geochemistry of tertiary igneous rocks of northern luzon philippines evidence for a back arc setting for alkalic porphyry copper gold deposits and a case for slab roll back oligocene to early miocene volcanic rocks are preserved in the central cordillera range and cagayan valley of northern luzon philippines basaltic and andesitic rocks of the pugo formation in the baguio district of the central cordillera were intruded by the 27 to 20 ma calc alkaline central cordillera diorite complex in the southern cagayan valley the subalkaline to alkaline late oligocene mamparang formation overlies the cretaceous caraballo formation and has been intruded by the didipio igneous complex the cordon syenite complex and the palali batholith the didipio complex comprises an early suite of diorites which were intruded by the strongly mineralized stocks of the dinkidi cu au porphyry deposit whole rock geochemical data for intrusive and extrusive rocks of the baguio district range from low k calc alkaline to shoshonitic basalts to dacites with rare earth element ree and high field strength element hfse characteristics of suprasubduction zone magmas and are all interpreted to have been sourced from the same parent melt samples from didipio display higher alkali contents but similar trace element characteristics new age dates for the didipio area range from 25 7 to 24 8 ma the potassic magmas of the cagayan valley are interpreted to have formed in a back arc coeval to the mainarc sequence that is preserved in the baguio miocene rocks this contradicts earlier models which invoke an early miocene arc reversal in the northern luzon archipelago with the switch from early westward subduction to later eastward subduction attributed to a variety of causes the lack of a single compelling trigger for arc reversal combined with the coeval emplacement of arc magmas in the west and back arc magmas in the east in northern luzon is best interpreted as the result of eastward subduction since the late oligocene the presence of 20 ma adakitic magmas in the baguio district may indicate that flattening of the downgoing slab resulted in a hiatus in magmatism and termination of back arc rifting

physical models of volcanic eruptions abstract our present understanding of the dynamics of volcanic eruptions is summarized a few important facts about volcanic eruptions are reviewed and outstanding problems are identified current research efforts are aimed at explaining why an eruption may alternate between and pass through different dynamical regimes the basic physics of eruptions are explained from a dynamical point of view with emphasis on the processes acting to fix the gas content of the erupted material the coupling between flow processes and pressure release leads to large changes of decompression rate during ascent in a volcanic conduit the decompression rate may reach very high values which suggests that the kinetics of degassing play an important role accurate values of the solubility of the major magmatic volatile species are required at pressures of

dynamics of magma flow in volcanic conduits with variable fragmentation efficiency and nonequilibrium pumice degassing numerical models of volcanic conduit dynamics assume that the foamy structure of magma is totally destroyed at fragmentation neglecting the presence of vesicular fragments in the above gas particle flow region the abundance of pumice among the volcanic pyroclasts is however the outstanding evidence that such a complete atomization assumption is an oversimplification the present one dimensional multiphase nonequilibrium magma ascent modeling rejects such an assumption taking into account the presence of pumice ash and loose crystals in the turbulent flow region above fragmentation a parametric study is conducted on the role of pumice abundance related to a fragmentation efficiency parameter in affecting the magma ascent dynamics additionally different hypotheses on pumice degassing behavior are evaluated the numerical results show that the presence of even relatively small amounts of pumice translates into a large decrease of continuous gas volume fraction while other flow quantities are mainly affected by the efficiency of pumice degassing a decreased degassing efficiency translates into significant velocity and pressure decrease mixture density increase and further gas volume fraction decrease that overall result in a decrease of the mechanical energy content of the erupted magma by an amount comparable to that due to water content decrease by several weight percent postprocessing of numerical results based on the dynamics of porous flow of gas in pumice allows one to relate the efficiency of pumice degassing to a measurable pumice texture parameter namely the degree of gas bubble coalescence which is therefore identified as a major factor affecting the ascent dynamics of silicic magma

viscous heating in silicate melts an experimental and numerical comparison 1 the transition from newtonian to non newtonian flow of silicate melts is commonly manifested as shear thinning at conditions of high stress and strain rate shear thinning may strongly influence the dynamics of magmatic flows but the details of its microscopic origins are not fully understood here we consider viscous heating and thermomechanical coupling as a potential cause of shear thinning we compare the results of laboratory uniaxial compression experiments of a silicate melt with the results of thermomechanical numerical simulations corresponding to the experimental setup both the experimental and numerical results concord and indicate that the reduction of the temperature dependent viscosity in flowing silicate melts is a result of viscous heating viscous heating was quantified for glasses with viscosities ranging from 108 to 1011 pa s and strain rates from 10 5 to 100 s 1 the results of 48 compression experiments indicate that the transition from newtonian to non newtonian flow in the silicate melt occurs at a brinkmann number i e ratio of heat gained to heat lost around 1 whereas brittle behavior dominates the melt deformation when the deborah number i e ratio of viscoelastic relaxation time to characteristic deformation time is larger than around 0 01 the observed viscous heating significantly contributes to the viscosity decrease observed in high stress strain rate experiments and questions our current understanding of the non newtonian deformation behavior of silicate melts

textural heterogeneities in pumices from the climactic eruption of mount pinatubo 15 june 1991 and implications for magma ascent dynamics the climactic event of mount pinatubo represents one of the most thoroughly studied eruptions of the century and has provided important insights into the dynamics of explosive volcanism we have performed detailed textural analyses of the white and gray pumices of the plinian and pyroclastic flow deposits and found that differences in color and clast density reflect different crystal and vesicle amounts and size distributions white pumice has higher vesicularity deformed and highly coalesced vesicles with thin walls euhedral phenocrysts and microlite free groundmass gray pumice shows lower vesicularity wider ranges in vesicle number density limited coalescence vesicles with thick walls that are less deformed phenocrysts and microphenocrysts with abundant solution pitting and groundmass containing ubiquitous microlites and crystal fragments the presence of white and gray pumice varieties and the broad range in vesicularity and vesicle number density that characterizes both of them appear to record the complexities of conduit processes such as magma vesiculation and fragmentation and the development of conduit regions marked by different rheological behaviors in particular the results of this study suggest the likely importance of intense shear and viscous dissipation at the conduit walls a mechanism that may be responsible for the creation and discharge of the gray pumice of this eruption along with the dominant white variety

in situ elemental and sr isotope characteristics of magmatic to hydrothermal minerals from the black mountain porphyry deposit baguio district philippines at the black mountain porphyry cu au deposit in the baguio district northern luzon philippines pre and synmineralized rocks preserve magmatic and hydrothermal minerals e g plagioclase amphibole titanite and epidote spanning the complete paragenesis of the deposit strontium isotope values in early crystallized plagioclase phenocrysts from all felsic porphyries can be divided into two types the type i plagioclase crystals show relatively homogeneous sr isotope values 0 7035 0 7038 1 0 0001 indicating crystallization from a relatively stable and long lived felsic magma chamber the type ii plagioclase grains have a wider range of sr isotope compositions 0 7032 0 7039 1 0 0001 indicating mafic and or felsic magma recharge in magmatic titanite nb ta values are higher than those in the whole rock while zr hf and y ho values are lower in hydrothermal titanite and epidote the ratios are similar to those in the whole rock these patterns reflect crystallization effects imposed during the magmatic stage but an absence of differentiation during the hydrothermal stage the consistent gradual decrease in total rare earth element y zr and u contents in both hydrothermal titanite and epidote from early to late stages indicates the effect of hydrothermal fluid evolution with decreasing temperature the variation of 87sr 86sr values in magmatic amphibole plagioclase and hydrothermal epidote in felsic and mafic rocks indicates the addition of mafic magma derived fluid into the felsic magma derived fluid one extra source of fluid probably derived from wall rock limestone was required to generate the highly radiogenic 87sr 86sr values of some epidote 0 7038 0 7053 thus in situ elemental and sr isotope variation in minerals from different paragenetic stages can be used to interpret formation process and source for both magmas and hydrothermal fluids

rheology of bubble bearing magmas the rheology of bubble bearing suspensions is investigated through a series of three dimensional boundary integral calculations in which the effects of bubble deformation volume fraction and shear rate are considered the behaviour of bubbles in viscous flows is characterized by the capillary number ca the ratio of viscous shear stresses that promote deformation to surface tension stresses that resist bubble deformation estimates of ca in natural lava flows are highly variable reflecting variations in shear rate and melt viscosity in the low capillary number limit e g in carbonatite flows bubbles remain spherical and may contribute greater shear stress to the suspension than in high capillary number flows in which bubble deformation is significant at higher ca deformed bubbles become aligned in the direction of flow and as a result contribute less shear stress to the suspension calculations indicate that the effective shear viscosity of bubbly suspensions at least for ca 0 5 is a weakly increasing function of volume fraction and that suspensions of bubbles are shear thinning field observations and qualitative arguments however suggest that for sufficiently large ca ca greater than about 1 the effective shear viscosity may be less than that of the suspending liquid bubbles reach their quasi steady deformed shapes after strains of order one for shorter times the continuous deformation of the bubbles results in continual changes of rheological properties in particular for small strains the effective shear viscosity of the suspension may be less than that of the liquid phase even for small ca results of this study may help explain previous experimental theoretical and field based observations regarding the effects of bubbles on flow rheology

calibration of shell δ18o from the common whelk buccinum undatum highlights potential for palaeoenvironmental reconstruction abstract the common whelk buccinum undatum is a commercially important gastropod found throughout the north atlantic one method of age and life history analysis for gastropod species is the use of oxygen isotope ratio 18o measurements from their shells which is a well established technique for the reconstruction of historical seawater temperatures at the time of shell biomineralization palaeotemperature calibrations have been developed for different types of calcium carbonate as well as species specific equations to produce the most accurate seawater temperature reconstructions here we investigate the four layer internal structure of b undatum shells and confirm an aragonite composition using micro raman spectroscopy mrs we then calibrate a species specific palaeotemperature equation for this gastropod species this was achieved through the isotopic analysis of shells from laboratory reared specimens of known provenance reared at specific seawater temperatures to produce the following t c 14 96 0 15 4 94 0 22 x 18oshell 18owater the calibrated equation differs significantly from previously published data derived from both aragonite and calcite an offset of 1 04 0 41 was discovered between observed 18oshell values and those expected under equilibrium suggesting a species specific vital effect the calibrated equation was used to reconstruct accurate high resolution historical seawater temperatures from three sites across the u k shetland the menai strait and jersey with this new accurate calibration both modern and fossil b undatum shells now have the potential to be employed as high resolution archives of recent and historical seawater temperature

along strike segmentation of the abanico basin central chile new chronological geochemical and structural constraints the andes of central chile are composed mostly of cenozoic volcanic rocks erupted during the opening and subsequent inversion of the intra arc volcano tectonic abanico basin until recently the internal segmentation of this inverted basin was poorly understood based on a combination of u pb geochronology u th he thermochronology whole rock geochemistry and structural data we propose that the abanico basin can be divided into two main segments separated by the nw striking piuquencillo fault and conjugate ne striking faults stratigraphic units defined in the northern segment abanico and farellones formations cannot be correlated with the rocks of the southern segment coya machali formation and teniente volcanic complex in terms of lithofacies depositional ages and geochemistry the northern and southern segments also show temporal differences in their tectonic evolution an early deformation event beginning at 22 ma affected only the northern segment and is associated with the formation of progressive unconformities between the abanico and farellones formations and also with crustal thickening as reflected in the geochemistry of the farellones formation a second stage of crustal thickening and exhumation began at 12 ma as suggested by a sharp increase of the la yb ratios in the northern segment in the southern segment this event is reflected by only a moderate increase of la yb ratios and by the transition between the coya machali formation and the teniente volcanic complex finally a third stage of exhumation and crustal thickening beginning at 7 ma affected both the northern and southern segments this last stage was the main exhumation event affecting the rocks of the andes of central chile and is recorded in the geochemistry of igneous rocks by a sharp increase in the la yb ratios in the southern segment

