loading of the san andreas fault by flood induced rupture of faults beneath the salton sea the southern san andreas fault terminates in a stepover zone several small faults that separate major fault segments beneath the salton sea analysis of movements on the stepover zone faults indicates that periodic flooding of the palaeo salton sea during the late holocene could have triggered earthquakes on the san andreas fault

eruption and magma crystallization ages of las tres virgenes baja california constrained by combined 230th 238u and u th he dating of zircon las tres virgenes volcano is a calc alkaline composite cone located near the main gulf of california escarpment on the e coast of the baja california peninsula high sensitivity ion microprobe u series 230 th 238 u ages for zircon from la virgen tephra average 121 10 12 ka 1 mswd 2 7 with discrete age peaks at 100 and 160 ka the noble gas mass spectrometric u th he zircon age corrected for disequilibrium and pre eruptive storage is 36 3 ka this result for the eruption age of la virgen tephra is significantly older than previously postulated historic or holocene ages that were based on an 18th century map reference and 14 c dating of accidental charcoal respectively the new u th he zircon age is consistent with a n26 4 ka age derived from cosmogenic he exposure dating of an overlying basaltic lava flow hausback b p and abrams m j 1996 plinian eruption of la virgen tephra volcan las tres virgenes baja california sur mexico eos transactions american geophysical union 77 46 suppl 813 814 u pb zircon analysis of ignimbrites erupted from the adjacent early pleistocene la reforma and el aguajito calderas yielded ages of 1 38 0 03 ma n 12 mswd 1 0 and 1 17 0 07 ma n 23 mswd 1 3 respectively no evidence for these ages is found among la virgen zircons whereas pre quaternary zircon xenocrysts are common the la virgen magma therefore evolved unrelated to early pleistocene magmatism in adjacent calderas but assimilated local basement rocks a gap between average th u and u th he zircon ages suggests that zircon crystallization was discontinuous in the la virgen magma chamber in addition partial resorption of zircon suggests episodic thermal rejuvenation most likely by basaltic recharge based on the zircon record the n100 ka lifetime of the thermal anomaly that sustained repeated intrusive pulses significantly exceeds the age of the last eruption this strengthens the view that tres virgenes has a potential for future eruptions

graphitization of carbonaceous matter during metamorphism with references to carbonate and pelitic rocks of contact and regional metamorphisms japan this study is an attempt to correlate the graphitization process of carbonaceous matter during metamorphism with metamorphic grade graphitization can be parameterized using crystal structure and chemical and isotopic compositions the extent of graphitization could be characterized mainly by temperature duration of metamorphism and rock composition we compared the graphitization trends for two metamorphic terrains a contact aureole of the kasuga area and a regional metamorphic terrain of high temperature low pressure type of the ryoke metamorphic terrain in northern kiso area central japan and for two different lithologies carbonate and pelite using x ray diffractogram dta tg analysis and chemical and stable isotope analyses during contact metamorphism graphitization and carbon isotopic exchange reactions proceeded simultaneously in pelitic and carbonate rocks the decreases in basal spacing d 002 of the carbonaceous matter in carbonate rocks is greatly accelerated at temperatures higher than about 400 c furthermore carbon isotopic ratios of graphite in carbonate rocks also change to 13c enriched values implying exchange with carbonates the beginning of this enrichment of 13c in the carbonaceous matter coincides with an abrupt increase of the graphitization processes carbon isotopic shifting up to 5 in pelites could be observed as metamorphic temperature increased probably by about 400 c carbonaceous matter in pelitic rocks is sometimes a mixture of poorly crystallized organic matter and well crystallized graphite detritus dta tg analysis is an effective tool for the distinction of detrital graphitic material two sources for the original carbon isotopic composition of carbonaceous matter in pelites in the kasuga contact aureole can be distinguished about 28 and 24 regardless of the presence of detrital graphite and were mainly controlled by depositional environment of the sediments graphitization in limestones and pelitic rocks in regional metamorphism proceeds further than in a contact aureole in the low temperature range the differences in extent of graphitization between the two metamorphic regions is large however at temperatures higher than 600 c the extent of graphitization in both regions is indistinguishable the degree of graphitization is different in limestones and pelitic rocks from the ryoke metamorphic terrain we demonstrate that the graphitization involves a progressive re construction process of the crystal structure the sequence of the first appearance of crystal inter planar spacing correlates with the metamorphic grade and indicates the crystal growth of three dimensional structured graphite

u th he zircon and archaeological ages for a late prehistoric eruption in the salton trough california usa u th and u th he zircon geochronology redefines the timing of volcanic activity in the salton trough southern california usa the subaerial extension of the incipiently oceanic gulf of california u series disequilibrium corrected u th he zircon analyses for a granophyre ejecta clast from the red island rhyolite dome indicate an eruption age of 2480 470 a calendric dates between 0 and 940 before common era bce error at 95 confidence this eruption age is supported by u th zircon crystallization ages for two obsidian bearing lavas red island the host for the granophyre and obsidian butte a prehistoric quarry for obsidian that is widely distributed in southern california and northern mexico archaeological sites lavas and granophyre display overlapping zircon crystallization age distributions that support field and compositional evidence that they are cogenetic and contemporaneous the u th he eruption age is younger and significantly more precise than previous ages for these volcanoes and is the first indication that the eruption of obsidian flows coincided with human presence in the region a late prehistoric eruption age agrees with the absence of the obsidian butte lithic source among early prehistoric cultural artifacts previously attributed to submergence of the quarry location during hypothesized persistent flooding by ancient lake cahuilla

influence of late quaternary climatic changes on geomorphic and pedogenic processes on a desert piedmont eastern mojave desert california radiocarbon dating of late quaternary deposits and shorelines of lake mojave and cation ratio numerical age dating of stone pavements dorn 1984 on the adjacent soda mountains piedmont provide age constraints for alluvial and eolian deposits these deposits are associated with climatically controlled stands of lake mojave during the past 15 000 yr six alluvial fan units and three eolian stratigraphic units were assigned ages based on field relations with dated shorelines and piedmont surfaces as well as on soil geomorphic data all but one of these stratigraphic units were deposited in response to time transgressive climatic changes beginning approximately 10 000 yr ago increased eolian flux rates occurred in response to the lowering of lake mojave and a consequent increase in fine sediment availability increased rates of deposition of eolian fines and associated salts influenced pedogenesis stone pavement development and runoff infiltration relations by 1 enhancing mechanical weathering of fan surfaces and hillslopes and 2 forming clay and silt rich surface horizons which decrease infiltration changes in alluvial fan source areas from hillslopes to piedmonts during the holocene reflect runoff reduction on hillslopes caused by colluvial mantle development and runoff enhancement on piedmonts caused by the development of less permeable soils inferred increased in early to middle holocene monsoonal activity resulted in high magnitude paleo sheetflood events on older fan pavements this runoff triggered piedmont dissection which in turn caused increased sediment availability along channel walls thus runoff infiltration changes during the late quaternary have occurred in response to eolian deposition of fines pedogenesis increased sheetflood activity in the holocene and vegetational changes which are related to many complicated linkages among climatic change lake fluctuations and eolian hillslope and alluvial fan processes

late holocene lacustrine chronology and archaeology of ancient lake cahuilla california freshwater lakes existed intermittently in the salton trough of southern california during the late holocene the lakes formed north of the subaerial colorado river delta whenever the colorado river flowed west into the trough instead of south to the gulf of california water filled the trough to a maximum altitude of 12 m stratigraphy radiocarbon dates and supplementary evidence document four lacustral intervals of lake cahuilla between a d 700 and 1580 archaeological sites are associated with the 12 m shoreline and their occupation correlates with these lacustral intervals

san andreas fault earthquake chronology and lake cahuilla history at coachella california the southernmost 100 km of the san andreas fault has not ruptured historically it is imperative to determine its rupture history to better predict its future behavior this paleoseismic investigation in coachella california establishes a chronology of at least five and up to seven major earthquakes during the past 1100 yr this chronology yields a range of average recurrence intervals between 116 and 221 yr depending on assumptions with a best estimate average recurrence interval of 180 yr the most recent earthquake occurred c 1690 more than 300 yr ago suggesting that this stretch of the fault has accumulated a large amount of tectonic stress and is likely to rupture in the near future assuming the fault follows a stress renewal model this study also establishes the timing of the past 5 6 highstands of ancient lake cahuilla since a d 800 we found that earthquakes do not tend to occur at any particular stage in the lake cycle

post glacial range of variability in the conejos river valley southern colorado usa fluvial response to climate change and sediment supply effective river management strategies require an understanding of how fluvial processes vary both spatially and temporally here we examine the natural range of variability in the conejos river valley southern colorado through documentation of terrace morphostratigraphic and sedimentological characteristics as well as through investigation of sediment contributions from headwaters hillslopes and tributary streams additionally soil development and radiocarbon ages together with local and regional paleoclimate reconstructions were used to infer the range of processes acting in this system since de glaciation the conejos river has fluctuated between episodes of bedrock strath formation aggradation and vertical incision morphostratigraphic relationships soil development and radiocarbon ages enable us to propose a chronology for periods of alluvial deposition around 8 9 7 6 ka 5 5 ka and from 3 5 to 1 1 ka separated by intervals of fluvial incision we infer potential forcing mechanisms by utilizing multiple working hypotheses specifically we discuss the potential for increases in sediment supply during periods of 1 para glacial adjustment 2 climatic cooling 3 increased frequency of climate change and 4 increased fire frequency or severity we also consider the effects of changes in stream discharge and extreme storm occurrence we conclude that combinations of these processes operating at different times have contributed to sediment mobilization since de glaciation stream and landform morphology also varies longitudinally due to the influence of remnant glacial topography in particular valley bottom overdeepening at tributary junctions has resulted in incision and strath formation into unlithified glacial deposits i e fill cut terraces rather than bedrock in some reaches overall the conejos fluvial system has varied significantly both temporally and spatially since de glaciation and appears to be sensitive to changes in sediment supply related to holocene scale climate fluctuations this natural range of variability must therefore be a key consideration in any future stream management policies copyright 2012 john wiley sons ltd

