ignimbrites of the roque nublo group gran canaria canary islands non welded lithic rich ignimbrites hereintermed the roque nublo ignimbrites are the most distinctive deposits of the pliocene roque nublo group which forms the products of second magmatic cycle on gran canaria they are very heterogeneous with 35 55 volume lithic fragments 15 30 mildly vesiculated pumice 5 7 crystals and 20 30 ash matrix the vitric components pumice fragments and ash matrix are largely altered and transformed into zeolites and subordinate smectites the roque nublo ignimbrites originated from hydrovolcanic eruptions that caused rapid and significant erosion of vents thus incorporating a high proportion of lithic clasts into the eruption columns these columns rapidly became too dense to be sustained as vertical eruption columns and were transformed into tephra fountains which fed high density pyroclastic flows the deposits from these flows were mainly confined to palaeovalleys and topographic depressions in distal areas close to the coast line where these palaeovalleys widened most of the pyroclastic flows expanded laterally and formed numerous thin flow units the combined effect of the magma water interaction and the high content of lithic fragments is sufficient to explain the characteristic low emplacement temperature of the roque nublo ignimbrites this fact also explains the transition from pyroclastic flows into lahar deposits observed in distal facies of the roque nublo ignimbrites the existence of hydrovolcanic eruptions generating high density pyroclastic flows unable to efficiently separate the water vapour from the vitric components during transport also accounts for the intense zeolitic alteration in these deposits

early mesozoic tectonics of the south china block insights from the xuefengshan intracontinental orogen abstract intracontinental orogens remain less understood than accretionary or collisional orogens that are related to plate margin interactions in the center of the south china block the xuefengshan belt provides a well exposed example of such an intracontinental orogen of early mesozoic age detailed field tectonic observations indicate that the xuefengshan belt can be divided into a western outer zone characterized by km scale box fold structures and an eastern zone separated from the western outer zone by the se dipping main xuefengshan thrust in the eastern zone nw verging folds coeval with a pervasive slaty cleavage and a nw se trending lineation are the dominant structures from west to east the dip of the cleavage surface exhibits a fan like pattern the bulk architecture of the xuefengshan belt results from polyphase deformation d 1 is characterized by a top to the nw ductile shearing d 2 corresponds to se directed back thrusting and folding d 3 consists of upright folds with vertical cleavage and lineation at depth a high strain zone characterized by greenschist facies metamorphic rocks and a top to the nw ductile shearing corresponds to a ductile decollement zone that accommodated the deformation of the neoproterozoic to early triassic sedimentary series kinematic compatibility suggests that the synmetamorphic ductile shearing was coeval with the d 1 event in the sedimentary cover the xuefengshan belt is interpreted as an early mesozoic intracontinental orogen which possibly originated from the se directed continental subduction of a piece of the south china block in response to northwestwards subduction of the pacific plate

conduction model for the thermal influence of lithic clasts in mixtures of hot gases and ejecta the thermal influence of lithic clasts in mixtures of hot gases and ejecta produced during explosive eruptions is an important aspect of the dynamics of eruption columns and pyroclastic flows in order to study this aspect we present a heat conduction model which considers both the temperature distributions of the lithic clasts and the exchange of thermal energy between the lithic clasts and other constituents gases and juvenile pyroclasts the model quantifies the thermal energy of the system by means of a characteristic energy function that describes the energy absorbed by each lithic clast using this energy function and considering some hypothetical arrangements of lithic clasts it is possible to calculate by means of heat balance the temperature variations in mixtures of hot gases and ejecta as a function of time volume fraction of particles to gas and thermal diffusivity initial temperature diameter and volume fraction of lithic clasts theoretical results show that in an eruption a volume fraction of 12 of cool lithic clasts 20 100 c whose size is less than 5 cm will cause the mixture of hot gases and ejecta to cool by 300 c we suggest therefore that the thermal effects of lithic clasts should be considered in numerical models of eruption columns and pyroclastic flows

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

constraints from zircon u pb ages o and hf isotopic compositions on the origin of neoproterozoic peraluminous granitoids from the jiangnan fold belt south china zircon textures and micro chemical compositions precisely record the origin and petrogenesis of granitoids which are crucial for evaluating crustal growth and reworking thermal and geodynamic evolution zircons in peraluminous granitoids from the three largest 820 ma complexes guibei yueyang and jiuling in the jiangnan fold belt in south china are used to constrain their sources and petrogenetic processes zircons in the guibei granitoids have complex internal structures nearly all magmatic and inherited zircons have similar ehf 6 8 to 5 6 and 18o values 8 8 11 6 and dominantly lie between ehf evolution vectors for a crust created between 1 7 and 2 1 ga suggesting that the guibei granitoids were produced by partial melting of recycled heterogeneous supracrustal material however the yueyang granitoids contain zircons with high ehf 0 5 to 9 7 and relatively low 18o values 5 9 8 4 and two stage model ages of 1 1 1 8 ga and thus may have been formed by melting of mafic rocks from the lower crust the jiuling granitoids and their enclaves contain more complex zircons with more variable ehf 7 2 to 9 7 and 18o values 7 0 10 6 and lie along the mixing trend between the above proposed infracrustal and supracrustal granitoids therefore the neoproterozoic peraluminous granitoids in the jiangnan fold belt were produced by melting and mixing of continental crust compared with extremely low 4 and negative 18o values of neoproterozoic igneous zircons formed in its northern active continental margin the high 18o peraluminous granitoids in the southeastern yangtze block are considered to have been formed by melting of hydrothermally unaltered continental crust triggered by asthenosphere upwelling in the nanhua back arc basin

the taurewa eruptive episode evidence for climactic eruptions at ruapehu volcano new zealand the ca 10 500 years b p eruptions at ruapehu volcano deposited 0 2 0 3 km3 of tephra on the flanks of ruapehu and the surrounding ring plain and generated the only known pyroclastic flows from this volcano in the late quaternary evidence of the eruptions is recorded in the stratigraphy of the volcanic ring plain and cone where pyroclastic flow deposits and several lithologically similar tephra deposits are identified these deposits are grouped into the newly defined taurewa formation and two members okupata member tephra fall deposits and pourahu member pyroclastic flow deposits these eruptions identify a brief ca 2000 year but explosive period of volcanism at ruapehu which we define as the taurewa eruptive episode this episode represents the largest event within ruapehu s ca 22 500 year eruptive history and also marks its culmination in activity ca 10 000 years b p following this episode ruapehu volcano entered a ca 8000 year period of relative quiescence we propose that the episode began with the eruption of small volume pyroclastic flows triggered by a magma mingling event flows from this event travelled down valleys east and west of ruapehu onto the upper volcanic ring plain where their distal remnants are preserved the genesis of these deposits is inferred from the remanent magnetisation of pumice and lithic clasts we envisage contemporaneous eruption and emplacement of distal pumice rich tephras and proximal welded tuff deposits the potential for generation of pyroclastic flows during plinian eruptions at ruapehu has not been previously considered in hazard assessments at this volcano recognition of these events in the volcanological record is thus an important new factor in future risk assessments and mitigation of volcanic risk at tongariro volcanic centre

