origin of maghaemite in chinese loess deposits aeolian or pedogenic maghaemite and magnetite common ferrimagnetic minerals in surficial soils have physical properties which are similar except for the thermal instability of the former utilising this exception a study of thermomagnetic properties has been made to systematically separate these two magnetic components in loess and palaeosol samples from the upper parts of the luochuan section of the central chinese loess plateau this investigation has shown that although maghaemite thermally unstable component occurs in greater amounts in palaeosols ms 0 0187 am2 kg 1 1 1 than in loess units ms 0 0156 am2 kg 1 1 1 in relative terms its contribution to the ferrimagnetic component of magnetisation is higher in the loess units ms 32 2 2 0 than the soils ms 23 5 3 7 such an observation suggests that much of the maghaemite component originates in the aeolian source area with some addition of pedogenic maghaemite occurring in the palaeosols

the paleoclimatic record provided by eolian deposition in the deep sea the geologic history of wind the mineral component of pelagic sediment is brought to the deep sea by transport in the wind extraction and analysis of this dust allows estimation of the past aridity of the eolian source region via flux determinations and of the intensity of the transporting winds from grain size data these two parameters the grain size and mass flux of dust vary independently there are three significant sources of modern dust eastern and central asia northwest africa and arabia all in the northern hemisphere as the rainfall associated with the intertropical convergence zone is an effective barrier to southerly transport of dust the northern hemisphere is an order of magnitude more dusty than is the southern an asymmetry that has characterized most of the cenozoic eolian flux data show that most of the northern hemisphere was more arid during glacial maxima with 3 to 5 times as much dust transported during glacial stages than during interglacials only northwestern south america varied in the opposite sense the periodicity of quaternary variation in both eolian flux and eolian grain size data is strongly influenced by the milankovitch cycles of orbital variability wind intensities vary on a shorter time scale than the general 100 kyr cycles of glaciation and general aridity eolian grain sizes display forcing at precessional 19 and 23 kyr tilt 41 kyr and at approximately 30 kyr periodicities as a result the generalization that winds are uniformly stronger during glacial times is not valid whole cenozoic records show that the largest change in dust flux an order of magnitude increase occurred in the northern hemisphere and reflects continental drying associated with the late pliocene onset of northern hemisphere glaciation southern hemisphere eolian records show no sign of paleoclimatic changes in the late pliocene the most important change in cenozoic atmospheric circulation was a severalfold reduction in wind intensity that occurred at the time of the paleocene eocene boundary before then latest cretaceous and paleocene winds were essentially as strong as those of the late cenozoic this shift appears to be one of several climatic responses to a change in the nature and amount of global heat transport at about 55 ma

age of amphibolitic metamorphism in the ophiolitic unit of the morais allochthon portugal implications for early hercynian orogenesis in the iberian massif structurally lower portions of the allochthonous morais complex are comprised of an ophiolitic unit exposed in two contrasting structural units separated by a ductile thrust fault the lower unit is dominated by variably foliated amphibolite whereas the upper unit is largely comprised of metaperidotite and metagabbro most of the ophiolitic unit has recrystallized synkinematically under amphibolite facies metamorphic conditions c 500 c and 5 kbar amphiboles formed during this metamorphism are typically magnesio hornblende an amphibole concentrate from the lower structural unit records a 39 ar 40 ar plateau age of 397 3 4 ma an amphibole concentrate from the upper structural unit displays an internally discordant 39 ar 40 ar age spectrum corresponding to a total gas age of 431 1 27 3 ma gas fractions evolved from each amphibole concentrate at intermediate and high experimental temperatures are characterized by similar apparent k ca ratios and yield well defined 36 ar 40 ar v 39 ar 40 ar isotope correlations which result in similar ages of 384 2 5 2 and 392 4 7 3 ma these are considered to be more reliable than the plateau ages and are interpreted as dating the last cooling through those temperatures required for intracrystalline retention of argon within constituent amphibole grains the associated tectonothermal event represented a first stage of hercynian orogenesis in the iberian massif it was probably related to oceanic closure and associated obduction of granulitic eclogitic massifs which at present structurally overlie the ophiolitic unit

