geochronology and geochemistry of late ordovician early devonian gneissic granites in the kumishi area northern margin of the south tianshan belt constraints on subduction process of the south tianshan ocean abstract the south tianshan belt has recorded the evolution of the south branches of the paleo asian ocean and is pivotal for understanding the formation of the southernmost part of the central asian orogenic belt in order to place constraints on the tectonic evolution of the south tianshan ocean three highly deformed granitic plutons monzonitic granites biotite k feldspar granites and two mica granites exposed in the kumishi area were collected for geochronological and geochemical studies zircon la icp ms u pb dating of the gneissic granites from the three plutons yielded crystallization ages at 446 2 2 2 ma 431 0 2 8 ma and 407 3 3 1 ma all of these gneissic granites have geochemical characteristics of calc alkaline igneous rocks the biotite monzonitic granites 446 ma are slightly peraluminous lree enriched and show relatively flat hree patterns mostly with negative eu anomalies the rocks have positive pb and negative nb ta ti anomalies in combination with their negative ehf t values 7 9 to 11 6 and the corresponding old tdmc ages 1 93 2 16 ga we suggest that the monzonitic granites were mainly derived from partial melting of the middle lower crust dominated by precambrian basement rocks the biotite k feldspar granites 431 ma are characterized by high k2o low fe2o3 and mgo contents they show ree patterns similar to the monzonitic granites as well as the nb ta ti depletions but have more enrichment in u pb zr and hf their depleted ehf t values 0 7 to 5 4 and young tdmc ages 1 07 1 37 ga imply significant input of mantle derived juvenile materials and thus suggest that the k feldspar granites were possibly derived from a k enriched crustal source mixed significantly with mantle derived juvenile materials the two mica granites 407 ma show a typical s type granite affinity with high al2o3 tio2 low cao na2o and k2o al2o3 ratios suggesting a pelitic source these characteristics together with their positive ehf t values 0 2 to 3 1 and relatively young tdmc ages 1 20 1 41 ga indicate that the two mica granites were most likely generated by partial melting of a pelitic source with less input of juvenile materials all of the samples from the three plutons show same geochemical characteristics of felsic magmatic rocks formed in an arc related tectonic setting together with the northward thrusting tectonics and the arc magmatic rocks exposed along the south nalati kawablak fault a northward subduction of the south tianshan ocean is suggested to interpret the formation of the three granitic plutons taking into account the distinct tectonic polarity and distribution of magmatic rocks in the western and eastern south tianshan belt we suggest that the south tianshan ocean probably underwent different subduction processes in its western and eastern parts a single northward subduction and a divergent bidirectional subduction are further proposed to interpret the formation of the western and eastern south tianshan belt respectively

significance of europium anomalies in silicate melts and crystal melt equilibria a re evaluation although europium speciation in silicate melts partly reflects prevailing oxygen fugacities melt composition and structure play the major role in determining eu2 eu3 ratios and europium partitioning into mineral phases experimental evidence by different investigators on the magnitude of the compositional effect on eu2 eu3 provides consistent results only if account is taken of the oxygen buffer system employed in the experiments the medium dependent reduction of europium can be understood in terms of the preferential stabilization of eu2 by a strong aluminosilicate complex in the melt phase and to a much lesser degree by metasilicate complexes the stability of these complexes increases as the field strength of the associated cation species decreases hence eu2 aluminosilicate complexes are preferentially stabilized relative to trivalent lanthanides in melts of appropriate composition and their presence minimizes the enthalpy of the melt the influence of these complexes is particularly pronounced in melts with a high feldspathic component and a strongly polymerized structure their petrogenetic influence is best documented through the behaviour of europium relative to the other lanthanides during anhydrous anatexis in high grade metamorphic terrains and in the anomalous europium partition coefficients of phenocryst phases which formed during the crystallization of highly silicic magmas

adakites from continental collision zones melting of thickened lower crust beneath southern tibet adakites are geochemically distinct intermediate to felsic lavas found exclusively in subduction zones here we report the first example of such magmas from southern tibet in an active continental collision environment the tibetan adakites were emplaced from ca 26 to 10 ma and their overall geochemical characteristics suggest an origin by melting of eclogites and or garnet amphibolites in the lower part 50 km of thickened tibetan crust this lower crustal melting required a significantly elevated geotherm which we attribute to removal of the tectonically thickened lithospheric mantle in late oligocene time the identification of collision type adakites from southern tibet lends new constraints to not only the himalayan tibetan orogenesis how and when the indian lithosphere started underthrusting asia can be depicted but also the growth of the early continental crust on earth that consists dominantly of the tonalite trondhjemite granodiorite suites marked by adakitic geochemical affinities

transition from island arc to passive setting on the continental margin of gondwana u pb zircon dating of neoproterozoic metaconglomerates from the se margin of the tepla barrandian unit bohemian massif abstract sedimentological petrological and geochemical data combined with shrimp and la icp ms u pb ages of pebbles of granitic rocks and detrital zircons from neoproterozoic vletice metaconglomerates of the se margin of the tepla barrandian unit were used to characterize the regional geotectonic setting at the proterozoic paleozoic boundary the metaconglomerates form several metres thick intercalations in metasediments of the svrchnice formation roof pendants of central bohemian plutonic complex which is considered to have been deposited in neoproterozoic cambrian times a sedimentological study indicates that the metaconglomerates are products of debris flows deposited in submarine canyons or proximal parts of submarine fans geochemistry of metaconglomerate matrix and petrography of pebbles revealed an obvious difference from the western and central part of the tepla barrandian unit proterozoic where pebbles eroded from uplifted parts of the basin and island arc sources are most common the abundance of quartz quartzite granite and crystalline limestone in pebbles together with the more mature composition of host metagreywackes indicate the presence of relatively evolved continental crust in the source area for the vletice metaconglomerates and shallowing of the neoproterozoic basin to the se in the present geographic co ordinate system the neoproterozoic to lowermost cambrian age of the svrchnice formation suggested by its comparison with similar rocks in the central part of the tepla barrandian unit was confirmed by u pb dating ages of detrital zircons from the metaconglomerate matrix cluster around two values archaean to paleoproterozoic u pb ages between ca 2580 and 1780 ma and neoproterozoic to early cambrian ages between 660 and 520 ma with a significant peaks at 600 and 544 ma suggesting local volcanic activity extending shortly beyond the proterozoic paleozoic boundary zircons separated from two trondhjemite pebbles from the metaconglomerate yielded a mean u pb age of 610 17 ma arguing for active arc magmatism during the latest neoproterozoic the overall age distribution of detrital zircons and the absence of grenvillian zircon ages favour a location of the tepla barrandian unit close to the west african craton at the neoproterozoic paleozoic boundary the rapid onset of a transtensional geotectonic regime and maturing of sediments is evidenced by the overlying cambrian shallow water clastics alternating with predominantly felsic calc alkaline to alkaline volcanics during the cambrian oblique plate convergence the active continental margin was transformed into belts of horsts and narrow pull apart or small rift basins after the ordovician break off of armorican terrane assemblage blocks from gondwana the extensional regime in the tepla barrandian unit continued until the beginning of variscan orogeny in the middle devonian

