late mesozoic bimodal volcanic rocks in the jinniu basin middle lower yangtze river belt yrb east china age petrogenesis and tectonic implications abstract late mesozoic intrusive and volcanic rocks are widespread in the southeast hubei province middle lower yangtze river belt yrb east china detailed in situ zircon u pb and hf isotope elemental and sr nd pb isotopic data are presented in this paper for late mesozoic volcanic rocks from the jinniu basin yrb aiming to constrain their age petrogenesis and tectonic implications the jinniu volcanic rocks show a bimodal distribution in composition with dominant rhyolite and dacite and subordinate basalt and basaltic andesite new shrimp and la icpms zircon u pb ages indicate that the volcanic rocks of three formations in the jinniu basin were erupted at quite a short age range of about 5 ma during the early cretaceous 130 125 ma the mafic rocks are moderately enriched in large ion lithophile elements lile e g ba th u and pb and light rare earth elements lree and are characterized by negative nb ta and ti anomalies and relatively high tio2 0 72 2 06 and nb 9 20 26 5 ppm contents these analyses indicate that the geochemical characteristics of the mafic rocks in the jinniu basin are similar to worldwide phanerozoic nb enriched basalt and andesites neba new in situ zircon u pb ages and field geological relationships demonstrate that neba in the southeast hubei province are not spatially or temporally associated with high silica adakitic rocks but were most likely derived from an enriched lithospheric mantle with assimilation of minor crustal materials and then fractional crystallization during the evolution of the magma overall the felsic rocks in the jinniu basin have geochemical characteristics and sr nd pb signatures and in situ zircon hf isotopic compositions similar to those of the mafic rocks compared with the mafic rocks the felsic rocks are characterized by enriched and variable concentrations of lile and ree e g ba 33 3 1372 ppm y 11 4 33 6 ppm ybc 5 07 18 7 and negative eu anomalies eu 0 22 0 98 as well as a wide range of radiogenic nd pb isotopic values with end t 10 2 to 2 4 206pb 204pb i 17 659 18 705 207pb 204pb i 15 478 15 663 and 208pb 204pb i 37 654 38 935 and in situ zircon hf compositions of ehf t 12 7 to 1 8 these features indicate that the genesis of felsic magma in the jinniu basin is consistent with extensive fractional crystallization and large amounts of crustal contamination from an evolved mafic magma sio2 55 the bimodal volcanic rocks in this study provide convincing evidence that early cretaceous volcanic rocks in the yrb developed in a back arc extensional tectonic regime

geobarometry geothermometry and late archean geotherms from the granulite facies terrain of south india geobarometry and geothermometry of garnet bearing chamockitic and metapelitic assemblages from the high grade terrain from south india indicate three fields of equilibration high pressure granulites p 8 3 pm 1 0 kb t 760 pm 40 circ c are restricted to the charnockites and mafic granulites of the north slopes of the nilgiris these place a minimum constraint of 30 km on the maximum thickness of late archean crust medium pressure granulites p 6 4 pm 1 0 kb t 735 pm 40 circ c characterize the charnockites of the central nilgiris and the madras granulites to the east low pressure granulites p 5 0 pm 1 0 kb t 700 pm 20 circ c characterize the charnockites and metapelites south of the bhavani shear zone although migmatites from the kodaikanal massif may reflect somewhat higher temperatures all three granulite fields lie on a metamorphic geotherm piezothermic array which is strongly convex to the temperature axis and characteristic of convective heat transfer

petrogenesis of coarse grained intrusives from tahiti nui and raiatea society islands french polynesia this study is based on a set of coarse grained igneous rocks collected from two zoned plutons located in the central part of tahiti nui and raiatea the ahititera pluton central depression of tahiti nui comprises a great diversity of rocks ranging from ultrabasic to felsic in composition it shows a concentric zonation with nepheline free rocks in its periphery and nepheline bearing rocks in its central part the faaroa pluton central depression of raiatea is entirely mafic and includes only gabbros and theralites the two plutons have variable nd sr isotopic signatures especially the ahititera rocks which are subdivided into three groups based on their mineralogy geochemistry and isotope composition the isotopic variability probably reflects local heterogeneities in the society mantle plume petrographic and isotopic data have been used to define two magmatic suites in ahititera identifiable from their degree of si undersaturation the evolution of the mildly si undersaturated suite is controlled by simple fractional crystallization whereas the strongly si undersaturated suite requires additional h2o influx the third isotopic group includes only theralites the rare earth element ree compositions of the mafic rocks from both plutons do not correlate with their isotopic signature the ree patterns of the most si undersaturated rocks are systematically characterized by steeper slopes such features are also observed in lavas from seamounts located within the present day hotspot area it appears that ree concentrations in society lavas and intrusives are probably mainly governed by variable degrees of partial melting of a garnet free mantle source and are independent of their isotopic signature

lateral extrusion of lower crust from under high topography in the isostatic limit where there is isostasy the rocks of the lower continental crust are subject to an effective lateral pressure gradient equal to the gradient of topographic load whether compensation is in crustal roots or in the mantle the result is a poiseuille flow planar channel flow in the weak lower crust which removes crust from under mountains and smooths and levels the topography assuming cubic power law creep the flux of crust is proportional to the third power of the topographic gradient to the usual arrhenius term to the tenth power of the absolute temperature of the lower crust and to the negative fifth power of the geothermal gradient the result is that any initial condition with an isolated high tends eventually toward a state where the topography is pancake shaped with a flat central plateau and steeper flanks spreading outward when adjacent pancakes merge most of the relief is eliminated flow may either roughen or smooth the moho depending on whether there are lateral density contrasts in the mantle lithosphere or not although it is difficult to find analytic solutions for this evolution it is easily simulated numerically using various published flow laws for plausible lower crustal rocks lateral extrusion is shown to be insignificant under the oceans marginally significant under shields and platforms important under elevated plains and dominant beneath high plateaus and or hot delaminated regions in particular the basin and range province of the western united states cannot maintain short wavelength 100 km moho relief of more than 1 km for times greater than 10 20 m y at most if the tibetan plateau of china has been delaminated as recently suggested then it may flatten even faster reducing 100 km wavelength topographic features to 200 m relief in no more than 0 03 0 13 m y and leaving only features with wavelengths over 400 km at the present therefore the lower crust of tibet may resemble a hydraulic reservoir as previously suggested because the flatness of the moho is self maintained in these regions information on past tectonics is continually lost and the present moho shape cannot be used to balance cross sections for times in the past the flatness of the moho today is not a constraint on the mechanisms of extension or shortening in these regions nor is it evidence for underplating by intrusions it also follows that very intense and inhomogeneous dilational strains may have occurred locally as in the metamorphic core complexes of the basin and range province

plutonic rocks of intermediate composition on gran canaria the missing link of the bimodal volcanic rock suite analysis of abundant plutonic fragments in the voluminous miocene rhyolitic basaltic composite ignimbrite p1 the initial felsic cooling unit overlying the basaltic shield on gran canaria helps to solve the problem of bimodal volcanism syenogabbros dominate the plutonic suite and provide evidence that magmas of intermediate composition formed abundantly at depth but did not erupt the entire suite of plutonic fragments comprises 1 pyroxenitic to gabbroic cumulates that directly influenced the liquid line of descent of p1 magmas 2 gabbro syenogabbro syenodiorite and quartz syenite fragments showing moderate degrees of hydrothermal alteration chloritization partial melting and 3 diverse xenoliths differing significantly in bulk rock composition and texture from the erupted p1 magmas the compositions of plutonic series 2 overlap with both bulk rock and mineral compositions of the erupted magmas whereas series 1 rocks show lower alkali and silica contents compositional variations among the plutonic rocks are compatible with fractional crystallization as the major petrogenetic process locally modified by magma mixing and selective element contamination variations in trace element concentrations of the plutonic rocks however are inferred to be the result of evolved interstitial melts penetrating into or draining out of crystalline mushes during slow intratelluric solidification the presence of f rich amphibole suggests that crystalline mushes were invaded by f bearing fluids overall compositional similarity and the fact that selective contamination effects are similar support the interpretation that the series 2 plutonic fragments represent solidified portions of the magmatic system that ultimately produced the erupted p1 magmas this system extended from the zone of underplating at the mantle crust boundary through the lower and into the upper crust of gran canaria and was roughly vertically zoned in composition as shown by geobarometric calculations

the structural framework of the southern margin of the limpopo belt south africa abstract the southern marginal zone is characterized by two prominent regional structural features i massive crustal wedges 20 km 50 km containing large oval shaped closed fold structures which are related to an early d 1 phase of deformation and which are bounded by ii major ductile shear zones that strike in a general e w direction the hout river shear zone forms the southern boundary of the southern marginal zone where granulites are juxtaposed against lower grade granite greenstones of the kaapvaal craton this fault system also acted as the sole thrust for the other high grade shear zones in the southern marginal zone and played a major role in the development of the zone of retrogression and the establishment of the orthoamphibole isograd in the southern marginal zone the hout river shear zone and related faults within the southern marginal zone clearly post date d 1 and a period of peak metamorphism m 1 they are composite structures showing dominantly reverse sense displacements and are responsible for the emplacement of the granulite terrane over and onto the kaapvaal craton in contrast to these older structures are the reactivated shear zones these structures clearly cross cut and displace the high grade shear zones in the southern marginal zone as well as the retrograde orthoamphibole isograd the fact that the hout river and related shear zones are in metamorphic equilibrium with the surrounding wall rocks suggests their age is that of the metamorphism i e 2670 ma barton and van reenen 1992

discovery of a trachyte ignimbrite sequence at hualālai hawaii ignimbrites are common in many intraplate ocean islands but have been missing from the known geological record in hawaii during a recent field campaign the remnants of a trachytic ignimbrite sequence have been discovered at hual lai volcano fortuitously preserved from subsequent basaltic lava flow cover we provide a preliminary description of these deposits as well as bulk and glass chemical analyses to determine their potential relationship with other nearby trachytes from pu u wa awa a pww and pu u anahulu pa the results suggest that these ignimbrites are from neither pww nor pa but instead may relate to trachytes that are found as maar wallrock blocks some 20 km distant therefore despite being rare overall in hawaii the ignimbrites and more generally trachytes were probably widespread around hual lai compared to other intraplate ocean islands the combination of a fast moving plate high magma supply and eruption rates underneath hawaiian volcanoes may explain the scarcity of ignimbrites preserved at the surface their presence at hual lai could reflect unusual conditions of edifice stress during the transition from shield to post shield volcanism

weakly coupled lithospheric extension in southern tibet a b s t r a c t west east extension is a prominent tectonic feature of southern and central tibet despite ongoing north south n s convergence between india and eurasia knowledge of deep structure beneath the n s trending rifts is key to evaluating models proposed for their origin including gravitational collapse oblique convergence along the arcuate plate boundary and mantle upwelling we model direct s and moho reflected sspmp phases at teleseismic distances to constrain variations in crustal thickness across the major rifts crossed by a 900 km long w e broadband array in the lhasa terrane crustal thicknesses are 70 80 km however moho depth decreases by 10 km within a horizontal distance of 100 km west of the yadong gulu rift ygr and nyainquentanghla mountains nqtl this moho uplift taken with deep extensional focal mechanisms and reduced seismic velocity in the upper mantle suggests that asthenospheric upwelling has significantly contributed to the pattern of extension across the ygr and nqtl the 100 km separation between surface rift and moho uplift is likely enabled by partial decoupling across a ductile middle crust 2015 the authors published by elsevier b v this is an open access article under the cc by nc nd license http creativecommons org licenses by nc nd 4 0