iron oxide characteristics of the chinese loess red clay sequences and their implications for the evolution of the east asian summer monsoon since the late oligocene abstract the most significant climatic change in east asia involved the formation and evolution of the coupled monsoon arid environmental system during the cenozoic which was dynamically linked to global cooling and the growth of the qinghai tibet plateau however our understanding of monsoon evolution since the late oligocene remains controversial due to differences in the interpretation of various environmental proxies from loess archives in this study the composition and relative abundance of pedogenic iron oxides of two eolian sequences from the lingtai section and the zhuanglang drill core were investigated using rock magnetic optical spectroscopy and geochemical analyses the results indicate that while the composition of iron oxides in the mio pliocene red clay and quaternary loess paleosol sequences is similar the concentrations of magnetite maghemite hematite and goethite are quite different between loess paleosol and red clay the combination of magnetic proxies magnetic susceptibility fed fet and color a values reveals that precipitation and temperature played varying roles in driving pedogenesis and chemical weathering since the late oligocene during the late oligocene to early miocene the occurrence of relatively strong pedogenesis and chemical weathering was likely the result of high temperatures rather than high precipitation the east asian summer monsoon intensified during the middle miocene climate optimum 16 5 14 5 ma in response to global climatic warming subsequently during the middle to late miocene the monsoon intensity gradually weakened due to global cooling after 8 ma the summer monsoon strengthened substantially and reached a maximum during 4 5 2 6 ma possibly as a result of the uplift of the northeastern margin of the qinghai tibetan plateau during the late miocene the monsoon intensity then commenced a decreasing trend in the early pleistocene but then strengthened again after 1 0 ma

estimating superparamagnetism in marine sediments with the time dependency of coercivity of remanence superparamagnetic grains ultrafine 50 nm particles that do not retain a magnetic remanence at room temperature can form through several biogeochemical pathways in marine sediments knowing the distribution of such particles therefore provides a means of better understanding the past diagenetic history of sediments and evaluating their potential record of environmental change herein we employ magnetic hysteresis measurements in a new way to determine the abundance of superparamagnetic sp grains we start with a phenomenological model that describes a dependency of coercivity of remanence hcr with measurement time as a function of superparamagnetic grain content calculated time dependencies of hcr agree with experimental data suggesting that the model can be used to derive quantitative sp estimates when applied to pelagic sediments of the western equatorial pacific ocean ocean drilling program site 805c the time dependencies of hcr reveal a 25 30 sp increase starting just above the modern iron redox boundary irb next we compare our approach with a previous method that detects sp grains through the thermal demagnetization of saturation remanence acquired at very low temperature 20 k we find this method fails to record the sp changes detected using the time dependency of hcr and suggest that the cause is a sensitivity of the low temperature data to small differences in magnetic mineralogy that accompany diagenesis specifically differential maghemitization causes varying suppression of the remanence changes associated with the verwey transition at 120 k the removal of maghemite shells on magnetic grains below the irb results in greater remanence changes at the verwey transition and a corresponding sp underestimate using the low temperature method in addition some iron oxyhydroxides paramagnetic at room temperature can become ferromagnetic at very low temperatures resulting in an sp overestimate in oxic sediments above the irb the increase of sp content in the zone of magnetic mineral reduction observed with the time dependency of hcr may be related to the breakdown of larger magnetic grains and the activity of dissimilatory iron reducing bacteria superparamagnetism measured by the time dependency of hcr may be useful for tracking the history of these biogeochemical processes in marine sediments

triassic 40ar 39ar ages from the sakaigawa unit kii peninsula japan implications for possible merger of the central asian orogenic belt with large scale tectonic systems of the east asian margin the 218 4 0 4 228 8 0 9 and 231 9 0 7 ma 40ar 39ar laser probe pseudo plateau ages 2 49 63 39ar release of very low grade meta pelitic whole rocks from the sakaigawa unit date high p t metamorphism we argue that this event occurred in a subduction accretion complex not along the east asian continental margin but on the pacific side of the proto japan superterrane proto japan was a permian magmatic arc presently dispersed in the japanese islands which also contained older subduction accretion complexes the arc system was fringing but not yet part of the eurasian continent the middle to late triassic high p t tectono metamorphic event was partly coeval with proto japan s collision with proto eurasia along the southward extension of the central asian orogenic belt causing the main metamorphism in the hida oki terrane it is possible that this system continued via the cathaysia block china to indochina the late permian to middle triassic indosinian event might stem from docking of pacific derived terranes with southeast asia s continental margin the concept of the proto japan superterrane implies that the qinling dabie sulu suture zone joined the central asian orogenic belt to the east of the north china craton and did not continue to japan as commonly assumed

