highly fractionated i type granites in ne china ii isotopic geochemistry and implications for crustal growth in the phanerozoic ne china is the easternmost part of the central asian orogenic belt caob the area is distinguished by widespread occurrence of phanerozoic granitic rocks in the companion paper part i we established the jurassic ages 184 137 ma for three granitic plutons xinhuatun lamashan and yiershi we also used geochemical data to argue that these rocks are highly fractionated i type granites in this paper we present sr nd o isotope data of the three plutons and 32 additional samples to delineate the nature of their source to determine the proportion of mantle to crustal components in the generation of the voluminous granitoids and to discuss crustal growth in the phanerozoic despite their difference in emplacement age sr nd isotopic analyses reveal that these jurassic granites have common isotopic characteristics they all have low initial 87 sr 86 sr ratios 0 7045f0 0015 positive end t values 1 3 to 2 8 and young sm nd model ages 720 840 ma these characteristics are indicative of juvenile nature for these granites other late paleozoic to mesozoic granites in this region also show the same features sr nd and oxygen isotopic data suggest that the magmatic evolution of the granites can be explained in terms of two stage processes 1 formation of parental magmas by melting of a relatively juvenile crust which is probably a mixed lithology formed by pre existing lower crust intruded or underplated by mantle derived basaltic magma and 2 extensive magmatic differentiation of the parental magmas in a slow cooling environment the widespread distribution of juvenile granitoids in ne china indicates a massive transfer of mantle material to the crust in a post orogenic tectonic setting several recent studies have documented that juvenile granitoids of paleozoic to mesozoic ages are ubiquitous in the central asian orogenic belt hence suggesting a significant growth of the continental crust in the phanerozoic d 2003 elsevier science b v all rights reserved

dehydration melting of amphibolite at 10 kbar the effects of temperature and time we have simulated the dehydration melting of a natural low k calcic amphibolite 67 4 hornblende 32 5 anorthite in piston cylinder experiments at 10 kbar and 750 1000 c for 1 9 days the solidus temperature is lower than 750 c garnet appears at 850 c the overall reaction is hb p l cpx al hb ca hb ga opx three stages of reaction are 1 melting dominated by the growth of clinopyroxene and garnet with little change in composition of liquid or garnet 2 a reversal of this reaction between 875 c and 900 c with decreases in the amounts of liquid and garnet and 3 a large increase in liquid along with the loss of hornblende and decrease of plagioclase while clinopyroxene and garnet increase garnet is enriched in pyrope and zoned from fe cores to mg edges range 3 mol pyrope liquid composition is enriched first in an to 950 c and then in ab the liquids are more calcic and aluminous than natural tonalites which is attributed to the plagioclase composition an90 the formation of peraluminous liquid from the metaluminous amphibolite is caused by anorthite not h2o saturated conditions the results are consistent with an amphibolite phase diagram with relatively high solidus temperatures in the garnet absent field 900 1000 c but with a solidus backbend at 7 9 kbar coincident with the garnet in boundary hornblende breakdown due to garnet formation in a closed system must make h2o available for h2o undersaturated melting right down to the h2o saturated solidus below 700 c which defines a large low temperature pt area where hydrous granitoid melts can be generated with residual garnet and hornblende

generation of the early cenozoic adakitic volcanism by partial melting of mafic lower crust eastern turkey implications for crustal thickening to delamination abstract early cenozoic 48 50 ma adakitic volcanic rocks from the eastern pontides ne turkey consist of calc alkaline and high k calc alkaline andesite and dacite with sio2 contents ranging from 56 01 to 65 44 wt this is the first time that early eocene volcanism and adakites have been reported from the region the rocks are composed of plagioclase amphibole quartz and mg rich biotite they have high and low mg values ranging from 55 to 62 and 13 to 42 respectively high mg rocks have higher ni and co contents than low mg samples the rocks exhibit enrichments in large ion lithophile elements including the light rare earth elements depletions in nb ta and ti and have high la yb and sr y ratios their relative high isr 0 70474 0 70640 and low end 50 ma values 2 3 to 0 8 are inconsistent with an origin as partial melts of a subducted oceanic slab combined major and trace element and sr nd isotope data suggest that the adakitic magmas are related to the unique tectonic setting of this region where a transition from a collision to an extension stage has created thickening and delamination of the pontide mafic lower crust at 50 ma the high mg adakitic magmas resulted from partial melting of the delaminated eclogitic mafic lower crust that sank into the relatively hot subcrustal mantle and its subsequent interaction with the mantle peridotite during upward transport leaving garnet as the residual phase elevates the mgo content and mg of the magmas whereas low mg magmas formed by the melting of newly exposed lower crustal rocks caused by asthenospheric upwelling which supplies heat flux to the lower crust the data also suggest that the mafic lower continental crust beneath the region was thickened between the late cretaceous and the late paleocene and delaminated during late paleocene to early eocene time which coincides with the initial stage of crustal thinning caused by crustal extensional events in the eastern pontides and rules out the possibility of an extensional regime before 50 ma in the region during the late mesozoic to early cenozoic

melting of continental crust during subduction initiation a case study from the chaidanuo peraluminous granite in the north qilian suture zone the chaidanuo granite batholith is a similar to 500 km 2 homogeneous high level intrusion in the north qilian oceanic suture zone three types of enclaves have been recognized including 1 supracrustal xenoliths 2 biotite gneiss that represents restite of the upper crustal melting and 3 coeval mantle derived mafic magmatic enclave mme the batholith consists dominantly of peraluminous biotite monzogranite with sio2 69 73 wt and a cnk 1 05 1 28 and shows geochemical affinity with the upper continental crust e g enrichment of large ion lithophile elements liles k rb u th pb depletion of high field strength elements hfses nb ta ti p eu and sr and enriched sr sr 87 sr 86 i 0 731 to 0 744 nd epsilon nd t 6 0 to 7 1 isotopes zircon u pb dating indicates that this batholith formed at 516 505 ma coeval with the mmes 510 ma which represent the early stage of seafloor subduction in the north qilian suture zone the hf isotopic composition of the mme epsilon hf t 6 0 to 2 1 is more depleted than that of the host granite epsilon hf t 14 8 to 4 7 suggesting mixing of mantle and crust derived melts the inherited zircon cores in the host granite yield an age peak at similar to 750 ma with a few detritals of 591 1683 ma zircons from a biotite gneiss enclave yield a protolith age of 744 ma and a metamorphic melting event at 503 ma sr nd isotopic modeling reveals that the batholith was generated by melting of neoproterozoic granitic rocks with the addition of 10 18 mantle derived magma therefore peraluminous granite can be produced by melting of crustal materials heated by mantle derived magmas during the early stage of subduction initiation at the site of a prior passive continental margin c 2014 elsevier ltd all rights reserved

