gold in madagascar abstract gold is commonly found throughout the precambrian basement exposed in the eastern two thirds of madagascar composed of archean and proterozoic greenstone belts and metasediment belts gold occurs in four main kinds of deposits as a diffuse component of the crystalline basement rocks in concordant quartz veins within the metamorphic rocks of the precambrian basement in recent discordant veins and in recent and ancient alluvial deposits the initial gold mineralisation is probably in relation with the greenstone belt formation and many have been formed and or reworked during different metamorphic and magmatic events it is therefore difficult to obtain an accurate age of the different kinds of deposits

spatial and temporal relationships between mid tertiary magmatism and extension in southwestern arizona cenozoic magmatism in southwestern arizona which is within the basin and range tectonic province occurred almost entirely between 15 and 25 ma volcanic rocks typically consist in ascending order of 1 a thin sequence of mafic to intermediate lava flows 2 voluminous felsic lava flows and pyroclastic rocks with minor to moderate amounts of intermediate to mafic lava flows and 3 basalt and andesite volcanic rock sequences rest disconformably on pre tertiary bedrock in most areas but locally overlie substantial coarse clastic debris that was deposited immediately before and during earliest magmatism prevolcanic clastic debris is interpreted as a consequence of local early normal faulting in most regions tilting related to extension began later and occurred during or after eruption of felsic volcanic rocks and before the end of younger mafic volcanism extension generally ended before about 17 ma except in a northwest trending belt adjacent to the relatively unfaulted and topographically elevated transition zone tectonic province which is adjacent to the colorado plateau rapid cooling of metamorphic core complexes and tilting of young basalts and coarse clastic rocks continued in this belt until as recently as 11 ma extension was extreme in this belt whereas it was generally moderate to slight in other parts of southwestern arizona large magnitude extension was not associated with areas of greatest igneous activity and rapid cooling and exhumation of core complexes postdated local magmatism these relationships are inconsistent with theories that relate genesis of metamorphic core complexes to magma intrusion in the upper crust except for young extension in this northwest trending belt there are no apparent regional migration trends for either magmatism or extension within southwestern arizona lack of substantial extension before magmatism and general lack of magmatism during youngest extension are inconsistent with the hypothesis that magmatism was the product of decompression melting during lithospheric extension the long duration and large magnitude of extension adjacent to the transition zone tectonic province and within an area of earlier crustal thickening are consistent with the hypothesis that extension was driven by the gravitational potential energy of elevated land mass and crustal roots regional magmatic heating apparently weakened the lithosphere and triggered extension but did not control extension locally

les gneiss rubanes a cordierite et grenat d ihosy un marqueur thermo barometrique dans le sud de madagascar resume le precambrien de madagascar represente une portion de la ceinture mobile mozambicaine certains auteurs considerent que le sud de l lle montre d est en ouest des formations de plus en plus recentes il s agit de la sequence androyenne ultrametamorphique a noyaux archeens a laquelle succedent la sequence du graphite et la sequence du vohibory les gneiss rubanes et leptynites a cordierite et grenat ont une repartition generale dans la sequence androyenne ces gneiss proviennent de la fusion anatectique de paragneiss les geothermometres et barometres bases sur les couples grenat plagioclase grenat cordierite grenat biotite nous permettent d estimer les conditions de cette migmatisation t 700 c p t 5 5 5 kbar p h 2 o 0 3 0 4 p t une certaine quantite de leptynites a cordierite et grenat represente vraisemblablement les leucosomes de cet episode migmatitique la presence quasi systematique dans les gneiss rubanes d un assemblage a sept phases quartz plagioclase feldspath potassique grenat biotite sillimanite cordierite qui d apres les donnees experimentales semble ne se former que dans un intervalle de pression et de temperature limite t 690 710 c p t 4 5 7 kbar p h 2 o 0 2 0 5 p t permet de considerer ces gneiss comme des niveaux marqueurs thermobarometriques tres precis dans le sud de madagascar cet episode migmatitique pourrait appartenir a l orogene panafricaine

late archaean to early proterozoic granitoid magmatism and high grade metamorphism in the central limpopo belt south africa the central zone of the limpopo belt in southern africa has previously been interpreted as a segment of archaean crust which experienced its main deformation and metamorphism around 2 7 ga ago we report new single zircon u pb and pb pb ages for granitoid gneisses supracrustal rocks and anatectically derived granitic melt patches in the area around messina south africa the sand river gneiss is a composite suite of tonalitic to trondhjemitic rocks with protolith ages between 2 6 and 3 2 ga the singelele gneiss a heterogeneous granodioritic to quartz monzonitic rock has protolith ages between 2 55 and 2 58 ga since both the sand river and singelele gneisses experienced polyphase high strain ductile deformation this must have occurred later than 2 55 ga ago granulite facies pelitic gneisses of the beit bridge complex contain abundant spherical multifacetted zircons which reflect new zircon growth near or at the peak of metamorphism these zircons provide ages with a mean at 2026 5 6 3 ma which is interpreted as reflecting a high p t event 10 kbar 825 25 c granitic melt patches in the metapelites as well as in the sand river gneiss and anatectic granites are probably related to rapid near isothermal decompression to below 3 5 kbar and 600 750 c these rocks contain new magmatic zircons which yielded a mean age of 2005 6 4 4 ma and probably reflect a crustal melting event resulting from rehydration of the granulitic assemblage our zircon data support previous suggestions for only one single granulite facies event in the central zone and we suggest that this event occurred c 2027 ma ago since most of the deformation seen in the gneisses of the messina area must have occurred later than 2 55 ga ago it is likely that the limpopo orogeny at least in the central zone is not an archaean event but took place in the early proterozoic

use of isotopic signatures for the determination of natural recharge and chemical characterization of groundwaters the case of horombe plateau area sw madagascar the necessity for accurate determination of groundwater recharge is an important issue for the smart mining of groundwater resources especially in hydrologically sensitive regions different techniques and methods have been widely reported in literature and isotope hydrology has found to be one of the major fields of the study this paper presents the combination of different techniques for analyzing the recharge discharge mechanisms within a large catchment area in sw madagascar by correlating different isotopic signatures piezometric conditions major ion water chemistry from ground and surface water samples including their physical characteristics from in situ measurements and also geological structures present within the study area it provides among others a detailed description on the geologic and hydrogeologic regime of the horombe plateau based on field data obtained from a well distributed monitoring network within the study area which is envisaged to serve as the conceptual model for future groundwater modeling studies

single zircon dating constraining the maximum age of the barberton greenstone belt southern africa one of the central issues in early precambrian crustal evolution is the age and genetic relationship between greenstone belt supracrustal assemblages and nearby high grade gneiss terranes the crucial question in most cases is which of the two is older we report zircon ages using the new single grain evaporation technique from a metaquartzite and a felsic volcanic flow of the onverwacht group in the barberton greenstone belt bgb south africa and swaziland that constrain the age of this basal part of the greenstone sequence between 3451 15 and 3438 6 ma this is almost 100 ma younger than previous age estimates assigned to the onverwacht and agrees well with recent sm nd pb pb and zircon ion microprobe dating tonalitic orthogneisses of the ancient gneiss complex agc adjacent to the bgb have ages up to 3644 4 ma and are thus demonstrably older than the greenstones also they cannot be derived from them by partial melting as has been proposed previously this age relationship as well as the presence of metaquartzites and thrust slices of pregreenstone tonalite in the onverwacht tectonic sequence rule out an entirely oceanic evolution for the early history of the bgb and suggest that continental crust was either below or nearby

fluid motion associated with tertiary mylonitization and detachment faulting 18o 16o evidence from the picacho metamorphic core complex arizona major crustal detachment faults of tertiary age in the picacho metamorphic core complex of southern arizona demark three tectonic plates in a structural section characterized by the transition from undeformed granitic basement through a mylonitic carapace to brecciated and hydrothermally altered counterparts a lower detachment fault defines the top of the lower plate and is coplanar with the overprinting of mylonites to chloritic breccias variably altered and fractured granite of an overlying middle plate is bounded by an upper detachment fault on which allochthonous miocene volcanics have been superposed the volcanics display intense oxidative potassic alteration this overall transition is accompanied by a 10 increase in 18o and a 400 c decrease of temperature undeformed oracle granite of the lower plate 18o 7 8 retains near magmatic mineral fractionations but these are disturbed in mylonitic equivalents where ambient temperatures were 520 c 18oh2o 4 and conditions of low water rock ratios prevailed chloritic breccias record a whole rock shift to 8 5 accompanied by diminished temperatures of 300 c and the infiltration of fluids where 18oh2o 3 1 under conditions of high water rock ratios a pronounced isotopic discontinuity occurs at the upper detachment fault where intensely oxidized and k metasomatized volcanics of the upper plate are shifted to 18 at temperatures that diminished to 150 c two distinct fluid reservoirs were involved in alteration of the lower two plates and the upper plate respectively the tectonic section recording an upward transition from high to low temperature low to elevated water rock ratios ductile creep to brittle fracturing and an interface of deep exchanged fluids with a shallow oxidized surface aqueous reservoir

