a review of magmatism and deformation history along the ne asian margin from ca 95 to 30 ma transition from the izanagi to pacific plate subduction in the early cenozoic abstract previous plate reconstruction in the paleo pacific domain indicates that the izanagi and pacific plates influenced the ne asian continental margin during the mesozoic and cenozoic respectively however it remains unclear how and when the transition from izanagi to pacific plate subduction occurred because most evidence came from the oceanic crust however the onshore geological records are poorly understood so in order to solve this problem we review recently published data and identify three magmatic episodes a magma gap and four deformation stages from ca 95 to 30 ma margin parallel izanagi pacific ridge subduction is proposed to explain the changing magmatic and deformation signatures and other regional events in the early cenozoic from ca 95 to 65 ma the n or nnw directed subduction of the izanagi plate triggered sinistral shearing along the nne striking faults arc magmas at that time incorporated older crustal materials from ca 65 to 55 ma the nww striking transform faults and ridge segments were subducted developing small slab windows and facilitating the input of juvenile magmas local extension was active above these windows during ca 55 to 45 ma the ridge was totally subducted and a margin wide slab window was opened resulting in an arc magma gap and extension massive asthenosphere upwelling caused the extensive accretion of juvenile materials from ca 45 30 ma the dextral nne trending faults resulted from the w directed subduction of the pacific plate new arc magmas were generated from juvenile materials and thus are isotopically depleted

borehole geophysical log signatures and stratigraphic assessment in a glacial basin southern ontario over the past two decades the geological survey of canada has used a standardized suite of slim hole geophysical tools to log 57 polyvinyl chloride cased boreholes drilled in the glacial sediments of southern ontario this article documents downhole tool responses natural gamma apparent conductivity magnetic susceptibility and seismic velocity in the context of mineralogical characteristics of the region and grain size data from 28 of the 57 boreholes characteristic geophysical properties and or patterns are identified within the units of a regional hydrostratigraphic framework in southern ontario the importance of a calibrated suite of tools is emphasized as stratigraphic units may have variable response from site to site the use of a high sensitivity magnetic susceptibility induction probe is shown to be an important tool in the log suite for lithostratigraphic interpretation and more broadly for provenance studies of source rock across the region ranges of compressional p and shear s

first finding of eclogite facies metamorphic event in south korea and its correlation with the dabie sulu collision belt in china abstract metabasite lenses occur within granitic gneiss at bibong in the southwestern part of the gyeonggi massif south korea bibong metabasites experienced an initial eclogite facies metamorphism 17 0 20 9 kbar 835 860 c which was succeeded by a granulite facies metamorphism 11 5 14 6 kbar 830 850 c and finally overprinted by an amphibolite facies metamorphism 6 7 11 0 kbar 570 740 c the metabasites have sm nd internal isochron ages at 225 258 ma and the major elements of the metabasites display an island arc affinity the petrochemical and geochronological data suggest the southwestern part of the gyeonggi massif as an extension of the triassic dabie sulu collision belt in china

tracking source materials of phanerozoic granitoids in south korea by zircon hf isotopes phanerozoic granitoids in south korea are classified into four primary spatiotemporal groups showing geochemical and isotopic diversity this study presents the first in situ hf isotope data for zircons extracted from representative outcrops of each granitoid group the core to rim variation in ehf values observed in some zircon grains provides evidence for open system processes influenced by the input of more primitive melts or interactions with pre existing crustal materials a general core to rim decrease in lu hf and th u ratios indicates a progressive compositional change in the melts during magmatic differentiation contrasting evolutionary paths demonstrated by zircon ehf values suggest that the neoproterozoic to palaeozoic crust including the permian granitoids was recycled during the cretaceous to palaeogene magmatism in south eastern korea whereas the palaeoarchaean to palaeoproterozoic crust provided major source material for the triassic to jurassic granitoids in central korea

tectonic and sedimentary evolution of a cretaceous continental arc backarc system in the korean peninsula new view abstract this paper focuses on the tectonic and sedimentary evolution of the cretaceous volcanic rocks and nonmarine successions in the southeastern part of the korean peninsula which constitute an ancient continental arc system the arc system gyeongsang arc system comprising an arc platform gyeongsang volcanic arc and a backarc basin gyeongsang backarc basin was a southwestward extension of the japanese arc formed by oblique northward subduction of the proto pacific izanagi plate under the asian continent the backarc basin was initiated in the southeastern part of the korean peninsula in the early cretaceous as a narrow ns trending trough the basin was bounded by a major fault in the northern part of the basin from which sediments largely emanated sediments were deposited in streamflow dominated alluvial fans adjacent to the fault bounded basin margin in the north and low gradient fluvial systems of braided channels that extended southward and southeastward for tens of kilometers sediments were also derived from the western highland margin draining precambrian to jurassic basement rocks the initially narrow trough progressively expanded toward the east resulting in eastward migration of depocenters that eventually generated a broad fluvio lacustrine plain fringing the volcanic arc platform the arc platform played an important role for the derivation of volcanogenic materials and accreted sediments into the backarc basin via extensive fluvial network pyroclastic density currents and landslides which originated from the arc platform also entered the basin in addition extrusion of basaltic volcanic rocks was continual within the basin during basin expansion the resultant succession of mixed sedimentary and volcanogenic rocks is generally indicative of a temporal increase in volcanic activity in the arc platform and in subsidence rate of the basin in the late cretaceous andesitic to rhyolitic volcanism became climactic in the arc platform producing a number of calderas or volcanotectonic depressions volcaniclastic aprons fringing the arc platform encroached upon the basin fill intraarc basins produced by sinistral shearing of the arc platform received sediments from active volcanoes and footwalls of contemporaneous faults forming small scale streamflow dominated alluvial fans and floodplains with later development of a deep lake overall the gyeongsang arc system formed under an extensional or transtensional sinistral strike slip stress regime suggesting that the subducting and the overriding plates were not strongly coupled both the volcanic arc and the backarc basin ceased to develop as volcanic activities shifted progressively eastward in the paleogene accompanied with rollback of the subduction of the izanagi plate

