underwater noise due to rain hail and snow the spectra of underwater noise generated by rain hail and snow have been measured in a lake at a depth of 35 m for a variety of atmospheric conditions rain noise spectra for light winds 1 2 m s 1 have a sharp peak at 13 5 khz with a steep falloff 60 db oct on the low frequency side and a more gradual falloff 9 db oct on the high frequency side a quasi flat spectral regime exists in the frequency interval 2 10 khz wind for speeds increasing above 1 2 m s 1 progressively rounds the peak the spectral level at 15 khz i e near the peak shows a linear dependence on the log of the rain rate with wind speed as a parameter correlation of the rain noise spectra with raindrop size distributions suggests that low frequencies are generated by the larger drops although this aspect of the problem needs further work hail noise spectra have rounded maxima appearing between 2 and 5 khz with an approximately 10 db falloff on each side the spectrum of underwater sound generated by gently falling s

early jurassic tectonism occurred within the basu metamorphic complex eastern central tibet implications for an archipelago accretion orogenic model abstract the basu metamorphic complex surrounded by ophiolitic melanges and intruded by a large volume of undeformed granitoid rocks along the eastern segment of the bangong nujiang suture holds one of the keys to understanding the pre cenozoic tectonic evolution of central tibet zircon u pb dating of rocks from the basu metamorphic complex reveals that meta igneous rocks yield early paleozoic crystallization ages of 500 492 ma and an early jurassic metamorphic age of 173 ma and that undeformed granitoid rocks yield crystallization ages of approximately 186 174 ma whole rock geochemical and zircon lu hf isotopic data indicate that the undeformed granitoid rocks originated mainly from partial melting of ancient crustal sources which may reflect a collisional orogenic setting 40 ar 39 ar dating of biotite from a sillimanite garnet biotite paragneiss shows cooling to 300 50 c at 165 ma these data indicate significant early jurassic tectonism during which most of the basu metamorphic complex was formed furthermore the age data resemble those of the amdo metamorphic complex located approximately 500 km to the west along the bangong nujiang suture together these complexes may represent a destroyed or unrecognized block i e the amdo tongka block which may be the eastern extension of the south qiangtang terrane based on the tectonic outlines of the multiple ophiolitic zones and magmatic belts we suggest a new archipelago accretion model that attributes the early jurassic tectonism to an arc continent micro continent collision this model further enables the reconstruction of the eastern tethyan ocean and the orogenic processes of central tibet during the mesozoic

geochronology petrogenesis and tectonic significance of the jitang granitic pluton in eastern tibet sw china abstract the jitang granitic pluton which is situated in the southern margin of the north qiangtang block in eastern tibet may provide critical information about the source and derivation of the north qiangtang block during gondwana breakup and dispersion in this paper we report relevant data such as zircon u pb age whole rock major and trace element abundances and sr nd isotopes for the jitang pluton the major rock types of the pluton are granodiorite and biotite granite whole rock major element data reveal that the jitang pluton is a peraluminous s type granitic pluton the u pb age of zircons from the pluton is 219 1 1 7 ma which is 10 ma younger than the age of high pressure metamorphism in the longmu co shuanghu collisional suture between the north and south qiangtang blocks the jitang granitoids show pronounced negative ba eu sr anomalies high initial 87sr 86sr ratios from 0 7266 to 0 7389 and low end t values for from 11 1 to 13 2 which are remarkably similar to the gneisses and meta sedimentary rocks from the indian craton as well as the north qiangtang block the results from this study indicate that the jitang granitoids formed by melts derived from a crustal source with sr nd isotopic compositions similar to those of the indian cratonic crust we concur with the previous interpretation based on detrital zircon records that the north qiangtang block was derived from the indian gondwana

