early neoproterozoic crustal evolution in northern yili block insights from migmatite orthogneiss and leucogranite of the wenquan metamorphic complex in the nw chinese tianshan abstract the northern part of the yili block located in the northwest of the chinese tianshan extends westward to joint with the aktau junggar domain in kazakhstan and is one of the major continental constituents of the central asian orogenic belt caob the nature and tectonic significance of the basement of the continental domains are important for understanding the continental evolution and geodynamic processes of the caob we investigated the wenquan metamorphic complex wmc that represents the basement and metamorphosed sedimentary cover of the northern yili block the wmc is mainly composed of gneissic s type granite migmatite associated with amphibolite paragneiss micaschist quartzite marble and intruding leucogranitic dykes the migmatites display banded schlieren and ptygmatic structures containing rootless lensoid or layered leucosome and leucocratic sills which are indicative of in situ partial melting of the country rocks and variable degrees of melt migration geochemical data suggest that the protoliths of the orthogneiss and gneissic k granite belong to peraluminous s type granites characterized by wide range of i sr values 0 68324 0 72365 low nd t values 1 9 to 4 4 and two stage nd model ages t md 2 1 56 1 82 ga the s type granites might have derived from both clay poor and clay rich source rocks that are probably meta sedimentary and meta volcanic rocks in the wmc the leucogranites show relatively narrow range of i sr values 0 70273 0 70419 more negative nd t values 6 9 to 9 1 and more consistent t md 2 ages 1 51 1 55 ga the leucosome and leucocratic sills show variable sr and nd isotopic compositions probably due to localized and different degree of partial melting our new zircon shrimp and la icpms u pb ages together with previously published data indicate that the migmatization occurred at 926 909 ma the gneissic s type granites emplaced during 919 862 ma and the leucogranites emplaced in the period of 909 845 ma inherited old zircons were found in all dated samples and may have come from their source rocks association of geochronologically overlapped and genetically linked migmatites gneissic s type granites and leucogranitic rocks is interpreted as the results of anatexis of the upper crustal rocks during an early neoproterozoic tectonic metamorphic event that needs to be further studied in the northern yili block such early neoproterozoic migmatization and s type granitic magmatism are considered as an important episode of reworking and cratonization of the continental crust of the yili and adjacent continental blocks in central asia

a magnetic fabric study of the aigoual saint guiral liron granite pluton french massif central and relationships with its associated dikes 1 in the southeastern french massif central the aigoual saint guiral liron pluton consists of porphyritic and microgranitic types the latter is encountered within dikes forming the northern end of the pluton both types show prefull crystallization microstructures indicating weak subsolidus deformations an anisotropy of magnetic susceptibility ams study has been carried out to determine the granite fabric biotite local hornblende and small grains of magnetite are the main carriers of ams in both types porphyritic granite and dikes display different ams patterns related to magma dynamics and regional deformation recorded during crystallization in the porphyritic type the ams lineation is mainly consistent with the regional nw se to e w trending extensional event coeval with emplacement and crystallization of the pluton indicating an influence of regional tectonics on the linear fabric development the dome shaped foliation pattern of the saint guiral massif likely corresponds to internal deformation related processes within the pluton in the microgranite dike swarm the nne ssw trending lineations with shallow plunges record magmatic flow processes within dikes i e magma injection and filling of dikes from a probable source located southwestward regional tectonics played a significant role in the pluton geometry and fabric development for example the ne sw trend of the dikes suggests that extensional fractures took place in the same extensional strain field as elsewhere in the pluton different fabric development modes were therefore responsible for the contrasted fabric patterns between the microgranite and the porphyritic granite

igneous microstructures in migmatites clear differences between the microstructures of leucosomes and mesosomes occur in some nebulitic migmatites in the proterozoic arunta block central australia the leucosomes show crystal faces of k feldspar microcline microperthite cordierite andalusite and plagio clase against quartz indicating crystallization of a melt in contrast the mesosomes show predominantly polygonal grain shapes modified by 001 crystal faces of biotite and rare crystal faces in some porphyroblasts of garnet cordierite and orthopyroxene indicating re crystallization in the solid state the occurrence of abundant crystal faces in minerals such as feldspar cordierite and andalusite which typically grow crystal faces in magmas but generally not in metamorphic rocks is evidence of former melt therefore igneous microstructures can be preserved in leucosomes that have not undergone penetrative subsolidus deformation and recrystallization the cores of many of the crystals of cordierite and k feldspar have small locally oriented inclusions similar to those in the same minerals in the mesosomes these indicate that the cores remained solid during crystallization of the leucosome which produced inclusion free euhedral rims these crystals provide criteria for recognizing possible restite in granitoid magmas

magmatic mn rich garnets in volcanic settings age and longevity of the magmatic plumbing system of the miocene ramadas volcanism nw argentina abstract the miocene corte blanco tuff rhyolite deposit is the product of a large volume and high intensity plinian eruption from the solitary and monogenetic ramadas volcanic centre central andes province of salta nw argentina the corte blanco tuff consists of vitreous tube pumices with rare euhedral sub millimetric mn garnet phenocrysts typically hosting inclusions of u phases as zircon and monazite here we present new textural major and trace elemental analyses of garnet zircon and glass that combined with in situ u th pb zircon and monazite dating are used to reconstruct the thermobaric environment of formation age and longevity of the magmatic plumbing system of the ramadas magma the results indicate to a crystallization path of a peraluminous rhyolitic melt at shallow crustal levels 6 km as sequentially tracked by the initial nucleation of zircon 780 c at 9 16 ma and garnet above or at ca 700 c to the final monazite growth 660 670 c at 8 70 ma in a water saturated h2o 3 5 wt environment shortly before the eruption started these data 1 define for the first time the primary magmatic origin of mn garnet in a rhyolitic volcanic setting 2 provide new partition coefficients of rare earth elements ree between natural garnet zircon and rhyolitic melts and 3 permit reconstruction of the magmatic processes that resulted in the ramadas eruption on a wider scale our results document the spatio temporal p t conditions timing and longevity time scales involved in the petrogenesis of a shallow peraluminous water saturated rhyolitic magmatic plumbing system that is able to generate the conditions for extremely explosive plinian eruptions

regional scale pressure shadow controlled mineralization in the principe orogenic gold deposit central brazil abstract the principe gold deposit situated in the natividade gold district resulted from establishment of the regional transbrasiliano strike slip system during the late neoproterozoic brasiliano event the host rocks however were formed in the paleoproterozoic with sm nd model ages from 2250 to 2500 ma therefore the principe gold deposit is epigenetic the local lithological association is a tonalitic granodioritic suite formed in an island arc environment that later experienced continental collision tectonics leading to the intrusion of the peraluminous principe granite with an age of 2169 42 ma by zircon u pb during the neoproterozoic these units underwent deformational greenschist facies metamorphism and hydrothermal alteration forming metamorphic tectonites inside the principe granite and hydrothermally altered rocks at the northern termination of the granite where the principe gold deposit is located a strong structural control is identified in the deposit with n10 w n20 e trending brittle ductile shear zones that are visible at all scales from thin sections to regional satellite images these shear zones played an important role in the formation of the deposit acting as pathways that promoted a channelized fluid flow to metal precipitation sites hydrothermal alteration processes of different grades occur along these structures mainly characterized by saussuritization biotitization chloritization sericitization carbonatization silicification and sulfidation reactions producing a neoformed mineral assemblage the principe granite acted as a regional scale porphyroclast for migration of the hydrothermal solutions as well as deformation forming two pressure shadows that acted as structural traps for the ore as stress release zones these structures concentrated hydrothermal solutions during seismic fault movement events under a transpressive regime they also changed the physicochemical conditions in the fluid by abruptly reducing the pressure and increasing the fluid rock interaction rate producing a structural framework characterized by a pervasively interconnected system of discontinuous and anastomosed quartz carbonate sulfide gold veins fieldwork petrographic analysis and mineral chemistry characterization suggest that the fluid had an aqueous carbonic nature and low salinity values thermometric studies suggest that gold mineralization occurred between 250 c and 400 c stable isotopes of s c and o indicate a metamorphic source for the fluid whole rock geochemistry and microprobe studies highlight hydrothermal alteration processes involving fe ca mg k na au and ag metasomatism the principe gold deposit belongs to the orogenic class but it is distinguished from other orogenic type occurrences in the natividade gold district and from most orogenic deposits of the world for not being located along a shear zone instead it is situated within a regional scale auriferous pressure shadow that was brought about by a short lived pressure drop during the brasiliano cycle

andalusite and na and li rich cordierite in the la costa pluton sierras pampeanas argentina textural and chemical evidence for a magmatic origin the la costa pluton in the sierra de velasco nw argentina consists of s type granitoids that can be grouped into three igneous facies the alkali rich santa cruz facies scf sio2 67 wt distinguished by the presence of andalusite and na and li rich cordierite na2o 1 55 1 77 wt and li2o 0 14 0 66 wt the anillaco facies sio2 74 wt with a significant proportion of mn rich garnet and the anjullon facies sio2 75 wt with abundant albitic plagioclase the petrography mineral chemistry and whole rock geochemistry of the scf are compatible with magmatic crystallization of na and li rich cordierite andalusite and muscovite from the peraluminous magma under moderate p t conditions 1 9 kbar and ca 735 c the high li content of cordierite in the scf is unusual for granitic rocks of intermediate composition

magnetic anisotropy of the redencao granite eastern amazonian craton brazil implications for the emplacement of a type plutons abstract a magnetic fabric study was performed on the redencao pluton in an attempt to understand its emplacement history the redencao pluton is part of the 1 88 ga anorogenic a type jamon suite that intruded 2 97 2 86 ga old archean granitoids of the rio maria granite greenstone terrane in the eastern amazonian craton northern brazil previous gravity survey indicates that the pluton is a 6 km thick tabular intrusion it is characterized by a concentric distribution of facies with rings of seriated and porphyritic granite that cut across the main facies of even grained monzogranites the whole set is intruded by leucogranites that occupy the center of the pluton petrographic examination magnetic susceptibilities coercivity spectra and thermomagnetic curves indicate that the magnetic fabric is primarily carried by coarse grained multidomain magnetite this is reinforced by the coincidence of magnetic susceptibility and remanence anisotropy principal axes the absence of solid state deformation features and the low anisotropy degrees indicate that the magnetic fabric is magmatic in origin the magnetic fabric displays a systematic pattern with all facies including the rings of porphyritic granite being characterized by concentric gently dipping foliations associated with gently plunging lineations only the central leucogranitic facies shows a slightly discordant pattern with steeply dipping fabrics at its northeastern sector an emplacement model by vertical stacking of successive magma batches is proposed for the construction of the redencao pluton which reconciles the tabular shape of the intrusion the petrographic and geochemical zoning and the magnetic fabric pattern initially two magma batches were emplaced as sills first the even grained monzogranite then the seriated and porphyritic granites which formed by mingling of a leucogranitic melt with the host biotite monzogranitic magma as attested by geochemical data and field evidence the final shape of the pluton was acquired after the intrusion and inflation of the central leucogranite giving raise to the concentric pattern of facies in map view

garnet geochemistry of tungsten mineralized xihuashan granites in south china abstract the xihuashan complex intrusion in south china which is emplaced at ca 154 ma mainly comprises medium grained porphyritic biotite granite medium grained biotite granite and fine grained two mica granite medium grained biotite and fine grained two mica granites are important tungsten bearing granites and contain an unusual amount of garnet garnets from this intrusion are dominated by almandine and spessartine which constitute 94 to 99 of the total molecular composition of the garnet these garnets display unusual compositional zoning the cores of these garnets are rich in heavy rare earth element hree y ca and contain abundant hree and y rich mineral inclusions their rims are free of mineral inclusions and have low of these elements two types of fluorite inclusions exist in garnet y fluorite and y poor fluorite garnet exhibits specific mn zoning with a relatively mn poor core but a relatively mn rich rim thus constituting a specific spessartine inverse bell shaped profile that belongs to typical magmatic garnets all analyzed garnets have high ree content and exhibit hree enriched and lree depleted patterns with strong negative eu anomalies the incorporation of ree into garnet is in part controlled by its crystal chemistry with ree3 following a coupled substitution of the type fe2 mn2 1viii ree3 1viii si4 1iv z3 1iv the texture and compositional zoning of garnet suggest that garnet growth is largely controlled by the pressure temperature composition condition of magmatic evolution internal crystal chemical parameters and kinetics during mineral growth the garnet core grows in near equilibrium with magmatic melt under a relatively high pressure temperature p t condition by contrast the garnet rim grows rapidly by the coexisting melt fluid phase and co2 rich volatile environment under a relatively low p t condition which is virtually unfavorable for the incorporation of ree into the magmatic garnet structure garnet fingerprints the magmatic hydrothermal transition during crystallization of these granites

