thermomechanical evolution of the crust during convergence and deep crustal pluton emplacement in the western province of fiordland new zealand 1 fiordland new zealand contains exposures of high pressure p 12 14 kbar granulite facies rocks that form one of earth s largest exposed lower crustal roots of an early cretaceous magmatic arc these exposures allowed us to examine the mechanisms and processes that controlled crustal thickening and large vertical displacements at the deepest levels of a deforming arc system we present structural and metamorphic data that show how the root of this arc was tectonically thickened by imbricate granulite facies thrust zones during and after the emplacement of sheeted plutons into the middle and lower crust the imbricate thrust zones form part of a well exposed 12 km wide two sided fold thrust belt that preferentially developed in crust that was thermally softened by magmatism changes in metamorphic mineral assemblages and microstructural data from the contact aureole of a composite arc batholith called the western fiordland orthogneiss record thrust and pluton emplacement conditions of p 7 9 kbar paleodepths of 25 30 km these data show that thrust imbrication and tectonic loading at 30 km depth is a viable mechanism of large vertical displacements and crustal thickening at the deepest levels of magmatic arcs this mechanism produced a characteristic up pressure metamorphic history that is similar to that observed in many other large magmatic belts worldwide

building an island arc crustal section time constraints from a la icp ms zircon study geochronological studies of samples from continuous crustal sections of fossil intra oceanic arcs are paramount for determining the precise timing of major processes that took place during the building of an oceanic island arc crust in this study laser ablation icp ms u pb zircon and conventional rb sr analyses pinpoint the timing of major events responsible for crustal growth of the kohistan paleo island arc northern pakistan inheritance in magmatic rocks is evidence for recycling of intra arc material and indicates that the kohistan arc may have formed as early as 135 4 ma one older inherited grain indicates that the arc developed on a young oceanic lithosphere whose remnants include a c 175 ma old component an interesting consequence is that intra oceanic arc magmas can yield inherited zircons which can be much older than the arc system itself without requiring recycling back into the mantle by subduction processes in the case of the kohistan arc ante arc neotethys oceanic lithosphere was tapped by arc magmas during their way upward into the arc crust the oldest magmatic ages measured in this study fall in the range 101 102 ma 100 9 0 6 and 102 1 0 4 ma this period corresponds to arc build up and thickening of the arc crust leucogranitic melts dated at 89 9 0 4 ma and 90 9 1 0 ma are considered as produced by dehydration melting reaction accompanying granulitisation of the thickened lower arc crust a maximum age for this event is 97 7 0 7 ma the age of emplacement of a garnet meta tonalite affected by granulite facies metamorphism magmatic activity was still ongoing during and after the granulitisation as testified by emplacement of a diorite and a gabbro dated at 89 1 0 6 and 88 2 0 9 ma respectively the upper part of the metaplutonic sequence contains diorite samples dated at 84 6 0 5 ma and 84 3 0 5 ma and as young as 81 1 0 7 ma during this period and based on the present day outcropping sequences the mean crustal growth rate varied from 32 to 65 km 3 km ma volume per unit width along the strike of the arc which is comparable to the range for present day arcs of the western pacific region the correspondence of crustal growth rates observed for present day island arcs and the good preservation of the crustal section in the kohistan arc make this area an exceptional natural laboratory to study arc related processes and to check models of continental crust formation by arc accretion

a magma loading model for barrovian metamorphism in the southeast coast plutonic complex british columbia and washington barrovian metamorphism linked to crustal thickening in the southeast coast plutonic complex of british columbia and washington is proposed in this paper to be the product of magmatic loading relevant observations and arguments are the following 1 isotopic ages coupled with structural and fabric relations document that throughout the region barrovian metamorphism and plutonism are broadly coeval 2 baric patterns in country rock indicate steep sided crustal loads originating within the high grade parts of the orogen 3 patterns of subsidence and uplift are localized and diachronous precluding a single event regional thickening mechanism 4 country rock fabrics are dominated by steep orogen parallel foliations and shallow strike parallel lineations features not easily reconciled with tectonic thickening mechanisms 5 abroad zone of schists flanking the large 25 x 100 km scuzzy batholith in british columbia bears metamorphic isograds and foliations that track the border of the pluton across regional structural trends implicating pluton emplacement as the cause of metamorphism 6 the crustal loading event in the vicinity of the scuzzy pluton is bracketed by mineral and textural features as being coeval with emplacement of the pluton which occurred over a period of 7 m y 7 the regionally developed chiwaukum schist in washington has petrologic and structural features and a metamorphic age that suggest it was formed by and represents the floor of an eroded extension of the scuzzy pluton the proposed magma loading model invokes the diapir and ballooning concepts of previous workers in which rising plutons displace country rock downward this mechanism results in a type of convective crustal overturn and at least partially solves the space problem related to batholithic emplacement as a result of this process relatively broad tracts of country rock acquire barrovian mineralogy following plutonism magmatically thickened crust is eroded and isostatically up lifted to expose the plutonic roots and high grade country rock basement

multi pulse cotectic evolution and in situ fractionation of calc alkaline tonalite granodiorite rocks sierra de velasco batholith famatinian belt argentina abstract the study of a magmatic series composed of qtz diorites to granites of the south part of the sierra de velasco batholith famatinian arc in argentina reveals that most rocks follow a coherent cotectic trend which is comparable to a cotectic line of liquids cll traced from experimental liquid compositions of calc alkaline andesitic systems the identification of cotectic relationships is crucial to interpret the meaning of geochemical variation trends in terms of phase equilibria we show the contrast between typical cll trends defined by rocks of the palanche pluton and rocks departing from cll array which are interpreted as resulting from local processes of bulk assimilation of a pelitic host zircon u pb age determinations with shrimp support a coeval zircon crystallization record from of about 480 to 450 ma of samples plotting on the cll the absence of intrusive contacts between these coeval cotectic granites implies that they were fractionated in situ at the level of emplacement other younger pulses 442 5 ma also fall on the cll denoting that uniform processes of melting and fractionation were operative for at least 4 0 ma at the same locus at the active continental margin of gondwana during this protracted magmatic activity sporadic pulses of mafic magma were injected e g at 456 7 ma revealing a complex process of pulse amalgamation in the building up of the batholith these mafic magma pulses may have contributed to basification of the host granitoid by enclave dissolution at the low pressure of emplacement the reported geochemical trends as true cll together with observed textures and crystallization sequences point to a parental intermediate system of andesite composition sio 2 55 wt with a water content of about 2 5 wt h 2 o similar to those formed by sediment basalt subducted melanges a sublithospheric origin for the mafic precursor is proposed in agreement with prediction by recently published thermo mechanical modeling and experimental phase equilibria

free core resonance parameters from strain data sensitivity analysis and results from the gran sasso italy extensometers summary we analyse eight years of discontinuous strain records from two crossed 90 m long laser interferometers extensometers operating in the gran sasso underground observatory central italy to investigate the fcn parameters at first inverting realistic synthetic tidal parameters amplitude and phase of eight diurnal tidal components we show that the resolving power of strain tides is comparable to that of gravity tides if tidal parameters are inverted minimizing the misfit as usually done both resolving powers improve if data are inverted minimizing the misfit and this improvement is particularly notable for gravity tides the analyses of strain records have been performed after correcting measured strain for local distortion of the regional strain field and ocean loading data are inverted minimizing the misfit parameter uncertainty assessment is performed both inverting synthetic tidal parameter sets obtained from the real one by adding a random noise and estimating the bayesian marginal probabilities from an ensemble of parameter space samples even if we analyse data from a single station our results for the period of the free core nutation about 429 sidereal days are robust and comparable to those from gravity tides obtained from the joint inversion of data from several stations the quality factor is not well constrained because of the large uncertainty on the 1 phase however our results are consistent with recently published values 20 000 the agreement between observations and predictions looks better than in any previous work that makes use of strain tides our inversions provide also an independent estimate of re h1 3l1 which is the dominating term in the resonance strength expression in agreement with iers conventions 2003

anatectic reworking and differentiation of continental crust along the active margin of gondwana a zircon hf o perspective from west antarctica the fosdick migmatite granite complex of west antarctica preserves evidence of two crustal differentiation events along a segment of the former active margin of gondwana one in the devonian carboniferous and another in the cretaceous the hf o isotope composition of zircons from devonian carboniferous granites is explained by mixing of material from two crustal sources represented by the high grade metamorphosed equivalents of a lower palaeozoic turbidite sequence and a devonian calc alkaline plutonic suite consistent with an interpretation that the devonian carboniferous granites record crustal reworking without input from a more juvenile source the hf o isotope composition of zircons from cretaceous granites reflects those same two sources together with a contribution from a more juvenile source that is most evident in the detachment hosted youngest granites the relatively non radiogenic ehf isotope characteristics of zircons from the fosdick complex granites are similar those from the permo triassic granites from the antarctic peninsula however the fosdick complex granites contrast with coeval granites in other localities along and across the former active margin of gondwana including the tasmanides of australia and the western province of new zealand where the wider range of more radiogenic ehf values of zircon suggests that crustal growth through the addition of juvenile material plays a larger role in granite genesis these new results highlight prominent arc parallel and arc normal variations in the mechanisms and timing of crustal reworking v crustal growth along the former active margin of gondwana

structure and evolution of the himalaya tibet orogenic belt the 1981 french chinese expedition to tibet focused on the lhasa block extending earlier coverage 400 km north of the tsangpo suture the lhasa block stood between 10 and 15 n latitude over most of the upper cretaceous and eocene and if gondwanian in origin had detached from gondwana by early permian seismic profiles reveal a complex moho topography resulting both from multiple continental thrusting and large scale strike slip faulting subduction related granitoids representing mixtures of mantle and crustal components and anatectic granitoids have been analysed and dated this study emphasizes the role of smaller blocks in the accretion of the continental mosaic

