are granitic intrusions scale invariant published length l and thickness t data on 135 laccolith and 21 granite intrusions define power law relationships of the form l kt a typical of systems exhibiting scale invariant fractal behaviour both data sets are characterised by an exponent a 1 0 88 0 1 for laccoliths and 0 80 0 20 for plutons that reflects an inherent preference for scale invariant tabular sheet geometries these power law size relationships can be explained in mechanical terms by the need for an incoming magma sheet to travel laterally some distance before vertical thickening can occur sheet thickness is a function of available magma pressure which for an intrusion fed by a feeder dyke is proportional to the vertical magma transport distance

phase equilibrium constraints on the viscosity of silicic magmas 1 volcanic plutonic comparison by using recently determined experimental phase equilibria we show that the viscosity of granitic magmas emplaced at upper crustal levels is approximately constant at 104 5 pa s irrespective of their temperature and level of emplacement magmas crystallizing as granitic plutons are not water poor and thus not more viscous than their extrusive equivalents instead comparison between pre eruption magma viscosities of extrusive silicic intermediate and intrusive granitic magmas shows that the former are on average slightly more viscous given the typical strain rates in silicic magma chambers magma rheological behavior is expected to be dominantly newtonian bubbles having a minor rheological influence at depth although exceptions can exist thus whether a silicic intermediate magma is erupted or frozen at depth depends primarily on the rheological properties of surrounding terranes or on external tectonic factors but not on the rheology of the magma itself however preemptive viscosities of extrusive magmas rarely exceed 106 pa s which suggests that crystal melt mushes with higher viscosities cannot leave the magma storage regions beneath volcanoes the narrow range of viscosities displayed by silicic intermediate magmas results from both the strong control that pressure exerts on volatile solubilities in silicate melts and thermal limitations required to produce acid magmas considerations of the relationships between magma crystallinities bulk sio2 and preemptive melt h2o contents show that the higher the melt h2o content is the higher the maximum crystallinity that a given magma will be while still being potentially erupted an empirical correlation is proposed that enables us to estimate preemptive melt h2o contents of erupted magmas by knowing their crystallinity and bulk sio2

initiation of subduction and the generation of slab melts in western and eastern mindanao philippines adakite found in both the eastern and western parts of mindanao island philippines is a rare rock type characterized by low heavy rare earth elements and y contents together with high sr y ratios and is considered to be the result of the melting of young subducted oceanic crust which leaves an eclogite residue pliocene quaternary adakites from western mindanao zamboanga peninsula are probably derived from the melting of the young miocene sulu sea crust which is currently subducting beneath zamboanga associated nb enriched basalts are thought to come from mantle metasomatized through interaction with adakitic melts in eastern mindanao pliocene quaternary cones and plugs of typical adakitic composition mark the trace of the philippine fault in surigao and north davao the underlying philippine sea crust is of eocene age and therefore cannot melt under normal subduction thermal conditions thermal models indicate that melting at the start of subduction can occur subduction of the philippine sea plate began 3 to 4 ma beneath eastern mindanao and probably accounts for the presence of adakites along the philippine fault

granitic melt viscosity and silicic magma dynamics in contrasting tectonic settings models for granitic melt viscosities reveal the influences of h 2 o temperature and tectonics on melt segregation ascent and emplacement low t magmas are wetter than those formed at high t so log 10 melt viscosity pa s is 3 2 6 3 for leucogranite to tonalite only 10 1000 times more viscous than basalts there should be no correlation between critical dyke width to prevent freezing and tectonic setting granitic magma ascent rates in dykes vary by

biotite dehydration partial melting and fluid composition experiments in the system kalo2 feo mgo sio2 h2o co2 biotite solid solutions are a significant h 2 o reservoir in the lithosphere and the assemblage bt opx kfs qtz is commonly used to estimate a h2o in high grade metamorphic and magmatic rocks here we report experimental constraints on subsolidus mineral equilibria involving biotite and orthopyroxene in the system kalo 2 mgo feo sio 2 h 2 o co 2 our experiments address the question of stability of biotite of a given x fe in the assemblage bt qtz or sa or the stability of the assemblage opx sa or qtz clemens 1993 and clemens et al 1997 concluded that co 2 has no effect other than to lower a h2o and thereby raise the solidus t our data at x fl h2o h2o values being too low although various models allow us to estimate x h2o in h 2 o co 2 fluids we are still unable to use biotite equilibria to estimate a h2o accurately during high grade metamorphism and magma crystallization we also consider qualitatively the effects fe mg biotite solid solution on partial melting equilibria in fluid poor rock dominated systems in which hydration dehydration reactions control the fluid composition

reply to comment by j l zhou on from passive margin to volcano sedimentary forearc the tonian to cryogenian evolution of the anosyen domain of southeastern madagascar by boger et al precambrian research volume 247 july 2014 pages 159 186

experimental evidence for high temperature felsic melts formed during basaltic intrusion of the deep crust basaltic intrusion into the continental crust was simulated with melting experiments on layered charges of high alumina olivine tholeiite haot and metapelite at 11 kbar rapid diffusion of cao and na 2 o into the metapelite and h 2 o k 2 o sio 2 and al 2 o 3 into the basalt resulted in spectacular changes in the phase assemblage garnet megacrysts crystallized profusely across the basalt layer cumulate assemblages closely resembled rocks found in underplating localities and glass compositions graded from dacite to rhyolite even at temperatures as high as 1150 c silicic melts can be generated during interaction of basalt with metapelite at temperatures well above the h 2 o saturated granite minimum calculations based on these results and comparison with natural occurrences suggest that high temperature felsic melts with 2 o can form and segregate at rates of 1 cm yr

the sulfur and strontium isotopic compositions of permian evaporites from the zechstein basin northern germany the sulfur isotopic composition of stratigraphically well constrained samples from five evaporitic cycles of the german zechstein upper permian displays no systematic lateral variation and no significant temporal trend with an average 34s value close to 11 results confirm the previously determined sulfur isotope minimum for phanerozoic seawater least radiogenic 87sr 86sr ratios for these sulfates define a clear temporal variation with a minimum value at 0 707008 a conclusive assessment of this temporal trend in comparison with other data sets for upper permian seawater is not possible due to substantial differences in bio stratigraphic assignments

