magma sources for mesozoic anorogenic granites of the white mountain magma series new england usa the magma sources for granitic intrusions related to the mesozoic white mountain magma series in northern new england usa are addressed relying principally upon nd isotopes many of these anorogenic complexes lack significant volumes of exposed mafic lithologies and have been suspected of representing crustal melts sm nd and rb sr isotope systematics are used to evaluate magma sources for 18 felsic plutons with ages ranging from about 120 to 230 ma the possibility of crustal sources is further examined with analyses of representative older crust including paleozoic granitoids which serve as probes of the lower crust in the region multiple samples from two representative intrusions are used to address intrapluton initial isotopic heterogeneities and document significant yet restricted variations 1 in end overall mesozoic granite plutons range in end t from 4 2 to 2 3 with most 2 to 0 and in initial 87sr 86sr from 0 7031 to 0 709 the isotopic variations are roughly inversely correlated but are not obviously related to geologic geographic or age differences older igneous and metamorphic crust of the region has much lower nd isotope ratios with the most radiogenic paleozoic granitoid at end 180 ma of 2 8 these data suggest mid proterozoic separation of the crust in central northern new england moreover the bulk of the mesozoic granites cannot be explained as crustal melts but must have large mantle components the ranges of nd and sr isotopes are attributed to incorporation of crust by magmas derived from midly depleted mantle sources crustal input may reflect either magma mixing of crustal and mantle melts or crustal assimilation which is the favored interpretation the results indicate production of anorogenic granites from mantle derived mafic magmas

depleted mantle source for the ulungur river a type granites from north xinjiang china geochemistry and nd sr isotopic evidence and implications for phanerozoic crustal growth abstract the ulungur river granites from north xinjiang china display two petrological trends 1 a peralkaline trend composed of alkali amphibole and alkali pyroxene bearing granites and dykes and 2 an aluminous trend represented by alkali granites monzogranites and aluminous granite dykes within the peralkaline granites rb sr whole rock isochron dating yielded an age of 300 ma for the emplacement of the peralkaline granites and 270 ma for the aluminous granites the ulungur river granites share all the geochemical features common to a type granites of the world the ulungur river granites are post orogenic and emplaced in a post collisional extensional environment the peralkaline and aluminous rocks have indistinguishable nd isotopic compositions they have unusually positive and relatively uniform nd t values ranging from 5 1 to 6 7 for the peralkaline rocks at 300 ma and from 5 5 to 5 9 for the aluminous rocks at 270 ma the initial 87sr 86sr ratios cannot be determined precisely by the whole rock isochron technique mainly because most samples are highly radiogenic and have high rb sr ratios in addition the large variation in sr isotopic compositions can partly be explained by crustal contamination during magma differentiation the nd isotopic data clearly indicate that both the peralkaline and aluminous rocks were derived from a long lived depleted mantle reservoir the formation of the ulungur river a type granites is hypothesized as follows 1 a depleted mantle was first metasomatised then subjected to partial melting leading to the generation of basic magmas now represented by the kalatongke mafic ultramafic complex in the ulungur river area 2 the basic magmas have undergone high degrees of fractional crystallisation with separation of biotite hornblende plagioclase potassium feldspar and other accessory phases such as apatite and ilmenite with the residual magmas forming the peralkaline granites during the magmatic differentiation the liquids were probably contaminated by crustal materials as evidenced by nb depletion and nd sr isotopic systematics on the other hand the formation of aluminous granites involved a replenishment of new mantle derived magmas into the differentiating magma chamber and followed by fractional crystallisation with retention of hydrous phases hornblende and biotite the petrological evolution of the ulungur river granites is thus from peralkaline to calc alkaline and not from calc alkaline to peralkaline as more often observed elsewhere the ulungur river granites in common with the voluminous granitoids occurring in the central asian orogenic belt represent a significant juvenile addition of the continental crust in the phanerozoic