abnormal reverse faulting above a depleting reservoir subsurface deformation is observed during pumping of some hydrocarbon fields deformation features include subsidence centered on the field and subsidence related centripetal horizontal displacements and faulting focal mechanisms yield reverse movements on steeply dipping faults in our sand silicone analogue model the reservoir is represented by a latex balloon or by undercompacted ground sand deflation of the reservoir results in formation of steeply dipping reverse faults bounding a downward opened cone the cone moves downward to follow the reservoir contraction faults along the cone are straight beneath a thick reservoir cover and tend to curve upwards with decreasing cover our results similar to natural structures observed around magma chambers allow us to reinterpret paul segall9s numerical model of poroelastic stresses caused by changes in the distribution of pore fluids and draw a new pattern of active faults

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

cenozoic k rich adakitic volcanic rocks in the hohxil area northern tibet lower crustal melting in an intracontinental setting it is generally accepted that the cenozoic potassic volcanic rocks of northern tibet were derived from a lithospheric mantle source here we report new chronological geochemical and isotopic data for the miocene ca 18 15 ma k rich adakitic volcanic rocks from the hohxil area of the songpan ganzi block in northern tibet we contend that these rocks were generated by partial melting of the mafic lower crust in an intracontinental setting unrelated to subduction of oceanic crust the hohxil rocks exhibit high sr y and la yb ratios high sr and la contents but low yb and y concentrations similar to adakites formed by slab melting associated with subduction however their relatively low e n d values 2 09 to 4 58 high 8 7 sr 8 6 sr 0 7072 0 7075 th u th ba and rb ba ratios and distinctive potassium enrichments k 2 o na 2 o are very different from the composition of typical adakites in addition those k rich adakitic rocks with the highest sio 2 contents 61 wt exhibit the lowest 8 7 sr 8 6 sr ratios and highest e n d values and are the oldest cenozoic volcanic rocks exposed in the songpan ganzi block suggesting that they were derived neither directly from a mantle source nor by differentiation of a shoshonitic magma taking into account the composition of lower crustal mafic xenoliths in tibet as well as the tectonic and geophysical evidence we conclude that the hohxil adakitic magmas were produced by partial melting of amphibole bearing eclogites with a k rich mafic bulk composition in the lower part 55 km of the thickened northern tibetan crust partial melting of the lower crust may have been triggered by dehydration release of fluids from sedimentary materials in the southward subducted continental lithosphere

high sr y and la yb ratios the meaning of the adakitic signature abstract the name of adakite is used to describe a far too large group of rocks whose sole common feature is high sr y and la yb ratios however such a signature can be achieved via different processes melting of a high sr y and la yb source deep melting with abundant residual garnet fractional crystallization or afc or interactions of felsic melts with the mantle causing selective enrichment in lree and sr over hree a database of the compositions of adakitic rocks including high silica and low silica adakites martin h smithies r h rapp r p moyen j f champion d c 2005 an overview of adakite tonalite trondhjemite granodiorite ttg and sanukitoid relationships and some implications for crustal evolution lithos 79 1 2 1 24 continental adakites and archaean adakites was assembled geochemical modeling of the potential processes is used to interpret it and reveals that 1 the classical model of slab melting provides the best explanation for the genesis of high silica adakites 2 low silica adakites are explained by garnet present melting of an adakite metasomatized mantle i e at depths greater than 2 5 gpa 3 continental adakites is a term encompassing a huge range of rocks with a corresponding diversity of petrogenetic processes and most of them are different from both low and high silica adakites 4 archaean adakites show a bimodal composition range with some very high sr y examples similar to part of the ttg suite reflecting deep melting 2 0 gpa of a basaltic source with a relatively high sr y while lower sr y rocks formed by shallower 1 0 gpa melting of similar sources comparison with the archaean ttg suite highlights the heterogeneity of the ttgs whose composition spreads the whole combined range of hsa and archaean adakites pointing to a diversity of sources and processes contributing to the ttg suite

neoproterozoic ages of the kuluketage diabase dyke swarm in tarim nw china and its relationship to the breakup of rodinia the widely exposed kuluketage diabase dyke swarm tarim block nw china has been considered to have been emplaced in permian times new precise zircon u pb shrimp ages for two samples from the dyke swarm yield neoproterozoic ages of 823 8 8 7 ma and 776 8 8 9 ma correlated with peaks of magmatism in south china and australia at c 825 ma and c 780 ma these two new ages provide significant information for palaeocontinental reconstructions the prolonged duration of the magmatic events combined with regional stratigraphic relationships imply that the tarim block may have been affected by a mantle plume during the breakup of rodinia

