testing the reliability of discrimination diagrams for determining the tectonic depositional environment of ancient sedimentary basins the lower palaeozoic rocks of the west of ireland caledonides provide a novel way to test the reliability of discrimination diagrams in sedimentary rocks a comparison between the geochemical signature of the tuff bands and the signature from the adjacent sedimentary rocks reveals that a combination of multi element diagrams and discrimination diagrams is best suited to identify the tectonic environment of deposition the tuff bands from the ordovician and silurian formations all carry an active continental margin signature only three of the sedimentary discrimination diagrams assigned more than 50 of the samples to this setting the results from the discrimination diagrams indicate an active margin setting for the ordovician mweelrea and rosroe formations and the silurian formations whilst the derrylea and sheeffry sedimentary samples have a passive margin signature overall the results show that in the case where felsic volcanism occurred on a quartz rich source region the active continental margin field on many established discrimination diagrams is not reliable in identifying this setting however the study highlights one of the key strengths of discrimination diagrams in their ability to be used as a lever to reveal significant information about the sediment source as in this case to identify sediment that has been transported from different tectonic settings and deposited in one sedimentary basin

protracted post collisional magmatism during plate subduction shutdown in early paleoproterozoic insights from post collisional granitoid suite in nw china abstract tectonic regimes display different styles worldwide during the early paleoproterozoic ranging from plate tectonic slowdown or shutdown to vigorous subduction related magmatism here we report a suite of early paleoproterozoic granitoids from the quanji massif in northwestern china that provides potential insights on the tectonic regime in this period the granitoids formed at 2 37 2 34 ga as evidenced by la icp ms zircon u pb ages these dioritic to granitic rocks with amphibole and biotite as the major mafic minerals display a wide range of sio2 55 8 66 2 wt low to moderate mgo 1 4 4 1 wt low to high k2o 1 6 5 9 wt and al2o3 16 3 21 5 wt with mg values in the range of 36 47 and relatively low cr 7 7 79 2 ppm and ni 2 3 19 3 ppm concentrations geochemically they correspond to high potassium calc alkaline i type granitoids with some showing dominantly adakitic but minor a type affinities both the i type and a type granitoids show moderately fractionated lree to hree and moderately to weakly negative eu anomalies however the adakitic granitoids display strongly fractionated ree patterns and weak negative to positive eu anomalies all the rock types from this suite show enrichment in liles and lrees and depletion in hfses ta nb the nd hf isotope data suggest that the granitoids were sourced from mesoarchean to neoarchean metamafic rocks in the lower crust the coeval nature of the complex genetic types of rocks in this suite suggest that their precursor magmas formed in an extensional tectonic regime similar to those previously reported for the 2 39 2 37 ga granitoids in central to eastern domains of the quanji massif our new results in conjunction with those from previous studies suggest protracted post collisional magmatism lasting for 50 myr along the periphery quanji massif following the cessation of plate subduction in the early paleoproterozoic

is taiwan the result of arc continent or arc arc collision abstract conventionally it has been accepted that the formation of taiwan results from the collision of the luzon arc with the eurasian continental margin we suggest that taiwan results from the collision of the luzon arc with the former ryukyu subduction zone before the collision the latter extended a few hundred kilometres southwest of its present day termination in the early to middle miocene the subduction became inactive in its southwest portion and the manila trench migrated northeastward giving rise to the formation of the luzon arc the collision started in the late miocene by closing the oceanic domain located between the luzon arc and the former ryukyu arc because the luzon arc moved northwestward with respect to the former ne sw trending ryukyu arc the oblique collision first resulted in the indentation on the south ryukyu arc west of 123 5 e and the narrowing of the corresponding portion of the okinawa trough prior to the uplift of taiwan today the hsuehshan and backbone ranges represent the uplifted portion of the backarc basin and the longitudinal valley corresponds to the suture zone between the former ryukyu and the luzon arcs in this model we propose to link although they are not of the same age two truncated backarc basins the active okinawa trough to the northeast and the former active tainan backarc basin to the southwest the former backarc basin extended from southwest of taiwan to southern japan and could have been initiated near the paleo location of taiwan as early as the eocene oligocene the result of the collision is the truncation of the ancient backarc basin associated with the shortening deformation and uplift of the ancient outer arc and related backarc region which provides the fundamental mechanism of the taiwan mountain building this model is in agreement with a counterclockwise rotation of the portion of taiwan located west of the coastal range belonging to the luzon arc and a clockwise rotation of the south ryukyu arc earthquake focal mechanisms geodetic observations and the distribution of the petrologic and structural features are also consistent with the proposed arc arc collision model

the indosinian orogeny a perspective from sedimentary archives of north vietnam the triassic stratigraphic framework for the song da and the sam nua basins north vietnam suffers important discrepancies regarding both the depositional environments and ages of the main formations they contain using sedimentological analyses and dating foraminifer biostratigraphy and u pb dating on detrital zircon we provide an improved stratigraphic framework for both basins a striking feature in the song da basin located on the southern margin of the south china block is the diachronous deposition over a basal unconformity of terrestrial and marine deposits the sedimentary succession of the song da basin points to a foreland setting during the late early to the middle triassic which contrasts with the commonly interpreted rift setting on the northern margin of the indochina block the sam nua basin recorded the activity of a proximal magmatic arc during the late permian up to the anisian this arc resulted from the subduction of a southward dipping oceanic slab that separated the south china block from the indochina block during the middle to the late triassic the song da and sam nua basins underwent erosion that led to the formation of a major unconformity resulting from the erosion of the middle triassic indosinian mountain belt built after an ongoing continental collision between the south china and the indochina blocks later during the late triassic as syn to post orogenic foreland basins in a terrestrial setting the song da and sam nua basins experienced the deposition of very coarse detrital material representing products of the mountain belt erosion

marsis remote sounding of localized density structures in the dayside martian ionosphere a study of controlling parameters enhanced topside electron densities in the dayside martian ionosphere have been repetitively observed in areas of near radial crustal magnetic fields for periods of tens of days indicating their long term spatial and temporal stability despite changing solar wind conditions we perform a statistical study of these density structures using the ionospheric mode of the mars advanced radar for subsurface and ionosphere sounding marsis onboard mars express we estimate the apparent extents of these structures relative to the altitude of the surrounding ionosphere the apex of the density structures often lies higher than the surrounding ionosphere median vertical extent of 18 km which indicates upwellings these structures are much wider than they are high with latitudinal scales of several degrees the radar reflector regions are observed above both moderate and strong magnetic anomalies and their precise locations and latitudinal extents match quite well with the locations and latitudinal extents of magnetic structures of given magnetic polarity oblique to vertical fields which happen to be regions where the field lines are open part of the time the majority of the density structures occur in regions where ionospheric plasma is dominant indicating closed field regions shielded from shocked solar wind plasma

alteration and geochemical patterns in the 3 7 3 8 ga isua greenstone belt west greenland abstract volcanic rocks of the 3 7 3 8 ga isua greenstone belt have undergone heterogeneous poly phase post magmatic alteration resulting in mobilisation of many elements including lree and th in contrast nb ta ti zr y and hree were relatively immobile during alteration processes the degree of element mobility appears to have been controlled by chemical composition primary volcanic structures carbonate and silica enrichment and proximity to ultramafic units and shear zones on the basis of the geochemical characteristics of the least altered samples two distinct associations have been recognised in structurally separated lithotectonic sequences these magmatic associations are a low hfse association tio2 0 20 0 40 wt zr 12 30 ppm nb 0 13 0 80 ppm y 6 14 ppm in the central arc sequence resembling tertiary boninites and a high hfse association tio2 0 50 1 14 wt zr 34 77 ppm nb 1 2 2 7 ppm y 11 28 ppm in the outer and inner arc sequences comparable to phanerozoic island arc basaltic to picritic volcanic rocks these two geochemically distinct sequences were likely to have been juxtaposed as a consequence of phanerozoic style plate tectonic processes operating in the early earth the isua volcanic rocks are characterised by sub chondritic nb ta 10 16 ratios post magmatic alteration can be ruled out as the cause of these low ratios rather these ratios reflect a compositional characteristic of the paleoarchean mantle source s the sub chondritic nb ta ratios may have resulted from either preferential sink of nb into the core as a weakly siderophile element or preferential retention of nb in the subducted oceanic crust during subduction zone petrogenetic processes alternatively the building blocks of the earth were not composed of ci like chondritic material

precise u pb ages and geochemistry of palaeoproterozoic mafic dykes from southern west greenland linking the north atlantic and the dharwar cratons palaeoproterozoic magmatism throughout the north atlantic craton nac has produced dyke swarms of diverse orientations and emplacement ages in southern west greenland previously identified swarms include the ca 2500 ma kilarsaarfik dykes ca 2215 ma boninitic norite bn dykes and the abundant ca 2040 ma kangamiut and coeval 2050 2030 ma md3 dykes additional insight into the distribution of swarms is provided by new baddeleyite u pb id tims dates interpreted as emplacement ages for six dykes the geochemistry for a total of 42 samples on 40 dykes including the dated dykes provides some constraints on their petrogenesis two e w trending dykes yield ages of 2365 2 ma and 2374 4 ma representing a hitherto unknown magmatic event we propose the name graedefjord dykes for dykes of this age in southern west greenland one ne sw trending dyke is dated at 2209 5 ma close to a previously published 2214 10 ma age for a n s trending dyke member of bn dykes three dykes yield md3 ages of 2053 2 ma e w trending 2049 6 ma n s trending and 2042 2 ma wnw ese trending ca 2 21 ga old and predominantly ne se and nne sse trending dolerites some of which are high al basalts were derived from a relatively depleted mantle and injected during a boninite dolerite magmatic event together with nearby bn dykes derived from a harzburgitic yet highly lree and lile enriched subcontinental lithospheric mantle sclm ca 2 05 ga kangamiut md3 dolerites were derived from a similar ambient asthenospheric mantle and subsequently experienced crustal assimilation during fractional crystallisation the ca 2370 ma dykes in the nac are coeval with the dharwar giant dyke swarm of the dharwar craton dc and based on this unique age match it is proposed that the dharwar and nac were neighbours in the palaeoproterozoic furthermore both blocks share 222 2 21 ga dykes and matching the trends of 2 2 2 1 and 2 37 ga swarms between the two blocks allows several plausible reconstructions c 2012 published by elsevier b v

an early paleoproterozoic high k intrusive complex in southwestern tarim block nw china age geochemistry and tectonic implications abstract systematic geochronologic geochemical and nd isotopic analyses were carried out for an early paleoproterozoic high k intrusive complex exposed in southwestern tarim nw china the results provide a better understanding of the paleoproterozoic tectonic evolution of the tarim block zircon u pb age dating indicates two paleoproterozoic magmatic episodes occurring at ca 2 41 ga and ca 2 34 ga respectively which were followed by a ca 1 9 ga metamorphic event the 2 41 ga granodiorite adamellite suite shares characteristics of late to post orogenic metaluminous a type granites in its high alkalinity na2o k2o 7 6 9 3 total ree 410 788 ppm zr 370 660 ppm and y 21 7 58 4 ppm contents end t values for the suite range from 3 22 to 4 71 and accordingly the nd modal ages t2dm vary between 3 05 ga and 3 17 ga based on geochemical data the 2 34 ga suite can be subdivided into two sub suites namely a type and s type however both types have comparable nd isotope compositions end t 0 41 to 2 08 and similar narrow t2dm ranges 2 76 2 91 ga geochemical and nd isotopic data for the high k intrusive complex in conjunction with the regional geological setting suggest that both the 2 41 ga suite and the 2 34 ga a type sub suite might have been produced by partial melting of the archean mafic crust in a continental rift environment the s type sub suite is thought to have formed by partial melting of felsic pelites and or metagreywackes recycled from archean crust ttg gabbro enclaves with positive end t value 2 15 have been found to be intermingling within the 2 34 ga suite ca 2 34 2 36 ga gabbroic dykes and adamellites have previously been documented in eastern tarim these observations indicate that the high k intrusions may reflect the emergence of depleted mantle upwelling beneath the tarim block at that time we suggest a three stages model for the precambrian crustal evolution in the tarim block 1 the formation of proto crust ttg by ca 2 5 ga 2 episodes of felsic magmatism possibly occurring in continental rift environments at ca 2 41 ga and ca 2 34 2 36 ga and 3 ca 1 9 ga metamorphism that may represent the solidification of the precambrian basement of the tarim block

age and tectonic implications of paleoproterozoic deo khe granitoids within the phan si pan zone vietnam abstract we report the first u pb zircon ages of 1855 1873 ma for the deo khe granitoids dkg in the phan si pan zone pspz of northern vietnam the dkg are medium grained two mica granitoids predominantly composed of quartz k feldspar and muscovite trace element analyses indicate that the dkg are enriched in large ion lithophile elements but depleted in high field strength elements zircons from the granitoids have negative e hf t values ranging from 23 6 to 17 5 the magmatic zircons from the dkg have single stage hf model ages t dm1 that range from 3 3 to 2 8 ga and their e hf t data all plot well below the evolution trend of 2800 ma average juvenile mantle observed hf model ages are contemporaneous with the emplacement of 2 90 2 84 ga tonalite trondhjemite granodiorite ttg gneiss observed in a nearby ca vinh complex suggesting that pspz in northern vietnam is a product of partial melting of archean crust a sequence of similar tectonic events including initial emplacement of ttg protolith at 2 8 2 9 ga metamorphic development of ttg gneiss at 1 9 2 0 ga and magmatic activity at 1 8 2 0 ga are now recognized both in northern vietnam and yangtze block which we interpret to indicate basement rocks in northern vietnam are similar to those found along southern china

