provenance of jurassic sediments in the hefei basin east central china and the contribution of high pressure and ultrahigh pressure metamorphic rocks from the dabie shan abstract the provenance of the jurassic sediments in the hefei basin is constrained by compositions of the detrital k white micas and garnets and shrimp dating of the detrital zircons which can help to understand the evolution and to reconstruct the paleogeographic distribution of hp uhp rocks in the jurassic dabie shan 1 for the oldest mesozoic sediments at the bottom of the fanghushan formation j 1 the predominance of the early paleozoic and luliang 1700 1900 ma zircons indicates a major source from the north china block however neoproterozoic zircons as the major component in other jurassic sediments indicate that the source rocks were mainly derived from the exhumed yangtze block in the dabie shan 2 the co occurrence of high si phengites and triassic zircons provides stratigraphic evidence that the first exposure of the uhp rocks at the earth s surface in the dabie shan occurred in the early jurassic during deposition of the fanghushan formation 3 from the east to the west of the hefei basin there is a spatial variation in the compositions for detrital micas and garnets and in the u pb ages of detrital zircons evidently hp uhp rocks were widely distributed at outcrop in the eastern dabie shan in contrast they were less important in the western dabie shan during the jurassic

tectonothermal evolution of the mayuan assemblage in the cathaysia block implications for neoproterozoic collision related assembly of the south china craton the mayuan assemblage in the cathaysia block southeast china consists of felsic paragneiss pelitic schist greenschist amphibolite marble calcsilicate and quartzite that underwent three episodes of deformation d1 d3 and four episodes of metamorphism m1 m4 in the early neoproterozoic the m1 assemblage consists of mineral inclusions defining an early foliation s1 within porphyroblasts represented by chlorite muscovite biotite plagioclase quartz in pelitic schist and actinolite chlorite epidote albite quartz enclosed in amphibolite m2 coincides with the development of the regional schistosity s2 and represents the formation of the porphyroblasts and growth of matrix minerals resulting in development of prograde metamorphic zones chlorite biotite garnet staurolite and kyanite zones m3 is simultaneous with the third phase of deformation d3 and produced sillimanite bearing mineral assemblages in pelitic schist and hornblende bearing assemblages in amphibo lite the last metamorphic episode m4 gave rise to the retrogressive assemblage chlorite muscovite in pelitic rock and actinolite chlorite epidote in amphibolite the sequence of mineral assemblages and history of metamorphic reactions built from the petrogenetic grid of pelites suggest a near isothermal decompression clockwise p t path for the mayuan pelitic schists using the tweequ software program the garnet biotite thermometer and garnet muscovite biotite plagioclase barometer yield p t conditions for m1 of 5 5 to 6 0 kb and 450 to 500 c and conditions for the garnet staurolite and kyanite zones of m2 of 6 0 to 7 0 kb and 550 to 600 c 6 0 to 7 5 kb and 600 c and 11 0 to 11 5 kb and 600 c the p t conditions of m3 were estimated at 570 to 625 c and 4 0 to 4 5 kb using the muscovite biotite thermometer and hornblende plagioclase geothermo barometer the garnet chlorite thermometer yields temperatures of 300 to 400 c for m4 but the pressures of m4 cannot be quantitatively estimated because of the lack of a suitable geobarometer these p t estimates also define an near isothermal decompression clockwise p t path which is involved in initial crustal thickening followed by rapid exhumation and final cooling and is related to amalgamation of the cathaysia and yangtze blocks to form the south china craton

preliminary study of the turner albright zn cu ag au co massive sulfide deposit josephine county oregon the turner albright zn cu ag au co deposit represents an important ophiolite hosted cyprus type massive sulfide occurrence in the west coast accreted terranes of the united states recent exploration efforts have located an estimated 3 300 000 tons of mineralization with an average grade of 3 33 percent zn 1 46 percent cu 0 44 oz ton ag 0 11 oz ton au and 0 06 percent co the deposit occurs as a series of pods in the basal basaltic pillow lava sequence within the josephine ophiolite complex the ophiolite represents a 157 m y old accreted terrane which comprises part of the western jurassic lithotectonic belt of the klamath mountain province in southwestern oregon and northern california the mineralized horizons known as the main upper and lower zones and the upper high grade pods consist in order of abundance of pyrite sphalerite chalcopyrite marcasite and native gold with minor to trace amounts of tetrahedrite galena arsenopyrite and pyrrhotite the mineralized areas occur as discrete pods containing massive 50 100 and semimassive 20 50 sulfides which commonly occur stratigraphically above zones of nonmassive 5 20 or veined sulfides the massive sulfides exhibit fine to medium grained granular to porous and massive textures locally interrupted by colloform banding and sulfide breccias semimassive sulfides typically exhibit brecciated textures with minor colloform banding nonmassive sulfides occur as veins and disseminations in pillow basalts below the other sulfides thin mudstones typically overlie the massive sulfides whereas brecciated chert and basaltic volcaniclastic units occur in association with the semimassive sulfides alteration of the basalts consists of minor to moderate chloritization accompanied by limited silicification and pyritization in the mineralized areas the turner albright deposit was formed by metal bearing fluids generated by the circulation of seawater through a pillow basalt and sheeted dike complex within a back arc rifting environment these fluids vented as sea floor hot springs resulting in the precipitation of semi massive and massive sulfides stringer zones of nonmassive sulfides are interpreted as conduits for exhalative fluids

uhp metamorphism and exhumation of the dabie orogen china evidence from shrimp dating of zircon and monazite from a uhp granitic gneiss cobble from the hefei basin a shrimp u pb study of zircons separated from an ultrahigh pressure uhp granitic gneiss cobble sample ds12 from the upper jurassic fenghuangtai formation of the hefei basin north of the dabie orogen has identified three different domains 1 cores some of which show straight boundaries and strong oscillatory zoning with th u ratios of 0 12 0 70 and an imprecise upper intercept age of 777 220 ma mswd 2 4 2 mantles variable in shape and cathodoluminescence cl intensity and containing many uhp mineral inclusions including coesite and omphacite and with th u ratios of 0 02 0 26 and a weighted mean 206pb 238u age of 244 5 ma mswd 4 7 and 3 rims which are more homogeneous and luminescent in cl than the mantles and contain fewer uhp mineral inclusions with th u ratios of 0 01 0 05 and a weighted mean 206pb 238u age of 226 2 ma mswd 0 65 monazites from the same sample do not contain coesite and omphacite inclusions but they do show sector zones and are composed of two contrasting domains the light domains in backscattered electron bse images have a th u ratio of 7 3 18 9 and a weighted mean 206pb 238u age of 221 2 ma mswd 0 94 the dark domains in bse have a th u ratio of 2 8 7 2 and have a weighted mean 206pb 238u age of 218 2 ma mswd 0 96 based on these data and a review of the literature it can be concluded that 1 the protolith of gneiss sample ds12 is a granitoid of neoproterozoic age 2 the ages of 244 and 226 ma for the mantle and rim domains of the zircons are time records of two discrete stages in the uhp evolution of dabieshan 3 the ages of 221 and 218 ma for the light and dark gray domains respectively of the monazites record the times of two stages of retrogressive metamorphism during exhumation of the uhp granitic gneiss to a higher crustal level 4 the exhumation rate of the gneiss is estimated to be 6 km ma and 5 the uhp metamorphic blocks of the dabie orogen provided significant detritus to the hefei basin in the jurassic

clinoenstatite bearing buchites possibly from combustion of hydrocarbon gases in a major thrust zone glenroy valley new zealand surficial and partial melts of tertiary sandstones overthrust by a nappe adjacent to the alpine fault have crystallised several high temperature minerals presumably derived from combustion metamorphism mineral assemblages include clinoenstatite wo 2 4 en 50 8 fs 46 6 labradorite cristobalite tridymite rutile hematite magnetite and mullite the rare pyroxene clinoenstatite is metastable and occurred in a naturally quenched environment no coal is exposed within 1 5 km of the buchite buchite within a shear zone at the base of a major thrust sheet overriding carbonaceous units suggests that combustion may have been due to ignition of hydrocarbon gas seepage derived initially by over pressuring and shear heating of dispersed carbonaceous material the thrust sheet is associated with the miocene recent alpine fault absence of fission tracks in zircon indicates a recent origin for the buchite

provenance of the beihuaiyang lower grade metamorphic zone of the dabie ultrahigh pressure collisional orogen china evidence from zircon ages the beihuaiyang zone of the dabie ultrahigh pressure collisional belt which was formed by the early mesozoic collision of the north and south china yangtze blocks is diversely interpreted as an active or a passive sedimentary sequence it comprises the luzhenguan and foziling complexes six granitoid rocks of the luzhenguan complex are dated at 770 ma to 720 ma and one schist sample also contains detrital zircons of 760 ma to 720 ma these data indicate that this complex was of the south china affinity and probably originated from the northern yangtze block two quartzite samples of the foziling complex contain abundant detrital zircons of archean to paleozoic ages clustering around 2 5 ga 1 9 1 8 ga 1 0 0 7 ga and 0 5 0 4 ga this age spectrum demonstrates a mixed source of these two blocks the youngest zircons suggest a paleozoic deposition along an active continental margin south of the north china block while these late proterozoic zircons are characterized for the yangtze block subsequently it is suggested that a terrain drifted from the northern yangtze block in early paleozoic must have been situated to the southern margin of the north china block to provide material source prior to the final collision in early mesozoic q 2003 elsevier ltd all rights reserved

the 1 75 1 68ga anorthosite mangerite alkali granitoid rapakivi granite suite from the northern north china craton magmatism related to a paleoproterozoic orogen abstract the amgrs is a suite of anorthosite mangerite alkali granitoid rapakivi granite in the northern north china craton ncc new zircon shrimp and laser ablation inductively coupled plasma mass spectrometry la icpms u pb dating of the damiao anorthosite changsaoying k feldspar granite lanying anorthosite and quartz syenite and the gubeikou k feldspar granite yielded emplacement ages of 1726 9 ma 1753 23 ma 1739 43 ma 1712 15 ma and 1692 19 ma respectively combined with previous geochronological data from the damiao norite and mangerite and shachang rapakivi granite it indicates that the whole amgrs was intruded between 1750 ma and 1680 ma the damiao anorthosites and norites exhibit a high mg and this decreases from mafic up to 63 to felsic t dm c ages ranging from 2 32 ga to 2 43 ga and 2 70 ga to 2 88 ga respectively this suggests that these rocks are the fractional crystallization products of one parental magma zircon hf and whole rock sr nd pb isotopic systematics combined with high la yb 10 and intermediate th ta ratios suggest that the parental magma of the amgrs could have been derived from em i lithosphere mantle enriched by recycled archean continent crustal materials accompanied by some assimilation of lower crustal components the mafic to felsic compositions of the amgrs are different from typical bimodal rocks formed in an anorogenic intra continent rift setting but resemble those emplaced during post collisional post orogenic extensional tectonic setting this is compatible with their age of