zircon u pb chronology of the nyingtri group southern lhasa terrane tibetan plateau implications for grenvillian and pan african provenance and mesozoic cenozoic metamorphism abstract the nyingtri group of the lhasa terrane in southern tibet consists dominantly of metasedimentary rocks and orthogneiss these rocks have a similar mineral paragenesis of plagioclase k feldspar biotite quartz sillimanite garnet staurolite muscovite amphibole indicating amphibolite facies metamorphic conditions inherited detrital zircons from the metasedimentary rocks show magmatic features and yield widely variable 206pb 238u ages ranging from 3300 to 50 ma the data define two prominent age populations 1200 1000 and 600 500 ma indicating that the source of the nyingtri group preserves the records of both grenville and pan african magmatic thermal events inherited magmatic zircon cores from the orthogneiss yield a crystallization age of 496 ma limiting the depositional age of the metasedimentary sequence to cambrian or older overgrowth rims on the detrital zircons from one metasedimentary rock yield a metamorphic age of 32 ma on the basis of these results together with the

late cretaceous to middle tertiary basin evolution in the central tibetan plateau changing environments in response to tectonic partitioning aridification and regional elevation gain located in the bangong suture zone between the lhasa and qiangtang terranes of central tibet the nima basin records cretaceous late miocene sediment accumulation during a period of drastically changing paleogeography and paleoelevation the bangong suture zone originally formed during late jurassic early cretaceous time as the lhasa terrane collided with the qiang tang terrane during early to middle cretaceous time the region lay at northern near equatorial paleolatitudes near sea level by aptian time the nima basin was above sea level and was strongly influenced by nearby volcanic activity and crustal shortening in the reactivated bangong suture zone in the southern nima basin an 50 m y late cretaceous eocene depositional hiatus correlates with major crustal shortening and ensuing voluminous ignimbrite eruptions in the lhasa terrane in the northern nima basin deposition continued during latest cretaceous time recording arid paleoclimate in evaporitic lacustrine and eolian dunefield deposits by oligocene time the nima basin comprised two independent depocenters that accumulated coarse grained alluvial fluvial lacustrine evaporitic and fan delta deposits in close association with reactivated thrusts in the bangong suture zone carbon and oxygen isotope data from oligocene paleosol carbonate reported elsewhere indicate that regional paleoelevation during the late oligocene was 4 6 km as it is today overall the depositional record of the nima basin combined with ongoing structural and geochronologic studies demonstrates that the bangong suture zone was reactivated during middle cretaceous and middle tertiary time that the intervening 50 m y interval was a time of regional upper crustal shortening in the lhasa terrane followed by regional ignimbrite eruptions and that arid paleoclimate and high paleoelevation were established by the late cretaceous and late oligocene respectively within the context of other data sets from the lhasa terrane the record of deposition in the nima basin is consistent with low angle subduction of neotethyan oceanic lithosphere and reactivation of the bangong suture zone during the early cretaceous followed by shortening within the gangdese retroarc and northern lhasa terrane thrust belts during the late cretaceous lithospheric delamination dripping and regional magmatic flare up during latest cretaceous through early tertiary time and underthrusting of indian lower crust and lithosphere as far north as the bangong suture zone during late oligocene time

superposition of replacements in the mafic granulites of the jijal complex of the kohistan arc northern pakistan dehydration and rehydration within deep arc crust abstract a deep level crustal section of the cretaceous kohistan arc is exposed in the northern part of the jijal complex the occurrence of mafic to ultramafic granulite facies rocks exhibits the nature and metamorphic evolution of the lower crust mafic granulites are divided into two rock types two pyroxene granulite orthopyroxene clinopyroxene plagioclase quartz 1 and garnet clinopyroxene granulite garnet clinopyroxene plagioclase quartz 2 two pyroxene granulite occurs in the northeastern part of the jijal complex as a relict host rock of garnet clinopyroxene granulite where the orthopyroxene rich host is transected by elongated patches and bands of garnet clinopyroxene granulite garnet clinopyroxene granulite together with two pyroxene granulite has been partly replaced by amphibolite hornblende garnet plagioclase quartz 3 the garnet bearing assemblage 2 is expressed by a compression dehydration reaction hornblende orthopyroxene plagioclase garnet clinopyroxene quartz h 2 o subsequent amphibolitization to form the assemblage 3 is expressed by two hydration reactions garnet clinopyroxene plagioclase h 2 o hornblende quartz and plagioclase hornblende h 2 o zoisite chlorite quartz the mafic granulites include pod and lens shaped bodies of ultramafic granulites which consist of garnet hornblendite garnet hornblende clinopyroxene 4 associated with garnet clinopyroxenite garnetite and hornblendite field relation and comparisons in modal chemical compositions between the mafic and ultramafic granulites indicate that the ultramafic granulites were originally intrusive rocks which dissected the protoliths of the mafic granulites and then have been metamorphosed simultaneously with the formation of garnet clinopyroxene granulite the results combined with isotopic ages reported elsewhere give the following tectonic constraints 1 crustal thickening through the development of the kohistan arc and the subsequent kohistan asia collision caused the high pressure granulite facies metamorphism in the jijal complex 2 local amphibolitization of the mafic granulites occurred after the collision

paleogene crustal anatexis and metamorphism in lhasa terrane eastern himalayan syntaxis evidence from u pb zircon ages and hf isotopic compositions of the nyingchi complex in the eastern himalayan syntaxis the southern lhasa terrane is dominated by middle to high grade metamorphic rocks nyingchi complex which are intruded by felsic melts u pb zircon dating and zircon hf isotopic composition of these metamorphic and magmatic rocks provide important constraints on the tectono thermal evolution of the lhasa terrane during convergent process between indian and asian continents u pb zircon data for an orthogneiss intruding the nyingchi complex yield a protolith magma crystallization age of 83 4 1 2 ma with metamorphic ages of 65 46 ma this orthogneiss is characterized by positive ehf t values of 8 3 and young hf model ages of 0 6 ga indicating a derivation primarily from a depleted mantle or juvenile crustal source zircons from a quartz diorite yield a magma crystallization age of 63 1 0 6 ma with ehf t values of 8 2 to 2 7 suggesting that this magma was sourced from partial melting of older crustal materials zircon cores from a foliated biotite granite show ages ranging from 347 to 2690 ma with age peaks at 347 403 ma 461 648 ma and 1013 1183 ma their zircon ehf t values range from 30 6 to 6 9 both the u pb ages and hf isotopic composition of the zircon cores are similar to those of detrital zircons from the nyingchi complex paragneiss implying that the granite was derived from anatexis of the nyingchi complex metasediments the zircon rims from the granite indicate crustal anatexis at 64 4 0 7 ma and subsequent metamorphism at 55 1 1 3 and 41 4 2 3 ma our results suggest that the late cretaceous magmatism in the southern lhasa terrane resulted from neo tethys oceanic slab subduction and we infer that paleocene crustal anatexis and metamorphism were related to the thermal perturbation caused by rollback of the northward subducted neo tethyan oceanic slab

mean crustal velocity a critical parameter for interpreting crustal structure and crustal growth abstract mean crustal velocity is a critical parameter for genesis of continental crystalline crust because it is a function of mean crustal composition and therefore may be used to resolve continental crustal growth in space and time although the best values of mean crustal velocity are determined from wide angle reflection measurements most studied here necessarily come from vertical averages in crustal refraction determinations the mode of 158 values of mean crustal velocity is 6 3 km s a velocity which corresponds to a mean crustal composition of granodiorite to felsic quartz diorite archean crust may be slightly more mafic mean crustal velocities range from 5 8 to 7 0 km s the lowest values invariably are found in thermally disturbed rift zones and the highest values correspond to velocities in gabbro velocities in island arcs may be as low as 6 0 km s but are typically 6 5 6 9 km s which corresponds to andesitic composition estimates of island arc composition are andesitic if values of mean crustal velocity are not biased this observation suggests that continental crust did not grow simply by addition of island arc material possibilities are that crust formed from fusion of island arcs and was later changed to more felsic composition by addition of material from the mantle or that the late archean episode of major crustal growth did not involve processes similar to younger island arcs some crustal blocks might be changed in composition and thickness by such processes as underplating interthrusting necking and sub crustal erosion specially designed experiments are suggested to determine this parameter so critical for understanding genesis of continental crust

mesozoic and cenozoic volcanic rocks from central and southern tibet 39ar 40ar dating petrological characteristics and geodynamical significance field relations radiometric ages and mineralogical and chemical characteristics of the orogenic volcanic rocks from the lhasa block tibet are studied volcanic activity started before the deposition of the albian aptian sedimentary units takena formation and xigaze group volcanism is volumetrically limited in the northern part of the lhasa block basalts andesites and dacites are present while ignimbrites are scarce radiometric ages of these rocks range between 110 and 80 ma conversely in the southern part of the lhasa block volcanism is largely developed gangdese belt there paroxysmal activity occurred later than in the north between 60 and 50 ma lingzizong formation this volcanism is mainly represented by andesites and ignimbrites a younger episode at about 15 10 ma took place in this southern region the mineralogical and geochemical characteristics indicate that as a whole the volcanic rocks from the lhasa block belong to a calc alkaline continental margin series enriched in k20 and other incompatible elements subordinate potassic alkaline lavas are present in the northern margin of the gangdese belt some basalts exhibit a tholeiitic affinity daquin area southern region the geodynamic significance of these cenozoic volcanic rocks lies in the relative motion of india and the lhasa block after accretion of the latter to the northern qantang block a geodynamic model based on changes of the type of subduction of the indian oceanic crust beneath the lhasa block for the period 120 40 ma is proposed to account for the abundance age and magmatic affinity of the volcanic products