generation of sodium rich magmas from newly underplated basaltic crust sodium rich rocks of trondhjemite tonalite dacite ttd or granodiorite ttg suites form much of precambrian continental crust1 they are thought to have formed by partial melting of subducted oceanic crust2 3 a process that would have been much more widespread early in earth history than at present owing to the higher thermal gradients prevailing at that time4 phanerozoic ttd suites do exist however and seem also to relate to subduction zones5 defant and drummond6 proposed that these suites form where young 25 myr hot oceanic lithosphere is subducted and melts thus locally simulating the conditions that led to widespread crustal growth in the archaean here we describe plutonic and volcanic rocks from the cordillera blanca complex in peru which have characteristics of the high al ttd suite but which were produced above a subduction zone containing a 60 myr old slab we present evidence that the complex formed by partial melting of newly underplated basaltic crust and argue that this mechanism should be considered more generally as an additional way of generating sodium rich arc magmas

the latest neoarchean paleoproterozoic evolution of the dunhuang block eastern tarim craton northwestern china evidence from zircon u pb dating and hf isotopic analyses abstract the dunhuang block in the easternmost segment of the tarim craton exposes precambrian mafic granulite and felsic gneiss to better understand the poorly known formation and evolution of the tarim craton we carried out detailed field based petrological and geochronological investigations on the major lithologies of the dunhuang block u pb dating and hf isotopic analyses on the mafic granulite and felsic gneiss ttg gneiss of the dunhuang block reveal two distinct age populations the latest neoarchaean 2 5 ga and the late palaeoproterozoic 1 82 1 85 ga the zircon magmatic cores and metamorphic rims of tonalitic gneisses yield similar ages of 2 5 ga supporting the existence of archean rocks in the dunhuang block the short time interval between latest neoarchean magmatism and high grade metamorphism suggests that they were related to the same latest neoarchean tectonothermal event the 2 5 ga zircons show hf depleted mantle model ages between 2 55 ga and 2 8 ga with a peak at 2 7 ga suggesting that 2 7 ga juvenile crust of the dunhuang block was reworked during a 200 300 m y period at the end of the neoarchean the age of 1 85 ga obtained from the mafic granulites and felsic gneisses is interpreted as the age of high pressure granulite facies metamorphism combined with the hf isotopic data these data suggest that the dunhuang block experienced 2 7 ga crustal growth a 2 5 ga magmatic metamorphic event and a 1 82 1 85 ga hp granulite facies metamorphic event this sequence of events is very similar to that of the north china craton implying that the evolutionary history of the dunhuang block was consistent with that of the north china craton from the neoarchean to paleoproterozoic

geochemistry nd and sr isotopes and u pb zircon ages of granitoid and metasedimentary xenoliths from the navajo volcanic field four corners area southwestern united states abstract proterozoic granitoid xenoliths from the navajo volcanic field nvf in the southwestern united states range in composition from tonalite diorite to granite with the latter greatly dominating there is no apparent relationship between chemical or isotopic composition and age geographic location or degree of alteration or deformation of the xenoliths two of the major ages of proterozoic plutonism recognized in the southwest 1 75 1 70 yavapai and 1 45 1 41 ga anorogenic are represented by the nvf xenoliths in some instances within a single diatreme we did not find any xenoliths in the 1 65 1 62 ma mazatzal range the xenoliths are dominantly i types and both nd and sr isotopes and trace element distributions indicate juvenile arc sources approximately 1 8 ga the granite magmas may have been produced by either vapor absent partial melting or fractional crystallization of a tonalite parent juvenile sources must have been available for melting in each of the three periods of magma production and hence some juvenile 1 8 ga crust in this region must have escaped melting in the first two episodes of magmatism and deformation metasedimentary xenoliths are chiefly biotite sillimanite schists garnet biotite schists and garnet paragneisses these three groups are strikingly similar in chemical and isotopic composition suggesting similar graywacke pelite protoliths the schists appear to be limited to the far northern part of the volcanic field consistent with a possible crustal boundary in the basement compositional and thermobarometric results suggest that the biotite schist xenoliths are not restites but that the paragneisses may be mixtures of melt and restite sedimentary protoliths may have been deposited in a forearc basin and later tectonically transported to the deep crust during one or more arc continent collisions in the late paleoproterozoic

paleozoic adakitic rocks in the northern altyn tagh northwest china evidence for progressive crustal thickening beneath the dunhuang block abstract to constrain the phanerozoic crustal evolution of the dunhuang block new whole rock geochemistry zircon u pb ages and hf isotopic data were determined for three paleozoic granitic plutons in the northern altyn tagh northwest china zircon u pb dating of these plutons including gneissic granite mylonitic granite and augen gneiss yielded middle devonian to early carboniferous crystallization ages of 358 4 ma 382 4 ma and 332 2 ma respectively the gneissic granites are characterized by a small sio 2 variation 69 11 70 33 wt with moderate k 2 o 3 07 3 55 wt na 2 o 3 61 4 08 wt low fe 2 o 3 t 2 25 2 84 wt and mgo 1 04 1 25 wt the granites are weakly peraluminous with moderate aluminium saturation indexes asi 1 02 1 08 and exhibit the geochemical features of high k calc alkaline igneous rocks although the major element contents of the mylonitic granites and the augen gneisses are different from those of the gneissic granites all three plutons show similar ree patterns and trace element variations rocks from these plutons are all lree enriched and show relatively flat hree patterns with pronounced depletions in ba nb ta ti and negative eu anomalies moreover the rocks are characterized by high sr 330 7 656 ppm low yb 0 513 1 521 ppm and y 5 199 13 73 ppm and thus have high sr y ratios 32 122 showing geochemical affinities of adakitic rocks their negative e hf t values low mgo cr co and ni contents indicate a continental crust origin without significant involvement of mantle derived materials the negative eu anomalies low yb and y contents of these granitic rocks reveal that plagioclase and garnet are major residue minerals in the magma source all the geochemical characteristics demonstrate that these adakitic plutons were produced by partial melting of thickened lower crust at relatively shallow depths under high amphibolite or granulite facies and did not reach the eclogite facies combining this with the formation ages we suggest that the southern margin of the dunhuang block was thickened during the late paleozoic the gradual variations in the la yb n ratios of these rocks indicate that the continental crust of the southern dunhuang block was progressively thickened from the late devonian to early carboniferous this thickening process was most likely triggered by continental collision between the dunhuang block and the central altyn massif