rb sr whole rock mineral isochron ages of plutonic rocks from the wanni complex sri lanka abstract granitoid plutons in different east gondwana fragments give evidence for vigorous felsic magmatism during the pan african period 800 to 500 ma the sri lankan basement which is mainly composed of proterozoic high grade metamorphic rocks was intruded by a few late to post tectonic syenitic and granitic plutons reliable geochronological data for these plutons are few and some of the available data are inconsistent with the ages of the surrounding metamorphic country rocks this report presents five rb sr whole rock mineral isochron wrmi ages and initial sr isotope ratios of three granitoid plutons from the wanni complex sri lanka namely the ambagaspitiya tonigala and kotadeniya granites two samples from the ambagaspitiya granite yielded ages of 520 5 and 502 15 ma with initial sr ratios of 0 7103 0 0003 and 0 7125 0 0009 respectively whereas ages of 467 27 and 497 11 ma and initial sr ratios of 0 7070 0 0004 and 0 7085 0 0008 were determined for two tonigala granite plutons a single sample from the kotadeniya granite gave an age of 533 19 ma and an initial sr ratio of 0 7202 0 0018 these ages are consistent with the geological relations between the granitoids and their country rocks which underwent granulite facies metamorphism between 650 and 550 ma and also with available u pb zircon ages 550 ma the present study gives improved chronological correlation of the felsic magmatism in east gondwana fragments in sri lanka india madagascar antarctica and western australia each of the sri lankan plutons exhibits different initial sr isotope ratios indicating that their magmas were derived from distinct source materials

proterozoic to mesozoic east gondwana the juxtaposition of india sri lanka and antarctica jurassic and cambro ordovician paleomagnetic pole positions deduced from rocks of sri lanka and antarctica generally conform when sri lanka is juxtaposed with the coast of lutzow holm bay east antarctica in a manner similar to a current gondwana reassembly the highland and southwest groups mostly proterozoic of sri lanka are correlated with the ongul and skallen groups of east antarctica on the basis of similar lithology structural characteristics structural trend tectonothermal history ranging from early proterozoic to mesozoic and the pressure temperature time path of the main metamorphism some neighboring lithotectonic units are also comparable between the two areas fracture lineament systems of sri lanka and antarctica formed after the latest silurian and during or before the jurassic conform in this reconstruction an improved fit of the 2000 m isobaths of india sri lanka antarctica and continuation of the latest archaean mobile belt from enderby land to peninsular india is obtained by fitting sri lanka into lutzow holm bay and enderby land into the embayment in the east coast of peninsular india this juxtaposition of peninsular india to east antarctica involves about 80 km northward movement and about 30 clock wise rotation of sri lanka relative to india the reassembly thus obtained accords well with data supporting the idea that east gondwana existed from the earliest proterozoic to middle mesozoic although some disruption collision events of limited scale are considered possible

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

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

blocky regularization schemes for full waveform inversion with ill posed inverse problems such as full waveform inversion regularization schemes are needed to constrain the solution whereas many regularization schemes end up smoothing the model an undesirable effect with fwi where high resolution maps are sought blocky regularization does not it identifies and preserves strong velocity contrasts leading to step like functions these models might be needed for imaging with wave equation based techniques such as reverse time migration or for reservoir characterization enforcing blockiness in the model space amounts to enforcing a sparse representation of discontinuities in the model sparseness can be obtained using the 1 norm or cauchy function which are related to long tailed probability density functions detecting these discontinuities with vertical and horizontal gradient operators helps constraining the model in both directions blocky regularization can also help recovering higher wavenumbers that the data used for inversion would allow thus helping controlling the cost of fwi while the cauchy function yields blockier models both 1 and cauchy attenuate illumination and inversion artifacts

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

kinematics and significance of a poly deformed crustal scale shear zone in central to south eastern madagascar the itremo ikalamavony thrust across the crystalline basement of madagascar late archaean rocks of the antananarivo block are tectonically overlain by proterozoic predominantly metasedimentary units of the ikalamavony and itremo groups of the southwest madagascar block the generally west dipping tectonic contact can be traced for more than 750 km from nw to se and is referred to here as the itremo ikalamavony thrust the basal units of the sw madagascar block comprise metasedimentary quartzites with the potential to preserve a multistage deformation history in their microstructures previous studies suggest contrasting structural evolutions for this contact including eastward thrusting top to the west directed extension and right lateral strike slip deformation during the late neoproterozoic ediacaran in this study we integrate remote sensing analyses structural and petrological fieldwork as well as microstructural investigations of predominantly quartz mylonites from the central southern segment of the contact between ankaramena and maropaika in this area two major phases of ductile deformation under high grade metamorphic conditions occurred in latest neoproterozoic early phanerozoic times a first andreaba phase produces a penetrative foliation which is parallel to the contact between the two blocks and contemporaneous with widespread magmatism a second ihosy phase of deformation folds andreaba related structures the investigated micro structures indicate that a juxtaposition of both blocks possibly already occurred prior to the andreaba phase b re activation with top to the east thrusting took place during the latest stages of the andreaba phase c the ihosy phase resulted in regional scale open folding of the tectonic contact and d reactivation of parts of the contact took place at distinctively lower temperatures post dating the major ductile deformations

new zircon ages for precambrian granites gneisses and granulites from central and southern madagascar significance for correlations in east gondwana institiif fiir geozuissenschnffen universitiit mainz 55099 mainz gevniany 2depnrtnienf of geology university of leicesfer leiccsfev le 1 7r h uk 3mnx plnnck lnsfitzit fiiv clzeniie postjnclz 3060 55020 mcriiiz geuniany 4depf of applzed geology ciirfin university of t c c z j i o o pevflz wa 6001 airstvaliii 5dc ppnrfeniei f des sciences nnfirreiles uiiiversitt de toliavn tolinrn mndngascav

geologic and geochemical study of the picacho gold mine california gold in a low angle normal fault environment the picacho gold deposit located in southeasternmost california is a low grade gold deposit in a nearly flat lying denudational fault of regional extent and probable oligocene age the deposit is hosted by intensely fractured and faulted mesozoic leucogranite and by chloritic augen gneiss and schist and is overlain unconformably and in fault contact by unmineralized late oligocene quechan volcanic rocks the deposit is structurally characterized by normal and normal oblique faults of low to high dip at shallow depths in the mine merging downward with a synchronous low dipping ore stage extensional fault system the chocolate mountains gatuna fault of probable oligocene age in deeper portions of the deposit the fault system was infiltrated during much of its active life by hot dilute highly exchanged meteoric water having temperatures of 170 210 c salinity 2 wt nacl equivalent and calculated 18ofluid between 2 6 and 5 2 this main stage fluid precipitated quartz pyrite and specular hematite accompanied by silicification and sericitization auriferous ore stage pyrite was precipitated late in the fault evolution probably by mixing of reducing ore fluid with relatively oxidized main stage fluid during regional oligocene extension on the chocolate mountains gatuna fault the picacho deposit is characterized by a gold arsenic antimony geochemical signature consistent with bisulfide complexing of gold in reducing fluid in contrast with typical denudation fault hosted base metal rich deposits associated with high salinity fluids elsewhere in the southwestern united states the deposit is overprinted by miocene normal faults having a wide range of dips these postore faults are associated with red earthy hematite precipitation pyrite oxidation and supergene enrichment of gold

gold occurrences in madagascar south india and sri lanka significance of a possible pan african event abstract gold occurs in various quantities mainly in the central and southern parts of sri lanka although gold is recovered mainly from river sediments at present it has been mined in the past also from a concordant quartz reef in central sri lanka and from other types of sediments such as marshy beds surrounding dykes of ultramafic composition in southern sri lanka the sedimentary gold has been interpreted as being derived from the mineralizations of the latest deformational features such as ductile and brittle shears and fractures consisting of quartz veins minor pegmatites and sulphide mineralizations mainly of pyrite pyrrhotite and chalcopyrite the mineralization of gold is exclusively in the highland complex rocks of sri lanka which represent archaean late proterozoic metamorphic complexes in madagascar gold is present in the precambrian basement rocks consisting of some greenstone belts and metasediments as a diffused mineral in the rocks concordant quartz reefs and more recent discordant quartz veins the concordant quartz veins and diffused gold are related to both archaean and proterozoic rock associations and the discordant veins which have been identified as permo triassic are found mainly in the north within the contact zone of proterozoic rocks and the phanerozoic cover gold is also recovered form ancient and recent alluvial accumulations and lateritic earth the mineralization of vein deposits is mainly gold quartz and gold sulphide associations the sulphides being galena chalcopyrite and pyrite in south india gold occurs in archaean metamorphic units comprising of granite greenstone complexes in the kolar classic gold field and in granulite terranes in more southerly occurrences such as the historic wynad fields in both cases gold occurs in gold quartz veins which occupy fissures and shears within the archaean metavolcanics gold also occurs in the attapadi area of kerala both in sediments and in primary epigenetic gold deposits which are hosted by precambrian amphibolites and granitic gneisses the attapadi lode veins are predominantly structurally controlled and are closely associated with the proterozoic bhavani shear zone gold is recovered both from the primary and placer deposits and the primary deposits contain gold quartz veins gold quartz carbonate veins and gold quartz sulphide veins the sulphide minerals are mainly galena pyrite chalcopyrite and arsenopyrite the rocks in the southern madagascar southern india and sri lanka belong to granulite terranes that have peak metamorphic imprints and tectonic development during the pan african the emplacement of gold mineralization mainly in madagascar and south india can be correlated in the archaean the gold deposition in the later deformational structures with concordant and discordant gold bearing veins can be correlated also with those in sri lanka as a pan african mineralizing event it is also evident that co2 rich fluids from a deep seated source has continued from archaean and were still active during the proterozoic being responsible for the pan african event of gold deposition particularly in the younger structures in these countries