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

thermal vesiculation during volcanic eruptions a textural examination of volcanic ash erupted from santiaguito volcano in guatemala coupled with an analysis of the geophysical signals indicates that rapid heating during fault friction can cause melting and vesiculation development of bubbles of hydrated silicic magma thus strongly affecting magma strength and eruptive behaviour

fracture healing and strength recovery in magmatic liquids cycles of fracture and healing in magma are important controls on outgassing time scales and repetitive seismicity at silicic volcanoes here we experimentally drove silicate melts at 109 1011 pa s to tensile failure measuring the strength during fracture of the otherwise liquid material we then took the same melts with parallel contact surfaces and closed the fracture under compressive stress and recorded the evolution of tensile strength of the interface healed for different times we provide a semi empirical model for fracture healing time scales useful for volcanic applications as the time available for healing is increased strength nonlinearly recovers toward that of unfractured glass we parameterized the healing kinetics as a three stage process 1 relaxation of the compressive stress 2 fracture surface surface wetting and 3 diffusive removal of the interface during welding of these surfaces in air we observed that micropores are trapped along the fracture plane which may inhibit complete healing and provide a textural record of relict fracture planes in volcanic glasses we conclude that at magmatic conditions fracture healing is efficient in crystal poor melts and it could rapidly seal outgassing pathways over eruptive time scales contributing to cyclic behavior associated with recurring gas and ash explosions and outgassing events

bubble deformation in magma under transient flow conditions abstract we investigate the transient behavior of bubble deformation in magma previous studies have used the relationships between capillary number and equilibrium shape for a single bubble under either steady simple shear or steady pure shear to estimate strain rates in pyroclasts and to calculate bubble shapes in a conduit flow we developed a deformation model for a bubble by adjusting the empirical functions used in an existing model for a droplet to fit experimental data for bubble deformation the modified model was used to calculate transient large deformations of a single bubble in arbitrary shear flows in this paper we demonstrate the significance of the modified model through two different applications firstly using this model we are able to simulate the entire history of the evolution of bubble shapes in a time dependent flow using the computational code conflow in this simulation we further include the effect of gas expansion on the bubble shape from this simulation we show that bubble deformation in conduit flow is controlled by the history of both capillary number and strain secondly by using this model to analyze bubble shapes observed in natural magma samples we are able to better estimate the strain rates and deformation durations we apply this model to certain transient features in the relationship between bubble shape and radius in the published data of obsidian clasts from newberry volcano and estimate a deformation duration of 2 7 4 0 min

formation of tube pumice structure under pure shear insights from extension tests of solidifying foam abstract tube pumice the bubbles of which are highly elongated in one direction is a common product of explosive silicic eruptions especially caldera forming eruptions to understand the conditions necessary to generate tube like bubbles we performed extensional experiments on polyurethane foam which experiences vesiculation deformation and solidification at room temperature the strain and strain rate of pure shear deformation were controlled by the pull length and pull rate of the extension respectively the shapes of bubbles in the solidified polyurethane foam were analyzed in three dimensions by x ray computed tomography the experimental results demonstrate that strain and capillary number control the degree of bubble deformation highly deformed tube like bubbles were found in the experiments with large strain and large capillary number in each experimental sample there is scatter in the relation between the degrees of bubble deformation and expected capillary numbers which we interpret to be the result of the inhomogeneous shear field around individual bubbles and bubble coalescence despite the large scatter the average deformation degree is predicted well by the dynamics of a single bubble where the viscous stress acting on the bubble surface is taken as the product of bulk strain rate and effective viscosity using this approach the average deformation degree of experimental bubbles in each sample was approximately calculated from the deformation model of a single bubble at least in the range of pure shear strain smaller than 1 5 small bubbles in the samples extended at low viscosities tend to be less elongated than those calculated from the deformation model due to shape relaxation the effect of shape relaxation is quantified by a new dimensionless number i e the quench number which represents the ratio of the timescale of increasing viscosity and shape relaxation our experiment suggests that bubble shapes in highly vesicular pumice can be used to analyze their viscosity and strain rate during the elongation process an analysis of the published data was presented as a test case

petrologic constraints on the decompression history of magma prior to vulcanian explosions at the soufriere hills volcano montserrat abstract a series of 88 vulcanian explosions occurred at the soufriere hills volcano montserrat between august and october 1997 conduit conditions conducive to creating these and other vulcanian explosions were explored via analysis of eruptive products and one dimensional numerical modeling of magma ascent through a cylindrical conduit the number densities and textures of plagioclase microlites were documented for twenty three samples from the events the natural samples all show very high number densities of microlites and 50 by number of microlites have areas 2 pre explosion conduit conditions and decompression history have been inferred from these data by comparison with experimental decompressions of similar groundmass compositions our comparisons suggest quench pressures 13 75 mpa each during ascent from chamber to surface values are consistent with field studies of the same events and statistical analysis of explosion time series data the microlite volume number density trend with depth reveals an apparent transition from growth dominated crystallization to nucleation dominated crystallization at pressures of 7 mpa and lower a concurrent sharp increase in bulk density marks the onset of significant open system degassing apparently due to a large increase in system permeability above 70 vesicularity this open system degassing results in a dense plug which eventually seals the conduit and forms conditions favorable to vulcanian explosions the corresponding inferred depth of overpressure at 250 700 m near the base of the dense plug is consistent with depth to center of pressure estimated from deformation measurements here we also illustrate that one dimensional models representing ascent of a degassing crystal rich magma are broadly consistent with conduit profiles constructed via our petrologic analysis the comparison between models and petrologic data suggests that the dense conduit plug forms as a result of high overpressure and open system degassing through conduit walls

large scale porphyry type mineralization in the central asian metallogenic domain a review abstract a series of porphyry type cu mo au deposits including some giant world class deposits such as kounrad aktogai kal makyr oyu tolgoi and chalukou occur in the central asian metallogenic domain camd the outline of the camd almost overlaps with that of the central asian orogenic belt caob which is located between the east europe siberia tarim and north china cratons the camd comprises sparsely exposed tectonic slices of precambrian high grade metamorphic basement rocks that are sandwiched between phanerozoic accretionary complexes and arc rocks according to geological metallogenic geochronological and tectonic setting characteristics the camd comprises three metallogenic provinces 1 the kazakhstan cu au mo 2 the mongolia cu au and 3 the northeast china mo cu metallogenic provinces early paleozoic 490 to 440 ma and late paleozoic 330 to 295 ma large scale porphyry type mineralization in the kazakhstan metallogenic province is intimately associated with the evolution of the central tianshan yili northern tianshan kokchetav chingiz paleozoic arc system which was terminated by the late permian closure of the western segment of the paleo asian ocean in contrast the 370 ma and 240 ma porphyry type mineralization of the mongolia metallogenic province is related to the tuva mongolia arc collage system that was formed during the evolution of the paleo asian ocean in the paleozoic and the mongol okhotsk ocean in the early mesozoic furthermore the northeast china metallogenic province was affected by three distinct geodynamic processes that were associated with the paleo asian mongol okhotsk and paleo pacific oceans during the paleozoic 250 ma the early mesozoic 200 ma and the late mesozoic 200 90 ma respectively except for the early paleozoic cu mineralization at duobaoshan 480 ma and bainaimiao 440 ma the porphyry type mineralization of this metallogenic province is predominantly characterized by widespread mesozoic mo mineralization in general porphyry type deposits of the camd have geological structural metallogenic alteration and fluid characteristics which are consistent either with those related to the oceanic subductional arc settings of the circum pacific metallogenic domain or with those of the continental collisional environment in the tethyan metallogenic domain the involvement of highly oxidized intermediate to felsic magmas is fundamental for the formation of porphyry deposits even though the prevalent mash and slab melting models can be employed to interpret the genesis of some porphyry cu deposits in the camd a here newly suggested model which is based on pre enriched mafic lower crust and the subduction of a relictic mid oceanic ridge is thought to explain the large scale copper mineralization in the balkhash west junggar metallogenic belt of the kazakhstan metallogenic province nevertheless most of the porphyritic suites associated with the mo deposits in the northeast china metallogenic province have relatively depleted mantle like neodymium and hafnium isotopic compositions suggesting that the ore forming granitic magma may have predominantly been derived from juvenile lower crust upwelling of asthenosphere may have triggered the melting of the lower crust during post collisional extension subsequent to the closure of the paleo asian ocean in the triassic the subduction of the paleo pacific ocean in the jurassic and the thinning of the lithosphere caused by rollback of the paleo pacific plate in the cretaceous ancient crustal components were only locally found to have been involved in the magma source of several medium large mo deposits

a new insight on the geometry of subducting slabs in northern luzon philippines abstract we used hypocentral and focal mechanism data in order to characterize the tectonic configuration of northern luzon and propose a model for describing the geometry of the subducted slab of the eurasian plate beneath the northern segment of the manila trench we took into consideration some of the observed bathymetric features i e the bend in the trench line of the manila trench at 20 n lat and the collision and subduction of an extinct mid oceanic ridge at 16 17 n lat and the intraplate deformation pattern in the north luzon ridge region in depicting a model that is consistent with most of the observed features and phenomena in the study area an earthquake catalog covering the period from 1619 to 1997 was used in studying the distribution of strain energy released while earthquakes covering the instrumental years 1963 1997 were used to analyze vertical distributions of earthquakes in sections we refined the location of some of the historical earthquakes using macroseismic data the inclusion of historical records despite their inaccurate locations could prevent misidentification of some regions as aseismic zones or gaps in seismicity especially along structures with long return periods a focal mechanism database that covers the period from 1963 to 1997 was used to decipher intraplate deformation pattern and subduction process of the slab the focal mechanism database consists of first motion solutions from previous authors and cmt solutions from harvard university seismicity strain energy release and focal mechanism maps and cross sections were drawn in order to have a three dimensional visualization of the geometry and stress regimes of seismogenic zones a new model of the subducted slab of the eurasian plate beneath the manila trench is proposed the model suggests the collision and subsequent partial subduction of a buoyant plateau at around 20 n lat to explain the sharp bend in the trench line the complicated deformation pattern on the overriding plate fronting the bend and the shallow dip of the subducted slab beneath this zone a tear in the slab is also inferred to be present as evidenced by the observed gap in strain energy release and the abrupt change in dip from shallow to steep south of 18 n lat the gap in seismicity and strain energy release 65 300 km depth at around 17 n lat may be used to infer the trajectory and location of the subducted extinct mid oceanic ridge mor the aseismic behavior is probably caused by the subducted ridge which is still hot and is deforming plastically this is supported by the heatflow data which shows high values along the extinct mor the subducted part of the mor may serve as the weakest zone where this tear could be localized the tear may also explain the cause of the abrupt termination of the eastern chain of volcanoes south of 18 n lat the above model which is a refinement of the model introduced by yang et al tectonophysics 258 1996 85 is consistent with the observed seismicity and deformation pattern observed bathymetric features spatial distribution and geochemical character of volcanism in northern luzon

plate kinematics origin and tectonic emplacement of supra subduction ophiolites in se asia abstract a unique feature of the circum pacific orogenic belts is the occurrence of ophiolitic bodies of various sizes most of which display petrological and geochemical characteristics typical of supra subduction zone oceanic crust in se asia a majority of the ophiolites appear to have originated at convergent margins and specifically in backarc or island arc settings which evolved either along the edge of the sunda eurasia and australian cratons or within the philippine sea plate these ophiolites were later accreted to continental margins during the tertiary because of fast relative plate velocities tectonic regimes at the active margins of these three plates also changed rapidly strain partitioning associated with oblique convergence caused arc trench systems to move further away from the locus of their accretion we distinguish relatively autochthonous ophiolites resulting from the shortening of marginal basins such as the present day south china sea or the coral sea and highly displaced ophiolites developed in oblique convergent margins where they were dismantled transported and locally severely sheared during final docking in peri cratonic mobile belts i e the philippine mobile belt we find a series of oceanic basins which have been slightly deformed and uplifted varying lithologies and geochemical compositions of tectonic units in these basins as well as their age discrepancies suggest important displacements along major wrench faults we have used plate tectonic reconstructions to restore the former backarc basins and island arcs characterized by known petro geochemical data to their original location and their former tectonic settings some of the ophiolites occurring in front of the sunda plate represent supra subduction zone basins formed along the australian craton margin during the mesozoic the philippine sea basin the huatung basin south of taiwan and composite ophiolitic basements of the philippines and halmahera may represent remnants of such marginal basins the portion of the philippine sea plate carrying the taiwan philippine arc and its composite ophiolitic continental crustal basement might have actually originated in a different setting closer to that of the papua new guinea ophiolite and then have been displaced rapidly as a result of shearing associated with fast oblique convergence

