classification of huminite iccp system 1994 abstract in the new classification iccp system 1994 the maceral group huminite has been revised from the previous classification iccp 1971 int handbook coal petr suppl to 2nd ed to accommodate the nomenclature to changes in the other maceral groups especially the changes in the vitrinite classification iccp 1998 the new vitrinite classification iccp system 1994 fuel 77 349 358 the vitrinite and huminite systems have been correlated so that down to the level of sub maceral groups the two systems can be used in parallel at the level of macerals and for finer classifications the analyst now has according to the nature of the coal and the purpose of the analysis a choice of using either of the two classification systems for huminite and vitrinite this is in accordance with the new iso coal classification that covers low rank coals as well and allows for the simultaneous use of the huminite and vitrinite nomenclature for low rank coals

zircon xenocrysts from the kambalda volcanics age constraints and direct evidence for older continental crust below the kambalda norseman greenstones abstract the hangingwall basalt at kambalda western australia contains zircons that have been shown by ion microprobe analyses to have very high u and th contents and a wide variety of crystallization ages nearly all of these zircons certainly are xenocrysts a few might relate to intrusive veinlets the age of the youngest xenocrysts 2693 50ma 2 shows that the eruptive age of the basalt cannot exceed 2743 ma this confirms that the apparent sm nd isochron giving 3200 ma 1 2 for kambalda mafic and ultramafic rocks is a mixing line 2 between unrelated components enriched and depleted in light rare earth elements mixing probably occurred at depth by erosion of 3200 3500 ma old felsic crust from the walls of the hwb conduits the zircon xenocryst ages are the first direct evidence for the presence of very old felsic crust in the eastern yilgarn block the latter implies that the kalgoorlie norseman greenstone sequences were formed in a continental rather than an oceanic environment

an extensive crustally derived 3325 to 3310 ma silicic volcanoplutonic suite in the eastern pilbara craton evidence from the kelly belt mcphee dome and corunna downs batholith abstract new sensitive high resolution ion microprobe zircon u pb dating indicates that the eastern pilbara craton contains an extensive suite of variably deformed 3325 to 3310 ma rhyolites high level quartz feldspar porphyries and granitoids these range in composition from na rich trondhjemite low k rhyolite to k rich monzogranite with silica rich 68 biotite hornblende bearing porphyritic granodiorites and monzogranites most abundant initial volcanism accompanied regional extension at 3325 ma and was followed by intrusion of high level porphyritic stocks and emplacement of massive to foliated batholithic granitoids at 3315 ma the compositions of this volcano plutonic suite suggest that the most likely source for parent magmas is older continental crust with tonalite trondhjemite granodiorite suite compositions crustally derived silicic volcanic rocks and granitoids have traditionally been considered rare in pre 3 0 ga granitoid greenstone terranes evidence from the eastern pilbara indicates that magmatic recycling of continental crust has occurred at least as early as 3 3 ga

the procedure used to develop a coal char classification commission iii combustion working group of the international committee for coal and organic petrology abstract this paper describes an assessment of char classification system by the combustion working group in commission iii of the international committee for coal and organic petrology iccp the work of the group culminated in the production of a char atlas after a final round robin exercise this round robin involved 21 analysts and was an electronic exercise using digitally captured images of individual char particles rather than actual char blocks a software program featuring 170 char images was specifically designed to allow operators to identify each char based on a classification system with 9 individual char types tenuisphere crassisphere tenuinetwork crassinetwork mixed porous mixed dense fusinoid solid and mineroid the program electronically recorded all decisions as well as the time taken for each decision to be made from 170 chars 128 chars were identified by a majority 70 and these chars were then compiled in a char atlas that is now available for download www nottingham ac uk eczehl charatlas as would be expected all analysts appeared to become more confident in identifying chars during the exercise taking less time per image but with no clear evidence of improvement without exception analysts took longer to make an incorrect decision

silurian orogenesis in the western penobscot bay region maine new 40ar 39ar mineral ages from rocks collected west of penobscot bay maine indicate this region was regionally deformed metamorphosed to amphibolite facies conditions and intruded by plutons in silurian times rather than in the devonian as previously assumed disturbed hornblende age spectra along with the presence of some devonian felsic plutons and extensive retrograde metamorphic textures do suggest however that these rocks were subsequently affected by low grade devonian thermal events in sharp contrast rocks west of the sennebec pond thrust fault a major tectono stratigraphic boundary in this region lack a significant silurian tectono thermal signature and instead record the effects of intense devonian deformation and high grade regional metamorphism the data suggest the two regions experienced very different pre devonian histories and were most likely juxtaposed by the sennebec pond thrust fault in latest silurian to early devonian time rocks now exposed east of the sennebec pond faul

polydiapirism of the archean mount edgar batholith pilbara block western australia structural analysis of the mount edgar batholith and the enveloping greenstone belt indicates that the batholith and rocks around its margin were subjected to similar deformation events that involved progressive and stepwise diapiric uplift of the batholith these events may be correlated on the basis of fold style orientation and fabric development early recumbent to reclined structures d1 including thrusts were buckled d2 by rounded upright folds prior to shearing d4 along normal faults parallel with the batholith margin d4 deformation was responsible for uplifting the batholith to its present structural position a deep seated linear deformation zone d3 within the batholith did not affect the supracrustal belt syntectonic granite intrusion amphibolite facies metamorphism and migmatisation were associated with d1 in the batholith whereas greenschist facies metamorphism was associated with d1 in the lowermost unit of the greenstone belt the talga talga subgroup amphibolite facies conditions were maintained within the batholith throughout the structural history but this grade was reached in the supracrustal belt only near the batholith margin progressive deformation d1 d2 and metamorphism occurred in response to early massive granite generation and emplacement producing a first order plutonic dome that reached full maturity during d2 solid state diapirism d4 followed producing a gneiss dome of similar dimension restriction of early recumbent folds and thrusts in the supracrustal belt to areas within or adjacent to the batholith is consistent with the model elsewhere deformation is minor the late stage diapirism brought a gneiss complex within the batholith into juxtaposition with lowermost greenschist facies rocks of the pilbara supergroup this complex represents in part sialic material that existed below the pilbara supergroup before diapirism began the complex closely resembles other archean high grade gneiss terranes which suggests that some of these terranes are deeper crustal exposures of granite greenstone terranes

oldest known terrestrial anorthosite at mount narryer western australia abstract fragments of a deformed and metamorphosed layered anorthosite gabbro ultramafic intrusion called the manfred complex occur as inclusions in 3650 and 3400 ma old banded granite gneisses at mount narryer in the western gneiss terrain of the yilgarn block the complex is typical of a world wide group of archaean anorthosites characterized by very large primary grain sizes of equidimensional calcic plagioclase this is the first recorded occurrence of these anorthosites in australia pb pb and sm nd whole rock and u pb zircon geochronology given in the accompanying two papers fletcher i r rosman k j r and libby w g 1988 precambrian res 38 343 354 kinny p d williams i s froude d o ireland t r and compston w 1988 precambrian res 38 325 341 shows that the manfred complex is about 3730 ma old it is therefore the oldest known terrestrial anorthosite complex and one of the oldest known rock units on earth

sm nd age of kambalda and kanowna greenstones and heterogeneity in the archaean mantle a precise sm nd age of 2 790 30 myr and an initial ratio corresponding to nd 3 4 0 6 has been determined for the kambalda sequence from the eastern yilgarn block of western australia the high initial 143nd 144nd implies a long lived light rare earth element depleted high sm nd source for the kambalda volcanics and therefore provides definitive evidence for ancient heterogeneities in the archaean mantle

controls on intra plate seismicity in southwestern australia abstract although the southwest seismic zone swsz located about 150 km to the east of perth in southwestern australia is one of the most seismically active areas in australia there is little understanding as to why the earthquakes are occurring an analysis of geophysical geological and geodetic data from the area suggests that the swsz coincides with a precambrian terrane boundary seismic data show that the terrane boundary zone dips at a shallow angle in a northeasterly direction reactivation of this zone of weakness in the contemporary stress field east west maximum horizontal stress is interpreted to be the first order control on seismicity in the region gravity data show that the terrane boundary is offset by near orthogonal structures which are interpreted as faults at least one of these trends corresponds with a linear zone of epicentres the temporal and spatial distributions of epicentres associated with the 1968 meckering earthquake ml 6 9 suggest that the second order distribution of seismicity in the swsz can be explained by the intersection model whereby stresses are amplified by space problems associated with displacements on crosscutting faults it is speculated that a zone of high density and high seismic velocity in the lower crust may also be a second order control on the local seismicity however confirmation awaits better delineation of the extent of this zone

the composition and evolution of the continental crust rare earth element evidence from sedimentary rocks the composition of the present day upper crust inferred from the uniformity of sedimentary rock r e e rare earth element patterns is close to that of granodiorite a revised andesite model is used to obtain total crustal composition the lower crust is the composition remaining assuming that the upper crust one third of the total is derived from intracrustal partial melting the upper crustal r e e pattern has pronounced eu depletion eu eu 0 64 the lower crustal pattern has eu enrichment eu eu 1 17 and the total crust has no eu anomaly relative to chondritic abundances the eu depletion in the upper crust is attributed to retention of eu in plagioclase in the lower crust because plagioclase is not stable below 40 km 10 kbar the anomaly is intracrustal in origin the archaean upper crust has a different r e e pattern to that of the present day upper crust being lower in total r e e and la yb ratios and lacking an eu anomaly these data are used to infer the archaean upper crustal composition which resembles that of the present day total crust except that ni and cr contents are higher the archaean crustal composition can be modelled by a mixture of tholeiites and tonalite trondhjemites the latter have steep light r e e enriched heavy r e e depleted patterns consistent with equilibration with garnet and hence probable mantle derivation there is little reason to suppose that the archaean lower crust was different in composition from the upper crust except locally where partial melting episodes occurred the r e e evidence is consistent with isotopic and geological evidence for a low continental growth rate in the early archaean a massive increase to about 70 of the total crust between about 3000 and 2500 ma b p and a slow increase until the present day the change from archaean to post archaean r e e patterns in the upper crust is not isochronous but is reflected in the sedimentary rock r e e patterns at differing times in different continents on the basis of a model composition for the mantle 36 of the potassium 30 of uranium 15 of lanthanum and 3 of ytterbium are concentrated in the present continental crust this enrichment is related to ionic size and valency differences from common mantle cations e g mg fe pre 3 9 ga b p crusts were obliterated by meteorite bombardment no geochemical evidence exists for primordial anorthositic sialic or mafic crusts

geochemistry of a silicified felsic volcaniclastic suite from the early archaean panorama formation pilbara block western australia an evaluation of depositional and post depositional processes with special emphasis on the rare earth elements abstract many elements in a suite of pervasively silicified dacitic volcaniclastic and volcanic rocks from the early archaean panorama formation warrawoona group western australia were strongly depleted or enriched relative to those in equivalent unaltered units during widespread post depositional metasomatism the present compositions of the altered rocks are dominated by al 2 o 3 k 2 o and sio 2 and lie on or close to a quartz sericite mixing line the al contents were probably inherited from the original dacitic rocks while si some k and possibly rb may have been introduced during metasomatism the elements mg ca na and to a lesser extent fe mn and sr have been depleted relative to al during alteration whereas silica has been greatly enriched the high and variable ba concentrations suggest that ba like si was introduced during silicification the ree and th were relatively immobile during alteration and were diluted during silicification in the same proportion as al most of the rocks represent detrital sand and silt sized felsic tephra debris eroded from felsic volcanic rocks or ash fall deposits that underwent a short interval of transport and deposition dominated by physical rather than chemical processes a few siltstones contain clay minerals that probably formed by the chemical weathering of warrawoona group basalts the siltstones have lower th and la concentrations lower la lu ratios and higher sc and cr concentrations than felsic rocks in the suite regional alteration pervasively silicified the original sediments shortly after deposition the replacement mineral assemblage indicates a maximum temperature for hydrothermal alteration of about 350 c because of the lack of chemical weathering and the low temperature of post depositional alteration ree abundances in volcaniclastic rocks of the panorama formation reflect their felsic volcanic provenance and are interpreted to have preserved a relict magmatic signature the lack of a negative eu anomaly and high la lu ratio chondrite normalized ratio is 7 to 39 suggest that magmas of the panorama formation formed by direct melting of an eclogite residue without subsequent crystallization of feldspar during the commencement of cratonization of the pilbara block this origin differs from the origin of some other silicic volcanic rocks of the duffer formation representing earlier felsic volcanic activity of the warrawoona group that were produced by fractional crystallization from basaltic parents this suggests a polygenetic origin for felsic magmas of the warrawoona group

