an isotopic mixing model for the origin of granitic rocks in southeastern australia a simple mixing process was a significant element in the genesis of many southeastern australian granitic rocks major and trace element abundances are near linearly correlated nd and sr initial isotopic compositions define a simple hyperbolic trend and many initial 87sr 86sr ratios are a regular function of rb sr ratios geological arguments and regression of geochemical variation diagrams place limits on the nature of the end members the high si end member is meta sedimentary derived magma comparable in composition to the cooma granodiorite the one pluton in the region surrounded by and clearly derived from regional metamorphic rocks the low si end member is basaltic material with similarities to rare gabbros also found in the area isotope dilution calculations based upon initial sr and nd isotopic compositions deduced for the end members yield realistic hypothetical sr and nd concentrations for the end members in a demonstration of the internal consistency of the model all of the granitic rocks from hornblende tonalites to cordierite granodiorites are a single broad family products of variable mixing between distinct batches of basaltic material and a uniform partial melt of the regional basement

origin of the late jurassic to early cretaceous peraluminous granitoids in the northeastern hunan province middle yangtze region south china geodynamic implications for the paleo pacific subduction the late mesozoic granitic belt in the northeastern hunan province situated in the south of the middle yangtze region represents the western front of the large magmatic province of se china in order to determine their ages and petrogenesis we carried out zircon u pb dating hf isotope and whole rock geochemical analyses for four granitic plutons namely taohuashan dayunshan mufushan wangxiang and lianyunshan our sims zircon u pb ages together with previously published data reveal that the magmatic activities in this area can be roughly subdivided into three phases at 151 146 ma 132 127 ma and ca 117 ma and the dayunshan mufushan batholith therein is a composite pluton these four plutons are mainly composed of weakly to strongly peraluminous biotite or two mica monzogranites with a minor amount of biotite granodiorites their geochemical features are similar to s type as well as fractionated s type granites with enrichment in lrees and negative ba sr nb p and ti anomalies all samples show negative zircon ehf t values ranging from 12 5 to 3 6 corresponding to crustal hf model tdmc ages of 1 4 2 0 ga it is inferred that these granitoids were derived from partial melting of metasedimentary rocks analogous to the neoproterozoic lengjiaxi group predominantly with psammitic component fractional crystallization probably played an important role in the magma evolution while input of mantle derived magma was insignificant combined with other geological evidence our new data allow us to propose that the cretaceous 132 127 ma and ca 117 ma magmatism might be response to episodic slab rollback of the paleo pacific plate while the early stage 151 146 ma magmatism that overlapped the epilogue of jurassic magmatic flare up and subsequent magmatic quiescence probably foreshadowed the transformation from foundering of a subducted flat slab to slab rollback alternatively slab foundering after a se directed intracontinental subduction in the central scb cannot be ruled out for geodynamic interpretation of the jurassic magmatism in se china

highly evolved juvenile granites with tetrad ree patterns the woduhe and baerzhe granites from the great xing an mountains in ne china abstract in ne china voluminous granitoids were emplaced in late paleozoic and mesozoic times we report here sr nd o isotopic and elemental abundance data for two highly evolved granitic plutons woduhe and baerzhe from the great xing an mountains they show a rather juvenile sr nd isotopic signature and a spectacular tetrad effect in their ree distribution patterns as well as non charac charge and radius controlled trace element behavior the emplacement ages are constrained at 130 4 ma for the woduhe and 122 5 ma for the baerzhe granites by rb sr and sm nd isotope analyses both granites are also characterized by low but imprecise initial 87sr 86sr ratios of about 0 703 the nd sr isotope data argue for their generation by melting of dominantly juvenile mantle component with subordinate recycled ancient crust this is largely compatible with the general scenario for much of the phanerozoic granitoids emplaced in the central asian orogenic belt the parental magmas for both the woduhe and baerzhe granites have undergone extensive magmatic differentiation during which intense interaction of the residual melts with aqueous hydrothermal fluids probably rich in f and cl resulted in the non charac trace element behavior and the tetrad effect of ree distribution both the woduhe and baerzhe granites show the characteristic trace element patterns of rare metal granites but their absolute abundances differ by as much as two orders of magnitude the oxygen isotope compositions of the two granites have been severely disturbed significant 18o depletion in feldspar but not so much in quartz suggests that the hydrothermal alteration took place in a temperature condition of 300 500 c this subsolidus hydrothermal alteration is decoupled from the late stage magma fluid interaction at higher temperatures despite the two distinct and intense events of water rock interaction the rb sr and sm nd geochronological systems seem to have maintained closed hence suggesting that the two events occurred shortly after the plutonic emplacements

geology geochronology geochemistry and ore genesis of the wangu gold deposit in northeastern hunan province jiangnan orogen south china abstract the wangu gold deposit in northeastern hunan south china is one of many structurally controlled gold deposits in the jiangnan orogen the host rocks slates of the lengjiaxi group are of neoproterozoic age but the area is characterized by a number of late jurassic cretaceous granites and ne trending faults the timing of mineralization tectonic setting and ore genesis of this deposit and many similar deposits in the jiangnan orogen are not well understood the orebodies in the wangu deposit include quartz veins and altered slates and breccias and are controlled by wnw trending faults the principal ore minerals are arsenopyrite and pyrite and the major gangue minerals are quartz and calcite alteration is developed around the auriferous veins including silicification pyritic arsenopyritic and carbonate alterations field work and thin section observations indicate that the hydrothermal processes related to the wangu gold mineralization can be divided into five stages 1 quartz 2 scheelite quartz 3 arsenopyrite pyrite quartz 4 poly sulfides quartz and 5 quartz calcite the lianyunshan s type granite which is in an emplacement contact with the ne trending changsha pingjiang fracture zone has a zircon la icpms u pb age of 142 2 ma the dayan gold occurrence in the changsha pingjiang fracture zone which shares similar mineral assemblages with the wangu deposit is crosscut by a silicified rock that contains muscovite with a ca 130 ma 40ar 39ar age the gold mineralization age of the wangu deposit is thus confined between 142 ma and 130 ma this age of mineralization suggests that the deposit was formed simultaneously with or subsequently to the development of ne trending extensional faults the emplacement of late jurassic cretaceous granites and the formation of cretaceous basins filled with red bed clastic rocks in northeastern hunan which forms part of the basin and range like province in south china empa analysis shows that the average as content in arsenopyrite is 28 7 atom and the mineralization temperature of the arsenopyrite pyrite quartz stage is estimated to be 245 20 c from arsenopyrite thermometry the high but variable au as molar ratios 0 02 of pyrite suggest that there are nanoparticles of native au in the sulfides an integration of s pb h o he ar isotope systematics suggests that the ore fluids are mainly metamorphic fluids originated from host rocks possibly driven by hydraulic potential gradient created by reactivation of the wnw trending faults initially formed in paleozoic with possible involvement of magmatic and mantle components channeled through regional fault networks the wangu gold deposit shares many geological and geochemical similarities as well as differences with typical orogenic epithermal and carlin type gold deposits and may be better classified as an intracontinental reactivation type as proposed for many other gold deposits in the jiangnan orogen

the trace element compositions of s type granites evidence for disequilibrium melting and accessory phase entrainment in the source within individual plutons the trace element concentrations in s type granites generally increase with maficity total iron and magnesium content and expressed as atomic fe mg in this study the degree of variability in trace element concentration also expands markedly with the same parameter the strongly peraluminous high level s type granites of the peninsular pluton cape granite suite south africa are the product of biotite incongruent melting of a metasedimentary source near the base of the crust leucogranites within the suite represent close to pure melts from the anatectic source and more mafic varieties represent mixtures of melt and peritectic garnet and ilmenite trace elements such as rb ba sr and eu that are concentrated in reactant minerals in the melting process show considerable scatter within the granites this is interpreted to reflect compositional variation in the source in contrast elements such as lree zr and hf which are concentrated within refractory accessory phases zircon and monazite show well defined negative correlations with increasing sio2 and increase linearly with increasing maficity this is interpreted to reflect coupled co entrainment of accessory minerals and peritectic phases to the melt leucocratic rocks cannot have evolved from the more mafic compositions in the suite by a process of fractional crystallisation because in this case they would have inherited the zircon saturated character of this hypothetical earlier magma trace element behaviour of granites from the peninsular pluton has been modelled via both equilibrium and disequilibrium trace element melting in the disequilibrium case melts are modelled as leaving the source with variable proportions of entrained peritectic phases and accessory minerals but before the melt has dis solved any accessory minerals thus the trace element signature of the melt is largely inherited from the reactants in the melting reaction with no contribution from zircon and monazite dissolution in the equilibrium case melt leaves the source with entrained crystals after reaching zircon and monazite saturation a significant proportion of the rocks of the peninsular pluton have trace element concentrations below those predicted by zircon and monazite saturation in the case of the most leucocratic rocks all compositions are zircon undersaturated whilst the majority of the most mafic compositions are zircon oversaturated however in both cases zircon is commonly xenocrystic thus the leucocratic rocks represent close to pure melts which escaped their sources rapidly enough that some very closely match the trace element disequilibrium melting model applied in this study zircon dissolution rates allow the residency time for the melt in the source to be conservatively estimated at less than 500 years

new insights into the origin of two contrasting himalayan granite belts the two parallel belts of miocene granite that extend along much of the himalaya differ in age petrogenesis and emplacement style we suggest that their origin is linked to shear heating on a continuously active decollement that cuts through previously metamorphosed indian supracrustal rocks that were transformed into basement during the initial stages of the indian asian collision numerical simulations assuming a shear stress of 30 mpa indicate that initiation of slip on the himalayan thrust at 24 ma could trigger discontinuous melting reactions leading to formation of the high himalayan granite chain from 24 to 20 ma and the north himalayan belt from 18 to 12 ma this model result agrees well with observation as do model predictions regarding emplacement style and location

multiple emplacement and exhumation history of the late mesozoic dayunshan mufushan batholith in southeast china and its tectonic significance 2 magnetic fabrics and gravity survey the late mesozoic magmatic province is a prominent feature of the south china block scb however the tectonic regimes associated with the magmatism are still elusive a combined anisotropy of magnetic susceptibility ams and gravity study has been carried out to determine the fabric patterns and shape at depth of the dayunshan mufushan composite batholith in the north central scb this is a companion paper to part 1 that presented the structural and geochronological data of this batholith the magnetic fabrics in the batholith interior predominantly reflect magma flow structures two distinct patterns of the magnetic lineations are defined around nne ssw and wnw ese trends for the early stage and late stage intrusions of the batholith respectively the gravity survey reveals that the early stage intrusion has a main feeder zone located below its northern part while several linear feeder zones trending nne ssw are inferred for the late stage intrusion integrating all results a two stage construction of the batholith with distinct tectonic regimes has been established it is concluded that the early stage intrusion experienced a southward magma transport during its emplacement partially assisted by far field compression from the north at ca 150 ma conversely the emplacement and exhumation of the late stage intrusion was accommodated by a nw se crustal stretching involving a lateral magma expansion above the multiple feeder zones likely corresponding to extensional fractures and ductile shearing during 132 95 ma localized mainly along the dayunshan detachment fault finally we discuss the geodynamic linkage between the paleo pacific subduction and the late mesozoic tectono magmatism in the scb