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

trace elements in continental margin magmatism part iii alkali granites and their relationship to cratonization summary alkali granites form as the final tectonic magmatic event in shield areas and in many phanerozoic orogenic belts they occur mostly as discordant post tectonic plutons plutons of various ages and geographic locations are lithologically extremely similar with an average composition of 72 5 sio 2 0 3 tio 2 14 1 a1 2 o 3 2 2 total fe as fe 2 o 3 0 4 mgo 1 3 cao 3 6 na 2 o and 4 8 k 2 o the alkali granite suites differ from the felsic rocks of most calcalkalic batholiths in having higher k 2 o mgo ratios and few rocks with less than 65 sio 2 alkali granite magmas may form by normal crystallization differentiation from calc alkalic adamellites quartz monzonites although not all alkali granite magmas have originated by that process most alkali granites do not show evidence of having been derived from a crustal source in some occurrences pre existing sial apparently was not present in the area at the time that the alkali granite magmas evolved thus alkali granites may be part of the process that causes development of stable cratons by addition of new sialic material to thicken the continental crust the higher concentration of k 2 o in younger shield areas and compensatory changes in abundances of other elements are caused by the higher abundance of alkali granites in younger cratons

aeromagnetic anomalies and tectonic trends in and around the benue trough nigeria four major fracture zones cutting the atlantic sea floor off the west coast of africa st paul s romanche chain and charcot have been delineated offshore using bathymetrie seismic gravity and magnetic data respectively1 but the extrapolation of these fractures onto the african continent is uncertain here we analyse recently compiled aeromagnetic data2 for the benue trough and adjacent areas of nigeria and demonstrate the existence of narrow magnetic lineaments parallel to the margins of the benue trough which can be matched with the oceanic fractures alkaline ring complexes which predate the opening of the atlantic ocean are concentrated along these lineaments suggesting that they represent pre existing zones of weakness in the african crust which may have influenced the rifting of the continental margins and the early development of atlantic fracture zones

the trace element and isotope geochemistry of the sabaloka igneous complex sudan the sabaloka igneous complex comprises 2 igneous centres of 9within plate9 magmatism intruded into precambrian gneisses in northern sudan the cauldron complex is the larger centre and was emplaced at 383 14 ma the earliest felsic magmas are highly fractionated peraluminous rhyolites with isotope and trace element characteristics indicative of either a biotite tonalite granodiorite source or of amphibole fractionation from a metalumi nous melt the rhyolites are overlain by ignimbrites which have similar trace element characteristics to the rhyolites but are less aluminous a mica granite boss intrudes both rhyolites and ignimbrites and has an unuaual trace element geochemistry which is ascribed to volatile loss during emplacement the cauldron complex is circumscribed by a microgranite ring dyke which has isotopic and trace element characteristics which allow the microgranite magma to be the source from which the rhyolites fractionated the microgranite itself requires a monzonite or syenite source the subsidiary tuleih complex was emplaced at 465 14 ma and is largely composed of a silica saturated marginally undersaturated syenite trace elements and isotopes from the syenite indicate a garnet eclogite source of mantle origin the sabaloka intrusions provide an example of several periods of magmatism contributing to the same igneous complex although not necessarily to the same igneous centre

the probable occurrence of interstitial al in hydrous f bearing and f free aluminosilicate melts experiments carried out on the system sio2 naalsi3o8 kalsi3o8 qz ab or at 1 kbar in the presence of h2o and f show that the quartz alkali feldspar field boundary is progressively displaced towards the feldspar join as f contents increase from 0 to 4 wt f increasing p h 2 o in the absence of f has already been shown to have a similar effect tuttle and bowen 1958 luth jahns and tuttle 1964 the increased size of the quartz field in the f bearing system compared to the hydrous system is believed to be caused by progressive removal of al from the tetrahedral network of the melt by complexing with f the residual network in the melt is thus enriched in si and this stabilizes precipitation of quartz rather than feldspar for certain bulk compositions the common presence of quench cryolite na3alf6 in certain experiments carried out with 4 wt f supports this interpretation and indicates that some al in the melt may be present in six fold coordination with f the effect of h2o in the absence of f may be similar with al being progressively removed from four fold coordination as more h2o is dissolved in the melt although a proportion of al in hydrous melts may occur in six fold coordination dry melts predominantly contain al in four fold coordination this major difference in al complexing may be one of the main causes for differences in the high pressure phase relations of wet and dry albearing silicate systems

