unexposed archean components and complex post archean accretion reworking processes beneath the southern yangtze block revealed by zircon xenocrysts from the paleozoic lamproites south china abstract integrated u pb dating and hf isotope analysis of zircon xenocrysts from the baifen lamproites of the zhenyuan area in the southeastern guizhou province offer a unique glimpse into the evolution of the deep continental crust beneath the southern part of the yangtze block south china a total of 148 zircons with 90 110 concordance yield u pb ages varying from 3458 9 ma to 252 2 ma they generally have high th u ratios 0 3 indicating a magmatic origin the two youngest magmatic zircons give a weighted mean 206pb 238u age of 253 0 2 8 ma which is interpreted as the emplacement age of the host baifen lamproites most zircons with pre eruption ages are xenocrysts derived from the deeper crust of the southern yangtze block they define two major paleoproterozoic age populations at 2 17 ga and 1 8 ga with similar hf model ages tcrust of 2 4 2 8 ga peak at 2 64 ga the oldest zircon has a 207pb 206pb age of 3458 9 ma with ehf t 1 47 and tcrust 3 7 ga this demonstrates the presence of archean crustal components or remnants as old as 3 7 ga beneath the southern yangtze block magmatic events involving significant addition of juvenile mantle derived materials mainly occurred at 2 2 2 1 ga and 2 0 1 8 ga but the latter event also involved reworking of the neoarchean crust and might represent the cratonization of the yangtze continent during the middle paleoproterozoic which could be correlated to the assembly of the columbia supercontinent this segment of the yangtze block became tectonically quiescent until 1 2 ga implying that the crustal stabilization lasted for ca 600 million years a few late mesoproterozoic zircon xenocrysts within the age range 1 3 1 0 ga suggest the occurrence of grenville aged events these neoproterozoic zircon xenocrysts 992 547 ma have a wide range of ehf t values 8 47 to 22 07 consistent with the zircon hf isotopic compositions of widespread coeval igneous rocks in the yangtze block the data show that the yangtze block has undergone important crustal growth and reworking during the neoproterozoic in response to the breakup of the rodinia supercontinent the early middle paleozoic zircon xenocrysts 513 357 ma have negative ehf t values 2 97 to 12 69 and tcrust hf model ages of 1 59 2 27 ga indicating reworking of the precambrian crustal basement without juvenile contributions during the caledonian orogeny the positive ehf t values 13 79 to 0 22 of twelve late paleozoic zircons 284 264 ma with a major age peak at ca 275 ma suggest that the precambrian basement beneath the southern yangtze block experienced an episode of strong rejuvenation during the permian we propose a possible linkage between the baifen lamproite magmatism 284 252 ma and the emeishan flood basalt volcanism

petrology mineral chemistry and geochemistry of late triassic ni cu ore bearing mafic ultramafic intrusions hongqiling northeastern china petrogenesis and tectonic implications the hongqiling magmatic ni cu sulfide deposit situated on the southern margin of the eastern central asian orogenic belt caob is composed of over 30 mafic ultramafic intrusions these ore beari

model for the evolution of the avalonian cadomian belt the inception and termination of the ca 670 550 ma avalonian cadomian orogenic cycle are interpreted in terms of the amalgamation and breakup of a late precambrian supercontinent synorogenic arc related volcano sedimentary and granitoid rocks are attributed to oblique southward directed subduction along the margin of the supercontinent after its amalgamation breakup of the supercontinent and inception of the iapetus cycle led to termination of subduction and its replacement by transform activity the resulting closure of late precambrian arc related basins and the development of early cambrian platform successions with acado baltic fauna accompanied the formation of iapetus but took place on the active margin of a preexisting ocean and do not record the evolution of its rifted european flank

model for the precambrian evolution of the avalon terrane in southern new brunswick canada the tectonostratigraphic record of the avalon terrane in southern new brunswick is interpreted as that of a wilson cycle that followed the grenvillian and preceded the opening of lapetus development of a late precambrian continental shelf is attributed to the cadomian ocean whose initial subduction at 800 ma led to platform collapse regional metamorphism and minor mafic to ultramafic plutonism in response to back arc extension and crustal separation oblique subduction of the resulting back arc basin at 600 630 ma produced calcalkaline plutonism and cogenetic volcanism that ended in dextral transtension and the emplacement of a bimodal dike complex initial rifting of lapetus which isolated this record from cratonic north america is recorded in eocambrian redbeds and bimodal volcanics and produced a cambro ordovician overlap sequence bearing acado baltic fauna

contrasting mantle crust melting processes within orogenic belts implications from two episodes of mafic magmatism in the western segment of the neoproterozoic jiangnan orogen in south china abstract partial melting of crust and mantle is the dominant mechanism controlling earth s differentiation the two melting processes are commonly associated because mantle derived magmas could simultaneously provide heat and volatiles to promote partial melting of crust to explore how the two melting regimes interact during the evolution of orogenic belts we evaluated the petrogenesis and possible relationships with the rodinia supercontinent of the two episodes of neoproterozoic mafic magmatism in the western segment of the jiangnan orogen in south china the first episode of mafic magmatism took place at ca 830 ma in the baotan area and was associated with widespread peraluminous granitoids in contrast the second formed at ca 770 ma in the longsheng area without significant granitic magmatism the ca 770 ma longsheng mafic ultramafic rocks show radiogenic and coupled nd end t values of 1 3 to 3 1 and hf isotopes ehf t values of 2 6 to 6 7 whereas the ca 830 ma baotan mafic rocks show unradiogenic and decoupled nd hf compositions end t 8 3 to 0 9 ehf t 6 3 to 2 0 crustal contamination cannot result in the enriched isotopic features and the arc like such as strong deletions of nb and ta geochemistry for the baotan rocks because of the absence of correlations of nd isotopes with incompatible elements and sio 2 contents but crust derived components may have been incorporated into the mantle source

neoproterozoic s type granites in the western jiangnan orogenic belt south china implications for petrogenesis and geodynamic significance abstract the neoproterozoic jiangnan orogenic belt in south china was formed by the amalgamation of the yangtze and cathaysia blocks however the timing of its formation is still debated in this study we present whole rock geochemistry u pb zircon ages lu hf and nd isotope systematics on the neoproterozoic yexijiang and baomuping granites occurring in the western segment of the jiangnan orogenic belt these granites mark the initiation of post collision extension environment zircon u pb age data identify 807 805 ma emplacement age for these granites they have low and variable ree concentrations 47 115 ppm and show moderate to highly fractionated ree patterns la sm n 2 83 3 78 with low gd yb n 1 34 2 31 and negative eu anomalies eu 0 34 0 57 geochemical features and presence of peraluminous minerals classify them as s type granites and p t parameters indicate low to moderate temperature 730 791 c and pressure middle crustal level conditions these granites show consistent negative whole rock nd t 5 7 to 6 6 and magmatic zircon hf t values from 0 3 to 3 8 while inherited zircon grains show variable hf t values 2 1 to 13 0 and two stage hf model ages 1 80 to 2 88 ga in particular one neoproterozoic 899 ma inherited zircon grain displays depleted hf hf t 5 4 and tdm2 1 34 ga isotopic characteristic these lines of evidence indicate a sedimentary source for yexijiang and baomuping granites possibly the archean to neoproterozoic crustal rocks and emplacement in a post collision tectonic environment triggered by mafic magma underplating during the amalgamation of the yangtze and cathaysia blocks we infer that convergence in the jiangnan orogenic belt occurred during 830 800 ma therefore unrelated to the global grenvillian orogeny

