one hundred million years of geomagnetic polarity history since 1968 absolute ages have been assigned to the late cretaceous cenozoic geomagnetic reversal time scale by fixing the ages of two or more calibration points in a composite marine magnetic anomaly profile and interpolating between or extrapolating beyond these points assuming constant spreading rates in each interval previously no more than 4 calibration points were used but it is now possible to specify 11 calibration points in addition to the 0 m y datum this improvement is based on magnetostratigraphic studies in italian pelagic limestones these studies closely tie the geomagnetic reversal sequence to the foraminiferal and coccolith zonations absolute ages of calibration points are provided by the best available dates on stage boundaries which are located from the biostratigraphic zonation the greatest changes from previous scales come in the late paleocene early eocene where the new ages are as much as 3 m y younger than in the 1977 scale of labrecque and others and as much as 1 8 m y younger than in the 1980 scale of ness and others

geochemistry and petrogenesis of intrusive and extrusive ophiolitic plagiogranites central anatolian crystalline complex turkey plagiogranites associated with the sarikaraman ophiolite of the central anatolian crystalline complex turkey closely resemble other plagiogranites from supra subduction zone type ophiolites of neotethys the ophiolite is remarkable in displaying a higher proportion of the plagiogranite suite ca 10 by volume than is usually associated with such bodies the sarikaraman plagiogranites are represented by intrusive sheets and netvein trondhjemites largely developed at the top of the upper gabbros and as multiphase dykes within the sheeted dyke complex the plagiogranite dykes are considered to feed extrusive silicified rhyolites associated with the basaltic lavas in the volcanic section of the ophiolite field relations suggest that the trondhjemites were probably generated from the roof section of a dynamic and evolving gabbroic magma chamber both the deep seated trondhjemites and the volcanic rhyolites constitute the sarikaraman plagiogranite suite geochemically there is complete overlap between the intrusive trondhjemites and extrusive rhyolites which are characterised by morb normalized low hfs element contents with small negative nb ta anomalies and variably enhanced lil element abundances unlike other plagiogranites however the sarikaraman suite is not characterized by consistently low k2o contents a feature that reflects the variable mobilization of the lil elements under lower greenschist facies conditions the ree are uniformly enriched relative to the basic components of the complex but have similar normalized patterns exhibiting mild light ree depletion in terms of their origin the initial or most primitive plagiogranite melts could have been generated by either fractional crystallization 70 85 of clinopyroxene feldspar amphibole or partial melting 5 15 batch melting of a gabbroic source material although only the first process can produce most of the range of the plagiogranite compositions as a group the plagiogranites exhibit some degree of internal variation which can be generated by further fractionation largely dominated by feldspar with minor apatite and amphibole

a vestige of earth s oldest ophiolite a sheeted dike complex within the 3 8 billion year old isua supracrustal belt isb in southwest greenland provides the oldest evidence of oceanic crustal accretion by spreading the geochemistry of the dikes and associated pillow lavas demonstrates an intraoceanic island arc and mid ocean ridge like setting and their oxygen isotopes suggest a hydrothermal ocean floor type metamorphism the pillows and dikes are associated with gabbroic and ultramafic rocks that together make up an ophiolitic association the paleoarchean isua ophiolite complex these sheeted dikes offer evidence for remnants of oceanic crust formed by sea floor spreading of the earliest intact rocks on earth

an exotic mesoarchean microcontinent the coorg block southern india abstract sandwiched between the dharwar craton in the north and the neoarchean proterozoic crustal blocks to the south the coorg block in southern india is composed dominantly of a suite of arc magmatic rocks including charnockites ttg tonalite trondhjemite granodiorite related granitoid suite and felsic volcanic tuffs together with minor accreted oceanic remnants along the periphery of the block coeval mafic and felsic magmatism with magma mixing and mingling in an arc setting is well represented in the block here we present the petrology geochemistry zircon u pb geochronology and lu hf isotopes of all the major lithologies from this block computation of metamorphic p t conditions from mineral chemical data shows consistent granulite facies p t conditions of 820 870 c and up to 6 kbar our geochemical data from major trace and ree on representative samples of the dominant rock types from the coorg block corroborate an arc related signature with magma generation in a convergent margin setting the zircon data yield weighted mean 207pb 206pb ages of 3153 4 9 to 3184 0 5 5 ma for syenogranites 3170 3 6 8 ma for biotite granite 3275 5 1 ma for trondhjemite 3133 12 to 3163 8 6 9 ma for charnockites 3156 10 to 3158 3 8 2 for mafic enclaves 3161 16 ma for diorite and 3173 16 ma for felsic volcanic tuff an upper intercept age of 3363 59 ma and a lower intercept age of 2896 130 ma on zircons from a charnockite as well as an evaluation of the th u values of the zircon domains against respective 207pb 206pb ages suggest that the mesoarchean magma emplacement which probably ranged from 3 3 to 3 1 ga was immediately followed by metamorphism at ca 3 0 to 2 9 ga the ages of magmatic zircons from the charnockites and their mafic granulite enclaves as well as those from the volcanic tuff and biotite granite are all remarkably consistent and concordant marking ca 3 1 ga as the peak of subduction related crust building in this block within the tectonic milieu of an active convergent margin the majority of zircons from the coorg rocks show hf isotope features typical of crystallization from magmas derived from juvenile sources their hf crustal model ages suggest that the crust building might have also involved partial recycling of basement rocks as old as ca 3 8 ga the crustal blocks in the southern granulite terrane in india preserve strong imprints of major tectonothermal events at 2 5 ga 2 0 ga 0 8 ga and 0 55 ga associated with various subduction accretion collision or rifting events however the coorg block is exceptional with our data suggesting that none of the above events affected this block importantly there is also no record in the coorg block for the 2 5 ga pervasive regional metamorphism that affected all the other blocks in this region the geochronological data raise the intriguing possibility that this block is an exotic entity within the dominantly neoarchean collage in the northern domain of the southern granulite terrane of india the mesoarchean arc related rocks in the coorg block suggest that the magma factories and their tectonic architecture in the early earth were not markedly different from those associated with the modern style plate tectonics

birth death and resurrection the life cycle of suprasubduction zone ophiolites 1 suprasubduction zone ssz ophiolites display a consistent sequence of events during their formation and evolution that suggests that they form in response to processes that are common to all such ophiolites this sequence includes the following 1 birth which entails the formation of the ophiolite above a nascent or reconfigured subduction zone this stage is typically characterized by the eruption of arc tholeiite lavas and the formation of layered gabbros and sheeted dike complex 2 youth during which is continued melting of refractory asthenosphere depleted during birth occurs in response to fluid flux from the subducting slab with extensional deformation of the older plutonic suite eruption of refractory lavas and the intrusion of wehrlite pyroxenite 3 maturity with the onset of semistable arc volcanism typically calc alkaline as the subduction zone matures and stabilizes and the intrusion of quartz diorite and eruption of silicic lavas and 4 death which is the sudden demise of active spreading and ophiolite related volcanism which in many cases is linked to collision with an active spreading center and the onset of shallow underthrusting of the buoyant spreading axis expressed as dikes and lavas with oceanic basalt compositions that crosscut or overlie rocks of the older suites 5 resurrection with emplacement by obduction onto a passive margin or accretionary uplift with continued subduction the early stages 1 3 may be diachronous and each stage may overlap in both time and space the existence of this consistent progression implies that ophiolite formation is not a stochastic event but is a natural consequence of the ssz tectonic setting

petrology and petrogenesis of cumulate peridotites and gabbros from the marum ophiolite complex northern papua new guinea

comparisons between tethyan anorthosite bearing ophiolites and archean anorthosite bearing layered intrusions implications for archean geodynamic processes

near station topographic masses correction for high accuracy gravimetric prospecting the expanding role of gravity prospecting in mineral and hydrocarbon exploration as well as seismic and volcanic risk studies will be related to its ability to provide high resolution anomalies to achieve this goal it is necessary to consider the errors resulting from the topographic corrections in particular near station effects such errors are relevant not only for severe topographies but also for relatively flat surfaces involving microgravity applications and large scale surveys indeed the errors introduced by low resolution digital elevation models can be of the same order of magnitude as the anomalies of interest this basic fact is demonstrated by tests on synthetic models the results of this analysis are summarized in an intuitive graph that can be used to estimate what near station topographic resolution is necessary for a specific survey for the vulcano island test site in southern italy we also compare results obtained using three different representations of the topographic surface with different horizontal resolutions and vertical precisions one from existing photogrammetric data the second from a digitized map and the third from a very high resolution laser scanning system among the three methods laser scanning is shown to obtain the highest resolution topographic reconstructions in the shortest time some weaknesses of the laser scanning methodology are discussed and we suggest ways of overcoming them

constrictive deformation in transpressional regime field evidence from the cauvery shear zone southern granulite terrain india abstract the paper describes a typical field example of rarely preserved complex structural patterns related to constrictional strain from the deeply eroded central part of the neoproterozoic crustal scale cauvery shear zone system southern india the area exposes intense migmatisation and a distinctive map pattern of a broad domal structure with complex strain patterns the field features such as interfering fold structures extreme hinge line variation domes and basins highly asymmetrical and amoeboid shape structures geometry of foliation trajectories and radial stretching lineations could be related to a single phase of d2 m2 constrictive deformation our observations suggest a close spatial and temporal relationship between transpressive deformation deep crustal melting assent of granite magmas and migmatisation synchronous with d2 m2