termination of the permo triassic songrim indosinian orogeny in the ogcheon belt south korea occurrence of ca 220 ma post orogenic alkali granites and their tectonic implications abstract the indosinian orogeny is defined in a broad sense as encompassing all the late paleozoic to early mesozoic tectonic events associated with the closure of the eastern parts of the paleo tethys ocean it includes the permo triassic continent continent collision between the north china and south china blocks that generated the qinling dabie sulu orogen containing the largest ultrahigh pressure uhp terrane in the world the korean peninsula situated at the easternmost end of the tethyan tectonic regime was also tectonically influenced by the indosinian orogeny often known as the songrim orogeny in korea the songrim orogeny characterized by intense regional medium to high pressure metamorphism extends from the imjingang belt through the gyeonggi massif to the ogcheon belt it was initiated as early as the early permian period ca 290 ma and peaked in the triassic period ca 255 to 225 ma this paper presents new geochemical and shrimp u pb zircon ages for two alkali granites that postdated the songrim orogeny in the ogcheon belt both granites are contemporaneous 219 3 3 3 ma and 219 6 1 9 ma and are geochemically post orogenic a type granites intruded in an extensional setting subsequent to a major collision the data provide a critical time constraint on the termination of the indosinian orogeny in the korean peninsula despite the absence of an uhp terrane in the korean peninsula similarities in the duration metamorphic characteristics and post orogenic magmatism between the qinling dabie sulu and songrim orogens as parts of the broad indosinian orogenic regime are clearly alike suggesting a genetic linkage in their time space characteristics

tectonic and sedimentary evolution of the korean peninsula a review and new view abstract this review focuses on the tectonics and sedimentation of major sedimentary basins and orogenic belts late proterozoic neogene in the korean peninsula the korean peninsula is part of the amuria plate and represents an important link between continental blocks of north and south china and the island arcs of japan the basement rocks exposed in the kyonggi and yongnam massifs consist of 2 7 to 1 1 ga high grade gneiss and schist these massifs are separated by the okchon fold belt which comprises metasedimentary rocks and bimodal meta volcanic rocks the stratigraphy of the okchon group is unclear at present the okchon basin was probably initiated as an intraplate rift prior to the late proterozoic the hwanggangri formation clast bearing phyllite most likely represents deposition by subaqueous debris flows in slope environments of an enclosed basin the stratigraphic relationship between the okchon group and the choson supergroup cambro ordovician of the taebaeksan basin is poorly constrained the choson supergroup unconformably overlies the yongnam massif and consists mainly of carbonate sequence that formed mostly in shallow marine and tidal environments reflecting numerous sea level fluctuations the sequence is disconformably overlain by siliciclastic sequence of pyongan supergroup carboniferous triassic which formed most likely in shallow marine deltaic and fluvial environments the imjingang belt is an east trending fold and thrust zone and consists of metasedimentary rocks and volcaniclastics devonian carboniferous underlain unconformably by proterozoic basement rocks late proterozoic amphibolites of oceanic affinity were metamorphosed at about 8 13 kbar and 630 740 c during the late permian to the early triassic the south vergent contraction and top down to the north normal faulting are suggestive of a suture belt between the north china block sino korea craton and the south china block yangtze craton an extension of sulu belt across the yellow sea entire peninsula experienced strong deformation and metamorphism during the suturing event namely songrim orogeny during this orogenic event the kyonggi massif and the okchon basin accreted to the yongnam massif and taebaeksan basin along the south korean tectonic line running northeast southwest a series of northward trending thrust formed along the boundary zone to the east kaktong and kongsuwon thrusts and others piggyback basins locally developed along the thrust faults forming the taedong group the crustal deformation resumed in the early to late jurassic daebo event under contractional setting dextral ductile shearing associated with thrusting and folding continued in the mid southern part of the peninsula it was due to orthogonal northwestward subduction of the izanagi plate under the asian continent the yongnam massif experienced continuous dextral offset along the honam shear zone in the early cretaceous the izanagi plate began to subduct northward and caused formation of strike slip basins in retroarc setting i e kyongsang basin in the southeastern part and a number of small scale basins in the mid southwestern part of the peninsula small scale alluvial fans and fluvial channel networks formed in the basin margin and were transitional to ephemeral lacustrine systems under semiarid to arid conditions extensive intrusion of granitoids occurred from triassic to early tertiary with a gap between 160 and 100 ma representing continental magmatic arc in the tertiary the southeastern margin of the korean peninsula experienced back arc opening pull apart basins formed in the miocene bounded by the yangsan and hupo faults the yonil group sedimentary fill of the pohang basin comprises more than 1 km thick siliciclastic sequence which represents deposition in fan delta systems on the hanging wall of the yangsan fault thick more than 10 km thick sediments in the ulleung basin margin were deformed in the late miocene due to the northward movement of kyushu block quarternary volcanic events in cheju island represent intraplate hot spots