granitic magma transport by fracture propagation granitic magmas commonly ascend tens of kilometres from their source terranes to upper crustal emplacement levels or to the earth s surface apart from its obvious bearing on the interpretation of the geology and geochemistry of felsic igneous rocks the magma ascent mechanism critically affects any modelling of the metamorphic evolution of the upper lithosphere as well as its rheology we propose that in general granitic magmas ascend via propagating fractures as dykes in extremely short time periods long distance diapiric transport of granitoid magmas through crustal sections is not viable on thermal or mechanical grounds and there is an apparent total absence of field evidence for diapiric rise of such magmas even in supposed type localities neither the shapes nor internal or external characteristics of granitic plutons necessarily reveal anything about the transport of their precursor magmas these are purely arrival phenomena dictated by local structure kinematics and stress states based on existing numerical treatments of the problem we show that granitic magmas are apparently sufficiently inviscid to travel through fractures to high crustal levels without suffering thermal death by freezing for example a 2000 km3 granitic batholith can be inflated by a 1 km 3m 20 km deep dyke system in less than 900 years the model proposed has numerous far reaching petrological and rheological consequences some of which are outlined

s type granitic magmas petrogenetic issues models and evidence abstract despite a perception that it represents a perverse divergence it is perfectly possible to believe in the existence of s and i type granites and the implications for the nature of their protoliths and to disbelieve in the applicability of the restite unmixing model for chemical variation in granitic magmas white and chappell erected the s i classification with impeccable validity the isotopic evidence demands contrasting source reservoirs for s and i type granitic magmas however the major advance was not the classification but the recognition that highly contrasting parental materials must be involved in the genesis of granitic magmas the restite unmixing model is commonly seen as a companion to the s i classification but it is really a separate issue this model implies that the compositions of granites image those of their source rocks in a simple way however there are other equally valid models that can explain the data and none of them represents a unique solution the most cogent explanation for the high grade metasedimentary enclaves in most s type granites is that they represent mid crustal xenoliths restitic enclaves are either rare or absent inherited zircons in s type rocks are certainly restitic however the occurrence of a substantial restitic zircon population does not imply an equally substantial restitic component in the rest of the rock zircon and zirconium behaviours are controlled by disequilibrium and kinetics and zr contents of granitic rocks can rarely be used to infer magma temperatures since the dominant ages among inherited zircons in lachlan fold belt lfb s type granites are ordovician and proterozoic it seems likely that crust of this age but geochemically different from the exposed rocks not only underlies much of the lfb but also forms a component in the granite magma sources the evidence is overwhelming that the dark microgranular enclaves that occur in both s and i type granites are igneous in origin they represent globules of quenched more mafic magma mingled and modified by exchange with the host granitic magma however magma mixing does not appear to be a significant process affecting the chemical evolution of the host magmas likewise the multicomponent mixing models erected for some granitic rock suites are mathematically nonunique and in some cases violate constraints from isotopic studies s and i type magmas commonly retain their distinct identities this suggests limited source mixing limited magma mixing and limited wall rock assimilation though intermediate types certainly exist they are probably relatively minor in volume crystal fractionation probably plays the major role in the differentiation of very many granitic magmas including most s types especially those emplaced at high crustal levels or in the volcanic environment minor mechanisms include magma mixing wall rock assimilation and restite unmixing isotopic variations within plutons and in granite suites could be caused by source heterogeneities magma mixing assimilation and even by isotopic disequilibrium however source heterogeneity coupled with the inefficiency of magma mixing is probably the major cause of observed heterogeneity normal geothermal gradients are seldom sufficient to provide the necessary heat for partial melting of the crust and crustal thickening likewise fails to provide sufficient heat generally the mantle must be the major heat source this might be provided through mantle upwelling and crustal thinning and possibly through the intra and underplating of mafic magmas upper crustal extension seems to have been common in regions undergoing granitic magmatism migmatites probably provide poor analogues of granite source regions because they are mostly formed by fluid present reactions granitic magmas are mostly formed by fluid absent processes where we do see rare evidence for arrested fluid absent partial melting the melt fraction is invariably concentrated into small shear zones veinlets and small dykes thus it seems likely that dyking is important in transporting granitic magma on a variety of scales and at many crustal levels however one major missing link in the chain is the mechanism by which melt fractions in small scale segregations occurring over a wide area can be gathered and focused to efficiently feed much wider spaced major magma conduits answers may lie in the geometry of the melting zones and in the tendency of younger propagating fractures to curve toward and merge with older ones self organization almost certainly plays a role

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

distinguishing strongly rheomorphic tuffs from extensive silicic lavas high temperature silicic volcanic rocks including strongly rheomorphic tuffs and extensive silicic lavas have recently been recognized to be abundant in the geologic record however their mechanisms of eruption and emplacement are still controversial and traditional criteria used to distinguish conventional ash flow tuffs from silicic lavas largely fail to distinguish the high temperature versions we suggest the following criteria ordered in decreasing ease of identification to distinguish strongly rheomorphic tuffs from extensive silicic lavas 1 the character of basal deposits 2 the nature of distal parts of flows 3 the relationship of units to pre existing topography and 4 the type of source as a result of quenching against the ground basal deposits best preserve primary features can be observed in single outcrops and do not require knowing the full extent of a unit lavas commonly develop basal breccias composed of a variety of textural types of the flow in a finer clastic matrix such deposits are unique to lavas because the chilled base of an ashflow tuff generally does not participate in secondary flow primary pyroclastic features are best preserved there massive flow banded bases are more consistent with a lava than a pyroclastic origin lavas are thick to their margins and have steep abrupt flow fronts ashflow tuffs thin to no more than a few meters at their distal ends where they generally do not show any secondary flow features lavas are stopped by topographic barriers unless the flow is much thicker than the barrier ash flow tuffs moving at even relatively slow velocities can climb over barriers much higher than the resulting deposit lavas dominantly erupt from fissures and maintain fairly uniform thicknesses throughout their extents tuffs commonly erupt from calderas where they can pond to thicknesses many times those of their outflow deposits these criteria may also prove effective in distinguishing extensive silicic lavas from a postulated rock type termed lava like ignimbrite the latter have characteristics of lavas except for great areal extents up to many tens of kilometers these rocks have been interpreted as ash flow tuffs that formed from low boiling over eruption columns based almost entirely on their great extents and the belief that silicic lavas could not flow such distances however we interpret the best known examples of lava like ignimbrites to be lavas this interpretation should be tested through additional documentation of their characteristics and research on the boiling over eruption mechanism and the kinds of deposits it can produce flow bands flow folds ramps elongate vesicles and probably upper breccias occur in both lavas and strongly rheomorphic tuffs and are therefore not diagnostic pumice and shards also occur in both tuffs and lavas although they occur throughout ash flow tuffs and generally only in marginal breccias of lavas dense welding secondary flow and intense alteration accompanying crystallization at high temperature commonly obliterate primary textures in both thick rheomorphic tuffs and thick lavas high temperature silicic volcanic rocks are dominantly associated with tholeiitic flood basalts extensive silicic lavas could be appropriately termed flood rhyolites

