crustal scale mass transfer geotherm and topography at convergent plate boundaries end member cases for the fate of the crust at convergent lithospheric plate boundaries accommodated by subduction are considered combining i plate kinematics slab retreat vs slab advance and ii mechanical coupling decoupling within the subducting slab slab advance is accommodated by thickening of the overriding lithospheric mantle whereas slab retreat is associated with thinning crust mantle mechanical coupling results in continental subduction whereas decoupling leads to continental accretion the thermal and topographic evolutions of the associated orogenic belts are functions of the crust mantle thickness ratio resulting from the combined effects of crust mantle coupling decoupling and slab advance retreat crustal accretion associated with slab advance results in crustal thickening and contributes to an increase in radioactive heat production and buoyancy which is impeded by concomitant thickening of the conductive and dense lithospheric mantle in contrast crustal accretion combined with slab retreat results in thickening simultaneous with thinning of the lithospheric mantle this peculiar situation is the most favourable for generating a high temperature and buoyant orogenic belt

thickening and partial melting of the northern qinling orogen china insights from zircon u pb geochronology and hf isotopic composition of migmatites the widespread migmatites in the precambrian basement of the north qinling belt are products of partial melting of the earth9s crust and record the tectono thermal response to late paleozoic collision along the shangdan suture the morphology and trace element geochemistry of zircons from the migmatites suggest a metamorphic origin related to partial melting the zircons from leucosome melanosome and banded migmatites yield u pb weighted mean ages of 408 2 403 2 and 400 2 ma respectively constraining the timing of migmatization the granitic dyke intruded into the migmatites gives a zircon u pb weighted mean age of 396 2 ma constraining the minimum time of migmatization zircons from the melanosome and banded migmatite yield upper intercept ages of 1088 1061 ma representing the protolith ages of the basement of the north qinling belt the zircon rims of the melanosome and banded migmatite predominantly display negative and positive ehf t values respectively suggesting the variable input of mantle material together with the regional geology and previous geochronological data we propose that the thickening and partial melting in the north qinling belt was induced by the collision along the shangdan suture between the north and south china blocks from c 408 to 400 ma supplementary material la icp ms u pb data lu hf isotopic compositions and trace element geochemistry data of the zircons and ti in zircon thermometry are available at https doi org 10 6084 m9 figshare c 4521389

granite ascent and emplacement during contractional deformation in convergent orogens based on a case study in the central maine belt of west central maine u s a it is proposed that crustal scale shear zone systems provide an eaective focussing mechanism for transfer of granite melt through the crust in convergent orogens during contractional deformation flow of melt in crustal materials at depths below the brittle plastic transition is coupled with plastic deformation of these materials the flow is driven by pressure gradients generated by buoyancy forces and tectonic stresses within the oblique reverse central maine belt shear zone system stromatic migmatite and concordant to weakly discordant irregular granite sheets occur in zones of higher strain which suggests percolative flow of melt to form the migmatite leuco somes and viscous flow of melt channelized in sheet like bodies possibly along fractures cyclic fluctuations of melt pressure may cause instantaneous changes in the eaective permeability of the flow network if self propa gating melt filled tensile and or dilatant shear fractures are produced due to melt enhanced embrittlement in homogeneous migmatite and schlieric granite occur in zones of lower strain which suggests migration of partially molten material through these zones en masse by granular flow and channelized flow of melt carrying entrained residue founded on the central maine belt case study we develop a model of melt extraction and ascent using the driving forces stress conditions and crustal rheologies in convergent especially transpressive orogens ascent of melt becomes inhibited with decreasing depth as the solidus is approached for intermedi ate a h2o muscovite dehydration melting the water saturated solidus occurs between 400 and 200 mpa near the brittle plastic transition during high t low p metamorphism where the balance of forces favors sub horizontal fracture propagation emplacement of melt may be accommodated by ductile flow and or stoping of wall rock and inflation may be accommodated by lifting of the roof at shallower crustal levels and or sink ing of the pluton floor the resultant plutons have sub horizontal tabular geometries with floors that slope down to the ascent conduits although these plutons may have locally discordant relations with country rock structures when viewed at the crustal scale granite ascent and emplacement in convergent orogens are syn tec tonic processes 1998 elsevier science ltd all rights reserved

the thermal structure of collisional orogens as a response to accretion erosion and radiogenic heating thermal models of collisional orogens generally predict temperature structures that are much cooler than those recovered by thermobarometric studies here we demonstrate that high temperature low pressure metamorphism and the development of inverted geotherms within collisional belts may be the result of accretion and erosion acting on crust enriched with heat producing elements a new two dimensional finite difference model described here incorporates the subduction of lithosphere with heat producing material in the upper crust accretion of crustal material from the subducting plate to the upper plate and surface erosion of the upper plate these processes result in the development of a wedge of heat producing material within the upper plate the rate of heat production within the wedge and maximum depth of the wedge are the most important parameters controlling the magnitude of upper plate temperatures our model yields inverted upper plate geotherms when heat production rates exceed 0 75 w m3 and the heat producing wedge extends to a depth greater than 35 km temperatures in excess of 500 c at depths of 20 30 km are computed when heat production rates are greater than 1 75 w m3 and the wedge extends to a depth 50 km other processes such as shear heating fluid flow or mantle delamination need not be invoked to explain geologic evidence of high temperatures or inverted thermal gradients in collisional systems

the lamongan volcanic field east java indonesia physical volcanology historic activity and hazards abstract the lamongan volcanic field lvf in east java indonesia consists of up to 61 basaltic cinder or spatter cones a minimum of 29 prehistoric maars and a central compound complex comprising three main vents including the historically active lamongan volcano the field lies on the sunda volcanic arc and abuts onto the currently active tengger caldera semeru complex to the west between 1799 and 1898 lamongan was persistently active and discharged up to 15 lava flows of basaltic to basaltic andesite composition which is probably more than what was erupted by any other indonesian volcano in that period several seismic crises have been recorded in the lvf since lamongan s last eruption there are no existing records of cinder cone or maar forming eruptions in the area despite the possible hazards posed by future activity in the lvf the history and structure of the field have been subject to little scrutiny this contribution presents the results of a detailed morphometric and volumetric investigation of prehistoric vents and historic lava flows in the lvf using field data historic records satellite imagery and modelling of erupted volumes a typical lvf maar is characterised as a crater with a diameter of 450 m generated by an eruption with a model ejecta volume of 0 016 km 3 dense rock equivalent dre and a model juvenile component volume of 0 0096 km 3 dre diameter depth ratios of some maars display a morphological freshness comparable to recently formed phreatomagmatic eruption craters elsewhere the median lvf cinder spatter cone has a basal diameter of 600 m a height of 75 m a volume of 0 009 km 3 and a model total erupted volume of 0 014 km 3 variation in cone basal diameter to height ratios indicates that the age of some prehistoric cones may not exceed a few centuries and morphological classification identifies the youngest prehistoric vents on the northern flanks of the central edifice historic activity from vents on the western flanks of lamongan produced at least 0 05 km 3 of basaltic to basaltic andesite lava resurfacing rates at lamongan in the 19th century imply an age of 900 2100 years for the surface of the volcano the steady state eruptive rate from 1843 1898 is evaluated as 0 025 m 3 s 1 which is low in comparison to estimates from many presently active systems using eruptive rate and volume data the age of the lvf is estimated as between 13 and 40 ka placing it among the youngest dated indonesian volcanoes complex regional tectonics in conjunction with magmatic pressure have probably been the major influences on the distribution of volcanism in the lvf the occurrence of maar forming eruptions may have been controlled by a porous substrate possibly a deposit from the holocene iyang argapura complex to the east volcanic hazards from future eruptions in the lvf include base surges ash flows ashfall and ejected blocks associated with phreatomagmatic activity and ashfall and ejecta from magmatic eruptions

morphology and emplacement of an unusual debris avalanche deposit at jocotitlan volcano central mexico a pre historic collapse of the northeastern flank of jocotitlan volcano 3950 m located in the central part of the trans mexican volcanic belt produced a debris avalanche deposit characterized by surficial hummocks of exceptional size and conical shape the avalanche covered an area of 80 km2 had an apparent coefficient of friction h l of 0 11 a maximum runout distance of 12 km and an estimated volume of 2 8 km3 the most remarkable features of the jocotitlan debris avalanche deposit are the several steep 29 32 conical proximal hummocks up to 165 m high large tansverse ridges up to 205 m high and 2 7 km long situated at the base of the volcano and the steep 15 50 m thick terminal scarp proximal conical hummocks and parallel ridges that can be visually fitted back to their pre collapse position on the mountain resulted from a sliding mode of emplacement steep primary slopes developed as a result of the accumulation of coarse angular clasts at the angle of repose around core clasts that are decameters in size distal hummocks are commonly smaller less conical and clustered with more diffuse outlines field evidence indicates that the leading distal edge of the avalanche spilled around certain topographic barriers and that the distal moving mass had a yield strength prior to stopping in the ne sector the avalanche was suddenly confined by topographically higher lacustrine and volcaniclastic deposits which as a result were intensely thrust faulted folded and impacted by large clasts that separated from the avalanche front post emplacement loading also induced normal faulting of these soft locally water rich sediments the regional tectonic pattern n ne direction of flank failure and the presence of a major normal fault which intersects the volcano and is parallel to the orientation of the acambay graben located 10 km to the n suggest a genetic relationship between the extensional tectonic stress regime and triggering of catastrophic slope failure the presence of a 3 m thick sequence of pumice and obsidian rich pyroclastic surge and fall tephra directly overlying the debris avalanche deposit indicates that magma must have been present within the edifice just prior to the catastrophic flank failure the breached crater left by the avalanche has mostly been filled by dacitic domes and lava flows the youngest pryroclastic surge deposits on the upper flanks of the volcano have an historical c14 age of 680 80 yearsbp ad 1270 80 thus jocotitlan volcano formerly believed to be extinct should be considered potentially active because of its close proximity to mexico city 60 km the most populous city in the world reactivation could engender severe hazards

tectonic evolution of a composite collision orogen an overview on the qinling tongbai hong an dabie sulu orogenic belt in central china abstract the formation of collisional orogens is a prominent feature in convergent plate margins it is generally a complex process involving multistage tectonism of compression and extension due to continental subduction and collision the paleozoic convergence between the south china block scb and the north china block ncb is associated with a series of tectonic processes such as oceanic subduction terrane accretion and continental collision resulting in the qinling tongbai hong an dabie sulu orogenic belt while the arc continent collision orogeny is significant during the paleozoic in the qinling tongbai hong an orogens of central china the continent continent collision orogeny is prominent during the early mesozoic in the dabie sulu orogens of east central china this article presents an overview of regional geology geochronology and geochemistry for the composite orogenic belt the qinling tongbai hong an orogens exhibit the early paleozoic hp uhp metamorphism the carboniferous hp metamorphism and the paleozoic arc type magmatism but the three tectonothermal events are absent in the dabie sulu orogens the triassic uhp metamorphism is prominent in the dabie sulu orogens but it is absent in the qinling tongbai orogens the hong an orogen records both the hp and uhp metamorphism of triassic age and collided continental margins contain both the juvenile and ancient crustal rocks so do in the qinling and tongbai orogens in contrast only ancient crustal rocks were involved in the uhp metamorphism in the dabie sulu orogenic belt without involvement of the juvenile arc crust on the other hand the deformed and low grade metamorphosed accretionary wedge was developed on the passive continental margin during subduction in the late permian to early triassic along the northern margin of the dabie sulu orogenic belt and it was developed on the passive oceanic margin during subduction in the early paleozoic along the northern margin of the qinling orogen three episodes of arc continent collision are suggested to occur during the paleozoic continental convergence between the scb and ncb the first episode of arc continent collision is caused by northward subduction of the north qinling unit beneath the erlangping unit resulting in uhp metamorphism at ca 480 490 ma and the accretion of the north qinling unit to the ncb the second episode of arc continent collision is caused by northward subduction of the prototethyan oceanic crust beneath an andes type continental arc leading to granulite facies metamorphism at ca 420 430 ma and the accretion of the shangdan arc terrane to the ncb and reworking of the north qinling erlangping and kuanping units the third episode of arc continent collision is caused by northward subduction of the paleotethyan oceanic crust resulting in the hp eclogite facies metamorphism at ca 310 ma in the hong an orogen and low p metamorphism in the qinling tongbai orogens as well as crustal accretion to the ncb the closure of backarc basins is also associated with the arc continent collision processes with the possible cause for granulite facies metamorphism the massive continental subduction of the scb beneath the ncb took place in the triassic with the final continent continent collision and uhp metamorphism at ca 225 240 ma therefore the qinling tongbai hong an dabie sulu orogenic belt records the development of plate tectonics from oceanic subduction and arc type magmatism to arc continent and continent continent collision

zircon u pb ages and geochemistry of migmatites and granites in the foping dome evidence for late triassic crustal evolution in south qinling china abstract migmatites provide a record of melt formation and crustal rheology in this study we present zircon u pb ages and geochemical composition of migmatites from the foping dome and granites from the wulong pluton u pb results from migmatite zircons indicate two episodes of partial melting rim domains from a leucosome in the longcaoping area yield an age of ca 209 ma migmatites collected from the foping dome yield u pb zircon ages of 2910 to 190 ma suggesting the involvement of meta sedimentary source components rim domains of the zircons with low th u ratios 2 o and sr contents than the low sr y granites they also lack negative eu anomalies and are depleted in hree compared to the low sr y granites initial 87 sr 86 sr ratios and e nd values of all the samples roughly overlap with those of neoproterozoic basement rocks exposed in south qinling including previous studies we propose that the high and low sr y granites formed by melting of thickened and normal crust respectively close temporal spatial relationship of the high and low sr y granites with the two stage migmatization events implies variation of crustal thickness and thermal overprints of the orogenic crust in post collisional collapse following the collision of south qinling and the yangtze block prior to 219 ma partial melting of the deep crust occurred the melts migrated upwards to form the high sr y granites this process occurred rapidly and caused collapse of the thickened crust and carried heat upwards leading to further partial melting within the shallower crust and formation of the low sr y granites

