new constraints on the timing of early proterozoic tectonism in the black hills south dakota with implications for docking of the wyoming province with laurentia a question regarding the 1900 1600 ma assembly of laurentia is whether the wyoming province west central united states was part of the hearne province west central canada prior to the early proterozoic trans hudson orogeny or a separate entity welded later to a hearne superior continent central canada new 40 ar 39 ar mica dates help to address this question by extending a middle proterozoic geochronologic front long established along the southern wyoming province into the black hills of south dakota this suggests that previously unexplained north directed fold nappes f 1 in the black hills resulted from island arc accretion to the south ca 1780 ma north northwest trending upright f 2 folds which formed during east west collision of the wyoming and superior provinces must therefore be younger new 40 ar 39 ar hornblende dates recently published age data and crustal heat flow considerations further suggest that this collision began at or before ca 1770 ma and culminated with posttectonic magmatism beginning ca 1715 ma the harney peak granite this tectonic magmatic interval is 50 60 m y younger than that reported for the hearne superior collision trans hudson orogeny in canada comparably young metamorphic dates 1810 1710 ma also typify the eastern and northern wyoming province periphery western dakotas and southwestern montana collectively these data suggest that the hearne and wyoming provinces were once separate continents that were ultimately welded to the superior province and to each other during distinct early proterozoic orogenies regional relationships further suggest that final docking of the eastern wyoming province with laurentia began during the ca 1780 1740 ma interval of island arc accretion along the southern margin of the growing craton

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

coesite bearing eclogites from the dabie mountains central china petrogenesis p t paths and implications for regional tectonics coesite and kyanite bearing eclogites are abundant in the southern part of the dabie mountains southern dabie terrane sdt two types of eclogites from the sdt types iii and iv were selected for detailed paragenetic study type iii eclogites most abundant in the northern part of the sdt occur as blocks in gneisses and marble and contain eclogitic assemblages of omphacite garnet phengite epidote coesite kyanite carbonate rutile ilmenite these minerals exhibit weak compositional zoning and contain few mineral inclusions type iv eclogites mostly in the southern part of the sdt occur as coherent layers interbedded with gneisses and amphibolites and have assemblages of omphacite garnet glaucophane kyanite epidote phengite quartz rutile ilmenite garnets of type iv eclogites exhibit a prograde compositional zoning and have mineral inclusions of paragonite phengite epidote quartz and rutile in the core and omphacite barroisite and mg katophorite in the rim prograd

volcanic biotite sanidine 40ar 39ar age discordances reflect ar partitioning and pre eruption closure in biotite the 40 ar 39 ar radioisotope system is widely used to date eruption and cooling of volcanic tephra marker horizons that commonly provide the only means of correlating and assigning numerical ages to stratigraphy in which they are contained this chronometer bridges the gap between 14 c and longer lived isotopic systems that are too imprecise for dating young samples however 40 ar 39 ar ages obtained from coevally erupted biotite and sanidine do not always match here we use an independent chronometer 238 u 230 th disequilibrium to demonstrate that 40 ar 39 ar age disparity is not caused by differences in pre eruption crystallization times our findings indicate that the presence of extraneous 40 ar in biotite and its absence in sanidine may result from violations of two assumptions implicit in 40 ar 39 ar geochronology on volcanic samples 1 prior to eruption minerals are devoid of 40 ar due to rapid loss to an infinite reservoir such as the atmosphere and 2 closure to volume diffusion is geologically instantaneous and coincident with eruption we propose a mechanism whereby the presence of extraneous ar in certain minerals is explained by the relative sequence of four events in a magmatic system 1 crystallization 2 mineral closure with respect to ar diffusion 3 isotopic equilibration of magmatic and atmospheric ar and 4 quenching of the system by eruption these data have potentially far reaching implications for studies that depend on geochronological data necessitating re evaluation of interpretations based solely on biotite with no independent age control particularly in young samples where the effects are most pronounced

mineralogy and petrography of the two lower eocene lacustrine coals sorgun and suluova turkey abstract coal reserves of lower eocene age are limited in turkey and the most important are situated in the sorgun and suluova basins a thick and laterally extensive coal seam occurs at the base of the celtek formation in the two basins the celtek formation was developed in a fluvial lacustrine facies and was unconformably overlain by transgressive units of middle upper eocene age the coal samples were studied mineralogically and petrographically and to define fluorescence emission spectra of sporinite and cutinite a total of 75 coal samples collected from the two basins are characterized on an air dried basis by relatively low moisture contents 0 69 9 31 wt av 3 90 wt a broad range of ash yields 2 4 67 8 wt av 19 9 wt total sulphur contents 0 55 7 60 wt av 2 15 wt and calorific values 1689 7480 kcal kg av 5458 kcal kg minerals of the coals from the two basins determined on an air dried basis by x ray diffraction method are mainly kaolinite quartz and pyrite kaolinite and pyrite contents of the sorgun coals are higher and quartz contents are lower than those of the suluova coals in addition a few samples in the sorgun coals contain epigenetic calcite whereas some samples in the suluova coals contain syngenetic siderite calcite and dolomite the coals in general are characterized by high contents of macerals of the huminite group av 69 0 vol and contain liptinite av 4 9 vol inertinite av 0 8 vol and mineral matter av 25 3 vol on the tpi and gi diagram the coals plot in the limno telmatic and telmatic areas in which the sorgun and suluova peats accumulated between the high and low water tables fluorescence emission spectra of sporinite and cutinite macerals are in general similar in the coals of both basins but the spectral maxima of sporinites and cutinites are between 594 658 nm and 592 671 nm respectively the values for the sporinites indicate a relatively broad range of coal rank however the range of values of the mean random ulminite mainly eu ulminite a reflectances 0 46 0 60 rr av 0 53 rr the proximate and ultimate analyses imply that the rank is a subbituminous a coal transitional to high volatile c bituminous coal according to the astm classification

alpe arami a peridotite massif from depths of more than 300 kilometers the abundance of fetio 3 and chromite precipitates in olivine of the alpe arami peridotite massif switzerland requires a much higher solubility for highly charged cations than is found in mantle xenoliths from depths of 250 kilometers three previously unknown crystal structures of fetio 3 were identified that indicate that the originally exsolved phase was the high pressure perovskite polymorph of ilmenite implying a minimum depth of origin of 300 kilometers these observations coupled with the unique lattice preferred orientation of olivine further suggest that the original composition of olivine may reflect that of a precursor phase perhaps wadsleyite that is stable only at depths greater than 400 kilometers

intercalibration of the hb3gr 40ar 39ar dating standard abstract the 40 ar 39 ar dating technique is based on the knowledge of the age of neutron fluence monitors standards recent investigations have improved the accuracy and precision of the ages of most of the phanerozoic aged standards e g fish canyon tuff sanidine fcs alder creek sanidine ga1550 biotite and lp 6 biotite however no specific study has been undertaken on the older standards i e hb3gr hornblende and nl 25 hornblende generally used to date precambrian high ca k and or meteoritic rocks in this study we show that hb3gr hornblende is relatively homogenous in age composition ca k and atmospheric contamination at the single grain level the mean standard deviation of the 40 ar 39 ar k f value derived from this study is 0 49 comparable to the most homogeneous standards the intercalibration factor which allows direct comparison between standards between hb3gr and fcs is r fcs hb3gr 51 945 0 167 using an age of 28 02 ma for fcs the age of hb3gr derived from the r value is 1073 6 5 3 ma 1 internal error only and 8 8 ma including all sources of error this age is indistinguishable within uncertainty from the k ar age previously reported at 1072 11 ma turner g huneke j c podosek f a wasserburg g j 1971 40 ar 39 ar ages and cosmic ray exposure ages of apollo 14 samples earth planet sci lett 12 19 35 the r value determined in this study can also be used to intercalibrate fcs if we consider the k ar date of 1072 ma as a reference age for hb3gr we derive an age of 27 95 0 19 ma 1 internal error only for fcs which is in agreement with the previous determinations altogether this shows that hb3gr is a suitable standard for 40 ar 39 ar geochronology

ultrahigh pressure minerals and metamorphic terranes the view from china abstract ultrahigh pressure uhp metamorphism refers to mineralogical modifications of continental and oceanic crustal protoliths associated mafic ultramafic rocks initially formed or emplaced in shallow levels of the lithosphere but which subsequently have experienced p t conditions within or above the coesite stability field 2 7 gpa 700 c typical products include eclogite garnet peridotite and uhp varieties of metapelite quartzite marble paragneiss and orthogneiss uhp metamorphic assemblages require relatively cold lithospheric subduction to mantle depths some recrystallization even occurs under forbidden p t conditions characterized by a geotherm of the current explosion of research on continental uhp terranes reflects their significance for mantle dynamics and the tectonics of continental subduction collision exhumation mantle slab interactions and geochemical recycling a further characterization of uhp phases and positive identification of uhp minerals requires new experimental studies coupled with state of the art analyses for example the very rare occurrence of microdiamond inclusions in zircons from dabie sulu uhp rocks may reflect high f o 2 attending recrystallization inasmuch as epidote is rather common rutile needles within garnets from sulu uhp eclogitic rocks may not be the result of exsolution so in such cases the apparent uhp pressure may have been over estimated hadean igneous microdiamond inclusions in jack hills detrital zircons could have originated from mantle xenoliths whereas abundant detrital phanerozoic diamonds containing inclusions of coesite and other eclogitic minerals from new south wales might have been derived from unexposed uhp metamorphic terranes micro mineral intergrowth and nano size minerals may hold important key to deciphering the actual p t paths of subduction and mantle return flow although most exhumed terranes have returned surfaceward relatively rapidly after short time of uhp condition the long duration of storage at great depth and slow exhumation for the largest uhp terranes remain as major problems

39ar recoil artifacts in chloritized biotite biotite separates from the metamorphosed granitic rocks of eastern taiwan yield disturbed 40ar39ar age spectra their disturbed profiles become more pronounced with increasing degree of chloritization and are attributed to mixed release of argon from sub micron scale biotite and chlorite intercalations 39ar internal recoil best explains the discordance of the age spectra chlorite is randomly intercalated with biotite at scales ranging from 10 a to several tens of microns both 40ar and k were evidently lost during the chloritization reaction during the neutron irradiation 39ar atoms recoil into the chlorite from the adjacent biotite intercalations the recoil and chloritization combine to produce a variable 40ar39ar spatial distribution in the intercalated biotite chlorite during 40ar39ar step heating analyses argon in biotite is released in two pulses centered at 650 c and 950 1050 c which represent two stages of dehydroxylation of the biotite chlorite out gasses in two pulses centered at 550 c and 700 900 c during dehydroxylation and then decomposes this differential thermal release of argon from each mineral phase with variable 40ar39ar ratios yields discordant incremental heating age spectra

u pb shrimp geochronology of th poor hydrothermal monazite an example from the llallagua tin porphyry deposit bolivia in situ u pb shrimp analysis of hydrothermal monazite virtually free of th and poor in u 0 2 ppm th 40 103 ppm u from the world class llallagua tin porphyry deposit in bolivia defines a mineralization age of 23 4 2 2 ma mswd 0 48 confirming earlier k ar sericite alteration age data these ages are however in contrast with a weighted mean single crystal 207pb 206pb evaporation age of 39 3 6 0 ma and a related pb pb inverse isochron age of 42 4 4 0 ma mswd 0 66 on zircon from a post porphyry dike as well as with an earlier single crystal sm nd apatite isochron age our data points to a significant time gap between emplacement of the ore hosting porphyry intrusion magmatism and its hydrothermal overprint tin mineralization suggesting long lived magmatic hydrothermal activity in this part of the andean back arc crust the decoupling of porphyry magmatism and hydrothermal activity may explain the unusual occurrence of relatively little fractionated felsic rocks together with extensive tin mineralization our study demonstrates the usefulness of the application of the u pb shrimp method to direct age determination of ore mineralization using th poor hydrothermal monazite even when dealing with geological young events the common assumption of synchronous magmatism and hydrothermal ore formation in porphyry systems may not always be warranted