regional geochemistry of tertiary igneous rocks in central chile implications for the geodynamic environment of giant porphyry copper and epithermal gold mineralization the giant porphyry copper molybdenum deposits of central chile formed within a thick sequence of cretaceous to pliocene volcanic rocks the predominantly calc alkaline basaltic andesites of the cretaceous las chilcas formation are characterized by la smn ratios of 1 8 to 2 5 and sm ybn of 1 8 to 2 8 the upper oligocene to lower miocene abanico formation variously defined as the los pelambres abanico or coya machali formations ranges from basalt to rhyolite in composition and exhibits a broad southward transition from calc alkaline to tholeiitic all samples from this formation are characterized by lree enrichment and moderately or locally strongly fractionated hree la smn 1 3 4 1 sm ybn 1 5 5 8 the basaltic andesites and andesites of the middle miocene salamanca formation have ree chemistry similar to that of the upper cretaceous strata la smn 1 5 2 7 sm ybn 1 6 2 7 the overlying middle miocene farellones formation ranges from tholeiitic to calc alkaline and from basalt to andesitic and has a similar lree enrichment and fractionated hree la smn 1 7 2 5 sm ybn 1 7 3 3 the pliocene la copa rhyolite complex however is strongly lree enriched and hree depleted la smn 3 8 3 9 sm ybn 4 2 4 7 overall the trace element geochemistry of the cretaceous to middle miocene volcanic rocks is characterized by enriched lree and negative nb anomalies consistent with an arc setting with only minor differences in the abundance of most elements crustal thickening during the miocene in central chile has been suggested to have been responsible for a transition from an amphibole to garnet dominated residual mineralogy resulting in the release of fluids that enabled the formation of giant copper porphyry deposits however the gradual increase in la yb through the early middle and late miocene reported in earlier studies and interpreted to be a response to crustal thickening is not observed in the regional data instead a rapid change in the geochemical signature between the end of the eruption of the farellones formation and the eruption of the high la yb la copa rhyolite complex implies a more abrupt change in the tectonic environment isotopic data broadly support this although lower epsilon nd values in the farellones formation imply a greater role for crustal contamination in younger suites in the absence of gradual crustal thickening it is suggested that the subduction of the juan fernandez ridge may have been the key geodynamic process responsible for the genesis of the three middle miocene to lower pliocene giant porphyry copper deposits in central chile possibly by promoting crustal scale faulting and even acting as a source of metals

evidence for magmatic hydrothermal fluids and ore forming processes in epithermal and porphyry deposits of the baguio district philippines the baguio district contains a diverse array of epithermal porphyry and skarn deposits together with a large broadly strata bound advanced argillic lithocap magmatism mineralization and alteration occurred in response to subduction of the south china sea plate and the scarborough ridge beneath northern luzon over the past 3 m y rapid uplift and exhumation resulted in epithermal veins overprinting several porphyry cu au deposits most of the epithermal au ag deposits of the baguio district including antamok and acupan the two largest au deposits are intermediate sulfidation state quartz carbonate adularia illite base metal sulfide veins that contain electrum and minor au ag tellurides in contrast high sulfidation mineralization at kelly includes enargite tennantite electrum and precious metal tellurides and is associated with advanced argillic alteration although the mineralizing fluids that formed the porphyry and epithermal deposits had distinct temperatures and salinities stable and radiogenic data provide evidence for direct magmatic contributions into each deposit type the epithermal mineralizing fluids were dilute generally 600 c hypersaline brines 30 to 70 wt nacl equiv and low density vapor sulfur isotope compositions of sulfides in the porphyry skarn and intermediate sulfidation epithermal veins of the southern and central baguio district are mostly between 1 and 6 per mil consistent with a predominance of h2s in the mineralizing fluids i e reducing conditions in contrast sulfides from the high sulfidation porphyry and intermediate sulfidation deposits located adjacent to the baguio lithocap mostly have negative sulfur isotope values 6 9 to 0 8 consistent with oxidizing so42 predominant mineralizing fluids intermediate sulfidation epithermal veins at acupan have crosscut a well mineralized porphyry cu au stock work at ampucao the two deposits cannot be distinguished on the basis of radiometric age determinations ampucao 0 51 0 26 ma acupan 0 65 0 07 ma and are interpreted to be cogenetic with telescoping of the two environments caused by the rapid uplift and exhumation associated with ridge subduction measured 34ssulfide 1 1 to 6 6 34ssulfate 10 4 to 31 8 values and initial strontium ratios of anhydrite 0 70378 0 70385 are consistent with identical and predominantly magmatic sources of these components for the ampucao porphyry and acupan epithermal veins helium isotopes provide further evidence of mantle derived components in the epithermal veins r ra values of 6 0 and 6 7 oxygen deuterium and carbon isotopes provide evidence for predominantly magmatic water at ampucao and for hybrid magmatic meteoric waters at acupan that precipitated precious metals due to boiling the proportion of magmatic water relative to meteoric water and precious metal grades both decreased with time during epithermal vein formation at acupan the common observation of cross cutting relationships between porphyry and epithermal veins observed throughout the baguio district imply that the evolution of porphyry style to intermediate sulfidation style mineralization was a common phenomenon in this region and contributed significantly to its rich metal endowment

thermal and dynamical regimes of single and two phase magmatic flow in dikes finite element calculations of magma flow in dikelike channels with length to width ratios of 1000 1 or more have been used to investigate the coupling between thermal and dynamical regimes due to temperature dependent viscosity and dissipation steady state solutions with realistic thermal and dynamical parameter values have been obtained the models show that the onset of solidification on the boundaries of a basaltic or andesitic dike as predicted by idealized laminar flow models can be prevented or significantly delayed by a small amount of transverse flow induced by rising bubbles boundary roughness or turbulence this effect will reduce the critical initial widths of dikes estimated by bruce and huppert 1989 in the absence of transverse flow the bulk temperature of the magma may actually increase slightly with distance along the dike as a result of viscous dissipation even while solidification is occurring on the walls of the dike with converging or necking dike walls it is found that boundary temperatures fall to a minimum and then increase with distance along a dike even if viscous heating is neglected for viscous heating to offset significant rates of heat loss 2 kw m2 in a plane parallel 1 m wide basaltic dike an average flow velocity of 2 7 m s driven by a pressure gradient of 1 7 mpa km is required a 7 void fraction caused by exsolution of volatiles or chamber gas in the magma will produce this pressure gradient the ease of producing such a gradient by reducing the density of the magmatic column with addition of a gas phase makes it likely that flows of basaltic magma could be maintained in dikes tens of kilometers long furthermore a gas phase may be important for the propagation of the fracture prior to the initial injection of magma during dike emplacement rapid transport of more silicic magmas through dikes is inhibited by the requirement of large driving pressure gradients exceeding several hundred megapascals per kilometer however the pressure gradient can be substantially reduced if the magma is a heterogenous mixture of a predominantly silicic component with a more mafic component pipe flow experiments involving molten polymers that exhibit dynamical similarity to magmas strongly suggest that unmixing occurs when a two component magma mixture in which the components have different viscosities rises within a dike within a few dike widths of the inlet the less viscous mafic component encapsulates the more viscous silicic part effectively lubricating the passage of the more viscous component compositional variations in the obsidian dome volcano support the occurrence of this pressure reduction or self lubrication mechanism such a self lubrication process often may be necessary to permit very viscous magmas to reach earth s surface if so chemical and lithologic zoning would be anticipated as a common feature of near surface intrusions or lava flows that are characterized by a high silica and or high crystal content such as at deadman dome in long valley california

dynamics and energetics of bubble growth in magmas analytical formulation and numerical modeling we have developed a model of diffusive and decompressive growth of a bubble in a finite region of melt which accounts for the energetics of volatile degassing and melt deformation as well as the interactions between magmatic system parameters such as viscosity volatile concentration and diffusivity on the basis of our formulation we constructed a numerical model of bubble growth in volcanic systems we conducted a parametric study in which a saturated magma is instantaneously decompressed to one bar and the sensitivity of the system to variations in various parameters is examined variations of each of seven parameters over practical ranges of magmatic conditions can change bubble growth rates by 2 4 orders of magnitude our numerical formulation allows determination of the relative importance of each parameter controlling bubble growth for a given or evolving set of magmatic conditions an analysis of the modeling results reveals that the commonly invoked parabolic law for bubble growth dynamics r t1 2 is not applicable to magma degassing at low pressures or high water oversaturation but that a logarithmic relationship r log t is more appropriate during active bubble growth under certain conditions a second aspect of our study involved a constant decompression bubble growth model in which an initially saturated magma was subjected to a constant rate of decompression model results for degassing of initially water saturated rhyolitic magma with a constant decompression rate show that oversaturation at the vent depends on the initial depth of magma ascent on the basis of decompression history explosive eruptions of silicic magmas are expected for magmas rising from chambers deeper than 2 km for ascent rates 1 5 m s 1

a model for calculating the viscosity of natural iron bearing silicate melts over a wide range of temperatures pressures oxygen fugacites and compositions a new general model that takes into account the pressure and redox state effect is presented to calculate melt viscosities of natural fe bearing melts this new model is applicable to melts that span a wide range of temperatures from 733 to 1873 k pressures 0 001 15 kbar h2o content from 0 to 12 3 wt and compositions from ultramafic mafic to silicic melts the accuracy of the model is calculated to be 0 23 log units of viscosity which is within or close to experimental uncertainty the transport properties including glass transition temperature and melt fragility can also be calculated from this model a spreadsheet to calculate the viscosity is provided in an electronic supplement

strain localization in vesicular magma implications for rheology and fragmentation the rheology of two or three phase magmas has been the focus of much interest because it controls magma ascent and eruption behavior research on magma rheology has typically considered homogeneous fl ow here we demonstrate based on natural examples that strain resulting from viscous fl ow preceding explosive fragmentation localizes into shear zones at a microscopic scale strain localization affects the rheological behavior of magmas modifying predictions based on experiments localization leads to high strain rate shear zones where elongated sheared vesicles and shear heating have a weakening effect surrounding regions of relatively low strain rate where subequant vesicles have a strengthening effect thus energy is dissipated more effi ciently into localized bands where strain rate increases through feedback effects and can lead to melt fragmentation

the influence of geometry on the ascent of magma in open fissures during steady eruption the flow conditions emitted mass flux exit velocity and exit pressure depend on the geometry of the conduit in which the eruption occurs this dependence is examined for the onedimensional isothermal ascent of a homogeneous basaltic magma with an aqueous volatile phase and newtonian rheology by fixing the geometry of the feeding fissure the mass flux flowing in steady conditions can be determined at any depth as well the magma pressure and vertical velocity flow behaviour is analysed for three fissure shapes constant width slowly upward narrowing and lenticular in all the cases examined the magma arrives at the earth s surface with a pressure greater than atmospheric the results are compared with those obtained when a lithostatic pressure gradient is assumed for the magma column some speculations are made moreover about the change in eruption style if conduit geometry varies during a non steady phase

trace element geochemistry of enargite in the mankayan district philippines we report the first laser ablation inductively coupled plasma mass spectroscopy la icp ms study of trace element substitution in enargite results indicate significant variability in the composition of enargite samples from a single ore system samples come from the mankayan district philippines which hosts the lepanto high sulfidation cu au deposit now mined out and the adjacent far southeast porphyry cu au deposit the genetic relationship between these deposits has been documented in previous studies la icp ms analyses indicate significant incorporation of sb and fe in enargite locally exceeding 1 wt other elements such as bi sn se te ag and zn occur at concentrations exceeding 0 1 wt percent the distribution of selected trace elements in enargite correlates with previously published variations in fluid inclusion homogenization and melting temperatures and gas compositions the spatial distribution of data indicates enargite is enriched in au and te close to the far southeast porphyry enargite is enriched in silver fe and pb in the center of lepanto and zn cd is enrich the ore distal to the far southeast porphyry locally elevated sb contents occur in samples from subsidiary branch vein structures and may indicate that mineralizing fluids along these structures were modified compared to those that formed the main orebody

the influence of viscous and latent heating on crystal rich magma flow in a conduit the flow dynamics of crystal rich high viscosity magma is likely to be strongly influenced by viscous and latent heat release viscous heating is observed to play an important role in the dynamics of fluids with temperature dependent viscosities the growth of microlite crystals and the accompanying release of latent heat should play a similar role in raising fluid temperatures earlier models of viscous heating in magmas have shown the potential for unstable thermal runaway flow as described by a gruntfest number using an arrhenius temperature dependence for the viscosity but have not considered crystal growth or latent heating we present a theoretical model for magma flow in an axisymmetric conduit and consider both heating effects using finite element method techniques we consider a constant mass flux in a 1 d infinitesimal conduit segment with isothermal and adiabatic boundary conditions and newtonian and non newtonian magma flow properties we find that the growth of crystals acts to stabilize the flow field and make the magma less likely to experience a thermal runaway the additional heating influences crystal growth and can counteract supercooling from degassing induced crystallization and drive the residual melt composition back towards the liquidus temperature we illustrate the models with results generated using parameters appropriate for the andesite lava dome forming eruption at soufriere hills volcano montserrat these results emphasize the radial variability of the magma both viscous and latent heating effects are shown to be capable of playing a significant role in the eruption dynamics of soufriere hills volcano latent heating is a factor in the top two kilometres of the conduit and may be responsible for relatively short term days transients viscous heating is less restricted spatially but because thermal runaway requires periods of hundreds of days to be achieved the process is likely to be interrupted our models show that thermal evolution of the conduit walls could lead to an increase in the effective diameter of flow and an increase in flux at constant magma pressure

the controlling effect of viscous dissipation on magma flow in silicic conduits nearly all volcanic conduit models assume that flow is newtonian and isothermal such models predict that during high flux silicic eruptions gradients in pressure with depth increase upward as magma accelerates and becomes more viscous leading to extremely low pressure and fragmentation at a depth of kilometers below the surface in this paper i show that shear heating also known as viscous dissipation dramatically reduces the pressure gradient required for flow and concentrates shear in narrow zones along the conduit margin the reduction in friction may eliminate the zone of low pressure predicted by isothermal models and move the fragmentation level up to the surface