evidence for a fold nappe in the high grade basement of central sri lanka terrane assembly in the pan african lower crust abstract structural analysis in the kadugannawa complex kc in central sri lanka reveals the presence of an early steep gneissic layering s 1 which has been overprinted by a younger event d 2 in the structurally deepest levels d 2 folds become progressively tighter towards the contact with the underlying highland complex hc the distribution of lithologies in the kc the distribution of steep s 1 and subhorizontal s 2 and the strain gradient towards the kc hc contact suggest that the kc is a d 2 fold nappe overlying the hc vergence of asymmetric sheath folds in sections parallel to a strong nnw sse lineation is towards nnw indicating a top to the nnw displacement granulite facies assemblages were formed during and after the final stages of d 2 in the kc d 2 was the main fabric forming event in the hc where meta igneous and metasedimentary gneisses show l s fabrics defined by the peak granulite facies assemblages the sense of shear deduced from asymmetric foliation boudinage is consistent with the top to the nnw displacement in the kc both units have been refolded around large scale upright d 3 folds the nappe model and nd model ages suggest that the kc and the eastern part of the hc may represent distinct lower crustal terranes juxtaposed during pan african high grade high strain events d 2 the contact between them is interpreted as a terrane boundary in a lower crustal environment the top to the nnw movement sense suggests that the kc may have been originally located south of the eastern highland complex this and the similarity in isotope signatures indicate a pre assembly link of the kc the western highland complex and the vijayan complex of eastern sri lanka

origin of compositional layering and mechanism of crustal thickening in the high grade gneiss terrain of sri lanka abstract the highland complex of sri lanka comprises a 7 km thick succession of banded para and orthogneisses at granulite grade or partly retrograded we interpret this complex as a tectonostratigraphic unit in which no primary stratigraphy is preserved this succession with a probable depositional age of 2 ga evolved from a rift basin into a foredeep and then into a fold and thrust belt prior to severe flattening and transport into the lower crust during the final stages of collision some 550 620 ma ago granitoid rocks were emplaced into the supra crustal sequence at various stages of the tectonic evolution between 1950 and 670 ma ago and the original intrusive contacts became obscured or severely modified during progressive non coaxial ductile deformation structures that predate the presently observed layering are preserved in rootless isoclinal folds boudins and microstructures and in deformed pre granulite metamorphic minerals preserved in garnet porphyroblasts the evolution of the highland complex is similar to that of other precambrian granulite terrains notably parts of the grenville province of canada and probably resulted from collisional tectonics during which the original supracrustal assemblage was detached from its archaean to palaeoproterozoic basement the paucity of structures reflecting the early deformational history is due to severe flattening and ductile deformation during predominantly non coaxial deformation when the highland rocks were transported into the lower crust

textural changes of graphitic carbon by tectonic and hydrothermal processes in an active plate boundary fault zone alpine fault new zealand graphitization in fault zones is associated both with fault weakening and orogenic gold mineralization we examine processes of graphitic carbon emplacement and deformation in the active alpine fault zone new zealand by analysing samples obtained from deep fault drilling project dfdp boreholes optical and scanning electron microscopy reveal a microtextural record of graphite mobilization as a function of temperature and ductile then brittle shear strain raman spectroscopy allowed interpretation of the degree of graphite crystallinity which reflects both thermal and mechanical processes in the amphibolite facies alpine schist highly crystalline graphite indicating peak metamorphic temperatures up to 640 c occurs mainly on grain boundaries within quartzo feldspathic domains the subsequent mylonitization process resulted in the reworking of graphite under lower temperature conditions 500 600 c resulting in clustered in protomylonites and foliation aligned graphite in mylonites in cataclasites derived from the mylonitized schists graphite is most abundant 50 as opposed to 10 elsewhere and has two different habits inherited mylonitic graphite and less mature patches of potentially hydrothermal graphitic carbon tectonic hydrothermal fluid flow was probably important in graphite deposition throughout the examined rock sequences the increasing abundance of graphite towards the fault zone core may be a significant source of strain localization allowing fault weakening

age of colluvium indicates accelerated late wisconsinan hillslope erosion in the upper mississippi valley colluvium on foot slopes in parts of north america and europe has been attributed to a major mass wasting episode during the last glacial period stratigraphic evidence and 14 c ages support this hypothesis for the northern part of the upper mississippi valley colluvium in this region grades laterally into or interfingers with fluvial sediment beneath the late wisconsinan savanna terrace colluvial foot slopes are truncated by fluvial surfaces postdating the incision that created the savanna terrace between 13 and 11 ka samples from within the colluvium have 14 c ages between 18 6 and 12 ka ages of 28 9 and 20 3 ka have been obtained from beneath colluvium and fluvial sediment inset into colluvial foot slopes has yielded ages between 12 5 and 9 8 ka widespread permafrost during the late wisconsinan glacial maximum may explain the onset of accelerated mass wasting that produced the colluvium although mass wasting apparently continued for some time during subsequent climatic warming the results described here imply major long term climatically driven fluctuations in sediment supply from hillslopes to the fluvial system in this region

the effects of secular disequilibrium on u th he systematics and dating of quaternary volcanic zircon and apatite the u th he dating method applied to u rich phases such as zircon and apatite has sufficient sensitivity and precision to be of potential use for dating relatively recent geologic events such as volcanic eruptions however in phases with crystallization ages less than 1 ma chemical fractionation within the 238 u decay series may modify the he ingrowth rate causing he ages computed from the secular equilibrium age equation to be incorrect the resulting systematic error depends on the 230 th 238 u activity ratio of the dated phase when it is erupted and on the eruption age zircons which exclude th relative to u will likely have secular equilibrium he ages that underestimate the eruption age by up to a few tens of decreasing with increasing eruption age apatites tend to accommodate u and th with little fractionation so apatite secular equilibrium he ages will be nearly concordant with eruption age if minerals are erupted immediately after crystallization the disequilibrium effect can be reasonably accounted for based on th u systematics however crystals are likely to reside for unknown but potentially long periods in a magma chamber such that the degree of secular disequilibrium will be reduced prior to the onset of he accumulation u th he analyses of co genetic phases that fractionate the u th ratio differently like apatite and zircon can be used to better constrain eruption age as well as to provide insights into magma chamber residence time we illustrate this approach with u th he analyses of zircons and apatites of the pleistocene age rangitawa tephra new zealand

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

co 2 melts and granulite metamorphism the discovery that granulites contain co2 rich fluid inclusions while amphibolites in the same terrane contain h2o rich or mixed co2 h2o inclusions1 has led to a debate over the processes that produce granulite metamorphism one theory2 4 maintains that granulites form as the result of a massive influx of co2 from lower crustal or mantle sources such an influx could be responsible for the marked depletion many granulite terranes show in large ion lithophile elements2 5 others however maintain that granulite facies metamorphism is a necessary complement of crustal melt ing6 9 recently lamb and valley10 11 have argued that granulite metamorphism may be a product of any one of three processes co2 streaming partial melting or recrystallization of originally dry rocks here we suggest that these processes need not be independent and that they all may be related to the passage of melts through the crust

hydrothermal graphite in new hampshire evidence of carbon mobility during regional metamorphism graphite precipitated from hydrothermal fluids pervades the sillimanite grade metasedimentary and plutonic rocks of new hampshire hydrothermal graphite occurs as microscopic veinlets halos of spherulites around shear zones and metre thick veins carbon isotope analyses of the graphite range from 25 to 9 13c pdb intermediate between the two biogenic crustal reservoirs of reduced organic matter and carbonate it is proposed that carbon was mobilized from sediments as co2 and ch4 during metamorphic devolatilization reactions the carbon bearing species were transported in aqueous fluids through hydraulic fractures graphite precipitated when aqueous fluids with different co2 ch4 ratios were mixed in fractures

carbon isotope stratigraphy in a single graphite crystal implications for the crystal growth mechanism of fluid deposited graphite a 2 cm long and 0 8 cm thick single graphite crystal embedded in quartz feldspar and orthopyroxene of a granulite facies metamorphic rock from southern india exhibits unique homogeneity in 13 c values along layers parallel to the 0001 surface and smooth unidirectional variation of about 1 along the c crystallographic axis the fluid precipitated graphite shows isotope homogeneity indicative of crystal growth along the 0001 plane the c isotope data along with the textural features suggest that the formation of graphite initiates in a layer perpendicular to the c axis possibly by a spiral growth mechanism the initial layer is followed layer after layer until the final stages of graphite growth a second stage of graphite growth unrestricted by surrounding minerals is inferred from the un deformed idiomorphic hexagonal form of the overgrown crystals the c isotope values in the overgrown rim of the crystal are lowered by about 2 suggesting a multistage precipitation morphologic and stable isotope studies thus confirm uniform crystal growth of graphite recording an isotope evolution pattern relating to fractionation from a fluid in an infinite reservoir during the main crystal formation spiral growth parallel to the 0001 face can explain the perfect uniformity of stable isotope composition in a single layer whereas the variation across the 0001 surface gives insights into the time integrated fluid evolution attending the crystal growth