petrographic composition and depositional environments of cretaceous coals and coal measures in the middle benue trough of nigeria abstract cretaceous coals and coal measure sequences in the area around the obi lafia part of the middle benue trough of nigeria were studied petrographically the turonian santonian awgu formation and the maastrichtian lafia formation constitute the sedimentary successions in both outcrop and borehole profiles of this area maceral analyses of 32 coal samples from this area allow subdivision of the coal beds into three different coal facies the vitrinite fusinite coal facies the trimaceritic coal facies and the shaly coal facies in the vitrinite fusinite coal facies vitrinite constitutes on average 67 3 liptinite 4 2 inertinite 17 1 and mineral matter 11 4 the trimaceritic coal facies has 42 3 vitrinite 22 4 liptinite 23 8 inertinite and 11 5 mineral matter in the shaly coal facies vitrinite constitutes 29 9 liptinite inertinite and mineral matter are 13 8 22 0 and 34 3 respectively correlation of the distribution of microfossils in the interseam sediments with the tissue preservation gelification groundwater and vegetation indices of the coal facies indicates that the vitrinite fusinite coal facies was deposited in wet forest swamp subenvironments along and within lagoons the trimaceritic facies was deposited in limno telmatic clastic marsh subenvironments in lower delta plains the shaly coal facies shows characteristics of various subenvironments but generally tends to be more marine than the other two facies

relationships between hornblende k ar ages chemical composition and hydrogen isotopes connemara western ireland evidence for a massive extinct hydrothermal system major element compositional analyses k ar ages d and 18 o values for 30 zoned and unzoned hornblendes from dalradian amphibolites and metagabbros mostly in north connemara are reported although the cooling ages are expected from previous u pb zircon studies to be c 475 450 ma the results obtained are from 556 6 to 410 9 ma with an average of 470 ma fluid movements probably at 275 15 c i e much below ar closure temperature for hornblende erratically reset the ages as is shown by a negative correlation of hornblende d and age and a wide scatter of ages even within 2 m the changes were implemented by h d exchange between fluid and hornblendes in which ionic porosity z influenced the loss of ar and possibly its gain from the fluid to give the excess ar found in some samples z is controlled by hornblende chemical composition high mg si and mg fe and low fe al ti na and particularly low k amphiboles giving low z values retained ar more firmly and gained ar more readily than compositions which had higher z values which gave younger ages these range down to c 400 ma being the age of the intrusion of the galway granite suite that initiated the fluid circulation the scatter of ages is a consequence of incomplete equilibration mainly because of the slow h d exchange rate below 350 c and partly because the fluid permeated erratically in different areas and down cracks of all kinds promoting irregular ar movement the meteoric fluid circulated through connemara the galway granite and at least some of the contiguous silurian sediments of the south mayo trough these overlying sediments may have contributed to the water circulated in north connemara which was slightly less negative d than in central connemara for hornblende k ar ages to be a reliable measure of times of uplift and cooling they need to be demonstrated to be free from the influence of hot fluids by showing no correlation of age with d

coal geology chemical and petrographical characteristics and implications for coalbed methane development of subbituminous coals from the sorgun and suluova eocene basins turkey

paleoproterozoic basement beneath the eastern cathaysia block revealed by zircon xenocrysts from late mesozoic volcanics abstract the eastern cathaysia block located to the western side of the pacific plate was mainly covered by late mesozoic intrusive and volcanic rocks and the age and composition of its basement remain enigmatic here we firstly report u pb and hf isotopes in zircon xenocrysts from late mesozoic mafic sub volcanic rocks in pingtan island the east coastal area of the eastern cathaysia block with surprising paleoproterozoic to archean u pb isotopic ages neither ca 1 8 ga nor 1 0 0 7 ga age peaks that are very typical in the outcropping rocks including the neoproterozoic sedimentary rocks in pingtan island in the south china block is identified the two main precambrian age peaks of 3 4 3 0 ga and 2 6 2 2 ga of these zircon xenocrysts are explained by contamination from the sediments in the unexposed basement the youngest two paleoproterozoic zircon xenocrysts may indicate the deposition of the basement close to ca 1 8 ga the identification provides clues to the existence of the paleoproterozoic basement in the eastern cathaysia block it may further suggest that the late mesozoic continental arc magmatism in the eastern cathaysia block was most likely developed on and contaminated by the paleoproterozoic basement

zircon from the mantle a new way to date old diamonds we report the first successful dating by the shrimp u pb method of a tiny single zircon inclusion in a natural diamond from a kimberlite at mbuji mayi zaire the mbuji mayi diamonds are a composite suite of contrasting composition probable age and origins which includes diamonds of the typical ultramafic and eclogitic parageneses as well as many diamonds featuring a fibrous coating that formed in equilibrium with a volatile rich fluid phase the zircon diamond pair is believed to be cogenetic and the 628 pm 12 ma age obtained from the inclusion is the youngest yet recorded for a diamond originating in the earth s upper mantle though still much older than the cretaceous eruption age of the kimberlite host from which it was recovered significantly the age does not appear to correspond to any known geological event affecting the congo craton or adjacent areas as recorded in the currently exposed rocks unlike zircon megacrysts from mbuji mayi and elsewhere the inclusion is u rich up to 650 p

determination of pyritic sulphur and organic matter contents in spanish subbituminous coals by x ray power diffraction abstract two analytical methods for the indirect determination of pyritic sulphur and ash contents in spanish subbituminous coals by means of x ray diffraction xrd are described an indirect analytical method for pyritic sulphur is used to determine this type of sulphur in high sulphur coals from the teruel and mequinenza mining areas northeastern spain the analytical method is based on the quantification by m eans of xrd of the hematite content resulting from the oxidation of iron sulphides after coal ashing at 750 c it has been possible to use this analytical method for the particular coals studied because they do not contain iron carbonates or organic fixed iron in appreciable amounts it can thus be assumed that the hematite in the ash proceeds exclusively from the oxidation of iron sulphide the method of determining the ash content in coal by means of xrd was devised in order to analyze small amounts of samples from density separations the small amount of sample available some fractions lower than 0 5g and the need to analyze various parameters sulphur mineral and trace element contents in the sample did not allow us to determine the inorganic content by ashing the method permits the total recovery of the sample for subsequent analyses and the determination of the different mineral phases the method devised is based on the quantification of scattering due to the organic matrix in xrd the organic matrix as an amorphous matrix increases the x ray diffraction background intensity scattering curve by amorphous scatterring to an extent proportional to the organic matter content the main advantages of this method is its speedy determination 4 min and the possibility of sample recovery for subsequent analyses

pan african granulite formation in the kabye massif of northern togo west africa pb pb zircon ages metamorphic zircons from seven granulite facies orthogneisses of tonalitic composition enderbites collected from four different nappes of the kabye massif in the dahomeyide belt of northern togo were dated by the pb pb evaporation method they yielded consistent neoproterozoic ages with a mean of 612 5 0 8 ma interpreted to reflect the peak of regional granulite facies metamorphism following pan african continental collision between the west african and benin nigerian plates these results support previous ages obtained by various chronometers on high grade rocks from the same suture zone and from surrounding units in togo benin and ghana they are also similar to zircon ages from granulites in the mozambique belt of souteastern africa these pan african metamorphic ages reflect continental amalgamation resulting in the formation of the gondwana supercontinent towards the end of the neoproterozoic

obtaining geologically meaningful 40ar 39ar ages from altered biotite biotite is the most used 40ar 39ar geochronometer yet two significant problems arise from ar ar step heating dating altered biotite can be problematic producing disturbed age spectra that reflect 39ar recoil however unaltered biotite can yield disturbed ages with apparently meaningful plateau ages as a result of mineral breakdown during stepped heating obtaining meaningful ages from such spectra is very difficult because alteration of biotite is common and widespread in nature and because many biotites affected by alteration are nonetheless unique geologically and or representative of key localities the capability to obtain reliable ages from altered material is extremely important in this study the effects of alteration progress on biotite age spectra were tested using both ir laser step heating and uv laser microprobe 40ar 39ar dating techniques our aims were to extract geologically meaningful ages from altered biotite and to identify cases where the ages had been influenced by alteration three variably altered biotites from the precambrian metamorphic terrain of southwestern montana were selected for argon isotopic analysis sample a is an unaltered rock containing pristine biotite sample b is a highly altered rock with chlorite and prehnite interlayers within biotite and sample c contains biotite with only incipient alteration for each sample the biotite ages obtained with ir and uv laser techniques were compared and the validity of the apparent ages was assessed ir step heating analysis of biotite from sample a yielded a well defined plateau of 1776 6 ma uv laser microprobe analysis of the same biotite yielded a concordant weighted mean age of 1771 8 ma based upon 65 spot analyses of eight grains that ranged in age from 1819 54 to 1722 62 ma in contrast ir step heating of biotite from sample b resulted in disturbed spectra with two separate fragments yielding total gas ages of 1505 12 and 1540 16 ma whereas uv laser microprobe analysis yielded 14 spot ages ranging from 1806 72 to 1565 78 ma uv laser analysis of sample c produced apparent ages ranging from 1772 52 to 1359 200 ma detailed analysis of age variations occurring perpendicular to 001 cleavage planes of biotite in sample b was accomplished by depth profiling using the uv laser microprobe depth profiling of single grains revealed considerable age variation among the biotite layers with profiles 1 and 2 yielding apparent ages ranging from 1730 226 to 1511 186 ma and from 1741 290 to 956 230 ma respectively in both samples b and c younger apparent ages correspond to higher 36ar 39ar ratios the youngest ages therefore are believed to have derived from altered layers whereas the oldest ages are related to layers with relatively little or no alteration in summary incremental step heating of altered biotite single grains yields 40ar 39ar age spectra that are compromised by recoil and variable release patterns however extraction of small samples via the uv laser microprobe provides a simpler pattern of apparent ages all younger than the true cooling age yet inversely correlated with atmospheric argon this result suggests that true biotite ages can be recovered from areas of pristine material if they are sufficiently large to be ablated by the uv laser

new interpretations of the facies of the rhenish brown coal of west germany abstract new ideas concerning the understanding of the facies of the rhenish brown coal of west germany are discussed these new interpretations are based on a significantly larger number of samples and refined procedures for analysis within the field of paleobotany palynology coal petrology and organic geochemistry than were available to p w thomson and m teichmuller the light and dark bands in the coal are mainly the result of different degrees of plant decomposition the influence of the peat forming plant communities plays a subordinate role in the petrographical composition of these particular strata of the rhenish brown coal

the origin and significance of crustal minerals in ophiolitic chromitites and peridotites abstract various combinations of zircon quartz corundum k feldspar plagioclase apatite amphibole rutile titanite almandine garnet kyanite andalusite and coesite have been recovered from podiform chromitites of the luobusa and dongqiao ophiolites in tibet the oman ophiolite and the ray iz ophiolite in the polar urals russia chromitites in all four ophiolites also contain moissanite and the luobusa and ray iz ophiolites contain in situ diamonds most of the recovered zircons are sub rounded grains with complex internal structures indicating polyphase growth trace element contents and a low pressure inclusion assemblage quartz muscovite k feldspar apatite ilmenite rutile indicate a continental crustal origin for the zircons they have sims u pb ages that are generally much older than the host body total range 90 to 2500 ma the presence of numerous crustal minerals particularly zircon suggests derivation from metasedimentary rocks subducted into the mantle their preservation in chromitites and peridotites implies effective isolation from the mafic melts that formed the ophiolites we suggest that most of these minerals were derived from the crustal parts of subducted slabs and were encapsulated into chromite grains precipitated from rising asthenospheric and suprasubduction magmas the chromite grains were carried in melt channels to shallow levels in suprasubduction mantle wedges and then deposited as podiform chromitites near the moho the rise of asthenospheric peridotites into suprasubduction zones was facilitated by subduction initiation and possibly by slab tear allowing mixing of the uhp highly reduced minerals and crustal minerals now found in ophiolitic chromitites and peridotites