crustal and mantle velocity models of southern tibet from finite frequency tomography 1 using traveltimes of teleseismic body waves recorded by several temporary local seismic arrays we carried out finite frequency tomographic inversions to image the three dimensional velocity structure beneath southern tibet to examine the roles of the upper mantle in the formation of the tibetan plateau the results reveal a region of relatively high p and s wave velocity anomalies extending from the uppermost mantle to at least 200 km depth beneath the higher himalaya we interpret this high velocity anomaly as the underthrusting indian mantle lithosphere there is a strong low p and s wave velocity anomaly that extends from the lower crust to at least 200 km depth beneath the yadong gulu rift suggesting that rifting in southern tibet is probably a process that involves the entire lithosphere intermediate depth earthquakes in southern tibet are located at the top of an anomalous feature in the mantle with a low vp a high vs and a low vp vs ratio one possible explanation for this unusual velocity anomaly is the ongoing granulite eclogite transformation together with the compressional stress from the collision eclogitization and the associated negative buoyancy force offer a plausible mechanism that causes the subduction of the indian mantle lithosphere beneath the higher himalaya our tomographic model and the observation of north dipping lineations in the upper mantle suggest that the indian mantle lithosphere has been broken laterally in the direction perpendicular to the convergence beneath the north south trending rifts and subducted in a progressive piecewise and subparallel fashion with the current one beneath the higher himalaya

a flow law for dislocation creep of quartz aggregates determined with the molten salt cell we have used the molten salt cell to conduct an experimental study on the rheology of a natural quartzite containing 0 15 wt water co axial deformation experiments were conducted at constant piston displacement rates approximating constant strain rates at low strain the strengths of our natural quartzite measured in the molten salt cell are approximately half those measured at the same conditions in solid media because unlike solid confining media molten salt does not contribute to the strength of the sample it reduces the friction on the moving piston and it allows clear identification of the hit point we have limited the experimental conditions to those required for dislocation creep and have used only steady state flow stresses measured during climb accommodated dislocation creep to calculate the flow law parameters two flow laws were determined one for samples containing minor amounts of melt 1 2 and one for melt free samples in both cases the power law stress exponent n is 4 0 0 9 which is greater than that previously reported in flow laws for dislocation creep of quartz aggregates determined in solid media the activation energy q is 137 34 kj mol 1 for samples with melt and 223 56 kj mol 1 for those without within the range of previously determined values for quartz aggregates containing 0 1 wt water the pre exponential term a is 1 1 10 4 2 mpa ns 1 for samples without melt and 1 8 10 8 2 mpa ns 1 for those with melt the lower strengths measured in the molten salt cell indicate that previous piezometer relations for quartz experimentally determined in solid media are not correct extrapolation of the flow law for melt free aggregates to natural strain rates predicts higher strengths than most previous quartz flow laws however accurate extrapolation requires determining the dependence of flow stress on fh2o and or ah

mode of cenozoic east west extension in tibet suggesting a common origin of rifts in asia during the indo asian collision cenozoic rifts in tibet were traditionally interpreted as a result of topographic collapse of the tibetan plateau reaching the maximum elevation that can be supported by its mechanical strength recent studies have emphasized possible similarities between rifting in tibet and extension in the basin and range of the western united states however when examined in detail one finds that spacing of long 100 km rifts in tibet 100 300 km is significantly greater than that in the basin and range 20 40 km from south to north rift spacing decreases systematically from 191 67 km in the himalaya south of the indus yalu suture to 146 34 km in south tibet between the indus yalu and bangong nujiang sutures and farther north to 101 31 km in central tibet between the bangong nujiang and jinsha sutures instability analysis suggests that the mantle lithosphere must have been involved in east west tibetan extension specifically the widely spaced rifts in the himalaya and tibet may have been related to the presence of a relatively light crust density 2 90 g cm 3 and a strong mantle lithosphere 40 km thick and a factor of 5 stronger than the upper crust the observed systematic decrease in rift spacing can be explained by the known decrease in the crustal thickness in tibet from 70 80 km in the himalaya in the south to 50 55 km in central tibet in the north a regional comparison of rifts in east asia suggests that both the involvement of the mantle lithosphere and the age of rift initiation are similar for tibetan rifts the baikal rift and the shanxi graben this implies that topographic collapse or a convective event in the mantle cannot be the sole cause for the development of the tibetan rifts a regional boundary condition applied throughout east asia must be required

cocorp deep seismic reflection traverse of the interior of the north american cordillera washington and idaho implications for orogenic evolution deep seismic reflection data collected by the consortium for continental reflection profiling along a transect of the interior of the northwestern u s cordillera indicate the presence of crustpenetrating faults these faults developed during an orogenic cycle consisting of jurassic to paleocene accretion related crustal thickening followed by eocene crustal thinning the reflection data also suggest that the moho is a young dynamic feature which developed a relatively smooth flat geometry beneath a complexly deformed crust probably during an eocene episode of crustal extension the profiles reveal prominent west dipping reflection sequences and associated diffractions interpreted as a mesozoic thrust system of crustal dimensions that linked accretion related shortening in the western cordillera with the thrust belt in the eastern cordillera east dipping reflections that crosscut the west dipping reflections in the middle and lower crust and are interpreted as ductile deformation zones that accommodated eocene crustal extension and essentially flat moho reflections at 33 35 km beneath complexly deformed crust these features are consistent with a sequence of 1 late mesozoic crustal imbrication of transitional crust beneath the easternmost accreted terrane which overrode the north american passive continental margin along a jurassic thrust 2 propagation of crust penetrating thrusts to shallower levels in the east where they probably formed the sole of the rocky mountain thrust belt 3 eocene extension that disrupted these thrusts and led to uplift of core complexes along detachments which fed into broad deformation zones in the lower crust and 4 development of a flat moho geometry during cenozoic extension and magmatism these data provide new information about the structure of the entire crust in the cordilleran interior and offer a working model both for evolution of cordilleran core complexes and possibly for other collisional orogens

structural geology and tectonic setting of the sutherland greenstone belt kaapvaal craton south africa abstract this paper presents the results of recent mapping in selected parts of the sutherland greenstone belt at the northern edge of the kaapvaal craton south africa structural studies in the northern and central parts of the belt have defined a tectonic regime dominated by north dipping ductile shear zones with an oblique to reverse sense of movement the fabric pattern related to this thrusting can be recognised in both the supracrustal rocks and the adjacent granitoid gneisses but is absent from the 2 65 ga granites bordering the southern margin of the belt this implies a minimum age for deformation in the sutherland belt coeval with that recognised in the immediately adjacent high grade limpopo belt the structures in the granitoid greenstone terrane are interpreted as a mechanical response on the kaapvaal craton to collision tectonics in the limpopo belt

the transformation of amphibolite facies gneiss to charnockite in southern karnataka and northern tamil nadu india amphibolite facies metamorphic grade gives way southward to the granulite grade in southern karnataka as acid gneisses develop charnockite patches and streaks and basic enclaves develop pyroxenes petrologic investigations in the transitional zone south of mysore have established the following points 1 the transition is prograde amphibole bearing gneisses intimately associated with charnockite at kabbal and several similar localities are not retrogressive after charnockite as proved by patchy obliteration of their foliation by transgressive very coarse grained charnockite high fluorine content of biotite and amphibole in gneisses and high large ion lithophile element contents in gneisses and charnockites these features are in contrast to very low fluorine in retrogressive amphiboles and biotites very low large ion lithophile element contents and zonal bleaching of charnockite in clearly retrogressive areas as at bhavani sagar tamil nadu 2 metamorphic temperatures in the transitional areas were 700 800 c pressures were 5 7 kbar and h2o pressures were 0 1 0 3 times total pressures based on thermodynamic calculations using mineral analyses dense co2 rich fluid inclusions in the kabbal rocks confirm the low h2o pressures at the first appearance of orthopyroxene farther to the south in the nilgiri hills and adjacent granulite massif areas peak metamorphic temperatures were 800 900 c pressures were 7 9 kbar and water pressures were very low so that primary biotites and amphiboles those with high f contents are rare 3 the incipient granulite grade metamorphism of the transitional areas was introduced by a wave of anatexis and k metasomatism this process was arrested by drying out under heavy co2 influx charnockites so formed are hybrids of anatectic granite and metabasite of metabasite and immediately adjacent gneiss or are virtually isochemical with pre existing gneiss despite gross recrystallization to granulite mineralogy these features show that partial melting and metasomatism are attendant rather than causative in charnockite development copious co2 from a deep crustal or mantle source pushed ahead of it a wave of more aqueous solutions which promoted anatexis granulite metamorphism of both neosome and paleosome followed the process is very similar to that deduced for the madras granulites by weaver 1980 the massif charnockites for the most part extremely depleted in lithophile minor elements show many evidences of having gone through the same process

evidence for metamorphic and igneous charnockites in the southern marginal zone of the limpopo belt abstract metamorphic and igneous charnoenderbite and enderbite can be distinguished on the basis of field relations and bulk rock geochemistry in the southern marginal zone of the limpopo belt termed here metamorphic charnockite and igneous charnockite respectively both varieties are petrographically similar they are massive homogeneous rocks which formed after the d1 fabric forming event associated with the limpopo orogeny and they at least experienced the entire retrograde portion of that orogeny the metamorphic charnockite probably developed first during peak metamorphism m1 and is often closely associated with a banded orthopyroxene bearing quartzo feldspathic gneiss the baviaanskloof gneiss the metamorphic charnockite baviaanskloof gneiss relations in the field are those of quiescent obliteration of foliation and the charnockite is often characterized by the presence of relict banding the chemical composition of the metamorphic charnockite is identical with that of the associated baviaanskloof gneiss the igneous charnockite locally retaining igneous textures is characterized by intrusive relationships with the baviaanskloof gneiss and in most cases by chemical compositions that are distinct from those of the older unit available geochronological data indicate that the metamorphic charnockite or its protolith formed at or after 2715 ma ago significantly before emplacement of the igneous charnockite of the matok complex at 2671 ma emplacement of igneous charnoenderbite and enderbite in the rest of the southern marginal zone and in the matok complex was not synchronous but occurred respectively during and after peak metamorphism m1 metamorphic charnockite with preserved relict banding probably formed by in situ biotite dehydration to orthopyroxene in the presence of a fluid phase with locally different activity of co2 h2o dilution through extraction of a granitic melt is considered to be unfeasable because of similar bulk chemistry the igneous charnockite crystallized from calc alkaline magmas derived by partial melting of lower crustal rocks similar to some phases of the baviaanskloof gneiss

