tertiary tectonics of the border ranges fault system chugach mountains alaska deformation and uplift in a forearc setting the border ranges fault system brfs locally separates supracrustal upper cretaceous tertiary rocks of the cook inlet matanuska valley forearc basin on the peninsular terrane from metamorphosed cretaceous subduction complex rocks of the chugach terrane to the south this 5 to 10 km wide zone of concentrated deformation along the inboard edge of southern alaska s accretionary prism has suffered a protracted history beginning with its inception as a megathrust by early cretaceous time and continuing into early tertiary time when it was transected by a system of steeply arcward dipping normal and dextral slip faults based on detailed geologic mapping structural analysis metamorphic mineral assemblage data and k ar and fission track geochronology the tertiary history of displacements along a central part of this forearc fault system has been divided into several phases of deformation these include a paleocene phase of regional uplift of the forearc basin together with block faulting along the brfs to rapidly uplift and subaerially expose low grade metamorphic rocks of the subduction complex upper cretaceous valdez group within 15 ma of their inferred underplating at deep structural levels of the accretionary prism continued normal separation faulting along the brfs in late paleocene early eocene time to further uplift the subduction complex was recorded by deposition of a syntectonic alluvial fan system chickaloon formation across the brfs and along the southern margin of the forearc basin a third mid eocene increment of normal separation faulting along the brfs juxtaposed greenschist facies fabrics of early eocene age in the subduction complex against the chickaloon formation this latter uplift event predated intrusion of early late eocene 43 48 ma zircon fission track minimum ages felsic dikes into the present terrane bounding fault and was accompanied by no more than a few tens of kilometers of dextral slip faulting along the brfs a conclusion that does not accord with recent suggestions for thousands of kilometers of displacement along that fault system in early middle tertiary time based on paleomagnetic data field observations in the study area indicate that the great oroclinal bend of southern alaska is postearly eocene and possibly prelate oligocene in age a final period of deformation in middle to late tertiary time was marked by periods of minor north south contraction of the forearc basin and by local reverse slip reactivation of earlier faults fission track age data on apatite indicate rapid cooling and probably uplift of both subduction complex and adjacent parts of the forearc basin in miocene time 17 24 ma episodic uplift of the subduction complex either locally by normal separation faulting along its backstop or by a more regional uplift also affecting the adjacent forearc basin overlapped in time or closely followed discrete pulses of large scale sediment underplating along the gulf of alaska subduction margin this coincidence suggests that major pulses of sediment underplating along southern alaska s convergent margin may have led to isostatic uplift of its critical taper accretionary prism an uplift that was in part accommodated by normal separation faulting along its mechanical backstop

alaskan type nature and pge mineralization of the wuxing mafic ultramafic complex in eastern part of the central asian orogenic belt abstract the wuxing mafic ultramafic complex is located in the eastern part of the central asian orogenic belt caob it consists of hornblende olivine clinopyroxenite hornblende clinopyroxenite clinopyroxenite gabbro and diorite hornblende occurs in all the rock types of the complex and is pargasite in composition clinopyroxene is diopsidic in composition with high cao and low tio2 contents the flat ree and sub parallel trace element patterns of both clinopyroxene and hornblende suggest that these rocks were formed by fractional crystallization from common magma with no significant crustal contamination enrichment in large ion lithophile elements relative to high field strength elements of the silicate minerals together with the variable sulfur isotope compositions 34s 2 55 11 01 of the sulfides indicate the involvement of subducted oceanic materials into the mantle source these petrological mineralogical and geochemical features strongly suggest the alaskan type nature of the wuxing complex the hydrous parental magma of the wuxing complex was derived from partial melting of the mantle wedge which had been metasomatized by subducted melts fluids the wuxing complex is a unique pt pd rich ni cu sulfide deposit in the caob the ore minerals which are dominated by pyrite pyrrhotite pentlandite chalcopyrite and platinum group mineral pgm mainly occur in clinopyroxenite and hornblende clinopyroxenite the sulfide ores are predominantly present as net textured and disseminated textures pentlandite and chalcopyrite associated with pyrrhotite commonly occur in the marginal zone of the pyrrhotite grains the pgms occur as interstitial grains at the contact between base metal sulfides and silicate minerals or along the crack of pyrrhotite or clinopyroxene or as inclusions in pyrrhotite the occurrences of sulfides and pgms in the wuxing complex together with sulfur isotope compositions and pge patterns reveal that the pge enrichment of the alaskan type complexes can be attributed to the high degree partial melting of the mantle source sulfur involvement in the source and pge transportation of hydrous melts

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

composition and structure of the central aleutian island arc from arc parallel wide angle seismic data new results from wide angle seismic data collected parallel to the central aleutian island arc require an intermediate to mafic composition for the middle crust and a mafic to ultramafic composition for the lower crust and yield lateral velocity variations that correspond to arc segmentation and trends in major element geochemistry the 3 d ray tracing 2 5 d inversion of this sparse wide angle data set which incorporates independent phase interpretations and new constraints on shallow velocity structure produces a faster and smoother result than a previously published velocity model middle crustal velocities of 6 5 7 3 km s over depths of 10 20 km indicate an andesitic to basaltic composition high lower crustal velocities of 7 3 7 7 km s over depths of 20 35 km are interpreted as ultramafic mafic cumulates and or garnet granulites the total crustal thickness is 35 37 km this result indicates that the aleutian island arc has higher velocities and thus more mafic compositions than average continental crust implying that significant modifications would be required for this arc to be a suitable building block for continental crust lateral variations in average crustal velocity below 10 km roughly correspond to trends in major element geochemistry of primitive mg 0 6 lavas the highest lower crustal velocities and presumably most mafic material are detected in the center of an arc segment between unmak and unalaska islands implying that arc segmentation exerts control over crustal composition