the plinian phase of the campanian ignimbrite eruption phlegrean fields italy evidence from density measurements and textural characterization of pumice textural characterization of pumice clasts from explosive volcanic eruptions provides constraints on magmatic processes through the quantification of crystal and vesicle content size shape vesicle wall thickness and the degree of interconnectivity the plinian fallout deposit directly underlying the campanian ignimbrite ci eruption represents a suitable case to investigate pumice products with different textural characteristics and to link the findings to processes accompanying conduit magma ascent to the crater the deposit consists of a lower lfu and upper ufu pumice lapilli bed generated by the sub steady eruption of trachytic magma with 0 90 with vesicle number density ranging from 107 108 cm 3 the degree of vesicle coalescence is high for all pumice types with interconnected vesicles generally representing more than 90 of the bulk vesicle population the results show a high degree of heterogeneous textures among pumice clasts from both phases of the eruption and within each eruption phase the different pumice types and also within each single pumice type fragment the origin of pumice clasts with different textural characteristics is ascribed to the development of conduit regions marked by different rheological behavior the conclusions of this study are that vesicle deformation degree of coalescence and intense shear at the conduit walls play a major role on the degassing process hence affecting the entire conduit dynamics

physics based models of ground deformation and extrusion rate at effusively erupting volcanoes 1 we present a model of effusive silicic volcanic eruptions which relates magma chamber and conduit physics to time dependent data sets including ground deformation and extrusion rate the model involves a deflating chamber which supplies newtonian magma through a cylindrical conduit solidification is approximated as occurring at fixed depth producing a solid plug that slips along its margins with rate dependent friction changes in tractions acting on the chamber and conduit walls are used to compute surface deformations given appropriate material properties and initial conditions the model predicts the full evolution of an eruption allowing us to examine the dependence of observables on initial chamber volume overpressure and volatile content employing multiple data sets in combination with a physics based model allows for better constraints on these parameters than is possible using kinematic idealizations modeling posteruptive deformation provides an improved constraint on the rate of influx into the magma chamber from deeper sources we compare numerical results to analytical approximations and to data from the 2004 2008 eruption of mount st helens for nominal parameters the balance between magma chamber pressure and frictional resistance of the solid plug controls the evolution of the eruption with little contribution from the fluid magma below the idealized crystallization depth while rate dependent plug friction influences the time dependent evolution of the eruption it has no control on the final chamber pressure or extruded volume

rheological behavior of bubble bearing magmas abstract the vesiculation of the rising magma in a volcanic conduit results in a large change in the viscosity of magma understanding the effect of bubbles on the viscous behavior of bubble melt mixtures over a wide range of bubble volume fractions is essential for accurate modeling of the volcanic processes in this study new equations for the relative viscosity of concentrated bubble bearing suspensions are developed using a differential scheme along with the solution of an infinitely dilute suspension of bubbles out of the four models developed in the paper two models predict the relative viscosity to be a function of only two variables capillary number and volume fraction of bubbles there are no adjustable parameters in these models the remaining two models include an additional parameter that is the maximum packing volume fraction of bubbles the proposed models are evaluated using published numerical and experimental data on the relative viscosity of concentrated suspensions of bubbles the viscosity models for bubble bearing suspensions developed in this study are useful in modeling of the volcanic processes

the trace metal chemistry of deep geothermal water palinpinon geothermal field negros island philippines implications for precious metal deposition in epithermal gold deposits palinpinon geothermal field negros island philippines is a high temperature liquid dominated geothermal system in an active volcanic island arc setting parallels have been drawn between the geologic setting and hydrothermal alteration styles recognized at palinpinon with those that characterize several types of magmatichydrothermal ore deposits e g porphyry skarn and high and intermediate sulfidation epithermal systems the neutral chloride palinpinon reservoir water chemistry is affected by boiling mixing and conductive cooling the occurrence of base and precious metal scale deposits in several geothermal wells demonstrates that the deep reservoir fluid is capable of transporting and depositing base and precious metals however trace metal analysis of the deep reservoir fluids shows they are undersaturated with respect to gold 1 4 ig kg chemical modeling of palinpinon geothermal water predicts that boiling should be the most effective mechanism for base and precious metal deposition producing sulfide assemblages similar to those present in well scale deposits mixing with acid bearing sulfate waters can also produce these sulfide assemblages but less efficiently and with gold and silver deposition occurring at lower temperatures i e 110 c despite indications that modern hydrothermal fluids are capable of metal transportation base and precious metal deposition is not strongly developed at palinpinon base and precious metal concentrations in drill core and drill cuttings are one to two orders of magnitude below ore grade 0 02 wt cu 0 03 wt pb 0 01 wt zn 0 01 wt mo 8 g t ag and 0 05 g t au and important features that characterize high grade ore zones of porphyry i e quartz stockwork veining and epithermal deposits i e residual vuggy quartz massive crustiform veins and or hydrothermal breccia complexes are apparently absent at palinpinon such features intimate that high grade ore deposition is promoted by large volumes of fluid and highly permeable wall rock their absence at palinpinon indicates that permeability fluid flux and or metal transport have been insufficient to form significant zones of base or precious metal enrichment

magma flow and interaction with waste packages in a geologic repository at yucca mountain nevada abstract the likelihood that a nuclear waste repository at yucca mountain could be intersected by igneous activity is very low but its potential consequences are nevertheless important to performance assessments an ongoing critical area of concern is the nature and magnitude of the thermal interaction of magma with tunnel walls high level nuclear waste packages and waste forms previous assessments consider a variety of dynamic scenarios but large uncertainties remain in understanding the rheological nature of the magma likely to be involved and its behavior within a repository drift here we specifically address the issue of magma rheology during degassing cooling and solidification as basaltic magma approaches earth s surface and enters a drift magma containing significant amounts 2 wt of dissolved water wet magma as is anticipated for this region at or near its liquidus temperature and saturated with water at 200 mpa is at a temperature near or below the 1 atm solidus temperature isentropic ascent from this near liquidus temperature promotes extensive solidification and or glassification exsolving water with approach to the surface promotes rapid vesiculation leading to fragmentation and tephra production with continued ascent the still water saturated magma traverses the solidification phase field and undergoes a combination of rapid crystallization and quenching becoming a glassy highly viscous 108 pa s mass of greatly reduced mobility this immobility is reflected in the high effective viscosity regulating flows from nearby cinder cones associated with wet basalt this also matches well with the experimentally determined rheology of dry basalt glass this rheology greatly restricts the mobility of basalt within repository drifts amounting to

rheological tests of polyurethane foam undergoing vesiculation deformation solidification as a magma analogue abstract in this study we examined the rheology of polyurethane foam puf as an analogue system of magma undergoing vesiculation deformation and solidification solid puf was formed by mixing two polymeric liquids followed by chemical reactions of foaming and curing the transient rheology during the processes was investigated using an instrument based on a stress controlled rheometer first the oscillatory rheology was measured applying small amplitude oscillations with cyclic frequency sweeps in the foaming stage the frequency dependence of the rheology depends on the dynamic capillary number in a similar way to the existing bubbly flow model in the curing stage the viscoelasticity of the liquid becomes dominant and the rheology of puf shows maxwell type viscoelasticity with relaxation time exponentially increasing with time when the reaction is rapid because of high temperature the viscoelasticity during curing is not scaled by the single relaxation time instead puf passes through a condition called gelation which is characterized by a power law relaxation spectrum next we examined the relationship between the dynamic viscosity measured by the small strain oscillation and the shear viscosity measured with a fixed shear rate at a large strain we found that puf obeys the cox merz rule which states that the two viscosities are equivalent when the shear rate and the angular frequency are equal specifically the shear viscosity tends to be smaller than the dynamic viscosity by a few tens of percent the difference was shown to be in the range expected for bubbly fluid in the foaming stage while it went beyond what can be explained by the bubbly fluid model when the viscoelasticity of the liquid becomes significant x ray computed tomography analyses revealed that samples in these cases contained large bubbles indicating bubble coalescence we infer that the difference is caused by bubble coalescence as well as non newtonian rheology during solidification including gelation all these processes might occur in actual magma during eruptions we show that the ranges of the capillary number comparing the deformation time and the bubble shape relaxation time and deborah number comparing the deformation time and the viscoelastic relaxation time in our experiments are comparable with their ranges expected for magma ascending in an eruptive conduit the characteristic time of viscosity increase of puf was also shown to be realizable for magma in natural volcanoes we conclude that puf is useful for simulating magma processes in eruptive conduits

repeated crustal thickening and recycling during the andean orogeny in north chile 21 26 s posttectonic granitoids 2 lower crustal p wave velocities of 7 3 7 7 km s 1 compatible with underplated mafic crust and 3 results from recent experimental petrologic work showing garnet stability in mafic mineralogies 12 kbar 40 km crustal thickness analogous to older andean magmatic episodes in north chile newly underplated basaltic crust may account for the remaining deficit in neogene crustal thickening similar evolutionary patterns in geochemistry and initial sr and nd isotopic characteristics of andean 200 ma to present magmatic rocks suggest that the andean orogeny in this region evolved by a combination of processes of repeated arc migration tectonic and magmatic crustal thickening and igneous recycling which was controlled by periodically changing plate convergence rates and obliquity and corresponding changes in the rheologic behavior of the continental crust index terms 1020 geochemistry composition of the crust 1030 geochemistry geochemical cycles 0330 8159 evolution of the earth rheology crust and lithosphere 9360 information related to geographic region south america keywords andean orogeny continental crust recycling adakite ttg crustal thickening

age and geochemistry of host rocks of the cobre panama porphyry cu au deposit central panama implications for the paleogene evolution of the panamanian magmatic arc abstract the cobre panama porphyry cu au deposit located in the petaquilla district of central panama is hosted by a sequence of medium to high k calc alkaline volcanic and sub volcanic rocks new crystallisation ages obtained from a granodiorite petaquilla batholith and associated mineralised diorite to granodiorite porphyry stocks and dikes at cobre panama indicate that the batholith was emplaced as a multi phase intrusion over a period of 4 million years from 32 20 0 76 ma to 28 26 0 61 ma while the porphyritic rocks were emplaced over a 2 million year period from 28 96 0 62 ma to 27 48 0 68 ma both the volcanic to sub volcanic host rocks and intrusive rocks of the cobre panama deposit evolved via fractional crystallisation processes as demonstrated by the major elements e g al2o3 fe2o3 tio2 and mgo displaying negative trends with increasing sio2 the petaquilla intrusive rocks including the diorite granodiorite porphyries and granodiorite batholith are geochemically evolved and appear to have formed from more hydrous magmas than the preceding host volcanic rocks as evidenced by the presence of hornblende phenocrysts higher degrees of large ion lithophile element lile and light rare earth element lree enrichment and heavy rare earth element hree depletion and higher sr y and la yb values however the degree of lree enrichment hree depletion and la yb values are insufficient for the intrusive rocks to be considered as adakites collectively the volcanic and intrusive rocks have lile ree and mobile trace element concentrations similar to enriched miocene age cordilleran arc magmatism found throughout central and western panama both the petaquilla and cordilleran arc magmatic suites are geochemically more evolved than the late cretaceous to eocene chagres bayano arc magmas from northeastern panama as they display higher degrees of lile and lree enrichment the geochemical similarities between the petaquilla and cordilleran arc magmas suggest that evolved calc alkaline arc magmatism may extend to the late eocene at least 10 million years earlier than previously estimated the crystallisation ages for intrusive rocks associated with mineralisation at cobre panama imply that the deposit formed in the early oligocene between a period of late cretaceous to eocene magmatism ca 66 42 ma chagres bayano arc and cordilleran arc magmatism 22 7 ma similarities in the timing of intrusive suite emplacement and the fingerprinting of magmatic fractionation processes between the cobre panama porphyry deposit and the cerro colorado porphyry deposit in western panama ca 5 3 ma suggest that these features provide favourable geodynamic and geochemical prerequisites for the formation of porphyry deposits along the panamanian magmatic arc during the cenozoic

statistical evidence of transitioning open vent activity towards a paroxysmal period at volcan santiaguito guatemala during 2014 2018 abstract long term eruptive activity at the santiaguito lava dome complex guatemala is characterised by the regular occurrence of small to moderate size explosions from the active caliente dome between november 2014 and december 2018 we deployed a seismo acoustic network at the volcano which recorded several changes in the style of eruption including a period of elevated explosive activity in 2016 here we use a new catalogue of explosions to characterise changes in the eruptive regime during the study period we identify four different phases of activity based on changes in the frequency and magnitude of explosions at the two ends of the spectrum of repose times we find pairs of explosions with near identical seismic and acoustic waveforms recorded within 1 10 min of one another and larger explosions with recurrence times on the order of days to weeks the magnitude frequency relationship for explosions at santiaguito is well described by a power law we show that changes in b value between eruptive regimes reflect temporal and spatial changes in rupture mechanisms likely controlled by variable magma properties we also demonstrate that the distribution of inter explosion repose times between and within phases is well represented by a poissonian process the poissonian distribution describing repose times changes between and within phases as the source dynamics evolve we find that changes in source properties restrict the extrapolation of explosive behaviour to within a given eruptive phase limiting the potential for long term assessments of anticipated eruptive behaviour at santiaguito