oblique reflections in the mars express marsis data set stable density structures in the martian ionosphere the mars advanced radar for subsurface and ionospheric sounding marsis onboard the european space agency s mars express mex spacecraft routinely detects evidence of localized plasma density structures in the martian dayside ionosphere such structures likely taking the form of spatially extended elevations in the plasma density at a given altitude give rise to oblique reflections in the active ionospheric sounder data these structures are likely related to the highly varied martian crustal magnetic field in this study we use the polar orbit of mex to investigate the repeatability of the ionospheric structures producing these anomalous reflections examining data taken in sequences of multiple orbits which pass over the same regions of the martian surface under similar solar illuminations within intervals lasting tens of days presenting three such examples or case studies we show for the first time that these oblique reflections are often incredibly stable indicating that the underlying ionospheric structures are reliably reformed in the same locations and with qualitatively similar parameters the visibility or lack thereof of a given oblique reflection on a single orbit can generally be attributed to variations in the crustal field within the ionosphere along the spacecraft trajectory we show that within these examples oblique reflections are generally detected whenever the spacecraft passes over regions of intense near radial crustal magnetic fields i e with a cusp like configuration the apparent stability of these structures is an important feature that must be accounted for in models of their origin

multi stage crustal accretion by magmatic flare up and quiescence intervals in the western margin of the sao francisco craton u pb hf and geochemical constraints from the almas terrane abstract the global plate tectonic regime in early paleoproterozoic times is highly debated the interval 2 45 2 2 ga is known for a minima in juvenile magmatism but this is not a global phenomenon new results of whole rock geochemistry and u pb hf analysis in igneous and detrital zircons allied with existing isotopic and geophysical data allow to identify and constrain the duration of magmatic flare up and quiescence events in the western sao francisco paleoplate igneous samples yield ages indicating three accretionary magmatic events an older with ages ca 2476 4 9 ma to 2462 13 ma an intermediate at 2390 14 ma and a younger from 2235 26 ma to 2201 5 ma all presenting magmatic arc geochemical signatures xenoliths of quartzite and volcanic tuff from the upper greenstone sequence morro do carneiro fm are hosted in the 2211 9 ma tonalite and the maximum depositional age of the morro do carneiro basin is dated 2234 12 ma indicating a syn orogenic setting for this basin detrital zircon u pb age distribution for quartzites of the greenstone sequence shows peaks at 2 65 2 47 2 39 2 27 and 2 23 ga altogether the studied rocks record an accretionary orogeny with four distinct episodes episode s1 2 52 2 46 ga hf t values from 0 57 to 6 36 episode s2 2 43 2 37 ga hf t values from 0 10 to 4 30 episode r1 2 32 2 26 ga hf t values from 1 61 to 7 23 from detrital zircons episode r2 2 24 2 20 ga hf t values from 0 39 to 2 73 these early paleoproterozoic accretionary orogenies mark the onset of amalgamation of the sao francisco continental paleoplate that surrounds the craton with accretions of an exotic micro block and continental magmatic arcs indicating evolution from dominant siderian juvenile magmatism to rhyacian crustal magmatism these patterns show striking similarities to the orogenies in the mineiro belt and north china craton

early crustal evolution of the yangtze craton south china new constraints from zircon u pb hf isotopes and geochemistry of ca 2 9 2 6 ga granitic rocks in the zhongxiang complex abstract we report here new zircon u pb age and hf isotope as well as geochemical analyses of the recently discovered archean paleoproterozoic zhongxiang complex in the northern central yangtze craton south china and interpret the early crustal evolution of the yangtze craton zircon la icp ms dating yielded magmatic crystallization ages of ca 2 90 2 87 ga for two monzogranites ca 2 77 ga for a trondhjemitic gneiss and ca 2 67 2 62 ga for three potassic granites the trondhjemitic gneiss also contains zircon rims that record a metamorphic event at ca 2 08 ga these results demonstrate that the zhongxiang complex is significantly older than previously thought and it contains the only potential ttg with ages of 2 8 2 7 ga for the yangtze craton these granitoids vary considerably in zircon hf isotopic and geochemical characteristics the ca 2 90 2 87 ga monzogranites correspond in composition to alkaline and calc alkalic peraluminous granites originated from mixing of ancient and juvenile crustal components at relatively shallow depths in contrast the ca 2 77 ga trondhjemitic gneisses are calc alkalic and peraluminous with relatively high sr y and la yb n ratios and slightly evolved zircon hf signatures but without significant eu anomaly that is typical of a medium pressure ttg component consequently they are interpreted to be the products of partial melting of a basaltic source with appreciable involvement of pre existing crustal components i e tonalites with garnet and amphibole in the residue the ca 2 67 2 62 ga potassic granites are alkaline faintly peraluminous and ferroan exhibiting a high silica a type granite affinity they are characterized by low al2o3 tio2 ratios with dominantly undepleted zircons suggesting their derivation from melting of meta sedimentary rocks under high temperature conditions with plagioclase as the main residual phases the zhongxiang complex thereby differs from the kongling complex and other archean provinces in the yangtze craton by preserving distinct magmatic pulses ca 2 90 2 87 ga ca 2 77 ga ca 2 67 2 62 ga with different nature and sources a comparative analysis of the crustal evolution of the zhongxiang complex and other archean provinces of the yangtze craton indicates a compositionally heterogeneous archean yangtze craton which likely comprises several archean terranes that accreted together to form a uniform craton during the late archean to paleoproterozoic time

ionospheric storms on mars impact of the corotating interaction region measurements made by the aspera 3 and marsis experiments on mars express have shown for the first time that space weather effects related to the impact of a dense and high pressure solar wind corotating interaction region on mars cause strong perturbations in the martian induced magnetosphere and ionosphere the magnetic barrier formed by pile up of the draped interplanetary magnetic field ceases to be a shield for the incoming solar wind large blobs of solar wind plasma penetrate to the magnetosphere and sweep out dense plasma from the ionosphere the topside martian ionosphere becomes very fragmented consisting of intermittent cold low energy and energized plasmas the scavenging effect caused by the intrusions of solar wind plasma clouds enhances significantly by a factor of 10 the losses of volatile material from mars

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

late paleoproterozoic sedimentary and mafic rocks in the hekou area sw china implication for the reconstruction of the yangtze block in columbia abstract the paleoproterozoic hekou group in the western yangtze block is a volcano sedimentary succession that is intruded by gabbroic plutons sedimentary rocks in the group include slates marble and meta siltstones interlayered with felsic metavolcanic rocks metabasalts and metatuffs both the volcanic rocks of the hekou group and gabbros have undergone upper greenschist to lower amphibolite facies metamorphism metatuff samples from different layers have average zircon u pb ages of 1697 ma slightly older than the intruding gabbroic plutons with zircon u pb ages of 1684 ma both metabasalts and metagabbros have similar elemental and isotopic compositions indicative of a co magmatic origin they are rich in tio2 mostly 2 5 wt zr 94 5 347 ppm ta 0 48 3 00 ppm and th 1 05 7 61 ppm with high nb y ratios mostly 0 6 and lree enriched chondrite normalized ree patterns resembling within plate mafic rocks their positive whole rock nd t 0 2 to 3 4 and zircon hf t values 3 3 to 8 4 suggest contributions from a depleted mantle source the large range of nd t and hf t values and variable degrees of nb ta anomalies ta lapn 0 17 1 75 are indicative of crust contamination during magma ascending both the metabasalts and metagabbros are considered to have formed in a continental rift setting detrital zircon grains from meta siltstones in the hekou group have u pb age populations mainly at 2070 1880 ma 2330 2250 ma and 2900 2700 ma these age populations are comparable to those of the north australian and north china cratons in the columbia supercontinent however in terms of geochemical features the 1 7 ga within plate mafic rocks at hekou are similar to those from the 1 7 ga leichhardt and calvert superbasins of the north australian craton but different from those from the 1 7 1 2 ga zhaertai bayan obo rift zone of the north china craton it is thus suggested that the yangtze block was more likely linked with the north australian craton in columbia during the late paleoproterozoic these rifting basins and mafic rocks may record the initial break up of the columbia supercontinent

peraluminous i type granites abstract s type granites always contain more al than the amounts of na ca and k in the rock required to form feldspars primarily owing to their derivation from source components that had previously been weathered those rocks are therefore always saturated in al or peraluminous many i type granites are also peraluminous despite i type source rocks typically not being saturated in al it has previously been suggested that this may result from the fractional crystallisation of amphibole however data from compositionally zoned high temperature plutons in the lachlan fold belt show that it is difficult to generate large quantities of peraluminous melt by removal of amphibole most of the i type granites 95 in the lachlan fold belt formed at lower temperatures and almost half of those rocks for which bulk chemical compositions are available are peraluminous among these granites there are 98 separate suites for which there are chemical data for two or more samples with 47 suites that include both metaluminous more mafic and peraluminous more felsic compositions the origin of those peraluminous compositions is fundamental to any understanding of i type granite petrogenesis in this region compositional variations caused by the assimilation or by partial melting of supracrustal rocks are very small as the isotopic variations within these rocks are dominantly between different suites not within suites the partial melting of more mafic source rocks rather than the fractional crystallisation of more mafic magmas is favoured for the origin of these rocks partial melting is the most likely process involved in the petrogenesis of felsic granites where broadly granodioritic monzogranitic batholiths are associated with lesser amounts of tonalite and very minor amounts of mafic rock experimental studies have shown that the melts generated by the partial melting of basaltic to andesitic rocks under crustal conditions are mostly peraluminous during the dehydrational melting of i type granite source rocks at pressures below the garnet stability field biotite and amphibole melt incongruently to yield pyroxenes the excess al is incorporated into the felsic liquid resulting in the generation of peraluminous melts in this instance the excess al in felsic i type granites is a function of the melting process and unrelated to the bulk composition of the source the observed gradation from peraluminous felsic granites to metaluminous compositions in less felsic rocks in largely isotopically closed systems could happen in two ways at higher temperatures of partial melting ca and other components of clinopyroxene could dissolve in the melt with the melt eventually becoming metaluminous alternatively minerals residual from partial melting dominantly pyroxenes and plagioclase could be incorporated in suspension in the melt so that the resulting bulk magma is metaluminous examples of these two extremes and of intermediate cases are well developed among the granites of southeastern australia

characterization and origin of aluminous a type granites from the lachlan fold belt southeastern australia the metaluminous to weakly peraluminous a type granites of the lachlan fold belt are a distinctive group of igneous rocks on the basis of chemical and mineralogical criteria those granites that contain 65 72 sio2 can be distinguished from other types on the basis of higher abundances of fetotal fetotal mg high field strength elements trivalent rare earth elements ga and zn mineralogically they contain fe rich hydrous mafic minerals and primary ilmenite and hence are reduced relative to the ni nio buffer however the extremely felsic a type granites sio2 72 have the same chemical and mineralogical characteristics as felsic fractionated i type granites recent analyses indicate that the lachlan fold belt a type granites have sc f alkali element trace transition element and h2o contents similar to those of other unfractionated i type granites rb sr and nd sm isotopic compositions are highly variable probably reflecting source region heterogeneity the metaluminous to weakly peraluminous a type granites of the lachlan fold belt are distinct from peralkaline rocks in terms of chemical composition petrography and field associations although these rocks have been grouped together as a single type in current classification schemes we propose that the metaluminous to weakly peraluminous a type granites such as those of the lachlan fold belt should be defined as aluminous a type granites and should not be grouped with peralkaline granites the lachlan fold belt aluminous a type granites have relatively high calculated zircon saturation temperatures we suggest that these granites were produced by high temperature partial melting of a felsic infracrustal source

