paleomagnetism of the middle proterozoic electra lake gabbro needle mountains southwestern colorado the electra lake gabbro is a small 1 435 ga pluton that intrudes 1 7 to 1 6 ga gneisses and schists of the needle mountains in southwestern colorado paleomagnetic samples were collected from the main phases of the gabbro diabase dikes granite and alaskite dikes that cut the gabbro and from a partially melted zone in gneiss along the southern margin of the pluton gabbro diabase and some melt zone samples have a single polarity characteristic magnetization of northeast declination d and moderate negative inclination i demagnetization behavior and rock magnetic characteristics indicate that the remanence is carried by nearly pure magnetite after correction for the minor west dip of overlying paleozoic strata we obtain a mean direction of d 32 1 i 41 9 k 94 95 3 3 n 21 sites and a paleomagnetic pole at 21 1 s 221 1 e k 89 a95 3 4 this pole is similar to poles from the middle proterozoic belt supergroup but is located at a higher southerly latitude than poles from other 1 47 1 44 ga plutons from north america most of which plot at equatorial latitudes the reason for this discrepancy is not clear but may result from a combination of factors including unrecognized tilting of the gabbro the failure of this relatively small pluton to fully average paleosecular variation and uncertainties in the overall reliability of other 1 5 1 4 ga poles of the north american apparent polar wander path

shrimp u pb zircon geochronology of the fuping complex implications for formation and assembly of the north china craton abstract the fuping complex located within the central zone of the north china craton is composed of amphibolite to granulite facies orthogneisses interleaved with minor supracrustal rocks at similar metamorphic grade the oldest components recognised are hornblende gneiss enclaves within the predominant biotite orthogneiss which have a shrimp u pb zircon age of 2708 8 ma we consider these enclaves to represent fragments of 2 7 ga continental materials incorporated in the biotite gneiss the biotite gneiss has a shrimp u pb zircon age of 2513 12 ma interpreted to be time of magmatic crystallisation of the igneous precursor based on the igneous characteristics of the zircons this indicates a major magmatic episode at 2 52 ga in the fuping complex identical to the age of felsic volcanism within the low grade wutai complex which crops out immediately to the west a gneissic granite that intrudes the biotite gneisses has a poorly defined 207pb 206pb age of 2045 64 ma this is within error of the age of 2097 46 ma obtained from a fine grained gneiss interlayered with amphibolite of the wanzi supracrustal suite wss interpreted to be volcanic in origin zircons from both these samples have strong oscillatory zoning and provide the first indication of a palaeoproterozoic magmatic event in the area again similar in age to magmatic events recently recognised in the adjacent wutai complex these data indicate a comparable history for the fuping and wutai complexes and support geochemical evidence that they had a common origin and formed part of a late archaean arc affected by later palaeoproterozoic re activation low uranium zircons without oscillatory zoning separated from a sample of biotite gneiss yield data clustered at 1817 26 ma which is interpreted to reflect a period of new zircon growth during a major metamorphic event these data support the recently proposed tectonic model that amalgamation of the north china craton occurred due to collision of the eastern and western blocks along the central zone at 1 8 ga ago