granite magma formation transport and emplacement in the earth s crust the origin of granites was once a question solely for petrologists and geochemists but in recent years a consensus has emerged that recognizes the essential role of deformation in the segregation transport and emplacement of silica rich melts in the continental crust accepted petrological models are being questioned either because they require unrealistic rheological behaviours of rocks and magmas or because they do not satisfactorily explain the available structural or geophysical data provided flow is continuous mechanical considerations suggest that far from being geologically sluggish granite magmatism is a rapid dynamic process operating at timescales of or 100 000 years irrespective of tectonic setting

petrogenesis of cretaceous adakitic and shoshonitic igneous rocks in the luzong area anhui province eastern china implications for geodynamics and cu au mineralization both adakitic and shoshonitic igneous rocks in the luzong area anhui province eastern china are associated with cretaceous cu au mineralization the shaxi quartz diorite porphyrites exhibit adakite like geochemical features such as light rare earth element lree enrichment heavy ree hree depletion high al2o3 mgo sr sr y and la yb values and low y and yb contents they have low end t values 3 46 to 6 28 and high 87sr 86sr i ratios 0 7051 0 7057 sensitive high resolution ion microprobe shrimp zircon analyses indicate a crystallization age of 136 3 ma for the adakitic rocks most volcanic rocks and the majority of monzonites and syenites in the luzong area are k rich or shoshonitic and were also produced during the cretaceous 140 125 ma they are enriched in lree and large ion lithophile elements and depleted in ti and nb and ba and exhibit relatively lower end t values ranging from 4 65 to 7 03 and relatively higher 87sr 86sr i ratios varying between 0 7057 and 0 7062 the shoshonitic and adakitic rocks in the luzong area have similar pb isotopic compositions 206pb 204pb 17 90 18 83 207pb 204pb 15 45 15 62 and 208pb 204pb 38 07 38 80 geological data from the luzong area suggest that the cretaceous igneous rocks are distributed along ne fault zones e g tanlu and yangtze river fault zones in eastern china and were likely formed in an extensional setting within the yangtze block the shaxi adakitic rocks were probably derived by the partial melting of delaminated lower crust at pressures equivalent to crustal thickness of 50 km i e 1 5 gpa possibly leaving rutile bearing eclogitic residue the shoshonitic magmas in contrast originated mainly from an enriched mantle metasomatized by subducted oceanic sediments they underwent early high pressure 1 5 gpa fractional crystallization at the boundary between thickened 50 km lower crust and lithospheric mantle and late low pressure 1 5 gpa fractional crystallization in the shallow 50 km crust the adakitic and shoshonitic rocks appear to be linked to an intra continental extensional setting where partial melting of enriched mantle and delaminated lower crust was probably controlled by lithospheric thinning and upwelling of hot asthenosphere along ne fault zones e g tanlu and yangtze river fault zones in eastern china both the shoshonitic and adakitic magmas were fertile with respect to cu au mineralization

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

an experimentally based model for the origin of tonalitic and trondhjemitic melts abstract the results of melting experiments have been used to establish equations relating melt composition to temperature t pressure p and water content of the source y h2o for tonalitic and trondhjemitic melts produced by partial melting of average archean tholeiite aat a normative classification diagram for these rocks has been translated into a t p y h2o diagram showing the required conditions of partial melting tonalites form at high temperatures low pressures and high water contents while trondhjemites form at lower temperatures higher pressures and lower water contents as pressure increases the k content increases but the melts never achieve the composition of a granodiorite or granite a method for estimating the conditions for the formation of natural tonalites and trondhjemites is proposed the method has been successfully tested against other experimental studies and natural rocks where the temperature pressure conditions have been constrained by traditional geochemical methods

amphibole out phase boundary in partially melted metabasalt its control over liquid fraction and composition and source permeability a number of experimental studies in recent years have demonstrated that partial melting of hydrated basalt at lower crustal to upper mantle pressures is capable of generating up to 40 high sio2 liquids closely comparable in many geochemical respects to tonalitic trondhjemitic granitoids ttg from these studies it is apparent that the initiation of melting coincides with the beginning of amphibole dehydration but that a broad reaction interval over which amphibole coexists with high sio2 liquids is defined by the wet basalt solidus on one side and the amphibole out phase boundary on the other the phase relations of an alkali rich tholeiitic metabasalt have been examined in the outer half of this region up to and beyond the amphibole out phase boundary results indicate that amphibole exerts a strong control over the amount and composition of coexisting liquid over this interval melt fraction increases slowly and gradually from 0 up to 20 30 between the wet basalt solidus and up to amphibole out but increases substantially from 30 to 50 across and immediately beyond it liquids are strongly to moderately peraluminous molar al2o3 cao na2o k2o a cnk 1 0 up to amphibole out neutral at the boundary a cnk 1 0 and increasingly metaluminous a cnk 1 0 beyond it up to amphibole out potassium is strongly partitioned into liquid relative to amphibole i e mineral melt kd 0 2 0 3 whereas titanium is strongly partitioned into liquid kd 2 0 6 0 physical and chemical criteria suggest that efficient melt segregation leading to ttg plutonism requires more than 20 30 batch melting of a garnet bearing basaltic protolith near or beyond the amphibole out phase boundary leaving dry residues of eclogite or garnet granulite and producing metaluminous liquids that are strongly depleted in heavy rare earth elements hrees and yttrium y lower degrees of melting may involve critical melting where melt is squeezed out of the residue by deformation of the crystalline matrix resulting in disequilibrium melt compositions

