the nilgiri enderbites south india nature and age constraints on protolith formation high grade metamorphism and cooling history abstract the trident shaped nilgiri highland massif represents the deepest level exposure 9 10 kbar c 35 km palaeo depth of granulite grade lower crust in southern peninsular india the crustal section is essentially composed of garnetiferous enderbitic granulites with minor kyanite bearing gneisses quartzites and banded magnetite quartzites its deeper northern part is intruded by intermediate and basic magmatic rocks now represented by non garnetiferous enderbites garnetiferous basic granulites gabbroic to anorthositic two pyroxene granulites and pyroxenites the garnetiferous enderbitic granulites are tonalitic in composition displaying a typical calc alkaline trend and exhibit chemical features of both i type as well as s type granitoids they show moderate enrichment of lree la n 35 70 no to weakly negative eu anomalies and fairly constant levels of hree yb n 6 5 13 0 and resemble the ree spectra of tonalitic trondhjemitic granitoids and greywackes of late archaean to palaeoproterozoic age sedimentary protoliths are indicated by negative df values ranging from 1 to 4 al 2 o 3 cao na 2 o k 2 o molar ratios 1 1 high 18 o whole rock values 10 12 which match the values reported for greywackes whole rock isotope data for garnetiferous enderbites define linear arrays in the rb sr and sm nd evolution diagrams rb sr system 2460 81 ma with sr i 0 7030 5 sm nd system 2506 70 ma with nd i 0 509363 60 implying a provenance of the sedimentary precursors that was dominated by felsic volcanic rocks or tonalitic trondhjemitic granitoids of uniform isotopic composition and early proterozoic age the u pb systematics of composite zircon grains with detrital cores and metamorphic overgrowths in accord with the sm nd and rb sr errorchron ages and sm nd garnet whole rock data suggest that formation of the provenance deposition of the sedimentary precursors and subsequent high grade tectono metamorphic processes must have occurred in the short time span between 2600 and 2500 ma based on the regional geological framework the petrological geochemical and isotopic data the nilgiri granulite terrain is interpreted as a metasedimentary allochthonous unit that was thrust onto the dharwar craton during palaeoproterozoic collisional tectonics

geothermobarometry of mafic granulites and metapelite from the palghat gap south india petrological evidence for isothermal uplift and rapid cooling the palghat gap region is located near the centre of the large southern indian granulite terrane at the northern edge of the kodaikanal charnockite massif the dominant rock types in the region are hornblende biotite orthopyroxene gneisses and charnockites along with minor amounts of intercalated mafic granulite metapelite and calc silicate the p t estimates from garnetiferous mafic granulites and metapelite samples are generally in the range 9 10 kbar and 800 900d c using both conventional thermobarometric methods and the tweequ thermobarometry program these p t estimates which should be taken as minimum values are among the highest yet reported for south indian and sri lankan granulites the occurrence of orthopyroxene plagioclase symplectites around embayed garnet grains in the mafic granulites and cordierite rims around garnet grains in metapelite suggest an isothermal decompression type path similarly a core rim p t trajectory indicates c 3 and 7 kbar decompression at high temperature in the mafic granulites and metapelite respectively in both rock types the key to the determination of the retrograde p t path was the recognition of small amounts of second generation plagioclase with a more anorthitic composition than the matrix plagioclase the preservation of high garnet pyroxene temperatures in the mafic granulites despite small garnet grain size suggests rapid cooling of the terrane calculated minimum cooling rates range from 8 to 80d c ma 1 such cooling rates are more rapid than those associated with normal isostatic processes and suggest that the terrane was tectonically exhumed at high temperature

monazite dating of granitic gneisses and leucogranites from the kerala khondalite belt southern india implications for late proterozoic crustal evolution in east gondwana two stages of granitic magmatism occurred during the pan african evolution of the kerala khondalite belt kkb in southern india granitic gneisses were derived from porphyritic granites which intruded prior to the main stage of deformation and peak metamorphism subsequently leucogranites and leucotonalites formed during fluid absent melting and intruded the gneiss sequences monazites from granitic gneisses leucogranites and a leucotonalite were investigated by conventional u pb and electron microprobe dating in order to distinguish the different stages of magma emplacement u pb monazite dating yielded a wide range of ages between 590 520 ma which are interpreted to date high grade metamorphism rather than magma emplacement the results of this study indicate that the kkb experienced protracted heating 50 ma at temperatures above 750 800 c during the pan african orogeny the tectonometamorphic evolution of the study area is comparable to southern madagascar which underwent a similar sequence of events earlier than the kkb the results of this study further substantiate previous assertions that the timing of high grade metamorphism in east gondwana shifted from west to east during the late proterozoic

nature of charnockite and closepet granite in the dharwar craton implications for the architecture of the archean crust abstract the architecture of the archean crust is crucial in understanding the geodynamics of cratonization the dharwar craton in the indian peninsula is well known for its continuous change of metamorphic grades from granulite facies in the south to amphibolite facies in the middle and greenschist facies in the north here the genetic relationship of different units is re analyzed to decipher the crustal architecture of the craton the major lithologic unit the granitoids can be subdivided into ttg charnockite closepet granite and anatectic granites migmatites the charnockite near salem has two types of occurrence one is massive garnet free and plutonic crystallized at 2560 ma and the other is gneissic garnet bearing and incipient metamorphosed at 2490 ma both types are compositionally close to tonalite granodiorite and generally show positive eu anomalies a large proportion of zircons of both types show positive hft values varying from 3 to 9 indicating a relatively juvenile parent 2900 ma crust at medium pressure in the middle crustal level with significant mantle contribution the genetic relationship of different units the spatial age difference of greenstone belts and the spatial variation of metamorphic grade in the dharwar craton suggest a trinity of archean crustal architecture sandwich structure with 2700 2500 ma dominated uppercrust 3400 2900 ma dominated middle crust and 2560 to 2500 ma anatectic lower crust this sandwich structure possibly indicates vertical growth of the crust in the archean the whole craton was tilted at 2500 ma with the southern end uplifted the most by an intra cratonic process and then it was shaped by later orogenic events resulting in the formation of the dominantly n s trending structures in the basement

mechanism of arrested charnockite formation at nemmara palghat region southern india abstract the formation of arrested charnockite is an excellent example of structurally controlled channellised fluid flow along specific sites accompanied by selective elemental mobility and mineralogical changes the present paper recognises and focuses study on three types of arrested charnockite formation from palghat region namely shear controlled foliation parallel and boudin neck types and address their spatio temporal relations to regional scale charnoenderbite the shear controlled and foliation parallel types post date deformation and migmatisation the boudin neck type on the contrary is coeval with partial melting and followed the path of cooling and decreasing water activity in the gneiss k feldspar veining around plagioclase and quartz symplectitic intergrowth of biotite quartz after orthopyroxene and k feldspar and fluid inclusion data suggests the presence of alkalic supercritical brine and low density co2 rich fluid during charnockite formation charnockite domains developed following the breakdown of hornblende biotite and quartz are characterised by a more or less pronounced depletion of fe ca mg and ti and trace elements y and zr compared to their counterpart gneiss ree spectra indicate a subtle depletion in the hree near the centre of the charnockite domain although close pair samples of gneiss charnockite are isochemical on a scale of a few millimetres bi directional element movement related to the formation of new mineral was noted it is postulated that arrested charnockite formation developed in situ on local scale within the granitic domains of the hornblende biotite gneiss in the presence of co2 rich fluids and alkalic supercritical saline brine this process post dated the time of regional granulite charnoenderbite and large regional scale retrogression and migmatisation

contrasting carbon and oxygen isotopic evolution in metacarbonates from the kerala khondalite belt southern india abstract the kerala khondalite belt is a proterozoic metasupracrustal granulite facies terrain in southern india comprising garnet biotite gneiss garnet sillimanite gneiss and orthopyroxene granulites as major rock types calc silicate rocks and marbles occurring as minor lithologies in the kerala khondalite belt show different mineral assemblages and reaction histories of which indicate a metamorphic p t fluid history dominated by internal fluid buffering during the peak metamorphism followed by external fluid influx during decompression the carbon and oxygen isotopic compositions of calcite from three representative metacarbonate localities show contrasting evolutionary trends the ambasamudram marbles exhibit carbon and oxygen isotope ratios 13c 0 and 18o 20 typical of middle to late proterozoic marine carbonate sediments with minor variation ascribed to the isotopic exchange due to the devolatilization reactions the 13c and 18o values of 9 and 11 respectively for calcite from calc silicate rocks at nuliyam are considerably low and heterogeneous the wollastonite formation here possibly corresponds to an earlier event of fluid infiltration during prograde to peak metamorphism which resulted in decarbonation and isotope resetting further petrologic evidence supports a model of late carbonic fluid infiltration that has partially affected the calc silicate rocks with subsequent isotope resetting more towards the contact between calc silicate rock and charnockite at korani only oxygen isotopes have been significantly lowered 18o 13 and the process involved might be a combination of metamorphic devolatilization accompanied by an aqueous fluid influx supported by petrologic evidence the stable isotope signatures obtained from the individual localities thus indicate heterogeneous patterns of fluid evolution history within the same crustal segment

carbonate platforms of passive extensional continental margins types characteristics and evolution abstract carbonate platforms of extensional margins may be grouped into several major categories homoclinal ramps have gentle slopes into deep water may have skeletal or ooid pellet sand shoal complexes that grade without break in slope into deep ramp nodular limestone and then into pelagic hemipelagic basin facies they generally lack significant slump and sediment gravity flow deposits in the deeper water facies distally steepened ramps differ from the above in having a marked increase in slope at the seaward edge of the deep ramp and abundant slumps slope breccias and turbidites however clasts of shallow platform margin facies are generally absent from breccias rimmed shelves have linear trends of shelf edge lime sands and reefs a marked increase in slope into deep water and foreslope and slope sands breccias with clasts of platform margin rocks and turbidites grading seaward into basin hemipelagic pelagic muds they may be divided into accretionary bypass and erosional margins isolated platforms are broad flat topped shallow platforms surrounded by deeper water few hundred meters to 4 km deep many are bypass margins but accretionary and erosional margins also occur finally incipiently to completely drowned or open platforms may develop by rapid submergence of ramps shelves or isolated platforms shallow platform margin facies are shifted landward and the earlier shallow water platform is covered with transgressive lags and deeper water blankets of hemipelagic or pelagic facies or open marine whole fossil wackestone the carbonate prisms commonly develop above a basal clastic phase ramps develop early and evolve into rimmed shelves however with drowning rimmed shelves may evolve up into ramps rimmed shelves may also develop into ramps where miogeoclinal faces prograde into filling marginal basins carbonate miogeocline deposition terminates when the shelf becomes a prograding clastic shelf or where it becomes part of a collisional orogen the various platform types may be recognized from continental margin sequences ranging from proterozoic to holocene in age