the gold flat tuff nevada insights into the evolution of peralkaline silicic magmas the gold flat tuff is the youngest 9 15 ma ash flow sheet erupted from the black mountain volcanic centre southwest nevada usa this paper explores some aspects of the very complex nature of the tuff s magmatic plumbing system the main body of the deposit is a mixed magma product comprising pantelleritic and comenditic melts derived from independently evolving reservoirs and antecrysts and enclaves derived from a range of basic to intermediate sources metre scale cognate xenoliths point to the presence of alkali feldspar accumulation zones the pantellerite contains phenocrysts of fluorite and chevkinite ce the inferred intermediate magma component contains perrierite ce phenocrysts the pantellerite has unusually high contents of f 2 2 wt f cl 2 9 wt and zro 2 1 04 wt the high halogen contents may have influenced the evolution of the strongly peralkaline magma the crystallization conditions are poorly constrained but those for the pantelleritic magma may have been close to water saturation 4 wt melt water at temperatures 740 c and fo 2 around fmq

the southern high grade margin of the dharwar craton the amphibolite facies to granulite facies transition at the southern margin of the dharwar craton has been studied in the krishnagiri satnur halaguru and kushalnagar areas of southern india in all three areas the transition appears to be a progressive metamorphic overprint on cratonal gneisses and their mafic and metasedimentary enclaves without major structural interruption in the kabbal satnur br hills section of karnataka a high grade charnockite massif with pronounced rb depletion is the culmination of an apparently continuous increase of metamorphic grade southward in this and the kushalnagar areas increase of paleopressure from near 6 to near 8 kbar with increasing grade indicates a depth zone relationship of the amphibolite and granulite facies incipient charnockite replacing peninsular gneiss first appears along n s shears parallel to the regional grain of the craton low p h 2 o high co 2 vapors were instrumental in the creation of orthopyroxene introduction from a deep so

tectono metamorphic evolution of the simatic basement of the pietersburg greenstone belt relative to the limpopo orogeny evidence from serpentinite abstract detailed studies of low grade ultramafic rocks are integrated with regional structural field observations in the pietersburg greenstone belt these results show that petrographic investigations of serpentinite can reveal more about the tectonometamorphic history of greenstone belts than has previously been appreciated the ultramafic rocks of the pietersburg greenstone belt were affected by an early deformation event d 1 which is reflected by the presence of a relic magnetite fabric s 1 in serpentinite the s 1 fabric is crosscut by the peak metamorphic assemblage of antigorite tremolite talc chlorite both s 1 and the peak metamorphic assemblage are deformed by a second deformation event d 2 on a macroscale d 2 is manifested as a series of northward verging thrust faults that affected the entire belt published u pb zircon data show that d 2 was active between 2 9 and 2 7 ga prior to uplift and southward thrusting of the limpopo belt lb major carbonatization of the simatic basement postdates peak metamorphism and occurred towards the end of d 2 coeval with the late evolution of the limpopo orogeny the alteration assemblages were subsequently deformed the sequence of events that affected the pietersburg greenstone belt is similar to that established for the high grade southern marginal zone of the limpopo belt but their absolute timing is different the results of this study demonstrate the important contribution that low grade ultramafic rocks can make to the understanding of the tectono metamorphic history of the archaean cratons and terrane accretion

cenozoic crustal scale duplexing and flat moho in southern tibet evidence from reflection seismology abstract although the lhasa terrane of southern tibet is an integral part of the tibetan plateau how the area has achieved its current high altitude of 5000 m remains poorly understood this is largely due to the fact that little is known about deep 15 km crustal structures and the depth and geometry of the moho below the terrane in this study we address this question by acquiring and analyzing high resolution seismic reflection data gathered from four high energy dynamite shots along a 137 km north south traverse spanning the southern lhasa terrane our new seismic data reveal the presence of north dipping reflectors in the crust a seismically weak reflection layer at the lowermost crust and a sub horizontal and locally discontinuous moho at depth of 69 72 km with a crustal velocity of 6 30 km s the discordant relationship between the crustal dipping reflectors the subhorizontal lower crustal layer and the flat moho suggests that the 12 km basal section of the lhasa terrane crust is a decoupling zone accommodating different styles of lithospheric scale deformation because the north dipping reflectors can be projected and or connected with the reflectors in the cenozoic himalayan thrust system and the southernmost lhasa terrane we propose that the dipping crustal reflectors are parts of a south directed crustal scale thrust duplex created by the cenozoic india asia collision the relatively discontinuous and horizontal geometry at the base of the crust indicates that the moho was reconstituted through ductile deformation in the lowermost crust caused by abnormally high temperature

predictive mapping of prospectivity for orogenic gold giyani greenstone belt south africa we present a mineral systems approach to predictive mapping of orogenic gold prospectivity in the giyani greenstone belt ggb by using layers of spatial evidence representing district scale processes that are critical to orogenic gold mineralization namely a source of metals fluids b active pathways c drivers of fluid flow and d metal deposition to demonstrate that the quality of a predictive map of mineral prospectivity is a function of the quality of the maps used as sources of spatial evidence we created two sets of prospectivity maps one using an old lithologic map and another using an updated lithological map as two separate sources of spatial evidence for source of metals fluids drivers of fluid flow and metal deposition we also demonstrate the importance of using spatially coherent or geologically consistent deposit occurrences in data driven predictive mapping of mineral prospectivity the best predictive orogenic gold prospectivity map obtained in this study is the one that made use of spatial evidence from the updated lithological map and spatially coherent orogenic gold occurrences this map predicts 20 of the ggb to be prospective for orogenic gold with 89 goodness of fit between spatially coherent inactive orogenic gold mines and individual layers of spatial evidence and 89 prediction rate against spatially coherent orogenic gold prospects in comparison the predictive gold prospectivity map obtained by using spatial evidence from the old lithological map and all gold occurrences has 80 goodness of fit but only 63 prediction rate these results mean that the prospectivity map based on spatially coherent gold occurrences and spatial evidence from the updated lithological map predicts exploration targets better i e 28 smaller prospective areas with 9 stronger fit to training gold mines and 26 higher prediction rate with respect to validation gold prospects than the prospectivity map based on all known gold occurrences and spatial evidence from the old lithological map

geochronology of the hout river shear zone and the metamorphism in the southern marginal zone of the limpopo belt southern africa abstract in this paper monazite u pb and zircon evaporation dates stepleaching pb pb results on garnet staurolite and kyanite and hornblende ar ar data are presented which constrain the timing of granulite facies metamorphism in the southern marginal zone of the limpopo belt and its thrusting onto the kaapvaal craton the southern marginal zone of the limpopo belt is considered to be a lower crustal equivalent of the northern kaapvaal craton granulite exhumation is associated with southward thrusting along the hout river shear zone which is a set of thrust and strike slip shear zones zircon ages for the matok intrusive complex which was emplaced within the zone during this thrusting charno enderbites 2671 4 ma granodiorites between 2667 and 2664 ma have previously been interpreted as evidence for rapid exhumation of the southern marginal zone within only 7 ma we have obtained a u pb date of 2691 7 ma for monazite from the bandelierkop quarry in the southern marginal zone interpreted as the age of high grade metamorphism a single zircon evaporation pb pb date of 2643 1 ma from a leucosome band at the same locality may indicate longer lasting metamorphism or decompression melting during exhumation anatexis of metapelitic xenoliths within the matok intrusive complex was dated at 2663 4 ma by u pb on monazite indistinguishable from existing zircon ages for this complex pb pb step leaching dates obtained on synkinematically grown garnet 2691 20 ma staurolite 2712 37 ma and kyanite 2672 51 ma from the khavagari hills in the giyani greenstone belt in the immediate footwall of the hout river shear zone indicate that early thrusting was contemporaneous with peak metamorphism in the southern marginal zone ar ar dating and geochemistry on syntectonic hornblende separates from the same shear zone system yielded disturbed spectra and indicated multiple populations probably reflecting repeated or continuous tectonic activity of the hout river shear zone up to about 2600 ma

detection of widespread fluids in the tibetan crust by magnetotelluric studies magnetotelluric exploration has shown that the middle and lower crust is anomalously conductive across most of the north to south width of the tibetan plateau the integrated conductivity conductance of the tibetan crust ranges from 3000 to greater than 20 000 siemens in contrast stable continental regions typically exhibit conductances from 20 to 1000 siemens averaging 100 siemens such pervasively high conductance suggests that partial melt and or aqueous fluids are widespread within the tibetan crust in southern tibet the high conductivity layer is at a depth of 15 to 20 kilometers and is probably due to partial melt and aqueous fluids in the crust in northern tibet the conductive layer is at 30 to 40 kilometers and is due to partial melting zones of fluid may represent weaker areas that could accommodate deformation and lower crustal flow

the boundary between the indian and asian tectonic plates below tibet the fate of the colliding indian and asian tectonic plates below the tibetan high plateau may be visualized by in addition to seismic tomography mapping the deep seismic discontinuities like the crust mantle boundary moho the lithosphere asthenosphere boundary lab or the discontinuities at 410 and 660 km depth we herein present observations of seismic discontinuities with the p and s receiver function techniques beneath central and western tibet along two new profiles and discuss the results in connection with results from earlier profiles which did observe the lab the lab of the indian and asian plates is well imaged by several profiles and suggests a changing mode of india asia collision in the east west direction from eastern himalayan syntaxis to the western edge of the tarim basin the indian lithosphere is underthrusting tibet at an increasingly shallower angle and reaching progressively further to the north a particular lithospheric region was formed in northern and eastern tibet as a crush zone between the two colliding plates the existence of which is marked by high temperature low mantle seismic wavespeed correlating with late arriving signals from the 410 discontinuity poor sn propagation east and southeast oriented global positioning system displacements and strikingly larger seismic sks anisotropy

hydration of cordierite and hypersthene and a description of the retrograde orthoamphibole isograd in the limpopo belt south africa

exhumation of limpopo central zone granulites and dextral continent scale transcurrent movement at 2 0 ga along the palala shear zone northern province south africa abstract the palala shear zone is part of a major tectonic lineament in southern africa which extends over 1000 km from central botswana to the soutpansberg in south africa the lineament is situated between the central zone of the limpopo belt and the kaapvaal craton at its main exposure in the koedoesrand window 50 km e of ellisras northern province the palala shear zone comprises a c 15 km wide mylonite zone which can be subdivided into a northern central and southern domain this study addresses the tectono metamorphic history of the palala shear zone in the koedoesrand window and its role in the tectonics of the limpopo belt in order to do this structural and petrographical work was combined with geochronology age data from relic mineral parageneses show that metapelites from the central domain underwent granulite facies metamorphism around 2 6 ga however because of an intense proterozoic overprint field relationships of the archean event are largely disrupted during a second high grade event at c 2 0 ga granulite facies metamorphism was reached in the northern domain and the adjacent limpopo central zone charnockites were emplaced in the northern domain after 2 1 ga and were affected by retrograde amphibolite and greenschist facies metamorphism around 1 97 ga post orogenic transcurrent faulting under sub greenschist facies conditions characterizes the central and southern domains field observations and microstructural investigations indicate a dextral shear sense for the retrograde mylonitisation these findings document that the palala shear zone played an important role during the exhumation of the proterozoic granulites in the limpopo central zone associated with a transpressive orogeny between 2 05 and 1 95 ga low grade post orogenic transcurrent faulting in the palala shear zone at c 1 9 ga may be associated with the transtensional opening of the soutpansberg and palapye grabens on the basis of the tectono metamorphic history in the koedoesrand window and regional geophysical geochemical and sequence stratigraphic data the palala shear zone is interpreted as a proterozoic suture zone along which the archean kaapvaal and zimbabwe provinces were juxtaposed