loess stratigraphy in central china abstract loess sequences in central china provide a detailed record of climate changes during the last 2 5 million years at least 44 major shifts from glacial to interglacial conditions occurred during this time in china prolonged loess deposition lasting more than 40 millennia occurred eleven times with only four of these intervals in brunhes chron the climate oscillations took place with increased frequency between 2 4 and 0 5 ma compared to the last half million years major shifts toward cooler and drier climates occurred at about 2 4 ma 1 2 ma and 0 5 ma while less pronounced shifts are dated to 1 65 ma 0 8 ma and 0 2 ma three of the last four reversals of the earth s magnetic field whose lithostratigraphic position has been detail are recorded in a glacial deposit the low field magnetic susceptibility of loess and soils which reflects the concentration of ultrafine magnetite is related to the degree of pedogenesis and serves as a proxy measure of climate the accumulation rate of the magnetite was fairly constant throughout the last 1 2 million years and can serve as a measure of time susceptibility records from localities near xifeng and luochuan located 160 km apart are compared and used to define informal lithostratigraphic units in the loess plateau of china

40ar 39ar mineral ages from the oki metamorphic complex oki dogo southwest japan implications for regional correlations abstract the oki metamorphic complex exposed in the oki dogo islands consists predominantly of psammitic and pelitic gneisses with subordinate amphibolite and rare calcareous gneiss the oki gneisses were regionally metamorphosed to general amphibolite facies conditions with local development of granulite facies assemblages peak metamorphic conditions of c 800 c have been suggested hornblende concentrates from amphibolites collected within the oki metamorphic complex record 40 ar 39 ar isotope correlation ages of 199 192 ma these are interpreted to date the post metamorphic cooling through temperatures required for intracrystalline retention of argon c 500 c muscovite concentrates record 40 ar 39 ar plateau ages of 167 168 ma these are interpreted to date post metamorphic cooling through appropriate closure temperature of muscovite c 400 375 c combined with the previously reported geochronological data the oki metamorphic complex appears to have experienced peak metamorphic conditions at c 250 ma subsequently it cooled and was exhumed at the earth s surface at c 90 ma with cooling rate of c 5 c ma the oki metamorphic complex records a similar prograde metamorphic event as the hida metamorphic complex exposed in central japan the cooling and exhumation rates of the hida metamorphic complex were significantly more rapid compared with the oki metamorphic complex and they were exhumed with extensively intruded jurassic granites funatsu granites

subdivision of the hida metamorphic complex central japan and its bearing on the geology of the far east in pre sea of japan time abstract petrological and structural characteristics of the regional metamorphic rocks in south west japan and in the korean peninsula make it possible to speculate on the geological correlation between japan and the asian continent prior to the opening of the sea of japan a typical marginal sea the hida metamorphic complex situated on the sea of japan side of southwest japan is subdivided into two distinct geological units the hida gneisses and the unazuki schists the hida gneisses are polymetamorphosed precambrian rocks while the unazuki schists occurring on its eastern and southern sides are low to medium grade metamorphic rocks originating from upper paleozoic deposits sedimentary facies and other geological features suggest that the hida gneisses were the basemen of the unazuki schists consequently the geotectonic framework of southwest japan is revised from north to south as follows hida gneiss region precambrian massif unazuki zone late permian intermed p t metamorphic belt circum hida tectonic zone mid to late paleozoic melange zone mino tanba terrain late paleozoic to mesozoic geosynclinal region and so on the unazuki zone is similar to the okcheon zone in the korean peninsula in respect of age lithology and biofacies of sedimentary rocks as well as the age and type of regional metamorphism furthermore the hida gneisses and the gyeonggi polymetamorphosed precambrian gneisses in the korean peninsula are similar in the apparent baric type of metamorphism radiometric ages and the relationship with the overlying metamorphosed formations the similarity of the geotectonic frameworks of southwest japan and the korean peninsula suggests that the unazuki zone and the okcheon zone once formed a continuous geotectonic unit thus we have a new geological coordinate in reconstructing the paleogeography prior to the opening of the sea of japan