neoarchean continental growth through arc magmatism in the nilgiri block southern india the formation and growth of continental crust in the archean have been evaluated through models of subduction accretion and mantle plume the nilgiri block in southern india exposes exhumed neoarchean lower crust uplifted to heights of 2500 m above sea level along the north western margin of the peninsula major lithologies in this block include charnockite with or without garnet anorthosite gabbro suite pyroxenite amphibolite and hornblende biotite gneiss ttg all these rock types are closely associated as an arc magmatic suite with diffuse boundaries and coeval nature the charnockite and hornblende biotite gneisses ttg show sio2 content varying from 64 to 73 wt the hornblende biotite gneisses ttg are high al type with al2o3 15 wt whereas the charnockites show al2o3 15 wt the composition of charnockite is mainly magnesian and calcic to calc alkaline the mafic ultramafic rocks show composition close to that of tholeiitic series the low values of k 2 o 3 wt k rb k2o 500 zr ti and trace element ratios like la yb n sr y y nb y nb rb y ta rb yb ta indicate a volcanic arc signature for these rocks the geochemical signature is consistent with arc magmatic rocks generated through oceanic plate subduction the primitive mantle normalized trace element patterns of these rocks display enrichment in large ion lithophile elements lile and comparable high field strength elements hfse in charnockite and hornblende biotite gneisses ttg consistent with subduction related origin primitive mantle normalized ree pattern displays an enrichment in lree in the chamockite and hornblende biotite gneisses ttg as compared to a flat pattern for the mafic rocks the chondrite normalized ree patterns of zircons of all the rock types reveal cores with high hree formed at ca 2700 ma and rims with low hree formed at 2500 2450 ma log transformed la th nb th sm th yb th discrimination diagram for the mafic and ultramafic rocks from nilgiri displays a transition from mid oceanic ridge basalt morb to island arc basalt iab suggesting a morb source the u pb zircon data from the charnockites mafic granulites and hornblende biotite gneisses ttg presented in our study show that the magma generation during subduction and accretion events in this block occurred at 2700 2500 ma together with the recent report on neoarchean supra subduction zone ophiolite suite at its southern margin the nilgiri block provides one of the best examples for continental growth through vertical stacking and lateral accretion in a subduction environment during the neoarchean c 2014 elsevier b v all rights reserved

plagiogranite from the andaman ophiolite belt bay of bengal india plagiogranites occur on the eastern margin of the southern part of south andaman in the outer sedimentary arc of the andaman nicobar islands a part of the sunda burmese double chain arc system in the bay of bengal they intrude gabbros that are associated with pillow basalt east coast volcanic rocks basalt basaltic andesite to acid differentiates radiolarian cherts conglomerate and grit the plagiogranite characterized by the occurrence of interstitial vermicular and micrographic intergrowths of quartz and plagioclase shows some geochemical similarities with ophiolitic plagiogranites but rb yb nb ta and y abundances in the andaman plagiogranite are characteristic of volcanic arc granites rather than the ocean ridge granites it is suggested that the andaman plagiogranite has been generated by low pressure crystal fractionation of a low k tholeiitic magma derived from a mantle source in a supra subduction zone tectonic setting further it is suggested that this plagiogranite intruded the gabbro of the andaman ophiolite probably during mid eocene times as the andaman ophiolitic rocks late cretaceous to early eocene are believed to have been obducted onto the leading edge of the eurasian continent during the mid eocene to late oligocene subduction event prior to the currently active andaman java subduction system that was initiated during the late miocene

age and tectonic evolution of neoproterozoic ductile shear zones in the southern granulite terrain of india with implications for gondwana studies 1 the high grade rocks of the southern granulite terrain sgt of peninsular india are bounded to the north by the archean dharwar craton another high grade terrane the mesoproterozoic eastern ghats occurs to the northeast of the sgt the tectonic relationship between these crustal domains is complex we present new geochronological and structural data that indicate a continuation of the dharwar craton into the southern granulite terrain as far south as a newly identified neoproterozoic shear zone here named the karur kamban painavu trichur shear zone kkptsz south of the kkptsz mesoproterozoic dates of the sgt are similar to those recorded in the eastern ghats and the two domains may have been conterminous thirty three new u pb th single zircon and monazite dates of samples from six structural transects across the regional shear zones indicate that the sgt has experienced at least seven thermo tectonic events at 2 5 ga 2 0 ga 1 6 ga 1 0 ga 800 ma 600 ma and 550 ma and two distinct episodes of metasomatism charnockitization between 2 50 2 53 and between 0 55 0 53 ga deformation along a number of major shear zones in the sgt is neoproterozoic to earliest paleozoic in age with an early phase d2 concentrated between 700 800 ma and a later phase d3 between 550 and 600 ma major charnockitization 530 550 ma post dates d3 and is in turn overprinted by granitization retrogression and uplift between 525 and 480 ma the kkptsz active between 560 and 570 ma is either a terrane boundary or a tectonized decollement between cover and archean basement rocks represented by predominantly paragneisses to the south and orthogneisses to the north respectively other regional neoproterozoic shear zones do not appear to separate allochthonous terranes as previously suggested on the basis of nd model ages and rb sr biotite whole rock dates the neoproterozoic cambrian tectonothermal history of the sgt and eastern ghats is similar to that recorded in parts of madagascar east africa and antarctica and is used to reconstruct parts of central gondwana here named the deccan continent with more robust confidence

gte a new software for gravitational terrain effect computation theory and performances the computation of the vertical attraction due to the topographic masses the so called terrain correction is a fundamental step in geodetic and geophysical applications it is required in high precision geoid estimation by means of the remove restore technique and it is used to isolate the gravitational effect of anomalous masses in geophysical exploration the increasing resolution of recently developed digital terrain models the increasing number of observation points due to extensive use of airborne gravimetry in geophysical exploration and the increasing accuracy of gravity data represents nowadays major issues for the terrain correction computation classical methods such as prism or point masses approximations are indeed too slow while fourier based techniques are usually too approximate for the required accuracy in this work a new software called gravity terrain effects gte developed to guarantee high accuracy and fast computation of terrain corrections is presented gte has been thought expressly for geophysical applications allowing the computation not only of the effect of topographic and bathymetric masses but also those due to sedimentary layers or to the earth crust mantle discontinuity the so called moho in the present contribution after recalling the main classical algorithms for the computation of the terrain correction we summarize the basic theory of the software and its practical implementation some tests to prove its performances are also described showing gte capability to compute high accurate terrain corrections in a very short time results obtained for a real airborne survey with gte ranges between few hours and few minutes according to the gte profile used with differences with respect to both planar and spherical computations performed by prism and tesseroid respectively of the order of 0 02 mgal even when using fastest profiles

stratigraphic and geochemical evidence for the depositional environment of the early archaean isua supracrustal belt southern west greenland the highly deformed c 3800 ma isua supracrustal belt is a fragment of a more extensive early archaean sedimentary and volcanic succession intruded by and tectonically intercalated with tonalitic and granitic amftsoq gneisses in the period 3800 3600 ma the supracrustal rocks recrystallised under amphibolite facies conditions between 3800 and 3600 ma in the late archaean and locally at c 1800 ma layered sequences of rock of sedimentary and probable volcanic origin form over 50 of the belt bodies of high mg al basic rocks and ultramafic rocks were intruded into the layered sequences prior to isoclinal folding and intrusion of amitsoq gneisses the layered rocks which are 1 km thick are divided into two sequences that are in faulted contact with each other the way up of these sequences has been determined from facing directions of locally preserved graded layering in felsic metasediments at several localities the overall upwards change in sedimentary succession is interpreted as showing change from dominantly basic to dominantly felsic volcanism which provided the major clastic component of the sediments clastic sedimentation took place against a background of chemical sedimentation shown by interlayers of banded iron formation metachert and calc silicate rocks throughout the sequences the felsic rocks locally preserve graded bedding and possible conglomerate structures indicating deposition from turbidite flows and possibly as debris flows nodules in the felsic rocks contain structures interpreted as fiamme there is an irregular enrichment in k2o na2o in many of the felsic rocks at constant sio2 and al2o3 content interpreted as owing to alteration of original andesitic to dacitic volcanic rocks banded iron formations locally contain conglomeratic structures suggesting sedimentary reworking possibly under shallow water conditions lithological and geochemical characters of the clastic components of the supracrustal sequences are consistent with derivation from felsic and basic volcanic rocks and do not require a continental source

the volcanic stratigraphy and petrogenesis of the oman ophiolite complex the volcanic stratigraphy and trace element geochemistry of the oman ophiolite complex indicate a multistage magmatic origin comprising 1 magmatism due to sea floor spreading in a marginal basin 2 magmatism associated with discrete submarine volcanic centres or seamounts 3 magmatism associated with crustal uplift and rifting and 4 magmatism associated with continent arc collision trace element petrogenetic modelling is used to investigate the nature of the mantle source region and the partial melting and fractional crystallization history for each magmatic event the petrogenetic pathway for the sea floor spreading lavas requires a high degree of melting of a mantle that was depleted in incompatible elements prior to subduction but subsequently selectively enriched in certain elements mostly lil elements and h2o from an underlying subduction zone it also requires magma mixing in an open system magma chamber prior to eruption the seamount lavas were probably derived by a similar degree of partial melting of a similar source but fractional crystallization was restricted to smaller high level magma chambers the rifting lavas were derived from a mantle source that was more depleted than the seamount lavas prior to subduction but which was later modified by a larger subduction zone component the syn collision lavas were however derived from an enriched mantle source which probably underlay the passive continental margin rather than the marginal basin complex results such as these may provide considerable insight into the petrogenetic changes accompanying the transitions from spreading to arc volcanism in a supra subduction zone setting