juxtaposition of allochthonous terranes in the central korean peninsula evidence from zircon u pb ages and o hf isotopes in jurassic granitoids abstract the eventual docking of an exotic continental mass over the course of a wilson cycle is marked by intracontinental sites of prograde metamorphism contractional deformation and syn or post collisional magmatism this study shows that inherited premagmatic zircon in subsequently emplaced granitoid rocks provides another line of evidence for the collisional orogeny following the continental collision between the north and south china cratons jurassic granitoids intruded widely throughout the central korean peninsula in association with the subduction of the paleo pacific plate the latest early to middle jurassic 177 167 ma granitoids occur throughout the cratonic gyeonggi massif whereas early jurassic 194 184 ma granitoids occur locally along the okcheon metamorphic belt to the southeast of the massif these two spatiotemporal groups differ profoundly in their age patterns and o hf isotopic signatures of inherited zircon the younger group yields a typically north china like archean paleoproterozoic mostly 2 5 1 8 ga zircon inheritance in contrast inherited zircons from the older group exhibit exclusively neoproterozoic ca 0 9 0 7 ga and paleozoic ca 0 4 ga cluster along with archean paleoproterozoic populations the exotic feature of the protolith of the older group is further indicated by significantly lower 18o 0 9 to 4 9 and positive ehf 3 to 15 values of the cryogenian zircon cores the results of zircon analysis demonstrate that south and north china like terranes were juxtaposed infracrustally along the southeastern margin of the gyeonggi massif during the pre jurassic collisional orogeny and then selectively provided major source material to the older and younger jurassic granitoids respectively

hydrocarbon migration and entrapment in the baltic syneclise abstract based on the data concerning the type quantity and thermal maturity of the organic matter several petroleum source rock complexes have been distinguished in the paleozoic succession of the baltic syneclise maturity of organic matter varies from immature in the ne part of the baltic syneclise to peak generation in the central part and postmature stage towards the teisseyre tornquist zone the isolated local postmature areas are most likely related to the occurrence of sills the oils of the baltic region have moderate gravities 26 42 api that decrease with present reservoir depth reflecting the thermal maturity quantitative modeling of the oil and gas migration and entrapment was performed by the means of the 3d software semi this confirmed that oil generation and migration was most intense during late silurian early devonian time based on modeling the major cambrian traps accumulated oil expelled from different stratigraphic source rock units the cambrian ordovician and silurian organic rich shales

mississippian bakken and equivalent formations as source rocks in the western canadian basin abstract the organic rich bakken formation of mississippian age has previously been identified as the source rock for mississippian reservoired oils in the williston basin north dakota examination of c15 hydrocarbon data including biomarker distributions steranes triterpanes and diterpanes and stable carbon isotope ratios of bakken formation core samples from williston basin and those of equivalent formations from western canadian basin alberta reveals similar types of extractable organic matter stable carbon isotope and biomarker data support strong positive correlations between the bakken formation core samples and mississippian oils from both basins in addition a positive correlation is found between bakken core samples and lower cretaceous heavy oil deposits from athabasca cold lake and peace river areas in alberta thus the mississippian bakken and equivalent formations are also important source rocks in the western canadian basin

application of aromatic compounds as maturity indicators in source rocks and crude oils concentration ratios of aromatic compounds such as naphthalenes phenanthrenes and dibenzothiophenes are at present attracting increasing attention as maturity indicators in sedimentary rocks and crude oils generally these indicators rely either on an increase with maturity in the degree of alkylation of a given parent compound or a shift in the isomer distribution of alkyl aromatic homologues towards thermally more stable isomers a combination of both concepts in the methylphenanthrene index mpi resulted in an excellent maturity parameter as demonstrated for rock extracts by an improved correlation with mean vitrinite reflectance rm between 0 65 and 1 4 i e in the zone of oil formation in cases that rm measurements were ambiguous this empirical relationship has proved useful in the calculation of theoretical vitrinite reflectance rc values from the mpi on the other hand for potential petroleum source rocks reaching maturity at relatively low temperatures the influence of the organic matter type on the aromatic distributions cannot be neglected consequently organic facies effects should be taken into account in the interpretation of aromatic distributions of such source rocks and related crude oils since the impact of the organic facies is strongly reduced with increasing maturity aromatic maturity parameters are particularly useful in the maturity evaluation of post mature crude oils and condensates in contrast to biological markers substantially higher concentrations of the key alkyl aromatics persist at elevated maturation levels thus aromatic maturity parameters should be considered as versatile tools in organic maturation studies and can complement established maturity indicators based on steranes and terpanes

the middle permian to triassic tectono magmatic system in the southern korean peninsula abstract the permo triassic adakitic magmatism in the southern korean peninsula preserves important tectonomagmatic records along the east asian continental margin in this study we present shrimp la mc icpms u pb zircon ages and hf isotope signature with whole rock geochemical compositions of the middle permian to middle triassic granitoid gneiss granitoid suites and the middle to late triassic plutonic rocks in the southeastern korean peninsula the former yields three age groups of ca 272 263 ma ca 258 251 ma and ca 243 240 ma respectively the latter group yields ca 237 216 ma ages indicating a long term magmatism that lasted from the middle permian to late triassic the granitoid gneiss granitoid suites have chemical compositions similar to high silica adakitic rocks showing high sio2 59 55 73 97 wt na2o 3 55 5 30 wt sr y 30 401 and la ybn 11 173 and low mgo 0 18 1 82 wt y 2 14 ppm and ybn 2 7 these rocks are peraluminous with high ba and sr and show relatively high k2o contents and k2o na2o ratios 0 7 with positive correlation between la ybn vs sr y ratios and high initial sr ratios these features reflect different contribution of the slab derived adakite melts that interacted with peridotitic mantle in the metasomatized mantle wedge and the middle paleoproterozoic lower crustal basement of the yeongnam massif on the contrary the triassic alkaline magmatism in the yeongnam massif is alternatively interpreted either by a tectonic switch to the extension dominated arc system or by a delamination of an overthickened arc the results from this study offer important evidence for a common subduction accretion system existed between the north and south china cratons and microcontinents between them along margins of the east asian continental blocks with different plate interaction in different locations generating distinct magmatic suites

