ultra high temperature metamorphism of mafic granulites from south altyn orogen west china a result from the rapid exhumation of deeply subducted continental crust

geochemical and zircon u pb and lu hf isotopic constraints on the origin of supracrustal rocks from the mid qilian terrane a comparison between supracrustal rocks on the two sides of the eastern segment of the altyn tagh fault abstract comparisons between the tectonic units on two sides of the sinistral ene striking altyn tagh fault atf are important tracers to find out the corresponding relationships among different terranes this study focuses on the tectonic units along the eastern segment of the atf the paleoproterozoic beidahe group in the mid qilian terrane south side of the eastern segment of the atf is mainly composed of metasedimentary rocks and amphibolites these rocks are petrologically and mineralogically similar to those from the dunhuang group north side of the eastern segment of the atf the detrital zircon u pb ages indicate that the protolith sediments of the beidahe metasedimentary rocks were mainly sourced from a provenance dominated by magmatic rocks in ages of 2 85 2 55 ga 1 70 1 55 ga and 1 42 1 03 ga this sedimentary sequence was deposited no earlier than 1 03 ga as passive continental margin deposits geochemical and zircon u pb and lu hf isotopic data suggest that the neoarchean mesoproterozoic basement rocks from the tarim craton most likely provide the majority of source materials for their protolith amphibolites from the beidahe group have high total fe2o3 and al2o3 contents but low mg values indicating a high degree of compositional evolution and contamination of the older crust they have flat ree patterns but positive lile and negative nb ta and ti anomalies suggesting a back arc basin basalts babbs affinity for their protolith the amphibolites have a crystallization age of 1 61 ga and zircon ehf t values ranging from 7 7 to 10 6 the zircon ehf t values and whole rock end t values indicate an involvement of juvenile and pre existing old materials in the formation of their parental magmas comparisons between the beidahe and dunhuang groups indicate that the metasedimentary rocks from these two groups are both sourced from the tarim craton but have different depositional ages and that the amphibolites have similar crystallization ages but different tectonic settings for their source rocks the field relationships and geochemical and geochronological characteristics of the metasedimentary rocks and amphibolites from the beidahe group support the interpretation that the observed recent proximity of these two rock complexes might be the result of tectonic juxtaposition rather than a primary intrusive contact

metamorphic evolution during deep subduction and exhumation of continental crust insights from felsic granulites in south altyn tagh west china

origin and mode of emplacement of lithic rich breccias at aso volcano japan geological paleomagnetic and petrological reconstruction abstract takajosan breccia rocks are distributed around the southwestern caldera rim of the aso volcano in japan they are characterized by coarse lithic breccias with a pumiceous matrix the proximal coarse lithic breccias are divided into the lower massive unit and the upper stratified unit the lower massive lithic breccias tend to transform laterally into tuff breccias and pumiceous lapilli tuffs paleomagnetic results showed that all of the deposits were deposited at high temperatures of 175 560 c this was also supported by geological characteristics such as spatter clasts clasts with a bread crust texture and weakly welded parts these features clearly show that the deposits originated from pyroclastic density currents pdcs the dense lithic rich lithofacies low vesicularity of pumice lack of plinian fall deposits and radial distribution indicate that the deposits were derived from boil over pdcs rather than plinian column collapse pdcs the sio2 contents of the matrix glasses of the proximal lower massive breccia showed a progressive decrease from the bottom toward the upper part we interpret that this chemical variation corresponds to chemical zonation of the magma chamber this indicates that the massive deposits aggraded progressively from the base upwards progressive aggradation rather than through en masse freezing the vertical lithofacies changes of the proximal breccias from the lower massive to the upper stratified units indicate that a sustained current in a quasi steady state switched to an unsteady current with the progression of the volcanic activity

geochemistry of fluids associated with the 1995 1996 eruption of mt ruapehu new zealand signatures and processes in the magmatic hydrothermal system abstract the 1995 1996 eruption of mt ruapehu has provided a number of insights into the geochemical processes operating within the magmatic hydrothermal system of this volcano both pre eruption degassing of the rising magma and its eventual intrusion into the convective zone of the hydrothermal system beneath the lake were clearly reflected in lake water compositions the eruptions of september october 1995 expelled the lake and provided the first ever opportunity to characterise gas discharges from this volcano the fumarolic discharges revealed compositions typical of andesite volcanoes and strong interaction with the enclosing meteoric and hydrothermal system fluids some 1 1 mt of so 2 gas was released from the volcano between september 1995 and december 1996 whereas ca twice this amount 2 2 mt equivalent so 2 was erupted as soluble i e leachable oxyanions of sulphur significantly more sulphur was released from the volcano over this period than can be accounted for from the magma volume actually erupted the evidence suggests that a sizable component of the evolved sulphur was remobilised from the long lived hydrothermal system within the volcano during the 1995 1996 activity

uniform permian 40ar 39ar detrital mica ages in the eastern qaidam basin nw china where is the source new single grain 40ar 39ar detrital white mica ages from the lulehe section at the eastern qaidam basin yield uniform permian ages between 250 3 and 279 3 ma throughout the whole cenozoic sequence this is inconsistent with the present hinterland which is composed of early palaeozoic metamorphic units with subordinate early palaeozoic and few permian granites the new data indicate that permian tectonic units are likely more widespread at the north eastern margin of the tibetan plateau as known at present particularly within the qilian mountains the preferred explanation is that the qaidam block represents a rigid indenter which indented during late tertiary times into early palaeozoic orogenic units this is consistent with recent findings of a nw trending sinistral permian ductile shear zone and a dextral nw trending tertiary fault system close to the north eastern margin of the qaidam basin

structure of the central altyn tagh fault revealed by magnetotelluric data new insights into the structure of the northern margin of the india asia collision abstract the altyn tagh fault atf is a left lateral strike slip fault that forms the northern margin of the tibetan plateau and plays a significant role in accommodating the convergence between the colliding indian and eurasian plates as a part of the fourth phase of the indepth project magnetotelluric mt data were collected across the central segment of the atf to determine the lithospheric scale structure of the fault system dimensionality analyses demonstrated that the mt data can be interpreted using two dimensional approaches but some localized 3 d effects are seen consequently both 2 d and 3 d inversions were carried out and a joint interpretation was made on the basis of these two types of models inversion models revealed two major conductors beneath the qaidam basin qb and altyn tagh range atr respectively the conductive region beneath the qb was interpreted as a ductile layer in the lower crust to upper mantle that might represent flow beneath the western margin of the qb whereas the large scale south dipping conductor beneath the atr is interpreted as a region with high fluid content formed by metamorphism associated with the oblique underthrusting of the tarim block beneath the northern tibetan plateau these fluids migrate upwards through the fault system and have formed serpentinized zones in the crust combining these interpretations a structural model compatible with diverse geophysical observations is proposed in which we suggest the competing end member rigid block model and continuum model are reconcilable with the continuum model locally dominant for the study region as evidenced by a thickened crust

did the altyn tagh fault extend beyond the tibetan plateau abstract the pre miocene northeastern termination of altyn tagh fault is a critical outstanding problem for understanding the mechanics of cenozoic deformation resultant from the indo asian collision and mechanisms of tibetan plateau formation structures beyond the widely accepted ne end of the altyn tagh fault near the town of yumen are needed in order to accommodate strike slip deformation related to plate like lateral extrusion tectonics but structures with the necessary slip magnitudes and histories have not been identified we report on a series of newly recognized and documented e to ene striking faults within the alxa block ne of the tibetan plateau that are visible on remotely sensed images and confirmed by field studies these structures are demonstrably left lateral faults based on offset geology and kinematic indicators such as striae and s c fabrics in fault gouge the faults have post cretaceous offsets of at least tens to possibly 150 km but limited post miocene displacement constrained by offset sedimentary basins these characteristics suggest that strike slip faults of the alxa region have a similar structural history as the central eastern altyn tagh fault and can provide a mechanism for accommodating oligocene early miocene extrusion along the altyn tagh fault

