adakite like potassic magmatism and crust mantle interaction in a postcollisional setting an experimental study of melting beneath the tibetan plateau

amphibole sponge in arc crust pressure temperature time paths followed by arc magmas ascending through the lithosphere dictate the phase assemblage that crystallizes and hence the compositions of liquid fractionates here we use la yb and dy yb versus sio 2 relationships from selected volcanoes to show that amphibole is an important mineral during differentiation of arc magma production of intermediate and silicic arc magmas occurs as magmas stall and cool in the mid lower crust where amphibole is stable because amphibole is rarely a phenocryst phase we term this cryptic amphibole fractionation if this process is as widespread as our investigation suggests then 1 amphibole cumulates may act as an effective filter for water dissolved in mantle derived magmas 2 amphibole cumulates may act as a fertile source of intracrustal melts and fluids and 3 recycling of amphibole cumulates has the potential to return incompatible trace elements and water to the mantle

chronology of deformation within the turbidite dominated lachlan orogen implications for the tectonic evolution of eastern australia and gondwana ar ar data from fabric forming white mica in slates syntectonic quartz veins and granitic mylonites constrain the timing of metamorphism deformation and exhumation in the lachlan orogen australia these data also help define the tectonic evolution of the tasmanides during paleozoic time the lachlan orogen formed by the progressive accretion of a thick turbidite fan sequence and volcanic terrains to gondwana during the closing of a small marginal ocean or back arc basin this tectonic setting was similar to the present western and southwestern pacific region accretion of the lachlan orogen to gondwana occurred by closing of the basin system by subduction accretion processes and some translation the process is typified in the western lachlan orogen by a major eastward migrating deformation involving chevron folding and faulting over an eastward propagating decollement melange zone and is recorded by ar ar mica ages ranging from 455 ma in the western part to 390 ma in the eastern part in the central lachlan orogen deformation migrated southwestward from 440 430 to 405 ma away from the high temperature wagga omeo metamorphic complex where deformation metamorphism occurred between 440 and 400 ma in the north 400 ma mica ages record deformation and inversion of structures in the cobar basin in the eastern lachlan orogen ar ar mica dates range from 450 to 340 ma ages of 455 445 ma are yielded by the narooma accretionary complex 405 390 ma ages are found along the major thrust faults bounding high grade metamorphic complexes and 360 340 ma cooling ages are found in the inverted extensional basins e g hill end and related structural zones the ar ar results also document periods of reactivation on early formed structures during later deformation elsewhere in the orogen

miocene high sr y magmatism south tibet product of partial melting of subducted indian continental crust and its tectonic implication abstract post collisional 26 10 ma high sr y and hree depleted intrusive rocks occur in an e w trending belt paralleling the yarlung zangbo suture in south tibet their petrogenesis has been a subject of debate here we present chronological geochemical and sr nd hf isotopic data for two newly discovered large volume high sr y granitoids the pagu granodiorite and the nanmuqie granite and three high sr y porphyries intruding the granitoids in the lhasa terrane la icp ms zircon u pb dating for the pagu granodiorite and nanmuqie granite yielded identical magma crystallization ages of 14 0 14 4 ma which is indistinguishable from their associated high sr y porphyries 14 2 14 6 ma the granitoid was intruded at middle crust depth whereas the porphyry was intruded at upper crust depth indicating that the lhasa terrane has experienced a rapid crustal uplift during the high sr y magma emplacement zircon hf isotopic and whole rock sr nd isotopic compositions for these granitoids and porphyries suggest that their magmas were dominantly derived from partial melting of crustal materials the granitoids and porphyries have ehf t values overlapping with the mafic granulites in the himalayan terrane indian plate their sr nd isotopic compositions show two endmember mixing between the himalayan mafic lower crust and the ultrapotassic lavas the lhasa lower crust we suggest that the high sr y magmas in the lhasa terrane could be derived from partial melting of subducted indian mafic lower crust with incorporation of the ultrapotassic lava and or the lhasa lower crust components our study suggests a new model for the high sr y magma generation in the lhasa terrane and provides a line of geochemical evidence that the indian continental crust was subducted beneath the southern lhasa terrane in the early middle miocene

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

early cleavage development in the late ordovician of northeast victoria australia abstract a pervasive cleavage s that subparallels bedding in the upper ordovician rocks of northeast victoria can be related to an incremental strain distribution in a shear environment during the establishment of the first set of regional tectonic folds and faults the cleavage is a strong preferred orientation of phyllosilicates which overprints sedimentary features is contemporaneous with early formed extension fractures and postdates the formation of clastic dykes the cleavage is attributed to tectonic and metamorphic processes rather than a static mimetic recrystallization of clay minerals during burial metamorphism there is little evidence to suggest that this is an early deformation originating during imbricate stacking of an ordovician accretionary prism

