experimental study of subsolidus phase relations and mixing properties of pyroxene in the system cao al2o3 sio2 abstract subsolidus phase relations in the system cao al2o3 sio2 cas were experimentally determined with tight reversals of several univariant curves and with 14 equilibration experiments containing the assemblage pyroxene anorthite where pyroxene is a binary solid solution of ca tschermak cats caal2sio6 and ca eskola caes ca0 5alsi2o6 endmembers reversals were obtained on the following reactions bar c 3an gr 2ky q p 22t 700 3an cor gr 3ky p 21 8t 950 3cats gr 2cor p 55t 53900 and 6cats 1 x caesx 2 1 2x gr 4 1 2x cor 9xan observed slopes indicate 9 8 j mol k of al si disorder in ca tschermak pyroxene and 5 3 j mol k of al si disorder in anorthite at 1300 c it is suggested that al si disorder in anorthite increases by 1 9 j mol k from 700 c to 1300 c compositions of cats caes pyroxene in equilibrium with anorthite and pbo rich liquid were experimentally determined at 1400 1430 c and 22 7 30 8 kbar microprobe measurements gave compositions which are consistent with an ideal pyroxene solution and the following parameters for the reaction 3an 2cats 2caes j bar k 2rtln xcats xcaes 60200 86 4t 5 06 13 10 7p p 0 resulting in h0j 39 8 kj mol and s0 461 8 j mol k for the ca eskola endmember at 1300 c the obtained properties of the ca eskola component are necessary for thermobarometry based on pyroxene bearing assemblages containing plagioclase quartz or kyanite

paleo tethyan evolution of tibet as recorded in the east cimmerides and west cathaysides the paleozoic early mesozoic geology of tibet was controlled by the rift drift seafloor spreading and subduction zone tectonics of a paleo tethyan realm which evolved between the west cathaysides wc and the east cimmerides ec different suture zones with ophiolites and ophiolitic melanges high pressure metamorphic belts magmatic arcs and accretionary prism complexes separating different terranes mark multiple subduction accretion systems within this paleo tethyan domain reminiscent of the modern western pacific ocean discrete basins separated by different continental blocks and magmatic arcs constituted a complex paleogeography of paleo tethys and these oceanic strands were closed as a result of subduction with different polarities during the late paleozoic triassic the longmu tso shuanghu changning menglian suture zone ls cms in tibet represents the main tectonic boundary between the wc and ec that developed in the devonian the east kunlun a nyemaqen oceanic slab was subducted northward beneath the east kunlun terrane in northern tibet whereas the sumdo oceanic slab was subducted northward beneath the south qiangtang north lhasa terrane in southern tibet the longmu tso shuanghu changning menglian and jinshajiang ailaoshan song ma ophiolites were developed and emplaced in subduction accretion systems with opposite polarities westward and eastward beneath the north qiangtang qamdo simao indochina terrane in central tibet the jinshajiang ailaoshan song ma oceanic slab was subducted westward beneath the north qiangtang simao indochina terrane along the jinshajiang and ailaoshan song ma trenches in a trench ridge trench triple junction plate configuration the emeishan mantle plume produced a large permian basaltic terrane developed on the western passive margin of the south china block the final closure of the paleo tethyan oceanic branches resulted in continental collisions and development of a vast indosinian orogenic collage in the latest triassic jurassic

palaeoenvironmental reconstruction and biostratinomic analysis of the jurassic yanliao lagerstatte in northeastern china abstract the middle upper jurassic yanliao lagerstatte contains numerous exceptionally preserved fossils of aquatic and land organisms including insects salamanders dinosaurs pterosaurs and mammaliaforms despite extensive study of the diversity and evolutionary implications of the biota the palaeoenvironmental setting and taphonomy of the fossils remain poorly understood we reconstruct both the palaeoenvironment of the daohugou area one of the most famous yanliao fossil areas and the biostratinomy of the fossils we use high resolution stratigraphic data from field investigation and excavations to document in detail the stratigraphic succession lithofacies facies associations and biostratinomic features of the lagerstatte our results show that frequent volcanic eruptions generated an extensive volcaniclastic apron and lake s in the studied area the frequent alternation of thin lacustrine deposits and thick volcaniclastic apron deposits indicates either that the studied area was located in the marginal regions of a single lake where the frequent influx of volcaniclastic apron material caused substantial fluctuations in lake area and thus the frequent lateral alternation of the two facies or that many short lived lakes developed on the volcaniclastic apron most terrestrial insects were preserved in the laminated normally graded siltstone claystone and tuff facies that form many thin intervals with deposits of graded sandstone siltstone and tuff in between within each interval the terrestrial insects occur in many laminae associated with abundant aquatic organisms but are particularly abundant in some laminae that directly underlie tuff of fallout origin most of these terrestrial insects are interpreted to have been killed in the area adjacent to the studied palaeolake s during volcanic eruptions their carcasses were transported by influxes of fresh volcaniclastic material primarily meteoric runoff and possibly minor distal pyroclastic flow into the palaeolake s and were buried in palaeolake deposits prior to extended decay probably due to a combination of rapid vertical settling ash fall and water turbulence

p t t evolution of ultrahigh temperature granulites from the saxon granulite massif germany part ii geochronology the granulites of the saxon granulite massif equilibrated at high pressure and ultrahigh temperature and were exhumed in large part under near isothermal decompression this raises the question of whether p t t data on the peak metamorphism may still be retrieved with confidence felsic and mafic granulites with geochronologically useful major and accessory phases have provided a basis to relate p t estimates with isotopic ages presented in a companion paper the assemblage garnet clinopyroxene in mafic granulite records peak temperatures of 1010 1060 c consistent with minimum estimates of around 967 c and 22 3 kbar obtained from the assemblage garnet kyanite ternary feldspar quartz in felsic granulite multiple partial overprint of these assemblages reflects a clockwise p t evolution garnet and kyanite in the felsic granulite were successively overgrown by plagioclase spinel plagioclase sapphirine plagioclase and biotite plagioclase most of this overprinting occurred within the stability field of sillimanite garnet clinopyroxene in the mafic granulite were replaced by clinopyroxene amphibole plagioclase magnetite the high p t conditions and the absence of thermal relaxation features in these granulites require a short lived metamorphism with rapid exhumation the ages of peak metamorphism 342 ma and shallow level granitoid intrusions 333 ma constrain the time span for the exhumation of the saxon granulites to about 9 ma

sedimentology of subaqueous volcaniclastic sediment gravity flows in the neogene santa maria basin california subaqueous tuff deposits within the lower miocene lospe formation of the santa maria basin california are up to 20 m thick and were deposited by high density turbidity flows after large volumes of ash were supplied to the basin and remobilized tuff units in the lospe formation include a lower lithofacies assemblage of planar bedded tuff that grades upward into massive tuff which in turn is overlain by an upper lithofacies assemblage of alternating thin bedded coarse grained tuff beds and tuffaceous mudstone the planar bedded tuff ranges from 0 3 to 3 m thick and contains 1 8 cm thick beds that exhibit inverse grading and low angle and planar laminations the overlying massive tuff ranges from 1 to 10 m thick and includes large intraclasts of pumiceous tuff and stringers of pumice grains aligned parallel to bedding the upper lithofacies assemblage of thin bedded tuff ranges from 0 4 to 3 m thick individual beds are 6 30 cm thick and display planar laminae and dewatering structures pumice is generally concentrated in the upper halves of beds in the thin bedded tuff interval the association of sedimentary structures combined with semi quantitative analysis for dispersive and hydraulic equivalence of bubble wall vitric shards and pumice grains reveals that particles in the planar bedded lithofacies are in dispersive not settling equivalence this suggests deposition under dispersive pressures in a tractive flow grains in the overlying massive tuff are more closely in settling equivalence as opposed to dispersive equivalence which suggests rapid deposition from a suspended sediment load the set of lithofacies that comprises the lower lithofacies assemblage of each of the lospe formation tuff units is analogous to those of traction carpets and subsequent suspension sedimentation deposits often attributed to high density turbidity flows grain distributions in the upper thin bedded lithofacies do not reveal a clear relation for dispersive or settling equivalence this information together with the association of sedimentary features in the thin bedded lithofacies including dewatering structures suggests a combination of tractive and liquefied flows absence of evidence for elevated emplacement temperatures e g eutaxitic texture or shattered crystals suggests emplacement of the lospe formation tuff deposits in a cold state closely following pyroclastic eruptions the tuff deposits are not only a result of primary volcanic processes which supplied the detritus but also of processes which involved remobilization of unconsolidated ash as subaqueous sediment gravity flows these deposits provide an opportunity to study the sedimentation processes that may occur during subaqueous volcaniclastic flows and demonstrate similarities with existing models for sediment gravity flow processes

alpe arami a peridotite massif from depths of more than 300 kilometers the abundance of fetio 3 and chromite precipitates in olivine of the alpe arami peridotite massif switzerland requires a much higher solubility for highly charged cations than is found in mantle xenoliths from depths of 250 kilometers three previously unknown crystal structures of fetio 3 were identified that indicate that the originally exsolved phase was the high pressure perovskite polymorph of ilmenite implying a minimum depth of origin of 300 kilometers these observations coupled with the unique lattice preferred orientation of olivine further suggest that the original composition of olivine may reflect that of a precursor phase perhaps wadsleyite that is stable only at depths greater than 400 kilometers

a pyroxene bearing meta ironstone and other pyroxene granulites from tonagh island enderby land antarctica further evidence for very high temperature 980 c archaean regional metamorphism in the napier complex a suite of granulites including a meta ironstone pyroxenites and spinel lherzolites from east tonagh island enderby land antarctica preserve exsolution recry stallization features consistent with a shared metamorphic evolution that involves marked cooling from initial metamorphic temperatures of nearly 1000 c reintegrated pre exsolution and pre reaction grain compositions in the meta ironstone indicate the former coexistence of metamorphic pigeonite wo12en38fs50 and ferroaugite wo35en31fs34 at temperatures in excess of 980 c for pressures of 7 kbar 0 7 gpa using pyroxene quadrilateral thermometry lindsley 1983 intra grain lamellae relationships indicate the exsolution of a second pigeonite wo12en35fs53 from the ferroaugite at temperatures in the range 930 970 c prior to the c 720 600 c exsolution of orthopyroxene and clinopyroxene 100 lamellae and later partial recrystallization at similar temperatures although pyroxenitic and iherzolitic granulites preserve a much less complete history reintegrated porphyroclast compositions in these yield temperature estimates which approach those inferred from the metaironstone pyroxene thermometry based on neoblast compositions suggests that recrystallization post dating a late low intensity deformation phase d3 occurred at temperatures greater than 600 c these results are consistent with the independent evidence obtained from studies of metapelitic and felsic rock types for very high temperature metamorphism throughout the napier complex followed by near isobaric cooling and later deformation under lower grade granulite facies conditions comparison with similar pyroxene data from fyfe hills sandiford powell 1986 demonstrates further the regional significance of these high temperatures and implies broadly isothermal metamorphic conditions over a large area 5000 km2 and thickness 6 9 km of lower crust at c 3070 ma

