high magnesium staurolite in a sapphirine garnet rock from the limpopo belt southern africa unusually mg rich staurolite with f fe fe mg 0 51 occurs in a peraluminous silica deficient sapphirine garnet gedrite spinel corundum phlogopite rock as relic inclusions within pyrope almandine garnet f 0 43 0 48 from which it is generally separated by sapphirine f 0 19 gedrite has f 0 24 0 27 phlogopite 0 19 and spinel 0 43 the gradual disappearance of staurolite is due to a shift of the 3 phase afm assemblage staurolite garnetsapphirine to more fe rich compositions according to probably prograde reactions between staurolite and garnet to form sapphirine together with either quartz or gedrite thus still more mg rich staurolite seems to have existed in the rock at earlier stages of metamorphism characterized by lower temperatures and perhaps higher pressures the local equilibria now frozen in indicate minimum pressures of 7 8 kbar and temperatures in excess of about 800 c

barred olivine chondrules in ordinary chondrites barred olivine chondrules were studied in h l ll3 6 chondrites no important differences were found between barred olivine chondrules in h3 l3 and ll3 chondrites however comparison of type 3 barred olivine chondrules with all chondrule types in h l ll3 ordinary chondrites show that barred olivine chondrules are enriched in feo al2o3 na2o and k2o and are depleted in sio2 and mgo mineral abundances of barred olivine chondrules show that they are enriched in the feldspathic component compared to most chondrules pyroxene is rarely absent and is sometimes a major component pyroxene rich barred olivine chondrules in type 3 chondrites correlate negatively with feldspathic glass rich barred olivine chondrules indicating that pyroxene formed at the expense of olivine bars barred olivine chondrules in ordinary chondrites decrease systematically in cr2o3 and tio2 with increasing petrologic type feo however shows a systematic decrease with petrologic type in h chondrites no change in l chondrites and a systematic increase in ll chondrites the conclusions of this study are 1 barred olivine chondrules are mainly intermediate in major element composition between the abundant fe mg chondrules and the rare al rich chondrules 2 systematic compositional changes in barred olivine chondrules from type 3 6 in h l ll chondrites may be the result of primary differences 3 precursor components that melted to form barred olivine chondrules were enriched in olivine rich and volatile feldspar and feldspathoid rich components the alkalies may have been condensed where alumina was already available

ultrahigh temperature metamorphism and decompression history of sapphirine granulites from rajapalaiyam southern india implications for the formation of hot orogens during gondwana assembly sapphirine bearing mg al granulites from rajapalaiyam in the southern part of the madurai block provide critical evidence for late neoproterozoic cambrian ultrahigh temperature uht metamorphism in southern india poikiloblastic garnet in quartzo feldspathic and pelitic granulites contain inclusions of fine grained subidioblastic to xenoblastic sapphirine associated with quartz suggesting that the rocks underwent t 1000 c peak metamorphism quartz inclusions in spinel within garnet are also regarded as clear evidence for a uht condition inclusions of orthopyroxene within porphyroblastic garnet in the sapphirine bearing rocks show the highest al 2 o 3 content of up to 10 3 wt suggesting t 1050 1070 c and p 8 5 9 5 kbar temperatures estimated from ternary feldspar and other geothermometers t 950 1000 c further support extreme thermal metamorphism in this region xenoblastic spinel inclusions in sapphirine coexisting with quartz suggest that the spinel quartz assemblage pre dates the sapphirine quartz assemblage probably implying a cooling from t 1050 c or an anticlockwise p t path the fmas reaction sapphirine quartz garnet orthopyroxene sillimanite indicates a cooling from the sapphirine quartz stability field after the peak metamorphism corona textures of orthopyroxene cordierite sapphirine orthopyroxene sapphirine cordierite and cordierite spinel around garnet suggest subsequent near isothermal decompression followed by decompressional cooling toward t 650 750 c and p 4 5 5 5 kbar the sapphirine quartz association and related textures described in this study have an important bearing on the uht metamorphism and exhumation history of the madurai block as well as on the tectonic evolution of the continental deep crust in southern india our study provides a typical example for extreme metamorphism associated with collisional tectonics during the late neoproterozoic cambrian assembly of the gondwana supercontinent

chondrules in h3 chondrites textures compositions and origins major and minor element bulk compositions of 90 individual chondrules and 16 compound chondrule sets in unequilibrated type 3 h group chondrites were determined in polished thin sections by broad beam electron probe analysis and the chondrules were classified petrographically into six textural types barred olivine porphyritic olivine porphyritic pyroxene barred pyroxene radiating pyroxene fine grained although analyses of individual chondrules scatter widely the mean composition of each textural type except barred pyroxene is rather distinct as verified by discriminant function analysis al2o3 tio2 and na3o are correlated in chondrules but al2o3 and cao do not correlate compound chondrule sets were found to consist almost entirely of chondrules or partial chondrules of similar texture and composition the data suggest that composition played a conspicuous role in producing the observed textures of chondrules though other factors such as cooling rates and degrees of supercooling prior to nucleation were also important if compound chondrules formed and joined when they were still molten or plastic then the data suggest that chondrules of each textural type could have formed together in space or time the correlation of al2o3 and tio2 with na2o and not with cao appears to rule out formation of chondrules by direct equilibrium condensation from the nebula we conclude that the most reasonable model for formation of the majority of chondrules is that they originated from mixtures of differing fractions of high intermediate and low temperature nebular condensates that underwent melting in space a small percentage of chondrules might have formed by impacts in meteorite parent body regoliths

the role of radiogenic heat production in the thermal evolution of a proterozoic granulite facies orogenic belt eastern ghats indian shield abstract the eastern ghats belt egb is a deeply eroded proterozoic orogenic belt juxtaposed against the archaean dharwar bastar singhbhum cratons in the indian shield the tectono metamorphic history of this belt is broadly similar to the grenvillian orogenic belts the egb exposes dominantly the granulite facies rocks such as charnockites khondalites and migmatitic gneisses with less abundant intermediate to mafic granulites the granulites are intruded by small plutons of granites syenites and anorthosites the northern and southern parts of the belt negb and segb have distinct crustal histories but 1600 1000 550 ma old tectono metamorphic events are common to both the belts in this research we attempt to understand the present day and past thermal state of this orogen on the basis of our radioelement k u and th measurements at 562 sites using in situ gamma ray spectrometry with other geological and geophysical constraints the dominant rock types of the negb and segb have similar radioelement abundances 3 k 3 ppm u and 30 ppm th and heat production heat production of charnockites gneisses khondalites and intermediate granulites of the negb is 2 9 2 8 2 9 and 1 1 w m 3 respectively in the segb they are 2 7 2 5 2 8 and 0 6 w m 3 respectively and the mafic granulites are the lowest in heat production 0 3 w m 3 on the basis of the heat production data and crustal petrologic models we show that the crustal contribution of the negb and segb is 54 and 45 mw m 2 respectively which are broadly in good agreement with the available surface heat flow data thermal models of the present day egb crust indicate that the temperature at the moho is 550 c radioelements and heat production of the eastern ghats granulites are higher than the other granulite belts in the indian shield the crustal radiogenic heat contribution and moho temperatures of the egb are higher than the adjoining archaean cratons this could also be responsible for the development of inverted metamorphic isograds in the tectonic boundary between the egb and the adjoining cratons during a collisional orogenic event at about 550 ma ago

gravity anomalies and associated tectonic features over the indian peninsular shield and adjoining ocean basins abstract a combined gravity map over the indian peninsular shield ips and adjoining oceans brings out well the inter relationships between the older tectonic features of the continent and the adjoining younger oceanic features the nw se ne sw and n s precambrian trends of the ips are reflected in the structural trends of the arabian sea and the bay of bengal suggesting their probable reactivation the simple bouguer anomaly map shows consistent increase in gravity value from the continent to the deep ocean basins which is attributed to isostatic compensation due to variations in the crustal thickness a crustal density model computed along a profile across this region suggests a thick crust of 35 40 km under the continent which reduces to 22 20 24 km under the bay of bengal with thick sediments of 8 10 km underlain by crustal layers of density 2720 and 2900 2840 kg m3 large crustal thickness and trends of the gravity anomalies may suggest a transitional crust in the bay of bengal up to 150 200 km from the east coast the crustal thickness under the laxmi ridge and east of it in the arabian sea is 20 and 14 km respectively with 5 6 km thick tertiary and mesozoic sediments separated by a thin layer of deccan trap crustal layers of densities 2750 and 2950 kg m3 underlie sediments the crustal density model in this part of the arabian sea east of laxmi ridge and the structural trends similar to the indian peninsular shield suggest a continent ocean transitional crust cotc the cotc may represent down dropped and submerged parts of the indian crust evolved at the time of break up along the west coast of india and passage of reunion hotspot over india during late cretaceous the crustal model under this part also shows an underplated lower crust and a low density upper mantle extending over the continent across the west coast of india which appears to be related to the deccan volcanism the crustal thickness under the western arabian sea west of the laxmi ridge reduces to 8 9 km with crustal layers of densities 2650 and 2870 kg m3 representing an oceanic crust

ultrahigh temperature metamorphism followed by two stage decompression of garnet orthopyroxene sillimanite granulites from ganguvarpatti madurai block southern india the mg al granulites from ganguvarpatti consist of orthopyroxene sillimanite garnet quartz as peak assemblage with a few porphyroblasts of cordierite and sapphirine these assemblages were strongly overprinted by late symplectites and coronas orthopyroxene inclusions in garnet and porphyroblast cores have the highest xmg 0 80 and al2o3 content 10 7 wt the estimated near peak metamorphic conditions 1 000 50 c and 11 kbar using garnet orthopyroxene geothermobarometry are consistent with those determined using a petrogenetic grid the proposed multi stage evolution process implies an initial decompression deduced from multi phase symplectites followed by cooling during biotite formation further late decompression is explained from the orthopyroxene rims on biotite this proposed p t path thus suggests a unique and complex evolution history for the uht granulites of southern india present results are comparable with similar adjacent terranes in the gondwana supercontinent but the lack of structural and geochronological data makes a link with any major metamorphic event uncertain

high pressure and ultrahigh temperature granulite facies metamorphism of precambrian high grade terranes case study of the limpopo complex

