evidence of isotopic equilibration between microgranitoid enclaves and host granodiorite warburton granodiorite lachlan fold belt australia abstract microgranitoid enclaves in the s type warburton granodiorite lachlan fold belt australia display a range in mineralogical and isotopic compositions many show obvious similarities to the host granodiorite they contain biotite as the sole ferromagnesian phase and have isotopic signatures similar to the host other enclaves contain cummingtonite actinolite and rare orthopyroxene mainly in the core of the enclaves these enclaves typically have higher nd i and lower sol 87 sr 86 sr i than the host rocks reversely zoned plagioclase megacrysts in some enclaves contain anhedral cores which are similar in composition to an poor plagioclase crystals in the host rock an45 these megacrysts are jacketed by an rich plagioclase an70 an80 the enclaves are interpreted to be globules of a more mafic magma which mingled with the host while it was still partially liquid megacrysts within the enclaves are thought to have been derived from the granodioritic magma the mafic enclave magma is likely to have been derived from a mantle source but equilibration between enclaves and host rock has changed the mineralogy and isotope geochemistry of all enclaves the equilibration process was incomplete for some enclaves resulting in mineralogical zoning and distinctions in isotopic signature between these enclaves and the host rock this equilibration process is likely to have been arrested as a result of rapid cooling of the pluton due to its small size and shallow emplacement level it is uncertain whether the compositions of the least equilibrated enclaves are truly representative of the original mafic enclave magma or whether their trace element signature has been affected by diffusional processes within a stratified magma chamber

structural geology and regional tectonic significance of the ramgarh thrust himalayan fold thrust belt of nepal 1 the ramgarh thrust is one of the major fault systems of the himalayan thrust belt in nepal and northern india the ramgarh thrust sheet is 0 2 2 0 km thick and can be traced along strike the entire length of the himalaya in nepal the fault generally places the oldest paleoproterozoic rocks in the lesser himalayan series upon younger lesser himalayan rocks or lower miocene foreland basin deposits regional balanced cross sections suggest that the ramgarh thrust had at least 120 km of initial south vergent displacement subsequently the frontal part of the thrust experienced further slip as the roof thrust for a large duplex in underlying lesser himalayan rocks ramgarh hanging wall strata are greenschist grade phyllite quartzite and augen gneiss all of which locally exhibit phyllonitic and mylonitic fabrics that indicate a top to the south sense of shear structural fabrics in the ramgarh thrust sheet are generally parallel to the fabrics in rocks above and below the thrust sheet regional and local mapping of the ramgarh thrust in nepal demonstrates that the fault always places a hanging wall flat upon a footwall flat except where local lateral ramps complicate its geometry similarly the structurally overlying main central thrust always places a hanging wall flat in greater himalayan series rocks upon the regionally flat ramgarh thrust sheet these geometric relationships preclude kinematic and thermal models that elevate greater himalayan and lower lesser himalayan rocks along high angle thrust ramps in the vicinity of the present traces of the ramgarh and main central thrust faults instead the corresponding footwall ramps for these thrusts must be located more than 100 km north of the current trace of the main central thrust the present steep dips of the ramgarh and main central thrust sheets can be attributed to tilting during emplacement of structurally lower thrust sheets within a large antiformal duplex that occupies most of the lesser himalayan zone the ramgarh thrust sheet overlaps a bed length of at least 100 km in lower miocene foreland basin deposits indicating that a significant amount of displacement on the thrust must have occurred after 15 ma growth of the lesser himalayan duplex and additional slip on the frontal part of the ramgarh thrust occurred from 12 to 5 ma the presence of a major greenschist grade metasedimentary thrust sheet composed of lesser himalayan rocks directly below the main central thrust suggests that the famous inverted metamorphism in this region is a result of structural inversion similarly the concept of a broad zone of intense shear strain related exclusively to emplacement of the main central thrust sheet is probably invalid in nepal