large scale fluid infiltration metasomatism and re equilibration of archaean basement granulites during palaeoproterozoic thrust belt construction ungava orogen canada abstract the metamorphic history of the archaean superior province crystalline basement in the palaeoproterozoic ungava orogen attests to the importance of structural and geohydrological controls on a retrograde amphibolite granulite transition two distinct metamorphic suites separated in age by nearly one billion years are recognized in extensively exposed tonalitic to dioritic metaplutonic gneisses the older suite comprises c 2 7 ga granulite facies assemblages orthopyroxene clinopyroxene hornblende plagioclase ilmenite biotite quartz that record moderate pressures 5 kbar and high temperatures 800 c a younger c 1 8 ga suite resulted from amphibolitization of the granulites and is characterized by regionally extensive amphibolite facies mineral zones that broadly parallel the basal decollement of the overlying proterozoic cape smith thrust belt deformation mineral growth relationships in the amphibolitized basement indicate that extensive hydration and re equilibration of the archaean granulites occurred during thrust belt deformation the transition from granulite facies to amphibolite facies assemblages is characterized by the growth of garnet hornblende quartz cummingtonite coronas between plagioclase and orthopyroxene clinopyroxene as well as titanite coronas on ilmenite multi equilibrium thermobarometry on the coronitic assemblages documents re equilibration of the granulitic gneiss to 7 7 kbar at 644 c in the south and 9 8 kbar at 700 c in the north the variably deformed amphibolite facies domain sandwiched between the coronitic garnet zone and the basal decollement is marked by significant metasomatic changes in major element concentrations within tonalite these changes are compatible with equilibrium flow of an aqueous chloride fluid down a temperature gradient the source of fluids for basement hydration metasomatism is interpreted to be dehydrating clastic rocks in the overlying thrust belt with fluid flow probably focused along the basal decollement

role of fluids in the exhumation of the southern marginal zone of the limpopo complex south africa abstract the limpopo complex lc of southern africa displays structural metamorphic and geochronological evidence for a complex exhumation controlled evolutionary history that followed after burial and attainment of peak metamorphic conditions p 9 kbar t 900 c in the root zone central zone cz of the orogen before 2 72 ga the southern marginal zone smz of the lc in addition provides evidence for thrust controlled emplacement of a hot granulite facies nappe onto the adjacent low grade granite greenstone terrane of the northern kaapvaal craton nkvc in the interval 2 69 2 62 ga a tectono metamorphic scenario in which initial near vertical channeling exhumation at 2 69 ga was transferred into horizontal channeling thrusting at 2 67 2 64 ga is interpreted to have commenced with in situ partial melting of crustal rocks in the root zone of the orogen cz this resulted in formation of melt weakened crust migmatized gneisses that with continuous shortening led to vertical transfer of lower crustal material accompanied by fluid fluxed decompression melting that resulted in production of large volumes of leucocratic granitic material this material detached to form buoyant diapirs that accompanied the emplacement at 2 65 ga of mega sheath folds at the middle crustal level throughout the entire cz of the lc this early stage of near vertical exhumation is reflected i by the fact that linear elements of sheath folds and shear zones plunge consistently at steep angles to the sw throughout the entire cz and ii by decompression cooling dc segments of p t paths constructed for metapelitic rocks dc paths in the far northern part of the smz that are associated with the steep north dipping portion of the hout river shear zone hrsz traversed from p 8 5 kbar t 850 c to p 6 5 kbar t 720 c early near vertical exhumation transferred into horizontal channel flow that controlled emplacement at the middle crustal level of the granulite facies nappe smz horizontal channeling thrusting is recorded in metapelitic granulite by near isobaric cooling p 6 kbar t 700 550 c segments of earlier dc p t paths that are associated with the shallow north dipping portion of the hrsz devolitilization of underthrusted greenschist due to thrusting hot granulite over cool greenschist released large volumes of high temperature aqueous fluids from the underthrusted rocks that constantly infiltrated the overriding granulite during southwards migration of the nappe immiscible co 2 rich and brine fluids thus derived persistently infiltrated into and reacted with cooling granulite and triggered the main pulse of anatexis at p 7 5 kbar t 750 c similar fluids established a regional retrograde isograd and associated zone of 4500 km 2 retrograde hydrated granulite at p 6 kbar t 600 630 c located in the hanging wall of the shallow north dipping hrsz that bounds the smz in the south shear zone hosted metasomatic alteration including formation of lode gold deposits occurred at t 600 630 c p 6 kbar in the zone of rehydrated granulite and at higher p t conditions in the granulite zone the granulite facies smz of the lc thus preserves clear evidence for a direct link between persistent fluid flux and fluid rock interaction as an integral component of a conveyor like tectonic mechanism driven by vertical tectonics that operated from the core region of the orogen cz this mechanism explains the important link between the thrust controlled emplacement of the smz onto the nkvc and deep crustal fluid flow

progressive metamorphism of mafic rocks from greenschist to granulite facies in the dharwar craton of south india in the archean dharwar craton of southern india n s trending belts of metabasic rocks are exposed which underwent regional metamorphism at about 2 5 ga ago the progressive changes in the assemblages and mineral chemistry of metabasites was studied in a n s traverse covering the chitradurga and nagaman gala belts the sargur area and the nilgiri hills towards the south with increasing metamorphic grade greenschists chl act ab ep pm carb qtz give way to amphibolites hbl plag pm qtz gar cumm and mafic granulites pyx plag gar pm hbl qtz the amphibole composition changes from actinolite in the greenschist zone to tschermakitic hornblende in the amphibolite zone and pargasitic hastingsitic hornblende in the granulite zone the ti content of the amphiboles systematically increases in this direction the plagioclase composition changes from albite in the greenschist zone to oligoclase andesine in the amphibolite zone and andesine labradorite in the granulite zone almandin

stable isotope profile across the orthoamphibole isograd in the southern marginal zone of the limpopo belt south africa abstract in the southern marginal zone of the limpopo belt the transition from granulite to amphibolite facies metamorphism is defined by a change from orthopyroxene to orthoamphibole bearing assemblages within metasedimentary rocks of the supracrustal bandelierkop formation oxygen isotope fractionations among constituent minerals within both amphibolite and granulite facies metasedimentary rocks are of similar magnitude implying a similar retrograde thermal history temperatures calculated from these fractionations can be arranged in the following order quartz garnet quartz orthoamphibole quartz pyroxene quartz plagioclase quartz biotite the observed temperature sequence is consistent with retrograde metamorphism under closed system conditions and oxygen closure of garnet orthoamphibole and orthopyroxene at or close to peak metamorphism while quartz plagioclase and biotite crystallized from and or re equilibrated via a melt fluid phase down to temperatures of 560 c small fluid rock ratios are implied by the simultaneous closure of quartz plagioclase and biotite given that quartz is the dominant oxygen bearing phase during retrogressive exchange and assuming garnet closure to oxygen diffusion at the time of formation then the quartz garnet fractionations may provide minimum peak temperature estimates of 736 52 c coarse grained concordant felsic veins within the metasediments texturally interpreted as anatectites are in isotopic equilibrium with the host rock the closed system defined here only on the basis of oxygen isotope systematics does allow for fluid melt extraction at high temperatures in contrast to the metasediments oxygen isotope systematics for mafic and ultramafic rocks of the bandelierkop formation suggest open system behaviour for at least some of the rocks such rocks are commonly cross cut by coarse grained felsic veins which are not in isotopic equilibrium with the host rock and contain quartz whose 18 o values are indistinguishable from those observed in the metasedimentary rocks the stable isotope data are consistent with a model involving anatexis of metasedimentary rocks at peak metamorphic temperatures partial extraction of the anatectic melts and their crystallization elsewhere in the sequence is suggested by the mineralogy and isotopic composition of large discordant pegmatites together with the isotopic disequilibrium and open system behaviour observed in some mafic and ultramafic rocks in the pelitic rocks the release of small amounts of fluid from residual melts and or somewhat larger amounts from partially collected melts during cooling may account for many of the retrograde mineralogic features observed for these rocks without perturbing the oxygen isotope systematics such retrograde features include the cordierite orthopyroxene symplectite after garnet and the incipient hydration of orthopyroxene and cordierite within the granulites and possibly the regional retrogression of granulites to form orthoamphibole gneisses retrograde kyanite within these rocks thus suggests an isobaric cooling path similar 13 c values of graphite separated from amphibolite and granulite facies metasediments together with significant oxygen and carbon isotope heterogeneity among charnockites metasediments banded iron formations and most mafic and ultramafic rocks are not consistent with a retrogressive influx of extraneous co 2 rich fluids as the cause of regional retrogression and orthoamphibole formation in the gneisses nor with charnockitization the data also argue against a pervasive co 2 infiltration as the cause of granulite facies metamorphism

regional geological setting of the limpopo belt abstract the 2 7 ga limpopo belt granulite terrane is situated between two lower grade granite greenstone cratons the kaapvaal craton in the south and the zimbabwe craton in the north this granulite facies terrane is subdivided into three zones each with a distinctive geological signature and separated from each other and from the adjacent cratons by well defined shear zones 1 the southern marginal zone composed of granite greenstone material very similar to that of the kaapvaal craton has retrogressed granulites on its southern margin in the hanging wall of the northward dipping terrane boundary hout river shear zone separating the granulite terrane from the adjacent kaapvaal craton 2 the northern marginal zone a granite greenstone terrane at granulite grade which is separated from the zimbabwe craton by a southward dipping terrane boundary and 3 the central zone a unique crustal element with distinctive carbonate platform lithologies and a metagabbroic layered complex which intruded before or at 3250 ma this zone cannot be related to the kaapvaal craton or to the zimbabwe craton it is separated from the two marginal zones by mylonitic shear zones with horizontal stretching lineations the entire limpopo belt is characterized by a distinctive deformational and metamorphic style which contrasts markedly from that of the adjacent granite greenstone terranes of the kaapvaal and zimbabwe cratons this distinctive pattern developed during the limpopo orogeny probably as a consequence of continental collision during the late archaean

the oxygen isotopic composition of granulites and retrogressed granulites from the limpopo belt as a monitor of fluid rock interaction abstract from the southern marginal zone of the limpopo belt three rock systems cordierite bearing metapelites mafic rocks and ultramafic rocks are studied by whole rock oxygen isotope analysis in order to decide wether the observed variation in the o isotopic composition of the metapelites is due to granulite facies fluid rock interactions or due to earlier processes a discrimination index pelite index pi is proposed in combination with the whole rock oxygen isotopic composition this index enables one to put some constraints on the origin of the precursor rocks in the case of the limpopo metapelites it is concluded that the cordierite anthophyllite bearing metapelites are only in part true metasediments a metasomatic alteration of mafic precursors in an ocean crust environment could be responsible for both low pi indices and high with respect to unaltered mantle material 18 o values fluid rock interactions in this environment are also thought to be responsible for the observed chemical and o isotopic variation of mafic and ultramafic rocks the data do not support significant large scale co 2 infiltration during granulite facies metamorphism the retrograde regionally extended anthophyllite formation in the southern marginal zone of the limpopo belt is certainly triggered by some fluid infiltration model calculations show that the amount of fluids necessary for this readjustment is too small to have an imprint on the whole rock oxygen isotope composition