from arc accretion to continental collision in the eastern jiangnan orogen evidence from two phases of s type granites abstract the ne jiangxi ophiolitic melange separates the huaiyu and jiuling terranes in the eastern jiangnan orogen of the south china block two phases of s type granites which are geochemically and mineralogically similar to the himalayan leucogranites are newly recognized in mantle peridotites of the ne jiangxi ophiolite the first phase a tourmaline bearing granite porphyry has a sims zircon u pb age of 904 4 4 8 ma and is strongly enriched in k2o 4 00 4 26 wt and strongly peraluminous a cnk 2 94 3 24 it further shows high rb sr ratios and highly fractionated ree patterns indicating an origin due to fluid absent melting of metapelites at relatively high temperatures the granite porphyry exhibits highly evolved zircon hf 6 0 to 1 0 and whole rock nd 4 2 to 5 1 isotopes which is in contrast to juvenile derived magmatic rocks in the huaiyu terrane we thus propose that the granite porphyry was formed by melting of the metapelites sourced from the yangtze block subsequent to the accretion of the huaiyu arc to the jiuling terrane the continental margin of the yangtze block the second phase a muscovite bearing quartz porphyry is strongly peraluminous a cnk 1 29 1 34 with low cao na2o ratios 0 11 0 14 and low feot mgo tio2 contents 2 39 2 75 wt similar to melts derived from fluid fluxed melting of metapelites zircon grains from the quartz porphyry yielded u pb ages of ca 803 801 ma with mainly positive ehf t values 1 9 to 8 7 which are younger than the depositional ages of the basement sequences and nearly identical to the accumulation of the overlying unmetamorphosed cover in the jiangnan orogen therefore it is suggested that the quartz porphyry was formed in a post collisional extensional setting subsequent to the final assembly of the yangtze and cathaysia blocks in conjunction with published data we conclude that multiple subduction accretion events may have been involved in the formation of the south china block during neoproterozoic times

constraints from zircon u pb ages o and hf isotopic compositions on the origin of neoproterozoic peraluminous granitoids from the jiangnan fold belt south china zircon textures and micro chemical compositions precisely record the origin and petrogenesis of granitoids which are crucial for evaluating crustal growth and reworking thermal and geodynamic evolution zircons in peraluminous granitoids from the three largest 820 ma complexes guibei yueyang and jiuling in the jiangnan fold belt in south china are used to constrain their sources and petrogenetic processes zircons in the guibei granitoids have complex internal structures nearly all magmatic and inherited zircons have similar ehf 6 8 to 5 6 and 18o values 8 8 11 6 and dominantly lie between ehf evolution vectors for a crust created between 1 7 and 2 1 ga suggesting that the guibei granitoids were produced by partial melting of recycled heterogeneous supracrustal material however the yueyang granitoids contain zircons with high ehf 0 5 to 9 7 and relatively low 18o values 5 9 8 4 and two stage model ages of 1 1 1 8 ga and thus may have been formed by melting of mafic rocks from the lower crust the jiuling granitoids and their enclaves contain more complex zircons with more variable ehf 7 2 to 9 7 and 18o values 7 0 10 6 and lie along the mixing trend between the above proposed infracrustal and supracrustal granitoids therefore the neoproterozoic peraluminous granitoids in the jiangnan fold belt were produced by melting and mixing of continental crust compared with extremely low 4 and negative 18o values of neoproterozoic igneous zircons formed in its northern active continental margin the high 18o peraluminous granitoids in the southeastern yangtze block are considered to have been formed by melting of hydrothermally unaltered continental crust triggered by asthenosphere upwelling in the nanhua back arc basin

petrology and origin of primitive lavas from the troodos ophiolite cyprus parental magmas to the troodos ophiolite are characterised by low tio2 and al2o3 and high sio2 extremely fresh and chemically primitive high mgo rocks are found within the upper pillow lavas and along the arakapas fault belt of cyprus and contain forsteritic olivine enstatite and groundmass clinopyroxene set in glass or plagioclase with accessory magnesiochromite and sometimes hornblende they are quartz normative and may have originally contained up to 3 wt h2o geochemically there are three distinct groups of primitive lavas based on tio2 and zr contents but also reflected by cao na2o and ree abundances these groups cannot be related by crystal fractionation and are considered to have been generated by incremental melting of a variably depleted source region the parental magma to the least depleted group group i was that of the major portion of the troodos plutonic complex and is similar to those postulated for other low ti ophiolites chemically it has close affinities with komatiitic basalts the most depleted lavas group iii all have u shaped ree profiles and variable 143nd 144nd ratios interpreted in terms of metasomatism of the source region by an incompatible element enriched component which was probably derived from a subducted slab these lavas represent an intermediate step in the development of boninite series rocks

provenance and tectonic setting of the proterozoic turbidites in hunan south china geochemical evidence the geochemical composition of the middle upper proterozoic turbidites in hunan south china is related to provenance and tectonic setting of the sedimentary basin the turbidites are characterized by moderate sio 2 contents al 2 o 3 sio 2 typically 0 1 0 3 and moderate k 2 o na 2 o ratios generally 0 1 0 3 but relatively high fe 2 o 3 mgo contents 5 10 wt and unusually low contents of cao typically 1 wt abundances of ferromagnesian trace elements and incompatible elements are moderate the ree patterns are uniform and similar to the upper continental crust and typical post archean shales with lree enrichment flat hree and significant negative eu anomalies in general the slates show higher contents of fe 2 o 3 mgo and ferromagnesian trace elements and lower ratios of la sc and th sc than the associated graywackes suggesting that more mafic materials were incorporated into the clay size fraction compared to the upper proterozoic banxi group the middle proterozoic lengjiaxi group has higher concentrations for most ferromagnesian trace elements and lower la sc ratios implying more mafic components in the provenance of the latter the relationship among alkali and alkaline earth elements the chemical index of alteration cia and the th u ratios indicate that the source area was affected by a moderate weathering history the main source was old continental crust the yangtze craton dominated by felsic rocks of magmatic origin with a variable admixture of mafic components from continental island arcs e g the fangjingshan dayong and the sibao yiyang island arc chains the flysch deposition took place in a back arc basin rather than a failed intracontinental rift as proposed by some workers we suggest that suturing between the yangtze and cathaysia blocks did not occur until late proterozoic time and that the final assembly of the supercontinent rodinia in south china was probably marked by the jingning orogeny at 0 8 ga

dating transcurrent terrane accretion an example from the meguma and avalon composite terranes in the northern appalachians the polygenetic minas geofracture separates the meguma and avalon composite terranes of the maritime appalachians dynamically recrystallized muscovite and biotite occur within d3 shear zones which developed during dextral movement along the geofracture these record 40ar 39ar plateau ages between circa 360 and 370 ma comparable plateau ages are recorded by micas within undeformed portions of granitic plutons and their low pressure contact metamorphic aureoles these are similar to u pb ages previously reported for igneous monazite therefore indicating relatively rapid postmagmatic cooling at shallow crustal levels the d3 fabrics were superimposed on earlier s1 structures which formed during initial amalgamation of the meguma and avalon composite terranes whole rock slate phyllite samples collected outside high temperature portions of the plutonic contact aureoles yield 40ar 39ar plateau ages which correspond to groupings of circa 400 395 ma 390 385 ma and 380 375 ma these are interpreted to date diachronous formation of the s1 fabrics and concomitant low grade regional metamorphism in the circa 25 ma between initial amalgamation and granitic plutonism it is inferred that there was a slow rise of isotherms through the crust as a result of overthrust tectonic thickening this led to crustal melting and the production of widespread granitic magmas which were emplaced immediately prior to d3 dextral shearing