fluids in high to ultrahigh temperature metamorphism along collisional sutures record from fluid inclusions abstract petrographic studies and microthermometric investigations on fluid inclusions associated with high to ultrahigh temperature metamorphic rocks in three major precambrian suture zones on the globe demonstrate the dominant occurrence of co 2 rich fluids these rocks form part of hot orogens developed along collisional plate boundaries the sapphirine quartz bearing mg al rich rock from the palghat cauvery suture zone a trace of the cambrian gondwana suture zone in southern india preserves evidence for a prograde high pressure event and subsequent peak ultrahigh temperature metamorphism along a clockwise path and contains abundant co 2 rich inclusions in corundum garnet and sapphirine most of the fluid inclusions are either primary or secondary and preserve low density co 2 rich fluids 0 569 0 807 g cm 3 similar low density co 2 rich fluid inclusions 0 853 0 953 g cm 3 are also present in pelitic granulites from the limpopo complex of southern africa a neoarchean granulite facies orogen formed by continent continent collision in contrast the garnet orthopyroxene granulite from tonagh island in the neoarchean napier complex in east antarctica contains very high density primary 1 095 1 129 g cm 3 and secondary 0 960 1 179 g cm 3 carbonic inclusions in garnet and quartz the calculated isochores for the fluid inclusions from the palghat cauvery suture zone and the limpopo complex yield significantly lower pressure estimates than those predicted from peak metamorphic conditions we interpret this as a result of significant density decrease due to rapid decompression along a clockwise p t trajectory in contrast the estimated isochores for primary inclusions in garnet orthopyroxene granulites from the napier complex are consistent with the peak p t conditions estimated from mineral phase equilibria for the tonagh island rocks suggesting that most of the fluid inclusions in these rocks did not undergo any marked effect of volume change and density decrease the contrasting fluid densities among the localities investigated in this study are probably related to the nature of the p t trajectory the tonagh island rocks had a near isochoric exhumation history whereas the metamorphic orogens in the other two sutures witnessed rapid decompression our results suggest that whereas the composition of the syn metamorphic fluids are preserved in all cases density reversal occurs within inclusions as a function of the tectonic history and exhumation style

thermo mechanical structure of the southern part of the indian shield and its relevance to precambrian basin evolution abstract the thermal and mechanical structures of the southern part of the precambrian indian shield have been estimated using available heat flow data and shear stress profiles from olivine rheology these and other geological geochronological and geophysical data including deep seismic studies dss profiles of proterozoic cuddapah basin on south indian shield are utilized to examine thermal models for the evolution of precambrian intracratonic platform basins on the archean lithosphere of indian shield evidence of mantle perturbations and cycles of thermal events are documented to be important in the cuddapah basin s evolution haxby et al s 1976 thermal model has been shown to explain the cuddapah basin s flexuring and magnitude of subsidence

compositional evidence regarding the origins of rims on semarkona chondrules abstract the compositions of the interiors and abraded surfaces of 7 chondrules from semarkona ll3 0 were measured by neutron activation analysis for nonvolatile elements the lithophile and siderophile element abundance patterns in the surfaces are generally similar to those in the corresponding interiors siderophile and chalcophile concentrations are much higher in the surfaces whereas lithophile concentrations are similar in both fractions most of the similarities in lithophile patterns and some of the similarities in siderophile patterns between surfaces and interiors may reflect incomplete separation of the fractions in the laboratory but for 3 or 4 chondrules the siderophile resemblance is inherent implying that the surface and interior metal formed from a single precursor assemblage metal and sulfide rich chondrule rims probably formed when droplets of these phases that migrated to the chondrule surface during melting were reheated and incorporated into matrix like material that had accreted onto the surface the moderately volatile to volatile elements k as and zn tend to be enriched in the surfaces compared with other elements of similar mineral affinity both enrichments and depletions are observed for other moderately volatile elements a small fraction of chondrules experienced fractional evaporation while they were molten

historical seismicity of peninsular india abstract well documented historical earthquake data from 1340 to 1983 has been analyzed and reported in this work so as to understand the seismicity and associated phenomena of peninsular india it has been the belief among earth scientists that the peninsular shield is aseismic as the region attained stability long ago however the earthquake at koyna 10 december 1967 bhadrachalam 13 april 1969 broach 23 march 1970 hyderabad 30 june 1983 etc altered this concept and it has now widely accepted that seismic activity is still continuing on a mild scale as such a need has arisen to take into consideration historical as well as recent seismological data to study the seismic status of peninsular india using the well known frequency magnitude relation of gutenberg and richter 1954 an attempt has been made to derive a and b and the ratio a b the values as obtained for peninsular india are 4 4 0 85 and 5 17 which are typical of moderately active zones the world over recurrence periods of earthquake magnitude 6 75 and 7 0 are calculated to be 21 and 35 yr respectively further the historical seismicity correlates well with the protocontinents such as dharwar aravalli and singhbhum in the peninsular shield it is observed that seismicity in a broad sense is defined by the protocontinents with low intracontinental seismicity while the lineaments and rift valleys are from zones of moderate seismicity

high pressure and ultrahigh temperature metamorphism at komateri northern madurai block southern india we report for the first time the evidence for prograde high pressure hp metamorphism preceding a peak ultrahigh temperature uht event in the northernmost part of the madurai block in southern india mg al rich grt ged rocks from komateri in karur district contain poikiloblastic garnet with numerous multi phase inclusions although most of the inclusion assemblages are composed of gedrite quartz and secondary biotite rare staurolite sapphirine and spinel quartz are also present the xmg mg fe mg of staurolite 0 45 0 49 is almost consistent with that reported previously from namakkal district in the palghat cauvery shear zone system xmg 0 51 0 52 north of the madurai block the hp event was followed by peak uht metamorphism at t 880 1040 c and p 9 8 12 5 kbar as indicated by thermobarometric computations in the grt ged rock and associated mafic granulite symplectic intergrowth of spinel xmg 0 50 0 59 zno 1 7 wt and quartz a diagnostic indicator of uht metamorphism probably formed by decompression at uht conditions the rocks subsequently underwent retrograde metamorphism at t 720 760 c and p 4 2 5 1 kbar the p t conditions and clockwise exhumation trajectory of the komateri rocks comparable to similar features recorded from the palghat cauvery shear zone system suggest that the madurai block and the palghat cauvery shear zone system underwent similar hp and uht metamorphic history probably related to the continent continent collision during the final stage of amalgamation of gondwana supercontinent

relationship among o mg isotopes and the petrography of two spinel bearing compound chondrules the petrological properties and o and al mg isotopic compositions of two spinel bearing chondrules from the allende cv chondrite were investigated using scanning electron microscopy and secondary ion mass spectrometry a coarse spinel grain in a barred olivine bo chondrule is less enriched in 16o 17o 5 17o 17o 0 52 18o whereas smaller spinel grains in a plagioclase rich chondrule member of a compound chondrule are extremely 16o rich 17o 17 and the spinels have a strongly serrated character the petrological features and 16o enrichments of the spinels in the plagioclase rich chondrule indicate that spinels originating in coarse grained ca al rich inclusions cais were incorporated into chondrule precursors and survived the chondrule forming event the degree of 16o excesses among minerals within each chondrule is correlated to the crystallization sequences this evidence suggests that the o isotopic variation among minerals may have resulted from incomplete exchange of o isotopes between 16o rich chondrule melt and 16o poor nebular gas aqueous alteration also has changed the o isotope compositions in the mesostasis the feldspathic mesostasis in the bo chondrule shows a disturbed mg al isochron indicating that the bo chondrule experienced secondary alteration while plagioclase in the plagioclase rich chondrule member of the compound chondrule shows slight 26mg excesses corresponding to 26al 27al 0 4 6 4 0 2 10 6 nepheline formed by secondary alteration shows no detectable excess the al mg isotopic system of these chondrules was disturbed by aqueous alteration and thermal metamorphism on the allende parent body

mineralogy and composition of the upper mantle seismic velocities are calculated for two petrological models of the upper mantle an olivine rich assemblage pyrolite and a garnet clinopyroxene rich olivine bearing assemblage piclogite these are compared with recent seismic profiles for various tectonic provinces the shield data is most consistent with a cold olivine and orthopyroxene rich lid the seismic lithosphere extending to 150 km followed by a high temperature gradient and or a change in mineralogy that serves to decrease the velocity from 200 to 400 km the velocities follow a 1400 c adiabat the rise tectonic mantle is much slower presumably hotter and is likely to be above the solidus to depths of at least 300 km the high v p v s ratio of the lower oceanic lithosphere in the western pacific is most consistent with eclogite

sapphirine and corundum bearing granulites from karur madurai block southern india we report new occurrences of sapphirine and corundum bearing granulites intercalated within orthogneisses at lachmanapatti and malappatty in the northern part of madurai block sapphirine in these localities occurs either as needle like intergrowth with cordierite and corundum in symplectites and medium to fine grained euhderal to subhedral crystals associated with cordierite and corundum lachmanapatti or in association with plagioclase corundum and gedrite malappatty the sapphirine from lachmanapatti is highly magnesian xmg 0 87 0 94 with higher cr content up to 0 9 pfu as compared with those in other localities in the madurai block the sapphirine corundum association reported in this study has important bearing on the ultrahigh temperature metamorphism and exhumation history of the northern madurai block as well as on the tectonic evolution of the continental deep crust in southern india

teleseismic delay time tomography of the upper mantle beneath southeastern india imprint of indo antarctica rifting summary a 3 d p velocity map of the crust and upper mantle beneath the southeastern part of india has been reconstructed through the inversion of teleseismic traveltimes salient geological features in the study region include the archean dharwar craton and eastern ghat metamorphic belt egmb and the proterozoic cuddapah and godavari basins the krishna godavari basin on the eastern coastal margin evolved in response to the indo antarctica breakup a 24 station temporary network provided 1161 traveltimes which were used to model 3 d p velocity variation the velocity model accounts of 80 per cent of the observed data variance the velocity picture to a depth of 120 km shows two patterns a high velocity beneath the interior domain dharwar craton and cuddapah basin and a lower velocity beneath the eastern margin region egmb and coastal basin across the array velocity variations of 7 10 per cent in the crust 0 40 km and 3 5 per cent in the uppermost mantle 40 120 km are observed at deeper levels 120 210 km the upper mantle velocity diverences are insignificant among diverent geological units the presence of such a low velocity along the eastern margin suggests significantly thin lithosphere 200 km beneath the eastern dharwar craton such lithospheric thinning could be a consequence of indo antarctica break up