the significance of 3000 ma granulite facies mafic dikes in the central zone of the limpopo belt southern africa abstract structural petrographic and chemical relationships indicate that a suite of mafic dikes possibly 3000 ma old were emplaced at high crustal levels deformed and metamorphosed epicontinental supracrustal rocks of the central zone of the limpopo belt these dikes were subsequently metamorphosed at granulite grade conditions probably during the limpopo orogeny about 2700 ma ago the earlier metamorphism was under conditions of at least amphibolite grade and isotopic data suggest that it may have occurred between about 3200 ma and 3100 ma ago these relationships indicate that the rocks comprising the central zone did not follow a single p t time loop but instead were involved in two distinct orogenies the orogenies were separated by a period of crustal stability during which the supracrustal rocks presently and originally exposed at the surface were again moved near to the surface this tectonic history also implies that the supracrustal rocks of the central zone and the sand river gneisses are of great antiquity if the sand river gneisses with their unique suite of older mafic dikes form a previously deformed basement to the supracrustal rocks then the sand river gneisses have been involved in three orogenies pb isotopic data indicate that the mafic dikes were derived from a mantle source with a high average ratio at some stage in its evolution prior to 3200 ma all archean lithologic units within the central zone of the limpopo belt are characterized by having been derived from rocks that had a large mu ratio at some time in their history and this relationship suggests that a link existed between the crustal rocks of the central zone and this mantle source from at least 3200 ma ago until it was destroyed by or after the limpopo orogeny the tectonic history of the central zone is different from those of the lithologies forming the adjacent terranes and suggests that it evolved separately from these terranes until they were juxtaposed during the limpopo orogeny

evolution of the northern high grade margin of the kaapvaal craton south africa the northern kaapvaal craton is divided into three distinctive crustal zones 1 the high grade southern marginal zone smz of the limpopo belt in the north 2 a transitional zone of technically stacked metamorphic rocks which includes the area from the sutherland belt in the north to the pietersburg and murchison belts in the south and 3 a low grade granite greenstone terrane farther to the south the change across the smz boundary is expressed by an overall increase in the grade of metamorphism a strip of retrogression related to a zone of thrusting abrupt changes in intensity of deformation rock densities magnetic lineament patterns and isotopic ages in compositionally similar rocks regional metamorphism in the northern kaapvaal craton is demonstrably not a continuum in the sense that the highest grade rocks underwent all earlier stages of metamorphism rather that rapid local tectonic processes must have occurred a proposed evolutionary path for the smz was initiated with crustal thicke

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

injection of indian crust into tibetan lower crust a two dimensional finite element model study we propose a model in which the stronger indian crust injects into the weaker lower crust of tibet if the viscosity of the tibetan lower crust is below a critical value the continuous advancing of the indian crust beneath tibet would produce negligible variation in stresses on the bottom of the stronger brittle tibetan upper crust and would result in a spatially uniform uplift and a virtually undisturbed tibetan plateau surface this process is analogous to that of a piston rising in a hydraulic jack as additional fluid is forced into the chamber we find that if the viscosity of the tibetan lower crust is assumed to be constant throughout the computed critical viscosity is 6 1018 pa s for the present day lower crustal thickness of 58 km in addition to uniform viscosity two hypothetical depth dependent viscosities are also tested which give rise to essentially the same distribution of stress along the bottom of brittle upper crust our model also interprets the 4 km elevation contrast between the tibetan plateau and the tarim block a 1 160 viscosity contrast between the tibetan and tarim lower crust is required in order to maintain the current thickening rate of the tibetan crust 1 2 mm yr while keeping the thickness of the tarim crust unchanged

himalayan tibetan analogies for the evolution of the zimbabwe craton and limpopo belt abstract the limpopo orogeny which lasted from 2700 to 2600 ma was characterised by nnw sse directed shortening this resulted in both crustal thickening accommodated by folding and nnw directed thrusting and associated crustal extrusion whereby a number of crustal blocks including the central zone of the limpopo belt was displaced to the west southwest along wsw ene trending shear zones this was part of a regionally distributed phase of orogenic deformation which affected the adjoining kaapvaal and zimbabwe cratons from the witwatersrand basin in the south to the harare greenstone belt in the north this deformation which migrated northwards with time postdated an earlier phase of crustal accretion during which the archaean crustal unit now marked by the kaapvaal and zimbabwe cratons and the limpopo belt grew towards the north northwest shortening of this unit during the limpopo orogeny may be regarded as the last stage of the crustal accretion process this tectonic process of continental growth by accretion followed by crustal shortening with associated crustal extrusion has many features in common with tibetan tectonics from the mesozoic to the present

incipient charnockite characterization at the type localities abstract fifty years after the discovery of incipient charnockite orthopyroxene bearing granitic rocks partially replacing amphibole biotite gneiss in southern india this phenomenon has been found in transitional amphibolite facies to granulite facies terrains on every continent and may be a characteristic feature of progressive deep crustal metamorphism the alteration is essentially a metasomatic transformation mass balance calculations for localities in southern india and sri lanka for which analytic data are most comprehensive show that the main reaction producing orthopyroxene in orthogneiss is breakdown of hornblende by reaction with biotite with loss of feo mgo tio 2 and cao as well as h 2 o from the host rock there is concomitant gain of sio 2 no significant change of alkalis can be demonstrated as essential to the transformation though advanced charnockitic alteration at the type locality of kabbal karnataka may also show large increase of k 2 o with removal of mafic constituents very similar orthopyroxene bearing veins in garnet biotite gneiss at the ponmudi type locality in kerala show definite but more subtle metasomatism with loss of feo and mgo and increase of sio 2 and alkalis both the kabbal and ponmudi alterations occurred at mid crustal levels 5 6 kbar and 700 750 c it is postulated that both types of charnockitic alteration may have been produced as open system processes by much the same kind of metasomatizing fluids nearly coeval co 2 rich and ultrasaline fluids based on fluid inclusion evidence the paragneiss alteration is mainly biotite breakdown with some involvement of garnet and leads to a distinct metamorphic facies type the presence of graphite in some of the garnet biotite gneisses at ponmudi would have resulted in lower oxygen fugacity and probably lower h 2 o activity than in the similar kabbal type metamorphism the fluid rock ratios for both types were small about 10 mass percent of the rocks and the fluids whatever their origin were largely rock governed the source of the metasomatizing fluids has not been definitely identified carbon isotopes of fluid inclusions seem to show an igneous or primitive mantle source at both type localities but ultrasaline fluids may be more plausibly explained by metasedimentary sources and calcareous metasediments have been identified as a source of co 2 in some occurrences much remains to be learned about incipient charnockite and its significance in processes of crustal evolution

cocorp arizona transect strong crustal reflections and offset moho beneath the transition zone the consortium for continental reflection profiling cocorp transect across the southwest margin of the colorado plateau in arizona reveals prominent subhorizontal zones of reflections here termed the bagdad reflection sequence brs which dominate the upper and middle crust in the transition zone tz between the colorado plateau and the basin and range province a cenozoic origin for the brs is considered most likely it may represent fragmented or sheared zones related to detachment faulting or a series of intrusions possibly emplaced along preexisting zones of weakness moho reflections observed beneath the tz and the basin and range province contrast with the nonreflective moho beneath the colorado plateau suggesting that extension and its associated igneous processes probably played a key role in the development of reflectors at the moho the moho and a prominent midcrustal reflector are offset by about 3 km in a normal sense in at least one place this is the best example of offset moho yet found by cocorp and contrasts sharply with cocorp observations of a smooth moho elsewhere this offset is probably a late tectonic expression of crustal extension and thinning across the tz

a no lid zone in the central chang thang platform of tibet evidence from pure path phase velocity measurements of long period rayleigh waves pure path phase velocities of fundamental mode rayleigh waves in the period range 30 80s are obtained for different paths across the tibet plateau using a two event method with events located in tibet and aligned with world wide standard seismograph network stations at teleseismic distance evidence is found for significant lateral variations of structure of the crust and uppermost mantle in tibet in particular phase velocities obtained on paths contained in the chang thang platform are higher than average in tibet at periods shorter than 60s while remaining very low at longer periods when interpreting these phase velocities in terms of simple layered s velocity models it is found that contrary to other areas in tibet they are incompatible with the existence of a high velocity lid s velocities of 4 4 4 5 km s are found immediately beneath the moho and the crust is somewhat thinner 50 60 km than average in tibet this indicates a possible upwelling of hot asthenospheric material compatible with landsat evidence for subrecent basaltic volcanism and independent seismic evidence such as inefficient transmission of sn waves on the other hand a 70 km thick crust and very high upper mantle s velocities are found on the western border of tibet karakorum hindu kush compatible with previous finer scale results concerning the associated zone of intermediate depth seismicity

metamorphic p t profiles from the zimbabwe craton to the limpopo belt zimbabwe abstract metamorphic p t profiles in the eastern and western parts of the limpopo belt zimbabwe were examined on the basis of mineralogic and petrologic data from areas which include the southernmost zimbabwe craton the northern marginal zone and the central zone the zimbabwe craton consists of a granite greenstone terrane of 2900 ma to 3600 ma old the northern marginal zone represents a high grade equivalent of the granite greenstone lithologies the central zone has suffered granulite facies metamorphism during the late archean metamorphic temperature of the zimbabwe craton adjacent to the northern boundary of the limpopo belt is calculated to be 650 680 c for amphibolite and 710 720 c for quartzofeldspathic gneiss in the eastern part these temperatures are slightly lower than that calculated from the same assemblages in the northern marginal zone and central zone peak metamorphic conditions in the eastern part of the northern marginal zone are estimated at about 740 790 c and 7 5 7 9 kbar from the assemblage garnet core orthopyroxene plagioclase quartz in pelitic gneiss these conditions were followed by a retrograde event of about 630 660 c and 3 8 4 5 kbar which were calculated from the assemblage garnet rim cordierite sillimanite quartz in the same rock the p t path is very similar to the retrograde path from the southern marginal zone it implies that both the northern marginal zone and southern marginal zone may have shared a similar metamorphic evolution garnet bearing mafic gneiss in the central zone indicates somewhat higher p t conditions than those in the northern marginal zone central zone metamorphic conditions are estimated at 790 890 c and 11 0 13 3 kbar for garnet clinopyroxene plagioclase quartz assemblages and 750 870 c and 7 6 8 8 kbar for garnet hornblende plagioclase quartz assemblages the difference of p t conditions between the northern marginal zone and central zone may be related to movement along the pre triangle shear zone the northern marginal zone appears to have been displaced along the shear zone relative to the central zone during uplift carrying the deeper crust to the surface

the archean craton of southern india metamorphic evolution and p t conditions the archaean craton of southern india is characterized by a highly complicated and not yet fully understood geological history comprizing several cycles of sedimentation and volcanism deformation and metamorphism in the span between about 3400 and 2500 m y the large scale regional variation in metamorphic grade observed today is essentially related to a metamorphic event at about 2600 m y ago which affected an older migmatite gneiss greenstone terrain 2900 3400 m y

an overview of charnockite abstract charnockite a quartzfeldspathic rock containing orthopyroxene is an important component of high grade deep crustal terrains the origin of charnockite by both igneous and metamorphic processes not yet clearly defined has major bearing on the evolution of continents