three s type volcanic suites from the lachlan fold belt southeast australia the concept that granitoids of the lachlan fold belt in southeast australia are derived from either igneous or sedimentary source rocks i or s type can be extended to volcanic rocks of the same age three suites of late silurian s type volcanics are described two of which can be matched quite closely with plutonic equivalents a large part of the paleozoic continental margin volcanic activity in southeast australia consisted of the magmatic recycling of old metasedimentary crust probably of late proterozoic age the three volcanic suites are moderately to strongly peraluminous with the corresponding presence of al rich minerals variation within the volcanic suites is ascribed chiefly to progressive removal of restite or source material residual from partial melting the most mafic suite the hawkins suite contains plagioclase cordierite orthopyroxene biotite and quartz as restite components with less abundant almandine these rocks are chemically equivalent to mafic biotite rich and cordierite bearing granitoids from parts of the berridale and murrumbidgee batholiths garnet is absent from the goobarragandra suite volcanics which are a little more felsic and close in compositon to granitoids of the young and maragle batholiths the s type character of the laidlaw suite is less pronounced although a sedimentary source seems to be established such a source would be less mature than those for the other two volcanic suites the laidlaw suite resembles but cannot be closely identified with felsic s type granitoids of the murrumbidgee and berridale batholiths low grade regional metamorphism in most of the volcanic rocks has resulted in much mineralogical alteration and mobility of alkali and alkaline earth elements despite this alteration unaltered rocks are present in all three suites compositional data for the phenocryst phases within the unaltered rocks has allowed estimation of some intensive parameters hawkins suite volcanics were extruded directly from their source region and preserve phenocryst equilibria established at 5 6 kbar at 800 c the laidlaw and goobarragandra suites reequilibrated at lower pressures than their source before extrusion the laidlaw suite reequilibrated at estimated temperatures of 725 730 c o2 of 10 15 5 to 10 14 bars and h2o of 2 to 2 5 kbar differing mineral compositions in the three suites are related to differing source rock compositons and oxygen fugacities relative biotite and orthopyroxene mg is possibly pressure dependent

the effects of rock uplift and rock resistance on river morphology in a subduction zone forearc oregon usa relationships between riverbed morphology concavity rock type and rock uplift rate are examined to independently unravel the contribution of along strike variations in lithology and rates of vertical deformation to the topographic relief of the oregon coastal mountains lithologic control on river profile form is reflected by convexities and knickpoints in a number of longitudinal profiles and by general trends of concavity as a function of lithology volcanic and sedimentary rocks are the principal rock types underlying the northern oregon coast ranges between 46 30 and 45 n where mixed bedrock alluvial channels dominate average concavity is 0 57 in this region in the alluviated central oregon coast ranges between 45 and 44 n values of concavity are on average the highest 0 82 south of 44 n however bedrock channels are common and 0 73 mixed bedrock alluvial channels characterize rivers in the klamath mountains from 43 n south 0 64 rock uplift rates of 0 5 mm a 1 mixed bedrock alluvial channels and concavities of 0 53 0 70 occur within the northernmost coast ranges and klamath mountains for rivers flowing over volcanic rocks 0 53 and 0 72 for reaches crossing sedimentary rocks whereas channel type and concavity generally co vary with lithology along much of the range rivers between 44 5 and 43 n do not follow these trends concavities are generally greater than 0 70 alluvial channels are common and river profiles lack knickpoints between 44 5 and 44 n despite the fact that lithology is arguably invariant moreover rock uplift rates in this region vary from low 0 5 mm a 1 to subsidence 0 mm a 1 these observations are consistent with models of transient river response to a decrease in uplift rate conversely the rivers between 44 and 43 n have similar concavities and flow on the same mapped bedrock unit as the central region but have bedrock channels and irregular longitudinal profiles suggesting the river profiles reflect a transient response to an increase in uplift rate if changes in rock uplift rate explain the differences in river profile form and morphology it is unlikely that rock uplift and erosion are in steady state in the oregon coastal mountains copyright 2006 john wiley sons ltd

glaciers and mass wasting processes their influence on the shaping of the kali gandaki valley higher himalaya of nepal abstract this paper addresses the question of the respective role of glaciers and mass wasting processes in the shaping of the high valleys of the central himalaya this is a region in which the himalayan range is at its narrowest and where it is cut by the deepest gorge in the world the kali gandaki valley following a brief description of the region and of the criteria used to discriminate between both sets of processes some specific sites that have given rise to divergent interpretations are discussed it is shown that undoubted evidence of past glaciation is rare and that most of the diamictons observed today are best interpreted as the products of deep seated landslides and or catastrophic debris flows of large magnitude and probably low frequency spanning the period from the last glacial maximum to the holocene the modalities of this evolution cast some doubt on some former reconstructions of past glacial extents moreover the continuous reworking of this material continues to influence the nature and current rates of erosion

