the petrogenesis of alkaline intrusives from arabia and northeast africa and their implications for within plate magmatism abstract trace element data from alkaline intrusions on both sides of the red sea indicate a within plate origin rb sr geochronology however demonstrates that alkaline magmatism in arabia was restricted to a post island arc emplacement of alkaline granite 650 500 ma whereas in northeast africa alkaline magmatism began with a similar phase but continued sporadically until the opening of the red sea at 30 ma with magmas becoming progressively undersaturated in silica with time isotope geochemistry of the earlier alkaline granites indicates a rb and lree depleted source region for the magmas and is consistent with a source in crustal rocks which had an age of less than 800 ma it is proposed that the time dependent undersaturation in silica resulted from the increasing role of asthenospheric volatiles and possibly magma in the petrogenesis of the younger alkaline magmas comparison between apparent polar wandering paths for africa inferred from recently published palaeomagnetic data and rb sr isochron ages from northeast africa and other alkaline provinces in africa active between 8 and 30 ma supports the theory that within plate magmatic emplacement is suppressed during periods of rapid lithospheric movement

plutonic rock types series discrimination of various granitoid series and related rocks abstract the streckeisen qap diagram discriminates four characteristic type series among the large variety of granitoids associated in intrusions three of them integrate varied compositions from basic to acid terms 1 the calcalkaline series with its low k or trondhiemite medium k or granodioritic high k or monzonitic variants 2 the alkaline series with its characteristic syenitic members 3 the tholeiitic series deprived of k feldspars the fourth 4 corresponds to mobilizates associated with migmatites of varied composition but always quartz rich the generalization in the qapf system allowing the plotting of undersaturated alkaline series provides an excellent visual display of the major associations and trends for the plutonic rocks

a facies model for a quaternary andesitic composite volcano ruapehu new zealand ruapehu composite volcano is a dynamic volcanic sedimentary system characterised by high accumulation rates and by rapid lateral and vertical change in facies four major cone building episodes have occurred over 250 ka from a variety of summit flank and satellite vents eruptive styles include subplinian strombolian phreatomagmatic vulcanian and dome related explosive eruptions and extrusion of lava flows and domes the volcano can be divided into two parts a composite cone of volume 110 km3 surrounded by an equally voluminous ring plain complementary portions of ruapehu s history are preserved in cone forming and ring plain environments cone forming sequences are dominated by sheet and autobrecciated lava flows which seldom reach the ring plain the ring plain is built predominantly from the products of explosive volcanism both the distal primary pyroclastic deposits and the reworked material eroded from the cone much of the material entering the ring plain is transported by lahars either generated directly by eruptions or triggered by the high intensity rain storms which characterise the region ring plain detritus is reworked rapidly by concentrated and hyperconcentrated streams in pulses of rapid aggradation immediately following eruptions and more gradually in the longer intervals between eruptions

paleoproterozoic late orogenic and anorogenic alkaline granitic magmatism from northeast brazil abstract during the transamazonian cycle 2 0 0 2 ga two silica saturated alkaline granite suites were intruded along the border of the sao francisco craton northeastern brazil the couro de onca intrusive suite cois is late tectonic relative to major deformational events and the serra do meio suite sms postdates this event being interpreted as anorogenic type deformed during the brasiliano cycle 0 65 0 52 ga the cois encompasses alkali feldspar granites syenogranites monzogranites with dykes of similar composition and age 2 157 5 ma they are metaluminous rocks of alkaline affinity whose evolution was probably controlled by mineral fractionation processes involving mainly plagioclase amphibole and fe ti oxide the sms whose probable age is 2 01 ga is composed of metaluminous and peralkaline granites with associated quartz syenites with ti aegirine transformed during the brasiliano deformation to riebeckite titanite aenigmatite the temperatures of two stages of metamorphic transformation overprinted during the brasiliano event are estimated at about 550 and 400 450 c both magmatic suites are derived from enriched mantle sources the metaluminous and peralkaline trends observed in the sms characterized by high concentrations of hfs and re elements controlled mainly by f activity reflect the evolution of different primary magmas produced by successive melting of the same mantle sources in an anorogenic setting

