late mesozoic bimodal volcanic rocks in the jinniu basin middle lower yangtze river belt yrb east china age petrogenesis and tectonic implications abstract late mesozoic intrusive and volcanic rocks are widespread in the southeast hubei province middle lower yangtze river belt yrb east china detailed in situ zircon u pb and hf isotope elemental and sr nd pb isotopic data are presented in this paper for late mesozoic volcanic rocks from the jinniu basin yrb aiming to constrain their age petrogenesis and tectonic implications the jinniu volcanic rocks show a bimodal distribution in composition with dominant rhyolite and dacite and subordinate basalt and basaltic andesite new shrimp and la icpms zircon u pb ages indicate that the volcanic rocks of three formations in the jinniu basin were erupted at quite a short age range of about 5 ma during the early cretaceous 130 125 ma the mafic rocks are moderately enriched in large ion lithophile elements lile e g ba th u and pb and light rare earth elements lree and are characterized by negative nb ta and ti anomalies and relatively high tio2 0 72 2 06 and nb 9 20 26 5 ppm contents these analyses indicate that the geochemical characteristics of the mafic rocks in the jinniu basin are similar to worldwide phanerozoic nb enriched basalt and andesites neba new in situ zircon u pb ages and field geological relationships demonstrate that neba in the southeast hubei province are not spatially or temporally associated with high silica adakitic rocks but were most likely derived from an enriched lithospheric mantle with assimilation of minor crustal materials and then fractional crystallization during the evolution of the magma overall the felsic rocks in the jinniu basin have geochemical characteristics and sr nd pb signatures and in situ zircon hf isotopic compositions similar to those of the mafic rocks compared with the mafic rocks the felsic rocks are characterized by enriched and variable concentrations of lile and ree e g ba 33 3 1372 ppm y 11 4 33 6 ppm ybc 5 07 18 7 and negative eu anomalies eu 0 22 0 98 as well as a wide range of radiogenic nd pb isotopic values with end t 10 2 to 2 4 206pb 204pb i 17 659 18 705 207pb 204pb i 15 478 15 663 and 208pb 204pb i 37 654 38 935 and in situ zircon hf compositions of ehf t 12 7 to 1 8 these features indicate that the genesis of felsic magma in the jinniu basin is consistent with extensive fractional crystallization and large amounts of crustal contamination from an evolved mafic magma sio2 55 the bimodal volcanic rocks in this study provide convincing evidence that early cretaceous volcanic rocks in the yrb developed in a back arc extensional tectonic regime

zircon u pb age hf isotopes and geochemistry of shuichang algoma type banded iron formation north china craton constraints on the ore forming age and tectonic setting abstract the shuichang iron deposit is an algoma type bif in the north china craton the shuichang bif is interlayered with plagioclase hornblende gneiss indicating that these two lithologies are temporally closely associated we use the content of major elements and their ratios in order to define the protolith of the plagioclase hornblende gneiss sims u pb dating of zircons from the plagioclase hornblende gneiss suggests that the shuichang bif was formed at ca 2547 7 ma and was subjected to metamorphism at 2513 4 ma the major elements and ree data of the ore as well as individual magnetite and chert bands indicate that the shuichang bif precipitated from hydrothermal fluids discharging on the sea floor and that iron and silica may be associated with submarine volcanic activity in situ lu hf analysis of zircons from the plagioclase hornblende gneiss yielded initial 176 hf 177 hf ratios from 0 280903 to 0 281197 and e hf t values from 10 5 to 0 4 with a weighted mean value of 8 72 0 38 the single stage hf model age ranges from 2836 ma to 3249 ma we infer that the protolith of the plagioclase hornblende gneiss originated from the enriched mantle mixed with a small amount of subducted crust materials the shuichang bif formed at 2 5 ga and is coeval with other bifs in north china and in other cratons in the world we propose that the shuichang bif was formed in a subduction related back arc basin or intra arc basin based on the geochemical signatures of the wall rocks we further point out that the 2 5 ga global bif formation peak not only predates the amalgamation of the north china craton but probably also record an ancient supercontinent event

early cretaceous bimodal volcanism in the duolong cu mining district western tibet record of slab breakoff that triggered ca 108 113 ma magmatism in the western qiangtang terrane abstract we report new zircon u pb ages and hf isotope compositions and whole rock major and trace element and sr nd isotope data for the meiriqiecuo formation mf bimodal volcanic rocks collected from the duolong cu mining district dcmd in the western qiangtang terrane qt western tibet these data provide important constraints on the petrogenetic evolution and geodynamic setting of early cretaceous magmatism in the dcmd the mf bimodal volcanic rocks are mainly basaltic andesite and andesite with subordinate rhyolite four mafic samples yielded zircon u pb ages of ca 108 2 113 0 ma and one silicic sample has an age of 109 3 2 2 ma indicating that the mafic and silicic eruptions were contemporaneous the mf bimodal volcanic rocks belong to the medium k calc alkaline to shoshonite series the rocks show arc type affinities characterized by significant enrichment in light rare earth lan ybn 7 74 12 60 and large ion lithophile elements rb cs k and pb but depletions in the high field strength elements nb ta and ti which geochemically resemble andean arc basalts therefore the mf bimodal volcanic rocks were likely emplaced at an andean type active continental margin and represent an early cretaceous magmatic arc that was located at the western qt margin moreover the mafic volcanic rocks have high initial sr isotopic ratios 0 705269 0 705413 and negative end t values of 1 5 to 0 6 compared with the silicic volcanic rocks 87sr 86sr i 0 704770 0 704903 end t 1 2 to 1 3 zircons from silicic samples have significantly higher ehf t values 11 6 to 15 5 and predominantly lower paleoproterozoic hf crustal model ages tdmc 180 428 ma than the mafic samples which have variable ehf t values of 3 4 to 13 0 and tdmc ages of 346 952 ma these results indicate that the mafic and silicic end members of the mf bimodal suite were generated from mantle and crustal sources respectively the basaltic andesite and andesite may have been derived from mantle enriched by the metasomatism of subducted fluids whereas the rhyolite could have been derived by partial melting of mafic juvenile crust that originated from an older and more depleted mantle in light of the geochemical characteristics and field relationships we propose that breakoff of the bangong nujiang oceanic lithosphere was responsible for the generation and emplacement of the mf bimodal volcanic rocks the fact that the mf bimodal volcanic arc magmatism was active at ca 108 113 ma indicates that it was associated with closure of the bangong nujiang ocean via an arc arc soft collision during the early cretaceous

petrogenesis and tectonic significance of late jurassic early cretaceous volcanic intrusive complex in the tianhuashan basin south china abstract the late mesozoic geological framework of south china is characterized by voluminous volcanic rocks and their intrusive counterparts however the ages petrogenesis and tectonic implications of the late mesozoic magmatism in south china especially those in the qinhang zone are poorly constrained the tianhuashan basin is one of the important volcanic basins along the southern margin of the qinhang zone and is mainly composed of the daguding and ehuling formations and their intrusive counterparts laser ablation inductively coupled plasma mass spectrometry la icpms u pb zircon data presented here in conjunction with our previously published data suggest that there are two distinct pulses of magmatism within the tianhuashan basin the granite porphyry was emplaced at 158 157 ma whereas large scale volcanic intrusive rocks were formed at 144 137 ma rather than in the late jurassic as previously regarded the tianhuashan volcanic intrusive rocks have similar whole rock element geochemical and sr nd and zircon hf isotopic compositions suggesting that they were probably derived from similar source regions these rocks exhibit clearly negative and variable bulk rock end t values in a range of 12 07 to 8 69 and low zircon ehf t values from 12 25 to 2 31 with paleo and mesoproterozoic two stage model ages for both nd and hf isotopes 1639 1915 ma and 1051 1970 ma respectively suggesting their derivation dominantly through partial melting of ancient crustal materials with involvement of a mantle derived component in variable proportions and followed by differential assimilation of upper crust during magma ascent an extensional setting associated with the roll back of the paleo pacific slab is favored to explain the petrogenesis of the tianhuashan volcanic intrusive complex the ehf t of zircon increases from late jurassic granite porphyry to early cretaceous magmatic rocks suggesting progressive crust mantle interaction in generating the episodic intermediate felsic magma eruption during late mesozoic our results suggest that the continental extension along the qinhang zone was weak during late jurassic and large scale lithospheric extension did not take place until early cretaceous with ongoing development of slab roll back