the rb sr geochronological evolution of the ras gharib segment of the northern nubian shield rb sr whole rock isochron ages have been determined for six igneous rock suites from the ras gharib these represent all major crustal components of the northern nubian shield and are from oldest to youngest diorite tonalite extrusive rocks granodiorite adamellite and leucogranite muscovite trondhjemite orogenic pan african suites dyke swarms and anorogenic peralkaline granites the ages obtained are 881 58 ma 620 16 ma 552 7 ma 516 7 ma 493 7 ma and 476 2 ma the initial 87 sr 86 sr ratios of the pan african suites and the dykes are in the range 0 7039 to 0 7047 the initial ratio of the anorogenic peralkaline granites is 0 7110 0 0012 higher than any previously reported for igneous rocks from the egyptian belt the 493 7 ma age of the dyke swarms represents a younger limit for the pan african compression accretion event and an upper older limit for an extensional tectonic regime linked with this is the anorogenic peralkaline granite magmatism 476 2 ma which is the last manifestation of igneous activity in this region the low initial ratios of the pan african suites provide no support for the existence of an older archaen sialic continental crust in the region the anorogenic peralkaline granite with the high initial ratio was produced by partial melting of earlier pan african suites the age data are used to construct a tentative correlation scheme for shield rocks of egypt sudan and saudi arabia

paleomagnetism and mesozoic cenozoic paleocontinental maps a method of making paleocontinental reconstructions based on ocean floor magnetic anomaly data and land based paleomagnetic data is briefly described and its limitations are assessed synthetic polar wander paths for the period 20 200 ma are presented for australia antarctica india africa eurasia north america and south america comparison is made with observed paths except for antarctica for which the data are too sparse there are no significant differences between observed and synthetic paths a plot of inclination anomaly against paleolatitude shows a large scatter that limits the precision of the paleogeographic grid on continental reassemblies the absence of any systematic trend to this scatter with time suggests that the field has had the same average behavior over the past 180 ma and that large scale expansion of the earth cannot have occurred in this period south america has undergone the least change in position relative to the poles in the past 160 ma

geochronology geochemistry and tectonics of the ne bayuda desert n sudan implications for the western margin of the late proterozoic fold belt of ne africa abstract five suites of rocks collected from the precambrian basement in the ne bayuda desert of central northern sudan give late proterozoic whole rock rb sr isochron ages the abu harik complex thought by some previous workers to be an older basement gives an age of 898 51 ma upper amphibolite facies metasediments give a metamorphic age of 761 22 ma the supposedly younger greenschist facies el koro volcanic series were erupted 800 83 ma ago these are chemically similar to the volcanics which unconformably overlie the sol hamed ophiolite in the red sea hills of ne sudan and to some modern island are volcanics the metasediments were intruded 678 43 ma ago by the diefallab granite which is itself deformed the younger weakly deformed amaki series with a basal conglomerate containing basement clasts overlain by purple grits is probably equivalent to the molasse type hammamat group of the eastern desert of egypt which was deposited between 616 and 596 ma ago finally the post tectonic shallal granite with within plate geochemistry was intruded 549 12 ma ago geochemical data suggest that the abu harik complex the el koro volcanic series and the diefallab granite are arc related magmatic rocks they were intruded into or thrust onto shallow water shelf sediments during subduction and then collision between c 900 and 550 ma the data presented here give no support to previous views that the high grade metasediments were metamorphosed prior to the late proterozoic events that they unconformably overlie a still older perhaps archaean basement or that they are unconformably overlain by younger late proterozoic low grade volcanics the precambrian rocks along the e side of the bayuda desert must now all be assigned to the late proterozoic and the boundary between this late proterozoic fold belt and an older craton known to crop out at jebel uweinat must lie farther to the w