hydrovolcanic explosions the systematics of water pyroclast equilibration the initial contact of external water with erupting magma and the mass ratio of water to magma govern the development of hydrovolcanic phenomena the phase relations of water within the runout system and the separation of vapor or liquid from the pyroclasts explain gradational transitions between some pyroclastic flows base surges mud hurricanes mudflows and sheetfloods

jiangnan orogen in south china developing from divergent double subduction abstract the jiangnan orogen is considered as a continent continent collisional belt resulting from the closure of a meso neoproterozoic ocean separating the southeastern margin of the yangtze block from the northwestern margin of the cathaysia block recent data indicate the existence of early neoproterozoic 1000 825 ma volcanic arc assemblages on both sides of the orogen suggesting that the ocean lithosphere between the yangtze and cathaysia blocks must have undergone divergent double sided subduction during the period of 1000 825 ma the divergent double subduction eventually resulted in the closure of the ocean basin at 825 ma leading to the soft collision of the yangtze and cathaysia blocks to form the jiangnan orogen without involvement of continental deep subduction high grade metamorphism of continental crust and uplift exhumation of high grade metamorphic rocks shortly after the collision the initial detachment of the ocean lithosphere from the overlying crust and sedimentary sections induced underplating of mantle magmas triggering partial melting of accretionary wedge strata to form some peraluminous s type granites in the period 825 815 ma finally the sinking of the oceanic slab pulled down the overlying strata to form some basins in which the banxi group and its equivalent strata including bimodal volcanic rocks were formed in the period 815 750 ma

structural development of the lower paleozoic belt of south china genesis of an intracontinental orogen abstract an extensive structural study of wuyishan and surrounding areas south china brings data on the structures formed prior to the devonian unconformity building the lower paleozoic belt an ordovician tectonic event was responsible for i south directed structures in the wuyishan proper and its southern border related to both thin and thick skinned tectonics accompanied by metamorphism and crustal thickening ii north directed structures to the west of ganjiang fault and north of jiangshan shaoxing fault where only thin skinned tectonics is visible the southward deformation accommodating the shortening includes emplacement of thrust sheets involving deep crustal material and some mantle peridotite and likely repetition of continental material both responsible for crustal thickening the orogeny was due to the underthrusting of the southern part of the south china block beneath the northern part of this block closing the pre existing nanhua rift created at around 850 800 ma and involving the rift sedimentary fill in the southward thrusting the syn metamorphic tectonic piling was followed by anatexis and granite emplacement at around 440 390 ma our new u th pb epma monazite dating gives ages of 433 9 ma and 437 5 ma early silurian for the main anatectic event and a younger age of 412 5 ma late silurian early devonian for a late re heating process the lower paleozoic belt is an intracontinental orogen without oceanic suture record it shows some similarities with the cenozoic european pyrenees chain with a strong asymmetry

cerro grande volcano the evolution of a miocene stratocone in the early trans mexican volcanic belt abstract cerro grande volcano is a well preserved low angle composite volcano that evolved between 11 and 9 ma it represents the beginning of volcanism in the eastern sector of the early trans mexican volcanic belt extensive field stratigraphic and petrologic work revealed a complex volcanic evolution that can be summarized in six principal stages i construction of a shield like lava cone ii generation of unusual lithic rich ignimbrites apparently related to powerful eruptions that caused rapid and significant vent erosion these eruptions could have resulted from shallow magma water interactions iii eruption of peripheral fissural lava flows following a nnw sse trending fissure system iv period of dome growth and explosive collapse v short period of repose followed by open vent explosive magmatic and hydromagmatic eruptions vi radial eruption of lava flows related to a ring fissural system geochemistry data of late miocene volcanics show a continental arc magmatic origin and show that crystal fractionation contributed strongly to magma differentiation several lava flows were erupted with a persistent nnw sse trending orientation and the entire late miocene geologic record is affected by highly dislocated normal faults with the same orientation this may indicate that late miocene volcanism was under tectonic control finally late miocene volcanics and the inception of the early trans mexican volcanic belt could be related to a shallow subduction angle influenced by a significant increase in the convergence rate along the middle american trench at about 20 ma

major and trace element and sr nd isotope constraints on the origin of paleogene volcanism in south china prior to the south china sea opening abstract paleogene volcanic rocks crop out in three sedimentary basins namely sanshui heyuan and lienping in the attenuated continental margin of south china lavas from the sanshui basin which erupted during 64 43 ma are bimodal consisting of intraplate tholeiitic basalt and trachyte rhyolite associations similar to cretaceous a type granites from the nearby region the felsic member shows peralkaline nature na2o k2o 10 12 na k al 0 98 1 08 general enrichment in the incompatible trace elements and significant depletion in ba sr eu p and ti although both types of the sanshui lavas have rather uniform nd isotope compositions nd t 6 to 4 that are comparable to late cenozoic basalts around the south china sea the felsic rocks possess apparently higher initial sr isotope ratios isr up to 0 713 and form a horizontal array to the right in the nd vs sr isotope plot closed system differentiation of mantle derived magmas in a double diffusive magma chamber is considered for the bimodal volcanism in which the trachytes and rhyolites represent a type melts after extensive crystal fractionation in the upper portion of the chamber such a type melts were later contaminated by small amounts 1 3 of upper crustal materials during ascent on the other hand composition of lavas in the other two basins varies from tholeiitic basalt to andesite their sr and nd isotope ratios isr 0 705 to 0 711 nd t 1 to 5 and generally correlative nb ta depletions suggest a distinct magma chamber process involving fractional crystallization concomitant with assimilation of the country rock we conclude that these paleogene volcanic activities resulted from the lithospheric extension in south china that migrated southwards and eventually led to opening of the south china sea during 30 16 ma

provenance and thermal history of the franciscan accretionary complex constraints from zircon fission track thermochronology the franciscan accretionary complex of california contains large belts of sediments that were subducted to depths of 10 to 30 km metamorphosed under high pressure and low temperature conditions and later returned to the surface reconnaissance zircon fission track ft data 21 samples place constraints on the sedimentary provenance thermal history and or exhumation history of various franciscan units in a transect across the northern coast ranges through mendocino pass metagraywacke samples from the valantine springs formation of the franciscan eastern belt experienced maximum temperatures tmax ranging from 250 to 310 c zircon ft and previously reported apatite ft data indicate that parts of the western eastern and eastern central belts cooled at about 100 70 ma from 300 c to 100 c considering the low paleogeothermal gradient of roughly 9 c km during franciscan subduction these rocks were apparently uplifted and exhumed from 30 to 10 km depth at this time one sample from the eyler mountain sequence in the northwestern diablo range similarly shows tmax 310 c and major cooling at 100 70 ma this uplift and exhumation may have reflected accelerated accretion at the base as well as the front of the franciscan wedge accompanied by extensional thinning at shallower levels as a result of massive sediment influx into the franciscan trench from the sierra nevada magmatic arc in the northern coast ranges the western part of the central belt and the coastal belt experienced lower tmax probably 200 c and retain source area detrital zircon ft ages burnt hills terrane samples in the diablo range also experienced tmax 230 c and retain detrital ages the detrital single grain age patterns from these areas are compatible with those expected from the sierra nevada magmatic arc and its country rocks this is consistent with derivation of franciscan sediments from arc terranes in california and provides no particular suggestion that these parts of the franciscan are highly allochthonous paleogene coastal belt sandstones also yield detrital ages compatible with derivation from the sierra nevada but also include early tertiary zircon populations compatible with reconstructions suggesting these sandstones were sourced partly from the idaho batholith

