zircon u pb chronology and elemental and sr nd hf isotope geochemistry of two triassic a type granites in south china implication for petrogenesis and indosinian transtensional tectonism abstract a detailed study utilizing zircon u pb dating major and trace element geochemistry and sr nd hf isotope geochemistry has been carried out for the caijiang granite in jiangxi province and the gaoxi granite in fujian province south china the new data indicate that the caijiang and gaoxi granites are triassic 228 230 ma and have the petrographic and geochemical characteristics of a type granites in both granites biotite occurs along the boundary of euhedral plagioclase and quartz which implies that the primary magma could have been anhydrous the two granites show high contents of total alkalis na 2 o k 2 o 7 81 12 15 high field strength elements e g zr 240 458 ppm y 16 8 38 0 ppm nb 13 5 33 8 ppm and zr nb ce y 382 604 ppm and rare earth elements total ree 211 373 ppm as well as high ga al ratios 10000 ga al 2 41 3 53 the lowest magmatic temperatures estimated from zircon saturation thermometer were 800 840 c for the caijiang granite and 820 850 c for the gaoxi granite respectively the caijiang granite has relatively high 87 sr 86 sr i ratios of 0 71288 to 0 72009 low e nd t values of 9 9 to 9 3 and low zircon e hf t values peak value of 7 5 whole rock nd isotopic model ages and zircon hf isotopic model ages mostly vary from 1 65 ga to 1 80 ga the gaoxi granite has also high 87 sr 86 sr i ratios of 0 71252 to 0 71356 low e nd t value of 13 8 and low zircon e hf t values peak value of 12 0 whole rock nd isotopic model ages and zircon hf isotopic model ages mostly vary from 1 95 ga to 2 10 ga according to these data we suggest that the two granites might have been derived from partial melting of precambrian crustal rocks that had been granulitized during an earlier thermal event our study of the caijiang and gaoxi granites together with previous studies on two triassic alkaline syenites tieshan and yangfang in fujian province and one a type granite wengshan in zhejiang province in south china indicate a wide transtensional tectonic environment in the cathaysia block that lasted at least from 254 ma to 225 ma combined with extant data for the indosinian granites and tectonic evolution in south china we suggest that the formation of a type granites was related to the local ne trending extensional faults probably caused by collision between the south china block and the indochina block or the north china block

pronounced zonal heterogeneity in eocene southern high latitude sea surface temperatures paleoclimate studies suggest that increased global warmth during the eocene epoch was greatly amplified at high latitudes a state that climate models cannot fully reproduce however proxy estimates of eocene near antarctic sea surface temperatures ssts have produced widely divergent results at similar latitudes with ssts above 20 c in the southwest pacific contrasting with ssts between 5 and 15 c in the south atlantic validation of this zonal temperature difference has been impeded by uncertainties inherent to the individual paleotemperature proxies applied at these sites here we present multiproxy data from seymour island near the antarctic peninsula that provides well constrained evidence for annual ssts of 10 17 c 1 sd during the middle and late eocene comparison of the same paleotemperature proxy at seymour island and at the east tasman plateau indicate the presence of a large and consistent middle to late eocene sst gradient of 7 c between these two sites located at similar paleolatitudes intermediate complexity climate model simulations suggest that enhanced oceanic heat transport in the south pacific driven by deep water formation in the ross sea was largely responsible for the observed sst gradient these results indicate that very warm ssts in excess of 18 c did not extend uniformly across the eocene southern high latitudes and suggest that thermohaline circulation may partially control the distribution of high latitude ocean temperatures in greenhouse climates the pronounced zonal sst heterogeneity evident in the eocene cautions against inferring past meridional temperature gradients using spatially limited data within given latitudinal bands

the trace element compositions of s type granites evidence for disequilibrium melting and accessory phase entrainment in the source within individual plutons the trace element concentrations in s type granites generally increase with maficity total iron and magnesium content and expressed as atomic fe mg in this study the degree of variability in trace element concentration also expands markedly with the same parameter the strongly peraluminous high level s type granites of the peninsular pluton cape granite suite south africa are the product of biotite incongruent melting of a metasedimentary source near the base of the crust leucogranites within the suite represent close to pure melts from the anatectic source and more mafic varieties represent mixtures of melt and peritectic garnet and ilmenite trace elements such as rb ba sr and eu that are concentrated in reactant minerals in the melting process show considerable scatter within the granites this is interpreted to reflect compositional variation in the source in contrast elements such as lree zr and hf which are concentrated within refractory accessory phases zircon and monazite show well defined negative correlations with increasing sio2 and increase linearly with increasing maficity this is interpreted to reflect coupled co entrainment of accessory minerals and peritectic phases to the melt leucocratic rocks cannot have evolved from the more mafic compositions in the suite by a process of fractional crystallisation because in this case they would have inherited the zircon saturated character of this hypothetical earlier magma trace element behaviour of granites from the peninsular pluton has been modelled via both equilibrium and disequilibrium trace element melting in the disequilibrium case melts are modelled as leaving the source with variable proportions of entrained peritectic phases and accessory minerals but before the melt has dis solved any accessory minerals thus the trace element signature of the melt is largely inherited from the reactants in the melting reaction with no contribution from zircon and monazite dissolution in the equilibrium case melt leaves the source with entrained crystals after reaching zircon and monazite saturation a significant proportion of the rocks of the peninsular pluton have trace element concentrations below those predicted by zircon and monazite saturation in the case of the most leucocratic rocks all compositions are zircon undersaturated whilst the majority of the most mafic compositions are zircon oversaturated however in both cases zircon is commonly xenocrystic thus the leucocratic rocks represent close to pure melts which escaped their sources rapidly enough that some very closely match the trace element disequilibrium melting model applied in this study zircon dissolution rates allow the residency time for the melt in the source to be conservatively estimated at less than 500 years

source requirements for the golden mile kalgoorlie significance to the metamorphic replacement model for archean gold deposits the golden mile at kalgoorlie represents a giant archean hydrothermal gold system localized by ductile shear zones and hosted mainly by a differentiated tholeiitic sill chlorite carbonate and pyrite alteration zones cover the whole mineralized area 1 km 3 km and calculations suggest that for the golden mile production around 1200 t au the amounts of components added to these alteration zones are 340 mt co2 20 mt k and 5 mt s if one adopts a metamorphic replacement model for gold mineralization in which all ore components derive from devolatilization of greenstones at amphibolite facies or above these data suggest that a source area involving a 5 km thick greenstone slab of area 8 km 8 km could produce the necessary co2 k s h2o and au this is considered a reasonable volume of greenstone belt and under such a model the minimum spacing of large gold deposits would be approximately 20 km along strike neither special au enriched source rocks nor unreasonably large volumes of greenstone b