s type granitic magmas petrogenetic issues models and evidence abstract despite a perception that it represents a perverse divergence it is perfectly possible to believe in the existence of s and i type granites and the implications for the nature of their protoliths and to disbelieve in the applicability of the restite unmixing model for chemical variation in granitic magmas white and chappell erected the s i classification with impeccable validity the isotopic evidence demands contrasting source reservoirs for s and i type granitic magmas however the major advance was not the classification but the recognition that highly contrasting parental materials must be involved in the genesis of granitic magmas the restite unmixing model is commonly seen as a companion to the s i classification but it is really a separate issue this model implies that the compositions of granites image those of their source rocks in a simple way however there are other equally valid models that can explain the data and none of them represents a unique solution the most cogent explanation for the high grade metasedimentary enclaves in most s type granites is that they represent mid crustal xenoliths restitic enclaves are either rare or absent inherited zircons in s type rocks are certainly restitic however the occurrence of a substantial restitic zircon population does not imply an equally substantial restitic component in the rest of the rock zircon and zirconium behaviours are controlled by disequilibrium and kinetics and zr contents of granitic rocks can rarely be used to infer magma temperatures since the dominant ages among inherited zircons in lachlan fold belt lfb s type granites are ordovician and proterozoic it seems likely that crust of this age but geochemically different from the exposed rocks not only underlies much of the lfb but also forms a component in the granite magma sources the evidence is overwhelming that the dark microgranular enclaves that occur in both s and i type granites are igneous in origin they represent globules of quenched more mafic magma mingled and modified by exchange with the host granitic magma however magma mixing does not appear to be a significant process affecting the chemical evolution of the host magmas likewise the multicomponent mixing models erected for some granitic rock suites are mathematically nonunique and in some cases violate constraints from isotopic studies s and i type magmas commonly retain their distinct identities this suggests limited source mixing limited magma mixing and limited wall rock assimilation though intermediate types certainly exist they are probably relatively minor in volume crystal fractionation probably plays the major role in the differentiation of very many granitic magmas including most s types especially those emplaced at high crustal levels or in the volcanic environment minor mechanisms include magma mixing wall rock assimilation and restite unmixing isotopic variations within plutons and in granite suites could be caused by source heterogeneities magma mixing assimilation and even by isotopic disequilibrium however source heterogeneity coupled with the inefficiency of magma mixing is probably the major cause of observed heterogeneity normal geothermal gradients are seldom sufficient to provide the necessary heat for partial melting of the crust and crustal thickening likewise fails to provide sufficient heat generally the mantle must be the major heat source this might be provided through mantle upwelling and crustal thinning and possibly through the intra and underplating of mafic magmas upper crustal extension seems to have been common in regions undergoing granitic magmatism migmatites probably provide poor analogues of granite source regions because they are mostly formed by fluid present reactions granitic magmas are mostly formed by fluid absent processes where we do see rare evidence for arrested fluid absent partial melting the melt fraction is invariably concentrated into small shear zones veinlets and small dykes thus it seems likely that dyking is important in transporting granitic magma on a variety of scales and at many crustal levels however one major missing link in the chain is the mechanism by which melt fractions in small scale segregations occurring over a wide area can be gathered and focused to efficiently feed much wider spaced major magma conduits answers may lie in the geometry of the melting zones and in the tendency of younger propagating fractures to curve toward and merge with older ones self organization almost certainly plays a role

geochemistry and tectonic setting of lewisian clastic metasediments from the early proterozoic loch maree group of gairloch nw scotland abstract the early proterozoic 2 0 ga loch maree group lmg of the lewisian complex consists of a supracrustal sequence of metasedimentary schists and associated tholeiitic metavolcanic amphibolites within the archaean basement the metasediments comprise three main mapped units the flowerdale schists kerrysdale schists and charlestown schists together with numerous narrower bands the dominant clastic metasediments within the schist units semipelites and psammites are chemically distinguished as a series of greywackes and lithic sandstones with variable lil and transition element abundances fractionated ree patterns and overall compositions similar to paas three chemical groups are recognized whose range of composition largely reflects the mixing between two main source components lewisian gneissose basement with an intermediate acid continental crust composition and local rift related amphibolites with a primitive tholeiitic composition which also incorporates components not preserved within the lmg the geochemistry of the clastic metasediments confirms a previous tectonic model for the lmg as an intracontinental extensional rift receiving clastic detritus from the adjacent uplifted basement as well as from internal volcanogenic sources

structural framework of the bashkirian anticlinorium sw urals the bashkirian anticlinorium of the southwestern urals shows a much more complex structural architecture and tectonic evolution than previously known pre uralian proterozoic extensional and compressional structures controlled significantly the uralian tectonic convergence a long lasting proterozoic rift process created extensional basement structures and a riphean basin topography which influenced the formation of the western fold and thrust belt with inversion structures during the uralian deformation a complete orogenic cycle during cadomian times including terrane accretion at the eastern margin of the east european platform resulted in a high level cadomian basement complex which controlled the onset of uralian deformation and resulted in intense imbrication and tectonic stacking in the subjacent footwall of the main uralian fault the uralian orogenic evolution can be subdivided into three deformation stages with differently oriented stress regimes tectonic convergence started in the late devonian with ophiolite obduction tectonic accretion of basin and slope units and early flysch deposits zilair flysch the accretionary complex prograded from the se to the nw continuous nw se directed convergence resulted finally in the formation of an early orogenic wedge thrusting the cadomian basement complex onto the east european platform the main tectonic shortening was connected with these two stages and although not well constrained appears to be of late devonian to carboniferous age in the permian a final stage of e w compression is observed throughout the sw urals in the west the fold and thrust belt prograded to the west with reactivation of former extensional structures and minor shortening in the east this phase was related to intense back thrusting the east european platform was subducted beneath the magnitogorsk magmatic arc during the late paleozoic collision the thick and cold east european platform reacted as a stable rigid block which resulted in a narrow zone of intense crustal shortening tectonic stacking and high strain at its eastern margin whereas the first orogenic wedge is of thick skinned type with the involvement of crystalline basement even the later west directed wedge is not typically thin skinned as the depth of the basal detachment appears below 15 km and the involvement of archean basement can be assumed

petrogenesis of the red mountain pluton laramie anorthosite complex wyoming implications for the origin of a type granite abstract the red mountain pluton the youngest intrusion in the laramie anorthosite complex is one of five monzonitic to granitic a type plutons intruded at 1 43 ga in the laramie mountains of southeastern wyoming the red mountain pluton ranges in composition from fayalite monzonite clinopyroxene quartz monzonite and biotite hornblende quartz syenite to granite the red mountain pluton is petrologically and geochemically distinct from the northern and southern sherman batholith the sybille intrusion and the maloin ranch pluton it has higher feo t feo t mgo and higher k 2 o is more strongly enriched in rees and at any given silica content it has lower abundances of tio 2 feo mgo cao and p 2 o 5 the fayalite monzonite in the red mountain pluton crystallized at temperatures around 1000 c and oxygen fugacities 1 2 log units below that of the fmq buffer similar to the crystallization conditions of the sybille and maloin ranch plutons the biotite hornblende quartz syenite crystallized at oxygen fugacities near or slightly below those of fmq most of the units in the pluton carry mantle like nd sr and pb isotopic compositions similar to the least contaminated anorthositic and ferrodioritic rocks of the lac indicating that the pluton evolved mainly via differentiation substantial crustal assimilation occurred only in the late dikes and the granite we suggest that all of the 1 43 ga a type intrusions in southeastern wyoming are ultimately derived from tholeiites extreme differentiation or partial melting produced monzonitic magmas that have assimilated varying amounts of siliceous crust the differences between the red mountain pluton and other a type granitoids in southeastern wyoming are related to the extent of differentiation prior to crustal assimilation the compositional ranges of a type granitoids in southeastern wyoming suggest that the geochemical spectrum of a type granites worldwide similarly are produced from evolved mantle derived melt that has assimilated varying proportions of crustal material

a palaeoproterozoic tectono magmatic lull as a potential trigger for the supercontinent cycle the geologic record exhibits periods of active and quiescent geologic processes including magmatism metamorphism and mineralization this apparent episodicity has been ascribed either to bias in the geologic record or fundamental changes in geodynamic processes an appraisal of the global geologic record from about 2 3 to 2 2 billion years ago demonstrates a palaeoproterozoic tectono magmatic lull during this lull global scale continental magmatism plume and arc magmatism and orogenic activity decreased there was also a lack of passive margin sedimentation and relative plate motions were subdued a global compilation of mafic igneous rocks demonstrates that this episode of magmatic quiescence was terminated about 2 2 billion years ago by a flare up of juvenile magmatism this post lull magmatic flare up is distinct from earlier such events in that the material extracted from the mantle during the flare up yielded significant amounts of continental material that amalgamated to form nuna earth s first hemispheric supercontinent we posit that the juvenile magmatic flare up was caused by the release of significant thermal energy that had accumulated over some time this flux of mantle derived energy could have provided a mechanism for dramatic growth of continental crust as well as the increase in relative plate motions required to complete the transition to modern plate tectonics and the supercontinent cycle these events may also be linked to palaeoproterozoic atmospheric oxygenation and equilibration of the carbon cycle earth experienced a lull in magmatic and tectonic activity about 2 3 billion years ago followed by a flare up of magmatism according to a compilation of existing geologic data these events might mark the transition to the supercontinent cycle

episodic crustal growth in the southern segment of the trans north china orogen across the archean proterozoic boundary abstract the dengfeng and taihua complexes are well exposed neoarchean to paleoproterozoic units in the southern segment of the trans north china orogen tnco zircon u pb dating shows that the dengfeng complex records two episodes 2568 11 ma and 2306 6 ma of tonalite trondhjemite granodiorite ttg magmatism all of the ttg rocks are characterized by high sio2 66 7 75 4 wt na2o 3 20 5 06 wt and relatively low mgo 0 40 1 78 wt the late neoarchean ttg gneisses have very low contents of hree ybn 0 69 2 75 and y 1 73 7 07 ppm with moderate la yb n 24 1 53 8 and high sr y 65 1 291 3 ratios the early paleoproterozoic ttg gneisses have low contents of hree ybn 2 93 6 37 and y 6 7 11 0 ppm with moderate la yb n 10 1 27 0 and sr y 10 6 52 1 ratios both suites show pronounced negative nb ta p and ti anomalies but positive sr and pb anomalies the late neoarchean ttg gneisses all have similar bulk rock nd and zircon hf model ages with mainly positive nd t and are interpreted as resulting from the melting of dominantly juvenile thickened lower crust with residual garnet and amphibole the early paleoproterozoic ttg gneisses have extremely variable nd t 6 23 to 4 23 and heterogeneous zircon hf t 3 3 to 3 1 which are also best interpreted as resulting from the partial melting of thickened lower crust with residual amphibole and garnet the taihua complex in the xiaoqinling area records three episodes of early paleoproterozoic ttg magmatism 2 48 ga at caotan 2 31 ga at houjiacun and 2 16 ga at bayuan younger than the taihua complex in the lushan area 2 85 2 72 ga all rocks have relatively low contents of hree ybn 1 03 8 32 and y 2 84 24 ppm with moderate la yb n 8 7 88 4 and sr y 19 8 125 8 ratios and show negative ta nb and ti anomalies and positive sr and pb anomalies the caotan gneisses at 2 48 ga and the houjiacun ttg gneisses at 2 31 ga have low mg 0 14 0 45 low cr the geochronology of the dengfeng complex in the dengfeng area and the taihua complex in the lushan xiong er and xiaoqinling areas reveals at least four magmatic episodes in the southern segment of the tnco from the late mesoarchean to early paleoproterozoic 2 85 2 72 ga 2 57 2 48 ga 2 34 2 30 ga and 2 20 2 07 ga the rocks of the two early episodes are dominantly of juvenile compositions with mostly positive whole rock nd t and zircon hf t values suggesting two episodes of crustal growth formed in a subduction tectonic setting the magmatic rocks of the third episode consist of both the juvenile and pre existing crustal materials with variable whole rock nd t and zircon hf t values which were generated in a subduction zone during the initial assembly of the ncc within the columbia supercontinent cycle the final episode of magmatism lacks juvenile materials with whole rock nd t and zircon hf t values being consistently negative these may have resulted from the orogenic collapse the episodic continental growth recorded in the southern segment of the tnco was caused by subduction and consequent orogeny consistent with global supercontinent cycles within the late archean and early paleoproterozoic

reworked old crust derived shoshonitic magma the guarany pluton northeastern brazil abstract the 572 ma guarany stock consists of magmatic epidote bearing hornblende monzodiorite to biotite granite that intruded paleoproterozoic orthogneisses about 10 km inland from the coast in northeastern brazil co magmatic diorite enclaves and dikes are abundant throughout the pluton the monzodiorite granite pluton and diorite enclaves are shoshonitic and display continuous trends in variation diagrams they display chemical and isotopic characteristics of crustal melts such as enrichment in incompatible elements high back calculated initial 87 sr 86 sr ratios avg 0 71253 negative end 0 57ga values avg 14 58 as well as high and variable 9 1 to 11 1 vsmow 18 o zircon values correlations between o isotope and whole rock silica contents as well as initial 87 sr 86 sr ratios with 1 sr concentrations suggest hybridization of a lower continental crustal melt with more felsic crustal rocks concomitant with fractional crystallization amphibole chemistry and whole rock zr tio 2 and p 2 o 5 contents suggest magma solidification at a pressure 7 kbar and near liquidus temperature 900 c the parental magma was likely formed by partial melting of old tdm 2 0 ga amphibolitic lower continental crustal rocks in a post collisional setting probably triggered by underplating of mantle derived mafic magma during the period of relaxation after collision