geochemistry and origin of archaean quartz cordierite gneisses from the godthabsfjord region west greenland we report new data on the major minor and trace element compositions of metasedimentary quartzcordierite gneisses qcg an important member of the archaean malene supracrustal suite found throughout the godthabsfjord region of west greenland the analyzed qcg contain assemblages of quartz cordierite biotite garnet anthophyllite gedrite staurolite sillimanite plagioclase with abundant accessory zircon and minor rutile monazite and allanite and broadly resemble cordierite orthoamphibole rocks found in a great number of other metamorphic terrains chemically the qcg are characterized by 1 high but variable sio2 59 87 wt relative enrichments in mgo feo and al2o3 mg 0 35 0 85 and depletions in na2o k2o and especially cao 2 low concentrations of sc cr co ni and sr 3 high concentrations of y nb zr hf ta th u and ree rare earth elements with prominent negative eu anomalies in each case 5 high concentrations of ga 18 55 ppm with variable ga al ratios that are significantly higher than average crustal material low cr and ni together with enriched and fractionated ree displaying negative eu anomalies distinguish the malene qcg from published accounts of most other archaean sedimentary rocks furthermore all of the above mentioned trace element characteristics distinguish the qcg from ordinary malene clastic metasediments quartzites psammites and pelites suggesting a separate origin for the qcg these data point towards chemically evolved felsic igneous rocks being the source of the qcg consequently we propose that the malene qcg represent metamorphosed felsic volcaniclastic sediments that underwent hydrothermal alteration by heated seawater prior to metamorphism which resulted in gain of mg and fe loss of alkalis and lime and possibly eu and sr the overall trace element characteristics of the qcg elevated ga zr nb ree etc are features shared by a type granites and their volcanic equivalents such igneous rocks may represent the ultimate source material for the qcg protolith

archaean calc alkaline volcanism in the pilbara block western australia abstract archaean calc alkaline volcanics occur in two distinct settings in the pilbara block within the ca 3500 ma old warrawoona group sequences of intermediate to acid volcanics are interlayered with an extensive sequence of komatiites and tholeiitic basalts in the western pilbara the whim creek group a younger sequence of intermediate to acid volcanics is associated with terrigenous sediments and minor mafic lavas both suites have typical calc alkaline mineralogy and chemical compositions the major and trace element chemistry of the most mafic lavas basalts and basaltic andesites is consistent with derivation by partial melting of the mantle and a spectrum of fractional crystallization trends involving the observed phenocryst phases appears to relate more evolved members to parent magmas the fractionation processes involved in the evolution of the calc alkaline suites operated at higher fluid pressures than those involved in the development of associated komatiitic and tholeiitic lava sequences the setting of volcanism in the pilbara has a number of broad similarities and some differences to modern volcanically active continental margins in which growth of new continental crust appears to be dominated by calc alkaline plutonism with calc alkaline volcanism playing a subordinate role however more detailed information is required before direct comparisons with modern environments can be made with confidence or rejected in favour of non uniformitarian models for archaean crustal evolution

the age and origin of younger granitic plutons of the shaw batholith in the archaean pilbara block western australia the whole rock pb pb method has been used to date four of the younger mainly adamellite late tectonic plutonic phases within the ca 3 5 ga shaw batholith of the archaean east pilbara block three suites give ages within error of 2966 ma porphyritic granites at 2948 50 ma leuco adamellites at 2943 46 ma and garden creek adamellite at 3007 48 ma the post tectonic cooglegong adamellite gives an age of 2847 34 ma the sm nd model isotopic systematics of all four suites indicate derivation from crust ranging between ca 3200 and 3600 ma in age the sources for these four younger plutonic phases were heterogeneous and although exhibiting some isotopic characteristics of the older 3 5 3 3 ga calc alkali plutonic suites were more depleted in the lil elements rb u and th in addition the garden creek adamellite and the cooglegong adamellite lack the very fractionated and hree depleted ree patterns characteristic of both the older calc alkali plutonic rocks and the porphyritic granites and leuco adamellites the crust underlying the shaw batholith at ca 2950 ma must have been both markedly heterogeneous and variably depleted a conclusion consistent with the complex tectonic and plutonic evolution of this region

structural setting and geochemistry of archaean i type granites in the bardoc coolgardie area of the norseman wiluna belt western australia abstract regional structures in the bardoc coolgardie area of the archaean norseman wiluna greenstone belt in western australia developed during early large scale thrusting followed by a transpressional regime characterized by enewsw shortening and large scale sinistral strike slip faults structural relationships between granites and greenstone successions indicate emplacement of granitic rocks throughout this deformation history emplacement mechanisms include solid or liquid state diapirism forceful dilation of country rocks and passive stoping i type granites are common and include early and late tectonic intrusions the widespread occurrence of i type granodiorite and monzogranite probably derived by partial melting of lower continental crust or subducted oceanic crust suggests a continental margin setting for the belt alkalic quartz diorite occurs as rounded enclaves in granodiorite and is enriched in ba rb sr ce la p and f which suggests a mantle derived volatile component these quartz diorite melts may have formed by partial melting of metasomatized mantle or alternatively mantle degassing may have lowered solidus temperatures in the source region and thus induced melting emplacement of hot volatilerich quartz diorite melts into the lower crust may have lowered solidus temperatures in the source rocks for granodiorite and monzogranite recent models for archaean gold mineralization suggest that auriferous fluids contained co 2 derived from lower crustal or mantle sources in the bardoc coolgardie area co 2 may have been flushed out during partial melting of the lower continental crust however the origin of the gold in the mineralizing fluids requires some other explanation because gold mineralization is generally a late stage event and took place over a considerably shorter time interval than the formation and emplacement of i type granites

silurian u pb zircon intrusive ages for the red river anorthosite northern cape breton island implications for the laurentia avalonia boundary in atlantic canada abstract current interpretations of the geology of cape breton island suggest that it exposes a complete cross section of the appalachians from laurentia across iapetan vestiges to avalonia crucial to this view is the presence of ca 1 ga plutons including anorthosites which have been regarded as correlatives of grenvillian basement a correlation that overlooks the fact that avalonia is also underlain by a ca 1 ga basement we analyzed zircons from the red river anorthosite blair river complex northwestern cape breton island previously dated as ca 1 1 ga they yielded 421 3 ma intrusive ages with older ages between 865 18 ma and 1044 20 ma inferred to be either xenocrysts derived from the country rock or from the source implications of these data suggest that the accompanying low pressure granulite amphibolite facies metamorphism of the blair river complex is either the root of a 440 410 ma magmatic belt produced during slab break off or relict ca 1 ga basement the blair river complex occurs in a nne ssw sinistral positive flower structure that progresses upwards from a neoproterozoic rifted arc through a low grade upper ordovician silurian overstep sequence to amphibolite facies fault slices capped by the low pressure granulite facies rocks blair river complex the correlation of neoproterozoic rifted arc units across most of cape breton island suggests it represents the deformed northwestern margin of avalonia intruded by a silurian lower devonian magmatic belt as the geological record in the blair river complex is similar to both grenvillian and avalonian basements its provenance is equivocal however pb isotopic data suggest the blair river complex has amazonian avalonia affinities thus cape breton island rather than representing a complete cross section of the appalachian orogen is part of pristine deformed avalonia with a positive flower structure exposing a cross section of avalonian crust

constraints on the age of the warrawoona group eastern pilbara block western australia abstract zircons from mafic and felsic volcanic rocks in the type area of the warrawoona group the basal archaean greenstone succession of the eastern pilbara block have been dated precisely using the ion microprobe shrimp the results allow two alternative time frames for the duration of the warrawoona group dependent on how the dated zircons are considered to relate to the volcanic rocks our favoured interpretation requires a hiatus of 135 5 ma between the duffer formation at 3 46 ga and the overlying felsic volcanic rocks of the wyman formation and a hydrothermal or later magmatic origin for zircons of age 3 33 ga within one duffer formation sample and the underlying metabasalts the alternative time frame requires a short time for deposition of the entire group less than 15 ma at 3 33 ga and a xenocrystic origin for the 3 46 ga zircons of the duffer formation outside the type area of the warrawoona group the age of an intrusive granodiorite requires that greenstones be older than 3 43 ga and the group formed over an interval of 120 ma visibly different zircons within one of the duffer formation samples were found to be palaeozoic in age and presumably constitute hydrothermal growth of new zircon within the rock at low temperature similar zircons were found within samples from other rock units but with a spread of proterozoic ages

u pb geochronological constraints on the evolution of the aspy terrane cape breton island implications for relationships between aspy and bras d or terranes and ganderia in the canadian appalachians new u pb zircon ages from nine samples of igneous and sedimentary rocks in the aspy terrane cape breton island show that neoproterozoic rocks form a major part of the terrane and confirm that the terrane was affected by a major silurian devonian tectonothermal event a rhyolitic crystal tuff a leucotonalite pluton and a felsic sheet yield ages of 618 8 0 6 ma 619 7 0 9 ma and 573 5 2 7 ma respectively a metasedimentary unit contains zircon ranging in age from ca 546 to ca 1520 ma a diorite has an age of 428 6 1 9 ma and another diorite unit and a quartz porphyry also have probable late silurian ages a third diorite yields an age of 373 0 0 5 ma part of a widespread bimodal igneous event represented by both volcanic and plutonic rocks throughout the aspy region geochronological results and field relationships indicate that the neoproterozoic rocks are similar to those in the bras d9or terrane and form the basement to the ordovician silurian rocks in the aspy terrane and that the aspy terrane is probably correlative with rocks in the hermitage flexure of southern newfoundland where silurian metasedimentary and metavolcanic rocks lie unconformably on similar neoproterozoic rocks likely correlative rocks also occur in the kingston terrane of southern new brunswick the results of this study support the idea that the aspy kingston terrane is part of ganderia and that neoproterozoic rocks in the bras d9or terrane and correlative rocks in southwestern newfoundland and elsewhere represent exposed basement to ganderia in the canadian appalachians

origin of granite greenstone patterns yilgarn block western australia the yilgarn block of western australia consists mainly of archean granite greenstone terrain with oval areas of granitoid rocks fringed by arcuate greenstone belts in a typical part of this terrain the outcrop pattern is shown to result from large scale dome and basin fold interference structures rather than diapiric emplacement of steep sided batholiths the greenstones and underlying granitoid rocks were initially deformed together in a subhorizontal tectonic regime as a result the granitoid rocks recrystallized and a gneissosity developed subparallel to the stratigraphy of the greenstones thick subhorizontal monzogranite sheets were later emplaced into the lower part of the greenstone sequence subsequent deformation generated two successive sets of folds with vertical axial surfaces at a high angle to each other these folds formed dome and basin interference structures ranging from a few centimetres to more than 50 km in diameter