correlations between the eastern block of the north china craton and the south indian block of the indian shield an archaean to palaeoproterozoic link abstract recent tectonometamorphic data reveal that the north china craton formed by amalgamation of an eastern block eb and a western block wb along the trans north china orogen tnco and the indian shield formed by amalgamation of the south indian block sib and the north indian block nib along the central indian tectonic zone citz the eb of the north china craton and the sib of the indian shield share many similar magmatic and sedimentary features the early archaean supracrustal sequences in both the eb and sib consist of 3 6 3 4 ga fuchsite quartzites pelitic gneisses calc silicates marbles banded magnetic quartzites and amphibolites called the caozhuang group in the eb and the older metamorphic group omg in the sib the middle archaean assemblages in both the blocks can be divided into an early 3 4 3 3 ga phase represented by the chentaigou supracrustal rocks and orthogneisses in the eb and the holenarsipur supracrustal rocks and the gorur orthogneisses in the sib and a late 3 1 2 9 ga phase represented by the qianan supracrustal rocks and the yangyashan orthogneiss in the eb and the sargur group and the peninsular gneiss in sib the late archaean taishan group and its equivalents in the eb are stratigraphically similar to the dharwar supergroup in the sib moreover both the eb and sib are characterised by near contemporaneity of late archaean granitoid intrusive and metamorphic events in addition dome and basin structures are dominant in both the eb and sib and anticlockwise p t paths charaterise the metamorphic evolution of the late archaean granulites in the two blocks the palaeoproterozoic sedimentary volcanic rocks in both blocks comprise lower clastic rich middle volcanic rich and upper clastic carbonate sequences the strong resemblance of the archaean to palaeoproterozoic magmatic and sedimentary records suggests that it is possible that the eb and sib are dispersed remnants of what was once a single continent a possible fit is proposed for the reconstruction of the eb and sib where the northern margin of the eb is placed adjacent to the western margin of the sib with the tnco and wb representing the continuations respectively of the citz and nib this fit suggests that the eb sib and the wb nib represent two discrete continental blocks that joined along the tnco citz orogen during a palaeoproterozoic collisional event fragmentation of the eb and sib may have occurred at the beginning of the mesoproterozoic and by 1 4 ga the eb and sib may have separated into two different continental blocks as evidenced by striking differences in the mesoproterozoic sedimentary successions and associated bimodal volcanics in the two blocks

paleomagnetism of the middle proterozoic laramie anorthosite complex and sherman granite southern laramie range wyoming and colorado we present the results of a combined paleomagnetic and 40ar 39ar geochronologic investigation of the middle proterozoic laramie anorthosite complex and sherman granite in the southern laramie range of wyoming and colorado anorthosites and monzosyenites of the laramie anorthosite complex yield a well defined characteristic magnetization of northeast declination d and moderate negative inclination i although antipodal normal and reverse polarity magnetizations are present at three sites a grand mean direction from 29 of 35 sites in the complex is d 44 6 i 48 7 k 77 4 95 3 1 alternating field af and thermal demagnetization behavior and rock magnetic experiments indicate that magnetization is carried by low ti titanomagnetite of single or pseudo single domain character that occurs as elongate to rod shaped inclusions in plagioclase and potassium feldspar the sherman granite contains a dual polarity magnetization that is less well defined than that of the laramie anorthosite complex but similar in declination and inclination d 53 1 i 48 1 k 46 5 95 7 6 n 8 14 sites rock magnetic data indicate the primary carrier of remanence in sherman granite is magnetite the 40ar 39ar geochronologic data from sherman granite hornblende biotite and microcline indicate that subsolidus cooling was moderate to relatively rapid through the range of temperatures over which magnetization was blocked and that the age of remanence is about 1415 ma microcline data indicate that the laramie anorthosite complex and sherman granite have probably not been thermally remagnetized paleomagnetic poles from the laramie anorthosite complex and sherman granite are indistinguishable at the 95 confidence level and individual virtual geomagnetic poles vgps from both units are combined to provide a mean pole at 215 0 e 6 7 s k 46 9 a95 3 5 n 37 vgps the location of this pole is similar to paleomagnetic poles derived from 1480 ma to 1450 ma intrusions elsewhere in north america but it plots significantly north of those from middle proterozoic sedimentary strata of the belt supergroup and sibley group in addition to the characteristic magnetization samples from some sites in sherman granite contain a remanence of southeast declination and shallow negative inclination d 154 9 i 16 0 k 90 6 95 9 7 n 4 sites this secondary magnetization was probably acquired during late paleozoic time