early cretaceous adakitic granites in the northern dabie complex central china implications for partial melting and delamination of thickened lower crust abstract to date few adakitic rocks have been reported in direct association with contemporary intra continental extensional structures which has cast doubt on genetic models involving partial melting of the lower crust this study presents early cretaceous 143 129 ma new sensitive high resolution ion microprobe shrimp zircon u pb ages adakitic granites which are directly associated with a contemporary metamorphic core complex i e the northern dabie complex in the dabie area these granites exhibit relatively high sr contents negligible to positive eu and sr anomalies high la yb and sr y ratios but very low yb and y contents similar to subducted oceanic crust derived adakites they are also characterized however by very low mgo or mg and ni values and nd sr isotope compositions end t 14 6 to 19 4 and 87sr 86sr i 0 7067 0 7087 similar to triassic continent derived eclogites subducted in the dabie sulu orogen additionally late granitic dikes in the adakitic intrusions exhibit low sr contents clearly negative eu and sr anomalies low la yb and sr y ratios but relatively high yb and y contents similar to 118 105 ma granites in the northern dabie complex based on composition and geochronology data of neoproterozoic amphibolites and orthogneisses triassic high to ultra high pressure metamorphic rocks and early cretaceous mafic ultramafic intrusive rocks and the constraints provided by experimental melt data for tonalites metabasaltic rocks and eclogites we suggest that the adakitic granites were most probably generated by partial melting of thickened amphibole or rutile bearing eclogitic lower crust as a consequence of triassic middle jurassic subduction and thrusting the late dikes probably originated from plagioclase bearing intermediate granulites moreover we suggest that late mesozoic delamination or foundering of thickened eclogitic lower crust is also a more plausible mechanism for the petrogenesis of early cretaceous mafic ultramafic intrusive rocks in the dabie area and probably involved partial melting of a mixed source comprising eclogitic lower crust that had delaminated or foundered into upper lithospheric or asthenospheric mantle peridotite asthenospheric upwelling in response to post collisional delamination of lithospheric mantle was likely to have provided the heat source for the cretaceous magmatism

peraluminous i type granites abstract s type granites always contain more al than the amounts of na ca and k in the rock required to form feldspars primarily owing to their derivation from source components that had previously been weathered those rocks are therefore always saturated in al or peraluminous many i type granites are also peraluminous despite i type source rocks typically not being saturated in al it has previously been suggested that this may result from the fractional crystallisation of amphibole however data from compositionally zoned high temperature plutons in the lachlan fold belt show that it is difficult to generate large quantities of peraluminous melt by removal of amphibole most of the i type granites 95 in the lachlan fold belt formed at lower temperatures and almost half of those rocks for which bulk chemical compositions are available are peraluminous among these granites there are 98 separate suites for which there are chemical data for two or more samples with 47 suites that include both metaluminous more mafic and peraluminous more felsic compositions the origin of those peraluminous compositions is fundamental to any understanding of i type granite petrogenesis in this region compositional variations caused by the assimilation or by partial melting of supracrustal rocks are very small as the isotopic variations within these rocks are dominantly between different suites not within suites the partial melting of more mafic source rocks rather than the fractional crystallisation of more mafic magmas is favoured for the origin of these rocks partial melting is the most likely process involved in the petrogenesis of felsic granites where broadly granodioritic monzogranitic batholiths are associated with lesser amounts of tonalite and very minor amounts of mafic rock experimental studies have shown that the melts generated by the partial melting of basaltic to andesitic rocks under crustal conditions are mostly peraluminous during the dehydrational melting of i type granite source rocks at pressures below the garnet stability field biotite and amphibole melt incongruently to yield pyroxenes the excess al is incorporated into the felsic liquid resulting in the generation of peraluminous melts in this instance the excess al in felsic i type granites is a function of the melting process and unrelated to the bulk composition of the source the observed gradation from peraluminous felsic granites to metaluminous compositions in less felsic rocks in largely isotopically closed systems could happen in two ways at higher temperatures of partial melting ca and other components of clinopyroxene could dissolve in the melt with the melt eventually becoming metaluminous alternatively minerals residual from partial melting dominantly pyroxenes and plagioclase could be incorporated in suspension in the melt so that the resulting bulk magma is metaluminous examples of these two extremes and of intermediate cases are well developed among the granites of southeastern australia

geochemistry and zircon u pb ages of adakitic rocks from the dulan area of the north qaidam uhp terrane north tibet constraints on the timing and nature of regional tectonothermal events associated with collisional orogeny abstract geochemistry and u pb ages of leucosomes and tonalites in a high pressure granulite unit of the dulan area have been determined to constrain the tectonothermal evolution related to collision and thickening of lower crust in the north qaidam mountains nqd leucosomes and tonalites show a marked chemical resemblance to adakites 1 high la yb and sr y and low y and hree 2 high al 2 o 3 and low mg values with obvious positive eu anomalies and 3 slightly positive e nd t values zircon u pb analysis of leucosomes and tonalites yielded 206 pb 238 u ages of 428 437 ma and 436 437 ma respectively which constrain the emplacement ages of the adakitic rocks petrological geochronological and geochemical characters indicate that the adakitic rocks may have been derived from partial melting of a thickened mafic lower crust 50 km suggesting a dominating source regime of synchronous high pressure mafic granulites contemporary magmatism in other units of the nqd shows evidence of a widespread tectonothermal event during early silurian 420 450 ma that includes metamorphism magmatism and anatexis related to collision and thickening of lower crust

syn collisional adakitic granodiorites formed by fractional crystallization insights from their enclosed mafic magmatic enclaves mmes in the qumushan pluton north qilian orogen at the northern margin of the tibetan plateau the qumushan qms syn collisional granodiorite which is located in the eastern section of the north qilian orogen at the northern margin of the greater tibetan plateau has typical adakitic characteristics and also contains abundant mafic magmatic enclaves mmes this recognition offers an unprecedented insight into the petrogenesis of both the adakitic host granodiorite and the enclosed mmes the mmes and their host granodiorites share many characteristics in common including identical crystallization age similar to 430 ma same mineralogy similar mineral chemistry and whole rock isotopic compositions indicating their genetic link the mmes are most consistent with being of cumulate origin formed at earlier stages of the same magmatic system that produced the qms adakitic granodiorite subsequent replenishment of adakitic magmas could have disturbed the cumulate piles as mmes dispersed in the adakitic granodiorite host during emplacement the geochemical data and petrogenetic modeling of trace elements suggest that the qms adakitic host granodiorite is most consistent with fractional crystallization dominated by the mineral assemblage of the mmes the parental magma for the qms granodiorite is best explained as resulting from partial melting of the ocean crust together with recycled terrigenous sediments during continental collision which may have also experienced interaction with mantle peridotite during ascent c 2016 elsevier b v all rights reserved

a hybrid origin for lachlan s type granites the murrumbidgee batholith example the murrumbidgee batholith is typical of other composite s type granite batholiths in the lachlan fold belt lfb of southeastern australia consisting of discrete peraluminous cogenetic granite suites however it contains a unique suite murrumbucka at its southern extremity that has chemical and petrological features transitional with metaluminous i type granites previous detailed structural and metamorphic studies have shown that the batholith is tilted northward exposing subvolcanic plutons in the north and root zones in the south originally located at depths of not vert similar10 km a transition from mafic foliated sheet like granites in the south to felsic generally less foliated homogeneous granites in the north is consistent with magma ascent via subvertical structurally controlled sheets to emplacement in an overlying magma chamber in the inferred root zones the murrumbucka suite hosts migmatitic metasedimentary and gabbroic rocks both of which have transitional contacts and show evidence for mafic felsic interaction with the murrumbucka suite the migmatites extend southward to become part of the high t low p cooma metamorphic complex which contains a core of heterogeneous remobilised diatexitic granite cooma suite lenses of which also occur throughout the southern deeper parts of the murrumbidgee batholith the composition of the most mafic rocks from the batholith lies on a chemical tie line between cooma suite granites and the gabbros for almost all elements this chemical coincidence is interpreted to reflect derivation of parental murrumbidgee s type granite magmas by bulk mixing between a felsic crustal and mafic mantle derived component consistent with sr and nd isotopic results and field observations based on the tie lines and simple numerical modelling the murrumbucka suite is estimated to be a 50 50 mix of mantle and crustal derived magmas whereas the more peraluminous and widespread clear range suite which is very typical of lachlan s type granites is modelled as a 40 60 mix chemical variation trends diverge from the mixing lines and suggest that much of the fractionation process operated after mixing mixing occurred within sheets during the ascent of the magmas whereas fractionation was the dominant process generating chemical diversity during emplacement in the magma chamber generation of the s type granites probably occurred in an extensional tectonic environment