the formation of the suyunhe large porphyry mo deposit in the west junggar terrain nw china zircon u pb age geochemistry and sr nd hf isotopic results abstract newly discovered suyunhe mo deposit 0 57 mt mo metal located in the barluk mountains of the west junggar terrain xinjiang nw china is the largest porphyry mo deposit discovered in xinjiang so far the suyunhe mo deposit is associated with the suyunhe intrusive complex which was emplaced into the volcano sedimentary sequences of the middle devonian barluk group the suyunhe complex consists of the deep granite intrusion and the shallow granite porphyry granodiorite porphyry monzonitic granite porphyry and tonalite porphyry stocks which are cut by the felsic dikes dominant mo mineralization is hosted in the barluk group volcano sedimentary rocks but is minor in the granite granite porphyry granodiorite porphyry and monzonitic granite porphyry of the suyunhe complex we report in situ zircon u pb ages whole rock geochemistry sr nd isotopic and zircon lu hf isotopic data for the ore bearing intrusive rocks from the suyunhe deposit secondary ion mass spectrometry sims zircon u pb dating indicates that the deep granite emplaced at 298 4 1 9 ma 295 3 3 ma and the shallow granodiorite porphyry intruded during 294 7 2 1 ma 293 7 2 3 ma the suyunhe granite intrusion including fine and medium grained granite belongs to the high k calc alkaline series most of them have low zr nb y ce 2 7 values showing a geochemical affinity to i type granite these granites are enriched in light rare earth elements lree and depleted in heavy rare earth elements hree and eu ba sr nb p and ti indicating that they have experienced advanced fractional crystallization they have low initial 87sr 86sr ratios 0 70318 0 70380 high whole rock end t values 4 4 to 6 0 high zircon ehf t values 10 8 to 14 6 and young whole rock nd and zircon hf model ages they also show low mg 0 35 these features demonstrate that the ore bearing granite formed by partial melting of the juvenile lower crust and experienced advanced fractional crystallization the suyunhe granite porphyry and monzonitic granite porphyry are compositionally similar to the granite indicating that they share a common magma source and process with the granite the suyunhe granodiorite porphyries belong to slightly high k calc alkaline series and have lree enrichment and hree nb and ti depletion they have low 87sr 86sr i ratios 0 70421 0 70444 high end t values 4 9 to 6 2 high zircon ehf t values 9 6 to 15 2 and young whole rock nd and zircon hf model ages they also show low mg

geochemical and geochronological studies of granitoid rocks from the western tianshan orogen implications for continental growth in the southwestern central asian orogenic belt abstract the western tianshan orogen in northwestern china is part of the southwestern margin of the central asian orogenic belt caob which represents the largest tectonic assembly of accretionary complexes and the most significant area of phanerozoic continental growth in the world granitoid rocks have a widespread occurrence and occupy about 30 of the whole exposure of the orogen ages obtained for the granitoids vary from 896 ma to 247 ma while initial 87 sr 86 sr ratios vary from 0 70329 to 0 72070 and e nd values from 14 1 to 7 3 the occurrence of neoproterozoic granitic gneisses implies the presence of precambrian basement in this area during oceanic subduction continental growth occurred as a result of several combined processes i e by addition of oceanic crustal melts adakites the intrusion of basaltic magmas derived by partial melting of the metasomatized depleted mantle wedge and the upwelling of granitic magma derived from a mixed source of basaltic magmas and old continental basement however during the post collisional period vertical accretion of underplated juvenile mantle material may have been accomplished during slab breakoff delamination this study suggests a two stage model of continental growth of syn subduction lateral accretion of arc complexes and post collisional vertical accretion of underplated mantle material which may also be relevant for the western segment of the caob in the phanerozoic the reworking and involvement of old crust are more obvious in the western tianshan orogen compared to other parts of the caob comprising the western junggar eastern junggar alataw and altay terranes