the kokchetav massif kazakhstan type locality of diamond bearing uhp metamorphic rocks abstract after the discovery of metamorphic coesite in crustal rocks from the western alps italy and the western gneiss region norway in the mid 1980s of the last century metamorphic diamond was observed only a few years later in situ in the kokchetav massif kazakhstan findings of such coesite and diamond bearing ultrahigh pressure metamorphic uhp rocks with protoliths formed or embedded in crustal levels and subsequently experienced pt conditions within or even higher than the coesite stability field have dramatically changed our geodynamic view of orogenetic processes these occurrences provide evidence that crustal rocks were subducted into mantle depths and exhumed to the surface recent studies even suggest continental subduction to depths exceeding 300 km these rocks have been extensively studied and many new and important observations have been made thus far more than 350 papers have been published on various aspects of kokchetav uhp rocks the kokchetav massif of northern kazakhstan is part of one of the largest suture zones in central asia and contains slices of hp and uhp metamorphic rocks classical uhp rocks mainly occur in the kumdy kol barchi kol and kulet areas and include a large variety of lithologies such as calcsilicate rocks eclogite gneisses schists marbles of various compositions garnet pyroxene quartz rocks and garnet peridotite most of them contain microdiamonds some of which reach a grain size of 200 m most diamond grains show cuboid shapes but in rare cases diamonds within clinozoisite gneiss from barchi kol occur as octahhedral form microdiamonds contain highly potassic fluid inclusions as well as solid inclusions like carbonates silicates and metal sulfides which favour the idea of diamond formation from a c o h bearing fluid nitrogen isotope data and negative 13c values of kokchetav diamonds indicate a metasedimentary origin pt estimates of kokchetav uhp rocks yield peak metamorphic conditions of at least 43 kbar at temperatures of about 950 1000 c some zircon separates show inherited proterozoic cores and 537 530 ma uhp metamorphic mantle zones several ar ar ages on micas scatter around 529 528 and 521 517 ma and reflect different stages of the exhumation history migmatization occurred during exhumation at about 526 520 ma isotopic studies on calcsilicate rocks confirm a metasedimentary origin 18o values of garnet and clinopyroxene of a layered calcsilicate rock rule out the possibility having a primitive mantle protolith similar studies on eclogites indicate their basaltic protolith having experienced water rock interaction prior to uhp metamorphism a number of unique mineralogical findings have been made on kokchetav uhp rocks k feldspar exsolutions in clinopyroxene demonstrate that potassium can be incorporated into the cpx structure under upper mantle pressures other significant observations are coesite exsolutions in titanite quartz rods in cpx the discovery of k tourmaline as well as new minerals like kokchetavite a hexagonal polymorph of k feldspar and kumdykolite an orthorhombic polymorph of albite the kokchetav uhp rocks represent a unique and challenging stomping ground for geoscientists of various disciplines from crystallography petrology and geochemistry to geophysics and geodynamics geotectonics it concerns all who are interested in the diverse metamorphic processes under upper mantle conditions

dating of zircon from ti clinohumite bearing garnet peridotite implication for timing of mantle metasomatism comment and reply reply we welcome the comments by yang who proposed that the kokchetav ultramafic rocks have an fe ti gabbroic protolith based on whole rock major element chemistry muko et al 2002 1 and have been subjected to a mg metasomatism at crustal depths the kokchetav ti clinohumite rich rocks described

laser probe 40 ar 39 ar dating of strain fringes mid cretaceous synconvergent orogen parallel extension in the interior of the sevier orogen 1 uv and co2 laser probe 40ar 39ar in situ analyses of phlogopite and muscovite in fibrous strain fringes from greenschist facies metamorphic rocks document mica growth ages at temperatures lower than their closure temperatures and therefore directly date deformation the new dates resolve the age of the earliest ductile fabric recorded in the raft river albion grouse creek metamorphic core complex of utah and idaho phlogopite was dated in quartz calcite phlogopite strain fringes around pyrite in pennsylvanian permian rocks from the grouse creek mountains utah using both the uv and co2 laser probe muscovite was dated in quartz muscovite strain fringes around pyrite in deformed jurassic sills from the black pine mountains idaho using the co2 laser probe phlogopite 40ar 39ar ages for individual strain fringes grouse creek mountains range from 92 ma to 110 ma with the most reliable ages ranging from 101 ma to 110 ma mean age 105 0 5 8 ma muscovite 40ar 39ar ages for individual strain fringes black pine mountains range from 97 ma to 112 ma mean age 104 7 5 8 ma strain fringes are associated with a subhorizontal foliation and a generally n trending elongation lineation exhibiting components of top to the north simple shear and coaxial strain accommodating n s extension and subvertical shortening midcrustal northward flow at 105 6 ma within the interior of the sevier orogen coeval with east directed shortening in the foreland and with plate convergence records orogen parallel synconvergent extension we favor gravitational relaxation of structural culminations resulting from focused crustal shortening as a driving mechanism for orogen parallel flow

radioisotopic dating of the tortonian global stratotype section and point implications for intercalibration of 40ar 39ar and astronomical dating methods in this paper we present new 40 ar 39 ar data of volcanic ash layers intercalated in the astronomically dated sections of monte dei corvi and monte gibliscemi italy to obtain better radioisotopic time constraints on the serravallian tortonian boundary and to confirm the intercalibration of radioisotopic and astronomical time proposed by kuiper et al 2004 fish canyon tuff fct sanidine at 28 21 0 03 ma the latter intercalibration is supported by astronomically calibrated fct sanidine ages for two ash layers at monte gibliscemi gif 1 28 28 0 04 gid 3 28 16 0 04 ma 1 se as a consequence our results support the astronomically calibrated age of 11 608 ma for the tortonian global stratotype section and point and hence the tuning of the serravallian tortonian boundary interval the ancona and respighi levels at monte dei corvi give a more diffuse picture possibly because of contamination with detrital or xenocrystic material and the inferior quality of biotite for intercalibration

the diamond bearing kokchetav uhp massif in northern kazakhstan exhumation structure in the kokchetav ultrahigh pressure uhp massif in northern kazakhstan diamond bearing uhp rocks occur exclusively in a western rhomb shaped domain that differs from an eastern transpressional domain with coesite bearing remnants indicating highest uhp conditions different mechanisms may have contributed to the early ascent of the uhp kokchetav massif the geometry and structure of the diamond bearing domain are interpreted as a sheath like fold coeval with early stage melting in contrast the coesite bearing domain has a sheet like geometry at mid crustal level this early difference in the ascending uhp wedge is reflected in a western rhomb horst and an eastern transpression structure respectively the latter extends to the east borovoye where the uppermost sequences of the uhp massif were defined by others sheath folding is postulated as a suitable mechanism contributing to the early buoyancy driven ascent of the subducted rocks and explains the selective spatial distribution of diamond and coesite bearing sequences preserved in the wedge

raman spectroscopic and microscopic criteria for the distinction of microdiamonds in ultrahigh pressure metamorphic rocks from diamonds in sample preparation materials natural diamond from three ultrahigh pressure metamorphic uhpm terranes erzgebirge massif germany kokchetav massif northern kazakhstan rhodope metamorphic province greece and synthetic diamond from cutting and polishing materials paste spray saw blade were studied by means of optical microscopy and raman microspectroscopy to constitute a new petrographic and spectroscopic data set that might be a useful tool for identifying and characterizing metamorphic diamond several criteria are established for distinguishing natural microdiamond identified in a rock thin section from the externally introduced ones i e diamond as residual particles contaminants from the cutting and polishing material such as the diamond size the presence of inclusions coatings or coexistent phases and two diamond raman band parameters i e the raman shift and the full width at half maximum height fwhm

polyphase zircon in ultrahigh temperature granulites rogaland sw norway constraints for pb diffusion in zircon shrimp u pb ages have been obtained for zircon in granitic gneisses from the aureole of the rogaland anorthosite norite intrusive complex both from the ultrahigh temperature uht 900 c pigeonite in zone and from outside the hypersthene in isograd magmatic and metamorphic segments of composite zircon were characterised on the basis of electron backscattered electron and cathodoluminescence images plus trace element analysis a sample from outside the uht zone has magmatic cores with an age of 1034 7 ma 2 n 8 and 1052 5 ma 1 n 1 overgrown by m1 metamorphic rims giving ages between 1020 7 and 1007 5 ma in contrast samples from the uht zone exhibit four major age groups 1 magmatic cores yielding ages over 1500 ma 2 magmatic cores giving ages of 1034 13 ma 2 n 4 and 1056 10 ma 1 n 1 3 metamorphic overgrowths ranging in age between 1017 6 ma and 992 7 ma 1 corresponding to the regional m1 sveconorwegian granulite facies metamorphism and 4 overgrowths corresponding to m2 uht contact metamorphism giving values of 922 14 ma 2 n 6 recrystallized areas in zircon from both areas define a further age group at 974 13 ma 2 n 4 this study presents the first evidence from rogaland for new growth of zircon resulting from uht contact metamorphism more importantly it shows the survival of magmatic and regional metamorphic zircon relics in rocks that experienced a thermal overprint of c 950 c for at least 1 myr magmatic and different metamorphic zones in the same zircon are sharply bounded and preserve original crystallization age information a result inconsistent with some experimental data on pb diffusion in zircon which predict measurable pb diffusion under such conditions the implication is that resetting of zircon ages by diffusion during m2 was negligible in these dry granulite facies rocks imaging and th u y systematics indicate that the main processes affecting zircon were dissolution reprecipitation in a closed system and solid state recrystallization during and soon after m1

geochronology and isotopic character of ultrahigh pressure metamorphism with implications for collision of the sino korean and yangtze cratons central china the collision zone between the sino korean and yangtze cratons in central china is marked by subduction related metamorphic rocks composed of crustal protoliths these range in metamorphic grade from ultrahigh pressure eclogite to low greenschist facies the paragenesis of the ultrahigh pressure rocks coesite and diamond bearing indicates subduction to depths greater than 120 km as a result of continental collision despite extreme metamorphic conditions zircons have not undergone pb loss but rather illustrate two periods of growth crystallization between 700 and 800 ma and ultrahigh pressure metamorphism at 218 2 5 ma previous studies have predicted a time transgressive collision but age data presented in this study indicate that the collision was approximately coeval along the length of the suture u pb zircon ages of these ultrahigh pressure metamorphic rocks are 218 4 2 5 ma and 217 1 8 7 ma for the dabie mountains and shandong peninsula respectively the similar ages imply that the tan lu fault which offsets the ultrahigh pressure metamorphic rocks is a secondary feature not related to collision or subduction whole rock isotopic data show that both rb sr and u pb were open systems during metamorphism but that sm nd probably remained closed nd initial isotopes display crustal signatures indicating that the crust subducted as a coherent slab rather than becoming intercalated with mantle material at depth sm nd data in conjunction with u pb zircon dating indicate that the ultrahigh pressure rocks are not typical yangtze craton basement 2 9 ga but that they originally crystallized in a rift environment between 700 and 800 ma discrepancies between 40ar 39ar plateaus sm nd isochrons and u pb zircon ages render calculation of a p t t path unrealistic