middle cambrian granites in the dunhuang block nw china mark the early subduction of the southernmost paleo asian ocean abstract the formation and evolution of the central asian orogenic belt caob were closely associated with the opening and closure of the so called paleo asian ocean pao the subduction of the southern pao is expected to have resulted in widespread paleozoic arc magmatism in the southernmost caob however the precise location and timing of such subduction remain uncertain the dunhuang block dhb in the southernmost caob is thought to have recorded such subduction in this contribution we report the results of the zircon u pb ages hf isotopes and whole rock geochemical data for the lianghu dapokouzi and xiaowan plutons in the northeastern dhb nw china zircon u pb dating reveals that these plutons formed at ca 510 ma the middle cambrian and may represent the oldest arc related magmatism in the dhb geochemically the dapokouzi pluton is characterized by high sio2 70 7 71 2 wt na2o 5 11 5 26 wt al2o3 15 5 15 6 wt and sr 730 733 ppm contents high sr y 380 398 ratios moderate mg 49 50 values low mgo 0 61 wt k2o 2 00 2 01 wt and y 1 84 1 93 ppm contents and weakly positive eu 1 39 1 44 anomalies indicating geochemical affinities of adakitic rocks they have positive ehf t values 11 2 to 13 9 close to the depleted mantle line and young two stage model ages tdm2 571 742 ma all the geochemical characteristics show that the dapokouzi pluton was derived from the remelting of juvenile crust with addition of minor metasomatized mantle derived materials and a residue phase dominated by garnet bearing amphibolite similarly the lianghu and xiaowan plutons have high sio2 70 2 74 4 wt and al2o3 12 8 14 3 wt low mgo 0 61 1 01 wt with mg values of 35 43 and k2o 0 43 1 55 wt contents compared to the dapokouzi pluton they exhibit low sr 121 155 ppm and y 15 3 23 1 ppm contents low sr y 5 25 9 54 ratios and cr ni co concentrations and pronouncedly positive ehf t values 11 0 to 14 7 indicating that the magmas originated from the remelting of juvenile lower crust followed by significant fractional crystallization at relatively shallow crustal depths data from this and previous studies convincingly suggest the dhb was located in a thickened continental arc setting during the early paleozoic probably related to the early subduction of the pao lithosphere crustal thickening as indicated by the geochemical signature of the dapokouzi pluton was most likely associated with the underplating of mantle derived mafic magmas during the slab subduction this study therefore highlights that the subduction of the southern pao might have initiated prior to 510 ma possibly dating back to the early cambrian

petrology geochemistry and genesis of high al tonalite and trondhjemites of the cornucopia stock blue mountains northeastern oregon tonalitic and trondhjemitic compositions span a narrow range of trondhjemite sio2 content 65 74 wt and exhibit characteristics of a highal tonalite trondhjemite granitoid ttg suite including light rare earth element lree enrichment low y nb and rb sr and high al2o3 and sr these compositions are consistent with an

3820 ma zircons from a tonalitic armisoq gneiss in the godthab district of southern west greenland abstract zircons from a sample of the tonalitic amitsoq gneisses near godthhb west greenland exhibit several stages of crystal growth measurements of the u th pb isotopic composition of 30 m areas within individual grains using the ion microprobe shrimp reveal that components of magmatic zircon from the original igneous protolith are at least 3822 5 ma l old slightly but significantly older than the age of zircons from a fclsic volcanic unit in the isua supracrustal belt as measured at high precision by the same technique and older than all ages previously determined by mineral and whole rock isotopic techniques for amitsoq gneisses at godthab and at isukasia overgrowths of younger zircon formed at ca 3630 ma coinciding with an episode of major pb loss from the older grains these results indicate that previous whole rock isochron ages refer to later isotopic redistribution and or that other phases of the amitsoq gneisses intruded during this later period and dominate the isotopic systematics comparison of the ion probe results with previous conventional u th pb data for the same zircon sample demonstrates the strong influence of discordant high uranium parts of zircon on the mean isotopic composition of multi grain samples

peraluminous granites frequently with mantle like isotope compositions the continental type murzinka and dzhabyk batholiths of the eastern urals murzinka and dzhabyk are continental type batholiths of the middle and southern east uralian domain they comprise mainly undeformed peraluminous k rich granites whose elemental composition is similar to some late variscan granites of western europe but with much more primitive sr and nd isotope ratios murzinka 254 5 ma is composed of silica rich granites forming two different rock series with a 87sr 86srinit of 0 709 and 0 704 respectively both series have enormous variations in end255 11 9 to 0 1 and 8 9 to 4 1 that reveal derivation from heterogeneous sources dzhabyk 291 4 ma also comprises two coeval magmas which yielded voluminous granites and quartz monzonites respectively with smaller differences in 87sr 86srinit and end290 0 7043 0 8 to 1 6 and 0 7049 0 0 to 0 8 despite their isotope compositions both batholiths lack evidence of genetic involvement of a mantle derived parental magma moreover we suggest that dzhabyk granitoids were generated by polybaric partial melting of paleozoic island arc material whereas murzinka granitoids derived from an extremely heterogeneous source consisting mainly of paleozoic and proterozoic metagreywackes this implies a relative fast reworking of juvenile arc crust and burial of the protoliths during the orogenic evolution of the urals since there is neither evidence of significant extension nor a direct link with subduction we suggest that the main cause for late orogenic anatexis was elevated heat production and fertility in the protolith perhaps combined with some additional heat from unexposed mafic intrusions

