depleted mantle source for the ulungur river a type granites from north xinjiang china geochemistry and nd sr isotopic evidence and implications for phanerozoic crustal growth abstract the ulungur river granites from north xinjiang china display two petrological trends 1 a peralkaline trend composed of alkali amphibole and alkali pyroxene bearing granites and dykes and 2 an aluminous trend represented by alkali granites monzogranites and aluminous granite dykes within the peralkaline granites rb sr whole rock isochron dating yielded an age of 300 ma for the emplacement of the peralkaline granites and 270 ma for the aluminous granites the ulungur river granites share all the geochemical features common to a type granites of the world the ulungur river granites are post orogenic and emplaced in a post collisional extensional environment the peralkaline and aluminous rocks have indistinguishable nd isotopic compositions they have unusually positive and relatively uniform nd t values ranging from 5 1 to 6 7 for the peralkaline rocks at 300 ma and from 5 5 to 5 9 for the aluminous rocks at 270 ma the initial 87sr 86sr ratios cannot be determined precisely by the whole rock isochron technique mainly because most samples are highly radiogenic and have high rb sr ratios in addition the large variation in sr isotopic compositions can partly be explained by crustal contamination during magma differentiation the nd isotopic data clearly indicate that both the peralkaline and aluminous rocks were derived from a long lived depleted mantle reservoir the formation of the ulungur river a type granites is hypothesized as follows 1 a depleted mantle was first metasomatised then subjected to partial melting leading to the generation of basic magmas now represented by the kalatongke mafic ultramafic complex in the ulungur river area 2 the basic magmas have undergone high degrees of fractional crystallisation with separation of biotite hornblende plagioclase potassium feldspar and other accessory phases such as apatite and ilmenite with the residual magmas forming the peralkaline granites during the magmatic differentiation the liquids were probably contaminated by crustal materials as evidenced by nb depletion and nd sr isotopic systematics on the other hand the formation of aluminous granites involved a replenishment of new mantle derived magmas into the differentiating magma chamber and followed by fractional crystallisation with retention of hydrous phases hornblende and biotite the petrological evolution of the ulungur river granites is thus from peralkaline to calc alkaline and not from calc alkaline to peralkaline as more often observed elsewhere the ulungur river granites in common with the voluminous granitoids occurring in the central asian orogenic belt represent a significant juvenile addition of the continental crust in the phanerozoic

fractionation hybridisation and magma mixing in the kialineq centre east greenland the igneous rocks of the kialineq centre on the coast of east greenland at 67 n include a number of quartz syenite and granite plutons intruded 35my bp these are subvolcanic bodies emplaced by cauldron subsidence and with ring dike and bell jar form associated with the major intrusions is an extensive acid basic mixed magma complex two liquid structures chilling of basic against acid magma pillows of basic in acid and net veining of basic by acid magma are superbly displayed the basic magma was of a transitional or alkaline type and underwent varying degrees of fractionation in a regime of repeated intrusions and diverse chambers heterogeneous hybrid rocks intermediate between basalt and quartz syenite are strongly developed and were formed by repeated mechanical mixing of contrasting magmas the energy for this mixing probably came in the main from cauldron block subsidence the quartz syenite magma which itself fractionated towards granite has initial 87sr 86sr ratios the same as the basic magma and is itself believed to be a fractionation product of alkali basalt magma

late paleozoic anorogenic magmatism of the gobi altai sw mongolia tectonic position geochronology and correlation with igneous activity of the central asian orogenic belt abstract geological and geochronological u pb zircon id tims and 40ar 39ar amphibole data were obtained for six anorogenic magmatic associations in the gobi altai southwestern mongolia in order to reveal its relationships with late paleozoic tectonothermal events in the southern part of the central asian orogenic belt caob in the gobi altai massifs of alkaline granites and grabens filled with bimodal volcanic suites occur as a chain 450 km long east southeast direction the massifs and volcanic fields are controlled by contacts of pre ediacaran continental crustal blocks with late neoproterozoic cambrian juvenile ophiolite and arc type complexes of the lake zone gobi altai alkaline granites bear sodic rich mafic silicate minerals and possess typical properties of ferroan a type granites our geochronological data constrain emplacement time of the gobi altai alkaline granites and bimodal volcanic rocks at 293 1 279 1 ma this age interval coincides with timing of the emplacement of i type granites with a few s type granites and gabbroic intrusions in the mongolian and chinese altai as well as with numerous massifs of a type granites layered mafic ultramafic intrusions and eruption of bimodal volcanic rocks between the southern edge of the siberia craton and the northern margins of the tarim and north china cratons hence the gobi altai anorogenic magmatic complexes were produced by a large scale late paleozoic magmatic event or events that encompassed a wide spectrum of terranes in caob the most realistic models explaining these magmatic processes are i the effect of the tarim mantle plume ii slab or slabs break off after subduction blocking due to accretion of terranes of various types