sm nd isotopic systematics as tectonic tracers an example from west avalonia in the canadian appalachians abstract the tectonothermal evolution of west avalonia in mainland nova scotia appalachian orogen provides an excellent example of the application of sm nd isotopic analyses to tectonic studies regional syntheses indicate that west avalonia originated in the neoproterozoic as one of a number of terranes collectively known as peri gondwanan terranes along the gondwanan margin these terranes are characterized by late neoproterozoic voluminous arc related magmatism west avalonia became detached from gondwana in the early ordovician was accreted to laurentia by the silurian and was deformed by post accretionary strike slip tectonics in the late devonian and carboniferous associated with the amalgamation of pangea all stages of this evolution can be recognized in the sm nd isotopic signature in various neoproterozoic and paleozoic volcanic and sedimentary successions in mainland nova scotia crustally derived felsic volcanic rocks of late neoproterozoic ca 615 ma early cambrian and early silurian ages define an envelope that provides gross constraints on the sm nd isotopic composition of the underlying basement source e nd values range from 0 78 to 2 92 in the neoproterozoic rocks from 2 0 to 5 0 in the cambrian rocks and from 0 1 to 2 5 in the silurian rocks t dm model ages for each of these suites are remarkably similar they typically range from 0 9 to 1 2 ga suggesting that the sm nd isotopic composition of these suites was profoundly influenced by repeated melting of the same basement source this sm nd envelope provides a reference by which other suites may be compared and interpreted for example turbidites in a late neoproterozoic volcanic arc basin have sm nd compositions that lie within this envelope suggesting that they are derived from the coeval volcanic rocks that flank the basin the genetic significance of the model ages can be investigated by the analysis of felsic and metasedimentary rocks that pre date the main arc phase two felsic complexes have compositions that lie within the envelope and have similar model ages the presence of this signature in early avalonian rocks suggests that the model ages represent a genuine tectonothermal event at ca 1 0 to 1 2 ga that formed juvenile proto avalonian crust by implication the main arc phase in avalonia recycled this crust sm nd isotopic analyses of the metasedimentary sequences are incompatible with previous interpretations of deposition in a passive margin setting that pre dated avalonian arc activity instead the data indicate derivation from two distinct sources ancient gondwanan basement and juvenile avalonian arc suggesting deposition in a western pacific type back arc environment this re interpretation has important implications for neoproterozoic reconstructions the sm nd isotopic composition of west avalonian basement contrasts with that of other peri gondwanan terranes for example similarly aged felsic rocks in the cadomian terrane of nw france are characterized by more negative e nd values and older t dm model ages 1 0 1 9 ga these data together with detrital u pb data suggest that west avalonia was developed along the amazonian margin whereas cadomia was developed along the west african margin of gondwana sm nd isotopic analyses of early silurian clastic rocks in mainland nova scotia show fundamental differences with unconformably underlying neoproterozoic and early paleozoic avalonian successions these data have e nd values ranging from 5 7 to 6 1 with t dm model ages of about 1 4 to 1 7 ga indicating that the clastic rocks cannot have been derived from underlying successions a variety of geologic data and regional constraints suggests that they may have been derived from the caledonide orogen of western europe thereby providing a minimum age for the accretion of west avalonia to laurentia baltica sm nd isotopic analyses from late devonian early carboniferous continental clastic rocks deposited along the southern flank of avalonia in mainland nova scotia also show little influence from avalonian rocks and are compatible with derivation from the meguma terrane immediately to the south these data are consistent with regional synthesis which indicate uplift of the meguma terrane relative to west avalonia during dextral transpression

late devonian carboniferous detachment faulting and extensional tectonics in western cape breton island nova scotia canada abstract shallow dipping detachment faults have played a key role in the evolution of the late devonian to carboniferous maritimes basin of western cape breton island the faults are responsible for large scale post orogenic crustal thinning and denudation of mid crustal levels tournaisian and westphalian clastic sedimentation can be related to detachment faulting along the margaree shear zone msz and the ainslie detachment ad respectively the msz consists of thick shallow dipping retrogressive mylonite overprinted by cataclastic horizons brittle detachment faults and chloritic breccia the shear zone crosscuts and transports low grade late devonian bimodal volcanic rocks and redbeds of the fisset brook formation in its hanging wall towards the west and juxtaposes them against exhumed medium to high grade metamorphic rocks and basement in its footwall locally the mylonite is 200 700 m thick and outcrops along strike from north to south for approximately 60 km tournaisian coarse clastic units and debris flow deposits of the horton group unconformably overlie ductile mylonite of the msz due to incision of brittle faults into the msz regional transgression and marine flooding of the extensional complex and exhumed units are marked by the thick visean carbonate and evaporite deposits of the windsor group the carbonates reflect a relatively quiescent tectonic regime and widespread subsidence was likely due to the cessation of extension related thermally induced buoyancy in a tectonically thinned crust the ainslie detachment nucleated as a regional flat lying fault along the carbonate evaporite contact at the top of the macumber formation forming calc mylonite near the base of the windsor group significant stratigraphic omissions in the windsor group above the detachment provide clear evidence of an extensional regime during faulting clastic deposits of the westphalian cumberland group are bounded by the detachment and sedimentation is related to listric normal faults which cut down section towards the west southwest the documentation of detachment faults has widespread implications for the interpretation of the post orogenic evolution of the northern appalachians an extensional regime can now be recognized deep reflection seismic data are interpreted in light of the new findings and can be related to late devonian carboniferous basin formation

mantle convection and early crustal evolution abstract the existence of peridotitic komatiites in the archaean suggests that the archaean mantle was significantly hotter than the modern mantle this evidence is contradicted by estimates of archaean continental geothermal gradients based on the pressure and temperature recorded in metamorphic rocks which suggest that there is no marked difference between archaean and modern continental geothermal gradients numerical modelling shows that small changes in the mantle temperature can have an important influence on convection if the average temperature of the upper mantle is increased by 200 c convection within the mantle becomes chaotic and an upper mantle partial melt zone encircles the globe the crust formed during this period will be komatiitic in composition but will be unstable and will be mixed back into the mantle by subduction later when the mantle temperature falls to 100 c above its present level the upper mantle partial melt zone contracts away from subduction areas it is suggested that the first primitive felsic magmas were generated at subduction zones the appearance of these magmas at 3 8 ga permitted the formation of buoyant continents and eventually led to crustal thickening as a consequence of this thickening the proto continents consisting of a bimodal suite of basalts and sodic granodiorites contained two types of latent energy 1 radioactive energy held in elements such as th k and u and 2 potential energy resulting from the elevation of the continents above sea level the potential energy of the continents led to sedimentation the increase in the rate of sedimentation during the archaean resulted from increased crustal buoyancy at the same time heat released by radioactive elements in the deep crust built up under the insulating blanket of the upper crust this caused a major metamorphic metasomatic and crustal melting event which produced the potassic granites of the late archaean once the radioactive elements had been removed from the lower crust that region of the continent become tectonically stable the proterozoic shelf sediments were deposited at the margins of these stable cratons convection models of the archaean mantle show hot diapirs rising from the boundary layer above the core mantle interface we suggest that these diapirs began to melt at a depth of 450 km giving rise to komatiitic magmas this model requires the average temperature of the archaean upper mantle to be 100 c above that of the modern mantle the similarity between archaean and modern continental geothermal gradients can be explained if archaean continents formed above subduction zones raising the temperature of the archaean mantle by 100 c 1 halves the thickness of the oceanic lithosphere 2 increases the oceanic geothermal gradient at the mid point of a convection cell 3 decreases the viscosity of the mantle by at least an order of magnitude the combination of these effects produces a marked decrease in the strength of the archaean lithosphere and mantle thus the form of archaean tectonics can be expected to have been very different from modern tectonics

deciphering the tectonic evolution of archaean greenstone belts the importance of contrasting histories to the distribution of mineralization in the yilgarn craton western australia abstract the distribution of archaean metallic mineral deposits reflects tectonic controls operating during the evolution of granitoid greenstone terranes tectonic analysis of these terranes not only allows recognition of parameters that may be important in interpreting the tectonic histories of greenstone belts but also the resultant tectonic control on mineral deposit distribution in the yilgarn craton of western australia the late archaean 2 7 ga norseman wiluna belt is most intensely mineralized with both komatiite associated ni cu deposits and epigenetic gold deposits this belt is dominated by 2 7 ga supracrustal successions and its tectonic history is that of an orogenic belt at an obliquely convergent craton or proto craton margin tectonostratigraphic associations preserved resemble younger volcanic arc and back arc basin crust and many represent the remains of a volcanic arc and back arc basin complex developed in response to westward dipping subduction these sequences were deformed metamorphosed and intruded by granitoids relatively soon 30 ma after the major period of volcanism with gold mineralization occurring late in the orogenic event possibly during uplift in contrast greenstone belts in the murchison and southern cross provinces are dominated by older supra crustal successions that contain 2 9 ga volcanosedimentary associations characterized by extensive oxide facies banded iron formations bifs bifs appear to have been deposited in shelf or platform environments in intra oceanic or passive craton margin settings komatiite associated ni cu mineralization and volcanogenic cu zn mineralization are locally hosted by favourable tectonostratigraphic associations the main period of deformation metamorphism and gold mineralization in these provinces was broadly synchronous with that in the norseman wiluna belt as was regional granitoid emplacement during this event strain was probably transmitted from the eastern margin of the craton via a yilgarn wide system of shear zones faults and fold belts gold mineralization is developed best in greenstone belts such as the meekathara mt magnet belt that contain large scale fault systems and internal granitoids similar tectonic controls on the distribution of mineral deposits can also be recognized in other archaean cratons and the heterogeneous distribution of archaean mineral deposits probably reflects a range of tectonic settings as well as different stages in the evolution of individual cratons

comment on tectonic burial thrust emplacement and extensional exhumation of the cabot nappe in the appalachian hinterland of cape breton island canada by gregory lynch

synthesis of palaeozoic deformational events and terrane accretion in the canadian appalachians plate tectonic theory predicts that most deformation is associated with subduction and terrane accretion with some deformation associated with transform transcurrent movements deformation associated with subduction varies between two end members 1 where the tectonic regime is dominated by subduction of oceanic lithosphere containing small terranes a narrow surface zone of accretionary deformation along the subduction zone starts diachronously on the subducting plate at the trench as material is transferred from the subducting plate to the over riding plate and 2 where continent continent collision is occurring a wide surface zone of accretionary deformation starts synchronously or with limited diachronism palaeozoic deformational events in the canadian appalachians correspond to narrow diachronous events in the ordovician and silurian whereas devonian carboniferous and permian deformational events are widespread and broadly synchronous along the western side of the canadian appalachians the taconian deformational event starts diachronously throughout the ordovician and corresponds to the north north west accretion of the notre dame ascot weedon st victor and various ophiolitic massifs volcanic arc and peri arc terranes over cratonic north america within the eastern half of the central mobile belt the late cambrian early ordovician penobscotian deformational event corresponds to the south easterly accretion of the exploits subzone various volcanic are and peri arc terranes over the gander zone continental rise in the centre of the orogen the late ordovician silurian beothukan deformational event corresponds to the south easterly accretion of the notre dame over the exploits gander subzones along the south eastern side of the central mobile belt the silurian ganderian deformational event corresponds to the north north east sinistral transcurrent accretion of the avalon composite terrane microcontinent over the gander exploits zones along the south eastern half of the orogen the late silurian middle devonian acadian deformation event corresponds to the westerly accretion of the meguma terrane intradeep or continental rise over the avalon composite terrane affecting the entire orogen the late devonian carboniferous and permian acadian alleghanian deformational events correspond to the east west convergence between laurentia and gondwana continent continent collision

superposed neoproterozoic and silurian magmatic arcs in central cape breton island canada geochemical and geochronological constraints isotopic and geochemical data indicate that intrusions in the eastern creignish hills of central cape breton island canada represent the roots of arcs active at 540 585 ma and 440 ma times of intrusion are closely dated by 1 a nearly concordant u pb zircon age of 553 2 ma in diorites of the creignish hills pluton 2 a lower intercept u pb zircon age of 540 3 ma that is within analytical error of 40 ar 39 ar hornblende plateau isotope correlation ages of 545 and 550 7 ma in the river denys diorite and 3 an upper intercept u pb zircon age of 586 2 ma in the melford granitic stock on the other hand 441 455 ma 40 ar 39 ar muscovite plateau ages in the host rock adjacent to the skye mountain granite provide the best estimate of the time of intrusion and are consistent with the presence of granitic dykes cutting the skye mountain gabbro diorite previously dated at 438 2 ma all the intrusions are calc alkaline the skye mountain granite is peraluminous trace element abundances and nb and ti depletions of the intrusive rocks are characteristic of subduction related rocks the 540 585 ma intrusions form part of an extensive belt running across central cape breton island and represent the youngest neoproterozoic arc magmas in this part of avalonia nearby they are overlain by middle cambrian units containing rift related volcanic rocks which bracket the transition from convergence to extension between 540 and 505 520 ma this transition varies along the avalon arc 590 ma in southern new england 560 538 ma in southern new brunswick and 570 ma in eastern newfoundland the bi directional diachronism in this transition is attributed to northwestward subduction of two mid ocean ridges bordering an oceanic plate and the migration of two ridge trench transform triple points following complete subduction of the ridges remnant mantle upwelling along the subducted ridges produced uplift gravitational collapse and the high temperature low pressure metamorphism in the arc in both southern new brunswick and central cape breton island the 440 ma arc magmatism in the creignish hills extends through the cape breton highlands and into southern newfoundland and has recently been attributed to northwesterly subduction along the northern margin of the rheic ocean