partial melting of the appin quartzite driven by fracture controlled h sub 2o infiltration in the aureole of the ballachulish igneous complex scottish highlands the ballachulish igneous complex consists of an outer quartz diorite and an inner granite emplaced at about 300 mpa initially at 1000 to 1050 c the contact aureole 0 5 2 km wide occurs in metapelites and metapsammites plus minor graphitic slates carbonate rocks and metaquartzites a textural examination of the arkosic appin quartzite which was previously believed to have melted only within a few metres of the intrusion demonstrates that partial melting occurred up to 500 m away from the vertical eastern contact coupling petrographic observations with qtz ab or h2o phase relations we determined both the amounts of actual melt and the maximum possible amounts of melt in the samples melting efficiency was everywhere less than 100 and decreased with distance from the intrusion though perhaps not the only possible source of fluid throughout the aureole simple models demonstrate that h2o evolution from the pluton would have been volumetrically sufficient and persisted long enough to account for the observed partial melting a time integrated fluid flux of 7000 kg m2 from the pluton is necessary to account for the observed amounts of partial melt in the appin quartzite from its inefficiency we infer that infiltration of the appin quartzite cannot have occurred along interconnected grain edge channels rather it was controlled by hydraulic fracturing with fracture density determining melting efficiency bulk rock permeability is calculated to be 10 20 m2 an order of magnitude lower than that necessary to permit pervasive flow of all the fluid exsolving from the pluton there is little difference between the calculated time integrated fluid flux through the appin quartzite on the eastern flank and an estimate of the infiltrating flux through the pelitic leven schist on the western flank in the time interval during which both rock types were above their solidus temperature despite differences in their equilibrium quartz h2o dihedral angles at temperatures immediately below the solidus and differences in the attitude of the contact the rates of h2o expulsion from the cooling pluton are consistent with highly efficient fracture dominated flow allowing insufficient time for textural equilibration

the role of synmetamorphic igneous rocks in the metamorphism and partial melting of metasediments northwest adirondacks field and petrologic studies along the adirondack lowlands highlands boundary near harrisville ny indicate that heat from the synmetamorphic intrusion of the diana syenite complex intrusion temperature of 1 050 c played a major role in the local metamorphic thermal regime and was responsible for extensive partial melting of adjacent metasedimentary units major paragneiss of engel and engel metamorphic temperatures inferred from two feldspar and spinel quartz assemblages decrease from 850 950 c along the diana metasediment contact to 650 700 c 2 3 km away from the contact metamorphic pressures are 7 0 5 kb as determined from coexisting plagioclase garnet sillimanite quartz kyanite sillimanite and garnet rutile ilmenite sillimanite quartz grail in the paragneiss migmatites consisting of quartz microcline perthite sodic plagioclase leucosomes are generally concordant with the melanosome consisting of biotite sillimanite garnet spinel plagioclase corundum cordierite qualitatively the amount of partial melt and occurrences of corundum bearing assemblages decrease away from the diana contact activity of h2o inferred from coexisting biotite sillimanite quartz garnet k feldspar ranges from 0 01 to 0 17 and is five to ten times lower in corundum bearing rocks

the 3d shear experiment over the natih field in oman the effect of fracture filling fluids on shear propagation this is the final paper in a series on the 3d multicomponent seismic experiment in oman in this experiment a 3d data set was acquired using three component geophones and with three source orientations the data set will subsequently be referred to as the natih 9c3d data set we present for the first time evidence demonstrating that shear waves are sensitive to fluid type in fractured media two observations are examined from the natih 9c3d data where regions of gas are characterized by slow shear wave velocities one is that the shear wave splitting map of the natih reservoir exhibits much larger splitting values over the gas cap on the reservoir this increase in splitting results from a decrease in the slow shear wave velocity which senses both the fractures and the fracture filling fluid using a new effective medium model it was possible to generate a splitting map for the reservoir that is corrected for this fluid effect secondly an anomaly was encountered on the shear wave data directly above the reservoir the thick fiqa shale overburden exhibits a low shear wave velocity anomaly that is accompanied by higher shear reflectivity and lower frequency content no such effects are evident in the conventional p wave data this feature is interpreted as a gas chimney above the reservoir a conclusion supported by both effective medium modelling and the geology with this new effective medium model we show that introduction of gas into vertically fractured rock appears to decrease the velocity of shear waves s2 polarized perpendicular to the fracture orientation whilst leaving the vertical compressional wave velocity largely unaffected this conclusion has direct implications for seismic methods in exploration appraisal and development of fractured reservoirs and suggests that here we should be utilizing s wave data as well as the conventional p wave data as a direct hydrocarbon indicator

fluids p t paths and the fates of anatectic melts in the earth s crust this paper presents a model for the longevity and physical behaviour of anatectic melts in the earth s crust it is assumed that a thermal anomaly has caused partial fusion of biotite plagioclase quartz rocks to produce a granitic melt and restitic solids the focus is on the potential chemical and physical outcomes as a function of the various p t fluid conditions rocks undergoing fluid present melting will sustain an increase in bulk density and will become more gravitationally stable diapiric up welling of such material should not occur in fluid absent melting the bulk density will decrease and foster gravitational instability stresses in the surrounding rocks could however lead to fracture and rapid evacuation of the melt with excess fluid and without melt segregation isobaric cooling will cause immediate freezing if t rises above the biotite quartz melting reaction freezing will still occur at the original reaction t diapiric up welling should not occur biotite bearing diatexites might be formed by large degrees of fusion but magma ascent is unlikely in isothermal decompression immediate freezing will ensue and the migmatite should contain orthopyroxene rather than biotite with excess fluid and melt segregation cooling will induce freezing at the fluid saturated granite solidus magma ascent could however occur even for xflh2o 1 in isothermal decompression melt will persist to the granite solidus for the imposed xflh2o limited magma ascent could occur all granites should contain primary igneous biotite for fluid deficient rocks without melt segregation freezing will occur at the t of the original melting if the reaction is overstepped solidification will occur at the original xflh2o magma formation is unlikely migmatites should contain either pyroxene retrogressed to biotite or biotite alone on decompression immediate freezing is likely if t remains at the biotite quartz reaction for higher t melt will survive decompression and freeze at the fluid saturated solidus for some increased xflh2o magma ascent is possible and the melt will freeze very close to the wet granite solidus in fluid deficient rocks with melt segregation cooling and crystallization will cause fluid saturation in situ segregated melt will freeze at the original xflh2o melt ascent will however cause exsolution of a co2 rich fluid with fluid escape melt will freeze close to the wet granite solidus an increase in melt proportion may accompany magma ascent or source up welling any low p granite intrusion should be charnockitic for fluid absent rocks without melt segregation freezing will occur on cooling to the t of the original reaction only transitory migmatites could be formed but melt will survive decompression almost to the h2o saturated granite solidus the rocks will reveal former melt presence only in the existence of veins of coarse retrograde biotite with isothermal decompression further melting may occur forming an orthopyroxene bearing granulite facies migmatite in fluid absent situations with melt segregation magma could be efficiently transported to higher crustal levels if the melt proportion were insufficient for magma formation granulite facies migmatites would result ascending melt would progressively dissolve any entrained restite the granites produced should be charnockitic to aid in interpretation of the modelling results we suggest that it may be slightly more common for the highest grade metamorphic terranes to exhibit isobaric cooling paths that fluid present anatexis is normally limited to the shallower parts of the crust and that fluid absent reactions are more likely at greater depths we conclude that the best models for natural fluid present anatexis are those dealing with essentially pure h2o fluid and that fluid deficient conditions will dominate over fluid excess we also believe that it is common for melts to be physically segregated on scales larger than the diffusion paths of many elements