detrital zircon reference for the north china block abstract u pb analyses of 250 single detrital zircons from upper proterozoic to ordovician strata collected from the zhuozi shan in north central china provide a detrital zircon reference for the north china block a major crustal entity in the asian tectonic collage the results which range in age from 1 72 to 2 97 ga shed new light on the age of the crystalline basement in north china much of which is covered by younger sedimentary units in addition this detrital zircon reference can be used to help determine the provenance of clastic sedimentary units and for assessing validity of paleogeographic and regional tectonic models that include the complex history of asian continental amalgamation terrane accretion and subsequent translation that is ongoing today

geophysical evidence of deep hydrocarbon flow in mottled zone areas dead sea transform zone the origin of unusual magnetic initially sedimentary rocks of the mottled zone mz in israel and jordan remained enigmatic for several decades until integrated characterization of the mz area was achieved by ground magnetic measurements and geologic observations along representative profiles in parallel with reprocessing and reinterpretation of available aeromagnetic and gravity data micromagnetic profiling was combined with gamma radioactivity measurements and representative samples were selected to determine rock physical properties results of field measurements reduction and transformation of geophysical fields anomaly inversion and forward modeling accompanied by geologic analysis suggest that two types of magnetic anomalies and local gravity minima in the mz areas are related to the same event i e deep hydrocarbon flow associated with fossil mud volcanism physicochemical interaction of deep hydrocarbon flow and surrounding sedimentary rocks caused widespread weak magnetization and correspo

early mesozoic unroofing pattern of the dabie mountains china constraints from the u pb detrital zircon geochronology and si in white mica analysis of synorogenic sediments in the jianghan basin abstract this paper presents the results of an integrated u pb detrital zircon geochronology and si in white mica analysis for synorogenic sediments in the jianghan basin to the south of the dabie orogen the results provide an improved understanding of the provenance of these sediments and the unroofing pattern of the early mesozoic dabie mountain si contents of detrital white micas range from 3 09 to 3 34 atoms pfu for the upper triassic sandstones whereas 3 06 to 3 59 atoms pfu for the lower and middle jurassic sandstones the majority of detrital white micas in the lower jurassic sandstones is phengitic and originated exclusively from the dabie high to ultrahigh pressure rocks the u pb dating results of the detrital zircons for seven samples suggest that these synorogenic sediments have a significant change of provenance from late triassic to early and middle jurassic for the upper triassic sandstone the u pb age clusters of these zircons are characterized by 420 450 ma 750 820 ma 1050 1200 ma and 2500 ma with minor luliangian 1700 2000 ma components in contrast the zircon ages of the jurassic sandstones are dominated by the luliangian 1700 2000 ma ages with only minor caledonian 420 450 ma and greenville 1050 1200 ma ages in combination with other available geological data it can be concluded that the dabie hp uhp rocks might initially be exposed to the surface at the beginning of early jurassic 190 ma the jiangnan terrain also named jiangnan old continental in chinese to the south of the jianghan basin provided the predominant supply of upper triassic sediments whereas the paleoproterozoic yangtze crustal materials overlying the present dabie complex at the time were the important provenance of the jurassic sediments in the jianghan basin

u pb dating of detrital zircons in the eastern guangdong basin south china and constraints on the tectonic transformation from the early to middle jurassic controversies exist regarding the mechanism of formation of basins located on the continental margin of south china as well as when they formed it was ascertained based on clastic petrology geoch

detrital record of mountain building provenance of jurassic foreland basin to the dabie mountains 1 the huangshi foreland basin developed on the southern margin of the dabie mountains as a result of tectonic loading during triassic collisional suturing between the north china and south china cratons modal and detrital zircon data for jurassic samples within the basin suggest a multicomponent source with input from both the south china craton and dabie orogen samples are predominantly quartz arenites derived on the basis of framework compositions from a recycled orogen source detrital zircons range in age from archean to triassic with a dominant component in the late paleoproterozoic between 1 9 1 7 ga and subsidiary components at 2 6 2 2 ga 0 8 0 7 ga 0 5 0 4 ga and 0 33 0 2 ga age data integrated with cathodoluminescence and trace element data for the zircons indicate that the archean and proterozoic detritus was derived from igneous and metamorphic sources that overlap with time equivalent pulses of such activity within the south china craton phanerozoic zircon ages overlap the times of the ordovician carboniferous and triassic high pressure metamorphism in the dabie mountains the provenance record integrated with paleocurrent and regional relations enables a paleogeographic reconstruction in which the huangshi basin was fed by a major axial flowing trunk river system carrying detritus from eastern and southern sources within the south china craton and was also fed by short south flowing tributaries supplying some detritus from the evolving dabie orogen the dominance of cratonic derived detritus within the provenance record of the huangshi basin contrasts with that of the hefei foreland basin that lies to the north of the dabie mountains which is dominated by neoproterozoic mesozoic detritus derived directly from the dabie mountains and lacks any significant older paleoproterozoic or archean components easterly extensions of the dabie sulu collisional suture and of the resultant huangshi basin occur in korea and japan over an along strike length of some 2000 km

zircon u pb age trace element and hf isotope composition of kongling terrane in the yangtze craton refining the timing of palaeoproterozoic high grade metamorphism u pb age trace element and hf isotope compositions of zircon were analysed for a metasedimentary rock and two amphibolites from the kongling terrane in the northern part of the yangtze craton the zircon shows distinct morphological and chemical characteristics most zircon in an amphibolite shows oscillatory zoning high th u and 176lu 177hf ratios high formation temperature high trace element contents clear negative eu anomaly as well as hree enriched patterns suggesting that it is igneous the zircon yields a weighted mean 207pb 206pb age of 2857 8 ma representing the age of the magmatic protolith the zircon in the other two samples is metamorphic it has low th u ratios low trace element concentrations variable hree contents 33 8 lun 2213 14 7 lun smn 354 and 176lu 177hf ratios 0 000030 0 001168 the data indicate that the zircon formed in the presence of garnet and under upper amphibolite facies conditions the metamorphic zircon yields a weighted mean 207pb 206pb age of 2010 13 ma these results combined with previously obtained palaeoproterozoic metamorphic ages suggest a c 2 0 ga palaeoproterozoic collisional event in the yangtze craton which may result from the assembly of the supercontinent columbia the zircon in two samples yields weighted mean two stage hf model tdm2 ages of 3217 110 and 2943 50 ma respectively indicating that their protoliths were mainly derived from archean crust

shrimp u pb zircon ages of granitoid rocks in the luliang complex implications for the accretion and evolution of the trans north china orogen abstract the luliang complex is situated in the central segment of the trans north china orogen tnco a continent continent collisional belt along which the discrete eastern and western blocks amalgamated to form the basement of the north china craton the complex consists of supracrustal and granitoid rocks of which the latter can be subdivided into pre tectonic ttg gneisses including the yunzhongshan guojiazhuang and chijianling guandishan gneisses syn tectonic gneissic granites represented by the huijiazhuang gneissic granite and post tectonic granites including the luyashan charnockite luchaogou porphyritic granite and tangershang guandishan massive granite the pre tectonic ttg gneisses are mostly calc alkaline and considered to have formed in a magmatic arc environment shrimp u pb analyses reveal that the yunzhongshan gneisses were emplaced at 2499 9 ma representing the earliest arc related magmatic event in the luliang complex this was followed by the intrusion of the guojiazhuang gneisses at 2375 10 ma the most widespread arc related magmatic event in the region was the emplacement of the chijianling guandishan granitoid gneisses of which the tonalitic granodioritic and monzogranitic gneisses were emplaced at 2199 11 ma 2180 7 ma and 2173 7 ma respectively metamorphic zircon overgrowth rims from a tonalitic gneiss yielded a weighted mean 207 pb 206 pb age of 1872 7 ma consistent with the metamorphic age range of 1880 1820 ma defined by metamorphic zircons from various high grade rocks in the tnco thus the evolution of the luliang complex involved emplacement of the yunzhongshan granitoids at 2499 ma the guajiazhuang granitoids at 2375 ma and the chijianling guandishan granitoids at 2199 2173 ma with the final collision between the eastern and western blocks in this area occurring at 1872 7 ma the whole series of magmatic events lasting nearly 650 ma this suggests that the trans north china orogen represents a long lived magmatic arc the new shrimp zircon data combined with structural and petrological studies also reveal the existence of both syn and post collisional granites in the luliang complex the former is represented by the huijiazhuang gneissic granite that was emplaced at 1832 11 ma whereas the latter include the 1815 5 ma luyashan charnockite 1807 10 ma luchaogou porphyritic granite 1798 11 ma guandishan massive granite and 1790 14 ma tangershan massive granite

ductile opening mode fracture by pore growth and coalescence during combustion alteration of siliceous mudstone opening mode fractures with blunt tips and large maximum apertures are characteristic of clinker that formed by combustion alteration of siliceous mudstone these fractures are inferred to result from pore growth and coalescence with initially circular pores inherited from the diatomaceous protolith circular pores grow preferentially in an en echelon arrangement and coalesce to elongate pores and blunt tipped fractures by thinning and rupture of bridges between pores coalescence of overlapping en echelon pores causes fracture propagation in a zig zag path that is considered indicative of significant inelastic deformation outside the immediate vicinity of the fracture tips this process of fracture formation by void growth and coalescence is inferred to result from solution mass transfer and possible bulk melt movement during partial melting of clinker chemical mass transfer provides a mechanism for extensive inelastic deformation in the surrounding host rock concurrent with fracturing that is considered characteristic of ductile fracture the preferred elongation of coalescing pores and local rupture of pore bridges is explained by a tensile sintering stress due to the thermodynamic tendency of the system for energy minimization of solid and liquid surfaces it is suggested that ductile fracture processes that are accompanied by extensive inelastic deformation lead to opening mode fractures with large apertures in a variety of crustal settings including metamorphic and magmatic systems as well as in reactive diagenetic environments thus affecting fracture controlled transfer of heat and mass and the rheology of the earth s crust