an insight into the breakup of gondwana identifying events through low temperature thermochronology from the basement rocks of madagascar 1 fission track analysis was applied to the precambrian suites of madagascar in order to identify the lower temperature cooling histories and their relationships to the phanerozoic events that affected the island apatite ages range from 431 to 68 ma and zircon ages range from 452 to 238 ma thermochronologically the island can be divided into a southern central and northern region each with a subdivision on an east west basis the southern region is sharply separated from the central region by strongly contrasting apparent apatite ages over the northwest southeast striking ranotsara shear zone rsz the change in apparent ages over the rsz is indicative of later reactivation along younger brittle faults the central region has the oldest ages of the island and has a diffuse contact to the third region northward along the entire western margin of the precambrian basement initial paleozoic exhumation was followed by heating burial by sediments during jurassic and cretaceous times a decrease in ages along the eastern margin from 119 to 68 ma coincides with the predicted positions of the marion hot spot after effects of erosion are considered on the other hand these ages may represent progressive opening of the margin in a southward direction together with associated denudation of the rift shoulder the eastern part of the central region has remained very stable since at least devonian times undergoing only long term very slow exhumation at rates of 1 5 m myr

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

magma mixing and gold mineralization in the maevatanana gold deposit madagascar abstract the maevatanana gold deposit in madagascar is hosted by archean metamorphic rocks in quartz sulfide veins that are structurally controlled by nnw sse trending shear zones fluid inclusion data show that the trapping conditions in quartz range from 0 87 to 2 58 kbar at temperatures of 269 362 c laser raman spectroscopy confirms that these inclusions consist of co 2 so 2 and h 2 o the 34 s values of the pyrites range from 1 7 to 3 6 with an average of 2 25 supporting a magmatic origin noble gases he ne ar ke xe are chemically inert thus will not be involved in chemical reactions during geological processes also due to the low concentration of he in the atmosphere and the low solubility of he in aqueous fluids the atmosphere derived he is unlikely to significantly affect he abundances and isotopic ratios of crustal fluids ensures that he production should have the typical crust 3 he 4 he ratios the 3 he 4 he ratios of fluid inclusions in pyrite from the deposit range from 0 06 to 0 12 ra while the 40 ar 36 ar ratios range from 6631 to 11441 we infer that the ore forming fluids could have been exsolved from a granitic magma the oxygen and hydrogen isotope compositions of the ore forming fluids 1 5 18 o h2o 7 8 72 d 117 indicate they were derived from a granitic magma four pyrite samples from the gold deposit yield a precise re os isochron age of 534 13 ma given that the post collisional granites in northern and central madagascar were derived by melting of sub continental lithospheric mantle and formed between 537 and 522 ma we can state that the gold metallogenesis was coeval with the crystallization age of these parental magmas these data could be accounted for the formation of the maevatanana gold deposit first the shear zones hosting the deposit formed around 2 5 ga when the madagascan micro continental blocks collided with other continental blocks triggering large scale tectono magmatic activity and forming nnw sse trending shear zones the gold mineralization at maevatanana is coeval with the crystallization age of the cambrian post collisional a type granitoid plutons in northern and central madagascar implying that this deposit is associated with extensional collapse of the east african orogen this extension in turn induced asthenospheric upwelling melting of sub continental lithospheric mantle these magmas underplated the lower crust generating voluminous granitic magmas by partial melting of the lower crust the mixing magma during tectono thermal reactivation of the east african orogen produced large volumes of volatiles that extracted gold from the granitic magma and produced au s complexes e g au hso 3 2 the shear zones which were then placed under extensional collapse of the east african orogen in the cambrian formed favorable pathways for the magmatic ore forming fluids these fluids then precipitated gold sulfides that form the maevatanana gold deposit

metamorphism and tectonics in southern madagascar an overview the scope of this paper is to briefly summarize the general tectonic pattern and the metamorphic evolution of the continental crust from southern madagascar after a presentation of the main geophysical features of the malagasy crust the brittle and the ductile strain patterns are established at a crustal scale the pan african metamorphic zonation is discussed and interpreted through a tectonic model related to a transpressive regime

are strongly peraluminous magmas derived from pelitic sedimentary sources igneous compositions that require the presence of a phase more aluminous than biotite are here designated as strongly peraluminous ps such compositions are common but subordinate in felsic rocks and exceedingly rare in more mafic rocks the term s type has been applied to many igneous rocks based largely or even solely upon their ps compositions neglecting or placing less emphasis on other criteria for sedimentary parentage in chappell and white s i and s type classification system the s type designation was in fact initially restricted to granites which were thought to have been derived from a chemically mature strongly weathered pelitic shaly source application of chappell s and white s criteria together with others based upon isotopic and phase equilibria considerations leads to the following conclusions about ps igneous rocks worldwide 1 very few ps rocks are derived entirely from pelitic sources 2 a majority are derived largely from intermediate to felsic crustal sources including

cooling of a late syn orogenic pluton evidence from laser k feldspar modelling of the carion granite madagascar since the late 1980s it has been hypothesized that the wide range of apparent argon ages seen within single k feldspar samples might be due to a distribution of diffusion domain sizes within the mineral to test and apply this idea an analytical technique that combines conventional laboratory degassing experiments resistance heating with numerical inversion procedures has been developed to extract cooling history information from feldspars a key part of the method involves careful control of temperature in the laboratory to constrain the diffusion parameters of the feldspar samples in our study we have k feldspar data from single crystals that mimic the types of data seen in classic resistance heater fusion experiments our step heating data are based on using a continuous argon ion laser with no direct control on temperature however with only a single added free parameter in the model we show that it is possible to analyze this data in the multi domain style and make some simple inferences on the nature of the cooling history of the carion pluton in central madagascar the carion granitic pluton in central madagascar was intruded into warm continental crust following orogenic events related to the final amalgamation of gondwana u pb shrimp dating of the pluton yields an emplacement age of 532 1 5 2 ma followed by relatively slow cooling as constrained by 40ar 39ar ages on hornblende biotite and k feldspar four hornblende samples yielded a mean 40ar 39ar age of 512 7 2 6 ma a biotite sample yielded an age of 478 9 1 0 ma and modeled k feldspar ages show cooling from 350 c at 466 ma to 100 c by 410 ma collectively the data suggest that the pluton cooled from 850 c at 532 1 5 2 u pb zircon ma to 500 c at 512 7 2 6 ma 40ar 39ar hornblende or approximately 18 c ma slowing to 4 c ma between 512 ma and 478 ma and finally to about 3 c ma between 478 and 410 ma

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

eocene gold ore formation at muteh sanandaj sirjan tectonic zone western iran a result of late stage extension and exhumation of metamorphic basement rocks within the zagros orogen a multidisciplinary study including field geology microstructure analysis 40ar 39ar dating and fluid inclusion microthermometry and raman spectrometry has been carried out on the muteh gold deposit located in a greenschist to amphibolite facies metamorphic rock complex of the sanandaj sirjan tectonic zone zagros orogen iran the muteh gold deposit has been previously interpreted as genetically related to precambrian granitic intrusions as an exhalative hot spring deposit related to paleozoic rhyolitic acidic tuffs and to local metamorphic processes host rocks of the gold deposit are predominantly schist and gneiss and subsidiary amphibolite and quartzite and are intruded by leucogranites in the vicinity of the gold deposit the metamorphic and granitic rocks display a subhorizontal mylonitic foliation containing a northeast oriented stretching lineation field investigations in one of the producing open pits show that the gold orebodies are controlled by northwest oriented normal faults and joints dipping to the northeast and the southwest and crosscutting the ductile fabric of the host rocks both the ductile fabric of the host rocks and the gold ore controlling brittle structures are interpreted to have formed within a single continuous extensional event which started with ductile deformation and gradually changed into brittle deformation hydrothermal alteration associated with ore formation consists of quartz muscovite pyrite dolomite ankerite and albite which crosscuts the ductile fabric and overprints the metamorphic minerals of the host rocks pyrite is the dominant opaque mineral and is the major phase associated with gold chalcopyrite marcasite bismuth galena sphalerite and pyrrhotite are subsidiary to rare phases 40ar 39ar incremental heating and in situ laser ablation age data reveal a coherent sequence of cooling and hydrothermal events in the metamorphic complex hosting the muteh gold deposit muscovite samples from the alteration zone and from one quartz vein from the orebodies yield 40ar 39ar ages between 55 7 and 38 5 ma and show that gold mineralization is the youngest among the different dated events this eocene age is consistent with the young structural setting of the gold orebodies revealing an emplacement along northwestoriented normal faults which can be correlated with tertiary extensional tectonic events reported by previous field investigations the data also indicate that gold ore formation is coeval with magmatism in the adjacent tertiary urumieh dokhtar magmatic belt and with intrusive activity within the sanandaj sirjan zone with a 40ar 39ar age of 54 ma near muteh according to our study the remaining 40ar 39ar data reveal a cretaceous to early tertiary metamorphic magmatic and deformation history of the complex hosting the muteh gold deposit the fluid inclusion study showed that distinct fluids were present during the regional geologic evolution predating ore formation and later eocene gold ore formation early regional fluids consist of an h2o co2 nacl fluid with subsidiary n2 and ch4 that underwent local unmixing the salinities of the liquid rich inclusions resulting from unmixing range between 2 2 and 17 5 wt percent nacl equiv and those of the co2 rich vaporrich inclusions range between 2 8 and 7 2 wt percent nacl equiv the high salinity of the regional liquid rich fluids is attributed to metamorphism of evaporite bearing sedimentary rocks magmatic intrusions retrograde hydration reactions metamorphism of impure marbles and migmatization the fluid inclusions spatially associated with the gold veins postdating regional metamorphism and ductile deformation of the host rocks contain a low salinity aqueous fluid with a dilute low density co2 component and maximum salinities between 2 6 and 5 8 wt percent nacl equiv gold deposition at muteh occurred during or after late stages of eocene brittle extension as a result of exhumation of the host metamorphic complex possibly during magmatic activity in the sanandaj sirjam zone it is most likely linked to the introduction of a distinct low salinity low co2 content aqueous fluid during crustal extension these fluids are younger than and therefore unrelated to the highly saline and co2 rich regional metamorphic magmatic fluids our results refute previous models advocating precambrian to paleozoic ages of the muteh gold deposit