migration of a late cretaceous fish late cretaceous sediments from the western interior of north america yield exceptionally well preserved fossils1 2 that serve as proxies for the rapidly changing climate preceding the cretaceous tertiary boundary about 67 65 myr ago 3 4 here we reconstruct the ontogenetic history of a maastrichtian age fish vorhisia vulpes5 by using the carbon oxygen and strontium isotope ratios of four aragonite otoliths collected from the fox hills formation of south dakota individuals of v vulpes spawned in brackish water about 70 80 seawater and during their first year migrated to open marine waters of the western interior seaway where they remained for 3 years before returning to the estuary presumably to spawn and die the mean 18o from the marine growth phase of v vulpes yields a seawater temperature of 18 c which is consistent with leaf physiognomy and general circulation model temperature estimates for the western interior during the latest maastrichtian4 6 7

petrogenesis of the ramba leucogranite in the tethyan himalaya and constraints on the channel flow model abstract the himalayan leucogranites are critical for understanding the geodynamic process of himalayan orogenesis in this article we present petrological geochronological and geochemical results of the ramba leucogranites that crop out in the center of the ramba dome within the tethyan himalaya u th pb dating of zircon monazite and xenotime revealed three episodes of leucogranitic magmatism in the ramba dome at ca 44 ma ca 28 ma and ca 8 ma respectively the ca 44 ma and ca 28 ma magmatisms are both represented by strongly deformed porphyritic two mica granite gneiss dykes that intruded into the margin of the dome these granite gneisses have high cao and sr contents relatively high na k sr y and low rb sr ratios and much lower initial 87sr 86sr ratios but higher end t and ehf t values than the ancient indian crustal material these results indicate that the granite gneisses were not derived from metasedimentary rocks of the high himalayan sequence but most likely were derived from partial melting of mafic rocks at a thickened crustal condition the ca 8 ma leucogranites consist of a two mica granite pluton in the core of the dome and garnet bearing granite dykes in the margin of the dome major and trace element compositions indicate that these miocene leucogranites are typical s type granites and were most likely derived from muscovite and or biotite dehydration melting of metapelites the relatively lower tio2 tfeo mgo and cao contents higher rb concentration and rb sr ratio and more strongly negative eu anomaly of the garnet bearing granite indicate that it was more evolved than the coeval two mica granite the non charac charge and radius controlled trace element behavior and the tetrad effect of the ree distribution of the garnet bearing granite further suggest that it is a highly fractionated granite besides our study detected some asian like materials in the ramba leucogranites such as mesozoic aged inherited zircons and the relatively depleted sr nd hf isotopic compositions of some leucogranite samples these features of leucogranites in the tethyan himalaya have been previously considered as direct geological evidence for the channel flow model however detailed geochronological and geochemical studies of the country rocks suggest that these asian like materials are most likely sourced from the country rock rather than the asian lithosphere thus they may not be used as solid evidence for the channel flow model

contrasting geochemical signatures of fluid absent versus fluid fluxed melting of muscovite in metasedimentary sources the himalayan leucogranites most of the himalayan cenozoic leucogranites are products of partial melting of metapelite sources in the malashan gyirong area southern tibet the geochemical compositions of leucogranites define two groups with distinct whole rock major elements large ion lithophile elements rare earth elements high field strength elements and sr and hf isotope ratios based on published experimental results that define generalized melting reactions of metapelitic sources we infer that these leucogranites are the products of two different types of crustal anatexis fluid fluxed melting and fluid absent melting of muscovite in metasedimentary sources as compared to the leucogranites derived from fluid absent melting those from fluid fluxed melting have relatively higher ca sr ba zr hf th and light rare earth element concentrations and zr hf eu eu and nd nd but lower rb nb ta and u concentrations rb sr and 87 sr 86 sr ratios and e hf t the geochemical differences can be explained by melting behaviors of major muscovite feldspar and accessory minerals zircon and monazite during different modes of crustal anatexis the systematic elemental and isotopic signatures of different types of crustal anatexis and in particular the coupling of major and trace elements that results from common influences on rock forming and accessory mineral behaviors provide tools with which to refine our understanding of the nature of crustal anatexis

characterization and origin of aluminous a type granites from the lachlan fold belt southeastern australia the metaluminous to weakly peraluminous a type granites of the lachlan fold belt are a distinctive group of igneous rocks on the basis of chemical and mineralogical criteria those granites that contain 65 72 sio2 can be distinguished from other types on the basis of higher abundances of fetotal fetotal mg high field strength elements trivalent rare earth elements ga and zn mineralogically they contain fe rich hydrous mafic minerals and primary ilmenite and hence are reduced relative to the ni nio buffer however the extremely felsic a type granites sio2 72 have the same chemical and mineralogical characteristics as felsic fractionated i type granites recent analyses indicate that the lachlan fold belt a type granites have sc f alkali element trace transition element and h2o contents similar to those of other unfractionated i type granites rb sr and nd sm isotopic compositions are highly variable probably reflecting source region heterogeneity the metaluminous to weakly peraluminous a type granites of the lachlan fold belt are distinct from peralkaline rocks in terms of chemical composition petrography and field associations although these rocks have been grouped together as a single type in current classification schemes we propose that the metaluminous to weakly peraluminous a type granites such as those of the lachlan fold belt should be defined as aluminous a type granites and should not be grouped with peralkaline granites the lachlan fold belt aluminous a type granites have relatively high calculated zircon saturation temperatures we suggest that these granites were produced by high temperature partial melting of a felsic infracrustal source

regional metamorphism at extreme conditions implications for orogeny at convergent plate margins abstract regional metamorphism at extreme conditions refers either to alpine type metamorphism at low geothermal gradients of 30 c km extreme pressures refer to those above the polymorphic transition of quartz to coesite so that ultrahigh pressure uhp eclogite facies metamorphism occurs at mantle depths of 80 km extreme temperatures refer to those higher than 900 c at crustal depths of 80 km so that ultrahigh temperature uht granulite facies metamorphism occurs at medium to high pressures while crustal subduction at the low geothermal gradients results in blueschist eclogite facies series without arc volcanism heating of the thinned orogenic lithosphere brings about the high geothermal gradients for amphibolite granulite facies series with abundant magmatism therefore uhp metamorphic rocks result from cold lithospheric subduction to the mantle depths whereas uht metamorphic rocks are produced by hot underplating of the asthenospheric mantle at the crustal depths active continental rifting is developed on the thinned lithosphere in response to asthenospheric upwelling and this tectonism is suggested as a feasible mechanism for regional granulite facies metamorphism with the maximum temperature depending on the extent to which the mantle lithosphere is thinned prior to the rifting while lithospheric compression is associated with subduction metamorphism in accretionary and collisional orogens the thinned orogenic lithosphere undergoes extension due to the asthenospheric upwelling to result in orogen parallel rifting metamorphism and magmatism thus the rifting metamorphism provides a complement to the subduction metamorphism and its operation marks the asthenospheric heating of the orogenic lithosphere because of the partial melting and melt extraction of the lower continental crust contemporaneous granite migmatite granulite associations may serve as a petrological indicator of rifting orogeny that is superimposed on precedingly accretionary and collisional orogens the uht metamorphic rocks have occurred since the archean suggesting that the hot underplating has operated very early in the earth s history in contrast the uhp metamorphic rocks primarily occur in the phanerozoic indicating that the thermal regime of many subduction zones has changed since the neoproterozoic for the cold subduction