geochronology and geochemical and nd sr isotopic characteristics of triassic plutonic rocks in the gyeonggi massif south korea constraints on triassic post collisional magmatism abstract sensitive high resolution ion microprobe shrimp zircon u pb ages and whole rock chemical and isotopic sr nd compositions have been measured on some middle triassic plutonic rocks associated with post collisional magmatism in the gyeonggi massif hongseong namyang and yangpyeong areas south korea plutonic rocks from the southwest hongseong and central yangpyeong gyeonggi massif and central okcheon belt cheongsan have ages of ca 227 226 231 and 225 ma respectively the plutonic rocks from hongseong are dominated by granite and quartz syenite from namyang by granite and from yangpyeong by quartz monzonite to quartz syenite and gabbro three rocks from the gyeonggi massif have the depletions in ta nb p and ti and enrichment in large ion lithophile elements lile characteristic of plutonic rocks that are subduction related or derived from an arc related crustal source the hongseong quartz syenite and yangpyeong quartz monzonite to quartz syenite have features of shoshonitic and high ba sr granitoids with lree enrichment and no significant eu anomaly the plutonic rocks from the gyeonggi massif have high initial 87 sr 86 sr ratios 0 7083 0 7144 and strongly negative e nd t 20 3 to 10 4 indicating derivation of the magmas from sources in the middle to upper crust these high k calc alkaline and shoshonitic plutonic rocks formed during periods of relaxation that separated the culminations of post orogenic uplift collapse in the transitions from orogenic to anorogenic events after the early middle triassic continent continent collision 249 229 ma between the north and south china blocks in south korea the plutonic rocks of the gyeonggi massif support therefore a possible eastern extension of the early middle triassic dabie sulu collision belt in china

geotectonic framework of permo triassic magmatism within the korean peninsula abstract this study presents sensitive high resolution ion microprobe shrimp u pb zircon ages and whole rock chemical and isotopic sr nd compositions of representative triassic plutons from south korea the plutons from the gyeonggi massif hongseong namyang yangpyeong and odesan the central okcheon belt baeknok and yongsan and the yeongnam massif sangju gimcheon hamyang and macheon yield zircon u pb ages of ca 232 226 ma 227 226 ma and 240 228 ma respectively among the triassic plutonic suite in south korea those within the gyeonggi massif are dominated by granite syenite monzonite monzodiorite and gabbro plutons within the okcheon belt are mainly by granite to quartz monzodiorite the yeongnam massif mainly incorporates granite to granodiorite and minor monzodiorite intrusions the geochemical signatures of the triassic plutons are characterized by ta nb troughs depletion of p and ti and enrichment of lile most plutons except macheon monzodioritic pluton show high initial 87 sr 86 sr ratios 0 708248 0 714678 and strongly negative e nd t 20 3 to 7 7 values suggesting contribution from middle to upper crust in contrast the macheon monzodioritic pluton in the yeongnam massif shows relatively low initial 87 sr 86 sr ratios 0 706547 0 706629 and negative e nd t 4 43 to 3 62 values the middle triassic syenite monzonite granite gabbro series in and around the gyeonggi massif possess high k calc alkaline and shoshonitic affinity suggesting a post collisional magmatic event following the permo triassic collision between the north and south china blocks the triassic plutons in the yeongnam massif and the okcheon belt together with a permian yeongdeok pluton in the gyeongsang basin show features typical of high to medium k calc alkaline magmatism with lree and lile enrichments this together with a depletion of y and hree suggests their formation in a subduction setting our results provide robust evidence to consider the gyeonggi massif as an extension of the qinling dabie sulu belt between the north and south china blocks in central china the okcheon belt and yeongnam massif in south korea together with the continental margin of south china are marked by a common permian to triassic magmatic episode probably related to the paleo pacific slab subduction

zircon u pb geochronological and hf isotopic constraints on the precambrian crustal evolution of the north eastern yeongnam massif korea abstract in situ u pb dating and hf isotopic analyses were conducted on zircons extracted from paleoproterozoic metasedimentary rocks and peraluminous meta granitoids in the north eastern yeongnam massif korea combined with previous results analyses from detrital zircons from the metasedimentary rocks yield a predominant age population of ca 2 5 ga with subordinate clusters at ca 2 7 2 3 and 2 1 ga and minor points older than 2 8 ga the detrital zircons frequently have textureless rims that were overgrown at ca 2 03 1 85 ga indicating post depositional thermal overprints associated with the intrusion of neighbouring granitoids and metamorphism the meta granitoids are divided into three lithologic groups of banded or augen biotite gneisses group i pyeonghae and buncheon gneiss massive cordierite or two mica granitic gneisses group ii icheonri and hongjesa granitic gneiss and a garnet bearing leucogranite group iii imwon leucogranite the best estimates of the timing of the emplacement of the first two groups are indistinguishable within their error ranges 1980 22 ma pyeonghae gneiss 1966 15 ma buncheon gneiss 1985 14 ma icheonri granitic gneiss and 1975 16 ma hongjesa granitic gneiss the upper intercept discordia ages of ca 1 86 ga indicated by the metamorphic overgrowth rims of zircons from the buncheon gneiss and the icheonri granitic gneiss agree with the emplacement age of the imwon leucogranite 1867 6 ma a close genetic link between the meta granitoids and metasedimentary rocks is demonstrated by the comparable age pattern of inherited zircon cores in the former with that of detrital zircons in the latter the lower intercept ages of zircons indicate repeated pb loss events in the neoproterozoic to paleozoic although their exact tectonic meaning is still unclear most zircons have negative hf values corresponding to two stage hf model ages t2dm from 3 4 to 2 7 ga demonstrating neoarchaean to paleoproterozoic reworking of the paleo to neoarchaean crust zircons from group i metagranitoids display a narrow t2dm range 2 74 0 09 ga the neoarchaean hf model ages are also reduced by high hf zircons from the metasedimentary rocks and group ii metagranitoids t2dm ca 2 75 ga and most zircons from the imwon leucogranite t2dm 2 62 0 06 ga the protoliths of group i metagranitoids are considered to be i type granites that was derived by infracrustal melting at depth in contrast the scattered hf model ages of zircons from group ii metagranitoids are suggestive of crystallisation from heterogeneous s type magmas derived from the partial melting of supracrustal rocks it is concluded that the neoarchaean era ca 2 75 2 62 ga marks the most important stage of crustal formation in the north eastern yeongnam massif the paleoproterozoic ca 2 50 1 98 ga magmas from which the zircons crystallised were principally a product of crustal reworking these hf isotopic features generally match those reported for zircons from the north china craton and the eastern part of the cathaysia block in the south china craton but the zircon ages determined here do not allow an indisputable correlation of the north eastern yeongnam massif with paleoproterozoic terranes in eastern china