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

an experimental investigation on the p t stability of mg staurolite in the system mgo al 2 o 3 sio 2 h 2 o the pressure temperature stability field of mg staurolite ideally mg4al18si8o46 oh 2 was bracketed for six possible breakdown reactions in the system mgo al2o3 sio2 h2o mash mg staurolite is stable at water pressures between 12 and 66 kbar and temperatures of 608 918 c requiring linear geotherms between 3 and 18 c km this phase occurs in rocks that were metamorphosed at high pressure low temperature conditions e g in subducted crustal material provided they are of appropriate chemical composition mg staurolite is formed from the assemblage chlorite kyanite corundum at pressures 24 kbar whereas at pressures up to 27 kbar staurolite becomes stable by the breakdown of the assemblage mg chloritoid kyanite corundum beyond 27 kbar the reaction mg chloritoid kyanite diaspore mg staurolite vapour limits the staurolite field on its low temperature side the upper pressure limit of mg staurolite is marked by alternative assemblages containing pyrope topaz oh with either corundum or diaspore at higher temperatures mg staurolite breaks down by complete dehydration to pyrope kyanite corundum and at pressures below 14 kbar to enstatite kyanite corundum the reaction curve mg staurolite talc kyanite corundum marks the low pressure stability of staurolite at 12 kbar mg staurolite does not coexist with quartz because alternative assemblages such as chlorite kyanite enstatite kyanite talc kyanite pyrope kyanite and mgmgal pumpellyite kyanite are stable over the entire field of mg staurolite

matrix material in type 3 chondrites occurrence heterogeneity and relationship with chondrules abstract matrix material in type 3 chondrites forms rims on chondrules metal sulfide aggregates ca al rich inclusions and chondritic clasts it also forms lumps up to a millimeter in size which may contain coarser silicates chondrules of all types were found with internal matrix lumps that appear to have entered the chondrules before the latter had crystallized mean concentrations of mg na al and ca in matrix occurrences show up to fivefold variations in a single chondrite variations between mean matrix compositions of individual type 3 ordinary chondrites are almost as large and partly reflect systematic differences between h l and ll matrices such variations are probably a result of nebular separation of feldspathic material and ferromagnesian silicates compositions of chondrules and their matrix rims are normally unrelated although rim compositions are correlated with those of matrix lumps inside chondrules a single chondrule was found with a composition nearly identical to that of its internal matrix lump suggesting that some chondrules may have formed from matrix material matrix lumps are as heterogeneous as chondrules but mean chondrule and matrix compositions differ even allowing for possible loss of metallic fe ni during chondrule formation since bulk compositions of matrix lumps and rims have probably not changed significantly since their formation except for fe mg exchange our matrix samples cannot represent typical chondrule precursor materials

anisotropy of the indian continental lithospheric mantle summary due to the paucity of seismological data available in the public domain the structure of the indian lithosphere is still little known we investigate the lithospheric structure and potential mechanical coupling between the crust and upper mantle along the himalayan arc and underneath peninsular india using seismic anisotropy shear wave splitting measurements are performed on core refracted phases for each event recorded at a given seismological station we measured the orientation of the polarization plane of the fast s wave phi assumed to be a proxy for the orientation of the a axis of olivine and the delay dt between the arrival time of the fast and slow s waves we present a very comprehensive data set recorded at 86 seismological stations deployed from the himalayas to the southern tip of the indian peninsula in a joint effort by the national geophysical research institute hyderabad india the university of cambridge and the indian institute of astrophysics the unprecedented data set we present sheds light on the mechanisms involved in the india eurasia continental collision in a region along the himalayan arc south of the indus tsangpo suture zone at the scale of the indian plate the majority of the stations show a nne ssw orientation of phi over hundreds of kilometres from sri lanka to the northern part of the dharwar craton this direction closely parallels the trend of the indian plate motion with respect to a fixed eurasian plate as defined through the nuvel1a plate model along the himalayan arc from ladakh in the northwest to bhutan and the shillong plateau in the east the orientation of phi rotates to become ew perpendicular to the plate motion as defined through nuvel1a unlike previous studies we do find strong evidence for seismic anisotropy south of the indus tsangpo suture zone a large number of null results have been computed with consistent orientation of the two fast polarization directions phi across the subcontinent we demonstrate the potential value of the too often neglected null measurements in the interpretation of seismic anisotropy from these results we infer the dominance beneath the indian lithosphere of the asthenospheric flow in aligning minerals in the sheared lithosphere asthenosphere boundary layer masking any compression induced anisotropy expected to be normal to this direction closer to the collision front in northern india the anisotropy may in part be due to the foliation planes of the himalayan fold and thrust belt aligning the a axis of olivine perpendicular to the compression axis but more likely to the turning of the relative asthenospheric flow along the strike caused by the downthrusting indian lithosphere acting as a barrier the continent wide consistency of results strengthens the understanding that the indian lithosphere has distinct anisotropic signatures contrary to the hitherto assumed isotropy and allows one to interpret the results in a coherent framework of indo eurasian convergence

ultrahigh temperature metamorphism in the achankovil zone implications for the correlation of crustal blocks in southern india abstract the achankovil zone of southern india a nw se trending lineament of 8 10 km in width and 100 km length is a kinematically debated crustal feature considered to mark the boundary between the madurai granulite block in the north and the trivandrum granulite block in the south both these crustal blocks show evidence for ultrahigh temperature metamorphism during the pan african orogeny although the exhumation styles are markedly different the achankovil zone is characterized by discontinuous strands of cordierite bearing gneiss with an assemblage of cordierite garnet quartz plagioclase spinel ilmenite magnetite orthopyroxene biotite k feldspar sillimanite the lithology preserves several peak and post peak metamorphic assemblages including 1 orthopyroxene garnet 2 perthite and or anti perthite 3 cordierite orthopyroxene corona around garnet and 4 cordierite quartz symplectite after garnet we estimate the peak metamorphic conditions of these rocks using orthopyroxene bearing geothermobarometers and feldspar solvus which yield 8 5 9 5 kbar and 940 1040 c the highest p t conditions so far recorded from the achankovil zone the retrograde conditions were obtained from cordierite bearing geothermobarometers at 3 5 4 5 kbar and 720 60 c from orthopyroxene chemistry we record a multistage exhumation history for these rocks which is closely comparable with those reported in recent studies from the madurai granulite block but different from those documented from the trivandrum granulite block an evaluation of the petrologic and geochronologic data together with the nature of exhumation paths leads us to propose that the achankovil zone is probably the southern flank of the madurai granulite block and not a unit of the trivandrum granulite block as presently believed post tectonic alkali granites that form an array of suturing plutons along the margin of the madurai granulite block and within the achankovil zone but are absent in the trivandrum granulite block suggest that the boundary between the madurai granulite block and the trivandrum granulite block might lie along the tenmalai shear zone at the southern extremity of the achankovil zone

extreme thermal metamorphism associated with gondwana assembly evidence from sapphirine bearing granulites of rajapalayam southern india

a deterministic seismic hazard map of india and adjacent areas a seismic hazard map of the territory of india and adjacent areas has been prepared using a deterministic approach based on the computation of synthetic seismograms complete with all main phases the input data set consists of structural models seismogenic zones focal mechanisms and earthquake catalogues there are few probabilistic hazard maps available for the indian subcontinent however this is the first study aimed at producing a deterministic seismic hazard map for the indian region using realistic strong ground motion modelling with the knowledge of the physical process of earthquake generation the level of seismicity and wave propagation in anelastic media synthetic seismograms at a frequency of 1 hz have been generated at a regular grid of 0 2 x 0 2 by the modal summation technique the seismic hazard expressed in terms of maximum displacement d m a x maximum velocity v m a x and design ground acceleration dga has been extracted from the synthetic signals and mapped on a regular grid over the studied territory the estimated values of the peak ground acceleration are compared with the observed data available for the himalayan region and are found to be in agreement many parts of the himalayan region have dga values exceeding 0 6 g the epicentral areas of the great assam earthquakes of 1897 and 1950 in northeast india represent the maximum hazard with dga values reaching 1 2 1 3 g the peak velocity and displacement in the same region is estimated as 120 177 cm s 1 and 60 90 cm respectively

discovery of sapphirine bearing mg al granulites in the north china craton implications for paleoproterozoic ultrahigh temperature metamorphism abstract we report here for the first time the occurrence of mg al granulites within the khondalite belt of the north china craton and provide robust evidence for extreme crustal metamorphism at ultrahigh temperature uht conditions in this region the uht indicator assemblages in these rocks include equilibrium sapphirine quartz high alumina orthopyroxene sillimanite quartz and high temperature perthites the surrounding khondalites also carry spinel quartz equilibrium assemblage the sapphirines occurring in various microstructural associations are markedly magnesian x mg 0 69 0 77 with a composition varying from 7 9 3 to 2 2 1 end members spinel is a solid solution of mg spinel and hercynite x mg 0 41 0 54 with zno content varying from 0 48 to 1 79 wt garnet shows a compositional range between pyrope and almandine x mg 0 39 0 48 with minor grossular 2 6 3 8 mol and spessartine 0 5 1 8 mol the orthopyroxenes display marked compositional variations in terms of al 2 o 3 content with the core of the medium grained orthopyroxene adjacent to sillimanite in the matrix showing the highest al 2 o 3 content 8 4 wt the history of prograde to peak metamorphism of these rocks is indicated by inclusions of sapphirine spinel quartz and sillimanite within garnet sapphirine formed during the peak stage through the reaction spl qtz sil spr occurrence of sapphirine and quartz inclusions trapped in sillimanite suggests the reaction spr qtz grt opx sil a series of retrograde reactions are observed in these rocks as corona and symplectite textures around early coarse grained minerals such as opx sil qtz crd grt sil qtz crd grt qtz crd opx grt o 2 crd qtz fe 2 o 3 in mag or ilm and finally opx kfs h 2 o bt qtz whereas the stability of sapphirine quartz and orthopyroxene sillimanite quartz in the matrix assemblage suggests t 1000 c and p 10 kbar peak metamorphism conventional geothermobarometric estimates confirm the ultrahigh temperature nature and also trace the retrograde conditions 930 970 c at p 10 kbar al in opx 900 1000 c perthite 930 990 c grt opx and 910 940 c sapphirine spinel electron microprobe dating of monazites in textural association with the uht assemblages yield a sharply defined isochron age of 1917 48 ma which correlates with the 1927 11 ma obtained in a previous study from monazites in the surrounding khondalites carrying spinel quartz assemblage an evaluation of the assemblages and textures using appropriate petrogenetic grids indicate that following peak ultrahigh temperature conditions the rocks underwent initial isobaric cooling and subsequent isothermal decompression and we interpret these trajectories to be part of an overall anti clockwise p t evolution the study area located within the northern margin of the north china craton defines a collisional orogenic belt where continental collision and deep subduction was possibly followed by extensional collapse of the orogen evidence for the extensional phase is provided by the several mafic magmatic intrusives of paleoproterozoic age occurring in the region which suggest a possible link between the thermal anomaly associated with extreme crustal metamorphism in this region with mantle derived magmas the uht metamorphism in north china craton coincided with the assembly of the paleoproterozoic supercontinent columbia

