temporal spatial distribution and tectonic implications of the batholiths in the gaoligong tengliang yingjiang area western yunnan constraints from zircon u pb ages and hf isotopes abstract considerable progress has recently been made regarding temporal and spatial distribution of magmatism in the lhasa terrane however the eastward and southeastward correlation of these tibetan magmatic suites in western yunnan and burma remains poorly constrained this paper reports zircon u pb dating and hf isotopic compositions of granites in the gaoligong tengliang yingjiang area west yunnan it reveals three episodes of plutonism and more importantly a southwestward magmatic migration the gaoligong batholiths in the northeast were mainly emplaced during early cretaceous 126 121 ma and comprised predominantly s type granites with negative zircon ehf values ehf 2 12 the tengliang granites situated southwest of the gaoligong belt were emplaced in late cretaceous 68 76 ma and also displayed a strong peraluminous affinity and negative ehf 5 14 indicating a provenance from a proterozoic sedimentary source with little mantle contribution the youngest phase of magmatism 52 66 ma occurred in yingjiang southwestmost of the study area it is composed of s type granites ehf 2 12 in east yingjiang and i type granites ehf 4 6 in west yingjiang near the china burma border the late cretaceous early cenozoic plutons in the tengliang and yingjiang area are thus considered as the northern continuation of the late cretaceous magmatic arc west which comprises i type granites and andesitic rocks and of the belt of predominant s type granites east in burma thailand and malaysia such a chemical polarity of the dual i type and s type granites is strongly reminiscent of the northern american cordillera indicating a cordilleran style continental margin during the late cretaceous early cenozoic while the magmatic arc was related to eastward subduction of the neo tethys beneath the asian continent the s type granites represented the melting products of thickened crust in the hinterland in response to subduction induced decrease in lithospheric strength and compressive plate convergence forces and to a less degree to the collision between burma and sundaland blocks the gaoligong early cretaceous granites which bear strong similarities in lithology geochemistry and emplacement age to those in the northern magmatic belt in the lhasa terrane are also the magmatic expression of crustal thickening this crustal thickening may have stemmed from the collision between the lhasa block and the qiangtang block in late jurassic and early cretaceous the magmatism in western yunnan thus recorded a long term subduction of the neo tethyan plate enhanced by continental collisions at different time

forty years of ttg research abstract ttgs tonalite trondhjemite granodiorite are one of the archetypical lithologies of archaean cratons since their original description in the 1970s they have been the subject of many studies and discussions relating to archaean geology in this paper we review the ideas concepts and arguments brought forward in these 40 years and try to address some open questions both old and new the late 1960s and the 1970s mark the appearance of grey gneisses ttg in the scientific literature during this period most work was focused on the identification and description of this suite and the recognition that it is a typical archaean lithology ttgs were already recognised as generated by melting of mafic rocks this was corroborated during the next decade when detailed geochemical ttg studies allowed us to constrain their petrogenesis melting of garnet bearing metamafic rocks and to conclude that they must have been generated by archaean geodynamic processes distinct from their modern counterparts however the geodynamic debate raged for the following 30 years as many distinct tectonic scenarios can be imagined all resulting in the melting of mafic rocks in the garnet stability field the 1990s were dominated by experimental petrology work a wealth of independent studies demonstrated that melting of amphibolites as well as of mafic eclogites can give rise to ttg liquids whether amphibolitic or eclogitic conditions are more likely is still an ongoing debate from 1990s onwards one of the key questions became the comparison with modern adakites as originally defined these arc lavas are reasonably close equivalents to archaean ttgs pending issues largely revolve around definitions as the name ttg has now been applied to most archaean plutonic rocks whether sodic or potassic irrespective of their hree contents this leads to a large range of petrogenetic and tectonic scenarios a fair number of which may well have operated concurrently but are applicable only to some of the rocks lumped together in the ever broadening ttg bin

plutonic rock types series discrimination of various granitoid series and related rocks abstract the streckeisen qap diagram discriminates four characteristic type series among the large variety of granitoids associated in intrusions three of them integrate varied compositions from basic to acid terms 1 the calcalkaline series with its low k or trondhiemite medium k or granodioritic high k or monzonitic variants 2 the alkaline series with its characteristic syenitic members 3 the tholeiitic series deprived of k feldspars the fourth 4 corresponds to mobilizates associated with migmatites of varied composition but always quartz rich the generalization in the qapf system allowing the plotting of undersaturated alkaline series provides an excellent visual display of the major associations and trends for the plutonic rocks

radiolarian age constraints on mesotethyan ocean evolution and their implications for development of the bangong nujiang suture tibet radiolarian discoveries indicate that deep marine conditions prevailed in central tibet from the early middle jurassic until well into the early cretaceous late hauterivian early aptian 131 121 ma and help to constrain the temporal extent of oceanic conditions along the bangong nujiang suture these new fossils occur in chert or siliceous mudstone blocks associated with the lagkor tso ophiolitic melange basin inversion associated with closure of mesotethys between the qiangtang and lhasa terranes was accompanied by melange formation and predated regionally widespread deposition of overlying shallow marine late aptian albian orbitolinid limestones

dissolution of tonalitic enclaves in ascending hydrous granitic magmas an experimental study dissolution of natural mafic magmatic enclaves in a hydrous leucogranitic synthetic melt has been tested experimentally results suggest that the mechanism of enclave dissolution is a potential hybridization process in granitic systems experiments performed in decompression simulating ascending magmas show interesting results from 10 kbar to 4 kbar for a given temperature enhanced dissolution of the tonalitic enclave into the melt has been observed compared with experiments at constant pressure furthermore the composition of the melt changed to higher cao feo and mgo contents dissolution textures on relict crystals from the tonalitic enclave were monitored these results have implications for the generation of peraluminous monzogranites and granodiorites and an example for the cabeza de araya caceres spain from the serie mixta of the iberian massif is given the tonalitic enclaves that are partially dissolved may be early or coeval intrusions into the granite magma or even into the source migmatitic area many mineralogical geochemical and isotopic features of monzogranites e g reverse zoning in plagioclase low sr isotopic ratios are accounted for by this mechanism of enclave dissolution during ascent and emplacement

mafic magmatic enclaves and mafic rocks associated with some granitoids of the central sierra nevada batholith california nature origin and relations with the hosts abstract the calc alkaline granitoids of the central sierra nevada batholith are associated with abundant mafic rocks these include both country rock xenoliths and mafic magmatic enclaves mme that commonly have fine grained and less commonly cumulate textures scarce composite enclaves consist of either xenoliths enclosed in mme or of mme enclosed in other mme with different grain size and texture enclaves are often enclosed in mafic aggregates and form meter size polygenic swarms mostly in the margins of normally zoned plutons enclaves may locally divert schlieren layering mafic dikes which also occur in swarms are undisturbed composite or largely hybridized in central sierra nevada with the exception of xenoliths that completely differ from the other rocks host granitoids mafic aggregates mme and some composite dikes exhibit a bulk compositional diversity and at the same time important mineralogical and geochemical including isotopic similarities mme and host granitoids display distinct major and trace element compositions however strong correlations between mme host granitoid pairs indicate interactions and parallel evolution of mme and enclosing granitoid in each pluton identical mafic mineral compositions and isotopic features are the result of these interactions and parallel evolution mafic dikes have broadly the same major and trace element compositions as the mme although variations are large between the different dikes that are at distinctly different stages of hybridization and digestion by the host granitoids the composition of the granitoids and various mafic rocks reflects three distinct stages of hybridization that occurred respectively at depth during ascent and emplacement and after emplacement the occurrence and succession of hybridization processes were tightly controlled by the physical properties of the magmas the sequential thorough or partial mixing and mingling were commonly followed by differentiation and segregation processes unusual mme that contain abundant large crystals of hornblende resulted from disruption of early cumulates at depth whereas those richer in large crystals of biotite were formed by disruption of late mafic aggregates or schlieren layerings at the level of emplacement mme and host granitoids are considered cogenetic because both are hybrid rocks that were produced by the mixing of the same two components in different proportions the felsic component was produced by partial melting of preexisting crustal materials whereas the dominant mafic component was probably derived from the upper mantle however in the lack of a clear mantle signature the origin of the mafic component remains questionable