petrogenesis of the greenhills complex southland new zealand magmatic differentiation and cumulate formation at the roots of a permian island arc volcano a permian 265 ma intrusive complex which formed as a magmatic feeder reservoir to an immature island arc volcano is fortuitously exposed in southern new zealand known as the greenhills complex this intrusion was emplaced at shallow crustal levels and consists of two layered bodies which were later intruded by a variety of dykes cumulates which include dunite olivine clinopyroxenite olivine gabbro and hornblende gabbro norite are related products of parent magma fractionation both primary magmatic and secondary platinum group minerals occur within dunite at one locality using the composition of cumulus minerals mafic dykes and melt inclusions we have determined that the parent magmas of the complex were hydrous low k island arc tholeiites of ankaramitic affinities progressive magmatic differentiation of this parent magma generated fractionated melt of high alumina basalt composition which is now preserved only as dykes which cut the complex field evidence and cumulus mineral profiles reveal that the magma chambers experienced turbulent magmatic conditions during cumulate rock formation recharge of the chambers by primitive magma is likely to have coincided with eruption of residual melt at the surface similar processes are inferred to account for volcanic rock compositions in other parts of this arc terrane and in modern island arc systems

accretion and subduction tectonics in the chugach mountains and copper river basin alaska initial results of the trans alaska crustal transect geologic seismic gravity and magnetic data from the northern chugach mountains and southern copper river basin alaska indicate that the chugach terrane cgt and the composite peninsular wrangellia terrane pet wrt are thin 10 km rootless sheets bounded on the south by north dipping thrust faults that sole into a shallow horizontal low velocity zone the cgt has been thrust at least 40 km beneath the pet wrt along the border ranges fault system brfs adjacent to the brfs uplift and erosion of 30 40 km since jurassic time have exposed blueschist facies rocks in the cgt and mafic and ultramafic cumulate rocks in the pet wrt four paired north dipping layers of low and high seismic velocities extend beneath the northern cgt and southern pet wrt and may be slices of subducted oceanic crust and upper mantle the upper two pairs may now be joined to the continental plate 15 references 5 figures

origin of andesite in the deep crust and eruption rates in the tancitaro nueva italia region of the central mexican arc the tancitaro nueva italia region 4400 km 2 is located within the west central mexican volcanic belt and spans 80 km of arc length along the volcanic front on the basis of 65 40 ar 39 ar ages it is shown that a total volume of 326 57 km 3 of magma has erupted from 200 vents since 1 2 ma nearly a third of this volume 103 5 km 3 was erupted as crystal rich andesite from a large stratovolcano volcan tancitaro whereas the remaining two thirds 223 52 km 3 span the complete range from 51 to 63 wt sio 2 are relatively crystal poor and were erupted from peripheral vents the total proportions of erupted lava types are 6 7 basalt 18 basaltic andesite and 74 76 andesite there is no rhyolite and only a trace of dacite there is no evidence that any significant magma differentiation occurred in the upper crust along this arc segment based on phenocryst abundances and modes major and trace element data as well as phase equilibrium experiments from the literature the following mechanisms for formation of andesite in this region can be ruled out 1 crystal fractionation of basalt basaltic andesite in the upper crust 2 magma mixing of basalt basaltic andesite with dacite rhyolite and 3 assimilation of granitoid by basalt basaltic andesite instead the same body of evidence indicates that the andesites were formed primarily in the deep crust through partial melting 20 of hornblende rich 40 gabbronorite the basaltic andesites are required owing to their elevated ni concentrations to be hybrids of the andesitic partial melts with basaltic residual liquids that have undergone variable two pyroxene fractionation in the deep crust

effects of chaotic advection on the timescales of cooling and crystallization of magma bodies at mid crustal levels we numerically define the thermochemical evolution of a subduction related crystal bearing magmatic mass at mid crustal levels 0 7 gpa 20 25 km two main dynamic mechanisms are considered 1 a pure buoyancy driven system where the convective flow is induced by density changes during magma cooling 2 a buoyancy driven convective system governed by chaotic advection the non newtonian rheology of natural magmas is taken into account linking the herschel bulkley formulation with the results of fractional crystallization experiments of magmas with the same composition and at the same conditions of temperature and pressure of the studied system the latent heat of crystallization is also considered in order to address the thermal release in the system induced by the crystallization results indicate that the development of chaotic advection generates a complex thermochemical evolution of the system speeding up the crystallization process and the timing required to reach the jamming condition relative to the pure buoyancy driven convective system nearly 2 times faster our results have important implications for both the rheological history of the magmatic body and the refilling of shallower magmatic systems in particular 1 a time dependent composition ranging from basalt to andesite can be extracted from an initial basaltic magmatic batch 2 at the attainment of the maximum packing fraction i e just before the jamming condition homogeneous andesitic melts can be potentially extracted from the system and 3 the development of chaotic advection within the system allows for the extraction of andesitic melt in shorter times compared to a buoyancy dominated system

adakite differentiation and emplacement in a subduction channel the late paleocene sabzevar magmatism ne iran this study describes the structural setting petrogenesis and geochronology of a suite of acidic magmatic rocks that are intruded in the metamorphic core of the tertiary ophiolitic suture zone of the sabzevar range ne central iran these intrusive bodies show tabular geometries with solid state fabrics documenting syntectonic emplacement during crustal shortening in the total alkalis silica tas diagram their compositions define a medium k calc alkaline suite spanning from basaltic andesite to the dacite and rhyolite fields they show characteristic low mgo 0 15 0 60 wt and ni 40 ar 39 ar white mica and amphibole geochronology constrains the sabzevar magmatism to the late paleocene at ca 58 ma genesis of the sabzevar magmatic suite is interpreted in terms of prograde wet amphibolite melting during oceanic subduction within a pressure temperature range between a plagioclase out and a hornblende out boundary magma differentiation and high pressure amphibole fractionation of pristine slab melts are proposed as the dominant factors that imparted the adakite signature in the sabzevar structural zone implications in terms of the regional tectonic scenario are discussed and framed within the advancing and retreating evolution of the neotethyan subduction during the mesozoic tertiary time span