magnetic enhancement and palaeoclimate study of a loess palaeosol couplet across the loess plateau of china summary the origin of the milankovitch palaeoclimatic signal so dramatically recorded in the chinese loess remains uncertain at least five hypotheses have been put forward to explain the well documented enhancement of magnetic susceptibility in palaeosols which formed when conditions were warmer and wetter we report here a magnetic study of a prominent loess soil couplet l4 s3 at three widely separated sites where present day mean annual rainfall and temperature range from 350mm to 700mm and from 6 c to 13 c the properties determined are laboratory remanences isothermal irm anhysteretic arm viscous vrm including viscous decay room temperature susceptibility and thermomagnetic behaviour at fields beyond 100 mt the irm spectra are all essentially identical both in shape and in absolute magnitude suggesting a uniform magnetic component widespread across the plateau subtracting the signal of this component isolates a second component the amount of which varies by an order of magnitude between sites the magnetic properties of this latter material are remarkably similar to those reported for bacterial magnetite in geological sediments and modern soils viscous decay of vrm takes place in two distinct stages one of which falls to zero in 10min we attribute magnetic enhancement which is the source of the palaeoclimatic signal to in situ pedogenetic formation of magnetite which we characterize in our interpretation as two separate forms of bacterial magnetofossils the most magnetic sample requires a mass fraction of about half of one per cent of this biogenic material

mossbauer spectroscopy analysis of iron containing minerals in the chinese loess chinese loess has been analyzed in bulk using mossbauer spectroscopy in order to investigate the effects of climate on the overall iron containing mineralogy strongly weathered loess is found to differ from weakly weathered loess in two significant respects it contains a much larger content of fine grained superparamagnetic hematite and the ratio of paramagnetic fe 3 fe 2 is substantially increased the suggestion is that weathering results in the liberation of fe from silicate minerals and the formation of fine grained hematite measurements of total iron content by atomic absorption spectrometry highlight that the strongly weathered loess has a higher content of iron than the weakly weathered loess and that this is too large an effect to be explained by the leaching of carbonate the indication is that the mineralogy of the dust input to the loess plateau varies as a function of climate one possibility is that the grain size and hence mineralogy of the dust fluctuates in response to vegetational cover in proximal source areas during interglacial times the vegetational cover would expand to stabilize these proximal sources resulting in a bias to finer dust from more distant sources thus the mechanisms linking climate and mineralogy in the chinese loess may be more complex than previously thought

mineral magnetic properties of loess paleosol couplets of the central loess plateau of china over the last 1 2 myr 1 we have conducted a multiparameter investigation of 15 loess paleosol couplets s0 l1 to s14 l15 from the jiaodao section in the central loess plateau of china using environmental magnetic approaches coupled with soil science techniques the magnetic parameters display systematic variations that seem to be closely related to paleoclimate variations and intensity of pedogenesis high temperature susceptibility curves of paleosols show a generally decreasing trend in reversibility from the base of the lishi formation to the holocene black loam possibly indicating a decrease in weathering intensity this may reflect a long term increase in aridity and or a general long term cooling trend of the interior of the asian continent from 1 2 ma to the present several samples display wasp waisted hysteresis loops these are most pronounced in moderately enhanced paleosols less pronounced in the practically unaltered loess and subdued in the well developed paleosols but wasp waistedness reappears in the most developed paleosols this wasp waistedness sequence suggests that the composition concentration and grain size of magnetic minerals all contribute to the hysteresis behavior of samples from the studied loess paleosol sequence but each factor has a different effect at different stages of pedogenesis