constraints on timing of peak and retrograde metamorphism in the dabie shan ultrahigh pressure metamorphic belt east central china using u th pb dating of zircon and monazite abstract the dabie shan ultrahigh pressure metamorphic uhpm belt occupies the suture between the yangtze and sino korean blocks in east central china the timing of uhpm in the dabie belt is controversial and most recent data come from dating of zircons monazite has recently been recognized as useful for dating of multiple tectonic events due to its preservation of multiple growth zones and monazite growth has been documented to occur during prograde peak and retrograde metamorphism zircons and monazites from uhp mafic rocks from maowu and a uhp jadeite quartzite from shuanghe were imaged in thin sections and in mineral separates and dated using a high resolution ion microprobe maowu mafic rocks are unique in that they contain high light rare earth element concentrations and therefore contain abundant monazite maowu eclogites and garnet pyroxenites contain zircons with mean 206 pb 238 u age of 230 4 ma and monazites from a clinopyroxenite have 208 pb 232 th ages of 209 4 ma multiple lines of evidence suggest that the measured ages represent the timing of new growth and recrystallization and are not cooling ages i e they do not correspond to the cessation of diffusional pb loss as rocks cooled below the closure temperature during exhumation shuanghe jadeite quartzites contain zircons with cores that define a discordia with an upper intercept age 1921 22 ma and lower intercept age of 236 32 ma interpreted to represent the ages of the source rocks and of peak metamorphism respectively zircon rims contain jadeite inclusions that restrict growth to pressures greater than 1 5 gpa their pooled age of 238 3 ma agrees well with the lower intercept defined by cores suggesting that pb loss from cores and growth of new rims occurred during uhpm at 235 240 ma monazite records multiple events during retrograde metamorphism maowu clinopyroxenite and shuanghe jadeite quartzite experienced monazite growth at 209 ma interpreted to reflect regional amphibolite facies overprinting resulting from pervasive retrograde fluid infiltration only the jadeite quartzite records growth at 223 1 ma estimated exhumation rates for maowu are 7 8 km ma

detrital zircon provenance of the late triassic songpan ganzi complex sedimentary record of collision of the north and south china blocks the majority of the songpan ganzi triassic flysch sequence is believed to have derived from denudation of the dabie and sulu ultrahigh pressure uhp metamorphic belt in eastern china e g nie et al 1994 1 zhou and graham 1996 2 however intense debate still exists on the sources of

late triassic volcanic activity in south east asia new stratigraphical geochronological and paleontological evidence from the luang prabang basin laos in south east asia sedimentary basins displaying continental permian and triassic deposits have been poorly studied among these the luang prabang basin north laos represents a potential key target to constrain the stratigraphic and structural evolutions of south east asia a combined approach involving sedimentology palaeontology geochronology and structural analysis was thus implemented to study the basin it resulted in a new geological map in defining new formations and in proposing a complete revision of the late permian to triassic stratigraphic succession as well as of the structural organization of the basin radiometric ages are used to discuss the synchronism of volcanic activity and sedimentation the luang prabang basin consists of an asymmetric ne sw syncline with ne sw thrusts located at the contact between late permian and late triassic deposits the potential stratigraphic gap at the permian triassic boundary is therefore masked by deformation in the basin the late triassic volcaniclastic continental deposits are representative of alluvial plain and fluvial environments the basin was fed by several sources varying from volcanic carbonated to silicic non volcanic u pb dating of euhedral zircon grains provided maximum sedimentation ages the stratigraphic vertical succession of these ages from ca 225 ca 220 to ca 216 ma indicates that a long lasting volcanism was active during sedimentation and illustrates significant variations in sediment preservation rates in continental environments from 100 m ma to 3 m ma anhedral inherited zircon grains gave older ages a large number of them at ca 1870 ma imply the reworking of a proterozoic basement and or of sediments containing fragments of such a basement in addition the late triassic carnian to norian sediments yielded to a new dicynodont skull attributed to the kannemeyeriiform group family from layers dated in between 225 and 221 ma carnian

the delimitation between the mature and juvenile crustal provinces in se asia insights from detrital zircon u pb and hf isotopic data for the salween drainage myanmar abstract to understand the geological control and tectonic forcing on the development of a major river system in se asia new detrital zircon u pb and hf isotopic data from the salween drainage myanmar is reported three samples with a range of deposition ages corresponding to different tectonic periods of the sibumasu terrane reveal a dramatic change in the drainage system differing from the detrital compositions of modern sediments from the salween river mouth a late triassic sandstone sample shows zircon age distribution implying that the salween drainage may have been connected at that time to the northeastern part of the songpan ganzi terrane and western part of the south china block u pb ages and hf t isotopic data of detrital zircons from other major rivers in se asia including the yangtze red and mekong rivers in the east and the irrawaddy river and the yalu tsangpo in the west are further synthesized a marked discrepancy in zircons younger than jurassic ages is identified similar to results from the salween samples these young zircons from the eastern rivers are overwhelmed by an old recycled crustal component exhibiting negative hf t values in contrast those from the western rivers are dominated by juvenile components that are characterized by high and positive hf t values the salween drainage therefore lies in the frontier with its western outline broadly depicting the boundary between mature and juvenile crustal provinces in se asia

mesozoic basin development and tectonic evolution of the dabieshan orogenic belt central china 1 the dabieshan orogenic belt is a zone of long lived shortening with late stage extension that formed during mesozoic time in central china regional basin analysis coupled with structural cross sections provides a means of reconstructing the paleogeography of the region from triassic continental collision through early tertiary extension the overall distribution of different basin types in the orogenic belt indicates that there was a prolonged period of shortening throughout the mesozoic that became less intense over time meanwhile extension became more common from jurassic into tertiary time either compression or extension was isochronous but limited to different geographic regions and or crustal levels or alternated repeatedly orogen wide through time nonetheless the orogen underwent gradual transition from overall shortening and crustal thickening to dominantly extension and rift basin formation although these events overlapped in time from the jurassic through early cretaceous penecontemporaneous compression and extension across the orogen may reflect long term 100 m y shortening and crustal thickening leading to gravitational spreading of the resultant thick crustal welt we suspect that this prolonged history of shortening across two nearby 100 km apart suture zones played some role in the exhumation of ultrahigh pressure rocks from beneath the dabieshan

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

oceanic faulting and fault controlled subseafloor hydrothermal alteration in the sheeted dike complex of the josephine ophiolite based upon detailed mapping 1 10 and 1 100 of a large water polished outcrop of the sheeted dike gabbro transition zone in the josephine ophiolite of nw california and sw oregon the following history of alternating episodes of magmatic structural and hydrothermal events has been documented using crosscutting relationships petrography geochemistry and strontium and oxygen isotopic data 1 crystallization of gabbro and later subvertical mafic dike injection 2 amphibolite facies metamorphism 3 extensional faulting and tilting of dikes 4 continued faulting tilting and dike injection associated with retrograde metamorphism at greenschist facies conditions 5 continued extensional faulting and tilting synchronous with the development of a variety of hydrothermal veins at decreasing temperature and increasing fluid rock ratios and 6 subvertical injection of a highly fractionated dike which truncates all previous features features 1 5 trace element geochemistry indicates the highly fractionated dike feature 6 is genetically related to the other dikes and thus was intruded at or near the paleospreading axis hence all previous events features 1 5 can be constrained to have occurred at the rift axis including large scale tilting 50 of the sheeted dikes and extensional faulting the 87sr 86sr initial ratios from recrystallized whole rocks and from hornblende epidote and prehnite separates from veins display a systematic increase with relative age from 0 7033 for altered gabbro screens and mafic dikes to 0 7049 for prehnite in the youngest oceanic fault rock calculated oxygen isotope fluid compositions for the same suite of samples range from 18ofluid 5 to 1 with time indicating a change to a seawater dominated hydrothermal system with time consistent with observed increased permeability due to seafloor extensional faulting the crosscutting relationships alteration mineral assemblages and isotopic data suggest 1 an early stage of high temperature 450 c hydrothermal alteration with low permeability i e grain scale flow followed by 2 a decrease in temperature 350to 200 c and an increase in permeability due to faulting and accompanied by tectonic tilting at the rift axis the consistency of these crosscutting relationships at similar pseudo stratigraphic levels at different localities in the josephine ophiolite suggests that alternating magmatic and structural extension with synchronous retrograde alteration is common in crust formed at similar rates of spreading slow to intermediate spreading centers such as the modern mid atlantic ridge

exhumation of the dabie shan ultra high pressure rocks and accumulation of the songpan ganzi flysch sequence central china the presence of coesite and diamond bearing ultra high pressure uhp metamorphic rocks in the dabie and sulu regions central china suggests that a 100 km thick crustal section 4 x 106 km6 in volume has been denuded this volume is comparable to that represented by the 10 15 km thick middle to upper triassic flysch rocks in the songpan ganzi region regional geology and radiometric dates are compatible with an interpretation that the majority of these sedimentary rocks were derived from denudation of the orogenic belt between north and south china blocks following their latest paleozoic to triassic collision this correlation implies a high denudation rate of 4 mm yr that may have been in part promoted by tropical precipitation and extreme topographic relief for as long as 25 m y

natural pseudowollastonite crystal structure associated minerals and geological context abstract pseudowollastonite an extremely rare constituent of ultrahigh temperature combustion metamorphic and igneous rocks has been found as a rock forming mineral in ca rich paralava veins of nabi musa fossil mud volcano dead sea area pseudowollastonite bearing paralavas are the products of combustion metamorphism associated with spontaneous burning of methane the melt began to crystallize at 1480 1500 c about the ambient pressure pseudowollastonite enters two mineral assemblages 1 rankinite larnite nagelschmidtite wollastonite 1 t gehlenite rich melilite ti rich andradite cuspidine and fluorapatite 2 parawollastonite 2 m wollastonite 1 t gehlenite rich melilite ti rich andradite fluorellestadite in this study we present the first single crystal structure determination of natural pseudowollastonite pseudowollastonite from nabi musa dome is stoichiometric casio 3 and belongs to the most widespread four layer polytype a 6 83556 10 a b 11 86962 18 a c 19 6255 3 a 90 6805 13 v 1592 21 4 a 3 space group c 2 c we argue that pseudowollastonite is so scarce in nature because its formation requires joint action of several uncommon factors availability of hot melts of t 1200 c that bear free calcium but are poor in mg and fe mostly as fe 3 and their crystallization in the shallow crust followed by quenching