miocene adakitic intrusions in the zhongba terrane implications for the origin and geochemical variations of post collisional adakitic rocks in southern tibet abstract post collisional adakitic rocks 26 9 ma have been recognized in southern tibet for almost 20 years however their origins and geodynamics remain highly controversial this paper reports geochronological and geochemical data for the lasa intrusion exposed in the zhongba terrane and proposes implications for the origins of the miocene adakitic rocks in southern tibet the lasa intrusion consists of granite and granodiorite porphyries with zircon u pb ages of ca 16 ma the porphyries have high sio2 65 14 67 61 wt al2o3 15 13 16 10 wt k2o 2 87 3 54 wt and sr 727 1046 ppm contents and low y 9 13 ppm and yb 0 82 1 19 ppm contents they are enriched in light rare earth elements lrees and depleted in heavy rare earth elements hrees the porphyries display relatively low mgo contents 1 39 1 98 wt high sr y 65 98 and la yb n 18 8 25 5 ratios initial 87sr 86sr i ratios of 0 70912 0 70805 end t values of 6 4 to 7 9 206pb 204pb t ratios of 18 7108 18 7665 207pb 204pb t ratios of 15 6837 15 7118 and 208pb 204pb t ratios of 39 1600 39 3034 these signatures indicate that the lasa porphyries are adakitic rocks derived from the partial melting of a thickened lower crust the miocene adakitic rocks in the zhongba terrane have similar sr nd isotopic compositions to those of western lhasa and the himalaya terrane which differ from coetaneous adakitic rocks of the eastern lhasa terrane in terms of sr nd isotopic compositions based on the spatial distributions and isotopic features of the post collisional adakitic rocks in southern tibet we suggest that all of the adakitic rocks were derived from the partial melting of thickened lower crust and that their geochemical variations are mainly caused by the heterogeneity of lower crust

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

igneous garnet and amphibole fractionation in the roots of island arcs experimental constraints on andesitic liquids to evaluate the role of garnet and amphibole fractionation at conditions relevant for the crystallization of magmas in the roots of island arcs a series of experiments were performed on a synthetic andesite at conditions ranging from 0 8 to 1 2 gpa 800 1 000 c and variable h2o contents at water undersaturated conditions and fo2 established around qfm garnet has a wide stability field at 1 2 gpa garnet amphibole are the high temperature liquidus phases followed by plagioclase at lower temperature clinopyroxene reaches its maximal stability at h2o contents 9 wt at 950 c and is replaced by amphibole at lower temperature the slopes of the plagioclase in boundaries are moderately negative in text t text x text h 2 text o space at 0 8 gpa garnet is stable at magmatic h2o contents exceeding 8 wt and is replaced by spinel at decreasing dissolved h2o the liquids formed by crystallization evolve through continuous silica increase from andesite to dacite and rhyolite for the 1 2 gpa series but show substantial enrichment in feo mgo for the 0 8 gpa series related to the contrasting roles of garnet and amphibole in fractionating fe mg in derivative liquids our experiments indicate that the stability of igneous garnet increases with increasing dissolved h2o in silicate liquids and is thus likely to affect trace element compositions of h2o rich derivative arc volcanic rocks by fractionation garnet controlled trace element ratios cannot be used as a proxy for slab melting or dehydration melting in the deep arc garnet fractionation either in the deep crust via formation of garnet gabbros or in the upper mantle via formation of garnet pyroxenites remains an important alternative despite the rare occurrence of magmatic garnet in volcanic rocks

pluton emplacement in a strike slip fault zone the doctors flat pluton victoria australia the siluro devonian doctors flat pluton lachlan fold belt australia is a hornblende biotite granite i type that was emplaced into the early silurian omeo metamorphic complex it lies along the ensay fault a major dextral strike slip fault a solid state foliation is developed in much of the pluton with the most intense development at the outer parts grading into massive granite at the centre this foliation is approximately parallel to trace of the ensay fault and to the borders of the pluton it grades into a mylonitic fabric along the ensay fault in the eastern part of the pluton a second fabric is developed giving rise to s c fabrics indicating dextral shear some microfabrics in the pluton are typical of relatively high temperature deformation consistent with syntectonic emplacement but lower temperature microstructures overprint them and are dominant a possible mechanism of intrusion involves the pluton being emplaced into a dilational section of the ensay fault when a jog in the fault opened up into a pull apart structure as intrusion continued the northeastern branch of the pull apart took up all the movement with later intrusive material being less deformed along the ensay fault lower grade assemblages of chlorite epidote albite sphene and cataclastic textures developed at lower temperatures as fault displacement continued during and after cooling of the pluton