improvements of precision and accuracy in in situ hf isotope microanalysis of zircon using the laser ablation mc icpms technique abstract we have developed a new analytical technique for precise and accurate in situ hf isotope ratio measurements for zircons by means of laser ablation multiple collector inductively coupled plasma mass spectrometry la mc icpms newly designed sample cell and n 2 mixing technique provided higher elemental sensitivity with smoother signal intensity profiles in order to improve the accuracy of the hf isotope ratio analysis previously reported correction methods for isobaric interferences on 176 hf by 176 yb and 176 lu were rigorously evaluated and we found that the use of independent mass bias factors for hf and yb in the isobaric interference correction resulted in more reliable hf isotopic data in order to test the reliability of the hf isotopic data from 35 m diameter ablation pits that are smaller than the typical size of most zircon grains lu hf isotopes for three zircons harvard 91500 antarctic zircon pma7 and acasta zircon ay199 were measured the present 176 hf 177 hf isotope ratio of 0 282321 46 2 s d and initial 176 hf 177 hf isotope ratio of 0 282315 47 2 s d were obtained for zircon standard 91500 the resulting present 176 hf 177 hf isotope ratio shows an excellent agreement with the previously reported data our in situ isotopic data revealed that the lu hf ratio varied significantly within the 91500 grain 176 lu 177 hf 0 000130 0 000345 suggesting the existence of slight isotopic heterogeneity in present 176 hf 177 hf approximately 15 ppm for antarctic zircon pma7 despite the smaller size of the single grain 50 100 m wide 80 300 m long and the presence of various mineral inclusions resulting analytical precision for the initial 176 hf 177 hf ratio was 128 ppm 0 281208 36 2 s d for acasta zircon ay199 despite the high yb hf 0 05 and lu hf 0 01 isotope ratios with the early archean crystallization age 3 74 ga resulting analytical precision for the initial 176 hf 177 hf ratio was 187 ppm 0 280160 52 2 s d these results demonstrate clearly that in situ hf isotope ratio microanalysis developed here has the potential to become a significant tool for zircon lu hf isotopic study

petroleum source rock evaluation of the alum and dictyonema shales upper cambrian lower ordovician in the baltic basin and podlasie depression eastern poland we present geochemical characteristics of the lower palaeozoic shales deposited in the baltic basin and podlasie depression in the study area this strata are represented by the upper cambrian lower ordovician alum shale recognized in southern scandinavia and polish offshore and a equivalent the lower tremadocian dictyonema shale from the northern estonia and the podlasie depression in poland geochemical analyses reveal that the alum shale and dictyonema shale present high contents of organic carbon these deposits have the best source quality among the lower palaeozoic strata and they are the best source rocks in the baltic region the bituminous shales complex has toc contents up to ca 22 wt the analysed rocks contain low sulphur oil prone type ii kerogen deposited in anoxic or sub oxic conditions the maturity of the alum and dictyonema shales changes gradually from the east and north east to the west and south west i e in the direction of the tornquist teisseyre zone samples located in the seashore of estonia and in the podlasie region are immature and in the initial phase of oil window the mature shales were found in the central offshore part of the polish baltic basin and the late mature and overmature are located in the western part of the baltic basin the alum and dictyonema shales are characterized by a high grade of radioactive elements especially uranium the enrichment has a syngenetic or early diagenetic origin the measured content of uranium reached up to 750 ppm and thorium up to 37 ppm

geochronological and geochemical constraints on the petrogenesis of mesozoic high k granitoids in the central korean peninsula abstract mesozoic granitoids are prevalent throughout the korean peninsula but precise geochronological and petrogenetic information of these granitoids is not available for this study 14 granitoid samples from the gyeonggi massif and okcheon belt in central korea were chosen for zircon and titanite u pb age determination these granitoids can be divided into two age groups 230 225 and 184 167 ma triassic granitoids are metaluminous shoshonitic diorite while jurassic granitoids range from metaluminous to peraluminous high k calc alkaline biotite granite nd isotopic data for both groups suggest that the granitoids were derived from younger lower crustal rocks instead of precambrian basement rocks in close proximity high alkali content within the triassic diorite implies a post collisional signature and cessation of indosinian songrim orogeny on the korean peninsula the geochemical transition in granitoids from triassic shoshonitic to a jurassic high k calc alkaline series suggests a change in tectonic environments from continent continent collision to active continental margin the temporal difference in triassic high k magmatism indicates that the extension after indosinian orogeny most likely began earlier in the korean peninsula than in the north china block which might be related to differences in geodynamics rather than a diachronous continental collision