tectonic environment and geodynamic significance of the neo cimmerian donqiao ophiolite bangong nujiang suture zone tibet the bangong nujiang suture zone is underlain by a discontinuous and roughly ew ophiolite belt around donqiao gyanco ophiolitic and metasedimentary palaeozoic series form nappes thrusted over jurassic flysch series all these series have undergone a late symmetrical folding accompanied by generally northward reverse faulting although strongly dismembered a complete ophiolitic sequence comprising harzburgites with large podiform type chromite deposits wehrlites cumulate gabbros sheeted dolerite dykes foliated garnet bearing amphibolites and pillow lavas has been found

fluorine geochemistry of basaltic rocks from continental and oceanic regions and petrogenetic application fluorine contents in about 300 samples of various types of basalts and related rocks from continental southwestern u s a zaire deccan and south africa and oceanic regions hawaii and mid atlantic ridge between 23 n and 40 n were determined by a selective ion electrode method

geochemistry and petrogenesis of a nepheline syenite carbonatite complex from the sudan jabal dumbeir is a nepheline syenite carbonatite complex of cambrian age 550 87 ma the earliest igneous event of the complex was the intrusion of a phlogopite sodalite nepheline syenite ditroite which was derived from the partial melting of the upper mantle k co 2 and f rich volatiles metasomatise both the ditroite and the gneissic country rock which results in an orthoclase rich undersaturated syenite orthoclasite carbonatitic breccias and sovite dykes were subsequently emplaced carrying high levels of f ree y and th with extremely high lree hree ratios ce n yb n 64 8 such carbonatiticmagmas may be derived by partial melting of an lree enriched upper mantle hydrothermal emplacement of fluorite quartz veins represent the final phase of igneous activity and these carry high levels of ree y th and u their emplacement was strongly controlled by a system of nne strike slip faults which have been reactivated in recent times

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

two phases of cretaceous tertiary magmatism in the eastern desert of egypt paleomagnetic chemical and k ar evidence abstract paleomagnetic and radiometric data confirm the existence of two phases of igneous activity in the late mesozoic and cenozoic history of eastern egypt in the mid to late cretaceous the earlier and more extensive episode produced the wadi natash alkaline basalts and numerous small trachytic intrusions the ring complexes of abu khrug and gebel el kahfa were also emplaced at this time the mean paleopole position for the intrusive rocks is at 65 3 n 111 2 w a 95 11 4 31 sites chemical analyses of the trachytes show that they are enriched in nb zr and y and depleted in sr ba cr and ni suggesting either prolonged fractionation or limited partial fusion the second phase of activity during oligo miocene time is represented by small tholeiitic basalt flows on the red sea coast and along the nile valley the mean paleopole position for the flows is at 68 2 n 101 5 e a 95 27 8 3 sites the wide scatter is interpreted as reflecting secular variation the chronological and chemical data are used with studies of sedimentary formations coeval with the igneous rocks to apply a general model of continental rifting to the northern red sea the earlier alkaline phase of igneous activity is related to regional uplift while the second tholeiitic phase is associated with widening of the rift valley

the geology and geochemistry of upper proterozoic granitoids from the red sea hills sudan granitoid plutons are common throughout the late proterozoic basement complex of north east sudan frequently accounting for 60 of the outcrop geological petrological and isotopic studies indicate that two distinct types of granitic plutonism can be recognized an older diorite to granodiorite assemblage was emplaced between c 815 and 724 ma and a younger predominantly alkali feldspar granite assemblage consisting of numerous discrete high level intrusions was emplaced between c 717 and 555 ma these granitoids have a calc alkaline character similar to the volcanic rocks of the basement complex to which they are spatially and temporally related the geochemical and isotopic data suggest a destructive plate margin rather than a within plate palaeotectonic association it is suggested that the granitoids represent an evolutionary trend from early diorite to granodiorite intrusions emplaced above a late proterozoic subduction zone to later post tectonic intrusions emplaced during a late stage of volcanic arc development the magmas of the early granitoids were probably mantle derived while the later post tectonic intrusions may have been derived from magmas produced by partial refusion of the volcanic arc protolith