experimental resetting of the u th pb systems in monazite abraded fragments 200 400 m of a large chemically homogeneous and non metamict brazilian monazite crystal characterised by a concordant u pb ages of 474 1 ma 208pb 206pb 19 5 were hydrothermally treated at varying temperatures with solutions of different compositions product monazites were analysed with scanning electron microscope sem electron microprobe emp secondary ion mass spectrometer sims and isotope dilution thermal ionisation mass spectrometer id tims experiments with pure water over a temperature range of 800 1200 c at 700 mpa and durations ranging from 5 to 60 days showed that even at 1200 c any dissolution and recrystallization of new monazite is confined to the outermost surface of the grain neither pb diffusion at the emp scale nor significant discordancy were observed we performed experiments at 800 and 1000 c for different durations using different fluid compositions at quartz saturation a 10 wt cacl2 fluid a 10 wt srcl2 fluid a 10 wt nacl fluid and a fluid containing nbs 982 pb standard 208pb 206pb 1 for all runs emp traverses revealed no pb diffusion profiles significant overgrowths of newly formed monazite are documented by sem analyses they occurred only in the 1000 c experiments when either cacl2 or pb bearing fluids were present in the cacl2 experiment two zones could be distinguished within the crystal a core possessing the initial monazite composition and a rim consisting of newly formed monazite produced by dissolution precipitation which was enriched in ca and pb free id tims dating of single grains treated with srcl2 and cacl2 solutions at 1000 c are significantly discordant experiments employing the nbs pb standard produced sub concordant monazite for which the 207pb 206pb apparent age has become older than prior to the experiment 544 ma at 800 c and 495 ma at 1000 c the newly grown monazite rim had obviously incorporated pb from the fluid none of our reaction products contained a detectable diffusion profile the only resetting mechanism we detected involved dissolution precipitation the extent of the dissolution precipitation process depends on fluid composition and is a more efficient mechanism than diffusion for controlling the resetting of monazite in natural rocks

geochronology and geochemistry of the triassic bimodal volcanic rocks and coeval a type granites of the olzit area middle mongolia implications for the tectonic evolution of mongol okhotsk ocean abstract the olzit volcanism in middle mongolia comprises a bimodal suite of basalts and peralkaline rhyolites adjacent to the main mongolia lineament the basalts are characterized by enrichment in lile and lree and depletion in hfse with typical sr nd isotopic signatures end t 2 50 to 0 38 and 87sr 86sr i 0 7058 0 7063 indicating they were likely derived from partial melting of an enriched lithospheric mantle modified by subducted slab derived fluids the rhyolites show a close affinity to a type granites with enrichment in lile and lree and depletion in nb ta and ti they also show a significant negative eu anomaly and have end t values ranging from 0 50 to 1 38 and initial 87sr 86sr ratios ranging from 0 7022 to 0 7200 suggesting the rhyolites stem from partial melting of crustal rocks rather than fractional crystallization of the basaltic melt the rhyolite porphyry yields a shrimp zircon u pb age of 207 2 ma mswd 1 42 indicating the bimodal volcanic suite formed in the late triassic the miarolitic per alkaline granite and biotite bearing granite which are associated with the bimodal volcanic rocks show typical a type granitic geochemical affinity with end t 0 89 0 91 and 87sr 86sr i 0 7021 0 7043 indicating they are likely generated by partial melting of crustal rocks similar to the rhyolitic end member of bimodal suite the miarolitic per alkaline granite and biotite bearing granite yielded shrimp zircon u pb ages of 209 2 ma mswd 0 91 and 213 3 ma mswd 1 65 respectively which are nearly coeval with the age of the bimodal volcanic suites in view of the new geochemical and chronological data in this study we suggest the olzit late triassic bimodal volcanic rocks together with the coeval a type granites represent a back arc basin extensional environment which probably related to the roll back of mongol okhotsk oceanic plate during the southward subduction under the central mongolia microcontinent

reconstructing multiple arc basin systems in the altai junggar area nw china implications for the architecture and evolution of the western central asian orogenic belt abstract the altai junggar area in northwestern china is a critical region to gain insights on the tectonic framework and geological evolution of the western central asian orogenic belt caob in this study we report results from integrated geological geochemical and geophysical investigations on the wulungu depression of the junggar basin to determine the basement nature of the basin and understand its amalgamation history with the chinese altai within the broad tectonic evolution of the altai junggar area based on borehole and seismic data the wulungu depression is subdivided into two nw trending tectonic units suosuoquan sag and hongyan high by southward vergent thrust faults the suosuoquan sag consists of the middle late devonian basaltic andesite andesite dacite tuff tuffaceous sandstone and tuffite and the overlying early carboniferous volcano sedimentary sequence with lava flows and shallow marine sediments from a proximal juvenile provenance zircon e hf t 6 0 14 9 compared to the late carboniferous andesite and rhyolite in the hongyan high zircon sims u pb ages for dacites and andesites indicate that these volcanics in the suosuoquan sag and hongyan high erupted at 376 3 ma and 313 4 ma respectively the middle late devonian basaltic andesites from well lc1 are calc alkaline and exhibit primitive magma like mgo contents 7 9 8 6 and mg values 66 68 with low initial 87 sr 86 sr 0 703269 0 704808 and positive e nd t values 6 6 7 6 and relatively high zr abundance 98 2 116 0 ppm and zr y ratios 5 1 5 4 enrichment in lrees and liles e g th and u and depletion in nb ta and ti suggesting that they were probably derived from a metasomatized depleted mantle in a retro arc extensional setting the well lc1 andesitic tuffs well l8 dacites well wl1 dacitic tuffs and well l5 andesites belong to calc alkaline and metaluminous to peraluminous a cnk 0 8 1 7 series and display low mg values 35 46 and variably positive e nd t 4 5 8 5 and e hf t 10 2 16 8 values as well as young isotopic model ages these devonian carboniferous intermediate felsic volcanics are interpreted as the products of partial melting of a juvenile lower crust with some contributions from mantle components in an evolved island arc setting from immature to mature island arc the basin filling pattern and the distribution of arc volcanics and their zircon hf model ages with the eruptive time suggest that the wulungu depression represents an island arc basin system with the development of a carboniferous retro arc basin in combination with previous work we propose that the northern junggar area comprises three arc basin belts from south to north the darbut luliang karamaili wulungu yemaquan and saur fuhai dulate such tectonic subdivisions are consistent with the regional gravity and magnetic anomaly data the recognition of the wulungu arc basin system demonstrates that the junggar basin is likely underlain by juvenile continental crust rather than ancient precambrian basement and also implies that the caob was built by amalgamation of multiple linear arcs and accretionary complexes

the metallogeny of late triassic rifting of the alexander terrane in southeastern alaska and northwestern british columbia a belt of unusual volcanogenic massive sulfide vms occurrences is located along the eastern margin of the alexander terrane throughout southeastern alaska and northwestern british columbia and exhibits a range of characteristics consistent with a variety of syngenetic to epigenetic deposit types deposits within this belt include greens creek and windy craggy the economically most significant vms deposit in alaska and the largest in north america respectively the occurrences are hosted by a discontinuously exposed 800 km long belt of rocks that consist of a 200 to 800 m thick sequence of conglomerate limestone marine clastic sedimentary rocks and tuff intercalated with and overlain by a distinctive unit of mafic pyroclastic rocks and pillowed flows faunal data bracket the age of the host rocks between anisian middle triassic and late norian late late triassic this metallogenic belt is herein referred to as the alexander triassic metallogenic belt the vms occurrences show systematic differences in degree of structural control chemistry and stratigraphic setting along the alexander triassic metallogenic belt that suggest important spatial or temporal changes in the tectonic environment of formation at the southern end of the belt felsic volcanic rocks overlain by shallow water limestones characterize the lower part of the sequence in the southern and middle portion of the belt a distinctive pebble conglomerate marks the base of the section and is indicative of high energy deposition in a near slope or basin margin setting at the northern end of the belt the conglomerates limestones and felsic volcanic rocks are absent and the belt is composed of deep water sedimentary and mafic volcanic rocks this northward change in depositional environment and lithofacies is accompanied by a northward transition from epithermal like structurally controlled discontinuous vein and pod shaped pb zn agba cu occurrences with relatively simple mineralogy to sulfosalt enriched vms occurrences exhibiting characteristics of vein diagenetic replacement and exhalative styles of mineralization and finally to cu zn co au occurrences with larger and more clearly stratiform orebody morphologies occurrences in the middle of the belt are transitional in nature between structurally controlled types of mineralization that formed in a shallow water near arc setting to those having a more stratiform appearance formed in a deeper water rift basin setting the geologic setting in the south is consistent with shallow subaqueous emplacement on the flanks of the alexander terrane northward the setting changes to an increasingly deeper back or intra arc rift basin igneous activity in the alexander triassic metallogenic belt is characterized by a bimodal suite of volcanic rocks and a previously unrecognized association with mafic ultramafic hypabyssal intrusions immobile trace and rare earth element ree geochemical data indicate that felsic rocks in the southern portion of the belt are typical calc alkaline rhyolites which give way in the middle of the belt to peralkaline rhyolites rhyolites are largely absent in the northern part of the belt throughout the belt the capping basaltic rocks have transitional geochemical signatures radiogenic isotope data for these rocks are also transitional basalts and gabbros end 4 9 and 87sr 86sr initial at 215 ma 0 7037 0 7074 together these data are interpreted to reflect variable assimilation of mature island arc crust by more primitive melts having the characteristics of either mid ocean ridge morb or intraplate within plate basalts wpb the ore and host rock geochemistry and the sulfosalt rich mineralogy of the deposits are strikingly similar to recent descriptions of active sea floor hydrothermal white smoker systems in back arcs of the southwest pacific ocean these data in concert with existing faunal ages record the formation of a belt of vms deposits and occurrences in a propagating intra to back arc rift tectonic setting during the late triassic a modern analogue having similar tectonic and metallogenic features is the southward projection of the lau basin from the active sea floor hydrothermal vents of the valu fa ridge to the taupo volcanic zone of the north island new zealand

