detrital zircon u pb geochronology and lu hf isotopic analysis of the neoproterozoic penglai group and their comparisons with coeval sedimentary strata of the southeastern north china craton provenance tectonic affinity and implications new u pb dating and hf isotope analyses of detrital zircons from the penglai group occurred in the jiaobei terrane ncc have been conducted the results display age populations of 1000 1300 1300 1500 1500 1700 1700 1800 and 1800 ma the penglai group has identical age spectra u and th variations th u ratios and hf isotopic compositions of detrital zircons similar litho and biostratigraphy and tectonic setting with the tonian strata of the southeastern ncc demonstrating that the penglai group has tectonic affinity of the ncc and be deposited during tonian given that the detrital zircon age populations are well consistent with the multiple magmatic and metamorphic thermal events in the ncc especially 1000 1300 ma medium acid and or acid igneous rocks have been more and more recognized in the korean peninsula we propose that the penglai group might be mainly sourced from the ncc the detrital zircon populations of 1000 1300 1300 1500 1500 1700 and 1700 1800 ma exhibit both positive and negative ehf t values combined with late paleo neoproterozoic basic magmatism represented by large volumes of diabase sills in the ncc suggesting that the ncc experienced strong crust mantle interactions which might trigger the continuously multi stage rifting activities of the ncc during late paleoproterozoic 1 8 ga neoproterozoic supplementary material https doi org 10 6084 m9 figshare c 4816317

a 1 78 ga large igneous province in the north china craton the xiong er volcanic province and the north china dyke swarm abstract the 1 78 ga xiong er volcanic province xvp and coeval north china giant mafic dyke swarm ncds are the most important magmatic events occurring after the amalgamation of the north china craton ncc the xvp consists of 3 7 km of extrusive volcanics and some feeder dykes sills located along the southern margin of the ncc and extending over an area 0 06 m km2 compositions vary from basalt to rhyolite but are predominantly intermediate in terms of silica content there are also minor sedimentary intercalations and pyroclastic units the sedimentary interlayers indicate an environment changing from continental facies to oceanic facies up section the xvp is characterized by fractional crystallization from an em i type mantle source and both continental arc andean type and rift environments have been proposed the ncds is widespread in the central ncc with an outcrop area 0 1 m km2 and are exposed at variable depths up to 20 km deepest in the north dyke compositions vary from basalt to andesite and dacite but are dominantly mafic and comprise two series of magmatism previous studies revealed that the ncds recorded assimilation and fractional crystallization of an em i type magma source with a minor dm contribution in the younger magmas both syn collisional and intra continental anorogenic environments have been proposed spatial and petrogenic correlations suggest a cogenetic relationship between the ncds and xvp and considered together they define a large igneous province lip of 0 1 m km2 in area and 0 1 m km3 in volume which is also notable for its continuous compositional range from mafic to felsic with no gap at intermediate compositions the petrology is explained by a common magma source that undergoes a silica poor and iron enriched fractionation trend at depth followed by a silica rich and iron poor fractionation trend in shallow level magma conduits dykes and surface lavas a mantle plume is favored as the cause of this 1 78 ga north china lip

4 d constrained depth conversion for reservoir compaction estimation application to ekofisk field ekofisk field in the norweigian sector of the north sea is a remarkable example of a strongly compacted chalk reservoir since the start of production in 1971 the water bottom has subsided by over 8 m as a result of reservoir compaction hermansen et al the field wide waterflood begun in 1989 was expected to halt pressure depletion and reservoir compaction after a few years of waterflooding ekofisk reservoir pressure began increasing but compaction and subsidence continued laboratory studies on chalk cores indicated that high porosity oil filled chalk compacts strongly when flushed with seawater these studies along with reservoir flow simulation and geomechanic modeling led to a model of a water weakening compaction sylte et al ongoing efforts deployed to monitor the reservoir compaction and seabed subsidence include twice a year compaction logging in a monitoring well daily gps measurements at the platforms and periodic bathymetry surveys nine surveys from 1970 to 1999

the 1 75 1 68ga anorthosite mangerite alkali granitoid rapakivi granite suite from the northern north china craton magmatism related to a paleoproterozoic orogen abstract the amgrs is a suite of anorthosite mangerite alkali granitoid rapakivi granite in the northern north china craton ncc new zircon shrimp and laser ablation inductively coupled plasma mass spectrometry la icpms u pb dating of the damiao anorthosite changsaoying k feldspar granite lanying anorthosite and quartz syenite and the gubeikou k feldspar granite yielded emplacement ages of 1726 9 ma 1753 23 ma 1739 43 ma 1712 15 ma and 1692 19 ma respectively combined with previous geochronological data from the damiao norite and mangerite and shachang rapakivi granite it indicates that the whole amgrs was intruded between 1750 ma and 1680 ma the damiao anorthosites and norites exhibit a high mg and this decreases from mafic up to 63 to felsic t dm c ages ranging from 2 32 ga to 2 43 ga and 2 70 ga to 2 88 ga respectively this suggests that these rocks are the fractional crystallization products of one parental magma zircon hf and whole rock sr nd pb isotopic systematics combined with high la yb 10 and intermediate th ta ratios suggest that the parental magma of the amgrs could have been derived from em i lithosphere mantle enriched by recycled archean continent crustal materials accompanied by some assimilation of lower crustal components the mafic to felsic compositions of the amgrs are different from typical bimodal rocks formed in an anorogenic intra continent rift setting but resemble those emplaced during post collisional post orogenic extensional tectonic setting this is compatible with their age of

geothermometry geochronology and mass transfer associated with hydrothermal alteration of a rhyolitic hyaloclastite from ponza island italy abstract a rhyolitic hyaloclastite from ponza island italy was hydrothermally altered producing four distinct alteration zones based on x ray diffraction mineralogy and field textures 1 nonpervasive argillic zone 2 propylitic zone 3 silicic zone and 4 sericitic zone the unaltered hyaloclastite is volcanic breccia with clasts of vesiculated obsidian in a matrix of predominantly pumice lapilli incomplete alteration of the hyaloclastite resulted in the nonpervasive argillic zone characterized by smectite and disordered opal ct the other three zones exhibit more complete alteration of the hyaloclastite the propylitic zone is characterized by mixed layer illite smectite i s with 10 to 85 i mordenite opal c and authigenic k feldspar akspar the silicic zone is characterized by i s with 90 i pure illite quartz akspar and occasional albite the sericitic zone consists primarily of i s with 66 i pure illite quartz and minor akspar and pyrite k ar dates of i s indicate hydrothermal alteration occurred at 3 38 0 08 ma oxygen isotope compositions of i s systematically decrease from zones 1 to 4 in the argillic zone smectite has 18o values of 21 7 to 22 0 and i s from the propylitic silicic and sericitic zones ranges from 14 5 to 16 3 12 5 to 14 0 and 8 6 to 11 9 respectively 18o values for quartz from the silicic and sericitic zones range from 12 6 to 15 9 by use of isotope fractionation equations and data from authigenic quartz hosted primary fluid inclusions alteration temperatures ranged from 50 to 65 c for the argillic zone 85 to 125 c for the propylitic zone 110 to 210 c for the silicic zone and 145 to 225 c for the sericitic zone fluid inclusion data and calculated 18owater values indicate that hydrothermal fluids were seawater dominated mass transfer calculations indicate that hydrothermal alteration proceeded in a relatively open chemical system and alteration in the sericitic zone involved the most extensive loss of chemical species especially si systematic gains in mg occur in all alteration zones as a result of i s clay mineral formation and systematic losses of na ca and k occur in most zones with the exception of ca calculations of mass transfer associated with hydrothermal alteration on ponza agree with chemical fluxes observed in laboratory experiments involving hydrothermal reactions of rhyolite and seawater the anomalous ca loss at ponza may be due to hydrothermal formation of anhydrite and later low temperature dissolution on the basis of mg enrichments derived from circulating seawater we estimate the following minimum water rock ratios 9 3 6 and 9 for the argillic propylitic silicic and sericitic zones respectively hydrothermal fluid ph for the propylitic and silicic zones was neutral to slightly basic and relatively acidic for the sericitic zone as a result of condensation of carbonic and perhaps other acids

crustally derived granites in the panzhihua region sw china implications for felsic magmatism in the emeishan large igneous province abstract in the panxi region of the late permian 260 ma emeishan large igneous province elip there is a bimodal assemblage of mafic and felsic plutonic rocks most emeishan granitic rocks were derived by differentiation of basaltic magmas i e mantle derived or by mixing between crustal melts and primary basaltic magmas i e hybrid the yingpanliangzi granitic pluton within the city of panzhihua intrudes sinian 600 ma marbles and is unlike the mantle derived or hybrid granitic rocks the shrimp zircon u pb ages of the yingpanliangzi pluton range from 259 8 ma to 882 22 ma younger ages are found on the zircon rims whereas older ages are found within the cores field relationships and petrography indicate that the yingpanliangzi pluton must be pm anomalies low end 260 ma values 3 9 to 4 4 and high i sr 0 71074 to 0 71507 consistent with a crustal origin the recognition of a crustally derived pluton along with mantle derived and mantle crust hybrid plutons within the panxi region of the elip is evidence for a complete spectrum of sources as a consequence the types of panxi granitoids can be distinguished according to their asi eu eu end t ehf t t zr c and nb ta pm values the diverse granitic magmatism during the evolution of the elip from 260 ma to 252 ma demonstrates the complexity of crustal growth associated with lips