u pb and 40ar 39ar mineral ages from cape north northern cape breton island implications for accretion of the avalon composite terrane isotopic studies have been used to determine protolith and tectonothermal ages in rocks of the cape north and money point groups and two plutons that form part of the avalon composite terrane colourless euhedral zircon crystals from low grade metarhyolite in the money point group record a concordant u pb age of 427 5 4 ma which is interpreted to date the time of extrusion this provides a maximum age of middle silurian for polyphase deformation and concomitant greenschist amphibolite facies metamorphism that affected the cape north area in the eastern half of a major sinistral positive flower structure monazite from the cape north granite intruded during peak metamorphic conditions yielded nearly concordant data with a 207pb 235u age of 414 3 ma this dates the time of crystallization of the cape north granite metamorphic hornblende from the cape north and money point groups records variably discordant 40ar 39ar age spectra that define plateau and or isotope correlation ages of ca 380 390

extensional structures during deposition of the 3460 ma warrawoona group in the eastern pilbara craton western australia abstract structures in the granite greenstone terrain of the eastern pilbara were mapped in both the coongan belt greenstones and shaw batholith granitoids important structures that are coeval with an early magmatic event in both the granitoids and the greenstones were identified in the coongan greenstones a brittle extensional fault array is syn kinematic with felsic volcanics of the duffer formation dated at 3452 16 ma pidgeon 1978a in the underlying shaw batholith a major ductile shear zone was mapped at the contact with the coongan belt although the foliation in the shear zone forms part of a domal structure the stretching lineations constantly plunge to the ene the sense of shear as recorded in the shear zone is consistently east up with subsidiary sinistral strike slip movement on the e w trending part and dextral movement on the n s trending part a wedge shaped granitoid intruded into the shear zone during deformation this granitoid is part of the north shaw suite which has been dated at 3467 6 ma mcnaughton et al 1988 the structures in the greenstones and in the batholith have therefore formed during the same event at different crustal levels the structural kinematic magmatic metamorphic and geochronological data are consistent with a model in which these structures represent the downdip part of a dome with a core complex type of geometry in turn this implies that the domal geometry of the shaw batholith was initiated at mid crustal levels at a very early stage namely during deposition of the warrawoona group in the evolution of the pilbara craton as extension occurred during intrusion and extrusion of large volumes of magma the original distribution and thickness of granitoid bodies and greenstones were probably largely determined by the extensional geometry the initial domal shape of the shaw batholith has been enhanced during later deformation phases which caused steepening of n s trending structures

a 3500 ma plutonic and volcanic calc alkaline province in the archaean east pilbara block variably foliated predominantly granodioritic plutonic rocks from the northern part of the shaw batholith in the east pilbara archaean craton are dated at 3 499 22 ma 2 errors by a whole rock pb pb isochron these rocks intrude the surrounding greenstone sequence and their age is indistinguishable from that sequence high strain grey gneisses which occupy much of the western and southern shaw batholith are chemically and isotopically similar to the north shaw suite and are inferred to have been derived from this suite by tectonic processes felsic volcanics within the greenstones together with a major portion of the granitic batholiths apparently formed in a calc alkaline volcanic and plutonic province at 3 500 ma this volcanic and plutonic suite is similar to modern calc alkaline suites on the basis of major element rare earh element and most other trace element contents the archaean suite contrasts with modern equivalents only in having lower concentrations of hree and higher concentrations of ni and cr

geological correlations between cape breton island and newfoundland northern appalachian orogen geological correlations between cape breton island and newfoundland are apparent both in surface geology and at deeper crustal levels based on similarities in sm nd isotopic signatures the mira terrane of southeastern cape breton island is part of the avalon terrane sensu stricto and is composed of neoproterozoic volcanic sedimentary plutonic belts and overlying cambrian rocks directly comparable to those in the western part of the newfoundland avalon terrane the bras d or terrane is also mainly of neoproterozoic age but shows lithological and isotopic contrasts with the mira terrane small areas of similar neoproterozoic rocks occur in southern newfoundland and to the north as inliers in the exploits terrane the bras d or terrane and similar rocks in newfoundland are interpreted to represent a peri gondwanan terrane where rocks of the gander terrane were later formed hence this area is part of the central mobile belt and distinct from avalon terrane sensu stricto the aspy terrane is a complex area

tectonic burial thrust emplacement and extensional exhumation of the cabot nappe in the appalachian hinterland of cape breton island canada silurian imbricate thrusting early devonian high grade metamorphic nappe emplacement and devonian carboniferous extensional denudation characterize deformation in the appalachian hinterland of cape breton island compressional deformation following early silurian arc volcanism features imbrication of cambrian precambrian basement rocks of gondwana derivation with ordovician silurian cover sequences across thick zones of mylonite during south directed transport high grade metamorphism and gneissic rocks of late silurian age in the region indicate that significant tectonic burial and crustal thickening occurred as a result of the thrusting partial denudation of the high grade assemblages occurred during early devonian thrust emplacement of the cabot nappe toward the northwest along the highlands shear zone the nappe is characterized by an amphibolitic gneiss and high grade schist complex defining a large folded klippe above silurian units kyanite is widespread within the nappe and a distinctive feature of the thrust sheet is the dynamothermal metamorphism of cooler greenschist grade footwall rocks producing inverted isograds staurolite is regionally distributed and occurs in pelitic units in the immediate footwall of the highlands shear zone forming a discontinuous halo around the klippe greenschist grade footwall rocks are exposed in structural windows as a result of folding and faulting shear sense indicators along the margins of the cabot nappe have been rotated into their present positions due to superposed folding providing apparent movement directions for the nappe complete exhumation to surface occurred during late devonian extension along the low angle margaree shear zone

sphysicochemical conditions of fluid wall rock interaction at amphibolite facies conditions in two archean hydrothermal gold deposits in the mt york district pilbara craton western australia synamphibolite facies archean gold mineralization in the mt york district pilbara craton western australia is hosted in metamorphosed banded iron formation main hill breccia hill prospect amphibolites and ultramafic schists zakanaka prospect mineralization at main hill occurs in quartz breccias with sulfide matrices and in altered wall rock adjacent to quartz biotite amphibole clinopyroxene veins alteration associated with quartz veins is zoned with biotite pyrrhotite vein selvedges and a distal calcic amphibole arsenopyrite lollingite zone hydrothermal biotite and actinolite have highest mg mg fe ratios where associated with abundant sulfarsenides in the distal alteratin zone whole rock geochemical analyses and calculated metasomatic reactions indicate the addition of k al s as au ag and ni during hydrothermal alteration mineralization at zakanaka is characterized by a broad wall rock alteration halo of biotite amphibole and zoned quartz calc silicate veins proximal to ore

sedimentology of archean greenstone belts signatures of tectonic evolution abstract stratigraphic styles in archean greenstone belts are compared to those of modern and phanerozoic depositional basins in order to test the conformity of tectonic style through time six lithological associations in greenstone belts are recognized 1 mafic ultramafic volcanic 2 calc alkaline volcanic 3 bimodal volcanic 4 quartz arenite iron formation or carbonate 5 conglomerate wacke and 6 conglomerate arenite examples of the associations are described from the kaapvaal superior and zimbabwe provinces and the pilbara block each association differs only slightly between the four regions thereby emphasizing a common tectonostratigraphic theme sedimentary rocks are a minor component of the mafic ultramafic volcanic association they were deposited in two types of volcanic basin environments one similar to barred lagoons and bays around oceanic volcanic islands and the other similar to sediment starved platforms adjacent to coalesced volcanoes in inter arc intra arc and back are basins in contrast sedimentary rocks in the calc alkaline volcanic association comprise thick wedges of epiclastic and volcaniclastic detritus deposited in elongate sedimentary basins and thick wedges of pyroclastic and volcaniclastic detritus deposited adjacent to volcanic centers sedimentary facies were identical to those of forearc trench and marine volcanoplutonic arc settings chemogenic lithofacies in both volcanic associations were deposited in shallow to deep marine waters although shallow water profiles were selectively preserved in some terrains sedimentary rocks of the bimodal volcanic association are thick wedges of siliciclastic and volcaniclastic deposits interbedded with subaerial to submarine erupted flows of komatiite basalt and rhyolite depositional environments ranged from braid plain and braid delta to coastal marine and submarine fan starved basin deposits include banded iron formation and sulfidic black shale the association is identical to that of cratonic extensional basins in arc continent and intracontinental rifts sedimentary rocks of the quartz arenite iron formation or carbonate association onlap weathered basement or bimodal volcanic association rocks depositional environments were either transitional from fluvial to siliciclastic and carbonate marginal marine to offshore pelagic hemipelagic or from inshore intrabasinal clastic and volcaniclastic to offshore pelagic hemipelagic siliciclastic deposits are supermature two tectonic sites are represented those with siliciclastic facies represent continent adjacent syn to post rift stable shelves and those dominated by banded iron formation with volcaniclastic deposits represent arc adjacent post rift stable shelves sedimentary rocks of the conglomerate wacke association are represented by thick wedges of siliciclastic or volcaniclastic sandstone with minor volcanic components these were deposited in braid plain braid delta and submarine fan environments within elongate sedimentary basins coastal plain and pelagic basin facies are also present siliciclastic detritus was derived from syndepositional magmatic and metasedimentary thrust belts two stratigraphic styles are preserved one that records a flysch like stage of deep to marginal marine environments and the other that records a molasse like stage of marginal marine to fluvial environments the association is identical to that of compressional foreland basins of arc continent collisional and compressional arc tectonic affinities the conglomerate arenite association is preserved in elongate structural basins that transgress crustal components and all other supracrustal units it is dominated by thick wedges of siliciclastic sandstone although bimodal volcanic and volcaniclastic rocks can also be present depositional environments ranged from talus and alluvial fan and braid plain to either lacustrine or coastal marine local basin margins often show lateral offsets from source terrains and vertically skewed facies patterns longitudinal facies geometries are typical the association is similar to that of strike slip collisional graben in hinterland tectonic escape and terrane accretion orogens the examples show that despite all the ramifications of secular geophysical geothermal and geochemical global change the stratigraphic style of sedimentary rocks in archean greenstone belts can be matched with modern tectonic analogues thereby emphasizing the conformity of stratigraphic style through time the inseparable link between stratigraphic and tectonic styles implies that the tectonic style of greenstone belts was not temporally unique major differences exist with regards to the chemical composition of certain magmatic and sedimentary products but these are compensated for by their constant stratigraphic function in terms of depositional processes depositional environments and tectonic sites

early archaean gneisses from the yilgarn block western australia the yilgarn block is an ancient crustal block which extends over 650 000km2 in the south west of western australia and consists of high grade gneiss and granite greenstone terrains1 being one of the largest segments of archaean crust in the world it is important to studies of early crustal evolution we report here a geochronological study of banded gneisses near mount narryer in the north west of the yilgarn block which has given a rb sr whole rock isochron age of 3 348 43 myr with an initial 87sr 86sr ratio of 0 7037 0 0005 the data indicate that the rocks had a prior crustal residence time of 200 myr the validity of this early archaean age is supported by model sm nd ages of 3 510 myr and 3 630 myr for two of the samples taken together these ages represent the oldest evolutionary sequence so far identified in the yilgarn block and are comparable with old ages from other archaean cratons elsewhere in the world

detrital zircon geochronology of a conglomerate in the northeastern cape breton highlands implications for the relationships between terranes in cape breton island the canadian appalachians cape breton island has been interpreted as consisting of four zones of pre carboniferous rocks but the relationships among them are controversial to help resolve the controversy we have dated detrital zircons from a conglomerate part of the cheticamp lake gneiss in the aspy terrane in the northeastern cape breton highlands using the u pb method the following ages were obtained 462 2 ma middle ordovician 492 488 ma 6 ages early ordovician 552 3 ma latest precambrian early cambrian 620 13 and 687 4 ma cadomian and 809 17 1423 10 1462 12 1605 14 1644 4 and 1911 5 ma proterozoic the middle ordovician age sets a maximum age limit for deposition of the conglomerate and supports an ordovician silurian age for the cheticamp lake gneiss the early ordovician latest precambrian early cambrian and cadomian ages match published ages from the bras d or terrane and its correlatives and the mira terrane and its correlatives and indicate provenance of the conglomer

