upper miocene to lower pliocene sigaboy formation turbidites on the pujada peninsula mindanao philippines internal structures composition depositional elements and reservoir characteristics abstract turbidites of the upper miocene to lower pliocene sigaboy formation on the pujada peninsula southeastern mindanao philippines potentially provide a superb record how a forearc basin may fill up investigation of the basin sequences allows the provenance of the material to be established and hence the data are useful for tectonic models of the sub region in addition information obtained from the basin may give insights that are useful for petroleum exploration which mindanao is an important area the sigaboy formation is exposed for 45 km along the western coast of pujada peninsula the unit directly overlies various levels within the pujada ophiolite various facies within the formation identified from the detailed stratigraphic logs of six major outcrops represent channelized and unchannelized deposits of submarine fans that were deposited largely by turbidity currents field and petrographic evidence suggest that the portions of the sangihe arc and its uplifted outer arc ridge now forming the pujada peninsula provided the main source of detritus that was shed into the forearc basin this event probably followed the initial subduction of the snellius plateau halmahera arc in mindanao latest middle miocene and prior to the initiation of the philippine trench in the region

monsoon variability and deep oceanic circulation in the western equatorial pacific over the last climatic cycle insights from sedimentary magnetic properties and sortable silt magnetic and grain size properties of a sediment core located in the western equatorial pacific off the southeastern tip of the philippine island of mindanao are presented in an effort to reconstruct past changes in the east asian monsoon and deep ocean circulation during the last 160 kyrs the sedimentary concentration of magnetic particles interpreted to reflect past changes in runoff from mindanao varies almost in antiphase with northern hemisphere insolation this suggests that precipitation was lower in the western equatorial pacific region during boreal insolation maxima and thus corroborates model results showing opposing trends in precipitation between land and the marine realm there variations in the grain size distribution of the inorganic sediment fraction as recorded by both the sortable silt mean size and the magnetic grain size provide a monitor of changes in sediment reworking by bottom currents the close correlation of this proxy of bottom current strength and the benthic delta 18 o record from the same site implies a tight coupling between deep water flow most likely antarctic intermediate water aaiw and global climate

mantle flow in regions of complex tectonics insights from indonesia 1 indonesia is arguably one of the tectonically most complex regions on earth today due to its location at the junction of several major tectonic plates and its long history of collision and accretion it is thus an ideal location to study the interaction between subducting plates and mantle convection seismic anisotropy can serve as a diagnostic tool for identifying various subsurface deformational processes such as mantle flow for example here we present novel shear wave splitting results across the indonesian region using three different shear phases local s sks and downgoing s to improve spatial resolution of anisotropic fabrics allows us to distinguish several deformational features for example the block rotation history of borneo is reflected in coast parallel fast directions which we attribute to fossil anisotropy furthermore we are able to unravel the mantle flow pattern in the sulawesi and banda region we detect toroidal flow around the celebes sea slab oblique corner flow in the banda wedge and sub slab mantle flow around the arcuate banda slab we present evidence for deep sub 520 km anisotropy at the java subduction zone in the sumatran backarc we measure trench perpendicular fast orientations which we assume to be due to mantle flow beneath the overriding eurasian plate these observations will allow to test ideas of for example slab mantle coupling in subduction regions

numerical simulation of metamorphic pressure temperature time paths and fluid production in subducting slabs in order to investigate metamorphic reactions and fluid production in subduction zones a 2 dimensional finite difference model has been constructed the model simulates heat transfer in a subduction zone with a 27 dip a spatial resolution of 1 km is used in the region of the subduction thrust in order to carefully monitor metamorphic reactions elsewhere the spatial resolution is 5 km in these calculations the oceanic crust is assumed to contain 2 wt bound volatiles distributed homogeneously within the 7 5 km thick crust fluids are released by continuous and discontinuous end members of three different dehydration models pressure sensitive temperature sensitive and amphibole negative dp dt dehydration subduction zone pressure temperature time p t t paths predicted by the model intersect the wet basaltic solidus only at the initiation of subduction in mature subduction zones and in the absence of significant frictional heating and induced mantle convection p t t paths encounter subsolidus conditions at depths of 100 150 km beneath magmatic arcs suggesting that slab dehydration reactions play an important role in arc magma genesis metamorphic dehydration reactions that consume 50 kj kg result in p t t paths that are only 5 k cooler than models that neglect such reactions in both the temperature and amphibole dehydration models the fluid production region moves to greater depths over time as the subduction zone cools within the oceanic crust fluid production starts at the top of the crust and migrates downward in the temperature and amphibole dehydration models this phenomenon coupled with the weakening effect of devolatilization reactions may result in the accretion of oceanic crust to the hanging wall of the subduction zone for a convergence rate of 3 cm yr average fluid fluxes out of the subducting slab range from 0 1 kg fluid m yr for the continuous reaction models to 1 kg fluid m yr for the discontinuous pressure and amphibole dehydration models vertical fluid fluxes on the order of 1 kg fluid m yr can substantially perturb the thermal structure of overlying rocks and can cause large scale hydration and metasomatism of the overlying mantle wedge in contrast the thermal effect of fluid flow parallel to the subduction thrust zone is insignificant compared to the thermal effect of subducting oceanic lithosphere

comparison of young volcanic associations of western and eastern anatolia formed under a compressional regime a review abstract turkey is characterised by widespread post oligocene volcanism in western anatolia volcanic activity began during the late oligocene early miocene in a compressional regime a widespread suite of andesitic and dacitic calc alkaline rocks was erupted in this period they were accompanied by locally developed high level granitic intrusions the abrupt change from n s compression to n s stretching in the middle miocene was accompanied by a gradual transition to alkali basaltic volcanism in eastern anatolia the collision related compressional tectonics and associated volcanic activity began in the late miocene to pliocene and continued almost without interruption into historical times one minor and two major volcanic phases may be distinguished an initial minor eruptive phase of basic and intermediate alkaline rocks was followed by widespread eruptions of andesitic and dacitic calc alkaline rocks during the pliocene partly penecontemporaneous with this a second more important major phase of volcanism involving alkaline and transitional lavas began and continued during the quaternary the calc alkaline lavas of both regions were erupted at a time when the compressional regime led to crustal thickening these lavas display many geochemical and isotopic signatures that may be interpreted in terms of contamination of magma involving continental crustal materials the alkaline lavas in both regions began during the final stage of the compressional regime and form two groups one group is geochemically and isotopically similar to the calc alkaline rocks and the other is more typically alkaline the volcanic evolution of these two regions of turkey followed a somewhat similar pattern producing highly contaminated calc alkaline lavas at the beginning and evolving towards uncontaminated alkaline lavas at the end

earthquake induced crustal gravitational potential energy change in the philippine area abstract the crustal gravitational potential energy change gpe caused by earthquakes in the philippine area from january 1976 to november 2011 was estimated in this study the active convergence between the philippine sea plate and the sundaland eurasian margin is reflected by the greatest gains in gpe along the philippine negros and cotabato trenches whereas the manila trench is covered by a gpe loss pattern although the philippine mobile belt pmb itself is actually affected by the ongoing collision and subduction processes almost the entire philippine fault zone is dominated by gpe loss revealing a slightly extensional environment along the fault the time evolution of the cumulated gpe for different segments along the philippine archipelago shows distinct patterns due to the numerous large underthrusting events that have occurred along the philippine trench the cumulated gpe is regularly increasing in its most southern segment however in the middle segments where the palawan block enters into collision with the pmb the increase in cumulated gpe is relatively small in the most northern segment where the north luzon is located a decrease of cumulated gpe demonstrates that the seismic characteristic of the manila trench is dissimilar from other subduction systems in the world we suggest that the collision of both the palawan block and the benham rise with the pmb promotes the rotation of the pmb and facilitates the northward escape of the northeastern luzon resulting in a decrease of cumulated gpe in the northern philippines