episodic growth of the chocolate mountains anticlinorium recorded by the neogene bear canyon conglomerate southeastern california u s a abstract between indian pass and picacho state recreation area southeastern california the neogene bear canyon conglomerate crops out in and around the chocolate mountains anticlinorium and can be subdivided into three unconformity bounded sequences that are designated from oldest to youngest i ii and iii each of these sequences is composed of interstratified debris flow channelized stream flow and sheet flood deposits debris flow deposits are poorly sorted inversely graded pebble to boulder paraconglomerates with subangular clast populations large clasts extruding into overlying layers are a characteristic of such beds in contrast sheet flood and channelized stream flow deposits are well sorted orthoconglomerates with subrounded to rounded clasts the bear canyon conglomerate unconformably overlies a lower miocene volcanic and epiclastic unit metasedimentary and metavolcanic rocks of the jurassic winterhaven formation a probable jurassic orthogneiss and the upper cretaceous to paleogene orocopia schist geochemical paleocurrent and clast count data suggest that alluvial material in the bear canyon conglomerate was derived from local exposures underlain by one or more of these units and then was funneled into an approximately north south trending structural depression in the axial trace of the chocolate mountains anticlinorium though the exact timing of deposition of the three sequences in the bear canyon conglomerate is difficult to constrain stratigraphic and published geochronological data when viewed in the context of regional tectonic patterns suggest that sequence i probably was deposited after 17 ma and before 9 45 ma in contrast sequence ii was deposited after sequence i and then was folded either sometime prior to or after 9 45 ma sequence iii is interstratified with the 9 45 ma basalts of black mountain and was folded prior to deposition of flat lying quaternary alluvial sediments the east west trending copper basin reverse fault cuts sequence iii in addition key components of a conjugate set of northwest and northeast trending dextral and sinistral slip faults respectively transect the copper basin fault the axis of the chocolate mountains anticlinorium and the 9 45 ma basalts of black mountain however neither the copper basin nor the conjugate faults disrupt flat lying quaternary alluvial sediments episodic growth of the east west trending chocolate mountains anticlinorium during deposition of the neogene bear canyon conglomerate and subsequent formation of the east west trending copper basin fault and the conjugate set of dextral and sinistral faults all record transient north south shortening within the margin of the north american plate these transient periods of north south shortening followed extensional deformation of lower miocene continental arc volcanic rocks and are strain compatible with the 12 10 ma to 6 ma development of the eastern california shear zone numerous other examples of late cenozoic east west trending folds and reverse faults are reported in the literature to have formed during development of this 100 110 km wide zone of dextral shear we therefore conclude that the geology and development of the chocolate mountains anticlinorium between indian pass and picacho state recreation area reflect changing continental margin strain patterns as the boundary between the farallon and north american plates was transformed from a paleogene convergent margin to a wholly neogene transform setting

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

k metasomatism and detachment related mineralization harcuvar mountains arizona the bullard detachment fault a regional low angle normal fault exposed in the harcuvar mountains of west central arizona separates lower plate mylonitic rocks and chloritic breccia from upper plate volcanic and sedimentary rocks areally extensive k metasomatism has converted upper plate mafic flows and felsic ash flow tuffs into rocks with 8 to 12 wt k 2 o 2 o and a simple k feldspar hematite quartz mineralogy the secondary k feldspar is very pure or 95 to 99 5 monoclinic and structurally similar to orthoclase differences in 18 o values between secondary k feldspar replacing sanidine phenocrysts 9 to 11 and k feldspar replacing groundmass 11 to 14 in the tuffs imply differential o exchange with migrating fluids whole rock 18 o values for tuffs 10 to 14 and mafic flows 6 to 9 do not therefore represent primary igneous values the rocks apparently became k metasomatized and 18 o enriched while interacting with low to moderate temperature neutral to alkaline oxidizing brines that accumulated in an extensional basin above the detachment fault cu au ag mineralization is concentrated in faults and fissures in metasomatized mafic flows above the detachment fault fluid inclusion studies show that mineralizing fluids had minimum temperatures of 290 to 330 c along the detachment fault 240 to 290 c in mafic flows above the detachment fault and 100 to 130 c in barite calcite mn oxide veins farther from the detachment fault the predominant mineralizing fluids near the detachment fault and in the mafic flows were saline brines with 13 to 17 equivalent wt naci more dilute brines with 6 to 12 equivalent wt naci formed the barite calcite mn oxide veins inferred 18 o values of the mineralizing fluids range from 3 for high temperature quartz sulfide veins to 5 for lower temperature barite calcite mn oxide veins the high salinities oxygen isotope compositions and geologic setting indicate that the mineralizing fluids were basinal brines the mineralizing fluids apparently evolved from early deep level reducing basin brines to a later stage marked by the influx of higher level oxidizing basin brines relatively minor amounts of less evolved lower 18 o meteoric water entered the system during the very late stages of mineralization paragenetic relations and geochemical and isotopic data indicate that mineralization was superimposed on previously k metasomatized rocks mineralization and k metasomatism may be indirectly linked however because both occurred during detachment faulting and both involved basinal brines specifically k metasomatism of upper plate units liberated elements such as cu pb zn mn sr and na that were incorporated into the mineralizing basin brines multiple fluid regimes during detachment faulting are indicated because basin brine dominated mineralization overprinted lower plate mylonitic rocks and breccia that had probably previously equilibrated with igneous or metamorphic fluids at deeper levels of the detachment system

extensional reactivation of the chocolate mountains subduction thrust in the gavilan hills of southeastern california the ne vergent chocolate mountains fault of south eastern california has been interpreted as either a subduction thrust responsible for burial and prograde metamorphism of the ensimatic orocopia schist or as a normal fault involved in the exhumation of the schist our detailed structural analysis in the gavilan hills area provides new evidence to confirm the latter view a zone of deformation is present at the top of the orocopia schist in which lineations are parallel to those in the upper plate of the chocolate mountains fault but oblique to ones at relatively deep levels in the schist both the orocopia schist and upper plate contain several generations of shear zones that show a transition from crystalloblastic through mylonitic to cataclastic textures these structures formed during retrograde metamorphism and are considered to record the exhumation of the orocopia schist during early tertiary time as a result of subduction return flow the gatuna fault which places low grade supracrustal metasediments of the winterhaven formation above the gneisses of the upper plate also seems to have been active at this time final unroofing of the orocopia schist occurred during early to middle miocene regional extension and may have involved a second phase of movement on the gatuna fault formation of the chocolate mountains fault during exhumation indicates that its top to the ne sense of movement provides no constraint on the polarity of the orocopia schist subduction zone this weakens the case for a previous model involving sw dipping subduction while providing support for the view that the orocopia schist is a correlative of the franciscan complex