depositional features and stratigraphic sections in granitic plutons implications for the emplacement and crystallization of granitic magma abstract many granitic plutons contain sheet like masses of dioritic to gabbroic rocks or swarms of mafic to intermediate enclaves which represent the input of higher temperature more mafic magma during crystallization of the granitic plutons small scale structures associated with these bodies e g load cast and compaction features silicic pipes extending from granitic layers into adjacent gabbroic sheets indicate that the sheets and enclave swarms were deposited on a floor of the magma chamber on granitic crystal mush and beneath crystal poor magma while the mafic magma was incompletely crystallized these structures indicate way up typically toward the interior of the intrusions and appear to indicate that packages of mafic sheets and enclave concentrations in these plutons are a record of sequential deposition hence these plutons preserve a stratigraphic history of events involved in the construction filling replenishment and crystallization of the magma chamber the distinctive features of these depositional portions of plutons allow them to be distinguished from sheeted intrusions which usually preserve mutual intrusive contacts and dike sill relations of different magma types the considerable thickness of material that can be interpreted as depositional and the evidence for replenishment suggest that magma chamber volumes at any one time were probably much less than the final size of the pluton thus magma chambers may be constructed much more slowly than presently envisaged the present steep attitudes of these structures in many plutons may have developed gradually as the floor of the chamber along with the underlying solidified granite and country rock sank during continuing episodes of magma chamber replenishment these internal magmatic structures support recent suggestions that the room problem for granites could be largely accommodated by downward movement of country rock beneath the magma chamber

cross strata and bedforms of probable transitional dune to upper stage plane bed origin from a late precambrian fluvial sandstone northern norway a late precambrian fluvial sandstone sequence in northern norway is dominated by large scale cross sets that show either lenticular or tabular geometries in the streamwise sections the lenticular sets interdigitate and in places show nearly symmetrical formsets the tabular sets are in places solitary but are mainly grouped in cosets in both cross set types the cross strata range from concave up to sigmoidal in shape with the latter variety comprising subhorizontal to gently inclined topset strata with parting lineation that merge uninterruptedly downflow into the steeper 10 2 foresets within the cross sets the geometry and dip azimuths of the foresets are conspicuously consistent although the concave up and sigmoidal strata commonly alternate downcurrent the cross strata characteristics suggest flood stage deposition from relatively high velocity steady currents heavily laden with suspended sand both cross set types are interpreted as representing bedforms generated by flow in the dune to upper stage plane bed transition the lenticular cross sets probably represent periodic dunes but it is far less clear whether the long bedforms represented by the tabular sets should be classified as dunes or rather as solitary to quasi periodic bars

petrogenesis and tectonic implications of the high k alamas calc alkaline granitoids at the northwestern margin of the tibetan plateau geochemical and sr nd hf o isotope constraints abstract the alamas granitoid pluton in the eastern part of the western kunlun orogen the northwestern margin of the tibetan plateau is composed of quartz diorite zircon separates from the pluton has sims u pb age of 446 ma rocks from the pluton have a narrow range of sio2 56 84 62 57 wt mgo 1 76 2 94 wt and total alkalis na2o k2o 5 14 9 59 wt and are metaluminous and high k calc alkaline to shoshonitic in composition they are enriched in light rare earth elements lrees with la yb n 14 25 and show weakly negative eu anomalies these rocks are relatively enriched in sr 472 676 ppm and ba 435 2388 ppm and depleted in nb ta th and ti their end t values range from 6 4 to 8 4 and 87sr 86sr i 0 7184 0 7200 zircons from the pluton show ehf t values of 1 4 to 8 8 and 18o 6 4 9 0 geochemical data indicate that the granitoids were likely derived from the reworking of an ancient deep crustal source influenced by a minor mantle derived component magmatic differentiation was dominated by the fractional crystallization of hornblende biotite and accessory minerals such as apatite allanite and fe ti oxides in summary the late ordovician alamas pluton is an i type granitoid that was emplaced in a post collisional environment suggesting that this tectonic stage had already initiated prior to 445 ma

hafnium isotopic heterogeneity in zircons from granitic rocks geochemical evaluation and modeling of zircon effect in crustal anatexis abstract we carried out a geochemical evaluation and modeled the mechanism responsible for varied hf isotopic ratios in magmatic zircons of a single granitic rock specimen five representative granitic samples were selected from southern china based on preliminary hf isotopic data our new dataset of zircon hf isotopes confirmed significant hf isotope variations 5 9 epsilon units for each sample and these zircons show roughly positive th u versus t crystallizing temperature correlations while the lu hf variation is independent from t in addition some zircons show significantly higher ti concentrations in the rims compared to the interiors implying reverse thermal zonation based on the ti in zircon thermometry these geochemical features in zircons suggest open system processes which may have resulted from frequent replenishment we modeled zircon dissolution during crustal anatexis to reveal the hf isotopic evolution in the extracted melts the model suggests that the extracted melts may have extremely variable hf isotope compositions 20 epsilon units if the bulk zr concentration in the source is initially above 100 ppm and the melting is rapid 10 4 yr 1 the decoupled release of zircon hf and non zircon hf from a single crust derived magma source can lead to significant hf isotope variations in a solidified granitic body this work provides an alternative explanation for the hf isotopic heterogeneity in magmatic zircons of granitic rocks which is in contrast to the common interpretation by mixing with mantle derived magmas

composition of the crust beneath southeastern china derived from an integrated geophysical data set 1 the nonuniqueness in determining the composition of the crust can be reduced by comparing the data for p wave velocity vp s wave velocity vs vp vs ratio mass density and lame impedances to data determined from laboratory measurements of these physical variables on a variety of crustal rock samples the composition of the crust along the tunxi wenzhou transect southeastern china is presented as a model based on a complete set of geophysical data involving seismic p wave and s wave velocity density gravity heat flow and temperature surveys which allow us to place tighter constraints on possible crustal models the yangtze and the cathaysia blocks have a crustal structure characterized by remarkable differences between the upper middle and lower crust integrated geophysics data are here interpreted to indicate 1 an average composition of granite gneiss for the upper crust with presence of mica quartz schist felsic granulite paragranulite and granite granodiorite beneath the yangtze block and basalt down to the upper part of the middle crust just over the jiangshan shaoxing fault followed laterally by granite gneiss beneath the songyang qintiang and wenping depressions 2 granite granodiorite and biotite gneiss for the upper half of the middle crust with presence of mica quartz schist beneath the cathaysia block 3 gabbro and in less proportion basalt for the lower part of the middle crust and 4 amphibolite for the lower crust with presence of mafic garnet granulite just over the jiangshan shaoxing fault and beneath the wenping depression

petrogenesis of late jurassic two mica granites and associated diorites and syenite porphyries in guangzhou se china abstract the south china block scb is considered to have undergone extensive reworking of continental crust however the mechanisms of this crustal reworking are unclear here we report on late jurassic 162 155 ma two mica granites and associated diorites and syenite porphyries from the huolushan longyandong area of guangzhou city southeastern china the two mica granites have high sio2 contents 69 6 73 6 wt and weakly peraluminous to strongly peraluminous compositions these rocks have enriched sr nd hf isotopic compositions 87sr 86sr i 0 7094 0 7145 end t 10 5 to 9 3 ehf t 12 7 to 4 2 the huolushan two mica granites have variable zircon o isotopic compositions 18o 6 8 10 4 whereas the longyandong two mica granites have homogeneous compositions 18o 8 6 9 9 the huolushan syenite porphyries have moderate sio2 59 6 60 1 wt and high alkaline k2o na2o 9 4 9 8 wt contents their sr nd hf o isotopic compositions 87sr 86sr i 0 7105 0 7119 end t 9 9 to 9 8 ehf t 14 1 to 5 9 18o 6 5 10 6 are similar to those of the huolushan two mica granites the huolushan diorites have moderate sio2 contents 59 9 60 4 wt and are high k calc alkaline and metaluminous with mg values of 43 2 43 8 their sr nd isotopic compositions 87sr 86sr i 0 7106 0 7109 end t 8 8 to 8 6 are less enriched than those of the two mica granites and syenite porphyries in addition the diorites have high zircon 18o values 7 8 8 9 and a wide range of zircon ehf t values 13 4 to 5 6 we propose that the huolushan two mica granites were derived from a metagreywacke dominated hybridized source with minor juvenile crustal rocks and that the longyandong two mica granites were generated by partial melting of metagreywackes the huolushan syenite porphyries were formed by the mixing of metasedimentary rock derived felsic magmas and minor enriched mantle derived alkaline magmas the huolushan diorites were most likely derived by partial melting of mafic lower crust with the dioritic magmas undergoing assimilation and fractional crystallization afc during ascent all of these intrusive rocks were likely formed in an extensional intraplate setting the mantle derived magmas which could have been triggered by lithospheric extension underplated the middle lower crust and provided heat for crustal reworking and materials for late mesozoic crustal growth

geochronology petrogenesis and tectonic settings of pre and syn ore granites from the w mo deposits east kounrad zhanet and akshatau central kazakhstan abstract there is significant debate regarding the mineralization ages of the east kounrad zhanet and akshatau w mo deposits of central kazakhstan and the petrogenesis and tectono magmatic evolution of the granites associated with these deposits to address these issues we present molybdenite re os dating zircon u pb dating whole rock geochemistry as well as sr nd pb and zircon o hf isotopic analyses on the pre mineralization and ore forming granites u pb dating of zircons from pre mineralization granitic rocks yield late carboniferous ages of 320 309 ma whereas ore forming granites have early permian ages of 298 285 ma molybdenite re os isotopic data indicate a mineralization age of 296 ma at east kounrad 294 ma at akshatau and 285 ma at zhanet the pre ore and ore forming granites are high k calc alkaline metaluminous to slightly peraluminous i type granites the pre mineralization granites are relatively unfractionated whereas the ore forming granites are highly fractionated the fractionating mineral phases are probably k feldspar apatite ti bearing phases and minor plagioclase the pre mineralization and ore forming rocks are characterized by similar sr nd pb hf o isotopic compositions 87 sr 86 sr i 0 70308 0 70501 e nd t 0 5 to 2 8 207 pb 204 pb 15 60 15 82 zircon e hf t 1 2 to 15 6 and 18 o 4 6 to 10 3 whole rock t dm c nd 840 1120 ma and zircon t dm c hf 320 1240 ma the isotopic characteristics are consistent with a hybrid magma source caused by 10 30 assimilation of ancient crust by juvenile lower crust the geochronology and geochemistry of these granites show that the late carboniferous pre mineralization granitic rocks formed during subduction whereas the early permian ore forming highly fractionated granite probably underwent significant fractionation with a restite assemblage of k feldspar apatite ti bearing phases and minor plagioclase and developed during collision between the yili and kazakhstan terranes commenced at the latest late carboniferous

formation of the jurassic south china large granitic province insights from the genesis of the jiufeng pluton abstract the magma source and thermal regime for the formation of large granitic provinces lgps have been the subjects of much debate as mineralogical geochemical and isotopic variations of plutonic rocks preserve records of crustal thermal evolution the jiufeng pluton was comprehensively studied to gain insights into material and heat sources and tectonic process in the formation of the south china lgp sims zircon u pb analyses of a biotite granite 158 9 1 3 ma three two mica granites stage i 159 7 1 2 ma stage ii 158 6 1 2 ma and 156 8 1 2 ma and a muscovite granodiorite 159 0 4 4 ma of the jiufeng pluton suggest that they are coeval and were emplaced in a short period of no more than 5 3 m y limited variations in whole rock nd isotope end t 12 6 to 9 5 and zircon hf o isotopes ehf t 12 0 to 6 3 18 o 6 8 9 4 suggest that the jiufeng pluton was mainly derived from melting of a common metasedimentary source possibly with a minor basaltic contribution we consider the geochemical variations of the jiufeng pluton are primarily a result of incremental assembly of magma batches produced from rapid step like transition from fluid saturated to fluid absent melting of the source the muscovite granodiorite with high na 2 o 3 80 wt k 2 o na 2 o 1 is interpreted to have been produced by fluid saturated melting at low temperature 650 c high p 2 o 5 0 09 0 17 wt zircon saturation temperature t zr 769 816 c and la yb ratios 8 4 55 8 of the stage ii two mica granite support its formation from high temperature 800 c biotite dehydration melting whereas lower p 2 o 5 zr 685 742 c and la yb