late carboniferous to early permian magmatic pulses in the uliastai continental margin linked to slab rollback implications for evolution of the central asian orogenic belt abstract the paleo asian ocean underwent a protracted closure history during late paleozoic here we investigate the magmatic evolution during this process based on a detailed study in the baiyinwula region along the uliastai continental margin the major rock types in this area are late carboniferous early permian volcanic sequences and coeval intrusions we identified four stages of magmatic evolution based on the diverse assemblages and their precise isotopic ages the first stage is represented by andesites with a zircon 206pb 238u age of ca 326 12 ma these rocks are metaluminous to weakly peraluminous high k calc alkaline and possess high na2o k2o ratios in the range of 1 23 to 2 45 they also display enrichment of large ion lithophile elements lile and depletion of high field strength elements hfse with markedly positive zircon ehf t varying from 8 1 to 15 6 the geochemical features of these andesites are similar to those of typical arc volcanic rocks the second stage includes granodiorites emplaced at 318 6 1 8 ma the rocks are high k calc alkaline with a cnk values ranging from 0 95 to 1 06 and show enrichment in lile and depletion in hfse they show geochemical affinities to adakites with high sr and low y and yb contents indicating magma derivation from thickened lower crust zircon grains from these rocks display positive initial ehf t values ranging from 11 1 to 14 6 with corresponding two stage hf model ages tdm2 of 394 622 ma the third stage consists of syenogranite together with a volcanic suite ranging in composition from rhyolite todacite which formed during 303 4 1 2 to 285 1 2 2 ma they possess elevated silica and alkali contents high feot mgo and ga al ratios low al2o3 mgo and cao contents and high rb y nb ce zr y and ga contents strong negative ba sr and eu anomalies showing i to a type granitic affinities zircons in these rocks show elevated hf isotopic compositions ehf t 9 9 to 14 6 with tdm2 varying from 324 to 673 ma the fourth magmatic pulse is represented by k feldspar granite with zircon u pb ages from 283 2 1 9 ma to 280 0 1 4 ma and typical alkalic a type granite geochemistry these rocks possess positive ehf t values in the range of 9 7 to15 2 and a restricted range of hf model age from 327 to 684 ma the magmatic rocks from the four stages show comparable ehf t and t2dm suggesting that the magmas were derived from the same evolving mantle derived source we propose a tectonic model linking the evolution of the magmatism with the closure of the paleo asian ocean that involved the following stages the andesites were formed during the initial oceanic subduction stage with magma sourced from the metasomatized lithospheric mantle stage 2 adakite like rocks were derived from subduction induced thickened crust subsequent slab rollback resulted in asthenospheric upwelling and melting of residual juvenile crust to generate the i and a type syenogranite rhyolite and dacite suite finally followed by the a type k feldspar granite

petrogenesis of the triassic bayan ulan alkaline granitic pluton in the north gobi rift of central mongolia implications for the evolution of early mesozoic granitoid magmatism in the central asian orogenic belt abstract the 221 ma bayan ulan bu granitic pluton occurs in the north gobi rift of central mongolia which was superimposed on the paleozoic central asian orogenic belt caob an area characterized by voluminous juvenile crust this large 1000 km 2 pluton is part of an early mesozoic granitoid province in the central part of the caob it consists mainly of alkali feldspar granites containing mesoperthite with interstitial quartz minor ca na and na amphiboles and rare fe rich biotite graphic texture is common the bu granites are fractionated leucocratic alkaline a type rocks that are enriched in incompatible elements particularly rb and th and depleted in ba sr eu and ti most have nd t values of 1 4 to 1 7 and t dm model ages 800 and 1200 ma and lie within the range of other a type granites of the caob granitoid province the granites are interpreted to have formed by the partial melting of underplated neoproterozoic mildly alkaline basalts in the lower crust followed by fractional crystallization dominated by feldspars the formation of the bu pluton and the coeval khentei batholith the largest late triassic intrusion in north central mongolia appears to be related either to a plume or to the closure of the mongol okhotsk ocean north gobi rift magmatism extends across the mongol okhotsk suture so a plume related origin is more consistent with available data this plume which would provide the heat source for magmatism in this area explains the eastward movement of large scale magmatism over time from the tarim traps 300 275 ma through khangai 270 240 ma to khentei 230 195 ma in central mongolia