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

microchondrule bearing clast in the piancaldoli ll3 meteorite a new kind of type 3 chondrite and its relevance to the history of chondrules in the piancaldoli ll3 chondrite we found a mm sized clast containing 100 chondrules 0 2 64 m in apparent diameter much smaller than any previously reported that are all of the same textural type radial pyroxene fs1 17 this clast like other type 3 chondrites has a fine grained ferich opaque silicate matrix sharply defined chondrules abundant low ca clinopyroxene and minor troilite and si and cr bearing metallic fe ni however the very high modal matrix abundance 63 8 vol unique characteristics of the chondrules and absence of microscopically observable olivine indicate that the clast is a new kind of type 3 chondrite most chondrules have feo rich edges and chondrule size is inversely correlated with chondrule core feo concentration the first reported correlation of chondrule size and composition chondrules acquired fe by diffusion from fe rich matrix material during mild metamorphism possibly before final consolidation of the rock microchondrules those chondrules 100 m in diameter are also abundant in another new kind of type 3 chondrite clast in the rio negro l chondrite regolith breccia in other type 3 chondrite groups microchondrule abundance appears to be anticorrelated with mean chondrule size viz 0 02 0 04 vol in h and co chondrites and 0 006 vol in l ll and cv chondrites microchondrules probably formed by the same process that formed normal sized droplet chondrules melting of pre existing dustballs because most compound chondrules in the clast and other type 3 chondrites formed by collisions between chondrules of the same textural type we suggest that dust grains were mineralogically sorted in the nebula before aggregating into dustballs the sizes of compound chondrules and chondrule craters which resulted from collisions of similarly sized chondrules while they were plastic indicate that size sorting of dustballs occurred before chondrule formation probably by aerodynamic processes in the nebula we predict that other kinds of type 3 chondrites exist which contain chondrule abundances size ranges and proportions of textural types different from known chondrite groups

petrology and u th pb geochronology of the sapphirine quartz bearing metapelites from rajapalayam madurai block southern india evidence for polyphase neoproterozoic high grade metamorphism abstract strongly migmatitic sapphirine bearing granulites occur as dm to m sized enclaves in garnetiferous leucogranites in the southeastern part of the madurai block mb the largest section of neoproterozoic crust in the southern granulite terrain of india sgt early mineral assemblages biotite sillimanite quartz garnet biotite quartz break down via successive dehydration melting reactions to high t and high p phase assemblages e g garnet melt orthopyroxene sillimanite melt orthopyroxene cordierite melt melt consuming reactions along the retrograde path led to partial resorption of garnet to orthopyroxene sillimanite orthopyroxene sapphirine and or orthopyroxene cordierite as well as orthopyroxene sillimanite assemblages to cordierite biotite quartz symplectites the occurrence of uht metamorphism is indicated by the presence of small inclusions of sapphirine quartz intergrowths in garnet this reaction sequence is interpreted in a kfmash partial petrogenetic grid taking both melt producing and melt consuming reactions into account which yields a clockwise heating cooling decompression p t path culminating at 8 11 kbar and 1000 1100 c in situ u th pb monazite dating has been performed to more precisely constrain the ages given by chemical electron microprobe on monazite in various textural positions monazite inclusions in garnet often yield a bimodal distribution of early 950 850 ma and late neoproterozoic 600 500 ma ages which is in good agreement with the results of u pb and epma zircon dating in other places of the southeastern madurai block by contrast monazite inclusions in other mafic minerals orthopyroxene cordierite biotite or as interstitial phase exclusively yield ages between 550 and 480 ma the early neoproterozoic population is interpreted to reflect the timing of migmatisation and peak uht metamorphism whereas the late neoproterozoic ages are inferred to date cooling and exhumation with granitic magmas to a higher crustal level of the madurai block

age and sedimentary provenance of the southern granulites south india u th pb shrimp secondary ion mass spectrometry abstract southern india lies at a junction in the gondwana forming orogenic belts between the east african orogen and the kuunga orogen it contains voluminous high grade metasedimentary gneisses that make up an important component of the record of collision and amalgamation of gondwana here we present u pb secondary ion mass spectrometry sims isotopic data from detrital zircon cores from throughout southern india that demonstrate dominant neoarchaean to palaeoproterozoic age components that are incompatible with the known ages of potential southern and central indian source regions the original sediments to the trivandrum block gneisses were deposited between 1900 and 515 ma whereas a sample from the achancovil unit and possible also a sample from the madurai block were deposited in neoproterozoic times we speculate that these rocks broadly correlate with southern and western malagasy metasedimentary rocks including the itremo and molo groups and formed an extensive basin or basins that lay on the west side present orientation of the neoproterozoic continent azania in addition metamorphic zircon from four samples yielded an age of 513 6 ma that is interpreted as dating high grade metamorphism throughout much of the southern granulite terrane

the matrices of unequilibrated ordinary chondrites implications for the origin and history of chondrites abstract the matrices of sixteen unequilibrated ordinary chondrites all witnessed falls were studied microscopically in transmitted and reflected light and analyzed by electron microprobe selected specimens were also studied by scanning electron microscopy these studies indicate that the fine grained opaque silicate matrix of type 3 unequilibrated chondrites is compositionally mineralogically and texturally distinct from the chondrules and chondrule fragments and may be the low temperature condensate proposed by larimer and anders 1967 1970 examination of the matrices of unequilibrated chondrites also shows that each meteorite has been metamorphosed with the alteration ranging in intensity from quite mild where the matrix has been only slightly altered to a more severe metamorphism that has completely recrystallized the opaque matrix most of the metamorphic changes in the matrix occurred without significant effects on the compositions or textures of the chondrules the metamorphic alteration probably resulted from a combination of processes including thermal metamorphism and the passage of shock waves the present appearance of each unequilibrated chondrite is a result of the particular temperature and pressure conditions under which it and its components formed plus the subsequent metamorphic alteration it experienced

intracanyon flows in the deccan province india case history of the rajahmundry traps the rajahmundry traps form small exposures of basaltic flows on either side of the godavari river near the east coast of india and they are thought to be late cretaceous early paleocene in age because they lie 400 km from the nearest surface exposures of the deccan traps their mode of formation remains unknown furthermore the rajahmundry traps show normal magnetic polarity indicating that they cannot be exact time equivalents of the large volumes of the deccan traps which show reversed magnetic polarity geochemical analyses reveal that the rajahmundry traps are of very uniform composition geochronological studies suggest an age of 64 ma marginally younger than the main sections western ghats of the deccan traps the only section of the main deccan province that may be stratigraphically equivalent on the basis of age geochemical signatures and magnetic polarity is the kolhapur formation at the southern end of the deccan province near the west coast 800 km from the rajahmundry traps we suggest that the rajahmundry traps and other basaltic material found near the mouth of the godavari river represent remnants of intracanyon flows that flowed eastward down the ancestral river systems of peninsular india away from the domed structure caused by the plume that led to the formation of the deccan traps similar intracanyon flows that have traveled 600 km from their source to the pacific coastline are well documented in association with the columbia river basalt province united states in the case of the rajahmundry traps the distance traveled would appear to be 1000 km

spinel quartz assemblage in granulites from the achankovil shear zone southern india implications for ultrahigh temperature metamorphism abstract we report the finding of equilibrium spinel quartz assemblage enclosed within garnet in garnet orthopyroxene cordierite granulites from pakkandom within the achankovil shear zone a region which is considered as the trace of an accretionary suture in recent tectonic models on southern india the spinel quartz bearing granulites are composed of poikiloblastic garnet and subidioblastic orthopyroxene in the matrix of quartz plagioclase biotite cordierite and fe ti oxides garnet contains numerous inclusions of sillimanite and biotite as well as spinel and quartz the spinel in direct contact with quartz has moderate x mg mg fe 2 mg 0 44 0 47 and is zn and fe 3 poor x zn zn fe 2 mg zn 0 027 0 036 fe 3 fe 2 fe 3 0 12 0 17 spinel is also present in the matrix surrounded by magnetite but the matrix spinel contains more zn x zn 0 067 0 072 and does not show any contact relationship with quartz such zn and fe 3 poor spinel in direct contact with quartz has been regarded as a diagnostic evidence of ultrahigh temperature uht metamorphism the high temperature stability of the spinel quartz is also supported by the results of geothermobarometric calculation of garnet orthopyroxene assemblages that provides robust evidence for peak uht metamorphism at 920 980 c and 8 10 kbar which was further confirmed by al in opx and magnetite ilmenite geothermometers 900 950 c and 1000 c respectively the peak uht event was followed by decompression down to 4 0 4 2 kbar and 640 670 c toward the stability of cordierite along a clockwise p t path similar spinel quartz assemblage enclosed in poikiloblastic garnet has also been reported from the palghat cauvery shear zone system the trace of a major suture zone within the gondwana amalgam with evidence for prograde high pressure p up to 20 kbar metamorphism followed by uht event the fine grained spinel quartz may thus indicate decompression from higher pressure conditions confirming the recent attributes of the achankovil shear zone being a collisional suture with evidence for subduction exhumation processes which accompanied the final collision and amalgamation of the gondwana supercontinent during the latest neoproterozoic