crustal growth as revealed by integrated u pb and lu hf isotope analyses of detrital zircons from the ganjiang river southeastern china the ganjiang river one of eight major tributaries of the yangtze river located in the western hinterland of the cathaysia block se china has a length of 823 km and a drainage area of 82 809 km 2 whose detrital zircons provide a valuable means to trace sediment provenances of the river and explore the crustal growth and evolution of the cathaysia block in the current study 389 concordia zircon u pb age spots and rare earth element ree contents in combination with 201 lu hf isotope analyses have been determined oscillatory zoning high th u ratios and ree distribution patterns indicate that most detrital zircon grains are of magmatic origin the age can be further divided into seven groups 130 185 ma with a peak at 153 ma 7 217 379 ma with a peak at 224 ma 16 390 494 ma with a peak at 424 ma 37 500 698 ma with a peak at 624 ma 5 716 897 ma with a peak at 812 ma 10 902 1191 ma with a peak at 976 ma 13 and 2232 2614 ma with a peak at 2471 ma 5 the sources of almost all the zircon age groups can be found from the exposed rocks in particular yanshanian hercynian to indosinian pan african grenvillian and palaeoproterozoic archaean zircons can be mainly sourced from the northern guangdong southern jiangxi western fujian region while caledonian zircons come from southern and central jiangxi and jinningian zircons are from central and northern jiangxi most determined zircon grains exhibit negative e hf t values and t dm2 ages of 797 to 4016 ma with a wide peak at 1500 2100 ma and a keen peak at 1824 ma suggesting that most zircons are sourced from the reworked ancient crustal materials or crust mantle mixing the zircon hf model age cumulative probability diagram shows that rapid crustal growth took place at the palaeo to mesoproterozoic and that about 90 of the crust of the cathaysia block was formed before 1 5 ga

are strongly peraluminous magmas derived from pelitic sedimentary sources igneous compositions that require the presence of a phase more aluminous than biotite are here designated as strongly peraluminous ps such compositions are common but subordinate in felsic rocks and exceedingly rare in more mafic rocks the term s type has been applied to many igneous rocks based largely or even solely upon their ps compositions neglecting or placing less emphasis on other criteria for sedimentary parentage in chappell and white s i and s type classification system the s type designation was in fact initially restricted to granites which were thought to have been derived from a chemically mature strongly weathered pelitic shaly source application of chappell s and white s criteria together with others based upon isotopic and phase equilibria considerations leads to the following conclusions about ps igneous rocks worldwide 1 very few ps rocks are derived entirely from pelitic sources 2 a majority are derived largely from intermediate to felsic crustal sources including

s type granites and their probable absence in southwestern north america criteria that have been and still are used to characterize s type granites of the lachlan fold belt lfb of southeastern australia are reviewed and comparisons are made with various peraluminous granites of southwestern north america some of which have been classified as s types on the basis of insufficient data virtually all of the vast volume of s type granites in the lfb are near surface batholithic granites that are commonly associated with s type volcanics and are not associated with regional metamorphic rocks and migmatites they are strongly peraluminous as shown by the presence of cordierite granites with primary muscovite are rare all are low in na ca and sr as a result of chemical weathering during formation of the sedimentary sources peraluminous granites of various ages in southwestern north america are distinctly different they rarely contain cordierite a mineral characteristic of lfb s types but some are highly evolved such that fe mn rich garnet has crystallized they are dominantly two mica granites indicating crystallization at higher water fugacities and greater depths than most peraluminous granites of the lfb cordierite bearing volcanics s types have not been reported sodium is generally high in the peraluminous granites of southwestern north america some of these rocks have trondhjemitic affinities the parent magmas seem more likely to have been produced by partial melting of altered basaltic rocks locally some peraluminous rocks marginal to metaluminous types may owe their compositions to high level contamination of i types these are not s type rocks no compelling evidence has been presented that any of the peraluminous granites of southwestern north america are s types

petrogenesis of triassic granite from the jintan pluton in central jiangxi province south china implication for uranium enrichment abstract numerous triassic biotite granites and two mica granites crop out in the interior of south china and some of them possess high u contents which have been regarded as the sources for later hydrothermal mineralization their petrogenesis is therefore crucial for constraining the possible origins of the u enrichment here we report new la icpms zircon u pb ages mineral geochemistry of biotite and muscovite whole rock geochemical results and sr nd and zircon hf isotope data from the jintan pluton in central jiangxi province south china la icpms zircon u pb dating indicates that both biotite granite bg and two mica granite tmg in the jintan pluton crystallized at 220 ma the tmg have higher u contents 7 85 to 48 90 ppm average18 44 ppm than thebg 4 99 to 17 72 ppm average 8 64 ppm both bg and tmg show negative whole rock end t and zircon ehf t values and contain some inherited zircons the tmg are strongly peraluminous a cnk 1 13 1 33 contain abundant primary muscovite and display s type affinity on plots of yvs rb and th vs rb suggesting that they are s type granites the bg also display s type granite affinities on plots of yvs rb and th vs rb the suites display similar sr nd isotope compositions bg initial 87sr 86sr values 0 711389 to 0 714225 and end t 9 91 to 9 16 tmg initial 87sr 86sr values 0 711832 and end t 10 02 and are spatially associated suggesting that the bg should also be classified as s type granites the tmg have higher zircon ehf t values 6 4 to 1 1 than the bg 8 7 to 3 7 indicating the tmg and bg might be derived from similar sediments but possibly with some distinct characteristics in their sources the bg exhibit linear covariations in chemical compositions with relatively high total ree and light ree contents and mgo contents while the tmg displays broader compositional variations but with relatively low total ree light ree and mgo contents biotite geochemistry indicates the tmg formed in a more reduced magmatic system than the bg the temperatures estimated by zircon saturation thermometry indicate the bg had distinctly higher magmatic temperatures than the tmg the tmg display relatively high al2o3 tio2 ratios and low cao na2o ratios than the bg but have higher sr y and la yb ratios the geochemical and petrological data suggest the bg were derived from clay poor psammite sources at deeper levels with higher temperatures and higher oxygen fugacity and underwent an extensive fractional crystallization while the tmg was derived from clay rich pelitic sources at higher levels and lower temperatures and oxygen fugacity with only limited fractional crystallization we conclude that the combination of u rich sources physical chemical conditions such as low partial melting temperature or low degrees of partial melting a reduced environment and low ree and lree contents of magmas controlled the u enrichment in tmg

precise determination of phanerozoic zircon pb pb age by multicollector sims without external standardization 1 zircon has long been recognized as the best geochronometer and the most important timekeeper in geosciences modern microbeam techniques such as sims and la icpms have been successfully applied to in situ u pb zircon age determinations at spatial resolutions of 20 30 m or less matrix matched calibration by external standardization of well characterized natural zircon references is a principal requirement for precise microbeam u pb zircon age determination due to fractionation effects between pb and u which usually result in an external age error exceeding 1 alternatively zircons with a closed u pb system can be directly dated by measurement of 207pb 206pb isotopic ratio without external standardization which has been a common practice for zircons older than 1 0 ga but not for relatively young 1 0 ga and particularly phanerozoic ones because of limitations of analytical precision we describe in this paper a method of 207pb 206pb measurement on phanerozoic zircons using a new generation of large radius magnetic sector multicollector cameca ims 1280 sims in combination with multicollector mode a nuclear magnetic resonance nmr magnet controller and oxygen flooding techniques we achieve precisions of 207pb 206pb ratio of 0 1 and 0 1 0 2 propagating to pb pb age errors 0 4 and 1 3 excluding u decay constant uncertainties for zircons of latest neoproterozoic and late paleozoic to mesozoic age respectively therefore the multicollector sims is capable of direct determination of zircon pb pb ages as young as mesozoic age with uncertainties of geological significance this technique is useful for direct dating of zircons in thin sections moreover it has significance for dating of some other u rich minerals i e baddeleyite and zirconolite that are not suitable for sims u pb dating by external standardization

generation of metaluminous a type granites by low pressure melting of calc alkaline granitoids melting experiments on two calc alkaline rocks a tonalite and a granodiorite demonstrate that shallow dehydration melting of hornblende and biotite bearing granitoids generates metaluminous a type granitic melts profuse crystallization of calcic plagioclase orthopyroxene during low pressure p 4 kbar incongruent melting explains all of the major and trace element characteristics of a type granites e g their low al ca mg sr and eu contents and high ga al and k na ratios as well as their common association with anorthositic and mangeritic rocks the residual assemblage produced during melting of the same rocks in the deep crust p 8 kbar is dominated by clinopyroxene and as a consequence most of the chemical features diagnostic of a type granites are lost the high temperatures 900 c and low h 2 o contents 4 wt that are characteristic of a type magmas are adequately explained by the h 2 o poor nature of the granitoid source materials and require neither a granulitic nor a f rich source previous attempts to explain the origin of a type granites have focused on the nature of the source material these experiments show that pressure of melting is at least as important as source composition in engendering a type melts

origin of enclaves in s type granites of the lachlan fold belt abstract the more mafic s type granites of the lachlan fold belt contain a distinctive assemblage of lithic inclusions of deep crustal origin two types predominate the schistose enclaves and the microgranular enclaves there is also a small proportion of fragments of local country rock all the features of the schistose enclaves are consistent with an origin as lithic restite fragments from the source their ubiquity in the mafic s type granites and absence from both felsic s type granites and i type granites conform with such an origin the argument that these enclaves are not in chemical equilibrium with the host granite because they do not complement its composition is not valid since they represent less fertile parts of the source that had different compositions that as a result melted to a lesser extent models that derived these enclaves from regions of the crust above the zones of partial melting so that they are not restites are complex unnecessary do not conform with their ubiquity in the more mafic s type granites and are not consistent with their chemical compositions the origin of the microgranular enclaves is controversial in most s type granites these enclaves have been recrystallized to a similar mineral assemblage to the host granite with a higher proportion of biotite in a few places such material is seen to be forming by recrystallization of material in the core of the enclave which has the assemblage quartz calcic plagioclase orthopyroxene cordierite biotite with accessory ilmenite sulfide and apatite all of the microgranular enclave cores show pseudo doleritic texture in which calcic plagioclase crystals an60 and orthopyroxene crystals project into or are enclosed by quartz large plagioclase crystals are zoned with some core compositions as calcic as an94 the low na contents of the microgranular enclaves are not consistent with an igneous origin we consider that these enclaves were derived by metamorphism of calcareous mudstones or argillaceous limestones the pseudo igneous textures result from the presence of a partial melt during metamorphism an analysis of a calc silicate lens of upper amphibolite grade from the wilsons group of victoria land antarctica is remarkably similar in composition to the microgranular enclaves from lachlan fold belt s type granites we suggest that rocks equivalent to that group which is of appropriate age and general composition were the source rocks for the s type granites of the lachlan fold belt

formation of high δ18o fayalite bearing a type granite by high temperature melting of granulitic metasedimentary rocks southern china the genesis of a type granites has been controversial fayalite granite is a member of the most reduced a type granites commonly thought to have been primarily sourced from tholeiitic rocks in this paper we report petrography whole rock geochemistry sr nd isotope and in situ zircon hf o isotope results for a fayalite bearing a type granite suite at jiuyishan in southern china high zircon 18 o 8 0 9 8 negative zircon ehf t 6 2 to 2 3 and evolved whole rock sr nd isotopes i sr 0 7151 0 7181 end t 7 4 to 6 6 indicate the reworking of old supracrustal rocks isotopic and geochemical results particularly downward inflections of zr and ba at 70 wt sio 2 point to fractional crystallization rather than magma mixing as the controlling process for the evolution of the igneous suite integrated petrological geochemical and isotopic studies present the first convincing case that reduced a type granites can also be generated by high temperature 960 c melting of granulitic metasedimentary rocks likely related to the upwelling of the asthenosphere and or underplating and intrusion of basaltic melts we emphasize that key factors for the genesis of this unique rock type are low oxygen fugacity f o 2 low f h 2 o and high temperature

