petrogenesis of late paleozoic volcanics from the zhaheba depression east junggar insights into collisional event in an accretionary orogen of central asia abstract the carboniferous to permian period marks an important transition from accretion to collision in the east junggar terrane nw china field and well fuqian 1 well core data from the zhaheba depression provide a better window for understanding the magmatic process during this period and its implications for the continental growth of central asia field investigations reveal structural features characterized by ne sw compression with lithology composed of basic and intermediate felsic volcanic rocks and pyroclastic rocks the core samples from well fuqian 1 show dominantly basalt basaltic andesite dacite and minor pyroclastic rocks we report zircon shrimp u pb ages of 276 0 279 8 ma from rhyolites and zircon la icp ms u pb ages of 315 4 317 4 ma from dacite and basaltic andesite our data suggest that the volcanic rocks from surface exposures and the well cores of well fuqian 1 formed in the early permian and the late carboniferous respectively the late carboniferous mafic rocks have geochemical characteristics similar to the intercalated early permian felsic rocks the mafic rocks show low initial 87 sr 86 sr 0 703162 0 703783 and high e nd t 5 5 7 5 enrichment in lree and some liles such as rb and th as well as hfses such as zr y and depletion in nb ta and ti furthermore they also display lower sm th 1 6 8 4 and higher th y 0 03 0 12 ratios than those of morb and variable th zr 0 004 0 016 ba th 61 839 ba la 6 13 48 77 and ba nb 10 101 ratios the geochemical data suggest that these rocks were derived dominantly from a 5 10 partial melting of a mainly garnet rich with minor spinel bearing iherzolite mantle source metasomatized by slab derived fluids the felsic rocks are rich in silica sio 2 57 43 78 07 and alkalis k 2 o na 2 o 5 33 9 28 and possess high tfe 2 o 3 0 70 6 95 contents and ga al ratios and low cao 0 18 5 11 and mgo 0 13 2 02 contents they represent typical high medium k calc alkaline a type rhyolite with enrichment of lree and hfse zr hf and depletion of ba sr eu nb and ta these rocks display high e nd t 6 9 7 3 and e hf t 10 3 14 6 values and young crustal model ages 348 557 ma integrating regional geological data from the east junggar terrane and the available data from the proximal ophiolite belt we propose that an arc arc or arc continent post collisional setting in the zhaheba area commenced at ca 317 ma the late carboniferous early permian felsic and mafic magmas were derived from the partial melting of the juvenile mafic lower crust and subcontinental lithospheric mantle metasomatized by pre carboniferous subduction related fluids through asthenospheric upwelling this post collisional event suggests that vertical accretion played an important role not only in the late carboniferous early permian continental growth of the east junggar but also in the final evolutionary phase of the accretionary orogen

holocene vegetation and climatic variations in central india a study based on multiproxy evidences palynology texture mineralogy geochemistry and magnetic susceptibility analysis of a 2 m deep sediment core from padauna swamp southeastern madhya pradesh infers that between 8600 and 7500 cal yr bp a warm and relatively less humid climate prevailed with open tree savannahs dominated by grasses followed by sedges artemisia and members of chenopodiaceae amaranthaceae with scanty trees viz schrebera aegle marmelos and sterculia urens this is well supported by lower organic to carbonate carbon ratio coarser texture having relatively low cia and magnetic susceptibility values and presence of some primary minerals between 7500 and 6250 cal yr bp the tree savannahs were succeeded by open mixed deciduous forests with the invasion of a few more trees viz madhuca indica holoptelea emblica officinalis mitragyna parvifolia and members of anacardiaceae in response to onset of a warm and humid climate a considerable rise in organic carbon generated from the degradation of plentiful biomass along with increase in clay content with signs of kaolinite and increase in immobile over mobile elements with slightly higher cia and magnetic susceptibility values also suggest climatic amelioration the presence of ruderal plants such as artemisia cannabis sativa and cheno am further infers initiation of human activities in the region between 6250 and 2800 cal yr bp the mixed deciduous forests became more diverse and dense subduing grasses and other herbaceous elements sporadic incursion of shorea robusta sal in forest floristic was recorded around 5000 cal yr bp the overall change in the vegetation mosaic reflects that a warm and more humid climate prevailed in the region probably on account of invigoration of southwest monsoon this observation is further corroborated by other proxy data showing a spurt in organic inorganic carbon ratio increase in clay content with matured mineralogy significantly higher cia and magnetic susceptibility values since 2800 cal yr bp onwards the modern sal dominated deciduous forests were established indicating continuation of warm and more humid climate including timely arrival of sw monsoon coinciding with the shedding of sal seeds as they are viable for a very short period