polyphase late paleozoic tectonothermal evolution of the southwestern meguma terrane nova scotia evidence from 40ar 39ar mineral ages 40ar 39ar incremental release ages of hornblende muscovite and biotite from a variety of granitic stocks and host metamorphic rocks suggest a complex late paleozoic tectonothermal evolution for the southwestern meguma terrane regional d1 folding with cleavage formation under greenschist lower amphibolite facies m1 metamorphic conditions occurred at ca 400 410 ma and was followed by emplacement of a series of granitic stocks ranging in age between ca 375 and 315 ma these were emplaced at relatively shallow crustal levels and developed contact metamorphic aureoles of variable grade these are locally superposed on m1 regional metamorphic assemblages and result in a complex isograd pattern 40ar 39ar mineral ages suggest episodes of contact metamorphism occurred at 1 360 375 ma possibly related to emplacement of the south mountain batholith or temporal equivalents 2 350 356 ma around the port mouton pluton and northeastern shelburne pluton 3 ca 315 325 ma near the wedgeport pluton and in

age and palaeoclimatic significance of the loess of lanzhou north china extending in a broad arc that abuts the sandy non gobi deserts the loess plateau of northern china1 3 is one of the most massive accumulations of loess in the world the loess sequence is typically characterized by an alternation of silty or sandy loess with more clay rich palaeosols these alternations in conjunction with their enclosed faunas and distinctive mineralogies have been interpreted as reflecting pleistocene glacial interglacial cycles3 8 because it holds implications for the climatic and anthropological history of china9 10 the definition of a reliable chronological framework for loess deposition is of great interest recent palaeomagnetic studies4 5 have indicated that loess deposition in shaanxi province commenced 2 4 myr ago to assess the synchrony of loess accumulation across the loess plateau we have dated a 330 m thick loess sequence near lanzhou gansu province the magnetostrati graphical results reported here indicate that the base of this loess succession dates from 1 3 my r ago this young age in comparison to the shaanxi sequence is attributed to uplift along the northern fringe of the tibetan plateau11 that precluded early pleistocene loess preservation in this mountainous region palaeosols in the basal loess occur on average once every 25 kyr suggesting that climates conducive to soil forming events may have been modulated by orbital precession in the early pleistocene

magnetic enhancement paths in loess sediments from tajikistan china and hungary saturation magnetization and saturation remanence of 139 loess and paleosol samples from tajikistan china and hungary increase linearly with low field susceptibility in all sections investigated thus magnetic enhancement is caused by increasing quantities of ferrimagnetic minerals the observed variations of high field susceptibility h are attributed to variations in the amount of clay minerals as h is linearly correlated with the 2 m grain size mineral fraction the magnetic enhancement process is compared with a theoretical mixing model based on two magnetic mineral assemblages with different but invariable coercivities the two component model provides a good approximation to the data from hungary and the magnetically enhanced paleosol samples in tajikistan and china discrepancies between model calculation and data arise in the behaviour of the loess samples in tajikistan here a three component model improves the agreement between data and mixing calculation the magnetic enhancement in the loess in tajikistan and china is different from the one in the paleosols and the link between paleoclimate and susceptibility is not as straightforward as in the hungarian loess paleosol sequence

progressive metamorphism of the unazuki pelitic schists in the hida terrane central japan four progressive mineral zones i chloritoid quartz ii staurolite chlorite muscovite iii kyanite biotite and iv sillimanite muscovite have been mapped in a well defined stratigraphic unit of ferruginous pelitic composition from north to south over a distance of about 15 km in the unazuki area hida terrane central japan textural and chemical evidence indicates that a given metamorphic mineral assemblage was formed from the mineral assemblage that precedes it in a spatial sequence of mineral zones