yardea dacite large volume high temperature felsic volcanism from the middle proterozoic of south australia the yardea dacite is a large volume felsic volcanic unit from the middle proterozoic gawler range volcanics of south australia it has been previously described as an ignimbrite however some samples contain no petrographic evidence for a pyroclastic origin but have characteristics compatible with final crystallization from a nonfragmented magma these samples may have erupted as lavas but others are likely to be extremely densely welded ignimbrites suggesting a compound nature for the unit geothermometry and phase equilibria indicate that the yardea dacite originated from a high temperature 1000 c felsic magma with a low water content 2 the yardea dacite is not associated with a known caldera of the valles type and shares many characteristics of recently described cenozoic felsic volcanic rocks from the western united states interpreted as rheoignimbrites or as unusually extensive lavas

granitoid magmas preserved as melt inclusions in high grade metamorphic rock this review presents a compositional database of primary anatectic granitoid magmas entirely based on melt inclusions mi in high grade metamorphic rocks although mi are well known to igneous petrologists and have been extensively studied in intrusive and extrusive rocks mi in crustal rocks that have undergone anatexis migmatites and granulites are a novel subject of research they are generally trapped along the heating path by peritectic phases produced by incongruent melting reactions primary mi in high grade metamorphic rocks are small commonly 5 10 m in diameter and their most common mineral host is peritectic garnet in most cases inclusions have crystallized into a cryptocrystalline aggregate and contain a granitoid phase assemblage nanogranitoid inclusions with quartz k feldspar plagioclase and one or two mica depending on the particular circumstances after their experimental remelting under high confining pressure nanogranitoid mi can be analyzed combining several techniques emp la icp ms nanosims raman the trapped melt is granitic and metaluminous to peraluminous and sometimes granodioritic tonalitic and trondhjemitic in composition in agreement with the different formula 1 conditions of melting and protolith composition and overlap the composition of experimental glasses produced at similar conditions being trapped along the up temperature trajectory as opposed to classic mi in igneous rocks formed during down temperature magma crystallization fundamental information provided by nanogranitoid mi is the pristine composition of the natural primary anatectic melt for the specific rock under investigation so far 600 nanogranitoid mi coming from several occurrences from different geologic and geodynamic settings and ages have been characterized although the compiled mi database should be expanded to other potential sources of crustal magmas mi data collected so far can be already used as natural starting point compositions to track the processes involved in formation and evolution of granitoid magmas 1 embed mml math 1 gif

jiangnan orogen in south china developing from divergent double subduction abstract the jiangnan orogen is considered as a continent continent collisional belt resulting from the closure of a meso neoproterozoic ocean separating the southeastern margin of the yangtze block from the northwestern margin of the cathaysia block recent data indicate the existence of early neoproterozoic 1000 825 ma volcanic arc assemblages on both sides of the orogen suggesting that the ocean lithosphere between the yangtze and cathaysia blocks must have undergone divergent double sided subduction during the period of 1000 825 ma the divergent double subduction eventually resulted in the closure of the ocean basin at 825 ma leading to the soft collision of the yangtze and cathaysia blocks to form the jiangnan orogen without involvement of continental deep subduction high grade metamorphism of continental crust and uplift exhumation of high grade metamorphic rocks shortly after the collision the initial detachment of the ocean lithosphere from the overlying crust and sedimentary sections induced underplating of mantle magmas triggering partial melting of accretionary wedge strata to form some peraluminous s type granites in the period 825 815 ma finally the sinking of the oceanic slab pulled down the overlying strata to form some basins in which the banxi group and its equivalent strata including bimodal volcanic rocks were formed in the period 815 750 ma

generation of metaluminous a type granites by low pressure melting of calc alkaline granitoids melting experiments on two calc alkaline rocks a tonalite and a granodiorite demonstrate that shallow dehydration melting of hornblende and biotite bearing granitoids generates metaluminous a type granitic melts profuse crystallization of calcic plagioclase orthopyroxene during low pressure p 4 kbar incongruent melting explains all of the major and trace element characteristics of a type granites e g their low al ca mg sr and eu contents and high ga al and k na ratios as well as their common association with anorthositic and mangeritic rocks the residual assemblage produced during melting of the same rocks in the deep crust p 8 kbar is dominated by clinopyroxene and as a consequence most of the chemical features diagnostic of a type granites are lost the high temperatures 900 c and low h 2 o contents 4 wt that are characteristic of a type magmas are adequately explained by the h 2 o poor nature of the granitoid source materials and require neither a granulitic nor a f rich source previous attempts to explain the origin of a type granites have focused on the nature of the source material these experiments show that pressure of melting is at least as important as source composition in engendering a type melts

formation of high δ18o fayalite bearing a type granite by high temperature melting of granulitic metasedimentary rocks southern china the genesis of a type granites has been controversial fayalite granite is a member of the most reduced a type granites commonly thought to have been primarily sourced from tholeiitic rocks in this paper we report petrography whole rock geochemistry sr nd isotope and in situ zircon hf o isotope results for a fayalite bearing a type granite suite at jiuyishan in southern china high zircon 18 o 8 0 9 8 negative zircon ehf t 6 2 to 2 3 and evolved whole rock sr nd isotopes i sr 0 7151 0 7181 end t 7 4 to 6 6 indicate the reworking of old supracrustal rocks isotopic and geochemical results particularly downward inflections of zr and ba at 70 wt sio 2 point to fractional crystallization rather than magma mixing as the controlling process for the evolution of the igneous suite integrated petrological geochemical and isotopic studies present the first convincing case that reduced a type granites can also be generated by high temperature 960 c melting of granulitic metasedimentary rocks likely related to the upwelling of the asthenosphere and or underplating and intrusion of basaltic melts we emphasize that key factors for the genesis of this unique rock type are low oxygen fugacity f o 2 low f h 2 o and high temperature