the position and nature of the gander avalon boundary southern new brunswick based on geochemical and isotopic data from granitoid rocks in southern new brunswick the gander avalon boundary is obscured by boundary parallel faults and various cover sequences siluro devonian granites which intrude unequivocal gander or avalon rocks display exclusively negative 1 9 1 0 and positive 1 9 0 7 end t signatures respectively such contrasting nd isotopic signatures combined with other geochemical differences between plutons are potentially valuable tools for terrane analysis nine small devonian plutons intruding the boundary zone fall into contrasting geochemical groups with la lu n 4 the former are topaz bearing granites while the latter are volcanic arc type granites except for one pluton with an end t signature of 2 0 end t values range from 0 4 to 0 7 spanning the gap between type avalon and type gander plutons these results suggest the plutons sampled either i stratigraphically overlapping or tectonically interleaved gander and avalon basement rocks or ii a distinct basement source beneath the b

the kingston terrane southern new brunswick canada evidence for an early silurian volcanic arc the fault bounded kingston terrane of southern new brunswick consists of felsic and less abundant mafic volcaniclastic rocks and flows and minor interbedded sedimentary rocks of the kingston group intruded by mainly granitic plutons and abundant younger mafic sheets the granitic plutons form about half of the terrane and are characterized by fine grain size and typically granophyric and porphyritic textures indicative of high level emplacement mafic sheets were emplaced in the granitic plutons and parallel to primary layering in the kingston group which is typically steeply dipping and trends northeast to north northeast hence in contrast to some earlier interpretations the kingston terrane does not consist of a bimodal dike complex the kingston group granitic rocks and mafic sheets contain mineral assemblages consistent with metamorphism to upper greenschist lower amphibolite facies new u pb zircon ages of 442 6 ma from a dacitic tuff unit and 437 10 ma from a granitic pluton corroborate previous u pb dates that had suggested that the volcanic and granitic rocks in the kingston terrane are early silurian rather than precambrian as had been earlier assumed the felsic volcanic and granitic rocks are interpreted to be comagmatic on the basis of their field relationships and similar u pb ages and chemical character compositions of both the felsic rocks and less abundant mafic and intermediate and mafic rocks are consistent with calc alkalic affinity and emplacement in a continental margin volcanic arc herein termed the kingston arc a u pb date of 435 5 ma was also obtained from compositionally similar granite that forms a dike in an adjacent fault bounded amphibolite facies metasedimentary unit the similarity in age and composition of the granite dike suggests a link between these metasedimentary rocks and the kingston terrane possibly as part of an accretionary complex if so its location southeast of the kingston terrane indicates that subduction was to the present northwest mafic sheets in the kingston terrane are tholeiitic but have some arc like characteristics consistent with their origin in a former arc setting but the sheets were likely emplaced in a subsequent extensional setting the compositional similarity of the mafic sheets to late silurian mafic volcanic rocks and plutons of the coastal maine magmatic province suggests that they may be related a possible model for evolution of the kingston terrane is proposed wherein the kingston group and related granite formed in an early silurian arc on the margin of ganderia as represented by the new river terrane of late precambrian and early paleozoic age northwestward subduction culminated in collision between the kingston arc and a continental block now represented by the brookville terrane to the southeast subsequent transpressive mo tions in the late silurian may have been responsible for the crustal scale extensional environment in which the coastal maine magmatic province was formed including emplacement of mafic dikes into the deformed former kingston arc transpressive motions related to juxtaposition of the avalon and meguma terranes with the previously accreted more inboard terranes continued through the devonian and into the carboniferous

tectonics of the acadian orogeny in new england and adjacent canada paleogeographic analysis of post taconian rocks in new england and adjacent parts of canada has revealed the existence of two volcanic arcs which shut off at the time of the acadian orogeny one was built on arc basement previously accreted to north america during the taconic orogeny the other on precambrian continental basement of avalonia in the intervening merrimack fredericton trough metamorphosed and poly deformed turbidites and black shales record deep water conditions in silurian following mckerrow and ziegler 1971 this tract is interpreted as the site of an ocean which closed in siluro devonian by simultaneous subduction beneath both continental margins in the molucca sea in indonesia a comparable arc arc collision is in an early stage of development moore et al 1982 suggested that an accretionary prism built against one arc is overthrusting its counterpart which developed on the other side of the ocean an identical geometry with an avalonian accretionary prism overriding the conve

u pb age and hf isotope composition of detrital zircons from neoproterozoic sedimentary units in southern anhui province south china implications for the provenance tectonic evolution and glacial history of the eastern jiangnan orogen abstract neoproterozoic sedimentary units at the southeastern margin of the yangtze block south china record a complex geological history including development of the jiangnan orogen during assembly of the yangtze and cathaysia blocks and later episodes of glacial activity the timing of these events is controversial and we have used u pb hf compositions of detrital zircons from the lantian type section to constrain their ages in southern anhui province at the eastern end of the jiangnan orogen this section comprises the xikou group below an angular unconformity and the xiuning formation and overlying glacial leigongwu formation above the unconformity detrital zircons in all three units are dominated by 2 6 2 4 ga 2 1 1 9 ga and 960 740 ma age populations suggesting erosion from similar source regions and an increasing proportion of older grains up through the sequence reflects a change in depositional environment from syn collisional to extensional pre neoproterozoic zircon grains were derived from the yangtze block basement much of which is now concealed by younger rocks while the neoproterozoic population was eroded from local igneous rocks in the jiangnan orogen zircon hf isotope compositions indicate that 2 6 2 4 ga source rocks were a mix of juvenile and reworked crust while 2 1 1 9 ga source rocks were dominated by reworked crust neoproterozoic sources show a switch from 960 860 ma juvenile crust to 860 740 ma juvenile and reworked crust reflecting a transition in the jiangnan orogen from subduction to collision and extension the youngest detrital grains in the xikou group and overlying xiuning formation indicate deposition after ca 810 ma and ca 732 ma respectively correlating closely with comparable sequences elsewhere in the yangtze block this demonstrates that the unconformity in south anhui is part of a regional erosion surface that formed more or less synchronously throughout the orogen at 830 800 ma consistent with it dating the end of collision between the yangtze and cathaysia blocks the age of the overlying glacial leigongwu formation is difficult to constrain from detrital zircons because of a paucity of post 720 ma source rocks but our data suggest the leigongwu diamictite in the lantian section is a single marinoan age glacial unit deposited after 649 13 ma it follows that a carbonate layer in the middle of the leigongwu diamictite in the lantian section most likely reflects later faulting rather than two separate glacial sequences although we cannot rule out the presence of both sturtian and marinoan diamictite elsewhere in south anhui

synthetic and natural tremolite in equilibrium with forsterite enstatite diopside and fluid the following equilibrium among tremolite forsterite diopside and orthorhombic enstatite has been investigated using either synthetic tremolite or natural amphibole in the starting materials ca2mg5si8o22 oh 2 mg2sio4 2 camgsi2o6 5mgsio3 h2o a significant increase in the stability of the reactants was observed with natural rather than synthetic tremolite for example in nearly pure h2o with the h2 content of the fluid buffered by nickel bunsenite at one kilobar 108 pascals the breakdown of the assemblage with synthetic amphibole occurs at 708 20 c the breakdown of the assemblage with natural amphibole ca2 16mg4 94fe0 03si7 92 al0 01o22 oh 2f0 03 occurs at 841 47 c the shift in the breakdown curve is attributed to variation in the properties of the amphiboles since all other factors were common in the experiments the reactions have also been investigated with hydrogen fugacity defined by the methane buffer and the nb oh xg coh buffer analysis of the experimental data by linear programming indicates that the enthalpy of reaction is tightly constrained when the calorimetrically determined entropy of 160 92 joules degree is used the resulting enthalpy of reaction is 113 96 1 82 kilojoules with the natural amphibole and 104 83 0 12 kilojoules with synthetic tremolite deviation of the natural amphibole from the ideal tremolite formula as well as a greater number of defects and dislocations in the synthetic amphibole may have contributed to the change in stability