the pan african reactivation of eburnean and archaean provinces in nigeria structural and isotopic data eastern nigeria experienced high grade metamorphism during the pan african collision event marked by a pervasive migmatization structural evidence demonstrates that anatexis was contemporaneous with regional deformation and emplacement of calc alkaline granite diorite plutons new pb evaporation data on single zircons from the migmatites confirm that partial melting and plutonism occurred synchronously around 580 10 ma the pan african reactivation is almost ubiquitous and therefore the protolith age is still debated however sm nd analysis of migmatitic gneisses from eastern nigeria yielded eburnean model ages of the protolith between 1 7 and 2 0 ga in agreement with previous model ages of 1 3 and 2 0 ga from granitoids and gneisses of the same area these results suggest that the protoliths in eastern nigeria are distinct from those from the western archaean province furthermore recent work has shown that further north in the tuareg shield the pan african belt consists of amalgamated terranes thus a terrane model could explain the contrast between eastern nigeria characterized by eburnean protoliths and calc alkaline granitic rocks and western nigeria characterized by archaean protoliths and units having tonalite trondhjemite granodiorite and greenstone affinities the contrasted terranes also correspond to a gold province in the west and to a tin province in the east of nigeria

downstream dilution of a lahar transition from debris flow to hyperconcentrated streamflow nearly instantaneous melting of snow and ice by the march 19 1982 eruption of mount st helens released a 4 106 m3 flood of water from the crater that was converted to a lahar volcanic debris flow through erosion and incorporation of sediment by the time it reached the base of the volcano over the next 81 km that it traveled down the toutle river the flood wave was progressively diluted through several mechanisms a transformation from debris flow to hyperconcentrated streamflow began to occur about 27 km downstream from the crater when the total sediment concentration had decreased to about 78 by weight 57 by volume the hyperconcentrated lahar runout flood wave transporting immense quantities of sand in suspension continued to experience progressive downstream dilution although turbulence was significantly dampened by the extremely high suspended load very large standing waves and antidune waves were observed the hyperconcentrated lahar runout flow deposited an unusual faintly stratified coarse sand which locally contained small isolated gravel lenses very similar deposits in the quaternary stratigraphy of mount st helens and other cascades volcanoes suggest that lahars may be more frequent than previously recognized

acidity salinity diagrams application to greisen and porphyry deposits to a first approximation alteration minerals in greisen and related fluorine rich porphyry deposits lie in the system k 2 o al 2 o 3 sio 2 h 2 o f 2 o sub 1 equilibria in this model system can be depicted on quartz and fluid saturated chemical potential diagrams whose axes are mu hf acidity where mu hf mu hcl mu sub fcl sub 1 and mu kf salinity where mu kf mu nacl mu sub kna sub 1 mu sub fcl sub 1 advantages of this approach are that it reflects the fact that the anion f unlike cl is incorporated in greisen minerals mainly topaz fluorite and micas and that mineral stabilities can be correlated with possible ore forming processes the most important of these processes is probably vapor phase separation from an aqueous brine or magma first or second boiling fluid inclusion studies have shown that boiling occurs in all porphyry and many but not all greisen deposits vapor phase separation from a magma pneumatolysis must similarly be responsible for the formation of greisenlike mineral assemblages in miarolitic topaz rhyolite lava flows and is presumably also responsible for the formation of gem pockets in more deeply seated pegmatites and granites the result of boiling is an increase in mu hf and a decrease in mu kf in the vapor phase and the opposite in the residual brine or magma in deep greisen veins the muscovite stability field is encountered between the k feldspar and topaz fields whereas in f rich relatively shallow porphyry deposits such as henderson colorado boiling causes aqueous fluids to pass directly from the k feldspar ore zone to the overlying topaz magnetite zone muscovite or f bearing mn fe garnet is only encountered farther out as the fluids are condensed neutralized and diluted by cooler meteoric waters when ca fe mn be w o 2 s 2 and co 2 are added to the greisen model system scheelite is shown to transform to wolframite with increasing mu hf and beryl to phenakite with increasing mu hf or mu kf biotite transforms to magnetite hematite siderite pyrite chlorite or fayalite beyond the k feldspar field its stability is enhanced by high mu hf and chlorite cordierite or mn fe garnet transform to an mn fe carbonate oxide or sulfide or biotite beyond the al silicate field the ca content of plagioclase coexisting with fluorite decreases with an increase in either mu hf or mu kf and thus provides an indicator of total fluorine in both acid and neutral species