experimental melts from crustal rocks a lithochemical constraint on granite petrogenesis abstract many studies of experimental petrology have devoted to partial melting of crustal rocks in order to provide lithochemical constraints on granite petrogenesis this paper presents a compilation and synthesis of available experimental data for the major element compositions of felsic melts derived from partial melting of natural or synthetic materials in the compositional range of crustal rocks the experimental melts are categorized into four types according to the species of hydrous minerals in starting materials i amphibole bearing ii amphibole and biotite bearing iii biotite bearing and iv biotite and muscovite bearing if dehydration melting takes place at normal crustal conditions p 5 10 kbar t 1000 c experimental melts are rich in sio 2 but poor in mgo feo t except those from amphibole bearing sources a comprehensive comparison of compositions between experimental melts and starting materials indicates that geochemical fractionation is variable for different major elements and their ratios source composition and melting temperature exert stronger controls on the compositional variations of experimental melts than pressure and fluid by comparing the experimental melts with natural granites the following insights into granite petrogenesis can be got 1 while peritectic assemblage entrainment may be the dominant mechanism for the compositional variations of garnet cordierite rich s type granites fractional crystallization of diverse melts from heterogeneous metasedimentary precursors probably governs the compositional variations of garnet cordierite poor s type granites 2 relatively k 2 o rich mafic to intermediate rocks are appropriate sources for calc alkaline i type granites the compositional variations of calc alkaline granites are jointly controlled by peritectic assemblage entrainment and subsequent fractional crystallization 3 while dehydration melting at t 950 c is appropriate for the production of ferroan and alkali rich granitic melts from intermediate magnesian tonalite or granodiorite it is also possible for ferroan alkali rich and fluorine rich granitic melts to be produced by dehydration melting of moderately magnesian mica bearing materials at t 900 c nevertheless the low t melts are more peraluminous than the high t ones therefore the composition of source rocks exerts the first order control on the composition of granitic melts in closed systems in addition the dehydration melting of crustal rocks under different conditions is also responsible for variations in the composition of granites

the influence of source rock type chemical weathering and sorting on the geochemistry of clastic sediments from the quetico metasedimentary belt superior province canada abstract the geochemistry of clastic sediments is controlled by at least four factors source rock composition weathering of the source rocks sorting of the weathered detritus and post depositional alteration during diagenesis and metamorphism in order to evaluate the relative importance of these factors major trace and rare earth element contents were determined for distal and proximal clastic turbidites from the archaean quetico metasedimentary belt the degree of metamorphism of the samples ranges from middle greenschist to upper amphibolite facies simple mixing of archaean mafic and acidic rocks of similar composition to those found in the adjacent granite greenstone terrains will account for the tio2 feot mgo cr sc y and hree contents of the clastic turbidites the metasediments are enriched in lree relative to the mafic acidic rock mixing line indicating that the acidic source was enriched in the lree compared to archaean terrains from other parts of the world some quetico turbidites have an additional fe rich source component possibly a chemical sediment incorporated at the time of turbidite slumping cao na2o k2o rb and sr form trends on bivariant plots that are oblique to the mafic acidic rock mixing line and hence cannot be accounted for by simple mixing of the source rocks chemical weathering of feldspars prior to deposition can account for cao na2o k2o rb and sr trends since cao na2o and sr are lost to the weathering solutions and k2o and rb are retained on the product clays the higher na 2 o cao k 2 o na 2 o rb k 2 o and sr cao ratios but lower cao content of the distal samples suggest that they were derived from a bulk sediment that had experienced a greater degree of chemical weathering than the proximal sediments sorting is the factor that most strongly influences the composition of individual samples within the proximal sample group the coarse grained metagraywackes with their higher quartz feldspar and heavy mineral contents are enriched in sio2 cao na2o sr lree zr and hf relative to the fine grained clay rich metapelites metapelites are enriched in tio2 al2o3 feot mgo k2o cr zn sc y hree cs and rb as a result of their higher clay contents thus the use of clastic sediment compositions in determining average crustal compositions e g for the ree can only give satisfactory results when all grain size fractions are sampled in their correct relative proportions the chemical differences observed between the high and low grade samples can be accounted for by source rock type chemical weathering of the source rocks or sorting post depositional element mobility does not appear to be an important factor in determining the composition of the quetico metasediments

large scale melt depletion in granulite terranes an example from the archean ashuanipi subprovince of quebec this study uses field petrographic and geochemical methods to estimate how much granitic melt was formed and extracted from a granulite facies terrane and to determine what the grain and outcrop scale melt flow paths were during the melt segregation process the ashuanipi subprovince located in the north eastern superior province of quebec is a large 90 000 km2 metasedimentary terrane in which 85 of the metasediments are of metagreywacke composition that was metamorphosed at mid crustal conditions 820 900 c and 6 7 kbar in a late archean dextral transpressive orogen decrease in modal biotite and quartz as orthopyroxene and plagioclase contents increase together with preserved former melt textures indicate that anatexis was by the biotite dehydration reaction biotite quartz plagioclase melt orthopyroxene oxides using melt orthopyroxene ratios for this reaction derived from experimental studies the modal orthopyroxene contents indicate that the metagreywacke rocks underwent an average of 31 vol partial melting the metagreywackes are enriched in mgo cao and feot and depleted in sio2 k2o rb cs and u have lower rb sr higher rb cs and th u ratios and positive eu anomalies compared to their likely protolith these compositions are modelled by the extraction of between 20 and 40 wt granitic melt from typical archean low grade metagreywackes a simple mass balance indicates that about 640 000 km3 of granitic melt was extracted from the depleted granulites the distribution of relict melt at thin section and outcrop scales indicates that in layers without leucosomes melt extraction occurred by a pervasive grain boundary porous flow from the site of melting across the layers and into bedding planes between adjacent layers in other rocks pervasive grain boundary flow of melt occurred along the layers for a few to tens of centimetres followed by channelled flow of melt in a network of short interconnected and structurally controlled conduits visible as the net like array of leucosomes in some outcrops the leucosomes contain very little residual material 5 biotite orthopyroxene indicating that the melt fraction was well separated from the residuum left in situ as melt depleted granulite only 1 3 vol percentage melt remained in the melt depleted granulites hence the extraction of melt generated by biotite dehydration melting in these granulites was virtually complete under conditions of natural melting and strain rates in a contractional orogen