permian bimodal volcanism in the zhangguangcai range of eastern heilongjiang province ne china zircon u pb hf isotopes and geochemical evidence abstract in this paper we report on zircon u pb dating hf isotopes major and trace elements and sr nd isotope data with the aim of constraining the petrogenesis and regional tectonic evolution of late paleozoic volcanic rocks in the zhangguangcai range of eastern heilongjiang province ne china located in the eastern segment of the central asian orogenic belt caob between the siberian and north china cratons the late paleozoic volcanic rocks in the zhangguangcai range are composed mainly of basalt basaltic andesite rhyolite and minor dacite the results of la icp ms zircon u pb dating for two basaltic rocks three rhyolites and one dacite indicate that they formed in the early permian ca 292 ma the mafic rocks have sio2 50 13 53 80 wt k2o 0 98 2 28 wt mg 0 51 0 71 cr 144 541 ppm ni 74 260 ppm 87sr 86sr i 0 7044 and end t 4 28 whereas the felsic rocks have sio2 69 12 77 98 wt k2o 3 09 5 33 wt mg 0 17 0 36 87sr 86sr i 0 7032 and end t 4 32 these data are typical of bimodal volcanism the mafic volcanic rocks are characterized by a strong enrichment in large ion lithophile elements liles such as rb ba sr and pb depletion in high field strength elements hfses such as nb ta and ti depletion in heavy rare earth elements hrees and weak negative eu anomalies eu eu 0 88 0 94 on the other hand the felsic rocks show a strong depletion in nb ta sr p and ti enrichment in th u and k and relatively large negative eu anomalies eu eu 0 28 0 95 taken together these data suggest that the mafic magma was derived from the partial melting of a depleted lithospheric mantle modified by subducted slab derived fluids and that the felsic magma originated by partial melting of newly accreted crust the early permian bimodal volcanic rocks together with the coeval a type granites indicate an extensional environment similar to a back arc basin such a setting was possibly related to subduction of the paleo asian oceanic plate beneath the jiamusi and songnen zhangguangcai range massifs

geochemistry of metamorphosed volcanic rocks in the neoarchean qingyuan greenstone belt north china craton implications for geodynamic evolution and vms mineralization abstract the qingyuan greenstone belt qgb situated in the northeastern section of the north china craton ncc is considered to be the unique metallogenic province in china that hosts both banded iron formations bifs and volcanogenic massive sulfide vms deposits traditionally the qgb is divided into the hunbei and hunnan terranes separated by the hunhe fault and grouped into the lower shipengzi middle hongtoushan and upper nantianmen formations which have undergone amphibolite to granulite facies metamorphism exposed supracrustal rocks are composed of plagioclase amphibolite and biotite plagioclase gneiss with minor amounts of bif marble and schist in this study detailed field and petrographic observations lithogeochemical u pb geochronological as well as whole rock sm nd isotope analyses were conducted on the metamorphosed mafic and felsic volcanic rocks i e plagioclase amphibolite and biotite plagioclase gneiss respectively in the qgb in order to discuss implications for tectonic evolution and comitant vms mineralization the supracrustal rocks of both hunbei and hunnan regions were formed synchronously from 2 57 to 2 53 ga and then had undergone 2 52 2 48 ga regional metamorphism specifically the lower section of the shipengzi formation is composed dominantly of transitional tholeiitic basalts with negative nb anomalies suggesting that they were likely generated by partial melting of a mantle source that was previously altered by variable amounts of slab derived fluids or melts in contrast the lithologic assemblages grade upward into interlayered bimodal volcanic rocks i e n morb type tholeiitic basalts and fi to fii type dacites of transitional to calc alkalic affinities representing a change in tectonic regime related to subduction from compression to extension among them all the basalts are characterized by consistently positive initial nd compositions end t 2 7 to 3 4 further indicate a depleted mantle source whereas those dacites possess negative to positive initial nd values end t 4 6 to 3 0 suggesting they were formed by partial melting of depleted mafic source likely lower mafic crust and accompanied with variable addition of old crustal material significantly vms deposits are genetically linked to this extensional event and closely associated with the fii type dacites that were generated by higher melting degree at high temperature conditions and shallow crustal level likely 10 15 km in all geochemical features e g negative nb anomalies and end value together with contamination from continental crusts for tholeiitic mafic rocks and inherited zircons from these volcanic rocks imply a continental arc setting where the qgb was evolved

shear zone hosted polymetallic sulfides in the south limousin area massif central france remobilized sulfide deposits related to variscan collisional tectonics and amphibolite facies metamorphism this paper investigates the relationship between deformation and mineralization developed in amphibolite facies shear zones in the south limousin area using structural microstructural and textural analyses the investigated shear zone is a ductile thrust fault that stacked low grade metamorphic rocks onto a highly metamorphosed basement with both being folded during a late orogenic tectonic phase the shear zone hosts sulfide bearing mylonitic quartzites composed of dynamically recrystallized quartz syntectonic pyrite pyrrhotite sphalerite chalcopyrite and late covellite mylonites display northwest southeast trending lineation and asymmetric microstructures including sigmoidal quartz aggregates and shear bands that indicate a northwestward movement along the shear zone sulfides occur mainly along shear bands and exhibit primary undeformed textures attesting to their syntectonic emplacement based on petrographic analyses of the metamorphic host rocks mineralization formed at p 5 0 to 5 5 kbars and t 300 to 600 c consistent with p t conditions for recrystallization of metapelites and metabasites involved in the shear zone all data indicate that ductile deformation and emplacement of mineralization were coeval the occurrence of similar pretectonic sulfide deposits in the hanging wall of the mineralized shear zone suggests that the sulfides in the thrust fault were remobilized from preexisting mineralization by fluid that migrated toward the thrust this is the first evidence for thrust zone hosted mineralization within the massif central and attests to the role of crustal scale thrust faults as occasional pathways for mineralized fluids during the upper paleozoic continental collision these results suggest that ductile thrust faults may be important sites of ore deposition in the variscan belt of western europe and in similar collisional belts elsewhere

formation age and tectonic setting of the shirengou neoarchean banded iron deposit in eastern hebei province constraints from geochemistry and sims zircon u pb dating abstract the shirengou iron deposit a typical bif deposit located in eastern hebei province in the north china craton is hosted in neoarchean metamorphic rocks the metamorphic rocks include hornblende plagiogneiss plagioclase amphibolite and magnetite quartzite we use contents and ratios of major elements for protolith reconstruction showing that protoliths of the hornblende plagiogneiss and plagioclase amphibolite should be dacite and basaltic andesite respectively based on geology and geochemistry of the hosting rocks we infer that the shirengou bif is an algoma type deposit formed in an arc related basin in the neoarchean constrains from sims u pb dating and oxygen isotopic analysis of zircon from wall rocks and interlayer of the shirengou iron bodies can be summarized as follow 1 igneous zircons from two samples of amphibolite and gneiss yield u pb ages of 2541 21 ma and 2553 31 ma respectively providing the age of the neoarchean protolith because the shirengou bif is an algoma type deposit determined u pb age 2541 2553 ma for zircons from hosting volcanic rocks can constrain the bif forming time 2 metamorphic overgrowth zircons from amphibolite and gneiss give two u pb ages of 2512 13 ma and 2510 21 ma respectively corresponding to the neoarchean matemorphism 3 the zircons with 18o values of 5 9 7 6 also indicate magmatic origin the zircons have 18o values of 6 8 9 9 suggesting that exotic high 18o fluids were involved during metamorphism based on the geology and geochemistry of bif and combing with zircon age and oxygen isotopic systematics we suggest that the shirengou iron deposit was formed in a submarine volcanic setting related to subduction of an oceanic slab