petrogenesis of calc alkaline shoshonitic and associated ultrapotassic oligocene volcanic rocks from the northwestern alps italy along the western alps there is geological evidence of late alpine oligocene magmatic activity which clearly postdates the lepontine eocene early oligocene metamorphism and related deformation of the alpine nappe pile this magmatic activity was notably delayed in relation to the most important convergent processes and may be related to buoyancy of lithosphere tensional tectonics and thermal updoming subsequent to the collision between the eurasian and african plates the geochemical features of the rocks and the geophysical characteristics of the alpine chain suggest that a shoshonitic and calcalkaline melts may have been generated by partial melting of metasomatized peridotitic material and subsequent fractional crystallization and crustal contamination silicic andesites and latites however could have been also derived from metasomatized eclogite or deep continental crust material b the ultrapotassic lamprophyres with high k p lree th zr u and high 87sr 86sr ratios were generated by partial melting of strongly metasomatized mantle the varied sr isotopic ratios may partially also reflect additional radiogenic component from the continental crust following magma segregation from the source

volcano stratigraphy and 40ar 39ar geochronology of the macusani ignimbrite field monitor of the miocene geodynamic evolution of the andes of southeast peru abstract a total of 1400 m of ignimbritic ash flow tuffs distributed in five cross sections and extending from the pre neogene base to the uppermost flows were studied in the miocene macusani ignimbrite basin southeast peru volcano stratigraphic interpolation and precise 40ar 39ar dating of sanidine biotite and rhyolitic glass from the ash flow tuffs give rise to a southwest northeast correlation chart subdivided into six volcanic cycles or fundamental cooling units i e 1 10 0 0 5 2 7 8 8 0 0 1 3 7 5 0 1 4 7 3 0 1 5 6 8 7 0 o 1 and 6 6 7 o 1 ma these delimit two brief e 10 1 and 7 1 ma which are sensibly synchronous with the quechua 2 and quechua 3 compressional events characterizing the tectonic regime in the central andes this close relationship between tectonic pulses and felsic magmatic activity adds a supplementary constraint to models of the miocene tectono magmatic evolution of the cordillera oriental the stratigraphic relationships of the uranium mineralization of the macusani field are defined for the first time the stratiform stratabound occurrences are restricted to three main cooling units dated at 7 8 7 5 and 6 9 6 8 ma which constitute a maximum age for uranium deposition

petrology of the coyaguayma ignimbrite northern puna of argentina origin and evolution of a peraluminous high sio2 rhyolite magma abstract the coyaguayma ignimbrite is a strongly peraluminous sp sillimanite garnet bearing silicic rhyolite which erupted in the northern puna segment of the central andean plateau in the upper miocene 11 ma a period that was characterized by the eruption of voluminous 100s to 1000s km3 dacitic ignimbrites of high k calc alkaline affinity in this region the sp magmatic rocks are both rare and small but their importance is potentially much greater as rocks of this type are usually interpreted as products of crustal melting and therefore useful for addressing mantle addition vs crustal recycling in the central andes the phenocryst assemblage of the coyaguayma ignimbrite comprises plagioclase an26 18 quartz ba rich and ba poor sanidine minor al rich ferromagnesian minerals alt and alvi rich biotite almandine spessartine garnet and sillimanite as well as accessory zircon and monazite textural relations suggest that the accessory and ferromagnesian phases crystallized before quartz and feldspars mineral equilibria suggest that crystallization of the rhyolite magma began at 5 kbar and 800 c and continued almost isobarically to 720 c causing the residual liquid to increase h2o contents from 4 5 to 7 5 before eruption most major features e g high sio2 a cnk 1 3 low cao mgo tio2 and feo and trace element patterns low ba sr th lree and eu eu high rb u y and nb along with the thermobarometric constraints on magmatic p t and water contents are consistent with an origin by mica dehydration melting of metapelitic sources e g typical biotite muscovite gneisses from the outcropping s puna basement however the relatively low initial 87sr 86sr 0 7125 and high 143nd 144nd 0 512200 ratios invalidate a pure crustal origin instead we propose contamination of calc alkaline dacitic magmas similar to typical puna ignimbrites by metapelite at mid crustal settings 18 km depth geochemical modeling that satisfies the chemical and isotopic data suggests mixing of 70 dacite and 30 of metapelite partial melts followed by extensive 70 fractionation of plagioclase k feldspar quartz and biotite with minor magnetite and apatite from the hybrid magma sillimanite in the coyaguayma rhyolite is interpreted as a restite mineral or a product of incongruent melting of the metapelite which was preserved intact in the hybrid melt due to local equilibrium this petrogenetic model explains most characteristics of crystal poor sp rhyolites from the puna plateau e g tocomar ramadas rhyolites and it may be more generally applicable to occurrences of sp magmas in andean type continental arcs dominated by calc alkaline magmatism