inclination flattening and the geocentric axial dipole hypothesis abstract william gilbert first articulated what has come to be known as the geocentric axial dipole hypothesis the gad hypothesis is the principle on which paleogeographic reconstructions rely to constrain paleolatitude for decades there have been calls for permanent non dipole contributions to the time averaged field recently these have demanded large contributions of the axial octupole which if valid would call into question the general utility of the gad hypothesis in the process of geological recording of the geomagnetic field earth filters distort the directions many processes for example sedimentary inclination flattening and random tilting can lead to a net shallowing of the observed direction therefore inclinations that are shallower than expected from gad can be explained by recording biases northward transport or non dipole geomagnetic fields using paleomagnetic data from the last 5 million years from well constrained lava flow data allows the construction of a statistical geomagnetic field model such a model can predict not only the average expected direction for a given latitude but also the shape of the distribution of directions produced by secular variation the elongation of predicted directions varies as a function of latitude from significantly elongate in the up down direction at the equator to circularly symmetric at the poles sedimentary inclination flattening also works in a predictable manner producing elongations that are stretched side to side and the degree of flattening depending on the inclination of the applied field and a flattening factor f the twin tools of the predicted elongation inclination relationship characteristic of the geomagnetic field for the past 5 million years and the distortion of the directions predicted from sedimentary inclination flattening allows us to find the flattening factor that yields corrected directions with an elongation and average inclination consistent with the statistical field model the method can be tested using sediments deposited in a known field application of the elongation inclination correction method to two magnetostratigraphic data sets from red beds in asia and pakistan brings the inclinations into agreement with those predicted from modern gps measurements and from global paleomagnetic data there appears to be no compelling reason at this time to abandon the geocentric dipole hypothesis which has provided such an excellent working model for so long

anorogenic alkaline granites from northeastern brazil major trace and rare earth elements in magmatic and metamorphic biotite and na mafic minerals abstract the anorogenic alkaline silica oversaturated serra do meio suite is located within the riacho do pontal fold belt northeast brazil this suite assumed to be paleoproterozoic in age encompasses metaluminous and peralkaline granites which have been deformed during the neoproterozoic collisional event preserved late magmatic to subsolidus amphiboles belong to the riebeckite arfvedsonite and riebeckite winchite solid solutions riebeckite winchite is frequently rimmed by ti aegirine ti aegirine cores are strongly enriched in nb y hf and ree which significantly decrease in concentrations towards the rims ree patterns of ti aegirine are strikingly similar to ti pyroxenes from the ilimaussaq peralkaline intrusion recrystallisation of mineral assemblages was associated with deformation although some original grains are still preserved magmatic annite was converted into magnetite and biotite with lower fe fe mg ratios recrystallised amphibole is pure riebeckite magmatic ti na bearing pyroxene was converted to low ti aegirine titanite astrophyllite aenigmatite the reaction riebeckite quartz aegirine magnetite quartz fluid is also observed biotite and na mafic minerals recrystallised under metamorphic oxidising conditions corresponding to temperatures of 600 c between the ninio and hm buffers

volcanic mixed avalanches a distinct eruption triggered mass flow process at snow clad volcanoes a generally unrecognized type of pyroclastic deposit was produced by rapid avalanches of intimately mixed snow and hot pyroclastic debris during eruptions at mount st helens nevado del ruiz and redoubt volcano between 1982 and 1989 these mixed avalanches traveled as far as 14 km at velocities up to 27 m s involved as much as 10 7 m 3 of rock and ice and left unmelted deposits of single flow units as thick as 5 m during flow downslope heat transfer from hot rocks to snow produced meltwater that partially saturated the mixtures apparently giving these mixed avalanches mobilities equal to or greater than those of dry debris avalanches of similar volume the avalanches studied for this report began as snow avalanches triggered during explosive phases of the eruptions by ballistic bombardment of snow slopes by hot pyroclastic debris erosion by the avalanches incorporated additional snow fragments of glacier ice and other rock debris mixed avalanche deposits were massive very poorly sorted and ungraded to inversely graded prior to melting although rock debris composed as much as 70 wt of the frozen samples the bulk volumetric content of rock debris was only 9 to 36 snow and ice composed between 36 and 72 of the frozen samples with void space making up the remainder of the volume deformation and consolidation after melting of the ice content reduced the deposits to loose porous layers that were only a fraction of their original thickness and that later gave little hint of their origin after melting and desiccation the deposits are highly susceptible to erosion and unlikely to be well preserved in the stratigraphic record

