formation and palaeozoic evolution of the gorny altai altai mongolia suture zone south siberia zircon u pb constraints on the igneous record abstract the altai sayan fold belt epitomises how phanerozoic island arc continental crust contributed to the growth of the palaeo eurasian continent its formation is dominated by the closure of the palaeo asian ocean pao when island arc systems and gondwana derived terranes accreted to the siberian margin relics of the pao related tectonic units and associated granitoids occur along the ophiolitic charysh terekta ulagan sayan suture ctuss between gorny altai and altai mongolia zircon la icp ms u pb dating of this igneous record constrains the multi stage geodynamic pao evolution primitive kuznetsk altai island arc crust formed at the siberian margin during the ediacaran early cambrian 525 555 ma this island arc matured during the middle late cambrian 510 ma and was consumed by pao subduction in the late cambrian early ordovician 480 490 ma forming the siberian early caledonian accretion collision belt south of the ctuss granitic magmatism occurred within gondwana derived altai mongolia during the middle late ordovician 450 470 ma palaeo kazakhstan assembly and during a silurian early devonian 400 425 ma andean type collision middle late devonian 360 395 ma granitoids were emplaced as a result of the collision of altai mongolia with siberia during final pangaea amalgamation the suture was strike slip reactivated associated magmatism 295 ma the youngest 220 255 ma sampled granitoids originated in an intra plate setting

zircon u pb and hf isotopic study of gneissic rocks from the chinese altai progressive accretionary history in the early to middle palaeozoic abstract gneissic rocks in the chinese altai mountains have been interpreted as either paleozoic metasedimentary rocks or precambrian basement this study reports geochemical and geochronological data for banded paragneisses and associated gneissic granitoids collected along a ne sw traverse in the northwestern chinese altai petrological and geochemical data suggest that the protoliths of the banded gneisses were possibly immature sediments with significant volcanic input and that the gneissic granitoids were derived from i type granites formed in a subduction environment three types of morphological features can be recognized in zircons from the banded gneisses and are interpreted to correlate with different sources zircons from five samples of banded paragneiss cluster predominantly between 466 and 528 ma some give neoproterozoic ages and a few yield discordant paleoproterozoic to archean ages zircon hf isotopic compositions indicate that both juvenile mantle and crust materials were involved in the generation of the source rocks from which these zircons were derived in contrast zircons occur ubiquitously as elongated euhedral prismatic crystals in the four samples of the gneissic granitoids and define single populations for each sample with mean ages between 380 and 453 ma the general absence of precambrian inheritance and positive zircon hf values for these granitoids suggest insignificant crustal contribution to the generation of the precursor magmas our data can be interpreted in terms of a progressive accretionary history in early to middle palaeozoic times and the chinese altai may possibly represent a magmatic arc built on a continental margin dominated by neoproterozoic rocks

role of melt supply in oceanic detachment faulting and formation of megamullions normal faults are ubiquitous on mid ocean ridges and are expected to develop increasing offset with reduced spreading rate as the proportion of tectonic extension increases numerous long lived detachment faults that form megamullions with large scale corrugations have been identifi ed on magma poor mid ocean ridges but recent studies suggest counterintuitively that they may be associated with elevated magmatism we present numerical models and geological data to show that these detachments occur when 30 50 of total extension is accommodated by magmatic accretion and that there is signifi cant magmatic accretion in the fault footwalls under these low melt conditions magmatism may focus unevenly along the spreading axis to create an irregular brittle plastic transition where detachments root thus explaining the origin of the enigmatic corrugations morphological and compositional characteristics of the oceanic lithosphere suggested by this study provide important new constraints to assess the distribution of magmatic versus tectonic extension along mid ocean ridges