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

u pb geochronology and isotope geochemistry of the archean and proterozoic rocks of north central madagascar abstract new u pb zircon ages and sm nd and rb sr isotopic data are presented for orthogneisses from north central madagascar including ile sainte marie alaotra beforona maevatanana and ambatolampy ambatomarina a migmatite tonalite gneiss from ile sainte marie is dated precisely at documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 3187 pm 2 end document ma and has a sm nd model age tdm of 3204 ma thereby establishing a middle archean age for the oldest juvenile gneisses in northeast madagascar dated orthogneisses int

carbon isotope compositions of fluid inclusions in charnockites from southern india models for the formation of crustal granulites which include the metamorphism of anhydrous lithologies1 2 the extraction of hygroscopic granite melts3 4 and fluxing by co25 7 constrain the thermal and geochemical evolution of the lower crust and its interaction with mantle derived volatiles here we present abundances and carbon isotope compositions of co2 in fluid inclusions in quartz and garnet from gneiss granulite pairs from the incipient char nockite quarries of southern india determined by a stepped combustion thermal decrepitation carbon extraction technique which quantifies problematic sample contamination and allows precise lithological control the results show that the co2 in incipient charnockite is more abundant and isotopically heavier than the co2 in associated gneiss providing evidence for the influx of co2 rich fluids during their formation the inferred carbon isotope compositions of these fluids are consistent with a mantle source and at ponmudi the isotopic composition of coexisting graphite and co2 implies fluid rock volume ratios in excess of 0 15

crustal evolution in south india constraints from nd isotopes nd isotope analyses from a range of lithologies including charnockites metasediments and granitoids that comprise the high grade terrain of south india define contrasting age provinces north of the palghat cauvery shear zone the karnataka craton and the granulite blocks of nilgiri and madras are characterized by model nd ages from 3 4 to 2 4 ga with significant crustal growth during the late archean south of the palghat cauvery shear zone a pan african granulite facies event resulted in sm nd fractionation as observed in several incipient charnockite localities model nd age calculations that allow for sm nd fractionation at 550 ma yield ages in the range of 2 9 to 1 3 ga this range compares closely with published model nd ages from the high grade terrain of sri lanka mid proterozoic crustal growth 1 1 ga observed in the vijayan complex of sri lanka may have provided source material for metasediments characterized by young model nd ages in southern peninsular india the palghat cauvery shear z

back arc extension in the okinawa trough the okinawa trough lying to the east of china is a back arc basin formed by extension within continental lithosphere behind the ryukyu trench arc system middle to late miocene uplift associated with normal faulting of the initially adjacent ryukyu nonvolcanic arc and the taiwan sinzi folded belt corresponds to the first rifting phase the timing of rifting is supported by the presence of marine sediments of corresponding age drilled in the northern okinawa trough the rifting occurred after a major early miocene change in the motion of the philippine plate with respect to eurasia and ceased during the pliocene a second rifting phase started about 2 m y ago at the plio pleistocene boundary and has continued until the present time it has proceeded to a more advanced stage in the middle and southern okinawa trough than it has farther north detailed bathymetric sea beam seismic reflection and magnetics data collected during the pop 1 cruise of the r v jean charcot reveal the principal features of the extensional processes the back arc spreading phase started very recently in the southern and middle okinawa trough as exemplified by several en echelon and in some cases overlapping active central graben oriented n70 e n80 e some of these depressions are intruded by volcanic ridges of fresh back arc basalt with associated large magnetic anomalies transform faults between these en echelon active rifts are not obvious we suggest that the major part of the southern okinawa trough is underlain by a thinned continental crust and that except for the system of en echelon rifts of the southern okinawa trough the back arc basin oceanic domain is limited to a width of a few tens of kilometers or less in the axial portion of the trough the system of axial back arc volcanic ridges that occur in the rifts ends at the latitude of okinawa island whereas active volcanoes in the ryukyu arc occur only north of okinawa island we refer to this transition between active arc and back arc volcanism as the volcanic arc rift migration phenomenon vamp globally back arc volcanism propagated from the southern okinawa trough to the okinawa vamp area rifting continues to occur in the northern okinawa trough but is not yet accompanied by associated volcanism the okinawa vamp area is characterized by a series of parallel basaltic ridges oriented n75 e with associated linear magnetic anomalies characteristic of dyke intrusions we suggest that the formation of the back arc oceanic domain took place along the axial back arc extensional zone trending n75 e and that this zone presently ends at the southern extremity of the active volcanic chain the initial phase of formation of back arc basin oceanic crust is non steady state and is characterized by the lack of a developed fracture zone pattern the termination of the vamp area in the direction of the volcanic zone of the arc is consistent with the suggestion of molnar and atwater that the volcanic arc is a fundamental line of weakness which determines where initial back arc spreading occurs apparently back arc extension initially occurred within the continental lithosphere located westward of the ryukyu arc along its whole length but the subsequent back arc volcanism was initiated in the southernmost portion of the region and then moved northward this migration was accompanied by the shutting down of volcanic activity along the abandoned portions of the arc it is this transfer of volcanism that we call the vamp process thus arc and back arc basin volcanism seem to be associated in such a manner that spreading tends to migrate simultaneously with a cessation in volcanic activity along the arc this interplay of arc and back arc activity is probably linked to changes in the parameters of plate convergence since the plate motion in the phillippine sea is oblique to the trench at least in the southern part of the okinawa trough we suggest that the oblique resisting force applied to the edge of the overriding plate engenders the development of en echelon extensional features within and behind the arc the motion of the ryukyu platelet with respect to eurasia is consequently an extensional strike slip motion trending roughly parallel to the okinawa trough

fluid evolution and exhumation path of the trivandrum granulite block southern india in charnockites and host garnet biotite gneis ses of the trivandrum granulite block tgb southern india fluid inclusions are mainly represented by co2 rich categories aqueous inclusions with low salt concentration 0 33 13 5 wt nacl equiv occur sub ordinately we recognize three generations of co2 rich inclusions which have contrasting fluid densities that correlate with the different stages of metamorphic evo lution established from mineral phase equilibria studies although some of these inclusions show variable depression in melting temperatures laser raman spec troscopic studies revealed no volatiles other than co2 we report here the occurrence of high density co2 rich inclusions for the first time from this region with homogenization temperatures in the range of 21 9 to 15 6 c q 1 041 1 012 g cm 3 the marked secondary character of these high density inclusions and their tex tural association with late brittle deformational features indicate their entrapment at low temperature conditions 250 300 c post dating charnockite formation regional metamorphism in tgb stage m1 took place at ca 700 c and 5 36 kbar although higher tempera tures prevailed in the northern part of the terrain a subsequent period of decompression of the order of 6 7 km was accompanied by partial recrystallization of host rocks and intensive processes of charnockite for mation m2 that extended to subisobaric cooling from 785 805 to 680 700 c the relatively small population of aqueous inclusions in charnockites and their low salinity preclude any major role for metasomatic pro cesses involving increase of potassium activity and confirm the important role of co2 rich fluids in stabi lizing low water activities during the final phase of exhumation of the region ca 9 km in depth additional heat input resulted in mineralogical alterations corre lated to stage m3 while the source of heat and fluids involved in charnockite formation is postulated to be magmas derived from sublithospheric sources the late heat and fluid transfer are correlated to late pan african ultrabasic and alkaline intrusives emplaced at shallow depths

zircon ion microprobe dating of high grade rocks in sri lanka the high grade gneisses of sri lanka display spectacular in situ granulitization phenomena similar to those observed in southern india and of current interest for evolutionary models of the lower continental crust the absolute ages of these rocks are poorly constrained and so using the shrimp ion microprobe we have analyzed small spots on zircons from upper amphibolite to granulite grade quartzitic and pelitic metasediments detrital grains from a metaquartzite of the highland group preserve premetamorphic u pb ages of between 3 17 and 2 4 ga and indicate derivation of the sediment from an unidentified archean source terrain the pb loss patterns of these zircons and the other samples suggest severe disturbance at ca 1100 ma ago which we attribute to high grade regional metamorphism two pelitic gneisses contain detrital zircons with ages up to 2 04 ga and also record an approx 1100 ma event that is also apparent from metamorphic rims around old cores and new zircon growth a granite intrusive into

late paleocene early eocene tethyan carbonate platform evolution a response to long and short term paleoclimatic change abstract the early paleogene experienced the most pronounced long term warming trend of the cenozoic superimposed by transient warming events such as the paleocene eocene thermal maximum petm the consequences of climatic perturbations and associated changes on the evolution of carbonate platforms are relatively unexplored today modern carbonate platforms especially coral reefs are highly sensitive to environmental and climatic change which raises the question how sub tropical reef systems of the early paleogene reacted to gradual and sudden global warming eutrophication of shelf areas enhanced co 2 levels in an ocean with low mg ca ratios the answer to this question may help to investigate the fate of modern coral reef systems in times of global warming and rising co 2 levels here we present a synthesis of tethyan carbonate platform evolution in the early paleogene 59 55 ma concentrating on coral reefs and larger foraminifera two important organism groups during this time interval we discuss and evaluate the importance of the intrinsic and extrinsic factors leading to the dissimilar evolution of both groups during the early paleogene detailed analyses of two carbonate platform areas at low egypt and middle spain paleolatitudes and comparison with faunal patterns of coeval platforms retrieved from the literature led to the distinction of three evolutionary stages in the late paleocene to early eocene tethys stage i late paleocene coralgal dominated platforms at low to middle paleolatitudes stage ii a transitional latest paleocene platform stage with coralgal reefs dominating at middle paleolatitudes and larger foraminifera dominated miscellanea ranikothalia assilina platforms at low paleolatitudes and stage iii early eocene larger foraminifera dominated alveolina orbitolites nummulites platforms at low to middle paleolatitudes the onset of the latter prominent larger foraminifera dominated platform correlates with the paleocene eocene thermal maximum the causes for the change from coral dominated platforms to larger foraminifera dominated platforms are multilayered the decline of coralgal reefs in low latitudes during platform stage ii is related to overall warming leading to sea surface temperatures in the tropics beyond the maximum temperature range of corals the overall low occurrence of coral reefs in the paleogene might be related to the presence of a calcite sea at the same time larger foraminifera started to flourish after their near extinction at the cretaceous paleogene boundary the demise of coralgal reefs at all studied paleolatitudes in platform stage iii can be founded on the effects of the petm resulting in short term warming eutrophic conditions on the shelves and acidification of the oceans hampering the growth of aragonitic corals while calcitic larger foraminifera flourished in the absence of other successful carbonate producing organisms larger foraminifera were able to take over the role as the dominant carbonate platform inhabitant leading to a stepwise tethyan platform stage evolution around the paleocene eocene boundary this szenario might be also effective for threatened coral reef sites

geochronological evolution of the moyar bhavani and palghat shear zones of southern india implications for east gondwana correlations abstract the amphibolite facies moyar bhavani and palghat shear zones separate the south indian dharwar craton dc from the nilgiri and madurai high grade crustal provinces we present sm nd and rb sr isotopic whole rock mineral and small slab data for remnant granulites mylonitic gneisses and granitoids that reveal a neoproterozoic tectonothermal imprint in these shear zones sm nd garnet crystallisation ages and rb sr mica cooling ages from reworked rocks suggest that pan african structurally controlled retrogression and successive cooling in the moyar shear zone msz garnet 624 591 ma muscovite 594 ma biotite 604 540 ma predates tectonometamorphism in the bhavani shear zone bsz garnet 552 ma biotite 521 491 ma and palghat shear zone psz garnet 521 ma biotite 488 485 ma a rb sr small slab isochron age of 624 ma indicates that pan african retrogression in the msz also led to isotopic reequilibration of the rb sr system on a cm scale a sm nd garnet crystallisation age of 513 5 ma reflects emplacement of a postdeformative pegmatite in the bsz during the late cambrian a sm nd garnet age of 2355 18 ma obtained from a granulite remnant in the msz records early palaeoproterozoic high grade metamorphism which may be related to the amalgamation of the nilgiri block and dc along the palaeo moyar zone a relict charnockite from the southern bsz yielded a reproducible sm nd garnet age of 1705 11 ma presumably recording late palaeoproterozoic metamorphism neoarchaean and palaeoproterozoic nd model ages 3 0 2 2 ga suggest that the shear zone rocks are mainly reworked material from the adjacent nilgiri and madurai crustal domains nd model ages of granites and tonalites which intruded the msz syntectonically provide evidence for 1 7 ga crust generation unsheared metasediments exposed between the bhavani and palghat high strain zones were subjected to pan african high grade metamorphism as constrained by sm nd age data of garnets grown during coaxial deformation at 610 560 ma sm nd whole rock data from these supracrustal rocks indicate average crustal residence ages of 1 9 ga which implies contributions from an unknown proterozoic crustal source and disproves their previously suggested correlation to the archaean sargur group these model ages provide an upper limit for the juxtaposition of dc and madurai block along the bsz and psz

