generation of palaeoproterozoic tonalites and associated high k granites in southwestern tanzania by partial melting of underplated mafic crust in an intracontinental setting constraints from geochemical and isotopic data abstract the southwestern part of the 2 0 1 8 ga palaeoproterozoic usagaran belt in the njombe area of sw tanzania is intruded by two types of synchronous granitic rocks with different chemical and petrological characteristics the first type consists of hornblende rich tonalites that have major element compositions similar to those of archaean ttg but differ significantly in their trace element composition the tonalites are spatially and closely associated with felsic high k i type granites some of which are gneissic and or aplitic u pb zircon geochronology shows that the emplacement of tonalites at 1887 11 ma was largely contemporaneous with emplacement of high k granitic gneisses at 1877 15 ma and aplitic granites at 1857 19 ma the data also reveal the presence of archaean crust of 2648 25 ma in the zircon cores of some samples in the otherwise palaeoproterozoic terrane the tonalites are characterized by mgo contents of 1 60 4 11 wt at a sio2 range of 58 1 67 9 wt the mg of 34 55 lower sr contents 220 462 ppm and less fractionated ree patterns la ybcn 3 55 12 9 compared to archaean ttg sr 500 ppm la ybcn 20 these features coupled with the end 1887 ma values of 0 37 to 0 66 as well as the associated mafic enclaves are suggestive of derivation of the tonalites by low pressure below the garnet stability partial melting of a mantle derived mafic underplate that was subsequently contaminated with small amounts of pre existing igneous crustal rocks the evolved nature of the high k granites mgo 0 20 1 30 wt sio2 65 5 73 9 wt mg 25 42 end 3 20 to 4 75 coupled with old tdm ages which are 200 1000 ma older than their emplacement age requires a higher degree of assimilation of older crustal material by the magma derived from partial melting of the underplated mafic crust which was subsequently followed by crystal fractionation involving plagioclase pyroxene and amphibole the close spatial and temporal association of the tonalites mafic enclaves and the high k granites and gneisses in the njombe area provides the first direct evidence of the role of magmatic underplating for the regional thermal anomaly that caused widespread crustal anatexis leading to the generation of the 1 8 1 9 ga granitic rocks in njombe area and or associated felsic volcanism in the usagaran in ndembera and ubendian in ngualla belts of sw tanzania

nd sr isotopic composition of granulites and constraints on the evolution of the lower continental crust in a statistical study of granulite facies rocks we have analysed 32 samples with ages ranging from 2 900 to 200 myr for sm nd rb and sr and 143nd 144nd and 87sr 86sr ratios the sm nd ratio is found to be constant compared with the highly variable rb sr ratio using the model age formalism we show that in many cases internal differentiation of continental crust follows external differentiation with a large time interval

timing of exhumation of a high pressure mafic granulite terrane of the paleoproterozoic ubende belt west tanzania abstract the ubendian belt west of the archean tanzanian craton is known as a prominent crustal structural weakness the belt is composed of different terranes set in a nw se oriented lateral shear pattern sharply outlined by steeply inclined inter terrane shear fault zones its predominant mafic terranes ubende and mbozi display different and more complex evolutionary characteristics than their adjacent felsic terranes ufipa metamorphic studies reveal that mafic and ultramafic high pressure granulites and rare eclogites subsist as rigid blocks and lenses within the highly sheared amphibolite facies country rock p t studies suggest two succeeding decompressional stages for the uplift of the high pressure mafic granulite terranes the second stage being identified on high pressure blastomylonites included in the latter are amphibolites mafic tectonites for which barroisite constitutes a p t marker near the southwestern boundary of the nw se oriented mafic terranes these barroisite bearing tectonites display a pronounced extensional shear banding considering that this tectonite has been formed during the second stage of uplift of the ubende high pressure mafic granulites timing of that stage is pursued ar ar stepwise heating analyses on different barroisite separates from this mafic tectonite have yielded a weighted average cooling age of 1848 6 ma for the argon fractions released at intermediate temperatures this age falls in the time range 1950 1850 ma associated with the second phase of the evolution of the ubende belt characterized by amphibolite facies metamorphism the isotopic composition of argon released at higher temperature steps from compositionally different phases in the separates yields apparent ages tentatively considered only as indicative for a possible older paleoproterozoic event based on geochronological and structural data the uplift of the high pressure granulite ubende terrane is interpreted as related to a late paleoproterozoic dextral lateral shear deformation in ductile amphibolite facies conditions this appears to be associated with the emplacement of granites subsequent proterozoic strike slip reactivation of the paleoproterozoic ubende shear belt did not fundamentally contribute to the near surface exhumation of the high pressure mafic granulites

neodymium isotopic constraints on the protolith ages of rocks involved in pan african tectonism in the mozambique belt of tanzania abstract nd isotopic data from samples collected across the tanzanian sector of the mozambique belt indicate that the bulk of the belt consists of early proterozoic crust which was tectonothermally reworked during pan african time the only evidence for younger crust is preserved in granulite complexes that are surrounded by older amphibolite facies gneisses the uniformity of crustal formation ages of the gneisses across most of the belt indicates that models explaining the mozambique belt as a tectonic collage of terranes with different pre pan african crustal histories are invalid

pollen analysis and 14c age of moss remains in a permafrost core recovered from the active rock glacier murtel corvatsch swiss alps geomorphological and glaciological implications within the framework of core drilling through the permafrost of the active rock glacier murtel corvatsch in the swiss alps subfossil stem remains of seven different bryophyte species were found at a depth of 6 m below surface and about 3 m below the permafrost table in samples from massive ice the composition of the moss species points to the former growth of the recovered mosses in the nearest surroundings of the drill site a total of 127 pollen and spores captured by the mosses and representing 23 taxa were determined the local vegetation during deposition time must be characterized as a moss rich alpine grassland meadow rich in cyperaceae poaceae chenopodiaceae and asteraceae comparable to today s flora present around the study site for l4 c analysis accelerator mass spectrometry had to be used due to the small sample mass about 0 5 mg carbon content the mean conventional 14 c age of 2250 100 years 1 variability corresponds to ranges in the calibrated calendar age of 470 170 bc and 800 bc to ad 0 at statistical probabilities of 68 and 95 respectively this result is compared with the present day flow field as determined by high precision photogrammetry and with information about the thickness vertical structure and flow of the permafrost from borehole measurements total age of the rock glacier as a landform is on the order of 10 4 years the development of the rock glacier most probably started around the onset of the holocene when the area it now occupies became definitely deglaciated the bulk of the ice rock mixture within the creeping permafrost must be several thousand years old characteristic average values are estimated for 1 surface velocities through time cm a 1 2 long term ice and sediment accretion rates mm a 1 on the debris cone from which the rock glacier develops 3 retreat rates 1 2 mm a 1 of the cliff which supplies the debris to the debris cone and rock glacier and 4 ice content of the creeping ice rock mixture 50 90 by volume the pronounced supersaturation of the permafrost explains the steady state creep mode of the rock glacier