the magnetostratigraphy and a 1780 ma palaeomagnetic pole from the red sandstones of the vazhinka river section karelia russia summary a palaeomagnetic study of the vepsian sandstones of the upper part of the shoksha formation has revealed a stable remanence rock magnetic and mineralogical studies suggest diagenetic haematite as the main carrier of this remanence two magnetic polarities with a regular stratigraphic zonation have been found the reversal test is positive the tilt corrected palaeomagnetic direction is n 36 d 354 3u i 21 6u k 22 3 a95 5 2u the corresponding palaeomagnetic pole of 39 7un 221 1ue dp 2 9u dm 5 5u is proposed as a key pole for 1790 1770 ma for fennoscandia

development of the north china craton during the late archaean and its final amalgamation at 1 8 ga some speculations on its position within a global palaeoproterozoic supercontinent abstract new data obtained over the past few years indicate that by the end of the archaean the north china craton consisted of two major crustal entities identified as the eastern and western blocks the formation of island and magmatic arcs along the western margin of the eastern block record the latest archaean events so far identified the location of the western block at this time is unknown however the occurrence of palaeoproterozoic passive margin sediments similar in age to those occurring in the eastern block indicates essentially similar conditions and possible separation by an ocean of unknown dimensions re initiation of east dipping subduction beneath the western margin of the eastern block resulted in closure of this palaeoproterozoic ocean bringing the western block into close proximity continued subduction resulted in a major continental continental collision and the western margin of the eastern block was again reactivated leading to extensive thrusting high pressure metamorphism and the generation of crustal melts this occurred between 1 88 and 1 79 ga ago resulting in the formation of the trans north china orogen and final amalgamation of the north china craton although information is sparse to non existent we have reviewed the possible location of the north china craton at the time of collision and note a similarity in terms of age lithology and distribution of major crustal components between the north china craton and baltica although speculative we present this model in the hope that it will stimulate debate and lead to further research in these areas

a group of rifting events in the terminal paleoproterozoic in the north china craton abstract rifting events of the terminal paleoproterozoic are well preserved in the north china craton they include an aulacogen and cratonic marginal rift basins mafic dyke swarms two anorthosite suites and rapakivi granites all of these events took place after metamorphism deformation and magmatism of the late paleoproterozoic between 2 0 ga and 1 85 ga or 1 80 ga the rifting events belong to an anorogenic event group reflecting breakup of a late paleoproterozoic united north china craton an intracratonic rift basin in the center of the north china craton is named the yanshan taihangshan aulacogen a stratigraphic thickness of 9193 m was deposited within the basin including three groups and twelve formations with time range ca 1 8 ga to 0 85 ga the oldest changcheng group of the sequence was deposited in a graben and is mainly composed of alluvial basal conglomerates terrestrial clastic rocks and shallow marine sandstone and carbonates with super high potassium basalt trachyte the marginal basins occur around the northern western and southern margins of the north china craton respectively located in langshan baiyun obo helanshan mountain and the adjacent regions between henan shanxi and shaanxi provinces the stratigraphic sequences of the basins are generally made up of clastic rocks in the lower parts and carbonates in the upper part with a thick volcanic sequence with u pb age of 1840 14 ma exposed in the southern marginal regions based on stratigraphic sequences isotopic data and volcanic features the initial rifts took place at about 1 85 ga to 1 80 ga mafic dyke swarms are largely developed in central parts of the north china craton where they intrude archean granulite and high amphibolite rocks a new single zircon u pb isotopic age of 1769 2 5 ma has been reported from a mafic dyke in east henshan mountain northwestern shanxi province an anorthosite suite agms is exposed along two deep fault belts in northern hebei province isochron ages of 1683 193 ma by rb sr and 1735 239 ma by sm nd were measured from an anorthosite body the rapakivi granites crop out along the miyun qiangzilu xinglong fault in an e w direction and are characterized by megacrysts of alkali feldspars mantled by plagioclase thus showing many of the petrographic features of classic rapakivi granites worldwide a typical rapakivi body preserved in shachang of miyun county beijing has a u pb age of 1716 31 ma super high potassium volcanics located in the upper changcheng group have been dated at 1683 67 ma and 1625 6 ma the time range for all magmatic events is between 1 72 ga and 1 62 ga