origin and evolution of a peraluminous silicic ignimbrite suite the violet town volcanics the violet town volcanics are a 373 ma old comagmatic s type volcanic sequence mainly comprising crystal rich intracaldera ignimbrites rock types vary from rhyolites to rhyodacites all containing magmatic cordierite and garnet phenocrysts variation in the suite is primarily due to fractionation of early crystallized quartz plagioclase and biotite plus minor accessory phases in a high level magma chamber prior to eruption early magmatic crystallization occurred at around 4 kb and 850 c with melt water contents between 2 8 and 4 wt this high temperature markedly water undersaturated restite poor granitic magma was generated by partial melting reactions involving biotite breakdown in a dominantly quartzofeldspathic source terrain leaving a granulite facies residue

geochemistry and u pb and 40ar 39ar geochronology of the man of war gneiss lizard complex sw england pre hercynian arc type crust with a sudeten iberian connection the man of war gneiss is a variably deformed meta igneous body that ranges in composition from gabbro to tonalite and comprises part of the basal structural slice of the lizard ophiolite complex sw england u pb dating of zircon demonstrates that the parental magmas crystallized in the earliest ordovician 499t s 3 ma but 40 ar 39 ar step heating analysis of amphiboles suggests that at least the northernmost zone of the gneiss was subjected to an amphibolite grade metamorphic event at c 374 ma probably associated with thrust emplacement of the ophiolite the gneiss exhibits major trace and rare earth element variations in conformity with an origin through partial melting of an amphibolitic source containing hornblende and plagioclase probably with minor garnet and zircon the elemental abundances and variations are characteristic of granitoids formed in supra subduction zone environments a granitoid sill within the adjacent old lizard head series schists previously inferred to be related to the man of war gneiss is trondhjemitic and exhibits higher sio 2 tio 2 and mgo but lower zr v ba and sr contents and therefore is probably not genetically related to the latter the sill may record local melting during ophiolite emplacement the man of war gneiss probably represents a structural inlier of pre hercynian basement incorporated in the hercynian nappes of sw england during nnw directed thrusting we emphasize the geological and geochronological similarities between variably deformed granitoid units of the lizard complex the sudeten massif of southern poland and the allochthonous complexes of northern portugal and nw spain these granitoid rocks may represent dispersed remnants of tremadoc arcs that formed oceanward from the northern margin of gondwana this is further convincing evidence that at this time eastern avalonia was separated from northern gondwana by an ocean basin that may have been connected with the tornquist sea

the miocene pliocene macusani volcanics se peru i mineralogy and magmatic evolution of a two mica aluminosilicate bearing ignimbrite suite

two mica rhyolitic tephra in the east pisco basin peru new age and dispersion constraints for the eruptions of the eastern cordillera of central andes wo mica biotite and muscovite volcanics are particularly rare in the geological record one of the several dozen volcanic ash layers from central andes volcanoes found in the upper miocene marine succession of the pisco formation ica desert peru contains juvenile biotite and muscovite sillimanite andalusite feldspars and rhyolitic glass 39ar 40ar dating on biotite and muscovite concordantly constrain an age of 7 96 ma for this two mica ash layer a second tephra in the pisco formation has a similar biotite composition and an age between 7 45 and 6 93 ma the peculiar mineral assemblage and the chemical composition of biotite indicate a strongly peraluminous composition of the erupted magmas and together with the 39ar 40ar ages suggest to consider a correlation of these ash layers to the eruptions of the miocene macusani peru or morococala bolivia volcanic complexes in the eastern cordillera of central andes the major and trace element composition of glass supports the correlation with macusani a provenance from morococala seems less likely given the large distances involved these results provide new data on the volcanic activity of the eastern cordillera revealing ash that dispersed to over 500 km to the west in the forearc marine basins this finding highlights that the exhumed forearc east pisco basin is highly promising as an archive of distal ash for the reconstruction of the volcanic activity of central andes during the miocene silicic flare ups