inactive and relict rock glaciers of the deboullie lakes ecological reserve northern maine usa lobate talus slopes in the deboullie lakes ecological reserve dler of northern maine exhibit parabolic profiles characteristic of inactive and relict talus derived rock glaciers vegetated rock glacier surfaces suggest that the landforms are no longer active and lobes comprising two dler rock glaciers document periods of past growth observations of perennial subsurface ice are supported by datalogger temperature measurements indicating that sporadic permafrost exists throughout the dler we compare the dler rock glaciers along with similar features elsewhere in new england and adjacent quebec to the modern alpine permafrost distribution results indicate that a mean annual temperature cooling of 68c is required to promote active rock glacier growth ages of plant remains recovered from the basal sediments of a local pond constrain deglaciation to before 11 320 14 c a bp and core stratigraphy and organic content reveal that a periglacial environment persisted during the early postglacial era thus we hypothesise that the dler rock glaciers were active during lateglacial time despite the lack of glacier activity in the region we take this to suggest that north eastern us rock glaciers formed in response to mean annual temperatures skewed towards the frigid winters of the younger dryas chronozone copyright 2009 john wiley sons ltd

on the sensitivity of holocene talus derived rock glaciers to climate change in the ben ohau range new zealand the morphology and surface ages of talus derived rock glaciers are investigated to establish the timing of rock glacier formation in the central southern alps samples of rock weathering rinds show that all rock glaciers studied were formed during the neoglacial period but differences exist between sites in the number of new rock glacier lobes formed by holocene climatic fluctuations a qualitative conceptual model is proposed to explain rock glacier formation in terms of two thresholds an external threshold relates to the presence of a cool climate capable of allowing internal ice to form within talus slopes an internal threshold relates to the presence of sufficiently thick talus at a site to generate a shear stress capable of overcoming internal friction within the talus ice mass the model produces a non steady state response to explain why unmodified talus single lobed and double lobed rock glaciers developed at adjacent sites under the same climatic regime individual landforms have different sensitivities to formation which depend partly on the previous history of talus accumulation and rock glacier activity at a site the model demonstrates how successive cool climate periods may be fully represented by rock glacier lobes at sensitive sites but under represented at insensitive sites sensitivity and therefore climatic representativeness is favoured by high rates of debris supply by implication the timing of formation of rock glacier lobes in regions of prolonged cool climate and low debris production is less likely to correspond to the timing of climatic cooling and more likely to follow the rules of deterministic chaos

rock control on microweathering of bedrock surfaces in a periglacial environment abstract microweathering of ice smoothed bedrock surfaces was investigated in the roldal area of hardanger plateau 60 southern norway postglacial rates of weathering were determined from surface lowering using quartz veins as reference surfaces weathering processes are inferred from assessment of weathering rind formation surface hardness and the preservation of small scale glacial erosional features surface lowering rates for a range of metamorphic rocks vary from 0 05 to 2 20 mm ka 1 and are broadly comparable with those obtained from crystalline rocks in other periglacial environments the mean rate of surface lowering at 0 55 mm ka 1 is low and demonstrates the relatively small impact of microweathering on postglacial landscape evolution variations in bedrock microweathering can be explained by lithological variation amphibolite and mica rich bedrock surfaces experience greater denudation and weakening least weathering rind formation and abundant preservation of glacial striae despite greater surface lowering conversely quartz rich bedrock surfaces are most resistant to denudation and weakening but have greater weathering rind formation and fewer preserved striae postglacial microweathering is achieved primarily through granular disintegration involving detachment and removal of mineral grains and weakening from increased porosity granular decomposition is manifest in the formation of weathering rinds analysis of interactions between weathering indices indicates that rind accumulation is limited by microerosion a conceptual model is proposed that illustrates the temporal interrelationships between in situ and erosional facets of microweathering in two contrasting mineral assemblages the model proposes that cyclic processes of in situ disintegration decomposition and erosion are at work the relative balance between these processes varies with lithology so that in more resistant quartz rich rocks the net effect is minimal surface lowering and accumulation of weathering rind in weaker amphibolitic and micaceous rocks the net effect is greater surface lowering and minimal accumulation of weathering rind the results of the research demonstrate the important influence of rock properties notably mineral composition in postglacial microweathering of crystalline bedrock in a periglacial environment

argon composition of metamorphic fluids implications for 40ar 39ar geochronology the concentration and isotopic composition of argon trapped in fluid inclusions within a mylonitic amphibolite were determined by combining electron microprobe sup 40 ar sup 39 ar laser microprobe and fluid inclusion analyses on the same samples in our study fluid in inclusions formed during ductile deformation was probably derived locally from the paleozoic caledonian sequences whereas brines emplaced in cross cutting fractures were derived from proterozoic footwall rocks several kilometers from the mylonite by utilizing sup 38 ar sub cl to correct for open quotes excess sup 40 ar close quotes we estimate a 370 ma closure age for the amphibole which is distinctly younger than the 440 480 ma plateau ages indicated by step heating spectra the open quotes excess sup 40 ar close quotes responsible for the anomalous plateaus probably resulted from diffusive re equilibration of the amphibole with the fluids present during ductile deformation of the mylonite the significantly younger cooling age determined by our method is concordant with ages determined from more recent geochronological studies in the region these ages indicate that the western gneiss terrain in our study area records deformation and cooling associated with siluro devonian tectonothermal metamorphism rather than an earlier caledonian tectonism as proposed in previous sup 40 ar sup 39 ar investigators 46 refs 8 figs

age and geochemistry of coeval felsic volcanism and plutonism in the palaeoproterozoic ndembera group of southwestern tanzania constraints from shrimp u pb zircon and sm nd data abstract the ndembera metavolcanic rocks represent a continuum of compositions ranging from intermediate to more siliceous calc alkaline trachyandesites dacites trachytes rhyolites which have been intruded by largely metaluminous calc alkaline i type granites in the palaeoproterozoic usagaran belt of southwestern tanzania shrimp u pb zircon age data show that the ndembera metavolcanic rocks were extruded at 1871 30 ma an event that was largely coeval with the emplacement of cross cutting microcline biotite rich granites at 1896 29 ma despite some compositional differences both the granites and metavolcanic rocks share similar geochemical features including very coherent ree patterns characterized by enrichment of the lree relative to the hree la ybcn 13 6 32 8 average 23 9 for metavolcanic rocks and la ybcn 10 8 38 2 average 21 8 for granites negative eu anomalies eu eu 0 45 1 01 average 0 83 for volcanic rocks and eu eu 0 48 0 70 average 0 60 for granites negative ta ti and nb anomalies nb lapm 0 14 0 29 average 0 20 for volcanic rocks and nb lapm 0 01 0 37 average 0 16 for granites the metavolcanic rocks have nd 1871 ma values of 0 03 to 4 89 and tdm ages of 2243 2714 which are broadly similar to those of the granites nd 1896 ma values of 3 56 to 4 59 and tdm ages of 2469 2646 ma the geochemical similarities between the ndembera volcanic rocks and granites coupled with their similar emplacement age suggest that the two rock suites were derived by the same processes in the same tectonic setting from an isotopically similar source these geochemical features coupled with the paucity of mafic rocks suggest the derivation of the pluto volcanic suite by crustal anatexis of basic meta igneous rocks mixed with a minor metasedimentary component in an intra continental setting the more siliceous compositions in the suite may have formed by subsequent fractionation involving hornblende plagioclase biotite titanite and apatite as indicated by the negative correlation between sio2 and mgo fe2o3 al2o3 cao tio2 and p2o5 low sr contents and negative eu and ti anomalies the 1 87 ga coeval felsic volcanic and granitic magmatic event documented in the ndembera group is also a feature of the nearby ubendian belt to the west pointing to a palaeoproterozoic regional thermal event that occurred in southwestern and western tanzania