decoupled ca and fe mg content of s type granites an investigation on the factors that control the ca budget of s type granites abstract the compositional variation of s type granites is thought to derive from source controlled processes whereby magmas acquire their composition as a result of entrainment of variable proportions of a peritectic rich residual mineral assemblage into the melt a corollary of this model is that the entrained phases keep the stoichiometric proportions they have in the partially melted source these proportions are controlled by the composition of the subsolidus minerals consequently it is expected that the ratio between maficity feot mgo and other compatible major elements in granites would be the same as those of the source this is true for ti but not for ca which shows systematically higher ca maficity ratios in order to explore the factors that cause this discrepancy thermodynamic calculations were used to study the partial melting of a representative metapelite and a metagreywacke under a range of crustal pressure and temperature conditions the modelling included segregation of melt or magma in pulses disequilibrium melting of plagioclase in which only a fraction of this phase participates in the melting system and produces two populations of plagioclase the unreacted cores with the original na rich composition and a second one with a ca rich composition and the entrainment of up to 40 wt minerals into the melt this entrained assemblage consists of two fractions one containing the minerals that are most likely to be surrounded by melt in the source and representing 80 wt of the total assemblage and another one consisting of the phases that are most likely to make up the solid framework with the minerals within each fraction keeping stoichiometric proportions the modelled magmas failed to reproduce the observed high ca maficity ratios this indicates that there are other processes that increase the ca content of granites several processes were evaluated including the presence of apatite enhanced availability of subsolidus plagioclase in the melting system disequilibrium ca rich melts and enhanced entrainment of ca rich plagioclase the last three processes were found to increase the ca content but only enhanced entrainment of ca rich plagioclase increases the ca maficity ratio to values similar to those of the granites a possible explanation for the preferential entrainment of ca rich plagioclase is that the density of this phase is different to that of garnet and ilmenite and similar to the density of the melt consequently a significant fraction of the plagioclase in granites may actually be of residual origin other relevant findings of this study are that metagreywackes seem to be the main source of s type granites as previous investigations suggested and that cordierite rich granitoids represent mineral rich magmas that are formed only from metapelites at pressures below 0 75 gpa this investigation emphasizes the importance of source controlled processes in the composition of s type granites and indicates that entrainment of a mineral assemblage into the melt is not a simple and straightforward process as previously thought

temperature and salinity of the late cretaceous western interior seaway the western interior seaway wis was a shallow and expansive body of water that covered the central united states during the late cretaceous attempts to reconstruct temperatures in the seaway using the oxygen isotopic composition of biogenic carbonates have suffered from uncertainty in the oxygen isotopic composition of seawater 18 o w in the semi restricted basin we present new reconstructed temperature and 18 o w data from marine and estuarine environments in the wis and freshwater environments in wis source rivers derived from clumped isotope analyses of bivalve and gastropod shells we find temperatures of 5 21 c 18 o w values below contemporaneous gulf of mexico marine sites and a strong correlation between 18 o w and environmental setting we propose that decreasing 18 o w values reflect decreasing salinity driven by an increasing contribution of continental runoff using a two end member salinity 18 o w mixing model we estimate salinities of 29 35 psu practical salinity units for the deep marine 20 32 psu for the shallow marine and 11 26 psu for the estuarine environments of the wis new climate model simulations agree with reconstructed temperatures and salinities and suggest the presence of salinity driven stratification within the seaway

highly fractionated late eocene 35 ma leucogranite in the xiaru dome tethyan himalaya south tibet abstract the xiaru dome is located in the middle section of the north himalayan gneiss domes belt in southern tibet the leucogranite which crops out in the core of the xiaru dome is a typical medium grained garnet tourmaline muscovite leucogranite u th pb dating of zircon and monazite from the leucogranite yielded ages of approximately 35 ma this finding supports a growing body of evidence indicating that an extensive magmatic event occurred during the late eocene in the himalayas this leucogranite is strongly peraluminous with a cnk values of 1 08 1 52 and characterized by evolved geochemical composition with high contents of sio 2 and alkali elements low levels of cao mgo tio 2 and feot enriched large ion lithophile elements such as rb and depleted of high field strength elements such as nb zr and hf the non charac charge and radius controlled trace element behaviors which are typical of a highly fractionated granite system were recorded in the whole rock and the accessory minerals of the xiaru leucogranite furthermore the magmatic zircon overgrowths have extremely high content of hf consistent with those from the highly fractionated aqueous like fluid system in addition whole rock geochemical fractionation trends were observed which can be explained by crystal fractionation of biotite k feldspar zircon xenotime and monazite these geochemical features indicate that the xiaru leucogranite is a typical highly fractionated granite the geochronological and geochemical features of the inherited zircons from the xiaru leucogranite show a close affinity to those of the country rocks suggesting a certain degree of assimilation from the country rocks during melt ascent and emplacement although a restricted range of e hf t values from 12 8 to 6 6 with hf t dm2 model ages of 1 2 1 6 ga was obtained from the late eocene zircons it is invalid to constrain the source of the parental magma due to the strong fractionation and assimilation processes

on ferroan a type granitoids their compositional variability and modes of origin we recognize eight types of ferroan granitoid that can be distinguished on the basis of major element chemistry these include alkalic granitoids that may be metaluminous or peralkaline alkali calcic granitoids that may be metaluminous peraluminous or peralkaline calc alkalic granitoids that may be metaluminous or peraluminous and rare calcic ferroan granitoids these granitoids may form by two distinct end member processes extreme differentiation of basaltic melts results in ferroan granitoids that are either peralkaline alkalic and alkali calcic or metaluminous alkalic alkali calcic and calc alkalic with alkalinity increasing with increasing pressure of differentiation partial melting of tonalitic to granodioritic crust produces alkali calcic to calc alkalic granitoids that are metaluminous at low pressures and peraluminous at high pressures it is likely that a combination of these two processes plays some role in the formation of most ferroan granitoids most granitoids that are referred to as ferroan however the term has become more confusing than edifying because it has been applied to a broad spectrum of granitoid compositions with varying petrogenesis for this reason we recommend that the term be discontinued and ferroan used in its stead

structural controls on gold mineralisation and the nature of related fluids of the paiol gold deposit almas greenstone belt brazil chemical analyses suggest that the metavolcanic rocks of the almas greenstone belt agb tocantins state brazil have a continental affinity possibly related to a continental rift environment they were metamorphosed to amphibolite facies during a regional tectono metamorphic event dn retrogressed to greenschist facies assemblages and then hydrothermally altered within dextral strike slip shear zones dn 1 fracture sets related to dn 2 intersect sn 1 the paiol gold mine is one of several mineralised zones within metabasic and meta intermediate rocks of the agb it exploits shoots of sulphide au quartz mineralisation that occupy dilational zones approximately perpendicular to an elongation lineation ln 1 within mylonitic foliation sn 1 sn 1 s within the s c fabric the dilational zones probably formed due to dextral displacement on sinistrally en echelon c surfaces minor amounts of gold may have been introduced or remobilised during dn 2 coexisting primary and pseudosecondary fluid inclusions in mineralised quartz veins from ore shoots comprise a high salinity three phase type type ii and a lower salinity two phase type type i homogenisation temperatures for type ii inclusions range from 200 to 410 c and type i from 90 to 320 c the inclusions and their temperature ranges are believed to reflect heat exchange and some mixing between the two fluid types under relatively constant ambient temperatures but variable though broadly declining fluid temperatures this took place late in dn 1 in conjunction with greenschist facies retrogression and localised hydrothermally induced metasomatism

