducting enger us slab

geochronologic and geochemical evidence for persistence of south dipping subduction to late permian time langshan area inner mongolia china significance for termination of accretionary orogenesis in the southern altaids the langshan area in inner mongolia is situated in the southern altaids between the beishan suture to the west and the solonker suture to the east this paper addresses the poorly known tectonic evolution that led to formation of the terminal solonker suture dating of deformed porphyries and undeformed dolerites and gabbros constrains the timing of the relevant ne e striking and north vergent deformation deformed granitic granodioritic porphyries in this area are characterized by high sio2 65 38 78 00 low tfe2o3 1 29 5 07 mgo 0 13 0 63 and variable k2o 0 53 4 14 and na2o 2 05 4 62 all samples have enriched lree la yb 6 18 and negative nb anomalies nbn thn0 09 0 48 but different eu anomalies eu

ridge subduction kinematics and implications for the nature of mantle upwelling discussion the authors e farrar and j m dixon in a recent paper on ridge subduction discuss the magmatic and tectonic effects of overridden sea floor spreading centres their paper provides a useful compilation of information on several ridge trench encounters and elucidates a certain type of plate interaction called eduction however much of their analysis is flawed because they 1 do not consider the geometry of subducted slabs and intervening slab windows 2 misrepresent the importance of slab windows to magmatism and tectonism in the overriding plate and to mantle currents and 3 incorrectly link the concept of slab window formation to a model of passive mantle upflow consequently their paper overlooks the controlling influence that slab windows have on asthenosphere lithosphere interactions when diverging plates are subducted since the authors present their material in the style of a comprehensive review their omissions and misrepresentations warrant critical response numerous authors have established that ridge trench encounters tend to affect the overriding plate in three main ways 1 tectonic erosion of forearc and related uplift of forearc and arc 2 change in magmatic regime from arc type volcanism to riftor plume type volcanism extending from forearc to retroarc and 3 broad thermally supported uplift high heat flow and extension stacey 1974 fox et al 1985 merritts and vincent 1989 thorkelson and taylor 1989 hole 1990 hole et al 1991 babcock et al 1992 hole and larter 1993 references cited in the authors paper the authors concur and appeal to active asthenospheric upwelling e g hess 1962 gough 1984 wilson 1988 beneath the overriding plate as a principal cause although the authors depiction of active mantle dynamics is plausible the geological context of their arguments is unacceptable specifically they appeal to ridge subduction and mantle upwelling without providing a framework of subducted slab geometry without full appreciation of slab geometry and the constraints imposed by slab windows their arguments are limited to a vague precept of mantle convection

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

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

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

isotope provinces mechanisms of generation and sources of the continental crust in the central asian mobile belt geological and isotopic evidence abstract the available geological geochronological and isotopic data on the felsic magmatic and related rocks from south siberia transbaikalia and mongolia are summarized to improve our understanding of the mechanisms and processes of the phanerozoic crustal growth in the central asian mobile belt camb the following isotope provinces have been recognised precambrian t dm 3 3 2 9 and 2 5 0 9 ga at the microcontinental blocks caledonian t dm 1 1 0 55 ga hercynian t dm 0 8 0 5 ma and indosinian t dm 0 3 ga that coincide with coeval tectonic zones and formed at 570 475 420 320 and 310 220 ma continental crust of the microcontinents is underlain by or intermixed with juvenile crust as evidenced by its isotopic heterogeneity the continental crust of the caledonian hercynian and indosinian provinces is isotopically homogeneous and was produced from respective juvenile sources with addition of old crustal material in the island arcs or active continental margin environments the crustal growth in the camb had episodic character and important crust forming events took place in the phanerozoic formation of the camb was connected with break up of the rodinia supercontinent in consequence of creation of the south pacific hot superplume intraplate magmatism preceding and accompanying permanently other magmatic activity in the camb was caused by influence of the long term south pacific plume or the asian plume damping since the devonian

