early devonian alkaline intrusive complex from the northern north china craton a petrological monitor of post collisional tectonics abstract laser ablation inductively coupled plasma mass spectrometry zircon u pb dating and geochemical data document the early devonian sandaogou alkaline complex 409 ma from the northern margin of the north china craton the rock suite includes pyroxene syenite quartz syenite and monzonite these rocks exhibit high contents of k 2 o 5 13 wt strong enrichments in large ion lithophile elements and light ree slightly negative eu anomalies and pronounced depletions in high field strength elements they are characterized by moderate 87 sr 86 sr i ratios of 0 7052 0 7071 low e nd t values of 12 7 to 17 9 and zircon e hf t values from 27 8 to 32 3 these geochemical features and quantitative isotopic modelling suggest that they might have been formed through simultaneous fractional crystallization and lower crustal assimilation of a metasomatized mantle derived alkali basaltic magma these trachytic rocks together with the middle devonian alkaline rocks and mafic ultramafic complex from neighbouring regions constitute a linear post collisional magmatic belt along the northern north china craton possibly formed under a tectonic regime of slab breakoff they serve not only as a magmatic milestone for marking the termination of early palaeozoic orogenic cycle around the northern north china craton but also as a spatial tracer for locating the position of the potential ancient slab breakoff at the surface supplementary material analytical methods mineral composition data field and zircon cathodoluminescence cl photographs are available at http www geolsoc org uk sup18403

episodic and short lived granitic pulses in a post collisional setting evidence from precise u pb zircon dating through a crustal cross section in corsica abstract in western corsica a large granitoid batholith was emplaced during carboniferous and permian times these rocks display an evolution through time from high mg k calc alkaline to calc alkaline then alkaline affinities moreover the magmatic processes can be observed at different crustal levels new and precise isotope dilution and thermo ionisation mass spectrometry id tims u pb dating suggests that granite magmatism in corsica did not simply reflect the continuous thermal evolution of a thickened crust a precise timing for the post collisional magmatism is documented with four distinct short lived episodes respectively at 345 338 305 and 280 ma separated by periods of inactivity these four magma pulses display variable nd isotopic signatures related to mantle crust interactions granite magma generation and emplacement is strongly connected to the thermal and tectonic activity of the lithosphere an increasing contribution of a depleted mantle to the magmatic processes is documented for the last igneous event this is interpreted to reflect the emplacement of depleted mantle materials in the lower crust which enhanced heat flow and triggered partial melting of previously underplated mafic rocks this increasing contribution of mantle materials marks the onset of a new geodynamic cycle characterised by an extensional tectonic regime preluding to the opening of the alpine orogenic tract

genesis of post collisional calc alkaline and alkaline granitoids in qiman tagh east kunlun china abstract the post collisional magmatism of qiman tagh is characterized by the intrusion of voluminous intermediate to felsic granitoids including syenogranite monzogranite granodiorite tonalite and diorite the granitoids can be divided into two magmatic suites calc alkaline ca and alkaline alk which were emplaced from 236 ma to 204 ma the ca suite contains metaluminous granodiorites and monzogranites typical qiman tagh ca granodiorites show moderately fractionated ree patterns la yb n 4 35 25 11 with significant negative eu anomalies eu eu 0 54 1 34 and the primitive mantle normalized spidergrams show strong depletion of nb and sr the qiman tagh ca monzogranites show similar fractionated ree patterns la yb n 2 70 13 5 with less prominent negative eu anomalies and the chondrite normalized spidergrams show strongly depleted ba nb and sr the alk suite including syenogranite is highly potassic k 2 o na 2 o 1 09 3 56 and peraluminous a cnk 0 91 1 06 compared to typical qiman tagh ca granodiorites the qiman tagh alk granitoids can be distinguished by their higher rb nb ga al feo mgo y sr and rb sr as well as their lower mg mgo cao al 2 o 3 sr co v eu eu ba nb la nb ba la and ce nb the qiman tagh ca rocks were most likely to be derived from the partial melting of garnet amphibolite facies rocks in the lower crust leaving behind anhydrous granulite facies rocks with plagioclase and garnet in the residue the alk rocks may have formed by the continued partial melting of granulite facies rocks at elevated temperatures 830 c

brazilian structural provinces an introduction abstract the territory of brazil coincides almost entirely with the south american platform the crystalline core of the continent its basement is composed of ancient metamorphic and igneous rocks and it has not suffered any tectonic regeneration since the beginning of the phanerozoic sedimentary rocks with almost horizontal bedding cover this crystalline basement this latter shows ages as old as early precambrian although ages between 500 and 1000 m y are conspicuously frequent the cratonic areas became consolidated more than 1700 m y ago whereas the fold belts formed essentially between 500 and 1700 m y ago the sedimentary cover accumulated from the early silurian in three large intracratonic basins until the platform became completely stabilized a late jurassic to early cretaceous reactivation caused by the break up of the ancient gondwana continent created another basin sequence chiefly along the atlantic continental margin based on the nature of the crystalline basement rocks and the sedimentary cover ten structural provinces can be distinguished 1 rio branco province in the north of the country occupied by the guyana shield and still only poorly known with an important fold belt developed during the so called transamazonian cycle 2000 200m y with high grade metamorphic rocks and only slight influence of later events 2 tapajos province in central brazil corresponding to the amazonas or guaporecraton showing chiefly precambrian crystalline rocks and behaving as an cratonic area during the phanerozoic 3 sa o francisco province located on the atlantic shield with its basement covered by rocks of different ages chiefly affected by the brasiliano cycle between 1000 and 500 m y and constituting another cratonic area 4 tocantins province between the amazonas and sa o francisco cratons with the oldest rocks in its centre ages over 2600 m y and at the eastern and western borders metamorphic sequences of various fold belts and almost no phanerozoic deposits 5 mantiqueira province located along the southern part of the atlantic coast affected chiefly by the brasiliano folding cycle 6 borborema province in the northeast brazilian fold belt affected by the brasiliano cycle in a very complex way and with important faulted zones it was reactivated in phanerozoic times when also sedimentary covers accumulated 7 amazonas province represented by the amazonas sedimentary basin syneclise and subdivided into four parts by three important arcs 8 parnaiba province coinciding with the piauf maranha o syneclise and filled with a rather thick sedimentary sequence 9 paranaprovince the sedimentary basin of southern brazil in which the well known late paleozoic glaciation features are found and which possesses an extensive cover of basaltic rocks of late jurassic to early cretaceous age 10 coastal province and continental margin the youngest structural unit developed during the separation of the continent and represented by rift valleys and coastal basins filled with mesozoic cenozoic deposits of various kinds

new chronological and geochemical constraints on the genesis and geological evolution of ponza and palmarola volcanic islands tyrrhenian sea italy abstract a new geochronological and geochemical study of the volcanic rocks of the ponza and palmarola islands pontine archipelago has been carried out this archipelago is located along the boundary between the italian continental shelf and the opening tyrrhenian basin it is a key area to study volcanism related to the opening of the tyrrhenian sea ponza is the oldest felsic magmatic manifestation in the central tyrrhenian area previous studies suggested that ponza volcanic activity began before 5 ma twenty five new k ar ages constrain the volcanic activity rhyolitic hyaloclastites and dykes to the last 4 2 ma with two episodes of quiescence between 3 7 and 3 2 ma and between 2 9 and 1 0 ma a new volcanic episode dated at 3 2 2 9 ma has been identified on the central and southern ponza with emplacement of pyroclastic units at 1 0 ma a trachytic episode ended the volcanic activity the near island of palmarola exhibits rhyolitic hyaloclastites and domes dated between 1 6 and 1 5 ma indicating that the island was entirely built during the early pleistocene in a short span of time of ca 120 ka although only 6 8 km apart the two islands display significantly different geochemical signatures ponza rhyolites show major and trace element compositions representative of orogenic magmas of subduction collision zones high k calc alkaline and metaluminous rhyolites agpaitic index ai and alumina saturation index asi 3 and nb ta negative anomalies in palmarola the orogenic character is also present but much less marked than in ponza rhyolites have a peralkaline character ai 1 lower lile hfse th ta 11 15 low lree hfse ratios la nb 1 2 close to those of anorogenic lavas and the nb ta negative anomalies are almost absent y nb ratios indicate different magmatic sources one similar to island arc or active continental margin basalts for ponza rhyolites and the others probably involving an oib type component for palmarola rhyolites and ponza trachytes palmarola volcanics represent a transitional magmatism although a preserved collisional geochemical imprint they show geochemical features approaching those of anorogenic lavas erupted in a within plate context the change of magmatism evidenced in this study can be related to the tectonic evolution of the area indeed hf ta and rb contents suggest that the oldest pliocene rhyolites of ponza would emplace in a syn to late collisional setting while the younger pleistocene rhyolites of palmarola would be emplaced in a post collisional setting in which the orogenic character th ta decreases and mantle influence nb ta increases geochemical modeling strongly suggests that the palmarola rhyolites represent the waning stages of a subduction related magmatism the k ar datings allow us to estimate precisely the transition of magmatism to last less than 1 3 ma the transitional magmas may be the result of the upwelling of asthenospheric mantle inducing melting of a metasomatized lithospheric mantle and the mixing between these two sources this upwelling could occur during the extension of the tyrrhenian basin caused by the slab retreat and steepening or during a process of slab break off starting in the pliocene

petrogenesis and tectonic implications of late carboniferous a type granites and gabbronorites in nw iran geochronological and geochemical constraints abstract carboniferous igneous rocks constitute volumetrically minor components of iranian crust but preserve important information about the magmatic and tectonic history of sw asia ghushchi granites and gabbronorites in nw iran comprise a bimodal magmatic suite that intruded ediacaran cambrian gneiss and are good representatives of carboniferous igneous activity precise sims u pb zircon ages indicate that the gabbronorites and granites were emplaced synchronously at 320 ma ghushchi granites show a type magmatic affinities with typical enrichments in alkalis ga zr nb and y depletion in sr and p and fractionated ree patterns showing strong negative eu anomalies the gabbronorites are enriched in lrees nb ta and other incompatible trace elements and are similar in geochemistry to oib type rocks granites and gabbronorites have similar e nd t 1 3 to 3 4 and 0 1 to 4 4 respectively and zircon e hf t 1 7 to 6 2 and 0 94 to 6 5 respectively the similar variation in bulk rock e nd t and zircon e hf t values and radiometric ages for the granites and gabbronorites indicate a genetic relationship between mafic and felsic magmas either a crystal fractionation or silicate liquid immiscibility process further work is needed to resolve petrogenetic details the compositional characteristics of the bimodal ghushchi complex are most consistent with magmatic activity in an extensional tectonic environment this extension may have occurred during rifting of cadomian fragments away from northern gondwana during early phases of neotethys opening

the cadomian orogeny in the north armorican massif a brief review the north armorican massif nam of nw france is the type locality of the late precambrian cadomian orogenic belt the nam largely escaped reworking during the upper palaeozoic variscan cycle with the result that earlier geological relationships are commonly preserved intact a large part of the cadomian belt of the nam comprises variably deformed and metamorphosed supracrustal rocks of the brioverian succession barrois 1895 1896 1908 graindor 1957 cogne 1962 le corre 1977 rabu et al 1982 1983 chantraine et al 1982 the cadomian orogeny which takes its name from cadomus caen in normandy was defined by bertrand 1921 as the late precambrian orogeny which in the nam resulted in the folding and uplift of the brioverian succession prior to the deposition of lower palaeozoic red bed sequences events which have been ascribed to the cadomian orogeny span the period c 700 425 ma during which time the brioverian was heterogeneously deformed and metamorphosed and intruded by calc alkaline igneous complexes the evolution of the belt has been interpreted in terms of the gradual cratonization of calc alkaline arc complexes and syn orogenic sediments in an andean type continental margin brown et al 1990 dupret et al 1990 this contribution reviews briefly the geology of the cadomian belt of the nam fig 1 and outlines some of the more controversial debates concerning the distribution and status of precadomian basement the stratigraphy of the brioverian succession the nature of cadomian magmatism the kinematic significance of major structures and the geotectonic setting of the component parts of the belt

i type granitoids as indicators of a late paleozoic convergent ocean continent margin along the southern flank of the central european variscan orogen a belt of carboniferous to permian i type granitoid bodies is in the basement thrust sheets of the alps which correspond to the southern flank of the central european variscan orogen these predominantly granodioritic plutons contrast with a contemporaneous belt of s type granites which extends to the north of the alps in the moldanubian section of the variscan orogen the observed regional duality of variscan plutonism resembles the plutons along active circum pacific continental margins in that there are outer i type plutons and inner s type granitoid belts therefore central europe could have undergone a cordilleran type orogenic event in late paleozoic time involving northwestward subdcution of the paleo tethys ocean