features of crustal structure under the canary islands abstract the data and interpretation of a seismic refraction project carried out in the canary islands in 1977 and 1979 are presented the moho is reached at about 15 km under fuerteventura gran canaria and tenerife whereas under lanzarote a value of 11 km has been found the velocity at the surface varies even within one island from 2 8 to 4 4 km s there are variations in the velocity of the main crustal layer under the volcanic deposits a continuous velocity depth function seems to be more appropriate for the island of tenerife contrary to fuerteventura lanzarote and gran canaria which seem to have a layered structure there is evidence for pronounced structural differences between lanzarote and fuerteventura the mantle velocity has a value of about 8 0 km s under the western islands whereas a very low value of 7 4 km s has been found under lanzarote and fuerteventura despite the clear structural differences between the islands our results leave little doubt about the oceanic character of all the islands the differences between the eastern and western group may be due to the islands being situated close to the african continental margin the seismic velocities under the volcanic deposits do not support the idea of a common basement for all the islands they rather suggest independent volcanic edifices finally a comparison of the structure of the crust under the canary islands with that under other volcanic islands reveals remarkable similarities

constructing the longmen shan eastern tibetan plateau margin insights from low temperature thermochronology 1 contrasting models of upper crustal shortening versus lower crustal flow have been proposed to explain the formation of thickened crust in the longmen shan lms eastern tibetan plateau tp margin these models require different structural kinematics along the lms whose structural geometry is defined by three parallel nw dipping fault zones from foreland southeast to hinterland northwest they are the guanxian anxian fault yingxiu beichuan fault ybf and wenchuan maowen fault wmf newly derived and previously published low temperature thermochronology data from the lms were synthesized to constrain the spatial exhumation and test previous models the results show that 1 exhumation increases abruptly across the range bounding ybf suggesting the fault being the main thrust boundary between the lms and the sichuan basin to the east 2 younger late cenozoic cooling ages are found on the hinterland wmf where a dichotomy of ages on the hanging wall versus footwall suggests late cenozoic thrust activity and 3 toward the hinterland to the west exhumation rates decrease twofold over a distance of 30 40 km this exhumation pattern indicates a westward decrease of tectonic uplift providing the regional topography approached a steady state whereby exhumation is in balance with tectonic uplift the observed exhumation estimates support an upper crustal configuration where thrusts in the lms merge gradually into a gentle detachment seated at a depth of 20 30 km results of this study support a revised upper crustal thrusting model

partitioning of the cretaceous pan yangtze basin in the central south china block by exhumation of the xuefeng mountains during a transition from extensional to compressional tectonics abstract the formation of a series of intermountain basins is likely to indicate a geodynamic transition especially in the case of such basins within the central south china block cscb determining whether or not these numerous intermountain basins represent a division of the cretaceous pan yangtze basin by exhumation of xuefeng mountains is key to understanding the late mesozoic to early cenozoic tectonics of the south china block scb here we present apatite fission track aft data and time temperature modeling in order to reconstruct the evolution history of the pan yangtze basin fourteen rock samples were taken from a ne sw trending mountain basin system within the cscb including from west to east the wuling mountains wuling shan the south and north mayang basins the xuefeng mountains xuefeng shan and the hengyang basin cretaceous lacustrine sequences are well preserved in the south and north mayang and hengyang basins and sporadically crop out in the xuefeng mountains whereas paleogene piedmont proluvial lacustrine sequences are only found in the south mayang and hengyang basins aft results indicate that the wuling and xuefeng mountains underwent rapid denudation post 84 ma whereas the south and north mayang basins were more slowly uplifted from 67 and 84 ma respectively following a quiescent period from 32 to 19 ma both the mountains and basins have been rapidly denuded since 19 ma both the aft data and sedimentary facies changes suggest that the cretaceous deposits that cover the south north mayang and hengyang basins through to the xuefeng mountains define the cretaceous pan yangtze basin integrating our results with tectonic background for the scb we propose that rollback subduction of the paleo pacific plate produced the pan yangtze basin which was divided into the south north mayang and hengyang basins by the abrupt uplift and exhumation of the xuefeng mountains from 84 ma to present apart from a period of tectonic inactivity from 32 to 19 ma this late late cretaceous to paleogene denudation resulted from movement on the ziluo strike slip fault which formed due to intra continental compression most likely associated with the eurasia indian plate subduction and collision sinistral transpression along the ailao shan red river fault at 34 17 ma probably transformed this compression to the extrusion of the indochina block and produced the quiescent window period from 32 to 19 ma for the mountain basin system in the cscb therefore the initiation of exhumation of the xuefeng mountains at 84 ma indicates a switch in tectonic regime from cretaceous extension to late late cretaceous and cenozoic compression

geochronology and stratigraphy of the roque nublo cycle gran canaria canary islands the roque nublo cycle of pliocene age is the second main cycle of subaerial volcanic activity in gran canaria during this cycle a stratovolcano at least 2500 m high developed in the central part of the island its volcanic products covered an area of about 540 km 2 the detailed stratigraphy of the roque nublo cycle together with a magnetostratigraphic study and six new k ar dates which complement 22 previously published ones have led to the reconstruction of the volcanic history of this cycle in which six main stages have been distinguished the revised chronology of the roque nublo cycle implies an overlap in age with the earliest rocks of the third cycle of subaerial activity in gran canaria no volcanic hiatus occurred between the second and third cycles as was previously thought to be the case

cooling and exhumation of the oldest sanqiliu uranium ore system in motianling district south china block the sanqiliu uranium deposit belongs to a uranium ore system in motianling district it is the oldest uranium deposit in south china primary uranium mineralization occurred almost simultaneously with the emplacement of the host granites and subsequent dykes and it has a relatively high grade of uranium 0 421 we clarify the age of mineralization and investigate the cooling history through new pitchblende u pb and apatite fission track thermochronology the pitchblende u pb results indicate that uranium mineralization occurred at 801 759 ma fractionation of uranium and lead at 374 295 ma is interpreted as remobilization and resetting of the original uranium the motianling area has apatite fission track ages of 57 to 18 ma by combining our results with previous work we conclude that the deposit cooled slowly and was exposed at the surface during the cenozoic the timing and depth of exhumation helped to preserve and avoid erosion of the uranium deposit and highlight the potential for regional uranium exploration

eastern asia and the western pacific timing and spatial distribution of rifting in china abstract a compilation of recognized rifts in china indicates that five distinct episodes of rifting have occurred these episodes occurred at approximately 2 3 ga 1 85 1 7 ga 1 0 0 7 ga 550 190 ma and 175 ma to present the timing and location of rifting has largely been a function of tectonic activity along the margins of the various chinese paleocontinents some of the oldest rocks in china are found in the sino korean and tarim cratons and most of the identified precambrian rifts are found within or on the margins of these cratons these cratons are viewed as the continental nucleus of china and fold belts and microcontinents have been accreted to this nucleus through time much of the locus of rifting has migrated away from this nucleus as has the location of the continental margin especially in the paleozoic beginning in the middle mesozoic rifting commenced along the entire eastern margin of china extension occurred concomitantly with subduction in a backarc setting this is the dominant cenozoic extensional regime in china and is still active today to date few cenozoic rifts have been found in western china where most tectonism is probably a consequence of the collision between india and china but it seems likely that more rifts will be discovered in this area as it becomes better studied