peralkaline granitoid magmatism in the mongolian transbaikalian belt evolution petrogenesis and tectonic significance article i nfo the central asian orogenic belt caob is well known for its massive generation of juvenile crust in the phanerozoic in eastern caob voluminous peralkaline and alkaline alkali feldspar granitoids and genetically related bimodal volcanics were emplaced in three stages from early permian to late mesozoic 295 270 ma 230 190 ma and 150 120 ma they occur as huge granitoid belts in extensional tectonic settings and form a complex network of about 12 million km 2 in area among them the 2500 km long mongolian transbaikalian belt mtb is the most spectacular and it comprises more than 350 granite syenite plutons and stocks with numerous co genetic volcanic fields the three stages of granitoids have similar chemical compositions but show temporal variation in nd isotopic composition initial end t values range from 1t o 5 for the early permian 0 to 4 for the early mesozoic and 2t o 3 5 for the late mesozoic granitoids the negative end t values observed in the mtb are not typical of the caob granitoids which are generally characterized by positive values however several pieces of evidence suggest that the mtb peralkaline and alkali feldspar granitoids were produced from enriched mantle derived sources the evidence includes 1 felsic and mafic rocks formed in the same stage have similar initial nd sr isotopic ratios and sm nd model ages tdm 2 the granitoid belts extend over thousands of kilometers and intersect distinct crustal provinces but no correlation is found in chemical compositions between the granitoids and country rocks 3 the abundant syenites are considered to be cogenetic or in some cases parental to the granites experimental and isotope data argue for the derivation of syenites from an enriched mantle source thus the granites and comendites are regarded as mantle derived 4 a study of melt inclusions in quartz phenocrysts of comendite indicates a high liquidus temperature of 1000 1100 c for the magma generation this suggests that the silicic magma was generated in an unusually high temperature condition which is likely produced by basaltic magma that underplated the lower crust the generation of voluminous peralkaline and alkaline alkali feldspar granitoids and genetically related bimodal volcanic rocks represents an important addition of juvenile crustal mass to the earth s continental crust during a time span of about 150 ma from late paleozoic to late mesozoic

geochronology and geochemistry of the triassic bimodal volcanic rocks and coeval a type granites of the olzit area middle mongolia implications for the tectonic evolution of mongol okhotsk ocean abstract the olzit volcanism in middle mongolia comprises a bimodal suite of basalts and peralkaline rhyolites adjacent to the main mongolia lineament the basalts are characterized by enrichment in lile and lree and depletion in hfse with typical sr nd isotopic signatures end t 2 50 to 0 38 and 87sr 86sr i 0 7058 0 7063 indicating they were likely derived from partial melting of an enriched lithospheric mantle modified by subducted slab derived fluids the rhyolites show a close affinity to a type granites with enrichment in lile and lree and depletion in nb ta and ti they also show a significant negative eu anomaly and have end t values ranging from 0 50 to 1 38 and initial 87sr 86sr ratios ranging from 0 7022 to 0 7200 suggesting the rhyolites stem from partial melting of crustal rocks rather than fractional crystallization of the basaltic melt the rhyolite porphyry yields a shrimp zircon u pb age of 207 2 ma mswd 1 42 indicating the bimodal volcanic suite formed in the late triassic the miarolitic per alkaline granite and biotite bearing granite which are associated with the bimodal volcanic rocks show typical a type granitic geochemical affinity with end t 0 89 0 91 and 87sr 86sr i 0 7021 0 7043 indicating they are likely generated by partial melting of crustal rocks similar to the rhyolitic end member of bimodal suite the miarolitic per alkaline granite and biotite bearing granite yielded shrimp zircon u pb ages of 209 2 ma mswd 0 91 and 213 3 ma mswd 1 65 respectively which are nearly coeval with the age of the bimodal volcanic suites in view of the new geochemical and chronological data in this study we suggest the olzit late triassic bimodal volcanic rocks together with the coeval a type granites represent a back arc basin extensional environment which probably related to the roll back of mongol okhotsk oceanic plate during the southward subduction under the central mongolia microcontinent

boundary lines within petrologic diagrams which use oxides of major and minor elements use of some petrologic diagrams applied to analyses of volcanic rocks is unnecessarily difficult due to lack of data for construction of discriminant lines between rock series coordinates are provided for sufficient points to enable accurate plotting of the boundary lines within seven diagrams viz 1 tas total alkalies na2o k2o vs sio2 2 k2o vs sio2 3 afm 4 jensen 5 ktp k2o tio2 p2o5 6 fms feo mgo vs sio2 and 7 taktip k2o na2o k2o vs tio2p2o5 different versions of these boundaries are collated to indicate their variable position and it is demonstrated that inter laboratory analytical precision suffices to account for almost all of their spread on the tas and k2o vs sio2 diagrams