genetic significance of multiple enclave types in a peraluminous ignimbrite suite lachlan fold belt australia the violet town volcanics lachlan fold belt australia are an s type ignimbrite suite containing microgranitoid enclaves basaltic andesite enclaves and enclaves of high silica rhyolite the microgranitoid enclaves are similar to those in peraluminous granites they typically have lower initial sr sr and higher e i than the host and represent globules of a mafic mantle derived magma ivhich was hybridized by mixing and diffusional exchange with the host magma the basaltic andesite enclaves were incorporated into the ignimbrite as xenoliths but their parental magma may have been similar to that of the microgranitoid enclaves they are isotopically less depleted than other mantle derived rocks from the lachlan fold belt reflecting contamination by crustal material or derivation from less depleted mantle sources the high silica rhyolite enclaves previously interpreted to be related to the ignimbrite by crystal fractionation have ejw values up to 3 units higher than their host and cannot be related to their host by crystal fractionation or assimilation fractional crystallization afc processes the coexistence of s type magmas and mantle derived magmas suggests that the latter may have played a role in the palaeozoic magmatism of the lachlan fold belt acting as a heat source for melting and perhaps also contributing chemical components to the crustally derived magmas

rock slope failure and the river long profile this study examines the geomorphic effects of large 10 6 m 3 rock slope failures on long profiles of rivers in the swiss alps and the new zealand southern alps regression of channel slope versus drainage basin area objectively highlights knickpoints separating incised from aggraded reaches that often correspond to locations of large rock slope failures for a fixed concavity index the highest values of the steepness index and erosion index along a given profile spatially coincide with breach channels cut into formerly river damming rockslide debris as old as 10 k y assuming that the knickpoints do not predate slope failure data show that high profile steepness and inferred specific stream power are not always the cause but often a result of large river blocking rock slope failure in mountain basins omission of rockslide data from slope area plots lowers the steepness index and increases the concavity index on average yet only in few cases more than one standard deviation

correlations of fluvial knickzones with landslide dams lithologic contacts and faults in the southwestern annapurna range central nepalese himalaya 1 we investigate the role of landslide dams spatial changes in lithology and rock uplift on faults in the formation of knickzones on bedrock rivers we focus our analysis in the southwestern annapurna range of the central nepalese himalaya where detailed geologic maps topographic data field observations and aerial photographs are available we identified knickzones in our study area from normalized river steepness indices ksn values extracted from river longitudinal profiles derived from a 25 m digital elevation model we interpolated from digitized topographic map contours we compared the location of these knickzones with 1 lithologic contacts and faults from a detailed geologic map of the modi khola valley and 2 inferred ancient landslide dam features mapped from field observations and aerial photographs the steepest location on the modi khola occurs near the same latitude as the steepest reach on the mardi khola located directly to the east potentially highlighting a major topographic transition across the annapurna however we find that landslide dams once blocked the flow of the modi khola and damming followed by incision after landslide breaching can explain the location of these knickzones without the need for active faulting near the main central thrust we also conclude that 1 knickzones do not correlate with any spatial changes in lithology and 2 knickzones generated by rock uplift on unmapped faults cannot be ruled out we emphasize that disentangling the processes responsible for knickzone formation remains challenging even when high resolution geologic and topographic data are available

rare element enriched s type ash flow tuffs containing phenocrysts of muscovite andalusite and sillimanite southeastern peru ash flow tuffs of neogene age exposed over 2 500 km 2 in the macusani region of southeastern peru are the volcanic equivalent of s type granites the strongly peraluminous tuffs contain phenocrysts of andalusite sillimanite and muscovite and have high 87 sr 86 sr i 0 7258 and 0 7226 and 18 o 11 elevated concentrations of li cs be sn b and other minor elements compare with those in tin granites mineral phase relations and composition are indicative of low magmatic temperatures and oxygen fugacities and high a hf a h 2 o the chemical isotopic and mineralogical features and regional geologic relations are consistent with models of magma generation involving the incorporation of large amounts of pelitic rock