a large volcanic debris avalanche in the pliocene roque nublo stratovolcano gran canaria canary islands abstract during the second magmatic cycle of gran canaria between 5 and 3 ma the roque nublo stratovolcano rns achieved its almost total development reaching a height of 2500 2600 m and spreading about 100 km 3 of products over a surface area of at least 250 km 2 in the last growth stage of the rns and probably in connection with the paroxysmic genesis of the roque nublo explosive caldera a sector of the volcanic edifice suffered a gravitational collapse of significant importance the data reported here show that the collapse was accompanied by a great avalanche the roque nublo debris avalanche deposit rndad composed mainly of block facies the last explosive emissions of the rns were restricted to ignimbrites and some explosive breccias that locally overlap the avalanche deposits the features of the outcrops along the course of the avalanche make it possible to distinguish between the proximal intermediate and distal deposits the debris avalanche 3 4 km 3 moved sw then s and finally sse part of the deposit mainly formed by large megablocks remained very near the source area halfway along the course the avalanche was channelled by broad and steep paleovalleys in the most distal areas the rndad opens into a fan shape reaching a distance of 28 30 km a simplified kinematical physical model is used to understand the movement of the rndad the model is based on the speculative reconstruction of the rns edifice based in turn on its scarce peripheral remains an arbitrary avalanche path was selected with a total travel distance from the source area top of the stratovolcano of 28 km with these assumptions it is possible to calculate the speed at each point v m 90 m s and other parameters that show that the rndad is very similar to other well known volcanic avalanches

geochemical and geochronological study of the sanshui basin bimodal volcanic rock suite china implications for basin dynamics in southeastern china abstract sanshui basin is one of the typical mesozoic cenozoic intra continental rift basins with voluminous cenozoic volcanic rocks in southeastern china thirteen cycles of volcanic eruptions and two dominant types of volcanic rocks basalt and trachyte rhyolite have been identified within the basin both basalt and trachyte rhyolite members of this bimodal suit have high values of end 2 3 to 6 2 and different sr isotopic compositions initial 87 sr 86 sr ratios are 0 70461 0 70625 and 0 70688 0 71266 for basalts and trachyte rhyolite respectively reflecting distinct magma evolution processes or different magma sources the results presented in this study indicate that both of the trachyte rhyolite and basaltic magmas were derived from similar independent primitive mantle but experienced different evolution processes the trachyte rhyolitic magma experienced significant clinopyroxene and plagioclase fractionational crystallization from deeper magma chamber with significant crustal contamination while the basaltic magmas experienced significant olivine and clinopyroxene fractionational crystallization in shallower magma chamber with minor crustal contamination new zircon u pb dating confirms an initial volcanic eruption at 60 ma and the last activity at 43 ma geologic geochemical and geochronological data suggest that the inception of the sanshui basin was resulted from upwelling of a mantle plume the sanshui basin widened due to subsequent east west extension and the subsequent volcanism constantly occurred in the center of the basin evidence also supports a temporal and spatial association with other rift basins in southeastern china the upwelling mantle plume became more active during late cenozoic time and most likely triggered opening of other basins including the young south china sea basin

origin and tectonic significance of a mesozoic multi layer over thrust system within the yangtze block south china abstract in the yangtze block south china a well developed mesozoic thrust system extends through the xuefeng and wuling mountains in the southeast to the sichuan basin in the northwest the system comprises both thin and thick skinned thrust units separated by a boundary detachment fault the dayin fault to the northwest the thin skinned belt is characterized by either chevron anticlines and box synclines to the northwest or chevron synclines to the southeast the former structural style displays narrow exposures for the cores of anticlines and wider exposures for the cores of synclines thrust detachments occur along silurian fs and lower cambrian fc strata and are dominantly associated with the anticlines to the southeast this style of deformation passes gradually into one characterized by chevron synclines with associated principal detachment faults along silurian fs cambrian fc and lower sinian fz strata there are however numerous secondary back thrusts therefore the thin skinned belt is like the valley and ridge province of the north american applachian mountains the thick skinned belt structurally overlies the thin skinned belt and is characterized by a number of klippen including the xuefeng and wuling nappes it is thus comparable to the blue ridge province of appalachia the structural pattern of this thrust system in south china can be explained by a model involving detachment faulting along various stratigraphic layers at different stages of its evolution the system was developed through a northwest stepwise progression of deformation with the earliest delamination along lower sinian strata fz analyses of balanced geological cross sections yield about 18 1 21 total 88 km shortening for the thin skinned unit and at least this amount of shortening for the thick skinned unit the compressional deformation from southeast to northwest during late jurassic to cretaceous time occurred after the westward progressive collision of the yangtze block with the north china block and suggests that the orogenic event was intracontinental in nature

climatic forcing of erosion landscape and tectonics in the bhutan himalayas a fundamental objective in studies of climate erosion tectonics coupling is to document convincing correlation between observable indicators of these processes on the scale of a mountain range the eastern himalayas are a unique range to quantify the contribution of tectonics and climate to long term erosion rates because uniform and steady tectonics have persisted for several million years while monsoonal precipitation patterns have varied in space and time specifically the rise of the shillong plateau the only orographic barrier in the himalayan foreland has reduced the mean annual precipitation downwind in the eastern bhutan himalaya at the miocene pliocene transition apatite fission track aft analyses of 45 bedrock samples from an e w transect along bhutan indicate faster long term erosion rates outside of the rain shadow in the west 1 0 1 8 mm yr than inside of it in the east 0 55 0 85 mm yr furthermore an aft vertical profile in the latter segment reveals a deceleration in erosion rates sometime after 5 9 ma in this drier segment of bhutan there are remnants of a relict landscape formed under a wetter climate that has not yet equilibrated to the present climatic conditions uplift and preservation of the paleolandscape are a result of a climate induced decrease in erosion rates rather than of an increase in rock uplift rate this study documents not only a compelling spatial correlation between long term erosion and precipitation rates but also a climatically driven erosion rate change on the scale of the eastern himalayas a change that in turn likely influences that region s recent tectonic evolution

quaternary sea level changes and aeolianites in the fernando de noronha archipelago northeastern brazil abstract finite and directional sea level indicators were found in the fernando de noronha archipelago brazil showing that in the mid to late holocene the sea level was similar to present and had been stable for at least the preceding 500 years this sea level behaviour is different from that observed in the nearest continental coast of northeastern brazil suggesting a slight sinking of the archipelago relative to the mainland the caracas formation that corresponds to the conspicuous aeolianites found on the archipelago was subdivided into two chronological units i and ii unit i is pleistocene formed before 50 000 bp and could correspond to an interglacial stage unit ii was formed during the holocene and was built up over the period from 10 700 to 5700 bp meaning that the sand dunes were transgressive and formed during the postglacial sea level rise prior to a mid holocene maximum at 7000 5000 bp

