low phosphorus sediments in a hypersaline marine bay abstract shark bay western australia is a large shallow hypersaline coastal lagoon with low nutrient input dissolved inorganic p in the water column decreased from 0 2 m in the oceanic region of the bay to undetectable limits in the hypersaline areas of the bay organic c total n and total p were measured in the sediments along the salinity gradient the sediment organic c was 1 3 mmol c g 1 and increased into the bay total n was positively correlated with c but p decreased from 15 mol p g 1 in the outer bay to less than 1 mol p g 1 in the hypersaline region sediment inorganic p comprised 80 90 of the total sediment p the molar c p ratio of the sediment organic phase increased from 1700 in the outer bay to 12 000 in the hypersaline region organic c and total p were also measured in the benthic plants p content of the plants also decreased as salinity increased resulting in an increase in the plant c p ratio which was similar to the increase in the c p ratio of the sediment organic phase p composition of the coastal sediments can change dramatically in a relatively short distance as a result of net uptake and sedimentation by benthic communities

how old are young lunar craters 1 the accurate definition of the lunar cratering chronology is important for deriving absolute model ages across the lunar surface and throughout the solar system images from the lunar reconnaissance orbiter narrow angle cameras and wide angle camera and the selene kaguya terrain camera provide new opportunities to investigate crater size frequency distributions csfds on individual geological units at lunar impact craters we report new csfd measurements for the copernican aged craters north ray tycho and copernicus which are crucial anchor points for the lunar cratering chronology we also discuss possible reasons for an age discrepancy observed between the impact melt and ejecta units our csfds for north ray and tycho crater ejecta deposits are consistent with earlier measurements however for copernicus crater and one of its rays we find significantly lower cumulative crater frequencies than previous studies our new results for copernicus crater fit the existing lunar absolute chronologies significantly better than the previous counts our derived model ages of the ejecta blankets of north ray tycho and copernicus agree well with radiometric and exposure ages of the apollo 16 17 and 12 landing sites respectively and are generally consistent with a constant impact rate over the last 3 ga however small variations of the impact rate cannot be resolved in our data and require further investigations

alha 81005 moon mars petrography and giordano bruno meteorite alha 81005 a glassy regolith breccia is beyond any reasonable doubt of lunar origin according to its petrographic chemical and isotopic characteristics the sample was shocked at most to 15 gpa during ejection and supports the possibility of some meteorites being of martian origin most lithic fragments are feldspathic granulitic impactites cataclastic anorthosites and impact melts all typical lunar lithologies mineral fragments are derived from plutonic troctolitic and noritic rocks varied mare basalt like extrusive rocks and the identified lithic fragment types neither petrographically nor chemically are there significant components in alha 81005 which were previously unknown a mare basalt component restricts the origin of the sample with remotely sensed data which eliminates most frontside as well as farside groundtrack regions it makes the probability of ejection from giordano bruno near the moon s ne limb far greater than from any other lunar crater

emplacement of the giant kunene amcg complex into a contractional ductile shear zone and implications for the mesoproterozoic tectonic evolution of sw angola massif type anorthosites are voluminous plagioclase dominated batholiths that occur worldwide but only during the proterozoic eon granitoids are closely associated with the anorthosites and they collectively may form an amcg suite i e anorthosite mangerite charnockite granite many fundamental questions regarding the origin of the amcg complexes remain unresolved including their tectonic setting and mode of emplacement here we present new data from reconnaissance structural mapping u pb la icp ms zircon dating and 40ar 39ar amphibole and mica dating from a 500 km2 area straddling the western margin of one of the world s largest 15 000 km2 but also least studied amcg suites the kunene complex kc in angola the oldest recognized pre kunene deformation event d1 produced a steep wnw ese striking s1 fabric in supracrustal rocks migmatite and paleoproterozoic 1800 1780 ma granodiorite before 1680 ma this was folded by the d2 event resulting in steep n s striking s2 leucosomes and gneissosity in the rocks surrounding the kc concordant magmatic layering and lamination and high to low temperature gneissic foliation in the marginal kunene anorthosite and coeval megacrystic granite are parallel with s2 fabrics in the paleoproterozoic country rocks mineral stretching lineations l2 in all rock types across intrusive contacts are steep and associated with west side up thrust shearing overlapping u pb zircon baddeleyite ages from the kunene megacrystic granite and kunene anorthosite with 40ar 39ar mica ages from the mantling paleoproterozoic country rocks attest to progressive contraction thrusting and exhumation of the paleoproterozoic country rocks onto the kunene anorthosite in the interval 1410 1380 ma we suggest that thermal softening of the country rocks caused by heat dissipation from the earliest phase 1500 1410 ma of ponding of hot anorthositic magma in the deep crust localized d2 deformation at the margin of the kc our data point to a tectonic model whereby anorthosite sheets were emplaced at a high angle to the shortening direction and along host rock s2 anisotropy deep in the crust and cooled during progressive ductile thrusting and exhumation of the country rocks these results question the current view that proterozoic amcg suites are emplaced in extensional environments as large diapiric bodies and favor formation in a continental arc setting

lu hf and u pb age determination of capivarita anorthosite in the dom feliciano belt brazil abstract the capivarita anorthosite formed in an intraplate environment and later metamorphosed under amphibolites facies conditions is exposed in the dom feliciano belt as part of the brasiliano magmatic arc and occurs as a roof pedant in or is even intruded by 0 6 ga post collisional granites in this work magmatic and metamorphic minerals were dated using the la mc icp ms in situ method u pb magmatic and metamorphic zircon dating yielded an age of 1573 21 ma and of 606 6 ma respectively whereas the igneous titanite dating yielded an age of 1530 33 ma and the metamorphic ages were 651 9 ma and 601 5 ma the lu hf model ages showed two clusters from 1 81 to 2 03 ga hf from 2 21 to 6 42 and 2 55 2 62 ga hf from 4 59 to 5 64 this intraplate magmatism can be connected to a very important episode of continental accretion in an extensional setting from the fragmentation of the supercontinent during the early mesoproterozoic

gld100 the near global lunar 100 m raster dtm from lroc wac stereo image data 1 we derived near global lunar topography from stereo image data acquired by the wide angle camera wac of the lunar reconnaissance orbiter camera lroc system from polar orbit tracks the lroc wac provides image data with a mean ground resolution at nadir of 75 m pixel with substantial cross track stereo overlap wac stereo images from the one year nominal mission and the first months of the science mission phase are combined to produce a near global digital terrain model dtm with a pixel spacing of 100 m the global lunar dtm 100 m or gld100 it covers 79 s to 79 n latitudes 98 2 of the entire lunar surface we compare the gld100 with results from previous stereo and altimetry based products particularly with the lunar orbiter laser altimeter lola altimetry which is the current topographic reference for the moon we describe typical characteristics of the gld100 and based upon the comparison to the lola data set assess its vertical and lateral resolution and accuracy we conclude that the introduced first version of the stereo based gld100 is a valuable topographic representation of the lunar surface complementary to the lola altimetry data set further improvements can be expected from continuative investigations