revisiting the irtish tectonic belt implications for the paleozoic tectonic evolution of the altai orogen abstract the irtish tectonic belt is one of the most important suture zones in the central asian orogenic belt caob the various models invoked to explain the tectonic evolution of this belt in relation to paleozoic continental growth include 1 syn subduction strike slip duplication of a single long lived arc and 2 collision of many arcs and micro continents here we report new results from detailed field studies geochemistry and zircon u pb geochronology on the major rock units along the irtish belt both in kazakhstan and in the chinese altay our new results together with information from published data suggest that the altai mongolia terrain witnessed several episodes of subduction the junggar terrain which formed by multiple collisions of intra oceanic arcs seamounts and ophiolite suites prior to 330 ma subsequently amalgamated with the altai mongolia terrain at around 320 ma along the irtish suture zone following the clockwise rotation of the siberian craton and the n directed movement of the junggar terrain the sinistral strike slip motion along the irtish suture zone resulted in ductile shearing partial melting of the crustal material and the formation of mylonites syn tectonic migmatite and gneissic granites the sinistral strike slip deformation of the irtish suture zone culminated at ca 275 ma the diverse 275 250 ma magmatism mafic ultramafic complexes bimodal intrusives mafic and silicic dykes a type and i a type granites is related to magma pulses generated by the permian tarim mantle plume our model of multiple episodes of subduction below the altai arc is different from previous models and proposes that the subduction continued until triassic with the permian subduction under the altai arc and a late permian large scale south directed thrusting

sedimentological and geochronological constraints on the carboniferous evolution of central inner mongolia southeastern central asian orogenic belt inland sea deposition in a post orogenic setting handling editor s j liu keywords central asian orogenic belt central inner mongolia carboniferous sedimentary detrital zircon sedimentological and geochronological analyses were performed on carboniferous strata from central inner mongolia china to determine the tectonic setting of the southeastern central asian orogenic belt caob sedimentological analyses indicate that the widespread late carboniferous strata in central inner mongolia were dominated by shallow marine clastic carbonate deposition with basal conglomerate above the precambrian basement and early paleozoic orogenic belts based on lithological comparison and fossil similarity five sedimentary stages were used to represent the carboniferous deposition the depositional stages include from bottom to top 1 basal molassic 2 first carbonate platform 3 terrigenous with coeval intraplate volcanism 4 second carbonate platform and 5 post carbonate terrigenous these five stages provide evidence for an extensive transgression in central inner mongolia during the late carboniferous detrital zircon geochronological studies from five samples yielded five main age populations 310 ma 350 ma 400 450 ma 800 1200 ma and some meso proterozoic to neoarchean grains the detrital zircon geochronological studies indicate that the provenances for these late carboniferous strata were mainly local magmatic rocks early paleozoic arc magmatic rocks and carboniferous intrusions with subordinate input of precambrian basement combining our sedimentological and provenance analyses with previous fossil comparison and paleomagnetic reconstruction an inland sea was perceived to be the main paleogeographic feature for central inner mongolia during the late carboniferous the inland sea developed on a welded continent after the collision between north china craton and its northern blocks

a new method for combining gravimetric and geological data abstract application of the gravity method to structural geology makes it possible to highlight local variations of the gravitational field that are caused by density heterogeneities related to the composition and structure of the crust these density heterogeneities and their boundaries which correspond to gravity discontinuities can be identified using the properties of the vertical and horizontal derivatives of the field gravity data on the huelgoat granite in the armorican massif france were subjected to a new two step geological analysis first the gravity discontinuities derived from bouguer anomaly data were automatically identified and second these discontinuities were combined with existing geological data the first step was made possible by a software program that defines and visualizes the gravity discontinuities through an automatic analysis of the horizontal and vertical gravity gradient functions the result is a map on which gravity discontinuities separate areas of different densities the vertical and horizontal derivatives of the field were computed from the gravity data using frequency domain transformations the second step corresponds to the geological interpretation of the gravity discontinuity map a petrostructural map is used which mainly shows the structure and the syntectonic internal fabric of the area along with fracture data obtained on the granite and its host rocks the latter illustrates late brittle deformation the petrostructural map is used to adjust and rearrange the gravity discontinuities in order to obtain a gravity image deduced from the fabric of the rocks a fabric gravity image after this a fracture gravity image is derived by using the fracture data as a filter to obtain priority directions in the gravity discontinuities superimposing the two images provides a structural sketch map that gives a more integrated insight into the ductile and brittle deformation of the area than can be obtained from field observations alone