palaeomagnetism and age of mid palaeozoic ring complexes in niger west africa and tectonic implications summary detailed af and thermal demagnetization has been performed on 185 oriented samples from 36 sites in a series of basic to acid subvolcanic ring complexes in central and southern niger published rb sr whole rock and mineral isochrons from acid members of the suite range north to south from 480 ma to 320 ma two distinct vectors have been isolated by linear regression of demagnetization data 1 a due north and negative inclination rarely reversed component is present in basic rocks from 12 sites in four distinct northern complexes published rb sr ages date from 480 ma to 420 ma this component is carried by single domain magnetite occasionally also by titanhaematite and is considered to be primary acquired during the terminal stages of this igneous episode 40ar 39ar dating of biotite from three samples used in the palaeomagnetic study yield 435 myr ages 2 a due north and positive inclination rarely reversed direction is present as a second component in some of the basic suite sites and is the predominant component in most sites in syenitic to granodioritic rocks primary fe ti oxides are rare in these acid igneous rocks and the feldspars within them are usually pervasively altered this component carried primarily by haematite is believed to be either a modern day vrm or a secondary tcrm acquired during hydro thermal activity associated with widespread quaternary volcanism in the southern air the primary component gives a mean south pole at 43 4s 08 6e n 12 k 50 95 6 2 located southwest of cape town this position requires a rapid southward excursion of the gondwana a p w path in early silurian time but it is generally consistent with most of the other silurian devonian palaeomagnetic data from gondwana these however are from the lachlan fold belt in southeast australia whose relation to the australian craton in early palaeozoic time is not clear integrating all stratigraphic palaeoclimatic tectonic and other palaeomagnetic data for early and middle palaeozoic time suggests that the proto atlantic closed during the silurian period if a late devonian early carboniferous gondwana pole in central africa is valid the ocean may have opened again before the final closure and reassembly of gondwana in the late carboniferous

evolution of a vent hosted hydrothermal system beneath ruapehu crater lake new zealand a two year chemical monitoring program of ruapehu crater lake shows that it has evolved considerably since the volcano s more active eruptive periods in the early 1970s the present ph 20 c of 0 6 is about one half unit more acid than the baseline values in the 1970s whereas s cl ratios have increased markedly owing in part to declining hcl inputs into the lake but also to absolute increases in so4 levels which now stand at the highest values ever recorded increases in k mg and na mg ratios over the 20 year period are attributed to hydrothermal reaction processes in the vent which are presently causing dissolution of previously formed alteration phases such as natroalunite these observations combined with results of a recent heat budget analysis of the lake have led to the development of hydrothermal convection model for the upper portion of the vent possible vent lake chemical reaction processes between end member reactants have been modelled with the computer code chiller the results are consistent with the view that variations in lake chemistry which are initiated by the introduction of fresh magmatic material into the vent reflect the extent of dissolution reaction progress on the magmatic material and or its alteration products the results also provide insights into the role of such vent processes in the formation of high sulfidation type ore deposits