eclogite facies relics and inferred ultrahigh pressure metamorphism in the north dabie complex central eastern china mineral assemblages and microstructures of a newly identified retrogressed eclogite sensu lato in the north dabie complex ndc central eastern china indicate early very high pressure metamorphic relics preserved in a dominant amphibolite facies host where no eclogite sensu stricto has been positively identified before the investigated eclogitic rock shows distinct multistage recrystallization with granulite and amphibolite facies assemblages overprinting eclogite facies relics the minimum temperature for the eclogite facies metamorphism is estimated to be 800 820 c a spectacular microstructure of oriented quartz needles 2 20 m wide 5 200 m long in matrix ca na clinopyroxene implies the prior existence of a non stoichiometric supersilicic omphacite stabilized at ultrahigh pressure uhp 25 kbar conditions although no coesite or coesite pseudomorphs have been found in the samples the absence of coesite may be due to the lack of free silica at uhp conditions or the consumption of silica during retrograde reactions the inferred uhp conditions metamorphism is further supported by sm nd ages that are equivalent to triassic metamorphic ages from uhp eclogites in the southeastern dabie mountains this finding expands the uhp terrane northward about 50 km spatial distribution of subduction collision related uhp rocks includes parts of the ndc

first finding of eclogite facies metamorphic event in south korea and its correlation with the dabie sulu collision belt in china abstract metabasite lenses occur within granitic gneiss at bibong in the southwestern part of the gyeonggi massif south korea bibong metabasites experienced an initial eclogite facies metamorphism 17 0 20 9 kbar 835 860 c which was succeeded by a granulite facies metamorphism 11 5 14 6 kbar 830 850 c and finally overprinted by an amphibolite facies metamorphism 6 7 11 0 kbar 570 740 c the metabasites have sm nd internal isochron ages at 225 258 ma and the major elements of the metabasites display an island arc affinity the petrochemical and geochronological data suggest the southwestern part of the gyeonggi massif as an extension of the triassic dabie sulu collision belt in china

deep water facies processes and models a review and classification scheme for modern and ancient sediments abstract a review of previous work on modern and ancient deep water facies processes and models is presented with a new classification scheme involving 40 distinct facies related to 15 conceptually distinct facies groups these facies are fixed points in a spectrum of facies generated in a process continuum from resedimentation processes through semi permanent bottom currents to pelagic settling in essence the scheme is descriptive of the sedimentary attributes of sediments although it is designed to aid interpretation of possible sediment transport deposition processes the classification scheme is three tier with facies classes groups and constituent facies and is hierarchical to allow flexibility in its use there are seven facies classes with classes a e defined largely on the basis of grain size differences class f on the basis of internal organization and class g on composition the facies classes are class a gravels muddy gravels gravelly muds and pebbly sands with 5 gravel grade class b sands with 80 sand grade and 80 mud 40 silt and 0 20 sand class e muds and clays with 95 mud grade for the purpose of large scale mapping or reconnaissance fieldwork either the level of facies classes or groups may be appropriate whereas for more detailed sedimentology the more detailed facies level will be necessary

ultrahigh pressure minerals and metamorphic terranes the view from china abstract ultrahigh pressure uhp metamorphism refers to mineralogical modifications of continental and oceanic crustal protoliths associated mafic ultramafic rocks initially formed or emplaced in shallow levels of the lithosphere but which subsequently have experienced p t conditions within or above the coesite stability field 2 7 gpa 700 c typical products include eclogite garnet peridotite and uhp varieties of metapelite quartzite marble paragneiss and orthogneiss uhp metamorphic assemblages require relatively cold lithospheric subduction to mantle depths some recrystallization even occurs under forbidden p t conditions characterized by a geotherm of the current explosion of research on continental uhp terranes reflects their significance for mantle dynamics and the tectonics of continental subduction collision exhumation mantle slab interactions and geochemical recycling a further characterization of uhp phases and positive identification of uhp minerals requires new experimental studies coupled with state of the art analyses for example the very rare occurrence of microdiamond inclusions in zircons from dabie sulu uhp rocks may reflect high f o 2 attending recrystallization inasmuch as epidote is rather common rutile needles within garnets from sulu uhp eclogitic rocks may not be the result of exsolution so in such cases the apparent uhp pressure may have been over estimated hadean igneous microdiamond inclusions in jack hills detrital zircons could have originated from mantle xenoliths whereas abundant detrital phanerozoic diamonds containing inclusions of coesite and other eclogitic minerals from new south wales might have been derived from unexposed uhp metamorphic terranes micro mineral intergrowth and nano size minerals may hold important key to deciphering the actual p t paths of subduction and mantle return flow although most exhumed terranes have returned surfaceward relatively rapidly after short time of uhp condition the long duration of storage at great depth and slow exhumation for the largest uhp terranes remain as major problems

supersilicic clinopyroxene and silica exsolution in uhpm eclogite and pelitic gneiss from the kokchetav massif kazakhstan abundant exsolved quartz rods occur in matrix clinopyroxene of eclogite from the kokchetav massif kazakhstan these rocks are diamond grade ultrahigh pressure uhp metamorphic rocks that recrystallized at p 6 gpa and t 1000 c zircon is an excellent container which effectively protects peak uhp metamorphic phases from retrogression therefore to ascertain the pre exsolution composition of the clinopyroxene we analyzed clinopyroxene inclusions in zircon of the eclogite and a diamond bearing biotite gneiss clinopyroxene in zircon has an excess of fe 3 vi al over iv al na k and calculated cation totals significantly less than 4 0 per six o atoms the stoichiometry of these pyroxenes can be reconciled if the ca eskola end member ca 0 5 0 5 alsi 2 o 6 is considered the zircon hosted clinopyroxene in the eclogite contains up to 9 6 mol of the ca eskola component and in the biotite gneiss up to 18 2 mol whereas the matrix clinopyroxene contains much less 1 3 mol on average recalculation of the composition of the matrix clinopyroxene prior to exsolution of quartz rods yields 6 8 mol ca eskola component which is consistent with the composition of the clinopyroxene inclusions in zircon we conclude that the ca eskola component in the peak metamorphic clinopyroxenes broke down by a retrograde reaction 2 ca 0 5 square 0 5 alsi 2 o 6 rightarrow caal 2 sio 6 3 sio 2 resulting in exsolution of the quartz rods in the matrix clinopyroxene subducted crustal and supracrustal rocks form the ca eskola clinopyroxene at high pressures and temperatures the vacancy containing clinopyroxene may have an important bearing on the physico chemical properties of the subducted slab at upper mantle depth

diamond from the dabie shan metamorphic rocks and its implication for tectonic setting diamond occurs in ultrahigh pressure metamorphic rocks from dabie shan anhui province eastern china diamond bearing rocks include eclogite gamet pyroxenite and jadeitite diamond occurs in a mineral assemblage with coesite and jadeite the diamonds and diamondiferous rocks of dabie shan are interpreted to be the products of ultrahigh pressure metamorphism in the underthrust basement of the yangtze continental plate during the early mesozoic at greater than 4 0 gigapascals and 900 c this interpretation is based on the distribution of rock units the stability field of diamond and isotopic data indicating a crustal origin for the rocks most diamonds occur as euhedral inclusions in garnets and are 10 to 60 micrometers across although some are up to 700 micrometers across

shallow water eruption fed mafic pyroclastic deposits along a paleoproterozoic coastline kangerluluk volcano sedimentary sequence southeast greenland abstract the 200 300 m thick volcano sedimentary sequence at kangerluluk is part of the psammite zone one of four major zones which constitute the 1 8 ga ketilidian orogen in south greenland three lithofacies are emphasized in the study 1 the conglomerate sandstone 2 the volcanic and 3 the pyroclastic lithofacies the 2 40 m thick conglomerate sandstone lithofacies represents a subaerial to subaqueous fan delta deposit matrix and clast supported conglomerates are interpreted as debris flow and longitudinal gravel bar deposits erosive based conglomerate channel fills attest to stream incision trough crossbedded sandstone interpreted as lunate megaripples planar bedded sandstone indicative of upper flow regime bar top sands and small scale trough crossbeds reflecting ripples follow up section form collectively with the conglomerate 0 40 2 50 m thick fining upward sequences the sandstone dominated unit up section from the conglomerates and composed of planar and low angle crossbeds minor ripples and graded beds as well as mudstone is indicative of a lower shoreface deposit below normal wave base the clastic sedimentary rocks are suggestive of a fan delta setting the 100 200 m thick volcanic lithofacies composed of pillowed and pillow brecciated lava flows is consistent with shallow water deposition interstratification of lava flows with both conglomerate sandstone and pyroclastic lithofacies intrusion of dykes into volcaniclastic rocks and peperite formation accentuate contemporaneity between volcanism and sedimentation and is a common feature of island arcs the 1 50 m thick pyroclastic lithofacies with sharp depositional contacts to the overlying volcanic and underlying conglomerate sandstone lithofacies was emplaced in a subaqueous setting the lithofacies is divided into a planar to crossbedded tuff lapilli tuff and a bedded lapilli tuff breccia whereby both deposits are inferred to result from shallow water surtseyan type eruptions the 5 15 m thick bedded lapilli tuff breccia with abundant bomb sag structures and graded beds is considered a result of subaqueous eruptions strong enough to form an insulating steam cupola characterized by ballistically emplaced bombs that rapidly collapsed allowing for transport via mass flow processes the deposits are considered proximal to the vent the 2 50 cm thick planar to crossbedded tuff lapilli tuff featuring abundant euhedral and broken crystals of feldspar 2cm and minor pyroxene 1 cm are massive graded crossbedded and stratified the planar but laterally discontinuous beds characterized by abundant low angle scours are interpreted as low to high concentration sediment gravity flows produced directly from subaqueous tephra jets that collapsed due to massive water ingestion local breccia size pyroclasts disrupting beds are interpreted as bomb sags the mafic eruption fed surtseyan type deposits postulated to be a subaqueous counterpart of cold subaerial base surges originate from subaqueous tuff cones formed along a rugged volcanic dominated shoreline featuring high energy fan deltas

understanding mesozoic accretion in southeast asia significance of triassic thermotectonism indosinian orogeny in vietnam results from a zircon u pb study of the metamorphic basement of vietnam reveal that a large part of the continental crust was affected by a short lived episode of ductile deformation and high temperature metamorphism between 258 6 ma and 243 5 ma although coincident with final stages of north south china collision qinling orogenesis the thermotectonism in vietnam was caused by accretion of sibumasu to indochina south china this accretion event indosinian orogeny has regional significance because it contributed to the final stages of north south china collision an aspect not explicitly recognized in qinling orogenic models

time scales of intra oceanic arc magmatism from combined u th and u th he zircon geochronology of dominica lesser antilles the island of dominica located in the intra oceanic lesser antilles arc has produced a series of intermediate mostly andesitic lava domes and ignimbrites since the early pleistocene u th he eruption ages from centers across the island range from 3 to 770 ka with at least 10 eruptions occurring in the last 80 ka three eruptions occurred near the southern tip of dominica plat pays volcanic complex in the past 15 ka alone zircon u th ages from individual centers range from near eruption to secular equilibrium implicating protracted storage and recycling of zircons within the crust overlapping zircon crystallization peaks within deposits from geographically separated vents up to 40 km apart indicate that magma associated with separate volcanic edifices crystallized zircon contemporaneously two lava domes from the southern sector of the island display exclusively young zircon rim ages 50 ka with narrow crystallization peaks consistent with the construction of a new magma reservoir the younging of eruption and crystallization ages implies that the magmatic foci leading to the construction of this reservoir have migrated southward arc parallel over time overall our data support geochemical models for the ongoing construction of a silicic intrusive complex consisting of varying amounts of crystal mush beneath the island u pb zircon ages 1 2 ma indicate that accumulation of this complex is entirely quaternary in age together zircon u th and u pb ages for dominica suggest that the magmatic processes and time scales operating in intra oceanic arcs are similar to those documented for continental arcs