direct observation of adakite melts generated in the lower continental crust fiordland new zealand adakites have a distinct chemistry that links them to melting of a mafic source at high pressure they have been attributed to melting of subducted oceanic crust or melting of the mafic crustal roots of thick continental arcs and are an important contrast to mantle wedge melting as a means of generating continental crust we report the first direct evidence for the generation of adakitic melts in mafic lower continental crust in an exhumed cretaceous arc in the south island of new zealand the lower crustal pembroke granulite has the bulk chemistry and partial melting textures involving peritectic garnet appropriate for a source region for an adakitic melt the melt migrated from the area through a fracture network now filled with trondhjemitic veins emplacement of the melt was in the upper crust of the cretaceous section illustrated by the presence of coeval adakites in the upper crustal nelson westland region

eocene k rich adakitic rocks in the central iran implications for evaluating its cu au mo metallogenic potential abstract the kal e kafi intrusive complex is located in the central iran magmatic zone and includes a series of granitoid rocks ranging in composition from gabbro to granite these granitoids show some affinities with adakites e g high sr y and la yb ratios coupled with low yb and y enrichment in lree and lile with no eu anomalies similar to products from all of which are compatible with derivation from slab melting however relatively their low end t values 0 21 to 2 35 and higher 87 sr 86 sr i 0 7041 0 7044 are inconsistent with an origin by slab melting with these rocks also showing shoshonitic affinities with high k 2 o na 2 o ratios geochemical characteristics indicate that the kal e kafi intrusive complex was probably derived by the partial melting of delaminated lower crust at pressures equivalent to crustal thicknesses of 40 km generated near the boundary between garnet bearing amphibolite or amphibole bearing eclogite melting moreover the kal e kafi adakitic magmas must have interacted with the surrounding metasomatized mantle peridotite during their ascent which elevated not only their mgo cr and ni contents but also the oxygen fugacity o 2 of the magma the high o 2 could have induced oxidation of metallic sulfides in the mantle resulting in the mobilization of chalcophile elements which are required to produce porphyry cu au and mo mineralization

origin of potassic c type adakite magmas experimental and field constraints the postcollisional mesozoic c type or high k adakitic intrusions k adakites of china lack either temporal or spatial association with subduction and have k2o na2o around unity otherwise their geochemistry is very similar to ordinary adakites new experimental data geological observations and theoretical considerations lead us to suggest that these k adakites were high t 1050 and probably 1075 c rather hydrous magmas 6 wt h2o produced by fluid absent partial melting of metatonalites meta andesites or possibly potassic metabasalts at pressures exceeding 2 gpa their peculiar chemistry is a consequence of both the elevated k2o na2o in the protolith and the very high pressure of partial melting the most likely tectonic setting is one of extreme crustal thickening followed by delamination of the eclogitic keel and partial melting of this continental crustal material at mantle depths with high mantle heat flow during orogenic collapse

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

trace element evidence for growth of early continental crust by melting of rutile bearing hydrous eclogite the early continental crust is composed dominantly of archean tonalite trondhjemite granodiorite ttg gneisses and is generally explained as the product of melting of metabasalts in the subducted crust however whether the melting occurs in shallow level amphibolite facies or in relatively deep eclogite facies is debated here i present experimental partition coefficients ds for 27 trace elements between garnet amphibole and tonalitic melts they are used together with published mineral melt trace element ds to model the melting of metabasalt in order to delimit the conditions for ttg production the results clearly show that model melts with trace element characteristics that completely mimic the ttg are in equilibrium with rutile bearing anhydrous and hydrous amphibole bearing eclogitic residues but not rutile free amphibole dominated residues rutile appears to be a necessary residual phase to account for the characteristic negative nb ta anomaly in the ttg these results thus suggest that the early continental material was produced under eclogite facies conditions based on the modeling results obtained using appropriate partition coefficients and the archean geotherm the preferred process for the ttg production is the melting of rutile bearing hydrous eclogite triggered by the release of h2o from the progressive breakdown of amphibole the pressure temperature p t conditions for ttg production via this process are constrained to 1 5 2 5 gpa 50 80 km and 850 1050 c by the p t stability boundaries of amphibole and rutile in the basalt system