structural constraints on reworking in the western ghats granulite belt india and the antarctic analogue abstract the granulite belt along the west coast of india in kerala tamil nadu is particularly important for the study of proterozoic events in the pre drift east gondwana super continent in antarctica structural petrological and geochronological lines of evidence points to a proterozoic reworking of archaean granulites in the western ghats granulite belt the late proterozoic pan african event is overwhelmingly dominant though some late archaean to early proterozoic granulites have been reported structural evidence presented here points to a late proterozoic reworking resulting in a break or discontinuity in the tectonic evolution of this granulite belt thus relicts or vestiges of probable late archaean to early proterozoic granulites are recognised in the largely pan african granulite belt of the western ghats

zirconolite and baddeleyite in an ultramafic suite from southern india early ordovician carbonatite type melts associated with extensional collapse of the gondwana crust abstract we report here the occurrence of rare zirconium bearing minerals zirconolite cazrti2o7 and baddeleyite zro2 from an unusual ultramafic suite within the achankovil shear zone acsz in southern india zirconolite occurs as inclusions within spinel in phlogopite graphite spinellite and shows characteristic development of radial cracks baddeleyite is commonly observed as an included phase within phlogopite from phlogopite dunite and graphite spinel glimmerite the mineral also occurs less commonly within spinel and graphite from graphite spinel glimmerite the composition of zirconolite is characterized by an enrichment of u and th over rare earth elements baddeleyite shows abundance of zr with minor hf ti and u the mode of occurrence along with the chemical composition of these minerals implies their formation as early stage crystallization products from a silica undersaturated melt that was enriched in carbonatite phile elements such as ca zr ti and volatiles co2 and h2o we report

zircon geochronology of anatectic melts and residues from a highgrade pelitic assemblage at ihosy southern madagascar evidence for pan african granulite metamorphism we report u pb and 207 pb 206 pb zircon ages for a granulite facies gneiss assemblage exposed in a large quarry at ihosy southern madagascar the granulites are derived from pelitic to arkosic sediments and attained equilibrium conditions at 650 700 c and 4 5 kbar higher p t conditions of 750 800 c and 6 kbar in the presence of low water activities have led to dehydration melting processes the formation of granitic melts which partly moved away from their source region intruded into upper parts of the metapelitic gneisses as small granitic veins and left behind granulitic garnet cordierite quartz bearing rocks detrital zircons in a sample of metapelite and a sample of quartzofeldspathic gneiss yielded ages between 720 and 1855 ma suggesting a chronologically heterogeneous source region and a depositional age of less than 720 ma for these rocks high grade metamorphism and anatexis are documented by zircon ages between 526 34 and 557 2 ma with a mean age of about 550 ma the broad lithologies metamorphic grades and ages recorded in the ihosy rocks are similar to those in the wanni complex of northwestern sri lanka and in high grade assemblages of southernmost india and support the contention that all these terrains were part of the mozambique belt which formed as a result of collision of east and west gondwana in latest precambrian time

nature of the crust below the southern granulite terrain sgt of peninsular india across the bavali shear zone based on analysis of gravity data abstract the bavali shear zone is a major proterozoic structure in the southern granulite terrain sgt of peninsular india a detailed gravity survey has been carried out in order to delineate the crustal structure and tectonics of the shear zone and adjacent areas the bouguer anomaly map of the region shows a number of features i an e w gravity gradient zone with an average gradient of 3 0 mgal km in the west and 0 4 mgal km in the east across the shear ii a broad gravity low with a relief of 20 mgal in the wynad plateau se of the shear iii a series of gravity lows in the southern part of the shear between kannur and vayittiri iv a broad gravity high of 30 40 mgal encircling the charnockite province in the nw part of the shear and v a sharp localised gravity high closure of 40 50 mgal towards coast at nileswaram spatially filtered gravity anomaly maps clearly exhibit contrasting gravity signatures on either side of the shear zone with relatively higher gravity values within the northern block crustal thickness estimates in general vary from 36 to 41 km with a thinning of 2 to 3 km towards the north below the shear zone using two dimensional crustal models determined from gravity anomaly profiles across the bavali shear zone it is interpreted that the shear zone extends downward close to a plane separating blocks of different crustal densities the shear zone dips steeply towards the south and its downward extension meets with a slight moho rise suggesting that the bavali shear zone is a crustal scale structure eastwards below the wynad plateau however the shear zone becomes less prominent suggesting local variations in its geometry the models also point out the presence of a thick high density body 2 98 g cc in the lower part of the crust below the northern charnockite province which narrows towards the wynad plateau region in the east the presence of such a high density body could be due to factors such as evolution together with the protoliths of overlying charnockitic rocks a dense lower crustal residual fraction during anorthosite genesis or emplacement during pan african later anorogenic events the models also indicate that the wynad plateau region is characterised by a relatively homogeneous crust

ultra high temperature metamorphism and multistage decompressional evolution of sapphirine granulites from the palni hill ranges southern india sapphirine granulites from a new locality in the palni hill ranges southern india occur in a small enclave of migmatitic highly magnesian metapelites mg 85 72 within massive enderbitic orthogneiss they show a variety of multiphase reaction textures that partially overprint a coarse grained high pressure assemblage of bt opx ky grt pl qtz the sequence of reactions as deduced from the corona and symplectite assemblages together with petrogenetic grid considerations records a clockwise p t evolution with four distinct stages 1 equilibration of the initial high p assemblage in deep overthickened crust 12 kbar 800 900 c was followed by a stage of near isobaric heating presumably as a consequence of input of extra heat provided by the voluminous enderbitic intrusives during heating kyanite was converted to sillimanite and biotite was involved in a series of vapour phase absent melting reactions which resulted in the ultra high temperature assemblage opx crd kfs spr sil grt qtz bt coexisting with melt equilibration at c 950 1000 c 11 10 kbar 2 subsequently as a result of decompression of the order of 4 kbar at ultra high temperature a sequence of symplectite assemblages opx sil spr spr crdopx spr crdopx crdopx crd spl crd spl developed at the expense of garnet orthopyroxene and sillimanite this stage of near isothermal decompression implies rapid ascent of the granulites into mid crustal levels possibly due to extensional collapse and erosion of the overthickened crust 3 development of late biotite through back reaction of melt with residual garnet indicates a stage of near isobaric cooling to c 875 c at 7 8 kbar i e relaxation of the rapidly ascended crust to the stable geotherm 4 a second period of near isothermal exhumation up to c 6 5 kbar 850 c is indicated by the partial breakdown of late biotite through volatile phase absent melting reactions available isotope data suggest that the early part of the evolutionary history stages 1 3 is presumably coeval with the early proterozoic metamorphism in the extended granulite terrane of the nilgiri biligirirangan and shevaroy hills to the north while the exhumation of the granulites from mid crustal levels stage 4 occurred only during the pan african thermotectonic event which led to the accretion of the kerala khondalite belt to the south

atmospheric circulation upwelling and organic rich rocks in the mesozoic and cenozoic eras abstract by using the basic principles of atmospheric circulation and analogies from present circulation patterns it is possible to construct qualitative atmospheric circulation models on paleogeographic maps the wind patterns predicted by these models then serve as the basis for predicting the locations of ancient upwelling zones atmospheric circulation and upwelling models for seven geologic stages in the mesozoic and cenozoic were constructed and the distribution of the upwelling zones compared with the distribution of organic rich rocks for all time periods together the correlation was statistically significant when the individual time periods were analyzed separately the following pattern emerged significant correspondence between upwelling zones and organic rich rocks in the triassic middle early cretaceous to late cretaceous and middle paleogene to neogene no significant correspondence was found for the intervening times upwelling does not account for all organic rich rocks so called anoxic oceanic events and transgressions play an important role in their deposition it is suggested that upwelling contributes significantly to the pattern of organic rich rock distribution and is important in determining organic facies

multistage metamorphic evolution of the trivandrum granulite block southern india abstract the trivandrum granulite block tgb southern india records evidence for three distinct stages of evolution m1 m3 during the pan african high grade metamorphism with possible temperature gradient from north to south of the terrain as detected from mineral phase equilibria thermobarometry in three classic localities namely nuliyam kunnanpara and nellikkala the charnockites both incipient and massive were formed during the first stage m1 at temperatures higher than their host rocks and at appreciably lower pressures charnockite formation was dominantly controlled by an increase in partial pressure of co 2 along structural locales during subisothermal decompression although an increase of potash activity could have also been an important factor in this process the charnockites at nellikkala in the northern margin of tgb were formed under appreciably more h 2 o rich conditions x h 2 o 0 53 0 03 than those at nuliyam x h 2 o 0 25 0 02 in the southern margin it is inferred that during the period between the metamorphic stages m1 and m2 the terrain experienced subisobaric cooling comparison of results from thermobarometry with data on absolute age determinations from geochronology of the metamorphic rocks in tgb allows the interpretation that the m1 metamorphic event took place during 540 600 ma m2 at about 530 ma and m3 in the interval of 440 470 ma mineralogic and thermobarometric evidence for earlier high grade metamorphic processes if any have been erased from these rocks the processes of charnockite formation and post peak retrograde metamorphism in the tgb took place under high geothermal gradients 40 150 km this probably testifies to the existence of a local heat source either magmas at depth or mantle plume beneath the region the general metamorphic cycle in the tgb is estimated to be ca 100 160 ma which is much shorter in time span than that in the other regions of southern peninsular india such as the karnataka craton and the eastern ghats mobile belt during this period the terrain experienced rapid exhumation of approximately 6 7 cm year with the total amplitude of vertical movements estimated to be about 16 17 km

the timescale and mechanism of granulite formation at kurunegala sri lanka incipient charnockite formation at kurunegala in sri lanka is characterized by the growth of orthopyroxene at the expense of amphibole and biotite in an originally homogeneous gneiss mineral equilibria in the charnockite assemblage record pressure temperature p t conditions of 738 60 c and 6 9 1 2 kbar at 17 0 1 2 log fo2 and ah2o 0 18 0 16 wholerock trace element and isotopic measurements show that charnockite formation was accompanied by a systematic depletion of sm rb pb u sr nd with a fractionation of rb sr sm nd and th u ratios and crystallization of the charnockite assemblage at 535 5 ma major element fe mg ca and sm nd equilibration between minerals occurred at 524 9 ma whereas pb and rb sr underwent continued exchange to 501 5 ma and 486 1 ma respectively trace element data for both amphibolite and charnockite minerals show that depletion on a whole rock scale can be accounted for either by changes in mineral modes or trace element abundances within the immediate area of dehydration the fractionation of sm nd on a whole rock scale is controlled by the breakdown of amphibole without the growth of a major new host phase for sm in the charnockite rubidium and sr are dependent on the relative behaviour of biotite plagioclase and alkali feldspar modelling of dehydration melting involving the breakdown of amphibole biotite and alkali feldspar reproduces the observed sm nd and rb sr fractionation and indicates the loss of small melt fractions on a cm scale from the charnockite these observations suggest that partial melting is the most plausible means of effecting both the dehydration and depletion that accompanies charnockite formation