geology petrology and isotope geochemistry of massif type anorthosites from southwest madagascar four massif type anorthosite bodies 25 100 km2 in area occur within high pressure granulite facies supracrustal gneisses in southwestern madagascar two of these bodies ankafotia and saririaky appear to have been pulled apart by 40 km in a ductile shear zone but structural features such as sub vertical stretching lineations indicate an origin by intense west directed flattening and pure shear country rocks graphite series include abundant graphite schist some with 60 graphite marble quartzite and minor amphibolite and leucogneiss comagmatic granitoids e g charnockites are conspicuously absent the anorthosite bodies are dominated by coarse grained anorthosites and leuconorites feldspars typically 3 5 cm up to 1 m minor norites and oxide rich ferrogabbros occur near the margins but ultramafic rocks are absent typical mineralogy of the anorthositic rocks is plagioclase an41 54 orthopyroxene en38 66 augite mg 32 68 ilmenite magnetite apatite high alumina to 6 1 wt al2o3 orthopyroxene megacrysts are widespread most have exsolutions of calcic plagioclase an72 85 but some contain garnet lamellae metamorphism has produced abundant recrystallization and sporadic coronitic garnet mg 12 36 clinopyroxene assemblages rb sr isotopic analyses of whole rocks and minerals reveal no meaningful age relationships the age of late neoproterozoic metamorphism is best constrained at 559 50 ma by a 6 point sm nd mineral isochron whole rock plag pyx ilm apat gar from a saririaky oxide rich gabbro the igneous crystallization age of the anorthosites is estimated at 660 60 ma by a 19 point combined whole rock and mineral sm nd isochron for samples from both the ankafotia and saririaky bodies initial isotopic ratios calculated at 0 66 ga among 13 whole rocks are nd 2 6 to 5 2 mean 3 7 and isr 0 70328 0 70407 mean 0 70347 indicating derivation of the malagasy anorthosites from a depleted mantle source and little if any contamination with archean crustal material one anorthosite sample with nd 1 4 and isr 0 70344 calculated at 0 66 ga probably reflects the effects of assimilation of early to middle proterozoic crustal basement but typical surrounding graphite schist nd 0 3 isr 0 70636 both at 0 66 ga tdm 1131 ma represents only a minor potential contaminant for the anorthosite bodies tdm model ages of the malagasy anorthosites 797 1280 ma mean of 14 samples 949 ma as those of most other massif type anorthosites are older than the true crystallization age because of crustal contamination effects our isotopic data together with recent u pb data from the anorthosites and surrounding country rocks are consistent with emplacement of the malagasy anorthosite bodies at or before the start of a protracted high grade metamorphic event or series of events between about 630 and 550 ma this period coincides with the collision between and amalgamation of east and west gondwana

u pb single zircon pb evaporation and sm nd isotopic study of a granulite domain in se madagascar madagascar is a poorly known precambrian domain at the eastern border of the mozambique belt strongly affected by the pan african orogeny this large island represents a key part of gondwanaland and the accurate dating of the major metamorphic event is critical for the reconstruction of that supercontinent in the southeastern zone of the island metamorphism has reached high temperature and low pressure conditions 850 c and 5 kbar with moderate retrogression a u pb conventional multigrain single zircon stepwise pb evaporation and sm nd geochronological study on meta felsic rocks has shown that basement of at least lower proterozoic age was strongly metamorphosed around 570 580 ma this tectometamorphic event nearly obliterated any pre pan african zircon isotopic memory the granulite facies metamorphism is associated with the syn metamorphic emplacement of the anosyan charnock ites and granites a strong channelized fluid contribution at 545 ma has produced the re crystallization of biotite apatite

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

paleomagnetism of the late archean nyanzian system western kenya abstract the nyanzian system lavas of western kenya are believed to be the oldest rocks of the tanzanian craton intrusive age relationships suggest an age 2850 ma although direct attempts at dating the nyanzian have produced disparate results our study involves a suite of samples collected from the nyanzian basalts pillow basalts andesites and rhyolites from sixteen sites in western kenya these rocks yield a tilt corrected paleomagnetic pole at 14 n 150 e k 59 dp 5 dm 7 this pole is constrained to be older than the first d1 deformation 2472 30 ma by positive fold conglomerate and reversal tests analysis of the paleomagnetic data base for three african cratonic nuclei tanzanian kaapvaal zimbabwe and west africa for the time period from 2 0 ga to 3 0 ga demonstrates a paucity of well dated poles although there are several poles from the kaapvaal zimbabwe and tanzanian cratons which allow spot readings of their relative positions we demonstrate based on these data that the kaapvaal zimbabwe and tanzanian cratons were drifting independently at 2875 ma 2700 ma and 2450 ma this independent motion of the tanzanian and kaapvaal zimbabwe cratons indicates that previously proposed models involving african cratonic coherence can no longer be considered valid for the time period from 2850 to 2500 ma