mesozoic episodic magmatism in south korea and its tectonic implication 1 ne trending granitoid batholiths in south korea are part of the huge phanerozoic granitoid belt in east asia the crystallization ages of granitoid plutons show three episodes of mesozoic magmatism in south korea triassic 248 210 ma jurassic 197 158 ma and late cretaceous early tertiary 110 50 ma with a late jurassic early cretaceous 50 m y hiatus the granitoid ages and kinematic analyses of the mesozoic structures in south korea suggest that the middle jurassic 180 170 ma dextral strike slip ductile shear zones formed by the northwest orthogonal subduction of the izanagi plate whereas the cretaceous sinistral faults formed by the northward oblique subduction of the same plate which generally agree with proposed reconstruction of plate motions albeit uncertain around east asia among the models proposed to explain magmatic lull in a continental arc shallow angle subduction associated with high convergence rate and subduction of oceanic plateaus might be the probable cause for the magmatic hiatus in south korea and japan the huge width 500 km of the east asian granitoid belt may be explained by oceanward retreat of subduction zone in the cretaceous in association with change in the movement direction of the subducting plate

the tectonic setting of the eastern margin of the sino korean block inferred from detrital zircon u pb age and nd isotope composition of the pyeongan supergroup upper palaeozoic lower triassic korea the upper palaeozoic succession pyeongan supergroup in central eastern korea is well correlated with the equivalent successions distributed in north china suggestive of the korean upper palaeozoic being part of the sino korean block detrital zircon u pb ages and sm nd isotope compositions of the pyeongan supergroup in the samcheok coalfield of the taebaeksan basin were analysed a single predominant zircon age peak at c 1 9 ga 70 is marked in all sedimentary units followed by varying amounts of minor late palaeozoic grains up to 30 the rarity of meso to neoproterozoic and silurian aged zircons confirms that sediment influx from the south china and qinling blocks was insignificant the 2 0 1 8 ga dominated zircon age pattern and the nd isotope composition average end 0 15 5 4 0 of the pyeongan supergroup most closely reflect the signature of the yeongnam massif basements which supports a previous hypothesis that the pyeongan supergroup was mostly derived from a palaeo orogen located to the east southeast relatively higher end 0 values 10 1 in the lowermost and the upper parts of the succession are closely matched by the increased occurrence of syn depositional aged zircons which indicates considerable mixing of juvenile materials at c 320 ma and 260 ma both arc related magmatic events are interpreted to have been related to oceanic subduction suggesting that the eastern margin of the sino korean block was an active continental margin during late palaeozoic times

the ratios of dibenzothiophene to phenanthrene and pristane to phytane as indicators of depositional environment and lithology of petroleum source rocks abstract the ratio of dibenzothiophene to phenanthrene and the ratio of pristane to phytane when coupled together provide a novel and convenient way to infer crude oil source rock depositional environments and lithologies such knowledge can significantly assist in identifying the source formation s in a basin thereby providing valuable guidance for further exploration the ability to infer this information from analysis of a crude oil is especially valuable as frequently the earliest samples in a new area may be shows and or drill stem test samples from exploratory wells which are characteristically drilled on structural highs stratigraphically remote from the source formation s a cross plot of dibenzothiophene phenanthrene versus the pristane phytane ratios measured on seventy five crude oils from forty one known source rocks ranging in age from ordovician to miocene consistently classified the oils into the following environment lithology groups marine carbonate marine carbonate mixed and lacustrine sulfate rich lacustrine sulfate poor marine and lacustrine shale and fluvial deltaic carbonaceous shale and coal the dibenzothiophene phenanthrene ratio alone is an excellent indicator of source rock lithology with carbonates having ratios 1 and shales having ratios the dibenzothiophene to phenanthrene and the pristane to phytane ratios can also be used to classify source rock paleodepositional environments the classification scheme is based on the premise that these ratios reflect the different eh ph regimes resulting from the significant microbiological and chemical processes occurring during deposition and early diagenesis of sediments the dibenzothiophene phenanthrene ratio assesses the availability of reduced sulfur for incorporation into organic matter and the pristane phytane ratio assesses the redox conditions within the depositional environment interpretation of these ratios has been aided by quantitative biomarker analysis and by carbon isotope data for pristane and phytane obtained by gas chromatography isotope ratio mass spectrometry

carbon isotope composition of lower palaeozoic kerogen effects of maturation abstract the carbon isotope composition of type i kerogen from a marine organic rich black shale deposit of late cambrian age the alum shale formation of southern scandinavia varies from 28 to 30 pdb the thermal maturity is rated by hi oi index t max pi and vitrinite reflectance values and published conodont alteration data the results range from immature conditions with regard to oil generation in central sweden to overmature and submetamorphic conditions in southern sweden denmark and norway in the immature areas the shale is found to be extremely hydrogen rich and oil prone hydrocarbon potential up to 85 kg per ton rock the observed 13 variations are not correlated with maturity it is therefore concluded that thermal maturation processes have no influence on the isotopic composition of a purely marine kerogen the observed range in lower palaeozoic 13 kerogen values 27 to 31 is in accordance with published results from other sedimentary basins