petrogenesis of the early paleozoic low mg and high mg adakitic rocks in the north qilian orogenic belt nw china implications for transition from crustal thickening to extension thinning abstract the petrogenesis and geodynamic implications of the early paleozoic adakitic rocks in the north qilian orogenic belt remain topics of debate in this study petrology zircon u pb age and lu hf isotopic composition whole rock geochemistry and sr nd isotopes were carried on both low mg and high mg adakitic rocks from the north qilian orogenic belt these results will provide significant constraints on the evolution of the north qilian orogenic belt in general the low mg adakitic granites mainly consist of plagioclase 50 55 alkali feldspar 20 25 quartz 20 25 and biotite 5 in contrast the high mg adakitic granodiorites are generally composed of alkali feldspar 30 35 plagioclase 30 45 quartz 20 25 and amphibole 10 the low mg adakitic granitoids 446 457 ma are characterized by high sio2 69 72 wt low mg 39 42 and low cr and ni contents however the high mg adakitic granitoids 430 431 ma have relatively lower sio2 65 66 wt higher mg 50 59 and higher cr and ni contents the low mg adakitic rocks have high initial 87sr 86sr ratios 0 7068 0 7080 negative end t 2 3 to 3 3 and ehf t values 6 8 to 1 9 with old zircon hf model ages 1 2 1 6 ga however the high mg adakitic rocks show lower initial 87sr 86sr ratios 0 7044 0 7066 higher end t 1 8 to 3 0 and positive ehf t values 2 3 7 7 with younger zircon hf model ages 0 9 1 1 ga these results suggest that the low mg rocks were generated by partial melting of thickened crust whereas the high mg rocks were derived from anatexis of delaminated lower crust which subsequently interacted with mantle magma upon ascent the data obtained in this study provide significant information about the geological and tectonic processes after the closure of the qilian ocean the continent continent collision and thickening probably occurred during 440 467 ma with formation of low mg adakitic rocks and the transition of the tectonic regime from compression to extension probably occurred at 430 ma with formation of high mg adakitic rocks

collision driven slab breakoff magmatism in northwestern anatolia turkey abstract postcollisional eocene magmatism in northwestern anatolia produced two e w trending linear belts of plutons along and north of the izmir ankara erzincan suture zone iaesz whose geochemical features and age relations support a slab breakoff model for their petrogenetic oinevolution the suture zone granitoids szgs in the southern belt have ages around 54 48 ma are intrusive into blueschist rocks of the iaesz and are composed of diorite quartz diorite granodiorite and syenite the marmara granitoids mgs in the northern belt are slightly younger 48 35 ma intrusive into the paleozoic mesozoic crystalline basement rocks of the sakarya continent and composed of monzogranite granite and granodiorite both szgs and mgs have moderately to highly evolved medium to high k calc alkaline compositions and are predominantly metaluminous i type granitoids nd sr isotope systematics documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepacka

are strongly peraluminous magmas derived from pelitic sedimentary sources igneous compositions that require the presence of a phase more aluminous than biotite are here designated as strongly peraluminous ps such compositions are common but subordinate in felsic rocks and exceedingly rare in more mafic rocks the term s type has been applied to many igneous rocks based largely or even solely upon their ps compositions neglecting or placing less emphasis on other criteria for sedimentary parentage in chappell and white s i and s type classification system the s type designation was in fact initially restricted to granites which were thought to have been derived from a chemically mature strongly weathered pelitic shaly source application of chappell s and white s criteria together with others based upon isotopic and phase equilibria considerations leads to the following conclusions about ps igneous rocks worldwide 1 very few ps rocks are derived entirely from pelitic sources 2 a majority are derived largely from intermediate to felsic crustal sources including

multi element geochemical modelling of crust mantle interactions during late archaean crustal growth the closepet granite south india the closepet granite in the dharwar craton of south india is a large late archaean magmatic body its composition can be explained as a result of interactions between mantle derived melts and pre existing continental crust ttg gneisses using geochemical modelling based upon major and trace element compositions the following petrogenetic model is proposed for the formation of the batholith i an enriched mantle garnet and amphibole bearing lherzolite melts to produce a basaltic liquid ii the basaltic liquid undergoes limited fractional crystallization of biotite and amphibole f 0 9 iii the differentiated liquid rises into the old continental crust and induces water saturated anatexis of the ttg gneisses k feldspar qtz plagioclase melt iv both mantle derived and anatectic liquid mix and give rise to magma compositions ranging from quartz monzonite to granite the origin of the mantle enrichment is discussed in the context of the regional geological setting the most likely possibility seems to be metasomatism by slab melts the metasomatism of a mantle wedge by slab melts this suggests a two stage evolution for the dharwar craton during the late archaean i a subduction related event with the formation of ttgs sanukitoids and with associated mantle metasomatism and ii re melting of the metasomatized mantle this evolutionary history implies that at least some of the k rich late archaean granites are juvenile rather than products of intracrustal reworking as frequently assumed

cenozoic thrust emplacement of a devonian batholith northeastern brooks range involvement of crystalline rocks in a foreland fold and thrust belt involvement of crystalline rocks in thrusting near the foreland basin of a fold and thrust belt is relatively uncommon in the northeastern brooks range the devonian okpilak batholith was thrust northward and structurally elevated above adjacent foreland basin deposits during cenozoic fold and thrust deformation the batholith may have acted initially as a regional structural buttress but a drop in the basal detachment surface to greater depth south of the batholith resulted in northward transport of the batholith shortening within the batholith was accommodated by 1 the development of discrete thrust slices bounded by ductile shear zones 2 simple shear and development of penetrative mesoscopic and microscopic fabrics throughout the batholith or both the mississippian kayak shale a regional detachment horizon at the base of the overlying cover sequence is depositionally thin or absent adjacent to the batholith thus most of the cover sequence remained structurally coupled to the batholith during thrusting and was shortened by the development of penetrative structures