petrographic characteristics of lignite gasification chars abstract the article presents the petrographic characteristics of lignite subjected to fluidized bed gasification and the residues after the process the gasifying agents were air oxygen and carbon dioxide the petrographic composition of the lignite is dominated by huminite group with 81 7 vol share the inertinite group is 3 6 vol while the liptinite group amounts to 7 0 vol the mineral matter in the tested samples is 7 7 vol the gasification temperature was in the range of 850 950 c therefore the study focused mainly on the pyrolysis of coal the dominant particles in the residues after the gasification process include inertoid 35 5 vol and crassinetwork 31 6 vol the total amount of char with different kinds of pores arising as a result of degassing tenuisphere crassisphere tenuinetwork crassinetwork and mixed porous is 52 3 vol of all components the macerals from inertinite group do not change form during the gasification process the particles with thicker walls crassinetwork and intertoids represent approximately 70 vol are dominant such char composition is related to the chemical structure of coal during gasification process the macerals from liptinite group such as resinite release carboxyl groups increased porosity and swelling a similar phenomenon is observed in the case of textinite macerals of low porosity mainly ulminite do not substantially increase their porosity and take the inertoid form mineroids represent about 8 vol of char but the mineral matter is also associated with coal particles taking into account the efficiency of gasification process it should be noted that only a small percentage

archean komatiites and geotherms solution to an apparent contradiction the high liquidus temperatures of archean komatiites and the low geothermal gradients in archean continents appear to give contradictory indications of temperatures in the archean mantle we resolve this contradiction in terms of an idealized model of convection in the archean mantle we suggest that temperatures in the archean mantle were no more than 100 c higher than at present that komatiites originate from the hot lower boundary layer of a non steady convection cell and that archean continents were formed near subduction zones where near surface temperature gradients were lower than elsewhere in the convecting mantle

upb zircon geochronology of archaean felsic units in the marble bar region pilbara craton western australia abstract archaean supracrustal rocks that are well exposed in the marble bar region pilbara craton have been assigned to the warrawoona group and younger sequences the predominantly volcanic warrawoona group previously dated at 3300 to 3500 ma is largely basaltic with locally intercalated thick felsic volcanic units the supracrustal rocks have been folded and are distributed around the margins of large roughly circular to ovoid granitic gneissic batholiths a u pb zircon geochronology study was undertaken to obtain precise age constraints for some of the ore deposits in the area especially the big stubby north pole barite and miralga creek zn pb cu au deposits in support of efforts to improve archaean lead isotope models the results also help significantly in interpreting stratigraphic relationships and crustal evolution in the area the new conventional zircon u pb data indicate that most of the warrawoona group from the duffer formation upwards was deposited over a period of 20 ma from 3470 ma to 3450 ma the age of the duffer formation is closely constrained by results of 3471 5 ma and 3465 3 ma and the age of the upper salgash subgroup appears to be established at 3454 1 ma the panorama formation at the southern margin of the north pole dome is 3458 1 9 ma unless the duffer formation was deposited over a period of at least 10 ma this casts doubt on a previously suggested panorama duffer correlation the age of the lower part of the north pole succession is greater than 3458 ma consistent with the correlation of the basal greenstones at north pole with the talga talga subgroup the north pole chert barite unit which is well known for its preservation of early archean stromatolites microfossils and evaporite sediments is clearly older than 3458 ma the 3325 ma age now established for the wyman formation shows that this unit must be excluded from the warrawoona group the age of the talga talga subgroup remains uncertain a felsic schist in the mount ada basalt which has been dated at 3449 3 ma is interpreted to be a sill probably related to the 3450 ma granitoid bodies an age 3724 ma for a zircon xenocryst in the panorama formation is the oldest obtained so far for zircon from the pilbara granite greenstone terrane and indicates the existence of pre pilbara supergroup sialic basement ages of 3465 3 ma and 3449 2 ma applied to previous pb isotope data for the big stubby and miralga creek deposits respectively in combination with comparable data for other syngenetic archaean sulphide deposits yield a general single stage pb evolution model with the parameters t0 4560 ma a0 9 0818 b0 9 9002 and c0 29 343 this gives model ages in accord with known constraints for many archean deposits a model lead age of 3490 ma for the north pole deposit which is older than 3457 ma may be too great but attempts to directly date this deposit have not yet succeeded there are striking parallels between the warrawoona group ages reported here and those recently obtained for the barberton region kaapvaal craton south africa the talga talga subgroup and lower onverwacht tjakastad subgroup sequences have approximately equivalent constraints and an age for the upper salgash subgroup agrees closely with those for the hoogenoeg formation of the upper onverwacht group geluk subgroup

reply to comment on tectonic burial thrust emplacement and extensional exhumation of the cabot nappe in the appalachian hinterland of cape breton island canada

40 ar 39 ar amphibole ages from the kingston complex new brunswick evidence for silurian devonian tectonothermal activity and implications for the accretion of the avalon composite terrane the kingston complex of southern new brunswick canada is a regional ne trending belt of mafic and felsic dikes bordered by dextral ductile shear zones the complex separates two contrasting late precambrian early paleozoic tectonostratigraphic assemblages near the nw margin of the avalon composite terrane rhyolitic dikes within the complex record an early silurian crystallization age dike orientations suggest that emplacement of the complex accompanied sinistral movement interpreted to reflect accretion of the avalon composite terrane to cratonic north america mafic dikes within the complex display greenschist to epidote amphibolite facies metamorphic assemblages that developed during dextral transpression amphibole concentrates from seven metamorphosed mafic dikes record late silurian to early devonian ca 416 390 ma 36 ar 40 ar vs 39 ar 40 ar isotope correlation ages interpreted to date closely the dextral transpressive deformation together with other recent age data the 40 a

late devonian palinspastic reconstruction of the avalon meguma terrane boundary implications for terrane accretion and basin development in the appalachian orogen abstract a late devonian early carboniferous palinspastic reconstruction of atlantic canada restoring subsequent motion along the avalon meguma terrane boundary and other major faults facilitates re examination of the spatial distribution of important accretionary and post accretionary events in the northern appalachian orogen the reconstruction suggests a genetic relationship between 415 and 360 ma tectonothermal activity in both these terranes and the collision between laurentia and gondwana in a zone of anomalously intense shortening opposite the st lawrence promontory in the appalachian orogen erosional products of this collision are an order of magnitude greater than the thin ca 1 km veneer of latest devonian early carboniferous clastic rocks that were deposited in intermontane basins across the orogen this missing detritus may be found in the thick deposits of the catskill clastic wedge in the central appalachians

isotopic dating and structural relationships of granitoids and greenstones in the east pilbara western australia a structural rb sr isotopic study has been made on two small areas in the eastern pilbara block western australia the sites were chosen because they showed good interrelationships of granitoid and greenstone units at warrery gap on the western side of the corunna granitic dome the acid volcanics of the duffer formation at the top of the lower talga talga subgroup of the warrawoona group show some updating but a good 3506 62 ma isochron with initial ratio ir of 0 7006 0 0011 was recognized consistent with the 3452 16 ma zircon measurement of pidgeon 1978b both a penetrative d1 deformation and the doming of d2 preceded intrusion of late tectonic granitoids dated at 3270 22 ma ir 0 7015 0 0003 into both the duffer formation and the overlying pillow lavas of the salgash subgroup thus the salgash subgroup is much older than suggested by glikson 1979 and the stratigraphic succession cannot contain a hiatus between the apparently conformable talga talga and salgash subgroups of the magnitude he proposed the granitic domes clearly owe their form to the d2 deformation rather than to batholitic intrusion but near horizontal structural lineations suggest that they were not formed by diapiric movements a granodiorite and pink feldspar granite from just within the corunna granitic dome are slightly deformed pooled isochrons indicate an age of 3232 27 ma but different irs of 0 7032 and 0 7009 respectively at tambourah in the eastern shaw granitic dome local d2 also deforms an intrusive microadamellite of age 3087 34 ma and ir 0 7103 0 0057 there is therefore a real spread in ages of d2 granitoids and d2 deformation between about 3300 and 3100 ma layered megacrystic gneiss at tambourah also intruded by the microadamellite contains a nebulous foliation argued to be local s1 layered and homogeneous megacrystic gneiss produce updated rb sr total rock isochrons of 2995 95 and 2779 38 ma respectively the primary age of these d1 gneisses is clearly greater than that of the d2 granitoids and is probably indicated by pidgeon s 1978c zircon age of 3417 40 ma from the shaw granitic dome if so hickman s 1975 migmatite suite contains both d1 gneisses and d2 granitoids separated in age at tambourah by ca 300 ma although neither appears to be older than the lower part of the preserved layered sequence the protocrust on which that sequence was deposited has yet to be directly identified

ree geochemistry and isotopic data of archean silicic volcanics and granitoids from the pilbara block western australia implications for the early crustal evolution eighteen silicic volcanic rocks of the warrawoona group and ten associated plutonic rocks from the pilbara block western australia have been chosen for geochemical and isotopic studies silicic volcanics of the unsb upper member of north star basalt are dated at 3 56 3 57 by both the rb sr and the sm nd methods the respective 1 initial isotopic composition values are 0 7005 5 sr and 0 50810 39 nd this age is consistent with the stratigraphic interpretation that the talgatalga subgroup in which the north star basalt occupies the lowermost position is overlain by the duffer formation whose age was earlier established at 3 45 by the zircon u pb method the new rb sr data on six silicic lava samples from the duffer formation yield an isochron of 3 23 0 28 2v though imprecise this age agrees with the zircon age within error limits rb sr ages of 2 3 2 4 obtained for the panorama rocks and the wyman formation do not correspond to their initial eruption ages chemical arguments suggest that these ages represent the time of metasomatism associated with the widespread thermal event in this region about 2 3 2 4 ago geochemically most of these analyzed rocks volcanic and plutonic are of tonalite trondhjemitegranodiorite ttg composition a typical feature found in many other archean terrains they generally show fractionated ree patterns except the panorama formation rocks furthermore the wyman formation rhyolites and the post tectonic adamellites show significant negative eu anomalies suggesting a similar mode of magma generation and a probable genetic link theoretical considerations suggest that most of these ttg rocks could have been generated by partial melting of amphibolitic or basaltic sources followed by fractional crystallization although the archean granitic gneisses often possess mantle like isr values the trace element data indicate that they could not have been derived by direct melting of upper mantle materials the immediate tectonic implication is that in any archean terrain the formation of na rich continental crust of ttg suite must be preceded by the presence of basaltic crust the occurrence of this basaltic crust is a matter of controversy such crust might have been totally destroyed by repeated melting processes or its remnants are now represented by some of the mafic ultramafic enclaves within the tonalite trondhjemite batholiths

petrogenesis and regional tectonic significance of late devonian mafic intrusions in the meguma zone nova scotia late devonian 377 368 ma 40ar 39ar 376 ma u pb mafic intrusions in the meguma zone crop out as dykes plugs and synplutonic bodies of gabbro diorite or lamprophyre all of the intrusions have similar lithologie characteristics and hydrous ferromagnesian mineral assemblages and they appear to represent a genetically related series of mafic bodies with similar petrogenetic histories in the crust of the meguma zone the intrusions show wide chemical variation of sio2 45 7 65 7 wt al2o3 8 9 26 5 wt mgo 2 8 26 5 wt cao 1 2 11 2 wt and k2o 0 1 4 4 wt and they have calc alkaline high k calc alkaline and shoshonitic characteristics large ion lithophile elements lile are present at variable but high concentrations e g ba 62 1920 ppm sr 176 2567 ppm relative to most high field strength element hfse abundances e g y 10 37 ppm zr 8 421 ppm and light rare earth elements lree have much higher concentrations than heavy rare earth elements hree la lu 24 3