partial melting of mantle wedge and evolution of island arc crust evolution of the crust in the northeast honshu arc is discussed on the basis of the melting experiments of a peridotite under hydrous conditions and the crustal compositions estimated from deep crustal xenoliths and seismic velocity data the chemical compositions of melts formed by partial melting of a hydrous peridotite within the pressure temperature conditions for shallow parts of the mantle wedge 1 2 2 0 gpa have been experimentally determined with the sandwich experiment technique the melts range from olivine tholeiitic to magnesian andesitic compositions sio2 48 56 wt mgo 9 14 wt beneath the volcanic front in the northeast honshu arc and are slightly less silicic beneath the coast of the sea of japan the crustal compositions of the volcanic zone in the northeast honshu arc estimated from deep seated crustal inclusions in volcanic rocks and the velocity structures of the crust are andesitic in the central zone and high alumina basaltic near the coast of the sea of japan the compositions of possible primary melts formed in the mantle wedge are significantly more mafic than these crustal compositions consequently if the major part of the crustal materials in the volcanic zone was formed by magmas generated in the mantle wedge thick up to 30 km ultramafic cumulates including olivine pyroxenite must have been formed beneath moho during fractional crystallization and would constitute the upper part of the mantle wedge the volume of the mantle wedge consisting entirely of even fertile peridotite was not sufficient to produce crustal materials and cumulates in the volcanic zone by such processes in the northeast honshu arc a static mantle wedge model is therefore not satisfactory for the growth of the crust in northeast honshu arc a dynamic model is suggested in which the mantle materials have been supplied into the mantle wedge from the back arc side by a convective flow

examination of the deep levels of an island arc evidence from the tonsina ultramafic mafic assemblage tonsina alaska the tonsina ultramafic and mafic assemblage located in the northern chugach mountains along the trans alaska crustal transect tact is interpreted to be the remains of a magma chamber that crystallized at the base of a mature interoceanic island arc it is one of a number of isolated masses of peridotite and gabbro that comprise the basal part of the peninsular terrane along the northern fringe of the chugach mountains in southern alaska these masses the roots of a mid jurassic island arc have been uplifted along the north dipping border ranges fault system and crop out sporadically for 700 km from kodiak island to eastern alaska the tonsina ultramafic mafic assemblage is a layered sequence whose basal rocks are dunite and harzburgite 1 km thick overlain by a narrow zone of websterite and clinopyroxenite above these ultramafic rocks is a 5 to 6 km thick layered gabbro that is garnet bearing at its base textural relationships and chemical trends throughout the series of dunite pyroxenite garnet bearing gabbro and spinel gabbronorite point to an origin by igneous accumulation from a hydrous tholeiitic parent magma at deep levels i e at the base of a mature island arc before being emplaced along the border ranges fault the rocks were crystallized from a melt at pressures between 9 5 and 11 kbar 28 to 33 km depth and equilibrated at temperatures of 800 to 90o c based on phase relationships in the cumulus assemblages pyroxene thermobarometry and olivine spinel thermometry transitional sequences from ultramafic to mafic rocks are characterized by increasingly al rich pyroxenes and spinels or garnet and the absence of olivine plagioclase assemblages deep level fractionation of al poor phases such as olivine and pyroxene at moderately high pressures and temperatures resulted in concentration of al in the evolving magma as crystallization continued the phases recorded this alumina concentration and the resultant gabbros are al rich this observed trend supports the theory of high alumina basalt generation by fractional crystallization of olivine and pyroxene comparison of the tonsina rocks with aleutian lower crustal xenoliths as well as other exposures of arc related lower crustal magma chambers in japan pakistan and western north america indicates that this process is a common mechanism for island arc magma evolution at depth

two stages of granulite facies metamorphism in the eastern himalayan syntaxis south tibet petrology zircon geochronology and implications for the subduction of neo tethys and the indian continent beneath asia the eastern himalayan syntaxis in southeastern tibet consists of the lhasa terrane high himalayan rocks and indus tsangpo suture zone the lhasa terrane constitutes the hangingwall of a subduction zone whereas the high himalayan rocks represent the subducted indian continent our petrological and geochronological data reveal that the lhasa terrane has undergone two stages of medium p metamorphism an early granulite facies event at c 90 ma and a late amphibolite facies event at 36 33 ma however the high himalayan rocks experienced only a single high p granulite facies metamorphic event at 37 32 ma it is inferred that the late cretaceous c 90 ma medium p metamorphism of the southern lhasa terrane resulted from a northward subduction of the neo tethyan ocean and that the oligocene 37 32 ma high p 1 8 1 4 gpa rocks of the high himalayan and coeval medium p 0 8 1 1 gpa rocks of the lhasa terrane represent paired metamorphic belts that resulted from the northward subduction of the indian continent beneath asia our results provide robust constraints on the mesozoic and cenozoic tectonic evolution of south tibet

india s hidden inputs to tibetan orogeny revealed by hf isotopes of transhimalayan zircons and host rocks abstract to better understand tibetan orogenesis which involves complex tectonic processes here we report the first integrated analysis of hf isotopes of zircon separates and their host rocks from the transhimalayan batholiths the rocks studied include 19 granitoids from different parts of the gangdese batholith the largest transhimalayan intrusive complex and five postcollisional adakites that occurred as plugs or dikes cutting the gangdese batholith in the southern lhasa terrane southern tibet published zircon hf isotope data from other transhimalayan granitoids and the linzizong volcanic rocks are also synthesized the magmatic zircons crystallizing between ca 200 and 15 ma show depleted mantle type hf isotopic characteristics throughout the mesozoic but shift markedly in the paleogene the hf isotopic shift is interpreted as tracking the evolving progress of himalayan sediment subduction driven by the approaching indian continent and thus signals the initiation of the india asia collision that we infer to have occurred by 55 ma our data furthermore indicate that southern tibet underwent significant crustal thickening during ca 45 and 30 ma before emplacement of the postcollisional adakites that exhibit hf nd isotopic systematics suggesting binary mixing of melts from the juvenile gangdese mafic crust and incorporated himalayan sediments in the petrogenesis

late cretaceous gangdese intrusions of adakitic geochemical characteristics se tibet petrogenesis and tectonic implications article i nfo the gangdese batholith emplaced from the cretaceous to eocene in southern tibet has been widely regarded as the major constituent of an andean type convergent margin resulting from northward subduction of the neo tethyan oceanic lithosphere under asia while the gangdese batholith consists predominantly of calc alkaline rocks we identify from the eastern part of the batholith a suite of epidote bearing granodiorites that shows adakitic geochemical characteristics marked with apparently higher la yb and sr y and lower y and hree than other gangdese rocks and common arc magmas shrimp zircon u pb analyses of two of the samples yielded 206 pb 238 u dates of 80 4 1 1 and 82 7 1 6 2 ma which constrain the emplacement ages of the adakitic rocks trace element modeling suggests that these rocks originated from partial melting of a garnet amphibolite source that on the basis of the sr and nd isotope data isr 0 7044 0 7048 end t 3 2 to 0 9 we interpret to be a newly underplated mafic lower crust rather than the subducted neo tethyan oceanic crust this juvenile crust was produced by cretaceous basaltic underplating above the mantle wedge and then thickened by the tectonic contraction owing to flattening of the neo tethyan subduction a process that also led to the adakitic magmatism our interpretation involving a late cretaceous flat slab subduction and related orogenesis in southern tibet is consistent with petrographic data such as the occurrence of magmatic epidote and muscovite rimmed with resorption texture in the granodiorites which indicate deep seated emplacement followed by rapid tectonic exhumation

origin of late oligocene adakitic intrusives in the southeastern lhasa terrane evidence from in situ zircon u pb dating hf o isotopes and whole rock geochemistry abstract we describe the whole rock geochemistry biotite ar ar and zircon u pb geochronology and in situ hf o isotopic compositions of the linzhi two mica granitoid in the southeastern lhasa terrane the granitoids are calc alkaline to high k calc alkaline in composition and they have geochemical affinities with adakites the whole rock geochemistry with high sio 2 and al 2 o 3 contents low mgo cr and ni contents low mg values negative e nd t 6 2 to 2 9 and low 87 sr 86 sr i 0 7059 0 7074 together with the in situ zircon hf o isotopic data e hf t 2 7 to 2 7 18 o 7 2 8 8 indicates that the granitoid was derived mainly from the partial melting of a rejuvenated lower crust underneath the lhasa terrane this lower crust was dominated by ancient crustal material with the minor involvement of juvenile mantle inputs 40 ar 39 ar ages for biotites and the presence of magmatic epidote suggest the granitoid was intruded at mid crustal depths indicating that the eastern lhasa terrane experienced a pulse of rapid uplift in the interval from 26 to 22 ma the geochemical features of the linzhi adakitic rocks are significantly different from those of eocene adakitic rocks from the north himalayan belt and this suggests that we can rule out the possibility of crustal thickening in the eastern part of lhasa terrane caused by impingement of the indian lower crust post collisional 26 10 ma adakitic rocks occur within an e w trending magmatic belt that runs parallel to the yarlung tsangpo suture in the lhasa terrane these rocks have distinctive variations in geochemistry along the strike of the lhasa terrane indicating that the lower crust is isotopically heterogeneous from west to east along the terrane these geochemical variations probably result from variable degrees of input along strike of juvenile mantle derived melts into an old lower crust the middle part of the lhasa terrane is marked by a juvenile lower crust that contains large amounts of mantle derived melt underplated during neo tethyan oceanic crust subduction in contrast the lower crust below the western and eastern parts of the terrane is relatively ancient therefore crustal thickening caused by underplating of mantle derived magmas could play an important role in the middle segment of the terrane whereas crustal compression and shortening as a result of the collision of the indian and asian plates could be the major cause for crustal thickening in the western and eastern segments

zircon u pb and hf isotope constraints on the mesozoic tectonics and crustal evolution of southern tibet the first in situ hf and u pb isotope analyses of zircon separates from mesozoic granites in southern tibet identify a significant previously unknown stage of magmatism igneous zircons n 34 from a granite within the gangdese batholith show a weighted mean 206 pb 238 u age of 188 1 1 4 ma and e hf t the parts in 10 4 deviation of initial hf isotope ratios between the zircon sample and the chondritic reservoir values between 10 4 and 16 8 suggesting predominantly early jurassic intrusive activity with a juvenile mantle contribution of 40 inherited zircons from two cretaceous s type granites in the northern magmatic belt 23 delineate a slightly older 206 pb 238 u age cluster between 188 and 210 ma these zircons have e hf t values from 3 9 to 13 7 yielding crustal hf model ages from ca 1 4 to 2 1 ga suggesting a major episode of crustal growth in proterozoic time and remelting of this crust in early jurassic time combining these with literature data we interpret the jurassic gangdese magmatism as an early product of the neo tethyan subduction that played a long lasting role in the tectonic evolution of southern tibet prior to the india asia collision