two stage decompression in garnet bearing mafic granulites from sostrene island prydz bay east antarctica mafic garnet bearing granulites from sostrene island 150 km southwest of davis station on the coast of prydz bay east antarctica exhibit two stage symplectic coronas on garnet formed after peak metamorphic conditions m1 an outer corona of opx mg66 pl an94 97 minor hbl mantles a finer grained inner corona of opx mg67 pl an95 96 spl mg36 both symplectites contain minor ilmenite magnetite intergrowths the finer grained symplectite also occurs along a fracture cleavage in the garnet the outer corona originated during a second metamorphic event m2 via the reaction grt cpx hbl sio2 opx pl 1 whereas the inner corona formed later in response to decompression and minor deformation resulting in the fracture cleavage in the garnet according to the reaction grt opx pl spl 2 the grossular content of the garent xgrs 0 168 is almost exactly that which is required for the stoichiometric breakdown by reaction 2 calculated xgrs 0 167 the mafic rocks are silica undersaturated and the sio2 for reaction 1 was most probably derived externally from the surrounding felsic gneisses preferred p t estimates for m1 based on garnet core prp40alm42grs17sps1 matrix opx cpx hbl pairs are c 10 kbar at 980 c the fine grained symplectite formed post peak m2 at c 7 kbar and 850 c the enclosing felsic gneisses yield pressure estimates of between 5 and 7 kbar which compare with conditions of c 6 kbar and 775 c in the nearby bolingen islands these lower p t estimates are considered to be representative of the widespread 1100 ma metamorphic event recognized in outcrops along the prydz bay coast the high p high t estimates derived from the garnet relics provide evidence for an earlier possibly archaean high grade metamorphic event

sedimentary record of explosive silicic volcanism in a cretaceous deep marine conglomerate succession northern antarctic peninsula lower cretaceous conglomeratic strata exposed on southern sobral peninsula were deposited on a deep marine apron in the back arc larsen basin close to its faulted boundary with the antarctic peninsula magmatic arc the succession is dominated by amalgamated beds of clast supported conglomerate which together with minor intercalated sandstones consist of varied but largely basaltic to andesitic volcanic material and clasts derived from the palaeozoic triassic meta sedimentary basement of the arc most of the volcanic clasts are thought to have been derived from lithified volcanic successions or older synvolcanic deposits rather than from sites of coeval eruption these mixed provenance strata enclose a number of intervals consisting mainly of inverse normally graded conglomerate and graded stratified pebbly sandstone in which the sand fraction is dominated by crystals and vitric grains considered to have been redeposited in the immediate aftermath of explosive silicic arc volcanism like syneruption deposits on non marine volcaniclastic aprons these intervals are more sand prone than the enclosing strata and appear to show evidence of unusually rapid aggradation plagioclase from one such interval has yielded 40ar 39ar ages concordant at 121 ma similar to those obtained from the non marine cerro negro formation deposited within the magmatic arc it is suggested that the two successions can be viewed as counterparts both recording a history of mainly basaltic to andesitic volcanism punctuated by relatively infrequent explosive silicic eruptions whereas the cerro negro formation consists mainly of syneruption deposits most of the volcaniclastic material delivered to the eruption distal deep marine apron appears to have been derived by normal degradation processes only rare silicic eruptions were capable of supplying pyroclastic material rapidly enough and in sufficient quantities to produce compositionally distinct syneruption intervals

submarine pyroclastic deposits formed at the soufriere hills volcano montserrat 1995 2003 what happens when pyroclastic flows enter the ocean the soufriere hills volcano montserrat west indies has undergone a series of dome growth and collapse events since the eruption began in 1995 over 90 of the pyroclastic material produced has been deposited into the ocean sampling of these submarine deposits reveals that the pyroclastic flows mix rapidly and violently with the water as they enter the sea the coarse components pebbles to boulders are deposited proximally from dense basal slurries to form steep sided near linear ridges that intercalate to form a submarine fan the finer ash grade components are mixed into the overlying water column to form turbidity currents that flow over distances 30 km from the source the total volume of pyroclastic material off the east coast of montserrat exceeds 280 106 m3 with 65 deposited in proximal lobes and 35 deposited as distal turbidites

silica precipitates in omphacite from eclogite at alpe arami switzerland evidence of deep subduction observations of oriented sio2 precipitates in omphacite from eclogite with tholeiitic basalt protolith bordering the alpe arami garnet peridotite massif ticino switzerland and petrological studies of the eclogitic mineral assemblages suggest that this rock was subjected to higher pressure metamorphism than previously realized we employed various calibrations of the fe2 mg exchange thermometer and calculations of equilibria with thermodynamic data considering the calcium tschermak s component caal2sio6 of garnet pyroxene pairs from these calculations it is concluded that the eclogitic lenses have recorded at least four stages of mineral growth corresponding to the following stage i prograde c 2 4 gpa 700 c stage iia maximum recorded grade c 7 0 gpa 1100 c stage iib retrograde c 3 7 gpa 900 c stage iii retrograde c 2 1 gpa 750 c because of the preservation of stage i a relatively rapid subduction and exhumation of alpe arami eclogite is suggested the exhumation path of the eclogitic rock is in good agreement with most exhumation paths inferred for the alpe arami garnet lherzolite proposed previously by several authors based upon a variety of different observations although the eclogite and peridotite exhumation paths may diverge at depths greater than 120 km

precipitates of α cristobalite and silicate glass in uhp clinopyroxene from a bohemian massif eclogite

a magmatic hydrothermal lacustrine exhalite from the permian lucaogou formation santanghu basin nw china the volcanogenic origin of fine grained clastic sedimentary rocks abstract shales in the middle permian lucaogou formation in the intracontinental santanghu rift basin have been considered as typical organic rich profundal shales for decades our study of well cores using petrographic microscope and scanning electron microscopy suggests an otherwise complex hydrovolcanic and hydrothermal origin this paper describes characteristics of a particular type of the shales composed of fine grained detrital minerals and lithic grains some of them are orthopyroxene calcite peralkaline feldspars and analcime that are interpreted as derived from peralkaline alkaline carbonatite pyroxenite analcime phonolite and andesite whereas others are quartz dolomite ankerite serpentine and calcite that were precipitated from syndepositional or penecontemporary hydrothermal fluids grain size ranges from 0 001 to 2 mm mostly 0 01 0 1 mm well developed laminae are mostly 0 5 3 mm thick and alternate with tuffaceous dolomicrite the rocks are interpreted as sublacustrine hydrovolcanic deposits which had been altered by syndepositional hydrothermal fluids the interpretation is substantiated by abundant cone shaped stratigraphic buildups on seismic sections in the basin this study shows an ancient example of volcanic hydrothermal deposits in a rift basin

two examples of subaqueously welded ash flow tuffs the visean of southern vosges france and the upper cretaceous of northern anatolia turkey abstract pyroclastic deposits interpreted as subaqueous ash flow tuff have been recognized within archean to recent marine and lacustrine sequences several authors proposed a high temperature emplacement for some of these tuffs however the subaqueous welding of pyroclastic deposits remains controversial the visean marine volcaniclastic formations of southern vosges france contain several layers of rhyolitic and rhyodacitic ash flow tuff these deposits include from proximal to distal settings breccia lapilli and fine ash tuff the breccia and lapilli tuff are partly welded as indicated by the presence of fiamme fluidal and axiolitic structures the lapilli tuff form idealized sections with a lower coarse and welded unit and an upper bedded and unwelded fine ash tuff sedimentary structures suggest that the fine ash tuff units were deposited by turbidity currents welded breccias interbedded in a thick submarine volcanic complex indicate the close proximity of the volcanic source the lapilli and fine ash tuff are interbedded in a thick marine sequence composed of alternating sandstones and shales presence of a marine stenohaline fauna and sedimentary structures attest to a marine depositional environment below storm wave base in northern anatolia thick massive sequences of rhyodacitic crystal tuff are interbedded with the upper cretaceous marine turbidites of the mudurnu basin some of these tuffs are welded as in southern vosges partial welding is attested by the presence of fiamme and fluidal structures the latter are frequent in the fresh vitric matrix these tuff units contain a high proportion of vitroclasis and were emplaced by ash flows welded tuff units are associated with non welded crystal tuff and contain abundant bioclasts which indicate mixing with water during flowage at the base basaltic breccia beds are associated with micritic beds containing a marine fauna the welded and non welded tuff sequences are interbedded in an alternation of limestones and marls these limestones are rich in pelagic microfossils the evidence above strongly suggest that in both examples tuff beds are partly welded and were emplaced at high temperature by subaqueous ash flows in a permanent marine environment the sources of the pyroclastic material are unknown in both cases we propose that the ash flows were produced during submarine fissure eruptions such eruptions could produce non turbulent flows which were insulated by a steam carapace before deposition and welding the welded ash flow tuff deposits of southern vosges and northern anatolia give strong evidence for existence of subaqueous welding

revised chronostratigraphy of recurrent ignimbritic eruptions in dominica lesser antilles arc implications on the behavior of the magma plumbing system ignimbritic eruptions represent catastrophic events due to the magma volume involved and the related consequences on earth s environment in relation with the released gases and the tephra dispersal dominica has been recognized as hosting one of the major ignimbritic eruptions of the last 200 ky in the lesser antilles arc called the roseau tuff but more recent works have evidenced several pumiceous events instead of a single large one here we propose a revised chronostratigraphy of the explosive activity that occurred in the last tens of thousands years based on three field trips new 14c ages detailed lithological and geochemical investigations in particular a precise characterization of trace element glass chemistry this eruptive history reconstruction is mainly based on outcrops along the coast and in the valley since the luxury vegetation in the center of the island mostly precludes sections close to the central volcanic centers we thus confirm that the roseau event has been overestimated and that we may recognize five main ignimbritic events grande savane layou 51 ka grand bay roseau 33 ka grand fond 24 ka we discuss the possible volcanic center at their origin in addition to correlations with some plinian events of lower magnitude that were identified in the roseau valley and in the southern part of dominica this study may help to better constrain the eruptive history of the most active volcanic island of the lesser antilles arc which has important implications on hazard mitigation