aleutian tholeiitic and calc alkaline magma series i the mafic phenocrysts diagnostic mafic silicate assemblages in a continuous spectrum of aleutian volcanic rocks provide evidence for contrasts in magmatic processes in the aleutian arc crust tectonic segmentation of the arc exerts a primary control on the variable mixing fractional crystallization and possible assimilation undergone by the magmas end members of the continuum are termed calc alkaline ca and tholeiitic th ca volcanic rocks e g buldir and moffett volcanoes have low feo mgo ratios and contain compositionally diverse phenocryst populations indicating magma mixing their ni and cr rich magnesian olivine and clinopyroxene come from mantle derived mafic olivine basalts that have mixed with more fractionated magmas at mid to lower crustal levels immediately preceding eruption high al amphibole is associated with the mafic end member in contrast th lavas e g okmok and westdahl volcanoes have high feo mgo ratios and contain little evidence for mixing evolved lavas represent advanced stages of low pressure crystallization from a basaltic magma these lavas contain groundmass olivine fo 40 50 and lack ca poor pyroxene aleutian volcanic rocks with intermediate feo mgo ratios are termed transitional tholeiitic tth and calc alkaline tca tca magmas are common e g moffett adagdak great sitkin and kasatochi volcanoes and have resulted from mixing of high al basalt with more evolved magmas they contain amphibole high and low al or orthopyroxene or both and are similar to the japanese hypersthene series tth magmas e g okmok and westdahl contain orthopyroxene or pigeonite or both and show some indication of upper crustal mixing they are mineralogically similar to the japanese pigeonite series high al basalt lacks mg rich mafic phases and is a derivative magma produced by high pressure fractionation of an olivine tholeiite the low pressure mineral assemblage of high al basalt results from crystallization at higher crustal levels

seismic reflection images of the crust of the northern part of the chugach terrane alaska results of a survey for the trans alaska crustal transect tact deep crustal seismic reflection data show strong reflections from the middle and lower crust of the convergent continental margin near the eastern end of the aleutian trench these data were collected across the border ranges fault system a major suture zone that separates the peninsular and chugach tectonostratigraphic terranes the chugach terrane consists of metamorphosed strongly deformed flysch subduction melange as well as blueschist and greenschist facies rocks even so highly reflective rocks lie at great depth within or beneath this terrane the shallow part of the seismic section 0 5 s reveals only a few reflections which undulate and interweave indicating that some of them originate outside of the plane of the seismic section other undulating events could reveal early cenozoic granitic plutons or antiformal stacks of rock imbricated along thrust faults in contrast to this shallow reflection pattern three reflection bands contain most of the events on the middle part of the seismic section 5 12 s or 12 34 km we interpret divergent reflections within the uppermost band 5 5 5 s as gently north dipping thrust faults and subparallel reflections truncated at the boundaries of the upper and middle bands may image a metamorphic foliation or thrust faults if the truncated reflections represent thrust faults then the reflection geometry suggests that subparallel roof and floor thrust faults bound imbricated rocks forming duplex structures the upper and middle bands are separated by an area on the seismic section that reveals few reflections this area correlates approximately in depth with a high velocity 7 5 km s layer evident in refraction models the poorly reflective high velocity unit might be volcanic rocks like those exposed in the structurally lowest part of the chugach terrane if so then the bottom of the poorly reflective rocks indicates the base of the chugach terrane rocks that cause the upper and middle reflection bands and the intervening poorly reflective volcanic rocks may form a midcrustal shear zone that is about 10 km thick the deepest band of subparallel reflections 30 35 km correlates closely in depth with the top of the wadati benioff zone associated with the underthrusting pacific plate the reflections probably stem from the decollement that separates lithospheric plates within the subduction zone this reflection band increases abruptly in thickness possibly indicating local underplating of subducted sediment no events from the oceanic moho have been recognized neither the border ranges fault system the suture between the chugach and peninsular terranes nor deep crustal layers of the peninsular terrane are evident in these seismic reflection data

early proterozoic oceanic crust in the northern colorado front range implications for crustal growth and initiation of basement faults 1 early proterozoic oceanic crust exposed in a shear zone in the northern colorado front range provides insight into the tectonic evolution of the colorado province tectonically intermingled mylonitic amphibolite locally pillowed metagabbro clinopyroxenite metachert and melange are preserved within the buckhorn creek shear zone west of fort collins whole rock geochemical data support formation of the igneous protoliths in an oceanic environment major and trace element data overlap samples from modern oceanic ridges and are distinct from modern arc trends metagabbros and clinopyroxenites preserve a subhorizontal lineation defined by igneous pyroxenes core and mantle microstructures indicate initial deformation at temperatures 900 c secondary amphiboles define both subhorizontal and subvertical lineations kinematic analysis indicates that the rocks experienced e w strike slip deformation prior to and during amphibolite facies metamorphism followed by n s shortening 40ar 39ar data from secondary hornblende yield age steps as old as 1 9 ga based on the chemical data and kinematic evidence for strike slip deformation we propose that the buckhorn creek rocks originated at an oceanic ridge transform intersection the high t pyroxene fabrics formed during ocean floor deformation prior to 1 7 ga regional deformation of the colorado province the buckhorn creek fragment likely represents original oceanic basement for 1 8 1 7 ga arc magmatism and sedimentation that ultimately formed the colorado province continental crust reactivation of original oceanic fracture zones may be responsible for the network of regularly spaced northeast trending shear zones in colorado once formed subvertical transform faults are likely to survive the transition from oceanic to continental crust and persist as long lived crustal discontinuities