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

ridge subduction kinematics and implications for the nature of mantle upwelling ridge subduction follows the approach of an oceanic spreading centre towards a trench and subduction of the leading oceanic plate beneath the overriding plate there are four possible kinematic scenarios 1 welding of the trailing and overriding plates e g aluk antarctic ridge beneath antarctica 2 slower subduction of the trailing plate e g nazca antarctic ridge beneath chile and pacific izanagi ridge beneath japan 3 transform motion between the trailing and overriding plates e g san andreas transform or 4 divergence between the overriding and trailing plates e g pacific north america in case 4 the divergence may be accommodated in two ways the overriding plate may be stretched e g basin and range province extension which has brought the continental margin into collinearity and therefore transform motion with the pacific north america relative motion or divergence may occur at the continental margin and be manifest as a change in rate and direction of sea floor

paleozoic multiple accretionary and collisional processes of the beishan orogenic collage in the paper by xiao and others american journal of science v 310 p 1553 1594 1 please substitute figure 16 reproduced here for figure 16 that appears between pages 1573 and 1574 the original figure 16 2 was printed at an improper size figure 3 1 lookup volpage 310

architecture and mineral deposit settings of the altaid orogenic collage a revised model abstract the altaids are an orogenic collage of neoproterozoic paleozoic rocks located in the center of eurasia this collage consists of only three oroclinally bent neoproterozoic early paleozoic magmatic arcs kipchak tuva mongol and mugodzhar rudny altai separated by sutures of their former backarc basins which were stitched by new generations of overlapping magmatic arcs in addition the altaids host accreted fragments of the neoproterozoic to early paleozoic oceanic island chains and neoproterozoic to cenozoic plume related magmatic rocks superimposed on the accreted fragments all these assemblages host important many world class late proterozoic to early mesozoic gold copper molybdenum lead zinc nickel and other deposits of various types in the late proterozoic during breakup of the supercontinent rodinia the kipchak and tuva mongol magmatic arcs were rifted off eastern europe siberia and laurentia to produce oceanic backarc basins in the late ordovician the siberian craton began its clockwise rotation with respect to eastern europe and this coincides with the beginning of formation of the mugodzhar rudny altai arc behind the kipchak arc these earlier arcs produced mostly cu pb zn vms deposits although some important intrusion related orogenic au deposits formed during arc arc collision events in the middle cambrian and late ordovician the clockwise rotation of siberia continued through the paleozoic until the early permian producing several episodes of oroclinal bending strike slip duplication and reorganization of the magmatic arcs to produce the overlapping kazakh mongol and zharma saur valerianov beltau kurama arcs that welded the extinct kipchak and tuva mongol arcs this resulted in amalgamation of the western portion of the altaid orogenic collage in the late paleozoic its eastern portion amalgamated only in the early mesozoic and was overlapped by the transbaikal magmatic arc which developed in response to subduction of the oceanic crust of the paleo pacific ocean several world class cu mo porphyry cu pb zn vms and intrusion related au mineral camps which formed in the altaids at this stage coincided with the episodes of plate reorganization and oroclinal bending of magmatic arcs major pb zn and cu sedimentary rock hosted deposits of kazakhstan and central asia formed in backarc rifts which developed on the earlier amalgamated fragments major orogenic gold deposits are intrusion related deposits often occurring within black shale bearing sutured backarc basins with oceanic crust after amalgamation of the western altaids this part of the collage and adjacent cratons were affected by the siberian superplume which ascended at the permian triassic transition this plume related magmatism produced various deposits such as famous ni cu pge deposits of norilsk in the northwest of the siberian craton in the early mesozoic the eastern altaids were oroclinally bent together with the overlapping transbaikal magmatic arc in response to the northward migration and anti clockwise rotation of the north china craton the following collision of the eastern portion of the altaid collage with the siberian craton formed the mongol okhotsk suture zone which still links the accretionary wedges of central mongolia and circum pacific belts in the late mesozoic a system of continent scale conjugate northwest trending and northeast trending strike slip faults developed in response to the southward propagation of the siberian craton with subsequent post mineral offset of some metallogenic belts for as much as 70 400 km possibly in response to spreading in the canadian basin india asia collision rejuvenated some of these faults and generated a system of impact rifts