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

neogene sediment thickness and miocene basin floor fan systems of the celebes sea abstract in the celebes sea basin the middle miocene turbidites were correlated from odp site 767 throughout the studied area differences in their regional thickness variations and distribution indicate two source areas the middle miocene turbidite fan complexes of the central and southern celebes sea basin are controlled by the paleo tarakan delta system the tectonic events and the basin floor morphology respectively the main source area for the time correlative turbidites along the southern sulu arc is assumed to be mindanao the correlation of the middle miocene to pleistocene sequences exhibit tentative ages for the development of the accretionary wedges along the cotabato trench and along the north sulawesi subduction a post middle miocene to pre pliocene age is inferred for the cotabato wedge and a plio pleistocene age is assumed for the north sulawesi wedge

complexities in the tectonic evolution of the west philippine basin abstract the oceanic west philippine basin can be subdivided into a main basin a northwest sub basin and a southern sub basin these basins have undergone different tectonic histories magnetic anomalies 21 17 are present in the main basin and indicate that it formed by two limbed seafloor spreading the now extinct spreading center is located within a zone of rough topography associated with the northwest trending central basin fault cbf the morphology of the cbf is primarily the result of post spreading tectonism the magnetic anomaly pattern of the main basin does not extend into the southern sub basin or the northwest sub basin geometrical constraints indicate that the southern sub basin did not form as part of the southern limb of the main basin the southern sub basin may have formed immediately before a ridge jump to the main basin spreading axis alternatively possible shallower crustal depth in the southern sub basin may indicate that the sub basin is younger than the main basin and formed separately from it in the northwest sub basin which originated as part of the main basin the dominant topographic trends are perpendicular to those in the main basin this difference as well as higher heat flow and shallower depth in the northwest sub basin indicate that it has undergone a deformational event which did not affect the main basin this deformation may be related to subduction along the east luzon margin in mid tertiary time the gagua ridge an uplifted sliver of oceanic crust along the western edge of the northwest sub basin may have originated during this event

isotopic geochemistry o sr pb of the golda zuelva and mboutou anorogenic complexes north cameroun mantle origin with evidence for crustal contamination abstract the annular 6 8 km diameter golda zuelva and mboutou anorogenic complexes of north cameroun are composed of a suite of alkaline plutonic rocks ranging from olivine gabbro to amphibole and biotite granite for the mboutou complex there are two overlapping centres in the golda zuelva complex the plutonic rocks are associated with a later hawaiite to rhyolite volcanic suite a rb sr whole rock isochron gives an age of 66 3 ma for the golda zuelva granites with initial 87 sr 86 sr ratio of 0 7020 and demonstrates that plutonism and volcanism were essentially contemporaneous and probably cogenetic for golda zuelva and the north mboutou centre 18 o 16 o 5 6 6 2 87 sr 86 sr 0 7030 0 7045 and pb isotopic ratios 207 pb 204 pb 15 60 15 64 support a mantle origin for the initial magmas unlike sr isotopes the o isotopic ratios of the granitic end members at golda zuelva 7 5 indicate crustal contamination post magmatic alteration was not significant for the younger south mboutou centre the o sr and pb isotopic data indicate more extensive magma crust interaction and in a different higher level crustal environment with 18 o granite 3 3 87 sr 86 sr ratios up to 0 706 and pb isotopic ratios more markedly displaced from the oceanic volcanic field the low 18 o granites probably record at least in part a magmatic process with subsequent minor post magmatic alteration effects the major and trace element systematics between the north and south mboutou centres are directly comparable the evolution of the magmas were dominated by fractional crystallisation and progressive crustal contamination processes

generation of sodium rich magmas from newly underplated basaltic crust sodium rich rocks of trondhjemite tonalite dacite ttd or granodiorite ttg suites form much of precambrian continental crust1 they are thought to have formed by partial melting of subducted oceanic crust2 3 a process that would have been much more widespread early in earth history than at present owing to the higher thermal gradients prevailing at that time4 phanerozoic ttd suites do exist however and seem also to relate to subduction zones5 defant and drummond6 proposed that these suites form where young 25 myr hot oceanic lithosphere is subducted and melts thus locally simulating the conditions that led to widespread crustal growth in the archaean here we describe plutonic and volcanic rocks from the cordillera blanca complex in peru which have characteristics of the high al ttd suite but which were produced above a subduction zone containing a 60 myr old slab we present evidence that the complex formed by partial melting of newly underplated basaltic crust and argue that this mechanism should be considered more generally as an additional way of generating sodium rich arc magmas

arc and backarc geochemical signatures of the proto philippine sea plate insights from the petrography and geochemistry of the samar ophiolite volcanic section abstract remnants of a cretaceous lithosphere are found at the peripheries of the west philippine basin these mesozoic fragments preserve arc affinity and include the amami plateau the east halmahera ophiolite and the ophiolites along the eastern margin of the philippine archipelago the eastern margin of the philippines is composed of early to late cretaceous ophiolites and ophiolitic complexes that exhibit strong subduction imprints the early late cretaceous samar ophiolite in the central philippines forms part of this eastern belt recent surveys in southern samar revealed the presence of peridotites gabbros and massive and pillowed flows of the samar ophiolite major trace and rare earth element signatures of the volcanic rock samples indicate moderate to strong subduction related influences and formation in an island arc setting in contrast to other similarly aged ophiolites along the eastern margin of the archipelago those to the north of samar island exhibit weak to almost mid oceanic ridge characteristics these differences in the geochemical signatures of the ophiolites and ophiolitic complexes along eastern philippines require a re evaluation of a previous model suggesting that these ophiolites originated from a single oceanic lithosphere possibly the proto philippine sea plate

magmatic relationships and ages between adakites magnesian andesites and nb enriched basalt andesites from hispaniola record of a major change in the caribbean island arc magma sources abstract located in the cordillera central of the dominican republic the late cretaceous tireo fm tf records a major change of the magma sources in the caribbean island arc it comprises a 3 km thick sequence of arc related volcanic and volcano sedimentary rocks with variable geochemical characteristics combined detailed mapping stratigraphy geochemistry and u pb ar ar geochronology show that the volcanic rocks of the tireo fm include two main volcanic sequences the lower volcanic sequence is dominated by monotonous submarine vitric lithic tuffs and volcanic breccias of andesite to basaltic andesite with minor interbedded flows of basalts and andesites fossil and u pb and 40ar 39ar geochronological data show that arc magmatism in the lower sequence began to accumulate before 90 ma from the aptian to turonian these rocks constitute an island arc tholeiitic suite derived from melting by fluxing of a mantle wedge with subduction related hydrous fluids the upper volcanic sequence is characterized by a spatial and temporal association of adakites high mg andesites and nb enriched basalts which collectivelly define a shift in the composition of the subduction related erupted lavas a dacitic to rhyolitic explosive volcanism with subaerial and episodic aerial eruptions and sub volcanic emplacements of domes characterize mainly this stratigraphic interval the onset of this volcanism took place at turonian coniacian boundary and continued in the santonian to lower campanian with minor events in the late campanian adakites represent melts of the subducting slab magnesian andesites the product of hybridization of adakite liquids with mantle peridotite and nb enriched basalts melts of the residue from hybridization we propose a model of oblique ridge subduction at 90 ma and possibly subsequent slab window formation as principal cause of magmatic variations recorded in the caribbean island arc above a southwestern dipping subduction zone