cretaceous ongonites topaz bearing albite rich microleucogranites from ongon khairkhan central mongolia products of extreme magmatic fractionation and pervasive metasomatic fluid rock interaction abstract ongonites were defined at their type locality at ongon khairkhan central mongolia as pristine magmatic topaz bearing albite quartz keratophyres with up to 4 wt f and containing phenocrysts of albite k feldspar quartz and rare mica and topaz hosted in a groundmass composed of the same minerals however detailed petrographic and sem eds studies indicate that these rocks underwent considerable subsolidus exchange with deuteric fluids as evidenced by the presence of albitic plagioclase ab 100 and end member orthoclase or 100 secondary li fe rich mica zinnwaldite enriched in rare metals sn w ta nb pitted feldspars containing fluid inclusions and disseminated fluorite the 120 ma old dyke rocks emplaced at a high structural level in the crust are strongly peraluminous leucogranites characterized by high al and alkalis that are also enriched in rb cs ga and ta depleted in mg ca zr ba sr and eu and have anomalous k rb rb sr zr hf and nb ta ratios compared to the average continental crust however the suite has nd isotopic ratios nd 120 1 similar to those of contemporaneous a type granites of the mongolian transbaikalian igneous province of the central asian orogenic belt the inferred primary 18 o 6 to 7 and pb isotopic values are consistent with a granitic parent magma and interaction with orthomagmatic fluids the ongonites and constituent minerals record 1 an extensive and protracted crystal fractionation history in part due to the presence of volatiles particularly f which depressed the solidus temperature of the felsic rocks and extended its duration of crystallization and 2 subsolidus exchange with fluids which includes late flux of heated meteoric water as indicated by modified whole rock 18 o values 0 5 to 2 7 the interaction of the ongonites with internally derived orthomagmatic fluids is considered to result in enrichment and or redistribution of several incompatible elements but not to have greatly modified their original major element chemistry which indicates that this suite represents the last stages of fractionation of a highly differentiated f rich granitic magma the escape of these evolved melts from the apical part of the underlying pluton is now represented by the ongonite dykes late stage magmatic water rich fluids enriched in incompatible elements including nb ta sn and w were responsible for the late to post magmatic alteration and associated w mineralization

microgranular enclaves as evidence of rapid cooling in granitoid rocks the case of the los pedroches granodiorite iberian massif spain microgranular enclaves from the los pedroches granodiorite lpg los pedroches batholith iberian massif spain have sr nd isotopic and mineral chemical compositions close to those of their host this similarity is not related to restite unmixing as indicated by the igneous textures of the enclaves a number of other geological and geochemical lines of evidence including the high ree and hfse contents of the microgranular enclaves relative to the host granitoid strongly suggest that this similarity cannot be explained by magma mixing alternatively a crystallization process by rapid cooling within the host granitoid magma could explain the geochemical and textural characteristics of the microgranular enclaves including shape grain size mineralogy texture chemistry and sr nd isotopic composition such a crystallization occurred at the walls of the magma conduits through which the granitic magmas were emplaced in the upper crust this process should be considered as an alternative hypothesis to magma mixing for the generation of some microgranular enclaves especially where no direct evidence exists for the presence of basic magmas coeval with granitoids and where there is a lack of isotopic contrast between hosts and enclaves as the process is favoured by feeder dyke related emplacement we suggest that abundance of microgranular enclaves can be related to the mechanism of emplacement of granitoid bodies

structures uplift and magmatism of the western myanmar arc constraints to mid cretaceous paleogene tectonic evolution of the western myanmar continental margin abstract knowledge of trans himalayan tectono magmatic evolution is critical to understanding the complex pre collisional history of southern eurasia active continental margin it has been proposed that magmatic rocks of the trans himalayan batholith extending from southern tibet to southeast asia are now exposed as the western myanmar arc and central granite belt in myanmar yet origin emplacement and relationships of the two juxtaposed belts remain poorly constrained in this study 2d seismic and drilling data for the western myanmar arc zircon u pb age and hf isotope and whole rock geochemical data for magmatic rocks from the arc have been applied our seismic profiles borehole stratigraphic sequences and zircon u pb data show that a typical arc basin system was well developed along the western myanmar continental margin the magmatic arc has experienced at least three igneous events in the mid cretaceous 110 90 ma latest cretaceous early paleocene 69 64 5 ma and eocene 53 38 ma as well as three associated uplift processes in the late cretaceous eocene and late oligocene whole rock geochemical characteristics and zircons showing variable but predominately positive e hf t values suggest a significant juvenile mantle source involving a proportion of ancient subducted sediments and juvenile crustal materials for these typical arc related magmatic rocks the identification of mid cretaceous to paleogene magmatic rocks having positive e hf t values from the western myanmar arc 1 indicates that the magmatism can be correlated with the gangdese arc within the lhasa terrane of the southern tibetan plateau 2 provides evidence for the proximal derived model that paleogene sediments in the central myanmar basin were from the western myanmar arc but were not delivered by the paleo yarlung tsangpo irrawaddy river system from the gangdese arc and 3 enables a model of eastward subduction of the neo tethyan indian oceanic crust to reflect onset of the magmatism at the mid cretaceous and a long existed back arc extension in western myanmar

u pb geochronology and geochemistry of the mccoy mountains formation southeastern california a cretaceous retroarc foreland basin the timing of deposition of fluvial sediments now forming the 7 km thick mccoy mountains formation is one of the key uncertainties in reconstructing the mesozoic paleogeography of southern california and western arizona ion microprobe u pb geochronologic data for individual zircons from nine sandstones from the mccoy mountains type section and six associated igneous rocks provide significant new constraints on the tectonic setting and the timing of deposition within the northwest trending mccoy basin u pb zircon data from a metavolcanic rock of the underlying dome rock sequence in the palen mountains confirm that the mccoy mountains formation was deposited after regional middle to late jurassic arc magmatism u pb zircon data from a late cretaceous granodiorite intruding the formation in the coxcomb mountains confirm that the formation was deformed and metamorphosed prior to 73 5 1 3 ma populations of detrital zircons vary systematically with both rock type and stratigraphic height lithic arkoses predominantly derived from the west have consistently more abundant younger zircons than do litharenite sandstones predominantly derived from the north and the youngest zircons yield maximum depositional ages that decrease from 116 ma near the base to 84 ma near the top of the section the detrital zircon data permit a late jurassic age for the basal comparatively quartz rich sandstone however the data further suggest that 90 of the formation was deposited between middle early and middle late cretaceous time these results are consistent with the hypothesis that most of the mccoy mountains formation represents a retroarc foreland basin deposited behind the active evolving cretaceous cordilleran continental margin magmatic arc that lay to the west and in the foreland of the actively deforming cretaceous maria fold and thrust belt

jurassic to miocene magmatism and metamorphism in the mogok metamorphic belt and the india eurasia collision in myanmar situated south of the eastern himalayan syntaxis at the western margin of the shan thai terrane the highgrade mogok metamorphic belt mmb in myanmar occupies a key position in the tectonic evolution of southeast asia the first sensitive high resolution ion microprobe u pb in zircon geochronology for the mmb shows that strongly deformed granitic orthogneisses near mandalay contain jurassic 170 ma zircons that have partly recrystallized during 43 ma high grade metamorphism a hornblende syenite from mandalay hill also contains jurassic zircons with evidence of eocene metamorphic recrystallization rimmed by thin zones of 30 9 plus or minus 0 7 ma magmatic zircon the relative abundance of jurassic zircons in these rocks is consistent with suggestions that southern eurasia had an andean type margin at that time mid cretaceous to earliest eocene 120 to 50 ma i type granitoids in the mmb myeik archipelago and western myanmar confirm that prior to the collision of india an up to 200 km wide magmatic belt extended along the eurasian margin from pakistan to sumatra metamorphic overgrowths to zircons in the orthogneiss near mandalay date a period of eocene 43 ma high grade metamorphism possibly during crustal thickening related to the initial collision between india and eurasia at 65 to 55 ma this was followed by emplacement of syntectonic hornblende syenites and leucogranites between 35 and 23 ma similar syntectonic syenites and leucogranites intruded the ailao shan red river shear belt in southern china and vietnam during the eocene oligocene to miocene and the wang chao and three pagodas faults in northern thailand that most likely link with the mmb were also active at this time the complex history of eocene to early miocene metamorphism deformation and magmatism in the mmb provides evidence that it may have played a key role in the network of deformation zones that accommodated strain during the northwards movement of india and resulting extrusion or rotation of indochina

late cretaceous protolith age and provenance of the pelona and orocopia schists southern california implications for evolution of the cordilleran margin the pelona and orocopia schists are southern members of a eugeoclinal terrane that structurally underlies a large part of southwestern north america ion microprobe u pb ages of 100 detrital zircons from three widely spaced samples of these two units indicate that deposition occurred after 70 80 ma moreover the distribution of zircon ages including a significant peak centered ca 1 7 ga implies a major contribution f detritus from the mojave desert and transverse ranges of southern california recrystallization of the schists at depths of 20 35 km occurred within 10 15 m y of deposition which requires underthrusting at minimum rates of approximately 4 12 mm yr for reasonable thrust dips considering that similar processes formed the more northern rand schist at somewhat earlier times results indicate a southward progression in timing of deformation for the schists protoliths this result is at odds with northward migration of deformation implied by the baja british columbia hypothesis the age and provenance of the schist s protoliths are consistent with models that derive the schists from either the franciscan complex or great valley group