petrogenesis and geodynamic significance of the 850 ma dongling a type granites in south china abstract the ongoing controversy on the timing of amalgamation of the yangtze and cathaysia blocks either prior to 890 ma or later than 830 ma impedes a proper understanding of geological evolution of the south china block and its role in the breakup of rodinia supercontinent in this study we report 850 ma la icp ms zircon u pb ages and whole rock geochemistry of the dongling granites from the southeastern yangtze block that did not receive much geoscientific attention as compared to the better studied 830 760 ma sedimentary and igneous rocks in the south china block the studied dongling granites have high k2o na2o 7 44 9 09 wt and low mgo 0 07 0 54 wt and cao 0 16 1 21 wt contents their high feot feot mgo 0 82 0 94 and ga al 2 6 values and hfse abundance zr nb ce y 443 965 ppm allow their grouping as a type granite in the chondrite normalized diagrams these granites display right inclined ree patterns and strong to moderate negative eu anomalies eu eu 0 32 0 70 the dongling granites have variable and negative whole rock end t 14 6 to 8 6 and zircon ehf t 12 8 to 4 4 with paleoproterozoic two stage nd 2 20 2 69 ga and hf model ages 2 01 2 41 ga zircon saturation parameters tzr suggest 850 c temperature and other geochemical characteristics reveal low melting pressure 350 to

a tectonomagmatic model for the genesis and emplacement of caledonian calc alkaline lamprophyres petrogenetically related caledonian calc alkaline lamprophyric and shoshonitic dykes in eastern ireland appear coeval with sinistral transtension these dykes are exposed in a 10 km wide zone trending sw from clogher head that strikes anticlockwise of the surface trace of the iapetus suture lamprophyre dykes intruded sinistral riedel shears with a mean strike trend at 040 riedel shears that host lamprophyre dykes strike anticlockwise of primary sinistral shears by 35 consistent with sinistral transtension primary shears strike 070 parallel to the mean strike trend of shoshonitic dykes sinistral primary shears may have controlled the emplacement of caledonian shoshonitic intrusive rocks using lamprophyres from clogher head as examples a new model is proposed for lamprophyre genesis and emplacement related to simple shear in this model transient decompression of metasomatized mantle during simple shear triggers release of volatile rich potassic magma which ascends to crustal levels along steep strike slip faults an association between lamprophyre magmatism and deep faulting suggests that relatively deep 60km seismicity may precede magma ascent this model can account for several paradoxes of caledonian magmatism

tectonic setting of avalonian volcanic and plutonic rocks in the caledonian highlands southern new brunswick canada the caledonian highlands of southern new brunswick consist of late proterozoic to cambrian rocks generally considered typical of the avalon terrane of the northern appalachian orogen mainly tuffaceous volcanic and sedimentary rocks of the broad river group and cogenetic dioritic to granitic plutons with ages ca 620 ma form most of the eastern caledonian highlands they have petrological features indicative of origin in a continental margin subduction zone significantly younger ca 560 550 ma dacitic to rhyolitic lapilli tuffs and flows laminated tuffaceous siltstone basaltic and rhyolitic flows and clastic sedimentary rocks of the coldbrook group form most of the western highlands and occur locally throughout the highlands the mainly tuffaceous lower part of the group has been intruded by gabbroic and syenogranitic plutons that are interpreted to be cogenetic with basaltic and rhyolitic flows in the upper part of the group this voluminous subaerial magmatism may have formed during postorogenic ex

post orogenic extension in the eastern part of the jiangnan orogen evidence from ca 800 760ma volcanic rocks we present a systematic geochronological and geochemical study on ca 800 760 ma volcanic rocks in the eastern part of the jiangnan orogen the xucun composite dykes are dated at ca 805 ma the mafic components have oib like trace element patterns and positive anomalies in zr and hf the least contaminated sample has relatively depleted nd isotopic features suggesting the xucun mafic dykes may have been generated from the partial melting of oib like asthenosphere with later crustal contamination the xucun felsic dykes have decoupled nd hf isotopes and the hf isotope compositions of zircons indicate that the dykes may be derived from the partial melting of the early neoproterozoic juvenile crustal materials with minor incorporation of paleoproterozoic crustal components the ca 800 790 ma shangshu volcanics include two compositional series calc alkaline and tholeiitic the shangshu calc alkaline volcanics in the minjiawu area have low abundances of lile hfse and high na2o contents and sr y ratios similar to adakitic rocks the evident arc like geochemical features and radiogenic nd isotopes nd t values of 3 7 to 4 8 suggest that these rocks may have been generated from the partial melting of juvenile lithospheric mantle metasomatized by na rich melts released from the subducted slab the tholeiitic mafic rocks from the shangshu bimodal volcanics represent two different magma sources the partial melting of metasomatized lithospheric mantle led to the formation of arc like basalts with low tio2 contents negative anomalies in zr and hf and high values of mg and nd t 6 2 whereas the partial melting of asthenospheric mantle generated volcanic rocks with high tio2 contents and low positive nd t 1 4 to 2 7 without negative anomalies of nb ta zr and hf the shangshu felsic rocks were formed by the reworking of early neoproterozoic juvenile arc crustal materials the ca 760 ma mafic rocks from the puling bimodal volcanics generally have low tio2 contents 0 9 wt nearly flat ree distributions and arc like trace element patterns they may have been generated from the high degree partial melting of metasomatized lithospheric mantle one sample has a high tio2 content 2 41 wt and high nd t 6 2 with overall oib like trace element patterns implying the local partial melting of asthenospheric mantle the occurrence of significant volumes of bimodal volcanics in the eastern part of the jiangnan orogen suggests an extensional setting in the period ca 800 760 ma the evident partial melting of newly metasomatized lithospheric mantle and subordinate partial melting of asthenosphere suggest that post orogenic extension shortly after the neoproterozoic orogenesis may be a better explanation for the genesis of the mid neoproterozoic magmatic rocks in the eastern part of the jiangnan orogen post orogenic extension may be diachronous along the whole orogenic belt and probably has no direct relationship with the rodinia rifting event a more detailed model is presented to illustrate the evolution of the eastern part of the jiangnan orogen