nature and origin of proterozoic a type granitic magmatism in the southwestern united states of america the mountain ranges of arizona and adjacent california and nevada contain large areas underlain by proterozoic anorogenic granites comprising the southwesternmost portion of a transcontinental belt of 1 4 1 5 ga old anorogenic complexes that extends across north america northeast into labrador of these a two mica monazite bearing granitic suite resides in central and southeastern arizona as part of a peraluminous subprovince that is bordered on the south southern arizona to sonora and west western arizona and adjacent portions of california and nevada by marginally metaluminous granites bearing biotite sphene hornblende and fluorite all of these 1 4 ga granites are distinctly more potassic iron enriched relative to mg and depleted in ca mg and sr in contrast to typical orogenic granitoids in general the large ion lithophile element enriched composition is a consequence of limited melting of a water deficient crustal source at depths greater than 25 37 km for the peraluminous granites this contrast is less extreme perhaps resulting from a larger degree of melting as a consequence of a greater metasedimentary component and water in its crustal source the anorogenic granitic magmas intruded into the upper crust at depths of 8 17 km or shallower at temperatures up to 790 c the most dramatic variation in the crystallization intensive parameters resides in the oxygen fugacity which spans three orders of magnitude relative to other anorogenic suites all of the magmas crystallized at elevated levels of o2 as reflected in their assignment to the anorogenic magnetite series yet a regionally significant rise in primary o2 levels unmatched elsewhere in the transcontinental belt occurs for plutons in western arizona including the holy moses and hualapai granites the most extreme case is the hualapai granite whose biotite fe fe mg ratios drop due to high o2 to a low of 0 27 down from more typical levels of 0 54 to 0 75 such extreme variations in primary levels of oxygen fugacity must be an indirect imprint of regional changes of the level of oxidation of the lower crust the high fo2 holy moses and hualapai plutons have intruded near the regional boundary between the metaluminous and peraluminous granites and appear to be imaging a major change in the level of oxidation of the lower crust this boundary is also approximately equivalent to significant changes in the nd and pb isotopic compositions of these granites and the metamorphic and magmatic character of the older orogenic terrane on a global scale the crust forming orogenies ended by 1 6 ga ago and the continents entered a long lived era dominated by localized extension and transcontinental intrusion of anorogenic potassic rapakivi granite mafic dike swarms charnockite and anorthosite the absence of orogenic deformation implies that plate consumption became intraoceanic during this time the profuse and widespread nature of the igneous activity has no phanerozoic analogue and is considered to be unique to the proterozoic a crustal overturn model ties the magmatism to heating within a largely undepleted subcontinental mantle the eventual rise of mantle plumes and the transfer of heat into the youthful undifferentiated proterozoic crust subsequent melting and rise of potassic granitic magmas from the lower crust leads to considerable crustal reorganization a process that would continue until both the mantle and crust reached a stable configuration

geochemistry of ttg and ttg like gneisses from lushan taihua complex in the southern north china craton implications for late archean crustal accretion the late archean taihua complex mainly composed of amphibolite and ttg and ttg like gneisses is volumetrically most important metamorphic rock suites scattered along the southern margin of the north china craton ncc zircon shrimp u pb dating shows two episodes 2765 13 and 2723 9 ma of archean magmatism in the lushan area with distinct geochemical features the early 2765 13 ma suite ttg like gneisses has low sio2 52 5 66 1 wt high mg 0 47 0 68 low hree ybn 3 0 5 4 and y 8 07 13 9 ppm with low to moderate la yb n 6 7 37 1 and sr y 25 9 119 3 the younger 2723 9 ma suite ttg gneisses has high sio2 63 5 74 3 wt low mg 0 13 0 52 very low ree ybn 1 1 20 2 43 both suites show pronounced negative nb ta and ti anomalies on the primitive mantle normalized spidergram the ttg like gneiss suite has similar bulk rock nd and zircon hf model ages 3 0 ga with end t 0 0 26 1 46 and is interpreted as resulting from melt of mantle interactions with the melts derived from partial melting of subducted ocean crust with a residual assemblage of garnet clinopyroxene rutile amphibole which favors subducted slab model for the late archean ttg the ttg gneiss suite has abundant relic zircons 2 95 2 80 ga with end t 0 1 31 to 0 23 which is best interpreted as derived from partial melting of thickened lower continental crust with a garnet amphibolite residue garnet amphibole rutile significant high pressure fractional crystallization garnet amphibole and accumulation plagioclase are also required in the petrogenesis the lushan amphibolite with nearly flat primitive mantle normalized trace element pattern is interpreted to represent early ocean crust metamorphism these observations suggest a possible model of late archean crustal accretion from ocean crust to continental terrain in the southern north china craton

multiple episodes of tectono thermal disturbances in the huayangchuan u nb pb polymetallic deposit in the xiaoqinling region central china and their significances on metallogeny abstract the huayangchuan u nb pb polymetallic deposit is hosted in a suite of neoarchean to paleoproterozoic gneisses in the xiaoqinling region central china the deposit is composed mainly of discrete small scale carbonatite veins emplaced largely along shallow lying brittle fractures various regimes of deformation from ductile flow to brittle fracturing are documented in the deposit and its adjacent areas these include an earliest phase of contracting deformation d1 manifested as well developed boudinages abundant relict rootless folds and penetrative compositional bands i e gneissosity generally trending e w a subsequent phase of disperse dextral ductile shear d2 along the e w trending gneissosity and a following phase of general ne sw compression d3 resulting in local contraction deformation bands and abundant sinistral ductile shear along the e w trending gneissosity all of which are affected by an overwhelming nne ssw compression d4 with hemi ductile to brittle deformation behaviors the deformation after d4 is generally brittle and may arise from an earlier regime of dominant compression nw se d5 and a later regime of extensive extension d6 multiple thermal events are superimposed on gneisses as revealed by 40ar 39ar dating on biotite collected from various lithologies in the deposit and its adjacent areas which yields ages ranging from late triassic 238 203 ma through earliest cretaceous 139 134 ma and middle cretaceous 117 92 ma to latest cretaceous 72 ma and even early cenozoic 53 ma this sequence of thermal disturbances unlikely predates the d4 deformation given that it is the last ductile deformation experienced by the precambrian rocks ore bearing carbonatite veins have their shallowly lying attitudes consilient with the d4 and d5 deformation and ages compatible with the first two thermal overprints accordingly a two stage mineralization model is proposed for the huayangchuan polymetallic deposit with an early phase of ore veining in the late triassic to latest jurassic during the nearly n s collision between the south and north china blocks and a late phase of ore veining or more possibly reworking in the early cretaceous related to the yanshanian orogeny due to a generally nw directed subduction of the paleo pacific oceanic plate beneath the eastern asia continent

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

fault controlled carboniferous a type magmatism in the proto andean foreland sierras pampeanas argentina geochemical constraints and petrogenesis abstract the intrusion of granitoids into the eastern sierras pampeanas in the early carboniferous took place after a long period of mainly compressional deformation that included the famatinian ordovician and achalian devonian orogenies these granitoids occur as small scattered plutons emplaced in a dominant extensional setting within older metamorphic and igneous rocks and many of them are arranged along a reactivated large shear zone a set of 46 samples from different granitic rocks huaco granitic complex san blas pluton and the la chinchilla stock from the sierra de velasco zapata granitic complex from sierra de zapata and the los arboles pluton from sierra de fiambala display high and restricted sio 2 contents between 69 2 and 76 4 wt on both feo feo mgo vs sio 2 and na 2 o k 2 o cao vs sio 2 plots the samples plot in the ferroan and alkaline calcic to calco alkaline fields feo feo mgo 0 88 1 0 na 2 o k 2 o cao 6 3 8 3 thus showing an a type granitoid signature the high concentrations for the high field strength elements hsfe such as y nb ga ta u th etc and flat ree patterns showing significant negative eu anomalies are also typical features of a type granites our petrogenetic model supports progressive fractional crystallization with dominant fractionation of feldspar and a source mineral assemblage enriched in plagioclase biotites have distinctive compositions with high feo mgo ratios 7 8 61 5 f 360 5610 ppm and cl 120 1050 ppm the feo mgo ratios together with the f and cl content of igneous biotites seem to reflect the nature of their parental host magmas and may be useful in identifying a type granitoids the isotopic data rb sr and sm nd confirm that the a type granites represent variable mixtures of asthenospheric mantle and continental crust and different mixtures lead to different subtypes of a type granite illustrating the lack of consensus about a type magma origin we conclude that prominent shear zones play an important role in providing suitable conduits for ascending asthenospheric material and heat influx in the crust a hypothesis that is in accord with other recent work on a type granites

comparison of proterozoic and phanerozoic rift related basaltic granitic magmatism abstract this paper compares the 1 67 1 47 ga rapakivi granites of finland and vicinity to the 1 70 1 68 ga rapakivi granites of the beijing area in china the anorogenic 130 ma granites of western namibia and the 20 15 ma granites of the colorado river extensional corridor in the basin and range province of southern nevada in finland and china the tectonic setting was incipient aborted rifting of paleoproterozoic or archean continental crust in namibia it was continental rifting and mantle plume activity that led to the opening of southern atlantic at 130 ma the 20 15 ma granites of southern nevada were related to rifting that followed the triassic paleogene subduction of the farallon plate beneath the southwestern united states in all cases extension related magmatism was bimodal and accompanied by swarms of diabase and rhyolite quartz latite dikes rapakivi texture with plagioclase mantled alkali feldspar megacrysts occurs in varying amounts in the granites and the latest intrusive phases are commonly topaz bearing granites or rhyolites that may host tin tungsten and beryllium mineralization the granites are typically ferroan alkali calcic metaluminous to slightly peraluminous rocks with a type and within plate geochemical and mineralogical characteristics isotope studies nd sr suggest dominant crustal sources for the granites the preferred genetic model is magmatic underplating involving dehydration melting of intermediate felsic deep crust juvenile mafic magma was incorporated either via magma mingling and mixing or by remelting of newly hybridized lower crust in namibia partial melting of subcontinental lithospheric mantle was caused by the tristan mantle plume in the other cases the origin of the mantle magmatism is controversial for the fennoscandian suites extensive long time mantle upwelling associated with periodic migrating melting of the subcontinental lithospheric mantle governed by heat flow and deep crustal structures is suggested

the source of a type magmas in two contrasting settings u pb lu hf and re os isotopic constraints the sources of post orogenic a type magmas from two distinct geodynamic settings are compared the end of the ca 514 480 ma delamerian orogeny southeastern south australia was marked by 10 myr of bimodal a type magmatism driven by convective removal of thickened lithosphere initial os and hf isotope ratios record a heterogeneous lithospheric mantle source with some input from aesthenospheric mantle mafic parental melts fractionated to produce the granites in contrast initial os isotope ratios of the a type magmas that comprise the ca 1598 1583 ma mesoproterozoic gawler felsic large igneous province central south australia record a dominant evolved lower crust component however initial hf isotope ratios from these samples are depleted indicating a mantle source for lithophile elements this voluminous bimodal magmatism lasted for 15 myr and ended the wartakan orogeny in both cases the homogenisation of chemical rheological heterogeneities inherited from terrain amalgamation and orogenic thickening strengthened the lithosphere the contemporaneous fusion of heterogeneous mantle crust may represent a common stabilising influence on the lithospheric column regardless of tectono magmatic setting