the origin and hydrothermal mobilization of carbonaceous matter associated with paleoproterozoic orogenic type gold deposits of west africa abstract the chemical and physical properties of carbonaceous matter cm were studied in paleoproterozoic metasediment hosted orogenic type gold deposits in burkina faso the inata deposit in mali the syama deposit and in ghana the obuashi and bogoso deposits two types of cm occur in all the studied deposits metamorphosed and hydrothermal metamorphosed cm prevails in all the deposits hydrothermal cm occurs in small veinlets in hydrothermally altered rocks and in quartz veins or forms irregular accumulations parallel or sub parallel to c type cleavage within the shear zones the origin of hydrothermal cm which occurs in paragenesis with au bearing arsenopyrite or pyrite seems to have been due to supersaturation of hydrothermal fluids with carbon at the deeper or middle crustal levels the isotopic composition of carbon in bulk cm 33 1 to 26 2 vpdb indicates its biogenic origin the isotopic composition of carbon in hydrothermal carbonates ranges from 14 5 to 4 4 vpdb which suggest mixing of carbon derived from a deep seated source with carbon derived from an organic source the interaction of hydrothermal fluids with metamorphosed cm could be one of the causes of the reduction of hydrothermal fluids and formation of the respective mineralization the optical properties and raman spectra of the metamorphosed cm particles in the individual studied mineral deposits differ considerably the temperatures calculated on the basis of the raman spectra of metamorphosed cm vary between 280 and 440 c depending on the thermometer used and correspond to temperatures of metamorphism of upper sub grenschist and greenschist facies the temperatures calculated for hydrothermal cm at the individual deposits are only slightly lower compared to the metamorphosed cm at the same deposits which indicates approximately the same temperature of the metamorphic and hydrothermal processes

synmetamorphic carbon mobility and graphite enrichment in metaturbidites as a precursor to orogenic gold mineralisation otago schist new zealand the macraes orogenic gold deposit is hosted by a graphitic micaceous schist containing auriferous porphyroblastic sulphides the host rock resembles zones of unmineralised micaceous graphitic pyritic schists derived from argillaceous protoliths that occur locally in background pelitic otago schist metasediments this study was aimed at determining the relationship between these similar rock types and whether the relationship had implications for ore formation argillites in the protolith turbidites of the otago schist metamorphic belt contain minor amounts of detrital organic matter 50 million years of evolution of the metamorphic belt from development of sheared argillite in the jurassic to postmetamorphic ductile extension in the cretaceous introduced graphite is structurally controlled and occurs with metamorphic muscovite and chlorite as veins and slicken sided shears with some veins having 50 noncarbonate carbon graphitic foliation seams in low grade micaceous schist and metamorphic quartz veins contain equant graphite porphyroblasts up to 2 mm across that are composed of crystallographically homogeneous graphite crystals graphite reflectance is anisotropic and ranges from 1 to 8 green light texturally similar porphyroblastic pyrite has grown in micaceous schist up to 10 wt s metamorphic quartz veins and associated muscovite rich shears these pyritic schists are weakly enriched in arsenic up to 60 ppm the low grade metamorphic mobility and concentration of graphite in micaceous schists is interpreted to be a precursor process that structurally and geochemically prepared parts of the otago schist belt for later more restricted gold mineralisation economic amounts of gold and associated arsenic were subsequently introduced to carbonaceous sulphidic schists in the macraes gold deposit by a separate metamorphic fluid derived from high grade metaturbidites fluid flow at all stages in these processes occurred at metamorphic rates mm year and fluids were broadly in equilibrium with the rocks through which they were passing

reconciling the roles of tectonism and climate in quaternary alluvial fan evolution tectonism and climate are the primary variables considered in conceptual models of alluvial fan evolution and basin fill architecture yet their roles and relative importance are far from resolved the madison range and madison river valley southwestern montana were affected during late quaternary time by varying degrees of tectonic activity as well as by climate change through multiple glaciations we are thus able to evaluate the impact of tectonism and climate on fan evolution two primary fan deposits were correlated along the range front they were deposited as proglacial outwash fans during glacial periods of increased sediment and water discharge during the present interglacial period these deposits were entrenched and secondary fans formed on lower fan areas temporal and stratigraphic relations indicate that climate was largely responsible for driving both fan aggradation and entrenchment in contrast stratigraphic relations between fan deposits and surface faults do not support a direct relation either spatial or temporal between local faulting and fan deposition because tectonism produces and maintains the relief necessary for fans to form its primary role is long term controlling the duration over which fan deposition may occur along a mountain front if quaternary alluvial fan evolution is representative of ancient alluvial fan evolution this study has important implications for interpreting primary controls on deposition of the ancient alluvial fan sequences

isotopic evidence for involvement of c0 2 bearing magmas in granulite formation it has been suggested1 2 that co2 bearing magmas play an important part in the formation of granulites these magmas crystallize directly into granulites and also expel fluids which promote the development of granulite facies mineral assemblages in adjacent country rocks direct evidence for carbonic fluids remains elusive however as granulite facies aureoles adjacent to charnockitic intrusives1 may result either from the influx of carbonic fluids derived from the intrusives or simply from extraction of water into a vapour absent melt here we report results of a detailed study of carbon isotope compositions of graphite associated with a charnockite dyke from the ponmudi quarry of south india higher graphite abundances and an anomalously heavy carbon isotopic composition 13cgr 8 1 0 at the dyke margins indicate that externally derived co2 rich fluids were transported into the ponmudi paragneisses by the intrusive and precipitated nearly quantitatively as graphite on encountering reducing country rocks the occurrence of large orthopyroxene crystals along the dyke margins suggests that these fluids also served to dehydrate the country rocks this dyke may thus be representative of the widespread transport of carbonic fluids in this terrain by felsic magmas

revised two feldspar geothermometry applied to sri lankan feldspars abstract two feldspar geothermometry is applied to granulite and amphibolite facies feldspars from sri lanka a procedure is suggested to correct for compositional changes due to retrograde resetting the feldspars derive from different geological units from different rocks involved in different acts of deformation and to feldspars which grew at different times in the temperature time deformation history of the parent rock string perthites from granulite facies rocks in the eastern highland complex hc which are the deepest in the pile yield highest temperatures of c 890 c 850 c feldspars that unmixed during or after f 4 give temperatures around 800 c 750 c in the south but 870 c 830 c in the north in the north near peak temperatures outlasted formation of the big late f 4 folds in the south temperature started to drop before f 4 temperatures obtained from mesoperthites of monzonitic to pegmatitic metamagmatic rocks from the west of the hc give 780 c 770 c interpreted as the minimum temperature of growth during f 4 and reflecting metamorphic equilibration a large metagranite in the nw vijayan nwv contains coarse string perthites yielding a temperature of metamorphic equilibration for phenocrysts and a temperature of metamorphic formation for coarse recrystallized grains of 880 c 840 c this proves that the nwv went through the same granulite facies deformation and metamorphism as the hc before it was swamped by strong migmatization at 751 c 710 c at a late stage whenever string perthites are deformed and recrystallize once or several times during falling temperature the original grains and first recrystallizates yield the highest the younger and younger generations of recrystallized grains successively lower temperatures of formation in the same sequence grains and plagioclase domains unmixed from them become successively less and finer thus there is a very good correlation between string perthite morphology and temperature of growth or equilibration preceding unmixing

holocene strath terraces climate change and active tectonics the clearwater river basin olympic peninsula washington state the 400 km 2 clearwater river basin located on the pacific flank of the actively uplifting olympic mountains of western washington state contains a well preserved flight of holocene fluvial terraces we have collected a large data set of numeric ages from these terraces that is used to elucidate the geomorphic fluvial active tectonic and climatic processes that operate at holocene spatial and temporal scales detailed field mapping reveals three prominent holocene straths and their overlying terrace deposits terrace ages fall into three broad ranges ca 9000 11 000 yr b p qt4 4000 8000 yr b p qt5 and 0 3000 yr b p qt6 terrace deposit stratigraphy sedimentology and age distributions allow us to consider two alternative models for their genesis the favored model states that the terrace ages are coincident with lateral incision of the clearwater channel emplacement of the terrace alluvium and the carving of the straths vertical incision of the clearwater channel was primarily relegated to the brief 1000 yr intervals when we have no record of terraces alternatively the straths were carved as the channel incised vertically during the brief time periods between dated terrace deposits and the terrace ages record a subsequent long time of alluviation atop the straths and concomitant termination of vertical incision in both models we envision a clearwater river channel at or near capacity with a temporally variable rate of both lateral and vertical incision small deviations from this at capacity condition are driven by variations in the liberation and delivery of hillslope sediment to the channel we consider several causes for variable hillslope sediment flux in this tectonically active setting including holocene climate change and ground accelerations related to earthquakes holocene rates of vertical incision are reconstructed along nearly the entire clearwater valley from the wide distribution of dated terraces incision rates clearly increase upstream mimicking a pattern documented for pleistocene terraces in the same basin however the rates are 2 3 times those determined for the pleistocene terraces the faster holocene incision rates may be interpreted in terms of an increase in the rates of rock uplift however we favor an alternative explanation in which the holocene rates represent a channel rapidly reacquiring its stable graded concavity following protracted periods of time in the pleistocene when it could not accomplish any vertical incision into tectonically uplifted bedrock because the channel was raised above the bedrock valley bottom by climatically induced alluviation these results illustrate how even in tectonically active settings representative rates of rock uplift inferred from studies of river incision should be integrated over at least one glacial interglacial cycle