geochemical differences between subduction and collision related copper bearing porphyries and implications for metallogenesis abstract porphyry cu mo au deposits occur not only in continental margin arc settings subduction related porphyry cu deposits such as those along the eastern pacific rim eprim but also in continent continent collisional orogenic belts collision related porphyry cu deposits such as those in southern tibet these cu mineralized porphyries which develop in contrasting tectonic settings are characterized by some different trace element e g th and y concentrations and their ratios e g sr y and la yb suggesting that their source magmas probably developed by different processes subduction related porphyry cu mineralization on the eprim is associated with intermediate to felsic calc alkaline magmas derived from primitive basaltic magmas that pooled beneath the lower crust and underwent melting assimilation storage and homogenization mash whereas k enriched collision related porphyry cu mineralization was associated with underplating of subduction modified basaltic materials beneath the lower crust with subsequent transformation into amphibolites and eclogite amphibolites and resulted from partial melting of the newly formed thickened lower crust these different processes led to the collision related porphyry cu deposits associated with adakitic magmas enriched by the addition of melts and the subduction related porphyry cu deposits associated with magmas comprising all compositions between normal arc rocks and adakitic rocks all of which were associated with fluid dominated enrichment process in subduction related cu porphyry magmas the oxidation state fo2 the concentrations of chalcophile metals and other volatiles e g s and cl and the abundance of water were directly controlled by the composition of the primary arc basaltic magma in contrast the high cu concentrations and fo2 values of collision related cu porphyry magmas were indirectly derived from subduction modified magmas and the large amount of water and other volatiles in these magmas were controlled in part by partial melting of amphibolite derived from arc basalts that were underplated beneath the lower crust and in part by the contribution from the rising potassic and ultrapotassic magmas both subduction and collision related porphyries are enriched in potassium and were associated with crustal thickening their high k2o contents were primarily as a result of the inheritance of enriched mantle components and or mixing with contemporaneous ultrapotassic magmas

weathering grade of rock masses as a predisposing factor to slope instabilities reconnaissance and control procedures weathering of rock masses often assumes importance as a predisposing factor to slope instability and it is possible to map it at various scales depending on the different purposes the effects of weathering processes are particularly intense on crystalline rocks plutonic and metamorphic these rocks are present in large areas of the globe and widespread in calabria the relationships between rock mass weathering grades and slope instabilities are analysed with reference to sectors 1 50 000 scale and areas 1 10 000 scale where crystalline rocks are strongly affected by weathering to this aim a reconnaissance procedure has been proposed to delimitate the zones with different weathering condition three macro classes at average scale 1 50 000 and six classes at detail scale 1 10 000 in this procedure first analysis of aerial photos and then field observations of representative situations have been used the reconnaissance procedure has been verified in a selected study area acri whose geological features are provided by the comparison with weathering maps obtained by means of a control procedure this last procedure consists of observations and index tests carried out in check points located in representative check sites discolouration sound when struck by geological hammer effect of the point of geological pick breaking with the hands rebound of schmidt hammer grain size analysis the results obtained confirm through quantitative data that the weathering of a rock mass can be assumed as a predisposing factor to slope instability at average scale 1 50 000 the reconnaissance procedure is able to give weathering maps representative for this type of evaluation the ratio between the landslides area in each weathering macro class and the whole landslide area goes from 67 to 14 for the macro class a and from 24 to 9 for the macro class b at detail scale 1 10 000 it is necessary to use a control procedure to obtain weathering maps indicative of predisposition to slope instabilities

topographic fingerprints of bedrock landslides bedrock landslides in mountainous regions may be triggered by either storms or earthquakes the dominant mechanism in a region affects both landscape evolution and landslide hazard we describe a simple observational test to distinguish between storm and earthquake triggers based on a probabilistic measure of hillslope morphology in areas that are dominated by storm triggered landslides steep topographic slopes are concentrated on the lowermost parts of the hillslopes storm triggers act primarily on the hillslope toes and landslides preferentially remove material from those locations giving rise to inner gorges areas where most landslides are earthquake triggered have more uniform spatial distributions of steep topographic slopes because coseismic shaking causes failures at both ridge crests and hillslope toes earthquake triggered landslides lead to planar hillslopes and rare or absent inner gorges

glass rich cordierite biotite rhyodacite valle ninahuisa puno se peru petrological evidence for hybridization of lachlan s type and potassic mafic magmas

giant landslides topography and erosion abstract distributions of slope angles in tectonically active mountain belts point to the development of threshold conditions where hillslopes attain a critical inclination or height at which they fail readily because of limitations in material strength it has been proposed that hillslopes adjust to rapid uplift and bedrock incision through an increase in the rate of relief limiting landsliding rather than gradual slope steepening here we test this concept by investigating the relationship between mean local relief h which we take to be a proxy of long term erosion rates e and the occurrence of over 300 of the largest v 10 8 m 3 terrestrial landslides on earth we find that nearly two thirds of these giant landslides have occurred in the steepest 5 of the earth s land surface where relief is close to its proposed upper strength limit they are primarily located in deeply incised valleys along fault bounded fringes of active mountain belts and in volcanic arcs this distribution coincides with areas of high long term erosion rates 4 mm yr 1 confirming that giant landslides contribute to rapid denudation of mountains most of the eroded volume is concentrated in the smallest but steepest parts of mountain belts and volcanic arcs first order estimates of minimum erosion rates accomplished by the largest landslides are 0 01 mm yr 1 these rates are between 1 and 10 of the late pleistocene to holocene mean erosion rates in a given area importantly the landslide erosion rates show a nonlinear increase with mean local relief suggesting that the contribution of giant landslides in total and per event increases significantly with increasing overall erosion rates however giant landslides also occur in areas of lower than average relief h 300 700 m irrespective of whether threshold hillslopes have developed or not factors contributing to these failures include soft rocks extensive low angle discontinuities high rates of fluvial bedrock incision and tectonically driven deformation and slope loading