the moho in the northern basin and range province nevada along the cocorp 40 n seismic reflection transect cocorp seismic reflection profiles across nevada at about 40 n image a prominent essentially continuous band of reflectors at a two way traveltime of 9 to 11 s the approximate correspondence of this reflection time with estimates of the two way traveltime to the moho in this area provided by seismic refraction data suggests that the prominent reflections are from the moho the relief on these reflectors the reflection moho beneath nevada across a latitudinal transect of

the significance of rb sr ages of biotite and phlogopite for the thermal history of the central and southern marginal zones of the limpopo belt of southern africa and the adjacent portions of the kaapvaal craton abstract generally rb sr biotite and phlogopite ages range from 2050 ma to 1950 ma between the murchison thabazimbi and triangle tuli sabi shear zones of southern africa in the rocks comprising the central and southern marginal zones of the limpopo belt and adjacent portions of the kaapvaal craton in some shear zones however similar ages ocurr as young as 800 ma in contrast within the kaapvaal craton south of the murchison thabazimbi shear zone rb sr biotite ages range between 3150 ma and 2100 ma and show no regular distribution in all cases the ages are interpreted to indicate the time elapsed since the biotite last cooled below its closure temperature approximately 320 c the regular pattern of 2050 ma to 1950 ma rb sr biotite ages north of the murchison thabazimbi shear zone suggests that the present erosional surface approximates a paleo isothermal surface this surface was exposed by uplift and erosion which was completed by 1900 ma to 1800 ma ago when the sedimentary and volcanic rocks of the soutpansberg group were deposited the regional uplift was accommodated by motion on the murchison thabazimbi and triangle tuli sabi shear zones and probably was related to tectonism associated with the emplacement of the anorogenic bushveld and phalaborwa complexes other anorogenic granitic magmatism accompanied uplift the depth represented by the paleo isothermal surface and the magnitude of the uplift that exhumed it cannot be constrained precisely it is suggested however that the majority of the 10 to 15 km by which crust in this area is thinner than that of the adjacent cratons is the manifestation of this uplift

petrogenesis of siluro devonian rhyolites of the tobique group in the northwestern appalachians northern new brunswick canada tectonic implications for the accretion history of peri gondwanan terranes along the laurentian margin

shear zones of the limpopo belt and adjacent granitoid greenstone terranes implications for late archaean collision tectonics in southern africa abstract granulite metamorphism in the 2 65 ga limpopo belt of southern africa is related to crustal thickening due to late archaean collision tectonics the central zone of the belt is a major thrust sheet of distinctive lithological structural and isotopic character separated from adjacent terranes by crustal scale shear zones and the northern and southern marginal zones are up thrust deep level granulite facies terranes of the adjacent cratons geometrically the marginal zones are mirror images and the limpopo belt is essentially symmetrical in n s section conventionally regarded as a n s collisional orogeny the gross structural and lithological differences between the central zone and the marginal zones kinematic data from shear zones and geophysical data showing the three dimensional geometry of the central zone are here used to suggest that the direction of emplacement of the central zone was also the collision direction this was from northeast to southwest with the upthrust outwardly directed marginal zones developing in mechanical response to this collision

the reflection moho beneath the southern canadian cordillera the transition from the crust to the mantle beneath the canadian portion of the north american cordillera varies in depth geometry and tectonic age across the orogen these variations are rarely spatially related to the positions of morphologic or tectonic belts based on surface geology nor to nearly 25 km of structural relief identified in outcrop and on seismic reflection data the moho in this region is thus interpreted to be a long lived feature perhaps as old as proterozoic in the eastern part of the cordillera that probably has been active as a structural boundary during periods of crustal contraction and subsequent crustal stretching recognition of the moho and lower crust as a zone of localized tectonic activity provides a partial explanation for the problem of where regional detachments that underlie the foreland thrust and fold belt go as they project westward to deep structural levels beneath the interior of the orogen they likely project to the base of the crust where they flatten and

trace element indicators of crystal accumulation in silicic igneous rocks abstract if fractional crystallization coupled with some crustal assimilation afc is the dominant process for generating evolved magmas in the earth s crust large quantities of igneous cumulates must be present in the rock record however despite some reported occurrences intermediate to silicic rocks characterized by an unambiguous crystal accumulation signature have remained elusive the signature is expected to be subtle in viscous silicic magmas and sensitive criteria need to be developed to test whether these cumulates are truly rare or simply difficult to discern we propose models that use specific trace element concentrations and ratios zr ba eu eu and zr hf within a well constrained framework for the physical extraction of liquid from crystals mostly occurring between 50 and 70 vol crystals to identify silicic cumulates the results of these models indicate that crystal accumulation or melt extraction has occurred in multiple natural examples around the world both in plutonic and volcanic realms however these examples were selected because they represent conditions that are most favorable high amount of melt extraction well constrained initial liquid compositions for unambiguously identifying cumulate residue in many cases we show that the trace element signature for crystal accumulation will remain largely concealed in addition the different liquid lines of descent related to the various tectonic settings on earth dry reduced in hot spots divergent margins and wetter more oxidizing in arc environments will lead to different trace element evolution and each criterion must be applied on a case by case basis despite these limitations the undeniable but subtle presence of silicic cumulates in the mid to upper crust lends support to the idea that igneous differentiation is dominantly driven by melt extraction from partially crystalline reservoirs

how the crust meets the mantle lithoprobe perspectives on the mohorovicic discontinuity and crust mantle transition application of regional geophysical and geological methods throughout two decades of canada s lithoprobe project provides new opportunities to analyze the mohorovicic discontinuity moho and crust mantle transition the transect format employed during lithoprobe in which 10 specified regions of canada were targeted for approximately a decade each between 1984 and 2003 permitted teams of scientists to focus on geological geophysical and tectonic issues for each transect as a primary objective was to enhance knowledge of the structure of the crust and lithosphere an obvious target in each transect was the nature and origin of the moho and crust mantle transition accordingly the combined results provide new perspectives on the moho and the relationship of the moho to the crust mantle transition perhaps the most important result is that the continental geophysical moho is a deceptively simple feature it has a variety of signatures at different scales that preclude a single universally applicable interpretation in methods that provide large scale information such as regional seismic studies it is a relatively abrupt refraction velocity contrast that often displays a dramatic downward decrease in seismic reflectivity however its origin in a geological or tectonic sense is perhaps best determined by careful analyses of structural details near the geophysical moho which are complex and varied in some areas within canada it appears that the geophysical moho may be old and perhaps remains from the time the crust formed in other areas it appears to be a relatively young feature that was superimposed onto older crustal fabrics

ignimbrite correlation using whole rock geochemistry an example from the sulcis sw sardinia italy ignimbrites are useful chronological markers in the geological record at local and regional scales they also provide information on the dynamics of the eruption that produced them making their study of great importance in terms of volcanic hazard assessment however their study is usually hampered by their lateral variation and discontinuity when stratigraphic and lithologic criteria are not sufficient for correlation purposes the use of multiple complementary correlation tools may be necessary to correctly determine their areal extension volume and facies variations whole rock geochemistry is considered one of the less reliable correlation techniques due to the pyroclastic nature of these deposits and their emplacement dynamics these may introduce vertical and horizontal geochemical heterogeneity in the final deposit in addition the occurrence of zoned ignimbrites due to magma supply of changing composition is common in this work we show that if appropriately used whole rock geochemistry can be a valid and highly useful tool for ignimbrite correlation we provide an example from the study of an ignimbrite sequence containing 18 units sensu lato in the sulcis region sw sardinia italy a protocol has been developed for unit recognition based on successive simple binary diagrams where the whole rock composition of a problem sample can be plotted immobile trace elements have been preferentially used to minimize effects of element mobilization associated with alteration and weathering the diagrams provided here are designed for the sulcis but the methodology followed to develop them may be applied to other study areas

fluid inclusions in rocks from the amphibolite facies gneiss to charnockite progression in southern karnataka india direct evidence concerning the fluids of granulite metamorphism fluid inclusion studies of rocks from the late archaean amphibolite facies to granulite facies transition zone of southern india provide support for the hypothesis that co2 rich h2o poor fluids were a major factor in the origin of the high grade terrain charnockites closely associated leucogranites and quartzo feldspathic veins contain vast numbers of large co2 rich inclusions in planar arrays in quartz and feldspar whereas amphibole bearing gray gneisses of essentially the same compositions as adjacent charnockites in mixed facies quarries contain no large fluid inclusions inclusions in the northernmost incipient charnockites as at kabbal karnataka occasionally contain about 25 mol of immiscible h2o lining cavity walls whereas inclusions from the charnockite massif terrane farther south do not have visibile h2o microthermometry of co2 inclusions shows that miscible ch4 and n2 must be small probably less than 10mol combined densities of co2 increase steadily from north to south across the transitional terrane entrapment pressures calculated from the co2 equation of state range from 5 kbar in the north to 7 5 kbar in the south at the mineralogically inferred average metamorphic temperature of 750 c in quantitative agreement with mineralogic geobarometry this agreement leads to the inference that the fluid inclusions were trapped at or near peak metamorphic conditions calculations on the stability of the charnockite assemblage biotite orthopyroxene k feldspar quartz show that an associated fluid phase must have less than 0 35 h2o activity at the inferred p and t conditions which agrees with the petrographic observations high tio2 content of biotite stabilizes it to lower h2o activities and the steady increase of biotite tio2 southward in the area suggests progressive decrease of ah2o with increasing grade oxygen fugacities calculated from orthopyroxene magnetite quartz are considerably higher than the graphite co2 o2 buffer which explains the absence of graphite in the charnockites the present study quantifies the nature of the vapours in the southern india granulite metamorphism it remains to be determined whether co2 flushing of the crust can by itself create large terranes of largeion lithophile depleted granulites or whether removal of h2o bearing anatectic melts is essential

partial melting and the origin of metapelitic granulites in the southern marginal zone of the limpopo belt south africa abstract two potentially realistic models exist to explain the origin of the granulite facies rocks of the southern marginal zone i co2 fluxing inducing the necessary high temperature dehydration reactions and ii melting of hydrous minerals to yield an h2o undersaturated granitic melt in equilibrium with a dehydrated granulitic assemblage petrographic and field evidence from the bandelierkop quarry in the southern marginal zone indicate a sequence of melting reactions involving muscovite and biotite relict textural evidence indicates that the reaction responsible for the generation of most of the melt bi sil qz plag melt ga ksp was fluid absent the melting reactions can be shown to have occurred during the prograde metamorphic path with the highest temperature reaction bi qz plag hy cord melt occurring at approximately peak metamorphic conditions 850 c and less than 9 5 kbar this suggests a fluid absent mode of granulite formation a subsequent retrograde reaction cord ksp h2o ky qz related to a rise in h2o during in situ crystallisation of the melt did not destroy the granulite assemblages and hypersthene remains stable in areas of the southern marginal zone that record pressures of greater than 6 5 kbar during retrogression

structures in granitoid gneisses and associated migmatites close to the granulite boundary of the limpopo belt south africa abstract river platform exposures of quartzofeldspathic gneiss and associated migmatites occur close to the boundary of the southern marginal zone of the limpopo belt with the adjacent low grade granite greenstone terrane of the kaapvaal craton the complex gneisses and migmatites are described and are considered to represent the tectono thermally reworked equivalents of the pietersburg sutherland greenstone belts and early tonalitic and trondhjemitic intrusive granitoids structural features associated with the hout river shear zone and the limpopo orogeny 2700 2650 ma old and which include subhorizontal overthrust fault slices produced as a result of lateral extrusion of rock material following transpressional displacement along a strike slip fault or thrust zone support structural metamorphic and geophysical evidence suggesting southwestwards overthrusting of the high grade southern marginal zone rocks onto lower grade rocks of the kaapvaal craton