the colima volcanic complex mexico part ii late quaternary cinder cones since the late pleistocene eleven cinder and lava cones have erupted on the floor of the southern colima graben ne and nw of the large active andesitic volcano colima scoria and lava samples from nine of the cones form a completely transitional basic alkalic series including basanites 9 leucite basanites 3 and minettes 15 the commonest variety of mica lamprophyre these samples represent primitive high temperature magmas with 47 6 50 3 sio2 7 4 15 3 mgo 2 5 4 4 k2o and 2 2 9 9 normative nepheline all members of this basic alkalic suite contain mg olivine fo75 94 chromite augite and late plagioclase and titanomagnetite the petrographic transition from basanite to minette is marked by the appearance of sanidine and the volatile bearing phases phlogopite apatite and analcime during late stages of crystallization as these phases increase in abundance presumably reflecting a rise in magmatic volatile content there are corresponding increases in the whole rock concentrations of 16 incompatible elements although these incompatible elements are relatively abundant even in the basanites many are highly concentrated in the minettes ba 4 200 ppm sr 3 100 ppm zr 550 ppm ce 190 ppm hf 18 ppm among the incompatible elements the degrees of enrichment in the minettes relative to the basanites decrease in the order h th ce la nd zr hf u ba sm eu pb p nb sr ti these enrichments may reflect the increasing importance of minor incompatible element rich mantle phases during partial melting the concentrations of alkali metals k na rb and cs do not correlate with these other elemental enrichments the leucite basanties have similar incompatible element contents to many minettes differing from them only in the presence of leucite rather than analcime and ti f rich groundmass phlogopite rather than hydrous phlogopite phenocrysts thus the leucite basanites represent relatively dry equivalents of minettes two of the eleven cinder cones are calc alkaline in nature and do not belong to the basanite minette group the easternmost cone is constructed of high al basalt and the northernmost of basaltic andesite the high al basalt 49 5 sio2 9 3 mgo 221 ppm ni closely approximates a parental magma to the post caldera andesitic suite of v colima 56 5 61 6 sio2 the basaltic andesite is relatively enriched in incompatible elements compared to the high al basalt v colima trend the ne normative basanite minette samples are highly enriched in incompatible elements while the contemporaneous hy qz normative calc alkaline suite encompassing the high al basalt and v colima s andesites is characterized by relatively low abundances of these elements no likely mineral assemblage can relate the alkaline and calc alkaline suites through crystal fractionation they probably represent fundamentally different melting events during the quaternary the main focus of andesitic volcanism in the southern colima graben has migrated southward with time volcan colima marks its present position 5 km south of the pleistocene volcano nevado de colima and another 15 km from the still older volcan cantaro the eruptions of basic alkalic magma probably occurred during the late stages of nevado s life and through the life of v colima they generally migrated from west to east with time towards v cantaro the most recent cone v apaxtepec is the only one east of the andesitic colima cantaro axis the oldest and the two youngest cones produced basanites while minettes dominated at cones of intermediate ages the cinder cone eruptions may have coincided with a phase of lamprophyre dike injection into plutons solidifying beneath the extinct volcanoes north of v colima the southern end of the colima graben can be viewed then as the volcanic analog of many classical post plutonic hypabyssal lamprophyre localities

petrogenesis of triassic granitoids in the fengxian taibai ore cluster western qinling orogen central china implications for tectonic evolution and polymetallic mineralization abstract the petrogenesis and tectonic setting of the triassic granitoids and their relationship with the coeval au and pb zn polymetallic mineralization in the western qinling orogen wqo in central china remain debated the fengxian taibai ore cluster ftoc in the eastern part of the wqo hosts large scale triassic granitoids and numerous polymetallic deposits providing an excellent opportunity to constrain the controversies mentioned above in this work we choose the huahongshuping granodiorite pluton guji biotite monzogranite pluton taoerling biotite monzogranite dike dongtangzi granite porphyry dike and dagou dioritic dikes in the ftoc for a combined study on the zircon u pb geochronology whole rock major trace elements and sr nd isotopes la icp ms zircon u pb dating yields ages of 225 ma 224 ma and 227 217 ma for the huahongshuping pluton guji pluton and granitoid dikes respectively all these granitoids have metaluminous and high k calc alkaline series affinities and are enriched in large ion lithophile elements lile light rare earth elements lree and depleted in high field strength elements hfse with a marked lree hree fractionation la yb n 10 66 32 42 fractionation of lree la sm n 2 72 6 60 and negative eu anomalies eu 0 45 0 99 they have initial sr isotopic ratios of 0 7062 to 0 7082 and negative end t values of 9 42 to 8 09 the above features indicate that the ftoc granitoids were mainly derived from the partial melting of the neoproterozoic meta volcanic rocks induced by the decrease in pressure and the addition of water dehydrated from biotite with direct contribution of mantle materials combined with previous data on sedimentation deformation and metamorphism and tectonics we suggest that the wqo underwent the early mesozoic tectonic evolution from the subduction at 248 235 ma through syn collision at 235 225 ma to post collision at 225 195 ma integrating with the ore geology geochronology mineralogy and quartz and carbonate h o and in situ s isotope data of sulfides from different mineralization stages at representative au and pb zn deposits in literatures we proposed a conceptive model that the post collision process and granitic magmatic activity played an important role in the polymetallic mineralization within the ftoc

neodymium isotopes in seawater from the barents sea and fram strait arctic atlantic gateways abstract the neodymium concentration c nd and isotopic composition e nd in seawater have been determined in the water column at five sites in the barents sea fram strait area where most of the water exchange between the arctic ocean and the north atlantic takes place in the main arctic ocean inflow branch across the barents sea the concentration and isotopic composition c nd 15 5 pmol kg and e nd 10 8 are similar to those reported for the northeastern nordic seas which is consistent with this region being a source area for the arctic inflow due to the addition of nd from svalbard shelf sediments the c nd in the surface waters above 150 m in the fram strait inflow branch is higher by a factor of 2 and the e nd is shifted to lower values 11 8 in the stratified nansen basin where cold low salinity water overlies warmer atlantic water the c nd and e nd do not vary with the vertical temperature salinity structure but are essentially constant and similar to those of the atlantic inflow throughout the entire water column down to 3700 m depth which indicates that the nd is to a large extent of atlantic origin compared to the atlantic inflow water the nd in the major arctic ocean outflow the fram strait show higher c nd in the surface waters above 150 m and a higher e nd 9 8 throughout the entire water column down to 1300 m depth sources for the more radiogenic nd isotopic composition in deep water of the fram strait outflow most likely involve boundary exchange with sediments on the shelf and slope as the water passes along the canadian archipelago river water is a possible source in the surface water but it also seems likely that pacific water nd modified by interactions on the shelf is an important component in the fram strait surface outflow changes in the relative proportions of inflow of river water and flow of pacific water through the arctic ocean could thus influence the isotopic composition of nd in the north atlantic

archean lamprophyre dikes of the superior province canada distribution petrology and geochemical characteristics shoshonitic lamprophyre dikes of late archean age are a volumetrically minor yet widespread component of magmatism in the superior province and preferentially occur along major structures that delineate subprovince boundaries lamprophyre swarms are most prominently developed in fault graben where they are associated with alkali gabbro and trachytes and molasse sediments mineralogically the dikes are characterized by zoned phlogopite or hornblende phenocrysts restriction of feldspar to the groundmass globular segregations of k feldspar and calcite panidiomorphic texture and the sporadic presence of crustal xenoliths compositionally fresh dikes have contents of sio2 47 51 wt tio2 0 55 0 60 and p2o5 0 36 1 58 and k2o na2o ratios 1 5 2 3 and mg numbers 78 80 comparable to phanerozoic shoshonitic lamprophyres unaltered dikes are characterized by extreme enrichments of the lithophile elements k rb ba cs th u and light rare earth elements lree and enhanced abundances of cr 650 920 ppm co 37 42 ni 264 355 and sc 22 28 mid ocean ridge basalt normalized plots display 1 consistent enrichments of k rb and ba relative to sr and lree 2 variable enrichments of rb and ba relative to k and 3 troughs at ta nb and ti compared with fresh rocks altered samples have systematically lower k rb ba and sr contents but similar ree distributions and profiles from th yb and sc ni on normalized diagrams suites of post archean ultramafic lamprophyres in the superior province are enriched in large ion lithophile elements lile lree and compatible elements relative to their archean counterparts and lack troughs at ta nb and ti emplacement of the lamprophyres was diachronous from north 2705 ma to south 2674 ma probably being related to localized extension during the successive accretion of individual subprovinces archean lamprophyre magmas were derived from a depleted mantle source variably enriched in lile and lree by sediments and altered oceanic crust with small variable degrees of crustal contamination whereas the post archean lamprophyres lacked a plate margin setting

partial melting of the south qinling orogenic crust china evidence from triassic migmatites and diorites of the foping dome abstract the qinling orogen was ultimately formed by suturing of the south qinling and yangtze blocks but the exact timing of the final amalgamation of the two blocks has not been well established so far partial melting of the qinling orogenic continental crust resulted in the generation of migmatites and such rocks may help to decipher the chronology of such event in this paper we report u pb ages trace element and hf isotopic compositions of zircons from migmatites and diorite gneisses of the foping dome south qinling zircons from migmatites form anhedral grains of variable sizes that are characterized by complex trace element compositions based on zircon u pb ages the migmatites can be subdivided into two groups group 1 migmatites mainly retain triassic zircons with u pb ages of 214 211 ma and hf model ages of 1 46 ga in core and rim domains zircons from group 2 migmatites record both triassic 210 ma and neoproterozoic u pb ages analogous to igneous rocks of the wudang and yaolinghe groups exposed in south qinling zircons from the diorite gneisses yield u pb ages of 216 210 ma with hf isotopic composition tdm2 ages of 1 46 ga similar to the migmatites evidence from whole rock nd isotopic analyses also points to a similar genesis between migmatites and diorite gneisses it is proposed that group 1 migmatites were derived by melting of triassic diorites while group 2 migmatites were derived from neoproterozoic igneous rocks a major basement lithology of south qinling partial melting of the orogenic crust took place at 214 210 ma approximately consistent with the retrograde metamorphism of granulites exposed along the suture zone between the south qinling and yangtze blocks we suggest that the collision of these two blocks occurred prior to 215 ma and that the foping dome resulted from rapid collapse of an overthickened crust followed by partial melting enhanced by asthenospheric influx

neodymium isotope constraints on chemical weathering and past glacial activity in svalbard abstract neodymium nd isotopes in leached authigenic components of marine sediments have been increasingly used as a tracer of past ocean water masses despite the general assumption that the nd isotopic composition of solutes released during chemical weathering fingerprints the source rocks on continents preferential dissolution of easily dissolvable phases may result in significant deviations in nd isotopic composition between the solutes and the source rocks with potential implications for the utility of nd isotopes in paleoenvironmental studies here we present the nd isotopic compositions of leached and detrital fractions separated from bedrock and marine sediment samples from the svalbard archipelago our goal is to further understand the behaviour of nd isotopes during chemical weathering in glacial catchments and evaluate how glacier fluctuations and associated weathering congruency may have affected the export of dissolved nd isotope signatures to seawater our results confirm that terrestrial weathering on svalbard causes considerable nd isotopic decoupling between the leached and detrital fractions of fjord sediments end resulting from the preferential dissolution of marine precipitates in glaciated catchments dominated by sedimentary rocks we also show that the degree of nd isotopic decoupling has fluctuated in response to climate variability on svalbard during the holocene which is also as suggested by the occurrence of generally higher end values during periods of glacier advances in sediment cores retrieved from two different fjords dicksonfjorden and woodfjorden we posit that the high end values can be ascribed to incongruent chemical weathering of fresh rock flour produced by glacial abrasion this finding suggests that the degree of nd isotopic decoupling could be used as a new proxy for tracing glacial fluctuations and associated glacier derived nutrient inputs to the marine realm

the mechanism of ascent and emplacement of granite magma during transpression a syntectonic granite paradigm we propose a model for syntectonic ascent and emplacement of granite magma based on structural relations in part of the northern appalachians in the study area in western maine strain was distributed heterogeneously during devonian acadian transpression metasedimentary rocks migmatites at high grades record two contrasting types of finite strain in zones that alternate across strike rocks in both types of zones have a penetrative moderately to steeply ne plunging mineral elongation lineation defined by bladed muscovite fibrolite sillimanite at high grades in straight belts of enhanced deformation rocks have s l fabrics that record apparent flattening to plane strain apparent flattening zones afzs but rocks between these belts have l s fabrics that record apparent constriction apparent constriction zones aczs at metamorphic grades above the contemporary solidus rocks in afzs developed stromatic structure in migmatite which suggests that percolative flow of melt occurred along the evolving flattening fabric stromatic migmatites are intruded by concordant to weakly discordant m scale composite sheet like bodies of granite to suggest magma transport in planar conduits through the afz rocks inhomogeneous migmatite is found in the intervening aczs which suggests migration of partially molten material through these zones en masse probably by melt assisted granular flow inhomogeneous migmatites are intruded by irregular m scale bodies of granite that vary from elongate to sub circular in plan view and seem cylindrical in three dimensions these bodies apparently plunge to the northeast parallel to the regional mineral elongation lineation to suggest magma transport in pipe like conduits through the acz rocks we postulate that the form of magma ascent conduits was deformation controlled and was governed by the contemporaneous strain partitioning magma ascent in planar and pipe like conduits through migmatites is possible because oblique translation during contraction displaces isotherms upward in the orogenic crust to form a thermal antiform within this hot corridor it is the difference in temperature between melt producing reactions in the anatectic zone and the wet solidus for granite melt that enables magma to migrate pervasively up through the orogenic crust without congealing heat advected with the migrating melt promotes amplification of the thermal antiform in a feedback relation that extends the zone of plastic deformation and pervasive melt migration to shallower levels in the crust at the wet solidus we suggest melt flows obliquely toward axial culminations in the thermal antiform which are sites of melt accumulation and perturbations from which magma may escape to form plutons batches of melt that escape from these perturbations may be trapped by tectonic structures higher in the crust or ascent may become inhibited with decreasing depth by thermal arrest and solidification if the rate of arrival of subsequent melt batches exceeds the rate of crystallization at the site of pluton construction melt pressure ultimately may lead to sub horizontal magma fracture or viscous flow of wall rocks may allow lateral spreading the resultant plutons have sub horizontal tabular geometries with floors that slope down to the ascent conduit as the thermal antiform decays the height of sub solidus crust that separates the deepest part of the pluton from the anatectic zone increases consequently pluton inflation declines and solidification leads to infilling of the magma feeder channel to form a root zone that passes downward into migmatite which may explain the difficulty of determining precisely the floor to these deeper segments of large plutons using gravimetry

cortlandtitic enclaves associated with calc alkaline granites from tapia asturias hercynian belt northwestern spain abstract petrographic and mineralogical characteristics of amphibole olivine and pyroxene bearing ultramafic rocks from asturias nw spain are dealt with in this paper these rocks are of cortlandtitic type and occur as small rare enclaves in basic rocks related to hercynian calc alkaline post tectonic epizonal granites in the northwest of the iberian peninsula these particular ultramafic enclaves are characterized by poikilitic cumulate microtexture olivine fo 77 81 spinel from chromite to pleonaste composition enstatite subordinated diopside and sulphides are included in large brown calcic amphibole crystals displaying an irregular zonation phlogopite and plagioclase are also found in a much lower proportion between the large amphibole crystals mineral assemblage and chemical composition of minerals indicate formation conditions of 1150 c 7 8 kbar of p total and p h 2 o p total these rocks could represent the earlier products of fractional crystallization from a hydrated high alumina basalt involved in the genesis of the calc alkaline granites this basic magma would start crystallizing at a relatively deep level carrying up the first products of its crystallization during its ascent