subsurface geometry of the san andreas fault in southern california results from the salton seismic imaging project ssip and strong ground motion expectations the san andreas fault saf is one of the most studied strike slip faults in the world yet its subsurface geometry is still uncertain in most locations the salton seismic imaging project ssip was undertaken to image the structure surrounding the saf and also its subsurface geometry we present ssip studies at two locations in the coachella valley of the northern salton trough on our line 4 a fault crossing profile just north of the salton sea sedimentary basin depth reaches 4 km southwest of the saf on our line 6 a fault crossing profile at the north end of the coachella valley sedimentary basin depth is 2 3 km and centered on the central most active trace of the saf subsurface geometry of the saf and nearby faults along these two lines is determined using a new method of seismic reflection imaging combined with potential field studies and earthquakes below a 6 9 km depth range the saf dips 50 60 ne and above this depth range it dips more steeply nearby faults are also imaged in the upper 10 km many of which dip steeply and project to mapped surface fault traces these secondary faults may join the saf at depths below about 10 km to form a flower like structure in appendix d we show that rupture on a northeast dipping saf using a single plane that approximates the two dips seen in our study produces shaking that differs from shaking calculated for the great california shakeout for which the southern saf was modeled as vertical in most places shorter period t 1 s shaking is increased locally by up to a factor of 2 on the hanging wall and is decreased locally by up to a factor of 2 on the footwall compared to shaking calculated for a vertical fault

evolution of calc alkaline to alkaline magmatism through carboniferous convergence to permian transcurrent tectonics western chinese tianshan continuous magmatic activity occurred in the western chinese tianshan central asia from the carboniferous to the permian i e before and after the late carboniferous amalgamation of junggar and the yili blocks zircon u pb la icpms and ar ar data reveal a coincidence in time between regional wrench faulting and granitoid emplacement permian post collisional granitoids crop out within or at the margins of large scale dextral strike slip shear zones some of them show synkinematic fabrics the whole rock geochemical features of the early middle permian granitoids indicate an evolution from high k calc alkaline towards alkaline series in other places of the north tianshan alkaline magmatism occurred together with deep marine sedimentation within elongated troughs controlled by wrench faults therefore in contrast with previous interpretations that forwarded continental rift or mantle plume hypotheses the coexistence of diverse magmatic sources during the same tectonic episode suggests that post collisional lithosphere scale transcurrent shearing tightly controlled the magmatic activity during the transition from convergent margin to intraplate anorogenic processes

a rift system in southeastern central asian orogenic belt constraint from sedimentological geochronological and geochemical investigations of the late carboniferous early permian strata in northern inner mongolia china abstract the late carboniferous early permian is the most controversial period for the tectonic evolution of the southeastern central asian orogenic belt caob in this study we performed sedimentological and detrital zircon dating on the clastic sediments as well as zircon u pb age and geochemical analysis on the rhyolitic rocks of the late carboniferous early permian strata from northern inner mongolia china the sedimentological analysis showed that the late carboniferous early permian sediments are composed of continental clastic and volcanic rocks with basal conglomerate overlying the devonian strata the detrital zircon study revealed three age peaks at 300 ma 440 ma and 530 ma indicating that the clastic rocks derived from paleozoic magmatic rocks and pre carboniferous sediments from northern inner mongolia the age data revealed that rhyolitic rocks from northern inner mongolia extruded in two stages at late carboniferous and latest carboniferous early permian the late carboniferous rhyolites show i to a type granitic affinities whereas the latest carboniferous early permian rhyolites have a type granitic characteristics the difference suggests a transition of tectonic setting from post collisional extension to intraplate rift the late carboniferous early permian strata from northern inner mongolia and south mongolia show similar sedimentary facies detrital zircon age distribution and geochemistry features hence they were probably deposited in the same tectonic setting a unified block was finally proposed in northern inner mongolia and south mongolia where a rift system provided an intra continental sedimentary basin and induced intense syn rift magmatic activities the intra continental basin and syn rift magmatism indicate that the xing an airgin sum songliao hunshandake and south mongolia blocks in southeastern caob have been welded into a single continental block before the late carboniferous