an example of ultrahigh temperature metamorphism orthopyroxene sillimanite garnet sapphirine quartz and spinel quartz parageneses in al mg granulites from in hihaou in ouzzal hoggar quartz al mg granulites exposed at in hihaou in ouzzal nw hoggar preserve an unusual high grade mineral association stable at temperatures up to 1050 c involving the parageneses orthopyroxene sillimanite garnet quartz sapphirine quartz and spinel quartz the phase relationships within the fmas system show that a continuum exists between the earlier prograde reaction textures and those of the later decompressive event the following mineral reactions involving sillimanite are deduced 1 grt qtzopx sil 2 opx silgrt spr qtz 3 grt sil qtzcrd 4 grt silcrd spr 5 grt sil sprcrd spl 6 grt silcrd spl 7 grt crd silspl qtz and 8 grt silspl qtz minerals in quartz al mg granulites display compositional variations consistent with the observed reactions the mg mg fe2 range of the main minerals is as follows cordierite 0 81 0 97 sapphirine 0 77 0 88 orthopyroxene 0 65 0 81 garnet 0 33 0 64 and spinel 0 23 0 56 the reaction textures and the evolution of the mineral assemblages in the quartz al mg granulites indicate a clockwise p t trajectory characterized by peak conditions of at least 10 kbar and 1050 c followed by decompression from 10 to 6 kbar at a temperature of at least 900 c

dynamic crystallization study of barred olivine chondrules abstract chondrules which comprise the bulk of most chondrites formed very early in the solar system and understanding their formation will elucidate early nebular processes barred olivine bo chondrules make up approximately 10 of chondrules and because of their unique dendritic textures represent a restricted set of crystal growth conditions and thus formation conditions bo textures have been replicated using dynamic crystallization techniques on olivine rich chondrule starting compositions in an attempt to determine these formation conditions one composition was modeled after the average bo chondrule composition in l chondrites determined by weisberg 1987 in a survey of bo chondrules chondrule droplets are presumed to form by the melting of crystalline material bo textures developed best if the crystalline precursor was melted slightly above to tens of degrees above the liquidus temperature eliminating all ready nuclei crystalline material embryos of subcritical nuclei size in the melts provide the nuclei when they reach critical nucleus size at degrees of supercooling which stabilize the plate dendritic crystal form of olivine the plate dendrite is the basic crystal unit of the bo texture multiple plate dendrites are the common texture in experimentally produced chondrules the classic single plate dendrite bo chondrule has not been reproduced in the experiments while the preferred bo composition of weisberg does produce bo textures over a broader range of melting temperatures any composition that has olivine on the liquidus for a significant temperature interval will develop bo texture over some range of melting temperatures the range of possible melting temperatures depends on the range of compositions available that have olivine on liquidus the low end of the range will be near 1500 c and the upper end near 1700 c the range of cooling rates that produced bo textures in the compositions studied is 500 to 2300 c h

deep crystal response to continental collision the limpopo belt of southern africa the limpopo belt is an example of an archean 2700 ma continental collisional zone in which the thickened crust 70 km has virtually attained equilibrium with the adjacent non thickened plates by a process of upward movement and lateral spreading of the underthrust high grade metamorphosed plates

sapphirine quartz corona around magnesian xmg 0 58 staurolite from the palghat cauvery suture zone southern india evidence for high pressure and ultrahigh temperature metamorphism within the gondwana suture abstract we report the first finding of equilibrium sapphirine quartz assemblage from the palghat cauvery suture zone pcsz in southern india providing unequivocal evidence for extreme crustal metamorphism at ultrahigh temperature uht conditions associated with the collisional assembly of the gondwana supercontinent in the late neoproterozoic cambrian the sapphirine and quartz occur as coronas around mg rich x mg 0 58 staurolite within poikiloblastic garnet in an mg al rich rock suggesting the progress of the prograde dehydration reaction staurolite garnet sapphirine quartz h 2 o although the occurrence of mg rich staurolite was previously noted from some localities in the pcsz no sapphirine quartz direct association has yet been reported the available experimental studies on mg rich staurolite indicate that the mineral is stable at p 15 kbar and suggest that the complex texture that we report here might correspond to a peak high pressure regime prior to the uht event the symplectic sapphirine quartz around staurolite probably implies decompression from p 15 kbar toward the stability of sapphirine quartz at temperatures of c 1000 c along a clockwise p t path the prograde high pressure metamorphism and following uht event correlate with the subduction collision tectonics associated with the final stage of amalgamation of gondwana supercontinent

spinel sapphirine quartz bearing composite inclusion within garnet from an ultrahigh temperature pelitic granulite implications for metamorphic history and p t path abstract we report here a multiphase mineral inclusion composed of quartz plagioclase k feldspar sapphirine spinel orthopyroxene and biotite in porphyroblastic garnet within a pelitic granulite from rajapalaiyam in the madurai granulite block southern india in this unique textural association hitherto unreported in previous studies sapphirine shows four occurrences 1 as anhedral mineral between spinel and quartz spr 1 2 subhedral to euhedral needles mantled by quartz spr 2 3 subhedral to anhedral mineral in orthopyroxene and 4 isolated inclusion with quartz spr 4 spr 1 spr 2 and spr 4 show direct grain contact with quartz providing evidence for ultrahigh temperature uht metamorphism at temperatures exceeding 1000 c associated orthopyroxene shows high mg fe mg ratio 0 75 and al 2 o 3 content up to 9 6 wt also suggesting t 1050 c and p 10 kbar during peak metamorphism coarse spinel spl 1 with irregular grain morphology and adjacent quartz grains are separated by thin films of spr 1 and k feldspar suggesting that spl 1 and quartz were in equilibrium before the stability of spr 1 quartz this texture implies that the p t conditions of the rock shifted from the stability field of spinel quartz to sapphirine quartz petrogenetic grid considerations based on available data from the fmas system favour exhumation along a counterclockwise p t trajectory the irregular shape of the inclusion and chemistry of the inclusion minerals are markedly different from the matrix phases suggesting the possibility that the inclusion minerals could have equilibrated from cordierite bearing silicate melt pockets during the garnet growth at extreme uht conditions

electron microprobe dating of monazites from high grade gneisses and pegmatites of the kerala khondalite belt southern india monazites of five samples one leptynitic garnet biotite gneiss one khondalite one augen gneiss and two pegmatites from the central and northern part ponmudi unit of the kerala khondalite belt kkb in southern india were analyzed with the electron microprobe dating technique monazites in the augen gneiss and the pegmatites yield grain sizes between 200 800 m th abundances are rather low 10 wt and the distribution of th u and pb within single grains is fairly homogeneous contrasting to this monazites in the leptynitic gneiss and the khondalite are small 150 m they often display very complex th u pb patterns and contain high th concentrations up to 20 wt the statistical treatment of individual ages from the investigated samples revealed three populations of lower proterozoic 1 9 ga upper proterozoic 580 ma and ordovician age 470 ma as well as mid proterozoic ages between 0 8 1 7 ga which are not regarded to be of geological significance lower proterozoic ages are preserved in the cores of monazites from leptynitic gneisses and khondalites they fairly agree with sm nd model ages for similar rocks of the kkb and give a minimum age for first monazite growth or complete homogenization the prominent pan african population with mean values between 540 and 580 ma is present in the leptynitic gneiss the khondalite and the augen gneiss and in line with published isotope ages for the kkb the ordovician population finally marks the emplacement of granitic pegmatites subsequent to the pan african high grade metamorphic event there is an obvious discrepancy between khondalites and leptynitic gneisses on the one hand and augen gneisses on the other concerning the presence of lower proterozoic ages while these are abundant in the former often rimmed by upper proterozoic ages they are completely absent in the latter it appears unlikely that lower proterozoic ages were completely reset during a pan african event exclusively in the augen gneisses while they were preserved in leptynitic gneisses and khondalites it is further concluded that the augen gneisses are of magmatic origin and were derived from porphyritic granites thus the upper proterozoic age of 605 37 ma calculated for the investigated sample approximates the time of magma emplacement which slightly precedes the peak stage of pan african high grade metamorphism in the kkb and of monazite crystallization from the granitic melt a characteristic feature of the investigated monazites is the resetting of lower proterozoic and pan african ages to significantly younger values due to partial pb loss monazites not shielded by other minerals e g garnet suffered selective mobilization of pb along fractures or at their rims while th and u concentrations remained almost unchanged the results presented in this study indicate that this was mainly due to fluid rock interaction it is concluded that thermal diffusion of pb even at the suggested temperatures of 900 c only had minor influence on the th u pb composition in monazite and that the closure temperature for this system must be significantly higher than previously assumed 750 c

first report of sapphirine bearing rocks from the palghat cauvery shear zone system southern india abstract silica deficient mg al granulites from paramati within the palghat cauvery shear system contain sapphirine in association with corundum spinel and sillimanite gedrite which occurs commonly in this locality coexists with cordierite corundum and sillimanite mineral assemblages and reaction textures indicate peak metamorphism at ultrahigh temperature conditions this new locality provides evidence for extreme crustal metamorphism along the archean proterozoic collision boundary in southern india

first evidence of ultrahigh temperature decompression from the granulite province of southern india sapphirine bearing orthopyroxene sillimanite garnet granulites of the palni hill ranges record evidence of ultrahigh temperature metamorphism during which primary mineral assemblages were partially overprinted by a succession of reaction coronas and symplectites that together suggest decompression model system petrogenetic grids allow constraints to be placed on the evolution of p t conditions as recorded by reaction relationships of successively developed mineral assemblages thermobarometry on garnet bearing assemblages allows minimum constraints to be placed on p t conditions for the primary mineral assemblages primary mineral assemblages suggest p t conditions of c 12 kbar and c 900 1000 c secondary mineral assemblages in reaction textures show a two stage decompression to probable p t conditions of c 5 kbar and c 850 c the inferred p t path is clockwise