tectonic and sedimentary evolution of basins in the northeast of qinghai tibet plateau and their implication for the northward growth of the plateau the northeast of the qinghai tibet plateau from the hoh xil basin to the hexi corridor basin demonstrates a basin ridge geomorphy that is the result of the long term uplift of the qinghai tibet plateau the geological evolution of the qinghai tibet plateau is recorded in its associated basins from the cenozoic sedimentary filling pattern in the hoh xil qaidam and jiuquan basins we find that the evolution of these basins is similar they began as strike slip basins evolved into foreland basins and then to intermontane basins foreland basins are the direct result of orogenic activity in the northeast of qinghai tibet plateau and can be documented to have formed over the following time periods 49 23 ma for the hoh xil foreland basin 46 2 45 ma for the qaidam foreland basin and 29 5 0 ma for the jiuquan foreland basin this northward migration in the timing of foreland basin development suggests that the northeastern part of tibet grew at least in part by accretion of cenozoic sedimentary basins to its northern margin

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

contact spectroscopy for determination of stratigraphy of snow optical grain size we present a technique for in situ measurement of the vertical and spatial stratigraphic distribution of snow optical grain size with a coupled contact illumination probe and field spectroradiometer accurate measurements of optical equivalent grain size are critical for modeling radiative properties of snow such as spectral albedo and microwave emission measurements of the spectral reflectance of the snow pit surface are made at 2 cm intervals in the vertical plane under constant illumination and view geometries we invert the integral of the continuum normalization of the ice absorption feature with maximum at 1 03 mm wavelength for optical equivalent grain size using the validated model of nolin and dozier 2000 that has accuracy of 10 50 mm across the grain size range 50 900 mm results are presented for an alpine site in southwest colorado usa across the ablation season and for a greenland ice sheet site at the onset of snowmelt these results suggest that traditional measurements of grain size using a hand lens are nearly accurate only for rounded grains r 2 0 41 rmse 160 mm for polycrystals and faceted grains the hand lens approach is very inaccurate r 2 0 03 and 0 24 rmse 1206 and 1010 mm respectively we demonstrate an order of magnitude improvement in modeling of shortwave spectral albedo and net shortwave flux with contact spectroscopy measurements of grain size stratigraphy over those from a hand lens

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

assembling north china craton within the columbia supercontinent the role of double sided subduction abstract an evaluation of recent s wave receiver functions s wave velocities and two versions of p wave tomographic images along various transects in the north china craton provides some clues on the subduction collision history of the different crustal blocks and their final amalgamation within the paleoproterozoic columbia supercontinent interpretation of a n s seismic section of the craton suggests thick slab debris sinking to various depths in the mantle the w e seismic corridors show the preservation of a thick 200 km lithospheric root tectosphere beneath the ordos block and its variable and extensive erosion towards the yanliao block eastern block this zone is characterized by layers with marked velocity contrast and suggests repeated stacking of the remnants of underplated and accreted paleoproterozoic oceanic lithosphere the present day lithosphere asthenosphere boundary beneath this region probably marks the erosional plane along which decratonization occurred through subduction erosion from the east and thermal and material erosion by upwelling asthenosphere from below resulting in the partial destruction of the tectosphere and its thinning towards the east within the asthenosphere below the yanliao block younger and thinner slabs predominate in the absence of any prominent thick high velocity layers these younger slabs define a westward polarity and constitute a mega scale duplex formed by underplating through phanerozoic subduction process particularly the pacific plate subduction from the east the lithologic associations within the inner mongolia suture zone dividing the yinshan block to the north and ordos block to the south correspond to an accreted ocean plate stratigraphic sequence with the tonalite trondhjemite granodiorite ttg gneisses charnockites and calc alkaline granites representing a continental arc built up through subduction from the north the seismic transects bring out a contrasting polarity in the subduction regime with an oblique east to southward subduction of the yinshan block and a westward subduction of the yanliao block here i propose a double sided subduction history for the ncc similar to the ongoing subduction process in the western pacific such double sided subduction is considered to promote rapid amalgamation of continental fragments within supercontinents and the subduction polarities and mantle dynamics of ncc are therefore considered to be critical in evaluating the final assembly of the paleoproterozoic supercontinent columbia

structural anatomy of the early paleozoic laohushan ophiolite and subduction complex implications for accretionary tectonics of the proto tethyan north qilian orogenic belt northeastern tibet