comparative mineralogy and geochemistry of gold bearing sulfide deposits on the mid ocean ridges a comparative study of the mineralogy and geochemistry of sulfide deposits on mid ocean ridges in the northeast pacific and the mid atlantic reveals common characteristics associated with primary gold enrichment average gold contents of 0 8 to 5 ppm au occur in sulfides from southern explorer ridge and axial seamount northeast pacific and from the tag hydrothermal field and snakepit vent field mid atlantic ridge the enrichment of gold in these deposits is consistently related to a phase of late stage low temperature 1 ppm au occur exclusively in pyritic assemblages and commonly with abundant fe poor sphalerite and a suite of complex pb sb as sulfosalts amorphous silica and locally barite or carbonate are important constituents of the gold rich precipitates but do not contain gold themselves high temperature 350 c black smoker assemblages consisting dominantly of pyrite chalcopyrite pyrrhotite isocubanite and abundant anhydrite are uniformly gold poor 0 2 ppm au to the extent that individual sulfides can be mechanically separated chemical analyses by neutron activation indicate that gold is most abundant in sphalerite up to 5 7 ppm au but also occurs in pyrite and marcasite samples of sphalerite with abundant inclusions of fine grained sulfosalts locally contain up to 18 ppm au suggesting that sulfosalts may be repositories for gold no free gold has been observed at 4000 magnification of polished specimens indicating that the gold is present only as submicroscopic inclusions or as a chemical constituent within the sulfides samples from gold rich deposits in the northeast pacific and mid atlantic are compared with similar but relatively gold poor sulfides from the galapagos rift and 13 n on the east pacific rise epr and with barren sulfides from 11 n epr 21 n epr the endeavour ridge and the southern juan de fuca ridge trace element analyses of more than 170 samples show that gold enrichment in almost all of the deposits is associated with high concentrations of ag as sb pb and zn and locally with high cd hg tl and ga in contrast gold is typically depleted in samples with high co se and mo the close association of au with ag as sb and pb may reflect the common behavior of these metals as aqueous sulfur complexes e g au hs 2 at low temperatures similar mineralogical and geochemical associations are observed in sulfide deposits from modern back arc settings and in the ancient geologic record

provenance of the triassic songpan ganzi flysch west china abstract the songpan ganzi basin of western china is a triangular structure on the northeast corner of the qinghai tibet plateau filled with a great thickness of triassic flysch surrounded to the north by the eastern kunlun orogenic belt that extends farther eastwards as the qinling dabie belt to the southwest by the ganzi litang ophiolite and yidun volcanic arc and to the east by the longmenshan thrust belt we have carried out the most extensive petrographic and geochemical study so far undertaken on typical turbidites from the songpan ganzi basin including major element trace element rb sr and sm nd isotopic analyses in the baryan har region on the northern side of the basin petrographic study identifies the sandstones as typical graywackes whose matrix is fine grained detrital material recrystallized by greenschist facies metamorphism chemical weathering indices confirm that the fine grained turbidites were little altered during transport and deposition so compositions accurately represent their source areas major trace and isotope analyses displayed on various discriminant diagrams indicate that the flysch from the baryan har region is derived from the eastern kunlun belt to the north a conclusion supported by published paleocurrent directions comparison of the baryan har turbidites with published results from western sichuan on the eastern side of the songpan ganzi basin suggests that the latter is derived from a source area of varied sedimentary rocks and a few igneous rocks the most likely source is south qinling to the northeast presently available data do not support derivation of material in the songpan ganzi basin from the yidun volcanic arc or the dabie ultra high pressure metamorphic belt the present provenance studies on the triassic songpan ganzi flysch support a westward opening model for the paleotethyan ocean and reveal different evolutional histories for the qilian qinling dabie and eastern kunlun orogens

the detrital record of orogenesis a review of approaches and techniques used in the himalayan sedimentary basins the sediment archive of material eroded from an active tectonic region and stored in adjacent basins can provide a valuable record of hinterland tectonism especially when information in the source region itself is obscured by later metamorphism or removed by tectonism or erosion using the sediment record to document tectonism is a well established approach but more recently there has been a burgeoning of the number of isotopic techniques which can be applied to detrital material in particular single grain analyses thus the scope for application of detrital studies to a number of different tectonic problems has widened considerably in this review the example of sediments eroded from the himalayan orogen and preserved in the suture zone basin foreland basin remnant ocean basins and deep sea fans is used to illustrate the approach techniques as diverse as petrography heavy mineral xrf and sr nd studies single grain dating by ar ar u pb and fission track methodologies and single grain sm nd and pb isotopic analyses are described the paper documents how the sediment record can be used to determine the thermal and tectonic evolution of the orogen constrain mechanisms of continental deformation exhumation rates and palaeodrainage

u pb dating on single detrital zircon grains from the triassic songpan ganze flysch central china provenance and tectonic correlations the songpan ganze flysch belt central china covers a huge triangular area of more than 200 000 km and is bounded by the continental blocks of south china north china and the tibetan plateau detrital zircons extracted from three flysch samples collected in the central part of the belt were analyzed grain by grain using the u pb method two samples of middle triassic sandstones collected at different locations in the belt provide identical results which suggests similar source regions the detrital zircons yield a wide range of ages and indicate their principal derivation from mid proterozoic 1 8 2 0 ga source rocks with minor contribution from late archean ca 2 5 2 6 ga material the discordance and pb loss patterns from low u zircons indicate disturbances during a subsequent event which may be of caledonian age as suggested by concordant zircon grains at ca 420 and 450 ma one sample collected within the palang shan pass zone provides concordant zircon grains at around 230 ma 231 1 ma and 233 1 ma these triassic ages are synchronous to flysch deposition and suggest intense geological activity calc alkaline volcanism at that time in the area close to the basin the data support an origin of the clastic material mainly from a northeastern landmass corresponding to the southern margin of the sino korean craton to a lesser degree inputs from the yangtze craton and possibly from the northern margin of the basin kunlun arc are also detected the age spectrum from the upper triassic sandstone is significantly different and shows predominance of sinian ca 760 ma grains probably derived from the yangtze craton this change in the source region is interpreted as reflecting the tectonic evolution of this area and in particular as being linked to the late triassic collision between south china and north china in the middle triassic while subduction of the songpan sea northward beneath the north china plate was still taking place continental subduction of south china in the dabie region was responsible for uplift of the overriding plate i e the sino korean craton which supplied large volumes of sediments during the late triassic clockwise rotation of the south china block uplifted the indo sinian part of the qinling belt and closed the basin as the accretionary wedge was thickening along the southern margin of north china detritus derived from this source region were unable to reach the flysch basin the age spectrum presented by detrital zircons indicates

chemical and microbial processes causing anomalous magnetization in environments affected by hydrocarbon seepage aero magnetic anomalies have been reported from several commercial hydrocarbon accumulations however the processes responsible for such anomalies are relatively poorly understood this paper conceptually discusses chemical and microbiological processes involved in generating anomalous magnetization related to hydrocarbon accumulations including hydrocarbon seepage environments based on thermodynamic criteria and microbiologic activity the formation and destruction of magnetic mineral assemblages can be predicted under the influence of hydrocarbons magnetite and pyrrhotite are the most important magnetic minerals formed and the most abundant magnetic mineral destroyed is hematite hence the invasion of hydrocarbons may result in positive absent or negative magnetic contrasts relative to the total magnetization prior to hydrocarbon invasion depending upon the amounts of authigenic magnetite and pyrrhotite formed relative to the amounts of hematite destroyed magnetism may be generated al

the generation of magnetic anomalies by combustion metamorphism of sedimentary rock and its significance to hydrocarbon exploration low temperature exothermic oxidation of fine pyrite in carbonaceous marine rocks can lead to spontaneous combustion as a result the organic material in the rocks is consumed as fuel but unlike burnt coal seams and lignite beds the host rock remains after combustion thermal metamorphism and in some cases partial to complete melting of the rock will result zones of combustion metamorphism are frequently associated with oil fields these occurrences can be related to either surface outcrops of source rock or leakage from reservoirs a study of the magnetic properties of a suite of combustion metamorphosed specimens of the monterey and sisquoc formations in southern california shows orders of magnitude increases in magnetic intensity and susceptibility for both thermally metamorphosed and fused samples these changes reflect both the production of new magnetic minerals derived mainly from oxidized iron sulfide and their acquisition of thermal remanence ground magnetic surveys across combustion metamorphism zones in three southern california oil fields reveal anomalies of up to 2 000 gammas associated with the metamorphosed and melted rock large scale aero magnetic anomalies are also associated with extensive combustion metamorphism zones in the marcelina formation of venezuela and the ghareb and taquiya formations the mottled zone in israel the recognition of magnetic anomalies caused by combustion metamorphism in areas of oil production suggests that closely spaced low altitude aeromagnetic surveying can be a useful petroleum exploration tool

the oldest known rocks in south western china shrimp u pb magmatic crystallisation age and detrital provenance analysis of the paleoproterozoic dahongshan group abstract the paleoproterozoic dahongshan group outcrops along the present day southwest margin of the yangtze craton it is the oldest rock found in the region and forms the basement for the late mesoproterozoic sedimentary successions the dahongshan group is a sequence of volcanic and sedimentary rocks which have undergone structural deformation and metamorphism to upper greenschist lower amphibolite facies some of the original sedimentary features such as unimodal cross bedding in quartzite and rounded conglomerate clasts are still preserved suggesting deposition in a fluvial environment abundant carbonate rocks also found within the sedimentary sequence indicate periods of shallow marine or lagoonal conditions a weighted mean shrimp 206pb 207pb age of 1675 8 ma for a tuffaceous schist unit confirms the paleoproterozoic formation age of the group shrimp and lam icp ms u pb analysis of detrital zircon grains from metasedimentary units indicate a late archean to paleoproterozoic ca 2780 1860 ma source region the age spectrum of the detrital zircons is similar to that of the late archean and early paleoproterozoic basement rocks previously identified in the yangtze craton but includes a population of paleoproterozoic grains ca 2400 2100 ma which have no known source region in the exposed yangtze craton