do phreatomagmatic eruptions at ubehebe crater death valley california relate to a wetter than present hydro climate of the region a novel application of 10 be surface exposure dating is presented to determine the timing of phreatomagmatic eruptions at ubehebe crater and to test the idea that volcanism may relate to a wetter than present hydro climate twelve of the fifteen ages obtained lie between 0 8 and 2 1 ka while three samples give older mid holocene ages the cluster between 0 8 and 2 1 ka is interpreted as encompassing the intervalof volcanicactivityduringwhich ubehebe crater was formed the remaining older ages are inferred to date eruptions at the older neighboring craters the main and most recent period of activity encompasses the medieval warm period an interval of prolonged drought in the american southwest as well as slightly wetter conditions prior to the medieval warm period phreatomagmatic activity under varied hydrologic conditions casts doubt on the idea that eruptive timing relates to a wetter hydro climate instead the presence of a relatively shallow modern water table suggests that sufficient groundwater was generally available for phreatomagmatic eruptions at the ubehebe site in spite of prevailing arid conditions this and the youth of the most recent activity suggest that the ubehebe volcanic field may constitute a more significant hazard than generally appreciated citation sasnett p b m goehring n christie blick and j m schaefer 2012 do phreatomagmatic eruptions at ubehebe crater death valley california relate to a wetter than present hydro climate geophys res lett 39 l02401

calcite graphite isotope thermometry a test for polymetamorphism in marble tudor gabbro aureole ontario canada graphitization and coarsening of organic material in carbonate bearing metasedimentary rocks is accompanied by carbon isotope exchange which is the basis of a refractory pressure independent geothermometer comparison of observed isotopic fractionations between calcite and graphite a13ccal gr with independent petrological thermometers provides the following empirical calibration over the range 400 800 c a13ccal gr 5 81 x lo6 x t k 2 61 this system has its greatest potential in marbles where calcite graphite is a common assemblage and other geothermometers are often unavailable the temperature dependency of this empirical calibration differs from theoretical calibrations reasons for this are unclear but the new empirical calibration yields temperature estimates in better agreement with independent thermometry from several terranes and is preferred for geological applications both calcite graphite isotopic thermometry and calcite dolomite solvus thermometry are applied to marble adjacent to the tudor gabbro in the grenville province of ontario canada the marble has undergone two metamorphic episodes early contact metamorphism and later regional metamorphism values of a13cca gr decrease regularly from c 8 0 in samples over 2 km from the pluton to values of 3 4 within 200m of the contact these samples appear to preserve fractionations from the early thermal aureole with the empirical geothermometer and indicate temperatures of 450 500 c away from the intrusion and 700 750 c near the gabbro this thermal profile around the gabbro is consistent with conductive heat flow models in contrast the distribution of mg between calcite and dolomite has been completely reset during later regional metamorphism and yields uniform temperatures of c 500 c even at the contact graphite textures are important for interpreting the results of the calcite graphite thermometer coarsening of graphite approaching the tudor gabbro correlates with the decrease in isotopic fractionations and provides textural evidence that graphite crystallization took place at the time of intrusion in contrast to isotopic exchange during prograde metamorphism which is facilitated by graphitization retrogressive carbon isotopic exchange appears to require recrystallization of graphite which is sluggish and easily recognized texturally resistance of the calcite graphite system to resetting permits thermometry in polymetamorphic settings to see through later events that have disturbed other systems

graphite bearing co2 fluid inclusions in granulites insights on graphite precipitation and carbon isotope evolution graphite in deep crustal enderbitic orthopyroxene garnet plagioclase quartz granulites 740 c 8 9 kb of nilgiri hills southern india were investigated for their spectroscopic and isotopic characteristics four types of graphite crystals were identified the first type gri which is interstitial to other mineral grains can be grouped into two subtypes gria and grib gria is either irregular in shape or deformed and rough textured with average 13c values of 12 7 0 4 n 3 a later generation of interstitial graphite grib shows polygonal crystal shapes and highly reflecting smooth surface features these graphite grains are more common and have 13c values of 11 9 0 3 n 14 both subtypes show well defined raman shifts suggesting a highly crystalline nature cores of interstitial graphite grains have on average lower 13c values by 0 5 compared to that of the rim the second type of graphite grii occurs as solid inclusions in silicate minerals commonly forming regular hexagonal crystals with a slightly disordered structure the third type of graphite griii is associated with solid inclusions up to 100 m that have decrepitation halos of numerous small 15 m satellite fluid inclusions of pure co2 with varying density 1 105 to 0 75 g cm3 the fourth type of graphite griv is found as daughter crystals within primary type co2 fluid inclusions in garnet and quartz these fluid inclusions have a range of densities 1 05 to 0 90 g cm3 but in general are significantly less dense than graphite free primary pure co2 fluid inclusions 1 12 g cm3 raman spectral characteristics of graphite inside fluid inclusions suggest graphite crystallization at low temperature 500 c the precipitation of graphite probably occurred during the isobaric cooling of co2 rich peak metamorphic fluid as a result of oxyexsolution of oxide phases the oxyexsolution process is evidenced by the magnetite ilmenite granular exsolution textures and the systematic presence of numerous micron sized rutile and other oxide inclusions in association with fluid inclusions within garnet plagioclase and quartz the carbon isotope compositions of coexisting co2 in fluid inclusions and graphite show a fractionation co2 gr co2 gr of 6 in garnet consistent with the existing theoretical estimates of co2 gr co2 gr at 800 c a subsequent generation of co2 inclusions trapped in matrix quartz and quartz segregation have higher 13c values 4 and 2 9 respectively graphite in quartz segregations also has higher 13c values 9 8 than those in enderbite 12 7 micro graphite crystals included in garnet quartz enderbite and quartz segregation have average 13c values of 11 1 10 4 and 8 7 respectively indicating progressive enrichment in 13c with a decrease in temperature of recrystallization of respective minerals this progressive enrichment is also observed in carbon isotope compositions of fluid inclusion co2 suggesting isotopic equilibrium during graphite precipitation from co2 fluids thus the carbon isotope record preserved in these rocks by the interstitial graphite co2 fluid in enderbite graphite microcrystals graphite in quartz segregation and co2 fluid in quartz segregation suggests a temperature controlled isotopic evolution this evolution is in accordance with a closed system rayleigh type graphite precipitation process which progressively enriched residual co2 in 13c

episodic holocene eruption of the salton buttes rhyolites california from paleomagnetic u th and ar ar dating in the salton trough ca five rhyolite domes form the salton buttes mullet island obsidian butte rock hill north and south red hill from oldest to youngest results presented here include 40ar 39ar anorthoclase ages 238u 230th zircon crystallization ages and comparison of remanent paleomagnetic directions with the secular variation curve which indicate that all domes are holocene 238u 230th zircon crystallization ages are more precise than but within uncertainty of 40ar 39ar anorthoclase ages suggesting that zircon crystallization proceeded until shortly before eruption in all cases except one remanent paleomagnetic directions require three eruption periods 1 mullet island 2 obsidian butte and 3 rock hill north red hill and south red hill borehole cuttings logs document up to two shallow tephra layers north and south red hills likely erupted within 100 years of each other with a combined 238u 230th zircon isochron age of 2 83 0 60 ka 2 sigma paleomagnetic evidence suggests this age predates eruption by hundreds of years 1800 cal bp rock hill erupted closely in time to these eruptions the obsidian butte 238u 230th isochron age 2 86 0 96 ka is nearly identical to the combined red hill age but its virtual geomagnetic pole position suggests a slightly older age the age of aphyric mullet island dome is the least well constrained zircon crystals are resorbed and the paleomagnetic direction is most distinct possible mullet island ages include ca 2300 5900 6900 and 7700 cal bp our results constrain the duration of salton buttes volcanism to between ca 5900 and 500 years

rhyolites at kerlingarfjoll iceland the evolution and lifespan of silicic central volcanoes kerlingarfjoll central volcano is iceland s second largest outcrop of quaternary rhyolite and is part of the icelandic western rift zone geochemical and ar ar age data show that at least 21 different rhyolite eruptions have taken place at kerlingarfjoll over the last 350 ka ar ar dating was carried out on samples of obsidian which showed variable reproducibility illustrating the difficulty in dating young icelandic volcanics nevertheless reasonable estimates of eruption age have been derived for a number of eruptive units that are consistent with observed stratigraphy enabling an understanding of the temporal evolution of kerlingarfjoll two rhyolite magma types are present the first is an older low nb rhyolite that was erupted episodically along a cryptic curved fracture system to form a discontinuous ring of rhyolite mountains between 350 and 250 ka this discontinuous ring is similar to structures observed at other volcanoes in iceland suggesting that the development of a curved fracture that acts as a pathway for episodic silicic eruptions is a feature of central volcano development the second magma is a younger high nb rhyolite that was erupted episodically between 250 and 68 ka in the northern part of kerlingarfjoll forming two clusters both of which have areas of intense hydrothermal activity repose periods for rhyolite volcanism are thought to be on the order of tens of thousands of years and it is possible that kerlingarfjoll will erupt rhyolite again in the future

alteration and remelting of nascent oceanic crust during continental rupture evidence from zircon geochemistry of rhyolites and xenoliths from the salton trough california abstract rhyolite lavas and xenoliths from the salton sea geothermal field southern california provide insights into crustal compositions and processes during continental rupture and incipient formation of oceanic crust salton buttes rhyolite lavas contain xenoliths that include granophyres fine grained altered rhyolites felsite and amphibole bearing basalts zircon is present in lavas and xenoliths surprisingly even in the basaltic xenoliths where it occurs in plagioclase rich regions interpreted as pockets of crystallized partial melt zircons in the xenoliths are exclusively late pleistocene holocene in age and lack evidence for inheritance u th isochron ages are 20 5 1 2 1 2 ka granophyres 18 3 3 5 3 6 ka felsite 30 1 12 4 14 1 ka and 9 2 6 6 7 0 ka basalts all errors 1 the dominant zircon population in the rhyolite lavas yielded u th ages between 18 and 10 ka with few pre quaternary xenocrysts present 18ozircon values are lower than typical crustal basement values thus ruling out rhyolite genesis by melting of continental crust moreover 18ozircon values are 0 5 1 0 lower than compositions achievable by zircon crystallization from residual melt in equilibrium with unaltered mid ocean ridge basalt suggesting that basaltic crust and silicic plutons in the subsurface of the salton sea geothermal field isotopically exchanged with meteoric waters this is evidence for deep reaching hydrothermal circulation and indicates rhyolite genesis by episodic remelting of altered basalts instead of fractional crystallization of unaltered basaltic magma