geochronological and geochemical constraints on sulfide mineralization in the qingmingshan mafic intrusion in the western part of the proterozoic jiangnan orogenic belt along the southern margin of the yangtze craton abstract the qingmingshan magmatic fe ni cu sulfide deposit is located in the southern margin of the yangtze craton in the western part of the proterozoic jiangnan orogenic belt sulfide mineralization occurs as disseminated zones in a dike like mafic body intruding early neoproterozoic meta sedimentary rocks the host intrusion can be further divided into an upper gabbronorite unit a middle dolerite unit and a lower gabbronorite unit each unit contains at least one disseminated sulfide zone sulfide mineral assemblages are composed of pyrrhotite pentlandite and chalcopyrite zircon u pb dating yields a crystallization age of 847 8 3 8 ma 2 for the intrusion the host rocks are characterized by enrichment in light rare earth elements ree relative to heavy ree and show pronounced negative nb ta anomalies similar to the characteristics of arc mafic igneous rocks worldwide mixing calculations indicate that the variations of th yb n and th nb n in the intrusion are consistent with between 10 and 20 wt crustal contamination of the parental magma the sulfide separates are characterized by elevated 34s values from 1 5 to 3 5 with a peak at 2 6 and by os values from 2 to 89 indicating addition of crustal sulfur and os to the parental magma the trace element data and s os isotopes support the premise that sulfide saturation in the qingmingshan magma was triggered by crustal contamination variations of pge tenors in different sulfide zones can be explained by variable r factors from 300 to 1000 during sulfide segregation from the magma mass balance calculations indicate that the parental magma was severely depleted in pge which could be a primary signature of arc basaltic magma formed by low degrees of partial melting or due to previous sulfide segregation at depth the lithological and mineralization zonation across the dike like qingmingshan mafic intrusion indicates that at least three separate pulses of sulfide charged magma were involved in the formation of the qingmingshan magmatic sulfide deposit the qingmingshan mafic intrusion is interpreted to be a part of a sub arc magma plumbing system coeval ultramafic intrusions formed from more primitive magma in the region are good exploration targets for magmatic sulfide deposits with higher ni cu ratios than the qingmingshan deposit

geochemistry of siluro devonian tobique volcanic belt in northern and central new brunswick canada tectonic implications siluro devonian volcanic rocks of the northwestern mainland appalachians are found mainly in the tobique belt of new brunswick where they consist predominantly of bimodal mafic felsic suites erupted in a continental rift environment the axis of the tobique rift trends north northeast south southwest obliquely to the regional northeast southwest trend of the appalachians these geometric relationships are interpreted as being the result of rifting in a sinistral shear regime produced during emplacement of the avalon terrene the basaltic rocks are continental tholeiites and transitional basalts derived from a heterogeneous upper mantle source that was enriched in incompatible elements relative to the primordial mantle the mantle source was probably affected by the subduction processes

the yanbian terrane southern sichuan province sw china a neoproterozoic arc assemblage in the western margin of the yangtze block abstract the magmatic and tectonic history of the yangtze block and its possible affinity with other neoproterozoic arc terranes are important elements in the reconstruction of neoproterozoic plate tectonics the yanbian terrane in the western margin of the yangtze block is a typical arc assemblage composed of a flysch type sedimentary sequence intruded by gabbroic and granodioritic plutons the sedimentary sequence consists chiefly of tuffaceous material with interlayered chert sandstone and pillow basalts laser ablation icp ms u pb dating of detrital zircons from the sandstones yield ages as young as 840 ma the gaojiacun and lengshuiqing mafic intrusions are dated at 812 3 ma and 806 4 ma respectively using the shrimp zircon u pb technique geochemical data show that both the gaojiacun and lengshuiqing intrusions have arc signatures with nd t values of 1 5 to 6 0 initial 87 sr 86 sr ratios of 0 705 0 706 and pronounced negative nb ta and zr hf anomalies their geochemical variations are best explained by fractional crystallization without major crustal contamination the yanbian terrane represents a typical arc assemblage formed on the western edge of the yangtze block during neoproterozoic time the sedimentary sequence was deposited in an oceanic setting probably in a back arc basin environment the depleted subduction modified lithospheric mantle wedge above the subduction zone was the source of melts from which the mafic plutons were crystallized this scenario suggests subduction of oceanic lithosphere eastward present day orientation underneath the yangtze block

neoproterozoic high mg basalts formed by melting of ambient mantle in south china abstract the southeastern yangtze block south china contains voluminous late neoproterozoic felsic plutonic rocks and minor mafic ultramafic volcanic rocks some ca 830 ma lavas hosted in the neoproterozoic strata have pillow structures and micro spinifex textures and thus previously were considered as komatiitic basalts derived from a mantle plume these rocks have high mgo 10 1 16 3 wt variable cao 6 57 11 2 wt restricted al2o3 11 2 13 4 wt and low tio2 0 50 0 61 wt p2o5 0 03 0 04 wt and incompatible trace elements they also have negative nd 5 4 to 2 9 and high 207pb 204pb 15 71 15 76 and initial 187os 188os ratios 0 1100 0 1931 indicating crustal contamination the most primitive samples show arc like trace elemental features which are characterized by enrichment of lree and lile relative to hfse in the primitive mantle normalized trace element diagrams suggesting derivation from enriched mantle sources primelt2 modeling shows that their primary magmas were produced by about 30 partial melting of a fertile peridotite source having a mantle potential temperature of 1440 1500 c such a potential temperature is in agreement with the estimates for ambient mantle during the neoproterozoic these evidence suggest that the high mg basalts are similar to boninites and probably be part of an ophiolite assemblage they were generated by melting of a hot ambient mantle source in relation to slab subduction during the amalgamation of the yangtze and cathaysia blocks

tectonic evolution of the southeastern margin of the yangtze block constraints from shrimp u pb and la icp ms hf isotopic studies of zircon from the eastern jiangnan orogenic belt and implications for the tectonic interpretation of south china abstract the jiangnan orogenic belt is a ne sw trending fold belt in the southeastern yangtze block of south china the eastern segment of the belt located in northeastern jiangxi and southern anhui provinces is predominated by greenschist facies metamorphosed volcanic sedimentary strata with minor ophiolite and their protoliths are considered to have formed in a back arc basin zircon shrimp u pb dating reveals that these rocks were deposited emplaced in the period 842 825 ma zircon la icp ms hf analyses show that most igneous zircons from the volcanic sedimentary rocks possess positive hf t values of 0 09 12 74 corresponding to depleted source model ages of 0 9 1 4 ga this suggests a significant neoproterozoic juvenile crustal growth event in the southeastern part of the yangtze block which is also documented by gabbro 833 ma from ophiolite with positive hf t values of 5 24 13 25 and hafnium model ages of 0 87 1 20 ga these new data combined with previous results indicate the presence of an arc and back arc basin system along the southeastern margin of the yangtze block at 842 825 ma collision of the arc with the proposed shuangxiwu composite terrane which had previously formed away from the southeastern margin of the yangtze block through possible subduction and collision led to the closure of the back arc basin and the formation of the jiangnan belt in the period 825 815 ma we suggest that south china is an accretionary orogen and that orogen parallel motion along terrane bounding shear zones may have played a significant role in its amalgamation

ca 825 ma komatiitic basalts in south china first evidence for 1500 c mantle melts by a rodinian mantle plume mantle plume or superplume activities have repeatedly been invoked as a cause for the breakup of the neoproterozoic supercontinent rodinia with supportive evidence including radiating dike swarms globally synchronous anorogenic igneous activity large scale lithospheric doming and unroofing and geochemical signatures similar to recent plume related magmatism however high temperature magmas such as picrite or komatiite a requisite product of mantle plume activities have not previously been identified for those events we present here geochronological and geochemical data from a suite of pillow lavas in central south china sensitive high resolution ion microprobe shrimp u pb dating of zircons from an evolved member of andesitic composition within the suite indicates that the lavas were erupted at 823 6 ma all but a few highly evolved crust contaminated basaltic rocks are characteristically high in mgo 10 2 17 5 ni 183 661 ppm and cr 677 1672 ppm but low in tio2 0 5 0 7 al2o3 10 6 12 7 and feot total fe as feo 7 4 10 5 typical of komatiitic basalts our geochemical modeling which removes the effect of olivine crystallization suggests that their primary magma has typical komatiitic compositions with mgo 20 feot 11 sio2 47 tio2 0 48 al2o3 10 ni 860 ppm and cr 1780 ppm such a high mgo content in the primary melts implies a melt temperature of 1500 c suggesting that the yiyang komatiitic basalts were generated by melting of an anomalously hot mantle source with potential temperature t p 260 50 c higher than the ambient mid oceanic ridge basalt morb source mantle similar to that of modern mantle plumes the yiyang komatiitic basalts are thus the first solid petrological evidence for the proposed ca 825 ma mantle plume that played a key role in the breakup of the supercontinent rodinia