sorption of methane on lignite from polish deposits abstract coal samples from miocene lignite deposits ortholignite ece un 2002 in belchatow adamow konin and turow poland were analyzed to determine the relationships between coal properties and gas capacity investigations presented here addressed the occurrence of methane sorbet in lignite deposits within poland s largest penetrated lignite deposits e g belchatow adamow turow and konin lignite samples collected from surface mines were detritic coal with variable contents of xylites the highest level of xylites up to 25 vol was found in carbon samples taken from the belchatow deposit samples from other mines contained no more than 10 vol xylites petrographic compositions were dominated by huminite group macerals 73 88 vol with atrinite densinite and texto ulminite a porous form of ulminite inertinite groups were less important components 4 8 vol isotherms were determined for methane sorption at 298 k with test pressure ranges below 1 2 mpa at a pressure of 1 0 mpa the largest gas capacity of approximately 1 7 dm3 stp kg was found in the sample from belchatow this result may have been attributable to differences in the porosity of the samples the porosity of the sample from belchatow was twice as high as the porosities of the other coal samples this variation in porosity resulted from the lithologic and maceral composition of the coal sample that contained substantial quantities of porous textinite and texto ulminite the thermal sorption equation was used to determine the limiting values of isosteric enthalpy of sorption which suggested weak interactions between methane and the lignite matrix the residual gas capacity of the tested samples was also determined all samples exhibited a high residual methane bearing capacity which may not only cause methane to be released from coal at a pressure of 1 bar but also may pose a gas risk during mining operations

formation of earth s early archaean continental crust abstract subduction of oceanic crust at an unusually low angle has been proposed as a model for the growth of continental crust older than about 2 5 ga at modern zones of low angle or flat subduction magmatic additions to new crust come from partial melting of both the subducting oceanic crust slab and the thin wedge of mantle above the slab evidence for both a slab and wedge source is preserved in most late archaean 3 0 2 5 ga terrains but we find little evidence that a mantle wedge contributed to crustal growth prior to 3 1 ga this lack of evidence in part reflects a dearth of exposed crust aged between 3 0 and 3 3 ga but also suggests that subduction enriched mantle source regions did not develop before 3 3 ga and possibly not before 3 1 ga in contrast to most modern terrains and some late archaean terrains early archaean 3 3 ga continental crust evolved through direct melting of thick mafic crust we invoke a process of subduction that does not include the development of a mantle wedge and call this process archaean flat subduction to distinguish it from modern low angle subduction

relation between archaean high grade gneiss and granite greenstone terrain in western australia abstract two major regions of archaean crust occur in western australia called the pilbara and yilgarn blocks all the pilbara and most of the yilgarn block consist of granite greenstone terrain with metamorphic grades in greenschist or low amphibolite facies but these terrains developed at different times in the two regions 3500 3000 ma ago in the pilbara and 3000 2600 ma ago in the yilgarn high grade gneisses are only known to occur in a belt along the western margin of the yilgarn block in a region called the western gneiss terrain direct observations of the relation between this high grade gneiss terrain and typical granite greenstone terrain can therefore only be made in this region two kinds of relation are seen in the north the high grade gneiss terrain developed between 3800 3300 ma ago it is distinctly older than and contains a different assemblage supracrustal rocks from the granite greenstone terrain the gneiss terrain could have formed a sialic basement to the greenstones but direct evidence of this relation is not preserved the gneiss terrain is intruded by granites similar to those of the granite greenstone terrain and the two terrains are deformed and metamorphosed together prograde metamorphism of the granite greenstone terrain increases from mainly greenschist facies in the east to high amphibolite facies in the west where it is associated with retrograde metamorphism of the gneiss terrain from granulite to high amphibolite facies in the south a portion of the western gneiss terrain also consists of gneisses which pre date the granite greenstone terrain however most of the gneiss terrain consists of younger granitoid intrusions similar to those that form a major component of the granite greenstone terrain but metamorphosed in granulite facies as in the north a regional eastward decrease in metamorphic grade suggests that the exposed crustal section is tilted eastwards the tectonic significance of the high grade gneiss and granite greenstone terrains remains uncertain regional relationships on the scale of the yilgarn block suggest three possible models 1 cratonic basinal model whereby large greenstone basins formed on gneissic basement 2 continental marginal model whereby greenstone terrains developed adjacent to pre existing protocontinent or 3 collision involving two unrelated segments of crust

lead ages reset rubidium strontium ages and implications for the archaean crustal evolution of the diemals area central yilgarn block western australia abstract the pb pb whole rock geochronology of archaean granitic and gneissic rocks from the diemals area in the central yilgarn granite greenstone terrain provides important constraints on crustal evolution the regionally extensive banded gneisses previously considered as candidates for basement to the greenstones give a pb pb whole rock age of 2700 97 ma 2 errors this is within error of previously published rb sr and sm nd gneiss ages and also within error of the sm nd ages on the greenstones in the eastern goldfields province two synkinematic plutons give pb pb whole rock ages 2737 62 ma and 2700 100 ma and pb isotopic compositions consistent with the hypothesis based on field and geochemical relations that these plutons were derived by partial melting of the precursors to the banded gneisses assuming this the combined data date the melting event at 2723 25 ma with a model source value of 8 18 0 02 this source value is close to the range postulated for mantle values and restricts the crustal history of the precursors to less than 200 ma a post kinematic pluton with a whole rock pb pb age of 2685 26 ma and value of 8 26 0 02 puts a younger limit on this relatively short lived crustal accretion differentiation event comparison of pb pb and rb sr whole rock dates for the plutons suggests that the latter became closed systems up to 200 ma after the pb pb ages and that the plutons gained or lost rb or sr at this time

volcanic sedimentary and tectonostratigraphic environments of the 3 46 ga warrawoona megasequence a review abstract the warrawoona megasequence comprises a 3 46 ga assemblage of tholeiitic and calc alkaline volcanic rocks interlayered with cherty sedimentary rocks deposition occurred in a range of shallow to deeper water environments ranging from the shoreline to distal deposits in sediment starved basins volcanism provided the dominant or only source of clastic sediment sedimentary rocks also contain evaporites probable stromatolites other evidence for the existence of microbial life and possible evidence of meteorite impact the tectonostratigraphic assemblage and its contained facies associations are most similar to those developed in younger volcanic arc or near arc settings the compositions of volcanic rocks are also comparable with those of younger tholeiitic and calc alkaline suites developed above subduction zones the calc alkaline suite has a complex petrogenesis which probably involves mantle derived basaltic melts intermediate to silicic melts derived from subducted mafic crust magma mixing and fractional crystallization intense hydrothermal alteration and a distinctive association of metal deposits are also compatible with the interpretation that the warrawoona megasequence contains examples of early archaean volcanic arc and near arc assemblages

40ar39ar laserprobe ages of metamorphic hornblendes from the coongan belt pilbara western australia abstract 40 ar 39 ar laser step heating analyses of metamorphic amphiboles and a whole rock sample from metavolcanic rocks of the archaean coongan greenstone belt in the south eastern pilbara in western australia provide evidence that these rocks with no previous age constraints belong to the ca 3460 ma old warrawoona group the western margin of the belt is formed by an east dipping ductile shear zone the eastern margin is undeformed and shows a narrow contact metamorphic zone the coongan belt shows a metamorphic gradient from amphibolite facies at the western margin to greenschist facies in the centre of the belt the rocks in the centre of the belt are transected by north south striking retrograde shear zones one of which has been constrained at ca 2940 ma and may be related to the ca 2940 ma intrusive event in the eastern pilbara foliated granodiorites from the ductile shear zone along the western margin of the belt yield dates of ca 3170 ma which are interpreted as the age of recrystallisation after deformation deformed blue green hornblendes from the adjacent foliated amphibolites however give younger and inconsistent dates of ca 3090 ma and ca 3020 ma these dates cannot be interpreted as meaningful ages and indicate an inhomogeneous distribution of argon in the shear zone age spectra from blue green hornblendes from the western part of the coongan belt constrain the age of a regional metamorphic event at 3240 ma which compares well with the metamorphic hornblende ages of ca 3200 ma obtained in the western shaw belt by wijbrans and mcdougall the same event is indicated by complex age spectra of partially metamorphosed mafic volcanic rocks which yield ca 3260 ma maximum ages in the higher temperature steps the age spectrum of contact metamorphic hornblendes from the eastern margin indicates that the cooling age of the undeformed granite in the south western corunna downs batholith is ca 3400 ma which is 70 90 ma younger than the north shaw granodiorites in the shaw batholith but considerably older than previously assumed that is ca 3300 ma all obtained ages fall within the expected range of ca 3 5 2 94 ga and generally increase from west to east through the coongan belt the effects of excess argon on the age spectra seems to be rather considerable in some of the experiments

petrography and provenance of silicified early archaean volcaniclastic sandstones eastern pilbara block western australia intense post depositional alteration has profoundly affected sandstones in the volcanic portions of early archaean 3 5 3 3 ga greenstone belts the mineralogy and bulk compositions of most grains have been completely destroyed by pervasive metasomatism but grain textures are commonly well preserved consequently microtextural information coupled with present alteration compositions as determined petrographically can be used to estimate original framework modes silicified early archaean volcaniclastic sandstones assigned to the panorama formation and duffer formation warrawoona group eastern pilbara block western australia were originally composed of volcanic vrf and sedimentary srf rock fragments volcanic quartz feldspar traces of ferromagnesian minerals and pumice only volcanic megaquartz remained stable during alteration all other primary components were replaced by granular microcrystalline quartz gmc and sericite in most areas the sandstones were composed of dacitic to rhyolitic vrfs now totally replaced by sericite poor gmc and recognized by preserved microporphyritic textures in a few areas quartz poor dacitic to andesitic vrfs dominated the detrital assemblage minor srfs and mafic vrfs now replaced by gmc are recognized on the basis of colour internal structures and internal textures including skeletal possible spinifex textures detrital feldspar is represented by blocky sericite rich grain pseudomorphs a semi quantitative point count scheme developed for the analysis of heavily altered sandstones indicates the following primary detrital mode ranges for panorama arenites quartz 0 28 feldspar 0 28 vrfs 58 86 and srfs 0 25 in about half the point counted samples feldspar could not be distinguished from rock fragments in such cases both were counted as one grain type lv which makes up from 84 to 100 of the framework modes of these rocks these sands were derived from a terrane composed largely of fresh felsic volcanic rocks and sediments but locally including minor mafic ultramafic and sedimentary rocks much but not all of the felsic volcaniclastic sand represents reworked pyroclastic debris there is no evidence for contributions from plutonic or metamorphic sources the panorama modal assemblage represents a provenance that is lithologically more restricted than that of archaean greywackes and other siliciclastic units common in the sedimentary portions of these same early archaean greenstone belts and younger greenstone belts 3 0 2 7 ga