potassic ultrapotassic intrusions at the southwestern margin of the yangtze craton southwestern china petrogenesis and implications for the metal and fluid source of non arc porphyry cu mo au deposits abstract magmatic rocks can reveal partial melting processes in the lower crust and mantle therefore the petrogenesis of both ore barren and ore bearing magmatic rocks can provide key insights into the deep geological processes that control porphyry cu mo au deposits pcds here we investigate the petrogenesis of potassic ultrapotassic igneous rocks in the southwestern yangtze craton swyc to constrain their magmatic sources and assess their role in the formation of pcds these igneous rocks can be divided into three suites ore barren granite porphyry lamprophyre and ore bearing quartz monzonite u pb dating of zircons indicates that these potassic ultrapotassic intrusions crystalized simultaneously between 35 4 and 33 6 ma the lamprophyres have low sio2 contents and high mgo and k2o contents exhibiting an ultrapotassic affinity they are depleted in high field strength elements hfses nb ta zr hf and ti and enriched in large ion lithophile elements liles rb k sr and ba the enriched sr nd and hf isotopic compositions of the ore barren lamprophyres suggest that they were derived from low degree partial melting of lithospheric mantle that had been metasomatized by subduction related fluids during the proterozoic the ore barren granite porphyries have high sio2 and k2o contents with high k calc alkaline to shoshonitic characteristics these rocks have low mgo cr ni and co contents all samples exhibit high sr y ratios and highly fractionated rare earth elements rees patterns with weak eu anomalies combined with their low total ree contents 75 81 ppm and ehf t values which are similar to neoproterozoic granulite xenoliths we conclude that the ore barren porphyries are adakites derived from thickened neoproterozoic lower crust the ore bearing intrusions show variable sio2 and high na2o k2o contents plotting in the monzonite to quartz monzonite fields in a sio2 vs na2o k2o diagram these samples have much higher total ree contents 346 671 ppm than the ore barren intrusions the homogeneous sr nd isotopic compositions and linear trends on harker diagrams suggest that the magma that formed the ore bearing rocks underwent fractional crystallization of mainly amphibole and accessory minerals water contents are estimated to have been 9 12 wt at 0 8 gpa on the basis of the retention of plagioclase and low zircon saturation temperatures which suggests that the primitive magmas of the ore bearing intrusions were derived from water fluxed melting rather than dehydration melting the additional water required to promote water fluxed melting was probably derived from underplated poassic ultrapotassic mafic magmas the high total ree contents sr y and la yb n ratios indicate that the primitive magmas of ore bearing intrusions were probably derived from partial melting of potassic ultrapotassic lower crust formed by the underplating of mantle derived potassic ultrapotassic magma these results combined with previous research show that mantle derived potassic ultrapotassic magmas can provide metals and fluids required for the formation of pcds

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

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

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

geochemistry and nd sr isotopic studies of late mesozoic granitoids in the southeastern hubei province middle lower yangtze river belt eastern china petrogenesis and tectonic setting abstract a geochemical and isotopic study was carried out for the mesozoic yangxin tieshan and echeng granitoid batholiths in the southeastern hubei province eastern china in order to constrain their petrogenesis and tectonic setting these granitoids dominantly consist of quartz diorite monzonite and granite they are characterized by sio 2 and na 2 o compositions of between 54 6 and 76 6 wt and 2 9 to 5 6 wt respectively enrichment in light rare earth elements lree and large ion lithophile elements lile and relative depletion in y concentrations ranging from 5 17 to 29 3 ppm and yb 0 34 2 83 ppm with the majority of the granitoids being geochemically similar to high sio 2 adakites hsa their initial nd e nd 12 5 to 6 1 and sr 87 sr 86 sr i 0 7054 0 7085 isotopic compositions however distinguish them from adakites produced by partial melting of subducted slab and those produced by partial melting of the lower crust of the yangtze craton in the late mesozoic the granitoid batholiths in the southeastern hubei province exhibit very low mgo ranging from 0 09 to 2 19 wt with an average of 0 96 wt and large variations in negative to positive eu anomalies eu eu 0 22 1 4 especially the tieshan granites and yangxin granite porphyry eu eu 0 22 0 73 geochemical and nd sr isotopic data demonstrate that these granitoids originated as partial melts of an enriched mantle source that experienced significant contamination of lower crust materials and fractional crystallization during magma ascent late mesozoic granitoids in the southeastern hubei province of the middle lower yangtze river belt were dominantly emplaced in an extensional tectonic regime in response to basaltic underplating which was followed by lithospheric thinning during the early cretaceous

geochemical evidence for the melting of subducting oceanic lithosphere at plate edges most island arc magmatism appears to result from the lowering of the melting point of peridotite within the wedge of mantle above subducting slabs owing to the introduction of fluids from the dehydration of subducting oceanic crust1 volcanic rocks interpreted to contain a component of melt not just a fluid from the subducting slab itself are uncommon but possible examples have been recognized in the aleutian islands baja california patagonia and elsewhere2 3 4 the geochemically distinctive rocks from these areas termed adakites are often associated with subducting plates that are young and warm and therefore thought to be more prone to melting5 but the subducting lithosphere in some adakite locations such as the aleutian islands appears to be too old and hence too cold to melt6 7 this implies either that our interpretation of adakite geochemistry is incorrect or that our understanding of the tectonic context of adakites is incomplete here we present geochemical data from the kamchatka peninsula and the aleutian islands that reaffirms the slab melt interpretation of adakites2 but in the tectonic context of the exposure to mantle flow around the edge of a torn subducting plate we conclude that adakites are likely to form whenever the edge of a subducting plate is warmed or ablated by mantle flow the use of adakites as tracers for such plate geometry may improve our understanding of magma genesis and thermal structure in a variety of subduction zone environments

geochronology geochemistry and hf isotope of late triassic magmatic rocks of qingchengzi district in liaodong peninsula northeast china abstract the initiation timing and mechanism of lithospheric thinning of the north china craton ncc was still controversial late triassic igneous rocks especially mantle derived mafic rocks would provide constrains on early mesozoic lithospheric mantle geodynamics and initiation of lithospheric thinning this paper reports late triassic magmatic rocks including lamprophyre diorite dykes and biotite monzogranite cropped out in qingchengzi district of liaodong peninsula northeastern ncc la icpms zircon u pb dating yield ages of 210 227 ma and 224 ma for lamprophyres and biotite monzogranite respectively lamprophyre is ultrapotassic strongly enriched in ree and liles depleted in hfses and negative hf isotopes which are discriminating signatures of crustal source but distinguishingly high compatible element contents indicate the primary magma originated from mantle source a fertile one lamprophyre derived from partial melting of an enriched lithospheric mantle which was modified by slab derived hydrous fluids melts associated with deep subduction between the yangtze craton and the ncc the diorite displays distinct features with relatively enriched nb ta hree and depleted th u which suggest it derived from a relatively depleted source the depletion was caused by break off of the yangtze slab during deep subduction introducing asthenospheric mantle into the source the biotite monzogranite shows adakitic affinity and originated from partial melting of the thickened lower crust with addition of small proportion of mantle material the recognition of late triassic magmatism implies extensional tectonic settings in liaodong peninsula and suggests initiation of lithospheric thinning of north china craton in eastern segment might begin early in late triassic