geochemistry of pelitic and psammopelitic precambrian metasediments from southwestern sri lanka implications for two contrasting source terrains and tectonic settings most of the pelitic and psammopelitic metasediments of southwestern sri lanka are compositionally transitional between shales and greywackes and are of two isotopically distinct types metasediments of the northern part have nd model ages of 1 5 ga and belong to the relatively younger nd model age province of wanni complex wc model ages of two southerly located samples are 2 8 ga being compatible with those of the highland complex hc in accordance with mostly mesoproterozoic to late archaean crustal residence ages cr ni la th and ree data also suggest that the sources of these metasediments are plagioclase fractionated post archaean upper crustal material crustal differentiation of source terrains corresponding to the hc and the wc probably occurred at distinctly different time intervals significant differences exist between the major and trace element chemistry of the metapelites from the wc north and the hc south suggesting derivation from two different source terrains metasediments of the south are slightlymmore siliceous low in sc and v and highly enriched in lree 220 ppm their sources have been chemically evolved felsic igneous rocks and or recycled sediments relatively high v sc feo mgo tio2 and slightly low sio2 in the rocks of the wc indicate contribution of more basic sources cia chemical index of alteration estimates and comparative alkali alkaline earth element patterns suggest relatively fresh source material the o isotope data indicate that the pre metamorphic isotope signatures are preserved and the clay component of the original sediments mainly consisted of illite typifying continental weathering conditions sediments of the south hc were probably deposited in a passive continental margin continental rift environment while those of the northern part wc were probably formed at an andean type active continental margin the two units were tectonically juxtaposed prior to pan african granulite metamorphism

structural evidence for northeastward movement on the chocolate mountains thrust southeasternmost california the late cretaceous chocolate mountains thrust of southeastern california and southwestern arizona places a block of proterozoic and mesozoic continental crust over the late mesozoic continental margin oceanic sedimentary and volcanic rocks of the regionally distinctive orocopia schist the chocolate mountains thrust is interpreted as a thrust burial subduction fault rather than a low angle normal exhumation unroofing uplift fault an important parameter required to understand the tectonic significance of the chocolate mountains and related thrusts is their sense of movement the chocolate mountains thrust zone contains sparse to locally abundant mesoscopic asymmetric folds fabric relations supported by regional geologic evidence indicate that these folds are an integral part of and coeval with the thrust zone on a lower hemisphere equal area plot representing the orientation and sense of asymmetry of 80 thrust zone folds from 36 localities spread over an area 60 by 10 km z folds plot northwest of and s folds plot southeast of a northeast southwest striking vertical plane of overall monoclinic symmetry the only sense of movement consistent with the collective asymmetry of the thrust zone folds is top to the northeast asymmetric microstructures studied at several localities also indicate top to the northeast movement paleomagnetic data suggest that the original sense of thrusting prior to neogene vertical axis tectonic rotation related to the san andreas fault system was northward the essential point is that movement of the upper plate of the chocolate mountains thrust evidently was continentward continentward thrusting suggests a tectonic scenario in which an insular or peninsular microcontinental fragment collided with mainland southern california the suture predicted by this model is elusive but the chocolate mountains thrust and underlying orocopia schist themselves may represent the suture at the present level of exposure alternative tectonic models involving subduction of the orocopia schist eastward beneath continental southern california circumvent the suture problem but are presently not supported by any direct structural evidence

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

dating post metamorphic cooling of the eastern granulites in the mozambique belt of northern tanzania using the garnet sm nd method abstract granulites from the usambara wami river and uluguru areas in the northern part of the mozambique belt in tanzania yield sm nd garnet whole rock ages of between 580 and 634 ma with a mean of 609 11 ma 2 this mean age is only slightly younger than the previously published peak metamorphic age of 641 2 ma suggesting that contrary to some earlier arguments garnet sm nd ages can be used to closely constrain the age of peak metamorphism even in slowly cooled terranes using published peak metamorphic temperatures of 810 c and cooling rates of 1 4 c ma the mean age translates into garnet closure temperatures of 690 to 780 c the similarity in garnet ages over widely separated areas coupled with the previously established similarity in equilibrium pt conditions indicate that isolated complexes that form the eastern granulites in the tanzanian sector of the mozambique belt share the same thermal history and were formed under the same geodynamic setting a few published garnet ages of between 525 and 545 ma indicate a younger less pervasive event of granulite facies metamorphism in the belt the bimodal distribution of garnet ages supports a previously published hypothesis that the assembly of gondwana took place in two stages the 610 ma old ages most likely date cooling from granulite facies metamorphism arising from regional crustal thickening associated with the amalgamation of india madagascar parts of eastern antarctica the kalahari craton the congo craton and the arabian nubian shield forming the imslek ans collage on the other hand the 525 545 ma ages may mark cooling from a thermal event associated with the collision of australo antarctica with the imslek ans collage

dip selective 2 d multiple attenuation in the plane wave domain in many geological settings strong reflections at the air water interface contribute to most of the multiple energy in the recorded seismograms here we describe a method for free surface multiple attenuation using a reflection operator model of a seismic record derived using the well known invariant embedding technique we implement this method in the 2 d plane wave domain where lateral variation of the geological structure of the earth is taken into account by the coupling of different ray parameters in situations where the lateral variations are smooth the data are well compressed in the 2 d plane wave domain and the resultant bandlimited matrices significantly reduce the computation cost one important feature of the proposed method is its flexibility which allows for the removal of multiples from selected reflections to generate multiple free data wave theory based multiple attenuation methods attempt to estimate either the source function or the subsurface reflectivity our method takes adv

microstructural evidence for northeastward movement on the chocolate mountains fault zone southeastern california microstructural analysis of rocks from the chocolate mountains fault zone gavilan hills area southeastern california shows unequivocal evidence for northeast directed transport of the upper plate gneisses over lower plate orocopia schists samples were taken from transects through the fault zone prefaulting fabrics in upper plate gneisses show a strong component of northeast directed rotational deformation under lower amphibolite facies conditions in contrast prefaulting lower plate orocopia schists show strongly coaxial fabrics minimum stretch value of 2 2 formed at greenschist grade mylonitic fabrics associated with the chocolate mountains fault are predominantly northeast directed shear bands that are unidirectional northeastward in the gneisses but bi directional in the schists suggesting a significant component of nonrotational deformation occurred in the orocopia schists during and after emplacement of the upper plate the kinematic findings are in agreement with dillon et al 1989 who found that the vergence of asymmetrical folds within the fault zone indicates overthrusting to the northeast toward the craton in this region the available evidence favors a single protracted northeastward movement on the chocolate mountains fault zone with temperatures waning as deformation proceeded

the role of fluids in the development of a granulite facies transition zone in s india the bodinayakkanur area of tamil nadu lies in the transition zone p 5 kbar t 700 c from amphibolite to granulite facies metamorphism in the archaean high grade terrain of s india within the zone 3 closely associated rock units occur 1 precursor grey gneiss 2 pink migmatitic and metasomatic gneiss 3 green charnockitic granulites field observation shows that the complex inter relations between these 3 rock types can be related to influx of fabric and layer guided co 2 and h 2 o k 2 o rich fluid phases derived from a deep seated source

late palaeoproterozoic depositional age for khondalite protoliths in southern india and tectonic implications abstract shrimp dating of detrital zircons from garnet sillimanite cordierite paragneisses khondalites of the trivandrum block southern india revealed nanometer scale pb isotopic inhomogeneity in most grains that we ascribe to annealing processes during ultra high temperature metamorphism at ca 570 ma our age data for zircons from six representative khondalite samples do not document any neo or mesoproterozoic detrital grains and we conclude from the concordant ages and discordant minimum 207pb 206pb ages that the khondalite precursor sediments were deposited more than 2 1 ga ago and were subsequently intruded by granitoid rocks at ca 1765 2100 ma some detrital zircons in the khondalites contain late palaeoproterozoic metamorphic domains suggesting that these grains are derived from an unknown crustal source that most likely experienced late palaeoproterozoic high grade metamorphism both the metasedimentary assemblage and granitoids were severely ductilely deformed metamorphosed and migmatized during the pervasive pan african event at ca 550 580 ma this caused many detrital zircons in the khondalites to become variably recrystallized and to develop metamorphic rims proper interpretation of cathodoluminescence images and zircon morphology is important in interpreting detrital populations the southern indian khondalites were part of an extensive palaeoproterozoic metasedimentary assemblage that may have extended from southern madagascar via the trivandrum block to the highland complex of sri lanka and seems to reflect a continental margin sequence that was deposited on an archaean to early palaeoproterozoic continental terrane possibly the southern margin of the indian dharwar craton the tectonic history of this terrane remains obscure due to the pervasive overprint during the pan african event