examination of the deep levels of an island arc evidence from the tonsina ultramafic mafic assemblage tonsina alaska the tonsina ultramafic and mafic assemblage located in the northern chugach mountains along the trans alaska crustal transect tact is interpreted to be the remains of a magma chamber that crystallized at the base of a mature interoceanic island arc it is one of a number of isolated masses of peridotite and gabbro that comprise the basal part of the peninsular terrane along the northern fringe of the chugach mountains in southern alaska these masses the roots of a mid jurassic island arc have been uplifted along the north dipping border ranges fault system and crop out sporadically for 700 km from kodiak island to eastern alaska the tonsina ultramafic mafic assemblage is a layered sequence whose basal rocks are dunite and harzburgite 1 km thick overlain by a narrow zone of websterite and clinopyroxenite above these ultramafic rocks is a 5 to 6 km thick layered gabbro that is garnet bearing at its base textural relationships and chemical trends throughout the series of dunite pyroxenite garnet bearing gabbro and spinel gabbronorite point to an origin by igneous accumulation from a hydrous tholeiitic parent magma at deep levels i e at the base of a mature island arc before being emplaced along the border ranges fault the rocks were crystallized from a melt at pressures between 9 5 and 11 kbar 28 to 33 km depth and equilibrated at temperatures of 800 to 90o c based on phase relationships in the cumulus assemblages pyroxene thermobarometry and olivine spinel thermometry transitional sequences from ultramafic to mafic rocks are characterized by increasingly al rich pyroxenes and spinels or garnet and the absence of olivine plagioclase assemblages deep level fractionation of al poor phases such as olivine and pyroxene at moderately high pressures and temperatures resulted in concentration of al in the evolving magma as crystallization continued the phases recorded this alumina concentration and the resultant gabbros are al rich this observed trend supports the theory of high alumina basalt generation by fractional crystallization of olivine and pyroxene comparison of the tonsina rocks with aleutian lower crustal xenoliths as well as other exposures of arc related lower crustal magma chambers in japan pakistan and western north america indicates that this process is a common mechanism for island arc magma evolution at depth

pge mineralization in andesite explosive breccias associated with the poperechny iron manganese deposit lesser khingan far east russia whole rock geochemical 190pt 4he isotopic and mineralogical evidence abstract we report in this paper an unusual occurrence of platinum group minerals in evolved explosive breccia associated with the poperechny iron manganese deposit lesser khingan range far east russia pgms in andesite breccia are represented by fe pt solid solutions 85 and pgm mostly os ir ru solid solutions sulfides and sulfarsenides 15 textural and compositional variations in pgm assemblages suggest that pt fe and os ir ru solid solutions as well as erlichmanite laurite series sulfides were formed during high temperature fractionation of mantle derived mafic parental melt similar to alaskan type complexes and were entrained in the evolved andesitic melt during its emplacement in the crust pd pt plumbostannide and copper gold solid solutions reflect late magmatic re crystallization and metasomatism early cretaceous 125 ma age of ferroplatinum in the explosive breccia suggests that pgm bearing ultramafic material could have been sampled during regional slab window tectonics related to the late mesozoic subduction of izanagi plate along southern margin of the north asian continent

hornblende and phlogopite bearing gabbroic xenoliths from volcan san pedro 36 s chilean andes evidence for melt and fluid migration and reactions in subduction related plutons

origin and age of the border ranges fault of southern alaska and its bearing on the late mesozoic tectonic evolution of alaska local geologic relationships in the western chugach mountains together with regional considerations suggest that in the early cretaceous a subduction zone was formed along the southern edge of the wrangellia peninsular alexander composite terrane talkeetna superterrane of csejtey et al 1982 of southern alaska the effects of this event include 1 a shattering of older pre cretaceous crystalline rocks along a complex fault system 2 emplacement of a tectonic melange beneath the shattered crystalline rocks and 3 a subgreenschist facies prograde metamorphism of the melange and retrograde metamorphism of the older crystalline rocks this event created a regionally mappable structural contact between broken crystalline rocks and the melange the border ranges fault of mackevett and plafker 1974 regional stratigraphy as well as radiometric ages of rocks predating and postdating the border ranges fault appear to bracket the age of the border ranges fault and its associated deformational effects to the interval between about 135 and 120 ma further regional tectonostratigraphic associations suggest that the deformation along the border ranges fault represents the nucleation of a north and or east dipping subduction zone beneath the talkeetna superterrane and that the magmatic arc associated with this juvenile subduction zone is the gravina nutzotin belt of southeast alaska mismatches in the distribution of different elements of this early cretaceous arc trench system are probably a result of late cretaceous or early tertiary strike slip motion but early cretaceous ridge trench interaction suggested by the occurrence of early cretaceous near trench plutons may have played a role as well the talkeetna superterrane is generally thought to be an exotic block that collided with the cordillera in the middle cretaceous coney et al 1980 the age data commonly cited as evidence for a middle cretaceous age for the collision are however misleading a model is proposed here in which the initiation of convergence along the trailing edge of the talkeetna superterrane records the time of initial impingement of two irregular margins whereas intense middle cretaceous deformation recognized along the leading edge of the talkeetna superterrane csejtey et al 1982 records destruction of a syncollisional flysch basin during the final phase of the collision

stability and composition relations of calcic amphiboles in ultramafic rocks calcic amphiboles are observed in ultramafic rocks that have equilibrated under a broad span of geological conditions and might prove to be good indicators of metamorphic grade if their stabilities could be determined as a function of their compositions experiments were performed on the stability of tremolite plus forsterite in the system h2o cao mgo sio2 from 5 to 20 kbar a univariant curve was fitted to the experimental brackets using volume water fugacity and heat capacity data the results indicate that the maximum stability of tremolite in the presence of forsterite is about 825 c at 5 kbar addition of al2o3 to this system increases the stability of tremolitic amphibole by only 20 40 c and induces solubility of 5 7 wt al2o3 in the amphibole as determined from quantitative sem analyses of individual amphibole crystals thus substitution of the tschermakite component ca2 mg3al2 si6al2 o22 oh 2 alone cannot lead to the greatly enhanced al2o3 contents or thermal stability of natural calcic amphiboles