paleozoic magmatism and porphyry cu mineralization in an evolving tectonic setting in the north qilian orogenic belt nw china abstract the nww striking north qilian orogenic belt records the paleozoic accretion collision processes in nw china and hosts paleozoic cu pb zn mineralization that was temporally and spatially related to the closure of the paleo qilian qinling ocean the wangdian cu deposit is located in the eastern part of the north qilian orogenic belt nw china copper mineralization is spatially associated with an altered early paleozoic porphyritic granodiorite which intruded tonalites and volcaniclastic rocks alteration zones surrounding the mineralization progress outward from a potassic to a feldspar destructive phyllic assemblage mineralization consists mainly of quartz sulfide stockworks and disseminated sulfides with ore minerals chalcopyrite pyrite molybdenite and minor galena and sphalerite gangue minerals include quartz orthoclase biotite sericite and k feldspar zircon la icpms u pb dating of the ore bearing porphyritic granodiorite yielded a mean 206pb 238u age of 444 6 7 8 ma with a group of inherited zircons yielding a mean u pb age of 485 12 ma consistent with the emplacement age 485 3 6 2 ma of the barren precursor tonalite rhenium and osmium analyses of molybdenite grains returned model ages of 442 9 6 8 ma and 443 3 6 2 ma indicating mineralization was coeval with the emplacement of the host porphyritic granodiorite rhenium concentrations in molybdenite 208 9 213 2 ppm suggest a mantle re source the tonalities are medium k calc alkaline they are characterized by enrichment of light rare earth elements lrees and large ion lithophile elements liles depletion of heavy rare earth elements hrees and high field strength elements hfses and minor negative eu anomalies they have ehf t values in the range of 3 6 to 11 1 with two stage hf model ages of 0 67 1 13 ga suggesting that the ca 485 ma barren tonalites were products of arc magmatism incorporating melts from the mantle wedge and the lithosphere in contrast the 40 m y younger ore bearing porphyritic granodiorite is sub alkaline and peraluminous they are enriched in lrees and liles depleted in hfses and show weak negative eu anomalies they display ehf t values of captured or inherited zircons in the range of 8 5 to 10 0 and younger two stage hf model ages of 0 78 ga and 0 86 ga similar to those of ca 485 ma tonalite the ca 445 ma zircons have ehf t values of 2 1 to 9 9 with two stage hf model ages of 0 75 1 27 ga moreover they have relatively high oxygen fugacity than that of the precursor barren tonalite the ca 445 ma magmas at wangdian thus formed in a subduction setting and incorporated melts from the subduction modified lithosphere that had previously been enriched by additions of chalcophile and siderophile element rich materials by the earlier magmatism and metasomatism during the paleo qilian qinling ocean subduction event

mineral leucosome trace element partitioning in a peraluminous migmatite a laser ablation icp ms study abstract low pressure peraluminous migmatites from the pena negra complex central spain stayed partially molten for 5 10 ma until the latest hercynian deformations allowed the segregation of melt as discordant leucosome veins due to long residence within its source the melt had time enough to equilibrate with residual phases other than accessories included within major refractory minerals we estimated crystal melt partition coefficients as the concentration ratios between leucosome samples appearing to be pure melts and mesosome minerals which were analyzed for trace elements with a laser probe coupled to an icp mass spectrometer our data reveal that when biotite is stable li rb cs tl sc v cr ni nb and ta become strongly compatible the role of biotite in fractionating y th u and the ree is insignificant cordierite strongly fractionates li and be and also has some effect on the hree and u garnet produces extreme fractionation of sc y and the hree the ree partition coefficients for ferromagnesian silicates increase with the atomic number this effect being progressively more important through biotite cordierite and garnet k feldspar strongly fractionates ba and pb but plays a secondary role to biotite for rb and cs k feldspar fractionation does not change the lree hree ratio but plagioclase fractionation produces a significant decrease in the lree hree ratio both feldspars greatly fractionate sr and eu monazite fractionation produces a dramatic depletion in ree th and u as well as a decrease in the lree hree and th u ratios apatite also fractionates the ree although in contrast with monazite it increases the lree hree ratio and does not affect the th u ratio zircon fractionation like apatite and garnet produces a strong depletion in the hree and a concomitant increase in the lree hree ratio in contrast with monazite zircon fractionation causes the th u ratio to increase

amphibolite dehydration melting sorting out the solidus amphibolite melts under vapour absent conditions by dehydration melting during which h 2o released from hornblende is transferred directly into h 2o undersaturated silicate liquid five independent 1991 studies on amphibolite dehydration melting differ from each other and from earlier conceptual treatments five solidi between 7 and 10 kb vary from 975 c to 740 c two solidi above 10 kb are near vertical near 975 c and 850 c some differences are due to different amphibolite compositions and mineralogy we use the results of earlier experiments involving garnet supported by our own new results to construct a closed system phase diagram for a simple fully hydra ted amphibolite hornblende plagioclase showing the solidus and the melting interval for hornblende garnet other minerals liquid the amphibolite vapour absent solidus is defined by the beginnings of two multivariant reactions 1 a near vertical curve large positive d p dt where the formation of augite dominates and 2 a near horizontal curve a t higher pressures where the formation of garnet dominates the solidus curve bends back estimated near 900 c 9 kb with slope changing to low positive dp dt the new phase diagram expands the field for liquid generation with garnet amphibolite residues to much lower temperatures and pressures than the other recent experimental results increasing the range from which small amounts of hydrous granitoid melts may be segregated by compaction the reversed near horizontal slope of the solidus near 10 kb has interesting petrological consequences

the fluid regime of high temperature metamorphism during granitoid magma genesis the strong negative correlation between the temperatures and melt h2o contents of granitoid magmas implies that the crustal melting reactions that produced the magmas were strongly buffered with t and ah2o co varying within a narrow band this observation can only be explained if the partial melting reactions that created the magmas were either fluid absent from the outset or evolved toward this condition as melting progressed since these melting reactions occur during upper amphibolite to granulite facies metamorphism it is reasonable to conclude that metamorphic events responsible for the generation of granitoid magmas generally occur in the absence of excess pervasive fluid

early paleozoic polyphase metamorphism in northern tibet china abstract the altun qilian kunlun orogenic system in the northern tibetan plateau is considered as the northernmost orogenic collage of the proto tethyan domain early paleozoic polyphase tectonothermal events related to the subduction of the proto tethyan ocean and subsequent collisional orogeny produced two dominant metamorphic belts the north altun nat north qilian nql hp lt metamorphic belt and south altun sat north qaidam nqd uhp metamorphic belt the nat nql hp lt metamorphic belt mainly consists of blueschist eclogite and hp metasediments eclogites record metamorphic conditions of 420 550 c and 21 25 kbar at 510 460 ma the hp lt metamorphic belt is associated with ophiolite subduction accretion complex and arc magmatic rocks suggesting that the nat nql is a typical early paleozoic accretionary orogenic belt in contrast the sat nqd uhp metamorphic belt is characterized by eclogite and garnet peridotite enclosed within continental orthogneiss and paragneiss these rocks record uhp metamorphism at t 700 c and p 27 kbar geochronology suggests that the uhp metamorphism occurred at 500 423 ma and eclogite protolith ages of 850 750 ma suggest that protoliths are related to neoproterozic continental rifting field relationships petrology and geochronology suggest that the sat nqd uhp metamorphic belt resulted from the deep subduction of continental crust moreover in the sat nqd uhp metamorphic belt two types of hp granulites have been recognized the first type of hp granulite was formed by overprinting of eclogites during exhumation the second type of hp granulite has a single metamorphic history and similar age to that of uhp eclogite penecontemporaneous metamorphic ages but different geothermal gradients between hp granulites and related uhp eclogite define a possible paired metamorphic belt which may be considered to be a diagnostic characteristic of collisional orogens high grade metamorphic rocks in the sat nqd commonly record barrovian type metamorphism although it remains unclear whether or not all of these rocks have an earlier u hp metamorphic history the low medium p barrovian metamorphism and associated magmatism are about 20 50 ma later than uhp metamorphism newly recognized eclogites in the northern part of the east kunlun mountains to the south of qaidam basin have a similar field relationship enclosed in continental gneiss and mineral assemblage to the nqd eclogites and a similar or slightly younger metamorphic age 410 430 ma the occurrence of similar continental type eclogites on two sides of the qaidam basin makes it necessary to reconsider the nature of the qaidam block the polyphase metamorphism in northern tibet can be linked to accretion and collisional orogenesis related to the evolution of prototethys through the early paleozoic