the precambrian of sri lanka a historical review early studies of the geology of sri lanka lasted from the beginning of the century until 1935 the modern phase began in the forties whereas intensive studies of the precambrian rocks took place in the last ten years the term vijayan gneiss first used for the gneisses around colombo was changed in 1947 to vijayan series for all rocks outside the central khondalite charnockite belt the latter was named the highland series in 1961 and the vijayan series was subdivided into an eastern and a western component and the name was altered to vijayan complex in 1978 in 1959 a southwestern group was recognized and in 1935 the kadugannawa gneiss was named at present there is a general agreement that the following nomenclature should be used by all workers on the precambrian of sri lanka highland complex hc formerly highland series and southwestern group an intimate association of interbanded ortho and paragneisses including pelitic gneiss metaquartzite marble and charnockitic gneiss which forms the broad belt of rocks extending from northeast to southwest through the central highlands of the island outliers of hc rocks tectonic klippen are found in the vijayan complex at kataragama and maligawila vijayan complex vc formerly eastern vijayan complex consisting of migmatites granitic gneisses granitoids and scattered metasediments and occurring in the east and south of the hc wanni complex wc formerly western vijayan complex consisting of migmatites granitic gneisses charnockitic gneisses minor metasediments and granitoids lying west of the hc kadugannawa complex kc partly the former kadugannawa gneiss for the hornblende and biotite hornblende gneisses within the elongate synformal basins arenas around kandy in the early sixties it was noted that the highland series rocks had been subjected to granulite facies metamorphism and the vijayan series rocks to amphibolite facies metamorphism charnockitic rocks in the highland series showed the characteristics of metamorphic rocks recently near peak metamorphic conditions in the hc were established at 770 900 c and 5 kbar in the west wc to 9 kbar in the east hc with a pronounced regional gradient in palaeopressure relict inclusions of staurolite and kyanite in garnet of hc pelitic gneisses have been shown to document a late stage of a prograde metamorphic p t path a retrograde p t path with coronitic assemblages of retrograde mineral phases like andalusite has also been recorded outliers of hc rocks in the vc are klippen thrust over the vc parts of kc have been interpreted as a metamorphosed layered basic complex in the late seventies three phases of deformation were recognized in the hc presently four phases are known to occur an early view was that the highland series was younger than the vijayan series the former sediments having been laid down on a vijayan floor the hc is now known to be older than the vc the boundary between them being a thrust contact the exact position and nature of the hc wc boundary however is still to be resolved recent geochronological data suggest that 1 hc sedimentation as well as igneous activity in the form of basic lava flows basic sills and dykes took place 2 0 ga ago 2 intrusion of the earliest granitoids into the hc took place 1 8 to 1 94 ga ago 3 orthogneisses of the wc were intruded between 0 67 and 1 1 ga ago 4 orthogneisses of the vc were intruded 1030 to 1040 ma ago and 5 high grade regional metamorphism of the hc vc and wc took place between 455 and 610 ma ago

upb geochronology of the sri lankan basement abstract u pb ages of zircons and monazites from ortho and paragneisses representing the major crystalline basement units of sri lanka establish a new geochronological framework which is consistent with previously published work on sm nd model ages rb sr whole rock ages and u pb mineral ages in the highland complex three newly dated orthogneisses one previously dated by baur et al 1991 and one dated by kroner and williams 1992 all intruded 1 85 to 1 9 ga ago this widespread intrusion event occurred soon after the end of sediment deposition which is estimated at 1 9 to 2 0 ga using upper intercept ages of zircons from metapelites in the highland complex within the boundaries of this complex established by sm nd model age mapping milisenda et al 1994 only one orthogneiss has been found which is substantially younger than 1 8 ga this is the metatonalite from polonnaruwa which intruded 670 ma ago baur et al 1991 three orthogneisses from the vijayan complex which lies to the east of the highland complex yield similarly consistent crystallization ages of 1023 8 1016 28 and 1033 121 ma in the wanni complex which lies to the west of the highland complex a sillimanite bearing gneiss which is apparently of metasedimentary origin but contains zircons of igneous appearance yields a primary crystallization age of 793 4 ma consistent with a previously dated orthogneiss from kurunegala of 770 ma baur et al 1991 in addition single zircon data by kroner et al 1994 indicate primary crystallization ages of 1 0 to 1 1 ga for orthogneisses from the vavuniya charnockite province which is considered to be part of the wanni complex these results confirm the age contrast between the highland complex on the one hand and the vijayan complex and wanni complex on the other as previously inferred from sm nd model ages milisenda et al 1988 1994 post tectonic granitoids and pegmatites intruded throughout all three complexes 550 ma ago the age of regional high grade metamorphism mostly granulite facies in the highland complex mostly amphibolite facies in the vijayan complex and both granulite and amphibolite facies in the wanni complex is constrained to be less than 670 ma the primary crystallization age of the polonnaruwa metatonalite it is more closely defined by lower intercept zircon ages which include one concordant age of 608 4 ma and one discordant but well defined age of 611 23 ma and by a nearly concordant monazite age of 592 2 ma this indicates that peak metamorphic conditions occurred 610 ma ago numerous other lower intercept ages between 590 and 550 ma and post tectonic intrusion ages of 550 ma set a lower age limit for high grade conditions and deformation the temperature history between 610 and 550 ma is not yet well resolved and it is not clear whether there was a separate thermal event 550 ma ago previously published whole rock rb sr and sm nd isochrons for gneisses from the highland complex yielding ages of 2 ga and greater and previously interpreted to date the time of granulite grade metamorphism must be reinterpreted in the light of the new data presented here as well as other recently published results e g burton and o nions 1990b baur et al 1991 and in view of the accumulated evidence for very limited element mobility during high grade metamorphism these whole rock ages are in fact identical or close to the ages of primary granitoid emplacement rather than the age of granulite facies metamorphism

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

age of metamorphism in the high grade rocks of sri lanka controversy concerning the age of regional granulite grade metamorphism in sri lanka stems from the fact that different isotopic systems apparently record different events published isotopic data show that zircons from igneous and sedimentary protoliths experienced considerable pb loss at 550 ma and that new growth of zircon monazite rutile and garnet between 539 pm 6 and 608 pm 3 ma probably dates near peak metamorphic conditions upper concordia intercept ages between 670 and 1950 ma for magmatic zircons from high grade orthogneisses and charnockites date igneous emplacement and also support the occurrence of granulite metamorphism after 670 ma a granulite of igneous origin from the kataragama klippe in southern sri lanka yielded discordant ion microprobe data that suggest emplacement of the protolith at 1770 1880 ma thus a rb sr whole rock age of 1930 pm 130 ma from the same suite is unlikely to reflect isotopic homogenization during granulite formation and we interpret this and oth

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

possible controls of base and precious metal mineralization associated with tertiary detachment faults in the lower colorado river trough arizona and california base and precious metal mineral deposits are located along and adjacent to tertiary detachment faults in western arizona and southeastern california mineralization is characterized by early copper and iron sulfides followed by massive specular hematite in turn followed by fracture filling chrysocolla and malachite mineralization occurred during faulting and was associated with unusually high geothermal gradients that were at least partially due to detachment fault movement that brought hot lower plate rocks up from great depth and juxtaposed them with cool near surface upper plate rocks mixing of ascending reduced metal bearing aqueous solutions with higher level oxidized fluids is proposed as an important process leading to oxide mineralization movement of the redox interface down normal fault ramps at the same rate as displacement of hanging wall rocks resulted in continuous mineralization at some upper plate sites over significant periods of geologic time this produced the largest orebodies aqueous solutions that precipitated iron oxides near the detachment fault carried manganese oxides to higher levels where vein and strata bound sedimentary manganese oxides were deposited

petrology of granulite facies metapelites of the highland complex sri lanka implications for the metamorphic zonation and the p t path in the pan african granulite facies highland complex of sri lanka the metamorphic grade increases from the central northwestern part towards boththe east and the southeast in metapelites the following prograde changes in metamorphic assemblage are recognized biotite ilmenite garnet or co cordierite sillimanite alkali feldspar quartz in the central northwestern part give way to garnet rutile sillimanite ilmenite or biotite alkali feldspar quartz in the e and se and are replaced by gar garnet cordierite biotite magnetite ilmenite alkali feldspar quartz bearing assemblages in the southwest cordierite coexisting with garnet is restricted to the western highland complex where metamorphic pressures of 5 7 kbar havebeen determined in the eastern highland complex where garnet clinopyroxene quartz is stable in metabasic rocks metamorphic pressures of 8 10 kbar have been obtained from barometers on metapelites phase relations of metapelites as well as geobarometry point to a continuous metamorphic transition between the eastern and the western part of the highland complex a clockwise p t path for the whole highland complex is indicated by the succession kyanite sillimanite andalusite an even earlier stage of the prograde metamorphism documented by sillimanite needles older than kyanite inclusions in garnet hhave been found in two samples of the eastern highland complex this points to a strong pressure increase at amphibolite facies conditions probably due to tectonic thickening of the crust during prograde metamorphism other inclusion assemblages in garnet and reaction textures indicate the breakdown of staurolite to kyanite spinel garnet and subsequent continued garnet growth in the sillimanite field at the expense of biotite sillimanite and quartz during further heating ternary feldspars in metapelites bear evidence of extreme metamorphic temperatures of about 900 c previously only indicated by coexisting pyroxenes in metabasic and charnockitic rocks within a few garnets inclusions of staurolite which are ti rich 1 34 wt tio2 and richer in fe xfe 0 745 than the enclosing garnet have survived the very high grade metamorphism the high grade stage of the prograde metamorphism was accompanied by strong flattening and a general n s stretching which is many metapelites ceased after peak temperatures but proceeded in others tectonic uplift of the whole highland complex occurred only after a phase of near isobaric cooling in the lower crust zircon 610 ma and biotite cooling ages 460 ma holzl et al 1991 as well as retrograde p t paths which passed the andalusite stability field are similar in the eastern and western part of the highland complex this suggests that the lower crustal cross section exposed in the highland complex was already tilted during cooling

sri lanka indian eastern ghats east antarctica and the australian albany fraser mobile belt gross geometry age relationships and tectonics in precambrian gondwanaland in the reassembly of precambrian gondwanaland many now dispersed precambrian mobile belts appear to be aligned and this geometrical relationship may be significant one example is the alignment of the sri lanka eastern ghats mobile belt of india east antarctica and the albany fraser province of southwest australia sl eg ea af belt although these terrains differ somewhat in detail they have gross similarities in lithology structure and possibly in their age relationships recent dating suggests a common late proterozoic charnockite event the structural and tectonic alignment of this proposed sl eg ea af belt in the precambrian gondwanaland reassembly suggests a once continuous metamorphic terrain geometrically this belt can be fitted to arc swaths of small circles with their center in the middle of antarctica which can be interpreted in a preliminary way as being controlled by transform like plate tectonic processes