giordano bruno small crater populations implications for self secondary cratering abstract giordano bruno is a lunar farside copernican age crater the relatively few superposed impact craters on its floor and ejecta blanket and extensive bright rays indicate its youth high resolution lunar reconnaissance orbiter camera images reveal that the frequency and the characteristics of the cumulative size frequency distributions of the small diameter impact craters and their morphology vary across the clastic ejecta and impact melt covered surfaces crater frequencies n 10 defined as 10 m km 2 vary by a factor of 10 s 100 s n 10 2 700 across the ejecta blanket and between the ejecta and the melt deposits numerous craters on the ejecta blanket are degraded and buried by debris and impact melt indicating that these partially buried craters formed during the deposition of the ejecta and prior to or during the emplacement of the impact melt from geologic relations and crater statistics we conclude that a significant fraction of the craters observed on the ejecta blanket and the melt were formed during the cratering process itself and represent self secondaries further we conclude that these craters do not represent an extra lunar primary impact production population self secondary craters are formed by material launched into near vertical trajectories and having velocities such that their flight time is sufficiently long that the bulk of the clastic ejecta and impact melt are deposited before that material impacts the surface the presence of a significant number of self secondary craters on the ejecta makes the determination of relative and absolute age dates problematic crater counts would indicate an inappropriately old age using data for craters on the melt surfaces and for small diameter bright ejecta craters an absolute model age of 1 ma is estimated this age is considerably younger than that estimated by other studies and probably represents a maximum age

the crustal tongue melting model and the origin of massive anorthosites recent detailed field studies in several anorthosite complexes have shown that anorthosites are frequently associated with weakness zones in the crust which may have favoured their emplacement at mid crust levels recent experimental data have shown that the parent magma compositions of various anorthosite massifs lie on thermal highs in the relevant phase diagrams at 10 13 kbar indicating that these magmas cannot be derived by fractionation of peridotitic mantle melts but by melting of gabbronoritic sources in the lower crust at 40 50 km depths in the sveconorwegian province terne boundaries have been traced in deep seismic profiles to moho offsets or to tongues of lower crustal material underthrust to depths higher than 40 km in southern norway we suggest that a lithospheric scale weakness zone the feda transition zone has channelled the rogaland anorthosites through linear delamination asthenospheric uprise and melting of a mafic lower crustal tongue

osmium isotopes and highly siderophile element fractionation in the massif type anorthosites of the mesoproterozoic kunene intrusive complex nw namibia abstract the behaviour of highly siderophile elements during differentiation of mafic melts and formation of cumulate rocks in the crust is little understood mostly because abundances of some of these elements are very low and reliable data is scarce data on massif type anorthosites and associated fe ti ore from the mesoproterozoic kunene intrusive complex nw namibia indicate the effects of fractional crystallisation crustal contamination cumulate formation and post cumulus re equilibration on the distribution of the highly siderophile elements and the osmium isotopic composition of the anorthositic cumulate rocks anorthosites and fe ti ore of the kunene intrusive complex are characterised by low concentrations and strongly fractionated chondrite normalised patterns of the highly siderophile elements the abundance of highly siderophile elements is dominated by fe ti oxides and traces of sulphides the osmium isotopic composition and highly siderophile element distribution of the anorthosites are consistent with their derivation from a mantle derived parental magma that was subject to fractionation of mafic minerals and variable extent of crustal contamination during its igneous evolution under conditions of the anorthosite formation re behaves as a compatible element in fractionating titanomagnetite the other highly siderophile elements are incorporated to a lesser extent into fe ti oxides during post cumulus re equilibration of fe ti oxides re is preferentially incorporated in magnetite whereas pt displays preference for ilmenite

meteor storm evidence against the recent formation of lunar crater giordano bruno it has been suggested that the formation of the 22 km diameter lunar crater giordano bruno was witnessed in june 1178 a d to date this hypothesis has not been well tested such an impact on the earth would be civilization threatening previous studies have shown that the formation of giordano bruno would lead to the arrival of 10 million tonnes of ejecta in the earth s atmosphere in the following week i calculate that this would cause a week long meteor storm potentially comparable to the peak of the 1966 leonids storm the lack of any known historical records of such a storm is evidence against the recent formation of giordano bruno other tests of the hypothesis are also discussed with emphasis on the lack of corroborating evidence for a very recent formation of the crater

petrogenesis of a mid proterozoic anorthosite mangerite charnockite granite amcg complex isotopic and chemical evidence from the nain plutonic suite the nain plutonic suite nps about 1 30 ga underlies some 19 000 km intruding the boundary between middle to late archean rocks in the east nain province and late archean and early proterozoic rocks in the west churchill province nd isotopic compositions in most igneous units of the nps reflect their geographic position relative to the inferred boundary between the nain and churchill provinces rare exceptions are local intrusions of olivine bearing basic rocks with highest epsilon nd 1 3 ga near 3 0 present in both eastern and western sectors of the nps anorthositic rocks have variable and significant contributions of crustal nd but notably lower i sr than granitoid rocks ferrodiorites are similar isotopically to the anorthositic group and have chemical and mineral compositions more akin to the anorthositic group than the granitoids aluminous orthopyroxene megacrysts in nps anorthositic rocks indicate crystallization pressures of 6 2 to 11 0 kbar consistent with lower cru

petrology and isotope geochemistry of the mesoproterozoic anorthosite and related rocks of the kunene intrusive complex nw namibia abstract the intrusion of the large c 18 000 km2 anorthosite body of the kunene intrusive complex angola namibia which is associated with minor marginal granite bodies and syenite syenodiorite dykes marked the beginning of mesoproterozoic igneous activity in nw namibia anorthosites display an igneous mineral assemblage of dominating plagioclase together with minor olivine orthopyroxene or clinopyroxene fe ti oxides whereas biotite and pargasite are late magmatic phases apatite is a common accessory phase in regionally associated felsic rocks k feldspar plagioclase clinopyroxene and or hastingsite crystallized first followed by interstitial quartz hastingsite fe ti oxides and titanite the general geochemical characteristics for the anorthositic rocks of the kunene intrusive complex i e high al2o3 cao sr and eu intermediate to high mg numbers of 0 37 0 74 positive nd t values of 3 0 to 1 0 and initial sr ratios of 0 7028 0 7041 are in excellent agreement with their derivation from fractionated basaltic liquids the 18o values obtained for plagioclase 5 88 0 19 18o support their development from mantle derived magmas relic crustal xenoliths of calc silicate rocks and sr and nd isotope data however suggest that the anorthosite magma underwent minor crustal contamination which was most prominent during the early intrusion stage a u pb zircon age of 1376 2 ma was obtained for a felsic rock associated with the 1385 25 ma anorthosites which suggests that the two suites were emplaced during the same igneous event the geochemical and isotope signatures of the granites syenites and syenodiorites provide convincing evidence against consanguinity of the anorthositic and the felsic rock suites i e 1 a compositional gap exists between the major and trace element contents of the two suites 2 trace element and sr and nd isotopic data of the felsic rocks point to a crustal source nd 2 0 to 0 4 initial sr ratio 0 7024 0 7063 3 the ree patterns of the felsic rocks do not exhibit ubiquitous negative eu anomalies which would be expected from fractionation products of melts that previously formed extensive plagioclase cumulates and 4 18o values of magmatic feldspar from the felsic rock suite fall in a restricted range 7 20 7 92 that is about 1 6 higher than the average igneous plagioclase 18o of the anorthosites and suggests a formation of the felsic melts by anatexis of the lower crust