late paleozoic tectonic evolution of the paleo asian ocean in the northern alxa block nw china the northern alxa block occupies a key position in the southern margin of central asian orogenic belt caob and records late paleozoic subduction and closure processes of the paleo asian oce

zircon shrimp u pb and in situ lu hf isotope analyses of a tuff from western beijing evidence for missing late paleozoic arc volcano eruptions at the northern margin of the north china block abstract zircon shrimp u pb and in situ lu hf isotopic analyses via laser ablation microprobe inductively coupled plasma mass spectrometer lam icpms of a tuff within the upper paleozoic from western beijing were carried out to give new constraints on volcano eruption ages and source area of the tuffs within the north china block ncb shrimp u pb zircon dating of the tuff yielded a 206 pb 238 u weighted mean age of 296 4 ma 95 confidence mswd 3 3 which is very similar to the emplacement age of the newly discovered carboniferous calc alkaline i type continental arc granitoid plutons in the inner mongolia paleo uplift impu on the northern margin of the ncb in situ lu hf analysis results of most zircons from the tuff yielded initial 176 hf 177 hf ratios from 0 282142 to 0 282284 and e hf t values from 15 9 to 10 7 these lu hf isotopic compositions are very similar to those of the late carboniferous granitoids in the impu but are very different to those of the central asian orogenic belt caob together with the sedimentary and tectonic analyses results we inferred that the source area of the tuffs within the ncb is the impu instead of the caob therefore some arc volcanoes once existed in the impu on northern margin of the ncb during the late carboniferous but they were entirely eroded due to strong exhumation and erosion of the impu during the late carboniferous to early jurassic

paleozoic multiple accretionary and collisional tectonics of the chinese tianshan orogenic collage abstract subduction related accretion in the junggar balkash and south tianshan oceans paleo asian ocean mainly in the paleozoic gave rise to the present 2400 km long tianshan orogenic collage that extends from the aral sea eastwards through uzbekistan tajikistan kyrgyzstan to xinjiang in china this paper provides an up to date along strike synthesis of this orogenic collage and a new tectonic model to explain its accretionary evolution the northern part of the orogenic collage developed by consumption of the junggar balkash ocean together with paleozoic island arcs northern ili issyk kul and chatkal located in the west which may have amalgamated into a composite arc in the paleozoic in the west and by addition of another two roughly parallel arcs dananhu and central tianshan in the east the western composite arc and the eastern dananhu and central tianshan arcs formed a late paleozoic archipelago with multiple subduction zones the southern part of the orogenic collage developed by the consumption of the south tianshan ocean which gave rise to a continuous accretionary complex kokshaal kumishi which separated the central tianshan in the east and other paleozoic arcs in the west from cratons tarim and karakum to the south cross border correlations of this accretionary complex indicate a general southward and oceanward accretion by northward subduction in the early paleozoic to permian as recorded by successive southward juxtaposition of ophiolites slices of ophiolitic melanges cherts island arcs olistostromes blueschists and turbidites which are mainly paleozoic in age with the youngest main phase being late carboniferous permian the initial docking of the southerly tarim and karakum cratons to this complicated late paleozoic archipelago and accretionary complexes occurred in the late carboniferous early permian in the eastern part of the tianshan and in the late permian in the western part which might have terminated collisional deformation on this suture zone the final stages of closure of the junggar balkash ocean resembled the small ocean basin scenario of the mediterranean sea in the cenozoic in summary the history of the altaids is characterized by complicated multiple accretionary and collisional tectonics

kinematics and age constraints of deformation in a late carboniferous accretionary complex in western junggar nw china abstract the karamay area situated in the eastern part of western junggar southern altaids contains an ophiolitic melange with ultramafic rocks gabbro basalt chert and limestone which show typical block in matrix structures and coherent turbidites and tuffs these lithological associations are interpreted as incoherent and coherent series formed in an accretionary complex on the basis of detailed field mapping and analyses of the asymmetry of imbricate thrusts duplexes tilted structures shear band cleavages and the nw verging inclined to overturned folds we conclude that the overall movement in the accretionary complex was top to the nw the youngest tuff involved in the deformation contains detrital zircons that have a u pb age la mc icp ms of 308 7 ma 39ar 40ar resistance furnace step heating of amphibole separates from a diorite dike which cuts the folded and imbricated rocks in the accretionary prism yielded a plateau age of 307 2 ma consequently the age of the deformation in the prism is tightly constrained at 307 308 ma implying that the deformation occurred in an extremely short time span during se ward subduction combined contemporaneous occurrence of baogutu adakite high mg sr enriched and y poor dioritic dikes miaoergou charnockite and maliya mafic rocks we further suggest the accretionary complex was cut by near trench volcanic rocks and plutons possibly due to interaction with a spreading ridge