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

constraints on the early mesozoic denudation of the qinling orogen from upper triassic lower jurassic successions in the zigui basin central china abstract the qinling mountains are situated in central china and mark the collisional assembly between the north and south china blocks to understand the early mesozoic evolution of the qinling orogenic belt we present new paleocurrent data sandstone petrology and detrital zircon u pb ages for upper triassic lower jurassic sedimentary successions in the zigui basin in the northern margin of the south china block the deltaic sandstones in the upper triassic jiuligang formation have high concentrations of quartz grains and the basal alluvial conglomerates of the lower jurassic tongzhuyuan formation contain abundant chert gravels with sandy matrixs rich in quartz grains both the upper triassic sandstones and lower jurassic conglomerate have two major detrital zircon u pb age groups of 2100 1700 ma and 950 700 ma with very few phanerozoic ages the overlying fluvial sandstones of the tongzhuyuan formation display relative enrichment in volcanic and metamorphic rock fragments they are characterized by primary zircon u pb age groups of 250 200 ma and 950 700 ma consistent with the northwestward paleocurrents zircon grains of 2100 1700 ma and 900 700 ma were likely derived directly or recycled from the paleoproterozoic and neoproterozoic basement of the south china block with the latter also possibly sourced from the qinling orogen the 250 200 ma age group is understood to be associated with volcanic rock fragments and considering the southward paleocurrents indicates a volcanic source within the qinling orogen along with the increased low grade metamorphic rock fragments in sandstones this significant provenance change in the early jurassic indicates the initial massive erosion and rapid southward propagation of the south qinling orogen

temporal and spatial patterns of sediment routing across the southeast margin of the tibetan plateau insights from detrital zircon the cenozoic deposits of the tibetan plateau s southeastern margin are often cited as part of a continental scale river system connecting the paleo yangtze river with the paleo red river confirming the purported connection and any subsequent drainage reorganization has garnered significant attention and varied proposed ages for reorganization this study presents detrital zircon u pb ages and paleocurrents in eocene to pleistocene sedimentary basin deposits distributed over a broad area of the southeast tibetan plateau margin within the area of proposed paleoriver connectivity when combined with previously published studies our u pb ages allow examination of the temporal and spatial distributions of provenance throughout the cenozoic we identify six key age components of the detrital u pb age distributions and use these to examine the patterns of sediment provenance for different cenozoic epochs detailed analysis of these components shows that provenance for both onshore and offshore deposits is best described by local bedrock sources and provides little to no evidence of regional changes in provenance this suggests that a stable fluvial system similar to the modern drainage network has existed since the eocene with no evidence for major provenance altering river capture paleoflow measurements taken throughout the se margin further corroborate the results of detrital zircon provenance the combination of u pb age components and paleocurrent directions does not support a cenozoic connection between the paleo yangtze and paleo red rivers

a field chemical and stable isotope study of subseafloor metamorphism of the josephine ophiolite california oregon the josephine ophiolite is a large complete ophiolite generated in a late jurassic back arc basin along western north america the ophiolite underwent subseafloor metamorphism under a steep thermal gradient as indicated by downward changes in mineralogy and 18o values characteristic mineral zonation includes mica orthoclase and hematite in the lower extrusives greenschist facies assemblages in the upper sheeted dikes and amphibolite facies assemblages in the lower sheeted dikes and high level gabbro cumulate gabbros are only incipiently altered but are depleted in 18o indicating high temperature alteration with fluids that evolved at low water w rock r ratios a second off axis circulation system is recorded by metalliferous sediments that occur 8 21 m above the ophiolite alteration in the upper sheeted dike complex and especially the extrusive sequence is heterogeneous at outcrop scale as indicated by large variations in mineralogy chemistry and 18o values within individual pillows and dikes the heterogenity is due to 1 variations in w r ratio and starting material during alteration by downwelling seawater e g pillow cores versus glassy rims and 2 localized discharge mineralization mostly epidosites superimposed on the background recharge alteration epidosites occur largely as dike parallel stringers irregular replacement of pillow and massive lava and replacement of interpillow and pillow breccia matrices the chemistry of most extrusives and sheeted dikes is characterized by loss of ca and gain in na and mg consistent with alteration by downwelling fluids in addition the sheeted dikes are depleted in k2o and the lower sheeted dikes and high level gabbros are depleted in zn and cu the 18o and d values for most samples from the extrusive sequence indicate alteration by seawater at 50 rotation of the entire crustal sequence at the spreading axis suggests that the ophiolite formed at a slow spreading center where magma chambers were episodic

geochronological and geochemical constraints on the petrogenesis of late triassic aluminous a type granites in southeast china abstract the late permian triassic granites in the south china block scb have important tectonic significance for the evolution of east asia most of the intrusive bodies in the central scb consist of s type granites here we report the first unambiguous discovery of wengshan aluminous a type granites in southwest zhejiang province southeast china shrimp and la icp ms zircon u pb analyses yielded similar ages about 224 ma for two samples from the wengshan pluton in concordance with peak time of late indosinian 234 205 ma tectonic magmatic activity in the scb the wengshan granites are peraluminous with a cnk values ranging from 1 09 to 1 24 enriched in k rb th ree and zr nb ce y have elevated feo tot mgo and ga al ratios and are depleted in mg ca mn ba sr p and ti in addition they show high zircon saturation temperatures 820 878 c their ree patterns are highly fractionated with la yb n ratios of 36 89 101 24 and strongly negative eu anomalies eu eu 0 28 0 53 in situ zircon hf isotopic analyses indicate that the wengshan granites have e hf t values ranging from 15 9 to 12 7 and two stage depleted mantle hf model ages from 2 05 to 2 26 ga sr nd isotope systematics are characterized by a high initial 87 sr 86 sr ratios of 0 7092 0 7218 and a low e nd t values of 15 0 to 12 1 with two stage depleted mantle nd model ages of 1 99 2 22 ga consistent with those of paleoproterozoic basement rocks in the area it is inferred that the wengshan granite magma formed by partial melting of paleoproterozoic metasedimentary sources in cathaysia block meanwhile the underplating of mafic magma in the lower crust may provide the heat source for the formation of the wengshan granites the data show that the wengshan granites unlike other s type granites in the scb are late triassic aluminous a type granite therefore they may indicate distinct geodynamic significance our study of the wengshan a type granites 224 ma together with jingju a type granites 215 ma in zhejiang province and gaoxi a type granites 215 ma xiaotao a type granites 222 ma tieshan 254 ma and yangfang 242 ma alkaline syenites in fujian province define important extensional events in triassic considering these available data we suggest that the late permian triassic a type granites and alkaline syenites in the coastal region of the scb probably formed under an extensional regime related to oblique subduction of the paleo pacific plate

crustal structure interpreted from magnetic anomalies this review discusses publications during the last quadrennium 1987 1990 that used aeromagnetic data marine magnetic data satellite magnetic data and rock magnetic and petrologic data to provide information on the sources of magnetic anomalies the publications reviewed reflect increased integration of rock magnetic property and petrologic studies with magnetic anomaly interpretation studies particularly in deep crustal magnetization exploration for hydrocarbons and inversion of marine magnetic anomalies interpretations of aeromagnetic data featuring image display techniques and using the horizontal gradient method for locating magnetization boundaries became standard

palaeozoic pre and post collision calc alkaline magmatism in the qinling orogenic belt central china as documented by zircon ages on granitoid rocks based on large scale reconnaissance mapping we identified two calc alkaline plutonic assemblages from the northern qinling orogenic belt central china the older assemblage of intrusions closely associated and deformed coevally with their host volcanic arc sequences seems to represent the fractionation product of basaltic arc magma it therefore predates the collision of the north china block with the central qinling island arc system that developed in a sw pacific type oceanic domain south of the north china block single zircon 207pb 206pb evaporation dating yielded early to middle ordovician ages for this assemblage with a relatively small range from 487 2 1 1 to 470 2 1 3 ma intrusions of the younger assemblage are largely undeformed and truncate structures shown in rocks of the older assemblage they are interpreted as post collisional calc alkaline granitoids single zircon dating provided an age of 401 8 0 8 ma for the younger assemblage consistent with earlier work that defines an age range from c 420 to 395 ma our data favour a tectonic model involving formation and amalgamation of island arc and microcontinent terranes between ca 490 and 470 ma ago to create the central qinling zone which subsequently collided with the north china block prior to c 400 ma ago a late precambrian age of 762 0 0 7 ma for a granitoid gneiss at the northern margin of the yangtze block supports a gondwana affinity for this large continental block

detrital zircon u pb geochronological and lu hf isotopic constraints on the precambrian magmatic and crustal evolution of the western yangtze block sw china abstract in situ u pb geochronological and lu hf isotopic studies of detrital zircon from a variety of precambrian strata along the western margin of the yangtze block of south china show that the rocks have maximum depositional ages of ca 1000 740 ma all of the strata probably formed in a subduction related tectonic setting the kunyang group heishantou formation and huili group tongan formation have maximum depositional ages of 960 1000 ma and contain zircons whose ages are dominantly paleoproterozoic 2500 2300 1900 1700 and 1650 1500 ma the yanbian group has a zircon population ranging mainly from 860 to 1000 ma different from that of the bikou group with ages from 745 to 890 ma ages of zircons from the lieguliu and guanyinya formations range from 740 to 900 ma the paleoproterozoic detrital zircons commonly have negative or slightly positive hf values from 8 0 to 2 3 whereas most of the 1000 740 ma zircons have moderately to significantly high positive hf values from 0 3 to 13 0 the new detrital zircon u pb and lu hf isotopic dataset suggests four major episodes of magmatism in the region 2500 2300 1900 1700 1650 1500 and 1000 740 ma and the presence of widespread archean basement of the western yangtze block the data also suggest an important period of juvenile magmatism at 1000 740 ma our results favor a model in which continuous subduction 1000 740 ma along the western margin of the yangtze block lasted for 260 million years

the paleozoic metamorphic history of the central orogenic belt of china from 40ar 39ar geochronology of eclogite garnet fluid inclusions abstract the pressure temperature time evolution of the uhp eclogites of dabie sulu in the eastern sector of the central orogenic belt of china shows a complex pattern of predominantly triassic and to a lesser extent early paleozoic ages 40 ar 39 ar stepwise crushing analysis of eclogitic garnets yielded consistently paleozoic ages because our initial results from the bixiling eclogite were of limited areal extent their significance required further testing data for garnet bearing gneiss and eclogites from new locations in northern and southern dabieshan shuanghe ganghe and hongmiao confirm the paleozoic garnet results obtained from bixiling eclogite we also tested the technique on a garnet sample of known paleozoic age from altyn tagh in the western central orogenic belt these new results indicate that light colored garnets from dabieshan and altyn tagh yield concordant 40 ar 39 ar ages ranging from 445 465 ma by in vacuo crushing and confirm that a paleozoic u hp metamorphic event took place in the central orogenic belt from altyn tagh in western china to dabieshan in eastern china in contrast dark colored garnets from mafic metavolcanics do not yield 40 ar 39 ar plateaux or isochrons ages