new insights into carbon fluid chemistry and graphite precipitation sims analysis of granulite facies graphite from ponmudi south india slow graphite carbon self diffusion rates and a strong dependence of 13 c 12 c fractionations on fluid chemistry can be combined with secondary ion mass spectrometry sims analyses of 13 c 12 c of graphite to fingerprint graphite precipitation mechanisms and carbon chemistry of crustal fluids the d 13 c variations in single graphite grains in granulite facies rocks from ponmudi south india approach 10 and are interpreted to reflect three distinct generations of graphite formation and fluid infiltration illustrating the potential of sims microdomain graphite analyses for deciphering crustal fluid influx histories our analyses provide insight into a longstanding debate in crustal metamorphism of south india indicating that the fluid rock interaction history included early middle and late interactions with c o h fluids 1999 elsevier science b v all rights reserved

chemical weathering of fluvial sediments during alluvial storage the macuapanim island point bar solimoes river brazil abstract modification of sediment composition by chemical weathering depends both on the overall intensity of weathering and on its duration at macuapanim island solimboes river amazon main channel brazil a number of alluvial deposits have been exposed to intense tropical weathering for differing lengths of time in the very fine sand fraction the detrital component shows a steady and marked increase in compositional maturity with increasing sediment age immature sands of arkosic to litharenitic composition show a progressive loss of lithic fragments feldspars and accessory minerals with a concomitant enrichment in quartz this process produces first cycle sands whose detrital component is of quartz arenite composition among the salts quartz is enriched over time relative to more nstable phases particularly plagioclase this enrichment is probably largely caused by the destruction of plagioclase through chemical weathering but silt size quartz may also be produced through the breakdown of sedimentary and metasedimentary lithic fragments in the sand fraction in the clay fraction an increase in the relative abundance of smectite accompanies the breakdown of volcanic lithic fragments in the sand fraction and probably reflects the degradation of volcanic glass the clay fractions of the oldest samples examined contain less smectite somewhat more kaolinite and markedly more vermiculite as compared to younger samples but an extensively leached cation poor clay fraction analogous to the quartz dominated silts and sands is never attained the compositional mat rity of the clay fraction is everywhere lower than that of associated silts and sands perhaps because of a continual influx into the clay fraction of immature cation rich clays derived from the physical breakdown of lithic fragments in coarser size fractions this effect is enhanced by the translocation of fines whereby younger less weathered fine material is juxtaposed with older more mature coarse material in the lower levels of the alluvial stratigraphy reworking of floodplains during channel migration results in the reincorporation of older material stored on the floodplains into the sediment load if sediments have been intensely weathered during storage on the floodplain then the result will be a net dilution and replacement of younger relatively immature sands transported by the river by older more mature material from the floodplain the relative proportion of the older sands in the bedload will increase downstream resulting in a progressive downstream increase in the compositional maturity of the bedload of the river

erosion rates in the southern oregon coast range evidence for an equilibrium between hillslope erosion and sediment yield the relationship of hillslope erosion rates and sediment yield is often poorly defined because of short periods of measurement and inherent spatial and temporal variability in erosion processes in landscapes containing hillslopes crenulated by alternating topographic noses and hollows estimates of local hillslope erosion rates averaged over long time periods can be obtained by analysing colluvial deposits in the hollows hollows act as local traps for a portion of the colluvium transported down hillslopes and erosion rates can be calculated using the age and size of the deposits and the size of the contributing source area analysis of colluvial deposits in nine oregon coast range hollows has yielded average colluvial transport rates into the hollows of about 35 cm3cm 1yr 1 and average bedrock lowering rates of about 007 mm yr for the last 4000 to 15000 yr these rates are consistent with maximum bedrock exfoliation rates of about 0 09 mm yr calculated from six of the hollows supporting the interpretation that exfoliation rates limit erosion rates on these slopes sediment yield measurements from nine coast range streams provide similar basin wide denudation rates of between 0 05 and 0 08 mm yr 1 suggesting an approximate steady state between sediment production on hillslopes and sediment yield in addition modern sediment yields are similar in basins varying in size from 1 to 1500 km2 suggesting that erosion rates are spatially uniform and providing additional evidence for an approximate equilibrium in the landscape

paleogene postcompressional intermontane basin evolution along the frontal cordilleran fold and thrust belt of southwestern montana the paleogene renova formation is the earliest record of postcompressional sedimentation within and adjacent to the helena salient of the cordilleran fold and thrust belt in southwestern montana paleocurrent and compositional data from basin margin facies document radiating paleodispersal away from high relief 2 km highlands coincident with modern mountainous areas source rocks within the paleohighlands included the same archean metamorphic proterozoic paleozoic and mesozoic sedimentary and mesozoic plutonic and vo l canic rocks as exposed in modern uplifts paleocurrent and compositional data from trunk fluvial conglomerates and sandstones document the existence of an interbasinal drainage system that connected the three forks western gallatin and townsend basins with headwaters farther to the west and southwest near the present day montana idaho border overall the distribution of paleogene mountainous areas and basins closely resembled modern geography and the paleogene drainage network was strikingly similar to the modern missouri river headwater system the renova formation records the early stages of decay of the cordilleran orogenic belt including the evolution of a complex intermontane basin network in southwestern montana high energy late cretaceous to early eocene fluvial systems carved deep large scale paleovalleys into the orogenic wedge along zones of structural and stratigraphic weakness at least a 5 km thickness of overburden was removed during this time incision was temporally correlative with early cenozoic regional uplift and subtropical climatic conditions subsequent deposition of the renova formation was temporally correlative with the cessation of uplift the initiation of crustal extension and climatic cooling however extension is not interpreted to have played a major role in earliest basin development

the role of fluids in granulite facies metamorphism as deduced from oxygen and carbon isotopic compositions abstract based on stable isotope data of whole rock samples the main fluid rock exchange reactions in the high grade rocks of the crystalline basement of sri lanka is deduced to have occurred prior to granulite facies metamorphism the influx of external fluids during and after granulite facies conditions was only channellized and did not give rise to the formation of interconnected fluid films in rock volumes larger than several cm3 fluid present conditions were thus only manifest for relatively short periods of time much less than 106 y so that the conditions of granulite facies metamorphism of the sri lankan basement rocks in general can be characterized as fluid absent

multistage graphite precipitation through protracted fluid flow in sheared metagranitoid digana sri lanka evidence from stable isotopes abstract the digana metagranitoid situated within the digana shear zone a critical junction between the highland and wanni complexes of sri lanka hosts a variety of graphite types with structural evidence showing formation during different stages prior to and after deformational events affecting this terrain here we discuss their occurrence association and stable isotopes the carbon isotopes of graphite reported in this study from the sri lankan high grade metamorphic terrain differ significantly from the composition of graphite reported elsewhere in other precambrian terrains the stable isotope composition from random crystals depicts an enriched carbon isotopic composition in the range 1 6 to 8 suggesting precipitation by reduction from co2 rich fluids intracrystalline microscale traverses made on graphite crystals indicate the 13c composition of the rims becoming progressively lighter in 13c in metagranitoid hosted samples with the variation reaching up to 1 5 consistent with the changing fluid regimes induced by decreasing temperature during the graphite growth and shows the preservation of core at higher temperatures a highly enriched carbon isotopic composition 0 6 with a rimward enrichment of 13c observed in migmatites contrary to the values in metagranitoid indicates preferential enrichment of heavy isotopes as a result of the partial melting and dissolution reprecipitation of graphite the oxygen isotopic composition of a quartz calcite vein show values in the range 18 5 to 20 typical for recycled crustal source the tight constraint in the isotopic data from different graphite types studied indicates a homogeneous fluid source for the deposits prolonged fluid flow in this region is envisaged in this study which is related to the multistage deformation of the crustal scale shear zone

ancient hillslope deposits missing links in the study of climate controls on sedimentation rare outcrops of miocene pliocene buried hillslopes and colluvium present in proximal basin fill exposures in the arid southwestern united states in settings free of tectonic influences reveal hillslope sediment production and delivery to piedmonts that vary as a function of climate change a petrologic weathering index for sand clay mineral data and stratigraphic relations indicates that episodes of greater production and delivery of colluvium from hillslopes coincide with more intense chemical weathering and thus greater effective moisture strata recording limited colluviation less intense weathering and piedmonts dominated by eolian processes very likely correspond to drier conditions the mode of climate driven hillslope and piedmont landscape responses evident in these older stratigraphic records corroborates the conceptual model developed by quaternary researchers for arid environments even though climate change was operating at higher amplitudes and different frequencies in the quaternary this evidence of long term continuity in the mode of landscape response to climate change provides a new level of support for using the geologic record to understand the effects of future climate change

microscale isotopic zoning in calcite and graphite crystals in marble chemical and isotopic zoning in minerals is a clear indication of disequilibrium and provides an important key for elucidating metamorphic processes here i present carbon and oxygen isotope data from calcite and graphite crystals in a marble from the hida metamorphic belt central japan showing isotopic zoning within individual crystals within 100 m of the calcite graphite grain boundary the 13c contents gradually decrease from the constant values of the crystal core to values 1 lower 18o contents in calcite decrease by up to 7 within 200 300 jim of the boundary at a calcite calcite grain boundary similar patterns of 13c and 18o depletions were found whereas the 13c contents of graphites remained constant except in a narrow zone at the calcite contact these isotopic patterns are interpreted as reflecting isotopic exchange with metamorphic fluids that were relatively depleted in 13c and 18o these results strongly suggest that the most accurate geothermometry will be obtained if only the cores of crystals are used and that isotopic zoning may provide another way to elucidate polymetamorphic processes