tracing magma mixing and crystal melt segregation in the genesis of syenite with mafic enclaves evidence from in situ zircon hf o and apatite sr nd isotopes abstract alkaline rocks contain essential information on post collisional intraplate extensional magmatic processes within the continental lithosphere the erhulai syenitic pluton in the liaodong peninsula north china craton consists of quartz syenite mafic enclaves and porphyritic k feldspar granite field observations and zircon u pb dating reveal that the quartz syenite and the hosted mafic enclaves were coeval with identical emplacement ages of 129 126 ma and slightly older than the porphyritic k feldspar granite ca 123 ma the mafic enclaves have relatively low sio2 and high mgo contents up to 3 0 and lack ba or eu anomalies whole rock analyses of the mafic enclaves and accessory minerals in the enclaves show relatively low initial 87sr 86sr ratios 0 7056 to 0 7078 and variable end t 11 2 to 2 0 and ehf t 15 8 to 5 5 values large proportion of zircons from the mafic enclaves possess mantle like 18o values these geochemical features suggest an enriched mantle source with the addition of crustal material the quartz syenites have relatively high sio2 and low cao and mgo contents as well as significantly positive k ba and eu anomalies all the quartz syenites show variable 87sr 86sr i ratios 0 7073 to 0 7081 and end t values 10 4 to 8 6 for the whole rocks and apatites ehf t values of 12 4 to 3 6 for the zircons and zircon 18o values of 4 4 to 7 0 mainly greater than 5 3 all these features can be isotopically produced by the partial melting of ancient crustal material with the involvement of mantle derived magma crystal fractionation process such as k feldspar crystallization and accumulation played an important role in the formation of the quartz syenite the porphyritic k feldspar granites possess relatively evolved geochemical compositions with high silica contents sio2 69 wt and remarkable negative ba and eu anomalies which are chemically complementary to the quartz syenite a crystal mush model was employed to establish the genetic link between the porphyritic k feldspar granite and the quartz syenite with the granite representing the interstitial melt extracted from the crystal mush and the quartz syenite representing the residual accumulation of crystals the variable isotopic compositions of the quartz syenite and the porphyritic granite were mainly the result of extensive interaction of isotopes between mantle and crust derived magmas during magma mixing in situ analyses of the sr nd hf and o isotopes of accessory minerals coupled with field observations and whole rock geochemical data for the erhulai syenitic pluton indicate a complex multi stage magmatic process involving magma mixing crystal melt segregation and wall rock assimilation our study also shows that in situ isotopic analyses of accessory minerals provide unique information on and are powerful geochemical tracers of mantle crust interactions and magmatic processes

volgo uralia the first u pb lu hf and sm nd isotopic evidence of preserved paleoarchean crust the crustal segment volgo uralia is the least known part of the east european craton its crystalline crust is hidden beneath a thick neoproterozoic to phanerozoic cover but disclosed by thousands of drill holes in conjunction with the recent tatseis reflection seismic profile we conducted the first isotopic study of the bakaly granitoid block in eastern volgo uralia which represents a subsurface section of the layered upper middle crust the study included whole rock sm nd and ion probe zircon u th pb sims and lu hf la icpms analyses of granitoids from seven drill cores the bakaly block was also targeted because its rocks have never been subjected to granulite facies metamorphism making it possible to date pristine pre metamorphic zircon our study showed that the four principal suites of granitoids in the bakaly block are different in age each corresponding to a particular stage of archean crustal evolution between 3 3 and 2 6 ga the tashliar monzonitic suite belonging to an alkaline series yielded zircon ages of 3 3 and 3 2 ga which are the oldest ages yet found in volgo urafia the epsilon hf t values of the dated zircon and the epsilon nd t values of their host rocks indicate that a paleo to eoarchean protolith with model t dm ages up to 3 8 ga had been involved in the formation of the tashliar melts three neoarchean rock suites one comprising quartz dioritic and tonalitic gneisses the bak 1 another k rich granodiorites granites and migmatites the bak 2 and the third monzonitic granitoids the aktanysh suite were formed sequentially between 2 72 and 2 60 ga the 2 72 ga bak 1 suite is chemically diverse it includes granitoids of the ttg type related to slab subduction melts as well as rocks formed by the re melting of older crust with whole rock nd t dm and hf t dm model ages of 3 4 to 3 2 ga the 2 69 to 2 65 ga bak 2 suite was probably associated with a major collisional event which defined the stacked structure of the archean crust in volgo uralia and its seismic layering our data suggest that the bak 2 melts originated partly from juvenile sources with epsilon hf t zircon values up to 4 8 as well as mixed crustal and juvenile mantle materials some crustal contamination of the melts appears to have occurred as evidenced by incorporated xenocrystic zircon the chemical compositions of bak 2 granitoids from the different plutons their zircon epsilon hf values and the hf and nd t dm ages all mirror a heterogeneous collisional crustal structure during post collisional extension at 2 6 ga the intrusion of aktanysh monzonitic granitoids took place these rocks also bear evidence of a long crustal pre history with nd and hf t dm model ages of 3 3 to 3 5 ga the aktanysh rocks are coeval with the tuymazy gabbro norite anorthosite intrusions which are widely distributed along post collisional shear zones in the bakaly block they could have provided the heat necessary to melt the crust at this stage altogether the isotopic evidence suggests several episodes of crustal growth and recycling possibly reaching back to 3 6 and 3 8 ga metamorphic zircon rims show that the archean crust in the bakaly block were subjected to several tectonothermal overprints in the paleoproterozoic between 2 4 and 1 9 ga ago less

geodynamic model for the palaeozoic crustal consolidation of western and central europe abstract the crystalline basement of western and central europe consists of a mosaic of crustal elements which were consolidated during pre grenvillian the grenvillian dalslandian morarian cadomian caledonian and hercynian orogenic cycles contemporaneous with the caledonian suturing of the precambrian laurentia greenland and fennosarmatian shields a number of gondwana derived cadomian micro cratons were accreted to the southern margin of laurasia following the late caledonian paroxysm the devonian and early carboniferous evolution of europe was dominated by continued northward subduction of the proto tethys plate at an arc trench system parallelling the southern margin of laurasia the accretion of additional gondwana derived continental fragments back arc rifting and a sinistral translation of major proportions between the laurentia greenland and the fennosarmatian sub plates the acadian and bretonian orogenies were of the pacific type the visean collision of gondwana with laurasia marked the onset of the himalayan type variscan orogeny during which collision related compressive stresses overpowered the devonian early carboniferous back arc rift systems and caused the development reactivation of a subduction zones the central armorican and saxothuringian successor basins became folded and destroyed during the latest visean whilst the variscan foredeep became scooped out in part by nappes involving the basement during the late westphalian major crustal shortening during the variscan diastrophism was accompanied by the anatectic remobilisation of subducted crustal and upper mantle material and a widespread syn and late orogenic magmatism the latest carboniferous early permian reorientation of the convergence direction between gondwana and laurasia induced the development of a complex wrench fault system transecting the variscan fold belt and an extensive post orogenic volcanism the hypotheses summarised here require confirmation by further palaeomagnetic and radiometric data and faunal analyses

pan african displaced terranes in the tuareg shield central sahara concepts developed in the recently published model of the air region eastern tuareg shield niger africa as a college of three displaced terranes integrated in a single geodynamic model lead us to propose a terrane map of the tuareg shield 500000 km 2 the 23 terranes recognized have their own lithological metamorphic magmatic and tectonic characteristics and are separated by subvertical strike slip megashear zones that can be traced for hundreds of kilometres or by major thrust fronts some of these boundaries have ophiolitic assemblages or molassic deposits the tuareg shield was shaped and partly accreted during the pan african orogeny 750 550 ma but not as a homogeneous body

evolution of the corsica sardinia batholith and late orogenic shearing of the variscides the corsica sardinia batholith formed in the late carboniferous permian along the northern gondwana margin one or more of the following processes may have raised the variscan geotherm enhancing melting of the crust and uppermost mantle such as i break off and detachment of the rheic oceanic slab ii mantle delamination due to gravitational re equilibration of thickened crust iii shear heating iv advection of mantle derived melts and v concentration of heat producing elements in this paper we present a simple one dimensional thermal model to explain the origin of the batholith in a geodynamic setting consistent with intraplate shearing between gondwana and cadomian avalonian blocks input parameters and boundary and initial conditions of the model crust are derived from a careful re examination of large petrological geochronological and structural datasets all parameters are varied within a range of geologically realistic values to reproduce thermal histories related to different tectonic processes and the reliability of models is quantitatively evaluated by comparing the simulated geotherms with a large dataset of pressure temperature time p t t constraints the best fit to p t t constraints is obtained for i break off in the late devonian early carboniferous ii decreasing exhumation rates from 340 to 280 ma and iii transpression at high average strain rates mostly coeval with the peak of high t low p metamorphism

early permian east ujimqin mafic ultramafic and granitic rocks from the xing an mongolian orogenic belt north china origin chronology and tectonic implications abstract the east ujimqin complex located north of the erenhot hegenshan fault north china is composed of mafic ultramafic and granitic rocks including peridotite gabbro alkali granite and syenite we investigated the tectonic setting age and anorogenic characteristics of the xing an mongolian orogenic belt xmob through field investigation and microscopic and geochemical analyses of samples from the east ujimqin complex and la mc icp ms zircon u pb dating of gabbro and alkali granite petrographic and geochemical studies of the complex indicate that this multiphase plutonic suite developed through a combination of fractional crystallization assimilation processes and magma mixing the mafic ultramafic rocks are alkaline and have within plate geochemical characteristics indicating anorogenic magmatism in an extensional setting and derivation from a mantle source the mafic ultramafic magmas triggered partial melting of the crust and generated the granitic rocks the granitic rocks are alkali and metaluminous and have high fe fe mg characteristics all of which are common features of within plate plutons zircon u pb geochronological dating of two samples of gabbro and alkali granite yielded ages of 280 8 1 5 and 276 4 0 7 ma placing them within the early permian the zircon hf isotopic data give inhomogeneous e hf t values of 8 2 14 7 for gabbroic zircons and extraordinary high e hf t values 8 9 12 5 for the alkali granite in magmatic zircons thus we consider the east ujimqin mafic ultramafic and granitic rocks to have been formed in an extensional tectonic setting caused by asthenospheric upwelling and lithospheric thinning the sources of mafic ultramafic and granitic rocks could be depleted garnet lherzolite mantle and juvenile continental lower crust respectively all the above indicate that an anorogenic magma event may have occurred in part of the xmob during 280 276 ma

petrogenesis and tectonic implications of the high k alamas calc alkaline granitoids at the northwestern margin of the tibetan plateau geochemical and sr nd hf o isotope constraints abstract the alamas granitoid pluton in the eastern part of the western kunlun orogen the northwestern margin of the tibetan plateau is composed of quartz diorite zircon separates from the pluton has sims u pb age of 446 ma rocks from the pluton have a narrow range of sio2 56 84 62 57 wt mgo 1 76 2 94 wt and total alkalis na2o k2o 5 14 9 59 wt and are metaluminous and high k calc alkaline to shoshonitic in composition they are enriched in light rare earth elements lrees with la yb n 14 25 and show weakly negative eu anomalies these rocks are relatively enriched in sr 472 676 ppm and ba 435 2388 ppm and depleted in nb ta th and ti their end t values range from 6 4 to 8 4 and 87sr 86sr i 0 7184 0 7200 zircons from the pluton show ehf t values of 1 4 to 8 8 and 18o 6 4 9 0 geochemical data indicate that the granitoids were likely derived from the reworking of an ancient deep crustal source influenced by a minor mantle derived component magmatic differentiation was dominated by the fractional crystallization of hornblende biotite and accessory minerals such as apatite allanite and fe ti oxides in summary the late ordovician alamas pluton is an i type granitoid that was emplaced in a post collisional environment suggesting that this tectonic stage had already initiated prior to 445 ma

geochronology petrogenesis and tectonic implications of permian felsic rocks of the central great xing an range ne china the hegenshan heihe suture hhs is an important suture zone between the xing an and songliao xilinhot blocks in the eastern central asian orogenic belt caob many studies have examined the northern and southern parts of this suture zone but the middle segment remains under researched and controversial this paper presents new research into permian felsic rocks along the middle segment of the hhs including alkaline granites rhyolites and granodiorites zircon u pb dating of these rocks yielded precise permian ages of 300 4 2 5 292 8 2 7 and 293 1 2 2 ma the alkaline granites and the rhyolites are a type rocks that were derived from lower crustal protoliths with the melting of these protoliths caused by asthenospheric upwelling associated with lithospheric delamination in comparison the granodiorites are i type granites were generated from a mantle derived magmatic source the petrogenesis of these granites also involved the assimilation of sedimentary rocks and fractional crystallization this permian magmatism occurred during post collisional extension following the amalgamation of the xing an and songliao xilinhot blocks

sr nd isotopic constraints on the petrogenesis of the central bohemian pluton czech republic the sr nd isotopic data for selected granitoids of the central bohemian pluton show a broad negative correlation with the total range of 87sr 86sr 330 0 7051 0 7129 and nd330 0 2 to 8 9 the older intrusions have more depleted sr nd compositions and calc alkaline geochemistry sazava suite whereas the younger intrusions shift towards k rich calc alkaline blatna suite and shoshonitic rocks icany and certovo b emeno suites with more evolved isotopic signatures the distribution of the data is interpreted as reflecting a diversity of sources and processes rather than a single progressive crustal contamination trend the sazava suite could have originated by partial melting of metabasites or of a mantle source with an isotopic composition close to bulk earth or by hybridization of crustally derived tonalitic and mantle derived magmas variation within the blatna suite is modelled by mixing between a moderately enriched 87sr 86sr 330 0 708 nd330 3 mantle component with either an isotopically evolved metasedimentary component or with more evolved magmas of the suite the icany suite was most probably produced by partial melting of peraluminous lithologies possibly of the adjacent moldanubian unit the certovo b emeno suite evolved from strongly enriched mantle derived magmas 87sr 86sr 330 0 7128 end330 7 either through closed system fractional crystallization or interaction with magma corresponding to leucogranites of the central bohemian pluton

pbsl dating of garnet and staurolite constraints on the paleoproterozoic crustal evolution of the eastern block north china craton abstract it is well accepted that the north china craton formed by assembly of the eastern and western blocks along the trans north china orogen with cratonization in the paleoproterozoic 1 85 ma however there is controversy about the formation of the eastern and western blocks themselves especially the timing of cratonization of the eastern block the jiao liao ji orogenic belt is the most important paleoproterozoic orogenic belt in the eastern block and consists mainly of the north and south liaohe groups and the liaoji granitoids the timing of metamorphism of the liaohe group is the key to understanding the evolution of the eastern block of the north china craton because the isotope systematics of metamorphic minerals can be used to date parts of the prograde thermal histories of metamorphic belts dating of minerals such as garnet and staurolite has the distinct advantage that time information can be related directly to the pressure temperature deformation history recorded by the same mineral in this paper we present results of precise 207pb 206pb stepwise leaching pbsl dating of garnet and staurolite from the metasedimentary rocks of the south liaohe group the primitive pb and radiogenic pb are selectively extracted from garnet and straurolite by leach steps and determined by multiple collector mc icp ms the garnet and staurolite record two groups of 207pb 206pb ages at 1 93 1 91 ga and 1 86 ga respectively these constrain the time of peak metamorphism of the liaohe group during collision between the longgang block in the north and the liaonan nangrim block in the south and a subsequent post collisional event respectively our data for the south liaohe group and previously published data for the liaoji granite combined with key lithological structural and metamorphic information indicate that the cratonization of the eastern block of the north china craton occurred at 1 93 1 91 ga