palaeomagnetic data late precambrian magnetostratigraphy and tectonic evolution of eastern china abstract in this paper middle and upper proterozoic 1900 1600 ma apw paths are presented for north and south china based on palaeomagnetic results obtained from the jixian yangzi gorges shennongjia and fuzhou areas in eastern china two or three loops are present the second of these has an age of about 1000 800 ma and is probably roughly equivalent to the sveconorwegian and grenville loops apw paths loops reversed polarity zones and pole positions combined with geological data permit definition of several markers that are useful in regional correlation these form the basis for a proposed stratigraphic subdivision of the middle and upper proterozoic in china a tentative palaeogeographic map has been drawn for the north china and yangzi blocks for the period from 1700 ma to 600 ma

tectonic implications of the 1419 ma nova guarita mafic intrusives paleomagnetic pole amazonian craton on the longevity of nuna abstract the nuna supercontinent was probably assembled in the paleoproterozoic but its paleogeography and the timing for its demise are still a matter of debate a paleomagnetic and geochronological study carried out on the mesoproterozoic nova guarita dyke swarm northern mato grosso state sw amazonian craton provides additional constraints on the duration of this supercontinent paleomagnetic af and thermal treatment revealed south southwest northeast magnetic directions with downward upward inclinations for 19 analyzed sites these directions are carried by psd magnetite with high unblocking temperatures as indicated by additional magnetic tests including thermomagnetic curves hysteresis loops and the progressive acquisition of isothermal remanence in selected samples a positive contact test with the host granite in one of the studied dykes further attests to the primary origin of the characteristic magnetic component a mean site direction was calculated at d m 220 5 i m 45 9 95 6 5 k 27 7 which yielded a paleomagnetic pole located at 245 9 e 47 9 s a 95 7 0 40 ar 39 ar dating carried out on biotites from four analyzed dykes yielded well defined plateau ages with a mean of 1418 5 3 5 ma the nova guarita pole precludes a long lived nuna configuration in which laurentia baltica north china and amazonia formed a long and continuous block as previously proposed for the paleoproterozoic it is nevertheless fully compatible with a samba amazonia baltica link at mesoproterozoic times

neoarchean to paleoproterozoic continental evolution and tectonic history of the north china craton a review rocks of archean age are widely distributed in the north china craton ncc and can be divided into high grade regions and granite greenstone belts even though most underwent amphibolite to granulite facies metamorphism quartzo feldspathic gneisses occupy 70 to 85 of the total exposure of archean rocks in the craton supracrustal rocks commonly consist of intercalated volcano sedimentary rocks including metabasites meta intermediate to acid rocks banded iron formations metapelites and minor marbles the oldest rocks outcrop over a small area of 4 km2 and are a quartzite amphibolite sequence that yield 3 8 3 5 ga isotopic ages chemical and geochronological data constrain the earliest main crust forming episode at ca 2 9 2 7 ga when most rocks were metamorphosed to upper amphibolite or granulite facies at moderate pressures mafic granulites can be classified into two textural types referred to as near isobaric cooling ibc and near isothermal decompression itd types high pressure granulites and retrograded eclogites occur mainly near the junction of the provinces of hebei shanxi and inner mongolia in the central ncc and in eastern shandong province of the eastern ncc as well as locally in henan province in the southern ncc these high pressure rocks show clockwise p t paths indicating decompression from 14 kb to 5 6 kb an exposed section interpreted as representing precambrian continental lower crust has also been identified in the northern part of the craton although some mafic ultramafic metamorphic rocks have been interpreted as remnants of ancient oceanic crust and mantle by a number of geologists geochemical and petrological evidence demonstrates that many supracrustal orthogneiss associations have island arc affinities therefore several tectonic models have been proposed including continent continent collision and island arc terrain accretion achieved by arc arc or arc microcontinent collision the ncc underwent multiple high grade metamorphic events of which the two most important took place during the intervals 2600 2450 ma and 1950 1750 ma which are considered to be related to early precambrian supercontinental cycles