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

geochronological and geochemical constraints on the petrogenesis of late triassic aluminous a type granites in southeast china abstract the late permian triassic granites in the south china block scb have important tectonic significance for the evolution of east asia most of the intrusive bodies in the central scb consist of s type granites here we report the first unambiguous discovery of wengshan aluminous a type granites in southwest zhejiang province southeast china shrimp and la icp ms zircon u pb analyses yielded similar ages about 224 ma for two samples from the wengshan pluton in concordance with peak time of late indosinian 234 205 ma tectonic magmatic activity in the scb the wengshan granites are peraluminous with a cnk values ranging from 1 09 to 1 24 enriched in k rb th ree and zr nb ce y have elevated feo tot mgo and ga al ratios and are depleted in mg ca mn ba sr p and ti in addition they show high zircon saturation temperatures 820 878 c their ree patterns are highly fractionated with la yb n ratios of 36 89 101 24 and strongly negative eu anomalies eu eu 0 28 0 53 in situ zircon hf isotopic analyses indicate that the wengshan granites have e hf t values ranging from 15 9 to 12 7 and two stage depleted mantle hf model ages from 2 05 to 2 26 ga sr nd isotope systematics are characterized by a high initial 87 sr 86 sr ratios of 0 7092 0 7218 and a low e nd t values of 15 0 to 12 1 with two stage depleted mantle nd model ages of 1 99 2 22 ga consistent with those of paleoproterozoic basement rocks in the area it is inferred that the wengshan granite magma formed by partial melting of paleoproterozoic metasedimentary sources in cathaysia block meanwhile the underplating of mafic magma in the lower crust may provide the heat source for the formation of the wengshan granites the data show that the wengshan granites unlike other s type granites in the scb are late triassic aluminous a type granite therefore they may indicate distinct geodynamic significance our study of the wengshan a type granites 224 ma together with jingju a type granites 215 ma in zhejiang province and gaoxi a type granites 215 ma xiaotao a type granites 222 ma tieshan 254 ma and yangfang 242 ma alkaline syenites in fujian province define important extensional events in triassic considering these available data we suggest that the late permian triassic a type granites and alkaline syenites in the coastal region of the scb probably formed under an extensional regime related to oblique subduction of the paleo pacific plate

general circulation model simulation of the δ18o content of continental precipitation in the middle cretaceous a model proxy comparison we use the genesis atmospheric general circulation model gcm with water isotopic transport and fractionation capabilities to quantify the influence of atmospheric co 2 sea level and elevation of the western cordillera on the 18 o of middle cretaceous precipitation the model predicts a systematic increase of nearly 3 in the 18 o of north american precipitation due to warming associated with an increase in co 2 from 2 to 12 times pre industrial levels in contrast the specification of lowstand conditions and a high ancestral western cordillera reduces the 18 o of north american precipitation locally by as much as 6 and 8 we compare the simulated 18 o of precipitation with the 18 o of paleosol siderite spherules and find good agreement only when the model includes lowstand conditions and a high ancestral western cordillera our results imply either that cretaceous high latitude paleosol 18 o was influenced by orographic precipitation and the western interior seaway or that the gcm9s hydrological cycle is deficient at high p co 2 additional paleosol data are needed to resolve this issue

experimental formation of brittle structural assemblages in oblique divergence abstract a series of experiments is reported in which brittle minor structures are initiated in narrow deformation zones in clay under conditions of kinematically controlled oblique divergent displacement nineteen settings of boundary displacement angle were used from pure wrench to pure divergence under conditions favouring either faults dry experiments or extension fractures wet experiments pure wrench produced the well known assemblage of riedel strike slip faults whereas experiments in pure divergence produced conjugate arrays of normal faults and extension fractures with a dihedral angle of 30 bisected by the direction of the zone as has been described in rift zones experiments with boundary displacements at intermediate settings show a continuum of structural orientations and dihedral angles between these two extremes a boundary between assemblages dominated by strike slip faults and extensional faults was found at a displacement angle of 45 from the deformation zone these results are interpreted kinematically in terms of 1 principal axes of infinitesimal incremental strain 2 material dilatancy control on shear structure dihedral angles and 3 whether the vertical strain in divergent wrench settings is a thickening strike slip assemblage or a thinning normal fault assemblage

origin of the fluorine rich highly differentiated granites from the qianlishan composite plutons south china and implications for polymetallic mineralization abstract many composite granite plutons occur in south china accompanied by large scale polymetallic mineralization each composite pluton is composed of main phase granite and late stage highly differentiated granite traditionally the highly differentiated granite is thought to be residual melt of the former via fractionation and ore forming materials and fluids are from granite magma itself we propose a different model for the origin of the granites and related mineralization based on petrological and geochemical studies on the qianlishan composite plutons that host the supergiant shizhuyuan w sn bi mo deposit the main phase granite shows features of normal granites while the highly differentiated granite is characterized by f rich water deficient low f o 2 alkalinity ree tetrad effect and modified behavior of some trace elements e g very high k ba and low k rb and zr hf ratios we suggest that the parent magma of the highly differentiated granite was derived from melting of dominantly lower crustal rocks triggered by underplating of a new pulse of basaltic magmas small amounts of basaltic magmas and volatiles such as fluorine were involved in the source of the granite addition of fluorine lowered the solidus temperature and viscosity of granite magma and thus prolonged the process of magma evolution this resulted in extreme fractional crystallization and intense interaction between melt and circulating waters from country rocks forming the unusual geochemical features of the granite the high temperature circulating waters along with metamorphic fluids released from deep crustal rocks subsequently extract ore forming metals from country rocks through forming f bearing complexation forming the polymetallic deposits

archaean and lower proterozoic units in central brazil the following main units have been recognized as archaean basement rocks in central brazil a high grade granite gneiss terrain composed predominantly by granodioritic and tonalitic gneisses these rocks are cut by amphibolite dykes and present several nucleii locally surrounded by an array of archaean greenstone belts b greenstone belts presenting basal komatiitic ultramafic mafic lavas with abundant spinifex textures followed by basalts and chemical sediments such as cherts limestones and graphitic pelites that are often interbedded with acid and intermediate volcanics metamorphism is within greenschist facies granulite mobile belt including the granulitized mafic ultramafic complexes of barro alto niquel ndia and brava the lower proterozoic is represented by a metasedimentary sequences overlying the granite greenstone terrains with several tin bearing granite intrusions b gabbro anorthosite layered plutons and their associated mafic volcanics which are followed by acid and intermediate volcanics and ocean floor sediments c mafic ultramafic layered intrusions in the granulite terrain