new geochemical constraints for the origin of ridge subduction related plutonic and volcanic suites from the chile triple junction taitao peninsula and site 862 leg odp141 on the taitao ridge abstract several features of neogene and quaternary magmatism in the region south of the present day chile triple junction ctj at 46 12 s are directly related to the migration of the triple junction due to the obliquity of the ridge orientation with respect to the subduction front the triple junction migrated from south to north during the last 14 ma the taitao peninsula the westernmost promontory of the chile coast and the taitao ridge a submarine promontory north of the taitao peninsula provide the most complete collection of ridge subduction related magmatic products in the region the emplacement of near trench volcanics the intrusion of a variety of plutonic rocks and the related hydrothermal activity at these two sites have been interpreted as resulting from magma interactions between subducted ridge segments of the chile spreading centre and the continental crust we present new field observations and geochemical data that help to better constrain the problem of the sources and evolution of the taitao magmas the new geochemical data were obtained on samples collected from the taitao peninsula during a field expedition in 1995 and from samples of the taitao ridge during leg odp141 site 862 which have been re sampled in 1996 by one of us selected major and trace element compositions of 20 volcanic rocks from the taitao ridge are discussed together with 53 analyses from different rock types from the taitao peninsula including 24 unpublished analyses nd and sr isotopic compositions were obtained from 5 whole rocks and separated minerals of the taitao peninsula together with the oxygen isotope composition of four separated clinopyroxenes six main magmatic types are identified 1 n type morb 2 e type morb 3 lree depleted morb showing some trace element features typical of arc basalts 4 moderately nb depleted e morb 5 calc alkaline andesites dacites and rhyolites and 6 andesites and dacites with adakitic signature chemical similarities exist between some forearc magmas of the taitao ridge and the taitao peninsula and magmas emplaced at the chile active spreading ridge one important result based on isotope data is that the lavas emplaced over the continental crust taitao peninsula did not originate from melting of continental crust nor from extensive assimilation of such a crust by mantle derived magmas the likely source of these basalts could be the hot convective oceanic mantle of the southern chile spreading ridge buried at moderate depth 10 30 km

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

carboniferous granitic plutons from the northern margin of the north china block implications for a late palaeozoic active continental margin we report four late palaeozoic zircon sensitive high resolution ion microprobe shrimp u pb ages for granitic plutons from the inner mongolia palaeo uplift on the northern margin of the north china block these cast a new light on the poorly understood tectonic history of the northern margin of the north china block and the central asian orogenic belt during the late palaeozoic the plutons have for a long time been considered to belong to the early precambrian basement of the north china block our new shrimp u pb zircon dating of four plutons at longhua daguangding boluonuo and hushiha has yielded intrusive ages of 311 2 ma 324 6 ma 302 4 ma and 310 5 ma respectively geochemical data suggest that these granitoids have a calc alkaline subduction related i type signature indicating the existence of an andean style continental arc along the northern margin of the north china block during the late palaeozoic our results also indicate that the palaeo asian ocean still existed during latest carboniferous earliest permian time and that the final collision between the southern mongolia composite terranes and the north china block occurred later than c 290 ma we suggest that the northern margin of the north china block was an active continental margin and the inner mongolia palaeo uplift is a deeply exhumed mid crustal root of a late palaeozoic andean style continental arc