complex upper mantle deformation beneath the dharwar craton inferred from high density splitting measurements distinct lateral variation from west to east abstract upper mantle anisotropy investigated using 172 core refracted sks skks seismic phases along a 660 km long profile at 10 to 20 km intervals from the west to the east coast of south india reveals significant lateral variations in its magnitude and direction this profile with 38 broadband seismic stations covers mid archean western dharwar craton wdc late archean eastern dharwar craton edc proterozoic cuddapah basin cb and the passive continental margins along the west and east coast the observed fast polarization directions fpds show lateral variability nw50o to nw5o beneath the wdc nw40o to ne30o beneath the edc and n5o to n85o beneath the cb and further east the delay time varies between 0 4 and 2 0 s with an average of 1 s however we are unable to fit a two layers anisotropy model for the region due to sparse azimuth coverage beneath the wdc the direction of the fast axis follows trends of shear zones and faults suggesting frozen in anisotropy in the lithosphere possibly established during the lithospheric evolution in mid late archean in the edc the fast axis does not only follow the plate motion direction but it deviates manifesting late archean to proterozoic deformation may still be present as fossil mantle anisotropy the splitting trend beneath the cb and eastern ghat eg follows the strike of the rift along with plate motion direction indicating anisotropy is influenced by the combination of frozen anisotropy due to continental rifting along the eastern margin of indian plate and active asthenospheric flow

paleoproterozoic uht metamorphism with isobaric cooling ibc followed by decompression heating in the khondalite belt north china craton new evidence from two sapphirine formation processes

continental scale body wave tomography of india evidence for attrition and preservation of lithospheric roots we assemble p and s waveforms of 2301 teleseismic earthquakes registered at 413 broadband seismic stations spanning the indian plate from the southern tip of india to the himalayan collision belt and generate an accurate data set of 52 050 p and 30 423 s arrival times through the multichannel cross correlation approach these traveltimes are then inverted to obtain 3 d p and s velocity structures of the subcontinent at a 2 2 lateral resolution the heterogeneous nature of the indian lithospheric mantle revealed in this study suggests that the lithospheric roots are not uniformly thick on a regional scale the key cratonic segments of the indian shield are characterized by pockets of high velocity anomalies 3 at shallow depths 300 km with the diamondiferous regions like wajrakarur revealing high shear wave anomalies down to 300 km in contrast to the southern deccan volcanic province dvp the northwestern dvp is underlain by low velocity anomalies at similar depths suggesting that the upper mantle retains imprints of deccan volcanism which was facilitated by the reactivation of the rift systems

garnet spinel corundum quartz bearing titanohematite veins in eclogite from the sulu ultrahigh pressure terrane imprint of a short lived high temperature metamorphic stage abstract eclogite hosted garnet spinel corundum quartz bearing titanohematite veins and lenses 10 20 cm in width are described for the first time in the sulu ultrahigh pressure metamorphic terrane some of the lenses were aligned parallel to the foliation of the host eclogite suggesting that they were the product of ductile deformation of one titanohematite layer a vein composed of titanohematite ilmenite hematite spinel garnet corundum quartz k feldspar albite was studied in detail ti fe oxides account for up to 80 and al rich phases for 15 of the total volume of this vein electron microprobe analyses show that the titanohematite solid solution was made up of 0 75 hematite 0 25 ilmenite the unusual mineral assemblage of garnet spinel corundum quartz implies that this vein could have experienced high temperatures 900 c although the garnets showed well defined mg and mn diffusion zoning in the rim as a result of the high temperature event slight mg and mn growth zoning was preserved in the core thus we suggest that the sulu uhp terrane could have experienced a short lived high temperature 900 c stage during exhumation garnets in the titanohematite vein were characterized by extremely low trace element contents petrological and geochemical features of the veins suggest that they could be metamorphic products of igneous cumulates composed of magnetite plagioclase clinopyroxene

meteorites evidence for the interrelationships of materials in the solar system of 4 55 ga ago abstract members of the five major chemical groups of chondrites are samples of rocks which have remained essentially unaltered for 4 55 ga the ordinary chondrites are often regarded as primitive in both chemical and temporal senses however the presence of noritic and anorthositic inclusions in a few ordinary chondrites of great age indicates that at least some unequilibrated ordinary chondrites are not primitive in a temporal sense the eucrites are differentiated basaltic rock types which have ages indistinguishable from those of the oldest chondrites members of three groups of iron meteorites too appear to be equally old it is concluded that the period of formation of ordinary chondrites continued until after differentiation had occurred within some planetary bodies such early melting may have been caused by the decay of 26 al some of the differentiated materials were probably incorporated as plastic rocks liquid chondrules and solid clasts into the accreting parent bodies of the ordinary chondrites

ultrahigh temperature metamorphism 1150 c 12 kbar and multistage evolution of mg al rich granulites from the central highland complex sri lanka mgand al rich granulites of the central highland complex sri lanka preserve a range of reaction textures indicative of a multistage p t history following an ultrahigh temperature metamorphic peak the granulites contain a near peak assemblage of sapphirine garnet orthopyroxene sillimanite quartz k feldspar which was later overprinted by intergrowth symplectite and corona textures involving orthopyroxene sapphirine cordierite and spinel biotite rims kornerupine and orthopyroxene rims on biotite are considered to be late assemblages thermobarometric calculations yield an estimated p t of at least 1100 c and 12 kbar for the near peak metamorphism isopleths of al2o3 in orthopyroxene are consistent with a peak temperature above 1150 c the p t path consists of four segments initial isobaric cooling after peak metamorphism segment a which produced the garnet sapphirine quartz assemblage was followed by near isothermal decompression at ultrahigh temperature segment b which produced the multiphase symplectites further isobaric cooling segment c resulted in the formation of biotite and kornerupine and late isothermal decompression segment d formed orthopyroxene rims on biotite this evolution can be correlated with similar p t paths elsewhere but there are not yet sufficient geochronological and structural data available from the highland complex to allow the tectonic implications to be fully assessed

multiple tectonothermal events in the granulite blocks of southern india revealed from epma dating implications on the history of supercontinents abstract we report age data on zircon monazite uraninite and huttonite from a suite of 29 samples covering four major granulite blocks in southern india using an electron microprobe technique the rocks analysed in this study cover all of the major lithounits in these terrains and include garnet bearing and garnet free charnockites garnet biotite gneisses khondalites calc silicate rocks and a suite of orthogneisses biotite gneiss biotite hornblende gneiss two pink metagranites representing the magmatic phase were also analysed zircons from the madras block yield well defined isochrons at 2 5 2 6 ga core to rim analyses of single zircon grains show age zoning with 2 6 2 9 ga igneous cores mantled by 2 4 2 5 ga rims detrital zircons show age up to ca 3 2 ga monazites in this block have cores and rims with 2 5 2 3 ga a suite of 19 samples from the madurai block brings out the multiple tectonothermal events in this terrain zircons from an orthogneiss yield well defined isochrons at 1 7 0 1 ga 0 82 0 05 ga and 0 58 0 04 ga from core inner rim and rim portions respectively zircon grains in other rocks preserve either core or secondary growth ages at 0 8 1 0 ga zircons in a pink metagranite from this block show sharply defined isochrons of 0 68 0 03 ga for the core and 0 57 0 01 ga for the secondary portion a late pan african overprint is observed throughout this block with zircon rims monazite uraninite and huttonite yielding age values in the range of 0 45 0 60 ga zircons from both the trivandrum and nagercoil blocks show a major tectonothermal event at 0 55 ga with faint indications of previous tectonothermal events during 0 8 1 0 and 1 7 2 0 ga monazite data from both the trivandrum and nagercoil blocks are essentially similar to those from the madurai block except for presence of relic monazite in the former our study confirms the notion that the palghat cauvery shear zone marks the major terrain boundary between an archean craton in the north and proterozoic terrains in the south it also strengthens the view of paleoproterozoic accretion and pan african reworking of blocks south of this shear zone the ages of production accretion and reworking in the terrains of southern india yield important information for the histories of columbia 1 8 1 5 ga rodinia 1 1 1 0 ga and gondwana 0 6 0 5 ga supercontinents the southern indian terrains formed part of a worldwide network of orogenic belts that is centered around 1 8 ga and outlines the configuration of columbia accretion of terrains in the madurai block to archean rocks north of the palghat cauvery shear zone at this time is consistent with the ages suggested to be the oldest metamorphic event in the eastern ghats orogen of eastern india and the rayner belt of coastal east antarctica our data confirm earlier evidence that all of southernmost india underwent resetting of isotopic systems during the final accretion of gondwana at the time of the pan african orogeny 0 5 ga the possibility that the trivandrum and nagercoil terrains accreted to the madurai block after 0 7 ga suggests that this pan african zone may also have involved accretion and closure of ocean basins if that happened then this zone may be the long sought suture between east and west gondwana

tectonic stress field in the indian subcontinent a map of maximum horizontal compressive stress orientation in the indian subcontinent has been prepared using orientations derived from three different stress indicators borehole elongation breakouts in situ hydraulic fracturing measurements and earthquake focal mechanisms most part of the subcontinent appears to be characterized by a compressional stress regime thrust and strikeslip faulting imposed by plate boundary forces although shmax orientations do not in general show clear correlation with the direction of motion of the indian plate four provinces are recognized on the basis of regionally consistent orientations these are the midcontinent stress province the southern shield the bengal basin and the assam wedge their boundaries have been determined taking into consideration regional tectonics and seismicity central and northern india including the shillong plateau stretching up to the great himalaya pakistan and nepal are included in a broad midcontinent stress province characterized by nne ene oriented shmax the mean orientation of shmax in this province is n23 e subparallel to the direction of compression expected to arise from the net resistive forces at the himalayan collision zone suggesting that it is largely determined by the tectonic collision processes much of southern india mysore plateau and the high grade metamorphic terrain south of the plateau appears to be part of a second stress province characterized by nw oriented shmax these appear close to those of the intraplate stress field prevailing in the central indian ocean a third stress province was recognized in the bengal basin including parts of west bengal tripura manipur and mizoram in northeastern india and most of bangladesh this province extends eastward from the marginal fault in the western margin of the bengal basin to the indo burma subduction zone and is bounded on the north by the e w striking dauki fault shmax within the sedimentary pile of the bengal basin is oriented in e w direction while p axes of earthquakes within the basement and the crust beneath the basin and within the subducted slab of the indian plate beneath the indo burman ranges generally trend north n30 e shmax orientations within the sedimentary pile of the basin are parallel to the local approximately e w direction of the convergence of indian and burmese plates suggesting a casual relation to the resistive forces at the subduction zone in the indo burma region interestingly the stress field in the basement and the crust beneath the bengal basin and in the subducted slab is similar to the one prevailing in the midcontinent stress province assam wedge stress region occupies the northeastern corner of the indian plate including upper assam arunachal pradesh and much of nagaland this region subducts beneath a sharply bent continental collision boundary consisting of the northeastern limb of the himalayan and northern limb of the indo burman fold belts as a result the stress field in this province is depth differentiated and most likely responsible for the absence of consistent shmax directions