cenozoic volcanic rocks of eastern china secular and geographic trends in chemistry and strontium isotopic composition abstract the cenozoic volcanic rocks of eastern china are subalkalic to alkalic basalts erupted in an early tertiary back arc rift environment and from scattered late tertiary and quaternary volcanic centers in a continental area crossed by active faults driven by subduction of the pacific plate and the collision of india and eurasia immobile trace elements and major elements conform very well to each other in classification of the 59 rocks for which complete data are reported and they correctly identify the tectonic setting lil element enrichments of the basalts lie between those of p morb and ocean island alkalic basalts and show a secular increase 87sr 86sr ratios of basalts vary from 0 7029 to 0 7048 alkalic basalts are systematically less radiogenic than geographically coextensive and contemporaneous tholeiitic basalts increase of radiogenic sr with increasing crustal thickness and crustal age and with silica enrichment of the magmas suggests crustal contamination but this is inadequate to explain the lil element enrichment patterns and variable lil element enrichments the preferred hypothesis is that the alkalic magmas come from a deeper source with long term lil element depletion and low rb sr ratio but relatively recent lil element enrichment conversely the tholeiitic magmas are melts of subcontinental mantle lithosphere that is more lil element depleted than the alkalic source at the time of magma genesis but has had an elevated rb sr ratio for much of its post consolidation history

reconstructing eroded paleovolcanoes on gran canaria canary islands using advanced geomorphometry abstract original volcanic edifices of two successive stages of gran canaria are reconstructed using a geomorphometric analysis of existent or restored paleosurfaces in the reconstruction surface fitting was applied preferably to planezes i e triangular facets of original volcano flanks and quasi planar surfaces qps those occurring on planezes or scattered slightly eroded portions derived from original cone surfaces with the help of red relief image map rrim analysis out of the long lasting mid miocene to holocene subaerial evolution of the island the late miocene fataga volcano and the subsequent pliocene roque nublo volcanoes were the largest and highest the eruptive center of fataga a composite edifice 12 2 8 8 ma that may have grown up excentrically with respect to the previous tejeda caldera is well defined by both two planezes named veneguera mogan and fataga tirajana and qps remnants its calculated original volume 1000 km3 is close to the largest stratovolcanoes on earth however its 3300 m elevation obtained by exponential fit may have been significantly lower due to the complex architecture of the summit region e g a caldera responsible for ignimbrite eruptions roque nublo a 3 7 2 9 ma stratovolcanic cone which was superimposed upon the fataga rocks 3 km west of the fataga center has left no considerable paleosurfaces behind due to heavy postvolcanic erosion yet its remnant formations preserved in a radial pattern unambiguously define its center moreover surface fitting of the outcropping rocks can be corrected taking the erosion rate for the past 3 ma into account such a corrected surface fit points to a regular shaped 3000 m high cone with a 25 km radius and ca 940 km3 original volume also comparable with the dimensions of the largest terrestrial stratovolcanoes

phreatomagmatism and its relevance abstract phreatomagmatic explosive volcanism results from the interaction of magma with external water groundwater or surface water close to or at the earth s surface magma of any composition acid to ultrabasic may be involved in this type of explosive activity at present the relevance of phreatomagmatism is realized increasingly because it takes place in many diverse environments and affects not only small monogenetic volcanoes but also large volcanoes at some volcanoes as for example at lake taupo new zealand a volume of several tens of km 3 of magma may have been mixed with several km 3 of external water phreatomagmatism has thus become an important aspect in volcanic hazard assessments other fields involving the interaction of water with a melt are the study of nuclear and other industrial safety hazards of magma mixing of diamond carrying volcanism and of hydrothermal ore deposits as well as geothermal and groundwater reservoirs experiments on the dynamics of magma water interactions provide increasing insight into the principles and hazards of phreatomagmatic explosions this type of research will become a new field in experimental petrology

tectonics of the middle triassic intracontinental xuefengshan belt south china new insights from structural and chronological constraints on the basal decollement zone in orogenic belts a basal decollement zone often develops at depth to accommodate the shortening due to folding and thrusting of the sedimentary cover in the early mesozoic intracontinental xuefengshan belt of south china such a decollement zone is exposed in the core of anticlines formed by the emplacement of the late orogenic granitic plutons our detailed multi scale structural analysis documents a synmetamorphic ductile deformation in the basal decollement the neoproterozoic pelite and sandstone and the intruding early paleozoic granites were deformed and metamorphosed into mylonites and orthogneiss respectively the metamorphic foliation contains a nw se stretching lineation associated with top to the nw kinematic indicators the ductile shearing of these high strained rocks can be correlated with nw verging folds and thrusts recognized in the neoproterozoic to early triassic sedimentary cover monazite u th pbtot chemical dating and zircon sims u pb dating provide age constraints of the ductile shearing between 243 and 226 ma and late orogenic granite emplacement around 235 215 ma in agreement with recent geochronological data these new results show that the xuefengshan belt is an early mesozoic orogen dominated by the nw directed shearing and thrusting at the southeastern boundary of the xuefengshan belt the chenzhou linwu fault separates the early mesozoic domain to the nw from the early paleozoic domain to the se the tectonic architecture of this belt was possibly originated from the continental underthrusting to the se of the south china block in response to northwest directed subduction of the paleo pacific plate

alteration processes of the roque nublo ignimbrites gran canaria canary islands abstract the non welded lithic rich ignimbrites of roque nublo are the most characteristic deposits of the second magmatic cycle pliocene in age on gran canaria they are very heterogeneous with 35 55 by volume of lithic clasts 15 30 of moderately vesiculated pumice 5 7 of crystals and 20 30 of ash matrix the juvenile components pumice fragments and ash matrix are largely altered the mineral assemblage that originated by the alteration of vitric components consists mainly of zeolites chabazite phillipsite and analcime and subordinate smectites the alteration does not show any lateral or vertical gradation in the deposits and does not change in intensity with respect to the distance from the emission center these alteration processes affect only the roque nublo ignimbrites their associated fall and surge deposits remain unaltered a geo autoclave mechanism is proposed to explain the alteration processes which devitrified the juvenile components of the roque nublo ignimbrites and gave the rocks their characteristic strong induration this implies that the eruptions which led to the formation of the roque nublo ignimbrites had an important phreatomagmatic component and that the resulting pyroclastic flows were too dense and too poorly expanded to permit the water vapour to be separated from the vitric fragments during transport the alteration took place in situ and immediately after the deposition of the deposits when they were still hot by the reaction between the vitric components and the condensed vapour and cannot be attributed to late diagenesis or weathering

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

cenozoic exhumation of the neoproterozoic sanfang batholith in south china the 1500 km long jiangnan tectonic belt in china is well known for large outcrops of proterozoic basement rocks although the exhumation history of the exposed basement is poorly constrained the thermochronology of the sanfang batholith in the southernmost section of this belt was used to constrain the timing and mechanism of exhumation using apatite fission track analysis of granite samples collected along three transects the results show that the ages of single apatite grains are much younger than the neoproterozoic crystallization age of the sanfang batholith the apparent age v elevation diagram shows two stage cooling during the paleogene and miocene the cooling stages are coincident with unconformities between the paleogene and underlying strata and between the neogene and the paleogene with movement on high angle normal faults a compilation of apatite fission track data from this and previous studies suggests that the batholith was exhumed in the cenozoic and the timing of rapid cooling is progressively younger to the east as a result of slab rollback leading to cenozoic back arc extension the exhumation of the sanfang batholith therefore indicates that cenozoic erosion and extensional tectonics played a key part in exhuming the basement rocks and forming the present architecture of the jiangnan tectonic belt