ophiolite genesis and global tectonics geochemical and tectonic fingerprinting of ancient oceanic lithosphere ophiolites and discussions on their origin and significance in earth s history have been instrumental in the formulation testing and establishment of hypotheses and theories in earth sciences the definition tectonic origin and emplacement mechanisms of ophiolites have been the subject of a dynamic and continually evolving concept since the nineteenth century here we present a review of these ideas as well as a new classification of ophiolites incorporating the diversity in their structural architecture and geochemical signatures that results from variations in petrological geochemical and tectonic processes during formation in different geodynamic settings we define ophiolites as suites of temporally and spatially associated ultramafic to felsic rocks related to separate melting episodes and processes of magmatic differentiation in particular tectonic environments their geochemical characteristics internal structure and thickness vary with spreading rate proximity to plumes or trenches mantle temperature mantle fertility and the availability of fluids subduction related ophiolites include suprasubduction zone and volcanic arc types the evolution of which is governed by slab dehydration and accompanying metasomatism of the mantle melting of the subducting sediments and repeated episodes of partial melting of metasomatized peridotites subduction unrelated ophiolites include continental margin mid ocean ridge plume proximal plume distal and trench distal and plume type plume proximal ridge and oceanic plateau ophiolites that generally have mid ocean ridge basalt morb compositions subduction related lithosphere and ophiolites develop during the closure of ocean basins whereas subduction unrelated types evolve during rift drift and seafloor spreading the peak times of ophiolite genesis and emplacement in earth history coincided with collisional events leading to the construction of supercontinents continental breakup and plume related supermagmatic events geochemical and tectonic fingerprinting of phanerozoic ophiolites within the framework of this new ophiolite classification is an effective tool for identification of the geodynamic settings of oceanic crust formation in earth history and it can be extended into precambrian greenstone belts in order to investigate the ways in which oceanic crust formed in the archean

petrology and zircon u pb geochronology of metagabbros from a mafic ultramafic suite at aniyapuram neoarchean to early paleoproterozoic convergent margin magmatism and middle neoproterozoic high grade metamorphism in southern india abstract several mafic ultramafic complexes occur within the palghat cauvery suture zone pcsz in southern india the pcsz is regarded in recent models as the zone along which crustal blocks were amalgamated during the late neoproterozoic cambrian 550 530 ma gondwana assembly here we report petrologic and zircon u pb geochronologic data from gabbros associated with the aniyapuram mafic ultramafic suite in the central domain of the pcsz geothermobarometry and pseudosection approach in the system ncfmashto for the metagabbro grt cpx opx hbl pl qtz ilm rt yield peak p t condition of 9 8 10 6 kbar and 730 790 c which was followed by decompression to 6 5 8 0 kbar and ca 750 c as inferred from the formation of opx pl symplectite around garnet probably along a clockwise p t path zircon u pb data analyzed by la icp ms plot along a well defined discordia with upper and lower intercepts in the concordia at 2436 22 ma and 731 11 ma respectively suggesting neoarchean early paleoproterozoic magmatic emplacement of the protolith and progressive pb loss related to the middle neoproterozoic cryogenian thermal event or high grade metamorphism these results closely compare with the available neoarchean magmatic ages of mafic ultramafic complexes e g sittampundi devanur agali hills and kanja malai and middle neoproterozoic magmatic event e g manamedu and kadavur in the pcsz and adjacent granulite blocks the 650 ma concordia ages obtained from unzoned zircons might indicate the timing of high grade metamorphism or post peak hydration event the p t conditions obtained from aniyapuram are significantly lower than the high pressure and ultrahigh temperature conditions of the 550 530 ma final collisional event p 14 kbar and t 950 c the middle neoproterozoic ca 730 ma or 650 ma high grade metamorphism in aniyapuram reported for the first time from the pcsz is possibly associated with magmatism in arc tectonic setting

u pb ages and hf o isotopes of zircons from late paleozoic mafic ultramafic units in the southern central asian orogenic belt tectonic implications and evidence for an early permian mantle plume abstract u pb age and hf o isotope data of zircons from the mafic ultramafic complexes and their related rhyolites and granites in the eastern tianshan and beishan regions were obtained by sims and la icpms the hf o isotopic composition and hf model age of the zircons are closely related to their tectonic setting and formation age the zircons from the mafic ultramafic complexes in the bogeda haerlike and jueluotage belts have higher ehf t 8 17 and lower 18o 4 6 whereas most of the zircons in the gabbros granites and rhyolite from the middle tianshan massif and beishan rift display lower ehf t 0 8 and higher 18o 5 8 the positive ehf t and relatively lower 18o values suggest that these mafic ultramafic complexes were derived from depleted mantle which was subjected to subduction related modification processes the hf isotopic composition of zircons in the granites has revealed that the growth of juvenile crust was also very significant in the southern margin of the central asian orogenic belt during the paleozoic the hf model ages of the analyzed zircons together with the regional geology suggest that the beishan area had a northward subduction possibly from ca 900 ma to ca 400 ma whereas the eastern tianshan had a south directed subduction most likely from ca 600 ma to ca 310 ma additionally zircons with ca 280 ma u pb ages display wider and more scattered ehf t and 18o variations than the relatively older and younger ones which further support the early permian mantle plume model

a brief review of ophiolites in china abstract ophiolites are widely distributed in western southwestern and northern china where they fall into four principal age groups neoproterozoic early paleozoic late paleozoic and mesozoic cenozoic neoproterozoic ophiolites are known only in the north qinling orogenic belt in ne jiangxi province and in western sichuan province phanerozoic ophiolites are grouped into the paleo asian tethyan and circum pacific series the paleo asian ophiolites crop out in the western and northern parts of china between the siberian and north china blocks and range in age from early to late paleozoic all of these ophiolites are of the franciscan formerly cordilleran type and many are superimposed on one another suggesting repeated accretion of arc assemblages in an environment similar to the present day western pacific ocean mediterranean type formerly tethyan type ophiolites are confined to sw china particularly tibet and yunnan province paleo tethyan ophiolites are characterized by morb type lavas and are typically bounded by old continental lithosphere suggesting that they formed in small intercontinental ocean basins neo tethyan ophiolites contain a range of lava types including morb iat and boninite indicating formation in a variety of suprasubduction zone ssz environments circum pacific ophiolites occur sporadically in taiwan and ne china where they form tectonic melanges composed mainly of metaperidotite gabbro and basalt

zircon u pb geochronology and hf isotope of felsic volcanics from attappadi southern india implications for neoarchean convergent margin tectonics abstract the attappadi area on the south western flanks of the archean dharwar craton in southern india is located along the e w trending bhavani shear zone which marks the trace of a neoarchean suture zone the dominant rock types in the area include meta ultramafics amphibolites ttg tonalite trondhjemite granodiorite gneisses metapelites and sulphidic banded iron formation bif here we report the occurrence of felsic volcanic rocks preserving primary textures from the anaikatti area in eastern attappadi the felsic volcanics are interbanded with the bif amphibolite metapelite metapyroxenite and hornblende gneisses the felsic volcanics are divided into two types based on their textures the type 1 rock is medium grained with gneissic texture and lack unequivocal primary volcanic textures type 2 felsic volcanics are thinly laminated fine grained and at places preserve relict soft sediment deformation structures they also contain relict volcanic clasts or lapilli and are interpreted as felsic tuff geochemically the attappadi felsic volcanics are rhyolitic in composition and have arc related trace element signatures they also possess a calc alkaline volcanic affinity we report la icpms u pb ages from zircons in four samples of the felsic tuffs which show weighted mean ages of 2567 18 ma and mswd 1 4 2499 19 ma mswd 0 57 2555 24 ma mswd 1 7 and 2576 64 ma mswd 5 8 the late neoarchean early paleoproterozoic ages obtained in our study correlate well with the zircon u pb ages reported in recent studies from ophiolites and other suprasubduction suites from attappadi and surrounding regions the zircon ehf values range from 11 1 to 7 6 suggesting heterogeneous source material involving both juvenile and older reworked components we build a tectonic model for the sw margin of the dharwar craton with an oceanic realm characterized by island arcs and widespread submarine tholeiitic as well as komatiitic ultramafic and mafic volcanism during the neoarchean this predominantly mafic volcanism on the ocean floor is represented by primitive komatiitic lavas oxide and sulphide facies bif the birth of volcanic arc at the convergent margin is marked by felsic volcanism and the deposition of felsic volcanics and volcano sedimentary successions in the final stage of ocean closure the ocean plate and continental arc assemblages were brought in juxtaposition including the accretion of the ophiolitic fragments our study confirms the recent models of arc arc and arc continental accretion to the southern margin of the dharwar craton and major continental growth at the end of the archean

cryogenian 830 ma mafic magmatism and metamorphism in the northern madurai block southern india a magmatic link between sri lanka and madagascar abstract the northern madurai block southern india lies directly south of and partly deformed by the palghat cauvery shear zone system pcss a potential suture of the neoproterozoic mozambique ocean the kadavur gabbro anorthosite complex lies south of the pcss in the northern madurai block and crystallized at 829 14 ma la icpms zircon data in a supra subduction zone setting the complex contains zircon e hf t values of 12 5 to 8 6 that represent palaeoproterozoic t dm model ages 2 3 2 5 ga these broadly agree with a whole rock neodymium t dm model age of 2287 ma oxygen isotope 18 o ratios range from 5 82 and 6 74 the parental magma for the gabbro anorthosites are interpreted to be derived from a juvenile neoproterozoic mantle contaminated by mesoarchaean igneous infra crustal sources the gabbro anorthosites intrude quartzites with dominantly palaeoproterozoic detrital zircons that contain neoarchaean and mesoarchaean hafnium model ages these quartzite zircons contain metamorphic rims that yield an age of 843 23 ma demonstrating the autochthonous nature of the gabbro anorthosite complex later felsic magmatism is recorded by the 766 8 ma crystallisation age of the protolith of a felsic gneiss cryogenian magmatism in the madurai block is interpreted to form part of an extensive arc magmatic province within the southern east african orogen that can be traced from central madagascar through southern india to the wanni complex of sri lanka this province is interpreted to have formed above a south west dipping subduction system as the mozambique ocean was subducted under the neoproterozoic continent azania