pre cretaceous crystalline rocks of the western chugach mountains alaska nature of the basement of the jurassic peninsular terrane a narrow belt of mesozoic and or paleozoic crystalline rocks exposed immediately north of the border ranges fault of southern alaska is the southern trailing edge of the peninsular terrane an exotic micro continental block that collided with north america during the cretaceous the tectonic significance of this crystalline belt has been debated because previous k ar geochronology suggested that the belt contains both early jurassic high p t metamorphic rocks an apparent subduction complex and early jurassic intermediate plutonic rocks an apparent magmatic arc three major observations from detailed mapping in the western chugach mountains pertain directly to this apparent problem of mixed rocks association first despite major postmetamorphic faulting along the border ranges fault all of the metamorphic rocks have been subjected to a regional apparently low p t metamorphism at the greenschist amphibolite facies transition the metamorphism is imprinted on rocks ranging from nearly undeformed ultramafic rocks to highly deformed amphibolites yet regardless of structural histories of fault bounded slices the culmination of metamorphism appears to postdate deformation second the metamorphic rocks are locally an argillite matrix melange in which main phase structural fabrics appear to have formed at relatively low grade but are overprinted by growth of postkinematic transitional greenschist amphibolite facies minerals third despite ambiguities in correlation across faults and equivocal absolute age relationships presumably due to cretaceous resetting of k ar ages the transitional greenschist amphibolite facies rocks are intruded by plutons that yield early jurassic k ar ages thus the metamorphic protolith is presumably pre jurassic in addition the close association of low p t metamorphic rocks with a large plutonic complex implies that the regional metamorphism is probably early jurassic in age and is associated with extensive plutonism new age data from the western chugach mountains imply that a belt of early jurassic plutonism and associated low p t metamorphism probably extends throughout the southern edge of the peninsular terrane the entire terrane probably represents a long lived magmatic arc because this early jurassic magmatic arc is separated from a parallel belt of early to middle jurassic plutons by an early jurassic andesitic volcanic pile in the western chugach mountains emplacement of alpine ultramafic bodies and development of chaotic rock bodies is premetamorphic presumably pre jurassic in age these observations of structural style together with lithologic associations and regional occurrences of blueschist facies rocks suggest that a pre jurassic subduction complex may have formed parts of the basement for the peninsular terrane arc as a corollary of this conclusion the blueschists of the southern peninsular terrane are probably pre jurassic in age and represent scraps of the pre jurassic basement not parts of an early jurassic high pressure metamorphic belt

devonian alaskan type ultramafic mafic intrusions and silicic igneous rocks along the southern altai orogen implications on the phanerozoic continental growth of the altai orogen of the central asian orogenic belt abstract in this contribution we have carried out an integrated study of the petrography geochronology geochemistry and mineral chemistry for the devonian alaskan type ultramafic mafic complexes and granitoids widely distributed along the southern altai orogen in order to provide a better understanding of the continental growth process and the regional tectonic evolution la icpms zircon u pb dating of three mafic samples one acidic volcanic rock sample and one granitic sample yielded the concordia ages of ca 400 ma olivine chemical compositions indicate these mafic intrusions crystallized from evolved mafic magmas and compositions of the clinopyroxene and hornblende show typical arc affinities whole rock compositions reveal their tholeiitic signature with enriched lrees and liles and intensive depleted hfses nb ta zr and hf leading to low hfse lree ratios nb la 0 1 0 4 isotopically these mafic intrusions have positive zircon e hf t 7 6 to 14 3 mostly 10 and positive whole rock e nd t 0 8 to 5 3 values which suggest that their parental magmas were derived from a time integrated depleted mantle source these features together with the presence of primary water bearing minerals biotite and or hornblende suggest that their primitive magmas were derived from a time integrated depleted lithosphere mantle which were metasomatized by fluids released from subduction slab these findings in combination with coeval voluminous granitoids and arc signature volcanic rocks indicate that the underplating of mafic magmas at the continental arc root leads to the partial melting of the continental crust it is inferred that mafic magmatic underplating in the altai orogenic belt significantly contributed to the phanerozoic vertical continental crust growth in the altai orogen

studies of an artificially shock loaded h group chondrite five samples of the naturally unshocked kernouve h6 meteorite were artificially shock loaded to pressures of 70 165 270 and 390 kbar and the silicates and metal examined optically by scanning and transmission electron microscopy and by thermoluminescence tl olivine deformation is closely comparable to that in naturally shocked meteorites producing dislocations with burgers vector 001 at pressures of 165 kbar these are formed in well defined slip planes at 270 kbar olivine develops optical mosaicism has high dislocation densities throughout and is also highly fractured recovery due to heating is minimal in orthopyroxene the deformation mechanism changes from the clino inversion to unit dislocation slip between 70 and 165 kbar in diopside 001 and 100 twinning was produced plagioclase is inferred to have been progressively converted to maskelynite but some is still present in 270 kbar sample the microhardness of the kamacite in the samples increases with shock pressure the transformation pressure in the kamacite is 30 40 kbar higher than observed for iron meteorites annealed kamacite displays incipient polycrystallinity and martensite and taenite sometimes contains slip lines troilite acquired cracks undulose extinction twins polycrystallinity and finally melted as the shock pressure increased at pressures over 200 kbar there was a systematic decrease in the natural tl and the tl sensitivity detailed considerations of changes in the natural tltl sensitivity ratio for various regions of the tl glow curve suggest that two processes were effective during shock thermal drainage of electron traps and a reduction in the effective trap density it is suggested that the latter process associated with the vitrification of feldspar the tl phosphor an additional sample was subjected to a shock pulse which was spiked instead of square very distinctive changes were apparent thermal effects are conspicuous and with widespread annealing 600 800 c of metal and sulfide glassy opaque veins were produced which are analogous to the black veins in shock lithified gas rich meteorites anomalous low temperature tl was induced suggesting that a new or modified phase or mineral has become the dominant tl phosphor

the border ranges ultramafic and mafic complex south central alaska cumulate fractionates of island arc volcanics a discontinuous elongate zone of mafic and ultramafic plutonic rock crops out in south central alaska for a distance of more than 1000 km intermediate and detailed scale geologic mapping petrographic study and compositional data suggest that the plutonic rocks are compositionally petrologically and mineralogically distinct from rocks in mid ocean ridge and back arc basin ophiolites the mafic and ultramafic rocks instead represent part of the plutonic core of an intraoceanic island arc the mafic ultramafic zone referred to as the border ranges ultramafic and mafic complex brumc is composed of ultramafic cumulates gabbronorite cumulates and massive gabbronorites a very minor amount of tectonized ultramafic rock of mantle origin is present in the southern part of the brumc a thick sequence of andesitic volcanic rocks the talkeetna formation of early jurassic age lies to the north of and structurally above the mafic ultramafic zone voluminous calcalkaline plutons composed of quartz diorite