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

uranium metallogenesis in south china and its relationship to crustal extension during the cretaceous to tertiary south china is rich in vein type hydrothermal uranium deposits hosted in granitic volcanic and carbonaceous and siliceous pelitic sedimentary rocks the uranium deposits are spatially associated with extensional structures and or mantle derived mafic dikes both the uranium deposits and mafic dikes are cretaceous to tertiary in age temporally coincident with the crustal extension carbon isotope analyses of calcite deposited in the main stage mineralization in the veins from 12 representative uranium deposits yield 13c values of ore forming fluids mainly from 4 to 8 per mil which are permissive of a mantle origin for the co2 in the ore forming fluids a mantle origin is consistent with the association of the deposits with mafic dikes and the 3he 4he ratios of ore forming fluids e g 0 10 2 02 ra for the volcanic hosted xiangshan uranium deposit isotopic compositions of h and o demonstrate that water in the ore forming fluids is predominantly meteoric in origin ore forming temperatures ranged approximately from 150 to 250 c uranium rich crustal rocks in south china may have been the sources for the uranium crustal extension and associated mafic magmatism are considered to have heated the rocks and allowed co2 possibly from mantle sources to migrate upward and to mix with co2 poor meteoric water the co2 rich hydrothermal fluids mobilized uranium from the source rocks and then the uranium was deposited in various host rocks to form the uranium deposits

roque nublo caldera a new stratocone caldera in gran canaria canary islands abstract an inferred elliptic structure 5 4 km in diameter has been identified as the caldera that brought the roque nublo pliocene 5 3 5 ma volcanic cycle in gran canaria to an end this buried caldera whose morphological expression is no longer evident is now filled with almost 400 m of interbedded lacustrine sediments and nephelinite lava flows ashes and tuffs mapping of the postcaldera sedimentary layers which comprise debris flow lobes turbiditic sandstones and muds has permitted the paleogeographic reconstruction of a saucerlike volcanic depression this has been confirmed by a gravimetric survey whose data clearly define a double gravity minimum in the area of the inferred caldera some postcaldera vents are located near the caldera rim but most of them define a nw se line across the caldera which apparently marks an important tectonic lineation the paucity of remains of roque nublo breccias in the caldera infilling suggests a powerful explosive terminal phase for the stratocone after which the center of the residual edifice collapsed to form the caldera the very early resumption of activity in the caldera is not a definite proof for a resurgent activity in the area whereas the roque nublo formation was formed after a quiescent period of almost four million years on gran canaria this event was soon followed by the third and last cycle of volcanic activity on the island

geodynamic scenario and structural styles of mesozoic and cenozoic basins in china the continental lithosphere of china is situated at the junction of the marginal pacific and tethys himalayan tectonic domains the most manifest tectonic movements responsible for the deformation of the continental lithosphere of china are taphrogenic collisional and shearing these movements played an important role in the mesozoic and cenozoic tectonic evolution of china and formed various types of mesozoic and cenozoic basins in china the marginal pacific crust in eastern china was attenuated by rifting and developed north northeast trending mesozoic and cenozoic basins e g the songliao bohai bay and jianghan basins rifting mainly followed preexisting lineaments or shear zones in the basement then basin range graben horst structures and large fault bounded basins developed extensional structural styles have prevailed in mesozoic and cenozoic basins in eastern china with listric faults and tilted blocks in the basement and detached faults growth faults and rollover anticlines or drape folds in the cover rocks these structures are favorable for hydrocarbon accumulation thus the famous daqing and zhongyuan oil fields formed in songliao basin and bohai bay basin respectively in western china the continental crust thickened by collision and formed a series of mesozoic and cenozoic intermontane and foreland basins such as the junggar dzungaria tarim and qaidam basins contraction structural styles are predominant in mesozoic and cenozoic basins in western china with shortened fault blocks and ramps in basement and thrust faults and folds in the cover rocks the kelamayi and laojunmiao oil fields formed in these compressional basins in addition many great wrench faults played an important role in the evolution of china such as the tan lu fault in eastern china and the altunshan and talaso fergana faults in western china many mesozoic and cenozoic basins of china were characterized by transtension and transpression the mesozoic and cenozoic basins of china have undergone numerous tectonic events the extensional structures have been changed into contractional structures or conversely thus tectonic inversion occurred in successive events and different structural styles may be found in different tectonic levels of these basins these superposed deformations and structures have complicated petroleum exploration but they also enlarge prospective exploration targets and potential hydrocarbon reserves in china

evidence of lateral asthenosphere flow beneath the south china craton driven by both pacific plate subduction and the india eurasia continental collision south china cenozoic basalts sccb are regionally distributed at the south east coastal area and can be grouped into three zones a western zone 38 ma a central zone 17 8 ma and an eastern zone 8 ma leaving a temporal gap at 38 17 ma between the western and central zones an eastward migration of the sccb could therefore be identified by the systematic decrease in the eruption age from the western inland to the eastern coast we propose that most sccb were associated with the lateral asthenospheric flows moving along the lithosphere asthenosphere boundary lab beneath south china and the subsequent decompressional partial melting the lateral asthenospheric flows had been pulled by the eastward retreat of the pan pacific plate subduction before 38 ma and have been pushed by the northward indentation of the indian plate into the eurasian plate since 17 ma

denudation history of onshore central vietnam constraints on the cenozoic evolution of the western margin of the south china sea abstract apatite fission track analysis is used to monitor the timing and rate of upper crustal cooling and denudation across central vietnam in response to rifting of the south china sea results show regional denudation in the syn and post rift phases of the south china sea occurred at a similar rate 40 10 m myr consistent with the sedimentation record in adjacent offshore basins this suggests that extension was not the dominant factor controlling cenozoic denudation probably because the study area was sufficiently distant from the actively rifted continental margin a significant change in regional cooling rate during the upper miocene inferred denudation rates also shift from 34 m myr to 390 500 m myr is linked to enhanced erosion and deposition of prograding sediments in adjacent offshore basins this shift in denudation is associated and contemporaneous with initiation of regional basaltic magmatism and development of topography

mesozoic tertiary exhumation history of the altai mountains northern xinjiang china new constraints from apatite fission track data abstract this study uses apatite fission track ft analysis to constrain the exhumation history of bedrock samples collected from the altai mountains in northern xinjiang china samples were collected as transects across the main structures related to palaeozoic crustal accretion events ft results and modeling identify three stages in sample cooling history spanning the mesozoic and tertiary stage one records rapid cooling to the low temperature part of the fission track partial annealing zone circa 70 10 c stage two records a period of relative stability with little if any cooling taking place between 75 and 25 20 ma suggesting the altai region had been reduced to an area of low relief support for this can be found in the adjacent junngar basin that received little if any sediment during this interval final stage cooling took place in the miocene at an accelerated rate bringing the sampled rocks to the earth s surface this last stage linked to the far field effects of the himalayan collision most likely generated the surface uplift and relief that define the present day altai mountains