evidence in cerro pampa volcanic rocks for slab melting prior to ridge trench collision in southern south america late miocene approx 12 ma hornblende bearing andesitic to dacitic documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage wasysym pagestyle empty declaremathsizes 10 9 7 6 begin document mathrm 63 to 68 sio 2 1 2 to 1 9 k 2 o end document adakite flows at the small cerro pampa center in patagonia 47 55 s 71 25 have some of the clearest slab melt geochemical signatures yet seen in a phanerozoic center on continental crust these magmas formed in response to melting of the hot thin slab that was subducting beneath south america prior to the collision of the chile rise at approx 6 ma or at approx 10 ma their n morb like 87 sr 86 sr 0 70285 0 70309 and low 206 pb 204 pb 18 44 18 59 ratios show that they could have been generated by

evolution of the southern segment of the philippine trench constraints from seismic tomography a high resolution model of three dimensional p wave tomography of the crust and upper mantle beneath the southern part of the philippine trench is determined by inverting 22 960 arrival times of 2 789 local earthquakes and 4 660 traveltime residuals of 751 teleseismic events simultaneously the tomographic method tomog3d is used to obtain the tomographic model that has a lateral resolution of 0 6 degrees or better as shown by extensive checkerboard resolution tests a model recovery synthetic test is also performed which indicates that main features of our tomographic images are quite robust our results show that the philippine sea slab has subducted down to depths of 450 600km with an overturned dip angle along the southern segment of the philippine trench indicating that the subduction of the philippine sea plate along the southern segment of the philippine trench initiated at 20 25ma contemporaneous with the sangihe trench the subduction initiation of the philippine sea plate at the central philippine trench resulted from the collision between the philippines and the palawan block at similar to 10ma the subduction completion of the molucca sea slab propagated from the south to the north at similar to 5ma which presumably resulted in the subduction reorganization of the philippine sea plate along the southern segment of the philippine trench and subduction initiation of the philippine sea plate along the northern segment of the philippine trench

initiation and propagation of subduction along the philippine trench evidence from the temporal and spatial distribution of volcanoes abstract k ar ages of 37 samples collected from the bicol peninsula the luzon island philippines were determined by the unspiked sensitivity method in order to constrain the timing of initiation of subduction along the philippine trench the measured k ar ages range from 0 to 7 ma with two old outliers of 27 and 43 ma together with k ar ages previously reported on volcanics in leyte and eastern mindanao subduction volcanism has likely propagated from north to south 6 6 ma in bicol and 3 5 ma in leyte and its vicinity the temporal and spatial distribution suggests that the subduction volcanism started earlier in the north than in the south this is consistent with the southern propagation of subduction along the philippine trench from 8 ma

formation of arcs and backarc basins inferred from the tectonic evolution of southeast asia since the late cretaceous abstract results of the geological and geophysical surveys in the daito ridges and basin in the northern west philippine basin suggest that the daito ridge was an arc facing toward the south from the late cretaceous to the early tertiary the late cretaceous and tertiary history of southeast asia is evaluated based on these data in the daito ridges and basins and reconstructed based on overall plate kinematics that have operated in this area during the late cretaceous the daito ridge and the east philippine islands were positioned along the boundary between the indian and pacific plates the western half of the philippines setting on the indian plate approached from the south and collided with the east philippine daito arc either during the latest paleocene or the earliest eocene it is inferred that the bulk of the philippine archipelago rotated clockwise and borneo spun counterclockwise during the tertiary from the reconstruction the formation of backarc basins and their spreading direction are assessed as a result some primary causes and significant characteristics are suggested for the opening of backarc basins in southeast asia first opening of some backarc basins commenced with or was triggered by collisions second backarc basins opened approximately parallel to oceanic plate motion third the formation of some backarc basins was triggered by the approach of a hot spreading center fourth the spreading mode or direction of backarc basins was greatly affected by the configuration of the surrounding continent and was also rearranged to spread approximately parallel to oceanic plate motion the formation of backarc basins and their spreading direction can be reasonably explained by plate kinematics however the generative force responsible for their formation is possibly within the subduction system particularly to form horizontal tensional force in backarc side

episodic rifting and subsidence in the south china sea the south china sea experienced at least three stages of rifting and two intervening stages of sea floor spreading since the early cretaceous its evolution can be described by an episodic model of tectonism one of thermal cooling and subsidence pulsed by temporally and spatially confined heating events analysis of regional geologic and geophysical data suggests episodes of rifting and associated thermal activities initiated during the late cretaceous the late eocene and the late early miocene the rift system corresponding to the first episode trends northeast southwest whereas those of the second and third trend east west these two trends coincide with the orientations of the major tectonic lineations within the basin age estimates from heat flow and bathymetric data suggest the oceanic crust in the southwest subbasin is considerably older 55 ma than that in the northwest 35 36 ma or east 32 ma subbasins the episodic tectonic model is supported by basin subsidence analysis subsidence especially subsidence rate curves derived from the regional well data demonstrates that unlike a classic atlantic passive margin thermal subsidence in the south china sea was punctuated by rapid subsidence events which are chronologically consistent with the rifting episodes in terms of hydrocarbon potential the episodes of rifting and drifting would be conducive to the development of overprinted structures and the deposition of several discrete transgressive packages of source rocks and reservoirs separated by widespread unconformities the thermal maturity of sedimentary organic matter affected by episodic rifting and subsidence may be greater than expected on a purely passive margin of equivalent age that had not experienced repeated heating

origin and evolution of the west philippine basin a new interpretation abstract combined analyses of the magnetic lineations and seafloor structures of the west philippine basin show that it was formed by spreading from the central basin spreading center in two distinctly different spreading phases before and after subduction was initiated along the palau kyushu trend at 45 ma b p these episodes are recognized by differences in the strike of the magnetic lineations spreading rates density and strike of fracture zones and basement relief from 60 45 ma b p spreading was ne sw relative to present orientations at a half rate of 44 mm a at about 45 ma b p the spreading direction changed to a more n s direction with a reconfiguration of the central basin spreading center into numerous short e w segments offset by closely spaced n s transform faults spreading slowed to a half rate of 18 mm a and ceased at 35 ma b p thus the west philippine basin originated at 45 ma b p by the trapping of normal ocean crust west of the initial subduction along the palau kyushu trend the 45 35 ma b p period represents the dying phase of spreading on the central basin spreading center following isolation of the west philippine basin from the plate driving forces of the pacific