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

an experimental investigation of the influence of water and oxygen fugacity on differentiation of morb at 200 mpa crystallization experiments were performed at 200mpa in the temperature range 1150 950 c at oxygen fugacities corresponding to the quartz fayalite magnetite qfm and mno mn3o4 buffers to assess the role of water and fo2 on phase relations and differentiation trends in mid ocean ridge basalt morb systems starting from a primitive mgo 9 8 wt and an evolved morb mgo 6 49 wt crystallization paths with four different water contents 0 35 4 7 wt h2o have been investigated in primitive morb olivine is the liquidus phase followed by plagioclase clinopyroxene amphibole is present only at water saturated conditions below 1000 c but not all fluid saturated runs contain amphibole magnetite and orthopyroxene are not stable at low fo2 qfm buffer residual liquids obtained at low fo2 show a tholeiitic differentiation trend the crystallization of magnetite at high fo2 mno mn3o4 buffer results in a decrease of melt feo mgo ratio causing a calc alkaline differentiation trend because the magnetite crystallization temperature is nearly independent of the h2o content in contrast to silicate minerals the calc alkaline differentiation trend is more pronounced at high water contents residual melts at 950 c in a primitive morb system have compositions approaching those of oceanic plagiogranites in terms of sio2 and k2o but have ca na ratios and feo contents that are too high compared with the natural rocks implying that fractionation processes are necessary to reach typical compositions of natural oceanic plagiogranites

characterization of hyperalkaline fluids produced by low temperature serpentinization of mantle peridotites in the oman and ligurian ophiolites a regional survey of alkaline springs in oman and ligurian ophiolites shows that the alkaline water compositions significantly vary from one ophiolite to the other and within the same ophiolite the first order correlation between the na and k and cl concentrations points to fluid compositions only partly due to evaporation the scatter around the evaporation line implies that na and cl may not be conservative during the alteration of the ultramafic rocks mg is almost entirely depleted at ph 10 5 as a result of serpentine formation within the ultramafic body and of brucite and minor hydrotalcite precipitation at the springs ca accumulates in the high ph fluids and is consumed by ca carbonate formation at the springs by mixing with river waters or by the co2 supply from the atmosphere thermodynamic calculations show that brucite saturation is reached at ph values around 10 5 which triggers major changes in the water composition the waters evolve from a quartz saturated low ph continental environment to a brucite dominated high ph serpentinizing system at low temperature the highest water salinities are found in springs located along the basal thrust plane of the ophiolite the highest al concentrations are found in some springs located on the crustal side of the mantle crust boundary this poses the question of the hydrologic pathways and of the role of the mineralogical composition of the altered formations key points new temperature and composition of the alkaline waters brucite formation at ph 10 5 triggers major chemical and mineralogical changes relationship between the water compositions and the hydrologic pathways

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

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

the death of an accretion zone as evidenced by the magmatic history of the sumail ophiolite oman abstract the ophiolitic crustal sequence of the salahi block in the northern part of oman shows a complex magmatic history the oceanic accretion episode led to the formation of a main plutonic sequence consisting of layered cumulates isotropic gabbros a sheeted dike complex and a first tholeiitic lava series v1 depleted in compatible elements the second magmatic episode immediately following the accretion stage is illustrated by predominantly ultrabasic intrusions crosscutting the main plutonic sequence and by a second volcanic suite v2 directly linked as can be seen from field evidence to more differentiated high level intrusive plutons this volcanic unit includes very primitive members and is in general characterized by depletion in incompatible elements a third volcanic event v3 exhibiting a transitional character enriched in lree s caps the ophiolite these v3 lavas are separated from the v2 unit by a 15 m thick pelagic sedimentary section a geotectonic evolution of the sumail ophiolite is proposed on the basis of field mineralogical and geochemical data after the oceanic accretion phase that gave birth to the main ophiolitic sequence a second stage melting of the upper mantle is suggested to explain the genesis of the second magmatic episode such a model accounts well for the depleted character of the v2 lavas the compression linked to the beginning of the intra oceanic detachment allows the extraction from the mantle of these second stage melts which are then emplaced in the crust and partly extruded the v3 volcanism postdates the intraoceanic detachment of the ophiolite but precedes its obduction onto the arabian continental margin it is interpreted as an intraplate seamount type magmatism