magnetic fabrics and microstructures of the post collisional aegirine augite syenite triunfo pluton northeast brazil abstract anisotropy of magnetic susceptibility ams and microstructures of the neoproterozoic triunfo alkaline pluton situated in northeast brazil have been investigated in order to characterize fabrics formed during its emplacement bulk magnetic susceptibility in the pluton varies between 0 15 10 3 and 25 10 t 3 si and the mean magnetic anisotropy ratio is around 1 04 aegirine augite and secondary magnetite are the main carriers of the magnetic susceptibility with the former controlling the directions of the anisotropy magnetic foliation and lineation are consistently oriented throughout the pluton foliation typically dips gently to the northeast and lineation plunges gently to the east these structures were mostly acquired at the magmatic and sub magmatic stages the shape of the magnetic ellipsoid is dominantly oblate a feature that may reflect either the uniaxial oblate ams ellipsoid of the aegirine augite crystals and or a vertical compaction of the magma in its final stages of crystallization the foliation pattern suggests the pluton has a tabular shape within the basement rocks magma emplacement is inferred to have occurred by its lateral migration along a flat lying crustal structure after fracture propagation and ascension of the magma from a lower crust mantle source late fine grained vertical dykes with the same mineral composition as the triunfo syenite cross cut the wall rocks of the pluton and close the alkaline magmatism in the area the ne trending alignment of alkaline plutons and the nne trending syenitic dykes indicate that the emplacement of such bodies occurred along a crustal extension event with a minimum compressive stress horizontal trending roughly e w

emplacement setting of the granite sheeted pluton of esperanca brasiliano orogen northeastern brazil abstract anisotropy of magnetic susceptibility ams and isotopic u pb sm nd data were combined to study the emplacement setting of the granite sheets that constitute the esperanca pluton in the borborema province northeastern brazil the sheets dip moderately to the se along the contact zone between the paleoproterozoic basement rocks and early neoproterozoic orthogneisses and metasediments granite fabrics were determined mainly using ams in 136 sites distributed within the central and western part of the pluton the sheets normally have susceptibility lower than 0 35 msi but locally where a ti poor magnetite appears with titanite the susceptibility increases up to 5 msi comparison between the silicate fabric and ams showed inconsistencies between the shape of mineral and magnetic ellipsoids despite of their orientations that fit fairly well to each other ams indicated the deformation was partitioned between the lower tonalite syenogranite and upper leucogranite and coarse porphyritic granite sheets in the lower sheets the curvilinear lineation trajectory is attributed to a dominant heterogeneous pure shear event that flattened laterally the still molten tonalite and syenogranite into the regional foliation the associated microstructures are typically magmatic zircon u pb data of the syenogranite yielded a crystallization age of 592 5 ma in the upper sheets the fabric recorded a component of simple shear deformation that displaced the coarse porphyritic granite and the top gneissic host rocks to the southwest microstructures are mostly of post full crystallization type tdm model ages and nd t 0 values indicate that the magma contaminated by partial melting of the regional host rocks sheet propagation at the emplacement level would have exploited the contact zone between crustal blocks of different rheologies when the melt pressures would be able to tensionally fail the anisotropy of the host rocks

different origins of garnet in high pressure to ultrahigh pressure metamorphic rocks abstract garnet in high pressure hp to ultrahigh pressure uhp metamorphic rocks in subduction zone commonly shows considerable zonation in major and trace elements as well as mineral inclusions which bears information on its growth mechanism via metamorphic or peritectic reactions in coexistence with relic minerals and metamorphic fluids or anatectic melts at subduction zone conditions it provides an important target to retrieve physicochemical changes in subduction zone processes including those not only in pressure and temperature but also in the durations of metamorphism and anatexis garnet from different compositions of hp to uhp metamorphic rocks may show different types of major and trace element zonation as well as mineral inclusions discrimination between the different origins of garnet provides important constraints on pressure and temperature and the evolution history for the hp to uhp metamorphic rocks magmatic garnet may occur as relics in granitic gneisses despite metamorphic modification at subduction zone conditions with spessartine increasing or flat major element profiles from inner to outer core and exceptionally higher contents of trace elements than metamorphic mantle and rim metamorphic garnet can grow at different metamorphic stages during prograde subduction and retrograde exhumation with spessartine decreasing from core to rim if the intracrystalline diffusion is not too fast the compositional profiles of metamorphic garnet in the abundances of grossular almandine and pyrope are variable depending on the composition of host rocks and co existing minerals peritectic garnet grows through peritectic reactions during partial melting of hp to uhp rocks with the composition of major elements to be controlled by anatectic p t conditions and the compositions of parental rocks and anatectic melts trace element profiles in garnet with different origins are also variable depending on the coexisting mineral assemblages the garnet forming reactions and the property of metamorphic fluids or anatectic melts mineral inclusions not only present key clues to identify the different origins of garnet but also serve as sound candidates for the temporal constraint on garnet growth

the ordovician las chacritas pluton sierra de humaya nw argentina origin and emplacement triggered by lateral shortening and magmatic stoping at mid crustal level field relationships and structural studies combined with in situ u th pb dating of monazite from las chacritas pluton lcp sierra de humaya provide insight into the emplacement of peraluminous magmas triggered by lateral shortening of the host rock and magmatic stoping at a mid crustal level of a retro arc zone in a convergent orogen modal and chemical compositions indicate that the lcp is composed of two main igneous units of peraluminous granitoids the predominant two mica granitoids were generated by interaction of crustal rocks with mafic or mafic derived magmas and or crystal rich magmas that entrained residual phases whereas less abundant leucocratic granitoids may have been originated by partial melting of metasedimentary rocks the calculated crystallization age of 474 4 ma is consistent with ordovician ages 477 470 ma of the high grade metamorphic rocks indicating concomitant magmatism and metamorphism during the famatinian orogeny the lcp was emplaced in the middle crust at a maximum depth of 14 5 km where attendant fracturing and ductile deformation were active field evidence shows strong temporal and spatial relationships between host rock ductile deformation and the emplacement of the pluton such as folding and strike deflection of the host rock layering and folded concordant leucocratic sheets with magmatic fabrics this suggests that material transfer processes like lateral wall rock displacement lateral shortening was a viable mechanism for the emplacement of the lcp however cross sectional restoration and field evidence such as wall rock xenoliths and intrusive truncations of the host rock foliation and fold traces suggest that magmatic stoping was a complementary mechanism to create the necessary space for the emplacement of the lcp this work supports previous studies showing that participation of multiple material transfer processes are the rule rather than the exception in the emplacement of plutons

precise determinations of the equilibria kyanite sillimanite and kyanite andalusite and a revised triple point for al 2 sio 5 polymorphs

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

in situ major and trace elements of garnet and scheelite in the nuri cu w mo deposit south gangdese tibet implications for mineral genesis and ore forming fluid records abstract garnet and scheelite are common hydrothermal silicate mineral and metal mineral in skarn or porphyry skarn deposits and can aid in constraining the composition of ore forming fluid based on in situ major and trace element analyses of garnet and scheelite from the nuri cu w mo deposit we discuss the factors and mechanisms controlling the incorporation of rare earth elements rees into the garnet and scheelite and investigate the associated hydrothermal fluid characteristics the garnets from the nuri deposit belong to the grossular andradite grandite solid solution series and range in composition from adr64grs34 to adr91grs8 the ree patterns of garnet are enriched in light rare earth elements lrees and depleted in heavy rare earth elements hrees the rees characteristics indicates that the incorporation of rees into the nuri garnet was controlled by the surface adsorption and element substitution in the mineral structure based on the cathodoluminescence cl images of scheelite two growth stages early and late stages are identified these two stages of scheelite belong to the scheelite powellite series and are enriched in lrees and depleted in hrees however the early scheelite exhibits negative or no eu anomalies 0 34 0 95 whereas the late scheelite presents positive eu anomalies 1 25 2 89 the mo content in the early scheelite is positively correlated with the eu value while the mo content in the late scheelite is negatively correlated with eu these characteristics indicate that the redox state of the ore forming fluid gradually changed from the oxidized state to the reduced state the preferred ree3 substitution for ca2 in the nuri scheelite is 3ca2 2ree3 ca where ca is a ca site vacancy the ree features of the scheelite are mainly controlled by the ree nature of the ore forming fluid according to the characteristics of trace elements in garnet and scheelite we suggest that the ore forming fluid of the nuri deposit is a magmatic fluid mixed with a slightly acidic external fluid chloride complexes may be the main transport ligand of the rees and the oxygen fugacity of the ore forming fluid gradually decreased

multiple injections of magmas along a hercynian mid crustal shear zone sila massif calabria italy abstract syn tectonic late hercynian granitoids emplaced at mid crustal level through multiple dyke injections are well exposed in a 1 km long section in the southern sila massif calabria italy the fieldwork study permitted us to reconstruct the following order of emplacement i granodiorites with euhedral kfs megacrystals grd1 in the main text and heterogranular granodiorites with coarse rounded pl and kfs in a fine grained biotite rich matrix grd2 ii granodiorites with a nearly homogeneous grain size grd3 iii tonalite and iv granite both in the wall rock represented by migmatitic paragneisses and in the deformed granitoids the foliation attitude is substantially homogeneous striking about n140 with a dip of 50 60 to the ne the magmatic lineation is defined by the preferred alignment of euhedral feldspars parallel to the mineral lineation observed in the host rocks micro and meso structural analyses indicate that foliation evolved from melt present to solid state conditions feldspar tiling s c structures type structures c axis quartz orientations commonly suggest a top to the west sense of shear during magma injection and deformation under decreasing temperatures age of magma emplacement is constrained by u pb datings at 304 300 ma coeval with the regional metamorphic peak foliated granitoids and wall rock are intruded by poorly foliated hbl gabbro and finally by undeformed leucogranite pegmatite and felsic porphyritic dykes new u pb datings of the last intrusion event indicate an age of emplacement of 281 8 ma providing a minimum estimate for the end of the shear strain at mid crustal level

early cretaceous extensional reworking of the triassic hp uhp metamorphic orogen in eastern china corresponding to the early mesozoic continental subduction between the north china block ncb and the south china block scb the tongbaishan hong an dabieshan sulu massifs are famous for their hp uhp metamorphism more than 50 of the hp uhp orogenic belt was significantly reworked by early cretaceous extensional tectonics this early cretaceous event with a fast cooling period at 130 120 ma superimposed on the early mesozoic hp uhp orogenic belt and intensively changed the architecture of this orogen each individual segment documents different early cretaceous extensional structures namely the central tongbaishan domain is a metamorphic core complex mcc represented by an a type non cylindrical antiform the central dabieshan domain is a typical cordilleran type migmatite cored mcc the southern sulu uhp domain is a wedge shaped structure exhumed by a simple detachment fault these late stage extensional structures expose the previous hp uhp orogenic belt as fragments along the ncb scb boundary the geodynamic setting of this early cretaceous extensional tectonics along the hp uhp orogen is a part of a 1000km scale crustal extension belt that is widespread in eastern eurasia continent from trans baikal to the central part of scb convective erosion or delamination of the mantle lithosphere might be considered as a possible mechanism for mantle removal