possible manifestations of slab window magmatisms in cretaceous southwest japan abstract the slab window system related to ridge subduction is studied to explain the conspicuous activity and the along arc migration of the magmatisms in the eastern margin of eurasian continent in the late mesozoic the trend of the granite ages is scrutinized with systems approach and analyzed by the statistical method in cretaceous southwest japan a reference field of the eurasian eastern margin the magmatic distribution of simultaneous activity was determined to be the v shape of the slab window excluding dating errors from the located age data the slab window magmatism has the most dominant zone of the acidic rocks at or near the centerline of the window the spreading center of two plates and also has the subdominant zone of the acidic to intermediate rocks on either side of the window as the slab window opens due to the plate motions the upwelling current is adiabatically induced in the asthenosphere to fill the window gap the partial melt is generated in the upwelling flux and transports heat to the lower crust to make greater granitic magma chambers by heat conduction to and assimilation with the crustal matter the younger and hotter slabs outside the window also play a role in partial slab melting and or dehydration to the asthenosphere and generate the subdominant granodioritic and adakitic and or high magnesian andesitic magmatisms as the pre and post activities of the main granite genesis the slab window formed by the subducted ridge between the kula and pacific plates was the strong heat source of the active magmatisms and migrated at the rate of 2 8 cm year from southwest to northeast about 8000 km along the continental margin the growth of the continent associated with the active acidic to intermediate magmatisms including the adakitic and or high magnesian andesitic activities is modeled by the slab window system

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

accordion vs quantum tectonics insights into continental growth processes from the paleozoic of eastern gondwana the early paleozoic lachlan fold belt of eastern australia is widely regarded as an ancient convergent plate margin beneath which paleo pacific panthalassic oceanic lithosphere was continuously subducted it is cited as the type example of a retreating accretionary orogeny however sandstone compositions the sedimentological nature and timing of chert accumulation and overall stratigraphic architecture are not necessarily consistent with this model we suggest an alternative explanation for growth of gondwanan continental margin oceanic lithosphere outboard of the passive gondwana continental margin was subducted beneath an extensive intra oceanic island arc that now crops out as an allochthonous element macquarie arc within the fold belt once intervening oceanic lithosphere was eliminated this arc collided with and was emplaced upon the gondwana margin recognition of four such events along this margin through the phanerozoic suggests it is a significant mechanism for continental growth

sources of phanerozoic granitoids in the transect bayanhongor ulaan baatar mongolia geochemical and nd isotopic evidence and implications for phanerozoic crustal growth abstract the central asian orogenic belt caob is renowned for massive generation of juvenile crust in the phanerozoic mongolia is the heartland of the caob and it has been subject to numerous investigations particularly in metallogenesis and tectonic evolution we present new petrographic geochemical and sr nd isotopic analyses on phanerozoic granitoids emplaced in west central mongolia the data are used to delineate their source characteristics and to discuss implications for the phanerozoic crustal growth in central asia our samples come from a transect from bayanhongor to ulaan baatar including three tectonic units the baydrag cratonic block late archean to middle proterozoic the eo cambrian bayanhongor ophiolite complex and the hangay hentey basin of controversial origin the intrusive granitoids have ages ranging from ca 540 to 120 ma the majority of the samples are slightly peraluminous and can be classified as granite s s including monzogranite syenogranite and alkali feldspar granite most of the rocks have initial 87sr 86sr ratios between 0 705 and 0 707 late paleozoic to mesozoic granitoids 250 ma are characterized by near zero end t values 0 to 2 whereas older granitoids show lower end t values 1 5 to 7 the data confirm the earlier observation of kovalenko et al geochemistry international 34 1996 628 who showed that granitoids emplaced outside of the pre riphean basement rocks are characterized by juvenile positive end t values whereas those within the pre riphean domain and the baydrag cratonic block as for the present case show a significant effect of contamination by precambrian basement rocks nevertheless mass balance calculation suggests that the granitoids were derived from sources composed of at least 80 juvenile mantle derived component despite our small set of new data the present study reinforces the general scenario of massive juvenile crust production in the caob with limited influence of old microcontinents in the genesis of phanerozoic granitoids