continental growth of northwest china the northwest corner of china is the most interior part of central asia and includes parts of the siberian tarim and kazakhstan plates where the junggar and tarim basins are separated by the east west trending tianshan range a preliminary terrane map divides the area into thirteen separate areas which are classified into four types l continental 2 oceanic crust 3 island arc 4 composite the boundaries for these terranes are faults that have been previously defined either by geologic mapping or holocene activity passive consolidation of the southern margin of paleo asia is marked by accretion and subduction of the paleo tethys oceanic basins and by development of volcanic arcs paleomagnetic data indicate that the major plates and associated smaller terranes did not reach final consolidation until the permian or later the stratigraphic record reveals development of intracontinental basins in the late carboniferous followed by the pan asian thermal event that gave rise to widespread intrusion of a type granites of late paleozoic age the frontal collision of india along the southern border of asia in the paleocene reactivated nearly all of the older major strike slip and thrust faults formed during the late paleozoic consolidation of paleo asia the complex nature of the continental growth of central asia has produced and destroyed many varieties of sutures and reconstruction of the original plate and terrane configurations therefore is enigmatic

aluminous subsolvus anorogenic granite genesis in the light of nd isotopic heterogeneity the problem of the genesis of anorogenic granites is far from being resolved and this may result from their petrogenetic variability several kinds of a type granites are indeed present in many anorogenic provinces a peralkaline and an aluminous trend commonly occur and the latter often comprises hypersolvus and subsolvus granites as an example of the aluminous subsolvus group we have studied major trace elements and nd sr isotopes of the tana granite sw corsica france along with the coeval mafic and intermediate rocks petrological and trace element constraints suggest that the associated mafic rocks are not representative of the magmatic precursor of the granite rather the intermediate rocks monzonites are more akin to the parental magma that yielded the granite through fractional crystallization the aluminous subsolvus anorogenic granite and related monzonite are characterized by variable nd isotopic compositions ndi from 4 7 to 1 3 and from 2 to 0 1 respectively while sr isotopes give good isochrons it is argued that the nd isotopic heterogeneity was not acquired at a late stage in the upper crust by contamination from partial crustal melts or f rich late to post magmatic fluids similarly the lack of correlation between trace element concentrations and nd isotopes in a single granitic body precludes a process of crustal contamination of mantle melts instead it is inferred that these low and very heterogeneous ndi s were probably inherited from the magma source region in the lower crust the preservation of this heterogeneity during magma ascent suggests that only limited mechanical and chemical mixing occurred in magma chambers it is more likely that separate batches of low viscosity a type magmas ascended individually mainly through fracture propagation in an extensional regime the possible derivation of granites from monzonites through fractional crystallization together with the examination of the corsican lower crust composition and already published dehydration melting experiments suggest that the aluminous subsolvus trend of a type granites may be generated through the partial melting of metamorphosed amphibole bearing and k2o rich mafic cumulates that have not undergone a previous partial melting event

petrogenesis and tectonic implications of neoproterozoic highly fractionated a type granites from mianning south china abstract the origin of neoproterozoic intrusions ca 860 750 ma along the western part of the yangtze craton has been the subject of debate in recent years with two competing models proposed the plume model argues for an extensional setting and emphasizes the role of a superplume in the rodinia breakup whereas the arc model argues for the presence of a subduction zone in the yangtze craton as a contribution to this animated dispute geochronologic and geochemical analyses have been carried out on the mianning granite which is the largest pluton 700 km2 in the northern kangdian rift of the western yangtze craton it is shown that the mianning granites were emplaced at ca 780 ma and display highly fractionated feature i e sio2 75 wt eu eu 0 03 0 50 enrichment of k rb th u zr hf y and rees depletion of nb ta ba sr p eu and ti they are metaluminous to strongly peraluminous a cnk 0 93 1 55 and contain abundant perthite and minor alkali riebeckite and sphene sharing the petrological and geochemical characters of a2 type granites positive nd t 2 97 5 24 and zircon hf t 9 2 12 1 values are consistent with a derivation by partial melting of a relatively young crust formed about 1000 900 ma given the general absence of a type granites in arc settings the mianning a type granites are suggestive of an anorogenic crustal extensional environment for the western yangtze craton during the neoproterozoic the data presented in this study are therefore consistent with an intracontinental rift model but are not sufficient to identify plume involvement in the neoproterozoic magmatism