high pressure granulites of east siberia in terms of archaean and proterozoic evolution abstract the southern borderland of the aldan shield comprises high pressure granulites with dominant mafic components combined into the kurulta series these granulites pertain to the archaean and palaeoproterozoic history of the lower crust of the marginal part of the siberia craton while the upper crustal layers are composed of largely amphibolite facies rocks of the stanovik series which together with the kurulta series constitute the stanovik fold area the evolution of the area is characterized by recurrent tectonic activity of the aldan early stanovik stanovik and jeltulac cycles the earliest aldan events at 3 5 3 1 ga gave rise to primary granulite assemblages formed under geothermal gradients of 32 28 km homogeneous over the entire shield at 2 7 3 0 ga the northern more stable terrains were separated from the mobile stanovik area by the sutam belt which both controlled the special magmatism and high pressure metamorphism these events were governed by a low geothermal gradient of 22 20 km high pressure granulites resulted from sub crustal accretion including the aldan assemblages and early stanovik basic and calc alkaline magmatic rocks recrystallized at 750 950 c and 9 12 kbar the following stanovik activity at 2 4 ga resulted in uplift and interstacking of altered granulites together with the stanovik series metamorphosed under almandine amphibolite facies conditions after all this the jeltulac orogenic movements at 2 2 2 1 ga were accompanied by further retrogression and tectonic ascent of granulite fragments along local tectonic zones these events anticipated the emplacement of mafic dykes in the tukuringra zone at 1 8 1 9 ga when high grade metamorphism was resumed at deep crustal levels

metamorphic fluids in the deep crust most of the inferences about fluid action in the deep crust are drawn from field petrographic and geochemical studies in the granulite facies terrains deductions on the nature of fluids are made from major and minor element distributions and isotopic patterns in metamorphic rocks calculations of fluid mineral equilibria based on observed assemblages fluid inclusions and field evidence of fluid pathways discussion of these features has focused attention on several major problems concerning the timing flow mechanisms and origin of the fluids a key issue concerns episodicity versus secular activity of fluids there are strong arguments that most fluid action in the deep crust is linked to thermal deformational events including magmatism and regional meta morphism discrete radiometric ages of high grade terrains often define relatively short periods of crystallization or recrystallization the late archean granulite facies metamorphism of southern india is an example where u pb systems of zircons buhl et al 1983 whole rock u pb data peucat et al 1 987 and sm nd systems of whole rocks and minerals bernard griffiths et al 1 987 all show a major high temperature recrys tallization event inferred by several workers to have involved pervasive fluid action centered closely around 2 5 ga another geochemical indi cation of episodic fluids comes from stable isotope studies of progressive metamorphic sequences as in the damara orogen namibia where whole rock b 180 increases slightly at isograds representing arrested dehydration events but is monotonous between isograds hoernes hoffer 1 985

metamorphism denudation and sea level in the archean and cooling of the earth abstract over long enough time scales an antagonistic balance acting between tectonics uplift and rifting and the sedimentary cycle erosion and deposition probably keeps continental surfaces close to sea level we argue that this balance constitutes the primary mechanism for keeping long term continental freeboard constant by regulating the continental thickness an important implication is that the long term uplift or down warp history of ancient continental segments provides direct information on secular changes in the isostasy of the adjacent ocean basins through time the amount of uplift can be estimated from the regional metamorphic grade the burial pressures at present exposure of ten undisturbed archean granite greenstone segments are estimated to be 1 5 0 5 kb from this we infer a maximum long term uplift and erosion of 5 2 km for stable continent since 3 0 ga implying a mean continental thickness of 46 km at the time of crustal stabilization for a constant mean ocean depth isostasy requires a palaeo oceanic crust 14 2 km thick in the middle archean alternatively if little continent existed then the oceans would have been shallower and the palaeo oceanic crust 21 2 km thick assuming plate spreading did occur in the archean potential temperatures in the shallow mantle at 3 ga were 90 30 c or 170 20 c for no continent hotter than those prevailing today these temperatures translate into time averaged cooling rates for the shallow mantle of 57 c ga 1 and are comparable to estimates of the present day cooling rate for the whole earth the low burial pressures are a strong argument against the existence of catastrophic mantle flush events in the geological past which have been predicted by some mantle convection models taken together these constraints suggest a plume related geological setting i e oceanic plateau like flood basalt for the initial stage of greenstone belt evolution

fine structure of the continental reflection moho comparisons of deep seismic reflection profiles from a variety of tectonic regions that span a large fraction of the earth s history lead to a threefold categorization of reflection patterns near the crust mantle transition type i patterns have no distinct reflections including no reflection moho at all type ii patterns have one or more subhorizontal reflections and type iii patterns have reflections that project from the crust into the mantle subdivisions within each of these are based upon the geometric relationships between the reflection moho and reflections immediately above and in some cases below hence type i examples may have essentially no reflectivity near the crust mantle transition or they may be characterized by crustal reflectivity that fades downward type ii examples may have a single prominent reflection or they may have several subhorizontal reflections type ii patterns may also have crustal reflections that are either listric downward or that appear to be truncated at or near the base of reflectivity type iii examples may have a single prominent reflection that projects below the moho as delineated by seismic refraction data or they may have a series of subparallel dipping reflections more than one type of reflection moho may be visible along a single profile sometimes changing within distances of only a few tens of kilometers this diversity of both geometry and character leads to the conclusion that the variable appearance of the reflection moho is a consequence of variations in the thermal and structural evolution of the crust mantle transition at many scales from a few tens to many hundreds of kilometers

tectonic model for the evolution of the limpopo belt abstract constraints on which any model for the tectonic evolution of the limpopo belt must be based include the following data crustal thickening to at least 65 km between 2700 and 2650 ma ago was responsible for the formation of the granulite terrane exposed in the limpopo belt today this crustal thickening probably resulted from the thrusting of the kaapvaal craton over the zimbabwe craton along the south dipping triangle tuli sabi shear zone other northwardverging thrusts related to this event occur in the kaapvaal craton south of the limpopo belt in the central and southern marginal zones and the kaapvaal craton this shortening is directed to the southwest while in the northern marginal zone it was directed to the north northwest peak metamorphism was superimposed on the thickened crust and then nearly isothermal decompression of several kbars occurred throughout the belt during this decompression rocks moved upward and spread outward onto the adjacent cratons from the zone of thickened crust along several inward directed shear zones creating a regional pop up high grade rocks were thrust over low grade rocks on the cratons producing along the terrane boundary retrograde metamorphism in the limpopo belt and prograde metamorphism on the cratons

emplacement at granulite facies conditions of the sesia lanzo metagabbros an early record of permian rifting the evolution of the pre alpine corio and monastero metagabbros points to strong chemical and mineralogical similarities with that of other permian gabbro bodies of the alps which are concentrated in the southalpine and austroalpine domains the structural and metamorphic pre alpine evolution of these gabbros records a re equilibration following the emplacement in the deep crust p 0 6 0 9 gpa and t 850 70 c exhumation through amphibolite facies conditions p 0 5 0 35 gpa and t 570 670 c followed by a greenschist facies imprint 0 25 p 0 35 gpa and t 550 c this retrograde p t evolution suggests that the exhumation occurred in a high thermal gradient regime such as that induced by upwelling of an asthenospheric plume during continental rifting this would be consistent with the crustal thinning known to have occurred in both the southalpine and austroalpine domains during permian times the gabbros and their country acid granulites are spatially associated with the serpentinised subcontinental mantle of the lanzo massif this lithologic association and the metamorphic evolution is similar to that of the fedoz gabbro austroalpine domain of the central alps and completely different from that observed in passive margins where no remnants of the lower crust occur and the upper granitic crust directly overlies the serpentinized lherzolites the location of permian gabbro bodies in the austroalpine and southalpine domains and their absence in the helvetic domain is evidence for asymmetric rifting

underplating in the himalaya tibet collision zone revealed by the hi climb experiment we studied the formation of the himalayan mountain range and the tibetan plateau by investigating their lithospheric structure using an 800 kilometer long densely spaced seismic array we have constructed an image of the crust and upper mantle beneath the himalayas and the southern tibetan plateau the image reveals in a continuous fashion the main himalayan thrust fault as it extends from a shallow depth under nepal to the mid crust under southern tibet indian crust can be traced to 31 n the crust mantle interface beneath tibet is anisotropic indicating shearing during its formation the dipping mantle fabric suggests that the indian mantle is subducting in a diffuse fashion along several evolving subparallel structures

magma mixing and crustal recycling recorded in ternary feldspar from compositionally zoned peralkaline ignimbrite a gran canaria canary islands miocene ignimbrite a on gran canaria contains three compositional endmember fiamme types two rhyolites and one trachyte each of which crystallized distinct feldspar various textural and compositional criteria are interpreted as reflecting a complex scenario within the magma chamber in which the crystals formed about 25 30 of the feldspar phenocrysts contain evidence for magma mixing in the form of 1 partial to severe dissolution resorption rims 2 distinct zones of drastically different compositions and 3 overgrowth textures on formerly resorbed crystals four major types of zoning in the oligoclase to anorthoclase feldspars of ignimbrite a include a normal and a reversely zoned type and two complexly zoned types the feldspars with normal and reverse zonation show only minor compositional amplitudes between individual zones ab or 4 whereas the complexly zoned types show compositional differences between zones of up to 18 mol ab and 20 mol or and are commonly associated with an internal dissolution surface complex zoning with large compositional amplitudes and dissolution textures is taken as evidence of crystal movements within the magma and across compositional boundaries between magma batches a multiple step cycle model involving growth and transport of a crystal into another magma batch and its return to the original host magma is suggested by the data moreover feldspars from one rhyolite compositional group are found to be substantially elevated in 18o suggesting an input of a high 18o component to this rhyolite the other endmember rhyolite appears to be related to the endmember trachyte by mainly crystal fractionation of anorthoclase feldspar this observation is consistent with trace element and rare earth element concentrations for the magma endmembers and their feldspars where contamination led to a depletion in incompatible trace elements and light rare earth elements in the contaminated rhyolite and its feldspar phenocrysts we suggest that the combination of textural and compositional variation in ternary feldspar of peralkaline rhyolitic systems is well suited to reconstruct dynamic processes such as magma mixing and contamination in evolving rhyolitic magma chambers