geochemistry geochronology and zircon hf isotopic study of peralkaline alkaline intrusions along the northern margin of the north china craton and its tectonic implication for the southeastern central asian orogenic belt abstract a giant permian alkaline magmatic belt has recently been identified in southern inner mongolia along the northern margin of the north china craton ncc this belt is mainly composed of syenite quartz syenite alkaline granite and mafic microgranular enclaves mme bearing granodiorite in order to study the petrogenesis and tectonic implications of these rocks we undertook zircon u pb dating and geochemical analysis of two permian alkaline plutons the first guangxingyuan pluton occurs in the hexigten area and is composed of mme bearing tonalite k feldspar granite and syenite the second durenwuliji pluton located in the xianghuangqi area comprises syenite quartz syenite and k feldspar granite zircon u pb dating on tonalite k feldspar granite syenite and quartz syenite from the two plutons yielded a tight range of ages from 259 to 267 ma the peralkaline alkaline rocks show high abundance of total alkalis k2o na2o 7 9 12 9 and k2o contents 3 9 8 0 enrichment in large ion lithophile elements lile and light rare earth element lree and depletion of high field strength elements hfse the associated tonalite and mmes display i type granitic geochemical affinity with less total abundance of trace elements than the peralkaline alkaline rocks zircon hf isotopic analysis of the guangxingyuan pluton yielded a large range of ehf t values from 15 5 to 6 7 and model ages tdmc from 781 to 2012 ma by contrast the hf isotopic data of the durenwuliji pluton shows a small range of ehf t from 6 2 to 8 9 and tdmc from 667 to 816 ma the geochemical and hf isotopic characteristics indicate that the parental magma was derived from a mixing of metasomatic mantle derived mafic magma with different amount of crust derived felsic magma and followed by fractional crystallization considering previous tectonic studies in inner mongolia a permian post orogenic extension was proposed to account for these peralkaline alkaline intrusions following the amalgamation of ncc and its northern blocks in the southeastern part of central asian orogenic belt

k rich mantle metasomatism control of localization and initiation of lithospheric strike slip faulting a conceptual model is proposed in which bulk transtension or local transtension during bulk simple shear resulting from mantle anisotropy contrasts or lithosphere rheology contrasts of heterogeneously enriched lithospheric mantle triggers localized k rich magmatism which focuses strain and causes nucleation of lithosphere scale transtensional or strike slip shear zones transtension triggered magmatism is most likely to be located at sites of maximum metasomatism of the lithospheric mantle magma generated fractures propagate upwards nucleating zones of lithospheric weakness which focus shear in narrow transcurrent faults or at basin margins in this way magmatism controls fault timing and location although volcanism will be coeval with fault development and volcanoes will appear fault controlled counterintuitively our model suggests that faults are in a sense volcanocontrolled we suggest that this new transtension k rich magmatism transcurrent faulting association represents a hitherto unrecognized genetic relationship as significant as for example the ocean island magma series

why do continents break up parallel to ancient orogenic belts the frequently observed parallelism between rifts and the pre existing orogenic fabric of continents suggests that the inherited tectonic fabric of the lithosphere influences the rupture of continents we propose that the existence of a pervasive fabric in the lithospheric mantle induces an anisotropie strength in the lithosphere that guides the propagation of continental rifts subcrustal mantle mechanical anisotropy is supported by i the anisotropie strength of olivine ii an ubiquitous tectonic fabric in exposed mantle rocks and iii measurements of seismic and electrical anisotropy during major episodes of continent rifting parallel to orogenic belts ocean opening through rifting and continent break up is frequently related to the occurrence ofhotspots there is howevcr a discrepancy between hot spots acting as pin point sources of heat and the linear extent of rifts over thou sands ofkilometres moreover rifts tend to parallel pre existing orogenic fab

reworked old crust derived shoshonitic magma the guarany pluton northeastern brazil abstract the 572 ma guarany stock consists of magmatic epidote bearing hornblende monzodiorite to biotite granite that intruded paleoproterozoic orthogneisses about 10 km inland from the coast in northeastern brazil co magmatic diorite enclaves and dikes are abundant throughout the pluton the monzodiorite granite pluton and diorite enclaves are shoshonitic and display continuous trends in variation diagrams they display chemical and isotopic characteristics of crustal melts such as enrichment in incompatible elements high back calculated initial 87 sr 86 sr ratios avg 0 71253 negative end 0 57ga values avg 14 58 as well as high and variable 9 1 to 11 1 vsmow 18 o zircon values correlations between o isotope and whole rock silica contents as well as initial 87 sr 86 sr ratios with 1 sr concentrations suggest hybridization of a lower continental crustal melt with more felsic crustal rocks concomitant with fractional crystallization amphibole chemistry and whole rock zr tio 2 and p 2 o 5 contents suggest magma solidification at a pressure 7 kbar and near liquidus temperature 900 c the parental magma was likely formed by partial melting of old tdm 2 0 ga amphibolitic lower continental crustal rocks in a post collisional setting probably triggered by underplating of mantle derived mafic magma during the period of relaxation after collision

geodynamics of the indosinian orogeny between the south china and indochina blocks insights from latest permian triassic granitoids and numerical modeling

geology re os age s and o isotopic composition of the lar porphyry cu mo deposit southeast iran abstract the lar porphyry cu mo deposit is located 20 km northeast of zahedan in the sistan suture zone ssz in the southeastern part of iran lar is a sub economic porphyry cu mo deposit with average grades of 0 16 cu 0 01 mo and 0 66 ppb au the cu mo mineralization developed in post collisional shoshonitic syenitic to monzonitic intrusions in an oligocene igneous complex known as the lar igneous complex lic the host and country rocks have undergone variable degrees of supergene and hypogene alteration including silicic potassic propylitic phyllic and argillic alteration the cu mo mineralization is mainly controlled by nnw striking faults and fractures and occurs as quartz veins and veinlets disseminated sulfides and stockworks the mineralization can be divided into two types hypogene and supergene hypogene mineralization is characterized by chalcopyrite pyrite bornite molybdenite and magnetite whereas supergene mineralization is characterized by enargite covellite chalcocite digenite malachite azurite and iron hydroxides sulfur isotope values for hypogene sulfides range from 0 28 to 1 76 and suggest an igneous source or mantle derived origin for sulfur the 18o values for quartz in mineralized quartz veins range from 9 6 to 11 5 these values would be in equilibrium with waters having calculated 18oh2o values of 9 3 11 2 suggesting magmatic water re os dating of two molybdenite samples from two different drill holes yields dates of 29 72 0 11 and 31 95 0 11 suggesting that the system was active for at least 2 0 m y

early permian a type granites from central inner mongolia north china magmatic tracer of post collisional tectonics and oceanic crustal recycling abstract a type granitoids constitute volumetrically minor but genetically important building blocks of most post archean continental landmasses on earth and their diversified genesis encapsulates key information for ascertaining critical geodynamic controls on continental crustal formation and differentiation this zircon u pb dating and geochemical study documents the early permian peralkaline granites from central inner mongolia of china eastern central asian orogenic belt caob these granites show an a type magmatic affinity with typical enrichment in alkalis ga zr nb and y depletion in sr and p and fractionated ree patterns showing strong negative eu anomalies they have meaningful initial 87 sr 86 sr ratio of 0 70594 e nd t of 4 25 to 6 05 zircon e hf t from 9 8 to 16 0 and 18 o values from 2 41 to 4 72 these elemental and isotopic characters argue for parental magmas from partial melting of a newly trapped charnockitic lower crustal protolith and their subsequent fractional crystallization with an inherent hydrothermally altered oceanic crustal heritage and possible tectonic linkage to post collisional slab breakoff this kind of a type granites could not only serve as an insightful temporal milestone for monitoring the amalgamation between various constituent terranes in the caob but also present a spatial magmatic proxy for marking the maturation stage of juvenile crustal reworking during the final phase of a large orogenic event

the plutonic rocks of the elbe valley zone germany evidence for the magmatic development from single zircon evaporation and kar age determinations abstract plutonic bodies of different geochemical characteristics and probably of different origin occur within the elbe valley zone e germany the complex of the so called meisen massif consists from rim to core of a nearly concentric sequence of diorite to monzodiorite monzonite and some granite intrusions field geological arguments point for an emplacement during the variscan orogenic cycle in contrast due to the lack of geochronological data the time of intrusion of the granodiorites from dohna and laas situated in the southeast and northwest of the massif is still under discussion according to 207 pb 206 pb ages of 355 12 ma produced by the single zircon evaporation technique all errors are given at the 1 level the pyroxene monzodiorite from groba represents an early stage of the variscan plutonic activity it is followed by different monzodiorites and monzonites of the meisen massif inherited zircon relics of 399 429 ma are comparable with similar zircon ages of some granulites and gneisses of the saxothuringian zone single grain determinations on zircons of the main granodioritic to monzogranitic intrusive rocks of the meisen massif gave only relic ages varying between 573 and 2222 ma these results are in agreement with the microscopic recognition of voluminous cores in the majority of the zircons based on k ar analyses the cooling of the main granitic intrusion below 350 50 c took place at 326 6 ma 207 pb 206 pb isotope ratios measured on 13 zircon grains yield implications for a cadomian emplacement of the granodiorites from dohna and laas 550 ma are interpreted as the most reliable minimum age of their intrusion in the case of laas a scattering of the data between 552 and 634 ma may be related to minor inherited components although no such indications were detected by microscopic investigations however a second group of zircons is characterized by the predominance of relictic domains showing strong crustal influences on these cadomian intrusions

formation and stabilization of the dunhuang block nw china constraints from the late paleoproterozoic a type granites of the dunhuang complex abstract the geochemical composition and temporal evolution of a type granites may hold key information regarding critical geodynamic processes that occurred during the evolution of the continental crust moreover these aspects may provide an indication of when orogenic processes ended and craton formation stabilization began in early precambrian blocks in this study we report the results of zircon u pb ages hf isotope data and whole rock geochemistry for the proterozoic huoyanshan potassic granites in the dunhuang block northwest china zircon u pb dating reveals that the huoyanshan granites were emplaced at ca 1791 1786 ma representing a late paleoproterozoic magmatism event the granites have high sio2 70 71 72 20 wt total alkali contents k2o na2o 8 81 8 96 wt k2o na2o 1 57 1 66 tio2 mgo ratios 2 19 2 57 low cao 0 77 1 27 wt mgo contents 0 12 0 16 wt and mg values 7 72 8 67 with a cnk ratios of 0 96 1 04 pointing to alkaline and metaluminous to weakly peraluminous affinities these granites are characterized by high total rare earth element concentrations ree 419 755 ppm and variable la yb n values 8 24 with significantly negative eu anomalies eu eu 0 31 0 42 the primitive mantle normalized trace element patterns suggest that the huoyanshan plutons are depleted in ba nb ta sr p and ti but enriched in rb th and pb additionally the granites have a high zirconium saturation temperature of 855 897 c with an average of 878 c these geochemical characteristics indicate that huoyanshan potassic granites are an aluminous a type granite and further classified as a2 type together with the zircon hf isotope data ehf t 11 21 to 2 76 indicates that these granites were generated from a crustal source the heterogeneous hf isotope compositions in zircons are principally caused by the disequilibrium melting of zr bearing sources in addition two stage hf model ages of zircons from the granites are ca 2 6 3 1 ga and plot on the average crustal evolution trend of archean tonalite trondhjemite granodiorite ttg gneiss 2 5 to 3 0 ga in the dunhuang block thus we propose that a type granites in the study area were generated from the disequilibrium partial melting of archean tonalitic to granodioritic rocks under high temperature and low pressure conditions according to their low al2o3 cao na2o k2o molar values and high 10000 ga al values 2 9 subsequently the magma experienced prominent fractional crystallization of k feldspar plagioclase orthopyroxene and or magnetite our results with available data suggest that the huoyanshan granites formed in a tectonic transform regime within a collisional to extensional setting and serve as a precursor for the formation and stabilization of the dunhuang block during the late paleoproterozoic furthermore it is probable that the post collisional magmatism event at ca 1 79 ga in the dunhuang block was closely related to the assembly of the columbia supercontinent

aluminous subsolvus anorogenic granite genesis in the light of nd isotopic heterogeneity the problem of the genesis of anorogenic granites is far from being resolved and this may result from their petrogenetic variability several kinds of a type granites are indeed present in many anorogenic provinces a peralkaline and an aluminous trend commonly occur and the latter often comprises hypersolvus and subsolvus granites as an example of the aluminous subsolvus group we have studied major trace elements and nd sr isotopes of the tana granite sw corsica france along with the coeval mafic and intermediate rocks petrological and trace element constraints suggest that the associated mafic rocks are not representative of the magmatic precursor of the granite rather the intermediate rocks monzonites are more akin to the parental magma that yielded the granite through fractional crystallization the aluminous subsolvus anorogenic granite and related monzonite are characterized by variable nd isotopic compositions ndi from 4 7 to 1 3 and from 2 to 0 1 respectively while sr isotopes give good isochrons it is argued that the nd isotopic heterogeneity was not acquired at a late stage in the upper crust by contamination from partial crustal melts or f rich late to post magmatic fluids similarly the lack of correlation between trace element concentrations and nd isotopes in a single granitic body precludes a process of crustal contamination of mantle melts instead it is inferred that these low and very heterogeneous ndi s were probably inherited from the magma source region in the lower crust the preservation of this heterogeneity during magma ascent suggests that only limited mechanical and chemical mixing occurred in magma chambers it is more likely that separate batches of low viscosity a type magmas ascended individually mainly through fracture propagation in an extensional regime the possible derivation of granites from monzonites through fractional crystallization together with the examination of the corsican lower crust composition and already published dehydration melting experiments suggest that the aluminous subsolvus trend of a type granites may be generated through the partial melting of metamorphosed amphibole bearing and k2o rich mafic cumulates that have not undergone a previous partial melting event