episodic crustal growth in the southern segment of the trans north china orogen across the archean proterozoic boundary abstract the dengfeng and taihua complexes are well exposed neoarchean to paleoproterozoic units in the southern segment of the trans north china orogen tnco zircon u pb dating shows that the dengfeng complex records two episodes 2568 11 ma and 2306 6 ma of tonalite trondhjemite granodiorite ttg magmatism all of the ttg rocks are characterized by high sio2 66 7 75 4 wt na2o 3 20 5 06 wt and relatively low mgo 0 40 1 78 wt the late neoarchean ttg gneisses have very low contents of hree ybn 0 69 2 75 and y 1 73 7 07 ppm with moderate la yb n 24 1 53 8 and high sr y 65 1 291 3 ratios the early paleoproterozoic ttg gneisses have low contents of hree ybn 2 93 6 37 and y 6 7 11 0 ppm with moderate la yb n 10 1 27 0 and sr y 10 6 52 1 ratios both suites show pronounced negative nb ta p and ti anomalies but positive sr and pb anomalies the late neoarchean ttg gneisses all have similar bulk rock nd and zircon hf model ages with mainly positive nd t and are interpreted as resulting from the melting of dominantly juvenile thickened lower crust with residual garnet and amphibole the early paleoproterozoic ttg gneisses have extremely variable nd t 6 23 to 4 23 and heterogeneous zircon hf t 3 3 to 3 1 which are also best interpreted as resulting from the partial melting of thickened lower crust with residual amphibole and garnet the taihua complex in the xiaoqinling area records three episodes of early paleoproterozoic ttg magmatism 2 48 ga at caotan 2 31 ga at houjiacun and 2 16 ga at bayuan younger than the taihua complex in the lushan area 2 85 2 72 ga all rocks have relatively low contents of hree ybn 1 03 8 32 and y 2 84 24 ppm with moderate la yb n 8 7 88 4 and sr y 19 8 125 8 ratios and show negative ta nb and ti anomalies and positive sr and pb anomalies the caotan gneisses at 2 48 ga and the houjiacun ttg gneisses at 2 31 ga have low mg 0 14 0 45 low cr the geochronology of the dengfeng complex in the dengfeng area and the taihua complex in the lushan xiong er and xiaoqinling areas reveals at least four magmatic episodes in the southern segment of the tnco from the late mesoarchean to early paleoproterozoic 2 85 2 72 ga 2 57 2 48 ga 2 34 2 30 ga and 2 20 2 07 ga the rocks of the two early episodes are dominantly of juvenile compositions with mostly positive whole rock nd t and zircon hf t values suggesting two episodes of crustal growth formed in a subduction tectonic setting the magmatic rocks of the third episode consist of both the juvenile and pre existing crustal materials with variable whole rock nd t and zircon hf t values which were generated in a subduction zone during the initial assembly of the ncc within the columbia supercontinent cycle the final episode of magmatism lacks juvenile materials with whole rock nd t and zircon hf t values being consistently negative these may have resulted from the orogenic collapse the episodic continental growth recorded in the southern segment of the tnco was caused by subduction and consequent orogeny consistent with global supercontinent cycles within the late archean and early paleoproterozoic

characterization of neoformed illite from hydrothermal experiments at 250 degrees c and p sub v soln an hrtem atem study solid products from hydrothermal experiments conducted at 250 c and p sub v soln were characterized by powder x ray diffraction xrd and atem hrtem experiments were conducted with muscovite kaolinite and quartz or amorphous silica in 2m kcl solutions for 43 to 176 d post experiment solution compositions lie either within the illite 0 88 k stability field or on the illite 0 88 k kaolinite or illite 0 88 k diaspore univariant boundaries in log a sub k sup a sub h sup vs log a sub h sub 4 sio sub 4 activity space transmission electron microscopy tem observations of muscovite grain edges reveal the neoformation of illite crystals with a range of compositions atem from 0 31 to 0 89 k o sub 10 oh sub 2 the range of k contents appears to narrow toward 0 88 k o sub 10 oh sub 2 with increased experiment duration hrtem suggests the presence of 2 to 11 layer fundamental particles composed of illitic layers with 10 a periodicity fundamental particle thicknesses increase toward an average of 8 layers particle with increased experiment duration in the longer duration experiments fundamental particle thicknesses were normally distributed about thicknesses of 4 and 8 layers whereas fundamental particles with thicknesses 4 layers were common in a shorter duration experiment the compositions and structure of the illites are consistent with the multiphase model which more states that the smectite to illite transition occurs through the step wise formation of solubility controlling phases consisting of fundamental particles with thicknesses of 1 2 4 and le 8 layers the increase in k content and fundamental particle thickness with the extent of reaction suggests that the illite crystals underwent a prograde reaction culminating in the formation of end member illite 0 88 k o sub 10 oh sub 2 this reaction in conjunction with the previously observed retrograde reaction from muscovite to end member illite demonstrates the stability of end member illite in the system k sub 2 o al sub 2 o sub 3 sio sub 2 h sub 2 o at 250 c less

geochronology and petrogenesis of gray gneisses from the taihua complex at xiong er in the southern segment of the trans north china orogen implications for tectonic transformation in the early paleoproterozoic the taihua complex in the xiong er area along the southern segment of the trans north china orogen tnco consists of gray gneisses with minor amphibolites zircon u pb dating of the gray gneisses reveals at least two episodes of early paleoproterozoic magmatism 2 30 2 32 ga and 2 07 2 19 ga each with distinct geochemical features the tieluping ttg gneisses of the early suite have very low hree ybn 0 58 4 75 and y contents 2 05 13 2 ppm with moderate to high la yb n 14 3 192 6 and sr y ratios 38 5 220 1 pronounced negative ta nb and ti anomalies but positive sr and pb anomalies the protoliths of these gneisses were most likely derived from partial melting of thickened lower crust with residual garnet and amphibole and underwent garnet amphibole fractionation plagioclase accumulation and or fractionation the tieluping ttg gneisses have variable zircon hf model ages tdm hfc 2 48 3 06 ga and whole rock end t values 4 89 4 98 and the youngest whole rock depleted mantle nd model age tdm 2 32 ga is close to the crystallization age of the ttg 2318 8 ma the protolith of the gneisses formed from a combination of pre existing continental crust and juvenile materials implying an andean type continental margin arc environment it is suggested that crustal thickening was the result of accretionary orogenesis accompanied by basaltic underplating the ganshugou potassium rich gneisses of the later suite have low na2o and high k2o and show fractionated lree and nearly flat hree patterns with strong negative ta nb and ti anomalies they are characterized by negative end t values 5 01 to 2 33 and heterogeneous zircon ehf t values 10 65 2 10 with decoupling of zircon hf model ages tdm hfc 2 64 3 23 ga and whole rock nd model ages tdm 2 55 2 81 ga the protolith of the ganshugou gneisses was a potassium rich igneous rock which was likely the result of partial melting of pre existing crustal materials at shallow depth compatible with a post collisional setting there is therefore a tectonic transformation from an accretionary orogenesis to an extensional regime as a consequence of post collisional uplift in the southern segment of the tnco in the early paleoproterozoic

a hybrid origin for the qianshan a type granite northeast china geochemical and sr nd hf isotopic evidence abstract major and trace element whole rock sr and nd isotope and zircon hf isotope data are reported for a suite of a type granites and mafic microgranular enclaves from the early cretaceous 126 2 ma qianshan pluton liaodong peninsula northeast china with the aim of investigating the sources and petrogenesis of a type granites the qianshan pluton includes hornblende alkali feldspar granite graphic biotite granite and mafic microgranular enclaves the hornblende alkali feldspar granites have high sio 2 fe 2 o 3t mgo k 2 o na 2 o rb zr and lree contents and low ba and sr concentrations with strongly negative eu anomalies their high rb sr 87 rb 86 sr 16 76 24 15 and initial 87 sr 86 sr ratios 0 7215 to 0 7283 negative e nd t values 14 1 to 16 5 and zircon e hf t values 18 9 to 11 5 indicate they were mainly derived from a crustal source but with involvement of high e nd t and e hf t materials graphic biotite granites have similar geochemical features and sr nd hf isotopic compositions to enclaves indicating they were the result of crystal fractionation of evolved mafic magmas but with involvement of low e nd t and e hf t materials the mafic enclaves have an igneous texture and contain acicular apatite suggesting quenching of mafic magmas that have co mingled with the host granites they have low initial 87 sr 86 sr ratios 0 7097 0 7148 negative e nd t 14 5 to 11 9 and zircon e hf t 17 1 to 6 9 values and are enriched in liles and lrees and depleted in hfses when coupled with the high mgo mg up to 54 this indicates derivation from an enriched lithospheric mantle source but contaminated by crustal materials geochemical and sr nd and zircon hf isotopic compositions rule out simple crystal liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks instead magma mixing of mantle derived mafic and crustal derived magmas coupled with crystal fractionation is compatible with the data this example shows that at least some a type granites formed through a complex process involving mantle and crustal derived magma mixing crystal fractionation and infracrustal melting

development of the north china craton during the late archaean and its final amalgamation at 1 8 ga some speculations on its position within a global palaeoproterozoic supercontinent abstract new data obtained over the past few years indicate that by the end of the archaean the north china craton consisted of two major crustal entities identified as the eastern and western blocks the formation of island and magmatic arcs along the western margin of the eastern block record the latest archaean events so far identified the location of the western block at this time is unknown however the occurrence of palaeoproterozoic passive margin sediments similar in age to those occurring in the eastern block indicates essentially similar conditions and possible separation by an ocean of unknown dimensions re initiation of east dipping subduction beneath the western margin of the eastern block resulted in closure of this palaeoproterozoic ocean bringing the western block into close proximity continued subduction resulted in a major continental continental collision and the western margin of the eastern block was again reactivated leading to extensive thrusting high pressure metamorphism and the generation of crustal melts this occurred between 1 88 and 1 79 ga ago resulting in the formation of the trans north china orogen and final amalgamation of the north china craton although information is sparse to non existent we have reviewed the possible location of the north china craton at the time of collision and note a similarity in terms of age lithology and distribution of major crustal components between the north china craton and baltica although speculative we present this model in the hope that it will stimulate debate and lead to further research in these areas