irradiation of organic matter by uranium decay in the alum shale sweden the alum shale of sweden contains black shales with anomalously high uranium concentrations in excess of 100 ppm syngenetic or early diagenetic origin of this uranium indicates that organic matter within these shales has been irradiated by decaying uranium for approximately 500 ma radiation induced polymerization of alkanes through a free radical cross linking mechanism appears to be responsible for major alterations within the irradiated organic matter specific radiation induced alterations include generation of condensate like oils at reduced yields from hydrous pyrolysis experiments decrease in atomic h c ratios of kerogens decrease in bitumen organic carbon ratios and a relative increase in low molecular weight triaromatic steroid hydrocarbons conversely stable carbon isotopes of kerogens reflectance of vitrinite like macerais oil generation kinetics and isomerization of 20r to 20s c29 steranes were not affected by radiation the radiation dosage needed to cause the alterations observed in the alum shale has been estimated to be in excess of 105 mrads with respect to organic carbon this value is used to estimate the potential for radiation damage to thermally immature organic matter in black shales through the geological rock record high potential for radiation damage is not likely in cenozoic and mesozoic black shales but becomes more likely in lower paleozoic and precambrian black shales

the pyeongan supergroup upper paleozoic lower triassic in the okcheon belt korea a review of stratigraphy and detrital zircon provenance and its implications for the tectonic setting of the eastern sino korean block abstract the tectonic setting of the okcheon belt in the korean peninsula has long been regarded as one of the important topics in discussing east asian tectonics the controversial tectonic setting of the okcheon belt before the triassic collision between north and south china limits proper understanding of the late paleozoic tectonics of east asia focusing on the upper paleozoic to lower triassic pyeongan supergroup coeval sedimentary records in different parts of the okcheon belt are reviewed this study has a clear advantage over previous assessments that build upon comparison between meta sedimentary units of unknown age in the region the pyeongan supergroup sediments show high similarity in both stratigraphy and detrital zircon u pb ages throughout the okcheon belt they are distinct from their age equivalents in south china while sharing characteristics with those in north korea and north china critical criteria include marine influence and enrichment of organic materials at each time period and relative proportion of mesoproterozoic silurian components in detrital zircon age population the combined results agree with the view that the entire okcheon belt was a single tectonic unit at least by the late carboniferous the results of this study favor the idea of the sino korean block encompassing the whole korean peninsula and cast doubts on the tectonic models that attribute the southern part of the korean peninsula to the south china block a retroarc foreland basin setting of the okcheon belt with detritus mainly provided from a subduction related orogen in the continent sided japan is inferred from the provenance characteristics of the detrital zircons

recurrent rare earth element mineralization in the northwestern okcheon metamorphic belt korea shrimp u th pb geochronology nd isotope geochemistry and tectonic implications abstract rare earth element ree mineralization is hosted within neoproterozoic alkaline metaigneous rocks in the northwestern part of the okcheon metamorphic belt omb a polymetamorphosed fold and thrust belt transecting the paleoproterozoic gyeonggi and yeongnam massifs in the southern korean peninsula the principal carrier phase of rees is allanite allanite grains can be subdivided into several types based on the texture and mineral assemblage including quartz k feldspar biotite britholite apatite fergusonite andradite magnetite zircon titanite and fluorite electron microprobe analysis of allanite clearly distinguishes sample to sample variations in total rees ca al fe and y but the textural varieties from each rock sample do not show marked differences in those elements sensitive high resolution ion microprobe dating of allanite and zircon reveals a complex history of multistage mineralization allanite grains from ree ores yielded late ordovician 444 6 8 0 ma permian to triassic ca 300 220 ma and early jurassic 199 183 ma 208pb 232th ages these multiple age components often coexist in single grains showing slight differences in backscattered electron brightness the ordovician components have distinctly higher th u than the younger domains in the same rock sample the cores and rims of zircon from a syenite hosting ree ore bodies yielded neoproterozoic 858 2 6 3 ma and early jurassic ca 190 ma 206pb 238u ages respectively the early jurassic ages 194 187 ma also obtained from zircon grains from granites taken from dykes occurring close to the ores and a drill core indicate the correspondence between granitic magmatism and ree mineralization the neoproterozoic zircon inheritance weighted mean 853 9 3 8 ma in these granites is in sharp contrast to the dominant paleoproterozoic inherited zircon from the widespread earliest middle jurassic granites enclosing the mineralized zone the geotectonic significance of the late ordovician event recorded in the allanite as well as in detrital zircon from the omb is still unclear but its temporal coincidence with intraplate volcanism and arc related igneous activity respectively reported from the southwestern edge of the adjacent taebaeksan basin and the southwestern gyeonggi massif is noteworthy the following permian triassic and early jurassic mineralization events are probably linked to the continental suturing between the north and south china blocks and subsequent post orogenic magmatism and arc magmatism resulting from the paleo pacific plate subduction respectively sub grain sm nd isotopic analyses of allanite by laser ablation multiple collector icpms yielded initial end values plotting along the nd isotopic evolution path of the neoproterozoic metaigneous rocks indicating that rees originating from the host rock have been recycled during multistage mineralization events the profound differences in inherited zircon ages and nd isotopic compositions between the early and middle jurassic granites may reflect the presence of a major thrust bounded crustal structure beneath the omb

mineral ages and zircon hf isotopic composition of the andong ultramafic complex implications for the evolution of mesozoic subduction system and subcontinental lithospheric mantle beneath se korea the phanerozoic subduction system of the korean peninsula is considered to have been activated by at least middle permian time the geochemically arc like andong ultramafic complex auc occurring along the border between the precambrian yeongnam massif and the cretaceous gyeongsang back arc basin provides a rare opportunity for direct study of the pre cretaceous mantle wedge lying above the subduction zone the tightly constrained shrimp u pb age of zircons extracted from orthopyroxenite specimens 222 1 1 0 ma is indistinguishable from the ar ar age of coexisting phlogopite 220 6 ma these ages represent the timing of suprasubduction zone magmatism likely in response to the sinking of cold and dense oceanic lithosphere and the resultant extensional strain regime in a nascent arc environment the nearly coeval occurrence of a syenite gabbro monzonite suite in the sw yeongnam massif also suggests an extensional tectonic setting along the continental margin side during late triassic time the relatively enriched hf range of dated zircons 6 2 to 0 6 at 222 ma is in contrast to previously reported primitive sr nd hf isotopic features of cenozoic mantle xenoliths from korea and eastern china this enrichment is not ascribed to contamination by the hypothetical palaeozoic crust beneath se korea but is instead attributable to metasomatism of the lithospheric mantle during the earlier subduction of the palaeo pacific plate most auc zircons show a restricted core to rim spread of hf values but some grains testify to the operation of open system processes during magmatic differentiation