characteristic mineralogy of arc related cumulate gabbros implications for the tectonic setting of gabbroic plutons and for andesite genesis several mineralogically distinct suites of gabbroic cumulates occur as xenoliths in basalt and andesite from active arc volcanos and in plutons exposed in deeply eroded arcs calcic an85 100 plagioclase and moderately fe rich fo60 80 olivine commonly occur together in arc cumulate gabbros but not in cumulate gabbros from mid ocean ridges ocean islands or tholeiitic layered intrusions compositions of coexisting olivine and plagioclase in gabbro cumulates can establish the tectonic setting of ancient volcano plutonic complexes in deeply eroded arcs gabbroic cumulates typically occur in zoned plutons that also contain gabbros diorites and quartz diorites of basaltic to andesitic composition in many cases it is clear that the cumulate gabbros are chemically and physically related to the gabbro diorite basalt andesite fractionation trends seen in the zoned plutons zoned plutons in deeply eroded arcs provide physical evidence that crystal fractionation plays an important role in andesite petrogenesis

fluids in deeply subducted continental crust petrology mineral chemistry and fluid inclusion of uhp metamorphic veins from the sulu orogen eastern china abstract the complex vein associations hosted in southern sulu ultrahigh pressure uhp eclogites contain quartz omphacite or jadeite kyanite allanite zoisite rutile garnet these minerals have chemical compositions similar to those of host eclogites inclusions of polycrystalline quartz pseudomorphs after coesite were identified in vein allanite and garnet and coesite inclusions were found in vein zircon these facts suggest that the veins together with host eclogites have been subjected to synchronous uhp metamorphism the vein minerals contain relatively high concentrations of rare earth elements ree high field strength elements hfse and transition metal elements tme a kyanite quartz vein has a whole rock composition similar to adjacent uhp metamorphic granitic gneisses abundant primary multi solid fluid inclusions trapped within uhp vein minerals contain complex daughter minerals of muscovite calcite anhydrite magnetite pyrite apatite celestite and liquid and gas phase of h 2 o with solids up to 30 70 of the inclusion volume the presence of daughter minerals anhydrite and magnetite indicates the subduction fluids were oxidizing and provides a possible interpretation for the high oxygen fugacity of subduction zone magmas these characteristics imply that the uhp vein minerals were crystallized from supercritical silicate rich aqueous fluids that were in equilibrium with peak uhp minerals and that the fluids in deeply subducted continental crust may contain very high concentrations of silicate as well as hree hfse and tme such fluids might have resulted in major fractionation between nb and ta i e the uhp fluids have subchondritic nb ta values whereas the host eclogites after extraction of the fluids have suprachondritic nb ta values therefore voluminous residual eclogites with high nb ta ratios may be the complementary suprachondritic reservoir capable of balancing the subchondritic depleted mantle and continental crust reservoirs

crustal thickening of the lower crust of the kohistan arc n pakistan deduced from al zoning in clinopyroxene and plagioclase the lower crustal rocks of the kohistan complex northern pakistan are mostly composed of metabasic rocks such as pyroxene granulites garnet granulites and amphibolites we have investigated p t trajectories of the relic two pyroxene granulites which are the protolith of the amphibolites within the kamila amphibolite belt aluminous pyroxene retains igneous textures such as exsolution lamellae developed in the core the significant amount of al in clinopyroxene is buffered by breakdown reactions of plagioclase accompanied by film like quartz as a product at grain boundaries between plagioclase and clinopyroxene distinct al zoning profiles are preserved in pyroxene with exsolution lamellae in the core and in plagioclase adjacent to clinopyroxene in pyroxene granulites in the northern part of the kamila amphibolite belt al in clinopyroxene increases towards the rim and abruptly decreases at the outer rim and anorthite in plagioclase decreases towards the rim and abruptly increases near the grain boundary between plagioclase and clinopyroxene in the southern part of the kamila amphibolite belt al in clinopyroxene and anorthite in plagioclase simply increase towards the margins of the grains the anorthite zoning in plagioclase is in agreement with the zoning profiles of ca tschermaks and jadeite components inferred from variations of al na ti and fe3 in clinopyroxene assuming that the growth surface between them was in equilibrium geothermobarometry based on al zoning in clinopyroxene coexisting with plagioclase indicates that metamorphic pressures significantly increased with increasing temperature under granulite facies metamorphism the peak of granulite facies metamorphism occurred at conditions of about 800 c and 800 1100 mpa these prograde p t paths represent a crustal thickening process of the kohistan arc during the early to middle cretaceous the crustal thickening of the kohistan arc was caused by accretion of basaltic magma at mid crustal depths

western fiordland orthogneiss early cretaceous arc magmatism and granulite facies metamorphism new zealand u pb isotopic analyses of zircons from a distinctive suite of previously undated granulite facies metaplutonic rocks here termed the western fiordland orthogneiss wfo in fiordland southwest new zealand indicate synkinematic magmatic emplacement between 120 and 130 ma ago these rocks were previously interpreted as possibly being of precambrian age initial pb and sr ratios are consistent with arc subduction related magmagenesis with little or no involvement of ancient continental crust subsequent high pressure 12 kb metamorphism of the wfo may reflect a major collision event involving crustal thickening by overthrusting of a 15 km thick sequence metamorphism ceased 116 ma ago based on206pb 238u ages of zircon from a retrogressed granulite u pb isotopic analysis of apatite along with previously published rb sr mineral ages indicate that final uplift and cooling to 300 400 c was largely completed by 90 ma the average uplift rate during this period is inferred to have been in excess of 1 mm yr unmetamorphosed gabbronorites of the darran complex in eastern fiordland inferred by some investigators to be the granulite protolith yield concordant u pb zircon ages of 137 1 ma u pb ages of apatite and previously published k ar mineral ages indicate that these rocks experienced a rapid and simple cooling history lasting only a few million years the high grade wfo and unmetamorphosed darran complex are now separated by a profound structural break however the ages and similarities in initial pb and sr isotopic ratios suggest that both suites are products of the same early cretaceous cycle of subduction related magmatism the timing of early cretaceous magmatism and metamorphism collision and resultant crustal thickening and subsequent great uplift and erosion in fiordland has important implications for terrane accretion and the evolution of relative plate motions along the new zealand segment of the gondwana margin

cretaceous high p granulites at milford sound new zealand metamorphic history and emplacement in a convergent margin setting granulite facies orthogneiss of the arthur river complex arc at milford sound western fiordland records a complex early cretaceous magmatic and orogenic history for the pacific gondwana margin that culminated in the emplacement and burial of a dioritic batholith the western fiordland orthogneiss wfo enstatite bearing mafic to intermediate protoliths of the arc and wfo intruded the middle to upper crust the early deformation history of the arc is preserved in the pembroke granulite where two pyroxene s1 assemblages that reflect p 750 c were only patchily recrystallized during later deformation s1 is cut by garnet bearing leucogabbroic to dioritic veins which are cut by distinctive d2 fractures involving anorthositic veins and garnet diopside plagioclase bearing reaction zones these zones are widespread in the arc and wfo and record conditions of p 14 kbar and t 750 c garnet clinopyroxene bearing corona reaction textures that mantle enstatite in both the arc and wfo reflect early cretaceous burial by approximately 25 km of continental crust most of the arc is formed from the milford and harrison gneisses which contain steeply dipping s4 assemblages that envelop the pembroke granulite and involve garnet hornblende diopside clinozoisite rutile and plagio clase with or without kyanite the p t history of rocks in western fiordland reflects pronounced early cretaceous convergence related tectonism and burial possibly related to the collision and accretion of island arc material onto the pacific gondwana margin

metamorphic rocks of the 1985 tibet geotraverse lhasa to golmud two examples of uplifted basement have been studied in the lhasa terrane of the tibetan plateau the nyainqentanglha orthogneisses are bounded by staurolite garnet schists to the north which record prograde metamorphism at 5 0 1 3 kbar 610 70 degrees c garnet sillimanite xenoliths within the orthogneiss suggest that peak temperatures reached at least 700 70 degrees c at 5 1 2 5 kbar these p t fields reflect high t low p metamorphism during eocene subduction and indicate that the syntectonic nyainqentanglha orthogneiss was emplaced at depths greater than 10 km sillimanite bearing assemblages from the amdo gneisses in the northern lhasa terrane provide evidence of crustal anatexis at temperatures 680 degrees c this event is poorly constrained in time but is probably cambrian or earlier within the kunlun terrane biotite and garnet isograds north of the xidatan fault indicate an increase in metamorphic grade from north to south reaching peak metamorphism at 470 30 degrees c 4 3 1 5 kbar synchronous with the emplacement of the triassic batholith regional metamorphism was followed by uplift of at least 2 km before emplacement of post tectonic early jurassic granites

kinematics and dynamics of the namche barwa syntaxis eastern himalaya constraints from deformation fabrics and geochronology abstract field observations deformation and fabric analyses and precise age data acquired by zircon shrimp la icp ms u pb and 40 ar 39 ar dating methods have yielded new constraints on the kinematics and dynamics of the namche barwa syntaxis nbs which is the eastern corner of the himalaya a two stage model has been established to explain the formation and evolution of the nbs the northward indentation of the indian plate beneath the lhasa terrane began at 55 40 ma and crustal materials at this corner were subducted to depths 70 km where they experienced hp uhp metamorphism since 40 ma large scale right lateral strike slip along the sagaing fault has accommodated the rapid northward movement of the eastern indian plate corner with respect to the indochina block this caused significant and progressive bending of the indus yarlung suture zone iysz such that it became the dongjiu milin left lateral strike slip shear zone dmsz in the west and the aniqiao motuo right lateral strike slip shear zone amsz in the east both zones underwent strong mylonitization meanwhile the hp uhp metamorphic rocks were rapidly exhumed first into the deep crust at 22 18 ma and then to the shallow crust to form an antiformal dome at 6 2 ma our model provides new insight into the processes of post collisional crustal thickening related to the formation of the himalayan orogenic belt