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

geochronological and geochemical constraints on the petrogenesis of the early paleoproterozoic potassic granite in the lushan area southern margin of the north china craton abstract we conducted a geochronological and geochemical study on the paleoproterozoic potassic granites in the lushan area southern margin of the north china craton ncc to understand the tectonic regime of the ncc at 2 2 2 1 ga this rock suite formed at 2194 29 ma the rocks are rich in sio2 76 10 77 73 wt and k2o 5 94 6 90 wt with high k2o na2o contents from 7 56 wt to 8 48 wt but poor in cao 0 10 0 28 wt p2o5 0 02 0 05 wt and mgo 0 01 0 30 wt mg 1 08 27 3 indicating they experienced fractional crystallization major element compositions suggest the potassic granites share an affinity with high k calc alkaline granite even though the lushan potassic granitic rocks have high a cnk ratios 1 11 1 25 which can reach peraluminous feature the very low p2o5 contents and negative correlation of p2o5 and sio2 ruling out they are s type granites different from peralkaline a type granites the lushan potassic granites have variable zr concentrations 160 344 ppm 226 ppm on average and 10 000 ga al ratios 1 76 3 00 together with high zircon saturation temperatures tzr 826 885 c indicating they are fractionated aluminous a type granites enriched lree la yb n 9 72 81 8 negative eu anomalies and low sr y with no correlations in sr y and sr zr versus cao suggest the possible presence of ca rich plagioclase and absence of garnet in the residual magmatic zircon grains have variable ehf t values 2 4 to 7 3 with zircon two stage hf model ages tdmc varying from 2848 ma to 2306 ma mostly around ca 2 5 ga and are plotted in the evolution line of crustal felsic rock we propose that the rocks mainly formed by partial melting of ca 2 50 ga tonalitic granodioritic crust as a result of upwelling mantle derived magmas which provided thermal flux and source materials in an intra continent rifting the ca 2 2 ga magmatism suggests that intra continental rifting occurred at 2 35 1 97 ga at least in the southern margin of the ncc after its final cratonization in the late neoarchean

f rich strongly peraluminous a type magmatism in the pre andean foreland sierras pampeanas argentina geochemical geochronological isotopic constraints and petrogenesis abstract the petrogenetic nature of a type granites is a controversial problem the vinquis batholith in the sierras pampeanas of argentina contains unusual f rich and strongly peraluminous a type monzogranites a new la mc icp ms u pb zircon crystallization age of 355 7 ma indicates emplacement in latest devonian or earliest carboniferous time overlapping with extensive metaluminous a type magmatism in the area the monzogranites have a restricted range of sio 2 content 71 5 74 8 they are poor in ca 0 54 1 4 cao and rich in feo t with relatively high feo t feo t mgo values ranging from 0 77 to 0 86 average 0 80 both feo t feo t mgo vs sio 2 and na 2 o k 2 o cao vs sio 2 plots indicate ferroan and alkali calcic signatures typical of a type granitoids the samples have mgo tio 2 1 2 and are moderately enriched in total alkalis average 8 18 with high k 2 o na 2 o values of 1 40 2 24 the granites are strongly peraluminous with asi molar al 2 o 3 cao na 2 o k 2 o values of 1 2 to 1 3 the high p 2 o 5 content 0 23 0 37 is distinctive and close to values reported for other paleozoic f rich peraluminous a type granites in the sierras pampeanas they have moderate contents of high field strength elements e g zr nb th y etc and moderately fractionated to flat ree patterns la yb n in the range 4 8 19 6 showing significant negative eu anomalies eu eu 0 41 biotite has a distinctive composition with relatively high fe 2 fe 2 mg ratios 0 61 0 74 and high f 0 55 1 42 wt content together with the whole rock chemistry this may be useful in identifying strongly peraluminous a type granites in addition the rb sr vs th zr ce diagram may be an appropriate discriminant between metaluminous and peralkaline a type granites strongly peraluminous a type granites and strongly peraluminous orogenic granites the geochemical evidence indicates that differentiation of the granitic rocks occurred by mineral fractionation from a f rich peraluminous parental magma dominant of plagioclase k feldspar biotite and accessory minerals such as zircon monazite xenotime and oxides the peraluminous composition and isotope data e hf 355 and e nd 355 ranging from 9 5 to 1 5 and 6 1 to 7 8 respectively together with abundant inherited ordovician and cambrian zircon strongly suggest a dominantly metasedimentary source the whole rock and biotite compositions indicate that the vinquis batholith crystallized under mainly oxidizing conditions whereas early carboniferous metaluminous to weakly peraluminous a type granites of the sierras pampeanas crystallized under dominantly reduced conditions

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

paleoproterozoic crustal growth in the western block of the north china craton evidence from detrital zircon hf and whole rock sr nd isotopic compositions of the khondalites from the jining complex detrital zircons from the khondalites in the western block of the north china craton were mainly derived from a 1 9 to 2 1 ga paleoproterozoic provenance most of the more than 200 detrital zircons analyzed in this study have positive initial ehf values and a wide range of hf depleted mantle model ages tdmc that range from 2 08 ga to 2 74 ga with a peak at 2 24 to 2 40 ga the whole rock nd isotopic analyses for the khondalite samples give quite uniform 143nd 144nd ratios from 0 511188 to 0 511858 corresponding to initial end 2 10ga values ranging from 1 6 to 0 6 in contrast sr isotopic compositions of these samples are more varied with initial 87sr 86sr ratios at 2 10 ga ranging from 0 7017 to 0 7076 in the end versus 87sr 86sr correlation diagram most samples plot near the mixing hyperbola of the depeleted mantle and crust with the contribution of depleted mantle about 65 to 75 percent these results suggest that a large amount of juvenile materials from the depleted mantle was accreted to the western block at 2 1 ga a period of magmatic quiescence in many cratons of the world

meso neoproterozoic strata and target source rocks in the north china craton a review abstract along with the breakup of the supercontinent nuna and rodinia during the mesoproterozoic and neoproterozoic a series of sedimentary basins formed within or at the periphery of the ancient cratons under the regime of breakup of the supercontinent these basins underwent different evolutionary history hence formed different geological elements constituting the petroleum systems in this contribution the distribution and the stratigraphic successions of the meso neoproterozoic basins in the north china craton ncc were examined as well as the retrospective review for the mechanisms of the basin formation the synthesized geological and geochronological data support a long term extensional environment for the basins in the northern ncc the mesoproterozoic basins in the yanshan western liaoning yan liao and the zhaertai areas formed at the intracratonic position later than that in the bayan obo which possibly deposited in the continental margin the further efforts still need to be devoted to decipher the huge controversial around the tectonic nature of the basins in the southern ncc as well as their stratigraphic correlations with the basins in the north and central the distribution and organic geochemical characteristics of the mesoproterozoic source rock were also summarized three sets of source rock in the central ncc namely the chuanlinggou hongshuizhuang and xiamaling formations developed favorable geological and geochemical conditions for the generation of the hydrocarbon while in the south the cuizhuang formation was the only interval feasible for potential source rock the progress in seismic geophysics and borehole drilling suggested the existence of the mesoproterozoic beneath deep ordos basin this open a new window for the reconstruction of the mesoproterozoic paleogeography as well as the evaluation of the source potential of the mesoproterozoic basins in the north china craton

interpretation of k ar dates of illitic clays from sedimentary rocks aided by modeling k ar dates of illitic clays from sedimentary rocks may contain mixed ages i e may have ages that are intermediate between the ages of end member events two phenomena that may cause mixed ages are 1 long lasting reaction during the burial illitization of smectite and 2 physical mixing of detrital and diagenetic components the first phenomenon was investigated by simulation of illitization reactions using a nucleation and growth mechanism these calculations indicate that values for mixed ages are related to burial history for an equivalent length of reaction time fast burial followed by slow burial produces much older mixed ages than slow burial followed by fast the type of reaction that occurred in a rock can be determined from the distribution of ages with respect to the thickness of illite crystals dating of artificial mixtures confirms a non linear relation between mixed ages and the proportions of the components vertical variation of k ar age dates from gulf coast shales can be modeled by assuming diagenetic illitization that overprints a subtle vertical trend presumably of sedimentary origin in detrital mineral content

a review of the relationships between granitoid types their origins and their geodynamic environments abstract granitoids are divided into several types according to their mineral assemblages their field and petrographical features and their chemical and isotopic characteristics this typology complements most of the recent classifications because it is not based solely on chemical and isotopic criteria but also on the field petrographical and mineralogical criteria it thus has the advantage of distinguishing the various granitoid types in the field in most cases the proposed classification shares many similarities with the twenty most used genetic classifications of granitoids both types of peraluminous granitoids are of crustal origin the tholeiitic alkaline and peralkaline granitoids are of mantle origin and both types of calc alkaline granitoids are of mixed origin and involve both crustal and mantle materials each granitoid type is generated and emplaced in a very specific tectonic setting each stage of the wilson cycle is characterised by typical associations of granitoids well typed and precisely dated granitoids can then complement structural approaches and indicate on the geodynamic environment with reference to some case studies the use of granitoids as tracers of the geodynamic evolution is also proposed and discussed