quality palynology and palaeoenvironmental interpretation of the ilgin lignite turkey the ilgin lignite field can be subdivided into the haramikoy and kurugol areas which are separated by an area of basement and a fault the lignite bearing sequence consists mainly of fluvial and lacustrine neogene deposits the ilgin lignite averaging 8 m up to 25 m thick is blackish brown and dark brown in colour the lignite contains abundant brown wood plant remains and white gastropod shells samples have an average of 38 9 moisture on an as received basis and a mean of 3 9 total sulphur on an air dried basis geochemical analysis using an icp ms was undertaken thirteen lignite core samples from three boreholes drilled in the kurugol area were ashed at 750 c the average trace element contents of the lignite show a higher concentration of u average 43 ppm when compared to the range for most coals in the world while the others ti p sc be mn co cu zn ga as rb sr zr nb mo cs ba y ta w tl pb bi th la ce pr nd sm eu gd tb dy ho er yb lu agree with the global ranges the ilgin lignite is identified as of middle miocene middle serravallian age and occurs within lacustrine sediments deposited under subtropical climatic conditions

petrographic facies analysis of lower and middle jurassic coal seams on the island of bornholm denmark abstract the lower middle jurassic succession on the island of bornholm denmark comprises a wide range of clastic depositional environments lacustrine fluvial coastal plain tidal flat and shallow marine the succession contains a number of coal seams and carbonaceous seams permitting 1 evaluation of the relationship between the different clastic environments and the associated environments of organic matter deposition and 2 assessment of facies changes within coal carbonaceous seams from different peat forming environments coal petrographic facies analysis was performed by maceral analysis of 61 coal samples from the lower jurassic ronne formation comprising the munkerup member sose bugt member and galgelokke member and from the upper lower middle jurassic baga formation the seams in the munkerup member and the sose bugt member are mainly allochthonous whereas those of the galgelokke member and the baga formation are mainly autochthonous the seams represent a broad spectrum of depositional environments from raised bogs to freshwater swamps and marine influenced swamps the facies are subaquatic to telmatic and terrestrial compositional differences between the seams are attributed to vegetation characteristics depositional facies and differences in the degradation of the organic matter the original peat forming vegetation was composed of cellulose rich shrub like plants tree ferns herbaceous plant communities and a varying amount of trees the peat forming environments represented by the carbonaceous seams in the munkerup and sose bugt members are compatible with the depositional environments indicated by the sediments however the coal seam in the galgelokke member represents deposition in a raised bog conflicting with sedimentary interpretation of a clastic tidal flat environment for the member the baga formation is considered to have formed under a lower upper delta plain or fluvial environment the composition of the coal seams supports the interpretation of a system of meandering streams in a upper delta plain to fluvial environment

dating of zircon from ti clinohumite bearing garnet peridotite implication for timing of mantle metasomatism garnet peridotites from the kokchetav ultrahigh pressure uhp massif contain abundant volatile and high field strength element hfse bearing minerals such as ti clinohumite and zircon these characteristics are interpreted to be related to fluid infiltrated mantle metasomatism from the oceanic lithosphere that had been subducted below the area the zircons from the peridotites were dated by using sensitive high resolution ion microprobe shrimp and yielded apparent u pb ages of 554 494 ma weighted mean age 528 ma that are mostly consistent with the timing of uhp metamorphism deduced from diamond bearing country rocks in this massif these zircons have an almost flat rare earth element ree pattern and very low ree concentrations these characteristics are similar to those observed in kimberlitic zircons inherited zircon cores although only rarely preserved yielded apparently proterozoic ages and have different trace element characteristics compared to the overgrowth rims these features indicate that the mantle metasomatic events and the recrystallization of ti clinohumite and zircon were due to hfse enriched fluid infiltration during the uhp metamorphism at great depths the metasomatized mantle may have been transported farther into the deep upper mantle and contributed to the source of intraplate magmas such as kimberlites and alkali basalts because these rocks have characteristically high volatile and hfse concentrations such as those of the ti clinohumite bearing garnet peridotites

discovery of clinoenstatite in garnet pyroxenites from the dabie sulu ultrahigh pressure terrane east central china transmission electron microscopy tem reveals that some enstatites from garnet pyroxenites in the chinese dabie sulu ultrahigh pressure uhp terrane consist of intimate intergrowths of orthoenstatite oren and clinoenstatite clen and some grains of pyroxene mgfe 2 si 2 o 6 exhibit inclined extinction these garnet pyroxenites are enclosed in coesite bearing eclogite or interlayered with harzburgite and omphacitite they contain garnet prp 54 to prp 73 enstatite en 85 to en 92 magnesite and titanomagnetite sulu sample or clinohumite dabie sample and recrystallized at about 4 0 6 5 0 2 gpa 750 50 c all clinoenstatite lamellae in the host orthoenstatite have even numbers of 9 a 100 fringes without twins and are oriented parallel to 100 of the host the lamellae and host phases have nearly identical mgfe 2 si 2 o 6 compositions the thickest observed clen lamellae are 0 4 m clinoenstatites from sulu have cell parameters a 9 67 7 a b 8 88 6 a c 5 22 3 a 107 7 2 and v 427 8 a 3 and those from dabie a 9 53 6 a b 8 88 6 a c 5 24 3 a 107 0 2 and v 424 8 a 3 the tem electron diffraction patterns indicate that both clinoenstatites possess p 2 1 c structures and are the low p polymorph of clinoenstatite the host orthoenstatites have cell dimension a 18 32 12 a b 8 88 6 a c 5 20 3 a for the sulu sample and a 18 24 12 a b 8 88 6 a c 5 18 3 a for the dabie sample clinoenstatites in garnet pyroxenites from the dabie sulu uhp terrane may have formed either by inversion from oren or by the displacive transformation from high p clinoenstatite during decompression

genesis of the banxi sb deposit south china constraints from wall rock geochemistry fluid inclusion microthermometry rb sr geochronology and h o s isotopes abstract the banxi sb deposit is located in the world class central western hunan sb belt in south china with quartz vein type sb only mineralization hosted by neoproterozoic epimetamorphic clastic sedimentary rocks in this contribution microthermometric rb sr geochronological and h o isotopic analyses on fluid inclusions trapped in quartz s isotopic studies on stibnite and major trace elemental measurements on fluid altered and unaltered wall rocks are presented to constrain the genesis of the deposit the fluid inclusions are characterized by low homogenization temperature 170 260 c and low salinity 3 7 wt nacl equiv with a low fluid density of 0 72 0 93 g cm3 and a ph of 5 59 the low dw fluid 2h 140 to 107 whereas positive 18ov smow 15 9 17 2 and 18ow fluid 18o 5 9 8 4 values in quartz indicate that the mineralizing fluid was generated from the low grade metamorphic basement rocks and received a certain input of meteoric water the rb sr isotopic compositions of quartz samples yielded an isochron age of 196 4 ma 1 mswd 0 70 for quartz stibnite ores a highly radiogenic 87sr 86sr value 0 72663 0 74002 initial 87sr 86sr 0 72640 indicates that ore forming materials are derived mainly from basement rocks and a direct material contribution from local granitic rocks can be ruled out the 34sv cdt values of stibnite range narrowly from 4 81 to 6 72 and are in the 34sv cdt range of the metamorphic rocks indicating that the sulfur was sourced from the basement rocks by sulfate reduction compositional differences between fluid altered decolorized and unaltered wall rocks are obvious the migrated sio2 so3 lree hf nb zr ta th u mo sn w into altered wall rocks suggests a deep basement metamorphic rocks derived si rich alkaline brine for the fluid rock interaction while the leached fe2o3 mgo mno tio2 ba cs rb as co v ni sb sc cu pb and zn from the altered rocks imply a material contribution of banxi group wall rocks during the sb mineralization process consequently we conclude that the quartz stibnite stage of the banxi sb deposit was formed during the late triassic orogeny in the xuefeng mountain region heated by granitic magmatism sb and s were leached from the basement rocks forming an initial si rich and alkaline ore bearing fluid precipitation of stibnite from the fluid resulted from interacting with wall rocks and mixing of meteoric water coupled by releasing of stress decreasing of temperature and lowering of the ph at higher crustal levels

geochemistry and age of ultrahigh pressure metamorphic rocks from the kokchetav massif northern kazakhstan isotopic and geochemical data of the zerenda series metamorphic rocks from the kokchetav massif are reported some of these rocks contain microdiamond inclusions in garnets and other indicators of ultrahigh pressure metamorphism p 40 kbar t 900 1000 c the diamond bearing rocks exhibit distinctive geochemical characteristics compared to typical crustal rocks the ree patterns range from lree depleted to slightly lree enriched chondrite normalized la yb n 0 1 5 4 with a negative eu anomaly they are depleted in incompatible elements e g sr ba u th with respect to the upper crust in contrast non diamondiferous rocks of the zerenda series exhibit normal crustal geochemistry all rocks of the zerenda series have very radiogenic lead isotopes the measured values 238u 204pb compared with those calculated for the interval between crust formation and ultrahigh pressure uhp metamorphism suggest a decrease by factors of up to 200 during the uhp metamorphism the sm nd mineral isochrons from the diamond bearing rocks and other rock types of the zerenda series give a middle cambrian 524 535 ma age of metamorphism the nd model ages show that crust formation occurred about 2 3 ga ago significant fractionation of sm and nd and loss of incompatible elements may be due to partial melting of the protoliths the ar ar age determinations of secondary biotite and muscovite from the diamond bearing rocks yield an age of 517 5 ma this cooling age requires a short time interval between uhp metamorphism and uplift to a crustal level ultrahigh pressure metamorphism might be a significant source of pb for the mantle we propose that the radiogenic pb of the oceanic array is the contamination traces of numerous uhp events beside the geological aspect we demonstrate a method of dating a high grade metamorphic terrain using nd isotopes we compare whole rock isochrons and mineral isochrons and in this way get some insight into the behaviour of the sm nd system during very high grade metamorphic events

fluid evolution and mineralization mechanism of the east kounrad porphyry mo w deposit in the balkhash metallogenic belt central kazakhstan abstract the east kounrad mo w deposit is located in the eastern part of the balkhash metallogenic belt central kazakhstan the intrusion in the east kounrad deposit is a permain complex that includes medium and fine grained granites both the primary magma and hydrothermal fluids of the east kounrad deposit are characterized by high fluorine and high oxygen fugacity which would be important for the formation of high grade large mo deposit four mineralization stages are identified in this study stage i quartz wolframite molybdenite fluorite veins which were associated with intensively greisen alteration were formed from fluids with high temperature 340 460 c high pressure 250 400 bars high low salinity 54 5 wt and 3 4 10 2 wt respectively and high oxygen fugacity with a fluid system of h2o nacl co2 stage ii quartz molybdenite pyrite veins which were accompanied by k feldspars muscovite fluorite alteration were formed from fluids with medium high temperature 260 400 c medium pressure 100 250 bars high low salinity 39 1 58 4 wt and 2 4 11 7 wt respectively and relatively low oxygen fugacity after the main stage of w and mo mineralization stage iii quartz pyrite molybdenite veins associated with quartz sericite alteration were formed from fluids with medium low temperature 160 340 c low pressure less than 100 bars various salinity 4 5 46 2 wt and high oxygen fugacity stage iv quartz polymetallic sulfide veins related to argillic alteration were formed under temperature of 120 220 c and salinity of 1 7 8 1 wt remarkably fluid boiling is the most important precipitation mechanism of molybdenite and wolframite in addition the decreasing oxygen fugacity and temperature also play a vital role in triggering deposition of molybdenite the pb isotopic composition of sulfides shows that the ore forming materials at east kounrad are dominantly derived from the lower crust

ultrarapid exhumation of ultrahigh pressure diamond bearing metasedimentary rocks of the kokchetav massif kazakhstan the diamond bearing ultrahigh pressure kokchetav massif recrystallized at eclogite facies conditions deep in the mantle at 180 km depth at 535f3 ma and yet new 40 ar 39 ar ages suggest that it may have been exhumed to crustal depths as indicated by closure of mica to ar loss by f529 ma these data indicate a possible exhumation rate of tens of kilometers per million years i e a rate that is comparable to rates of horizontal plate motion and subduction d 2003 elsevier b v all rights reserved