kwangsian crustal anatexis within the eastern south china block geochemical zircon u pb geochronological and hf isotopic fingerprints from the gneissoid granites of wugong and wuyi yunkai domains abstract gneissoid granites were traditionally thought to be the components of the precambrian basement in the eastern south china block but twenty four gneissoid granite samples from the wugong northern wuyi southern wuyi and yunkai domains gave zircon u pb ages of 424 456 ma 410 457 ma 426 438 ma and 415 450 ma respectively the existence of the abundant kwangsian gneissoid granites which have incorrectly been assigned to the precambrian basement indicates that the so called precambrian stratigraphic sequence of the cathaysia block should be termed complex rather than group these gneissoid granites are peraluminous granites with relatively high al 2 o 3 mgo tio 2 feot and cao na 2 o but low cao al 2 o 3 mgo feot and rb sr ratios they exhibit strongly negative ba sr nb p and ti and positive pb anomalies in primitive mantle normalized diagrams and have initial 87 sr 86 sr ratios ranging from 0 70924 to 0 72935 and negative end t values from 6 4 to 11 4 similar to those of paragneissic enclaves and precambrian sedimentary rocks in the eastern south china block the zircons crystallized in the kwangsian gneissoid granites gave ehf t values ranging from 2 4 to 19 4 with the peak at 4 almost all grains clustering at 1 to 12 and hf model ages of 1 1 2 1 ga the synthesis of these geochemical and in situ zircon hf isotopic data indicates that the kwangsian gneissoid granites dominantly originated from proterozoic metapelitic and meta igneous components with insignificant input of juvenile mantle derived magmas a model of two stage crustal anatexis is proposed for the kwangsian granitic magma in the eastern south china block with 1 the formation of 460 430 ma granite through the breakdown of hydrous minerals under the condition of the doubly thickened crust and 2 the generation of the 430 400 ma granite accompanying promoted melting along a path of isothermal decompression due to the increasing thermal weakening for the collapse of the thickened crust the crustal anatexis has probably occurred in an intracontinental tectonic regime that was geodynamically linked to the far field response to the assembly of the australian indian plate with the cathaysia block during middle paleozoic kwangsian time

tectonics of south china key to understanding west pacific geology abstract south china is not a post caledonian platform but a composite of orogenic belts this late proterozoic mesozoic orogen is a collage of three continental fragments the three blocks are yangzi huanan and dongnanya and they are separated by the banxi nanpanjiang formerly xianggangzhe and gunanhai suture zones yangzi was separated from gondwanaland during late precambrian when an open ocean called banxi was present between the two continents tectonic processes at an active margin during sinian and early paleozoic time led to the genesis of an accretionary wedge complex the banxi melange and the huanan flysch nappes on the northern margin of huanan which was then the northern margin of the gondwana continent reorganization of plates during the devonian suspended subduction at the huanan active margin huanan was uplifted and unconformably overlain by transgressive deposits of devonian and or carboniferous age a remnant ocean the nanpanjiang sea still existed between yangzi and huanan and deep water sedimentation continued both at the southern margin of yangzi and at the northwestern margin of huanan the latter again became an active margin during late paleozoic when permian and triassic flysch sediments were deposited in foredeeps ahead of advancing nappes huanan and yangzi collided during the triassic and resulted in the deformation of the passive margin of the latter to form the yangzi deformed belt huanan was separated from a more southerly continent dongnanya in the devonian by seafloor spreading which created a late paleozoic and early mesozoic ocean gunanhai ocean a continuously deposited sequence ranging in age from devonian to triassic was laid down on the southern passive margin of huanan on the other side of the ocean the dongnanya permian strata include glacial marine deposits of gondwanaland affinity dongnanya became separated from gondwanaland during the late permian when it marched northward to be reunited with huanan the huanan passive margin sequence was deformed by folding and overthrusting after the late mesozoic collision of huanan and dongnanya scattered outcrops of the suture melange resulting from this collision occur at a few localities in coastal fujian dongnanya in our model was the mesozoic continent of southeast asia peripheral to mainland asia this may or may not have been the eastern continuation of the microcontinent sibumasu siam thailand burma malaysia sumatra the collision of dongnanya and asia gave rise not only to the gunanhai melange of coastal fujian but also to the ophiolite melanges parts of which are present in taiwan tailuko the philippines and western malaysia

reappraisal of the ages of neoproterozoic strata in south china no connection with the grenvillian orogeny the jiangnan fold belt separates the yangtze and cathaysia blocks in south china and has long been considered grenvillian in age in order to place south china in central rodinia it consists of deformed early neoproterozoic strata that are unconformably overlain by undeformed late neoproterozoic strata and intruded by undeformed and unmetamorphosed granitic plutons zircons from the early neoproterozoic strata yield u pb ages as young as 830 ma and one granitic pluton has a zircon u pb age of ca 827 ma the 830 ma mafic rocks along the southeastern margin of the yangtze block have arc affinity geochemical characters whereas mafic rocks younger than 830 ma have typical ocean island basalt oib like compositions thus we suggest that the early neoproterozoic strata were deposited on an active continental margin prior to amalgamation of the yangtze and cathaysia blocks at ca 830 ma the overlying late neoproterozoic strata were deposited in the intracontinental rifted nanhua basin at 820 730 ma and probably reflect backarc spreading above the long lived 950 735 ma oceanic subduction zone along the northern and western margin of the yangtze block this model is consistent with the secular tectonic evolution of south china during the neoproterozoic the jiangnan fold belt is therefore not a grenvillian feature as previously suggested and there is no evidence to place south china in central rodinia instead we believe that south china was located in a marginal position relative to this supercontinent

chemical and textural controls on the formation of sepiolite palygorskite and dolomite in volcanic soils abstract a 3 m deep soil profile from gran canaria canary islands has been studied the profile is interesting because soil development on mafic tephra produced a pattern of clay mineral and carbonate distribution carbonates precipitated abundantly increasingly towards the bottom making up 20 90 wt of the soil sepiolite and calcite are dominant at the top whereas palygorskite and dolomite are the major components at the lower part of the profile quartz is present in low amounts in most of the profile and smectite and non crystalline partially altered tephra occur in low amounts towards the bottom mineralogical chemical and textural investigations indicate that sepiolite and calcite precipitated in the space between the original tephra grains from ca mg and si dissolved in situ and transported in the runoff palygorskite and smectite precipitated within tephra grains from in situ tephra weathering as well as dolomite the distribution of the clay minerals is due to clay composition and ion mobility sepiolite consisting only of si and mg precipitated outside tephra grains where the mobile si and mg ions were abundant in the interstitial waters palygorskite and smectite contain si al fe and mg and only precipitated within the tephra grains where the immobile al and fe were available calcite precipitation in solution is kinetically favored over that of dolomite and thus calcite precipitated between the grains dolomite precipitated within altering tephra grains and spherical clay structures because tephra and clay generated a viscous medium where dolomite supersaturation increased and the kinetic barrier for precipitation was overcome these results are relevant to the dolomite problem as they illustrate how dolomite can precipitate in surface conditions and to co 2 sequestration because dolomite can immobilize higher co 2 amounts both ca and mg are involved and not only ca and in a much more stable manner dolomite is eight orders of magnitude less soluble than calcite