a strontium and neodymium isotopic investigation of the laramie anorthosites wyoming usa implications for magma chamber processes and the evolution of magma conduits in proterozoic anorthosites abstract a strontium and neodymium isotopic investigation of plagioclase rich cumulate rocks in the 1 43 ga laramie anorthosite complex lac provides new insights into the evolution of magma chambers and underlying magma plumbing systems during the crystallization of proterozoic anorthosites the 725 km2 lac intruded the boundary between archean rocks of the wyoming province in the north and proterozoic island arc terranes in the south the cheyenne belt at pressures of 3 4 kilobars initial strontium and end isotopic ratios from the three large composite anorthositic intrusions poe mountain chugwater and snow creek and late troctolitic intrusions range from 0 7034 0 7055 and 0 9 to 4 9 respectively the isotopic ratios in the cumulates vary as a result of 1 the isotopic composition of the parental mantle derived high al gabbroic magmas 2 the amount of contamination by archean rocks during ascent through the crust and 3 mixing of replenishing magmas and resident magmas with different isotopic compositions in the magma chamber at the final level of emplacement strontium isotopic and normative an variations across the 5 7 km thick stratigraphic section of layered plagioclase rich cumulates in the 200 km2 poe mountain anorthosite indicate that multiple inputs of magma are needed to construct even relatively small intrusions in proterozoic anorthosites the earliest and stratigraphically lowest intrusion was emplaced as a plagioclase rich magma that crystallized large volumes of compositionally homogeneous anorthosite at least three subsequent chamber wide episodes of magma replenishment followed by mixing are indicated by abrupt shifts in isr and or normative an in the stratigraphically higher levels of the poe mountain anorthosite strontium and neodymium isotopic disequilibrium between a high al clinopyroxene megacryst and the layered cumulates is consistent with a high pressure origin for al rich pyroxene megacrysts in proterozoic anorthosites the megacryst crystallized in a magma chamber at pressures of 10 12 kilobars from relatively uncontaminated basaltic parent magma high al gabbro and was transported through the crust in progressively contaminated anorthositic magma the range of observed isotopic compositions in the anorthositic rocks is nearly identical to that of high al gabbroic dikes in the lac supporting the proposition that high al gabbros are parental to anorthosite mitchell et al 1995 the isotopic data require that the anorthositic parental magmas were contaminated during ascent through the crust by archean orthogneisses and or metapelitic rocks decreasing isr and increasing end with a relative decrease in age of the intrusions snow creek poe mountain chugwater troctolites indicates that the magma conduit became increasingly insulated from crustal contamination over time this study indicates that the strontium and neodymium isotopic systems can be used to distinguish between processes that occurred at lower crustal upper mantle pressures during the ascent of magma diapirs and within individual magma chambers in proterozoic anorthosites and underscores the need for stratigraphic control when addressing the origin of plagioclase rich cumulates

characterization of space weathering from lunar reconnaissance orbiter camera ultraviolet observations of the moon we investigate the effects of space weathering at ultraviolet wavelengths using a near global seven band 321 689 nm mosaic from the lunar reconnaissance orbiter camera lroc wide angle camera wac we confirm that for moderate to high iron compositions 5 wt feo the steeply positive uv slope at wavelengths 415 nm shallows with increasing exposure to space weathering we measure these differences in lroc wac data as variations in the 321 415 nm ratio which has low values for fresh craters in the mare and moderate iron highlands for low iron highland compositions the break in slope occurs at shorter wavelengths and it is instead the 321 360 nm ratio that increases with exposure to the space weathering environment whereas the 321 415 nm ratio appears to be largely controlled by the degree of shock experienced during the impact the effects of shock may be more important at highland craters because modest shock pressures result in the solid state transformation of plagioclase to its glass equivalent maskelynite and can help distinguish between primary shocked ejecta and locally exposed fresh material in rays while all of the fresh craters we examined have uv spectral properties consistent with substantial alteration due to space weathering the uv spectra of lunar swirls magnetically shielded from the solar wind are consistent with exposure of immature crystalline material together these results suggest that lunar space weathering is dominated by the solar wind and saturates in the uv at is feo values of 40 submature

the kunene complex angola namibia a composite massif type anorthosite complex the c 15000 km2 c 2 ga kunene complex of southern angola and northern namibia is one of the world s largest occurrences of anorthositic rocks rivalled only perhaps by the lac st jean massif of the grenville province in quebec we report here the results of a detailed field and laboratory study of a 100 km2 area in the northern part of the complex coarse grained av 1 2 cm anorthosite and leucotroctolite are the predominant rock types although colour index varies between 0 and 50 averaging about 10 ultramafic rocks are totally absent typical mineralogy is plagioclase an57 76 olivine fo64 71 orthopyroxene en65 74 clinopyroxene fe ti oxides apatite textures are dominantly massive although weak impersistent magmatic lamination with nearvertical dip and unsystematic strike is also present other magmatic features include plagioclase and orthopyroxene megacrysts block structure mortar texture and anorthositic dykes metamorphic effects are minimal or absent all of these attributes are similar to those found in typical massif type anorthosites since comparable features are present over large areas elsewhere in the complex we suggest that kunene should be considered analogous to a large composite massif type anorthositic intrusive complex rather than to a large single or composite mafic layered intrusion such as bushveld as has been previously suggested or assumed this interpretation is supported by satellite imagery which shows the outlines of several individual anorthositic plutons one of which clearly encompasses our field area