geodetic versus geophysical perspectives of the gravity anomaly summary a gravity anomaly is essentially the difference between the gravitational acceleration caused by the earth s masses and that generated by some reference mass distribution however there are numerous subtleties to the definition and moreover to the practical realization of a gravity anomaly an attempt is made here to clarify the definition of a gravity anomaly from the geodetic and geophysical perspectives point out some of the key differences in terminology and philosophy and to identify some of the problems remaining in its practical realization from a variety of observation types it is argued that if the gravity anomaly is defined and realized in a rigorous and consistent manner this may lead to the improvement of its use in both geodesy and geophysics

the mafic ultramafic complex of aniyapuram cauvery suture zone southern india petrological and geochemical constraints for neoarchean suprasubduction zone tectonics abstract several precambrian mafic ultramafic complexes occur along the cauvery suture zone csz in southern granulite terrain india their origin magmatic evolution and relationship with the associated high grade rocks have not been resolved the aniyapuram mafic ultramafic complex amuc the focus of the present study in southern part of the csz is dominantly composed of peridotites pyroxenites gabbros metagabbros mafic granulites hornblendites amphibolites plagiogranites felsic granulites and ferruginous cherts the rock types in the amuc are structurally emplaced within hornblende gneiss ttg basement rocks and are highly deformed the geochemical signature of the amphibolites indicates tholeiitic affinity for the protolith with magma generation in island arc setting n morb normalized pattern of the amphibolites show depletion in hfs elements p zr sm ti and y and enrichment of lil elements rb ba th sr with negative nb anomalies suggesting involvement of subduction component in the depleted mantle source and formation in a supra subduction zone tectonic setting our new results when correlated with the available age data suggest that the lithological association of amuc represent the remnants of the neoarchean oceanic lithosphere

optimization in gravimetric terrain corrections terrain corrections for gravimetric measurements require topographical data of appropriate accuracy and density the situation is clearly dependent on the ruggedness of the topography and the accuracy specifications for the terrain corrections from the optimization point of view this computational problem has been investigated and computer software developed for the near intermediate and distant zones surrounding the gravity stations test results indicate that the general accuracy objective of 1 0 1 5 mgal 10 15 m s 2 for the gravimetric terrain corrections is easily achievable in practice with appropriate topographical data for geophysical applications higher accuracies are certainly achievable with the method even in rugged areas

recognition of ocean plate stratigraphy in accretionary orogens through earth history a record of 3 8billion years of sea floor spreading subduction and accretion abstract ocean plate stratigraphy ops is a term used to describe the sequence of sedimentary and volcanic rocks deposited on oceanic crust substratum from the time it forms at a spreading center to the time it is incorporated into an accretionary prism at a convergent margin in this study we review the major geological characteristics of relict cenozoic to early archean oceanic crust and ops persevered in alaska japan california franciscan complex central asia british isles canada slave province australia pilbara craton and greenland isua and ivisaartoq belts an assessment of ops in accretionary orogens spanning the duration of earth s rock record shows remarkable similarities between ops of all ages in terms of structural style major rock components sequence of accretion and trace element geochemical signatures volcanic rocks preserved in the orogenic belts are characterized predominantly by oceanic island arc basalts island arc picrites mid ocean ridge basalts back arc basalts oceanic plateau basalts ocean island basalts and boninites with extremely rare komatiites this demonstrates that sea floor spreading lateral movement of oceanic plates with accompanying sedimentation over the oceanic substratum and accretion at convergent margins have been major earth processes since at least 3 8 ga ago there have been some secular changes in the rock types in ops such as changes in carbonates and radiolarian cherts whose sources were in the biota in existence in phanerozoic times but absent in the precambrian but overall there have been few changes in the style of ops accretion with time komatiites and banded iron formations occur predominantly in archean orogenic belts reflecting higher mantle temperatures and less oxic seawater composition respectively before 2 5 ga this is clear documentation that plate tectonics including the lateral movement of oceanic lithosphere has been a major heat loss mechanism on earth since the early precambrian

the cauvery shear zone southern granulite terrain india a crustal scale flower structure abstract the cauvery shear zone csz is a crustal scale shear system within the southern granulite terrain along the southern margin of the archaean dharwar craton structural interpretation of satellite data and field observations reveal four major shear zones within the csz system they show dextral shear kinematics synchronous with a major neoproterozoic tectono metamorphic event d2 associated with intracrustal melting and migmatisation the disposition geometry and contemporaneity of shear fabrics of the csz system are modelled in terms of a crustal scale flower structure akin to transpressional and collisional orogens in the light of recent seismic evidence for a displaced moho structure and a mid to lower crustal low velocity zone the flower structure across the csz may extend to mantle depths

structural anatomy of a dismembered ophiolite suite from gondwana the manamedu complex cauvery suture zone southern india abstract detailed geological and structural mapping of the manamedu ophiolite complex moc from the south eastern part of the cauvery suture zone csz within the gondwana collisional suture in southern india reveals the anatomy of a dismembered ophiolite succession comprising pyroxenite actinolite hornblendite hornblendite gabbro norite gabbro anorthosite amphibolite plagiogranite mafic dykes and associated pelagic sediments such as chert magnetite bands and carbonate horizons the magmatic foliation trajectory map shows inward dipping foliations and a variety of fold structures structural cross sections of the moc reveal gentle inward dips with repetition and omission of different lithologies often marked by curvilinear hinge lines the succession displays imbricate thrust sheets and slices of dismembered ophiolite suites distributed along several localities within the csz the moc can be interpreted as a deformed large duplex structure associated with south verging back thrust system consistent with crustal scale flower structure the nature and distribution of ophiolitic rocks in the csz suggest supra subduction zone setting associated with the lithospheric subduction of the neoproterozoic mozambique ocean followed by collision and obduction during the final stage of amalgamation of the gondwana supercontinent in the end precambrian

proterozoic orogens in southern peninsular india contiguities and complexities abstract the precambrian terranes of southern peninsular india have been central to discussions on the history of formation and breakup of supercontinents of particular interest are the proterozoic high grade metamorphic orogens at the southern and eastern margins of the indian shield skirting the 3 4 ga dharwar craton which not only preserve important records of lower crustal processes and lithospheric geodynamics but also carry imprints of the tectonic framework related to the assembly of the major neoproterozoic supercontinents rodinia and gondwana these proterozoic orogens are described as southern granulite terrane sgt in the southern tip and the eastern ghats mobile belt egmb in the eastern domains of the peninsula the contiguity of these orogens is broken for a distance of 400 km and disappears in the bay of bengal these orogens expose windows of middle to lower crust with well preserved rock records displaying multiple tectonothermal events and multiphase exhumation paths recent studies in these orogens have led to the recognition of discrete crustal blocks or terranes separated by major shear zone systems some of which represent collisional sutures the sgt and egmb carry several important features such as fold thrust tectonics regional granulite facies metamorphism of up to ultrahigh temperature conditions in some cases multiple p t paths development of lithospheric shear zones emplacement of ophiolites presence of alkaline and anorthositic complexes development of crustal scale flower structures transpressional strains and reactivation tectonics a heterogeneous distribution of different metamorphic and magmatic assemblages with distinct spatial and temporal strain variations in shaping the fabric elements in different blocks is identified both egmb and sgt share a common transpressional deformation history during the latest neoproterozoic characterized by the steepening of the initial low angle crustal scale structures leading to a subvertical grain conducive to reactivation tectonics our synthesis of the spatial distribution geometry kinematics and the transpressional strain of the shear zone systems provides insights into the tectono metamorphic history of the proterozoic orogens of southern india and their contiguity and complexities recent understanding of subduction accretion and collisional history along these zones together with a long lived transpressional tectonic regime imply that these orogens witnessed identical tectonic regimes at different times in earth history although the major and common structural architecture was built during the final assembly of the gondwana supercontinent

biostratigraphy and palaeobiogeography of east africa and the iberian peninsula abstract the aim of this paper is to examine the mammalian faunas of the neogene of spain and of east africa with a view to understanding the relationships between the neogene faunas of the two regions the available data indicates that during the neogene faunal similarities between iberia and east africa varied greatly at times the resemblances being relatively great high number of genera in common at others the similarities being weak low number of genera in common changes in faunal similarity appear to be related to latitudinal fluctuations in the position of the boundaries of the proto ethiopian and proto palaearctic zoogeographic realms periods of greater faunal similarity coincide with a northerly position of the boundary zone while periods of greater faunal dissimilarity occur when the boundary zone is located in a southerly position as it is today we related these changes in the position of the boundary zone between zoogeographic realms to geotectonic climatic and astronomic changes which affected the globe