architecture of vertically stacked fluvial deposits atane formation cretaceous nuussuaq central west greenland excellent exposures of thick multistorey fluvial deposits from the deltaic atane formation on south east nuussuaq central west greenland show the architecture of up to 100 m thick continuously aggrading fluvial depositional complexes the succession comprises vertically stacked channel belt sandstones separated by thin floodplain deposits with little to no incision between storeys architectural elements and palaeocurrent patterns of channel deposits indicate deposition in large relatively stable low sinuosity rivers probably located within an incised valley gradual transitions from channel to floodplain deposits accompanied by a gradual change from floodplain to spillover sand suggest avulsion on the floodplain as a possible mechanism for the vertically alternating channel and floodplain deposits despite its relative proximity to contemporaneous sea level ca 35 km upstream from the palaeo shoreline the depositional complex is entirely non marine the aggrading nature of the deposits suggests a continuously rising base level coupled with a high and steady sediment supply vertical alternations between floodplain and channel deposits may be forced by subtle interruptions in this balance or autocyclic mechanisms on the floodplain this study provides an example of aggrading lowstand non marine transgressive systems tract deposits

magmatic garnet in the cordilleran type galileia granitoids of the aracuai belt brazil evidence for crystallization in the lower crust abstract magmatic garnet together with epidote is a rare mineral association in cordilleran i type granitoids and of special petrogenetic significance the metaluminous to slightly peraluminous asi 0 97 1 07 galileia batholith brazil is a large ca 30 000 km 2 neoproterozoic ca 632 570 ma weakly foliated calc alkaline granitoid body characterized by the widespread occurrence of garnet grossular 25 43 mol and epidote pistacite 9 3 22 7 mol field petrographic and mineral chemical evidence indicates that garnet epidote biotite as well as white mica crystals low si phengite are magmatic there is no difference in bulk rock major and trace element composition between the galileia granitoids and other garnet free cordilleran type granitoids worldwide this evidence strongly suggests that the origin of the uncommon garnet epidote parageneses is related to the conditions of magma crystallization such as pressure temperature and water content comparison between the mineral assemblages and mineral compositions from this study and those recorded in crystallization experiments on metaluminous calc alkaline magmas as well as within garnet bearing metaluminous volcanic rocks and granitoids indicates that the supersolidus coexistence of grossular rich garnet epidote and white mica is consistent with magma crystallization at pressures greater than 0 8 gpa above 25 km depth and at temperatures below 700 c i e near the water saturated solidus furthermore resorption textures around garnet plagioclase quartz coronas and epidote suggest that these minerals have been partially consumed prior to complete crystallization these findings demonstrate that at 630 ma the crust underneath the aracuai orogen was already at least 25 30 km thick and relatively cool however this contrasts with the marked high heat flow registered from the neighbour carlos chagas batholith located 50 km to the east in fact such granitoids record granulite facies metamorphism at the same pressure and time ca 570 ma of galileia granitoids crystallization thus a more suitable geodynamic scenario is required in order to explain these two contrasting thermal regimes within the same orogen eventually field petrographic and mineral chemical analogies with similar garnet bearing granitoids located in the fore arc settings of the british columbia subduction zone possibly imply that the galileia granitoids represent rare garnet and epidote bearing metaluminous cordilleran i type granites which can only form in a fore arc setting

petrogenesis of the late jurassic peraluminous biotite granites and muscovite bearing granites in se china geochronological elemental and sr nd o hf isotopic constraints biotite granites and muscovite bearing granites are dominant rock types of the widespread granites in se china however their petrogenesis has been enigmatic a combined study of zircon u pb dating and lu hf isotopes whole rock element geochemistry and sr nd o isotopes was performed for three late mesozoic granitic plutons xinfengjie jiangbei and dabu in central jiangxi province se china all the plutons are composed of biotite granites and muscovite bearing granites that have been poorly investigated previously the new data not only allow us to assess their sources and magma evolution processes but also helps us to better understand the genetic link to the large scale polymetallic mineralization in se china la icp ms zircon u pb dating shows that three plutons were emplaced in the late jurassic 159 148 ma and that the muscovite bearing granites are almost contemporaneous with the biotite granites the biotite granites have sio2 contents of 70 3 74 4 wt and are weakly to strongly peraluminous with asi from 1 00 to 1 26 and show a general decrease in asi with increasing sio2 they have relatively high zircon saturation temperatures t zr 707 817 c most 745 c and show a general decrease in t zr with increasing sio2 they have high initial 87sr 86sr ratios 0 7136 to 0 7166 and high 18o values 9 1 12 8 most 9 5 and clearly negative e nd t 9 5 to 11 8 and e hf t in situ zircon 13 1 to 13 5 the muscovite bearing granites have high sio2 contents 74 7 78 2 wt they are also weakly to strongly peraluminous with asi of 1 04 1 18 but show a general increase in asi with increasing sio2 they have relatively low t zr 671 764 c most 9 5 and e hf t in situ zircon of 11 3 to 13 1 geochemical data suggest the origin of the biotite granites and muscovite bearing granites as follows partial melting of precambrian metasedimentary rocks mainly two mica schist in the lower crust at temperatures of ca 820 c generated the melts of the less felsic biotite granites such primary crustal melts underwent biotite dominant fractionation crystallization forming the felsic biotite granites progressive plagioclase dominant fractionation crystallization from the evolved biotite granites produced the more felsic muscovite bearing granites thus the biotite granites belong to the s type whereas the muscovite bearing granites are highly fractionated s type granites we further suggest that during the formation of the muscovite bearing granites the fractional crystallization was accompanied by fluid fractionation and most likely the addition of internally derived mineralizing fluids that is why the large scale polymetallic mineralization is closely related to the muscovite bearing granites rather than biotite granites in se china this is important to further understand the source and origin of biotite granites and muscovite bearing granites in se china even worldwide

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method

small scale hf isotopic variability in the peninsula pluton south africa the processes that control inheritance of source 176 hf 177 hf diversity in s type granites directly transfer to their magmatic overgrowths smallscale modeling of zircon dissolution and re precipitation in a static magma generates sub mm melt domains having variable zr content and hf isotope composition the composition of these domains is controlled by the size and isotope composition of the nearest dissolving zircon crystals and the cooling rate of the magma these results suggest that in magma systems with a substantial inherited zircon load zircon crystals within the same rock should record variable 176 hf 177 hf in the magmatic zircon fraction

geochronological and geochemical study of palaeoproterozoic gneissic granites and clinopyroxenite xenoliths from nw fujian se china implications for the crustal evolution of the cathaysia block abstract u pb zircon dating of gneissic granite samples in northwestern fujian province southeast china gave neoarchean to palaeoproterozoic ages 2 6 1 9 ga for the rounded zircon cores and palaeoproterozoic upper intercept ages 1851 21 ma 1857 29 ma respectively for zoned rims that have oscillatory growth zoning and yield e hf t values between 8 8 and 3 7 and t dm model ages between 2 1 and 2 6 ga these gneissic granites are peraluminous a cnk 0 94 1 28 characterized by high sio 2 68 72 wt al 2 o 3 14 15 wt and low cao mgo feo t tio 2 and p 2 o 5 contents they also possess relatively low ree contents and moderate lree hree fractionation and display pronounced negative anomalies in nb ta sr p and ti all these features indicate that the precursor magmas were likely formed by partial melting of sedimentary rocks clinopyroxenite xenoliths with gneissosity structures are found in these gneissic granites and they have high mgo ni and cr but low contents of tio 2 and are characterized by high cao al 2 o 3 4 1 5 3 low al 2 o 3 tio 2 ratios 8 7 9 5 and hree depletion these features are similar to the typical al depleted type komatiites their geochemical characteristics such as high gd yb n and low hree y zr and hf suggest partial melting of the upper mantle source with residual garnet the negative nb ta and ti anomalies indicate a strong arc related affinity our results and recently published data for granitic and mafic basement rocks in this region reveal that 1 85 ga was an important period for the evolution of the cathaysia block possibly related to the columbia supercontinent assembly large amount of granitic magma was emplaced probably as a result of partial melting of old neoarchean to palaeoproterozoic materials

late jurassic sn metallogeny in eastern guangdong se china coast evidence from geochronology geochemistry and sr nd hf s isotopes of the dadaoshan sn deposit abstract the newly discovered dadaoshan sn deposit is located in the eastern guangdong sn w province coastal se china the sn mineralization hosted in jurassic porphyritic granite and the lower jurassic jinji formation sedimentary wall rocks is considered to be granite related in this study the porphyritic granite was la icp ms zircon u pb dated to be 153 2 1 2 ma consistent with the syn mineralization molybdenite re os age of 152 6 1 8 ma the porphyritic granite samples are weakly peraluminous a cnk 1 0 1 1 and high k calc alkaline the rocks contain high sio2 72 9 75 6 wt moderate rb sr 5 9 and low ree 136 223 ppm they are enriched in f li rb and sn depleted in ba sr p zr th nb and y and have distinct negative eu anomalies eu 0 09 0 18 suggesting that the porphyritic granite is highly fractionated i type granite the calculated initial 87sr 86sr 0 711582 0 715173 relatively low nd t 9 48 to 8 54 tdm2 1638 1814 ma and the zircon ehf t 14 2 to 5 1 two stage model ages 1528 2103 ma all suggest that the granite was mainly crustal derived with little mantle input sulfur isotopic compositions for the sulfides arsenopyrite and chalcopyrite 34s 1 1 to 1 4 average 0 1 imply a dominantly magmatic sulfur source the calculated zircon ce4 ce3 and eun eun ratios of the dadaoshan granite range from 1 0 to 112 mean 31 7 and from 0 04 to 0 37 mean 0 14 respectively indicating a low oxygen fugacity for the magma the reducing and highly fractionated nature of the dadaoshan granitic magma may have played a key role in the sn mineralization it was previously argued that the jurassic sn w mineralization and its causative magmatism were largely confined in the south china interior e g the nanling range our new data suggest that the late jurassic sn w mineralization and its causative magmatism actually extended to the se china coastal area the dadaoshan granite may have been generated from partial crustal melting led by underplating of mantle derived magmas in an extensional environment regional extension may have been related to the west directed flat slab subduction and delamination of the paleo pacific izanagi plate beneath the south china block another suite of early cretaceous sn w bearing granitic rocks in eastern guangdong may have mainly been crustal derived with minor mantle input and likely occurred under back arc extensional setting led by the paleo pacific subduction rollback