petrology of an unusual orthopyroxene bearing minette suite from southeastern peru eastern andean cordillera al rich lamproites contaminated by peraluminous granites abstract the puno minettes eastern andean cordillera southern peru are phlogopite phyric rocks with a groundmass consisting mainly of phlogopite alkali feldspar and apatite microphenocrysts and or microlites they display the strong large ion lithophile element enrichments that characterize potassic to ultrapotassic rocks of the lamprophyre clan rb ba sr up to 103 primordial mantle ree up to 6 102 ci chondrites in addition to their close association with granitoids the rocks of this series show many features of minettes occurring in arc related collisional zones i e absence of mantle xenoliths up to 30 vol felsic xenocrysts and strong negative nb and ti anomalies however they are distinguished by unusual phenocryst phases whole rock compositions and groundmass mineralogy some minettes contain only a few per cent of biotite quartz plagioclase xenocrysts immersed in a typical minette groundmass whereas those richer in xenocrysts display the complete liquidus paragenesis of the coexisting peraluminous granite biotite quartz plagioclase alkali feldspar cordierite zircon pyrite in a high k latitic to trachydacitic groundmass field relations radiometric ages whole rock data and felsic xenocrysts all indicate that the puno minettes result from a mixing process involving al rich lamproites and peraluminous monzogranites the most common phenocryst assemblage highly magnesian phlogopite olivine titanian cr spinel ca rich enstatite magnesian ilmenite al and ti poor diopside is similar to that reported in al rich phlogopite lamproites from southeastern spain likewise the host rocks have diagnostic features of spanish phlogopite lamproites e g k2o al2o3 ratios 9 cao contents

tibetan plateau river incision inhibited by glacial stabilization of the tsangpo gorge much attention has focused on how and when the tibetan plateau formed but far less research has been done on the controls on river incision into the plateau itself oliver korup and david montgomery address this issue which also relates to the question of why the tibetan plateau is still there at all despite hosting one of the deepest gorges on earth the plateau edge is extremely well defined and according to widely accepted stream power theory should be heavily dissected by one of asia s most powerful rivers the yarlung tsangp or yarlung zangbo korup and montgomery propose that the southeastern edge of the plateau was preserved in part by the presence of glaciers that were large enough during the holocene to stall aggressive river downcutting along the plateau margin a considerable amount of research has focused on how and when the tibetan plateau formed in the wake of tectonic convergence between india and asia1 although far less enquiry has addressed the controls on river incision into the plateau itself2 widely accepted theory3 predicts that steep fluvial knick points river reaches with very steep gradients in the eastern himalayan syntaxis at the southeastern plateau margin should erode rapidly4 driving a wave of incision back into the plateau preservation of the plateau edge thus presents something of a conundrum that may be resolved by invoking either differential rock uplift matching erosional decay5 6 7 or other mechanisms for retarding bedrock river incision8 9 in this region where high stream power excludes the potential for aridity as a simple limit to dissection of the plateau10 here we report morphologic evidence showing that quaternary depression of the regional equilibrium line altitude where long term glacier mass gain equals mass loss was sufficient to repeatedly form moraine dams on major rivers such damming substantially impeded river incision into the southeastern edge of the tibetan plateau through the coupled effects of upstream impoundment and interglacial aggradation such glacial stabilization of the resulting highly focused river incision centred on the tsangpo gorge could further contribute to initiating and accentuating a locus of rapid exhumation known as tectonic anaeurysm6