sedimentary evidence against large strike slip translation on the northern altyn tagh fault nw china abstract whether the oblique left reverse northern altyn tagh fault has large cumulative offset 120 km is one of the most controversial issues concerning the deformation mechanics for the northern tibetan plateau in order to obtain material constraints on its total displacement sedimentologic and bedrock provenance analysis of oligocene pleistocene strata on the northern side of the fault was conducted lithofacies analysis and 970 paleocurrent measurements show that these strata were products of fluvial systems originating from south of the fault making it possible to constrain the fault s slip history by establishing cross fault sediment source linkages identification of 5217 conglomerate clasts in the field indicates that major clast types are quartzite 30 50 schist gneiss increasing upsection from 10 to 40 50 and marble decreasing upsection from 20 30 to 30 km strike slip translation on the northern altyn tagh fault and support the model that strain induced by indian indentation has been partitioned into left slip on the altyn tagh fault and reverse dip slip on the northern altyn tagh fault

island arc volcanic rocks in the north qaidam uhp belt northern tibet plateau evidence for ocean continent subduction preceding continent continent subduction abstract a suite of early paleozoic island arc volcanic rocks consisting mainly of basaltic lavas and some intermediate silicic rocks crops out in the north qaidam basin northeastern tibet plateau these rocks have been extensively metamorphosed under greenschist facies conditions and hence differ from the older proterozoic intermediate mafic volcanic rocks which have generally experienced amphibolite facies metamorphism the early paleozoic volcanic rocks are divided into three groups based on their geochemical characteristics 1 vtg i consists of island arc tholeiitic basalts basaltic andesites and andesites that crop out in jilusu and shuangkoushan 2 vtg ii which are high al alkaline and sub alkaline basalts in jilusu and 3 vtg iii an assemblage of highly depleted morb type basalts andesites and dacites that occurs mostly in shuangkoushan groups vtg i and ii are considered to be the products of mature island arcs produced in two stages in the first stage following initiation of ocean continent subduction the island arc tholeiites were generated by partial melting of the subsiding oceanic crust and mantle wedge beneath the island arc as subduction progressed and accelerated calc alkaline and high al basalts were formed and the crust beneath the island arc was thickened the highly depleted morb type lavas vtg iii have boninitic affinities and are believed to have been generated from depleted mantle material modified by subduction zone fluids possibly in the fore arc region the island arc tholeiites are dated at 514 2 8 5 ma whereas uhp eclogite in the north qaidam belt has been dated at 494 ma these ages suggest that ocean continent subduction took place in the north qaidam basin in the early paleozoic and was followed shortly by continent continent collision and subduction which produced the uhp rocks

metamorphic evolution mineral chemistry and thermobarometry of orthogneiss hosting ultrahigh pressure eclogites in the north qaidam metamorphic belt western china abstract many uhp terranes contain abundant felsic gneiss with volumetrically minor mafic and ultramafic blocks the mafic rocks typically preserve the high pressure assemblages whereas felsic gneisses preserve low pressure mineral assemblages in the absence coesite it can be difficult to confirm that felsic lithologies experienced high pressure conditions this has lead to uncertainty over the regional extent of uhp terranes we conducted a detailed study of textures mineral parageneses and compositions of a suite of eclogite bearing orthogneisses in the luliang shan from the north qaidam uhp metamorphic belt two distinctive gneisses were observed volumetrically dominant uniform leucocratic orthogneiss and mica rich gneiss which occurs only in proximity to eclogite blocks two pseudosections have been constructed to constrain the p t conditions recorded by the mineral assemblages and mineral zoning observed in the orthogneiss relic high pressure inclusions and predicted conditions of garnet growth suggest that the orthogneiss followed a metamorphic p t path comparable to the eclogite it encloses with peak pressure at 26 kbar very close to the quartz coesite phase boundary

the dynamic evolution of the palaeozoic geography of eastern asia abstract new palaeogeographical reconstructions are presented for eleven time intervals through the palaeozoic of the eastern asia region from the middle cambrian at 510 ma to the end of the permian at 250 ma they centre on the continental blocks of north china south china and annamia indochina and their relationships with northeastern gondwana which was united to form part of pangea from the late carboniferous onwards also shown is the continent of tarim during the lower palaeozoic as well as the hutag uul songliao and khanka jiamasu bureya terranes both of which straddle the russian mongolian and chinese borders today from silurian times onwards we conclude that annamia and south china were united as a single continent throughout the lower palaeozoic and early devonian and were translocated by major strike slip faulting along the northeastern gondwana margin during that period from off afghanistan to outboard of the sibumasu and australian sectors of the superterrane they left the gondwana marginal area together during the lower devonian opening of the palaeotethys ocean but very shortly afterwards they themselves divided into the two separate continental blocks that we recognise today not to reunite until the triassic the various cambrian to permian rocks found in japan largely represent active volcanic arcs which originally lay to the southeast of south china although the carboniferous was more quiescent there the neotethys ocean opened during the permian dividing sibumasu and the tibetan terranes from gondwana and the palaeotethys ocean started to close progressively in the upper palaeozoic as most of the east asian continents and smaller terranes moved towards siberia the positions of the various continents and terranes have been deduced from a mixture of palaeomagnetic and faunal data the positions of large igneous provinces and kimberlites and the need to provide kinematic continuity between maps of successive ages however many uncertainties remain

the september 14 2015 phreatomagmatic eruption of nakadake first crater aso volcano japan eruption sequence inferred from ballistic pyroclastic density current and fallout deposits abstract an explosive eruption occurred at nakadake first crater aso volcano in central kyushu southwestern japan on september 14 2015 the sequence and causes of the eruption were reconstructed from the distribution textures grain size component and chemical characteristics of the related deposits and video record the eruptive deposits are divided into ballistics pyroclastic density current and ash fall deposits a large number of ballistic clasts mostly

cenozoic tectonic evolution of the qaidam basin and its surrounding regions part 3 structural geology sedimentation and regional tectonic reconstruction the qaidam basin is the largest topographic depression inside the tibetan plateau because of its central position understanding the tectonic origin of the qaidam basin has important implications for unraveling the formation mechanism and growth history of the tibetan plateau in order to achieve this goal we analyzed regional seismic reflection profiles across the basin and a series of thickness distribution patterns of cenozoic strata at different time slices the first order structure of the basin is a broad cenozoic synclinorium which has an amplitude ranging from 16 km in the west to 48 in the west to 15 s 1 to 1 3 10 17 s 1 the eastward decrease in upper crustal shortening requires a progressive shift in crustal thickening mechanisms across qaidam basin from dominantly upper crustal shortening in the west to dominantly lower crustal shortening in the east although sedimentation began synchronously at 65 50 ma across the entire basin the initiation ages of the southern and northern basin bounding structures are significantly different deformation started at 65 50 ma in the north and at 29 24 ma in the south this information and the existing inference that the uplift of the eastern kunlun range south of qaidam basin began after 30 20 ma imply that the paleogene 65 24 ma qaidam and hoh xil basins on both sides of the eastern kunlun range may have been parts of a single topographic depression 500 km wide in the north south direction between the qilian shan and fenghuo shan thrust belts in the north and south the development of this large paleogene basin in central tibet and its subsequent destruction and partitioning by the neogene uplift of the eastern kunlun range requires a highly irregular sequence of deformation possibly controlled by preexist ing weakness in the tibetan lithosphere