petrology of the coyaguayma ignimbrite northern puna of argentina origin and evolution of a peraluminous high sio2 rhyolite magma abstract the coyaguayma ignimbrite is a strongly peraluminous sp sillimanite garnet bearing silicic rhyolite which erupted in the northern puna segment of the central andean plateau in the upper miocene 11 ma a period that was characterized by the eruption of voluminous 100s to 1000s km3 dacitic ignimbrites of high k calc alkaline affinity in this region the sp magmatic rocks are both rare and small but their importance is potentially much greater as rocks of this type are usually interpreted as products of crustal melting and therefore useful for addressing mantle addition vs crustal recycling in the central andes the phenocryst assemblage of the coyaguayma ignimbrite comprises plagioclase an26 18 quartz ba rich and ba poor sanidine minor al rich ferromagnesian minerals alt and alvi rich biotite almandine spessartine garnet and sillimanite as well as accessory zircon and monazite textural relations suggest that the accessory and ferromagnesian phases crystallized before quartz and feldspars mineral equilibria suggest that crystallization of the rhyolite magma began at 5 kbar and 800 c and continued almost isobarically to 720 c causing the residual liquid to increase h2o contents from 4 5 to 7 5 before eruption most major features e g high sio2 a cnk 1 3 low cao mgo tio2 and feo and trace element patterns low ba sr th lree and eu eu high rb u y and nb along with the thermobarometric constraints on magmatic p t and water contents are consistent with an origin by mica dehydration melting of metapelitic sources e g typical biotite muscovite gneisses from the outcropping s puna basement however the relatively low initial 87sr 86sr 0 7125 and high 143nd 144nd 0 512200 ratios invalidate a pure crustal origin instead we propose contamination of calc alkaline dacitic magmas similar to typical puna ignimbrites by metapelite at mid crustal settings 18 km depth geochemical modeling that satisfies the chemical and isotopic data suggests mixing of 70 dacite and 30 of metapelite partial melts followed by extensive 70 fractionation of plagioclase k feldspar quartz and biotite with minor magnetite and apatite from the hybrid magma sillimanite in the coyaguayma rhyolite is interpreted as a restite mineral or a product of incongruent melting of the metapelite which was preserved intact in the hybrid melt due to local equilibrium this petrogenetic model explains most characteristics of crystal poor sp rhyolites from the puna plateau e g tocomar ramadas rhyolites and it may be more generally applicable to occurrences of sp magmas in andean type continental arcs dominated by calc alkaline magmatism

the south china block indochina collision where when and how this study uses new field observations and existing studies to shed new light on the origin and significance of two nw se striking orogens in nw and ne vietnam we conclude that the architecture of each belt is a stack of ne directed nappes formed either under deep ductile synmetamorphic conditions or under shallow depth in the sw and ne parts respectively the song ma zone and song chay ophiolitic melange represent two ophiolitic sutures however the late permian song da and babu mafic rocks are not ophiolites but intraplate basalts related to the emeishan plume a late triassic unconformity the 225 205 ma postorogenic plutonism and the 250 230 ma syntectonic metamorphism support an early to middle triassic age for these tectonic events both nw and ne belts are due to sw directed subduction with arc magmatism ocean closure and continental collision though two contemporary s dipping subduction zones might explain the structural evolution of the two belts a single convergent system offset by the tertiary red river fault is preferred as this s directed subduction better accounts for the late permian intraplate magmatism this scenario is discussed in the general geodynamic framework of se asia

mississippian volcanism in the south central andes new u pb shrimp zircon geochronology and whole rock geochemistry abstract in the northern extension of the famatina and the southern puna nw argentina prominent rhyolitic volcanic rocks traditionally referred to as ordovician are exposed resting on metamorphic basement and covered by thick late paleozoic siliciclastic successions we report new u pb shrimp ages from these rhyolites that show them to be of mississippian 348 342 ma age thus identifying a previously unknown volcanic event in this portion of western gondwana whole rock geochemistry and sr nd isotopic analyses suggest a crustal source for these rocks but with a juvenile input end t between 2 91 and 0 3 and tdm values between 1 09 and 1 1 ga this is different from the early paleozoic magmatism of western argentina where crustal recycling took place without any involvement of mantle material the carboniferous magmatism is compatible with an extensional environment developed along the terra australis accretionary orogen as a result of tectonic switching processes these rhyolites may be related to the coeval mississippian a type granites exposed to the east in the sierras pampeanas confirming the regional character of this magmatism

f rich strongly peraluminous a type magmatism in the pre andean foreland sierras pampeanas argentina geochemical geochronological isotopic constraints and petrogenesis abstract the petrogenetic nature of a type granites is a controversial problem the vinquis batholith in the sierras pampeanas of argentina contains unusual f rich and strongly peraluminous a type monzogranites a new la mc icp ms u pb zircon crystallization age of 355 7 ma indicates emplacement in latest devonian or earliest carboniferous time overlapping with extensive metaluminous a type magmatism in the area the monzogranites have a restricted range of sio 2 content 71 5 74 8 they are poor in ca 0 54 1 4 cao and rich in feo t with relatively high feo t feo t mgo values ranging from 0 77 to 0 86 average 0 80 both feo t feo t mgo vs sio 2 and na 2 o k 2 o cao vs sio 2 plots indicate ferroan and alkali calcic signatures typical of a type granitoids the samples have mgo tio 2 1 2 and are moderately enriched in total alkalis average 8 18 with high k 2 o na 2 o values of 1 40 2 24 the granites are strongly peraluminous with asi molar al 2 o 3 cao na 2 o k 2 o values of 1 2 to 1 3 the high p 2 o 5 content 0 23 0 37 is distinctive and close to values reported for other paleozoic f rich peraluminous a type granites in the sierras pampeanas they have moderate contents of high field strength elements e g zr nb th y etc and moderately fractionated to flat ree patterns la yb n in the range 4 8 19 6 showing significant negative eu anomalies eu eu 0 41 biotite has a distinctive composition with relatively high fe 2 fe 2 mg ratios 0 61 0 74 and high f 0 55 1 42 wt content together with the whole rock chemistry this may be useful in identifying strongly peraluminous a type granites in addition the rb sr vs th zr ce diagram may be an appropriate discriminant between metaluminous and peralkaline a type granites strongly peraluminous a type granites and strongly peraluminous orogenic granites the geochemical evidence indicates that differentiation of the granitic rocks occurred by mineral fractionation from a f rich peraluminous parental magma dominant of plagioclase k feldspar biotite and accessory minerals such as zircon monazite xenotime and oxides the peraluminous composition and isotope data e hf 355 and e nd 355 ranging from 9 5 to 1 5 and 6 1 to 7 8 respectively together with abundant inherited ordovician and cambrian zircon strongly suggest a dominantly metasedimentary source the whole rock and biotite compositions indicate that the vinquis batholith crystallized under mainly oxidizing conditions whereas early carboniferous metaluminous to weakly peraluminous a type granites of the sierras pampeanas crystallized under dominantly reduced conditions

dynamic growth of garnet in granitic magmas in order to improve understanding of how accessory garnet crystallizes in igneous rocks and evaluate it as a mineral recorder of magma history we analyzed 18 o of garnets from the hallowell and togus plutons in south central maine united states by laser fluorination and in situ by ion microprobe two types of garnet are recognized magmatic and locally derived peritectic traverses of some single crystals show both gradual and abrupt changes of 18 o garnet commonly 1 while other garnet grains are isotopically homogeneous rimward increase of 18 o in many crystals indicates that garnet grew while high 18 o metamorphic wall rocks were assimilated peritectic grains have a complementary record of the transfer of high 18 o melts to the plutons in some rocks 18 o varies among neighboring grains evidence that crystals grew episodically or were juxtaposed from different sources during magma mixing garnet faithfully records changing magmatic 18 o and is a valuable tool to decipher magma petrogenesis

geology geochemistry and emplacement conditions of the vega intrusive complex an example of large scale crustal anatexis in north central norway

magmatic evolution of the penon rosado granite petrogenesis of garnet bearing granitoids garnet is an uncommon accessory mineral in igneous rocks but is petrologically significant the penon rosado granite 469 4ma at cerro asperecito is an s type granite that contains an unusual amount of magmatic garnet combined petrology chemistry mineralogy and whole rock geochemistry indicates that the magma was produced by partial melting of the surrounding metasedimentary rocks and subsequent differentiation by fractional crystallization during emplacement with garnet occurring throughout the crystallization sequence three facies are recognised prg1 sio2 65 70 represents cumulates prg2 sio2 70 88 represents a differentiated melt and prg3 sio2 74 59 a residual melt the fractionation of mn in garnet and the proportion of garnet crystallizing are roughly controlled by the evolving composition of the different granitic facies geothermobaric calculations reveal an initial crystallization temperature of 764 792 cand a pressure of 5 9 6 0 kb indicating that the parental magma was emplaced atmiddle crustal depths 1920 km in moderate low magmatic temperature conditions major cao na2o k2o and trace element rb sr ba contents in the penon rosado granite strongly suggest anatexis was the outcome of h2o fluxed melting of metagreywacke with heat input from a major metaluminous suite our studies reveal that garnet formed by direct crystallization from peraluminous magma in equilibrium with solid phases such as biotite and white mica we confirm previous suggestions that zoning in garnet is strongly temperature dependent thus magmatic garnets in granitic rocks crystallized above 700 c have spessartine inverse bell shaped profile or are unzoned whereas garnet exhibiting spessartine bell shaped profile must be considered of metamorphic origin i e xenocrystic or formed in very felsic magmas sio2 73 76 crystallizing below 700 c 2006 elsevier b v all rights reserved

magmatic garnet in the cordilleran type galileia granitoids of the aracuai belt brazil evidence for crystallization in the lower crust abstract magmatic garnet together with epidote is a rare mineral association in cordilleran i type granitoids and of special petrogenetic significance the metaluminous to slightly peraluminous asi 0 97 1 07 galileia batholith brazil is a large ca 30 000 km 2 neoproterozoic ca 632 570 ma weakly foliated calc alkaline granitoid body characterized by the widespread occurrence of garnet grossular 25 43 mol and epidote pistacite 9 3 22 7 mol field petrographic and mineral chemical evidence indicates that garnet epidote biotite as well as white mica crystals low si phengite are magmatic there is no difference in bulk rock major and trace element composition between the galileia granitoids and other garnet free cordilleran type granitoids worldwide this evidence strongly suggests that the origin of the uncommon garnet epidote parageneses is related to the conditions of magma crystallization such as pressure temperature and water content comparison between the mineral assemblages and mineral compositions from this study and those recorded in crystallization experiments on metaluminous calc alkaline magmas as well as within garnet bearing metaluminous volcanic rocks and granitoids indicates that the supersolidus coexistence of grossular rich garnet epidote and white mica is consistent with magma crystallization at pressures greater than 0 8 gpa above 25 km depth and at temperatures below 700 c i e near the water saturated solidus furthermore resorption textures around garnet plagioclase quartz coronas and epidote suggest that these minerals have been partially consumed prior to complete crystallization these findings demonstrate that at 630 ma the crust underneath the aracuai orogen was already at least 25 30 km thick and relatively cool however this contrasts with the marked high heat flow registered from the neighbour carlos chagas batholith located 50 km to the east in fact such granitoids record granulite facies metamorphism at the same pressure and time ca 570 ma of galileia granitoids crystallization thus a more suitable geodynamic scenario is required in order to explain these two contrasting thermal regimes within the same orogen eventually field petrographic and mineral chemical analogies with similar garnet bearing granitoids located in the fore arc settings of the british columbia subduction zone possibly imply that the galileia granitoids represent rare garnet and epidote bearing metaluminous cordilleran i type granites which can only form in a fore arc setting

generation of syntectonic calc alkaline magnesian granites through remelting of pre tectonic igneous sources u pb zircon ages and sr nd and pb isotope data from the donkerhoek granite southern damara orogen namibia abstract the 541 4 ma old magnesian weakly peraluminous calc alkalic donkerhoek onanis granite is part of the ca 6000 km2 large donkerhoek batholith in the southern zone of the damara orogen of namibia linear major and trace element variations and decreasing mgo feo al2o3 cao k2o na2o ba and sr concentrations with increasing sio2 indicate that this part of the batholith represent a coherent mass and underwent fractional crystallization processes the donkerhoek onanis granites are isotopically evolved initial end 4 7 to 12 3 initial 87sr 86sr 0 7099 0 7157 with moderately radiogenic pb isotope ratios 206pb 204pb 17 26 18 22 207pb 204pb 15 59 15 67 208pb 204pb 37 60 38 06 beside heterogeneities imparted by the sources an evaluation of lree fractionation and nd isotope data suggests that afc processes also modified some samples based on the chemical and isotope data the donkerhoek onanis granites cannot be derived by partial melting of al and fe rich metasedimentary rocks of the kuiseb formation in which they intruded instead melting of meta igneous crustal sources with proterozoic crustal residence ages is more likely three igneous to meta igneous rock suites from the area matchless amphibolites proterozoic mafic to felsic gneisses from the southern kalahari craton basement syn tectonic salem granodiorites to granites are potential sources an evaluation of chemical and isotope data suggests that remelting of early syn orogenic salem type granites is the most likely process which would also explain the existence of ca 563 4 ma old zircon in the donkerhoek onanis granites comparison of the donkerhoek onanis granites with experimentally derived melt compositions from an intermediate igneous parent indicates temperatures between 800 and 850 c it is suggested that the pan african igneous activity in this part of the damara belt was a moderate temperature intra crustal event although there are some compositional similarities with juvenile granites generated in subduction zones unradiogenic pb isotope ratios and moderately radiogenic sr and unradiogenic nd isotopes suggest that reprocessed crustal rocks are more likely sources previously obtained high 18o values of the donkerhoek onanis granites ranging from 11 8 to 13 6 covering the range of 18o values obtained on salem type granites from the area 12 5 13 3 confirm this view in contrast to igneous processes along active continental margins that produce juvenile batholiths with calc alkaline affinities this igneous event was not a major crust forming episode and the donkerhoek onanis granites represent reprocessed crustal material