geochemistry and intrusive directions in sheeted dikes in the troodos ophiolite implications for mid ocean ridge spreading centers 1 sheeted dikes at mid ocean ridge volcanoes represent the link between deep magma production and storage processes and shallow processes such as volcanism and hydrothermal activity as such they are crucial for the interpretation of many observations at mid ocean ridges or other volcanoes with pronounced rift zones including topography hydrothermal systems petrology and geochemistry we carried out a structural magnetic and chemical investigation of a 4 10 km sheeted dike section in the troodos ophiolite cyprus on the basis of major and trace element geochemistry we distinguish dikes that may be correlated with the basal high ti series hts lavas from those of the overlying low ti series lts lavas all dikes studied are nearly parallel to each other with vertical or steeply dipping planes whose strike likely indicates the orientation of the spreading center anisotropy of magnetic susceptibility measurements suggests that the hts and lts dikes intrude in fundamentally different ways hts dikes reflect the intrusive behavior of dikes in the vicinity of a magma supply system and define ridge parallel intrusive sheets that radiate out from the magma chamber lts dikes show a bimodal orthogonal set of intrusive directions one shallow and one near vertical near lateral propagating dikes provide a means for delivery of magma into distant portions of a rift system and near vertical dike propagation directions are probably associated with feeder dikes to down rift surface flows our study suggests that the types of dike intrusive behavior in the troodos ophiolite may also be typical for normal mid ocean ridges or other major shield volcanoes with well developed rift zones

compositions of chromite associated minerals and parental magmas of podiform chromite deposits the role of slab contamination of asthenospheric melts in suprasubduction zone environments abstract podiform chromitites in the mantle sections of ophiolites belong to either high cr metallurgical or high al refractory varieties their highly variable compositions are reflected by different cr s 100cr cr al and cr2o3 and al2o3 contents of the chromite falling in the boninitic and morb fields respectively parental magmas of high cr chromitites have higher sc mn co and ni and lower ti v zn and ga concentrations than morb melts their trace element patterns are similar to those of boninites except for ni and zn in contrast high al chromitites have parental magmas characterized by generally flat morb normalized patterns showing slight enrichments in v mn and co and depletion in ni and zn regardless of their compositions both types of chromitites have chondrite normalized platinum group element pge patterns showing enrichment in ipge and depletion in ppge a variety of platinum group minerals are typically present in both types occurring either as euhedral inclusions or along fractures in chromite grains these minerals have a wide span of re os isotopic compositions reflecting a variety of origins there is a diversity of unusual minerals and mineral inclusions associated with podiform chromitites the presence of these minerals suggest that grains of amphibolite plagioclase amphibole and zircon and eclogite coesite kyanite and garnet were present in the magmas from which chromite crystallized multiphase mineral inclusions demonstrate that podiform chromitites form from hydrous mafic magmas in suprasubduction zone environments ssz we propose a new model in which chromitite formation was involved in intra oceanic subduction zones initiated in closing oceanic basins continued subduction carries oceanic and possibly continental crustal materials to deep levels where they are metamorphosed under greenschist amphibolite and eclogite facies conditions the tearing and breakoff of the subducted slab possibly along the transitional contact between amphibolites and eclogites create a slab window through which the underlying asthenosphere rises and melts to generate cr rich mafic magmas these upward migrating magmas pass through the subduction zone and assimilate the subducted slab as a result of slab contamination these magmas become more siliceous more oxidized and more hydrous rapidly triggering chromite crystallization minute grains of chromite are suspended in the upward moving magmas as they migrate through the overlying metasomatized mantle wedge such chromite bearing magmas eventually deposit chromite in magma conduits in the uppermost mantle close to the moho where the upward flow changes from vertical to subhorizontal and velocity is greatly reduced highly reduced and ultrahigh pressure minerals including diamonds are reported in literature both in podiform chromitites and host peridotites of ophiolites some of these minerals in association with host peridotites may have been brought by the uprising asthenosphere at mid oceanic ridges due to the mantle convection it is also possible that some diamonds may have formed in the subducted slab below about 150 km some minerals of subducted slabs are preserved because they are encapsulated in chromite grains where they are protected from the ssz melts some of these ssz mantle wedges are emplaced on land to become podiform chromitite bearing ophiolites

the eurasian arabian and african continental margin from iran to greece the continental margin between eurasia and arabia can be traced as a continuous thrust zone for c 2000 km from the gulf of iran westwards through iran and turkey towards the mediterranean sea in this sector it forms a coherent geological whole with increasing structural complexity from s to n across the junction this clarity is terminated 125 km ne of the gulf of iskenderun on the mediterranean sea by the n s trending levant fault system lfs w of the lfs for 1400km to the w of greece the boundary significantly changes to curved micro plates which are not directly observable as they are beneath the sea their location is in doubt for c 650 km from the lfs westwards to the eastern margin of the aegean plate at the anaximander mountains over this sector several major tectonic units occur which obscure and complicate such a margin to the w the aegean plate continues for 750 km with the characteristics of an island arc

short paper plagiogranitic differentiates in morb type sills of the faeroe shetland basin the association of certain types of low potassium acid igneous rocks known as plagiogranites with oceanic crust and ophiolite complexes is well known both this association and experimental studies have suggested that oceanic plagiogranites could have originated by differentiation of magmas of mid ocean ridge basalt morb type however no actual example appears to have been recorded in which the progressive differentiation of morb type magma can be demonstrated to generate integrally associated plagiogranite the occurrence in the faeroe shetland sill complex of intrusions containing plagiogranite within tholeiitic olivine dolerite of morb type composition provides direct evidence of this process for the first time the occurrence and composition of oceanic plagiogranites have been reviewed by coleman donato 1979 they describe plagiogranites as leucocratic rocks which occur within ophiolite complexes in the upper parts of cumulate gabbros and within the sheeted dyke complexes and which have also been occasionally recovered by sea floor dredging and drilling see also hekinian 1982 plagiogranites characteristically differ from most continental granitic rocks by having very low k 2 o and rb contents having rare earth element ree distributions that resemble those of morb light ree depleted and typically exhibiting interstitial vermicular and or micrographic intergrowths of quartz and plagioclase the main models proposed for the origin of plagiogranites are 1 fractional crystallization of morb type magma 2 liquid immiscibility and 3 anatexis of basic rocks some workers e g saunders et al 1979 ashley et al 1983 have concluded that although fractionation of basic liquids was the dominant process by which plagiogranites originated in the ophiolites

the origin of oceanic plagiogranites from the karmoy ophiolite western norway both field relationships and geochemical characteristics indicate two suites of plagiogranitic and related rocks coexisting in the higher parts of the karmoy ophiolite of western norway the plutonic zone of this ophiolite can be subdivided into three complexes the east karmoy igneous complex the visnes high level complex and the veavagen igneous complex and plagiogranitic rocks are well developed in the first two of these within the east karmoy igneous complex plagiogranites are associated with high temperature pre basic dyke shear zones rare earth element modelling indicates that these plagiogranites were derived by anatexis of amphibolite hydrated diabase assuming a starting material consisting of 40 hornblende and 60 plagioclase and that batch melting occurred within the stability field of hornblende in comparison plagiogranite occurs in a number of bodies in the upper part of the visnes high level complex and forms a sandwich horizon together with biotite diorites and epidosites between a roof assemblage of dykes microgabbros and magnetite gabbros and a floor assemblage of layered and non layered gabbros the r e e modelling of the petrogenesis of this series of plagiogranites indicates that they were derived by filter pressing of a differentiated interstitial liquid to the vari textured gabbros although the distribution of highly hygromagmatophile elements such as k rb ba etc cannot be explained satisfactorily by this model alone depletion in these elements appears to be an autometasomatic effect

a neoarchean dismembered ophiolite complex from southern india geochemical and geochronological constraints on its suprasubduction origin abstract ophiolites the remnants of ancient oceanic lithosphere have been described from collisional sutures of various ages with only few examples from archean terranes here we report the discovery of a neoarchean ophiolite suite from the southern margin of the dharwar craton in india tectonically intercalated within a neoproterozoic suture zone the metamorphosed and variably dismembered ophiolite suite exposed around devanur comprises altered ultramafic units websterite gabbros mafic dykes amphibolites trondhjemites and pegmatites associated with ferruginous metachert structural and petrographic studies indicate that the rocks represent a highly sheared and metamorphosed suite emplaced as a thrust sheet the major and trace element geochemistry of the mafic dykes indicate derivation from basaltic andesitic magmas with tholeiitic to calc alkaline characteristics the rocks display negative nb anomalies with enrichment of lile k rb ba th and depletion in hfse ti nb hf tb the tectonic discrimination of these rocks based on various geochemical plots suggests that they were generated in a suprasubduction zone setting we present new shrimp zircon u pb data for two trondhjemite samples from this complex which yield 238u 206pb ages of 2528 61 and 2545 56 ma the neoarchean age from the trondhjemites obtained in our study is closely comparable to similar ages obtained in recent studies from magmatic zircons in charnockites and orthogneisses in the area the suprasubduction zone assemblages and arc magmas suggest a neoarchean ocean closure along the southern margin of the dharwar craton

mineral chemistry of isotropic gabbros from the manamedu ophiolite complex cauvery suture zone southern india evidence for neoproterozoic suprasubduction zone tectonics abstract the dismembered units of the neoproterozoic manamedu ophiolite complex moc in the cauvery suture zone southern india comprises a well preserved ophiolitic sequence of ultramafic cumulates of altered dunites pyroxenites mafic cumulates of gabbros gabbro norites and anorthosites in association with plagiogranites isotropic gabbros metadolerites metabasalts amphibolites and thin layers of ferruginous chert bands the isotropic gabbros occur as intrusions in association with gabbroic anorthosites plagiogranite and metabasalts amphibolites the gabbros are medium to fine grained with euhedral to subhedral orthopyroxenes clinopyroxenes and subhedral plagioclase together with rare amphiboles mineral chemistry of isotropic gabbros reveal that the clinopyroxenes are diopsidic to augitic in composition within the compositional ranges of en 42 59 fs 5 12 wo 31 50 they are ca rich and na poor na2o