geochemistry of high grade metamorphic rocks of the north qaidam mountains and their geological significance the high grade metamorphic basement in the north qaidam mountains nqd is composed mainly of jinning age metasedimentary rocks and crustally derived granites possibly accompanied by minor paleoproterozoic and archaean material based on their geochemistry the metasedimentary rocks are divided into two groups type i which makes up the bulk of the high grade metamorphic basement in the nqd is characterized by high k2o low na2o relatively high total rare earth elements treez114 357 ppm and weak fractionation of light lree and heavy hree rare earth elements la yb nz8 27 these rocks also have strong negative eu anomalies eu eu z0 3 0 6 type ii rocks crop out mainly in the wulan area and are characterized by relatively high mgo 2 2 4 1 wt lower total ree and strong fractionation of lree and hree la yb nz29 48 both types of metasedimentary rocks have similar nd isotope compositions with tdmz1 9 2 2 ga and 3ndzk4 1 to k8 5 the granites are compositionally similar to the type i metasedimentary rocks the high grade metamorphic basement of the nqd is similar to that in the qilian orogenic belt and the yangtze platform the central qilian nqd terrane was separated from the dunhuang alxa terrane before the jinning period and the two terranes belong to the south china block and north china craton respectively q 2006 elsevier ltd all rights reserved

subaqueous geothermal activity revealed by lacustrine sediments of the acidic nakadake crater lake aso volcano japan abstract lacustrine sediments were sampled from the inaccessible acidic ph 0 43 nakadake crater lake of aso volcano japan by a simple method the sediments contain an extremely high content 74 wt of sulfur which exits as elemental sulfur gypsum and anhydrite the abundant elemental sulfur is likely formed by the reaction of so 2 and h 2 s gases and by the so 2 disproportionation reaction in magmatic hydrothermal system below the crater lake based on the sulfur content of sediments and measurements of elevation change of the crater bottom the sulfur accumulation rate at the nakadake crater lake was calculated as 250 tonne day which is comparable with the so 2 emission rate 200 600 tonne day from the nakadake crater the sediments include a small amount 9 of clear glass shards that are apparently not altered in spite of the high reactivity of hyperacid lake water this finding suggests that the clear glass shards are fragments of recently emitted magmas from fumaroles on the bottom of the crater lake and the magma emissions continuously occur even in quiescent periods

shear wave splitting vp vs and gps during a time of enhanced activity at aso caldera kyushu 1 we performed shear wave splitting analyses and calculated vp vs ratios on local earthquakes recorded at seven stations around aso volcano japan between 2001 and 2008 the mean vp vs of 1 61 0 01 from all stations is among the lowest average values found anywhere in the world a temporal change of shear wave splitting fast polarization is observed at one station additionally spatial variations of shear wave splitting delay times and vp vs ratios show a correlation at three stations we suggest that stress controlled anisotropy dominates at these stations gps baselines and 2d strain reveal extension in the caldera in 2003 and compression before and after 2003 strain shortening axes indicate either a variation in regional stress or a local stress source that overprints the regional field in 2003 baseline changes correlate with periods of increased low frequency seismic activity located deeper than 15 km compaction and degassing of a previously reported sill at approximately 15 km depth could lead to constantly high pressure of volcanic gases over a wide area and explain the generally low vp vs ratios and the observed compression in the caldera we propose that a small volume of magma intruded into the sill in 2003 caused extension at the surface and triggered ash eruptions between 2003 and 2005 associated localized migration of gases in overpressured cracks with aspect ratios potentially 0 5 in the upper crust with porosities 0 1 could be the source of changes in seismic anisotropy and explain the slight variations observed in vp vs

petrography and geochemistry of the lower silurian sandstones from the angzanggou formation in the north qilian belt china implications for provenance weathering and tectonic setting the north qilian orogenic belt is on the northeastern margin of the tibetan plateau it is connected with the alxa block in the north and the tethyan orogenic assemblage in the south the lower silurian angzanggou formation deposited in the northern area of the north qilian orogenic belt is the most important for revealing the architecture and orogenic processes of the north qilian belt provenance analysis of the angzanggou formation can reveal not only the tectonic evolution of the central orogenic belt of china but also palaeozoic asia plate reconstructions petrographic analysis indicated that the compositional and textural maturity of the sandstones was low the detrital composition of the angzanggou formation samples consists of quartz 8 14 feldspar 6 29 and lithic fragments 56 86 the sandstones could be classified as litharenites or feldspathic litharenites the detrital modal composition suggests that these sandstones were probably deposited in a fore arc basin the element ratios and some discrimination diagrams based on geochemistry indicate that felsic and intermediate rocks were the main source rocks the sio 2 al 2 o 3 ratio the index of chemical variability and the th sc versus zr sc discrimination diagram suggest that the compositional maturity and degree of recycling were moderate to low the index of alteration cia and the a cn k diagram indicate the intensity of weathering was moderate the discrimination diagrams based on major and trace elements and petrographic discrimination diagrams imply that the angzanggou formation rocks were derived from a continental island arc and a sedimentary cover probably overlaid the volcanic arc therefore we infer that during early silurian time the north qilian belt sediments accumulated in a fore arc basin

a unique omphacite amphibole and graphite bearing clast in queen alexandra range que 99177 a metamorphosed xenolith in a pristine cr3 chondrite abstract a metamorphosed unshocked chondritic clast containing approximately equal volumes of matrix and chondrules has been identified in the primitive cr3 chondrite que 99177 the most striking features of this clast are the presence of ferroan omphacitic pyroxenes amphiboles pargasite and pargasitic hornblende and coarse grained up to 70 m graphite laths the clast also contains relatively equilibrated feo rich olivine low ca pyroxene and sulfides pentlandite and troilite the clast s matrix contains coarse interlocking crystals the chondrules are extensively recrystallized contain no mesostasis or metal and present poorly defined boundaries based on its compositional and crystallographic features omphacite appears to have formed at high pressures 6 gpa amphibole formation probably postdated graphite formation and occurred by reaction of olivine clinopyroxene and omphacite in the presence of an oxidizing na rich fluid at high temperatures 700 c the absence of metal suggests that this fluid was also rich in s

monitoring of volcanic eruptions at yugama crater lake by aqueous sulfur oxyanions abstract variations of polythionates sulfane disulfonates and sulfate in the yugama crater lake japan have been monitored for more than 25 years just before the 1982 eruption at the crater lake polythionate ions decreased to zero from the normal level of about 2000 ppm and sulfate ions increased from 2500 to 5000 ppm during the 1982 eruption polythionate and sulfate ions varied inversely in concentration and the variations exactly coincided with the frequency of volcanic earthquakes and subsequent explosions these observations are interpreted in terms of aqueous reactions of fumarolic so2 h2s gases resulting in precipitation of alunite the behavior of polythionate and sulfate ions strongly suggests that they are useful indicator for prediction of impending volcanic hazards from active crater lakes