episodic exhumation of hp rocks inferred from structural data and p t paths from the southwestern massif central variscan belt france abstract in the southwestern massif central france hp rocks are found in lower allochthonous units i e hp units sandwiched between upper allochthonous units i e lp units and the parautochthonous unit both devoid of hp rocks hp rocks were formed at minimum conditions of 1 7 gpa 700 c in response to the northward subduction of gondwana beneath armorica during the silurian 430 390 ma their exhumation results from an 80 ma long sequence of tectonic processes effective at various scales and active specifically at each main stage of the evolution of the gondwana lithosphere hp rocks were first exhumed by upward southwestward extrusion during the early devonian 390 380 ma in relation to late stage of subduction in the northern area exhumation is achieved by rifting during the upper devonian 365 ma the most obvious event corresponds to the northwestward thrusting of lp units above hp units during early stages of continental collision 355 ma hp rocks were exhumed for a second time during orogen parallel transpression in response to late stage of continental collision 350 345 ma exhumation in internal zones is achieved by crustal scale normal faulting and erosion during the middle carboniferous 335 315 ma synorogenic extension while compression was still active in the southern external zones exhumation of the whole region is completed by regional uplift and erosional denudation during the upper carboniferous 310 290 ma postorogenic extension subduction and collision have played a key role for driving hp rocks to the surface but their efficiency and the resulting strain pattern depend on the angle of convergence upper devonian rifting and the subsequent early carboniferous crustal thickening prevents to consider exhumation of variscan hp rocks as a single and continuous process and open new questions on the related paleozoic lithosphere history

a mantle derived bimodal suite in the hercynian belt nd isotope and trace element evidence for a subduction related rift origin of the late devonian brevenne metavolcanics massif central france in the brevenne series ne massif central a low grade bimodal association of metabasalts and metarhyolites is exposed together with intrusive trondhjemite bodies zircon u pb dating constrains their magmatic emplacement at 366 5 ma and 358 1 ma respectively the metabasalts are characterized by a distinct enrichment in incompatible elements e g th and lree and positive ndi from 5 to 8 combined isotope and trace element systematics rule out crustal contamination of mafic melts as a suitable cause of the lile large ion lithophile element enrichment rather a mixing process between a component similar to mid ocean ridge basalts and an enriched end member with ndi 5 is suggested an enriched mantle source of ocean island basalt affinity is precluded by the relative depletion of high field strength elements especially nb which shows negative anomalies in chondrite normalized patterns on the contrary a subduction related origin for the lile enrichment would be more consistent it may be inferred that arc like melts enriched in th and lree light rare earth elements and depleted in nb with ndi 5 were produced through partial melting of a depleted mantle source to which a small amount of crustally derived component had been added the metarhyolites are enriched in lile and have a close genetic relationship with the metabasalts as evidenced by their high ndi from 4 7 to 6 8 although the chemical evidence remains ambiguous it is suggested that fractional crystallization accompanied by subordinate assimilation is the petrogenetic process most consistent with the data the trondhjemites are isotopically distinct from the metarhyolites their ndi values from 1 0 to 2 2 reflect an important contribution of continental crust to their genesis and disprove their inferred cogenetism with the felsic volcanics a review of modern environments in which such bimodal suites are exposed shows that settings involving incipient rifting of a volcanic arc fringing a continental margin or built upon young thin continental crust might provide suitable analogues geodynamic reconstructions are complicated by subsequent tectonic events which disrupted the initial patterns and by mesozoic cenozoic sedimentary cover however this subduction related magmatism enlarges the growing body of evidence for southward subduction processes until the late devonian during the evolution of the northern flank of the european variscides as a general implication it is suggested that the combined use of the sm nd system with incompatible elements relatively resistant during alteration and low grade metamorphism ree th zr nb may provide diagnostic criteria for recognizing the tectonic setting of bimodal metaigneous suites in ancient orogenic belts

early middle paleozoic volcanic rocks from the ereendavaa terrane tsarigiin gol area ne mongolia with implications for tectonic evolution of the kherlen massif abstract we report our newly obtained geochronological and geochemical data of volcanic rocks in the tsarigiin gol area ereendavaa terrane to better understand of the early middle paleozoic tectonic evolution of the kherlen massif ne mongolia la icp ms zircon u pb dating reveals two stages of magmatism in the southern part of the ereendavaa terrane one in late ordovician 462 to 455 ma and the other in early devonian 418 ma the late ordovician tsarig volcanic rocks are medium to high k calc alkaline and intermediate to felsic in composition and characterized by relative enrichment of lree and liles e g rb and u and also by relative depletion of hfses e g nb ta p and ti a typical feature of subduction related magmas these rocks have mostly positive whole rock nd t 1 65 to 1 35 and zircon hf t 0 1 to 4 07 values with mesoproterozoic nd and hf model ages of 1 59 1 09 ga they were likely generated in a continental arc setting and their parental magmas were originated from a common source as the product of extensive fractional crystallization of mafic melts and their interaction with arc crust the early devonian chandmana volcanic rocks consist of rhyolite and minor trachybasalt displaying a typical bimodal distribution the trachybasaltic rocks are characterized by low lan smn 2 08 2 60 tbn ybn 1 42 1 50 and la yb 7 51 8 96 ratios and relatively flat mree to hree patterns with nd t values being close to zero 1 36 to 0 13 the rhyolites are high k calc alkaline and possess variably negative hf t 4 96 to 0 29 and nd t 2 14 to 0 57 values exhibiting a strong affinity to a2 type granites considering the distinct compositional gap between the end members and geochemical and isotopic signature of the bimodal suite the felsic magma is interpreted to have derived from partial melting of a hybrid crustal source composed of both pre neoproterozoic and juvenile crustal materials while the mafic rocks were probably derived from a mantle source previously metasomatized by slab derived melt the two phases of magmatism in the southern part of the ereendavaa terrane reflected two stages of tectonic evolution early middle paleozoic subduction of the paleo asian ocean plate that gave rise to the late neoproterozoic early cambrian undur khaan island arc and late cambrian to late ordovician ereendavaa active continental margin silurian amalgamation of the ereendavaa and idermeg terranes kherlen massif caused the break off of the subducted slab and the early to middle devonian extension in the area

very shallow melting of oceanic crust during spreading ridge subduction origin of near trench quaternary volcanism at the chile triple junction 2nd 2 9 to 3 8 and d 18 o values 6 4 to 6 9 respectively consistent with mixed magma sources that include a morb type component and sediments we propose that the high si dacites are derived from the hydrous melting of a mixture of morb and sediments at high temperatures 800 900 c under low pressures 0 8 gpa the low si dacites originate from the melting of a similar source under higher pressures consistent with depths of 25 45 km two scenarios accounting for the near trench position of these latter rocks are envisioned the first invokes rapid tectonic erosion and changes in sedimentary wedge geometry the second one postulates that parts of the slab are subducted rapidly to depths of 20 30 km right under the trench index terms 3640 mineralogy and petrology igneous petrology 8120 tectonophysics dynamics of lithosphere and mantle general 9360 information related to geographic region south america keywords slab melting chile trench ridge subduction forearc magmatism geochemistry calc alkaline magmatism

late paleoproterozoic tectonic setting of the northern margin of the north china craton constraints from the geochronology and geochemistry of the mangerites in the longhua and jianping areas abstract the tectonic setting of the late paleoproterozoic suite of anorthosite mangerite charnockite and alkali feldspar granite amcg on the northern margin of the north china craton ncc is controversial this paper reports on the petrology geochronology and geochemistry of the beibaiqi beipuzi and shibalitai mangerites in the longhua area and the shinao mangerite in the jianping area zircon u pb dating reveals that these mangerites formed 1768 1725 ma similar to the other lithologies 1753 1673 ma of the amcg suite on the northern margin of the ncc the mangerites have intermediate sio2 contents 57 2 66 8 wt and high k2o 4 04 7 05 wt and na2o 3 75 5 50 wt contents which belong to alkaline series rocks they are enriched in light rare earth elements rees and large ion lithophile elements liles of rb ba and k and they are depleted in heavy rees and the high field strength elements hfses of th u nb ta ti and p the zircon hf t values of these mangerites are negative 8 3 to 1 7 with depleted mantle model ages tdm and average crustal two stage modal ages t2dm from 2250 ma to 2533 ma and 2573 ma to 2959 ma respectively these geochemical characteristics suggest that these mangerites originated from the early precambrian lower crust of the ncc compared with the mangerites the other lithologies of the amcg suite display different evolutionary trends of their major elements and different distribution patterns of their rees and trace elements indicating that those lithologies were not formed through fractional crystallization by the same primary magma the anorthosites share similar distribution patterns of rees and trace elements and zircon hf isotopic composition with associated norites the data suggest that the anorthosites were formed by noritic magmas through fractional crystallization at the base of the lower crust the alkali granites and rapakivi granites display the geochemical features of a type granites and might have originated from partial melting of the mid upper crust combined with regional geological data these findings suggest that the amcg suite occurred in a post collisional post orogenic extensional setting rather than an anorogenic continental rift setting