hotspots in the southern oceans an absolute frame of reference for motion of the gondwana continents abstract the geometry and geochronology of aseismic ridges and oceanic islands in the southern oceans provide a good test of the proposition that hotspots remain fixed over long periods of time that is motion of an order of magnitude less than the relative motion between plate pairs in most cases it is concluded that inter hotspot movement cannot be discerned for the period 100 m y to present and that widely distributed hotspots in the atlantic and indian oceans provide a frame of reference for plate motions following the disintegration of gondwanaland which is independent of paleomagnetism this frame of reference is absolute in that it gives the motion of the lithosphere with respect to the mantle hotspots the absolute motion model indicates that africa and antarctica are now moving only very slowly that there has been significant relative movement between east and west antarctica since the cretaceous and prescribes the relative motion between the somali and african plates

old model nd ages in namibian pan african rocks the nd isotope analytical technique is a powerful tool for studying many geological processes particularly the evolution of the upper mantle as seen through the isotope and trace element geochemistry of mantle derived volcanic rocks1 3 and magmatic processes along destructive plate margins4 5 however previous studies of continental crust6 8 have been largely restricted to archaean examples and concerned primarily with dating their time of derivation from the upper mantle in the present study we investigated the nd and sr isotope characteristics of rocks involved in a relatively young 800 450 myr orogenic event such events represent critical stages in the evolution of most of the earth s continental crust new material is added from the mantle and pre existing crust is remobilized by erosion and sedimentation deformation and magmatic activity nd and sr isotopes were used to outline the range in age and geochemical characteristics of rock sequences and provinces within both the upper mantle and the pre existing crust which were sampled during the orogeny and hence provided the major components of what is now a stable segment of continental crust

the geological evolution of the tibetan plateau the geological evolution of the tibetan plateau is best viewed in a context broader than the india eurasia collision zone after collision about 50 million years ago crust was shortened in western and central tibet while large fragments of lithosphere moved from the collision zone toward areas of trench rollback in the western pacific and indonesia cessation of rapid pacific trench migration 15 to 20 million years ago coincided with a slowing of fragment extrusion beyond the plateau and probably contributed to the onset of rapid surface uplift and crustal thickening in eastern tibet the latter appear to result from rapid eastward flow of the deep crust probably within crustal channels imaged seismically beneath eastern tibet these events mark a transition to the modern structural system that currently accommodates deformation within tibet

significance of contrasting magmatism in north east africa and saudi arabia the red sea is a north south rift that split the arabian nubian shield into eastern arabian and western egypt and sudan segments by seafloor spreading less than 30 myr ago1 because geological studies in the region have generally have confined by national frontiers a comparative study of magmatic evolution on either side of the red sea has not so far as we are aware previously been undertaken and current models for arabian nubian shield evolution suggest broadly similar histories for the basement on both sides of the red sea however although the igneous rock geochronology on both sides of the red sea indicate that precambrian magmatic histories were broadly similar since then they have been quite distinct in that the african crust has been invaded episodically by quartz syenites nepheline syenites and peralkaline granites between 500 and 80 myr no such igneous activity is evident in arabia fig 1 we propose here that the red sea line has had structural significance from at least the end of the cambrian though whether it defines a line of contrasting upper mantle or lower crust geochemistry or of contrasting crustal rigidity cannot yet be established

tectonics and structural zonation of southern tibet china the yarlung zangbo suture zone is composed of several thrust sheets with distinct stratigraphies states of deformation and metamorphic grades successive deformations have been produced by persistent north south shortening since obduction of the ophiolites by late cretaceous palaeocene times a possible decoupling of the crust may be associated with the intracontinental thrust system which has formed in the subducted indian plate this article emphasizes the role of shear deformations in the building of a collision belt