petrography and zircon u pb isotopic study of the bayanwulashan complex constrains on the paleoproterozoic evolution of the alxa block westernmost north china craton abstract the bayanwulashan metamorphic complex bmc exposes along the eastern margin of the alxa block the westernmost part of the north china craton ncc bmc is principally composed of metamorphic rocks with amphibole plagiogneiss biotite plagioclase gneiss and granitic gneiss our research has been focused on the petrography and zircon u pb geochronology of the bmc to better understand the evolution of the alxa block and its relationship with the ncc evidences from field geology petrography and mineral chemistry indicate that two distinct metamorphic assemblages the amphibolite and greenschist facies had overprinted the preexisting granitic gneiss and suggest that the bmc experienced retrograde metamorphic episodes the la icp ms zircon u pb ages reveal that the primary magmatic activities of bmc were at ca 2 30 2 24 ga and the two metamorphic events were at ca 1 95 1 91 ga and ca 1 88 1 85 ga respectively these ages indicate that bmc initially intruded during paleoproterozoic not as previously suggested at archean period the early paleoproterozoic metamorphic records and the magmatic thermochronological data in bmc exhibit different evolution paths between the alxa block and the ncc the alxa block was most likely an independent early paleoproterozoic terrain following different amalgamation processes the alxa block combined with western block at ca 1 95 ga and then united with ncc at ca 1 85 ga

slab breakoff a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens we present a model proposing that oceanic lithosphere detaches from continental lithosphere during continental collision slab breakoff allowing an explanation of syn to post collisional magmatism and metamorphism continental collisions are preceded by subduction of dense oceanic lithosphere and followed by attempted subduction of buoyant continental lithosphere this situation of opposing buoyancy forces leads to extensional deformation in the subducted slab a narrow rifting mode of deformation will result if there is strain localization slab breakoff results we have assessed the plausiblity of this process by quantitatively evaluating an upper bound for the strength of the lithosphere and have compared it with the change in buoyancy force during continental sbduction whether breakoff will occur and the depth at which it will occur is a strong function of temperature and hence the subduction velocity for a subduction velocity of 1 cm yr breakoff could occur at depths of between 50 and 120 km while at higher velocities it is still likely to occur but at deeper depths as a result of the rifting during breakoff the asthenosphere upwells into the narrow rift and following breakoff it impinges on the mechanical lithosphere of the overriding plate the resulting conducted thermal perturbation leads to melting of the metasomatised overriding mantle lithosphere producing basaltic magmatism that leads to granitic magmatism in the crust dry asthenospheric mantle will melt only if breakoff occurs at a depth shallower than ca 50 km breakoff removes the force at the downdip side of the continental crust while the enhanced heating leads to a reduction of the strength of the underlying crust both effects facilitate the freeing of buoyant crustal sheets which can then rise towards the surface leading to the rapid exhumation of eclogite facies continental crust the cessation of subduction and replacement of the cold oceanic lithosphere by asthenosphere leads to rapid uplift of the orogen we have tested the variety of predicted expressions of slab breakoff in the geological record of the alps the aegean islands and the dabie shan a comparison of the various belts highlights 1 that the magmatism and metamorphism are found near the suture and the centre of the orogen demonstrating the general steepening of suture during collision 2 that a vague inverse correlation exists between maximum depth of metamorphism and volume of syn orogenic magmatism and 3 that the melts can be emplaced in both compressive and extensional environments we suggest that slab breakoff is an important step in the evolution of many orogenic belts and it allows an explanation for the combined presence in collisional orogens of magmatism with a mantle parentage and ultra high pressure metamorphics

geology geochronology and isotopic geochemistry of the xiaoliugou w mo ore field in the qilian orogen nw china case study of a skarn system formed during continental collision this study is financially supported by the national natural science foundation of china 41502068

late silurian early devonian adakitic granodiorite a type and i type granites in nw junggar nw china partial melting of mafic lower crust and implications for slab roll back abstract late silurian early devonian magmatism of the nw junggar region in the central asian orogenic belt provides a critical geological record that is important for unraveling regional tectonic history and constraining geodynamic processes in this study we report results of zircon u pb ages and systematic geochemical data for late silurian early devonian largely granitic rocks in nw junggar aiming to constrain their emplacement ages origin and geodynamic significance the magmatism consists of a variety of mafic to felsic intrusions and volcanic rocks e g adakitic granodiorite k feldspar granite syenitic granite gabbro and rhyrolite u pb zircon ages suggest that the granitoids and gabbros were emplaced in the late silurian early devonian 420 405 ma adakitic granodiorites are calc alkaline characterized by high sr 407 532 ppm low y 12 2 14 7 ppm yb 1 53 1 77 ppm cr mostly

continuity of the north qilian and north qinling orogenic belts central orogenic system of china evidence from newly discovered paleozoic adakitic rocks abstract adakitic intrusive rocks of 430 450 ma were discovered in the north qilian orogenic belt the western section of the central orogenic system cos in china these adakitic rocks were lower crust melts rather than slab melts as indicated by their crustal ce pb nb u ti eu and nd sm ratios and radiogenically enriched 87sr 86sr i of 0 7053 0 7066 and end t of 0 9 to 1 7 while they are all characterized by low yb 65 and la yb n 13 7 ratios these adakitic rocks are classified into the low mgo ni cr and high mgo ni cr groups the low mgo samples were derived from partial melting of thickened lower crust whereas the high mgo samples were melts from delaminated lower crust which subsequently interacted with mantle peridotite upon ascent adakitic rocks from the adjacent north qinling orogenic belt also originated from thickened lower crust at 430 ma in addition the north qilian and north qinling orogenic belts both consist of lithological assemblages varying from subduction accretionary complexes at south to central arc assemblages which include adakitic rocks then to backarc phases at north such a sequence reflects northward subduction of the qilian and qinling oceans in these two orogenic belts the occurrence of adakitic rocks of common origin and ages together with the similarities in tectonic configurations and lithological assemblages are considered to be the evidence for the continuity between eastern qilian and western qinling forming a 1000 km early paleozoic orogenic belt in such a tectonic configuration the qilian and qinling oceans that subducted from south possibly represent parts of the large proto tethyan ocean this inference is supported by the coexistence of early paleozoic coral and trilobite specimens from asia america and australia in the north qilian orogenic belt post 400 ma volcanic rocks occur in the north qinling orogenic belt but are absent in the north qilian orogenic belt indicating that these two orogenic belts underwent distinct evolution history after the closure of the proto tethyan ocean 420 ma