hirm variations in the chinese red clay sequence insights into pedogenesis in the dust source area abstract although extensive rock magnetic studies have been performed on the chinese loess paleosol sediments few comparable studies have been applied to the underlying red clay sediments because of this paucity of data our understanding of the mineral magnetic properties of the red clay sequence and the paleoenvironmental information encoded in them is limited here we report hard isothermal remanent magnetization hirm variations from 8 1 to 1 2 ma in the lower part of the loess paleosol and the underlying red clay sequences of the chaona section on the chinese loess plateau the results indicate that there is a negative correlation between hirm and frequency dependence of magnetic susceptibility fd fd lf hf where lf and hf are magnetic susceptibility measured at 470 hz and 4700 hz respectively from 3 4 to 2 6 ma and a positive correlation between these two proxies from 8 1 to 3 4 ma and 2 6 to 1 2 ma we argue that the hirm variations from 8 1 to 3 4 ma and from 2 6 to 1 2 ma reflect hematite availability in the dust source region determined by the size of the area and climatic conditions of the dust source region the negative correlation between hirm and fd between 3 4 and 2 6 ma suggests that magnetic enhancement during this time interval may be related to the alteration of hematite goethite to magnetite maghemite we also find that hirm data between 8 1 and 3 4 ma show high coherence with the 100 kyr eccentricity signals suggesting eccentricity forcing of concentration variations of hematite in the dust source area higher resolution samples are needed to determine if concentration variations of hematite in the dust source area are coherent with the orbital obliquity and precession forcing signals

glacial deposits at wylfa head anglesey north wales evidence for late devensian deposition in a non marine environment the nature and origin of glacial sediments at wylfa head are described and their significance with regard to sedimentary environments during late devensian deglaciation of the irish sea basin is discussed recent models of deglaciation under glaciomarine conditions are challenged the quaternary sequence at wylfa consists of eroded and glaciotectonically deformed bedrock locally derived lodgement till calcareous silt rich lodgement till containing northern erratics discontinuous units of orange brown silty sand of possible aeolian origin and grey laminated freshwater silts filling a small kettle hole the till units thicken to the south where the surface is drumlinised it is concluded that the landforms and deposits result from a warm based irish sea glacier which moved towards the southwest spatial variation in basal water pressure resulted from localised drainage through zones of more heavily jointed bedrock rapid glacial erosion occurred in areas where subglacial water pressure was relatively high while deposition of the resulting basal sediment took place where water pressures were reduced the glacier also carried basal calcareous silty till onshore which was deposited by lodgement processes none of the deposits at wylfa are interpreted as glaciomarine in origin and there is no evidence at this site for an isostatically induced marine transgression prior to deglaciation

a multi proxy study of argentina loess marine oxygen isotope stage 4 and 5 environmental record from pedogenic hematite abstract a 12 m loess paleosol section la angostura from tafi del valle tucuman argentina was studied using rock magnetic measurements geochemistry mossbauer effect particle size and nd isotopes in order to elucidate changes in paleoenvironment at the site combining the results of these experiments allowed us to separate the effects of the two major controls parent material and pedogenesis on the measured parameters two behavior regimes were identified in the sediment 1 in the lower half of the section pedogenic processes though weak dominate the variation of the sediment s physical properties and 2 for the upper half of the section subtle changes in parent material control the majority of the measured parameters even those typically controlled by pedogenesis e g clay fraction applying rock magnetic mossbauer and bulk geochemistry measurements in concert we calculated an iron budget for the loess 80 of the iron in the sediment is in paramagnetic minerals 20 of the iron is in hematite 1 6 of the sediment s mass and 1 of the total iron is present as magnetite maghemite 0 1 of the sediment s total mass neither the pedogenic formation nor the depletion of ferrimagnetic material with increasing pedogenesis was consistently observed in this sediment throughout the entire section the concentration of hematite is argued to be sensitive to in situ alteration and it is further surmised to be sensitive to the available moisture at the site finally we present a curiosity within the error of our chronology the concentration of hematite seems to correlate with the coeval climate record from antarctica vostok it is possible that the sedimentary magnetic record from la angostura is a proxy for the strength of the south american summer monsoon