neoproterozoic paleozoic and mesozoic granitoid magmatism in the qinling orogen china constraints on orogenic process abstract the qinling orogen is one of the main orogenic belts in asia and is characterized by multi stage orogenic processes and the development of voluminous magmatic intrusions the results of zircon u pb dating indicate that granitoid magmatism in the qinling orogen mainly occurred in four distinct periods the neoproterozoic 979 711 ma paleozoic 507 400 ma and early 252 185 ma and late 158 100 ma mesozoic the neoproterozoic granitic magmatism in the qinling orogen is represented by strongly deformed s type granites emplaced at 979 911 ma weakly deformed i type granites at 894 815 ma and a type granites at 759 711 ma they can be interpreted as the products of respectively syn collisional post collisional and extensional setting in response to the assembly and breakup of the rodinia supercontinent the paleozoic magmatism can be temporally classified into three stages of 507 470 ma 460 422 ma and 415 400 ma they were genetically related to the subduction of the shangdan ocean and subsequent collision of the southern north china block and the south qinling belt the 507 470 ma magmatism is spatially and temporally related to ultrahigh pressure metamorphism in the studied area the 460 422 ma magmatism with an extensive development in the north qinling belt is characterized by i type granitoids and originated from the lower crust with the involvement of mantle derived magma in a collisional setting the magmatism with the formation age of 415 400 ma only occurred in the middle part of the north qinling belt and is dominated by i type granitoid intrusions and probably formed in the late stage of a collisional setting early mesozoic magmatism in the study area occurred between 252 and 185 ma with the cluster in 225 200 ma it took place predominantly in the western part of the south qinling belt the 250 240 ma i type granitoids are of small volume and show high sr y ratios and may have been formed in a continental arc setting related to subduction of the mianlue ocean between the south qinling belt and the south china block voluminous late stage 225 185 ma magmatism evolved from early i type to later i a type granitoids associated with contemporaneous lamprophyres representative of a transition from syn to post collisional setting in response to the collision between the north china and the south china blocks late mesozoic 158 100 ma granitoids located in the southern margin of the north china block and the eastern part of the north qinling belt are characterized by i type i to a type and a type granitoids that were emplaced in a post orogenic or intraplate setting the first three of the four periods of magmatism were associated with three important orogenic processes and the last one with intracontinental process these suggest that the tectonic evolution of the qinling orogen is very complicated

isotope studies of granitoids from the bangenhuk formation ny friesland caledonides svalbard the caledonian hecla hoek succession in ny friesland eastern svalbard has been interpreted for many decades to be a continuous stratigraphic sequence early palaeozoic and neoproterozoic strata

the system tonalite peridotite h 2 o at 30 kbar with applications to hybridization in subduction zone magmatism we present data on the phase relationships of mixtures between natural tonalite and peridotite compositions with excess h2o at 30 kbar and on the composition of the piercing point where the peridotite tonalite mixing line intersects the l ga opx reaction boundary these data in conjunction with earlier analogous data along peridotite granite and basalt granite mixing lines permit construction of a pseudoternary liquidus projection that is relevant to interaction of peridotite with slab derived magmas knowledge of the liquidus phase and temperature for a range of compositions within this projection enables us to map primary crystallization fields for quartz garnet orthopyroxene clinopyroxene and olivine and to estimate the distribution of isotherms across the projection using this projection we explore the consequences of peridotite assimilation by mafic to intermediate basalt to dacite hydrous slab derived melts progressive assimilation under isothermal conditions results in garnet precipitation as the melt composition traverses the garnet liquidus surface and then garnet orthopyroxene crystallization once the melt reaches the l ga opx field boundary the melt is constrained to remain on this field boundary and further assimilation of peridotite simply results in continued precipitation of garnet orthopyroxene until the melt is consumed the product is a hybrid solid assemblage consisting of ga opx it is noteworthy that this process drives the melt composition in a direction nearly perpendicular to the mixing line between peridotite and the initial melt if assimilation occurs with increasing temperature as might occur if a slab derived magma rises into the hotter mantle wedge intermediate magmas e g andesites will again precipitate garnet until they reach the l ga opx reaction boundary at which point ga re dissolves and orthopyroxene precipitates as the melt composition moves up temperature along this boundary the product of this process is a hybrid solid assemblage with garnet subordinate to orthopyroxene for more mafic initial compositions e g basalts originally plotting in the cpx field it appears possible to avoid field boundaries involving garnet and shift in composition more directly toward peridotite if assimilation is accompanied by a sharp increase in temperature considering published ree evidence arguing against garnet playing a significant role in the genesis of many subduction related magmas in light of our results it appears unlikely that peridotite assimilation by intermediate magmas under conditions of constant or increasing temperature is an important process in subduction zones however if assimilation is accompanied by an increase in temperature our data do permit the derivation of high mg basalts from less refractory precursors e g high al basalts by peridotite assimilation

deep subduction of continental crust in accretionary orogen evidence from u pb dating on diamond bearing zircons from the qinling orogen central china abstract accretionary orogenic belts are considered the most important sites for the generation of continental crust however it is still not very clear whether in such settings the continental crust has been significantly transported back to the mantle accompanied with its lateral growth continental uhp rocks the best indicators of the recycling of the continental crust have rarely been discovered in accretionary orogenic belts which hinder the exploration of this essential process the possible occurrence of early paleozoic uhp metamorphism in the qinling orogen provides an excellent opportunity to address this question this article reports an integrated study of u pb age trace element and mineral inclusion of zircon from an amphibolite sample in the qinling orogen the zircon crystals show typical metamorphic growth zoning low th u ratios flat hree patterns and insignificant eu anomalies they give a weighted mean u pb age of 490 4 5 8 ma most importantly an in situ diamond inclusion was identified from one of the zircon crystals therefore the obtained 490 4 5 8 ma age was taken as registering the peak uhp metamorphism the discovery of in situ diamond inclusion provides important evidence for the uhp metamorphism of the qinling orogen and indicates that the north qinling microcontinent was subducted to mantle depths of 120 km when it collided with the erlangping arc attached to the southern margin of the north china block combining with previous results we suggested that the north qinling uhp terrane underwent fast exhumation from 120 km to ca 30 km within 10 myr intervals as a few examples of uhp metamorphism have been discovered in some accretionary orogens more attention needs to be paid to the occurrence of continental uhp metamorphism during accretionary orogenic process furthermore evident crustal growth has been documented in the north qinling orogen during the early paleozoic it is inferred that the accretionary orogenic belts may play important roles in recycling of continental crust into the mantle besides generation of continental crust

neoproterozoic tectonic evolution of south qinling china evidence from zircon ages and geochemistry of the yaolinghe volcanic rocks abstract the south qinling terrain is located at the northern margin of the yangtze block neoproterozoic volcanic sedimentary sequences of the yaolinghe group play the most significant role in studying the tectono magmatic evolution of the south qinling based on temporal and spatial geochemical variations the yaolinghe group can be divided into three volcanic cycles basaltic lavas of cycle 1 crystallized at 847 8 ma and exhibit typical island arc basalt characteristics with enrichment of liles th u and lrees depletion of hfses nb ta zr hf ti and homogeneous initial 143nd 144nd ratios ranging from 0 51162 to 0 51172 corresponding to initial nd values of 1 6 to 3 5 these features suggest that they were derived from a depleted mantle source during a subduction period volcanic rocks of cycle 2 show a complete lithological evolution from basalt through andesite to rhyolite without a significant gap of sio2 content they formed between 731 11 ma and 768 7 ma and display trace element signatures similar to arc volcanics with positive lile and negative hfse anomalies these basalts have lower initial 87sr 86sr ratios from 0 7030 to 0 7052 and higher initial 143nd 144nd ratios from 0 51172 to 0 51194 with positive initial nd values ranging from 0 9 to 5 3 whereas intercalated felsic rocks show lower initial 143nd 144nd ratios from 0 51150 to 0 51177 with initial nd values varying from 4 5 to 1 9 suggestive of metasomatic alteration of lithospheric mantle by slab derived fluids melts cycle 3 lavas show undepleted hfses nb ta zr hf and slightly enriched liles th u and ti concentrations they were generated in a rift related setting between 680 ma and 650 ma geochronological and geochemical evidence from lavas of the yaolinghe group suggest that the panxi hannan arc system extended to northern south qinling and the magmatism in the northern margin of the yangtze block evolved from arc to rift magmatism during the neoproterozoic

generation and emplacement of triassic granitoids within the songpan ganze accretionary orogenic wedge in a context of slab retreat accommodated by tear faulting eastern tibetan plateau china the eastern songpan ganze accretionary orogenic wedge northern tibetan plateau is characterized by extensive magmatism coeval with middle to upper triassic closure of the paleotethys ocean along a double subduction system traced by the kunlun anyemaqen suture to the north and the jinsha litang suture to the south new field petrographic and geochemical data on mesozoic plutons intrusive into triassic metasediments of the eastern songpan ganze wedge are presented and integrated in a review of available data for the region three types of granitoids are distinguished i high k calc alkaline granite to granodiorite yanggon maoergai markam sheng meng xue sheng taiyanghe menggu manai dusong xian tagong and jiulong plutons ii high k alkaline granite nyanbaoyeche and rilong plutons and the niuxingou shoshonitic syenite and iii peraluminous s type granite markam and manai leucogranites elemental and isotopic signatures of these granitoids are consistent with the implication of both crustal yangtze craton and songpan ganze metasediments and mantle asthenospheric mantle and metasomatised lithospheric mantle sources based on i the ages of the plutons that are concomitant with the end of the northern and southern subductions of the paleo tethys ii the position of the plutons intrusive in the crustal tectonically accreted units decoupled from the downgoing plate of this double subduction system iii the diversity of their petrological and geochemical signatures we propose that magma emplaced in the songpan ganze accretionary orogenic wedge were generated at the end of the paleotethys closure in a context of slab retreat accommodated by a tear fault along the passive margin of the south china block indeed we argue that this context is the most favourable to trigger synchronous partial melting of i the suprasubduction enriched mantle wedge owing to lowering of the mantle solidus by metasomatism ii the upwelled undepleted asthenospheric mantle into the tear fault owing to decompression iii the metasomatized lithospheric continental mantle of the yangtze craton owing to the temperature increase associated to the asthenospheric upwelling and iv the rocks of the songpan ganze accretionary wedge composed of metasediments and yangtze continental margin basement owing to the combined effects of increased radioactive heat production and mantle heat flux we propose that the concentration of plutons along the eastern margin of the songpan ganze results from focussed migration and emplacement of the granitic magmas guided by the development of a tear fault along the former passive eastern margin of the yangtze craton as a consequence of the paleotethys slab retreat

mineralogy petrology u pb geochronology and geologic evolution of the dabie sulu classic ultrahigh pressure metamorphic terrane east central china the dabie sulu triassic collisional orogen in eastern asia was created by northward subduction of the yangtze continental crust capped plate beneath the sino korean craton eclogites garnet peridotites and surrounding country rock gneisses and marbles were all subjected to in situ uhp metamorphism as indicated by the presence of rare but widespread coesite inclusions in eclogitic minerals and in zircon crystals in the country rocks as well as by virtually identical metamorphic ages of various uhp rock types metamorphic p t estimates combined with investigations of mineral exsolution textures and high p polymorphs indicate that recovered depths of continental subduction may have exceeded 200 km parageneses of mineral inclusions in zoned zircon domains combined with u pb ages delineate a well constrained p t time path suggesting exhumation rates of 5 10 km myr a similar p t time trajectory has been established for the microdiamond bearing kokchetav massif thus far however diamond inclusions have not been confirmed from coesite bearing zircon domains of dabie sulu uhp rocks despite numerous detailed studies oxygen isotopes of minerals from many outcrop samples and the chinese continental scientific drilling ccsd project main hole cores indicate that delta 18 o depletion took place in a volume of proterozoic protoliths exceeding 100 000 km 3 along the northern edge of the yangtze craton evidently passive margin sediments and bimodal igneous rocks that had formed during rifting and breakup of the supercontinent rodinia were subjected to extensive meteoric water rock interactions attending terminal neoproterozoic snowball earth conditions such hydrothermal alteration volatilized and depleted c from the relatively oxidized protoliths accounting for the rare occurrences of graphite and apparent lack of microdiamond in dabie sulu uhp rocks

petrology and p t evolution of garnet peridotites from central sulawesi indonesia alpine type orogenic garnet bearing peridotites associated with quartzo feldspathic gneisses of a 140 115 ma high pressure ultra high pressure metamorphic hp uhpm terrane occur in two regions of the indonesian island of sulawesi both exposures are located within nw se trending strike slip fault zones garnet lherzolite occurs as 28 kbar peak t of c 760 c meta morphic basement terrane which was recrystallized and uplifted in a n dipping continental collision zone at the southern sundaland margin in the mid cretaceous the low t low p and metasomatized spinel lherzolite precursor to the garnet lherzolite probably represents mantle wedge rocks that were dragged down parallel to the slab wedge interface in a subduction collision zone by induced corner flow ductile tectonic incorporation into the underthrust continental crust from various depths along the interface probably occurred during the exhumation stage and the garnet peridotites were subsequently uplifted within the hp uhpm nappe suvering a similar decompression history to that experienced by the regional schists and gneisses final exhumation from upper crustal levels was clearly facilitated by entrainment in neogene granitic plutons and or oligocene trans tension in deep seated strike slip fault zones

the petrology of poas volcano lavas basalt andesite relationship and their petrogenesis within the magmatic arc of costa rica abstract basalt andesite and dacite from poas volcano exhibit a distinct petrogenetic evolution the basalts al 2 o 3 16 17 4 k 2 o 1 43 1 61 are hypersthene normative tholeiites with phenocrysts of labradorite olivine and two pyroxenes orthopyroxene en 67 56 is generally rimmed by clinopyroxene fs 10 18 wo 39 48 in all basalts and in a few andesites pyroxene phenocrysts have al and ti rich salite rims that surround cores of orthopyroxene or augite having lower values the al z values of tetrahedral site occupied by al for these augites are above 6 thus defining on the al z vs tio 2 diagram an intermediate trend between arc related cumulates and rift related tholeiites and ophiolite layered cumulates moreover the presence of olivines with coronae of wormy intergrowths of orthopyroxene and ti magnetite similar to those occurring in metagabbros of the granulite facies and characteristic of arenal gabbroic xenoliths together with anorthite megacrysts an 94 91 suggest a possible contamination origin for these basalts sr isotope ratios for the basalts are between 0 703816 and 0 703849 whereas andesites and dacites have lower ratios 0 703676 0 703715 these relationships together with the mineral compositions suggest that the basalts may have formed from basaltic andesites by contamination with olivine gabbros and anorthosites of the lower crust pearce element ratio analysis confirms that the basalts had a different origin from the rest of the suite and that the dacite may be derived by crystal fractionation of the less silicic basaltic andesite with removal mainly of clinopyroxene and plagioclase