erosion of deccan traps determined by river geochemistry impact on the global climate and the 87sr 86sr ratio of seawater abstract the impact of the deccan traps on chemical weathering and atmospheric co2 consumption on earth is evaluated based on the study of major elements strontium and 87sr 86sr isotopic ratios of the main rivers flowing through the traps using a numerical model which describes the coupled evolution of the chemical cycles of carbon alkalinity and strontium and allows one to compute the variations in atmospheric pco2 mean global temperature and the 87sr 86sr isotopic ratio of seawater in response to deccan trap emplacement the results suggest that the rate of chemical weathering of deccan traps 21 63 t km2 yr and associated atmospheric co2 consumption 0 58 2 54 106 mol c km2 yr are relatively high compared to those linked to other basaltic regions our results on the deccan and available data from other basaltic regions show that runoff and temperature are the two main parameters which control the rate of co2 consumption during weathering of basalts according to the relationship f r f c 0 exp ea r 1 t 1 298 where f is the specific co2 consumption rate mol km2 yr rf is runoff mm yr c0 is a constant 1764 mol l ea represents an apparent activation energy for basalt weathering with a value of 42 3 kj mol determined in the present study r is the gas constant and t is the absolute temperature k modelling results show that emplacement and weathering of deccan traps basalts played an important role in the geochemical cycles of carbon and strontium in particular the traps led to a change in weathering rate of both carbonates and silicates in carbonate deposition on seafloor in sr isotopic composition of the riverine flux and hence a change in marine sr isotopic composition as a result deccan traps emplacement was responsible for a strong increase of atmospheric pco2 by 1050 ppmv followed by a new steady state pco2 lower than that in pre deccan times by 57 ppmv implying that pre industrial atmospheric pco2 would have been 20 higher in the absence of deccan basalts pco2 evolution was accompanied by a rapid warming of 4 c followed after 1 myr by a global cooling of 0 55 c during the warming phase continental silicate weathering is increased globally since weathering of continental silicate rocks provides radiogenic sr to the ocean the model predicts a peak in the 87sr 86sr ratio of seawater following the deccan traps emplacement the amplitude and duration of this spike in the sr isotopic signal are comparable to those observed at the cretaceous tertiary boundary the results of this study demonstrate the important control exerted by the emplacement and weathering of large basaltic provinces on the geochemical and climatic changes on earth

pillow lava basalts with back arc morb affinity from the usagaran belt tanzania relics of orosirian ophiolites the paleoproterozoic usagaran orogenic belt in east africa is an exemplary site of preserved relics of paleoproterozoic volcanic arc magmas and subducted and displaced oceanic crust this paper describes the geochemical characteristics of displaced pillow basalts from the usagaran belt konse group our data indicate that the konse pillow basalts have tholeiitic composition and are overprinted by greenschist facies metamorphic conditions but their primary geochemical signatures are preserved by ree and fluid immobile elements the la sm n ratios 0 62 1 09 and ree patterns point to normal mid ocean ridge basalt n morb and transitional morb t morb mantle source the analysis of high valency elements and trace element patterns points to a mixed signature of morb tholeiites and island arc tholeiites with elevation of ba th u eu and sr this composition is similar to that of phanerozoic back arc suprasubduction zone ophiolites therefore usagaran belt pillow basalts are an analogue of the tethyan type suprasubduction zone ophiolite evolution and emplacement in the precambrian the emplacement of the konse pillow basalts is older than the neighbouring 1920 1870 ma volcanic arc magmas and is probably coeval with the formation of the 2000 ma yalumba eclogites with morb affinity thus the konse pillow basalts might have been displaced from their suprasubduction zone tectonic setting of igneous construction within the yalumba ocean basin around 2000 ma thematic collection this article is part of the tethyan ophiolites and tethyan seaways collection available at https www lyellcollection org cc tethyan ophiolites and tethyan seaways

the triangle shearzone zimbabwe revisited new data document an important event at 2 0 ga in the limpopo belt abstract the limpopo belt in southern africa has been used to demonstrate that modern style continent continent collision operated during the late archaean 2 6 2 7 ga we have studied the age and pt conditions of strike slip tectonism along the important right lateral triangle shearzone our results substantiate existing proterozoic metamorphic mineral age data of prior uncertain significance using the pb pb and sm nd garnet chronometers and the ar ar step heating technique for amphibole we have dated pre and syn tectonic metamorphic minerals at 2 2 and 2 0 ga thus the triangle shearzone can now be regarded as an important proterozoic suture examination of corresponding high grade pt conditions reaching 800 c at 9 kbar indicates a clockwise metamorphic evolution with pronounced isothermal uplift although the evidence that thrusting of the marginal zones of the limpopo belt over the adjoining cratons occurred during the late archaean clearly remains it is now very uncertain to which event the various pt paths obtained in the limpopo belt may be assigned therefore the question of whether the 2 6 2 7 ga tectonism fits on its own a modern style continental collision model remains open and has to be reassessed

lignin land plants and fungi biological evolution affecting phanerozoic oxygen balance as dominance shifted from lycopsids and pteridophytes in the paleozoic to gymnosperms in the mesozoic to angiosperms in the tertiary plant architecture became more sparing in its use of lignin lignin degrading organisms were rare or absent in the paleozoic but diverse and abundant in the tertiary thus the terrigenous organic carbon cycle has quickened over time the fraction of terrestrial primary production preserved in coals and kerogens has declined and terrestrial production has been able to increase over time without concomitant rises in atmospheric o sub 2

southern carpathian rock glaciers inventory distribution and environmental controlling factors abstract rock glaciers are valuable diagnostic landforms indicating permafrost creeping during their genesis and activity based on the analysis of high quality air orthophoto and field work a first polygon based inventory of rock glaciers from the southern carpathians has been elaborated in total 306 rock glaciers were included in the inventory comprising 79 debris and 227 talus rock glaciers most of these landforms were classified as relict 258 while only 48 covering 2 81 km 2 were considered intact the size of rock glaciers considered as a proxy for past environmental conditions and the relationships with the predictor variables lithology aspect contributing area geographic coordinates elevation and slope range were analysed using bivariate statistics analysis of variance anova and various post hoc tests the statistical analysis revealed that the rock glaciers occurring in the highest mountain ranges in areas composed of granites and granodiorites are considerably larger than the others because their duration of activity is greater strong dependences between rock glacier size and other topographic attributes contributing area aspect and slope range were also confirmed the rock glacier distribution in the southern carpathians is clearly controlled by topography lithology and debris availability the abundance of rock glaciers increases with altitude but their size decreases slightly in mountain units where granites and granodiorites predominate retezat and par ng mountains the density of rock glaciers and the mean specific area covered by these spectacular landforms are considerably higher than in other areas the higher continentality effects of the southern carpathians enabled the formation of rock glaciers at substantially lower elevations than in the alps the mean altitude of intact rock glaciers front which could be used as a morphological indicator of discontinuous permafrost is located at 2088 m