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

paleozoic peralkaline a type magmatism of the tongbai orogen central china petrogenesis and tectonic implications abstract the precise timing petrogenesis and geodynamic significance of the paleozoic magmatic rocks in the southern qinling tongbai orogenic belt qtob central china are poorly constrained we performed an integrated study of petrology zircon u pb ages geochemistry and hf o nd isotopic compositions of the huangyangshan pluton from the southern qtob to constrain its genesis and tectonic implications a granite sample from the pluton yields a weighted mean zircon 206pb 238u age of 445 6 ma representing the age of the pluton the huangyangshan pluton is dominated by alkaline granite and quartz syenite has a peralkaline signature a nk 0 82 0 96 and belongs to the shoshonite and high k calc alkaline series both the alkaline granite and quartz syenite have geochemical signatures similar to a type granite with high sio2 and total alkali na2o k2o contents high fe numbers and tio2 mgo and ga al ratios and high field strength elements hfses along with low mgo cao and p2o5 contents these samples are enriched in light rare earth elements lrees and have flat heavy rare earth element hree patterns and strongly negative eu anomalies they are enriched in most large ion lithophile elements liles and hfses with significant negative eu ba sr and ti anomalies in primitive mantle normalized trace element diagrams the samples have relatively high zr contents 175 776 ppm yielding minimum magmatic temperatures of 768 c 891 c the huangyangshan pluton has initial zircon ehf t values of 6 6 to 10 0 and whole rock end t values of 2 86 to 3 62 the analyzed zircons yield a mean 18o value of 5 14 0 26 indicating negligible incorporation of hydrothermally altered crustal materials the geochemical signatures of the huangyangshan pluton indicate that the magmas were derived from prompt partial melting of juvenile crust rocks combined with previous results we suggest that the paleozoic huangyangshan peralkaline a type granites resulted from the rifting of a passive continental margin in the northern south china block followed by the opening of the paleotethyan ocean

petrogenesis and tectonic setting of volcanic rocks in the xiaoshan and waifangshan areas along the southern margin of the north china craton constraints from bulk rock geochemistry and sr nd isotopic composition abstract as part of the xiong er volcanic belt along the southern margin of the north china craton volcanic rocks in the xiaoshan and waifangshan areas have a compositional range from the basaltic andesite andesite dacite to rhyolite which display consistent variation trends in terms of their major and trace elements and sr nd isotopic compositions the variable yb contents with nearly constant la yb and tb yb ratios of volcanic rocks in two areas suggest that the fractional crystallization may have played an important role in the differentiation from the basaltic andesite through andesite and dacite to rhyolite the volcanic rocks in these two areas are characterized by the lile and lree enrichments and negative hfse anomalies implying hydrous melting of a mantle wedge in a subduction zone variable sr nd ratios of the basaltic andesite and andesite are interpreted as a result of the fluid addition from a subducting slab non radiogenic nd isotopic compositions as well as high zr y and nb y ratios suggest that the volcanic rocks in these areas were derived from an enriched mantle source on the other hand the volcanic rocks of the basaltic andesite and andesite possess markedly higher fe ti and hfse concentrations than those of typical intra oceanic arcs implying that the mantle source from which the volcanic rocks were derived was metasomatised by siliceous melts during the archean to paleoproterozoic subduction collision in the trans north china orogen these data suggest that in the paleo mesoproterozoic the southern margin of the north china craton was most likely an andean type continental arc in which slab dehydration not only induced the melting of a pre existing metasomatised mantle source but also released lile enriched fluids into the mantle source masking the inherent hfse enriched characteristics of the volcanic rocks along the southern margin of the craton the results of this study indicate that the north china craton like many other continental components e g north america greenland baltica amazonia australia etc of the supercontinent columbia nuna also underwent a subduction related outgrowth along its southern margin during the paleo mesoproterozoic time

characterization of the waukesha illite a mixed polytype illite in the clay mineral society repository the waukesha illite is an excellent example of the illites found in argillaceous rocks typical for paleozoic shales that have undergone significant burial diagenesis during their geologic history it consists of a mixture of detrital 2 m 1 interpreted to be a residuum of karstification within silurian carbonates and diagenetic 1 m and 1 m d illite the chemistry and the age of the illite polytypes are different extrapolating to 100 the 1 m and 1 m d polytypes have an apparent diagenetic age between 295 and 325 ma the chemistry of the 1 m polytype could not be determined because of its low abundance the approximate chemical composition of the 1 m d polytype is 0 67 k 3 6 si and 1 9 al per half unit cell the 2 m 1 polytype has an apparent detrital age between 440 and 520 ma and an approximate chemical composition per half unit cell of 0 78 k 3 4 si and 2 1 al all within our margin of error x ray diffraction xrd results of both random powder and oriented preparations both indicate that the waukesha illite consists of a mixture of illites the xrd patterns of the random powder preparation indicate it is a physical mixture of three different illite polytypes this result was confirmed using 3 different methods 1 by measuring illite polytype specific reflections 2 by mixing illite polytype reference samples and 3 by mixing wildfire calculated xrd patterns decomposition of the illite 001 xrd peak from oriented preparations also indicates mixtures of illites however the proportions of the three illitic components derived from the oriented 001 peak decomposition differ from those results derived from the analysis of the random powder data therefore the shape of the 001 reflection of the waukesha illite cannot be explained by mixing the three different illite polytypes

1700 ma shachang complex northeast china proterozoic rapakivi granite not associated with paleoproterozoic orogenic crust proterozoic rapakivi granites are found on all continents and are characteristically associated with paleoproterozoic crustal domains that predate the granites by

episodic mineralization of hydrothermal illite in the soultz sous forets granite upper rhine graben france episodic and localized illite mineralization is documented in the hydrothermally altered soultz sous forets granite upper rhine graben france separated grain size fractions of altered granite and argillite vein samples contain mixtures of 2m1 and 1m trans vacant illite varieties the platy pseudohexagonal 2m1 illite phases dominate the vein fillings whereas the 1m illite occurs largely as a fibrous pore filling variety which is particularly abundant in the granite matrix multiple phases of fluid injections into the granite body have resulted in different illite assemblages each sample containing a mixture of polytype generations formed during different crystal growth events on the basis of mineralogical and k ar isotopic constraints the ages of these vein mineralizing events are determined by plotting the k ar values of the various grain size fractions against polytype abundance and the fitted volume weighted crystallite thickness distributions the results suggest a permian age for the formation of the studied argillite veins characterized by successive injections of hydrothermal fluids secondary episodes of illite crystallization occurred during jurassic and cretaceous or even younger times in both the veins and the granite matrix there are indications that the polytype structure and composition of illite were strongly influenced by variations in fluid chemistry and the degree of fluid rock interaction as the granite was progressively sealed during post variscan episodic hydrothermal activity

apatite dissolution into peraluminous haplogranitic melts an experimental study of solubilities and mechanisms apatite ap dissolution and solubility in the peraluminous haplogranite system were investigated in three sets of experiments at 750 c and 200 mpa ph2o as functions of ap grain size abundance melt ap ratio and melt asi alumina saturation index mol al2o3 na2o k2o cao between an asi of 1 1 1 metaluminous to mildly peraluminous the solubility of ap as measured by p2o5 in the melt is low 0 1 wt p2o5 which agrees with previous work in equilibrium runs with more strongly peraluminous compositions however ap solubility increases linearly with asi to p2o5 0 63 wt in melt at asi 1 3 and can be denned by a simple equation p2o5 3 4 3 1 asi r 0 833 which agrees within error with previous calculations reported in the literature in experiments where ap grains are sparce and widely separated i e simulating ap distribution in an ap bearing peraluminous protolith undergoing anatexis the dissolution of ap involves transient disequilibrium that promotes higher solubility of p2o5 up to nearly six times the equilibrium values adjacent to ap at a given bulk asi of melt and this increases the al si of melt in the ap dissolution aureole the approach to equilibrium between ap and melt is hastened by increasing the ap melt ratio within the local melt pool e g the capsule or by decreasing ap grain size for a similar melt ap ratio diffusivity calculations using inverse error function methods for the normal diffusion of p and ca yield dp 10 11 cm2 s and dca 10 10 cm2 s confirming prior results that diffusion of p away from ap surfaces is the rate limiting factor in ap dissolution these results combined with previous work at higher temperatures but on similar hydrous melts yield an activation energy e for p diffusion of 16 5 kcal mol and a frequency factor d0 of 1 4 10 3 cm2 s phosphorus diffusion through the melt is accompanied by uphill diffusion of al resulting in an increase of up to 2 wt al2o3 in melt near the ap within the ap dissolution aureole melt contains one additional al cation for every p cation i e al p 1 1 which is suggestive of an associated alpo4 complex the exchange reaction in melt is represented by alpsi 1 in which 2 to4 clusters are created in the place of 1 this exchange should be accompanied by discernable changes in melt properties e g density molar volume viscosity the results have applications to equilibrium and local disequilibrium effects during anatexis of apbearing metapelites and to the saturation limits of ap during crystallization of peraluminous magmas the dissolution of flux bearing minerals such as ap e g those containing p b or f may influence the stability and solubility of other trace element bearing minerals during anatexis of metapelites texture e g grain size and distribution and spacial relationship to other minerals plays an important role in reaction kinetics and at least temporarily in mineral solubilities the disequilibrium behavior of accessory minerals has critical consequences for ree modelling of granites

earth s middle age this research was funded through natural environment research council grant ne j021822 1 and the apc was paid through the rcuk oa block grant

u pb and hf isotopic study of detrital zircons from the wulashan khondalites constraints on the evolution of the ordos terrane western block of the north china craton abstract detrital zircons from the wulashan khondalites in the ordos terrane western block of the north china craton give u pb ages between 1 84 and 2 32 ga showing that their provenance was dominated by paleoproterozoic rocks much younger than those exposed in the eastern block these zircons have oscillatory growth zoning and possess e hf values between 8 and 9 suggesting derivation of their precursor magmas from old crust and juvenile materials from the mantle the lowest e hf values of zircons with different ages give a line that intersects the depleted mantle line at about 2 6 ga in a e hf vs time diagram this may imply that the precursor magma source in the deep level of the crust was largely separated from the mantle 2 6 ga ago and therefore 2 6 ga may be an important crustal formation period for the terrane one khondalite sample has detrital zircons with a single age population at 2 0 ga and positive e hf values from 1 to 9 clearly recording the significant addition of juvenile materials at 2 0 ga in the source region this may inspire us to seek whether there was a connection between a mantle superevent and the formation of columbia supercontinent data of this study exclusively demonstrate that the ordos terrane was developed independently from the eastern block until assembly of the columbia supercontinent therefore tectonic models involving that the whole north china craton was cratonized 2 5 ga ago should be dismissed

origin of the late paleoproterozoic low δ18o a type granites on the southern margin of the north china craton and their geodynamic mechanism abstract a type granites with 18o depletion signatures are rather enigmatic as regards not only their typology and petrogenesis but also their rock forming geodynamic mechanism here we report a suite of late paleoproterozoic low 18o a type granites from the longwangzhuang pluton on the southern margin of the north china craton sims zircon u pb dating showed that the longwangzhuang pluton was emplaced at 1613 4 ma combined with previous data it is clear that the mineral components of longwangzhuang granites are variable and consist of perthite quartz plagioclase biotite and amphibole calcic hastingsite ferroedenite and sodic ferrogedrite geochemically they are metaluminous to peraluminous and have high alkali contents k2o na2o 8 76 wt k2o na2o 1 feot feot mgo 0 95 ratios zr nb ce y 993 ppm contents and 10000 ga al 4 31 ratios resembling a type granites in situ oxygen isotope analysis displayed low zircon 18o values between 4 0 and 4 9 with a weighted average of 4 4 based on the archean two stage zircon hf and whole rock nd model ages coupled with the low la yb cn ratios negative anomalies of eu and sr and high zr saturation temperatures and the presence of low pressures 3 3 5 7 kbar amphiboles the longwangzhuang a type granites were generated by self cannibalizing pre existing low 18o old rocks i e the taihua complex and emplaced at shallow depth combined with regional geological observations i e coeval tuffites a passive rift setting was the most plausible explanation for the generation of the late paleoproterozoic low 18o a type granites on the southern margin of the north china craton such a rifting could likely have lasted at least till 1 6 ga