late vendian early palaeozoic tectonic evolution of the baltic basin regional tectonic implications from subsidence analysis abstract subsidence analysis was performed on 43 boreholes penetrating the upper vendian lower palaeozoic sedimentary succession of the baltic basin the results were related to lithofacial and structural data to elucidate subsidence mechanisms and the regional tectonic setting of basin development tectonic subsidence patterns are consistent throughout the basin for the time period studied an extensional tectonic subsidence event possibly of two phases is indicated from the late vendian to the beginning of the middle cambrian this event is seen in the southwestern part of the baltic basin peri tornquist zone until the earliest cambrian after which it is also observed in the sw ne trending baltic depression part of the basin basin development during this time is interpreted as recording the latest stages of break up of the precambrian super continent rodinia and ultimately the formation of the tornquist sea the late middle cambrian to middle ordovician tectonic subsidence pattern of the baltic basin is characteristic of post rift thermal subsidence of the newly formed passive continental margin of baltica developed along its southwestern edge a gradual increase in subsidence rate is observed from the middle late ordovician and throughout the silurian particularly for ludlow and pridoli times creating subsidence curves with convex shapes typical of foreland basin development the rate of late silurian tectonic subsidence increases significantly towards the southwest margin of the baltic basin adjacent to the present location of the north german polish caledonides the baltic basin therefore appears to have developed primarily as a flexural foreland basin during silurian oblique collision of baltica and eastern avalonia a foreland setting is supported by the influx of distal turbidites into the basin from southwest sources in the late silurian compressional deformation structures of early devonian lochkovian age are seen in seismic sections in the central part of the baltic basin lithuania these together with a change in subsidence pattern mark the end of the caledonian stage of basin development of the baltic basin

reflectance of vitrinite like macerals as a thermal maturity index for cambrian ordovician alum shale southern scandinavia the middle cambrian to lower ordovician alum shale in southern scandinavia contains kerogen macerals resembling vitrinite the age of this rock unit excludes terrestrially derived vitrinite as an origin for these macerals nevertheless the vitrinite like macerals respond to heating in a way similar to that of suppressed vitrinite in younger shales this similarity is demonstrated both for the natural system where alum shale maturity ranges from immature to low grade metamorphic conditions and for hydrous pyrolysis experiments where an immature sample of alum shale was isothermally heated at temperatures ranging from 280 to 360 degrees c for 72 hr reflectance values from the naturally matured alum shale show an increase in standard deviations i e anisotropy with i creasing maturity and a negative relationship with atomic h c ratios of the total kerogen these relationships are similar to those observed in post ordovician vitrinites comparisons with other vitrinites subjected to hydrous pyrolysis indicate the vitrinite like macerals in the alum shale follow the suppressed vitrinite pathway rather than the coal vitrinite pathway the occurrence of vitrinite like macerals in the alum shale indicates that traditional vitrinite reflectance measurements are useful in evaluating thermal maturity of pre silurian rocks

organic geochemistry depositional history and hydrocarbon generation modelling of the upper permian kupferschiefer and zechstein limestone strata in south west poland abstract the upper permian zechstein limestone carbonates in the south western part of the polish zechstein basin contain small quantities of organic matter usually less than 0 1 wt toc and have poor hydrocarbon potential in contrast the kupferschiefer shales have excellent hydrocarbon potential with toc as high as 10 8 wt predominantly algal type ii kerogen generally thermal maturity of the organic matter is 0 6 0 9 vitrinite reflectance while locally is as high as 1 45 the kimmerian and laramian tectonic events significantly influenced the regional burial and thermal history the main subsidence of the kupferschiefer zechstein limestone complex took place during the late permian late triassic period with additional burial during the jurassic and late cretaceous hydrocarbon generation from the kupferschiefer source rocks occurred from middle triassic to late jurassic times reaching maximum rates during the jurassic the cretaceous subsidence had only a minor influence on the maturity and transformation of organic matter expulsion started between late triassic in the western part of the study area and early jurassic in the eastern part then ceased by the late jurassic early cretaceous erosion the total amount of generated and expelled hydrocarbons from source rocks was small because their thickness was limited therefore kupferschiefer shales cannot be considered as source rocks for liquid hydrocarbons but may be a potential source for gas accumulated in the zechstein limestone and rotliegend reservoirs

the isotopic composition of yb and the precise and accurate determination of lu concentrations and lu hf ratios by isotope dilution using mc icpms 1 we report yb isotopic data for the purpose of accurately correcting measurements of lu and lu hf ratios during multicollector inductively coupled plasma mass spectrometer mc icpms analysis the yb isotopic ratios agree with recent results of chu et al 2002 and amelin and davis 2004 when corrected for mass fractionation to the same reference values protocols for lu measurement involving simultaneous subtraction of yb interference and application of a fractionation correction based on yb are described it is shown that lu concentrations and lu hf ratios determined by mc icpms using this methodology are more precise and potentially more accurate than results based on analysis of lu by thermal ionization mass spectrometry tims a number of mc icpms lu hf isotopic analyses of standard rocks and other geological samples previously analyzed by tims are presented these document reproducibility of lu hf within 0 2 where most discrepancies can be assigned to the unknown lu mass fractionation during tims analysis