landscape human shaping and spatial mobility of agropastoral practices in the chaine des puys during historical times massif central france abstract the chaine des puys is a great collection of extinct volcanoes that cover the hercynian basement and belongs to the auvergne volcanoes natural regional park both the sedimentary record and the historical archives of this area are numerous however the cores studied hitherto do not offer a good knowledge of the historical dynamics because most of the wetlands have dried out during historical times causing gaps in the sedimentary record and pollen oxidization moreover all the studied cores are located on the crystalline basement or on an old volcanic one predating the chaine des puys thus they do not directly provide information about the volcanoes this article presents the only core obtained from a site a small maar located on a volcano the sequence is short but does not have the same sedimentary gaps as the others owing to physical grain size chemical c n p k ca mg n no 3 n nh 4 palaeoecological pollen and non pollen palynomorphs and historical analyses this study gives a more precise recording of the landscape changes during the last 500 years in the volcanic area and contributes to the knowledge of the chaine des puys landscape it illustrates the mobility of the historical landscape in particular the progression and regression of the forest and the erosive crises related to more intensive grazing activity the observations are linked with the social and economical changes especially with the consequences of the french revolution and the rural depopulation this landscape history gives data about the direction of changes to the landscape preservationists they demonstrate that even if the chaine des puys landscape looks natural it has been shaped by a very specific socio economical system inherited from the middle ages these aspects could be integrated in a management plan

geochemistry of silurian devonian volcanic rocks in the coastal volcanic belt machias eastport area maine evidence for a pre acadian arc the coastal volcanic belt in easternmost maine consists of silurian devonian volcanic rocks related to the convergence of avalonia with the leading edge of composite laurentia just before and during early stages of the acadian orogeny new major and trace element analyses from the machias eastport area indicate that the early to late silurian volcanic rocks in the dennys edmunds and leighton formations formed in a continental arc environment in contrast to some earlier interpretations these rocks have a continuous compositional range from basalt through andesite to rhyolite the mafic rocks in these formations are calc alkaline and show subduction related geochemical signatures on various discrimination diagrams whereas the felsic rocks have geochemical characteristics of volcanic arc granites in contrast the thick volcanic deposits of the younger late silurian eastport formation are bimodal and tholeiitic and have transitional to within plate geochemical signatures thus the transition over time as the acadian seaway closed from subduction and arc settings to continental extension can been seen in this rock sequence and it constrains the time of avalonia s docking regionally volcanic rocks in the kingston terrane of southern new brunswick are likely correlatives of the earlier arc volcanic rocks dennys edmunds and leighton formations while eastport formation analogues are found in the coastal maine magmatic province cranberry island series and probably the passamaquoddy bay volcanic series which similarly appear to have formed after the transition to an extensional environment occurred although other extensional settings cannot be entirely ruled out basalts in the post acadian perry formation indicate continental extension was also active in the late devonian

geochemical complexities preserved in the volcanic rocks of the zambales ophiolite philippines the geochemical characteristics of the volcanic rocks from the zambales ophiolite luzon philippines indicate that the oceanic crust now preserved in the ophiolite must have been generated in an oceanic subduction zone environment the volcanic rocks and dikes range in composition from magnesian basalt including mid ocean ridge basalt morb island arc tholeiite and boninite like lavas to andesite and dacite and have trace element abundances which suggest derivation from a mantle source which was heterogeneous on both regional and local scales the sources are mixtures of components variably depleted with respect to a morb source differences exist between respective exposures of the ophiolite from north to south the following geochemical patterns in the basaltic rocks emerge sm nd decreases with corresponding decreases in 143nd 144nd rare earth element ree patterns range from extremely light ree lree depleted with generally low abundances of all ree 1 5x chondritic to patterns which are slightly lree depleted similar to morb patterns throughout the range sr isotopic abundances are relatively high 87sr 86sr 0 7035 0 7040 perhaps more significant than the regional geochemical trends however is that all volcanic provinces display ranges of rock compositions and the spatial distribution of basalts with such variable geochemical affinities indicate superposition interfingering and possible mixing of different lavas we suggest that the array of geochemical data from the zambales ophiolite can be explained in terms of processes observed in present day convergent plate margins such as the marianas or lau basin in the western pacific complicated plate boundaries which have existed for long periods of time including closely opposing and changing subduction systems the rifting of arcs and the formation of backarc basins may result in the superposition of one lava type on another or may produce many small domains in the upper mantle sources for subduction related lavas some of which become extremely depleted or secondarily enriched magmas derived from such a heterogeneous mantle will display ranges in geochemical characteristics possibly similar to those observed in the zambales ophiolite indeed recent studies from present day backarc systems suggest that young arc volcanism is superimposed on morb like crust and that basalt compositions of certain backarc basins are zoned inward from arc like to morb like these studies indicate that relationships between arc and morb like crust in young subduction complexes is extremely complex hawkins and melchior 1985 volpe et al 1987 1990 we extrapolate arguments from studies of zoned backarc systems to the zambales ophiolite and in turn use data from the zambales ophiolite as a ruler for defining the scale of variation in these complex supra subduction zone systems

tectonic setting of eocene boninite magmatism in the izu bonin mariana forearc abstract middle eocene boninites were simultaneously generated over a large region during the early history of the izu bonin mariana ibm arc however widespread boninite magmatism is not recognised in younger subduction zones of similar dimensions this suggests that an additional tectonic or thermal factor influenced the generation of the ibm boninite suite regional uplift ocean island basalt style magmatism and high heat flow also characterised the northern philippine sea plate psp at the start of the middle eocene these features are similar to those observed in large volume basaltic volcanic provinces such as the early tertiary north atlantic and suggest that the ibm boninite suite may have been produced because there was already a thermal anomaly in the mantle beneath the psp the reconstructed middle eocene location of the ibm arc and west philippine basin lies close to the present day manus basin where petrological and geochemical evidence indicate the presence of a mantle plume a calculated plume track linking these locations through time also passes close to the eauripik rise an aseismic ridge on the caroline plate during the oligocene and miocene therefore we propose that a thermal anomaly or mantle plume influenced the magmatic and tectonic development of the western pacific from the middle eocene until the present day

komatiites and picrites evidence that the plume source is depleted high temperature magmas such as picrites and komatiites are candidates for the material in the hot axial region of mantle plumes in current plume theories these magmas should have an enriched ocean island basalt oib type chemistry high mgo magmas however are often more like depleted mid ocean ridge basalts morb in trace element and isotopic chemistry this is a paradox for deep mantle plume or primitive mantle theories but is expected if hotspots and morb share a common depleted source and the transition to oib chemistry involves shallow level cooling crystal fractionation and contamination ancient komatiites appear to have come from the same deep depleted reservoir as modern picrites and morb