a model for continental crust genesis by arc accretion rare earth element evidence from the irish caledonides abstract the formation of continental crust is a complex problem with a paradox at its center continental material is believed to form by arc magmatism a model that does not reconcile the bulk mafic and light rare earth element lree depleted composition of intra oceanic arcs with the andesitic lree enriched composition of continents we present evidence supporting an arc origin for continental crust by demonstrating significant changes in magmatic chemistry during arc continent collision we use as an example the connemara region of the western irish caledonides where metamorphism and orogenesis of dalradian sedimentary sequences record the collision of the oceanic lough nafooey arc with the passive margin of laurentia at 475 ma arenig this arc was mafic and lree depleted during its oceanic activity during collision rhyolites more lree enriched than the surrounding continental crust were erupted while the mid crust was intruded by similarly lree enriched gabbro and diorite crystal fractionation caused lree enrichment to exceed that expected by assimilation of continental crust alone we propose that subsequent loss of the corresponding lower crustal cumulates into the mantle removed this lree depleted material from the net crust added to the continent the combined processes of crystal fractionation and lower crustal loss during arc continent collision drive the average bulk arc composition toward that of average continental crust

initial argon in amphiboles from the chugach mountains southern alaska the 40ar 39ar step heating analyses of three hornblende samples from the border ranges ultramafic mafic assemblage yield plateaus with slightly discordant dates of 180 2 0 7 177 5 0 7 and 181 1 1 7 ma a plot of the plateau temperature data for all three hornblende samples on an 36ar 40ar versus 39ar 40ar diagram reveals the presence of a non atmospheric initial argon component with an apparent 40ar 36ar of 379 16 and yields a date of 175 7 1 0 ma a potassium poor hastingsite from the same complex yields a complicated spectra with dates as high as 900 ma these older dates reflect contamination by argon with an isotopic composition similar to that found for the initial argon in the hornblende and do not record a paleozoic component in the age of the border ranges ultramafic mafic complex hornblende from an eocene tonalite sill in the nearby chugach metamorphic complex also contains an initial argon component 40ar 36ar 314 4 the composition of this initial argon is presumably dictated by the argon isotopic composition of the host rock fluid at the time of amphibole fluid equilibration approximately 2 10 7 cm3 g of this initial argon is sufficient to perturb the 40ar 39ark model plateau dates although 40ar 39ar stepwise degassing at low temperatures removes an additional atmospheric argon component the initial argon in these samples is released at a rate that closely coincides with the degassing rate of radiogenic argon from the amphibole and only through application of the 36ar 40ar versus 39ar 40ar diagram can this nonatmospheric argon component be detected for the border ranges ultramafic mafic and chugach metamorphic complexes the range in k ar amphibole dates could be interpreted as recording differential uplift and an extended duration of intrusive activity respectively the results suggest however that nonatmospheric initial argon contributes significantly to the observed age variations

the variscan french massif central a new addition to the ultra high pressure metamorphic club exhumation processes and geodynamic consequences abstract this paper presents the first documentation of coesite bearing eclogites in the eastern french massif central monts du lyonnais unit and discusses the exhumation processes for these very high pressure rocks a combination of mineralogical and geochronological datasets allows us to quantify a depth time path and related exhumation rates high pressure metamorphism is constrained to 400 420 ma minimum 28 kbar or ca 90 km by 360 380 ma the rocks were exhumed to 30 km depth these kinetic results conform to the geological constraints extracted from the tectonic and sedimentary record of the eastern french massif central these multidisciplinary approaches provide new information on paleozoic orogeny and allow us to discuss the relative roles of subduction and collision in exhumation of very high pressure rocks we suggest that a significant amount of exhumation of these rocks occurred during subduction prior to continental collision continental collision itself was responsible only for the final stages of exhumation under a transpressive regime

native metals and intermetallic compounds in subduction related ultramafic rocks from the stanovoy mobile belt russian far east implications for redox heterogeneity in subduction zones abstract plutonic associations in orogenic belts are potent indicators of multi stage fractionation of primitive arc magmas and growth of island arc crust under evolving redox conditions in ancient subduction zones ildeus lucha ultramafic mafic complex ilc was emplaced at 232 233 ma within the mesozoic stanovoy convergent margin and subsequently underwent multi stage hydrothermal alteration and greenschist to amphibolite facies metamorphism 140 ma followed by adakite and k lamprophyre magmatism 114 117 ma dunite wehrlite pyroxenite and gabbro in the ilc are composed of olivine clinopyroxene orthopyroxene plagioclase and late magmatic amphibole mineral compositions in the ilc are typical of island arc plutonic complexes characterized by early crystallization of orthopyroxene plagioclase intercumulus and presence of magmatic amphibole ultramafic mafic rocks display high field strength hfs element depletions coupled with large ion lithophile lil element enrichments indicating formation via crystal fractionation of mafic primary magma derived from a subduction related mantle source some ultramafic rocks from the ilc contain native metals w pt au ag zn bi and intermetallic compounds cu au ag pt rh pd cu sn zn etc included in silicate minerals or observed as discrete phases in the finer grained silicate oxide sulfide matrix textural evidence and association with either primary magmatic or secondary metasomatic or metamorphic mineral phases suggest either magmatic high temperature or metasomatic low temperature origin of metals and metallic compounds the following models of their formation can be proposed on the basis of the presented data a crystallization w and possibly pt from metal rich arc magmas under unusually reduced conditions in subduction related lithosphere b formation from magmatic sulfides during pervasive serpentinization and release of abiogenic hydrocarbons and c precipitation from chlorine rich saline aqueous fluids associated with collision related metasomatism of the stanovoy island arc crust