petrogenesis of the carboniferous ghaleh dezh metagranite sanandaj sirjan zone iran constraints from new zircon u pb and 40ar 39ar ages and sr nd isotopes

kar systematics of clay to mica minerals in a multi stage low grade metamorphic evolution the behavior of the ar isotopic system of clay to mica fractions was studied in cambrian greywacke samples along a transect of northwestern morocco these rocks were affected by a progressively increasing metamorphic overprint spanning from diagenetic to epizonal metamorphic conditions k ar isotopic data of 0 2 10 m size fractions recovered from seven samples allowed to recognized three episodes of illite crystallization 1 during an upper ordovician thermal event at 445 ma which is characterized by an illite chlorite mineral paragenesis in the northernmost part of the transect 2 during a carboniferous low grade overprint at 305 ma which is characterized by a similar mineral paragenesis and 3 during a triassic to liassic hydrothermal activity at 205 ma which generated a smectite dominated paragenesis the ages were examined in combining x ray mineralogic and k ar isotopic data in terms of linear arrays in both isochron and harper diagrams to differentiate true isochrons from mixing lines it could also be shown that mixtures of two generations of micaceous minerals can induce different trends in the k ar dates depending whether the first generation of minerals was either or not affected by the event inducing the later generation

the xiong er volcanic belt at the southern margin of the north china craton petrographic and geochemical evidence for its outboard position in the paleo mesoproterozoic columbia supercontinent abstract the xiong er volcanic belt covering an area of more than 60 000 km 2 along the southern margin of the north china craton has long been considered an intra continental rift zone and recently interpreted as part of a large igneous province formed by a mantle plume that led to the breakup of the paleo mesoproterozoic supercontinent columbia however such interpretations cannot be accommodated by lithology mineralogy geochemistry and geochronology of the volcanic rocks in the belt lithologically the xiong er volcanic belt is dominated by basaltic andesite and andesite with minor dacite and rhyolite different from rock associations related to continental rifts or mantle plumes which are generally bimodal and dominated by mafic components however they are remarkably similar to those rock associations in modern continental margin arcs in some of the basaltic andesites and andesites amphibole is a common phenocryst phase suggesting the involvement of h 2 o rich fluids in the petrogenesis of the xiong er volcanic rocks geochemically the xiong er volcanic rocks fall in the calc alkaline series and in most tectono magmatic discrimination diagrams the majority of the xiong er volcanic rocks show affinities to magmatic arcs in the primitive mantle normalized trace element diagrams the xiong er volcanic rocks show enrichments in the lile and lree and negative nb ta ti anomalies similar to arc related volcanic rocks produced by the hydrous melting of metasomatized mantle wedge nd isotope compositions of the xiong er volcanic rocks suggest that 5 15 older crust has been transferred into the upper lithospheric mantle by subduction related recycling during archean to paleoproterozoic time available shrimp and la icp ms u pb zircon age data indicate that the xiong er volcanic rocks erupted intermittently over a protracted interval from 1 78 ga through 1 76 1 75 ga and 1 65 ga to 1 45 ga though the major phase of the volcanism occurred at 1 78 1 75 ga such multiple and intermittent volcanism is inconsistent with a mantle plume driven rifting event but is not uncommon in ancient and existing continental margin arcs taken together the xiong er volcanic belt was most likely a paleo mesoproterozoic continental magmatic arc that formed at the southern margin of the north china craton similar paleo mesoproterozoic continental magmatic arcs were also present at the southern and southeastern margins of laurentia the southern margin of baltica the northwestern margin of amonzonia and the southern and eastern margins of the north australia craton which are considered to represent subduction related episodic outbuilding on the continental margins of the paleo mesoproterozoic supercontinent columbia therefore in any configuration of the supercontinent columbia the southern margin of the north china craton could not have been connected to any other continental block as proposed in a recent configuration but must have faced an open ocean whose lithosphere was subducted beneath the southern margin of the north china craton

age and geochemistry of the early mesoproterozoic a type granites in the southern margin of the north china craton constraints on their petrogenesis and tectonic implications abstract a type granites are a spectrum of especial felsic rocks that are produced commonly within an extensional environment they are rare before middle paleoproterozoic but present abundantly since the late paleoproterozoic probably indicating a causal change of tectonics and geodynamic mechanism in the continental evolution of the earth an early mesoproterozoic a type granite belt was recognized in the southern margin of the north china craton ncc in recent decades the occurrence of this type of rocks indicates a possible spatial temporal link between the tectonic evolution of the ncc and the breakup of the columbia supercontinent during the mesoproterozoic the zhangjiaping granitic pluton is located in the southern margin of the ncc and one part of the a type granite belt it can be divided into three zones according to their textural and compositional differences of the rocks with a gradually increasing degree of evolution inwards wall zone intermediate zone and core zone la icp ms zircon u pb dating shows that it was emplaced at ca 1 53 ga petrologically the zhangjiaping granites contain perthite and calcic amphibole but are lack of alkali mafic minerals geochemically all the zhangjiaping granitic rocks show an affinity to fractionated aluminous a type granites with enrichment in silicon and total alkali and depletion in mgo cao and p2o5 with negative eu sr and ti anomalies as well as high feot mgo tio2 mgo and ga al ratios and high hfses i e zr nb ce y concentrations such signatures along with their low ehf t values 12 5 to 4 0 of all the granites from the zhangjiaping pluton obviously indicate that the zhangjiaping aluminous a type granites were derived from partial melting of the crustal calc alkaline granitoids i e the granodiorites of the taihua complex in the study area and experienced fractional crystallization during magmatic evolution their generation would be related to the underplating of mafic magmas in an extensional regime integrated with regional data the zhangjiaping aluminous a type granites were the products of anorogenic magmatism likely corresponding to the breakup of the columbia supercontinent

shrimp and la icp ms zircon geochronology of the xiong er volcanic rocks implications for the paleo mesoproterozoic evolution of the southern margin of the north china craton the paleo mesoproterozoic xiong er volcanic rocks along the southern margin of the north china craton are lithologically and geochemically similar to those formed in subduction related continental margin volcanic arcs the volcanic rocks are primarily composed of basaltic andesites and andesites with minor dacites and dacitic rhyolites traditionally the xiong er volcanic rocks have been divided from lower to upper into the xushan jidanping and majiahe formations but the ages of volcanic rocks in these formations have not been well constrained which has hindered further understanding the tectonic significance of the xiong er volcanic belt at the southern margin of the north china craton shrimp and la icp ms u pb zircon analyses combined with cathodeluminescence cl images have enabled resolution of xenocrystic and magmatic zircons that can be directed toward determination of the ages of the xiong er volcanic rocks shrimp and la icp ms u pb analyses on magmatic zircons from two basaltic andesite samples one dacite sample and one rhyolite sample of the xushan formation known as the lowest sequence of the xiong er volcanic rocks indicate that the volcanic eruption of this formation occurred at 1 78 ga but most xenocrystic inherited zircons in these samples yielded 207pb 206pb ages ranging from 2 55 ga to 1 91 ga of three samples collected from the jidanping formation two rhyolite samples 05xe015 and 05xe100 yielded weighted mean 207pb 206pb ages of 1778 5 5 ma and 1751 14 ma respectively similar to the ages of the volcanic rocks in the xushan formation whereas one dacite sample 05xe066 gave a weighted mean 207 pb

discrete clay diagenesis in a very low permeable sequence constrained by an isotopic k ar and rb sr study multimethod analyses of several size fractions of clays were used to reconstruct the diagenetic history of the shallow buried claystones within the paris basin a systematic decrease occurred in k ar dates relative to the decrease in size of the clay fractions signifying higher amounts of newly formed clay material in the finer fractions we suggest that the authigenic clay minerals occurring in the fine fractions had an al montmorillonite composition by assuming that the isotopic k ar and rb sr dates obtained on a bentonite layer in the sedimentary sequence stand for pure authigenic clay minerals one may interpret all k ar dates as mixtures of one authigenic and two detrital end members the results imply that a period of low sea level favoured diagenetic smectite type clay formation about 10 15 million years after deposition of the sediments signatures of limited scale chemical and isotopic homogenisation mean that the rock volumes affected by the diagenetic modifications had to be quite limited the study of clay minerals extracted from some stylolites further suggests that any overpressure related to the origin of stylolites had no effect on clay authigenesis

petrogenesis of the igneous mucajai amg complex northern amazonian craton geochemical u pb geochronological and nd hf o isotopic constraints abstract the ca 1525 ma igneous mucajai anorthosite monzonite granite amg complex in northern brazil is a rare manifestation of mesoproterozoic intraplate magmatism in the northern amazonian craton the complex comprises a two phase rapakivi granite batholith with subordinate quartz fayalite monzonites and syenites and the closely associated repartimento anorthosite zircon u pb id tims geochronology reveals that the anorthosite 1526 2 ma monzonite 1526 2 ma and the main phase biotite hornblende granite 1527 2 ma of the complex intruded the paleoproterozoic 1 94 ga country rocks simultaneously at 1526 ma and that the more evolved biotite granite is marginally younger at 1519 2 ma intraplate magmatism in the mucajai region was relatively short lived and lasted 12 million years 1529 1517 ma at maximum the nd whole rock id tims e nd from 1 9 to 2 8 hf zircon lam icp ms e hf from 2 0 to 3 1 and o zircon sims 18 o from 6 1 to 7 0 isotopic compositions of the studied rocks are fairly uniform but still reveal a small degree of isotopic heterogeneity in the paleoproterozoic crust enclosing the complex the small isotopic differences observed in the two types of rapakivi granites biotite hornblende granite and biotite granite may result either from an isotopically heterogeneous lower crustal source or more likely from contamination of the granitic magma derived from a lower crustal source during prolonged residence at upper crustal levels