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

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

post collisional plutonism with adakite like signatures the eocene saraycik granodiorite eastern pontides turkey the post collisional saraycik granodiorite intruded into a late paleocene to early eocene nappe pile that formed during collision of the pontides in the north and the anatolide tauride platform in the south leading to the formation of the izmir ankara erzincan suture a relatively shallow pluton intrusion depth 5 to 8 km was estimated from al in hornblende geobarometry and contact metamorphic assemblages the emplacement age is tightly constrained to 52 ma by two ar ar plateau and total fusion ages on biotite the main mass of the pluton consists of metaluminous to peraluminous biotite granodiorite and hornblende biotite granodiorite in addition up to 10 m thick dacitic and 25 cm thick aplitic dikes occur granodiorites and dacites show many close compositional similarities to high silica adakites from supra subduction zone settings but tend to be slightly more felsic and to have a higher aluminium saturation index chondrite normalized cn rare earth element patterns are characterized by high ratios of la yb cn concave upward shapes of the hree and a lack of significant eu anomalies in conjunction with relatively high abundances of ba and sr as well as low abundances of y hree and sc these patterns suggest a feldspar poor garnet amphibole rich fractionating mineral assemblage residue all samples have very similar nd sr isotopic characteristics regardless of rock type initial end values range from 0 3 to 1 2 and initial 87sr 86sr ratios from 0 70491 to 0 70529 it is suggested that the magmas formed by partial melting of mafic lower crust at elevated pressures 1 to 2 gpa

age and composition of dikes in southern tibet new constraints on the timing of east west extension and its relationship to postcollisional volcanism controversy exists over whether east west extension in southern tibet is related to plateau uplift or to the accommodation of plate boundary forces relationships between the onset of extension plateau uplift and the thermal state of the lithosphere are critical to this debate we present new data on postcollisional north south trending dikes in southern tibet their ages range from 18 3 2 7 ma to 13 3 0 8 ma and define the onset of regional east west extension in southern tibet dikes are compositionally indistinguishable from postcollisional lavas in southern tibet being either ultrapotassic having a source in the subcontinental lithospheric mantle or calc alkaline with a dominantly crustal origin the ultrapotassic dikes demonstrate that east west extension and subcontinental lithospheric mantle derived magmatism were temporally and spatially linked supporting models that relate the latest phase of plateau uplift to subcontinental lithospheric mantle thinning thus the onset of extension by 18 3 2 7 ma represents the time at which the potential energy of the plateau exceeded convergent boundary forces this places a new age limitation on the attainment of high elevation in southern tibet with implications for models that relate cenozoic monsoon intensification to plateau uplift

seismic anisotropy and shear wave splitting in lower crustal and upper mantle rocks from the ivrea zone experimental and calculated data to quantify the relationships between anisotropy s wave splitting and tectonics we determined the seismic properties of lower crustal and upper mantle rocks outcropping in the ivrea zone northern italy we obtained p and s wave seismic velocities by laboratory direct velocity measurements and or by calculations based on the modal compositions of the rocks the lattice preferred orientations lpos and the single crystal stiffness coefficients measured p and s wave velocities 6 0 7 5 km s 1 and 3 6 4 2 km s 1 are typical of the lower crust the p wave anisotropy is in the range 0 10 shear wave birefringence is in the range 0 0 0 6 km s 1 with typical values between 0 0 and 0 2 km s 1 in many cases the birefringence is clearly related to fabric elements foliation lineation mafic rocks such as anorthosite or pyroxene bearing gabbros exhibit low p wave anisotropies 5 and low shear wave birefringences less than 0 1 km s 1 in contrast the seismic properties of felsic rocks such as biotite bearing gneisses and mafic rocks such as amphibolites display high vp anisotropy 10 and strong birefringence 0 3 km s 1 biotite and amphibole preferred orientations clearly control seismic anisotropy and particularly shear wave splitting in these rocks maximum splitting is observed in directions parallel to the foliation with the fast split shear wave polarized parallel to the foliation plane to have an overview of the seismic properties of this lower crustal section at a broader scale we calculated from our data the anisotropic seismic properties of several hypothetical samples that are perhaps more representative of the regional anisotropy than each sample individually for instance the average lower crustal sample displays an anisotropy of 5 5 for p waves and a birefringence around 0 14 km s 1 for s waves propagating parallel to the foliation we observe little splitting for waves propagating at high angle to the foliation

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

rock buffered fluid rock interaction in deformed quartz rich turbidite sequences eastern australia quartz vein 18o 16o ratios across a 500 km transect through the lachlan fold belt of southeastern australia are remarkably uniform 1 1 5 permil at both local centimeter to meter and regional over 104 km2 scales they define isotopic zones that correlate with the tectonic divisions of the cambrian through devonian dominantly ordovician quartz rich turbidites determined by regional mapping from west to east these divisions are 1 stawell 18o 14 7 1 1 47 samples 2 bendigo ballarat 18o 17 5 1 3 204 samples 3 melbourne 18o 19 0 1 6 80 samples 4 tabberabbera 18o 16 3 1 9 12 samples and 5 omeo 18o 14 4 1 0 26 samples isotope profiles across zone boundaries and intrazone fault zones particularly within the bendigo ballarat zone show steps indicating abrupt changes across faults with little or no evidence of fluid mixing within the fault zones 18o values of veins are insensitive to relative age type of vein and immediate host rock lithologies the 18o values of coexisting vein and host rocks show nonequilibrium relationships which can be explained in terms of rock buffering under conditions of low fluid rock ratios water rock 1 limited d h determinations on fluid inclusions fall mainly in the range 70 and 100 with one value as low as 140 these low deuterium values when considered in the context of paleolatitude may have been inherited from deutrium depleted detrital minerals and do not necessarily require the direct penetration of meteoric fluids to midcrustal depths the work suggests that vein formation is possible in regions where the integrated water rock w r ratio is very low w r 1 as long as a pervasive fluid phase is present this fluid appears to achieve a steady state isotopic composition on scales of hundreds of meters and once a quasi steady state has been reached the isotopic compositions of the resultant quartz veins are rather insensitive to the diachronous and nonisothermal conditions under which vein growth occurs advective cycling of the fluid on this scale is by episodic dilatancy pumping in fracture networks associated with localized faulting