identification of ca 2 65 ga ttgs in the yudongzi complex and its implications for the early evolution of the yangtze block abstract in this contribution we carry out an integrated study of petrology whole rock geochemistry as well as zircon morphology trace element u pb and lu hf isotopes for granitic gneisses and leucogranites from the yudongzi complex a poorly studied archean outcrop in the northwestern yangtze block the granitic gneisses have the ttg affinity characterized by relatively high na2o k2o and sr y ratios and fractionated ree pattern with relatively high lree but low hree concentrations zircon grains in the granitic gneisses show magmatic cores and metamorphic rims u pb dating on zircon core of four granitic gneiss samples yielded their formation ages as 2660 2632 ma with a weighted average of 2650 21 ma mswd 1 6 n 4 their average ehf t values range from 0 6 to 0 5 and two stage model ages from 3 20 to 3 10 ga suggesting that the granitic gneisses were likely generated by remelting of thickened mesoarchean proto mafic crust the metamorphic zircon rims have relatively high th u ratios and identical hf isotope compositions to their corresponding cores indicating that the rims were formed via solid state recrystallization process the metamorphic zircons recorded a metamorphic event at ca 2 5 ga u pb dating and lu hf isotopes on magmatic zircons from a leucogranite sample revealed a magmatic event at 2477 18 ma the recognization of the 2 65 ga ttg magmatism and the 2 48 ga intercrustal remelting event in the yudongzi complex demonstrates the widespread neoarchean magmatism in the yangtze block combined with the age spectra of previous detrital zircons we show that the yangtze block has experienced a major tectonothermal event in the late archean the yudongzi complex has a distinct crustal evolution history compare with the paleoarchean continent nucleus in the eastern part of the yangtze block suggesting that the continent nucleus might have subjected to lateral growth by accretion processes

generation of sodium rich magmas from newly underplated basaltic crust sodium rich rocks of trondhjemite tonalite dacite ttd or granodiorite ttg suites form much of precambrian continental crust1 they are thought to have formed by partial melting of subducted oceanic crust2 3 a process that would have been much more widespread early in earth history than at present owing to the higher thermal gradients prevailing at that time4 phanerozoic ttd suites do exist however and seem also to relate to subduction zones5 defant and drummond6 proposed that these suites form where young 25 myr hot oceanic lithosphere is subducted and melts thus locally simulating the conditions that led to widespread crustal growth in the archaean here we describe plutonic and volcanic rocks from the cordillera blanca complex in peru which have characteristics of the high al ttd suite but which were produced above a subduction zone containing a 60 myr old slab we present evidence that the complex formed by partial melting of newly underplated basaltic crust and argue that this mechanism should be considered more generally as an additional way of generating sodium rich arc magmas

geochronology and geochemistry of igneous rocks from the kongling terrane implications for mesoarchean to paleoproterozoic crustal evolution of the yangtze block abstract the kongling terrane is a high grade metamorphic complex exposed in the northern part of the yangtze block it mainly consists of ttg gneisses amphibolites migmatites and metasedimentary rocks the amphibolites occur as enclaves or layers in the ttg gneisses and both the amphibolites and the ttg gneisses are intruded by granitic and diabasic dykes u pb ages of zircons from the amphibolite migmatite granite and diabase are determined by la icp ms igneous zircons from an amphibolite have an upper intercept age of 2945 13 ma mswd 1 2 while metamorphic zircons yield an upper intercept age of 2870 13 ma mswd 0 9 igneous zircons from a migmatite and a granite have weighted average 207pb 206pb age and upper intercept age of 1987 5 ma mswd 1 7 and 1985 6 ma mswd 1 3 respectively igneous zircons from a diabase dyke yield an upper intercept age of 1783 23 ma mswd 0 86 the protoliths of the amphibolites are tholeiitic basalts and exhibit slightly depleted to variably enriched lree patterns with moderate enrichment of rb and ba and depletion in nb and zr similar to oceanic island arc volcanics they have high cr co ni mgo and feot content with negative nd t values indicating that the tholeiitic basalts resulted from high degree batch melting of enriched mantle wedge the granites are strongly peraluminous with a cnk 1 1 and they have relatively low hree and y and negative nd t values detailed elemental and isotopic data suggest that they formed by partial melting of recycled products of the ttg gneisses amphibolites and older crustal material in a continental collisional setting the diabases are characterized by relatively high al2o3 feot and tio2 contents and low zr ba ratios their high initial 87sr 86sr ratios and negative nd t values indicate that they were derived from enriched lithospheric mantle in extensional setting therefore the kongling terrane occurred as an oceanic island arc at ca 2 95 ga and experienced earliest metamorphism at ca 2 87 ga which probably contributed to the formation of the high grade metamorphic terrane this terrane experienced a strong collisional event at 2 0 ga which may have led to cratonization over all of the yangtze block and also experienced lithospheric extension during 1 85 1 78 ga the global 2 0 ga paleoproterozoic collisional events were related to the assembly of supercontinent columbia

a hybrid origin for the qianshan a type granite northeast china geochemical and sr nd hf isotopic evidence abstract major and trace element whole rock sr and nd isotope and zircon hf isotope data are reported for a suite of a type granites and mafic microgranular enclaves from the early cretaceous 126 2 ma qianshan pluton liaodong peninsula northeast china with the aim of investigating the sources and petrogenesis of a type granites the qianshan pluton includes hornblende alkali feldspar granite graphic biotite granite and mafic microgranular enclaves the hornblende alkali feldspar granites have high sio 2 fe 2 o 3t mgo k 2 o na 2 o rb zr and lree contents and low ba and sr concentrations with strongly negative eu anomalies their high rb sr 87 rb 86 sr 16 76 24 15 and initial 87 sr 86 sr ratios 0 7215 to 0 7283 negative e nd t values 14 1 to 16 5 and zircon e hf t values 18 9 to 11 5 indicate they were mainly derived from a crustal source but with involvement of high e nd t and e hf t materials graphic biotite granites have similar geochemical features and sr nd hf isotopic compositions to enclaves indicating they were the result of crystal fractionation of evolved mafic magmas but with involvement of low e nd t and e hf t materials the mafic enclaves have an igneous texture and contain acicular apatite suggesting quenching of mafic magmas that have co mingled with the host granites they have low initial 87 sr 86 sr ratios 0 7097 0 7148 negative e nd t 14 5 to 11 9 and zircon e hf t 17 1 to 6 9 values and are enriched in liles and lrees and depleted in hfses when coupled with the high mgo mg up to 54 this indicates derivation from an enriched lithospheric mantle source but contaminated by crustal materials geochemical and sr nd and zircon hf isotopic compositions rule out simple crystal liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks instead magma mixing of mantle derived mafic and crustal derived magmas coupled with crystal fractionation is compatible with the data this example shows that at least some a type granites formed through a complex process involving mantle and crustal derived magma mixing crystal fractionation and infracrustal melting

early paleoproterozoic magmatism in the yangtze block evidence from zircon u pb ages sr nd hf isotopes and geochemistry of ca 2 3 ga and 2 1 ga granitic rocks in the phan si pan complex north vietnam abstract our understanding of the early evolution of the yangtze block is limited by the sparsely dispersed nature of pre neoproterozoic exposures new integrated petrographic zircon u pb age and hf nd isotope analyses and whole rock geochemical data for early paleoproterozoic granites in the phan si pan complex provides new insights into the evolution of the yangtze block as well as its role in the pre nuna supercontinent la icp ms zircon u pb dating of magmatic zircons from quartz monzonite and gneissic granite yielded 207pb 206pb ages of 2306 12 ma and 2096 15 ma respectively zircons from the quartz monzonite have ehf t values ranging from 4 1 to 2 1 corresponding to tdm2 model ages of 3002 2890 ma whereas zircons in the gneissic granite have ehf t values between 0 95 and 1 72 and corresponding tdm2 model ages of 2660 2516 ma which are consistent with their whole rock nd isotope values geochemically the quartz monzonites are i type granites combined with their relatively high sr y ratios and low y concentrations as well as fractionated ree patterns with relatively high lree but low hree concentrations they were probably generated by partial melting of the thickened middle lower crust under elevated temperature geochemical and isotopic signatures suggest that the ca 2 1 ga gneissic granites are high k calc alkaline ferroan a type granites formed by partial melting of juvenile crustal source at high temperature and low pressure with little involvement of ancient crustal material the phan si pan complex has a distinct early paleoproterozoic crustal evolution history compared with the other crustal provinces of the yangtze block suggesting independent histories that were not unified until the late paleoproterozoic during the assembly of nuna moreover the magmatism and tectonic evolution of the north vietnam region is broadly similar to that of the arrowsmith orogen of the rae craton in laurentia suggesting a potential spatial linkage the geologic record of the yangtze block does not support an early paleoproterozoic shutdown of plate tectonics

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

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

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

a possible buried paleoproterozoic collisional orogen beneath central south china evidence from seismic reflection profiling abstract the south china craton consisting of the yangtze and cathaysia blocks has figured importantly in the neoproterozoic supercontinent of rodinia however its lack of grenville age structures and metamorphism and the presence of paleoproterozoic high grade metamorphism and s type magmatism raise questions about the timing and nature of the yangtze cathaysia collision here we present results of a 400 km long high resolution seismic reflection profile across the purported suture between the yangtze and cathaysia blocks the seismic profile reveals folded and thrust imbricated seismic reflectors which we interpret to represent the relics of the yangtze cathaysia collisional orogen the inferred orogen was extended in the neoproterozoic resulting in its burial by neoproterozoic flysch strata geochronological data suggest that this buried orogen was formed in the paleoproterozoic 2 0 1 9 ga likely associated with the assembling of the columbia supercontinent these results call for major revision of the models for the formation of the south china craton and its role in the assembling of the rodinia and the columbia supercontinents

maven observations of energy time dispersed electron signatures in martian crustal magnetic fields energy time dispersed electron signatures are observed by the mars atmosphere and volatile evolution maven mission in the vicinity of strong martian crustal magnetic fields analysis of pitch angle distributions indicates that these dispersed electrons are typically trapped on closed field lines formed above strong crustal magnetic sources most of the dispersed electron signatures are characterized by peak energies decreasing with time rather than increasing peak energies these properties can be explained by impulsive and local injection of hot electrons into closed field lines and subsequent dispersion by magnetic drift of the trapped electrons in addition the dispersed flux enhancements are often bursty and sometimes exhibit clear periodicity suggesting that the injection and trapping processes are intrinsically time dependent and dynamic these maven observations demonstrate that common physical processes can operate in both global intrinsic magnetospheres and local crustal magnetic fields

first evidence for archean continental crust in northern vietnam and its implications for crustal and tectonic evolution in southeast asia southeast asia or in a more strict sense the indochinese continent was previously considered to be composed entirely of proterozoic to phanerozoic rocks and reliable evidence for archean crust was lacking gneisses from the cavinh complex south of the red river shear zone northern vietnam however show archean nd model ages of 3 4 3 1 ga zircon separates from the rocks yielded u pb dates of 2 8 2 5 ga the first convincing evidence for the presence of a late archean complex in southeast asia given that the red river shear zone was propagating in the south china block with a left lateral offset of 600 km the cavinh complex can be correlated with the late archean kangding complex in the western margin of the yangtze craton southwestern china the cavinh complex therefore represents one of the oldest crustal nuclei of the south china block

petrogenesis of the late archean 2 5 ga na and k rich granitoids in the zhongtiao wangwu region and its tectonic significance for the crustal evolution of the north china craton abstract petrogenesis of coexisting late archean 2 5 ga na and k rich granitoids in the zhongtiao wangwu region is significant for understanding the crustal evolution of the north china craton ncc in this paper we present new zircon u pb age and whole rock geochemical and isotope data on a 2 5 ga na and k rich granitoid suite from the zhongtiao wangwu region southern ncc the na rich granitoids ttg and k rich granitoids a type granites are zircon u pb dated to be 2 45 ga and 2 49 ga respectively integrated geochemical data indicate that the ttg were formed by partial melting of the mafic lower crust while the a type granites were formed by partial melting of a ttg like source in the shallower crust the petrogenesis likely occurred under lithospheric delamination and the subsequent asthenospheric upwelling and mafic magma underplating we proposed that the coeval occurrence of na and k rich granitoids in the southern ncc is probably ascribed to the partial melting of continental crust at different depths which could be an important way to induce the archean crustal reworking and compositional changes at 2 5 ga

dynamic response of the martian ionosphere to an interplanetary shock mars express and maven observations multipoint observations from the mars advanced radar for subsurface and ionosphere sounding marsis instrument on board mars express and the mars atmosphere and volatile evolution maven mission reveal a dynamic response of the martian ionosphere to abrupt variations in the upstream solar wind plasma on 2 february 2017 maven located upstream from the martian bow shock encountered a cir related interplanetary shock with a sudden enhancement in the dynamic pressure marsis operating in the upper ionosphere at 478 km altitudes and 78 solar zenith angles observed a sharp increase in the local magnetic field magnitude 1 min after the shock passage at maven the time lag is roughly consistent with the expected propagation time of a pressure pulse from the bow shock to the upper ionosphere at the fast magnetosonic speed subsequently remote soundings recorded disturbed signatures of the topside ionosphere below mars express