structure and u pb geochronology of central hoggar algeria a reappraisal of its pan african evolution discovery of large scale deep seated thrusts in central hoggar with a plurifacial evolution ranging from lower amphibolite facies to upper greenschist facies conditions and linked to a regional refoliation has led us to reconsider the pan african tectonic and metamorphic history in that region two areas are described and a review of other thrusts leads to an interpretative cross section of a large portion of reactivated continental crust the age and kinematics of this structural reworking have been approached using u pb zircon dating in the tamanrasset region despite the difficulty of estimating the age of the initiation of the assumed intracontinental a type subduction the results provide a time span of 30 40 m y between the climax of granitoid emplacement and a late retrogressive offset along the thrust planes some key ages were determined 1 2075 30 ma is the age given by granulite facies remnants which escaped from the refoliation the corresponding lower intercept at 530 70 ma confirms the pan african imprint 2 615 5 ma reflecting the age of syntectonic to late tectonic granitoids emplaced in reworked gneisses and in preserved granulites 3 580 ma the concordant age of sphenes and monazites from the same granitoids which is interpreted as corresponding to the end of medium grade conditions no evidence has been found of a 1000 ma age a kibaran event does not appear to exist in central hoggar the age similarity between the observed deep intracontinental evolution of central hoggar and the collision related tectonics of western hoggar and iforas suggests a common origin for both phenomena

devonian a type granitic magmatism on the northern margin of the north china craton shrimp u pb zircon dating and hf isotopes of the hongshan granite at chifeng inner mongolia china abstract we report first data for the devonian hongshan a type granite along the northern margin of the precambrian north china craton the granite is rich in silica sio2 76 07 77 62 wt and alkalis na2o k2o 8 45 8 78 wt but poor in calcium cao 0 28 0 70 wt and magnesium mgo 0 08 0 20 wt chondrite normalized ree patterns for the samples from the hongshan granite show significant negative eu anomalies eu eu 0 13 0 19 they contain high rb ga y nb and zn but low ba and sr contents and in the multi element spider diagrams normalized to primitive mantle they show a similar pattern to that of a type granites such as negative ba nb ta sr p and ti anomalies geochemical plots also show an affinity with a type granites and suggest a post orogenic setting a2 type granite shrimp u pb zircon dating yielded a concordant weighted mean 206pb 238u age of 387 4 ma which defines a devonian magmatic event the zircons have negative ehf t of 8 4 to 11 5 and give old hf model ages of tdm2 2548 2820 ma mean 2667 ma most 176hf 177hf and ehf values display hf isotope evolutional line of the lower continental crust these results suggest that the hongshan granite originated from melting of archaean lower crust beneath the north china craton in a post orogenic extensional setting during the devonian

assimilation of felsic crust by basaltic magma thermal limits and extents of crustal contamination of mantle derived magmas basaltic magma is the most widely used probe of mantle chemistry and structure chemical characterization of the sources for mantle derived magma is possible only after quantitative evaluation of possible contamination by crustal wall rocks the most commonly invoked contamination mechanisms are the combined processes of crustal assimilation and magmatic fractional crystallization afc because of the energetic link between these processes predicting assimilation and crystallization progress requires modeling both heat and mass transfer in the magma country rock system using the melts software package we modeled isenthalpic heat balanced afc between two common types of basaltic magma and several types of felsic crust at 1 kbar our simulations show that during the early stages of isenthalpic afc the ratio of rates of assimilation to crystallization r may be substantially greater than one 2 0 2 7 allowing assimilation of a mass of country rock up to 5 18 of the initial magma mass with only 3 7 crystallization the second stage of afc beginning with plagioclase and or pyroxene saturation is characterized by lower r 0 5 1 0 the initial high r stage results from suppression of crystallization associated with the change in magma composition as assimilation progresses under certain conditions even small degrees of crystallization of olivine alone coupled with cooling of the magma can accommodate relatively large amounts of crustal assimilation and cause large shifts in isotopic and trace element geochemical indices with little differentiation

petrogenesis and tectonic implications of late carboniferous continental arc high k granites in the dongwuqi area central inner mongolia north china abstract central inner mongolia is located in the southeastern segment of the central asian orogenic belt caob and controversy has long surrounded its late paleozoic tectonic evolution here we present an integrated study of zircon u pb ages whole rock major and trace elements and zircon hf isotopes for five less studied alkali feldspar granitic plutons in the dongwuqi area to the north of the erenhot hegenshan heihe suture zone zircon u pb dating by laser ablation inductively coupled plasma mass spectrometry la icp ms indicates that these granitic plutons were emplaced in the late carboniferous 315 300 ma the late carboniferous alkali feldspar granites belong to high k calc alkaline series and are evolved i type granites they are high in sio2 73 22 77 36 and low in mgo 0 07 0 49 with enrichments in light rare earth elements lrees zr hf rb th and u and depletions in ba nb ta sr and eu the dongwuqi granitic rocks are characterized by high positive zircon ehf t values 7 6 to 15 1 and relatively young two stage hf model tdm2 ages 364 835 ma these geochemical data together the common miarolitic texture indicate that they were derived from partial melting of juvenile arc related mafic to intermediate rocks in the lower crust and eventually emplaced in low pressure conditions or at shallow crustal levels with involvement of extensive fractional crystallization during magma evolution our new results along with the published regional data delineate a ne trending carboniferous continental arc belt more than 1700 km in the northern accretionary zone of the erenhot hegenshan heihe suture accordingly we invoke that the dongwuqi late carboniferous high k granites were emplaced in an active continental margin setting which was probably related to the northwestward subduction of the hegenshan oceanic plate beneath the xing an uliastai nuhetdavaa block

geochronology petrogenesis and tectonic implications of triassic granitoids from beishan nw china abstract five triassic granitic plutons have been identified from the beishan orogen by zircon u pb dating that defines two age groups 240 238 ma and 225 217 ma they belong to the high k calc alkaline and shoshonitic series and have alkalic and alkali calcic characteristics and are weakly peraluminous metaluminous these granitoids are mainly highly fractionated i type granite and a type granite they have high i sr values 0 7049 0 7096 and negative e nd t values of 0 9 to 2 8 with nd model ages of 0 89 1 13 ga suggesting a crustal contribution they also exhibit relatively high zircon e hf t values 1 3 to 6 1 and two stage hf model ages of 1 34 0 87 ga suggesting a mixture of mantle and various amounts of crustal components the mantle component probably originated from underplated magmas produced by partial melting of lithospheric mantle metasomatized by fluids or melts possibly derived from pre existing subducted materials the mid triassic 240 238 ma pluton has relatively high sr y 15 9 41 8 ratios and mgo 1 22 3 77 contents with negligibly to weakly negative eu anomalies eu 0 57 0 80 indicating adakite like features this was likely the result of partial melting of crustal components in response to voluminous underplated mantle derived magmas probably linked to lithospheric delamination and asthenospheric upwelling the late triassic 217 225 ma granitic plutons show strong negative anomalies of eu ba nb ta sr p and ti these were likely the result of fractional crystallization and crustal contamination for the granitic magma emplaced in a post orogenic setting

formation of laminite deposits in anaerobic dysaerobic marine environments abstract vraconian sediments from the bandol area var southeastern france are characterized by alternations of thin dark laminae in which organic matter is concentrated and light calcareous silt grained laminae the dark laminae black shales are regarded as the result of autochthonous i e in place hemipelagic sedimentation the light laminae calcareous silt result from the partial reworking of turbiditic influxes by deep marine currents comparison with modern marine environments suggests that the vraconian laminite deposits from the bandol area were formed by sedimentation from deep marine circulations the laminated structures formed in a deep marine environment subjected to periodic variations in the physicochemical conditions organic productivity and thus the nature of organic matter the dark laminae were deposited in an anaerobic marine environment in which the organic matter was preserved during the deposition of the light laminae the depositional environment changed to dysaerobic conditions allowing the development of a peculiar benthic fauna responsible for the decreasing organic matter content source rock characteristics of these deposits are inferred from their laminated structure and fine texture coupled with the occurrence of organic rich layers

systematics of internal zircon morphology in major variscan granitoid types the internal morphologies of zircon crystals from different types of granitoids alkaline calcalkaline and anatectic are revealed by cathodoluminescence imaging and are described in terms of growth rates of the crystal faces relative to each other zircons in the alkaline granitoids are characterized by high and constant growth rates of 010 relative to the pyramidal forms and by symmetric grwoth of 011 zircons in the calcalkaline and anatectic granitoids are characterized by fluctuating or gradually decreasing relative growth rates of 010 by asymmetric and highly variable growth of 011 and by a tendency of 110 to become grwoth inhibited corrosion events are interspersed during zircon growth in the calcalkaline magmas in the calcalkaline and anatectic magmas a discontinuity breaks the morphological evolution at late stages of crystallization the discontinuity coincides with a sharp drop in cathodoluminescence the growth behaviour of each crystal form is analysed and compared with predictions made by the periodic bond chain pbc theory it is argued that the relative growth rate of 010 depends on supersaturation that the growth rates of 011 faces are changed in response to different ratios of adsorbing cations na k al and that 110 faces become growth inhibited by the adsorption of h2o or trace elements enriched in the residual liquid morphological and chemical discontinuities at late stages of crystallization are reasonably explained by the formation of larger growth units from smaller ionic entities in the residual liquid important factors controlling the zircon morphology in different types of granitoids are high cooling rates alkaline magmas magma mixing calcalkaline magmas enrichment of h2o and trace elements in residual liquids calcalkaline and anatectic magmas and the major element chemistry of the magma possibly the ratio of na and k to al agpaicity

the pb isotopic systematics during crustal contamination of subcrustal magmas the hercynian magmatism in the serie dei laghi southern alps italy abstract the pb isotopic composition and pb u and th concentrations of a series of hercynian granite plutons and mafic intermediate stocks and dykes belonging to the serie dei laghi western southern alps have been analyzed the initial pb isotope ratios of both the granites and the dykes are typical of the lower crust and are spread over a significant range 206 pb 204 pb 280 17 883 18 530 207 pb 204 pb 280 15 597 15 710 and 208 pb 204 pb 280 38 077 38 708 four dykes all at the same locality display evidence of post emplacement fluid circulation with mobilization of u and pb in spite of the pb isotopic crustal signature of the studied rock series the 87 sr 86 sr ratios reach values as low as 0 704 in the dykes and as low as 0 707 in the granites suggesting a mantle contribution in the genesis of both lithologies indeed when compared with the metasediments from the same area the magmatic rocks have somewhat lower initial 207 pb 204 pb ratios which weakly trend towards the less radiogenic pb isotopic composition of the mantle the relationships between the pb isotopic composition and the pb content along with the pb and sr isotopic covariation suggest that the suite under examination was generated from a mantle derived primary magma that underwent extensive crustal contamination a primary magma similar to the neighbouring ivrea mafic complex has been inferred further assimilation of crustal material is particularly evident in the dykes while the granites appear to have been generated mainly by crystal fractionation processes from the intermediate magma already contaminated

chemical evolution of a large mafic intrusion in the lower crust ivrea verbano zone northern italy the ivrea verbano and adjacent strona ceneri zones have been described collectively as a section through the continental crust while resident in the lower crust amphibolite to granulite facies paragneiss of the ivrea verbano zone was intruded by huge volumes of mafic to intermediate plutonic rocks grouped as the mafic complex growth of the mafic complex involved hypersolidus deformation in an extensional environment isotopic and trace element variations close to the axis of this structure indicate crystallization from mantle derived melts that were extensively contaminated by crustal material previous investigations determined that the contaminant was fingerprinted by 87sr 86sr 0 71 18o 10 12 5 and a positive eu anomaly in the present study the contaminant is also shown to have been enriched in ba with respect to rb and k charnockites associated with paragneiss septa in the lower part of the mafic complex have the appropriate chemistry to be samples of the contaminating material these chemical features can be explained by melting of granulite facies paragneiss which had previously been depleted in k and rb by an earlier melting event the ba enrichment in the core of the mafic complex can be modeled by a replenishment tapping fractional crystallization rtf process operating within a small magma chamber is repeatedly replenished by mantle melts and contaminated by ba rich charnockite very high ba k in the lower part of the complex are tentatively attributed to chemical exchange between the cumulate framework and infiltrating anatectic melts from underlying paragneiss septa in contrast to the mafic complex the chemistry of coeval granites in the adjacent strona ceneri zone reflect a component derived from crustal rocks that had not been significantly depleted by a previous melting event significantly the incompatible trace element abundances in the mafic complex and strona ceneri granites are similar to model compositions for the lower and upper crust respectively