paleomagnetic evidence for high temperature emplacement of the 1883 subaqueous pyroclastic flows from krakatau volcano indonesia the paroxysmal 1883 eruption of krakatau volcano in indonesia discharged at least 6 5 km3 dense rock equivalent of pyroclastic material into the shallow waters of the sunda straits within a 15 km radius of the volcano progressive thermal demagnetization studies of individually oriented pumice clasts from a core sample of the submarine pyroclastic deposits show that 41 out of 47 clasts exhibit single component remanence with mean inclination of 2 4 the partial thermoremanent magnetization components of both pumice and lithic clasts are well grouped in orientation indicating that substantial cooling of clasts must have occurred following deposition estimated subaqueous emplacement temperature for such clasts is greater than 500 c rare two component lithic fragments exhibit inflection points on vector endpoint diagrams that mark the temperature below which the fragments acquired magnetization of similar orientation these inflection points range from 350 to 550 c indicating a minimum subaqueous emplacement temperature of 350 c paleomagnetic evidence for high emplacement temperature supports the hypothesis that proximal 1883 submarine pyroclastic deposits resulted from entrance of hot subaerially generated pyroclastic flows into the sea similar deposits have been interpreted from the geologic record but this is the first documented example of submarine pyroclastic flows from a historic eruption the krakatau deposits thus serve as an important modern analog for the study of pyroclastic flow seawater interactions

multiple breakdown and chemical equilibrium of silicic clinopyroxene under extreme metamorphic conditions in the kontum massif central vietnam clinopyroxene in ultrahigh temperature mafic granulites from the kontum massif in central vietnam records multiple metamorphic stages manifested as exsolution textures quartz rods and orthopyroxene hornblende plagioclase needles and as symplectitic intergrowths involving clinopyroxene plagioclase these textures suggest a metamorphic evolution characterized by decompression and subsequent cooling from eclogite facies to amphibolite facies conditions through ultrahigh temperature conditions quartz rods in clinopyroxene and clinopyroxene plagioclase symplectites were formed under eclogite conditions prior to ultrahigh temperature metamorphism the orthopyroxene hornblende plagioclase needles in clinopyroxene are regarded as cooling products after ultrahightemperature metamorphism recalculated compositions of precursor clinopyroxene show supersilicic composition during the metamorphic evolution the chemical composition varies from silicic ca eskola rich via sodic jadeite rich to aluminous ca tschermak rich compositions presence of supersilicic clinopyroxene suggests that the granulite decompressed from possible ultrahigh pressure conditions ca 800 900 c at 2 3 gpa preceding the ultrahigh temperature stage 1050 c at 1 3 gpa which provide strong constraints on the tectonic evolution of the indochina region and it also provides insights on crustal exhumation at a continental collision zone another significant aspect of this study is that the breakdown textures of clinopyroxene and its chemical variations may provide important information in establishing pre and post peak evolution especially for extremely high temperature or high pressure metamorphic rocks

large scale phreatomagmatic silicic volcanism a case study from new zealand abstract pyroclastic deposits of the 20 000 year old wairakei formation occur in the region surrounding lake taupo in the central north island of new zealand recent advances in the correlation of both proximal and distal equivalents of the wairakei deposits show that they were the result of one large eruption the ash layers once covered most of new zealand they are also present in deep sea cores from the sw pacific and on the chatham islands the dispersal and grain size parameters of the ash fall members indicate that their vent area was within the taupo volcano tectonic depression which for a long period has contained a large lake the wairakei eruption is interpreted to have been phreatomagmatic throughout each phase of the eruption sequence generated its own characteristic deposits the sequence of events can be summarized as follows 1 eruption and deposition of a white fine grained bedded fall deposit 2 eruption and deposition of a medium grained pumice fall deposit 3 generation of ultra fine wet ash clouds with a fall deposit accumulating largely as aggregates and accretionary lapilli accompanied by base surges 4 formation of water cooled pyroclastic flows and deposition of an extensive ignimbrite 5 return to a dominantly vertical eruption column and deposition of a medium grained bedded ash fall immediately followed by another wet ultra fine fall deposit charged with accretionary lapilli finally 6 an ignimbrite less widespread than that of phase 4 was produced co ignimbrite fall deposits of fine vitric dust and medium grained pumice were laid down over a huge area there may have been a short hiatus in activity between phases 4 and 5

petrology of ultrahigh pressure rocks from the southern su lu region eastern china abstract in the su lu ultrahigh p terrane eastern china many coesite bearing eclogite pods and layers within biotite gneiss occur together with interlayered metasediments now represented by garnet quartz jadeite rock and kyanite quartzite in addition to garnet omphacite rutile coesite other peak stage minerals in some eclogites include kyanite phengite epidote zoisite talc nyboite and high al titanite the garnet quartz jadeite rock and kyanite quartzite contain jadeite quartz garnet rutile zoisite apatite and quartz kyanite garnet epidote phengite rutile omphacite assemblages respectively coesite and quartz pseudomorphs after coesite occur as inclusions in garnet omphacite jadeite kyanite and epidote from both eclogites and metasediments study of major elements indicates that the protolith of the garnet quartz jadeite rock and the kyanite quartzite was supracrustal sediments most eclogites have basaltic composition some have experienced variable crustal contamination or metasomatism and others may have had a basaltic tuff or pyroclastic rock protolith the su lu ultrahigh p rocks have been subjected to multi stage recrystallization and exhibit a clockwise p t path inclusion assemblages within garnet record a pre eclogite epidote amphibolite facies metamorphic event ultrahigh p peak metamorphism took place at 700 890 c and p 28 kbar at c 210 230 ma the symplectitic assemblage plagioclase hornblende epidote biotite titanite implies amphibolite facies retrogressive metamorphism during exhumation at c 180 200 ma metasedimentary and metamafic lithologies have similar p t paths several lines of evidence indicate that the supracrustal rocks were subducted to mantle depths and experienced in situ ultrahigh p metamorphism during the triassic collision between the sino korean and yangtze cratons

coesite in clinopyroxene in the caledonides and its implications for geodynamics coesite is reported for the first time from the caledonide orogen it occurs as inclusions in clinopyroxene in the dolomite eclogite at grytting norway and provides valuable new evidence to support the hypothesis of extremely high pressure 30 kbar in certain norwegian eclogites this finding has implications for the geodynamics and kinetics of the concepts of microscale intracrystalline tectonic overpressure and of macroscale local or regional tectonic overpressure and for the hypotheses of subduction and obduction involving a more than 100km vertical component of transport

metamorphic evolution of high pressure and ultrahigh temperature granulites from the highland complex sri lanka abstract sapphirine quartz bearing pelitic granulites garnet clinopyroxene quartz bearing mafic granulites and quartzo feldspathic granulites with corundum garnet quartz constitute rare but important members of the highland complex in sri lanka peak metamorphic conditions from the highland complex generally have been considered to be up to 850 900 c and 8 10 kbar however this study on the above mentioned rocks indicates that ultrahigh temperature and high pressure conditions 1100 c and 12 kbar were attained during peak conditions a metamorphic evolution of the ultrahigh temperature metamorphic rocks was determined from careful analyses of shifts in divariant assemblages and reaction textures this shows a clockwise p t path from more high pressure conditions 1000 c and 17 kbar stage 0 as part of the prograde metamorphic path to lower pressure and temperature conditions 950 c and 9 kbar stage 4 as part of the retrograde path through the peak metamorphic conditions stage 1 widespread lower pressure and temperature granulite facies metamorphic rocks surround the ultrahigh temperature granulites and are interpreted to have formed by the strong effect of the retrograde metamorphism and deformation the widely reported pan african metamorphic ages derived from similar granulite facies metamorphic rocks in the gondwana fragments highland complex in sri lanka lutzow holm complex in east antarctica etc may also be the result of retrograde metamorphism of ultrahigh temperature metamorphic rocks there remains a possibility that this early ultrahigh temperature high pressure granulite facies metamorphism in the highland complex as well as that in the lutzow holm complex might pre date pan african metamorphism

sedimentology of the minoan deep sea tephra layer in the aegean and eastern mediterranean the minoan eruption of santorini resulted in deposition of pyroclastic material over a large area of the aegean sea and eastern mediterranean the eruptive activity commenced with a plinian phase which was followed by a phreatomagmatic phase and ended with numerous pyroclastic flows we present a sedimentological study of the minoan ash deposits found in cores taken throughout the region the cores are divided into three groups 1 those from the sub marine slopes immediately surrounding santorini 2 those from the aegean sea where the topography consists of many steep sided basins and 3 those from the eastern mediterranean where the topography is relatively flat and smooth this last group is designated the distal ash zone the deposits from the aegean sea have been remobilised as turbidity currents grain size distribution analysis reveals that these deposits are bimodal the mdo of the coarse population increasing away from source whilst that of the fine population remains uniform irrespective of distance from source the bimodality is caused by a combination of aggregation of fine particles in ash clouds produced during the last two phases of the eruptive activity and mixing of the coarse plinian and fine phreatomagmatic and co ignimbrite deposits within the turbidity current an unusual feature found in two of the cores is a coarse density sorted layer beneath the bouma a division of several turbidite units this is interpreted as an example of a flow head deposit as described by sparks and wilson 1983 the deposits from the distal ash fall zone are fine grained and unimodal containing phreatomagmatic and co ignimbrite material they show only very minor remobilisation of the original air fall ash due to the flat topography of the area the mdo of the deposit remains uniform irrespective of distance from source and is very similar to that of the fine population found in the bimodal aegean deposits this is interpreted as the result of aggregation of fines in the phreatomagmatic and co ignimbrite ash clouds the deposits in both areas have been extensively bioturbated a study of dispersed ash in the cores to reconstruct the original thickness of the ash layers suggests that ash layers less than 0 62 cm thick will be completely dispersed in the surrounding sediment whilst those thicker than 2 5 cm will be preserved as discrete layers the base being unaffected by bioturbation the deposits on the submarine slopes of santorini were not originally deposited as air fall ash but are derived from the erosion of coastal exposures of non welded ignimbrite they form storm sand layers and are typical of the type of deposit that may be formed by rapid erosion of fresh pyroclastic material after the eruption of an island arc volcano

quartz exsolution in clinopyroxene is not proof of ultrahigh pressures evidence from eclogites from the eastern blue ridge southern appalachians u s a oriented quartz needles in clinopyroxene have become one of the diagnostic indicators of ultrahigh pressure uhp metamorphism the presence of apparently exsolved quartz is taken as evidence of decompression of a non stochiometric ca eskola component ca0 5 0 5alsi2o6 caes that is presumed to be stable only at uhp conditions eclogite from the eastern blue ridge north carolina contains clinopyroxene jd20cats5ac5caesdi65hd5 with oriented needles of quartz and calcic amphibole that appear to have exsolved together the quartz amphibole intergrowths are surrounded by 1 5 m haloes of neoformed pyroxene jd10cats10ac5caesdi70hd5 the modes of quartz amphibole and clinopyroxene haloes were determined using bse images and reintegrated with the host clinopyroxene viewing the quartz and amphibole needles down the c axis of the pyroxene host provides a better estimate of their proportions than in prismatic sections reintegrated pyroxene compositions were nearly identical to the analyzed host pyroxene with no caes component clinopyroxene with caes solid solution has been repeatedly synthesized at uhp conditions however examination of the phase equilibria usually cited as evidence for caes stability at conditions of 25 kbar shows that clinopyroxene with 10 mol caes is stable well within the quartz field and provides a pressure minimum similar to the albite jadeite quartz barometer exsolution of quartz and associated amphibole is commonplace in clinopyroxene from the blue ridge eclogite that lacks coesite or other evidence for uhp metamorphism the presence of a diluted 5 10 caes component in clinopyroxene does not require uhp conditions