40ar 39ar analyses of artificially mixed micas and the treatment of complex age spectra from samples with multiple mica populations abstract artificial mixtures of two age populations of muscovite and of biotite were prepared at relative weight percents of 3 1 1 1 and 1 3 and analyzed by the 40ar 39ar vacuum furnace step heating method the original micas are late jurassic and late cretaceous samples that yield flat age spectra with plateau ages defined by 97 100 of the gas release and no evidence for excess argon the age spectra from mica mixtures yield patterns that systematically decrease with decreasing jurassic mica content and increasing cretaceous component however the mixed muscovite spectra are relatively flat consistent with the two original samples showing similar degassing patterns during vacuum furnace heating in contrast the mixed biotite spectra are highly discordant with an overall decrease in age during the step heat analysis this is consistent with the degassing patterns of the original mica samples which indicate the jurassic biotite high fe mg outgases at lower temperatures and the cretaceous biotite low fe mg remains retentive until the higher temperature steps the individual degassing patterns and compositions show consistencies with known compositional controls on argon diffusivity indicating that micas may maintain argon retention characteristics rooted in crystal chemistry during vacuum furnace heating all but one of the mixed samples failed to yield ages reflecting the age of the original micas these results imply that complex age spectra obtained for multiply deformed rocks with multiple mica populations may define geologically meaningless ages as a result of simultaneous degassing i e mixing of sample reservoirs during heating comparison of laboratory degassing rates and crystal chemistry indicates compositional controls on argon retention are likely preserved during vacuum furnace step heating and micas may degas retain argon in a predictable manner these conclusions support assertions that recovery of fossil age gradients in slowly cooled samples may be possible using furnace step heating

the geology and coal petrology of a miocene lignite profile at meliadi mine katerini greece abstract the present study describes vertical variations in petrographic composition of a miocene lignitebearing succession from the moschopotamus basin greece the lignite beds are described megascopically in terms of lithotypes and microscopically by maceral contents examination of lithotype variations within the seams and the distribution of facies critical macerals such as humotelinite and humodetrinite and the relatively low inertinite contents suggest a formation of the miocene lignites as reed marsh to wet forest type swamps in a lower delta plain depositional setting reflectances measured on the maceral type eu ulminite b range from 0 36 to 0 42 r random placing the meliadi coals into a transition zone from lignite to subbituminous coal

nitrogen aggregation in metamorphic diamonds from kazakhstan microdiamonds that occur in high grade metamorphic rocks of the kokchetav massif in kazakhstan have been interpreted as part of the metamorphic mineral assemblage implying previously unrecorded pressures for the metamorphism of crustal rocks analysis of the infrared and visible absorption spectra from kokchetav microdiamonds extracted from recent stream sediments draining the metamorphic rocks shows roughly 48 63 aggregation of single n substitution to a centres but no measurable further aggregation calculated residence times assuming peak metamorphic temperatures of 800 900 c are from 0 5 ma to 40 ma and support the petrographic evidence that these diamonds formed during the high pressure metamorphism rather than being inherited from premetamorphic sediments

zircon the metamorphic mineral a mineral that forms under conditions as variable as diagenesis to deep subduction melt crystallization to low temperature alteration and that retains information on time temperature trace element and isotopic signatures is bound to be a useful petrogenetic tool the variety of conditions under which zircon forms and reacts during metamorphism is a great asset but also a challenge as interpretation of any geochemical data obtained from zircon must be placed in pressure temperature deformation fluid context under which condition and by which process zircon forms in metamorphic rocks remains a crucial question to answer for the correct interpretation of its precious geochemical information in the last 20 years there has been a dramatic evolution in the use of zircon in metamorphic petrology with the advent of in situ dating techniques zircon became relevant as a mineral for age determinations in high grade metamorphic rocks since then there has been incredible progress in our understanding of metamorphic zircon with the documentation of growth and alteration textures its capacity to protect mineral inclusions zircon thermometry trace element patterns and their relation to main mineral assemblages solubility of zircon in melt and fluids and isotopic systematics in single domains that go beyond u pb age determinations metamorphic zircon is no longer an impediment to precise geochronology of protolith rocks but has become a truly indispensable mineral in reconstructing pressure temperature time fluid paths over a wide range of settings an obvious consequence of its wide use is the rapid increase of literature on metamorphic zircon and any attempt to summarize it can only be partial in this chapter reference to published works are intended as examples and not as a compilation this chapter approaches zircon as a metamorphic mineral reporting on its petrography and texture deformation structure and mineral chemistry including trace element and isotopic systematics linking this information together highlights the potential of zircon as a key mineral in petrochronology

unusual hf contents in metamorphic zircon from coesite bearing eclogites of the dabie mountains east central china implications for the dating of ultrahigh pressure metamorphism metamorphic zircon from coesite bearing eclogites in the dabie mountains encloses high p phases and may have formed at the peak of ultrahigh pressure uhp metamorphism morphologically the metamorphic zircon typically occurs as small multi faceted near spherical grains with homogeneous internal structure and weak backscattered electron bse luminescence geochemically it is characterized by extremely high and relatively constant contents of hafnium hf and very low contents of y u and th reflecting the contraction of the zircon lattice under the uhp conditions high contents of hf may be characteristic of zircon formed during uhp metamorphism which has important consequences for interpretation of geochronological results we propose that the metamorphic zircon extremely enriched in hf may be used to date the peak of uhp metamorphism that produced the coesite bearing eclogites in the dabie mountains and potentially in other uhp terranes

zircon response to diamond pressure metamorphism in the kokchetav massif ussr ultrahigh pressure metamorphism of the diamond bearing gneisses in the kokchetav massif ussr has been dated at 530 7 ma 2 sgr by shrimp ion microprobe analysis of metamorphic zircons zircon xenocrysts as old as 2000 ma have retained original isotopic compositions through the extreme metamorphic conditions and indicate that the protolith that was buried to diamond forming pressures included early proterozoic components highly variable reset ting of u pb isotopic systems in zircon xenocrysts at the time of metamorphism suggests that diffusive lead loss had commenced but was incomplete a disequilibrium observation consistent with relatively transient exposure of the rocks to extreme crustal depths

40ar 39ar dating of a langhian biotite rich clay layer in the pelagic sequence of the conero riviera ancona italy a nearly complete and undisturbed miocene carbonate sequence is present in the easternmost part of the umbria marche basin italy which is ideal for detailed and integrated stratigraphic investigations of the miocene epoch in this study we were trying to obtain evidence for the presence or absence of distal ejecta from the 15 ma ries impact structure in southern germany located about 600 km to the north northwest of the umbria marche basin the first step is to find coeval strata in the umbria marche sequence at the la vedova section conero riviera we dated a volcaniclastic biotite rich clay layer the aldo level which is situated within planktonic foraminiferal zone n8 at 14 9 0 2 ma using the 40ar 39ar method together with detailed geologic and stratigraphic information about the aldo level the resulting age can be used confidentially to calibrate the langhian stage besides providing new constraints on miocene geochronology this age can now be used for impact stratigraphic studies to directly correlate the biotite ages of the la vedova section with rocks from the ries impact event ries impact glass was also analyzed and found to be coeval although unrelated to this impact event the biotite rich clay layer should help in the search for evidence of distal ejecta related to the ries crater

dating of zircon from ti clinohumite bearing garnet peridotite implication for timing of mantle meta somatism comment and reply comment katayama et al 2003 1 dated zircon in some ultramafic rocks dominated by garnet and titanian clinohumite hereafter ti clinohumite rocks from the kokchetav ultrahigh pressure uhp metamorphic terrane and interpreted that the zircon cores formed in the mantle in the proterozoic and the

metamorphic diamonds mechanism of growth and inclusion of oxides we report a detailed series of electron microscope observations of metamorphic microdiamonds included in and separated from garnets and zircons from a single specimen of garnet biotite feldspar gneiss from the kokchetav massif kazakhstan the morphology of the diamonds ranges from skeletal forms composed of thin 111 plates through cuboid and octahedral forms included within the diamonds is a diverse suite of nanometric oxides suggesting that the c o h fluid from which the diamonds grew may have carried chemical components derived from both the sediments and the mantle the spectrum of morphologies and their abundant tiny inclusions can all be explained by a simple model based on the ratio of the rate at which 111 plates grow and the rate of random nucleation of new plates at their edges

40ar 39ar u pb and sm nd constraints on the timing of metamorphic events in the maksyutov complex southern ural mountains new 40 ar 39 ar u pb and sm nd data from 13 metamorphic samples constrain the formation and exhumation of the high pressure maksyutov complex in the southern uralide orogen the maksyutov complex records the highest pressure metamorphism in the southern uralides and is part of the thrust stack that composes the footwall beneath the main ural fault the new isotopic data from eclogite in the lower unit of the maksyutov complex include 40 ar 39 ar data from phengite u pb data from rutile and apatite and sm nd data from garnet clinopyroxene rutile and apatite these data indicate that eclogite facies metamorphism occurred in the mid devonian epoch c 380 ma corresponding with eastward subduction of the east european craton beneath the magnitogorsk island arc after partial exhumation the lower unit was juxtaposed c 360 ma based on 40 ar 39 ar white mica data with the middle and upper units at mid crustal levels during the development of a d 3 shear zone within the subduction zone by the late carboniferous epoch the maksyutov complex was exhumed to upper crustal levels compatible with sedimentologic evidence that at this time the foreland thrust and fold belt began to form and material was shed off it into the foreland basin

geochemistry of garnetiferous ti clinohumite rock and talc kyanite phengite almandine schist from the kokchetav uhp terrane kazakhstan an insight to possible origins of some chemically unusual uhp rocks abstract major element trace element and o isotope analyses were carried out for garnetiferous ti clinohumite rocks and eclogites from the kumdy kol area as well as talc kyanite phengite almandine schists previously named as whiteschists and eclogites from the kulet area of the kokchetav ultrahigh pressure uhp terrane to explore their possible origins both garnetiferous ti clinohumite rocks and talc kyanite phengite almandine schists are possibly genetically related to eclogites as they occur in close association with eclogites enclosed within gneiss metapelite in these two areas most eclogites from the kumdy kol and the kulet areas exhibit chemical characteristics similar to that of ocean floor basalt although a few show arc affinity with slight ta and nb depletions garnetiferous ti clinohumite rocks from the kumdy kol region are characterized by very high mg low si ca na and k contents compared with eclogites they also exhibit lree depleted patterns with negative eu anomalies their o isotope compositions are less than 5 6 on the other hand the rocks show comparable al ni cr co and hree contents as eclogites the mg enrichment of these garnetiferous ti clinohumite rocks is suggested to have originated from their mafic protoliths that had been subjected to mafic rock ultramafic rock fluid interactions talc kyanite phengite almandine schists from the kulet area are typically low in ca and na but high in mg k rb ba cs th u and o isotope compositions 6 to 10 relative to eclogites this peculiar chemical composition can be accounted for if the basaltic protoliths had been subjected to sequential high temperature enrichment of mg and low temperature enrichment of k rb ba cs th u and o isotope compositions seawater basalt interactions one talc kyanite phengite almandine schist sample klw 7 is distinctly high in si and k but low in mg and fe which can be attributed to more severe low temperature hydrothermal alterations however except for mg the enrichment of rb ba cs th u and o isotope compositions in these schist samples could also be enhanced by or otherwise be interpreted as a result of fluid influx sourced from enclosing metapelite during subduction