vapor absent melting at 10 kbar of a biotite and amphibole bearing tonalitic gneiss implications for the generation of a type granites vapor absent melting experiments on a biotite and amphibole bearing archean tonalitic gneiss agc150 at 10 kbar and 875 to 1050 c show that amphibole breaks down from 900 to 950 c producing garnet orthopyroxene and granitic melt biotite dehydration melting produces 10 wt melt up to 950 c via incongruent melting reactions that produce garnet orthopyroxene and titanomagnetite widespread biotite dehydration melting occurs between 950 and 975 c and produces orthopyroxene magnetite titanomagnetite and 20 wt fluorine rich melt up to 0 31 wt f minor f rich 2 7 wt biotite is present even at 1000 c our experiments show that under vapor absent conditions intrusion of hot mantle derived magmas into the lower crust is necessary to initiate widespread biotite dehydration melting in rocks with compositions like agc150 we propose that the high thermal stability of biotite in agc150 suggests that this rock is residual after a previous episode of partial dehydroxylation that left behind somewhat f enriched biotite we show that dehydration melting of such f enriched biotite produces f rich granitic liquids with compositions within the range of a type granite and leaves behind a granulitic residue consisting of orthopyroxene plagioclase quartz titanomagnetite and magnetite

diagenetic xenotime dating to constrain the initial depositional time of the yan liao rift abstract in the mesoproterozoic and neoproterozoic the north china craton ncc underwent long lived sedimentation with minor volcanism in the yan liao rift forming the mesoproterozoic changcheng jixian and qingbaikou groups in sequence the basal boundary age of the lower changcheng group indicates the initial time of the ncc as a stable cratonic block and can be used to make correlations among several representative basins worldwide recent studies have debated the initial depositional time of the changcheng system which is the key to understanding the proterozoic tectonic evolution of the ncc this study documented the xenotime and monazite from the lowermost changzhougou and chuanlinggou formations of the changcheng group the monazite grains from the chuanlinggou formation yielded u pb ages between 1810 ma and 2500 ma which are comparable to the detrital zircon u pb ages in the same formation indicating their detrital origin xenotime mostly occurs as overgrowths on the detrital zircon grains supporting a post depositional origin the low u content of the changzhougou formation s xenotimes indicates a hydrothermal origin its peak age of 1570 15 ma may indicate a hydrothermal event the xenotime from the chuanlinggou formation is characterized by a high u content and a ree pattern with a nonexistent or weak negative eu anomaly and a higher gd yb ratio implying a diagenetic origin these xenotime overgrowths yielded a wide age variation even on a single grain due to prolonged xenotime precipitation the oldest population of 1716 3 ma is interpreted as the minimum depositional age of the chuanlinggou formation the lowest boundary of the changchengian system must be older than 1716 ma this work demonstrates that diagenetic xenotime could better constrain the depositional time however extensive systematic research is required especially with higher spatial resolution and more diagenetic xenotime grains

permian peralkaline peraluminous and metaluminous a type granites in the panxi district sw china their relationship to the emeishan mantle plume abstract numerous permian granitic intrusions plutons in the panxi region sw china are spatially and temporally associated with the emeishan large igneous province elip granites from the 260 ma panzhihua and taihe plutons are peralkaline asi 0 76 1 12 na k al 0 78 1 2 and have high fe values 0 92 0 99 with negative eu anomalies eu eu 0 22 0 98 contrasted with the peralkaline plutons syenites from the 260 ma woshui pluton are metaluminous asi 0 84 1 0 na k al 0 91 1 01 and have moderately high fe 0 79 0 81 and positive eu anomalies eu eu 0 82 1 91 both the peralkaline and metaluminous rocks have mantle isotopic signatures end t 1 3 3 2 and were derived from the elip mantle plume source the peralkaline rocks were likely derived by fractional crystallization of the temporally spatially and chemically associated layered mafic intrusions which host giant magmatic fe ti v oxide deposits the metaluminous granitic rocks although spatially related to the peralkaline granites and layered gabbros were likely generated by partial melting of underplated mafic rocks granites from the 251 ma ailanghe pluton are peraluminous asi 0 97 1 29 and have high fe 0 82 0 91 values and negative eu anomalies eu eu 0 10 0 37 their crustal isotopic signature end t 5 7 to 6 7 indicates possible derivation from melting of yangtze block basement rocks the ailanghe pluton is contemporaneous with other a type granites in the region supporting the view that elip magmatism was sporadic for at least 9 million years the coexistence of three chemically exclusive a type granites in the elip suggests that a variety of a type granites can be generated in the same tectonic setting

reduced rapakivi type granites the tholeiite connection reduced rapakivi type granites are the most iron enriched and reduced i e least oxidized of the anorogenic granite association the low oxygen fugacity and chemical composition of these granites severely limit their sources in this paper we argue that reduced rapakivi type granites and their eruptive equivalents high potassium fayalite rhyolites are derived from mafic sources because tholeiitic magmas and their derivatives have the required low oxygen fugacity reduced rapakivi type granites are produced either by extreme differentiation of basaltic melts or by partial melting of underplated basalts and their differentiated equivalents they form in extensional environments where the asthenosphere is present at shallow depths we envision three stages in the origin of these rocks 1 tholeiitic melts are emplaced at the base of the crust 2 continued introduction of heat partially remelts these tholeiitic rocks and 3 the hot dry melts so produced migrate into the middle crust to produce rapakivi batholiths or erupt as rhyolites partial melting of felsic continental crust may accompany the intrusion of rapakivi type magmas thereby producing the other metaluminous and peraluminous granite compositions of the anorogenic suite

composition and evolution of the archaean crust in central henan china the upper archaean in central henan province is composed chiefly of rocks of the dengfeng complex and the taihua complex rocks of the dengfeng complex show essential features of a granite greenstone terrane they consist of greenstone belts and surrounding granitic and migmatite gneiss terranes when the late superposed structures are removed the archaean conspicuously exhibits alternating gneiss domes and synformal basins which are occupied by granitic rocks and greenstone belts respectively the succession of the greenstone belts is divided into three groups 1 1 the lower group consists of ultramafic complexes which are only locally exposed 2 2 the middle group which comprises most of the greenstone belts contains tholeiitic metabasalts and felsic volcanics which are bimodal in character 3 3 the upper sedimentary group consists of molasse and turbidite formations the metamorphic grade in the dengfeng complex is up to the medium pressure almandine amphibolite facies the mafic volcanics are characterized by flat to slightly enriched ree patterns that resemble the th2 type of condie in most other elements the rocks are also similar to the tholeiitic metabasalts in typical greenstone belts throughout the world however they possess certain unique characteristics as for the granitic rocks the early plutons tend to be na rich and are geochemically similar to tonalitic rocks while the later intrusives tend to be k rich the taihua complex is composed primarily of felsic gneisses and metamorphosed supracrustal rocks interbedded with mafic ultramafic rocks the metamorphic grade in the complex ranges from granulite to upper amphibolite facies geochemical characteristics of the rocks are also similar to those of other high grade terranes throughout the world in general an overall structural grain characterizes the taihua complex which constitutes an elongate fold belt with characteristic features of linear mobile belts that are frequently found in high grade terranes the extensive dengfeng granite greenstone terrane and the linear taihua high grade gneissic terrane are bounded by thrusts or contacts of unknown nature and together make up the late archaean crust in central henan they may have developed more or less contemporaneously on a pre existing sialic crust in this region controlled by shallow convection cells in the upper mantle their development may have occurred in two different tectonic settings the rift type and the subsiding type setting respectively

petrogenesis of late paleozoic diorites and a type granites in the central eastern tianshan nw china response to post collisional extension triggered by slab breakoff abstract geological evolution of the eastern tianshan in the late paleozoic is significant to understanding the multiple accretions and final formation of the southern central asian orogenic belt here we report new whole rock geochemical and sr nd isotopic data and in situ zircon u pb ages for three typical a type granitic intrusions syenogranite k feldspar granite and monzonitic granite and one dioritic pluton in the central eastern tianshan nw china in order to constrain their petrogenesis tectonic setting and geodynamic evolution zircon la icp ms u pb ages indicate that the syenogranites and k feldspar granites were both formed during a short time interval in the early permian 284 286 ma whereas the diorites were crystallized at 293 6 ma the monzonitic granites were generated at 307 3 ma earlier than the other two granitic intrusions the three kinds of granites syenogranite k feldspar granite and monzonitic granite have high hfse hree and y nb ratios 1 68 2 84 and remarkably low sr and ba contents with the absence of aluminium rich minerals and show metaluminous to weakly peraluminous natures indicating geochemical characteristics of aluminous a2 type granites these granites show negative end t 3 32 to 4 40 values and mesoproterozoic two stage nd model ages tdm2 1 36 to 1 43 ga which suggest a relatively old crustal source for these rocks the sr nd isotopic compositions imply that the mesoproterozoic metamorphic basement of the central tianshan block can be considered as a potential magmatic source for these aluminous a2 type granites in addition they have geochemical characteristics similar to the experimental melt that was derived from partial melting of crustal metaigneous rocks at the depth of middle to lower crust levels therefore the granites were most likely produced by partial melting of metaigneous rocks in a post collisional extensional setting the diorites have low sio2 and high mgo with uniform 87sr 86sr i values 0 7055 negative end t values 4 11 to 4 59 and mesoproterozoic two stage nd model ages tdm2 1 41 to 1 45 ga they have higher ti zr 42 8 46 7 and ti y 280 359 ratios than continental crust and show negative nb ta ti anomalies which can be explained by partial melting of a relatively fertile mantle modified by old crustal materials of the central tianshan block the formation of the dioritic pluton was induced by an upwelling asthenosphere in an extensional geodynamic setting based on the zircon saturation temperatures of these a type granites and the distribution of late carboniferous to early permian mantle derived intrusions a slab breakoff model is suggested to interpret the formation of the studied a type granites and diorites we infer that the slab breakoff was initiated in the late carboniferous and ended in the early permian