petrology and geochemistry of metamorphosed komatiites and basalts from the sula mountains greenstone belt sierra leone the sula mountains greenstone belt is the largest of the late archaean greenstone belts in the west african craton it comprises a thick 5 km lower volcanic formation and a thinner 2 km upper metasedimentary formation komatiites and basalts dominate the volcanic formation and komatiites form almost half of the succession all the volcanic rocks are metamorphosed to amphibolite grade and have been significantly chemically altered two stages of alteration are recognised and are tentatively ascribed to hydrothermal alteration and later regional amphibolite facies metamorphism ratios of immobile trace elements and ree patterns preserve for the most part original igneous signatures and these are used to identify five magma types these are low ti komatiites depleted in light ree low ti komatiites with flat ree patterns high ti komatiitic basalts with flat ree low ti basalts depleted in light ree high ti basalts with flat ree patterns much of the variation between the magma types can be explained in terms of different melt fractions of the mantle source although there were two separate mantle sources one light ree depleted the other not the interleaving of the basalts and komatiites produced by this melting indicates that the two mantle sources were melted simultaneously the simplest model with which to explain these complex melting processes is during melting within a rising mantle plume in which there were two different mantle compositions the very high proportion of komatiites in the sula mountains relative to other greenstone belts suggests either extensive deep melting and or the absence of a thick pre existing crust which would have acted as a filter to komatiite eruption

components and episodic growth of precambrian crust in the cathaysia block south china evidence from u pb ages and hf isotopes of zircons in neoproterozoic sediments abstract detailed lam icpms u pb dating and lam mc icpms lu hf isotope analysis were carried out on zircons from nine samples of basement metamorphic rocks in the southern cathaysia block south china the chemical compositions of these metamorphic rocks and zircon morphology indicate that their protoliths were sedimentary zircon u pb dating results show they mainly formed during the late neoproterozoic the precambrian crust of the cathaysia block can be divided into two distinct tectonic domains the wuyishan terrane to the northeast and the nanling yunkai terrane to the southwest the nearly e w boundary between these two terranes is also supported by geophysical petrological and geochemical evidence and the basement rocks of the two terranes are comprised of different components the wuyishan terrane is characterized by dominant paleoproterozoic 1 86 ga and lesser neoarchean magmatism and the southern central part of the terrane suffered strong neoproterozoic and early paleozoic tectonothermal reworking the zircon data suggest that there were five important episodes of juvenile crust generation 3 6 ga 2 8 ga 2 6 2 4 ga 1 85 ga and 0 8 0 7 ga in the wuyishan terrane strong paleoproterozoic 1 85 ga magmatism in northern wuyishan and neoproterozoic 0 8 0 7 ga magmatism in central and southern wuyishan both principally involved the remelting of old crust with minor input of juvenile material in contrast the nanling yunkai terrane contains abundant neoarchean and grenvillian zircons some evidence of mesoproterozoic activity and rare paleo to meso archean and neoproterozoic zircons u pb ages and hf isotopic compositions of the detrital zircons from the late neoproterozoic to early paleozoic meta sedimentary rocks in the nanling yunkai area suggest that the growth of juvenile crust mainly occurred at 3 6 ga 3 3 ga 2 5 2 6 ga 1 6 ga 1 0 ga and 0 8 0 7 ga neoarchaean 2 6 2 5 ga grenvillian 1 0 ga and neoproterozoic 0 8 0 7 ga magmatism mainly involved the recycling of old crust minor remnants of 4 1 ga crust may remain in the terrane this crustal history is distinct from that of the wuyishan terrane and analogous to those recorded for the eastern india east antarctic domain of northern gondwanaland suggesting that they probably were once linked

major and minor element mobilization with implications for nicufe sulphide remobilization during retrograde metasomatism at the vammala mine southwest finland abstract granitic melt derived from migmatitic host rocks intruded a d 3 related fracture system in a 1 89 ga ni cu fe sulphide bearing peridotite sheet near vammala southwest finland zones of wall rock alteration mainly comprising amphibole phlogopite chlorite developed in the peridotite adjacent to the leucocratic dykes the minerals in the alteration zones and in the gneissic host rocks together with the production and intrusion of granitic melt are consistent with metamorphic conditions of at least 600 c and 4 5 kbar later hydration under condtions of lower temperature and pressure resulted in partial serpentinization of the peridotite and dykes this later transition precluded direct investigation of the earlier transition that developed the medium to high grade alteration zones in the peridotite at the interface with the leucocratic dykes because serpentinization was largely accommodated by volume increase rather than mass loss of mgo and sio 2 the relative concentrations of most components in the serpentinized peridotite should mirror those in the peridotite based on this contention two methods are used to document the composition volume behaviour of the transition from now serpentinized peridotite to alteration zones apparent transition and hence infer the mass balance trends of the igneous peridotite to alteration zone transition real transition the methods collectively suggest that in the alteration assemblages k 2 o rb 2 o p 2 o 5 tio 2 and al 2 o 3 form a group of relatively enriched components and that al 2 o 3 was particularly mobile an estimated volume increase of 10 10 accompanied serpentinization and a calculated volume decrease of 10 15 accompanied the apparent transition thus a volume increase of real transition for best fit volume factors gresens equation demonstrates that the apparent transition involved mass gains in sio 2 al 2 o 3 k 2 o p 2 o 5 and tio 2 and mass losses in feo mgo cao ni s and volatiles loss on ignition loi these amounting to net losses of 32 5 kg of volatiles and 10 8 kg of other components per 1000 kg of serpentinized peridotite the real transition involved similar relative gains and losses except for sio 2 gain slightly less mgo loss slightly more and loi loss negligible it respectively records inward transfer of components from a feldspar rich source to the peridotite and outward transfer to that source this is consistent with metasomatic exchange between the peridotite and the probably supra solidus granitic intrusions during the real exchange ni cu fe sulphides were retained and redistributed internally remobilized in the outer alteration zones but transferred into the dyke from the zone abutting it externally remobilized the same genetic interpretations probably apply to similar dyke and alteration systems in other ni cu fe sulphide deposits hosted in ultramafic rocks in southwest finland