tectonics of the neoproterozoic southern granulite terrain south india geological correlations between south india and adjacent gondwana fragments are hampered by the lack of a coherent structural framework for india s southern granulite terrain sgt this paper is an attempt at delineating its large scale geometry and neoproterozoic kinematics and tectonics the basic regional pattern is that of a high grade gneissic layering produced during several deformation stages grouped as d1 which became refolded during two regionally separated phases of upright folding d2a nnw trending d2b ne trending and reworked in e to ese trending zones of increased strain d3 of these the palghat cauvery shear zone pcsz which marks the boundary between archaean terrains in the north and proterozoic belts in the south may represent a major crustal shear zone whereas the achankovil zone az is better considered as a high strain zone characterized by flattening with a reverse component d1 structures developed close to the peak of pan african high grade metamorphism d2a and d2b occurred on the retrograde path partly at supersolidus and partly at subsolidus conditions d3 high strain zones show subsolidus textures in view of the general clockwise nature with a clear decompressional component of all published p t paths we propose that d2 d3 occurred during partial exhumation of the terrain correlations with madagascar are weakened as the az is no longer considered a major crustal shear zone whereas correlations with sri lanka are strengthened by the presence of a similar syntactical bend possibly produced by an indentor

submarine hydrothermal mineralization in the okinawa trough sw of japan an overview abstract the okinawa trough is a heavily sedimented rifted back arc basin formed in an intracontinental rift zone submarine hydrothermal activity is located within the six back arc rifts located in the middle and southern okinawa trough and its distribution is controlled principally by tectonic factors subduction of the daito and gagua ridges beneath the ryukyu arc has resulted in fracturing of the brittle lithosphere beneath the okinawa trough hydrothermal activity is strongest in the volcanic arc rift migration phenomenon vamp area plus the jade site and southernmost part of the okinawa trough spot area which form the prolongation of these two ridges these areas are characterized by extremely high heat flow locally submarine hydrothermal fluids from the okinawa trough tend to be strongly influenced by interaction of the hydrothermal fluids with organic matter in the sediment resulting in high alkalinity and nh 4 concentrations of the fluids the fluids also contain high concentrations of co 2 of magmatic origin submarine hydrothermal mineralization in the trough is diverse the clam site consists principally of carbonate chimneys interaction of the hydrothermal fluid with organic matter in the sediment is particularly strong at this site this is most probably a sediment hosted deposit in which sulphide minerals have deposited within the sediment column leaving spent ore fluids to emerge at the seafloor the jade site consists of active and inactive sulphide sulphate chimneys and mounds the zn pb rich sulphides at this site contain the highest concentrations of pb ag and au so far recorded in submarine hydrothermal sulphide deposits at minami ensei knoll and hatoma knoll active and inactive chimneys consist principally of anhydrite and barite as a result of phase separation of the hydrothermal fluids beneath the seafloor an intense black smoker has recently been discovered at yonaguni knoll in the spot area if it is confirmed that sulphide mineralization is dominant at this site this could be a highly prospective area the most prospective areas for economic grade minerals in the okinawa trough appear to be the jade site and the spot area

the application of single zircon evaporation and model nd ages to the interpretation of polymetamorphic terrains an example from the proterozoic mobile belt of south india the technique of single zircon dating from the thermal evaporation of 207pb 206pb kober 1986 1987 provides a means of dating successive periods of growth and nucleation of zircons in polymetamorphic assemblages in contrast nd model ages may provide a measure of the period of crustal residency for the sample or its protolith these two techniques have been combined to elucidate the tectonic history of the proterozoic mobile belt of southern india exposed south of the palghat cauvery shear zone that marks the southern boundary of the archaean craton of karnataka the two main tectonic units of this mobile belt comprise the madurai and trivandrum blocks both of which are characterised by massive charnockite uplands and low lying polymetamorphic metasedimentary belts that have undergone a complex tectonic history throughout the proterozoic evidence for early palaeoproterozoic magmatism is restricted to the madurai block where single zircon evaporation ages from a metagranite 2436 4 ma are similar to model nd ages from a range of lithologies suggesting crustal growth at that time the trivandrum block to the south of the achankovil shear zone is comprised of the kerala khondalite belt the nagercoil charnockites and the achankovil metasediments single zircon evaporation ages together with conventional zircon and garnet chronometry suggest that all three units underwent upper amphibolite facies metamorphism at 1800 ma an event unrecorded in the metagranite from the madurai block this implies that the madurai and trivandrum blocks represent distinct terrains throughout the palaeoproterozoic model nd ages from the achankovil metasediments are much younger 1500 1200 ma than those from the adjacent kerala khondalite belt and madurai blocks 3000 2100 ma but there is no evidence for zircon growth in these metasediments during the mesoproterozoic hence the comparatively young model nd ages of the metasediments are indicative of a mixed provenance rather than a discrete period of crustal growth zircon overgrowths from the madurai block 547 17 ma and achankovil metasediments 530 21 ma suggest that all tectonic units of the proterozoic mobile belt of south india shared the same metamorphic history from the early palaeozoic this event has been recognised in the basement lithologies of sri lanka and east antarctica confirming that the constituent terrains of east gondwana had assembled by this time

a reappraisal of the pressure temperature path of granulites from the kerala khondalite belt southern india abstract the kerala khondalite belt kkb of south india is a pan african ca 600 500 ma regional granulite terrain that is composed of charnockites khondalites and migmatitic leptinitic gneiss peak assemblages in the kkb are characterized by orthopyroxene garnet k feldspar magnetite biotite plagioclase quartz in felsic to intermediate rocks and by orthopyroxene cordierite plagioclase quartz or garnet cordierite sillimanite feldspars quartz in semipelites and pelites postpeak reaction textures in these assemblages often involve the production of cordierite and are consistent with a predominantly decompressional pressure temperature history at temperatures in excess of 700 c pressure temperature estimates based on garnet orthopyroxene thermobarometry are in the range of 6 5 7 5 kbar and 860 920 c for localities close to and within the achankovil shear zone a prominent high grade high strain zone on the northeastern margin of the kkb for this northeastern subarea peak t

rare earth elements distribution in granulite facies marbles a witness of fluid rock interaction abstract the rare earth element ree distribution of marbles from tranomaro se madagascar shows that the marbles have interacted with syn metamorphic fluids during the panafrican granulite facies metamorphism the tranomaro area is characterized by an extensive development of pyroxenites skarns variably mineralized in uranothorianite and enriched in ree zr across a meter scale marble pyroxenite contact the ree content increases from the most remote marble sample to the contact with the pyroxenite la from 19 8 to 129 ppm ree patterns in enriched marbles display a strong negative eu anomaly similar to those of pyroxenites this peculiar type of ree distribution results from fluid infiltration on a regional scale some marbles have similar ree characteristics progressive development of eu anomaly together with ree enrichment showing that some of them have interacted with fluids infiltration was hardly recognizable using c and o isotopic signatures because large isotopic variations were introduced during the pre granulitic history then ree distribution in high grade marble may be helpful in monitoring syn metamorphic fluid flow in the present case it is a more reliable tracer of fluid infiltration than stable isotopic compositions

evolution of the continental margin of western india new evidence from apatite fission track dating apatite fission track dating in southwestern india yields middle triassic to middle jurassic ages that correlate with the breakup of eastern gondwanaland and africa the later india madagascar split is not recorded in the apatite ages the apatite ages are not a reflection of thermal events in the present location of this region but are associated with uplift and erosion in the upper paleozoic early mesozoic eastern gondwana africa separation track length measurements indicate that cooling through the 100 c isotherm began in the upper paleozoic time temperature modeling suggests that a period of slow cooling during the mesozoic was followed by relatively rapid cooling to the present

isotopic chronometry of zoned garnets growth kinetics and metamorphic histories basic information on the chronological and pressure temperature evolution of regional metamorphic terrains may in principle be derived from metamorphic garnets because of the similarly low diffusivities of sm nd and major cations in this mineral we report here smnd and rbsr isotopic and major element data on prograde garnets from regionally metamorphosed pelites from newfoundland the garnets preserve a prograde major element zonation as well as a sympathetic variation in sm nd ratio smnd data for separated portions of the garnet from core to rim provide both upper limits on the time for garnet growth and demonstrate synchronous growth of different garnet grains on a hand specimen scale the rbsr data on the same garnet fractions are in general agreement with these results but in some cases cannot be interpreted in terms of growth a minimum heating rate of 3k ma 1 is derived by combining the estimates for garnet growth time with the apparent temperature interval over which the garnet grew deduced from the major element zonation this value is similar to the minimum suggested by theoretical models for the thermal evolution of thickened continental crust the growth rate is within the range of 1 3 19 mm ma 1 set respectively by the isotopic data and the likely upper limit for heating rate during regional metamorphism these growth rates appear too slow to be controlled by surface reaction and suggest that other factors such as transport may be rate limiting in this case the limits set on the effective diffusion coefficient for material transport to the growth site 0 4 6 1 10 17 m2 s 1 suggest that grain boundary diffusion is probably the transport mechanism for supply of material to the growing garnet

constraints on the application of carbon isotope thermometry in high to ultrahigh temperature metamorphic terranes nine marble horizons from the granulite facies terrane of southern india were examined in detail for stable carbon and oxygen isotopes in calcite and carbon isotopes in graphite the marbles in trivandrum block show coupled lowering of 13c and 18o values in calcite and heterogeneous single crystal 13c values 1 to 10 for graphite indicating varying carbon isotope fractionation between calcite and graphite despite the granulite facies regional metamorphic conditions the stable isotope patterns suggest alteration of 13c and 18o values in marbles by infiltration of low 13c 18o bearing fluids the extent of alteration being a direct function of the fluid rock ratio the carbon isotope zonation preserved in graphite suggests that the graphite crystals precipitated recrystallized in the presence of an externally derived co2 rich fluid and that the infiltration had occurred under high temperature and low fo2 conditions during metamorphism the onset of graphite precipitation resulted in a depletion of the carbon isotope values of the remaining fluid calcite carbon reservoir following a rayleigh type distillation process within fluid rich pockets pathways in marbles resulting in the observed zonation the results suggest that calcite graphite thermometry cannot be applied in marbles that are affected by external carbonic fluid infiltration however marble horizons in the madurai block where the effect of fluid infiltration is not detected record clear imprints of ultrahigh temperature metamorphism 800 1000 c with fractionations reaching 1000 c and would be consistent with uht metamorphism

dehydration melting phenomena in leptynitic gneisses and the generation of leucogranites a case study from the kerala khondalite belt southern india

constraints on the timing of pan african granulite facies metamorphism in the kerala khondalite belt of southern india shrimp mineral ages and nd isotopic systematics abstract the supracrustal kerala khondalite belt kkb southern india constitutes an important but little understood terrain in the evolution of east gondwana here we report shrimp u pb zircon and titanite ages for a suite of metasedimentary rocks from the kkb metamorphic ages at documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 530 2 pm 4 4 end document and documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd us

pbpb dating of apatite by a stepwise dissolution technique abstract apatites from different geologic settings have been dated by a new sequential dissolution method the procedure is based on stepwise exposure of apatite to weak intermediate acids a pegmatite sample from the kerala khondalite belt in southern india yielded an age of 509 25 ma apatites from an enderbite sample from the northern marginal zone of the limpopo belt in southern africa yielded an age of 1970 26 ma which is in agreement with the high grade metamorphism at around 2 0 ga reported from the triangle shear zone and amphibolite facies metamorphism in the transition zone apatites from an amphibolite facies gneiss from the transition zone south of the nmz yielded an age of 1878 18 ma this age is somewhat younger than a sphene age on the same rock of 1971 17 ma and is believed to reflect the time of retrograde metamorphism in the presence of a fluid phase the sequential dissolution method is a new tool that allows dating of apatites which have relatively unradiogenic pb ratios because the spread can be significantly increased and no correction for common lead is needed this study further demonstrates that apatites from magmatic and high grade metamorphic rocks yield ages very close to those of monazite zircon or garnet unless the rock subsequently interacted with a fluid phase

evolution of the continental crust in the kerala khondalite belt southernmost india evidence from nd isotope mapping u pb and rb sr geochronology in order to decipher crustal genesis and evolution as well as to refine the metamorphic history of the kerala khondalite belt kkb southernmost india we have applied u pb monazite rb sr biotite feldspar dating and nd isotope geochemistry the kkb belonged to an eastern gondwana mobile belt that underwent ultra high temperature metamorphism at pan african times it is dominated by nd model ages ranging between 2 0 and 3 0 ga except at its northernmost boundary the achankovil unit au where middle proterozoic nd model ages were recognised 1 3 1 6 ga the new data show that these ages are not restricted to cordierite bearing metasediments alone but can be found in other lithologies of the achankovil unit as well peak metamorphism occurred between 590 and 550 ma as recorded by u pb dating of monazite from charnockites and garnet biotite gneisses rb sr ages of minerals in pegmatite intrusions record cooling to ca 400 500 c by 490 470 ma based on a systematic regional sampling this contribution confirms and refines former models based on smaller datasets the major issue is that the kerala khondalite belt must have undergone an extremely slow cooling history 3 6 c ma comparable to the one described for sri lanka and madagascar still such ultra high temperatures associated to slow cooling processes cannot be satisfactorily fit into a common tectonic context 2004 published by elsevier b v