chilled margin fragmentation as a trigger for transition from strombolian to phreatomagmatic explosive activity at cova de paul crater santo antao cape verde islands sudden transitions from effusive or mildly explosive eruptions to violent phreatomagmatic explosive activity can occur at high elevation vents on oceanic island volcanoes here we describe near vent deposits around the large cova de paul crater on santo antao cape verde islands which provide insights into the transition from strombolian magmatic to violently explosive phreatomagmatic activity during one such eruption the strombolian deposits change from welded spatter to bedded clinkery scoria to angular blocky scoria and contain a distinct population of strongly banded low vesicularity angular sub glassy clasts with strongly prolate vesicles these sub glassy clasts also form a discrete layer of larger blocks at the transition to the overlying phreatomagmatic deposits they are compositionally identical to the strombolian scoria and are interpreted to be fragments of flow banded chilled margins from the walls of the eruptive conduit the layer of large sub glassy blocks records disintegration and expulsion of the chilled margins at the transition to phreatomagmatic activity we interpret the sequence from spatter through clinkery scoria to blocky scoria as recording an increase in groundwater inflow into the conduit from pressurised aquifers but at coolant input ratios below those required for explosions the shattering of the chilled margins to produce the bed of large sub glassy blocks appears to have allowed a much greater water flow into the conduit leading to the phreatomagmatic explosive phase of the eruption

the role of the east asian active margin in widespread extensional and strike slip deformation in east asia east asia is a region of widespread deformation dominated by normal and strike slip faults deformation has been interpreted to result from extrusion tectonics related to the india eurasia collision which started in the early eocene in east and se china however deformation started earlier than the collision latest cretaceous to palaeocene suggesting that extrusion tectonics is not the only driving mechanism for east asia deformation it is suggested that the east asian active margin has influenced deformation in east asia significantly along the margin cenozoic back arc extension took place behind several adjoining arcs implying eastward rollback of the subducting slab and collapse of the overriding plate towards the retreating hinge line we show that extension took place along a c 7400 km long stretch of the east asian margin during most of the cenozoic physical models are presented simulating overriding plate collapse and back arc extension the models reproduce important aspects of the strain field in east asia for geometrical and rheological conditions scaled to represent east asia modelling shows that the active margin can be held responsible for deformation in east asia as far west as the baikal rift zone located c 3300 km from the margin

stratigraphy structure and geochronology of the las canadas caldera tenerife canary islands after a long period of subaerial fissure fed extrusions of basaltic magmas 12 to 3 ma volcanic activity was then concentrated in the central part of tenerife phonolitic magma chambers formed and a central volcanic complex was constructed the las canadas edifice the formation of a large depression the las canadas caldera truncated the top of the edifice the active twin strato cones teide pico viejo are sited in this depression the history of the las canadas caldera and edifice are established from stratigraphy geochronology k ar dates and volcanological studies two different groups are recognized separated by a major unconformity the lower group is dated at 2 to 3 ma and includes the products of several volcanic centres which together represent several cycles the upper group ranges from 1 56 to 0 17 ma and includes three different formations representing three long term 100 to 300 ka volcanic cycles the periods of dormancy between each formation were of 120 to 250 ka duration the las canadas caldera is a multicyclic caldera which formed over the period 1 18 0 17 ma each cycle of activity represented by a formation culminated in caldera collapse which affected different sectors of the las canadas edifice geological observation and geochronology support an origin by collapse into a magma chamber the minimum volume of pyroclastic ejecta is substantially greater than the present caldera depression volume 45 km3 but approaches the inferred volume of the original caldera depression 140 km3 after the formation of the caldera sector collapses could also occur at the northern flank of the volcano causing the disappearance of the northern side of the caldera wall

tectonothermal history of the ne jiangshan shaoxing suture zone evidence from 40ar 39ar and fission track thermochronology in the chencai region abstract the jiangshan shaoxing suture zone a component of the neoprotozoic orogenic belt between the yangtze and cathaysia blocks in south china has experienced a complex history of tectonic reactivation previous investigations of this significant tectonic boundary have focused on earlier stages of its tectonic history pre dating the early paleozoic wuyi yunkai orogeny with more recent episodes rarely addressed we here improve understanding of the later reactivation history of this suture zone through new 40ar 39ar and fission track thermochronological constraint derived for the ductilely strained chencai complex and the lipu huangshan quartz diorite together with sedimentological investigations of the proximal wuzao formation this combined constraint demonstrates the occurrence of as many as five discrete episodes of tectonic reactivation and deformation of the jiangshan shaoxing suture zone the 431 4 ma 40ar 39ar biotite cooling age of the lipu huangshan quartz diorite records retrograde cooling of the chencai region following high grade metamorphism at c 460 450 ma during the early paleozoic wuyi yunkai orogeny consistent younger 40ar 39ar ages of the gneissic chencai complex suggest that this retrograde cooling episode may extend to c 390 400 ma with later ductile strain and or thermal overprinting of biotite at c 386 ma suggesting that the terminal stage of the early paleozoic wuyi yunkai orogeny may have continued significantly later than previously assumed although not reflected directly in the thermotectonic records compiled for the chencai complex lithoclasts of these distinctive ductilely deformed gneisses represent a significant component of the conglomeratic gravels appearing in the late triassic wuzao formation demonstrating substantial uplift and erosion of the suture zone at this time coincident with the indosinian orogeny zircon fission track thermochronology of the samples from both lipu huangshan quartz diorite and chencai complex yield much younger cretaceous cooling ages of 144 8 to 103 6 ma with apatite fission track ages ranging from 66 4 to 41 3 ma and mean apatite fission track lengths between 12 1 and 13 1 m thermal history modeling of these low temperature controls demonstrates significant cooling during the cenozoic these later activities reflect brittle deformation in the upper crust and erosional exhumation of these rocks coincident with the yanshanian extension event and himalayan tectonism these results together imply that the jiangshan shaoxing suture zone represents a significant and long lived lithospheric weakness that was preferentially reactivated during at least two episodes of tectonic activity in regional far field domains examining the complex reactivation history of the fossil suture zone thus provides a useful window into the timing of episodes of broader tectonic activity across the wider region

a users guide to fission track dating calibration abstract misconceptions regarding the fission track dating method prompt the description of five handling procedures and the derivation of commonly used system calibrations problems of registration geometry inhomogeneous uranium distribution accumulated radiation damage and anisotropic etching largely invalidate all but the population and external detector methods and impose strict limitations upon even their routine application complexities of unique absolute thermal neutron dosimetry are illustrated by the calibration and use of the nbs pre irradiated glass srm 962 the 20 disparity in measured f decay constant values cannot be isolated from neutron fluence measurement and calibration frequently a calibration ratio f is evaluated against an age standard and then split into its component parts to illustrate this interdependence of f and zircon fission track ages in agreement with independent k ar ages are obtained by calculating the same track count data with each of the preferred values of f f 7 03 10 17 yr 1 and8 46 10 17 yr 1 together with appropriate selected neutron dosimetry schemes an alternative approach is presented formally relating unknown ages of samples to known ages of standards either by direct comparison of standard and sample track densities or by the repeated calibration of a glass against age standards practical recommendations are given for the reporting and critical assessment of fission track data