chlorine rich amphibole in deep layered gabbros as evidence for brine rock interaction in the lower oceanic crust a case study from the wadi wariyah samail ophiolite sultanate of oman abstract hydrothermal veins and dykelets that cross cut layered olivine gabbros deep in the plutonic section of the samail ophiolite sultanate of oman point towards the occurrence of hydrothermal circulation in the deep oceanic crust and these features record interactions between rock and high temperature seawater derived fluids or brines deep penetration of seawater derived fluids down to 100 m above the moho transition zone and the consequent interactions with the host rock lead to hydrothermal alteration from granulite facies grading down to greenschist facies conditions here we present a study of veins and dykelets formed by hydrothermal interaction cutting layered gabbro in the wadi wariyah using petrographic microanalytical isotopic and structural methods we focus on amphiboles which show a conspicuous compositional variation from high ti magnesiohastingsite and pargasite via magnesiohornblende and edenite to cl rich ferropargasite and hastingsite up to 1 5 a p f u cl and actinolite these minerals record a wide range of formation conditions from magmatic to hydrothermal and reveal a complex history of interactions between rock and hydrothermal fluid or brine in a lower oceanic crustal setting large variations in cl content and cation configurations in amphibole suggest formation in equilibrium with fluids of different salinities at variable fluid rock ratios the presence of subsolidus amphibole extremely enriched in chlorine implies phase separation and brine rock interactions 87sr 86sr values of 0 7031 to 0 7039 and stable 18o isotopic compositions of 4 1 to 5 6 of the different amphibole types suggest a rock dominated environment i e with low fluid rock ratios however the slight departure from mean oman isotope values may indicate there was some influence of seawater in the aforementioned fluid rock interactions our study provides new petrological data for the subsolidus evolution of gabbro hosted amphibole rich veins in the presence of a seawater derived fluid

synmagmatic normal faulting in the lower oceanic crust evidence from the oman ophiolite structural and petrological study of the oman ophiolite shows that normal faults at oceanic spreading centers can root in incompletely crystallized cumulates and form at the same time as melt is being supplied to the magma chamber in the vicinity 200 m of a ridge facing normal fault located a couple of kilometers off a former axial mantle diapir blocks of layered cumulates were tilted at high angles relative to the moho block rotation was accommodated by a swarm of anastomosed fault planes connected to the main fault these planes are underlain by screens of gabbroic micropegmatites former melt layers and by ptygmatic folds pointing to viscous deformation of a compacting crystal mush flat lying undeformed cumulate layers from the same parent melts as the deformed ones settled directly over the tilted blocks water introduction into the melt is evidenced by the crystallization of anomalously high anorthite an 90 95 cumulus plagioclase regardless of the degree of evolution of the magma shearing continued at subsolidus temperatures along some but not all fault planes attested to by the local development of plastic deformation structures the subsolidus development of amphibole spinel coronas around olivine grains points to pervasive percolation of high temperature hydrothermal fluids through the cumulates as the crust cooled further water penetration progressively focused within the main fault zone where the crust was fractured and eventually altered in greenschist facies conditions

moist morb axial magmatism in the oman ophiolite the evidence against a mid ocean ridge origin the oman ophiolite has an axial volcanic suite and associated sheeted dike complex similar in composition to modern mid oceanic ridge basalt morb its internal structure is regarded by many as being directly comparable to ocean lithosphere from the east pacific rise however there has long been controversy over the geodynamic setting in which the ophiolite formed and the extent to which the analogy can be drawn because the morb like axial volcanics are overlain by lavas that include depleted arc tholeiites and boninites to some this implies that the entire ophiolite formed above a subduction zone others maintain that it formed at a true open ocean mor and that the water required to generate the arc like magmas derived from descending near axis hydrothermal fluids or from ancient subduction a popular compromise posits that the axial suite formed at a true mor and the later magmatism documents the initiation of obduction we test these models by reexamining the morb like character of the early axial lavas and dikes we show that fractionation trends require the presence of water at concentrations significantly higher than any open ocean morb instead trends are identical to those of backarc basin and intraoceanic forearc volcanics by showing that the entire ophiolite formed in a hydrous system we rule out all models in which the oman ophiolite was generated at an open ocean mor instead it formed at a submarine spreading center above a probably newly initiated subduction zone

dehydration melting of ultrahigh pressure eclogite in the dabie orogen evidence from multiphase solid inclusions in garnet several types of multiphase solid ms inclusions are identified in garnet from ultrahigh pressure uhp eclogite in the dabie orogen the mineralogy of ms inclusions ranges from pure k feldspar to pure quartz with predominance of intermediate types consisting of k feldspar quartz silicate plagioclase or epidote barite the typical ms inclusions are usually surrounded with radial cracks in the host garnet similar to where garnet contains relict coesite barite aggregates display significant heterogeneity in major element composition with total contents of only 57 73 and highly variable sio2 contents of 0 32 25 85 that are positively correlated with bao and so3 contents the occurrence of ms inclusions provides petrographic evidence for partial melting in the uhp metamorphic rock the occurrence of barite aggregates with variably high sio2 contents suggests the coexistence of aqueous fluid with hydrous melt under hp eclogite facies conditions thus local dehydration melting is inferred to take place inside the uhp metamorphic slice during continental collision this is ascribed to phengite breakdown during hot exhumation of the deeply subducted continental crust as a consequence the aqueous fluid is internally buffered in chemical composition and its local sink is a basic trigger to the partial melting during the continental subduction zone metamorphism