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

magnitude and symmetry of seismic anisotropy in mica and amphibole bearing metamorphic rocks and implications for tectonic interpretation of seismic data from the southeast tibetan plateau we calibrated the magnitude and symmetry of seismic anisotropy for 132 mica or amphibole bearing metamorphic rocks to constrain their departures from transverse isotropy ti which is usually assumed in the interpretation of seismic data the average bulk vp anisotropy at 600 mpa for the chlorite schists mica schists phyllites sillimanite mica schists and amphibole schists examined is 12 0 12 8 12 8 17 0 and 12 9 respectively most of the schists show vp anisotropy in the foliation plane which averages 2 4 for phyllites 3 3 for mica schists 4 1 for chlorite schists 6 8 for sillimanite mica schists and 5 2 for amphibole schists this departure from ti is due to the presence of amphibole sillimanite and quartz amphibole and sillimanite develop strong crystallographic preferred orientations with the fast c axes parallel to the lineation forming orthorhombic anisotropy with vp x vp y vp z effects of quartz are complicated depending on its volume fraction and prevailing slip system most of the mica or amphibole bearing schists and mylonites are approximately transversely isotropic in terms of s wave velocities and splitting although their p wave properties may display orthorhombic symmetry the results provide insight for the interpretation of seismic data from the southeast tibetan plateau the n s to nw se polarized crustal anisotropy in the sibumasu and indochina blocks is caused by subvertically foliated mica and amphibole bearing rocks deformed by predominantly compressional folding and subordinate strike slip shear these blocks have been rotated clockwise 70 90 around the east himalayan syntaxis without finite eastward or southeastward extrusion in responding to progressive indentation of india into asia

late paleozoic underplating in north xinjiang evidence from shoshonites and adakites abstract shoshonitic series volcanic rocks ssvr and adakites are widely distributed in the permian terrestrial volcanic strata of the yishijilike awulale range of west tianshan north xinjiang china isotopic dating yields permian ages of 280 250 ma the ssvr include absarokite shoshonite and banakite which are characterized by enrichment of alkalis particularly in k combined with lower ti higher al a nkc 0 70 0 99 metaluminous and fe2o3 feo the ssvr that are rich in lile with high ree contents and eu eu range from 0 59 to 1 30 they are rich in lree la yb n 2 15 11 97 and depleted in nb ta and ti tnt negative anomalies the adakites are metaluminous to weakly peraluminous a nkc 0 85 1 16 and belong to the high sio2 type of adakite hsa sio2 62 71 they are characterized by lower ree with strong lree enrichment la yb n 13 35 pronounced positive eu anomalies eu eu 1 02 1 27 very low yb contents and distinct tnt negative anomalies are evident the ssvr have end t 1 28 to 4 92 and 87sr 86sr i 0 7041 0 7057 that are similar to adakites in the regions which are characterized by end t 0 95 to 5 69 and 87sr 86sr i 0 7050 0 7053 trace element ree and sr nd isotopic compositions suggest that both ssvr and adakites possess similar source regions associated with underplated mantle derived basaltic materials lithosphere extension driven by magmatic underplating was responsible for the generation of both the ssvr and adakites this magmatism serves as a petrological indicator of underplating during the permian obviously thickened crust 62 52 km a complex moho discontinuity high heat flow 100 mw m 2 widespread contemporary alkali rich granites basic dike swarms k ar ages of 187 271 ma ar ar ages of 174 270 ma and rb sr ages of 255 28 ma end t 1 84 to 10 1 87sr 86sr i 0 7035 and 0 7065 and basic granulites shrimp u pb age of 268 279 5 6 ma provide additional evidences for the underplating event in this area during permian