a gigantic bezymianny type event at the beginning of modern volcan popocatepetl the history of volcan popocatepetl can be divided into two main periods the formation of a large primitive volcano approximatively 30 km wide on which is superimposed a modern cone 6 8 km in diameter and 1700m high a major event of bezymianny type marks the transition between these two dissimilar periods the activity of the primitive volcano was essentially effusive and lasted several hundred thousands of years the total volume of products ejected by the volcano is of the order of 500 600 km3 its last differentiated magmas are dacitic a gigantic debris flow d f spread on the southern side is related to the bezymianny type event which destroyed the summit area of the ancient edifice an elliptical caldera 6 5 11 km wide was formed by the landslide its deposits with a typical hummocky surface cover 300 km2 for a volume of 28 30 km3 numerous outcrops belonging to this debris flow show slabs of more or less fractured and dislocated rocks that come from the primitive volcano these deposits are compared to two studied debris flows of similar extent and volume the mount shasta and colima s d f this eruption takes a major place in the volcanologic and magmatic history of popocatepetl pyroclastic products of surge type with laminites and crude layers ashflows and pumiceous airfall layers are directly related to this event and begin the history of the modern volcano probably less than 50 000 years ago in addition a second andesitic and dacitic phase rose both from the central vent forming the basis of modern popo and from lateral vents the terminal cone is characterized by long periods of construction by lava flows alternating with phases of destruction the duration of these episodes being 1000 to 2000 years the cone is composed of two edifices the first volcan el fraile began with effusive activity and was partly destroyed by three periods of intense explosive activity the first period occurred prior to 10 000 years b p the second from 10 000 to 8000 years b p and the third from 5000 to 3800 years b p each period of destruction shows cycles producing collapsing pyroclastic flows or nuees of the st vincent type related to the opening of large craters plinian air fall deposits and minor lava flows the second edifice the summit popo produced lava flows until 1200 years b p and since that time entered into an explosive period two cataclysmic episodes each including major pyroclastic eruptions occurred 1200 and 900 1000 years ago during the pre hispanic and historic times effusive activity was restricted entirely to the summit area alternating with plinian eruptions nevertheless despite the quiet appearance of the volcano the last period of pyroclastic activity which started 1200 years ago may not have ended and can be very dangerous for the nearby populations

the pleistocene cinder cones surrounding volcan colima mexico re visited eruption ages and volumes oxidation states and sulfur content located at the volcanic front in the western mexican arc in the colima rift is the active volcan colima which lies on the southern end of the massive 450 km3 colima nevado volcanic complex along the margins of this andesitic volcanic complex is a group of 11 scoria cones and associated lavas which have been dated by the 40ar 39ar method nine scoria cones erupted 1 3 km3 of alkaline magma basanite leucite basanite minette between 450 and 60 ka with 99 between 240 and 60 ka two additional cones both the oldest and calc alkaline erupted 0 003 km3 of basalt 0 5 ma and 0 003 km3 of basaltic andesite 1 2 ma respectively cone and lava volumes were estimated with the aid of digital elevation models dems the eruption rate for these scoria cones and their associated lavas over the last 1 2 myr is 1 2 km3 myr which is more than 400 times smaller than that from the andesitic colima nevado edifice in addition to these alkaline colima cones two other potassic basalts erupted at the volcanic front but 200 km to the ese near the historically active volcan jorullo and were dated at 1 06 and 0 10 ma these potassic suites reflect the tendency in the west central mexican arc for magmas close to the volcanic front to be enriched in k2o relative to those farther from the trench

caldera formation at volcan colima mexico by a large large holocene volcanic debris avalanche abstract about 4 300 years ago 10 km 3 of the upper cone of ancestral volcan colima collapsed to the southwest leaving a horseshoe shaped caldera 4 km in diameter the collapse produced a massive volcanic debris avalanche deposit covering over 1550 km 2 on the southern flanks of the volcano and extending at least 70 km from the former summit the avalanche followed a steep topographic gradient unobstructed by barriers resulting in an unusually high area volume ratio for the colima deposit the apparent coefficient of friction fall height distance traveled for the colima avalanche is 0 06 a low value similar to those of other large volume deposits the debris avalanche deposit contains 40 75 angular volcanic clasts from the ancestral cone a small proportion of vesicular blocks that may be juvenile and in distal exposures rare carbonate clasts plucked from the underlying surface by the moving avalanche clasts range in size to over 20 m in diameter and are brecciated to different degrees pulverized and surrounded by a rock flour matrix the upper surface of the deposit shows prominent hummocky topography with closed depressions and surface boulders a thick coarse grained compositionally zoned scoria fall layer on the upper northeastern slope of the volcano may have erupted at the time of collapse a fine grained surge layer is present beneath the avalanche deposit at one locality apparently representing an initial blast event most of the missing volume of the ancestral volcano has since been restored at an average rate of 0 002 km 3 yr through repeated eruptions from the post caldera cone as a result the southern slope of volcan colima may again be susceptible to collapse over 200 000 people are now living on primary or secondary deposits of the debris avalanche and a repetition of this event would constitute a volcanic disaster of great magnitude ancestral volcan colima grew on the southern trenchward flank of the earlier and larger volcano nevado de colima trenchward collapse was favored by the buttressing effect of nevado the rapid elevation drop to the south and the intrusion of magma into the southern flank of the ancestral volcano other such trenchward younging paired volcanoes are known from mexico guatemala el salvador chile and japan the trenchward slopes of the younger cones are common sites for cone collapse to form avalanche deposits as occurred at colima and popocatepetl in mexico and at san pedro volcano in chile

compositional polarity of triassic granitoids in the qinling orogen china implication for termination of the northernmost paleo tethys abstract the qinling orogen was formed from the closure of the northernmost paleo tethys sea and the tectonic suturing of the yangtze and north china cratons the timing of this collision and the tectonic framework are debated the widely developed triassic granitoids in the western qinling orogen offer a key to understand the tectonic evolution of this region here we compile the geological geochemical and geochronological data of the triassic granitoids from the qingling orogen and conclude that the granitoids north of the mian lue suture were emplaced in an active continental margin related to the northward subduction of the mian lue oceanic plate during 248 200 ma the granitoids can be classified into i and s types with the former constituting the major variety northward from the mian lue fault an s type granite belt appears followed by and locally overlapping with an i type granite belt the i type granite belt can be subdivided into four sub belts from south to north with increasing contents of k2o k2o na2o sio2 th and u and the ratios of k2o na2o rb sr and 87sr 86sr i but decreasing contents of na2o al2o3 mg and ehf t except for the northernmost belt which occurs in the huaxiong block of the north china craton this geochemical polarity of i type granitoids and the zoned distribution of s and i types granitoids cannot be explained through continental collision orogeny including syn to post collision but can be well interpreted by considering the progressive subduction of the northernmost branch of the paleo tethys as represented by the mian lue ocean thus the termination of the northernmost paleo tethys and the onset of the continental collision between the yangtze and the north china plates are considered to have occurred at about 200 ma during the transition from triassic to jurassic our model is also supported by the available data from other studies and provides a revised framework for the timing and tectonics of assembly of the yangtze and the north china cratons

lamprophyric lavas in the colima graben sw mexico the colima graben located in sw mexico is one of three grabens which intersect about 50 km ssw of guadalajara forming a triple junction the 90 km long 20 60 km wide colima graben represents a n s rift of the e w trending mexican volcanic belt since the early pliocene the colima graben has served as a locus for the eruption of alkaline lavas the most recent of which are basanites and minettes erupted from late pleistocene cinder cones luhr and carmichael 1981 in this paper we report on older alkaline lavas which crop out in the graben s walls these rocks include phlogopite and hornblende bearing lamprophyres a phlogopite kalsilite ankaratrite and high k andesites these lavas crop out throughout the colima graben area and are intimately associated with calc alkaline lavas in the field compared to these the alkaline rocks are strikingly enriched in the incompatible elements particularly ba sr p and the lree unlike the younger late pleistocene alkaline cinder cone lavas most of the graben wall lamprophyres and the high k andesites represent magmas that appear to have undergone significant evolution since their generation including fractionation crustal contamination and possible magma mixing least squares modeling indicates that the cinder cone minettes represent reasonable parental magmas for the graben lamprophyres the occurrence of these alkaline lavas in an active calc alkaline volcanic arc is unusual and we suggest that they are a manifestation of the rifting processes which produced the colima graben

migmatites granites and orogeny flow modes of partially molten rocks and magmas associated with melt solid segregation in orogenic belts abstract this paper presents a model for the genesis of migmatites and granites during orogenic evolution based on the analysis of several phanerozoic crustal segments and a review of the physical properties of partially molten rocks and magmas this model inventories the modes of bulk flow of partially molten rocks and magmas and the mechanisms of melt solid segregation for each mode partial melting of rocks is associated with a strength decrease of two to three orders of magnitude leading to strain partitioning expressed by channel flow driven either by forces related to plate tectonics vertical channel flow or by the gravity force associated with lateral variations of crustal thicknesses horizontal channel flow although the structural characteristics of migmatites indicate that deformation plays a role in melt migration the emplacement of laccoliths of leucogranites above migmatites attests to the efficiency of the buoyancy force another decrease in apparent strength of about ten orders of magnitude is associated with the loss of continuity of the solid framework marking the transition from partially molten rocks to magmas the breakdown of the solid framework also allows for settling of the solid in suspension increasing the buoyancy of the remaining magma accordingly domes cored by diatexites former heterogeneous magmas and mantled by metatexites former partially molten rocks are interpreted as gravitational instabilities driven by the relative buoyancy of the magma and permitted by the weakness of the partially molten rocks this model provides a first order framework to elucidate the development of a crustal scale horizontal layering during the evolution of orogenic belts that are affected by partial melting in this case the middle crust is dominated by migmatites with domes cored by diatexites and mantled by metatexites that correspond to a partially molten and magmatic zone respectively the granitic dikes and sills and the associated laccoliths of leucogranites correspond to an intrusive zone overlying the partially molten zone the refractory lower crust is potentially in part formed by accumulation of solids segregated from the overlying heterogeneous magmas

timing of collision of the north and south china blocks controversy and reconciliation the qinling orogen was formed by the joining of the north and south china blocks but the timing of their integration has been debated for more than a decade the controversies obviously stem from different approaches to reconstruction of the integration history two contrasting lines of evidence yield two different ages for collision of the north and south china blocks middle paleozoic and late triassic the shangdan suture within the qinling was regarded in previous studies as the trace along which the north and south china blocks collided our studies however demonstrate that there are two sutures within the qinling the well documented shangdan suture and the newly discovered mianlue suture we show in this paper that the late proterozoic to early mesozoic evolution of the qinling involved interactions between the north china block the north and south qinling orogens and the south china block the middle paleozoic collision along the shangdan suture as constrained by some evidence accreted only the south qinling orogen to the southern part i e the north qinling of the north china block contemporaneous rifting of the south china block and subsequent drifting separated the south qinling from the south china block during the middle to late paleozoic the separation of the south from the north china blocks is supported by other evidence in particular geomagnetic data evidently it was the late triassic collision of the south china block with the south qinling orogen along the mianlue suture that led to final integration of the north and south china blocks

experimental constraints on plagioclase crystallization during h2o and h2o co2 saturated magma decompression experiments simulating magma decompression allow the textures of volcanic rocks to be calibrated against known eruptive conditions interpretation of natural samples may be complicated however by both the decompression path and the composition of exsolving volatiles which affect the time evolution of crystal textures here we present the results of decompression experiments at elevated temperature and pressure designed to assess the effects of degassing path on crystallization of mount st helens rhyodacite three families of experiments were employed to simulate varied ph2o t trajectories single step h2o saturated decompression ssd continuous h2o saturated decompression cd continuous h2o co2 saturated decompression quantitative textural data crystal abundance number density and size are used to calculate plagioclase nucleation and growth rates and assess deviations from equilibrium in run products these are the first experiments to quantify feldspar nucleation and growth rates during h2o co2 saturated decompression we find that reducing the initial melt water content through addition of co2 increases nucleation rates relative to the pure water case an effect most pronounced at low dp dt moreover these early formed textural distinctions persist at the lowest pressures examined suggesting that deep h2o co2 fluids could leave a lasting textural fingerprint on erupted magmas crystals formed prior to decompression during the annealing process also modulate sample textures and growth on pre existing crystals contributes significantly to added crystallinity at a wide range of experimental conditions the phase assemblage itself is a dynamic variable that can be used in conjunction with textural data to infer conditions of magma ascent and eruption finally quantitative textural data from experimental samples are compared with those of natural pyroclasts erupted during the summer 1980 explosive effusive transition at mount st helens this comparison supports a model of magma ejected from multiple storage regions present in the upper crust following the may 18 plinian eruption such that subsequent eruptions tapped magmas that experienced varied decompression and degassing histories

the oldest known rocks in south western china shrimp u pb magmatic crystallisation age and detrital provenance analysis of the paleoproterozoic dahongshan group abstract the paleoproterozoic dahongshan group outcrops along the present day southwest margin of the yangtze craton it is the oldest rock found in the region and forms the basement for the late mesoproterozoic sedimentary successions the dahongshan group is a sequence of volcanic and sedimentary rocks which have undergone structural deformation and metamorphism to upper greenschist lower amphibolite facies some of the original sedimentary features such as unimodal cross bedding in quartzite and rounded conglomerate clasts are still preserved suggesting deposition in a fluvial environment abundant carbonate rocks also found within the sedimentary sequence indicate periods of shallow marine or lagoonal conditions a weighted mean shrimp 206pb 207pb age of 1675 8 ma for a tuffaceous schist unit confirms the paleoproterozoic formation age of the group shrimp and lam icp ms u pb analysis of detrital zircon grains from metasedimentary units indicate a late archean to paleoproterozoic ca 2780 1860 ma source region the age spectrum of the detrital zircons is similar to that of the late archean and early paleoproterozoic basement rocks previously identified in the yangtze craton but includes a population of paleoproterozoic grains ca 2400 2100 ma which have no known source region in the exposed yangtze craton

triassic diorites and granitoids in the foping area constraints on the conversion from subduction to collision in the qinling orogen china abstract the western qinling is characterized by triassic granitic plutons which are keys to constrain the timing of subduction and collisional processes between the north china and south china blocks this paper presents a set of new la icpms u pb zircon geochronology major and trace element geochemistry for several representative diorite and granitoid plutons in the foping area the diorites are characterized by high mgo al2o3 y and yb contents but low sio2 sr y and la yb ratios as well as depletion of nb ta p and ti suggesting derived from a subduction related setting la icpms zircon u pb age of 216 9 1 5 ma mswd 0 69 from the diorite constrains the time of the subduction event the medium and fine grained monzogranites as well as the biotite granites exhibit relative high sio2 mgo and rb low y and yb contents and low sr y and la yb ratios as well as various depletions of nb ta p and ti these features indicate a derivation from lower continental crust but had been influenced by slab released fluids in a syn collisional setting the medium grained monzogranite and biotite granite yield la icpms zircon u pb ages of 207 3 2 1 ma mswd 1 8 and 201 6 1 2 ma mswd 1 04 respectively proposing the syn collision during ca 207 201 ma geochemical and geological evidences suggest that the granodiorites intruding in the syn collisional monzogranites were related to the post orogenic collapse after the collision between the north china and south china blocks the la icpms u pb zircon age of 189 3 2 9 ma mswd 5 6 for the granodiorite constrains the time of the post orogenic collapse our geochronological and geochemical data provide important constraints on the triassic evolution from subduction to collision then up to post collisional collapse along the qinling orogen