40ar 39ar age constraints on the timing of tertiary crustal extension and its temporal relation to ore forming and magmatic processes in the eastern rhodope massif bulgaria we quantify the timing of the tertiary crustal extension in the eastern rhodope massif of south bulgaria using 40 r 39 r geochronology to constrain the temporal evolution of exhumation of the metamorphic domes 40 r 39 r analyses of mineral phases with different closure temperatures extracted from metamorphic rocks collected in the footwall and the hanging wall of the extensional system reveal the low temperature cooling history towards shallow crustal levels the results reveal that subsequent to regional amphibolite facies metamorphism i the hanging wall gradually cooled between 500 and 300 c during the paleocene to late eocene 64 34 ma starting at a low rate of 25 c ma and increasing to 50 67 c ma during 38 34 ma ii cooling below 400 300 c of the footwall beneath the extensional detachments occurred in the middle late eocene 39 35 5 ma at an average rate of 35 c ma 40 r 39 r data from the metamorphic basement has been combined with 40 r 39 r data from the volcanic and hydrothermal rocks in the vicinity to the extensional domes to determine the temporal relationships between extensional tectonics ore forming and magmatic processes in the region extension of the high grade basement the ore formation and volcanism overlaps within a 5 ma lasting time interval especially during the cooling and exhumation of the footwall the eastern rhodope massif records an early stage of eocene extension within the aegean extensional province where distinct crustal scale processes occurred simultaneously implying the presence of cause and effect processes and thus represents a key example of coeval continental extension magmatism and hydrothermal activity in the late stage evolution of the orogens

multiple removal results from delft university at the workshop comparison of seismic multiple attenuation techniques held in conjunction with seg s 1997 annual meeting three data sets were distributed to a number of participants a gulf of mexico subsalt data set an offshore western australia data set and seg s 2 d synthetic salt model data set

pb loss patterns in zircons from a high grade metamorphic terrain as revealed by different dating methods upb and pbpb ages for igneous and metamorphic zircons from northern sri lanka abstract we report u pb and 207 pb 206 pb ages for multigrain fractions single grains and grain domains in zircons from highgrade igneous and sedimentary rocks of the vavuniya charnockite province vcp in northern sri lanka using the ionmicroprobe shrimp a new vapour digestion technique vdt and evaporation charnockitic enderbitic and mangeritic gneisses are derived from i type tonalitic syenitic granodioritic and granitic precursors for which the u pb systematics of zircons as determined by shrimp suggest crystallization ages between 1000 and 1100 ma many translucent round and multifacetted zircons in these rocks are not of metamorphic but of igneous origin and acquired their shape through partial zircon dissolution and recrystallization the large majority of zircons exhibit severe and variable pb loss some 540 580 ma ago which we ascribe to granulite grade metamorphism almost immediately followed by local retrogression most of the zircons lost pb from the entire grain with no domains remaining that reflect the original pb isotopic composition it was therefore impossible to obtain meaningful 207 pb 206 pb evaporation ages for such grains and this cautions against use of this method in zircons that experienced severe pb loss however zircons formed during high grade metamorphism apparently retain their primary pb isotopic composition and were dated successfully by the evaporation technique yielding ages between 553 26 and 560 30 ma conventional multigrain analysis and small fractions following vdt for two samples although much more precise than the shrimp data yielded only strongly discordant data points with patterns reflecting variable pb loss these data alone would not have made it possible to obtain reliable primary zircon crystallization ages 207 pb 206 pb evaporation ages for detrital zircons from two garnetiferous gneiss samples of sedimentary origin in the centre of the vcp revealed a maximum age of 1329 18 ma whereas a metaquartzite near the southern margin of the vcp contained zircons with 207 pb 206 pb ages between 1857 14 and 2543 14 ma we interpret the latter rock as belonging to the highland complex which occurs adjacent to the vcp our zircon data in combination with nd model ages milisenda et al 1994 suggest the vcp to be part of the wanni complex a distinct crustal domain in northern sri lanka characterized by relatively young ages in comparison with much older ages in the adjacent highland complex

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

whole grain evaporation for 207 pb 206 pb age investigations on single zircons using a double filament thermal ion source a technique has been developed and tested to analyse 207pb 206pb apparent ages by thermal evaporation of radiogenic lead directly from untreated whole zircon grains 0 3 mm the evaporation analyses are performed in the double filament arrangement of a thermal ion mass spectrometer thims the method is a powerful tool to distinguish between different lead components occurring in the same grain because differing activation energies of the competing lead components cause their sequential evaporation from the zircons the evaporation of test samples results in 207pb 206pb apparent ages in good agreement with u pb ages known from literature single zircons from a granite of the marble mountains california yield an age of crystallization of 1 410 30 ma ceylon zircons from heavy mineral bearing gravels yield 560 40 ma as age of crystallization of the pegmatitic gravel sources individuals from a heterogeneous zircon population of a diatexite from the southern schwarzwald sw germany indicate metamorphic zircon formation around 500 ma and the existence of middle proterozoic relics 1 95 0 05 ga

african linkage of precambrian sri lanka new age and isotopic data show that the high grade basement rocks of sri lanka were not linked to the archaean granulite domain of southern india but experienced their main structural and metamorphic development during the pan african event some 950 to 550 ma ago this occurred when west gondwana and east gondwana collided to form one of the longest collisional structures in the supercontinent the mozambique belt that extends from mozambique to ethiopia and sudan a major tectonic boundary interpreted as a thrust zone divides the highland southwestern complex in the central part of sri lanka from the vijayan complex in the e and se the former is interpreted to represent the remnant of a once extensive passive margin extending west in a gondwana reconstruction via madagasgar to tanzania and mozambique the vijayan complex may have been part of a separate continental margin plutonic assemblage and its collision with the highland southwestern complex marks the final amalgamation of east and west gondwana into a supercontinent some 550 ma ago the sri lankan granulites cannot be correlated with the distinctly older granulites of the eastern ghats belt of india and this suggests that sri lanka was situated close to the se coast of madagascar in a gondwana reconstruction

determination of volume loss and element mobility patterns associated with the development of the copper basin fault picacho state recreation area se california usa abstract the fault zone architectural elements of the copper basin fault include a fault core inner damage zone and outer damage zone utilizing yb as a reference frame element significant elemental mass changes were documented across all three architectural zones with the greatest change in the fault core porosity and volumetric strain both increase toward the principal slip surface reaching a maximum in the inner damage zone and then dropping significantly in the fault core in the damage zone and unaffected wall rocks smectite is the most prominent clay species whereas illite dominates the fault core suggesting that temperatures may have reached 100 150 c or more during the production of the fault core a number of studies have interpreted similar mass changes and clay mineral reactions to indicate high water rock ratios such interpretations imply that during rupture permeability increased within the fault core allowing it to act as a pathway for hot chemically reactive fluids then during and after rupture co 2 and sio 2 rich fluids moved into the inner and outer damage zones where calcite and quartz precipitation healed the fractured and permeable rock

unsupported radiogenic pb in zircon a cause of anomalously high pb pb u pb and th pb ages ion microprobe u th pb isotopic analyses of zircons from a granodioritic orthogneiss from the napier complex mount sones enderby land antarctica have identified an unambiguous example of unsupported radiogenic pb in a 3 950 ma old crystal at one 40 m spot on the crystal an unusually large heterogeneity in pb content was found the concentration of radiogenic pb ranging from 5 to 50 percent higher than could have been generated in 3 950 ma by radioactive decay of the co existing u and th this relative excess of radiogenic pb is attributed to pb gain rather than to u and th loss because first the pb content varied by more than the u or th contents and secondly changes in the pb u pb th and pb isotopic composition correlated directly with changes in the pb concentration the individual 207pb 206pb apparent ages ranged from 4 000 ma to 4 145 ma all greater than the inferred age of the crystal a correlation between 207pb 206pb and pb u shows that the pb excess has not resulted from recent pb movement the spot apparently gained radiogenic pb about 2 500 ma ago at the same time as the majority of the other zircons in the rock suffered substantial pb loss the pb movement occurred in response to a discrete geologic event reverse discordance is a phenomenon that must be considered when interpreting zircon u pb ages especially 207pb 206pb ages of single crystals or portions of crystals