the geology of north korea an overview abstract the korean peninsula kp occupies a junction area of three big tectonic domains that are the paleo central asian orogenic belt paleo tethyan orogenic belt and the western pacific orogenic belt therefore the kp is a key area to understand the northeast asian and even global geotectonic evolution in terms of geological unit division the kp has long been thought to be closely tied with the mainland of china with using collective term the sino korea craton for the north china craton ncc however such correlation suffered from the absence of relevant geological information in north korea nk to bridge this knowledge gap our group carries out a long term collaborative study between the two sides of the kp and china and organized united field investigation in the north korea south korea sk and china as possible this paper introduces general geology in the north korea and simply make comparison with the ncc and south korea these pioneering works culminated in the following summaries 1 the kp comprises three major precambrian massifs i e rangnim gyeonggi and yeongnam massifs the rangnim and gyeonggi massifs are separated by the imjingang structural belt while the gyeonggi and yeongnam massifs are separated by the ogcheon structural belt 2 the basement of the rangnim massif is correlated with that of the ncc the characteristic metamorphic rocks of high temperature high pressure granulite facies including meta sedimentary rocks kondalites and meta sandstones and marbles with 1 9 1 8 ga metamorphic ages occur in the three massifs similar to the ncc 3 the proterozoic rift sedimentary sequences are developed in the ncc and north korea but lower paleoproterozoic strata are rare and neoproterozoic strata are more developed in north korea relative to the ncc 4 two main paleozoic basins within the rangnim massif and within the gyeonggi and yeongnam massifs are the phyongnam basin and taebaeksan basin both of them have a similar paleozoic tectono stratigraphy to the ncc 5 although the nature and tectonic significance of the imjingang belt remains controversial it doesn t show characteristics and natures of collisional orogenic belt 6 the eclogite bearing hongseong complex hsc is located in the southwestern part of the gyeonggi massif in south korea but no similar rock unit has been found in north korea 7 mesozoic igneous rocks are widespread throughout the northern and southern parts of the kp triassic magmatism is related to indosinian orogeny jurassic and cretaceous igneous rocks are different and irregular from those in the ncc on age and spatial temporal distribution this paper also suggests several key and controversial academic issues that are worthwhile to study in the future

compositions of micas in peraluminous granitoids of the eastern arabian shield compositions and pleochroism of micas in fourteen peraluminous alkali feldspar granites in the eastern part of the late proterozoic arabian shield are unlike those of micas principally biotite in most calc alkaline granitoid rocks compositions of these micas are distinguished by elevated abundances of li2o f and numerous cations and by low mgo abundances these micas constituents of highly evolved rare metal enriched granitoids represent an iron lithium substitution series that ranges from lithium poor siderophyllite to lithium rich ferroan lepidolite the eastern arabian shield also hosts six epizonal granitoids that contain colorless micas compositions of these micas mostly muscovite and their host granitoids are distinct from those of the iron lithium micas and their host granitoids compositions of the analyzed micas have a number of petrogenetic implications the twenty granitoids containing these micas form three compositional groups that reflect genesis in particular tectonic regimes mica compositions define the same three groups the presence of magmatic muscovite in six of these shallowly crystallized granitoids conflicts with experimental data indicating muscovite stability at pressures greater than 3 kbar muscovite in the arabian granitoids probably results from its non ideal composition the presence of muscovite cannot be used as a pressure indicator finally mineral matrix partition coefficients are significantly greater than 1 0 for a number of cations the rare earth elements in particular in many of the analyzed iron lithium micas involvement of these types of micas in partial melting or fractionation processes can have a major influence on silicate liquid compositions

decoupled ca and fe mg content of s type granites an investigation on the factors that control the ca budget of s type granites abstract the compositional variation of s type granites is thought to derive from source controlled processes whereby magmas acquire their composition as a result of entrainment of variable proportions of a peritectic rich residual mineral assemblage into the melt a corollary of this model is that the entrained phases keep the stoichiometric proportions they have in the partially melted source these proportions are controlled by the composition of the subsolidus minerals consequently it is expected that the ratio between maficity feot mgo and other compatible major elements in granites would be the same as those of the source this is true for ti but not for ca which shows systematically higher ca maficity ratios in order to explore the factors that cause this discrepancy thermodynamic calculations were used to study the partial melting of a representative metapelite and a metagreywacke under a range of crustal pressure and temperature conditions the modelling included segregation of melt or magma in pulses disequilibrium melting of plagioclase in which only a fraction of this phase participates in the melting system and produces two populations of plagioclase the unreacted cores with the original na rich composition and a second one with a ca rich composition and the entrainment of up to 40 wt minerals into the melt this entrained assemblage consists of two fractions one containing the minerals that are most likely to be surrounded by melt in the source and representing 80 wt of the total assemblage and another one consisting of the phases that are most likely to make up the solid framework with the minerals within each fraction keeping stoichiometric proportions the modelled magmas failed to reproduce the observed high ca maficity ratios this indicates that there are other processes that increase the ca content of granites several processes were evaluated including the presence of apatite enhanced availability of subsolidus plagioclase in the melting system disequilibrium ca rich melts and enhanced entrainment of ca rich plagioclase the last three processes were found to increase the ca content but only enhanced entrainment of ca rich plagioclase increases the ca maficity ratio to values similar to those of the granites a possible explanation for the preferential entrainment of ca rich plagioclase is that the density of this phase is different to that of garnet and ilmenite and similar to the density of the melt consequently a significant fraction of the plagioclase in granites may actually be of residual origin other relevant findings of this study are that metagreywackes seem to be the main source of s type granites as previous investigations suggested and that cordierite rich granitoids represent mineral rich magmas that are formed only from metapelites at pressures below 0 75 gpa this investigation emphasizes the importance of source controlled processes in the composition of s type granites and indicates that entrainment of a mineral assemblage into the melt is not a simple and straightforward process as previously thought

a review of the famatinian ordovician magmatism in southern south america evidence of lithosphere reworking and continental subduction in the early proto andean margin of gondwana abstract along the proto pacific margin of gondwana from venezuela to northeastern patagonia the early middle ordovician famatinian orogeny was the first orogenic event following assembly of the supercontinent previous isotope studies of the igneous and meta sedimentary rocks of southwestern gondwana yield ambiguous implications for the role of juvenile mantle addition during the early crustal growth at the supercontinental margin to interpret the geological and tectonic evolution of the orogen and the magma sources in different episodes we look at evidence from a large area of southern south america including the 700 600 km type sector of the orogen in the sierras pampeanas 27 33 s the precordillera and northeastern patagonia previous geological geochemical and geochronological results are reviewed together with new u pb shrimp crystallization ages 177hf 176hf and 18o 16o data for dated zircon and whole rock sr and nd isotope compositions four geological domains are recognized in the sierras pampeanas western central eastern and foreland famatinian domains magmatism is mostly restricted to the interval 463 4 to 486 7 ma with the most intense period of emplacement between 468 and 472 ma constituting a magmatic flare up granitoid emplacement in both northeastern patagonia and the cordon de lila puna altiplano chile was effectively synchronous with that in the sierras pampeanas defining a continuous belt combined geochemical and isotopic data whole rock sr nd hf o in zircon indicate that the source of calcic metaluminous suites is the subcontinental lithosphere both mantle and mafic lower crust with variable contamination by the early paleozoic metasedimentary country rocks the lithospheric mantle involved is assumed to underlie the outcropping 1330 1030 ma age basement of the western domain which exhibits tectonic characteristics of active continental margin in the north and oceanic arc back arc in the south the latter sector is the potential source of some minor famatinian igneous rocks with less evolved isotopic compositions although a restricted asthenospheric addition cannot be discarded in this case minor peraluminous granites are spatially associated with the metaluminous sequence but major highly peraluminous batholiths occur on the eastern flank of the central domain field relations and geochemical isotopic evidence indicate that the most obvious source of these crustal melts was the very thick post early cambrian metasedimentary sequence comprising the host country rocks episodic tectono magmatic evolution of the famatinian magmatic belt in two overlapping stages is invoked to explain different characteristics in the four recognized domains in the type sector ca 474 486 ma roll back stage this is a mainly extensional interval involving asthenospheric upwelling and thinning of the subcontinental mantle full development of the marine ensialic basins and early emplacement of both metaluminous granites and highly peraluminous batholiths in the central and eastern famatinian domains trondhjemite plutons with an adakitic signature were emplaced in the foreland domain ca 468 472 ma slab break off stage steepening of the oceanic slab and arc migration to the southwest ended with slab break off due to subduction of continental crust during continental collision with the precordillera terrane this stage produced voluminous metaluminous magmatism at the western edge of the central domain the flare up episode k bentonites in the precordillera leucogranites in the western domain and scattered metaluminous and peraluminous plutons in all famatinian domains both slab roll back and break off stages developed during a high t regime typical of hot orogens although asthenospheric mantle was a necessary heat source for lithospheric melting its material contribution to the growth of early paleozoic crust was apparently very minor recycling of mesoproterozoic lithosphere including the subcontinental mantle coupled with crustal melting of early paleozoic metasedimentary sequences accounts for most famatinian magmatism comparable results from the central andes and east antarctica confirm that the early stages of the terra australis orogen in sw gondwana were dominated by lithospheric reworking processes

compositional variations caused by phenocryst sorting at cerro negro volcano nicaragua cerro negro volcano in nicaragua first erupted in 1850 it has erupted at least ten more times the last in february 1971 it is therefore a polygenetic parasitic cinder cone that may eventually evolve into a composite cone lavas produced during the five important eruptions since 1923 have been sampled these recent lavas from cerro negro are unusually phenocryst rich basalts 29 43 phenocrysts by volume with phenocrysts of plagioclase an 96 85 olivine fo 81 72 clinopyroxene en 45 wo 38 fs 16 en 41 wo 45 fs 14 and subordinate titanian magnetites usp 16 20 all of the basalts have relatively high al 2 o 3 and low k 2 o and other incompatible element contents there have been gradual but significant changes in lava composition with time all multiply sampled flows show significant intra lava inhomogeneity the fe mg contents of analyzed olivines indicate disequilibrium between the olivines and host lavas all of the inter lava compositional variation most of the mineral lava disequilibrium and most of the intra lava inhomogeneity are the result of differential olivine and clinopyroxene accumulation during magma ascent the magma originated from an essentially homogeneous magma chamber phenocrysts in the basalts could have experienced much of their growth during ascent ascent velocities were probably on the order of 10 1 10 5 cm sec increasing efficiency of crystal settling with time suggests that a narrow feeder probably a north south oriented dike may be widening beneath the cone

the collision between the yili and tarim blocks of the southwestern altaids geochemical and age constraints of a leucogranite dike crosscutting the hp lt metamorphic belt in the chinese tianshan orogen article i nfo a ca 600 m long 0 5 20 m wide nw se trending granite dike crosscuts the high pressure low temperature hp lt tianshan metamorphic belt the foliation of which is parallel to the main ene regional trend in the chinese south tianshan orogen it is mainly composed of plagioclase k feldspar quartz muscovite biotite and secondary chlorite while fluorite zircon and xenotime occur as accessories the immediate country rock is a quartz biotite plagioclase schist which grades several tens of meters away from the granite dike into a chlorite mica albite schist the latter schist is intimately intercalated with blueschist layers and boudins the a cnk value of the granite dike samples varies from 1 15 to 1 27 indicating a strongly peraluminous composition cao na2o ranges from 0 06 to 0 17 and al2o3 tio2 from 240 to 525 similar to the ratios of strongly peraluminous sp granites exposed in high pressure collision zones such as the himalayas a zircon u pb age of 285 ma was obtained for the granite dike thus constraining the upper limit for the age of hp lt metamorphism the petrological and geochemical data suggest that the sp leucogranite dike intruded during the exhumation of overthickened crust in the post collisional setting between the yili central tianshan and tarim blocks the dataset presented here in conjunction with previously published data corroborate that the hp lt metamorphism must have occurred earlier than the permian in the tianshan orogen therefore the collision between the yili central tianshan and tarim blocks and the final amalgamation of the southwestern altaids must have been terminated in late paleozoic and not in triassic times as previously suggested