precambrian iron formations from the cauvery suture zone southern india implications for sub marine hydrothermal origin in neoarchean and neoproterozoic convergent margin settings abstract thick horizons of iron formations including banded iron formations bifs and banded silicate formations bsfs occur as e w trending bands in the eastern part of cauvery suture zone csz in the sothern granulite terrane of india some of these occur in close association with the neoarchean neoproterozoic suprasubduction zone complexes where as some others are associated with metamorphosed accretionary sequences including pyroxene granulites and other high grade rocks the iron formations are highly deformed and metamorphosed under amphibolite to granulite facies conditions and are composed of quartz magnetite hematite goethite garnet pyrite together with grunerite and pyroxene here we report the geochemical characteristics of twenty representative samples from the iron formations that reveal a widely varying composition with fe 2 o 3 t 22 65 wt as total iron total fe 2 o 3 tio 2 205 6532 mno tio 2 0 25 12 66 and sio 2 33 85 wt broadly representing the two types of iron formations these formations also show very low al al fe mn ratio 0 001 0 01 al 2 o 3 0 07 0 76 wt al 2 o 3 tio 2 ratio 2 7 21 mgo 0 01 4 41 wt cao 0 1 1 24 wt na 2 o 0 01 0 05 wt and k 2 o 0 01 wt together with low total ree 3 38 31 63 ppm the trace and ree elemental distributions show wide variation with high ni 274 ppm and zn contents up to 87 ppm when compared to mafic volcanics of the adjoining areas tectonic discrimination plots indicate that the iron formations of the cauvery suture zone are of hydrothermal origin their chondrite normalized patterns show slight positive eu anomaly eu eu up to 1 77 and relatively less fractionation of ree with slight lree enrichment compared to hree however the paas post archean average of australian sediments normalized ree patterns display significant positive eu anomaly eu eu up to 2 32 with well represented negative ce anomalies ce ce 0 66 1 28 the above results together with petrological characteristics and available geochronology of the associated lithologies suggest that the iron formations can be correlated to algoma type the fe and si were largely supplied by medium to high temperature sub marine hydrothermal systems in neoarchean and neoproterozoic convergent margin settings

plagiogranites from the ocean crust and ophiolites rock types similar to those found in ophiolites have been recovered from the floors of ocean basins1 3 this is critical evidence for the theory that ophiolites represent the ocean crust4 5 aumento6 has described diorites quartz diorites and trondhjemites dredged from the mid atlantic ridge mar near 45 n and noted that these were the first rocks analogous to the low k acid differentiates in ophiolites collectively referred to as oceanic plagiogranites 7 8 to be recognized amongst those recovered from the modern ocean floor only a few plagiogranites from the ocean floor ocean crust plagiogranites have since been described these were dredged from widely spaced localities on the mar9 11 the amani plateau and the kyushu palau ridge north west pacific 12 however the kyushu palau ridge could be the product of destructive plate margin magmatism13 as are the potassic granites from the aves ridge caribbean 14 plagiogranite xenoliths also occur in extrusives on surtsey and askja iceland 15 that only some of the petrological compositions of ophiolite plagiogranites are represented in the ocean crust is not surprising in view of the small sample of ocean crust plagiogranites it is less obvious however if ophiolites represent the ocean crust why certain acidic rock types from the oceans have not been found in ophiolites i show here that the difference between the two groups of plagiogranites is evidence that ophiolites do not represent all kinds of ocean crust and that a greater range of magma compositions and or conditions of magmatic differentiation is represented by the ocean crust

gravimetric survey terrain correction using linear analytical approximation various methods for computing the terrain correction in a high precision gravity survey are currently available the present paper suggests a new method that uses linear analytical terrain approximations in this method digital terrain models for the near station topographic masses are obtained by vectorizing scan images of largescaled topographic maps and the terrain correction computation is carried out using a fourier series approximation of discrete height values distant topography data are represented with the help of digital gtopo30 and shuttle radar topography mission cartographic information we formulate linear analytical approximations of terrain corrections for the whole region using harmonic functions as the basis of our computational algorithm stochastic modelling allows effective assessment of the accuracy of terrain correction computation the perm krai case study has shown that our method makes full use of all the terrain data available from topographic maps and digital terrain models and delivers a digital terrain correction computed to a priori precision our computer methodology can be successfully applied for the terrain correction computation in different survey areas

a biostratigraphic zonation of the middle and upper siwaliks of the potwar plateau of northern pakistan abstract two biostratigraphic reference sections are designated for the middle and upper siwalik formations of northern pakistan the stratigraphic ranges of 41 mammalian taxa are established in the reference sections and the two sections are correlated to each other and to the standard magnetic polarity time scale by the magnetic polarity stratigraphy the resulting stratigraphic and magnetostratigraphic framework is used to define four contiguous biostratigraphic interval zones the proposed interval zones are bounded by five important faunal events which are identified in the reference sections the four proposed interval zones and the approximate ages in the reference sections of their lower and upper boundaries are 1 the hipparion s l interval zone 7 4 9 5 m y b p 2 the selenoportax lydekkeri interval zone 5 3 7 4 m y b p 3 the hexaprotodon sivalensis interval zone 5 3 2 9 m y b p and 4 the elephas planifrons interval zone 2 9 1 5 m y b p the paleomagnetic and biostratigraphic evidence indicates that the tatrot beds from which pilgrim developed his tatrot faunal zone are between 2 5 and 3 4 m y b p with a preferred maximum age of 2 9 m y b p the biostratigraphic evidence indicates the sethi nagri locality y311 the primary source of pilgrim s nagri faunal zone is in the hipparion s l interval zone the paleomagnetic evidence suggests an approximate age of 9 m y b p a major faunal event at 9 5 m y b p introduced equids suids and large giraffes from eurasia into the indain subcontinent a second more diffuse faunal event between 5 3 and 2 9 m y b p introduced several african taxa as well as equus and cervids from eurasia

chronostratigraphic assignment of volcanopelagic strata above the coast range ophiolite the chronostratigraphic assignments of radiolarian cherts tuffaceous mudstones and volcaniclastic strata referred to herein as volcanopelagic succession overlying the coast range ophiolite in the coast ranges of california have been refined on the basis of additional field data and recent revision of the pessagno and others radiolarian biostratigraphic zonation for the middle and upper jurassic of north america revision of this zonation scheme accomplished through the integration of radiolarian data with ammonite biostratigraphy and geochronometry has enabled close correlation of radiolarian assemblages in north america europe and japan however major differences still exist in the stage assignments of these assemblages among different zonations the pessagno and others scheme is utilized herein to discuss the current chronostratigraphic assignments at five localities in the coast ranges point sal stanley mountain cuesta ridge and llanada in the southern coast ranges and black mountain geyser peak area in the northern coast ranges of california stage assignments for volcanopelagic successions in these areas range from uppermost callovian lower oxfordian to upper tithonian

crustal architecture and tectonic evolution of the cauvery suture zone southern india abstract the cauvery suture zone csz in southern india has witnessed multiple deformations associated with multiple subduction collision history with incorporation of the related accretionary belts sequentially into the southern continental margin of the archaean dharwar craton since neoarchean to neoproterozoic the accreted tectonic elements include suprasubduction complexes of arc magmatic sequences high grade supracrustals thrust duplexes ophiolites and younger intrusions that are dispersed along the suture the intra oceanic neoarchean neoproterozoic arc assemblages are well exposed in the form of tectonic melanges dominantly towards the eastern sector of the csz and are typically subjected to complex and multiple deformation events multi scale analysis of structural elements with detailed geological mapping of the sub regions and their structural cross sections geochemical and geochronological data and integrated geophysical observations suggest that the csz is an important zone that preserves the imprints of multiple cycles of precambrian plate tectonic regimes

magmatic and metamorphic imprints in 2 9 ga chromitites from the sittampundi layered complex tamil nadu india abstract the ca 2 9 ga old sittampundi layered complex slc of south india belongs to a special group of deformed and metamorphosed archaean rocks where chromitites are interlayered with anorthosite detailed field studies have revealed that the slc comprises of a sequence of magmatic rocks consisting of clinopyroxenite anorthosite and mafic rocks of tholeiitic composition minor bif banded iron formations intercalated with the mafic rocks are the only sedimentary rock within the slc chromitite occurs as continuous to discontinuous bands a few mm up to 6 m thick within clinopyroxenite and highly calcic anorthosite an 99 100 but never in the enclosing mafic rocks chromite occurs broadly in three textural types a cumulus grains in chromitite layers b disseminated grains in clinopyroxenite and anorthosite and c small grains included in clinopyroxene and plagioclase a suite of felsic magma intruded the slc at ca 2 51 ga subsequently the entire rock sequence underwent polyphase deformation and metamorphism at ca 2 50 2 45 ga granulite facies and at 0 72 0 50 ga amphibolite facies during the latter event extensive alteration of clinopyroxene to amphibole and chlorite of plagioclase to clinozoisite and of chromite to cr poor green spinel took place chromite compositions show large variations cr 26 60 mg 4 45 fe 3 2 24 depending upon textural type and nature of the associated phases the most conspicuous compositional variation took place when chromite was metasomatically replaced by green spinel cr 3 21 mg 45 61 during amphibolite facies metamorphism on a cr vs mg plot pristine magmatic compositions of chromite define an array that suggest increasing cr with concomitant decrease in mg presumably during magmatic fractionation highly calcic plagioclase in anorthosite and fe al rich chromite in chromitite in the slc are consistent with a hydrous parental magma of tholeiitic composition this hydrous magma was generated in an oceanic arc environment and crystallized close to the surface of the earth in terms of mineralogy and field relations chromitite bearing rocks of the slc show remarkable similarity with the archaean anorthosite hosted chromitite deposits in layered magmatic complexes at fiskenaesset greenland and messina south africa but contrasts sharply with the chromitite deposits of proterozoic and phanerozoic age a change in the style of arc magmatism appears to be the controlling factor for marked change in chromite compositions across the archaean proterozoic boundary