pink sapphire from southern kerala s india implications on india madagascar correlation within gondwana assembly abstract we report here the occurrence of pink sapphires in association with a variety of gemstones from the trivandrum granulite block south of the achankovil shear zone in southern india the mineralization is associated with pegmatites or veins emplaced within granulite facies aluminous supracrustals the sapphires show near pure a1 0 composition 98 43 99 48 wt with traces of cr o 0 02 0 12 wt and feo 0 01 0 12 wt the available radiometric age of 513 2 ma for gem quality zircon associated with pink sapphire in the melankode locality confirms that the mineralization is of late pan african age pink sapphires have been widely reported from a number of localities in southern madagascar including betroka illakaka antranondambo and ambossary sapphires of various hues also occur in the ratnapura gem district in the southwestern part of sri lanka the pink sapphire occurrences in southern madagascar and southern kerala provide strong evidence for india madagascar juxtaposition in the gondwana assembly with the ranotsara shear zone in southern madagascar extending into the achankovil shear zone in southern india

neoproterozoic alkaline magmatism at sivamalai southern india abstract the sivamalai alkaline complex lies at the southern margin of the cauvery shear system that separates the archaean and proterozoic domains of the southern granulite terrain in india u pb tims dating of zircon from a pegmatitic syenite sample in the complex yields a concordant age of 590 2 1 3 2 ma which is interpreted to date the intrusion of the alkaline rocks a lower concordia intercept at 168 210 ma defined by two grains with high common lead may indicate post magmatic disturbances due to recrystallisation which is also evident in the cl images of the zircons epma dating of monazite from a post kinematic pegmatite which intrudes the crystalline basement hosting the alkaline rocks yields an age of 478 29 2 ma and provides a lower bracket for the main phase of tectonism in this part of the southern granulite terrain the pan african high grade metamorphism and ductile deformation has thus most likely affected the alkaline rocks this is supported by the presence of a metamorphic foliation and extensive recrystallisation textures seen in the rocks the major and trace element concentrations measured on selected samples reveals the presence of both enriched and depleted rock types the enriched group includes ferrosyenite and nepheline syenite while the depleted group has only nepheline syenites the trace element depletion of some nepheline syenites is interpreted to be a result of fractional crystallization involving the removal of accessory phases like zircon titanite apatite and allanite

alkali granites with pan african affinities from kerala s india in the kerala region of the south indian shield alkali granite plutons ranging in age from 550 to 740 ma intrude precambrian charnockites and migmatites all of them are spatially related to regional fault lineaments their geochemistry is similar to pan african post kinematic plutons from the afro arabian shield they are of granitic to adamellitic composition with high sio 2 k 2 o up to 12 and na 2 o and low cao feo and mgo their major element composition together with a low 87 sr 86 sr i 0 703 from one pluton denote a type affinities however local charnockites have such low rb and radiogenic 87 sr that the granites could have formed by crustal melting their overall trace element characteristics are typical of alkaline granites in general but the details show that most resemble more recent volcanic arc granites and one pluton is typical of a within plate setting the presence of co 2 rich fluid inclusions supports the notion that the enrichment of rare earth an

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

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

fluid rock interaction in the granulites of madagascar and lithospheric scale transfer of fluids abstract the nature of synmetamorphic fluids and their flow is examined in the granulitic lower crust of madagascar part of a precambrian crustal scale network of vertical ductile shear zones based on three independent data sets field and satellite mapping c o and h isotope geochemistry and gravimetry this crust is divided into three zones outside of shear zones minor shear zones 350 km long and 20 35 km wide the major shear zones are rooted in and are controlled by the mantle they tapped mantle derived co2 with carbon fluxes of the same order of magnitude as oceanic ridge degassing one major shear zone shows abundant phlogopite diopside apatite calcite mineralizations a well known paragenesis in mantle metasomatism due to mantle fluid infiltration and their interaction with the crust carbonatitic magmas possibly collected in the major shear zones at the base of the crust and may be the source for co2 upwellings as well as other metasomatic agents small scale minor shear zones were controlled by crustal deformation processes and focused crustally derived h2o rich fluids pervasive fluid circulation was restricted to the vicinity