metamorphic evolution and geochronology of the dunhuang orogenic belt in the hongliuxia area northwestern china abstract garnet bearing mafic granulites and amphibolites from the hongliuxia area of the southern dunhuang orogenic belt northwestern china commonly occur as lenses or boudinages enclosed within metapelite or marble which represent the block in matrix feature typical of orogenic melange three to four generations of metamorphic mineral assemblages are preserved in these rocks in the high pressure amphibolites prograde mineral assemblages m1 occur as inclusions hornblende plagioclase quartz chlorite epidote ilmenite preserved within garnet porphyroblasts and formed at 550 590 c and 7 7 9 2 kbar based on geothermobarometry the metamorphic peak mineral assemblages m2 are composed of garnet hornblende plagioclase quartz clinopyroxene as well as titanite zircon rutile apatite as accessory minerals in the matrix and are estimated to have formed at 640 720 c and 14 1 16 0 kbar the first retrograde assemblages m3 are characterized by white eye socket symplectites hornblende plagioclase quartz biotite epidote magnetite rimming garnet porphyroblasts which formed at the expense of the garnet rims and adjacent matrix minerals during the decompression stage under p t conditions of 610 630 c and 5 6 11 8 kbar the second retrograde assemblages m4 are intergrowths of actinolite and worm like quartz produced by the breakdown of the matrix hornblendes and formed under p t conditions of 490 c and 2 8 kbar for the high pressure mafic granulites the prograde assemblages m1 are represented by plagioclase quartz preserved within the garnet porphyroblasts the metamorphic peak assemblages m2 are garnet matrix minerals clinopyroxene plagioclase quartz hornblende rutile zircon and were estimated to have formed at 680 c and 15 4 kbar the retrograde assemblages m3 are characterized by fine grained patches of hornblende plagioclase quartz rimming the garnet porphyroblasts as well as hornblende rimming clinopyroxene in the matrix and were inferred to have formed at 620 c and 4 2 kbar for the metapelitic gneiss the metamorphic peak assemblages are the garnet porphyroblasts plus the matrix minerals biotite plagioclase quartz ilmenite zircon which were estimated to have formed at 630 c and 8 9 kbar the mafic granulites and amphibolites record fairly similar clockwise p t paths that include nearly isothermal decompression processes which suggest that they experienced subduction and subsequent rapid tectonic exhumation sims and la icp ms u pb dating of zircons and 40ar 39ar dating of hornblende suggest that the metamorphism occurred at 430 390 ma field occurrences different protolith ages of the mafic granulites and amphibolites and the considerable gap in peak p t conditions between the amphibolite and mafic granulite boudinages and their country rock may suggest a melange accumulation process during the paleozoic caused by the silurian devonian orogeny which is possibly associated with the closure of the liuyuan ocean a branch of the paleo asian ocean near the southern central asian orogenic belt

variety and sustainability of volcanic lakes response to subaqueous thermal activity predicted by a numerical model we use a numerical model to investigate the factors that control the presence or absence of a hot crater lake at an active volcano we find that given a suitable pair of parameters e g the enthalpy of subaqueous fumaroles and the ratio of mass flux of the fluid input at the lake bottom to lake surface area hot crater lakes can be sustained on relatively long timescales neither a high rate of precipitation nor an impermeable layer beneath the lake bottom are always necessary for long term sustainability the two controlling parameters affect various hydrological properties of crater lakes including temperature chemical concentrations and temporal variations in water levels in the case of low temperature crater lakes increases in flux and enthalpy which are a common precursor to phreatic or phreatomagmatic eruptions result in an increase in both temperature and water level in contrast a decrease in water level accompanied by a rise in temperature occurs at high temperature lakes furthermore our model suggests that crater geometry is a key control on water temperature for lakes with a conical topography a perturbation in the water level due to trivial non volcanic activity such as low levels of precipitation can cause persistent increases in water temperature and chemical concentrations and a decrease in the water level even though subaqueous fumarolic activity does not change such changes in hot crater lakes which are not caused by changes in volcanic activity resemble the volcanic unrest that precedes eruptions

mass change calculations in altered rock series a technique is presented whereby precursors of altered rocks in a fractionated rock series can be identified and changes in chemical components mass and volume accurately calculated the procedure utilizes elements with high degrees of immobility in hydrothermal and associated interactive water rock systems igneous rock series and sedimentary and metamorphic rocks with primary continuity of chemical composition are amenable to this treatment in the procedure for igneous rocks best fit fractionation lines are established for an immobile igneous compatible incompatible element pair the latter element functioning as the fractionation monitor alteration paths are linear and pass through the origin intersections of the fractionation and alteration lines yield the immobile element concentrations in the precursors to the altered rocks mobile components are determined from fractionation lines and monitor values total mass changes are proportional to the displacement of altered samples from the fractionation line masses of other elements in an altered sample reconstructed values are recalculated to the concentration of the monitor element in the precursor chemical changes are the differences between the calculated precursor compositions and the reconstructed altered rock compositions errors are mainly a function of the positioning of the fractionation lines

geological constraints on the 2003 2005 ash emissions from the nakadake crater lake aso volcano japan abstract the active crater of nakadake at aso volcano southwestern japan has been occupied by a lake during its dormant periods multiple ash emissions were observed from the crater lake between july 2003 and august 2005 the largest events occurred on 10 july 2003 and 14 january 2004 on 10 july 2003 ash 41 tons was dispersed throughout an area extending 14 km east northeast of the vent and on 14 january 2004 ash 32 tons extended 8 km to the east southeast thereafter small ash emissions were frequent at the crater lake in which the water level fell considerably from april to august 2005 during this period major ash emitting events producing mostly white aggregated ash occurred on june 10 12 june 21 4 2 tons and july 25 1 2 tons ash emissions at the nakadake crater in 2003 2005 were classified into three types gentle release of white aggregated ash from fumaroles inside the crater lake e g daytime of 14 april 10 12 june and 25 july 2005 emission of black ash from an almost dried up vent 21 june 2005 and short time ca 20 s small explosion triggered gas and ash emission through the crater lake 10 july 2003 14 january 2004 and 14 april 20h41m 2005 all products from these ash emissions consisted of fine grained

steady subsidence of a repeatedly erupting caldera through insar observations aso japan the relation between unrest and eruption at calderas is still poorly understood aso caldera japan shows minor episodic phreatomagmatic eruptions associated with steady subsidence we analyse the deformation of aso using sar images from 1993 to 2011 and compare it with the eruptive activity although the dataset suffers from limitations e g atmospheric effects coherence loss low signal to noise ratio we observe a steady subsidence signal from 1996 to 1998 which suggests an overall contraction of a magmatic source below the caldera centre from 4 to 5 km depth we propose that the observed contraction may have been induced by the release of the magmatic fluids feeding the eruptions if confirmed by further data this hypothesis suggests that degassing processes play a crucial role in triggering minor eruptions within open conduit calderas such as at aso our study underlines the importance of defining any eruptive potential also from deflating magmatic systems with open conduit