magnetic study of metamorphosed sedimentary rocks of the hatrurim formation israel summary the hatrurim formation is a mottled zone complex of metamorphosed carbonate rocks which is known as a complex subjected to combustion metamorphism cm its magnetic characteristics and their relationships to the cm were studied using magnetic measurements by different techniques and on different scales the magnetic features revealed are characterized by different spectral components and anisotropy determined at scales from hundreds of metres to a few centimetres a very short wave of relatively high magnetization is associated with shallow small bodies of high grade metamorphosed rocks of the hatrurim formation the magnetic susceptibility o fhigh grade rocks is highly variable with marked heterogeneity and anisotropy the frequency and magnitude of ground magnetic anomalies up to 4000 nt and the of high grade rocks up to 5600 10 5 si are higher than those reported from other areas of cm an approximate field method and conventional palaeomagnetic studies of oriented samples showed similar results namely a natural remanent magnetization of highgrade mottled zone rocks is heterogeneous and b the remanent magnetization of rocks within the hatrurim basin usually exceeds their induced magnetization and has a similar direction relatively low and homogeneous magnetization is found in another type of slightly metamorphosed rock of the hatrurim formation and in some cases also in adjacent outcrops of the underlying mishash formation this type appears to be the product of other sources not related to the cm process such sources can cause observed local aeromagnetic maxima that show some correlation with geological structure the weak magnetization is also related to hydrothermally altered and weathered rocks of the hatrurim formation thus the magnetization of the mottled zone is due to a combination of near surface surface magnetic bodies similar to the cm rocks in other regions and magnetization induced by hidden magnetic sources possibly related to hydrocarbon fluxes or magmatic activity along the faults and fractures

mineralization alteration and hydrothermal metamorphism of the ophiolite hosted turner albright sulfide deposit southwestern oregon the turner albright sulfide deposit part of the josephine ophiolite formed on and below the seafloor during late jurassic volcanism at a back arc spreading center ore fluids were probably localized by faults which were active on the seafloor at the time of sulfide deposition the uppermost massive sulfide formed on the seafloor at hydrothermal vents similar to sulfide depositing hot springs on modern spreading centers the bulk of the sulfide mineralization formed below the seafloor within olivine basalt hyaloclastite erupted near the time of mineralization infiltration of hydrothermal fluid into the hyalo clastite altered the rock to quartz fe chlorite and quartz sericite fe chlorite intense alteration resulted in replacement of the protolith by quartz sulfide the fluid responsible for the hydrothermal alteration was evolved seawater with low ph and mg and high fe sulfide 534s values average 4 7 and indicate contribution of sulfur from both basalt and seawater sulfate the average value of sulfide 34s and the difference between sulfide and contemporaneous seawater sulfate 34s values are similar to ophiolite hosted sulfide deposits in cyprus chlorite rich hydrothermally altered basalt has 18o values of 3 0 6 8 due to high temperature 250 350 c reaction with a seawater derived ore fluid quartz in silicified basalt has higher 18o values of 12 3 15 5 probably reflecting lower temperature recrystallization of amorphous silica formed during initial alteration of basalt mudstone and clinopyroxene basalt above the sulfide horizons were not altered by the ore transporting hydrothermal fluid but these rocks were hydrothermally metamorphosed by altered seawater heated by deep circulation into hot oceanic crust this subseafloor metamorphism produced a mineral assemblage typical of prehnite pumpellyite facies metamorphism exchange with altered seawater increased the whole rock 18o of the basalts to values of 9 4 11 2

a middle silurian early devonian magmatic arc in the qinling mountains of central china an early to middle paleozoic magmatic arc was discovered in the qinling orogenic belt of central china that marks the boundary between the north china craton and the south china craton the arc was built on the deformed southern passive continental margin of the north china craton which has been modified by the accretion of an oceanic island arc in early paleozoic time u pb analyses of zircons from granites indicate short lived arc activity from middle silurian to early devonian time trace element analyses as well as ree analyses of i type granites are consistent with the interpretation that the magmatic arc developed above a north dipping subduction zone termination of magmatic activity was probably caused by the initiation of collision between the north china craton and the south china craton this interpretation is consistent with faunal data and places new constraints on the paleozoic tectonic evolution of the qinling belt

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

detrital provenance of early mesozoic basins in the jiangnan domain south china paleogeographic and geodynamic implications abstract detrital provenance analysis is an effective way to understand paleogeographic change and geodynamics in this paper we present petrological whole rock geochemical and detrital zircon u pb geochronological analysis of early and middle jurassic terrestrial clastic rocks in the jingdezhen basin and the huangshan basin in the jiangnan domain south china petrology and whole rock geochemistry show that the source rocks are dominated by intermediate to acid component the chemical index of alteration ranges from 69 to 86 suggesting a moderate weathering history for the source rocks the early middle jurassic sediments in the jingdezhen and huangshan basins were mostly sourced from magmatogenic greywackes and felsic magmatic rocks respectively detrital zircons have seven age peaks at 240 ma 430 ma 1390 ma 1880 ma 2500 ma 3200 ma and 788 999 ma a wide peak provenance analysis indicates that the source rocks are in the jiangnan domain the northwest zhejiang basin and the wuyishan domain combining these with previous results and paleocurrent directions we infer that the ne trending wuyishan and xuefengshan domains and the nearly e w jiangnan domain and nanling tectonic belt were orogenic uplifts and watersheds during the late triassic to middle jurassic the early mesozoic geodynamics in the south china block was related to the westward subduction of the paleo pacific plate and the northward continent continent collision following the closure of the paleo tethys ocean

u pb zircon ages of granitoid rocks in the north dabie complex eastern dabie shan china the north dabie complex ndc comprises a high temperature metamorphic suite dominated by amphibolite facies granitoid orthogneiss intruded by voluminous granites u pb zircon dating on these rocks yields the following new results 1 two of the granitoid orthogneiss samples have early cretaceous protolith emplacement ages of 133 7 2 3 ma and 134 0 2 8 ma 2 an undeformed granodiorite intrusive into granitoid gneiss yields an emplacement age at 125 6 0 3 ma these early cretaceous ages entirely post date late triassic early jurassic continental subduction and exhumation evident in the neighboring ultrahigh and high pressure metamorphic terrains to the south in the dabie shan hence they do not support previous tectonic models that infer paleozoic or triassic protolith emplacement ages for these granitoid orthogneisses 3 a mylonitic granite from the northern boundary of the ndc yields a concordant age of 756 6 0 8 ma which is interpreted as magma emplacement age the concordance of this late

tectonics of the qinling central china tectonostratigraphy geochronology and deformation history abstract the qinling orogen preserves a record of late mid proterozoic to cenozoic tectonism in central china high pressure metamorphism and ophiolite emplacement songshugou ophiolite assembled the yangtze craton including the lower qinling unit into rodinia during the 1 0 ga grenvillian orogeny the lower qinling unit then rifted from the yangtze craton at 0 7 ga subsequent intra oceanic arc formation at 470 490 ma was followed by accretion of the lower qinling unit first to the intra oceanic arc and then to the sino korea craton subduction then imprinted a 400 ma andean type magmatic arc onto all units north of the northern liuling unit oblique subduction created silurian devonian wnw trending sinistral transpressive wrench zones e g lo nan shang dan and late permian early triassic subduction reactivated them in dextral transpression lo nan shang xiang shang dan and subducted the northern edge of the yangtze craton exhumation of the cratonal edge formed the wudang metamorphic core complex during dominantly pure shear crustal extension at 230 235 ma post collisional south directed shortening continued through the early jurassic cretaceous reactivation of the qinling orogen started with nw se sinistral transtension coeval with large scale early cretaceous crustal extension and sinistral transtension in the northern dabie shan it presumably resulted from the combined effects of the siberia mongolia sino korean and lhasa west burma qiangtang indochina collisions and pacific subduction regional dextral wrenching was active within a ne sw extensional regime between 60 and 100 ma an early cretaceous andean type continental magmatic arc with widespread early cretaceous magmatism and back arc extension was overprinted by shortening related to the collision of yangtze indochina block with the west philippines block strike slip and normal faults associated with eocene half graben basins record paleogene nne ssw contraction and wnw ese extension the neogene is characterized by normal faults and nne trending sub horizontal extension pleistocene quaternary nw se extension and ne sw contraction comprises sinistral strike slip faults and is part of the nw se extension imposed across eastern asia by the india asia collision

detrital zircon u pb geochronology hf isotopes and geochemistry constraints on crustal growth and mesozoic tectonics of southeastern china abstract in situ u pb geochronological hf isotopic and ree geochemical analyses of detrital zircons from mesozoic sandstones in central jiangxi and east hunan of the south china block scb are used to provide not only information about crustal evolution process but more importantly new constraints on sedimentary provenances as well as mesozoic basin evolutions in central scb a total of 417 concordant zircon analyses define five u pb age populations at ca 2 6 2 3 ga ca 2 0 1 7 ga ca 1 0 0 7 ga ca 500 400 ma and ca 300 200 ma integrated analyses of zircon u pb ages and hf isotopes of detrital zircons reveal five episodes of juvenile continental crust growth ca 2 5 ga ca 1 7 ga ca 850 ma ca 440 ma and ca 230 ma with all but the ca 2 5 ga episode likely represent that of the scb none of the three samples from t3 j1 strata showed strong ca 1850 ma and ca 230 ma peaks suggesting that the t3 j1 sediments probably sourced from the yangtze block in contrast stronger peaks of ca 1850 ma and ca 250 230 ma in post j1 strata relative to that of t3 j1 strata suggest a dominantly local cathaysian provenance in addition the distinct low ehf t values for zircons of ca 430 ma from t3 j1 strata in comparison with higher ehf t values for that from post j1 strata also support aforementioned viewpoint such sharp changes between the pre j1 and post j1 strata coincide with the remarkable change in regional palaeogeography from a broad shallow marine basin in the late triassic early jurassic time to a basin and range style province in the middle jurassic the characteristics and time evolution of detrital zircons from the studied area are consistent with the flat slab subduction model which involves the development of a broad sag basin during the t3 j1 time and a rapid regional uplift in the cathaysia block caused by the reinitiating of normal subduction along the coastal region at ca 190 ma