the petrology and geochemistry of alkaline granites from nigeria abstract the nigerian younger granites as well as the petrologically similar but chronologically older northern centres in air and south niger constitute the best example of epidosically migrating mid plate magmatism the nigerian anorogenic ring complexes provide a classic example of alkaline granitoid evolution each anorogenic centre represents the exposed roots of an eroded volcano volcanic assemblages preserved by caldera collapse at some centres confirm that early erupted liquids evolved from olivine basalt through hawaiite mugearite and trachyte although there are volumetrically abundant rhyolitic ignimbrites there are no volcanic equivalents of biotite granite however at the subvolcanic level biotite granites are extensively mineralized with rich deposits of columbite sphalerite cassiterite and wolframite related peralkaline granites contain enriched concentrations of ore elements but apart from pyrochlore thorium rich monazite and topaz most potential ores are contained in alkali amphiboles or alkali pyroxene fluid interaction in the granite cupolas has caused subsolidus mineral growth and modified original element abundances in some instances fluid reaction has been so intense that original granite has been locally converted to albite microclinite and greisen these reactions can be monitored by the new mineral assemblages the major element chemistry trace elements e g the rare earths and isotopically by the initial strontium isotopic ratios

fluid inclusion rb sr isochrons for dating mineral deposits the precise dating of mineral deposits has important implications for understanding the lifetime of ore forming systems and their temporal relationship to various geological events not all deposits provide material suitable for conventional radiometric dating or data which are amenable to unequivocal interpretation nor is it easy to verify that such samples have remained isotopically closed since their formation fluid inclusions provide an alternative closed system approach by analysing the isotopic composition of rband sr in inclusions within quartz from the carrock fell tungsten vein deposits cumbria we demonstrate here the feasibility of obtaining rb sr fluid inclusion isochrons the age for the mineralization 392 5 myr is in good agreement with the mean k ar age 387 6 myr for the vein micas and confirms the significance of fluid inclusions as precise rb sr geochronometers

internal fabric and strike slip emplacement of the pan african granite of solli hills northern nigeria the solli hills pluton 250 km2 in northern nigeria is an example of a pan african granite emplaced circa 580 ma ago in the jos adamawa block its micro structures and internal fabrics have been studied in order to help constrain the kinematics of the pan african orogeny in this block we used field observations and measurements petrographic examinations and measurements of the magnetic susceptibility and anisotropy of magnetic susceptibility of 124 regularly spaced stations most samples are ferromagnetic i e belong to the magnetite series granites and present predominantly magmatic microstructures particularly remarkable are the asymmetrical zoning of the petrographic types the susceptibility magnitudes and the planar linear magnetic fabrics the magnetic foliations and lineations are very consistent in the pluton around n 20 e 60 w and n 205 e 10 respectively field and microscopic observations reveal that deformation occurred everywhere in the pluton and immediate country rock envelope during a dextral strike slip synplutonic episode it is suggested that the solli hills pluton was emplaced in a late pan african dextral tear fault system the striking fabric asymmetry of the pluton strongly suggests a shear zone termination emplacement model

changes in whangaehu river lahar characteristics during the 1995 eruption sequence ruapehu volcano new zealand abstract during the 1995 ruapehu eruptive sequence multiple lahars occurred in the whangaehu river which drains ruapehu s crater lake during the earlier phreatic and phreatomagmatic eruptions lahars were generated by expulsion of waters from the lake but once the lake had emptied lahars were formed by remobilisation of seasonal snowpack laden with saturated freshly erupted tephra four types of lahars occurred during the eruptive sequence 1 initial snow slurry lahars composed of granular snow and ice incorporated by eruptively expelled crater lake waters which left behind frozen deposits with 2 5 20 clastic sediment 2 large dilute lahars generated as the volumes of ejected lake water increased and removed much of the readily available snow at least one third of the pre eruption crater lake volume was expelled during one day producing the largest lahars of the series these lahars were hyperconcentrated flows for up to 84 km from source leaving extensive deposits along the channel margins 3 concentrated lahars smaller volume lahars generated as the frequency of eruptions and volumes of expelled water declined these lahars were able to maintain high sediment concentrations measured at 46 52 by volume suspended sediment at 42 km from source their high sediment concentrations were maintained by erosion and incorporation of sand from the deposits of earlier flows which were lining the channel margins 4 remobilised tephra lahars generated following the two largest tephra eruptions of the sequence seasonal snowpack was covered by water saturated tephra warmer spring temperatures and heavy rainfall events caused collapse and remobilisation of snow and tephra producing several lahars in catchments draining eastern ruapehu