anorthosites in the eastern granulites of tanzania new sims zircon u pb age data petrography and geochemistry several occurrences of anorthosites are known in the neoproterozoic mozambique belt in tanzania these are tectonically incorporated into a suite of enderbitic rocks and migmatitic orthogneisses of the eastern granulites two larger anorthosite bodies and associated rocks from the pare mountains and the uluguru mountains have been chosen for a comparative study regarding their formation age age of subsequent metamorphic processes mineral chemistry and rock chemistry all the investigated magmatic bodies were overprinted by neoproterozoic high pressure granulite facies metamorphism and deformation documented by similar petrography and mineralogy metamorphic textures and deformational characteristics however mineral chemistry geochemistry and geochronology reveal major differences between the two occurrences the pare anorthosite contains igneous zircons that yield archean formation ages of ca 2 64 ga and a more calcic mineral and rock chemistry that is typical for archean type anorthosites extremely narrow metamorphic rims around zircons could not be dated but may have grown during the neoproterozoic metamorphic overprint in contrast the uluguru anorthosite contains igneous zircon cores with u pb ages of 880 820 ma a broad metamorphic rim was dated at ca 640 650 ma the adjacent migmatitic basement gneisses yield formation ages of ca 986 ma followed by pb loss early neoproterozoic events can also be inferred from both anorthosite suite and basement samples of the mahenge mountains there u pb ages from zircon cores cluster around 730 800 ma well developed metamorphic rims yield neoproterozoic ages of 650 ma the basement gneisses are related to a destructive plate margin setting the anorthosites may have formed when parts of central madagascar rifted off east africa at the time considered

crater degradation on the lunar maria topographic diffusion and the rate of erosion on the moon landscape evolution on the moon is dominated by impact cratering in the post maria period in this study we mapped 800 m to 5 km diameter craters on 30 of the lunar maria and extracted their topographic profiles from digital terrain models created using the kaguya terrain camera we then characterized the degradation of these craters using a topographic diffusion model because craters have a well understood initial morphometry these data provide insight into erosion on the moon and the topographic diffusivity of the lunar surface as a function of time the average diffusivity we calculate over the past 3 ga is 5 5 m2 myr with this diffusivity after 3 ga a 1 km diameter crater is reduced to approximately 52 of its initial depth and a 300 m diameter crater is reduced to only 7 of its initial depth and craters smaller than 200 300 m are degraded beyond recognition our results also allow estimation of the age of individual craters on the basis of their degradation state provide a constraint on the age of mare units and enable modeling of how lunar terrain evolves as a function of its topography

optical maturity of ejecta from large rayed lunar craters lucey et al 2000 have developed a methodology for extracting an optical maturity parameter omat from multispectral clementine images the omat parameter characterizes the overall maturity of lunar soils and crater ejecta by changes in reflectance spectra using these omat images we surveyed large craters 20 km diameter on the moon that had previously been mapped as possessing or possibly possessing rayed ejecta we generated average radial profiles of omat values for rays of these large craters from these profiles we classified the craters into three relative age groups 1 older than copernicus inferred age of 810 myr 2 intermediate and 3 as young or younger than tycho inferred age of 109 myr we suspect that there is a bias to our classification scheme such that the omat profiles of smaller craters look like that of larger but older craters nevertheless some large craters such as eudoxus 67 km and aristillus 55 km are now known from this study to have optically mature ejecta and therefore are suspected to be older than copernicus this is consistent with an age of 1 3 gyr suggested for aristillus by ryder et al 1991 such craters were included by mcewen et al 1997 when estimating the density of craters younger than or contemporaneous with copernicus therefore the case for a modest increase in the cratering rate in the past 800 myr versus the previous 2 4 gyr indicated from that work has been weakened grier and mcewen 2001 given current constraints on dating large and recent lunar craters we cannot support or disprove the hypothesis that there has been a significant increase in the rate of large terrestrial impact events in the past 100 400 myr

u pb monazite ages of the kabanga mafic ultramafic intrusions and contact aureoles central africa geochronological and tectonic implications mafic ultramafic rocks of the kabanga musongati alignment in the east african nickel belt occur as bushveld type layered intrusions emplaced in metasedimentary sequences the age of the mafic ultramafic intrusions remains poorly constrained though they are regarded to be part of ca 1375 ma bimodal magmatism dominated by voluminous s type granites in this study we investigated igneous monazite and zircon from a differentiated layered intrusion and metamorphic monazite from the contact aureole the monazite shows contrasting crystal morphology chemical composition and u pb ages monazite that formed by contact metamorphism in response to emplacement of mafic ultramafic melts is characterized by extremely high th and u and yielded a weighted mean 207pb 206pb age of 1402 9 ma which is in agreement with dates from the igneous monazite and zircon the ages indicate that the intrusion of ultramafic melts was substantially earlier by 25 m y 95 confidence than the prevailing s type granites calling for a reappraisal of the previously suggested model of coeval bimodal magmatism monazite in the metapelitic rocks also records two younger growth events at ca 1375 ma and ca 990 ma coeval with metamorphism during emplacement of s type granites and tin bearing granites respectively in conjunction with available geologic evidence we propose that the kabanga musongati mafic ultramafic intrusions likely heralded a structurally controlled thermal anomaly related to nuna breakup which culminated during the ca 1375 ma kibaran event manifested as extensive intracrustal melting in the adjoining karagwe ankole belt producing voluminous s type granites the grenvillian aged ca 990 ma tin bearing granite and related sn mineralization appear to be the far field record of tectonothermal events associated with collision along the irumide belt during rodinia assembly since monazite is a ubiquitous trace phase in pelitic sedimentary rocks in contact aureoles of mafic ultramafic intrusions and in regional metamorphic belts our study highlights the potential of using metamorphic monazite to determine ages of mafic ultramafic intrusions and to reconstruct postemplacement metamorphic history of the host terranes georef subject absolute age africa east africa gabbros igneous rocks intrusions mesoproterozoic metamorphic rocks metamorphism metasedimentary rocks monazite neoproterozoic nesosilicates norite orthosilicates phosphates plutonic rocks precambrian proterozoic silicates tanzania u pb ultramafics upper precambrian zircon zircon group

a new lunar digital elevation model from the lunar orbiter laser altimeter and selene terrain camera abstract we present an improved lunar digital elevation model dem covering latitudes within 60 at a horizontal resolution of 512 pixels per degree 60 m at the equator and a typical vertical accuracy 3 to 4 m this dem is constructed from 4 5 10 9 geodetically accurate topographic heights from the lunar orbiter laser altimeter lola onboard the lunar reconnaissance orbiter to which we co registered 43 200 stereo derived dems each 1 1 from the selene terrain camera tc 1010 pixels total after co registration approximately 90 of the tc dems show root mean square vertical residuals with the lola data of