causes and spatial distribution of anomalous magnetization in hydrocarbon seepage environments in diagenetic environments containing hydrocarbons the most important magnetic minerals formed appear to be magnetite and pyrrhotite whereas the most important magnetic mineral dissolved or replaced is hematite hence hydrocarbon seepage and migration may result in positive absent or negative anomalous magnetization magnetic contrasts relative to the total magnetization prior to hydrocarbon invasion depending on the amounts of magnetite and pyrrhotite formed relative to the amounts of hematite destroyed where anomalous magnetization is formed by hydrocarbon seepage the magnetic contrasts are located in relatively reducing geochemical plumes such plumes are located above leaking subsurface hydrocarbon accumulations if buoyancy is the predominant driving force for hydrocarbons commonly however migration of the escaping hydrocarbons is dominated by advection hydrocarbon seepage therefore may be vertical or may have any direction other than vertical depending on the position of the leaking hydrocarbon accumulation in the prevailing flow system and on the distribution of high permeability pathways such as carrier beds and faults in these cases magnetic contrasts are deflected or displaced and they are not likely to be vertically above the hydrocarbon source anomalous magnetization magnetic contrasts has been documented from some hydrocarbon seepage environments and can be used for hydrocarbon exploration in association with other surface exploration methods similar magnetic contrasts should also be detectable in core and could be used for evaluation of subsurface proximity to hydrocarbon traps the magnitude and spatial distribution of such magnetic contrasts whether at the surface or in the subsurface depend on geochemical microbial geophysical sedimentologic and hydrogeologic processes these factors must be taken into consideration in aeromagnetic and ground based prospecting for hydrocarbons in particular the use of anomalous magnetization as an exploration tool should always be accompanied by a hydrogeologic study the results of this study suggest that magnetic contrasts could also be used in environmental studies for example to detect hydrocarbons leaking from storage tanks or waste disposal sites on the other hand magnetic contrasts may be generated also by natural and anthropogenic processes that have no genetic relationships to hydrocarbons consequently anomalous magnetization may or may not be genetically related to natural or anthropogenic hydrocarbon accumulations

magnetic properties of k t and e o microspherules origin by combustion abstract magnetic spherules and spheroids are found in association with ir anomalies at the cretaceous tertiary k t boundary and near to the eocene oligocene e o boundary detailed magnetic analyses have been performed on these particles and on bulk k t boundary clay from marine and continental sections coercivity spectra and blocking and curie temperatures of the spheroids and k t boundary clays have been determined and natural remanence and susceptibility measured for the bulk clay and associated strata marine k t boundary clays are strongly magnetic with remanent coercivities similar to those of the spheroids in contrast the continental clays are diamagnetic except for the thin overlying impact layer which contains abundant paramagnetic jarosite along with shocked quartz and feldspar the morphologies mineralogies and magnetic properties of the k t magnetic spheroids are closely analogous to magnetic fly ash spherules this similarity may be related to the presence of carbonaceous soot particles in some marine k t boundary clays and the recent identification of plerospheres and cenospheres in continental boundary clay in contrast tektites and microtektites differ from the k t magnetic spherules in a number of fundamental ways the late eocene particles are magnetically distinct from the k t spheroids both in their coercivity and thermal spectra inaa results on upper cretaceous combustible shales indicate that the anomalous abundances of certain chalcophile and lithophile elements at the k t boundary may be explained in terms of the natural combustion of bituminous sediments

timing of synorogenic granitoids in the south qinling central china constraints on the evolution of the qinling dabie orogenic belt abstract the ca 400 km long granitoid belt in the south qinling is believed to be a synorogenic product of the collision between the north and south china blocks along the qinling dabie orogenic belt in central china single and multigrain zircon u pb dating of six of these granitoid bodies indicate that the granitoids were formed between documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 220 pm 1 end document and documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage m

an early mesozoic transcontinental palaeoriver in south china evidence from detrital zircon u pb geochronology and hf isotopes detrital zircon geochronology reveals that late triassic early jurassic fluvial sandstones from the major basins of the south china craton have similar age patterns and define four populations at 2 6 2 4 ga 2 0 1 7 ga 850 700 ma and 480 210 ma the late palaeoproterozoic group is predominant in all of the five samples and yielded remarkable age peaks at c 1 85 ga these zircons have hf t values between 22 5 and 3 6 suggesting derivation from reworked archaean crust and minor juvenile crustal additions in the late palaeoproterozoic these characteristics differ from those of the yangtze block but correlate well with those of samples from the eastern cathaysia block palaeocurrent analysis of the early mesozoic sandstones shows predominant west and nw directed palaeoflows supporting derivation of the sediments from the cathaysia block the remarkable similarities in provenance signatures and spatial changes of lithofacies of the triassic jurassic around the south china craton delineate an east west trending sedimentary zone extending from korea to west china accumulation of these sediments was probably related to the development of an active continental margin produced by westward subduction of the palaeo pacific plate a c 2000 km long westerly draining transcontinental palaeoriver probably had existed in the early mesozoic and fed the basins in korea south china and west china supplementary material sensitive high resolution ion microprobe and laser ablation inductively coupled plasma mass spectrometry zircon u th pb and hf isotope data are available at www geolsoc org uk sup18514

trace element signature and u pb geochronology of eclogite facies zircon bergen arcs caledonides of w norway secondary ion mass spectrometry sims u pb and trace element data are reported for zircon to address the controversial geochronology of eclogite facies metamorphism in the lindas nappe bergen arcs caledonides of w norway caledonian eclogite facies overprint in the nappe was controlled by fracturing and introduction of fluid in the proterozoic sveconorwegian granulite facies meta anorthosite norite protolith zircon grains in one massive eclogite display a core rim structure sveconorwegian cores have trace element signatures identical with those of zircon in the granulite protolith i e 0 31 th u 0 89 heavy rare earth element hree enrichment and negative eu anomaly weakly zoned to euhedral oscillatory zoned caledonian rims are characterized by th u 0 13 low lree content minimum normalized abundance for pr or nd variable enrichment in hree and no eu anomaly a decrease of ree towards the outermost rim especially hree is documented this signature reflects co precipitation of zircon with garnet and clinozoisite in a feldspar absent assemblage and consequently links zircon to the eclogite facies overprint the rims provide a mean 206pb 238u crystallization age of 423 4 ma this age reflects eclogite forming reactions and fluid rock interaction this age indicates that eclogite facies overprint in the lindas nappe took place at the onset of the scandian silurian collision between laurentia and baltica

components and episodic growth of precambrian crust in the cathaysia block south china evidence from u pb ages and hf isotopes of zircons in neoproterozoic sediments abstract detailed lam icpms u pb dating and lam mc icpms lu hf isotope analysis were carried out on zircons from nine samples of basement metamorphic rocks in the southern cathaysia block south china the chemical compositions of these metamorphic rocks and zircon morphology indicate that their protoliths were sedimentary zircon u pb dating results show they mainly formed during the late neoproterozoic the precambrian crust of the cathaysia block can be divided into two distinct tectonic domains the wuyishan terrane to the northeast and the nanling yunkai terrane to the southwest the nearly e w boundary between these two terranes is also supported by geophysical petrological and geochemical evidence and the basement rocks of the two terranes are comprised of different components the wuyishan terrane is characterized by dominant paleoproterozoic 1 86 ga and lesser neoarchean magmatism and the southern central part of the terrane suffered strong neoproterozoic and early paleozoic tectonothermal reworking the zircon data suggest that there were five important episodes of juvenile crust generation 3 6 ga 2 8 ga 2 6 2 4 ga 1 85 ga and 0 8 0 7 ga in the wuyishan terrane strong paleoproterozoic 1 85 ga magmatism in northern wuyishan and neoproterozoic 0 8 0 7 ga magmatism in central and southern wuyishan both principally involved the remelting of old crust with minor input of juvenile material in contrast the nanling yunkai terrane contains abundant neoarchean and grenvillian zircons some evidence of mesoproterozoic activity and rare paleo to meso archean and neoproterozoic zircons u pb ages and hf isotopic compositions of the detrital zircons from the late neoproterozoic to early paleozoic meta sedimentary rocks in the nanling yunkai area suggest that the growth of juvenile crust mainly occurred at 3 6 ga 3 3 ga 2 5 2 6 ga 1 6 ga 1 0 ga and 0 8 0 7 ga neoarchaean 2 6 2 5 ga grenvillian 1 0 ga and neoproterozoic 0 8 0 7 ga magmatism mainly involved the recycling of old crust minor remnants of 4 1 ga crust may remain in the terrane this crustal history is distinct from that of the wuyishan terrane and analogous to those recorded for the eastern india east antarctic domain of northern gondwanaland suggesting that they probably were once linked

tectonics of south china key to understanding west pacific geology abstract south china is not a post caledonian platform but a composite of orogenic belts this late proterozoic mesozoic orogen is a collage of three continental fragments the three blocks are yangzi huanan and dongnanya and they are separated by the banxi nanpanjiang formerly xianggangzhe and gunanhai suture zones yangzi was separated from gondwanaland during late precambrian when an open ocean called banxi was present between the two continents tectonic processes at an active margin during sinian and early paleozoic time led to the genesis of an accretionary wedge complex the banxi melange and the huanan flysch nappes on the northern margin of huanan which was then the northern margin of the gondwana continent reorganization of plates during the devonian suspended subduction at the huanan active margin huanan was uplifted and unconformably overlain by transgressive deposits of devonian and or carboniferous age a remnant ocean the nanpanjiang sea still existed between yangzi and huanan and deep water sedimentation continued both at the southern margin of yangzi and at the northwestern margin of huanan the latter again became an active margin during late paleozoic when permian and triassic flysch sediments were deposited in foredeeps ahead of advancing nappes huanan and yangzi collided during the triassic and resulted in the deformation of the passive margin of the latter to form the yangzi deformed belt huanan was separated from a more southerly continent dongnanya in the devonian by seafloor spreading which created a late paleozoic and early mesozoic ocean gunanhai ocean a continuously deposited sequence ranging in age from devonian to triassic was laid down on the southern passive margin of huanan on the other side of the ocean the dongnanya permian strata include glacial marine deposits of gondwanaland affinity dongnanya became separated from gondwanaland during the late permian when it marched northward to be reunited with huanan the huanan passive margin sequence was deformed by folding and overthrusting after the late mesozoic collision of huanan and dongnanya scattered outcrops of the suture melange resulting from this collision occur at a few localities in coastal fujian dongnanya in our model was the mesozoic continent of southeast asia peripheral to mainland asia this may or may not have been the eastern continuation of the microcontinent sibumasu siam thailand burma malaysia sumatra the collision of dongnanya and asia gave rise not only to the gunanhai melange of coastal fujian but also to the ophiolite melanges parts of which are present in taiwan tailuko the philippines and western malaysia

the crust of cathaysia age assembly and reworking of two terranes abstract u pb dating and hf isotope data from detrital zircons have been used to analyse the crustal evolution of the eastern and western parts of the cathaysia block in se china zircons from the oujiang river in eastern cathaysia indicate that the basement is dominantly paleoproterozoic 1850 1870 ma 2100 2400 ma in age with minor archean components it was extensively reworked in jurassic cretaceous time 100 155 ma to produce the widespread yanshanian magmatic suite both the 1850 1870 ma event and the yanshanian magmatism show wide ranges in hf isotope composition consistent with mixing between crustal and juvenile magmas marked downstream changes in the relative proportions of zircon age populations emphasize the care required in using detrital zircon data to estimate continental growth rates zircons from the north river indicate that the crust of western cathaysia was generated mainly during neoproterozoic time although it contains some archean 2500 3500 ma to mesoproterozoic components this crust was strongly reworked during caledonian ca 450 ma indosinian ca 240 ma and early yanshanian ca 160 ma thermal events there is little evidence for juvenile crustal growth in any of these events although the yanshanian granitoids of the eastern and western parts of the cathaysia give similar mean hf model ages consistent with published nd model ages this is a coincidence they have different crustal sources the distinct patterns of crustal evolution suggest that eastern and western cathaysia may represent separate microcontinents accreted to the older yangtze craton and transposed by extensive strike slip faults