age assessment and implications of late quaternary periglacial and paraglacial landforms on muckish mountain northwest ireland based on schmidt hammer exposure age dating shd abstract schmidt hammer exposure age dating shd was applied to a variety of late quaternary periglacial and paraglacial landforms composed of coarse rock debris on muckish mountain northwest ireland landform ages were determined using a linear high precision age calibration curve derived from young and old control surfaces of known age on the same rock type the shd ages represent maximum estimates of the time elapsed since the boulders stabilised and the landforms became inactive most ages are also minimum estimates for the start of landform development because older boulders are buried beneath the sampled surface boulders ages and 95 confidence intervals obtained for blockfield boulder lobes and talus indicate these features were likely active during several of the early holocene cold events evidenced in greenland ice cores and north atlantic sediment records activity ceased at different times 9 7 ka bp these landforms are the first indication of a geomorphological response to early holocene cooling in the oceanic mountains of ireland late holocene ages obtained for rock slope failure run out debris and debris cone boulders overlap with shifts to cooler and or wetter conditions including the little ice age geomorphological impacts associated with these changes in climate have not previously been recorded in the irish uplands the shd results indicate that previously implied timings for the stabilisation of some accumulations of coarse rock debris on mountain slopes are in need of revision

roots rock and regolith biomechanical and biochemical weathering by trees and its impact on hillslopes a critical literature review the role of trees and forests as a critical component of the biosphere and critical zone and of the earth system more generally is widely appreciated less known and acknowledged are the geomorphological functions of tree roots although their importance has been widely referred to in soil studies paleopedology and paleobotany tree roots and their impact on weathering processes and soil production were incorporated in the devonian plant hypothesis and tree root casts served as a key evidence of recognition of past soils in geology sedimentology and paleopedology however knowledge of biomechanical and biochemical weathering induced by vascular plant roots mainly trees has been rarely utilized in geomorphic studies biogeomorphic and pedologic studies in recent decades have highlighted the importance of tree uprooting in which roots play a primary role in soil development regolith disturbance and bedrock mining other important functions of roots were also recognized e g soil displacement by growing roots infilling of stump holes and root cavities root groove development direct and indirect effects taking place in the rhizosphere and mycorrhizosphere mainly biochemical weathering of minerals support by microbial communities and symbiotic fungi and changes in porosity permeability and hydrology of soils in the root zone however further studies are urgently needed because many aspects of biochemical and biomechanical weathering are not well understood this is especially true with respect to taxa specific impacts variations in root architectures edaphic settings ecological relationships and geographic ranges result in substantially different biogeomorphic impacts of different tree species additionally the same species in different environmental settings may have different effects

dating post metamorphic cooling of the eastern granulites in the mozambique belt of northern tanzania using the garnet sm nd method abstract granulites from the usambara wami river and uluguru areas in the northern part of the mozambique belt in tanzania yield sm nd garnet whole rock ages of between 580 and 634 ma with a mean of 609 11 ma 2 this mean age is only slightly younger than the previously published peak metamorphic age of 641 2 ma suggesting that contrary to some earlier arguments garnet sm nd ages can be used to closely constrain the age of peak metamorphism even in slowly cooled terranes using published peak metamorphic temperatures of 810 c and cooling rates of 1 4 c ma the mean age translates into garnet closure temperatures of 690 to 780 c the similarity in garnet ages over widely separated areas coupled with the previously established similarity in equilibrium pt conditions indicate that isolated complexes that form the eastern granulites in the tanzanian sector of the mozambique belt share the same thermal history and were formed under the same geodynamic setting a few published garnet ages of between 525 and 545 ma indicate a younger less pervasive event of granulite facies metamorphism in the belt the bimodal distribution of garnet ages supports a previously published hypothesis that the assembly of gondwana took place in two stages the 610 ma old ages most likely date cooling from granulite facies metamorphism arising from regional crustal thickening associated with the amalgamation of india madagascar parts of eastern antarctica the kalahari craton the congo craton and the arabian nubian shield forming the imslek ans collage on the other hand the 525 545 ma ages may mark cooling from a thermal event associated with the collision of australo antarctica with the imslek ans collage

re os molybdenite pyrite and chalcopyrite geochronology lupa goldfield southwestern tanzania tracing metallogenic time scales at midcrustal shear zones hosting orogenic au deposits a detailed re os molybdenite pyrite and chalcopyrite geochronology at five shear hosted au occurrences kenge mbenge porcupine konokono and dubwana in the lupa goldfield southwestern tanzania is reported in this paper au occurrences within the lupa goldfield share many geologic similarities with the orogenic au deposit type and are situated within a paleoproterozoic magmatic arc that intruded the archean tanzanian cratonic margin pyrite chalcopyrite molybdenite bearing fault fill veins and mylonitic shear zones crosscut granitic host rocks and are associated with the highest au grades re os sulfide ages are deemed a suitable proxy to constrain the timing of au based on the occurrence of au bearing minerals as inclusions within pyrite and chalcopyrite whereas au bearing minerals filling pyrite fractures may represent a younger and undated metallogenic event molybdenite at kenge occurs as ultrafine disseminations within fault fill veins 1953 6 ma n 3 that possess nominally older weighted average re os ages than molybdenite hosted by stylolite like veins 1937 8 ma n 7 both sample sets are ca 70 m y older than a weighted average re os pyrite age from the mylonitic shear zones at kenge and mbenge 1876 10 ma n 13 which contain fault fill veins and record the timing of mylonitization molybdenite at porcupine occurs as ultrafine disseminations within quartz veins and mylonitized granite samples 1886 6 ma n 4 that are broadly equivalent in age to weighted average re os ages of molybdenite occurring as stylolite like veins 1873 5 ma n 6 and pyrite within oblique extension veins 1894 45 ma n 2 weighted average re os pyrite model ages at konokono 1880 14 ma n 9 and dubwana 1905 25 ma n 2 are also consistent with the ca 1 88 ga event observed at kenge mbenge and porcupine gold occurrences in the lupa goldfield therefore record a protracted hydrothermal history 1 95 1 87 ga comprising at least three temporally distinct hydrothermal events ca 1 95 1 94 and 1 88 ga which are each represented in detail by a complex vein history that occurred at a time scale less than the resolution of the re os method the sampling of broadly contemporaneous sulfides from five shear zones suggest that mylonitic shear zones represented an interconnected network of midcrustal permeable fluid conduits at ca 1 88 ga that permitted the transportation and deposition of gold comparison between re os sulfide and high precision u pb zircon ages for the granitic host rocks provides unequivocal evidence for sulfidation concomitant with magmatism however the range of re os ages argues against an intrusion related deposit model whereby metallogenic fluids are solely derived from an individual intrusion the regional ca 1 88 ga metallogenic event identified as part of this study occurred concurrently with eclogite facies metamorphism during the ubendian orogenic cycle and provides one of earth s earliest temporal links between subduction zone processes and orogenic au deposit formation during the paleoproterozoic