geochronology and geochemistry of late paleozoic magmatic rocks in the lamasu dabate area northwestern tianshan west china evidence for a tectonic transition from arc to post collisional setting abstract voluminous late paleozoic igneous rocks and associated cu au mo deposits occur in the northwestern tianshan district xinjiang west china however the tectonic setting and petrogenesis of these rocks remain controversial this paper reports zircon u pb and hf cuu isotopic data major and trace elements and sr nd pb isotopic data for the intrusive rocks and minor dacites in the lamasu dabate area of northwestern tianshan adjacent to the cu au mo deposits la icpms u pb zircon analyses suggest that the lamasu porphyries were formed at 366 3 ma and contain 907 738 ma inherited zircons the dabate dacites were formed at 316 4 ma and granite porphyries were formed at 289 3 ma with 319 ma inherited zircons the lamasu porphyries consist of plagioclase granite and granodiorite and are geochemically similar to adakites e g having high al2o3 14 54 19 75 wt and sr 308 641 ppm and low y 7 84 16 9 ppm contents with fractionated rare earth element ree patterns and slightly positive sr anomalies however they have variable initial ratios of 87sr 86sr 0 7072 0 7076 and 206pb 204pb 18 139 18 450 and variable end t 5 6 to 0 8 and positive ehf t 1 4 to 10 6 values they also have variable mg 100 mg2 mg2 fe2 41 73 and low th 3 13 8 09 and th ce 0 14 0 28 values we suggest that the lamasu adakitic magmas were generated through partial melting of southward subducted junggar oceanic crust with subsequent melt mantle interaction and assimilation of basement rocks the dabate dacites show typical arc like geochemical characteristics e g enrichment of large ion lithophile elements lile and strong negative anomalies of ta nb p and ti with variable end t 0 1 to 3 3 they were probably generated by melting of juvenile basaltic lower crust as a result of magma underplating the dabate granite porphyries are geochemically similar to a2 type granites e g high sio2 75 6 77 6 wt and alkalis na2o k2o 8 27 8 70 wt low cao 0 28 0 34 wt and mg 2 10 and obvious negative eu ba and sr anomalies they have variable values of end t 1 7 to 0 and 206pb 204pbi 18 567 18 721 and zircon ehf t 0 5 to 11 1 the dabate a type granite porphyries contain late carboniferous residual zircon cores suggesting that their source rock possibly contained carboniferous arc igneous rocks taking into account all available data from late paleozoic magmatic rocks and cu mo au mineralization in the northern tianshan district we suggest that the dabate lamasu area was a continental arc during the late devonian carboniferous but had entered a post collisional stage by the early permian 290 ma

two phase southward subduction of the mongol okhotsk oceanic plate constrained by permian jurassic granitoids in the erguna and xing an massifs ne china abstract geodynamics of the mongol okhotsk oceanic plate southward subduction are still pending problems this paper presents new zircon la icp ms u pb age and whole rock geochemical data for the middle permian to middle jurassic granitoids in the western erguna and central xing an massifs 267 264 ma 241 ma and 173 ma i type granites and 216 ma a type granites were identified in the erguna and xing an massifs ne china the i type granites were produced by partial melting of the lower mafic crust the 216 ma a type granites were derived from partial melting of crustal materials with tonalitic to granodioritic compositions the 267 264 ma and 241 ma i type granites were generated in an andean type arc setting wheras the 216 ma a type and 173 ma granites were formed in supra subduction extensional setting we summarized previous age data of the middle permian to middle jurassic magmtaic rocks in the erguna and xing an massifs and identified two isolated phases of magmatic activity including the ca 267 225 ma and ca 215 165 ma periods with a significant magmatic gap at ca 225 215 ma these middle permian to middle jurassic magmatic rocks are closely related to the southward subduction of the mongol okhotsk ocean a two stage tectonic evolutionary model was proposed to account for these geological observations in the erguna and xing an massifs involving permian to middle triassic continuous southward subduction of the mongol okhotsk oceanic plate and late triassic to jurassic slab rollback and supra subduction extension

petrogenesis of the early eocene i type granites in west yingjiang sw yunnan and its implication for the eastern extension of the gangdese batholiths abstract this study reports new zircon u pb and hf isotopes and whole rock elemental and sr nd isotopic data for the gneissic granite and leucogranite from the nabang metamorphic zone yingjiang area west yunnan sw china the metamorphosed granitoids crystallized during the early eocene 55 50 ma with zircons showing ehf t values from 11 to 5 3 and crustal model ages of 1 5 to 0 42 ga comparable to those of coeval i type granitoids from the gangdese batholith southern lhasa the rocks are characterized by metaluminous and weakly peraluminous hornblende bearing gneissic granites with a cnk 0 95 1 09 na2o k2o coupled with low initial sr isotopic values of 0 7049 0 7070 and high end t values from 1 1 to 7 1 the rocks were derived from crustal materials involving ancient upper crust sedimentary and juvenile mantle derived rocks together with available data from nearby regions it is proposed that the early eocene granitoids in the nabang and tengliang area can be correlated to the gangdese granitoids and represent the southeastward continuation of the magmatic arc resulting from the neotethyan subduction in southern tibet the petrogenesis of early eocene granitoids in western yunnan was probably related to the rollback of the subducting neotethyan slab that caused the remelting of the crustal materials newly modified by the underplated basaltic magma

shrimp zircon u pb geochronological and whole rock geochemical evidence for an early neoproterozoic sibaoan magmatic arc along the southeastern margin of the yangtze block abstract it has been generally accepted that the south china block was formed through amalgamation of the yangtze and cathaysia blocks during the proterozoic sibaoan orogenesis but the timing and kinematics of the sibao orogeny are still not well constrained we report here shrimp u pb zircon geochronological and geochemical data for the taohong and xiqiu tonalite granodiorite stocks from northeastern zhejiang southeastern margin of the yangtze block our data demonstrate that these rocks dated at 913 15 ma and 905 14 ma are typical amphibole rich calc alkaline granitoids formed in an active continental margin combined with previously reported isotopic dates for the 1 0 ga ophiolites and 0 97 ga adakitic rocks from northeastern jiangxi the timing of the sibao orogenesis is thus believed to be between 1 0 and 0 9 ga in its eastern segment it is noted that the sibao orogeny in south china is in general contemporaneous with some other early neoproterozoic 1 0 0 9 ga orogenic belts such as the eastern ghats belt of india and the rayner province in east antarctica indicating that the assembly of rodinia was not finally completed until 0 9 ga

slab flattening trigger for isotopic disturbance and magmatic flare up in the southernmost sierra nevada batholith california the san emigdio schist of the southwestern sierra nevada batholith snb permits examination of partial melting and devolatilization processes along a late cretaceous shallow subduction zone detrital and metamorphic zircon of the structurally highest and earliest subducted portions of the san emigdio schist constrain the depositional age to between ca 102 and 98 ma zircon oxygen isotope data from both lower plate schist and upper plate batholithic assemblages reveal a 18 o shift of 1 5 between igneous 5 5 and metamorphic 7 domains these results taken with previous zircon and whole rock 18 o measurements provide evidence for massive devolatilization of the san emigdio schist and fluid traversal of upper plate batholithic assemblages thereby altering the isotopic composition of overlying material furthermore the timing of fluid rock interaction in the southwestern snb is coincident with eastward arc migration and an associated pulse of voluminous magmatism we posit that during flattening of the farallon slab the schist was rapidly emplaced beneath the magmatic source where ensuing devolatilization triggered a magmatic flare up in the southeastern snb this short lived 15 myr high flux event was followed by the termination of arc magmatism as the shallow subduction zone approached thermal equilibrium

zircon shrimp u pb ages and in situ hf isotopic analysis for the mesozoic intrusions in south taihang north china craton evidence for hybridization between mantle derived magmas and crustal components abstract the north china craton ncc was stabilized in paleo proterozoic times and the eastern part of it was remobilized in the mesozoic as is indicated by the presence of voluminous mesozoic magmas in eastern ncc in particular the lithospheric mantle beneath eastern ncc was significantly thinned during the intense mesozoic tectono magmatic activity which makes the ncc different from most other ancient cratons the mesozoic magmas may thus provide crucial evidence regarding the processes that transformed the ncc from craton to a tectonically active region the south taihang magmatic complex lies in the western part of the mesozoic magmatic zone in the ncc the complex falls into three main plutonic rocks the hongshan syenite the xishu gabbroic diorite monzonite and the wu an monzonitic rocks zircon u pb dating reveals that the three plutonic rocks were emplaced contemporaneously at 132 ma in situ hf isotopic analyses for the dated zircons indicate that the hongshan syenites show homogeneous hf t values 10 to 13 which are slightly lower than those 9 0 for the enriched subcontinental lithospheric mantle sclm as revealed by zircons from mafic rocks that originated from melting of sclm this suggests that the parental magma to the syenites was likely derived from melting of the enriched sclm and contaminated slightly by old lower continental crustal material during ascent the xishu and wu an gabbroic diorites to monzonitic rocks however show significantly varied hf isotopic compositions with hf t 9 2 to 16 and 15 5 to 24 respectively the wide range of hf t values for a single sample suggests that the xishu and wu an plutons formed through a process of hybridization between enriched mantle derived mafic magma and crustal components in variable proportions this is supported by the textural and compositional disequilibrium of plagioclase phenocrysts from the two plutons the wu an monzonitic rocks show lower hf t values than the xishu gabbroic diorite monzonite suggesting more incorporation of crustal components in the source the sudden surge of magmatic activity in the south taihang orogen took place as a result of lithospheric extension that caused partial fusion of the enriched portions of sclm generating voluminous mafic magmas underplating of the mafic magmas in the lower crust consequently caused partial melting of the old basement rocks to produce granitic melts which was followed by mixing between the mafic and granitic magmas