late carboniferous bimodal volcanic rocks and coeval a type granite in the suman khad area southwest mongolia implications for the tectonic evolution abstract the volcanic rocks in the suman khad area in southwest mongolia form a bimodal suite consisting mainly of peralkaline rhyolites with subordinate basalts the rhyolite sample collected from the bimodal suite yielded a shrimp zircon u pb age of 314 5 ma mswd 1 41 n 12 which was interpreted to represent formation time of the bimodal volcanic suite the basalts were characterized by enrichment in lile and lree and depletion in hfse indicating their formation was related to subduction processes these features together with their positive end t values 6 3 6 7 suggest that the basalts were likely derived from a depleted mantle source metasomatized by subduction related fluids in various tectonic discrimination diagrams the basalts exhibited a transition from true arc basalts to intraplate basalts and thus were suggested to from in a back arc tectonic setting the rhyolites show a close affinity to a type granites with enrichment in lile and lree depletion in nb ta and ti and positive end t values 6 0 6 4 considering the observed distinct compositional gap between the endmembers of the bimodal suite the rhyolites are proposed to originate from partial melting of juvenile basaltic crustal rocks rather than fractional crystallization of basaltic melt the granite associated with the bimodal volcanic rocks yielded a shrimp zircon u pb ages of 312 5 ma mswd 0 75 n 13 indicating that the granite is contemporaneous with the bimodal volcanic suite the granite samples showed typical a type granitic geochemical affinities and are considered to have been formed by partial melting of crustal rocks in a within plate tectonic setting based on a combination of the available data we suggest that the late carboniferous bimodal volcanic suite together with the coeval a type granites in the suman khad region probably document a back arc basin extensional environment which probably related to the roll back of the paleo asian oceanic plate during the northward subduction under the central mongolia microcontinent

protracted coeval crust and mantle melting during variscan late orogenic evolution u pb dating in the eastern french massif central the late stages of the variscan orogeny are characterized by middle to lower crustal melting and intrusion of voluminous granitoids throughout the belt which makes it akin to hot orogens these processes resulted in the development of large granite migmatite complexes the largest of which being the 305 300 ma old velay dome in the eastern french massif central fmc this area also hosts a wide range of late variscan plutonic rocks that can be subdivided into four groups i cordierite bearing peraluminous granites cpg ii muscovite bearing peraluminous granites mpg iii k feldspar porphyritic calc alkaline granitoids kcg and iv mg k rich monzo diorites and lamprophyres vaugnerites new results of la sf icp ms u pb zircon and monazite dating on 33 samples from all groups indicate that both granites and mafic rocks emplaced together over a long period of 40 million years throughout the carboniferous as shown by intrusion ages between 337 4 1 0 and 298 9 1 8 ma for the granitoids and between 335 7 2 1 and 299 1 1 3 ma for the vaugnerites low zircon saturation temperatures and abundant inherited zircons with predominant late ediacaran to early cambrian ages indicate that the cpg and mpg formed through muscovite or biotite dehydration melting of ortho and paragneisses from the lower gneiss unit the kcg and vaugnerites contain very few inherited zircons if any suggesting higher magma temperatures and consistent with a metasomatized lithospheric mantle source for the vaugnerites the kcg can be explained by interactions between the cpg mpg and the vaugnerites or extensive differentiation of the latter the new dataset provides clear evidence that the eastern fmc was affected by a long lived magmatic episode characterized by coeval melting of both crustal and mantle sources this feature is suggested here to result from a lithospheric scale thermal anomaly triggered by the removal of the lithospheric mantle root the spatial distribution of the dated samples points to a progressive southward delamination of the lithospheric mantle perhaps in response to rollback following continental subduction or to retro delamination owing to the retreat of a south verging subduction zone

timing and tectonic setting of the sijiaying banded iron deposit in the eastern hebei province north china craton constraints from geochemistry and sims zircon u pb dating the north china craton ncc one of the oldest continental blocks in asia has a complicated evolutionary history with the age of the old crust up to 3 8 ga and records the important geological events of the earth the sijiaying bif the largest banded iron formation located in eastern hebei province in the east part of ncc records an important thermo tectonic event previous studies focused on the geological description but the timing and tectonic setting have been unclear so far the sijiaying bif is hosted in neoarchean metamorphic rocks which includes biotite leptynite hornblende plagioclase gneiss and biotite plagioclase gneiss using major element contents and ratios of the host rocks the protoliths of the hornblende plagioclase gneiss and biotite plagioclase gneiss are shown to be dacite whereas those of the biotite leptynites are sedimentary rocks based on geology and geochemistry of the host rocks we infer that the sijiaying bif is an algoma type deposit sims zircon u pb dating shows igneous zircons from the biotite plagioclase gneiss and hornblende plagioclase gneiss have u pb ages of 2535 8 ma and 2543 14 ma respectively zircons from the biotite leptynite have an age of 2537 13 ma we infer that the ages of 2543 2535 ma represent the time of the sijiaying bif paas normalized rey profiles of the ore samples are characterized by lree depletion and hree enrichment positive la and eu anomalies and an average y ho weight ratio of 32 indicating a mixture of submarine hydrothermal fluids and deep seawater ree data was combined with the 18o analyzed by previous researcher of the ore from individual magnetite bands to infer that the sijiaying bif precipitated from hydrothermal fluids discharging on the sea floor the hornblende plagioclase gneiss and biotite plagioclase gneiss that are enriched in lile and lree and depleted in high field strength elements hfse nb ta and ti have geochemical signatures that are similar to those of island arc volcanic rocks the biotite leptynite shows the characteristics of continental island arc based on the geology and geochemistry of associated wall rocks and ore samples we suggest that the sijiaying bif formed in a back arc basin in neoarchean

derivation of a1 type granites by partial melting of newly underplated rocks related with the tarim mantle plume the granitic rocks of the tarim large igneous province lip are temporally and spatially related to mafic intrusions however their tectonic setting and genetic relationship are debated here we report geochemical and zircon u pb hf isotopic results for three alkali feldspar granitic plutons in the halajun area western margin of the tarim block zircon u pb ages suggest these plutons were emplaced at 268 275 ma coeval with the neighbouring mafic ultramafic complexes and syenitic rocks these granitic rocks have high contents of sio 2 alkalis rb th zr and ree except eu and high ratios of feo mgo and ga al and show strong depletions in ba sr eu which are commonly observed in the a1 type granites zircon hf isotopes reveal a limited range of e hf t values from 1 0 to 3 5 for different samples from three granitic plutons obviously higher than those mostly