adakite like granitoid porphyries in the eastern pontides ne turkey potential parental melts and geodynamic implications abstract the tectonic setting of the eastern pontides during the late mesozoic to early cenozoic remains a subject of debate petrogenesis of adakite like granitoid porphyries plays a critical role in determining the nature of the lower continental crust and mantle dynamics during orogenic processes in the region here we describe for the first time the late paleocene to early eocene adakite like granitoid porphyries from the northern part of the eastern pontides although their counterparts in the southern part have recently been found the adakitic porphyries which emplaced into the subduction related turonian santonian volcanics from northern part of the region consist of i type calc alkaline quartz monzonite tonalite sio2 62 89 65 07 wt and high k calc alkaline granodiorite granite associations sio2 69 06 70 43 wt the former displays peraluminous to metaluminous signatures whereas the latter shows peraluminous geochemical character the granite granodiorite porphyries have high k2o 3 32 3 84 wt and low na2o 3 48 4 61 wt and mgo 0 91 1 04 wt relative to the quartz monzonite tonalite association k2o 1 50 1 92 wt na2o 4 08 6 45 wt mgo 1 44 2 07 wt ar ar geochronology studies on the amphibole separates reveal that the adakite like porphyries have a crystallization ages of 51 34 0 27 to 53 55 0 34 ma here we contend that these rocks were formed by partial fusion of a mafic lower continental crust in a collisional phase but not in a subduction setting all the samples exhibit the typical geochemical characteristics of adakite that is high sr 250 1141 ppm high sr y ratios 16 147 low y 6 8 14 8 ppm and low hree concentrations they are similar to adakites formed by slab melting associated with the subduction zone however the rocks exhibit heterogeneity in isotopic composition with isr ranging from 0 70554 to 0 70986 end 51 ma from 8 5 to 0 9 and nd model ages from 0 72 to 1 26 ga the samples are characterized by relatively high th th u and no significant eu anomalies implying that garnet was stable in their source during partial melting the compositional diversity between rock groups is probably related to partial melting of heterogeneous lower crustal source all of the features are inconsistent with a slab melting origin and slab related petrogenetic model but instead they favor an origin by melting of heterogeneous lower continental crust due to a thermal anomaly that was induced by the upwelling of the asthenosphere through a slab break off in a collisional setting these interpretations argue against the evidence for the presence of an early cenozoic arc setting in the eastern pontides instead the early cenozoic time in the region may be attributed to a geodynamic response to a post collisional uplift phase that occurred along the continent continent collision between the pontide and the anatolide tauride blocks

a genetic linkage between subduction and collision related porphyry cu deposits in continental collision zones the genesis of continental collision related porphyry cu deposits pcds remains controversial the most common hypothesis links their genesis with magmas derived from subduction modified arc lithosphere however it is unclear whether a genetic linkage exists between collision and subduction related pcds here we studied jurassic subduction related cu au and miocene collision related cu mo porphyry deposits in south tibet the jurassic pcds occur only in the western segment of the jurassic arc which has depleted mantle like isotopic compositions e g 87 sr 86 sr i 0 7041 0 7048 e nd t as high as 7 5 and e hf t as high as 18 by contrast no jurassic pcds have been found in the eastern arc segment which is isotopically less juvenile e g 87 sr 86 sr i 0 7041 0 7063 e nd t 4 5 and e hf t 12 these results imply that incorporation of crustal components during underplating of jurassic magma induced copper accumulation as sulfides at the base of the eastern jurassic arc inhibiting pcd formation at this time miocene pcds are spatially confined to the jurassic arc and the giant miocene pcds cluster in its eastern segment where no jurassic pcds occur this suggests that the arc segment barren for subduction related pcds could be fertile for collision related pcds miocene ore forming porphyries have young hf model ages and sr nd hf isotopic compositions overlapping with those of the jurassic rocks in the eastern segment whereas contemporaneous barren porphyries outside the jurassic arc have abundant zircon inheritance and crustlike sr nd hf isotopic compositions these data suggest that remelting of the lower crustal sulfide bearing cu rich jurassic cumulates triggered by cenozoic crustal thickening and or subsequent slab break off led to the formation of giant miocene pcds the spatial overlap and complementary metal endowment between subduction and collision related magmas may be used to evaluate the mineral potential for such deposits in other orogenic belts

recycling of metal fertilized lower continental crust origin of non arc au rich porphyry deposits at cratonic edges recent studies argue that subduction modified cu fertilized lithosphere controls the formation of porphyry cu deposits in orogenic belts however it is unclear if and how this fertilization process operates at cratonic edges where numerous large non arc au rich deposits form here we report data from lower crustal amphibolite and garnet amphibolite xenoliths hosted by cenozoic stocks that are genetically related to the beiya au rich porphyry deposits along the western margin of the yangtze craton china these xenoliths are thought to represent cumulates or residuals of neoproterozoic arc magmas ponding at the base of arc at the edge of the craton that subsequently underwent high pressure metamorphism ca 738 ma the amphibolite xenoliths are enriched in cu 383 445 ppm and au 7 12 ppb and a few garnet amphibolite xenoliths contain higher au 6 16 ppb with higher au cu ratios 2 10 4 to 8 10 4 than normal continental crust these data suggest that metal fertilization of the base of an old arc at the edge of the craton occurred in the neoproterozoic via subduction modification and has since been preserved the whole rock geochemical and zircon hf isotopic data indicate that melting of the neoproterozoic cu au fertilized low crustal cumulates at 40 30 ma provided the metal endowment for the au rich porphyry system at the cratonic edge we therefore suggest that the reactivated cratonic edges triggered by upwelling of asthenosphere have the potential to host significant au ore forming systems especially non arc au rich porphyry deposits