zircon u pb geochronology and elemental and sr nd hf o isotopic geochemistry of post collisional rhyolite in the chiang khong area nw thailand and implications for the melting of juvenile crust volcanic rocks are widely exposed within the chiang khong lampang tak igneous zone in nw thailand a representative rhyolite sample from the chiang khong area yielded a zircon u pb age of 230 7 1 1 ma n 20 mswd 0 98 the chiang khong rhyolites are characterized by low tio2 0 29 0 62 wt and mgo 0 04 0 82 wt with a cnk values of 0 95 1 06 one outlier at 1 47 and can be classified as highly fractionated i type rhyolites they are enriched in liles and lrees and depleted in hfses two representative samples have 87sr 86sr i ratios of 0 70497 and 0 70527 and the end t values fall at 1 1 and 1 3 respectively ehf t and 18o in zircon are between 4 7 to 12 0 and 5 3 to 5 9 respectively our geochemical data suggest that the chiang khong rhyolites formed by partial melting of juvenile mafic lower crust in a post collisional setting deep crustal anatexis was probably induced by upwelling asthenospheric mantle shortly after slab detachment subsequent to closure of the paleo tethys

geodynamics of the indosinian orogeny between the south china and indochina blocks insights from latest permian triassic granitoids and numerical modeling

generation of sodium rich magmas from newly underplated basaltic crust sodium rich rocks of trondhjemite tonalite dacite ttd or granodiorite ttg suites form much of precambrian continental crust1 they are thought to have formed by partial melting of subducted oceanic crust2 3 a process that would have been much more widespread early in earth history than at present owing to the higher thermal gradients prevailing at that time4 phanerozoic ttd suites do exist however and seem also to relate to subduction zones5 defant and drummond6 proposed that these suites form where young 25 myr hot oceanic lithosphere is subducted and melts thus locally simulating the conditions that led to widespread crustal growth in the archaean here we describe plutonic and volcanic rocks from the cordillera blanca complex in peru which have characteristics of the high al ttd suite but which were produced above a subduction zone containing a 60 myr old slab we present evidence that the complex formed by partial melting of newly underplated basaltic crust and argue that this mechanism should be considered more generally as an additional way of generating sodium rich arc magmas

a hybrid origin for the qianshan a type granite northeast china geochemical and sr nd hf isotopic evidence abstract major and trace element whole rock sr and nd isotope and zircon hf isotope data are reported for a suite of a type granites and mafic microgranular enclaves from the early cretaceous 126 2 ma qianshan pluton liaodong peninsula northeast china with the aim of investigating the sources and petrogenesis of a type granites the qianshan pluton includes hornblende alkali feldspar granite graphic biotite granite and mafic microgranular enclaves the hornblende alkali feldspar granites have high sio 2 fe 2 o 3t mgo k 2 o na 2 o rb zr and lree contents and low ba and sr concentrations with strongly negative eu anomalies their high rb sr 87 rb 86 sr 16 76 24 15 and initial 87 sr 86 sr ratios 0 7215 to 0 7283 negative e nd t values 14 1 to 16 5 and zircon e hf t values 18 9 to 11 5 indicate they were mainly derived from a crustal source but with involvement of high e nd t and e hf t materials graphic biotite granites have similar geochemical features and sr nd hf isotopic compositions to enclaves indicating they were the result of crystal fractionation of evolved mafic magmas but with involvement of low e nd t and e hf t materials the mafic enclaves have an igneous texture and contain acicular apatite suggesting quenching of mafic magmas that have co mingled with the host granites they have low initial 87 sr 86 sr ratios 0 7097 0 7148 negative e nd t 14 5 to 11 9 and zircon e hf t 17 1 to 6 9 values and are enriched in liles and lrees and depleted in hfses when coupled with the high mgo mg up to 54 this indicates derivation from an enriched lithospheric mantle source but contaminated by crustal materials geochemical and sr nd and zircon hf isotopic compositions rule out simple crystal liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks instead magma mixing of mantle derived mafic and crustal derived magmas coupled with crystal fractionation is compatible with the data this example shows that at least some a type granites formed through a complex process involving mantle and crustal derived magma mixing crystal fractionation and infracrustal melting

some additional observations on inclusions in the granitic rocks of the sierra nevada microgranular quartz diorite and diorite inclusions are widespread in central sierra nevada granitoid rocks and are almost exclusively restricted to hornblende bearing rocks most commonly felsic tonalites and mafic granodiorites strontium 87 strontium 86 values of the inclusions and host granitoids from individual plutons generally plot on single isochrons that agree closely with previously determined ages neodymium 143 neodymium 144 values of five inclusions ranged from 0 51229 to 0 51248 and yield end values that correlate closely with values of their respective hosts the nd sm and rb sr systematics indicate that most inclusions were in isotopic equilibrium with enclosing materials at the time of formation silica contents of inclusions and granitoids are contiguous but inclusions generally contain less than and granitoids more than 60 sio2 linear continuity of the femic oxide and h2o trends relative to sio2 suggests many inclusions formed as concentrations of hydrous mafic minerals variation of other major element oxides and trace elements support this inference the aforementioned data are generally inconsistent with a direct genetic link between the mafic inclusions and the mantle derived basalt however many initial sierra nevada magmas were produced by mixing of mafic mantle derived melts and felsic crustal materials resulting in isotopically varied tonalitic and granodioritic magmas most inclusions thus represent fragmented crystal accumulations of early formed near liquidus minerals generated from these previously mixed magmas

the relationship between closure pressures from fluid injection tests and the minimum principal stress in strong rocks abstract closure pressures measured during injection tests such as mini fracs are normally considered an accurate measure of the minimum in situ principal stress magnitude this paper presents stress strength and image log data from the australian cooper basin which suggests that in reservoirs with high in situ stress high tensile strength and weak geological fabrics interpreted closure pressures may be significantly greater than the minimum principal stress closure pressures interpreted from mini frac injection tests in the cooper basin suggest the minimum principal stress varies from 12 4 27 2 mpa km 0 55 1 2 psi ft to better understand the reasons for this variation in closure pressure image logs and mini frac data from 13 treatment zones and core from seven of these treatment zones were analysed the analysis revealed that treatment zones with high measured closure pressures 18 1 mpa km 0 8 psi ft high treating pressures 31 6 mpa km 1 4 psi ft and high measured hydraulic fracture complexity existed in reservoirs with high tensile rock strength 7 mpa 1015 psi and geological fabrics planes of weakness including natural fractures conversely treatment zones with lower measured closure stress 19 mpa km 0 84 psi ft and low hydraulic fracture complexity occurred in reservoirs with lower tensile strength and or no geological fabrics we suggest that closure pressures in rocks with high tensile strength and weak geological fabrics may not be representative of the minimum principal stress magnitude in the cooper basin where they are associated with hydraulic fracture complexity rather they reflect the normal stress incident on pre existing weaknesses that are exploited by hydraulic fluid during the mini frac injection