glacial interglacial variation in denudation rates from interior texas usa established with cosmogenic nuclides abstract the brazos colorado and trinity rivers of texas drain a tectonically quiescent non glaciated and low relief landscape inland from the gulf of mexico where long term 10 3 10 5 a changes in denudation rates are probably driven largely by climate change here we use cosmogenic 10be to obtain spatially averaged denudation rates for these river catchments primarily from terrace deposits associated with glacial or interglacial intervals over the past half million years the denudation rates are 30 35 higher during interglacial periods than during glacial periods and correlate broadly with temperature the results are consistent with predictions from the bqart sediment flux model and support the hypothesis that increased weathering rates associated with warmer climates will accelerate landscape erosion furthermore by analyzing 26al 10be in these deposits we can estimate the bed load sourced from up catchment surfaces the stored coastal plain fraction varies from 10 to 30 and is greater during times of relatively lower sea level the results indicate that although sediment flux is moderated by coastal plain storage increased up catchment flux during warmer interglacial periods outpaces evacuation of stored sediment during glacial periods resulting in a net increase in sediment flux to the ocean during warm intervals if this temperature sediment flux relationship is valid beyond the plio pleistocene transition then global sediment flux to the ocean from passive non glaciated and low relief landscapes would have been greater during the pliocene than in the cooler quaternary

paleoproterozoic granulites from the xinghe graphite mine north china craton geology zircon u pb geochronology and implications for the timing of deformation mineralization and metamorphism abstract the xinghe graphite deposit is located in the north central part of the north china craton ncc where archean gray gneiss metasedimentary rocks khondalites and mafic high pressure granulites constitute the dominant rock types in this study we report results of detailed geological and structural mapping around the xinghe graphite mine we identify three stages of deformation d 1 d 3 and the graphite mineralization is correlated with d 1 or between d 1 and d 2 the first stage of deformation represented by rootless intrafolial folds whose axial surface cuts across the regional penetrative gneissosity s 2 of d 2 was constrained at ca 1 95 1 90 ga by u pb age of zircon grains from a mafic high pressure granulite the second stage of deformation is represented by tight folds with formation of regional penetrative foliation and strong ductile shearing when garnet broke down to plagioclase pyroxene this stage is dated as ca 1 85 ga from u pb analysis of zircons in leucosome from a migmatitic gneiss the third stage of deformation is represented by large scale folding which probably formed between 1 85 and 1 80 ga as constrained from the ca 1 80 ga emplacement age of an undeformed pegmatite that cuts across the host metasediments prior to the high pressure granulite metamorphism voluminous ca 2 15 2 0 ga granites were emplaced within the khondalites the graphitization postdates the emplacement of these granites as deduced from the presence of flaky graphites in the metagranites the protolith of the mafic high pressure granulite dike that intruded the khondalite at ca 2 0 ga was also metamorphosed at 1 95 1 90 ga and is part of the exploited graphite ore body we therefore constrain the timing of graphite mineralization to be between ca 1 95 and 1 85 ga our geological and geochronological data indicate that all the rock units in the xinghe graphite mine were subjected to high pressure granulite facies metamorphism at ca 1 95 1 90 ga

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

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

excess argon in melt inclusions in zero age anorthoclase feldspar from mt erebus antarctica as revealed by the method abstract historically erupted 1984 anorthoclase phenocrysts from mt erebus yield k ar and 40 ar 40 ar apparent ages as old as 700 ka indicating the presence of excess argon 40 ar 39 ar furnace step heating results from anorthoclase reveal a positive correlation between the cl k ratio and apparent age because chlorine up to 1700 ppm is present in melt inclusions but not in the anorthoclase crystal lattice this correlation suggests that the excess argon is associated with melt inclusions trapped within the anorthoclase during rapid crystal growth confirmation of the source of excess argon comes from step heating experiments on multiple anorthoclase aliquots separated from two phenocrysts and one glass aliquot prepared from the matrix of a volcanic bomb the anorthoclase phenocrysts were crushed and hf etched to yield aliquots containing 30 10 and 1 melt inclusions the step heated anorthoclase with 30 and 10 melt inclusions yielded the highest cl k ratios and apparent integrated ages of 640 30 ka and 179 16 ka respectively the anorthoclase with i melt inclusions yielded significantly lower cl k ratios and apparent integrated ages of 48 8 ka and 50 30 ka the step heated volcanic glass yielded the least variable cl k ratios and a total gas age of 101 16 ka argon released from the anorthoclase and the trapped melt inclusions can be distinguished by differences in their degassing behavior allowing geologically more reasonable ages to be obtained melt inclusions exposed on the exterior of anorthoclase grains principally degas during furnace extraction at temperatures less than 1200 c inclusions entirely within anorthoclase grains principally degas at temperatures greater than 1200 c when incongruent melting of the anorthoclase allows melt inclusion hosted excess argon to escape anorthoclase aliquots prepared with less than 1 inclusions can be fitted with a plateau for heating steps below 1200 c to yield ages as young as 8 2 ka whereas steps above 1200 c yield ages in excess of 100 ka however anorthoclase aliquots containing 10 30 melt inclusions yield ages in excess of 200 ka for heating steps below 1200 c minimizing the effects of the excess argon from melt inclusions relies on sample preparation and step heating fine crushing and treatment with hydrofluoric acid removes many of the larger melt inclusions small melt inclusions which remain within the anorthoclase degas primarily at temperatures above 1200 c temperatures below 1200 c yield the most accurate ages attempts at post analytically correcting for the chlorine correlated excess argon are hindered by the variations in 40 ar e cl within and between samples elevated 40 ar e cl ratios in bubbles within the melt inclusions as deduced from in vacuo crushing experiments are the most likely cause for some or all of a sample s total 40 ar e cl variation in addition relative solubilities of argon and chlorine within phonolitic melts may be partly responsible for variations in 40 ar e cl

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

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

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

u th he geochronology of single zircon grains of known tertiary eruption age we performed a systematic study of u th he ages of volcanic zircons of known eruption age to establish a reliable protocol for zircon he dating and to validate the resulting ages using laser he extraction and dissolution by lithium borate flux melting we analyzed many aliquots of single zircon grains separated from four rapidly cooled rocks fish canyon tuff accepted age 27 8 0 7 ma buluk tuff 16 3 0 2 ma tardree rhyolite 58 4 0 7 ma and utaosa rhyolite 2 52 0 02 ma he ages of fish canyon tuff buluk tuff and utaosa rhyolite zircons are in good agreement with their reference ages mean emission corrected ages were 28 6 1 4 1 16 1 0 8 and 2 61 0 18 ma respectively however the zircon he ages of tardree rhyolite are irreproducible and consistently too old with a mean he age of 78 8 7 0 ma spontaneous fission track densities of tardree rhyolite zircons indicate that about 80 of these zircons are strongly zoned in u with high track density i e high u content in the core and low density on the rim this causes an overestimation of the ejection correction that likely explains the erratic and excessively high he ages from these zircons assessment of the degree and style of u zonation within a zircon population is thus prudent before application of the u th he method step heating experiments indicate that for most zircons he extraction is 99 complete after 1 h at 1300 c however for as yet unknown reasons occasional grains seem to retain substantial amounts of he under these conditions

eccentricity forcing of pliocene early pleistocene climate revealed in a marine oxygen isotope record milankovitch theory that climate is controlled by variations in the earth s orbital parameters has gained wide acceptance for its ability to account for two climate cycles a 23 kyr cycle that is phase locked to the precession driven insolation cycle and a 41 kyr cycle that is phase locked to the obliquity driven insolation cycle1 6 but explaining the observed 100 kyr climate cycle in terms of milankovitch theory especially for the late pleistocene ice age cycle remains controversial in spite of a strong correlation with the 100 kyr cycle in the earth s orbital eccentricity5 one problem is that eccentricity affects insolation mainly by modulating the precession cycle its direct contribution to radiation change is too small 0 1 to effect the observed climate change directly5 7 another is the absence of a late pleistocene ice volume cycle in oxygen isotope records to match the 404 kyr component of the eccentricity cycle5 8 here we examine an oxygen isotope record spanning the interval 1 2 to 5 2 million years ago before the late pleistocene ice age regime we find 404 kyr and 100 kyr climate cycles which are coherent with eccentricity and which have amplitudes that are similar to the coexisting 23 kyr cycle analysis of these low frequency cycles suggests that they originate through an asymmetrical response mechanism that preferentially introduces variance into the climate system from the warmer portions of the eccentricity modulated precession cycle our data thus support eccentricity s role in the origin of low frequency oxygen isotope cycles before the late pleistocene ice age

primary igneous graphite in ultramafic xenoliths ii isotopic composition of the carbonaceous phases present in xenoliths and host lava at tissemt eggere alegerian sahara