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

zircon u pb chronology hf isotope analysis and whole rock geochemistry for the neoarchean paleoproterozoic yudongzi complex northwestern margin of the yangtze craton china abstract the archean paleoproterozoic high grade basement of yudongzi complex is a key to understanding the early precambrian crustal evolution of the yangtze craton it comprises mainly orthogneiss paragneiss and amphibolite whose protoliths are tonalitic trondhjemitic granitic ttg sedimentary and basic intermediate volcanic rocks respectively the ttg gneiss amphibole plagiogneiss and biotite plagiogneiss yield magmatic zircon la icp ms u pb ages of 2815 18 ma mswd 0 92 2692 26 ma mswd 0 59 and 2449 4 ma mswd 0 94 respectively metamorphic overgrowths on zircon from amphibolite have an age of 1848 5 ma mswd 0 71 ttg gneisses show medium sr y and variable high la yb n ratios with low y and yb contents they are characterized by positive eu anomaly and distinct depletion of hree together with negative nb ta and ti implying amphibole garnet and minor rutile as residual phases their positive e hf t values of 2 1 to 8 1 with t dm2 of ca 2 80 3 10 ga suggest significant reworking of juvenile crust amphibole plagiogneisses display a strong enrichment of lrees and depletion of nb ta and ti additionally a relative enrichment of ba rb pb and zr as well as high cr and ni contents and mg values imply a mantle source with the addition of continental crust material zircon e hf t values vary between 0 9 and 3 9 suggesting a proportionally significant input of juvenile material and therefore interaction between the mantle and pre existing continental crust biotite plagiogneisses show negative e hf t values between 3 4 and 0 1 with a few positive e hf t values ranging from 0 1 to 1 5 together with t dm2 ages of ca 2 80 3 00 ga these e hf t values suggest that these rocks were mainly generated by reworking of ancient crust thus the yudongzi complex exposed in the northern part of the yangtze craton has experienced significant reworking of juvenile crust at ca 2 80 ga and subsequent crustal growth at ca 2 70 ga followed by a second stage of reworking of ancient crust at ca 2 45 ga during the late paleoproterozoic the yudongzi complex was probably involved in the amalgamation of the paleoproterozoic supercontinent columbia and affected by a post collisional metamorphic event at ca 1 85 ga

triassic continental subduction in central tibet and mediterranean style closure of the paleo tethys ocean the qiangtang metamorphic belt qmb in central tibet is one of the largest and most recently documented high pressure hp to near ultrahigh pressure near uhp belts on earth lu hf ages of eclogite and blueschist facies rocks within the qmb are 244 223 ma indistinguishable from the age of uhp metamorphism in the qinling dabie orogen results of a u pb detrital zircon study suggest that protoliths of the qmb include upper paleozoic qiangtang continental margin strata and sandstones that were derived from a paleozoic arc terrane that developed within the paleo tethys ocean to the north we attribute qmb hp metamorphism to continental collision between the qiangtang terrane and a paleo tethys arc terrane this collision and the coeval south china north china collision may have slowed convergence between laurasia and gondwana derived terranes and initiated mediterranean style rollback and backarc basin development within much of the remnant paleo tethys ocean realm

on ferroan a type granitoids their compositional variability and modes of origin we recognize eight types of ferroan granitoid that can be distinguished on the basis of major element chemistry these include alkalic granitoids that may be metaluminous or peralkaline alkali calcic granitoids that may be metaluminous peraluminous or peralkaline calc alkalic granitoids that may be metaluminous or peraluminous and rare calcic ferroan granitoids these granitoids may form by two distinct end member processes extreme differentiation of basaltic melts results in ferroan granitoids that are either peralkaline alkalic and alkali calcic or metaluminous alkalic alkali calcic and calc alkalic with alkalinity increasing with increasing pressure of differentiation partial melting of tonalitic to granodioritic crust produces alkali calcic to calc alkalic granitoids that are metaluminous at low pressures and peraluminous at high pressures it is likely that a combination of these two processes plays some role in the formation of most ferroan granitoids most granitoids that are referred to as ferroan however the term has become more confusing than edifying because it has been applied to a broad spectrum of granitoid compositions with varying petrogenesis for this reason we recommend that the term be discontinued and ferroan used in its stead

filling in the juvenile magmatic gap evidence for uninterrupted paleoproterozoic plate tectonics abstract despite several decades of research on growth of the continental crust it remains unclear whether the production of juvenile continental crust has been continuous or episodic throughout the precambrian models for episodic crustal growth have gained traction recently through compilations of global u pb zircon age frequency distributions interpreted to delineate peaks and lulls in crustal growth through geologic time one such apparent trough in zircon age frequency distributions between 2 45 and 2 22 ga is thought to represent a pause in crustal addition resulting from a global shutdown of magmatic and tectonic processes the 2 45 2 22 ga magmatic shutdown model envisions a causal relationship between the cessation of plate tectonics and accumulation of atmospheric oxygen over the same period here we present new coupled u pb hf and o isotope data for detrital and magmatic zircon from the western churchill province and trans hudson orogen of canada covering an area of approximately 1 3 million km2 that demonstrate significant juvenile crustal production during the 2 45 2 22 ga time interval and thereby argue against the magmatic shutdown hypothesis our data is corroborated by literature data showing an extensive 2 22 2 45 ga record in both detrital and magmatic rocks on every continent and suggests that the operation of plate tectonics continued throughout the early paleoproterozoic while atmospheric oxygen rose over the same time interval we argue that uninterrupted plate tectonics between 2 45 and 2 22 ga would have contributed to efficient burial of organic matter and sedimentary pyrite and the consequent rise in atmospheric oxygen documented for this time interval

zircon u pb ages and hf isotopic compositions of alkaline silicic magmatic rocks in the phan si pan tu le region northern vietnam identification of a displaced western extension of the emeishan large igneous province abstract in situ zircon u pb and hf isotope analyses were carried out for alkaline silicic magmatic rocks from the phan si pan tu le region in northern vietnam to constrain their possible sources and to determine their petrogenetic relationship with the emeishan large igneous province elip sw china nine granites and nine rhyolites yield zircon 206pb 238u ages from 262 ma to 249 ma coinciding with the timing of silicic magmatism in the panxi area of the elip the zircon ehf t values 14 to 3 of these granites and rhyolites suggest a moderately depleted mantle source and overlap with those of peralkaline and metaluminous granites in the panxi area i e ehf t 14 to 4 the zircon hf isotope ratios show that the zircons probably record the original source characteristics whereas whole rock nd isotope data indicate an evidence for crustal contamination that may have occurred at lower temperatures during magma emplacement the synchroneity coupled with petrological and geochemical similarities indicate that silicic rocks from the phan si pan tu le region are cogenetic with the panxi silicic plutonic rocks and that they are likely derived by similar petrogenetic processes i e fractionation of mafic magmas or partial melting of mafic rocks therefore we propose that the phan si pan tu le region represents a displaced portion of the elip inner zone

oldest volcanic hosted submarine iron ores in south china evidence from zircon u pb geochronology and geochemistry of the paleoproterozoic dahongshan iron deposit abstract the dahongshan iron deposit is hosted in the paleoproterozoic submarine metavolcanic rocks of the dahongshan group in the yangtze block south china la icp ms dating of hydrothermal zircon grains from the genetically associated albitite and dolomite albitite show ca 2008 ma ages that are consistent with the zircon ages from the host metavolcanic rocks ca 2012 ma and postdated the post ore diabase dike ca 1724 ma marking the dahongshan iron deposit as the oldest submarine volcanic hosted deposit so far as known the ore hosting metavolcanic rocks in the dahongshan deposit have low ni 9 1 77 4 ppm cr 1 0 63 0 ppm and co contents 5 6 62 9 ppm suggesting the fractionation of olivine clinopyroxene and plagioclase within the magma chamber the major and trace element features of the alkaline to tholeiitic metavolcanic rocks are consistent with high degree partial melting of the mantle wedge metasomatized by melts enriched in high field strength elements hfses which were derived from the subducted slab in volcanic arc setting based on an evaluation of the morphology of orebody ore fabrics petrology and melt fluid inclusions as well as the geochemical characteristics of the major ore mineral magnetite we correlate the iron mineralization in the dahongshan deposit with hydrothermal process induced by the high temperature high salinity and fe rich brines derived through magmatic exsolution the similar characteristic of ce and eu anomalies of the dahongshan iron deposit and banded iron formations bifs suggest that the dahongshan deposit was formed in reducing environment although the two types of iron ores were generated through distinct processes with hydrothermal processes dominating for the submarine volcanic hosted iron deposits whereas the bifs were formed through chemical precipitation

mars ionospheric interaction with comet c 2013 a1 siding spring s coma at their closest approach as seen by mars express on 19 october 2014 mars experienced a close encounter with comet c 2013 a1 siding spring using data from the mars advanced radar for subsurface and ionosphere sounding marsis on board mars express mex we assess the interaction of the martian ionosphere with the comet s coma and possibly magnetic tail during the orbit of their closest approach the topside ionospheric electron density profile is evaluated from the altitude of the peak density of the ionosphere up to the mars express altitude we find complex and rapid variability in the ionospheric profile along the mex orbit not seen even after the impact of a large coronal mass ejection before closest approach large electron density reductions predominate which could be caused either by comet water damping or comet magnetic field interactions after closest approach a substantial electron density rise predominates moreover several extra topside layers are visible along the whole orbit at different altitudes which could be related to different processes as we discuss

geochemical characteristics of the triassic tethys turbidites in northwestern sichuan china implications for provenance and interpretation of the tectonic setting the triassic tethys graywacke slate turbidites in the northwestern sichuan china have been analysed for major and trace elements and petrographically studied compared to phanerozoic turbidites the turbidites in this study are quartz intermediate average 55 in composition and are characterized geochemically by their moderate fe2o 3 mgo 9 28 2 84 tio2 0 72 0 17 contents and al2o3 sio2 0 25 0 10 ratios moderate abundances of ferromagnesian trace elements co 15 8 ppm cr 113 72 ppm ni 14 9 ppm sc 12 4 ppm etc and moderate contents of incompatible elements such as th 11 3 ppm u 3 0 7 ppm zr 170 64 ppm hf 4 1 ppm and total rees 159 33 ppm in general the slates show systematically higher fe2o 3 mgo sc co and eueu but lower sollasc and thsc than the associated graywackes suggesting that various provenance components may separate into different grain size fractions during sedimentary sorting processes that is the more mafic materials tended to incorporate into the sedimentary record for the clay size fraction framework modes and geochemical data indicate that the turbidites were mainly derived from a recycled orogenic provenance characterized chiefly by sedimentary metasedimentary rocks and granite gneisses similar to the upper continental crust but with a variable admixture of continental island arc volcanic components flysch deposition took place in a back arc basin situated between an active continental margin the kunlun qinling fold belt and a continental island arc the yidun island arc weathering conditions in the source area significantly influenced the composition and distribution of elements in the sediments with the elapse of time during sedimentation the degree of chemical weathering in the provenance became intense while the tectonic activity decreased gradually

age geochemistry and tectonic setting of buqingshan ophiolites north qinghai tibet plateau china abstract the buqingshan ophiolite complex is a sector of the a nyemaqen ophiolite belt in the east kunlun southern marginal suture zone in the northern part of the qinghai tibet plateau the wnw trending ophiolite complex consists of metaperidotite gabbro diabase pillow basalt massive basalt and pelagic sedimentary rocks including radiolarian chert we have determined that early paleozoic and early carboniferous early permian ophiolites are present in this ophiolite complex that were previously thought of permian middle triassic age a zircon u pb age of 467 2 0 9 ma for gabbro and a rb sr isochron age of 481 130 ma for diabase and gabbro have been obtained from the early paleozoic ophiolite slice and middle late ordovician acritarchs have been found in the melange matrix that envelopes the sliver we interpret the first two ages to represent the formation age of the ophiolite granodiorite tonalite plutons with a zircon u pb age of 402 24 ma intrude the slice early carboniferous early permian radiolarians have been discovered in cherts and argillaceous cherts in the second ophiolite slice geochemistry indicates that metaperidotites in the first slice represent a depleted ocean lithosphere mantle most mafic rocks in the early paleozoic ophiolite are of n morb type with small amount of t morb and they resulted from the magma from the depleted oceanic lithosphere mantle with little crustal contamination the basalts in the early carboniferous early permian ophiolite have similar characteristics to the mafic rocks and also show dupal anomaly and a mixed source of dmm and em ii the original formation environments of the two stage ophiolites are both mid ocean ridges these lines of evidence suggest that a mature early paleozoic kunlun qilian qinling ocean basin and a mature paleotethyan ocean basin once existed in the study area

the petrogenesis of a type magmas from the amram massif southern israel the 550 530ma alkaline magmatic suite of the amram massif southern israel was emplaced during the transition from an orogenic to an intra plate tectonic setting in the northeastern arabian nubian shield ans the suite ranges from 45 6t o 788 wt sio2 and consists of rhyolites alkali quartz syenites quartz syenites monzonites and co magmatic mafic to felsic alkaline dikes these rocks define a continuous chemical evolutionary trend and reveal a correlation between decreasing stratigraphic age and increasing silica content the felsic members of the suite display a type characteristics and are genetically linked through fractionation to the more mafic ones moderately positive initial end values a2 05 low initial 87 sr 86 sr values 07036 2 high mgo and fe2o3 concentrations 410 895 and 100 125 wt respectively and relatively flat rare earth element patterns la yb na 64 09 in the amram mafic dikes 456 495 wt sio2 suggest their derivation from the sub continental lithospheric mantle above the garnet stability zone the melts program was used to quantitatively model the chemical evolution of the suite extensive anhydrous fractionation 490 of plagioclase alkali feldspar clinopyroxene olivine and minor timagnetite and apatite from parental mafic magmas represented by the amram mafic dikes produced the rhyolitic compositions as well as the intermediate members of the suite this suggests the presence of a large unexposed body of cumulate rocks at depth as well as fusion of a large source region equivalent to an 5km layer in the lithospheric mantle regarded as a representative example for similar a type outcrops in this region this petrogenetic model further suggests that neoproterozoic early cambrian a type magmatism in the northeastern ans represents a significant post orogenic addition of mantlederived material to the juvenile crust this magmatic episode was of a similar magnitude to that of the cenozoic extensionrelated alkaline volcanism of the arabian plate