zircon u pb geochronology and hf isotopic constraints on petrogenesis of the gangdese batholith southern tibet abstract during the mesozoic cenozoic northward neotethyan subduction and subsequent india asia collision gave rise to the extensive transhimalayan magmatism that stretches from burma and western yunnan through southern tibet to the ladakh and kohistan complexes to understand the age distribution and petrogenesis of the gangdese batholith the largest intrusive exposure along the transhimalayan magmatic belt fifty granitic samples were selected for in situ zircon u pb and hf isotopic analyses the u pb data suggest four discrete stages of magmatic activity i e 205 152 109 80 65 41 and 33 13 ma respectively with the 65 41 ma stage being the most prominent the hf isotopic data indicate that the gangdese batholith is overwhelmed by positive e hf t values which are comparable to those of the kohistan ladakh batholiths in the west but differ markedly from those of the chayu burma batholiths in the east most of the gangdese granites show similar and young hf model ages 1000 300 ma indicating their derivation from juvenile crust however those formed in the 65 41 ma stage exhibit more heterogeneous hf isotopic ratios with negative e hf t values being observed in some granites younger than 50 ma suggesting the involvement of old indian continental crust in their petrogenesis this age may thus mark the onset of the india asia collision the newly established zircon u pb age and hf isotope database of the gangdese batholith can be used as a powerful tracer or fingerprint when studying the source to sink relation of the sediments eroded from the southern tibetan plateau

the nature and timing of crustal thickening in southern tibet geochemical and zircon hf isotopic constraints from postcollisional adakites abstract rising as the roof of the world the tibetan plateau is now underlain with the thickest continental crust on earth how and when was this crust formed which would have exerted pivotal controls to the formation of the plateau has long been an issue of hot debates this paper reports zircon u pb ages and hf isotope data for postcollisional 30 9 ma adakites in the southern lhasa terrane southern tibet a comparative analysis of whole rock rare earth element geochemistry and zircon hf isotopes between the adakites and associated gangdese igneous rocks suggests that the tibetan crust underwent a major phase of tectonic thickening between ca 45 and 30 ma in the region the lower part of the thickened crust consisted prevailingly of mafic lithologies which we argue to have resulted from intense basaltic underplating and subsequent remelting that took place during the late cretaceous and eocene time related to the neotethyan subduction processes including breakoff of the subducted slab at ca 50 ma in the early stage of the india asia collision these processes were responsible for not only the juvenile crust formation but also for the creation of a thermally softened lithosphere in the southern lhasa terrane the indentation of india consequently caused distributed lithospheric thickening with formation of an orogenic root beneath southern tibet root foundering during the oligocene gave rise to the adakitic magmatism regional topographic uplift and onset of northward underthrusting of the indian plate that has since played a key role in forming the entire tibetan plateau

tracking the influence of a continental margin on growth of a magmatic arc fiordland new zealand using thermobarometry thermochronology and zircon u pb and hf isotopes 1 geothermobarometric radiogenic isotopic and thermochronologic data are used to track the influence of an ancient continental margin western province on development of an adjacent carboniferous cretaceous magmatic arc outboard median batholith in fiordland new zealand the data show a record of complicated mesozoic gondwana margin growth paragneiss within the outboard median batholith is of carboniferous to jurassic age and records burial to middle crustal depths in late jurassic early cretaceous during subduction related plutonism and arc thickening in contrast western province metasedimentary rocks in the area of study immediately west of the outboard median batholith are late cambrian early ordovician in age recrystallized at the amphibolite facies in the late devonian early carboniferous and exhibit no evidence for mesozoic textural or isotopic reequilibration a phase of deformation between 128 and 116 ma deformed exhumed and cooled the outboard median batholith to greenschist facies temperatures while large parts of the western province underwent 9 kbar metamorphic conditions zircon grains from mesozoic inboard plutons are isotopically more evolved hf t 2 3 to 4 0 than those in the outboard median batholith hf t 9 4 to 11 1 the contrasting zircon hf isotope ratios absence of s type plutons or proterozoic early paleozoic inherited zircon and the apparent absence of early paleozoic metasedimentary rocks indicates that the outboard median batholith is unlikely to be underlain by the western province continental lithosphere the new data are consistent with the outboard median batholith representing an allochthonous although not necessarily exotic arc that was juxtaposed onto the gondwana continental margin along the intervening grebe mylonite zone

new seismological constraints on growth of continental crust in the izu bonin intra oceanic arc the process by which continental crust has formed is not well understood though such crust mostly forms at convergent plate margins today it is thus imperative to study modern intra oceanic arcs such as those common in the western pacific ocean new seismic studies along the representative izu bonin intra oceanic arc provide unique along strike images of arc crust and uppermost mantle to complement earlier cross arc lithospheric profiles these reveal two scales 1000 10 km scale of variations one at the scale of the izu versus bonin thick versus thin arc crust the other at the intervolcano 50 km scale these images show that 1 the bulk composition of the izu bonin arc crust is more mafic than typical continental crust 2 the middle crust with seismic velocities similar to continental crust is predominantly beneath basaltic arc volcanoes 3 the bulk composition beneath basaltic volcanoes changes little at thick and thin arc segments and 4 a process to return lower crustal components to the mantle such as delamination is required for an arc crust to evolve into continental crust continued thickening of the izu bonin crust accompanied by delamination of lowermost crust can yield velocity structure of typical continental crust

metagabbros of the gangdese arc root south tibet implications for the growth of continental crust abstract the gangdese batholith of the southern tibet is regarded as a principal component of the pre himalayan collision andean type continental margin however the gangdese arc root remains poorly understood because most of previous studies concentrated on granitoids from the middle and upper crust of the arc in this paper we report new petrochemical and geochronological characteristics of metagabbros occurred as a part of the late cretaceous lilong batholith in the eastern gangdese arc the gabbros with protolith ages of 82 95 ma underwent slightly late 68 77 ma granulite facies metamorphism and nearly coeval anatexis typical of lower arc crusts the metagabbros and diorites of the lilong batholith show geochemical features of juvenile crust in continental magmatic arcs and an original magmatic differentiation trend with gabbros formed as the cumulates whereas diorites as the crystalline products of evolved magmas the lilong batholith forms an arc crustal section with a possible thickness of 20 30 km and may have an overall andesitic composition providing a support for the andesite model of continental crust growth

high density carbonic fluids in a slab window evidence from the gangdese charnockite lhasa terrane southern tibet abstract charnockites occurring in the well studied gangdese batholith in the southern lhasa terrane eastern himalayan orogen consist of orthopyroxene clinopyroxene plagioclase and minor quartz and k feldspar and show adakitic affinity here we present a systematic study on the fluid inclusions in gangdese charnockites which reveals that the primary fluid inclusions occur isolated or randomly distributed and as trails along intragranular fractures in quartz and that the secondary ones occur along healed mircofractures and coexist commonly with mineral inclusions of calcite magnetite and hematite within the host plagioclase and quartz the fluid inclusions contain dominantly near pure co 2 with traces of n 2 most of the primary fluid inclusions have low co 2 homogenization temperatures and with densities of 1 138 1 013 g cm 3 suggesting trapping pressures of 0 7 1 0 gpa at temperatures of 850 950 c we propose a model in which the southward subduction of neo tethyan mid oceanic ridge beneath the lhasa terrane resulted in the release of heat and co 2 from the upwelling asthenosphere through a slab window providing high temperatures ht and dry co 2 rich fluids for the formation and stabilization of the adakitic charnockite as well as the associated ht granulite facies metamorphic rocks in the late cretaceous prior to the final collision of the indian plate with the asian continent our study provides new insights into the geodynamics of andean type orogeny in the southern tibetan plateau during the late mesozoic

the significance of cenozoic magmatism from the western margin of the eastern syntaxis southeast tibet the major and trace element geochemistry sr nd bulk rock isotopes u pb zircon chronology and lu hf isotopic compositions are described for three granitic bodies which intrude the nyingchi gneisses lhasa terrane along the western margin of the eastern himalayan syntaxis the bayi two mica granite and lunan granite granodiorite were intruded at 22 1 and 25 4 0 3 ma respectively whereas the confluence biotite granite was emplaced at 49 1 0 4 ma all share strong depletions in y and hree requiring a garnet bearing source both during and following the eocene collision of the indian plate with the lhasa terrane the isotope geochemistry of these intrusives e nd t 3 to 5 87sr 86sr t 0 706 0 707 indicates a crustal source within the lhasa terrane sr nd systematics of the garnet bearing nyingchi gneisses together with the u pb and lu hf isotopic ratios of detrital zircons recovered from this unit identifies it as a potential melt source the combined element and isotope geochemistry of the plutons indicate a mixed source the gneisses provide the older component whereas the gangdese batholith provides a younger siliceous component the involvement of garnet bearing crustal material in melt sources from the cretaceous 80 ma to the miocene 20 ma is consistent with the presence of a thicker continental crust in the eastern lhasa terrane as is the presence of magmatic epidote in several plutons which indicates a regional deepening level of exposure eastwards post collision crustal melting is synchronous with proposed slab break off during the early miocene suggesting advective heating by rising asthenospheric melts