generation of anorthositic magma by h2o fluxed anatexis of silica undersaturated gabbro an example from the north norwegian caledonides the skattora migmatite complex in the north norwegian caledonides consists of migmatized slightly nepheline normative metagabbros that are net veined by numerous up to 90 anorthositic and leucodioritic dykes the average chemical composition of 17 anorthosite dykes is wt 58 4 sio 2 0 2 tio 2 23 al 2 o 3 1 8 feo t 0 7 mgo 6 3 cao 7 8 na 2 o 0 2 k 2 o a migmatite leucosome and a dyke have been dated by the u pb method on titanite to 456 4 ma in low melt fraction areas minor leucosomes are orientated parallel to the foliation more intense anatexis formed stromatic to schlieric migmatites the leucosomes are commonly connected to dykes suggesting that melt segregated and left its source dyke thicknesses range from a few centimetres up to several metres in general early dykes are parallel to the foliation in the host rock while the later dykes cut the foliation plagioclase an 20 50 is the dominant mineral 85 100 in the dykes and the leucosome but 0 15 amphibole is generally present field relations geochemistry and preliminary melting experiments strongly suggest that the anorthosites originated by h 2 o fluxed anatexis of the gabbroic host rock

growth and evolution of precambrian continental crust in the southwestern tarim terrane new evidence from the ca 1 4 ga a type granites and paleoproterozoic intrusive complex abstract the precambrian tectonic evolution and growth of continental crust in the southwestern tarim craton in nw china remain enigmatic in this contribution we report petrography zircon u pb ages geochemistry and nd hf o isotope data on the ca 1 4 ga granitic pluton in southwestern tarim terrane we also present zircon u pb ages and hf isotope data on the paleoproterozoic intrusive complex from this area and use the combined data to evaluate precambrian tectonic evolution of the region in relation to the columbia supercontinent the ca 1 4 ga azibailedi pluton is mainly composed of biotite monzogranite with contents sio2 72 8 75 1 wt the major element chemistry of the pluton reveals alkali calcic and ferroan affinities and metaluminous to peraluminous feature with a cnk ranging from 0 86 to 1 15 the granite shows high total ree contents with variable enrichment in lree la yb n 8 5 12 2 both ree and other incompatible elements features define a type affinities the initial epsilon nd values vary from 3 5 to 2 2 and zircon hf t and 18o values range from 4 1 to 6 1 and 7 1 to 8 6 integrating geological geochemical and nd hf o isotope data we suggest that the azibailedi a type granites were derived from pre existing early paleoproterozoic mafic lower crust with involvement of variable amount of paleoproterozoic juvenile crust possibly trigged by basaltic magma upwelling during the breakup of the columbia supercontinent u pb geochronology and hf isotopic analyses of zircons from the paleoproterozoic heluositan intrusive complex show crustal growth episodes during 2 2 4 2 ga with peaks at 2 3 2 5 ga 3 3 ga and 3 6 3 8 ga distinct from the other precambrian terranes of the tarim craton a comprehensive synthesis of the records of assembly and breakup of columbia supercontinent from different precambrian terranes in the tarim craton show significant differences the temporal distinction in tectonothermal events and the marked difference in the continental crust growth processes suggest that the early precambrian basement of the tarim craton was possibly composed of discrete terranes which were fragmented from different ancient cratonic nuclei

orogenic to post orogenic origin for the 1 76 ga horse creek anorthosite complex wyoming usa the age and inferred tectonic setting of the 1 76 ga horse creek anorthosite complex hcac in the laramie mountains of southeastern wyoming place important constraints on the origin of middle proterozoic anorthosite complexes the 100 km2 hcac consists of strongly recrystallized anorthosite and two small intrusions of monzonite and granite u pb crystallization ages from euhedral zircons in anorthosite and monzonite are 1761 5 2 ma and 1754 5 2 2 ma respectively an additional period of zircon growth in the anorthosite occurred at 1753 4 2 ma as represented by a small population of anhedral zircon we attribute the origin of this second morphological variety of zircon in the anorthosite to the loss of zr from ilmenite during reaction with plagioclase to form sphene this reaction took place in response to heat and fluid influx during intrusion of the adjacent monzonite the hcac and the younger 1 43 ga laramie anorthosite complex to the north were intruded along or near a paleoproterozoic suture

some phase equilibrium constraints on the origin of proterozoic massif anorthosites and related rocks experimental data in the range of 1 bar to 13 kbar enable us to introduction map the liquidus equilibria relevant to proterozoic massif anorthere is hardly a consensus on the petrogenesis of thosites and related mafic rocks massif anorthosites are widely proterozoic massif anorthosites but many investigators believed to have formed by accumulation of plagioclase into highbelieve in a petrogenetic scheme that involves at least al basaltic liquids mantle derived basaltic liquids fractionating two major stages 1 extensive crystallization asat pressures sufficiently high 10 13 kbar to crystallize the highly similation of a mantle derived magma ponded at or aluminous orthopyroxene megacrysts typically observed in anorthosite near the base of the crust that produces suspensions of massifs reach plagioclase saturation at low normative silica contents plagioclase in fe rich high al gabbroic liquids followed peritectic like equilibria e g liq opx plag cpx sp by 2 intrusion of these suspensions into the mid upper and a thermal divide on the plagioclase pyroxene liquidus surface crust where they form complex magma chambers e g ensure that mantle derived liquids become nepheline normative with duchesne 1984 emslie 1985 ashwal 1993 wiebe further crystallization and crustal assimilation at depth such 1994 a key component of the scheme is the presence liquids cannot produce the full range of troctolitic noritic to trocin most massifs of high al orthopyroxene megacrysts tolitic gabbroic mineral assemblages observed in anorthosite massifs haom emslie 1975 that appear to preserve a record without extensive low pressure granite assimilation conversely the of the high pressure stage maquil duchesne 1984 array of plausible anorthosite parental liquids not only lies along longhi et al 1993 the structures of massifs vary from the trace of the plagioclase two pyroxene cotectic from 10 to 13 those that are composites of multiple diapiric intrusions kbar but also straddles the thermal divide on the plagioclase harp lake emslie 1980 rogaland duchesne 1984 pyroxene liquidus surface this condition requires mafic source to intrusive bodies with well developed layering of regions such as lower continental crust or foundered mafic plutons leucocratic troctolites and leuconorites or leucogabbros for liquids parental to massif anorthosites and associated mafic overlain by masses of anorthosite michikamau emslie intrusions 1970 laramie frost et al 1993 so there may be

weathering indices as climate proxies a step forward based on congo and sw african river muds abstract despite the influence of other geological and geomorphological factors chemical weathering at the earth s surface is strongly controlled by climate thus a measure of weathering intensity determined from soils or sediments should provide information about the climatic conditions associated with their formation available geochemical and mineralogical data on modern fluvial and marine muds from different regions of southern africa and its atlantic continental margin are used to review the links between sediment composition and climatic properties together with the possible causes of variance although river muds may not be generated exclusively in a single sedimentary cycle and erosion and weathering processes do not necessarily take place in a spatially homogeneous way significant relationships between mineralogical and geochemical signatures of river mud and rainfall in the corresponding catchment area were recognised our study shows that the composition of clay is strongly influenced by climatically driven weathering whilst coarser mud fractions tend to be more affected by provenance grain size hydraulic sorting and recycling in the marine environment the climatic signal may be lost even in clay because of hydraulic fractionation authigenic mineral growth and mixing with foreign particles given the ubiquitous character of fluvial muds and the easy and non expensive methods available for separating and analysing clay fractions their geochemical fingerprints represent a most precious source of information concerning climate any geochemical parameter used as a regional proxy of climate however still requires that the diversity of geological geomorphological and biological factors that affect its value are cautiously considered