implications of volcanism in coastal california for the neogene deformation history of western north america 1 the geologic record of coastal california includes evidence of numerous volcanic centers younger than 30 ma that do not appear to have erupted in an arc setting by correlating these volcanic centers with specific slab windows predicted from analysis of magnetic anomalies on the pacific plate we add new constraints to tectonic reconstructions since 30 ma our correlations such as erupting the morro rock islay hill complex south of the pioneer fracture zone and the iversen basalt south of the mendocino fracture zone require larger displacements within western north america than advocated by most previous authors specifically we infer at least 315 km of motion between the sierra nevada and rigid north america at an azimuth of about n60 w and at least 515 km between baja california and rigid north america in a similar direction a consequence of inferring a large displacement of baja california is that the pacific north american plate boundary must have developed most of its current form prior to 10 ma we interpret a slab window developing between cocos and monterey plates after 19 ma that reconstructs under nearly all of the southern california volcanic centers dated at 18 14 ma most of the sedimentary basins associated with volcanic rocks show brief periods of rapid subsidence synchronous with volcanism followed by slow subsidence of variable but often extended duration consistent with rapid extension of cold lithosphere over recently introduced hot asthenosphere

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

slab tearing following ridge trench collision evidence from miocene volcanism in baja california mexico abstract neogene magmatic activity in central baja california underwent a major change at ca 12 5 ma when the pacific farallon active oceanic ridge collided with the trench east of vizcaino peninsula the calc alkaline magmatism which built the comondu volcanic arc vanished and was replaced by unusual volcanic associations which were erupted within six late miocene to quaternary volcanic fields jaraguay san borja san ignacio santa rosalia santa clara la purisima delineating a 600 km array along the baja california peninsula new fieldwork k ar datings and geochemical analyses on jaraguay and san borja lavas combined with previous data allow us to show that these associations include 1 adakites emplaced between 12 5 and 8 2 ma in the santa clara santa rosalia and jaraguay volcanic fields 2 niobium enriched basalts neb in santa clara and santa rosalia 11 2 to 7 4 ma 3 tholeiitic basalts and basaltic andesites 11 3 to 7 2 ma displaying a very weak subduction imprint in la purisima and san ignacio 4 alkali trachybasalts in mesas san carlos and santa catarina northwest of jaraguay 9 3 to 7 5 ma and finally 5 basalts and associated magnesian basaltic andesites and andesites locally referred to as bajaites the latter lavas display very specific geochemical characteristics including very high sr and ba contents they were emplaced in all the above mentioned volcanic fields between 14 6 and 5 3 ma the origin of these late miocene volcanics has been considered linked either to the opening of an asthenospheric window through which they ascended tholeiites and alkali trachybasalts or to the melting of its edges due to thermal erosion adakites and the subsequent reaction between adakitic melts and the supraslab mantle neb and bajaites however the identification of the remnants of the pacific farallon fossil ridge south of vizcaino peninsula indicates that active ridge subduction was not responsible for the opening of the asthenospheric window we propose that the emplacement of slab window related volcanic rocks between 13 and 7 ma from ca 25 n to 30 n was due to a process of slab tearing which started when the active ridge collided with the trench east of vizcaino peninsula the oldest part of the downgoing plate sunk into the mantle leading to the opening of a tear in the slab which likely started from the southern limit of the already existing southern california slab window it propagated parallel to the trench over about 600 km and developed into an asthenospheric window

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

carboniferous mantle derived felsic intrusion in the chinese altai nw china implications for geodynamic change of the accretionary orogenic belt abstract this paper reports whole rock sr nd and zircon u pb and hf isotopic compositions as well as major and trace element data for the granodiorites and their mafic microgranular enclaves mmes in the tuokesalei area northwestern chinese altai the granodiorites are characterized by peraluminous compositions a cnk 1 01 1 45 with relatively high sio 2 64 4 to 70 9 wt and low mgo 0 75 to 1 97 wt contents these rocks possess low total rare earth element ree contents 67 187 ppm with weakly negative eu anomalies and mantle like whole rock nd sr isotopic compositions e nd t 2 5 to 4 5 87 sr 86 sr i 0 7038 to 0 7048 and zircon e hf t values 5 93 to 12 9 the mmes contain abundant magmatic amphiboles and needle like apatites have low sio 2 51 7 to 54 3 wt and high mgo 2 86 to 3 75 wt and ree contents 162 298 ppm and are characterized by moderately negative eu anomalies eu eu 0 40 0 60 and slightly depleted to enriched lree la nd n 0 74 1 41 and flat hree gd yb n 1 16 1 39 patterns the mmes have whole rock nd sr isotopic compositions e nd t 2 6 to 2 7 87 sr 86 sr i 0 7034 0 7050 similar to those of the host granodiorites their similarities in crystallization age mineral assemblage and isotopic composition document that the mmes may represent an early crystallized phase which was broken into pieces and brought to the shallow level during the host magma ascent and emplacement their precursor magmas were probably derived from an oceanic lithospheric mantle which was characterized by pronounced nd ta depletions mineral compositions of mmes suggest that amphibole was a major early fractional mineral which effectively differentiated the mantle derived basaltic liquids to silica rich melts zircon u pb dating yielded an age of 313 5 ma for the host granodiorites and 313 13 ma for their mmes the geochemical data of this study records an important episode of magmatism in an environment transitional from a subduction related regime to a post tectonic extension results of this study suggest that the accretionary orogenic process did not completely terminate until the late carboniferous to permian in this region