mineralogical assemblages forming at hyperalkaline warm springs hosted on ultramafic rocks a case study of oman and ligurian ophiolites 1 we report on the mineralogical assemblages found in the hyperalkaline springs hosted on liguria and oman ophiolites based on exhaustive x ray diffraction and scanning electron microprobe analyses in liguria hyperalkaline springs produce a thin brownish calcite precipitate that covers the bedrock due to the concomitant atmospheric co2 uptake and neutralization of the hyperalkaline waters no brucite and portlandite minerals are observed the discharge of alkaline waters in oman ophiolite forms white orange precipitates calcium carbonate minerals calcite and or aragonite are the most abundant and ubiquitous precipitates and are produced by the same mechanism as in liguria this process is observed as a thin surface crust made of rhombohedral calcite morphological features of aragonite vary from needle bouquet dumbbell spheroidal like habitus according to the origin of carbon temperature and ionic composition of the hyperalkaline springs and the biochemical and organic compounds brucite is observed both at hyperalkaline springs located at the thrust plane and at the paleo moho the varying mixing proportions between the surface runoff waters and the hyperalkaline ones control brucite precipitation the layered double hydroxide minerals occur solely in the vicinity of hyperalkaline springs emerging within the bedded gabbros finally the dominant mineralogical associations we found in oman ca bearing carbonates and brucite in a serpentinizing environment driven by the meteoric waters are surprisingly the same as those observed at the lost city hydrothermal site in a totally marine environment

the biostratigraphy of sediments in the volcanic unit of the samail ophiolite sedimentary rocks of pelagic and ocean ridge affinity interbedded with or overlying the volcanic sequence at the top of the oman ophiolite contain datable radiolarian faunas that indicate 1 that the lavas were erupted in an oceanic environment during a narrow time interval ranging from the early cenomanian to the early turonian and 2 that the age progression within this volcanic sequence matches its development from a ridge axis to an off axis setting emplacement of the ophiolite has been dated as late campanian early maestrichtian glennie et al 1974 thus the time gap between formation and emplacement was very narrow

interplay between faults and lava flows in construction of the upper oceanic crust the east pacific rise crest 9 25 9 58 n the distribution of faults and fault characteristics along the east pacific rise epr crest between 9 25 n and 9 58 n were studied using high resolution side scan sonar data and near bottom bathymetric profiles the resulting analysis shows important variations in the density of deformational features and tectonic strain estimates at young seafloor relative to older sediment covered seafloor of the same spreading age we estimate that the expression of tectonic deformation and associated strain on old seafloor is 5 times greater than that on young seafloor owing to the frequent fault burial by recent lava flows thus the unseen volcanically overprinted tectonic deformation may contribute from 30 to 100 of the 300 m of subsidence required to fully build up the extrusive pile layer 2a many longer lava flows greater than 1 km dam against inward facing fault scarps this limits their length at distances of 1 2 km which are coincident with where the extrusive layer acquires its full thickness more than 2 of plate separation at the epr is accommodated by brittle deformation which consists mainly of inward facing faults 70 faulting at the epr crest occurs within the narrow 4 km wide upper crust that behaves as a brittle lid overlying the axial magma chamber deformation at greater distances off axis up to 40 km is accommodated by flexure of the lithosphere due to thermal subsidence resulting in 50 inward facing faults accommodating 50 of the strain on the basis of observed burial of faults by lava flows and damming of flows by fault scarps we find that the development of layer 2a is strongly controlled by low relief growth faults that form at the ridge crest and its upper flanks in turn those faults have a profound impact on how lava flows are distributed along and across the ridge crest