isotope provinces mechanisms of generation and sources of the continental crust in the central asian mobile belt geological and isotopic evidence abstract the available geological geochronological and isotopic data on the felsic magmatic and related rocks from south siberia transbaikalia and mongolia are summarized to improve our understanding of the mechanisms and processes of the phanerozoic crustal growth in the central asian mobile belt camb the following isotope provinces have been recognised precambrian t dm 3 3 2 9 and 2 5 0 9 ga at the microcontinental blocks caledonian t dm 1 1 0 55 ga hercynian t dm 0 8 0 5 ma and indosinian t dm 0 3 ga that coincide with coeval tectonic zones and formed at 570 475 420 320 and 310 220 ma continental crust of the microcontinents is underlain by or intermixed with juvenile crust as evidenced by its isotopic heterogeneity the continental crust of the caledonian hercynian and indosinian provinces is isotopically homogeneous and was produced from respective juvenile sources with addition of old crustal material in the island arcs or active continental margin environments the crustal growth in the camb had episodic character and important crust forming events took place in the phanerozoic formation of the camb was connected with break up of the rodinia supercontinent in consequence of creation of the south pacific hot superplume intraplate magmatism preceding and accompanying permanently other magmatic activity in the camb was caused by influence of the long term south pacific plume or the asian plume damping since the devonian

zircon u pb geochronology and hf isotopic composition of granitiods in russian altai mountain central asian orogenic belt the central asian orogenic belt caob consists of many tectonic terranes with distinct origin and complicated evolutionary history understanding of individual block is crucial for the reconstruction of the geodynamic history of the gigantic accetionary collage this study presents zircon u pb ages and hf isotopes for the granitoid rocks in the russian altai mountain range including gorny altai altai mongolian terrane and ctus suture zone between them in order to clarify the timing of granitic magmatism source nature continental crustal growth and tectonic evolution our dating results suggest that granitic magmatism of the russian altai mountain range occurred in three major episodes including 445 429 ma 410 360 ma and 241 ma zircons from these granitoids yield comparable positive ehf t values and neoproterozoic crustal model ages which favor the interpretation that the juvenile crustal materials produced in the early stage of the caob were probably dominant sources for the paleozoic magmatism in the region the inference is also supported by widespread occurrence of short lived juvenile materials including ophiolites seamount relics and arc assemblages in the northern caob consequently the paleozoic massive granitic rocks maybe do not represent continental crustal growth at the time when they were emplaced but rather record reworking of relatively juvenile proterozoic crustal rocks although mantle derived mafic magma was possibly involved to serve as heat engine during granitic magma generation the early triassic granitic intrusion may be a product in an intra plate environment as the case of same type rocks in the adjacent areas the positive ehf t values 1 81 7 47 and corresponding hf model ages 0 80 1 16 ga together with evidence of petrology are consistent with the interpretation that the magma source of the triassic granitic intrusion was derived from enriched mantle and melted under an usually high temperature condition likely due to basaltic magma that underplated the lower crust our data combined with evidence of the regional geology enable us to conclude that the gorny altai and altai mongolian terranes possibly have similar tectonic natures but represent two separate accretionary systems before devonian collision the accretion and amalgamation processes resulted in the paleozoic granitoid magmatism and caused the two terranes to merge as a composite tectonic domain at the siberian continental margin

chemical gradient at diffusive interfaces in magma chambers the results of high pressure experiments on diffusion and soret separation in natural silicate melts show that the diffusive behaviour between natural silicic and mafic magmas can be approximately modelled as if the system were a binary mixture of sio2 and other components such as mgo feo cao steady state compositional profiles across a diffusive interface between silicic and mafic magma layers are calculated on the basis of phenomenological relationships for the fluxes of chemical species and heat in the binary mixtures using the experimental data of diffusion and soret coefficients in natural silicate melts the compositional profiles show a curvature with a minimum sio2 value within the interface due to the soret effect and temperature dependence of diffusion coefficient the compositional gradient at the lower half of the diffusive interface is similar to that resulting from the soret separation of a mafic melt regardless of the composition of the silicic magmas these results suggest that picritic magma can be formed in the interfacial region between the mafic and silicic magma layers the compositional gradient explains chemical variation of mafic to picritic inclusions in a mixed andesite of the abu volcano group japan