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

pan african granulite facies assemblages in kodaikanal anaimalai ranges tamil nadu and mt abu balaram areas of gujarat india madagascar india connection in eastern gondwana assembly

late paleozoic post collisional magmatism in the eastern tianshan belt northwest china new insights from geochemistry geochronology and petrology of bimodal volcanic rocks abstract the late carboniferous to permian marks an important phase of terrane amalgamation through accretion collision in the central asian orogenic belt caob the tianshan belt forms part of this sequence where the post collisional event generated an e w trending bimodal volcanic zone that extends for ca 500 km from the southern bogda mt to the easternmost tianshan belt here we report the petrology geochemistry and geochronology of the bimodal volcanic rocks to characterize the post collisional tectonic evolution in late paleozoic the major element chemistry of the basalt and rhyolite samples indicates a bimodal high k calc alkaline affinity the basalts are characterized by high abundance in tio2 th u and pb and depletion in nb ta and p together with slight enrichment in lree contents and low hfse lree ratios the rhyolites show a close affinity to aluminous a type granites with enrichment in k na zr ce y and depletion in p nb ta and ti they also exhibit fractionated ree patterns with prominent negative eu anomalies the petrographic and geochemical data suggest that this bimodal rock series was generated in a post collisional setting the mafic rocks are likely derived from a metasomatized lithospheric mantle assimilation of different crustal components played a minor role in the genesis of these basaltic rocks the la icp ms analysis on zircons from four rhyolites and two basalts yielded similar ages ranging from 295 8 2 8 to 293 3 1 7 ma suggesting an early permian event zircons from the basalts show a wide ehf t range from 0 49 to 13 whereas those from the rhyolites show two groups of ehf t values from 5 to 3 and from 6 to 12 suggesting a mixing of crust mantle material the magmatic zircons from the bimodal volcanic rocks yielded two peaks of tdm2 between 600 820 ma and 1160 1360 ma probably representing two significant episodes of continental crust growth the bimodal volcanic rocks are correlated with the early stage of post collision our results suggest that vertical crustal growth through mantle derived magmas in post collisional setting also played a major role in the construction of the caob

juxtaposition of india and madagascar a perspective abstract proterozoic terrains in south india and madagascar provide important clues in understanding the gondwanaland tectonics especially the assembly of this mega continent during the pan african period the archaean terrains in both madagascar and india are characterized by n s trending greenstone belts occurring within gneissose granitic rocks in the northern part extensive development of k rich granitic rocks of ca 2 5 ga is also characteristic in both areas such a broad age zonation of younger dharwar ca 2 6 3 0 ga in the north and the older sargur ca 3 0 3 4 ga in the south as in south india remains to be identified in future studies from madagascar the occurrence of greenschist facies rocks in the northeastern part and higher grade rocks in most of other parts in the north central terrain of madagascar is comparable with the general tendency of increasing metamorphic grade from northwestern to southern areas ranging from greenschist to granulite facies in south india the proterozoic crystalline rocks in both continents show pronounced lithological similarity with the wide occurrence of graphite bearing khondalite in association with charnockitic rocks while the archaean proterozoic boundary is well defined in southern india by the palghat cauvery or the kkpt shear zones as recently identified this boundary is ill defined in madagascar due to extensive pan african overprinting as well as the development of the proterozoic cover sequence the itremo group there is also a possible general correlation between the mesoproterozoic cover sequences in madagascar and india such as between the itremo group of west central madagascar and the kaladgi and cuddapah sequences of south india the pan african granulite facies metamorphism of ca 0 5 ga extensively developed in both india and madagascar is generally comparable in intensity and extent p t conditions and p t t paths also appear comparable with the general range of ca 700 1000 c and 6 9 kb and near isothermal decompressional paths a type granite plutons and alkaline rocks including anorthosites and mafic plutonic rocks of ca 500 800 ma develop in both terrains provide strong basis for the correlation of both terrains and define a pan african igneous province within east gondwanaland major shear zones in both continents are expected to play a critical role in the correlation albeit are still poorly constrained detailed elucidation of the tectonic history of the shear zones and the timing of various events along the shear zones would provide important constraints on the correlation of the two continental fragments