eocene volcanism above a depleted mantle slab window in southern alaska the caribou creek volcanic fi eld lies along the continent side edge of forearc basin rocks in south central alaska and consists of over 1000 m of shallow dipping basalt and andesite lavas with minor mafi c pyroclastic deposits dacite and rhyolite lavas along with shallow intrusions form dome complexes with associated pyroclastic deposits that overlie and crosscut the basalt and intermediate lavas the basalts are tholeiitic and strongly depleted in the light rare earth elements la yb 0 18 1 5 with concentrations of high fi eld strength elements e g zr hf ti y similar to mid ocean ridge basalt and with variable enrichment in fl uid mobile elements e g cs ba and pb intermediate and felsic rocks show enrichment in the rare earth elements and fl uid mobile elements plus rb and k but retain low la yb ratios 0 48 3 6 a few andesite and dacite samples are strongly depleted in the heavy rare earth elements and are geochemically similar to adakites e g sr y up to 52 ten 40 ar 39 ar ages for the caribou creek volcanic rocks range from 49 4 2 2 to 35 6 0 2 ma an adakite like tuff beneath the other volcanic rocks yields an age of 59 0 0 4 ma caribou creek basalts were derived from mid ocean ridge like depleted mantle that was emplaced beneath the southern margin of alaska through a slab window following spreading ridge subduction caribou creek volcanism was coeval with oblique subduction oroclinal bending and right lateral strike slip faulting in south central alaska all of which could have induced crustal extension to allow adiabatic melting of the depleted mantle reservoir to form basaltic magmas the basalts then evolved by fractional crystallization with moderate to high degrees of assimilation of jurassic arc basement rocks to form the intermediate and felsic magmas enrichment of the basaltic parent magmas in fl uid mobile elements occurred by contamination from the jurassic arc rocks and or by contamination with metasomatic mantle remnant from preceding subduction high heat fl ow through the slab window induced partial melting of garnet bearing mafi c parts of the jurassic arc basement to form the adakite like rocks the caribou creek volcanic rocks demonstrate that slab windows can directly infl uence magmatism inboard of accretionary prism and forearc basin settings given a suitable deformation regime e g crustal extension and that the infl uence of a slab window on continental margin magmatism can be long lived 20 m y

mineral chemistry of ultramafic cumulates from the north arm mountain massif of the bay of islands ophiolite evidence for high pressure crystal fractionation of oceanic basalts the mineral chemistry of ultramafic cumulates forming the basal portions of the plutonic section of the north arm mountain massif bay of islands ophiolite complex is not consistent with crystal liquid fractionation of a primitive mid ocean ridge basalt at low pressures the appearance of clinopyroxene and orthopyroxene having very high mg numbers 100 mg mg fe 85 to 92 and the absence of plagioclase as early fractionating phases coprecipitating with forsteritic olivine fo86 92 suggest that moderate to high pressure phase relationships are applicable we propose on the basis of mineral chemistry crystallization sequences and structural relationships that the basal ultramafic cumulates on the north arm massif represent the products of high pressure 10 kbar polybaric crystal fractionation of primary basaltic melts generated within the mantle beneath a mid ocean spreading center these basal ultramafic cumulates were probably precipitated due to in situ crystallization processes along the margin of a vertical conduit extending downward from a crustal level magma chamber to mantle depths that exceed 30 km after their formation the cumulates were transported to the near surface along the axis of upwelling and divergence as the lithosphere was separating beneath a mid ocean ridge

the age and origin of miocene pliocene fault reactivations in the upper plate of an incipient subduction zone puysegur margin new zealand

postsubduction porphyry cu au and epithermal au deposits products of remelting of subduction modified lithosphere porphyry cu mo au and some epithermal au deposits are formed from hydrothermal fluids exsolved from cooling water rich calc alkaline magmas emplaced in volcanoplutonic arcs above subduction zones these magmas originate by partial melting of the metasomatized asthenospheric mantle wedge however there is increasing evidence for the existence of a suite of porphyry cu au and epithermal au deposits related to magmas generated after subduction beneath the arc has ceased associated magmas tend to be mildly alkaline relatively sulfur poor and emplaced as isolated complexes rather than in voluminous volcanoplutonic arcs they are likely formed by remelting of previously subduction modified arc lithosphere triggered by postsubduction lithospheric thickening lithospheric extension or mantle lithosphere delamination metasomatized mantle lithosphere or hydrous lower crustal cumulates residual from first stage arc magmatism contain small amounts of chalcophile and siderophile element rich sulfides and constitute a fertile source for hydrous au rich but relatively sulfur poor magmas during later remelting the recognition that porphyry cu au and related epithermal au systems can also form in postsubduction and collisional tectonic settings expands the range of geological environments and geographical terranes that are prospective for such deposits

late cretaceous infant intra oceanic arc volcanism the central pontides turkey petrogenetic and tectonic implications abstract a tectonic slice of an arc sequence consisting of low grade metavolcanic rocks and overlying metasedimentary succession is exposed in the central pontides north of the izmir ankara erzincan suture separating laurasia from gondwana derived terranes the metavolcanic rocks mainly consist of basaltic andesite andesite and mafic cognate xenolith bearing rhyolite with their pyroclastic equivalents which are interbedded with recrystallized pelagic limestone and chert the metasedimentary succession comprises recrystallized micritic limestone with rare volcanogenic metaclastic rocks and stratigraphically overlies the metavolcanic rocks the geochemistry of the metavolcanic rocks indicates an arc setting evidenced by depletion of hfse ti p and nb and enrichment of fluid mobile lile identical trace and rare earth elements compositions of basaltic andesites andesites and rhyolites suggest that they are cogenetic and derived from a common parental magma the arc sequence crops out between an albian turonian subduction accretionary complex representing the laurasian active margin and an ophiolitic melange absence of continent derived detritus in the arc sequence and its tectonic setting in a wide cretaceous accretionary complex suggest that the kosdag arc was intra oceanic zircons from two metarhyolite samples give late cretaceous 93 8 1 9 and 94 4 1 9 ma u pb ages these ages are the same as the age of the supra subduction ophiolites in western turkey which implies that that the kosdag arc may represent part of the incipient arc formed during the generation of the supra subduction ophiolites the low grade regional metamorphism in the kosdag arc is constrained to 69 9 0 4 ma by 40 ar 39 ar muscovite dating indicating that the arc sequence became part of a wide tethyan cretaceous accretionary complex by the latest cretaceous non collisional cessation of the arc volcanism is possibly associated with southward migration of the magmatism as in the izu bonin mariana arc system