evidence for prograde metamorphic evolution of sri lankan pelitic granulites and implications for the development of continental crust petrological evidence for prograde metamorphic evolution is obtained for pelitic granulites of the newly defined highland complex in sri lanka it includes the occurrences of relict kyanite quartz staurolite corundum kyanite hercynite kyanite and sapphirine kyanite spinel within garnet in the sillimanite quartz rich rocks of special interest is the mineralogy of khondalites graphitic garnet sillimanite k feldspar quartz gneisses the matrix of which is free of hydrous minerals indicating that dehydration reactions have been completed in this kind of rocks however relict hydrous minerals such as al ti rich more than 58 wt al2o3 and more than 1 wt tio2 staurolite and extremely ti rich up to 9 8 wt tio2 biotine occur as inclusions within garnet the local relict mineral assemblages within garnet in khondalites indicate the following continuous staurolite breakdown reactions which may have progressed as temperature increased and pressure decreased staurolite quartz kyanite garnet h2o staurolite corundum kyanite garnet h2o staurolite kyanite hercynite garnet h2o staurolite sillimanite hercynite garnet h2o some of these dehydration reactions are inferred to have also taken place in other widespread pelitic granulites of the highland complex moreover garnet in the highland complex pelitic granulites occasionally preserves growth zoning though modified slightly to extensively by post growth volume diffusion based on the evidence a clockwise prograde p t path of evolution is inferredfor meta sedimentary granulites belonging to the highland complex on the other hand no evidence for prograde metamorphism has so far been obtained in lower grade rocks of the wanni complex to the northwest of the highland complex such a difference between these complexes may be the first petrological feature which supports the new geotectonic division of the sri lankan basement rocks based virtually on the distinctly different neodymium model ages it suggests that these two complexes were amalgamated during the very last part of prograde metamorphism of the highland complex rocks that is during decompression and heating probably by the lower crustal lateral flow after tectonic overthickening and delamination the clockwise prograde p t t path followed by the meta sedimentary granulites of the highland complex is in marked contrast with the near isobaric cooling path for meta igneous granulites suggested by some authors it is proposed that both types of p t t paths albeit for different rock types are possible and resulted from diverse deep crustal processes such as underplating of continental crust by sediments and magmas or near convergent plate boundaries

chronology of early archaean granite greenstone evolution in the barberton mountain land south africa based on precise dating by single zircon evaporation abstract we report precise 207 pb 206 pb single zircon evaporation ages for low grade felsic metavolcanic rocks within the onverwacht and fig tree groups of the barberton greenstone belt bgb south africa and from granitoid plutons bordering the belt dacitic tuffs of the hooggenoeg formation in the upper part of the onverwacht group yield ages between 3445 3 and 3416 5 ma and contain older crustal components represented by a 3504 4 ma old zircon xenocryst fig tree dacitic tuffs and agglomerates have euhedral zircons between 3259 5 and 3225 3 ma in age which we interpret to reflect the time of crystallization a surprisingly complex xenocryst population in one sample documents ages from 3323 4 to 3522 4 ma we suspect that these xenocrysts were inherited during the passage of the felsic melts to the surface from various sources such as greenstones and granitoid rocks now exposed in the form of tonalite trondhjemite plutons along the southern and western margins of the bgb and units predating any of the exposed greenstone or intrusive rocks several of the granitoids along the southern margin of the belt have zircon populations with ages between 3490 and 3440 ma coeval with or slightly older than onverwacht felsic volcanism while the kaap valley pluton along the northwestern margin of the belt is coeval with fig tree dacitic volcanism these results emphasize the comagmatic relationships between greenstone felsic volcanic units and the surrounding plutonic suites some of the volcanic and plutonic units contain zircon xenocrysts older than any exposed rocks these indicate the existence of still older units possibly stratigraphically lower and older portions of the greenstone sequence itself older granitoid intrusive rocks or bodies of older unrelated crustal material our data show that the onverwacht and fig tree felsic units have distinctly different ages and therefore do not represent a single tectonically repeated unit as proposed by others unlike the late archaean abitibi greenstone belt in canada which formed over about 30 ma exposed rocks in the bgb formed over a period of at least 220 ma the complex zircon populations encountered in this study imply that conventional multigrain zircon dating may not accurately identify the time of felsic volcanic activity in ancient greenstones a surprising similarity in rock types tectonic evolution and ages of the bgb in the kaapvaal craton of southern africa and greenstones in the pilbara block of western australia suggests that these two terrains may have been part of a larger crustal unit in early archaean times

geology of the high grade proterozoic terrains of sri lanka and the assembly of gondwana an update on recent developments abstract nd model ages determined for the high grade rocks of sri lanka delineate three crustal units viz the highland complex hc the wanni complex wc and the vijayan complex vc the distribution of these three units differs considerably from the three geological divisions demarcated previously on the basis of geological mapping the centrally located hc comprises mainly granulite grade charnockitic rocks and metasediments characterized by older nd model ages 2 0 3 4 ga the highland sedimentary pile was thickened by intermittent granitoid intrusions most of which are now charnockitic gneiss and granulites and basaltic sills and dikes all these metaigneous rocks now occur as conformable bands or layers due to intense polyphase deformation the hc is bounded on the east by the amphibolite grade vc composed mainly of granitic gneisses basic gneisses and migmatites and they have younger nd model ages 1 1 1 8 ga the isotopic and geochemical characteristics identify the precursors to the vijayan rocks as i type calc alkaline granitoids originated at an arc related tectonic environment thus the earlier interpretation that the vijayan rocks represent reworked hc was rejected the granulite inliers within the vc earlier considered as resisters to re working are now shown as overthrust klippen or rotated rafts of the hc the wc demarcated on the basis of nd model ages 1 1 1 8 ga similar to those of the vc lies west of the hc it consists mainly of granitic gneisses charnockitic gneisses and migmatites and the metamorphic grade ranges from amphibolite to granulite comprehensive geothermobarometric surveys constrain the p t evolution of the three crustal units and indicate that both the hc and wc underwent near isobaric cooling followed by a decompression with decreasing temperature extensive isotopic studies u pb pb pb sm nd rb sr have established a new geochronological framework for these high grade rocks of sri lanka the new framework has bracketed the age of high grade metamorphism in the three crustal units at 550 600 ma the recent advances in knowledge of the geology of sri lanka favour a strong geological correlation of the hc and the vc of sri lanka respectively with the lutzow holm complex and the yatmato belgica complex in the east antarctica the geology of the wc suggests a possible correlation with madagascar and east africa the amalgamation of the three crustal units of sri lanka is apparently related to the two distinct orogenic events that resulted in the assembly of the gondwana supercontinent

u pb isotopic systematics of zircons from prograde and retrograde transition zones in high grade orthogneisses sri lanka we present u pb zircon isotopic data from locally restricted prograde arrested in situ charnockitization and retrograde metamorphic transition zones which are well exposed in proterozoic orthogneisses tectonically interbanded with granulite facies supracrustal rocks of the highland group in sri lanka these granitoid rocks yield apparent ages of 1942 pm 22 ma 770 ma 660 ma and 560 ma all samples show severe pb loss some 550 560 ma ago the main phase of granulite formation could not be dated unambiguously but is bracketed between 660 ma and 550 ma the pervasive pb loss event around 550 560 ma reflects the end of this period of high grade metamorphism and was associated with widespread igneous activity and retrogression this is constrained by the 550 pm 3 ma intrusion age for a post tectonic granite we relate this late phase of thermal activity to crustal uplift of the sri lankan granulites our data unambiguously prove the high grade history of the sri lanka gneisses to be a late

late neoproterozoic ordovician and carboniferous events recorded in monazites from southern central madagascar abstract the evolution of the basement of southern madagascar north and south of the ranotsara shear zone was investigated using u th pb electron probe monazite age dating in combination with petrographic constraints several monazite grains show a stepwise progression of younger ages towards the rim indicating partial and complete resetting during tectonic metamorphic and or fluid events the oldest ages ranging from 630 2400 ma occur relatively rare in relic cores a first clear age population is dated at 550 560 ma most ages fall in two populations at 420 460 and 490 500 ma which in some samples overlap in error we interprete these ages as dating low pressure and high temperature metamorphism we have also clear evidence for carboniferous 300 310 ma monazite overgrowth rims which can not directly be related to macroscopic structures or metamorphic parageneses in combination with literature data we propose that the observed monazite age populations are related to gondwana amalgamation and subsequent rifting events during the break up of gondwana our study confirms that only the electron or ion microprobe yields sufficient spatial resolution to date individual shells of multiple zoned monazites in the polymetamorphic basement of madagascar

gold graphite association in granulite terrains implications for ore genesis abstract the gold occurrences of sri lanka are closely associated with the graphite deposits and are genetically related the quartz graphite margins of the vein graphite are shown to be metal accumulating the movement of co 2 rich fluids formation of graphite and the concentration of au are closely related whereas the co 2 rich fluids may well be the carbon carriers in the formation of graphite the original source of co 2 is still debated hydrothermal solutions when in contact with the graphite tend to activate the carbon thereby producing sites for the enrichment of gold the genetic link between the graphite veins and gold deposition has useful implications in the investigation of ore genesis in high grade metamorphic terrains and the exploration for gold