setting of zn cu au ag massive sulfide deposits in the evolution and facies architecture of a 1 9 ga marine volcanic arc skellefte district sweden the skellefte mining district occurs in an early proterozoic mainly 1 90 1 87 ga svecofennian magmatic province of low to medium metamorphic grade in the baltic shield in northern sweden the district contains over 85 pyritic zn cu au ag massive sulfide deposits and a few vein au deposits and subeconomic porphyry cu au mo deposits the massive sulfide deposits mainly occur within and especially along the top of a regional felsic dominant volcanic unit attributed to a stage of intense extensional continental margin arc volcanism from facies analysis we interpret the paleogeography of this stage to have comprised many scattered islands and shallow water areas surrounded by deeper seas all the major massive sulfide ores occur in below wave base facies associations however some ores occur close to stratigraphic intervals of above wave base facies associations and the summits of some volcanoes that host massive sulfides emerged above sea level intense marine volcanism was superceded at different times in different parts of the district by a stage of reduced volcanism uplift resulting in subregional disconformities and then differential uplift and subsidence resulting in a complex horst and graben paleogeography uplift of the arc is attributed to the relaxation of crustal extension and the emplacement of granitoids to shallow crustal levels a few massive sulfide ores formed within the basal strata of this second stage the horst and graben system was filled by prograding fluvial deltaic sediments and mainly mafic lavas and during this stage the skellefte district was a transitional area between renewed arc volcanism of more continental character to the north and subsidence and basinal mudstone turbidite sedimentation to the south this whole volcanotectonic cycle occurred within 10 to 15 m y we define 26 main volcanic sedimentary and intrusive facies in the skellefte district the most abundant facies are 1 normal graded pumiceous breccias which are interpreted as syneruptive subaqueous mass flow units of pyroclastic debris 2 porphyritic intrusions and 3 mudstone and sandstone turbidites facies associations define seven main volcano types which range from basaltic shields to andesite cones and rhyolite calderas despite this diversity of volcano types most massive sulfide ores are associated with one volcano type subaqueous rhyolite cryptodome tuff volcanoes these rhyolite volcanoes are 2 to 10 km in diameter 250 to 1 200 m thick at the center and are characterized by a small to moderate volume rhyolitic pyroclastic unit intruded by rhyolite cryptodomes sills and dikes massive sulfide ores occur near the top of the proximal near vent facies association the remarkable coincidence in space and time between the ores and this volcano type indicates an intimate genetic relationship between the ores and the magmatic evolution of the volcanoes many of the massive sulfide ores occur within rapidly emplaced volcaniclastic facies and are interpreted to have formed by infiltration and replacement of these facies some of the ore deposits have characteristics of both marine massive sulfides and subaerial epithermal deposits we suggest that massive sulfides in the skellefte district span a range in ore deposit style from deep water sea floor ores to subsea floor replacements to shallow water and possibly subaerial synvolcanic replacements facies models are provided for the mineralized rhyolite volcanoes and volcanological guides are provided for exploration for blind ores within these volcanoes

dehydration and incipient charnockite formation a phase equilibria and fluid inclusion study from south india a 3 m traverse across a charnockite lens in a biotite garnet graphite gneiss quarry from southern india has provided mineral equilibria that indicate charnockite formation occurred under fluid conditions of x h 2 o 0 2 to 0 3 at p 5 5 pm 1 9 kb and t 700 to 750 c dehydration of biotite loss of garnet and growth of orthopyroxene are related to a decrease in x h 2 o in the fluid phase within the charnockite lens isotopic analysis of the fluid entrapped in quartz from the charnockite indicates delta 13 c 8 2 0 3 0 heavier than delta 13 c values from smaller volumes of co 2 entrapped in quartz from the gneiss although the increase in fluid inclusion abundance in the charnockite can be related to oxidation of graphite without influx of co 2 an external source of co 2 is required in order to account for the shift to heavier carbon isotope values observed in both graphite and entrapped co 2 within the charnockite carbon isotope data across the gne

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

were aspects of pan african deformation linked to iapetus opening the convergence recorded in some pan african deformational belts sensu lato in south america africa madagascar southern india sri lanka and antarctica is temporally correlated with opening of the iapetus ocean we propose a model in which continent continent collision and closure of the adamastor ocean between the amazon west african rio de la plata cratons and the sao francisco congo kalahari cratons in the late neoproterozoic are linked to rifting and orthogonal spreading between laurentia and the south american cratons by the early cambrian the cratons in south america and africa were assembled as west gondwana closure of the mozambique ocean which appears to have extended across antarctica between lutzow holm bay and the shackleton range resulted in continued convergence between the congo kalahari queen maud land block and east gondwana in the cambrian coeval deformation in the transantarctic mountains may be related to the obliquity of the antarctic margin relative to iapetus spreading directions initiation of voluminous arc magmatism along the paleo pacific margin of gondwana in the early cambrian is broadly synchronous with the cessation of intra gondwana pan african deformation possibly reflecting a change in plate motions at the time of final gondwana assembly the new subduction regime along the gondwana margin in the early cambrian may be linked to the closure of the iapetus ocean basin