break away of south china from gondwana insights from the silurian high nb basalts and associated magmatic rocks in the diancangshan ailaoshan fold belt sw china abstract the diancangshan ailaoshan fold belt yunnan sw china represents the remnants of the once vast ailaoshan ocean a branch or back arc basin of the eastern paleotethys however when and how this ocean back arc basin opened and rifted south china away from gondwana are still unresolved in this study ordovician silurian high nb basalts hnbs 420 ma mg rich andesite 435 ma high mg basalt 446 ma and i type granite 423 ma were newly identified in the diancangshan ailaoshan fold belt the hnbs are alkaline with high tio2 1 8 1 9 wt and nb 16 9 23 1 ppm contents high nb u 36 8 39 1 and nb th pm 1 17 1 21 ratios positive nb ta anomalies primitive mantle normalized and positive end t values 3 2 the hnbs were likely derived from an oib arc mixed mantle source in the garnet stability field the mg rich andesites are medium k calc alkaline andesite with negative end t values 10 1 and negative nb ta anomalies the magma was likely derived from a mixture of depleted mantle peridotites and subducted sediments under low pressure and high h2o conditions the high mg basalts are high k calc alkaline and characterized by high mgo contents 9 3 9 4 wt mg 70 high cr 369 379 ppm and low al2o3 13 6 13 9 wt contents they were likely derived from an enriched mantle source metasomatized by marine sediment derived fluids the i type granites show medium sio2 content 63 6 wt low a cnk ratio 0 9 and negative end t value 9 8 and may have formed by the partial melting of meta basalts these hnb mg rich andesite high mg basalt and i type granite were likely formed in an intracontinental back arc rift setting integrated with our published detrital zircon u pb age and lu hf isotope evidence we propose that south china may have commenced its break up from gondwana in response to the late ordovician silurian 446 420 ma proto tethyan subduction which opened a back arc basin along the ailaoshan fold belt

secular changes in tonalite trondhjemite granodiorite composition as markers of the progressive cooling of earth archean tonalite trondhjemite granodiorite associations ttg are classically thought to generate through partial melting of hydrous metabasalts however the chemical composition of the least differentiated ttg parental magmas evolved from 4 0 to 2 5 ga during this interval the mg as well as the ni and cr contents increased which is interpreted as reflecting increased interactions between felsic melts generated by metabasalt melting and mantle peridotite similarly cao na2o and sr also increased over time thus reflecting an increase in the abundance of plagioclase in the melt residue the presence or absence of residual plagioclase is interpreted in terms of melting depth the demonstrated interaction between ttg parental magmas and the mantle rules out their genesis by fusion of previously underplated metabasalt and favors the melting of subducted slab material at 4 0 ga earth s geothermal gradient was sufficiently high to allow slab melting at shallow depths where plagioclase was stable consequently due to the small thickness of the overlying mantle wedge felsic magmas interacted little with the mantle at 2 5 ga however owing to lower geothermal gradients the melting depth was greater and plagioclase became no longer stable in the thick mantle wedge overlying the subducted slab as a result felsic magmas reacted strongly with the mantle peridotite the changes of ttg composition during archean time can be thus interpreted as reflecting the progressive cooling of earth

polystage deformation of the gaoligong metamorphic zone structures 40ar 39ar mica ages and tectonic implications abstract the gaoligong metamorphic zone is located southeast of the eastern himalayan syntaxis in western yunnan china the zone is characterized by four stages of deformation d1 d4 d1 structures record early compressive deformation during the indosinian orogeny which formed tight to isoclinal f1 folds of bedding with a penetrative s1 foliation developed parallel to fold axial planes mid crustal horizontal shearing during d2 resulted in overprinting of d1 structures d1 and d2 structures are associated with granulite facies metamorphism d3 doming resulted in late crustal thickening and the development of a regional nw se trending f3 antiform synchronous with or slightly subsequent to d3 deformation the zone experienced d4 ductile strike slip shearing resulting in its exhumation to shallow crustal levels and retrograde metamorphism granitic d4 mylonites predominantly yield 40 ar 39 ar mica ages of 15 16 ma indicating that d4 dextral strike slip shearing occurred in the miocene weakly deformed leucogranite and protomylonite yield 40 ar 39 ar ages of 10 11 ma suggesting that ductile strike slip shearing continued to the late miocene the new 40 ar 39 ar data indicate that escape related deformation along the gaoligong strike slip shear zone occurred in the miocene in association with recent geophysical studies and on the basis of the structural crystal preferred orientation cpo and geochronological data presented in this paper we suggest that the gaoligong metamorphic zone formed in response to intracontinental transpression in the southeast of tibet characterized as intense deformation and metamorphism at middle upper crustal levels

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

geochemical evidence for origin of magma mixing for the triassic monzonitic granite and its enclaves at mishuling in the qinling orogen central china abstract whole rock major trace elements and sr nd isotopes mineral chemistry and zircon u pb and lu hf isotope compositions are reported for i type monzogranite and its mafic microgranular enclaves from the triassic mishuling granite in the qinling orogen central china zircon u pb dating yields an age of 212 5 ma for the mafic enclaves and 213 3 ma for the host monzogranite whole rock major trace elements sr nd isotopes and zircon hf isotopes preclude simple crystal fractionation or restite unmixing for a genetic link between the enclaves and the monzogranite mixing between mafic and felsic magmas is compatible with the geochemical data the monzogranite displays an enriched lile and lree pattern high 87sr 86sr i ratios of 0 7068 to 0 7071 and low end t values of 9 2 to 5 7 suggesting a crustal origin its high mg 47 6 to 50 7 cr and ni contents clearly indicate its derivation from mafic refractory lower crust most of the zircons in the monzogranite have negative ehf t values of 12 5 to 0 1 and two stage hf model ages of 1049 to 1683 ma and some zircons in the host monzogranite have positive ehf t values of 0 3 to 4 8 and single stage hf model ages of 665 to 845 ma this indicates that the monzogranite is most likely to be formed by partial melting of a proterozoic lower continental crust the mafic enclaves have fine grained igneous textures and contain abundant acicular apatite that indicates rapid cooling they also have a high 87sr 86sr i ratio of 0 7071 and a low end t value of 6 5 suggesting a derivation from ancient subcontinental lithosphere mantle sclm zircons in the mafic microgranular enclaves have ehf t values of 1 6 to 5 1 and single stage hf model ages of 700 to 910 ma suggesting a possible origin from partial melting of the neoproterozoic sclm that may form in association with contemporaneously supercontinental rifting in the south china block therefore the mishuling i type granite may form by anatexis of meso neoproterozoic lower crust with mixing between the mafic and felsic magmas in particular we interpret the positive ehf t values as being derived from reworking of neoproterozoic juvenile lithosphere in response to tectonic extension in the late stage of the triassic continental collision between the south china block and the north china block

miocene rapakivi granites in the southern death valley region california usa abstract rapakivi granites in the southern death valley region california include the 12 4 ma granite of kingston peak the ca 10 6 ma little chief stock and the 9 8 ma shoshone pluton all of these granitic rocks are texturally zoned from a porphyritic rim facies characterized by rapakivi textures and miarolitic cavities to an equigranular aplite core these granites crystallized from anhydrous and peraluminous to metaluminous magmas that were more oxidized and less alkalic than type rapakivi granites from southern finland chemical and isotope nd sr pb data suggest that rapakivi granites of the southern death valley region were derived by partial melting of lower crustal rocks possibly including mesozoic plutonic component with some mantle input as well they were emplaced at shallow crustal levels 4 km in an actively extending orogen

early cretaceous highly fractionated i type granites from the northern tengchong block western yunnan sw china petrogenesis and tectonic implications abstract western yunnan an important constituent of the southeastern segment of the east tethyan tectonic domain lies along a transformed orientation from the nww trending himalayan tethyan segment to the northerly trending southeast asian segment however the geodynamical setting of the early cretaceous tectonothermal magmatism along the bangong nujiang lushui luxi ruili belt as the tethyan branch in western yunnan sw china remains controversial the donghe granitoid which is located between the gaoligong and tengliang belts in the northern tengchong block reveals its petrogenesis and its tectonics both of which play a vital role in resolving previous disputes our zircon laser ablation inductively coupled plasma mass spectrometry u pb dating of granites from the donghe batholith yields ages of 119 9 0 9 130 6 2 5 ma these granites display features typical of highly fractionated i type granites high sio 2 contents 71 wt high k contents k 2 o 3 88 5 66 wt calc alkaline character slight peraluminosity a cnk 1 02 1 16 and a highly differentiated index ranging from 83 6 to 95 6 in addition as sio 2 contents increase the rare earth element ree abundances especially heavy ree abundances and ree pattern slopes change gradually but the negative eu anomalies increase sharply while the degree of enrichment in rb th u and pb and depletion in ba nb sr p and ti are enhanced these features indicate that k feldspar plagioclase biotite amphibole apatite sphene garnet and fe ti oxides such as ilmenite play the major role in the fractional crystallization process the high initial 87 sr 86 sr 0 7067 and 0 7079 and negative e nd t values 8 6 to 10 1 with t 2dm ranging from 1 39 to 1 49 ga indicate that the sources were mainly derived from the mature ancient middle to lower crust and minor mantle derived materials the initial 206 pb 204 pb 207 pb 204 pb and 208 pb 204 pb ratios of 18 462 18 646 15 717 15 735 and 38 699 39 007 respectively signify that some subduction related material such as ocean island volcanic rocks and mature arc primitive rocks may be involved as sources based on an analysis of similar zircon saturated temperature and geochemical characteristics of typical highly fractionated i type granites in se and sw china and consideration of the regional geological setting we suggest that the parent magma may be derived from the ancient middle to lower continental crust and mantle derived basaltic magma these were generated in the setting of a westward subducted lushui luxi ruili llr tethyan oceanic slab where mantle wedge derived sources provided enough heat and material to melt the middle to lower ancient crust taking into account the temporal spatial distributions of early cretaceous magmatic rocks in the region we further suggest the existence of an andean type active continental margin from the lhasa block to the tengchong block along the bangong nujiang llr tethys ocean during the early cretaceous