petrogenesis of a type granites associated with sn nb zn mineralization in ririwai complex north central nigeria constraints from whole rock sm nd and zircon lu hf isotope systematics abstract we report a combined study of whole rock major and trace element geochemistry sm nd isotope composition zircon u pb dating and lu hf systematics of peralkaline and aluminous a type granites from the ririwai ring complex in north central nigeria the ririwai peralkaline and aluminous a type granites are strongly ferroan alkalic to alkali calcic and enriched in hf zr ga rb y and rees they were emplaced between 176 2 3 ma and 169 6 0 75 ma the peralkaline granites yield relatively higher nd t 2 3 to 1 2 and zircon hf t values 5 8 to 1 7 than the aluminous granites nd t 3 6 to 3 3 zircon hf t 7 8 to 2 4 in addition inherited zircons in the aluminous granites yield pan african 590 ma ages and low hf t values 14 0 taken together these data suggest that the granites formed from extensive fractional crystallization of a transitional basaltic melt derived from an enriched oib mantle source the depletions of mgo cao ti2o sr and ba in the granites indicate that mg rich olivine calcic pyroxene calcic amphibole feldspars and fe ti oxides were the major fractionated phases during magma evolution in addition several types of evidence e g moderately negative nd t and hf t values and the presence of inherited zircons imply that the parental melt was modified by assimilation of pan african upper crust into which the granites were emplaced a transtensional regime generated prior to late jurassic breakup of gondwana which led to reactivation of shear zones and opening of associated transcurrent faults paved the way for emplacement of the a type suite the sn nb zn mineralization of the ririwai a type suite was probably linked to complex magmatic evolutionary processes involving extensive fractional crystallization coupled with crustal assimilation and late stage hydrothermal fluid activity

repeated s i a type granite trilogy in the lachlan orogen and geochemical contrasts with a type granites in nigeria implications for petrogenesis and tectonic discrimination

geological geochemical and geochronological characteristics of caledonian w sn mineralization in the baiganhu orefield southeastern xinjiang china abstract the newly discovered large scale baiganhu w sn orefield consisting of the kekekaerde baiganhu bashierxi and awaer deposits is located in ruoqiang county southeastern xinjiang china these deposits comprise mainly three types of w sn mineralization early stage skarn type middle stage greisen type and late stage quartz vein type in this study we classified seven major vertical zones on the basis of petrographic characteristics roughly from the bottom of the parental granitic intrusions upward as a fresh syenogranite b argillic alteration c muscovite dominated greisenization d tourmaline dominated greisenization e marginal facies including k feldspar pegmatite and fine grained granite f aplitic apophysis and g1 skarn or g2 infilled silification zones according to the alteration mineralization assemblages and cross cutting relationships five stages of mineralization are recognized in the orefield i skarn stage ii greisen stage iii quartz vein stage iv argillic alteration stage and v supergene stage and reverse alteration zonation in the altered intrusion is also observed the w sn deposits are spatially associated with syenogranite which is part of the caledonian bashierxi magmatic series laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb dating of the syenogranite yields a weighted mean 206 pb 238 u age of 413 6 2 4 ma mswd 0 36 n 30 hydrothermal muscovite from three samples associated with w sn mineralization yields plateau 40 ar 39 ar ages of 411 7 2 6 ma mswd 0 21 412 8 2 4 ma mswd 0 22 and 413 8 2 6 ma mswd 0 22 which is consistent with the zircon u pb age thus indicating a temporal link between the emplacement of the syenogranite and the w sn mineralization our age data and previously published ages along with geochemical data for the granitoids in the bashierxi magmatic series show a nearly contemporaneous evolution of a and s type granites which were emplaced in a post orogenic setting at ca 432 413 ma as compared with the a type granites the syenogranites with s type affinities probably resulted from a lower degree of partial melting of metagreywackes which was more likely to be enriched in the ore forming elements w and sn as well as volatiles such as b and h 2 o in addition the syenogranites exhibit low oxidation states and underwent high degrees of factional crystallization both of which favor post magmatic w sn mineralization we suggest that more attention should be given to buried syenogranites of s type affinities during mineral exploration in this area and that the proposed model of a vertical alteration zoning can act as a guide to the targeting of similar ore systems