depositional ages and provenance of the neoproterozoic damara supergroup northwest namibia implications for the angola congo and kalahari cratons connection abstract the damara orogen is composed of the damara kaoko and gariep belts developed during the neoproterozoic pan african orogeny the damara belt contains neoproterozoic siliciclastic and carbonate successions of the damara supergroup that record rift to proto ocean depositional phases during the rodinia supercontinent break up there are two conflicting interpretations of the geotectonic framework of the damara supergroup basin i as one major basin composed of the outjo and khomas basins related to rifting in the angola congo kalahari paleocontinent or ii as two independent passive margin basins one related to the angola congo and the other to the kalahari proto cratons detrital zircon provenance studies linked to field geology were used to solve this controversy u pb zircon age data were analyzed in order to characterize depositional ages and provenance of the sediments and evolution of the succession in the northern part of the outjo basin the basal nabis formation nosib group and the base of the chuos formation were deposited between ca 870 ma and 760 ma the upper chuos berg aukas gauss auros and lower brak river formations formed between ca 760 ma and 635 ma it also includes the time span recorded by the unconformity between the auros and lower brak river formations the ghaub upper brak river karibib and kuiseb formations were deposited between 663 ma and 590 ma the geochronological data indicate that the main source areas are related to i the angola congo craton ii rift related intrabasinal igneous rocks of the naauwpoort formation iii an intrabasinal basement structural high abbabis high and iv the coastal terrane of the kaoko belt the kalahari craton units apparently did not constitute a main source area for the studied succession this is possibly due to the position of the succession in the northern part of the outjo basin at the southern margin of the congo craton comparison of the obtained geochronological data with those from the literature shows that the abbabis high forms part of the kalahari proto craton and that angola congo and kalahari cratons were part of the same paleocontinent in rodinia times

re os isotopic evidence for a lower crustal origin of massif type anorthosites massif type anorthosites are large igneous complexes of proterozoic age they are almost monomineralic representing vast accumulations of plagioclase with subordinate pyroxene or olivine and fe ti oxides the 930 myr old rogaland anorthosite province in southwest norway1 represents one of the youngest known expressions of such magmatism the source of the magma and geodynamic setting of massif type anorthosites remain long standing controversies in precambrian geology with no consensus existing as to the nature of the parental magmas or whether these magmas primarily originate in the earth s mantle or crust at present massif type anorthosites are believed to have crystallized from either crustally contaminated mantle derived melts that have fractionated olivine and pyroxenes at depth2 or primary aluminous gabbroic to jotunitic melts derived from the lower continental crust3 here we report rhenium and osmium isotopic data from the rogaland anorthosite province that strongly support a lower crustal source for the parental magmas there is no evidence of significantly older crust in southwest scandinavia and models invoking crustal contamination of mantle derived magmas fail to account for the isotopic data from the rogaland province initial osmium and neodymium isotopic values testify to the melting of mafic source rocks in the lower crust with an age of 1 400 1 550 myr

imaging of lunar surface maturity the physical evolution of the lunar surface with exposure to the space environment is termed maturation and maturity is the degree to which a particular lunar soil possesses quantitative characteristics consistent with that exposure several quantitative measures or indices of maturity have been proposed and employed including the abundance of solar wind gas abundance of various types of agglutinates various measures of grain size and is feo among the changes attendant with space exposure are striking changes in the optical characteristics of soils mature lunar soils are dark red and exhibit reduced spectral contrast relative to immature soils this paper presents an optical maturity index that quantifies the spectral effects of maturation we show that this optical maturity index correlates with other maturity indices about as well as the accepted maturity indices correlate among themselves and is only weakly coupled to composition the modest correlations among maturity indices suggest important controls on individual maturity indices other than age uncorrelated variations in the rates of accumulation of individual indicators or variations in depths over which indicators are emplaced in addition to mixing these effects conspire to reduce the equivalence of maturity and duration of surface exposure optical maturity illustrates some of these effects showing that ejecta of large and small craters mature at different rates and that the interiors and ejecta of large craters exhibit systematically different optical maturities the same or analogous effects are likely to influence other maturity indices

mesoproterozoic sm nd and u pb ages for the kunene anorthosite complex of sw angola abstract the kunene complex kc of sw angola covering an area of 15 000 km2 is one of the largest anorthosite bodies in the world although considered by several authors as a layered complex field evidences demonstrate that it is a massive anorthosite in the study area a northern and a southern kc were distinguished on the basis of a clear difference in anorthite contents the boundary between them corresponds to a belt of coeval granitoids analyses of two multigrain zircon populations and several single grains evaporation and vapour dissolution methods from a late stage mangerite vein inferred to be cogenetic with the kc yielded a concordant age of 1371 2 5 ma implying a mesoproterozoic age for the kc because the anorthosites are unmetamorphosed cooling after emplacement was also constrained by sm nd mineral isochrons of 1470 25 ma for a troctolite near quihita and 1319 28 ma for a hyperporphyritic megacrystic anorthosite dyke considered to represent a quenched plagioclase crystal mush sampled near lufinda the discrepancy between the two data can be interpreted in two ways 1 the sm nd isochrons are geologically meaningful and thus the kc is constituted by various plutons with different mesoproterozoic ages 2 most likely the sm nd systematic was cryptically perturbed at mineral scale and thus only the u pb age of the mangerite vein yields to a reliable intrusion age of the kc the initial 143 nd 144 nd of the quihita sample 0 510734 28 as well as the high anorthite content of northern kc suggests that parent magma of kc derived from a mantle source and thus that the emplacement of kc anorthosites reflect an episode of continental accretion rather than a within continent differentiation the initial 143 nd 144 nd of the lufinda dyke 0 510766 24 implies instead a minor addition of crustal material into the parental magma collectively the emplacement of the kunene complex requires an extensional setting and a significant thermal anomaly at the margin of the congo craton during the early kibaran cycle