118 115 ma magmatism in the tethyan himalaya igneous province constraints on early cretaceous rifting of the northern margin of greater india abstract understanding the dynamics of large igneous provinces lips is critical to deciphering processes associated with rupturing continental lithosphere microcontinental calving the rifting of microcontinents from mature continental rifted margins is particularly poorly understood here we present new insights into these processes from geochronological and geochemical analyses of igneous rocks from the tethyan himalaya early cretaceous mafic dikes are widely exposed in the eastern and western tethyan himalaya but no such rocks have been reported from the central tethyan himalaya here we present an analysis of petrological geochronological geochemical and sr nd hf os isotopic data for bimodal magmatic rocks from the center east tethyan himalaya zircon u pb dating yields six weighted mean concordant 206pb 238u ages of 118 1 2 to 115 1 3 ma mafic rocks display morb like compositions with flat to depleted lree trends and positive end t 2 76 to 5 39 and ehf t 8 0 to 11 9 values the negative nb anomalies and relatively high 187os 188os ratios 0 15 0 19 of these rocks are related to variable degrees up to 10 of crustal contamination geochemical characteristics indicate that mafic rocks were generated by variable degrees 2 20 of partial melting of spinel lherzolites in shallow depleted mantle felsic rocks are enriched in th and lree with negative nb anomalies and decoupling of nd end t 13 39 to 12 78 and hf ehf t 4 8 to 2 0 suggesting that they were derived mainly from garnet bearing lower continental crust the geochemical characteristics of the bimodal magmatic associations are comparable to those of associations that form in a continental rift setting results indicate that early cretaceous magmatism occurred across the whole tethyan himalaya named here as the tethyan himalaya igneous province separation of the tethyan himalaya from the indian craton may have occurred during ongoing early cretaceous extension related to the kerguelen mantle plume during the nascent stages of a global plate reorganization event if this is the case our findings provide clues to the nature of the tethyan himalaya challenging traditional view of the india asia single stage collision model

early cretaceous morb type basalt and a type rhyolite in northern tibet evidence for ridge subduction in the bangong nujiang tethyan ocean abstract new zircon u pb ages major and trace element data and hf isotopic compositions are presented for bimodal volcanic rocks of the zhaga formation zf in the western middle segment of the bangong nujiang suture zone bnsz northern tibet the genesis of these rocks is described and implications for late stage evolution of the bangong nujiang tethyan ocean bnto are considered detailed studies show that the zf bimodal rocks which occur as layers within a typical bathyal to abyssal flysch deposit comprise morb type basalt that formed at a mid ocean ridge and low k calc alkaline a type rhyolite derived from juvenile crust the combination of morb type basalt calc alkaline a type rhyolite and bathyal to abyssal flysch deposits in the zf leads us to propose that they formed as a result of ridge subduction the a type zf rhyolites yield la icp ms zircon u pb ages of 118 112 ma indicating formation during the early cretaceous data from the present study combined with regional geological data indicate that the bnto underwent conversion from ocean opening to ocean closure during the late jurassic early cretaceous the eastern segment of the bnto closed during this period while the western and western middle segments were still at least partially open and active during the early cretaceous accompanied by ridge subduction within the bangong nujiang tethyan ocean

the rand granite in the southern schwarzwald and its geodynamic significance in the variscan belt of sw germany the rand granite is a heterogeneous metamorphosed granitoid rock complex with numerous wallrock inclusions situated in the moldanubian zone at the southern margin of the central schwarzwald gneiss complex it is a largely mylonitized elongated body and is thrust over the badenweiler lenzkirch zone forming a nappe with shear zones along its northern and southern boundaries it comprises meta granites meta trondhjemites and biotite augen gneisses derived from monzogranites to granodiorites mineral behaviour indicates that the magmatic body has been deformed under upper greenschist facies conditions nappe thrusting which also affected the south schwarzwald gneiss complex occurred in visean time during high temperature low pressure metamorphism kinematic indicators in the mylonites document e to ese directed nappe transport highly transpressive relative to the trend of the nappe boundaries and the foliation the trondhjemites formed at 351 5 4 ma predating the variscan ht metamorphism they have initial end values of 6 6 to 6 7 and relatively low initial 87sr 86sr ratios 0 7042 to 0 7063 indicating a predominant mantle origin the granites and protoliths of the biotite augen gneisses probably crystallised between 436 and 377 ma suggested by u pb zircon model ages they are different from the trondhjemites with low initial end values 4 7 to 3 3 and higher initial 87sr 86sr ratios 0 7068 0 7077 indicating that large part of the rand granite originated from anatexis of continental crust internal structure of zircons from the rand granite reveals mixing of magmas derived from both mantle and crust sources these new data support an interpretation that the rand granite developed along an active continental margin and therefore represents a possible root of a variscan magmatic arc

carboniferous arc magmatism in the qiangtang area northern tibet zircon u pb ages geochemical and lu hf isotopic characteristics and tectonic implications abstract the longmu co shuanghu suture zone lssz which lies between the southern qiangtang baoshan block and northern qiangtang qamdo block on the tibetan plateau represents remnants of the paleo tethys ocean previous investigations have focused on the ophiolite high pressure zone and metamorphic belt in this zone whereas few studies have considered the arc magmatism the present study examines a suite of carboniferous arc magmatic rocks in the western region of the lssz that consist of acidic to basic volcanic rocks including rhyolite dacite andesite basaltic andesite basalt and pyroclastics zircon u pb dating of three samples two andesites and one dacite by laser ablation inductively coupled plasma mass spectrometry demonstrates that these rocks were emplaced at ca 351 346 ma the basalts have low cr and ni abundances indicating that they were generated by varying degrees of partial melting of an enriched lithospheric mantle that was metasomatized by subduction derived components zircons in the felsic rocks have positive e hf t values of 0 8 14 8 indicating an origin by varying degrees of partial melting of juvenile basaltic lower crust consisting mainly of underplated magmas with similar compositions to the basalts geochemical data show that the volcanic rocks are enriched in light rare earth elements and large ion lithophile elements e g th u and pb but are strongly depleted in high field strength elements e g nb ta and ti have high al 2 o 3 and low zr concentrations and are calc alkaline these magmas are therefore subduction related volcanic arc rocks indicating that paleo tethys subduction began in the early carboniferous

a multidisciplinary study of a syntectonic pluton close to a major lithospheric scale fault relationships between the montmarault granitic massif and the sillon houiller fault in the variscan french massif central 2 gravity aeromagnetic investigations and 3 d geologic modeling 1 new gravity and aeromagnetic investigations have been carried out to understand the emplacement mechanisms of a granitic pluton and the relationships with a nearby lithospheric scale fault this paper concerns the second part of a methodological multidisciplinary study and complements previous geochronologic and anisotropy of magnetic susceptibility ams studies on the same pluton in the northern part of the variscan french massif central fmc the montmarault massif crops out along the sillon houiller fault shf bouguer and aeromagnetic anomaly maps imply thickening of the pluton along the shf and suggest laccolitic spreading northwestward on the basis of petrophysical measurements direct two dimensional 2 d joint gravity and magnetic modeling has been performed along 10 cross sections in order to quantitatively constrain the 3 d pluton geometry and its relationships with surrounding geologic units these geophysical cross sections new structural information field and ams measurements and petrophysical data have been integrated into a regional 3 d geological and geophysical model altogether the results obtained from geochronology petromagnetic fabrics part 1 gravity and aeromagnetic investigations as well as 3 d modeling part 2 demonstrate that the montmarault pluton was emplaced during the namurian period along the shf our results further show that at that time in response to a nw se regional extension if the shf existed it behaved as a normal fault mylonites attesting for synmagmatic normal motion on the northeastern part of the montmarault pluton strengthen this tectonic scenario during the late carboniferous the fmc experienced ne sw extension along the shf by 80 km of brittle left lateral wrench offset this second tectonic event is well recorded in the stephanian coal basins which were formed along nw se listric brittle faults and constrain the present day shape of the montmarault pluton

the variscan french massif central a new addition to the ultra high pressure metamorphic club exhumation processes and geodynamic consequences abstract this paper presents the first documentation of coesite bearing eclogites in the eastern french massif central monts du lyonnais unit and discusses the exhumation processes for these very high pressure rocks a combination of mineralogical and geochronological datasets allows us to quantify a depth time path and related exhumation rates high pressure metamorphism is constrained to 400 420 ma minimum 28 kbar or ca 90 km by 360 380 ma the rocks were exhumed to 30 km depth these kinetic results conform to the geological constraints extracted from the tectonic and sedimentary record of the eastern french massif central these multidisciplinary approaches provide new information on paleozoic orogeny and allow us to discuss the relative roles of subduction and collision in exhumation of very high pressure rocks we suggest that a significant amount of exhumation of these rocks occurred during subduction prior to continental collision continental collision itself was responsible only for the final stages of exhumation under a transpressive regime

kinematic constraints on buckling a lithospheric scale orocline along the northern margin of gondwana a geologic synthesis abstract the paleozoic variscan orogeny was a large scale collisional event involving amalgamation of multiple continents and micro continents existing data suggests oroclinal buckling of an originally near linear convergent margin during the last stages of variscan deformation in the late paleozoic closure of the rheic ocean resulted in e w shortening present day coordinates in the carboniferous producing a near linear n s trending east verging belt subsequent n s shortening near the carb permian boundary resulted in oroclinal buckling this late stage orogenic event remains an enigmatic part of final pangea amalgamation the present day arc curvature of the variscan has inspired many tectonic models with little agreement between them while there is general consensus that two separate phases of deformation occurred various models consider that curvature was caused by dextral transpression around a gondwana indentor strike slip wrench tectonics or a change in tectonic transport direction due to changing stress fields more recent models explain the curvature as an orocline with potentially two opposite facing bends caused by secondary rotations deciphering the kinematic history of curved orogens is difficult and requires establishment of two deformation phases an initial compressive phase that forms a relatively linear belt and a second phase that causes vertical axis rotation of the orogenic limbs historically the most robust technique to accurately quantify vertical axis rotation in curved orogens is paleomagnetic analysis but recently other types of data including fracture geochemical petrologic paleo current and calcite twin data have been used to corroborate secondary buckling a review of existing and new variscan data from iberia is presented that argues for secondary buckling of an originally linear orogenic system together these data constrain oroclinal buckling of the cantabrian orocline to have occurred in about 10 ma during the latest carboniferous which agrees well with recent geodynamical models and structural data that relate oroclinal buckling with lithospheric delamination in the variscan