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

u pb zircon geochemical and sr nd hf isotopic constraints on age and origin of jurassic i and a type granites from central guangdong se china a major igneous event in response to foundering of a subducted flat slab abstract the mesozoic geology of se china is characterized by widespread jurassic to cretaceous igneous rocks consisting predominantly of granites and rhyolites and subordinate mafic lithologies however the tectonic regime responsible for the inland jurassic granites remains controversial we report here u pb zircon ages geochemical and sr nd hf isotopic data for the nankunshan alkaline granite and the fogang granitic batholith in central guangdong mineralogical and geochemical features suggest that the fogang and nankunshan rocks are i and aluminous a type granites respectively shrimp u pb zircon analyses yield consistent ages ranging from 159 3 ma to 165 2 ma for four samples from the fogang batholith and an age of 158 5 ma for the nankunshan granite the fogang granites having i sr 0 7098 0 7136 e nd t 4 3 to 12 2 and e hf t 11 5 to 3 1 for the magmatic zircons were derived from paleoproterozoic mafic intermediate igneous protolith with minor addition of mantle derived magmas the nankunshan rocks have relatively lower i sr 0 706 0 708 higher e nd t 0 3 to 2 4 and e hf t 5 7 to 1 1 for the magmatic zircons and some oib like trace element ratios they were likely generated through extensive fractional crystallization of mantle derived alkaline parental magma associated with crustal assimilation these 160 ma i and a type granites in central guangdong were emplaced coeval with the widespread 165 155 ma i and a type granites and syenites following the initiation of intraplate basaltic and or bimodal igneous magmatism at 180 170 ma in the adjacent regions we interpret these jurassic igneous rocks as anorogenic magmatism formed during a major igneous event in response to foundering of an early mesozoic subducted flat slab beneath se china continent

paleo position of the north china craton within the supercontinent columbia constraints from new paleomagnetic results abstract several new paleomagnetic and geological studies focused on the reconstruction of the north china craton ncc within the paleo mesoproterozoic columbia supercontinent in spite of these new data there are still widely divergent opinions regarding supercontinental reconstructions in addition to qualitative correlations of orogenic belts rift basin ages stratigraphy and distribution of igneous provinces paleomagnetic data can provide key constraints on the positioning of individual cratons on the globe in this paper we report a detailed paleomagnetic study on the coeval 1780 ma mafic dyke swarm and xiong er volcanic province which extended for more than one thousand kilometers in the central ncc rock magnetic studies including thermomagnetic curves hysteresis loops and the progressive acquisition of isothermal remanence conducted in selected samples indicate that the dominant magnetic carriers are psd magnetite stepwise thermal demagnetization isolated higher temperature components directed to nne ssw with shallow inclinations from 37 sampling sites 16 sites in yinshan area 13 sites in taihang area and 8 sites in xiaoshan area a baked contact test conducted on two yinshan dykes intruded by a younger dyke demonstrates the magnetization in the yinshan dykes pre dates 1320 ma the existence of dual polarity magnetizations in both taihang and xiong er areas support our contention that the chrm was acquired during the cooling of the magma the primary origin of the chrm is also supported by a positive fold test on the xiong er data and a coherent regional test between the results from the taihang and xiong er areas two different site mean directions were compiled from these new results along with previous publications the first direction from the taihang and xiong er areas yields declination d inclination i 12 4 3 7 20 5 95 4 3 n 57 sites the second from the yinshan area is at d 36 7 i 12 4 86 8 95 2 7 n 32 sites we argue that the difference is due to mesozoic and or cenozoic vertical axis rotation of the taihang and xiong er areas with respect to the fixed yinshan ordos basin the corresponding paleopoles for the yinshan dykes falls at 245 2 e 35 5 n a95 2 4 a comparison between the ncc laurentia siberia and baltica is consistent with possible links between these four blocks in a perhaps even larger continent the proximity of australia and india to the ncc is also evaluated

a type medina batholith and post collisional anatexis in the aracuai orogen se brazil abstract the medina batholith and its host granitic migmatites record intriguing plutonic processes in the northern aracuai orogen se brazil this orogen shows a long lasting 630 480 ma succession of granite production events from the earliest pre collisional plutons to the latest post collisional intrusions the medina batholith includes granite intrusions ascribed to the post collisional stage they show high alkali and halogen contents low cao at sio2 71 na2o k2o 7 to 9 cao 1 6 and high feot feot mgo ratios 0 78 to 0 92 the medina granites are metaluminous to weakly peraluminous with asi molecular ratio al ca 1 67p na k values of 1 76 to 2 07 and have high concentrations of high field strength elements zr nb ce y 700 ppm as well as high ga al ratios accordingly the medina intrusions are typical ferroan a type granites u pb ages from zircon 501 2 ma and monazite 497 2 ma constrain the emplacement timing of the medina batholith surprisingly all monazite ages from host rocks also cluster around 500 ma despite their nature and distance from the batholith suggesting that they would have shared a same thermal process the studied host rocks are granitic migmatites varying from patch metatexite to nebulitic diatexite comprising paleosome of foliated sillimanite garnet biotite metagranite to gneiss and non foliated garnet cordierite neosome poor to free of biotite a metatexite r14 located relatively far from the medina batholith and a diatexite m26 found at the batholith contact were sampled for detailed studies the paleosome of foliated metagranite r14a only shows zircon grains with igneous features and th u ratio from 1 64 to 0 26 although the spreading of zircon spots the main cluster yields a concordia age at 556 6 ma constraining the protolith magmatic crystallization a minor cluster furnishes a concordia age at 499 7 ma in agreement with the u pb monazite age at 501 2 ma extracted from the same metatexite sample the non foliated garnet cordierite neosome r14b shows both igneous and metamorphic zircon domains with th u ratios ranging from 1 47 to 0 00 again the u pb spots cluster at two distinct concordia ages 562 3 ma and 499 3 ma the youngest of them fitting with the monazite age 495 3 ma constrains melt crystallization while the oldest age suggests paleosome inheritance the nebulitic diatexite m26 shows monazite 497 2 ma and zircon th u 1 7 to 0 0 concordia ages at 564 2 ma and 507 3 ma populations similar to the metatexite neosome also with the youngest ages bracketing the melt crystallization process around 500 ma accordingly all those ages at around 500 ma disclose a partial melting episode coeval with the medina batholith emplacement phase equilibrium modeling on a garnet cordierite neosome furnished p t conditions of 750 840 c at 2 4 3 5 kbar for that post collisional anatexis evidence for such a late thermal event are common in the aracuai orogen even far from the post collisional batholiths thus a possible major heat source can be envisaged like a mantle plume triggering crustal anatexis and regional fluid circulation during the gravitational collapse of the aracuai orogen

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method

water rock interaction processes in the triassic sandstone and the granitic basement of the rhine graben geochemical investigation of a geothermal reservoir saline fluids have been collected in the rhine graben over the last two decades both from the triassic sandstone aquifer and the granitic basement down to a depth of 3500m their salinities and location are compared in order to distinguish the respective influences of temperature and host rock mineralogy in the water rock interaction processes the comparison shows that sulphates in the sedimentary formations were dissolved by the fluids which also led to br enrichment mica dissolution has strongly increased the rb and cs contents which then provide an indication of the degree of water rock interaction the sr isotopic ratios are used to compare the fluids with the granite minerals two relationships are revealed for the fluids in the sandstone and the granite one related to widespread mica dissolution which could have affected both the buntsandstein and the granite and the other to subsequent plagioclase dissolution which is observed only in the granite computations showed that 12 5g of mica and 1 658 of plagioclase per liter of fluid have been dissolved the nature of these two relationships suggests two different evolutions for the fluids and the individualization of the two reservoirs during the graben s history the cation concentrations are mainly controlled by temperature and are independent of the type of host rock equilibrium with the rock mainly caused ca and k concentration variations which has induced clear ca k and ca 18o k 18o correlations geothermometric computations indicate that with increasing depth the cations the silica and the 18o so4 geothermometers evolve towards a value close to 230 c this demonstrates the existence of a hot reservoir in the granite of the graben at a depth estimated at 4 5 5 km

combined u pb hafnium and oxygen isotope analysis of zircons from meta igneous rocks in the southern north china craton reveal multiple events in the late mesoarchean early neoarchean abstract we report the results of a comprehensive isotopic investigation of zircons from metamorphosed tonalites and amphibolites from lushan in the far south of the trans north china orogen of the north china craton that reveal the oldest rocks yet dated from the orogen a detailed investigation of the internal structures of zircons combined with targeted sims u pb and oxygen isotope analyses and icp ms hf zircon analyses has resulted in the discovery of previously unknown geological events in the north china craton magmatic zircon from the tonalite and amphibolite was found to be essentially coeval with a small spread of ages from 2829 18 ma to 2832 11 ma in the tonalites and from 2838 35 ma to 2845 23 ma in amphibolite hafnium isotope data suggest derivation of both tonalite and amphibolite from a depleted mantle source with minor crustal contamination oxygen isotope ratios in zircons range from 5 1 to 7 3 vsmow with high 18 o results recorded only from metamorphic domains the earliest stage of metamorphic zircon growth present in all four samples analyzed in this study occurred between 2772 17 22 in the tonalites and 2776 20 and 2792 12 ma in the amphibolites a younger metamorphic event at 2638 61 in the tonalite and between 2671 13 and 2651 13 ma in the amphibolites is recognized in zircons from three of the four samples investigated this is the first time that metamorphic events at either of these times have been recorded in the north china craton importantly there is no evidence for the 1 8 ga high grade metamorphic event that is manifest in the northern and central parts of the trans north china orogen

further evidence for 1 85 ga metamorphism in the central zone of the north china craton shrimp u pb dating of zircon from metamorphic rocks in the lushan area henan province abstract this paper reports shrimp zircon u pb dating of precambrian supracrustal and granitic rocks from the lushan area henan province in the southern portion of the central zone also referred to as the trans north china orogen of the north china craton a graphite garnet sillimanite gneiss sample tw0006 1 of the shangtaihua group gives a range of inherited zircon ages from 2 73 to 2 26 ga and a metamorphic zircon age of 1 84 0 07 ga a garnet bearing gneissic granitoid sample twj358 1 which is considered to intrude the shangtaihua group gives a magmatic zircon age of 2 14 0 02 ga and a metamorphic zircon age of 1 87 ga the metamorphic zircon ages of 1 87 1 84 ga obtained in this study indicate that an important tectonothermal event occurred at the end of the paleoproterozoic in the lushan area this supports the southern continuation of a central zone in the north china craton that workers have recently considered to result from continent continent collision it is also evident that the shangtaihua group was formed during the paleoproterozoic between 2 26 and 2 14 ga and not during the archean as previously considered