u pb hf and o isotope evidence for two episodes of fluid assisted zircon growth in marble hosted eclogites from the dabie orogen abstract a combined study of internal structure u pb age and hf and o isotopes was carried out for metamorphic zircons from ultrahigh pressure eclogite boudins enclosed in marbles from the dabie orogen in china cl imaging identifies two types of zircon that are metamorphically new growth and recrystallized domain respectively the metamorphic zircons have low th and u contents with low th u ratios yielding two groups of 206 pb 238 u age at 245 3 to 240 2 ma and 226 4 to 223 2 ma respectively anomalously high 18 o values were obtained for refractory minerals with 9 9 to 21 4 for garnet and 16 9 for zircon this indicates that eclogite protolith is sedimentary rocks capable of liberating aqueous fluid for zircon growth during continental subduction zone metamorphism most of the zircons are characterized by very low 176 lu 177 hf ratios of 0 000001 0 000028 indicating their growth in association with garnet recrystallization a few of them falling within the older age group have comparatively high 176 lu 177 hf ratios of 0 000192 0 000383 suggesting their growth prior to the formation of garnet in the late stage of subduction the variations in the lu hf ratios for zircons can thus be used to correlate with garnet growth during eclogite facies metamorphism in either case the zircons have variable e hf t values for individual samples suggesting that their protolith is heterogeneous in hf isotope composition with localized fluid availability in the bulk processes of orogenic cycle nevertheless a positive correlation exists between 206 pb 238 u ages and lu hf isotope ratios for the metamorphically recrystallized zircons suggesting that eclogite facies metamorphism in the presence of fluid has the identical effect on zircon lu hf and u th pb isotopic systems we conclude that the zircons of the older group grew in the presence of fluid during the subduction prior to the onset of peak ultrahigh pressure metamorphism whereas the younger zircons grew in the presence of fluid released during the initial exhumation toward high pressure eclogite facies regime

early cretaceous lower crustal reworking in ne china insights from geochronology and geochemistry of felsic igneous rocks from the great xing an range this paper presents new zircon la icp ms u pb ages and whole rock geochemical data for two granitic plutons and rhyolites of the baiyingaolao formation in the western xing an range ne china the two syenogranite granitic plutons yield identical zircon u pb age of 142 1 ma and the baiyingaolao rhyolites yield zircon u pb age of 138 2 ma the granites contain some hornblendes and show low zr and zr nb ce y contents and low a cnk 0 98 1 11 mg 6 55 and feot mgo values rhyolite samples show similar geochemical characteristics with a cnk of 0 99 1 10 and mg of 14 21 in combination with the high k2o contents 4 43 5 61 wt and negative correlations between p2o5 and sio2 both the granites and rhyolites were classified as high k calc alkaline i type granitoids all samples give high zirconium saturation temperature of 794 964 c with few initially inherited zircons and belong to high temperature i type granitoids they were generated by dehydration melting of biotite muscovite from sub alkaline meta basalts in lower crust depth leaving garnet amphibole and plagioclase as the major residual minerals the syenogranites and rhyolites are likely formed in mongol okhotsk oceanic subduction setting incorporating other lower crust originated felsic rocks in erguna and xing an massifs and songliao basin it is argued that lower crustal reworking is pronounced in ne china during early cretaceous

crustal contamination in the petrogenesis of a calc alkalic rock series josephine mountain intrusion california the josephine mountain intrusion is a cretaceous calc alkalictonalite granite pluton emplaced at 22 km depth in a continental margin arc variable uplift of adjacent terranes in southern california since mid cretaceous time allows us to reconstruct the local crustal column and evaluate its role as a contaminant of mantle derived arc magmas in this region the parental magma of the intrusion was high alumina basalt whose isotopic signature 87 sr 86 sr 0 7087 18 o 7 5 nd 10 cannot have been generated by intracrustal assimilation of known or inferred rock types in the middle or lower crust such a signature could have resulted from high pressure fractionation of primary low alumina basalt coupled with assimilation of felsic pelitic lower crust partial melting of enriched subcontinental mantle followed by high pressure fractionation or a combination of these processes tonalite of the intrusion was formed by fractionation of the parent magma coupled with assimilation of local felsic wall rocks or by crustal melts similar to slightly younger granite assessment of the magnitude of crustal contamination is hampered by uncertainty regarding the existence and role of partial melting of previously enriched subcontinental mantle in generating the parental basaltic magma leading to concomitant uncertainty in the fraction of new continental crust created by such arc plutonism

tethys tectonic evolution and its bearing on the distribution of important mineral deposits in the sanjiang region sw china abstract the sanjiang region in se tibet plateau and nw yunnan is known to have formed by amalgamation of gondwana derived continental blocks and arc terranes as a result of oceanic subduction followed by continental collision from paleozoic to mesozoic in this paper we provide a synthesis of tectonic evolution magmatism and metallogeny in the region based on data from literatures early paleozoic ophiolites 473 439 ma in the changning menglian belt indicate the existence of a proto tethys ocean in this region two episodes of subduction related magmatism in the early paleozoic one occurred in the baoshan and tengchong blocks at 502 455 ma and the other occurred in the simao block at 421 401 ma are regarded as evidence for two different events of subduction of the proto tethys ocean at different locations the proto tethys was succeeded in early devonian by the paleo tethys which comprised the main ocean and three branches ailaoshan jinshajiang and garze litang the changning menglian main ocean existed from middle devonian to middle triassic the remnants of the oceanic crust are preserved in a few places in the longmu tso shuanghu suture as well as in the changning menglian ophiolite belt the eastward subduction of the main oceanic plate from early permian to early triassic formed a prominent arc terrane stretching 1500 km from yunnan to eastern tibet from the waning stage of subduction to post subduction numerous s type granite plutons with ages varying between 230 and 219 ma such as the lincang batholith in yunnan were emplaced at or close to the suture this event produced several hydrothermal w sn deposits in the region the tectonic evolution and associated magmatism of the jinshajiang and ailaoshan branch oceans are generally comparable to those of the main ocean however the branch oceans were subducted westward instead the garze litang branch ocean also underwent westward subduction from middle devonian to late triassic arc related high sr y porphyry intrusions and associated porphyry skarn cu mo au deposits are common in the jinshajiang ailaoshan region especially in the yidun arc which formed prior to jurassic the vms deposits in the sanjiang region formed in diverse tectonic settings including middle silurian back arc basins carboniferous oceanic islands paleozoic subduction zones and triassic post subduction rifting environments the mesozoic and early cenozoic evolution of the baoshan and tengchong blocks was largely influenced by eastward oceanic subduction of the meso and neo tethys from late permian to middle cretaceous and from late cretaceous to 50 ma respectively abundant early cretaceous granitoids and associated skarn type pb zn and sn fe deposits in the baoshan and tengchong blocks were produced in the background of the shan boundary oceanic slab subduction to the west and the break off of the nujiang bitu oceanic slab to the north the subduction of the neo tethys oceanic plate beneath the tengchong block from late cretaceous to paleogene formed abundant s type granitoids and many skarn type and greisen type sn w deposits granitoids formed at 105 to 81 ma and contemporaneous hydrothermal w mo ag and au deposits which temporally coincided with the subduction of the neo tethys are common in the yidun arc terrane