an introduction to the sangihe arc volcanism accompanying arc arc collision in the molucca sea indonesia in the molucca sea region of northeastern indonesia the sangihe and halmahera arcs are presently in the process of colliding the earth s only example of a collision between facing volcanic arcs the collision event is more advanced in the northern molucca sea where back arc thrusting occurs along the cotobato and philippine trenches and volcanic centers are inactive and dissected this paper describes the sangihe arc the western arc of the collision the sangihe are is approximately 500 km in length extending from the ne tip of sulawesi to mindanao philippines the arc comprises over 25 quaternary volcanic centers the eight presently active volcanoes being confined to the southern half of the arc the active volcanic front lies 100 110 km above the top of the west dipping benioff zone and volcanoes extend up to 70 km behind the front thereby defining a volcanic arc overlying earthquakes 100 to 180 km deep rock types range from basalt to rhyolite andesites are dominant pyroxene andesites are common on sulawesi and at or near the volcanic front from ruang to buhias pahepa and buang to mamanuk hornblende andesites are common at the volcanic front between mahengetang and sangihe island and in centers located well behind the volcanic front tholeiitic th suites defined on the basis of relative degrees of iron enrichment are confined to the southern volcanic front lack hornblende are low k k55 1 05 have low rb zr ratios 0 10 0 17 low ba y ratios 3 4 4 9 low rb cs ratios 8 14 and have the highest 87sr 86sr ratios 0 7039 0 7042 calcalkaline ca suites occur throughout the arc are low to high k k55 0 6 2 0 have variable rb zr ratios 0 15 0 65 and ba y ratios 4 2 12 4 high rb cs ratios 14 82 and low 87sr 86sr ratios 0 7035 0 7040 zr and y distribution coefficients are lower in th suites dzr 0 0 dy 0 2 0 5 than in ca suites dzr 0 0 0 8 dy 0 3 1 2 ti initially increases with increasing sio2 in some th suites while decreasing with sio2 in all ca suites with the exception of bukide ti zr 138 ti zr ratios of basalts exceed 100 only in th suites 76 130 lil element contents and lil hfs element ratios increase 2 to 3 times across the are away from the volcanic front and 2 to 3 times from south to north along the arc 87sr 86sr ratios decrease away from the volcanic front the south to north increase in lil element contents and lil hfs element ratios without corresponding changes in sr isotopic compositions are interpreted as resulting from decreasing degrees of partial melting northward this reflects the gradual cessation of volcanism in the north due to collision

partial melting of subducting oceanic crust the conditions under which partial melting of subducting oceanic crust occurs can be determined by combining a partial melting model for basaltic compositions with two dimensional thermal models of subduction zones for porosities of 1 containing h20 the amount of partial melt generated at the wet basaltic solidus is limited to 100 mpa can be maintained by rocks close to or above their melting temperatures in the absence of high shear stresses substantial melting of the oceanic crust will only occur during subduction of very young 5 ma oceanic lithosphere partial melting of hydrated basalt amphibolites derived from the mid ocean ridge has been proposed e g 1 3 as being responsible for the generation of certain recent high al andesitic to dacitic volcanic rocks adakites three of these volcanic suites mount st helens southern chile and panama occur in volcanic arcs where oceanic crust 25 ma is being subducted at rates of 1 3 cm yr and the calculated thermal regime is several hundreds of degrees hotter than more typical subduction zone environments however oceanic lithosphere is not currently being subducted beneath baja and new guinea where recent adakites are also present suggesting that some adakite magmas may form by water undersaturated partial melting of underplated mafic lower crust or previously subducted oceanic crust further experimental work on compositions representative of oceanic crust is required to define the depth of possible adakite source regions more accurately

tertiary and quaternary magmatism in mindanao andleyte philippines geochronology geochemistry and tectonic setting abstract a set of 230 igneous rock samples from leyte and mindanao philippines has beenstudied for their major and trace element chemistry mineralogy and geochronology 40k 40ar dating several volcanic sectors have been distinguished according to their geographic position tectonic framework and geologic history a in leyte the ophiolitic basement at 50 ma is morb type but characterized by negative nb 0 anomalies typical of back arc basalts miocene 20 and 11 ma and late pliocene to quaternary magmas 2 5 0 4 show typical island are talc alkaline characteristics in eastern mindanao eocene 47 46 ma magmatic rocks are arc tholeiitic while those of miocene 18 6 ma and early pliocene 4 5 ma age are generally talc alkaline dacites of 12 ma age have adakitic compositions the young 3 ma volcanics in surigao and north davao are mostly adakitic although talc alkaline basalts and basaltic andesites dated at 0 9 ma overlie adakites in surigao b in daguma oligocene 30 ma arc tholeiitic diorites intrude older arc sequences miocene0 17 0 7 7 ma dikes and flows in both daguma and sarangani peninsula are mostly talc alkaline andesites although minor adakites dated back to 18 ma have been found overlying undated breccias in daguma in southeast daguma the tale alkaline volcanic substratum of pliocene recent volcanoes is 8 6 ma old quaternary 1 83 ma to present andesitec and dacitic volcanics in mt parker and mt matutum southwest daguma are either talc alkaline or adakitic while mt blit northwest daguma is built up by 2 5 ma old hfse enriched basalts 1 ma old normal talc alkaline andesites and 0 8 ma old adakites c in central mindanao miocene andesites yield ages of 20 and 16 ma pliocene 2 5 ma 0 volcanism is dominated by talc alkaline and hfse enriched basalts and basaltic andesites quaternary lavas i ma to present range in character from tale alkaline to shoshonitic and exhibit wide variations in their trace element behavior d miocene lavas of 19 11 ma age in zamboanga are mostly talc alkaline though a lone 0 18 ma old adakite was also encountered pliocene quaternary basalts and basaltic andesites dated back to 2 2 1 ma are hfse enriched such that some display positive nb anomalies in spidergrams adakites of 3 8 0 7 ma age are located nearer the sulu trench the compositional variety of tertiary recent island arc magmatism in leyte and mindanao couldbe linked to the tectonic settings prevailing during each magmatic phase and is here utilized to refine paleokinematic reconstruction of the tectonic history of the philippine archipelago the identification of adakites at certain time periods is used to date timings of subduction initiations and arc polarity reversals major and abrupt changes in geochemical compositions are well exhibited by the pliocene pleistocene volcanism involving the collision related shift from talc alkaline to shoshonitic volcanism in central mindanao and adakite production in all the volcanic sectors defined the origin of the hfse enrichments among lavas younger than 3 ma could be attributed to the presence in their mantle source of an oib component or alternatively to the enrichment of this mantle source through percolation of slab derived melts

the mechanisms of petrogenesis of the archaean continental crust comparison with modern processes the petrographic and chemical composition of magmatic rocks generated during the archaean appears to be different from that of post archaean rocks komatiites are widespread before 2 5 ga and rarely occur afterwards in addition the archaean continental crust is primarily ttg tonalitic trondhjemitic and granodioritic in composition exhibiting typical trondhjemitic differentiation trends whereas modern equivalents are granodioritic to granitic following classical calc alkaline differentiation trends this distinction becomes more prominent when rare earth elements ree are taken into account archaean ttg are yb poor ybn 8 5 and have high layb ratios 5 layb n 150 in comparison the post 2 5 ga granitoids emplaced in subduction zone geodynamic environments have high yb content 4 5 ybn 20 with very low layb n ratios 20 theoretical calculations and experimental petrology have shown that the ttg can be produced by partial melting of an archaean tholeiite transformed into garnet bearing amphibolite consequently the low heavy ree content of the ttg is explained by the influence of both residual garnet and hornblende in their source after 2 5 ga the role of these minerals in calc alkaline magma genesis becomes progressively less important which is interpreted in terms of a cooling earth model in modern subduction zone environments the subducted oceanic slab is relatively old and cold and the geothermal gradient along the benioff plane in low ca 10 c km consequently the down going lithosphere undergoes dehydration before partial melting is able to occur the liberated fluids are light ree and lile enriched and ascend into the overlying mantle wedge where they induced partial fusion the produced magmas separate from their mantle source region leaving a residue mainly composed of olivine and pyroxenes mantle derived magmas typically exhibit high yb contents due to low kdyb values for olivine and pyroxenes during the archaean the subducted lithosphere was relatively young and hot providing high geothermal gradients along the benioff zone thus partial melting of the subducted slab was possible at lower temperatures before dehydration would take place garnet and hornblende are the main residual phases accounting for the low yb contents of the archaean ttg this model can be tested using a modern analogue of archaean like subduction processes in south chile an oceanic ridge has subducted and all thermodynamic calculations indicate that this creates locally high geothermal gradients along the benioff zone thus in very small areas archaean like environments may be simulated in modern subduction zones the modern andesites produced in this environment show archaean geochemical characteristics with low ybn 5 whereas the majority of andesites along the andean arc have modern patterns with ybn ranging from 8 to more than 17 this conclusion was generalised to all young subducted lithospheres all over the world in conclusion it appears that since the archaean there has been a change in the site of continental crust genesis the location of calc alkaline magma source in subduction zone environments has migrated through time from the subducted slab to the mantle wedge this is a direct consequence of the progressive cooling of the earth