zircon u pb and hf isotopic constraints from eastern transhimalayan batholiths on the precollisional magmatic and tectonic evolution in southern tibet abstract before the indian collision with asia northward subduction of the neo tethyan oceanic lithosphere produced an andean type convergent margin in south asia characterized by arc magmatism starting from the early jurassic and lasting until the eocene the magmatic arc is now represented by widespread intrusive bodies or the so called transhimalayan batholiths in the lhasa terrane of southern tibet that have been divided into two main magmatic suites i e the northern plutonic belt and the southern gangdese batholith their temporal distribution concerning how exactly the magmatic suites correlate eastwards and then southeastwards around the eastern himalayan syntaxis however remains poorly constrained here we report the first combined zircon u pb and hf isotopic study of the transhimalayan batholiths from the eastern part of the lhasa terrane 95 97 5 e and 28 5 30 n se tibet zircon u pb dating results of 24 granitoids indicate that the rocks were emplaced principally in the early cretaceous 133 110 ma and subordinately in the paleocene 66 57 ma long after an older granite intrusion in the earliest jurassic 198 ma the zircon ehf t values range from 5 to 20 yielding hf crustal model ages tdmc between 0 8 and 2 4 ga that peak at 1 7 ga suggesting a major episode of crustal growth in the proterozoic and a predominantly or even exclusively crustal source for the granitoid petrogenesis these zircon u pb and hf isotopic constraints together with whole rock geochemical characteristics allow us to correlate them to s type granitoids in the northern plutonic belt rather than to the i type gangdese batholith from the central part of the southern lhasa terrane in contrast to the gangdese magmatism that formed with a significant juvenile mantle input related to the neo tethyan subduction these s type granitoids show geochemical similarities to the north american cordilleran interior batholiths that did not form directly from a subduction setting thus we attribute the petrogenesis of the widespread early cretaceous granitoids in the northern belt and eastern himalayan batholiths to a postcollisional regime due to the late jurassic early cretaceous continental collision between the lhasa and qiangtang terranes under this framework the s type magmatism was generated in response to collision induced crustal thickening which may have capabilities of causing crustal anatexis by itself in the region however continued interplay with the neo tethyan subduction zone processes such as back arc extension could also have played a crucial and long lasting role in the magma generation

early late cretaceous ca 93 ma norites and hornblendites in the milin area eastern gangdese lithosphere asthenosphere interaction during slab roll back and an insight into early late cretaceous ca 100 80 ma magmatic flare up in southern lhasa tibet abstract at more than 500 km in length the mainly jurassic early eocene gangdese batholith is one of the most important constituents of the southern lhasa sub block and provides an ideal site for study of tibetan orogenesis recent studies on gangdese intermediate felsic intrusive rocks mainly granites demonstrate that remarkable crustal growth as well as an early late cretaceous ca 100 80 ma magmatic flare up event occurred in southern tibet however the mechanism that drove this magmatic event and its relationship to crustal growth event are not yet clear here we report detailed petrological geochronological geochemical and sr nd hf isotopic data for recently identified norites and hypersthene bearing hornblendites in the milin area southern lhasa sub block these mafic rocks are dated at early late cretaceous ca 93 ma and are characterized by relatively uniform sr nd hf isotopic compositions 87sr 86sr i 0 7042 to 0 7047 end t 2 9 to 3 6 and ehf t zircon 10 9 to 17 0 suggesting that they evolved from similar parental magmas the milin norites and hornblendites are likely to be the products of mineral fractionation and accumulation from a common parental magma during the early and late stages of the magma evolution respectively thermometric calculations indicate that pyroxenes from the milin norites have high crystallization temperatures 1240 1349 c the parental magmatic compositions calculated from pyroxene trace element compositions in the milin norites show slightly flat to enriched light rare earth element lree patterns la yb n 2 9 3 4 la gd n 2 1 2 9 with variable negative nb anomalies nb la n 0 18 0 81 indicating their dual or hybrid geochemical characteristics we suggest that their parental magmas may have been generated by the interaction of upwelling asthenospheric and metasomatized lithospheric mantle taking into account the spatial and temporal distribution of the mesozoic magmatic rocks and regional paleomagnetic data we further suggest that the early late cretaceous magmatic flare up in the southern lhasa sub block was also triggered by the asthenospheric upwelling which resulted from roll back of subducted tethyan oceanic slab

kyanite paragonite bearing assemblages northern fiordland new zealand rapid cooling of the lower crustal root to a cretaceous magmatic arc fiordland new zealand exposes the lower crustal root of an early cretaceous magmatic arc that now forms one of earth s most extensive high p granulite facies belts the arthur river complex a dioritic to gabbroic suite in northern fiordland is part of the root of the arc and records an early cretaceous history of emplacement tectonic burial and high p granulite facies metamorphism that accompanied partial melting of the crust late random intergrowths of kyanite quartz and plagioclase partially pseudomorph minerals in the earlier high t assemblages of the arthur river complex indicating high p cooling of an over thickened crustal root by c 200 c the kyanite intergrowths are themselves partially pseudomorphed by paragonite commonly in the presence of phengitic white mica biotite plagioclase intergrowths that partially pseudomorph phengitic white mica and diopside plagioclase intergrowths that partially pseudomorph jadeitic diopside combined with published thermochronology results are consistent with later rapid decompression a short duration anticlockwise p t path may be explained by the high p juxtaposition of comparatively cool upper crustal rocks following their tectonic burial and under thrusting during the waning stages of early cretaceous orogenesis this was then followed by the decompression giving the rapid exhumation within 20 myr of peak metamorphism as suggested by the isotopic data

structure and composition of the aleutian island arc and implications for continental crustal growth we present results of a seismic reflection and refraction investigation of the aleutian island arc designed to test the hypothesis that volcanic arcs constitute the building blocks of continental crust the aleutian arc has the requisite thickness 30 km to build continental crust but it differs strongly from continental crust in its composition and reflectivity structure seismic velocities and the compositions of erupted lavas suggest that the aleutian crust has a mafic bulk composition in contrast to the andesitic bulk composition of continents the silicic upper crust and reflective lower crust that are characteristic of continental crust are conspicuously lacking in the aleutian intraoceanic arc therefore if island arcs form a significant source of continental crust the bulk properties of arc crust must be substantially modified during or after accretion to a continental margin the pervasive deformation intracrustal melting and delamination of mafic to ultramafic residuum necessary to transform arc crust into mature continental crust probably occur during arc continent collision or through subsequent establishment of a continental arc the volume of crust created along the arc exceeds that estimated by previous workers by about a factor of two

transformation of two pyroxene hornblende granulite to garnet granulite involving simultaneous melting and fracturing of the lower crust fiordland new zealand granulite facies gabbroic and dioritic gneisses in the pembroke valley milford sound new zealand are cut by vertical and planar garnet reaction zones in rectilinear patterns in gabbroic gneiss narrow dykes of anorthositic leucosome are surrounded by fine grained garnet granulite that replaced the host two pyroxene hornblende granulite at conditions of 750 uc and 14 kbar major and trace element whole rock geochemical data indicate that recrystallization was mostly isochemical the anorthositic veins cut contacts between gabbroic gneiss and dioritic gneiss but change in morphology at the contacts from the anorthositic vein surrounded by a garnet granulite reaction zone in the gabbroic gneiss to zones with a septum of coarse grained garnet surrounded by anorthositic leucosome in the dioritic gneiss the dioritic gneiss also contains isolated garnet grains enclosed by leucosome and short planar trains of garnet grains linked by leucosome partial melting of the dioritic gneiss mostly controlled by hornblende breakdown at water undersaturated conditions is inferred to have generated the leucosomes the form of the leucosomes is consistent with melt segregation and transport aided by fracture propagation limited retrogression suggests considerable melt escape dyking and melt escape from the dioritic gneiss are inferred to have propagated fractures into the gabbroic gneiss the migrating melt scavenged water from the surrounding gabbroic gneiss and induced the limited replacement by garnet granulite

crustal evolution by arc magmatism arc magmas generated at depths near 100 km by dehydration of subducting slabs are olivine rich melabasalts but the magmas that reach the surface in mature continental magmatic arcs have an average composition near that of rhyodacite depth varying fractionations and equilibrations profoundly modify the initial magmas plate tectonics has operated throughout proterozoic and phanerozoic time much as it does now and so many deeply eroded terrains must expose products of arc magmatism most exposed middle and deep continental crust consists of igneous rocks plus older rocks equilibrated at magmatic temperatures variably deformed and metamorphosed subsequently each crustal level displays a typical assemblage of magmatic rock types which are here deduced to be products mostly of arc magmatism pre arc mantle consists of depleted dunite and harzburgite rising arc magmas precipitate much additional olivine and add the equivalent of 10 or 15 of basalt in clinopyroxene orthopyroxene garnet and spinel the seismic m discontinuity may be controlled primarily by the shallow limit of the depth zone in which crystallization is mostly of ultramafic components and the fractionated magmas that reach the crust are mostly of gabbroic to granodioritic compositions the lower crust is characterized by differentiated layered complexes which often contain mafic or ultramafic basal cumulates medial floated plagioclase anorthosite and upper quartz poor pyroxene mesoperthite granites and by other magmatic rocks magmatic rocks are more voluminous than pre existing rocks supracrustal rocks are in middle granulite facies and much granitic material has been melted from them by magmatic heat the magmatically modified middle crust consists primarily of migmatites in lower granulite facies in the deeper part and upper amphibolite facies in the shallower part much dissociation of hydrous mineral assemblages is caused by the magmatic heat producing water rich aluminous magmas assimilation and anatexis the high water contents restrict rise of the equilibrated magmas voluminous pegmatites are expelled into the wall rocks and crystallization is forced sheets of two mica granites characterize the upper part of the middle crust the comparatively dry magmas that rise into the upper crust are mostly tonalite to adamellite these magmas spread out in steep sided batholiths above the migmatites erupt as ash flow sheets from calderas and produce voluminous far travelling volcanic ash inverted metamorphic gradients and outward verging structures are produced beneath the spreading batholiths magmatic arcs are extensional at all crustal levels