zircon u pb hf isotopes and whole rock geochemistry of gneissic granites from the jitang complex in leiwuqi area eastern tibet china record of the closure of the paleo tethys ocean abstract previous investigations have indicated that the changning menglian cmsz and longmu co shuanghu lssz suture zones represent the paleo tethys ocean of northern tibet and western yunnan of china respectively however our understanding of the paleo tethys suture zone in eastern tibet seems to be incomplete the north lancangjiang suture zone nlsz has been hypothesized to mark the paleo tethys ocean in eastern tibet the present study reports new zircon u pb ages and hf isotope data and whole rock major and trace element data from the gneissic granites from the jitang complex in the leiwuqi area of the nlsz in order to verify this hypothesis six samples of gneissic granites yielded weighted mean 206 pb 238 u ages of 249 237 ma the geochemical data show that the gneissic granites are characterized by high sio 2 70 79 77 75 wt k 2 o 2 00 6 13 wt low a cnk values hf t values from 2 7 to 2 5 and t c dm model ages 1447 1009 ma indicate that they were generated by the partial melting of the meso proterozoic juvenile crust these gneissic granites were interpreted to have formed in the post collisional tectonic setting related to the closure of the paleo tethys ocean by combining previously published and the new data we propose that the nlsz connects the lssz and cmsz in the tibetan plateau and marks a paleo tethys ocean basin in eastern tibet

geochemistry and petrogenesis of el gezira ring complex egypt a monzosyenite cumulate derived from fractional crystallization of trachyandesitic magma abstract el gezira ring complex grc is a high level permo triassic intrusion related to the alkaline province of southern egypt it comprises an older outer ring of alkali basalt flows and agglomerates and an inner ring of monzosyenite crescent shaped bodies of alkali gabbros separate the two rings the core of the complex is dominated by monzosyenite and volcanic flows which range in composition from trachyandesite to trachydacite the monzosyenite constitutes a major part of the complex and rarely grades to mafic rich segregations mg rich augites are the dominant mafic minerals in most rock types of grc olivine is common in basic rocks whereas secondary amphiboles and annitic biotite are more abundant in intermediate felsic rocks the rocks of grc are metaluminous and exhibit geochemical features of a type anorogenic magmatism such as high fe mg ratio total alkalis nb zr and y contents with concomitant low values of ca mg and sr the basic rocks have flat primitive ree patterns with no eu anomalies suggesting that they probably represent the parental basaltic melt from which the complex was evolved the trachyandesite trachydacite and the monzosyenite all have steep lree enriched patterns with no enrichment in hree pronounced positive eu anomalies eu eu 1 83 3 50 are characteristic for the monzosyenites on which together with remarkably high ba contents indicate the role of feldspar accumulation the trachyandesite and trachydacite exhibit large negative eu anomalies eu eu 0 45 0 75 0 72 indicating the dominant separation of feldspars field and geochemical features of the complex supported by mass balance calculations of major elements and ree modelling suggest two distinct stages of fractional crystallization in the evolution of the grc the first stage includes crystal melt fractionation dominated by clinopyroxene calcic plagioclase and olivine from the basaltic source to yield the trachyandesitic magma further fractional crystallization of the trachyandesitic melt with the alkali feldspar as dominant separating phase and minor contribution of clinopyroxene biotite fe ti oxides and apatite yielded the more evolved felsic trachydacite stage ii the cumulus mineralogy and its modelled ree abundances of stage ii closely match the modal and ree compositions of the monzosyenite which reinforces the cumulate origin of the rock