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

carboniferous and triassic eclogites in the western dabie mountains east central china evidence for protracted convergence of the north and south china blocks shrimp u pb dating and laser ablation icp ms trace element analyses of zircon from four eclogite samples from the north western dabie mountains central china provide evidence for two eclogite facies metamorphic events three samples from the huwan shear zone yield indistinguishable late carboniferous metamorphic ages of 312 5 307 4 and 311 17 ma with a mean age of 309 3 ma one sample from the hong an group 1 km south of the shear zone yields a late triassic age of 232 10 ma similar to the age of ultra high pressure uhp metamorphism in the east qinling dabie orogenic belt ree and other trace element compositions of the zircon from two of the huwan samples indicate metamorphic zircon growth in the presence of garnet but not plagioclase namely in the eclogite facies an interpretation supported by the presence of garnet omphacite and phengite inclusions zircon also grew during later retrogression zircon cores from the huwan shear zone have ordovician to devonian 440 350 ma ages flat to steep heavy ree patterns negative eu anomalies and in some cases plagioclase inclusions indicative of derivation from north china block igneous and low pressure metamorphic source rocks cores from hong an group zircon are neoproterozoic 780 610 ma consistent with derivation from the south china block in the western dabie mountains the first stage of the collision between the north and south china blocks took place in the carboniferous along a suture north of the huwan shear zone the major triassic continent continent collision occurred along a suture at the southern boundary of the shear zone the first collision produced local eclogite facies metamorphism in the huwan shear zone the second produced widespread eclogite facies metamorphism throughout the dabie mountains sulu terrane and a lower grade overprint in the shear zone

eclogites preserved as pebbles in jurassic conglomerate dabie mountains china abstract two types of eclogite pebbles were discovered in middle to upper jurassic conglomerates from the hefei basin north of the dabie ultrahigh pressure uhp terrain china type a eclogite pebbles are characterized by idioblastic garnet with well preserved chemical zonation si content in phengite is lower than 3 5 per formula unit pfu the maximum metamorphic pressure is lower than 2 5 gpa and the temperature is below 600 c type b eclogite pebbles contain coesite pseudomorphs in xenoblastic garnet si content in phengite is higher than 3 5 pfu the maximum metamorphic pressure is 2 8 4 0 gpa at 700 c indicating uhp metamorphism types a and b eclogite pebbles are comparable with eclogites occurring in the southern portion of the dabie uhp terrain based on the petrologic similarities and northeastwards directed paleocurrents we infer that the eclogite pebbles were eroded from the dabie uhp terrain sandstones containing detrital phengite with si content higher than 3 5 pfu are also derived from uhp rocks these petrologic and stratigraphic data place time constraints on exhumation and erosion history of the dabie uhp terrain

early archean to middle jurassic evolution of the korean peninsula and its correlation with chinese cratons shrimp u pb zircon age constraints abstract u pb zircon ages of tuffs and sandstones of the daedong supergroup bansong and nampo groups in the korean peninsula were determined using a sensitive high resolution ion microprobe shrimp in order to constrain their age of sedimentation and to unravel discrete geologic events as recorded in detrital zircons the ages of four tuffaceous samples from the bansong group imply that the daedong supergroup formed at ca 187 172 ma in association with the early middle jurassic orogeny these data are in marked contrast with paleomagnetic arguments suggesting that the bansong and nampo groups are precollisional early middle triassic deposits that are correlative with the north and south china blocks respectively detrital zircons of the daedong supergroup define seven age components 1 early middle archean 3 64 2 97 ga 2 late archean middle early proterozoic 2 63 2 33 ga 3 late early proterozoic 1 98 1 75 ga 4 middle late proterozoic 1 2 0 6 ga 5 devonian 400 355 ma 6 early

oblique closure of the northeastern paleo tethys in central china a branch of the paleo tethys ocean once separated the north china plate from the south china plate however the mode of closure of the northeastern paleo tethys ocean during the late paleozoic to early mesozoic has been debated one reason for this debate is that the collisional suture zone was later buried by large scale thrust faults in the southern qinling dabieshan orogen which made it difficult to reconstruct the amalgamation of the supercontinent in central china new regional geologic mapping provides stratigraphic and structural constraints on the mechanism of this ocean closure our results indicate that dextral transpressional suturing in the southern qinling dabieshan foreland fold thrust belt resulted in the formation of the northern yangtze foreland basin where the stratigraphy precisely shows the time transgressive closure of the ocean and the orogenic sediments shed over 1000 km westward from eastern china to the closing paleo tethys therefore we propose an oblique subduction model to describe the closure of the paleo tethys ocean our findings suggest that prolonged slab pull during the oblique subduction of the oceanic plate continued to drive deep continental subduction thereby forming high and ultrahigh pressure metamorphic rocks and leading to sustained ocean closure

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

paleomagnetic and rock magnetic study of combustion metamorphic rocks in israel paleomagnetic and rock magnetic studies were combined with the mineralogy and metamorphic petrology of a sequence of combustion metamorphic cm rocks in israel samples from 12 sites from the three major outcrops of the hatrurim formation mottled zone were analyzed these are originally cretaceous bituminous chalks and marls which have undergone spontaneous combustion during the late neogene the analyses included natural remanent magnetization nrm intensity alternating field af and thermal demagnetization and isothermal remanent magnetization irm measurements mineralogical phases were identified by x ray diffraction and electron microprobe all metamorphosed rocks have a nrm intensity far above normal sedimentary rocks this increase in intensity is observed even in rocks where no mineralogical indications of metamorphism are observed the major contributors to the rock magnetism in these rocks are maghemite hematite and rare magnesioferrite the magnetic properties were acquired during the burning process as a consequence of oxidation of fe sulfides the in situ direction of the mean magnetic vector is dec 8 2 inc 49 2 k 40 and 95 5 2 whereas the stratigraphically corrected mean is dec 25 1 inc 46 9 k 26 and 95 6 5 the stratigraphic mean direction is in accordance with the pliocene expected field direction for this area as derived from stable africa poles therefore combustion must postdate tectonic tilt or vertical axis rotation associated with late cretaceous major folding and faulting and rifting associated with the dead sea transform normal and reverse polarity and paleosecular variation indicate that combustion at the different sites was not simultaneous

further evidence for 1 85 ga metamorphism in the central zone of the north china craton shrimp u pb dating of zircon from metamorphic rocks in the lushan area henan province abstract this paper reports shrimp zircon u pb dating of precambrian supracrustal and granitic rocks from the lushan area henan province in the southern portion of the central zone also referred to as the trans north china orogen of the north china craton a graphite garnet sillimanite gneiss sample tw0006 1 of the shangtaihua group gives a range of inherited zircon ages from 2 73 to 2 26 ga and a metamorphic zircon age of 1 84 0 07 ga a garnet bearing gneissic granitoid sample twj358 1 which is considered to intrude the shangtaihua group gives a magmatic zircon age of 2 14 0 02 ga and a metamorphic zircon age of 1 87 ga the metamorphic zircon ages of 1 87 1 84 ga obtained in this study indicate that an important tectonothermal event occurred at the end of the paleoproterozoic in the lushan area this supports the southern continuation of a central zone in the north china craton that workers have recently considered to result from continent continent collision it is also evident that the shangtaihua group was formed during the paleoproterozoic between 2 26 and 2 14 ga and not during the archean as previously considered

shrimp u pb zircon geochronology and geochemistry of metavolcanic and metasedimentary rocks in northwestern fujian cathaysia block china tectonic implications and the need to redefine lithostratigraphic units abstract northwestern fujian province is one of the most important pre palaeozoic areas in the cathaysia block of south china metavolcano sedimentary and metasedimentary rocks of different types ages and metamorphic grades granulite to upper greenschist facies are present and previously were divided into several formations and groups tectonic contacts occur between some units whereas deformed unconformities have been reported between others new shrimp u pb zircon ages presented here indicate that the original lithostratigraphy and the old group and formation terminology should be abandoned thus the tianjingping formation was not formed in the archaean or palaeoproterozoic as previously considered but must be younger than its youngest detrital zircons 1790 ma but older than regional metamorphism 460 ma besides magmatic zircon ages of 807 ma obtained from metavolcano sedimentary rocks of the nanshan formation and 751 728 ma for the mamianshan group many inherited and detrital zircons with ages ranging from 1 0 to 0 8 ga were also found in them these ages indicate that the geological evolution of the study area may be related to the assembly and subsequent break up of the rodinia supercontinent the new zircon results poorly constrain the age of the mayuan group as neoproterozoic to early palaeozoic 728 458 ma and not palaeoproterozoic as previously thought many older inherited and detrital zircons with ages of 3 6 2 8 2 7 2 6 2 5 2 0 1 8 and 1 6 ga were found in this study a 3 6 ga detrital grain is the oldest one so far identified in northwestern fujian province as well as throughout the cathaysia block nd isotope tdm values of eight volcano sedimentary and clastic sedimentary rock samples centre on 2 73 1 68 ga being much older than the formation ages of their protoliths and thus showing that the recycling of older crust played an important role in their formation these rocks underwent high grade metamorphism in the early palaeozoic 458 425 ma during an important tectono thermal event in the cathaysia block