ultrahigh pressure metamorphism and multistage exhumation of eclogite of the luotian dome north dabie complex zone central china evidence from mineral inclusions and decompression textures abstract the studied eclogite is located in the luotian dome in the southwestern part of the north dabie complex zone ndz central china and is a portion of deeply subducted mafic lower continental crust of the south china block petrologic analysis suggests that the eclogite underwent ultrahigh pressure uhp and high pressure eclogite facies metamorphism and subsequent hp granulite facies overprinting and amphibolite facies retrogression during continental subduction and exhumation as a result multiple decompression textures were produced suggesting a multistage exhumation history from eclogite to granulite to amphibolite facies conditions a striking feature of this eclogite is the widespread exsolution of different phases oriented needles of rutile clinopyroxene amphibole apatite in garnet quartz amphibole hypersthene sodic plagioclase in clinopyroxene and pyrrhotite in apatite most importantly we provide for the first time conclusive evidence of quartz pseudomorphs after coesite in garnet and relic coesite confirmed by in situ raman spectroscopy in zircon in addition there are two groups of apatite one is fluor f apatite that occurs as inclusions in garnet or in matrix which is rich in f 3 wt lakes exsolution features and formed at uhp metamorphic conditions the second is associated with the oriented pyrrhotite exsolution and formed during decompression microtextural and petrologic analysis suggests that the eclogites from the southwestern segment of the ndz are similar to eclogites from the northern segment of the ndz e g huangweihe and baizhangyan and underwent uhp metamorphism with a peak pressure 5 7 gpa these results combined with published geochronological data imply that the ndz experienced triassic uhp metamorphism as a coherent unit

late quaternary explosive silicic volcanism on st lucia west indies many explosive eruptions of dacitic magmas have occurred on st lucia during the late quaternary these have produced widespread aprons and fans of pumice flow and ash flow deposits radiating around the central highlands with co eruptive air fall and surge layers interbedded with palaeosols and epiclastic deposits vents in the highlands have not been located because of the dense tropical jungle but we suspect they are now plugged by lava domes surrounded by aprons of block and ash flow deposits young magmatically related dacitic lava domes have been extruded in the qualibou depression the pumice succession can be divided into older quartz poor deposits forming the choiseul pumice and younger crystal rich deposits with abundant large quartz which are called the belfond pumice the choiseul pumice groups together scattered remnants of the products of many eruptions of different low silica dacitic magma types the belfond pumice is the product of several eruptions of a high silica magma type and 14 c ages have dated these between 20900 to 34200 years b p the pumice flow deposits occur as small volume valley fills a granulometric study of belfond pumice flow deposits shows them to be strongly depleted in finer ash and vitric components it is suggested that the narrow winding and vegetated valleys on the island locally induced turbulence and the flows moved with large highly fluidized and inflated heads resulting in substantial loss of fine vitric ash one ash flow deposit which is extremely rich in crystals and carbonized vegetation is highly depleted in fines and shows enhanced vitric losses this flow may have been a much more violent ash hurricane or blast which surmounted topography ingesting large amounts of lush vegetation ignition of this released the large quantities of gas needed to elutriate most of the fines a model is suggested for the recent volcanic activity on st lucia in which separate batches of silicic magma each having a distinctive petrological and chemical character rose into high level chambers over a large area eruptions of volatile rich magma led to highly explosive pumice forming activity from vents in the central highlands degassed and more crystal rich magma was extruded later from the same vents or in the attenuated flank of the qualibou depression to from lava dome complexes

stratigraphy sedimentology and physical volcanology of the henik group central ennadai rankin greenstone belt northwest territories canada late archean paleogeography of the hearne province and tectonic implications abstract extensive areas of the hearne and rae provinces are underlain by archean metavolcanic and metasedimentary rocks in the rae province supracrustal sequences with prominent continental to shallow water quartz arenites have been interpreted by others as rift and passive margin deposits in the hearne province to the southeast the ennadai rankin greenstone belt is exposed in an area ca 700 200 km forming the second largest greenstone belt in canada it comprises mainly greenschist facies grade mafic and felsic volcanic siliciclastic chemogenic and volcaniclastic rocks stratigraphic sedimentologic and physical volcanologic data based on 1 50 000 scale mapping in the central part of the belt indicate deposition in a subaqueous mafic volcanic plateau slope basin system that was well removed from a continental influence water depth was below storm wave base as indicated by preservation of delicate lamination in pelitic and iron formation units absence of features suggestive of subaerial exposure oscillatory flow or tidal currents and lack of interfingering rocks that could represent laterally adjacent shelf coastal or continental environments reconnaissance level regional geologic and geochronologic data indicate an overall pattern of supracrustal deposition in the hearne province consisting of a large mafic volcanic plain isolated felsic volcanic centres with lenses and belts of deep water sedimentary rocks at least two speculative yet testable tectonic models are consistent with the inferred paleogeography for both the rae province is considered to be the western continental hinterland to the ensimatic hearne province the rae and hearne provinces may record late archean continental extension sea floor spreading oceanic volcanic arc growth diachronous back arc spreading and collapse and arc continent collision with thrusting of oceanic rocks on to attenuated rae crust alternatively the hearne province may represent northwestward accretion of multiple volcanic arc trench pairs

ignimbrite depositional facies the anatomy of a pyroclastic flow a model is presented for the depositional regimes in a pyroclastic flow to explain the origin of facies in the taupo ignimbrite and to compare these with other published examples in this model a pyroclastic flow consists of a head a body and a tail the head which is where fluidization caused by air ingestion occurs generates layer 1 deposits on lithological grounds these are divided into layers 1 p which is fines depleted and rich in pumice and 1 h which is fines depleted and rich in lithics and crystals layer 1 p represents material thrown forwards from the flow head and is termed the jetted deposits layer 1 h represents material sedimented from within the flow head and is termed the ground layer the body represents the bulk of the flow and the tail is its trailing part which is slowed by ground friction these parts generate the layer 2 deposits which include the valley pond ignimbrite and its associated ignimbrite veneer deposit localized depositional modes within the body and tail generate distinctive coarse pumice concentration zones and pumiceous lee side lenses behind obstacles at its outer limits all of the flow on interfluves is affected by air ingestion fluidization producing the distant facies which combines features of both layer 1 and 2 deposits one important conclusion of this work is that the thicknesses and compositions of the various facies are not related in any simple way to the thickness and composition of the parent flow

mihi breccia a stack of lacustrine sediments and subaqueous pyroclastic flows within the taupo volcanic zone new zealand abstract the taupo volcanic zone tvz new zealand encompasses a wide variety of arc related strata although most of its small volume non caldera forming eruptions are poorly exposed and extensively hydrothermally altered the mihi breccia is a stratigraphic sequence consisting of interbedded rhyolitic pyroclastic flows and lacustrine sediments with eruption ages of 281 18 to at least 239 6 ka uncertainties at 2 in contrast to other small volume rhyolitic eruptions within the tvz mihi breccia is relatively well exposed within the paeroa fault block and contains minimal hydrothermal alteration pyroclastic flow characteristics and textures include 1 prismatically jointed juvenile clasts 2 lack of welding 3 abundant ash rich matrix 4 lack of fiamme and eutaxitic textures 5 lack of thermal oxidation colors 6 lack of cooling joints 7 exclusive lacustrine sediment lithic clasts and 8 interbedding with lacustrine sediments all indicating that mihi breccia strata originated in a paleo lake system this ephemeral paleo lake system is inferred to have lasted for 50 kyr based on mihi breccia age constraints and referred to as huka lake mihi breccia pyroclastic flow juvenile clast geochemistry and petrography correspond with similar aged 264 8 263 10 and 247 4 ka intra caldera rhyolite domes filling the reporoa caldera source of the 281 ka kaingaroa formation ignimbrite these exposed intra caldera rhyolite domes as well as geophysically inferred subsurface domes are proposed to be source vents for the mihi breccia pyroclastic flows soft sediment deformation associated with mihi breccia strata indicates either seismic shock rapid sediment loading during pyroclastic flow emplacement or both thus the mihi breccia reflects a prolonged series of subaqueous rhyolite dome building and associated pyroclastic flows accompanied by seismic activity emplaced into a large paleo lake system within the tvz

timing of magmatism and ultrahigh to high grade metamorphism in the kannak complex kon tum massif vietnam magmatic activity and its tectonic implications abstract detailed geochronological and petrological investigations of metamorphic and igneous rocks can significantly improve our understanding of the tectonic processes that occur in orogenic belts the kon tum massif central vietnam includes various types of magmatic and metamorphic rocks the latter including ultrahigh to high temperature units the late permian early triassic metamorphic rocks in the massif correspond to the metamorphic characteristics of a collision between the indochina and south china blocks in terms of their metamorphic evolution and timescales of deformation u pb geochronological analyses of zircon obtained from two samples of garnet granite in the massif yielded magmatic ages of 239 6 3 9 ma and 243 5 3 4 ma this permitted the reevaluation of previous th u pb electron microprobe analyses of monazite from the garnet granite and garnet orthopyroxene gneiss and previously published u pb dating these data reveal the metamorphic and magmatic history of the western part of the massif the metamorphic rocks show a thermal evolution from 260 ma to 240 ma magmatic activity started at 260 ma and ceased at 240 ma considering this in terms of the regional scale igneous activities in the kon tum massif the base of the massif already thickened by the collision was situated above underplating plume related magma such magmatic activity resulted in high to ultrahigh temperature metamorphism and crustal melting during the late permian to early triassic between 260 and 240 ma

submarine volcaniclastic rocks in this chapter we are concerned with underwater volcaniclastic eruptions and products as well as pyroclastic flows and debris generated on land and transported by mass flowage into the sea submarine fallout deposits derived from land based eruptions are treated in chapter 7 clastic materials redistributed as mass flows or turbidity currents originating from the rapid build up of tephra and epiclastic volcanic debris along coastal parts of volcanoes are also discussed in various places throughout the text e g chap 13

sedimentary signatures of the entrance of coarse grained volcaniclastic flows into the sea the example of the breccia units of the las palmas detritic formation mio pliocene gran canaria eastern atlantic spain abstract the transformations of volcaniclastic gravity flows entering the sea remain poorly understood they are recorded by sedimentary signatures in the volcaniclastic sedimentation of the mio pliocene las palmas detritic formation lpdf of the volcanic island of gran canaria these gravity flows are contemporaneous with an erosive period on the island succeeding the emplacement of the fataga volcanic group and the beginning of the evolution of the roque nublo stratovolcano the sedimentary section that crops out at the mouth of a paleobarranco at las cuevas del guincho comprises subtidal calcareous and clastic sediments affected by wave and storm action units 1 and 3 beach gravels unit 4 volcaniclastic deposits represented by reworked phonolitic pyroclastic deposits unit 1 breccia deposits that contain juvenile and phonolitic gravels and blocks mainly incorporated at the shoreline unit 2 and a debris avalanche deposit at the top unit 5 the breccia deposits display a reversely graded base which disappears distally as a result of seawater incorporation and liquidization of the volcaniclastic mass and mixing with marine sediment this breccia unit is interpreted as having been emplaced by a granular mass flow after transformation of a block and ash flow deposit that entered the sea in the debris avalanche deposits thermal quenching of juvenile basaltic lava clasts and large inclined clastic intrusions of underlying marine sediments suggest that the debris avalanche entered the sea rapid subaqueous emplacement of the granular mass flow and of the debris avalanche deformed the sedimentary substratum water incorporation reduced the influence of the volcaniclastic flows on the substratum ams data confirm the effect of water ingestion and correlative loss of cohesion and allow also reconstruction of paleoflow directions that were normal to the paleoshoreline of the northern coast of gran canaria all these volcaniclastic gravity flow deposits and their textural characteristics are particularly useful in the reconstruction of processes that occur at the subaerial submarine transition and for the location of ancient shorelines