fractionation and incipient self granulitization during deep crust emplacement of lower ordovician valle fertil batholith at the gondwana active margin of south america abstract large granite batholiths were emplaced at the gondwana active margin during lower ordovician in south america these have contributed to crustal growth by net addition of silicic rocks to the continental crust new u pb shrimp zircon age determinations together with thermobarometric and geochemical data yield that batholith magma intrusion is the responsible of heating and self granulitization of early gabbro pulses partially molten granulitic gabbros which appear as either early intruded into the metasedimentary host or as large inclusions within the batholith forming qtz diorites contain opx bearing trondhjemitic leucosomes surrounding hbl opx pl mafic mesosomes forming typical agmatitic structures hornblende plagioclase equilibria applied to mineral pairs of granoblastic aggregates in textural equilibrium of metagabbro mesosomes yield temperatures in the range 850 910 c for core to core pairs and in the range 1000 1075 c for rim to rim pairs at pressures of about 0 7 gpa shrimp zircon age revealed that the whole batholith was emplaced over a narrow time interval of 20 ma from 465 to 485 ma with most ages clustered at about 470 ma the age of metagabbros is 473 7 ma for older zircons and 454 4 for younger zircons these ages are almost coincident within error with the age of host migmatites 477 5 ma and those of batholith intrusion of 476 9 ma and 475 3 ma for qtz diorites and 475 5 ma for granites zircon overgrowths of these intrusive rocks yield sages clustered around 450 ma revealing a protracted thermal history more complex than previously believed the geochemical study reveals that qtz diorites tonalites and granodiorites form a continuous trend produced by magmatic fractionation from a parental dioritic magma a weak adakitic tendency with sr y 15 in several samples implies the presence of grt in the source or magma chamber at a minimum pressure of 1 0 gpa higher than the depth of emplacement at 0 7 gpa the high temperature of magma emplacement which induced the incipient self granulitization of early magmatic pulses together with the cotectic like fractionation linking coeval qtz diorites tonalites and granodiorites is compatible with fractionation at the lower crust of a deep generated infracrustal sublithospheric intermediate magma

origin and evolution of multi stage felsic melts in eastern gangdese belt constraints from u pb zircon dating and hf isotopic composition this integrated study of whole rock geochemistry zircon u pb dating and hf isotope composition for seven felsic rocks from the nyingchi complex in eastern himalayan syntaxis has revealed a complex magmatic history for the eastern gangdese belt this involves multiple melt sources and mechanisms that uniquely identify the tectonic evolution of this part of the himalayan orogen our u pb zircon dating reveals five stages of magmatic or anatectic events 165 81 61 50 and 25 ma the jurassic granitic gneiss 165 ma exhibits ehf t values of 1 4 to 3 5 the late cretaceous granite 81 ma shows variable ehf t values from 0 9 to 6 2 indicating a binary mixing between juvenile and old crustal materials the paleocene granodioritic gneiss 61 ma has ehf t values of 5 4 to 8 0 suggesting that it originated from partial melting of a juvenile crustal material the eocene anatexis is recorded in the leucosome which has hf isotopic composition similar to that of the jurassic granite indicating that the leucosome could be derived from partial melting of the jurassic granite the late oligocene biotite granite 25 ma shows adakitic geochemical characteristics with sr y 49 3 56 6 the presence of a large number of inherited zircons and negative ehf t values suggest that it sourced from anatexis of crustal materials in contrast to the gangdese batholiths that are mainly derived from juvenile crustal source in central tibet the old crustal materials play an important role for the magma generation of the felsic rocks suggesting the existence of a crustal basement in the eastern gangdese belt these correspond to specific magmatic evolution stages during the convergence between india and asia the middle jurassic granitic gneiss resulted from the northward subduction of the neo tethyan oceanic slab the late cretaceous magmatism is probably related to the ocean ridge subduction the paleocene eocene magmatism metamorphism and anatexis are interpreted to result from roll back and break off of the subducted neo tethyan slab that occurred in the early stage of the india asian collision respectively the late oligocene adakitic rocks resulted from the break off of the subducted indian continental crust starting at 25 ma

hot orogens tectonic switching and creation of continental crust many granulite terrains were too hot to have formed during continental collision rather along with many high grade metamorphic terrains that typify continental crust most formed in accretionary orogens during tectonic switching when prolonged lithospheric extension was interrupted by intermittent transient contraction based on modern and ancient examples tectonic switching occurs when slab retreat induces upper plate extension causing arc splitting formation of microcontinent slivers and backarc basins then intermittent arrival of buoyant oceanic plateaus induces transient flat subduction or slab flip and crustal thickening during slab retreat basaltic magmas produced from decompressed asthenosphere advect into the extending orogen causing granulite facies metamorphism and granite generation but subsequent thickening during flat subduction cools the region thickening is focused in the thermally softened backarc region and if sediment filled a hot short lived 10 m y narrow 50 100 km orogenic belt forms such thickening is often misleadingly ascribed to arc or microcontinent collision once slab retreat mode is reestablished lithospheric extension recommences and a new arc backarc system forms generally outboard arrival of another plateau will reverse the procedure and another short lived hot orogen will form within the orogenic system cycles of tectonic switching efficiently produce continental crust

late cretaceous crustal growth in the gangdese area southern tibet petrological and sr nd hf o isotopic evidence from zhengga diorite gabbro abstract recent studies of gangdese granitic magmatism demonstrate a mantle contribution to crustal growth in southern tibet during the jurassic early eocene however the specific mechanism for adding such juvenile crust has been disputed owing to a lack of reliable evidence for contemporaneous deep mantle geodynamic processes here we report on the zhengga diorite gabbro suite from the gangdese area they consist of plagioclase labradorite and anorthite amphibole clinopyroxene biotite and minor magnetite epidote zircon and apatite la icp ms zircon u pb dating for two samples gives a ca 94 ma age for the zhengga intrusive rocks i e the late cretaceous apart from one high sio 2 52 2 wt diorite sample with slightly high k 2 o 1 72 wt and initial 87 sr 86 sr 0 7068 and low e nd t 5 6 values the gabbro samples are geochemically characterized by low sio 2 39 8 50 1 wt and k 2 o 0 3 1 1 wt strongly negative nb ta and positive sr anomalies and uniform initial 87 sr 86 sr 0 7043 0 7048 the gabbros can be divided into two groups group i gabbros with relatively low total rare earth element ree rb and sio 2 contents and positive eu anomalies and group ii gabbros with slightly higher total ree rb and sio 2 contents and negative eu anomalies the group i gabbros have e nd t 1 7 to 4 1 and zircon e hf t 6 5 to 11 1 and 18 o 5 89 to 7 24 values which are slightly different to those of the group ii gabbros 0 2 to 2 0 2 9 to 6 5 and 6 24 to 7 05 trace element compositions of amphibole and clinopyroxene grains suggest that the zhengga mafic magmas contained a significant fluid transported component probably released from subducted oceanic lithosphere we suggest that the parental magmas of the zhengga gabbros were generated by the hydrous partial melting of lithospheric mantle metasomatized by sediment melts fluids the group i gabbros were likely generated by the fractional crystallization of olivine or clinopyroxene from such parental magmas with insignificant crustal contamination whereas the group ii gabbros were probably produced by assimilation and fractional crystallization afc processes from mafic magmas that were geochemically similar to the group i gabbros pre collisional underplating of mantle derived mafic magmas likely played an important role in crustal growth and supplied the source materials for some late late cretaceous cenozoic granitoids of the gangdese batholiths this study also demonstrates that the hydrous partial melting of mantle wedge triggered by the dehydration in a subduction setting has a capacity to create significant volumes of juvenile continental crust

paleoproterozoic tectonic history of the cerbat mountains northwestern arizona implications for crustal assembly in the southwestern united states structural mapping metamorphic analysis and u pb geochronology from the cerbat mountains of northwestern arizona place new constraints on the tectonic significance and timing of the boundary between the mojave and yavapai proterozoic crustal provinces in the southwestern united states the cerbat mountains are the easternmost range that exhibits wholly mojave pb isotopic signatures yet their paleoproterozoic structural history is strikingly similar to that of the yavapai province two pervasive deformational events are recognized in the cerbat mountains d1 fabrics are well preserved in the northern cerbat mountains and are characterized by a north northwest striking moderately northeast dipping foliation s1 that is axial planar to macroscopic recumbent folds that we interpret to be west to southwest vergent similar structures have been documented locally in the yavapai province d1 is bracketed between 1740 and 1721 ma in the cerbat mountains on the basis of u pb zircon ages of the big wash granite 1737 4 3 ma and the diana and chloride granites 1719 1 2 and 1721 2 4 ma respectively d2 fabrics dominate the central and southern cerbat mountains and include a subvertical northeast striking foliation that truncates and transposes s1 d2 is bracketed between 1721 and 1682 ma on the basis of u pb ages of the diana and chloride granites and post d2 granite dikes in the eastern cerbat mountains 1682 5 2 4 ma the structural style orientation and timing of d1 and d2 fabrics in the cerbat mountains are identical to those of the yavapai province supracrustal rocks within the cerbat mountains were subjected to two periods of metamorphism that correspond temporally to the two periods of deformation the early phase occurred at 675 725 c and 5 6 kilobars the later phase occurred at 650 700 c and 3 5 4 5 kilobars we propose that the mojave and yavapai provinces were sutured together between ca 1740 and 1720 ma during regional d1 deformation collisional orogenesis in the eastern mojave province may have resulted in the tectonic burial of rocks in the cerbat mountains to depths of 20 km d2 between 1720 and 1682 ma corresponds to the widespread yavapai orogeny and is interpreted to record accretion of the amalgamated mojave yavapai terrane to proterozoic terranes that are now located in colorado