rapid late miocene rise of the bolivian altiplano evidence for removal of mantle lithosphere abstract the oxygen isotopic compositions of carbonates deposited in the northern altiplano demonstrate a rapid change in late miocene time which we attribute to an increase in elevation rainfall samples over an elevation transect adjacent to the altiplano of northern bolivia display a systematic decrease in mean annual 18 o values and define a local 18 o vs altitude gradient r gonfiantini m a roche j c olivry j c fontes g m zuppi the altitude effect on the isotopic composition of tropical rains chem geol 181 2001 147 167 which we apply to paleo meteoric water values determined from carbonates age constraints from magnetostragraphy and paleoelevation estimates suggest surface uplift of 2 5 to 3 5 km occurred between 10 3 ma and 6 8 0 4 ma when the altiplano rose to its current elevation only the removal of dense eclogitic lower crust and mantle lithosphere can generate a change of this magnitude and rapidity this change coincides with an 30 decrease in the rate of convergence between the south america and nazca plates and the propagation of deformation into the eastern subandean zone which we infer to have resulted from the increase in the force per unit length applied by the elevated andean plateau to the surrounding lowlands

cenozoic carbon cycle imbalances and a variable weathering feedback abstract the long term stability of earth s climate and the recovery of the ocean atmosphere system after carbon cycle perturbations are often attributed to a stabilizing negative feedback between silicate weathering and climate however evidence for the operation of this feedback over million year timescales and in response to tectonic and long term climatic change remains scarce for example the past 50 million years of the cenozoic era are characterized by long term cooling and declining atmospheric co2 p co 2 during this interval constant or decreasing carbon fluxes from the solid earth to the atmosphere suggest that stable or decreasing weathering fluxes are needed to balance the carbon cycle in contrast marine isotopic proxies of weathering i e 87sr 86sr 7 li and 187os 188os are interpreted to reflect increasing weathering fluxes here we evaluate the existence of a negative feedback by reconstructing the imbalance in the carbon cycle during the cenozoic using the surface inventories of carbon and alkalinity only a sustained 0 25 0 5 increase in silicate weathering is necessary to explain the long term decline in p co 2 over the cenozoic we propose that the long term decrease in p co 2 is due to an increase in the strength of the silicate weathering feedback i e the constant of proportionality between the silicate weathering flux and climate rather than an increase in the weathering flux this increase in the feedback strength which mirrors the marine isotope proxies occurs as transient 1 million year timescales remains invariant to match the long term inputs of carbon over the cenozoic this results in stable long term weathering fluxes even as p co 2 decreases we attribute increasing feedback strength to a change in the type and reactivity of rock in the weathering zone which collectively has increased the reactivity of the surface of the earth increasing feedback strength through the cenozoic reconciles mass balance in the carbon cycle with the marine isotopic proxies of weathering and may explain why there are periods when the earth system is more sensitive to carbon cycle perturbations

rock glaciers and related periglacial landforms in the sierra nevada ca usa inventory distribution and climatic relationships abstract rock glaciers and related periglacial rock ice features rifs are abundant yet overlooked landforms in the sierra nevada california where they occur in diverse forms we mapped 421 rifs from field surveys and grouped these into six classes based on morphology and location these categories comprise a greater range of frozen ground features than are commonly described in rock glacier surveys mapped features extended from 2225 to 3932 m modern mean 3333 m occurred mostly on nnw to nne aspects and ranged in apparent age from modern to relict late pleistocene many of the smaller features mapped here are not readily discernible with remote e g air photo observation field surveys remain the best approach for their detection we interpreted the presence of outlet springs basal lakes suspended silt in outlet streams and fringing phreatophytic vegetation in addition to morphologic indications of current rock movement as evidence for interstitial ice either persistent or seasonal the six classes were distinct in their geographic settings and morphologic conditions indicating process level differences to assess modern climate we intersected mapped locations with the 30 arcsec prism climate model discriminant analysis indicated significant differences among the climate means of the classes with the first three canonical vectors describing 94 of the differences among classes mean annual air temperatures maat for modern features ranged from 0 3 to 2 2 c mean precipitation ranged from 1346 to 1513 mm we calculated differences between modern and pleistocene temperatures in two ways one based on elevation differences of modern and relict rifs 662 m and standard lapse rate the other using prism estimates for the first we estimate the difference in maat as 3 9 c range 2 2 to 7 9 c from prism the difference was 3 3 c range 1 0 to 6 1 c in that persistent snowfields and glaciers are retreating in the sierra nevada under warming climates rifs will likely become increasingly important in prolonging water storage during the warm season and providing small but distributed water reserves for biodiversity and runoff their presence and water contributions would benefit by further hydrologic study

application of schmidt hammer relative age dating to late pleistocene moraines and rock glaciers in the western tatra mountains slovakia abstract in order to evaluate the potential of the schmidt hammer for relative age dating of late pleistocene moraines and rock glaciers rebound r values were collected at 38 sites in two formerly glaciated valleys in the western tatras in northern slovakia a large statistical population of measurements obtained from moraine and rock glacier surfaces was used to analyze the variability of mean r values in the same lithology and overall southern valley exposition four separate sets of glacial periglacial morphosystems are present at different positions in the western tatras starting from valley outlets to the highest cirques which represent successive phases of deglaciation r value means and 95 confidence intervals for selected last glacial maximum lgm and late glacial lg 1 lg 2 and lg 3 stages respectively 40 1 1 1 46 0 5 53 5 1 0 and 60 5 0 3 are significantly statistically different and values for each age category are comparable for the two analyzed valleys the results of weathering indexes used in conjunction with geomorphological relationships were taken together to reconstruct the pattern of deglaciation on southern tatra slopes it is stated that the schmidt hammer technique may be successfully used as a relative age dating tool for late pleistocene glacial and periglacial deposits and with this method it is possible to differentiate between late glacial moraines and rock glacier systems of different age

relative surface age dating of rock glacier systems near holar in hjaltadalur northern iceland schmidt hammer rebound values and photogrammetric measurements of surface displacement were used to date two active polymorphic high altitude and two relict monomorphic low altitude rock glaciers near holar in hjaltadalur trollaskagi peninsula northern iceland 65 40 n 19 00 w both low altitude rock glaciers are of late weichselian or preboreal origin ca 10 3 9 2 k yr bp with minor later reshaping until the early neoglacial period vatnsdalur i ca 5 2 k yr bp both high altitude rock glaciers consist of an upper lobe overriding a lower lobe corresponding to initiation during different cool phases the initiation of the lower lobes can possibly be attributed to the gh 8 2 event 8 6 8 0 k cal yr bp they reached their terminal position at the end of the vatnsdalur i stage the upper lobes of the high altitude rock glaciers were initiated sometimes between the vatnsdalur i but substantially before the vatnsdalur ii ca 3 0 3 2 k yr bp stages and underwent modification during each following holocene glacier expansion episode culminating the last time in the little ice age thus it is suggested that similar looking rock glaciers occurring elsewhere in the trollaskagi peninsula may have a similar complex history therefore they may not be seen as rather simple landforms developed during the little ice age copyright 2007 john wiley sons ltd