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

metamorphism and isotopic evolution of granulites of southern india reference to neoproterozoic crustal evolution abstract granulites are important component of high grade metamorphic rocks reflecting intense conditions observed for crustal rocks in terms of temperature and pressure this review paper demonstrates how these high grade granulites are critical to understanding the evolution of the lower continental crust with special reference to southern india geothermobarometric traverse across different granulite blocks in southern india shows wide ranging p t conditions of metamorphism 700 1000 c and 5 10 kbar the sapphirine orthopyroxene sillimanite and spinel bearing quartz deficient granulites recognised from parts of southern granulite terrain ganguvarpatti kiranur and palani hill ranges etc show oriented sillimanite aggregates pseudomorph after course twinned kyanite staurolite kyanite assemblages and corroded blebs of gedrite within orthopyroxene suggesting a prograde stage of a clockwise p t evolution evidence of itd history comes from the textures in which an early mg rich garnet xmg 52 60 with orthopyroxene up to 10 al2o3 involving sillimanite breakdown forming variety of symplectites having combinations of orthopyroxene sapphirine cordierite and spinel these spectacular reaction textures and mineralogic sensors from the high grade rocks establish a prograde clockwise p t t path with notable decompressive history itd in the southern granulite terrain the inferred p t t paths have been further integrated with the recent geochronological and isotopic data to constrain the timing and duration of metamorphism emplacement of the magmatic protolith for characterising the evolution of the granulites and their bearing on the geodynamic implications based on the emerging evidence for neoproterozoic tectonothermal imprints in the southern granulite terrain history of the assembly of dispersed fragments is also addressed within the east gondwana framework

rb sr isotope dating of neoproterozoic malani group magmatism from southwest rajasthan india evidence of younger pan african thermal event by 40ar 39ar studies abstract this paper reports rb sr isotope ages of the neoproterozoic volcanics and associated granitoids of the trans aravalli belt of northwestern india all these rocks along with the earlier reported 779 10 ma old felsic volcanics from diri and gurapratap singh of pali district rajasthan constitute the malani group the study indicates that different rock suites belonging to the malani group represent a polyphase igneous activity which spanned for about 100 ma ranging from 780 to 680 ma the granitoids of the malani group i e peraluminous jalore type and peralkaline siwana type were emplaced around 730 and 700 ma ago respectively these plutonic suites represent two different magmatic episodes within a short time interval the initial sr ratios of these granitoids suggest lower crustal derivation of the magma the peralkaline granitoids and the associated peralkaline rhyolites pantellerites are coeval and cogenetic the ultrapotassic rhyolite exposed at manihari of pali district represents the youngest magmatic activity at 681 20 ma having a very high initial sr ratio of 0 7135 0 0033 the high initial sr ratio of these rocks may be due to incorporation of radiogenic 87 sr from the country rock by assimilation or fusion into the residual fraction of the magma in the crust which gave rise to other differentiated rocks of the group 40 ar 39 ar studies of two jalore granite samples indicate presence of post crystallisation thermal disturbance between 500550 ma ago the timing of this thermal overprinting on the malani rocks is related to the widespread pan african thermo tectonic event which is witnessed and magmatically manifested in different part of the indian shield

a nomenclature system for granular phosphate rocks according to depositional texture abstract because of their striking textural similarities to carbonates a nomenclatural scheme for the classification and interpretation of phosphate sediments is proposed based on the dunham embry and klovan classification for carbonate rocks allochemical phosphate rocks and phosphorites comprise orthochems and allochems matrix cement ooids bioclasts intraclasts and lithoclasts to distinguish such components and their resulting rocks as being phosphatic a phos prefix is proposed i e phosclast mudstone phosclast wackestone phosclast packstone etc this classification nomenclature emphasizes fabrics as well as palaeoenvironmental criteria

a tectonic perspective of incipient charnockites in east gondwana abstract examples of spectacular occurrences of charnockite in an arrested stage of formation in crustal segments of east gondwana comprising india sri lanka and antarctica are evaluated to constrain the broad tectonic framework under which the charnockite formed field structures petrologic features and information including fluids trapped within inclusions in minerals suggest that the conversion of upper amphibolite facies gneisses to charnockites was brought about by the local desiccation induced by the infiltration of co2 rich fluids through faults and shears under brittle quasi brittle conditions the stable isotopic composition of these fluids is consistent with a juvenile sublithospheric source available geochronologic data indicate multiple events of the incipient charnockite formation at 2500 1000 and 500 ma in these crustal domains in each case incipient charnockite formation post dates regional metamorphism hence the process of local desiccation is inferred to have operated in cratonized crustal segments the p t t paths characterized by isothermal decompression are consistent with incipient charnockite development during the waning stages of collisional mobile belts when the tectonic regime changed from compressional to extensional crustal distension triggers the release of co2 rich fluids from depth which are transported to mid crustal levels through magmatic conduits a notion supplemented by the occurrence of alkaline intrusives of pan african age degassing of these magmas could have supplied the co2 required for stabilizing the anhydrous assemblages in the charnockites

microseismicity of back arc rifting in the middle okinawa trough a one month microearthquake survey was conducted at iheya knolls in the middle okinawa trough in 1992 to reveal microearthquake characteristics of back arc rifting we deployed five ocean bottom seismometers and observed over 1700 events during 30 days hypocenters were determined for 48 earthquakes with good resolution from this survey we conclude that the survey area with 100km 100km has high seismicity many earthquakes occur in the western part of iheya knolls and few occur in the central part the earthquakes are distributed in an en echelon shape and throughout the entire depth of the crust there is no evidence for the existence of transform faults suggesting that the middle okinawa trough is in a stage of back arc rifting

himalaya the northern frontier of east gondwanaland abstract the himalayan province which represents the northern platform of the peninsular india belonging to east gondwanaland was strongly affected by pan african diastrophism 500 25 ma ago this brought to an end the protracted purana cycle of sedimentation throughout the peninsular india and the lesser himalaya and interrupted basin filling in its northern tethyan domain but the sea returned in the early permian along a narrow depression formed due to rifting of the himalayan crust in what is today the southern lesser himalaya in the rift valley was deposited tillites by glaciers of the gondwana continent under grip of refrigeration along with the glacigene conglomerates were emplaced diamictites generated by submarine slides triggered presumably by earthquakes originating from faults delimiting the rift the diamictities are admixed with lava agglomerate and tuff indicating widespread volcanism in the rift valley the rifting culminated in the breaking away of the tibetan part of the himalaya in the late permian and formation of neotethys between the gondwanaland and the cimmerian microcontinent embodying tibet iran and turkey rivers of the northern peninsular india flowed in the northerly directions since the middle proterozoic through early eocene in the late eocene there was a drastic drainage reversal to south and southeast when the himalaya emerged in the northern front of the gondwanaland

carbonate platform facies models various types of carbonate platforms are characterized by distinctive profiles facies and evolutionary sequences ramps may be homoclinal or distally steepened and may have fringing or barrier shoal water complexes of ooid pellet sands or skeletal banks homoclinal ramps pass seaward into deeper water without major break in slope and lack deep water breccias distally steepened ramps may be low energy and characterized by widespread shallow subwave base mud blankets or high energy with coastal beach dune complexes and widespread skeletal sand blankets slope facies may contain abundant breccias of slope derived clasts rimmed shelves have relatively flat tops and marked break in slopes at the high energy shallow shelf edge where they pass into deep water such shelves may be aggraded with peritidal facies extending over much of the shelf or incipiently drowned depending on magnitude of sea level fluctuations they may be accretionary or bypass types that include gullied slope escarpment and high relief erosional forms intrashelf basins occur on some shelves controlling distribution of reservoir and source beds isolated platforms are surrounded by deeper water and may be located on rifted continental margins or on submarine volcanoes most have high relief rimmed margins platforms that have been subjected to rapid sea level rise may be incipiently drowned and characterized by raised rims elevated patch or pinnacle reefs and widespread subwave base carbonate or fine clastic blankets completely drowned shelves develop where the shelf is submerged to subphotic depths terminating shallow water deposition and commonly resulting in blanketing of the shelf by deeper water facies some margins show extensive down to basin faulting that is contemporaneous with carbonate deposition or associated with thick prograding clastic sequences the various types of platforms change in response to variations in sedimentation subsidence or sea level rise and may form distinctive evolutionary sequences the relatively few models presented appear to accommodate most geological examples some of which contain major reservoir facies

rb and sr diffusion in alkali feldspars with implications for cooling histories of rocks volume diffusion of sr and rb have been measured in orthoclase feldspar as well as sr diffusion in albite using isotope exchange hydrothermal solutions and analysis by ion microprobe for water pressures of 1000 bars the kinetics of diffusion parallel to the c axis are given by the arrhenius relations strontium transport in the or is isotropic to within a factor of four the measurements use a new procedure in which only the elements in question are present in the starting fluid cooling rates of igneous rocks can now be determined from closure temperatures material balance and the rb sr dating method based on the departures of the mineral data from the whole rock isochron for cooling rates less than approximately 100 c my

the phosphorus cycle phosphogenesis and marine phosphate rich deposits abstract phosphorus in the form of phosphate is an essential nutrient and energy carrier on many different levels of life and a key element in mediating between living and lifeless parts of the biosphere one of the most important aspects of the phosphorus cycle is its vital role in governing productivity thereby interacting with the exogenic part of the carbon cycle which in turn is important in regulating earth s climate phosphorus is a prime element to be traced in earth s history because it allows for the reconstruction of long term feedback mechanisms between climate environment and ecology and of global change as such marine sedimentary phosphate deposits are particularly suited to study aspects of the phosphorus cycle because in the case of ubiquity their origin may result from a general acceleration of the global phosphorus cycle sources of sedimentary phosphate are microbial breakdown of buried organic matter and redox driven phosphate desorption from iron and manganese oxyhydroxides dissolved sea water phosphate represents an additional source which may become important in the formation of phosphatic hardgrounds the main locus of phosphogenesis is near the sediment water interface but phosphogenesis also occurs at greater sediment depths current induced winnowing and transport processes along the sea floor concentrate phosphate precipitates into deposits which exhibit internal stratification patterns typical for the prevailing hydraulic energy regime in a sequence stratigraphic context phosphate deposits preferentially occur along marine or maximum flooding surfaces consequent sedimentary reworking may result in the transfer of phosphates to highstand or lowstand deposits bio chemical weathering on continents represents the most significant source of bioavailable phosphorus this implies that long term changes in marine phosphorus levels and with these changes in marine ecology productivity rates and ratios of exported carbonate carbon and organic carbon are a response to changes in continental weathering rates a compilation of marine sedimentary phosphorus burial rates for the last 160 myr suggests that natural variations have occurred that span one order of magnitude for the late jurassic cretaceous and most of the paleogene the phosphorus cycle appears to have been accelerated in times of climate warming which was most likely due to the spreading of zones of humid climate and more intense continental weathering in the neogene the phosphorus cycle appears to have responded to changes in glacially induced weathering this suggests that uniform interpretations with respect to the emplacement of major phosphorite deposits should be treated with caution integrated analyses of the sedimentary and biogeochemical context of phosphorite occurrences may help to identify paleoenvironmental conditions as well as to improve our understanding of periods of enhanced phosphate accumulation periods which were usually characterized by steep gradients in the development of climate and environment with regard to the complexity of feedback mechanisms between the phosphorus cycle and the biosphere the present day input rates of phosphate into the world s oceans should be of great concern they are more than doubled by anthropogenic means and affect ecological systems on a rapidly increasing scale