cooling and inferred uplift erosion history of the grenville orogen ontario constraints from 40ar 39ar thermochronology stepwise 40ar 39ar degassing experiments of 57 mineral separates of hornblende muscovite biotite and perthitic microcline have been used in conjunction with petrologic observations to place regional constraints on the postmetamorphic cooling and the inferred uplift and erosion history of the grenville orogen in ontario the 40ar 39ar data support an interpretation of slow nearly uniform cooling 1 4 c m y from temperatures of 500 c to below 150 c in the central gneiss belt cgb hornblendes cooled through ar closure between 930 and 1025 ma whereas in the central metasedimentary belt cmb hornblendes record the following range in 40ar 39ar cooling ages 1104 ma in the frontenac terrane 1007 1067 ma in the sharbot lake terrane 919 1026 ma in the elzevir terrane and 972 ma in the central metasedimentary belt boundary zone regional uplift erosion rates of 0 03 0 14 km m y have been estimated for the grenville orogen in ontario based on the 40ar 39ar data a model retrograde p t path for rocks of the cgb and an upper time constraint provided by flat overlying cambrian and ordovician sediments these erosion rates are consistent with rates estimated for other proterozoic or archean granulite terranes but are an order of magnitude slower than active orogens such as the alps and himalayas a regular variation in hornblende 40ar 39ar cooling ages is observed in rocks that traverse highly strained often mylonitic shear zones that separate the four major terranes of the cmb the pattern of 40ar 39ar ages is interpreted to reflect late tectonic extension consistent with field observations in the central metasedimentary belt boundary zone and elsewhere in the cmb up to 13 km of vertical displacement is inferred for some rocks in the cmb between the time they cooled below closure to argon diffusion in hornblende 500 c and their exposure at the surface 25 c

middle precambrian detrital monazite and zircon from the hida gneiss on oki dogo island japan their origin and implications for the correlation of basement gneiss of southwest japan and korea abstract monazite and zircon from seven paragneiss samples of the upper amphibolite facies grade from oki dogo island in the hida terrane western japan were analyzed for th u and pb on an electron microprobe most monazite grains are chronologically unzoned and give unequivocal chime chemical th u total pb isochron ages of 250 ma but some grains show a distinct core overgrowth relationship the cores retain evidence of pre 250 ma ages and are interpreted to be of detrital origin suggesting that monazite can be more immune to diagenetic and low grade metamorphic decompositions than had been thought previously zircon grains rounded to nearly euhedral give chime ages ranging from 3000 to 350 ma of particular interest are several rounded 3000 ma recycled zircons which perhaps originated from an archean terrain in the sino korean craton the youngest age of detrital zircons constrains that the gneiss protolith deposited after the 350 ma event the chime monazite and zircon ages for the paragneisses from oki dogo island and from the main part of the hida terrane reveal that the gneisses formed from permo carboniferous clastic sediments during a single metamorphism at ca 250 ma the results are in marked contrast to the current view that 1 the hida gneiss is a polymetamorphosed precambrian complex 2 the central part hida gneiss region and the eastern marginal part unazuki schist region of the hida terrane are correlative with the precambrian kyonggi massif and the paleozoic okchon zone in the korean peninsula respectively instead we consider that the okchon zone alone can be correlatable with the paleozoic hida terrane

40ar 39ar mineral age constraints for the tectonothermal evolution of the sambagawa metamorphic belt central shikoku japan a cretaceous accretionary prism abstract the sambagawa metamorphic belt exposed in central shikoku japan is tectonostratigraphically represented by the oboke and structurally overlying besshi nappe complexes oboke units consist of metamorphic rocks largely derived from clastic protoliths sandstone and conglomerate whereas besshi units are dominated by rocks with oceanic protolith affinities shale chert and greenstone whole rock schist samples from low grade oboke units record intermediate temperature 40 ar 39 ar plateau ages of 70 77 ma which are interpreted to closely date attainment of maximum metamorphic conditions whole rock schist samples from besshi units chlorite zone record plateau ages of 85 94 ma suggesting an earlier metamorphic climax within this structural level amphibole within the albite biotite zone in besshi records 36 ar 40 ar vs 39 ar 40 ar plateau isotope correlation ages of 84 87 ma which are interpreted as dating the post metamorphic cooling through temperatures required for intracrystalline retention of argon a slightly older isotope correlation age 94 ma is recorded by amphibole from the higher grade oligioclase biotite zone contrasts in amphibole cooling ages are interpreted as reflecting thrust related metamorphic inversion and resultant earlier cooling of higher grade structural units muscovite from all structural levels of the besshi nappe complex records 40 ar 39 ar plateau ages of 76 89 ma these are interpreted to date post metamorphic cooling through appropriate closure temperatures following structural assembly of the besshi nappe complex protoliths of the besshi units originated at approx 130 ma and attained maximum metamorphic conditions during entrainment in an accretionary complex at approx 90 100 ma higher grade sequences developed at deeper levels along the hanging wall of the accretionary complex during subsequent structural uplift to shallower crustal levels the besshi nappe complex was tectonically emplaced over oboke units