petrogenetic constraints on magma fertility in the baguio mineral district philippines probing the mineralization potential of the igneous host rocks in the sangilo epithermal deposit abstract in the baguio mineral district bmd porphyry cu au deposition and associated epithermal mineralization are attributed to the highly evolved magmatism during the pliocene it has been well documented that the interaction between silicic crustal melts and primitive mantle melts formed water rich oxidized magmas that resulted to hydrothermal mineralization however there are very few studies on the early to middle miocene calc alkaline magmatism which is considered to be barren of mineralization this magmatic event is represented by phases of the central cordillera diorite complex ccdc which also serve as host rocks to the sangilo epithermal deposit the sangilo quartz carbonate veins in the bmd are hosted by an early miocene hornblende diorite 22 33 0 63 ma intruded by a middle miocene quartz diorite 15 91 0 6 ma which are in turn penetrated by pliocene basaltic andesite dikes the miocene magmatic units with hybrid crust mantle source affinity were formed from varying degrees of interaction within the mash mixing assimilation storage and homogenization zone during the formation of the ccdc the basaltic andesite dikes part of the pliocene mafic dike complex represent direct differentiates of basaltic melts that experienced ponding at the base of the lower crust before ascending to shallow crustal levels based on the assessment of the physico chemical conditions three distinct magmatic events were identified a barren early miocene event a fertile middle miocene event and a fertile pliocene event the middle miocene fertile magmatism is attributed to further development of the mash zone under the luzon arc from the early to middle miocene on the other hand the enhanced fertility during the pliocene is associated with the subduction of the scarborough ridge

the nature of magmatism at palinpinon geothermal field negros island philippines implications for geothermal activity and regional tectonics the palinpinon geothermal field negros island philippines is a high temperature liquid dominated geothermal system in an active island arc volcanic setting this paper presents a regional context for the palinpinon geology discusses the petrogenetic evolution of magmatism in the district and assesses the genetic relationships between intrusion and geothermal circulation the oldest rock formation the lower puhagan volcanic formation middle miocene is part of a volcanic sequence that is traceable throughout the visayas region and is related to subduction of the sulu sea oceanic basin in a southeasterly direction beneath the sulu arc late miocene to early pliocene times mark a period of regional subsidence and marine sedimentation a thick sequence of calcareous sediments okoy formation was deposited during this period magmatism in early pliocene to recent times coincided with commencement of subduction at the negros sulu arc this produced basaltic andesites and andesites belonging to the southern negros and cuernos volcanic formations during this time the puhagan dikes and the nasuji pluton intruded middle miocene late miocene and early late pliocene formations based on radiogenic 40ar 39ar dating of hornblende the puhagan dikes are 4 1 4 2 ma and the nasuji pluton 0 3 0 7 ma this age difference confirms these intrusions are not genetically related the early pliocene age of the puhagan dikes also confirms they are not the heat source for the current geothermal system and that a much younger intrusion is situated beyond drill depths igneous rock formations in southern negros are the products of regional island arc magmatism with medium k calcalkaline basaltic to dacitic compositions their adakitic affinity implies that the melting of subducted oceanic basalt has influenced magmatism in this region considering the regional tectonic history the most likely scenarios for the generation of slab melts are 1 during the middle miocene by the melting of relatively young less than 20 ma oceanic crust 2 during early pliocene times by the initiation of subduction along the negros sulu trench and 3 during late pliocene times by the melting of young less than 10 20 ma oceanic crust the adakitic composition of the magmas at palinpinon has promoted the formation of a porphyry copper style magmatic hydrothermal system that is comparable to mineralised porphyry deposits elsewhere in the philippines

non newtonian flow of bubbly magma in volcanic conduits the dynamics of magma ascent along volcanic conduits toward the earth s surface affects eruptive styles and contributes to volcanic hazard the rheology of ascending magmatic mixtures is known to play a major role on mass flow rate as well as on pressure and exit velocity at the vent even determining effusive versus explosive eruptive behavior in this work we explore the effects of bubble induced non newtonian rheology on the dynamics of magma flow in volcanic conduits we develop a quasi 2 d model of magma ascent that incorporates a rheological constitutive equation describing the strain dependent effect of gas bubbles on the viscosity of the multiphase magma non newtonian magma flow is investigated through a parametric study where the viscosity of the melt and the water content are varied over natural ranges our results show that non newtonian rheology leads to greater exit velocity mass flow and density the pressure distribution along the conduit remains very similar to the newtonian case deviating only at the conduit exit plug like velocity profiles develop approaching the conduit exit when mixture velocity is high and are favored by smaller liquid viscosity since the mass flow rate the density and the velocity of the mixture exiting from the conduit are fundamental for quantifying and assessing the transport and emplacement dynamics neglecting that the non newtonian effect of bubble bearing magmas may result in misinterpretation of the deposit and consequently eruptive behavior

magma mixing origin for the aolunhua porphyry related to mo cu mineralization eastern central asian orogenic belt abstract many cu mo au deposits are considered to be related to adakitic porphyries formed in non arc settings e g in collisional orogenic zones and intra plate environments but their genesis is still under discussion the aolunhua porphyry complex and its related mo cu deposit from the eastern central asian orogenic belt caob provide important insights into this issue the porphyries are characterized by high sr 496 705 ppm and sr y and la yb ratios similar to those of typical adakitic rocks and low i sr ratios 0 7049 0 7052 and positive e nd t 0 5 to 1 4 and e hf t 3 5 to 9 8 values these features along with the occurrence of mafic microgranular enclaves mmes compositional and textural disequilibrium of plagioclase phenocrysts and relatively high mg values 45 52 indicate that they were derived from mixing of felsic magma from partial melting of a juvenile arc type lower crust and mafic magma from a lithospheric mantle previously metasomatized by subduction zone fluids melts high sr y and la yb ratios are indicative of contribution from enriched mantle derived materials with high liles e g sr la which were strengthened by subsequent fractionation of ferromagnesian phases such as pyroxene and hornblende mmes hosted by the ore bearing porphyry have zircon u pb ages of ca 132 ma similar to those of the host rocks the enclaves have elevated mg 56 63 liles e g sr 660 891 ppm lree la n 68 150 la sm n 3 0 4 0 la yb n 12 0 19 6 and ratios of radiogenic isotopes of nd and hf e nd 0 7 to 1 6 e hf 3 3 to 10 9 suggesting that their parental magmas were derived from the metasomatized mantle source the mo cu mineralization was probably related to the high water content high oxygen and sulfur fugacity of hybrid magma formation of the adakitic porphyries and related mo cu deposits of the eastern caob could be related to the early cretaceous lithospheric extension caused by the subduction of the paleo pacific plate and its induced reactivation of juvenile arc type lower crust

thermodynamic and rheological properties of rhyolite and andesite melts the heat capacities of a rhyolite and an andesite glass and liquid have been investigated from relative enthalpy measurements made between 400 and 1800 k for the glass phases the experimental data agree with empirical models of calculation of the heat capacity for the liquid phases the agreement is less good owing to strong interactions between alkali metals and aluminum which are not currently accounted for by empirical heat capacity models the viscosity of both liquids has been measured from the glass transition to 1800 k the temperature dependence of the viscosity is quantitatively related to the configurational heat capacity determined calorimetrically through the configurational entropy theory of relaxation processes for both rhyolite and andesite melts the heat capacity and viscosity do not differ markedly from those obtained by additive modeling from components with mineral compositions

fault textures in volcanic conduits evidence for seismic trigger mechanisms during silicic eruptions it is proposed that fault textures in two dissected rhyolitic conduits in iceland preserve evidence for shallow seismogenic faulting within rising magma during the emplacement of highly viscous lava flows detailed field and petrographic analysis of such textures may shed light on the origin of long period and hybrid volcanic earthquakes at active volcanoes there is evidence at each conduit investigated for multiple seismogenic cycles each of which involved four distinct evolutionary phases in phase 1 shear fracture of unrelaxed magma was triggered by shear stress accumulation during viscous flow forming the angular fracture networks that initiated faulting cycles transient pressure gradients were generated as the fractures opened which led to fluidisation and clastic deposition of fine grained particles that were derived from the fracture walls by abrasion fracture networks then progressively coalesced and rotated during subsequent slip phase 2 developing into cataclasite zones with evidence for multiple localised slip events fluidisation and grain size reduction phase 2 textures closely resemble those formed on seismogenic tectonic faults characterised by friction controlled stick slip behaviour increasing cohesion of cataclasites then led to aseismic distributed ductile deformation phase 3 and generated deformed cataclasite zones which are enriched in metallic oxide microlites and resemble glassy pseudotachylite continued annealing and deformation eventually erased all structures in the cataclasite and formed microlite rich flow bands in obsidian phase 4 overall the mixed brittle ductile textures formed in the magma appear similar to those formed in lower crustal rocks close to the brittle ductile transition with the rheological response mediated by strain rate variations and frictional heating fault processes in highly viscous magma are compared with those elsewhere in the crust and this comparison is used to appraise existing models of volcano seismic activity based on the textures observed it is suggested that patterns of long period and hybrid earthquakes at silicic lava domes reflect friction controlled stick slip movement and eventual healing of fault zones in magma which are an accelerated and smaller scale analogue of tectonic faults

bubbledrive 1 a numerical model of volcanic eruption mechanisms driven by disequilibrium magma degassing abstract we have developed a numerical model for magma dynamics in volcanic conduits that involves conduit flow driven by a detailed treatment of bubble growth the bubbledrive 1 model can be characterized by several unique aspects including treatment of the transient problem specific eruption triggers disequilibrium degassing a full scale bubble growth module and steady state eruption in the case of constant recharge from below the general conditions for the model are homogeneous two phase flow a simple fragmentation threshold pre existing bubbles and parameters corresponding to magma compositions of dacite and rhyolite the advantage of using a full scale bubble growth model in our treatment of conduit flow makes it possible to track the level of oversaturation throughout the conduit in space and time the model accounts for dynamic decompression in which the evolving interaction between decompression rate due to magma ascent and diffusive addition of gas into bubbles due to decreased solubility with decreasing pressure serves as the driving force of magma flow in the conduit the ability of the model to run the transient case enables us to explore specific triggers to initiate an eruption which proceeds with duration and eruption energy depending on conduit geometry and several other factors our numerical study revealed several notable features of volcanic systems that may shed additional light on natural systems each of the model runs was directed at elucidating specific aspects of eruptions and the suite of runs indicated some common features that may play a key role in all silicic eruptions we find that conduit geometry is critical in determining the nature of eruption conduits lacking a magma chamber have smooth and relatively quiet eruption styles conduits that include a magma chamber involve eruptions that start out like those without chambers but then evolve into a long and uniform eruption that ends with a violent explosion as the chamber is emptied the uniform phase is maintained by vesiculation within the chamber and the blast occurs when the fragmentation level descends down into the magma chamber resistance to flow imposed by the narrow conduit above the chamber acts as a buffer stabilizing the eruption during the steady state phase until the chamber is depleted in material discharge and vent velocity are proportional to the conduit chamber depth and are controlled by the initial concentration of dissolved volatiles maximum oversaturation is observed immediately below the fragmentation level of the magma column oversaturation at the vent remains remarkably constant at 1 wt of h2o for all cylindrical conduits with no recharge from below oversaturation at the vent is much more complex for conduits that contain magma chamber at 2 km depth our numerical study indicates that the primary factors that control eruption behavior are magma composition and conduit shape this suggests that future observation programs could provide valuable information by determining sub edifice structure so that better constraints on geometry can be derived