petrogenesis of mesozoic granitoids in the dabie uhp complex central china trace element and nd sr isotope evidence post collisional magmatism in the dabie orogen is characterized by the near contemporaneous intrusion of voluminous granitoids and minor mafic magmas in the late mesozoic 135 110 ma the granitoids are represented by three major batholiths in northern dabie zhubuyuan baimajian and tiantangzhai the largest zhubuyuan batholith comprises hornblende bearing intermediate rocks penghe suite and hornblende free granitic rocks huangbai suite the huangbai granites show limited variation in chemical composition but are highly variable in textures ranging from medium to coarse grained they have rather homogeneous sr and nd isotopic compositions with 87sr 86sr i 0 708 0 709 and end t 19 to 24 the penghe intermediate suite covers a wide spectrum of rock types including diorite monzodiorite quartz monzonite to monzogranitic rocks and all of them contain hornblende and mafic enclaves the existence of compositional and textural disequilibrium e g calcic and resorbed cores of plagioclase and the nature of chemical variation in these rock types indicate that magma mixing between an evolved mafic and a granitic magma was involved in their genesis fractional crystallization fc could have coupled with the process of magma mixing but with minor contamination by the upper continental crust during magma ascent isotopically the intermediate rocks show a variation in end t values from 13 7 to 18 while their 87sr 86sr i ratios vary from 0 7074 to 0 7088 the genesis and evolution of the dabie post collisional magmatic rocks may be summarized as follows intrusion of mantle derived mafic ultramafic magmas into thickened continental crust induced dehydration melting of the lower crust rocks with intermediate compositions and subsequent production of large amounts of granitic melts huangbai suite extensive interaction between the crustal and mantle derived melts through mixing produced a hybrid parental magma which subsequently underwent a fc process en route to higher crustal levels producing a wide variety of rock types as shown by the penghe intermediate rock suite injection of successive pulses of basic magma into upward mobile magma chambers could have resulted in the formation of the mafic enclaves as well as mafic dykes

time constraints on the emplacement of klippen in the longmen shan thrust belt and tectonic implications abstract a belt of klippen and nappes was emplaced in the longmen shan thrust belt mostly lying between the yingxiu beichuan and the pengguan fault zones timing of the allochthon emplacement is hotly debated but quantitative age constraints are rarely documented this study presents the first isotopic dating of authigenic illite isolated from a 30 m wide fault gouge zone developed in reverse faults in the leading edge of the largest baishi goujia klippe a representative of the klippen and nappe belt in the longmen shan thrust belt the results show that the authigenic illite formed 180 215 ma ago combined with apatite and zircon fission track dating of the hanging wall of the yingxiu beichuan fault zone the emplacement of the baishi goujia klippe is constrained at 171 180 ma in the middle jurassic the thickness of a successive strata unit of the baishi goujia klippe measures 3 km and of the tangwangzhai nappe is over 5 km this implies that the elevation of the longmen shan was at least of 3 5 km a s l during the indosinian orogeny

can slab melting be caused by flat subduction slab melting has been suggested as a likely source of adakitic arc magmas i e andesitic and dacitic magmas strongly depleted in y and heavy rare earth elements existing numerical and petrologic models however restrict partial melting to very young 5 ma oceanic crust typically at 60 80 km depth paradoxically most of the known pliocene quaternary adakite occurrences are related to subduction of 10 45 ma lithosphere which should not be able to melt under normal subduction zone thermal gradients we propose an unusual mode of subduction known as flat subduction occurring in 10 of the world s convergent margins that can produce the temperature and pressure conditions necessary for fusion of moderately old oceanic crust of the 10 known flat subduction regions worldwide eight are linked to present or recent 6 ma occurrences of adakitic magmas observations from chile ecuador and costa rica suggest a three stage evolution 1 steep subduction produces a narrow calc alkaline arc typically 300 km from the trench above the asthenospheric wedge 2 once flat subduction begins the lower plate travels several hundred kilometers at nearly the same depth thus remaining in a pressure temperature window allowing slab melting over this broad distance and 3 once flat subduction continues for several million years the asthenospheric wedge disappears and a volcanic gap results as in modern day central chile or peru the proposed hypothesis which reconciles thermal models with geochemical observations has broad implications for the study of arc magmatism and for the thermal evolution of convergent margins

migmatization and secondary granitic magmas effects of emplacement and crystallization of primary granitoids in southern bohemia austria the sauwald and muhl zones of the prebatholithic moldanubian middle crust in northern austria contain metapelites and metaluminous to weakly peraluminous metagreywackes respectively both zones were affected by low pressure high temperature metamorphism and anatexis the metapelites of the sauwald zone became in situ diatexites probably by fluid absent reactions involving the breakdown of muscovite and the partial breakdown of biotite the biotite plagioclase quartz gneisses of the muhl zone experienced only slight melting following this event and while the mid crust was still hot additional heat was locally advected into the muhl zone by the intrusion of the weinsberg granite this brought about fluid present partial melting of the biotite plagioclasequartz gneisses producing relatively large volumes of metaluminous to weakly peraluminous i type schlieren granite this cool wet restite rich magma remained close to its site of generation thus infracrustal i type granitoids may be formed anywhere in the crust and not always at high t under special circumstances the heat and fluids from granitic magmas can spawn secondary granites also relatively low initial 87sr 86sr values of around 0 707 in i type rocks do not necessarily indicate either lower crustal magma sources or mixing with mantle derived magma the weinsberg granite magma came from the lower crust p probably 700 mpa where widespread fluid absent breakdown of biotite plagioclase quartz assemblages occurred the necessary high heat flow was probably provided by newly underplated mafic magmas however these seem not to have mixed or mingled with the crustally derived weinsberg magmas deep equivalents of the muhl zone metagreywackes may have formed the weinsberg protolith fluid absent experiments show that the melting temperature probably exceeded 850 c and that a garnet bearing orthopyroxene rich residue should be present in the lower crust fluid present experiments demonstrate that the availability of free h2o can radically alter the characteristics of the partial melts from apparent s type mineralogy with fluid absent melting to i type mineralogy with wet melting