tectonic setting and significance of 2 3 2 1ga magmatic events in the trans north china orogen new constraints from the yanmenguan mafic ultramafic intrusion in the hengshan wutai fuping area abstract a detailed study of the 2 2 ga yanmenguan mafic ultramafic intrusion in the hengshan area is presented with the aim of clarifying the tectonic history of the trans north china orogen in the north china craton during the period 2 3 2 1 ga shrimp u pb dating of magmatic zircons from a gabbro indicates emplacement at 2193 ma the whole rock and mineral geochemistry of this intrusion are akin to alaskan type complexes and are typical of subduction related magmas the parental magma likely originated from the depleted mantle wedge above a subduction zone and experienced fractional crystallization of olivine and clinopyroxene the continental arc represented by the mafic ultramafic intrusions and contemporary granitic and volcanic rocks together with the luliang back arc basin constitute a marginal arc back arc basin system that evolved between 2 3 and 2 1 ga based on geochronological datasets and available data on magmatism metamorphism and structural deformation a comprehensive new tectonic model for the evolution of the tnco during the neoarchean to paleoproterozoic is constructed 1 2 5 ga intra oceanic arc back arc basin system and its accretion at the end of the neoarchean 2 a long hiatus in magmatism and tectonism between 2 5 and 2 3 ga 3 2 3 2 1 ga marginal arc back arc basin magmatism developed in the accreted hengshan wutai fuping arc terranes and the luliang terrane 4 2 1 ga collision resulting in closure of the 2 3 2 1 ga luliang back arc basin accretion of the luliang hengshan wutai fuping arc terranes and their attachment to the western block of the north china craton 5 the intrusion of 1970 1910 ma post collisional mafic dykes in an extensional event 6 final collision between the western and eastern blocks between 1 9 and 1 8 ga resulting in extensive magmatism tectonism and metamorphism and lastly 7 post 1 8 ga crustal extension

geochemistry and geochronology of meta igneous rocks from the tokat massif north central turkey implications for tethyan reconstructions located in the eastern pontides of the sakarya zone in north central turkey the tokat massif records the closure of both the paleo tethyan karakaya complex and neo tethyan ocean basins meta igneous samples collected from the region were studied to determine their sources and ages we find significant geochemical differences between metagabbros of the karakaya and neo tethyan units in terms of their trace elements neo tethyan rocks are consistent with generation in an island arc setting whereas karakaya assemblages were likely generated in an oceanic spreading center environment karakaya metagabbros also contain glaucophane consistent with subduction subsequent to formation small 2 50 m zircon and baddeleyite grains from four karakaya metagabbros were dated in thin section using an ion microprobe the results demonstrate the reliability of the method to directly constrain the tectonomagmatic history of these types of assemblages the rocks yield late permian early triassic 238u 206pb crystallization ages of 258 14 ma 1 zircon and 254 8 ma 1 baddeleyite and an early cretaceous minimum metamorphic age of 137 8 ma 1 zircon some zircon grains and baddeleyite grains with zircon overgrowths yield early to middle jurassic ages here we present a model in which metamorphism and deformation in this region occurred during northward subduction and closure of a paleo tethyan ocean basin and accretion of the karakaya units to the laurasian continental margin this was followed by the onset of closure of the neo tethys during the campanian paleocene and accretion of island arc units to the tokat region

continental margin tectonics forearc processes recent studies of convergent plate margins and the structural development of forearc terranes are summarized in a critical review of u s research from the period 1987 1990 topics addressed include the geometry of accretionary prisms coulomb wedge taper and vertical motion in response to tectonic processes offscraping vs underplating or subduction the response to oblique convergence fluids in forearc settings the thermal framework and the effects of fluid advection and serpentinite seamounts also included is a comprehensive bibliography for the period

petrology and geochemistry of the island of sarigan in the mariana arc calc alkaline volcanism in an oceanic setting isotopic studies of rocks from oceanic island arcs such as the marianas indicate that little if any recycling of continental material e g oceanic sediments occurs in these arcs because oceanic arcs are on the average more mafic than the dominantly andesitic continental arcs an important question is whether the andesites of continental arcs are produced by a fundamentally different more complex mechanism than the lavas of oceanic arcs an excellent opportunity for study of this question is provided by the island of sarigan in the mariana active arc on which calc alkaline andesites including hornblende bearing types are exposed along with more mafic lavas

phase relations of a high mg basalt from the aleutian island arc implications for primary island arc basalts and high al basalts many volcanic centers in the aleutian islands have erupted lavas that range in composition from high mg basalt mgo 9 wt to more fractionated and voluminous high al basalts and basaltic andesites the petrogenetic relationships between these rock types and the composition of primary magmas has been vigorously debated the phase relations of a typical high mg basalt from the makushin volcanic field on unalaska island provide important constraints on petrogenetic models results of one atmosphere and moderate to high pressure 5 20 kb anhydrous experiments are similar to results obtained from primitive morb at low pressures olivine is the liquidus phase joined by plagioclase and clinopyroxene at progressively lower temperatures clinopyroxene is the second phase to crystallize at pressures greater than 5 kb and replaces olivine on the liquidus at approximately 10 kb above 10 kb the liquidus pyroxene is aluminous augite and orthopyroxene is the second phase to crystallize glasses in equilibrium with olivine and clinopyroxene at intermediate pressure 5 to 10 kb are similar in composition to high al basalt plagioclase is not involved and most likely does not become a liquidus phase until the liquid has evolved significantly although our studies do not confirm the primary nature of high mg basalts they do support a model in which high al basalts are generated by moderate amounts of crystal fractionation from more primitive high mg mg fe lower al2o3 basaltic magmas near the arc crust mantle boundary

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