isotopic and ree characteristics of the intrusive charnoenderbite and enderbite geographically associated with the matok pluton limpopo belt southern africa abstract intrusive charnoenderbite and enderbite in the southern marginal zone of the limpopo belt are genetically linked with peak metamorphism m1 during the 2700 ma to 2650 ma limpopo orogeny they occur geographically associated with and are intruded by the granodiorite and granite of the matok pluton which were emplaced between the response phase m2 and the rehydration phase m3 of that orogeny rb sr pb ree and sm nd elemental and isotopic data for whole rock samples and u pb data for populations of zircon suggest that the charnoenderbite and enderbite were intruded 2671 ma ago and were derived from a crustal precursor the granodiorite and granite of the matok pluton were intruded at approximately the same time between 2667 ma and 2664 ma ago they also were derived from a crustal precursor nd model ages and ree patterns for the charnoenderbite and enderbite and the granodiorite are distinct this relationship may imply that they are not cogenetic alternatively these rock types could be cogenetic if the magma from which they were derived was assimilating older crustal material as it evolved nd isotopic data indicate that the magma giving rise to the charnoenderbite and enderbite formed by partial melting of rocks compositionally similar to some portions of the baviaanskloof gneiss data for the granodiorite are consistent with the possibility that this unit formed in a similar manner from rocks compositionally similar to other portions of the baviaanskloof gneiss the close temporal relationship between the emplacement of the charnoenderbite and enderbite and the granitic rocks of the matok pluton indicates that the retrograde portion of the limpopo orogeny beginning 2670 ma ago occurred rapidly perhaps over 7 ma

direct current resistivity and lotem model for the deep structure of the northern edge of the kaapvaal craton south africa abstract the transition from the high grade metamorphic southern marginal zone smz of the limpopo belt to the low grade kaapvaal craton was studied by means of geoelectrical techniques the d c data showed conclusively that the high grade rocks were thrust over the low grade rocks this contradicts earlier geological ideas but agrees with recent seismic reflection data the area where high grade rocks occur above low grade strata was mapped out and the southern boundary of the smz was established the conductor that occurs at lower crustal depths in the kaapvaal craton was traced into the smz using the lotem method the conductor shallows from 30 to 22 km over 12 km on the surface but does not reflect the difference in metamorphic depth of 30 km seen at the surface this indicates that the conductor is a transient feature of the crust the conductor is not associated with a seismically reflective crust the cause of the conductor remains enigmatic

seismic studies in the limpopo belt abstract the structure of the archaean limpopo belt of southern africa has been investigated by means of a deep seismic reflection profile and the recording of the traveltimes of mining induced tremors at regional distances interpretation of the traveltimes suggests that a seismic low velocity zone exists in the upper mantle of the northern kaapvaal craton and limpopo belt beginning at a depth of about 42 km the seismic reflection profile provides evidence for a northward dip of the contact between the kaapvaal craton and the southern marginal zone of the limpopo belt no mid crustal decollement zone is detected the tectonic model of van reenen et al 1990 is in accord with these observations the lower crust is essentially seismically transparent and no reflection moho is observed

shear zones bounding the central zone of the limpopo mobile belt southern africa abstract contrary to previously suggested north directed thrust emplacement of the central zone of the limpopo mobile belt we present evidence indicating west directed emplacement the central zone differs from the marginal zones in rock types structural style and isotopic signature and is an allochthonous thrust sheet it is bounded in the north by the dextral tuli sabi shear zone and in the south by the sinistral palala shear zone which are crustal scale lateral ramps published gravity data suggest that the lateral ramps are linked at depth and they probably link at the surface in a convex westward frontal ramp in the vicinity of longitude 26 30 e in eastern botswana two phases of movement the first between 2 7 and 2 6 ga and the second between 2 0 and 1 8 ga occurred on both the tuli sabi and the palala shear zones

open system processes in the genesis of silica oversaturated alkaline rocks of the rallier du baty peninsula kerguelen archipelago indian ocean abstract the rallier du baty peninsula forms the southwestern part of the kerguelen archipelago indian ocean whose magmatic activity is related to the long lived 115 ma kerguelen plume the peninsula is mostly made of alkaline rocks constituting two well defined ring complexes this paper focuses on the northern ring complex which is not yet known recent field studies have revealed seven discrete syenitic ring dykes ranging in age from 6 2 to 4 9 ma and two later volcanic systems 40ar 39ar dating of a trachytic ignimbrite linked to the dome carva volcano complex yields an age of 26 3 ka this represents the last major eruptive event on the kerguelen archipelago the volcanism is bimodal with trachybasalts and trachyandesites constituting the mafic lavas and trachytes and rhyolites constituting the felsic lavas the volume of erupted felsic magma is by far the larger and is represented by abundant pyroclastic deposits and lava flows boulders of plutonic rocks are found to the northwest of dome carva and represent intermediate rocks i e monzogabbros and monzonites that are not present at the surface basic rocks are mostly trachybasalts and trachyandesites while true basalts are scarce their mineralogy consists chiefly of plagioclase olivine diopside and oxides sieve textured plagioclase is common as well as corroded olivine and diopside phenocrysts peralkaline commenditic trachytes are the most abundant type of acid volcanic rocks they consist of abundant sanidine augite and magnetite phenocrysts and interstitial quartz aegerinic pyroxenes and na amphiboles ring dykes of quartz poor alkali feldspar syenites display the same mineralogy except hornblende is common and replaces diopside hornblende is particularly abundant in intermediate monzogabbros major and trace element variations of volcanic rocks emphasise the predominant role of fractional crystallisation with a general decrease of mgo cao p2o5 tio2 feo ba sr and ni from basic to felsic rocks however the scattering of the data from the basic rocks indicates that other processes have operated the overall evolution from trachyte to rhyolite is in agreement with the fractionation of sanidine as the major control an increase of incompatible elements from trachyte to rhyolite is observed the felsic lavas display an increase of 87sr 86sr i without any significant variations in the nd isotopic composition the genesis of the basic rocks is complex and reflects concomitant processes of fractional crystallisation mixing between different basic magmas and probable assimilation of ba rich oceanic crust major and trace element modelling confirms the possibility of producing the trachytes through continuous differentiation from a basaltic alkaline parent discrepancies observed for some trace elements can be explained by the crystallisation of amphibole at an intermediate stage of magma evolution the overall evolution from trachyte to rhyolite is thought to be controlled by crystal fractionation high 87sr 86sr i of the trachytes is interpreted to reflect interaction with an ocean derived component probably during assimilation of hydrothermally altered oceanic crust boulders of amphibole bearing monzonites and monzogabbros found to the northwest of dome carva are thought to represent intermediate magma composition that formed at depths but did not erupt

constraints on the composition of fluids involved in retrograde anthophyllite formation in the limpopo belt south africa abstract orthoamphibole occurs widely in the metapelitic gneisses and other lithologies within the southern marginal zone of the limpopo belt in southern africa petrographic relations indicate that the orthoamphibole formed as a retrogressive replacement of the granulite facies minerals cordierite and orthopyroxene the existence of a discrete retrograde orthoamphibole isograd that can be mapped for over 150 km has led previous workers to suggest that the orthoamphibole formed as a result of equilibrium high temperature reactions which occurred in response to regional influx of fluids at the close of the limpopo orogeny 2670 ma ago however in order to substantiate this interpretation it is necessary to show from phase equilibrium analysis that the metapelitic minerals and their retrograde products comprise assemblages stable at elevated pressure and temperatures rather than disequilibrium assemblages formed at unspecified lower temperatures by monotropic reactions some of the properties of the infiltrating fluids may in turn be specified if the alteration products represent equilibrium assemblages it is shown that the presence of anthophyllite and the absence of talc in two analysed metapelites from the retrograde isograd require an associated vapour phase to have been relatively low in water ah2o 0 7 and temperatures to have been below 620 c when the anthophyllite developed from the orthopyroxene the fast kinetics of formation of talc and the relatively sluggish kinetics of formation of anthophyllite in experimental systems make it highly unlikely that the orthoamphibole could have formed metastably at low temperatures and high water activities a more general phase equilibrium analysis based on the coexistence of cordierite garnet biotite enstatite plagioclase and quartz with retrograde anthophyllite and kyanite indicates that this assemblage was in equilibrium at temperatures between 450 to 650 c for pressures of 5 to 7 kbars and water activities between 0 6 and 0 1 the temperatures pressure and water activities are highly correlated and lower pressures and water activities are required for equilibrium at lower temperatures the reduced water activities calculated require that a substantial quantity of another volatile be present in the vapour phase in the absence of stable scapolite or carbonates this volatile cannot be confirmed to be co2 the required infiltration of late orogenic fluids with reduced water activities is apparently a major feature of the limpopo event and may have parallels in other large precambrian high grade terrains in as much as orthoamphibole replacing orthopyroxene is a commonly observed mineral assemblage

when was the limpopo orogeny abstract the term limpopo orogeny refers to the common high grade metamorphic and tectonic event that affected the archaean rocks of the central and southern marginal zones of the limpopo belt and its influence on the adjacent portions of the kaapvaal craton available geochronological data from muscovite kyanite prograde metamorphic assemblages zircon in igneous charnoenderbite and enderbite muscovite in syntectonic pegmatite bodies formed in shear zones partial melts and syn and post tectonic plutons of granodiorite and granite indicate that this orogeny occurred during the time interval from 2700 ma ago to 2650 ma ago the retrograde phase of this orogeny from the onset of the response phase m 2 until the rocks were subjected to rehydration m 3 was very rapid probably lasting 7 ma from 2671 ma to 2664 ma ago similar but limited data from the northern marginal zone and adjacent portions of the zimbabwe craton suggest that rocks there were subjected to a more recent interval of high grade metamorphism and tectonism extending from 2600 ma until 2550 ma

cocorp new perspectives on the deep crust summary relict sutures from colliding continents regions characterized by a young mono layering and faulting throughout the crust mid crustal magma traps and seismic bright spots which suggest deep crustal fluids are among recent cocorp findings in addition new studies of signal penetration noise mitigation recording geometry and coherency filtering have yielded better understanding of and substantial improvements in data quality amplitude anomalies or bright spots in the basin and range may be due to magma at mid crustal levels in one case a normal fault appears to link the deep magma with young surface volcanics another bright spot 15 km deep in southeastern georgia has a flat geometry that suggests a gas liquid interface perhaps within fluids underthrust along an appalachian suture the mono continues to appear relatively undisturbed in many regions of past deformation suggesting that its formation post dates these major tectonic episodes the diversity of reflection patterns from the u s cordillera casts further doubt on the generality of the common model of a reflective layered lower crust underlying a transparent upper crust

a low velocity belt beneath northern and eastern tibetan plateau from pn tomography 1 new pn tomography on tibetan plateau tp and neighboring region is conducted with data assembled from various sources the improved pn tomographic image delineates several distinct geophysical regimes that are approximately separated by geological boundaries at surface dense distribution of the stations in the eastern half of the plateau allows us to infer a 3d view of crustal thickness variation of the region the most striking feature is the bended low velocity belt lvb which extends from west to east beneath qiangtang block and songpan ganzi fold belt sgfb in northern tp and then turns consistently southward with sgfb beneath eastern tp and further south to indo china the consistency between geological boundaries on surface and sharp velocity contrast boundaries in uppermost mantle may suggest strong deformation coupling between crust and at least uppermost mantle if not whole lithosphere a north south striking lvb is also observed cutting through the southern tp coinciding with the yadong gulu rift at surface our tomographic results are generally consistent with dynamic models that involve with underthrusting in the southern tp shortening in the northern tp and extrusion to the east and the southeast furthermore our results suggest significant east west variations in the structure and dynamics

lhasa block and bordering sutures a continuation of a 500 km moho traverse through tibet a north south wide angle fan profile through the yarlung zangbo suture across the lhasa block and through the bangong nujiang suture shows several significant and sharp changes in crustal thickness both sutures appear as vertical 20 km steps in the moho and may have been the loci for eastward strike slip motion of the tibetan lithosphere