rutile saturation in hydrous siliceous melts and its bearing on ti thermometry of quartz and zircon the tio2 solubility of rutile saturated hydrous siliceous melts has been investigated at p 1 gpa and t 650 1000 c for several representative felsic compositions the dissolution of a rutile crystal into a tio2 undersaturated melt provides information on both tio2 solubility and ti diffusion results of this study confirm that tio2 solubility is strongly dependent on both temperature and melt composition but not on the amount of h2o present for a given t t io2 content decreases as the melts become more felsic the solubility of tio2 is given by log ti ppm 7 95 5305 t 0 124fm

folding of angular unconformable sequences and effects on early folds tabberabbera district eastern victoria australia abstract the tabberabbera district of the southeast lachlan fold belt has a good example of a complex fold pattern produced by folding of angular unconformable sequences ordovician quartz flysch hotham group contains subhorizontal upright tight easterly trending f 1 folds of early to middle silurian age these have been reoriented by a locally intense middle devonian deformation that has formed an upright isoclinal syncline mitchell syncline in the unconformably overlying emsian clastic sequence wentworth group many of the f 1 folds in the hotham group immediately west of the mitchell syncline have steep northwest trending axial surfaces and are steeply plunging east of the syncline f 1 folds are upright with shallow plunges and gradually become inclined to recumbent as the unconformity is approached a folded foam layer analogue matching the geometry of the mitchell syncline shows that the reorientation of early fold axes and axial surfaces produced by the unconformity folding event is identical to that predicted by the model

character and kinematics of faults within the turbidite dominated lachlan orogen implications for tectonic evolution of eastern australia reply fault zones within turbidite dominated orogenic systems typified by the lachlan orogen of eastern australia are characterised by higher than average strain and intense mica fabrics transposition foliation and isoclinal folds poly deformation with overprinting crenulation cleavages and steeply to moderately plunging meso and micro folds they have a different character compared to the brittle ductile fault zones of classic foreland fold and thrust belts such as the appalachians and the canadian rocky mountains multiple cleavages and transposition layering record a progressive shear related deformation history an intense mica fabric evolves initially during shortening of the overlying sedimentary wedge but is progressively modified during rotation and emplacement to higher structural levels along the steep parts of inferred listric faults the deformed wedge outside the fault zones generally undergoes one phase of deformation shown by a weak to moderately developed slaty cleavage which is parallel to the axial surface of upright subhorizontally plunging chevron folds other faults within the turbidites of the lachlan orogen include the steep zones of ductile strike slip deformation that bound a centrally located high t low p metamorphic complex characterised by s c mylonites these ductile shear zones indicate a southward passage of the metamorphic complex as a crustal wedge with emplacement to higher structural levels along a leading edge ductile thrust fault ar ar dating constrains the timing of regional deformation to be mostly late ordovician through silurian across the lachlan orogen faults in the low grade turbidite sequences record the kinematic evolution of the developing lachlan orogen and indicate progressive deformation associated with simultaneous eastward propagating and migrating deformation fronts in both the western and eastern parts of the fold belt these deformation fronts are related to accretionary style deformation at the leading edges of overriding plates in an inferred southwest pacific type subduction setting from the late ordovician to the mid devonian along the former gondwana margin the fault zones effectively accommodate and preserve movements within the structurally thickening migrating and prograding accretionary wedge

earth s first stable continents did not form by subduction phase equilibria modelling of rocks from western australia confirms that the ancient continental crust could have formed by multistage melting of basaltic parents along high geothermal gradients a process incompatible with modern style subduction tim johnson et al perform phase equilibria modelling of the coucal basalts from western australia and confirm their suitability as parent rocks of the archaean continental crust the authors suggest that these early crustal rocks were produced by 20 30 per cent melting along high geothermal gradients they conclude that the production and stabilization of the first continents required a protracted multistage process when coupled with the high geothermal gradients this suggests that the continents did not form by subduction instead it favours a stagnant lid regime in the early archaean eon in which a single rigid plate lay over the mantle the geodynamic environment in which earth s first continents formed and were stabilized remains controversial1 most exposed continental crust that can be dated back to the archaean eon 4 billion to 2 5 billion years ago comprises tonalite trondhjemite granodiorite rocks ttgs that were formed through partial melting of hydrated low magnesium basaltic rocks2 notably these ttgs have arc like signatures of trace elements and thus resemble the continental crust produced in modern subduction settings3 in the east pilbara terrane western australia low magnesium basalts of the coucal formation at the base of the pilbara supergroup have trace element compositions that are consistent with these being source rocks for ttgs these basalts may be the remnants of a thick more than 35 kilometres thick ancient more than 3 5 billion years old basaltic crust4 5 that is predicted to have existed if archaean mantle temperatures were much hotter than today s6 7 8 here using phase equilibria modelling of the coucal basalts we confirm their suitability as ttg parents and suggest that ttgs were produced by around 20 per cent to 30 per cent melting of the coucal basalts along high geothermal gradients of more than 700 degrees celsius per gigapascal we also analyse the trace element composition of the coucal basalts and propose that these rocks were themselves derived from an earlier generation of high magnesium basaltic rocks suggesting that the arc like signature in archaean ttgs was inherited from an ancestral source lineage this protracted multistage process for the production and stabilization of the first continents coupled with the high geothermal gradients is incompatible with modern style plate tectonics and favours instead the formation of ttgs near the base of thick plateau like basaltic crust9 thus subduction was not required to produce ttgs in the early archaean eon