cenozoic tectonic evolution of asia a preliminary synthesis abstract asia has been a major testing ground for various competing models of continental deformation due to its relatively well understood plate boundary conditions in the cenozoic exceptional exposure of active structures and strain distribution and widespread syn collisional igneous activity as a proxy for the thermal state of the mantle and crust two cenozoic orogens dominate the continent the himalayan tibetan orogen in the east induced by the india asia collision and the turkish iranian caucasus orogen in the west induced by the arabia asia collision the development of the two orogens was accomplished by shortening in the early stage followed by strike slip faulting and extension in the late stage in the himalayan tibetan orogen shortening across two discrete thrust belts at 55 30 ma in southern and northern tibet created a large intracontinental basin the paleo qaidam basin in between subsequent crustal thickening and a possible thermal event in the mantle e g convective removal of central tibetan mantle lithosphere may have raised the elevation of this early intra plateau basin up to 2 3 km to its current height collision between india and asia also caused lateral extrusion of southeast asia between 32 ma and 17 ma the latest stage of the india asia collision was expressed by north trending rifting and the development of trench facing v shaped conjugate strike slip faults in central mongolia central tibet eastern afghanistan and southeast asia in the turkish iranian caucasus orogen early crustal thickening in the orogenic interior began at or prior to 30 20 ma this style of deformation was replaced by strike slip faulting at 15 5 ma associated with further northward penetration of arabia into asia westward extrusion of the anatolia turkey block and rapid extension across the sea of crete and sea of aegean the late stage extension in both orogens was locally related to extensional core complex development the continental margin extension of east asia was developed in two stages initially in a widely distributed zone that has an east west width of 500 800 km during 65 35 ma which was followed by localized extension and opening of back arc basins associated with the development of spreading centers at 32 17 ma e g japan sea or east korea sea bohai bay and south china sea opening of the back arc basins could be induced by 1 rapid eastward migration of the western pacific trench system or 2 oblique subduction of pacific plate beneath asia that had produced a series of en echelon right slip primary shear zones linking with back arc spreading centers oriented obliquely to the strike of the nearby trench since 15 ma the eastern margin of asia became contractional in the east west direction as indicated by the collapse of back arc basins in the western pacific and the development of fold thrust belts along the eastern continental margin coeval with the contraction is widespread east west extension in siberia north china and the tibetan plateau the above observations can be explained by a change in boundary condition along the eastern margin of asia that allowed the thickened asian continent to spread eastward causing east west extension in its trailing edge and east west compression in its leading edge in west asia continental margin extension started at about 25 20 ma in the aegean and cretan regions which was associated with a rapid southward retreat of the hellenic arc the complex evolution of cenozoic deformation in asia may be explained by a combined effect of temporal changes in plate boundary conditions thermal evolution of the upper mantle perturbed by collisional tectonics and the built up of gravitational energy through crustal thickening and thermal heating although the past research in asia has treated the india asia and arabia asia convergence as separate collisional processes their interaction may have controlled the far field cenozoic deformation in asia the most pronounced result of this interaction is the creation of a northeast trending 300 400 km wide and 1500 km long zone of northwest striking right slip faults which extends from the zagros thrust belt in the south to western mongolia in the north and links with the active tian shan and altai shan intracontinental orogens cenozoic deformation and coeval igneous activity spatially overlap with one another in the himalayan tibetan and turkish iranian caucasus orogens a large cenozoic magmatic gap exists between tibet in the south and mongolia in the north where cenozoic deformation has not been associated with any coeval igneous activity finally cenozoic igneous activity is always associated with jurassic cretaceous magmatic arcs suggesting a causal relationship between the early arc magmatism and later syn collisional magmatism

rapid slip along the central altyn tagh fault morphochronologic evidence from cherchen he and sulamu tagh consistent with the d 18 o record from the guliya ice cap in the west kunlun the features of interest were all formed by glacial and fluvial processes subsequent to marine isotope stage 5e with the youngest features having formed during the early holocene optimum this near field morphochronological slip rate is averaged over many earthquake cycles and is hence little affected by interseismic strain it is kinematically consistent with other somewhat lower geomorphic slip rate measurements to the east the average rate and lower bounds obtained from alternate interpretational models 18 4 mm yr cannot be reconciled with the most rece geodetic measurements 7 mm yr suggesting that interseismic strain and interactions with adjacent faults may lead to disparate geologic and geodetic rate estimates this late pleistocene holocene morphochronologic rate would imply that at this longitude the altyn tagh fault on the north edge of tibet might absorb almost as much of india s convergence relative to siberia as the himalayan main frontal thrust does on the southern edge of the plateau index terms 1035 geochemistry geochronology 1208 geodesy and gravity crustal movements intraplate 8110 8102 tectonophysics continental contractional orogenic belts 8107 tectonophysics continental neotectonics keywords slip rates cosmogenic dating indo asian collision

geological setting of the rapu rapu gold rich volcanogenic massive sulfide deposits albay province philippines gold rich fe cu zn volcanogenic massive sulfide deposits occur within strata of probable jurassic age on rapu rapu island in albay province philippines massive sulfides at the ungay malobago and hixbar deposits are spatially associated with dacitic volcanic rocks within a highly deformed sequence of mafic volcanic and quartzofeldspathic sedimentary rocks the massive sulfide deposits formed at the stratigraphic contact between footwall dacites and hangingwall mafic volcanic and quartzofeldspathic rocks the deposits and their host strata have undergone regional metamorphism with strong penetrative deformation metamorphic mineral assemblages and textural evidence suggest that peak metamorphism was upper greenschist to lower amphibolite grade and syn d1 deformation based on the age of regional metamorphism deformation is inferred to be mid tertiary in age deformation at rapu rapu resulted in reorientation of the strata into a broad antiform with strong shallow plunging elongation fabrics overturning of the volcanic sequence that hosts the ungay malobago deposit and complex folding of the mineralized zones the present highly linear form of the ungay malobago deposit is mainly a product of this ductile strain

magmatic history of the northeastern tibetan plateau 1 the northeastern margin of the tibetan plateau is underlain by the qaidam and qilian terranes which consist primarily of mid proterozoic through lower paleozoic oceanic and arc type assemblages that have been accreted to the southern margin of the tarim sino korean craton most previous models suggest that these assemblages formed along a northeast dipping subduction system constructed along the margin of the tarim sino korean craton during early paleozoic time the main components are interpreted to have formed either as an archipelago of volcanic arcs and back arc basins or as a broad expanse of accretionary complexes our geochronologic data support a model suggested by sobel and arnaud 1999 in which the qaidam and qilian terranes are separated from the tarim sino korean craton by a mid paleozoic suture that closed along a southwest dipping subduction zone the basement to these terranes consists of oceanic assemblages that were amalgamated into a coherent crustal fragment prior to emplacement of 920 930 ma granitoids early paleozoic arc type magmatism occurred between 480 and 425 ma apparently sweeping southwestward across much of the qilian and qaidam terranes accretion related magmatism along the inboard margin of the qilian terrane occurred between 423 ma and 406 ma following silurian devonian accretion the region has experienced late paleozoic and mesozoic uplift and erosion and has been severely overprinted by tertiary thrusting uplift and strike slip motion along the altyn tagh fault correlation of geologic features and magmatic histories between the altun shan and the nan shan suggests that the eastern altyn tagh fault has a total left lateral offset of 375 km

discovery of an eclogite belt in the lhasa block tibet a new border for paleo tethys abstract a newly discovered eclogite belt in the eastern part of the lhasa block tibet is about 500 1000 m wide and at least 60 km long in an e w direction the eclogites occur as tectonic slices in garnet bearing mica quartz schist they are generally fresh and form thick massive layers that consist chiefly of garnet grt omphacite omp phengite phe rutile rut quartz qtz p t calculations based on the grt omp phe mineral assemblage yielded peak metamorphic conditions of 2 7 gpa and 730 c close to the phase boundary between coesite and quartz and thus the eclogites can be regarded as part of a very high pressure metamorphic belt petrochemical data suggest that the eclogite protoliths were typical morb basalts derived from depleted mantle shrimp u pb dating of zircons from the eclogite yielded metamorphic ages ranging from 242 15 to 292 13 ma with an average value of 262 5 ma the morb eclogites are interpreted to be remnants of paleo tethyan oceanic lithosphere the eclogites along with permian island arc volcanic rocks to the north are believed to mark a carboniferous permian suture zone dividing the lhasa block into a northern and southern segment this newly identified suture zone suggests that the border of the paleo tethyan ocean jumped southward from north of the bangong nujiang suture to within what is now the lhasa block