triassic turbidites in the west qinling mountains nw china part of the collisional songpan ganzi basin or an active forearc basin abstract lower to middle triassic clastic rocks in the west qinling mountains along ne margin of the qinghai tibet plateau are generally regarded as part of the songpan ganzi flysch basin however relatively little attention has been paid to the age and provenance of these units new petrological and geochemical results demonstrate that these sediments accumulated along an active continental margin and are dominated by feldspathic litharenite and lithic arkose with low mineral and compositional maturity they were derived primarily from a continental arc source dominated by intermediate to felsic igneous rocks with a minor contribution from older metamorphosed and sedimentary sources turbidite samples yielded two primary detrital zircon u pb age populations of ca 273 ma and ca 435 ma which is different from the early triassic ca 252 ma and middle silurian ca 427 ma age populations that dominate the songpan ganzi basin these data together with paleocurrent results indicate that the south qilian belt was the primary source origin because this belt contains both early paleozoic andean type igneous and magmatic rocks and has a basement of precambrian metamorphosed rocks regionally an andean type arc traverses the south qilian belts and extends into the west qinling mountains and kunlun orogen which formed by north directed subduction of the paleo tethyan ocean during the early to middle triassic voluminous detritus that originated from the qilian and kunlun orogens as they were uplifted and eroded was transported to the south and deposited in the forearc area in front of the a nimaqen mianlue suture

radar soundings of the ionosphere of mars we report the first radar soundings of the ionosphere of mars with the marsis mars advanced radar for subsurface and ionosphere sounding instrument on board the orbiting mars express spacecraft several types of ionospheric echoes are observed ranging from vertical echoes caused by specular reflection from the horizontally stratified ionosphere to a wide variety of oblique and diffuse echoes the oblique echoes are believed to arise mainly from ionospheric structures associated with the complex crustal magnetic fields of mars echoes at the electron plasma frequency and the cyclotron period also provide measurements of the local electron density and magnetic field strength

geochemical and zircon u pb hf o isotopic evidence for a coherent paleoproterozoic basement beneath the yangtze block south china abstract the early history of the yangtze block is not well constrained owing to the scarce outcrops of the archean to paleoproterozoic rocks here we report an integrated study of zircon u pb and hf o isotopes as well as whole rock major and trace elements and nd isotopes for the orthogneisses of the dongling complex in the eastern yangtze block la icp ms zircon u pb dating yields a 207 pb 206 pb age of ca 1 85 ga for the protolith age of the orthogneiss revealing a reliable paleoproterozoic basement in the eastern yangtze block zircons from the orthogneiss have e hf t values ranging from 13 4 to 9 0 corresponding to t hf dm2 model ages of 3 03 2 80 ga the zircon 18 o values range between 6 5 and 8 8 with an average of 7 8 in addition the whole rock e nd t values of the orthogneisses range from 9 3 to 8 0 and t nd dm2 model ages from 2 98 ga to 3 05 ga the orthogneisses have relatively high sio 2 k 2 o na 2 o rb ga zr nb y and ce contents and show low cao eu ba and sr concentrations their 10 000 ga al ratios range between 3 0 and 3 5 all these geochemical and nd hf o isotopic signatures suggest a type granite protolith for orthogneisses in the dongling complex they were generated by partial melting of archean crustal materials in a post collisional tectonic setting when our data is combined with the existing geochronological data in other parts of the yangtze block it demonstrates a coherent paleoproterozoic basement beneath the yangtze block representing an important part of the columbia supercontinent the ca 1 85 ga a type magmatism may have led to craton stabilization over all of the yangtze block in addition a potential precambrian dual basement structure may exist in the eastern yangtze block which is supported by paleoproterozoic metamorphic basement and overlaying neoproterozoic sedimentary cover

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

marsis observations of the martian nightside ionosphere dependence on solar wind conditions despite the absence of solar radiation on the martian nightside a weak irregular and variable ionosphere is produced there the nightside ionosphere is thought to be maintained by two main sources dayside nightside plasma transport and electron precipitation observations by mars express mex and mars global surveyor mgs have shown that these plasma sources are either hindered or favored by the presence of strong crustal magnetic fields and that these effects are modulated by external parameters such as the solar wind dynamic pressure and the orientation of the interplanetary magnetic field imf these external drivers are expected to influence the supply of plasma to the nightside and thus the formation of the irregular nightside ionosphere we here present a statistical study of the martian ionosphere at solar zenith angle greater than 107 from november 2005 to may 2006 using remote measurements of ionospheric echoes with the mars advanced radar for subsurface and ionospheric sounding marsis radar sounder onboard mex and using mgs based proxies for the solar wind dynamic pressure and the imf clock angle we find that the peak densities increase with the dynamic pressure and also that cases of very high peak density are almost always associated with westward imf orientation we find that using mex aspera 3 electron data these cases often seem to be linked to accelerated electrons plasma transport is known to be important in the near nightside on the other hand electron precipitation prevails when the dynamic pressure is high enough to compress the ionosphere and in vertical field regions where the imf orientation matters

geochemistry and zircon u pb geochronology of paleoproterozoic arc related granitoid in the northwestern yangtze block and its geological implications abstract the early history of the yangtze block has not been well constrained yet due to the scarce outcrops of archean to paleoproterozoic rocks in this study we report an integrated study of zircon u pb age major and trace element data and sr nd hf isotope compositions for gray gneisses from the houhe complex in the northwestern part of the yangtze block zircon u pb dating yields a weighted mean 207 pb 206 pb age of 2081 9 ma for a gneiss this age is interpreted as the formation age of the houhe complex and thus the houhe complex represents the oldest rocks found in the western part of the yangtze block the gray gneisses from the houhe complex range in sio 2 contents from 58 63 to 68 59 and na 2 o from 3 88 to 5 28 and have relatively high fe 2 o 3 contents of 2 86 6 69 al 2 o 3 of 16 01 18 88 and mgo of 0 97 2 65 these rocks show low sr 149 390 ppm cr 9 07 45 1 ppm and ni 4 97 21 3 ppm contents but high y 12 9 32 7 ppm and yb 0 95 2 25 ppm they are characterized by a relative enrichment in liles and lrees but a depletion in hfses these features are similar to those of calc alkaline granitoids suggesting that their formation might be related to a subduction related process or remelting of preexisted arc rocks compiled age spectra of archean to paleoproterozoic zircon grains reveal that the western and eastern parts of the yangtze block have similar archean and early paleoproterozoic age ranges indicative of the occurrence of an old continental nucleus in both parts on the other hand there are large amounts of ca 2050 2400 ma zircon grains revealed in the western part of the yangtze block implying that there might be a microcontinent with an active type continental margin during the paleoproterozoic times in the western part of the yangtze block because of the poor outcrops of archean to paleoproterozoic rocks in the yangtze block it is no possible to establish the exact nature of such an active margin yet the occurrence of ca 2 0 ga khondalitic and metasedimentary rocks has been documented in the eastern part of the yangtze block implying that there might be a passive type continental margin in the eastern part of the yangtze block at ca 2 0 ga the collision between the western microcontinent and the eastern part may have resulted in the final arc continental collision and the amalgamation of the yangtze block

first measurements of the ionospheric plasma escape from mars before the phobos mission the magnetosphere of mars was relatively unexplored basically all data on the charged particle environment of mars had come from the soviet mars missions mars 2 3 4 and 5 here we report the results of an ion composition experiment on board the soviet phobos 2 spacecraft which allowed us to determine the loss of plasma from the ionosphere of mars because mars has a very small intrinsic magnetic field the ion outflow had been expected to be similar to that of venus surprisingly there were many similarities between the ionospheric outflow from mars and the earth the ion loss from mars results from both ion pick up that results from mass loading of the solar wind in the martian boundary layer and ionospheric o beams of energies up to several kev possibly from upward acceleration processes similar to those observed above the earth s auroral oval a preliminary estimate of the ionospheric outflow from mars indicates that the planet is losing oxygen at a rate of 3 x 1025 ions s 1 this corresponds to an evacuation of its present total atmospheric oxygen content contained in co2 and o2 in less than 100 million years

enclaves from the a type granite of the megantic complex white mountain magma series clues to granite magmagenesis the subsolvus biotite hornblende granite at megantic contains mafic to leucocratic enclaves the groundmass of leucocratic enclaves is microgranitic that of mafic enclaves is meladioritic apatite is commonly acicular in the enclaves suggesting quenching of mafic magmas commingled with the granitic host mafic enclaves contain mafic clots hornblende cores biotite rims some with relic cores of vial cr rich diopsidic augite similar to phenocrysts in associated basaltic dykes the clots are interpreted to be pseudomorphs after mafic phenocrysts the enclaves also contain granitic microxenoliths xenocrysts of quartz with hornblende reaction rims partially resorbed oligoclase and perthite megacrysts and biotite xenocrysts the xenocrysts record hybridization between a mafic enclave magma and a granitic magma and or rock enclaves define mixing lines on many variation diagrams however rb sr ba zr nb y ree zn pb and f do not fit this pattern and appear to have been redistributed by residual granitic melts or exsolved volatiles the data suggest that the granite is anatectic heat being provided by crystallization of associated basalts mixing calculations suggest that as little as 5 hybridization with basalt or hawaiite can account for the sr isotopic heterogeneity of the granites

the 2 65 ga a type granite in the northeastern yangtze craton petrogenesis and geological implications abstract the presence of old residual continental crustal rocks in cratons is crucial for investigation of their early evolution due to the poor exposure of archean rocks the early evolution of the yangtze craton has not been well constrained yet in this study a combined study of zircon u pb and hf o isotopes as well as whole rock major and trace elements and nd isotopes have been conducted for a granite pluton at huji town in the northern part of the yangtze craton u pb dating for magmatic zircons yielded a weighted mean age of 2656 6 ma revealing another archean outcrop in the yangtze craton besides the kongling terrane the granite samples show relatively evaluated k 2 o na 2 o cao tfeo mgo ratios and zr nb ce ga and y contents sharing the same features as the a type granite they have pronounced eu sr and ba negative anomalies on the primitive mantle normalized trace element spidergram implying the residual of plagioclase in the magma sources their high zr contents of 440 529 ppm correspond to initial magma temperatures of 930 962 c the zircons have negative hf t values of 2 7 to 0 1 and hf model ages of ca 3 2 ga respectively the 18 o values of the zircons range between 4 94 and 5 72 and give an average of 5 26 0 14 suggesting no contribution of supracrustal rock into the host magma therefore the hf model ages indicate the generation of the huji granite by reworking of ca 3 2 ga juvenile crustal rocks combined with previous studies the finding of the neoarchean huji granites with ca 3 2 ga hf model ages suggests the widespread of archean basement in the yangtze craton the occurrence of the ca 2 65 ga huji granites likely records the thermally stable lithosphere and thus heralds the craton stabilization of the eastern part of the yangtze block

the 2 40ga ringvassoy mafic dykes west troms basement complex norway the concluding act of early palaeoproterozoic continental breakup abstract a north to northwest trending mafic dyke swarm of gabbronoritic and gabbroic composition makes up a significant part of the archean basement on the island of ringvassoy in northern norway u pb geochronology of zircon and baddeleyite in a gabbronorite provides an age of emplacement of 2403 3 ma metamict zircon in a plagioclase phyric dyke yield data that are discordant but consistent with the age of the gabbronoritic dyke titanite indicates a metamorphic overprint at 1768 4 ma the two types of dyke show some distinct chemical characteristics they are both tholeiitic enriched in lrees and lile elements but depleted in hfs elements including nb their nd isotopic composition yields nd values of 1 5 to 1 8 for gabbronorites and 0 4 to 1 3 for the plagioclase phyric dykes the chemical and isotopic constraints are typical of continental basalts the ringvassoy mafic dykes correlate broadly with a global palaeoproterozoic magmatic event that formed extensive bimodal intrusive and extrusive suites in most archaean cratons including the northeastern fennoscandian shield in detail the 2403 3 ma ringvassoy dykes postdated most episodes of magmatism at this time on the regional scale there is a distinct trend from a 2505 2490 ma suite present in the kola peninsula over a second 2460 2440 ma suite present both in kola and further south in karelia to the 2403 ma dykes on ringvassoy this variation suggests that the locus of maximum extension and magmatic activity may have been shifting with time