neoarchean convergent margin ni cu mineralization axis lake and nickel king ni cu deposits in the south rae craton of the canadian shield abstract the southern portion of the rae craton in the canadian shield is host to multiple ca 2 63 ga ni cu rich metamorphosed mafic intrusions with noritic noritic anorthositic and pyroxenitic compositions including those hosting the undeveloped axis lake and nickel king deposits found in northern saskatchewan and southeast northwest territories in order to understand the setting of these intrusions and their concomitant mineralization we investigated their geochemical and isotopic characteristics mineralized intrusions at axis lake and nickel king are metamorphosed up to granulite facies with local partial melting the intrusions preserve whole rock major element geochemistry consistent with a sub alkaline basaltic composition e g sio2 46 54 wt tio2 the nd isotope values of the axis lake intrusion ndt2630 0 3 to 2 1 are more juvenile and closer to the depleted mantle value relative to those of the nickel king deposit ndt2630 2 4 to 0 4 this agrees with their also distinctive th yb and nb yb distributions suggesting oceanic and continental arc like settings respectively the axis lake and nickel king nd isotope compositions are indistinguishable from other arc like 2 61 to 2 58 ga mafic to felsic intrusions and volcanic rocks recognized in the rae craton and the neighboring chesterfield block the age spatial distribution and geochemistry of the studied intrusions coincide with the initial stages of a recently defined 2 63 2 60 ga continental arc the roots of which are found in the south rae craton our data indicate that neoarchean intrusions with an arc like geochemistry may be a significant exploration target in the rae craton this broadens possible host suites beyond the established paleoproterozoic rift related rocks most commonly considered as the prime target for ni cu pge mineralization

petrology of upper ordovician lower silurian rocks of the antigonish highlands nova scotia upper ordovician to lower silurian rocks in the antigonish highlands consist of interlayered basalts rhyodacites arkoses and conglomerates overlain by a thick sequence of marine clastic rocks and minor rhyolites the stratigraphy documents a marine transgression the volcanic rocks were deposited in a within plate continental extensional environment the basalts display alkalic and tholeiitic affinities and the rhyodacites were formed by anatexis of the crust the origin of the younger rhyolites is not clear they are compositionally distinct from the rhyodacites but may be related to them as late stage differentiates at present it is not possible to evaluate whether the tectonic setting and magmatic affinities are regionally or locally controlled the geological history is very similar to that of lower silurian rocks immediately north of the antigonish highlands at arisaig in the simplest sense this indicates these areas may have been juxtaposed prior to the late ordovician and limits cumulative

evidence for silurian sinistral accretion of avalon composite terrane in canada the kingston dyke complex forms a regionally developed belt of sheeted bimodal dykes within the avalon composite terrane of southern new brunswick canada dyke orientation relative to the belt suggests their injection accompanied sinistral movement and the belt has previously been attributed to transform activity within the late precambrian magmatic arc regime typical of the avalon composite terrane in the northern appalachians however rhyolite dykes of the sheeted complex yield a u pb age of 435 5 1 5 ma that is interpreted to date their emplacement the complex therefore provides evidence of early silurian sinistral displacement in the northern appalachians such movement is suggested by palaeomagnetic data and may record the accretion of the avalon composite terrane to cratonic north america

a rhythmic source change of the neoproterozoic basement meta sedimentary sequences in the jiangnan orogen implications for tectonic evolution on the southeastern margin of the yangtze block abstract the amalgamation of south china block and its role in the assembly and breakup of rodinia supercontinent have been hotly debated in recent decades however the intensity of accreted arcs around south china and the following exhumation were poorly constrained here we present combined u pb dating and hf isotopic analyses of detrital zircons for a stratigraphic section of the upper shuangqiaoshan group in the eastern part of the jiangnan orogen jo u pb dating results suggest a general younging trend of the maximum depositional ages from the anlelin to the xiushui formations of the upper shuangqiaoshan group indicating a co magmatic sedimentation in association with continental arc magmatism along the se margin of the yangtze block the anlelin and xiushui formations were constrained to form in the periods of ca 845 836 ma and ca 835 815 ma respectively provenance analysis suggests two sources for these neoproterozoic basement sequences neoproterozoic zircons with positive ehf t values represent detritus derived from the arc related magmatic rocks to the southeast whereas 1000 ma zircons and the other neoproterozoic detrital zircons with negative ehf t values are proposed to be sourced from the interior of the yangtze block to the northwest simultaneously the hf isotope results of detrital zircons indicate that there was a rhythmic source change for the sediments in the upper shuangqiaoshan group with the amounts of two separate sources from the yangtze block to the west and from the arc terranes to the east increasing alternatively this provides solid evidence for a bidirectional water flow and supports a back arc basin setting for the deposition of the sediments combined with the published u pb and hf isotopic data for detrital zircons from the neoproterozoic folded sedimentary sequences in the jo we calculated the relative contribution of arc derived crustal materials in the basement sequences of the jo the calculations suggest that the proportion increases progressively from west to east and the back arc basins represented by the folded sedimentary sequences could be divided into three regions plus an arc terrane region to the east this comparison further suggests that the orientation of neoproterozoic accretion on the southeastern margin of the yangtze block was in a w e direction and the jo in neoproterozoic may not have extended 1500 km as what we see today

reappraisal of the ages of neoproterozoic strata in south china no connection with the grenvillian orogeny the jiangnan fold belt separates the yangtze and cathaysia blocks in south china and has long been considered grenvillian in age in order to place south china in central rodinia it consists of deformed early neoproterozoic strata that are unconformably overlain by undeformed late neoproterozoic strata and intruded by undeformed and unmetamorphosed granitic plutons zircons from the early neoproterozoic strata yield u pb ages as young as 830 ma and one granitic pluton has a zircon u pb age of ca 827 ma the 830 ma mafic rocks along the southeastern margin of the yangtze block have arc affinity geochemical characters whereas mafic rocks younger than 830 ma have typical ocean island basalt oib like compositions thus we suggest that the early neoproterozoic strata were deposited on an active continental margin prior to amalgamation of the yangtze and cathaysia blocks at ca 830 ma the overlying late neoproterozoic strata were deposited in the intracontinental rifted nanhua basin at 820 730 ma and probably reflect backarc spreading above the long lived 950 735 ma oceanic subduction zone along the northern and western margin of the yangtze block this model is consistent with the secular tectonic evolution of south china during the neoproterozoic the jiangnan fold belt is therefore not a grenvillian feature as previously suggested and there is no evidence to place south china in central rodinia instead we believe that south china was located in a marginal position relative to this supercontinent

neoproterozoic tourmaline bearing peraluminous granitoids in the western jiangnan orogen south china geochemistry petrogenesis and tectonic implications abstract neoproterozoic granitoids are widespread in the jiangnan orogen in south china that was formed by assemblage between the yangtze and cathaysia blocks the peraluminous s type granitoids from the guibei region in the western jiangnan orogen are composed of quartz alkali feldspar and plagioclase with subordinate biotite and muscovite they show large variable feo mgo 1 9 12 8 and k2o na2o cao 2 84 ratios indicating the rocks experienced fractional crystallization their end values 9 31 to 5 45 and two stage nd model ages 1 83 2 34 ga suggest that the guibei granitoids were derived from a mature source that was mainly composed of recycled supracrustal materials the tourmalines occurring as disseminated and nodular grains show high na k 0 56 0 77 apfu low ca 0 01 0 08 apfu and x site vacancy 0 18 0 51 apfu and belong to the alkali group their high fe 1 64 2 28 apfu low mg 0 14 0 80 apfu and mg mg fe 0 016 0 325 suggest that the minerals are dominated by schorl end member in the schorl dravite solid solution series the tourmalines have constant and strong negative 11b values 13 4 to 9 5 and the nodular tourmalines are isotopically lighter 11b 10 0 to 13 4 than the disseminated tourmalines 11b 9 5 to 12 5 the disseminated tourmalines are suggested to have been crystallized in melts at the early magmatic stage whereas the nodular tourmalines formed in fluid rich immiscible melts at the late magmatic stage the boron isotopic variations within or among the tourmaline generations are mainly controlled by rayleigh fractionation in combined with the regional geology the peraluminous granitoids in this study are considered to be partial melts of the supracrustal rocks in the post orogenic setting