metamorphic evolution of diamond bearing and associated rocks from the kokchetav massif northern kazakhstan representative diamond bearing gneisses and dolomitic marble eclogite and ti clinohumite bearing garnet peridotite from unit i at kumdy kol and whiteschist from unit ii at kulet eastern kokchetav massif northern kazakhstan were studied diamond bearing gneisses contain variable assemblages including grt bt qtz pl kfs zo chl tur cal and minor ap rt and zrn abundant inclusions of diamond graphite chlorite or calcite phengite clinopyroxene k feldspar biotite rutile titanite calcite and zircon occur in garnet diamond bearing dolomitic marbles consist of dol di grt phl inclusions of diamond dolomite graphite biotite and clinopyroxene were identified in garnet whiteschists carry the assemblage ky tlc grt rt garnet shows compositional zoning and contains abundant inclusions of talc kyanite and rutile with minor phlogopite chlorite margarite and zoisite inclusions and zoning patterns of garnet delineate the prograde p t path inclusions of quartz pseudomorphs after coesite were identified in garnet from both eclogite and gneiss other ultrahigh pressure uhp indicators include na bearing garnet up to 0 14 wt na2o with omphacitic cpx in eclogite occurrence of high k diopside up to 1 56 wt k2o and phlogopite in diamond bearing dolomitic marble and cr bearing kyanite in whiteschist these uhp rocks exhibit at least three stages of metamorphic recrystallization the fe mg partitioning between clinopyroxene and garnet yields a peak temperature of 800 1000 c at p 40 kbar for diamond bearing rocks and about 740 780 c at 28 35 kbar for eclogite whiteschist and ti bearing garnet peridotite the formation of symplectitic plagioclase amphibole after clinopyroxene and replacement of garnet by biotite amphibole or plagioclase mark retrograde amphibolite facies recrystallization at 650 680 c and pressure less than about 10 kbar the exsolution of calcite from dolomite and development of matrix chlorite and actinolite imply an even lower grade greenschist facies overprint at c 420 c and 2 3 kbar a clockwise p t path suggests that supracrustal sediments together with basaltic and ultramafic lenses apparently were subjected to uhp subduction zone metamorphism within the diamond stability field tectonic mixing may have occurred prior to uhp metamorphism at mantle depths during subsequent exhumation and juxtaposition of many other tectonic units intense deformation chaotically mixed and mylonitized these lithotectonic assemblages

the behaviour of subaerially produced pyroclastic flows in a subaqueous environment evidence from the roseau eruption dominica west indies abstract the roseau tephra 30 000 yrs b p with its source on the island of dominica represents the largest eruption in the caribbean in the last 200 000 years pyroclastic flows generated by the eruption flowed offshore into the grenada basin giving rise to a widespread layer of sediment gravity flow deposits on land the flow deposits are valley ponded and their granulometric characteristics allow them to be classified as weakly fines depleted ignimbrites offshore the characteristics of the sediment gravity flow deposits are consistent with deposition by numerous small scale events rather than a single large scale event a comparison of the on land and offshore deposits reveals an increase in the crystal lithic ratio in the latter this change can be best explained by the fragmentation of hot pyroclastic material as the subaerial flows mixed with water mixing and fragmentation occurred not at the air water interface but as flows descended the steep submarine slopes of the island of dominica

calderas or gravity slide structures in the lesser antilles island arc abstract large arcuate depressions up to about 10 km in diameter on the back arc side of st lucia dominica and st vincent in the lesser antilles arc are discussed the qualibou depression on st lucia has been previously described as a caldera partly because it was believed to be the source of ignimbrites on the island remapping and stratigraphic studies of these ignimbrites show they were not erupted from the depression but from central vents in the highland rainforests and are much younger dominica is geologically similar and has two scarped depressions at the southern end of the island soufriere volcano st vincent has a prominent arcuate scarp opening to the west the southern half of which has been buried under the youngest volcanic products of the volcano we interpret these structures as the root zones of gravity slides perhaps eruption related and triggered by magmatic inflation post slide activity lava domes and fumaroles has occurred within them due to reduction in lithostatic load and pressure release on the flanks of these island volcanic complexes the presence of these structures only on the western coasts is correlated with the steeper 9 slopes into the grenada basin in contrast to the 1 submarine slopes of the atlantic side

primary or secondary distal volcaniclastic turbidites how to make the distinction an example from the miocene of new zealand mahia peninsula north island abstract miocene marine volcaniclastic deposits occur in mahia peninsula north island new zealand and were sedimented in a forearc setting related to the hikurangi trench subduction system these deposits are interbedded with hemipelagic marls and correspond to simple or amalgamated centimetric to metric thick turbiditic sequences volcaniclastic material is mainly composed of vitric particles with crystals quartz plagioclase with minor biotite amphibole pyroxene and oxides which are well represented in the coarse grained fraction the glass shards are mainly rhyolitic in composition three types of volcaniclastic turbidites were distinguished with geochemical data because distinction is impossible on sedimentary characteristics 1 primary monomagmatic turbidites contain both magmatic bubble wall pumice and shards and phreatomagmatic blocky shards with few vesicles and hydroclastically fragmented pyroclasts vitric particles the chemical compositions of the vitric particles and the crystals are very homogeneous suggesting a cogenetic origin these turbidites directly result from unique eruptive events and are probably related to the entrance of hot subaerial pyroclastic flows into the sea which also led to their transformation into subaqueous gravity flows 2 secondary monomagmatic turbidites never contain phreatomagmatic pyroclasts and the glass compositions display a trend from andesites to rhyolites there is a strong linear correlation in the compositions that suggest that the glass particles are derived from the same magma crystals also show a compositional homogeneity these deposits reflect the succession of several eruptions related to a unique magmatic system and result of the reworking of volcaniclastic material after relatively short storage on the shelf 3 secondary multimagmatic turbidites do not display compositional homogeneity of their vitric and mineral components this implies that the volcaniclastic material has been stored during a relative long period on the shelf before remobilization and that this sedimentation records the volcanic activity of multiple magmatic sources consequently it has been possible to distinguish primary volcaniclastic turbidites that are directly related to the volcanic activity from secondary turbidites that result from reworking of previously deposited material on the shelf monomagmatic turbidites can be used as stratigraphic and magmatic markers whereas multimagmatic cannot multimagmatic turbidites however record the evolution of the volcanic arc during longer periods this leads to the conclusion that the mahia peninsula volcaniclastic turbidites distally record the evolution of the source volcanic arc

characteristics of submarine pumice rich density current deposits sourced from turbulent mixing of subaerial pyroclastic flows at the shoreline field and experimental assessment this study investigates the types of subaqueous deposits that occur when hot pyroclastic flows turbulently mix with water at the shoreline through field studies of the znp marine tephra in japan and flume experiments where hot tephra sample interacted with water the znp is a very thick pumice rich density current deposit that was sourced from subaerial pyroclastic flows entering the japan sea in the pliocene notable characteristics are well developed grain size and density grading lithic rich base pumice rich middle and ash rich top preponderance of sedimentary lithic clasts picked up from the seafloor during transport fine ash depletion in coarse facies and presence of curviplanar pumice clasts flume experiments provide a framework for interpreting the origin and proximity to source of the znp tephra on contact of hot tephra sample with water steam explosions produced a gas supported pyroclastic density current that advanced over the water while a water supported density current was produced on the tank floor from the base of a turbulent mixing zone experimental deposits comprise proximal lithic breccia medial pumice breccia and distal fine ash experiments undertaken with cold water saturated slurries of tephra sample and water did not produce proximal lithic breccias but a medial basal lithic breccia beneath an upper pumice breccia results suggest the characteristics and variations in znp facies were strongly controlled by turbulent mixing and quenching proximity to the shoreline and depositional setting within the basin presence of abundant curviplanar pumice clasts in submarine breccias reflects brittle fracture and dismembering that can occur during fragmentation at the vent or during quenching subsequent transport in water supported pumiceous density currents preserves the fragmental textures careful study is needed to distinguish the products of subaerial versus subaqueous eruptions

relict coesite exsolution in omphacite from western tianshan eclogites china exsolution rods of relict coesite together with quartz were identified in omphacite in eclogites from western tianshan china they are oriented along the c axis of the host clinopyroxenes and have grain size up to 30 m long and 2 3 m wide raman spectra of exsolved lamellae yield consistent but weak bands at 521 270 181 151 and 118 cm 1 typical for coesite in addition to those of quartz and the host omphacite such occurrences together with textured observations suggest a two stage evolution of sio 2 exsolution rods in omphacite lamellae of coesite were apparently exsolved from supersilicic omphacite at p 5 0 gpa and the transformation from coesite to quartz occurred during retrograde metamorphism

eclogites in the makuti gneisses of zimbabwe implications for the tectonic evolution of the zambezi belt in southern africa the gneisses of the makuti group in north west zimbabwe are characterized by complex geometries that resulted from intense non coaxial deformation in a crustal scale high strain zone that accommodated extensional deformation along the axis of the zambezi belt at c 800 ma within low strain domains in the makuti gneisses undeformed metagabbroic lenses preserve eclogite and granulite facies assemblages which record a part of the metamorphic history that predates pan african events eclogitic rocks can be subdivided into 1 corona textured metagabbros that preserve igneous textures and 2 garnet omphacite rocks in which primary textures are destroyed the lenses of eclogitic rocks are enveloped in a mantle of garnet clinopyroxene hornblende gneiss which is a common rock type in the makuti gneisses the eclogites preserve multi staged domainal symplectic reaction textures that developed progressively as the rocks experienced loading followed by decompression heating in the metagabbros the original clinopyroxene plagioclase and olivine domains acted separately during the peak of metamorphism with plagioclase being replaced by garnet and kyanite and olivine being replaced by orthopyroxene and possibly omphacite the peak assemblage was overprinted by 1 the multi mineralic corona assemblage pargasite orthopyroxene spinel plagioclase replacing garnet kyanite clinopyroxene possibly at c 19 kbar 760 25 c 2 orthopyroxene pargasite plagioclase scapolite coronas replacing orthopyroxene 15 1 5 kbar 750 50 c and 3 moats of orthopyroxene plagioclase replacing garnet 10 1 kbar 760 50 c the garnet omphacite rocks record similar peak conditions 15 1 1 kbar 760 60 c garnet clinopyroxene hornblende plagioclase gneisses envelop the eclogites and record matrix conditions of 11 1 5 kbar at 730 50 c using assemblages that are oriented in the regional fabric these rocks are characterized by decompression heating textures reflecting temperature increases during exhumation of the makuti gneisses the eclogite facies rocks formed during a collisional event prior to 850 ma their formation could be related to a suture zone that developed along the axis of the zambezi belt during the formation of rodinia between 1400 and 850 ma the main deformation metamorphism in the makuti gneisses occurred around 800 ma and involved extension and exhumation of the high p rocks break up of rodinia which experienced a high t metamorphic overprint around 550 500 ma a collisional event associated with the formation of gondwana resulted in renewed burial and metamorphic recrystallization of the makuti gneisses