late paleozoic calc alkaline to shoshonitic magmatism and its geodynamic implications yuximolegai area western tianshan xinjiang article i nfo continuous late paleozoic magmatic activity in the western tianshan xinjiang china temporally overlapped the late carboniferous collision between the tarim and the kazakhstan yili plates isotopic dating whole rock geochemistry and isotopic characteristics of a suite of calc alkaline to shoshonitic rocks from yuximo legai area in the eastern end of the awulale mountains help define the tectonic evolution of this part of the tianshan zircon u pb ages of a quartz diorite and a quartz syenite porphyry are 310 8 2 1 ma and 284 4 3 6 ma respectively the geochemical and isotopic characteristics of late carboniferous igneous rocks including the quartz diorite as well as basaltic andesite and k rich trachyandesite indicate an evolu tion from calc alkaline towards shoshonitic series this was generated by the partial melting of the hydrated mantle wedge induced by changing subduction angles the younger quartz syenite porphyry however was more likely derived from the partial melting of a thickened crust in a post collisional setting or the partial melting of hot juvenile basalt in the lower crust during early permian delamination of the subcontinental lithospheric mantle therefore the generation of the calc alkaline to shoshonitic igneous rocks in yuximole gai area is genetically linked to the geodynamic evolution of the western tianshan from convergent subduc tion to collision during the late paleozoic

cao mgo al 2 o 3 sio 2 h 2 o to 35 kb amphibole talc and zoisite dehydration and melting reactions in the silica excess part of the system and their possible significance in subduction zones amphibolite melting and magma fractionation

mio pliocene adakite generation related to flat subduction in southern ecuador the quimsacocha volcanic center abstract new geochemical and geochronological data on the miocene pliocene quimsacocha volcanic center qvc have led to the recognition of adakitic lavas generated by slab melting related to the flat slab subduction in southern ecuador and northern peru the qvc located in the presently inactive southern part of the ecuadorian arc was built up during three distinctive volcanic phases the first phase generated a basal edifice with mainly andesitic lava flows while the second phase is characterized by the emplacement of cryptodomes domes and related outflow breccias comprised of andesites and some dacites the last phase released rhyolitic ignimbrites associated with the formation of a large caldera which was later partly filled by dacitic rhyolitic domes geochemical data for the qvc indicate higher al2o3 tio2 na2o zr and sr contents and lower fe2o3 mgo y mree and hree abundances compared to other eruptive rocks of the plio quaternary volcanic front of ecuador such geochemical features as well as the frequent presence of an associated epithermal gold deposit are characteristic of the involvement of slab melts also known as adakites 1 2 in the generation of these magmas after a calc alkaline arc magmatism phase slab horizontalization in response to the subduction of a buoyant oceanic plateau results in increased involvement of a slab melting component in the magmas produced however pristine adakites were generated and emplaced during a relatively short period as indicated by zircon fission track ages then volcanic activity stopped and a volcanic gap formed the identification of these adakites their location and age support a model of slab melting associated with flat slab subduction m a gutscher et al geology 28 2000 535 538

contrasting granitic magmatism of the kalba fold belt east kazakhstan evidence for late paleozoic post orogenic events abstract the paper presents new original data and a review of previous studies on the late carboniferous early permian granitoids of the kunush and kalguty intrusive complexes from the kalba fold belt east kazakhstan these rocks formed at the initial post collisional stage of the irtish zaisan orogen in the western central asian orogenic belt caob the granitoids form isometric or nw linear intrusions inside the late devonian early carboniferous metasediments which overlap the accretionary lithology of the kalba fold belt in front of the altai active margin of the siberian continent these granitoids contain zircon grains with u pb ages of ca 308 291 ma synchronous with the peak post orogenic magmatic event at 300 ma in the caob the kunush high na granitoids with high sio2 67 72 and al2o3 15 18 contents and the na2o k2o ratio 1 94 6 43 and low hrees yb 0 22 0 93 ppm but moderate sr y ratios 47 179 mg 35 55 and ni cr are generally common to high al ttg series with non subduction geochemical signatures the kalguty granitoids mostly belong to calc alkalic to subalkalic high k series have non corundum cipw norm compositions and mainly weakly peraluminous asi 0 97 1 09 characteristics their fe 0 7 1 at 62 74 wt sio2 and relatively high y nb 2 4 and rb nb 9 18 ratios are similar to transitional compositions reported for fractionated i and a2 type post orogenic granites worldwide the kunush high na and kalguty high k granitoids formed at different depths in the crust and the parent melts were derived from metabasaltic morb p 10 15 kbar and metagranitic ttg like p

late triassic granitoids of the eastern margin of the tibetan plateau geochronology petrogenesis and implications for tectonic evolution abstract late triassic granitoids in the songpan garze fold belt sgfb on the eastern margin of the tibetan plateau formed at 230 to 220 ma and can be divided into two groups group 1 are high k calc alkaline rocks with adakitic affinities k adakites with sr 400 ppm y n 32 105 and high k 2 o na 2 o 1 group 2 are ordinary high k calc alkaline i types with lower sr 18 ppm and weakly fractionated ree patterns la yb n 0 50 to 0 94 and initial 87 sr 86 sr 0 70528 to 0 71086 but group 1 rocks have higher e nd t 1 01 to 4 84 than group 2 3 11 to 6 71 calculated initial pb isotope ratios for both groups are 206 pb 204 pb 18 343 to 18 627 207 pb 204 pb 15 610 to 15 705 and 208 pb 204 pb 38 269 to 3759 group 1 magmas were derived through partial melting of thickened and then delaminated ttg type eclogitic lower crust with some contribution from juvenile enriched mantle melts group 2 magmas were generated by partial melting of shallower lower crustal rocks the inferred magma sources of both groups suggest that the basement of the sgfb was similar to the exposed kangding complex and that the sgfb was formed in a similar manner to the south china basement here passive margin crust was greatly thickened and then delaminated all within a very short time interval 20 myr such post collisional crustal thickening could be the tectonic setting for the generation of many adakitic magmas especially where there is no spatial and temporal association with subduction