geochemistry and zircon u pb hf isotopic systematics of the ningshan granitoid batholith middle segment of the south qinling belt central china constraints on petrogenesis and geodynamic processes abstract the ningshan granitoid batholith ngb is located in the middle segment of the south qinling belt sqb section of the qinling orogenic belt in shaanxi province china and consists of the laocheng yanzhiba and lanbandeng intrusions the laocheng intrusion is dominated by quartz diorites granodiorites and monzogranites with the yanzhiba intrusion being granodiorite dominated and the lanbandeng intrusion predominantly consisting of two mica monzogranites la icp ms zircon u pb isotopic dating indicates that the ngb formed during two distinct episodes of magmatism with an earlier episode at 222 216 ma and a later episode at 210 ma whole rock geochemistry and geochronology data have allowed us to divide the ngb into three groups with group 1 rocks consisting of laocheng intrusion quartz diorites and granodiorites and yanzhiba intrusion granodiorites that were produced in the early magmatic episode group 1 samples have high k calc alkaline compositions and are characterized by high mg values high sr cr and ni concentrations and high la yb n and sr y ratios but low yb and y concentrations implying that these rocks were formed from a mixed magma derived from melting of neoproterozoic crustal basaltic rocks and magmas sourced from a region of depleted mantle group 2 rocks are composed of the mid k calc alkaline monzogranites in the laocheng intrusion that were formed in the later magmatic episode and are characterized by lower mg values and lower cr ni and co concentrations than the group 1 samples but with high sr concentrations and la yb n and sr y ratios low y and yb concentrations negligible eu anomalies and a significant range in zircon ehf t values the highest zircon ehf t values of the group 2 samples plot close to the depleted mantle evolutionary line indicating that these rocks were derived from a mixed magma with both crustal and mantle derived components group 3 samples consist of high k calc alkaline two mica monzogranites of the lanbandeng intrusion that were formed during the later magmatic episode these rocks are characterized by high sio2 al2o3 and k2o concentrations with low mgo and sr concentrations low la yb n and sr y ratios nb ta p and ti depletions and significantly negative eu anomalies implying that the magmas from which these rocks formed were derived from partial melting of sedimentary rocks and minor amount of basalt the early magmatic episode associated with formation of the ngb occurred in a continental arc environment at 222 216 ma with 210 ma magmatism during a geodynamic transition from syn collisional to post collisional tectonics across the sqb

age differences between rbsr whole rock and upb zircon ages of syn and postorogenic svecokarelian granitoids in sottunga sw finland abstract the u pb age on pooled data of zircnos from the postorogenic svecokarelian mosshaga granite is 1788 11 ma 2 the corresponding rb sr whole rock age is 1742 34 ma 2 the mosshaga granite intrudes a synorogenic granodiorite which has a u pb zircon age of 1889 10 ma and a rb sr whole rock age of 1850 28 ma the age differences between the u pb zircon ages and rb sr whole rock ages are concluded to be geologically significant it is shown that the mosshaga pluton cooled rapidly and as a consequence the rb sr isotopic system closed at a time very close to the crystallization of the zircons a commonly observed feldspar and biotite alteration in svecokarelian granitoids indicates migration of rb and sr isotopes in the whole rock system which may lead to apparently younger rb sr ages

metamorphic growth and recrystallization of zircon distinction by simultaneous in situ analyses of trace elements u th pb and lu hf isotopes in zircons from eclogite facies rocks in the sulu orogen simultaneous in situ analyses of trace elements u th pb and lu hf isotopes were carried out on distinct domains of zircons in ultrahigh pressure uhp eclogite facies metamorphic rocks from the main hole of the chinese continental scientific drilling ccsd in the sulu orogen for the first time trace elements are directly linked to lu hf isotopes in metamorphic zircons with reference to their u pb dates this enables methodological integration to distinguish four types of metamorphic zircon solid state replacement and dissolution recrystallizations of protolith zircons and new growth from the aqueous fluid metamorphically grown zircons are characterized by concordant u pb ages for the metamorphism flat hree patterns typical of the garnet effect low contents of ree especially hree y nb ta and th u high contents of hf low lu gd n lu hf and th u 0 1 ratios and elevated 176hf 177hf ratios relative to solid state recrystallized zircons this suggests the effects of both garnet and fluid on the growth of metamorphic zircons in contrast metamorphic recrystallization has reset the u th pb isotope system of protolith zircons to different extents depending on the extents of fluid action during metamorphism solid state recrystallized zircons exhibit the lowest degrees of resetting and thus almost inherit all geochemical features from the protolith zircons which are characterized by discordant u pb ages close to or below the protolith age steep mree hree patterns typical of magmatic origin high contents of trace elements and their ratios and low 176hf 177hf ratios on the other hand dissolution recrystallized zircons show the highest degrees of reworking and thus have concordant or nearly concordant u pb ages for the metamorphism steep mree hree patterns lowered contents of trace elements such as ree th u y nb ta and ti relative to the protolith zircons and almost unchanged hf isotope ratios replacement recrystallized zircons display intermediate degrees of reworking and thus have their many features of elements and isotopes in between while the metamorphic growth in the presence of both garnet and fluid is characterized by both depletion of hree with flat pattern and the low contents of trace elements the metamorphic recrystallization in the presence of aqueous fluid is indicated by gradual decreases of mree to hree without the flat hree pattern therefore the simultaneous in situ analyses of metamorphic zircons have the advantage over single term analyses in making distinction between the new growth and the different types of recrystallization

zircon ages and nd isotopic and chemical compositions of orthogneisses from the black forest germany evidence for a cambrian magmatic arc single zircon u pb dating combined with 207pb 206pb ages obtained by the evaporation method constrains the emplacement of tonalitic trondhjemitic and granodioritic orthogneisses of the moldanubian zone in the black forest between 500 and 510 ma two detrital zircon populations of 1 9 and 1 6 ga indicate early middle proterozoic material in the former setting of the basement the initial end values range from 0 1 to 3 4 and mean crustal residence ages of 1 0 1 4 ga are consistent with involvement of early middle proterozoic crust and a subordinate juvenile component probably originating from subduction related melting of the mantle the orthogneisses have fractionated ree patterns and slightly higher k2o na2o ratios than typical low k tonalite trondhjemite granite suites the chemical data are interpreted as evidence for melting of amphibolite and contributions from evolved crust the emplacement of the orthogneisses was superceded by a high temperature metamorphic event at 480 ma which we interpret as a result of lithospheric thinning in a marginal basin behind a cambrian magmatic arc

upper mantle and crustal fragments in the ithaca kimberlites xenocrysts and xenoliths from three ithaca kimberlite localities are consistent with derivation from mantle depths of less than 150 km mantle mineral assemblages reconstructed from xenocrysts and rare 2 4 mineral fragments include spinel and garnet peridotites and garnet pyroxenites the cr rich garnets pyroxenes and spinels and the subcalcic diopsides commonly found in mantle xenoliths from other kimberlites are apparently absent secondary pargasitic amphibole attributed to mantle metasomatism is found associated with pyrope garnet al rich pyroxenes and spinels suggest that al spinel lherzolites of the type commonly found in alkali basalts but rarely found in kimberlites occur some of the spinels are the most aluminous ever reported from kimberlites these al spinel lherzolites suggest either a fossil high geothermal gradient or a very undepleted mantle in the ithaca region fragments of presumed crustal origin include a suite of mg rich minerals which may belong to a granulite carbonate assembl

tholeiitic and calc alkaline magma series at adatara volcano northeast japan 2 mineralogy and phase relations abstract low alkali low k by gill 1978 or island arc by jakes and gill 1970 tholeiitic magma suites are closely associated with medium k by gill 1978 calc alkaline suites in the outer volcanic zone called nasu volcanic zone nvz of northeast japan chemistry and phase relations of phenocrysts are studied for representative 5 low alkali tholeiitic and 8 calc alkaline rock samples of adatara volcano in nvz in order to examine the mineralogical evolutionary mechanisms of the two series of magmas also clarified are differences in the magmatic temperature and oxygen fugacity between the two magma series the correlations between whole rock chemical compositions and mineralogy in the tholeiitic suite at this volcano are well documented supporting the argument that the members of this suite are related through fractional crystallization of observed phenycrystic phases part 1 fujinawa 1988 mineralogical observations in the calc alkaline suite are compatible with the views based on their whole rock chemical compositions although fractional crystallization process is dominant in the evolution of the calc alkaline magma intermittent mixing of small amounts of magnesian high temperature magmas operates concurrently temperature estimates with the two pyroxenes geothermometer range from 1080 c to 970 c in the tholeiitic suite whereas from 990 to 850 c in the calc alkaline suite temperature oxygen fugacity trends indicate that the low alkali tholeiitic magma evolves under conditions of higher temperature and lower oxygen fugacity than the calc alkaline magma in view of the fact that the hyperstene and augite phenocryst cores in the calc alkaline samples are compositionally distinguishable from the relevant phases in the tholeiitic samples it is unlikely that magma mixing played an essential role in the generation of the calc alkaline suite at adatara volcano

granite magma generation ascent and emplacement within a transpressional orogen crustal thickening during transpressive orogenesis may produce anatectic granites which may then localize deformation leading to transcurrent movement granites may be transported from sites of generation through the mid crust in dyke like channelways within relatively narrow strike slip shear zones which link to major fault zones in the upper crust extensional jogs within fault systems provide developing sites for the assembly of plutons from magma arriving from below the model is based upon observations from the cadomian belt of nw france which exposes sections through middle and upper levels of the late precambrian crust within different elements of the orogen the mechanism provides a favourable alternative to diapirism and explains the common collocation of granites and shear zone fault systems within orogenic belts

magma mixing origin for the post collisional adakitic monzogranite of the triassic yangba pluton northwestern margin of the south china block geochemistry sr nd isotopic zircon u pb dating and hf isotopic evidences petrogenesis of high mg adakitic rocks in intracontinental settings is still a matter of debate this paper reports major and trace element whole rock sr nd isotope zircon u pb and hf isotope data for a suite of adakitic monzogranite and its mafic microgranular enclaves mmes at yangba in the northwestern margin of the south china block these geochemical data suggest that magma mixing between felsic adakitic magma derived from thickened lower continental crust and mafic magma derived from subcontinental lithospheric mantle sclm may account for the origin of high mg adakitic rocks in the intracontinental setting the host monzogranite and mmes from the yangba pluton have zircon u pb ages of 207 2 and 208 2 ma respectively the mmes show igneous textures and contain abundant acicular apatite that suggests quenching process their trace element and evolved sr nd isotopic compositions 87sr 86sr i 0 707069 0 707138 and end t 6 5 indicate an origin from sclm some zircon grains from the mmes have positive ehf t values of 2 3 8 2 with single stage hf model ages of 531 764 ma thus the mmes would be derived from partial melts of the neoproterozoic sclm that formed during rift magmatism in response to breakup of supercontinent rodinia and experience subsequent fractional crystallization and magma mixing process the host monzogranite exhibits typical geochemical characteristics of adakite i e high la yb and sr y ratios low contents of y 9 5 14 5 ppm and yb no significant eu anomalies eu eu 0 81 0 90 suggesting that garnet was stable in their source during partial melting its evolved sr nd isotopic compositions 87sr 86sr i 0 7041 0 7061 and end t 3 1 to 4 3 and high contents of k2o 3 22 3 84 and th 13 7 19 0 ppm clearly indicate an origin from the continental crust in addition its high mg 51 55 cr and ni contents may result from mixing with the sclm derived mafic magma most of the zircon grains from the adakitic monzogranite show negative ehf t values of 9 4 to 0 1 with two stage hf model ages of 1 043 1 517 ma some zircon grains display positive ehf t of 0 1 3 9 with single stage hf ages of 704 856 ma these indicate that the source region of adakitic monzogranite contains the neoproterozoic juvenile crust that has the positive ehf t values in the triassic thus the high mg adakitic granites in the intracontinental setting would form by mixing between the crustal derived adakitic magma and the sclm derived mafic magma the mafic and adakitic magmas were generated coevally at late triassic temporally consistent with the exhumation of deeply subducted continental crust in the northern margin of the south china block this bimodal magmatism postdates slab breakoff at mantle depths and therefore is suggested as a geodynamic response to lithospheric extension subsequent to the continental collision between the south china and north china blocks

repetitive mixing events and holocene pyroclastic activity at pico de orizaba and popocatepetl mexico the holocene volcanic activity which built up the present terminal cones of pico de orizaba and popocatepetl in eastern mexico was characterized by repeated pyroclastic saint vincent type eruptions radiocarbon data show that these paroxysmal events occurred at more or less regular intervals and were followed by moderate activity producing ash and pumice falls and andesitic lava flows from the summit craters

late quaternary volcanic record from lakes of michoacan central mexico this paper describes the initial stages of the development of a tephrochronology for the region of the michoacan guanajuato volcanic field mgvf in central mexico there are two elements to this the geochemical characterisation of volcanic glass and the linkage of tephra deposits to eruptions of known age the mgvf is dominated by cinder cones and shield volcanoes which erupt only once there are only two stratovolcanoes multiple eruptions which are common elsewhere in the trans mexican volcanic belt tephras were sampled from sub aerial sites close to cones of known age and from lake sediment cores from the zirahuen patzcuaro and zacapu basins in the state of michoacan multiple samples were collected to ensure that each tephra was well represented the glass was analysed by electron microprobe and found to be calc alkaline in composition sio2 abundances varied from 52 to 75 full results are available at http www geo ed ac uk tephra the ages of the dated cones ranged from the 20th century ad to ca 17 000 14c years bp tephras from eruptions of el jabali 3840 14c years bp jorullo 1759 1774 and paricutin 1943 1952 have been identified in lake cores these provide a means of correlating between basins and have the potential to provide a basis for understanding the volcanic history of this area and for dating a wider range of sediment sequences

geochemistry of micas from the finero spinel lherzolite italian alps the finero lherzolite is distinct amongst the tectonically emplaced slices of mantle in the ivrea zone italian alps for its abundant coarse phlogopite an average composition sio2 39 9 tio2 0 97 al2o3 16 0 cr2o3 1 16 feo 2 73 mgo 24 5 nio 0 16 bao 0 31 na2o 0 58 k2o 8 7 rb2o 0 056 cl 0 03 f 0 10 wt is similar in fe cr ni ba and f cl to primary textured micas from coarse garnet lherzolite xenoliths from s africa but is higher in ti na rb and al and lower in halogens the distinct values of ti and fe for five specimens of finero peridotites demonstrate local spatial variation the overall ranges of tio2 0 5 1 7 and feo 2 3 3 6 fall within the range for secondary textured micas in peridotite xenoliths from s africa the finero micas are lower in both k rb and k ba than the primary and secondary micas from s africa and their mean values of k rb 110 220 and k ba 15 39 are lower than for almost all bulk rocks but fit well with the ranges of 109 180 and 12 49 for the high k lavas of the roman region