commingling and mixing of s type peraluminous ultrapotassic and basaltic magmas in the cayconi volcanic field cordillera de carabaya se peru abstract the cayconi district of the cordillera de carabaya se peru exposes a remnant of an upper oligocene lower miocene 22 2 24 4 ma volcanic field comprising a diverse assemblage of s type silicic and calc alkaline basaltic to andesitic flows members of the picotani group of the central andean inner arc basaltic flows containing olivine plagioclase clinopyroxene ilmenite and glass and glassy rhyolitic agglutinates with phenocrystic quartz cordierite plagioclase sanidine ilmenite and apatite respectively exhibit mineralogical and geochemical features characteristic of medium k mafic and lachlan s type silicic lavas cordierite bearing dacitic agglomerates and lavas however are characterized by dispersed melanocratic micro enclaves and phenocrysts set in a fine grained quartzo feldspathic matrix they contain a bimodal mica population comprising phlogopite and biotite as well as complexly zoned sieve textured plagioclase grains sector zoned cordierite sanidine quartz irregular patches of replaced olivine clinopyroxene and orthopyroxene and accessory phases including zircon monazite ilmenite and chromite the coexistence of minerals not in mutual equilibrium and the growth dissolution textures exhibited by plagioclase are features indicative of magmatic commingling and mixing trachytic textured andesite flows interlayered with olivine plagioclase glomerophyric calc alkaline basalts have a phenocrystic assemblage of resorbed orthopyroxene and plagioclase and exhibit melanocratic groundmass patches of microphenocrystic phlogopite ca rich sanidine ilmenite and aluminous spinel the mineralogical and mineral chemical relationships in both the dacites and the trachytic textured andesites imply subvolcanic mixing between distinct ultrapotassic mafic melts not represented by exposed rock types and both the s type silicic and calc alkaline mafic magmas such mixing relationships are commonly observed in the oligo miocene rocks of the cordillera de carabaya suggesting that the s type rocks in this area and by extension elsewhere derive their unusually high k 2 o ba sr cr and ni concentrations from commingling and mixing with diverse mantle derived potassic mafic magmas

the iugs systematics of igneous rocks in order to create a sustainable classification of igneous rocks which all geologists might use an international body was set up by the iugs the iugs subcommission on the systematics of igneous rocks in the course of creating the classification the subcommission has established ten principles for its construction and for defining an appropriate nomenclature the principles are 1 use descriptive attributes 2 use actual properties 3 ensure suitability for all geologists 4 use current terminology 5 define boundaries of rock species 6 keep it simple to apply 7 follow natural relations 8 use modal mineralogy 9 if mode not feasible use chemistry 10 follow terminology of other iugs advisory bodies these principles and their rationale have not previously been enunciated the classification separates and individually classifies the pyroclastic carbonatitic melititic lamprophyric and charnockitic rocks before entering the main qapf classification for plutonic and volcanic rocks which is based on the modal mineral proportions of quartz q alkali feldspar a and plagioclase p or of alkali feldspar a plagioclase p and feldspathoids f rocks with mafic content 90 have their own classification if the mineral mode cannot be determined as is often the case for volcanic rocks then a chemical classification of total alkalis versus silica tas is used the nomenclature for these classifications necessitates only 297 rock names out of the c 1500 that exist

shallow seated controls on styles of explosive basaltic volcanism a case study from new zealand abstract the pyroclastic deposits of many basaltic volcanic centres show abrupt transitions between contrasting eruptive styles e g hawaiian versus strombolian or dry magmatic versus wet phreatomagmatic these transitions are controlled dominantly by variations in degassing patterns magma ascent rates and degrees of interaction with external water we use crater hill a 29 ka explosive effusive monogenetic centre in the auckland volcanic field new zealand as a case study of the transitions between these end member eruptive styles the crater hill eruption took place from at least 4 vents spaced along a nne trending 600 m long fissure that is contained entirely within a tuff ring generated during the earliest eruption phases early explosive phases at crater hill were characterised by eruption from multiple unstable and short lived vents later dominantly extrusive volcanism took place from a more stable point source most of the crater hill pyroclastic deposits were formed in 3 phreatomagmatic p and 4 dry magmatic m episodes forming in turn the outer tuff ring and maar crater p1 m1 p2 and scoria cone 1 m2 m4 this activity was followed by formation of a lava shield and scoria cone 2 purely wet activity is represented by the bulk of p1 and p2 and purely dry activity by much of m2 m4 however m1 and parts of m2 and m4 show evidence for simultaneous eruptions of differing style from adjacent vents and rapid variations in the extent and timing of magma water interaction at each vent the nature of the wall rock lithics and these rapid variations in inferred water magma ratios imply interaction was occurring mostly at depths of 80 m and the vesicularity patterns in juvenile clasts from these and the p beds imply that rapid degassing occurred at these shallow levels we suggest that abrupt transitions between eruptive styles in time and space at crater hill were linked to changes in the local magma supply rate and patterns and vigour of degassing during the final metres of ascent