petrogenesis of the late mesozoic guposhan composite plutons from the nanling range south china implications for w sn mineralization voluminous granite plutons were emplaced in the late mesozoic in the nanling range south china accompanied by large scale polymetallic deposits most plutons are composite composed of major phase granites and highly differentiated late stage granites previous studies have proposed fractionation of both granite types and polymetallic deposits from a common magma we choose to study the ages chemistry and nd hf isotopes of the guposhan complex from the range and propose a different model for the origin of the granite and relationship with associated polymetallic mineralization in the nanling range the major phase granites of the guposhan complex contain medium coarse grained biotite granite and hornblende bearing biotite granite with mafic microenclaves mmes emplaced at 161 1 ma on the contrary the late stage granite is fine grained with apparent zircon ages of 154 3 ma the major phase granites show large variation in chemical compositions and in whole rock nd and zircon hf isotope ratios with end t 6 0 to 1 9 and ehf t 4 5 to 1 5 respectively we suggest that their origin is attributable to a magma mixing process between felsic magma derived from melting of proterozoic basement rocks and mantle derived mafic magma the mafic magma could be represented by the parental magma of the mmes within the plutons which have low sio2 high mgo and high end t values up to 1 8 the late stage granite shows end t and ehf t values similar to the major phase granites but are chemically more evolved sio2 75 80 more reduced and water deficient and in particular are fluorine bearing numerical geochemical modeling and the petrological data indicate that the late stage granite was produced during a new thermal event rather than being produced due to fractional crystallization of the earlier major phase granite the polymetallic deposits are genetically linked with the late stage granite we suggest that a break off of the flat subducted pacific slab beneath the cathaysia block in the late mesozoic resulted in the upwelling of asthenosphere and formation of basaltic magma that underplated and triggered partial melting of the basement rocks forming felsic melts and followed by mixing of the mafic and felsic magmas in an extensional regime

geochemistry and origin of the archaean beit bridge complex limpopo belt south africa the beit bridge complex bbc in the central zone of the limpopo belt in southern africa includes metasedimentary rocks of archaean age chiefly metapelites quartzites and carbonates amphibolites chiefly mafic dykes and sills and granitoids which comprise 50 of the complex the amphibolites are tholeiites with relatively unfractionated ree patterns and pronounced subduction zone geochemical signatures in terms of relatively immobile incompatible elements tonalites and granites which also exhibit such a signature can be formed by batch melting of a mafic source with residual garnet or amphibole most granites have negative eu anomalies and appear to represent residual liquids bbc metapelites have negative eu anomalies and other immobile element distributions similar to phanerozoic shales one group of metapelites that shows significant depletion in ca na and sr appears to have been derived from intensely weathered sources the negative eu anomalies in bbc metapelites suggest intracrustal melting of their sources and thus the presence of evolved continental crust in this part of southern africa by about 3000 ma bbc quartzites are quartz rich sandstones that are depleted in hfs elements ba and sr relative to phanerozoic counterparts a feature that probably reflects less recycling during the early archaean bbc banded iron formation bif is similar to other archaean bif and shows large positive eu anomalies bbc carbonates are dolostones depleted in ree hfs elements u th ba sr ni sc v and y and enriched in fe mn na rb and cs relative to phanerozoic marine carbonates

early precambrian crustal development changing styles of mafic magmatism the work of sutton watson 1951 was one of the cornerstones in the interpretation of the evolution of early precambrian archaean and early proterozoic gneiss complexes and in the documentation of mafic dyke swarms that intruded recently accreted archaean cratons since then the concept of geological terranes and terrane accretion has been extended back to the archaean and different events can be distinguished by increasingly precise isotopic dating techniques there is growing evidence that with certain clear provisos the principle of uniformitarianism can be applied as equally to the early precambrian as it can through the rest of the geological record in terms of mafic magmatism there are similarities between some ancient and modern suites both intrusive and extrusive but there are also some major differences such as the presence of komatiites in the archaean and the formation of large scale layered intrusions and extensive dyke swarms at the end of the late archaean and in the early proterozoic the mantle has evolved continuously with the progressive extraction of mafic magmas but allowing for this continental tholeiites appear to have changed little with time ancient mid ocean ridge basalts are elusive because of their poor preservation record the geochemistry of mafic volcanics occurring in many greenstone belts commonly suggests a continental contamination component and that these volcanic rocks are most closely akin to those from present day arc systems the relative abundance of early precambrian intrusive mafic suites parallels the crustal growth curve reflecting the global crustal accretion differentiation superevent towards the close of the archaean bimodal tholeiitic noritic mafic magmatism associated with many of these intrusive suites stems from this rapid turnover in the mantle mafic crust continental crust system this in turn gives a possible link between the respective evolutions of archaean to recent komatiitic noritic and boninitic suites

origin of peraluminous a type granites from appropriate sources at moderate to low pressures and high temperatures abstract four triassic plutons from south china exhibit compositional features of s type granites high a cnk values enriched sr nd isotopic compositions and high zircon 18o values but also of a type granites alkali rich and ferroan compositions high zr nb ce y contents and high ga al ratios the s type like features suggest a metasedimentary source however the exact anatectic processes are poorly known here phase equilibrium modelling on the melting of five representative metasedimentary rocks with contrasting compositions at 0 4 1 2 gpa and fmq 1 to fmq 1 indicate that the most likely source should have a composition of mg 1 14 k2o na2o 5 two neoproterozoic volcanic sedimentary rocks adjacent to the pags have compositions that match these requirements and modelled melts at p 0 8 gpa and t 850 c match the main compositional variables of the plutons this study highlights that source composition and anatectic p t conditions are the main factors controlling the generation of peraluminous a type granites

the climate of the late cretaceous new insights from the application of the carbonate clumped isotope thermometer to western interior seaway macrofossil abstract we apply the carbonate clumped isotope thermometer 47 to macrofossils from the baculites compressus 73 5 ma and the hoploscaphites nebrascensis 67 ma ammonite zones of the western interior seaway wis of north america and nearby coeval terrestrial and open marine environments the carbonate clumped isotope thermometer is based on a single phase isotope exchange equilibrium that promotes the clumping of two heavy isotopes together within a single carbonate molecule as temperature decreases due to the thermometer s isotopic independence from water coupled measurements of 47 and the bulk oxygen isotopic composition of a carbonate 18 o c enable the reconstruction of both paleotemperature and the isotopic composition of the water in which the organisms grew before applying the technique to the aragonite shells of fossil marine organisms mostly ammonites but also some gastropods bivalves and one belemnite we measure the clumped isotopic composition of modern nautilus and cuttlefish two of the nearest living relatives to the cretaceous ammonites modern cephalopods exhibit disequilibrium isotope effects with respect to 47 but not 18 o c therefore a simple correctional scheme is applied to the late cretaceous macrofossil data before reconstructing paleotemperatures diagenesis is also assessed by visual preservation and previously measured sr concentrations cochran et al 2003 temperatures reconstructed for the late cretaceous western interior seaway range from 16 4 3 5 c for an offshore interior seaway environment from the h nebrascensis zone to 24 2 0 4 c for the b compressus ammonite zone the seaway itself has an isotopic composition of approximately 1 relative to vsmow the expectation for an ice free global ocean average while a nearby freshwater environment has an isotopic composition approaching 20 we compare the attributes of the reconstructed climate to predictions based on late cretaceous climate models and previous reconstructions of the seaway and also assess the sensitivity of our results to the modern cephalopods correction by comparisons to suitable modern analogs finally our clumped isotope data are consistent with cooling between the late campanian and maastrichtian as also seen in benthic foraminfera 18 o