observations and numerical simulation of atmospheric cellular convection over mesoscale topography radar and satellite images provide observations of convective rolls and other structures in the convective boundary layer cbl but the data are intermittent and neither radar nor satellite gives a complete picture of roll circulation in the observed cases as a consequence numerical modeling is a useful complement to the observations to investigate the temporal and spatial details of convective rolls in this paper observations of convective rolls over the south of england are described numerical simulations have been performed to investigate these rolls using the boundary layer above stationary inhomogeneous uneven surfaces blasius model a relatively simple boundary layer code for flow over topography the numerical results indicate that most of the features of the convective structures can be successfully reproduced notably the roll orientation and spacing and the basic features of the cloud field these features are in good agreement for two case studies one with distinct rolls and the other with more dispersed convective structures and a time dependent basic state the model tends to predict the initial occurrence of rolls later than observed and this time of occurrence is found to be influenced by model resolution the presence of low topography with maximum height on the order of 30 of the cbl depth may have a small influence on the average orientation and spacing and the time of initial occurrence of modeled rolls local flow anomalies related to the hills are much more pronounced these anomalies appear to be related to coherent patterns in the model cloud fields with a tendency for more cloud cover upstream and over hills and cloud clearing in the lee as a result of descent suppressing convective eddies when the satellite imagery is combined with topography data this kind of orographic control of the shallow convection by the topography is evident the cbl height varies considerably in the early stages of cbl evolution over hilly topography but when the convection is fully developed the cbl height is almost constant over the domain

age and evolution of a late archean to paleoproterozoic upper to lower crustal section in the wutaishan hengshan fuping terrain of northern china abstract the taihangshan wutaishan area forms part of the central north china craton and consists of three main components the hengshan and fuping complexes containing predominantly ductilely deformed late archean to paleoproterozoic high grade granitoid orthogneisses intruded by mafic dykes of gabbroic composition and the low grade late archean wutai greenstone belt developed between the high grade terrains and consisting of bimodal volcanic rocks and metasediments associated with coeval granitoids zircon dating shows that both the hengshan and fuping complexes were intruded by major granitoid bodies between 2 52 and 2 48 ga with rare occurrences of 2 7 ga gneisses wutai granitoids adjacent to the fuping complex also contain 2 7 ga xenocrysts all possibly reflecting the same older basement rare granitic gneisses in the hengshan have emplacement ages of 2 35 ga and show the same structural features as the older rocks indicating that the main deformation occurred after 2 3 ga deposition of supracrustal rocks and emplacement of mafic dykes occurred in the paleoproterozoic together with intrusion of granitic bodies at 2 1 2 0 ga and all underwent regional metamorphism at 1 88 1 85 ga we interpret the hengshan and fuping granitoid gneisses as the lower plutonic part of a late archean to early paleoproterozoic japan type magmatic arc with the upper volcanic part represented by the wutai complex components of this arc may have evolved at a continental margin as indicated by the 2 7 ga zircons virtually all magmatic events are recorded in all three complexes however the 1 85 ga metamorphic event is only recorded in the higher grade portions of the wutai since low grade and presumably high crustal level precluded metamorphic zircon growth shear zones in the hengshan are interpreted as major lower crustal discontinuities post dating the peak of high pressure granulite metamorphism we suggest that they formed during orogenic collapse and uplift of the hengshan complex in the late paleoproterozoic 1800 ma