geochronology and geochemistry of high pressure granulites of the arthur river complex fiordland new zealand cretaceous magmatism and metamorphism on the palaeo pacific margin the arthur river complex is a suite of gabbroic to dioritic orthogneisses in northern fiordland new zealand the arthur river complex separates rocks of the median tectonic zone a mesozoic island arc complex from palaeozoic rocks of the palaeo pacific gondwana margin and is itself intruded by the western fiordland orthogneiss new shrimp u pb single zircon data are presented for magmatic metamorphic and deformation events in the arthur river complex and adjacent rocks from northern fiordland the arthur river complex orthogneisses and dykes are dominated by magmatic zircon dated at 136 129 ma a dioritic orthogneiss that occurs along the eastern margin of the complex is dated at 154 4 3 6 ma and predates adjacent plutons of the median tectonic zone rims on zircon cores from this sample record a thermal event at c 120 ma attributed to the emplacement of the western fiordland orthogneiss migmatitic palaeozoic orthogneiss from the arthur river complex 346 6 ma is interpreted as deformed wall rock very fine rims 5 20 lm also indicate a metamorphic age of c 120 110 ma a post tectonic pegmatite 81 8 1 8 ma may be related to phases of crustal extension associated with the opening of the tasman sea the arthur river complex is interpreted as a batholith emplaced at mid crustal levels and then buried to deep crustal levels due to convergence of the median tectonic zone arc and the continental margin

spectral ratio estimates for site effects on the horst graben system in west turkey recordings of micro and moderate size local earthquakes have been used to quantify site effects in the central west turkey which contains one of the world s best examples of a rapid intra continental extension with its high population and industrial potential we analyzed 436 earthquakes with local magnitudes ranging between 2 0 and 5 6 using three component digital recordings from 32 stations site functions were obtained using two different spectral ratio approaches horizontal to vertical spectral ratio hvsr and standard spectral ratio ssr hvsr estimates of transverse and radial s waves were compared with one another epicentral distance magnitude and back azimuth dependencies of site functions were also evaluated in general hvsr values from transverse and radial s waves are similar within a factor of 2 the back azimuth dependencies of transverse s wave hvsr results are more significant than distance and magnitude dependencies on the other hand averaging of transverse and radial s wave hvsr results eliminates systematic back azimuth dependencies caused by source radiation effects distributions of hvsr estimates along n s linear array which traversed main grabens in the region with a station spacing of 3 4 km reflect subsurface geological complexities in the region the sites located near the basin edges are characterized by broader hvsr curves broad hvsr peaks could be attributed to the complexity of wave propagation related to significant 2d 3d velocity variations at the sediment bedrock interface near the basin edges the results also show that even if the site is located on a horst the presence of weathered zones along the surface could cause moderate frequency dependent site effects comparison of hvsr and ssr estimates for the stations on the graben sites showed that ssr estimates give larger values at lower frequencies which could be attributed to lateral variations in regional velocity and attenuation values caused by basin geometry and edge effects

cretaceous tertiary shortening basin development and volcanism in central tibet the geologic map pattern of the qiangtang terrane in central tibet defines a 600 km long and up to 270 km wide east plunging structural culmination it is characterized by early mesozoic blueschist bearing melange and upper paleozoic strata in the core and mainly triassic jurassic strata along the limbs in the western qiangtang terrane 84 e the culmination is unconformably overlain by weakly deformed mid cretaceous volcanic flows and tuffs along the bangong suture to the south 32 n 84 e mid cretaceous nonmarine red beds and volcanic rocks lie unconformably on jurassic suture zone rocks these relationships demonstrate that west central tibet was above sea level during the mid cretaceous and experienced significant denudation prior to mid cretaceous time growth of the qiangtang culmination is inferred to have initiated during southward emplacement of a thrust sheet of early mesozoic melange and upper paleozoic strata during the early cretaceous lhasa qiangtang collision the north south width of the inferred thrust sheet provides a minimum slip estimate of 150 km at 84 e decreasing eastward to 70 km at 87 e paleogene deformation in the qiangtang terrane is characterized by widely distributed mainly north dipping thrust faults that cut eocene oligocene red beds and volcanic rocks in their footwall along the bangong suture the north dipping shiquanhe gaize amdo thrust system cuts 64 and 43 m y old volcanic tuffs in its footwall and accommodated 40 km of post mid cretaceous shortening the tertiary south dipping gaize siling co backthrust bounds the southern margin of the bangong suture and marks the northernmost limit of mid cretaceous marine strata in central tibet cretaceous deformation and denudation in central tibet is attributed to northward underthrusting of the lhasa terrane beneath the qiangtang terrane along the bangong suture this model implies that 1 cretaceous strata along the bangong suture and in the northern lhasa terrane were deposited in a flexural foreland basin system and derived at least in part from the qiangtang terrane and 2 the central tibetan crust was thickened substantially prior to the indo asian collision although its magnitude is poorly known tertiary shortening in the qiangtang terrane is more prevalent than in the lhasa terrane this difference may be attributed to the presence of underthrust melange in the deeper central tibetan crust which would have made it weaker than the lhasa terrane during the indo asian collision

the origin and pre cenozoic evolution of the tibetan plateau different hypotheses have been proposed for the origin and pre cenozoic evolution of the tibetan plateau as a result of several collision events between a series of gondwana derived terranes e g qiangtang lhasa and india and asian continent since the early paleozoic this paper reviews and reevaluates these hypotheses in light of new data from tibet including 1 the distribution of major tectonic boundaries and suture zones 2 basement rocks and their sedimentary covers 3 magmatic suites and 4 detrital zircon constraints from paleozoic metasedimentary rocks the western qiangtang amdo and tethyan himalaya terranes have the indian gondwana origin whereas the lhasa terrane shows an australian gondwana affinity the cambrian magmatic record in the lhasa terrane resulted from the subduction of the proto tethyan ocean lithosphere beneath the australian gondwana the newly identified late devonian granitoids in the southern margin of the lhasa terrane may represent an extensional magmatic event associated with its rifting which ultimately resulted in the opening of the songdo tethyan ocean the lhasa northern australia collision at similar to 263 ma was likely responsible for the initiation of a southward dipping subduction of the bangong nujiang tethyan oceanic lithosphere the yarlung zangbo tethyan ocean opened as a back arc basin in the late triassic leading to the separation of the lhasa terrane from northern australia the subsequent northward subduction of the yarlung zangbo tethyan ocean lithosphere beneath the lhasa terrane may have been triggered by the qiangtang lhasa collision in the earliest cretaceous the mafic dike swarms ca 284 ma in the western qiangtang originated from the panjal plume activity that resulted in continental rifting and its separation from the northern indian continent the subsequent collision of the western qiangtang with the eastern qiangtang in the middle triassic was followed by slab breakoff that led to the exhumation of the qiangtang metamorphic rocks this collision may have caused the northward subduction initiation of the bangong nujiang ocean lithosphere beneath the western qiangtang collision related coeval igneous rocks occurring on both sides of the suture zone and the within plate basalt affinity of associated mafic lithologies suggest slab breakoff induced magmatism in a continent continent collision zone this zone may be the site of net continental crust growth as exemplified by the tibetan plateau c 2012 international association for gondwana research published by elsevier b v all rights reserved

isotopic evidence for rapid continental growth in an extensional accretionary orogen the tasmanides eastern australia the trace element signature of earth s continental crust resembles that of arc lavas but the continents may have formed during ancient igneous pulses that are hard to reconcile with supra subduction zone magmatism we explore the role of coupled arc back arc accretionary processes in crust generation by considering the tectonic context of whole rock nd isotope and zircon hf o isotope data from igneous rocks of the australian tasmanides 515 230 ma which is thought to have evolved by the repeated opening and closure of sediment filled back arc basins behind a long lived subduction zone the significance of this process for continental crust formation has yet to be evaluated from an isotopic perspective granitic rocks in this area define striking secular end ehf 18o trends that correlate with the pattern of deformational events and register changes in magma source during tectonic activity these trends reveal that juvenile magmatic input was enhanced during extensional back arc rifting episodes that immediately followed crustal thickening suggesting a relationship between slab rollback and continental growth interaction between juvenile magma and sedimentary units deposited during a preceding back arc rifting cycle was integral to the formation of stable continental material this highlights the importance of back arc environments for both the generation and differentiation of continental crust the juvenile component within the tasmanide igneous rocks increased from the cambrian to the triassic consistent with a diminished input from cratonderived metasedimentary material as the subduction zone migrated outboard of the gondwana margin subduction zone retreat formed large tracts of new crust in eastern australia at comparable rates to crust generation at modern island arcs providing a mechanism for rapid continental growth at convergent margins using isotopic tracers to link tectonic evolution and crust generation in modern and ancient orogens can lead to a better understanding of the geodynamic controls on planetary differentiation

partitioning of nb and ta between rutile and felsic melt and the fractionation of nb ta during partial melting of hydrous metabasalt abstract in order to fully assess the role of rutile in fractionation of nb ta during partial melting of hydrous metabasalt we have measured rutile felsic melt partition coefficients d values for nb and ta with tonalitic to trondhjemitic compositions at 1 5 3 5 gpa 900 1350 c and 5 0 20 wt h2o dnb dta and dnb dta range from 17 1 to 246 13 34 2 to 232 25 and 0 51 0 04 to 1 06 0 13 respectively for the compositions investigated melt composition appears to have no observable effect on the partitioning the effect of pressure is also slight whereas temperature and h2o have marked effects dnb dta and dnb dta increase with decreasing temperature and h2o content showing a reversal of dnb dta from 1 0 using the data that approached equilibrium and obeyed henry s law expressions describing the dependences of dnb dta and dnb dta on temperature pressure and melt h2o content were obtained 1 ln d nb rutile melt 2 846 0 453 9621 470 t 0 207 0 101 p 0 042 0 009 h 2 o 2 ln d ta rutile melt 0 775 0 398 6954 413 t 0 140 0 089 p 0 009 0 007 h 2 o 3 ln d nb d ta rutile melt 2 075 0 289 2657 301 t 0 075 0 065 p 0 033 0 006 h 2 o where t is in k p is in gpa and h2o are in wt these expressions are applicable to calculate nb and ta partitioning coefficients between rutile and felsic highly siliceous melts and nb ta fractionation during partial melting of metabasalt the derived models suggest that fluid present partial melting of metabasalt always leads to dnb dta lower than 1 0 whereas dehydration melting may lead to dnb dta higher than 1 0 at temperatures