enhanced atmospheric oxygen outflow on earth and mars driven by a corotating interaction region 1 solar wind controls nonthermal escape of planetary atmospheric volatiles regardless of the strength of planetary magnetic fields for both earth with a strong dipole and mars with weak remnant fields the oxygen ion o outflow has been separately found to be enhanced during corotating interaction region cir passage here we compared the enhancements of o outflow on earth and mars driven by a cir in january 2008 when sun earth and mars were approximately aligned the cir propagation was recorded by stereo ace cluster and mars express mex during the cir passage cluster observed enhanced flux of upwelling oxygen ions above the earth s polar region while mex detected an increased escape flux of oxygen ions in the martian magnetosphere we found that 1 under a solar wind dynamic pressure increase of 2 3 npa the rate of increase in martian o outflow flux was 1 order higher than those on earth and 2 as a response to the same part of the cir body the rate of increase in martian o outflow flux was on the same order as for earth the comparison results imply that the dipole effectively prevents coupling of solar wind kinetic energy to planetary ions and the distance to the sun is also crucially important for planetary volatile loss in our inner solar system

crustal origin of early proterozoic syenites in the congo craton ntem complex south cameroon two petrographically different syenite suites can be distinguished in the mostly archean ntem complex of cameroon a suite of two pyroxene syenites and a suite of clinopyroxene syenites the syenites occur as discrete intrusions surrounded by archean gneisses based on structural relationships the syenites are of late to post orogenic origin the pb pb zircon evaporation ages indicate that the syenites intruded at ca 2 3 ga and thus postdate significantly the archean episode of crust formation and differentiation that generated the surrounding gneiss terrane the isotope signatures and the trace elements provide unambiguous evidence that the two syenite suites are of crustal origin the syenite suites do not show simple chemical trends have highly variable trace element contents and display only a small variation in their major element contents particularly in their sio2 contents the isotope characteristics including 143nd 144nd 87sr 86sr and common pb point to a heterogeneous source or multiple sources for the syenites the sr isotope compositions calculated for the time of intrusion vary over a wide range from an unrealistically low 87sr 86sr ratio of 0 690 to high values of 0 713 in contrast the initial 143nd 144nd ratios are more homogenous with end 2320 from 5 8 to 7 5 the pb isotopes from leached k feldspars indicate that the sources of the syenites had a history of u depletion and th u enrichment the syenites show the same isotope characteristics as the surrounding gneiss terrane thus the deep continental crust of the ntem complex itself is the most likely source for the syenite melts this conclusion is also supported by the heterogeneity of the trace elements in the syenites that exclude a homogenous source in addition inherited zircons from the syenites yield pb pb evaporation ages that are consistent with ages obtained for zircons extracted from the surrounding gneisses in contrast to many other syenites worldwide the syenites in the ntem complex do not contain juvenile mantle material but are the sole products of crustal reworking thus the syenites did not contribute to the crustal growth in the ntem complex however this conclusion may not preclude the possibility that the syenites were generated in the deep crust during a time of addition of mantle material to the continental crust the cause for the generation of these large syenite bodies in the ntem complex is most likely an episode of deep seated thermal activity that was so far not known from this part of the congo craton

petrogenesis of archean ttgs and potassic granites in the southern yangtze block constraints on the early formation of the yangtze block abstract the southern yangtze is underlain by mesoarchean igneous rocks that were emplaced in two pulses including trondhjemites at ca 3092 3071 ma followed by potassic granites at 2920 ma these represent the oldest ttg suites identified in the southern yangtze and the oldest potassic granites in the entire yangtze block hf isotopic analysis reveals ehf t values of 2 49 7 49 and hf crustal model ages tdmc of 3601 3000 ma for zircons from the trondhjemites whereas those from the potassic granites show ehf t values of 1 79 4 18 and tdmc ages of 3453 3043 ma the trondhjemites have high sio2 mg and low al2o3 and k2o na2o and are highly evolved with low ni cr and co the trondhjemites are characterized by low sr y eu gd and la yb n and high y and hree typical of high hree and low pressure ttgs their geochemical and isotopic compositions indicate that they are derived from melting of juvenile mafic crust with little contribution from older crustal materials consistent with a plagioclase bearing basaltic source the multielement patterns of potassic granites are similar to that of the trondhjemites albeit with lower hree and y contents their high k2o values strongly fractionated ree patterns and low y contents indicate the presence of garnet and or amphibole as residual phases in the source region which indicates they formed at higher pressure than the trondhjemites the isotopic and geochemical data imply that the formation of the potassic granites could be related to reworking of older ttgs the increasing k2o na2o of the 3092 3071 ma trondhjemites to potassic 2920 ma granites corresponds with stabilization maturation and thickening of the archean continental crust in the southern yangtze the geochemical data of the ttgs in the kunming region show important differences from that of contemporaneous ttgs from the kongling complex suggesting that southern yangtze is composed by an independent old nucleus which is exotic to the northern yangtze a synthesis of petrogenesis geochronology geochemistry and isotope data of archean granites of the yangtze block provides important insights into the architecture of its archean crust we propose that the yangtze block can be subdivided into discrete northwestern northeastern and southern domains but which are all affected by late paleoproterozoic tectonothermal events suggesting that they had been assembled together by then to form the united yangtze block

long term variability of mars exosphere based on precise orbital analysis of mars global surveyor and mars odyssey a long term perspective on mars exosphere variability at 405 km is provided by merging together density data derived from precise orbit determination of the mars global surveyor and mars odyssey mo satellites extending from 2001 to 2010 these data are heavily weighted toward afternoon local times at high latitudes in the southern hemisphere clear long term solar and annual variations are well captured by empirical formulas residuals from the empirical fit show evidence for relative depletions in exosphere density around mars closest approach to earth which would be consistent with a scavenging mechanism that is dependent on solar wind dynamic pressure superimposed on this variation with mars sun distance are positive density residuals during mars year my 25 my27 and my29 that are apparently due to elevated dust levels in mars middle atmosphere however during my24 my26 and my28 there are dust level increases without any corresponding increase in exosphere density we suspect that this inconsistency is related to a variable ability to sense the response to dust related effects imposed by the high latitude limitations of our measurements combined with interference between the mechanisms that translate middle atmosphere heating to an exosphere response evidence also supports the hypothesis that winter helium bulge effects contributed to the inferred interannual density variability during the 2007 2009 solar minimum period when the o he transition height likely resided near the 405 km orbit of mo

precambrian tectonothermal evolution of south qinling and its affinity to the yangtze block evidence from zircon ages and hf nd isotopic compositions of basement rocks abstract south qinling is a tectonic block in the mesozoic qinling orogenic belt central china which is generally believed to have had tight affinity with the yangtze block during precambrian times in recent years it has been recognized that the south qinling block hosts several precambrian basement complexes here we report u pb ages and hf isotopic data for zircons and sr nd isotopic data for whole rocks of the douling complex the oldest rock unit exposed in south qinling to reveal the early evolution of this basement the douling complex mainly consists of paragneisses and granitic orthogneisses zircons of magmatic origin from three orthogneiss samples give u pb ages of c 2 5 ga suggesting an early paleoproterozoic magmatic activity zircon hf isotopic compositions imply the growth of juvenile crust since c 3 0 ga the magmatic activity at c 2 5 ga points to differences in the precambrian history between south qinling and the archean kongling terrene the oldest basement of the yangtze block three paragneiss samples contain complex detrital zircons of different ages and origins their u pb ages mainly cluster around 2 5 ga 2 0 ga 1 5 ga and 800 ma zircon grains with ages of 2 0 ga and 800 ma have low th u ratios and no oscillatory zoning implying two episodes of metamorphic overprint such paleoproterozoic metamorphic event has also been traced in the yangtze block confirming its close relationship with south qinling the record of neoproterozoic amphibolite facies metamorphism in detrital zircons suggest that the tectonic transition from convergent to extensional environments in the yangtze block must have occurred later than 800 ma ago

neoarchean and paleoproterozoic k rich granites in the phan si pan complex north vietnam constraints on the early crustal evolution of the yangtze block abstract precambrian igneous and metamorphic rocks of the phan si pan complex north vietnam constitute the southern extension of the yangtze block and provide a valuable record of the early evolution of the continental crust we present results of u pb zircon geochronology and geochemistry for precambrian granites in this complex to constrain their emplacement age and genesis granites from three plutonic bodies yielded ages of 2848 15 ma 2768 19 ma and 1869 30 ma which represent newly recognized late archean to paleoproterozoic potassic granite plutonism in the southern yangtze block the average ehf t values range from 6 2 to 0 1 for the 2 85 to 2 77 ga granitic rocks and 13 1 to 9 2 for the ca 1 86 ga granitic rocks with two stage model ages of 3 64 to 3 20 ga and 3 31 to 3 07 ga respectively suggesting derivation from partial melting of paleoarchean and mesoarchean crust the late archean potassic granites exhibit high k2o and high sr y and la yb n ratios with negligible eu anomalies indicating derivation from melting of the thickened lower crust which is inferred to have occurred in an active margin setting the late paleoproterozoic alkali feldspar granites are characterized by high feot feot mgo 0 96 0 99 and 10000 ga al 2 75 2 94 ratios showing an affinity of a type granite these a type granites exhibit flat chondrite normalized hree patterns and strong negative eu anomalies and low sr y and la yb n ratios corresponding to melting at a shallow depth probably in a post collisional extension setting comparison of the rock units and events recorded by the phan si pan complex with other archean to paleoproterozoic complexes houhe dongchuan yudongzi douling zhongxiang and kongling complexes in the yangtze block indicate spatially distinct histories of crustal growth and thus may reflect independent terranes the ca 1 86 ga post collisional magmatism which succeeds a 2 0 1 9 ga metamorphic event is distributed throughout the yangtze block including the phan si pan complex suggesting assembly of the disparate terranes and final cratonization of the yangtze block overlaps with and may be related to assembly of the nuna supercontinent

neoproterozoic granitoids from the phan si pan belt northwest vietnam implication for the tectonic linkage between northwest vietnam and the yangtze block abstract several neoproterozoic granitic intrusions including the phin ngan intrusion and some smaller ones from the sin quyen cu mine have been identified in the phan si pan belt northwest vietnam whole rock geochemical and sm nd isotopic data and zircon u pb age and hf o isotopes have been determined for these intrusions in order to constrain their ages petrogenesis and tectonic implications the phin ngan intrusion mainly composed of syenogranite was emplaced at 824 4 ma rocks from this intrusion have high sio 2 72 1 73 5 wt and k 2 o 5 14 5 52 wt with a cnk values ranging from 1 02 to 1 06 they have negative whole rock e nd t 8 4 to 5 2 and zircon e hf t values 8 1 to 5 5 and high zircon 18 o values 9 7 10 9 these geochemical features suggest that the phin ngan intrusion was derived from ancient k rich crustal sources granitic intrusions from the sin quyen mine occur as stocks or dykes intruding ores and ore hosting rocks they are composed of monzogranite and granodiorite emplaced between 736 8 and 758 6 ma they have high sio 2 68 2 76 7 wt and k 2 o na 2 o 6 93 8 54 wt and low mgo 0 19 0 87 with a cnk values ranging from 0 91 to 1 14 their whole rock e nd t values range from 6 6 to 0 mostly clustered between 6 6 and 4 3 their zircon e hf t values vary from 6 7 to 11 1 mostly lower than 2 0 such geochemical data indicate that these intrusions were produced mainly through partial melting of ancient crustal materials coupled with subordinate involvement of mantle juvenile crust derived components the phin ngan intrusion and intrusions from the sin quyen mine are sub alkaline in nature and have arc like trace element compositions compiled with other broadly synchronous igneous rocks with arc like affinities in the region the neoproterozoic magmatism in the phan si pan belt was generated in a subduction related setting the neoproterozoic intrusions in the phan si pan belt are geochronologically and geochemically correlated with the neoproterozoic subduction related igneous rocks along the western yangtze block thus the phan si pan belt represents the southern most part of the giant arc system of the western yangtze block

martian magnetic storms the response of mars to the major space weather events called interplanetary coronal mass ejections icmes is of interest for both general planetary solar wind interaction studies and related speculations on their evolutionary consequences especially with respect to atmosphere escape various particle and field signatures of icmes have been observed on phobos 2 mars global surveyor mgs mars express mex and now mars atmosphere and volatile evolution maven of these maven s combined instrumentation and orbit geometry is particularly well suited to characterize both the event drivers and their consequences however maven has detected only moderate disturbances at mars due in large part to the general weakness of the present solar cycle nevertheless the strongest event observed by maven in march 2015 provides an example illustrating how further insights can be gained from available models here we first look more closely at what previously run bats r us mhd simulations of the combined maven observations tell us about the march 2015 event consequences we then use analogous models to infer those same responses including magnetic field topology changes and ionospheric consequences to a hypothetical extreme icme at mars based on stereo a measurements in july 2012 the results suggest how greatly enhanced yet realistic solar wind pressure magnetic field and convection electric field combine to produce strong magnetospheric coupling with important consequences for upper atmosphere and ionosphere energization