a molecular and isotopic geochemical study of meso to neoproterozoic 1 73 0 85 ga sediments from the jixian section yanshan basin north china abstract this paper systematically describes the molecular and isotopic characteristics of meso to neoproterozoic 1 73 0 85 ga sediments from the jixian section yanshan basin north china a correlation is made between molecular fingerprints and paleoenvironmental and paleontologic data acquired from these strata the agreement between these parameters suggest that the biomarkers preserved in these proterozoic sediments are good indicators of the evolution of the paleoenvironment and early life in the yanshan basin the fact that extended regular isoprenoids c20 were detected in most of the studied samples suggest a sustained lipid input to sedimentary organic matter from halophiles and other possible archaebacteria through meso to neoproterozoic two dehydroabietin isomers i e 18 and 19 norabieta 8 11 13 trienes were unexpectedly present in these samples which provides evidence suggesting that the compounds with abieta structure could be derived from primitive organisms such as algae the stable carbon isotope characteristics of kerogen and coexisting carbonate show a rapid decline in atmospheric co2 concentration from 1 73 to1 58 ga and a stable concentration afterwards the difference c in isotopic composition between carbonate and associated kerogen lies in zonal variations along the stratigraphic sequence which can be attributed to the alternations of marine transgressions and regressions that occurred in the yanshan basin during the proterozoic the carbon isotope relationships between kerogen and alkanes from these strata are consistent with the hypothesis that before 1 4 ga both n alkanes and isoprenoids had been derived mainly from heterotrophic halophiles and other archaebacteria but after that only isoprenoids originated from archaebacteria whereas n alkanes were possibly derived from photosynthetic organisms changes in the source of n alkanes around 1 4 ga may suggest a decline of heterotrophic reworking to primary lipids which can be related to the formation of a quiet shallow water environment of an epicontinental sea at this stage in the yanshan basin

fluorite bearing early and middle proterozoic granites okak bay area labrador geochronology geochemistry and petrogenesis three discrete age groups of relatively fractionated fluorite bearing post tectonic proterozoic granitoid rocks have been identified in the okak bay area of labrador on the basis of u pb zircon geochronology two of the groups occur entirely within the archean nain province the third occurs largely within the early proterozoic churchill province torngat orogen none of the three episodes of granitoid magmatism can be directly linked to orogenic activity in its immediate surroundings although all three granite groups are broadly similar petrologically and chemically to the felsic rocks of the anorogenic elsonian 1450 1290 ma nain plutonic suite nps only one of the dated units can be correlated confidently with elsonian magmatism the other two groups represent significantly older anorogenic or postorogenic magmatism the oldest unit the wheeler mountain granites of the nain province has yielded a u pb age of about 2135 2134 31 2137 2 ma a major tectonothermal event of this age has not been previously identified in northern labrador and the origin of the granites is uncertain three small intrusions on offshore islands white bear saddle opingiviksuak in the nain province were intruded about 1775 1774 21 1776 42 1774 21 ma these granites may have been intruded post kinematically into a stable archean foreland representing a delayed effect of a preceding collisional event torngat orogeny between nain and churchill province to the west the youngest unit dated at about 1318 1319 2 1316 23 ma is umiakovik lake batholith which crops out largely within the eastern churchill province and is one of the largest granitoid intrusions of the nps the age confirms that it formed an integral part of the elsonian magmatic event but both umiakovik lake batholith and nearby 1320 ma makhavinekh lake pluton are significantly older than some basic members of the nps in the coastal area to the southeast nd and sr isotopic data demonsrate that archean protoliths formed significant components of the source materials for all three of these proterozoic granitoid magmas this source was supplemented by one or more proterozoic episodes of lower crustal intrusion and underplating by juvenile mantle derived magmas that occurred in the region

early paleozoic paleomagnetic poles from the western part of the north china block and their implications abstract we collected 917 oriented drill core samples 124 sites from 22 units spanning the early cambrian to middle ordovician formations of the north china block ncb to construct an apparent polar wander path apwp of the early paleozoic for the ncb sampling areas are located in the helanshan tongchuan hancheng and yuncheng luliang regions along the margin of the ordos basin characteristic remanent magnetizations for five periods between the cambrian and ordovician are isolated from 281 samples by careful demagnetization directions cluster around northwesterly direction with upward shallow inclination and its antipode after tilt correction reliability of these five directions is ascertained through positive fold and reversal tests the five cambrian and ordovician paleopoles plot in the present atlantic ocean which is consistent with previously reported paleomagnetic poles a newly constructed apwp between the cambrian and ordovician indicates that the ncb was located in the southern hemisphere around 15 s during cambro ordovician time motion of the ncb between the early and middle cambrian comprised a counterclockwise rotation of 22 3 7 0 and a northward latitudinal displacement of 7 2 6 8 combining the paleobiogeographic and paleoclimatic data with the paleomagnetic implies that the ncb was probably located in east gondwana in the early cambrian and may have separated from gondwana in post early cambrian time the ncb is also inferred to have been located close to siberia and north america during the middle ordovician