silurian clastic sediments in the north qilian shan nw china chemical and isotopic constraints on their forearc provenance with implications for the paleozoic evolution of the tibetan plateau abstract the north qilian shan is an accretion collision orogenic belt in the northeastern margin of the tibetan plateau that connects the north china plate to the north with the tethyan orogenic assemblage to the south its early paleozoic temporal spatial tectonic evolution has been extensively studied but the nature and tectonic setting of its silurian deposition is poorly understood our new petrological analysis suggests that the clast composition of fan conglomerates is complex in the danbanshan and zhongdabanshan sections abundant lenticular conglomerate beds contain mainly subrounded rounded metamorphic clasts whereas the tongziba and haichaoba sections contain volcanic chert limestone ultramafic and granitoid clasts however in the sunan area clast types are relatively simple dominated by basalt and granitoids and at tongziba gabbro and basalt clasts are subrounded and angular indicating two different types of provenance ultramafic limestone and abundant chert clasts in the haichaoba and tonghe conglomerates indicate an ophiolitic source whereas subrounded rounded quartzite and granitoid gneiss clasts were probably derived from the central qilian basement geochemical data from the volcanic and granitoid clasts suggest an arc related source and their zircon u pb ages of 515 429 ma suggest that the arc magmatism did not end in the mid silurian sandstone petrology and detrital modes suggest that the silurian sediments were deposited in a forearc basin paleogeographic reconstructions inferred from nnw nne paleocurrent data indicate that an island arc was present off the southern margin of the silurian basin and that southward subduction of oceanic crust between north china and the qilian shan was active or continued in the silurian

geochronology of the initiation and displacement of the altyn strike slip fault western china abstract 40ar 39ar dating studies have been carried out along the dangjin pass transect across the altyn strike slip fault assf the samples gave ages of 445 2 454 3 ma in the northern belt 164 3 178 4 ma in the mesozoic shear zone and 26 3 36 4 ma in the cenozoic shear zone using the piercing point of the bashikaogong fault and the cangma heihe fault an offset of 350 400 km along the assf has been estimated the 40ar 39ar dating of the syntectonic growth or syntectonic resetting minerals from the samples within the assf belt and offset estimations from different age piercing points suggest that the assf should be initiated in the middle jurassic 178 4 160 ma combined with previously reported ages our studies show that the assf is characterized by multi phase re activation during 85 100 25 40 and 8 10 ma following its initiation in the middle jurassic in the regional tectonic setting of convergence between the indian and eurasian continents

chemical evolution and volcanic activity of the active crater lake of poas volcano costa rica 1993 1997 abstract concentrations of chloride and sulfate and ph in the hot crater lake laguna caliente at poas volcano and in acid rain varied over the period 1993 1997 these parameters are related to changes in lake volume and temperature and changes in summit seismicity and fumarole activity beneath the active crater during this period lake level increased from near zero to its highest level since 1953 lake temperature declined from a maximum value of 70 c to a minimum value of 25 c and ph of the lake water increased from near zero to 1 8 in may 1993 when the lake was nearly dry chloride and sulfate concentrations in the lake water reached 85 400 and 91 000 mg l 1 respectively minimum concentrations of chloride and sulfate after the lake refilled to its maximum volume were 2630 and 4060 mg l 1 respectively between january 1993 and may 1995 most fumarolic activity was focused through the bottom of the lake after may 1995 fumarolic discharge through the bottom of the lake declined and reappeared outside the lake within the main crater area the appearance of new fumaroles on the composite pyroclastic cone coincided with a dramatic decrease in type b seismicity after january 1996 between may 1995 and december 1997 enhanced periods of type a seismicity and episodes of harmonic tremor were associated with an increase in the number of fumaroles and the intensity of degassing on the composite pyroclastic cone adjacent to the crater lake increases in summit seismic activity type a b and harmonic tremor and in the height of eruption plumes through the lake bottom are associated with a period of enhanced volcanic activity during april september 1994 at this time visual observations and remote fumarole temperature measurements suggest an increase in the flux of heat and gases discharged through the bottom of the crater lake possibly related to renewed magma ascent beneath the active crater a similar period of enhanced seismic activity that occurred between august 1995 and january 1996 apparently caused fracturing of sealed fumarole conduits beneath the composite pyroclastic cone allowing the focus of fumarolic degassing to migrate from beneath the lake back to the 1953 1955 cone changes in the chemistry of summit acid rain are correlated changes in volcanic activity regardless of whether fumaroles are discharging into the lake or are discharging directly into the atmosphere

early paleozoic to devonian multiple accretionary model for the qilian shan nw china abstract the qilian shan located in the northern tibetan plateau of nw china has an excellent record of early to middle paleozoic subduction accretion that resulted from convergence between the alax and qaidam blocks but there is no consensus about its detailed tectonic history this paper summarizes its tectonic divisions and discusses its tectonic evolution from the cambrian to devonian the belt has the following tectonic divisions in the far north the southern passive margin of the alax block is juxtaposed against an early cambrian to ordovician marianan type intra oceanic arc north qilian the north qilian arc is separated from a japanese type arc central qilian to the south by a high pressure metamorphic belt composed mostly of ophiolitic slices and oceanic crusts the central qilian is bounded to the south by a wide melange zone south qilian consisting of ophiolitic slices and continental margin sequences farther to the south lies the oulongbuluk microcontinent that is separated from the qaidam block and farther south by an ultrahigh pressure metamorphic belt tectonostratigraphic analysis together with geochemical geochronological and geophysical data indicates a complex evolution by subduction accretion processes from the cambrian to the devonian before final amalgamation and docking to the northern alax block this model solves the long lasting discussion on the polarity of subduction in paleozoic time this multiple subduction accretion history sheds light on the continuity of paleozoic sutures along the qilian shan and the nature of the altyn tagh fault and thus contributes to an improved understanding of the tectonic architecture of central asia

diverse ages and origins of basement complexes luzon philippines geological field investigations and 40ar 39ar ages from two basement complexes in southeast luzon document the first known occurrences of pre late cretaceous age rocks in the eastern philippines however individual components within the two complexes vary in age from late jurassic caramoan basement complex to early cretaceous and early miocene camarines norte calaguas islands basement complex these and other data show that southeast luzon basement complexes are genetically diverse and they indicate that the concept of an old autochthonous basement in the philippines is open to question this supports the hypothesis that the philippine archipelago is an amalgamation of allochthonous mesozoic and cenozoic island arc ocean basin and continental fragments that were assembled during the tertiary

slowing extrusion tectonics lowered estimate of post early miocene slip rate for the altyn tagh fault abstract determination of long term slip rate for the altyn tagh fault is essential for testing whether asian tectonics is dominated by lateral extrusion or distributed crustal shortening previous slip history studies focused on either quaternary slip rate measurements or pre early miocene total offset estimates and do not allow a clear distinction between rates based on the two the magmatic and metamorphic history revealed by shrimp zircon dating of clasts from miocene conglomerate in the xorkol basin north of the altyn tagh fault strikingly matches that of basement in the southern qilian shan and northern qaidam regions south of the fault this match requires that the post early miocene long term slip rate along the altyn tagh fault cannot exceed 10 mm year supporting the hypothesis of distributed crustal thickening for post early miocene times this low long term slip rate and recently documented large pre early miocene cumulative offset across the fault support a two stage evolution wherein asian tectonics was dominated by lateral extrusion before the end of early miocene and since then has been dominated by distributed crustal thickening and rapid plateau uplift