transitional i s type characteristic in the main range granite peninsular malaysia the dominantly triassic main range granite of peninsular malaysia that occurs west of the paleo tethyan bentong raub suture zone was regarded exclusively as an s type granite the main range dominantly biotite granites are of batholithic proportion and host one of the world s largest tin provinces the s type characteristics include high initial 87sr 86sr isotope ratios 0 710 a narrow range in silica presence of ilmenite and occasional cordierite and andalusite and the presence of pelitic or quartzose meta sedimentary xenoliths however the present review shows that the main range granites also have many features that are more characteristic of i type granites such as the very large scale of plutonism the presence of primary titanite and amphibole occurrence of hornblende bearing mafic enclaves increasing peraluminosity towards the more differentiated end members of the suite and decreasing p2o5 with increasing sio2 contents the moderately peraluminous nature of the bulk main range granite without containing cordierite fe mg garnet or sillimanite is consistent with derivation from a meta sedimentary protolith that was itself undersaturated with respect to al2sio5

crustal contamination in the petrogenesis of a calc alkalic rock series josephine mountain intrusion california the josephine mountain intrusion is a cretaceous calc alkalictonalite granite pluton emplaced at 22 km depth in a continental margin arc variable uplift of adjacent terranes in southern california since mid cretaceous time allows us to reconstruct the local crustal column and evaluate its role as a contaminant of mantle derived arc magmas in this region the parental magma of the intrusion was high alumina basalt whose isotopic signature 87 sr 86 sr 0 7087 18 o 7 5 nd 10 cannot have been generated by intracrustal assimilation of known or inferred rock types in the middle or lower crust such a signature could have resulted from high pressure fractionation of primary low alumina basalt coupled with assimilation of felsic pelitic lower crust partial melting of enriched subcontinental mantle followed by high pressure fractionation or a combination of these processes tonalite of the intrusion was formed by fractionation of the parent magma coupled with assimilation of local felsic wall rocks or by crustal melts similar to slightly younger granite assessment of the magnitude of crustal contamination is hampered by uncertainty regarding the existence and role of partial melting of previously enriched subcontinental mantle in generating the parental basaltic magma leading to concomitant uncertainty in the fraction of new continental crust created by such arc plutonism

what controls chemical variation in granitic magmas abstract consideration of the models that have been applied to explain the chemical variations within granitic rock suites shows that most are inadequate to account for the main variations this stems from a variety of model deficiencies ranging from physical or energetic inadequacies to incompatibility with the chemical data or internal inconsistency between models based on for example isotope or trace element data and major element data we contend that any model that fails any of these tests of internal consistency cannot be considered further thus although we can point to examples in which many of the traditionally accepted mechanisms have played secondary roles in producing variation there presently remains but one viable choice for the primary mechanism by which most granitic magmas acquire compositions beyond the range defined by the compositions of crustal melts that primary mechanism is peritectic assemblage entrainment pae we infer that once a partial melt has formed in a crustal protolith it may segregate from its complementary solid residue carrying small crystals of the peritectic phase assemblage formed in the melting reaction and that the ratios of individual peritectic minerals in the entrained assemblage remains fixed in the ratio decreed by the stoichiometry of the melting reaction for those elements with low solubilities in granitic melts pae in varying degrees accompanied by co entrainment of accessory minerals is responsible for most of the primary elemental variation in granitic magmas in contrast the concentrations of elements with high solubilities in silicic melts reflect the protolith compositions in a simple and direct way the source is the primary control on granite magma chemistry it dictates what is available to dissolve in the melt and what will be formed as the entrainable peritectic assemblage the apparent complexity in granitic rock suites is largely a consequence of these processes in the source all other mechanisms contribute only as a secondary overlay

magmatic history of the northeastern tibetan plateau 1 the northeastern margin of the tibetan plateau is underlain by the qaidam and qilian terranes which consist primarily of mid proterozoic through lower paleozoic oceanic and arc type assemblages that have been accreted to the southern margin of the tarim sino korean craton most previous models suggest that these assemblages formed along a northeast dipping subduction system constructed along the margin of the tarim sino korean craton during early paleozoic time the main components are interpreted to have formed either as an archipelago of volcanic arcs and back arc basins or as a broad expanse of accretionary complexes our geochronologic data support a model suggested by sobel and arnaud 1999 in which the qaidam and qilian terranes are separated from the tarim sino korean craton by a mid paleozoic suture that closed along a southwest dipping subduction zone the basement to these terranes consists of oceanic assemblages that were amalgamated into a coherent crustal fragment prior to emplacement of 920 930 ma granitoids early paleozoic arc type magmatism occurred between 480 and 425 ma apparently sweeping southwestward across much of the qilian and qaidam terranes accretion related magmatism along the inboard margin of the qilian terrane occurred between 423 ma and 406 ma following silurian devonian accretion the region has experienced late paleozoic and mesozoic uplift and erosion and has been severely overprinted by tertiary thrusting uplift and strike slip motion along the altyn tagh fault correlation of geologic features and magmatic histories between the altun shan and the nan shan suggests that the eastern altyn tagh fault has a total left lateral offset of 375 km

petrogenesis and tectonic implications of the early paleozoic granites in the western segment of the north qilian orogenic belt china abstract early paleozoic granitic magmatism in the north qilian orogenic belt records a complete wilson cycle and provides critical geological clues for unraveling the regional tectonic history in this study we report the results of zircon u pb ages hf isotopic analysis and systematic whole rock geochemical data for the late ordovician hongliuhe granite and early silurian qingshan monzogranite in the western segment of the north qilian orogenic belt to constrain their emplacement ages petrogenesis and regional evolution history u pb dating reveals that the hongliuhe granite was emplaced around 453 452 ma and the qingshan monzogranite was emplaced about 440 438 ma a geochemical study shows that the two granites belong to the calc alkaline to high k calc alkaline series the hongliuhe granite shows adakitic and peraluminous features while the qingshan monzogranite belongs to metaluminous to weak peraluminous granites zircons in the hongliuhe granite show ehf t values ranging from 15 1 to 11 7 with two stage hf model ages tdm2 of 687 2398 ma whereas zircons in the qingshan monzogranite show ehf t values ranging from 5 7 to 11 0 with two stage hf model ages from 814 to 1057 ma the geochemical characteristics indicate that the hongliuhe granite was a transitional i s type granite and was generated from a thickened lower crust with the addition of minor paleo to mesoproterozoic crustal materials which left a rutile garnet pyroxene plagioclase residue the qingshan monzogranite formed from the partial melting of mafic crust with minor mantle derived materials and the fractionation of ti bearing phases apatite and pyroxene occurred during the magma s evolution which left an amphibole and plagioclase residue we infer that the hongliuhe granite formed during the northward subduction of the north qilian ocean while the qingshan monzogranite was generated during the post collision stage between the qilian and alxa blocks this observation indicates that syn collision stage of the north qilian orogenic belt began before 453 ma and that the extension regime began prior to 444 ma