architecture of pre vegetation sandy braided perennial and ephemeral river deposits in the paleoproterozoic athabasca group northern saskatchewan canada as indicators of precambrian fluvial style abstract the late paleoproterozoic athabasca basin contains more than 1 5 km of predominantly sandy strata most of which are of braided fluvial origin in the eastern part of the basin at mcclean lake sandstones and minor conglomerates of the read formation at the base of the succession are preserved within a steep walled valley system they consist predominantly of meter scale sheet elements characterized by massive and flat laminated fine to very coarse grained sandstone with minor discontinuous cobble and boulder conglomerate along lower set boundaries these are interpreted as sheet flood and stream flood deposits of a terminal dry land system deposited in an arid climatic setting analysis of closely spaced drill core indicates that late stage run off was confined to small shallow channels typically only a few tens of meters wide and a few meters deep overlying strata of the bird member of the manitou falls formation are predominantly coarse to very coarse grained sandstones with abundant small scale cross stratification and minor granule and small pebble conglomerate at mcclean lake these appear to have been deposited as sheet elements by semi ephemeral to perennial braided rivers under more humid conditions at key lake 160 km to the southwest architectural analysis of a 184 m wide section indicates that at least part of the bird member was deposited in deeper sandy braided rivers characterized by seasonally varied flow the closest modern analogue is the south saskatchewan river in which large simple flow transverse bedforms become exposed and dissected during falling stage and act as nuclei for sand flat development the presence of numerous low relief 4th order surfaces suggests continued reworking of bar tops during rising stage the incremental character of downstream accretionary elements suggests periodic migration of barforms during peak flood stages separated by periods of low flow this indicates similarities with modern perennial braided systems and is counter to the idea that all pre devonian fluvial systems should consist of stacked sheets formed by individual flood events thin gravel layers accumulated as lags on fourth order surfaces with discontinuous mud layers suggesting deposition within temporary ponds in channel thalwegs after major floods strata in overlying units indicate a return to semi perennial conditions

observations on the origins and ascent mechanisms of granitic magmas this review presents a summary of most current beliefs about the origins of granitic magmas together with calculations of some important properties of granitic melts and magmas turning attention to magma ascent mechanisms diapirism and dyking represent two competing end member models the differences in processes and products between the two are fundamental and the implications of the differences are profound for the evolution of the continental crust the weight of evidence favours dyke ascent for the majority of granitic magmas and modelling shows that dyke ascent is both rapid and thermally effcient furthermore the structural and petrological features expected in crust through which granitic diapirs have passed have not been discovered in nature granites in the upper crust commonly have forms and structures indicating that diapirism could not have been the emplacement mechanism in contrast granitic dykes are common suggesting that it is reasonable to consider the ascent of granitic magmas through fracture systems many granitic batholiths are tabular and some have been fed by large dykes the rate limit in granitic plutonism is not magma ascent but either the rate of melting and segregation near the magma source or the rate of deformation at the emplacement site

petrogenesis and tectonic significance of a mesozoic granite syenite gabbro association from inland south china article i nfo a granite syenite gabbro association from inland south china has been studied for zircon u pb ages and hf isotopic compositions as well as whole rock elemental and sr nd isotopic compositions to constrain their sources and tectonic settings these rocks have distinctive crystallization ages the longyuanba biotite granites have a mean age of 240 ma the longyuanba two mica granites pitou alkali feldspar granites and amphibole bearing alkali feldspar granites tabei and huangbu syenites and chebu and chenglong gabbros have a similar age of 178 ma whereas the dafengnao syenite gives a mean age of 165 ma the longyuanba biotite granites and two mica granites exhibit s type characteristics probably derived from a neoproterozoic crustal source the chenglong gabbro has oib like trace element and highly depleted isotope compositions ehf t 10 0 1 3 end t 5 2 suggesting its parental melt to be of asthenospheric origin with insignificant crustal assimilation however crustal contamination is required to explain the isotopic compositions of the chebu gabbro the syenites are shoshonitic in composition and have depleted sr nd hf isotopic signatures which we interpret to have resulted from a mixed source of asthenospheric mantle and metasomatized lithospheric mantle as expected the signals of crustal assimilation are conspicuous in the petrogenesis of the more evolved syenites the pitou alkali feldspar granite and amphibole bearing alkali feldspar granite exhibit i type and a type characteristics respectively they are isotopically more depleted than the s type longyuanba granites we suggest that they may have formed through magma mixing of predominantly mantle derived melts with the neoproterozoic crust the jurassic granite syenite gabbro association was the product of asthenosphere lithosphere crust interactions which records the primary role of asthenospheric mantle upwelling in magma generation both in the mantle and in the crust in the early yanshanian time in south china they probably occurred in an intraplate rift like environment as a tectonic response to far field stress at plate margins during the early stage of the paleo pacific plate subduction the tectonic transition from the tethys orogenic regime to the paleo pacific regime was accomplished in the early jurassic and the early yanshanian magmatism should be genetically associated with the paleo pacific tectonic regime

a lower cretaceous clastic slope succession livingston island antarctica sand body characteristics depositional processes and implications for slope apron depositional models the early cretaceous fill of the forearc intra arc byers basin includes a 600 to 900 m thick interval of marine slope apron deposits the president beaches formation this is a predominantly argillaceous succession within which coarser grained deposits are largely confined to lenticular packages of low width thickness ratios the entire formation was deposited in mid to late berriasian times coincident with a pulse of regional arc expansion at minimum mean accumulation rates of 120 225 mm 1000 years 1 the mudstones are finely laminated with a restricted benthic macrofauna and minimal bioturbation indicating relatively inhospitable sea floor environments sand rich packages occur as 7 to 30 m thick channel fill units composed chiefly of classical medium grained turbidites in some cases associated with complex high concentration turbidity current deposits and minor mud rich debrites these sand bodies are apparently elongate along normal to the nw facing palaeoslope implied by slump fold axes and the strike of the volcanic arc similarly palaeocurrent indices show a consistent arc parallel ne directed trend suggesting that transport processes were strongly influenced by the structural grain of the irregular slope morphology slope instability is recorded by widespread slump and soft sediment collapse features promoted by a combination of steep sea floor gradients and relatively high rates of sedimentation a lack of systematic vertical facies trends indicates that this was not a progradational or well organized system despite high rates of sediment supply however the strong systematic relationship between palaeocurrents and palaeoslope suggests a promising basis for evaluating organization in otherwise poorly ordered slope apron depositional systems

braidplain sedimentation in a proterozoic rift basin the sao joao da chapada formation southeastern brazil abstract the sao joao da chapada formation 100 200 m thick is the basal unit of the middle proterozoic espinhaco supergroup this up to 3000 m thick and slightly metamorphosed sedimentary succession is exposed in the central part of the southern serra do espinhaco southeastern brazil detailed lithofacies studies were carried out to characterize the sedimentation and paleogeography of the sao joao da chapada formation six lithofacies are present horizontal to low angle cross stratified sandstones lithofacies sh sl trough cross stratified sandstones lithofacies st planar cross stratified sandstones lithofacies sp ripple cross laminated sandstones lithofacies sr massive to horizontally stratified conglomerates lithofacies gm and fine grained rocks lithofacies f these are usually arranged in fining and thinning upward successions characterizing waning flood cycles which are always bounded by erosional surfaces no complete cycles have been found however and the cycles are normally composed only of lithofacies sh sl and st the other lithofacies are rare the observed sedimentary structures and their arrangement the geometry of the bodies mainly sheet like to slightly lenticular the low relief morphology of the erosion surfaces bounding the waning flood cycles the coarse and poorly sorted texture of the sediments and the measured paleocurrent pattern unidirectional with little dispersion throughout the study area point to deposition on a wide ephemeral braidplain characterized throughout by relative uniformity of facies and paleocurrent directions water depths on this subsiding braidplain were maintained by high discharge streams debouching onto it from the upland catchment areas as these rivers spread onto the depositional plain the changes in gradient and substratum transformed them into channels of remarkable width and very large width depth ratios these rivers ran in braided tracts and floods were temporarily confined within banks in these tracts channel switching events associated with tilting and subsidence of the basin floor resulted in the accumulation of extensive sandstone sheets interfingering laterally with one another and succeeding one another vertically which built up a braidplain succession extending up to 35 km transverse to the paleoslope the sao joao da chapada braidplain flanked the west margin of a n s elongated rift basin

late neoarchean subduction related crustal growth in the northern liaoning region of the north china craton evidence from 2 55 to 2 50 ga granitoid gneisses abstract the north china craton ncc dominated by 2 6 2 5 ga tectonothermal events provides a natural laboratory to study neoarchean crustal growth and geodynamic evolution late neoarchean granitoid gneisses are well exposed in the northern liaoning province located north of the ancient anshan benxi terrane along the northeastern margin of the eastern block eb of the ncc la icpms zircon u pb isotopic data reveal that granitoid gneisses in the qingyuan area can be grouped into two major episodes i e 2559 2534 ma strongly gneissic quartz dioritic and tonalitic to trondhjemitic gneisses and 2529 2495 ma weakly gneissic to massive quartz monzodioritic and monzogranitic gneisses with subordinate tonalitic to trondhjemitic gneisses the late magmatic episode was accompanied by regionally high grade metamorphism 2510 2495 ma most granitoid gneisses display highly depleted zircon ehf t2 values 4 2 to 8 1 whereas one monzogranitic gneiss shows negative values of 4 7 to 1 0 indicating late neoarchean crustal growth with minor involvement of ancient continental materials probably sourced from the anshan benxi terrane geochemical and petrogenetic studies reveal that the quartz dioritic magmas were derived from partial melting of plagioclase poor garnet amphibolites or eclogites metamorphosed from oceanic slab materials with slab melts contaminated by mantle wedge peridotites during ascent the tonalitic to trondhjemitic magmas stemmed from partial melting of mainly juvenile metabasaltic rocks with minor metagreywackes of lower arc crust in comparison the quartz monzodioritic and monzogranitic magmas were derived respectively from partial melting of depleted mantle sources metasomatized by slab derived fluids and metagreywackes with different crustal resident ages at middle to lower crustal levels combined with previous studies of supracrustal metavolcanic rocks the northern liaoning province records late neoarchean crustal growth evolving from mid ocean ridge through initiation and maturation of an intra oceanic arc to arc continent collision arc continent accretion and possibly slab rollback processes may have triggered reworking of both juvenile arc crust and minor ancient continental margin materials generating the magmatic precursors for the monzogranitic gneisses overall the intense late neoarchean crustal growth of the eb was controlled mainly by arc continent accretion possibly linked to global assembly of cratonic fragments

recharge in volcanic systems evidence from isotope profiles of phenocrysts strontium isotope ratios measured from core to rim across plagioclase feldspar crystals can be used to monitor changes in the isotope composition of the magma from which they grew in samples from three magma systems from convergent margin volcanoes sudden changes in major element composition petrographic features and strontium isotope composition were found to correspond to discrete magmatic events most likely repeated recharge of more mafic magma with lower ratios of strontium 87 to strontium 86 into a crustally contaminated magma

middle to late jurassic felsic and mafic magmatism in southern hunan province southeast china implications for a continental arc to rifting abstract extensive middle to late jurassic felsic and mafic magmatism occurred in the southern hunan province southeast china shrimp zircon u pb dating mineral chemical element geochemical and sr nd hf isotopic data have been determined for these rocks the results indicate that the middle jurassic 178 170 ma changchengling and ningyuan basaltic rocks belong to tholeiitic series and alkaline series respectively and were formed by fractional crystallization of the parent magmas that were derived from partial melting of asthenospheric mantle triggered by the slab released fluids of the subducted paleo pacific plate the late jurassic 152 146 ma daoxian basalts and guiyang lamprophyre dikes are low ti high mg potassic rocks they were derived directly by partial melting of shallower 60 100 km lithospheric mantle composed of amphibole and phlogopite bearing lherzolite in response to thermal perturbations associated with the rifting of continental arc the middle jurassic 164 ma tongshanling granitic rocks belong to calc alkaline series and are strongly peraluminous they were formed by partial melting of palaeoproterozoic metasedimentary basement in the lower crust plus additional input from coeval basaltic magma the late jurassic 156 ma jinjiling and xishan granitic rocks show a2 subtype affinity they were generated by partial melting of granulitized palaeoproterozoic metamorphic basement in the lower crust in response to injection of coeval low ti high mg potassic magmas a moderate degree 56 58 of fractionation of these pure crustal melts could account for more felsic end member of granitic rocks detailed petrologic and geochemical data for the middle to late jurassic felsic and mafic rocks from the southern hunan imply that during the middle jurassic time se china on the southeast of the shi hang zone was a continental arc coupled with the subduction of the palaeo pacific plate and since the beginning of late jurassic time an intra arc rift has been formed along the shi hang zone as a consequence of slab roll back