geodynamic transition from subduction to extension evidence from the geochronology and geochemistry of granitoids in the sangsang area southern lhasa terrane tibet the northward subduction of the neo tethys oceanic lithosphere and subsequent collision between india and asia continents gave rise to the tibetan plateau however how and when oceanic subduction started to transform to an intraplate environment are still open questions the granitoids distributed in lhasa terrane of south tibet offer a unique chance for us to investigate the deep geodynamic processes here we present zircon u pb hf isotope whole rock major and trace element and sr nd isotope data of granitoid intrusions in the sangsang area of the southern lhasa terrane the sangsang granodiorites and granites were crystallized at ca 54 ma while the emplacement age of the quartz monzonites is ca 47 ma the granodiorites are characterized by relatively high mg values 35 3 41 1 and fe2o3t 5 16 6 26 wt contents and low na2o k2o contents 6 4 6 9 wt and a cnk values 0 91 0 99 which are similar to the geochemical characteristics of i type high k calc alkaline rocks they have high 87sr 86sri ratios 0 706455 0 706490 and low end t 3 58 to 2 96 and zircon ehf t 3 4 to 0 3 values indicating they were derived from a hybrid source of ancient mafic crust and juvenile lower crust the coeval granites have lower mg values 22 5 27 25 and similar zircon ehf t values 2 6 to 1 1 suggesting they were probably differentiation productions of the granodiorites the quartz monzonites have higher na2o k2o contents 9 18 9 59 wt and a cnk values 0 98 1 03 higher zircon ehf t values 2 2 to 2 6 and more depleted sr and nd isotopes than the granodiorites and granites the quartz monzonites were probably produced by melting of mixed juvenile crustal materials and metagreywacke the new geochronological and geochemical data help constrain the geodynamic processes in the lhasa terrane during the early cenozoic as the sangsang granitoids represent the change from subduction termination to intraplate extension at the southern margin of the lhasa terrane

post orogenic extension in the eastern part of the jiangnan orogen evidence from ca 800 760ma volcanic rocks we present a systematic geochronological and geochemical study on ca 800 760 ma volcanic rocks in the eastern part of the jiangnan orogen the xucun composite dykes are dated at ca 805 ma the mafic components have oib like trace element patterns and positive anomalies in zr and hf the least contaminated sample has relatively depleted nd isotopic features suggesting the xucun mafic dykes may have been generated from the partial melting of oib like asthenosphere with later crustal contamination the xucun felsic dykes have decoupled nd hf isotopes and the hf isotope compositions of zircons indicate that the dykes may be derived from the partial melting of the early neoproterozoic juvenile crustal materials with minor incorporation of paleoproterozoic crustal components the ca 800 790 ma shangshu volcanics include two compositional series calc alkaline and tholeiitic the shangshu calc alkaline volcanics in the minjiawu area have low abundances of lile hfse and high na2o contents and sr y ratios similar to adakitic rocks the evident arc like geochemical features and radiogenic nd isotopes nd t values of 3 7 to 4 8 suggest that these rocks may have been generated from the partial melting of juvenile lithospheric mantle metasomatized by na rich melts released from the subducted slab the tholeiitic mafic rocks from the shangshu bimodal volcanics represent two different magma sources the partial melting of metasomatized lithospheric mantle led to the formation of arc like basalts with low tio2 contents negative anomalies in zr and hf and high values of mg and nd t 6 2 whereas the partial melting of asthenospheric mantle generated volcanic rocks with high tio2 contents and low positive nd t 1 4 to 2 7 without negative anomalies of nb ta zr and hf the shangshu felsic rocks were formed by the reworking of early neoproterozoic juvenile arc crustal materials the ca 760 ma mafic rocks from the puling bimodal volcanics generally have low tio2 contents 0 9 wt nearly flat ree distributions and arc like trace element patterns they may have been generated from the high degree partial melting of metasomatized lithospheric mantle one sample has a high tio2 content 2 41 wt and high nd t 6 2 with overall oib like trace element patterns implying the local partial melting of asthenospheric mantle the occurrence of significant volumes of bimodal volcanics in the eastern part of the jiangnan orogen suggests an extensional setting in the period ca 800 760 ma the evident partial melting of newly metasomatized lithospheric mantle and subordinate partial melting of asthenosphere suggest that post orogenic extension shortly after the neoproterozoic orogenesis may be a better explanation for the genesis of the mid neoproterozoic magmatic rocks in the eastern part of the jiangnan orogen post orogenic extension may be diachronous along the whole orogenic belt and probably has no direct relationship with the rodinia rifting event a more detailed model is presented to illustrate the evolution of the eastern part of the jiangnan orogen

contrasting late carboniferous and late permian middle triassic intrusive suites from the northern margin of the north china craton geochronology petrogenesis and tectonic implications two contrasting intrusive suites have been identified from the northern margin of the north china craton a late carboniferous dioritegranodiorite suite mainly made up of quartz diorite diorite granodiorite tonalite and hornblende gabbro and a late permian middle triassic suite of granitoid intrusions consisting of monzogranite syenogranite and quartz monzonite plutons from the late carboniferous suite exhibit variable sio2 contents and calc alkaline or high k calc alkaline metaluminous geochemical features most have low negative whole rock nd t values where t is the crystallization age of 17 1 to 11 5 and zircon hf t values of 38 3 to 11 2 indicating that they were derived mainly from anatectic melting of the ancient lower crust with some involvement of mantle materials however an older pluton in the suite exhibits higher nd t values of 11 5 to 9 9 nd model ages of 1 82 1 64 ga lower initial 87sr 86sr ratios of 0 7046 0 7048 and it contains some zircon grains that are characterized by high negative to positive zircon hf t values of 8 7 to 1 2 indicating strong involvement of juvenile materials derived from the lithospheric mantle the late carboniferous plutons are interpreted as subduction related and to have been emplaced in an andean style continental margin arc during the southward subduction of the paleo asian oceanic plate beneath the north china craton rocks from the late permian middle triassic intrusive suite display geochemical signatures ranging from highly fractionated i type to a type they exhibit higher zircon hf t values of 14 9 to 6 7 whole rock nd t values of 10 6 to 8 8 and younger hf and nd model ages than most of the late carboniferous plutons indicating that they could have been produced by extreme fractional crystallization of hybrid magmas resulted from mixing of coeval mantle and crust derived melts they are interpreted as postcollisional postorogenic granitoids linked to lithospheric extension and asthenosphere upwelling due to slab break off and subsequent sinking after final collision and suturing of the mongolian arc terranes with the north china craton these two contrasting intrusive suites suggest that the final closure of the paleo asian ocean and collision between the mongolian arc terranes and the north china craton occurred during the late permian and these events were followed by postcollisional postorogenic extension large volume magmatism and significant continental growth no significant syncollisional crustal thickening high pressure metamorphism or s type granitoid magmatism occurred during the collision process

early permian post collisional high k granitoids from liuyuan area in southern beishan orogen nw china petrogenesis and tectonic implications abstract early permian magmatism is one of the most important tectonothermal events in the central asian orogenic belt caob early permian granitic magmatism has been recognized from southern beishan nw china and they were emplaced between 275 ma and 279 ma according to la icpms zircon u pb dating they are mainly metaluminous i type and belong to the high k calc alkaline and shoshonitic series the granitoids have calc alkalic and alkali calcic features their sri values range from 0 7028 to 0 7047 end t values from 2 5 to 1 2 with nd model ages tdm of 1 06 1 25 ga suggesting a mixed magma source of juvenile material with old continental crust furthermore some granitoids have weak heterogeneous zircon ehf t values 1 7 to 9 6 and hf model ages tdm2 0 84 1 57 ga that are also indicative of juvenile components with a small involvement of old continental crust based on geochemical and isotopic features these high k granitoids were derived from melting of heterogeneous crustal sources or through mixing of old continental crust with juvenile components and minor afc assimilation and fractional crystallization the juvenile components probably originated from underplated basaltic magmas in response to asthenospheric upwelling these early permian high k calc alkaline granitoids in the southern beishan orogen were probably emplaced in a post collisional extensional setting and suggest vertical continental crustal growth in the southern caob which is the same or similar to most granitoids in caob this study provides new evidence for determining the post accretionary evolution of the southern caob

petrogenesis of the middle triassic erenhot granitoid batholith in central inner mongolia northern china with tectonic implication for the triassic mo mineralization in the eastern central asian orogenic belt abstract voluminous post orogenic magmatism holds a key to monitoring important geodynamic processes and crustal evolution within an extensional accretionary orogen the middle triassic erenhot granitoid batholith from central inner mongolia northern china is tectonically located in the eastern central asian orogenic belt this batholith consists of monzogranite syenogranite alkali feldspar granite and albite granite with minor mafic to felsic enclaves and dykes the formation of this composite batholith involved distinct mafic and felsic magma end members concomitant episodic mafic felsic interaction and intra felsic magma evolution mafic diabasic and high mg intermediate dioritic dykes show enrichment in large ion lithophile elements depletion in high field strength elements and heterogeneous isotopic compositions indicative of a metasomatized lithospheric mantle source inherited from prior oceanic slab subduction the monzogranites and aplites have arc like elemental features and variable isotopic compositions isr t 0 706132 end t 0 4 zircon ehf t 2 8 to 7 4 zircon 18o 5 32 to 6 74 clearly suggesting that the felsic magma end member of a dual heritage was derived from dominant newly underplated mafic crust and subordinate ancient lower crust hybridization processes between mantle derived mafic magma and crust derived felsic magma result in the formation of monzonitic enclaves isr t 0 704850 end t 0 1 with an isotopic resemblance to the host granites fractional crystallization of the monzogranitic magmas endows the differentiated syenogranites and alkali feldspar granites with highly evolved elemental characteristics we further deduce that the magmatic immiscibility between k rich and na rich residual melts in a deep reservoir produces albite granites with distinct deformation imprints variable k2o contents and wide range of crystallization temperatures combining with coeval regional igneous episodes as well as concomitant sedimentary and structural events in the northern china mongolia tract the triassic erenhot composite batholith can be considered as post orogenic magmatic products possibly within a coupled geodynamic scenario between lithospheric dripping and concurrent gravitational collapse therefore the documentation of this large scale triassic post orogenic magmatism could not only lead to the characterization of the eventual closure of the paleo asian ocean but also capture a snapshot into the tectonic background for the widespread porphyry type mo mineralization in the eastern central asian orogenic belt during early mesozoic

post collisional permian volcanism in northwestern corsica an assessment based on mineralogy and trace element geochemistry abstract within the western mediterranean permo triassic province the scandola senino area represents one of the best sites for the study of the successive volcanic episodes and their relationships with this purpose new mineralogical and geochemical data on basic and intermediate rocks are presented the first volcanic episode including andesites dacites and rhyolitic ignimbrites of early permian age is calc alkaline with a strong lile enrichment particularly in k u and th andesite evolution is controlled by fractional crystallization with an important crustal component subsequent permian volcanism corresponds to the emplacement of bimodal volcanic products basalt ignimbritic rhyolite into a caldera structure the basalts are interpreted as derived from a mantle source still modified by subduction component the last volcanic episode is represented by doleritic dykes derived from a within plate or oib type source devoid of any arc signature no ta anomaly permian magmatism in corsica is interpreted as the progressive fading out of an arc signature in favour of a primitive mantle source the magmatic features and the structural setting agree with an inferred post orogenic geotectonic regime during the period between the hercynian and early alpine orogenic events

the atlas east variscan elbe shear system and its role in the formation of the pull apart late palaeozoic basins the variscan orogeny occurred as a result of the late devonian to late carboniferous collision and accretion of gondwana derived microcontinents and continental masses with those of laurussia the irregular boundaries of the colliding continents caused isochronous transpressional and transtensional tectonics accompanied by a complex pattern of intracontinental shear zones at the scale of the southern european variscides these shear zones and their configuration controlled the subsequent evolution of permian to middle triassic paleogeography the geographic distribution from morocco to the eastern alps of the late carboniferous permian up to triassic basins most of which are considered as pull apart basins was related with the development of the late palaeozoic intracontinental shear network our analysis of the stratigraphic tectonic and volcanic features of the late carboniferous permian continental basins across the laurussia gondwana boundary reveals the role of the east variscan shear zone during this time as a precursory lineament for the development of the permian to triassic rifting of pangaea and the following opening of oceanic basins e g the neothetyan ocean

zircon u pb ages and geochemistry of the wenquan mo bearing granitioids in west qinling china constraints on the geodynamic setting for the newly discovered wenquan mo deposit abstract east qinling is the largest porphyry molybdenum province in the world these mo deposits have been well documented in west qinling however few mo deposits have been discovered although granitic rocks are widespread recently the wenquan porphyry mo deposit has been discovered in gansu province which provides an insight into mo mineralization in west qinling in this paper we report pb isotope compositions for k feldspar and sulfides s isotope ratios for sulfides the results obtained from petrochemical study and from in situ la icp ms zircon u pb dating and hf isotopes the granitoids are enriched in lile and lree with ree and trace element patterns similar to continental crust suggesting a crustal origin the mg 40 05 to 56 34 and cr and ni contents are high indicating a source of refractory mafic lower crust the e hf t values of zircon grains from porphyritic monzogranite range from 2 9 to 0 6 and from granitic porphyry vary from 3 3 to 1 9 the zircons have t dm2 of 1014 to 1196 ma for the porphyritic monzogranite and 954 to 1224 ma for the granitic porphyry implying that these granitoids were likely derived from partial melting of a late mesoproterozoic juvenile lower crust the pb isotope compositions of the granitoids are similar to granites in south china showing that the magma was sourced from the middle lower crust in the southern qinling tectonic unit the pb isotopic contrast between the mo bearing granitoids and ores shows that the pb in the ore forming solution was derived from fractionation of a triassic magmatic system 34 s values of sulfides are between 5 02 and 5 66 similar to those associated with magmatic hydrothermal systems la icp ms zircon u pb dating yields crystallization ages of 216 2 1 7 and 217 2 2 0 ma for the granitoids consistent with a previously reported molybdenite re os isochron age of 214 4 7 1 ma this suggests that the mo mineralization is related to the late triassic magmatism in the west qinling orogenic belt in view of these geochemical results and known regional geology we propose that both granitoid emplacement and mo mineralization in the wenquan deposit resulted from the triassic collision between the south qinling and the south china block along the mianlue suture since triassic granitoid plutons commonly occur along the qinling orogenic belt the triassic wenquan mo bearing granitoids highlight the importance of the triassic tectono magmatic belt for mo exploration in order to apply this metallogenic model to the whole qinling orogen further study is needed to compare the wenquan deposit with other deposits