the formation of pangea abstract the making of pangea is the result of large scale amalgamation of continents and micro continents which started at the end of the neoproterozoic with the formation of gondwana as pieces were added to gondwana on its south american antarctica and australia side ribbon like micro continents were detached from its african and south chinese side cadomia in the late neoproterozoic avalonia and hunia in the ordovician galatia in the devonian and cimmeria in the permian cadomia was re accreted to gondwana but the other ribbon continents were accreted to baltica north china laurussia or laurasia finding the origin of these numerous terranes is a major geological challenge recently a global plate tectonic model was developed together with a large geological geodynamic database at the lausanne university covering the last 600 ma of the earth s history special attention was given to the placing of gondwana derived terranes in their original position using all possible constraints we propose here a solution for the variscan terranes another paper deals with the altaids the galatian super terrane was detached from gondwana in the devonian during the opening of paleotethys and was quickly separated into four sub terranes that started to by pass each other the leading terranes collided at the end of the devonian with the hanseatic terrane detached from laurussia in the carboniferous gondwana started to impinge onto the amalgamated terranes creating the variscan chain and the pangean super continent east of spain paleotethys remained opened until the triassic subducting northward under laurasia roll back of the paleotethyan slab triggered the collapse of most of the european variscan orogen which was replaced by series of permian rifts some of them becoming oceanized back arc basins during the triassic major force changes at the pangean plate limits at the end of the triassic provoked its break up through the opening of the proto caribbean central atlantic alpine tethys oceanic seaways

exploration and monitoring geothermal activity using landsat etm images a case study at aso volcanic area in japan abstract thermal activity monitoring in and around active volcanic areas using remote sensing is an essential part of volcanology nowadays three identical approaches were used for thermal activity exploration at aso volcanic area in japan using landsat etm images first the conventional methods for hydrothermal alteration mapping were applied to find the most active thermal region after exploring geothermal indicator minerals second we found some thermally highly anomalous regions around nakadake crater using land surface temperature estimation then the stefan boltzmann equation was used for estimating and also monitoring radiative heat flux rhf from the most active region of about 8 km 2 in and around nakadake crater in the central part of the aso volcano to fulfill the required parameter in the stefan boltzmann equation for radiative heat flux the ndvi normalized differential vegetation index method was used for spectral emissivity and the mono window algorithm was used for land surface temperature of this study area the ndvi value was used to divide land cover in the study area into four types water bare ground mixed and vegetated land the bare land was found within the most active region vegetation coverage area showed an inverse relationship with total rhf in this study as health of thermally stressed vegetation supports this relationship the spatial distribution of spectral emissivity ranged from 0 94 to 0 99 in our study land surface temperature was estimated using a mono window algorithm and was highest lst in 2008 and lowest in 2011 the results of rhf showed that the highest pixel rhf was found to be about 296 w m 2 in 2008 total rhf was obtained of about 607 mw in 2002 and the lowest was about 354 mw in 2008 the rhf anomaly area was found the highest in 2002 and was lowest in 2011 the highest total heat discharge rate hdr obtained about 3918 mw in 2002 and lowest total hdr about 2289 mw in 2008 from this study area but in the case of nakadake crater alone the higher thermal activity was observed in 2008 and was less in 2004 the study showed that landsat thermal infrared is the best option for thermal activity exploration and monitoring at aso volcano as well as in any active volcano region considering high efficiency and low cost

provenance of eclogitic metasediments in the north qilian hp lt metamorphic terrane western china geodynamic implications for early paleozoic subduction erosion abstract in this contribution we present new petrological geochemical zircon u pb and hf isotopic data for eclogitic metasediments from the north qilian orogenic belt in which early paleozoic ophiolitic sequences and hp lt metamorphic rocks have been previously recognized the studied metasediments contain eclogite facies assemblages reflecting p t conditions of 450 560 c and 1 9 2 4 gpa consistent with those of adjacent eclogites ree compositions of the eclogitic metasediments overlap those of average upper continental crust the shrimp and la mc icpms u pb data of zircons from metasediments demonstrate input from sources with major age components about 500 ma and 1600 1900 ma peak at 1800 ma with a smaller population at 660 800 ma peak at 750 ma and minor 1 9 ga zircon ages the youngest detrital zircons suggest a maximum depositional age of ca 475 ma and combined with p t conditions and previously determined metamorphic age suggest a burial rate of 0 6 0 7 cm yr zircon hf isotopic analyses show that 500 ma zircons have positive ehf t mainly between 8 and 16 by contrast meso to paleoproterozoic detrital zircons show a broad spectrum of ehf t mainly between 5 and 10 with t dm hf of 1800 2500 ma these data suggest that eclogitic metasediments are derived from a mixture of proterozoic continent crust and juvenile early paleozoic oceanic and or island arc crust and their protoliths likely were deposited in a terrigenous dominated continental margin basin rather than a pelagic oceanic crust environment the new results are consistent with subduction erosion of the active continental margin during south dipping subduction as these sediments formed in a fore arc environment close to the qilian block to the south were transported in the subduction zone to 60 70 km depth prior to their exhumation

using crater lake chemistry to predict volcanic activity at poas volcano costa rica monitoring of crater lake chemistry during the recent decline and disappearance of the crater lake of poas volcano revealed that large variations in so4 cl f cl and mg cl ratios were caused by the enhanced release of hcl vapor from the lake surface due to increasing lake temperature and solution acidity variation in the concentration of polythionic acids h2sxo6 x 4 6 was the most reliable predictor of renewed phreatic eruptive activity at the volcano exhibiting sharp decreases three months prior to the initiation of phreatic eruptions in june 1987 polythionic acids may offer a direct indicator of changing subsurface magmatic activity whereas chloride based element ratios may be influenced by surface volatilization of hcl and subsequent recycling of acidic fluids in crater lake volcanoes

volcanic lake systematics i physical constraints volcanic lakes have a wide range of characteristics and we make an attempt to delineate the limiting physical conditions for several lake classes the ratio between heat input and heat dissipation capacity of a lake constrains the temperature for perfectly mixed steady state volcanic lakes poorly mixed lakes are also conditioned by this ratio but their temperature structure is also strongly influenced by the hydrodynamics resulting from different mechanisms of heat transfer the steady state temperatures of volcanic lakes are largely determined by the magnitude of the volcanic heat influx relative to the surface area of the lake small lakes have only a small capacity for heat dissipation and their temperature rises quickly with only small heat inputs large lakes are buffered against variations in heat input both the heat dissipation and meteoric water input into a lake are functions of lake surface area and therefore each lake water temperature demands a certain precipitation rate for mass conservation independent of lake size the results of energy mass balance modeling shows that under common atmospheric conditions most steady state volcanic lakes are unlikely to maintain a temperature in excess of 45 50 c validation of the volcanic lake model was performed using published data from yugama lake japan and the keli mutu lakes indonesia also the model was applied to 24 natural systems to provide a baseline assessment of energy fluxes under the model assumptions so future work on those systems can identify nuances in individual systems that deviate from the simple model conditions we recommend the model for use in assessing temperature variations and volcanic lake stability in settings with known physical and atmospheric conditions application of the energy mass balance calculations of model lakes provides a genetic classification scheme largely based on physical process parameters