serpentinization of the acoje massif zambales ophiolite philippines hydrogen and oxygen isotope geochemistry abstract oxygen and hydrogen isotope ratios in serpentine lizardite chrysotile from the acoje chromite mine zambales ophiolite philippines and in regional meteoric water from the zambales and sierra madre mountains have been measured to help determine the physical conditions and source of fluids during serpentinization the measured range of 18 o in serpentine is 2 3 to 5 9 and that of d is 119 to 83 the range of d in regional meteoric water including published analyses from the baguio gold district and the tongonan leyte geothermal system is 75 to 34 the isotopic data for acoje serpentines are consistent with serpentinization by regional meteoric water at temperatures between 30 and 350 c thermal stability limit of serpentine brucite the range of serpentine compositions suggests a serpentinization process involving 1 episodic infiltration by isotopically distinct meteoric waters and or 2 progressive infiltration by a water having constant initial isotopic composition that undergoes isotopic fractionation as a function of reaction progress

geology of the boyongan and bayugo porphyry cu au deposits an emerging porphyry district in northeast mindanao philippines the boyongan and bayugo porphyry cu au mineral deposits discovered under postmineralization cover during the previous decade are part of an emerging belt of intrusion centered au rich cu mineral deposits and prospects in the surigao district of northeast mindanao philippines since their formation in the early pleistocene exhumation and weathering of these deposits have led to the development of a 600 m thick oxidation profile at boyongan and a modest 30 70 m oxidation profile at bayugo debris flows volcanic material and fluviolacustrine sediments accumulating in the actively extending mainit graben subsequently covered the weathered deposits preserving their supergene profiles the mineral deposits formed in association with a composite diorite complex containing at least 12 discrete intrusive stages three premineralization diorite porphyry stocks and a silt sand matrix breccia complex represent early stages of magmatism and brecciation significant cu and au introduction followed these events and occurred in association with small early mineralization diorite porphyry stocks at boyongan and bayugo within the diorite complex the two mineral deposits are spatially distinct separated by approximately 1 km of premineralization diorites inter and late mineralization intrusions were emplaced as the magmatic hydrothermal system waned a characteristic progression of vein and k silicate alteration styles affected each of the synmineralization intrusions rare comb quartz unidirectional solidification textures stage 0 mark the transition from magmatic to hydrothermal conditions quartz poor wispy magnetite biotite k feldspar veinlets characterize stage 1 stage 2 veins consist of quartz with selvage and or disseminated magnetite or biotite and k feldspar halos stage 3 quartz veins have k feldspar halos but generally lack magnetite and biotite stage 4 veins consist of massive bornite chalcopyrite and chalcopyrite pyrite with k feldspar halos stage 3 quartz veins and stage 4 sulfide veins host cu au mineralization of the greatest volumetric significance reflecting the general paucity of sulfide in the earlier vein stages despite the simplicity of this sequence detailed paragenetic reconstructions reveal that this characteristic progression of veining and k silicate alteration was repeated with the emplacement of each synmineralization intrusive event revealing multiple magmatic hydrothermal cycles of alteration and mineralization at boyongan and bayugo intense and pervasive illite alteration in association with pyrite chalcopyrite and tetrahedrite tennantite developed in narrow structures crosscutting quartz k feldspar veins debris flows in the burial sequence above and adjacent to the boyongan bayugo complex also contain abundant clasts with intense pervasive illite and alunite pyrophyllite dickite kaolinite alteration assemblages that have overprinted k silicate style quartz veins in both mineral deposits cu and au are associated with intense quartz vein stockworks composed primarily of k silicate stage 3 veins despite this association not all of these quartz vein stockworks contain cu and au to the same tenor quartz vein stockworks in the eastern high grade of boyongan have been intersected over a vertical interval of 800 m having affected much of the early mineralization stock however hypogene cu grades exceed 0 5 by weight only in the upper 300 m of the stockwork in the cupola of the early mineralization stock superior grade development in high grade zones at boyongan locally exceeding 2 cu and 3 g t au developed where fertile vein stages from two or more magmatic hydrothermal cycles affected the same wall rock the documented paragenetic relationships demonstrate multiple discrete cycles of k silicate stage veining and alteration associated with each synmineralization intrusive event all such events predate formation of feldspar destructive illite smectite chlorite illite pyrite and quartz alunite clay assemblages existing geochronological constraints on the timing of magmatism and hydrothermal activity demonstrate that these repeated cycles supporting superior grade development transpired extremely rapidly in a period of less than 200 000 years 2 3 2 1 ma geologic and geochronological constraints on the life cycle of boyongan and bayugo describe an extremely dynamic history of emplacement exhumation weathering and burial over a period of 2 3 m y the study illustrates the spectrum of metallogenic processes operative over a geologically brief period and highlights some of the key elements responsible for formation of superior grades and for deposit preservation in an extensional setting

deformation and mantle flow beneath the sangihe subduction zone from seismic anisotropy abstract subduction of oceanic lithosphere is the most direct feedback between the earth s surface and deep interior however the detail of its interaction with the broader convecting mantle is still unclear mantle flow around subduction zones can be constrained using seismic anisotropy but despite many such studies a simple global picture is lacking the sangihe subduction zone where the molucca sea microplate is subducting westward beneath the eurasian plate is part of the tectonically complex sulawesi philippine region and an ideal natural laboratory to study complex subduction processes we investigate the anisotropic structure of the sangihe subduction zone with shear wave splitting measurements of local s and sks phases at two stations mni in sulawesi dav in the philippines as well as downgoing s phases at five stations at teleseismic distances combining different phases allows a better vertical resolution of anisotropic fabrics than is possible with a single phase the broad depth distribution of local events 60 630 km allows us to observe a change in splitting behaviour at 380 km depth above fast directions are trench parallel and delay times t are 0 34 0 53 s with no increase with depth we suggest this anisotropy is caused by aligned cracks possibly melt filled beneath the volcanic arc and fossil anisotropy in the overriding plate below 380 km is predominantly trench normal and t are slightly higher 0 53 0 65 s as no correlation is observed with inferred distance travelled inside the slab we attribute this anisotropy to shear layers atop the slab which are coherent from 200 to 400 km depth and perhaps extend into the transition zone sks and source side measurements show larger t 1 53 and 1 33 s respectively and trench parallel since these phases predominantly sample sub slab mantle we consider along strike lateral flow associated with the double sided subduction of the molucca sea microplate to be the most likely explanation we thus infer three dominant regions of anisotropy at the sangihe subduction zone one within the overriding lithosphere one along the slab wedge interface and one below the subducting molucca sea slab the mantle wedge above 200 km depth and the slab itself do not seem to contribute notably to the measured anisotropy this study demonstrates the insight seismic anisotropy can provide into mantle dynamics even in tectonically complex subduction systems