mesozoic porphyry cu au mineralization and associated adakite like magmatism in the philippines insights from the giant atlas deposit the atlas porphyry cu au deposit is located in central cebu island in the philippines with a proven mineral reserve of 1420 mt at 0 45 cu 0 24 g t au 0 018 g t mo and 1 8 g t ag it is associated with the lutopan quartz diorite porphyry stock that was emplaced into the cretaceous volcano sedimentary sequences of the cansi formation two stages of hypogene alteration have been identified at atlas stage i quartz magnetite chalcopyrite pyrite veins associated with potassic and propylitic alteration and stage ii anhydrite pyrite chalcopyrite specularite veins associated with phyllic alteration supergene gypsum veins are also present mineralization is mainly associated with the stage i and stage ii veins the lutopan quartz diorite porphyry yielded a zircon u pb age of 108 5 1 6 ma and is characterized by high sr y 54 69 with the corresponding magma having high oxygen fugacity avg fmq 2 7 avg ce4 ce3 of zircon 439 and h2o concentrations presence of amphibole and biotite phenocrysts and low magmatic temperatures avg ti in zircon thermometry 706 c the relatively low la yb ratios 9 11 listric shaped normalized ree patterns with weak or absent eu anomalies together with zircon ehf t values of 3 4 to 10 0 whole rock isr values of 0 70373 0 70382 end t values of 2 4 3 4 and mg numbers of 49 53 suggest that the lutopan quartz diorite porphyry was likely derived from partial melting of young lower continental crust or interaction between asthenospheric melts and lower crust melts that evolved to adakite like trace element compositions through amphibole fractionation stage i veins yielded a pyrite re os isochron age of 110 2 5 6 ma making atlas the only mesozoic porphyry deposit discovered in the philippines

philippine porphyry and epithermal deposits an introduction the philippines is one of the most richly endowed parts of the southwest pacific with many porphyry copper gold and epithermal gold silver deposits scattered along the archipelago e g sillitoe and gappe 1984 mitchell and leach 1991 fig 1 chinese mining artifacts discovered in the baguio and masbate districts suggest that gold and silver mining dates back to at least the 14th century several placer gold mining regions were active during the spanish regime 1521 to 1898 bureau of mines and geosciences 1986 since world war ii gold and silver production has risen fairly steadily from 2 0 t au and 1 4 t ag in 1946 to 37 t au and 34 t ag in 2009 bureau of mines and geosciences 1986 fig 2 between 2005 and 2009 the philippines accounted for approximately 1 5 and 0 1 percent of global gold and silver production respectively when approximately 37 t au and between 14 and 34 t ag were produced each year george 2011 brooks 2011 fig 2 fig 1 a distribution of active copper gold silver zinc chromium and nickel mines in the philippines circa mid 2011 modified from philippines mines and geoscience bureau http www mgb gov ph b study site locations for this special issue fig 2 philippines gold and silver production in tonnes from 1910 to 2009 and copper production in millions of tonnes from 1947 to 2009 data sources wu 1984 bureau of mines and geosciences 1986 lyday 1989 edelstein 1994 1999 2004 2011 lucas 1994 reese 1994 amey 1999 hilliard 1999 brooks 2004 brooks 2011 and george 2011 the first large scale copper mining operations in the philippines commenced at

porphyry and epithermal deposits and 40ar 39ar geochronology of the baguio district philippines the baguio district is located in the central cordillera of northern luzon philippines it contains numerous mineralized porphyry copper gold epithermal gold silver and skarn gold lead zinc deposits the district is floored by cretaceous eocene metavolcanic and metasedimentary rocks which are overlain by marine to terrestrial sedimentary and volcanic rocks of early miocene to pliocene ages tertiary arc magmatism related to east directed subduction of the south china sea plate along the manila trench beneath northern luzon produced a major batholith the central cordillera intrusive complex which defines the eastern boundary of the baguio district the intrusive complex was emplaced in the early miocene based on 40ar 39ar age determinations for the lucbuban gabbro 22 6 0 5 ma and virac granodiorite 20 23 0 38 and 20 2 0 7 ma recent exploration has led to the discovery of several new porphyry copper gold and skarn prospects in the western baguio district mineralization was preceded by the intrusion of a suite of hornblende megacrystic andesite dikes in the central part of the district from 4 55 0 15 to 3 45 0 19 ma porphyry copper gold and skarn deposits at black mountain and mexico on the western side of the district formed between 3 09 0 15 and 2 81 0 24 ma the santo tomas ii cu au pd porphyry deposit was emplaced into the central southern part of the district at about 1 5 ma based on secondary biotite 40ar 39ar ages of 1 48 0 05 ma and 1 47 0 05 ma on the eastern side of the district porphyry style mineralization at the hartwell and ampucao porphyry cu au prospects returned 40ar 39ar ages of 1 09 0 10 and 0 51 0 26 ma respectively quartz carbonate base metal sulfide style epithermal gold silver veins are well developed on the eastern side of the baguio district and crosscut porphyry style mineralization at the acupan baguio gold and nugget hill deposits epithermal veining is inferred to have occurred in the past million years based on a reported k ar age determination of 0 65 0 07 ma from the acupan gold mine a large advanced argillic alteration zone the baguio lithocap crops out in the northwest part of the district and formed between 1 4 and 0 9 ma based on previous k ar dating pliocene pleistocene mineralization in the baguio district was triggered by the east directed subduction of the scarborough ridge ridge subduction caused the subduction angle along the manila trench beneath northern luzon to decrease which facilitated crustal thickening and exhumation ridge subduction also promoted the development of northwest trending faults in the upper plate that interacted with major arc parallel north and arc normal northeast trending fault sets associated with the philippine fault system fertile magmas were emplaced into transtensional strike slip relay basins resulting in the accumulation of more than 35 moz of gold and several million tonnes of copper over approximately 3 m y

vesiculation of may 18 1980 mount st helens magma the may 18 1980 eruption of mount st helens washington produced both white and gray pumice similar in composition but varying in phenocryst microlite and vesicle content the white pumice has fewer phenocrysts no microlites and higher vesicularity and is thus less dense than the gray in addition vesicles in the white pumice are larger and more interconnected than those in the gray both white and gray have effective crystal free vesicularities 85 7 and 72 2 respectively close to the traditionally accepted fragmentation vesicularity of 75 77 given that the two pumice types were erupted concurrently for several phases of the eruption and therefore probably had similar eruption paths we suggest that differences in vesicularity and vesicle size distributions result from the presence or absence of microlites the presence of microlites in the gray pumice not only increased the magma viscosity and effective vesicularity but appears to have aided bubble nucleation and hindered bubble expansion and coalescence thus magmas with microlites may fragment at a lower bulk vesicularity than those without microlites fragmented microlite bearing clasts are also likely to expand less after fragmentation and therefore more closely preserve the bubble distribution and structure at the time of magma fragmentation

the constitutive equation and flow dynamics of bubbly magmas 1 a generalized constitutive equation for bubbly liquids is presented which successfully reproduces the expected viscosity response for both steady flows with varying capillary number ca a measure of the bubble deformation and unsteady flows with varying dynamic capillary number cd a measure of the steadiness of the flow previously given in separate studies the constitutive equation is given in terms of observable material and flow parameters and is

subduction arc reversal and the origin of porphyry copper gold deposits in island arcs the tertiary quaternary porphyry copper gold deposits of the southwest pacific rim particularly in luzon mainland papua new guinea bougainville guadalcanal and fiji mostly formed after a reversal of arc polarity where this reversal has not occurred e g new zealand and japan porphyry copper gold deposits are absent or scarce hence older porphyry copper gold deposits may be useful clues to tectonic history enrichment of magmas in gold may have resulted from two stage melting the tectonic setting of porphyry copper gold deposits differs from that inferred for volcanic hosted massive sulfide deposits

geology geochronology and fluid inclusion studies of the xiaorequanzi volcanogenic massive sulphide cu zn deposit in the east tianshan terrane china the xiaorequanzi cu zn deposit is in the westernmost part of east tianshan terrane in northwestern china the deposit is unique in the region being a volcanogenic massive sulphide vms deposit loc

physicochemical processes in the magma chamber under the black mountain porphyry cu au deposit philippines insights from mineral chemistry and implications for mineralization the black mountain porphyry cu au deposit in the baguio district northern luzon philippines is associated with late miocene to pliocene intrusive rocks that retain primary crystallization signatures providing an excellent natural laboratory to study the details of magma chamber processes each igneous phase contains disequilibrium textures including resorptive textures in plagioclase and distinct core rim textures in amphibole high crystallization temperatures and pressures in felsic 880 c and 2 8 kbar and mafic 1 030 c and 7 7 kbar rocks recorded by amphibole phenocrysts reflect crystallization in the magma chamber amphibole in felsic rocks aged 6 39 0 49 to 2 83 0 23 ma have similar crystallization temperatures and pressures suggesting a long lived and hot felsic magma chamber under the black mountain deposit four independent oxybarometers including amphibole coexisting magnetite ilmenite magmatic anhydrite and zircon consistently show a high oxidation state of nno 1 for the felsic magma and nno 0 7 for the mafic magma element profiles across phenocrystic plagioclase display repeated oscillatory variations of ca fe and sr contents with large anorthite variations and a positive correlation between cao and feo the amphibole is classified as group 1 mg hornblende to tschermakite hornblende in felsic rocks and group 2 mg hastingsite in mafic rocks the group 1 amphiboles show a variety of major and trace element variations from core to rim in disequilibrated grains and transitional compositions to group 2 mg hastingsite both mineral textures and chemistry recorded by phenocrysts of plagioclase and amphibole indicate repeated mafic magma recharge in the felsic magma chamber the common occurrence of rounded or elongated sulfide inclusions in the megacrystic amphibole from the mafic rocks 2 81 0 15 ma suggests a greater enrichment of cu in mafic rocks relative to the ore forming porphyries 2 91 2 83 ma this is also consistent with higher cu contents 3 4 0 9 ppm in the megacrystic amphibole from the mafic rocks relative to amphibole in felsic rocks 1 0 0 2 ppm and may indicate up to 3 4 times higher cu in the mafic magma relative to felsic magma therefore large scale mafic magma recharge particularly at 2 8 ma likely introduced ore forming metals to the long lived felsic magma chamber further contributing to the fertility of the felsic rocks and ultimately to cu au mineralization at the black mountain deposit

geodynamic evolution of the baguio mineral district unlocking the cenozoic record from clastic rocks abstract geochemical studies done on the baguio mineral district had dominantly looked at the igneous rocks a major gap is the scarcity of studies dealing with the sedimentary rock geochemistry for this district which this paper intends to address the petrographic data and major and trace element compositions of the siltstones sandstones from the lower member of the late oligocene to early miocene zigzag formation show that they are generally quartz rich and have high k2o th and la sc contents the chondrite normalized ree pattern exhibits a negative eu anomaly enrichment in lrees and flat hrees similar to typical post archean shales taken together the data indicate derivation of the lower member of the zigzag formation from intermediate to acid igneous rock sources generated in an active margin setting a likely candidate source is the cordon syenite complex palali formation in the northern sierra madre caraballo mountains derivation from mafic source rocks in an oceanic island arc setting is inferred for samples of the middle to late miocene klondyke late miocene to early pliocene amlang and late pliocene cataguintingan formations these samples are characterized by low k2o th and la sc but high cr th values unroofing of the pugo metavolcanics and the younger plutons in the central cordillera provided the materials which eventually produced the klondyke and amlang formations continuous uplift and shallowing of the basin resulted in the deposition of the cataguintingan formation the petrography geochemistry and geological features of the sedimentary rocks clearly define the change in sediment provenance from quartz rich during the oligocene to quartz deficit in the miocene this offers additional constraints in understanding the geological evolution of the mineral district

viscous dissipation in explosive volcanic flows 1 most models for magma flow in volcanic conduits during explosive eruptions assume isothermal ascent conditions due to the high heat capacity of the magma here we present a non isothermal axisymmetrical flow model that accounts for the temperature dependent viscosity of magma and viscous dissipation of heat significant changes in velocity and temperature profiles result in strong reduction of the conduit friction leading to an order of magnitude increase in discharge rate in comparison with the isothermal case this allows high intensity eruptions to occur from significantly narrower volcanic conduits and so helps to resolve inconsistencies between conduit dimensions inferred from models and observations for a given conduit diameter fragmentation can occur at much shallow depths than for an isothermal model