fluid present melting of ocean crust in subduction zones phase equilibria were investigated in the pressure range 400 2000 mpa on the pinatubo dacite a representative example of natural cenozoic adakites slab melts to constrain the temperature versus wt h 2 o conditions of magma genesis the experimental results suggest that water contents of the melt were at least 10 wt and temperatures were 900 c during magma ascent from the slab natural aluminous amphiboles present in the dacite crystallized at pressures of 1000 mpa fixing an upper temperature limit of 950 c for the slab melts at the crust mantle boundary amphiboles experimentally crystallized at 1000 mpa are identical to those found in melt inclusions in metasomatized mantle xenoliths these natural inclusions represent samples of slab melts and contain glasses with a dacitic bulk composition which requires an h 2 o content in the melt of 10 wt at 1000 mpa and 900 c for conditions of magma generation of 800 900 c at 2000 2300 mpa melt h 2 o contents close to 15 wt are needed to generate melts with dacitic bulk composition overall these constraints are inconsistent with slab melts being produced by dehydration melting of amphibolite a mechanism unable to produce dacite melt compositions that are both hydrous and cool slab melting in modern subduction zones must occur under fluid present conditions at temperatures below 900 c in agreement with the thermal regime of subducted oceanic crust as deduced from numerical simulation of heat transfer using the present day mantle geotherm at such low temperatures ti bearing phases are stable during melting thus any metasomatizing slab melt will be strongly depleted in nb ta and ti a major characteristic of arc magmas produced in the mantle wedge

petrogenesis of adakitic porphyries in an extensional tectonic setting dexing south china implications for the genesis of porphyry copper mineralization the dexing adakitic porphyries quartz diorite granodiorite porphyries associated with giant porphyry cu deposits are located in the interior of a continent south china they exhibit relatively high mgo cr ni and sr contents high la yb and sr y ratios but low yb and y contents similar to adakites produced by slab melting associated with subduction however they are characterized by bulk earth like nd sr isotope compositions end t 1 14 to 1 80 and 87 sr 86 sr i 0 7044 0 7047 and high th 12 6 27 2ppm contents and th ce 0 19 0 94 ratios which are different from those of cenozoic slab derived adakites sensitive high resolution ion microprobe shrimp geochronology studies of zircons reveal that the dexing adakitic porphyries have a crystallization age of 171 3ma this age is contemporaneous with middle jurassic extension within the shi han rift zone and within plate magmatism elsewhere in south china indicating that the dexing adakitic porphyries were probably formed in an extensional tectonic regime in the interior of the continent rather than in an arc setting their high th contents and th ce ratios and middle jurassic age argue against an origin from a neoproterozoic 1000ma stalled slab in the mantle taking into account available data for the regional metamorphic magmatic rocks and the present day crustal thickness 31km in the area we suggest that the dexing adakitic porphyries were most probably generated by partial melting of delaminated lower crust which was possibly triggered by upwelling of the asthenospheric mantle due to the activity of the shi hang rift zone moreover the dexing adakitic magmas must have interacted with the surrounding mantle peridotite during their ascent which elevated not only their mgo cr and ni contents but also the oxygen fugacity fo2 of the mantle the high fo2 could have induced oxidation of metallic sulfides in the mantle and mobilization of chalcophile elements which are required to produce associated cu mineralization therefore the cu metallogenesis associated with the dexing adakitic porphyries is probably related to partial melting of delaminated lower crust similar to the metallogenesis accompanying slab melting

partial melting of subducting oceanic crust the conditions under which partial melting of subducting oceanic crust occurs can be determined by combining a partial melting model for basaltic compositions with two dimensional thermal models of subduction zones for porosities of 1 containing h20 the amount of partial melt generated at the wet basaltic solidus is limited to 100 mpa can be maintained by rocks close to or above their melting temperatures in the absence of high shear stresses substantial melting of the oceanic crust will only occur during subduction of very young 5 ma oceanic lithosphere partial melting of hydrated basalt amphibolites derived from the mid ocean ridge has been proposed e g 1 3 as being responsible for the generation of certain recent high al andesitic to dacitic volcanic rocks adakites three of these volcanic suites mount st helens southern chile and panama occur in volcanic arcs where oceanic crust 25 ma is being subducted at rates of 1 3 cm yr and the calculated thermal regime is several hundreds of degrees hotter than more typical subduction zone environments however oceanic lithosphere is not currently being subducted beneath baja and new guinea where recent adakites are also present suggesting that some adakite magmas may form by water undersaturated partial melting of underplated mafic lower crust or previously subducted oceanic crust further experimental work on compositions representative of oceanic crust is required to define the depth of possible adakite source regions more accurately

the nature and timing of crustal thickening in southern tibet geochemical and zircon hf isotopic constraints from postcollisional adakites abstract rising as the roof of the world the tibetan plateau is now underlain with the thickest continental crust on earth how and when was this crust formed which would have exerted pivotal controls to the formation of the plateau has long been an issue of hot debates this paper reports zircon u pb ages and hf isotope data for postcollisional 30 9 ma adakites in the southern lhasa terrane southern tibet a comparative analysis of whole rock rare earth element geochemistry and zircon hf isotopes between the adakites and associated gangdese igneous rocks suggests that the tibetan crust underwent a major phase of tectonic thickening between ca 45 and 30 ma in the region the lower part of the thickened crust consisted prevailingly of mafic lithologies which we argue to have resulted from intense basaltic underplating and subsequent remelting that took place during the late cretaceous and eocene time related to the neotethyan subduction processes including breakoff of the subducted slab at ca 50 ma in the early stage of the india asia collision these processes were responsible for not only the juvenile crust formation but also for the creation of a thermally softened lithosphere in the southern lhasa terrane the indentation of india consequently caused distributed lithospheric thickening with formation of an orogenic root beneath southern tibet root foundering during the oligocene gave rise to the adakitic magmatism regional topographic uplift and onset of northward underthrusting of the indian plate that has since played a key role in forming the entire tibetan plateau