partitioning of fluorine and chlorine between biotite and granitic melt experimental calibration at 200 mpa h 2 o experiments from 640 to 680 c 200 mpa h2o at fo2 nno employing a natural f rich vitrophyric rhyolite from spor mountain utah assessed the effect of variable mg 100mg mg mn fe on the partitioning of fluorine and chlorine between biotite bt and melt over this temperature interval bt fluorite quartz is the sole liquidus phase partition coefficients for fluorine between biotite and glass dfbt melt show a strong dependence on the mg of bt dfbt melt varies from 1 5 to 7 2 over the range of mg from 21 to 76 a strong linear correlation between dfbt melt and mg has a slope of 9 4 and extrapolates through the origin i e dfbt melt 0 at mg 0 an annite siderophyllite solid solution in these experiments dclbt melt values 1 to 6 in the same experiments vary inversely with mg the al content of biotite does not vary with the aluminum saturation index asi molar al2o3 alkali and alkaline earth oxides of melt but two exchange mechanisms involving al appear to operate in these micas 1 alvi aliv siiv mgiv and mgiv 2aliv 2siiv iv the effects of other components such as li or other intensive parameters including fo2 have yet to be evaluated systematically at comparable mg of bt however the spor mountain rhyolite yields higher dfbt melt values than an li rich strongly peraluminous melt previously investigated the results indicate that the mg of bt exerts the principal control on halogen partitioning with asi and t as second order variables the experimental partition coefficients compare well with other experimental results but not with most volcanic rocks magmatic bt from most rhyolites records higher dfbt melt due to reequilibration with degassed h2o depleted magma and perhaps with f2o 1 exchange that may accompany oxidation fe3 o fe2 oh 1 this behavior is evident in magmatic biotite from a zoned peraluminous rhyolite complex near morococala bolivia bt is sharply zoned with f rich rims but bt core melt inclusion pairs fall on our experimental curve for dfbt melt these experimental data can be used in part to assess the preservation of magmatic volatile contents in plutonic or volcanic silicic rocks for plutonic rocks the actual f content of melt not a relative activity ratio involving hf species can be reasonably estimated if the mica has not undergone subsolidus reequilibration this information is potentially useful for some shallow level ca poor magmas that are thought to be rich in f e g a and s type granites but do not conserve f well as rocks

crust and mantle contributions to granite genesis an example from the variscan batholith of corsica france studied by trace element and ndsro isotope systematics the corsican batholith formed during the variscan orogeny was studied with particular emphasis on the respective roles played by crustal and mantle derived material in granite genesis major and trace element data identify the various types of magma and their genetic relationships the magmatic processes that gave rise to the observed rocks and the primitive liquids o sr nd isotope systematics constrain the origin of primitive liquids trace element models provide data on the various source materials and their possible geodynamic setting the evolution of two main calc alkaline plutonic associations is compared an older high mg k association u1 with both ultrapotassic mafic and silicic rocks the latter ranging from monzonite to leucosyenogranite and a younger calc alkaline composite association u2 involving a mafic cumulate sequence and granodiorite to leucomonzo granite both types of granite are accompanied by mafic units that were non cogenetic with surrounding granitic melts trace element calculations indicate that a primitive liquid of monzodioritic composition gave rise to the u1 mg k granite by fractional crystallization amphibole and titanite playing a major role which with increasing crystallization gave decreasing ree contents without a strong negative eu anomaly the u2 calc alkaline suite was formed by fractional crystallization involving feldspar and lree rich minerals i e monazite from a liquid of monzogranitic composition field and petrographical data identify magma mingling in both u1 and u2 between mafic and silicic rocks but geochemical data only indicate how magma mixing processes led to u2 granodiorite geochemical modelling shows that a single protolith of calculated graywacke composition yielded the two associations under different melting conditions the high mg k monzodiorite melt was formed after partial melting of a 30 non modal batch of granulite facies metamorphic protolith low ph2o but the calc alkaline monzogranite was formed by the same process in an amphibolite facies source of similar composition higher ph2o the primitive granite magmas of both associations show the same crustal characteristics i e sri 0 706 0 707 nd t 4 3 to 2 2 and 18o 7 to 8 this provides evidence that composition of both u1 and u2 granite melts probably was controlled by physico chemical fusion rather than protolith composition parameters the u1 ultrapotassic mafic rock is thought to be of mantle origin i e a deep source containing phlogopite garnet zone refining led to a significant increase of incompatible trace element contents during ascent of the magma mantle crust interaction lowered the la yb and 143nd144nd ratios but the 87sr86sr ratio increased nevertheless interaction with associated u1 mg k granite was not important and the crustal component is thought to be different from the associated granitic magma in u2 the mafic cumulate sequence clearly of mantle origin has e morb characteristics and seems the result of 10 non modal partial melting of a heterogeneous mantle of spinel or amphibole peridotite without garnet some interaction occurred with the associated granitic melt the e morb character of the mafic rock indicates that a large part of the batholith and at least the u2 magmatic association was generated under post collisional extensional conditions

seismic expression of shear zones insights from 2 d point spread function based convolution modelling abstract shear zones are common strain localization structures in the middle and lower crust and play a major role during orogeny transcurrent movements and rifting alike our understanding of crustal deformation depends on our ability to recognize and map shear zones in the subsurface yet the exact signatures of shear zones in seismic reflection data are not well constrained to advance our understanding we simulate how three outcrop examples of shear zones holsnoy norway cap de creus spain borborema brazil would look in different types of seismic reflection data using a 2 d psf based convolution modelling psf point spread functions being the elementary response of diffraction points in seismic imaging we explore how geological properties e g shear zone size and dip and imaging effects e g frequency resolution illumination control the seismic signatures of shear zones our models show three consistent seismic characteristics of shear zones 1 multiple inclined reflections 2 converging reflections and 3 cross cutting reflections that can help interpreters recognize these structures with confidence

some geodynamic and compositional constraints on postorogenic magmatism postorogenic magmatic suites are common to many orogens in many cases apparently just postdating the cessation of deformation they differ from preceding orogenic suites in that they have higher temperatures more primitive isotopic signatures and bimodal natures and thus are compositionally similar to suites found in extensional regimes we propose that thinning of the lithospheric mantle which may be an automatic response to orogenic lithospheric thickening is responsible for these magmatic suites mantle lithospheric thinning moves the asthenospheric lithospheric thermal boundary higher in the lithospheric column thereby raising the overall thermal budget of the orogen and the likelihood of basaltic magmatism this removal of much of the dense lithospheric mantle root of the orogen also invokes uplift capable of producing horizontal orogenic buoyancy forces that will oppose and potentially terminate deformation extreme fractionation of the magmas is promoted by high temperatures and a noncompressional or tensional lithospheric stress regime to produce accompanying felsic a type magmas this model explains why the cessation of deformation is coincident with high temperature bimodal magmatism and inferred uplift as well as the long noted similarity and confusion between postorogenic and extensional magmatic suites this arises because both reflect thinning of the lithospheric mantle not because both reflect extensional tectonics we suggest that these magmatic episodes may be important in transferring lithospheric mantle material and its compositional signatures into the crust

late orogenic magmatism and sedimentation within late carboniferous to early permian basins in the balkan terrane bulgaria geodynamic implications the orogenic balkanid belt which developed between the moesian plate and the moravian rhodopi thracian massifs was affected by the late carboniferous and early permian opening of w e oriented graben structures the progressive tectonic rejuvenation of the basins is demonstrated by the deposition of repeated re gional sedimentary cycles associated with volcanism that was mostly localised along the tectonic boundaries in an intramontane setting the late carboniferous volcanism is represented by rhyodacitic explosive products and hyaloclastites and by andesitic flows during the early permian subvolcanic rhyodacitic and rhyolitic bodies and the explosive prod ucts prevailed in the western sectors whereas rhyolitic ignimbrites occur to the east the tectonically active basins are interpreted due to late orogenic collapse and the alternation of extensional tectonics and minor compressional phases is consistent with the regional transtensional regime active along the variscan suture of pangaea the volcanic activity asso ciated with the evolution of the basins matches the pet rogenetic features and the evolution from early dacitic andesitic to late rhyolitic activity in the southern euro pean segment of the variscan system these late carboniferous early permian sedimentary and tectono magmatic events in bulgaria are character ized and compared with the homologous permo car boniferous sequences along some western european seg ments of the variscan belt

petrogenesis of syntectonic granites emplaced at the transition from thrusting to transcurrent tectonics in post collisional setting whole rock and sr nd pb isotope geochemistry in the neoproterozoic quatro ilhas and mariscal granites southern brazil abstract the neoproterozoic post collisional period in southern brazil 650 580 ma is characterized by substantial volumes of magma emplaced along the active shear zones that compose the southern brazilian shear belt the early phase syntectonic magmatism 630 610 ma is represented by the porphyritic high k metaluminous to peraluminous quatro ilhas granitoids and the younger heterogranular slightly peraluminous mariscal granite quatro ilhas granitoids include three main petrographic varieties muscovite biotite granodiorite mbg biotite monzogranite bmz and leucogranite lcg that although sharing some significant geochemical characteristics are not strictly comagmatic as shown by chemical and sr nd pb isotope data the most primitive muscovite biotite granodiorite was produced by contamination of more mafic melts possibly with some mantle component with peraluminous crustal melts the biotite monzogranite although more felsic has higher ca mgo tio 2 and ba and lower k 2 o feo t sr and rb contents possibly reflecting some mixing with coeval mafic magmas of tholeiitic affinity the leucogranite may be derived from pure crustal melts the mariscal granite is formed by two main granite types which occur intimately associated in the same pluton one with higher k 5 6 5 wt k 2 o high rb and lower cao na 2 o ba and zr as compared to the other 3 5 wt of k 2 o the two mariscal granite varieties have compositional correspondence with fine grained granites fgg that occur as tabular bodies which intruded the quatro ilhas granoitoids before they were fully crystallized and are inferred to correspond to the mariscal granite feeders an interpretation that is reinforced by similar u pb zircon crystallization ages the initial evolution of the post collisional magmatism marked by the emplacement of the quatro ilhas granitoids varieties activated sources that produced mantle and crustal magmas whose emplacement was controlled both by flat lying and transcurrent structures the transition from thrust to transcurrent related tectonics coincides with the increase in the proportion of crustal derived melts the transcurrent tectonics seems to have played an essential role in the generation of mantle derived magmas and may have facilitated their interaction with crustal melts which seem to be to a large extent the products of reworking of orthogneiss protoliths

lower carboniferous post orogenic granites in central eastern sierra de velasco sierras pampeanas argentina u pb monazite geochronology geochemistry and sr nd isotopes the central eastern part of the sierra de velasco sierras pampeanas nw argentina is formed by the large huaco 40 30 km and sanagasta 25 15 km granite massifs and the small la chinchilla stock 2 2 km the larger granites intrude into ordovician metagranitoids and crosscut devonian mylonitic shear zones whereas the small stock sharply intrudes into the huaco granite the two voluminous granites are biotitic muscovitic and biotitic porphyritic syeno to monzogranites they contain small and rounded tonalitic and quartz dioritic mafic microgranular enclaves the small stock is an equigranular zinnwaldite and fluorite bearing monzogranite the studied granites are silica rich sio2 70 potassium rich k2o 4 ferroan alkali calcic to slightly calk alkalic and moderately to weakly peraluminous a cnk 1 06 1 18 huaco granite 1 01 1 09 sanagasta granite 1 05 1 06 la chinchilla stock they have moderate to strong enrichments in several lil li rb cs and hfs nb ta y th u elements and low sr ba and eu contents u pb monazite age determinations indicate lower carboniferous crystallization ages 350 358 ma for the huaco granite 352 7 1 4 ma for the sanagasta granite and 344 5 1 4 ma for the la chinchilla stock the larger granites have similar nd values between 2 1 and 4 3 whereas the younger stock has higher nd of 0 6 to 1 4 roughly comparable to the values obtained for the carboniferous san blas granite 1 4 to 1 7 located in the north of the sierra the huaco and sanagasta granites have a mainly crustal source but with some participation of a more primitive possibly mantle derived component the main crustal component can be attributed to ordovician peraluminous metagranitoids the la chinchilla stock derives from a more primitive source suggesting an increase with time in the participation of the primitive component during magma genesis the studied granites were generated during a post orogenic period in a within plate setting possibly as a response to the collapse of the previous famatinian orogen extension of the crust and mantle upwelling they are part of the group of middle devonian lower carboniferous granites of the sierras pampeanas the distribution and u pb ages of these granites suggests a northward arc parallel migration of this mainly post orogenic magmatism with time

geochronological study of zircons from continental crust rocks in the frido unit southern apennines zircon crystals have been separated from gneisses and metagranitoids of the pollino area southern apennines in order to unravel the origin of these crustal slices within the ophiolite bearing frido unit the morphology of the zircon has been investigated by sem and the internal structure was revealed by cathodoluminescence data obtained by u pb dating have been used to deduce the age and significance of the different crystallization stages of zircon connected to the evolutionary stages of the continental crust late paleozoic early mesozoic zircons in gneisses are characterized by inherited cores of magmatic origin bordered by metamorphic rims inherited zircons generally show paleoproterozoic to ordovician ages indicating the provenance of the sedimentary protolith from different sources the exclusive presence of late neoproterozoic zircon cores in leucocratic gneisses may suggest a different magmatic source possibly connected to pan african events late carboniferous early permian ages are found mainly in zircon rims of metamorphic origin these are similar to the emplacement ages of protolith of the metagranites in the middle crust portion late carboniferous early permian metamorphism and magmatism testify the extensional collapse of the hercynian belt recorded in european particularly in the corsica sardinia block and in calabria late permian triassic ages have been detected in zircon rims from gneisses and metagranitoids these younger ages appear related to deformation and emplacement of albite quartz veins in both lithologies and are related to an extensional episode predating the middle triassic to middle jurassic rifting in the tethyan domain followed by middle to late jurassic spreading