formation of composite dykes by contact remelting and magma mingling the shaluta pluton transbaikalia russia abstract a unique opportunity to study the source areas from which composite dykes were injected occurs in the shaluta pluton transbaikalia russia the major quartz syenite pluton was intruded by several synplutonic gabbro bodies of various sizes investigations of the contact zones between gabbro and host syenite showed that liquid basalt magma intruded the incompletely crystallized coarse grained quartz syenite with t 700 720 c and caused contact remelting of the silicic rock at about 900 950 c mechanical interaction between newly formed silicic melt and partially crystallized mafic magma resulted in extensive magma mingling chemical interaction was exhibited by migration of mgo cao feo sr h2o and cl from the basalt magma whereas silica alkalis rb and ba migrated from the silicic refusion zone into the crystallized gabbro presence of melt inclusions with homogenization temperature ranging from 640 to 790 c in quartz and attaining 850 900 c in late clinopyroxene indicates that at least part of newly formed minerals crystallized from the hybrid melt mingled magmatic material was squeezed out inwards into the host solid quartz syenite pluton and formed dyke like apophyses that can be traced for a distance of 60 70 m from the contact zone apophyses have the same dimensions structure and composition as typical composite dykes that are common in the roof pendant over the gabbro bodies and nearby the gabbro exposures

rare earth elements in the sediments of lake baikal lake baikal is the deepest and probably oldest lake on earth its water column is pervasively oxic and sedimentation rates are very low which leads to the formation of a dynamic iron fe and manganese mn enrichment below the mn ii o2 boundary these often massive accumulations can be buried within the reducing part of the sediments and give rise to complex and cryptic redox cycles the mobility of rare earth elements rees is influenced by the dissolution and reduction dynamics of the ferromanganese oxides the present study offers an overview of the ree chemistry in lake baikal and its catchment area and more specifically ree distribution in five 11 to 26 cm long sediment cores situated across the lake at different water depths we analysed and discussed normalised ree patterns and their consequential cerium ce anomalies while particulate ree concentrations are mainly influenced by processes above or near the surface of lake baikal such as the development of a widespread negative ce anomaly processes occurring during early diagenesis in the sediment are most reflected in pore water rees the dissolution of ferromanganese oxides at the mn ii o2 boundary remobilizes significant amounts of ree into the pore water whereby some are likely adsorbed onto colloidal fe oxides however besides the tendency of ce being associated with mn oxides pore water rees fractionate predominantly around the buried fe mn accumulation where light rees preferentially adsorb onto fe oxides

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

rare earth elements in tributaries of lake baikal siberia russia abstract ree dissolved in waters from sixty five sampled tributaries of lake baikal show diversity in abundances ree 0 02 2 16 g l and distribution patterns la yb n 0 20 5 58 la tb n 0 30 2 59 total ree contents vary seasonally and yearly while ree patterns remain generally similar the ree patterns in stream waters correlate with those in the drained rocks and hence seem to be predominantly controlled by watershed lithology rivers having small watersheds around lake baikal belong to six lithological provinces with different stream water ree patterns the olkha plateau and the ranges of primorsky baikalsky barguzin ulan burgassy and khamar daban the khamar daban province in turn is lithologically heterogeneous and comprises four subprovinces with different respective patterns of dissolved ree rock water interaction in subsurface aquifers that feed the baikal inlets provides much stronger lithological control of ree than the weak interaction of short and rapid streams with rocks on the surface ree fractionation in stream waters show negative ce anomalies in almost all samples ce anom 0 12 0 97 except for those of large rivers selenga turka barguzin upper angara and kichera the reason is that the great length relatively slow current and the presence of streambed fine alluvium make the water chemistry more uniform and the ree patterns less dependent on local variations in watershed lithology the sampled tributaries represent a range of physico chemical and geomorphological conditions which can be used as reference for calibrations in future studies of sediment sources and possibly as a valuable tool for mineral exploration

shoshonitic liquid line of descent from diorite to granite the late precambrian post collisional tismana pluton south carpathians romania abstract the post collision late kinematic tismana pluton belongs to the shoshonitic series it is part of a late precambrian basement within the alpine danubian nappes of the south carpathians romania this pluton displays an exceptionally complete range of compositions from ultramafic to felsic rocks granites widespread mingling mixing relationships at all scales give rise to a variety of facies a liquid line of descent from the diorites to the granites is reconstructed by considering the variation in major and trace elements ree sr rb ba nb zr hf zn v co cr u th ga pb from 33 selected samples as well as mineral melt equilibrium relationships the first step of fractional crystallization is the separation from a monzodioritic parent magma of a peridotitic cumulate similar to the ultramafic rock found in the massif a possible contamination by lower crustal mafic component takes place at this stage the second step marks the appearance of apatite and fe ti oxide minerals as liquidus phases and the third step saturation of zircon mixing by hybridisation of magmas produced at different stages of the evolution along the liquid line of descent is also operating endo hybridisation as depicted by nd and sr isotopes fractional crystallization was combined to an important early contamination by a mafic lower crust in a deep seated magma chamber and to a later and mild contamination by a felsic medium crust in an intermediate chamber the mingling essentially occurred during the final emplacement in the high level magma chamber the monzodioritic parent magma identified by major and trace element modelling is shown by sr and nd isotopes to have its source in the lithospheric mantle or in a juvenile mafic lower crust derived from it the necessarily recent enrichment in k2o and associated elements of the lithospheric mantle is likely to be related to the preceding pan african subduction period the partial melting of this newly formed deep source has to be linked to a major change in the thermal state of the plate