anatexis and metamorphism in tectonically thickened continental crust exemplified by the sevier hinterland western north america the generation of granitoid magmas by partial melting of crustal rocks during continental thickening events is well documented in many geological provinces throughout the world including the late mesozoic sevier belt of western north america we present a thermal and petrologic model of anatexis and metamorphism in regions of crustal thickening where the only mantle contribution is the normal conductive supply of heat through the base of the lithosphere i e advection of mass and energy are excluded we distinguish between formation of migmatites and generation of mobile granitoid magmas and examine the temporal and spatial relationships between these two distinct anatectic processes between anatexis and regional deformation and between anatexis and metamorphism a fundamental conclusion is that if protoliths rich in hydrous minerals are present regional anatexis is the end product of classical barrovian metamorphism in thickened continental crust even in the absence of a free water rich fluid phase barrovian metamorphic facies series are predicted with thickening ratios maximum crustal thickness attained initial crustal thickness as low as 1 3 but mobile granitoid magmas are not formed if this ratio is less than approximately 1 5 above these lower bounds barrovian metamorphism and anatectic granitoid magmatism occur independently of the magnitude of thickening and of the way in which thickening is accomplished both processes are sensitive to a diminished heat supply lowering either moho heat flow or crustal radioactive heat production results in blueschist eclogite metamorphism and inhibits the formation of mobile granitoid magmas we model anatexis under fluid absent conditions and show that with such a constraint migmatization is always a syn kinematic process relative to the crustal thickening event whereas generation of mobile granitoid magmas is in most cases post kinematic relative to crustal thickening but can be syn kinematic if thickening takes more than approximately 50 myr the typical time intervals for melting are consistent with geological observations mobile granitoid magmas are predicted by most of our models within approximately 10 myr of the end of the crustal thickening event this incubation period results primarily from the temperature increase required for the dehydration melting reactions capable of producing large melt fractions to occur the energetic requirements of anatexis are relatively minor compared to conductive crustal thermal budgets as shown by the fact that once the necessary p t conditions are attained melting reactions are completed within time intervals on the order of 1 myr i e 1 2 orders of magnitude smaller than the characteristic time scales of the tectonic processes involved in crustal thickening

origin of mesozoic adakitic intrusive rocks in the ningzhen area of east china partial melting of delaminated lower continental crust to the best of our knowledge modern adakites have not been documented in a nonarc environment we report geochemical and isotopic data for early cretaceous anjishan adakitic intrusive rocks that are in a continental setting unrelated to subduction the anjishan adakitic intrusive rocks which are exposed in the ningzhen area of east china have high sr y and la yb ratios coupled with low yb and y as well as relatively high mgo contents and mg numbers mg 0 4 0 6 similar to products from slab melting however low n d t values 6 8 to 9 7 and high 8 7 sr 8 6 sr i 0 7053 0 7066 are inconsistent with an origin by slab melting the tectonics and geochemistry lead us to conclude that adakitic magmas were most likely derived from partial melting of mafic material at the base of the continental crust high sr y and la yb ratios of the adakitic intrusive rocks suggest that garnet was stable as a residual phase during partial melting implying that the crustal thickness exceeded 40 km in the early cretaceous the present thickness of the crust in the ningzhen area is only 30 km and therefore the crust appears to have been thinned by at least 10 km since the early cretaceous the relatively high mgo contents and mg of the anjishan intrusive rocks suggest that adakitic magmas interacted with mantle rocks possibly coinciding with lower crustal delamination which would also account for the observed thinning

a 9 year time series of planktonic foraminifer fluxes and environmental change in the bering sea and the central subarctic pacific ocean 1990 1999 abstract a 9 year study of planktonic foraminifer fluxes was conducted in the bering sea station ab and in the central subarctic pacific station sa results clearly reflected variations of the water mass characteristics in the upper layers the 9 year means of total foraminifer fluxes were the same 1400 shells m 2 d 1 at both stations however total foraminifer flux at station ab tended to show its primary maximum during fall october december and its secondary maximum in spring april june whereas the primary maximum appeared in spring and the secondary maximum in fall at station sa seasonal variation was more apparent at hemipelagic station ab than at pelagic station sa planktonic foraminifers found at both stations were of six species neogloboquadrina pachyderma globigerina umbilicata globigerinita glutinata globigerina quinqueloba globorotalia wilesi and orbulina universa the foraminifer assemblages at the two stations reflected the temperature difference in the surface waters the variable g umbilicata tended to be high in the warm surface waters during the summers the temporal and geographical variation of g quinqueloba indicated that this taxon prefers regions with relatively low diatom fluxes a notable appearance of o universa occurred in 1997 at station sa during this period other measured biogenic particle fluxes such as those of diatoms were low this unusual 1997 event may be a reflection of global climatic change that happened to be observed in the central subarctic pacific ocean

dehydration melting of a basaltic composition amphibolite at 1 5 and 2 0 gpa implications for the origin of adakites this study presents the results of dehydration melting experiments on a basaltic composition amphibolite under conditions appropriate to a hot slab geotherm 1 5 and 2 0 gpa and temperatures of 850 to 1150 c dehydration melting produces an omphacitic augite and garnet bearing residue coexisting with rhyolitic to andesitic composition melts at 1 5 gpa the amphibolite melts in two stages between 800 and 1025 c the 2 0 gpa data also define two melting stages at 2 0 gpa the first stage involves nearly modal melting of the original amphibolite minerals qtz pl amp to produce melt cpx grt during the second stage the eclogite restite melts non modally 0 86 cpx 0 14 grt 1 melt the experimental results were combined with data from the literature to generate a composite p t phase diagram for basaltic composition amphibolites over the 800 to 1100 c temperature range for pressures up to 2 0 gpa comparison of the major element compositions of the experimentally produced melts with compositions of presumed slab melts adakites shows that partial melting of amphibolite at conditions appropriate to a hot slab geotherm produces melts similar to andesitic and dacitic adakites except for significant mgo and cao depletions trace element modelling of amphibolite dehydration melting using the 2 0 gpa melting reactions produces ree abundances similar to those of adakites at 10 15 wt batch melting but the models do not reproduce the high sr y ratios characteristic of adakites taken together the major and trace element results are not consistent with the derivation of adakites by dehydration melting of the subducted slab with little or no interaction with the mantle wedge or crust if adakites are partial melts of the subducted slab they must undergo significant interaction with the mantle and or crust during which they acquire a number of their distinctive characteristics