collision propagation in papua new guinea and the solomon sea the mountain system in papua new guinea png is growing by accretion of australian margin strata to the front of the papuan fold and thrust belt in the south and by accretion of exotic terranes along its ne margin presently the finisterre terrane is being accreted along the ne margin of png and the collision point is migrating eastward within the solomon sea east of the collision point the new britain trench marks the tectonic front between the solomon sea and south bismarck sea plates whereas to the west the tectonic front lies in the southern part of the papuan fold and thrust belt present day sedimentary environments within the western solomon sea are inferred from interpretation of seismic reflection data similar facies are observed accreted to the front of the finisterre terrane in the markham valley the frontal leron formation is largely fluvial and shallow marine and the erap complex contains elements of turbidites and pelagic deposits there appears to be a smaller volume of coarse proximal marine strata accreted to the front of the finisterre terrane than is expected from interpretation of reflection profiles in the western solomon sea this difference may be attributed to a combination of submarine erosion in the markham canyon partial incorporation of these deposits into the erap complex and a difference in depositional rates and products carried to the solomon sea in the past relative to what is observed today based on plate motions alternative collision geometries and the youngest dates available for the erap complex we determine a rate of progression of the collision point of about 212 km m y during the past 1 m y

identification of an early cretaceous ophiolite in the camarines norte calaguas islands basement complex eastern luzon philippines abstract new geological studies of the camarines norte calaguas islands cnci basement complex have resulted in the identification of a pre late cretaceous ophiolite assemblage underlying much of the eastern part of the complex dominant ophiolite lithologies include harzburgites gabbros diabasic and basaltic dikes and pillow lavas a stratigraphically complete igneous section is lacking due to extensive structural disruption of the ophiolite by westward directed thrusting and displacement along high angle faults despite this disruption geochemical la yb n 1 la ta 15 and mineralogical data as well as field relationships support a cogenetic n morb origin for the basalt and gabbro units a minimum crustal age of 100 ma is inferred from recent 40 ar 39 ar analyses of amphibolites associated with the gabbros other lithologies occurring within the basement complex include medium to high grade garnet hornblende mica schists low grade metaigneous rocks and dioritic and granodioritic intrusives geological and geochemical data on these units indicate that they are not genetically related to the ophiolite but have been in close spatial proximity since the middle miocene together these studies raise new questions concerning the nature of philippine basement complexes and extent of fundamental pre tertiary basement in the eastern philippines

ancient subcontinental mantle a source for k rich orogenic volcanics active island arc volcanoes west of the arc continent collision between the sunda arc and northwest australia are erupting mafic volcanics that range from calc alkaline basalts through shoshonitic trachybasalts to leucitites these mafic arc volcanics display a threefold enrichment in k rb sr ba la and nb contents along the arc from bali to flores increasing toward the collision zone the enrichment is very well correlated with increasing 87sr 86sr and decreasing 143nd 144nd values the geochemical and isotopic data imply that the arc volcanics are being generated from two source materials one yielding material poor in k group elements with relatively low 87sr 86sr 0 707 values and low 143nd 144nd values the k rich material appearing in the arc volcanoes is being derived from ancient subcontinental mantle involved in the collision zone eastern sunda k rich mafic volcanism first appeared after the collision began before the collision ancient northwest australian mantle erupted k rich diamond bearing ultramafics with high 87sr 86sr values and low 143nd 144nd values these form part of a typical ultrapotassic continental volcanic association yet they have ba la and ba nb values characteristic of arc volcanism showing their parent mantle to be geochemically and isotopically capable of supplying the k rich material appearing in the eastern sunda arc volcanoes terranes possibly underlain by similar gondwanaland mantles surround the collision zone which is the site of large scale mixing between oceanic and continental lithospheres other petrogenetic theories seem incapable of accounting for the regional variations in eastern sunda arc geochemistry striking geochemical and isotopic resemblances between eastern sunda leucitites and k rich orogenic volcanics elsewhere in the world such as those of the roman region imply similar origins involving k rich subcontinental mantle material where a k h correlation occurs it might only be a signal that subduction of a lithospheric slab beneath a continental margin is inducing melting in old mantle deep in the continental lithosphere

mineralization parameters and exploration targeting for gold copper deposits in the baguio luzon and pacific cordillera mindanao mineral districts philippines a review abstract the baguio mineral district in luzon philippines is known to host several world class epithermal gold porphyry copper deposits the interplay of tectonic setting magma composition structural control and hydrothermal systems contributed to the generation of these deposits ridge subduction scarborough seamount resulting to flat subduction and a transpressional regime could also be related to the formation of epithermal gold porphyry copper deposits in baguio subduction processes leading to the formation of calc alkaline rocks associated with high water pressure oxygen fugacity and late sulfur saturation are almost always associated with the gold copper deposits in the district compared to the baguio mineral district less exploration work mine development and production were done in the pacific cordillera mineral district mindanao in southern philippines it is worth noting however that both mineral districts show similarities and overlapping features in terms of geological geophysical and geochemical characteristics this leads one to conclude that the pacific cordillera mineral district has ore deposits waiting to be discovered

k ar dating young rhyolitic rocks a case study of the sierra la primavera jalisco mexico this study demonstrates the feasibility of dating by the k ar method an eruptive sequence of silicic volcanic rocks with ages close to the limits of conventional 14 c dating forty six conventional k ar analyses of sanidine and glass from rhyolites of the sierra la primavera mexico yielded dates of 5 to 150 ka thousand years one sigma errors range from 3 to 39 ka although the samples contain small amounts of extraneous argon most of the dates are consistent with relative ages determined in the field the exceptions are dates on immediately postcaldera eruptive units which are older than the dates for the caldera producing ash flow tuff a similar result has been obtained on dating tuffs and lavas of some other pleistocene calderas this effect is attributed to incomplete outgassing of the larger amount of extraneous argon dissolved in magma that occupied a position deeper in the chamber than the ash flow tuff magma and that erupted immediately after caldera collapse we present two new error equations one a simple extension of the commonly used equation demonstrates the importance of including a term for the uncertainty in the spectrometer mass discrimination factor in the error equation the second equation is a more rigorous formulation of the propagation of errors in the calculation of the age it yields meaningful error estimates for all proportions of radiogenic 40 ar and because it expresses the error in absolute rather than relative terms it can be used to put an upper limit on the age of a sample when no radiogenic 40 ar has been resolved