crustal anatexis during the influx of mantle volatiles abstract many data show that large masses of silicic magma can be formed by crustal anatexis under the influence of volatiles possibly enriched in primary water derived from a mantle source in the present paper a model of crustal anatexis accompanied by a limited influx of volatiles is suggested the water influx is not excessive no independent fluid water phase appearing in the magma convective diffusion is assumed to be the dominant mechanism of volatile transport within the silicic melt this mechanism increases the volatile flux by several orders of magnitude compared with the diffusion flux in a non convecting system a high convective flux may be generated only at specific stages of magma chamber formation in this paper a mathematical formulation of the conditions favouring this type of anatexis is given the most plausible source of deep seated water bearing volatiles beneath large silicic crust derived magma masses are mantle derived magmas which maintain vigorous convection it is shown that convective diffusion influx of volatiles from a lower layer of mafic magma into an upper layer of silicic magma leads to a quasi equilibrium situation transient two liquid eguilibrium watson 1976 in this situation the distribution coefficient of water between adjacent silicic and mafic magmas is proportional to the ratio of water solubilities in these magmas and amounts to about 1 4 by mass the silicic magma overlying the mafic magma can contain up to 6 wt water which is particularly true of the latest stages of cryatallization of the mafic magma when its water content rises and diffusion of water across the mafic silicic melt boundary becomes more efficient on the basis of our results the different stages of formation of a silicic magma are discussed the geological consequences are analyzed and some new regularities in the interpretation of geological and petrological data relevant to granitoid petrogenesis are proposed

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

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

genesis of post collisional granitoids and basement nature of the junggar terrane nw china nd sr isotope and trace element evidence the post collisional magmatism of the junggar terrane is characterized by intrusion of large amounts of granitoids and minor basic ultrabasic rocks the granitoids comprise two magmatic suites calc alkaline and alkaline which were emplaced contemporaneously at about 294 ma the calc alkaline rocks are typically sodium rich na2o k2o 1 1 3 5 and metaluminous a nk 1 0 a cnk 0 78 1 04 they show mildly fractionated ree patterns la yb n 15 and spidergrams with strong depletion of nb ti and to a lesser extent sr the alkaline granites have high contents of sio2 75 78 alkalis nb hree y sn f and high feo mgo ratios and huge sr and eu depletion in the spidergrams moreover they display characteristic tetrad ree patterns and non charac trace element behaviour the two rock suites have similar initial nd and sr isotopic compositions with end t in the range 5 2 to 7 1 and isr mostly in the range 0 7031 0 7041 this points to a predominance of juvenile components in their sources the calc alkaline rocks are most probably derived by dehydration melting of a basic lower crust leaving behind a granulite residue the process was probably triggered by underplating of mantle derived basic magmas in an extensional regime the alkaline granites are considered to have formed by differentiation of the calc alkaline granitoids our study argues for a juvenile continental crust for the basement of the junggar terrane which is likely dominated by early paleozoic oceanic crust and arc complex that were deeply buried during the late paleozoic subduction and accretion

genesis of alkaline and peralkaline syenite granite series the kharitonovo pluton transbaikalia russia the kharitonovo pluton is located in the central part of the large mongolian transbaikalian province of late paleozoic alkaline syenites and granites the province stretches for a distance of almost 2000 km and contains over 350 plutons the kharitonovo pluton occupies an area of 230 km 2 it is made up of a type granitoids forming two successive alkaline and peralkaline syenite granite series syenites largely predominate over granites rocks of both series are characterized by abundant mesoperthitic feldspar about 90 vol in syenites more than 60 in granites while plagioclase is almost absent in the peralkaline series mafic minerals are riebeckite arfvedsonite kataphorite and aegirine with secondary annitic biotite in the alkaline series edenite and iron rich biotite are the major mafic minerals in the alkaline series coeval mafic rocks commonly occur in the form of synplutonic composite dikes they indicate at least two stages of mafic magma injection into the silicic magma chamber major