the dunhuang block is a paleozoic orogenic belt and part of the central asian orogenic belt caob nw china abstract the dunhuang region is located in the east of the tarim craton and has long been considered as part of a precambrian terrane of the tarim or north china craton aiming to further understand the tectonic evolution of the dunhuang region and its relations with the adjacent tectonic units this study is to investigate the metamafic rocks in the sanweishan area dunhuang region and in turn to summarize the tectono thermal events and re examine the tectonic characteristics of the region the metamafic rocks in the sanweishan area include three types of amphibolites they are garnet clinopyroxene amphibolites garnet free amphibolites and garnet mica amphibolites the whole rock major and trace element compositions suggest that their protoliths probably are e morb type tholeiitic basalts with some crustal addition the la icp ms zircon u pb data of the garnet clinopyroxene amphibolite and the garnet free amphibolite yielded lower intercept ages of 410 11 ma and 403 8 ma respectively while the garnet mica amphibolite gave three metamorphic ages of 441 3 ma 408 12 ma and 370 2 ma three suites of metamorphic conditions have been estimated at 1 14 1 55 gpa and 458 548 c 0 67 0 82 gpa and 590 600 c and 0 47 0 66 gpa and 557 586 c for garnet clinopyroxene amphibolite using different mineral thermobarometers which illustrate a collisional orogeny related clockwise p t path integrated with the study on the high pressure granulite in the mogutai area it can be inferred that the metamafic rocks in the sanweishan area obviously experienced amphibolite facies metamorphism at ca 410 400 ma and they could also underwent high grade metamorphism at ca 440 430 ma therefore we suggest that the e w trending collisional orogeny of ca 440 400 ma could sweep cross the dunhuang region the dunhuang region records multiple tectono thermal events occurring at ca 3 1 2 5 ga ca 2 3 1 6 ga ca 440 400 ma and ca 370 310 ma however compared with the limited exposure of the precambrian complex the early silurian late carboniferous ca 440 310 ma orogeny related metamorphic and magmatic rocks are widely distributed in the dunhuang region making up nearly 70 of the total exposure thus it is proposed that the geological bodies in the dunhuang region were mainly produced by a paleozoic orogeny and represent a paleozoic orogen while the limited precambrian complex probably is a microcontinental fragment or part of the margin of the tarim or north china cratons that was involved in this paleozoic orogeny during the early silurian late carboniferous we infer that the dunhuang orogenic belt is part of the central asian orogenic belt caob and thus the caob probably extends beyond the beishan orogenic collage and southward into the dunhuang region

paleoproterozoic crustal evolution of the tarim craton constrained by zircon u pb and hf isotopes of meta igneous rocks from korla and dunhuang abstract the tarim craton is one of three large cratons in china presently there is only scant information concerning its crustal evolutionary history because most of the existing geochronological studies have lacked a combined isotopic analysis especially an in situ lu hf isotope analysis of zircon in this study precambrian basement rocks from the kuluketage and dunhuang blocks in the northeastern portion of the tarim craton have been analyzed for combined in situ laser ablation icp mc ms zircon u pb and lu hf isotopic analyses as well as whole rock elements to constrain their protoliths forming ages and magma sources two magmatic events from the kuluketage block at 2 4 ga and 1 85 ga are revealed and three stages of magmatic events are detected in the dunhuang block i e 2 0 ga 1 85 ga and 1 75 ga the 1 85 ga magmatic rocks from both areas were derived from an isotopically similar crustal source under the same tectonic settings suggesting that the kuluketage and dunhuang blocks are part of the uniform precambrian basement of the tarim craton zircon hf model ages of the 2 4 ga magmatism indicate that the crust of the tarim craton may have been formed as early as the paleoarchean period the 2 0 ga mafic rock from the dunhuang block was formed in an active continental margin setting representing an important crustal growth event of the tarim craton in the mid paleoproterozoic that coincides with the global episode of crust formation during the assembly of the columbia supercontinent the 1 85 ga event in the kuluketage and dunhuang blocks primarily involved the reworking of the old crust and most likely related to the collisional event associated with the assembly of the columbia supercontinent while the 1 75 ga magmatism in the dunhuang block resulted from a mixture of the reworked archean crust with juvenile magmas and was most likely related to a post collisional episode

amphibolite facies metamorphism and geochronology of the paleoproterozoic aketashitage orogenic belt northwestern china abstract garnet amphibolite and semi pelitic gneiss preserving three stages of metamorphic mineral assemblages i e the prograde metamorphic inclusion assemblage m1 preserved in the garnet porphyroblast metamorphic peak assemblage m2 and retrograde symplectic assemblage m3 have been recognized in the aketashitage area northwestern china geothermobarometric computation shows that the m1 m2 and m3 assemblages record the metamorphic p t conditions of 600 700 c 4 2 5 3 kbar 730 790 c 6 3 7 1 kbar and 770 c 5 6 kbar respectively these three metamorphic stages define clockwise p t paths including nearly isothermal decompression itd process indicating typical orogenic metamorphism the peak metamorphic p t conditions reach the transition zone between the high amphibolite facies and granulite facies and belong to intermediate p t facies series the shrimp and la icp ms u pb dating of metamorphic zircon implies that the subduction might occur as early as 1 97 ga and last as late as 1 84 ga combined with literature data it is concluded that the aketashitage area was a paleoproterozoic orogenic belt which is possibly related to the assembly of the basements of the columbia supercontinent