the cenozoic growth of the qilian shan in the northeastern tibetan plateau a sedimentary archive from the jiuxi basin sedimentary deposits in tibetan basins archive the spatial temporal patterns of the deformation and surface uplift processes that created the area s high topography during the cenozoic india asia collision in this study new stratigraphic investigation of the caogou section from the jiuxi basin in the northeasternmost part of tibetan plateau provides chronologic constraints on the deformation and northward growth of the plateau magnetostratigraphic analysis results suggest that the age of the studied 1000 m thick section spans from 24 2 ma to 2 8 ma detailed sedimentology and apatite fission track aft analyses reveal that variations in the clast provenance lithofacies sediment accumulation rates and aft lag times occurred at 13 5 10 5 ma we interpret these changes as in response to the initial uplift of the north qilian shan in addition paleomagnetic declination results from the section indicate a clockwise rotation of the jiuxi basin before 13 5 ma which was followed by a subsequent counterclockwise rotation during 13 5 9 ma this reversal in rotation direction may be directly related to left lateral strike slip activity along the easternmost segment of the altyn tagh fault combined with previous studies we suggest that movement on the western part of the altyn tagh fault was probably initiated during the oligocene 30 ma and that fault propagation to its eastern tip occurred during the middle late miocene

combined rutile zircon thermometry and u pb geochronology new constraints on early paleozoic hp uht granulite in the south altyn tagh north tibet china abstract hp granulite and garnet peridotite constitute a hp metamorphic unit in the south altyn tagh northern tibet previous data suggest that hp granulites experienced high pressure ultrahigh temperature metamorphism 870 1050 c 19 25 kbar followed by a medium pressure granulite facies overprint 780 820 c 9 5 12 kbar previous zircon u pb geochronology of hp granulites gave an age of ca 500 ma but it is unclear if this age reflects the metamorphic peak or the post peak overprint in this contribution new shrimp analyses indicate that the zircons associated with the peak metamorphic assemblage in hp felsic granulite are characterized by flat heavy ree patterns and 30 62 ppm ti yielding a temperature of 892 1002 c by ti in zircon thermometry suggesting that the growth of the zircons occurred under or close to peak ultrahigh temperature conditions rather than under post peak conditions these zircons gave a shrimp u pb age of 495 4 ma representing the age of peak metamorphism in contrast oscillatory zoned zircon cores yield magmatic ree patterns ti in zircon temperatures of 720 820 c and neoproterozoic u pb ages 936 22 ma this indicates that the protoliths of the felsic granulites were formed in the early neoproterozoic and were taken to extreme temperatures and pressures during the early paleozoic orogenic event in situ la icpms and emp analyses indicate that rutiles are characterized by broad scatter and bimodal distribution in both zr concentration and u pb age which are interpreted as a result of recrystallization or growth rather than volume diffusion the first rutile u pb age cluster is at ca 485 ma with a zr concentration of ca 3000 ppm yielding ca 950 c at 20 kbar implying that they grew at or near the peak metamorphic conditions the second rutile u pb age cluster is at ca 450 ma with a zr concentration of ca 1000 ppm corresponding to a temperature around 780 c at 10 kbar which is related to post peak reheating under medium pressure medium temperature granulite facies conditions these results demonstrate that hp granulites in the south altyn tagh experienced a peak ultrahigh temperature and high pressure metamorphism at ca 495 ma followed by medium pressure medium temperature granulite facies overprint before ca 450 ma

cenozoic tectonic evolution of qaidam basin and its surrounding regions part 1 the southern qilian shan nan shan thrust belt and northern qaidam basin cenozoic qaidam basin the largest active intermountain basin inside tibet figures importantly in the debates on the history and mechanism of tibetan plateau formation during the cenozoic indo asian collision to determine when and how the basin was developed we conducted detailed geologic mapping and analyses of a dense network of seismic reflection profiles from the southern qilian shan nan shan thrust belt and northern qaidam basin our geologic observations provide new constraints on the timing and magnitude of cenozoic crustal thickening in northern tibet specifically our work shows that the southernmost part of the qilian shan nan shan thrust belt and contractional structures along the northern margin of qaidam basin were initiated in the paleocene early eocene 65 50 ma during or immediately after the onset of the indo asian collision this finding implies that stress was transferred rapidly through tibetan lithosphere to northern tibet from the indo asian convergent front located 1000 km to the south the development of the thrust system in northern qaidam basin was driven by motion on the altyn tagh fault as indicated by its eastward propagation away from the altyn tagh fault the eastward lengthening of the thrust system was spatially and temporally associated with eastward expansion of qaidam basin suggesting thrust loading was the main control on the basin formation and evolution the dominant structure in northern qaidam basin is a southwest tapering triangle zone which started to develop since the paleocene and early eocene 65 50 ma and was associated with deposition of an overlying southwest thickening growth strata sequence recognition of the triangle zone and its longevity in northern qaidam basin explains a long puzzling observation that cenozoic depocenters have been located consistently along the central axis of the basin this basin configuration is opposite to the prediction of classic foreland basin models that require the thickest part of foreland sediments deposited along basin edges against basin bounding thrusts restoration of balanced cross sections across the southern qilian shan nan shan thrust belt and northern qaidam basin suggests that cenozoic shortening strain is highly inhomogeneous varying from 20 to 60 both vertically in a single section and from section to section across the thrust belt the spatially variable strain helps explain the conflicting paleomagnetic results indicating different amounts of cenozoic rotations in different parts of qaidam basin the observed crustal shortening strain also implies that no lower crustal injection or thermal events in the mantle are needed to explain the current elevation 3000 3500 m and crustal thickness 45 50 km of northern qaidam basin and the southern qilian shan nan shan thrust belt instead thrusting involving continental crystalline basement has been the main mechanism of plateau construction across northern qaidam basin and the southern qilian shan nan shan region

u pb zircon geochronology of coesite bearing eclogites from the southern dulan area of the north qaidam uhp terrane northwestern china spatially and temporally extensive uhp metamorphism during continental subduction coesite bearing eclogites from 100 km 2 in the southern dulan area north qaidam mountains nqm of western china contain zircon that records protolith crystallization and ultra high pressure uhp metamorphism sensitive high resolution ion microprobe mass spectrometer and laser ablation inductively coupled plasma mass spectrometry u pb analyses from cathodoluminescence cl dark zircon cores in a coesite bearing eclogite yield an upper intercept age of 838 50 ma and oscillatory zoned cores in a kyanite bearing eclogite gave a weighted mean 206 pb 238 u age of 832 20 ma these zircon cores yield steep heavy rare earth element hree slopes and negative eu anomalies that suggest a magmatic origin thus c 835 ma is interpreted as the eclogite protolith age unzoned cl grey or bright zircon and zircon rims from four samples yield weighted mean ages of 430 4 438 2 446 10 and 446 3 ma flat hree patterns without eu anomalies and contain inclusions of garnet omphacite rutile phengite and rare coesite these ages are interpreted to record 16 5 myr of uhp metamorphism these new uhp ages overlap the age range of both eclogite and paragneiss from the northern dulan area suggesting that all uhp rock types in the dulan area belong to the same tectonic unit our results are consistent with slow continental subduction but do not match oceanic subduction and diapiric exhumation uhp model predictions these new data suggest that similar to eclogites in other hp uhp units of the nqm and south altyn tagh protoliths of the eclogites in the dulan area formed in a continental setting during the neoproterozoic and then subducted to mantle depth together with continental materials during the early palaeozoic