phase relations of a high mg basalt from the aleutian island arc implications for primary island arc basalts and high al basalts many volcanic centers in the aleutian islands have erupted lavas that range in composition from high mg basalt mgo 9 wt to more fractionated and voluminous high al basalts and basaltic andesites the petrogenetic relationships between these rock types and the composition of primary magmas has been vigorously debated the phase relations of a typical high mg basalt from the makushin volcanic field on unalaska island provide important constraints on petrogenetic models results of one atmosphere and moderate to high pressure 5 20 kb anhydrous experiments are similar to results obtained from primitive morb at low pressures olivine is the liquidus phase joined by plagioclase and clinopyroxene at progressively lower temperatures clinopyroxene is the second phase to crystallize at pressures greater than 5 kb and replaces olivine on the liquidus at approximately 10 kb above 10 kb the liquidus pyroxene is aluminous augite and orthopyroxene is the second phase to crystallize glasses in equilibrium with olivine and clinopyroxene at intermediate pressure 5 to 10 kb are similar in composition to high al basalt plagioclase is not involved and most likely does not become a liquidus phase until the liquid has evolved significantly although our studies do not confirm the primary nature of high mg basalts they do support a model in which high al basalts are generated by moderate amounts of crystal fractionation from more primitive high mg mg fe lower al2o3 basaltic magmas near the arc crust mantle boundary

a new age constraint on the onset of the neoproterozoic glaciations in the yangtze platform south china abstract a sample from the top of the banxi group early neoproterozoic in hunan province south china was dated by the shrimp zircon u pb method the resulting age of 725 10 ma probably solves the debate between the synglacial and the preglacial correlation of the liantuo formation and indicates that the liantuo formation most likely correlates with the preglacial banxi group since the banxi danzhou group is upwardly transitional to the glacial jiangkou group the age becomes a new maximum age constraint on the onset of the jiangkou glaciation which therefore most likely correlates with the sturtian glaciation

structural and tectonic evolution of the acatlan complex southern mexico its role in the collisional history of laurentia and gondwana 1 correlation of deformational phases and thermal events in the acatlan complex permits definition of nine major tectonic events seven events are related to the evolution of the iapetus and rheic oceans by the early ordovician the xayacatlan suite of laurentian affinity was metamorphosed to eclogite facies and exhumed before colliding with the el rodeo suite from the late ordovician to silurian the upper ordovician ixcamilpa suite with peri gondwanan affinity was metamorphosed to blueschist facies exhumed and subsequently was overthrust by the xayacatlan el rodeo block part of the esperanza suite is an early silurian intra iapetian continental arc that collided with and subducted beneath continental crust generating a silurian eclogitic event by the late devonian collision of the three high p t suites closed the iapetus ocean the resulting composite terrain was thrust over the gondwana bordering cosoltepec suite to close the rheic ocean during the earliest mississippian the collision of the acatlan complex and the oaxaca terrane uplifted and deformed the composite terrane by the early permian deposition of the chazumba suite occurred in a pull apart basin a latest early permian event produced thrust shear zones with n s directed shortening during the middle jurassic plume rise caused regional doming magmatism and amphibolite facies metamorphism during the late cretaceous paleogene folds and thrust faults overprint the acatlan complex most events in the acatlan complex correlate with events in the appalachian chain suggesting that the acatlan complex was located near the appalachians until the jurassic when it was displaced by the gulf of mexico opening

petrogenesis of the early silurian dashuang high mg basalt andesite dacite in eastern south china origin from a palaeosubduction modified mantle the early paleozoic kwangsian event the first compressive orogeny after the rodinia assemblage in the south china block scb is characterized by the angular unconformity between the devonian and pre devonian sequences and widespread granitoids and small volume mafic rocks however the mechanism triggering this event is highly debated and the associated mantle source is poorly known our recent investigations identified the occurrence of early silurian high mg basalt andesite dacite in the dashuang area along the northwestern yunkai domain in the scb this study presents a set of new zircon u pb geochronological whole rock geochemical and sr nd isotopic data for these volcanic rocks they yield weighted mean 206 pb 238 u ages of c 430 443 ma similar to the age peaks of early middle paleozoic magmatism and metamorphism in the scb these volcanic rocks have high mgo cr and ni contents and mg numbers they also exhibit arc like geochemical affinities with crustal like sr nd isotopic compositions similar to the neoproterozoic high mg basalt andesite in the eastern scb such signatures combined with other geological observations suggest that the dashuang high mg rocks were derived from an ancient probably early neoproterozoic mantle wedge source the early paleozoic intra continental reactivation in response to late orogenic extension triggered the melting of this palaeosubduction modified mantle which led to the formation of these volcanic rocks

back arc basin basalt systematics the mariana east scotia lau and manus back arc basins babs have spreading rates that vary from slow 100 mm yr and extension axes located from 10 to 400 km behind their island arcs axial lava compositions from these babs indicate melting of mid ocean ridge basalt morb like sources in proportion to the amount added of previously depleted water rich arc like components the arc like end members are characterized by low na ti and fe and by high h2o and ba la the morb like end members have the opposite traits comparisons between basins show that the least hydrous compositions follow global morb systematics and an inverse correlation between na8 and fe8 this is interpreted as a positive correlation between the average degree and pressure of mantle melting that reflects regional variations in mantle potential temperatures lau manus hotter than mariana scotia this interpretation accords with numerical model predictions that faster subduction induced advection will maintain a hotter mantle wedge the primary compositional trends within each bab a positive correlation between fe8 na8 and ti8 and their inverse correlation with h2o 8 and ba la are controlled by variations in water content melt extraction and enrichments imposed by slab and mantle wedge processes systematic axial depth as a proxy for crustal production variations with distance from the island arc indicate that compositional controls on melting dominate over spreading rate hydrous fluxing enhances decompression melting allowing depleted mantle sources just behind the island arc to melt extensively producing shallow spreading axes flow of enriched mantle components around the ends of slabs may augment this process in transform bounded back arcs such as the east scotia basin the re circulation by mantle wedge corner flow to the spreading axes of mantle previously depleted by both arc and spreading melt extraction can explain the greater depths and thinner crust of the east lau spreading center manus southern rifts and mariana trough and the very depleted lavas of east scotia segments e8 e9 the crust becomes mid ocean ridge mor like where the spreading axes further away from the island arc and subducted slab entrain dominantly fertile mantle

thermal and geochemical evolution of the mantle wedge in the northeast japan arc 1 contribution from experimental petrology melting phase relations at high pressures have been examined for four mg rich basalts from both the trench and the backarc sides of the miocene ne japan arc all these basalts crystallize olivine and orthopyroxene on the liquidus at pressures lower and higher than the point of multiple saturation with those two phases respectively the pressure of multiple saturation is rather constant for all samples 1 0 1 4 gpa suggesting that 1 there was no across arc variation in the depth of magma segregation in the miocene ne japan arc 2 the depth of magma segregation remains constant since 20 ma in the trench side of the volcanic arc whereas it has changed from 30 40 km to 60 km beneath the backarc side of the arc this temporal variation may be caused by increasing thickness of the lithosphere beneath the backarc region associated with cooling of the mantle wedge after the opening of the japan sea backarc basin in the early miocene