40ar 39ar ages of detrital muscovite within lower cambrian and carboniferous clastic sequences in northern nova scotia and southern new brunswick implications for provenance regions detrital muscovite from lowermost cambrian sequences exposed in the avalon composite terrane in nova scotia and new brunswick record 40ar 39ar plateau ages of ca 625 600 ma these are interpreted to date times of cooling in source areas the regional distribution of coarse grained detrital muscovite in lower cambrian rocks of avalonian overstep sequences suggests a source region of dimensions considerably larger than any presently exposed in appalachian segments of the avalon composite terrane late proterozoic tectonic reconstructions locate the avalon composite terrane adjacent to northwestern south america thereby suggesting a possible source within late proterozoic panafrican brasiliano orogens detrital muscovite from clastic sequences of the proximally derived lower carboniferous tournaisian horton group and the more distal upper carboniferous westphalian d stephanian pictou group in nova scotia records 40ar 39ar spectra that define plateau ages of ca 390 380 ma horton group and and ca 370 ma pictou group finer grained fractions from samples of the horton group display more internally discordant age spectra defining total gas ages of ca 397 395 ma a provenance for the finer muscovite may be found in southern nova scotia where cambrian ordovician turbidites of the meguma group display a regionally developed micaceous cleavage of this age the ca 390 380 ma detrital muscovites probably were derived from granite stocks presently exposed in proximal areas of northernmost cape breton island a more distal source for the ca 370 ma detrital muscovites in the pictou group is suggested by its original extensive distribution although a local possibly recycled source may also have been present the presence of only 400 370 ma detrital muscovite suggests a rapidly exhumed orogenic source with characteristics similar to those of crystalline rocks presently exposed in the cape breton highlands and or the meguma terrane

smnd geochronology of greenstone belts in the yilgarn block western australia abstract sm nd geochronology has been used to date 3 widely spaced metavolcanic greenstone belts in the yilgarn block the measured isochron ages are 2 78 0 03 ga initial nd 2 5 0 3 for greenstones at kanowna in the eastern goldfields province 3 05 0 10 ga initial nd 0 9 0 7 for diemals marda in the southern cross province and 2 98 0 12 ga initial nd 0 7 1 2 for the warriedar fold belt in the murchison province these data show that substantial crustal components of age 3 0 ga exist in the southern cross and murchison provinces and support published evidence that the eastern goldfields province contains no crustal material 2 8 ga these ages are all significantly younger than ages commonly observed in the western gneiss terrain and so support the suggestion of a progressive crustal age trend across the yilgarn block the measured age values depend strongly on the inclusion of data for felsic components of the greenstone belts data for mafic and ultramafic units allow that eruption in the 3 localities could have been contemporaneous at 2 8 ga the age range of metavolcanic rocks within each province is possibly comparable to the maximum possible age difference between study areas 0 2 ga so the documented lithological variations between the greenstone successions of the 3 provinces cannot be attributed to depositional age differences leaching experiments on altered warriedar samples indicate that both alteration and isotopic homogenisation occurred early in the evolution of the system probably prior to the 2 6 ga regional metamorphic event the initial nd values do not conform to previously suggested mantle depletion curves when considered with published data for greenstone belts they suggest complex mantle heterogeneity with end diverging over time to both positive and negative values

origin and early growth rate of continents abstract data suggest that early continents formed over mantle sinks and grew rapidly between 2 7 and 3 0 ga ago nd isotopic results indicate that archean tonalites which are largely responsible for early continental growth are derived chiefly from ancient depleted mantle sources trace element and oxygen isotope data however indicate enriched sources for such tonalites these constraints can be explained if the mantle is fractionated into depleted dmr and enriched emr reservoirs during early degassing of the earth 4 4 ga blocks of emr are recycled back into the mantle and the emr dmr mixtures serve as sources for archean basalts these basalts are enriched in incompatible elements by deep sea alteration metasomatism in sinks or by other mantle processes and then undergo wet partial melting to produce tonalite magmas enrichment must occur not more than 150 ma prior to tonalite production basalt plateaus accumulate at sinks and as cooling continues geotherms at sinks pass into the eclogite stability field dense eclogite roots form on the basalt plateaus which pulls them into the mantle between 2 7 and 3 0 ga ago resulting in rapid tonalite production and thus in rapid continental growth

precambrian tectonic history of the west australian craton and adjacent orogens this review is a synthesis of the precambrian geology of the west aus tralian craton and its fringing orogens it describes the evidence that has recently been reinvestigated and reinterpreted to reveal a long dynamic history of repeated aggregation and dispersal of crustal fragments cratons are large composite geological units of continental crust that have survived intact for significant periods of geological time they are generally tabular bodies with great areal extent millions of square kilo meters and lithospheric thickness 200 400 km orogens are narrow steep zones of tectonic and metamorphic activity associated with the deposition of sediments and the intrusion of plutonic rocks they occur within or along the margins of continents and are generally a few hundred kilometers wide thousands of kilometers long and extend to the base of the litho sphere most orogens are initially associated with compressive tectonics crustal thickening and the transient formation of mountain belts cratons and orogens may survive for very long periods of time but as part of a thin crust on a dynamic mantle they are inherently mobile and travel as rafts for great distances around the surface of the earth they may be torn apart dispersed and reassembled with other continental fragments the boundaries of these crustal fragments once established remain zones of crustal weakness that may be reactivated as intra continental mobile belts or lines along which cratons are assembled and dismembered cf hoffman 1 988

the genesis and tectonic control on archaean gold deposits of the western australian shield a metamorphic replacement model abstract gold deposits occur in greenstone belts world wide and contribute to anomalously high gold production from archaean terranes as in other cratons archaean gold mineralization of western australia represents a complex array of deposit styles despite this most deposits are clearly epigenetic and large deposits have a number of features in common including their strong structural controls distinctive wallrock alteration fe sulphide k mica albite ca mg fe carbonates consistent metal associations au ag as sb w b low base metals commonly fe rich host rocks great depth extension and lack of appreciable vertical zonation these shared characteristics combined with their ubiquitous occurrence indicate that archaean gold deposits had a common origin related to the tectonic evolution of greenstone belts auriferous hydrothermal systems were broadly synchronous with regional metamorphism and emplacement of synkinematic granitoids and felsic porphyry intrusions although these gold systems involved low salinity lowdensity reduced near neutral h 2 o co 2 fluids carrying gold as reduced sulphur complexes the origin of the fluids is equivocal most timing evidence and stable isotope data cannot distinguish metamorphic from magmatic granitoid or felsic porphyry orggins but the lack of consistent spatial relationships between specific volumetrically significant intrusive phases and large gold deposits in a number of cratons strongly favours metamorphic derivation of fluids the metamorphic replacement model for gold mineralization involves devolatilization of the lower portions of the greenstone pile with high geothermal gradients inhibiting significant melting co 2 possibly formed by the decarbonation of early alteration related to mantle degassing along crustal scale synbasinal fault zones auriferous fluids were channelled along greenstone scale faults in part developed during reactivation of crustal scale faults in a strike slip regime gold deposition occurred largely under greenschist facies conditions about 300 400 c 1 2 kb in response to decreasing gold solubility with declining temperature however a major control on gold deposition was fluid wallrock interaction many large deposits formed by sulphidation of fe rich host rocks with synchronous deposition of fe sulphides and gold however the variable nature of gold depositing reactions including lowering of fo 2 and ph allowed a multitude of small and some large deposits to form wherever that fluid circulation occurred in consequence several of the relatively small deposits currently worked from open pit are hosted by ultramafic and felsic rocks there are few constraints on the source of components au s k co 2 added to gold deposits but even giant deposits such as the golden mile kalgoorlie could have formed from a realistic greenstone source volume ca 8 8 5 km convective circulation of fluids could have contributed to the generation of high fluid rock ratios on the regional scale the markedly heterogeneous distribution of large gold deposits gold productivity and host rocks to deposits can be accommodated by the metamorphic replacement model the most favourable conditions for development of auriferous hydrothermal systems operated in younger ca 2 7 0 1 ga rift phase greenstones where greatest extension and crustal thinning produced high geothermal gradients crustal scale synbasinal faults and rapid extrusion and burial of volcanics including abundant komatiites iron rich tholeiitic basalts and dolerites were preferred host rocks for large gold deposits the least favourable conditions existed in older ca 3 5 3 4 ga platformphase greenstones where gentle sagging on submerged continental crust produced eruption of mainly mafic volcanics with few komatiites commonly in very shallow water environments this allowed intense synvolcanic alteration of both gold source rocks and potential host rocks the generally smaller gold deposits formed mainly in ultramafic or greywacke hosts younger ca 3 0 ga platform phase greenstones appear intermediate in nature but unlike other greenstones have significant epigenetic gold deposits in originally oxide facies bif which were deposited on relatively deep water platforms similar controls appear to exist on a world scale with gold mineralization peaking at ca 2 7 0 1 ga in response to development of major rift zones in thickened relatively mature continental crust interestingly the giant witwatersrand goldfield formed at about the same time

palaeozoic within plate volcanic rocks in nova scotia canada reinterpreted isotopic constraints on magmatic source and palaeocontinental reconstructions palaeozoic volcanism in the avalon terrane of northern nova scotia occurred during three time intervals cambrian early ordovician late ordovician early silurian and middle late devonian in the meguma terrane of southern nova scotia palaeozoic volcanism is limited to the middle ordovician geochemical data show that most of these volcanic rocks are bimodal within plate suites initial e nd signa tures range from 5 4 to 1 9 in the rhyolites and 6 8 to 2 7 in the basalts a difference attributable to the absence or presence respectively of a significant crustal component the data and regional tectonic settings of the avalon and meguma terranes suggest that the volcanism was generated in three different within plate settings 1 cambrian early ordovician volcanism related to thermal decay of late proterozoic arc magma tism during transtensional deformation 2 middle ordovician early silurian volcanism during sinistral tele scoping between laurentia and gondwana where extensional bends in the appalachians produced rifting and 3 devonian volcanism resulting from lithospheric delamination during dextral transpression and tele scoping in each setting active faults served as conduits for the magmas nd isotopic data indicate that the source of the palaeozoic felsic volcanic rocks is isotopically indistinguishable beneath southern and northern nova scotia and did not substantially change with time this crustal source appears to have separated from the mantle during the proterozoic a conclusion consistent with the hypothesis that the palaeozoic rocks in nova scotia were deposited upon a late proterozoic oceanic cratonic volcanic arc terrane the nd data when combined with published faunal palaeomagnetic and u pb isotopic data suggest that the avalon terrane was peripheral to gondwana off northwestern south america during neoproterozoic and early palaeozoic times

petrographic composition of polish lignite and its possible use in a fluidized bed gasification process abstract the main objectives of the study include the estimation of the proportion of lithotypes and macerals in lignite deposits and the determination of petrographic criteria for the assessment of quality and possible use of lignite in fluidized bed gasification process measures taken to meet the objectives included field research and laboratory studies the study included results from 293 samples of low rank coal from polish deposits the data used in the study included currently exploited abandoned and prospective lignite deposits lignite deposits were divided into seven regions a western b legnica c wielkopolska d konin e od belchatow f north and g radom polish lignite deposits include mainly humic coal with four basic lithotypes identified xylitic coal xylo detritic coal detro xylitic coal and detritic coal dominating lithotypes include detritic coal and xylo detritic coal fibrous xylites present only in minor amounts in polish lignite have a negative influence on coal gasification process grinding the highest content of fibrous xylites is observed in the konin region petrographic composition of coal was determined on the basis of maceral group and the sum of gelified macerals it has been found that the influence of macerals from the huminite group on fluidized bed gasification is significant the average content of this component in tested coal is 89 meanwhile average content of macerals from the liptinite group in polish coal is 8 while for macerals from the inertinite group it is 3 the content of mineral matter in lignite deposits is a highly variable parameter according to the proposed scheme in order to be suitable for fluidized bed gasification the maceral composition of the coal should contain at least 80 huminite while the sum of the macerals from the inertinite group and mineral matter should be less than 20 taking into account the whole region it should be noted that petrographic composition of lignite in almost all of the deposits allows its use in the process of fluidized bed gasification however limited usefulness in this process can be observed in some of the deposits containing bituminiferous lignite