relationship between structure morphology and carbon isotopic composition of graphite in marbles implications for calcite graphite carbon isotope thermometry carbon isotope exchange between calcite and graphite is a useful and reliable geothermometer for medium to high grade marbles however in rare instances such as at naxos greece apparent disequilibrium carbon isotope fractionation between calcite and graphite has been previously reported in this study new results are presented on the morphological features x ray diffraction studies raman spectroscopic studies and carbon isotope studies of graphite three morphologically distinct graphite types are identified the first type is fine grained with distinct polygonal crystal aggregates having smooth pinacoid faces the second type consists of large botryoidal aggregates with typical crystal overgrowth features and the third type is normal large platy crystals associated with phlogopite graphite morphological features using an fe sem suggests that the botryoidal aggregates are composed of complexly intergrown and stacked graphite layers comprised of cone helix structures different types of graphite display distinct lc 002 and dg values as well as a sharp first order raman peak at 1580 cm 1 with disordered bands the presence of raman disordered bands in fine grained well crystallized graphite is attributed to edge effects carbon isotope analysis of graphite reveals that marble with fine grained well crystallized graphite show a consistent isotope fractionation with the calcite which perfectly match temperature estimates based on the mineral isograds in contrast coarse botryoidal graphite gives heterogeneous carbon isotope values about a few per mill and show anomalous carbon isotope fractionation between the calcite and graphite the graphite associated with phlogopite also shows variations in the carbon isotope values combining morphologic crystallographic and carbon isotopic data we conclude that the variations in carbon isotopic composition were caused by the overgrowth of graphite on the preexisting grain and that morphological observations and structural characterizations of graphite crystals is a key for predicting isotopic equilibration during metamorphism

ion microprobe analysis of 231pa 235u and an appraisal of protactinium partitioning in igneous zircon ion microprobe u pa measurements in zircon crystals from a holocene rhyolite salton buttes california are utilized to assess the partitioning of pa and u between zircon and melt the relative sensitivity factor rsf for ion microprobe u pa zircon analysis was calibrated on a natural high u secular equilibrium zircon from buff peak nevada zircon crystals from salton buttes rhyolite show evidence for excess 231 pa with a weighted average 231 pa 235 u activity ratio of 1 9 from this ratio model zircon melt partition coefficient ratios d pa d u 0 9 2 2 are obtained assuming plausible limits for 231 pa 235 u melt and correcting for the age of zircon crystallization 15 ka these values roughly fit lattice strain models for tetravalent cations but are more than one order of magnitude lower than predictions for pentavalent pa based on this appraisal of pa partitioning initial 231 pa disequilibrium caused by zircon melt fractionation alone can produce minor discordance or excess 207 pb in late cenozoic zircon but is insignificant for older u pb zircon ages

deformation of a high grade gondwana fragment sri lanka abstract the sri lankan gondwana fragment is a high grade gneiss terrain consisting of at least three crustal units namely the wanni complex wc the highland complex hc and the vijayan complex vc which were juxtaposed during the pan african collision at least six phases of ductile deformation were recognized in the smallest gondwana fragment sri lanka there is microscopic and mesoscopic evidence for a first deformation d 1 which occurred during prograde metamorphism but before the main granulite facies metamorphism this first deformation is represented by straight to curved and crenulated inclusion trails in garnet porphyroblasts microfolds in some metapelites and quartzo feldspathic gneisses and by a s 1 foliation preserved in crenulation folds and some rootless intrafolial folds d 2 has two main stages of which the second phase is the most intense and dominant deformation which produced most part of the major composite and transposed compositional layering as well as the prominent stretching lineation l 2 the first stage of d 2 may have been responsible for the formation of a crenulation cleavage of s 1 as seen in garnet porphyroblasts in some metapelites the compositional layering s 2 in metapelites and metasemipelites appears to have formed through the development of a crenulation cleavage f 2 folds are mesoscopic isoclinal rootless and occur as intrafolial folds and their limbs are highly attenuated major part of the deformation during the latter stage of d 2 was non coaxial many minor and large scale recumbent isoclinal folds f 3 were produced during d 3 both d 2 and d 3 were coeval with the peak conditions of granulite facies metamorphism d 4 produced very large gentle nearly e w upright folds probably during the beginning of uplift of the entire high grade assemblage from deeper crustal levels still under granulite facies conditions d 5 is the second strongest deformation and was responsible for the large scale upright folds f 5 which control the present map pattern of sri lanka the superimposition of d 5 folds on d 4 folds gave rise to a large scale type i fold interference pattern of ramsay 1967 d 6 deformation caused local refolding of the f 5 folds and was accompanied by amphibolite facies conditions during uplift the deformation represented by the majority of vein type structures such as patchy charnockite veins cordierite bearing veins and graphite veins clearly post date d 5 it is argued that the wc the hc and the vc in sri lanka may be three distinctly different proterozoic crustal units brought into contact by tectonism in late proterozoic at two different stages

dating young basalt eruptions by u th he on xenolithic zircons accurate ages for young e g pleistocene volcanic eruptions are important for geomorphic tectonic climatic and hazard studies existing techniques can be time consuming and expensive when many ages are needed and in the case of k ar and 40ar 39ar dating extraneous ar often can limit precision especially for continental basalts erupted through old lithosphere we present a new technique for dating young basaltic eruptions by u th he dating of zircons zhe from crustal xenoliths single crystal zhe dates generally have lower precision than typical 40ar 39ar dates but can be determined relatively easily on multiple replicate grain aliquots we dated zircons from xenoliths from four volcanic centers in western north america little bear mountain british columbia 157 3 5 2 2 ka weighted 95 confidence interval ci mean square of weighted deviates mswd 1 7 and prindle volcano alaska 176 16 8 9 ka mswd 13 in the northern cordilleran volcanic province and fish springs 273 23 8 6 mswd 43 and oak creek 179 8 1 4 5 ka mswd 12 in the big pine volcanic field california all zhe ages are either equivalent to or younger than previously determined k ar or 40ar 39ar ages indicating the possibility of inherited 40ar in some of the previous measurements zircons from upper crustal xenoliths in the oak creek and fish springs vents show poorer reproducibility and multiple apparent age distribution peaks consistent with either intracrystalline u th zonation or 99 99 he degassing assuming ca 100 ma pre entrainment zhe ages of some zircons during magmatic entrainment removal of clear outliers in the older age distribution peaks of the upper crustal xenoliths most of which have extremely high u compared to other zircons of the same xenolith improve the reproducibilities of fish springs to 4 7 95 ci mswd 4 8 and oak creek to 3 4 95 ci mswd 6 2 coupled thermal and he diffusion modeling using appropriate xenolith sizes and magma temperatures and assuming published diffusion kinetics for zircon indicate that incomplete he degassing would require entrainment times 1 h however the observation of extremely high u in most zircons with older ages raises the possibility that zircons with high radiation dosages may have more retentive he diffusion characteristics

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

the origin of graphite of sri lanka abstract a model for the origin of graphite occurrences of sri lanka is discussed in the light of recent views on the geologic history of sri lanka the three modes of graphite occurrences in sri lanka are i as disseminated flakes in the country rocks ii as lenses pods and vugs and iii as veins occupying fractures in rocks the source of carbon of all three types is shown to be organic matter from microorganisms now known to have lived in precambrian sedimentary basins the highland series of sri lanka in which graphite is found in abundance is considered to be a precambrian sedimentary basin filled by sandstones limestones and shales the continuous filling of the basin followed by deformation resulted in intense folding accompanied by metamorphism during this process the hitherto disseminated carbonaceous matter was mobilized in a semi fluid state with the aid of magmatic fluids known to have pervaded the sediments during the intense folding the vein graphite deposits were formed in the axial regions of the anticline while the graphite not pressed into pre existing faults and fractures remained in the disseminated form

petrologic phase equilibria and stable isotope fractionations of carbonate silicate parageneses from granulite grade rocks of sri lanka abstract the calc silicate rocks and marbles of central and southern sri lanka can be characterized by three groups of parageneses cc dol phl fo spl di acc in the northern and eastern part of the highland complex di scp spn cc acc following to the west and wo di scp spn cc acc in the most southwestern part this distribution mirrors the compositional variation of the sedimentary precursors and the regional pressure gradient during prograde metamorphism decarbonation with internal fluid buffering increased xco2 to 0 75 graphite precipitation at near peak metamorphic conditions at constant oxygen fugacity reduced xco2 to 0 5 the variation of 18o 26 to 9 and 13c 4 to 5 of carbonates can largely be explained by primary differences in the isotopic composition and in the silicate carbonate contents of the protoliths the particular light isotopic compositions are due to fractionation processes during decarbonation of the rocks the dolomite free scapolite bearing rocks in general are isotopically lighter than dolomite bearing parageneses calcite graphite temperatures in the range of 750 900 c were calculated using the experimentally derived thermometer of scheele 1991 calcite silicate temperatures 150 900 c were calculated with the increment method hoffbauer et al 1994 these temperatures reflect near peak equilibrium as well as different degrees of retrograde isotope resetting

metamorphism of reduced granulites in low co 2 vapour free environment pervasive flooding of co2 has been proposed as the cause of granulite facies metamorphism that is capable of producing many distinctive characteristics of the deep continental crust reduced water activity orthopyroxene bearing assemblages depletion of large ion lithophile lil elements and dehydration1 6 the calculations presented here of c o h fluid composition for conditions of granulite facies metamorphism show that oxygen fugacity f o2 estimates from many terranes are sufficiently low that the addition of co2 rich fluid causes graphite to precipitate for values of pressure p and temperature t common to granulites and with f o2 slightly below the quartz fayalite magnetite buffer qfm the addition of co2 sufficient to grow 10 vol orthopyroxene requires the precipitation of 1 5 vol graphite as 0 1 vol graphite is readily recognizable but is not reported in most low f o2 granulites these rocks have not been flooded by co2 and low f h2o is probably due to extraction of a magma or recrystallization of an already dry rock7 8 granulite terranes may thus result from a combination of these three processes and the dominance of any one cannot now be demonstrated on a regional basis