distribution of trace elements in soils surrounding the el teniente porphyry copper deposit chile the influence of smelter emissions and a tailings deposit in the area surrounding the el teniente giant porphyry copper deposit eight soil sites were sampled at three depth levels in the summer 2004 the sites were selected for their theoretical potential of being influenced by past so2 emissions from the smelter and or seepage from a now idle tailings impoundment the soil mineralogy grain size distribution total organic matter contents major element composition cation exchange capacity and cu mo pb zn as and so4 2 concentrations were determined for all samples after nitric acid extraction and separate leaches by ammonium acetate ph 7 and sodium acetate ph 5 for water rinses only cu could be determined with the analytical set up used cu and so4 2 enrichment in topsoils was found at six sites either downwind from the smelter or within the combined influence of the smelter and the tailings impoundment both elements were released partially by ammonium and sodium acetate extractions due to the scarce background trace element concentrations of soil and rock outside the immediate mine area assessment of trace element mobility for mo zn pb and as was difficult arsenic was found to be concentrated in soil horizons with high smectite and or organic matter contents mo appears to be linked to the presence of windblown tailings sediment in the soils mobilization of mo zn and as for the acetate extractions was minimal or below the detection limits for the aas technique used the presence of windblown tailings is considered to be an additional impact on the soils in the foothills of the el teniente compound together with the potential of acidity surges and cu mobilization in topsoils after rainfalls two sites located at the western limit of the former so2 saturated zone with strongly zeolitized soils and underlying rock did not show any cu or so4 2 enrichment in the topsoils and remaining total trace element concentrations were below the known regional background levels

a mesoproterozoic paleomagnetic pole from the yangzhuang formation north china and its tectonics implications abstract a new paleomagnetic study has been carried out on mesoproterozoic sediments in the jixian area north china detailed stepwise thermal demagnetization allows us to isolate three components from 141 oriented drill core samples 14 sites a low temperature component component a identified from most of samples falls close to the present local earth field direction a middle temperature component component b is found only in 21 samples from four sites the component b directions after bedding correction yield a paleopole position at 336 0 e 10 2 n with a 95 17 1 which is close to the lower cambrian pole of the north china block characteristic remanent directions with dual polarities obtained from high temperature component c pass fold test the directions yield a pole position at 190 4 e 2 4 n with a 95 11 9 this result indicates that the north china block was located in low latitude at ca 1 35 ga recently several studies suggested that laurentia baltica and siberia were also situated at low latitudes from well dated paleopoles between ca 1 5 and 1 25 ga this would imply that the north china block laurentia and baltica might drift together or close in the mesoproterozoic

revisiting the paleomagnetism of the 1 476 ga st francois mountains igneous province missouri 1 a paleomagnetic investigation of the st francois mountains igneous province in southeastern missouri provides a key 1476 16 ma paleomagnetic pole for laurentia the pole 13 2 s 219 0 e dp 4 7 dm 8 0 is considered primary on the basis of positive conglomerate inverse baked contact and fold tests an analysis of 1470 1430 ma poles from laurentia highlights key differences between poles obtained from the belt supergroup electra lake gabbro and cratonic north america paleolatitudes based on the lower belt supergroup poles are enigmatic as two previous studies yielded a difference of 10 our new pole combined with an analysis of previous results favors the higher latitude interpretation for the lower belt supergroup paleolatitudes from the younger belt rocks indicate lower latitudes than coeval rocks from elsewhere in laurentia for which there has been no adequate explanation a comparison of the st francois mountain pole with similar age poles from baltica siberia and australia allow first order tests of proposed continental configurations paleomagnetic data from australia are compatible with proposed rodinia reconstructions whereas paleomagnetic data from baltica are not we are unable to rigorously test the alternative suggestion that places siberia against the western margin of laurentia due in part to large errors associated with siberian paleomagnetic data