pb diffusion in zircon abstract chemical diffusion of ti under anhydrous conditions at 1 atm and under fluid present elevated pressure 1 1 1 2 gpa conditions has been measured in natural zircon the source of diffusant for 1 atm experiments was a zro2 tio2 zircon mixture with experiments run in crimped pt capsules diffusion experiments done in the presence of h2o co2 fluid were run in a piston cylinder apparatus using a source of ground tio2 zrsio4 and sio2 with oxalic acid added to produce h2o co2 vapor and partially melt the solid source material yielding an assemblage of rutile zircon melt vapor nuclear reaction analysis nra with the resonant nuclear reaction 48ti p 49v was used to measure diffusion profiles for both sets of experiments the following arrhenius parameters were obtained for ti diffusion normal to cover the temperature range 1350 1550 c at 1 atm d ti 3 33 10 2 exp 754 56 kj mol 1 r t m 2 s 1 ti diffusivities were found to be similar for experiments run under fluid present conditions a fit to all of the data yields the arrhenius relation d 1 34 102 exp 741 46 kj mol 1 rt m2 s 1 these data suggest that zircon should be extremely retentive of ti chemical signatures indicating that the recently developed ti in zircon crystallization geothermometer watson e b harrison t m 2005 zircon thermometer reveals minimum melting conditions on earliest earth science 308 841 844 will be quite robust in preserving temperatures of zircon crystallization titanium diffuses somewhat faster in zircon than larger tetravalent cations u th and hf but considerably more slowly than pb the ree and oxygen hence ti crystallization temperatures may be retained under circumstances when radiometric ages or other types of geochemical information are lost

metamorphic evolution of diamond bearing and associated rocks from the kokchetav massif northern kazakhstan representative diamond bearing gneisses and dolomitic marble eclogite and ti clinohumite bearing garnet peridotite from unit i at kumdy kol and whiteschist from unit ii at kulet eastern kokchetav massif northern kazakhstan were studied diamond bearing gneisses contain variable assemblages including grt bt qtz pl kfs zo chl tur cal and minor ap rt and zrn abundant inclusions of diamond graphite chlorite or calcite phengite clinopyroxene k feldspar biotite rutile titanite calcite and zircon occur in garnet diamond bearing dolomitic marbles consist of dol di grt phl inclusions of diamond dolomite graphite biotite and clinopyroxene were identified in garnet whiteschists carry the assemblage ky tlc grt rt garnet shows compositional zoning and contains abundant inclusions of talc kyanite and rutile with minor phlogopite chlorite margarite and zoisite inclusions and zoning patterns of garnet delineate the prograde p t path inclusions of quartz pseudomorphs after coesite were identified in garnet from both eclogite and gneiss other ultrahigh pressure uhp indicators include na bearing garnet up to 0 14 wt na2o with omphacitic cpx in eclogite occurrence of high k diopside up to 1 56 wt k2o and phlogopite in diamond bearing dolomitic marble and cr bearing kyanite in whiteschist these uhp rocks exhibit at least three stages of metamorphic recrystallization the fe mg partitioning between clinopyroxene and garnet yields a peak temperature of 800 1000 c at p 40 kbar for diamond bearing rocks and about 740 780 c at 28 35 kbar for eclogite whiteschist and ti bearing garnet peridotite the formation of symplectitic plagioclase amphibole after clinopyroxene and replacement of garnet by biotite amphibole or plagioclase mark retrograde amphibolite facies recrystallization at 650 680 c and pressure less than about 10 kbar the exsolution of calcite from dolomite and development of matrix chlorite and actinolite imply an even lower grade greenschist facies overprint at c 420 c and 2 3 kbar a clockwise p t path suggests that supracrustal sediments together with basaltic and ultramafic lenses apparently were subjected to uhp subduction zone metamorphism within the diamond stability field tectonic mixing may have occurred prior to uhp metamorphism at mantle depths during subsequent exhumation and juxtaposition of many other tectonic units intense deformation chaotically mixed and mylonitized these lithotectonic assemblages

provenance of permian triassic and ordovician metagraywacke terranes in new zealand evidence from 40ar 39ar dating of detrital micas single crystal 40 ar 39 ar ages of detrital micas from greenland group ordovician low grade metagraywackes of the west coast south island new zealand indicate a source area having muscovite age patterns of 490 515 ma the detrital muscovite ages are characteristic of granitoid and gneiss terranes of the ross adelaide fold belt of antarctica therefore the source area for the greenland group sedimentary rocks was the immediately adjacent pacific margin of gondwana similar age determinations for rakaia terrane permian triassic torlesse supergroup metagraywackes of the wellington and canterbury regions of new zealand indicate that their source area included rocks having prominent muscovite cooling ages populations of 210 290 ma 80 and 410 460 ma 15 the torlesse detrital muscovite age data thus suggest that the source area of the metagraywackes was not within the lachlan fold belt of southeast australia and analogues in antarctica nor in the adjacent ross adelaide fold belt rather the rakaia terrane of the torlesse metagraywackes is a true suspect terrane derived from a source area within the hunter bowen fold belt of northeast australia and the immediately adjacent older orogens of eastern queensland

using u pb ages of miocene tufa for correlation in a terrestrial succession barstow formation california sedimentary rocks and associated fossils are the direct record of past life and its environmental context few methods allow direct and precise dating of this important archive making it diffi cult to correlate climate biologic and tectonic events preserved in terrestrial records we sampled lacustrine tufa calcite from the middle member of the miocene barstow formation to further test the feasibility of uranium lead dating of terrestrial carbonates five samples yield u pb ages 2 of 14 81 0 39 ma 15 30 0 25 ma 15 39 0 15 ma 16 14 0 40 ma and 16 24 0 23 ma these precise ages are consistent with existing ages from k ar and 40 ar 39 ar dating of intercalated volcanic ashes our most precise u pb age on tufa has an uncertainty of 1 150 ka which is equivalent to uncertainties for k ar ages from the same time period our results indicate that u pb dating of carefully sampled and screened sedimentary carbonates provides much needed age constraints in purely terrestrial sequences and can be a powerful tool for correlation even in deposits with structural complexity and rapid facies changes

palaeoenvironmental reconstruction of the cardak coal seam sw turkey abstract the study focuses on the firstly documented cardak coal seam cored in the neogene acigol basin denizli province being formed in a late stage of alpine orogeny in sw turkey the aim is to contribute reconstructing the palaeoenvironmental conditions before during and after coal formation on the basis of coal petrology mineralogical and palaeontological data combined with the results of previous studies the basin infilling consists of alluvial fluvial and lacustrine sediments their deposition depended on the water level in the basin which in turn was controlled by the interplay between tectonics and climate coal seam was probably formed in latest miocene early pliocene under the influence of basin s fluvial regime in oxbow and terminal lakes the coal seam comprises alternating inorganic and coal layers logged between depths of 599 00 to 627 00 m beneath surface the coal being lignite to subbituminous in rank displays moderate to high ash yield 21 31 46 11 wt on dry basis and high total sulphur content up to 13 30 wt on dry ash free basis the maceral composition points to an origin from mainly herbaceous helophytes along with woody species in the upper part of the core the presence of salinity tolerant ostracod species along with gypsum and halite are documented the palaeontological and mineralogical data obtained imply changes of climate water level and lake chemistry in the basin

mesozoic cooling across the yidun arc central eastern tibetan plateau a reconnaissance 40ar 39ar study abstract 40 ar 39 ar dating of hydrous minerals from igneous and metamorphic rocks was undertaken with the aim of characterizing the thermal history of the yidun arc eastern tibetan plateau regional metamorphism of palaeozoic rocks of the western yidun arc occurred during the early triassic collision with the qiangtang block along the jinsha jiang suture hornblende muscovite and biotite 40 ar 39 ar ages from syn tectonic igneous rocks within this collision zone suggest that the region had cooled to below 300 c by the late triassic muscovite samples from mica schists within this collision zone yield variably discordant age spectra with generally triassic 40 ar 39 ar ages consistent with cooling of the region to below muscovite closure temperatures by the close of the triassic hornblende and biotite 40 ar 39 ar ages from granite and granodiorite plutons in the eastern yidun arc suggest post emplacement cooling of these plutons occurred rapidly consistent with these granites being high level intrusions in sum these results suggest that by the beginning of the jurassic the yidun arc as a whole had cooled to below 300 c two cretaceous granites in the central yidun arc also experienced rapid cooling suggesting they too were emplaced at shallow crustal depths post triassic deformation of the region must have occurred at or below the brittle ductile transition within the crustal levels now exposed

evidence of fluid inclusions in metamorphic microdiamonds from the kokchetav massif northern kazakhstan abstract microdiamonds from garnet clinopyroxenites of the kokchetav massif northern kazakhstan and associated alluvial diamonds have been investigated using infrared spectroscopy all diamonds have high nitrogen contents from 747 up to 4488 20 at ppm and are of type ib iaa nitrogen data are consistent with a diamond formation over a narrow temperature range microdiamonds from garnet clinopyroxenites are characterized by water and carbonate inclusions similar to those ocurring in fibrous coated diamonds from kimberlite which suggest that diamonds grew from a c h o fluid in contrast alluvial microdiamonds do not contain any carbonate inclusions and h2o is absent or present in only minor amounts alluvial diamonds are considerably more n rich but have a similar range of nitrogen aggregation states it is suggested that alluvial diamonds belong to a separate diamond population compared with the microdiamonds from garnet clinopyroxenites the kokchetav microdiamonds are distinct from kimberlitic fibrous diamonds in that they contain abundant ib centres have high nitrogen contents and show different populations depending on host rock type these characteristics support a metamorphic origin for the microdiamonds from the uhpm rocks of the kokchetav massif

p t t paths and differential alleghanian loading and uplift of the bronson hill terrane south central new england late paleozoic u pb ages of sphene and 4 0 ar 3 9 ar cooling ages of amphibole and muscovite from rocks of the bronson hill terrane in connecticut and central massachusetts reflect a late paleozoic alleghanian overprinton acadian metamorphic rocks prograde alleghanian sphenes crystallized during the late pennsylvanian and eliminate the possibility that amphibole ages reflect delayed permian cooling from devonian acadian metamorphism fourteen new amphibole ages from connecticut form a north to south trend of decreasing age from 294 to 245 ma while in massachusetts four new amphibole ages together with three others from the literature produce a random carboniferous pattern seven new muscovite ages support existing data indicating uniform cooling throughout the bronson hill terrane through 350 c in the early triassic the rate of permian cooling defined by amphibole muscovite pairs increases from 4 c my in northern connecticut to 50 c my near long island sound hinged loading and hinged but delayed exhumation in the southern part of the bronson hill terrane with the hinge in central connecticut explain these ages and cooling rates as well as a southerly increasing metamorphic field gradient one dimensional thermal modeling indicates that loading of bronson hill rocks must have begun by the late mississippian the time of peak alleghanian metamorphic temperature decreases southward from early permian in northern connecticut to late permian to the south these results demonstrate that the metamorphic effects of the alleghanian orogeny are not restricted to the avalon terrane of southeastern new england on the contrary the alleghanian orogeny reset 4 0 ar 3 9 ar mineral ages recrystallized minerals partially melted felsic rocks and transposed fabrics at least as far west as the bronson hill terrane in south central new england

formation of the kokchetav subduction collision zone northern kazakhstan insights from zircon u pb and lu hf isotope systematics abstract the kokchetav subduction collision zone is located in the western part of the ancestral central asian orogenic belt this zone is built up by the precambrian kokchetav microcontinent which includes a hp uhp metamorphic belt and the north kokchetav tectonic zone nktz which represents an accretionary complex between the kokchetav microcontinent and the adjacent stepnyak island arc the entire region is widely intruded by palaeozoic granitoids which were emplaced after the collision of the stepnyak island arc with the kokchetav microcontinent we present zircon u pb ages and lu hf systematics in zircon to better characterize the tectonic evolution of the kokchetav subduction collision zone the lu hf results indicate that the kokchetav basement rocks are derived from late neoarchaean early palaeoproterozoic 2 5 ga crust for the granite gneiss basement of the kokchetav microcontinent early mesoproterozoic grenville age 1 17 1 14 ga zircon u pb crystallization ages were obtained for the nktz two main age components were recognized 1 an oldest mesoproterozoic age component 1 20 1 05 ga similar as for the kokchetav microcontinental zircons and 2 a younger early cambrian 540 520 ma or late cambrian early ordovician 490 480 ma age component th u ratios 0 1 and are typical of a magmatic protolith the distinction between both zircon types is supported by characteristic cathodoluminescence images these results confirm previous observations indicating early palaeozoic high grade metamorphism 540 520 ma and collisional deformation 490 480 ma of the mesoproterozoic kokchetav zone as a result of subduction accretion and collision of the stepnyak island arc to the kokchetav microcontinent for two additional samples of the balkashin granitic complex early devonian 415 395 ma zircon crystallization ages magmatic th u ratios were obtained the occurrence of a mesozoic xenocryst within these leucogranites may indicate that they were emplaced in a continental arc setting