palaeoproterozoic volcanism and granitic magmatism in the ngualla area of the ubendian belt sw tanzania constraints from shrimp u pb zircon ages and sm nd isotope systematics abstract secondary ion microprobe u pb zircon data and sm nd isotopic compositions are presented for the volcanic and associated granitic rocks in the northern part of the lupa terrane in the ngualla area of the ubendian belt southwestern tanzania the u pb data reveals two distinct episodes of volcanism in the area a 1943 ma volcanism involving the eruption of basaltic andesites and a 1871 ma event during which porphyritic dacites and minor rhyolites erupted the data further shows that the last episode of volcanism was coeval with the emplacement of potassic granites at 1878 ma an older age of 2682 ma was obtained from zircons inherited from the pre existing archaean crust in a potassic granite the basaltic andesites exhibit nd 1943 ma values of 3 33 to 6 24 and t dm ages of 2521 2883 ma whereas the porphyry dacites show nd 1871 values of 5 72 to 5 94 and t dm ages are 2546 2573 ma these t dm ages are 600 700 ma older than the emplacement ages of both the basaltic andesites and porphyry dacites the potassic granites exhibit the most negative nd 1878 values of all the rocks 6 00 to 11 2 in the area and their t dm ages are 2568 3283 ma which are 700 1400 ma older than their emplacement age the nd t values for both the ngualla volcanic and granitic rocks are much lower than the corresponding mantle value of 6 13 at 1871 ma the emplacement of 1 87 ga coeval felsic plutonic and volcanic rocks in the ngualla area of the ubendian belt is also a feature of the nearby usagaran belt and this corroborates previous evidence that plutonism and volcanism in both the usagaran and ubendian belts took place at the same time the presence of 1 87 ga granites in both the usagaran and the ubendian belt implies that granitic magmatism of this age was a regional thermal event that covered a significant portion of the proterozoic terranes of southeastern and southwestern tanzania inherited archaean zircon crystals coupled with negative nd and nd model ages 600 1400 ma older than their respective emplacement ages all suggest palaeoproterozoic crust is derived in part by re worked archaean crust at the southwest corner of the tanzanian craton

geochemical mapping in aichi prefecture japan its significance as a useful dataset for geological mapping abstract since 1994 a geochemical mapping project has been undertaken in aichi prefecture central japan with the aim of establishing a database to contribute to environmental assessment in the region to date more than 1200 stream sediments have been collected and analyzed the samples were analyzed for 10 major elements by x ray fluorescence spectrometry and loss on ignition the analyses show that stream sediments from areas with sedimentary bedrock are enriched in sio 2 and mafic elements such as tio 2 and fe 2 o 3 compared to areas with granitic bedrock the sio 2 enrichment is mainly due to the amount of quartz and the enrichment in mafic elements is due to biotite both of which are derived from the sedimentary bedrock in contrast sediments derived from granitic bedrock are characterized by high abundances of elements contained in feldspars such as na 2 o and cao this contrast in elemental distribution e g na 2 o tio 2 ratio can be useful for geological mapping in the areas covered by vegetation and with poor bedrock exposure the relative abundances of ca na and k in the stream sediments suggest that the inagawa granite one of the main granite bodies in the study area can be classified into four distinct types this classification is distinct from previous classifications based on geological and petrochemical studies it is confirmed that analysis of geochemical compositions of stream sediments has considerable potential as a tool in geological studies including lithological mapping

rock glaciers on the faeroe islands the north atlantic three sites with alleged relict rock glaciers are described from southern faeroe islands in the north atlantic ocean the rock glaciers represent both talus derived and glacier derived types and were most likely initiated during the late weichselian one of the rock glaciers apparently became unstable at some point during degradation of permafrost and was subsequently transformed by a landslide the age of the rock glaciers are not known precisely but a younger dryas age is suggested partly by considering contemporary local equilibrium line altitudes the climatic background for rock glacier initiation on the faeroe islands during the younger dryas is investigated using various types of palaeoclimatic information the potential of using relict rock glaciers in palaeoclimatic reconstructions is discussed and their implications for estimates on air temperature precipitation permafrost rock weathering and the contemporary extent of the weichselian faeroe ice cap is outlined 1998 john wiley sons ltd

nd and sr isotopic mapping of the archaean proterozoic boundary in southeastern tanzania using granites as probes for crustal growth abstract granitoids of two different generations occur across the archaean proterozoic boundary in southeastern tanzania the first generation which is confined to the archaean tanzania craton yields rb sr whole rock isochron ages of 2600 ma and low mantle like initial 87sr 86sr ratios 0 702 0 704 these coupled with positive or near zero e nd values 0 2 to 2 0 suggest that these granitoids represent juvenile mantle material which was added to the continental crust at about 2600 ma the second generation of granitoids intrudes the palaeoproterozoic usagaran belt and is characterised by rb sr whole rock isochron ages of about 1900 ma these granitoids show sm nd crustal formation ages which are 200 to 600 ma older than their emplacement ages and negative e nd values 2 2 to 6 2 mixing calculations suggest that the proterozoic granitoids may have formed by partial melting of 2000 ma usagaran mantle derived material which had incorporated between 15 and 45 of an archaean component from the adjacent tanzania craton

heterogeneous sources of the triassic granitoid plutons in the southern qinling orogen an e w tectonic division in central china 1 the qinling orogen is an important orogenic belt formed by the collision between the north and south china blocks along the mianlue suture during the triassic the orogen is customarily divided into the western and eastern qinling terranes however the boundary has long been a matter of debate there are many triassic granitoid plutons in the orogen especially in south qinling along the southern part of the shangdan suture systematic analysis of u pb ages and hf isotopes in zircons frxom six granitoid plutons including guangtoushan gaoqiaopu xiba laocheng dongjiangkou and zhashui from west to east allows us to trace their formation ages and source regions all plutons yield ages that vary only from 218 to 211 ma however zircon hf isotope data for these triassic plutons cluster in two distinct groups granitoids from the western segment i e guangtoushan gaoqiaopu and xiba of the south qinling belt have negative ehf t values 20 9 to 5 2 and relatively old two stage hf model ages 1 58 to 2 57 ga in contrast those from the eastern segment i e laocheng dongjiangkou and zhashui show higher ehf t values 5 4 to 6 8 and younger two stage hf model ages 0 82 to 1 60 ga integrating these analyses with published sr nd pb hf isotopic data we suggest that the division between the western and eastern south qinling segments is located near the taibai chenggu line between the baoji chengdu railway and 108 e longitude the integrated data suggest that the western south qinling segment separated from the north china block during the paleoproterozoic and early mesoproterozoic and then switched into continental convergence with the eastern south qinling segment northern margin of the yangtze block during the late mesoproterozoic to early neoproterozoic finally the two segments amalgamated during the late neoproterozoic

rock glacier dynamics and paleoclimatic implications many rock glaciers contain massive ice that may be useful in paleoclimate studies interpreting geochemical ice core records from rock glaciers requires a thorough understanding of rock glacier structure and dynamics high precision surface velocity data were obtained for the galena creek rock glacier absaroka mountains wyoming surface velocities range from 0 to 1 00 m yr and vary across the rock glacier in a manner similar to true glaciers we used glen s flow law to calculate the thickness of the deforming ice layer the modeled ice thickness ranges from 0 to 50 m and is confirmed by direct observations this agreement shows that rock glacier movement can be entirely explained by deformation of massive ice within the rock glacier neither basal sliding nor deformation of basal debris is necessary recovered ice cores to depths of 25 m contain thin debris layers associated with summer ablation in the accumulation zone the ages of four samples of organic material removed from several debris layers inthe southern half of the rock glacier range from 200 40 to 2250 35 14c yr b p demonstrating that the rock glacier formed well before the little ice age and may contain ice dating to the middle holocene or earlier