formation of metamorphic and metamorphosed garnets in the low t uhp metagranite during continental collision in the dabie orogen abstract a combined study of major and trace elements in garnet was carried out for low t uhp metagranite in the dabie orogen the results show different fashions of element zoning in the garnet suggesting occurrence of both metamorphic and metamorphosed garnets three generations of garnet growth are distinguished on the basis of the assumption that ca contents mn contents and fe mg ratios of garnet in metagranites are a function of pressure and temperature the first generation of garnet grt i occurs in the core of a skeletal garnet showing homogeneously low x grs values and fe mg ratios but high x sps values this implies that the core garnet grew at the highest temperature but the lowest pressure representing a metamorphosed residue of magmatic garnet in protolith granite the second generation of garnet grt ii occurs in the mantles and cores of many garnet grains exhibiting increased x grs values but decreased fe mg ratios this is ascribed to a continuous increase in temperature and pressure till the peak pressure corresponding to metamorphic growth or overgrowth during the prograde subduction the third generation of garnet grt iii occurs in the rims of all garnet grains displaying decreased x grs values and fe mg ratios in response to a pressure decrease but a temperature increase till the peak temperature these rims overgrew subsequent to the peak pressure with continuous heating during the initial exhumation grt i shows steep mree hree patterns and profoundly negative eu anoalies consistent with growth from granitic melt this kind of metamorphosed garnet from protolith granite has still preserved very high contents of many trace elements such as ree rb ba sr pb th u nb and ta despite the low t uhp metamorphism grt ii and grt iii in one sample exhibit steep mree hree patterns with a continuous decrease in ree contents this suggests their growth from the almost same matrix of mineral assemblages plagioclase k feldspar muscovite during metamorphism however grt ii in the other sample displays flat to steep mree hree patterns with an increase of ree contents from core to mantle this implies that the matrix of mineral assemblages for grt ii changes from common rock forming minerals e g feldspar muscovite and biotite to ree rich minerals e g epidote allanite zircon amphibole apatite and titanite grt iii in the two samples all displays significantly lowered ree contents compared to those of grt i and grt ii this may be due to a decrease in pressure and an increase in temperature during the initial exhumation therefore the two stage growth of metamorphic garnet is evident in the low t uhp metagranite during the continental subduction zone metamorphism in addition the highest pressure occurs in the cores or mantles whereas the highest temperature occurs in the rims this suggests that the peak pressure p max did not occur contemporarily at the peak temperature t max corresponding to the hot exhumation

late mesozoic compressional to extensional tectonics in the yiwulushan massif ne china and its bearing on the evolution of the yinshan yanshan orogenic belt part i structural analyses and geochronological constraints abstract with a cratonic nucleus the north china craton ncc experienced a complex tectonic evolution with multiphase compressional and extensional events during mesozoic times along the northern part of the ncc the yinshan yanshan fold and thrust belt was a typical intraplate orogen jurassic and cretaceous continental sedimentation magmatism widespread intraplate characterize the yinshan yanshan orogenic belt the geodynamic significance of these tectonic events is still in dispute in the western part of the liaoning province the yiwulushan massif crops out at the eastern end of the yinshan yanshan orogenic belt the yiwulushan massif presents an elliptical domal shape with a ne sw striking long axis the structural evolution of this massif brings new insights for the understanding of the mesozoic plutonic tectonic history of the ncc a multidisciplinary study involving structural geology geochronology anisotropy of magnetic susceptibility ams and gravity modeling have been carried out the presentation of the new results splits into two parts part i this paper deals with field and laboratory structural observations and presents the main geochronological results the ams gravity modeling data will be provided in a companion paper part ii the early compressional deformation d1 corresponds to a late jurassic to early cretaceous southward thrusting the subsequent deformation is related to the early cretaceous exhumation of the yiwulushan massif a detailed structural analysis allows us to distinguish several deformation events d2 d3 and d4 the cretaceous extensional structures such as syntectonic plutons bounded by ductile normal faults metamorphic core complexes and half graben basins are recognized in many places in east asia these new data from the yiwulushan massif constitute a link between transbaikalia mongolia north china and south china indicating that nw se extensional mesozoic tectonics occurred throughout the entire region

the role of manganese in the paragenesis of magmatic garnet an example from the old woman piute range california a reply strongly peraluminous plutons of the old woman piute range southeastern california display the differentiation sequence muscovite biotite granite rightarrow muscovite garnet biotite granite rightarrow garnet muscovite granite rightarrow garnet muscovite aplite and pegmatite the garnets show no evidence of reaction but biotites in garnet bearing rocks are rimmed by muscovite metamorphic assemblages in the country rocks suggest pressures of emplacement less than 4 5 kb the plutons become relatively enriched in manganese with differentiation whole rock ratios of molar mn fe mg increase from 0 01 in least felsic garnet free rocks to 0 3 in most felsic garnet rich rocks mno concentrations show a corresponding rise in biotites 0 4 to 1 8 wt and less clearly in garnets 9 to 25 the differentiation sequence textural relations and mineral and whole rock compositions suggest that garnet paragenesis was controlled by the reaction biotite mn rich liquid rightarrow garnet

early neoproterozoic gneissic granitoids in the southern yili block nw china constraints on microcontinent provenance and assembly in the sw central asian orogenic belt abstract the central asian orogenic belt caob was built by amalgamation of diverse geologic units including several microcontinents the nature and geodynamic evolution of the precambrian basement of these units are essential to constrain their origin and thus the geodynamic and paleogeographic reconstruction of the accretionary system the yili block located in the west of the chinese tianshan orogen is one of these precambrian based continental constituents field investigations zircon u pb dating and lu hf isotopic analysis whole rock geochemical and sr nd isotopic studies were conducted on the gneissic granitoids from the giergelang area southern yili block these rocks yielded early neoproterozoic zircon u pb ages 962 934 ma containing substantial number of paleo to mesoproterozoic inherited zircons 2403 1268 ma their protoliths belong to peraluminous high k granite series displaying right declining ree patterns and eu negative anomalies together with lile k rb u th enrichment and depletions in ba nb sr p ti they have negative end t values ranging from 2 5 to 4 7 and old nd model ages varying from 1650 ma to 1910 ma they also exhibit highly variable zircon ehf t values 12 1 to 4 4 and hf in zircon model ages 1558 2574 ma integrating all these results it is suggested that the primary magma of gneissic granitoids was generated by partial melting of supracrustal rocks with involvement of old crustal materials a comparison of data from the yili block and its neighboring continental units reveals that the neoproterozoic magmatic event is an important episode of crustal reworking for the major continental blocks within the caob similar neoproterozoic magmatic events and comparable upper neoproterozoic to lower paleozoic stratigraphic sequences of the yili chinese central tianshan and tarim blocks suggest that they had a similar crustal evolution and close tectonic and paleogeographic relation during neoproterozoic and were likely derived from one single continental plate

early evolution of the proto andean margin of south america from a detailed study of a 500 km transect in the sierras pampeanas central west argentina two pre silurian tectono magmatic episodes are recognized and defined each culminating in micro continental collisions against the proto andean margin of gondwana the pampean orogeny started in early cambrian time with short lived subduction indicated by ca 535 ma calc alkaline granitoids following pampean terrane collision burial to granulite facies conditions ca 9 kbar generated widespread migmatites and ca 520 ma highly peraluminous granites in the eastern sierras pampeanas after brief quiescence a second major episode the famatinian orogeny started with subduction ca 490 ma forming a wide continental arc and ensialic backarc basin this heralded the approach of laurentia to gondwana during which the precordillera terrane separated from the southern appalachian region finally colliding with gondwana in silurian devonian time

granitoid emplacement during a thrusting event structural analysis microstructure and quartz c axis patterns an example from hercynian plutons in the french massif central in the french massif central the rouergue albigeois area consists of three tectonic units stacked during the hercynian orogeny the structural analysis of the units and particularly the quartz c axis textural and structural observations in the syntectonic pinet type plutons allow us to determine the deformation history a first thrusting event d1 is responsible for a southwestward emplacement of the high grade levezou nappe above the lower gneiss unit itself overlying the para autochthonous micaschists in late devonian early carboniferous this stack of nappes experienced a second event d2 characterized by a top to the nw emplacement under medium pressure medium temperature conditions of the para autochtonous unit upon the lower gneiss unit during d2 the pinet type plutons were deformed by a shearing consistent with the kinematics recorded in the country rocks in contrast to previous works which considered the pinet type plutons emplacement to be coeval with the d2 event we argue that the emplacement of the plutons occurred during the top to the sw d1 nappe stacking this work emphasizes the importance of the quartz c axis and microstructure analysis for the identification of the early structural history in polyphase deformed terranes

gravity inversion ams and geochronological investigations of syntectonic granitic plutons in the southern part of the variscan french massif central magnetic fabric analyses gravity inversion associated with chemical u th pb dating and structural observations are carried out to elucidate relationships between faulting and magmatic processes this multidisciplinary study has been undertaken on late carboniferous plutons situated in the southern part of the variscan french massif central fmc the glenat omps and boisset plutons crop out on both sides of the crustal scale sillon houiller fault shf the anisotropy of magnetic susceptibility ams measurements and structural observations show that i the plutons acquired their final structure during the magma crystallization and record a nw se maximum stretching trend ii in the boisset pluton post magmatic fabrics predominate with a nnw sse trending lineation the structural pattern deduced from the ams study is thus consistent with the nw se late orogenic extensional tectonic regime that has been documented in other parts of the fmc during late carboniferous the 3d geological modelling refined by 3d gravity inversion does not show any evidence of rooting of the granites along the shf therefore despite the apparent cartographic relationship between the shf and the three plutons our study does not support a genetic link between fault and plutons it also questions the existence of the shf in this part of the massif central at the time of pluton emplacement and emphasises the dominant role of the regional tectonic framework rather than local faulting as a factor controlling pluton emplacement

paleoproterozoic porphyries and coarse grained granites manifesting a vertical hierarchical structure of archean continental crust beneath the yangtze craton abstract granites generated by anatexis are ideal crustal probes to decipher the differentiation processes within deep continental crust in this study the successive emplacement of granites including the porphyries and the coarse grained granites was found in the nucleus the kongling complex of the yangtze craton the porphyries intruding at 1840 17 ma shrimp ages occur as veins and contain phenocrysts of k feldspar 15 quartz 5 plagioclase 4 and garnet 1 setting in a fine grained groundmass 75 of k feldspar plagioclase quartz and biotite the coarse grained granites emplacing at 1821 8 ma shrimp ages are outcropped as pluton and comprise k feldspar 60 quartz 30 biotite 5 plagioclase 5 they are similar to a type granites with relatively high sio2 69 2 71 5 wt 72 6 74 5 wt high fe2o3t mgo 21 5 27 7 5 9 19 5 104 ga al ratios 3 6 4 3 2 7 3 5 respectively and enriched hfses th u pb ga the porphyries contain higher total trace element contents and zirconium saturation temperature mean 892 c than those of the coarse grained granites mean 803 c magmatic zircons from the porphyries and coarse grained granites have ehf t values of 17 6 to 11 7 and 21 0 to 14 8 which yield two stage hf model ages of 3 6 3 3 ga and 3 8 3 4 ga respectively both falling into the evolutionary trend of the 2 9 2 8 ga kongling ttgs it implies both granites are anatectic crustal reworking derivatives from partial melting of the archean ttgs under sustaining extension in paleoproterozoic applying trace element modelling we further identify a unique tonalitic source for both granites which is analogous to the exposed natural kongling tonalite and find the porphyries slightly affected by the juvenile crust based on the intrusion of porphyry veins into granitic gneisses we envisage the crustal structure beneath the craton is layered from older 2 94 ga tonalites upward to younger 2 87 ga granitic gneisses such hierarchical structure could shed new light on the imperceptible vertical differentiation process as cratonization