ultrahigh temperature metagabbros from wynad implications for paleoproterozoic hot orogen in the moyar suture zone southern india abstract the wynad region is located along the confluence of the mercara and moyar suture zones in southern india which mark major zones of amalgamation of microcontinental blocks during neoarchean to paleoproterozoic here we report garnet and clinopyroxene bearing metagabbros from this region and present petrological geochemical and zircon u pb and lu hf isotopic data the prograde peak and retrograde mineral assemblages of the metagabbro are garnet clinopyroxene plagioclase ilmenite hornblende quartz garnet clinopyroxene plagioclase ilmenite and garnet clinopyroxene plagioclase ilmenite hornblende orthopyroxene respectively the geochemical data showing nearly flat or slightly depressed lree and constant hree patterns with the absence of negative nb anomaly in primitive mantle normalized plots suggest n morb affinity for the protolith indicating remnants of oceanic lithosphere accreted during the subduction collision event associated with the microblock amalgamation the metamorphic conditions of the rock were estimated using phase equilibria modeling in the system ncfmashto which yield p t conditions of 960 c and 12 8 kbar suggesting high pressure and ultrahigh temperature conditions the prograde p t conditions are inferred as 7 8 8 3 kbar at 600 c and 10 6 11 8 kbar at 900 c whereas the retrograde p t conditions computed based on hornblende and orthopyroxene bearing mineral assemblages show p t path is inferred from the p t phase equilibria modeling suggesting rapid heating and cooling in the lower crustal level this p t path is in contrast to those reported from other meta mafic rocks in southern india the absence of any decompression texture involving garnet also supports near isobaric cooling of the rock our zircon u pb data suggest neoarchean 2 51 ga magmatism and early paleoproterozoic 2 36 2 22 ga high grade metamorphism zircon lu hf data indicate that the protolith magma was derived from meso to neoarchean juvenile components with very limited reworked crustal materials our study reveals paleoproterozoic ultrahigh temperature metamorphism associated with the hot orogen formed during the collision between the coorg and nilgiri blocks

palynology of the late miocene oreopithecus bearing lignite from baccinello italy abstract this paper presents the results of an analysis of pollen and spores in 7 lignite samples from the vertebrate bearing v1 lignite at baccinello italy the aim of the study is to reconstruct the vegetation associated with the vertebrate fauna and to integrate this evidence with data from geological paleontological and paleobotanical research in order to provide an improved understanding of the paleoenvironmental context the v1 lignite consisting of a 3 m seam near the base of the local geological sequence is late miocene tortonian in age it is dated at 8 5 9 0 ma european mammal zone mn 12 based on faunal correlations and on radiometric age determinations of an overlying tuffaceous layer the fossil fauna comprises a small number of species many of which are unique to the coal deposits of tuscany including the controversial hominoid oreopithecus bambolii the high level of endemism low taxonomic diversity preponderance of ruminants tendency for gigantism among some of the micromammals and absence of cursorial carnivores all point to an insular environment evidence from structural geology confirms that a tectonically unstable island arc bordered the eastern side of the tyrrhenian sea during the late miocene by a combination of island sweepstake events and ephemeral land connections intermittent migrations of mammals from continental europe and africa led to the development of a unique fauna on islands of the northern tethys sea the reconstructed vegetation is of lowland mixed mesophytic forest the community of broad leaved deciduous angiospermous and evergreen gymnospermous trees contained a rich understory of small trees and shrubs and an herbaceous ground cover the abundance of aquatic seed plants and algae as well as the preponderance of ferns and arboreal taxa that commonly grow on moist substrates suggest that the lowland areas were poorly drained upland or montane habitats appear to have been present but were located at some distance from the depositional basin the rarity of grasses and chenopods indicates that extensive tracts of open country were of limited occurrence the closest modern analogue to the vegetation is the mixed mesophytic forest of the yangtze river valley of east central china it seems reasonable to infer that comparable warm temperate to subtropical conditions with high precipitation especially in the summer months characterized the late miocene climate at baccinello

coupling geophysical investigation with hydrothermal modeling to constrain the enthalpy classification of a potential geothermal resource abstract an appreciable challenge in volcanology and geothermal resource development is to understand the relationships between volcanic systems and low enthalpy geothermal resources the enthalpy of an undeveloped geothermal resource in the karckar region of armenia is investigated by coupling geophysical and hydrothermal modeling the results of 3 dimensional inversion of gravity data provide key inputs into a hydrothermal circulation model of the system and associated hot springs which is used to evaluate possible geothermal system configurations hydraulic and thermal properties are specified using maximum a priori estimates limited constraints provided by temperature data collected from an existing down gradient borehole indicate that the geothermal system can most likely be classified as low enthalpy and liquid dominated we find the heat source for the system is likely cooling quartz monzonite intrusions in the shallow subsurface and that meteoric recharge in the pull apart basin circulates to depth rises along basin bounding faults and discharges at the hot springs while other combinations of subsurface properties and geothermal system configurations may fit the temperature distribution equally well we demonstrate that the low enthalpy system is reasonably explained based largely on interpretation of surface geophysical data and relatively simple models

sm nd and rb sr ages and p t history of the archean sittampundi and bhavani layered meta anorthosite complexes in cauvery shear zone south india evidence for neoproterozoic reworking of archean crust sittampundi and bhavani archean layered meta anorthosite complexes occur as tectonic lenses within the cauvery shear zone csz a crustal scale shear dividing the precambrian granulite crust of south india into late archean 2 5 ga and proterozoic c 0 55 ga blocks they and their host supracrustal gneiss rocks record at least two stages of tectonometamorphic history the first is seen as regional scale refolded isoclinal folds and granulite metamorphism d1 m1 while the second stage is associated with dominantly e w dextral transcurrent shearing and metamorphic recrystallisation d2 mcsz whole rock sm nd isochrons for several comagmatic rocks of the layered complexes yield concordant ages sittampundi 2935 60 ma nd 1 85 0 16 and bhavani 2899 28 ma nd 2 18 0 14 2 errors our sm nd results suggest that 1 the magmatic protoliths of the sittampundi and bhavani layered complexes were extracted from similar uniform and lree depleted mantle sources 2 m1 metamorphism occurred soon after emplacement at c 3 0 ga ago p t estimates on garnet granulites from the sittampundi complex characterise the mcsz as a high p event with metamorphic peak conditions of c 11 8 kbar and 830 c minimum the mcsz is associated with significant isothermal decompression of the order of 4 5 3 5 kbar followed by static high temperature rehydration and retrogression around 600 c the timing of mcsz is inferred to be neoproterozoic at c 730 ma based on a whole rock garnet plagioclase hornblende sm nd isochron age for a garnet granulite from the sittampundi complex and its agreement with the 800 600 ma published age data on post kinematic plutonic rocks within the csz these results demonstrate that the cauvery shear zone is a zone of neoproterozoic reworking of archean crust broadly similar to the interface between the napier and rayner complexes of the east antarctic shield in a model proterozoic gondwana supercontinent

age of the baula pge mineralization orissa india and its implications concerning the singhbhum archaean nucleus abstract two types of platinum group element pge mineralization are identified in the mafic facies of the baula complex intruding the gabbro anorthosite unit of the singhbhum archaean nucleus the first pt dominated is magmatic the pgm are included in the magmatic silicates of a gabbro norite intrusion the bangur gabbro cutting the complex s ultramafic facies and are found preferentially where the gabbro incorporates clasts of dunite and chromitite near the gabbro ultramafics contact the second pd dominated is hydrothermal the pgm are associated with hydrous silicates located in the gabbro matrix of a magmatic breccia in which the blocks are from the baula ultramafic facies the originating fluids for the second type of mineralization were exsolved from the magma and concentrated pge and volatile elements we see no evidence of remobilization or external fluid contamination a structural study has demonstrated that a the breccia was provoked by the intrusion of the upper part of the bangur gabbro and b the breccia matrix corresponds to the bangur gabbro consequently both types of pge mineralization are associated with the same magmatic event a zircon age of 3122 5 ma has been obtained for the main bangur gabbro intrusion and similar ages 3123 7 and 3119 6 ma have been obtained on the pge mineralized gabbro matrix of the breccia this supports the hypothesis of a genetic link between the bangur gabbro with its pt dominated pge and the breccia with its pd dominated pge giving a mean weighted age of 3121 3 ma for the mineralization sm nd data on eight gabbro samples of the baula complex yielded a similar age of 3205 280 ma the dating of these events places them close to the broad scale stabilization of the singhbhum archaean nucleus estimated at 3 1 ga and defines one of the oldest pge mineralizing events ever recorded the bangur gabbro shows many affinities with the gabbro anorthosite unit of the singhbhum archaean nucleus in terms of age of emplacement and isotopic composition it may be therefore that the bangur gabbro actually forms part of the gabbro anorthosite unit and that a fundamental control of its two types of pge mineralization is the presence of ultramafic clasts

strain pattern and deformational history in the eastern part of the cauvery shear zone southern india abstract regional structural mapping guided by remote sensing interpretation in the eastern part of the cauvery shear zone csz southern granulite terrain reveals a structural framework of two major boundary shear zones interlinked by a set of sigmoidal shear belts the strain pattern and deformational history are manifest in terms of two major episodes d1 and d2 while d1 structures are well preserved in low strain areas the d2 fabrics are well reflected in high strain belts enveloping the low strain domains our multiscale structural observations can be modelled in terms of a plan view duplex showing a cross sectional flower structure related to regional transpression resulting from late neoproterozoic collisional processes in the eastern gondwana