ultrapotassic dykes in the moldanubian zone and their significance for understanding of the post collisional mantle dynamics during variscan orogeny in the bohemian massif abstract we report mineral textures geochemistry and age relations of two ultrapotassic dykes from a dyke swarm in the western part of the moldanubian zone at contact to the tepla barrandian block the dykes have orientation almost perpendicular to the nne ssw trending central bohemian plutonic complex and cross cut metamorphic foliation in basement gneisses and migmatites based on mineral compositions and geochemistry the dykes show close relations to mg k syenite plutons in the moldanubian zone the two dykes are vaugnerite and syenite in compositions and contain talc pseudomorphs after olivine within a fine grained matrix that consists of k feldspar phlogopite with small amounts of clinopyroxene and accessory quartz apatite titanite and sulphides of fe cu ni the syenite porphyry dyke cross cuts the vaugnerite it contains quartz phenocrysts and comparing to vaugnerite has lower modal content of talc pseudomorphs the vaugnerite and syenite porphyry have high k 2 o 6 to 7 wt and mg number 0 66 0 74 but low cao and na 2 o contents the vaugnerite is markedly rich in p 2 o 5 2 wt and comparing to syenite porphyry has higher amount of mantle incompatible elements e g rb cs ba pb th u v and cr id tims analyses on titanite in vaugnerite and on zircon in syenite porphyry yielded 338 59 0 68 and 337 87 0 21 ma respectively mineral and bulk rock chemistry of the dykes indicates that the source magma was formed by a low degree of partial melting of a highly anomalous domain in the upper mantle which subsequently fractionated and was contaminated with crustal material during its ascent formation of ultrapotassic magma is explained by transcurrent shear zones in the mantle lithosphere that occurred due to block rotation and post collisional mantle dynamics initiated by slab break off and asthenosphere upwelling into the moldanubian accretionary complex during the variscan orogeny

zircon age and nd hf isotopic composition of the yunnan tethyan belt southwestern china the baoshan block of the tethyan yunnan southwestern china is considered as northern part of the sibumasu microcontinent basement of this block that comprises presumably greenschist facies neoproterozoic metamorphic rocks is covered by paleozoic to mesozoic low grade metamorphic sedimentary rocks this study presents zircon ages and nd hf isotopic composition of granites generated from crustal reworking to reveal geochemical feature of the underlying basement dating results obtained using the single zircon u pb isotopic dilution method show that granites exposed in the study area formed in early paleozoic about 470 ma pingdajie granite and in late yanshanian about 78 61 ma late cretaceous to early tertiary huataolin granite the early paleozoic granite contains archean to mesoproterozoic inherited zircons and the late yanshanian granite contains late proterozoic to early paleozoic zircon cores both granites have similar geochemical and nd hf isotopic charateristics indicating similar magma sources they have whole rock tdm nd values of around 2 000 ma and zircon tdm hf values clustering around 1 900 1 800 and 1 600 1 400 ma the nd hf isotopic data imply paleoproterozoic to mesoproterozoic crustal material as the major components of the underlying basement being consistent with a derivation from archean and paleoproterozoic terrains of india or nw australia both granites formed in two different tectonic events similarly originated from intra crustal reworking temporally the late yanshanian magmatism is probably related to the closure of the neotethys ocean the early paleozoic magmatism traced in the baoshan block indicates a comparable history of the basements during early paleozoic between the se asia and the western tethyan belt such as the basement outcrops in the alpine belt and probably in the european variscides that are considered as continental blocks drifting from gondwana prior to or simultaneously with those of the se asia

mixed magma pyroclastic surge deposits associated with debris avalanche deposits at colima volcanoes mexico the transition between the terminal cones and the ancestral edifices of nevado de colima and fuego de colima volcanoes is marked by the deposits of gigantic volcanic debris avalanches of the mount st helens msh or bezymianny type unusual mafic juvenile fragments and cauliflower bombs as well as juvenile fragments of mixed and more evolved composition are abundant in dune bedded pyroclastic surge deposits directly associated with these catastrophic events at both volcanoes at nevado these mafic juvenile fragments represent the most primitive magma ever erupted by the volcano sio2 52 50 the lavas directly preceding and following the debris avalanche event are silicic andesites sio2 59 at fuego these juvenile fregments have 56 sio2 the lavas from the upper parts of the caldera wall are dacites 65 sio2 whereas the terminal cone is composed of andesites 57 to 62 sio2 at nevado petrologic evidence for interaction of mafic magma with andesitic or dacitic magma in a high level magma chamber just before the eruption of pyroclastic surge deposits consists of 1 banded juvenile bombs of intermediate composition 2 the range of composition of these bombs from sio2 52 to 58 3 the presence of highly magnesian olivine with reaction rims 4 inverse zoning in clinopyroxene with strong mg enrichment towards the rim 5 resorption of plagioclase and 6 significant compositional heterogeneity in the vitric phase volcanic debris avalanche events at nevado and fuego de colima may thus correspond with major breaks in the petrological evolution of the volcanoes and the start of a new magmatic cycle injection of mafic magma into the presently perched viscous surface dome of the active fuego cone as occurred in 1818 and 1913 could enhance the likelihood of southward collapse of the flank of an already unstable edifice and it must be considered in future hazard assessment of this active volcano risk to life and property for the entire colima region associated with such catastrophic phenomena would be immeasurably greater in comparison with hazards related to the last explosive outburst in 1913 which resulted in emplacement of pyroclastic flows over uninhabited areas of the upper flanks of the volcano

pressure temperature dependent compositional variation of phlogopitic micas in upper mantle peridotites compositional variation of phlogopitic micas in upper mantle peridotites is reviewed phlogopitic micas in garnet peridotites are systematically lower in al or eastonite component than those in spinel peridotites the core of phlogopite megacryst and phenocryst of kimberlite is always lower in al than the rim it is apparent that al al si ratio or eastonite component in phlogopitic micas in ultramafic rocks is controlled by the equilibrium pressure and temperature in the upper mantle peridotites containing garnet or spinel the al al si ratio of phlogopitic mica decreases with increasing pressure at constant temperature phlogopitic mica is a potential thermo barometer in both garnet and spinel peridotite facies

na rich character of metasomatic metamorphic fluids inferred from preiswerkite in chromitite pods of the khoy ophiolite in iran role of chromitites as capsules of trapped fluids abstract we report a new occurrence of preiswerkite a rare na and al rich trioctahedral mica na analog of eastonite from a high al cr of chromite 0 4 chromitite pod of the khoy meta ophiolite in northwestern iran the preiswerkite is euhedral and associated with ca cr al garnet clinochlore titanite and calcite preiswerkite and associated minerals fill cavities in the chromitite pod and are absent in the surrounding peridotites the preiswerkite rich aggregate is possibly representative of aqueous fluid supplied to the mantle section of the khoy ophiolite peraluminous conditions which are indispensable for preiswerkite formation were established only in the pool of fluid trapped by the high al chromitite the fluid and preiswerkite precipitates were isolated from the peridotite by the chromitite capsule the na rich fluid was a precursor of the serpentinization fluid the fluid is an analog of the fluid released from the subducting slab which can precipitate jadeite under high pressure conditions

the colima volcanic complex mexico iii a 34 meter section of ash and scoria fall units has been studied on the upper ne flank of the active volcan colima charcoal and soil horizons are restricted to the topmost 12 m nine 14c dates show a smooth progression from 235 years near the surface to 8 300 years at 10 meters depth indicating a post pleistocene accumulation rate of 1 3 m 1 000 years at 6 km from the vent this figure allows an estimate of the magmatic eruption rate for fallmaterial of 0 31 km3 1 000 years less than 15 of colima s lava eruption rate an unusually thick and coarse grained scoria fall horizon at about 4 m depth in the section appears to have been produced by the caldera forming eruption of colima some 4 300 years ago

paragenesis and composition of laurite from chromitites of othrys greece implications for os ru fractionation in ophiolitic upper mantle of the balkan peninsula os rich laurite and erlichmanite are the dominant pgm inclusions in chromitites of the othrys ophiolite complex close association of the pgm sulfides with enstatite na rich pargasite clinopyroxene phlogopite and cu ni sulfides indicates crystallization at high temperature in the presence of an alkali rich fluid phase under relatively high sulfur fugacity because of the predominance of ru os ir phases the pgm assemblage of othrys is similar to that of chromitites located in the mantle unit of other ophiolite complexes of the balkan peninsula vourinos skyros island rhodope although it is distinguished because of the os rich composition of laurite and the presence of erlichmanite comparison among literature data for these complexes indicates that the os ru ratios vary consistently in laurite and bulk chromitite with respect to the chondritic os ru value this suggests that fractionation between the two elements occurred being apparently registered in the composition of laurite inclusions among other factors fluctuation of sulfur fugacity during fractionation as well as variation of the os ru ratio in the parent melts of the chromitites might be invoked to explain the ru os decoupling

zircon u pb ages and geochemistry of the wenquan mo bearing granitioids in west qinling china constraints on the geodynamic setting for the newly discovered wenquan mo deposit abstract east qinling is the largest porphyry molybdenum province in the world these mo deposits have been well documented in west qinling however few mo deposits have been discovered although granitic rocks are widespread recently the wenquan porphyry mo deposit has been discovered in gansu province which provides an insight into mo mineralization in west qinling in this paper we report pb isotope compositions for k feldspar and sulfides s isotope ratios for sulfides the results obtained from petrochemical study and from in situ la icp ms zircon u pb dating and hf isotopes the granitoids are enriched in lile and lree with ree and trace element patterns similar to continental crust suggesting a crustal origin the mg 40 05 to 56 34 and cr and ni contents are high indicating a source of refractory mafic lower crust the e hf t values of zircon grains from porphyritic monzogranite range from 2 9 to 0 6 and from granitic porphyry vary from 3 3 to 1 9 the zircons have t dm2 of 1014 to 1196 ma for the porphyritic monzogranite and 954 to 1224 ma for the granitic porphyry implying that these granitoids were likely derived from partial melting of a late mesoproterozoic juvenile lower crust the pb isotope compositions of the granitoids are similar to granites in south china showing that the magma was sourced from the middle lower crust in the southern qinling tectonic unit the pb isotopic contrast between the mo bearing granitoids and ores shows that the pb in the ore forming solution was derived from fractionation of a triassic magmatic system 34 s values of sulfides are between 5 02 and 5 66 similar to those associated with magmatic hydrothermal systems la icp ms zircon u pb dating yields crystallization ages of 216 2 1 7 and 217 2 2 0 ma for the granitoids consistent with a previously reported molybdenite re os isochron age of 214 4 7 1 ma this suggests that the mo mineralization is related to the late triassic magmatism in the west qinling orogenic belt in view of these geochemical results and known regional geology we propose that both granitoid emplacement and mo mineralization in the wenquan deposit resulted from the triassic collision between the south qinling and the south china block along the mianlue suture since triassic granitoid plutons commonly occur along the qinling orogenic belt the triassic wenquan mo bearing granitoids highlight the importance of the triassic tectono magmatic belt for mo exploration in order to apply this metallogenic model to the whole qinling orogen further study is needed to compare the wenquan deposit with other deposits

tectonics of the qinling central china tectonostratigraphy geochronology and deformation history abstract the qinling orogen preserves a record of late mid proterozoic to cenozoic tectonism in central china high pressure metamorphism and ophiolite emplacement songshugou ophiolite assembled the yangtze craton including the lower qinling unit into rodinia during the 1 0 ga grenvillian orogeny the lower qinling unit then rifted from the yangtze craton at 0 7 ga subsequent intra oceanic arc formation at 470 490 ma was followed by accretion of the lower qinling unit first to the intra oceanic arc and then to the sino korea craton subduction then imprinted a 400 ma andean type magmatic arc onto all units north of the northern liuling unit oblique subduction created silurian devonian wnw trending sinistral transpressive wrench zones e g lo nan shang dan and late permian early triassic subduction reactivated them in dextral transpression lo nan shang xiang shang dan and subducted the northern edge of the yangtze craton exhumation of the cratonal edge formed the wudang metamorphic core complex during dominantly pure shear crustal extension at 230 235 ma post collisional south directed shortening continued through the early jurassic cretaceous reactivation of the qinling orogen started with nw se sinistral transtension coeval with large scale early cretaceous crustal extension and sinistral transtension in the northern dabie shan it presumably resulted from the combined effects of the siberia mongolia sino korean and lhasa west burma qiangtang indochina collisions and pacific subduction regional dextral wrenching was active within a ne sw extensional regime between 60 and 100 ma an early cretaceous andean type continental magmatic arc with widespread early cretaceous magmatism and back arc extension was overprinted by shortening related to the collision of yangtze indochina block with the west philippines block strike slip and normal faults associated with eocene half graben basins record paleogene nne ssw contraction and wnw ese extension the neogene is characterized by normal faults and nne trending sub horizontal extension pleistocene quaternary nw se extension and ne sw contraction comprises sinistral strike slip faults and is part of the nw se extension imposed across eastern asia by the india asia collision