small scale lunar farside volcanism abstract we identified and mapped 28 mare basalt occurrences between the australe and south pole aitken basins on the southern lunar farside and determined their absolute model ages amas by performing crater size frequency distribution csfd measurements our study area can be subdivided into seven major mare basalt occurrences in and around bolyai roche v rosseland pauli and roche craters south of both rosseland and coblentz craters as well as mare patches between eotvos and roche craters the amas of the mare basalts in and around bolyai crater range from 2 1 ga to 3 5 ga two units varying drastically within short distances the mare patches south of coblentz crater contain nine units that have amas ranging from 2 1 ga to 3 8 ga the mare basalts in roche v crater show an ama of 2 2 ga we mapped seven volcanic units in rosseland crater and derived amas for five of these units the mare basalts in rosseland crater show the youngest amas found in our study area ranging from 1 5 ga to 2 9 ga the mare basalt occurrence south of rosseland crater shows significantly older ages with an ama of 3 3 ga the mare basalts in pauli and roche craters show amas from 1 7 ga to 3 1 ga the mare patches between eotvos and roche craters show a similar range of amas from 1 9 ga to 3 1 ga the amas of the mare basalts in our study show that the lunar farside was volcanically active for nearly as long as the lunar nearside 1 2 ga ago or at least longer than previously thought 2 5 ga ago and predicted by models of the ascent and eruption of lunar basalts in addition we calculated thicknesses and volumes of the investigated mare basalts with thicknesses between 21 m and 172 m and volumes from 0 1 km3 to 379 km3 the mare basalts in our study area show a wide range of dimensions similar to other mare basalts of the lunar near and farsides

the lunar crater giordano bruno as seen with optical roughness imagery abstract images acquired by the lro nac allow 3d phase ratio imagery of several areas in the crater giordano bruno this is a new optical remote sensing technique that allows a determination of the optical roughness of the lunar surface our study confirms complicated impact melt movement on the flanks and the floor of the crater in many cases however flow structures seen on the inner wall can be attributed to regolith debris taluses rather than impact melt flows it was found that the whirlpool like formation seen near the western side of the crater giordano bruno has a small central depression that can be interpreted as either a vortex cavity or a feature which resembles such a vortex formed by viscous flows coming from the crater flanks we discuss several features that confirm the young age of the crater concluding however that it was not formed within human history its age is likely on the order of 1 my

fe ti v p ore deposits associated with proterozoic massif type anorthosites and related rocks abstract magmatic rocks containing economic concentrations of iron titanium vanadium and phosphorous are commonly associated with massif type anorthosites and related rocks this rock association is part of the anorthosite mangerite charnockite rapakivi granite suites that are restricted to the proterozoic understanding the geochemistry and emplacement mechanisms of ilmenite magnetite and apatite ore deposits is crucial for exploration efficient mining operations and ore processing this review discusses the controlling factors on the grade of an ore its mineralogy and its major and trace element distribution we present petrogenetic models of currently mined deposits lac tio tellnes damiao and discuss the characteristics of minor ore bodies from anorthosite provinces worldwide grenville north china craton east european craton rogaland laramie models of formation of anorthosite and related rocks are presented as well as the nature of the possible parental magmas of the suite a mineralogical classification of fe ti ores is proposed 1 gabbro noritic ilmenite ore apatite magnetite 2 ti magnetite dominated ore 3 nelsonite fe ti oxides apatite and 4 rutile ilmenite ore the stability of ilmenite and magnetite is then critically reviewed and the influence of various factors particularly oxygen fugacity and crystallization pressure is examined we discuss liquidus compositions of fe ti oxides and the behavior of important trace elements such as cr and v both of which are sensitive to fo2 variations post cumulus evolution of both oxides can occur due to re equilibration with trapped liquid re equilibration with ferromagnesian silicates exsolution oxidation reaction between ilmenite and magnetite and metamorphic overprinting these various processes are described and their effects on the oxide geochemistry are emphasized several potential ore forming processes have been invoked and can explain the formation of huge concentration of ilmenite magnetite apatite fractional crystallization can be combined with crystal sorting and plagioclase buoyancy to produce relative enrichment of dense ore minerals silicate liquid immiscibility can segregate conjugate si rich and fe rich melts the latter being enriched in fe ti p magma mixing can produce hybrid magmas located in a single phase field of the phase diagram and precipitate a pure ilmenite cumulate alternative processes are also described such as ejection of fe ti enriched residual melts by filter pressing and compaction solid state remobilization of ilmenite in veins and hydrothermal transport of fe and ti from the host anorthosite followed by concentration in veins and lenticular ore bodies the magnetic properties of fe ti ore deposits present contrasting signatures depending on whether the natural remanent magnetization is dominated by hemo ilmenite or multi domain magnetite micro and macro scale deformation features of ore rocks are intimately correlated with magma emplacement and with ballooning of the anorthosite diapir associated with gravitational sagging of dense ore bodies exploration perspectives show that oxide apatite gabbronorites are interesting targets because ilmenite in these rocks is poorer in cr and mg and because the ti resource may be combined with apatite and vanadiferous magnetite

formation age of the lunar crater giordano bruno using the terrain camera onboard the japanese lunar explorer selene kaguya we obtained new high resolution images of the 22 kilometer diameter lunar crater giordano bruno based on crater size frequency measurements of small craters 200 m in diameter superposed on its continuous ejecta the formation age of giordano bruno is estimated to be 1 to 10 ma this is constructive evidence against the craters medieval age formation hypothesis

pb loss patterns in zircons from a high grade metamorphic terrain as revealed by different dating methods upb and pbpb ages for igneous and metamorphic zircons from northern sri lanka abstract we report u pb and 207 pb 206 pb ages for multigrain fractions single grains and grain domains in zircons from highgrade igneous and sedimentary rocks of the vavuniya charnockite province vcp in northern sri lanka using the ionmicroprobe shrimp a new vapour digestion technique vdt and evaporation charnockitic enderbitic and mangeritic gneisses are derived from i type tonalitic syenitic granodioritic and granitic precursors for which the u pb systematics of zircons as determined by shrimp suggest crystallization ages between 1000 and 1100 ma many translucent round and multifacetted zircons in these rocks are not of metamorphic but of igneous origin and acquired their shape through partial zircon dissolution and recrystallization the large majority of zircons exhibit severe and variable pb loss some 540 580 ma ago which we ascribe to granulite grade metamorphism almost immediately followed by local retrogression most of the zircons lost pb from the entire grain with no domains remaining that reflect the original pb isotopic composition it was therefore impossible to obtain meaningful 207 pb 206 pb evaporation ages for such grains and this cautions against use of this method in zircons that experienced severe pb loss however zircons formed during high grade metamorphism apparently retain their primary pb isotopic composition and were dated successfully by the evaporation technique yielding ages between 553 26 and 560 30 ma conventional multigrain analysis and small fractions following vdt for two samples although much more precise than the shrimp data yielded only strongly discordant data points with patterns reflecting variable pb loss these data alone would not have made it possible to obtain reliable primary zircon crystallization ages 207 pb 206 pb evaporation ages for detrital zircons from two garnetiferous gneiss samples of sedimentary origin in the centre of the vcp revealed a maximum age of 1329 18 ma whereas a metaquartzite near the southern margin of the vcp contained zircons with 207 pb 206 pb ages between 1857 14 and 2543 14 ma we interpret the latter rock as belonging to the highland complex which occurs adjacent to the vcp our zircon data in combination with nd model ages milisenda et al 1994 suggest the vcp to be part of the wanni complex a distinct crustal domain in northern sri lanka characterized by relatively young ages in comparison with much older ages in the adjacent highland complex