990 and 1100 ma grenvillian tectonothermal events in the northern oaxacan complex southern mexico roots of an orogen inliers of f1 0 1 3 ga rocks occur throughout mexico and form the basement of the oaxaquia microcontinent in the northern part of the largest inlier in southern mexico rocks of the oaxacan complex consist of the following structural sequence of units from bottom to top which protolith ages are 1 huitzo unit a 1012f12 ma anorthosite mangerite charnockite granite amcg suite 2 el catro n unit z1350 ma orthogneiss migmatized at 1106f6 ma and 3 el marquez unit z1140 ma para and orthogneisses these rocks were affected by two major tectonothermal events that are dated using u pb isotopic analyses of zircon a the 1106f6 ma olmecan event produced a migmatitic or metamorphic differentiation banding folded by isoclinal folds and b the 1004 978f3 ma zapotecan event produced at least two sets of structures z1 recumbent isoclinal class 1c 3 folds with gently nw plunging fold axes that are parallel to mineral and stretched quartz lineations under granulite facies metamorphism and z2 tight upright subhorizontal wnw to nne trending folds accompanied by development of brown hornblende at upper amphibolite facies metamorphic conditions cooling through 500 jc at 977f12 ma is documented by 40 ar 39 ar analyses of hornblende fold mechanisms operating in the northern oaxacan complex under zapotecan granulite facies metamorphism include flexural and tangential longitudinal strain accompanied by intense flattening and stretching parallel to the fold axes subsequent phanerozoic deformation includes thrusting and upright folding under lower grade metamorphic conditions the zapotecan event is widespread throughout oaxaquia and took crustal rocks to a depth of f25 30 km by orogenic crustal thickening and is here designated as zapotecan orogeny modern analogues for zapotecan granulite facies metamorphism and deformation occur in middle to lower crustal portion of subduction and collisional orogens contemporaneous tectonothermal events took place throughout oaxaquia and in various parts of the genvillian orogen in laurentia and amazonia d 2003 elsevier science b v all rights reserved

the hulan group its role in the evolution of the central asian orogenic belt of ne china the hulan group of central jilin province northeastern ne china consists of felsic volcanic and sedimentary rocks that underwent sub greenschist to amphibolite facies metamorphism and multiple phases of deformation during formation of the central asian orogenic belt young nd model ages of 1100 ma obtained in this study do not support earlier models that the hulan group was either rifted from the north china craton to the south or from the jiamusi block to the north since both these blocks have much older nd model ages detrital zircon u pb ages from a meta sedimentary rock of the hulan group define a maximum age of deposition of 287 6 ma not early palaeozoic as previously believed rb sr mineral isochron data indicate that metamorphism of the hulan group occurred at 250 ma this conclusion is further supported by an age of 248 4 ma for the dayushan granite which intruded the hulan group these new isotopic data together with evaluation of the regional geology indicate that final oceanic closure in the chinese segment of the central asian orogenic belt took place in the late permian the hulan area is located along the solonker xra moronchangchun suture that extends from inner mongolia across to central jilin province it is this zone that marks the final ocean closure resulting in the amalgamation of the north china and siberian cratons 2007 elsevier ltd all rights reserved