two stage deformation of the mont louis andorra granite pluton variscan pyrenees inferred from magnetic susceptibility anisotropy the mont louis andorra granodiorite pluton eastern pyrenees france andorra and spain was the subject of a thorough study of its internal structures principally the measurement of the anisotropy of its magnetic susceptibility and the classification of its microstructures regularly spaced sampling 281 sites allowed the precise mapping of the susceptibility magnitudes consistent with mapped rock types the magnetic structures fall into two distinct families which occur in separate parts of the pluton and have different microstructures family i is observed almost everywhere and has close to horizontal lineations remarkably constant in ne sw azimuth magmatic to submagmatic microstructures characterize these rocks in contrast family ii is confined within nw se trending corridors where moderate to steep nw plunges of the lineations are recorded and subsolidus microstructures are observed magnetic structures of family i are argued to equate with the kinematic framework of the deforming magma during its emplacement family ii structures represent a late to post consolidation reworking and therefore record a second stage of deformation in the pluton it is concluded that magma emplacement was related to a ne sw trending strike slip and or thrust fault system coeval with the sw verging thrust episode documented in the pyrenees the second stage corresponds to dextral shear movements with a thrust component on nw se striking planes corridors containing family ii structures are shear zones responsible for dissection of the massif into three blocks the eastern block representing the former base and the western block the top of the pluton restoration of the pluton9s shape along these corridors demonstrates the importance of the early variscan ne sw kinematic direction because the family ii structures that form shear zones affecting the pluton have similar orientation and kinematics as the compressive steep main phase of deformation ubiquitous in the pyrenees the pluton of mont louis andorra was probably emplaced before this main phase of deformation

the montagne noire migmatitic dome emplacement french massif central new insights from petrofabric and ams studies in the southern french massif central the montagne noire axial zone is a ne sw elongated granite migmatite dome emplaced within visean south verging recumbent folds and intruded by syn to late migmatization granitoids the tectonic setting of this dome is still disputed thus several models have been proposed in order to better understand the emplacement mechanism of this dome petrofabric and anisotropy of magnetic susceptibility ams studies have been carried out in the granites and migmatites that form the dome core magmatic texture and to a lesser extent weak solid state texture are dominant as a paramagnetic mineral biotite is the main carrier of the magnetic susceptibility on the basis of 135 ams sites the magnetic fabrics appear as independent of the lithology but related to the dome architecture coupling our results with previous structural and geochronological studies allows us to propose a new emplacement model between 340 325 ma the palaeozoic series underwent a compressional deformation represented by nappes and recumbent folds involving the thermal event leading to partial melting until 325 310 ma the dome emplacement was assisted by diapiric processes an extensional event took place at 300 ma after the emplacement of the late to post migmatitic granitic plutons in the northeast side of the dome a brittle normal dextral faulting controlled the opening of the graissessac coal basin

magnetic fabrics in the cabanas granite ne brazil interplay between emplacement and regional fabrics in a dextral transpressive regime the late neoproterozoic cabanas granite northeastern brazil is a tabular body that was emplaced south of the dextral east pernambuco shear zone system anisotropy of magnetic susceptibility ams measurements reveal that the cabanas granite has fabric patterns more complex than usually reported in plutons intruded in similar settings most structures have ew to ene trending magnetic lineations consistent with the overall regional structural direction in the north these structures parallel the mylonitic fabric encountered in shear zones that bound or cross cut the pluton in the central and southern portions of the pluton magnetic foliation trajectories the overall banded disposition of the iso susceptibility lines and shape parameters of the ams ellipsoids suggest that the cabanas granite was subjected to ene trending folding along with its underlying and enclosing country rocks it is concluded that emplacement took place during regional dextral transpression a secondary but well developed family of structures consists of n to nw plunging lineations which is interpreted to represent the trace of the vertical conduits that fed the pluton this study emphasizes the utility of the ams to unravel both the internal structure of low anisotropy plutons and the interplay between emplacement and regional straining during magma crystallization

a preliminary description of the gan hang failed rift southeastern china abstract the gan hang failed rift as defined by present day topography extends at least 450 km in length and 50 km in width it is a northeast southwest trending series of features spanning from hangzhou bay in zhejiang province into jiangxi province through fuzhou city southwest of fuzhou the rift splits into two portions one continuing along the southwestern trend and the other diverging westward the total extent of the rift cannot be defined at this time the rift is superimposed upon a major suture zone of caledonian or early mesozoic age the suture represents the fusing of the south china huanan and yangtze cratons perhaps in late triassic but for sure by late middle jurassic time the rifting was initiated and followed this older suture in part this time corresponds roughly to the middle stage of the yanshanian orogeny and to the subduction of the postulated pacific kula ridge southeast of the continental margin the total thickness of the sediments and volcanics filling the rift valley reaches more than 10 000 m peak intensity of extension was between late middle jurassic and middle to late cretaceous sedimentation within the rift was not continuous and is marked with periodic unconformities sediments within the rift include red beds sandstones siltstones mudstones conglomerates breccias tuffs and ignimbrites vertebrate fossils and dinosaur eggs are also found contemporaneous volcanics within and flanking the rift include basalts rhyolites granites gabbros dacites and andesites silicic volcanics are mostly attributed to caldera systems early basalts are tholeiitic and later change to alkaline olivine basalt bimodal volcanism is recognized peak intensity of volcanism ranges between 135 and 75 ma in early cenozoic time the area was a topographic low paleocene eocene sediments and evaporites are the last rocks to be deposited in the rift today the rift is delineated by major high angle faults the pingxiang guangfeng deep fault belt red beds and volcanically derived u deposits scismic study of p wave velocities has produced an earth model which shows substantial crustal thinning under the gan hang rift the region has been included in the southeastern china mesozoic fault depression system and has not generally been recognized as a distinct failed rift

structural and stratigraphical constraints on the kinematics history of the southern tan lu fault zone during the mesozoic anhui province china abstract the tan lu fault zone tlfz extends in a nne ssw direction for more than 2000 km in eastern china it has been considered either as a major sinistral strike slip fault as a suture zone or as a normal fault we have conducted a structural analysis of the southern segment of this fault zone stlfz in the anhui province the ages triassic to palaeocene of the formations affected by the faults have been re appraised taking into account recent stratigraphical studies to better constraint the ages of the successive stages of the kinematics of the stlfz subsequently the kinematics of the faults is presented in terms of strain stress fields by inversion of the striated fault set data finally the data are discussed in the light of the results obtained by previous workers we propose the following history of the stlfz kinematics during the mesozoic at the time of collision a nne orientated tan lu margin probably connected two margins located north of the dabie and sulu collision belts during the middle late triassic the scb has been obliquely subducted below the ncb along this margin which has acted as a compressional transfer zone between the dabie and sulu continental subduction zones the stlfz has been initiated during the early jurassic and has acted as a sinistral transform fault during the jurassic following which the ncb scb collision stopped a nw trending extension related to metamorphic domes was active during the basal early cretaceous 135 130 ma it has been followed by a nw se compression and a ne sw tension during the middle late early cretaceous 127 to 105 ma possibly 95 ma at that time the tlfz was a sinistral transcurrent fault within the eastern part of the asian continent during the late cretaceous palaeocene the stlfz was a normal fault zone under a wnw ese tension

the link between migmatites and s type granites in the turku area southern finland abstract metamorphism and compositions of metasedimentary migmatites and related small scale granites are investigated in the turku area sw finland phase assemblages and mineral compositions suggest peak p t conditions of 800 c and 6 kbar the portion of melt produced in the migmatites by dehydration melting is estimated to be 20 wt the partial melt is k 2 o rich and water undersaturated 4 wt h 2 o the leucosomes and small scale granites have almost the same proportions of felsic minerals but the mafic minerals grt crd bt are enriched in the granites along with the contents of tio 2 feo tot mgo and hfse the leucosomes have low total ree values and strongly positive eu anomalies the ree patterns of the granites are characterized by relatively high lree low hree and distinct negative eu anomalies the distribution of major elements and trace element signatures suggests that leucosomes and granites are related as cumulates and evolved residual melts respectively the decimeter to meter scale leucosomes are interpreted to be the product of crystal fractionation which occurred during near isobaric cooling and the granites are the crystallized residual melts which could move further on and form larger bodies within and outside of the migmatite complex

a multivariate discrimination scheme of detrital garnet chemistry for use in sedimentary provenance analysis abstract garnet chemistry provides a well established tool in the discrimination and interpretation of sediment provenance current discrimination approaches however i suffer from using less variables than available ii subjective determination of discrimination fields with strict boundaries suggesting clear separations where in fact probabilities are converging and iii significant overlap of compositional fields of garnet from different host rock groups the new multivariate discrimination scheme is based on a large database a hierarchical discrimination approach involving three steps linear discriminant analysis at each step and the five major host rock groups to be discriminated eclogite a amphibolite b and granulite c facies metamorphic rocks as well as ultramafic d and igneous rocks e the successful application of statistical discrimination approaches requires consideration of the a priori knowledge of the respective geologic setting this is accounted for by the use of prior probabilities three sets of prior probabilities priors are introduced and their advantages and disadvantages are discussed the user is free to choose among these priors which can be further modified according to the specific geologic problem and the level of a priori knowledge the discrimination results are provided as integrated probabilities of belonging to the five major host rock groups for performing calculations and results a supplementary excel spreadsheet is provided the discrimination scheme has been tested for a large variety of examples of crystalline rocks covering all of the five major groups and several subgroups from various geologic settings in most cases garnets are assigned correctly to the respective group exceptions typically reflect the peculiarities of the regional geologic situation evaluation of detrital garnets from modern and ancient sedimentary settings of the western gneiss region norway eastern alps austria and albertine rift uganda demonstrates the power to reflect the respective geologic situations and corroborates previous results as most garnet is derived from metamorphic rocks and many provenance studies aim at reconstructing the tectonic and geodynamic evolution in the source area the approach and the examples emphasize discrimination of metamorphic facies i e temperature pressure conditions rather than protolith composition

a calculated petrogenetic grid for the system k2o feo mgo al2o3 sio2 h2o with particular reference to contact metamorphosed pelites a petrogenetic grid is presented for the system kfmash k o feo mgo a1 0 sio h o including biotite muscovite k feldspar chlorite chloritoid staurolite cordierite garnet ortho amphibole orthopyroxene spinel andalusite sillimanite kyanite quartz and corundum with h o in excess which was calculated using the computer program thermocalc and the powell and holland internally consistent thermodynamic dataset by removing the normal constraint of having quartz in excess both quartz bearing and quartz absent equilibria are shown quartz absent equilibria are particularly relevant at high t and low p conditions because of their common occurrence at these conditions the calculated mineral assemblage and mineral compositional variations in terms of femg and fe mg siall exchange vectors are broadly compatible with observations on natural rocks particularly when non kfmash components are taken into account

garnet and scheelite as indicators of multi stage tungsten mineralization in the huangshaping deposit southern hunan province china abstract the huangshaping w mo pb zn deposit in southern hunan province south china contains multiple generations of garnet and scheelite in skarn and sulfide carbonate altered rocks optical characteristics and chondrite normalized rare earth element reen patterns obtained by in situ laser ablation inductively coupled plasma mass spectrometry analysis were used to distinguish different generations of garnet and scheelite these data show a clear correspondence of garnet reen patterns to major element zonation with hrees e g gd lu being depleted overall coarse grained garnets in the huangshaping deposit have extreme hree depletions and significant lree e g la eu enrichment particularly for ce pr and nd this indicates that rees in these garnets are the result of coupled substitutions ca2 viii 1 ree3 viii 1 and fe2 iv 1 al3 iv 1 medium grained garnets have reen patterns showing significant lree enrichment and depleted hrees with high sn contents this suggests that substitution of rees in these garnets occurs as ca2 viii 1 ree3 viii 1 sn4 iv 1 al3 iv 1 the fact that medium grained garnets have high sn contents indicates mineralizing fluids were oxidizing which is consistent with significant positive eu anomalies however hump shaped reen patterns for garnet rims suggest substitution by ca2 viii 2 na viii 1 ree3 viii 1 where nd tb are preferentially incorporated into the garnet lattice over other rees group 1a scheelite has black cores in cathodoluminescence images and reen patterns showing extreme lree enrichment and hree depletion group 1b scheelite has cores with fine oscillatory zoning and enriched lrees with depleted hrees similar to the reen patterns of group 2a scheelite that occur as rims with bright cl surrounding both group 1a and 1b scheelite the substitution mechanism for rees in these three types of scheelite is ca2 viii 3 viii 1 ree3 viii 2 with being a ca site vacancy the influence of ree speciation in the hydrothermal fluid dominates the reen patterns of these types of scheelite however for group 2b bright rims of scheelite rees are incorporated as ca2 viii 2 na viii 1 ree3 viii 1 similar to the garnet rims finally scheelite mo contents and eu values that decrease from group 1a to 2b support a temporal decrease in oxygen fugacity of the mineralizing fluids ratios of y ho and mo contents that decrease from group 1a and 1b to group 2a and 2b scheelites are similar to those in porphyry related skarn w mo and quartz vein au w deposits respectively our studies also suggest that all these scheelites in this deposit formed from magmatic fluids at the huangshaping deposit medium grained garnets associated with group 1a scheelite precipitated from evolved magmatic fluids during prograde metamorphism as indicated by their complementary y ho ratios paragentically younger scheelite particularly group 2b may have formed from dilute magmatic fluids that underwent large scale hydrothermal circulation the characteristics of group 1b and 2a scheelite likely reflect a transitional environment and fluid mixing during tungsten mineralization in the polymetallic huangshaping deposit