enhanced boulder weathering under late lying snowpatches a schmidt test hammer was employed to assess the aggregate surface hardness of samples of boulders dug out from under late lying snowpatches at sites in switzerland scotland and norway the results were compared with an equivalent set of readings made on boulders at nearby snow free control sites the results in every case reveal that the aggregate surface hardness of boulders buried by late lying snow is significantly less at the 0 001 level than that of boulders at the corresponding control sites this result indicates that late lying snow significantly enhances rock weathering probably because boulder and rock surfaces are exposed to prolonged wetting by percolating meltwater under late lying snowbeds

isotopically heterogeneous initial pb and continuous 222 rn loss in fossils the u pb systematics of brachiosaurus brancai abstract diagenetic incorporation of uranium into bone material establishes a u pb system that provides a minimum age for the fossil loss of rn and ra from the 238u decay series recent mobility of u and pb and incorporation of radiogenic pb from an old basement which was available during diagenesis disturbed the u pb system all data fall in a 206pb 238u 207pb 235u concordia diagram to the right of the concordia while samples that do not experience redistribution of u pb 222rn and 226ra should define a linear array that intersects the concordia curve at the age of diagenesis and at the age of the source of inherited radiogenic pb loss of 222rn and 226ra has led to displacement of the present data set to lower 206pb 238u and therefore to younger intercept ages and to older 207pb 206pb ages the 207pb 204pb 208pb 204pb systematics demonstrates that 1 three out of 19 samples experienced significant postdiagenetic fractionation of u and pb and that 2 the diagenetic pb addition was isotopically heterogeneous the 235u 207pb 206pb 207pb and 206pb 238u 207pb 235u systematics combined demonstrate that 3 the samples experienced 5 to 18 222rn loss for a 1 9 ga old source of the diagenetically introduced pb and correspondingly more for a younger pb source and that 4 there is no unique solution for the age of the source of diagenetic pb since pb from a younger source appears in the 235u 207pb 206pb 207pb diagram in the same way as higher 222rn loss no precise age can be determined from the data although the most consistent solution falls in the range of 140 to 150 ma addition of radiogenic pb to the fossil bone is obvious only through its anomalous highly radiogenic character and the isotopic heterogeneity in 208pb 204pb no th i e no in situ 208pb growth the effect of unrecognized diagenetic additions of radiogenic pb is an overestimation of the age whereas unrecognized 222rn loss results in a too young discordant age since fossil material formed at times for which the discordia only shows a small curvature anomalies originating from diagenetic pb additions and 222rn loss may not always be recognized

solifluction rates processes and landforms a global review abstract field data on the rates of solifluction and associated parameters are compiled from the literature in an attempt to evaluate factors controlling the spatial variability in solifluction processes and landforms with special attention on the climate solifluction relationship the analyzed data originate from 46 sites over a wide range of periglacial environments from antarctic nunataks to tropical high mountains solifluction broadly defined as slow mass wasting resulting from freeze thaw action in fine textured soils involves several components needle ice creep and diurnal frost creep originating from diurnal freeze thaw action annual frost creep gelifluction and plug like flow originating from annual freeze thaw action and retrograde movement caused by soil cohesion the depth and thickness of ice lenses and freeze thaw frequency are the major controls on the spatial variation in solifluction processes near the warm margin of the solifluction affected environment diurnal freeze thaw action induces shallow but relatively rapid movement of a superficial layer 5 10 cm thick on average often creating the thin stone banked lobes typically seen on tropical high mountains in addition to diurnal movement annual frost creep and gelifluction may occur on slopes with soil climates of seasonal frost to warm permafrost dislocating a soil layer shallower than 60 cm at a rate of centimeters per year and eventually producing medium size solifluction lobes in high arctic cold permafrost regions two sided freezing can induce plug like flow of a soil mass 60 cm or thicker the correlation between process and landform suggests that the riser height of lobes is indicative of the maximum depth of movement and prevailing freeze thaw type climate change may result in new different ground freezing conditions thereby influencing the surface velocity and maximum depth of soil movement soil moisture and topography also control solifluction high moisture availability in the seasonal freezing period enhances diurnal freeze thaw action and subsequent seasonal frost heaving the latter contributes to raising the moisture content of the thawed layer and promotes gelifluction during the thawing period the slope angle defines the upper limit of the surface velocity of solifluction a diagram correlating the potential frost creep with the actual surface velocity permits an inter site comparison of the relative magnitude of solifluction components physically based modelling of periglacial slope evolution requires synthetic and more detailed field monitoring and laboratory simulations of solifluction processes

montane forest root growth and soil organic layer depth as potential factors stabilizing cenozoic global change tree roots and their symbiotic fungal partners are believed to play a major role in regulating long term global climate but feedbacks between global temperature and biotic weathering have not yet been explored in detail in situ field data from a 3000 m altitudinal transect in peru show fine root growth decreases and organic layer depth increases with the cooler temperatures that prevail at increased altitude we hypothesize that this observation suggests a negative feedback as global temperatures rise the soil organic layer will shrink and more roots will grow in the mineral layer thereby accelerating weathering and reducing atmospheric co2 we examine this mechanism with a process based biological weathering model and demonstrate that this negative feedback could have contributed to moderating long term global cenozoic climate during major cenozoic co2 changes linked to volcanic degassing and tectonic uplift events

nd isotopic mapping of the archaean proterozoic boundary in southwestern tanzania implication for the size of the archaean tanzania craton abstract nd isotopic data are presented for the granitoids that straddle the archaean proterozoic a p boundary in southwestern tanzania along the itigi makongolosi road traverse on the basis of their nd depleted mantle tdm ages two groups of granitoids can be identified those which show tdm ages of 2541 2894 ma and are therefore belonging to the archaean and those which have proterozoic tdm ages of 2031 2430 ma archaean granitoids are deformed grayish in colour and consist of plagioclase biotite and quartz and include a few microcline rich rare biotite pink alkali granites whereas the proterozoic ones include mainly undeformed microcline rich pink alkali granites although there are several compositional overlaps between the two proterozoic granitoids differ from those of the archaean in showing enrichment in incompatible elements and overall higher abundances of the rare earth elements both archaean and proterozoic granitoids show negative to slightly positive eu anomalies these geochemical characteristics are attributed to the involvement of plagioclase in their magmagenesis indicating their generation at low pressures and shallow depths with the proterozoic granitoids being derived from a more felsic protolith nd isotopic data coupled with petrography and lithological field relationships as well as major and trace elements geochemistry of the granitoids places the a p boundary approximately 150 km inside from the southern traditionally accepted boundary near lake rukwa in southwestern tanzania this implies that the size of the archaean tanzania craton is smaller than hitherto understood small vestiges domains showing archaean ages within the proterozoic regions could be explained as being slivers of tectonically interleaved archaean material found within the proterozoic terrane in southwestern tanzania

palaeo altimetry of the late eocene to miocene lunpola basin central tibet the elevation history of the tibetan plateau provides direct insight into the tectonic processes associated with continent continent collisions here we present oxygen isotope based estimates of the palaeo altimetry of late eocene and younger deposits of the lunpola basin in the centre of the plateau which indicate that the surface of tibet has been at an elevation of more than 4 kilometres for at least the past 35 million years we conclude that crustal but not mantle thickening models combined with plate kinematic solutions of india asia convergence are compatible with palaeo elevation estimates across the tibetan plateau