the interpretation of fold axial data from regions of polyphase folding a region with two interfering fold phases is considered if it is assumed that the early fn and later fn 1 fold phase are cylindrical and that fn 1 formed by shear folding then it is shown how the a b directions of fn 1 can be determined a technique for determining the original orientation of fn and sn is presented provided that similar amounts of simple shear occurred on opposing limbs of fn 1 folds the cause of variability in fold axial orientations is investigated and information to be extracted from fold axial distributions is presented

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

genesis of pristine adakitic magmas by lower crustal melting a perspective from amphibole composition genesis of adakites in arc and nonarc tectonic settings plays an important role in magmatic processes and material recycling along convergent margins however little is known about characteristics of pristine adakitic melts due to late stage magma evolutionary processes that dilute or obscure primary melt features and thus the genesis of adakites remains controversial here we present a detailed analysis of amphibole composition from early permian awulale postcollisional adakitic diorite and granodiorite porphyries in the core of tianshan orogen the central asian orogenic belt two distinct populations of amphiboles with markedly different aluminum contents are observed in the adakitic rocks these are 1 high al amphiboles crystallizing as an early mineral phase at about 1 gpa and 2 low al amphiboles as a late mineral phase at 220 400 mpa estimated on the basis of experimental phase equilibria data trace element modeling indicates that melts in equilibrium with the high pressure amphiboles are of adakitic composition suggesting that the awulale pristine magmas already had an adakitic nature before the amphibole crystallization this is consistent with the mafic lower crustal melting model rather than a high pressure basaltic melt fractionation for adakite petrogenesis our results lend new insights into how amphibole composition can be used to constrain the geochemical characteristics of pristine adakitic magmas

melting and dynamic metasomatism of mixed harzburgite glimmerite mantle source implications for the genesis of orogenic potassic magmas abstract tectonically young orogenic settings are commonly the sites of post collisional silica rich ultrapotassic magmas with extreme k2o contents of up to 9 wt and k2o na2o 2 many experimental studies investigating the generation of these melts have concentrated on melting of homogenous phlogopite bearing peridotites whereas geochemical signatures indicate the involvement of at least two types of source rocks ultra depleted and k and trace elements enriched ones we report the results of melting experiments at 1 2 gpa of mixed glimmerite and harzburgite in which these rock types make up two halves each capsule melting begins in the glimmerite and its metasomatic effects on the harzburgite are apparent at 1100 c even before melt pools are visible the first melts are na rich seen in zoning of olivines and as growth of clinopyroxene in the harzburgite but change at higher degrees of melting to produce a typical lamproite like melt with k2o 10 wt a major advantage of this study is the preservation of distinct melts in different parts of the capsule which reflect a process of dynamic metasomatism within the harzburgite matrix the infiltrating melt derived from melting of the glimmerite changes consistently with the distance of travel through the harzburgite enabling quantification of the metasomatic effects as an increase in sio2 and k2o this results principally from assimilation of orthopyroxene which increases the ol opx ratio of the residual harzburgite the effects of quench olivine growth are recognizable and can be quantified due to a step change in composition at the glimmerite harzburgite border the large total surface area of olivine and small melt fraction mean that the amount of quench olivine is high within the harzburgite but negligible in the almost completely molten glimmerite melts of the glimmerite contain up to 8 10 wt k2o and 53 wt sio2 which increase to 55 56 wt after interaction with the harzburgite mediterranean lamproites resemble melts of glimmerite whereas melts that have interacted with harzburgite are more similar to less potassic but more sio2 rich shoshonites of the mediterranean region