what controls chemical variation in granitic magmas abstract consideration of the models that have been applied to explain the chemical variations within granitic rock suites shows that most are inadequate to account for the main variations this stems from a variety of model deficiencies ranging from physical or energetic inadequacies to incompatibility with the chemical data or internal inconsistency between models based on for example isotope or trace element data and major element data we contend that any model that fails any of these tests of internal consistency cannot be considered further thus although we can point to examples in which many of the traditionally accepted mechanisms have played secondary roles in producing variation there presently remains but one viable choice for the primary mechanism by which most granitic magmas acquire compositions beyond the range defined by the compositions of crustal melts that primary mechanism is peritectic assemblage entrainment pae we infer that once a partial melt has formed in a crustal protolith it may segregate from its complementary solid residue carrying small crystals of the peritectic phase assemblage formed in the melting reaction and that the ratios of individual peritectic minerals in the entrained assemblage remains fixed in the ratio decreed by the stoichiometry of the melting reaction for those elements with low solubilities in granitic melts pae in varying degrees accompanied by co entrainment of accessory minerals is responsible for most of the primary elemental variation in granitic magmas in contrast the concentrations of elements with high solubilities in silicic melts reflect the protolith compositions in a simple and direct way the source is the primary control on granite magma chemistry it dictates what is available to dissolve in the melt and what will be formed as the entrainable peritectic assemblage the apparent complexity in granitic rock suites is largely a consequence of these processes in the source all other mechanisms contribute only as a secondary overlay

marine volcaniclastics of the hidden lake formation coniacian of james ross island antarctica an enigmatic element in the history of a back arc basin

bed shear stress over subaqueous dunes and the transition to upper stage plane beds plane beds that replace small bedforms e g ripples on the backs of subaqueous dunes are dynamically similar to upper stage plane beds local bed shear stresses at the position on dunes where plane beds develop may be two to three times larger than on a completely plane bed under the same general hydraulic conditions these observations help explain the transition from dunes to a wholly upper stage plane bed

geochemical and sm nd isotopic study of amphibolites in the cathaysia block southeastern china evidence for an extremely depleted mantle in the paleoproterozoic abstract geochemical and sm nd isotopic results are reported for late paleoproterozoic mafic amphibolites from sw zhejiang and nw fujian parts of the cathaysia block of se china two suites of contemporaneous amphibolites are distinct in their geochemical characteristics group 1 samples from nw fujian have chemical compositions of transitional and alkali basalts show lree enriched patterns and plot mainly in the field of within plate basalt on a number of trace element discrimination diagrams group 2 rocks from sw zhejiang have tholeiitic compositions and are characterized by flat to lree depleted patterns and fall into the morb field all the amphibolite samples have high end t values of 5 6 to 8 5 t 1766 19 ma a positive correlation between end t and nb th suggests possible mixing of a mantle derived magma and a crustal component with the least contaminated samples having very high end t values 8 8 5 and nb th ratios of 13 the geochemical and isotopic characters and close temporal relationship of these two suites of amphibolites suggest that their magmatic precursors were likely formed in an environment similar to an ensialic rift developing into a proto oceanic basin e g the gulf of tadjoura the exceptionally high end t values of up to 8 5 for some of the amphibolites suggest the presence of a time integrated extremely depleted mantle source beneath cathaysia during the paleoproterozic

uniformly mantle like δ18o in zircons from oceanic plagiogranites and gabbros lower ocean crust is primarily gabbroic although 1 2 felsic igneous rocks that are referred to collectively as plagiogranites occur locally recent experimental evidence suggests that plagiogranite magmas can form by hydrous partial melting of gabbro triggered by seawater derived fluids and thus they may indicate early high temperature hydrothermal fluid circulation to explore seawater rock interaction prior to and during the genesis of plagiogranite and other late stage magmas oxygen isotope ratios preserved in igneous zircon have been measured by ion microprobe a total of 197 zircons from 43 plagiogranite evolved gabbro and hydrothermally altered fault rock samples have been analyzed samples originate primarily from drill core acquired during ocean drilling program and integrated ocean drilling program operations near the mid atlantic and southwest indian ridges with the exception of rare distinctively luminescent rims all zircons from ocean crust record remarkably uniform 18o with an average value of 5 2 0 5 2sd the average 18o zrc would be in magmatic equilibrium with unaltered morb 18o wr 5 6 5 7 and is consistent with the previously determined value for equilibrium with the mantle the narrow range of measured 18o values is predicted for zircon crystallization from variable parent melt compositions and temperatures in a closed system and provides no indication of any interactions between altered rocks or seawater and the evolved parent melts if plagiogranite forms by hydrous partial melting the uniform mantle like 18o zrc requires melting and zircon crystallization prior to significant amounts of water rock interactions that alter the protolith 18o zircons from ocean crust have been proposed as a tectonic analog for 3 9 ga detrital zircons from the earliest hadean earth by multiple workers however zircons from ocean crust are readily distinguished geochemically from zircons formed in continental crustal environments many of the 3 9 ga zircons have mildly elevated 18o 6 0 7 5 but such values have not been identified in any zircons from the large sample suite examined here the difference in 18o in combination with newly acquired lithium concentrations and published trace element data clearly shows that the 3 9 ga detrital zircons did not originate by processes analogous to those in modern mid ocean ridge settings

lower flow regime bedforms rational classification a classification of lower flow regime bedforms is proposed based upon consideration of the effect of water flow on the shape and size of bedforms which is more rational than other systems presently in use it is suggested that a more quantitative system of bedform classification may lead to a better understanding of the occurrence and behaviour of bedforms the assumptions on which the proposed classification is based are shown to agree with published data relating bedform characteristics to mean flow depth and velocity the proposed classification offers a rational explanation for several hitherto puzzling aspects of bedform behaviour in laboratory and field situations since much presently available data is unsuited to testing these proposals further extensive experimental work on bedforms is required

relict proterozoic basement in the nanling mountains se china and its tectonothermal overprinting 1 the basement beneath the caledonian belt in se china has been overprinted by indosinian and yanshanian events and is poorly known in situ u pb and hf isotope analysis of zircons from granites and surrounding rocks in the nanling mountains area of the fold belt reveal that the lanhe felsic gneiss is an outcrop of mesoproterozoic basement probably with a partly late archean protolith this is consistent with nd model ages of granites in this region and exposures of cathaysia basement in the coastal area of se china in the nanling mountains this basement was reactivated to generate granitoid rocks during the caledonian indosinian and yanshanian tectonothermal events the caledonian overprint is documented in extensive exposures of the zhuguang migmatites 425 470 ma and by previous u pb zircon dating of adjacent granites the jiangnan granite may be of similar age but strongly reworked the indosinian overprint resulted in the formation of granites such as the luxi and xiazhuang bodies 233 246 ma in the guidong complex the yanshanian overprint produced the most widely distributed granites circa 160 ma such as the jiufeng and fogang bodies and parts of the guidong complex hf isotope data show that all of the phanerozoic magmatic rocks were derived from the mesoproterozoic basement and minor archean components without obvious contributions from juvenile sources the presence of zircons with archean age 2 5 2 7 ga in both the lanhe gneiss and devonian sediments indicates that the badu and mayuan groups may not be the oldest basement of the cathaysia block some archean basement circa 2 5 2 7 ga may still be exposed but unrecognized in this region and cathaysia may include archean microcontinental fragments

geochronological and geochemical results from mesozoic basalts in southern south china block support the flat slab subduction model the debate on the mesozoic tectonics of the south china block scb featuring a broad orogenic belt and a large continental magmatic province has been rejuvenated in the past years there are a number of competing tectonic models each predicting different time space and compositional evolutional trends for the mesozoic igneous rocks including sporadic basalts in this paper we report high precision 40 ar 39 ar ages and geochemical and sr nd isotopic data for basaltic rocks in central eastern scb these results together with a data compilation indicate three evolutionary stages for the basalts stage 1 195 160 ma basalts occur only in inland scb they exhibit low initial 87 sr 86 sr ratios 0 7038 to 0 7078 high e nd t values 1 5 to 6 0 and ocean island basalt oib like geochemical characteristics such as low la nb ratios 0 6 to 1 4 implying negligible to minor lithosphere contamination stage 2 160 110 ma basalts mostly from the same inland region with minor occurrence closer to the coast are characterized by variable initial 87 sr 86 sr ratios 0 7053 to 0 7102 end t values 6 0 to 5 0 and low la nb ratios 0 9 to 1 8 indicating an enhanced asthenosphere lithosphere interaction stage 3 110 to 80 ma basalts can be further divided into two sub groups based on their geographic and geochemical variations basalts in the coastal regions are characterized by low end t values 8 1 to 3 7 and variably high la nb ratios up to 4 8 indicating a strong arc signature on the other hand basalts in the inland regions have high e nd t values 1 9 to 6 8 and low la nb ratios 0 6 to 1 2 that are similar to the stage 1 basalts we interpret these features as results of geodynamics processes related to the break up foundering and retreating of an early mesozoic flat subducted oceanic slab

u pb age and hf o isotopes of detrital zircons from hainan island implications for mesozoic subduction models abstract a compilation of magmatic ages from the mesozoic south china block suggests a number of magmatic quiescence periods at ca 205 195 ma ca 150 140 ma and ca 125 115 ma casting doubt on tectonic models that suggest ongoing andean type subduction along the south china continental margin however sims u pb analyses on two detrital zircon samples from the cretaceous lumuwan formation on hainan island southeast china reveal three major age peaks at ca 120 ma ca 155 ma and ca 235 ma zircons of these ages are mostly euhedral and show typical magmatic oscillatory zoning suggesting short distance transport from nearby magmatic sources the extremely rare occurrence of ca 120 ma magmatic records onshore suggests that detrital zircons of this age population may be derived from a source proximal to hainan island but presently missing therefore our data provide new evidence for ongoing magmatic activity in late mesozoic south china in situ hf and o isotope analyses of the mesozoic detrital zircons reveal large variations in both ehf t 21 2 to 10 5 and 18o 4 4 to 13 6 values a general negative correlation between them suggests the reworking of old supracrustal materials average crustal residence age of ca 2 0 ga by juvenile mantle derived magmas the progression of increasing ehf t and decreasing 18o values of zircons from the triassic to the cretaceous suggests progressive crustal growth during the mesozoic the results are consistent with hybridization at an active continental margin we briefly review tectonic models for the indosinian orogeny and suggest that the petrologic evidence indicates that mesozoic magmatism was part of the circum pacific accretionary orogens that formed along the continental margin of east asia no later than ca 250 ma and continued at least to the late cretaceous

supracrustal input to magmas in the deep crust of sierra nevada batholith evidence from high δ 18 o zircon oxygen isotope ratios of zircon zc from intrusives exposed in the tehachapi mountains southern california reveal large inputs of high 18 o supracrustal contaminant into gabbroic and tonalitic magmas deep 30 km in the cretaceous sierra nevada batholith high 18 o zc values 7 8 0 7 predominate in the deep parts of the batholith but lower values 6 1 0 9 occur in shallower parts this indicates a larger gradient in 18 o with depth in the batholith than occurs from west to east across it oxygen sr and nd isotope data show that the supracrustal contaminant was likely young paleozoic or mesozoic hydrothermally altered upper oceanic crust or volcanic arc sediments such rocks were subducted or underthrust beneath the sierran arc during accretion of oceanic terranes onto north america this component yielded high 18 o magmas that were added to the base of the batholith on average gabbros in the southern sierra contain at least 18 of the subducted supracrustal component some tonalite and granodiorite magmas were additionally contaminated by kings sequence metasedimentary rocks as evidenced by 18 o zc and initial 87 sr 86 sr that trend toward values measured for the kings sequence besides high 18 o values in the southern sierra xenoliths in the central sierra also have elevated 18 o which confirms the widespread abundance of supracrustal material in the sub arc lithospheric mantle in contrast to 18 o zc whole rock 18 o values of many samples have undergone post magmatic alteration that obscures the magmatic contamination history of those rocks such alteration previously prevented confident determination of the mass of young hydrothermally altered mantle rocks that contributed to sierran granitoids