on the upper precambrian sinian suberathem in china abstract this paper gives a general outline of the upper precambrian sinian suberathem in china including its geographic distribution development stratigraphic sequence and classification the sinian suberathem with a time range of 1950 615 ma constitutes the greater part of the proterozoic it is characterized by the development of stromatolites appearance of eukaryotes and the presence of metazoa in its upper part its sedimentological characteristics are quite similar to those of the overlying paleozoic on the basis of the biological evolution isotopic dating and stages of tectonic development the sinian suberathem can be divided into four systems in ascending order the changcheng ca 1950 1400 ma jixian ca 1400 1000 ma qingbaikou ca 100 800 ma and sinian ca 800 615 ma systems the stratotype for the first three systems is located in jixian county and the type section for the sinian system in the yangzi gorges area the contact between the sinian and cambrian systems is in some places a continuous transition with abundant small shelly fauna above the precambrian cambrian boundary e g in the yangzi gorges yunnan and sichuan with regard to sedimentological characteristics the rocks of the sinian suberathem may be subdivided into three main facies such as 1 carbonate facies of stable region 2 volcanic sedimentary facies of mobile zones and 3 argillo arenaceous carbonate facies of transitional zones progress in the study on the succession of the sinian suberathem has been made in recent years in regard to palaeontology stratigraphy isotopic geochronology magnetostratigraphy and palaeoglaciology they provide important evidence for the stratigraphic correlation of the upper precambrian of china and other continents

the paleomagnetic significance of new u pb age data from the molson dyke swarm cauchon lake area manitoba u pb geochronology paleomagnetism and petrography indicate that the molson dyke swarm along the western margin of the superior craton is a composite of at least two ages of intrusion the more extensive younger dyke set the molson swarm sensu stricto generally has a 030 trend is mainly pyroxenitic to noritic with subordinate diabase and has been related to rifting in a back arc environment during closure of the manikewan ocean at about 1920 1800 ma which culminated in the trans hudson orogen a u pb zircon age from one of these dykes located at cauchon lake indicates emplacement at 1877 7 150 4 similar to two previous u pb age determinations on molson dykes another dyke from cauchon lake yields a baddeleyite zircon u pb date of 2091 2 ma and appears to be part of an older mainly diabasic suite of east northeast trending dykes that may represent a continental rifting episode that preceded the opening of the manikewan ocean the new u pb age data require a revision to the interpretation of

the direction correction tilt test an all purpose tilt fold test for paleomagnetic studies abstract the tilt or fold test in paleomagnetism is used to infer whether paleomagnetic remanence was acquired before or after tectonic tilting while several tilt test formulations have been proposed none fully satisfies the requirements of statistical validity applicability to all bedding geometries and ease of use this paper introduces the direction correction dc test which examines the relationship between the paleomagnetic site mean directions and their corresponding bedding tilt corrections the dc test is similar to mcfadden s correlation test because both test whether or not the site directions contain information about the bedding tilts however the dc test gives greater weight to sites with greater counter bedding the dc test is similar to watson s numerical tilt tests because they determine the degree of untilting which gives optimal concentration of site directions however the dc test uses analytical rather than numerical methods graphical output aids the researcher in recognizing problem sites using both real and simulated data the dc test is demonstrated to be more discriminating than other tilt test formulations for all bedding geometries the simulations show that the power of the tilt test is inversely proportional to the 95 confidence interval 95 of the overall mean as a rule of thumb paleomagnetists should attempt to sample sufficient sites to obtain an 95 less than 1 6 of the bedding attitude difference