palaeotsunamis and tsunami hazards in the eastern mediterranean the dominant uncertainties in assessing tsunami hazard in the eastern mediterranean are attached to the location of the sources reliable historical reports exist for five tsunamis associated with earthquakes at the hellenic plate boundary including two that caused widespread devastation because most of the relative motion across this boundary is aseismic however the modern record of seismicity provides little or no information about the faults that are likely to generate such earthquakes independent geological and geophysical observations of two large historical to prehistorical earthquakes in crete and rhodes lead to a coherent framework in which large to great earthquakes occurred not on the subduction boundary but on reverse faults within the overlying crust we apply this framework to the less complete evidence from the remainder of the hellenic plate boundary zone identifying candidate sources for future tsunamigenic earthquakes each such source poses a significant hazard to the north african coast of the eastern mediterranean because modern rates of seismicity are irrelevant to slip on the tsunamigenic faults and because historical and geological data are too sparse there is no reliable basis for a probabilistic assessment of this hazard and a precautionary approach seems advisable

primary carboniferous and permian paleomagnetic results from the yili block nw china and their implications on the geodynamic evolution of chinese tianshan belt in order to better understand the tectonic role of the yili block on the paleozoic evolution of the chinese tianshan belt we performed a primary paleomagnetic study on carboniferous and permian rocks from different areas in the yili block nw of china more than 320 sedimentary and volcanic samples were collected from 39 sites except for the ordovician samples showing a weak and unstable magnetic remanence the majority of this collection presents characteristic remanent magnetization carried by magnetite and hematite in the study area though positive fold test has been observed on the early carboniferous rocks a general remagnetization of these rocks has been identified and attributed to the late carboniferous magmatism moreover all early and late carboniferous samples from the interior of the yili block yield stable and coherent magnetic directions with exhaustively reverse magnetic polarity the late carboniferous c2 is considered as the magnetic remanence age since these rocks are covered or intruded by synchronous magmatic rocks of the yili arc which lasted until to 310 ma the c2 paleomagnetic pole is therefore calculated at 68 6 n 290 6 e with 95 6 4 and n 15 the late carboniferous rocks located close to a deformation zone present a consistentmagnetic inclination but significant different declination with respect to other areas and are suspected to have probably experienced a local rotation although no fold test can be performed due to the monoclinal bedding stable magnetic components are isolated from late permian p2 red beds in the interior of the yili block with also a solo reverse magnetic polarity the p2 paleomagnetic pole of the yili block has been therefore calculated from the characteristic remanent magnetization 79 7 n 172 0 e with 95 11 3 and n 5 keeping important uncertainties in mind comparisons of the c2 and p2 paleomagnetic poles of the yili block with available coeval poles of junggar tarim and siberia indicate 1 no significant relative motion between the yili and junggar blocks since the late carboniferous 2 no significant or weak latitudinal relative motion occurred since the late carboniferous among these blocks but 3 the 46 2 15 1 and the 31 6 15 1 counterclockwise rotations of the yili junggar blocks with respect to tarim and siberia took place during c2 to p2 these rotations are accommodated by the permian dextral strike slip faults along the northern and southern sides of tianshan belt and sinistral strike slip faulting along the erqishi fault of altay belt resulting in about 1000 km and 600 km lateral displacements in the tianshan and altay belts respectively

reply to comment by f boudier and a nicolas on dating the geologic history of oman s semail ophiolite insights from u pb geochronology by c j warren r r parrish m p searle and d j waters

late paleozoic faults of the altai region central asia tectonic pattern and model of formation abstract the present kinematic and dynamic analysis of large scale strike slip faults which enabled the formation of a collage of altai terranes as a result of two collisional events the late devonian early carboniferous collision of the gondwana derived altai mongolian terrane and the siberian continent resulted in the formation of the charysh terekta system of dextral strike slip faults and later the kurai and kuznetsk teletsk bashkauss sinistral strike slip faults the late carboniferous permian collision of the siberian and kazakhstan continents resulted in the formation of the chara irtysh and north east sinistral strike slip zones the age of deformation of both collisional events becomes younger toward the inner areas of the siberian continent in the same direction the amount of displacement of strike slip faulting decreases from several thousand to several hundred kilometers the width of the late paleozoic zone of deformation reaches 1500 km these events deformed the accretion collision continental margins and their primary paleogeographic pattern