arc magmatism in the delhi fold belt shrimp u pb zircon ages of granitoids and implications for neoproterozoic convergent margin tectonics in nw india abstract the northwestern region of peninsular india preserves important records of precambrian plate tectonics and the role of indian continent within proterozoic supercontinents in this study we report precise shrimp zircon u pb ages from granitoids from the sirohi terrane located along the western fringe of the delhi fold belt in rajasthan nw india the data reveal a range of neoproterozoic ages from plagiogranite of peshua foliated granite of devala and porphyritic granite of sai with zircon crystallization from magmas at 1015 4 4 ma 966 5 3 5 and 808 3 1 respectively the plagiogranite shows high sio2 na2o and extremely low k2o rb ba comparable with typical oceanic plagiogranites these rocks possess low lree and hree concentrations and a relatively flat lree hree slope a well developed negative eu anomaly and conspicuous nb and ti anomalies compared to the plagiogranite the foliated devala granite shows higher sio2 and moderate na2o together with high k2o and comparatively higher rb ba sr and ree with steep ree profiles and a weak positive eu anomaly in contrast to the plagiogranite and foliated granite the porphrytic sai granite has comparatively lower sio2 moderately higher na2o extremely high y zr nb and elevated ree the geochemical features of the granitoids hfse depletion and lile enrichment nb and ta negative anomalies and their plots in the fields of volcanic arc granites and those from active continental margins in tectonic discrimination diagrams suggest widespread neoproterozoic arc magmatism with changing magma chemistry in a protracted subduction realm our results offer important insights into a long lived active continental margin in nw india during early and mid neoproterozoic consistent with recent similar observations on cryogenian magmatic arcs widely distributed along the margins of the east african orogen and challenge some of the alternate models which link the magmatism to extensional tectonics associated with rodinia supercontinent breakup

proto tethys magmatic evolution along northern gondwana insights from late silurian middle devonian a type magmatism east kunlun orogen northern tibetan plateau china abstract the east kunlun orogen records the geological evolutions of the neoproterozoic early paleozoic proto tethyan ocean and late paleozoic mesozoic paleo tethys ocean along northern gondwana however the late stage evolution of the proto tethyan ocean and the configuration of peri gondwana microcontinents during the silurian devonian is under debate here we report new geochronological and geochemical data of a type granites from the western wulonggou and the eastern gouli areas in the east kunlun orogen to deepen our understanding of these problems zircon la icp ms u pb data reveal that the danshuigou monzogranite and shenshuitan syenogranite from the western wulonggou area were emplaced simultaneously at 418 3 ma while the niantang syenogranite from the eastern gouli area was emplaced at 403 2 ma all these rocks display high k calcic alkalic to shoshonitic and metaluminous to slight peraluminous signatures with relatively low cao al2o3 mgo and sr and high feot mgo ga al zr and nb indicating their a type affinity their moderate whole rock end t 5 3 to 0 6 and zircon ehf t 6 3 6 4 are different from those of depleted mantle and old basement rocks but similar to those of the ordovician silurian granitoids in the east kunlun orogen these chemical signatures together with the anhydrous low pressure and high temperature characteristics of the magmas indicate that partial melting of the ordovician silurian granitoids generated these a type granites regionally these a type granites and previously reported a type granites in the east kunlun orogen compose a late silurian middle devonian a type granite belt this belt together with the regionally coeval molasse formation and mafic ultramafic rocks indicate a post collisional extensional regime for the east kunlun orogen during the late silurian middle devonian given that extensive contemporaneous post collision related magmatic rocks have also been revealed in the neighboring west kunlun altyn qilian and qinling blocks terranes we contend that the neoproterozoic early paleozoic proto tethyan ocean that separated these blocks terranes from gondwana had closed by the late silurian middle devonian which resulted in the re welding of the above blocks terranes to northern gondwana or gondwana derived microcontinents

geochronology and geochemistry of late pan african intrusive rocks in the jiamusi khanka block ne china petrogenesis and geodynamic implications abstract to constrain the early paleozoic tectonic evolution of the jiamusi khanka block and its relationship to the late pan african event in gondwana we undertook zircon u pb dating and geochemical analyses major and trace elements and hf isotopic compositions of early paleozoic intrusive rocks in the jiamusi khanka block ne china la icp ms zircon u pb age data demonstrate that these intrusive rocks were emplaced at three stages during the late pan african event represented by 540 ma syenogranite 515 ma quartz syenite and 500 ma monzogranite and gabbro geochemically the 500 ma gabbros in the jiamusi khanka block have low sio 2 50 26 51 21 wt relatively high mgo 4 08 5 67 wt ni 13 1 14 1 ppm and cr 28 4 56 0 ppm and are slightly enriched in liles e g ba k and lrees and depleted in zr hf nb ta and p the e hf t values of zircons in the gabbro range from 2 6 to 6 4 all these geochemical features indicate that the gabbros were likely produced by the partial melting of a depleted mantle that had been metasomatized by fluids derived from a subducted slab in contrast the ca 540 500 ma granites and quartz syenites contain high sio 2 64 49 72 20 wt and low mgo 0 40 0 75 wt cr 1 69 6 88 ppm and ni 1 26 3 26 ppm they have relatively low 176 hf 177 hf ratios of 0 282247 0 282599 with hf two stage model ages of 1173 2280 ma and most of the magmatic zircons have positive e hf t values varying from 0 2 4 8 indicating that these granites and quartz syenites were probably derived from a dominantly paleo mesoproterozoic old crustal source with possible different degrees of addition of juvenile materials according to the geochemical data and global geological investigations we propose that the 541 498 ma intrusive rocks in the jiamusi khanka block formed in a post collisional or post orogenic extensional setting linked to the collapse of a late pan african orogen associated within the gondwana

geology and ore fluid geochemistry of the jinduicheng porphyry molybdenum deposit east qinling china abstract jinduicheng deposit is a giant mesozoic porphyry mo system deposit in the east qinling molybdenum belt shaanxi province china the mineralization is associated with the i type jinduicheng granite porphyry both the porphyry stock and country rocks underwent intense hydrothermal alteration the alteration with increasing distance from the parent intrusion changes from silicification through potassic and phyllic assemblages carbonation to propylitic assemblages molybdenite the dominant ore mineral occurs in veinlets most of which are hosted by the altered country rocks with less than 25 of the ore in the porphyry body the hydrothermal system comprises four stages including pre ore quartz and k feldspar two ore stages of quartz k feldspar molybdenite and pb and zn bearing sulfides and post ore quartz and carbonate six main types of primary fluid inclusions are present in hydrothermal quartz including two phase aqueous one phase aqueous three phase co 2 bearing co 2 dominated fluid inclusions gas inclusions and melt inclusions the homogenization temperatures of fluid inclusions range from 210 to 290 c in the pre ore stage 150 310 c in ore stage i 150 360 c in the ore stage ii and 195 325 c in the post ore stage quartz estimated salinities of the ore forming fluids range from 6 9 to 13 5 4 3 to 12 3 6 2 to 12 4 and 3 4 to 9 9 wt nacl equiv in stages 1 4 respectively the 34 s values of pyrite in the two ore stages range from 2 8 to 4 3 whereas the 34 s values of molybdenite range from 2 9 to 6 2 the data suggest both magmatic and crustal sources of sulfur the d and 18 o values for the hydrothermal fluids are 57 2 to 84 4 and 8 0 to 3 2 respectively the fluid inclusion and stable data indicate that the pre ore hydrothermal fluids were mostly of magmatic origin but the fluids responsible for ore deposition were mixed magmatic and meteoric and eventually meteoric water dominated the system in the post ore stage

petrogenesis of jurassic fractionated i type granites in southeast china constraints from whole rock geochemical and zircon u pb and hf o isotopes abstract whole rock geochemical and sr nd hf isotopic data and zircon in situ u pb ages and hf o isotopes are reported for twelve granitic intrusions from inland to coastal areas of the cathaysia block southeast se china in order to constrain their magma sources and petrogenesis they are mainly composed of biotite granites with minor granodiorites zircon u pb dating by la icpms and sims methods gave emplacement ages of 165 154 ma for these rocks which are metaluminous to weakly peraluminous a cnk 0 95 1 10 with variable sio 2 contents and high k 2 o contents they are all depleted in high field strength elements hfses and enriched in light rare earth elements lrees and large ion lithophile elements liles and show negative sr ba and eu anomalies in the spidergrams petrographic and geochemical features suggest the granodiorites that contain hornblende to be typical of i type while the biotite granites without hornblende are fractionated i type granites their variable major and trace element concentrations are due to fractional crystallization of feldspar biotite hornblende and accessory minerals including apatite allanite and monazite moreover our data indicate significant variations in isotopic compositions of these granitoids from inland to coastal areas the inland granites from the lapu fogang xinfengjiang and baishigang plutons have relatively high initial 87 sr 86 sr ratios 0 7105 0 7155 and low e nd t 11 2 to 7 3 and e hf t 10 9 to 6 3 values for whole rocks and variable 18 o values of 7 69 9 25 and e hf t values of 13 5 to 6 7 for zircons in contrast the coastal granites from the longwo wushikeng lianhuashan shigushan chiliao hulutian mantoushan and fenghuang plutons show relatively low initial 87 sr 86 sr ratios 0 7056 0 7110 and high e nd t 7 6 to 6 2 and e hf t 6 5 to 2 8 values for whole rocks together with e hf t values of 7 2 to 3 9 and 18 o values of 6 69 8 38 for zircons the spatial isotopic variations suggest that these jurassic granitoids were generated by different percentage mixing of mantle and ancient paleoproterozoic crust derived magmas with subsequently minor crustal assimilation our results highlight that jurassic fractionated i type granites occur widely from inland to coastal areas of se china arguing against the previous view that jurassic granites occur only in inland while coastal granites are mainly cretaceous the widespread occurrence of jurassic granitoids in se china is attributed to ascending of mantle derived magmas which provide heat for partial melting of lower crustal materials in an extensional setting

late palaeozoic deformation of post variscan lower crust shear zone widening due to strain localization during retrograde shearing the santa lucia shear zone slsz corsica is a granulite facies permian shear zone that developed after the emplacement of a deep seated gabbroic intrusion new structural data shows that the slsz results from the juxtaposition of three spatially distinct mylonite belts which are the product of the interaction between magmatism metamorphism and shearing over a temperature range from 800 to 400 c during the earlier high grade deformation stage which was accompanied by decompression from 7 to 5 kb at 800 c the slsz has accommodated high finite strain on a shear zone 1 km wide strain became increasingly localized as temperature decreased but rather than reactivating pre existing shear zones as commonly expected younger mylonites expanded into previously unsheared rock extending the total width of the shear zone the zonation of different fabrics across the slsz suggests that pre existing compositional and grain size heterogeneities in the starting material played a key role in governing superposed generations of shear zones

petrogenesis of late triassic intrusive rocks in the northern liaodong peninsula related to decratonization of the north china craton zircon u pb age and hf o isotope evidence abstract major and trace element whole rock sr nd and hf isotopes and zircon u pb age and hf o isotope data have been determined for mafic to felsic intrusive rocks from the late triassic mayihe myh longtou chaxinzi xiaoweishahe lcx and nankouqian xidadingzi nx intrusions in the northern liaodong peninsula ne china in order to investigate their sources and petrogenesis related to decrationization of the north china craton the rocks include clinopyroxene diorite monzodiorite quartz diorite granodiorite monzogranite and mafic microgranular enclaves laser ablation inductively coupled plasma mass spectrometry la icpms and secondary ion mass spectrometer sims u pb analyses of zircon from myh lxc to nx intrusions yield late triassic ages of 224 218 ma establishing that the mafic and felsic magmas were coeval the clinopyroxene diorites from the myh pluton have high mgo concentrations at low silica contents with positive whole rock e nd t up to 3 8 and e hf t up to 6 2 and zircon e hf t values and some mantle like 18 o values they are enriched in large ion lithophile liles and light rare earth elements lrees and depleted in high field strength elements hfses which can be explained as melts derived from a depleted mantle with some subsequent crustal contamination however the diorites from the lxc and nx intrusions are high k rocks and have high mgo concentrations and mantle like 18 o values but with negative whole rock e nd t 12 9 to 7 6 and e hf t 14 0 to 5 6 and zircon e hf t values distinct from those of myh pyroxene diorites indicating that their parental magmas were derived from partial melting of an ancient lithospheric mantle the granitoids from three plutons have high sio 2 contents and low mgo concentrations suggesting they were mainly derived from crustal sources however the myh monzogranite and granodiorite have positive whole rock e nd t and e hf t up to 3 0 and zircon e hf t values indicating a juvenile crustal source whereas the lxc and nx granitoids have strong negative and variable whole rock e nd t and e hf t and zircon e hf t values indicating that they were derived from partial melting of ancient lower crustal materials with involvement of mantle components field observations geochronology geochemistry sr nd hf isotopic and zircon hf o isotopic compositions point to a complex petrogenesis where mantle and crust derived magma mixing was coupled with crystal fractionation thus explaining the genetic link between mafic and felsic rocks identification of four components i e depleted and enriched mantle and juvenile and ancient crust in the late triassic magmatism in the northern liaodong peninsula suggests a strong mantle crust interaction process related to decratonization of the north china craton

mantle hotspots plumes and regional tectonics as causes of intraplate magmatism in continental areas it is often difficult to determine the cause of intraplate magmatism large volumes of magma high eruption rates and the presence of a hotspot trace on the adjacent ocean floor are all evidence for the presence of an anomalously hot mantle however in many continental magmas there are chemical variations with time which are inferred to reflect changes from asthenospheric to predominantly lithospheric source regions or vice versa it is argued that these chemical characteristics constrain whether magmatism was triggered by the emplacement of a mantle plume or by lithospheric extension