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

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

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

petrogenesis of syenite granite suites from the bryansky complex transbaikalia russia implications for the origin of a type granitoid magmas abstract two syenite granite suites metaluminous and peralkaline that form the bryansky complex in transbaikalia russia have been studied with the aim to constrain the existing models of a type granitoid magma generation the bryansky complex is a large intrusive body of about 1600 km2 emplaced in the central part of the mongolian transbaikalian granitoid belt which extends for more than 2000 km and is 200 300 km wide the belt comprises about 350 a type granitoid plutons and numerous volcanic fields u pb and rb sr isotope dating revealed that all the intrusive rocks of the complex and closely associated comendites were emplaced within a narrow time span 279 285 ma the isotope characteristics are rather similar for all main rock types the metaluminous suite has a 87sr 86sr t value of 0 7050 0 001 end t from 1 9 to 3 0 and the peralkaline suite has 87sr 86sr t 0 7053 0 0008 end t 2 1 and 2 4 comendites and trachyandesites have similar end t values from 2 2 to 3 5 but a slightly higher 87sr 86sr t value of 0 7062 0 0002 the systematic change in chemical and mineralogical composition from syenitic to granitic rocks in both suites and the similar isotopic ratios suggest that the granites were formed by fractional crystallization of the syenite magmas several lines of evidence suggest that metaluminous syenite is the parental magma for the whole bryansky complex study of melt inclusions in quartz phenocrysts from the peralkaline granite and in pyroxene from the nordmarkite indicates that fractional crystallization has resulted in significant f enrichment in the granitic magma up to 1 5 1 7 wt the syenite magmas crystallized at rather high temperature 940 c whereas the near liquidus temperature of the peralkaline granite was lower 760 790 c very high homogenization temperatures of the melt inclusions in quartz phenocrysts from comendites 1000 1100 c suggest that the alkali rich silicic magma formed at a depth of 50 60 km far exceeding the normal crust thickness the sr nd isotope data advocate the main role of mantle derived material in the source region from which the alkali rich syenitic and granitic magmas were produced

composite dikes in four successive granitoid suites from transbaikalia russia the effect of silicic and mafic magma interaction on the chemical features of granitoids abstract in the study area of 60 by 15 km located south of lake baikal coeval mafic rocks form small intrusive bodies synplutonic dikes and composite mafic felsic dikes in four successive late paleozoic and early mesozoic syenite and granite suites mafic rocks in all suites are monzogabbro with similar mineral and chemical composition this attests that mantle derived mafic magmas produced at post collisional and within plate tectonic settings were not noticeably distinguished in chemical composition within a time interval of about 60 65 m y detailed study of the interaction zones between synplutonic mafic magma and host syenite at two key localities allowed recognition of syenite remelting stage at about 900 950 c remelting was accompanied by sr and ba enrichment by diffusion from the adjacent mafic magma diffusive transport of ba also caused enhanced bao content 0 37 0 54 wt in alkali feldspars of the newly formed syenite remelting of the host syenite was followed by mingling chemical interaction and mixing of mafic and silicic magmas within the contact zone the results of petrographic and geochemical study of the composite and synplutonic dikes are used to interpret the chemical features of the large about 1000 km2 essentially syenitic ust khilok pluton two varieties of syenite are recognized in the pluton one with ba

the mongol okhotsk ocean subduction related permian peraluminous granites in northeastern mongolia constraints from zircon u pb ages whole rock elemental and sr nd hf isotopic compositions abstract northeastern mongolia represents a unique area that experienced evolution of both the mongol okhotsk ocean moo and the central asian orogenic belt caob in order to better understand the evolution of moo and its effect on the caob we performed zircon u pb dating and hf isotope and whole rock elemental and sr nd isotopic analyses on three permian peraluminous granitic plutons in northeastern mongolia zircon dating result revealed that granitoids of east monhhaan south monhhaan and tuvshinshiree plutons were emplaced at 254 3 265 5 and 273 3 ma respectively the result provides permian ages for the permian triassic volcanic plutonic belt in northeastern mongolia geochemical analysis suggests that all the granitoids are of peraluminous s type granites elemental and sr nd hf isotopic data indicate that these s type granites were generated from mainly clay poor greywacke with some contribution of juvenile mantle component the permian s type granites emplaced likely in an active margin related to the subduction of the mongol okhotsk ocean the protracted intermittent extension during the long lived compression was considered as a likely geodynamic mechanism for the upwelling of mantle derived basaltic magma and melting of crustal materials