exploration tools for linked porphyry and epithermal deposits example from the mankayan intrusion centered cu au district luzon philippines the mankayan mineral district of northern luzon philippines hosts several significant ore deposits and prospects of various types within an area of similar to 25 km 2 including the far southeast porphyry cu au deposit the lepanto high sulfidation epithermal cu au deposit the victoria intermediate sulfidation epithermal au ag vein deposit the teresa epithermal au ag vein deposit the guinaoang porphyry cu au deposit and the buaki old palidan porphyry cu au prospects all having formed in a period of about 2 m y from similar to 3 ma the geologic units include 1 a basement composed of late cretaceous to middle miocene metavolcanic rocks volcaniclastic rocks 2 the miocene 12 to 13 ma tonalitic bagon intrusive complex 3 the pliocene similar to 2 2 to 1 8 ma imbanguila dacite porphyry and pyroclastic rocks and 4 postmineralization cover rocks including die similar to 1 2 to 1 0 ma bato dacite porphyry and pyroclastic rocks and the similar to 0 02 ma lapangan tuff extensive advanced argillic alteration crops out for similar to 7 kin along the inconformity between the basement rocks and the imbanguila dacite formation consists of quartz alunite pyrophyllite or diaspore with local zones of silicic alteration and a halo of dickite kaolinite the alteration and its subhorizontal geometry indicate that it is a lithocap or coalesced lithocaps the northwest striking portion is similar to 4 km long and hosts the lepanto enargite au ore deposit also controlled by the lepanto fault the lepanto epithermal deposit is related to the underlying far southeast porphyry the quartz alunite alteration halo of lepanto is contemporaneous with the similar to 1 4 ma potassic alteration of the porphyry there are also silicic advanced argillic alteration patches similar to 600 m above the far southeast orebody at the present surface these are interpreted to be perched alteration there is no systematic mineralogical or textural zoning in the lepanto lithocap that indicates direction to the intrusive source most surface samples of the lithocap contain less than 50 ppb au despite many being less than a few hundred meters from underground cu au ore this study found that several characteristics of the lepanto lithocap change systematically with distance from the causative intrusion the alunite absorption peak at similar to 1 480 nm in the short wavelength infrared swir spectrum shifts to higher wavelengths where the sample is closer to the intrusive center due to higher na and lower k content in the alunite published experimental studies indicate that high na na k is related to higher formation temperature high ca alunite including huangite also occurs at locations proximal to the intrusive center alunite mineral composition analyzed by laser ablation inductively coupled plasma mass spectrometry la icp ms indicates that the pb content decreases toward the intrusive center whereas sr la sr ph and la pb increase markedly whole rock compositions using only nonmineralized taken as cu similar to 350 m above the upper extent of the veins an airborne geophysics survey indicates that the far southeast orebody is associated with a wide zone of demagnetization due to extensive magnetite destructive phyllic alteration such low magnetic anomalies on the margin of a large lithocap elsewhere may deserve attention the directional indicators and mineralization signatures found in this study have the potential to indicate direction to the intrusive center during exploration of similar porphyry epithermal districts

giant porphyry deposits characteristics distribution and tectonic controls more than half of the 25 largest known porphyry copper deposits defined in terms of contained copper metal formed during three time periods the paleocene to eocene eocene to oligocene and middle miocene to pliocene these giant deposits are clustered within three provinces central chile northern chile and southwest arizona northern mexico other giant deposits occur in montana utah panama peru argentina irian jaya mongolia and iran compressive tectonic environments thickened continental crust and active uplift and erosion were associated with the formation of many of these deposits calc alkalic magmas are most favorable for the formation of giant porphyry copper deposits although several of the largest systems are associated with high k calc alkalic intrusions the 25 largest gold rich porphyry deposits are concentrated in the southwest pacific and south america with other occurrences in eurasia british columbia alaska and new south wales many of the deposits formed in the last 13 m y the largest of the deposits are associated with high k calc alkalic intrusions many calc alkalic porphyritic intrusions have also produced giant gold rich porphyries in the last 20 m y the formation of giant porphyry copper molybdenum and copper gold deposits in the circum pacific region has been closely associated with subduction of aseismic ridges seamount chains and oceanic plateaus beneath oceanic island and continental arcs in several examples these tectonic perturbations have promoted flat slab subduction crustal thickening uplift and erosion and adakitic magmatism coeval with the formation of well endowed porphyry and or epithermal mineral provinces similar tectonic features are inferred to be associated with the giant porphyry copper molybdenum provinces of northern chile eocene oligocene and southwest united states cretaceous paleocene topographic and thermal anomalies on the downgoing slab appear to act as tectonic triggers for porphyry ore formation other factors such as sutures in the overriding plate permeability architecture of the upper crust efficient processes of ore transport and deposition and in some cases formation and preservation of supergene enrichment blankets are also vital for the development of high grade giant ore deposits a low grade geochemical anomaly may be the final product of mineralization if ore forming processes do not operate efficiently even in the most favorable geodynamic settings

on magma fragmentation by conduit shear stress evidence from the kos plateau tuff aegean volcanic arc abstract large silicic explosive eruptions are the most catastrophic volcanic events yet the intratelluric mechanisms underlying are not fully understood here we report a field and laboratory study of the kos plateau tuff kpt 161 ka aegean volcanic arc which provides an excellent geological example of conduit processes that control magma vesiculation and fragmentation during intermediate to large scale caldera forming eruptions a prominent feature of the kpt is the occurrence of quite unusual platy shaped tube pumice clasts in pyroclastic fall and current deposits from the early eruption phases preceding caldera collapse on macroscopic and sem observations flat clast faces are elongated parallel to tube vesicles while transverse surfaces often occur at 45 to vesicle elongation this peculiar pumice texture provides evidence of high shear stresses related to strong velocity gradients normal to conduit walls which induced vesiculation and fragmentation of the ascending magma either an increasing mass discharge rate without adequate enlargement of a narrow central feeder conduit or a developing fissure like feeder system related to incipient caldera collapse provided suitable conditions for the generation of plate tube pumice within magma volumes under high shear during the pre climactic kpt eruption phases this mechanism implies that the closer to the conduit walls where the stronger are the velocity gradients the larger was the proportion of plate vs conventional lensoid juvenile fragments in the ascending gas pyroclast mixture consequently plate pumice clasts were mainly entrained in the outer portions of the jet and convecting regions of a sustained plinian type eruption column as well as in occasional lateral blast currents generated at the vent as a whole plate pumice clasts in the peripheral portions of the column were transported at lower altitudes and deposited by fallout or partial collapse closer to the vent relative to lensoid ones that dominated in the inner column portions the plate tube pumice proportion decreased abruptly up to disappearance during the emplacement of the main pyroclastic currents and lithic rich breccias related to extensive caldera collapse at the eruption climax as a consequence of an overall widening of the magma feeder system through the opening of multiple conduits and eruptive vents along with fissure erosion concomitant to the disruption of the collapsing block

geochemistry and tectonic setting of the luzon arc philippines miocene 10 ma to recent volcanism is associated with eastward subduction along the manila trench for some 1 200 km from the coastal range in taiwan south to min doro we suggest calling this the luzon arc there are five distinct segments along this arc mindoro bataan northern luzon babuyan and taiwan the bataan and mindoro segments in the south are separated by a northeast southwest zone of volcanoes the northern luzon segment consists of pliocene to pleistocene necks and plugs that run along the entire western section of northern luzon the babuyan segment 10 ma to recent is made up of a larger number of active volcanoes than are other segments of the luzon arc the taiwan segment is composed entirely of extinct volcanoes in the coastal range and on the islands of lanhsu and lutao late miocene to 1 8 ma the nature of volcanism throughout the luzon are is clearly subduction related with calc alkaline affinities high th la ratios and negative nb ta and ti anomalies the earliest phase of volcanism so far recorded 10 ma however is tholeiitic island of calayan in the babuyan segment with mid oceanic ridge basalt like morb like tight rare earth element and large ion lithophile element concentrations and low th u ratios there are clear differences in sr and nd isotopic ratios among the samples from the luzon are samples from the southern segments mindoro through bataan fall within the mantle array with higher sr and lower nd isotopic values than those of morb pb isotope data indicate sediment involvement the isotopic values have been interpreted to result from source contamination by slab derived fluids bataan segment and by subducted crustal material from collision of the mindoro palawan terrane mindoro segment with the manila trench in contrast babuyan segment samples north have unusual island arc nd and sr isotopic ratios that fall below the mantle array they appear to be the result of sediment source contaminants or a unique mantle composition incorporation of sediments analyzed from eastern china and taiwan into a morb type mantle can explain the nd and sr isotopic ratios of the babuyan segment

textural and micro petrological variations in the eruptive products of the 2006 dome forming eruption of merapi volcano indonesia implications for sub surface processes the interplay between magma ascent degassing and changing magmatic properties are widely recognized as critical factors controlling the style of silicic volcanic eruptions microlite textures in samples from the prolonged dome forming eruption of merapi in 2006 provide a record of changing magmatic ascent conditions and shallow conduit processes throughout the eruption analysis of microlite textural parameters including measurements of areal number density na mean microlite size crystal aspect ratio and groundmass crystallinity combined with the monitoring record and field observations indicate that magma ascent paths change between continuous ascent at varying rates from a deeper magma storage region to ascent being temporarily stalled at shallow depths in the latter stages of the eruption supporting the idea of an ephemeral shallow magma storage region at merapi plagioclase microlite compositions show evidence of decompression induced degassing often displaying rims of anorthoclase and more k rich alkali feldspar sanidine anorthite contents also support the textural data of later erupted magma being temporarily stalled at shallow depths crystal size distributions csds are interpreted to show that both growth dominated and nucleation dominated crystallisation regimes existed during the 2006 eruption resulting from changing conditions of undercooling t during variable magma ascent paths by contrast microlite textural analysis and feldspar microlite compositions of samples from the fast growing lava dome of the second phase of the 2010 eruption prior to the cataclysmic events on 5 november indicate faster ascent rates a crystallisation regime more strongly dominated by nucleation due to high t and interaction of the 2010 magma with more hotter magma from depth

a double island arc between taiwan and luzon consequence of ridge subduction analysis of geomorphological geochronological geochemical and geophysical features in the segment of the taiwan luzon arc between taiwan and luzon the bashi segment allows the recognition of a double arc structure the two volcanic chains are separated by 50 km just north of luzon 18 n and converge near 20 n islets in the western chain are older and largely composed of volcanic rocks of miocene to pliocene age they all show low relief lateritic platforms and wave cut terraces and are covered by massive recrystallized limestone in contrast all active volcanoes in this segment of the taiwan luzon arc belong to the eastern chain where most islets are quaternary in age the volcanoes have well developed cone shapes and well preserved deposits of near vent facies magmas of the eastern chain have higher ksi la n la yb n and lowerend than their counterparts at the same latitude in the western chain therefore the magmas erupted in the eastern chain were derived from more enriched mantle sources than the magmas erupted in the western chain moreover the available seismological data seem to suggest an abrupt increase of the dip angle from 30 at 18 n to 80 at 20 n thus the double arc structure is located in the region where the benioff zone suddenly changes in analogy with the lesser antilles arc we propose a geodynamic model in which the double arc in the bashi strait is the tectonic manifestation of the subduction of the aseismic scarborough seamount chain the extinct mid ocean ridge of the south china sea before that ridge reached the manila trench the western chain was the volcanic front when the ridge reached the subduction zone at 5 4 ma its buoyancy temporarily interrupted the subduction thus causing a time gap in magmatic activity furthermore this ridge arc collision was probably also responsible for regional uplift causing extensive sub aerial weathering and erosion as well as massive reef formation in the western chain when subduction started again the dip angle became shallower in response to the extra buoyancy of the downgoing ridge if the depth of magma generation remained constant the shallower dip angle would have naturally led to an eastward shift of the volcanic front thus producing the younger eastern chain moreover we speculate that the abrupt change of the dip angle may have torn the downgoing slab thus allowing more enriched continental lithospheric material to invade the mantle wedge from the northwest thus imprinting a geochemically more enriched signature on the magmas of the eastern chain

petrological architecture of a magmatic shear zone a multidisciplinary investigation of strain localisation during magma ascent at unzen volcano japan shearing of magma during ascent can promote strain localisation near the conduit margins anymechanical and thermal discontinuities associated with such events may alter the chemical physicaland rheological stability of the magma and thus its propensity to erupt lava spines can record suchprocesses preserving a range of macroscopic and microscopic deformation textures attributed toshearing and friction as magma ascends through the viscous brittle transition here we use a multi disciplinary approach combining petrology microstructures crystallography magnetics and experi mentation to assess the evidence role and extent of shearing across a marginal shear zone of the1994 1995 lava spine at unzen volcano japan our results show that crystals can effectively moni tor stress conditions during magma ascent with viscous remobilisation crystal plasticity and com minution all systematically increasing towards the spine margin accompanying this we find anincrease in mineral destabilisation in the form of pargasitic amphibole breakdown displaying tex tural variations across the shear zone from symplectitic to granular rims towards the spine margin in addition the compaction of pores chemical and textural alteration of interstitial glass and mag netic variations all change systematically with shear intensity the strong correlation between thedegree of shearing crystal deformation and disequilibrium features together with distinct magneticproperties implies a localised thermal input due to shear and frictional processes near the conduitmargin during magma ascent this was accompanied by late stage or post emplacement fluid andgas induced alteration of the gouge as well as oxidation and glass devitrification understandingand recognising evidence for strain localisation during magma ascent may therefore be vital whenassessing factors that regulate the style of volcanic eruptions which may provide insights into thecryptic shifts from effusive to explosive activity as observed at many active lava domes