the stability of sapphirine quartz and hypersthene sillimanite quartz assemblages an experimental investigation in the system feo mgo al 2 o 3 sio 2 under h 2 o and co 2 conditions phase relations and mineral chemistry involving the phases garnet gt spinel sp hypersthene hy sapphirine sa cordierite cd sillimanite sil and quartz qz have been experimentally determined in the system fmas feo mgo al2o2 sio2 under low fo2 and for various h2o co2 conditions several compositions were studied with 100 mg mg fe ratio ranging from 64 to 87 with excess quartz and sillimanite our data do not show any differences in gt cd stability and composition as a function of h2o co2 and h2o co2 ch4 content in good agreement with a previous experimental study at lower temperature aranovich and podlesskii 1983 at 1 000 c and 11 kbar under co2 saturated conditions cordierite grew from a crystalline mix unseeded with cordierite thus under water absent conditions cordierite will have a high p stability field in the presence of co2 if water has a pressure stabilizing effect on cordierite then our results would indicate that the effects of h2o and co2 are of the same magnitude at high temperature our data support the theoretical p t grid proposed by hensen 1986 for high t metapelites and are largely consistent with the high temperature experimental data of hensen and green 1973 the univariant boundary gt cd hy sil qz which marks the disappearance of hy sil qz assemblages has a negative dp dt slope above 1 000 c and a positive one below this temperature extrapolation of our data to iron free systems shows that the high p breakdown limit of mg cordierite has a negative slope in the range 1 025 1 300 c and probably positive below 1 000 c this indicates a maximum of stability for mg cordierite at around 1 000 c and 13 kbar because of the curvature of the univariant reactions en sil py qz mg cd en sil qz and gt cd hy sil qz the iron free invariant point involving the phases py en cd sil and qz probably does not exist sapphirine qz bearing assemblages are stable only at temperatures above 1 050 c at 1 075 c the joint gt sa is stable up to 11 kbar at higher pressure garnet sapphirine and quartz react according to the reaction gt sa qz hy sil reequilibrated sapphirines are more aluminous than the theoretical endmember mg2al4sio10 due to alal mgsi substitutions 100 al2o3 al2o3 feo mgo in experimental sapphirines ranges from 50 5 to 52 2 sapphirine in the assemblage sa cd sil qz shows a decrease in al content with decreasing temperature and pressure such that the alumina isopleths for sapphirine have a slight negative dp dt slope a similar decrease in al content of sapphirine with temperature is also observed in sa sil qz assemblages

chemical isotopic and mineralogical evidence for the origin of matrix in ordinary chondrites abstract we report the first combined chemical isotopic and mineralogical study of fine grained opaque matrix material from a type 3 ordinary chondrite allan hills a77299 h3 7 electron microprobe and instrumental neutron activation analysis of a large matrix lump show that it has a major element composition typical of matrix material in type 3 chondrites unlike chondrules it shows no siderophile element depletions and is remarkably unfractionated relative to ci chondrites suggesting that it is primitive solar system material the matrix lump has an unique oxygen isotopic composition which lies below the terrestrial fractionation line and differs markedly from the composition of fine grained matrix lumps and chondrule rims from semarkona grossman et al 1987 transmission electron microscope tem studies of microtomed samples of the fine grained fraction m of the matrix lump show that it consists of rare angular clastic grains 1 m of pyroxene and olivine embedded within a groundmass of rounded fine grained m olivines the mineralogy and textures of the matrix lump are typical of other occurrences of fine grained matrix studied by tem in other type 3 ordinary chondrites we conclude that the matrix lump in alha 77299 consists largely of material that formed by annealing of amorphous presolar dust or nebular condensates and could not have been derived from chondrules by any reasonable mechanism however the observed compositional differences between the matrix lump and chondrules in ordinary chondrites are consistent with the idea that some chondrules formed by melting of matrix like material accompanied by loss of siderophile and volatile elements

chondrules and other components in c o and e chondrites similarities in their properties and origins we describe three types of chondrules that are common to h3 ll3 cm2 co3 and cv3 chondrites low and high feo porphyritic olivine chondrules contain olivine with fa0 3 8 and fa5 50 respectively and can easily be distinguished petrographically poikilitic pyroxene chondrules have 1 20 vol olivine fa0 2 8 enclosed by low ca pyroxene fs0 5 7 and also occur in e chondrites these three types formed in separate nebular regions which had dimensions and spacings such that a few percent of the chondrules that collided to form compound chondrules were of different types sorting of chondrule precursor material and mixing of chondrule types probably account for most variations in bulk and mineral chemistry among chondrite groups metallic fe ni grains containing 0 1 1 cr si and p are present in low feo olivine chondrules from all type 2 and least metamorphosed type 3 chondrites metal compositions reflect reduction during chondrule formation in the nebula not nebular condensation opaque matrices in type 3 ordinary and carbonaceous chondrites are somewhat similar in composition and mineralogy and probably have related origins we conclude that chondrules in all known chondrite groups share similar nebular origins

precambrian tectonics and crustal evolution in south india about 3 4 ga ago voluminous calc alkaline magmas represented by the granitoid gneisses of southern india were emplaced into a now poorly preserved non continental crust unstable ensialic basins initiated at about 3 0 ga were filled with volcanic and sedimentary rocks up to about 2 6 ga this basement cover sequence was deformed at the close of the archaean first by northward accretion and thickening of several crustal slabs formerly separated by prisms of stable shelf sediments structures produced by this episode were refolded and dislocated by large n s strike slip shear belts to impart intense steep planar fabrics to large volumes of the crust fluids rich in co possibly derived from sedimentary material driven beneath the crustal slabs by the thickening mechanism purged the deep crust of h o to form granulites upward migration of hot h o rich fluids encouraged partial melting at intermediate crustal levels these late archaean tectonic and thermal events began soon after the close of basin f

eclogite facies metamorphism preserved in tectonic blocks from a lower crustal shear zone central transantarctic mountains antarctica in the geologists and miller ranges of the central transantarctic mountains mafic and ultramafic tectonic blocks occur in an early cambrian amphibolite to granulite facies lower crustal transpressional shear zone mafic blocks consist of garnet clinopyroxene symplectite hornblende plagioclase quartz ilmenite orthopyroxene biotite mineral compositions in these blocks generally reflect equilibration under upper amphibolite to lower granulite facies conditions t 700 c p 8 12 kbar these conditions are the same as those indicated by peak syn kinematic assemblages in the enclosing shear zone tectonites mineral and textural evidence for an earlier higher pressure p 12 25 kbar eclogite facies metamorphism includes 1 plagioclase reaction rims around anhedral garnet 2 complex clinopyroxene intergrowths consisting of augite plagioclase quartz hornblende orthopyroxene interpreted as exsolved na rich clinopyroxene omphacite and 3 unusual magnesian staurolite atomic mg mg fe 0 58 inclusions in pyrope almandine garnet we suggest that the relict eclogite facies metamorphism records an earlier period of crustal thickening that occurred prior to formation of the transpressional shear zone perhaps as an early stage in ross orogenesis

hot orogens and supercontinent amalgamation a gondwanan example from southern india abstract the southern granulite terrane in southern india preserves evidence for regional scale high to ultrahigh temperature metamorphism related to the amalgamation of the supercontinent gondwana here we present accessory mineral zircon and monazite geochronological and geochemical datasets linked to the petrological evolution of the rocks as determined by phase equilibria modelling the results constrain the duration of high to ultrahigh temperature 900 c metamorphism in the madurai block to be c 40 ma with peak conditions achieved c 60 ma after the formation of an orogenic plateau related to the collision of the microcontinent azania with east africa at c 610 ma a 1d numerical model demonstrates that the attainment of temperatures 900 c requires that the crust be moderately enriched in heat producing elements and that the duration of the orogenic event is sufficiently long to allow conductive heating through radioactive decay both of these conditions are met by the available data for the madurai block our results constrain the length of time it takes for the crust to evolve from collision to peak p t i e the prograde heating phase then back to the solidus during retrogression this evolution illustrates that not all metamorphic ages date sutures

the origin and history of the metal and sulfide components of chondrules abstract fourteen siderophile and other non lithophile elements determined in 31 semarkona ll3 0 chondrules by neutron activation analysis are severely fractionated relative to lithophile elements their chondrule whole rock abundance ratios vary by factors of up to 1000 the mean ratio is 0 2 non refractory siderophile abundance patterns in ni rich chondrules are smooth functions of volatility and in ni poor chondrules patterns are more irregular refractory siderophile elements are often fractionated from ni they covary confirming the presence of a refractory metal component the chalcophile element se correlates with br and siderophile elements zinc is uniformly low and uncorrelated with other elements most metal and sulfide in chondrules was probably present in the solar nebula before chondrule formation most siderophile and chalcophile elements were in these materials some fe was also in silicates as were minor amounts of ni co au ge and possibly se the amount of metal formed by reduction during chondrule melting was minor the common metal component in chondrules is similar to and may be the same as the common component involved in the metal silicate fractionation of the ordinary chondrite groups chondrules are depleted in metal chiefly because they sampled metal poor precursor assemblages metal segregation during the molten period and subsequent loss was a minor process that may be responsible for most surface craters on chondrules