paleozoic sanweishan arc in the northern dunhuang region nw china the dunhuang block is a phanerozoic orogen not a precambrian block abstract the dunhuang region is located to the south of the central asian orogenic belt and connecting the tarim craton westerly and north china craton easterly respectively it is an important position for understanding the geodynamics and tectonic framework of central asia however the issue about whether it is a phanerozoic orogen or a precambrian block is still controversial magmatic arcs are the most prominent and laterally persistent structural elements of orogenic belts and used to reconstruct now disrupted orogenic belts this paper concerns the petrogenesis and tectonic setting of andesitic dacitic rhyolitic volcaniclastic rocks and dacitic porphyries in the sanweishan area northern dunhuang region volcaniclastic rocks overlie the metasedimentary rocks by fault and dacitic porphyries intrude into the metasedimentary rocks the calc alkaline signature and relative enrichment of large ion lithophile elements of volcaniclastic rocks and dacitic porphyries seemingly favor a subduction setting zircon u pb dating results reveal the deposition at 424 414 ma for volcaniclastic rocks and emplacement at ca 364 ma for dacitic porphyries the coeval granitoid plutons in the sanweishan area show arc geochemical signature and are originated from partial melting of the subducted sediments based on the available data paleozoic volcanic subvolcanic rocks and plutons in this area probably represent a magmatic arc developed on the accretionary complex resembling the arc magmatic rocks formed in the nankai complex sw japanese island arc our results provide constraints on the existence of the paleozoic magmatic arc in the northern dunhuang region and suggest the so called dunhuang block is virtually a phanerozoic orogen rather than a precambrian block

underplating related adakites in xinjiang tianshan china abstract intermediate to acidic porphyries and dacites in the awulale and sanchakou area of the xinjiang tianshan region china conform to the definition of high sio2 adakite hsa these volcanic or subvolcanic rocks are characterized by high na2o na2o k2o high sr y 51 327 and strong depletions in hree and hfse nb ta and ti despite these characteristics they are distinct from a suite of adakites in the same area that are inferred to be derived from partial melting of subducted slab differences between the two suites are evident mainly in their rock association isotopic ages and compositions of sr and nd isotopes the subducted slab related adakites are associated with common island arc rocks niobium enriched basalt neb and high mg andesite hma formed in the late carboniferous 320 334 ma and are characterized by relative high nd t 3 4 11 6 and low 87sr 86sr i ratios 0 7032 0 7063 in contrast the awulale and sanchakou adakites were generated during the middle to late permian 260 278 ma possess a wide range of low nd t 0 75 5 69 a greater range of 87sr 86sr i 0 7039 0 7054 and have young nd model ages t2dm 472 699 ma in addition their mg 35 56 and compatible element contents mgo 0 93 2 22 wt cr 4 95 16 41 ppm ni 2 9 25 8 ppm are relative low compared to the subducted slab related adakites mg 55 71 mgo 1 22 6 78 wt cr 24 132 ppm ni 2 28 45 61 ppm based on these characteristics the regional association of igneous rocks and evidence from a global geoscience transect ggt the awulale and sanchakou adakites represent a genetically distinct suite in the xinjiang region we proposed that the source of these adakites was basaltic rock underplated to the base of the lower crust that partial melted at the rutile bearing amphibole eclogite facies 650 c and 1 5 gpa minimum pressure 50 km

adakites without slab melting high pressure differentiation of island arc magma mindanao the philippines abstract new geochemical data for pleistocene magmatic rocks from the surigao peninsula eastern mindanao the philippines demonstrate typical adakitic traits including elevation of sr y and depletion of the heavy rare earth elements 87sr 86sr and 143nd 144nd ratios of the adakites do not support melting of the subducted philippine sea plate but resemble pliocene arc lavas generated in the same subduction zone excepting the heavy rare earth elements the adakites and arc lavas also possess similar ratios of incompatible elements suggesting that the adakites were ultimately derived from melting of the mantle wedge the wide range of sio2 in the adakites and its strong correlation with trace element concentrations and ratios indicate two possible mechanisms for generating the adakitic signature 1 adakitic melt was produced from basaltic arc magma by fractional crystallisation of a garnet bearing assemblage 2 solidified basaltic rock containing garnet melted to yield adakitic magma in either case the basaltic precursor was generated from fluid modified mantle then differentiated within the garnet stability field in surigao this requires differentiation within mantle the surigao example suggests that any subduction zone has the potential to produce adakitic magma if basalt crystallises at sufficient depth this has important implications for the geodynamics of modern and ancient subduction zones that have generated similar rocks

granitoid associated orogenic intrusion related and porphyry style metal deposits in the archean yilgarn craton western australia abstract the yilgarn craton hosts three types of archean granitoid associated metallogenic systems orogenic gold intrusion related and porphyry systems these systems may occur in the same terrane and share many broad characteristics including spatial coincidence with granitoids gangue silicate mineralogy metal associations and local structural controls features of archean orogenic gold systems spatially associated with granitoids include i a broad range of granitoid host compositions ii gold mineralization is mostly late with respect to yilgarn wide granitoid emplacement peak metamorphism and deformation iii orebodies are structurally controlled and iv gold deposition occurs over a large range of temperatures pressures and crustal depths 2 to 15 km from a co 2 rich low to moderate salinity reduced ore fluid that was derived from a metamorphic and or distal magmatic fluid source these systems do not display deposit scale vertical or lateral zonation of metals away from granitoid contacts host granitoid age composition or oxidation state are not important in controlling gold mineralization processes whereas structural setting and fluid flux are paramount archean intrusion related au mo w and mo au systems in the yilgarn craton are commonly small 2 bearing moderate to high salinity fluid during ore deposition archean porphyry cu mo au cu mo and cu au systems are spatially and temporally associated with volumetrically small pervasively altered felsic plutons and dikes surrounded by altered and mineralized supracrustal countryrock lateral zonation of metals away from porphyry centers commonly includes a decrease in cu au and ag au ratios with distance from the source pluton microthermometry studies on fluid inclusions in gold bearing veins suggest that early ore fluids were high temperature and high salinity and that mineralization occurred at 2 o 3 feo are demonstrated in phanerozoic terranes but are presently ill defined in the yilgarn craton crustal depth estimation is important in defining areas likely to have preserve shallow forming archean porphyry systems