metasedimentary melting in the formation of charnockite petrological and zircon u pb hf o isotope evidence from the darongshan s type granitic complex in southern china abstract charnockites are opx bearing igneous rocks commonly found in high grade metamorphic terranes despite being volumetrically minor they show a wide range in both bulk geochemistry and intensive parameters they form a characteristic component of the amcg anorthosite mangerite charnockite granite suite but their association with typical s type granites is less well known the darongshan s type granitic complex dsgc in guangxi province southern china contains granites varying in mafic silicate mineral assemblages from bt crd darongshan suite to opx grt bt crd jiuzhou suite and opx crd bt taima suite corresponding to a geochemical transition from magnesian calc alkalic to ferroan calc alkalic however its genesis even the accurate age of intrusion remains highly contentious despite intensive research in order to understand the genesis of charnockite and its genetic relationship with s type granite here we first determined zircon u pb ages of each suite using a sims on the basis of a detailed petrological study zircon u pb ages show that all suites of the complex were emplaced contemporaneously at ca 249 ma monazite apparent u pb ages are indistinguishable from zircon u pb ages within analytical error further in situ zircon hf o isotope analyses reveal that the granitic complex was dominantly derived from reduced melting metasedimentary rocks 18ozircon ca 11 ehf t zircon ca 10 log fmq 0 mn in apatite oxybarometer with rare material input from the mantle the variation in 18o 7 8 12 9 is more likely a result of hybridization whereas that in ehf t 31 9 to 1 8 is a result of both hybridization and disequilibrium melting the variation in mineralogy and geochemistry may be interpreted as a result of entrainment of peritectic garnets from biotite dehydration melting nevertheless heat input from mantle through basaltic intrusion underplating is considered to play a major role in high temperature 850 c melting at mid crustal levels i e the cordierite stable field for generation of the granitic complex we interpret that the granites were intruded in a back arc setting and basaltic magmatism was directly associated with slab roll back and tearing during the latest permian and early triassic times

multiple origins for the middle jurassic to early cretaceous high k calc alkaline i type granites in northwestern fujian province se china and tectonic implications abstract a comprehensive study of zircon u pb dating and in situ hf isotopes whole rock major and trace element geochemistry and sr nd isotopes was carried out for three late mesozoic granitic plutons waitun shipi and taiyuan in northwestern fujian province se china we assess the origin of the granites and their relationship to the evolution of the late mesozoic volcanic intrusive complex belt in se china la icp ms zircon u pb dating shows that three plutons were emplaced in the middle jurassic to early cretaceous 168 109 ma in which the waitun and shipi plutons are intrusive complexes all the plutons are composed of high k calc alkaline i type granites with a great diversity in elemental and isotopic compositions the granites have sio 2 contents of 68 3 78 5 wt showing a gradual decrease in initial 87 sr 86 sr 0 7181 to 0 7091 and increase in e nd t 16 7 to 8 1 and e hf t in situ zircon 20 6 to 6 9 with decreasing emplacement ages geochemical data suggest that the middle jurassic 168 ma waitun granites are of purely crustal origin derived by partial melting of a mixed source of paleoproterozoic metaigneous 78 and metasedimentary 22 rocks at a depth of 30 40 km triggered by underplating of basaltic magma mixing of such crustal melts with about 10 basaltic magma could account for the origin of the late jurassic 161 ma waitun granites the late jurassic 156 ma shipi and early cretaceous 134 ma taiyuan granites were produced by extensive fractional crystallization of primary crustal melts the source of which show relatively high proportion 82 of metaigneous rocks the early cretaceous 109 ma shipi granites were generated by partial melting of a mixed source of paleoproterozoic metaigneous 92 and metasedimentary 8 rocks at a depth of 30 km plus additional 15 input from coeval basaltic magma the granites were formed in a continental arc setting induced by northwestward subduction of the paleo pacific plate progressive slab rollback since the middle jurassic from inland to the coastal area is the most likely mechanism for the generation of the granites

u pb geochronology and hf nd isotopic geochemistry of the badu complex southeastern china implications for the precambrian crustal evolution and paleogeography of the cathaysia block abstract the oldest rocks of the cathaysia block south china comprise the badu complex and paleoproterozoic granites in the wuyishan area southern zhejiang and northwestern fujian provinces new zircon u pb ages hf isotopes and trace elements for metamorphic rocks from the badu complex and bulk nd isotope compositions of these rocks and granites in the wuyishan area provide important constraints on the precambrian crustal evolution of the cathaysia block inherited cores of zircon grains from the metamorphic rocks are of magmatic origin predominantly formed at ca 2 5 ga while overgrowth rims reflect two episodes of granulite facies metamorphism related to collisional orogeny at 1 89 1 88 ga and 252 234 ma the unimodal age distribution 2 5 ga of detrital zircons and the positive hf t of most neoarchean zircons suggest that the detritus of these sedimentary protoliths of the badu complex came from a proximal volcanic arc and that they were deposited in an arc basin synchronously with 2 5 ga volcanism zircon u pb ages and hf isotopes with whole rock nd isotopes suggest that the juvenile crust of the eastern cathaysia block was generated mainly at 2 5 ga and 2 8 ga and minor at 3 5 3 3 ga with some evidence for the generation at 3 7 3 6 ga and 4 0 ga a strong 1 9 ga orogeny and the 3 3 3 0 ga thermal event only involved the reworking of older crust material paleoproterozoic 1 89 1 86 ga granitic magmatic activity and high grade metamorphism in the eastern cathaysia block were synchronous with the assembly of the columbia supercontinent using integrated geochronological hf nd isotopic and petrologic data as a barcode we compare the cathaysia block with other paleoproterozoic orogens worldwide and argue that its closest affinity is with the south korean peninsula and the lesser himalaya of nw india consequently the eastern cathaysia block and the south korean massif as the united cathay south korea terrane was close to the lesser himalaya terrane in the paleoproterozoic configuration of the columbia supercontinent the spatial linkage was maintained for ca 1 ga until the fragmentation of the rodinia supercontinent during neoproterozoic time

petrogenesis of jurassic fractionated i type granites in southeast china constraints from whole rock geochemical and zircon u pb and hf o isotopes abstract whole rock geochemical and sr nd hf isotopic data and zircon in situ u pb ages and hf o isotopes are reported for twelve granitic intrusions from inland to coastal areas of the cathaysia block southeast se china in order to constrain their magma sources and petrogenesis they are mainly composed of biotite granites with minor granodiorites zircon u pb dating by la icpms and sims methods gave emplacement ages of 165 154 ma for these rocks which are metaluminous to weakly peraluminous a cnk 0 95 1 10 with variable sio 2 contents and high k 2 o contents they are all depleted in high field strength elements hfses and enriched in light rare earth elements lrees and large ion lithophile elements liles and show negative sr ba and eu anomalies in the spidergrams petrographic and geochemical features suggest the granodiorites that contain hornblende to be typical of i type while the biotite granites without hornblende are fractionated i type granites their variable major and trace element concentrations are due to fractional crystallization of feldspar biotite hornblende and accessory minerals including apatite allanite and monazite moreover our data indicate significant variations in isotopic compositions of these granitoids from inland to coastal areas the inland granites from the lapu fogang xinfengjiang and baishigang plutons have relatively high initial 87 sr 86 sr ratios 0 7105 0 7155 and low e nd t 11 2 to 7 3 and e hf t 10 9 to 6 3 values for whole rocks and variable 18 o values of 7 69 9 25 and e hf t values of 13 5 to 6 7 for zircons in contrast the coastal granites from the longwo wushikeng lianhuashan shigushan chiliao hulutian mantoushan and fenghuang plutons show relatively low initial 87 sr 86 sr ratios 0 7056 0 7110 and high e nd t 7 6 to 6 2 and e hf t 6 5 to 2 8 values for whole rocks together with e hf t values of 7 2 to 3 9 and 18 o values of 6 69 8 38 for zircons the spatial isotopic variations suggest that these jurassic granitoids were generated by different percentage mixing of mantle and ancient paleoproterozoic crust derived magmas with subsequently minor crustal assimilation our results highlight that jurassic fractionated i type granites occur widely from inland to coastal areas of se china arguing against the previous view that jurassic granites occur only in inland while coastal granites are mainly cretaceous the widespread occurrence of jurassic granitoids in se china is attributed to ascending of mantle derived magmas which provide heat for partial melting of lower crustal materials in an extensional setting

early silurian granitic rocks and associated enclaves as evidence of rapid cooling in a cognate magma system the case of the xuehuading panshanchong pluton south china block

architectural analysis of fluvial sequences in the northwestern part of kyongsang basin early cretaceous se korea abstract the northwestern part of kyongsang basin early cretaceous largely comprises interbedded sandstones and mudstones of fluvial depositional environments with local basin margin conglomeratic deposits of alluvial fan systems these non marine successions are divided into 5 successive stratigraphic units each of which is unique in facies assemblages and architecture of sandstone bodies one stratigraphic unit sinpyong anpyong unit is documented in this paper with detailed architectural analysis of sandstone bodies the deposits of the sinpyong anpyong unit are divided into three major components thick sandstone thin sandstone and mudstone dominated bodies the thick sandstone bodies 2 47 m thick comprise stacked element groups bounded by laterally persistent erosional surfaces probably representing superposition of channel belts within individual element groups elements are laterally superposed indicating growth and superposition of bars and movement of associated channels the thickness of bar deposits and dip angles of laterally inclined strata suggest that channels were a few to 10 m in depth and 9 m to several tens of meters in width paleocurrent data indicate southward or southeastward draining channel systems the thin sandstone bodies generally the thick sandstone bodies formed in major channels and channel belts are characterized by lateral superposition of numerous bar and channel deposits limited extent of individual lateral accretion sets and the presence of mid channel bar deposits suggesting deposition in braided rivers detailed reconstruction of bars and channels reveals a range of processes including growth coalescence and erosion of bars and migration switching and filling of channels lateral accretion of bars and the associated channel migration might have been principal processes in the sinpyong anpyong river systems bars were commonly coalesced by lateral superposition forming large bar complexes

carbon cycling at the sediment water interface in a eutrophic mountain lake jezero na planini pri jezeru slovenia abstract the biogeochemical processes that dominantly influence carbon cycling at the sediment water interface in a eutrophic mountain lake jezero na planini pri jezeru slovenia were studied for 2 years using measurements of the stable isotopic composition of carbon as well as chemical analyses of the pore waters and sediment according to the isotopic mass balance the major contribution of the most important aquatic carbonate species dissolved inorganic carbon dic is derived from methanogenesis 67 92 and much less from decomposition of sedimentary organic carbon with other electron acceptors 8 23 the contribution of calcite dissolution was observed only in 1999 when it varied throughout the year from 3 16 and did not influence dic production appreciably the lake sediment is an important sink of organic carbon that accumulates at the bottom of the lake