sinking intrusion model for the emplacement of garnet bearing peridotites into continent collision orogens garnet and probably spinel bearing ultramafic rocks that form tectonic lenses within high metamorphic grade gneisses in mountain core zones may intrude by moving downward or laterally into deeply subducted continental crust from the overlying mantle wedge this model is most credible for garnet peridotites associated with ultrahigh pressure metamorphic terranes in mountains formed by continent continent collision the extremely high pressures implied by ultrahigh pressure metamorphism indicate that continental crust was subducted to mantle depths 100 120 km where the mantle is in the garnet peridotite facies exchange between mantle and crust seems likely particularly the transfer of mantle from the hanging wall into the crustal footwall the mantle wedge overlying the subducted crustal terrane creates a denser over lighter configuration that is gravitationally unstable if the interface becomes hot or wet enough so that peridotite can deform by ductile flow it would sink as mantle blobs into the underlying crustal rocks if not the exchange presumably occurs through a brittle mechanism the intruded bodies will be transported to the surface passively as the host crustal terrane is exhumed buoyantly toward the surface this mechanism is proposed in particular to explain the intrusion of garnetiferous peridotites in the ultrahigh pressure western gneiss region of the norwegian caledonides during the caledonian orogeny

a fault controlled newly discovered eskisehir alpu coal basin in turkey its petrographical properties and depositional environment abstract recent findings suggest that turkey contains about 15 billion tons of coal within its borders a small proportion of it nw part is bituminous but the rest is sub bituminous and lignite coal about 80 of the coals are utilized in power generation the newly found coal field alpu basin contains about 1 3 billion tons of coal the coal basin is a fault controlled and deeply buried basin which is close to the industrial areas in turkey in the studied area there are two coal horizons in the middle miocene aged m2 unit the average mineable coal seam thickness is approximately 13 14 m in order to determine the coalification degree petrographic composition and depositional environment of the coals petrographic and xrd analyses were conducted on coal samples taken from three boreholes which were drilled by mta in the eskisehir alpu basin according to the astm classification the rank of the coals is sub bituminous alpu coals contain two prominent types of maceral assemblages either containing high amounts of funginites macrinites ulminite and gelinites or high amounts of liptinites associated with detrital huminites attrinite and densinites the first type exhibits rather more homogeneous features but the second has a more complex mixed appearance with detrital macerals intensive lineation laminations and considerably higher pyrite contents the alpu coals contain montmorillonite heulandite quartz pyrite gypsum kaolinite illite illite montmorillonite orthoclase and clay minerals which indicate variation in the ph conditions the tissue preservation index tpi and gelification index gi indicate a limnic environment the sample points are located between open marsh and clastic marsh areas on the tpi gi diagram this case indicates that the water table was variable during the coal formation

distribution of environmental sensitive trace elements in the eocene sorgun coals turkey abstract the distribution of trace elements in the lower eocene coal seam mined in the yeniceltek kucukkohne and ayridam coal mines from the sorgun basin was investigated in relation to ash content and maceral composition the coal seam is mainly composed of huminite in the present study 35 samples from five seam sections were collected on the basis of megascopic characteristics results were determined using an energy dispersive polarised x ray fluorescence edp xrf spectrometer on a whole coal dry basis most of the major and trace elements studied are enriched in high ash samples while ba br mn and w show relative enrichments in low ash samples most of elements studied such as ga ce la th nb rb zr v cu u pb sb cs sn cr se y and zn are primarily associated with mineral matter clay minerals arsenic and a part of zn se and sb are probably concentrated in pyrites in the samples element concentrations show statistically significant negative correlations with many macerals and positive relationships with only attrinite that is mainly mixed with mineral matter clay minerals and small quartz grains in the samples nine trace elements as cr mn ni pb sb se th and u considered as potentially hazardous air pollutants are present in low to moderate concentrations the mean values of trace element concentrations display relative enrichments in se 2 8 ppm th 21 ppm and w 26 ppm in the investigated samples in comparison with other coals in the world

oldest lamproites from peninsular india track the onset of paleoproterozoic plume induced rifting and the birth of large igneous province abstract potassic and ultrapotassic magmatism from deep lithospheric sources in intra cratonic settings can be the signal of subsequent voluminous mafic magmatism and the formation of large igneous provinces lips triggered by mantle plumes here we report for the first time precise zircon u pb age data from a suite of lamproites in the bastar craton of central india that mark the onset of paleoproterozoic rifting and culminating in the formation of extensive mafic dyke swarms as the bar codes of one of the major lip events during the precambrian evolution of the indian shield the lamproites from the nuapada field occur as dismembered dykes and are composed of phenocrysts and microphenocrysts of altered olivine together with microphenocrysts of phlogopite and magnetite within a groundmass of chlorite and calcite with accessory rutile apatite and zircon the rocks compositionally correspond to olivine phlogopite lamproite and phlogopite lamproite geochemical features of the lamproites correlate with their counterparts in peninsular india and other similar suites elsewhere in the world related to rift settings and also indicate oib like magma source the associated syenite shows subduction related features possibly generated in a post collisional setting magmatic zircon grains with high th u ratios in the syenite from the nuapada lamproite form a coherent group with an upper intercept age of 2473 8 ma representing the timing of emplacement of the magma zircon grains in three lamproite samples yield four distinct age groups at ca 2 4 ga 2 2 ga 2 0 ga and 0 8 ga the 2 4 ga group corresponds to xenocrysts entrained from the syenite whereas the 2 2 ga group is considered to represent the timing of emplacement of the lamproites the ca 2 0 ga zircon grains correlate with the major thermal imprint associated with mafic magmatism and dyke emplacement in southern bastar and the adjacent dharwar cratons a few young zircon grains in the syenite and lamproites show a range of early to middle neoproterozoic ages from 879 to 651 ma corresponding to younger thermal event s as also represented by granitic veins cutting across these rocks and extensive silicification zircon lu hf isotope data suggest magma derivation from a refertilized paleo mesoarchean sub continental lithospheric mantle or oib type sources the differences in hf isotope composition among the zircon grains from different age groups indicate that the mantle sources of the lamproite are heterogeneous at the regional scale a combination of the features from geochemical and zircon hf isotope data is consistent with asthenosphere lithosphere interaction during the lamproite magma evolution the timing of lamproite emplacement in central india correlates with the global 2 2 ga record of lips we link the origin of the related mantle plume to the recycling of subducted slabs associated with the prolonged subduction accretion history prior to the neoarchean cratonization as well as the thermal blanket effect of the earth s oldest supercontinent pulsating plumes and continued rifting generated voluminous dyke swarms across the bastar and dharwar cratons forming part of a major global rifting and lip event

thermal equation of state of natural ti bearing clinohumite the natural occurrence of clinohumite in metabasalts and hydrothermally altered peridotites provides a source of water rich minerals in subducted slabs making knowledge of their phase relations and crystal chemistry under high p t conditions important for understating volatile recycling and geodynamic process in the earth s mantle here we present a synchrotron based single crystal x ray diffraction study on two natural ti bearing clinohumites up to 28 gpa and 750 k in order to simulate conditions within subducted slabs no phase transition occurs in clinohumite over this p t range pressure volume relationships of both compositions at room temperature were fitted to a third order birch murnaghan equation of state eos with v0 650 4 3 a3 kt0 141 4 gpa and kt0 4 0 6 for ti poor clinohumite 0 07 ti per formula unit pfu and v0 650 8 3 a3 kt0 144 4 gpa and kt0 3 6 7 for ti rich clinohumite 0 21 ti pfu both clinohumites exhibit anisotropic compression with b c a we also refined p v t equation of state parameters using the high temperature birch murnaghan htbm eos yielding kt0 t p 0 040 10 gpa k and t 5 1 6 10 5 k 1 for ti poor clinohumite kt0 t p 0 045 11 gpa k and t 5 7 6 10 5 k 1 for ti rich clinohumite ti poor and ti rich clinohumites display similar equations of state but are 20 more incompressible than mg pure clinohumite and display 5 higher bulk sound velocity than olivine at upper mantle conditions our results provide constraints for modeling geodynamic process related to the subduction and transport of potentially water rich slabs in the mantle

comparison of 40ar 39ar conventional and laser dating of biotites from the north tregor batholith bulk samples and single grains of biotites from the north tregor batholite armorican massif france were analysed by the 40ar39ar method apparently pure single biotite grains analysed by the step heating procedure with the laser probe displayed plateau ages or slightly disturbed age spectra one strongly chloritized single biotite grain gave a very disturbed age spectrum with younger ages the bulk samples of biotites of the same rocks displayed disturbed age spectra characterized by younger low temperature ages and a hump at intermediate temperature steps these disturbances are explained by 39ar recoil from the biotite into the chlorite which is clearly evidenced by hrtem observations and not by excess 40ar the more concordant results obtained on single biotites are probably due to a more careful selection of the minerals in most cases we obtain good concordance of the ages 600 610 ma measured on the apparently pure single biotite grains and the high temperature ages of both the bulk samples of amphiboles and biotites from the same rocks

the relationship between tectono metamorphic evolution and argon isotope records in white mica constraints from in situ 40ar 39ar laser analysis of the variscan basement of sardinia the basement of sardinia represents a nearly complete section of a segment of the variscan belt that experienced a polyphase tectonometamorphic evolution and barrovian metamorphism this basement is well suited to investigate the relationship between tectono metamorphic evolution and argon isotope records in white mica micaschists from the garnet zone maximum t of up to 520 560 c contain two texturally and chemically resolvable generations of white mica 1 deformed celadonite rich flakes defining a relict s1 foliation preserved within the main s2 foliation or enclosed in rotated albite porphyroblasts 2 celadonite poor white micas aligned along the main s2 foliation the s1 foliation developed earlier and at a deeper crustal level with respect to that at which the thermal peak was reached from the staurolite zone t of up to 590 625 c to the sillimanite k feldspar zone white mica is nearly uniform in composition muscovite and is predominantly aligned along the s2 foliation or is of later crystallization in situ ar ar laser analyses of white mica yielded ages of 340 315ma in the garnet zone and 320 300ma in the staurolite and sillimanite k feldspar zones results highlight a close link between argon isotope records in white mica and both textures and structure forming major elements the oldest ages were detected in samples where the earlier syn d1 white mica generation did not texturally and chemically re equilibrate at upper crustal levels this study suggests that the white micas that escaped recrystallization retain argon isotope records of an earlier metamorphic stage that survived a later event characterized by temperatures higher than 500 c

diamond inclusions in garnets from metamorphic rocks a new environment for diamond formation diamonds commonly occur in kimberlites lamproites and alluvial sediments derived from these rocks more recently diamonds or their graphite pseudomorphs have been discovered in ultramafic massifs1 and picrites2 here we report the occurrence of diamonds in situ in crustal rocks highly retrograded high pressure metamorphic garnet pyroxene and pyroxene carbonate garnet rocks biotite gneisses and schists from the kokchetav massif northern kazakhstan ussr the diamonds are cubo octahedral averaging 12 m in size and occur in zircons and with euhedral graphite as inclusions in unzoned garnets we believe that the zircon and garnet matrices protected these diamonds from retrogressive transformation to graphite mica rutile titanite clinopyroxene kyanite and zircon also occur as inclusions in garnet often intergrown with the diamonds equilibration relations of inclusions and host garnets indicate that both diamonds and graphite crystallized from a fluid phase under static conditions at pressures of 40 kbar and temperatures 900 1 000 c