the mount gharib a type granite nubian shield petrogenesis and role of metasomatism at the source the mount gharib peralkaline a type complex 476 2 ma located in the nubian shield of egypt contains sodic calcic to sodic amphiboles accessory astrophyllite zircon fluorite apatite allanite aenigmatite elpidite and ilmenite this within plate hypersolvus suite is enriched in large ion lithophile lil and high field strength hfs elements and characterized by a fractionated ree pattern ce yb 49 and a significant negative eu anomaly a fine grained acicular amphibole bearing roof facies shows further enrichment in the lil and hfs elements the suite was emplaced in a pan african granodiorite adamellite host which it locally metasomatized the affected rocks contain hydrothermal albite end member arfvedsonite astrophyllite and levels of the lil and hfs elements intermediate between those in the peralkaline granite and the roof facies trace element and isotopic modeling of this a type granite with its high initial 87sr 86sr value 0 7110 documents an active role of the lithosphere in magma generation lithospheric extension expressed by regional dyke swarms was caused by cooling fracturing and relaxation of the thin newly formed pan african crust localized partial melting took place in an open system possibly as a result of an influx of alkali rich fluid derived from a sublithospheric source metasomatic reactions similar to those observed in the metasomatized wallrocks are considered to have played an important role just prior to the onset of anatexis and generation of the a type melt

late pan african tectonics marking the transition from subduction related calc alkaline magmatism to within plate alkaline granitoids adrar des iforas mali boullier a m liegeois j p black r fabre j sauvage m and bertrand j m 1986 late pan african tectonics marking the transition from subduction related talc alkaline magmatism to within plate alkaline granitoids adrar des iforas mali in b johnson and a w bally editors intraplate deformation characteristics processes and causes tectonophysics 132 233 246 the pan african trans saharan belt resulting from oceanic closure and oblique collision between the west african craton and the tuareg nigerian shields around 600 ma ago displays in the iforas a rapid switch from subduction and collision related talc alkaline magmatism to typical a type granitoids 560 540 ma the transition is clearly seen in associated spectacular acid dyke swarms early talc alkaline e w trending sets being cut by alkaline n s trending arrays isotopic evidence shows that the change in chemistry involves a different more primitive mantle source structural observations indicate that the transition from talc alkaline to alkaline magmatism is related to reversals in the stress field during and after collision in the model proposed the first major reversal from d with ei ca 13s0n lloon to ds with ui ca 50 n 60 n is thought to have been responsible for the slicing of the subducted plate thereby allowing the rise to shallow depth beneath the continental lithosphere of asthenospheric mantle believed to be the source of the alkaline magmatism the tapping of this new source occurred between ds and d4 events contemporaneous with a rotation of the maximum compressive stress back from 60 n to 105 n

late ediacaran 605 580 ma post collisional alkaline magmatism in the arabian nubian shield a case study of serbal ring shaped intrusion southern sinai egypt abstract the serbal pluton is a late neoproterozoic 605 580 ma post collisional a type granites in southern sinai egypt northernmost arabian nubian shield ans it is characterized by discontinuous ring shaped outcrops dislocated by later faulting the pluton intrudes late neoproterozoic metamorphic and high k calc alkaline rocks the serbal pluton mostly comprises an outer zone of alkali feldspar granite surrounding a core of peralkaline granite gradational and sharp contacts in the serbal granites suggest that they were emplaced with a very short time interval still before complete crystallization of the earlier batch serbal granites are highly evolved 75 98 78 52 wt sio2 and display the typical geochemical characteristics of post collisional a type granites namely high sio2 na2o k2o feo mgo ga al zr nb ga and y and low cao mgo ba and sr they are rich in ree and have extreme eu negative anomalies eu eu 0 01 0 23 the chemical characteristics indicate that the peralkaline granite shares many features of granites with the tetrad ree effect the serbal pluton evolved through fractional crystallization of a parental magma derived through partial melting of a juvenile crustal protolith that had been extracted from a source having mantle geochemical and isotopic characteristics the crystallization temperatures using fe ti oxides of the serbal granites point to their formation at high temperatures up to 650 850 c at a shallow depth of emplacement