petrogenesis and tectonic implications of the triassic volcanic rocks in the northern yidun terrane eastern tibet abstract triassic paleo tethyan subduction related volcanic rocks are extensively distributed in the xiangcheng and changtai regions northern yidun terrane eastern tibet the 228 ma volcanic rocks in the xiangcheng region have intermediate felsic composition 60 1 to 64 7 wt sio 2 with relatively low y 14 18 ppm and high sr 630 1830 ppm yielding high sr y ratios 43 94 they have lree enriched and hree depleted ree patterns la yb 19 26 typical of adakites sr nd isotopic compositions of these volcanic rocks are similar to those of the contemporaneous high silica adakitic rocks in the southern yidun terrane the 231 230 ma volcanic rocks in the changtai region are composed of basalts andesites dacites and rhyolites with sio 2 ranging from 47 7 to 81 0 wt basalts in the changtai region have initial 87 sr 86 sr ratios from 0 7052 to 0 7058 and e nd t from 1 1 to 1 5 they display oib like geochemical signatures and were likely derived from low degrees of partial melting of an oib like mantle source with subordinate input of subduction components indicated by increasing th content andesites and dacites show elemental and isotopic compositions that evolved from the basalts indicative of derivation via crustal contamination and fractional crystallization afc of basaltic magmas rhyolites have lower initial 87 sr 86 sr ratios and higher e nd t values than andesites dacites inconsistent with an afc process and are suggested to have been derived from anatexis of crustal materials emplacement of volcanic rocks in the changtai and xiangcheng regions could be attributed to the subduction of the ganzi litang ocean a branch of the paleo tethys ocean volcanic rocks in the changtai and xiangchang regions have zircon u pb ages about 4 6 myr older than that of arc granites in the eastern yidun terrane the spatio temporal distribution of the volcanic rocks indicates that the subduction was initiated under the southern yidun terrane in the middle triassic and extended to the northern yidun terrane at the beginning of the late triassic

chronology and geochemistry of neoarchean bif type iron deposits in the yinshan block north china craton implications for oceanic ridge subduction abstract the dongwufenzi iron deposit is a typical algoma type banded iron formation bif in the yinshan block of the north china craton the iron orebodies occur as layers interlayered with plagioclase amphibolite la icp ms u pb dating of zircons from the plagioclase amphibolite yielded protolith crystallization age of 2538 9 ma mswd 1 01 followed by metamorphism at 2452 7 ma mswd 0 68 the geochemical characteristics of the ores including low al 2 o 3 tio 2 and hfse contents pronounced positive eu anomalies and the y ho ratios 26 37 suggest that the bifs precipitated from hydrothermal fluids discharged on the seafloor the geochemical features of the plagioclase amphibolite suggest the protolith magma to be high fe basalt sm nd analyses of the plagioclase amphibolite yielded end t t 2 54 ga and single stage depleted mantle nd model ages t dm in the range of ca 1 8 to 3 0 and 2 68 to 2 81 ga respectively suggesting an enriched mantle source we link the source of fe in the bifs to the komatiitic complex in the same greenstone belt and the source of si with the high fe basalt integrated geochronological and geochemical data from the other archean rocks in this area suggest that the dongwufenzi bif formed at 2 5 ga coeval with the other bifs in the north china craton ncc as well as similar occurrences in other parts of the world we propose a geodynamic model involving ridge subduction and slab window for the formation of the bifs and the associated rock suites in the greenstone belt

early cretaceous bimodal magmatism related epithermal mineralization a case study of the gaosongshan gold deposit in the northern lesser xing an range ne china abstract low sulfidation ls gold mineralization is partially associated with bimodal volcanism in origin sources of ore mineralized fluids and relevant dynamic processes remain poorly understood to better understand these issues a study including deposit geology petrology zircon u pb dating and element and sr nd hf isotope geochemistry of bimodal volcanic rocks at gaosongshan was conducted the bimodal volcanic rocks are composed of andesitic rocks basalt andesite andesite trachyandesite andesitic pyroclastic of the meifeng formation and overlying rhyolite lava of the fuminhe formation cut by equivalent porphyry stocks which contain pyrite bearing granular textured quartz cavity with a sugary appearance with trapped single phase magmatic fluids the andesitic suite formed at approximately 106 108 ma prior to eruption of the younger rhyolitic magmas 105 6 1 7 ma n 13 mswd 1 2 the andesites show calc alkaline affinities which is a subduction related characteristic and show moderate la ybn 7 91 11 36 and weakly negative europium anomalies 0 80 1 02 indicating a source in the asthenospheric mantle with varying contamination by both subduction and continental materials a type rhyolitic rocks are geochemically comparable to the andesitic suite and their higher la ybn 13 01 30 05 and lower eu eu 0 66 0 73 indicate evolved products of andesitic magmas the estimated oxygen fugacity of dry 1 2 water rhyolite lava is fmq 1 87 whereas rhyolite porphyry is more oxidized fmq 0 12 and water rich 2 5 2 8 with probably being water saturated at 2 0 2 4 km depths therefore it is assumed that the essential constraints of bimodal magmatism on epithermal gold mineralization are sequential processes from ascending and gradual differentiation of crustal level mantle derived basaltic magmas to releases of hydrothermal solutions from the silica rich magma reservoir at shallower levels most importantly the oxygen fugacity of and the fluid fluxes from dry rhyolitic melts are critical restricting parameters for the limited reserves at gaosongshan thus this research contributes objective evidence for investigations of deposit genesis and prospecting targets

gravity and magma induced spreading of mount etna volcano revealed by satellite radar interferometry 1 mount etna underwent a cycle of eruptive activity over the past ten years here we compute ground displacement maps and deformation time series from more than 400 radar interferograms to reveal mount etna s average and time varying surface deformation from 1992 to 2001 we find that during this time interval it experienced magmatic inflation and radial spreading to the west south and east steady relative motion between the west and south flanks and between the east and north flanks during this time interval suggests they are related to gravitational spreading of the volcanic edifice time series analysis shows that growth of a southeastern basal anticline began with the end of magma recharge in 1995 thus showing a direct link between deep seated magma intrusions and edifice spreading these observations support a complex mode of radial gravitational collapse underlain by deeper magma driven basal spreading although ultimately the two must be related

zircon u pb geochronology and geochemistry of low grade metamorphosed volcanic rocks from the dantazi complex implications for the evolution of the north china craton abstract the late neoarchean witnessed the cratonization of the north china craton ncc through amalgamation of several micro blocks to form a coherent basement the archean orthogneisses and supracrustal rocks in this craton have experienced various grades of metamorphism ranging up to upper amphibolite and granulite facies at 2500 ma recently a suite of low grade metamorphosed greenschist to lower amphibolite facies volcanic rocks was discovered in the late neoarchean dantazi complex in northern hebei province these meta volcanic rocks consist of bimodal basalt andesite and trachyte dacite with a sio2 gap between 54 4 wt and 60 7 wt here we report shrimp zircon u pb ages of 2490 19 ma mswd 2 0 and 2502 8 ma mswd 0 83 from the meta mafic and meta felsic volcanics respectively representing the timing of igneous activity all the meta mafic volcanic rocks display coherent trace element and ree patterns which are characterized by enriched lile and lree but depleted hfse and hree la yb n 6 29 15 10 combining these trace element features with the positive zircon ehf t values 1 3 to 6 6 we propose that the mafic rocks were likely derived from partial melting of a previously metasomatized lithospheric mantle in the primitive mantle normalized diagram the felsic rocks display uniform patterns enriched in lile but depleted in nb and ta similar to those of lower crust furthermore their strongly fractionated ree la yb n 15 24 61 20 lower hree concentrations yb 0 47 1 65 ppm and positive zircon ehf t values 1 6 to 5 3 suggest that they were derived from partial melting of the lower crust with garnet in the residue this coeval occurrence of metasomatized mantle derived mafic magmas and potassic felsic magmas from different source regions reflects an intracontinental extensional setting during the late neoarchean to earliest paleoproterozoic following the cratonization of the ncc our new data combined with previous published data allow us to speculate that the intracontinental extensional regime might have prevailed in the ncc from late neoarchean onwards