ela icp ms u pb zircon geochronology of regional volcanism hosting the bajo de la alumbrera cu au deposit implications for porphyry related mineralization ela icp ms u pb zircon geochronology has been used to show that the porphyritic intrusions related to the formation of the bajo de la alumbrera porphyry cu au deposit nw argentina are cogenetic with stratigraphically well constrained volcanic and volcaniclastic rocks of the late miocene farallon negro volcanic complex zircon geochronology for intrusions in this deposit and the host volcanic sequence show that multiple mineralized porphyries were emplaced in a volcanic complex that developed over 1 5 million years volcanism occurred in a multi vent volcanic complex in a siliciclastic intermontane basin the complex evolved from early mafic intermediate effusive phases to a later silicic explosive phase associated with mafic intrusions zircons from the basal mafic intermediate lavas have ages that range from 8 46 0 14 to 7 94 0 27 ma regionally extensive silicic explosive volcanism occurred at 8 0 ma 8 05 0 13 and 7 96 0 11 ma which is co temporal with intrusion of the earliest mineralized porphyries at bajo de la alumbrera 8 02 0 14 and 7 98 0 14 ma regional uplift and erosion followed during which the magmatic hydrothermal system was probably unroofed shortly thereafter dacitic lava domes were extruded 7 95 0 17 ma and rhyolitic diatremes 7 79 0 13 ma deposited thick tuff blankets across the region emplacement of large intermediate composition stocks occurred at 7 37 0 22 ma shortly before renewed magmatism occurred at bajo de la alumbrera 7 10 0 07 ma the latest porphyry intrusive event is temporally associated with new ore bearing magmatic hydrothermal fluids other dacitic intrusions are associated with subeconomic deposits that formed synchronously with the mineralized porphyries at bajo de la alumbrera however their emplacement continued from 7 10 0 06 to 6 93 0 07 ma after the final intrusion at bajo de al alumbrera regional volcanism had ceased by 6 8 ma 6 92 0 07 ma

clays and other minerals in coal seams of the moura baralaba area bowen basin australia abstract the clays and other minerals in a succession of late permian coals of high volatile bituminous to semi anthracite rank have been identified using low temperature oxygen plasma ashing and x ray diffraction and evaluated to identify the relative roles in mineral matter formation of detrital input early diagenesis in the peat swamp and late diagenesis associated with rank advance although well ordered kaolinite of probable early diagenetic origin is abundant throughout the succession the uppermost and lowermost seams of the sequence regardless of rank contain relatively abundant illite and or interstratified illite smectite along with a small but significant proportion of chlorite these clays are thought to be essentially of detrital origin washed or blown into the peat deposit in relative abundance during the establishment and subsequent overwhelming of an extensive and long lived swampy environment quartz is also abundant in the lower seams of the sequence especially close to the regional sediment source area illite is unusually abundant in the topmost seam in both high and low rank parts of the succession and thus appears to represent detrital input from a particular source material although significant changes are reported in the clays of the associated strata due to rank advance the principal effect of rank advance on the minerals in the coal itself appears to be the development of an ammonium illite and possibly some additional fine grained chlorite in the semi anthracite material isolation within the organic matter of the coal is thought to have inhibited access for ions such as k which might otherwise have become involved in metamorphic reactions and given rise to mineralogical changes commonly found in non coal sedimentary successions

microdiamonds frontier of ultrahigh pressure metamorphism a review abstract this is a comprehensive review paper devoted to microdiamonds from ultrahigh pressure metamorphic uhpm terranes incorporated in orogenic belts formed at convergent plate boundaries in paleozoic mesozoic alpine time when in 1980 the first small diamonds were discovered within amphibolite granulate facies metamorphic rocks it came as a great surprise that buoyant continental crust could be subducted to depths of hundreds of kilometers and then subsequently exhumed since then much progress has been made in understanding the mechanism of these diamonds formation and the number of new diamond bearing uhpm terranes was significantly increased especially within european orogenes moreover new variations in tectonic settings in which uhp rocks can be formed and exhumed came to the attention of geologists simply due to the finding of diamonds in places previously forbidden for their formation e g oceanic islands ophiolites and forearc environments over the past decade the rapidly moving technological advancement has made it possible to examine microdiamonds in detail and to learn that part of them has a polycrystalline nature that they contain nanometric multiphase inclusions of crystalline and fluid phases and that they keep a crustal signature of carbon isotopes scanning and transmission electron microscopy focused ion beam techniques synchrotron infrared spectroscopy micro x ray diffraction and nano secondary ion mass spectrometry studies of these diamonds provide evidence that they keep traces of fluid originated from both crustal and mantle reservoirs and that they probably interacted with deep mantle plumes hypotheses proposed for diamond formation in subduction zones founded on both analytical and experimental studies are discussed the paper also emphasizes that the discovery of these microdiamonds as well as coesite triggered a major revision in the understanding of deep subduction processes leading to a clear realization of how continental materials can be recycled into the earth s mantle and geochemically rejuvenate it

detrital geochronology of unroofing magmatic complexes and the slow erosion of oligocene volcanoes in the alps abstract tectonic reconstructions and quantitative models of landscape evolution are increasingly based on detailed analysis of detrital systems since the definition of closure temperature in the 1960s mineral ages of low temperature geochronometers are traditionally interpreted as the result of cooling induced by erosion whose rate is a simple unique function of age patterns such an approach can lead to infer paradoxically high erosion rates that conflict with compelling geological evidence from sediment thickness in basins this indicates that tectonic and landscape models that solely interpret mineral ages as due to cooling during exhumation may not be valid here we propose a new approach that takes into account the effects of both crystallization and exhumational cooling on geochronometers from u pb on zircon to fission tracks on apatite we first model the mechanical erosion of an unroofing magmatic complex and the resulting accumulation and burial of the eroded units in reverse order in the basin detrital mineral ages follow a regular pattern downsection some mineral ages such as e g u pb ages of zircons cluster around the magmatic age i e the crystallization of the magma its value is constant along the stratigraphic column in the sedimentary basin we refer to this behavior as stationary age peak some other mineral ages such as e g apatite fission track ages are often younger than the magmatic age when they vary smoothly with depth they define a moving age peak which is the only possible effect of undisturbed cooling during overburden removal and can therefore be used to calculate an erosion rate the predictions of our model were tested in detail on the extremely well studied bregaglia bergell orogenic pluton in the alps and on the sedimentary succession derived from its erosion the gonfolite group the consistency between predicted and observed age patterns validates the model our results resolve a long standing paradox in quantitative modelling of erosion sedimentation namely the scarcity of sediment during apparently fast erosion starved basins are the observational baseline and modelling must be tuned to include a correct analysis of detrital mineral geochronology in order to reconcile perceived discrepancies between stratigraphical and geochronological information in addition our data demonstrate that volcanoes were active on top of the growing oligocene alps this study illustrates rigorous criteria for detrital mineral geochronology that are applicable to any geological setting including magmatic arcs and collision orogens and provides fundamental interpretive keys to solve complex puzzles and apparent paradoxes in geological reconstructions

re evaluating the geochronology of the permian tarim magmatic province implications for temporal evolution of magmatism the permian tarim magmatic province has 118 published ages ranging from 358 to 205 ma but the timing of mafic magmatism is not well constrained we report two new secondary ion mass spectrometry u pb zircon dates on the halahatang trachydacite and wajilitag olivine clinopyroxenite which are 287 2 2 0 ma and 283 2 2 0 ma respectively the trachydacite overlies the uppermost basalt and constrains the latest eruption age of basalt in northern tarim the latter is the first high resolution date for the wajilitag mafic layered intrusion by screening all published ages we identified 22 robust ages ranging from 290 9 4 1 to 261 7 1 8 ma the robust ages together with our new data reveal a protracted period of mafic magmatism at c 283 and c 267 ma silicic magmatism occurred from 291 to 272 ma although the current known volume of tarim basalt is too small to qualify as a large igneous province the eroded and intrusive components as well as pyroclastic deposits and silicic lavas may increase the estimated volume further work is required to refine the duration of magmatism and the volume estimate of the province supplementary material a list of all the published dates raw data of published 40 ar 39 ar dates raw data of published u pb dates published geochemistry data used for calculation and complete details of the data evaluation are available at http www geolsoc org uk sup18862

potassium and argon loss patterns in weathered micas implications for detrital mineral studies with particular reference to the triassic palaeogeography of the british isles abstract potassium argon ages are reported for 16 muscovite and 14 biotite samples separated from triassic sandstones of the manx furness carlisle solway firth and lough neagh arran basins disturbance of the k ar isotopic system in these minerals during weathering erosion transport and lithification is discussed and loss of potassium shown to be as important a parameter as argon loss published data from tropical weathering profiles are used to establish the pattern of loss for these two elements during the formation of clay minerals from biotite the data being displayed in a potassium argon loss diagram the loss curve so produced is then used to interpret our new biotite data and previously published age data for detrital biotite from the cheshire basin the limit of interdigitation of northerly and southerly derived materials in the irish sea basins is shown to lie north of the fylde district of lancashire age data from detrital muscovite are used to establish a potassum argon loss curve for muscovite the features of the two curves are markedly similar little effect on the apparent k ar age is observed until about 20 loss of potassium occurs at this point massive argon loss takes place with little loss of potassium due to either potassium stabilisation or structural expansion the suitability of micas as k ar chronometers may be a consequence of the correlated potassium argon loss rather than intrinsic high argon retentivity if both mica curves are applied to detrial micas of the namurian millstone grit series a source area of age of ca 900 ma is indicated for both muscovite and biotite indicating a probable pre cambrian age source area probably in scandinavia

metamorphic zirconology of continental subduction zones abstract zircon is widely used to date geological events and trace geochemical sources in high pressure hp to ultrahigh pressure uhp metamorphic rocks of continental subduction zones however protolith zircons may be modified by three different types of metamorphic recrystallization via mechanisms of solid state transformation metasomatic alteration and dissolution reprecipitation new zircon growth may be induced by dehydration reactions below the wet solidus of crustal rocks metamorphic zircon or peritectic reactions above the wet solidus peritectic zircon as a consequence there are different origins of zircon domains in high grade metamorphic rocks from collisional orogens thus determining the nature of individual zircon domains is substantial to correct interpretation of their origin in studies of isotopic geochronology and geochemical tracing we advocate an integrated study of zircon mineragraphy internal structure and external morphology u pb ages mineral inclusions trace elements and lu hf and o isotope compositions only in this way we are in a position to advance the simple zircon applications to metamorphic zirconology enabling discrimination between the different origins of zircon and providing constraints on the property of fluid activity at subduction zone conditions the metamorphic recrystallization of protolith zircons and the new growth of metamorphic and peritectic zircons are prominent in hp to uhp metamorphic rocks of collisional orogens these different types of recrystallized and grown zircons can be distinguished by their differences in element and isotope compositions while the protolith nature of metamorphosed rocks dictates water availability the p t conditions of subduction zones dictate the property of subduction zone fluids the fluids of different properties may be produced at different positions of subducting and exhuming crustal slices and they may physically and chemically mix with each other in continental subduction channels such fluids can act as an important agent not only for the physical transport of protolith zircons but also for the chemical transport of element zr and other fluid mobile incompatible trace elements from the subducted crust to the mantle wedge therefore the discrimination between the different types of zircons provides a powerful means to decipher the role of fluids in subduction zone processes