petrology and mineralogy of type ii feo rich chondrules in semarkona ll3 0 origin by closed system fractional crystallization with evidence for supercooling abstract the petrology of type ii porphyritic olivine chondrules in semarkona ll3 0 has been studied in detail olivines in these chondrules are euhedral fe richand are strongly zoned from cores to rims of grains in feo fa10 30 cr2o3 0 2 0 6 wt mno 0 2 0 7 wt and cao 0 1 0 4 wt interstitial mesostasis is rich in si aland ca and is glassy with abundant microcrystallites minor minerals include troilite fe ni metaland chromite some olivine grains contain euhedral fayalite rich cores that are probably produced during initial supercooling of the chondrule melt rare relict grains of forsteritic olivine have compositions very similar to olivines in type ia chondrules in semarkona and may result from disaggregation of such chondrules apart from these relics all properties of type ii chondrules can be described by closed system fractional crystallization of droplets which were essentially entirely molten the chondrules did not originate as aggregates of large olivine grains and are unlikely to be fragments of precursor igneous rocks cooling rates of the order of 1000 c h are indicated from peak temperatures of around 1600 c the chondrules show considerable evidence for supercooling and large departures from equilibrium during crystallization there is no evidence for chondrule nebula exchange either during or after crystallization or any secondary processes such as aqueous alteration or thermal metamorphism bulk composition data suggest that precursor material included a refractory ca al ti rich component similar to one of the precursor components of type ia chondrulesand a non refractory fe and si rich component type ia chondrules may have formed from type ii chondrules by loss of fe and volatiles alternatively the two chondrule types may have formed in regions of considerable diversity in the solar nebula from precursor materials with different fe mg ratios

first report of sapphirine quartz assemblage from southern india implications for ultrahigh temperature metamorphism we report here for the first time the occurrence of sapphirine quartz assemblage in textural equilibrium from quartzo feldspathic and pelitic granulites from southern india the sapphirine bearing rocks occur as layered gneisses associated with pink granite within massive charnockite in rajapalaiyam area in the southern part of madurai block sapphirine occurs in three associations i fine grained subhedral mineral associated with quartz enclosed in garnet ii intergrowth with al rich orthopyroxene up to 9 7 wt al2o3 and iii in symplectitic intergrowth with orthopyroxene al2o3 5 9 6 7 wt and cordierite surrounding garnet the sapphirine in association with quartz is slightly magnesian xmg 0 79 0 80 and low in si content 1 55 1 56 pfu as compared with those associated with orthopyroxene and cordierite xmg 0 77 0 79 si 1 59 1 63 pfu the sapphirine quartz assemblage suggests that the granulites underwent t 1050 c peak metamorphism cores of porphyroblastic orthopyroxene in the sapphirine bearing rocks shows high al2o3 content of up to 9 7 wt suggesting t 1040 1060 c and p 8 kbar fmas reaction of sapphirine quartz garnet sillimanite cordierite indicates a cooling from sapphirine quartz stability field after the peak ultrahigh temperature metamorphism slightly lower temperature estimates from ternary feldspar and sapphirine spinel geothermometers t 950 1000 c also support a post peak isobaric cooling corona textures of orthopyroxene cordierite sapphirine orthopyroxene sapphirine and cordierite spinel around garnet suggest subsequent decompression the sapphirine quartz association and related textures reported in this study have important bearing on the ultrahigh temperature metamorphism and exhumation history of the madurai block as well as on the tectonic evolution of the continental deep crust in southern india

the stability and origin of sodicgedrite in ultrahigh temperature mg al granulites a case study from the gondwana suture in southern india mg al rich rocks from the palghat cauvery shear zone system pcsz within the gondwana suture zone in southern india contain sodicgedrite as one of the prograde to peak phases stable during t 900 990 c ultrahigh temperature metamorphism gedrite in these samples is mg rich mg fe mg x mg 0 69 0 80 and shows wide variation in na2o content 1 4 2 3 wt naa 0 33 0 61 pfu gedrite adjacent to kyanite pseudomorph is in part mantled by garnet and cordierite the gedrite proximal to garnet shows an increase in naa and aliv from the core naa 0 40 0 51 pfu aliv 1 6 1 9 pfu to the rim naa 0 49 0 61 pfu aliv 2 0 2 2 pfu suggesting the progress of the following dehydration reaction ged ky na ged grt crd h2o this reaction suggests that as the reactants broke down during the prograde stage the remaining gedrite became enriched in na to form sodicgedrite which is regarded as a unique feature of high grade rocks with mg al rich and k si poor bulk chemistry we carried out high p t experimental studies on natural sodicgedrite and the results indicate that gedrite and melt are stable phases at 12 kbar and 1 000 c however the product gedrite is na poor with only 0 13 wt na2o naa 0 015 0 034 pfu in contrast the matrix glass contains up to 8 5 wt na2o suggesting that with the progressive melting of the starting material na was partitioned into the melt rather than gedrite the results therefore imply that the occurrence of sodicgedrite in the uht rocks of the pcsz is probably due to the low h2o activity during peak p t conditions that restricted extensive partial melting in these rocks leaving na partitioned into the solid phase gedrite the occurrence of abundant primary co2 rich fluid inclusions in this rock which possibly infiltrated along the collisional suture during the final amalgamation of the gondwana supercontinent strengthens the inference of low water activity

deep seismic sounding in the godavari graben and godavari coastal basin india abstract deep seismic sounding investigations along two profiles one along and the other across the godavari graben delineate the northeastern extension of the bapatla ridge which separates the godavari graben from the godavari coastal basin the godavari graben is filled with a maximum of 2 8 km thick lower gondwana upper permian to lower triassic sediments it is inferred on the basis of p wave velocities that the upper precambrian pakhal formation does not continue south of paloncha the upper gondwana upper jurassic to lower cretaceous rocks with marine incursions appear to continue towards the southeast for about 15 km from the exposures below the rajahmundry sandstone mio pliocene two dimensional velocity modelling of the upper crust indicates a major interface at 3 5 km depth in the upper crust where the velocity increases from 5 4 5 5 km s 1 to 6 2 6 4 km s 1 this interface shows a dpmal upwarp across the godavari graben which may be an indication of ne sw crustal extension in this region the deeper part of the crust consists of a large number of almost horizontal small reflector segments two dimensional crustal velocity modelling for some correlatable wide angle reflections yielded a three layered structure below 10 km depth with the moho at about 41 to 43 km

anticlockwise evolution of ultrahigh temperature granulites within continental collision zone in southern india abstract we report three new localities of corundum and sapphirine bearing hyper aluminous mg rich and silica poor ultrahigh temperature granulites formed during late neoproterozoic cambrian times within the palghat cauvery shear zone system in southern india from petrologic characteristics mineral chemistry and petrogenetic grid considerations the peak metamorphic conditions of these rocks are inferred to lie around 950 1000 c as suggested by al in orthopyroxene thermometer at pressures above 10 kbar as indicated by the equilibrium orthopyroxene sillimanite gedrite quartz assemblage these rocks preserve several remarkable reaction textures the most prominent among which is the triple corona of spinel sapphirine cordierite on corundum with the whole textural assembly embedded within the matrix of gedrite suggesting the reaction ged crn spl spr crd the formation of sapphirine sillimanite assemblage symplectite associated with relict corundum and porphyroblasitc cordierite is explained by the reaction crd crn spr sil the association of sapphirine cordierite symplectite with gedrite sillimanite assemblage as well as with aluminosilicate boundaries indicates the gedrite consuming reaction ged sil spr crd extensive growth of sapphirine cordierite observed on the rim of gedrite porphyroblasts with spinel occurring as relict inclusions within the sapphirine indicates the reaction ged spl spr crd the pressure temperature p t path defined from the observed mineral assemblages and reaction texture is characterized by anticlockwise trajectory with a prograde segment of initial heating and subsequent deep burial followed by retrograde near isothermal decompression such an anticlockwise trajectory is being reported for the first time from southern india and has important tectonic implications since these rocks were developed at the leading edge of the crustal block that was involved in collisional orogeny and subsequent extension during the final phase of assembly of the gondwana supercontinent we propose that the rocks were subjected to deep subduction and rapid exhumation and the extreme thermal conditions were attained either through input from underplated mantle derived magmas or convective thinning or detachment of the lithospheric thermal boundary layer during or after crustal thickening

a new correlation of sapphirine granulites in the indo antarctic metamorphic terrain late proterozoic dates from the eastern ghats province of india abstract uranium lead and thorium lead ages were obtained from perrierite and zircons in two samples from the eastern ghats province of south india a sapphirine bearing granulite from anakapalle and a charnockite from visakhapatnam the isotopic data are concordant or close to concordant at 1 ga which is interpreted to be the age of granulite facies metamorphism and charnockite plutonism in the vicinity of anakapalle and visakhapatnam and provides the first report of a late proterozoic granulite facies event in the eastern ghats province in reassemblies of gondwanaland based on dutoit s original proposal the eastern ghats province is juxtaposed against a high grade terrain in east antarctica affected by a granulite facies event and charnockitic plutonic activity 800 1100 ma ago the ages reported here suggest that a portion of the eastern ghats province represents an extension of the late proterozoic terrain from antarctica into india the radiometric results on the anakapalle rocks are the first evidence in south india for sapphirine formation during the late proterozoic other south indian sapphirine localities appear to be archaean in age a review of sapphirine localities in the indo antarctic metamorphic terrain indicates that sapphirine developed under a range of pressure temperature conditions during the archaean and proterozoic evidence for a systematic change in peak metamorphic conditions with time is obscured by these regional variations characterization of the entire pressure temperature path in addition to estimates for the peak conditions is needed to distinguish the archaean and proterozoic metamorphic regimes

nitrogen rich fluid in the upper mantle fluid inclusions in spinel dunite from lanzarote canary islands fluid inclusions ranging from pure n2 to pure co2 occur in olivine porphyroclasts in spinel dunite xenoliths chrome diopside suite from two localities within the quaternary to historic alkaline lavas of lanzarote canary islands this is the first report of fluid inclusions containing major amounts of n2 in mantle xenoliths the nitrogen rich fluid inclusions predate at least one generation of nitrogen free carbon dioxide inclusions textural evidence indicates that the inclusions were trapped within the upper mantle some of the nitrogen rich fluid inclusions are intimately associated with solid inclusions of spinel the nitrogen rich fluid was most likely produced in situ by oxidation dehydration reactions destabilizing ammonium bearing silicate minerals phlogopite amphibole increasing oxygen fugacity or possibly increasing temperature of the mantle this process could be related to an event of co2 and silicate melt injection at 6 8 kbar neumann et al in press or to some earlier event in the evolution of the mantle beneath lanzarote the existence of a n2 rich fluid phase in at least some mantle lithology ies at certain conditions is demonstrated by these data this discovery has consequences for the understanding of the evolution of the mantle below the canary islands as well as for the global nitrogen budget