titanium solubility in coexisting garnet and clinopyroxene at very high pressure the significance of exsolved rutile in garnet exsolution microstructures including ilmenite garnet in clinopyroxene and rutile in garnet are common in clinopyroxenite and eclogite from the sulu ultrahigh pressure uhp terrane in order to understand the phase relations and ti solubility in both garnet and clinopyroxene in a natural tio2 bearing system several experiments at 5 15 gpa 1000 1400 c were carried out using the multianvil high pressure apparatus the hujianlin ilmenite rich garnet clinopyroxenite showing exsolution microstructure was selected as starting material because it closely approaches a composition lying in the tio2 cao mgo feo al2o3 sio2 system except for minor melt in one experiment at 1400 c and 5 gpa other run products contain majoritic garnet clinopyroxene ilmenite or rutile and exhibit neoblastic texture with increasing pressure ti and ca mg and si contents of neoblastic garnet increase with decreasing al the principal coupled substitutions are ca2 ti4 2al3 and si4 mg2 2al3 responding to majorite component increase titanium solubility 0 8 4 5 wt as tio2 in garnet and grtti cpxti ratio have a pronounced positive correlation with pressure between 5 and 15 gpa on the other hand the coexisting clinopyroxene contains low ti 0 17 0 53 wt as tio2 and shows no significant pressure effect rutile exsolution in garnet is coupled to that of pyroxene exsolution both are exsolved from majoritic garnet on decompression therefore the amount of such exsolved lamellae is a potential indicator of high pressure metamorphism in exhumed rocks whereas the tio2 content of clinopyroxene coexisting with garnet is not sensitive to pressure change

tectonic evolution of high grade metamorphic terranes in central vietnam constraints from large scale monazite geochronology abstract several metamorphic complexes in southeast asia have been interpreted as precambrian basement characterized by amphibolite to granulite facies metamorphism in this paper we re evaluate the timing of this thermal event based on the large scale geochronology and compositional variation of monazites from amphibolite to granulite facies metamorphic terranes in central vietnam most of the samples in this study are from metamorphic rocks n 38 and granitoids n 11 in the kontum massif gneisses n 6 and granitoids n 5 from the hai van migmatite complex and the truong son belt located to the north of the massif were also studied two distinct thermal episodes 245 230 ma and 460 430 ma affected kontum massif gneisses while a single dominant event at 240 220 ma is recorded in the gneisses from the hai van complex and the truong son belt monazites from granitoids commonly yield an age of 240 220 ma mesoproterozoic ages 1530 1340 ma were obtained only from monazite cores that are surrounded by c 440 ma overgrowths thermobarometric results combined with concentrations of y 2 o 3 ce 2 o 3 and heavy rare earth elements in monazite and recently reported pressure temperature paths suggest that triassic ages correspond to retrograde metamorphism following decompression from high to medium pressure temperature conditions ordovician silurian ages reflect low pressure temperature metamorphism accompanied by isobaric heating during prograde metamorphism some samples were affected by both metamorphic events we conclude that high grade metamorphism observed in so called precambrian basement terranes in central vietnam occurred during both the permian triassic and the ordovician silurian while peraluminous granitoid magmatism is triassic additionally our preliminary analyses for u pb zircon age and whole rock chemistry of granitic gneisses from the truong song belt suggests the presence of the ordovician silurian volcanic arc magmatism in the region based on the pressure temperature time protolith evolutions metamorphic rocks from central vietnam provide a continuous record of subduction accretion collision tectonics between the south china and indochina blocks in the ordovician silurian the region was characterized by active continental margin tectonics followed by continental collision during the late permian to early triassic and subsequent exhumation during the late triassic the results also suggest that the timing of metamorphism and protolith formation as well as the geochemical features in other southeast asian terranes should be verified to achieve a better understanding of the precambrian to early mesozoic tectonic history in asia

fluidization of wet sediments during the emplacement and cooling of various igneous bodies during the emplacement and cooling of igneous bodies fluidization of wet sediment by water vapour may occur by heating or by pressure relief during the opening of fractures momentary fluidization causes sediment reconstitution and local transport continuous fluidization can result in substantial sediment displacement theoretical considerations show that fluidization due to heating is not likely to occur at depths where pressure is much greater than 312 bars studies of the emplacement of sills of andesite ayrshire fluid rhyolite ramsey island and viscous rhyolite moelwyn hills recognize a wide range of phenomena attributable to wet sediment fluidization and in places modify existing interpretations the transgressive bases of the subaqueously welded ash flow tuffs of the capel curig volcanic formation are consistent with emplacement attended by fluidization of the subjacent sediment evidently fluidization is a means by which large volumes of wet sediment can be replaced by igneous material with minimal disturbance of the remaining host also fluidization can result in exceptionally complex contact relationships between sedimentary host and intrusive rocks and can cause fractures formed during cooling to be thoroughly pervaded by sediment fluidization is commonly associated with the formation of peperites

subaqueous pyroclastic flows and ignimbrites an assessment an assessment of the literature on subaqueous pyroclastic flows and their deposits shows that the term pyroclastic flow is frequently used loosely to describe primary hot gas rich pyroclastic flows mass flows which resulted from the transformation of gassupported flows into water supported ones and secondary mass flows carrying redeposited pyroclastic debris based on subaerial pyroclastic flows the term pyroclastic flow should be restricted to demonstrably hot gas rich mass flows of pyroclastic debris using this definition very few examples of subaqueous pyroclastic deposits with evidence for hot emplacement and of having been wholly submerged have been described in the majority of these cases the evidence for a hot state of emplacement and for the subaqueous nature of the host depositional environment is inadequate the only unequivocal cases of hot pyroclastic flow deposits with adequate supporting evidence are the ordovician nearshore shallow marine ignimbrites of ireland and wales and miocene ignimbrites of southwest japan resulting from the passage of subaerially erupted pyroclastic flows into shallow water other possible examples are near vent dense clast deposits in the donzurobo formation of japan possible submarine intra caldera ponded ignimbrite successions in california and wales and near vent pumiceous deposits of ramsay island wales all other purported cases are either clearly the result of water supported mass flow transportation and deposition debris avalanches debris flows turbidity currents or lack adequate supporting evidence regarding the heat state or the palaeoenvironment only the shallow marine ignimbrites of ireland and wales show adequate evidence of welding but even these could have been nearly wholly exposed above sea level when welding occurred we conclude that when pyroclastic flows enter water they are generally disrupted explosively and or ingest water and transform into water supported mass flows and we suggest the various scenarios in which this occurs there is no evidence to suggest that welding in wholly subaqueous environments is common

anatomy of a submarine pyroclastic flow and associated turbidity current july 2003 dome collapse soufriere hills volcano montserrat west indies the 12 to 13 july 2003 andesite lava dome collapse at the soufriere hills volcano montserrat provides the first opportunity to document comprehensively both the sub aerial and submarine sequence of events for an eruption numerous pyroclastic flows entered the ocean during the collapse depositing approximately 90 of the total material into the submarine environment during peak collapse conditions as the main flow penetrated the air ocean interface phreatic explosions were observed and a surge cloud decoupled from the main flow body to travel 2 to 3 km over the ocean surface before settling the bulk of the flow was submerged and rapidly mixed with sea water forming a water saturated mass flow efficient sorting and physical differentiation occurred within the flow before initial deposition at 500 m water depth the coarsest components 60 of the total volume were deposited proximally from a dense granular flow while the finer components 40 were efficiently elutriated into the overlying part of the flow which evolved into a far reaching turbidity current

quantitative 3d measurement of ilmenite abundance in alpe arami olivine by confocal microscopy confirmation of high pressure origin a critical aspect of the debate about the origin and conditions of metamorphism of the alpe arami aa peridotite is the disagreement over how much ilmenite is contained in the older generation of olivine and therefore how much tio 2 might have been dissolved at high pressure and temperature we have now determined quantitatively the 3 dimensional distribution of ilmenite in aa olivine by confocal laser scanning microscopy clsm the clsm measurements show an average concentration of 0 31 vol of ilmenite in olivine with individual grains containing up to 1 2 vol this translates into average and maximum concentrations of 0 23 and 0 9 wt tio 2 in olivine respectively and confirms the original estimation of maximum concentration of 1 vol tio 2 the vast majority of ilmenite in aa olivine is distributed randomly although topotactically oriented and in all cases is accompanied by chromite in a ratio of 4 1 these observations are consistent with an origin of the ilmenite and chromite by exsolution from an olivine solid solution at p 9 12 gpa and temperatures above the stability field of titanian clinohumite but are not consistent with suggested breakdown of titanian clinohumite combining these results with other recent findings suggests that exsolution followed deformation under relatively high fugacity of h 2 o and that the high solubility of tio 2 is probably explained by pressure induced accommodation of ti in the tetrahedral site of silicates

isolated pods of subaqueous welded ash flow tuff a distal facies of the capel curig volcanic formation ordovician north wales acidic welded tuffs of the capel curig volcanic formation were deposited from ash flows that transgressed from a proximal subaerial to a distal submarine environment isolated pods of welded tuff enveloped by marine sediments represent a distal facies of the formation deposited beyond the limits of the continuous ash flow tuff sheets various mechanisms for the separation of the tuff pods from a parent flow are discussed and it is concluded that it is most likely that the formation of the pods was due to disruption of a subaqueous ash flow during transport rather than by downslope collapse at the front of a previously deposited ash flow tuff sheet

tephra deposits associated with a large lava dome collapse soufriere hills volcano montserrat 12 15 july 2003 abstract the 12 13 july 2003 dome collapse at soufriere hills volcano montserrat was the largest event of its kind during the eruption thus far 1995 2005 involving the removal of 210 million m 3 of the lava dome complex over 18 h less than 2 of the total volume of material involved in the dome collapse was deposited on land a pyroclastic density current deposit alongshore and inland from the tar river fan was generated from a single blast originating at the shoreline the blast was caused by the interaction of pyroclastic flows with seawater we propose that at the peak of the lava dome collapse a sharp increase in the volume flux of pyroclastic flows caused substantial displacement of seawater from the shoreline followed by inrush of seawater when the flux decreased a few minutes later the tsunami allowed penetration of seawater into the main body of the pyroclastic flow at the shoreline which led to explosive fragmentation of pyroclastic blocks tephra fall deposits accumulated at a high rate on montserrat causing extensive damage to vegetation and buildings their stratigraphy recorded fallout from high co pyroclastic flow clouds from a vulcanian explosion cloud at the peak in collapse rate caused by the fragmentation of degassed lava dome and from four vulcanian explosion clouds after the dome collapse caused by fragmentation of bubbly magma in the conduit the total tephra fall volume is estimated at 10 20 million m 3