the geology of damavand volcano alborz mountains northern iran damavand volcano located in northern iran is a large 400 km 3 composite cone that is currently dormant it shows fumarolic activity near the summit but no evidence of eruption in the past 1000 yr the volcano represents an isolated focus of magmatism of uncertain tectonic affinity although geophysical and geochemical constraints point toward a local hotspot plume origin possibly associated with lithospheric delamination rather than any association with subduction new u th he and 40 ar 39 ar geochronological constraints indicate that the present cone young damavand has been constructed over 600 k y on an older eroded edifice of indistinguishable composition younger than 1 8 ma damavand activity has been characterized by the eruption of radially directed trachyandesite lava flows almost exclusively from summit vents limited pyroclastic activity has yielded thin fallout pumice lapilli layers and a few pyroclastic flows only one significant pyroclastic event is recognized in the remnants of a welded ignimbrite ponded and preserved along the haraz river drainage relatively short periods of volcanic eruptive activity were interspersed with longer periods of erosion in which volcanic products were transported particularly as hyperconcentrated flows into the surrounding drainage systems to be further reworked into epiclastic deposits occasional catastrophic events punctuated this interplay between volcanism and erosion at least one sector collapse is signified by the presence of a large debris avalanche deposit and the regional drainage systems appear to have been frequently dammed by incursions of volcanic material

vein graphite deposits geological settings origin and economic significance graphite deposits result from the metamorphism of sedimentary rocks rich in carbonaceous matter or from precipitation from carbon bearing fluids or melts the latter process forms vein deposits which are structurally controlled and usually occur in granulites or igneous rocks the origin of carbon the mechanisms of transport and the factors controlling graphite deposition are discussed in relation to their geological settings carbon in granulite hosted graphite veins derives from sublithospheric sources or from decarbonation reactions of carbonate bearing lithologies and it is transported mainly in co2 rich fluids from which it can precipitate graphite precipitation can occur by cooling water removal by retrograde hydration reactions or reduction when the co2 rich fluid passes through relatively low fo2 rocks in igneous settings carbon is derived from assimilation of crustal materials rich in organic matter which causes immiscibility and the formation of carbon rich fluids or melts carbon in these igneous hosted deposits is transported as co2 and or ch4 and eventually precipitates as graphite by cooling and or by hydration reactions affecting the host rock independently of the geological setting vein graphite is characterized by its high purity and crystallinity which are required for applications in advanced technologies in addition recent discovery of highly crystalline graphite precipitation from carbon bearing fluids at moderate temperatures in vein deposits might provide an alternative method for the manufacture of synthetic graphite suitable for these new applications

pleistocene geomorphology and geochronology of eastern grand canyon linkages of landscape components during climate changes we report new mapping soils survey and geochronologic luminescence u series and cosmogenic nuclide data from pleistocene deposits in the arid setting of eastern grand canyon the result is a stratigraphic framework of inset fill gravels and associated terraces that provide a record of the responses of hillslopes tributary streams and the colorado river to the last 400 kyr of glacial interglacial climate change the best preserved last 80 kyr of this record indicates a stratigraphic chronologic disconnect between both deposition and incision along the colorado river versus along the trunks of local tributaries for example the colorado river finished aggrading and had already begun incising before the main pulse of aggradation in the trunks of local catchments during marine isotope stage 3 and then tributary incision followed during the millennial scale fluctuations of the last glacial epoch potentially concurrent with mainstem aggradation the mainstem record appears to broadly correlate with regional paleoclimate and upstream geomorphic records and thus may be responding to climatic hydrologic changes in its mountain headwaters with aggradation beginning during full glacial times and continuing into subsequent interglacials the contrasting lag time in responses of the dryland catchments within grand canyon may be largely a function of the weathering limited nature of hillslope sediment supply r 2005 elsevier ltd all rights reserved

40ar 39ar dating into the historical realm calibration against pliny the younger laser incremental heating of sanidine from the pumice deposited by the plinian eruption of vesuvius in 79 a d yielded a40ar 39ar isochron age of 1925 94 years ago close agreement with the gregorian calendar based age of 1918 years ago demonstrates that the 40ar 39ar method can be reliably extended into the temporal range of recorded history excess 40ar is present in the sanidine in concentrations that would cause significant errors if ignored in dating holocene samples

late quaternary weathering erosion and deposition in nahal yael israel an impact of climatic change on an arid watershed in their seminal paper in 1979 bull and schick proposed a conceptual model for the geomorphic response to pleistocene to holocene climate change based on the hyperarid nahal yael watershed in the southern negev desert in this model the change from semiarid late pleistocene to hyperarid early holocene climates reduced vegetation cover increased the yield of sediment from slopes and accelerated aggradation of terraces and alluvial fans the model is now over 30 yr old and during this time chronologic paleoenvironmental and hydrogeomorphic research has advanced here we reevaluate the model using data acquired in nahal yael over the 30yr since the original model was proposed recent studies indicate that the late pleistocene climate was hyperarid and a transition from semiarid to hyperarid climates did not occur the revised chronology reveals a major 35 20 ka episode of accelerated late pleistocene sediment production on slopes with lower rates probably already at ca 50ka due to increased frequency of wetting drying cycles caused by frequent extreme storms and floods between 35 and 27 ka without lag time these sediments were transported and aggraded in depositional landscape components fluvial terraces and alluvial fans this intensified sediment production and delivery phase is unrelated to the pleistocene holocene transition the depositional landforms were rapidly incised between 20 and 18 ka since and or soon after this last glacial maximum lgm incision most material leaving the basin originated from sediments stored in depositional landforms and was not produced from bedrock using these new data we propose a revision to the bull and schick model in this hyperarid environment our revision suggests that the model should include the frequent storms and floods responsible for a late pleistocene pulse of intense weathering due to numerous cycles of wetting and drying on slopes and coeval sediment transport to fluvial terraces and alluvial fans we also discuss the common use and pitfalls of using the bull and schick conceptual model to explain observations in diverse arid environments usually without sufficient data on basin specific stratigraphic chronologic paleoenvironmental and paleoclimatic information

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

mineralogical characterization of graphite deposits from thodupuzha kanjirappally belt madurai granulite block southern india abstract graphite from deposits occurring in the high grade metamorphic rocks and their larteritized equivalents of the thodupuzha kanjirappally belt in madurai granulite block southern india is structurally fully ordered crystallite size lc 002 ranging from 469 to 749 a possess high degree of graphitization dg value ranging from 105 to 267 a and reflect crystallization at high temperature 700 100 c raman spectra of graphite display profiles corresponding to high crystallinity and high structural ordering the high temperature crystallinity characteristics of graphite were not obliterated during retrogression of granulites to amphibolite facies gneisses preliminary carbon stable isotope results show a spread in isotope values from 11 8 to 26 8 which suggest more than one sources for carbon the lighter carbon isotope values are suggestive of biogenic origin whereas the heavier ones are probably fluid precipitated graphite

carbon isotopes as biogeochemical recorders of life over 3 8 ga of earth history evolution of a concept ever since pioneering studies in the late 1930s had shown that the conversion of inorganic carbon into biogenic substances entails sizeable redistributions of the stable carbon isotopes biologically mediated 13c 12c fractionations have come to be recognized as a common corollary of biochemical reactions meanwhile it is firmly established that the universal bias in favour of 12c characterizing biological materials primarily derives from a kinetic isotope effect that is imposed on the first carbon fixing enzymatic carboxylation reaction in the primary metabolism of co2 fixing autotrophic organisms this preference for 12c has turned out to be one of the most enduring relics of the ordered state of the biological precursor substances that may be preserved in fossil organics over billions of years with the currently known sedimentary record at hand it can be stated with confidence that biological carbon isotope fractionations have persisted throughout 3 8 ga of recorded earth history indicating that microbial prokaryotic and archaeoprokaryotic ecosystems had been prolific already on the archean earth while for the time span 3 5 ga the isotopic evidence is unequivocal the information encoded in the preceding record is commonly blurred by a metamorphic overprint this holds particularly for the metasediments of the 3 8 ga old isua supracrustal belt of west greenland which apart from widespread metasomatism have suffered amphibolite grade metamorphism it is known that 13c 12c exchange occurs in organic kerogenous rock constituents during both amphibolite and granulite facies metamorphism if a second carbon partner is available as either fluids or carbonate with isotopic re equilibration often only partially achieved due to the sluggish kinetics of the exchange reaction thermodynamic equilibria predict however that 13c 12c ratios in kerogen and graphite increase during this process hence high t exchange equilibria are always bound to drive 13c values in positive direction the lowermost values encountered being consequently the least exchanged and most pristine with the lowest values of reduced graphitic carbon obtained in early isua studies falling into the range 22 to 28 pdb we had straightforward evidence since the late 1970s that carbon constituents with the isotopic composition of biogenic matter were indeed present in the pre metamorphic isua suite it was therefore by no means surprising that the results of recent isotope work performed on apatite hosted carbonaceous microdomains in isua banded iron formation utilizing advanced techniques of instrumental microanalysis had prompted similar conclusions hence the mainstream of the sedimentary carbon isotope record can be best interpreted as the geochemical manifestation of the isotope discriminating properties of the principal co2 fixing reactions s in biological carbon assimilation suggesting an extreme degree of evolutionary conservatism in the biochemistry of autotrophic carbon fixation as a consequence biological modulation of the geochemical carbon cycle had been established at least 3 8 ga ago having been fully operative by the time of formation of the earth s oldest sediments

obsidian hydration dating of volcanic events abstract obsidian hydration dating of volcanic events had been compared with ages of the same events determined by the 14c and k ar methods at several localities the localities ranging in age from 1200 to over 1 million yr include newberry craters oregon coso hot springs california salton sea california yellowstone national park wyoming and mineral range utah in most cases the agreement is quite good a number of factors including volcanic glass composition and exposuretemperature history must be known in order to relate hydration thickness to age the effect of composition can be determined from chemical analysis or the refractive index of the glass exposure temperature history requires a number of considerations enumerated in this paper