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

petrogenesis and tectonic implications of the early paleozoic granites in the western segment of the north qilian orogenic belt china abstract early paleozoic granitic magmatism in the north qilian orogenic belt records a complete wilson cycle and provides critical geological clues for unraveling the regional tectonic history in this study we report the results of zircon u pb ages hf isotopic analysis and systematic whole rock geochemical data for the late ordovician hongliuhe granite and early silurian qingshan monzogranite in the western segment of the north qilian orogenic belt to constrain their emplacement ages petrogenesis and regional evolution history u pb dating reveals that the hongliuhe granite was emplaced around 453 452 ma and the qingshan monzogranite was emplaced about 440 438 ma a geochemical study shows that the two granites belong to the calc alkaline to high k calc alkaline series the hongliuhe granite shows adakitic and peraluminous features while the qingshan monzogranite belongs to metaluminous to weak peraluminous granites zircons in the hongliuhe granite show ehf t values ranging from 15 1 to 11 7 with two stage hf model ages tdm2 of 687 2398 ma whereas zircons in the qingshan monzogranite show ehf t values ranging from 5 7 to 11 0 with two stage hf model ages from 814 to 1057 ma the geochemical characteristics indicate that the hongliuhe granite was a transitional i s type granite and was generated from a thickened lower crust with the addition of minor paleo to mesoproterozoic crustal materials which left a rutile garnet pyroxene plagioclase residue the qingshan monzogranite formed from the partial melting of mafic crust with minor mantle derived materials and the fractionation of ti bearing phases apatite and pyroxene occurred during the magma s evolution which left an amphibole and plagioclase residue we infer that the hongliuhe granite formed during the northward subduction of the north qilian ocean while the qingshan monzogranite was generated during the post collision stage between the qilian and alxa blocks this observation indicates that syn collision stage of the north qilian orogenic belt began before 453 ma and that the extension regime began prior to 444 ma

late mesozoic tectonic evolution and growth of the tibetan plateau prior to the indo asian collision abstract the elevation of the tibetan plateau is a revolutionary event in the earth history which resulted in change not only of cenozoic regional and global climate but also of monsoon intensity a critical aspect for the development of the tibetan plateau that remains open to intense debate is whether the collision among the tibetan continental blocks or the subduction of the tethys prior to the indo asian collision in the cenozoic contributed to the plateau growth we propose here that an andean type orogen could have been present in southern tibet from the jurassic to early cretaceous but collapsed during the middle cretaceous based on a synthesis of existing structural sedimentary geochronological and geochemical data the orogeny is marked by north verging jurassic early cretaceous thrusting in the lhasa block a result of flat subduction of the yarlung zangpo neo tethys that is recorded by a narrow belt of 205 174 ma adakitic rocks along the southernmost margin of the lhasa block denudation of the orogen produced a thick up to 15 km sequence of jurassic turbidites and molasse type sediments on the northern lhasa block widespread middle cretaceous 135 100 ma postorogenic strongly peraluminous to calc alkaline magmatism and extensional deformation in the lhasa block and an extensive marine transgression in tibet indicate that the orogen broke down due to extension resulting from lithosphere delamination and asthenosphere upwelling meanwhile the north dipping subduction of the bangong meso tethys produced a wide jurassic middle cretaceous magmatic arc along the southern margin of the qiangtang block findings of 132 108 ma aged ophiolites rich with middle cretaceous radiolarians in central tibet indicate that the bangong meso tethys did not close until the late cretaceous repeated flat slab subduction of the yarlung zangpo neo tethys occurred during the late cretaceous 90 78 ma as indicated by the adakites in the southernmost lhasa block which together with the ongoing qiangtang lhasa collision could have contributed to the growth of the tibetan plateau

tectonic evolution of the mogok metamorphic belt burma myanmar constrained by u th pb dating of metamorphic and magmatic rocks 1 the mogok metamorphic belt mmb extends for over 1500 km along the western margin of the shan thai block from the andaman sea north to the eastern himalayan syntaxis previous geochronology has suggested that a long lasting jurassic early cretaceous subduction related event resulted in emplacement of granodiorites and orthogneisses 171 120 ma and a poorly constrained tertiary metamorphic event on the basis of new u pb isotope dilution thermal ionization mass spectrometry and u th pb laser ablation multicollector inductively coupled plasma mass spectrometer geochronology presented here we propose two tertiary metamorphic events affected the mmb in burma the first was a paleocene event that ended with intrusion of crosscutting postkinematic biotite granite dikes at 59 ma a second metamorphic event spanned late eocene to oligocene at least from 37 possibly 47 to 29 ma this resulted in the growth of metamorphic monazite at sillimanite grade growth of zircon rims at 47 43 ma sillimanite muscovite replacing older andalusite and synmetamorphic melting producing garnet and tourmaline bearing leucogranites at 45 5 0 6 ma and 24 5 0 7 ma these data imply high temperature sillimanite muscovite metamorphism peaking at 680 c and 4 9 kbar between 45 and 33 ma to around 606 656 c and 4 4 4 8 kbar at 29 3 0 5 ma the later metamorphic event is older than 24 5 0 3 ma the age of leucogranites that crosscut all earlier fabrics our structural and geochronological data suggest that the mmb links north to the unexposed middle or lower crust rocks of the lhasa terrane south tibet and east to high grade metamorphic core complexes in northwest thailand

neodymium and strontium isotope content of microdiorite enclaves points to mantle input to granitoid production microdiorite enclaves are arguably the most poorly understood of the more prominent features of granitoids surprisingly they have been invoked as support for completely different models for the origin of chemical zonation in granitoids depending on whether they are interpreted as restites syn plutonic blobs of magma remobilized cumulate or basified country rock xenoliths here we present the first detailed neodymium and strontium isotope study of suites of such enclaves from two plutons in both cases we find relatively high nd values most readily explained if enclaves represent syn plutonic injections of mafic magma into the host granitoid magma as such they are the tangible remains of a very important process of mantle derived heat and mass transfer operating in the generation of granitic magmas

permian tectonic framework and palaeogeography of se asia abstract the principal continental lithospheric terranes now comprising se asia and that existed in the permian are the south china indochina super terrane and the sibumasu simao west burma and sw borneo semitau terranes the simao terrane is here regarded a separate terrane derived from south china by back arc spreading in the lower carboniferous rather than as an extension of indochina the main palaeo tethys ocean in thailand is identified as being represented by the chiang mai and sra kaeo sutures and the eastern boundary of the sibumasu terrane is thus interpreted to lie farther west in thailand than previously inferred the nan uttaradit suture is interpreted as representing a marginal back arc basin and contiguous with the ailaoshan and jinshajiang sutures in sw china the south china indochina super terrane and the simao terrane were located within the palaeo tethys in equatorial latitudes during the permian where they together with north china exhibit cathaysian floras and faunas the sibumasu terrane was located on the nw australian margin of gondwana until the sakmarian major shifts in faunal affinities from gondwana province faunas to an independent sibumasu province and then to assimilation into the cathaysian province records the separation and rapid northward drift of sibumasu after the late sakmarian the principal permian palaeogeographic models for the region are assessed for internal consistencies and conflicts

petrogenesis of late triassic granitoids and their enclaves with implications for post collisional lithospheric thinning of the liaodong peninsula north china craton abstract major and trace element sr nd and hf isotope zircon u pb and hf isotope data are reported for mafic microgranular enclaves and host granitoids from the xiuyan pluton in the liaodong peninsula of the eastern north china craton in order to investigate their sources petrogenesis and tectonic implications the zircon u pb age of the enclaves 211 1 ma is identical to that of the host granite 210 1 ma establishing that the mafic and felsic magmas were coeval the enclaves have high mgo concentrations at low silica contents and positive ehf t values they are enriched in large ion lithophile and light rare earth elements and depleted in high field strength elements which can be explained as melts derived from a depleted asthenospheric mantle with some crustal contamination the host granitoids have high sio2 contents variable initial 87sr 86sr ratios and highly negative end t and ehf t values suggesting they were mainly derived from an ancient crustal source heated by the input of asthenospheric mantle derived magmas field observations geochronology geochemistry sr and nd isotopic and zircon hf isotopic compositions point to a complex petrogenesis where asthenospheric mantle and crust derived magma mixing was coupled with crystal fractionation thus explaining the genetic link between enclaves and host rocks identification of an asthenospheric mantle component in the xiuyan granitoids suggests asthenospheric mantle upwelling and decompressional melting of the mantle indicating that post collisional lithospheric thinning in the liaodong peninsula was initiated in the late triassic

overlapping sr nd hf o isotopic compositions in permian mafic enclaves and host granitoids in alxa block nw china evidence for crust mantle interaction and implications for the generation of silicic igneous provinces abstract in general the mantle provides heat and or material for the generation of the silicic igneous provinces sips the rarity of mafic microgranular enclaves mmes however hampers understanding of the mantle s role in generating sips and the process of crust mantle interaction the widespread distributed mmes in the newly reported alxa sip provide an opportunity to study these processes this study integrates in situ zircon u pb age and hf o isotope analyses whole rock geochemistry and sr nd isotope results for the mmes and host granitoids in the alxa block sims zircon u pb dating reveals that there are two generations of mmes and host granitoids the mmes in the bayannuoergong batholith were formed at ca 278 ma similar to the age 280 ma of host granitoids and the mmes and host granitoids in the yamaitu pluton were formed at ca 272 270 ma all mmes have relatively low sio 2 50 7 61 4 wt and th 0 8 2 8 ppm but relatively high mgo 2 6 4 9 wt cr 23 146 ppm and ni 6 38 ppm contents compared to the host granitoids with sio 2 63 6 77 5 wt th 5 2 41 ppm mgo 0 23 2 1 wt cr 10 38 ppm and ni 5 14 ppm all mmes have whole rock sr nd and zircon hf o isotope compositions similar to their corresponding host granitoids the 280 ma mmes have lower whole rock e nd t 13 5 and higher initial 87 sr 86 sr values 0 7095 and zircon 18 o values 6 3 compared to the e nd t 11 5 initial 87 sr 86 sr values 0 7070 and zircon 18 o values 5 6 of the 270 ma mmes the occurrences of quartz xenocrysts k feldspar megacrysts corroded feldspars and acicular apatites indicate that the mmes are the products of the mixing between mantle and crust derived magmas the striking similarities in the zircon hf o isotopic compositions in both mme host granitoid pairs indicate that the granitoids and mmes have similar sources the granitoids are proposed to be mainly sourced from magmas generated by remelting of newly formed mafic rocks which were generated by earlier basaltic magmas underplating or injected into the mid to lower crust the continuing basaltic magma underplating and remelting of earlier formed mafic rocks in the mid to lower crust indicate that the mantle provided both heat and materials for the generation of the alxa sip the scenario also suggests that basaltic magma underplating may play a more important role in the crustal growth in the alxa sip than it is generally recognized in other sips