mid permian rifting in central china record of geochronology geochemistry and sr nd hf isotopes of bimodal magmatism on ne qinghai tibetan plateau abstract little has been known regarding 200 myr long geologic history of central china during the late paleozoic between two major continental collisional events marked by 480 440 ma north qinling north qaidam and 230 ma dabie sulu ultrahigh pressure uhp belts respectively the gonghe basin is juxtaposed with major chinese continental blocks across several outstanding tethyan sutures in central china and may hold answers to many critical questions about the tectonic amalgamation of china in this paper geochronological laser ablation inductively coupled plasma mass spectrometry la icp ms mineral chemistry major and trace element and sr nd hf isotopic data are presented for newly discovered mid permian magmatic rocks in the gonghe basin on the northeastern qinghai tibetan plateau the data indicate that magmatism was active at 270 ma and shows a bimodal composition including gabbro diabase and granite major oxides and trace elements are obviously correlated with sio2 between the mafic and felsic rocks they are both characterized by enrichments in light rare earth elements lrees and large ion lithophile elements liles negative eu anomalies and depletion in high field strength elements hfses together with similar sr nd hf isotopic signature suggesting that they likely shared the same magma chamber and that the felsic rocks were produced by fractional crystallization of the mafic rocks the high initial 87sr 86sr ratios 0 7061 to 0 7094 and low end t values 6 4 to 3 7 as well as nb ta deletions indicate that they were likely derived from enriched mantle metasomatized by subduction related fluids during the early paleozoic in view of absence of coeval arc related magmatism in contrast to broad seamount mafic magmatism in the gonghe basin the bimodal magmatism is ascribed to an intracontinental rifting event that could have been caused by transtension in central china since the mid permian which could have been responsible for the initiation of the gonghe and songpan ganzi basins

late paleozoic intrusive rocks from the southeastern lhasa terrane tibetan plateau and their late mesozoic metamorphism and tectonic implications abstract the lhasa terrane in southern tibet experienced late paleozoic and mesozoic cenozoic composite orogenesis this work reports a study on the petrology geochemistry zircon u pb chronology and hf isotopes of late paleozoic and late mesozoic intrusive rocks from the southeastern lhasa terrane the late paleozoic intrusive rocks crystallized in the late devonian early carboniferous of 371 to 355 ma representing a bimodal igneous association formed in the back arc extensional setting the mafic end member originated from the enriched mantle and experienced contamination of crustal materials characterized by a slight enrichment of lree positive anomalies of u k and pb and negative anomalies of th nb ta and ti the felsic end member was derived from the partial melting of the ancient continental crust characterized by metaluminous positive anomalies of th zr and hf negative anomalies of ba sr nb ta and ti and negative e hf t values of zircon with t dm2 ages from 1 90 to 1 40 ga the late cretaceous ca 107 ma mafic intrusions along with the late paleozoic intrusive rocks underwent nearly syn intrusion amphibolite facies metamorphism under p t conditions of 0 56 to 0 69 gpa and 692 to 735 c during the andean type orogeny correlated with the subduction of the neo tethyan oceanic slab beneath the lhasa terrane this study provides a new insight into the pre cenozoic tectonic evolution of the lhasa terrane

estimation of s wave scattering coefficient in the mantle from envelope characteristics before and after the scs arrival 1 examining seismogram envelopes of regional earthquakes in central asia with focal depths deeper than 150 km in 1s 20s periods for a wide lapse time range up to 2000s we found an offset in each coda envelope before and after the scs arrival at 10s and 15s periods the scs wave has its own tail that is scattered waves of scs waves sometimes dominate over the original coda waves composed of scattered s waves and scattered surface waves for 1s 2s and 4s periods such an offset is not clear however there is a change of coda decay gradient associated with the scs arrival in a layered velocity and attenuation structure according to the prem scattering process of s waves can be simulated by using the monte carlo method based on the radiative transfer theory for isotropic scattering process comparing numerical simulation for a two layered scattering model with observed envelopes we estimated the total scattering coefficients as 1 129 10 3 km 1 and 6 230 10 4 km 1 at 4s and 4 510 10 4 km 1 and 2 710 10 4 km 1 at 10s for the lithosphere and upper mantle and for the lower mantle respectively in the lower mantle scattering loss is dominant in the total attenuation of s waves

highly depleted oceanic lithosphere in the rheic ocean implications for paleozoic plate reconstructions the rheic ocean formed at ca 500 ma when several peri gondwanan terranes e g avalonia and carolinia drifted from the northern margin of gondwana and were consumed during the late carboniferous collision between laurussia and gondwana a key event in the formation of pangea several mafic complexes ranging in age from ca 400 330 ma preserve many of the lithotectonic and or chemical characteristics of ophiolites they are characterized by anomalously high end values that are typically either between or above the widely accepted model depleted mantle curves these data indicate derivation from a highly depleted hd mantle and imply that i the mantle source of these complexes displays time integrated depletion in nd relative to sm and ii depletion is the result of an earlier melting event in the mantle from which basalt was extracted the extent of mantle depletion indicates that this melting event occurred in the neoproterozoic possibly up to 500 million years before the rheic ocean formed if so the mantle lithosphere that gave rise to the rheic ocean mafic complexes must have been captured from an adjacent older oceanic tract the transfer of this captured lithosphere to the upper plate enabled it to become preferentially preserved possible mesozoic cenozoic analogues include the capture of the caribbean plate or the scotia plate from the pacific to the atlantic oceanic realm our model implies that virtually all of the oceanic lithosphere generated during the opening phase of the rheic ocean was consumed by subduction during laurentia gondwana convergence

age petrogenesis and tectonic implications of early devonian bimodal volcanic rocks in the south altyn nw china abstract in this paper we report zircon u pb dating hf isotopes geochemical and sr nd isotopic data with the aim the petrogenesis and regional tectonic evolution of early devonian bimodal volcanic rock in the south altyn nw china new la icpms zircon u pb isotopic data constrained them at ca 406 ma the mafic samples are characterized by high fe cr and ni contents low ti and mg contents slightly enriched lree patterns and low la yb n la nb and la ta ratios and positive e nd t values 3 3 to 3 4 indicating that they were likely derived from strong batch melting of the asthenosphere in the spinel facies field the felsic rocks show an a type affinity with high alkalis fe ga zr nb ce and y contents low mg sr content high rb sr and ga al ratios enrichment in lile e g rb k th u and lree and depletion in ba sr nb ta p and ti and fractionated ree patterns with very strong negative eu anomalies these features along with distinct e nd t values 0 5 to 2 3 and mostly positive e hf t 0 29 to 5 18 indicate that the felsic rocks were mainly generated by partial melting of the crust in low pressure and high temperature conditions and simultaneously underwent slight magma mixing of such melts with mantle magma according to the petrogenetic schemes and geological background of the early devonian bimodal volcanic rocks tholeiite and a type dacite rhyolite they should have formed in a post collisional extensional setting moreover on the basis of spatial and temporal distribution and formation mechanism the tectonic magmatic evolution of the early paleozoic south altyn tagh could be divided into three stages 505 472 ma continental collision the magmatite formed under high pressure conditions due to the deep subduction and initial tearing of continental slab 467 450 ma continental slab break off the magmatite formed at high temperature and low pressure in virtue of felsic upper crust uplifting and mantle magma underplating and 432 385 ma post collisional extension the magmatite consists of a type granites and bimodal volcanic rocks which are the products of the interaction between mantle and crust and asthenosphere upwelling

the moldanubian zone in the french massif central vosges schwarzwald and bohemian massif revisited differences and similarities in order to portray the main differences and similarities between the northeastern variscan segments french massif central fmc vosges black forest and bohemian massif bm we review their crustal scale architectures the specific rock associations and lithotectonic sequences as well as the ages of the main magmatic and metamorphic events this review demonstrates significant differences between the moldanubian domains in thebmand the fmc on this basis we propose distinguishing between the eastern and western moldanubian zones while the vosges black forest mountains are an intermediate section between the bm and the fmc the observed differences are the result of first the presence in the french segment of an early large scale accretionary system prior to the main variscan collision and second the duration of saxothuringian armorican subduction which generated long lived magmatic arc and back arc systems in the bohemian segment while the magmatic activity in the fmc was comparably short lived