petrogenesis and tectonic setting of the roman volcanic province italy abstract the volcanism in the roman province of italy can be modelled by the partial melting of heterogeneously enriched mantle sources the heterogeneity was created by materials derived from a subducted slab which can still be traced geophysically beneath the central apennines new petrographical and chemical data are presented for the high k calc alkaline and the shoshonitic volcanics of the campania region primary magmas are present only locally the existence of spatial zonation in the volcanism of campania is documented for the first time the shoshonitic leucite basanitic and leucititic volcanics of the phlegraean fields procida ischia and the somma vesuvius areas are at similar degrees of evolution about two times richer in nb and ba than those of northwestern campania and the latium part of the roman province accordingly distinct north western and south eastern subprovinces can be defined the evaluation of enrichment factors that is the abundance ratio between the average contents of each element in the relatively primitive lavas of the low k and the high k suites shows that the mantle sources prior to k metasomatism were different in the two sub provinces of the roman province in the north western one they resembled the sources of ocean island tholeiites and moderately enriched mor basalts in the south east they were similar to those of ocean island alkaline lavas and enriched morb s modelling based on k p ce sr rb ba th sm eu gd y nb and 87 sr 86 sr was carried out it indicates that the range of mantle sources of the volcanics in northwestern campania and latium can be modelled by the addition of 3 to 20 of materials derived by partial melting of carbonaceous pelites to a sr enriched mantle wedge comparable to the honolulu mantle source least enriched in nb the production of sr enriched mantle wedge requires either the action of fluids produced by dehydration of subducted oceanic crust or a small amount of metasomatism caused by the presence of carbonatite melts the near absence of ti ta nb yb and the highly fractionated ree in the metasomatizing component requires the presence of residual garnet and accessory ti rich minerals during the partial melting of the subducted sediments the writers propose that the mantle wedge overlying the subducted slab was hybridized by melts produced by partial fusion of subducted material derived from the continental crust probably sediments this process played a dominant role in the generation of the mantle sources from which the high k calc alkaline shoshonitic leucite basanitic and leucititic magmas of the roman province were derived

genesis of collision volcanism in eastern anatolia turkey late cenozoic volcanism in eastern anatolia extends in a broad sw ne trending belt across the arabia eurasia collision zone from the arabian foreland basin in the southwest and to the kars plateau and lesser caucasus in the northeast foreland volcanism is dominated by basaltic shield and fissure eruptions of transitional tholeiitic alkaline composition volcanism on the thickened crust north of the bitlis thrust zone varies from the mildly alkaline volcano nemrut and older mus volcanics in the south through the transitional calc alkaline alkaline volcanoes bingol and suphan and the alkaline volcano tendurek to the calc alkaline volcano ararat and older kars plateau volcanics in the north isotope sr nd and trace element systematics indicate that the lavas from the foreland were derived from the mantle lithosphere of the arabian continent which had been enriched by small volumes of asthenospheric melts over a period of time and that lavas from the alkaline volcanic area around mus and the volcanoes nemrut and tendurek north of the bitlis thrust zone were derived from a lithospheric source of similar composition either from the same underthrust arabian continent or from the bitlis massif microcontinent by contrast the transitional lavas from bingol and suphan and the calc alkaline lavas from ararat and kars were derived from lithosphere carrying a distinct subduction signature inherited from pre collision subduction events positive correlations between 87sr 86sr and sio2 and rb nb and sio2 in the alkaline and transitional lavas suggest that combined assimilation and fractional crystallization was an important process within at least part of the thickened crust of the collision zone trace element covariation diagrams such as y rb indicate the importance of hornblende crystallization at depth and orthopyroxene at shallow levels within the calc alkaline provinces in contrast to the consistently anhydrous crystallization sequences of the alkaline lavas trace element diagrams based on the covariation of compatible and incompatible elements point to moderate low degrees of partial melting with residual clinopyroxene throughout and residual garnet in the foreland province consideration of mineral stabilities mantle solidi and geothermal gradients before and after collision suggest that lithospheric thickening should both increase the thickness of metasomatized lithosphere and depress the metasomatized zone to greater depths probably beneath the amphibole and dolomite breakdown curves perturbation of the thickened lithosphere by delamination of the thermal boundary layer perhaps coupled with local stretching associated with pull apart basins on strike slip fault systems is then sufficient to generate melt the composition of that melt being largely dependent on the enrichment history of the lithosphere in question in eastern anatolia volcanism appears to have started between about 8 and 6 ma ago some 5 ma after the start of rapid uplift of the east anatolian plateau

adakites the key to understanding lile depletion in granulites abstract for more than 20 years it has been argued that granulites of the lower continental crust are depleted in fluid mobile elements such as cs rb u and th either because they were removed by melting of the lower crust or through dehydration we argue that there is little evidence for a simple relationship between granulite facies metamorphism and element depletion and propose that for felsic orthogneisses the depletion may be a primary feature of crust generation processes modern adakites show a range of fluid mobile element ratios and adakites from the austral volcanic zone avz chile show fluid mobile element ratios very similar to those found in lower crustal felsic granulites this suite of adakites has a strong slab derived component we propose that modern adakites of the type found in the austral volcanic zone are analogous in their genesis to lower crustal felsic granulites and that their mafic precursors were already depleted in fluid mobile trace elements prior to melting an incremental melting model in which fluid mobile elements are released from an eclogitic slab prior to partial melting can explain the fluid mobile element behaviour of both modern adakites and lower crustal felsic granulites

mount st helens potential example of the partial melting of the subducted lithosphere in a volcanic arc mount st helens 50 km to the west of mount adams and the main cascade volcanic chain is only 80 km above the subducting oceanic lithosphere the elevated temperatures off the subducting slab because of the close proximity of the juan de fuca ridge to the trench may induce slab melting at a depth of 80 km dacites from mount st helens have geochemical compositions off magmas that are derived by direct partial melting of metamorphosed basalts at high pressure i e relatively high ai al2o3 15 at 70 sio2 low y and yb because of garnet and amphibole stability in the source low sc and high sr and eu trace element modeling of the partial melting of mid oceanic ridge basalt morb from the juan de fuca ridge that yields a hornblende eclogite residue can reproduce the mount st helens data results off the model are quite distinct from data derived from the mount adams volcanic rocks in contrast mount adams is 135 km above the subducting slab and is associated with normal arc magmatism believed to be derived from the mantle above the subducting plate the cascade are has been active in its present locality because of oblique subduction for the past 7 m y the major volcanoes along the arc have existed for at least 500 ka but mount st helens has existed for 40 ka we suggest that the subducting plate may have reached elevated temperatures because of the approach of north america to the juan de fuca ridge at 40 ka which initiated melting of the slab

benham rise unveiled morphology and structure of an eocene large igneous province in the west philippine basin abstract the benham rise is an oceanic large igneous province at the western margin of the philippine sea it has an ocean island basalt geochemistry a surface area of 0 11 mkm2 and a volume of 0 13 mkm3 a crustal thickness of 15 km is estimated from gravity anomalies volcanism was most active in the early stages of its formation during the lutetian from 47 9 ma to 41 3 ma although volcanic activity extended to 26 ma multibeam bathymetry with almost 100 coverage has revealed the morphology of benham rise for the first time it consists of a main body 310 km by 330 km with the narra loro and molave spurs extending 100 200 km from its eastern side the main body is built on a shield platform from 5200 m to 3800 m sub sea the platform flanks consist of 3 15 km wide terraces with scarps as high as 100 m to 300 m the platform is surmounted by a crest exhibiting caldera morphology at an average depth of 2500 m the crest is named apolaki caldera and may be the world s largest known caldera with a diameter of 150 km features like a breached rim intra caldera benches and a resurgent dome indicate a multi phase volcanic history consisting of both quiet and explosive eruptions the bathymetry and publicly available geological and geophysical data record three main stages in the tectonic history of benham rise shield building caldera formation and post caldera late stage volcanism the platform base is interpreted to have formed over a hotspot beneath the central basin spreading center during the shield building phase between 47 9 ma to 42 5 ma the arrowhead shaped molave spur is interpreted to have formed late in this stage along a ridge that propagated away from the hotspot the caldera formation followed and is interpreted to have occurred before 41 3 41 5 ma when benham rise was still joined with urdaneta plateau the narra spur was formed by late stage hotspot related volcanism as benham rise and urdaneta plateau separated and ended with the eruptions of the vinogradov seamount at 26 ma