a review of permian stratigraphy palaeobiogeography and palaeogeography of the qinghai tibet plateau the permian marine lithostratigraphy biostratigraphy and faunal compositions among all the major tectonic blocks in the qinghai tibet plateau are synthesised based on published literature authors unpublished data and field observations palaeobiogeographically the permian marine faunas in the qinghai tibet plateau can be assigned to four biotic provinces cathaysian indoralian cimmerian and himalayan provinces as well as a broad south transitional zone for parts of the permian the qamdo block clearly belongs to the cathaysian province as it is characterised by a lack of glacial deposits and a dominance of warm water faunas during the whole permian the qiangtang block lhasa block and the himalaya tethys zone in southern tibet can be grouped into the south transitional zone during the cisuralian which is characterised by the combination of the bivalve eurydesma the brachiopods cimmeriella bandoproductus and the fusuline monodiexodina the south transitional zone gave birth to the cimmerian province as shown in the qiangtang and lhasa blocks from roadian to capitanian during which its faunas were characterised by an admixture of cold and warm water elements supplemented by a limited number of endemic taxa such as the fusuline eopolydiexodina the non fusuline foraminifer shanita hemigordiopsis assemblage and the coral thomasiphyllum the formation and development of the south transitional zone and the cimmerian province is interpreted to be closely linked to and driven by the northward drift of the cimmerian continent superimposed with a gradual global warming in the wake of gondwanan deglaciation the consequently combined effects of these events forced the climate of the drifting peri gondwana blocks to ameliorate and its marine faunas to transform from an essentially cold water gondwanan type in the early cisuralian through a transitional stage to a predominantly warm water cathaysian type by the lopingian the himayala tethys zone in southern tibet is assigned to the himalayan province from late guadalupian to lopingian and identified by persistent occurrences and dominance of cold water brachiopods and corals both the permian sedimentary sequences and faunal successions are strongly different between the qamdo block and the qiangtang block this is taken to suggest that the longmu co shuanghu lancangjiang suture between the two blocks most likely represents the main branch of the palaeotethys suture in tibet a comparison and correlation of permian sequences and faunas between tibetan blocks and adjacent blocks in western yunnan indicates that the qiangtang block should be aligned with the baoshan block whereas the lhasa block more closely related to the tengchong block further comparison and correlations of permian stratigraphy and faunas of all major peri gondwana blocks suggest that they probably constituted three different continental slices rather than one as perceived in some previous literature the first slice is composed of central pamir the qiangtang block and the baoshan block the second slice consists of south pamir karakorum south afghanistan central iran and turkey these two continental slices are both characterised by a continuously active rifting event from artinskian to guadalupian by contrast the third slice is interpreted to consist of the lhasa block the tengchong block and the sibumasu block none of which has late cisuralian rift related basalts the baoshan block and the lhasa block are thought to have initially early cisuralian intersected probably at a triple junction point close to western australia and were then rifted off from gondwana at different times during the permian

early cretaceous na rich granitoids and their enclaves in the tengchong block sw china magmatism in relation to subduction of the bangong nujiang tethys ocean abstract the na rich intermediate to felsic granitic rocks provide insights into the generation of magmas in subduction zones this paper presents zircon la icp ms u pb ages as well as whole rock geochemical mineral chemical and in situ zircon hf isotopic data on na rich granitic rocks from the tengchong block sw china the granodiorites and associated mafic magmatic enclaves mmes from the menglian batholith yield zircon u pb ages of 116 1 0 8 to 117 8 0 6 ma and 117 7 0 7 ma respectively both host granodiorites and enclaves show calc alkaline and sodium rich nature enrichment in large ion lithophile elements liles and variable depletion in zircon hf isotopic compositions euhedral amphiboles in both granodiorites and associated enclaves are magnesian hornblende with high mg and ca and contain euhedral plagioclase inclusions of labradorite to andesine an36 57 composition the granodiorite was most likely derived through the mixing of partial melts derived from juvenile basaltic lower crust and a minor evolved component of ancient crustal sources the quartz monzodiorite granodiorites and associated mmes from the xiaotang mangdong batholith yield zircon u pb ages of 120 3 1 3 to 122 6 0 8 ma and 120 7 1 5 ma these rocks are also sodium rich and show calc alkaline trend with negative zircon hf isotopic compositions 5 55 to 0 58 the mmes in the host intrusions are monzogabbro with variable and depleted zircon hf isotopic compositions the amphiboles in the both host intrusions and the enclaves show al rich ferro tschermakite composition we infer that the quartz monzodiorite granodiorites were derived from magmas generated by the melting of ancient basaltic rocks in the lower arc crust induced by the underplating of mantle derived mafic magmas the formation of the different types of na rich granitic rocks is correlated to the subduction of bangong nujiang tethyan ocean a comparison with magmatism in the northern magmatic belt suggests that mantle derived magmas played an important role in the genesis of early cretaceous intrusions from tengchong to lhasa blocks although crustal melting is the dominant contributor

petrogenesis of mantle derived lile enriched archean monzodiorites and trachyandesites sanukitoids in southwestern superior province reply in southwestern superior province diorite monzodiorite and trachyandesite sanukitoids occurring within syn to post tectonic intrusive complexes and within greenstone belts have the following chemical characteristics 55 60 wt sio2 mgo 6 wt mg 0 60 ni and cr both 100 ppm na2o k2o 6 wt sr and ba both 600 1800 ppm and rare earth element ree patterns that are strongly light rare earth element lree enriched cen 80 250 ybn 4 10 and show no eu anomalies sanukitoids and their granodioritic derivatives constitute at least 5 of the exposed crust in the study area the sanukitoids cannot be derived by melting fractionation or crustal contamination of basalts or lamprophyres that are coeval with the sanukitoids crustal contamination of komatiites fails to explain the high large ion lithophile element lile contents of the sanukitoids rather we suggest that the sanukitoids were derived by hydrous melting of lile enriched mantle peridotite at pressures between 10 and

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

high mg andesites from the gila bend mountains southwestern arizona evidence for hydrous melting of lithosphere during miocene extension miocene andesites and basaltic andesites from the eastern gila bend mountains have high mg values most 66 72 high concentrations of ni and cr and olivine phenocrysts as mg rich as fo90 cores of euhedral olivine phenocrysts in some andesites are in equilibrium with the whole rock mg values and the compositions of chromian spinel inclusions in olivine phenocrysts are similar to those from boninites and other primitive magnesian silicic volcanic rocks the gila bend lavas are interpreted to have evolved from a variety of primary mantle derived melts some as silicic as approximately 56 sio2 all the lavas have strong lithospheric geochemical characteristics ba nb ratios range from 60 to 170 initial 87sr 86sr is from 0 7063 to 0 7073 and nd is from 5 to 8 previous experimental studies have established that the generation of primary andesitic magmas from mantle peridotite requires low pressure 15 kbar hydrous conditions and a refractory source the high mg andesites erupted near the end of miocene extension in the gila bend region because of their primitive nature and tectonic setting the gila bend lavas can be used to test three general types of models that have been proposed recently to relate magmatism extension and lithospheric geochemical character of mid cenozoic volcanic rocks of the basin and range models that require crustal contamination of the magmas or that produce the magmas by melting of basaltic material within mantle lithosphere are unsuitable for the origin of the gila bend lavas however some elements of a model involving hydrous melting of mantle lithosphere during extension are consistent with the chemistry and tectonic setting of the lavas the high mg andesites are interpreted to reflect hydrous melting of harzburgite near the base of mantle lithosphere at unusually low pressure the low pressure was the result of tectonic thinning of the lithosphere during extension and the source of the volatiles was most likely contemporaneous subduction although the nd isotopic composition of the gila bend lavas originated from proterozoic mantle lithosphere the pb isotope ratios mainly reflect the influence of mesozoic and cenozoic subduction