early paleozoic to devonian multiple accretionary model for the qilian shan nw china abstract the qilian shan located in the northern tibetan plateau of nw china has an excellent record of early to middle paleozoic subduction accretion that resulted from convergence between the alax and qaidam blocks but there is no consensus about its detailed tectonic history this paper summarizes its tectonic divisions and discusses its tectonic evolution from the cambrian to devonian the belt has the following tectonic divisions in the far north the southern passive margin of the alax block is juxtaposed against an early cambrian to ordovician marianan type intra oceanic arc north qilian the north qilian arc is separated from a japanese type arc central qilian to the south by a high pressure metamorphic belt composed mostly of ophiolitic slices and oceanic crusts the central qilian is bounded to the south by a wide melange zone south qilian consisting of ophiolitic slices and continental margin sequences farther to the south lies the oulongbuluk microcontinent that is separated from the qaidam block and farther south by an ultrahigh pressure metamorphic belt tectonostratigraphic analysis together with geochemical geochronological and geophysical data indicates a complex evolution by subduction accretion processes from the cambrian to the devonian before final amalgamation and docking to the northern alax block this model solves the long lasting discussion on the polarity of subduction in paleozoic time this multiple subduction accretion history sheds light on the continuity of paleozoic sutures along the qilian shan and the nature of the altyn tagh fault and thus contributes to an improved understanding of the tectonic architecture of central asia

mid pleistocene lavas from the seguam volcanic center central aleutian arc closed system fractional crystallization of a basalt to rhyodacite eruptive suite in contrast to adjacent volcanic centers of the modern central aleutian arc seguam island developed on strongly extended arc crust k ar dates indicate that mid pleistocene late pleistocene and holocene eruptive phases constitute seguam this study focuses on the petrology of the mid pleistocene 1 07 07 ma turf point formation tpf which is dominated by an unusual suite of porphyritic basalt and basaltic andesite lavas with subordinate phenocryst poor andesite to rhyodacite lavas increasing whole rock feo mgo from basalt to dacite the anhydrous plag ol cpx opx mt phenocryst assemblage groundmass pigeonite and the reaction ol liq opx preserved in the mafic lavas indicate a tholeiitic affinity thermometry and comparison to published phase equilibria suggests that most tpf basalts crystallized plag ol cpx mt at 1160 c between about 3 5 kb 1 2 h2o andesites crystallized plag cpx opx mt at 1000 c between 3 4 kb with 3 5 h2o and dacites crystallized plag cpx opx mt at 1000 c between 1 2 kb with 2 3 h2o all lavas crystallized at f o 2 close to the nno buffer mineral compositions and textures indicate equilibrium crystallization of the evolved lavas petrographic evidence of open system mixing or assimilation is rare mgo cao al2o3 cr ni and sr abundances decrease and k2o na2o rb ba zr and pb increase with increasing sio2 50 71 lree enrichment ce yb n 1 7 0 2 characterizes most tpf lavas total ree contents increase and eu anomalies become more negative with increasing sio2 relative to other aleutian volcanic centers tpf basalts and basaltic andesites have lower k2o na2o tio2 rb ba sr zr y and lree abundances 87sr 86sr ratios 0 70361 0 70375 and ratios of 206pb 204pb 18 88 18 97 207pb 204pb 15 58 15 62 208pb 204pb 38 46 38 55 are the highest measured for any suite of lavas in the oceanic portion of the aleutian arc conversely end values 5 8 to 6 7 are among the lowest from the aleutians sr nd and pb ratios are virtually constant from basalt through rhyodacite whereas detectable isotopic heterogenity is observed at most other aleutian volcanic centers major and trace element ree and sr nd and pb isotopic compositions are consistent with the basaltic andesitic andesitic dacitic and rhyodacitic liquids evolving from tpf basaltic magma via closed system fractional crystallization alone fractionation models suggest that removal of 80 wt cumulate 61 plag 17 cpx 12 opx 7 ol and 3 mt can produce 20 wt rhyodacitic residual liquid per unit mass of parental basaltic liquid petrologic and physical constraints favor segregation of small batches of basalt from a larger mid crustal reservoir trapped below a low density upper crustal lid in these small magma batches the degree of cooling crystallization and fractionation are functions of the initial mass of basaltic magma segregated the thermal state of the upper crust and the magnitude of extension tholeiitic magmas erupted at seguam evolved by substantially different mechanisms than did calc alkaline lavas erupted at the adjacent volcanic centers of kanaga and adak on unextended arc crust these variable differentiation mechanisms and liquid lines of descent reflect contrasting thermal and mechanical conditions imposed by the different tectonic environments in which these centers grew at seguam intra arc extension promoted eruption of voluminous basalt and its differentiates unmodified by interaction with lower crustal or upper mantle wallrocks

blueschists in major suture zones of china blueschist facies rocks and c type eclogites occur as allochthonous terranes or as small tectonic blocks in almost all the accretionary fold belts and major suture zones within the interior of china and along the circum pacific and tethys himalayan belts at least forty blueschist localities have been described blueschists around the pacific margins and the himalaya alpine belts are mostly mesozoic in age only those in taiwan and new caledonia are cenozoic intracratonal blueschists of china are older than late permian when the collision amalgamation presumably occurred several alleged precambrian blueschist localities ye 1987 are shown in the recently published metamorphic map of china a precambrian blueschist belt in anhui hubei central china extends for about 2300 km however except for the pre sinian blueschist terrane in aksu the age determination for precambrian blueschist metamorphism elsewhere has not been confirmed characteristic features of chinese blueschists include the following 1 blueschist facies metamorphism predates the continental collision amalgamation 2 most blueschists have undergone multistage metamorphism commonly with increasing temperature and or decreasing pressure for later events 3 assemblages include sodic amphibole epidote albite quartz phengite chlorite sphene lawsonite jadeitic pyroxene and aragonite are not common and 4 protoliths are mainly mafic pelagic and clastic rocks blueschist facies metamorphism in china may be divided into two types on the basis of the imposed geothermal gradient hence the progressive change in mineral assemblage the rare lawsonite bearing blueschists produced at lower temperatures and higher pressures are confined to the inner mongolia and yarlung zangbo suture zones where aragonite and possibly jadeitic pyroxene quartz occur and the blueschists are associated with subgreenschist facies rocks most other blueschists of china formed at higher temperatures along the transitional blueschist greenschist facies boundary where epidote sodic and calcic amphiboles are ubiquitous and the blueschists are interlayered with greenschist facies rocks the common occurrence of intracratonal blueschists in major suture zones of china and elsewhere in eurasia indicates pre mesozoic subduction accretion of oceanic crust and flysch sediments these zones of accretion represent growth around the periphery of major precambrian cratons this fact together with the distribution of ophiolite belts and available paleomagnetic data provides evidence for an extremely mobile history of plate movement in eurasia however the paleozoic intracontinental blueschists and ophiolites of china have formed before rather than during final closure between cratons they do not usually mark the location of terminal sutures but are the result of earlier accretion and continental growth by subduction underplating and imbrication of oceanic materials similar to the accretion history of westernmost north america