the geochemical characteristics of granitoids in contrasting arcs and comments on magma sources geochemical variations in modern subduction related igneous suites with respect to arc maturity in time and space are illustrated using data for both volcanic suites basalt andesite and dacite and intrusive granitoid suites diorite tonalite granodiorite and granite from circum pacific arcs using trace element data we suggest that two groups of processes control the natural variation in the parental magmas of both suites a subduction zone enrichment of lithospheric mantle locally coupled with crustal assimilation allied with fractional crystallization afc in zones of thickened crust all of which yield magmas with enhanced concentrations of the large ion lithophile lil elements k rb th u lrees etc b with increasing distance from the active trench contributions from within plate sub continental lithosphere producing mantle derived magmas with enhanced levels of high field strength hfs elements among which nb ta hf and y are particularly distinctive thus even for the evolved granitoids of intrusive arc series ratios of hfs lil elements not significantly affected by crystal fractionation such as ta nb k rb la may throw some light on the origin of mafic intermediate precursor magmas in terms of these elements we suggest the following groupings for interpreting the tectonic associations of granitoid suites 1 primitive calcic arc granitoids with low lil and hfs element abundances 2 normal calc alkaline continental arc granitoids with enhanced lil element abundances and low hfs lil ratios 3 mature alkali calcic arc granitoids with high levels of lil and hfs elements and higher hfs lil ratios 4 back arc anorogenic alkaline granitoids with the highest levels of hfs elements

magmatism of the eastern red sea margin in the northern part of yemen from oligocene to present the northern part of yemen during the last 30 ma has been the site of an intense magmatic activity which can be divided into three main episodes the first episode lasted from 30 to 18 ma and was characterized by two climaxes huge flood basalt eruptions between 30 to 26 ma were succeeded by massive ignimbrite sheet deposits and intrusion of silicic subvolcanic masses 22 to 20 ma ago contemporaneously along the present day western margin of the yemen plateau a basaltic dike swarm was emplaced additionally alkaline and peralkaline granites were intruded at the end of the first climax and throughout the second after a long quiescence magmatic activity occurred again in late miocene time about 10 ma with extrusion of lava flows in scattered areas e g al harf wadi as sirr maswar al hada jabal an nar the third magmatic episode has been concentrated in three well defined areas it began about 6 5 ma ago in the dhamar rada area and later in the san a amram and sirwah marib areas new chemical and isotopic data on the magmatic products of north yemen indicate that extensive crustal contamination together with simple crystal fractionation processes have been operating during the magmatic evolution the most primitive products of each of the three episodes share isotopic and petrological features suggesting a similar mantle source isotope geochemistry indicates an important asthenospheric component in the mantle source for the magmas of all three episodes finally a comparison between the coeval igneous rocks of the yemen and ethiopia plateaus is attempted