proterozoic sedimentation and pan african geodynamic development in the himalaya abstract the purana or proterozoic lower cambrian continental margin sedimentary succession of the lesser himalaya comprises three lithological divisions a lower argillo arenaceous group a middle carbonate succession and an upper argillo calcareous assemblage the poorly sorted basal sediments consisting of wacke slate alternations of the lower group were emplaced by currents including turbidity currents which in the early stage flowed northwards from the aravalli and satpura mountains of peninsular india lenticular deposits of diamictite within the flysch succession represent submarine slides penecontemporaneous basaltic lavas and associated tuffs interbedded with shallow water arenites making up the bulk of the upper part of the lower group were related to rifting of the basins the overlying dolomites and dolomitic limestones were laid down on a very shallow stable shelf and are characterised by columnar branching stromatolites and lenticular units of crystalline magnesite and associated talc lenticular diamictites towards the top of the upper group of slates and limestone are harbingers of tectonic movements culminating towards the end of the early cambrian in an upheaval which abruptly terminated sedimentation throughout the lesser himalaya and peninsular india and caused a pronounced interruption of sedimentation in the tethyan domain in the northern himalaya the cambro ordovician 500 25 ma granites intruding the lesser himalayan thrust sheets of lower proterozoic metamorphic rocks and the upwarped northern front of the himalaya signify an important tectonic event that terminated the purana proterozoic lower cambrian sedimentation in the indian subcontinent this seems to be related to the tectonic movements that occurred towards the close of the late proterozoic or early cambrian

fluid rock history of granulite facies humite marbles from ambasamudram southern india an extensive humite bearing marble horizon within a supracrustal sequence at ambasamudram southern india was studied using petrological and stable isotopic techniques to define its metamorphic history and fluid characteristics at peak metamorphic temperatures of 775 73 c based on calcite graphite carbon isotope thermometry the mineral assemblages suggest layer by layer control of fluid compositions clinohumite calcite bearing assemblages suggest xco2 0 9 at 790 c and local wollastonite scapolite grossular bearing zones formed at xco2 of c 0 3 retrograde reaction textures such as scapolite quartz symplectites after feldspar and calcite and replacement of dolomite diopside or tremolite dolomite after calcite forsterite or calcite clinohumite are indicative of retrogression under high xco2 conditions calcite preserves late proterozoic carbon and oxygen isotopic signatures and the marble lacks evidence for extensive retrograde fluid infiltration while during prograde metamorphism the possible infiltration of aqueous fluids did not produce significant isotopic resetting isotopic zonation of calcite and graphite grains was likely produced by localized co2 fluid infiltration during retrogression contrary to the widespread occurrence of humite marbles related to retrograde aqueous fluid infiltration the ambasamudram humite marbles record a prograde to peak metamorphic humite formation and retrogression under conditions of low xh2o

regional carbonate alteration of the crust by mantle derived magmatic fluids tamil nadu south india regional carbonate alteration of the crust associated with major shear zones provides direct evidence for co 2 rich fluid mobility a good example occurs within the attur lineament one of numerous proterozoic shear zones which crosscut charnockitic gneisses and other lithologies of the southern indian craton at this locality widespread carbonate alteration of silicate rocks has involved growth of ankerite and other carbonate minerals which replace preexisting silicates some of the ankerite was subsequently recrystallized to fine grained calcite and iron oxide accompanied by sericitization of feldspar carbon oxygen and strontium isotope analyses of samples of ankerite and calcite together with some of the coexisting silicate minerals have been used to constrain the conditions of formation of the carbonate alteration and the origin of the fluids involved delta 13 c values of ankerite are relatively homogeneous ranging between 6 5 and 3 9 with a mean of 5 3 delta 18 o is also

the crustal deformation of the ilan plain acted as a westernmost extension of the okinawa trough abstract we analyze the current deformation in ilan plain of northeastern taiwan considered as the western extension of okinawa trough based on 34 geodetic sites observed between 2002 and 2006 the station velocities are between 1 9 mm yr and 43 0 mm yr in a direction between 38 and 143 relative to stable continent margin of eurasian plate this largest velocity is moving southeastward 143 at a rate of 43 mm yr was enhanced by a seismic doublet that occurred on march 6 2005 the station velocities at the northern margin of ilan plain are insignificant with a magnitude of 1 9 mm yr to 7 5 mm yr on the contrary the station velocities increase approximately southward along the southern margin of ilan plain the stations are moving roughly e w with a magnitude of 5 8 mm yr to 21 8 mm yr further southward the station velocities increase to a value of 43 0 mm yr in a direction of 143 the most prominent feature of the strain distribution patterns in ilan plain are the remarkable extensional strain rates observed in central and southern flank of the ilan plain the largest extensional rate is found 2 66 strain yr in 147 with a shortening rate in the perpendicular direction 1 28 strain yr in 57 indicating transtensional deformation mode in the northeastern flank of ilan plain insignificant or minor deformations are observed overall the extensional rates in this area clearly increase from north to south the extension directions in study area mostly trend e w to nw se directions using the velocity field of 34 gps stations and its strain rate filed we suggest the lateral extrusion process towards mechanically weak domain adjacent ryukyu subduction zone the lateral extrusion is facilitated by the opening of the okinawa trough we conclude that the northeast taiwan is subjected to both the compressional force exerted by the indentation of philippine sea plate and the tensile force induced by trench retreat related to the suction force at the ryukyu trench due to the interaction of two forces the stress regime rapidly changes from pure compression in the taiwan collision belt to transtension near ilan plain and its offshore area

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

microfacies zonation and spatial evolution of a carbonate ramp marginal moroccan phosphate sea during the paleogene the paleoenvironment of the southwestern portion of the paleogene moroccan phosphate sea has been reconstructed in detail by microfacies studies for a complete transgressive regressive cycle in front of the anti atlas forming the southern border of the atlantic gulf a bioclastic and phosphatic carbonate ramp extended to the north the ramp was a homoclinal very gentle dipping surface four stages of ramp evolution can be distinguished they are 1 the marginal marine initial stage 2 the marine transgressive stage 3 the marine regressive stage and 4 the final silting up stage the ramp is characterized by a distinct facies zonation a sabkha plain documented by mudstones and dolocretes was situated between the arid hinterland and the ramp nearshore open lagoons existed populated by chlorophycean algae and miliolid foraminifera these environments were protected from the open sea by a chain of oyster banks a belt of echinoid enrichments probably marked the wave base sea areas with deeper and only temporarily highly agitated water are characterized by a benthic faunal assemblage of varied molluscs with bryozoans becoming more abundant to the east the carbonates are intensively bioturbated and interfinger with phosphorite sands which dominated in the deepest part of the ramp all phosphorites studied are clastics poor in matrix and are not associated with phosphate source rocks a process of extensive reworking transport and redeposition which is related to transgressions and regressions has to be assumed

first report of pan african sm nd and rb sr mineral isochron ages from regional charnockites of southern india mineral and whole rock isotope data for a massive charnockite from kottaram in the nagercoil block at the southern tip of peninsular india yield sm nd and rb sr ages of 517 26 ma and 484 15 ma respectively the nd model age calculated for the charnockite is c 2100 ma our study reports the first pan african mineral isochron ages from regional charnockites of peninsular india which are in good agreement with the recently obtained ages of incipient charnockites in the adjacent blocks as well as alkaline plutons within the same block our results indicate that the pan african tectonothermal event in the granulite blocks south of the palghat cauvery shear zone was regional with terrain wide rejuvenation these results correlate with similar pan african tectono thermal events reported from sri lanka and east antarctica and have an important bearing on gondwana reconstructions

the nagercoil charnockite a magnesian calcic to calc alkalic granitoid dehydrated during a granulite facies metamorphic event the paleoproterozoic nagercoil charnockite massif in southern india consists of garnet absent and garnet bearing varieties with garnet occurring only in those areas where the charnockite incorporates supracrustal lithologies there is a late neoproterozoic cambrian granulite facies metamorphic overprint on the charnockite massif this study examines whether orthopyroxene in the charnockites crystallized during the original magmatic stage or formed during the later high grade metamorphic event the textural association of orthopyroxene and relic garnet grains and reaction intergrowths of garnet and or biotite with orthopyroxene similar to those reported in dehydration zones within the gneisses surrounding the charnockite massif show that orthopyroxene in the charnockite massif formed during the late high grade metamorphic event from dehydration reactions and is not part of the original igneous assemblage metasomatic textures such as thin k feldspar rims together with the presence of co2 rich fluid inclusions in these rocks suggest that co2 metasomatism widely prevalent in the gneisses surrounding the massif probably resulted in the low water activity necessary to form orthopyroxene during the metamorphic overprint an oxygen isotope traverse along a metapelite enclave charnockite contact within the massif shows a distinct variation in d 18 o values with the charnockite away from the contact having the lowest d 18 o value 8 9o similar to that of typical metaluminous granitic rocks the charnockite at the contact shows an intermediate d 18 o value 10 3o and the surrounding metapelite has the highest d 18 o value 12o the intermediate d 18 o value indicates partial assimilation of metapelite with a high d 18 o value into the charnockite with a low d 18 o value both the garnet absent and garnet bearing charnockites have similar compositional ranges to tonalite granodiorite granite ttg suites irrespective of whether garnet is present or not the sr contents of the charnockites show a bimodal distribution a high sr group which is magnesian calcic to calc alkalic metaluminous showing ttg affinities and a low sr group which is dominantly ferroan calcic to calc alkalic metaluminous showing similarities to the orthopyroxene bearing dehydration zones petrogenetic characterization suggests that the protoliths of the nagercoil charnockites were high temperature melts of hydrous basalt

pan african charnockite formation in kerala south india sm nd mineral ages of gneisses and associated granulites from the ponmudi incipient charnockite locality south india indicate that granulite metamorphism occurred at or shortly after 558 ma proterozoic ages recorded by garnet separates reflect a detrital age or an earlier metamorphic event preserved by inclusions within garnet the age of post metamorphic uplift 440 460 ma is constrained by sr isotope equilibration between biotite and plagioclase since charnockite formation and subsequent uplift north of the palghat cauvery shear zone had terminated by earliest proterozoic time these results confirm two distinct periods of granulite formation in south india and suggest that the palghat cauvery shear zone represents the boundary between two blocks of strongly contrasting geological histories both incipient charnockite formation and subsequent uplift at ponmudi may be correlated with the tectonothermal evolution of the highlands group in sri lanka the similarity between nd and sr model ages for charnockites and gneisses from ponmudi indicates that no significant rb sr fractionation has occurred during the crustal history of these incipient charnockites pb isotopic ratios suggest that th u ratios were fractionated during charnockite formation at about 500 ma in contrast to charnockites found north of the palghat cauvery shear zone fractionation of u pb during the archaean did not occur in the ponmudi granulites