an overview of the geochemistry of eoarchean to mesoarchean ultramafic to mafic volcanic rocks sw greenland implications for mantle depletion and petrogenetic processes at subduction zones in the early earth abstract this study reviews the geochemical characteristics of eoarchean to mesoarchean ultramafic to mafic volcanic rocks now amphibolites in sw greenland and compares them with those of cenozoic oceanic island arc basalts to evaluate archean subduction zone petrogenetic processes emphasis is placed on the th ree hfse zr ti and nb systematics of the ca 3800 and ca 3700 ma arc suites in the isua greenstone belt the ca 3075 ma ivisaartoq ujarassuit greenstone belt and amphibolites associated with the ca 2970 ma fiskenaesset layered anorthosite complex on n morb normalized diagrams the isua ivisaartoq ujarassuit and fiskenaesset volcanic rocks are all characterized by depletion of nb relative to th and lree consistent with a supra subduction forearc arc backarc geodynamic setting similarly on the th yb nb yb projection these suites plot within the field of cenozoic oceanic island arc basalts on log transformed immobile trace element ratio nb th la th sm th and yb th diagrams they display a trend projecting from morb mid ocean ridge basalt to iab island arc basalt on the iab crb continental rift basalt oib ocean island basalt morb diagram as for cenozoic oceanic island arc basalts accordingly these trace element compositions are interpreted as reflecting the enrichment of archean depleted upper mantle morb source by subduction derived melts and fluids following the initiation of intra oceanic subduction and arc migration concentrations of mgo and ni in sw greenland archean basalts overlap with but extend to 2 to 4 times higher than those in cenozoic oceanic island arc counterparts in contrast the majority of archean basalts have ree and hfse concentrations 2 to 4 times lower than cenozoic oceanic island arc basalts consistent with more depleted sub arc mantle wedge peridotites in the archean than in cenozoic counterparts such depletion reflects the extraction of large volumes of mafic to ultramafic melts from hotter archean mantle we infer that less refractory compositions of the present day depleted upper mantle the source of morb and conservative hfse in arc basalts have resulted from re mixing with the less depleted to enriched deeper mantle material transferred to shallower depths by mantle convection and plumes in post archean times archean basalts in sw greenland share the negative nb and ti anomalies of cenozoic oceanic island arc basalts on average archean basalts however have lower th nb la nb sm tio2 and gd tio2 ratios than cenozoic oceanic island arc counterparts given that rutile controls the nb and ti budgets in arc magmas the lower th nb la nb sm tio2 and gd tio2 ratios in sw greenland basalts are attributed to the mobility of nb and ti in slab derived melts involving rutile fusion in archean subduction zones these elements are less mobile in fluids originating from cenozoic subducting oceanic crust where rutile appears to be generally stable accordingly it is suggested that higher geothermal gradients in the archean may have provided optimized conditions for slab melting and metasomatism of the sub arc mantle wedge by slab derived melts

isua supracrustal belt greenland a vestige of a 3 8 ga suprasubduction zone ophiolite and the implications for archean geology abstract the mafic ultramafic units of the 3 8 ga isua supracrustal belt isb in greenland occur in a two armed arcuate zone eastern and western arms and are grouped into two major tectonostratigraphic units based on their lithological and geochemical characteristics 1 undifferentiated amphibolites ua and 2 garbenschiefer amphibolites ga the ua contains all major lithological units of a typical penrose type complete ophiolite sequence the ga is composed dominantly of volcaniclastic and volcanic rocks commonly found in immature island arcs the available geochemical data from ua and ga show distinct differences between the two units compared with the geochemical evolution of some of the well known phanerozoic ophiolites the pillow lavas and associated dikes of the ua show a compositional range that is similar to typical morb type ligurian ophiolites in the western alps apennines and those displayed by lip type caribbean ophiolites the ga is characterized by island arc tholeiite iat to boninite like rocks and defines a magmatic evolution that is comparable to that of suprasubduction zone ssz ophiolites in the mediterranean region our proposed geodynamic model for the isb suggests that the ua was built by primary to differentiated mantle generated melts during seafloor spreading little to moderately affected by subduction processes and that the iat to boninitic like rocks of the ga formed at a later stage by melting from a strongly subduction affected depleted and hydrated mantle our interpretation of the isb is that the ua and ga represent early and late stages respectively in the formation of a ssz ophiolite this implies that phanerozoic type plate tectonic processes such as seafloor spreading and subduction were operating by 3 8 ga in the palaeoarchean

neoarchean suprasubduction zone arc magmatism in southern india geochemistry zircon u pb geochronology and hf isotopes of the sittampundi anorthosite complex abstract the sittampundi anorthosite complex sac in southern india is one of the well exposed archean layered anorthosite gabbro ultramafic rock associations here we present high precision geochemical data for the various units of sac coupled with zircon u pb geochronology and hf isotopic data for the anorthosite the zircon ages define two populations the older yield a concordia age of 2541 13 ma which is interpreted as the best estimate of the magmatic crystallization age for the sittampundi anorthosite a high grade metamorphic event at 2461 15 ma is suggested by the upper intercept age of the younger zircon population a neoproterozoic event at 715 180 ma resulted in pb loss from some of the metamorphic zircons the magmatic age of the anorthosite correlates well with the timing of crystallization of the arc related 2530 ma magmatic charnockites in the adjacent salem block while the metamorphic age is synchronous with the regional metamorphic event the geochemical data suggest that the rocks were derived from a depleted mantle source sub arc mantle metasomatism of slab derived fluids and subsequent partial melting produced hydrous aluminous basalt magma the magma fractionated at depth to produce a variety of high alumina basalt compositions from which the anorthositic complex with its chromite rich and amphibole rich layers formed as cumulates within the magma chamber of a supra subduction zone arc the coherent initial176hf 177hf ratios and positive ehf values 1 7 4 5 of the magmatic zircons in the anorthosite are consistent with derivation of a rather homogeneous juvenile parent magma from a depleted mantle source our study further confirms that the southern part of the dharwar craton was an active convergent margin during the neoarchean with the generation and emplacement of suprasubduction zone arc magmas which played a significant role in continental growth

standardization in gravity reduction gravity reduction standards are needed to improve anomaly quality for interpretation and to facilitate the joining together of different data sets to the extent possible data reduction should be quantitative objective and comprehensive leaving ambiguity only to the interpretation process that involves qualitative subjective and geological decisions the term bouguer anomaly describes a field intended to be free of all nongeologic effects not modified by a partial geologic interpretation measured vertical gradients of gravity demonstrate considerable variation but do not suggest as often reported that the normal free air gradient is in error or needs to be locally adjusted such gradients are strongly influenced by terrain and to a lesser extent by the same geologic sources which produce bouguer anomalies a substantial body of existing literature facilitates the comprehensive treatment of terrain effects which may be rigorously implemented with current computer technology although variations in topographic rock density are a major source of bouguer anomalies a constant density appropriate to the area under investigation is normally adopted as a data reduction standard leaving a treatment of the density variations to the interpretation a field example from british columbia illustrates both the variations in vertical gravity gradients which can be encountered and the conclusion that the classical approach to data reduction is practically always suitable to account for the observed effects standard data reduction procedures do not and should not include reduction to datum the interpreter must be aware however that otherwise smooth regional bouguer anomalies caused by regional sources do contain high frequency components in areas of rugged topography

les bovidae artiodactyla mammalia du miocene du sous continent indien de la peninsule arabique et de l afrique biostratigraphie biogeographie et ecologie abstract supported by the study of miocene bovids mainly from the indian subcontinent the arabian peninsula and africa a provisional synthesis of the bovid biogeographical history is provided different aspects are examined successively the initial bovid radiation the evolution and biogeography of the boselaphini the intercontinental faunal exchanges the biostratigraphic correlations the emergence of the ethiopian fauna and some of the major characteristics of the evolution of the palaeoenvironments two successive bovid phases characterizing their early history are accepted the first one took place during the mammal units nm 4 to 6 and concerns mainly the caprinae and the antilopini the boselaphini seem to have appeared later and during the second phase their initial radiation took place apparently in the indian subcontinent there the success of the boselaphini is linked both to geographical isolation and to the persistence of more or less closed environments south of the himalayas during this second phase nm 7 8 the specific diversity of the bovids is increasing especially in asia and africa secondly the problems concerning the early miocene terrestrial faunal exchanges after the establishment of the land connection between the afro arabian plate and asia sensu stricto are discussed new discoveries in saudi arabia provide evidence of the occurrence of two successive faunal exchange phases the earlier preceding the time of the dam deposits in eastern saudi arabia nm 5 the second one being probably posterior to the langhian transgression nm 6 afterwards during the middle and late miocene nine bovid taxa are involved in the migration between asia and africa while the immigration into eurasia seems to have been restricted the immigration into africa was more extensive and therefore affected the bovid fauna more profoundly the emergence of the true ethiopian fauna took place between 11 10 m y and 7 m y the bovids from the grosseto lignites in tuscany are here considered as being of african origin lastly from an ecological point of view the concordance of the climatic deterioration with the appearance of mosaic savanna biotopes at the beginning of the middle miocene between the arabian peninsula and the lower latitudes near the present day equator must be noticed towards the end of the miocene the saharo arabian belt no longer constituted an obstacle to latitudinal dispersal for larger mammals north africa may at that time have been a refuge for late surviving eurasian forms