quantitative pollen based climate reconstruction in central japan application to surface and late quaternary spectra abstract climate reconstruction in japan by the best modern analogues method was tested with 285 surface pollen spectra and then applied to the 47 000 years pollen profile from lake mikata central japan both pollen taxa percentages and plant functional type affinity scores were used in the reconstruction of six climate variables the reconstructed values were compared with the estimated modern climate at each surface pollen site in order to evaluate the accuracy of the pollen based reconstruction the reconstruction of thermal parameters is very reliable correlation coefficient r between 0 86 and 0 89 the reconstruction of annual precipitation is less accurate r 0 7 we also proposed a new technique to calibrate reconstructed values into both probability distribution and or confidence regions of observed values the specialized programs for this calibration polcalib and polhist are available at qsr website http www elsevier nl locate quascirev the reconstruction of the late quaternary climate changes around lake mikata shows that the summer and winter temperatures were 5 c lower than present around 40 uncal kyr bp reached a minimum around 20 uncal kyr bp 10 c lower than present and became similar to present day at 8 uncal kyr bp the estimates for annual precipitation show that it was driest around 20 uncal kyr bp

schmidt hammer exposure age dating shd of rock glaciers in the schoderkogel eisenhut area schladminger tauern range austria schmidt hammer rebound values r values enable relative age dating of landforms with r values relating to degree of weathering and therefore length of exposure this method recently termed as schmidt hammer exposure age dating shd was applied to date five rock glaciers size range 0 01 0 12 km2 and one recent rockfall deposit at the study area schoderkogel eisenhut in the schladminger tauern range 14 03 e 47 15 n austria the rock glaciers consist of gneiss or high metamorphic series of mica schist that are comparable in their r values four of them are relict permafrost absent and one is intact containing patches of permafrost on each of the five rock glaciers shd was carried out at 4 6 sites 50 measurements per site along a longitudinal transect from the frontal ridge to the root zone results at all five rock glaciers are generally consistent with each other sharing statistically significant r values along transects the range between the highest and the lowest mean r value at ea

convergent cenozoic co2 history reconstructions of atmospheric carbon dioxide concentrations over the past 65 million years are heading towards consensus it is time for systematic testing of the proxies against measurements and against each other

structural controls on orogenic au mineralization during transpression lupa goldfield southwestern tanzania au mineralization in the western lupa goldfield southwestern tanzania was associated with transpression and reverse sinistral slip along a network of steeply s dipping shear zones with non andersonian geometries slip was accommodated by 1 frictional failure and sliding during emplacement of quartz au bearing veins and 2 crystal plasticity and fluid assisted diffusive mass transfer the kenge mineral system is situated along a nw se trending shear zone and is characterized by 10 m thick au bearing fault fill veins hosted by well developed phyllosilicate rich mylonites the broadly contemporaneous porcupine mineral system is situated along an ene wsw to e w trending shear zone which is characterized by narrow discontinuous mylonitic shear zone within a silicified and nonfoliated granitoid protolith au mineralization at porcupine occurs within steeply dipping fault fill and subhorizontal extension oblique extension veins three dimensional frictional reactivation theory provides a self consistent explanation for the different vein styles at kenge and porcupine and extends the classic fault valve model to the general case of oblique slip along multiple arbitrarily oriented shear zones analysis of the differential stress required for frictional reactivation suggests the following 1 the kenge shear zone was intrinsically weaker than the porcupine shear zone consistent with the lack of well developed mylonites at porcupine and 2 frictional reactivation of the kenge shear zone occurred under suprahydrostatic but sublithostatic pore fluid pressures whereas frictional reactivation of the porcupine shear zone occurred under near lithostatic fluid pressures we hypothesize that near lithostatic pore fluid pressures relieved effective normal stresses at grain grain contacts helping to preserve intragranular and fracture porosity at the porcupine orebody as such these pore spaces may be important microstructural sites for au mineralization low effective normal stresses can also explain the poorly developed phyllosilicate rich mylonites and limited degree of shear zone weakening at porcupine

dating basaltic volcanism in the neoarchaean sukumaland greenstone belt of the tanzania craton using the sm nd method implications for the geological evolution of the tanzania craton abstract metabasalts from the sukumaland greenstone belt of north western tanzania yield a whole rock sm nd isochron age of 2823 44 ma initial e nd 2 7 mswd 1 24 this age which is interpreted as dating the eruption of the oldest mafic volcanics in the belt is at the 95 confidence level indistinguishable from 2780 3 and 2808 3 ma single zircon u pb ages previously reported from stratigraphically higher rhyolitic pyroclastic rocks from the southern margin of the belt the age equivalency suggests that the entire 5 7 km thick greenstone sequence that has been traditionally classified into a predominantly mafic lower part overlain by an upper part in which felsic volcanics and bif predominate was emplaced within a relatively short time interval not exceeding 44 ma the sm nd age of the metabasalts is significantly older than a published zircon u pb age of 2680 3 ma obtained from a migmatitic gneiss on the southernmost fringe of the sukumaland greenstone belt this corroborates previous evidence that high grade metamorphism in the tanzania craton postdates emplacement of the greenstones and is most likely associated with the regional emplacement of the large granitic bodies that intrude and flank the greenstones

rock glacier development in the northern calcareous alps at the pleistocene holocene boundary abstract relict rock glaciers provide information on past discontinuous permafrost and former mean annual air temperatures a lack of records showing former permafrost distribution along the northern alpine fringe prompted the investigation and numerical dating of a belt of relict rock glaciers in the karwendel mountains of the northern calcareous austrian alps in two neighbouring cirques that were still glaciated during the early younger dryas eleven 36cl exposure ages from boulder surfaces were obtained the ages imply the onset of rock glacier activity around 12 3 ka with subsequent stabilization and permafrost melt out no later than 10 1 ka hence rock glacier formation coincided with glacier retreat in the cirques around the mid younger dryas and continued into the early holocene as permafrost induced features the rock glacier termini indicate the local past lower limit of discontinuous permafrost in open cirque floors at 2000 m asl which is around 400 m lower than during the mid twentieth century at comparable locations in the karwendel mountains thus a mean annual air temperature reduction of 2 6 to 3 8 c relative to the mid twentieth century is inferred based on a minimum glacier equilibrium line altitude in the cirques a summer temperature reduction of less than 2 6 to 1 8 c is shown suggesting an increased seasonality at the time of rock glacier activity

rock glaciers in the northern japanese alps palaeoenvironmental implications since the late glacial in order to determine the palaeoclimatic and palaeo permafrost conditions in the northern japanese alps in central japan the ages of rock glaciers were investigated by relative age dating techniques such as weathering rind thickness and schmidt hammer measurements the results of the relative age dating suggest that the formation of the investigated rock glaciers may have started during the early phase of the late glacial or around the onset of the holocene the lower limit of current discontinuous permafrost in the northern japanese alps which is indicated by the terminus of the lowest active inactive rock glacier lies at 2530 m a s l while that of discontinuous permafrost during the late glacial or early phase of the holocene which is indicated by the terminus of the lowest relict rock glacier lies at 2220 m a s l therefore the lower limit of discontinuous permafrost during these periods would have been at least about 300 m lower than that of the current discontinuous permafrost climatic and geomorphological conditions during the late glacial led to a change in the environment from a glacial environment to a periglacial permafrost environment in the current alpine zone of the northern japanese alps a large number of cirques were deglaciated and several of them were occupied by active rock glaciers around the onset of the holocene copyright 2005 john wiley sons ltd

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