on the occurrence trace element geochemistry and crystallization history of zircon from in situ ocean lithosphere we characterize the textural and geochemical features of ocean crustal zircon recovered from plagiogranite evolved gabbro and metamorphosed ultramafic host rocks collected along present day slow and ultraslow spreading mid ocean ridges mors the geochemistry of 267 zircon grains was measured by sensitive high resolution ion microprobe reverse geometry at the usgs stanford ion microprobe facility three types of zircon are recognized based on texture and geochemistry most ocean crustal zircons resemble young magmatic zircon from other crustal settings occurring as pristine colorless euhedral type 1 or subhedral to anhedral type 2 grains in these grains hf and most trace elements vary systematically with ti typically becoming enriched with falling ti in zircon temperature ti in zircon temperatures range from 1 040 to 660 c corrected for a tio2 0 7 a sio2 1 0 pressure 2 kbar intra sample variation is typically 60 150 c decreasing ti correlates with enrichment in hf to 2 wt while additional hf enrichment occurs at relatively constant temperature trends between ti and u y ree and eu eu exhibit a similar inflection which may denote the onset of eutectic crystallization the inflection is well defined by zircons from plagiogranite and implies solidus temperatures of 680 740 c a third type of zircon is defined as being porous and colored with chaotic cl zoning and occurs in 25 of rock samples studied these features along with high measured la cl s ca and fe and low sm la n ratios are suggestive of interaction with aqueous fluids non porous luminescent cl overgrowth rims on porous grains record uniform temperatures averaging 615 26 c 2sd n 7 implying zircon formation below the wet granite solidus and under water saturated conditions zircon geochemistry reflects in part source region elevated hree coupled with low u concentrations allow effective discrimination of 80 of zircon formed at modern mors from zircon in continental crust the geochemistry and textural observations reported here serve as an important database for comparison with detrital xenocrystic and metamorphosed mafic rock hosted zircon populations to evaluate provenance

alkali calcic and alkaline post orogenic po granite magmatism petrologic constraints and geodynamic settings abstract the end of an orogenic wilson cycle corresponds to amalgamation of terranes into a pangaea and is marked by widespread magmatism dominated by granitoids the post collision event starts with magmatic processes still influenced by subducted crustal materials the dominantly calc alkaline suites show a shift from normal to high k to very high k associations source regions are composed of depleted and later enriched orogenic subcontinental lithospheric mantle affected by dehydration melting and generating more and more k and lile rich magmas in the vicinity of intra crustal magma chambers anatexis by incongruent melting of hydrous minerals may generate peraluminous granitoids bearing mafic enclaves the post collision event ends with emplacement of bimodal post orogenic po suites along transcurrent fault zones two suites are defined i the alkali calcic monzonite monzogranite syenogranite alkali feldspar granite association characterised by biotite plagioclase fractionation and moderate lile hfse enrichments and ii the alkaline monzonite syenite alkali feldspar granite association characterised by amphibole alkali feldspar fractionation and displaying two evolutionary trends one peralkaline with sodic mafic mineralogy and higher enrichments in hfse than in lile and the other aluminous biotite bearing marked by hfse depletion relative to lile due to accessory mineral precipitation alkali calcic and alkaline suites differ essentially in the amounts of water present within intra crustal magma chambers promoting crystallisation of various mineral assemblages the ultimate enriched and not depleted mantle source is identical for the two po suites the more primitive lile and hfse rich source rapidly replaces the older orogenic mantle source during lithosphere delamination and becomes progressively the thermal boundary layer of the new lithosphere present rock compositions are a mixture of major mantle contribution and various crustal components carried by f rich aqueous fluids circulating within convective cells created around magma chambers in favourable areas po suites pre date a new orogenic wilson cycle

u pb zircon geochronology of mesoproterozoic postorogenic rocks and implications for post ottawan magmatism and metallogenesis new jersey highlands and contiguous areas usa abstract postorogenic rocks are widespread in grenville terranes of the north central appalachians where they form small discordant largely pegmatitic felsic intrusive bodies veins and dikes and also metasomatic calcic skarns that are unfoliated and postdate the regional 1090 to 1030 ma upper amphibolite to granulite facies metamorphism related to the grenville ottawan orogeny zircons from magmatic and nonmagmatic rocks from northern new jersey and southern new york were dated to provide information on the regional tectonomagmatic and metallogenic history following ottawan orogenesis we obtained u th pb zircon ages of 1004 3 ma for pegmatite associated with the 1020 4 ma mount eve granite near big island new york 986 4 ma for unfoliated discordant pegmatite that intrudes supracrustal marble at the buckwheat open cut franklin new jersey 990 ma for a silicate borate skarn layer in the franklin marble at rudeville new jersey and 940 2 ma for a calc silicate skarn layer at lower twin lake new york this new data together with previously published ages of 1020 4 to 965 10 ma for postorogenic rocks from new jersey and southern new york provide evidence of magmatic activity that lasted for up to 60 ma past the peak of high grade metamorphism postorogenic magmatism was almost exclusively felsic and involved relatively small volumes of metaluminous to mildly peraluminous melt that fractionated from an a type granite parent source field relationships suggest the melts were emplaced along lithosphere scale fault zones in the highlands that were undergoing extension and that emplacement followed orogenic collapse by least 30 ma postorogenic felsic intrusions correspond to the niobium yttrium fluorine nyf class of pegmatites of cern 1992a geochronologic data provide a temporal constraint on late stage hydrothermal activity and a metallogenic event in new jersey at 990 to 940 ma that mineralized pegmatites with subeconomic to economic deposits of magnetite u th rare earth element ree and formed metasomatic calcic skarn bodies in marble and reactive carbonate rocks mineralization associated with this event overlaps the timing of pegmatite emplacement suggesting a petrogenetic relationship coeval metallogeny at 975 to 950 ma in the new york hudson highlands and 980 to 937 ma in the canadian grenville province implies that this event was widespread following the ottawan phase of the grenville orogeny

origin and evolution of overlapping calc alkaline and alkaline magmas the late palaeozoic post collisional igneous province of transbaikalia russia abstract the late palaeozoic voluminous magmatism in transbaikalia russia a territory of 600 000 km 2 to the east of lake baikal is highly diverse and complex of special interest are 1 the significant overlap in time between magmatic suites commonly ascribed to post collisional and within plate settings and 2 the provenance of the coeval but distinct granitoid magmas that are closely spaced within a large region magmatic activity lasted almost continuously from 330 ma to 275 ma and included five igneous suites occupying a total area of 200 000 km 2 1 the barguzin suite of high k calc alkaline granite 330 310 ma 2 and 3 the coeval chivyrkui suite of low silica calc alkaline granitoids and the zaza suite of high k calc alkaline to alkaline granite and quartz syenite which were emplaced between 305 and 285 ma and 4 and 5 the partially overlapped in time lower selenga monzonite syenite suite 285 278 ma and the early kunalei suite of alkali feldspar and peralkaline quartz syenite and granite 281 275 ma the overall increase in alkalinity of the granitoids with time reflects the progress from post collisional to within plate settings however a 20 m y long transitional period during which both calc alkaline and alkaline granitoids were emplaced indicates the coexistence of thickened batholiths and thinned rift crustal tracts sr nd o isotope and elemental geochemical data suggest that the relative contribution of mantle derived components to the generation of silicic magmas progressively increased with time the high k calc alkaline granite magmas that formed the angara vitim batholith were generated by high degree melting of supracrustal metamorphic rocks end t 5 7 to 7 7 18 o qtz 12 with minor contribution of h 2 o and k from the underplated mafic magma the convective diffusion model the coeval calc alkaline chivyrkui suite and the transitional to alkaline zaza suite formed as a result of mixing of crustal silicic and mantle derived basic melts in roughly equal proportions in so doing the former crystallized immediately from the hybrid magmas whereas the latter zaza suite formed by fractional crystallization of the hybrid melts following magma mixing finally the partly overlapping in time monzonite syenite lower selenga suite and highly alkaline syenite granite early kunalei suite were produced from the similar k rich basalt source for the former suite the magma generation process was dominated by fractional crystallization of k rich basalt magma in contrast the highly alkaline granitoid magmas were likely generated by partial melting 20 of k rich mafic rocks in the lower crust

the mount gharib a type granite nubian shield petrogenesis and role of metasomatism at the source the mount gharib peralkaline a type complex 476 2 ma located in the nubian shield of egypt contains sodic calcic to sodic amphiboles accessory astrophyllite zircon fluorite apatite allanite aenigmatite elpidite and ilmenite this within plate hypersolvus suite is enriched in large ion lithophile lil and high field strength hfs elements and characterized by a fractionated ree pattern ce yb 49 and a significant negative eu anomaly a fine grained acicular amphibole bearing roof facies shows further enrichment in the lil and hfs elements the suite was emplaced in a pan african granodiorite adamellite host which it locally metasomatized the affected rocks contain hydrothermal albite end member arfvedsonite astrophyllite and levels of the lil and hfs elements intermediate between those in the peralkaline granite and the roof facies trace element and isotopic modeling of this a type granite with its high initial 87sr 86sr value 0 7110 documents an active role of the lithosphere in magma generation lithospheric extension expressed by regional dyke swarms was caused by cooling fracturing and relaxation of the thin newly formed pan african crust localized partial melting took place in an open system possibly as a result of an influx of alkali rich fluid derived from a sublithospheric source metasomatic reactions similar to those observed in the metasomatized wallrocks are considered to have played an important role just prior to the onset of anatexis and generation of the a type melt

a type granites in northeastern china age and geochemical constraints on their petrogenesis abstract a type granites are widely distributed in northeastern china ne china they were emplaced during three major episodes the permian late triassic to early jurassic and early cretaceous and evolved in different tectonic regimes according to their mineralogical and geochemical characteristics two subgroups of a type granites aluminous and peralkaline can be recognized the peralkaline subgroup contains alkali mafic minerals such as riebeckite arfvedsonite and sodic pyroxene while the aluminous subgroup contains annite and fe rich calcic or sodic calcic amphibole with respect to the aluminous subgroup the peralkaline granites contain higher rb ga and total rare earth elements ree but lower mgo cao al 2 o 3 ba and sr based on the discrimination criteria of eby geology 20 1992 641 the permian and late triassic to early jurassic a type granites belong to the a 2 post orogenic type whereas the early cretaceous granites are of the a 1 anorogenic type nd isotopic compositions of these a type granites indicate their derivation from a dominantly juvenile crustal source their origin is thought to have involved partial melting of an underplated lower crustal source because the generation of a type granites requires high melting temperature we propose models involving slab break off lithospheric delamination and extension the permian a type granites have the same age range as those in eastern junggar southern mongolia and central inner mongolia they occur along a major suture and form a narrow belt between the north china and siberian cratons we suggest that their formation was associated with post collisional slab break off the late triassic to early jurassic a type granites are likely to be the product of lithospheric delamination after the final collision of the major crustal blocks in the late paleozoic to early triassic the early cretaceous a type granites have an anorogenic affinity and were possibly associated with rifting in eastern china at this time associated with the onset of paleo pacific subduction consequently we conclude that the a type granites in ne china were generated at three different times involving multiple processes operative in different tectonic environments

allanite and chevkinite in a type granites and syenites of the graciosa province southern brazil abstract allanite and chevkinite perrierite are important accessory phases in the granitic and syenitic a type rocks of the graciosa province southern brazil allanite ce is the main ree phase in the granites of the aluminous association which includes metaluminous to weakly peraluminous granites while chevkinite ce is the typical ree phase in the metaluminous syenites and alkali feldspar syenites and the peralkaline alkali feldspar granites of the alkaline association allanite shows complex zoning patterns but is always rich in the ferriallanite molecule and in tio 2 a compositional trait that distinguishes allanite in a type aluminous associations from allanite in calcic alkali calcic and calc alkaline granitoids normalized ree patterns show strong fractionation of heavy over light ree with la n nd n ratios between 3 and 5 and la n y n ratios between 200 and 400 primary allanite is variably affected by post magmatic transformations due to the interaction with a fluid phase and metamictization caused by radioactive decay chevkinite ce is always primary and is only present in the alkaline association the early crystallization of chevkinite indicates that the parental magmas were probably close to ree saturation the ree y abundance in chevkinite 43 49 wt is almost twice that observed in primary allanite yet the normalized patterns are similar in terms of fractionation between light and heavy ree as indicated by similar la n nd n ratios in chevkinite and allanite la n y n ratios are on average lower in chevkinite 90 300 chevkinite compositions were altered by interaction with post magmatic fluids which led to marked increase in tio 2 and a reduction in ree and fe t post magmatic allanite is present in both the alkaline and aluminous associations and corresponds to ferriallanite ce in the alkaline association and allanite y in the aluminous association compared to primary allanite post magmatic allanite ce shows smaller ree fractionation 2 n nd n 5 times chondrite the mineralogy of the rocks indicates crystallization conditions close to the titanite magnetite quartz amphibole ilmenite tmqai buffer for the aluminous association and close to the fayalite quartz magnetite fqm buffer for the alkaline association and temperatures between 800 and 900 c liquidus and ca 750 c solidus the experimentally determined stability fields for chevkinite and allanite are in agreement with the textural evidence for early chevkinite crystallization and late allanite crystallization even though allanite and chevkinite only rarely coexist in the alkaline association wherever allanite and chevkinite coexist they reveal reaction textures such that it is possible that only allanite or chevkinite is stable at any given condition the distribution of allanite and chevkinite in the alkaline association suggests that the composition of the magmas is an important control on the stability fields of these minerals a fact that is supported by the presence of primary allanite in rocks formed by processes of mixing and mingling of magmas in this sense the importance of composition on the stability of allanite and chevkinite may be larger than previously suggested

chemical variation modal composition and classification of granitoids pdf file same as figure 11 of main text using sio2 diagram is defined as al na k ca in atomic proportions i e the parameter such that al ca na k thus a cnk 1 corresponds to 2 and a nk 1 to 0 see main text