zircon geochronology geochemistry and stable isotopes of the wang ershan gold deposit jiaodong peninsula china abstract the early cretaceous gold deposits of the jiaodong peninsula eastern china define china s largest gold province with an overall endowment estimated more than 3000 t au the wang ershan gold deposit at the northern margin of the jiaolai basin in jiaodong peninsula of eastern china is located in the central southern segment of the wang ershan fault zone the major mesozoic intrusions exposed in this region are the linglong granite and the guojialing granodiorite to the northeast of the deposit both these intrusions show high k and alkaline signature the linglong granite displays peraluminous feature whereas the guojialing granodiorite is metaluminous zircon u pb dating of the linglong granite yields 206 pb 238 u weighted mean age of 149 0 1 3 ma the muscovite from the alteration zones associated with mineralization in the linglong granite yields a plateau age of 130 35 0 96 ma representing the initial phase of the hydrothermal activity the 34 s values of pyrite from the gold mineralized veins range from 6 to 8 3 with the mean value of 7 66 the 3 he 4 he ratios of pyrite are in the range 1 58 2 71 ra with a mean value at 2 168 ra the 40 ar 36 ar ratios show variation from 1220 3 to 1625 7 with an average of 1483 8 the 18 o values of the mineralizing fluids show a range of 2 13 to 7 5 with an average of 2 4 the 18 d w values are in the range of 97 5 to 61 4 with a mean at 82 6 detailed elemental and isotopic data suggest that the hydrothermal fluids in early stage were mainly magmatic and derived from the guojialing granodiorite and the ore forming fluids in the main ore forming stages evolved into a mixture of magmatic and meteoric water the ore forming materials were primarily derived from crust with minor input of mantle components the ore forming fluids might be related to the subduction of the paleo pacific slab beneath the north china craton and the associated asthenosphere upwelling and consequent lithospheric thinning

sedimentology of the krakatau 1883 submarine pyroclastic deposits the majority of tephra generated during the paroxysmal 1883 eruption of krakatau volcano indonesia was deposited in the sea within a 15 km radius of the caldera two syneruptive pyroclastic facies have been recovered in scuba cores which sampled the 1883 subaqueous pyroclastic deposit the most commonly recovered facies is a massive textured poorly sorted mixture of pumice and lithic lapilli to block sized fragments set in a silty to sandy ash matrix this facies is indistinguishable from the 1883 subaerial pyroclastic flow deposits preserved on the krakatau islands on the basis of grain size and component abundances a less common facies consists of well sorted planarlaminated to low angle cross bedded vitric enriched silty ash entrance of subaerial pyroclastic flows into the sea resulted in subaqueous deposition of the massive facies primarily by deceleration and sinking of highly concentrated deflated components of pyroclastic flows as they traveled over water the basal component of the deposit suggests no mixing with seawater as inferred from retention of the fine ash fraction high temperature of emplacement and lack of traction structures and no significant hydraulic sorting of components the laminated facies was most likely deposited from low concentration pyroclastic density currents generated by shear along the boundary between the submarine pyroclastic flows and seawater the krakatau deposits are the first well documented example of true submarine pyroclastic flow deposition from a modern eruption and thus constitute an important analog for the interpretation of ancient sequences where subaqueous deposition has been inferred based on the facies characteristics of encapsulating sedimentary sequences

depositional controls and characteristics of subaqueous bedded volcaniclastics of the lower devonian snowy river volcanics abstract the mt johnson member represents a relatively deep quiet marine enclave within the largely subaerial lower devonian snowy river volcanics it is intersected by seven diamond drill holes allowing a detailed analysis of the constituent sediment sequences normal sedimentation is represented by a mudstone facies which consists of interbedded black mudstones and turbiditic siltstones however this fine grained deposition was periodically interrupted by the emplacement of thick amalgamated volcaniclastic mass flow sequences six mass flow sequences are defined and two types can be recognized according to their overall bedding thickness and grainsize profile upward thinning and fining sequences are characterized by thickly bedded basal deposits of juvenile magmatically fragmented pyroclastic debris emplaced as cold water supported mass flows these are interpreted to be essentially syn eruptive arising either from flow transformation as pyroclastic flows ingested water upon entering the sea or from immediately post eruptive slumping of unreworked pyroclastic debris ponded at the shoreline these juvenile deposits are overlain by intervals of thinner bedded mass flows which are of a similar provenance but are considerably depleted in fines the second type of mass flow sequence the complex bedding thickness sequences has a basal zone of bedded fines depleted mass flow deposits with juvenile deposits in the mid to upper portions these mass flow sequences are interpreted to have arisen either from volcanic eruptive events which did not initially deposit primary pyroclastic debris subaqueously or from epiclastic events which eroded previously erupted unconsolidated volcaniclastic debris the depletion of fines in much of the volcaniclastic debris within the mt johnson member indicates that it was subjected to a degree of epiclastic reworking since this cannot have occurred in the deep quiet marine host environment it indicates an external source for the parental magmatic eruptions the eruptions probably occurred in an adjacent subaerial hinterland drained by an alluvial fluvial deltaic or fan deltaic system this system would have been a natural site for ponding and storage of freshly erupted debris and would have allowed the complex interaction between eruptive and epiclastic events which appear to have been responsible for the mass flow sequences

petrological and geochronological constraints on high pressure high temperature metamorphism in the snowbird tectonic zone canada the upper deck of the east athabasca mylonite triangle eamt northern saskatchewan canada contains mafic granulites that have undergone high p t metamorphism at conditions ranging from 1 3 to 1 9 gpa 890 960 c coronitic textures in these mafic granulites indicate a near isothermal decompression path to 0 9 gpa 800 c the godfrey granite occurs to the north adjacent to the upper deck high p t domain well preserved corona textures in the godfrey granite constrain igneous crystallization and early metamorphism in the intermediate pressure granulite field opx pl at 1 0 gpa 775 c followed by metamorphism in the high pressure granulite field grt cpx pl at 1 2 gpa 860 c u pb geochronology of zircon in upper deck mafic granulite yields evidence for events at both c 2 5 ga and c 1 9 ga the oldest zircon dates are interpreted to constrain a minimum age for crystallization or early metamorphism of the protolith a population of 1 9 ga zircon in one mafic granulite is interpreted to constrain the timing of high p t metamorphism titanite from the mafic granulites yields dates ranging from 1900 to 1894 ma and is interpreted to have grown along the decompression path but still above its closure temperature indicating cooling following the high p t metamorphism from c 960 650 c in 4 10 myr zircon dates from the godfrey granite indicate a minimum crystallization age of 2 61ga without any evidence for 1 9 ga overgrowths the data indicate that an early granulite facies event occurred at c 2 55 2 52 ga in the lower crust c 1 0 gpa but at 1 9 ga the upper deck underwent high p t metamorphism then decompressed to 0 9 1 0 gpa juxtaposition of the upper deck and godfrey granite would have occurred after or been related to this decompression in this model the high p t rocks are exhumed quickly following the high pressure metamorphism this type of metamorphism is typically associated with collisional orogenesis which has important implications for the snowbird tectonic zone as a fundamental boundary in the canadian shield

build up and depositional dynamics of an arc front volcaniclastic complex the miocene tepoztlan formation transmexican volcanic belt central mexico volcanic terrains such as magmatic arcs are thought to display the most complex surface environments on earth ancient volcaniclastics are notoriously difficult to interpret as they describe the interplay between a single or several volcanoes and the environment the early miocene tepoztlan formation at the southern edge of the transmexican volcanic belt belongs to the few remnants of this ancestral magmatic arc and therefore is thought to represent an example of the initial phase of evolution of the transmexican volcanic belt based on geological mapping detailed logging of lithostratigraphic sections palaeocurrent data of sedimentary features and anisotropy of magnetic susceptibility mapping of two dimensional panels from outcrop to field scale and geochronological data in an area of ca 1000 km 2 three periods in the evolution of the tepoztlan formation were distinguished which lasted around 4 myr and are representative of a volcanic cycle edifice growth phases followed by collapse in a magmatic arc setting the volcaniclastic sediments accumulated in proximal to medial distances on partly coalescing aprons similar to volcanic ring plains around at least three different stratovolcanoes these volcanoes resulted from various eruptions separated by repose periods during the first phase of the evolution of the tepoztlan formation 22ae 8t o 22 ae2 ma deposition was dominated by fluvial sediments in a braided river setting pyroclastic material from small andesitic dacitic composite volcanoes in the near vicinity was mostly eroded and reworked by fluvial processes resulting in sediments ranging from cross bedded sand to an aggradational series of river gravels the second phase 22ae 2t o 21 ae3 ma was characterized by periods of strong volcanic activity resulting in voluminous accumulations of lava and tuff which temporarily overloaded and buried the original fluvial system with its detritus continuous build up of at least three major volcanic centres further accentuated the topography and in the third phase 21ae 3t o 18 ae8 ma mass flow processes represented by an increase of debris flow deposits became dominant marking a period of edifice destruction and flank failures

entrance of hot pyroclastic flows into the sea experimental observations the entrance of hot pyroclastic flows into water has been observed in a series of experiments in which shooting granular flows of hot ignimbrite ash 403 c of bulk density near that of water run down a smooth chute and enter a water filled tank at an angle of 26 flows of relatively cool ash 250 c mixing across the water surface over some distance from shore generates steam explosions forming fountains of wet and dry ash and convectively rising fine ash plumes steam explosions increase in strength and lateral extent towards higher ash temperatures and mass fluxes the explosions generate water waves that remain driven by massive fountain fallout across some distance from shore the ash fountains feed pyroclastic surges which advance down tank over water at high speeds underwater plumes of sediment falling from the ash fountains and surges drop coarse load onto the floor whereas the finer load forms a turbidity current high temperature and poor size sorting of pyroclastic flows are key parameters that determine the processes of interaction with water and the associated hazards poor sorting allows for the formation of ash cloud surges moving over water even from cool flows high temperatures cause littoral explosions and facilitate extensive mass transport over water all flows generate tsunami waves by different mechanisms

application of actualistic models to unravel primary volcanic control on sedimentation taveyanne sandstones oligocene northalpine foreland basin abstract this work is focused on the taveyanne sandstones gres de taveyanne an oligocene volcaniclastic turbidite sequence cropping out in the northern alpine molassa between se france and central switzerland with the aim to investigate the temporal relationship between volcanic activity and sediment supply detailed stratigraphic sedimentological and petrographic xrd analyses on mudstones and point counts on sandstones studies conducted on three sections col de l oulette and flaine in se france taveyanne in sw switzerland allow a discrimination of three main facies among which only one is extremely enriched in volcaniclastic detritus and characterized by features similar to those of disaggregated pyroclastic density current deposits the other two facies are characterized by variable to no volcanic detritus but supplied by crystalline and sedimentary detritus such sediment trends are similar to those of modern volcanically controlled source to sink systems this allows a reinterpretation of the taveyanne sandstones as a syn volcanic turbidite system episodically supplied by large amounts of volcanic detritus which periodically modified the drainage paths moreover the well known temporal and spatial persistence of such modifications in modern settings leads to conciliate the syn volcanic supply with the location of the volcanic centers in the internal part of the alps without invoking particular climatic and tectonic conditions controlling foreland sedimentation