rapid crystallization of the animikie red ace pegmatite florence county northeastern wisconsin inclusion microthermometry and conductive cooling modeling we evaluated the crystallization regime of a zoned pegmatite dike and the degree of magma undercooling at the onset of crystallization by analyzing coeval fluid and melt inclusion assemblages the liquidus temperature of the pegmatite magma was 720 c based on re melting of crystallized melt inclusions in heating experiments the magma crystallized sequentially starting with a thin border zone which formed in less than one day at an average temperature of 480 c based on primary fluid inclusions meaning 240 c undercooling the primary inclusions from the outer zones were postdated by secondary inclusions trapped between 580 and 720 c representing fluid exsolved from hotter still crystallizing inner pegmatite units the huge temperature contrast between the pegmatite s inner and outer zones was simulated by conductive heat numerical modeling a 2 5 m wide dike emplaced in 220 c rocks cools entirely to 400 c in less than 50 days unidirectional and skeletal textures also indicate rapid disequilibrium crystallization

mafic injection as a trigger for felsic magmatism a numerical study 1 the origin of crustal scale silicic magmatism remains a matter of debate and notable uncertainty exists concerning the physical mechanisms that drive ascent and emplacement of felsic magmas in upper crustal regions a 2 d numerical model demonstrates that injection of mantle derived mafic magma into a partially molten hot zone in the lower crust can drive felsic magma ascent and intrusion into upper crustal levels the injection of mafic magma induces overpressure in the reservoir which increases crustal stresses and triggers development of brittle plastic shear zones and can drive significant surface uplift the emerging topography causes a nonuniform overpressure distribution in the reservoir and can trigger felsic magma ascent along crustal shear zones based on systematic numerical experiments we investigate the influence of crustal strength and injection rate the initial upper crustal strength controls the degree of crustal faulting and surface uplift and therefore whether felsic magma ascent can be initiated or not the final upper crustal strength influences the depth and final style of felsic intrusion the injection rate of mafic magma determines the time scale of overpressure growth and surface uplift stage in contrast the duration of the subsequent felsic ascent and intrusion emplacement stages remains nearly constant our results imply that mafic underplating and intrusion into the lower crust may not only be a prime control for the generation of felsic magmas in the lower crust but may also be an important physical driving mechanism for felsic magma ascent and intrusion into upper crustal levels

amphibole and lower crustal seismic properties abstract measurements of seismic anisotropy frequently offer insights of the deformation kinematics in situ within the earth in the continental crust seismic anisotropy is commonly ascribed to regionally aligned lattice preferred orientation lpo of mica crystals however determinations of composition suggest that the deep continental crust contains rather little mica in this contribution the role of amphibole a far more plausible constituent especially for deep continental crust of basic composition is assessed a field analogue approach is adopted where the lpo are measured correlated with the strain state of the rocks and used to calculate the seismic properties the seismic properties may be up scaled through the specimen using the modal proportions of the constituent mineral phases to examine the role of composition in controlling the intensity of seismic anisotropy seismic properties are computed using an array of modal compositions so called rock recipes the case history comes from the central block of the lewisian of nw scotland where a regional basic dyke swarm the scourie dykes is locally deformed within amphibolite facies shear zones conditions that are representative of the deep crust shear strain profiles across a 100 m wide shear zone using deflected banding in the host gneisses yield values of 0

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

overthrust terranes in the lachlan fold belt southeastern australia the paleozoic history of the lachlan fold belt involves terrane translation and accretion of discrete allochthonous fragments four major terranes developed adjacent to the gondwana continental margin and were overthrust in the middle devonian to form a partly composite crust for southeastern australia

partitioning of nb and ta between rutile and felsic melt and the fractionation of nb ta during partial melting of hydrous metabasalt abstract in order to fully assess the role of rutile in fractionation of nb ta during partial melting of hydrous metabasalt we have measured rutile felsic melt partition coefficients d values for nb and ta with tonalitic to trondhjemitic compositions at 1 5 3 5 gpa 900 1350 c and 5 0 20 wt h2o dnb dta and dnb dta range from 17 1 to 246 13 34 2 to 232 25 and 0 51 0 04 to 1 06 0 13 respectively for the compositions investigated melt composition appears to have no observable effect on the partitioning the effect of pressure is also slight whereas temperature and h2o have marked effects dnb dta and dnb dta increase with decreasing temperature and h2o content showing a reversal of dnb dta from 1 0 using the data that approached equilibrium and obeyed henry s law expressions describing the dependences of dnb dta and dnb dta on temperature pressure and melt h2o content were obtained 1 ln d nb rutile melt 2 846 0 453 9621 470 t 0 207 0 101 p 0 042 0 009 h 2 o 2 ln d ta rutile melt 0 775 0 398 6954 413 t 0 140 0 089 p 0 009 0 007 h 2 o 3 ln d nb d ta rutile melt 2 075 0 289 2657 301 t 0 075 0 065 p 0 033 0 006 h 2 o where t is in k p is in gpa and h2o are in wt these expressions are applicable to calculate nb and ta partitioning coefficients between rutile and felsic highly siliceous melts and nb ta fractionation during partial melting of metabasalt the derived models suggest that fluid present partial melting of metabasalt always leads to dnb dta lower than 1 0 whereas dehydration melting may lead to dnb dta higher than 1 0 at temperatures