nd isotope evidence for crust mantle interaction in the generation of a type granitoid suites in labrador canada early proterozoic a type granite suites in labrador northeastern canada have initial nd isotopic compositions that lie between those of a depleted mantle reservoir and the local basement rocks one of these a type granite suites consists of a linear array of discrete plutons that crosses the boundary between an archean craton and an adjoining early proterozoic crustal block the nd of these granites at formation t 1720 ma changes from 8 to 4 across this boundary reflecting changes in the nature and antiquity of the continental crust however in both domains the granites cannot be derived entirely by anatexis of the local basement rocks and apparently require significant amounts of juvenile mantle derived material a type granites have been widely interpreted as anatectic derivatives of lower crustal rocks that were depleted by previous melt extraction the nd isotopic data suggest that these labrador examples were instead mixtures of mantle derived magmas and overlying lower crustal rocks and that new material may have been the volumetrically dominant component 50 90 in most cases as an alternative to the prevalent residual source model it is proposed that the distinctive geochemical characteristics of these a type granites are a function of the high liquidus temperatures and primitive mg rich character of subcrustal magma fluxes in post orogenic and anorogenic environments where such magmas are emplaced into stabilized continental crust further nd isotope studies of other a type granite suites are required to ascertain if models invoking crust mantle interaction are more widely applicable

isotopic studies bearing on the tectonics of the west junggar region xinjiang china isotopic investigations from the hercynian age fold belt between the kazakhstan and siberian cratons in the west junggar region determine the timing of tectonic evolution and the petrogenesis of the granitic rocks of the region sphene from the leucogabbro phase of the tangbale ophiolite melange the oldest member of the ophiolite sequences in the fold belt yields an isotopic pb pb age of 523 2 7 2 ma zircon from a postcollision alkali granite yields slightly discordant isotopic u pb ages that indicate magma crystallization at 321 4 6 7 ma dating it in the lower carboniferous period radiometric dating thus documents a time span of circa 200 ma for igneous activity in the area petrogenetic studies were made to test whether precambrian crustal rocks might underlie the junggar sedimentary basin initial lead isotope ratios determined from potassium feldspars from five alkali granites show clear affinity with ratios from mid ocean ridge basalts in pb isotope correlation diagrams sm nd data from sphene and apatite from one of the granites yield an initial end t 6 1 the granite sources are depleted mantle rocks of oceanic affinity that show no involvement of recycled aged granitic crustal rocks

petrogenesis and tectonic setting of the tuyeh darvar granitoid northern iran constraints from zircon u pb geochronology and sr nd isotope geochemistry abstract tuyeh darvar granitoid which outcrop ca 45 km sw of damghan city in the eastern alborz zone comprise mainly the pluton emplaced into the barut formation of lower cambrian ages zircon u pb ages show carboniferous ages 325 3 ma for the formation of this granitoid the granitoid is mostly metaluminous ferroan and alkalic monzonite to monzodiorite these rocks have high values of feot mgo and ga al high concentrations of k2o na2o low abundances of mgo and transitional elements plots normalized to chondrite and primitive mantle compositions show strong enrichments of lree relative to hree and of lile relative to hfse accompannied by negative anomalies of nb and sr they contain fe rich hydrous mafic minerals and magnetite these features are typical of a type granites sr nd isotopic geochemistry with initial nd values from 1 1 to 1 5 and initial 87sr 86sr ratios between 0 70562 and 0 70678 are consistent with magmatic differentiation from mafic melts produced from an enriched mantle source however other models such as melting of mafic crust or mixing of components from depleted mantle and continental crust cannot be discarded on the basis of the u pb zircon age 325 3 ma and the known magmatic tectonic regime in iran during the paleozoic it is suggested that the pluton formed in a rift environment related to extensional structures of the alborz block in early carboniferous time

lithospheric structure in the southern canadian cordillera from a network of seismic refraction lines lithospheric velocity structure and its relationship to regional tectonics and development of the southern canadian cordillera are derived from a synthesis of interpretations from nine in line seismic refraction wide angle reflection profiles and broadside data recorded during the lithoprobe southern cordillera refraction experiment score and other refraction experiments across southern british columbia and one profile in northwestern washington consistency of the score two dimensional models at their intersection positions is achieved through application of a simultaneous inversion of all relevant traveltime data the cross sectional and map presentations demonstrate the strong degree of three dimensional heterogeneity within the crust and upper mantle a first order characteristic is the continuous increase in crustal velocities westward from the foreland belt to the insular belt the variations do not correlate with the morphogeological belts they do correspond with large scale geological and or