accretionary orogenesis of the chinese altai insights from paleozoic granitoids zircon u pb dating and whole rock major oxide trace element and nd sr isotope compositions have been determined for four representative granitic intrusions in the sw chinese altai in order to understand the orogenesis and history of crustal growth in the central asian orogenic belt caob the ashile and halatas bodiesare relatively small intrusions formed in the late carboniferous 318 6ma andpermian 267 4ma respectively thelargertarlang tb andkeketuohaibatholiths kb havemagmaticagesof 359 to 412 ma the granitoids consist chiefly of tonalite granodiorite and granite and are metaluminous to weakly peraluminous in composition asi 0 8 1 1 the ashile pluton is characterized by relatively high mgo 2 73 3 54 wt and sr 395 456 ppm contents with low k2o na2o ratios 0 31 0 42 and mantle like nd sr isotopic compositions endt 2 85 to 3 26 initial 87 sr 86 sr 0 7047 0 7051 it probably formed by fractional crystallization of a mantle derived magma with limited assimilation of oldcrustalmaterial theotherintrusionshavenear zeroornegative endtvalues 0 67to 4 41 andhighersrisotopiccompositions initial 87 sr 86 sr 0 7067 0 7092 indicating a mixture of juvenile and old components in their genesis these intrusions are characterized by relatively low k rb cs contents and low rb sr ratios mostly b0 6 and generally exhibit moderate to high heavy rare earth element hree concentrations ybn 8 26 and weak hree fractionation dy ybnb1 5 with or without negative europium anomalies eu eu 0 5 1 0 these characteristics are consistent with an origin by dehydration melting of a hornblendebearing mid crustal source above the garnet stability field dominated by mafic to intermediate rocks this suggests that the deep crust of the chinese altai may contain a considerable proportion of juvenile material mantle derived magmatism probably in an active extensional continental margin setting played an important role in the formation of the chinese altai acting not only as a heat sourceforcrustalmeltingbutasasourceofjuvenilecomponents therelativelysmallvolumesanddistinctgeochemicalcompositions of the ashile and halatas plutons may imply a substantial change in the geodynamic setting in the late paleozoic 2007 elsevier b v all rights reserved

age provenance and tectonic setting of paleoproterozoic quartzite successions in the southwestern united states new field studies combined with u pb zircon geochronology constrain the ages of deposition and sedimentary provenance of paleoproterozoic quartzite successions exposed in the southwestern united states orthoquartzites were deposited in short lived basins at two times ca 1 70 and 1 65 ga during crustal assembly of southern laurentia the more voluminous ca 1 70 ga successions occur in southern colorado northern new mexico and central arizona and are interpreted here to be time correlative though not necessarily deposited in the same basins detrital zircon from quartzites and metaconglomerates exposed in southern colorado and northern new mexico is characterized by a single population with a relatively narrow range of ages 1 80 1 70 ga and minimal archean input 5 of grains analyzed peak detrital zircon ages 1 76 1 70 ga vary slightly from location to location and mimic the age of underlying basement unimodal detrital populations suggest local sources and a first cycle origin of the orthoquartzites within a short time interval 1 70 1 68 ga during unroofing of local underlying basement the maximum age of quartzite exposed at blue ridge colorado is constrained by the 1705 1698 ma coarse grained granitoid basement on which quartzite was deposited unconformably the minimum age of ortega formation quartzite new mexico is constrained by ca 1680 1670 ma metamorphic monazite overgrowths these dates agree with direct ages on the lower mazatzal group arizona and suggest that orthoquartzite deposition occurred over a wide region during and soon after the ca 1 70 ga yavapai orogeny regional structural arguments and the thrust style of quartzite deformation suggest that the metasedimentary successions were deformed during the ca 1 66 1 60 ga mazatzal orogeny thus making them important time markers separating the yavapai and mazatzal orogenic events our model for syntectonic deposition involves extensional basin development followed by thrust closure possibly due to opening and closing of slab rollback basins related to outboard subduction the first cycle origin of orthoquartzites near the end of the arc collisions of the yavapai orogeny seems to contrast sharply with their extreme compositional maturity this can be explained in terms of protracted extreme diagenesis and or special environmental influences that enhanced chemical weathering but were unique to the transitional atmosphere and ocean chemistry of the proterozoic similarities among quartzites exposed throughout the southwestern united states and along the laurentian margin suggest that they represent a widespread regional and perhaps global episode of sedimentation involving a distinctive syntectonic setting and unique climatic conditions a combination that might make these units a signature lithology for paleoproterozoic time