accreted terranes in the northern part of the philippine archipelago the philippine archipelago consists of a complex array of ophiolites continental fragments and island arc elements that can be identified as accreted terranes similar to those increasingly recognized as components of orogenic zones the northern philippines including luzon mindoro and nearby smaller islands can be divided into at least six of these tectonic elements from west to east across central luzon there are the neogene accretionary prism of the west luzon arc the eocene zambales ophiolite the cretaceous angat ophiolite and a late cretaceous early paleogene volcanic arc built on an older metamorphic basement this arc terrane is repeated by offset along the philippine fault to the south the western luzon terranes are juxtaposed against the metamorphic basement block of mindoro which itself is colliding with the north palawan microcontinental fragment the ophiolites appear to have originated in back arc basins rather than in oceanic plateaus and the metamorphic belts have protoliths indicating deformation along active margins perhaps deep within accretionary prisms the philippine terranes are interpreted as fragments that originated within the complex pacific eurasian plate boundary rather than as intra pacific basin units these terranes have been assembling at least since the oligocene by strike slip and convergent displacements strike slip displacement either along transcurrent faults or as a component of convergence played a very important if not dominant role in this assembly most terranes identified in the northern philippine appear to be plate fragments with attached crust and upper mantle their characteristics contrast with those usually interpreted for terranes in the north american cordillera where near orthogonal convergence and shallow dipping terrane sutures are favored

low potassium hydrothermal alteration in low sulfidation epithermal systems as detected by irs and xrd an example from the co o mine eastern mindanao philippines abstract detailed hydrothermal alteration investigations including petrography infrared reflectance spectroscopy irs and xrd of the low sulfidation epithermal co o mine located in eastern mindanao philippines revealed that both distal and intermediate hydrothermal alteration zones contain dominantly illite and chlorite whereas the proximal alteration zone comprises mainly illite chalcopyrite and pyrite the gold bearing veins and the proximal hydrothermal alteration zone display a distinct absence of k rich hydrothermal alteration minerals such as k feldspar adularia gold mineralization in the co o mine is controlled by an extensive quartz breccia vein system which is characterized by three distinct stages of vein incl breccias formation gold is mainly observed in stages 2 and 3 veins stage 1 veins appear as fragments in stage 2 veins and display boiling textures such as quartz pseudomophs after bladed calcite these veins further display colloform to crustiform banding and contain pyrite chalcopyrite and minor gold located in the colloform bands and between bladed quartz pseudomorphs stage 2 veins comprise mostly banded to massive quartz and contains sulfides parallel to bands or disseminated these veins are fine grained with mosaic jigsaw quartz and contain calcite blebs and or fragments of stage 1 veins gold is in textural equilibrium with chalcopyrite sphalerite and locally pyrite stage 3 veins consist of quartz and carbonate locally mn rich and display irregular banded and comb textures in auriferous veins of this stage gold is in textural equilibrium with chalcopyrite and pyrite with local abundance of sphalerite other sulfide minerals observed with gold in stages 2 and 3 are galena acanthite and locally jalpaite the xrd and irs provide inconsistent results regarding the abundance of k rich clays e g illite associated with auriferous veins illite with possibly interlayered swelling clays such as al smectite was identified in auriferous vein stages 2 and 3 using irs but could not be confirmed by xrd comparative analysis of the results of these techniques with respect to the ordering of micaceous minerals suggest less ordered white mica proximal to the veins vein textures such as banded quartz the absence of k feldspar and the abundance of illite interlayered al smectite suggest relatively low temperatures of formation of the hydrothermal alteration system and point to a potential boiling horizon located deeper or marginal to the currently exploited levels of the co o mine the absence of k feldspar may also be related to relatively low temperatures of the hydrothermal fluid the medium potassium rich magma series of the host rocks and or a relatively low oxidation state of the hydrothermal fluid

volcano growth versus deformation by strike slip faults morphometric characterization through analogue modelling abstract volcanoes display diverse morphologies as a result of the complex interplay of several constructive and destructive processes scaled analogue models offer a tool to isolate and characterize the different processes here we investigate the interplay between volcano growth and deformation caused by an underlying strike slip fault through simple analogue models in particular we analyze the morphometry of analogue volcanoes resulting from different growth to deformation ratios deformation elongates the volcano edifice at an angle of 10 45 from the fault trace along the extensional quadrants and generates a summit graben structure oriented perpendicular to the edifice elongation the overall steepness of the edifice decreases but steep slopes are preserved or increased on the lower compressional flanks commonly related to small avalanches growth can partially to totally mask these features depending on the growth rate to strike slip velocity ratio the summit graben is easily masked even by low growth rates whereas edifice elongation has the best preservation potential scaling of the experiments suggests that at volcanoes with growth rate km3 yr to strike slip velocity km yr ratios 3 8 km3 km deformation features should be clearly preserved whereas at volcanoes with ratios 15 km3 km deformation features should be completely masked the typical growth rates of volcanoes 0 01 to 1 km3 ka and the typical velocities of strike slip faults 1 to 20 mm yr suggest that in nature growth rate to strike slip velocity ratios can range over 3 orders of magnitude spanning both types of end members using examples of both active and inactive volcanoes located on strike slip faults with variable elongation intensities and orientations we highlight that the analogue models account for some of the morphometric variability observed at volcanoes in nature although the role of vent distribution can be a key factor

paleogeographic reconstruction and origin of the philippine sea abstract our reconstruction of the philippine sea suggests that it formed by two distinct episodes of back arc spreading each of which resulted from seaward retreat of the trench in the first episode the protoizu bonin trench retreated northward and the west philippine basin formed behind the northern half of the palau kyushu ridge in the second episode the izu mariana trench retreated eastward and the shikoku and parece vela basins formed behind it during the last 17 ma the philippine sea basin has been moving northwestward with respect to eurasia shifting the ttt triple junction off central japan westward by about 50 km the motion of the philippine sea with respect to eurasia at the triple junction changed from north northwestward to west northwestward 10 5 ma ago for the period before 17 ma ago we construct two models retreating trench model and anchored slab model the izu bonin trench migrated from south to northeast rotating in a clock wise sense since 48 ma ago in the retreating trench model in the anchored slab model the trench has been fixed with respect to eurasian margin since 43 ma ago we prefer the retreating trench model because the deformation of the plate boundary along the eastern margin of eurasia during 30 17 ma ago is much simpler for this model than for the anchored slab model furthermore rotations of the bonin mariana islands are consistent with those predicted from the retreating trench model the 48 ma ages of the northern part of the palau kyushu ridge and of chichi jima of the bonin islands indicate that there was subduction beneath the northern half of the ridge beginning at least 48 ma ago from this and the subparallelism in trend between the northern part of the palau kyushu ridge and the central basin ridge we propose that the major part of the west philippine basin formed by back arc spreading in a n s direction behind the northern part of the palau kyushu ridge the pacific plate was moving northward with respect to hot spots from 48 to 43 ma ago which implies that the pacific plate is not likely to have been subducting beneath the west philippine basin during this time we speculate that another plate existed south of the pacific plate and thai it was subducting beneath both tile west philippine basin and australia the annihilation of this plate might he a cause for the sudden change of the pacific plate motion at 43 ma ago