late mesozoic compressional to extensional tectonics in the yiwulushan massif ne china and their bearing on the yinshan yanshan orogenic belt part ii anisotropy of magnetic susceptibility and gravity modeling abstract granitoids play an important role in deciphering both crustal growth and tectonic evolution of earth in the eastern end of the yinshan yanshan belt of north china craton the yiwulushan massif is a typical region that presents the tectonic evolution features of this belt our field work on the host rocks has demonstrated two phases of opposite tectonics compressional and extensional however the deformation is almost invisible in the intrusive rocks to improve the understanding of the tectonic evolution of the yiwulushan massif and the late mesozoic tectonics of east asia a multidisciplinary study has been carried out in this study anisotropy of magnetic susceptibility ams and gravity modeling have been applied on these jurassic plutons lushan jishilazi and guanyindong which intrude into the yiwulushan massif according to laboratory measurements and microscopic observations on thin sections the ams of the yiwulushan massif is characterized by secondary fabrics indicating that the intensive post solidus deformation has reset the primary magmatic magnetic fabrics a relatively gentle nw dipping magnetic foliation has been identified with two distinct groups of magnetic lineations of n34 e and n335 e orientations namely lm1 and lm2 relatively gravity modeling reveals a southward thinning of the massif with a possible feeding zone rooted in the northern part of the massif integrating all results from structural observation geochronological investigation ams measurement and gravity modeling two tectonic phases have been identified in the yiwulushan massif posterior to the jurassic 180 160 ma magmatism in the yinshan yanshan area the early one concerns a late jurassic early cretaceous 141 ma compressional event with a top to the south to southwest sense of shear the second one shows an early cretaceous 126 ma nw se ductile extensional shearing at that time sedimentary basins widened and jurassic plutons started to be deformed under post solidus conditions in fact the nw se trend of the maximum stretching direction is a general feature of east asian continent during late mesozoic

anatomy of garnets in a jurassic granite from the south eastern margin of the north china craton magma sources and tectonic implications abstract the late jurassic jingshan granite located at the south eastern margin of the north china craton contains abundant garnets which can be subdivided into three types based on texture and composition i euhedral garnet in mafic biotite and garnet rich enclave grt i ii coarse grained garnet grt ii in the host granite and iii small euhedral garnet in aplite grt iii in general grt i has higher feo cao and lower mno contents than grt ii grt iii has higher mn but lower ca contents than others grt i has lower mree and hree contents than grt ii grt iii has prominent and distinctly negative eu anomaly as well as higher mree composition compared to the others systematic variations in oxygen isotope compositions are observed among the three garnet types with 18 o values of 4 7 in grt iii some of the grt ii and grt iii display two distinct zonings with cores having similar major and trace element compositions to grt i cathodoluminescence cl images revealed that the zircons from different garnet bearing samples possess fine scale oscillatory zoned magmatic rims with inherited cores in situ zircon u pb dating and trace element analyses show that the dark luminescent magmatic rims all have jurassic concordia ages 160 ma and similar trace element patterns most of the inherited cores also display similar triassic ages of 210 236 ma which is similar to the ages of ultrahigh pressure uhp metamorphic rocks of the dabie sulu orogen 230 ma in addition jurassic concordia ages were also found in a zircon inclusion in grt i implying that the grt i was formed shortly before the main magmatic event the age data suggest that the three different garnet types may be genetically related and modified by cogenetic magmatic events based on the zircon u pb ages from different garnet bearing samples the major element trace element oxygen isotope and zoning textures of the three kinds of garnet we suggest that grt i may be peritectic garnet whereas grt ii and iii are probably the results of magmatic dissolution precipitation processes and re equilibration of garnets with changing magmatic conditions during melting differentiation crystallization and cooling within the granite we conclude from the oxygen isotopic character of the garnets and ages of the zircons that the source rocks for the jingshan granites are from dabie sulu orogen representing the south china craton

components and episodic growth of precambrian crust in the cathaysia block south china evidence from u pb ages and hf isotopes of zircons in neoproterozoic sediments abstract detailed lam icpms u pb dating and lam mc icpms lu hf isotope analysis were carried out on zircons from nine samples of basement metamorphic rocks in the southern cathaysia block south china the chemical compositions of these metamorphic rocks and zircon morphology indicate that their protoliths were sedimentary zircon u pb dating results show they mainly formed during the late neoproterozoic the precambrian crust of the cathaysia block can be divided into two distinct tectonic domains the wuyishan terrane to the northeast and the nanling yunkai terrane to the southwest the nearly e w boundary between these two terranes is also supported by geophysical petrological and geochemical evidence and the basement rocks of the two terranes are comprised of different components the wuyishan terrane is characterized by dominant paleoproterozoic 1 86 ga and lesser neoarchean magmatism and the southern central part of the terrane suffered strong neoproterozoic and early paleozoic tectonothermal reworking the zircon data suggest that there were five important episodes of juvenile crust generation 3 6 ga 2 8 ga 2 6 2 4 ga 1 85 ga and 0 8 0 7 ga in the wuyishan terrane strong paleoproterozoic 1 85 ga magmatism in northern wuyishan and neoproterozoic 0 8 0 7 ga magmatism in central and southern wuyishan both principally involved the remelting of old crust with minor input of juvenile material in contrast the nanling yunkai terrane contains abundant neoarchean and grenvillian zircons some evidence of mesoproterozoic activity and rare paleo to meso archean and neoproterozoic zircons u pb ages and hf isotopic compositions of the detrital zircons from the late neoproterozoic to early paleozoic meta sedimentary rocks in the nanling yunkai area suggest that the growth of juvenile crust mainly occurred at 3 6 ga 3 3 ga 2 5 2 6 ga 1 6 ga 1 0 ga and 0 8 0 7 ga neoarchaean 2 6 2 5 ga grenvillian 1 0 ga and neoproterozoic 0 8 0 7 ga magmatism mainly involved the recycling of old crust minor remnants of 4 1 ga crust may remain in the terrane this crustal history is distinct from that of the wuyishan terrane and analogous to those recorded for the eastern india east antarctic domain of northern gondwanaland suggesting that they probably were once linked

the magmatic to magmatic hydrothermal evolution of the el laco deposit chile and its implications for the genesis of magnetite apatite deposits

petrogenesis of jurassic tungsten bearing granites in the nanling range south china evidence from whole rock geochemistry and zircon u pb and hf o isotopes abstract the nanling range nlr is the largest tungsten metallogenic province in china and perhaps in the world the tungsten mineralization is believed to be related to jurassic granitic magmatism however the petrogenesis of these granites and their relation to the tungsten mineralization are still debated whole rock geochemical and sr nd hf isotopic data and zircon in situ u pb ages and hf o isotopes are reported for w bearing granitic intrusions from the southern jiangxi province in the nlr in order to constrain their magmatic sources and petrogenesis the nlr granites include biotite granites two mica granites and garnet muscovite granites sims and la icpms u pb dating of zircons and monazites give emplacement ages of 161 154 ma for these rocks the granites are metaluminous to strongly peraluminous with high sio 2 72 3 wt and high k 2 o 3 7 wt petrographic and geochemical features show that they are highly fractionated i type granites the biotite granites are enriched in light rare earth elements lrees relative to heavy rees have weakly negative eu anomalies and are depleted in nb ba p and ti in contrast the two mica and garnet bearing muscovite granites have tetrad type ree patterns with strongly negative eu anomalies and are extremely depleted in ba nb sr p and ti magmatic garnets are mainly almandine and spessartine and have low mn cores and high mn rims their y hree contents are high and generally decrease from core 1 2 wt to rim average 4955 ppm all of these granites are characterized by variable whole rock initial 87 sr 86 sr 0 7053 0 8000 e nd t 12 6 to 9 4 and e hf t 12 3 to 8 5 as well as variable zircon e hf t and 18 o with values of 16 3 to 7 4 and 7 6 to 10 0 respectively they contain abundant zircon xenocrysts and xenoliths of micaceous schist all of these features are consistent with a process of crystal fractionation of crustally derived magmas coupled with strong assimilation of sedimentary rocks the assimilation of meta sedimentary materials increased the concentrations of tungsten alumina and manganese in the magmas whereas high degrees of crystal fractionation further enriched them in the evolved granitic rocks the w enriched magmatic fluids formed the tungsten bearing quartz veins revealing the genetic relationship between tungsten mineralization and emplacement of garnet bearing two mica or muscovite granites in the nlr

geochronology of polygenetic monazites constrained by in situ electron microprobe th u total lead determination implications for lead behaviour in monazite abstract monazite grains are generally concordant and with an electron microprobe give th u total pb isochron ages in agreement with conventional u pb data monazites from four samples of post archean migmatite from the ivory coast record two main ages an archean age at 2 80 ga partly overprinted by a late migmatization event at 2 03 ga the in situ electron microprobe determination has also shown a third event that systematically appears at 2 72 ga for all the monazites from two of the four studied rocks the three events at 2 80 2 72 and 2 03 ga recorded by the th u total pb system in monazite were each obtained on independent homogeneous parts of a single grain with no sign of pb diffusion thus the oldest monazite remained closed during a high grade resetting migmatization at a temperature estimated at around 700 c and also remained closed during the growth of secondary monazite conventional u pb data isotope dilution on monazite from the same rocks did not allow precise age determinations because the representative points were highly up to 83 discordant and did not fit well along a chord in the concordia diagram the u pb isotope dilution method gave a lower intercept age of 2029 25 ma in agreement with the electron microprobe age although with an unusually high uncertainty due to a poor fit of the data the pb evaporation method gave an old archean age at 2830 7 ma and proterozoic ages ranging from 2417 10 to 2074 7 ma thus although the oldest archean age obtained with the pb evaporation and electron microprobe methods is similar within the analytical error the proterozoic ages obtained by the pb evaporation method are significantly older than the th u total pb proterozoic age obtained with the electron microprobe this discrepancy is presumed to be due to the influence in the pb evaporation method of radiogenic lead contained within archean zones of the monazite grains in other words it means that migmatization took place later than 2074 7 ma and probably around 2030 ma the second archean event at 2 72 ga revealed by high spatial resolution of the electron microprobe could not be determined by other methods the th u total pb method appears to be an inexpensive alternative method for dating simple monazites with a precision close to 20 ma even if the u pb isotopic dilution technique remains the reference method moreover owing to the very high spatial resolution 1 m of the electron microprobe it appears to have no competitor for dating complex polygenetic monazites especially when more than two events are recorded homogeneous zones of different ages coexist in a single grain with no major indication of lead diffusion a procedure for age and error calculations is defined

origin of late miocene peraluminous mn rich garnet bearing rhyolitic ashes in the andean foreland northern argentina abstract the ramadas volcanic center on the eastern margin of the central andean puna plateau along the olacapato el toro lineament in argentina erupted a rare strongly peraluminous mn rich garnet bearing rhyolitic tuff in the late miocene the voluminous ashes from this eruption which are distinctive in having euhedral spessartine almandine garnets alm70 72sps22 26grs2 4prp0 5 1 as their only phenocryst are widely dispersed in the andean foreland among these tuffs are those in the guanaco formation foreland basin sediments along the xibi xibi and los alisos rivers in the rio grande de jujuy basin and the metan valley some 100 200 km east of the ramada volcanic center the co occurrence of tubular to cellular pumice fragments and blocky glass shards in an ash matrix in these tuffs is interpreted as indicating that they erupted in an initial vent opening event with pulsating pyroclastic surges at the initiation of the strong plinian eruption of the ramada volcanic center new ar ar ages from the guanaco fm glass shards agree with fossil ages in placing the eruption at 6 3 0 3 ma a number of distinctive chemical isotopic and mineralogical features including mg rich biotite and mg hastingsite xenocrysts of the guanaco formation and ramadas volcanic center tuffs are consistent with the melt having been derived by extensive crystallization of a mantle derived mafic shoshonitic series magma contaminated by assimilation dehydration melts of metapelitic sediment and the puna crust distinctive chemical features include whole rock sio2 contents of 75 76 wt a cnk indices 1 2 low ca mg ti and fe concentrations steep ree patterns with extreme negative eu anomalies low ba sr lree and high cs rb u concentrations and recalculated initial ratios of 87sr 86sr at 0 7119 and 143nd 144nd of 0 5123 at 6 3 ma the erupted magma has a transitional chemical character between those of the 11 ma mn rich garnet bearing coyaguayma ignimbrites to the north and the 6 ma cerro galan ignimbrites to the south unlike these crystal rich ignimbrites the ramadas tuff records the extraction of an extensively fractioned melt from a plagioclase k feldspar quartz and biotite bearing mush with accessory titanomagnetite and apatite in line with existing experimental studies on mn rich garnets and comparisons with the coyaguayma ignimbrite pre eruption crystallization of the rhyolite segregated from the mush likely occurred at 800 to 720 c at a depth of no