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

discussion on giant versus small porphyry copper deposits of cenozoic age in northern chile adakitic versus normal calc alkaline magmatism by oyarzun et al mineralium deposita 36 794 798 2001

the altar porphyry cu au mo deposit argentina a complex magmatic hydrothermal system with evidence of recharge processes altar 31 29 s 70 28 w is a large porphyry cu au mo deposit with associated epithermal au ag cu veins located in the cordillera principal of southwest san juan province argentina altar is a complex magmatic hydrothermal system formed from several magmatic and hydrothermal pulses during the middle late miocene new la icpms u pb ages in zircons from the altar porphyries indicate four discrete events of intrusions over an extended magmatic life time of ca 3 m y it comprises a premineralization porphyry 11 75 0 24 ma three mineralized porphyries 11 62 0 21 and 11 68 0 27 ma 11 13 0 26 ma 10 35 0 32 ma related to hydrothermal breccias two postmineralization intrusions and a postmineralization breccia 8 9 0 4 ma the three mineralized porphyries porphyries 2 3 and 4 were emplaced within 0 7 to 1 3 m y amphibole phenocrysts from the porphyries crystallized from oxidized magmas f o2 nno 1 to 2 at temperatures of 780 to 850 c and pressures between 0 9 and 1 8 kbars corresponding to depths of 4 to 7 km anorthite and fe rich rims in the plagioclase phenocrysts suggest that the magmatic chambers were episodically recharged by a less evolved magma the middle late miocene intrusions are interpreted to have been derived from a deeper and relatively large magmatic reservoir that supplied magmas to smaller chambers located in the upper crust the focused magmatic output to shallow levels during a period of a few million years in the altar area has been a main requirement in the formation of this large porphyry copper deposit

insights into magma histories through silicate oxide crystal clusters linking the hiltaba suite intrusive rocks to the gawler range volcanics gawler craton south australia the roxby downs granite rdg host to the supergiant olympic dam deposit and the gawler range volcanics grv in south australia both contain clusters of touching silicate and oxide minerals here we report on the geochemistry and petrography of these rocks and their contained mineral clusters before comparing them to similar occurrences elsewhere in the world to gain a better understand of their genesis and the links between the rdg and other members of the hiltaba suite and the grv crystal clusters in the rdg comprise magnetite apatite titanite biotite zircon which are typically enveloped in a plagioclase crystal matrix in the grv crystal clusters comprise pyroxene titanomagnetite plagioclase apatite zircon display variably glomeroporphyritic equigranular poikilitic and loosely packed habits and are enclosed by texturally varied groundmass of the grv lavas studied pyroxenes and titanomagnetite show systematic trends in major and trace element compositions conformable with the stratigraphic position of their hosts this suggests the lavas are related through differentiation in the upper crust and represent the inversion of a normally zoned reservoir there are no compositional contrasts between phenocrysts and crystals in grv clusters and mg of pyroxenes are consistent with those predicted from whole rock compositions implying that all analysed crystals including those in clusters were formed within or were at least equilibrated to the host lavas precursors rather than entire histories as isolated crystal clusters entrained by melt flow intergrowth textures and mafic magmatic enclaves suggest that the majority of grv crystal clusters were liberated from closely packed crystal masses through magma rejuvenation and associated crystal resorption consistent fe ti oxide apatite zircon plagioclase assemblages and textures in both the grv and rdg suggest a shared genetic scenario although an extended model is needed to account for the transformation of grv type to hs type crystal clusters for hot and dry magmas such as those that formed the grv and rdg cooling causes pyroxene fe ti oxide plagioclase in grv type crystal clusters to react with each other and the melt forming hs type magnetite titanite biotite feldspar crystal clusters rejuvenation events implicated in the liberation of grv type crystal clusters may be important drivers of magmatic hydrothermal iron oxide deposit formation

comparison between thermal viscous coupling and frictional sliding abstract we investigated the similarity between thermal viscous coupling tvc and frictional sliding proposed by kameyama and kaneda pure appl geophys 159 2002 2011 we consider a one dimensional layer composed of viscous material which is sandwiched and sheared by two thick elastic layers the rate of viscous deformation depends on the temperature t c in the viscous layer as well as shear stress the temperature t c changes owing to heating by viscous dissipation and conductive cooling we carried out velocity stepping tests for the steady state deformation both numerically and analytically and compared the temporal evolution of small perturbations with that of the spring block model with rate and state dependent friction rsf we found that as is the case of frictional slip stability the manner of temporal evolution is classified into four regimes depending on whether it is stable or not and whether it is monotonous or oscillatory with time by further interpreting tvc in terms of general rsf theory by ruina j geophys res 88 1983 10359 we obtained the relations between the parameters appearing in the phenomenological rsf law and the nondimensional parameters which characterize the nature of tvc a further improvement of this approach might be important for estimating the actual values of frictional constitutive parameters at the deeper portion of seismogenic faults of interplate or inland earthquakes where a ductile deformation is expected to be significant

transient effects of magma ascent dynamics along a geometrically variable dome feeding conduit abstract the transient dynamics of magma ascent during dome forming eruptions were investigated and the effects of magma chamber pressure perturbations on eruption rate are illustrated the numerical model domeflow developed by the authors for this work is applied to the problem domeflow is a transient 1 5d isothermal two phase flow model of magma ascent through an axisymmetric conduit of variable radius which accounts for gas exsolution bubble growth crystallization induced by degassing permeable gas loss through overlying magma and through conduit walls as well as viscosity changes due to crystallization and degassing for runs in which chamber pressure increases the time required to reach the new steady state transition time is a complex function of the pressure perturbation while for decreasing chamber pressure transition time is a monotonic function of the magnitude of the pressure perturbation the transition to the new steady state is mainly controlled by magma compressibility travel time time required for one parcel of magma to travel from chamber to surface and the time over which the pressure perturbation occurs results of many runs 300 were analyzed using dimensional analysis to reveal a general relationship which predicts the temporal evolution of magma effusion rate for a given sudden increase in chamber pressure the product of the change in steady state extrusion rate and the time required to reach the new steady state is linearly proportional to the normalized change in chamber pressure the volume of the conduit and the ratio of top and bottom conduit radii and inversely proportional to the cubic root of volatile fraction this relationship is used to interpret observed variations in two ongoing dome building eruptions the soufriere hills volcano montserrat and merapi volcano indonesia

igneous geochemistry of mineralized rocks of the baguio district philippines implications for tectonic evolution and the genesis of porphyry style mineralization the baguio district of the philippines is one of the world s premier mineral provinces containing 35 million ounces moz of gold and 2 7 million metric tons mt of copper in epithermal porphyry and skarn deposits that formed in the last 3 5 m y pliocene and pleistocene magmatic rocks of the baguio district that are spatially and temporally associated with mineralization can be broadly subdivided into an intermediate to felsic suite of mineralized low to medium k intrusions some of which have adakitic affinities and a suite of mafic to intermediate medium k to shoshonitic hornblende phyric dikes the geochemical and isotopic characteristics of the dikes are consistent with primitive mantle derived melts that underwent minimal crustal contamination as they ascended through the arc crust in contrast the intermediate to felsic suite has been contaminated by young arc crust suggesting ponding and fractionation within shallow crustal magma chambers the philippine arc has formed in a complex tectonic environment and is currently sandwiched between two active subduction zones eastward directed subduction of the scarborough ridge along the manila trench is currently associated with flattening of the downgoing slab the formation of the mafic dike complex is broadly coeval with the onset of subduction of the scarborough ridge and slab flattening the extinct scarborough ridge would have been younger than the downgoing plate and consequently more susceptible to melting these melts can account for the isotopic recharge of the pliocene subarc mantle as well as the generation of the primitive melts and adakitic rocks found within the baguio district the interaction between primitive mafic melts and the more felsic calc alkaline rocks has

ascent and decompression of viscous vesicular magma in a volcanic conduit during eruption lava domes and flows may become unstable and generate dangerous explosions fossil lava filled eruption conduits and ancient lava flows are often characterized by complex internal variations of gas content these observations indicate a need for accurate predictions of the distribution of gas content and bubble pressure in an eruption conduit bubbly magma behaves as a compressible viscous liquid involving three different pressures those of the gas and magma phases and that of the exterior to solve for these three different pressures one must account for expansion in all directions and hence for both horizontal and vertical velocity components we present a new two dimensional finite element numerical code to solve for the flow of bubbly magma even with small dissolved water concentrations gas overpressures may reach values larger than 1 mpa at a volcanic vent for constant viscosity the magnitude of gas overpressure does not depend on magma viscosity and increases with the conduit radius and magma chamber pressure in the conduit and at the vent there are large horizontal variations of gas pressure and hence of exsolved water content such variations depend on decompression rate and are sensitive to the exit boundary conditions for the flow for zero horizontal shear stress at the vent relevant to lava flows spreading horizontally at the surface the largest gas overpressures and hence the smallest exsolved gas contents are achieved at the conduit walls for zero horizontal velocity at the vent corresponding to a plug like eruption through a preexisting lava dome or to spine growth gas overpressures are largest at the center of the vent the magnitude of gas overpressure is sensitive to changes of magma viscosity induced by degassing and to shallow expansion conditions in conduits with depth dependent radii

age tectonic setting and metallogenic implication of phanerozoic granitic magmatism at the eastern margin of the xing an mongolian orogenic belt ne china abstract the eastern margin of the xing an mongolian orogenic belt is characterised by widespread phanerozoic granitic magmatism some of which is closely related to significant ore mineralisation this paper presents new geochronological petrogenetic and tectonic data for selected intrusions zircon u pb geochronology for five granitoid plutons indicates they were emplaced during the middle late permian 264 255 ma and cretaceous 106 94 ma and thus granitic magmatism occurred throughout the phanerozoic permian 268 252 ma early middle triassic 248 240 ma early jurassic 183 ma and cretaceous 112 94 ma the permian granitoids consist of monzogranite granodiorite tonalite and quartz diorite characterised by enrichment in na2o 3 60 4 72 wt depletion in k2o 0 97 2 66 wt and a negative correlation between p2o5 and sio2 together with the presence of hornblende these geochemical features are indicative of an i type affinity the permian granitic magmatism is associated with quartz vein type tungsten deposits 252 ma unpublished sm nd isochron age which formed in an active continental margin setting related to subduction of the palaeo asian ocean the cretaceous quartz diorites have an adakitic affinity having relatively high sr 374 502 ppm low yb 0 51 0 67 ppm and y 8 7 10 7 ppm and high sr y 39 4 46 8 and la yb n values 16 2 34 7 suggesting that they were related to the partial melting of subducted oceanic crust in addition they are associated with porphyry au cu deposits we conclude that the cretaceous granitic rocks and associated porphyry au cu mineralisation occurred in an extensional tectonic setting related to the subduction of the palaeo pacific plate beneath the eurasian plate in addition the large scale early middle triassic syn collisional granite belt at the eastern margin of the xing an mongolian orogenic belt extends from the middle of jilin province to the wangqing hunchun region constraining the timing of the final collision between the north china craton and the jiamusi khanka massif and suggesting that the xra moron river changchun suture likely extends eastward into the eastern hunchun region this collision caused the middle triassic mesothermal lode gold mineralisation

insights into volcanic conduit flow from an open source numerical model 1 numerical models that calculate the fluid dynamics of explosive volcanic eruptions have been used with increasing frequency to understand volcanic processes and evaluate volcanic hazards yet those who develop such models rarely make them publicly available so that they can be verified used and possibly improved by other scientists in this paper i present a visual interactive open source numerical model that calculates steady state flow of magma and gas in vertical eruptive conduits and contains user friendly utilities for quickly determining physical thermodynamic and transport properties of silicate melts h2o gases and melt gas crystal mixtures the model represents an advance over previously published conduit models by incorporating a non arrhenian viscosity relation for hydrous silicate melts a relation between viscosity and volume fraction of gas that depends on capillary number and adiabatic temperature changes in the mixture using established thermodynamic relations for melts and h2o gas respectively volcanologists who have not had access to conduit models have frequently approximated conduit flow using an analytical equation for incompressible laminar newtonian pipe flow which predicts that the mass flux is proportional to the fourth power of conduit radius and inversely proportional to mixture viscosity the model presented here which is not much more difficult to use than a back of the envelope calculation shows that the pipe flow approximation significantly overestimates the sensitivity of mass flux to both conduit radius and mixture viscosity results from the model also show that viscous heating in the lower conduit which is not considered in most other models may increase the mass flux of large silicic eruptions by several percent and decrease the viscosity of the mixture at the fragmentation depth by a few tens of percent

sustained blasts during large volcanic eruptions we carried out numerical simulations to investigate magma ascent in wide conduits during large explosive eruptions wide conduits allow high discharge rates low frictional pressure losses and shallow levels of explosive fragmentation of the magma within the conduit in contrast with the commonly modeled lower rate eruptions during which magma fragments inside the conduit at depth and feeds a vertical eruptive jet we find that for sufficiently high discharge rates 10 10 kg s 1 the fragmentation level may rise up to the surface gas rich unfragmented magma reaches the surface at high pressure and feeds a sustained volcanic blast geologic evidence for very high discharge rate eruptions wide conduits and shock waves in large pyroclastic flows supports the occurrence of this type of explosive eruption