40 ar 39 ar mineral ages from the southern cape breton highlands and creignish hills cape breton island canada evidence for a polyphase tectonothermal evolution 40 ar 39 ar ages for hornblende and muscovite from plutons and metamorphic country rocks in the southern cape breton highlands and creignish hills together with previously published u pb rb sr and 40 ar 39 ar ages document a complex tectonothermal history that included 1 episodes of plutonic activity at ca 600 565 560 555 500 490 470 and 410 395 ma all of which cooled through 500 400 c within ca 20 m y and 2 deformation in the late precambrian 550 ma and the late silurian middle devonian 415 380 ma the latter was associated with evelopment of a positive flower like structure during maintenance of medium pressure greenschist amphibolite facies regional metamorphic conditions that may be traced northward along the length of the cape breton highlands it may be correlated with the kingston complex and associated mylonite zone exposed in the avalon composite terrane of southern new brunswick northward it merges with a major high grade poly deformed belt within the gan

shallow water volcaniclastic deposition in the early archean panorama formation warrawoona group eastern pilbara block western australia abstract the panorama formation of the 3 3 3 5 ga warrawoona group in the eastern pilbara block western australia is a regional unit composed largely of silicified felsic volcaniclastic sandstone and tuff deposited at the close of a major period of early archean felsic volcanism it is divided into eight lithofacies a volcaniclastic tuff and tuff breccia b chert clast conglomerate c well bedded tuff and tuff breccia d cross stratified volcaniclastic sandstone e laminated and cross laminated siltstone f banded chert g silicified evaporite and h felsic lava wave formed features occur in lithofacies b c d and e the eight lithofacies occur in four facies associations 1 sequences of banded chert with lesser amounts of tuff representing clastic starved probable basinal environments 2 sequences of siltstone with variable amounts of tuff representing lower shoreface and prodelta environments 3 sequences of cross stratified sandstone with one or more other facies representing upper shoreface fan delta front and braided stream environments and 4 sequences of turbiditic and ash fall tuff representing subaqueous settings under the direct influence of pyroclastic volcanism in the north pole dome the panorama formation is developed directly over basalt and consists of shoaling upward sequences from base to top of associations 1 2 and 3 interpreted as sedimentation in prograding fan delta complexes flanked laterally by shoreface environments in other areas associations 3 and 4 are developed over thick sections of felsic lava and breccia of the duffer formation felsic volcanism controlled sediment composition sediment size and the rate of sediment supply to surrounding epiclastic systems deposition took place under regionally developed shallow water to subaerial conditions in which deep water distal facies were not well developed despite the abundance of wave influenced shallow water environments evidence for tides is rare probably because deposition took place under regionally microtidal marine conditions

deep crustal structure in the southwestern yilgarn craton western australia abstract the archaean rocks of the yilgarn craton comprise the largest geological entity in western australia and one of the largest areas of archaean crust anywhere in the world however poor outcrop has prevented a thorough understanding of the structure of the crust in the region a subject of particular interest being the relationship between high grade metamorphic rocks in the west and lower grade granitoid greenstone terrains further east interpretation of previously unpublished seismic refraction data combined with the re interpretation of published data allows crustal velocity structure in the southwest of the craton to be resolved overall the structure is similar to shield areas elsewhere in the world the data defining a two layered crust with an average thickness of about 35 km however significant lateral variations in velocity structure are also observed and these coincide with a terrane boundary previously postulated on the basis of geological observations and seismic reflection data the terrane boundary dips to the east extends through the entire crust and is notable for an area of anomalously high seismic velocity in the lower crust two possible interpretations of the high velocity zone are proposed firstly it is due to the presence of mafic to ultramafic intrusions secondly and the preferred interpretation the high velocity zone is a fault bounded mega sliver which may be a suspect terrane in its own right perhaps with oceanic affinities the results of the seismic experiment strongly support tectonic models for yilgarn craton based on terrane tectonics the possibility that the terrane boundary is being reactivated by contemporary stresses also provides a plausible explanation for the local intra plate seismicity

on the metamorphic history of an archaean granitoid greenstone terrane east pilbara western australia using the40ar 39ar age spectrum technique abstract age spectrum analyses of blue green hornblendes from amphibolites from the western shaw belt east pilbara western australia indicate an age of at least 3200 ma for early regional metamorphism ages on hornblende and muscovite from the narrow contact zone with the adjacent yule batholith probably date updoming of the granitoid gneiss terranes at 2950 ma hornblendes from within the shaw batholith and from a contact zone of a post tectonic granitoid yield ages of 2840 2900 ma indicating either prolonged high temperatures within the granitoid gneiss terranes or a separate thermal pulse associated with the intrusion of post tectonic granitoids the preservation of very old hornblendes in a narrow greenstone belt surrounded by massive granitoid gneiss domes indicates that remarkable contrasts in metamorphic geotherms existed over short distances during the late archaean suggesting that updoming occurred during a period of rapid tectonism

lewisian gneiss geochemistry and archaean crustal development models abstract the geochemistry of lewisian amphibolite facies gneisses from northwest scotland is described with particular reference to the rare earth elements ree and compared with the geochemistry of lewisian granulite facies gneisses the results show that there are no significant differences between laxfordian amphibolite facies and scourian granulite facies gneisses in terms of ree and other immobile trace elements at equivalent silica levels although the mobile radioactive heat producing elements k rb th u are significantly lower in the granulites in both types the basic gneisses have moderately fractionated ree patterns while the intermediate and acid gneisses have strongly fractionated ree patterns with low heavy ree abundances and decreasing levels of total ree with increasing sio 2 the most silicic gneisses develop large positive europium anomalies these gross chemical similarities between gneisses from intermediate amphibolite facies and lower granulite facies crustal levels constrain models for the evolution of the archaean crust the depletion of k rb cs th and u in granulites but not other incompatible trace elements cannot be explained by magmatic processes the positive eu anomaly in the more siliceous gneisses of both facies is a function of the primary processes of crustal generation and not secondary processes such as intracrustal melting or fractional crystallisation fractionation of radioactive heat producing elements from other trace elements is a result of granulite facies metamorphism with these elements being removed by an active fluid phase the apparent lack of partial melting in lower crustal granulites suggests a model for archaean crustal growth largely through underplating by primary tonalitic magmas

chemical evolution of miocene wood example from the belchatow brown coal deposit central poland abstract miocene conifer wood samples from the belchatow brown coal deposit in poland were studied in order to discuss a range of chemical variations that occur as a result of biochemical coalification petrographic analysis ultimate analysis electron microprobe technique and ftir spectroscopy were used in this study our data show several progressive trends in functional groups distribution that take place during the wood transformation from group 1 to group 4 such as an overall increase in aromaticity an increase in lignin cellulose ratio and an increase in oxygen functionalities other observations include an increase in aliphatic stretching and bending functionalities from groups 1 to 3 followed by a decrease in the wood of group 4 appearance of aliphatic out of plane bands in group 3 and increase in group 4 an increase in ch2 ch3 in group 4 compared to the other groups and decrease in o h groups in group 4 compared to other groups these observations together with other chemical and petrological observations indicate that the progressive elimination of cellulose and modification of lignin are dominant processes of the wood transformation

geochemical and isotopic characteristics of early silurian clastic sequences in antigonish highlands nova scotia canada constraints on the accretion of avalonia in the appalachian caledonlde orogen avalonia is a terrane that accreted to laurentia baltica during the development of the appalachian caledonide orogen interpretations of the timing of accretion have been constrained by comparing faunal affinities overstep sequences age and kinematics of inferred accretionary deformational events and controversial paleomagnetic data we show that the time of accretion of avalonia may also be constrained by contrasts in the geochemical and isotopic signatures of its igneous rocks which reflect the characteristics of the underlying continental basement and mantle and sedimentary rocks which reflect provenance early silurian clastic sedimentary rocks of the beechill cove formation antigonish highlands nova scotia were deposited on avalonian crust the formation predominantly consists of approximately 80 m of siltstones and shales deposited in a nearshore environment and derived from the northeast their age is constrained by paleontological data and by directly underlying late ordovician early s

lithophile element systematics of archean greenstone belt au ag vein deposits implications for source processes reply potassic alteration domains of greenstone belt lode gold deposits are characterized by systematic partitioning between lithophile elements k rb and ba are generally co enriched and linearly correlated over almost three orders of magnitude in abundance where k rb 220 400 and k ba 30 85 k cs 104 and k tl 2 104 are also correlated though more weakly lithium abundances and rb sr ratios are erratic in altered rocks these interelement trends collectively are present in deposits variously hosted by ultramafic mafic or felsic volcanic rocks and sediments or granitoids magmatic processes involving crystal fractionation of biotite k feldspar and plagioclase generate trends to systematically diminished k rb 50 k li k cs and k tl but enhanced k ba 8 103 and rb sr in most late stage differentiates such late stage trends are the rule in magmatophile deposits including the archean cadillac molybdenite deposit phanerozoic cu and mo porphyry deposits sn w greisens and most

an open system fractional crystallization model for very early continental crust formation abstract a model is discussed which seeks to explain the origin of archaean tonalitic trondhjemitic granodioritic ttg crust by an open system fractional crystallization mechanism following the assumption that in the pre and early archaean the melt which formed above sites of large scale mantle upwelling was too voluminous and too hot to solidify an almost global near surface partially molten zone could form the magma in this layer would converge above mantle sinks and undergo fractional crystallization recharged fractionating reservoirs would form at sites of convergence the major element composition of the magma in such reservoirs would evolve towards one resembling that of ttg rocks provided that amphibole was a significant fractionating phase from such reservoirs their incompatible as well as compatible trace element abundances would also become similar to those of ttg terrains it is proposed that such terrains resulted from the consolidation of these recharged fractionating reservoirs it is shown that the reservoirs would remain indistinguishable from the primitive mantle in terms of radiogenic isotopes so that model ages cannot be greater than the age of their consolidation consideration of the main aspects of greenstone belts and archaean high grade terrains indicates no contradictions with the model the intrusion of late archaean adamellites and granites is interpreted as an indirect effect of pressure induced breakdown of amphibole in the cumulate underlying the ttg crust which in turn resulted from crustal shortening cratonization is seen as the result of consolidation not only of the crust but also of the underlying cumulate as subcontinental lithosphere

the archaean whim creek belt an ensialic faultbounded basin in the pilbara block australia abstract the whim creek belt in the western pilbara block australia is an elongate structural basin 80 km long and up to 15 km wide it contains a c 2950 ma old sequence of bimodal volcanics volcaniclastics and terrigenous sediments which unconformably overlies granitoids and older deformed and metamorphosed supracrustal sequences the volcano sedimentary sequence was deposited in terrestrial to marine environments in a fault bounded basin with approximately the same configuration as the present whim creek belt alluvial and subaqueous fanglomerate facies were deposited along the northern margin of the basin with submarine fan and basinal facies deposited longitudinally in the centre of the basin the basin margin faults are now part of a regional pattern of strike slip faults which were active both prior to and after basin formation and probably had a significant lateral component during basin evolution the whim creek belt is thus interpreted as an archaean rift or pull apart basin similar basins containing bimodal volcanics and terrigenous sediments occur at the head of the present gulf of california e g salton trough although a direct tectonic analogy cannot be assumed this interpretation implies that the archaean tectonic pattern in the western pilbara involved externally imposed deformation and was not solely the result of gravity driven uprise of granitic batholiths

40ar 39ar dating in the lochaber mulgrave area northern mainland nova scotia implications for timing of regional metamorphism and sediment provenance in the late devonian early carboniferous horton group 40ar 39ar dating of whole rock samples and muscovite separates using age spectrum analysis and of single muscovite grains using total fusion analysis yields new insights into the timing of regional metamorphism and sediment provenance in the late devonian early carboniferous horton group in the lochaber mulgrave area of nova scotia the time of regional metamorphism is constrained to ca 340 335 ma by whole rock spectra from well cleaved slate and shale samples from the lowermost clam harbour river and overlying tracadie road formations of the horton group this ca 340 335 ma event may have been the result of burial and deformation of the horton group by older volcanic and sedimentary rocks of the guysborough group which were overthrust from the south as the result of development of a positive flower structure at a restraining bend along the cobequid chedabucto fault system the boundary between the meguma and avalon terranes detrital muscovite ages of ca 410 380 and ca 500 ma were obtained from

granitoid associated orogenic intrusion related and porphyry style metal deposits in the archean yilgarn craton western australia abstract the yilgarn craton hosts three types of archean granitoid associated metallogenic systems orogenic gold intrusion related and porphyry systems these systems may occur in the same terrane and share many broad characteristics including spatial coincidence with granitoids gangue silicate mineralogy metal associations and local structural controls features of archean orogenic gold systems spatially associated with granitoids include i a broad range of granitoid host compositions ii gold mineralization is mostly late with respect to yilgarn wide granitoid emplacement peak metamorphism and deformation iii orebodies are structurally controlled and iv gold deposition occurs over a large range of temperatures pressures and crustal depths 2 to 15 km from a co 2 rich low to moderate salinity reduced ore fluid that was derived from a metamorphic and or distal magmatic fluid source these systems do not display deposit scale vertical or lateral zonation of metals away from granitoid contacts host granitoid age composition or oxidation state are not important in controlling gold mineralization processes whereas structural setting and fluid flux are paramount archean intrusion related au mo w and mo au systems in the yilgarn craton are commonly small 2 bearing moderate to high salinity fluid during ore deposition archean porphyry cu mo au cu mo and cu au systems are spatially and temporally associated with volumetrically small pervasively altered felsic plutons and dikes surrounded by altered and mineralized supracrustal countryrock lateral zonation of metals away from porphyry centers commonly includes a decrease in cu au and ag au ratios with distance from the source pluton microthermometry studies on fluid inclusions in gold bearing veins suggest that early ore fluids were high temperature and high salinity and that mineralization occurred at 2 o 3 feo are demonstrated in phanerozoic terranes but are presently ill defined in the yilgarn craton crustal depth estimation is important in defining areas likely to have preserve shallow forming archean porphyry systems