u pb th pb and ar ar geochronology from the southern sierras pampeanas argentina implications for the palaeozoic tectonic evolution of the western gondwana margin

peraluminous grenvillian ttg in the sierra de pie de palo western sierras pampeanas argentina petrology geochronology geochemistry and petrogenetic implications abstract combined petrological geochemical isotopic and geochronological data shed light on the origin and evolution of a peraluminous garnet bearing two mica granitoid el tigre granitoid etg cropping out in southwestern sierra de pie de palo 31 31 30 s 68 15 12 w and to constrain the age and petrogenetic conditions of this intrusive event etg experienced amphibolite to greenschist facies metamorphism after igneous crystallization followed by strong deformation restricted to narrow mylonite zones etg shear zone and partial dynamic recrystallization under lower t conditions a dextral shear sense is compatible with kinematic observations registered along the nne striking regional las pirquitas overthrust active at 473 10 ma k ar on 1 1 normative corundum low cao values between 1 72 and 2 41 plotting mostly in the granite trondhjemite fields of the ab an or diagram the trace element contents show a relatively low abundance of rb hfs elements such as y nb ta ga and zr and high concentrations of ba sr and lree the chondrite normalized ree pattern has a high slope with la yb n 9 48 55 32 and a negative or absent europium anomaly relationships between trace elements suggest the classical setting of granitoids produced in a convergent plate setting a u pb shrimp crystallization age on zoned igneous zircon of 1105 5 4 1 ma suggests that the etg could be part of the magmatic complex forming the grenvillian basement of the western sierras pampeanas 87 sr 86 sr 1105 values of 0 70543 and nd of 4 2 indicate a rather immature source for its origin with similar initial isotopic ratios to those found in orthogneisses from elsewhere in the western sierras pampeanas on the other hand sm nd model ages t dm for the etg range from 1 20 to 1 39 ga geochemical and isotopic signatures of the etg could be explained by low pressure partial melting from a basaltic source under high geothermal gradient conditions this thermal anomaly could be associated with the 1 1 ga global period of enhanced mantle plume activity developing widespread global magmatism

occurrence and origin of andalusite in peraluminous felsic igneous rocks andalusite occurs as an accessory mineral in many types of peraluminous felsic igneous rocks including rhyolites aplites granites pegmatites and anatectic migmatites some published stability curves for and sil and the water saturated granite solidus permit a small stability field for andalusite in equilibrium with felsic melts we examine 108 samples of andalusite bearing felsic rocks from more than 40 localities world wide our purpose is to determine the origin of andalusite including the t p x controls on andalusite formation using eight textural and chemical criteria size compatibility with grain sizes of igneous minerals in the same rock shape ranging from euhedral to anhedral with no simple correlation with origin state of aggregation single grains or clusters of grains association with muscovite with or without rims of monocrystalline or polycrystalline muscovite inclusions rare mineral inclusions and melt inclusions chemical composition andalusite with little significant chemical variation except in iron content 008 171 wt feo compositional zoning concentric sector patchy oscillatory zoning cryptically reflect growth conditions compositions of coexisting phases biotites with high siderophyllite eastonite contents aliv 268 007 atoms per formula unit muscovites with 057 401 wt feo and 002 285 wt tio2 and apatites with 353 018 wt f coexisting muscovite biotite pairs have a wide range of f contents and fbt 1612fms 0015 most coexisting minerals have compositions consistent with equilibration at magmatic conditions the three principal genetic types of andalusite in felsic igneous rocks are type 1 metamorphic a prograde metamorphic in thermally metamorphosed peraluminous granites b retrograde metamorphic inversion from sillimanite of unspecified origin c xenocrystic derivation from local country rocks and d restitic derivation from source regions type 2 magmatic a peritectic water undersaturated t associated with leucosomes in migmatites b peritectic water undersaturated t as reaction rims on garnet or cordierite c cotectic water undersaturated t direct crystallization from a silicate melt and d pegmatitic watersaturated t associated with aplite pegmatite contacts or pegmatitic portion alone type 3 metasomatic water saturated magma absent spatially related to structural discontinuities in host replacement of feldspar and or biotite intergrowths with quartz the great majority of our andalusite samples show one or more textural or chemical criteria suggesting a magmatic origin of the many possible controls on the formation of andalusite excess al2o3 water concentration and fluid evolution high be b li p high f high fe mn ti and kinetic considerations the two most important factors appear to be excess al2o3 and the effect of releasing water either to strip alkalis from the melt or to reduce alumina solubility in the melt of particular importance is the evidence for magmatic andalusite in granites showing no significant depression of the solidus suggesting that the and sil equilibrium must cross the granite solidus rather than lie below it magmatic andalusite however formed is susceptible to supra or sub solidus reaction to produce muscovite in many cases textural evidence of this reaction remains but in other cases muscovite may completely replace andalusite leaving little or no evidence of its former existence

an early extensional event of the south china block during the late mesozoic recorded by the emplacement of the late jurassic syntectonic hengshan composite granitic massif hunan se china continental scaled extension is the major late mesozoic jurassic and cretaceous tectonic event in east asia characterized by faulting magmatic intrusions and half grabens in an area with a length of 5000 km and a width of 1000 km numerous studies have been conducted on this topic in the south china block scb however the space and time ranges of the compressional or extensional regimes of the scb during the jurassic are still unclear partly due to the lack of structural data the emplacement fabrics of granitic plutons can help determine the regional tectonic background in this study a multidisciplinary approach including anisotropy of magnetic susceptibility ams macro and micro structural analyses quartz c axis preferred orientation gravity modeling and monazite epma dating was conducted on the hengshan composite granitic massif in scb that consists of the triassic nanyue biotite granitic pluton and the late jurassic baishifeng two mica granitic pluton the magnetic fabrics are characterized by a consistent nw se oriented lineation and weakly inclined foliation a dominant high temperature deformation with a top to the nw shear sense is identified for both plutons the deformation increasing from the center of the baishifeng pluton to its western border is associated to the development of the west hengshan boundary fault whbf the gravity modeling shows a saw tooth shaped ne sw oriented structure of the baishifeng pluton which may be considered as ne sw oriented tension gashes formed due to the nw se extension all results show that the triassic nanyue pluton was deformed under post solidus conditions by the whbf coeval with the emplacement of the late jurassic baishifeng pluton all these observations comply with the nw se extensional tectonics coeval with the emplacement of the baishifeng pluton which argues that the nw se crustal stretching started since the late jurassic at least in this part of the scb

geochemistry of granitoids and their minerals from rebordelo agrochao area northern portugal abstract deformed hercynian peraluminous granitoids ranging from tonalite to granite crop out in the rebordelo agrochao area northern portugal and some of them contain tonalitic and granodioritic enclaves variation diagrams of major and trace elements of the rocks biotites and sphenes show fractionation trends the most and the least deformed samples of granite and their biotites also define fractionation trends there is decrease in all rare earth element ree contents and increase in the eu anomaly in ree patterns from the most to the least deformed samples of granite all the granitoids define a whole rock rb sr errorchron a whole rock rb sr isochron for the least deformed samples of granite yields an age of 357 9 ma and an initial 87sr 86sr ratio of 0 7087 0 0007 geochemical modelling suggests that the tonalitic magma evolved by afc fractional crystallization of magnesiohornblende plagioclase quartz biotite and ilmenite and assimilation of metasediments to originate tonalitic and granodioritic enclaves granodiorite and granite 18o values support this mechanism the tonalite is hybrid and derived by interaction of a mantle derived magma and crustal materials

geochemistry and ar ar muscovite ages of the daraban leucogranite mawat ophiolite northeastern iraq implications for arabia eurasia continental collision abstract daraban leucogranite dykes intruded discordantly into the basal serpentinized harzburgite of the mawat ophiolite kurdistan region ne iraq these coarse grained muscovite tourmaline leucogranites are the first leucogranite dykes identified within the mawat ophiolite they are mainly composed of quartz k feldspar plagioclase tourmaline muscovite and secondary phologopite while zircon xenotime corundum mangano ilemnite and cassiterite occur as accessories the a cnk value of the granite dyke samples varies from 1 10 to 1 22 indicating a strongly peraluminous composition cao na2o ranges from 0 11 to 0 15 and al2o3 tio2 from 264 to 463 similar to the strongly peraluminous sp granites exposed in high pressure collision zones such as the himalayas ar ar muscovite step heating dating yields 37 57 0 25 and 38 02 0 53 ma plateau ages for two samples which are thought to reflect either their magmatic emplacement or resetting during collision related metamorphism mineral chemistry shows evidence of both primary and secondary types of muscovite with cores favouring the magmatic interpretation and slight effects of a late syn serpentinization fluid seen at the rims geochemical features of daraban leucogranite dykes favour a syn collisional tectonic setting they probably formed in response to the continental collision between eurasia and arabia during the initial stage of the opening of the gulf of aden at 37 ma the muscovite ages and geochemical features of daraban leucogranite are strong evidence for the timing of the continental collision between northeastern arabia and eurasia in kurdistan region of iraq

garnet compositions and their use as indicators of peraluminous granitoid petrogenesis southeastern arabian shield garnet an uncommon accessory mineral in igneous rocks occurs in seven small peraluminous granitoid plutons in the southeastern arabian shield textural equilibrium between garnet and other host granitoid minerals indicates that the garnets crystallized from their host magmas compositions of the garnets form three groups that reflect host granitoid compositions which in turn reflect source compositions and tectonic regimes in which the host magmas were generated garnets from the seven plutons have almandine rich cores and spessartine rich rims this reverse zoning depicts host magma compositional evolution i e rimward spessartine enrichment resulted from progressive host magma manganese enrichment the garnets are heavy rare earth element enriched lu la n ranges from 13 to 355 and one of the garnets contains spectacularly elevated abundances of y ta th u zn zr hf sn and nb involvement of garnets with these trace element characteristics in magma genesis or evolution can have dramatic effects on trace element signatures of the resulting magmas other researchers suggest that mn enriched magmas are most conducive to garnet nucleation although the garnetiferous granitoids discussed here are slightly mn enriched other genetically similar peraluminous arabian granitoids lack garnet mn enrichment alone does not guarantee garnet nucleation the presence of excess alumina in the magma may be a prerequisite for garnet nucleation

early paleozoic accretionary orogenies in nw argentina growth of west gondwana abstract two early paleozoic orogenic cycles in the gondwana margin of nw argentina were driven by subduction and interrupted by collision of continental ribbons of laurentian affinity subduction started at the passive margin of gondwana possibly in response to the end of amalgamation of the supercontinent the passive margin was underlain by a hyper extended continent ocean transition and low angle subduction gave rise to a wide and hot fore arc region during the pampean orogenic cycle the arrival of the first continent ribbon ended the pampean cycle steepened the subduction further heating the fore arc and restarting arc magmatism after a magmatic lull 250 300 km trenchwards initiating the famatinian orogenic cycle this cycle started with a 30 ma long period of extension and marine sedimentation followed by the arrival of the second continent ribbon and inversion the back arc initiating a 20 ma long shortening event culminating with the shutting down of the arc this event gave rise to a 300 km wide low topography hot orogeny thus the many peculiarities of the two paleozoic orogens of nw argentina result from a subduction history that efficiently transferred heat to a 300 400 km wide belt of turbidite dominated sediments that was first part of the pampean fore arc and then part of the famatinian retroarc developed at the extended continental margin of gondwana the two orogenies with continued high heat flux lasted 110 ma giving rise to two calc alkaline arcs separated by a 300 km belt of high t low p migmatites and peraluminous granites formed by anatexis of sediments deposited on the passive margin as well as those deposited in the wide pampean fore arc these turbidite dominated sequences were metamorphosed and melted to form a continental crystalline basement and accreted to the cratonic margin adding 500 km of crystalline rocks to the margin of gondwana at the same time that a similar process was happening in eastern australia along the same continental margin interestingly this wide accretionary orogen has now become the region where the andean system developed a wide orogeny above flat slab subduction