multi stage granitic magmatism during exhumation of subducted continental lithosphere evidence from the wulong pluton south qinling abstract detailed petrology and zircon u pb dating data indicate that the wulong pluton is a zoned granitic intrusive formed from successive increments of magmas an age range of at least 30 ma is recorded from the 225 235 ma quartz diorite on the pluton margin the ca 218 ma granodiorite in the intermediate zone and the ca 207 ma monzogranite at the pluton center all the granitoids display evolved sr nd pb isotopic compositions with 87 sr 86 sr i of 0 7044 0 7062 unradiogenic nd e nd t values of 6 1 to 3 0 nd model ages of 1 1 1 3 ga and moderately radiogenic pb compositions 206 pb 204 pb i 17 500 17 872 207 pb 204 pb i 15 513 15 549 208 pb 204 pb i 37 743 38 001 in combination with variations in zircon hf isotopic compositions with e hf t values in each stage span 12 units and the hf isotopic model ages of 800 1600 ma these features suggest that the granitoids might have been derived from the reworking of an old lower crust mixed with paleozoic and proterozoic materials the rocks also display an adakitic affinity with sr 479 973 ppm high sr y ratios mostly 60 and negligible eu anomalies eu eu 0 78 0 97 but low rb sr ratios low y 4 6 17 ppm hree yb 0 95 1 7 ppm yb lu 6 7 and dy yb 1 9 2 4 ratios suggesting the absence of plagioclase and presence of garnet amphibole in their residue considering a large gap among their crystallization ages we propose that the geochemical evolution from pluton margin to center was controlled mainly by melting conditions and source compositions rather than fractional crystallization mafic enclaves that were hosted in the quartz diorite and granodiorite are mainly syenogabbroic to syenodioritic in composition and are metaluminous and enriched in lree and liles but are depleted in hfse and display an evolved sr nd pb isotopic composition suggesting that they may have been derived from the partial melting of an enriched mantle lithosphere which was metasomatized by adakitic melts and fluids from a subducted continental crust in combination with the results of the triassic ultra high pressure metamorphic rocks in the dabie orogenic belt we apply a model involving the exhumation of subducted continental crust to explain the formation of the wulong pluton at the first stage a dense and refractory mafic lower crust that was trapped at mantle depth by continental subduction witnessed melting under high temperature conditions to produce the quartz diorite magma characterized by low sio 2 60 65 63 98 wt and high tio 2 0 39 0 86 wt the magma subsequently interacted with mantle peridotite leading to high mg 57 67 and the metasomatism of the overriding mantle wedge at the second stage an asthenosphere upwelling that was probably caused by slab break off at ca 220 ma melted the enriched sub continental lithospheric mantle sclm to produce mafic magmas represented by the mafic enclaves that are hosted in the quartz and granodiorite resulting in the partial melting of the shallower subducted crust and generating the granodiorite that is distinguished by high sio 2 69 16 70 82 wt high al 2 o 3 15 33 16 22 wt and a cnk values mostly 1 05 at the third stage the final collapse of the triassic qinling dabie orogenic belt at ca 215 205 ma caused extensive partial melting of the thickened orogenic lower crust to produce the monzogranite which is characterized by high sio 2 67 68 70 29 wt low tio 2 mostly

gneiss domes and orogeny many gneiss domes record positive feedback between decompression and partial melting of orogenic middle crust exhumed orogens are riddled with gneiss domes cored by migmatites that underwent dehydration melting during decompression the decreasing buoyancy associated with increasing melt fraction drives further decompression at near isothermal conditions as the partially molten crust rises diapirically this combination of processes may explain the generation and retention of large volumes of crustally derived melt recorded in many deep seated migmatite terranes and inferred for active orogens in exhumed orogens the signature of the rapid ascent of partially molten crust is a gneiss dome cored by migmatite granite the large volume of material involved in the vertical transfer of partially molten crust indicates that the formation of gneiss domes is an efficient mechanism for heat advection during orogenesis

ultramafic blocks from the ocean floor southwest of australia abstract samples dredged from the ocean floor near the junction of the naturaliste fracture zone and the diamantina zone 300 km southwest of cape leeuwin western australia contain 3 to 10 cm blocks of cr spinel lherzolites and al spinel olivine clinopyroxenites both assemblages show textural evidence for deformation and recrystallization with extensive development of kaersutite in one clinopyroxenite the blocks are enclosed by clay and fe mn oxide rich fragmental material which also contains a diverse detrital mineral suite and microfossils diopsides in the lherzolites are na and cr rich with marked similarities to those of lherzolite nodules in alkaline basaltic suites the al and ti rich diopsides of the clinopyroxenites are more magnesium than those of common pyroxene rich nodules the lherzolites are interpreted as upper mantle residues while the clinopyroxenites probably represent partly recrystallized cumulates from high temperature basaltic magmas traversing the lherzolite mantle both are thought to have been derived from an ultramafic body emplaced into the shallow crust near the cretaceous early tertiary continent ocean boundary off australia

particle sizes of andesitic ash fallout from vertical eruptions and co pyroclastic flow clouds volcan de colima mexico we report particle size distributions for andesitic ash fall from vertical eruptions and clouds above block and ash fl ows at volcan de colima mexico during 2004 2006 we analyzed 17 samples using laser diffraction 11 from vertical eruptions and 6 from block and ash fl ows copyroclastic fl ow ash vertical eruptions produce well sorted fall deposits whereas co pyroclastic fall deposits are poorly sorted with high proportions of very fi ne grained ash 30 m statistical analysis shows particle size distributions of vertical eruptions are more leptokurtic peaked than co pyroclastic samples deconvolution of grain size histograms shows that copyroclastic samples have at least one subpopulation with a mode of 8 3 8 7 2 4 3 1 m estimates of the number of particles in different size ranges show that co pyroclastic samples contain much greater numbers of very fi ne particles than vertical eruption samples our results provide no direct evidence that milling or comminution produces hazardous fi ne ash particles in pyroclastic fl ows but are consistent with that interpretation

heterogeneous sources of the triassic granitoid plutons in the southern qinling orogen an e w tectonic division in central china 1 the qinling orogen is an important orogenic belt formed by the collision between the north and south china blocks along the mianlue suture during the triassic the orogen is customarily divided into the western and eastern qinling terranes however the boundary has long been a matter of debate there are many triassic granitoid plutons in the orogen especially in south qinling along the southern part of the shangdan suture systematic analysis of u pb ages and hf isotopes in zircons frxom six granitoid plutons including guangtoushan gaoqiaopu xiba laocheng dongjiangkou and zhashui from west to east allows us to trace their formation ages and source regions all plutons yield ages that vary only from 218 to 211 ma however zircon hf isotope data for these triassic plutons cluster in two distinct groups granitoids from the western segment i e guangtoushan gaoqiaopu and xiba of the south qinling belt have negative ehf t values 20 9 to 5 2 and relatively old two stage hf model ages 1 58 to 2 57 ga in contrast those from the eastern segment i e laocheng dongjiangkou and zhashui show higher ehf t values 5 4 to 6 8 and younger two stage hf model ages 0 82 to 1 60 ga integrating these analyses with published sr nd pb hf isotopic data we suggest that the division between the western and eastern south qinling segments is located near the taibai chenggu line between the baoji chengdu railway and 108 e longitude the integrated data suggest that the western south qinling segment separated from the north china block during the paleoproterozoic and early mesoproterozoic and then switched into continental convergence with the eastern south qinling segment northern margin of the yangtze block during the late mesoproterozoic to early neoproterozoic finally the two segments amalgamated during the late neoproterozoic

eclogite origin and timings in the north qinling terrane and their bearing on the amalgamation of the south and north china blocks the amalgamation of south scb and north china blocks ncb along the qinling dabie orogenic belt involved several stages of high pressure hp ultra high pressure uhp metamorphism the new discovery of uhp metamorphic rocks in the north qinling nq terrane can provide valuable information on this process however no precise age for the uhp metamorphism in the nq terrane has been documented yet and thus hinders deciphering of the evolution of the whole qinling dabie sulu orogenic belt this article reports an integrated study of u pb age trace element mineral inclusion and hf isotope composition of zircon from an eclogite a quartz vein and a schist in the nq terrane the zircon cores in the eclogite are characterized by oscillatory zoning or weak zoning high th u and 176lu 177hf ratios pronounced eu anomalies and steep heavy rare earth element hree patterns the zircon cores yield an age of 796 13 ma which is taken as the protolith formation age of the eclogite and implies that the nq terrane may belong to the scb before it collided with the ncb the hf t values vary from 11 3 to 3 2 and corresponding two stage hf model ages are 2402 to 1495 ma suggesting the protolith was derived from an enriched mantle in contrast the metamorphic zircon rims show no zoning or weak zoning very low th u and 176lu 177hf ratios insignificant eu anomalies and flat hree patterns they contain inclusions of garnet omphacite and phengite suggesting that the metamorphic zircon formed under eclogite facies metamorphic conditions and their weighted mean 206pb 238u age of 485 9 3 8 ma was interpreted to date the timing of the eclogite facies metamorphism zircon in the quartz vein is characterized by perfect euhedral habit some oscillatory zoning low th u ratios and variable hree contents it yields a weighted mean u pb age of 480 5 2 5 ma which registers the age of fluid activity during exhumation zircon in the schist is mostly detrital and u pb age peaks at c 1950 to 1850 1800 to 1600 1560 to 1460 and 1400 to 1260 ma with an oldest grain of 2517 ma also suggesting that the nq terrane may have an affinity to the scb accordingly the amalgamation between the scb and the ncb is a multistage process that spans c 300 myr which includes the formation of the erlangping intra oceanic arc zone onto the ncb before c 490 ma the c 485 ma crustal subduction and uhp metamorphism of the nq terrane the c 430 ma arc continent collision and granulite facies metamorphism the 420 to 400 ma extension and rifting in relation to the opening of the palaeo tethyan ocean the c 310 ma hp eclogite facies metamorphism of oceanic crust and associated continental basement and the final 250 to 220 ma continental subduction and hp uhp metamorphism

late triassic high mg diorites of the wulong pluton in the south qinling belt china petrogenesis and implications for crust mantle interaction abstract high mg diorites provide important insights into crust mantle interaction during tectonic evolution of orogenic belt an integrated study including zircon u pb dating whole rock geochemistry and isotopic compositions of sr and nd was carried out on high mg diorites of the wulong pluton located in the narrowest part of the south qinling belt crystallized ages of 218 210 ma are obtained in combination with the published data these rocks are characterized with relatively high mgo 2 76 7 17 cr 68 7 310 ppm and ni 21 3 76 3 ppm contents most diorites exhibit high sr y and la yb ratios with negligible eu anomaly isotopic compositions of whole rock show enriched attributes 87sr 86sr i 0 7050 0 7071 end t 7 56 to 3 45 geochemical models indicate that these rocks did not result from melting of subducted slab magma mixing fractional crystallization or melts derived from delaminated lower crust reacted with mantle instead high mg diorites were generated through basaltic melts derived from asthenosphere mantle assimilating crustal components with crystallized phase of garnet bearing amphibolite afc together with regional geological setting of the south qinling belt we propose that the parental magma of these high mg diorites was probably originated from upwelled asthenosphere mantle at the base of thinned crust after delamination of thickened crust during late triassic the continental crust of the south qinling belt was thickened as a result of collision with the yangtze block during middle triassic then eclogitized lower crust collapsed and foundered into asthenosphere mantle due to partial melting of thickened crust during late triassic in response the upwelling and underplating of asthenosphere mantle took place at the base of continental crust moreover upwelling of hot mantle may provide the heat source for migmatites and initiate the doming of foping area

evolution of orogenic wedges and continental plateaux insights from crustal thermal mechanical models overlying subducting mantle lithosphere the links between an early phase of orogenesis when orogens are commonly wedge shaped and a later phase with a plateau geometry are investigated using coupled thermal mechanical models with uniform velocity subductionboundary conditions applied to the base of the crust and simple frictional plastic and viscous rheologies models in which rheological properties do not change with depth or temperature are characterized by growth of back to back wedges above the subduction zone wedge taper is inversely dependent on the ramberg number rm gravity stress basal traction increasing convergence velocity or crustal strength produces narrower and thicker wedges models that are characterized by a decrease in crustal viscosity from c to b with depth or temperature leading to partial or full basal decoupling of the crust from the mantle display more complex behaviour for models with a moderate viscosity ratio b c 10 1 the crustal wedges have dual tapers with a lower taper in the central region and a higher taper at the edges of the deformed crust a redaction in the viscosity ratio b c 10 2 is sufficient to cause a transition of the central wedge region to a plateau this transition depends on the basal traction therefore the thickness of the weak basal layer also affects the transition further reduction of the viscosity ratio b c 10 4 leads to full basal decoupling and the development of plateaux in all cases considered in most models the plateaux grow laterally at constant thickness between characteristic edge peaks associated with the transitions from coupled to decoupled lower crust where the crust is fully decoupled large scale model geometries for both depth and temperature dependent rheologies are similar with gravity driven flow concentrated in the low viscosity region however strong lateral temperature gradients within these models controlled by the interaction of horizontal and vertical thermal advection diffusion and heterogeneous thickening of the radioactive crustal layer lead to differences in the velocity and deformation fields between the two cases particularly at the plateau margins the results suggest that simple depth dependent viscosity models may be reasonable approximations for describing the large scale geometry of fully developed plateaux but that they are not appropriate for describing the internal features of large orogenic systems or the transition from wedge to plateau geometry

sector collapse of the sw flank of volcan de colima mexico the 3600 yr bp la lumbre los ganchos debris avalanche and associated debris flows abstract some 3600 years ago the sw flank of volcan de colima collapsed generating a debris avalanche that travelled 25 km to the armeria river and stopped against the cerro grande limestone escarpment the debris avalanche deposit covers an area of 48 km2 has an average thickness of 35 m and a volume of 1 7 km3 with an h l ratio of 0 13 along the la lumbre ravine the deposit forms a series of flat terraces up to 65 m thick a pyroclastic flow deposit overlies the debris avalanche deposit suggesting that the colima collapse was accompanied by a magmatic component the debris avalanche deposit dammed the armeria river forming a temporary lake that accumulated a water volume of 0 19 km3 the subsequent breaking of the dam formed a debris flow that travelled at least 20 km along the armeria river emplacing 15 70 m thick deposits the debris flow deposits cover a surface of 6 4 km2 and have a volume of 0 2 km3 the avalanche deposit contains fragments of carbonized wood that where dated at 3600 120 yr bp by komorowski et al 1997 this deposit underlies a younger debris avalanche 2505 45 yr bp and a debris flow deposit with pottery shards and charcoal dated at 1940 90 yr bp these pottery shards likely related to the capacha people 820 300 yr bc indicating that the region has been inhabited during the last 3000 years a future collapse of volcan de colima in the same direction may affect several towns for a total population of circa 40 000

eruptive and magmatic cycles at fuego de colima volcano mexico the fuego de colima volcano displays a pattern of eruptive cyclicity characterized by lava flows and or slow effusions in an open crater alternating with short explosive events historical accounts field investigations and petrological data allow us to refine our understanding of the relationships between recurrent acidic acidic andesite and mafic andesitic products and this eruptive behaviour the second to last 1818 1913 and present 1913 present day cycles are instructive in this respect the short explosive events in 1818 and 1913 correspond to the initial mixing stage of a differentiated magmatic body with a new mafic input mathematical modeling shows that the pyroclastic products of 1913 result from the mixing between an acidic andesite 61 5 sio2 and an olivine andesite 56 sio2 in various ratios the final andesite mixture is made of 38 basaltic andesite and 62 acidic andesite the long effusive extrusive phases correspond to the ensuing differentiation stage of the new magmatic body for example the 1961 lava flows were derived from the final mixed and homogenized magma after the eruption of 1913 by fractionation of 7 plagioclase 9 pyroxene clinopyroxene orthopyroxene and 0 7 titanomagnetite the fractionation of 11 plagioclase 9 pyroxene and 1 4 titanomagnetite is required to produce the composition of the 1986 summit lava dome from the same parent in conclusion at fuego de colima an eruptive cycle begins with a short violent explosive event related to a mixing process and continues with a long effusive phase characterized by magmatic differentiation this scheme differs from others that have been proposed which consider that the eruptive cycles end with an explosive event