climatic and geomorphologic cycles in a semiarid distributive fluvial system upper cretaceous bauru group se brazil abstract studies of distributive fluvial systems and their preserved successions envisage the distribution and orientation of architectural elements to be primarily controlled by channels radiating outward from fan apices in many cases along an elongate basin margin conceptual models for such systems account for the downstream dynamics of the fluvial network but with limited consideration of temporal geomorphic variations resulting vertical organisation of architectural elements or of the interplay of factors controlling system dynamics to understand the external and internal architecture of distributive fluvial systems and the factors that influence their sequential facies organisation a sedimentary succession of the proximal portion of an upper cretaceous semiarid distributive fluvial system located at the north eastern margin of the bauru basin southeast brazil has been analysed in detail three fining and thinning upward fluvial sequences are identified forming an interval separated at the top and the bottom by two palaeosol profiles each sequence is formed of channel and floodplain deposits two types of channel deposits are identified one is composed of stacked sets of small scale dune deposits suggesting perennial and steady fluvial regime associated with more humid climate periods the other is composed of large scale sets indicative of flattened dunes associated with unsteady and fast changing fluvial flow formed in quasi supercritical flow regime conditions associated with drier climate periods the vertical alternation of these two types of channel deposits records the accumulation of a fluvial succession that responded to high frequency climate induced cyclic change in boundary conditions two palaeosol profiles at the top and at the bottom of the succession indicate temporary interruptions and cessation of the fluvial sedimentation likely related to avulsion of the fluvial belt thus the studied succession reveals high frequency climate induced allogenic sedimentary cycles that occur within a long period autogenic geomorphologic induced sedimentary cycle this work suggests that the internal architecture of the channel deposits can be used as a climate proxy and that climate and geomorphology act jointly as notable factors to control the vertical organisation of distributive fluvial systems

sedimentary and structural evolution of a relict subglacial to subaerial drainage system and its hydrogeological implications an example from anglesey north wales uk subglacial drainage systems exert a major control on basal sliding rates and glacier dynamics however comparatively few studies have examined the sedimentary record of subglacial drainage this is due to the paucity of modern analogues the limited recognition and preservation of upper flow regime deposits within the geological record and the difficulty of distinguishing subglacial meltwater deposits from other meltwater sediments e g glacier outburst flood deposits within this study the sedimentological and structural evolution of a subglacial to subaerial ice marginal proglacial drainage system is examined particular emphasis is placed upon the genetic development and preservation of upper flow regime bedforms and specifically recognising them within a subglacial meltwater context facies are attributed to subglacial meltwater activity and record sedimentation within a confined but progressively enlargening subglacial channel system produced under dune to upper flow regime conditions bedforms include rare large scale sinusoidal bedding with syn depositional deformation produced by current induced traction and shearing within the channel margins subglacial sedimentation culminated with the abrupt change to a more ephemeral drainage regime indicating channel abandonment or a seasonal drainage regime retreat of the ice margin led to the establishment of subaerial drainage with phases of sheet flow punctuated by channel incision and anastomosing channel development under diurnal ablation related seasonal discharge the presence of extensive hydrofracture networks demonstrate that proglacial groundwater levels fluctuated markedly and this may have influenced later overriding of the site by an ice stream

proterozoic braided ephemeral fluvial deposits an example from the dhandraul sandstone formation of the kaimur group son valley central india abstract the dhandraul sandstone formation of the middle proterozoic kaimur group the vindhyan supergroup son valley india has been reinterpreted to represent a product of braided ephemeral fluvial environment the previous interpretation was shallow marine barrier bar evidences in favour of braided river deposition of the dhandraul sandstone are a cosets of low angle planar cross bed sets are closely associated with cosets of parallel laminated sets with particle streaming on bedding plane surfaces b vertical recurrence of planar laterally impersistent erosional surfaces that are invariably carpeted by red coloured intraformational shale pebble conglomerate c profuse development of ripple marks d sedimentary structures indicative of critical to supercritical flow conditions and e consistent palaeocurrent patterns

origin of cenozoic alkaline potassic volcanic rocks at konglongxiang lhasa terrane tibetan plateau products of partial melting of a mafic lower crustal source abstract it is generally believed that cenozoic potassic and ultrapotassic volcanic rocks of the tibetan plateau were generated by partial melting of enriched mantle this paper reports new data regarding the elemental and isotopic compositions of the konglongxiang volcanic rocks kvr k2o na2o 2 and low mgo contents in the middle western lhasa terrane tibetan plateau ar ar isotope dating of a sample from the kvr yields an age of 21 3 ma indicating that the kvr lavas erupted during the early miocene the kvr consists mainly of trachyte and rhyolite a comparison of the kvr with cenozoic potassic volcanic rocks of the tibetan plateau and the results of melting experiments involving k rich volcanic rocks as well as other k rich volcanic rocks from throughout the world reveals that the kvr trachytes originated from a mafic lower crust containing k rich minerals rather than from a mantle source the kvr rhyolites were probably derived from the middle upper crust based on age data compositional data and the regional tectonic setting we propose that the formation of the kvr and associated n s grabens was triggered by early miocene e w regional extension in the area of the southern tibetan plateau

u pb zircon geochemical and sr nd hf isotopic constraints on age and origin of jurassic i and a type granites from central guangdong se china a major igneous event in response to foundering of a subducted flat slab abstract the mesozoic geology of se china is characterized by widespread jurassic to cretaceous igneous rocks consisting predominantly of granites and rhyolites and subordinate mafic lithologies however the tectonic regime responsible for the inland jurassic granites remains controversial we report here u pb zircon ages geochemical and sr nd hf isotopic data for the nankunshan alkaline granite and the fogang granitic batholith in central guangdong mineralogical and geochemical features suggest that the fogang and nankunshan rocks are i and aluminous a type granites respectively shrimp u pb zircon analyses yield consistent ages ranging from 159 3 ma to 165 2 ma for four samples from the fogang batholith and an age of 158 5 ma for the nankunshan granite the fogang granites having i sr 0 7098 0 7136 e nd t 4 3 to 12 2 and e hf t 11 5 to 3 1 for the magmatic zircons were derived from paleoproterozoic mafic intermediate igneous protolith with minor addition of mantle derived magmas the nankunshan rocks have relatively lower i sr 0 706 0 708 higher e nd t 0 3 to 2 4 and e hf t 5 7 to 1 1 for the magmatic zircons and some oib like trace element ratios they were likely generated through extensive fractional crystallization of mantle derived alkaline parental magma associated with crustal assimilation these 160 ma i and a type granites in central guangdong were emplaced coeval with the widespread 165 155 ma i and a type granites and syenites following the initiation of intraplate basaltic and or bimodal igneous magmatism at 180 170 ma in the adjacent regions we interpret these jurassic igneous rocks as anorogenic magmatism formed during a major igneous event in response to foundering of an early mesozoic subducted flat slab beneath se china continent

large igneous provinces lips definition recommended terminology and a hierarchical classification abstract this article is an appeal for the adoption of a correct and appropriate terminology with respect to the so called large igneous provinces lips the term lip has been widely applied to large basaltic provinces such as the deccan traps and the term silicic large igneous province slip to volcanic provinces of dominantly felsic composition such as the whitsunday province however neither term lip slip has been applied to the large granitic batholiths of the world e g andes to which both terms are perfectly applicable lip has also not been applied to broad areas of contemporaneous basalt magmatism e g indochina mongolia and sizeable layered mafic intrusions e g bushveld which in many significant respects may also be considered to represent large igneous provinces here i suggest that the term lip is used in its broadest sense and that it should designate igneous provinces with outcrop areas 50 000 km 2 i propose a simple hierarchical classification of lips that is independent of composition tectonic setting or emplacement mechanism i suggest that provinces such as the deccan and whitsunday provinces should be called large volcanic provinces lvps whereas large intrusive provinces mafic ultramafic intrusions dyke sill swarms granitic batholiths should be called large plutonic provinces lpps lvps and lpps thus together cover all lips which can be felsic mafic or ultramafic of sub alkalic or alkalic affinity and emplaced in continental or oceanic settings lvps are subdivided here into four groups i the dominantly wholly mafic large basaltic provinces lbps e g deccan ontong java ii the dominantly felsic large rhyolitic provinces lrps e g whitsunday sierra madre occidental iii the dominantly andesitic large andesitic provinces laps e g andes indonesia cascades and iv the bimodal large basaltic rhyolitic provinces lbrps e g snake river high lava plains the intrusive equivalents of lrps are the large granitic provinces lgps e g the andean batholiths although an equivalent term for intrusive equivalents of lbps is not necessary or warranted the accuracy and usefulness of the terms flood basalt plateau basalt and trap are also examined the largest lbp lvp and lip is of course the bulk of the ocean floor it is contended that the proposed lip nomenclature and classification will lead to more accurate and precise terminology and hence better understanding of the wide variety of large igneous provinces

rethinking the emplacement and evolution of zoned plutons geochronologic evidence for incremental assembly of the tuolumne intrusive suite california new u pb geochronologic data indicate that the tuolumne intrusive suite california was assembled over a period of at least 10 m y between 95 and 85 ma and that the half dome granodiorite intruded over a period approaching 4 m y simple thermal considerations preclude the possibility that a magma chamber the size of the half dome pluton could have existed as a liquid at shallow crustal depths for that long rather field evidence for sheeting along the margins of the suite the range of ages and the regular decrease of ages toward the center of the suite and within individual plutons suggest incremental assembly geochronologic evidence for incremental assembly is consistent with the failure of geophysical methods to detect large magma chambers with more than 20 melt even in active volcanic areas because it is unlikely that the individual plutons composing the tuolumne ever coexisted as liquid rich magmas the chemical evolution of the suite cannot be the result of simple fractionation and or mixing between exposed units but instead must reflect processes occurring during magma generation

early paleozoic intracontinental granites in the guangzhou region of south china partial melting of a metasediment dominated crustal source abstract whether the formation of early paleozoic granites in the south china block was related to intracontinental orogenic processes or oceanic subduction continues to be debated here we present whole rock major element trace element and sr nd isotopic data as well as secondary ion mass spectrometry and laser ablation inductively coupled plasma mass spectrometry zircon u pb geochronological and hf o isotopic data from biotite two mica and muscovite granites in the baiyunshan and maofengshan areas of guangzhou city south china zircon u pb ages reveal that these rocks were formed during the early paleozoic 446 438 ma these muscovite and biotite bearing granites have high sio2 69 0 80 4 wt and k2o 3 19 5 32 wt contents and low mgo 0 46 1 17 wt contents the granites are enriched in rb th u and pb and depleted in ba sr nb and ti the baiyunshan muscovite granites and maofengshan biotite granites are slightly to moderately enriched in light rare earth elements lrees la yb cn 2 27 15 1 with slight to pronounced negative eu anomalies eu eu 0 27 0 72 whereas the baiyunshan two mica granites are significantly enriched in lrees la yb cn 19 6 32 1 with moderate negative eu anomalies eu eu 0 42 0 58 these muscovite and biotite bearing granites have enriched whole rock nd and zircon hf isotopic compositions end t 12 1 to 7 6 ehf t 11 4 to 0 8 and high zircon 18o values 8 5 10 0 we suggest that the baiyunshan granites were formed by partial melting of metasedimentary rocks as they contain primary muscovite and have high zircon 18o values the maofengshan granites were formed by partial melting of a hybridized crustal source containing metasedimentary rocks with subordinate amphibolites as they have high zircon 18o values variable whole rock sio2 contents and biotite compositions similar to those crystallized in i and s type magmas these granites along with the widespread early paleozoic s type granites in the wuyi yunkai orogen are distributed across a broad area rather than in a narrow belt in addition early paleozoic s type granites in the south china block generally do not show younging trends from east to west or south to north moreover the baiyunshan maofengshan granites show geochemical characteristics similar to syn or post collisional granites and different from volcanic arc granites we therefore propose that these muscovite and biotite bearing granites were formed in an intracontinental orogenic setting