multiple origins for the middle jurassic to early cretaceous high k calc alkaline i type granites in northwestern fujian province se china and tectonic implications abstract a comprehensive study of zircon u pb dating and in situ hf isotopes whole rock major and trace element geochemistry and sr nd isotopes was carried out for three late mesozoic granitic plutons waitun shipi and taiyuan in northwestern fujian province se china we assess the origin of the granites and their relationship to the evolution of the late mesozoic volcanic intrusive complex belt in se china la icp ms zircon u pb dating shows that three plutons were emplaced in the middle jurassic to early cretaceous 168 109 ma in which the waitun and shipi plutons are intrusive complexes all the plutons are composed of high k calc alkaline i type granites with a great diversity in elemental and isotopic compositions the granites have sio 2 contents of 68 3 78 5 wt showing a gradual decrease in initial 87 sr 86 sr 0 7181 to 0 7091 and increase in e nd t 16 7 to 8 1 and e hf t in situ zircon 20 6 to 6 9 with decreasing emplacement ages geochemical data suggest that the middle jurassic 168 ma waitun granites are of purely crustal origin derived by partial melting of a mixed source of paleoproterozoic metaigneous 78 and metasedimentary 22 rocks at a depth of 30 40 km triggered by underplating of basaltic magma mixing of such crustal melts with about 10 basaltic magma could account for the origin of the late jurassic 161 ma waitun granites the late jurassic 156 ma shipi and early cretaceous 134 ma taiyuan granites were produced by extensive fractional crystallization of primary crustal melts the source of which show relatively high proportion 82 of metaigneous rocks the early cretaceous 109 ma shipi granites were generated by partial melting of a mixed source of paleoproterozoic metaigneous 92 and metasedimentary 8 rocks at a depth of 30 km plus additional 15 input from coeval basaltic magma the granites were formed in a continental arc setting induced by northwestward subduction of the paleo pacific plate progressive slab rollback since the middle jurassic from inland to the coastal area is the most likely mechanism for the generation of the granites

genesis of the two main types of peraluminous granitoids two mica monzogranites to leucogranites and biotite rich cordierite bearing tonalites to monzogranites form two distinct groups of peraluminous granitoids they can be distinguished by mineral and rock associations and by the variation of their peraluminosity during differentiation except for the rare muscovite bearing granitoids produced by extreme fractionation or local contamination of metaluminous magmas the majority of peraluminous granitoids are produced by partial melting of crustal rocks production of either muscovite bearing granites or biotite rich cordierite bearing granitoids does not depend only on the nature of the sources but is also controlled by the physical parameters of partial melting and consequently by the way anatexis of a thickened crust is enhanced biotite rich cordierite bearing granitoids are generated where mantle derived magmas are injected into or have underplated crustal rocks two mica granites are generated where thickened crust is affected by major crustal shears or thrusts correlations between differentiation and peraluminosity indicate the dominant role of either restite unmixing or fractional crystallization in the production and evolution of the various types of peraluminous granitoids

a cold early palaeozoic subduction zone in the north qilian mountains nw china petrological and u pb geochronological constraints the north qilian high pressure hp low temperature lt metamorphic belt is composed mainly of blueschists eclogites and greenschist facies rocks it formed within an early palaeozoic accretionary wedge associated with the subduction of the oceanic crust and is considered to be one of the best preserved hp lt metamorphic belts in china here we report new lawsonite bearing eclogites and eclogitic rocks enclosed within epidote blueschists in the north qilian mountains five samples contain unaltered lawsonite coexisting with omphacite and phengite as inclusions in garnet indicating eclogite facies garnet growth and lawsonite pseudomorphs were observed in garnet from an additional 11 eclogites and eclogitic rocks peak pressure conditions estimated from lawsonite omphacite phengite garnet assemblages were 2 1 2 4 gpa at temperatures of 420 510 c in or near the stability field of lawsonite eclogite and implying formation under an apparent geothermal gradient of 6 8 c km 1 consistent with metamorphism in a cold subduction zone shrimp u pb dating of zircon from two lawsonite bearing eclogitic metabasites yields ages of 489 7 ma and 477 16 ma respectively cl images and mineral inclusions in zircon grains indicate that these ages reflect an eclogite facies metamorphism an age of 502 16 ma is recorded in igneous cores of zircon grains from one lawsonite pseudomorph bearing eclogite which is in agreement with the formation age of early ordovician for some ophiolite sequences in the north qilian mountains and may be associated with a period of oceanic crust formation the petrological and chronological data demonstrate the existence of a cold early palaeozoic subduction zone in the north qilian mountains

effects of desferrioxamine b on the release of arsenic from volcanic rocks abstract microorganisms play an important role in as mobilization into groundwater by directly influencing as speciation or indirectly inducing solubilisation from as bearing phases such as fe mn and al oxides iron oxide dissolution could also be induced by siderophores small molecule compounds produced by microorganisms to favour fe uptake well waters exceeding the potable water limit of 10 g as l 1 0 133 m have been widely reported in geothermal areas mechanisms responsible for these high as concentrations have not yet been thoroughly elucidated and the complexity of as mobilization in volcanic aquifers is still open to multiple interpretations the present study was based on batch release experiments aimed at verifying and quantifying the effect of siderophores on as mobilization from volcanic rocks lava tuff peperino and fallout deposit at different ph and ligand concentration in the experiments the siderophore trihydroxamate desferroxamine b dfob was used and its effect on as release from volcanic rocks was manifest after the first days the most favourable ph for as release was ph 6 while concentrations above 250 m dfob considerably enhanced as and fe concentrations in solution the as release from rocks was between 2 0 10 at ph 6 and 2 4 8 8 at ph 8 the as fe ratio in solution changed with time suggesting different release mechanisms and higher mobility of as compared to fe during the first phase of the experiment the presence of siderophore increased fe dissolution rates up to 10 orders of magnitude the as release correlated with al mn fe si v ga and sb and the release of all these elements increased with increasing dfob concentration in alkaline environments also cu zn and pb were mobilized the presence of siderophores represents a possible trigger for as mobilization from iron binding minerals to the water phase with interesting implications for groundwater quality plant uptake and bacterial communities

magma sources and petrogenesis of the early middle paleozoic backarc granitoids from the central part of the qilian block nw china abstract the petrology geochemistry geochronology and sr nd hf isotopes of the backarc granitoids from the central part of the qilian block are studied in the present work both s and i type granitoids are present in petrographic classification they are granite alkali feldspar granite felsic granite diorite quartz diorite granodiorite and albite syenite the shrimp ages are 402 447 ma for the s type and 419 451 ma for the i type granitoids they are mostly high k calc alkaline granitoids the s type granitoids are weakly to strongly peraluminous and are characterized by negative eu anomalies eu eu 0 18 0 79 the i type granitoids are metaluminous to weakly peraluminous and are characterized mostly by small negative to small positive eu anomalies eu eu 0 71 1 16 the initial 87 sr 86 sr values are 0 708848 0 713651 for the s type and 0 704230 0 718108 for the i type granitoids the e nd 450 ma values are 8 9 4 1 and 9 7 1 9 for the s type and i type granitoids respectively the t dm values are 1 5 2 4 ga for the s type and 1 0 2 3 ga for the i type granitoids for the qilian block the backarc granitoid magmatism took place approximately 60 million years after the onset of the southward subduction of the north qilian oceanic lithosphere and lasted approximately 50 million years partial melting of the source rocks consisting of the neoproterozoic metasedimentary rocks of the huangyuan group and the intruding lower paleozoic basaltic rocks could produce the s type granitoid magmas partial melting of basaltic rocks mixed with lower continental crustal materials could produce the i type granitoid magmas major crustal growth occurred in the late archean and meso paleoproterozoic time for the qilian block the magma generation was primarily remelting of the crustal rocks with only little addition of the mantle materials after 1 0 ga for the qilian block