rare earth element geochemistry of australian paleozoic graywackes and mudrocks provenance and tectonic control abstract in the paleozoic graywackes of eastern australia there is an increase in the total abundance of rare earth elements ree a light to heavy ree ratio and a decrease in the chondrite normalized eu anomaly with the increase in sio2 al2o3 and k2o na2o ratios this is due to the change in the dominant source rocks from andesite to dacite to granite gneiss and sedimentary rocks similarly an increase in total ree and a decrease in eu anomaly is also seen in the associated mudrocks with the increase in total clay content this correlation in paleozoic sedimentary rocks suggests that the ree characteristics of terrigenous sedimentary rocks reflect the depositional tectonic setting of the basin the oceanic island arc type graywackes are characterised by their lower total ree abundance only slight enrichment of lree over hree and the absence of a negative eu anomaly on chondrite normalised plots the continental island arc type graywackes are discriminated by their higher ree abundance and la yb ratio the presence of a small negative eu anomaly on the chondrite normalised plots and the significant enrichment of eu compared to paas post archean average australian shale the sedimentary rocks deposited on andean type active continental margins passive margins platform and cratonic basins are all characterised by the high enrichment of lree over hree and the presence of a pronounced negative eu anomaly on chondrite normalised plots and can not be discriminated from each other by ree patterns alone

mars plasma system response to solar wind disturbances during solar minimum this paper is a phenomenological description of the ionospheric plasma and induced magnetospheric boundary imb response to two different types of upstream solar wind events impacting mars in march 2008 at the solar minimum a total of 16 mars express orbits corresponding to five consecutive days is evaluated solar terrestrial relations observatory b stereo b at 1 au and mars express and mars odyssey at 1 644 au detected the arrival of a small transient interplanetary coronal mass ejection icme like on the 6 and 7 of march respectively this is the first time that this kind of small solar structure is reported at mars s distance in both cases it was followed by a large increase in solar wind velocity that lasted for 10 days this scenario is simulated with the wang sheeley arge wsa enlil cone solar solar wind model at mars the icme like event caused a strong compression of the magnetosheath and ionosphere and the recovery lasted for 3 orbits 20 h after that the fast stream affected the upper ionosphere and the imb which radial and tangential motions in regions close to the subsolar point are analyzed moreover a compression in the martian plasma system is also observed although weaker than after the icme like impact and several magnetosheath plasma blobs in the upper ionosphere are detected by mars express we conclude that during solar minimum and at aphelion small solar wind structures can create larger perturbations than previously expected in the martian system

the arrowsmith orogeny geochronological and thermobarometric constraints on its extent and tectonic setting in the rae craton with implications for pre nuna supercontinent reconstruction abstract geochronological and thermobarometric data for two widely spaced and previously unstudied areas of the rae craton canada provide important temporal and spatial links between previously investigated areas in the northern and southern rae craton that had been interpreted to have been reworked during the ca 2 5 2 3 ga arrowsmith orogeny on southern boothia peninsula upper amphibolite to granulite facies metamorphism dated by in situ shrimp u pb analysis of monazite occurred at ca 2 64 2 60 2 54 2 50 2 49 2 44 and 2 37 2 34 ga with retrograde monazite growth at 2 27 ga shrimp u pb ages of zircon in two metasedimentary rocks provide a further indication of metamorphism at ca 2 54 and 2 50 ga which also corresponds to the age determined for a k feldspar augen granite textural relationships indicate that the regional foliation formed between ca 2 54 ga and 2 50 2 49 ga peak metamorphic conditions of 700 c and 5 6 kbar are interpreted to have been reached during the 2 37 2 34 ga event in the mccann mcarthur lake region of the southwestern rae craton upper amphibolite to granulite facies metamorphism is dated by in situ shrimp monazite ages of ca 2 53 2 47 and 2 37 2 28 ga textural relationships indicate deformation initiated by ca 2 37 ga and terminated by ca 2 30 ga metamorphic conditions of 5 kbar and 700 735 c were quenched at ca 2 3 ga following peak conditions of 7 kbar and 800 c the evidence for regional deformation at 2 54 2 49 ga on southern boothia peninsula supports an andean type accretionary margin setting for the arrowsmith orogeny a collisional event may have terminated high grade metamorphism in the northern region by ca 2 34 ga while metamorphism associated with continental arc magmatism continued in the south until at least ca 2 28 ga these combined data demonstrate the continuity and broad extent of the arrowsmith orogen along the western flank of the rae craton extending 1500 km from northern saskatchewan to northern baffin island the broad extent of this globally rare orogenic event provides an important piercing point for supercontinent reconstructions which are discussed in a companion paper the inferred convergent tectonic activity between ca 2 54 and 2 28 ga is interpreted to have occurred on the flank of a growing supercontinent a setting that could account for an apparent slowdown but not shutdown in global tectonics

a type granites in northeastern china age and geochemical constraints on their petrogenesis abstract a type granites are widely distributed in northeastern china ne china they were emplaced during three major episodes the permian late triassic to early jurassic and early cretaceous and evolved in different tectonic regimes according to their mineralogical and geochemical characteristics two subgroups of a type granites aluminous and peralkaline can be recognized the peralkaline subgroup contains alkali mafic minerals such as riebeckite arfvedsonite and sodic pyroxene while the aluminous subgroup contains annite and fe rich calcic or sodic calcic amphibole with respect to the aluminous subgroup the peralkaline granites contain higher rb ga and total rare earth elements ree but lower mgo cao al 2 o 3 ba and sr based on the discrimination criteria of eby geology 20 1992 641 the permian and late triassic to early jurassic a type granites belong to the a 2 post orogenic type whereas the early cretaceous granites are of the a 1 anorogenic type nd isotopic compositions of these a type granites indicate their derivation from a dominantly juvenile crustal source their origin is thought to have involved partial melting of an underplated lower crustal source because the generation of a type granites requires high melting temperature we propose models involving slab break off lithospheric delamination and extension the permian a type granites have the same age range as those in eastern junggar southern mongolia and central inner mongolia they occur along a major suture and form a narrow belt between the north china and siberian cratons we suggest that their formation was associated with post collisional slab break off the late triassic to early jurassic a type granites are likely to be the product of lithospheric delamination after the final collision of the major crustal blocks in the late paleozoic to early triassic the early cretaceous a type granites have an anorogenic affinity and were possibly associated with rifting in eastern china at this time associated with the onset of paleo pacific subduction consequently we conclude that the a type granites in ne china were generated at three different times involving multiple processes operative in different tectonic environments

genesis of the two main types of peraluminous granitoids two mica monzogranites to leucogranites and biotite rich cordierite bearing tonalites to monzogranites form two distinct groups of peraluminous granitoids they can be distinguished by mineral and rock associations and by the variation of their peraluminosity during differentiation except for the rare muscovite bearing granitoids produced by extreme fractionation or local contamination of metaluminous magmas the majority of peraluminous granitoids are produced by partial melting of crustal rocks production of either muscovite bearing granites or biotite rich cordierite bearing granitoids does not depend only on the nature of the sources but is also controlled by the physical parameters of partial melting and consequently by the way anatexis of a thickened crust is enhanced biotite rich cordierite bearing granitoids are generated where mantle derived magmas are injected into or have underplated crustal rocks two mica granites are generated where thickened crust is affected by major crustal shears or thrusts correlations between differentiation and peraluminosity indicate the dominant role of either restite unmixing or fractional crystallization in the production and evolution of the various types of peraluminous granitoids

petrogenesis of the huashanguan a type granite complex and its implications for the early evolution of the yangtze block abstract an integrated study of petrography zircon u pb ages hf o isotope compositions as well as whole rock geochemistry was carried out for a paleoproterozoic granite complex in the northern yangtze block to investigate its petrogenesis and geological significances petrographically the huashanguan complex is a granite complex comprised of monzogranite in the marginal facies and k feldspar granite in the central facies u pb dating of magmatic zircons from the monzogranite and k feldspar granite yielded ages of 1816 50 ma and 1843 9 ma respectively the more precise age of 1843 9 ma is suggested to represent the formation age of the huashanguan complex the granites have uniform negative zircon e hf t values of 20 2 to 15 8 with corresponding t dm2 ages of ca 3 6 3 5 ga the 18 o values average at 6 80 0 09 these indicate that the granites were generated by partly melting of archean rocks implying the widespread of archean basement in the yangtze block the monzogranites display low feo t and mgo contents and evolved trace element features suggesting their formation by more fractional crystallization from similar parent magma of the k feldspar granites the occurrence of the huashanguan granite complex indicates that the yangtze block has experienced a tectonic transition from tectonic convergence to extension at ca 1 85 ga possibly due to the collapse of the ca 2 0 ga orogeny

contrasting geochemical and sm nd isotopic compositions of archean metasediments from the kongling high grade terrain of the yangtze craton evidence for cratonic evolution and redistribution of ree during crustal anatexis abstract twenty three clastic metasediments from the kongling high grade terrain of the yangtze craton south china were analyzed for major trace and rare earth elements and sm nd isotopic ratios associated dioritic tonalitic trondhjemitic dtt and granitic gneisses as well as amphibolites were also analyzed in order to constrain provenance the results show that the clastic metasediments can be classified into 3 distinct groups in terms of mineralogical geochemical and sm nd isotopic compositions group a is characterized by having no to slight negative eu anomalies eu eu 0 82 1 07 being high in cr 191 396 ppm and ni 68 137 ppm and low in th 3 3 7 8 ppm and ree ree 99 156 ppm these characteristics are similar to those of metasediments from archean greenstone belts in addition the group a metasediments have the value of the chemical index of alteration ciw close to felsic gneisses their sm nd isotopic ree and trace element compositions can be interpreted by mixtures of the dtt gneisses and amphibolites dating of detrital zircons from 2 group a samples by shrimp reveals a major concordant age group of 2 87 3 0 ga which is identical to the age of the trondhjemitic gneiss these results strongly suggest that group a was principally the first cycle erosion product of the local kongling dtt gneiss and amphibolite moreover the higher than amphibolite cr content and slight eu depletion exhibited by some samples from this group infer that ultramafic rocks like komatiite and granite of probably 3 0 3 3 ga in age also played a role group b is characterized by the presence of graphite and shows a more evolved composition similar to post archean shales with a prominent negative eu anomaly eu eu 0 48 0 77 and high ciw on paired cr th vs la co and co th plots group b samples conform to a two end member mixing line of the kongling granitic gneiss and amphibolite however data on nd model age and ciw suggest that the granite component should be younger than the sampled granitic gneiss and derived from a distal source both groups a and b exhibit a clear positive correlation between ciw and tdm and a negative one between ciw and eu eu these correlations point to the crustal evolution of the yangtze craton towards coupled increasing ciw and eu depletion with decreasing age this in turn reflects the change of granitoid magmatism from local na rich dioritic tonalitic trondhjemitic rocks to widespread k feldspar granite the change led to the intracrustal differentiation stabilization and growth of the craton group c is restite and contains abundant sillimanite and garnet and unusually high ilmenite 7 11vol which can be seen to be dehydration melting products of biotite under the microscope this group shows extremely varied ree distributions from lree enriched to depleted and from negative to strong positive eu eu 1 63 eu anomalies compared to groups a and b group c is severely depleted in na2o k2o lree rb and ba whereas tio2 co v sc and hree and y are considerably enriched this is accompanied by anomalous high sm nd 0 21 0 28 147sm 144nd 0 1361 0 1738 and 143nd 144nd 0 511589 0 511958 ratios tdm correlates clearly with sm nd ratio and 2 out of 3 samples give significantly older to unrealistic tdm 3 9 4 9 ga the results document redistribution of ree and an open behavior of the sm nd isotope system during the biotite dehydration melting of metasediments

zircon u pb age and trace element evidence for paleoproterozoic granulite facies metamorphism and archean crustal rocks in the dabie orogen abstract zircon u pb ages and trace elements were determined for granulites and gneiss at huangtuling which are hosted by ultrahigh pressure metamorphic rocks in the dabie orogen east central china cl images reveal core rim structure for most zircons in the granulites the cores show oscillatory zoning relatively high th u ratios and hree enriched patterns consistent with a magmatic origin they gave a weighted mean 207pb 206pb age of 2766 9 ma interpreted as dating magma emplacement of the protolith the rims are characterized by sector or planar zoning low th u ratios negative eu anomalies and flat hree patterns consistent with their formation under granulite facies metamorphic conditions zircon u pb dating yields a weighted mean 207pb 206pb age of 2029 13 ma which is interpreted to record a metamorphic event possibly during assembly of the supercontinent columbia the gneiss has a protolith age of 1982 14 ma which is younger than the zircon age of the granulite facies metamorphism suggesting a generally delay between ht metamorphism and the intrusion of post collisional granites a few inherited cores with igneous characteristics have 207pb 206pb ages of 2 90 3 28 and 3 53 ga suggesting the presence of mesoarchean to paleoarchean crustal remnants in the yangtze craton a few cretaceous metamorphic ages were also obtained suggesting the influence of post collisional collapse in response to cretaceous extension of the dabie orogen it is inferred that the recently discovered archean basement of the yangtze craton occurs as far north as the dabie orogen

transient layers in the topside ionosphere of mars 1 radar soundings from the marsis instrument on board the mars express spacecraft have shown that distinct layers can occur in the topside ionosphere of mars well above the main photo ionization layer these layers appear as cusps or sometimes steps in plots of the time delay as a function of frequency usually only one topside layer is observed typically at altitudes from 180 to 240 km however occasionally an additional layer occurs at even higher altitudes the layers are transient features and are present about 60 of the time near the subsolar point decreasing with increasing solar zenith angle to less than 5 at the terminator and the nightside the transient nature of the layers suggests that they are produced by a dynamical process most likely involving an interaction with the solar wind in the upper levels of the ionosphere