nd isotopic evidence for the position of southernmost indian terranes within east gondwana abstract sm nd model ages of orthopyroxene bearing massif charnockites from the cardamom hills massif and adjoining supracrustal rocks from the kerala khondalite belt in southernmost india are used to infer some of the relationships within these rocks and between them and neighboring areas most of these rocks have model ages of 2 1 2 8 ga with most charnockites in the range 2 2 2 6 ga thus 3 0 3 4 ga archean rocks to their north did not contribute material to either suite and the two suites may have been juxtaposed after formation of the supracrustal rocks the similarity of sm nd isotope systems in the two units studied here supports an argument that the massif charnockites were the primary sole source of the detritus incorporated into the supracrustal rocks a cordierite gneiss representative of a relatively minor lithology in the supracrustal belt has a model age of 1 3 ga the protolith of this gneiss not only formed from much younger material than the rest of the belt but also formed significantly after the other metasedimentary rocks the source material of the gneiss protolith may have been located in the wanni and vijayan complexes of sri lanka the overlap of the model ages of rocks in this area and those in the highland complex of sri lanka supports the notion that these two sets of rocks were joined to each other in gondwana they belong to a belt that ran from antarctica through sri lanka and india into madagascar this belt was involved in pan african tectono metamorphism as reflected in the 550 ma age of the last granulite forming event throughout the belt

arrested charnockite formation in southern india and sri lanka arrested prograde charnockite formation in quartzofeldspathic gneisses is widespread in the high grade terrains of southern india and sri lanka two major kinds of orthopyroxene producing reactions are recognized breakdown of calcic amphibole by reaction with biotite and quartz in tonalitic granitic gray gneiss produced the regional orthopyroxene isograd manifest in charnockitic mottling and veining of mixed facies exposures as at kabbal karnataka and in the kurunegala district of the sri lanka central highlands chemical and modal analyses of carefully chosen immediately adjacent amphibole gneiss and charnockite pairs show that the orthopyroxene is produced by an open system reaction involving slight losses of cao mgo and feo and gains of sio2 and na2o rb and y are depleted in the charnockite another kind of charnockitization is found in paragneisses throughout the southern high grade area and involves the reaction of biotite and quartz garnet to produce orthopyroxene and k feldspar although charnockite formation along shears and other deformation zones at such localities as ponmudi kerala is highly reminiscent of kabbal close pair analyses are not as suggestive of open system behavior this type of charnockite formation is found in granulite facies areas where no prograde amphibole bearing gneisses exist and connotes a higher grade reaction than that of the orthopyroxene isograd metamorphic conditions of both kabbaltype and ponmudi type localities were 700 800 c and 5 6 kbar lower p h2o in the ponmudi type metamorphism was probably the definitive factor

reaction textures in scapolite wollastonite grossular calc silicate rock from the kerala khondalite belt southern india evidence for high temperature metamorphism and initial cooling abstract scapolite wollastonite grossular bearing calc silicate rocks from the vellanad area in the kerala khondalite belt kkb of southern india preserve a number of reaction textures which help to deduce their p t fluid history textures include calcite plagioclase quartz symplectites after scapolite grossular quartz coronas between wollastonite and plagioclase grossular coronas between wollastonite and plagioclase calcite that replace former scapolite and grossular blebs replacing anorthite calcite quartz pseudomorphs of scapolite garnet coronas are also observed between clinopyroxene and wollastonite or scapolite or plagioclase the reactions apart from those involving clinopyroxene can be modelled in the simple cao al 2 o 3 sio 2 co 2 system and interpreted using partial reaction grids constructed for the activities of end members in the analysed phases the reaction topologies produced are good approximations for the peak as well as retrograde mineral assemblages and reaction textures for the compositions of the phases present in this study the medium pressure calc silicate assemblages are defined by the stable pseudo invariant points qtz mei and grs the textural features interpreted using these activity corrected grids indicate a phase of isobaric cooling from about 835 c to 750 c at 6 kbar in the vellanad area this is inconsistent with earlier studies on other lithologies from the kkb most of which imply a post peak p t path involving near isothermal decompression however as the temperatures obtained for the kkb from the calc silicates are higher than those previously deduced from metapelites and garnet orthopyroxene assemblages the phase of near isobaric cooling reported here is inferred to have proceeded prior to the onset of the decompression documented from studies of other rock types

postcrystallization disturbance in the neodymium and lead isotope systems of metabasalts from the ramagiri schist belt southern india archean metabasaltic rocks from three separate blocks that constitute the north south trending ramagiri schist belt of the eastern dharwar craton have large variations in nd2750 even samples of the same rock type from one outcrop that are less than 50 cm apart from each other show a range of up to 5 5 nd2750 units the trends of the fields defined by the three suites in smnd vs nd2750 space intersect at smnd 0 1 to 0 2 and nd2750 8 to 12 suggesting that these rocks have lost lrees and have interacted with and exchanged at least nd with a common extraneous phase the isotopic composition of this phase suggest derivation of its neodymium from sources which had a long term history of lree enrichment this phase may have derived its isotopic composition from older continental crust one of the unaltered dikes cutting the three blocks in the belt and the surrounding gneisses gives a 147sm144nd 143nd144nd age of 2454 100 ma a minimum for the time of block assembly and alteration of rees including nd while a set of samples from the central block lie along an isochron with a pb pb age of 2746 64 ma the samples from the eastern and western blocks show large uncorrelated variations in 207pb204pb with 206pb204pb residues after acid leaching of sample powders have lead isotope ratios that are distinct for samples from each of the block and are compatible with their derivation from archean mantle the lead isotope composition of the acid leaches suggest disturbance of the metabasalts by an isotopically extraneous phase that had variable and high 207pb204pb and 208pb204pb ratios for a given 206ph204pb relative to expected archean mantle values both lead and neodymium isotope disturbances are best explained as a result of alteration of these rocks by an extraneous phase which had derived its isotopic composition from ca 3 5 ga continental crust

evolution of the continental crust in the proterozoic eastern ghats belt india and new constraints for rodinia reconstruction implications from sm nd rb sr and pb pb isotopes abstract for this study nd sr and pb isotope compositions were analyzed for ortho and paragneisses from the eastern ghats belt of india in order to determine its crust formation and crustal evolution this belt represents a proterozoic orogen that extends along the east coast of peninsular india and forms part of the mobile belts in east gondwana and rodinia the eastern ghats belt was affected by mesoproterozoic granulite facies metamorphism in the western segment western charnockite zone and a grenvillian regional scale high grade event in the central and eastern segments western khondalite zone charnockite migmatite zone and eastern khondalite zone as well as a local pan african overprint the results of the isotope studies are used for the large scale reconstruction of the indo antarctic part of the rodinia supercontinent based on nd model ages and pb isotope ratios from leached feldspars four crustal domains can be distinguished in the eastern ghats belt these domains can in part be correlated with the lithological division of the belt 1 the western charnockite zone south of the godavari graben is characterized by nd model ages between 2 3 and 2 5 ga for orthogneisses and 2 6 and 2 8 ga for metasediments domain 1 the pb isotopes are primitive indicating reworking of dominantly archean and mixing with minor proterozoic material 2 north of godavari graben nd model ages for orthogneisses are significantly higher with values ranging from 3 2 to 3 9 ga the pb isotopes are strongly retarded 3 the north eastern parts of the charnockite migmatite zone and western khondalite zone form a distinct and almost homogeneous crustal domain domain 3 with nd model ages between 1 8 and 2 2 ga 4 between the isotopically homogeneous terranes stretches a broad transition zone domain 2 enclosing parts of the western khondalite zone charnockite migmatite zone and eastern khondalite zone the nd model ages for metasediments 2 1 2 5 ga are younger than paragneiss ages of the adjoining western charnockite zone the nd model ages for orthogneisses 1 8 3 2 ga display a large spread which is consistent with the pb isotope signatures that indicate mixing of archean with proterozoic material the border zone between domain 3 and the archean eastern indian craton forms a second transition zone domain 4 characterized by metasediments with nd model ages between 2 2 and 2 8 ga and orthogneisses with model ages around 3 2 ga reworking of archean crustal material is most intense along the border zones of the belt and juvenile material is more dominant away from the orogenic front this scenario is indicative of an active continental margin setting for the two proterozoic episodes of orogenesis in the eastern ghats belt a correlation of domain 3 with the rayner complex and the prydz bay region antarctica for the early crustal evolution is supported by the similarity of the isotope signatures the napier complex is very different to the eastern ghats belt and an early joint evolution of these terranes is ruled out on the basis of the pb pb and sm nd systematics

isotopic mapping of age provinces in precambrian high grade terrains sri lanka nd model ages of amphibolite and granulite grade rocks in sri lanka form a simple regional pattern that broadly correlates with mappable geological units and is in effect an isotopic map of the island s basement the granulite grade units of the highland group and southwest group have model ages of 2 2 3 0 ga indicating derivation mainly from late archean sources they are bounded to the east and west by late proterozoic gneisses of the vijayan complex with model ages of 1 1 2 0 ga the isotopic data identify three distinct crustal provinces and are not consistent with earlier suggestions that the vijayan gneisses are retrograde equivalents of the highland granulites sri lanka is not a direct continuation of the archean dharwar craton of southern india identification of vijayan type juvenile crustal terrains in other gondwana fragments may play a key role in determining the precise attachment of southern india sri lanka in eastern gondwana

quantitative meso cenozoic development of the eastern central atlantic continental shelf western high atlas morocco abstract the well exposed late paleozoic to cenozoic succession in the western high atlas morocco documents the early rift to mature drift development of the eastern central atlantic continental shelf basin vertical sections depositional geometries and unconformities have been used to reconstruct the basin architecture prior to atlasian inversion two dimensional reverse basin modeling has been performed to quantitatively analyze the development of the continental shelf between the latest paleozoic to early cenozoic basin evolution stages include i early rift late permian to anisian ii rift climax ladinian to carnian iii sag norian to early pliensbachian iv early drift late pliensbachian to tithonian v mature drift berriasian to cenomanian vi mature drift with initial atlasian deformation turonian to late eocene vii atlasian deformation late eocene to early miocene viii atlasian uplift and basin inversion early miocene to recent the late permian to late cretaceous basin development comprises eight subsidence trends of 10 35 myr duration trends were initiated by changes in thermo tectonic subsidence which in turn triggered positive and negative feedback processes between sediment flux flexural and compaction induced subsidence plate tectonic reconfigurations in the atlantic domain controlled the thermo tectonic subsidence history and the basin development of the agadir segment of the northwest african passive continental margin 1 major shifts in the sea floor spreading axis 2 significant decreases in sea floor spreading rates 3 the stepwise migration of crustal separation and sea floor spreading in and beyond the central atlantic 4 african eurasian relative plate motions and convergence rates during the early pliensbachian toarcian to cenomanian the first three plate tectonic reconfigurations triggered changes in ridge push forces and modulated the extensional stress field of central atlantic plate drifting since the turonian african eurasian relative plate motions and convergence rates represented the dominant control on the thermo tectonic subsidence history in the agadir basin major variations in sediment flux and total subsidence characterize the development of the northwest african passive continental margin the explanation of typical stratigraphic sequences as caused predominantly by sea level fluctuations and rough assumptions on sediment input production and subsidence is not necessarily applicable to passive continental margins the methodology applied in this study including the newly developed tool of compositional accommodation analysis allows to develop more rigorous genetic models for the development of continental shelf basins