magnetic fabrics and rock magnetism of the xiong er volcanic rocks and their implications for tectonic correlation of the north china craton with other crustal blocks in the nuna columbia supercontinent abstract the tectonic background of the paleoproterozoic xiong er volcanic rocks xvr is important for understanding the tectonic evolution of the north china craton ncc as well as its paleogeographic position during the assembly of the nuna columbia supercontinent here we report the results of the first anisotropy of magnetic susceptibility ams study of the xvr and use the interpreted flow directions to constrain the emplacement mechanism and to assess its geological significance for the reconstruction of the nuna columbia supercontinent thirty lavas were sampled from three sections in western henan province detailed rock magnetic analyses including measurements of hysteresis loops magnetization versus temperature curves and first order reverse curves were performed to identify the main magnetic phases and grain sizes the inferred directions from the ams results reveal a radial flow pattern with an eruption center probably located near xiong er mountain our data suggest that the xvr may have been emplaced in a triple conjugated continental rift on the south margin of the ncc probably initiated from a paleoplume based on this interpretation a comparison of geological and paleomagnetic results among the proposed crustal blocks in the nuna columbia supercontinent suggests a close linkage of the ncc with sao francisco congo rio de la plate and siberia

the growth of large mafic intrusions comparing niquelândia and ivrea igneous complexes abstract the niquel ndia complex brazil is one of the world s largest mafic ultramafic plutonic complexes like the mafic complex of the ivrea verbano zone it is affected by a pervasive high t foliation and shows hypersolidus deformation structures contains significant inclusions of country rock paragneiss and is subdivided into a lower and an upper complex in this paper we present new shrimp u pb zircon ages that provide compelling evidence that the upper and the lower niquel ndia complexes formed during the same igneous event at ca 790 ma coexistence of syn magmatic and high t subsolidus deformation structures indicates that both complexes grew incrementally as large crystal mush bodies which were continuously stretched while fed by pulses of fresh magma syn magmatic recrystallization during this deformation resulted in textures and structures which although appearing metamorphic are not ascribable to post magmatic metamorphic event s but are instead characteristic of the growth process in huge and deep mafic intrusions such as both the niquel ndia and ivrea complexes melting of incorporated country rock paragneiss continued producing hybrid rocks during the last vanishing stages of magmatic crystallization this resulted in the formation of minor late stage hybrid rocks whose presence obscures the record of the main processes of interaction between mantle magmas and crustal components which may be active at the peak of the igneous events and lead to the generation of eruptible hybrid magmas

proterozoic massif type anorthosites as the archetypes of long lived 100 myr magmatic systems new evidence from the kunene anorthosite complex angola abstract proterozoic massif type anorthosites are voluminous plagioclase dominated plutonic suites with characteristic intermediate compositions an50 10 that represent mantle derived magmas ponded at moho depths and crystallized polybarically until emplacement at mid crustal levels we present new id tims and la icpms u pb ages as well as detailed field observations from the kunene anorthosite complex kac of sw angola our main objective in this work is to contextualize a variety of important petrographic and geochronological features of the kac that have implications for proterozoic anorthosite petrogenesis and the geological evolution of the sw congo craton high precision u pb zircon and baddeleyite ages for anorthositic rocks from across the complex n 11 range between 1376 and 1438 ma a period of 60 myr showing that such systems may be remarkably long lived dating of magmatic zircons from a comagmatic pegmatoidal enclave in the kac which also contains high al orthopyroxene megacrysts haom gives an age of 1500 ma indicating as much as 120 myr between some of the earliest and latest crystallization products of the system these enclaves provide a snapshot of the liquid line of descent of the anorthosites and are compelling evidence for the comagmatic nature of the haom field evidence for block structure magma mingling and extreme textural heterogeneity in the kac supports emplacement as multiple influxes of magma of various volumes carrying variable proportions of crystals and autoliths from below we infer that the kac represents a long lived mushy magmatic system that was likely emplaced in distinct pulses these observations refine our understanding of proterozoic anorthosite petrogenesis they also provide evidence for an array of magmatic processes operating at various levels of crystallinity that may create textural and chemical heterogeneity in evolving magma products at lower to mid crustal levels

mesoproterozoic 1 4 ga a type gneissic granites in the xilinhot terrane ne china first evidence for the break up of columbia in the eastern caob abstract the precambrian evolution of the central asian orogenic belt caob in ne china remains enigmatic in this paper we report petrography zircon u pb ages and major and trace element data of the precambrian gneissic granites from the xilinhot complex in the xilinhot terrane of the eastern caob zircons in four gneissic granite samples yielded weighted mean 207 pb 206 pb ages of 1397 1360 ma suggesting that the xilinhot terrane is a microcontinental fragment with precambrian basement rather not a paleozoic accretionary complex as previously thought the gneissic granites contain high sio 2 73 7 78 6 wt and k 2 o 4 1 4 9 wt but low al 2 o 3 9 5 12 1 wt with depleted sr ba ti p fractionated ree patterns and strong negative eu anomaly these features are comparable to those of a type granites related to extensional environments this first discovery for the occurrence of 1 4 ga a type granites suggests that the xilinhot terrane in the caob was likely originated from part of the columbia supercontinent and re assembled after the breakup of columbia during the mesoproterozoic probably at ca 1 4 ga

the provenance of northern kalahari basin sediments and growth history of the southern congo craton reconstructed by u pb ages of zircons from recent river sands the southern congo craton is widely overlain by meso to cenozoic sediments of the northern kalahari basin which hamper any correlation of basement units the latter are represented by the archaean angola and kasai blocks while the southern cratonic margin is framed by several meso to neoproterozoic orogenic belts for provenance analysis of the sedimentary cover and reconstruction of the main zircon forming events we studied zircons from recent sediments of the largest rivers at the southern margin of the congo craton u pb zircon ages suggest a major amount of the sediments to originate from e lufilian and kibaran belts while input from the s damara belt seems to increase to the w ages related to the angola block were only noticed in the westernmost parts of the working area which is not in accordance with the se trending drainage pattern proposed to have been onset during the cretaceous thus it is assumed that the meso to cenozoic sedimentary cover extended further west than today prior to the mesozoic to neogene uplift of the angola block and that subsequent erosion exhumed the basement stepwise from west to east a recurrent destabilisation of the southern margin of the congo craton at 2 7 1 9 1 0 and 0 6 ga is supposed to be represented by major peaks in the age distribution pattern of the total amount of concordant zircons this is in accordance with similar studies in adjacent areas additionally the obtained data fit well to several hypothesised major events during the supercontinent cycle