estimating the concentration of aluminum substituted hematite and goethite using diffuse reflectance spectrometry and rock magnetism feasibility and limitations this study was supported by the national natural science foundation of china 41374073 and 41430962 the national program on global changes and air sea interaction gasi 04 01 02 and the chinese continental shelf deep drilling program gzh201100202 pengxiang hu was further supported by the china scholarship council 2013 3009 david heslop and andrew roberts were supported by australian research council discovery project dp110105419

the role of maghemite in the enhancement of magnetic signal in the chinese loess paleosol sequence an extensive rock magnetic study combined with citrate bicarbonate dithionite treatment we have conducted a rock magnetic study of selected chinese loess samples before and after citrate bicarbonate dithionite cbd treatment our study is more extensive than previous ones and reveals new details of the magnetic carriers which are related to paleoclimate change three pairs of paleosol s1 the top paleosol layer in the lishi formation and loess l2 underlying s1 from three localities xining xifeng and jixian were investigated by using a variety of magnetic methods after the paleosols were treated with cbd which removes all maghemite grains and the finest grains of magnetite magnetic susceptibility decreased from 77 10 8 to 20 10 8 m3 kg in xining from 165 10 8 to 30 10 8 m3 kg in xifeng and from 180 10 8 to 25 10 8 m3 kg in jixian anhysteretic remanent magnetizations arm were reduced to less than one tenth of the original values in the three sites and saturation magnetization js decreased by about 60 in xining and 70 in xifeng and jixian these reduced values in the paleosols after cbd treatment are similar to those of the corresponding underlying loess samples which however did not show significant decrease after cbd treatment room temperature frequency dependence of susceptibility high temperature dependence of susceptibility and low temperature sirm demagnetization of the paleosol and loess samples before and after cbd treatment confirm that in addition to superparamagnetic sp particles magnetite and or maghemite the cbd removable magnetic carriers also include maghemite of grain sizes larger than sp

palaeoclimatic and sedimentary history from magnetic susceptibility of loess in china the magnetostratigraphic age of the oldest loess sediments in the central chinese loess plateau investigated so far has recently been placed at ca 2 4 million years b p thus glaciations in central asia which are believed to provide the detritus for the formation of chinese loess commenced at the same time as the first ice rafted debris sank into the ocean basins of the northern hemisphere shackleton et al 1984 lithologic palaeontologic geochemical and especially rock magnetic variations throughout the loess column are indicative of palaeoclimatic fluctuations during loess deposition the variations of magnetic low field susceptibility in loess closely resemble those of the marine oxygen isotope record shackleton and opdyke 1976 and establish for the first time a continuous record of the pleistocene glacial and interglacial stages in a continental section spectral analysis of the rock magnetic time series indicates slow apparent sedimentation during interglacial periods which are characterized mainly by calcium dissolution and related compaction of the soils variable aeolic sediment influx as well as erosion and or polygenetic soil formation may also strongly influence the sedimentary history

the magnetic properties of particle sized samples from the luo chuan loess section evidence for pedogenesis abstract measurements of magnetic susceptibility and of laboratory remanences have been carried out on particle size fractions of loess and palaeosol samples from luo chuan china they show that the sub micron clay fraction of the palaeosol is greatly enriched in fine grained

new contributions to chinese plio pleistocene magnetostratigraphy abstract a new collection of over 500 orientated hand samples of which 180 are red clays from a 195 m loess red clay section near xian 109 12 e 34 12 n china has been dated by magnetic stratigraphy biostratigraphic and previous magnetostratigraphic investigations gave a maximum age for the chinese loess of 2 4 ma implying that the underlying red clay formation is pliocene in age the present study yields a clearly defined magnetic polarity stratigraphy in good agreement with a polarity time scale developed by earlier workers an interpretation of this magnetostratigraphy suggests a basal age for the loess series in this region of 2 5 ma and a minimum age for the base of the red clay formation which is also the base of the section of 5 0 ma