middle and late cenomanian oceanic anoxic events in shallow and deeper shelf environments of western morocco the response of shallow water sequences to oceanic anoxic event 2 and mid cenomanian events 1a and 1b was investigated along the west african margin of morocco north of agadir azazoul and correlated with the deep water sequence of the tarfaya basin mohammed beach based on biostratigraphy mineralogy phosphorus and stable isotopes in the deeper mohammed beach section results show double peaks in 13corg for mid cenomanian events 1a and 1b rotalipora reicheli biozone lower cc10a biozone the characteristic oceanic anoxic event 2 13c excursion rotalipora cushmani extinction top of cc10a biozone and laminated anoxic black shale in the shallow environment north of agadir a fluctuating sea level associated with dysoxic brackish and mesotrophic conditions prevailed during the middle to late cenomanian as indicated by oyster biostromes nannofossils planktonic and benthonic foraminiferal assemblages anoxic conditions characteristic of oceanic anoxic event 2 for example laminated black shales did not reach into shallow water environments until the maximum transgression of the early turonian climate conditions decoupled along the western margin of morocco between mid cenomanian event 1b and the cenomanian turonian boundary as also observed in eastern tethys north of agadir alternating humid and dry seasonal conditions prevailed whereas in the tarfaya basin the climate was dry and seasonal this climatic decoupling can be attributed to variations in the intertropical convergence zone and in the intensity of the north east trade winds in tropical areas

crustal and upper mantle structure in the ryukyu island arc deduced from deep seismic sounding summary in 1984 an extensive geophysical investigation was conducted in the northernmost part of the ryukyu island arc south of kyushu island japan the aim of this investigation was to obtain new information on the nature of crust and upper mantle at the continental margin through various geophysical measurements seismic profiling gravity magnetism sea beam and heat flow this paper presents the crust and upper mantle structure deduced from ocean bottom seismographic profiling conducted along two lines one line 190 km long was taken behind the northernmost part of the ryukyu island arc parallel to the major tectonic units i e the okinawa trough ryukyu arc and ryukyu trench the other 295 km long was perpendicular to the tectonic units along these profiles we deployed ocean bottom seismograms spaced 10 20km apart as a controlled source we used both explosives and an airgun array the experiment was successful and provided quite important information on the tectonics of a trenchisland arc back arc system the velocity structure obtained for the profile behind the ryukyu islands has continental properties the sediment thickness is 3 4 km although it shows lateral variations due to basement undulation the velocities of the upper and lower crust are 5 8 6 2 and 6 6 6 8 km s respectively the total crustal thickness decreases southwestward from 27 30 to 23 24 km this is direct evidence for crustal thinning associated with the process of back arc spreading the velocity structure along the profile from the trench to the island arc clearly shows the subduction accretion and deformation at this margin the crust beneath the trench has oceanic properties the sediment thickness is 1 3 3 0 km beneath which the igneous basement shows severe undulation the total crustal thickness is 7 8 km the p velocity is slightly less than 8 0 km s our seismic data revealed a huge sedimentary wedge located 50 150 km landward of the trench the maximum thickness of this wedge exceeds 12 km the origin of this wedge may be oceanic because the p wave velocity in its eastern half is almost comparable to that obtained for the trench area the velocity structure landward of the wedge is continental and similar to that obtained for the profile behind the island arc

dehydration reaction and isotope front transport induced by co2infiltration at nuliyam south india the field relations from a quarry at nuliyam south india illustrate dehydration of an amphibolite facies gneiss to granulite facies charnockite by co2 influx over a scale of 30 m both the calc silicate source of the fluids and the full extent of their penetration into the gneiss are preserved in a continuous section fluid flow is by a hydraulic fracture mechanism but is thought to be pervasive the sharp reaction front predicted by the continuum mechanical theory for advective fluid transport is not observed the front spreading is on too large a scale for either diffusive or dispersive control and is due to local kinetic disequilibrium between the fluid and rock although the divariant nature of the reaction may also have a limited effect the time integrated fluid flux varies from the instantaneous porosity at the fluid front to 20 vol adjacent to the calc silicate carbon isotope budgets suggest that decarbonation of the calc silicate by a rayleigh fractionation process provides a sufficient source for the co2 influxing into the gneiss graphite abundances vary from 0 01 to 0 1 by weight it is principally derived by precipitation from the fluid and may be modelled from phase equilibria carbon isotope fronts coincide with the reaction front on the scale of sampling although isotopic disequilibrium between graphite and inclusion co2 also implies local fluid rock disequilibrium

melt producing and melt consuming reactions in the achankovil cordierite gneisses south india migmatitic cordierite gneisses within the achankovil zone az of southern pan african india record melt producing and subsequent melt consuming mineral reactions early mineral assemblages bt sil qtz and bt sil spl deduced from inclusion textures in garnet prophyroblasts break down via successive dehydration melting reactions to high t phase assemblages e g grt crd liq opx liq spl crd liq later back reactions between the restite and the in situ crystallizing melt resulted in thin cordierite coronas separating garnet from the leucosome and partial resorption of garnet to opx crd or crd bt qtz symplectites leucosomes generally display a moderate low strain gneisses to strong high strain gneisses depletion of alkali feldspar attributed to mineral melt back reactions partly controlled by the degree of melt segregation using a kfmash partial petrogenetic grid that includes a melt phase and qualitative pseudosections for microdomains of high and low al si ratios the successive phase assemblages and reaction textures are interpreted in terms of a clockwise p t path culminating at about 6 7 kbar and 900 950 c this p t path is consistent with but more detailed than published results which suggests that taking a melt phase into account is not only a valid but also a useful approach comparing p t data and lithological and isotopic data for the az with adjacent east gondwana fragments suggests the presence of a coherent metasedimentary unit exposed from southern madagascar via south india az and sri lanka wanni complex to the lutzow holm bay in eastern antarctica

smnd isotopes and ree geochemistry of madras granulites india an introductory statement abstract major and trace elements including ree as well as sm nd isotope analyses of eight granulite samples from the madras area are presented the data provide evidence for the origin of some of the granulites but in other cases demonstrate metasomatic changes in major and trace element contents including lree the data furthermore demonstrate that the analysed granulites are not cogenetic however when plotted in a sm nd isochron diagram the samples yield a linear array which corresponds to an age t 2555 140 ma and nd t 1 1 4 this age of 2555 140 ma could represent the time of granulite facies metamorphism immediately preceded by the formation of the protolith the nd t 1 value may be indicative of a near chondritic mantle source or alternatively of mixing between a depleted mantle and a crustal component

charnockite formation at ponmudi in southern india charnockites constituents of most precambrian high grade terrains1 2 are essential for understanding the evolution of the early continental crust the arrested development of charnockite in shear veins at kabbaldurga3 in the state of karnataka in india suggests flow of co2 rich water deficient fluids through deep seated rocks as a mechanism of granulite grade metamorphism4 5 we report here an occurrence of arrested charnockite formation well south of kabbaldurga in the khondalite belt of southern kerala where rocks with amphibolite facies give way to the vast southern india sri lanka charnockite terrain indicating that metamorphism due to co2 rich fluids carbonic metamorphism may have operated over a large area in southern india if such localities prove to be widespread the present level of exposure throughout much of the high grade terrain probably does not extend far beyond an isofacial surface marking the boundary between upper and lower crust in the late archaean

pre metamorphic arar ages from biotite inclusions in garnet two populations of ar ar ages in biotite inclusions occluded by garnets from an indian polymetamorphic gneiss have been measured using uv laser ablation one population yields a reproducible age of 455 my identical to biotite ages in the adjacent groundmass and is interpreted as slow cooling following high grade metamorphism during the pan african event the second population yields ages in the range 1800 2100 ma and corresponds closely with zircon ages from the same rock these results imply that the ar ar system in a small population of biotite inclusions remained closed during high grade metamorphism 750 c and 5 kb the results of this first experiment accord with results for helium diffusion in garnet and suggest a mechanism for the long term storage of heavy noble gases in the deep crust

transition from oblique subduction to collision earthquakes in the southernmost ryukyu arc taiwan region tectonic characteristics of the region between taiwan and the southernmost ryukyu arc are inferred from a detailed analysis of local seismicity and source parameters of 62 recent earthquakes of 5 5 mb 6 6 five major seismogenic structures can be delineated the collision seismic zone csz the interface seismic zone isz the wadati benioff seismic zone wbsz the lateral compression seismic zone lcsz and the okinawa seismic zone osz in the csz located along the east coast of taiwan and offshore earthquake focal mechanisms show horizontal p axes distributed in two directions 287 10 and 333 16 possibly reflecting a strain partition associated with the relative plate convergence between the eurasia plate and the philippine sea plate the corresponding seismic strain tensor indicates a maximum compressive strain rate of 1 2 10 7 yr 1 along 293 and a comparable extension in vertical direction presumably resulted from plate collision in the region the geometry of the isz which is distorted significantly at its westernmost end can be approximated by a north dipping plane that is gradually pushed northward with increasing dip the seismogenic portion of the interface spans a short depth range from 10 km to 35 km a clear pattern of earthquake slip partition is observed the average slip vector residual is as large as 35 seismic strain patterns within the subducted philippine sea slab show predominantly downdip extension between 80 and 120 km and downdip compression at 270 km different from the pattern of strain segmentation observed for the rest of the ryukyu arc where the northern and southern portions are dominated by downdip extension and compression respectively owing to the large convergence obliquity the slab is descending at a rate significantly slower near taiwan than in the southern ryukyu thus we interpret the appearance of downdip extension within the subducted lithosphere as a combined result of oblique subduction and the slab s negative buoyancy a number of thrust or oblique strike slip earthquakes between isz and wbsz show a consistent pattern of lateral compression with p axes oriented roughly parallel to the local strike of the trench arc system they are probably due to the compressive strain originated from the collision and transmitted laterally within the lithosphere shallow normal faulting earthquakes show successive rotation of t axes from approximately n s in the okinawa trough to approximately e w in northeast taiwan possibly as a result of interaction between the extension from the opening of the okinawa trough and the compression from collision one normal event january 18 1991 mb 5 9 occurred in the central range with t axis roughly parallel to the structural trend of taiwan implying that the nature of the orogeny in taiwan has changed from thin skinned deformation to lithospheric collision involving the whole crust and uppermost mantle

arrested charnockite formation at kottavattam southern india at kottavattam southern kerala india late proterozoic homogeneous leptynitic garnet biotite gneisses of granitic composition have been transformed on a decimetric scale into coarse grained massive charnockite sensu stricto along a set of conjugate fractures transecting the gneissic foliation charnockitization post dates the polyphase deformation regional high grade metamorphism and anatexis and evidently occurred at a late stage of the pan african tectonothermal history geothermobarometric and fluid inclusion data document textural and chemical equilibration of the gneiss and charnockite assemblages at similar p lith t conditions 650 700 degrees c 5 6kbar in the presence of carbonic fluids internally buffered by reaction with graphite and opaque mineral phases x co2 0 7 0 6 x h2o 0 2 0 3 x n2 0 1 logf o2 l7 5 mineralogical zonation indicates that charnockitization of the leptynitic gneiss involved first the breakdown of biotite and oxidation of graphite in narrow outward migrating transition zones adjacent to the gneiss followed by the breakdown of garnet and the neoblastesis of hypersthene in the central charnockite zone compared to the host gneiss the charnockite shows higher concentrations of k na sr ba and zn and lower concentrations of mg fe ti v y zr and the hree with a complementary pattern in the narrow transition zones of biotite breakdown the p lith t x h2o data and chemical zonation patterns indicate charnockitization through subsolidus dehydration reaction in an open system subsequent residence of the carbonic fluids in the charnockite resulted in low grade alteration causing modification of the syn charnockitic elemental distribution patterns and the properties of entrapped fluids we favour an internally controlled process of arrested charnockitization in which during near isothermal uplift the release of carbonic fluids from decrepitating inclusions in the host gneiss into simultaneously developing fracture zones led to a change in the fluid regime from fluid absent in the gneiss to fluid present in the fracture zones and to the development of an initial fluid pressure gradient triggering the dehydration reaction