dissolution reprecipitation of zircon at low temperature high pressure conditions lanzo massif italy abst r a c t an eclogite facies meta plagiogranite from the lanzo massif western alps italy contains crystals of zircon intimately associated with allanite zircon displays different microtextures ranging from pristine euhedral and magmatic to fractured porous varieties with mosaic zoning and pervasive recrystallization into euhedral microcrystals fractures and voids in the recrystallized zircon microcrystals are mainly filled by high pressure na rich pyroxene electron backscattered diffraction analysis revealed a similar crystallographic orientation for primary magmatic zircon crystals and microcrystals with less than 2 misorientation among neighboring microdomains the textural change is coupled with chemical and isotopic modifications recrystallized zircon domains contain significantly less th and light to mid ree but are richer in sr than magmatic zircon crystals magmatic zircon preserves the protolith u pb age of 163 5 1 7 ma whereas zircon microcrystals have a mean age of 55 1 ma the coexisting allanite also contains inclusions of na rich pyroxene and has chemical features elevated sr and ni contents and lack of eu anomaly indicating formation at high pressure despite being associated texturally with zircon allanite yields a younger th pb age of 46 5 3 0 ma suggesting that the lanzo unit remained at relatively high pressure conditions for 8 m y zircon recrystallization proceeded with volume reduction and loss of material to an alkaline metamorphic fluid that acted as the agent for a coupled dissolution reprecipitation process recrystallization occurred with minimum transport in a low strain environment and was not significantly enhanced by metamictization the source of the fluid for zircon recrystallization is most probably related to prograde devolatilization reactions in the surrounding serpentinite

whole grain evaporation for 207 pb 206 pb age investigations on single zircons using a double filament thermal ion source a technique has been developed and tested to analyse 207pb 206pb apparent ages by thermal evaporation of radiogenic lead directly from untreated whole zircon grains 0 3 mm the evaporation analyses are performed in the double filament arrangement of a thermal ion mass spectrometer thims the method is a powerful tool to distinguish between different lead components occurring in the same grain because differing activation energies of the competing lead components cause their sequential evaporation from the zircons the evaporation of test samples results in 207pb 206pb apparent ages in good agreement with u pb ages known from literature single zircons from a granite of the marble mountains california yield an age of crystallization of 1 410 30 ma ceylon zircons from heavy mineral bearing gravels yield 560 40 ma as age of crystallization of the pegmatitic gravel sources individuals from a heterogeneous zircon population of a diatexite from the southern schwarzwald sw germany indicate metamorphic zircon formation around 500 ma and the existence of middle proterozoic relics 1 95 0 05 ga

triassic granitoids of the qinling orogen central china genetic relationship of enclaves and rapakivi textured rocks abstract granitoids varying in composition from biotite amphibole granodiorite to monzogranite with rapakivi textures have recently been recognized in the qinling orogen central china the granitoids are magnesian calc alkalic and metaluminous a cnk 0 71 0 95 they contain many mafic to intermediate microgranular enclaves the enclaves are quenched mafic melts in granitic host magma as evidenced by fine grained sinuous margins against or transitional contacts with the host granitoids as well as disequilibrium textures and mineral assemblages u pb zircon dating of the granitoids yields 209 2 ma and 212 2 ma and the enclaves 210 2 ma and 211 2 ma respectively the host granitoids and enclaves show obvious silica gap but similar ree and trace element patterns the whole rock 87sr 86sr t end t and zircon ehf t values for the granitoids vary from 0 70516 to 0 70634 2 3 to 4 4 and 0 7 to 7 6 and for the enclaves from 0 70467 to 0 70600 1 1 to 4 0 and 1 2 to 8 8 respectively showing overlapped end t and zircon ehf t values all these suggest that the two magmatic systems represented by the host rock and enclave are probably derived from distinct sources with sufficient interaction or common origin but underwent different degrees of crustal contamination mineralogical and geochemical data point to enclave dissolution in silicic magma as a major process accounting for the origin of the rapakivi texture the possible bimodal magmatism suggested by the coeval granitoids and lamprophyre dykes combined with the structural pattern geochemical features and regional tectonics suggest a post collision setting for the qinling rapakivi textured granitoid plutons

primitive calc alkaline and alkaline rock types from the western mexican volcanic belt since the early pliocene small volumes of alkaline magmas have erupted in the western mexican volcanic belt mvb in close association with the volumetrically dominant calc alkaline magmas both suites include relatively rare primitive types characterized by high mg cr and ni contents primitive hypersthene normative basalts parental to the calc alkaline suite are found along the volcanic front but are absent at greater distances from the trench the volcanic front calc alkaline suites are typically hornblende bearing indicating relatively high water contents associated primitive alkaline magmas at volcanic front locations also contain hydrous minerals these nepheline normative suites include basanites phlogopite bearing minettes and other hornblende bearing lamprophyres these are probably the youngest and freshest lamprophyres yet discovered on earth and ideal samples for addressing the origin of this exotic class of rocks on extended morb normalized elemental plots all lamprophyres show patterns similar to the calc alkaline basalts but have overall enrichments of 2 to 25x in ti k p ba sr light rare earth elements and other incompatible elements the lamprophyres show the same strong relative enrichments of ba k and sr and depletions in ti and nb that characterize the calc alkaline rocks of western mexico and all subduction zones these similarities in relative element abundances support a common source region for all primitive magmas of the western mvb the primitive calc alkaline and alkaline rocks of western mexico show narrow overlapping ranges in sr nd and pb isotopic ratios also consistent with derivation of all magmas from a common source the presence of the relatively cold subducted cocos and rivera plates below the volcanic front demands that all primitive magmas originated in the depth range 30 to 75 km these magmas are enriched in ba k sr and other elements probably transported by hydrous fluids rising from the subducted slab several lines of evidence support the presence of phlogopite in the source region of western mvb magmas probably in the form of phlogopite rich veins cutting asthenospheric peridotite the first liquids formed upon partial melting of this veined source region will concentrate the vein component and generate the lamprophyres these lamprophyric melts rarely erupt in arc settings more commonly forming dikes in the arc crust eruption of lamprophyric magmas in the western mvb is favored by through crustal extensional fracture systems related to active rifting of the jalisco block from the n american plate larger volumes of calc alkaline basalt are generated by melting of the same source which dilutes the vein component with larger proportions of peridotitic wall rock

eruptive history of the colima volcanic complex mexico abstract the evolution of the colima volcanic complex can be divided into successive periods characterized by different dynamic and magmatic processes emission of andesitic to dacitic lava flows acid ash and pumice flow deposits fallback nuees ardentes leading to pyroclastic flows with heterogeneous magma plinian air fall deposits scoriae cones of alkaline and calc alkaline nature four caldera forming events resulting either from major ignimbrite outbursts or mount st helens type eruptions separate the main stages of development of the complex from the building of an ancient shield volcano 25 30 km wide up to two summit cones nevado and fuego the oldest caldera c1 7 8 km wide related to the pouring out of dacitic ash flows marks the transition between two periods of activity in the primitive edifice called nevado i the first one which is at least 0 6 m y old was mainly andesitic and effusive whereas the second one was characterized by extrusion of domes and related pyroclastic products a small summit caldera c2 3 3 5 km wide ended the evolution of nevado i two modern volcanoes then began to grow the building of the nevado ii started about 200 000 y ago it settled into the c2 caldera and partially overflowed it the other volcano here called paleofuego was progressively built on the southern side of the former nevado i some of its flows are 50 000 y old but the age of its first outbursts is not known however it is younger than nevado ii these two modern volcanoes had similar evolutions each of them was affected by a huge mount st helens type or bezymianny type event 10 000 y ago for the paleofuego and hardly older for the nevado ii the landslides were responsible for two horseshoe shaped avalanche calderas c3 nevado and c4 paleofuego each 4 5 km wide opening towards the east and the south in both cases the activity following these events was highly explosive and produced thick air fall deposits around the summit craters the nevado iii formed by thick andesitic flows is located close to the southwestern rim of the c3 caldera it was a small and short lived cone volcan de fuego located at the center of the c4 caldera is nearly 1500 m high its activity is characterized by an alternation of long stages of growth by flows and short destructive episodes related to violent outbursts producing pyroclastic flows with heterogeneous magma and plinian air falls the evolution of the primitive volcano followed a similar pattern leading to formation of c1 and then c2 the analogy between the evolutions of the two modern volcanoes nevado ii iii paleofuego fuego is described their vicinity and their contemporaneous growth pose the problem of the existence of a single reservoir or two independent magmatic chambers after the evolution of a common structure represented by the primitive volcano

contrasting geochemical and sm nd isotopic compositions of archean metasediments from the kongling high grade terrain of the yangtze craton evidence for cratonic evolution and redistribution of ree during crustal anatexis abstract twenty three clastic metasediments from the kongling high grade terrain of the yangtze craton south china were analyzed for major trace and rare earth elements and sm nd isotopic ratios associated dioritic tonalitic trondhjemitic dtt and granitic gneisses as well as amphibolites were also analyzed in order to constrain provenance the results show that the clastic metasediments can be classified into 3 distinct groups in terms of mineralogical geochemical and sm nd isotopic compositions group a is characterized by having no to slight negative eu anomalies eu eu 0 82 1 07 being high in cr 191 396 ppm and ni 68 137 ppm and low in th 3 3 7 8 ppm and ree ree 99 156 ppm these characteristics are similar to those of metasediments from archean greenstone belts in addition the group a metasediments have the value of the chemical index of alteration ciw close to felsic gneisses their sm nd isotopic ree and trace element compositions can be interpreted by mixtures of the dtt gneisses and amphibolites dating of detrital zircons from 2 group a samples by shrimp reveals a major concordant age group of 2 87 3 0 ga which is identical to the age of the trondhjemitic gneiss these results strongly suggest that group a was principally the first cycle erosion product of the local kongling dtt gneiss and amphibolite moreover the higher than amphibolite cr content and slight eu depletion exhibited by some samples from this group infer that ultramafic rocks like komatiite and granite of probably 3 0 3 3 ga in age also played a role group b is characterized by the presence of graphite and shows a more evolved composition similar to post archean shales with a prominent negative eu anomaly eu eu 0 48 0 77 and high ciw on paired cr th vs la co and co th plots group b samples conform to a two end member mixing line of the kongling granitic gneiss and amphibolite however data on nd model age and ciw suggest that the granite component should be younger than the sampled granitic gneiss and derived from a distal source both groups a and b exhibit a clear positive correlation between ciw and tdm and a negative one between ciw and eu eu these correlations point to the crustal evolution of the yangtze craton towards coupled increasing ciw and eu depletion with decreasing age this in turn reflects the change of granitoid magmatism from local na rich dioritic tonalitic trondhjemitic rocks to widespread k feldspar granite the change led to the intracrustal differentiation stabilization and growth of the craton group c is restite and contains abundant sillimanite and garnet and unusually high ilmenite 7 11vol which can be seen to be dehydration melting products of biotite under the microscope this group shows extremely varied ree distributions from lree enriched to depleted and from negative to strong positive eu eu 1 63 eu anomalies compared to groups a and b group c is severely depleted in na2o k2o lree rb and ba whereas tio2 co v sc and hree and y are considerably enriched this is accompanied by anomalous high sm nd 0 21 0 28 147sm 144nd 0 1361 0 1738 and 143nd 144nd 0 511589 0 511958 ratios tdm correlates clearly with sm nd ratio and 2 out of 3 samples give significantly older to unrealistic tdm 3 9 4 9 ga the results document redistribution of ree and an open behavior of the sm nd isotope system during the biotite dehydration melting of metasediments

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method

petrogenesis of late mesozoic high ba sr granites in the qiushuwan cumo orefield implications for the distribution of porphyry mo mineralization in the east qinling area of central china abstract the qinling dabie orogenic belt of china is well known for its large mo resources there have been many studies of the genesis of the mo deposits but the reasons for the intensive mineralization during the late mesozoic and the spatially uneven distribution of the resulting ore deposits are not fully understood deposits with reserves of 6 mt of mo occur predominantly in the southern margin of the north china craton ncc and are mostly late mesozoic porphyry deposits in comparison a few relatively insignificant and cu dominated porphyry ore deposits have been identified in the north and south qinling terranes this study presents new whole rock geochemical isotopic and zircon u pb age data for ore bearing granites associated with the qiushuwan cu mo deposit the largest porphyry deposit in the north qinling nq terrane which provide insights into the asymmetric distribution of mo mineralization in this region the rocks display high k calc alkaline to shoshonitic affinities and are metaluminous to weakly peraluminous with high ba and sr contents low rb nb and ta contents and depletion in heavy rare earth elements and y which are features typical of high ba sr granites zircon u pb dating indicates that the granites crystallized at ca 145 ma their narrow ranges of whole rock initial 87sr 86sr ratios 0 7056 0 7069 and end t 7 4 to 9 7 zircon ehf t 2 49 to 5 13 and 18o 5 58 to 6 49 values together with the presence of significant nd hf isotopic decoupling hf 7 61 to 11 3 indicate that the parent magma was derived from partial melting of enriched subcontinental lithospheric mantle with minor assimilation of the lower crust in contrast late mesozoic granitic rocks in the southern margin of the ncc including both mineralized and barren intrusions are geochemically associated with the crust of the northern margin of the yangtze block yb the yb contains mo rich shales that are thought to have been subducted beneath the southern margin of the ncc during the triassic melting of these mo rich black shales would have mobilized mo a process that enabled the formation of the extensive mo mineralization associated with the late mesozoic granites of the southern margin of the ncc this indicates that the extensive mo mineralization within the southern margin of the ncc is genetically related to a mo rich source that was generated by the stagnation of subducted yb continental crust beneath the southern margin of the ncc in comparison the less significant mo mineralization within the east qinling orogenic belt might reflect the variable removal of the subducted yb continental slab during post collisional intracontinental orogenesis

tectonics and volcanism of sierra chichinautzin extension at the front of the central trans mexican volcanic belt abstract because of its recent activity and position at the southern magmatic front of the trans mexican volcanic belt tmvb the sierra chichinautzin volcanic field scn is a key area for the understanding of this controversial volcanic province volcanic activity has built more than 220 monogenetic volcanoes shields scoria cones thick lava flows and hydromagmatic structures during the last 40 000 years for a total volume of about 470 km 3 the scn basalts are geochemically similar to oibs while the intermediate and felsic volcanic rocks show a calc alkaline trend and abundant evidence for magma mixing the structural analysis of this volcanic field and surrounding areas has been based on field data satellite images and a method for detecting volcanic center alignments the tectonic data together with geophysical evidence confirm active general n s extensional conditions with a strike slip component for the scn area the same structural setting that prevails in the rest of the central tmvb extensional tectonics a negative regional bouger gravity anomaly a low velocity mantle high heat flow and shallow seismicity suggest a rift type setting involving the upwelling of anomalous mantle beneath the central tmvb the combined petrological structural and geophysical arguments support that the scn volcanism is rift related and rule out processes involving the subduction of the cocos plate which casts further doubts on the standard subduction model for the tmvb volcanism