a vendian cambrian island arc system of the siberian continent in gorny altai russia central asia an extended vendian cambrian island arc system similar to the izu bonin mariana type is described in the gorny altai terrane at the margin of the siberian continent three different tectonic stages in the terrane are recognized 1 a set of ensimatic active margins including subducted oceanic crust of the paleo asian ocean the uimen lebed primitive island arc oceanic islands and seamounts the set of rocks is assumed to be formed in the vendian 2 a more evolved island arc comprising calc alkaline volcanics and granites a fore arc trough in middle late cambrian time was filled with disrupted products of pre middle cambrian accretionary wedges and island arcs 3 collision of the more evolved island arc with the siberian continent folding metamorphism and intrusion of granites occurred in late cambrian early ordovician time in the late paleozoic the above mentioned caledonian accretion collision structure of the siberian continent was broken by large scale strike slip faults into several segments this resulted in the formation of a typical mosaic block structure

neoproterozoic middle paleozoic tectono magmatic evolution of the gorny altai terrane northwest of the central asian orogenic belt constraints from detrital zircon u pb and hf isotope studies abstract the gorny altai terrane ga is a key area in understanding the crustal evolution of the central asian orogenic belt caob this paper reports u pb and hf isotope data for detrital zircons from cambrian to early devonian sedimentary sequences to constrain their provenance as well as the tectono magmatic events and crustal growth in this region nearly all the detrital zircons are characterized by euhedral to subhedral morphology high th u ratios ca 0 1 1 6 and typical oscillatory zoning indicating a magmatic origin the three samples from the gorny altai group middle cambrian to early ordovician yield detrital zircon populations that are composed predominantly of 530 464 ma grains followed by a subordinate group of 641 549 ma old the silurian and devonian samples exhibit similar major zircon populations 555 456 ma and 525 463 ma respectively but a significant amount of additional 2431 772 ma zircons occur in the early devonian sample our results suggest that detritus from the nearby kuznetsk altai intra oceanic island arc served as a unitary source for the cambrian silurian sedimentary sequences but older detritus from other sources added to the early devonian sequence the low abundance of ca 640 540 ma detrital zircons may testify that this island arc was under a primitive stage in this period when mafic volcanic rocks probably dominated in contrast the dominant population of ca 530 470 ma zircons may indicate an increased amount of granitic rocks in the source area suggesting that the kuznetsk altai island arc possibly evolved into a mature one in the cambrian to early ordovician the ca 530 470 ma detrital zircons are almost exclusively featured by positive e hf t values and have two stage hf model ages of ca 1 40 0 45 ga indicating that the precursor magmas were sourced predominantly from heterogeneous juvenile materials we conclude that the late neoproterozoic to early paleozoic magmatism in the kuznetsk altai arc made a significant contribution to the crustal growth in the caob the absence of middle ordovician to early devonian detrital zircons possibly implies that the area changed to a passive margin in the middle ordovician of special significance are the 2431 772 ma zircons in the early devonian sample which are quite comparable to the precambrian detrital zircons from the altai mongolian terrane am but distinct from those from the siberian continent this indicates that the am is a plausible source for the 2431 772 ma zircons our data therefore imply that the ga possibly amalgamated with the am before the early devonian rather than in the late devonian to early carboniferous as previously suggested

early middle paleozoic granitoids in gorny altai russia implications for continental crust history and magma sources abstract major element and trace element compositions as well as nd isotope ratios and protolith model ages of early to middle paleozoic granitoids from the russian gorny altai are summarized and discussed in terms of the compositions and tectonic settings the observed geographic variations of compositions in the granitoid assemblages correlate with three groups of late neoproterozoic to middle paleozoic terranes corresponding to oceanic island arc and turbidite basin settings plutonic rocks in the oceanic and island arc terranes are commonly low alkali metaluminous tonalite and trondhjemite that formed by melting of oceanic crust the island arc terranes also contain tonalite trondjemite granodioritic and tonalite granodiorite granitic rocks with higher alkali and potassium contents which result from anatectic melting of volcanics and sediments granitoids in the turbidite basins are potassic peraluminous monzogranite produced by anatectic melting of upper crustal turbidites as well as subalkaline tonalite granodiorite and granite derived from upper crustal metasediments and metamorphosed mafic oceanic crust underlying the turbidite basins alkaline granitoids emplaced during two major tectonomogamatic events early and late devonian and are present in terranes of all three settings their source is of crustal origin like in other groups of granitoids but it was a mixture of crustal and mantle material thus the compositional trends of early and middle paleozoic granitoids in the gorny altai record the history of continental crust